Handbook of Carbon Nanotubes 9783030913458

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Jiji Abraham Sabu Thomas Nandakumar Kalarikkal Editors

Handbook of Carbon Nanotubes

Handbook of Carbon Nanotubes

Jiji Abraham • Sabu Thomas • Nandakumar Kalarikkal Editors

Handbook of Carbon Nanotubes With 803 Figures and 86 Tables

Editors Jiji Abraham Department of Chemistry Vimala College (Autonomous) Thrissur, Kerala, India

Sabu Thomas International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, India

Nandakumar Kalarikkal School of Pure and Applied Physics Mahatma Gandhi University Kottayam, Kerala, India

ISBN 978-3-030-91345-8 ISBN 978-3-030-91346-5 (eBook) https://doi.org/10.1007/978-3-030-91346-5 © Springer Nature Switzerland AG 2022 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

Carbon nanotubes (CNT) have been found to be one of the most intriguing materials with unusual properties over the last two decades. It was discovered by great Japanese Scientist Iijima in 1991. Carbon nanotubes are often thought of as molecular-scale graphitic carbon tubes. They are classified as single-walled or multi-walled nanotubes depending on the number of carbon layers. Nanotubes have outstanding mechanical and electrical capabilities due to their unique structure. The exceptional features of these materials have opened up new and exciting study avenues in nanoscience and nanotechnology. Carbon nanotubes are a subject of intense research, with strong relevance to both science and technology. The aim of this Handbook was to provide an overview of the science, technology, and engineering aspects of CNT. Discovery, production of impactful carbon research, transition between research fields, and novel and emerging applications are included in this book. The book has been divided into three parts. Volume 1 deals with Carbon nanotube: Fundamentals and fascinating attributes. Volume 2 covers CNT-based polymer composites: Fabrication and characterization. Volume 3 includes recent advances in carbon nanotube structures for potential applications. Volume 1 begins with an introduction about the history and development of CNT. Characterization techniques of CNT, different properties of CNT, novel approaches for functionalization, and the current market for carbon nanotubes are explained well in this volume. Volume 2 focusses on CNT-based polymer nanocomposites. The first part of this volume deals with the structure-property relationship and fabrication of CNT-based polymer nanocomposites. Following this, Volume 2 fully focuses on the various techniques adopted for the characterization of CNT-based polymer nanocomposites. This includes morphological, thermal, mechanical, electrical, optical, dielectric and EMI shielding characteristics, etc. The last volume deals with the various emerging applications of CNT. Carbon nanotube research developments including published scientific documents and patents, synthesis, and production as well as risk associated with CNT research area are also discussed in the last part of this volume. Our book is a cutting-edge interdisciplinary book specifically focused on carbon nanotubes and related aspects. This book will be a very valuable reference source for graduates and postgraduates, engineers, research scholars (primarily in the field of material science, polymer chemistry, polymer physics, nanoscience and v

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nanotechnology, and biomedical field), material scientists, polymer engineers, and polymer technologists from industries. Almost all the analytical techniques are discussed in the book. Hence, it will be a first-rate reference for professors, students, industrialists, and scientists. This book will assist them to solve fundamental and applied problems in CNT-based polymer composites and devices for innovative applications. Many skilled carbon nanotube researchers collaborated on the creation of this book. The book does not present material in a linear fashion; rather, each chapter can be read as a mini-book on its own subject. We recommend that the reader read this book in the order of their interest in the subject matter. Kottayam, India September 2022

Jiji Abraham Sabu Thomas Nandakumar Kalarikkal

Contents

Volume 1 Part I Carbon Nanotube: Fundamentals and Fascinating Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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History of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Namburu Srikanth and Anitha C. Kumar

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2

Synthesis Methods of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . Atike Ince Yardimci and Nesli Yagmurcukardes

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Carbon Nanotube Growth Mechanisms . . . . . . . . . . . . . . . . . . . . . Takahiro Maruyama

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Chemistry and Physics of Carbon Nanotube Structures . . . . . . . . Yoshitaka Fujimoto

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Innovative Approaches in Characterization of Carbon Nanotube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Olusola Olaitan Ayeleru, Helen Uchenna Modekwe, Nyam Tarhemba Tobias, Matthew Adah Onu, Messai Adenew Mamo, Kapil Moothi, Michael Olawale Daramola, and Peter Apata Olubambi

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Optical Properties of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . V. S. Abhisha and Ranimol Stephen

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Thermal Properties of Carbon Nanotube . . . . . . . . . . . . . . . . . . . . Elham Abohamzeh and Mohsen Sheikholeslami

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Electronic Transport and Electrical Properties of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prabhakar R. Bandaru

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Electrical Properties of Carbon Nanotubes . . . . . . . . . . . . . . . . . . Xoan F. Sánchez-Romate, Alberto Jiménez-Suárez, and Alejandro Ureña

173 213

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Field Emission from Carbon Nanotube Systems: Material Properties to Device Applications . . . . . . . . . . . . . . . . . . . . . . . . . . M. Sreekanth, S. Ghosh, and P. Srivastava

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Physical Properties of Carbon Nanotubes K. C. Sivaganga and Titto Varughese

...................

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Functionalization of Carbon Nanotube . . . . . . . . . . . . . . . . . . . . . Abhinav Omprakash Fulmali, Sunil Kumar Ramamoorthy, and Rajesh Kumar Prusty

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Carbon Nanotubes: Dispersion Challenge and How to Overcome It . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mohsen Mohammad Raei Nayini and Zahra Ranjbar

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Covalent Functionalization of Carbon Nanotube . . . . . . . . . . . . . . Ritu Yadav, Krishan Kumar, and Pannuru Venkatesu

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Noncovalent Functionalization of Carbon Nanotubes . . . . . . . . . . Monika Matiyani, Mayank Pathak, Bhashkar Singh Bohra, and Nanda Gopal Sahoo

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Double-Walled Carbon Nanotubes: Synthesis, Sorting, and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anchu Ashok, Afdhal Yuda, Ibrahim M. Abu-Reesh, and Anand Kumar

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Heteroatoms-Doped Carbon Nanotubes for Energy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diab Khalafallah, Rajib Sarkar, Muslum Demir, Khalil Abdelrazek Khalil, Zhanglian Hong, and Ahmed A. Farghaly

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Carbon Nanotube–Based Hybrid Materials . . . . . . . . . . . . . . . . . . Vindhyasarumi, Akhila Raman, A. S. Sethulekshmi, Saritha Appukuttan, and Kuruvilla Joseph

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Growth Mechanisms in Carbon Nanotube Formation . . . . . . . . . . K. Raji and C. B. Sobhan

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Experimental and Theoretical Aspects of the Fragmentation of Carbon’s Single- and Multiwalled Nanotubes . . . . . . . . . . . . . . . . Sumera Javeed and Shoaib Ahmad

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The Current Market for Carbon Nanotube Materials and Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Helen Uchenna Modekwe, Olusola Olaitan Ayeleru, Matthew Adah Onu, Nyam Tarhemba Tobias, Messai Adenew Mamo, Kapil Moothi, Michael Olawale Daramola, and Peter Apata Olubambi

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Novel Approaches to Synthesis of Double-Walled Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Marianna V. Kharlamova

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Volume 2 Part II Carbon Nanotube based Polymer compositesFabrication and Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

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Structure–Property Relationships in Polymer Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Seval Hale Guler, Omer Guler, and Burak Dikici

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Manufacturing Techniques for Carbon Nanotube-Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. J. Rosemary

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Carbon Nanotube Composites: Critical Issues . . . . . . . . . . . . . . . . Nidhi Sharma and Bankim Chandra Ray

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Dispersion and Alignment of Carbon Nanotubes in Polymer Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Li-Zhi Guan and Long-Cheng Tang

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Semi-crystalline Thermoplastic/Carbon Nanotube–Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P. Russo and E. Gallo

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Thermoset/Carbon Nanotube-Based Composites . . . . . . . . . . . . . . A. M. Shanmugharaj

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Phase Selective Wetting of Carbon Nanotubes (CNTs) and Their Hybrid Filler System in Natural Rubber Blends . . . . . . . . . Hong Hai Le, Xuan Tung Hoang, and Sven Wiessner

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Latex-Based Carbon Nanotube Composites . . . . . . . . . . . . . . . . . . Esma Ahlatcioglu Ozerol, Michael Bozlar, Cem Bulent Ustundag, and Burak Dikici

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Morphological Characterizations Carbon Nanotube-Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G. L. Devnani, P. Lodhi, and Dhananjay Singh

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Carbon Nanotube–Based Nano-composites: Introduction, Mechanism, and Finite Element Analysis . . . . . . . . . . . . . . . . . . . . Piyush Kumar Patel and Vidya

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Carbon Nanotubes Embedded in Polymer Nanofibers by Electrospinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Priyanka Rani, M. Basheer Ahamed, and Kalim Deshmukh

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X-Ray Scattering Investigation of Carbon-Nanotube-Based Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sitaraman Krishnan

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Neutron Scattering Investigation of Carbon Nanotube-Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1017 Nidhi Joshi, Jagadeshvaran P L, Aishwarya Vijayan Menon, and Suryasarathi Bose

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Structural Investigation of Carbon Nanotube-Polymer Composites by FTIR, UV, NMR, and Raman Spectroscopy . . . . . . . . . . . . . . . 1043 Swetapadma Praharaj and Dibyaranjan Rout

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Mechanical Properties of Carbon Nanotube–Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1067 H. N. Dhakal and J. Jefferson Andrew

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Crystallization Behavior of Carbon Nanotube Polymer Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1089 Kumari Sushmita, Tanyaradzwa S. Muzata, Sankeerthana Avasarala, Poulami Banerjee, Devansh Sharma, and Suryasarathi Bose

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Self-Healing and Shape Memory Effects of Carbon Nanotube–Based Polymer Composites . . . . . . . . . . . . . . . . . . . . . . 1113 Sujasha Gupta and Bankim Chandra Ray

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Thermal Characterizations Carbon Nanotube-Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1147 Muhammad Safdar, Muhammad Zakiullah Shafique, Muhammad Suleman Tahir, Misbah Mirza, Sadia Zafar Bajwa, and Waheed S. Khan

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Nanocomposites Based on Polymer Blends and CNT Manan Tyagi and G. L. Devnani

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Dielectric and Electrical Conductivity Studies of Carbon Nanotube-Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1209 Anjaly Sivadas, H. Akhina, M. S. Mrudula, and Nithin Chandran

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EMI Shielding Studies of Carbon Nanotube-Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1235 Krishnendu Nath and Narayan Ch. Das

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Characterization of the Dynamic Response of CNT-ReinforcedPolymer-Composite (CNTRPC) Materials Based on a Multiscale Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1261 Rajamohan Ganesan and Jorge Palacios

. . . . . . . . . . 1187

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Biomedical Applications and Biosafety Profile of Carbon Nanotubes-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1301 Mansab Ali Saleemi and Eng Hwa Wong

Volume 3 Part III Recent Advances in Carbon Nanotube Structures for Potential Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Carbon Nanotubes: General Introduction . . . . . . . . . . . . . . . . . . . 1321 Sehrish Ibrahim, Shumaila Ibraheem, Ghulam Yasin, Anuj Kumar, Mohammad Tabish, and Tuan Anh Nguyen

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Carbon Nanotubes for Mechanical Applications . . . . . . . . . . . . . . 1335 Elham Abohamzeh, Mohsen Sheikholeslami, and Fatemeh Salehi

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Carbon Nanotubes for Energy Conversion and Storage . . . . . . . . 1369 Elham Abohamzeh and Mohsen Sheikholeslami

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Carbon Nanotube for Water Splitting and Fuel Cell . . . . . . . . . . . 1391 Lakshmanan Karuppasamy, Lakshmanan Gurusamy, and Jerry J. Wu

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Carbon Nanotubes for Solar Cells and Photovoltaics . . . . . . . . . . 1419 Elham Abohamzeh, Mohsen Sheikholeslami, Zainab Al Hajaj, and M. Ziad Saghir

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Carbon Nanotubes for Sensing Applications . . . . . . . . . . . . . . . . . 1451 Çağrı Ceylan Koçak, Şükriye Karabiberoğlu, and Zekerya Dursun

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Carbon Nanotube–Based Nanofluids . . . . . . . . . . . . . . . . . . . . . . . 1501 Mohamed Abubakr, Hussien Hegab, Tarek A. Osman, Farida Elharouni, Hossam A. Kishawy, and Amal M. K. Esawi

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Carbon Nanotubes for Nanoelectronics and Microelectronic Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1533 Anju K. Nair, Paulose Thomas, Kala M. S, and Nandakumar Kalarikkal

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Carbon Nanotubes for Photonics Applications . . . . . . . . . . . . . . . . 1557 Parvathy Nancy, K. V. Ameer Nasih, Sabu Thomas, and Nandakumar Kalarikkal

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Carbon Nanotubes Applications in Agriculture . . . . . . . . . . . . . . . 1579 Silvy Mathew and Cristiane P. Victório

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Carbon Nanotubes for Piezo Electric Applications Sherin Joseph, Anshida Mayeen, and Honey John

. . . . . . . . . . . . 1595

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Carbon Nanotubes: Thermal Applications . . . . . . . . . . . . . . . . . . . 1613 A. Shajkumar

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Carbon Nanotubes for Tissue Engineering Scaffold Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1629 R. Rajakumari, Sabu Thomas, and Nandakumar Kalarikkal

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Carbon Nanotubes for Drug Delivery Applications . . . . . . . . . . . . 1651 Sonali Batra, Sumit Sharma, and Neelesh Kumar Mehra

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Carbon Nanotubes for Bio-imaging Applications . . . . . . . . . . . . . . 1665 K. Sapna, J. Sonia, B. N. Kumara, A. B. Arun, and K. S. Prasad

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Carbon Nanotubes in Regenerative Medicine . . . . . . . . . . . . . . . . 1687 R. Krishnaveni, M. Naveen Roobadoss, S. Kumaran, A. Ashok Kumar, and K. Geetha

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Carbon Nanotubes in Cancer Therapy . . . . . . . . . . . . . . . . . . . . . . 1739 Ammu V. V. V. Ravi Kiran, Garikapati Kusuma Kumari, Praveen T. Krishnamurthy, Pavan Kumar Chintamaneni, and Sai Kiran S. S. Pindiprolu

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Carbon Nanotube as a Multifunctional Coating Material . . . . . . . 1773 Amir Rezvani Moghaddam and Zahra Ranjbar

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Hydrogels and Aerogels of Carbon Nanotubes . . . . . . . . . . . . . . . . 1827 Anju Paul and Arunima Reghunadhan

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Carbon Nanotubes for Environmental Remediation Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1845 Abdelmageed M. Othman and Alshaimaa M. Elsayed

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Antimicrobial (Antibacterial) Properties and Other Miscellaneous Applications of Carbon Nanotubes (CNTs) . . . . . . . . . . . . . . . . . . 1875 Olawumi Oluwafolakemi Sadare, Chioma Nnaji Frances, and Michael Olawale Daramola

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Carbon Nanotubes as Antimicrobial Agents: Trends and Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1903 Felipe de Jesús Barraza-García, Sandra Pérez-Miranda, José Gil Munguia-Lopez, Florentino Lopez-Urias, and Emilio Muñoz-Sandoval

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Multifunctional Applications of Carbon Nanotube–Based Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1923 S. S. Godara and Navneet Sharma

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Carbon Nanotube Research Developments: Published Scientific Documents and Patents, Synthesis, and Production . . . . . . . . . . . . 1937 Claudio Ernani Martins Oliveira, Edelma Eleto da Silva, Evandro Augusto de Morais, and Viviany Geraldo

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Assessment of the Risks Associated with Carbon Nanotubes . . . . . 1975 Divya Praveen Ottoor

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Advanced Applications of Carbon Nanotubes in Engineering Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2001 Antonella D’Alessandro and Filippo Ubertini

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Carbon Nanotube-Based Membranes for Filtration . . . . . . . . . . . . 2039 Arunima Reghunadhan, K. C. Nimitha, and Jijo Abraham

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2071

About the Editors

Dr. Jiji Abraham works as an Assistant Professor of Chemistry in Vimala College (Autonomous) Thrissur. She completed her PhD from International and InterUniversity Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam. Her research interests include polymer nanocomposites, synthesis of nanomaterials, etc. Dr. Abraham has published over 60 research articles, which includes 33 journal papers and 32 book chapters. She has edited a book entitled Rheology and Processing of Polymer Nanocomposites published by Wiley. The H-index of Dr. Abraham is 15, and she has more than 800 citations. Dr. Abraham was a visiting research student in Institut Charles Sadron CNRS UPR 22, University de Strasbourg, France, from 10 December 2014 to 30 June 2015. She has presented papers at the European Polymer Congress 2015, during 21–26 June 2015 at Dresden, Germany; Malaysia Polymer International Conference (MPIC 2017), during 19–20 July 2017 at Universiti Kebangsaan, Malaysia; and at the International Polymer Characterization Conference POLY-CHAR 2019 (Kathmandu, Nepal) Polymer for Sustainable Development, 19–23 May 2019. Dr. Abraham has also delivered many presentations in national/international meetings.

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About the Editors

Prof. Sabu Thomas is currently Vice Chancellor of Mahatma Gandhi University, Kottayam, Kerala, India. Prof. Thomas is a highly committed teacher and a remarkably active researcher well-known nationally and internationally for his outstanding contributions in polymer science and nanotechnology. He has published over 1200 research articles in international refereed journals and has also edited and written 165 books with an H-index of 122 and total citation of more than 72,000. He has received a large number of international and national awards and recognitions. Under the leadership of Prof. Thomas, Mahatma Gandhi University has been transformed into a top university in the country where excellent outcome-based education is imparted to students for their holistic development. Prof. Nandakumar Kalarikkal is currently a Senior Professor at the School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala, India. His areas of interest are nanostructured materials and applications. He has received several national and international research grants. Prof. Nandakumar worked as professor@Lorraine, France; CNRS Professor@ILM Lyon, Claud Bernard University, France; and Visiting Professor at various international institutions in Europe. He has supervised 25 PhD and 30 Master’s theses. He has 6 patents and more than 30 books to his credit. In addition, he has more than 400 research publications in peer reviewed journals and over 6000 citations, with an H-index of 43. Prof, Kalarikkal received his MSc in Industrial Physics and PhD in semiconductor physics from Cochin University of Science and Technology, Kerala, India.

Contributors

V. S. Abhisha Department of Chemistry, St. Joseph’s College (Autonomous), Devagiri, Kozhikode, India Elham Abohamzeh Department of Energy, Materials and Energy Research Center (MERC), Karaj, Iran Jijo Abraham National Graphene Institute/School of Chemical Engineering and Analytical Science, University of Manchester, Manchester, UK Mohamed Abubakr Mechanical Design and Production Engineering Department, Cairo University, Giza, Egypt Department of Mechanical Engineering, The American University in Cairo (AUC), New Cairo, Egypt Ibrahim M. Abu-Reesh Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar M. Basheer Ahamed Department of Physics, B. S. Abdur Rahman Crescent Institute of Science and Technology, Chennai, India Esma Ahlatcioglu Ozerol Department of Bioengineering, Yildiz Technical University, Istanbul, Turkey Shoaib Ahmad National Center for Physics, Islamabad, Pakistan H. Akhina Department of Chemistry, MSM College, Kayamkulam, Kerala, India K. V. Ameer Nasih International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India J. Jefferson Andrew Department of Mechanical Engineering, Khalifa University, Abu Dhabi, UAE Saritha Appukuttan Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, Kerala, India A. B. Arun Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, India xvii

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Contributors

Anchu Ashok Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar Sankeerthana Avasarala Department of Materials Engineering, Indian Institute of Science, Bengaluru, India Sadia Zafar Bajwa Nanobiotechnology Group, Industrial Biotechnology Division, National Institute for Biotechnology and Genetic Engineering (NIBGE), Faisalabad, Pakistan Prabhakar R. Bandaru Department of Mechanical and Aerospace Engineering, Materials Science Program, University of California, San Diego, CA, USA Poulami Banerjee Department of Materials Engineering, Indian Institute of Science, Bengaluru, India Sonali Batra Department of Pharmaceutical Sciences, Guru Jambheshwar University of Science & Technology, Hisar, Haryana, India Bhashkar Singh Bohra Prof. Rajendra Singh Nanoscience and Nanotechnology Centre, Department of Chemistry, Kumaun University, Nainital, Uttarakhand, India Suryasarathi Bose Department of Materials Engineering, Indian Institute of Science, Bengaluru, India Michael Bozlar Department of Mechanical & Aerospace Engineering, University of Texas, Arlington, TX, USA Cem Bulent Ustundag Department of Bioengineering, Yildiz Technical University, Istanbul, Turkey Nithin Chandran Department of Chemistry, MES College, Marampally, Aluva, Kerala, India Pavan Kumar Chintamaneni Department of Pharmaceutics, Raghavendra Institute of Pharmaceutical Education and Research (RIPER), Anantapuramu, AP, India Antonella D’Alessandro Department of Civil and Environmental Engineering, University of Perugia, Perugia, PG, Italy Michael Olawale Daramola Department of Chemical Engineering, Faculty of Engineering, Built Environment and Information Technology, University of Pretoria, Pretoria, South Africa Narayan Ch. Das Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur, India Muslum Demir Department of Chemical Engineering, Osmaniye Korkut Ata University, Osmaniye, Turkey Kalim Deshmukh New Technologies – Research Center, University of West Bohemia, Plzeň, Czech Republic

Contributors

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G. L. Devnani Chemical Engineering Department, Harcourt Butler Technical University, Kanpur, India H. N. Dhakal Advanced Polymers and Composites (APC) Research Group, School of Mechanical and Design Engineering, University of Portsmouth, Portsmouth, UK Burak Dikici Department of Metallurgical and Materials Engineering, Ataturk University, Erzurum, Turkey Zekerya Dursun Department of Chemistry, Science Faculty, Ege University, Izmir, Turkey Farida Elharouni Department of Mechanical Engineering, The American University in Cairo (AUC), New Cairo, Egypt Alshaimaa M. Elsayed Molecular Biology Department, Genetic Engineering and Biotechnology Division, National Research Centre, Giza, Egypt Amal M. K. Esawi Department of Mechanical Engineering, The American University in Cairo (AUC), New Cairo, Egypt Ahmed A. Farghaly Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL, USA Chemistry Department, Faculty of Science, Assiut University, Assiut, Egypt Chioma Nnaji Frances Department of Histology, Southend University Hospital, Southend-on-Sea, UK Yoshitaka Fujimoto Department of Physics, Tokyo Institute of Technology, Tokyo, Japan Abhinav Omprakash Fulmali FRP Composites Laboratory, Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, India E. Gallo Corning Optical Communications, Berlin, Germany Rajamohan Ganesan Department of Mechanical, Industrial and Aerospace Engineering, Concordia Centre for Composites (CONCOM), Concordia University, Montreal, QC, Canada K. Geetha Nanotechnology Division/Department of ECE, PMIST, Thanjavur, India Viviany Geraldo Universidade Federal de Itajubá (Unifei), Campus de Itabira, Rua Irmã Ivone Drummond, Itabira, MG, Brazil S. Ghosh Department of Physics, Indian Institute of Technology Delhi, New Delhi, India S. S. Godara Mechanical Engineering Department, University Departments, Rajasthan Technical University, Kota, Rajasthan, India

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Contributors

Li-Zhi Guan College of Material, Chemistry and Chemical Engineering, Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou, People’s Republic of China School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Singapore Seval Hale Guler Department of Metallurgical and Materials Engineering, Mersin University, Mersin, Turkey Omer Guler Department of Metallurgical and Materials Engineering, Mersin University, Mersin, Turkey Sujasha Gupta School of Medicine, Johns Hopkins University, Baltimore, MD, USA Mechanical and Aerospace Engineering, The Ohio State University, Columbus, OH, USA Lakshmanan Gurusamy Department of Environmental Engineering and Science, Feng Chia University, Taichung, Taiwan Zainab Al Hajaj Department of Mechanical Engineering, Australian College of Kuwait, Kuwait City, Kuwait Hussien Hegab Machining Research Laboratory, University of Ontario Institute of Technology, Oshawa, ON, Canada Xuan Tung Hoang Department Elastomers, Leibniz-Institute of Polymer Research Dresden Germany, Dresden, Germany Zhanglian Hong State Key Laboratory of Silicon Material, School of Materials Science and Engineering, Zhejiang University, Hangzhou, China Shumaila Ibraheem Institute for Advanced Study, Shenzhen University, Shenzhen, Guangdong, China College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China Sehrish Ibrahim College of Life Science and Technology, Beijing University of Chemical Technology, Beijing, China Sumera Javeed Pakistan Institute of Nuclear Science and Technology (PINSTECH), Islamabad, Pakistan Felipe de Jesús Barraza-García División de Materiales Avanzados, IPICYT, San Luis Potosí, S.L.P., Mexico Alberto Jiménez-Suárez Materials Science and Engineering Area, University Rey Juan Carlos, Móstoles, Spain

Contributors

xxi

Honey John Inter University Centre for Nanomaterials and Devices, Cochin University of Science and Technology, Kochi, Kerala, India Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Kochi, Kerala, India Kuruvilla Joseph Department of Chemistry, Indian Institute of Space Science and Technology, Valiyamala, Kerala, India Sherin Joseph Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Kochi, Kerala, India Nidhi Joshi Department of Materials Engineering, Indian Institute of Science, Bengaluru, India Nandakumar Kalarikkal School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala, India Şükriye Karabiberoğlu Department of Chemistry, Science Faculty, Ege University, Izmir, Turkey Lakshmanan Karuppasamy Department of Environmental Engineering and Science, Feng Chia University, Taichung, Taiwan Diab Khalafallah State Key Laboratory of Silicon Material, School of Materials Science and Engineering, Zhejiang University, Hangzhou, China Mechanical Design and Materials Department, Faculty of Energy Engineering, Aswan University, Aswan, Egypt Khalil Abdelrazek Khalil Mechanical and Nuclear Engineering Department, College of Engineering, University of Sharjah, Sharjah, United Arab Emirates Waheed S. Khan Nanobiotechnology Group, Industrial Biotechnology Division, National Institute for Biotechnology and Genetic Engineering (NIBGE), Faisalabad, Pakistan Marianna V. Kharlamova Moscow Institute of Physics and Technology, State University, Dolgoprudny, Russia Institute of Materials Chemistry, Vienna University of Technology, Vienna, Austria Hossam A. Kishawy Machining Research Laboratory, University of Ontario Institute of Technology, Oshawa, ON, Canada Çağrı Ceylan Koçak Dokuz Eylül University, Bergama Vocational School, Izmir, Turkey Praveen T. Krishnamurthy Department of Pharmacology, JSS College of Pharmacy, JSS Academy of Higher Education & Research, Ooty, The Nilgiris, Tamil Nadu, India

xxii

Contributors

Sitaraman Krishnan Department of Chemical and Biomolecular Engineering, Clarkson University, Potsdam, NY, USA R. Krishnaveni Nanotechnology Thanjavur, India

Division/Department

of

ECE,

PMIST,

Anand Kumar Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar Anitha C. Kumar Department of Chemistry, Acharya Nagarjuna University, Guntur, Andhra Pradesh, India Department of Applied Chemistry, Cochin University of Science and Technology, Kochi, Kerala, India Anuj Kumar State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, China Nano-Technology Research Laboratory, Department of Chemistry, GLA University, Mathura, Uttar Pradesh, India A. Ashok Kumar Department of Biotechnology, PMIST, Thanjavur, India Krishan Kumar Department of Chemistry, University of Delhi, Delhi, India B. N. Kumara Nanomaterial Research Laboratory (NMRL), Nano Division, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, India S. Kumaran Department of Biotechnology, PMIST, Thanjavur, India Garikapati Kusuma Kumari Department of Pharmacology, JSS College of Pharmacy, JSS Academy of Higher Education & Research, Ooty, The Nilgiris, Tamil Nadu, India Hong Hai Le Department Elastomers, Leibniz-Institute of Polymer Research Dresden Germany, Dresden, Germany P. Lodhi Chemical Engineering Department, Institute of Engineering and Technology, Lucknow, India Florentino Lopez-Urias División de Materiales Avanzados, IPICYT, San Luis Potosí, S.L.P., Mexico Kala M. S. St Teresas’s College (Autonomous), Ernakulam, India Messai Adenew Mamo Research Centre for Synthesis and Catalysis, Department of Chemical Science, Doornfontein Campus, Faculty of Science, University of Johannesburg, Johannesburg, South Africa Takahiro Maruyama Department of Applied Chemistry, Nanomaterial Research Center, Meijo University, Nagoya, Aichi, Japan

Contributors

xxiii

Silvy Mathew Post Graduate Department of Botany, Vimala College (Autonomous), Thrissur, India Monika Matiyani Prof. Rajendra Singh Nanoscience and Nanotechnology Centre, Department of Chemistry, Kumaun University, Nainital, Uttarakhand, India Anshida Mayeen Inter University Centre for Nanomaterials and Devices, Cochin University of Science and Technology, Kochi, Kerala, India Neelesh Kumar Mehra Pharmaceutical Nanotechnology Research Laboratory, Department of Pharmaceutics, National Institute of Pharmaceutical Education & Research (NIPER), Hyderabad, Telangana, India Aishwarya Vijayan Menon Department of Materials Engineering, Indian Institute of Science, Bengaluru, India Misbah Mirza Department of Physics, The Women University of Multan, Multan, Pakistan Helen Uchenna Modekwe Department of Chemical Engineering, Faculty of Engineering and the Built Environment, University of Johannesburg, Johannesburg, South Africa Kapil Moothi Department of Chemical Engineering, Faculty of Engineering and the Built Environment, University of Johannesburg, Johannesburg, South Africa Evandro Augusto de Morais Universidade Federal de Itajubá (Unifei), Campus de Itabira, Rua Irmã Ivone Drummond, Itabira, MG, Brazil M. S. Mrudula School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India José Gil Munguia-Lopez Faculty of Dentistry, McGill University, Montreal, QC, Canada Department of Bioengineering, McGill University, Montreal, QC, Canada Emilio Muñoz-Sandoval División de Materiales Avanzados, IPICYT, San Luis Potosí, S.L.P., Mexico Tanyaradzwa S. Muzata Department of Materials Engineering, Indian Institute of Science, Bengaluru, India Anju K. Nair St Teresas’s College (Autonomous), Ernakulam, India Parvathy Nancy International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India School of Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Krishnendu Nath Rubber Technology Centre, Indian Institute of Technology Kharagpur, Kharagpur, India

xxiv

Contributors

Mohsen Mohammad Raei Nayini Department of Printing Inks Science & Technology, Institute for color science and technology, Tehran, Iran Tuan Anh Nguyen Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Viet Nam K. C. Nimitha Department of Chemistry, Vimala College, Thrissur, Kerala, India Olusola Olaitan Ayeleru Centre for Nanoengineering and Tribocorrosion (CNT), University of Johannesburg, Johannesburg, South Africa Claudio Ernani Martins Oliveira Universidade Federal de Itajubá (Unifei), Campus de Itabira, Rua Irmã Ivone Drummond, Itabira, MG, Brazil Peter Apata Olubambi Centre for Nanoengineering and Tribocorrosion (CNT), University of Johannesburg, Johannesburg, South Africa Matthew Adah Onu Centre for Nanoengineering and Tribocorrosion (CNT), University of Johannesburg, Johannesburg, South Africa Tarek A. Osman Mechanical Design and Production Engineering Department, Cairo University, Giza, Egypt Abdelmageed M. Othman Microbial Chemistry Department, Genetic Engineering and Biotechnology Division, National Research Centre, Giza, Egypt Divya Praveen Ottoor Department of Chemistry, Savitribai Phule Pune University, Pune, India Jagadeshvaran P. L. Department of Materials Engineering, Indian Institute of Science, Bengaluru, India Sandra Pérez-Miranda Laboratorio de Microbiología de productos naturales, CIIDZA, IPICYT, San Luis Potosí, S.L.P., Mexico Jorge Palacios Department of Mechanical, Industrial and Aerospace Engineering, Concordia Centre for Composites (CONCOM), Concordia University, Montreal, QC, Canada Piyush Kumar Patel Department of Physics, Maulana Azad National Institute of Technology Bhopal, Bhopal, Madhya Pradesh, India Mayank Pathak Prof. Rajendra Singh Nanoscience and Nanotechnology Centre, Department of Chemistry, Kumaun University, Nainital, Uttarakhand, India Anju Paul Department of Chemistry, Sree Sankara Vidyapeedom College, Eranakulam, Kerala, India Sai Kiran S. S. Pindiprolu Department of Pharmacology, Aditya Pharmacy College (affiliated to JNTUK), East Godavari, AP, India Swetapadma Praharaj Department of Physics, School of Applied Sciences, KIIT Deemed to be University, Bhubaneswar, Odisha, India

Contributors

xxv

K. S. Prasad Nanomaterial Research Laboratory (NMRL), Nano Division, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, India Centre for Nutrition Studies, Yenepoya (Deemed to be University), Mangalore, India Rajesh Kumar Prusty FRP Composites Laboratory, Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, India Center for Nanomaterials, National Institute of Technology, Rourkela, India R. Rajakumari International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India K. Raji Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, VIC, Australia Sunil Kumar Ramamoorthy Department of Textile, Engineering and Economics, University of Borås, Borås, Sweden Akhila Raman Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, Kerala, India Priyanka Rani Department of Physics, B. S. Abdur Rahman Crescent Institute of Science and Technology, Chennai, India Zahra Ranjbar Department of Surface Coatings and Novel Technologies, Institute for Color Science and Technology, Tehran, Iran Center of Excellence for Color Science Technology, Tehran, Iran Ammu V. V. V. Ravi Kiran Department of Pharmacology, JSS College of Pharmacy, JSS Academy of Higher Education & Research, Ooty, The Nilgiris, Tamil Nadu, India Bankim Chandra Ray Department of Metallurgical and Materials Engineering, National Institute of Technology Rourkela, Rourkela, India Arunima Reghunadhan School of Energy Materials, Mahatma Gandhi University, Kottayam, Kerala, India Department of Chemisrty, TKM College of Engineering, Karicode, Kollam, India Amir Rezvani Moghaddam Faculty of Polymer Engineering, Sahand University of Technology, Tabriz, Iran M. Naveen Roobadoss Nanotechnology Division/Department of ECE, PMIST, Thanjavur, India M. J. Rosemary Medical Devices Lab, Corporate R & D Centre, HLL Lifecare Ltd, Thiruvananthapuram, Kerala, India Dibyaranjan Rout Department of Physics, School of Applied Sciences, KIIT Deemed to be University, Bhubaneswar, Odisha, India

xxvi

Contributors

P. Russo Institute for Polymers, Composites and Biomaterials – National Research Council, Pozzuoli, NA, Italy Olawumi Oluwafolakemi Sadare Department of Chemical Engineering, Faculty of Engineering, Built Environment and Information Technology, University of Pretoria, Pretoria, South Africa Muhammad Safdar Department of Chemistry, Khwaja Fareed University of Engineering and Information Technology (KFUEIT), Punjab, Pakistan M. Ziad Saghir Department of Mechanical Engineering, Ryerson University, Toronto, ON, Canada Nanda Gopal Sahoo Prof. Rajendra Singh Nanoscience and Nanotechnology Centre, Department of Chemistry, Kumaun University, Nainital, Uttarakhand, India Mansab Ali Saleemi School of Biosciences, Taylor’s University Lakeside Campus, Subang Jaya, Selangor, Malaysia Fatemeh Salehi School of Engineering, Macquarie University, Sydney, NSW, Australia Xoan F. Sánchez-Romate Materials Science and Engineering Area, Escuela Superior de Ciencias Experimentales y Tecnología (ESCET), Universidad Rey Juan Carlos, Móstoles, Spain K. Sapna Nanomaterial Research Laboratory (NMRL), Nano Division, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, India Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, India Rajib Sarkar Department of Chemistry, Virginia Commonwealth University, Richmond, VA, USA A. S. Sethulekshmi Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, Kerala, India Muhammad Zakiullah Shafique Department of Chemistry, Khwaja Fareed University of Engineering and Information Technology (KFUEIT), Punjab, Pakistan A. Shajkumar International and Interuniversity Center for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India A. M. Shanmugharaj Centre for Energy and Alternative Fuels, Department of Chemistry, Vels Institute of Science, Technology and Advanced Studies (VISTAS), Pallavaram, Chennai, Tamil Nadu, India Devansh Sharma Department of Materials Engineering, Indian Institute of Science, Bengaluru, India Navneet Sharma Mechanical Engineering Department, University Departments, Rajasthan Technical University, Kota, Rajasthan, India

Contributors

xxvii

Nidhi Sharma Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, India Sumit Sharma Chitkara College of Pharmacy, Chitkara University, Rajpura, Punjab, India Mohsen Sheikholeslami Department of Mechanical Noshirvani University of Technology, Babol, Iran

Engineering,

Babol

Renewable Energy Systems and Nanofluid Applications in Heat Transfer Laboratory, Babol Noshirvani University of Technology, Babol, Iran Edelma Eleto da Silva Universidade Federal de Itajubá (Unifei), Campus de Itabira, Rua Irmã Ivone Drummond, Itabira, MG, Brazil Dhananjay Singh Chemical Engineering Department, Institute of Engineering and Technology, Lucknow, India Anjaly Sivadas School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India K. C. Sivaganga Department of Chemistry, Christ College (Autonomous), Affiliated to University of Calicut, Irinjalakuda, India C. B. Sobhan School of Materials Science and Engineering, National Institute of Technology, Calicut, India J. Sonia Nanomaterial Research Laboratory (NMRL), Nano Division, Yenepoya Research Centre, Yenepoya (Deemed to be University), Mangalore, India M. Sreekanth Centre for Nano Science and Engineering, Indian Institute of Science, Bengaluru, India Namburu Srikanth Department of Chemistry, Acharya Nagarjuna University, Guntur, Andhra Pradesh, India P. Srivastava Department of Physics, Indian Institute of Technology Delhi, New Delhi, India Ranimol Stephen Department of Chemistry, St. Joseph’s College (Autonomous), Devagiri, Kozhikode, India Kumari Sushmita Centre for Nanoscience and Engineering, Indian Institute of Science, Bengaluru, India Mohammad Tabish State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, China Muhammad Suleman Tahir Department of Chemical Engineering, Khwaja Fareed University of Engineering and Information Technology (KFUEIT), Punjab, Pakistan

xxviii

Contributors

Long-Cheng Tang College of Material, Chemistry and Chemical Engineering, Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou, People’s Republic of China Paulose Thomas Department of Physics, Mar Thoma College for Women, Perumbavoor, India Sabu Thomas International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Nyam Tarhemba Tobias Centre for Nanoengineering and Tribocorrosion (CNT), University of Johannesburg, Johannesburg, South Africa Manan Tyagi Chemical Engineering Department, Harcourt Butler Technical University, Kanpur, India Filippo Ubertini Department of Civil and Environmental Engineering, University of Perugia, Perugia, PG, Italy Alejandro Ureña Materials Science and Engineering Area, University Rey Juan Carlos, Móstoles, Spain Titto Varughese Department of Chemistry, Christ College (Autonomous), Affiliated to University of Calicut, Irinjalakuda, India Pannuru Venkatesu Department of Chemistry, University of Delhi, Delhi, India Cristiane P. Victório Fundação Centro Universiáario Estadual da Zona Oeste (UEZO), Rio de Janeiro, Brazil Vidya Department of Physics, Maulana Azad National Institute of Technology Bhopal, Bhopal, Madhya Pradesh, India Vindhyasarumi Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, Kerala, India Sven Wiessner Department Elastomers, Leibniz-Institute of Polymer Research Dresden Germany, Dresden, Germany Eng Hwa Wong School of Medicine, Taylor’s University Lakeside Campus, Subang Jaya, Selangor, Malaysia Jerry J. Wu Department of Environmental Engineering and Science, Feng Chia University, Taichung, Taiwan Ritu Yadav Department of Chemistry, University of Delhi, Delhi, India Nesli Yagmurcukardes Department of Material Science and Nanotechnology Engineering, Engineering Faculty, Uşak University, Uşak, Turkey Atike Ince Yardimci Technology Transfer Office, Uşak University, Uşak, Turkey

Contributors

xxix

Ghulam Yasin Institute for Advanced Study, Shenzhen University, Shenzhen, Guangdong, China College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, China Afdhal Yuda Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar

Part I Carbon Nanotube: Fundamentals and Fascinating Attributes

1

History of Carbon Nanotubes Namburu Srikanth and Anitha C. Kumar

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discovery and History of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pre-History of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Single Walled CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Multi Walled CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production of Carbon Nanotubes from Bio-hydrocarbon Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization, Properties, and Applications of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Disadvantages of CNT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Health Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 6 9 11 11 13 13 16 18 20 20 20 21 21

Abstract

Carbon nanotubes (CNTs) and nano fibers (CNFs) are some of the most gifted materials from nanotechnology. CNTs are considered the materials of the twentyfirst century. It is a nanostructured allotrope of carbon with a length-to-diameter ratio of more than 1,000,000. The products with CNTs used in commerce have increased greatly in the last decade. Many techniques, including arc discharge, laser ablation, and chemical vapor deposition, have been developed to produce

N. Srikanth Department of Chemistry, Acharya Nagarjuna University, Guntur, Andhra Pradesh, India A. C. Kumar (*) Department of Chemistry, Acharya Nagarjuna University, Guntur, Andhra Pradesh, India Department of Applied Chemistry, Cochin University of Science and Technology, Kochi, Kerala, India © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_51

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N. Srikanth and A. C. Kumar

nanotubes in sizeable quantities. In 2004, graphene, with a single-layered honeycomb structure, was discovered and became the “mother” of all carbon systems. All these discoveries are supposed to complete the carbon family. Their nanoscale size and extraordinary mechanical, electronic, transport, electrical and optical properties make them suitable for a range of applications. Electronics, engineering, optoelectronics, medicine, the defense industry, and molecular and biological systems are the main applications of CNTs. The global production of CNTs has increased from 30 tons in 2008 to nearly 3,400 tons in 2010, and the projected market for CNTs is predicted to worth $13.5 billion by 2026.

Introduction Nanoscience is the study of materials on the nanoscale, i.e., understanding, control of phenomena, and manipulation. It is the study of atoms, quantum dots, molecules, macromolecules, and assemblies of macromolecules. Nanotechnology is an emerging area of technology development that involves science and engineering of matter on the nanoscale of approximately 1–100 nm. Materials from the carbon family play a major function in today’s science and technology. The basis of organic chemistry is the self-catenation property of carbon, which forms endlessly different combinations of rings and chains. Products using carbon nanotubes (CNTs) and carbon nano fibers (CNFs) in commerce have increased greatly in the last two decades (Thostenson et al. 2001). CNTs contain very small cylinders of graphite, closed both ends. with six pentagonal ring caps. It is an allotrope of carbon-like graphite, diamond, fullerene, and a comparison of its structures is given in Fig. 1. The structure of CNTs can be demonstrated by considering that the CNT can be formed by placing a graphene cylinder between the two halves of a C60 molecule. Nanotubes generally are nearly one-dimensional structures because they have a large aspect ratio (length:diameter) of about 1000. If we are going deeper into the history of CNTs, more evidence can be found that the origin of CNTs might even be pre-historic in nature and is shown in Fig. 2. The issue of the natural occurrence of CNTs is hotly debated, as it has legal effects concerning the CNT intellectual property regime. The progress of CNT-based applications in a large variety of products is expected to offer great social advantage and it is important that they be developed responsibly to achieve that benefit. The strength of the sp2-hybridized C-C bonds present in CNTs gives them very good mechanical properties. No other material has before displayed the combination of excellent thermal, electronic, and mechanical properties. CNT densities are 1/6th of that of stainless steel (1.3 g/cm3). The stiffness of CNTs (Young’s modulus) is greater than 1 TPa, which is 5 times higher than steel and superior to all carbon fibers. They conduct electricity better than copper (Yu et al. 2000) and are able to survive repeated bending, twisting, and buckling. When compared with silicon and other known materials, they are superior semiconductors and good heat transporters (Service 1998). Scientists from all areas have

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History of Carbon Nanotubes

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Fig. 1 Structural comparison of allotropes of carbon: (a) graphite, (b) diamond (adamantane), (c) fullerene, and (d) carbon nanotubes

been attracted to the properties of CNTs. Their amazing electronic properties, their potential as ‘nano test-tubes’, their wonderful stiffness, strength, and resilience are applauded by physicists, chemists, and materials scientists respectively. Owing to these properties, CNTs have enormous commercial potential in applications, which are discussed later. Worker’s health safety is a keystone of responsible growth of arising technology. It is reported that CNTs have effects on the environment and on health. This is mainly because of its hydrophobic nature and many surface modifications are suggested to reduce these drawback of CNTs. It is reported that the global market for CNTs in 2015 was about $2.26 billion (Thostenson et al. 2001) and in 2009 it was ~ $ 1.24 billion (45% growth). The reason for growth was the growing potential of CNTs in many applications such as energy storage, electronics, and plastics. The projected market of CNTs is expected to be ~ $ 14 billion in 2026 (Pratik et al. 2021). CNTs have been accepted as the material of the twenty-first century.

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Fig. 2 Proof footprints of carbon nanotube in nature and (inset) their discovery year

Discovery and History of Carbon Nanotubes In the nineteenth century organic chemists started to create new molecules that do not exist in nature. As said above, the basis of organic chemistry is the selfcatenation property of carbon and forms varied combinations of chains and rings. It is reported that carbon fibers were scientifically studied in the 1950s and have been industrially fabricated since 1963. In the late 1950s, Radushkevich and Lukyanovich found straight and hollow tubes of carbon, a new carbon fiber. Its size was 50 nm in diameter and they reported it in 1952 (Radushkevich and Lukyanovich 1952). The tubes were a one-dimensional filamentary form of carbon with a length diameter of more than 100 nm and are shown in Fig. 3. In 1880, Thomas Edison used carbon fibers in incandescence light bulbs. Later, tungsten filaments were used for light bulb applications and nanostructured CNTs

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Fig. 3 The low-resolution transmission electron microscopy images of carbon nanotubes reported by Radushkevich and Lukyanovich in 1950 (MAG 20 000)

Fig. 4 Size and structural difference of conventional carbon fiber, nanofiber, and nanotubes (Endo and Dresselhaus 2003)

were introduced. CNTs are not single molecules; they are distinct from carbon fibers, i.e., strands of layered-graphite sheets. The structural difference and size comparison of conventional carbon fiber, nano fiber and nanotubes are shown in Fig. 4.

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N. Srikanth and A. C. Kumar

Morinobu Endo and colleagues reported that CNTs were produced by a vaporgrowth technique in the 1970s (Endo et al. 1997). In 1979, CNTs were produced using arc discharge on carbon anodes by John Abrahamson et al. (1999). In 1981, multiwalled carbon nanotubes (MWCNTs) were productively observed by a group of Soviet scientists using thermo catalytical oxidation and reduction of carbon monoxide (Kroto et al. 1985). In 1984, the first patent for preparing nanometer-sized carbon filaments, termed “carbon fibrils,” that is, significantly free of pyrolytically deposited thermal carbon was filed by Howard G. Tennent (1987). Later studies showed that these fibrils had the same morphology as MWCNTs, which was not identified at the time of invention. By using the thermal decomposition of hydrocarbon for preparing carbon filaments, T.V. Hughes’s team filed a patent in 1886 (Hughes and Chambers 1889). These studies could not find the presence of CNTs owing to the resolution limitations of the then available microscopic tools. In 1991, a well-known electron microscopy specialist, Sumio Iijima, working at that time in the NEC laboratories in Japan, found needle-shaped matter that appeared near fullerene using the arc discharge method. Iijima was very much interested by Krätschmer-Huffman’s paper on the lab-scale production of C60 and by using a high-resolution transmission electron microscope (HRTEM) he analyzed the soot produced by their method provided by Ando. This discovery of Iijima is a turning point of further studies on CNT. This needle-shaped material was later known as “carbon nanotube,” the name given by Iijima. Initially, he considered many other names including microtubules, tubulin, NEC tubes, and Iijima tubes (Iijima 1991). The tubes contained two or many layers (MWCNTs) and the outer diameter was within the range 3 nm to 30 nm. It was observed that these tubes were closed at both ends. The beautiful images (Fig. 5) of Iijima’s CNTs, shown for the first time in October 1991 and published a month later in Fig. 5 Transmission electron micrographs of Iijima’s tubes. The tube consists of (a) five graphite sheets, 6.7 nm in diameter, (b) two graphite sheets, 5.5 nm in diameter, and (c) seven graphite sheets, 6.5 nm in diameter

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Nature. These images were shown in a meeting at Richmond, Virginia, USA, and encouraged fullerene scientists the world over to look again at the used graphite cathodes that had been previously discarded as junk. It was known that the discovery of CNTs was accidental, but Iijima’s vast accumulated experience and knowledge from electron microscopy studies of various types of carbon materials played a crucial role in identifying CNTs in the graphite cathode soot produced by arc evaporation. The early tubes were not very interesting because of their imperfect structure and it did not have many noteworthy properties. In 2000, 9 years later, using the arc discharge method, Professor Tang Zhang and Wang Ning synthesized single-walled carbon nanotubes (SWCNTs), which are 75,000 times thinner than a human hair and 0.4 nm in diameter, the world’s narrowest CNT (Wang et al. 2000). Today, almost 25 years after the dissemination of CNTs on the scientific stage, all over the world, a number of ongoing research projects on CNTs are ongoing (Yang et al. 2008). The researchers are unlocking the potential of CNTs for various applications.

Pre-History of Carbon Nanotubes The discovery and early history of CNTs are shown in Fig. 6. From Russia, Ponomarchuk et al. reported that in igneous rocks, the occurrence micro and nano carbon tubes formed about 250 million years ago (Ryabov et al. 2012). They recommended that the formation of CNTs might take place during the magmatic processes and explained that in the presence of metal elements, condensation and decomposition of hydrocarbon occurs when the movement of hydrocarbon fluids through the remaining melt of the rock groundmass produced gas-saturated areas, forming micro and nano carbon tubes. Using X-ray fluorescence spectroscopy it was identified that a large group of elements such as Fe, Ti, V, Cr, K, Mn, Ni, Pt, Zn, Ga, Br, Ge, Sr, Zr, and Pb have been recognized in the nanoscale structures of igneous rocks that contain natural globules of graphite. The carbon nanostructures were formed in the presence of these elements as natural catalysts by a similar process of chemical vapor phase deposition and the presence of CNTs were identified using electron microscopy. Using transmission electron microscopy (TEM) studies, evidence for naturally occurring CNTs in the Greenland ice core in 7990 BC was reported by Esquivel and Murr (2004). These 10,000-year-old ice core samples were convincing evidence for naturally occurring CNTs (MWCNTs) and they suggested that MWCNTs could have been formed during the natural combustion of natural gas/methane. Raman spectral studies showed the resemblance of naturally occurring CNTs to commercially available MWCNTs. Scientists from Germany used igneous rock to synthesize CNTs and these rocks were obtained from Mount Etna lava and act as a catalyst and support. The natural iron oxide present in Etna lava rock helped the growth and immobilization of nano carbons. At 700
C and a reduced atmosphere, a mixture of hydrogen and ethylene was passed over the crushed rocks and the iron particles catalyzed the decomposition of ethylene to elemental carbon. This carbon is deposited on the lava rock in the form of tiny fibers and tubes. It is clear from this study that if a carbon source is available, at moderate temperatures, CNFs/

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Fig. 6 Discovery and early history CNTs [3]

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CNTs can grow on a mineral. This study also explains the possibility of CNT development in active sub-oceanic volcanoes or even in interstellar space, where atomic hydrogen, methane, metallic iron, and carbon oxides are present. The electron microscopy studies conducted by Peter Paufler and colleagues at Dresden’s Technical University (Reibold et al. 2006) on the legendary medieval Damascus sword dating back as far as 900 AD found the presence of CNTs in the microstructure of wootz steel, the material from which the swords were fabricated. The presence of cementite (Fe3C) nanowires was found on the Damascus steel sword from the initial studies. A hard and brittle compound was formed of iron and carbon steel. When a piece of the sword was exposed to hydrochloric acid, the microstructure made of cementite nanowires dissolved and the CNTs were revealed.

Classification of Carbon Nanotubes As discussed earlier, the ideal CNTs are nanoscale graphene cylinders, closed at each end by half-fullerene. CNTs are classified based on the number of graphite sheets that the cylinders are wrapped in to form the tube: SWCNTs, which consist of a single graphitic sheet, double-walled CNT (DWCNTs), which consist of two graphitic sheets, and MWCNTs, which consist of more than two graphitic sheets. The diameters of single-walled nanotubes are within the range 1–2 nm, generally narrower than the multi-walled tubes, and have a tendency to be curved rather than straight. The thickness of SWCNTs is only one atom and ten atoms around the perimeter. The MWCNTs consist of many single-walled tubes stacked one inside the other and are larger than SWCNTs. The molecular representations of SWCNTs and MWCNTs with usual transmission electron micrographs are shown in Fig. 7 (Donaldson et al. 2006). In the history of CNTs, SWCNTs should be distinguished from MWCNTs.

Single Walled CNTs It is reported that the development of SWCNTs was first reported in Nature (17 June 1993 issue) by two independent research groups. One paper submitted by Iijima and Ichihashi (Boehm 1997), affiliated to NEC, Japan, at the time, and the second one by Bethune et al., IBM, California, USA (Ijima and Ichihashi 1993). Nanotubes, which Iijima described in 1991, regularly contain a minimum of two graphitic layers, and usually have approximately an inner diameter of 4 nm. In 1993, the synthesis of SWCNT was a novel development. Three different types of CNTs are possible with two different basic structures. They are armchair CNTs, zigzag CNTs, and chiral CNTs. Depends on the “rolling up” of the graphite sheets during the secretion process these different CNTs are formed. The closing cylinder radius and the rolling axis relative to the hexagonal network of the graphene sheet are the two factors for different types of SWCNTs.

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Fig. 7 Single-walled carbon nanotube (top left), double-walled carbon nanotube (top middle) and multi-walled carbon nanotube (top right). Molecular representations (Ahmad et al. 2012) with transmission electron microscopy images (below, scale bar10 and 5 nm) (Donaldson et al. 2006)

Fig. 8 (a) Formation of armchair, zigzag, and chiral tubes by rolling of a graphene sheet along lattice vectors – schematic representation. and (b) the different types of CNTs

Carbon nanotubes are only described by the pair of integers (n, m) which is related to the chiral vector. The CNT is called “zigzag” when m ¼ 0; when n ¼ m is called “armchair,” and all other configurations when n 6¼ m the CNTs are called chiral tubes (0
< h < 30
). Figure 8 shows the armchair, zigzag, and chiral types of SWCNTs (Zhang and Li 2009). Iijima’s team from Japan submitted their paper on 23 April 1993 and the US team on 25 May 1993 to Nature. But in both cases the discovery was accidental, because

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SWCNTs were created in unsuccessful attempts to make MWCNTs filled with transition metals. From a scientific point of view it is reasonable to attribute the recognition for the finding of SWCNTs to both teams. It was also found that an image showing a nanotube resembling an SWCNT can be found in a figure from a paper published in 1976 by Oberlin et al. (1976).

Multi Walled CNTs In a MWCNT, concentric cylinders are formed by rolling up many number of graphene sheets. Each nanotube is with ~0.7 nm diameter and length of this molecule can be tens of micrometers. They are big and consist of several SWCNTs stacked one inside the other. If the outer diameter of Multi walled CNT is more than 15 nm, then the tubes are called carbon nano fibers. Carbon fibers are strands of layered-graphite sheets and different than CNTs. The comparison between SWCNTs and MWCNTs are shown in Table 1.

Synthesis of Carbon Nanotubes Even now, the growth mechanism of CNT is a topic of study, and it is assumed that more than one mechanism might be working for the development of CNTs. Tip growth (Ijima and Ichihashi 1993) and root growth (Saito et al. 1995) are the most accepted mechanisms for CNT growth. In the first method, a tubule tip is open and Table 1 SWCNT Single layer of graphene Catalyst is required for synthesis Bulk synthesis is difficult as it requires proper control over growth and atmospheric condition Not fully dispersed, and form bundled structures Resistivity usually within the range 104–103 Ωm Purity is poor. Typical SWCNT content in as-prepared samples using the chemical vapor deposition method is about 30–50 wt%. However, high purity up to 80% has been reported by using the arc discharge synthesis method The chance of a defect is greater during functionalization Characterization and evaluation is easy It can be easily twisted and is more pliable

MWCNT Multiple layer of graphene Can be produced without catalyst Bulk synthesis is easy Homogeneously dispersed with no apparent bundled formation Resistivity usually within the range 1.8  105–6.1  105 Ωm Purity is high. Typical MWCNT content in as-prepared samples using the chemical vapour deposition method is about 35–90 wt%

The chance of a defect is lower, especially when synthesized using the arc discharge method It has a very complex structure It cannot be easily twisted

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carbon atoms could be added to its perimeter. The metallic catalyst helps the growth reaction and prevents the closure of the tubule tip. One study by Hafner et al. proposed that the growth nuclei of CNT are fullerene molecules and the diameter of the tubule will determine the CNT size (Hafner et al. 1998). The root-growth model is based on the phase diagram of a metal and carbon. Still, number of research projects on the mechanism of CNT growth are ongoing. There are many theories on the growth mechanism for CNTs. One of the most accepted theories is that the metal catalyst particles float on graphite or another substrate (Sinnot et al. 1999). It presumes that the deposition will take place on spherical or pear-shaped catalyst particles. Thus, the deposition takes place only on the lower curvature side for the pear-shaped particle, i.e., one half of the surface. The diffusion of carbon is alongside the concentration gradient and on the opposite half, below and around the bisecting diameter it precipitates. The characteristic of these filaments is the hollow core, which is formed because the carbon does not precipitate from the apex of the hemisphere. Filaments can form for supported metals, either by extrusion or by tip growth. From the metal particles the CNTs grow upwards in extrusions that stay attached to the substrate. In tip growth, the particles separate and progress at the head of the growing CNT. A SWCNT or a MWCNT is grown, depending on the catalyst particle size (Fig. 9). Chemical modification, functionalization, filling, and doping are the different production methods for CNTs. Manipulation, separation, and characterization are now possible for individual CNTs. Every year the number of publications and patents on CNT synthesis rapidly increases even though many challenges concerning the synthesis of CNTs remain. At present, in the nanotube synthesis field there are four main challenges:

Fig. 9 Possible CNT growth mechanism: visualization

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1. Synthesis of large-scale, low-cost, high-quality CNTs: mass production. 2. Management of the structure and each property of the CNTs produced: selective production. 3. Control over the orientation and location of the CNTs produced on a substrate: organization. 4. A clear understanding of the nanotube growth processes: mechanism. Many methods have been used effectively for the production of CNTs: laser ablation (Guo et al. 1995), arc discharge (Qin et al. 2000), chemical vapor deposition (CVD), and spray pyrolysis (Yang et al. 2008; Xiao 2007; Ghosh et al. 2008). There are advantages, drawbacks, and challenges to each method. Laser ablation produces good quality SWCNTs with diameter and length within the range 5–20 nm and 10–100 mm respectively. A laser beam with high power impinges on a volume of methane or carbon monoxide, i.e., carbon-containing feedstock gas. Under an inert atmosphere a piece of graphite is vaporized by laser irradiation. The product is soot-containing CNTs and it is cooled in the quartz tube (at the walls). In terms of cost, this method is not effective because it requires highpurity graphite, high laser power, and a high energy process. In the arc discharge technique, by using an arc discharge between two carbon electrodes with or without a catalyst, a vapor is created and from the carbon vapor CNTs self-assemble. This method produces a mixture of MWCNTs, DWCNTs, and SWCNTs with diameters within the range 1.2–5.7 nm (Xiao 2007). CNT samples produced by this method contain carbon soot with fullerene molecules, amorphous nanofibers, amorphous carbon, and nanoparticles. The yield of CNTs from arc discharge is very low, but it is cheaper than the laser ablation method. It was found that compared with arc discharge method, laser ablation produced CNT samples that are purer, cleaner, have better properties, and a smaller amount of amorphous carbon. Generally, both methods create long, tangled, and impure CNTs and it is difficult to purify them (Ghosh et al. 2007). It is also observed that CNTs produced by arc discharge and laser ablation are also associated with the carbon phase, and are nontubular, and the removal of these carbon (non-nanotubes) is a very costly process (Robertson 2004). To synthesis CNT chemical vapor deposition (CVD) is the next method, which is accomplished by introducing hydrocarbon gases such as methane, ethane, benzene, or acetylene into the chamber. It is a well-known, economical method of synthesizing CNTs as it is suitable for mass production. The results of this method are MWCNTs with a diameter of approximately 10–20 nm or poor-quality SWCNTs with a large diameter range. Thermal chemical vapor deposition (TCVD) and plasma-enhanced chemical vapor deposition (PECVD) techniques have effectively synthesized MWCNTs and SWCNTs. Tsai et al. reported vertically aligned CNTs of a few millimeters in length grown on a large substrate with or without a catalyst using the TCVD method (Tsai et al. 2009). However, Zhang (2007) reported that MWCNTs produced using the TCVD method possess local curvature and are entangled with each other. Therefore, this is similar to CNT synthesis by arc discharge and laser ablation. The main drawbacks of the CNTs produced using

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this method are the low quality of CNTs and a high processing temperature, which easily damage the substrate (Zhang 2007). To overcome this problem, i.e., to synthesize aligned CNTs, a particular catalyst is needed (Tsai et al. 2009), or a lower substrate temperature using the PECVD method. To reduce the temperature during the synthesis of CNT, plasma (ionized gas) can be used and various plasma sources are used for the growth of CNTs: direct current, microwave, and inductive coupled radiofrequency. However, it is difficult to generate uniform plasma for a large area from these sources. For large area deposition the capacitive coupled radiofrequency the PECVD (rf-PECVD) method has been established. The quality is comparatively very low because it is covered with large amount of carbon soot, it has a low aspect ratio, and the CNTs have a stacked-cone structure when synthesized using the rf-PECVD method (Abrahamson et al. 1999). The diameter of the CNT synthesized by PECVD is larger (200 nm) than the CNT synthesized by TCVD and possesses a bamboo structure that is considered defective for CNT tubular structure. Controlling the length is also difficult; usually, it is about 25 μm (Zhang 2007). Spray pyrolysis is another promising method of synthesizing CNTs but is not very well known for CNT synthesis. This technique has recently attracted attention because of the possibility of obtaining long CNTs on a commercial scale (Yang et al. 2007). In this method, the carbon feedstock pyrolysis and the deposition of CNT simultaneously occur and is similar to CVD method. It is a very simple, adaptable, and cost-effective process with regard to equipment costs, low temperature process, and high-quality chemicals or substrates are not needed (Yang et al. 2007). One more advantage of spray pyrolysis is that easy scaling into an industrial-scale process, by continuously increasing reactants feeding into the furnace. In 1996, Smalley’s group described another method of synthesizing SWCNTs (Thess et al. 1996). These highly uniform tubes form aligned bundles, which Smalley named ‘ropes.’ From the initial experiments it was found that a very high proportion of nanotubes present in the rope samples with a specific armchair structure. This is a very good result because one major problem with nanotube samples up to that time was the presence of different structures. The results of later work have suggested that the rope samples are less homogeneous than expected. Table 2 shows some of the major synthetic methods of CNTs and their efficiency.

Production of Carbon Nanotubes from Bio-hydrocarbon Sources The synthetic methods of CNTs such as CVD, PECVD, TCVD, and spray pyrolysis use an expensive carbon source such as ethanol, hexane, benzene, toluene, xylene, and acetylene. Because of this high cost of raw materials, the production and industrial application of CNTs are limited. Mukhopadhyay (Kumar 2004) used a bio-hydrocarbon source – camphor (an extract of a tree) – to prepare various kinds of nano-carbons. This approach gives the idea to expand nanotechnology to be more environmentally and economically safe. It is observed that the use of a bio-hydrocarbon source on synthesizing CNT has been increased.

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Table 2 A summary of major production methods and efficiency of CNT Method Process

Arc discharge Connect two graphite rods to a power supply, place them a few millimeters apart. At 100 amps, carbon vaporizes and forms hot plasma

Condition

Low-pressure inert gas (helium)

Typical yield SWCNT

30–90% Short tubes with diameters of 0.6–1.4 nm

MWCNT

Short tubes with an inner diameter of 1–3 nm and an outer diameter of approximately 10 nm Pure graphite

Carbon source Cost Advantage

Disadvantage

High Can easily produce SWNT, MWNTs. SWNTs have few structural defects; MWNTs without catalyst, not too expensive, open air synthesis possible Tubes tend to be short with random sizes and directions; often needs a lot of purification

Laser ablation Blast graphite with intense laser pulses; use the laser pulses rather than electricity to generate carbon gas from which the CNTs form; try various conditions until hitting on one that produces prodigious amounts of SWNTs Argon or nitrogen gas at 500 Torr Up to 70% Long bundles of tubes (5–20 microns), with individual diameters from 1 to 2 nm Not very much interest in this technique, as it is too expensive, but MWNT synthesis is possible Graphite

High Good quality, higher yield, and narrower distribution of SWNTs than arc discharge

Costly technique, because it requires expensive lasers and high power, but it is improving

Chemical vapor deposition Place substrate in the oven, heat to a high temperature, and slowly add a carbon-bearing gas such as methane. As the gas decomposes it frees up carbon atoms, which recombine in the form of NTs

High temperatures within 500–1000
C at atmospheric pressure 20–100% Long tubes with diameters ranging from 0.6 to 4 nm Long tubes with diameters ranging from 10 to 240 nm

Fossil-based hydrocarbon and botanical hydrocarbon Low Easiest to scale up to industrial production; long length, simple process, SWNT diameter controllable, and quite pure

Often riddled with defects

By using the spray pyrolysis method CNTs have been effectively synthesized from turpentine and eucalyptus oil, which are bio-hydrocarbon precursors (Ghosh et al. 2008; Ghosh et al. 2007). Other precursors such as camphor are used to prepare CNTs using the CVD method (Kumar et al. 2004), palm oil by spray pyrolysis, and the TCVD method (Suriani et al. 2009). In addition, using the TCVD method CNTs

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have been prepared from different bio-hydrocarbon sources such as neem (Paul and Samdarshi 2011), coconut, olive, corn, sesame oil (Singaravelu et al. 2017), castor (Awasthi et al. 2011), and palm olein (Zobir et al. 2012). Suriani et al. reported that food industry waste materials such as cooking palm oil (Suriani et al. 2009) and chicken fat (Suriani et al. 2013) can be used as the precursor to the synthesis of CNT using the TCVD method. It is a good effort that the waste materials are used as carbon precursors for the production of CNTs because it helps the environment. In all the synthetic CNT methods, many impurities come with the CNT and the amount and type depend on the method used. The main impurities are carbonaceous materials – graphite sheets, fullerenes, and metals. The properties of the CNTs may change with the presence of impurities. There are many techniques used to purify CNTs which are: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Oxidation (Hajime et al. 2002; Borowiak-Palen et al. 2002) Acid treatment (Hajime et al. 2002; Borowiak-Palen et al. 2002) Ultrasonication (Hajime et al. 2002) Annealing and thermal treatment (Borowiak-Palen et al. 2002) Micro-filtration (Borowiak-Palen et al. 2002) Magnetic purification (Thien-Nga et al. 2002) Chemical or mechanical cutting (Gu et al. 2002) Functionalization (Niyogi et al. 2001) Chromatography (Farkas et al. 2002)

The purification helps to remove amorphous carbon, increase the surface area, change the pore volume, decompose the functional groups, which block the pore entrance, or induce extra functional groups.

Characterization, Properties, and Applications of CNTs To find out the quality, quantity, and properties of CNTs, the characterization is very important. Its application will need certification of function and properties (Kingston 2007). There are not so many techniques available for fully characterizing CNTs, but there are techniques that are useful for individual characterization. Carbon nanotubes have different optical absorptions, especially SWCNTs owing to their unique electronic structure. Thus, the absorption spectroscopic technique is very useful for characterizing the purity of CNTs. SWCNTs are either metallic or a semiconductor in nature. The energy gap of the semiconducting tube is related to the chirality and inverse of the tube diameter. Photoluminescence from the combination of an electron hole pair is observed. Carbon atoms in CNTs have p-electrons, which give a large π-electronic system. The luminescence spectra seem to be very useful for sensing the presence of chemical defects and the purity of CNTs. To obtain the information about the chemical structure, X-ray photoelectron spectroscopy (XPS) can be used. This technique proves that CNTs are like carbon oxides rather than graphite. It is most widely used to refer to the structural modification of the CNT walls. Lee et al. studied the sidewall

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functionalization of SWCNTs by fluorination using XPS and concluded that the C1s of undoped SWCNTs show an sp2 carbon peak at 284.3 eV, an sp3 carbon peak at 285 eV, and carbonyl groups at 288.5 eV (Lee et al. 2003). The structural defects on the electronic structure of MWCNTs are also identified by using this technique. To obtain direct information about the size, shape, and structure of the CNTs electron microscopy – both scanning and transmission, is an essential tool. To study the intershell spacing of MWCNTs, high-resolution TEM images were used (Kiang et al. 1998). The three-dimensional morphology of CNTs are directly obtained from scanning tunneling microscopy (STM). Both atomic structure and electronic density of states can be simultaneously resolved by STM (Sattler 1995). To obtain information about interlayer spacing, the structural strain, and impurities, a sample nondestructive technique, i.e., X-ray diffraction technique can be used (Kuzmany et al. 2001). The structural features such as bond length and possible distortion of the hexagonal network can be determined by neutron diffraction (Burian et al. 2004). The qualitative and quantitative information about diameter, electronic structure, crystallinity, and purity of the CNTs can be obtained using Raman spectroscopy. This is considered an extremely powerful technique for characterizing SWCNT. It is reported by Arepalli et al. that the purification of CNTS is studied using thermal gravimetric analysis (TGA) (Arepalli et al. 2004). The amazing properties of CNTs are due to the strength of the carbon–carbon bond. There was no material that previously displayed the combination of superlative thermal, mechanical, and electronic properties attributed to them. The densities of CNTs are one-sixth of that of stainless steel (1.3 g/cm3). Its Young’s modulus (measure of material stiffness) value is greater than 1 TPa, which is five times higher than steel, are higher than those of all carbon fibers (Andrews and Weisenberger 2004). CNTs are strong materials: the highest measured tensile strength for a CNT is around 50 times higher than that of steel (63 GPa) (Andrews and Weisenberger 2004). It can be 10 times as strong as steel and 1.2 times as rigid as diamond. They also have good environmental and chemical stability and high thermal conductivity (3000 W/m/K, comparable with diamond). The properties of CNTs are high thermal and electrical conductivity, aspect ratio, elasticity (elongation to failure ~18%), very high tensile strength, high flexibility (it can bend significantly without damage), low thermal expansion coefficient, and good electron field emission properties. These properties make them good for many applications. High-quality CNTs are preferred for basic and technological applications. These properties and lightness of CNTs have great potential in applications such as aerospace and nanoscale electronic devices. CNTs are already used in many areas of technology, including scanning probe microscopes, flat panel displays, fuel cells, and sensing devices (Yu et al. 2000). We reviewed the analytical applications of CNTs. CNTs are used for gas sensors, enzymatic biosensors, DNA probes, voltammetry, and immune sensors. The removal of metal ions such as Cd2+, Ni2+, Cu2+, Zn2+, and Pb2+ from an aqueous solution using different kinds of CNTs were summarized by Rao et al. (2007). It was reported that CNTs are promising adsorbents for natural organic matter in aqueous solutions; thus, CNT can be used for water treatment (Lu and Su 2007).

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Because of its large surface area and a tendency toward surface functionalization, CNTs can be used as carriers for the pharmaceutical nano-delivery of drugs and vaccines and for biosensing. In the biomedical field more applications of CNTs are reported. Some of them are biosensors, the preparation of unique biomaterials, protein delivery (Kam and Dai 2005), drug delivery (Bianco et al. 2005), gene delivery (Panhuis 2003), and vaccine delivery (Liu et al. 2005). It also used to determine food poisoning by mycotoxins, which are fungal bodies (Yao et al. 2006). It is reported that CNTs also have a role in agriculture. It is reported that water soluble CNTs increase the media’s solubility when dispersed in it. CNTs help the penetration of plant seeds and roots, which results in a considerable boost in the growth of plants. Researchers suggest that the use of water-soluble CNTs can play an important role in dry area plant growth (Jackson et al. 2013).

Disadvantages of CNT Even though CNTs have many applications in many fields, it has also some disadvantages for environmental and human health. The National Toxicology Program at the National Institute of Environmental Health Sciences (NIEHS) and the National Institute of Occupational Safety and Health (NIOSH) helps to find out and bring about awareness of the health effects of CNTs. Different nanoparticles and the impurities present in them can both have a specific effect on the toxicity of CNTs.

Environmental Effects Carbon nanotubes are highly stable, least biodegradable and bio-persistent in nature. Because of its hydrophobic nature, it should not be dispersed in water as it often aggregates. Studies show that there is a chance for bioaccumulation (Petersen et al. 2011). For example, Khodakovskaya et al. reported that during germination and growth, CNTs penetrate an array of plant seeds and plant roots (Khodakovskaya et al. 2009). They were also able to migrate small concentrations from the plant roots to other parts such as the leaves and fruit. It is reported that, in the digestive tract of organisms in the lower levels of the ecological pyramid, CNTs can remain and move up the food chain as these organisms are consumed (Kim et al. 2010).

Health Effects Studies showed that the inhalation of CNTs can cause mutations in lung cells that could lead to cancer. Zeni et al. (2008) reported that with the interaction of SWCNTs there was a potential toxic effect on human peripheral blood lymphocyte cells. Depending on the chemicals attached to the functional groups, the cytotoxicity of the functionalized CNT will vary. The toxicity of CNTs still not completely known. It is related to the impurities produced and maybe to their mass fragment constitution.

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To reduce toxicity, it is essential to control the extreme hydrophobicity of CNTs. Owing to its high hydrophobicity it aggregates in polar liquids. The surface modification with covalent or noncovalent groups can reduce its hydrophobicity or increase its hydrophilicity. Hydrophilic substituents can be incorporated through covalent linkage into the exterior CNT sidewalls (Wepasnick et al. 2010). Noncovalent modification involves adsorption of surfactants such as NaDDBS onto the CNT without affecting its basic CNT structure (Cheng et al. 2008). For consumer applications, covalent and noncovalent modifications have been used to prepare CNTs. Among the different types of strategies for covalent surface modification, purposely grafting oxygen-containing functional groups at the sidewalls and open ends of CNTs is an accepted and adaptable approach (Wepasnick et al. 2010). In fact, this strategy is frequently used to produce CNTs that disperse in water (Smith et al. 2009). Accidental surface oxidation is also possible after CNTs are released into the environment. This may be due to exposure to natural oxidizing agents such as ozone and hydroxyl radicals (Vione et al. 2006), or a common water treatment process that creates UV irradiation and ozonolysis (Savage et al. 2003). There are different methods available to incorporate surface oxygen into CNTs: wet chemical oxidation (Li et al. 2008), plasma treatments (Yang et al. 2008), and rational functionalization strategies (Gromov et al. 2005). The oxidative treatments also help to remove impurities such as amorphous carbon and metallic impurities from CNTs (Bergeret et al. 2008), in spite of the fact that a large variety of oxidizing conditions are used to modify CNTs. Researchers at different institutions such as Safer Nano materials and Nano Manufacturing Initiative at the Oregon Nanoscience and Microtechnologies Institute (ONAMI), Center for Green Chemistry and Green Engineering at Yale, and the Sustainable Nanotechnology Organization are helping to find green nanotechnology methods for developing CNTs. Through this sustainable development the toxicity of CNTs may be controlled.

Conclusions In this chapter the history, synthesis, and applications of CNTs are discussed. CNTs are examples of real nanotechnology. The size is in nanometric diameters and can be manipulated physically and chemically. CNTs were discovered many years ago, even though the properties and production of CNT remain debatable subjects. In spite of the fact that many researchers have been working on CNTs and its current technological applications, it is well known that the production of CNTs with controlled properties is difficult. Here, the synthetic methods of CNTs have been explained. They have unique physical, electronic, chemical, and mechanical properties and they have incredible applications.

References Abrahamson J, Wiles PG, Rhoades BL (1999) Structure of carbon fibers found on carbon arc anodes. Carbon 37:1873–1874 Ahmad A, Kholoud MM, El-NourbReda A, Ammarc AA, Al-Warthan A (2012) Carbon nanotubes, science and technology part (I) structure, synthesis and characterization. Arab J Chem 5:1–23

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Andrews R, Weisenberger MC (2004) Carbon nanotube polymer composites. Curr Opin Solid State Mater Sci 8(1):31–37 Arepalli S, Nikolaev P, Gorelik O, Hadjiev VG, Holmes W, Files B, Yowell L (2004) Protocol for the characterization of single-wall carbon nanotube material quality. Carbon 42:1783–1791 Awasthi K, Kumar R, Raghubanshi H, Awasthi S, Pandey R, Singh D, Srivastava ON (2011) Synthesis of nano-carbon (nanotubes, nanofibres, graphene) materials. Bull Mater Sci 34: 607–614 Bergeret C, Cousseau J, Fernandez V, Mevellec J-Y, Lefrant S (2008) Spectroscopic evidence of carbon nanotubes’ metallic character loss induced by covalent functionalization via nitric acid purification. J Phys Chem C 112:16411–16416 Bianco A, Kostarelos K, Prato M (2005) Applications of carbon nanotubes in drug delivery. Curr Opin Chem Biol 9:674–679 Boehm HP (1997) The first observation of carbon nanotubes. Carbon 35:581–584 Borowiak-Palen E, Pichler T, Liu X, Kunpfer M, Graff A, Jost O, Pompe W, Kalenczuk RJ, Fink J (2002) Reduced diameter distribution of single-wall carbon nanotubes by selective oxidation. Chem Phys Lett 363:567–572 Burian A, Koloczk J, Dore J, Hannon AC, Nagy JB, Fonseca A (2004) Radial distribution function analysis of spatial atomic correlations in carbon nanotubes. Diam Relat Mater 13:1261–1265 Chang TE, Jensen LR, Kisliuk A, Pipes RB, Pyrz R, Sokolov AP (2005) Microscopic mechanism of reinforcement in single-wall carbon nanotube/polypropylene nanocomposite. Polymer 46(2): 439–444 Cheng J, Fernando KAS, Veca LM, Sun Y-P, Lamond AI, Lam YW et al (2008) Reversible accumulation of PEGylated single-walled carbon nanotubes in the mammalian nucleus. ACS Nano 2(10):2085–2094 Donaldson K, Aitken R, Tran L et al (2006) Carbon nanotubes: a review of their properties in relation to pulmonary toxicology and workplace safety. Toxicol Sci 92:15–22 Endo M, Dresselhaus MS (2003) Carbon fibers and carbon nanotubes, Shinshu University, Japan & MIT, file:millie-science-endo99.tex Endo M, Saito R, Dresselhaus MS, Dresselhaus G (1997) Carbon fibers to carbon nanotubes. CRC Press, Boca Raton Esquivel EV, Murr LE (2004) A TEM analysis of nanoparticulates in a polar ice core. Mater Charact 52:15–25 Farkas E, Anderson ME, Chen ZH, Rinzler AG (2002) Length sorting cut single wall carbon nanotubes by high performance liquid chromatography. Chem Phys Lett 363:111–116 Ghosh P, Afre RA, Soga T, Jimbo T (2007) A simple method of producing single-walled carbon nanotubes from a natural precursor: eucalyptus oil. Mater Lett 61:3768–3770 Ghosh P, Soga T, Afre RA, Jimbo T (2008) Simplified synthesis of single-walled carbon nanotubes from a botanical hydrocarbon: turpentine oil. J Alloys Compd 462:289–293 Gromov A, Dittmer S, Svensson J, Nerushev OA, Perez-Garcia SA, Licea-Jimenez L et al (2005) Covalent amino-functionalisation of single-wall carbon nanotubes. J Mater Chem 15(32): 3334–3339 Gu Z, Peng H, Hauge RH, Smalley RE, Margrave JL (2002) Cutting single-wall carbon nanotubes through fluorination. Nano Lett 2(9):1009 Guo T, Nikolaev P, Thess A, Colbert DT, Smalley RE (1995) Catalytic growth of single walled nanotubes by laser vaporization. Chem Phys Lett 243:49–54 Hafner JH, Bronikowski MJ, Azamian BR, Nikolaev P, Rinzler AG, Colbert DT, Smith A, Smalley RE (1998) Catalytic growth of single-wall carbon nanotubes from metal particles. Chem Phys Lett 296:195–202 Hajime G, Terumi F, Yoshiya F, Toshiyuki O (2002) Method of purifying single wall carbon nanotubes from metal catalyst impurities. Honda Giken Kogyo Kabushiki Kaisha Hughes TV, Chambers CR (1889) Manufacture of N Filaments. US Patent No.:405480 Iijima S (1991) Helical microtubules of graphitic carbon. Nature 354:56–58 Ijima S, Ichihashi T (1993) Single-shell carbon nanotubes of 1-nm diameter. Nature 363:603–605 Jackson P, Jacobsen N, Baun A et al (2013) Bioaccumulation and ecotoxicity of carbon nanotubes. Chem Cent J 13(7):154

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Kam NWS, Dai HJ (2005) Carbon nanotubes as intracellular protein transporters: generality and biological functionality. J Am Chem Soc 127:6021–6026 Khodakovskaya M, Dervishi E, Mahmood M et al (2009) Carbon nanotubes are able to penetrate plant seed coat and dramatically affect seed germination and plant growth. ACS Nano 3(10): 3221–3227 Kiang C, Endo M, Ajayan P, Dresselhaus G, Dresselhaus M (1998) Size effects in carbon nanotubes. Phys Rev Lett 81:1869 Kim K, Klaine S, Lin S et al (2010) Acute toxicity of a mixture of copper and single-walled carbon nanotubes to Daphnia magna. Env Toxicol Chem 29(1):122–126 Kingston C (2007) Challenges in the characterization of carbon nanotubes: the need for standards. Molecular and nanomaterial architectures group tri-national workshop on standards for nanotechnology Kroto HW, Heath JR, O’Brien SC, Curl RF, Smalley RE (1985) C60: Buckminsterfullerene. Nature 318:162–163 Kumar M, Kakamu K, Okazaki T, Ando Y (2004) Field emission from camphor-pyrolyzed carbon nanotubes. Chem Phys Lett 385:161–165 Kuzmany H, Plank W, Hulman M, Kramberger C, Gruneis A, Pichler T, Peterlik H, Kataura H, Achiba Y (2001) Determination of SWCNT diameters from the Raman response of the radial breathing mode. Eur Phys JB 22(3):307–320 Lee Y, Cho T, Lee B, Rho J, An K, Lee Y (2003) Surface properties of fluorinated single-walled carbon nanotubes. J Fluor Chem 120:99–104 Li M, Boggs M, Beebe TP, Huang CP (2008) Oxidation of single-walled carbon nanotubes in dilute aqueous solutions by ozone as affected by ultrasound. Carbon 46:466–475 Liu Y, Wu DC, Zhang WD, Jiang X, He CB, Chun TS, Goh SH, Leong KW (2005) Polyethylenimine grafted multi-walled carbon nanotubes for secure noncovalent immobilization and efficient delivery of DNA. Angew Chem Int Ed Engl 44:4782–4785 Lu C, Su F (2007) Adsorption of natural organic matter by carbon nanotubes. Sep Purif Technol 58: 113–121 Niyogi S, Hu H, Hamon MA, Bhowmik P, Zhao B, Rpzenzhak SM, Chen J, Itkis ME, Meier MS, Haddon RC (2001) Chromatographic purification of soluble single-walled carbon nanotubes (s-SWNTs). J Am Chem Soc 123(4):733–734 Oberlin A, Endo M, Koyama T (1976) Filamentous growth of carbon through benzene decomposition. J Cryst Growth 32:335–349 Panhuis MIH (2003) Vaccine delivery by carbon nanotubes. Chem Biol 10:898–899 Paul S, Samdarshi SK (2011) A green precursor for carbon nanotube synthesis. New Carbon Mater 26:85–88 Petersen E, Zhang L, Mattison N et al (2011) Potential release pathways, environmental fate, and ecological risks of carbon nanotubes. Environ Sci Technol 45(23):9837–9856 Pratik M, Ashwin P, Prasad E (2021) Carbon nanotubes market Qin LH, Zhao X, Hirahara K, Miyamoto Y, Ando Y, Iijima S (2000) Materials science: the smallest carbon nanotube. Nature 408:50–51 Radushkevich LV, Lukyanovich VM (1952) On the carbon structure formed during thermal decomposition of carbon monoxide in the presence of iron’ (in Russian). Zh Fizich Khim 26:88 Rao GP, Lu C, Su F (2007) Sorption of divalent metal ions from aqueous solution by carbon nanotubes: a review. Sep Purif Technol 58:224–231 Reibold M, Paufler P, Levin AA et al (2006) Materials: carbon nanotubes in an ancient Damascus sabre. Nature 444:286 Robertson J (2004) Realistic applications of CNT. Mater Today 7:46–52 Ryabov VV, Ponomarchuk VA, Titov AT, Semenova DV (2012) Micro- and nanostructures of carbon in Pt-low-sulfide ores of the Talnakh deposit (Siberian platform). Dokl Earth Sci 446:1193–1195 Saito Y, Okuda M, Tomita M, Hayashi T (1995) Extrusion of single-wall carbon nanotubes via formation of small particles condensed near an arc evaporation source. Chem Phys Lett 236: 419–426 Sattler K (1995) Scanning tunneling microscopy of carbon nanotubes and nanocones. Carbon 33: 915–920

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Savage T, Bhattacharya S, Sadanadan B, Gaillard J, Tritt TM, Sun Y-P et al (2003) Photoinduced oxidation of carbon nanotubes. J Phys Condens Matter 15:5915–9521 Service RF (1998) Superstrong nanotubes show they are smart too. Science 281(5379):940–942 Singaravelu V, Schreiber M, Muthuramkumar S, Misra M, Mohanty AK (2017) Carbon nanotubes from renewable feedstocks: a move toward sustainable nanofabrication. Appl Polym Sci 134:44255 Smith B, Wepasnick K, Schrote KE, Cho H-H, Ball WP, Fairbrother DH (2009) Influence of surface oxides on the colloidal stability of multi-walled carbon nanotubes: a structure–property relationship. Langmuir 25(17):9767–9776 Suriani AB, Azira AA, Nik SF, Md Nor R, Rusop M (2009) Synthesis of vertically aligned carbon nanotubes using natural palm oil as carbon precursor. Mater Lett 63:2704–2706 Suriani AB, Dalila AR, Mohamed A, Mamat MH, Salina M, Rosmi MS, Rosly J, Md Nor R, Rusop M (2013) Vertically aligned carbon nanotubes synthesized from waste chicken fat. Mater Lett 101:61–64 Tennent HG (1987) Carbon fibrils, method for producing same and compositions containing same, US Patent No.: 4663230 A, Assignee: Hyperion Catalysis International, Inc. Thess A, Lee R, Nikolaev P, Dai H, Petit P, Robert J, Xu C, Lee YH, Kim SG, Rinzler AG, Colbert DT, Scuseria GE, Tománek D, Fischer JE, Smalley RE (1996) Crystalline ropes of metallic carbon nanotubes. Science 273:483 Thien-Nga L, Hernadi K, Ljubovic E, Garaj S, Forro L (2002) Mechanical purification of singlewalled carbon nanotube bundles from catalytic particles. Nano Lett 2(12):1349–1352 Thostenson ET, Zhifeng R, Tsu-Wei C (2001) Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 61:1899–1912 Tsai TY, Tai NH, Chen KC, Lee SH, Chan LH, Chang YY (2009) Growth of vertically aligned carbon nanotubes on glass substrate at 450
C through the thermal chemical vapour deposition method. Diam Relat Mater 18:307–311 Vione D, Maurino V, Minero C, Pelizzetti E, Harrison MAJ, Olariu RI et al (2006) Photochemical reactions in the tropospheric aqueous phase and on particulate matter. Chem Soc Rev 35: 441–453 Wang N, Tang ZK, Li GD, Chen JS (2000) Materials science: single-walled 4 Å carbon nanotube arrays. Nature 408:50 Xiao J (2007) Study of factors affecting the synthesis of carbon nanotubes by spray pyrolysis, Master Thesis, University of Texas, El Paso Yang D-Q, Sacher E (2008) Strongly enhanced interaction between evaporated Pt Nanoparticles and functionalized multiwalled carbon nanotubes via plasma surface modifications: effects of physical and chemical defects. J Phys Chem C 112:4075–4082 Yang Z, Chen X, Nie H, Zhang K, Li W, Yi B, Xu L (2008) Direct synthesis of ultralong carbon nanotube bundles by spray pyrolysis and investigation of growth mechanism. Nanotechnology 19:085606 Yao D-S, Cao H, Wen S, Liu D-L, Bai Y, Zheng W-J (2006) A novel biosensor for sterigmatocystin constructed by multi-walled carbon nanotubes (MWNT) modified with a flatoxin – detoxifizyme (ADT Z). Bioelectrochemistry 68:126–133 Yu MF, Files BS, Arepalli S, Ruoff RS (2000) Tensile loading of ropes of single wall carbon nanotubes and their mechanical properties. Phys Rev Lett 84(24):5552–5555 Zeni O, Palumbo R, Zeni L, Sarti M, Scarfi MR (2008) Cytotoxicity investigation on cultured human blood cells treated with single-wall carbon nanotubes. Sensors 8:488–499 Zhang Y (2007) Physical properties investigation of nanostructure material and their application, Ph.D. Thesis, University of California, Santa Cruz Zhang M, Li J (2009) Carbon nanotube in different shapes. Mater Today 12(6):12–18 Zobir SAM, Suriani AB, Abdullah S, Zainal Z, Sarijo SH, Rusop M (2012) Raman spectroscopic study of carbon nanotubes prepared using Fe/ZnO-palm olein-chemical vapour deposition. J Nanomater 2012:1–6

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Synthesis Methods of Carbon Nanotubes Atike Ince Yardimci and Nesli Yagmurcukardes

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNT Synthesis Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arc Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laser Ablation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sonochemical/Hydrothermal Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Liquid Phase Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flame Synthesis of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plastic Pyrolysis Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Vapor Deposition (CVD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Carbon nanotubes (CNTs) with their extraordinary mechanical, electrical, and thermal properties have built broad interest in most areas of science and engineering and are utilized for many applications in the nanotechnology area. There are many approaches for CNT synthesis. While arc discharge, laser ablation, and chemical vapor deposition are the most common methods, lots of rarely utilized CNT production methods were reported in the literature. In this chapter, a detailed discussion about CNT synthesis methods with their experimental process, advantages, and disadvantages are provided. Especially, the CVD method, which is the most promising CNT synthesis method, has been reviewed with its subtypes. A. I. Yardimci (*) Technology Transfer Office, Uşak University, Uşak, Turkey e-mail: [email protected] N. Yagmurcukardes Department of Material Science and Nanotechnology Engineering, Engineering Faculty, Uşak University, Uşak, Turkey © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_52

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CNT synthesis method directly defines the type and properties of nanotubes. Therefore, researchers are still working on new methods and the development of currently used methods. Literature studies indicate that different synthesis methods come to the fore for different features such as SWCNT or MWCNT production, vertically aligned nanotubes, high yield synthesis, large-area synthesis, etc. In this chapter, a general overview and comparison of these methods are made. Keywords

Carbon nanotube · Synthesis · Synthesis methods · Growth parameters · High quality CNT

Introduction Intensive researches in the synthesis of high-quality CNTs have been carried out since the first discovery of CNTs using arc synthesis techniques by Iijima in 1991. To obtain high quality and high yield of CNTs in a cost-effective way, a lot of methods have been tried and compared to each other. In this chapter, various carbon nanotubes synthesis (CNTs) methods are discussed in detail with the support of current literature. Among the discussed techniques gas-phase methods such as arc discharge, laser ablation, and chemical vapor decomposition (CVD) are found as the most promising methods for the synthesis of CNTs. In addition to gas-phase CNT synthesis methods, some condensed phase synthesis methods have been developed for carbon nanomaterial production. Molten salt systems and organic liquid systems such as electrochemical and sonochemical methods are some examples of condensed phase methods. Liquid phase synthesis of CNTs by applying an electrical current to a conductive substrate in organic solvents is also an alternative method. For each CNT synthesis technique, different process parameters such as gas flow rates, pressure, growth time, growth temperature, catalyst type, catalyst pretreatment process, substrates, carrier gases, and type of carbon precursors define the CNT growth mechanism and influence CNT properties. Each CNT synthesis method has some advantages and disadvantages that add different properties to the nanotubes, which is of paramount importance when choosing a specific method for the preparation of CNTs with the required properties. The most promising three synthesis methods are arc discharge, laser ablation, and chemical vapor deposition (CVD). First CNT synthesis was carried out with the Arc discharge method; however, in this method CNTs are found together with a large concentration of amorphous carbon, and CNTs are not aligned. The laser ablation method is the second method and it can provide arrays of ordered CNTs. Besides, arc discharge and laser ablation methods require a large amount of energy as well as a source of graphite, which makes them uneconomical processes for large-scale production. The CVD technique is the most widespread method to produce CNTs, which is appropriate for scaling up. This method provides low-cost CNT production

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using a simple setup. Also, it enables the production of a large yield of CNTs with high purity at a lower temperature compared to the previous methods. The properties of CNTs in terms of alignment, diameter, and length could be controlled by the CVD method. While arc discharge and laser ablation methods require a solid target and very high temperatures to evaporate it, the CNTs obtained from both arc-discharge and laser ablation methods are tangled and thus purification is not evident for nanotubes synthesized by these methods. All of the CNT synthesis methods have some advantages and disadvantages, each technique will be explained below. There are a lot of CVD types that are determined by factors such as reactants used during the process, maximum tolerable air leakage amount into the system, purity of the materials used, size, shape, and properties of the substrate, and so on. Detailed information about CVD types is provided in this chapter.

CNT Synthesis Methods Arc Discharge The Arc-discharge method is the first method to utilize for producing CNTs by Iijima (Iijima et al. 1992),.MWCNTs were obtained in 1991, and SWCNTs were obtained in 1993 firstly by this method. In this method, CNT growth carries out on carbon electrodes by applying direct current (DC) to obtain a non-fluctuating arc, for which closed-loop automation is utilized to adjust the gap automatically during arc-discharge evaporation of carbon. The process occurs in an inert buffer gas atmosphere such as argon, helium, hydrogen, nitrogen, or their mixtures in order to avoid the oxidation of carbon at higher temperatures (Popov 2004). Utilized two electrodes are high purity graphite electrodes; one of them (anode) contains catalysts such as Fe, Co, Ni while the other one (cathode) is pure graphite (Iijima and Ichihashi 1993). The anode surface is heated to a higher reaction temperature between 3000 and 6500  K, which provides anode to vaporize and condense on the cathode surface; therefore nanotube formation has occurred. The carbon vapors precipitate in the gas phase and drift toward the cathode where it cools down due to the temperature gradient. After an arc application time of few minutes, the discharge is stopped and cathodic deposition that contains CNTs along with the soot is collected from the walls of the chamber. The obtained product is then purified for observation under an electron microscope to investigate its morphology. Methane is the best gas for the growing of MWNTs (Ando 1994). However, it is indicated that the CNTs can also be grown in the liquid nitrogen atmosphere or water. Another important parameter is the pressure of inert gas during the arc-evaporation process, which has a strong influence on SWCNTs. The stability of electric arc (Harris 2007; Arora and Sharma 2014) and the type of power supply (Arora and Sharma 2014) also affect the morphology and quality of the CNTs. DC arc current is applied across the electrodes in the arc-discharge method, and continuous emission of electrons from the cathode bombards the anode at high velocity.

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The diameter of the cathode is usually larger than that of the anode, and it results in a lower current density of the cathode than the anode. Therefore, a high-temperature gradient across the electrodes is observed, which sublimes the carbon precursor filled inside the anode and results in the formation of CNTs. However, when AC arc current is applied, the current changes periodically across the electrodes, and deposition does not occur at either side of the electrodes. Other important arc discharge parameters are growth temperature, electrode dimension, geometry, shape of electrodes, and pulse duration. Especially, growth temperature directly influences the diameter of SWCNTs, and growth temperature, type of catalyst, and carbon precursor affect the diameter of MWCNTs. This method allows the production of SWCNTs or MWCNTs easily and enables the synthesis of carbon particles in the open air with optimum energy usage and high reaction temperature, which provides CNTs with high crystallinity. The most important advantage of this technique is its high efficiency and the capability to produce CNTs in a specific area.

Laser Ablation The laser ablation method was firstly employed to produce CNTs by Smalley’s Group in 1995 in a double-pulse laser oven and it is mostly utilized to develop SWCNT bundles with a narrow diameter distribution (Guo et al. 1995). The working principle of the laser ablation method is similar to that of the arc-discharge method. Firstly, a piece of graphite target is vaporized by utilization of various varieties of lasers under high temperature in an inert gas atmosphere, and vaporization of carbon gas is followed by condensation process. CNT growth temperature in this method is nearly 1200  C and the generally used inert gases are argon, neon, or helium. At this high temperature, a continuous or a pulsed laser is employed to vaporize the target that is located in the center of the quartz tube furnace to produce fullerenes and CNTs. The target also consists of a mixture of graphite and metal catalysts, such as Fe, Co, and Ni (Yudasaka et al. 1999; Kataura et al. 2000). Bi-metallic catalysts also could be appropriate for CNT growth by laser ablation. For materials that have high boiling temperatures, the laser can be more suitable than the other vaporization devices because of the energy density of the laser (Guo et al. 1995). By-products obtained during the CNT synthesis process are α-C, graphite particles, graphitic polyhedrons with enclosed metal particles, polyhedral, and fullerenes. The principle of obtaining a CNT is by following these steps: Firstly, the target is vaporized and then a cloud occurrs, which contain C, C2, C3, and rapidly catalyst vapors are formed. With the condensation of the cloud, the small weight of carbon molecules comes together to form a larger one. The vaporized catalysts are included in this large molecule to prevent the closing of these carbon molecules into a cage structure. The growth process is finalized with a SWNT. When the system is cooled up to low temperature, this process is finished (Baddour and Briens 2005). In literature, the pulsed arc technique and the pulsed laser technique are found to be similar in terms of nucleation, growth mechanism, and SWCNT yields and

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morphologies. In the laser ablation method, different types of lasers, pulse width, and laser wavelength were tried to enhance the quality of nanotubes, production rate, and consistency in the production rate. Different laser systems such as dual-laserpulsed system and continuous laser system were employed in many studies and detailed investigations indicate that laser ablation method seems to be the most reliable, environmentally friendly, and promising SWCNTs synthesis method. It provides high purity, yield, and easy post-processing as compared to other CNT synthesis techniques. In this method, the size of carbon agglomerates, catalyst cluster growth mechanism, and carbon-metal catalyst interactions define nanotube properties. Other advantages of this method are: high-quality SWCNT synthesis (MWCNTs synthesis with this method could be performed only in some special growth conditions), diameter controllability and narrow diameter distribution, tunable duration of laser vaporization, and high crystallinity of the CNT (Soni et al. 2020). Besides the advantages of laser ablation, the high temperatures and using a laser to produce CNTs make this method uneconomical compared to other CNT synthesis methods.

Electrolysis The capture of carbon dioxide (CO2) is an important issue because of the negative impact of this gas on climate change. The concentration of CO2 in the atmosphere continues to increase with fossil fuel combustion, deforestation, and massive industrialization. Therefore, the conversion of CO2 is a significant topic. In the electrolytic synthesis method of CNTs, which was developed in the Institute of General and Inorganic Chemistry (Novoselova et al. 2006), CO2 is the main reactant and is introduced in chloride melt under optimum parameters. Cathodic reduction of CO2 to elemental carbon in metallic electrodes forms the basis of the electrolysis method. By electrochemical reactions, a new condensed carbon phase is produced on the cathode from a liquid molten salt phase containing dissolved carbonic acid gas. CO2 indicates a low solubility in the chloride melting, so the only possible way to increase the cathode process rate is to create an extreme gas pressure. The only cathodic reaction product of wide current density and potential ranges is carbon and the anodic reaction product is oxygen. CO2 is separated into elemental carbon (C) and O2 electrochemically and the graphite anodes utilized in this electrolysis method can react with isolated oxygen and produce carbon dioxide. Therefore, a continuous process is provided. If the correct combinations of electrode materials, electrolyte, temperature, and electrical current were provided; CNTs have been obtained from CO2 by the electrolysis method (Novoselova et al. 2008). Carbon could be converted in a compact and stable form by the electrolysis method and carbon storage in this form is possible. Generally, the CNT growth process occurs by the decomposition of carboncontaining precursor or a hydrocarbon gas on the surface of a metal catalyst such as Fe, Ni, Co, Mo, and their oxides forms and alloys. However, in electrochemical

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techniques, the most common catalysts are Ni or Ir anode and steel cathode for the formation of carbon nanotubes or fibers. Ni erosion is assumed to be the source of metal catalyst particles (Novoselova et al. 2008). The synthesized CNTs are usually MWCNTs in this method and the possibility to obtain MWCNTs by this method is with content up to 40 vol% in the cathode product. MWCNTs have generally curved forms and they frequently create bundles and most of the nanotubes are partially filled with the electrolyte salt. Besides, some different carbon phases are produced with CNTs and these phases contain nanoscalesized metallic impurities. Metallic impurities are placed inside and on the ends of the CNTs. The increase of CNT percentage in the overall volume of the carbon product was detected in the samples where the austenite phase was fixed. As a result, the metallic phase acts as a catalyst for nanotube formation in the electrolytic synthesis method of CNTs from molten salts. The advantages of the electrolytic method are simple experimental set-up, controllable synthesis process by the electrolysis modes, cheap raw materials, controllable CNT structures, and morphologies and carbon phases doping in one step by means of optimization of electrolysis conditions and electrolytic bath composition (Chen and Fray 2003). Synthesis of CNTs by electrolysis method is a cheap method compared to conventional CNT synthesis methods. Considering, the disadvantages of this method, there are two main problems with this method. First is the cracking problem of graphite cathode during the electrolysis process and the second is the accumulation of electrolysis products such as chlorine gas (anode), alkaline metal (cathode), and carbon nanomaterials (cathode) in the bath. These two problems cause instability in the process and prevent continuous performance.

Sonochemical/Hydrothermal Synthesis The sonochemical/hydrothermal synthesis technique is an appropriate method to produce different types of carbon nanostructures such as nano-onions, nanorods, nanowires, nanobelts, and MWCNTs. There are three important advantages of this method compared to other CNT synthesis methods. Firstly, the starting materials are easy to obtain and are stable in ambient temperature; secondly, even at low temperatures about 150–180  C, CNT growth is carried out in this method, lastly, it does not require a hydrocarbon gas or carrier gas for the CNT growth process. For the production of MWCNTs by sonochemical/hydrothermal method, Gogotsi et al. utilized a mixture of polyethylene and water with a Ni catalyst, and the mixture was heated to 700–800  C under 60–100 MPa pressure (Gogotsi et al. 2000) and both closed- and open-end MWCNTs with the wall numbers from several to more than 100 carbon layers were obtained. CNTs synthesized by sonochemical/hydrothermal method have a small wall thickness and large inner core diameter, 20–800 nm. In another study, Manafi et al. studied low temperature sonochemical/hydrothermal MWCNT synthesis using reduction of CH2Cl2 by metallic Li in the presence of Co catalyzer at a low CNT growth temperature of about 150–160 C for 24 h. CH2Cl2, CoCl2, and metallic Li were utilized as starting materials in 5 mol/L NaOH aqueous solution. In this study, MWCNTs with

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high purity and lengths of 2–5 μm and diameters of 60  20 nm were obtained at such a low-temperature value (Manafi et al. 2008). In this study, the starting solution was pretreated ultrasonically and this pretreatment process provided uniform catalyst particle distribution and this process was concluded with the formation of MWCNTs. Razali et al. synthesized CNTs by hydrothermal method at growth temperature of 200  C (Razali et al. 2016). They used ferrocene with sulfur to form ferrous sulfide particles that act as the catalyst material for CNT growth. They synthesized well-graphitized nanotubes with a diameter range of 10–12 nm and hundreds of nanometers in length. In general, CNTs obtained by sonochemical/hydrothermal method have wall thickness between 7 and 25 nm and with an outer diameter of 50–150 nm. During the CNT growth process, a mixture of CO, CO2, H2O, H2, and CH4 are sent to the tube. Multi-walled carbon nanocells and MWCNTs have been artificially grown in hydrothermal fluids from amorphous carbon at temperatures below 800  C without utilizing any metal catalysts by interconnecting multi-walls of graphitic carbon causes the formation of carbon nanocells at 600  C (Moreno et al. 2000). The average diameter of obtained nanocells was smaller than 100 nm, and their outer diameters and diameter of internal cavities were in the range of 15–100 nm and 10–80 nm, respectively. Short nanotubes and nano-onions formation by hydrothermal synthesis were also reported in the literature from nanoporous carbon utilizing elemental Cs at a growth temperature of 50  C. It was indicated that the synthesized carbon nanoparticles were more ordered. At his low growth temperature some of the obtained carbon nanostructures were nanopolyhedra, tubes, and onions. At growth temperature 350 and 500  C, the same carbon nanostructures were found at larger numbers compared to 50  C for elemental Cs usage (Stevens et al. 1998).

Liquid Phase Synthesis Liquid phase synthesis of CNTs is an appropriate method to produce aligned CNTs by applying electrical heating to a substrate in organic liquids. Zhang et al. studied liquid phase CNT synthesis on Si substrates coated with Fe thin films (Zhang et al. 2002). The substrates were electrically heated to 500–1000  C in organic liquid and the results indicated aligned MWCNTs growth and well-stick nanotubes on the Si substrates by utilizing methanol and ethanol. The top ends of the obtained nanotubes closed and some different shapes of CNTs such as a special kind of coupled CNT containing a left hand rotated and a right hand rotated CNT with bridging chains between these two nanotubes were also observed in microscopic studies. Therefore, this method is appropriate especially for the synthesis of hollow MWCNTs. CNT growth in organic liquids is based on the adsorption and decomposition of organic radicals at the surface of catalyst material under thermodynamic equilibrium. The liquid phase CNT synthesis experimental set-up consists of an external watercooled glass liquid chamber, a DC power supply utilized for the application of a controlled current to the catalyst substrate.

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After an organic liquid is filled into the chamber, an inert gas is introduced into the chamber and the catalyst is heated to the desired CNT growth temperature, which is generally in the range of 500–1000  C and then a constant electrical current is applied through the catalyst substrate. As a result of the application of the electric current, gas bubbles are formed and flow upward from the catalyst substrate surface, and they condense at the top of the liquid. During this process, the water-cooled system provides control in such a way that the temperature of the organic liquid in the chamber is lower than the boiling point. To measure the temperature of the catalyst substrate, an optical pyrometer is utilized. Catalyst-free CNT growth is also possible in liquid phase synthesis. Yamagiwa et al. studied simple preheating effects by using stainless steel substrates and obtained highly pure and aligned CNT arrays from ethanol without the addition of a catalyst or its precursor (Yamagiwa et al. 2016). Their results indicated that preheating temperature influenced directly the morphologies of the MWCNTs. Besides, since no catalysts are used during production in this method, there are no metal nanoparticle impurities on the walls of the nanotubes afterward and it is not necessary to purify CNTs. Alcohols such as methanol and ethanol are frequently utilized as the organic liquid in liquid phase CNT synthesis. Especially CNTs produced with methanol indicate a high purity and perfect alignment (Yamagiwa et al. 2007). However, if ethanol is utilized in the synthesis, the liquid-phase method is also one of the most environmentally friendly methods for aligned CNT arrays synthesis. The main advantages of liquid phase synthesis methods are the simple production process, low cost, and a one-step process, which does not require a vacuum process or catalyst preparation step.

Flame Synthesis of CNTs The main challenge in CNT synthesis is to obtain a high yield of products with a high crystalline structure. The scaled-up and reliable production of CNTs offers a wide range of possibilities to synthesize CNT for various applications such as memory devices, MEMs/NEMs, hydrogen storage, electrochemical actuators, and sensor applications. Especially, for CNT synthesis with metal catalysts, furnace methods have some important advantages. Chemical vapor deposition (CVD) is the dominant method to produce CNTs because of its high efficiency. However, these methods do not allow large-scale synthesis on large surfaces; these processes require a lot of energy and long processing times. Flames are utilized for the production of solid black and printing ink. This method is useful for growing to determine the rate of growth as well as the structure of CNTs. The production of CNTs with flames is similar to the CVD method. Flames are scalable and are commercially used for the production of solid carbon forms such as carbon black and printing ink. This method is useful to grow to determine the development rate and also the structure of CNTs. CNTs develop in a similar way as the CVD method. Flame synthesis of CNTs, which is an alternative

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method to provide large area synthesis with elevated process temperatures and the hydrocarbon reactant for the synthesis of a metal catalyst supported CNTs, is an autothermal process in which it can maintain the temperature to achieve the desired production parameters. The evaporated catalyst can be injected into a flame (Vander Wal 2000). The use of flame creates a suitable environment to allow various carbon forms to be synthesized at nano-level at the same time. Large substrates could be covered readily by a flame restoring of multiple flame sources. This method provides a controllable residence time within the desired flame region as well. Besides these advantages, the diffusion flame generally utilizes a complicated burner with some other apparatus, and in addition, gaseous fuels must be stored safely and carefully. In this method, a transition metal precursor is dispersed within a support matrix such as Al, Si, or TiO or a mixture of these materials. A thinner oxide layer is an advantage; it provides great dispersion and the small size of metal nanoparticles. As a hydrocarbon source, diffusion flame synthesis of CNTs utilizes nitrogen diluted acetylene (C2H2), ethylene (C2H4), methane (CH4), or ethanol (C2H5OH), etc., as hydrocarbon sources with an oxidizer to create a gas mixture that includes carbon dioxide (CO2), water vapor (H2O), carbon monoxide (CO), hydrogen (H2) saturated and unsaturated hydrocarbons (C2H2, C2H4, C2H6), and radicals (Rathinavel et al. 2021). Establishing and maintaining the precise balance between the optimized carbon supply and flame temperature in the flame synthesis of CNTs is very important. Oversupply of carbon causes amorphous carbon formation. The complex relationship between fuel, oxidizer, and temperature in the combustion process, along with other important parameters in CNT synthesis such as catalyst type and composition are parameters that are still being studied by researchers.

Plastic Pyrolysis Method Today, the continuous increase of plastic wastes is a major environmental problem. This has led to an increase in studies on the recycling of plastic waste. Traditional recycling methods were tried to solve this waste problem; however, low-cost products were obtained and so these methods were not economically viable. Polypropylene, polystyrene, and polyethylene make up the majority of plastic waste. Therefore, the recycling of plastic waste for producing high-quality products such as CNTs is a good way to be used in commercial and industrial applications. The plastic pyrolysis method is utilized to synthesize CNTs from plastic wastes. Abul-Enein et al. studied the pyrolysis of different plastic wastes such as low- and high-density polyethylene, polypropylene, polyethylene terephthalate, and polystyrene for producing MWCNTs using a two-step process (Aboul-Enein et al. 2018). The first step was the cracking of plastic wastes to produce hydrocarbon gases at a temperature of 700  C and the second step was the decomposition of hydrocarbon gases that were decomposed on the surface of the Ni-Mo/Al2O3 catalyst to form CNTs at a growth temperature of 650  C. The results indicated that thermal degradation of the wastes provided the synthesis of different products including hydrocarbon gases, liquid,

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and char. The yield of the gas products depends on the type of plastic waste. Low density polyethylene and polypropylene gave the largest yields among the plastic wastes and the smallest value was revealed for polystyrene. This method allows to produce MWCNTs and the yield of nanotubes was related to the amount of gas produced. As expected, the highest yield of MWCNTs was obtained by utilizing low density polyethylene and polypropylene wastes and the obtained MWCNTs for these two plastic waste showed the highest quality and purity among the used plastics in Raman spectra while polyethylene terephthalate and polystyrene were found not to be appropriate for CNT formation. The most frequently used catalyst materials for CNT growth from waste polymers are Ni and its bimetallic catalyst forms, for example, Ni/Zn, Ni/Ca, Ni/Mg, Ni/Ce, Ni/Al, Ni/Mo/Mg. In general, the metal particles are impregnated in the surface of catalyst support material such as alumina, silica, or magnesia and the interaction between catalyst nanoparticles and support material is significant for the growth process. Increasing amounts of metals impregnated on the support surface results in a decrease in the inner diameter of CNT, as well as the ratio of different metals is also an important CNT growth parameter. It was also found that the filamentous carbon production was higher using polyethylene than polypropylene or polystyrene. Besides, waste plastics and hydrocarbon gases, pyrolysis oils also could be utilized for CNT synthesis. Pyrolysis oil could be obtained from a printed circuit board and CNTs could be prepared through pyrolysis oil-based resin (Borsodi et al. 2016). During CNT production with plastic pyrolysis, as a by-product hydrogen production is also provided. As conclusion, plastic pyrolysis is an important CNT synthesis method because the formation of CNTs from waste plastics provides resource conservation and environmental protection.

Chemical Vapor Deposition (CVD) Chemical vapor deposition (CVD) is a widely used material processing technology whereby solid materials are formed on a heated substrate by a chemical reaction of vapor precursors in the form of a thin film, powder, or single crystal. It is possible to obtain a wide range of material properties by varying experimental conditions such as substrate type, substrate temperature, the composition of the reaction gas mixture, and total gas flow pressure. CVD technology has many advantages over other vapor deposition techniques. For example, the reaction can be controlled in such a way that deposition may occur only in certain regions of substrate or material that may cover a rough surface relatively uniform with high quality. Another advantage of CVD is that source materials are flown into chambers from external reservoirs that can be refilled without contamination of the growth environment. Deposition requires low vacuum levels and has more tolerance for precision in the process conditions, making it a popular technology for electronics and, optoelectronics, surface modification, and biological applications. However, CVD generally requires high temperatures and highly toxic source gases, which

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need great attention during the operation. But still, its benefits predominate these problems for microelectronic fabrications. The type of the CVD system is determined by factors such as reactants used during the process, maximum tolerable air leakage amount into the system, purity of the materials used, size, shape, and properties of the substrate, and so on. To achieve high-quality thin films, parameters such as temperature, substrate type, pressure in the chamber, gas flow amount, and concentration are essential parameters. Therefore, suitable CVD equipment is required. Several designs are possible to satisfy the needs of the researchers including heating methods, gas flow controls, loading type of substrate, etc. A CVD system is constructed basically in three main modules: reaction gas delivery system, a reactor with gas flow components and exhaust system with pressure controllers, and vacuum pumps. During the CVD processing high-pressure, toxic, flammable, and explosive gas, liquid, and solid precursors are used. Therefore, it is essential to design safe and precisely controlled systems. The gaseous reactants are stored in high-pressure gas bottles with fitted metal regulators to control the outlet pressure. Valves and metal gasket seals are used in the connection of gas pipes to the inlet part of the reaction chamber. Mass flow controllers are used in controlling the gas flow rate according to a set flow rate sent as an electric signal without being affected by conditions and gas pressure fluctuations. While liquid precursors are introduced by heating them above the boiling temperature, solid source precursors are dissolved in a suitable solvent or sublimated into the gas phase. The main degradation problem in CVD processes is contaminations caused by reactions between gasses and the materials of gas delivery systems and by air leakage. The contamination level in the process may be improved by using purified reactants, obtaining a low air leak rate by improving vacuum levels at connection parts, and using degassed vacuum seal O-rings, using nonreactive carrier gasses during vaporization, checking the material-gas compatibility. The geometry and the type of reaction chambers are varied due to the type of process and the sample characteristics such as size, shape, and number. Reaction chambers may be designed as horizontally (Fig. 1a) or vertically (Fig. 1b) (Huang et al. 2020; Sun et al. 2021). The quartz tube is a widely used reaction chamber due to its resistance to high temperatures and to rapid temperature changes. Metal flanges are used to connect gas inlets and outlets to quartz tubes with cooling components. The substrate is usually placed at the middle part of the chamber close to thermocouples by using a quartz or graphite sample holder. Targeted deposition characteristics such as thickness, composition, and structure are strongly dependent on the sample holder configuration and optimized experimental conditions. To have reproducible products, it is important to have precise control on annealing ramping rate, gas flow rate, gas composition, temperature, pressure, and cooling rate. There exist various techniques for heating the substrates. Conductive substrates may be heated by resistive heating or radiofrequency induction heating whereas nonconductive substrates are heated by optical heating with tungsten filaments or lasers, thermal radiation heating. The shape and the type of materials that exist in the substrate have an influence on the heat transfer mechanism.

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Fig. 1 Schematic diagram of a typical (a) horizontal and (b) vertical chemical vapor deposition system (CVD) with reactor chambers, gas delivery systems, vacuum systems, energy systems, and control systems. (Reprinted with permission from (Saeed et al. 2020; Huang et al. 2020))

The temperature measuring systems and heating components are essential for thermal CVD processes. The widely used heating measurement devices are thermocouples. They are usually placed between the quartz tube and the heating zone. There are many types of thermocouples, each with its characteristics varied by temperature range, durability, vibration resistance, chemical resistance, and compatibility. Type J, K, T, and E thermocouples are the most common types of thermocouples called “Base Metal.” Type R, S, and B thermocouples are “Noble Metal” thermocouples that are used in applications where higher temperatures are required.

Catalytic CVD (CCVD) The chemical vapor deposition is based on the decomposition of hydrocarbons in the presence of a suitable transition metal catalyst. For obtaining CNTs with high purity on a large scale, the CVD method is a confidential synthesis method. During the synthesis of CNTs, catalysts may be introduced in the form of gas particulates or as solid precursors. The morphology and the growth yield of the nanotubes are directly

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related to the type of the selected metallic catalyst. As the catalyst plays a critical role in the nucleation and the growth of CNTs, a large number of studies focused on the catalyst type, preparation, composition, morphology, and size to figure out its influence on the growth of CNTs. The most popular catalysts for catalytic CVD synthesis of CNTs are Fe, Ni, and Co transition metals due to their high carbon solubility and easy carbon diffusion. The catalytic activity of Fe, Co, and Fe/Co on alumina or silica supports is investigated and the best yield of MWNTs was obtained at 7008  C on alumina synthesized from hydrolysis of aluminum isopropoxide with the mixture of Fe and Co (Nagaraju et al. 2002). The catalytic activity of Fe, Co, and Ni transition metals are compared using laser-treated vanadium plates as catalytic supports in the decomposition of acetylene at 7208  C. While CNTs with high density and small diameters (10–15 nm) were obtained with an iron catalyst, on nickel and cobalt catalysts carbon sources resulted in amorphous or fiber-like material. Lee et al. also studied the effect of the same catalysts (Ni, Fe, and Co) and it is revealed that nanotube growth rate is directly related with catalyst type and highest rate obtained with nickel catalyst whereas the best crystallinity is obtained over an iron catalyst (Lee et al. 2002). It is possible to produce well-aligned, highly pure MWCNTs with tungstenbased catalysts. Nonmetallic catalysts are also studied in the synthesis of CNTs. Well-aligned CNTs with high density are synthesized using magnetic fluids of magnetite nanoparticles that spin-coated on Si substrates. In addition, uniform distribution of particles obtained by mixing the magnetic fluid with PVA gave the opportunity to control nanoparticle density. To increase the productivity of the catalysts, some preparation routes may be applied. For example, Kong et al. patterned catalytic islands on Si wafers (Kong et al. 1998). The dispersion amount of the catalyst material directly influences the efficiency of the catalyst. While well-dispersed particles resulted in narrow-sized CNTs, large particles or aggregates failed in nanotube growth. SEM and TEM images of a MWCNT sample are displayed in Fig. 2.

Fig. 2 TEM and SEM images of MWCNTs grown on Co-Mo/MgO catalyst by CCVD method

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Floating Catalyst Chemical Vapor Deposition (FCCVD) Large-scale production of CNTs with optimized properties that are comparable to the individual CNT remained a challenge for a long time. Floating catalyst chemical vapor deposition (FCCVD) is the most scalable technique with mass production, low cost, high purity, and ease of fabrication. In the FCCVD technique, a catalyst is injected into the gas phase with the carbon source. During the growth, the first catalyst decomposes into small nanoparticles, which then collapse with each other to form bigger particles in size, then the coating of sulfur appears on nanoparticles and the growth process of CNTs is started. Sulfur has a significant role in stabilizing the growing edge of CNTs by removing blocking groups. In FCCVD, various hydrocarbon sources may be used in the growth of CNTs and their chemical structure and pyrolysis temperature directly affect the morphology of the product. For example, in the study of Gspann et al., toluene and methane hydrocarbon sources were compared, and results revealed that CNTs synthesized by toluene were stronger and purer than CNTs that were synthesized by methane (Gspann et al. 2014). The carrier gas flow rate has an impact on the retention time, which is the total time required for catalyst nanoparticles to collide to form biggersized nanoparticles and the growth of CNTs occurs. While the low flow rate leads to a larger diameter of catalyst nanoparticles, the higher flow rate results in a smaller diameter. In the FCCVD technique, metallocenes are widely used catalyst sources. A metallocene is composed of two cyclopentadienyl anions (Cp-ring) bound to a central metal atom that is in the oxidation state II. Ferrocene is the most popular organometallic compound due to its low cost, low toxicity, and good stability under ambient conditions. Alcohol Catalyst Chemical Vapor Deposition (ACCVD) In the alcohol catalyst chemical vapor deposition (ACCVD) technique, alcohol is used as a carbon source molecule. It is easier to control ACCVD as solid carbon formation is less with alcohol frame when compared to hydrocarbon flame. Low-cost and easier scale-up productions of SWNTs are possible with ACCVD due to its lower reaction temperatures (lower than 600  C) and higher purity features. The low reaction temperature in ACCVD also enables the direct growth of CNTs on patterned semiconductor substrates. Cobalt films with a thickness of 1 nm are widely used catalysts for CNT synthesis using ethanol supported on aluminum oxide films. Aligned CNTs with heights of up to hundreds of micrometers, in high quality and high purity, is possible. Murakami et al. performed ACCVD on quartz substrates using Co and Mo catalysts and obtained vertically aligned SWCNTs (VA- SWCNTs) with a thickness of ~1 μm (Murakami et al. 2004). The thickness of the VA-SWNTs increased up to 20 μm by adding a small amount of C2H2 to ethanol; however, the diameters of the SWCNTs exceed 1 nm even with the usage of Al2O3 buffer layers or Mo cocatalysts. Pt- group metal catalysts are also studied in the growth of SWCNTs by ACCVD and it was investigated that diameters of SWCNTs decreased with decreasing growth temperature, but vertical growth was not achieved. Vertical alignment was obtained by ACCVD with an Ir catalyst in a cold-wall CVD system.

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The length of SWCNTs increased with increasing growth time and the thickness of VA-SWCNT reached 5 μm. Additionally, the diameters of the SWCNTs were measured between 0.8 and 1.1 nm, which is much smaller than CNTs grown with Fe and Co catalysts.

Thermal CVD The thermal CVD technique generally requires high temperatures (~ 800 to 2000  C), which may be obtained by resistance heating, induced high-frequency heating, radiant heating, hot plate heating, etc. Thermal CVD may be categorized according to how heat is supplied to the substrates: hot-wall and cold-wall. Both can be designed either horizontally or vertically. The horizontal reactor is the most common one where the substrates may be placed horizontally, vertically, or with any tilt angle according to the gas flow. The vertical reactor is advantageous in terms of material uniformity and growth rate. A hot-wall reactor is surrounded by a tube furnace that is often heated by resistance elements. The walls of the reactor and the substrates all have the same temperature. As the growth takes place on substrate, by the time the walls of the reactor may be coated with materials. Thus, contamination may be a risk for purity with the reaction of formerly grown material and vapor. In some cases, substrates may be loaded vertically to reduce particle contamination and increase the loading capacity. Hot-wall reactors have the main advantage of more precise temperature control. In a cold-wall reactor, only the substrate is heated, and therefore, the reactor wall remains cold. This structure gives the opportunity for rapid heating and cooling. Heating in cold-wall CVD may be provided by resistance, hot plates, or induction. The cold-wall CVD will be discussed in more detail in further sections. In the thermal CVD method, catalyst-coated substrate is placed in a thermally heated atmosphere. During the exposure of one or more hydrocarbon precursor gases, CNTs are formed by reaction or decomposition on the substrate. Vertically aligned CNTs on Co/Ni patterned and pretreated Si substrates were synthesized using C2H2 precursor with the 15–40 sccm flow rate for 10–20 min. at 800–900  C (Murakami et al. 2004). The same technique was used to obtain aligned bambooshaped CNTs on Fe-coated Si substrates (Lee and Park 2000). Tripathi et al. used Al2O3 substrates to grow CNTs by the only decomposition of C2H2 at 800  C, without using any catalyst and the diameter distribution of CNTs were between 6–8 nm (Tripathi et al. 2014). Aligned and Fe-Co alloy-filled MWCNTs was synthesized on Si substrates by pyrolysis of ferrocene/cobaltocene mixture at 980  C. The diameters of encapsulated metal nanowires were measured as 10–20 nm with a length of up to a few micrometers. Rapid thermal chemical vapor deposition (RTCVD) is a type of thermal CVD method where a rapidly heated substrate is exposed to precursors. The infrared lamps are used for rapid heating and cooling. Here, the typical duration of growth is much less compared to normal thermal CVD. After heating lamps are energized, the irradiation passes through the tube and is absorbed by the substrate only. Thus, its surroundings have remained at room temperature.

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RTCVD was used to synthesize SWCNTs and MWCNTs on Ni/Al2O3 substrates by thermal decomposition of C2H2/H2 at 600  C. It is concluded that CNT growth was the result of a liquid metastable: dissolution of carbon, its oversaturation, and diffusion. In another study, catalysts were placed inside the pores of AlPO4-5 and L-type zeolites, and CNTs were synthesized at 800  C with the mixture of CH4 and H2 in RTCVD system (Martin et al. 2008). The effect of vacuum and H2 pretreatment of catalyst on growth morphology of carbon structures was examined.

Plasma-Enhanced CVD (PECVD) The plasma-enhanced CVD (PECVD) was first emerged due to the need for microelectronic processing. High substrate temperatures at thermal CVD was a serious problem for photoresist used patterned wafers. Therefore, PECVD allows to process with lower wafer temperatures between room temperature to 100  C in IC manufacturing. The low temperature is possible with PECVD because highly energetic electrons are used to create glow discharge plasma. In this type of plasma, the temperature of electrons is higher than the temperature of ions, which provides maintenance of discharge plasma at low temperatures. Precursor gas decomposition is provided by these high-energy electrons by transferring their energy into a gas mixture. As a result of this interaction, reactive radicals, ions, neutral atoms, and molecules, and other exciting species are produced, some of which interact with the substrate for either etching or deposition process. Since the high energy of electrons is mostly transferred for chemical reactions, the gas itself is stayed cooler, and growth occurs on substrates at lower temperatures compared to temperatures in conventional CVD systems. Plasma-enhanced CNT growth has several advantages over thermal growth. For example, CNTs can be easily aligned or even free-standing SWCNTs may be formed due to the presence of an electric field. The modification of the nucleation process is also possible with the present electric field. Additionally, narrow chirality distributions are observed through CNTs produced by PECVD. The rectors of PECVDs are classified according to the type of plasma source that is used to generate the gas discharge of feedstock. DC-PECVD uses DC power to achieve glow discharge plasma between the anode and the cathode that are fixed parallel to each other inside the reactor. Plasma and the strong electric field are produced with the applied negative DC voltage to the cathode. In the study of Tanemura et al., DC-PECVD of aligned CNTs on Co/Ni-coated tungsten wires with mixtures of C2H2 and NH3 were optimized (Tanemura et al. 2001). The effects of wire temperature, wire diameter, gas pressure, and sample bias were investigated. It is concluded that alignment of CNTs strongly depends on the catalyst and its support material and the sample bias affects the morphology of CNTs. Free-standing single CNT arrays were grown on Ni dot arrays by PECVD and it is revealed that vertical alignment was directly dependent on the location of the catalyst metals. VACNTs were grown on Ni-coated SiO2/Si substrate at low temperature (120  C) enabling the growth of CNTs onto substrates such as plastics and sensitive nanoelectronic devices that cannot sustain high temperatures.

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It is important to mention that as DC sources use high power to accelerate the ions, the high applied voltage may lead to substrate damage due to the strong ion bombardment in the DC-PECVD process.

Oxygen-Assisted CVD CVD is the most popular and reliable method for carbon nanostructure growth. However, in CVD processes, the low carbon diffusion rate results in the passivation of catalyst, and amorphous carbon formation has occurred on the surface of the catalyst. This is the main result of poisoning the carbon nanostructures formed at low temperatures. On the other hand, studies revealed that moderate concentration of oxygen, air, and water with the precursor gases improve the catalytic activity and help to remove the amorphous carbon layer leaving the pure carbon nanostructure at the growth temperature. High-purity SWCNTs were grown by cobalt nanoparticles with small diameters by using oxygen-assisted CVD. Various precursor gases such as CH4, H2, and C2H4 were introduced into chamber at 900  C growth temperature. With the small amount of oxygen flow, amorphous carbons and residual catalysts are evacuated from the chamber. It was concluded that high purity SWCNTs without any distinct defect were obtained by the oxygen-assisted CVD process. Besides dramatic increment in the purity, small amount of oxygen also increased the yield of the CNTs. In the study of Chou et al. carbon materials, i.e., CNTs, multilayer graphene (MLG), double-layer graphene (DLG), and single-layer graphene (SLG) on a Ni foil substrate were synthesized by oxygen-assisted CVD at ultralow pressure (20 mTorr) (Chou et al. 2015). Although oxygen seems to be undesirable in carbon nanomaterial synthesis, this study showed that the growth of NTs, MLG, DLG, and SLG can be maintained by adjusting oxygen concentration during the CVD process. Oxygen assessment also provides an easy way to control the layer of grown graphene. The effects of hydrogen and oxygen on the synthesis of CNTs by the CVD method were discussed in the study of Zhang et al. (2005). In hydrocarbon-based growth, highly reactive hydrogen species were damaging the formation of sp2-like SWNTs in a diameter-dependent manner, and with the addition of the oxygen, H species were purified and precise control over the C-H ratio was provided for SWNT growth. The oxygen assistance in field emission treatment improved the uniformity of CNT pixel arrays. The addition of oxygen gas during the field emission process improved the uniformity of emission area and the brightness of a CNT pixel array by 83% and 90%, respectively. Emission stability was preserved during the process. These phenomena were explained with the oxidation of highly emitting CNTs resulted in burning out. As a result, emitting CNTs with high current were removed and more emitting CNTs with weak current were excited at a higher field, leading to a balanced emission from each pixel in the array. Water-Assisted CVD In 1996, it was revealed that a water sonication of raw CNTs made by the arc-discharge technique was an effective method for purifying SWCNTs. Based on

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this information, Hata et al. firstly reported the water-assisted CVD technique used in the synthesis of vertically aligned CNT growth in millimeter scale (Hata et al. 2004). During the CVD process, a small amount of water (~ 20–500 ppm) was added into the feedstock and above 99.98% carbon purity was achieved. The assistance of water in CNT synthesis enhances the activity and the lifetime of the catalyst. The effect of water in the selectivity of SWCNTs over multi-walled nanotubes was confirmed and in the following studies, nanocatalyst stability through CO2/H2O mixtures was discussed. As a result, it was clarified that as a kinetic process, the water-generated surface-bound hydroxyl species might prevent the growth of the particle leading to an effective growth yield and improved overall control over CNT self-dispersion. Water-assisted CVD growth of vertically aligned SWCNTs is also called “super growth.” Water vapors of 20–500 ppm are passed through the reaction chamber by flowing carrier gas mixtures such as Ar or He with H2 through a water bubbler. The bubbler principle is rather simple and robust. The water is stored inside a stainless steel canister (bubbler), and an inert carrier gas such as Ar or N2, is passed through the liquid to bubble. The precursor vapor saturates the atmosphere in the bubbler and the vapor then enters into the heated substrate surface. The delivery of reactants depends on the temperature of the bubbler, the carrier gas flow rate, and the pressure over the surface of the liquid. Generally, ethylene or acetylene is used as carbon precursor along with various catalysts i.e. Fe, Al/Fe, Al2O3/Fe, Al2O3/Co. Wu et al. reported the one-step water-assisted CVD growth of metal oxide nanosheets/carbon nanotubes (CNTs) composites and helical CNTs were grown with the CVD growth process that prolonged to 1 h. The typical fishbone type CNTs were also observed by HRTEM (Wu et al. 2019). The CVD process was carried at the atmosphere pressure and as carrier gases, H2 and Ar, as a carbon precursor methane and as a metal catalyst ferrocene (0.05 g) were used. The reactor was heated to the desired temperature in the range 1020–1100  C at a ramp rate of 25.5–27.5  C/min under an ambient flow rate of Ar (30 sccm).

Microwave Plasma-Enhanced CVD (MPECVD) The deposition processes carried at too high temperatures are unsuitable for electronic device fabrication as most connections are made of materials like alumina, which have melting points below 700  C. These challenges forced scientists to explore alternative deposition techniques carried at low temperatures. At low pressures, microwave plasma is a kind of low-temperature plasma due to the nonequilibrium state between electrons and other heavy particles full of active species. Therefore, microwave-plasma-enhanced chemical vapor deposition (MPECVD) is a good candidate for low-temperature synthesis processes of carbon nanostructures. Okai et al. synthesized CNTs on Ni and Fe-Ni-Cr alloy substrates along with methane and hydrogen gases by MEPCVD and investigated their structures. The total gas pressure was maintained at 250–300 Pa and growth was carried for 30–60 min under 10–20 sccm methane/80–90 sccm hydrogen gas mixture flow. Plasma was generated by a 2.45 GHz microwave generator with 500 W power, and the growth temperature was measured as 650  C at the sample stage. A negative voltage was applied to the substrates during the process. TEM results showed that

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CNTs had a piled-cone structure with metal particles on top by determining the diameter of nanotubes. The diameters of CNTs were ranged between 60 nm and 80 nm. Additionally, in the case of Ni substrates, metal particles were composed of Ni; and in the case of Fe-Ni-Cr alloy substrates, they were composed of both Fe and Ni metals (Okai et al. 2000). The vertically aligned CNTs were grown on sputtered Ni-coated Si substrates by using MEPCVD at 700  C with gas mixtures of CH4 (~20%) and H2 (~80%). The applied microwave power and the pressure during the growth of CNTs were 400 W and 10 Torr, respectively. The growth time of CNTs was maintained for 5 min. The results revealed that the diameter, growth rate, and density of CNTs are directly related to the grain size and morphology of the Ni films, which may be altered by the rf power density during the rf magnetron sputtering process. The diameters of CNTs were varied between 10 nm and 35 nm. The growth rate and the density of CNTs increased with decreasing rf power density. The effect of microwave plasma on the alignment of CNTs was investigated by Bower et al. and it was shown that nanotubes were always aligned normal to the local substrate surface (Bower et al. 2000). The alignment was induced by the electric field applied on the substrate surface due to the plasma ambient. When the plasma source was switched off, the plasma-grown straight nanotubes and thermally grown curly nanotubes were formed without any alignment. During the plasma process, CNTs were grown at a significantly high rate of 100 nm/s, which gave the opportunity for large-scale production of CNTs. The catalyst synthesis of CNTs using CH4–CO2 precursors was carried successfully by MPECVD. As catalysts Fe, Ti, and Fe/Ti nanoparticles were used and a significant difference in morphologies in carbon deposition was observed between Fe and Ti. By optimizing the growth parameters of precursors, a high yield of vertically aligned CNTs was obtained on Fe-deposited substrate. On the other hand, Ti was found unsuitable as a catalyst for CNT production. Vertically aligned MWCNTs were synthesized on Fe-deposited Si substrate at low temperatures below 330  C by MPECVD using CH4 and CO2 gas mixture. The DC bias voltage was varied from 150 V to 200 V, at 300 W microwave power, at 1.3–2.0 kPa range of total gas pressure, and substrate temperatures were measured between 300  C and 350  C. The diameter of MWCNTs was measured as about 15 nm and the highest yield of about 50% was obtained at low temperatures (below 330  C) by MPECVD with an optimized rate of CH4/CO2 gas mixture (Chen et al. 2002).

Radiofrequency CVD (RF-CVD) Radiofrequency CVD (RF-CVD) uses RF power to generate plasma. Power of 13.56 MHz is supplied to the reactor by an impedance network between the supply and the plasma. The RF discharges are still dynamic at sub-Torr pressure levels and the bias voltage measured on the electrode smaller than the bias voltage of DC discharges. RF plasma discharges are divided into two categories due to the method of coupling the power supply. In capacitively coupled plasma (CCP) system, electrode configuration is like that of DC-PECVD system but uses RF power supply. In an inductively coupled plasma (ICP) system, a coil is placed outside of the reactor and RF power is generated to the inside of the reactor by forming a coil through a

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dielectric window (Sengupta 2018). The CCP RF-PECVD system is used in the synthesis of vertically aligned carbon nanofibers at 560  C with Ni catalyst on Si substrates. Nanofibers were observed as “herring-bone” and “bamboo-lie” structures. ICP RF-PECVD was used in the growth of bamboo-like CNTs with repeating arrowhead shapes. Aligned MWCNTs fully filled with Fe, Co, and Ni nanoparticles were synthesized by ICP RF-PECVD using nanowires (Fe, Co, and Ni) as catalysts. The length of the nanowires was 420  20 nm, which is suitable for the formation of a fully filled structure of CNTs. The significant effect of the type of catalyst on the alignment and the crystallinity of CNTs were observed. While the CNTs catalyzed by Ni and Fe have better alignment and a higher degree of graphitization, those from Co show a little curled character in the tips and somewhat tangle between CNTs. SWCNTs were grown by a home-built radiofrequency (RF,13.56 MHz) 4-in. remote PECVD system at 600  C. The nanotubes were of high quality and high-performance field effect transistors were obtained by RF-PECVD nanotubes. Electrical characterization revealed that nearly 90% of the nanotubes were semiconductors and thus highly preferential growth of semiconducting over metallic tubes were achieved in the PECVD process.

Hot-Filament CVD (HFCVD) The hot-filament CVD (HFCVD) system is composed of heated coiled wire that decomposes the precursors, and deposition takes place on a substrate placed near the filament at lower temperatures. A typical HFCVD system used in CNTs growth is shown in Fig. 3. The first HFCVD technique was used in 1979 to produce amorphous silicon films with low substrate temperatures under silane gas flow and a high Fig. 3 Schematic diagram of the hot-filament-enhanced chemical vapor. Deposition system used in the deposition of MWCNT growth. (Reprinted with permission from (Ono et al. 2004), © Copyright (2004) The Japan Society of Applied Physics)

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deposition rate was achieved. During the HFCVD processing, various types of metal filaments such as tungsten, tantalum, and rhenium are resistively heated and their temperatures are maintained at 1800–2300  C. The filament temperature plays a critical role in the production as it influences the type and quality of the grown carbon materials. While little or no diamond structure is observed at filament temperatures below 1800  C, higher temperatures up to 2300  C results in higher growth rates and high-quality polycrystalline diamonds. However, the higher the filament temperature, the faster the filaments become carburized and brittle. It is important to select a suitable filament that can resist processing temperatures. The substrate temperature, which is determined by the residence time, mobility of reactant molecules, and radicals absorbed at the substrate surface, is also critical for the synthesis of the target material. The residence time is inversely proportional to the total gas mass flow rate in the reactor with laminar flow. In the HFCVD chamber, the substrate temperature is essentially affected by the filament temperature and its power, distance to the filament, and total pressure. Thus, precise temperature control of the substrate is needed to optimize the growth parameters and quality of the product materials. CNTs were grown by HFCVD on Cu substrates with the substrate temperature of 900  C and passing 2.0% methane in the hydrogen gas mixture. The gas mixture of 100 sccm was flown through the reactor and activated by a heated Re filament placed at 8 mm above the substrate. The temperature range of the resistively heated filament was measured between 2300 and 2500  C. It was revealed that bambooshaped CNTs were obtained with diameters of 50–100 nm at variable lengths. Well-aligned bamboo-shaped CNTs by HFCVD were grown on Ni film-coated Si substrate using C2H4/NH3 precursors with a flow rate of 25/100 sccm. Diametercontrolled growth of SWCNTs was achieved by using Fe catalyst and C2H2 as precursor at 590  C. Aligned CNTs on Inconel sheets were grown by HFCVD with a gas mixture of CH4 and H2 and optimum alignment was achieved at the bias of 2500 V. Hot filament effects on vertically aligned CNT growth were investigated with respect to precursor composition, filament temperature, and filament types. Mixtures of CH4 and H2 gases were used as precursors. It was revealed that growth rate increased with the increasing concentration of CH4 in the mixture and independent of filament type and temperature. Carbon filament was used to synthesize SWCNTs and MWCNTs with C2H2 and H2 mixture on silica-supported Fe-Co catalysts. It was found that formation of the SWCNTs was favored at low C2H2 concentration and low ambient pressure. The planarly configurated vertically aligned CNTs with 48 μm length and micropatterned array with 36 μm lengthened CNTs on Al substrates were produced using CH4 as precursors (Sengupta 2018).

Fluidized-Bed CVD (FBCVD) The fluidized-bed CVD (FBCVD) system is designed to inject a fluidizing gas with suitable gas or vapor precursor. As the heterogeneous powder catalysts are used, the gases penetrate through the powder and react to form thin films or nanomaterials on the surface. In the FBCVD, gas/solid contact is optimized and there is no thermal gradient.

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Therefore, optimal mass and heat transfer is achieved in the reaction zone. It is also a user-friendly system that flexible operating parameters such as precursor concentration, gas mixture, and temperature can be easily applied. It is possible to work in either batch or continuous mode. These properties make FBCVD an attractive tool for the homogeneous and selective production of carbon nanomaterials. In the early 2000s, the first studies in FBCVD growth of MWCNTs and graphite nanofibers were carried out and nowadays this technique is used for the synthesis of SWCNTs. In a typical FBCVD system, a reactor is attached within a high-temperature furnace, which is maintained at suitable temperature, pressure, and gas flow ratios. The processing parameters are controlled by a data logging system (Fig. 4) (See and Harris 2007).

Fig. 4 Sketch of a typical fluidized-bed CVD reactor setup. (Reprinted with permission from (See and Harris 2007). © Copyright (2007) American Chemical Society)

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One-step preparation of highly dispersed metal-supported catalysts followed by synthesis of CNTs was achieved using FBCVD. This combined process helps to eliminate drying, calcination, and reduction problems and thus reduces the aggregation or crystalline size increase of supported metal nanoparticles. Mass production of CNTs was achieved by FBCVD using CaCo3 as a soluble support material and similar qualities were obtained through MWCNTs. Three-walled CNTs with a diameter of 6–10 nm and a length of 0.4 mm with 99 wt.% purities were synthesized by FBCVD using acetylene as a precursor. Catalysts were initially immobilized on ceramic beads and CNT growth occurred. Then CNTs and beads were separated by switching gases at a fixed temperature. MWCNTs were produced from H2–C2H4 mixtures on Fe-SiO catalysts by FBCVD. The parameters of catalyst preparation, the residence time, the run duration, the temperature, precursor concentration, and the amount of metal deposited on the support were studied and optimized. Homogeneously deposited CNTs with diameters of 10–20 nm were achieved (Venegoni et al. 2002).

Fixed-Bed CVD (FBCVD) In the production of CNTs, the FB-CVD technique offers numerous advantages such as an optimized gas/solid contact compared to fixed-bed reactors and the absence of any thermal gradient so that optimal mass and heat transfer are reached in the reaction zone (Lee et al. 2002). This is also a flexible process in terms of operating conditions and parameters. In the production of CNTs, the FB-CVD technique offers numerous advantages such as an optimized gas/solid contact compared to fixed-bed reactors, and the absence of any thermal gradient so that optimal mass and heat transfer are reached in the reaction zone (Lee et al. 2002). The fixed-bed CVD process is the most traditional and simplest technique for the synthesis of CNTs. It usually uses a crossflow setup employed inside a horizontal furnace. The catalyst amount should be kept at less than 1 g as the boat-shaped quartz holder has a fixed surface area. Increasing the catalyst amount would only increase the bed depth resulting in diffusion limitation and decrease the catalyst activity. In the study of Zeng et al., the same amount of catalyst was used in a single and two quartz boats in order to investigate the effect of the doubled contact area of the catalyst with the same amount (Zeng et al. 2002). As a result, sufficient mixing was achieved to eliminate diffusion limitations and CNTs yield increased over three times. Cold-Wall CVD The type of the reactor is defined according to size, shape, and the number of substrates based on the selected process. In the cold-wall reactor, heat is only applied to the substrate and its mounting fixture. Therefore, the walls of the reactor remain unheated and no deposition occurs on the walls. This also eliminates the risks of contamination. Temperature gradient near the substrate may affect the uniformity of the thickness and the microstructure but cold-wall reactors are frequently used in microelectronic fabrication due to their flexibility, cleanliness, high deposition rates, and fast cooling rates.

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Generally, in cold-wall CVD, substrates are mounted on a graphite holder and a tube with a radiofrequency coil surrounds it. As graphite is partially conductive, it absorbs radiofrequency from the coil. As a result, electron currents are induced in the graphite to heat the substrate uniformly. Aligned CNTs were synthesized on porous alumina substrates using ethanol as precursor and Co-Mo as catalysts. Two types of CVD synthesis methods were used to compare the effects: conventional thermal CVD and cold-wall CVD. While in conventional CVD frontward carrying gas flow direction was used, in cold-wall tangential flow direction was used toward the substrate. High density CNTs aligned perpendicular to the substrate were achieved with cold-wall reactor due to the precursor flow directed frontally. Mu et al. enhanced the cold-wall CVD reactor by adding a top heater similar to that in hot-wall CVD. Therefore, a more uniform temperature profile during the growth was achieved and precursor gas flow was preheated before its projection onto the catalyst. As a result, horizontally aligned SWCNTs were deposited directly on quartz by cold-wall CVD, which is a very promising application for the fabrication of future nanoelectronic devices (Mu et al. 2016).

Electron Cyclotron Resonance CVD (ECR-CVD) Electron cyclotron resonance (ECR) plasma sources are used for a variety of material processing applications such as synthesis of carbon nanomaterials, etching, and deposition of semiconductors. ECR sources operate at low neutral gas pressures. At low pressures, ion collisions in the substrate sheaths are reduced for anisotropic etching of increasingly high aspect ratio features in integrated circuits. ECR sources can operate in high ion density, which implies higher ion flux-driven processing rates. Due to the low plasma potential of ECR plasma sources (15–30 eV), there is no need for any substrate biasing. The ECR-CVD method is widely used in the synthesis of CNTs due to its advantages of high dissociation percentage of the precursor gases, high and uniform distribution of plasma energy, enabling large-area film production. Aligned hydrogenated amorphous CNTs were grown on porous anodic alumina by ECR-CVRD using acetylene and argon as precursor gases. The composite film with the aligned hydrogenated CNTs is prepared with a large area with advantages of high plasma density at low temperatures, less ionic damage, high deposition rate, and contamination-free growth. The pore size of anodic alumina can be adjusted to control the diameter size in the range of 30–230 nm. In order to use CNTs as connectors in microelectronic device technologies, the large-area horizontally aligned CNTs on 4-inch Si substrates were synthesized by ECR-CVD with CH4 and H2 precursor gases and Co catalysts. It is possible to obtain vertical and horizontal alignment of CNTs by manipulating the electric field applied on the substrate and thus adjusting the flow direction of gases (Hsu et al. 2002). Polymer Pyrolysis CVD (PP-CVD) In the polymer pyrolysis CVD (PP-CVD) method, carbon precursor gases are produced by the pyrolysis of polymers such as Polyethylene Glycol (PEG). In situ growth of uniform CNT/Al composite powders by the catalytic pyrolysis of PEG and

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Al nanoflake powders is possible. Al nanoflakes were chosen as the matrix due to their large surface area, which is very helpful and increases the amount of homogeneously dispersed CNTs on the Al surface. As PEG has a low decomposition temperature, it can be easily cross-linked with citric acid to form a uniform film on the Al nanoflake surfaces. Their results revealed that the as-grown graphitic CNTs were homogeneously dispersed on Al nanoflakes and CNT/Al composites’ compressive strength of 380 MPa can be achieved. The homogeneously dispersed CNTs in Al nanoflakes were also synthesized with PEG as a precursor and cobalt nitrate as a catalyst by PP-CVD. During the processing, parameters such as synthesis temperature, residence time, and mass ratio of PEG to citric acid to cobalt nitrate were optimized. The PP-CVD process used to synthesize CNT/Al composite powders consists of three main steps: preparation of precursor solution, adsorption of precursor film on Al nanoflakes, and polymer pyrolysis CVD growth of CNTs on Al nanoflakes. These steps are shown graphically in Fig. 5. It is obvious that the structure, size, and amount of CNTs strongly depend on the synthesis parameters. Additionally, the batch reaction mode and the lower synthesis temperature, as 600  C, make the CVD method a safe and easy way to scale up the synthesis of CNT/Al composite powders for industrial production. The different growth temperatures of CNT-reinforced aluminum composite powder prepared by PP-CVD were analyzed. The morphologies, structure, phase composition, and elemental content of the synthesized CNT-Al composite powders were investigated. The highest crystallinity of CNTs was obtained at 600  C with a reinforcement content of 7 wt%. The powder manufactured in this investigation

Fig. 5 Diagrams of polymer pyrolysis CVD process for the synthesis of CNT/Al composite powders: (a) impregnation of Al nanoflakes in a precursor solution, (b) CNT growth on Al nanoflakes within a closed batch reactor, and (c) the two-step heat treatment regime for CNT growth. (Reprinted with permission from (Tang et al. 2013))

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has excellent potential to be used in powder metallurgy, cold spraying of CNT-Al composite coatings, and additive manufacturing of composite free-forms.

Direct Liquid Injection CVD (DLI-CVD) In direct liquid injection CVD (DLI-CVD) reactors, liquid delivery units are used to provide carbon sources for the deposition of carbon nanomaterials. It is easier to control precise liquid flow rate rather than that of powders. In some cases, the liquid is vaporized in an evaporation unit and reaches to the substrate in a vapor phase while in other cases it reaches the substrate in a liquid phase and vaporized prior to decomposition. If the precursor is in the liquid droplet form on a substrate, the deposition method is named a spray or aerosol pyrolysis. The essential advantage of DLI-CVD is using the precursors with low thermal stability and low vapor pressure due to keeping the solution under pressure at room temperature. Additionally, it is possible to dissolve powder precursors in a solvent to be used in DLI-CVD. The bubbler technology can be used as a liquid delivery system. This system is preferred as vapor pressures are optimal, not too high, or too low. CNTs were synthesized by DIL-CVD using polyoxometalate cluster as a catalyst and ethanol or toluene as precursors. Although the purity of CNTs was higher at higher temperatures, the maximum CNT yield was obtained at 900  C with fast solution injection rates. Toluene precursor is more efficient at 800  C. The MWCNTs were synthesized by DIL-CVD using ethanol as precursor gas and Co nanoparticles as catalysts at 750  C growth temperature. The nanotubular structure was grown throughout the surface and the minimum crystallite size was measured as 14 nm. The grown CNTs were then processed to be used as CO2 sensor, which achieved ultrasensitive detection of CO2 gas at room temperature. Template-Based CVD The template-based CVD is a popular technique in CNT synthesis where the formation of CNTs occurs by depositing carbon on a sacrificial template and thermal decomposition of precursor gases. Appropriate template selection through templates, such as glass capillaries and anodized aluminum oxide (AAO) membranes, enables the precise control of outer diameter size, length of the tubes, and other structural properties. Like other CVD methods, in the template-based CVD, several parameters have an effect on grown CNTs. These variables include the type, geometry, and dimensions of the template, relative size, and location of the template in the furnace, type of precursor gases, gas concentration and flow rate, and growth temperature and time. The significant effects of the deposition time, synthesis temperature, and flow rate of the precursor gas on the wall thickness, deposited carbon mass, and carbon morphology were investigated. The different types of CNTs were fabricated on an anodized AAO template with acetylene precursor by using catalytic CVD. It was revealed that the structure of CNTs directly depends on the quality of catalyst deposition in the pores, the pore diameter of the AAO template, and the size of the metallic catalyst. In Table 1 the formation of various kinds of CNTs are listed regarding the different CVD synthesis methods and different reaction conditions.

Aligned, bamboo-like, arrowhead shape Bamboo-shaped CNTs

Vertically aligned MWCNTs

Vertically aligned CNT

Piled cone structured

MicrowavePECVD MicrowavePECVD MicrowavePECVD Radiofrequency CVD Hot-filament CVD

Vertically aligned SWCNTs Outer diameter: 25 nm Wall thickness: 6 nm Diameter: 60–80 nm Diameter: 10–35 nm Diameter: 15 nm yield: 50% Length: 420  20 nm Diameter: 50–100 nm

MWCNTs

Oxygen-assisted CVD Water-assisted CVD

MONs/CNTs composites

Length: 2.5 mm

Aligned CNTs

DC-PECVD

Water-assisted CVD

Length: 5 μm diameter: 20–130 nm Diameter: 3 nm

Aligned CNTs CNTs

Thermal CVD Thermal CVD

Characteristics Length: 5 μm diameter: 200 nm Bamboo-shaped Diameter: 6–8 nm

Product Vertically aligned CNTs

Method Thermal CVD

Table 1 Summary of studies on CNT synthesis with different types of CVD

Fe, Co, Ni nanoparticles Cu substrates

Methane in hydrogen

Fe-deposited Si substrate

Ni and Fe-Ni-Cr alloy substrates Ni-coated Si substrates

Fe, Al/Fe, Al2O3/Fe, Al2O3/Co on Si wafer Ferrocene

Ni foil substrates

Fe-coated Si substrates Al2O3 substrates without catalyst Co/Ni-coated tungsten wires

Catalyst Co/Ni

Methane-carbon dioxide Methane-nitrogen

Methane

Methane

Methane

Ethylene

Acetylene

Acetylene, ammonia

Acetylene Acetylene

C Source Acetylene

Synthesis Methods of Carbon Nanotubes (continued)

Sengupta (2018)

Sengupta (2018)

Bower et al. (2000) Chen et al. (2002)

Okai et al. (2000)

Wu et al. (2019)

Hata et al. (2004)

Chou et al. (2015)

Lee et al. (2000) Tripathi et al. (2014) Tanemura et al. (2001)

References Lee et al. (1999)

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Polymer pyrolysis CVD

ECR-CVD

Multi-walled CNTs

Fluidized-bed CVD Fixed-Bed CVD

Vertically and horizontally aligned CNTs CNT/Al composite powders

Multi-walled CNTs

Product Aligned bamboo-shaped

Method Hot-filament CVD

Table 1 (continued)

Length: 200–800 nm diameter: 10–20 nm

Characteristics Length: 10 μm diameter: 20–80 nm Diameter: 10–20 nm Diameter: 9–30 nm Length: 3–350 μm Polyethylene Glycol (PEG)

Methane-hydrogen

Acethylene-hydrogen

Ethylene-hydrogen

C Source Ethylene-ammonia

Cobalt nitrate

Co

Fe–Ni, Ni, and Fe

Fe-SiO2

Catalyst Ni-coated Si substrate

Tang et al. (2013)

Hsu et al. (2002)

Venegoni et al. (2002) Zeng et al. (2002)

References Sengupta (2018)

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Conclusion In conclusion, CNTs with their excellent mechanical, electrical, and chemical properties find use in different applications. Some of them are actuators, electronics, biotechnology and biomedical, microwave absorption, filtering, and composite materials. High efficiency and high-quality nanotube production is of great importance for all these applications. Each method indicates different advantages and disadvantages. For example, arc discharge and laser ablation methods provide single-walled and high crystalline CNT production. However, these methods consume a lot of energy for nanotube production and are not suitable for high-efficiency production. Another CNT synthesis method is electrolysis, which allows a controllable synthesis process by the electrolysis modes, cheap MWCNT production, controllable CNT morphologies, and carbon phases doping in one step. However, the cracking problem of graphite cathode during the electrolysis process and accumulation of electrolysis products cause instability in the process and prevent continuous performance for this method. The sonochemical/hydrothermal synthesis technique is a suitable method to produce different types of carbon nanostructures such as nano-onions, nanorods, nanowires, nanobelts, and MWCNTs. In this method, the starting materials are easy to obtain and are stable in ambient temperature, and even at low temperatures about 150–180  C CNT synthesis could be carried out, and there is no need a hydrocarbon gas or carrier gas for the CNT growth process in sonochemical/ hydrothermal synthesis of CNTs. The liquid phase CNT synthesis method is appropriate especially for the synthesis of aligned CNT arrays MWCNTs and it has a simple one-step production process and does not require a vacuum environment or catalyst preparation step. Besides, it is an environmentally friendly CNT synthesis method. The flame synthesis method is emerging as an alternative to furnace methods. While furnace methods do not allow large-scale synthesis on large surfaces because of the requirement of a lot of energy and long processing times, the flame synthesis method is suitable for large surface MWCNT synthesis. The plastic pyrolysis method is another method to utilize for the synthesis of CNTs from plastic wastes such as polypropylene, polystyrene, and polyethylene. With the flame synthesis method, MWCNTs production is possible and this method works similar to the catalytic CVD method and it is an important method in terms of environmental protection and recycling of waste. CVD method is based on the decomposition of hydrocarbons in the presence of a suitable transition metal catalyst. For obtaining CNTs with high purity on a large scale, the CVD method is a confidential synthesis method and capable of both SWCNT and MWCNT production with high crystallinity and high yield. It is a low-cost method compared to other methods suitable for SWCNT production. Overall, various nanotubes synthesis methods have been developed since the discovery of CNTs in 1991, and among these methods, CVD is currently the most widely used method.

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References Aboul-Enein AA, Awadallah AE, Abdel-Rahman AA-H, Haggar AM (2018) Synthesis of multiwalled carbon nanotubes via pyrolysis of plastic waste using a two-stage process. Fullerenes Nanotubes Carbon Nanostruct 26(7):443–450 Ando Y (1994) The preparation of carbon nanotubes. Fullerenes Nanotubes Carbon Nanostruct 2(2):173–180 Arora N, Sharma N (2014) Arc discharge synthesis of carbon nanotubes: comprehensive review. Diam Relat Mater 50:135–150 Baddour CE, Briens C (2005) Carbon nanotube synthesis: a review. Int J Chem React Eng 3:R3 Borsodi N, Szentes A, Miskolczi N, Wu C, Liu X (2016) Carbon nanotubes synthetized from gaseous products of waste polymer pyrolysis and their application. J Anal Appl Pyrolysis 120: 304–313 Bower C, Zhou O, Zhu W, Werder D, Jin S (2000) Nucleation and growth of carbon nanotubes by microwave plasma chemical vapor deposition. Appl Phys Lett 77(17):2767–2769 Chen G, Fray D (2003) Recent development in electrolytic formation of carbon nanotubes in molten salts. J Mining Metallurgy B Metallurgy 39(1–2):309–342 Chen M, Chen C-M, Chen C-F (2002) Preparation of high yield multi-walled carbon nanotubes by microwave plasma chemical vapor deposition at low temperature. J Mater Sci 37(17): 3561–3567 Chou Y-C, Wu H-C, Hsieh C-K (2015) From graphene to carbon nanotube: the oxygen effect on the synthesis of carbon nanomaterials on nickel foil during CVD process. Jpn J Appl Phys 55(1S):01AE12 Gogotsi Y, Libera JA, Yoshimura M (2000) Hydrothermal synthesis of multiwall carbon nanotubes. J Mater Res 15(12):2591–2594 Gspann T, Smail F, Windle A (2014) Spinning of carbon nanotube fibres using the floating catalyst high temperature route: purity issues and the critical role of Sulphur. Faraday Discuss 173:47–65 Guo T, Nikolaev P, Thess A, Colbert DT, Smalley RE (1995) Catalytic growth of single-walled manotubes by laser vaporization. Chem Phys Lett 243(1–2):49–54 Harris PJ (2007) Solid state growth mechanisms for carbon nanotubes. Carbon 45(2):229–239 Hata K, Futaba DN, Mizuno K, Namai T, Yumura M, Iijima S (2004) Water-assisted highly efficient synthesis of impurity-free single-walled carbon nanotubes. Science 306(5700):1362–1364 Hsu CM, Lin CH, Chang HL, Kuo CT (2002) Growth of the large area horizontally-aligned carbon nanotubes by ECR-CVD. Thin Solid Films 420:225–229 Huang X, Sun S, Tu G (2020) Investigation of mechanical properties and oxidation resistance of CVD TiB2 ceramic coating on molybdenum. J Mater Res Technol 9(1):282–290 Iijima S, Ichihashi T (1993) Single-shell carbon nanotubes of 1-nm diameter. Nature 363(6430): 603–605 Iijima S, Ajayan P, Ichihashi T (1992) Growth model for carbon nanotubes. Phys Rev Lett 69(21): 3100 Kataura H, Kumazawa Y, Maniwa Y, Ohtsuka Y, Sen R, Suzuki S, Achiba Y (2000) Diameter control of single-walled carbon nanotubes. Carbon 38(11–12):1691–1697 Kong J, Soh HT, Cassell AM, Quate CF, Dai H (1998) Synthesis of individual single-walled carbon nanotubes on patterned silicon wafers. Nature 395(6705):878–881 Lee CJ, Kim DW, Lee TJ, Choi YC, Park YS, Lee YH, Choi WB, Lee NS, Park GS, Kim JM (1999) Synthesis of aligned carbon nanotubes using thermal chemical vapor deposition. Chem Phys Lett 312:5 Lee CJ, Park J (2000) Growth model of bamboo-shaped carbon nanotubes by thermal chemical vapor deposition. Appl Phys Lett 77(21):3397–3399 Lee CJ, Park J, Jeong AY (2002) Catalyst effect on carbon nanotubes synthesized by thermal chemical vapor deposition. Chem Phys Lett 360(3–4):250–255 Manafi S, Nadali H, Irani H (2008) Low temperature synthesis of multi-walled carbon nanotubes via a sonochemical/hydrothermal method. Mater Lett 62(26):4175–4176

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Carbon Nanotube Growth Mechanisms Takahiro Maruyama

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brief History of Synthesis Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Vapor Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Growth Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Early Growth Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Growth Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Growth Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Growth Process from Catalyst Particle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalyst Particle Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalyst Metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Growth Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theoretical Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In Situ Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison Between Theory and Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Selective Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conductive Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chirality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Topics Related to Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vertically Aligned CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Horizontally Aligned CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Support Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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T. Maruyama (*) Department of Applied Chemistry, Nanomaterial Research Center, Meijo University, Nagoya, Aichi, Japan e-mail: [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_53

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Abstract

This chapter reviews the growth mechanisms of carbon nanotubes (CNTs). A brief overview of CNT synthesis methods including arc discharge, laser ablation, and chemical vapor deposition (CVD) is first provided, followed by a more in-depth discussion of the CVD method, which is currently the most important technique for CNT growth. The growth models of CNTs in the early stages for these synthesis methods are presented, and the various growth modes that are commonly used to describe CNT growth by CVD are introduced. Comparisons of tip growth vs. root growth, vapor–liquid–solid mode vs. vapor–solid–solid mode, and tangential mode vs. perpendicular mode are provided. Next, focusing on single-walled carbon nanotube (SWCNT) growth by CVD, the growth process of a SWCNT from a catalyst particle is described in detail. Theoretical simulations and in situ experimental analysis performed to elucidate the growth process of SWCNTs are reviewed and compared. In particular, focusing on the physical and chemical states of catalyst particles during SWCNT growth, the current understanding of the growth mechanism is summarized. Finally, the important issue of selective growth of SWCNTs, which can enable the growth of CNTs with desirable properties, is covered. Some important topics related to CNT growth such as vertically and horizontally aligned growth and the effect of a support layer on the catalyst activity are also discussed. Keywords

Growth · CVD · VLS · In situ analysis · Catalyst · Support layer

Introduction Carbon nanotubes (CNTs) are used in various applications in a wide range of fields owing to their outstanding properties. CNTs’ properties depend on their structure. In particular, the electronic state of single-walled carbon nanotubes (SWCNTs) is determined by their structural parameters, such as diameter and chirality. Therefore, selective growth of SWCNTs with desired chirality is an important issue in the field of CNTs. To achieve selective growth of SWCNTs, various growth techniques have been developed. Moreover, various efforts have been made for several years to understand the growth mechanism from both experimental and theoretical perspectives. Herein, following an overview of CNT synthesis techniques, a brief history of the growth model of CNTs is presented. The current state of understanding of the growth mechanism is then detailed from both experimental and theoretical progress.

Synthesis Methods Brief History of Synthesis Methods In the early stages, arc discharge and laser ablation were the main growth techniques used to produce carbon nanotubes (CNTs) (Prasek et al. 2011). In arc discharge, an arc is ignited between two graphite electrodes in a gaseous background

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(usually argon/hydrogen). The arcing evaporates the carbon; meanwhile, it cools and condenses such that some of the product forms as filamentous carbon on the cathode. Arc discharge has been developed into an excellent method for producing both highquality multiwalled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) (Iijima and Ichihashi 1993). MWCNTs produced by arc discharge are very straight, indicative of their high crystallinity. For the growth of SWCNTs, the mixing of a metal catalyst such as Fe, Co, Ni, Y, or Mo into the graphite anode is necessary. In the laser ablation method, high-power pulsed lasers are used to evaporate graphite (Lee et al. 1997). The evaporated material condenses to yield fullerenes; however, with the incorporation of a metal catalyst in the carbon target, SWCNTs with small diameters are grown. During laser ablation, a flow of inert gas is passed through the growth chamber to carry the grown nanotubes downstream to be collected on a cold finger. The produced SWCNTs are mostly in the form of ropes consisting of tens of individual nanotubes close-packed into hexagonal crystals via van der Waals interactions. Chemical vapor deposition (CVD) produces solid materials via chemical processes from flowing precursor gases (Kumar and Ando 2010). For CNT growth, CVD techniques involve the decomposition of a gaseous or volatile compound of carbon, catalyzed by metallic nanoparticles, which also serve as nucleation sites for the initiation. Because this method can be easily scaled up to industrial production levels, it has become the most important commercial method for SWCNT production. After the discovery of MWCNTs in 1991 (Iijima 1991), several groups achieved MWCNT growth via CVD using a combination of various types of carbon precursors and metal catalysts. To date, 15 transition metals have been found to act as catalysts for SWCNT growth (Maruyama 2018). Through the development of both the fabrication of catalysts and carbon feedstock supply, the temperature currently used in SWCNT growth by CVD is generally 700–1000
C, which is much lower than that for arc discharge and laser ablation. Thus, CVD is suitable for low-temperature CNT growth, which means that CVD is a cost-effective technique and applicable to various substrates.

Chemical Vapor Deposition Many types of CVD systems have been used for CNT growth (Kumar and Ando 2010; Maruyama 2018). The CVD method can be categorized based on the reactor type, the supply of the catalyst, and the excitation system. The categories of CVD methods are depicted in Fig. 1. Depending on the reactor type, CVD can be classified into hot-wall CVD or cold-wall CVD. In hot-wall CVD, the quartz tubes are generally used as reactors and are surrounded by heaters. The substrates on which the catalyst particles are deposited are placed in the quartz furnace; then, feedstock gas is introduced into the furnace. During CNT growth, entire furnaces are heated; hence, this method is called “hot-wall” CVD. The great advantage of hot-wall CVD is temperature uniformity. In addition, compared with cold-wall CVD, the equipment cost for hot-wall CVD is relatively low; hence, it is suitable for mass production of CNTs. However, because of the use of quartz furnaces, vapor could

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Fig. 1 Classification of CVD system

chemically react with the reactor wall when the reactor wall temperature is quite high, which would deposit contamination in the grown product. Hot-wall CVD is widely used and can be utilized for both catalyst-supported CVD and floatingcatalyst CVD. In cold-wall CVD, only the substrates are heated during CNT growth. In addition, metallic chambers are generally used for cold-wall CVD. Hence, by evacuating them using turbo molecular pumps, the base pressure can be much lower than that of a hot-wall CVD system, where a quartz furnace is used. Therefore, fine control of feedstock gas flow is possible under quite low residual gas pressure. In addition, compared with hot-wall reactors, shorter heating–cooling times and smaller growth periods are possible, which enables expansion of the growth conditions. However, the temperature on the substrate surface is not as homogeneous as that in hot-wall reactors, and its temperature gradient is sometimes a concern. Furthermore, another significant difference between hot-wall CVD and cold-wall CVD is the decomposition process of the feedstock gas. In hot-wall CVD, the feedstock gas passes inside the furnace heated at high temperature before reaching the catalysts. Hence, some portions of feedstock molecules are excited or dissociated by thermal energy, and CNT growth generally proceeds with the reaction between such excited species and catalysts. For example, in alcohol catalytic CVD (ACCVD), where ethanol gas is used as the feedstock, some portions of ethanol molecules are decomposed into C2H2, C2H4, and other species, and CNT growth occurs via their reaction on catalysts. However, in cold-wall CVD, only the substrates are heated; therefore, the feedstock gas reacts with the catalysts directly without predecomposition. From the perspective of the catalyst supply, CVD can be classified into two types: floating-catalyst CVD and catalyst-supported CVD. In floating-catalyst CVD, gas-phase catalysts are injected into a reactor with the feedstock gas. Both the catalysts and feedstock are in the gas phase; hence, this method is suitable for mass production. Some groups call this method “aerosol synthesis” because growth occurs on catalyst particles suspended in the gas phase. In the floating-catalyst CVD, ferrocene and iron pentacarbonyl are often used as a floating catalyst for Fe. Catalyst-supported CVD utilizes catalyst particles supported on substrates. SiO2/Si substrates, Al2O3 layers deposited on Si substrates, metal foils, and micrometer-sized oxide powders are generally used as support materials. To grow CNTs, nanometer-sized metal particles are generally used as catalysts. Fe and Co are

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commonly used as catalysts because of their high activity and low cost. In addition, cocatalysts are often used, where a catalytically inactive metal catalyst is deposited with Fe or Co catalysts to suppress aggregation of the catalyst metals. Co–Mo is a representative example, and SWCNTs grown from Co–Mo catalysts are available for commercial use. There are two main methods used to form catalyst particles on support materials: physical vapor deposition and the solution technique. For the former, sputtering and electron beam deposition are commonly used. By reducing the thickness of a catalyst metal below a few atomic layers, nanometer-sized metal particles can be formed on a substrate. The typical nominal thickness of metal catalysts is 0.1–1.0 nm. In some cases, catalyst particles are formed on the substrate by annealing after deposition of a thin film. In the solution technique, aqueous or ethanol solutions, where the metal complex such as metal acetate or metal nitrate are dissolved, are used to coat catalyst metals. After coating the solution onto the substrate surface by either spin coating or dip coating, catalyst particles are formed on the substrate by postannealing. The solution technique is available even for powder-type support materials. For the feedstock, many types of molecules have been used as carbon precursors, including hydrocarbons (CH4, C2H2, C2H4), alcohol (CH3OH, C2H5OH), aromatic hydrocarbons (C6H6), and CO. Furthermore, catalyst-supported CVD can be classified by the excitation system. When a traditional heat source, such as resistive, inductive, or infrared heater, is used without any excitation system for the carbon source, it is called “thermal CVD.” In particular, when alcohol gas such as ethanol is used as the feedstock, it is called ACCVD. When the feedstock gas is excited by a plasma source, it is called “plasmaenhanced CVD (PECVD).” If the feedstock is excited by a heated filament, it is called “hot-filament CVD.” Thermal CVD is the most common method to grow CNTs. Hydrocarbon, alcohol, aromatic hydrocarbon, and CO are used as the feedstock. If the feedstock is liquid at room temperature, such as alcohol or benzene, either vaporization by heating or bubbling with an inert carrier gas such as N2 or Ar is used to supply feedstock vapor into the reactor. PECVD employs electrical energy to create glow discharge plasma, where the electron temperature is much higher than the ion temperature. The high-energy electrons promote dissociation of gas molecules by which energy is transferred into gas molecules, forming reactive radicals, ions, neutral atoms, and other highly excited species. These species show high reactivity with catalysts; hence, CNT growth could occur at relatively low temperature. CH4 is the most common feedstock in PECVD. In addition, PECVD can be classified by the type of plasma source used to generate gas discharge of the feedstock: direct current (DC), RF and microwave (MW) PECVD, and remote plasma CVD. In PECVD, a high electric field is generated in the sheath region by the potential difference between the plasma and substrates, which is sometimes effective to obtain aligned CNTs, and several groups have reported vertically aligned (VA) CNT growth by PECVD. In catalyst-supported CVD, decomposition of the feedstock gas on catalyst particles is essential, and the decomposition rate increases with increasing temperature. However, with increasing temperature, aggregation of catalysts is enhanced, which hinders CNT growth; in particular, SWCNT growth is prevented. In addition,

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catalyst support materials such as SiO2 begin to sublimate upon approaching high temperature (typically 1000
C). Therefore, there is an optimal temperature to obtain the highest yield of CNTs by CVD. Conventional thermal CVD is performed at 800–1000
C depending on feedstock molecules and catalysts.

Growth Models Early Growth Models Except for MWCNT growth by arc discharge methods, CNT growth is generally performed using metal catalysts. In particular, metal catalyst nanoparticles are indispensable for SWCNT growth in all synthesis methods. In general, depending on the location of the catalyst particle, the growth mode is classified into two cases: tip growth mode, where a catalyst particle is included at the tip of a CNT during growth, and root growth mode, where a catalyst particle is anchored onto the substrate surface, from which a CNT is grown (Fig. 2). The latter is often observed for CNT growth by “catalyst-supported CVD.” These two modes have been observed for both MWCNT and SWCNT growth. In the early stage, several growth mechanisms were proposed as nascent models, including the “lip–lip interaction model” for MWCNT growth by arc discharge (Nardelli et al. 1998; Popov 2004), the “scooter model” for SWCNT growth (Thess et al. 1996), and the “sea-urchin model” and “Yarmulke model” for CNT growth from catalyst particles. The “lip–lip interaction model” proposed that the lip– lip interaction between the open edges of neighboring concentric shells on a Fig. 2 Schematics of (a) tip growth and (b) root growth of CNTs

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MWCNT effectively stabilizes the open edges and facilitates the CNT growth (Fig. 3), and it was used to explain the growth mechanism of MWCNTs. In the “scooter model,” metal atoms “scoot” around the open edge of the cluster, preventing the formation of carbon pentagons and the dome closure at the CNT tip (Fig. 4). This model was mainly applicable for CNT growth by arc discharge and laser ablation methods. The “sea-urchin model” is derived from the root growth of CNTs (Fig. 5a). In early studies using transmission electron microscopy (TEM), several groups Fig. 3 Lip–lip growth mechanism scheme showing a top view of a double-walled CNT with an open zigzag edge. The adatoms occupying sites between doubly coordinated edge atoms of adjacent walls stabilize opentube growth. (Reproduced with permission from (Nardelli et al. 1998); the figure is modified from the original one)

Fig. 4 Schematics of the scooter model

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Fig. 5 Schematics of (a) sea-urchin model and (b) Yarmulke model

observed catalyst particles from which a number of CNTs were grown by CVD. Based on these observations, the growth mode where a number of CNTs are grown from one catalyst particle or mass is called the “sea-urchin model” because of the similar morphology. In addition, a growth mode where only one CNT is grown from one catalyst particle has also been reported. Previously, such a growth mode was often described by the “Yarmulke model” (Fig. 5b). Compared with CNT growth based on the sea-urchin model, the catalyst particles for CNT growth described by the Yarmulke model were relatively small, and the particle sizes were typically one to a few nanometers. In this model, one CNT grows from one catalyst particle, and such a one-to-one correspondence much be considered for the growth mechanism of CNTs; the simplicity of this model is beneficial and suitable for fundamental research on CNT growth, in particular, for theoretical simulation. Therefore, the focus has been on growth processes where a CNT is grown from one catalyst particle to elucidate the growth mechanisms to date. As a successor of the “Yarmulke model,” in the early stage, the vapor–liquid– solid (VLS) model was widely used to explain the growth process of CNTs by CVD (Kukovitsuky et al. 2000), in analogy with the growth of nanowires of semiconductor materials. In the VLS mode, catalyst particles are considered to be in the liquid phase at the growth temperature (Fig. 6a). With the supply of feedstock gas, carbon atoms dissolve into the liquid catalyst particles and precipitate onto the catalyst particles after supersaturation. As a result, CNTs are formed from segregated carbon atoms on the catalyst particles. In CVD growth, the feedstock is in the gas phase, and CNTs are of course solid; therefore, this growth process is called the “VLS mode.” This model has been widely used to discuss the growth mode of CNTs. However, in the past two decades, in situ analysis techniques have been developed, which have revealed that catalyst particles can be in the solid phase during CNT growth. In this case, the growth mode is called “vapor–solid–solid (VSS) mode” (Fig. 6b).

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Fig. 6 Schematics of growth model of CNTs for (a) VLS mode and (b) VSS mode

Growth Modes From the appearance of the CNTs, the growth mode of CNTs can be classified into several types. As mentioned in the previous section, for catalyst-supported CVD, the growth mode can be classified by the location of the catalyst particles: “tip growth,” where the catalyst particles are located and included at the tip of CNTs, and “root growth,” where the catalyst particles are located on the substrates (Fig. 2b). “Root growth” is sometimes called “base growth” because the catalyst particles are located on the bottom of the CNTs. The growth type that is preferable is determined by the strength of the interaction between the catalysts and substrates. When the interaction between the catalyst particle and substrate is strong enough, the catalyst particles remain stuck to the substrate during CNT growth; that is, “root growth” occurs. However, if the adhesion strength is weak, the catalyst particles are detached from the substrate, resulting in “tip growth.” These two growth modes are observed for MWCNT and SWCNT growth by catalyst-supported CVD. From the viewpoint of the physical phase of the catalyst particles, the growth mode for CNTs can be classified into VLS and VSS modes, as discussed in the previous section (Fig. 6). In the VLS mode, catalyst particles become liquid during CNT growth, whereas they are solid in the VSS mode. In the VLS mode, carbon atoms are dissolved into the liquid catalyst particles, precipitating to form CNTs. In the VSS mode, catalyst particles are in the solid phase, and carbon atoms are not necessarily dissolved into the catalyst particles; instead, CNTs could be formed by surface diffusion of carbon atoms on the catalyst particles. In some cases, catalyst particles are transformed into solid carbides, forming CNTs by precipitating carbon atoms. In general, the preferred growth mode depends on the catalyst element and growth conditions, in particular, the melting point of the catalyst metal, the carbon solubility, and the growth temperature. CNT growth using high-melting-point metals as catalysts or CNT growth at low temperature would proceed via the VSS mode, whereas CNT growth with low-melting-point metal catalysts or at high temperature

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Fig. 7 Schematics of growth model of SWCNTs in (a) tangential mode and (b) perpendicular mode

would proceed via the VLS mode. The classification of the VLS and VSS modes is applicable for SWCNTs or few-walled CNTs. For SWCNT growth, the growth mode is also classified by the size correlation between the SWCNT diameter and its seeding catalyst particle. In SWCNT growth, it has been widely accepted that a size correlation exists between the SWCNT diameter and its seeding catalyst particle; as the catalyst particle size increases, the SWCNT diameter increases (Jeong et al. 2005). TEM observation has shown that there are two configurations of a tube wall on a catalyst particle: the tangential mode, where the SWCNT diameter is almost equal to the particle size, and the perpendicular mode, where the SWCNT diameter is much smaller than the particle size (Fig. 7). These two growth modes are strongly related to the diameters of the grown SWCNT, and control of these two modes is important to obtain smalldiameter SWCNTs. Theoretical simulation has shown that the wettability of a graphene sheet on the catalyst metal surface determines which growth mode is apt to occur, indicating that the carbon concentration of catalyst particles affects the wettability (He et al. 2018).

Growth Processes Overview Thus far, various types of growth modes have been discussed for CNTs; however, in recent years, the growth process of a CNT from a catalyst particle has been mainly investigated because of its importance in controlling the CNT structure in CVD

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growth. In particular, SWCNT growth has been a main subject because of its simplicity and importance for application. Here, the growth process of a SWCNT from a seeding catalyst particle is focused on. The growth of SWCNTs from catalyst particles is mainly divided into five stages: decomposition of the feedstock on catalyst particles, formation of nanocarbon fragments induced by either precipitation or migration of carbon atoms on the catalyst particles, formation of carbon nanocaps, lift-off of carbon nanocap from catalyst particle, and continuous growth of CNTs (Fig. 8). The performance of the catalyst metal in each stage affects the catalyst activity, which is related to the resultant CNT yield.

Growth Process from Catalyst Particle For SWCNT growth, hydrocarbon, alcohol, aromatic hydrocarbon, and CO are generally used as feedstock in CVD, and in some cases, the feedstock gas is excited by plasma or a hot filament to enhance the growth rate. These feedstock gases are supplied onto the catalyst particles, and they are dissociated on the catalyst surface. The metals that have been reported to act as catalysts for SWCNT growth are shown in Fig. 9. Thus far, 15 transition metals and several nonmetal particles such as Si, Ge, SiC, Al2O3, and nanodiamond have been reported to act as catalysts. Among them, Fe, Co, and Ni catalysts are commonly used for SWCNT growth (Maruyama 2018). In particular, Fe is most widely used because of its high activity and low cost.

Fig. 8 Schematics of growth process of SWCNT from catalyst particle

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Fig. 9 Periodic table. In the lower portion, the catalytic activity toward CNT growth for each group in the periodic table is shown. The elements from which SWCNT growth have been reported are shown in bold. The activity of transition metals to carbon atoms is related to the number of d electrons; as a result, the relative activity of catalyst metals follows a volcano-shaped pattern across the periodic table

In the first stage of the SWCNT growth process (Fig. 8), the feedstock gas is decomposed on catalyst particles, and its activity is dependent on the electronic structure of the catalyst metal. In general, the dissociation efficiency of a transition metal is related to the number of d electrons and the energy position of the d band center (Jourdain and Bichara 2013). For Au, Ag, and Cu, the outermost d shell is fully occupied, and the d band center is far below the Fermi level (typically 3–4 eV below the Fermi level). In this case, the adsorption energy is relatively low and the dissociation efficiency is quite low; as a result, these metals do not act as good catalysts for SWCNT growth. However, if the d band center is near the Fermi level, the adsorption energy is high and the dissociation efficiency is also relatively high. In fact, the maximum dissociation barrier of ethanol molecules on Cu (with a d-band center far below the Fermi level) is 1.9–3.4 eV, whereas that on Co (with a d-band center near the Fermi level) is 0.63–2.7 eV (Wang et al. 2009). In the second stage, nanocarbon fragments are formed on the catalyst surface, induced by either precipitation or migration of carbon atoms on the catalyst particles. The timescale of this stage is very short; therefore, experimental analysis is quite difficult, although some theoretical simulations have been conducted to investigate this stage. The results revealed that pentagons as well as hexagons are formed on the catalyst particle surface, which combine with each other, leading to carbon nanocap formation. In the third stage, a “carbon nanocap” is formed on the catalyst particle, which is a dome-shaped graphene sheet, that is, a type of hemispherical fullerene. To form tubular structures, the formation of carbon nanocaps is essential. Furthermore, carbon nanocaps determine both the diameters and chirality of the grown SWCNTs. Therefore, it is important to control the size and shape of the catalyst particles on

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which the carbon nanocaps are formed. In general, when the catalyst particle size is 1–3 nm, SWCNTs are apt to form; however, MWCNTs tend to grow from catalyst particles with sizes greater than ~6 nm. Compared with flat graphene flakes, the formation energy of spherical fullerene C60 is higher by 0.135 eV because of the inclusion of pentagons (Fan et al. 2003). However, if a carbon nanocap is placed on a metal surface, the dangling bonds of the carbon nanocap form bonds with metal atoms, reducing the total energy. Fan et al. compared the total energy on a Ni(100) surface between a graphene flake (carbon atom; 60) and a carbon nanocap and observed that the latter was more stabilized because of the formation of bonds between the carbon atoms in the periphery of the carbon nanocap and Ni atoms (Fan et al. 2003) (Fig. 10). Once carbon nanocaps are formed, the continuous growth of the cylindrical parts of SWCNTs is necessary to obtain a SWCNT with a certain length. On one hand, if the interaction between a SWCNT and a catalyst particle is too weak, the SWCNT will detach, and continuous growth is impossible. On the other hand, if the interaction is too strong, it is difficult to lift up the carbon nanocap, and in some cases, encapsulation of the catalyst particles occurs. Therefore, an appropriate adhesion strength between the SWCNT and catalyst particle is necessary to lift up a carbon nanocap, leading to continuous growth of SWCNTs (Fig. 11). Several studies have pointed out the importance of bond strength between the metal surface and a carbon nanocap. Using MD simulation, Burgos et al. demonstrated that the adhesion strength between the surface of a catalyst particle and nascent carbon structures must be weak enough to realize lift-off of the carbon nanocap from catalyst particles (Burgos et al. 2010). In addition, they showed that if the adhesion energy is too high, catalyst particles were encapsulated, poisoning the catalysts. They pointed out that the adhesion energy suitable for SWCNT growth depends on the catalyst particle size, and the maximum strength of interaction between a graphite sheet and catalyst atoms for SWCNT growth decreases from ~200 to ~60 meV/atom when the particle diameter increases from 0.8 to 1.8 nm. Silvearv et al. calculated the strength between a SWCNT and a metal surface for many transition metals and pointed out that there is a “Goldilocks zone” in the bond

Fig. 10 Schematics of (a) graphene flake and (b) carbon nanocap (hemispherical fullerene (C30)) on metal surface

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Intermediate Temperature

Unsaturated

High Temperature

Highly Supersaturated Supersaturated

Fig. 11 Detailed VLS model of SWNT growth at different temperatures. (Reproduced with permission from Ding et al. (2004c))

strength for SWCNT growth (Silvearv et al. 2015). Their simulation demonstrated that Fe, Ni, and Co are within the Goldilocks zone, indicating that these metals have bond strengths preferable for CNT growth. Rh and Pt are also in the Goldilocks zone, and Ir and Ru are on its edge, all of which act as catalysts for SWCNT growth. Therefore, their simulations are considered to be relatively consistent with experimental results (Fig. 12).

Catalyst Particle Behavior For SWCNT growth, nanosized metal particles are generally used. Typical growth temperatures are between 700 and 1000
C for CVD growth; hence, a high melting point is necessary for catalyst metals. Thus, transition metals between group 3 and 11 in the periodic table are candidates for catalysts for CNT growth because most of the transition metals have melting points higher than 800
C. However, as the metal particle size decreases, the ratio of the surface area to volume increases, and when the diameter of the particles becomes a few nanometers, approximately half of the constituent atoms are located at the surface, which causes instability of the surface atoms. As a result, the equilibrium vapor pressure increases significantly, and the equilibrium vapor pressure of a particle of radius r in contact with a vapor is approximately expressed by the Gibbs–Thomson equation, as below: p ¼ p0 exp

  2σ sg V , kB Tr

where p and p0 are the equilibrium vapor pressures over curved and flat surfaces, respectively; σ sgis the surface tension at the particle–gas interface; V is the volume of

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Fig. 12 Bulk cohesive energies per bond at 0 K and 1 atm, calculated from experimental thermodynamic data. The shaded region marks the “Goldilocks zone,” where SWCNTs were easy to grow. (Reproduced with permission from Silvearv et al. (2015); the figure is modified from the original one)

an atom in the particle; kB is the Boltzmann constant; and T is the temperature (Jourdain and Bichara 2013). This equation confirms that the particle becomes unstable as the particle size increases. As a result, the melting point of a nanoparticle decreases with decreasing particle size, as can be approximately expressed by   4σ sl T m ðr Þ ¼ T bulk 1  , m H f ρr where Tm(r) and T bulk m are the particle and bulk melting points, respectively; σ sl is the surface tension at the solid–liquid interface; Hf is the bulk latent heat of fusion per volume unit; and ρ is the density of the particle. This indicates that the melting point of a particle decreases as the particle size is reduced. Using MD simulation, Neyts et al. calculated the melting point of Ni clusters whose diameters were between 1.23 and 1.99 nm and showed that the melting point decreases in inverse proportion to the particle size (Neyts and Bogaerts 2009). They also analyzed the Lindemann index of the temperature dependence of each Ni cluster and observed that melting begins to occur in the subsurface region, when the particle size decreased from 1 to 2 nm, and pointed out that the inside of the particle remains a solid phase, even after the melting occurred at the surface. It should be noted that catalyst particles might be deformed from a spherical shape on substrates by the catalyst-supported CVD growth. Ding et al. simulated the effect of bond strength between a Fe cluster and the substrate on the cluster shape and showed that the cluster shape becomes flat as the bond becomes strong, leading to the higher melting point (Ding et al. 2006).

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During SWCNT growth by CVD, feedstock gas continues to be supplied onto catalyst particles at high temperature; hence, there is a possibility that carbon atoms dissolve into catalyst particles, leading to the formation of carbides. The details of the solubility of carbon in nanoparticles have not yet been clarified; however, experimentally, there are many reports that carbon solubility increases with the reduction of particle size. In addition, thermodynamic models generally predict that the solubility of impurities inside nanoparticles increases as the particle size decreases. Using grand canonical Monte Carlo simulations, Diarra et al. calculated the adsorption isotherms of carbon on Ni nanoparticles with various sizes (Diarra et al. 2012). They observed that with the reduction of Ni nanoparticle size, a larger fraction of carbon is dissolved into the particles owing to the larger carbon solubility in the subsurface than in the bulk. They showed that in a Ni405 cluster, carbon atoms exist in the subsurface at 800 K; however, they dissolved into the inside of the cluster at 1400 K. In addition, Ding et al. performed MD simulation for Fe clusters and showed that the melting point of the Fe cluster decreases by approximately 100 K when approximately 10% of the carbon atoms dissolved (Ding et al. 2004a). During CNT growth by catalyst-support CVD, aggregation of catalyst particles is generally observed. This phenomenon is regarded as Ostwald ripening, by which the migration mechanism refers to the diffusion of atoms between immobile nanoparticles. As the particle size decreases, the equilibrium vapor pressure increases; as a result, the smaller catalyst particles disappear while the larger ones are enlarged. In general, as the melting point becomes higher, the equilibrium vapor pressure decreases, and the Ostwald ripening is apt to be suppressed. Using MD calculation, Ding et al. demonstrated that coalescence of Fe clusters occurs even below the melting point of Fe, which reproduced the Ostwald ripening by the simulation (Ding et al. 2004b). In addition, the density and size distribution of catalyst particles and the substrate materials affect the Ostwald ripening.

Catalyst Metal Thus far, SWCNT growth has been reported from 15 transition-metal catalysts. In general, the ability of transition metals to form bonds with carbon atoms decreases with the number of d electrons. Metals with completely filled d orbitals such as Au, Ag, and Cu show a negligible affinity for carbon, and metals with a few d vacancies such as Ni, Fe, and Co exhibit a finite carbon solubility, whereas transition metals with many d vacancies such as Ti, Mo, and W can form strong chemical bonds with carbon and stable carbide compounds. This tendency indicates that for transition metals, the affinity for carbon increases from the right to the left in the periodic table (Jourdain and Bichara 2013). Similarly, there is a general trend for the strength of the adsorbate–substrate bond, that is, the heat of adsorption on transition metal surfaces. The heat of adsorption of adsorbates generally decreases from the left to the right for transition metals in the periodic table. This trend can be explained by the well-established chemisorption model proposed by Nørskov; molecules adsorbed on transition-metal surfaces interact with the d states and give rise to bonding and antibonding states (Nørskov 1982).

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When the d state of a metal is located near the Fermi level, the bonding states are generally formed below the bonding state of the molecule before adsorption, that is, the adsorption is stabilized. However, when the d state of a metal is far below the Fermi level, the antibonding state is generally formed below the Fermi level after desorption, leading to the unstable adsorption. In general, the binding energy of the d state becomes higher as the number of d electrons increases; as a result, the strength of the bond between the adsorbate and transition-metal surface decreases from the left to the right in the periodic table. This finding suggests that the feedstock gas is not apt to dissociate on transition-metal surfaces as it progresses from the left to the right in the periodic table, that is, the dissociation ability of a transition metal increases in that order. However, the strong bond between carbon atoms and the metal surface means that the adhesion strength is high, which prevents lift-off of a carbon nanocap from the catalyst particle, impeding CNT growth. For these reasons, the catalyst activity of transition metals for CNT growth shows a volcano-shaped pattern across the periodic table and transition metals having 6–8 d electrons, such as Fe, Co, and Ni, act as good catalysts.

Growth Mechanisms Overview In the early stage, SWCNT growth was described based on the VLS model, and carbon nanocaps were believed to be formed via the segregation of carbon atoms from liquid catalyst particles. The typical growth temperature of SWCNTs in conventional CVD (700–1000
C) is much lower than the melting point of conventional catalyst metals such as Fe, Co, and Ni. However, it is possible that the catalyst particles are in the liquid phase, considering the melting-point depression for nanoparticles (Teijingen et al. 2020). In addition, many theoretical simulations reproducing the growth process of a SWCNT from a catalyst particle suggest that the crystallinity of catalyst particles is not maintained at the growth temperature. However, recent in situ TEM observation of SWCNT growth has shown that the crystallinity of catalyst particles is maintained during SWCNT growth from Fe and Co catalysts. These findings suggest that the VSS model can also be applied to describe the growth process of SWCNTs from a catalyst particle. In addition, Fe and Co catalysts were shown to form carbides during CNT growth. Taking these findings into account, the catalyst state during CNT growth can be classified as shown in Table 1. Table 1 Classification of relationship between catalyst state and carbon dissolution Dissolution of carbon Dissolution No dissolution

Catalyst Solid (crystalline) VSS model (solid solution) VSS model

Liquid VLS model (Negligible solubility in liquid catalyst)

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Theoretical Simulation The ability to obtain SWCNTs with a specific chirality has been one of the most significant objectives in the nanocarbon field. To realize this, understanding of the growth mechanism of a SWCNT is essential. In addition, the growth process of tubular-structure nanocarbon materials from metal particles has also attracted attention for advancements in fundamental science. In CVD, a SWCNT is grown from a catalyst metal particle; therefore, it is important to clarify the physical and chemical states of catalyst particles during SWCNT growth. However, the catalyst size is only one to a few nanometers, and practically, they show a size distribution at the growth temperature caused by aggregation, even when catalyst particles with completely homogeneous size are prepared. The physical and chemical properties of nanoparticles such as the melting point and carbon solubility strongly depend on the particle size. Therefore, it is quite difficult to determine the accurate physical and chemical states of nanoparticles during CNT growth. From the theoretical viewpoint, MD simulations have been performed to clarify the growth mechanism of a SWCNT from a catalyst particle. Ding and his coworkers reproduced the growth process of a CNT from an Fe catalyst particle between 600 and 1600 K by MD simulation (Ding et al. 2004c) (Fig. 13). Under a supply of carbon atoms with sufficiently low rate on the catalyst particles, they demonstrated that carbon atoms are dissolved into catalyst particles; they are then highly supersaturated, and carbon atoms precipitate on them, forming carbon nanocaps. They noted that, irrespective of the temperature, carbon atoms are dissolved into a Fe particle, and Fe carbides are formed (Ding et al. 2004d). In addition, they showed that the physical state of Fe particles depends on the growth temperature by calculating the Lindemann index for particles of various sizes for each temperature (Ding et al. 2005) (Fig. 14). Their results indicate that when the temperature is sufficiently high (>1000 K), the particle is in the liquid phase. In this case, carbon

a. 5 ns

b. 5.5 ns

c. 6.25 ns

d. 6.5 ns

Fig. 13 Nucleation of a SWCNT from an Fe150 cluster. The diameter of the graphitic cap increases during the growth process until it equals the diameter of the cluster. The temperature was 1100 K. (Reproduced with permission from Ding et al. (2004c))

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Fig. 14 Lindemann indexes of Fe (open circles) and carbon (solid circles) atoms in Fe300C60 cluster as a function of their average distance from the cluster center of mass, d. (Reproduced with permission from Ding et al. (2005))

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Lindemann index 0.4 0.3 0.2

T=1100 K

0.1

Lindemann index (a. u.)

0.0 0.4 0.3 0.2

T=1000 K

0.1 0.0 0.4 T=900 K

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0.3 0.2 0.1 0.0 0.0

0.2

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d (nm) C,

Fe

atoms diffuse rapidly to the center of the particle and are homogenously distributed in the particle. However, when the temperature is relatively low (727
C) VSS (727
C)

In situ analysis Carbide VSS (600
C) Carbide VSS (625
C) Metal VSS (650
C)

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Fig. 17 Schematics of the relationship between the temperature and pressure condition in previous in situ analysis and typical growth conditions for CNT growth

Selective Growth Conductive Type A SWCNT can be either metallic or semiconducting depending on its chirality. A (n, m) SWCNT is metallic if n  m ¼ 3c (c is an integer); otherwise, it is semiconducting. Therefore, if SWCNT growth occurs randomly, one-third of as-grown SWCNTs are generally metallic and two-thirds are semiconducting. Therefore, realization of selective growth of metallic or semiconducting SWCNTs is an important objective in the nanotube field. In the early days, selective growth of semiconducting SWCNTs by PECVD was reported. However, it has been noted that plasma damage might affect the conducting property of grown SWCNTs. Nevertheless, there have been many reports on the control of the conducting property of SWCNTs by modifying the growth conditions such as the feedstock, catalyst preparation, and addition of water to the feedstock. Ding et al. performed ACCVD growth with Cu catalysts and obtained SWCNTs, including ~95% semiconducting SWCNTs, by controlling the ratio of the feedstock gas, that is, the ratio of methanol to ethanol (Ding et al. 2009). Selective growth was also achieved by ACCVD growth using isopropyl alcohol with Fe catalysts, with ~90% of the SWCNTs being semiconducting SWCNTs. In addition to ACCVD, selective growth of semiconducting SWCNTs has also been achieved by CVD using CH4 with Fe catalysts supported on CeO2 (Qin et al. 2014). CeO2 can offer active oxygen owing to the transformation of Ce4+ to Ce3+, providing a stable and proper oxidative environment for etching of m-SWCNTs during the tube nucleation. By optimizing the pretreatment process of catalysts and the growth time, ~95% semiconducting SWCNTs were obtained.

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In contrast to semiconducting SWCNTs, metallic SWCNTs have density of states near the Fermi level and show high reactivity with various gas molecules. Therefore, etching of only metallic SWCNTs during growth was attempted to increase the ratio of metallic SWCNTs during growth. By adding hydrogen in the floating-catalyst method using ferrocene and CH4, selective growth of metallic SWCNTs with diameters of 1–2 nm was achieved.

Chirality Control The ultimate target of CNT synthesis is to grow SWCNTs with a specific chirality. Currently, to grow SWCNTs with an intended chirality is impossible; however, SWCNT growth with a unique chirality has been achieved for several chiralities (Yang et al. 2020). There have been several reports that have shown that the growth conditions such as the feedstock, growth temperature, catalyst elements and their composition, and pretreatment affect the chirality distribution of grown SWCNTs. He et al. performed CVD with a Fe catalyst and showed that the chiral angles of grown SWCNTs depend on the feedstock; when CO was used, the chiral angle became larger (approaching that of armchair SWCNTs). Later, they obtained (6, 5) SWCNTs dominantly by CVD at 600
C using CO with an FeCu catalyst (He et al. 2010). Another group obtained (6, 5) and (8, 4) SWCNTs dominantly using CH4 with FeRu catalysts in CVD growth at 600 and 850
C, respectively. Instead of modification of the growth conditions in CVD with conventional metal catalysts, other approaches have been attempted to realize selective growth of SWCNTs with a unique chirality: (1) growth from nanocarbon materials as a seed and (2) growth from a catalyst particle as a template. For the former, three methods have been reported: growth from a seed SWCNT (“cloning”), a nanocarbon ring, and a carbon nanocap with a specific structure. For the approach by the “cloning” technique, regrowth of SWCNTs was reported by using open-end SWCNTs. After CVD growth using CH4, (7, 6), (6, 5), and (7, 7), SWCNTs were regrown with chiralities that were exactly the same as those of their seed SWCNTs. In addition, growth of uniform CNTs was attempted by ACCVD using carbon nanorings (cyloparaphenilenes) as seeds, resulting in the successful growth of CNTs with diameters close to those of the carbon nanorings. As for utilizing a carbon nanocap as a seed, Sanchez-Valencia et al. grew (6, 6) SWCNTs from carbon nanocaps with a specific structure, which were synthesized via an organic chemical synthesis technique (Sanchez-Valencia et al. 2014). This method can be applied to synthesize SWCNTs with another chirality. Using another approach with a template catalyst, Li’s group reported (12, 6), (16, 0), and (14, 4) SWCNTs using W6Co7 catalyst particles, depending on the pretreatment condition of the catalyst. The melting point of W6Co7 is quite high and is stable at the growth temperature (Yang et al. 2014). In addition, Co is an efficient catalyst to grow SWCNTs. Hence, the surface of this alloy catalyst acts as a template for SWCNT growth, and SWCNTs with a specific chirality are grown from a W6Co7 catalyst.

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Topics Related to Growth Vertically Aligned CNTs In the CNTs grown by the catalyst-supported CVD, the morphology of the CNTs varied depending on their density. When the density was low, web-like or spaghettilike CNTs were observed; however, when the density was sufficiently high (typically ~1012 cm1), CNTs grew leaning on each other and grew vertically. In this case, the CNTs were grown nearly perpendicular to the substrate, that is, vertically aligned CNTs (VA-CNTs) that are self-organizing were produced, which are expected to be useful for via wiring, emitters, membranes, cathodes of Li-ion batteries, supercapacitors, and photoabsorbers. In addition, CNT ropes can be formed by spinning vertically grown CNTs. VA-CNTs were first reported by Li et al. for MWCNTs in 1997 (Li et al. 1997). For SWCNTs, VA-SWCNTs were first fabricated by ACCVD using a Co catalyst. Most VA-SWCNTs have been grown from Fe catalysts deposited on Al2O3 support layers. Hata et al. achieved growth of VA-SWCNTs of 2.5 mm in length after only 10-min growth by adding a small amount of water to feedstock C2H4 (Hata et al. 2004). Their growth technique with remarkably fast growth rate is called “supergrowth.” In addition to Fe, Co catalysts on Al2O3 support layers have also been used as catalysts to grow VA-SWCNTs by ACCVD. Recently, Maruyama et al. succeeded in VA-SWCNT growth by ACCVD using Ir catalysts (Maruyama et al. 2020). The diameters of most SWCNTs grown from Ir catalysts have been below 1.1 nm, which is much smaller than those of VA-SWCNTs grown from Fe and Co catalysts. In addition, using Ir catalysts, VA-SWCNTs were grown directly on SiO2/Si substrates. The density of the SWCNTs of the VA-SWCNTs grown from Fe catalysts is typically 1011–1012 cm1, and a SWCNT density of ~1013 cm2 was reported as a maximum value, where SWCNTs were considered to be grown from most catalyst particles (Esconjauregui et al. 2010).

Horizontally Aligned CNTs When CNTs are grown parallel to each other and also parallel to the substrate in catalyst-supported CVD, they are called “horizontal-aligned (HA)” CNTs. For application in integrated circuits, it is important to realize high-density horizontally aligned semiconducting SWCNTs because high current drive becomes enabled in each transistor. Simulation results have demonstrated that when the lithography width is 16 nm and the SWCNT density is 50 μm1, the drive speed is 3.5 times faster than that of Si-FETs, and the energy consumption is approximately 2.5 times lower than the latter. The typical target density is considered to be 125 μm1. The methods to realize HA-CNTs can be classified into two categories: (1) direct synthesis of HA-CNTs on a substrate and (2) postgrowth alignment of CNTs. Recently, wafer-scale monodomain films of HA-SWCNTs were realized using slow vacuum filtration as a postgrowth process. The SWCNT density was

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~1  106 in a cross-sectional area of 1 μm2; that is, highly packed aligned SWCNTs were realized. However, direct synthesis of HA-CNTs is fascinating because the process is simple and cost effective. Thus far, for direct synthesis methods, mainly four types of strategies have been used to align CNTs on the substrate surface: (a) use of the gas flow direction (“kite growth”), (b) application of a DC electric field, and use of (c) steps and (d) atomic rows on the substrate surface. In the kite growth method, tip growth is used to align the CNTs along the direction of gas flow; hence, high heating rates or catalyst particles with low adhesion energy to the substrates are used to lift up the tip of the CNTs. HA-SWCNTs by kite growth have been achieved on Si substrates by CVD using CO with FeMo catalysts. HA-SWCNT growth has also been performed using CVD with application of DC electric field between Mo electrodes on Si substrates, resulting in HA-SWCNTs with lengths of 10 μm. In addition, PECVD has been used to align CNTs by using an electric field gradient in the plasma. By setting the substrates where the DC electric field was parallel to the substrates, HA-MWCNTs were obtained. HA-SWCNT growth achieved using steps or atomic rows on the crystal surface is a fascinating route to obtain high-density SWCNTs. Thus far, HA-SWCNT growth using this approach has been attempted on sapphire and quartz substrates. HA-SWCNT growth using steps on the vicinal surface of c-face sapphire was achieved by CVD using C2H4 with Fe catalysts. In addition, HA-SWCNT growth was realized on Y-cut quartz surfaces using steps by CVD using CH4 with Fe catalysts, where the average length of SWCNTs was approximately 100 μm, and the density was 10 μm1. However, the density of HA-SWCNTs obtained by utilizing surface steps was not so high; therefore, the use of atomic rows on the crystal is more effective to obtain high-density HA-SWCNTs. Thus far, HA-SWCNTs along atomic rows have been realized on sapphire and quartz surfaces. HA-SWCNT growth was performed on m- and r-face sapphires by CVD using CH4 and C2H4 with ferritin as catalysts, where the SWCNT density was 40 μm1. Later, it was pointed out that the potential valley along the atomic row on r-face sapphire caused SWCNT alignment. HA-SWCNTs were also realized on the a- and r-faces of sapphire by CVD using CH4 with CoMo catalysts. In addition, HA-SWCNTs were also realized on a quartz surface by several groups. HA-SWCNT growth was performed on ST- and Y-cut quartzes, and the density of SWCNTs was 10–20 μm1. Later, it was pointed out that ST-cut quartz is composed of vicinal R-face, and, because of the atomic row on the R-face, the SWCNTs were horizontally aligned on the surface of the ST-cut quartz.

Support Layer In catalyst-supported CVD growth, metal catalyst particles are generally supported on oxide substrates, oxide layers on substrates, or oxide powders. This is because oxides are relatively inactive with catalyst metals and are not apt to significantly affect the catalyst activity. An oxide layer formed on a substrate is sometimes called a “buffer layer.” Various oxides have been used as the support layer for CNT growth,

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including Al2O3, TiO2, SiO2, MgO, CeO2, and ZrO2. Because of their thermal resistance and inactivity against the feedstock gas, Al2O3, SiO2, and MgO are widely used (note that the oxides used for support layers might not be stoichiometric, that is, they may be expressed as AlxOy, SixOy, MgxOy,; however, here they are expressed as stoichiometric compounds). Several groups compared the effect of different support layers on CNT growth. MWCNT growth was performed on Al, Al2O3, TiN, and TiO2 by CVD using C2H2 with Fe catalysts to compare the support layer effect. The CNT yield was observed to decrease in the order of Al2O3, TiN, and TiO2, and no CNTs were grown from a Fe catalyst on an Al support layer (de los Acros et al. 2004). Amama et al. used Al2O3 and TiN prepared by sputter deposition and MgO and ZrO2 prepared by electron-beam deposition as catalyst support layers for Fe catalysts in CVD (Amama et al. 2012). They obtained VA-MWCNTs on Al2O3 and TiN support layers, whereas CNTs were not grown on the MgO and ZrO ones. They attributed this result to the subsurface diffusion of catalysts on MgO and ZrO, which reduced effective catalysts on them. Among the various oxide support layers, Al2O3 is recognized as one of the most effective support layers to enhance catalyst activity, and most VA-CNTs have been obtained from Fe and Co catalysts on Al2O3 support layers. By utilizing Al2O3 as buffer layers, CNT growth was realized on metal foils such as stainless steel and Inconel. The typical growth temperature of CNT growth by CVD is 700–1000
C, at which catalyst particles are apt to aggregate, that is, Ostwald ripening occurs. For CNT growth, catalysts must maintain particle-like shapes, in particular, SWCNTs are generally grown from catalyst particles with sizes of 1–3 nm. Therefore, an important role of the support layer is to suppress Ostwald ripening of catalyst metals as well as reaction with them. However, subsurface diffusion of catalysts into a support layer occurs at growth temperature, which reduces the amount of effective catalyst particles and thus the CNT yield. Amama et al. investigated VA-SWCNT growth on various Al2O3 support layers prepared by several techniques, sputter deposition, electron-beam deposition, atomic layer epitaxy, and sapphire substrates (Amama et al. 2010). They investigated the roughness and density of the Al2O3 support layers and their effect on SWCNT growth in detail. They showed that an amorphous Al2O3 layer is suitable to suppress Ostwald ripening of Fe catalysts, and as its density increases, the subsurface diffusion of Fe catalysts is suppressed.

Summary The current state of understanding of the growth mechanisms of CNTs was reviewed, and several significant topics related to the growth mechanisms were introduced. In the early stages, various growth models were proposed for CNT growth for arc discharge, laser ablation, and CVD methods. Among them, the VLS mode has been used as a basis to discuss the growth model for CNT growth from catalyst particles, in particular, CNT growth by CVD. However, with the progress in theoretical simulations and in situ experimental analysis techniques, the VSS model has also been shown to be applicable, especially for high-melting-

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point catalysts and low-temperature growth. Furthermore, in situ experimental analysis has shown that catalyst particles can become carbides even for CNT growth in the VSS mode. Despite the steady progress in this field, in situ experimental analysis under conventional growth conditions for CVD has not yet been performed. To establish a solid growth mechanism surveying various growth conditions, further studies including the details of chemical states of catalyst particles such as their crystallinity, carbon solubility, and carbon distribution in the particles are necessary. Acknowledgments This work was partly supported by the Meijo University Research Branding Project for Cultivation and Invention of New Nanomaterials under the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Private University Research Branding Project (Global Development Category). This work was also partly supported by JSPS KAKENHI Grant Number 19H02563.

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Chemistry and Physics of Carbon Nanotube Structures Yoshitaka Fujimoto

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Nitrogen Doping and Nitrogen-Vacancy Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Substitutional Doping with Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Growth Process to Nitrogen-Vacancy Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Energy Band Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Scanning Tunneling Microscopy Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Carbon Nanotube and Graphene-Based Molecular Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Adsorption Properties of Hydrogen Atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Adsorption Properties of Environmentally Polluting and Toxic Molecules . . . . . . . . . . . . . . . . 100 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Abstract

Carbon nanotubes (CNTs) have been investigated widely from fundamental physics and chemistry to applicable systems since they show unique and outstanding properties such as high electrical conductivity and optical transparency. This chapter reviews the first-principles density-functional study that reveals formation, stability, reactivity, and electronic properties of defects and impurities in CNT. This chapter begins with discussion of the stabilities and the electronic properties of various defect configurations in CNT induced by substitutional doping. Then, it also discusses adsorption effects of various molecules including toxic and environmentally polluting molecules on energetics, electronic properties, and transport of CNT and the possibilities for detecting those molecules individually. Furthermore, the curvature effects of nanotubes are revealed by comparing CNT with graphene. Y. Fujimoto (*) Department of Physics, Tokyo Institute of Technology, Tokyo, Japan e-mail: [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_54

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Keywords

Defects · Molecular sensors · Electronic transport · Density functional theory

Introduction Carbon nanotube (CNT) has been one of the most active research subject since its discovery (Iijima 1991) because it offers the novel and excellent properties as well as the potential applications in nano-electronic devices such as field effect transistors, field emission displays, and gas sensors (Kong et al. 2000; Collins et al. 2000). It is known that CNTs show the metallic and semiconducting properties, which depends on how graphene is rolled up into the cylindrical shape, i.e., the spiral conformation of the nanotubes (Hamada et al. 1992). Specifically, the semiconducting nanotube, together with fascinating properties such as high carrier mobility and long mean free path (Dürkop et al. 2004), has been considered to be one of the most promising materials for the semiconducting device applications. To realize the high performance nanotube-based devices, it is of great importance to control the electronic properties of the nanotube such as carrier type and concentration. Substitutionally doping with foreign atoms is generally an efficient method to modify the electronic properties of the semiconductors. The boron (B) and nitrogen (N) atoms are good dopants for carbon-based materials since their elements show similar structural properties (Fujimoto et al. 2014; Fujimoto and Saito 2016a, b, c). Actually, the introduction of the B atom into CNTs and graphene can modify the electronic properties and change the electrical conductivity (Fujimoto and Saito 2016d, 2019). On the other hand, the substitutional doping with N atoms can give rise to a variety of N-defect configurations in CNTs (Czerw et al. 2001; Min et al. 2008; Fujimoto and Saito 2011a, b), and therefore the N-doped CNT shows rich electronic properties such as p-type and n-type doping properties depending on their N-defect configurations (Fujimoto and Saito 2011a, 2014). Furthermore, substitutional doping can also improve the sensitivity to adsorbates. It is shown that the existence of impurity dopants can enhance the adsorption energies of hydrogens on graphene and CNTs (Fujimoto and Saito 2011b, 2014). Thus, the substitutional doping with B and N atoms can tune the electronic properties as well as enhance the chemical reactivity. The high sensitivity to adsorbates and the modification of the electronic properties of doped CNTs would provide novel nanoelectronics devices as well as high performance sensing devices (Cao and Rogers 2009; Schnorr and Swager 2011; Park et al. 2013; Shulaker et al. 2013; De Volder et al. 2013). This chapter provides a review of a first-principles density-functional study to reveal atomic structures, stabilities, and electronic properties of carbon nanotubes induced by dopings with B and N atoms. The chapter mainly consists of two parts. In the first part, we examine various nitrogen defect formation in CNT and discuss the energetics associated with the growth process of nitrogen-vacancy complexes in CNT. We also show the electronic structures of various nitrogen defects in CNT. In the next part, we investigate the adsorption properties of various molecules and

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discuss the possibility to be applied to CNT-based and graphene-based molecular sensors. We first study the adsorption properties of hydrogen atoms on N-doped CNT and discuss the possibility of the hydrogen sensors. Next, we examine the adsorption properties of toxic and environmentally polluting molecules and the electronic transport properties of the B-doped CNT and graphene. Furthermore, we also discuss the possibility to detect those molecules individually.

Methodology First-principles electronic structure calculations are performed in the density-functional theory (DFT) (Hohenberg and Kohn 1964). The norm-conserving TroullierMartins pseudopotentials are used for the description of the interactions between the ions and the valence electrons (Troullier and Martins 1991), and exchangecorrelation effects are treated using the local density approximation (LDA) parameterized by Perdew and Zunger (Kohn and Sham 1965; Cerperley and Alder 1980; Perdew and Zunger 1981). For the calculations of the energetics and electronic structures of carbon nanotubes, the supercell models include two unit cells for a zigzag (10,0) nanotube. The optimized structures of (10,0) carbon nanotubes are depicted in (Fig. 1), where a carbon atom is replaced with a nitrogen atom. In (Fig. 1a), we show the (10,0) CNT with the substitutional nitrogen defect. For (10,0) nanotube with the pyridine-type nitrogen defect, four different structures are considered: monomeric, dimerized, trimerized and tetramerized nitrogen defects, which are shown in (Fig. 1b–e), respectively. In (Fig. 1f and g), undoped (10,0) nanotubes with the monovacancy and the divacancy defects are shown, respectively. To calculate energetics and electronic structures of B(N)-doped graphene, we use a 4
4 supercell along the directions parallel to the graphene sheet (Fujimoto and Saito 2016d, 2019). Wave functions in the Kohn-Sham equations are expanded in terms of the plane-wave basis set with the cutoff energy of 50 Ry (Yamauchi et al. 1996). The supercell lattice constant along the direction perpendicular to graphene is set to be 20 Å. The Brillouin-zone integration (BZ) is performed with 6
6
1 k-points sampling. Upon the geometry optimization, atomic configurations are updated until Hellmann– Feynman forces acting on all atoms are less than 0.05 eV/ Å. The STM images are calculated using the Tersoff–Hamann approximation (Tersoff and Hamann 1985). This method is known to be valid for many systems despite its simplicity (Okada et al. 2001; Fujimoto et al. 2001, 2003). In this approach, the tunneling current I(r) is assumed to be proportional to the local density of states ρ(r, ε) (LDOS) of the surface at the tip position integrated over a range of an energy ε restricted by the applied bias voltage V, i.e., ð EF~þeV I ðr Þ ρðr, ϵ Þdϵ EF

ð1Þ

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Fig. 1 Optimized atomic structures of (10,0) carbon nanotubes with (a) substitutional nitrogen (C79N), (b) monomeric (C78N), (c) dimerized (C77N2), (d) trimerized (C76N3), and (e) tetramerized nitrogen formations in the pyridine-type defect (C74N4), (f) monovacancy (C79), and (g) divacancy (C78). (a)–(d) and (f)-(g) Reproduced with permission from Fujimoto and Saito (2011a). Copyright 2011, the Elsevier. (e) Reproduced with permission from Fujimoto and Saito (2020). Copyright 2020, the Institute of Physics

where EF is the Fermi energy. Images obtained with negative and positive voltages can reflect the occupied and unoccupied electronic states, respectively. To calculate electronic transport properties of various nanotube and graphene systems, a periodic boundary condition is assumed along a zigzag direction of a CNT and graphene. For the calculations of the electrical conductance, the scattering wave functions of CNT and graphene with and without several molecules between two semi-infinite CNTs and graphene sheets are constructed from the overbridging boundary-matching (OBM) method (Fujimoto and Hirose 2003a, b;

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Fujimoto et al. 2005a, b) using the real-space finite-difference approach (Chelikowsky 1994; Tsukamoto et al. 2001; Hirose 2005). We calculate the conductance G(E) associated with the transmission coefficient T(E,kx) by the Landauer– Büttiker formula (Büttiker 1985): 2e2 G ð EÞ ¼ h

ð π=Lx

dkx T ðE, kx Þ 2π=L x π=Lx

ð2Þ

where e and h denote the electron charge and Planck’s constant, respectively.

Nitrogen Doping and Nitrogen-Vacancy Complexes To discuss the stability of the various nitrogen defects in N-doped (10,0) CNTs, we evaluate the total energies of the several kinds of N-doped (10,0) CNTs in two different manners: formation energy and relative energy. The formation energy is obtained using the reference energies of nitrogen atom in gas phase and carbon atom in the pristine CNT. Therefore, the formation energy defined later indicates whether or not the reaction of the nitrogen doping into the CNT will be favored energetically. On the other hand, the relative energy is derived from comparing directly the total energies of the pyridine-type nitrogen configurations with that of the substitutional configuration. In the calculation of the relative energy, we consider the energetical preferences in the presence of the vacancy in the nanotubes. In addition, we also discuss the relative energy corresponding to the growth process of monomeric and dimerized pyridine-type defects into the dimerized and the trimerized ones, respectively.

Substitutional Doping with Nitrogen We here examine the formation energies Ef calculated by E f ¼ Etot  mC μC  mN μN

ð3Þ

where Etot is the total energy of N-doped (10,0) CNT, and μC and μN are the chemical potentials of the C atom in the pristine (10,0) CNT and the N atom in the N2 molecule, respectively. mC and mN are the numbers of the C atoms and the N atoms, respectively. Table 1 lists the formation energies for various N-defect configurations in N-doped (10,0) CNTs. The formation energy of the substitutionally N-doped (10,0) CNT is found to be the lowest, 0.41 eV (C79N). This implies that the substitutional N defect is the most plausible configuration when the N atom is doped into the (10,0) CNTs. For N-doped (10,0) CNTs with a vacancy, the tetramerized pyridine-type defects are found to be the most stable structure (Ef ¼ 1.78 eV), while the formation energy of the trimerized defects is 2.16 eV. The formation

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Table 1 Formation energy Ef (eV) for various N-defect formation in N-doped (10,0) CNT. See (Fig. 1) for N-defect formation. Reproduced with permission from Fujimoto and Saito (2020). Copyright 2020, the Institute of Physics Formation energy (eV) C78N C79N 0.41 3.79

C77N2 2.91

C76N3 2.16

C74N4 1.78

energies of the tetramerized and the trimerized configurations are much lower than those of the monomeric and the dimerized ones, implying the existence of the tetramerized and the trimerized configurations (Fujimoto and Saito 2011a, c). It has been reported that the atomic vacancies are often observed experimentally in CNTs and graphene (Hashimoto et al. 2004; Amorim et al. 2007). The pyridine-type defects are energetically favorable than the substitutional N defects in the presence of the atomic vacancy in the CNTs and the graphene. Furthermore, the formation energies of the N-doped CNTs with the pyridine-type defects decrease as the number of the N atoms around the monovacancy in the N-doped (10,0) CNTs increases (see Fig. 1b–d). Therefore, the trimerized and the tetramerized pyridine-type defects would be plausible configurations if there exist the pyridine-type defects in the (10,0) CNT. We further discuss the difference of the formation energies between the N-doped (10,0) CNTs and N-doped graphene. Interestingly, the formation energy of the substitutionally N-doped (10,0) CNT (C79N) is higher than that of the substitutionally N-doped graphene, where the formation energy of the substitutionally N-doped graphene is reported to be 0.32 eV (Fujimoto and Saito 2011c). For the pyridine-type defects in the N-doped (10,0) CNTs, the formation energy of the N-doped (10,0) CNT with the tetramerized pyridine-type defect (C74N4) is lower than that with the trimerized one (C76N3). On the other hand, for the N-doped graphene, the formation energy of the N-doped graphene with the tetramerized pyridine-type defects is slightly higher than that with the trimerized one (Fujimoto and Saito 2011c). Thus, it could be concluded that the behavior of the formation energies for the N-doped (10,0) CNTs is considerably different from that for the N-doped graphene.

Growth Process to Nitrogen-Vacancy Complexes We now consider the relative energies associated with growth processes from substitutional N defects to pyridine-type defects, which are defined as E1 ¼ EðC76 N 3 Þ þ EðC80 Þ
3  ½EðC79 N Þ
3 þ EðC79 Þ

ð4Þ

E2 ¼ EðC76 N 3 Þ þ EðC80 Þ
3  ½EðC79 N Þ
3 þ EðC78 Þ þ EðC1 Þ

ð5Þ

E3 ¼ EðC76 N 3 Þ þ EðC80 Þ
2  ½EðC79 N Þ
2 þ EðC78 N Þ

ð6Þ

E4 ¼ EðC76 N 3 Þ þ EðC80 Þ  ½EðC79 N Þ þ EðC77 N 2 Þ

ð7Þ

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Table 2 Calculated relative energies of nitrogen-doped (10,0) CNT defined in Eqs. 4, 5, 6, and 7. Reproduced with permission from Fujimoto and Saito (2011a). Copyright 2011, the Elsevier Relative energy (eV) E1 4.86

E2 3.10

E3 2.44

E4 1.15

Here, E(C80) is total energy of the pristine nanotube consisting of two unit cells of the (10,0) CNT, E(C78N ), E(C77N2), and E(C76N3) are total energies of monomeric (Fig. 1b), dimerized (Fig. 1c), and trimerized (Fig. 1d) nitrogen formations in the pyridine-type (10,0) nanotube, respectively, E(C79N ) is total energy of substitutional nitrogen-doped (10,0) CNT (Fig. 1a). E(C79) and E(C78) are total energies of undoped (10,0) CNTs with a monovacancy (Fig. 1f) and a divacancy (Fig. 1g), respectively, and E(C1) is the energy per carbon atom of the pristine (10,0) nanotube. Table 2 shows the results of the relative energies of the nitrogen-doped (10,0) CNT. From the result of the relative energy E1, the pyridine-type configuration (Fig. 1d) is energetically favored by 4.86 eV than the substitution-type configuration when there is a monovacancy defect in the nanotube. Although a monovacancy is often used as simple example in the theoretical works (Ma et al. 2004; Berber and Oshiyama 2006), the transmission electron spectroscopy measurements indicate the existence of the multivacancies rather than the monovacancies (Hashimoto et al. 2004), and the theoretical calculations show that the formation energy of the divacancy in the nanotube is much lower than that of the monovacancy (Amorim et al. 2007). Therefore, we examine the relative energy when there is a divacancy in the nanotube instead of the monovacancy. The relative energy E2 shows that the pyridine-type configuration (Fig. 1d) is also stable in energy by 3.10 eV compared to the substitutional nitrogen formation because of the existence of the divacancy in the nanotube. We next consider the relative energies of the monomeric, the dimerized, and the trimerized nitrogen formations of the pyridine-type structure. The monomeric configuration is less stable than the trimerized one by 2.44 eV. In the case of the dimerized formation, there exist two types as possible configurations of two nitrogens arranged in the pyridine-type structure: symmetric and asymmetric configurations along the tube axis (see Fig. 1c). We calculate the total energies of symmetric and asymmetric formations in the dimerized nitrogen of the pyridinetype structure, and have found that the total energy of the asymmetric formation as illustrated in (Fig. 1c) is lower than the symmetric one. The dimerized nitrogen formation in the pyridine-type (10,0) CNT is determined in this way. We calculate the relative energy E4 of the dimerized nitrogen formation and find that the dimerized nitrogen configuration is also less stable than the trimerized one by 1.15 eV. Thus, the trimerized nitrogen structure in the pyridine-type defect is the most stable among the possible formations of the pyridine-type defect. Similar behaviors are reported in the case of the N-doped graphene (Fujimoto and Saito 2011c).

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Energy Band Structure We here discuss the effect of the pyridine-type defects on the electronic properties of the CNTs. Figure 2 shows the calculated band structures of the (10,0) nanotubes. The pristine (10,0) CNT has semiconducting properties with a band gap of ~0.72 eV (Fig. 2a). Figure 2b and c show the band structures of monomeric and trimerized formations in the pyridine-type (10,0) CNT. By introducing the pyridine-type defect into the pristine (10,0) CNT, the impurity level appears near the valence-band edge of the nanotube and the Fermi level resides within the valence-band region although it is associated with the nitrogen impurity. This is because the number of the electrons in the nanotube with the pyridine-type configuration is deficient compared to the pristine nanotube. The electron deficiency induces acceptor levels and the pyridine-type nanotube becomes p-type semiconductors, as shown in (Fig. 2b and c) (Fujimoto and Saito 2011a). Although the pyridine-type CNTs with the monomeric and the trimerized nitrogen defects have the acceptor states near the valence-band edge, there are differences in the electronic structures. The band gaps of the pyridine-type CNTs with the monomeric and the trimerized nitrogen defects are ~0.73 eV and ~ 0.56 eV, respectively. The widths of the acceptor states of the monomeric and the trimerized nitrogen formations in the pyridine-type defects are 0.27 eV and 0.21 eV, respectively. The dispersion of the acceptor state of the trimerized nitrogen formation is somewhat narrower than that of the monomeric one. In the case of the monomeric formation, there are almost degenerated states at Γ point and these two states are half-filled (see Fig. 2b). On the other hand, only the localized state related to the nitrogen impurity is half-filled in the case of the trimerized formation, as shown in (Fig. 2c). These differences may affect the transport properties of the semiconducting nanotubes.

Fig. 2 Energy band structures of (a) pristine (10,0) nanotube (C40), pyridine-type (10,0) nanotubes with (b) monomeric (C78N ), and (c) trimerized (C76N3) nitrogen defects. The Fermi energy is set to be zero. Reproduced with permission from Fujimoto and Saito (2011a). Copyright 2011, the Elsevier

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Scanning Tunneling Microscopy Images Scanning tunneling microscopy (STM) is a powerful tool to observe electronic structures of surfaces of semiconductors and metals at atomic levels. Figure 3a shows the STM image of the substitutional N defect in N-doped (10,0) CNTs. The STM image around the N atom in the substitutionally N-doped CNT is different from that at the C atoms since the STM image of the substitutional N defects mainly reflects the donor states below the conduction band (Fujimoto and Saito 2015a, b, 2017). This behavior is also seen in the graphene and the hexagonal boron-nitride layers (Fujimoto et al. 2014; Fujimoto and Saito 2015a, b). Therefore, the N atom in the substitutionally N-doped CNT could be identified in the STM images. Figure 3b and c also illustrate the STM images of the trimerized and the tetramerized pyridinetype defective (10,0) CNTs, respectively. It is found that the N atoms in the pyridinetype defects would appear to be individual regions in the STM images, which depends on the number of the N atoms in the pyridine-type defects (see Fig. 3b and c). These STM images are similar to those of the N-doped graphene (Fujimoto and Saito 2011c). Thus, the STM images of the N-doped (10,0) CNTs with the substitutional N, the trimerized pyridine-type, and the tetramerized pyridine-type defects are distinguishable from one another and those three types of N-defect configurations would be observable by the STM measurements (Zhao et al. 2013).

Carbon Nanotube and Graphene-Based Molecular Sensors Adsorption Properties of Hydrogen Atoms Adsorption Energy and Atomic Structure To discuss the stability of hydrogen atom adsorbed on the N-doped (10,0) CNT, we calculate the adsorption energies Ea defined as

Fig. 3 Simulated STM images of N-doped (10,0) CNTs with (a) substitutional nitrogen, (b) trimerized pyridine-type, and (c) tetramerized pyridine-type defects. The STM images are generated at applied bias voltage of 0.5 eV. The yellow balls denote the positions of N atoms in N-doped (10,0) CNTs. Reproduced with permission from Fujimoto and Saito (2020). Copyright 2020, the Institute of Physics

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Ea ¼ Etot  ECNT  1=2EH2

ð8Þ

where Etot and ECNT are total energies of N-doped (10,0) CNT with and without hydrogen atom, respectively, and EH2 is the total energy per atom of the H2 molecule. In Table 3, we list the adsorption energy of hydrogen atom at three sites in N-doped CNT. The adsorption of H atom on the N atom (site A) is energetically unfavorable since the positive adsorption energy corresponds to endothermic adsorption (Ea > 0), whereas each H atom adsorbed on two different C atoms next to the N atom (sites B and C) is expected to be stable due to their exothermic behavior (Ea < 0). The adsorption energy at site C is somewhat lower than that at site B, which should be the curvature effect of N-doped CNT. We thus find that the adsorption of H atom upon N-doped CNT is energetically favored when H atom is adsorbed on the C atom rather than on the N atom. We next study the atomic geometries of N-doped (10,0) CNT without and with H atoms, as shown in (Fig. 4b and c), respectively. The bond angles θ1 and θ2 of N-doped CNT without H atom are 114.2 and 119.2 , respectively, and the N atom resides in the sp2bonding configuration (Fig. 4b). After H atom is adsorbed on N-doped CNT, the C-H bond with the length of 1.11 Å is formed, and the bond angles θ1 and θ2 change into 106.4 and 112.3 , respectively. It can be seen that the C atom bonded with H atom protrudes from the original surface of the nanotube and the C-H pair forms a covalent bond (see Fig. 4c). By the adsorption of H atom, the configuration of the C atom in N-doped CNT transforms from sp2 into sp3-hybridized configurations. We next consider the adsorption of H atom around the substitutional nitrogen defect in graphene (Fujimoto and Saito 2014). As adsorption sites of the H atom, we Table 3 Calculated adsorption energies (Ea) for N-doped (10,0) CNT with hydrogen atom at three different sites. Three sites A, B, and C are shown in (Fig. 4). Reproduced with permission from Fujimoto and Saito (2011b). Copyright 2011, the Institute of Physics Position Adsorption energy (eV)

A 0.96

B 0.28

C 0.29

Fig. 4 Optimized atomic structures of N-doped (10,0) carbon nanotubes: (a) top view with three possible hydrogen adsorption sites labeled as A, B, and C, and (b) its side view, and (c) N-doped (10,0) nanotube with hydrogen atom adsorbed on the C site. θ1 and θ2 denote bond angles around carbon atom labeled as C. Reproduced with permission from Fujimoto and Saito (2011b). Copyright 2011, the Institute of Physics

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Fig. 5 (a) Optimized atomic structure of substitutionally N-doped graphene. The dotted lines denote the supercell. The labels A1-A3 denote the adsorption sites of H atoms. (b) Adsorption energies at three different sites A1-A3 in substitutional N defective graphene. Reproduced with permission from Fujimoto et al. (2014). Copyright 2014, the American Institute of Physics

consider three different sites: the N atom (A1), the C atoms next to the N atom (A2), and the second-nearest neighbor the C atoms (A3) (see Fig. 5a). The adsorption energies at three sites (A1-A3) are summarized in (Fig. 5b). The adsorption energy at A2 site is found to be the lowest among three adsorption sites but it is still energetically unfavorable (Ea ¼ 0.27 eV) as in the case of the pristine graphene. It is noticed that the result in adsorption energy is different from that of (10,0) CNT. In the case of substitutionally N-doped (10,0) CNT, when one H atom is adsorbed outside the CNT, the adsorption energy is 0.29 eV and becomes favorable in energy, whereas the H atom adsorption becomes energetically unfavorable by the adsorption energy of 0.78 eV when adsorbed inside the CNT (Fujimoto and Saito 2011b). It is interesting that the average of these two adsorption energies is almost the same value as the adsorption energy of the N-doped graphene. Thus, the difference in the adsorption energy between the graphene and the CNT should arise from the curvature effect, indicating that the hydrogen favors the sp3-bonding configuration rather than the sp2-bonding one.

Electronic Band Structure We examine the effect of hydrogen-atom adsorption on the electronic structures of N-doped (10,0) CNT. Figure 6a–c show the calculated energy-band structures of pristine (10,0) CNT, N-doped (10,0) CNT without and with H atom, respectively. The pristine (10,0) CNT exhibits semiconducting properties with a band gap of ~0.72 eV, as shown in (Fig. 6a). When carbon atom in pristine (10,0) CNT is substituted by nitrogen atom, the impurity state associated with the N atom appears below conduction-band minimum and would act as a donor state (Fig. 6b). This is because N atom possesses extra one electron compared with C atom. When H atom is adsorbed on the C site in N-doped (10,0) CNT, the originally partially filled impurity state is completely filled with electrons, and the donor state induced by nitrogen doping disappears (Fig. 6c). Thus, the adsorption of H atom causes dramatic changes in the electronic structure of N-doped CNT. Thus, the N-doped CNT

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Fig. 6 Energy bands of (a) pristine (10,0) CNT, (b) N-doped (10,0) CNT, and (c) N-doped (10,0) CNT with hydrogen atom. The H atom is adsorbed at C site of N-doped (10,0) CNT in (Fig. 4a). The pristine (10,0) CNT considered here consists of 40 carbon atoms. The Fermi level is set to be zero. Reproduced with permission from Fujimoto and Saito (2011b). Copyright 2011, the Institute of Physics

is useful for the hydrogen sensor since the electronic properties of the N-doped CNT change from n-type semiconducting to intrinsic one.

Adsorption Properties of Environmentally Polluting and Toxic Molecules Energetics and Structure The adsorption properties of various molecules on B- and N-doped graphenes are discussed here. The adsorption energy is defined as Ea ¼ Etot  Egra  Emol ,

ð9Þ

where Etot and Egra are the total energies of B(N)-doped graphene with and without the adsorption of gas molecules, respectively, Emol is also total energy of an isolated molecule. Table 4 shows the adsorption energies (Ea) and the distances (d) between the molecule and the dopant atom for the adsorption of NO, NO2, CO, CO2, O2, and N2 molecules on B-doped and N-doped graphenes. In the case of the N-doped graphene, it is found that all six types of molecules are not chemically but rather physically adsorbed at the N site with relatively small adsorption energies and long distances between the molecule and the dopant atom (d > 2.6 Å). In the case of the B-doped graphene, CO, CO2, O2, and N2 molecules are also found not to be adsorbed chemically as in the case of the N-doped one, while NO and NO2 molecules are adsorbed chemically with the large adsorption energies (|Ea| > 1.1 eV) as well as the short distances (d < 2 Å). It is reported that B-doped and N-doped bilayer graphenes

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show the similar results (Fujimoto and Saito 2016d, 2020). Therefore, irrespective of the number of layers below the doped surface layer, NO and NO2 molecules would bind chemically on B-doped graphene layers. Figure 7a and b show the optimized atomic structures of NO and NO2 molecules adsorbed on B-doped graphene, respectively. Interestingly, the B atom protrudes from the planar graphene sheet when the NO2 molecule binds with a chemical bond between the O atom in the NO2 molecule and the B atom in the B-doped graphene (Fig. 7b), while it still resides in the planar sheet when the NO molecule binds

Table 4 Adsorption energy Ea (eV) and distance d (Å) between molecule and B(N) atom for various molecules adsorbed on B(N)-doped graphene. Reproduced with permission from Fujimoto and Saito (2019). Copyright 2019 the Institute of Physics Boron Nitrogen

Ea D Ea D

NO 1.23 2.15 0.35 2.62

NO2 1.16 1.59 0.74 2.66

CO 0.12 2.89 0.14 2.94

CO2 0.03 2.84 0.11 2.73

O2 0.20 1.83 0.32 2.69

N2 0.27 2.93 0.30 2.87

Fig. 7 Side views of optimized atomic structures of (a) NO and (b) NO2 molecules adsorbed on B-doped graphene. Reproduced with permission from Fujimoto and Saito (2019). Copyright 2019, the Institute of Physics. (c) Side view of optimized atomic structure of CO molecule adsorbed on B-doped (8,0) CNT. Reproduced with permission from Fujimoto and Saito (2020). Copyright 2020, the Elsevier

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chemically between N atom in the NO molecule and the B atom in the graphene layer (Fig. 7a). As discussed above, the adsorption energy of NO2 molecule on B-doped graphene is found to be somewhat smaller than that of NO molecule. The difference between the adsorption energies of NO and NO2 molecules would be mainly attributed to the electronegativity. For the NO2-molecule adsorption case, the electronegativity of the O atom is much larger than that of the B atom, and thereby the B atom in the graphene approaches to the O atom in the NO2 molecule. As a result, the B atom moves away from the planar graphene layer by about 0.64 Å. The total energy of this deformed B-doped graphene without the NO2 molecule is found to be higher by ~0.70 eV, compared with that of the B-doped graphene before the adsorption of the NO2 molecule. In addition, the N-O bond near the B atom in the NO2 molecule is stretched by ~0.12 Å after the adsorption although the remaining N-O bond length of 1.19 Å is almost unchanged compared with that of the isolated NO2 molecule. Thus, the interactions between the B atom and the NO2 molecule give rise to sizable energy gains. On the other hand, structural deformations of the B-doped graphene and the NO2 molecule cause energy costs. As a result, the total energy gain upon the adsorption becomes 1.16 eV (|Ea| in Table 4). For the NO-molecule adsorption case, the N atom in the NO molecule just approaches the B atom without noticeable structural modification either in the B-doped graphene layer or in the NO molecule. The B-N distance in this case is much longer than that in the NO2 case and the energy gain due to the adsorption becomes smaller. In the case of the adsorption of the NO molecule, however, there exists no energy cost due to the structural deformation, and the total energy gain (|Ea | ¼ 1.23 eV) becomes larger than that of the NO2 case. Similar discussion has been reported in the case of the B-doped bilayer graphene (Fujimoto and Saito 2016d). In the above discussion, the B-doped graphene can bind strongly with NO and NO2 molecules, while it does not bind strongly with a toxic CO molecule. We here study the adsorption properties of a toxic CO molecule on the B-doped (8,0) CNT (Fujimoto and Saito 2020). We also examine the adsorption energy of a CO molecule on the B-doped (8,0) CNT. Figure 7c illustrates the optimized atomic structure of the CO molecule adsorbed on the B-doped (8,0) CNT. It is found that the adsorption energy of the CO molecule is 0.62 eV and the distance between the CO molecule and the B-doped CNT is 1.53 Å. Thus, the B-doped (8,0) CNT can bind strongly with a CO molecule due to the curvature effects of the nanotube.

Electron Transport The adsorptions of the NO and NO2 molecules would modify the conductivity of the graphene, and it is of great importance to detect the variation of the conductivity for sensor applications. Here we discuss how the adsorption of molecules affects the electronic transport properties of graphene. Figure 8 shows the conductances of pristine graphene and B-doped graphenes with and without NO and NO2 molecules calculated by using Eq. 2. The conductance of the pristine graphene exhibits a linear dispersion, which agrees well with experimentally observed results (Novoselov et al. 2004). When a B atom is doped into graphene, electrons are scattered by the B-atom

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Fig. 8 Conductances of pristine graphene and B-doped graphenes with and without NO and NO2 molecules as a function of energy E. The Fermi energy is set to zero. Reproduced with permission from Fujimoto and Saito (2019). Copyright 2019, the Institute of Physics

impurity, which reduces the conductance by ~30%, compared with that of the pristine graphene at the energy E ¼ +0.5 eV. This reduction rate seems to be mostly independent of the energy studied, which is important for sensor applications. When a NO2 molecule is adsorbed on the B-atom impurity in graphene, the conductance of the graphene diminishes by ~40%. Furthermore, the adsorption of a NO molecule on the B-atom impurity reduces the conductance considerably (about 50%). In the case of the energy E ¼ 0.5 eV, the conductance of the B-doped graphene without an adsorbate is less by ~30% than that of the pristine graphene, which shows almost the same reduction rate as that at E ¼ 0.5 eV. The adsorption of a NO2 molecule diminishes the conductance of the graphene by over 50%. For the adsorption of a NO molecule, the conductance decreases by ~35%. The adsorption of molecules thus could dramatically reduce the electrical conductance of graphene, depending on the type of adsorbate. In addition, the variation of the conductance could depend on the impurity concentration and gate voltage as for field-effect transistors. Accordingly, the variation of the electrical conductance induced by the adsorption of NO and NO2 molecules could be utilized for sensor applications such as field-effect transistors. Furthermore, a large variation upon the adsorption of molecules in the conductance under low bias voltage (less than 0.5 eV) would be achieved. Therefore, the low power consuming sensor devices could be realized by using graphene-based field-effect transistors. Furthermore, the electronic transport of the pristine and the B-doped (8,0) CNTs with and without the CO molecule are studied as shown in (Fig. 9). The conductance of the pristine (8,0) CNT shows the step-like structures with a band gap of ~0.61 eV, which shows quite different behaviors from that of the graphene. The conductances of the B-doped (8,0) CNT diminish sizably in the energy ranges of E < 0.3 eV,

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Fig. 9 Conductances of pristine and B-doped (8,0) CNTs with and without the CO molecule. The Fermi energy is set to zero. Reproduced with permission from Fujimoto and Saito (2020). Copyright 2020, the Elsevier

while those are almost unchanged in the ranges of +0.3 < E < +0.7 eV. The adsorption of the CO molecule on the B-doped (8,0) CNT modifies largely the conductance spectrum. The conductance of the B-doped CNT with the CO molecule changes about 30%, compared with that of the B-doped one without any adsorbates near E ¼ 0.5 eV. Thus, B-doped (8,0) CNT is a good candidate for sensing device materials to detect toxic CO molecules.

Summary The chapter has reviewed the substitutionally doping and molecular adsorption effects on the formation, the stability, and the electronic properties of the various CNT systems on the basis of the first-principles density-functional study. In the former part, the plausible N-defect formations in CNT have been shown for the substitutional dopings of N atoms. The formation energy calculations suggest that the substitutional N defect is energetically the most possible structure among various N defects in CNT. The present relative energy calculations suggest that the pyridinetype defects are energetically favored rather than the substitutional N defect in the presence of the atomic vacancy in CNT. Furthermore, the trimerized formations as well as the tetramerized formations of the pyridine-type defects in CNT are expected to be plausible atomic structures among various nitrogen-vacancy complexes because of their small difference in the formation energy. The STM images of those N defects in CNT are demonstrated, and their N defects could be identified by STM experiments at atomic level. In the latter part, the adsorption effects on the atomic structures, the energetics, and the electronic properties of B(N)-doped CNTs and graphenes have been shown for various molecules including environmentally polluting and toxic molecules.

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The results of the adsorption energies and the electronic structures indicate that N-doped CNT is useful for detection of H atoms. Furthermore, the B-doped CNT and the B-doped graphene can bind strongly with toxic CO molecule and environmentally polluting NO (NO2) molecules, respectively. The adsorptions of these molecules change sizably the electronic transport properties of the B-doped CNT and the B-doped graphene. Therefore, the B-doped CNT and B-doped graphene could behave as sensing device materials for detecting toxic CO molecule and environmentally polluting NO and NO2 molecules. Acknowledgments This work was partly supported by MEXT Elements Strategy Initiative to Form Core Research Center through Tokodai Institute for Element Strategy (Grant Number JPMXP0112101001), JSPS KAKENHI Grant Numbers JP17K05053 and JP21K04876. Computations were partly done at Institute for Solid State Physics, the University of Tokyo.

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Innovative Approaches in Characterization of Carbon Nanotube Olusola Olaitan Ayeleru, Helen Uchenna Modekwe, Nyam Tarhemba Tobias, Matthew Adah Onu, Messai Adenew Mamo, Kapil Moothi, Michael Olawale Daramola, and Peter Apata Olubambi

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNTs Synthesis Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electric-Arc Discharge Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Laser Ablation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Vapor Deposition (CVD) Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structures of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Novel Characterization Techniques for Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electron Microscopic Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transmission Electron Microscopy (TEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scanning Electron Microscopy (SEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scanning Probe Microscopies (SPMs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . X-Ray Diffraction (XRD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Neutron Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spectroscopic Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy-Dispersive X-Ray Spectroscopy (EDX) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XPS (X-Ray Photoelectron Spectroscopy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoluminescence (PL) Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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O. Olaitan Ayeleru (*) · N. T. Tobias · M. A. Onu · P. A. Olubambi Centre for Nanoengineering and Tribocorrosion (CNT), University of Johannesburg, Johannesburg, South Africa e-mail: [email protected] H. U. Modekwe · K. Moothi Department of Chemical Engineering, Faculty of Engineering and the Built Environment, University of Johannesburg, Johannesburg, South Africa M. A. Mamo Research Centre for Synthesis and Catalysis, Department of Chemical Science, Doornfontein Campus, Faculty of Science, University of Johannesburg, Johannesburg, South Africa M. O. Daramola Department of Chemical Engineering, Faculty of Engineering, Built Environment and Information Technology, University of Pretoria, Pretoria, South Africa © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_55

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Ultraviolent-Visible (UV-Vis) and Near-Infrared (NIR) Spectroscopies . . . . . . . . . . . . . . . . . . . Atomic Emission and Absorption Spectroscopy (AEAS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Properties of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermogravimetric Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Differential Scanning Calorimetry (DSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fourier Transform Infrared (FTIR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Carbon nanotubes (CNTs) are promising modern nanostructured materials with extraordinary mechanical, chemical, and good thermal stability properties as well as high aspect ratio. These features make them useful materials in various applications. However, pristine CNTs tend to form aggregates and bundles making them difficult to use. Superficial modification/treatment of CNTs with organic or inorganic materials results in improvement of nanotubes’ useful properties and applicability, giving them the required magnetic, catalytic, electronic properties, etc. Therefore, characterization of CNTs is carried out to check and ascertain its features such as type, surface structure, and morphology (diameter, length), dispersion, configuration, size, etc. In this chapter, current innovative approaches widely utilized in CNTs characterization are classified and discussed with highlights on their principle as well as most of their merits and demerits. Keywords

Carbon nanotubes · Characterization techniques · Electron microscopic techniques · Thermal properties · Diffraction methods

Introduction Carbon nanotubes (CNTs) are basic allotropic form of carbon with sp2 hybridized configurations belonging to the fullerene family (Raju et al. 2019; Zhu et al. 2017) Since their discovery by Iijima in 1991 (Herrero-Latorre et al. 2015), these novel materials have been studied to possess various tremendous properties, for instance, excellent mechanical, thermal stability, chemical, and electrical properties; these features made them useful materials for quite a number of application potentials in different fields (He et al. 2013; Alturaif et al. 2014; Mehra et al. 2015). CNTs have high length-to-diameter (aspect ratio) ratio in a range of 1,000,000 and with highly available surface area with minimal resistivity in addition to appreciable stability (He et al. 2013; Liao and Tan 2011; Alturaif et al. 2014; Rahman et al. 2019). CNTs have the potential of improving the electrical, mechanical, and thermal properties of materials (Chua et al. 2013). CNTs have been described as the strongest and stiffest materials due to their tensile strength (up to 100 GPa) and Yound’s modulus

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(over 1 TPa). A recent study has shown that CNTs have tensile strength of about 16 times higher compared to stainless steel and thermal conductivity of 5 times more than copper (Alturaif et al. 2014; Rahman et al. 2019). The sp2 bonding structure of CNTs gives it its unique strength hence their application in advanced structural applications. CNTs have low densities; in addition to their remarkable mechanical strengths they are suitable for use as filler and reinforcement additives in polymer composite materials. Besides, they can be used in the development of strong composites to achieve weight reduction (Karimi et al. 2017; Arunkumar et al. 2020). The length of CNTs vary from one billionth meters to thousands of micrometers giving its tremendously high aspect ratio (Caetano et al. 2017; Xu et al. 2019; Wang et al. 2020). Furthermore, numerous chemical storage facilities have been developed using CNTs for hydrogen storage due to their absorbing capacity for different gases under moderate conditions (Arunkumar et al. 2020). These properties vary for different kinds of nanotubes as described by their diameter, length, twist, nature, and number of walls. CNTs could be single-walled carbon nanotube (SWCNT) or multi-walled carbon nanotubes (MWCNT) depending on their exceptional surface area, stiffness, strength, and resilience, which contribute to their applicability in different fields. Moreover, SWCNTs are classified into metallic, or semiconductors depending on their tube length, diameter, and the hexagonal arrangement of their rings (Karimi et al. 2017). Some other notable fields of CNTs’ application include pharmaceuticals, electronics and electrical, biomedical, biosensor, solar cells and supercapacitors, medical and cancer therapy, catalysis, wastewater treatment and gas separation, and in the development light-weight composite materials (Herrero-Latorre et al. 2015; Liao and Tan 2011; Alturaif et al. 2014; Rahman et al. 2019; Arunkumar et al. 2020; Ma et al. 2017). However, the applicability of CNTs in these fields requires that CNTs is dispersed in organic or inorganic medium by attaching different functional groups given the inert nature of pristine CNTs (Herrero-Latorre et al. 2015). Furthermore, since CNTs are produced together with impurities such as amorphous carbon and other non-crystalline carbon, residual metal catalysts, etc. (Belin and Epron 2005), it is important that concentration, quality and purity, and other features of CNTs are evaluated. The following section outlines some of the innovative characterization techniques of CNTs; however, no single technique could give complete information about all the features of CNTs.

CNTs Synthesis Methods Several techniques have been adopted to grow CNTs, namely: arc discharge, catalytic chemical vapor deposition, laser ablation, flame synthesis, saline solution, and spray pyrolysis (Veisi et al. 2019; Kennedy et al. 2017; Rahman et al. 2019). Among these methods, widely adopted methods are arc discharge, laser ablation, and chemical vapor deposition (CVD). Different types of CNTs in addition to other nanostructured materials and other impurities could be produced using any of these methods (Herrero-Latorre et al. 2015).

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Electric-Arc Discharge Technique This technique is one of the oldest methods used in the production of CNTs. In this method, CNTs are produced when electric-arc (plasma) vaporizes graphite electrodes (two carbon electrodes) at extremely high temperature produced from a direct current (DC) between 2000 and 3000
C in an inert atmosphere maintained at about 50–600 Torr (Iijima 1991). Short structured CNTs are mostly produced with diameter of 0.6–1.4 nm and 10 nm for SWCNTs and MWCNTs, respectively. Impurity content in arc-produced CNTs are usually high due to the formation of high carbon soot while CNTs yield up to 30% could be achieved using this method. However, irregularity in length, shapes, and wall structure of tubes were also reported (Veisi et al. 2019; Kennedy et al. 2017; Rahman et al. 2019).

Laser Ablation In this method, graphite rod is gasified by laser irradiation and vaporized carbon is further deposited onto a substrate in an inert environment. The optimal yield of CNTs especially for SWCNTs depend on the temperature (up to 1200
C) and substrate (transition metal: Fe, Ni, and Co) used while defective and low-quality tubes are produced at low temperature below 1200
C. The yield of nanotubes produced using this technique are very small (Veisi et al. 2019; Kennedy et al. 2017; Rahman et al. 2019).

Chemical Vapor Deposition (CVD) Technique The CVD technique involves depositing hydrocarbon molecules in their gas phase over the surface of heated transition metal catalyst. The CVD process is based on vapor-liquid-solid mechanism. Both MWCNTS and SWCNTs with better alignment, diameter, and length can be produced using this method. CVD technique is relatively simple, less energy intensive (600–1000
C) and can be industrially scaled up (Veisi et al. 2019; Kennedy et al. 2017; Rahman et al. 2019). The type, yield, and quality of produced nanotubes depend on process variables such as reaction temperature, quality, and quantity of metal catalyst/substrate, reaction time, gas flow rate, etc. Controlling and maneuvering any of these parameters could affect the nanotube yield, quality, and purity (due to posed difficulty in purification) and formation of totally different type and structure of nanostructured material (Herrero-Latorre et al. 2015).

Structures of CNTs The different forms of CNTs that have been successfully synthesized include waved, straight, coiled, branched, and commonly bent structures (Rahman et al. 2019; Omrani et al. 2019). Armchair, zigzag, and chiral are the unique geometries of

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SWCNTs as stated in some studies (Srivastava et al. 2019; Rahman et al. 2019). Figure 1 shows the various geometries of CNTs. The sheets of CNTs may be singlewalled (SWCNTs), double-walled (DWCNTs), or multi-walled (MWCNTs). SWCNTs in most cases possess the diameter ranging from 0.4 to >3 nm and the length can be many times greater than its diameter, while in MWCNTs, the space between the sheets has been estimated to be the same with the interlayer space of graphene sheets in graphite and the diameter is in the range of 1.4 to at least 100 nm (Wang et al. 2017; Rahman et al. 2019). The structures of SWCNTs and MWCNTs are presented in Fig. 2. The wrapping of layer of graphene into a tubular shape gives the formation of the SWCNTs and the several concentric tubes of graphene make up MWCNTs as shown in Fig. 2 (Karimi et al. 2017; Rahman et al. 2019). MWCNTs are also produced from

Fig. 1 Geometries of carbon nanotubes (This work is shared under a Creative Commons Attribution-Non-Commercial-No Derivative Works License and copyright has been granted by the authors) (Source: Rahman et al. (2019), no permission is required)

Fig. 2 (a) Structure of SWCNT; (b) MWCNT (This work is shared under a Creative Commons Attribution-Non-Commercial-No Derivative Works License and copyright has been granted by the authors) (Source: Rahman et al. (2019), no permission is required)

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substrate of porous silicon that is developed at a 90
angle to substrate giving them their characteristic formation (Rahman et al. 2019).

Novel Characterization Techniques for Carbon Nanotubes In recent times, the need to replace the conventional materials has given room for the invention of novel cutting-edge materials such as CNTs and these materials are being characterized using several innovative characterizations methods. These methods are discussed in detail in the following subsection.

Electron Microscopic Techniques The extremely small size of CNTs requires that highly and extremely sensitive approaches are employed to evaluate their characteristic features, qualities, and structure. Electron microscopy has emerged as one of the most exciting groundbreaking development in the imaging technology permitting nanoscale and submicron sized material characterization up to their atomic level (Williams and Carter 2009). For about three decades, the power of electron microscopes has enabled nanoscale characterization of the exceptional properties of CNTs. Various considerable signals are emitted when primary incident beam (high-energy) of electrons interacts with thin film sample as shown in Fig. 3. This forms the basis for most electron microscopic characterization approaches. Signals direction represented in Fig. 3 for each generated signal relatively shows where such signal is detected and not essentially its physical direction (Williams and Carter 2009). Electron microscopic methods employed in characterizing CNTs are classified as scanning tunneling microscopy (STM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Transmission Electron Microscopy (TEM) TEM is a dynamic and sophisticated method, which can generate and detect signals. It can also evaluate internal microstructure and measure and manipulate nanoscale materials in 1D, 2D, or 3D dimensions. TEM is also a very useful technique applied in analyzing size, crystallographic structure, morphology, and chemical composition of carbon nanomaterials such as CNTs. TEM can produce spatial resolution up to broad nanometer size range between 1 and 100 nm. TEM could readily produce 2D images, thereby allowing easy evaluation of the state of CNTs aggregation (Belin and Epron 2005). In conventional TEM, high energy electrons from 100 to 300 keV are used to acquire high spatial resolution image of the CNTs. Higher resolution TEM is employed to evaluate number of layers, distance between layers, and CNTs diameter (outer and inner). Higher resolution images of nanoscale materials are captured at a shorter wavelength of radiation. Therefore, by accelerating the electron

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Fig. 3 Representation of different signals emitted when primary beam of electrons interacts with sample (Chirayil et al. 2017)

voltage, shorter wavelengths of electrons are produced. Apart from high-resolution TEM (HRTEM), other forms of TEM are scanning transmission electron microscopes (STEM), analytical electron microscopes (AEM), HVEMs (high-voltage electron microscopes), and intermediate voltage electron microscopes (IVEMs) (Williams and Carter 2009). A typical TEM image of purified MWCNT is shown in Fig. 4. Furthermore, spectroscopic tools such as energy-dispersive spectroscopy (EDS) and electron energy-loss spectroscopy (EELS) could be incorporated into TEM for elemental analysis and examination of chemical bond, respectively. TEM could also be coupled with electron diffraction to determine the crystallographic structures of CNTs and helicity of SWCNTs within a bundle using scattered area electron diffraction (SAED) (Colomer and Van Tendeloo 2003). However, there are some clear drawbacks to using conventional TEM in characterizing CNTs given that TEM specimen needs to be sufficiently electron transparent (or, adequately thin) such that transmitted electrons have enough intensity to produce interpretable image at a given time, since the high spatial resolution of TEM depend on the acceleration voltage of the primary electrons. Again, the dependability of TEM analysis on minute portion of the bulk sample transferred on the specimen carrier (TEM grid) could be influenced by the TEM-specimen preparation step. The intense high voltage beam of electrons from the source can damage the sample and at the same time possible

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Fig. 4 TEM micrograph of MWCNTs

radiation-leaks could generate radiations levels that can kill human tissues (Belin and Epron 2005; Williams and Carter 2009).

Scanning Electron Microscopy (SEM) SEM is a versatile, nondestructive process widely employed to analyze surface topography and morphology of CNTs. SEM also offers direct application in the investigation of nanotube alignment degree, size distribution/density, and tube length (Inkson 2016). SEM is based on the principle that 1–40 keV high energy primary electron beam produced and released from the electron gun interacts with near-surface area of sample to some specified depth, resulting in signals being emitted. Emitted signals (usually the SE, BSE, or X-rays) are received by detectors (electron detectors) and further manipulated by computer to obtain the desired image (topography and morphology) of the sample. In SEM, resolution is usually in the range of 1–20 nm (Akhtar et al. 2018; Cao et al. 2001; Inkson 2016). Figure 5 gives a typical diagrammatic representation of SEM and its components.

Scanning Probe Microscopies (SPMs) This technique offers high-resolution dimensional surface images in the range of 0.01–0.1 nm. SPMs are commonly classified under scanning tunneling microscopy (STM) and atomic force microscopy (AFM). AFM is a high-resolution (lower than 1 nm) method used in CNTs characterization; 3D structural images of CNTs are obtained using AFM due to the interaction between CNTs surface and cantilever tip (Herrero-Latorre et al. 2015). In STMs, because electrons pass through vacuum created between sharp metal wire tip and CNTs surface, very small-scale images of CNTs surface are obtained

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Fig. 5 Schematic representation of scanning electron microscope. SE and BSE are Secondary electron and Backscattered electron detectors, respectively

once an electrical voltage is applied to the sample or on the tip of the metal wire very close to the sample surface. Images of CNTs at atomic level are obtained using STM. Again, information on the electronic density of states (DOS) and local electronic density of states (LDOS) are obtained from STMs. Hence, STMs could be employed in differentiating between metallic and semiconducting CNTs as well as differentiating between the chirality and electronic properties of CNTs. Major drawbacks to the application of AFM and STM techniques lie on the reliability of image on the curvature and control of the probe tip. Moreover, its sample preparation is more complex and relatively difficult compared to other microscopic techniques (HerreroLatorre et al. 2015; Kumar et al. 2019). Figure 6 illustrates the block diagram and all components of a typical atomic force microscope.

Diffraction X-Ray Diffraction (XRD) XRD is a nondestructive characterization method employed in evaluating the crystal formation, impurities, orientation, interlayer spacing, and structural strain of nanotubes (Allaf et al. 2011). Structural characteristics of CNTs as per spatial links between atoms within a single graphene sheet could be obtained from XRD analysis. Information

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Fig. 6 Schematic diagram of atomic force microscope (Source: https://commons. wikimedia.org/wiki/File: Atomic_force_microscope_ block_diagram.png#filelinks, no permission is required)

relating to the nature of interlayer arrangement, the number of graphene layers, tube length, outer and inner diameter, and chirality of the nanotubes could be also evaluated by XRD (Koloczek et al. 2005). In XRD, diffraction patterns emanate from interference of scattered beam of electrons from the carbon atoms in the CNTs. X-rays are produced in cathode-ray tube; only characteristics radiation with the highest intensity are used while other residual radiations are removed using appropriate monochromators or filters. When the incident monochromatic radiation hits onto sample atomic planes, diffracted, transmitted, refracted, and adsorbed beams are created. Diffracted X-rays with the atomic planes within the CNTs crystal lattices at specific wavelength of the incident beam are processed and counted, resulting in diffraction peaks. Diffracted peaks can be identified and indexed by comparing with reference databases such as JCPDS (Joint committee on powder diffraction standards) or computing an ideal pattern and comparing with experimental ones. The (002) peak intensity is used to evaluate the nanotubes diameter using Debye–Scherrer equation. The intensity of (002) peak decreases substantially with higher nanotube alignment (Cao et al. 2001); therefore, alignment degrees of CNTs could be evaluated by the peak intensity of the (002) peak. Furthermore, Peak reflections are created from honeycomb structure of graphene layers, which are generated due to the curvature and stacking layers of CNTs. Another peak, also referred to miller indices, is produced due to diminishing atomic planes where 3D structure of nanotube layers is to be established (Cao et al. 2001; Das et al. 2015). Major limitations to XRD technique arise from the complexity of using statistical tools in interpreting the various diffraction patterns. Detailed interpretation of pattern seems ambiguous because of the incapability of this technique to give 3D crystallinity of CNTs structure. Again, because of the inherent nature of CNTs, XRD pattern of CNTs is analogous to that of graphite and are difficult to separate (Belin and Epron 2005; Das et al. 2015; Dore et al. 2000; Epp 2016). Hence,

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this technique is not effective in differentiating between the structures of graphite and CNTs. Moreover, in powder analysis of CNTs, diffraction pattern originating from interferences of scattered beam of electrons by the carbon atoms in the CNTs could be related to the intensity scattered by N carbon atoms and expressed by the following relation (Eq. 1) according to (Koloczek et al. 2005): XN XN IðK Þ ¼ f ðK Þ i¼1 j¼1 ðiq:rjÞ

ð1Þ

where, K denotes the scattering vector; f(K) the X-ray atomic scattering factor; rj is the inter-atomic vector defined as the difference between the position of ith and jth atoms, q is the diffraction vector, K is the magnitude of the scattering vector and related to the scattering angle 2θ as given in Eq. 2: K ¼ jK j ¼

4π sin θ λ

ð2Þ

where λ is the wavelength. This above relation is necessary in studying different diffraction amplitude of SWCNTs. MWCNTs diffraction pattern could be obtained by integrating the successive diffraction amplitudes obtained for SWCNTs (Belin and Epron 2005; Burian et al. 2005; Koloczek et al. 2005).

Neutron Diffraction Neutron diffraction technique is widely used to determine carbon–carbon bond length and distortion or disorder degree of the hexagonal carbon framework of the CNTs. Large-scale structural features of CNTs are investigated resulting in large arrays of scattering vector (K). Atomic arrangements within graphene layer as well as the stacking arrangement of graphene sheets for MWCNTs are obtained (Belin and Epron 2005). In neutron diffraction, diffraction patterns are formed when there is coherent interference of wave scattering from all atoms in the sample (CNTs) with the intermittent lattice resulting in several Bragg peaks corresponding to the characteristics components of the spatial distribution. For CNTs, the periodicity in the atomic arrangement is influenced by the curvature of the graphene sheets. At large scattering vector values (K-values), relatively large amount of sample is required in order to obtain accurate statistical information on the structural features of CNTs. This forms a huge limitation to the use of the technique (Belin and Epron 2005; Dore et al. 2000). However, neutron diffraction technique could be used to distinguish between the armchair, zigzag, and chiral of small diameter carbon nanotubes.

Spectroscopic Techniques Spectroscopy involves the measurement and interpretation of distinct spectra produced from the interaction between the absorption and emission of light or other radiations with respect to the wavelength of radiation.

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Energy-Dispersive X-Ray Spectroscopy (EDX) This method is applicable in evaluating elemental compositions of synthesized CNTs, identifying phases or elements present in functionalized or modified CNTs with other elements, for example, in composites, CNTs encapsulated with metals, etc. (Inkson 2016). EDX is based on detectors converting individually generated X-ray to electronhole pairs resulting in an electron current. Each emitted electron signal is commensurate to the energy of the inbound X-ray. Chemical composition (elemental composition) of nanotubes is obtained using this tool. However, low atomic number elements are difficult to detect using EDX. Besides, elements like Fe, Mn, Ti, Mo, etc., display overlapping peaks, which can result in erroneous data (Kaliva and Vamvakaki 2020). In EDX, characteristic X-ray generated during bombardment with electron within atoms in the sample is collected by noting the number of X-rays reaching the detector with dissimilar energies when the beam of electrons is focused on the specimen. The detector can quantify each emitted electron signal at any given location on the specimen. These characteristic X-ray peaks are collected, allocated to the specific elements in the sample, and their respective energies compared with standard elements. In principle, the repulsiveness of the atomic nuclei is proportional to their positive charge, which is established by the number of protons (element Z-number); the backscattered electrons (BSE) signal is also comparative to the average proton number in the specimen (Inkson 2016). Hence, X-ray spot spectra (mappings) could be obtained at different spots or locations in a sample allowing for elemental distribution across a sample. EDX are commonly coupled to TEM or SEM.

XPS (X-Ray Photoelectron Spectroscopy) XPS has tremendous surface-sensitive spectroscopic tool commonly employed to provide information on the chemical structure of CNTs. Detailed information on the sidewall structural functionalization of CNTs or defect because of chemical interface or bonding with organic compounds or adsorbed gases such as nitrogen, boron, etc., are also obtained using XPS (Belin and Epron 2005; Davies et al. 2021). Basically, X-ray source interacts with CNTs surface; electrons with fixed binding energy are emitted from the surface of X-ray-irradiated CNTs. The kinetic energy and the quantity of escaped electrons from the CNTs surface are measured using an electron energy analyzer. The energy that binds the electrons can be estimated as shown in Eq. (3) according to Kaliva and Vamvakaki (2020): Ebinding ¼ Ephoton  ðEkinetic þ ΦÞ

ð3Þ

where, Ebinding is the binding energy of the emitted electrons; Ephoton is the X-ray photon electron energy; Ekinetic is the kinetic energy of the emitted electrons estimated by the electron energy analyzer and Φ is the work done by the electron energy analyzer.

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The oxidation state of the elements and their elemental composition can be obtained from the binding energy spectrum since the chemical state of the atoms in the sample influences the binding energy resulting in a chemical shift in the location of the XPS peaks in the spectrum. For example, the photoemission measurements of the C 1 s peaks of undoped CNTs (both SWCNTs and MWCNTs) are reported to be between 284.1 and 284.7 eV (Holbrook et al. 2015; Susi et al. 2015).

Photoluminescence (PL) Spectroscopy Photoluminescence (PL) spectroscopy is an important tool utilized in the quantitative estimation of the (n,m) division of semiconducting SWCNTs. In PL, electron in a semiconducting SWCNTs absorbs photoexcitation and electrons in the Van Hove singularities (that is, the energy levels with prominently high density of state (DOS) arising from rolling up of the 2D graphene sheet into 1D CNTs) of the valence band; subsequently, they are raised to the next energy level in the conductance band generating an electron-hole pair (exciton). Both electron and hole pair relax from c2 to c1 and from v2 to v1 correspondingly. Photoluminescence is then produced following the coalescing of electron-hole pairs at the bandgap. These created firstand second-order optical transitions expressed as S11 and S22 for the semiconducting SWCNTs as shown in Fig. 7. No photoluminescence is produced in metallic SWCNTs (Belin and Epron 2005). However, the limitations to the use of this technique is the inability of PL to measure metallic SWCNTs and also on the effect of varying yield between (n,m) species in zigzag-type nanotubes resulting in difficulty of PL-based quantitative measurement of their specific (n,m) species.

Fig. 7 Schematic illustration of the electronic density of states of a semiconducting (left) and (right) metallic SWCNTs with their valence and conductance Van Hove singularities

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Ultraviolent-Visible (UV-Vis) and Near-Infrared (NIR) Spectroscopies UV-Vis and NIR spectroscopies are based on absorptive and reflectance spectroscopy in the ultraviolent-visible-near-infrared region. Information regarding the physical and electronic properties of SWCNTs is obtained using UV-Vis-NIR. The wavelength of light required for electronic transitions within the ultraviolet region and the visible light region of the electromagnetic spectrum are 200–390 nm and 390–780 nm, respectively. The complexity of SWCNTs suspension, for example, requires the determination of both the concentration of the dispersed and individually dispersed SWCNT in aqueous solution. These concentrations are calculated using the absorbance and the resonance ratios from the UV-Vis-NIR absorption spectrum. The electronic band formation of CNTs is estimated using NIR spectroscopy. This technique could be carried out on thin films or aqueous samples. However, one limitation to its application is the difficulty in the sample preparation because of the possibility of nanotubes aggregation and formation of more bundles (Herrero-Latorre et al. 2015). UV-Vis absorption is a powerful nondestructive characterization tool employed in measuring CNTs dispersion, functionalization, and its electronic purity in aqueous suspension. Optical characteristics of SWCNTs are evaluated from solubilized samples. The UV-Vis technique detects individual (n,m) species by correlating the intensity of absorption at a specific wavelength to the concentration of CNTs dispersed in the solution using the Beer–Lambert law. Beer–Lambert law is expressed as in Eq. 4. A ¼ log ðI0 =IÞ ¼ ε CL

ð4Þ

where, A denotes the absorbance, I0 is the intensity of incident light, I is the intensity of light leaving the sample cell, C is the solute concentration, L is the length of sample cell, while ε is the molar absorption coefficient or absorptivity (Chirayil et al. 2017; Kaliva and Vamvakaki 2020). In UV-vis spectroscopy, when beam of light goes through a solution, some portion of the beam may be absorbed while others are transmitted through the solution. The energy of absorbed radiation is the same with the energy disparity among the ground state and the higher energy state of the electron. The degree of dispersion of CNT in liquid media is realized by recording the emitted UV-vis spectra at specific wavelength. The obtained spectra are characterized as a set of peaks for CNTs of different diameter, type, and chirality. Samples are prepared by ultrasonicating CNTs in aqueous surfactants such as sodium dodecyl sulfate (SDS) and subsequent centrifugation to remove large particles that are not dispersed. Ultrasonication is necessary because it provides the needed energy to overcome high Van der Waals and π-π stacking attractions between individual SWCNTs (Kaliva and Vamvakaki 2020). Figure 8 shows the schematic diagram of UV-Vis spectrophotometer.

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Fig. 8 Schematic illustration of UV-Vis spectrophotometer (Source: Kumar et al. 2019 with permission from Elsevier)

Atomic Emission and Absorption Spectroscopy (AEAS) CNTs are commonly synthesized using transition elements Ni, Fe, or Co, which results to some extent impurities in the synthesized CNTs due to catalyst (metal) residues especially for chemical vapor deposition (CVD)-synthesized CNTs. AEAS is usually employed to evaluate the degree of impurities as relates to the transition metals present in the bulk samples of synthesized and/or purified CNTs. This technique is also based on Beer–Lambert law by establishing a connection between the measured absorbance and the concentration of the element (metal) in the CNTs sample with standard of known metal concentration (Chirayil et al. 2017). Techniques like inductively coupled plasma mass spectrometry (ICP-MS) and solidsample graphite furnace atomic absorption spectrometry (SS-GFAAS) are used to obtain the level of Ni, Fe, and Co in CNTs. ICP-MS gives reliable and accurate measure of elemental composition of impurities due to metallic nanoparticles in CNTs samples, based on their mass-to-charge ratio since mass spectroscopy can distinguish charged particles with different masses by vaporizing, atomizing, and ionizing elements in the CNTs (Herrero-Latorre et al. 2015).

Thermal Properties of Carbon Nanotubes Thermogravimetric Analyses Themogravimetric analyses (TGA) are required to establish the thermal stability of materials. TGA is carried out in thermobalance, which is the instrument that permits uninterrupted measurement of weight of the sample that is dependent on temperature

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and time (Porwal et al. 2007; Rahman et al. 2019). TGA can be used to establish the effect of experimental conditions of CNTs as it gives an easy method of approach (Rahman et al. 2019). Recent studies by Fredi et al. (2017) and Zhang et al. 2017 presented that a paraffin wax was stabilized in its shape using 10 wt.% of CNTs in various concentrations of epoxy resins to build up a novel blend of storage capacity with good thermal energy. Thus, TGA revealed that CNTs improved the thermal solidity of paraffin (Fredi et al. 2017; Zhang et al. 2017). Porwal et al. (2007), in their study, observed that CNTs synthesized under various investigational conditions of catalytic chemical vapor deposition (CCVD) technique showed variation in their thermal stability and crystallinity. It has also been reported that the TGA in air medium is a good method for characterizing yield and thermal stability of CNTs (Porwal et al. 2007). Thus, thermal treatment allows the oxidization of contaminants in the CNTs in the form of amorphous carbon and surface films defect of CNTs. Air, CO2, O3, O2, and acids are the various oxidizing agents that can be used (Porwal et al. 2007). TGA provides information on the thermal and oxidative stabilities of materials; composition of multi-component systems, projected life span of products, putrefaction kinetics, effect of reactive atmosphere, and moisture and volatile content of materials (Szufa et al. 2020). There is reduction in mass when a sample is heated and then allowed to cool or maintained at a constant regulated condition as determined by TGA (Leyva-Porras et al. 2020). Figure 9 showed the overall configuration of thermal analysis device.

Fig. 9 Overall configuration of thermal analysis device

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Differential Scanning Calorimetry (DSC) Differential scanning calorimetry (DSC) has been reported to be a vital characterization technique commonly used in a number of applications ranging from various research progresses of emerging materials and quality monitoring in industries (Zheng et al. 2019). DSC is a thermo-analytical method that measures the variation in the quantity of heat needed to raise the temperature of a sample and reference, which is time and temperature dependent (Leyva-Porras et al. 2020). The various techniques usually used for phase transitions characterization are TGA, dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), and DSC. In comparing the different methods, it has been reported that DSC is the most preferred one due to its ability to detect transition within large temperature range of 90–550
C and the simplicity of assay analysis of the transition (LeyvaPorras et al. 2020). Figure 10 describes a simple diagram of heat flow DSC measuring cell. In DSC, both the sample and the reference heat flow are determined in relation to time and temperature. When a sample is subjected to a particular temperature, the heat energy released or absorbed is measured by the calorimeter (Zheng et al. 2019). The calculation of the variation of heat flow rates to sample and reference follows Fourier’s law and is given by Eq. 5. Φ ¼ ΦS  ΦR ¼

Fig. 10 A diagrammatic representation of simple heat flow DSC measuring cell (Source: Zheng et al. 2019 with permission from ACS Publications)

TSTR △T ¼ Rth Rth

reference

sample

ð5Þ

furnace

disk temperature sensors sample carrier

purge gas

heating elements

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where Φ is the DSC signal, the heat flux to the sample and reference (ΦS and ΦR), sample and reference temperatures (TS and TR), the sensor thermal resistance (Rth), and ΔT is change in temperature.

Raman Spectroscopy Raman Spectroscopy is one of the most practical apparatus employed for the analysis of 2D materials (Sarycheva and Gogotsi 2020). Raman spectroscopy has witnessed major improvement recently in its capacity to probe through turbid media such as biological tissues (Nicolson et al. 2021). Various studies have shown that Raman spectroscopy is usable as a mesoscopic means for the estimation in the general status of CNTs composites (Chen et al. 2019). A study has also shown that the development of the line shape, frequencies of samples enhanced with D and 2D modes semiconducting, and metallic nanotubes in contrast with the radial breathing mode could be investigated by Raman spectroscopy (Laudenbach et al. 2017). The Raman experiment is said to be a fast, easy, noninvasive, and noncritical characterization method, which can be done at ambient conditions. Furthermore, the technique has the feature of tremendous sensitivity for probing the alterations in the nanotubes features via a variety of measures and conditions (Rahman et al. 2019).

Fourier Transform Infrared (FTIR) Infrared spectroscopy is used as a fast and accurate tool for characterizing chemical compounds (Hou et al. 2018). FTIR has been described by researchers as a model method of analyzing the chemical basic constituents of ordinary materials, as the occurrences of numerous vibration modes of organic and inorganic molecules are found to be energetic in the infrared (Lopes et al. 2018). The FTIR characterization has a peculiar advantage of its ability to determine materials, the processing and aging as well as some other special applications (Hou et al. 2018). FTIR is one of the most effective techniques that present clear understanding of the chemical and surface chemistry of different types of membranes (Mohamed et al. 2017).

Conclusion In this study, the innovative approaches in the characterization of carbon nanotubes (CNTs) have been discussed extensively. CNTs as allotropes of carbon are currently used in divergent applications due to their varied properties. These materials have the potentials of replacing/supplementing the usual nanofillers when developed into multidimensional polymer composites. In view of the varied properties, CNTs would continue to gain attention in many disciplines in the future. Acknowledgments The authors appreciate the University Research Council (URC) of the University of Johannesburg for the funding support.

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Optical Properties of Carbon Nanotubes V. S. Abhisha and Ranimol Stephen

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Optical Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saturable Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Raman Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quantum Computing and Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Tunable optical properties of Carbon nanotubes have the potential to revolutionize the field of advanced materials. The non linear optical properties of carbon nanotubes find applications in lasers, LEDs, optoelectronic memory devices etc. Raman scattering and optical absorption of carbon nanotubes enable its indirect application in spectroscopic characterization methods also. This chapter discusses the optical properties of carbon nanotubes in terms of atomic and electronic structure and its potential applications. Keywords

Optical properties of Carbon nanotubes · Optical absorption · Saturable absorption · Photoluminescence · Raman scattering · Applications

V. S. Abhisha · R. Stephen (*) Department of Chemistry, St. Joseph’s College (Autonomous), Devagiri, Kozhikode, India e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_57

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Introduction The reputation of carbon nanotubes in the field of material science and advanced materials is due to its potentially revolutionizing effects owing to the unique interaction of carbon nanotubes with electromagnetic radiation. The exceptional optical properties of CNT are characterized by its peculiar absorption, Raman spectra, and photoluminescence. Unique, highly ordered atomic, and electronic structure possessed by CNT is responsible for its highly anisotropic and tunable optical properties. CNTs are made up of layers of graphene sheets rolled up into cylinders with diameter in the orders of nanometers. Single-walled carbon nanotube (SWCNT) is fabricated form of rolledup single graphene sheets, while multiwalled carbon nanotube (MWCNT) constitutes two or more layers of graphene sheets with diameter ranging from 3 nm to 30 nm. SWCNT can be considered as a long one-dimensional material with end caps and diameter ~1 nm and tube length ~1 μm. CNT can be also classified as metallic or semiconducting or insulating (Saito et al. 1992a, b, 1998). SWCNT is characterized by the chiral indices (n, m) where n and m are integers, diameter (d) of the tube, and chiral angle (θ). Chiral angle is the angle relative to the main symmetry axes of the hexagonal graphene lattice and varies from 0
to 30
. Chiral vector, C, in the graphene sheet is given by C ¼ na1 þ na2

ð1Þ

where a1 and a2 are unit vectors. Figure 1 illustrates the chiral vectors in carbon nanotubes. Depending on the chiral vectors, CNT’s are classified as armchair (where n ¼ m and chiral angle ¼ 30
), zig-zag (where m ¼ 0 and chiral angle ¼ 0), and chiral (n, m) nanotubes (Matsuda 2013). When two-dimensional graphene sheet is Fig. 1 Illustration of chiral vector in carbon nanotubes (Sakharova et al. 2017)

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folded to a one-dimensional CNT, electron confinement around the CNT circumference results in additional quantization. The electronic structure of CNT is responsible for its unique optical properties including optical activity, circular dichroism, and second harmonic generation. Different theoretical tools are employed to study the exceptional optical properties of CNT including excitonic effects in semiconducting carbon nanotubes. The geometric symmetry of carbon nanotubes affects its optical properties. Theoretical studies consider nanotubes as periodic infinite one-dimensional materials in which π conjugation extends across the rigid sp2 C-C bond network. The electrons, phonons, and excitons are extended across a continuum band of states which leads to optical transport properties. Carbon nanotubes demonstrate extraordinarily high carrier mobility (>100,000 cm2 V1 s1) in semiconducting nanotubes (Getty et al. 2004), room temperature ballistic electron conductivity (Du et al. 2008), and high thermal conductivity in metallic nanotubes and high exciton diffusion length (~550 nm) (Crochet et al. 2012). Excitons: Electrons in the semiconducting carbon nanotubes absorb light and produce excitons. Excitons, electrically neutral quasiparticles, is a bound state of electron-hole pairs held together by electrostatic Coulomb force. Relaxation of excitons bring about fluorescence, photoluminescence, and electroluminescence in the near-infrared with high photo-stability. Intense excitation by high power lasers produces bi-excitons comprising two electrons and two holes. Bi-excitons are highly stable in nanotubes and are detectable in pump-probe spectroscopy techniques (Ma et al. 2006). The structural defects in the carbon nanotubes as identified by scanning probe and electron microscopy studies are found to affect the overall performance of the nanomaterials by altering its intrinsic physical and chemical properties. Defects present include oxygenated and dangling bonds at the nanotube ends, carbon vacancies, sp3 point defects, and rotated bonds (Stone-Wales defects) (Charlier 2002; Fan et al. 2005; Hersam 2008; Wang et al. 2017). Both native defects and synthetic defects can serve as the focal point of electrons, phonons, excitons, and spin coupling. The optical properties are highly sensitive to the surrounding conditions like temperature and pH (Berger et al. 2007; Cognet et al. 2007). Nonlinear optics/Nonlinear Kerr effect: Carbon nanotubes permit elastic scattering of light. Nonlinear refractive index properties of carbon nanotubes make scattering of light possible only in one direction. Nonlinear refractive index change in carbon nanotubes is mainly due to the nonlinear polarization of electrons in the sp2 lattice, which is termed as nonlinear Kerr effect. Nonlinear Kerr effect is large in carbon nanotubes (Torres-Torres et al. 2015; Herrera et al. 2018). Unique optical properties have obliged the researchers for the theoretical study of structural and electronic aspects of CNT, in order to acquire in depth knowledge for the proper manipulation and engineering of CNT-based applications. In 1994, Lin and Shung have investigated the basic optical properties of the carbon nanotubes by means of evaluating their dielectric function ε (ω) (Lin and Shung 1994). Gradient approximation method and tight binding method were employed, respectively, for evaluating dielectric function and band structure. They found that dielectric function

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exhibits many divergent structures, due to the divergences in its density of states (DOS) leading to many peak structures in electron-energy-loss spectrum. The collective excitations of the m band electrons are reflected as a prominent peak in the spectrum. These collective oscillations of free electrons are called plasmons. Due to the unique one-dimensional band structure of carbon nanotubes, the plasmon is insensitive to the diameter and chiral angle. These theoretical findings of Lin and Shug are in agreement with the experimental results. In 1998, Tasaki et al. have studied π-band contribution in the optical properties like optical absorption, optical rotatory power, and circular dichroism based on tight binding model (Tasaki et al. 1998).

Optical Absorption Optical absorption can be used as one of the characterization techniques of carbon nanotubes as its electronic transitions offer sharp peaks unlike bulk threedimensional materials. Optical absorption spectroscopy (OAS) is associated with the interband electronic transitions in SWCNT. OAS data gives information about the diameter distribution, purity, and metallic and semiconducting nature of SWCNT (Kataura et al. 1999; Itkis et al. 2003). Diameter of SWCNT with known (n,m) chirality integers is given by pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a n2 þ nm þ m2 d¼ π

ð2Þ

where a ¼ 0.246 nm. Relation for transition energy of semiconducting and metallic SWCNT is computed using tight binding calculation. The band gap of semiconducting tubes is inversely proportional to the tube diameter due to quantum confinement along the circumference. The energy distance between Van Hove Singularities is given by (Saito et al. 1998; Kataura et al. 1999; Liu et al. 2002) Esc,m ¼ 2i

γ 0 aCC d

ð3Þ

With i ¼ 1 for S11, i ¼ 2 for S22, i ¼ 3 for M11, γ0 ¼ 2.9 eV is the nearest C-C interaction energy, aC  C ¼ 0.144 nm is the nearest neighbor C-C distance, and d is the tube diameter. Modifications of the above equations were made by Fantini et al. (2004) and Nugraha et al. (2010). The number of electrons which are available at a given energy range for CNTs is defined by the density of states (DOS). In one-dimensional systems like carbon nanotubes, DOS is proportional to the inverse of the square root of energy. The decaying tail between the maxima in DOS is called Van Hove Singularities. Figure 2 shows the density of states (DOS) of metallic (9, 0) CNT and semiconducting (10, 0) CNT. Additional level of quantization in CNT results in Van Hove Singularities, which gives sharp peaks at conduction band and valence bands

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Fig. 2 (a) and (b) band structures and (c) and (d) DOSs of metallic and semiconducting carbon nanotubes (Yamashita 2019)

(Fig. 2c and d). In metallic CNT, the band structure of the tube is given by the cutting lines marked on the 2D cone-shaped energy dispersion around the K point of graphene lattice. K point is the high symmetry point in the Brillouin zone of graphene lattice. Van Hove Singularity in DOS is demonstrated by each line shown as local maxima except for the two lines crossing at K point. The metallic character of the nanotube is represented by the nonzero DOS at the Fermi level resulting from the line crossing at the K point as given in Fig. 2. DOS near Fermi level in metallic CNT is constant unlike linear relation in graphene. Optical transitions occur from valence band to conduction band labeled as E11, E22. . .etc. The first band gap energy of E11 can be calculated according to the equation (Yamashita 2019) E11 ¼ 2i

γ 0 aCC 0:835 ¼ eV d d

ð4Þ

This approach is called single particle model and here contribution only from electrons is taken into account. When excitons are also considered, the optical bandgap is smaller than that derived from the single particle model. However, excitonic band gap remains inversely proportional to the tube diameter d for semiconducting tubes and the DOS is zero at the Fermi level, where no crossing occurs at the K level. Optical absorption can be measured by the spectrophotometer and photothermal deflection, spectroscopy (PDS). OAS is commonly measured in room temperature, but high temperature measurements are also done (Roch et al. 2014). The inert atmosphere for measurements helps to avoid side effects of adsorbates, unwanted contributions from substrate clamping, and functionalization effects from oxygen and water (Itkis et al. 2003; Fantini et al. 2004). One-dimensional Van Hove Singularity determined by chiral indices gives spike like density of states for carbon nanotubes. Optical transitions occur between mirror image spikes of density of states. Absorption of light in the UV-Visible range corresponds to the Van Hove transitions E33 and E22. The generated mobile excitons

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rapidly decay within ~40 fs to the lower-lying E11 excited state and recombine in the near IR region(~800–1600 nm) (Manzoni et al. 2005). The binding energy of nanotube excitons is on the order of several 100 millielectron volts and hence can be considered as quasi-particles (Wang et al. 2005). Three characteristic peaks with fine spectra are observed in the OAS spectra of SWCNT. Semiconducting SWCNTs constitute first two peaks and metallic SWCNTs constitute the third one. Metallic and semiconducting SWCNTs can be identified by the difference in energy gap between spikes of nanotubes; wider energy gap is characteristic of metallic SWCNT. OAS spectra have broad absorption spectra due to π plasmon in which the three peaks are superimposed. The energy of Van Hove Singularities depends on the diameter distribution of the synthesized CNT samples. Broad diameter distribution results in the smudging out of VHSs, while narrow distribution range offers sharp peaks in OAS. Figure 3 shows the OAS spectrum of SWCNTs with diameter distribution in the range 2–1.3 nm. Synthesized CNT sample may contain different proportions of metallic and semiconducting tubes: typically 30% metallic tubes and 70% semiconducting tubes. The equation used to estimate the metallic tubes is given in the inset of Fig. 3 (Scheibe et al. 2011). Kataura et al. studied the optical absorption of SWCNT synthesized by electric arc method using NiY catalyst using PDS with carbon black as black body reference (Kataura et al. 1999). The spectrum reflects the difference in electronic state between the carbon nanotube and carbon black. The electron hole pair generated by the absorption of light generates heat which is directly measured. In 2020, Wei et al. (2021) unveiled a potential research area of study of ability of the CNTs to achieve a full spectrum of colors, their mechanisms, and color controllability backing up with theoretical and experimental data. A correlation with OAS spectrum and color exhibited by CNTs is obtained. The semi-empirical coloration model developed by

Fig. 3 OAS spectrum of SWCNTs. (Inset: Equation used for the estimation of metallic tubes content) (Scheibe et al. 2011) (Copy-right permission required) (open acess journal, contacted the editorial office for copy-right permission)

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Wei et al. exposed light into the underlying mechanism of full spectrum of strong colored pigments presented by CNTs.

Saturable Absorption The presence of inversion symmetry in the honeycomb carbon structure is responsible for the absence of second order nonlinear susceptibility in the CNT tubes. The third order nonlinearity is very large in CNT and is accountable for the nonlinear Kerr effect and nonlinear absorption changes like saturable absorption (SA) and multiphoton absorption. Changes in refractive index (nonlinear Kerr effect) and optical absorption are dependent on the incident optical intensity. Saturable absorption is the phenomenon of reduced optical absorption in high intensity light. SA is observed universally in all materials that have optical absorption due to the electronic transitions between two energy levels. SA occurs when all the allowed states in the conduction band are fully populated and valence bands are vacant at high optical intensities. CNT also possess fast saturable absorption responses that enable the generation of ultrashot pulses (Nicolais and Narkis 1971).

Photoluminescence Photoluminescence of semiconducting SWCNT was first observed in 2002. Photoluminescence originates from the lowest energy band exciton state designated as E11. Selection of appropriate (n, m) chiral vectors and altering diameter enables to tune their emission wavelength in near infrared region, that is, 850 nm to 2500 nm. Optical properties of SWCNTs up to room temperature are ruled by the multiple mobile excitons along the tube with huge binding energy in the order of several hundreds of MeV. Here major contributions are made by the Coulomb interactions in the one-dimensional confinement of nanotubes. Hence, photoluminescence quantum yield remains almost constant up to room temperature even though low lying dark states exist. Figure 4 shows the photoluminescence spectra of nanotubes in room temperature and cryogenic temperature. The broadened photoluminescence spectrum is due to the electron–phonon coupling owing to the one-dimensional confinement of nanotubes (Galland et al. 2008). This broadening can be eliminated by the proper freezing of low energy phonons (Vialla et al. 2014; Hao et al. 2008). Luminescence is polarized along the nanotube axis. Electrically excited SWCNTs can also serve as incandescent and electroluminescent light sources (He et al. 2018). Emission behavior of SWCNT can be studied using photoluminescent photonic devices. In spite of the intrinsic and unique optical properties, organic functionalization can produce tunable sp3 quantum defects. These synthetic defects in semiconducting SWCNTs fluoresce brightly in the shorter IR frequency and emits pure single photons at room temperature leading to many technological applications. The allowed electronic transitions in the synthetic defects are responsible for the bright photoluminescence and chemical tunability unlike the

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intrinsic defects which acts as quenching traps for excitons. These sp3 quantum defects are otherwise known as fluorescent quantum defects or organic color centers. Figure 5 represents the organic color centers in semiconductor host. The study of chemically tailored synthetic defects has great impact as it allows the better understanding of electron scattering, phonon coupling, excitons diffusion and trapping, and spin-orbit coupling (Brozena et al. 2019). According to Quantum theory, the spin degeneracy of SWCNT excitons creates four singlet and twelve triplet excitonic states and intervalley Coulombic interactions between the electron and the hole. There is only one optically allowed transition from singlet state which has higher energy than majority of the singlet or triplet dark states (Zhao and Mazumdar 2004; Ando 2006). Hence, the bright excitons readily

Fig. 4 Photoluminescence spectra at room temperature and cryogenic temperature(He et al. 2018) (Reproduced with permission from Springer Nature)

Fig. 5 Schematic representation of organic color centers in semiconductor host (Brozena et al. 2019) (Reproduced with permission from Springer Nature)

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decay into the dark singlet or triplet states with lower energy. Then the excitation energy is lost as heat, which causes low quantum yield of photoluminescence. The excitons diffuse along the carbon nanotubes up until their recombination. The defects formed during chemical processing or functionalization can degrade the graphitic structure which is hypothesized to be responsible for the quenching of mobile excitons, which causes low photoluminescence quantum yield (Dukovic et al. 2004; Cognet et al. 2007; Hersam 2008; Hertel et al. 2010; Harrah and Swan 2011).

Raman Scattering Raman spectroscopy is one among the key characterization tool for carbon nanotubes. Study of Raman active phonons appreciates the vibrational properties, symmetry, optical (excitonic) transitions, electron-phonon coupling, and phase transitions. Raman spectroscopy provides information about the quality of the carbon nanotubes, microscopic structure of the tube, and phonon-electron confinement. The dependence of number of CNTs on phonon scattering intensity aids to quantify the nanotubes and also gives information about number of CNT walls, chirality diameter, and electronic structure (Kataura et al. 1999; Anoshkin et al. 2017; Jorio and Saito 2021). Study of in phase radial movement of carbon atoms (radial–breathing mode) at ~100–200 cm1 along with resonant excitation describes the nanotube microscopic structure. Defect-induced modes of vibration result in the Raman peak characteristic of graphite structure that corresponds to the phonons from Brillouin zone. D band at ~1300–1400 cm1 is used for defect identification and quantification. G band at ~1584 cm1 and G’ band at ~2600–2800 cm1 can be used for strain, doping, and isotope characterization. G’ band is the overtone of D band and hence it is also known as 2D band, even though this band is not associated with any defects. High energy Raman spectra distinguishes metallic and semiconducting nanotubes. Strong electron-phonon coupling of high energy phonons leads to anomalies in metallic carbon nanotubes. This generates broader G band for metallic SWCNT. Semiconducting SWCNT has larger energy gap between valence and conduction bands than phonon energy. Raman intensity mapping technique based on Raman spectroscopy allows sample imaging and carries local functional information. Micro-Raman imaging and tip-enhanced Raman spectroscopy (TERS) are employed for nano-Raman imaging (Jorio and Saito 2021). Figure 6 gives the Raman spectra of SWCNT and other carbon allotropes. The radial breathing mode (RBM) is the unique Raman active mode present in carbon nanotubes. This vibrational mode exists because of the cylindrical shape of nanotubes in which the radius of the tube is oscillating as if the tube is “breathing.” RBM arises due to the out of plane stretching mode of vibration in which frequency is inversely proportional to the diameter of the carbon nanotube. The C-C in plane bond stretching mode of the hexagonal lattice gives G band at 1585 cm1 and so it is

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Fig. 6 Raman spectra of various carbon allotropes (Dresselhaus et al. 2010) (Reproduced with permission from ACS publishers)

common sp2-bonded carbon materials. The strain effect associated with curvature or electron phonon coupling leads to the splitting of G band into G+ and G peaks. Defects in the carbon nanotubes cause G band broadening due to bond inhomogeneity and shortening of phonon lifetime. G’ band or 2D band present in SWCNT is common in all sp2-bonded carbon materials depending on the excitation laser frequency (Pimenta et al. 2000; Saito et al. 2001, 2003; Grüneis et al. 2002; Souza Filho et al. 2003). G’ peak is associated to the Raman scattering of vibrational mode of breathing of six hexagonal carbon in the hexagonal lattice. However, the hexagonal breathing mode is not Raman active in first order Raman scattering. Nevertheless its first overtone is Raman allowed and appears at ~2600–2800 cm1 (Maultzsch et al. 2001). If the hexagonal sp2 network exhibits a defect, the first order component of the hexagonal breathing mode is activated, which is combined with an elastic scattering of a photo-excited electron by the defect as double resonance Raman peak and appears at ~1300–1400 cm1 (Pimenta et al. 2007). G’ and D are double resonance Raman peaks involving the scattering of the photo-excited electron in the Brillouin zone (Saito et al. 2001, 2003). The double resonance phenomenon was introduced by Thomsen and Reich to introduce D band (Thomsen and Reich 2001).

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The exceptional optical properties allow the use of carbon nanotubes as nanofluids in solar cell collectors. The absorption, transmittance, scattering, and extinction coefficient depend on size and volume fraction of nanoparticles. It is found that absorption coefficient increases with the increase of volume fraction and particle size. Though, in some cases, the overall enhancement in optical properties is independent of particle size in nanometer. However, path length has some remarkable effects over optical absorption in nanofluids (Ahmad et al. 2017). In CNT-based sensors, Raman spectroscopy assists as a significant metric owing to the sensitivity of the method. Raman spectroscopic method has the additional advantage of highly efficient and selective resonance effects (Jorio et al. 2010). The Resonant effect makes it possible to identify isolated carbon nanotube and can even obtain information from Raman inactive vibrations (Jorio et al. 2001; Saito et al. 2001). Spectroscopic techniques such as inelastic neutron or X-ray scattering or electron energy loss spectroscopy (EELS) are required to access Raman inactive phonons which are present in the interior of the Brillouin zone to gather information about several static and dynamic properties of particles. Such techniques require expensive infrastructure and single crystal particles, which makes it unsuitable for nanoparticle characterization. Raman spectroscopic technique can be employed for such phonon characterizations to understand strain, doping, defects, and interactions of nanoparticles with the surrounding materials. Resonance effect in Raman spectra of SWCNT is attributed to its one-dimensional electronic structure. Resonance Raman results in the enhancement of Raman signal in the orders of 103 when either the incident or the scattered light matches the optical transition energy. Therefore, Raman spectra from CNT sample are dominated by the SWCNT in resonance.

Applications Carbon nanotube optics is a potential research area for material scientists as it promises several innovative quantum technological applications. The nonlinear optical properties find applications in lasers, high field emitters, saturable absorbers, and electro-optic modulators. The tuneability and wavelength selectivity of carbon nanotubes enable practical applications including LEDs, bolometers, optoelectronic memory devices, etc. Optical properties of CNTs also enable its indirect application in spectroscopic characterization methods. Carbon nanotube-based single photon emission sources can be a crucial development in the quantum computing and quantum communications technology.

Defects Single photon sources (SPSs) are optical devices that emit solo photon of near unit probability on single excitation pulse (Srinivasan and Zheng 2017). Color center defects in SWCNTs enable emission in the gamma region and room temperature

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operation which serve as the basis of on demand SPSs (He et al. 2017). Chemical modification of nanotubes alters the electronic structure and thereby affects the photon emission process also. Weisman et al. have found that reaction of ozone with SWCNT produces oxygen color centers with mobile exciton emission 100–200 meV red shifted from that of pristine SWCNTs (Ghosh et al. 2010; Abdelsalam et al. 2019). Similarly high fluorescence yield is obtained for the organic color centers formed by the reaction of SWCNT with aryl diazonium compounds (Piao et al. 2013). Photon antibunching phenomena in the system have made the carbon nanotubes as budding source of SPSs which was earlier observed at cryogenic temperatures (Högele et al. 2008). This is due to the exciton localization in the shallow traps formed in the nanotube structure. Deliberately introduced oxygen color centers also cause exciton localization at room temperature itself but with limited single-photon purity and emission stability (Ma et al. 2015). Chemical modification of SWCNT allows the wavelength tenability and quality improvement. Aryl-based organic color centers are formed by the reaction of different chiral forms of SWCNT with aryl diazonium compounds with emission wavelength from 1.1 μm to 1.55 μm in the telecomm Oand C-bands (He et al. 2017). Various chiral SWCNTs as indicated by different (n, m) chiral indices can be functionalized by (3,5-dichlorobenzenediazonium (Cl2-Dz) or 4-methoxybenzenediazonium (OCH3-Dz)). SWCNTs with diameters D ¼ 0.76 nm, 0.83 nm, and 0.96 nm, respectively, produce single photon emission, respectively, at λ ¼ 1.2 μm, 1.3 μm, and 1.5 μm (Fig. 7). The chemical modifications on carbon nanotubes allow the creation of strong localized excitons with high grade SPSs in the broad spectral range. Organic color centers produce single photon of purity ~99% (Srinivasan and Zheng 2017). SWCNT-based organic color centers have promising technological applications as they exhibit several advantages like improved stability and brightness, possibility of electrically driven emissions, coupling to photonic cavities, and controlled nanotube assembly. Carbon nanotubes parade reduction in unwanted multiphoton emission

Fig. 7 Functionalized SWCNTs with organic color centers that produce single photon emission with different wavelengths (He et al. 2017) (Reproduced with permission from Springer nature)

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even while approaching saturation. After surpassing the saturation limit, CNTs turn transparent (Khasminskaya et al. 2016; Jeantet et al. 2016). Even though SWCNT-based color centers exhibit high purity by suppression of multiphoton emission and enhanced brightness, emission of indistinguishable photons with identical spectro-temporal properties to facilitate the quantum interference of photons is yet to be accomplished. In theory, SWCNT can possess a single, chemically controlled defect with well-defined molecular configuration that can facilitate chemical tuning for improved quality of photon emission. However, doping can cause multiple molecular configurations that cause single photon emission to occur at different wavelengths (He et al. 2017). SWCNT-hosted organic color centers are promising candidates in quantum technological field as it allows engineering at various levels including synthesis and chemical functionalization. Precise control of photon emission by modification of electromagnetic environment of carbon nanotubes opens up the path for the development of high quality quantum emitters that can be coupled with other optical elements and systems for novel opto-electronic devices.

Quantum Computing and Communications Carbon nanotube-based SPSs can be incorporated into photonic devices which can provide better control over the emission properties like time control and wavelength. Chemically modified CNTs with deliberately introduced defects demonstrate photon emission in the room temperature and in telecom wavelength region. This has high impactful effect in the quantum computing and quantum cryptography field.

Biomedical Applications Exceptional optical properties of SWCNTs enable its use in the biomedical field for sensors, imaging, drug delivery, and as optical probes. Excellent photostability and emission in the near infrared region of carbon nanotubes makes them the appropriate candidates in optical probes and biosensors. SWCNT can fluorescence and the emission properties are highly sensitive to the local environment. Hence, chemically engineered CNTs can be used in optical biosensor for detecting biomolecules in different biological matrices. Chemical functionalization with suitable organic molecules makes CNTs biocompatible and can be employed for medical diagnosis based on intracellular sensing (Farrera et al. 2017). CNT-based biosensors detect changes in fluorescence, quenching, and electro-optical properties of in vivo systems and complex biological fluids.

Molecular Sensors Modified SWCNTs can be used to detect even molecular level perturbations at the surface. Suitable designing and engineering of SWCNT-based sensors permits the

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control of wavelength so that desired colors in the emission can be obtained. The emission also depends on the dielectric constant of the SWCNT environment. Molecular sensors based on SWCNTs have the advantage of high sensitivity and can be employed for multiplexed imaging. Near-infrared emission ensures maximum tissue penetration as needed for in vivo applications (Farrera et al. 2017). Surface functionalization of SWCNT allows to make target-specific molecular sensors. SWCNTs modified by suitable oligonucleotides can sense the presence of DNA with complementary protein structure. Changes in dielectric constant during DNA hybridization induce changes in fluorescence which in turn can be detected by the functionalized SWCNTs (Jena et al. 2017). However, addressing of areas like biocompatibility, toxicity, and biodegradability of the SWCNT-based sensors is of at most importance before its use in biological systems. In vitro studies are employed to understand the toxicity of SWCNT sensors preceding in vivo studies. Figure 8 gives the important steps in the development of SWCNT for biomedical applications. Synthesis and functionalization is the first step towards the development of CNT-based biosensors. In vitro testing is essential before assessing the biodistribution, biocompatibility, and biodegradability of biosensors in in vivo applications. Biosensors of high sensitivity, selectivity, and efficiency can be fabricated by formulating appropriate strategy to efficiently utilize the unique electrooptical properties of CNT and properties incorporated by surface functionalization of CNT.

Fig. 8 Development of SWCNT for biomedical applications (Farrera et al. 2017) (Reproduced with permission from ACS)

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Conclusion Carbon nanotubes possess impressive optical properties, which is attributed to its unique electronic band structure. Optical spectroscopy and Raman spectroscopy serve as the widely used characterization techniques of CNT. Saturable absorption property of CNT is conducive for the generation of ultrashot pulses. One-dimensional confinement in nanotubes generates broadened photoluminescence spectra. The promising optical properties of carbon nanotubes have unlocked array applications in lasers, high field emitters, and electro optic modulators. Unparalled features of carbon nanotubes demand a potential research in this field to facilitate the generation of novel quantum technological applications.

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Thermal Properties of Carbon Nanotube Elham Abohamzeh and Mohsen Sheikholeslami

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Conductivity of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental Methods for the Measurement of the Thermal Conductivity . . . . . . . . . . . . . . . . Molecular Dynamics Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Different Parameters Affecting Thermal Conductivity of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Diffusivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental Methods for Measuring Thermal Diffusivity of CNTs . . . . . . . . . . . . . . . . . . . . . . The Influence of Temperature on Thermal Diffusivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Specific Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental Techniques for Measuring Heat Capacity of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of Temperature on Heat Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

There has been an unexpected rise in energy demand along with the rapid technological advancements all around the world, leading to increasing nonrenewable energy consumption and environmental damages. As a result, researchers have been looking for new ways to get energy from renewable sources like wind or solar while minimizing energy losses. A parallel attempt has been the development of strategies for increasing the overall thermal performance of devices utilizing materials with superior thermal properties. This led E. Abohamzeh Department of Energy, Materials, and Energy Research Center (MERC), Karaj, Iran e-mail: [email protected] M. Sheikholeslami (*) Department of Mechanical Engineering, Babol Noshirvani University of Technology, Babol, Iran Renewable Energy Systems and Nanofluid Applications in Heat Transfer Laboratory, Babol Noshirvani University of Technology, Babol, Iran e-mail: [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_58

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scientists to explore novel materials with enhanced properties to minimize energy losses and consumption. In this regard, carbon nanotubes have gained much interest since their discovery nearly two decades ago and are reported to be promising candidates for a wide range of applications considering their exceptional mechanical, thermal, and electrical properties. Due to the extraordinary thermal properties of CNTs, several research studies have been conducted to explore more about these unique materials and examine their possible application in heat transfer-related systems. The thermal properties of carbon nanotubes can be explored by the measurement of specific heat capacity, thermal diffusivity, and thermal conductivity, which will be discussed in the present chapter. The developed experimental methods for conducting the measurements are also reviewed, and the effects of several parameters on the thermal characteristics of nanotubes are explained according to the previous studies in this field. Keywords

Carbon nanotubes · Specific heat capacity · Thermal diffusivity · Thermal conductivity · Thermal properties

Introduction Carbon nanotubes (CNTs) possess high aspect ratios and an extremely wide range of electronic, optical, chemical, mechanical, and thermal properties. They are referred to as “wonder nanomaterials” and have gained much attention from researchers in multidisciplinary fields due to their unique and remarkable properties. TEM images of carbon nanotubes were observed first by Oberlin et al. (1976) in 1976. However, Iijima (1991), a Japanese physicist has been often cited as the inventor of CNTs in 1991. Iijima (1991) used the arc-discharge evaporation method to demonstrate the synthesis of needle-like carbon tubes in nano-dimension with diameters in the range of 4–30 nm. Two different research groups, Bethune et al. (1993) and Iijima and Ichihashi (1993), reported the growth process for single-walled CNTs later in 1993. Carbon nanotubes have attracted the attention of both industry and academics since their discovery in the early 1990s, owing to their unique characteristics and possible applications in a variety of fields such as energy conversion, optical, electrical, automotive, and aerospace (Terrones 2003; De Volder et al. 2013). CNTs can act like semiconductors or metals and conduct heat and electricity more efficiently than diamond and copper, respectively. As a result, it is possible to employ them in nanoelectronics like transistors and diodes, in lithium-ion batteries, and as sensors and electromechanical actuators in supercapacitors, and in composite materials as fillers (Coleman et al. 2006; Lee et al. 2007; De Volder et al. 2013). The extremely high thermal conductivity of CNTs (2000–6600) is their remarkable thermal property, which varies considerably based on their synthesis method, form, and structure (Murshed and De Castro 2014). The value of their thermal conductivity can be an order of magnitude greater in comparison with oxide or metallic nanomaterials like

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aluminum oxide (40 W/m K) and aluminum (237 W/m K), which are normally utilized for intensifying heat transfer (Berber et al. 2000). Carbon nanotubes possess a high aspect ratio, and their dispersion activity in the most widely used solvents has been observed to be excellent. Thus, the research on these novel materials has attracted attention to explore their thermal properties accurately and find their possible thermal applications. The aim of the present chapter is to address and review the recent research progress in thermal properties of carbon nanotubes. Three significant thermal properties, namely, thermal conductivity, specific heat capacity, as well as thermal diffusivity of these materials are included and the important parameters that affect these properties are discussed. In addition, the widely used experimental techniques for measuring these properties are presented and the related studies are reviewed.

Thermal Conductivity of CNTs The phonon conduction mechanism is assumed to be the dominant phenomenon for the thermal energy transport in CNTs, same as other nonmetallic materials (Marconnet et al. 2013). Despite the metallic characteristics of some CNTs, which raises the expectation of conducting thermal energy by means of electrons (Han and Fina 2011), the transportation is, however, primarily via collective vibrations in atoms (Balandin 2011). Different processes like inelastic Umklapp-scattering (electron-phonon scattering and anharmonic phonon-phonon processes), the free path length of phonons, boundary surface scattering, and phonon active modes number affect the phonon conduction in CNTs (Maultzsch et al. 2002; Ishii et al. 2007).

Experimental Methods for the Measurement of the Thermal Conductivity For the evaluation of materials’ thermal properties, several parameters should be considered, making these measurements challenging (Kumanek and Janas 2019). Two significant developed methods for measuring the thermal conductivity of CNTs are the 3ω method and the T-type probe method (Qiu et al. 2020).

T-Type Probe Technique For nanotubes, one efficient technique for the measurement of thermal conductivity (k) is the T-type probe technique. The measurement of k for a single CNT was performed by Fujii et al. (2005). Applying electron beam etching, they fabricated a sample holder containing suspended platinum nanofilm sensors on a multilayer film. The individual CNT was suspended on the nanosensors electrode under the scanning electron microscope (SEM), and locally focused electron beam irradiation was used for fixing. With the measurement of nanosensors’ heat generation rate and average temperature increase, the thermal conductivity of CNT was obtained. Along with increasing diameter, the thermal conductivity of the individual CNT decreased.

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A value of more than 2000 W/m K was achieved for CNTs with 9.8 nm diameter (CNT I). For individual CNTs grown by the similar process and with a diameter of 16.1 nm (CNTII), a value above 1600 W/m K was achieved for thermal conductivities, while the thermal conductivity was reduced to 500 W/m K for individual CNTs grown by similar processes but having diameters of 28.2 nm (CNT III). Furthermore, after reaching a temperature of 320 K, no further increase in thermal conductivity was observed, indicating the dominance of phonon scattering in thermal transport (Qiu et al. 2020).

3v Technique For measuring thermal characteristics of bulk materials and films such as CNTs, the 3ω technique is often utilized. This technique is on the basis of a metal strip with a micrometer size, which is utilized as a temperature sensor and also heater. A temperature fluctuation of 2ω is induced in the sample with alternating currents with ω frequencies used for heating the sample. The strip resistance varies with a 2ω frequency in a small temperature range. The sample thermophysical characteristics are included in the resulting third harmonic voltage (U3ω). A schematic of the setup of the 3ω method is illustrated in Fig. 1 (Choi et al. 2006). Choi et al. (2005, 2006) were the first who used this approach for calculating the thermal conductivity of CNTs. They applied two-point measurements on an individual MWNT (650 W/mK). The results were compared with that obtained for MWNT coated with a platinum layer (830 W/mK). They used a four-point 3-ω measurement to define the thermal conductivity value for an individual MWNT in their next publication. k ¼ 300  20 W/mK was the conductivity value. It is one order of magnitude smaller compared to the value that was calculated theoretically for single SWNTs, as was demonstrated by authors using intertube phonon dispersion and nonharmonic Umklapp’s scattering as the primary scattering process in

1ω, TTL

Keithley 6221

Reference IN

LOCK-IN AMP

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1ω, AC current A 1ω, 2ω, 3ω

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Fig. 1 A schematic of 3ω method setup (Choi et al. 2006)

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MWNTs under room temperature. Their chapter could be regarded as a guideline for using this approach for the measurements of CNT films (Kumanek and Janas 2019).

Molecular Dynamics Simulations Despite the fact that advanced thermophysical measuring methods have been taken for characterizing thermal conductivity on CNTs (Yu et al. 2005), present experimental observations cannot adequately explain the thermal transport processes of the heat carriers. As a result, in order to explore the mechanism of thermal transport in materials, simulation at the molecular level has been regarded as a significant tool (Din and Michaelides 1997). Nonequilibrium Green’s function, Boltzmann transport equation (BTE), molecular dynamics (MD), and thermal diffusion equation are examples of representative simulation approaches. In a system, the momentum and position space trajectories of particles are described by Newton’s law, and the MD approach is based on it. It follows the ergodic theorem and can calculate large numbers of particles. MD is the most widely used approach for atomic-level simulations in terms of the computational domain that accurately represents material thermal behavior. CNTs have recently drawn the interest of researchers due to their excellent thermal transport properties and nanoscale nature. The unique processes behind the extraordinary thermal transport phenomenon have been revealed using a variety of simulation techniques. Through the modifications of the molecular structures with higher phonon mode couplings, Xu et al. (Xu and Buehler 2009) discovered a significantly increased thermal conductance between individual CNTs. According to this report, an extraordinarily high thermal conductance can be yielded by the phonon–electron coupling impact at heterogeneous interface of metal and carbon in comparison with other interfaces with nanoscales. A novel threedimensional carbon network made up of CNTs and graphene sheets were suggested by Varshney et al. (2010), where the CNTs act as pillars between the graphene layers. The thermal transport was governed by the length of the CNT pillars because of phonon scattering at the junctions of graphene and carbon. Wang et al. (2021) performed MD simulations to evaluate k of a designed CNT/Cu/CNT nanotubes. They observed 37.5 times enhancement in thermal conductivity of single-walled CNTs compared to Cu nanowire with the radius of 5 nm, and utilizing triple-walled CNT on the outer side, the improvement factor was increased to 58.2. Moreover, the concentration of thermal stresses around the interface of Cu and CNT was manifested by applying the atomic stress analysis. The great improvement factors and stabilities under high temperature suggest that these CNT/Cu/CNT nanotubes can be used for high-temperature applications in electronics and thermal management. Boroushak et al. (2021) performed several MD simulations to determine the thermal conductivity of defective and perfect SWNTs functionalized with carbene, applying an approach introduced by Muller-Plathe. The findings showed that thermal conductivity reduced by functionalization. In addition, with the increase of functional group weight percentage, the k of functionalized SWNT decreased. Furthermore, it has been shown that the sensitivity of k decreased by reducing the

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weight percentage. In comparison with pure SWCNTs, k of functionalized SWNTs was not affected by simulation temperature, based on obtained results. In conclusion, it was reported that k of functionalized SWNTs reduced with the presence of vacancy defects.

Different Parameters Affecting Thermal Conductivity of CNTs There are several parameters that influence the thermal conductivity of CNTs, such as the presence of impurities, the morphology and structural defects number, tube length and diameter, and also the atomic arrangement (Kasuya et al. 1996; Maeda and Horie 1999; Popov 2004).

Morphology and Structure Nanotube Morphology Multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) are two main types of nanotubes. SWNTs are fabricated by rolling an individual graphene sheet and have diameters of 1–2 nm, while MWNTs consist of multiple rolled-up sheets bonded together with weak Van der Waals forces (Breuer and Sundararaj 2004). Therefore, the diameter of SWNTs is substantially smaller in comparison with MWNTs, so thermal characteristics might be totally different. Yu et al. (2005) evaluated the individual SWNT thermal conductivity employing a suspended microdevice, where the chemical vapor deposition technique was used for the growth of a single tube. Thermal conductivities were estimated to be more than 2,000 W/m.K, although there were uncertainties about the diameter of the actual CNT. It should be mentioned that the measurement of the thermal conductivity of single SWNTs is usually performed on SWNT mats since they aggregate in bundles (Sauvajol et al. 2002). For instance, thermal conductivities of SWNT were estimated to be between 1,750 and 5,800 W/mK, according to the study performed by Hone et al. (1999). This result was reported based on measuring the k of SWNT mats at room temperature. For SWNTs and at room temperature, thermal conductivities were reported to be about 2,800 ~ 6,000 W/mK, based on simulation studies (Lindsay et al. 2009). Berber et al. (2000) conducted combined nonequilibrium and equilibrium molecular dynamics simulations to evaluate the dependence of an isolated SWNT thermal conductivity on temperature. Based on their outputs, the value of thermal conductivity at room temperature was 6,600 W/mK, and this value increased with lowering the temperature. An uncommon value of 37,000 W/mK was observed for k at 100 K. In the temperature range of 100–500 K, a maximum value was discovered for the SWNT conductivity by Osman et al. (Osman and Srivastava 2001). The results were based on molecular dynamics simulations. The complete phonon dispersion relations was used by Gu et al. (Gu and Chen 2007) for calculating SWNT thermal conductivity and reported the value of 474 W/m.K for k of SWNT at 300 K. MWNTs consisting of nested cylinder of graphene arranged coaxially around a hollow core in the center with approximately 0.34 nm interlayer

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separations indicated graphite interplane spacing. Small et al. (2003) discussed the dependency of MWNTs thermal conductivity on temperature. They noticed the monotonous decrease in k at temperatures lower than room temperature. A value higher than 3000 W/m.K for an isolated MWNT at room temperature was reported by Kim et al. (2001), which was defined by a suspended microdevice. Other values ranging between 200 and 3,000 W/mK were obtained for thermal conductivity for isolated MWNTs at room temperature, based on experiments (Yang et al. 2004). Large and numerous thermal contact resistance of MWNTs are considered to be the reason for the high disagreement between bulk and single-tube measurements (commonly utilizing macroscopic mats or films) (Prasher 2008). For k of MWNT films, the value of 25 W/mK was reported when the self-heating 3ω technique was applied (Yi et al. 1999), while this value was 15 W/mK using the pulsed photothermal reflectance method (Zhang et al. 2002). Li et al. (2009a) applied the Raman shift technique for measuring k of individual CNTs. Based on their measurements, the values of 1,400 W/mK and 2,400 W/mK were achieved for k of MWNTs and SWNTs, respectively. Two factors were mentioned to explain the lower conductivity of MWNTs: (1) thermal transport is assumed primarily by the outermost wall, and (2) the processes of intertube Umklapp scattering. Moreover, comparatively small defect density and a considerable number of phonon vibrational modes are shown by SWNTs in comparison with MWNT, resulting in a greater conductivity (Dimitrakopulos et al. 1997, Dresselhaus et al. 2002; Mingo and Broido 2005a). Nanotubes conductivity can be influenced by the other nanotubes around them. Experimental studies have comfirmed the theoritical calculations. For CNTs, the thermal conductivity decreases following an increase in bundle sizes (Feng et al. 2018), which is a consequence of increasing the rate of scattering with neighboring CNTs. Moreover, the thermal conductivity of nanotube films is less than that of CNT bundle, which can be explained by the fact that nanotubes are primarily parallel in bundles, whereas the crossbar structure in films negatively influence the phonons conductivity. Atomic Arrangement Nanotubes’ atomic structure can be defined on the basis of tube chirality. The chiral vector defines chirality Ch ¼ na1 + ma2 (Thostenson et al. 2001). Step numbers along the unit vectors of hexagonal lattices (a1 and a2) are represented by the integers (n, m) (Rahman et al. 2010). Using the (n, m) naming schemes, three forms for carbon atom orientations can be identified along the CNT circumferences. When m ¼ 0, they are “zigzag,” when n ¼ m, the nanotubes are “armchair,” and else, nanotubes are “chiral.” Considering the fact that thermal transport by electrons and phonons is intensely dependent on the materials band gaps, the mechanism of heat transfer in CNTs has been revealed to be firmly dependent on chirality, which defines electronic properties and also band gaps size (Thorpe et al. 2000; Grujicic et al. 2004). Nanotubes with (n,m) chiral indices representing the chiral vector satisfies the condition: |nm| 3 p, and p is an integer (Ando 2003) having the highest bandgap (on the order of 1.5 eV). The bandgap of armchair nanotubes (n ¼ m) is significantly lower than that of other kinds of nanotubes. Nanotubes that have (n,m) chiral indices

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have been found to possess the greatest bandgap (about 1.5 eV) (Ando 2003). For armchair nanotubes (n ¼ m), the bandgap is significantly lower for other sorts of nanotubes. Therefore, for metallic CNTs with narrow band gaps, the contribution of electronic into thermal conductivity is considerable (Odom et al. 2000), while the phonon component governs chiral CNT thermal conductivity (Ando 2004). Zhang et al. (2004) found that CNT phonon thermal conductivity depends on the chirality. Homogeneous nonequilibrium Green–Kubo technique on the basis of the Brenner potential was applied for calculating the temperature dependences of the thermal conductivities of (20, 0), (10, 13), (11, 11) nanotubes having almost the same radii. The thermal conductivities of these nanotubes appear to possess the same temperature dependences. For temperatures between 100 K and 400 K, the conductivity of the (20, 0) nanotube was higher than the conductivity of (11, 11) nanotube, whereas the lowest values were observed for the (10, 13) nanotube. Although several studies have been conducted on phonon thermal conductivity, there have been few reports regarding electron thermal conductivity of CNTs in the case of metallic CNTs. Even if phonons are thought to govern thermal transport in CNT, electronic states might have a profound contribution to the heat transport of metallic CNTs. For example, high values of 41,000 W/mK at 104 K and 200,000 W/mK at 80 K were reported by Mensah et al. (2004) for chiral SWNT’s electron thermal conductivity, which was more than the value of phonon thermal conductivity. Although thermal conductivity decreases when the temperature rises above 100 K, the value obtained for the room temperature was approximately 11,000 W/mK, which is yet extremely high. Nevertheless, considering the fact that an insignificant portion of CNTs crystalline ropes is metallic in the experiment, the thermal conductivity is governed by phonons (Yamamoto et al. 2004). Defects in the Topology The theoretical studies are mainly conducted on perfect structures, but it is almost impossible to have samples of CNTs that are totally perfect and without defects. In the growth process of nanotubes, the occurrence of different types of defects is unavoidable. Some of these defects can influence the properties of the CNTs specifically. Ebbesen and Takada (1995) considered a cylindrical sheet of graphene consisting of just hexagons and having tips with minimum defects for forming a closed seamless structure as a perfect nanotube. They categorized defects into three classifications: incomplete bonding, rehybridization defects, topological defects, and other defects. The formation of topological defects such as vacancy-related or non-hexagonal carbon rings defects might be throughout the growth process of CNT or after synthesis, for instance, by irradiation by charged particles or chemical purification (Mera et al. 2009). The mechanism of forming topological defects was studied in theory, in particular for SWNTs (Sternberg et al. 2006). One of the main defects in nanotubes is the Stone-Wales (SW) defect. Nanotubes are always created with defects, even if very accurate reaction control systems are used. Phonons meant free path is decreased by scattering caused by topological defects (Omari et al. 2020). The rotation of the C–C bond for about 90 is the cause of SW defect, leading to transforming four hexagons into two

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heptagons and two pentagons (Meng et al. 2003; Liu et al. 2004). The nanotube’s chirality can be changed, and a seamless junction can be formed following the introduction of a pentagon-heptagon pair on SWNT (Meunier and Lambin 2000). Furthermore, there are always some kinds of extraneous contaminations in CNTs. These contaminations have to be eliminated so that, at high performance, no interference with thermal energy transport occurs. However, the structure of CNTs is often affected by the removal of these undesirable extraneous contaminations (Xie et al. 2018). Releasing carbon atoms in nanotubes under irradiation of ion or electron can endanger vacancies in SWNTs (Yuan and Liew 2009). It is possible to reconstruct local structures around single vacancies (Lu and Pan 2004). It was observed with MD simulations that the reconstruction of surface and decrease of size occurred with non-hexagonal ring formation, dangling bond saturation, and 5–7 defects in the lattice (Ajayan et al. 1998). The prediction of MWNT defects theoretically seems to be more complicated compared to SWNT, and has been seldom reported. However, high-resolution transmission electron microscopy (HRTEM) was used to observe an extremely complicated structure and several MWNT morphologies. A detailed report of the MWNT complicated structure with many defects, like internal caps, irregular layer spacings, and slipplanes, was presented by Lavin et al. (2002). Defective sites contained approximately 5% of carbon atoms (Mawhinney et al. 2000). A considerable effect might be exerted on the thermal conductance of CNTs resulting from this substantial number of defect sites on CNTs graphene walls. Applying the nonequilibrium Green’s Function method, the influence of SW defects and vacancies was studied by Yamamoto and Watanabe (Yamamoto and Watanabe 2006). They reported a defect-dependent k of CNTs with the comparison of the thermal conductance ratio κvac(sw)/κp for CNT with SW defect of vacancy and perfect CNT. Vacancies induced more strong scattering of incident phonons compared to SW defects and in thin CNTs, detect scattering had a more substantial influence on thermal conductivity, when compared with thick CNTs. Che et al. (2000) revealed with the increase of defect concentration, thermal conductivity decreases. They also reported that vacancy has a greater scattering effect in comparison with SW defects. During the growth process and following the incorporation of non-hexagonal carbon rings into the nanotube structure, regularly coiled or branched CNTs can be formed (Osváth et al. 2007). By introducing the junction on CNTs, the local thermal resistance commonly increases and thermal conductivity decreases since at the junction there are lattice defects in non-hexagonal carbon rings forms (Cummings et al. 2004). For instance, in comparison with straight nanotubes, a decrease of 20–80% was observed for the thermal conductivity of X-shaped junctions (Meng et al. 2007). Qiu et al. (2016a) reported that for thermal conductivity, MWNTs are better than SWNTs since, with the increase of nanotubes diameter, more optical phonon modes are available, which might be exposed to excitation and participate in thermal transport. The presence of defects in the structure of SWNT has more stronger influence on conductivity properties compared with MWNTs, since new additional channels can be created for phonons in MWNTs, which is not possible for SWNTs.

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Dimensional Factor Nanotubes are considered to have a mean free path of phonons that are comparatively long: longer than 500 nm in SWNTs and 500 nm in MWNTs (Yu et al. 2005; Donadio and Galli 2007; Chang et al. 2008; Prasher 2008). Considering this point, the structural dimension of CNTs can be an important parameter. CNTs are well known to be distinguished by a great surface area and a high aspect ratio. For the description of CNTs, length and diameter are two main factors, which have a direct influence on the thermal conductivity of carbon nanotubes. The Length of Carbon Nanotubes With the increase of the length of CNTs, the thermal conductivity improves until it would be equal to the phonons’ mean free path, that is 500 nm for MWNTs and longer for SWNTs (Han and Fina 2011). The effect of length on the intrinsic k of CNT was predicted by performing simulations. For SWNTs with (5,5) and (10,10) chiral indices, it was observed through the molecular dynamics method that thermal conductivity improved when the tube length has increased from 6 to 404 nm (Maruyama 2002). Moreover, reverse nonequilibrium MD simulations were used to reveal that for SWNTs with several indices, the thermal conductivity at room temperature is dependent on the length (Alaghemandi et al. 2009) With the increase of tube length, for length parameters (L) from 5 to 350 nm, the calculated k followed a Lα law and increased, with values of α between 0.77 (L < 25 nm) and 0.54 (100 nm < L < 350 nm). These phenomena can be clarified by the variable ratio between the CNT length and the phonon mean free path (Chiu et al. 2005). It is anticipated, according to these outputs, that when the length of the tube is too larger than the energy-carrying phonons’ mean free path, the thermal conductivity becomes constant. The influence of ordering and structure of nanotubes in materials was comprehensively studied in research works by Aliev et al. (2007, 2009). They considered sheets of MWNT with different layer numbers, different lengths, and under various temperatures and applied the 3-ω method for the measurements. They also conducted measurements for different CNT sets, aligned MWNT sheets, freestanding MWNT sheets, bundled MWNTs, and single MWNTs. Based on the outputs, the length of MWNT sheets is not important for the thermal conductivity value up to 150 K, which might be because of a great decrease in radiation. But the value of thermal conductivity for the sample with a larger length was higher above. The Diameter of CNTs Cao et al. (2004) stated that, in theory, SWNTs with larger diameters should have lower thermal conductivity. Based on their outputs, the thermal conductivity of SWNT had an inverse relation with the SWNT diameter at 300. Fujii et al. (2005) used suspended sample-attached T-type nanosensors for measuring k of an individual MWNT. With the decrease of MWNT diameter and at room temperature, its thermal conductivity increased, i.e., with the decrease of walls number, thermal conductivity increased. For the diameters of 28 nm, the value of 500 W/m.K, and for the diameter of 10 nm, the value of 2069 W/m.K were obtained.

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Temperature The first step for gaining a proper understanding of the effect of temperature on the k of CNTs is the examination of a single nanotube. A linear increase in thermal conductivity was observed by rising temperature in the low ranges of temperature close to absolute zero (Mingo and Broido 2005b). With the additional rise in temperature, k initiates to be mediated through extra phonon modes, so thermal conductivity increases until it reaches the highest value (near room temperature (Berber et al. 2000, Osman and Srivastava 2001)). Consequently, the dominance of photon scattering processes occurs due to the temperature increase (Pop et al. 2006), resulting in the reduction of thermal conductivity. Duzynska et al. (2014, 2015, 2016) examined the thermal conductivity of SWNT and defined the temperature that provides the highest possible thermal conductivity. An SWNT film was prepared, and the effect of enhanced temperature on thermal conductivity variations was evaluated applying Raman spectroscopy. Based on their study, at temperatures of 30–-450 K, thermal conductivity reduced from 26.4 to 9.2 W/mK. The reduction in conductance for the SWNT film with rising temperature was explained by the increase of phonon scattering processes with higher order. Tambasov et al. (2020) formed SWNT thin films applying vacuum filtration. They applied the 3ω technique for investigating the effect of temperature up to 450 K and thickness on the thermal conductivity of these films. It was observed that the k of films was strongly affected by temperature and thickness. A very sharp increase was noticed for thermal conductivity when the thickness was increased from 11 to 65 nm. Additionally, with increasing temperature from 300 to 450 K, a rapid reduction in thermal conductivity from about 211 to 27.5 W/m.K was reported for a film with a thickness of 157 nm. Density The thermal conductivity of CNTs has been reported to be strongly affected by both packing density and specific density of CNTs. With increasing the junctions numbers, thermal transport inside the samples facilitated, leading to increasing thermal conductivity. The change of thermal conductivity with the density of CNT packing in bucky papers was evaluated by Zhang et al. (2012). For pressing the sample, a range of pressures from 20 to 30 Mpa was applied, and different densities of 0.8 to 1.39 g/cm were considered for the samples. Applying steady-state methods, they measured the values of thermal and observed that thermal conductivity was increased from 472 to 766 W/mK. They reported that with increasing density, thermal conductivity increased, which is due to higher CNTs packing in a sample, leading to faster thermal penetration (Gong et al. 2004; Wu et al. 2018).

Thermal Diffusivity The ability of materials for transferring heat is represented by thermal conductivity (k), while the heat transfer rate in materials is represented by thermal diffusivity (α). So it can be regarded as the diffusion rate in the material from the hot side to the cold side.

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Thermal diffusivity is a transient value and does not depend on the energy value during heat conduction. Considering the known relationship between α and k, the thermal conductivity of the material can be measured by defining thermal diffusivity, indicating the importance of experimental and theoretical research on α. Equation (1) presents the relationship between k, ρ, and cp (Qiu et al. 2020): α¼

k ρ:cp

ð1Þ

For larger values of α, the medium ability is stronger for balancing its internal temperature equilibrium, and hence thermal diffusivity indicates the medium ability for propagating energy. It is difficult and relatively rare to get a direct theoretical calculation of thermal diffusivity. Theoretically, we use Eq. (2) to obtain α from the calculated values of k and cp. Since there are only very few purely theoretical investigations of α, experimental techniques are commonly used to investigate thermal diffusivity and the involved transport mechanisms. Direct measurements of thermal diffusivity are described in Eq. (2), and it is based on another form of thermal diffusivity, describing the fundamental relation between temperature and time: @T ¼ αΔ2 T @t

ð2Þ

Considering the fact that the thermal transport rate from a heat source to a heat sink is described by α, it is required to use a rapid heating method for the measurements. The laser flash technology was utilized for the earliest characterization of thermal diffusivity, where a laser pulse heated the sample, and temperature differences in the small distance from the heating position were measured (Parker et al. 1961). Thermal diffusivity at 1D adiabatic conditions is commonly determined as: α ¼ 0:1388  d2 =t0:5

ð3Þ

wherein t0.5is the half max of time, and d is the thickness. Considering the fact that rapid heating is required for measuring α, laser heating is used in most measuring techniques, like the laser flash Raman spectroscopy technique. Some common methods are discussed in the following section.

Experimental Methods for Measuring Thermal Diffusivity of CNTs Experimental measurement techniques for defining the thermal diffusivity of carbon nanotubes are discussed in this section. Two widely used methods for measuring α are the transient electrothermal (TET) and the laser flash Raman spectroscopy techniques.

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Laser Flash Technique Parker et al. (1961) were the first who proposed a classical method for measuring thermophysical properties, the laser flash technique, in 1961. It is a noncontact, transient, and high sensitivity measuring method, which is commonly used for measuring the thermal diffusivity of homogeneous materials (Blumm et al. 2007). The test is conducted by irradiating a short laser pulse into the sample front side, and then in real time, the increase of the temperature on the backside of the sample is recorded. By fitting the profile of temporal temperature, thermal diffusivity can be achieved. The possibility of easily adjusting the width and energy of the pulsed laser is regarded as a significant feature of this method. The thermophysical characteristics of vertically aligned MWNTs were defined by Lin et al. (2012) according to this measurement principle. They considered some assumptions in their study. An ideal one-dimensional heat flow was assumed for the thin gold coating that is used for facilitating the quick planar heat distribution when the laser was irradiated on the sample surface. Because of the gold coating on the sample, the surface was opaque, and therefore a negligible penetration depth was assumed for the incident laser. So, the independence of thermal diffusivity from the sample thickness was confirmed. The accuracy of the measurement was improved by the coating of metal on the MWCNT film bottom. Using nanosecond pulsed lasers, noncontact heating of MWNT arrays was achieved by Xie et al. (2007). They acquired the thermal diffusivity by fitting for temperature against time. At room temperature, the MWNT arrays were reported to have a thermal diffusivity value of 4.6104 m2/s, which was significantly greater compared to several well-known outstanding thermal conductors like silver and copper. Akoshima et al. (2009) prepared a 1 mm long highly pure SWNT forest. The thermal diffusivity was found to be comparable to that of isotropic graphite and strongly dependent on temperature. The value of thermal diffusivity at room temperature ranged 0.47–0.77  104 m2/s. The Transient Electrothermal (TET) Method An efficient technique for the measurement of thermal diffusivity that can overcome the drawbacks of other techniques, such as weak signals and long test times, is the transient electrothermal technique (TET) (Xing et al. 2017). In brief, the suspended fiber placed on the heat sink is exposed to a direct current. The fiber temperature rises as a result of the induced Joule heat, and at each moment, the transient temperature response is utilized for determining the thermal diffusivity of the material. Guo et al. (2007) neglected heat losses on the sample surface when developing the thermal conduction model for the transient electrothermal technique and considered constant heating. With the increase of the circuit impedance in the experimental setup, the sample heating power was kept stable. In this study, the average thermal diffusivity of SWNT bundles was obtained equal to 2.73  105m2/s. Utilizing a polyester fiber with a gold coating, the thermal diffusivity of the nonconductive material was defined to be 5.26107 m2/s 5.26  107m2/s.

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Photothermal Resistance Method Hou et al. (2006a) were the first who proposed the photothermal resistance method in 2006. This technique can be applied for characterizing thermal properties of one-dimensional nonconductive and conductive materials with nano- and microscales. Briefly, a periodic laser beam is used to irradiate the testing sample suspended on two electrodes. A periodic variation is observed for the sample temperature because of the laser beam heating, and thus periodic variation is produced by the sample electrical resistance. Voltage variations reflect the change in electrical resistance with exposing the sample to a direct current, utilized for deriving the thermophysical characteristics factors of the sample. Hou et al. (2006b) used the measurement principle, which is discussed above to verify this method’s reliability with the measurement of a platinum wire with a diameter of 25.4μm as a reference sample and found the compatibility of the measured thermal conductivity for the platinum wire with the reference value. Consequently, the thermal diffusivity of clothing fibers, human hair, and three SWNT bundles were measured. Considering the nonconductivity of clothing fibers and human hair, they deposited a metal film on nonconductor samples for ensuring the accurate extraction of temperature following the application of the electrical voltage signal. The values of 6.64  105 m2/s, 4.41  105 m2/s, and 2.98  105 m2/s were obtained for the thermal diffusivity of three SWNT bundles. The measured thermal diffusivity for clothing fibers and human hair was at the level of 106 m2/s indicating the great thermal characteristics of carbon nanotubes in comparison with the conventional materials (Qiu et al. 2020).

The Influence of Temperature on Thermal Diffusivity Generally, the temperature-dependent thermal diffusivity is based on two parameters, which are the free electron and phonon transfer energies. The possibility of collisions between phonons increases with the rise of the temperature, leading to more intensified lattice scattering and decreasing phonon mean free path, and thus, the influence of phonons on thermal diffusivity decreases. Reduced graphene oxide (rGO)/CNT was prepared by Pan et al. (2017) to define how its thermal diffusivity can be affected by temperature. Based on the results, the values of cross-plane and in-plane thermal diffusivity increased with the rise of temperature, considering the fact that at higher temperatures, faster thermal transport occurs due to larger amplitudes and greater phonon vibration frequencies (Hummel 2011). Nanocomposites based on CNTs have outstanding thermal properties. The thermal diffusivity of CF/CNT composites was analyzed by Jackson et al. (2016). With the rise of temperature, α tends to reduce. Under a constant temperature, higher values were observed for α at higher volume fractions of the CNT. A research study was performed by Xie et al. (2007) for measuring the thermal diffusivity of MWNT at temperatures from 55 to 200  C. Their outputs confirmed that the thermal diffusivity augments with the increase of temperature between 55 and 70  C. This trend has been similar to

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what was earlier discovered using the 3ω technique (Yi et al. 1999). However, the values measured for thermal diffusivity have been an order of magnitude greater in comparison with that obtained via 3ω. This discrepancy is due to the use of longer carbon nanotubes in Ref. (Yi et al. 1999), which usually contain more defects, resulting in a higher probability of phonon scattering and hence a lower capability of thermal transport. When the values of temperature are between 70 and 200  C, there are small changes in α. This is attributed to the fact that the mechanics of phonon scattering have not been clear and needed to be clarified further (Yi et al. 1999; Qiu et al. 2020).

Specific Heat Specific heat is also a significant thermal property concerning the applications of CNTs. The specific heat C(T) of the substance probes the low-energy excitations. The dominant excitations in nanotubes are phonons, and under most temperatures, C (T) is dominated by the phonon-specific heat Cph. The phonon-specific heat is dependent on the state phonon’s density, ρ(ω), and is determined through integrating ρ(ω) with a convolution factor, which is dependent on temperature that accounts for each phonon state’s temperature-dependent occupation (Ashcroft and Mermin 1976). ð Cph ¼ kB



ℏω kB T

2

  ℏω kB T

e

ρðωÞdω   !2 e

ℏω kB T

ð4Þ

1

A decrease in the convolution factor is observed from 1 to 0.1 at ω ¼ 0 and ℏω ¼ kBT/6, respectively. As a result, there is no appreciable contribution to the specific heat by phonons above this energy. Generally, a numerical evaluation of Eq. (4) is needed. Under low temperatures, the phonon-specific heat only probes the phonons with the lowest energy, which are acoustic modes with dispersion that can be presented as ρ(ω)αkα. For a single such mode, it is possible to simplify Eq. (5) as: Cph αT ðd=αÞ

ð5Þ

In the above equation, d is the system dimensionality (Hone 2004).

Experimental Techniques for Measuring Heat Capacity of CNTs Experimental techniques have been developed for measuring the heat capacity of CNTs. In this section, these approaches are discussed.

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Thermal Relaxation Method For determining the heat capacity of nanotubes, thermal relaxation has been a widely applied technique (Mizel et al. 1999). This calorimetry-based approach utilizes several samples of CNT, which are firstly pressed before being transmitted to calorimeter pellets. These preliminary treatments alter the porosity of the CNTs, which can result in deviations in the measurement of heat capacity. The specific heat of SWNT ropes and MWNTs samples was investigated by Mizel et al. (1999) applying the thermal relaxation method. The values obtained for MWNT are observed to be broadly consistent with that of graphite. At low temperatures, the specific heat of SWNT ropes is observed to surpass that of graphite, and the peak of C(T )/T2 was observed at a lower temperature compared to graphite. Sumarokov et al. (2019) applied the thermal relaxation technique to measure the specific heat of MWNTs with low inorganic impurity content and defectiveness in the temperature range of 1.8–275 K. Energy dispersion X-ray spectroscopy, and Scanning electron microscopy was used to determine the morphology and elemental composition of MWNTs. Chemical catalytic vapor deposition was used for the preparation of MWNTs with lengths in tens of microns and mean diameters ranging between 7 and 18 nm. The purity of MWNTs was higher than 99.4 at.%. Based on the results, at low temperatures, it was discovered that the dependency of the MWNTs’ specific heat on temperature is greatly different from that of other carbon materials such as diamond, graphite, bundles of SWNTs, and graphene (Ruan et al. 2011). 3v Method The self-heating 3ω method (Yi et al. 1999) is the second technique for the measurement of the specific heat capacity of CNTs. For the calculation of the heat capacity, after the suspension of the CNTs bundle on electrodes and heating them with a sinusoidal wave, the 3ω part of the response is measured. Getting CNT bundles is needed in this approach prior to measurement, which can result in errors and CNT damages (Ruan et al. 2011). Under vacuum conditions, Lu et al. (2001) measured the heat capacity of MWNT arrays in the temperature ranges between 10 and 300 K. The findings revealed the linear dependency of cp on the temperature. At room temperature, a value of 500 J/kg K was obtained for cp, which is primarily due to phonon behavior. A value of 312 J/kg K was obtained for the specific heat capacity of vertically aligned CNT arrays with a thickness of 13 μm, at room temperature, in the work performed by Hu et al. (2006). They observed that with the increase of temperature to 325 K, the specific heat capacity raised to 375 J/kg K. Compared to the previously conducted studies (Zhang et al. 2000; Lu et al. 2001; Qiu et al. 2018), the obtained values were extremely lower, which was associated with the influence of the very low heat capacity of the air that is trapped in the CNT array. Qiu et al. (2016b, 2018, 2019b) applied the 3ω approach for the direct measurement of k and α to determine the heat capacity of Au nanoparticle-decorated CNT fibers, iodine molecule-decorated CNT fibers, and densified and functionalized CNT fibers. According to the findings, the values of specific heat capacity for the decorated fibers were about 685 J/kg K, which was similar to the specific heat capacity of pristine fibers (780 J/kg K) and previous studies (Zhang et al. 2000;

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Lu et al. 2001). A recent study conducted by Qiu et al. (2019a) was a significant progress regarding the thermophysical properties of the assembled fiber structures. This study notes only the outermost few layers of bundles at the surface of CNT fiber are associated with real radiation. A new theoretical model was developed based on the experimental findings, which utilized the actual circumference of the surface of fiber for replacing the diameter of the fiber. This advancement is important for the precise assessment of the thermal characteristics of assembled fiber structures.

AC-Calorimetric Technique Another approach for measuring the specific capacity of CNTs is to use the ac-calorimetric technique. The separation of CNTs from the substrate and its further processing before measurements are also required using this form (Pradhan et al. 2009). It should be mentioned that the accurate measurement of the CNT thermal properties utilizing conventional calorimetry is challenging considering some issues such as the uncertainties of sample geometries, the surface roughness, the pretreatment-induced state changes, and damages (Ruan et al. 2011). Pradhan et al. (2009) used bulk graphite powder as a reference sample to calculate the cp of randomly aligned SWCNT and MWCNT. The findings showed that the MWNT and graphite powder has the most consistent values of heat capacity, whereas the weakest temperature dependence was observed for the heat capacity of MWNT because of its macroscopic arrangements. Muratov et al. (2012) measured the cp of MWNTs using the the adiabatic calorimetry technique. MWNT had a diameter lower than 30 nm and the experiments were conducted in the temperature range between 60 and 300 K. At temperatures of 80–90 K, the anomaly was observed in heat capacity. A forest distribution was noticed for the cases with diameters of 17 nm (Jorge et al. 2010) and 25 nm (Jorge et al. 2009) prepared by the acetylene catalytic decomposition on Fe nanoparticles. In these samples, there was no internal bamboo-like structure. At 33 K, for samples with bamboo-like internal structures and larger tube diameters, the dimensional behavior crossover was noticed, and for samples with no bamboo-like internal structures and with a diameter of 25 and 17 nm, this was observed at 55 K (Sumarokov et al. 2019).

Effect of Temperature on Heat Capacity The temperature dependence for the heat capacity of CNTs was studied in several previous studies. For SWNTs ropes, Hone et al. (2000) investigated the dependency of heat capacity at low temperatures. The heat capacity was observed to be linearly correlated with temperature for the temperature range of 100–200 K. For temperatures less than 50 K, the heat capacity was proportional to T2, which was in compliance with Debye’s law. Li et al. (2009b) also observed similar outputs in their work. Furthermore, previous research has shown that at low temperatures, the specific heat capacity of both MWNTs and SWNTs had a linear dependency on T at low temperatures (Yi et al. 1999), which can be explained with the consideration of quite weaker wall-to-wall coupling in MWNTs in comparison with graphite.

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Fig. 2 The influence of changing temperature on specific heat (Kumaresan and Velraj 2012)

Moreover, it was observed that, for SWNTs, the contribution of phonons to heat capacity was higher compared to electrons (Lasjaunias et al. 2002; Li et al. 2009b), and a cubic polynomial describes the relationship between temperature and heat capacity. The specific heat of DIW/EG/MWNT nanofluid was studied by Kumaresan and Velraj (2012). They observed a continuous rise in specific heat with increasing  temperature up to30 C. However, it decreased with augmenting MWNT concentration. In addition, the specific heat of this nanofluid containing 0.3% CNT did not  increase with the rising temperature above 30 C, but rather decreased slightly with  increasing temperature, as illustrated in Fig. 2. In the same way, at 35 C, the specific heat of the nanofluid containing 0.45 and 0.15% CNTs did not rise with increasing temperature, and the specific heat of such nanofluids marginally reduced at 45 and   50 C. Similarly, with increasing temperature up to 35 C, an increase was observed in the base fluid (EG:DIW/30:70) specific heat, and after this temperature it decreased with increasing temperature (Shahrul et al. 2014).

Conclusion In this chapter, the thermophysical properties of carbon nanotubes are reviewed considering experimental and computational approaches. Three significant thermophysical properties of carbon nanotubes, namely, specific heat capacity, thermal

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diffusivity, and thermal conductivity, are analyzed, and different experimental approaches for examining these properties are discussed in detail. Moreover, based on previous computational and experimental studies, the most important parameters, which affect the thermal characteristics of CNTs, are presented. Based on previous literature, CNTs have been proved to have excellent thermal properties making them promising candidates for thermal applications. However, there are still several challenges that should be addressed for further development of these unique materials. One major obstacle that limits the applications of CNTs is their cost. The technique used for synthesizing CNTs are expensive, and prepared materials are commonly defective or contaminated. As a result, the cost of fabrication is needed to be lowered. Moreover, the methods for the measurement of nanoscale materials are imperfect and still under development. As a result, it is challenging to directly measure the thermal properties of CNTs. Despite the fact that recent decades have witnessed important progress in research on CNTs, there are still many important questions to be answered. The values reported for thermophysical properties of CNTs by different research groups are widely scattered due to the considerable dissimilarities between different synthesized samples. This is caused by differences in quality and geometric parameters, especially the inevitable presence of defects, which must be comprehended. Without a doubt, in order to meet a wide range of applications, prepared CNTs must have predictable and identical thermophysical properties. Although the developed experimental techniques and computational methods have aided our understanding of thermal transfer mechanisms for CNTs, much more study should be conducted in this regard.

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Electronic Transport and Electrical Properties of Carbon Nanotubes Prabhakar R. Bandaru

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Electronic Attributes of Single-Walled Nanotubes (SWNTs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wrapping of a Graphene Sheet to Form Metallic or Semiconducting SWNTs . . . . . . . . . . . . Armchair (AC) and Zigzag (ZZ) Modalities for SWNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Doping Characteristics of Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electric Field Profiles in Doped NTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical Conductivity and Resistance in Nanotubes: Applications to Devices . . . . . . . . . . . . . . Electrical Contacts to Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characteristic Features of Electrical Contacts to CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNT-Based Field Effect Transistors (FETs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control of CNT Device Electrical Conductance Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . Device to Device Variability in Measured Electrical Characteristics of the CNTs . . . . . . . . . Electrical Capacitance and Inductance in Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrostatic and Quantum Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electromagnetic and Quantum/Kinetic Inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental Measurements of the Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Low Frequency Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High Frequency Electrical Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Multiwalled CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Relation of MWNTs to Individual Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical Transport Characteristics of MWNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnetoresistance in MWNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Superconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

174 176 176 176 179 181 181 183 185 187 188 191 192 192 193 193 194 196 199 199 200 201 202

P. R. Bandaru (*) Department of Mechanical and Aerospace Engineering, Materials Science Program, University of California, San Diego, CA, USA e-mail: [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_59

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Applications of CNT-Based Electronics: Advantages and Issues to Be Overcome for Broad-Scale Utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNT-Based Interconnect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNT-Based Transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical Sensing and Biosensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Outlook for the Future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The wrapping of a graphene sheet in specific configurations into tubular structures of ~1 nanometer diameter, can embody the resulting Carbon Nanotubes (CNTs) with metallic or semiconducting character. The CNTs may either be single-walled or multi-walled and may approach centimeters in length. The article focuses on the related electrical properties and applications of the CNTs. While the length scales and large surface area to volume ratio are useful for high sensitivity, the one-dimensional characteristic of the CNTs imparts unique quantum mechanical functionality. The ballistic nature of electron transport in the CNTs, due to lack of scattering from the other two dimensions promises the application of CNTs in high frequency electronics. The harness of such attributes demands a thorough understanding of the constituent impedances that yield the electrical current response with respect to an applied voltage. In this context, the article discusses issues related to electrical contacts and the dimensional crossover from three-dimensional contacts to the one-dimensional CNTs. A consideration of both the electrostatic and quantum capacitances and inductances is necessary for understanding the limits of CNT-based electronics, such as transistors. The article then considers the possible applications of CNTs, with emphasis on (i) interconnect between devices, (ii) frontier field effect transistors extending Moore’s law, and (iii) electrochemical biosensors. It is finally concluded that a harness of the desirable and attractive features of CNTs, for widespread utilization, yet demands that significant technological and manufacturing hurdles be overcome. Keywords

Carbon nanotubes · Electrical properties · Metal vs. semiconductors · Ballistic transport · Contacts · Quantum · Capacitance · Kinetic inductance · Electrochemical properties · Superconductivity · Interconnect · Transistors · Sensors

Introduction Carbon nanotubes (CNTs) are unique materials both from a one-dimensional point of view (manifesting characteristic quantum mechanical attributes) and the very large length (in centimeters) to diameter (in nanometers) aspect ratio. Consequently, CNTs are being considered as potential candidates for a whole host of applications ranging

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from nanometer scale electronics to large scale structures, e.g., as fillers in composites. In this chapter, we review some of the unique electrical properties of CNT-based materials, peculiar to their low dimensionality, and consider the related utility. In addition to the advantages conferred by miniaturization, carbon-based nanoelectronics promise greater flexibility compared to conventional silicon electronics, arising from the extraordinarily large variety of carbon structures exemplified in organic chemistry. Consequently, nanotubes are also considered as candidates for molecular-level electronics. A clear understanding of the unique electrical properties of CNT-derived structures, enables integration of a diverse set of electrical phenomena into a multifunctional CNT-based device, incorporating logic, memory, and sensor-based modalities. While large-scale assembly of CNTs, at a level that would be impressive to a systems designer, is still challenging, substantial progress has been made recently, e.g., through the realization of a CNT-based computer (Shulaker et al. 2013). Such an application brings to the forefront power savings, radiation hardness, and reduced heat dissipation – all of which are major issues in modern electronics. In this chapter, firstly, the structure that impacts the electrical properties of the basic CNT morphologies, such as single-walled and multi-walled CNTs (SWNTs and MWNTs, respectively) are discussed. While the former variety may be either metallic or insulating, the latter are mostly metallic. Basic issues involving doping to control CNT properties, methods of contact from higher dimensional contacts/ environment, and sensitivity to ambient conditions are relevant topics of concern. A few examples which explicitly involve the electrical and electrochemical characteristics of CNTs, such as a change in electrical conductivity for biomolecular sensing, will be discussed. A most exciting aspect, the application of CNTs to nano-devices incorporating high frequency (THz scale) electronics will then be probed. In this context, the possibility of using CNTs as antennae to transmit/receive information at the nanoscale is an important consideration. The review will conclude with an outlook for the application of the electrical properties of the CNTs in future technology. While electrical properties have been measured on nanotubes synthesized through a variety of methods, this review does not specifically address methods of synthesis and characterization of nanotubes. Briefly, SWNTs and MWNTs have been synthesis by both physical and chemical methods. The former method incorporates arc discharge and laser ablation methods, which seem to have a higher degree of structural perfection, due to the high temperatures (>3000
C) involved in the synthesis. Chemical Vapor Deposition (CVD) enables NT growth at a lower temperature ( EF or (ii) kBT < EF. Case (i), suitable for high temperatures, corresponds to low doping while low temperatures (case (ii)) are typify strong doping conditions. In metallic nanotubes the results of the shifts in the Fermi level are masked by a higher density of states and doping effects are less marked. When connected to external contacts, semiconducting SWNTs may be measured to be either p-type or n-type. Such characteristics could be induced by the work function (WF) of the contact whereby holes (/electrons) could be generated in the nanotube due to electron transfer from (/to) the NT to (/from) the contact. The device characteristics, p-type or n-type are then determined by the relative heights of the Schottky barrier for electron and hole injection at the metal electrode-CNT interface. Given a CNT work function of ~4.5 eV, a high WF metal contact, such as Pt (WF ~ 5.5 eV) or Au (WF ~ 5.3 eV) promotes p-type character in the SWNT. Alternately, a low WF metal such as Er (WF ~ 3.0 eV) or La (WF ~ 3.5 eV) yields a n-type SWNT. It is to be noted that a larger Schottky barrier is present for lower diameter CNTs, with a desirable sharper threshold between the on- and off-states. The maximum electrical current that can be obtained is limited by the thermionic emission over the barrier. It was determined that the annealing in vacuum, through oxygen removal (or doping the surface with alkali metals) converts p-type devices to n-type, which results in a shift in the EF from the valence band to mid-gap. Interestingly, exposure to oxygen resulted in reversion to p-type characteristics. Evidence for charge transfer in doped car- bon nanotube bundles exposed to electron donor (K, Rb) and electron acceptor (Br, I) atoms was also seen through Raman spectroscopy investigations (Rao et al. 1997) through a vibrational mode shift. Generally, intrinsic semiconducting SWNTs cannot be produced whenever there is exposure to oxygen ambient. The effect of oxygen on nanotubes is plausibly not just due to doping, as is conventionally understood but could be related more to the effects on the contacts, considering that the physisorption of the oxygen with CNTs

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is weak and is unlikely to result in charge transfer. A phenomenological model was advanced to explain the p- to n-conversion: Fig. 2, where the concentration of oxygen is proportional to, and determines the position of, the EF at the metal-CNT interface. Such an effect changes the line-up of the bands at the interface but does not involve the bulk of the CNT. When EF at the junction is close to the center of the band gap, the barrier allows tunneling and ambipolar transport is observed. With Au contacts in air, only holes can be injected into the device, while annealing/removal of oxygen results in only

a

VGS < 0

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ANNEALED N-FET electron injection for VGS > 0

STARTING P-FET hole injection for VGS < 0

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N-DOPED no current at any VGS

HIGHLY N-DOPED electron tunneling through the barrier

Fig. 2 (From Derycke et al. (2002)) The effect of ambient conditions on the electronic band structure of a CNT-metal contact. (a) Initially the doping is p-type due to the higher work function of the metal and extrinsic oxygen doping, (b) When the oxygen is driven out, e.g., through an annealing treatment, an n-type behavior is evident in the transistor characteristics, through electron injection into the CNT. However, (c) true n-type behavior is obtained through elemental doping, e.g., using Group I elements such as K, (d) At higher doping levels, electron tunneling occurs through a thin barrier

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electrons being injected due to the high hole injection barriers: Fig. 2b. Subsequently exposing the SWNTs to nitrogen and alkali metal dopants (Derycke et al. 2002) resulted in n-doping. However, in the latter case the strong oxidizing characteristics of the dopants are undesirable, and stable doping can instead be obtained through functionalization by amine-rich polymers (Chattopadhyay et al. 2005) such as polyethyleneimine (PEI). However, given that C is in Group IV of the periodic table, both acceptor type impurities, e.g., B from Group III, and donor type impurities, e.g., As or P from Group V, may also be deployed for producing p- type and n-type doping in the more conventional sense. Additionally, intercalating Group I elements, such as Li, Na, or K have been used for n-type doping, for their electron donating characteristics. Generally, such extrinsic impurity introduction is difficult to control and also have a deleterious effect on the mobility characteristics of the CNTs. Consequently, the ambipolar attributes of CNTs, where either electrons or holes can be passed into the CNT channel, through metal contacts of low and high work functions, respectively (see also sections “Electrical Contacts to Carbon Nanotubes” and “Characteristic Features of Electrical Contacts to CNTs”) are exploited for p- and n-type devices.

Electric Field Profiles in Doped NTs Both p-type and n-type attributes are desirable in a semiconducting SWNT from the point of view of possible construction of a diode with rectifying characteristics. In a SWNT fabricated as a p-n junction, the depletion region is a dipole ring (Leonard and Tersoff 1999), c.f., as opposed to a dipole sheet in a planar junction. Consequently, the depletion width (W) of a nanotube device at low doping levels is found to depend exponentially on the inverse doping fraction ( f ). This factor needs to be considered in device design, where tunneling through the device occurs at f > 2103 while in the opposite case, i.e., with f < 2104, a large depletion width is formed. Even outside the depletion region, the charge distribution is not cut-off abruptly, but has a logarithmic decay with distance along the NT. The consequence is that the depletion region may extend to microns, which makes nanoscale device fabrication challenging. To circumvent such issues, the placement of an intrinsic region between the p- and the n-regions was advocated (Leonard and Tersoff 1999). Interestingly, such issues bring to the forefront the necessity for new modes of thought when materials with intrinsic quantum effects, as related for example to carrier confinement, are used in nanoscale devices. One must then move beyond conventional electrostatics in modeling.

Electrical Conductivity and Resistance in Nanotubes: Applications to Devices The number of carriers (n), as manifested through the density of states as well as the carrier mobility (μ) determine the electrical conductivity (σ) ¼ neμ, formally defined as the ratio of the electrical current density (J ) to the voltage gradient/electric field

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(E) through Ohm’s Law. From the Drude theory, the σ 2D for
the graphene sheet that  2 constitutes a nanotube surface may be derived to be: σ 2D ¼ 2eh ℏvEF le . The elastic scattering length (le) of the carriers is proportional to the electron-phonon scattering and generally increases with decreasing temperature. The electrical conductivity is characterized with respect to temperature, in two regimes: 1. Low temperatures (kBT < EF), where in the conductivity equation above, the energy (E) is replaced by EF. The conductivity in this regime is metallic - a finite zero-temperature value, the magnitude of which is determined by the static disorder, is obtained. 2. High temperatures (kBT > EF), where in the conductivity equation, the E is replaced by kBT. The conductivity, and the carrier density, is then directly proportional to T. At the very outset, it is not trivial to measure the intrinsic resistance of a SWNT. Any contact in addition to those at the two ends of the tube (say, for a four-terminal measurement) can destroy (Datta 1995) the one-dimensional nature of the SWNT and make a true interpretation difficult. For a strictly one-dimensional system the Landauer formula an intrinsic resistance (Rinto), independent of the length,  h  predicts 1 to be equal to e2 T E which translates to a resistance of 25.8 kΩ assuming perfect ð FÞ transmission through ideal Ohmic contacts, i.e., T (EF) equal to one. This contact resistance arises from an intrinsic mismatch between the external contacts to the wire (which are of higher dimensionality) and the one-dimensional NT system and is always present. When one takes individually into account both the two-fold spin and   band degeneracy of a nanotube the intrinsic resistance is now ¼ 4eh2 T 1E which ð FÞ again seems to be length independent. However, in the above discussion, we have not yet considered the contribution of the external contacts. When we consider the transmission (T ) through the contacts into the one-dimensional channel and then to le the next contact, T ¼ Lþl where L is the length of the one-dimensional conductor. The e net resistance, including that from the contacts (Rc) is now equal to:  Rnet ¼

   h h L þ Rc ¼ Rint þ ROhmic þ Rc þ 4e2 4e2 le

ð2Þ

The Rint is intrinsic to the CNT, while the ROhmic denotes an Ohmic resistance associated with scattering. In the presence of dynamically scattering impurities, such as acoustical or optical phonons, which are inevitably present at any temperature above 0 K, the Ohmic resistance should definitely be considered. It is interesting to consider the limiting cases of a large mean free path (le) or a small tube (L ➔ 0) i.e., the ballistic regime, where the Ohmic resistance is seen to vanish. The origin of reduced scattering is related to the reduced phase space for phonon scattering of the electrical carriers in lower dimensional structures. However, the resistance from the contacts contributes the additional term: Rc and may be of the order of 10 kΩ/CNT.

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Such considerations imply that a minimum resistance of (6.5 kΩ) is present in a SWNT with a single channel of conduction. In practice however, imperfect contacts (which lead to a T < 1) and the presence of impurities lead to larger resistance values, while deviations from strict one-dimensionality or multiple channels of conduction (as in a MWNT) could lead to smaller numbers for the resistance. These observations primarily account for the large discrepancy of the numerical value of the resistances in literature. It should also be noted that Eq. (2) applies to a linear regime, with small applied voltages. At higher electrical fields, the electrical resistance arises from the scattering of the electrons from acoustic phonons as well as optical phonons, with a consequent modification, through an effective increase, of the mean free path. Indeed, given that it has generally been considered that for a CNT channel length Lm, then the carrier transfer is complete while on the other hand, if Lm > Lcon, then the transmission probability would be reduced. The Lm is a function of the inhomogeneous and homogeneous lifetime broadening of the CNT states under the metal, which is again related to the contact metal- CNT coupling, the DOS of the metal states, etc. The scaling of the conductance with the Lcon was also noted (Anantram et al. 2000) for the metal contact coupling with the CNTs (considering that a larger diameter nanotube was akin to a graphene sheet), where chirality was observed to play a role as well with better coupling to an armchair type CNT compared to a zigzag variety CNT. Interestingly, disorder was posited to enhance the coupling due to the possibility of a greater variety of coupling mechanisms. If a 1-D characteristic of the CNT-contact is considered, then from Ohm’s law, where the current density (J) is proportional to the electrical field (E), we see that J ¼ I (the electrical current) and E ¼ V/Lcon, with V as the voltage drop. The conductivity (σ ¼ 1/ρ, with ρ being the resistivity) is the proportionality constant, i.e., J ¼ σ E. With ratio of V/I – the resistance (in Ω), the units of ρ1D are Ω/cm. Compared to ρ3D with Ω.cm units, then ρ1D ¼ ρ3D/Area. The relevant area normalization may be implemented through the product of the contact length with the nanotube diameter, i.e., the Lcdt. The Rc could alternately be indicated by the ratio of a contact resistivity: ρC (in units of Ω.nm2) divided by Lcdt. As indicated previously CVD synthesized CNTs offer enhanced carrier mobilities, e.g., to those synthesized through low temperature-based solution processing methods. The latte methods subject the CNTs to a greater variety of purification and suspension processes, all of which are hypothesized to add defects. It was seen, for instance through a comparison of the two types of CNTs, that the μ of the CVD synthesized CNTs could be in the range of 1000–3000 cm2/V.s while “high quality” (Cao et al. 2012) solution processed SWNT mobility would be ~200 cm2/V.s. Consequently, the measured ρ1D was of the order of 20 kΩ/μm for the CVD varieties of metallic SWNTs, and an order of magnitude lower than that seen for the solutionbased moieties. However, the contact resistance (Rc) still seems similar for both kinds, indicating the independence of the end from the body of the CNT.

Characteristic Features of Electrical Contacts to CNTs As was seen through Eq. (2), the Rc is a major variable in determining the power capability of CNT-based devices, such as FETs. For providing higher current drive, many CNTs are placed in parallel, e.g., of the order of 200 CNTs/μm, implying a

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Fig. 4 (From Franklin et al. (2014) (a) The inter-relationship between the desired array resistance (Rc-DEV) as function of the individual CNT contact resistance (Rc), (b) Typical array arrangement of CNTs, where a large array density (increased number of CNTs/μm) is desirable for high current drive. However, at such large densities, the sensitivity to diameter and lithographic variations as well as the mutual charge screening are factors of concern in device operation

CNT spacing of ~5 nm. The Rc for the array is then determined by ratio of Rc for a single CNT divided by the number of CNTs in the array. The intrinsic resistance (of ~6.4 kΩ) for a single CNT is also incorporated into the measured contact resistance. A target value for the array resistance (Rc-DEV) – see Fig. 4, has been indicated to be ~100 Ω.μm, implying that with 200 CNTs/μm, that the Rc for one contact to a single CNT should be ~20 kΩ, and the array resistance would then be ~100 Ω. However, even such a value is difficult to realize in practice for small contact sizes, say at the 10 nm scale, mainly due to inefficient current transfer from the contact to the CNT, arising from dimensional incompatibility. Indeed, the Rc-DEV is barely being met presently with Lcon of the order of 100 nm with substantial enhancement with increased contact length scaling. The enhancement of the metal contact - CNT resistance with smaller contact lengths, is a major issue, especially at the sub-20 nm length scale (Franklin et al. 2014). For instance, six different metals were considered for the contacts, i.e., Ti (WF ~ 4.3 eV), Ni (WF ~ 5.2 eV), Rh (WF ~ 5.0 eV), Au (WF ~ 5.3 eV), Pt (WF ~ 5.7 eV), and Pd (WF ~ 5.6 eV), in conjunction with the WF of the CNT (~ 4.5 eV). It is to be noted that CNT WF is weakly dependent on the radius, i.e., varying inversely with the radius. For relatively large Lcon (> 100 nm) it was indicated that low WF metals such as Ti form Schottky barriers to p-type CNTs, while outstanding Ohmic contacts may be obtained from Pd or Au. However, with Lcon < 20 nm, it was experimentally observed that Rh could prove to be a better choice, for providing a lower Rc – which is not apparent as the Rc with Rh for longer channels is twice as large as that obtained with Pd. The influence of the transfer

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length (Lt), where the applied potential is more efficiently dropped over a smaller length scale, was speculated as one reason for the more efficient scaling with Rh. Such contact scaling studies also need to be done for n-type CNTs, e.g., with Er metal, which only seem to be work up an Lcon of ~70 nm (Franklin et al. 2014), presumably due to severe oxidation related issues. Conventional contact pads to CNTs are placed on top of the CNTs, i.e., sidebonded, and may not be planar. Additionally, considering the curvature of the CNT as well as the aspect that the ends of the CNT there would be dangling bonds may indicate the necessity for a new methodology for placing the contacts. The atomistic aspects related to passing current into the CNT extend to the metal contact, which could be “end-bonded” (Cao et al. 2015). For this purpose, Molybdenum (Mo) – a good carbide former was chosen, so that MoxC is the effective CNT contact material. In spite of the relatively low WF of Mo (~4.7 eV), no evidence of a Schottky barrier was observed, as verified through temperature-dependent measurements. An Rc of the order of 30 kΩ/CNT was extracted through IDS – VDS measurements. Assuming a junction area of ~2 nm2, due to the end contacting, the contact resistivity (ρC) is of the order of 61010 Ω. cm2 – a value comparable to state-of-the-art electrical resistances even in Silicon technology. In retrospect, end-contacts are also desirable as they could ensure high electrical fields, for charge transfer, at the contact-CNT interface.

CNT-Based Field Effect Transistors (FETs) The promise of kΩ) and measurement apparatus – which is typically configured at 50 Ω, it would be advantageous to have a wireless contact through electromagnetic radiation. The radiation pattern for a CNT is comparable to that of a small dipole as the size is small relative to the freespace wavelength (λo). However, the lack of a clearly defined skin depth, in the case of the SWNTs, where conduction only happens along the sheet, implies that traditional electromagnetic considerations would have to be revised. Moreover, the smaller wave velocity in a NT (due to the larger LK) of the order of 106 m/s implies

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a smaller wavelength along the NT and a related momentum mismatch. Considering the resistive losses that are dominant in NTs further diminishes the radiation efficiency from a NT. In summary, a utilization of the NT in the manner of a traditional antenna is inefficient. Alternate notions of a CNT radio (Jensen et al. 2007) rely on mechanical resonances, which may extend to the 100 MHz range, and not on electromagnetic resonances.

Multiwalled CNTs MWNTs are composed of coaxial nanotube cylinders, of different helicities, with a typical spacing of 0.34 nm which corresponds closely to the inter-layer distance in graphite. These adjacent layers are generally non-commensurate (different chirality) with a negligible inter-layer electronic coupling and could alternate randomly between metallic and semiconducting varieties. The layers, constituting the individual cylinders, are found to close in pairs at the very tip of a MWNT, and the detailed structure of the tips plays an important role, e.g., in the electronic and field emission properties of nano- tubes. While there was debate on whether the individual nanotubes close on themselves or the tubes are scrolled, it is presently accepted through the evidence of high-resolution electron microscopy studies that the latter, i.e., a Russian doll model is more likely to be true. Typical MWNT diameters grown by the arc-discharge method are 20 nm, while CVD grown nanotubes can have much larger diameters of up to 100 nm. In literature, a wide variety of filamentous/segmented/ noncontinuous carbon nanostructures are often classified under the category of nanotubes, but which should really be called nanofibers. Larger diameter tubes are found to have a greater density of defects, i.e., vacancies or interstitials.

The Relation of MWNTs to Individual Nanotubes As MWNTs are composed of several coaxial SWNTs, it might be expected that they are not strictly one-dimensional conductors. However, a pseudo-gap was observed in I–V measurements with a power scaling law for the conductance, characteristic of Luttinger liquid (section “Wrapping of a Graphene Sheet to Form Metallic or Semiconducting pffiffiffiffi SWNTs”) like behavior. It has been found that the LL parameter g scales as N where N is the number of tubules screening the charge in the MWNT. It was also determined (Schonenberger et al. 1999) that the current flow only occurs through the outer most nanotube cylinder (which could also result from the contact geometry with deposited metal electrodes). Consequently, many features peculiar to reduced dimensionality can be studied in MWNTs as well. While the mutual interaction between the adjacent coaxial cylinders might be very small, it cannot be completely neglected, and makes for a richer band structure in contrast to SWNTs and comparable to multi-layer graphene, where the inter-plane coupling (which depends inversely with the MWNT diameter) can significantly affect the band

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structure. For instance, in a two-wall nanotube, one metallic and the other semiconducting low energy properties characteristic of metallic tubes predominate while if both the constituent tubes are metallic, a much more complicated situation in terms of band crossings can arise. As an example, it was shown that (Kwon and Tomanek 1998) in a n-wall armchair nanotube n2 avoided band crossing can occur leading to the formation of pseudo-gaps in the density of states. The consequent strong coupling between the electronic states at the Fermi level (EF) and the phonon modes may then cause superconductivity. Additionally, strong band structure modifications can also be expected if the constituent undoped nanotubes were of different chirality. However, in the case of different chirality (say, arm-chair and zig-zag varieties) the total DOS would just be the sum of the individual density of states.

Electrical Transport Characteristics of MWNTs The electrical conductivity of MWNTs can be modeled as compatible to that of independent graphene sheets. When the tube diameter (dt) is smaller than the elastic mean free path (le), the one-dimensional ballistic transport predominates, while, if dt is larger than le, the current flow could be described as diffusive/two-dimensional transport. Another quantity of importance is the phase coherence length (lpc) which was determined, through an elegant experiment (Bachtold et al. 1999) exploiting the Aharonov-Bohm effect, to be 250 nm, even larger than the diameter of the MWNT! However, the value of lcp inferred from direct I–V measurements was 20 nm; the discrepancy could arise from poor Ohmic contacts. Another source of difference could also be due to the quality of the MWNTs; higher temperature growth processes (say, arc-growth) synthesize cleaner MWNTs and exhibit metallic temperature dependence, where the resistance linearly decreases with temperature. Coulomb Blockade effects, which are almost always observed in SWNTs at low temperatures, do not seem to be particularly relevant for MWNTs. Quantized conductance, corresponding to integer fractions of the conductance 2 quantum: Go ¼ 2eh and related ballistic transport, was measured (Frank et al. 1998), at room temperature, in a single MWNT mounted on a scanning probe microscope (SPM) tip and dipped into liquid mercury metal. While a conductance of 2Go should have been observed in the absence of magnetic fields, it was assumed for the interpretation of the experimental results, that the spin degeneracy was resolved through electron-lattice structure coupling. In yet another experiment, where the MWNT was grown in situ on a tungsten contact, for better contact resistance, and probed with a W tip, a conductance of up to 490 Go (corresponding to a current of 8 mA in a 100 nm diameter MWNT) was observed (Li et al. 2005), characteristic of multi-channel quasi-ballistic transport. Generally, measurements of nanotubes placed on/below metal electrode contacts on substrates suffer from nonreliable Ohmic contacts – a recurring theme in electrical characterization of nanoscaled materials. In measurements, using contacting electrodes patterned through STM lithography, it was shown (Langer et al. 1995) that at low temperatures (20 mK)

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electron interference effects typical of disordered conductors are present in the transport characteristics. A logarithmic decrease of the conductance with temperature followed by a saturation was taken to be the evidence for two-dimensional weak localization (Datta 1995) effects. Further evidence of localization phenomena is manifested through the observation of a negative magnetoresistance (MR) and a low value (1. Nevertheless, many research studies have discussed about this simple statistical model to correct them taking several other aspects into account, such as corrections about the dimensions (waviness of the CNT, as it is not always completely straight) and the interaction of CNTs themselves which leads to find lower percolations threshold than predicted from the statistical percolation model,

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which is commonly referred as kinetic percolation. This kinetic percolation assumes the movement of CNT in the matrix that leads to interact and create electrically conductive pathways at lower contents (Bauhofer and Kovacs 2009).

Critical Parameters for Electrical Conductivity of CNT Nanocomposites The determination of the percolation threshold, thus, is a key parameter to understand the electrical properties of CNT/polymer nanocomposites. However, there are other key factors that determine the final electrical properties of the nanocomposites: • The volume fraction of CNTs in the nanocomposite, as explained previously in the graph of Fig. 10. • Morphological aspects such as CNT length, diameter, and the relation between them, which is known as aspect ratio, as it is directly correlated to the percolation threshold, estimated from Eq. (14): Λ ¼ L=d

ð14Þ

being Λ is the aspect ratio of the nanoparticle, L its length, and d its diameter. • CNT/polymer interface, as it might affect the electrical conductivity, mainly when tunnelling conduction is more involved in the overall electrical conductivity. • The CNT dispersion reached into the polymer matrix, meaning not only the size of CNT aggregates but also the homogeneity of CNT distribution through the matrix (X. F. Sánchez-Romate et al. 2019). Taking all these aspects into account, the type of CNT used, CNT additional treatment and dispersion technique used during processing will affect all the abovementioned parameters that would lead to a different electrical conductivity reached in the final nanocomposite.

Morphology of CNTs One of the most important aspects regarding the electrical conductivity reached in the nanocomposite is the aspect ratio of the CNT which can be strongly increased by increasing the length of the CNT. The use of SWCNTs instead of MWCNTs also increases the aspect ratio of the nanofiller because of the diameter reduction, and, moreover, at the same volume fraction, they allow to have higher number of individual nanoparticles available to create the electrically conductive pathways. However, an increase of the aspect ratio usually leads to an increase of the entanglement of the CNTs. The entanglement can be qualitatively and quantitatively described by the CNT waviness. In this regard, an increase of the CNT waviness reduces the effective aspect ratio of the nanoparticles and, thus, leads to an increase

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Fig. 11 Estimated waviness ratio obtained for different CNT types as a function of CNT content. (Reproduced with permission from X. X. F. Sánchez-Romate (2019))

of the percolation threshold and, consequently, to a decrease of the electrical conductivity of the final nanocomposite (X. F. Sánchez-Romate et al. 2016). Furthermore, the morphology of the CNTs is not only affected by their structure (i.e., SWCNT or MWCNT) but also is affected by the possible interactions between neighboring nanoparticles. In this regard, an increasing amount of CNTs leads to an increase of CNT-CNT interactions that may lead to a higher entanglement and, thus, higher CNT waviness. Here, Fig. 11 shows the estimated waviness ratio from the model developed in (X. F. Sánchez-Romate et al. 2016). This increasing waviness ratio with nanoparticle content is observed for every type of CNT. In addition, it can be stated, as commented before, that the CNTs with higher aspect ratio also show a higher entanglement. Moreover, the effect of CNT functionalization also has a deep impact in the electrical properties, and it will be discussed in the next section.

CNT/Matrix Interphase Modification As it has been previously mentioned, two main effects take place when talking about the effect of interphase matrix/CNT on electrical properties of the final nanocomposite: the contact between adjacent CNTs and the tunnelling transport between neighboring CNTs. The latter is related to the formation of a polymer film strongly adhered to the CNTs covering them completely. They can be chemically bonded, creating an interphase between them that reduces the possibilities of direct contact between nanotubes and increasing the possibility of tunnelling effect based conductive mechanisms with a barrier between them. Therefore, the chemical functionalization may play a dominant role in this type of interactions.

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Guadagno et al. (2011) observed that, despite having the same intrinsic electrical conductivity for functionalized and non-functionalized MWCNTs, differences between the electrical conductivity at nanocomposite level were noticeable. Percolation threshold arose from 0.1 wt.% for non-functionalized MWCNT to 0.5 wt.% for COOH-functionalized CNTs and the electrical conductivity above the percolation threshold increased slower when using this last type of CNTs, requiring almost 4 wt. % of CNTs to reach maximum electrical conductivity which was three orders of magnitude lower than that for non-functionalized MWCNT at 0.5–1 wt.%. The only explanation found for this behavior was the change in tunnelling resistance between nanotubes due to insulating films with different thickness wrapping the CNTs. Tunnelling resistance suffers an exponential increase when the thickness of the insulating film does, thus, reducing the overall electrical conductivity. Moreover, as it is mentioned before, percolation threshold calculations for these systems are usually based on kinetic percolation, that is, supposing filler movement which in the case of functionalized MWCNTs is limited, thus, increasing the experimental percolation threshold from the one predicted. Furthermore, the inclusion of functional groups may also induce radial distortions on the CNTs (Milowska et al. 2012) that could cause an increase in the waviness ratio and thus, in the percolation threshold. In fact, this effect was observed in the graph of Fig. 11 where the nanocomposites manufactured with functionalized CNTs showed higher estimated waviness ratios than the non-functionalized ones. Taking all these aspects into account, functionalization of MWCNT is usually found to be detrimental for the electrical properties leading to higher percolation thresholds because of the formation of thicker insulating films surrounding the CNTs, explained by the formation of structural defects in the graphitic lattice when they are high enough and because the mechanical distortions induced during the chemical functionalization process. Other aspect such as the CNT dispersion are also affected and will be discussed in the next section.

CNT Dispersion The dispersion process carried out to incorporate the CNTs into the hosting matrix has a very prevalent effect on the electrical properties of the final nanocomposites. There are multiple aspects that are affected by the dispersion procedure, but they can be divided mainly on changes in dispersion level (size, number, and distribution of aggregates) and changes in CNT morphology (waviness, length, and, consequently, aspect ratio). Focusing on the dispersion level reached, the addition of different types of CNTs with the same dispersion procedure can be used to study this aspect as the dispersion state will be different because of the different specific surface and, consequently, different tendency to keep entangled. In this regard, several studies have explored the differences among MWCNTs, DWCNTs, and SWCNTs in the electrical properties of the final nanocomposite. It has been proved that the higher specific surface of SWCNTs results in a poor dispersion and, consequently, in a lower electrical conductivity and higher percolation threshold than nanocomposites based on MWCNTs, which reach better dispersion levels (Cortés et al. 2021). Figure 12

X. F. Sa´nchez-Romate et al.

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a

b

d

Fig. 12 (continued)

c

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clearly illustrates this statement showing the relation between the dispersion level reached by means of transmitted light optical microscopy (TOM) images and the aggregate size distribution, and, on the other hand, the electrical conductivity of the respective nanocomposites manufactured with each type of nanocomposites. As commented before, functionalization may also play a prevalent role during dispersion procedure. It has been noticed that the incorporation of amine groups to the MWCNT influences the dispersion level reached after three-roll milling processing, consisting in a progressive reduction of the gaps between rolls, due to stronger interaction between CNTs. It was clearly observed that the disaggregation of this type of CNTs was only partially reached while non-functionalized CNTs were more disentangled. The higher number of aggregates when using amine functionalized CNTs was also correlated for the same type of CNTs in (X. F. Sánchez-Romate et al. 2016), assuming that CNTs present higher waviness inside the aggregates leading to lower effective aspect ratio, as previously stated. On the other hand, dispersion techniques used may lead to changes in the morphology/geometry of the CNTs, thus, leading to variations in the electrical properties induced by this modification of their length. One of these approaches is the ball milling treatment of CNTs to break and reduce the number of aggregates. Nevertheless, this treatment can affect the length and, consequently, the aspect ratio of CNTs leading to an increase of the percolation threshold observed when ball milling is applied (Krause et al. 2011). The balance between the disaggregation caused and the damage/shortening of CNTs induced will determine the electrical properties of the nanocomposite. Increasing energy of the milling process, by an increase of the frequency during milling or increasing the milling may cause excessive damage, thus, reducing the electrical conductivity. Azizi et al. (2019) showed this behavior with a slight increase observed at lower times and milling energy but causing excessive damage when the time and milling frequency were increased.

CNT Orientation Orientation of CNTs into the polymer matrix is also an important parameter that governs the electrical conductivity of the nanocomposites. This is a fact that has been widely explored by several authors to create anisotropic materials with improved properties in preferred directions. The two main approaches are the use of or electric fields taking advantage of the magnetic susceptibility of CNTs or by the incorporation of additional functional groups either to increase the response to the electric or magnetic field applied. Tanabi and Erdal (2019) applied a magnetic field of 0.2 T to mixtures or pristine CNT with epoxy during the curing process. The presence of a residual catalyst of CNT production such as Fe, Ni, or Co, increased the response of CNTs to the low

ä Fig. 12 Dispersion degree and properties of printed nanocomposites: (a) TOM micrographs, (b) fractional area occupied by CNTs, (c) aggregate size distribution, and (d) Tg of the nanocomposites. (Reproduced with permission from Cortés et al. (2021))

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intensity magnetic field applied, thus, promoting a preferential CNT orientation. By scanning electron microscopy, they checked the preferential orientation of CNTs. This preferential orientation was employed for determining the electrical conductivity obtaining that percolation probability should take place at lower amount of CNTs added as the contact between oriented CNTs is favored. Experimental results were based on electrical conductivity measurements at a fixed CNT concentration over the percolation threshold and they corroborate that preferential orientation decreased the electrical resistivity of the samples when a magnetic field was applied during curing. Several experiments were also carried out to promote CNT orientation by magnetic fields (Prolongo et al. 2016). Here, the intensity of the magnetic field and the use of pristine MWCNT or magnetite modified MWCNTs were studied as parameters of the process. It was concluded that moderate magnetic fields of 0.3 T induced partial orientation of pristine MWCNT but when they had magnetite nanoparticles anchored preferentially on their ends, they showed higher orientation capability and higher electrical conductivity. Also, it was proved that higher magnetic fields (≈0.5 T) could even promote movement of the MWCNTs in the polymer matrix, at least at low concentration which means moderate viscosity, and which results in areas of higher electrical conductivity (higher MWCNT concentration) and insulating ones (with lower content of MWCNT, below the percolation threshold).

Applications of CNTs Therefore, the commented exceptional physical properties, especially the electrical ones, makes CNT suitable for a wide range of applications that are described below.

CNT for Electrical Applications CNTs can be used as optically transparent electrodes (OETs) that are essential components of organic solar cells. Here, CNTs offer the advantage of a higher flexibility in comparison to the conventional ones, based on rare metals. The CNTs must be processed into a film with strong CNT-CNT junctions. Moreover, the extremely high surface area of CNTs makes them also very suitable as electrodes in Li-ion batteries, promoting a high intercalation of the Li ions and thus, enhancing the specific capacitance, a key parameter in this type of applications. CNTs can also be used in a wide range of electronic applications, such as rectifiers, that are two-terminal devices that only allow the current flow for one polarity of applied voltage. Field effect transistors are other of the best applications of the CNTs. They are usually formed by a CNT bridging two Pt electrodes and sitting on SiO2 substrates. Here, the device can change from low to high conductance depending on the gate voltage, thus, providing the desired behavior of the transistor. Finally, their excellent electromechanical properties can be used for the development of electromechanical oscillators. These devices consist in a CNT bridging two

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metallic electrodes and suspended. Here, when an AC current is applied, it causes the CNT to mechanically oscillate. This effect may lead to a change in the capacitance between the nanotube and the gate. Furthermore, their very high piezoresistive response makes them an interesting alternative for strain sensing purposes. Here, they can be used for detecting small strains at the nanoscale, as previously explained, by correlating the changes in the intrinsic electrical conductivity with the applied strain. However, to date, their interest as strain sensors is mainly devoted to the manufacturing of polymer nanocomposites.

CNT/Polymer-Based Strain Sensors As commented before, the addition of CNTs into a polymer matrix induces an enhancement of the electrical conductivity due to the creation of preferential electrical pathways. Here, the strain sensing mechanisms are based on the intrinsic piezoresistive response of the CNTs themselves jointly with the direct contact and tunneling transport between adjacent or neighboring CNTs. More specifically, the tunneling transport plays a dominant role in the electrical network in the nanocomposites, as stated before. In this regard, the electrical resistance associated to the tunneling transport, also called tunneling resistance, Rtunnel, has a linear-exponential correlation with the tunneling distance, t, as stated by the well-known Simmons formula (Simmons 1963): Rtunnel ¼

Ae2



h2 t 4πt pffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi exp 2mφ h 2mφ

ð15Þ

being h the Planck’s constant, m and e the electron mass and charge, A the area in which electrical transport takes place, also called tunneling area, and φ the height barrier of the matrix. Therefore, the tunnelling resistance varies in a linear-exponential way with the applied strain, a fact that prevails for strain sensing purposes. In this regard, this type of materials can be used for structural health monitoring (SHM) applications, where they must deal with structural resins, or as wearable strain sensors, by using much more flexible hosting systems.

Structural Health Monitoring Applications CNTs for SHM purposes not only involve the manufacturing of CNT/polymer-based nanocomposites but also the development of multiscale composite materials. Here, multiscale composites are defined as fiber-reinforced composites with matrices doped with CNTs. The multiscale definition refers to the presence of a microscopic reinforcement (the fiber) and a nanoscopic one (the CNTs). In this regard, their interest lies in the fact that the CNT addition also acts as a mechanical reinforcement of multiscale materials, improving some mechanical properties such as the interlaminar shear strength or the fracture energy.

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There are a lot of studies proving the SHM capabilities of CNT multiscale composites. Here, a key parameter is the GF of this system, which is usually far below the intrinsic GF value of CNTs due to piezoresistive effect, which was highly discussed in previous sections. In multiscale composites and nanocomposites, GF depends on the volume fraction of the CNTs but also on their disposition inside the hosting polymer matrix. In this regard, two main statements can be made: • As a general fact, the GF decreases with increasing the CNT amount, as observed in the graph of Fig. 13a This fact is explained by the less interparticle distance between neighboring particles, which induces a reduction of the tunnelling resistance. Therefore, the highest sensitivities are usually achieved for CNT volume fractions near the percolation threshold. • CNT dispersion also plays a dominant role. Usually, a good CNT dispersion leads to a prevalence of tunnelling conducting mechanisms, maximizing the sensitivity of the system (Fig. 13b). On the opposite side, a higher aggregation leads to a higher preference for conducting mechanisms through the aggregates, which barely varies with applied strain (X. F. Sánchez-Romate et al. 2019). Therefore, the sensitivity of the manufactured systems proves their applicability for SHM purposes of structural components.

Flexible Wearable Sensors One of the most interesting application of CNTs, nowadays, lies in the development of flexible wearable sensors. Here, the main difference with the structural components is correlated to the higher flexibility of the hosting matrix. This higher flexibility can be used for the detection of a wide range of strains. More specifically, most of the developed flexible sensors are used for human motion monitoring. Here, CNT/polymer strain sensors have proved an excellent sensitivity (Amjadi et al. 2016). Therefore, they can be used for detecting articulation motion such as fingers, shoulders, or knee motion and, even at very low strain levels, where they have shown very high sensitivity for vocal cord’s monitoring, for example (Wang et al. 2014). The reason for the extremely high sensitivity of these systems lies in the fact of the role of the hosting matrix in the electromechanical properties of the final nanocomposite. It has been observed that, generally, flexible systems present higher potential barriers. Therefore, the tunneling resistance change with applied strain would be higher, as expected from the expression of Eq. (15) and, thus, the sensitivity would be increased. In addition, the presence of a more flexible hosting system may allow the CNTs to be more strained, increasing the contribution of the intrinsic piezoresistive effect to the global electrical properties of the nanocomposite. Furthermore, another key parameter for their potential application as wearable sensors is the high reversibility of the electrical response of these systems making them very reliable for human motion monitoring.

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a

'Rtunnel/R0

Low CNT volume fraction (high sensitivity)

High CNT volume fraction (low sensitivity)

tunelling distance

b

Fig. 13 (a) Electrical resistance change with tunneling distance, indicating the influence of CNT volume fraction on the sensitivity of the system and (b) influence of dispersion state on the GF values were φ denotes an aggregation parameter that decreases with increasing GF. (Reproduced with permission from X. F. Sánchez-Romate et al. (2019))

CNTs as Resistive Heaters CNTs can be also used as resistive heaters taking advantages of the Joule’s effect heating induced when applying an electric field, as observed in the schematics of Fig. 14. Here, the current flow generates heat accordingly to Joule’s law:

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Fig. 14 Schematics of Joule’s heating effect in a CNT network

Q ¼ i2 R t

ð16Þ

where Q is the heat generated, i the current flow, R the electrical resistance, and t the time the current is applied. This generated heating can be used for a wide range of applications such as resistive heaters or deicing systems. The latter has a great interest in the wind industry, for example, where the ice formation in the wind blades may lead to a detriment on the efficiency of the system. In this regard, there are some studies exploring the deicing capabilities of CNT/polymer coatings, proving that, even at very severe ambient conditions, these systems work well for deicing purposes (Cortés et al. 2020).

Conclusions The electrical properties of intrinsic CNTs and CNT bundles have been widely explored. It has been observed that the CNT structure, their electronic nature, and the measurement type have a significant effect on their electrical properties. Apart from the determination of the intrinsic electrical conductivity, other effects such as the contact resistance have been also explored, providing some ways to reduce them, as it is an important factor to be removed to determine the actual intrinsic electrical properties of CNTs. Piezoresistive response of CNTs has been studied and the influence of chirality, electronic nature, and type of induced strain has been determined. It has been observed that, depending on the CNT structure, they can exhibit a positive, negative, or near-zero piezoresistive response to uniaxial and torsional strain. Furthermore, the electrical properties of CNT yarns have been also explored. Here, the CNT-CNT contacts and the porosity of the yarn play a dominant role on the electrical and electromechanical properties. For this reason, the sensitivity of CNT yarns is usually significantly lower than the sensitivity of the intrinsic CNTs. Moreover, the manufacturing of CNT-based nanocomposites and their electrical properties have been widely studied, as they have gained a great deal of attention in the last decades. Here, the intrinsic electrical conductivity of CNTs does not play a significant role, but the contact and tunneling resistance between neighboring nanoparticles, which depends on the morphology of CNTs, dispersion procedure,

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or other factors such as functionalization and preferential orientation of nanoparticles. Finally, some interesting electrical applications of CNTs have been described since their use as electrodes to their use in CNT/polymer strain monitoring or deicing systems, proving the huge potential and applicability of these fascinating materials.

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Field Emission from Carbon Nanotube Systems: Material Properties to Device Applications

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M. Sreekanth, S. Ghosh, and P. Srivastava

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Field Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . History of Field Emitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNT-Based Field Emitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methods Used to Enhance the Field Emission Properties of Carbon Nanotubes . . . . . . . . . . . . . . Various Synthesis Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pattern Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Low Work Function Material Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interlayer Between Substrate and CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNT Field Emitter-Based Device Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Electron sources are indispensable for many scientifically and technologically important applications like electron microscopy, e-beam lithography, X-ray generators, microwave amplifiers, etc. There is a need to replace thermionic electron sources due to their high power consumption, lower emission currents, and less durability caused by long-term emitter heating. Among several nanostructures explored in pursuit of suitable field emitters, carbon nanotubes (CNTs) are one of the best field emitters till date due to their fascinating properties such as high aspect ratio, high electrical conductivity, good thermal and chemical stabilities, high mechanical strength, and functionalizability. Despite their outstanding field emission properties in terms of turn-on and threshold fields, local field M. Sreekanth (*) Centre for Nano Science and Engineering, Indian Institute of Science, Bengaluru, India e-mail: [email protected] S. Ghosh · P. Srivastava Department of Physics, Indian Institute of Technology Delhi, New Delhi, India © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_61

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enhancement factor, and electron emission currents, there are still certain drawbacks associated with CNT based field emitters like high contact resistance and poor adhesion between CNTs and bare substrates, relatively higher work function of their tips, nanotube tip damage caused by the ion bombardment, high electric field screening among adjacent CNT emitters on large area substrates and poor temporal stability. To overcome these drawbacks, several pre-/post-synthesis methods were developed such as various synthesis methods, the lithography-assisted deposition, the low work function material coating, an interlayer/buffer layer deposition between the catalytic layer and the substrate, structural modification of CNT films, etc. In this chapter, firstly the basics of electron emission and its types, the brief history of field emitters and CNT-based field emission systems were discussed. Next, the methods employed to enhance the field emission properties of CNTs were explored. Finally, some important CNT-based FE applications were mentioned. Keywords

Field emission · Carbon nanotubes · Chemical vapor deposition · Work function · Contact resistance · Lithography

Introduction Field Emission In general, an electron emission takes place from the material surface in different ways such as thermionic emission, electric field assisted emission or field emission, photoemission, optical emission, etc. In field emission, the electron emission is induced from a material into vacuum in the presence of a strong applied electric field. For thermionic and photo-emission processes, an electron has to cross the surface potential barrier or should possess the work function energy, but the field emission is a quantum mechanical tunneling process where emission of electrons takes place from the Fermi level of an emitter. For flat metal surfaces, the electron emission is induced at electric fields of 107–108 V/m. Although the electron emission-related research was traced back to the late eighteenth century (Winkler 1744), it was intensified only after the discovery of electron by J.J. Thomson in 1897. In 1914, for the first time, Walter Schottky tried to develop the theory of thermionic emission (Schottky 1914) but later on, it was formulated by Richardson in 1921 (Richardson 1921). By calling it “auto electron emission,” Lilienfeld was the first one to report the field emission process in 1922 (Lilienfeld 1922). Afterward, various research groups, especially led by Millikan, Gossling, Lauritsen, etc., were the notable ones which contributed extensively in this area. In the process of developing the field emission theory, Millikan and Lauritsen proposed that an emitter’s surface atoms are responsible for the field-induced electron tunneling whereas Oppenheimer proposed that this tunneling is the reason for the field ionization of atoms in 1928 (Millikan and Lauritsen 1928). During the

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Fig. 1 Schematic representation of the field electron emission process

same time, Fowler and Nordheim proposed the theory of field emission on the basis of Fermi-Dirac statistics and Sommerfeld’s free electron model (Fowler and Nordheim 1928). This theory demonstrates that field emission is a quantum mechanical tunneling. This theory explains the field emission theory proposed by Millikan and Lauritsen and also the temperature-independent electron emission in the presence of an applied bias/electric field. Figure 1 shows the schematic of the field emission mechanism. In the presence of the applied field, the potential barrier becomes triangular as shown in Fig. 1, where electrons from the material surface tunnel through. As per Fowler-Nordheim theory, the field emission current density of an emitter is given by J¼

 3 Aβ2 E2 Bϕ2 exp  ϕ βE

where A (1.544  106 AeVV2) and B (6.83  109 eV3/2Vm1) are constants, β is the field enhancement factor (β-factor), E is the applied electric field between electrodes, and φ is the work function of the emitters. Figure 2 represents a basic high vacuum field emission measurement system in which both the electrodes are located in a high vacuum chamber and separated by a fixed distance. These electrodes are connected to a high voltage power supply and an electrometer to generate the electric field between anode and cathode, and measure the field emission current, respectively. The vacuum chamber is evacuated using the roughing (e.g., rotary pump) and high vacuum pumps (e.g., turbo molecular pump) which are connected in series with the chamber.

History of Field Emitters The first generation field emitters were based on the metallic emitters namely, Tungsten (W) and Molybdenum (Mo) emitters. For the first time, the etched W

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Fig. 2 Schematic representation of a basic field emission system

wire was used as the field emitter by Irwin Müller in 1937 (Müller 1937), and the same was used for very first designed field emission as well as field ion microscopes. Afterward, C.A. Spindt fabricated thin-film field emitters made of Mo cones in 1968 (Spindt 1968), and these emitters were applied for vacuum microelectronic applications (Spindt et al. 1991). F.C.K. Au also reported the thin-film-based field emitters using laser-ablated silicon (Si) nanowires in 1999 (Au et al. 1999). Later in 1970, J. Nishida fabricated silicon carbide whiskers based field emitters (Nishida 1967). Again in 1972, F.S. Baker demonstrated a new field emission electron source made of carbon fibers (Baker et al. 1972). Afterward, the field emission properties of various diamond structures were studied extensively because of their intriguing properties like low or negative electron affinity, good chemical stability, and ability to make p- and n-type diamond structures through doping (Wang et al. 1991; Geis et al. 1991). After their discovery in 1991 by S. Iijima (Iijima 1991), carbon nanotube (CNT) based field emitters have been extensively investigated due to their intriguing properties such as high aspect ratio, nanoscale tip, good electrical and thermal conductivities, good chemical stability, and their ability to grow directly on substrates. Apart from these properties, they also possess various other important properties like good elasticity, high mechanical strength, high flexibility, low thermal expansion co-efficient, etc. Overall, they have been investigated for wide variety of applications extensively for more than two decades due to their outstanding properties.

CNT-Based Field Emitters For the first time, A.G. Rinzler et al. and W.A. de Heer et al. demonstrated field emission from single/individual CNTs and multi-walled CNT films, respectively (Rinzler et al. 1995; de Heer et al. 1995). Within a few years, although CNT-based field emitters emerged as one of the best field emitters in terms of turn-on and threshold fields, local field enhancement factor, and emission currents, there are

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some drawbacks such as poor adhesion and high contact resistance between CNTs and substrate, high work function, field screening between adjacent CNTs, emitter tip deterioration due to ion bombardment, lack of device stability due to unstable emission, and expensive synthesis techniques. Several pre- and post- CNT synthesis techniques have been employed to address these drawbacks and improve the overall field emission properties of CNT systems.

Methods Used to Enhance the Field Emission Properties of Carbon Nanotubes Various Synthesis Methods Several synthesis techniques were developed to enhance the field emission properties of carbon nanotubes (CNTs) by improving the tube quality. In this regard, vertically aligned CNTs (VACNTs) are better field emitters than randomly grown CNTs (RGCNTs) as the defects and amorphous carbon present in RGCNTs adversely affect the field emission properties. In the field emission process, during the electron transportation across the emitters, the defects act as electron scattering centers (Pandey et al. 2013), and the amorphous carbon present at emitter tips increases the work function, both of which hinder the electron emission significantly (Tanaka et al. 2004). The very first reports on CNT film emitter or field emitter array used aligned CNTs. These vertically aligned films were made by drawing CNT suspension using a ceramic filter having a pore size of 0.2 μm (de Heer et al. 1995). Field emission properties of self-oriented arrays of CNTs grown on iron catalyst patterned porous silicon were investigated. Here, porous silicon increases the growth rate of nanotubes and strongly supports the catalyst particles present in the pores (Fan et al. 1999). Field emission properties of well-aligned CNTs grown on glass substrate patterned with nickel-based lines using microwave plasma chemical vapor deposition (CVD) were reported (Murakami et al. 2000). Similarly, field emission characteristics of vertically well-aligned CNTs grown on Nickel coated glass substrate using hot filament plasmaenhanced CVD were also reported (Han et al. 2000). Inspired by a simple and costeffective pyrolysis technique for the VACNT growth developed by Mahanandia et al. (Mahanandia and Nanda 2008), the field emission properties of these VACNTs grown on Si substrate using a solution based precursor without any carrier gas have been studied in diode geometry (Sreekanth et al. 2018a, b).

Pattern Substrates During the field emission process, CNT field emitters experience the field screening effect between the adjacent nanotubes, which results in the lowering of an average local field enhancement factor at their tips. As per Fowler-Nordheim theory, the field emission current density (J) depends on β-factor exponentially as J increases when

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β-factor increases. This means, the electric field screening adversely affects the field emission properties, especially in the case of large area or dense CNT emitters. To circumvent this drawback of CNT field emitters, various approaches including catalyst patterning for the growth of different CNT geometries, dielectric filling between emitters, etc., were studied. For the first time, the electric field screening or shielding effects were studied for dense CNT thin film emitters, especially for randomly oriented multi-walled CNTs grown by microwave plasma-enhanced CVD using the gaseous mixture of 2% CH4 in H2 gas (Gröning et al. 2000). The reasons for observing high field enhancement factors for lower dense CNT regions/films as compared to relatively high dense regions/films are high aspect ratio and low electric field screening among neighboring CNTs. This was explained using an electrostatic field distribution around the CNT electrode (cathode) through numerical calculations using MACSIMION Ver. 2.04 by considering the tube length/height and radius of curvature were 1 μm and 2 nm, respectively. It is clear from Fig. 3a that β-factor rapidly increases and becomes highest at 2 μm of inter-tube distance, that is, twice the length of the tubes but β-factor saturates for higher inter-tube distances (d>2 μm). Similarly, from Fig. 3b, it is well evident that the current density is maximum for the inter-tube distance of about 2 μm, that is, close to twice the length of the tubes again. Beyond 2 μm of inter-tube distance, the current density is decreasing although β-factor remains constant and is due to the lowering of the emitter tube density. For an inter-tube distance of below 2 μm, the current density is dramatically decreasing as the β-factor decreases rapidly. This shows that the inter-tube distance plays a crucial role in the field emission properties of large-area CNT films. Initially, the micro- and macro-scopic field emission analyses of patterned CNTs grown on Si substrate were investigated using printed catalytic inks of Fe (NO3)3.9H2O in the presence of a mixture of C2H2 and N2 (Nilsson et al. 2000). A large area phosphor screen as anode and Pt-Ir tip of radius 2–5 μm as cathode were used for the macroscopic and microscopic analyses of the emitters, respectively. From a large area anode based measurements, it was observed that no significant change was obtained for patterned CNT films of different tube densities as all these films start emitting around 2–3 V/μm but the emission is not homogeneous. This is attributed to a few high aspect CNTs present on the sample surface where low aspect ratio CNTs are not detected. Scanning field emission tip was employed for thorough field emission characterization of patterned CNT films with three different densities as shown (see SEM images) in Fig. 4. For a highly dense CNT film, an inhomogeneous emission pattern is observed, and similar and even sparse emission sports are seen for a low-density CNT film. But, a very clear and relatively homogeneous pattern is seen for a medium dense CNT film. Here, the poor field emission from the dense film is due to the electrostatic field screening among adjacent emitters whereas in the case of low dense films, poor emission is attributed to the short, bent, and non-protruding structures of emitters. But for medium dense films, both inter-tube distance and

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tube density are sufficient enough to give better field emission and a homogeneous pattern. Using microscopic analysis, it was observed that the nanotube density which governs the β-factor plays a significant role in the field emission properties of nanotubes. These experimental findings were verified again through electrostatic field calculations between electrodes as shown in Fig. 5 similar to the previous report (Gröning et al. 2000). In a similar way, the field emission properties of VACNTs in terms of tube diameter, length, and density for both macroscopic and microscopic analyses were investigated (Chhowalla et al. 2001). Later, the effect of field screening on field emission properties of highly ordered CNTs grown on anodic aluminum oxide (AAO) template was investigated (Suh et al. 2002). Field emission properties were studied under the fixed inter-tube distance by

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Fig. 4 SEM images of patterned CNTs with (a) high, (b) medium, and (c) low density along with their field emission image patterns (d)-(f). (Reproduced from (Nilsson et al. 2000), with the permission of AIP Publishing)

altering the tube heights. Figure 6 shows the SEM images of uniform CNTs with different inter-tube distances, 104 nm and 65 nm, respectively. Field emission measurements were performed for CNTs of five different heights at each inter-tube distance as shown in Fig. 7. From J-E characteristics, it is evident that better field emission is achieved for films with tube heights of 121 nm and 64 nm, and from the plot of β-factor versus tube height, as shown in Fig. 8, better

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Fig. 5 Simulated electrostatic field penetration between adjacent nanotubes with different inter-tube distances. (Reproduced from (Nilsson et al. 2000), with the permission of AIP Publishing)

field emission is expected from CNTs having tube heights 99 nm and 95 nm as they can have relatively high β-factors. It was concluded that under the constant/fixed inter-tube distance, the tube height of ordered CNTs plays a crucial role to get better field emission properties by reducing the field screening among neighboring emitters. Apart from studying the large area CNT bundle and catalyst patterned individual CNT films, efforts have also been employed on investigating field emission properties

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Fig. 6 SEM images of CNT films synthesized on AAO templates grown in (a) oxalic and (b) sulfuric acids. (Reproduced from (Suh et al. 2002), with the permission of AIP Publishing)

of VACNT pillars or arrays and their post-treated structures. Similar to achieving the enhanced field emission properties by optimizing the inter-tube distance and tube height in terms of reducing the field screening effect, the field emission properties of VACNT pillar arrays were investigated (Katayama et al. 2004). In this work, CNT pillars were synthesized on Si substrate using patterned Fe catalyst (through photolithography) and thermal CVD. Figure 9 shows the CNT pillar film with marked inter-pillar distance (R) and pillar height (H). For pillar emitters also, as seen for individual CNTs, the better field emission properties are achieved when inter-pillar distance is twice the pillar height, that is, R/H¼2. Despite the large pillar diameter (50 μm), their β-factor was estimated to be 14000. This high β-factor was attributed to the highly concentrated electric field on CNTs present at the periphery of the pillars and thus, these CNTs act as major emission sites.

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Fig. 7 Field emission J-E characteristics of CNT films with diameters (a) 38 nm and (b) 19 nm for five different tube heights as mentioned in the plots. (Reproduced from (Suh et al. 2002), with the permission of AIP Publishing)

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intrinsically good field emitters (except having a few drawbacks) due to their outstanding properties, still great efforts were made to further improve their field emission properties by reducing their high work function (~5 eV). Earlier, cesium (Cs), a low work function (~2.1 eV) alkali metal was coated on Mo tips and diamond films to improve their field emission properties (Macaulay et al. 1992; Geis et al. 1995). Upon intercalation of Cs in SWCNTs with work function 4.8 eV, the work function of the composite was found to be a two-fold lower than the work function of SWCNTs, that is, 2.4 eV (Suzuki et al. 2000). Inspired by these works, for the first time, A. Wadhawan et al. (Wadhawan et al. 2001) reported the improved field emission properties of SWCNTs upon Cs deposition. Figure 10 shows the schematic of low work function material coating of CNTs. In this work, a slurry made of SWCNT bundles mixed in toluene was deposited on Si substrate followed by the deposition of Cs using Cs-metal dispenser housed in a field emission ultra-high vacuum chamber. Cs was deposited on SWCNT films for four different times, 1, 2, 3, and 4 min and their field emission properties were

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Fig. 10 Schematic representation of decoration/coating of carbon nanotube tips with low work function materials

Fig. 11 (a) and (b) F-N plots of clean CNT bundles exposed to Cs metal for 1, 2, 3, and 4 min at the interelectrode separation of 250 μ and 150 μ, respectively. The insets show the field emission I-V characteristics of the samples. (Reproduced from (Wadhawan et al. 2001), with the permission of AIP Publishing)

studied for different anode-cathode separations (d ¼ 250, 150, 75, and 20 μm). Figure 11 shows the F-N plots and respective I-V characteristics (in insets) of the samples at electrode separations of 250 μm and 150 μm, respectively. The linear behavior of F-N plots indicates the electric field-assisted electron emission of the emitters as explained by F-N theory. For 4 min Cs deposited film as shown in Fig. 11, the turn-on voltage is reduced significantly (from 190 to 90 V) by a factor of 2.1

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times along with a six-fold increment in field emission current (1012–106 A) as compared to its pristine counterpart. It was also observed that there is no significant dependence of field emission current on anode-cathode separation. They also studied field emission properties of 4 min Cs deposited film at d ¼ 250 μm in the presence of N2 and O2 gases. From Fig. 12b, it is clear that although there is a fluctuation in field emission current in the presence of N2 but the magnitude of current has not been changed whereas in the case of O2 presence, field emission current is reduced but not recovered after O2 removal. This deterioration indicates the damage of nanotubes (Dean and Chalamala 1999). No degradation is observed in field emission current in UHV conditions. Overall, 4 min Cs deposited films showed superior field emission properties when compared to pristine SWCNT films. After this successful effort, many investigations have been employed to get superior field emission properties of CNTs using various low work function functional coatings in terms of reducing the CNT work function, protecting the CNT tip

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from the ion bombardment during the field emission process, increasing the emission site density, etc. Field emission properties of wide bandgap materials (WBGMs), especially diamond and diamond derivatives, were studied extensively after the initial report on diamond-based cold cathodes (Geis et al. 1991). These WBGMs possess either low positive or negative electron affinity which can effectively lower the work function of an emitter. Because of their low positive or negative electron affinity and high robustness, they were considered for low work function coatings as well as protection layers (to stop ion etching, and settling of adsorbates on the emitting tip) on CNT emitters. Based on this, W. Yi et al. demonstrated the enhanced field emission properties of low positive electron affinity WBGMs, SiO2, and MgO coated CNTs (Yi et al. 2002). In this work, firstly, MWNTs were grown using hot-filament plasmaenhanced CVD and subsequently, grown nanotubes were coated with SiO2 and MgO for various thicknesses 5, 10, 20, 45, 100 nm and 12, 18, 40 nm, respectively. It was observed from field emission measurements that the coated CNTs with an optimum thickness (10–12 nm, 10 nm thick SiO2 coated and 12 nm thick MgO coated CNTs) showed superior field emission properties including favorable temporal stability when exposed to O2 as compared to uncoated/pristine CNTs. Based on the obtained field emission energy distribution spectra and their dependence on the applied voltage, it was determined that the electron emission takes place from the conduction band of these WBGM coated CNT systems. For lower thickness coated CNTs as compared to optimum thickness coated films, field emission took place with low emission area of the tips whereas for high thickness coated CNT films, field emission got deteriorated due to the charge accumulation at the interface between the CNT tip and thick oxide coating since the electron transport is limited due to an insulating nature of the thick oxide. Further, the field present at the emitting tips is reduced for these high thickness coated films and this reduction in the field is proportional to the oxide film thickness and its dielectric constant. The effect of O2 on the field emission property of pristine and MgO coated CNTs was studied. It is observed that the fluctuation in field emission current increases upon the introduction of O2 for all films but uncoated CNTs fail to recover their initial field emission currents upon the evacuation of O2 in the chamber. However, 12 nm and 18 nm thick MgO coated CNTs could recover well to their initial currents whereas 40 nm thick MgO coated CNTs couldn’t recover. A similar observation was also made for SiO2 coated CNTs. The decrease in field emission current of uncoated CNTs is due to the reactive sputter etching of nanotubes (Dean and Chalamala 1999) which decreases the tube height whereas 10–20 nm thick WBGM coated CNTs can tolerate reactive O2 plasma due to the robust WBGM coating, thus they could recover to give their initial currents after O2 removal. Interestingly, the irreversible damage was seen for higher thick WBGM coated CNTs (40 nm MgO coated and 45 nm and 100 nm SiO2 coated CNTs) and was explained on the basis of the charging effects of the insulating nature of the WBGMs. During the field emission process in O2 plasma, positive ions start bombarding the oxide-coated CNT tips. There is no charge build-up at the surface of the WBGM coating for lower thick oxide coated films as positive ions can easily combine with electrons emitting from

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the thin coating whereas for higher thick coatings, a large amount of charge gets accumulated at the surface of the coating. This positive charge accumulation develops a large potential drop and at a certain point of time, this drop becomes large enough to create a breakdown in the WBGM coating. This degrades the thick coating on CNT tips, which makes CNTs not able to recover their initial field emission property upon O2 removal. Due to the outstanding properties of transition metal carbides (TiC, ZrC, HfC, NbC, TaC, etc.) such as low work function (~3.5 eV), good conductivity, high melting point, and good chemical stability, their electron emission properties were investigated. Among these metal carbides, HfC has shown better emission properties (Mackie et al. 1992). Based on this, HfC was coated on Si-based field emitters to enhance their field emission properties (Sato et al. 2003). Inspired by these works, for the first time, J. Zhang et al. reported the improved field emission properties of CNTs upon low work function HfC coating (Zhang et al. 2005). Firstly, CNTs were grown on iron catalyst coated Si by radio frequency plasma enhanced CVD using C2H2 and H2 followed by 5 nm thick Hf deposition on them by ion beam assisted deposition. At the end, these films were annealed at high vacuum, 8  106 Pa and high temperatures, 800  C and 1200  C for 4 h. Field emission properties of an unannealed sample with coating showed very poor emission characteristics even when compared to pristine CNTs. Although both metal-coated samples with annealing showed the improved emission properties but the 1200  C annealed sample shows superior field emission properties as compared to the rest of the samples. To understand the role of the coating and annealing induced changes in terms of electronic and crystal structures on field emission properties, XPS and XRD studies were performed. From XPS studies, it was observed that carbon bands, C-O and C-C, are present on the surface whereas for 5 nm surface sputtering, the C-O band disappeared but a new band emerged at 280.9 eV. This band is attributed to the HfC band and disappeared for 15 nm sputtered film. This suggests that HfC formation took place during the sample annealing at 1200  C through a solid-solid reaction between CNTs and Hf coating. Small-angle XRD measurements also confirmed the formation of HfC for this sample but it is not observed for other samples. Although bulk HfC structures can’t be made from direct reactions but the present case, HfC formation at a relatively lower temperature than usual may be attributed to nanoscale coating. Since β-factor doesn’t change much for coated films, the dramatically improved field emission properties of 1200  C annealed film are attributed to the low work function of HfC film. The role of various metal coatings on the electronic structure of CNTs was investigated using both ab initio calculations and field emission measurements (Cho et al. 2008). Selection of metals for coating the nanotubes and the field emission properties of coated tubes were studied/carried out using density functional calculations. Armchair nanotube, (5, 5) and metals, Ti, Co, W, Pd, Ru were used for a theoretical model system. Firstly, binding energy between various coated metals and CNTs was estimated to understand the metal adhesion on nanotubes. From the analysis, it was found that ruthenium and titanium have relatively higher binding

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energies which states that these metals are strongly bound to the metallic (5, 5) nanotube surface and similar results were obtained for semiconducting (8, 0) nanotube. It is also observed that these metals are strongly bound to the nanotube as compared to oxygen molecules. Afterwards, the effect of coating on work function of metal-coated CNTs was studied as it is known that work function is a crucial parameter that can impact the field emission property significantly. As per F-N theory, the field emission current increases exponentially as the work function decreases. For these selected metal coatings on nanotubes, work function got lowered and the lowest work function is obtained for Ti coated nanotubes. Work function got further reduced from 4.29 eV to 4.25 eV when a single Ti atom is replaced by a tetrahedral shaped Ti4 cluster. Further, band structure and charge density of these metal-coated CNT systems were studied to get a detailed analysis. For Ti adsorbed CNTs, band splitting took place near the Fermi level due to the mixing of d- and π-orbitals of Ti and CNTs, respectively. Work function of Ti coated CNTs is lower than pristine CNTs which is due to the upward shift in the Fermi level, and also density of states (DOS) is improved near the Fermi level for Ti coated CNTs because of d-orbital mixing. So, the lowered work function and increased DOS near the Fermi level improved field emission properties of metal-coated CNTs. It was concluded that the variation in the degree of orbital mixing leads to the change in work function for various metalcoated CNTs. To understand field emission properties of metal-coated CNTs, electronic structure calculations were performed under an applied electric field. With an applied field, DOS near the Fermi level is enhanced for Ti coated CNTs when compared to pristine CNTs as shown in Fig. 13, and this enhancement is due to adsorbed metal atoms. Along with this, these metal atoms can also act as extra emission sites in addition to CNT emitting tips. From all metal-coated CNTs, it is observed that relatively higher DOS near the Fermi level is obtained for Ti coated CNTs. By considering binding energy, work function, and DOS near the Fermi level, Ti coating is the suitable one among all selected metal coatings to achieve superior field emission properties. Based on the theoretical results, Ti-coated CNTs were synthesized by chemical reduction method using titanium trichloride and thin MWCNTs as precursors and sodium borohydride as a reduction agent, and demonstrated the field emission display application. CNT cathodes were prepared on ITO-coated glass using a screen printing method. Finally, field emission and its temporal stability measurements were carried out in diode geometry. Figure 14 revealed that both turn-on and threshold fields got reduced significantly for coated samples as compared to pristine ones, and in overall, these improved properties of coated CNTs are in good agreement with the theoretical predictions discussed earlier. During the metal coating on CNTs, there is a possibility that metal gets oxidized. Here, authors considered that the formation of an oxide layer took place only on the surface of the coating which means the interface between CNTs and the coating is constituted by Ti metal due to its higher binding energy with CNTs when compared to O2 binding energy with CNTs. Thus, field emission models were represented with metal coating on

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nanotubes unlike oxide coating. Temporal stability measurements revealed the longer lifetime of coated nanotubes which is almost 3.5 times of pristine nanotubes, and also, the fluctuation in the field emission current of coated tubes is lower than 10% as seen in 100 h stability measurement. Thermogravimetric analysis (TGA) performed in an O2 environment showed that Ti coating protects nanotubes from O2 induced degradation as the burning temperature of coated tubes got increased by 20  C. Additional calculations were also performed to understand the protection mechanism offered by Ti metal coating by considering O2 ambient and found that O2 molecules bind strongly and preferentially with Ti metal than CNT surface, which in turn reduces the nanotube tip damage from O2. Further, it is found from local strain measurements that the strain in local bonding of all adsorption sites on the coating is in the range, 1.9–2.2%. This indicates Ti coating protects the CNT surface without creating any adverse effect in the form of strain. As seen from EDX results, the partial oxidation of Ti is expected during the synthesis/fabrication process. So, in this regard, the investigation of role of TiOx on work function is required. For the sake of the simplistic model, nanotube-TiO2 interface was considered for the model since the modeling of interfaces of oxide nanoparticles with nanotubes is very complex. This model reports a higher work function by 0.6 eV for nanotube-TiO2 composite when compared to nanotube-Ti composite and also, the work function reduction for oxide composite through charge transfer is unlikely as it is known that oxide nanoparticles cannot interact as strongly as metal nanoparticles with nanotubes. Hence, it is obvious that the role of the

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Fig. 14 Field emission (a) J-E characteristics and (b) time stability of both pristine and coated CNTs. The inset in Fig. 14b shows the fluctuation in emission current measured for Ti coated CNTs. (Reproduced with permission from (Cho et al. 2008), copyright (2008) American Chemical Society)

nanotube-metal interface in the improvement of the field emission properties of metal-coated nanotube samples is very significant. Authors also discussed this way of selective adsorption of ambient gases may open a new approach to minimize the electrical noise of CNT based devices. Due to the presence of localized electrons at defects, the positive Ti ions favor these defect sites during the chemical reduction process. It was found earlier that TiO2 nanoparticles were used to detect the defect distribution, as a detector, since these particles were adsorbed selectively at the defects present on the CNT (Li et al. 2003). Hence, metals or metal-oxides can be used to passivate the defects present on CNTs. This can lower the resistive or Joule heating, which can eventually increase the lifespan of CNT based electronic devices. In a similar way, the improved field emission properties were also achieved for CNTs coated with In2O3 and In where coatings modified the top surface of CNTs from a lower to higher sp3/sp2 ratio (Lee et al. 2014; Sreekanth et al. 2018a, b). This indicates the surface behaves more like a diamond having negative electron

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surface where electrons can easily tunnel through the potential barrier. Like most other WBGMs, indium oxide (In2O3) is also a suitable coating material on CNTs due to its relatively low positive electron affinity, good chemical inertness, good sputter resistance and its ability to become n-type material through doping. Firstly, cleaned SWNTs were spray-coated on ITO substrate followed by their annealing at 200  C for 20 min in an N2 atmosphere to get better adhesion between SWNTs and ITO substrate. These CNT samples were coated with In2O3 using a successive ionic layer adsorption and reaction process for different cycles. From Fig. 15, it is clearly seen that the field emission current density increases as the number of coating cycles increases and the highest current density is achieved for a five-cycle oxide-coated CNT sample and further, the field emission current decreased for a six-cycle oxide-coated sample and its current density is equal to three-cycle coated sample. This shows that a five-cycle coated sample gives superior field emission properties when compared to pristine as well as lower and higher thick oxide-coated CNT samples. Here, field emission is suppressed for higher thick films due to the charge build-up at the interface between conducting SWNTs and higher thick insulating coating. To understand the improved field emission properties of coated films, X-ray photoelectron spectroscopy (XPS) was employed to investigate the role of electronic structure. As shown in Fig. 16, C 1s peak shifted significantly as the number of coating cycles increased and a peak shift of 0.62 eV was observed for the five-cycle coated film as compared to pristine film. Further, C 1s peak was de-convoluted into three peaks where binding energies, 284.7, 285.6, and 289 eV correspond to sp2, sp3, and oxygen-related peaks, respectively. A decrease in ratio, sp2/sp3, was observed with an increase in the number of cycles and this ratio was reduced to 1.14 for a fivecycle coated sample as compared to pristine CNT’s ratio, 1.72. This shows that sp3carbon content increases with an increase in the number of cycles due to the bending of tube walls caused by the coating. As it is known that, as per Fowler-Nordheim theory, the field emission properties of large-area emitters depend on work function, β-factor, and emission site density.

Fig. 15 (a) Field emission J-E characteristics and (b) corresponding F-N plots of pristine and In2O3 coated CNT films. (Reproduced from (Lee et al. 2014), copyright (2014), with permission from Elsevier)

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Fig. 16 XPS (a) C 1s and (b) O 1s core-level spectra of pristine and In2O3 coated CNT films. The insets in (a) and (b) represent the deconvolution of C 1s and O 1s peaks, respectively. (c) Valence band spectra of pristine and In2O3 coated CNT films. (Reproduced from (Lee et al. 2014), copyright (2014), with permission from Elsevier)

As seen from SEM images, there is not much change observed in geometrical features and texture. This suggests that the improved field emission is due to the lower work function of the coated emitters. This lower work function is attributed to the partial re-hybridization of sp2-C to sp3-C. This formation of sp3-C atoms constitutes a diamond like surface rather than a graphitic structure. The electron emission property of these surfaces resembles with electron emission of a diamond like surface having negative electron affinity. During the emission process, electrons experience a relatively lower potential barrier as electrons emit from the conduction band of the coating present on the CNT tips. In the case of the field emission properties reported for In coated RGCNT and VACNT films (Sreekanth et al. 2016, 2018a, b), In metal was decorated on CNT films using a thermal evaporation process. In has a low work function (ϕ ¼ 4.1 eV). Firstly, the coated RGCNT films showed a 250% enhancement in current density as compared to their pristine counterparts and this enhancement was primarily attributed to the presence of DOS close to the Fermi level. Afterward, In metal with different thicknesses (4, 12, 23, and 50 nm) was decorated on VACNTs to understand their field

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emission properties, and also it is expected that coated VACNTs would give better field emission properties as compared to coated RGCNTs as it is known that VACNTs are better field emitters than RGCNTs. The 4 nm In coated VACNT film showed more than a double-fold (217%) increment in current density than its pristine counterpart but for higher thickness coated films, the field emission currents deteriorated as thickness increased as shown in the J-E plot of Fig. 17 and are even lower than the field emission current of pristine CNTs. Here, the correlation between conventional field emission parameters like work function and β-factor, and the current density of the films couldn’t be explained as the changes in work function and β-factor are not significant to explain the improved field emission properties of 4 nm In coated CNT films. Although micro-crystalline behavior of the films estimated from Raman spectroscopy measurements suggested that there is no significant change observed in bulk CNT structure upon In metal coating but XPS C 1s core-level spectra revealed the coating induced changes on the surface of the films in terms of sp2/sp3 ratio. From de-convoluted C ls spectra shown in Fig. 18, it is observed that there is a dramatic improvement in sp3-content from pristine (18.6%) to coated films, 50.4% and 36.6% for 4 and 12 nm coated CNT films. It is known that CNTs purely comprise sp2-carbons (C ¼ C), so sp3-carbons or C-C bonds present in CNTs are treated as defective carbons. This dramatic increase in sp3-C content of coated films may be attributed to bending of the nanotube walls caused by In coating (Lee et al. 2014). These sp3-C or C-C bonds present at the curved regions on the CNT surface caused by coating formulate a diamond negative electron affinity surface which can reduce the effective potential barrier for electron emission for coated films when compared to their pristine counterparts. From the valence band spectra of the films shown in Fig. 19, it is seen that DOS is enhanced significantly for coated films and also, their enhancement is almost same for both the coated films. Apart from this, there is a spectral weight shift by 0.9 eV

Fig. 17 (a) Field emission J-E characteristics and (b) corresponding F-N plots of pristine and In coated CNTs. (Reproduced from (Sreekanth et al. 2018a, b), copyright (2018), with permission from Springer Nature)

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Fig. 18 XPS (a) In 3d spectrum of 4 nm thick In coated CNT film and C 1s spectra of (b) pristine, (c) 4 nm thick In coated and (d) 12 nm thick In coated CNT films. Significant changes are observed in C 1s spectra upon In coating. (Reproduced from (Sreekanth et al. 2018a, b), copyright (2018), with permission from Springer Nature)

towards the Fermi level for coated films which can also increase the electron emission as electrons present near the Fermi level can be easily emitted due to their lower binding energy. Despite having high sp3-C content and also, enriched DOS, the field emission properties of 12 nm thick In coated CNT film are inferior to the field emission properties of both pristine and 4 nm thick In coated CNTs. This is attributed to relatively high thickness coating present on 12 nm thick In coated CNT film which covers the emitting sites in the film and the same is reflected in β-factor with lower values for this film as well as other high thickness In-coated films. This is why, the field emission properties of these films are much inferior to 4 nm thick In coated as well as pristine CNT films. The importance of interface analysis of copper oxide species (CuO and Cu2O) with VACNT and their electronic structure was highlighted to understand the field emission properties of Cu coated VACNTs before and after oxidation (Sreekanth et al. 2020). In this work, the variation in conventionally monitored field emission parameters like work function and field enhancement factor does not explain the experimentally obtained improved field emission properties. However, (i) the

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Fig. 19 Valence band spectra of pristine and 4 nm and 12 nm thick In coated CNT films. The dramatic enhancement in DOS is observed for metal coated films. (Reproduced from (Sreekanth et al. 2018a, b), copyright (2018), with permission from Springer Nature)

electronic structure in terms of a closed shell (3d10) and an open-shell (3d-O 2p) configurations of Cu2O and CuO, respectively, along with CuO/Cu2O ratio at the CNT-oxide interface; and (ii) the presence of barrier height at the CNT-oxide interface could explain the experimentally determined results for all films. Firstly, VACNTs grown by thermal CVD were coated/decorated with copper for two different thicknesses (3 nm and 10 nm) followed by their oxidation at 250  C for 2 h. When compared to pristine CNTs, field emission properties were slightly improved for 3 nm Cu-coated film (10.6–12.8 mA/cm2), but they were significantly improved after oxidation (i.e., 10.6–20.1 mA/cm2) as shown in Fig. 20. Also, field emission properties of both pristine and 10 nm coated CNT films got further deteriorated from 10.6 to 4.8 mA/cm2 and 9.0 to 5.3 mA/cm2, respectively, after oxidation. Here, from pristine to 3 nm coated film, there is a little improvement in current density and this may be due to the slightly improved β-factor of the coated film, whereas, from 3 nm Cu-coated film to its oxidized counterpart, current density was enhanced significantly although β-factor was reduced. From β-factor estimation, it is understood that there is no one to one correlation between current density and βfactor. Based on this, the electronic structure of the films was studied to understand the change in current density. Unlike CNT/In2O3 and CNT/In systems, there is no considerable variation observed in fractions of sp2 and sp3-carbons. From XPS Cu 2p spectral analysis, it is found that significantly high Cu2O content is present on CNTs with a lower fraction of CuO, whereas upon oxidation, a large fraction of Cu2O was converted into CuO. In this way, correlation was found primarily between the current density and fraction, CuO/Cu2O for the films. In addition to this, the thickness of the oxide layer on CNTs also plays an important role in the field emission properties of the films as discussed earlier. The 10 nm Cu-coated CNT film obtained a lower current density upon oxidation. This is due to the fact that above certain critical thickness, the oxide layers behave as insulators.

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Fig. 20 Field emission J-E characteristics in (a) linear and (b) semi-log format, and (c) corresponding F-N plots of pristine and metal-coated CNTs (A0, A3, A10) and their oxidized counterparts (B0, B3, B10). (d) Temporal stability of pristine CNT film (A0) and 3 nm metal-coated CNT film (A3) and its oxidized counterpart (B3). (Reproduced from (Sreekanth et al. 2020), copyright (2020), with permission from Elsevier)

From the valence band analysis, the spectra of Cu coated CNT films before oxidation resemble previously reported Cu2O spectra. Besides this, the hump present at 5 eV (Fig. 21) indicates the presence of a small fraction of CuO. The DOS close to the Fermi level is reduced, and also, the peak broadening is observed with shifting of spectral weight towards higher binding energy. The hybridization between Cu 3d and O 2p bands leads to the broadening of the peak, and this confirms the formation of CuO. Among these two oxides, Cu2O has a full 3d shell (3d10) where electrons are localized whereas for another oxide, CuO, Cu 3d-O 2p shells are hybridized due to which the electrons present in these shells are itinerant/delocalized. It is expected that itinerant electrons can easily participate in the field emission process, which results in better field emission properties for CNT-CuO films. Hence, higher the fraction of Cu2O on CNTs lower the current density of the films and vice versa. The role of interfaces, CNT/Cu2O and CNT/CuO, on the field emission tunneling process is also discussed with the use of energy band diagrams shown in Fig. 22. The Schottky junction formation is expected for both the interfaces as CNT is a

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Fig. 21 Valence band spectra of pristine and metal-coated CNTs (A0, A3, A10) and their oxidized counterparts (B0, B3, B10). (Reproduced from (Sreekanth et al. 2020), copyright (2020), with permission from Elsevier)

Fig. 22 Energy band diagrams of CNT-CuO and CNT-Cu2O systems before and after the contact. (Reproduced from (Sreekanth et al. 2020), copyright (2020), with permission from Elsevier)

good 1D conductor (having work function ~4.8 eV) and both Cu2O and CuO are p-type semi-conductors (having bandgaps, 2.17 eV and 1.4 eV and work functions, 5.27 eV and 5.31 eV). Due to the bandgap difference, the larger Schottky barrier is

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formed at the CNT-Cu2O interface as compared to the CNT-CuO interface and the difference between the two barrier heights is ~1 eV. So, higher barrier height also affects the field emission properties of CNT/Cu2O films adversely. The highest current density was achieved (20 mA/cm2) for 3 nm thick Cu-coated CNTs upon oxidation, and this is the highest field emission current density obtained as compared to any metal-oxide coated/decorated CNTs so far. The highly enhanced field emission properties of this film are attributed to the presence of CuO on CNTs which have itinerant/delocalized electrons close to the Fermi level and also a relatively lower barrier at the CuO-CNT interface. Although there are already various studies on the correlation between the field emission and electronic structure related to metal/metal-oxide coated/decorated carbon nanotube systems, still there needs to be more clarity and generic understanding on how electronic structure influences the field emission properties of various metal/metal-oxide coated/decorated CNTs in terms of closed or open shells, mixing of orbitals, and band alignment between CNTs and the coating, etc.

Interlayer Between Substrate and CNTs At elevated temperatures, the CNT growth on Si substrate is hindered by the formation of silicide layer due to the reaction between the catalytic particles (Fe, Co, Ni) and exposed Si substrate, and these silicide layers formed deactivate the nucleation sites present on the substrate and thus, affecting the growth of nanotubes. To overcome this drawback, an interlayer was employed between the Si substrate and catalytic layer as a buffer that reduces the diffusion of catalytic particles from the catalytic layer into the substrate which eventually leads to the better growth of CNTs (De los Arcos et al. 2002). Apart from stopping the insulating silicide layer formation, an interlayer can also improve the adhesion and reduce the contact resistance between grown CNTs and the substrate. Improved adhesion and lowered contact resistance can significantly improve the field emission properties of CNTs as well as their temporal stability. Various interlayers, Cr, Ti, W, Al, Au, Pt, Pd, Cu, etc., have been studied to understand how they influence the growth of CNTs and their field emission behavior at various growth temperatures for different interlayer thicknesses using various growth methods and catalysts (Srividya et al. 2010; Shah et al. 2013). Figure 23 shows the schematic of the CNT growth on an interlayer.

Fig. 23 Schematic representation of the CNT deposition on the interlayer/buffer deposited between the substrate and catalyst layer

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The effect of 30 nm thick Ti interlayer on the field emission properties of patterned grown CNTs was investigated (Srividya et al. 2010). The Ti buffer layer was deposited on an n-type Si substrate by sputtering followed by deposition of a thin iron (Fe) catalyst layer. CNTs were grown using C2H2 as a carbon source by thermal chemical vapor deposition (T-CVD). From SEM analysis, it is observed that the nanotube density is lowered for Ti interlayered substrate compared to bare Si substrate. The lowered tube density is attributed to relatively less available catalytic particles for the nanotube growth as some portion of the catalytic particles were utilized for reacting with the Ti interlayer. This interaction between catalytic particles and Ti led to the better adhesion between CNTs and the substrate, which eventually reduces the peeling of CNTs during the field emission process at higher applied electric fields. This is useful to generate high current densities and thus, this interlayered CNT cathode may be applied to various field emission-based device applications. Field emission measurements from Fig. 24 show that the interlayered film obtained a three-fold increment in current density (30 mA/cm2) compared to its without interlayer film (10 mA/cm2) at an applied electric field of 4 V/μm. This increment was attributed to less densely grown CNTs on the interlayer and also, their reduced tube diameters. Both of these factors can significantly contribute to improve the field enhancement factor. Apart from this, the improved adhesion of the interlayered CNT film would also help to maintain the good field emission stability. The probable reasons for the better adhesion are (i) the strong bonding between CNT roots and Ti, (ii) good adhesion between Ti and Si substrate, and (iii) Possibility of titanium carbide formation at the interface which can protect the nanotube emitters from the problem posed by ionized molecules. In this work, the contact resistance between the substrate and CNTs was assessed indirectly by monitoring vacuum present in the field emission chamber at higher applied electric fields. It is reported earlier by the same group that when the field is increased beyond 5 V/μm, there was a fluctuation observed in chamber vacuum and

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Fig. 24 Field emission J-E characteristics of pristine and Ti interlayered CNT films. (Reproduced from (Srividya et al. 2010), copyright (2010), with permission from Elsevier)

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was attributed to substrate heating (Verma et al. 2007). This fluctuation can also happen if there is any damage to CNTs or a substrate in terms of local evaporation, burning, and melting (Sowers et al. 1999; She et al. 2003). Here, there was no noticeable change observed in vacuum at higher applied electric fields for the interlayered film which means that neither there was any significant amount of Joule heating produced from the film nor any structural damage happened to the film during the electron emission process. So, this confirms that the interlayered CNT film has lower contact resistance between the substrate and CNTs than CNT film on Si, and this eventually improves the field emission properties of interlayered films. It was also observed that the stable emission was achieved for more than 50 min at 4 V/μm. Followed by this work, the role of Ti interlayer thickness on the field emission properties of CNTs grown by microwave plasma-enhanced CVD was studied in detail (Sharma et al. 2011). The improved field emission properties of interlayered films were explained in terms of a double barrier model related to substrate-CNT and CNT-vacuum interfaces, and a conducting channel formed between CNTs and substrate. Three different thicknesses (5, 10, and 15 nm) of interlayer were used. It is reported that the interlayer thickness also plays an important role and the Al interlayer was effective only from the interlayer thickness of 10 nm (Teng et al. 2008). Also, the interaction between CNTs, catalysts, and substrate can also influence the field emission behavior significantly (Wang et al. 2008). Another important study reported that the non-linear behavior of F-N plot is related to the nature of interfacial and surface barriers (Chen et al. 2009). Due to its negative interfacial energy, Ti interacts strongly with CNTs and may form stable compounds (He et al. 2010). As seen usually from the XPS analysis, Ti develops a native oxide immediately after the deposition, but it is altered through reduction by forming a metal carbide during the CNT growth (Esconjauregui et al. 2009). This titanium carbide acts as a conducting channel for electron transport at the interface between the substrate and CNTs, and thus, reducing the contact resistance significantly which in turn enhances the electron emission from nanotubes. For very thin interlayered films, titanium oxide (TiOx) formation is inevitable since the bonding between C and Ti is not strong. This means that electrons experience another barrier, apart from the usual CNT-vacuum interface, in the form of titanium oxide-CNT interface. This is why the field emission behavior of the films is explained on the basis of the doublebarrier model. With an increase in the thickness of the interlayer, there is no considerable non-linearity in F-N plot which indicates the double barrier model is not applicable (Sharma et al. 2011). This shows that, for higher thicknesses of an interlayer, there is no significant interfacial effect on the field emission behavior. From this, one expects the formation of a thin conducting titanium carbide layer between the substrate and CNTs. On further increasing the thickness, it is expected to have a strong interaction between Ti and CNTs which can further improve the electron emission. Similarly, the role of Al interlayer (having a thickness of 300 nm and 40 nm) on the field emission properties of CNTs grown on Si substrate was investigated in detail (Shah et al. 2013; Sreekanth et al. 2015). Both the studies reported the improved structural and field emission properties of CNTs in terms of crystallinity or lower defect density, low

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turn-on and threshold fields, and higher current density. Good temporal stability of the field emission current was also reported for CNTs grown on 40 nm thick Al interlayer. Apart from the above-discussed methods, there are also other important methods employed to improve the field emission properties of pristine CNTs like structural modification of nanotubes (in terms of various gas plasma treatments (Wang et al. 1997; Zhi et al. 2002), alloy or metal coating/decoration (Lee et al. 2008; Sridhar et al. 2014), high-temperature thermal annealing (Sun et al. 2014), etc.), CNT doping (Charlier et al. 2002; Wang et al. 2002), etc. From various successful approaches used to enhance the field emission properties of CNTs, it can be understood that the incorporation of more than one approach simultaneously on a CNT film, for example, low work function material coating on the interlayered CNT system or deposition of low work function element-doped CNTs on the interlayered substrate, etc., would further enhance its field emission properties.

CNT Field Emitter-Based Device Applications As discussed previously, due to their serious drawbacks, thermionic electron sources need to be replaced by field emission electron sources. Because of their outstanding field emission properties, CNT based field emission electron sources have been developed for various device applications such as field emission displays (FEDs), electron microscopes, microwave amplifiers, compact X-ray sources, vacuum gauges, etc. The first CNT based field emission or flat panel display was demonstrated using a matrix addressable diode display with an electron source made of epoxy-CNT composite (Wang et al. 1998). After this work, the high brightness and fully sealed CNT-based FED was demonstrated using single-walled CNT-organic binders (Choi et al. 1999). CNT-based field emission lighting element was also developed and commercialized (Saito et al. 1998). CNT field emitters were also used to fabricate prototype microwave amplifiers as they can generate high emission currents which is a prerequisite for microwave amplifiers (Zhou 2000). The CNT-based field emission X-ray tube which generated a better-resolved image than the thermionic X-ray tube was demonstrated for both non-biological and biological samples (Sugie et al. 2001). Afterward, many efforts have been made to develop different types of X-ray sources for various applications (Yue et al. 2002; Cheng et al. 2004). Pristine and doped CNT-based ionization vacuum gauges were also realized (Dong and Myneni 2004; Choi and Woo 2005; Li et al. 2015; Su et al. 2013).

Conclusions This chapter primarily deals with field emission and its mechanism, the history of various field emitter materials, and carbon nanotube-based field emitters. Among these, the major emphasis is given to CNT-based emitters, especially methods

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employed to improve their field emission properties in terms of using various synthesis methods, catalyst patterning on the substrates, coating/decoration of low work function and other materials (mainly metals and metal-oxides), the introduction of an interlayer between the catalyst layer and substrate, etc. At the end, a few important applications of CNT based field emitters were also included.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Elastic Behavior of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Young’s Modulus (E) of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shear Modulus and Poisson’s Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Strength of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Carbon nanotubes are remarkable objects that look set to revolutionize the technology world. The fascinating properties of CNTs including physical, optical, and electrical properties made them one of the promising materials for the future research. This chapter mainly focuses on the physical properties of CNTs and provides an overview of factors affecting physical properties such as structural difference, purity, tube diameter, and density. Also, an attempt is made in this chapter to correlate and analyze both theoretical and experimental results with respect to Young’s modulus (E), shear modulus (G), bulk modulus (K), and Poisson’s ratio from the existing literature dealing with the physical and mechanical properties of CNTs.

K. C. Sivaganga · T. Varughese (*) Department of Chemistry, Christ College (Autonomous), Affiliated to University of Calicut, Irinjalakuda, India e-mail: [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_62

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Introduction Carbon nanotubes have unique physical properties which are essentially depend on the purity, uniformity in diameter, and their density. The bonding in CNTs is sp2 in nature with a planar carbon framework and hence considered to be rolled up graphene sheets. The physical and mechanical properties of CNTs arise due to this rolled up planarity of carbons and its unique strength could be understood by the fact that CNTs are stronger than the sp3 bonded diamonds. Under high pressure, nanotubes can merge together, trading some sp2 bonds for sp3 bonds, giving the possibility of producing strong, unlimited length wires through high-pressure nanotube linking (Dai 2002a, b, 1035; Hirlekar et al. 2009). Based on the number of graphene layers present in a single nanotube, they are either called single-walled carbon nanotubes [SWCNTs] or multiwalled carbon nanotubes [MWCNTs] (Figs. 1 and 2). Single-walled carbon nanotubes do not form in straight lines but as curled and curved strands. This way the graphene sheet wraps around to form three types SWCNTs, namely zigzag nanotubes, armchair nanotubes, and chiral nanotubes, with dissimilar properties such as optical activity, mechanical strength, and electrical conductivity (Dekker 1999).

Fig. 1 Carbon nanotube structures of armchair, zigzag, and chiral configurations

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Fig. 2 Schematic representation of single-walled carbon nanotube (SWCNT) and multiwalled carbon nanotube (MWCNT)

Fig. 3 Different forms of SWCNTs

The grapheme sheet wrapping in different types of CNTs are denoted using a pair of indices (n, m) called the chiral vector. Zigzag CNTs have graphene sheet rolled up that makes it parallel to the row of bonds in the hexagonal structure with values of n ¼ 0 or m ¼ 0 and chiral angle exactly equal to 0 . Armchair nanotubes have graphene sheet rolled up at an angle that is perpendicular to the bonds in the hexagonal lattice arm chair with any equal values of n and m (n ¼ m) and chiral angle exactly equal to 30 . Chiral nanotubes, on the other hand, have sheets aligned along the cylinder at some chiral angle other than armchair or zigzag. (i.e., any n and m values excluding n ¼ m and chiral angles lie between 0 and 30 ) (Fig. 3). Multiwalled carbon nanotubes (MWCNTs) consist of multiple layers of graphene rolled in on themselves to form a tube shape with an interlayer spacing of 3.4 Å. The outer diameter of MWCNTs may range from 1–50 nm while the inner diameter is usually of several nanometers. In fact two distinct models – namely Russian Doll and Parchment type – can be used to describe MWCNT. Russian Doll model describes the structures of MWCNTs in such a way that the smaller sheets of graphene are arranged in concentric cylinders within larger ones. Parchment model describes a

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MWCNT as a wrapping of a single graphene sheet to many folds around itself. The interlayer distance in MWCNTs is similar to the distance between graphene layers in graphite (Dai 2002a, b, 218). The bulk synthesis of pure SWCNTs is difficult as it requires proper control over growth and atmospheric condition. Chances of defect formation during functionalization process are very high. Bulk synthesis of MWCNTs with high purity is easy. Also, chances of defects during functionalization process are very low. However, the SWCNTs show far more interesting properties than the MWCNTs. One interesting application of SWCNTs is in the development of the first intramolecular field-effect transistors (FETs) and intramolecular logic gate (Martel et al. 2001) to create a logic gate, a p-FET and an n-FET are required. Because SWCNTs are p-FETs when exposed to air and n-FETs when unexposed to oxygen, they were able to protect half of a SWCNT from oxygen exposure, while exposing the rest to oxygen. This results in a single SWCNT that acts as a NOT logic gate with both pand n-type FETs within the same molecule.

Elastic Behavior of CNTs Elasticity is an important physical property of any structural material. Structural properties of the materials are largely based on elastic constants. Elastic constants are the parameters expressing the relationship between the stress and the strain on the materials within the stress range that the materials exhibits its elastic behavior. There are three elastic constants, namely Young’s modulus or modulus of elasticity (E), shear modulus or modulus of rigidity (G), and bulk modulus (K). “E” calculated as the ratio of tensile stress to tensile strain, “G” calculated as the ratio of shear stress to shear strain, and “K” calculated as the ratio of volumetric stress to volumetric strain. Modulus of elasticity (E) is a measurement of a material’s elasticity. It refers to the amount of stress a material has for an amount of elastic strain. The higher the elastic modulus (E), the more resistant is the composite material to deformation within the elastic range (Table 1). In other words, it is a measure of how easily a material is bended or stretched without deformation. The shear modulus (G) is a measure of deformation of a solid when it experiences a force parallel to one of its surfaces while its opposite face experiences an opposing force. “G” is a measure of the ability of a Table 1 Young’s modulus, tensile strength, and density of carbon nanotubes compared with other materials (Osmani et al. 2014)

Material MWCNT SWCNT Steel Wood Epoxy

E (GPa) 1200 1054 208 16 3.5

σ (GPa) 150 150 0.4 0.008 0.005

Density (g/cm3) 2.60 – 7.80 0.60 1.25

E Young’s modulus, σ Tensile strength, GPa Giga Pascal, MWCNT multiwalled carbon nanotubes, SWCNT single-walled carbon nanotubes

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material to resist transverse deformations and is a valid index of elastic behavior only for small deformations. Bulk modulus (K) of a substance is defined as the ratio of infinitesimal pressure increase to a decrease of the volume. It is known as the modulus of elasticity of liquids. Bulk modulus is meaningful only for a fluid substance. Another important parameter used in material property is Poisson’s ratio. It is defined as the ratio of change in width per unit width of a material to the change in its length per unit length, as a result of strain. The Poisson’s ratio of a stable, isotropic, linear elastic material must be between 1.0 and +0.5. Most materials have Poisson’s ratio values ranging between 0.0 and 0.5. Rubber has a Poisson’s ratio of nearly 0.5, cork has Poisson’s ratio equal to 0, auxetic materials shows negative value, and anisotropic materials can have values above 0.5. A negative Poisson’s ratio indicates that when a material is stretched, they become thicker perpendicular to the applied force and vice versa. A zero value indicates that there is no change in the thickness of material perpendicular to the applied force. Poisson’s ratio values of SWCNs are generally predicted to be in the range of 0.15 and 0.34, which is similar to common solids. The designing of structural materials for practical applications needs to be based on the safe loading parameter of the material which usually set less than half of its elastic limit (Harris and Bunsell 1977). The elastic modulus directly related to the cohesion of the solid and therefore to the chemical bonding of the constituent atoms in the material. Thus for a covalently bonded ideal crystalline solid (without any point defects or dislocations and having grain boundaries), modulus of elasticity is a function of potential energy of a pair of atoms in the solid and the inter-particle separation between them. A molecular solid with only weak van der Waals bonds acting on it usually has a low E value of less than 10 GPa. Covalently bonded solids like graphite, diamond, etc. show high E value above 100 GPa. Moreover, in each class of solids (defined by the nature of the bonding) elastic properties show inverse relationship with their lattice parameter. Even small variations of the lattice parameter of a crystal may induce significant changes in its elastic constants. The small diameter of a carbon nanotube has an important effect on the mechanical properties, compared with traditional micron-size graphitic fibers (Dresselhaus et al. 1988). Perhaps the most striking effect of CNTs is its high flexibility and strength along with high stiffness – a difficult combination for a material to achieve. This associative property is absent in simple graphite fibers. The small-tube CNTs does have superior greater physical and mechanical properties due to its very low defects per unit length (low density of defects) compared with larger structures like graphite. Strength of a material refers to the maximum force it can withstand per unit area and toughness measurements indicate the elastic energy stored or absorbed by a material before failure. CNTs are the strongest and stiffest materials on earth, in terms of tensile strength and elastic modulus, respectively. The planar honeycomb lattice carbon atoms of graphene in CNTs are responsible for these high-strength fibers. SWNTs are stiffer compared to steel and are extremely resistant to damage from physical forces. When the tip of a nanotube is pressed, it bends without causing any damage to the tip, and on the removal of the force, the tip returns to its original

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state. Due to this property, CNTs are very useful as probe tips for very high-resolution scanning probe microscopy.

Young’s Modulus (E) of CNTs There are two approaches commonly used to assess the elastic properties of CNTs: experimental and computational. The conventional extensiometers used for the Young’s modulus measurement of a material require a large quantity of material for it to be shaped into a rod. The precision of this method is high, and experimental observations are normally better than theoretically predicted values. However, it is difficult to precisely measure the modulus using conventional techniques owing to the nanometer size of CNTs. Also, the second-order effects on modulus due to the curvature and the helicity cannot be precisely determined by the extensiometers (Salvetat et al. 1999, 255). Hence in situ techniques based on atomic force microscopy (AFM)- and transmission electron microscopy (TEM)-based experimental methods are used for measuring the elastic modulus of CNTs (Salvetat et al. 1999, 944; Hall et al. 2006; Kallesøe et al. 2012; Wang et al. 2013). The first measurement of the Young’s modulus of MWCNTs came from Treacy and coworkers (Treacy et al. 1996). TEM was used to measure the mean square vibration amplitudes of arc-grown MWNTs over a temperature range from RT to 800 C. The average Young’s modulus value for 11 tubes using this technique was 1.8 TPa. Later Yu and coworkers developed a method using AFM-based tensile test for measuring the modulus of CNTs (Yu et al. 2000a, b). The Young’s modulus of MWNTs experimentally found to be in the range of 1.28 TPa. The variation in the measurement of Young’s modulus is generally attributed to the different experimental techniques used. It is observed that the modulus measurements of different MWCNTs do not correlate to their diameter but strongly correlates to the amount of disorder in their nanotube walls. The method of measuring thermal vibration amplitudes by TEM has been extended to measure SWNTs at room temperature (Krishnan et al. 1998). The average of 27 tubes yielded a value of E ¼ 1.3 TPa. AFM technique is more commonly used for modulus measurement of CNTs. In general, the experimentally found Young’s modulus values of SWNTs are around 1 TPa. All measured values of “E” for nanotubes indicate that it may be higher than the currently accepted value of the in-plane modulus of graphite. Though all experimental studies showed CNTs with unparalleled mechanical properties, due to the complexity of the characterization of nanomaterial at the atomic scale, the experimental results reported in the literature were inconsistent with respect to each other (Mielke et al. 2004; Hou and Xiao 2007). The reason of the scattering results can be attributed to the defects in the CNT’s structure: It is almost impossible to produce carbon nanotubes with a perfect structure. The theoretical predictions are far more superior in the case of CNTs as the nanometer size allows molecular dynamic simulations to be performed and compared directly with experimental data. The theoretical approaches for the modeling and characterization of the CNTs’ mechanical behavior can be grouped into three main

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categories: the atomistic approach, the continuum mechanics (CM) approach, and the nanoscale continuum modeling (NCM) approach, also called molecular structural mechanics (MSM). Initially, atomistic modeling is used for studying the mechanical behavior of CNTs, which calculated the position of atoms based on their interactive forces and boundary conditions (Yakobson et al. 1996; Kudin et al. 2001). In recent years, the atomistic approaches, due to their big computation cost, have been gradually replaced by continuum approaches. The basic assumption of the continuum mechanics (CM) approach consists of the modeling of CNTs as a continuum structure, concerning the distribution of mass, stiffness, etc., i.e., the real discrete structure of the nanotubes is neglected and replaced by a continuum medium (Chang 2010; Gupta and Batra 2008). The nanoscale continuum modeling [NCM/MSM] approach consists of replacing the carbon–carbon (C–C) bond with a continuum element. As a result, continuum mechanics theories can be used at the nanoscale, i.e., the connection between molecular configuration and solid mechanics is considered. NCM is an efficient modeling approach for simulation of the CNT’s behavior, which does not require intensive computation and can be applied to complex systems without limitation of length scales, when comparing with atomistic modeling (Rafiee and Moghadam 2014). Elaborate discussion on the theoretical studies is available in the review by Sakharova et al. (Sakharova et al. 2017). Theoretically the “E” value has been predicted to be 1.22–1.26 TPa based on the chirality and size. Kalamkarov et al. correlated the Young’s modulus of different CNTs with respect to tube diameter of SWCNTs, outer diameter of the double-walled CNTs, and the number of tubes in MWCNTs (Kalamkarov et al. 2006). The Young’s modulus of single-walled nanotube varied from 0.96–1.04 TPa as the nanotube diameter varied from 4–35 A . For the same diameter, the Young’s modulus of zigzag single-walled carbon nanotubes is higher than that of armchair nanotubes and is sensitive to tube diameter at lower tube diameters. This sensitivity decreases remarkably with increasing tube diameter. Young’s modulus of double-wall carbon nanotubes as a function of outside tube diameter (Fig. 4). The calculation of Young’s modulus of double-wall carbon nanotubes resulted in values in the range of 1.32–1.39 TPa and is sensitive to the increase in tube diameter, and the values predicted for zigzag configuration are higher than those of their armchair counterparts (Fig. 5). It shows the variation of Young’s modulus for typical MWCNTs with increasing number of walls (or tubes) for both zigzag and armchair configurations. For multiwalled carbon nanotubes, the Young’s modulus varied from 1.39–1.58 TPa and the largest increase in modulus (about 35%) occurs when CNTs move from singlewalled to double-wall carbon nanotubes. However, subsequent increase in number of tubes results in a smaller increase of the elastic modulus (Fig. 6).

Shear Modulus and Poisson’s Ratio The shear modulus can be determined directly from torsion tests for isotropic materials (whose properties remain the same when tested in different directions) based on the values of Young’s modulus and Poisson’s ratio (the phenomenon in which a material

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Fig. 4 Young’s modulus of single-walled carbon nanotubes as a function of tube diameter (Kalamkarov et al. 2006)

Fig. 5 Young’s modulus of double-wall carbon nanotubes as a function of outside tube diameter (Kalamkarov et al. 2006)

tends to expand in directions perpendicular to the direction of compression.). Young’s modulus (E), shear modulus (G), and Poisson’s ratio (υ) are related in isotropic material by the equation [G ¼ E/2(1 þ υ)]. However, if the material is anisotropic (changes with direction along the object), the shear modulus is an independent material property and cannot be determined based on two other elastic parameters.

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Fig. 6 Variation of Young’s modulus with number of tubes in MWCNT (Kalamkarov et al. 2006)

Shear modulus “G” of single-walled carbon nanotube has been evaluated experimentally by measuring the torsional properties (Hall et al. 2006). Theoretically, shear modulus has been predicted using simulation results of the torsion tests (Ghavamian et al. 2013), the torsional response (Wu et al. 2006), and from the results of the tensile test and resorts to the relationship between the Young’s modulus and the Poisson’s ratio, under isotropic conditions (Jin and Yuan 2003). A robust methodology for evaluating the shear modulus from results of tensile, bending, and torsion tests was proposed by Pereira et al. (Pereira et al. 2016). The discrepancies observed in shear modulus values in the literature are due to the same as that of Young’s modulus data: (i) different assumptions for the value of the CNT’s wall thickness, (ii) modeling approaches (MD, CM, and NCM), (iii) potential functions, (iv) force fields constants, and (v) theoretical formulations for shear modulus determinations. Theoretically predicted shear modulus for SWCNTs was generally in the range of 0.3–0.5 TPa while the experimental observation was 0.41 TPa. A comprehensive data on Poisson’s ratio (between 0.15–0.35 for SWCNTs) and shear modulus is tabulated by Sakharova et al. (Sakharova et al. 2017). Kalamkarov et al. correlated the shear modulus of different CNTs with respect to tube diameter of SWCNTs, outer diameter of the double-walled CNTs, and the number of tubes in MWCNTs (Kalamkarov et al. 2006). Shear modulus values for SWCNTs were found to be in the range of 0.14–0.47 TPa which is about 35–50% less than that of the corresponding E values. As in the case of Young’s modulus, the shear modulus of zigzag nanotubes is higher than that of their armchair counterparts. Moreover, the shear modulus is more sensitive to the tube diameter than the Young’s modulus (Fig. 7). The shear modulus of the double-wall CNTs has been found to be in the range of 0.37–0.62 TPa and is sensitive to the increase in tube diameter, and the values predicted for zigzag configuration are higher than those of their armchair counterparts (Fig. 8).

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Fig. 7 Shear modulus of single-walled carbon nanotubes as a function of tube diameter (Kalamkarov et al. 2006)

Fig. 8 Shear modulus of double-wall carbon nanotube as a function of outside tube diameter (Kalamkarov et al. 2006)

The variation of shear modulus for typical MWCNTs varied from 0.44–0.47 TPa with increasing number of walls (or tubes) for both zigzag and armchair configurations. It can be observed that the largest increase in shear modulus (about 35%) occurs when it moves from single-walled to double-wall carbon nanotubes. However, subsequent increase in number of tubes results in a smaller increase of the shear

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Fig. 9 Variation of shear modulus with number of tubes in MWCNT (Kalamkarov et al. 2006)

modulus. The shear modulus is more sensitive to an increase in the number of tubules than the Young’s modulus (Fig. 9).

Strength of CNTs The strength of a material can be simply stated as a material’s ability to withstand an applied load without failure. A load applied to a material will induce internal forces within it is called stresses. The applied loads can be categorized into three types. (i) Transverse loadings where forces applied perpendicular to the longitudinal axis of the material. Transverse loading causes bending and deflection to induce shear deformation. (ii) Axial loading where applied forces are collinear with the longitudinal axis of the member. These forces cause the member to either stretch or shorten (tensile or compressive). (iii) Torsional loading causes twisting action by a pair of externally applied equal and oppositely directed force on parallel planes of the material or by a single external force applied to a member that has one end fixed against rotation. The stress acting on the material causes deformation of the material in various manners including breaking them completely. Deformation of the material is called strain when it is measured per unit basis. Compression strength is the capacity of a structural material to withstand loads tending to reduce size. Tensile strength is the capacity of a structural material to withstand loads tending to elongate. In other words, compressive strength resists being pushed together, whereas tensile strength resists it being pulled apart. The strength of a brittle solid depends on the size of the sample material. For example, the strength of graphite whiskers is around 20 GPa, but for the larger fibers, the strength reduces to around 1 GPa (Dresselhaus et al. 1988). This behavior is due to the fact that the number of flaws in smaller sized whiskers is considerably lower

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than the large fiber material. Griffith showed that in brittle solids, fracture occurs through local de-cohesion at the tip of an extending sharp crack instead of simultaneous bond breaking across the whole fracture plane. This is why real strengths are orders of magnitude lower than theoretical ones. The material strength depends on the type of the material, atmospheric conditions, temperature, pressure, and different types of measuring system. Structural defects and imperfections normally present in the solid materials also affect the strength and approaching the theoretical limit of material strength is always difficult because of that. It is usual to distinguish between two kinds of solids associated with two different types of stress/strain curves. The first one is typical of brittle conditions and is characterized by the absence of plasticity, i.e., the rupture occurs in the elastic regime. Ceramics and glasses usually exhibit a brittle breaking mechanism. The second behavior is typical of ductile conditions and is encountered in metals and simple ionic solids. The strength and the breaking mechanisms of a material depend largely on the mobility of dislocations and their ability to relax stress (Harris and Bunsell 1977). Strength of a material is temperature dependent. The CNTs exhibit brittleness at low temperature, and flexibility in room temperatures and higher temperatures. The CNTs are brittle at low temperature, irrespective of its diameter and helicity. The flexibility of CNTs at room temperature is not due to any plastic deformation but to their high strength and to the unique capability of relaxing stress through its hexagonal network. At higher temperatures, the strained CNTs show spontaneous formation of double pentagon–heptagon pairs with respect to applied stress due to the thermally activated motion of nanotubes (Lauginie and Conard 1997; Yakobson 1998). Initially CNTs form a single pentagon–heptagon pair and it further act as the nucleation center for further dislocations and deformations (Nardelli et al. 1998). At higher temperatures, when a tensile strain is applied along the tube axis, the orientation of the easy-gliding line of CNTs depends upon the diameter of nanotubes, helicity, and symmetry (Nardelli et al. 1998). The carbon nanotubes have interesting mechanical and deformational properties under different loading conditions, such as compression, tension, torsion, bending, and hydrostatic pressure. CNTs under progressive axial compressive forces tend to bend, twist, kink, and finally buckle. The tubes, however, do not break under the compressive loads. This behavior is consistent with the Euler limit. The Euler limit specifies the point at which a straight tube will buckle. Since the buckling behavior of nanotube is elastic, the tube returns to its original shape when the load is removed (Motevalli et al. 2012). The bending strength of NTs can be determined by measuring maximum strain observed at the pinning site of tube at the initial buckling point. The initial buckling point is taken as a measure of bend strength because after that the stiffness drops substantially (Fig. 10). The bending strength of large-diameter MWNTs was measured to be around 14.2 GPa (Wong et al. 1997). This is a large value compared to graphite fibers, where the value is around 1 GPa (Dresselhaus et al. 1988). The high bending strength of CNTs is associated with their high flexibility (Despres et al. 1995; Iijima et al. 1996; Falvo et al. 1997; Lourie et al. 1998) However, one must be aware of identifying the

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Fig. 10 Elastic buckling mechanism of carbon nanotubes

bending strength with the tensile maximum stress or the fracture stress in such an anisotropic material. One reason for this is the inhomogeneous stress that results from the rippling and buckling of the graphite layer on the compressed side.

Conclusion In conclusion, the SWCNTs, with its hollow center and chicken-wire-like structure, is about 100 times stronger than steel. Due to its structural characteristics, SWCNT is only one-sixth the weight of copper and steel, and about half the weight of aluminum (Smalley and Hackerman 2004). However, it is extremely difficult to quantify the tensile strength of CNTs and an exact numerical value has not been agreed upon. Atomic-force-microscope-based measurement of the tensile strength values ranging from 13–52 GPa were reported for SWCNTs (Yu et al. 2000a, b, 5552) while that of MWCNTs (Yu et al. 2000a, b, 637), the values ranging in 11–63 GPa. Also, it is found that only the outermost layer of MWCNTs breaks during the loading process of tensile testing. Overall, CNTs seem to behave as ideal carbon fibers that can be stiff yet flexible, associating very high modulus with very high strength. Therefore, CNTs have great potential for applications requiring high-modulus high-strength materials (Salvetat et al. 1999). It is now well established that carbon nanotubes are ideal model systems in one-dimensional solids and have significant potential as building blocks for various practical nanoscale devices. To take full advantages of CNTs as a component in composite material, it needs to show very high load transfer efficiency. The load transfer efficiency in CNT composite is based on how effectively the applied load is transferred from the low strength matrix to high strength CNTs, which again depends

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solely on the adhesion between the matrix and the CNTs. Interfacial bonding can be improved either through an increase in the surface roughness or through the surface reactivity in traditional fibers. The functionalization of CNTs cost-effectively without creating too many defects is still an ongoing research area.

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Contents Introduction to Carbon Nanotubes (CNTs) and Their Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dispersion of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nature of Difficulties for CNTs Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNTs Dispersion Through Mechanical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNTs Dispersion Through Functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Noncovalent Functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Alignment of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNTs Alignment Using Van der Waals Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNTs Alignment Using Magnetic Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNTs Alignment Using Electric Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNTs Alignment Using Shear Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNTs Alignment Using Extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNTs Alignment Using Pulling Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Properties of CNTs-Embedded Polymer Composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Role of Dispersion Technique on Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Role of Alignment Technique on Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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A. O. Fulmali FRP Composites Laboratory, Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, India S. K. Ramamoorthy Department of Textile, Engineering and Economics, University of Borås, Borås, Sweden e-mail: [email protected] R. K. Prusty (*) FRP Composites Laboratory, Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, India Center for Nanomaterials, National Institute of Technology, Rourkela, India e-mail: [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_63

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Abstract

One-dimensional carbon nanotubes (CNTs) have outstanding mechanical properties, making them a good candidate for reinforcement application in polymer and fiber-reinforced polymer composites. Superior properties of the CNTs are exploited regularly by reinforcing these nanotubes in a polymer matrix. However, strong Van der Waals interaction energy of tube-tube contact, high electrostatic interaction between the tubes, small tube size, and large surface area of the tubes render CNT dispersion a problematic task. Therefore, to improve its dispersion and alignment in the composite, researchers have developed innovative techniques to strengthen the properties of the composite. For achieving optimum and reproducible mechanical properties in a composite, fine dispersion of CNTs, their alignment, and strong interfacial adhesion with polymer is a demand to be guaranteed. In this chapter, the principles and techniques for uniform dispersion and alignment of CNTs in the polymer and fiber-reinforced polymer composite are discussed. Keywords

Carbon nanotube (CNT) · Dispersion · Alignment · Functionalization · Mechanical properties

Introduction to Carbon Nanotubes (CNTs) and Their Properties CNTs are a one-dimensional nanostructured allotrope of carbon that differs from other allotropes such as diamond, fullerene, and graphite in structure. Depending on the number of concentric tubes (single or multiple) of graphite planes, CNTs can be classified into two types, single-walled CNTs (SWCNTs) (Fig. 1a) and multiwalled CNTs (MWCNTs) (Fig. 1a). Simultaneously, these concentric tubes are held in

Fig. 1 TEM images of (a) SWCNTs (Kong et al. 1998) and (b) MWCNTs (De et al. 2020)

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position due to inter Van der Waals forces. They have a nanoscale diameter and can have an even more significant aspect ratio than 1000 (Thostenson et al. 2001). One or both ends of this tube nanostructure can be closed with capped shape fullerene structure. The angle in which the graphene sheet is rolled decides the chiralities of the CNTs: chiral one, zigzag, and armchair. Chiral vector is used to define the tube chirality, Ch ¼ na1 + ma2 (Fig. 2). In the hexagonal lattice of CNTs, n and m are the number of steps along the a1 and a2 unit vectors (Thostenson et al. 2001). The carbon atoms orientation in the CNTs circumference is decided by the n and m, which differentiate the CNTs into three types. The CNTs are called “armchair” if n ¼ m and “zigzag” if m ¼ 0. In other cases, CNTs are called “chiral.” The transport properties of CNTs, especially the electronic properties, are majorly decided by their chirality. The CNTs will have metallic nature if the (2n + m) is a multiple of 3, or else it will be a semiconductor. The prediction of MWCNT’s mechanical properties can be challenging compared to SWCNTs due to the possibility of different chiralities in each graphene layer. Experimentally, the CNTs are normally less perfect than the idealized versions shown in Fig. 2. The CNTs are entirely made up of carbon atoms chemically bonded together by sp2 hybridization. Due to these sp2 bonding, CNTs get their exceptional mechanical properties. This sp2 bonding structure even exceeds in strength compared to sp3 bonds present in a diamond. This allows CNT to outperform any other known material in terms of mechanical properties. However, researchers are yet to agree on the CNT’s explicit mechanical properties, but the theoretical and experimental evaluation showed exceptional Young’s modulus and tensile strength of about 1.2 TPa and 50–200 GPa, respectively (Ma et al. 2010). With such properties, CNT holds the title of strongest and stiffest material known to humankind. In addition to superior mechanical properties, CNTs have other magnificent physical properties, as tabulated in Table 1. As observed in Table 1, CNTs hold an advantage in thermal and electrical properties compared to other considered carbon materials. These properties make CNT a promising material for various applications

Fig. 2 (a) Schematic diagram demonstrating CNT formation by rolling a hexagonal sheet of graphene with different chiralities ((b) armchair, (c) zigzag, and (d) chiral) (Thostenson et al. 2001)

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Table 1 Physical properties of MWCNT, SWCNT, fullerene, diamond, and graphite (Hwang et al. 2011) Property Specific gravity (g/cm3) Thermal conductivity (W/(m K)) Coefficient of thermal expansion (K1) Thermal stability in air ( C) Electrical conductivity (S/cm) Electron mobility (cm2/ (V s))

Carbon material MWCNT SWCNT 1.8 0.8 2000 6000

Fullerene 1.7 0.4

Diamond 3.5 900–2320

Graphite 1.9–2.3 298p, 2.2c

Negligible

Negligible

6.2105

>600

>600

~600

(1 ~ 3) 106 ω1), a necessary extensive shear rate is produced (Fig. 9c). Due to the high shear rate at the thin gap between feed and center roller, CNTs or other nanofillers present in viscous polymer get pre-dispersed. Then the material is transported to another gap as it sticks to the central roller and flows down. In the final gap between the center and apron roller, the desired degree of dispersion is obtained. While leaving the device through the final gap, the intensive shear force is exerted on the material due to the apron roller’s higher angular velocity than the center roller. A scraper blade is used to sweep processed polymer suspension off the apron roller, and it is collected in the apron. Multiple milling cycles can be conducted to achieve fine dispersion. The combined effect of a narrow gap and different angular speeds of the rollers create high shear force locally with the advantage of short residence time. Furthermore, the width of the gap can be easily controlled using a hydraulic and mechanical method, which can ensure fine dispersion of nanofillers in the viscous polymer. For more than a decade calendering technique is successfully employed to achieve fine CNTs dispersion in a polymer composite as per different reports (Jiménez-Suárez et al. 2020). With this technique, the application of high shear stress can disentangle and disperse CNTs into the viscous polymer, and a short residence time will limit the chances of nanotubes breakage. One more reason for using this technique is its alignment effect on the CNTs, which will be explained later in this chapter. Still, various concerns restrict this technique’s vast applicability for CNT dispersion: Such as the minimum width of the gap can be maintained between 1 and 5 μm, which is substantially more significant than the diameter of discrete CNTs but closely equivalent to its length. Due to this significant dimensional variation, the

Fig. 9 (a) Calendering equipment and (b and c) its schematic diagram illustrating the configuration and working technique (Akpan et al. 2019; Thostenson and Chou 2006)

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calendering technique can effectively disperse large CNT agglomerates compared to smaller agglomerates with few CNT. Furthermore, the primary condition for implementing this technique is that CNT dispersion can only be carried out in materials that remain in a viscous state during processing. This condition does not allow thermoplastic matrices, for example, polystyrene, polypropylene, and polyethylene, for dispersing CNTs. On the contrary, liquid oligomer or monomer of thermosetting polymers can effectively use this tool for CNT dispersion.

Extrusion Extrusion is extensively used for easy CNT disperse into most thermoplastic polymers by feeding a mixture of polymer pellets and CNTs into the equipment hopper. This technique is famous for making CNT/polymer compounding and mixing. Extrusion tool is more convenient for industrial use for producing polymer melt compounding due to the lesser processing step required. The high-speed rotation of twin screws produces the necessary high shear flow for mixing and dispersing CNTs in the polymer melt (Fig. 10a). One significant advantage of this equipment’s modular design is its ability to be tuned explicitly for the desired formulation of the material being processed. For instance, the two screws can be operated in intermeshing or nonintermeshing, as well as co-rotating or counter-rotating configuration. Further flexibility is to vary the layout of the screw by using kneading blocks, forward conveying components, reverse conveying components, or any other part intended for unique CNT/polymer mixture properties (Fig. 10b). Generally, this technique is convenient for the production of nanocomposites with high filler contents. However, utmost care needs to be taken to avoid CNT damage because of extreme shear stresses inflicted during the extrusion process. The factors deciding the dispersion quality are feeding, screw configuration, screws angular speed, torque, and temperature across die and barrel.

Fig. 10 (a) Twin-screw extruder and (b) different options for the layout of the screw (Hwang et al. 2011)

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CNTs Dispersion Through Functionalization Functionalization of CNTs has become very popular to reduce interaction between adjacent nanotubes to improve its dispersion and stabilization in the polymer matrix. In CNT functionalization, the stable carbon atoms on CNTs surface are attached with a polar functional group. These functional groups are responsible for reducing the Van der Waals interaction between adjacent CNTs and producing strong chemical bonding with the polymer matrix. Functionalization can be categorized into covalent and noncovalent functionalization. However, one has to remember that the CNTs are assembled as bundles or ropes, and there could be some catalyst residues, amorphous carbon, spheroidal fullerenes, and other forms of impurities in as-grown CNTs. For this reason, researchers prepare the CNTs before functionalization through various steps such as purification, disentangling or cutting, and activation.

Covalent Functionalization In covalent functionalization, CNTs surface carbon atoms are changed from sp2 to sp3 hybridization by covalent bond formation with the functional group. To attain this covalent interaction, defect-group and sidewall functionalization are plausible strategies (Fig. 11). Generally, to achieve these surface modifications of CNTs, they are treated with different oxidizing agents such as reactive gas, oxygen, nitric acid, sulfuric acid, their mixtures, etc. to create carboxylic, ester, hydroxyl, or carbonyl groups on the CNTs surface. These processes are termed silanization, hydrogenation, bromination, chlorination, carbene and nitrene incorporation, cycloaddition, and fluorination. Fourier transform infrared spectroscopy (FTIR) can confirm oxidation and presence of different functional groups on the CNTs surface. As shown in Fig. 12a, b, comparing FTIR spectrum of pristine and carboxyl functionalized CNTs (FCNTs), the occurrence of a peak at 1240 cm1 and 1740 cm1 confirms the presence of C─O and C═O bond stretching due to the carboxyl (-COOH) group (Prusty et al. 2017). The oxidizing agent and condition decide the quantitative number of functional groups generated during the oxidation process. During the oxidation process, CNTs ends can be opened to remove amorphous carbons and residual catalysts. With the help of functional groups, CNTs can form chemical bonding with the polymer by in situ polymerization. During in situ polymerization, monomers free radicals form a linkage with each other and the functional group as shown in Fig. 12c–f. This type of chemical link of polymer with the FCNTs is way stronger than weak Van der Waals interaction with pristine CNTs; in exchange, it will improve the composite’s mechanical, thermal, and electrical performance. Different functional groups show varying degrees of interaction between the CNTs and polymer matrix. Furthermore, the hydrophobic nature of pristine CNTs is changed to hydrophilic by functionalization, which results in improved wettability and strong interaction at the interface. In our earlier work, we observed better dispersion (Fig. 13) and mechanical properties from glass fiber/epoxy (GE) composite reinforced with carboxylated CNTs than pristine CNTs using the in situ polymerization method (Fulmali et al. 2020).

R

F

RMgBr/RLi

Amino substitution

Carbene

Nitrene

H (a)

(b)

O OH

Polymer grafting

23

Grafting from2 Monomer 1

to1

O-(M-M)n

(M1)n-(M2)m

(M1)n

O

Si-R Silanization

Metal nanoparticles

Metal ions (Au, Ag, etc) Thiolation

SH-R

Grafting

O

O

OR Esterification

Amidation

NH-R-NH2

2

Monomer CNTs react with reactive polymers; Functionalized CNTs act as initiators to initiate polymerization; 3 Living polymerization, CNT-copolymer can be obtained; n, m: Degree of polymerization.

1 Carboxylic

R: Alkyl, aryl, etc

SWCNT or MWCNT

O

O

Fig. 11 Strategies for covalent functionalization of SWCNTs: (a) sidewall and (b) defect-group functionalization (Ma et al. 2010)

Radical (R·) attachment

R

EtOOC EtOOC n Nucleophilic Diels-Alder cyclopropanation Cycloaddition

Cl Cl

ROOC-N

Hydrogenation

T>450 °C Dehydrogenation

Fluorination and derivative reactions

NH2

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a Intensity (a.u.)

UCNT O-H C=C 500

1000

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2500

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4000

−1

Wave number (cm )

b Intensity (a.u.)

FCNT O-H C-O

500

C O H

c

1000

C=O C=C 1500 2000 2500 Wave number (cm−1)

3000

3500

4000

Polymer Pristine CNT

d

Physical bonding

e

Carboxyl (-COOH) CNT

f

Chemical bonding

Fig. 12 (a and b) FTIR spectrum of pristine and carboxylated CNTs and schematic representation of (c and d) physical and (e and f) chemical interaction between the CNTs and polymer matrix (Prusty et al. 2017)

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Noncovalent Functionalization Even with the advantage of improved dispersion and mechanical performance, covalent functionalization comes with its drawback. Covalent functionalization has a detrimental effect on the physical properties of CNTs, due to damage inflicted to CNTs sp2 hybridization. High energy imparted during the oxidation process by ultrasonication can result in defects and breakage of CNTs. Furthermore, the use of concentrated acids utilized in this process is harmful to the environment. In noncovalent functionalization, surfactants are adsorbed on the CNTs surface, which provides electrostatic repulsion for improved dispersion into the polymer without compromising the nanotubes’ integrity (Fig. 14a). Surfactants have a hydrophilic and hydrophobic part, prior form a chemical bond with the polymer and later interact with the CNTs. The degree of interaction between surfactant and CNTs and their dispersion in the polymer matrix is determined by the type, length, headgroup size, and charge of its hydrophobic and hydrophilic regions (Ma et al. 2010). Another effective method for noncovalent functionalization of CNTs is the polymer wrapping on the CNTs surface by π-stacking and Van der Waals interactions, as shown in Fig. 14b (Hirsch 2002). For this wrapping method, polymers chains with aromatic rings are usually preferred. SWCNTs wrapped in regioregular poly(3-alkylthiophene) exhibited enhanced dispersity and electron mobility of 12 cm2 V1 s1(Lee et al. 2011).

Fig. 13 SEM images of (a) pristine CNTs and (b) carboxylated CNTs dispersion in epoxy polymer (Fulmali et al. 2020)

Fig. 14 Schematic of noncovalent functionalization with (a) surfactants, (b) polymer, and (c) endohedral functionalization with C60 (He et al. 2013)

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In another noncovalent functionalization procedure called an endohedral method, molecules or guest atoms are placed inside the nanotubes hollow channel using capillary action, as shown in Fig. 14c. Sidewalls or ends of CNTs with localized defects are used as a gateway for this insertion. Small biomolecules and inorganic nanoparticles, like DNA, proteins, Pt, Au, Ag, and C60, are widely used for endohedral functionalization (Georgakilas et al. 2007). With this approach, hybrid materials with integrated properties of guest molecules and CNTs for molecular scale devices, nanotechnology, energy storage, and catalysis application can be achieved. Refer (Bilalis et al. 2014) for more noncovalent functionalization methods. In Tables 3 and 4, the above-discussed dispersion techniques through mechanical and functionalization are compared. This comparison will help the selection of suitable dispersion techniques for the preparation of CNT-embedded polymer composite. However, researchers should note that the described techniques do not cover all the available methods. Most of the studies are observed to conduct a combination of discussed dispersion methods, for example, ultrasonication with stirring (Huang and Terentjev 2010), extrusion with ultrasonication (Mirjavadi et al. 2018), and ball milling with ultrasonication (Carneiro et al. 2020). For further improvement in dispersion and interfacial properties, functionalization and mechanical dispersion techniques are also combined.

Alignment of CNTs The particular orientation of CNTs is crucial for prompting specific nano effects in the polymer and fiber-reinforced composites. CNT’s successful orientation in the polymer matrix can build or destroy photoelastic response and Van der Waals interaction between them. On the contrary, some of the CNT’s properties include diffusion, as well as electrical and thermal conductivity, etc. which are directional, means depends on their orientation. To extract functional properties from CNT-embedded polymer and FRP composite, designing a material with a background of these effects will be helpful. With tremendous opportunities and challenges with the alignment of CNTs in the composite, some successful alignment methods are discussed as follows. CNTs alignment can be briefly categorized into two techniques called the in situ and ex situ alignment process, where the alignment of CNTs during growth and post-growth is achieved. Some of these techniques’ principles and influence on polymer and FRP composites’ mechanical properties are reported in the following section.

CNTs Alignment Using Van der Waals Interaction Commonly, the chemical vapor deposition (CVD) technique is employed to fabricate CNTs. Aligning these CNTs during their fabrication process is called in situ alignment technique. As shown in Fig. 15a, b, vertically aligned CNTs (VACNTs) are growing at a particular location on the substrate during the CVD process. This

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Table 3 Comparison of different mechanical techniques for dispersion of CNTs in a polymer matrix (Akpan et al. 2019) Dispersion technique High shear stirring

Factor Damage to CNTs No

Ultrasonication

Yes

Ball milling

Yes

Calendering

No, may align CNTs

Twin-screw extrusion

No

Suitable matrix Oligomers or monomers of thermosetting polymer in a viscous or liquid state Oligomers or monomers of thermosetting polymer in a viscous or liquid state Thermoplastic and thermosetting polymers in a solid or liquid state Oligomers or monomers of thermosetting polymer in a viscous or liquid state Solid thermoplastics

Controlling factors Rotating time, speed, and shape of blades

Solvent Required

Applicability Laboratory and industrial production

Required

Only for laboratory

Sonication duration and power

Not required

Laboratory and industrial production

Ball/CNTs ratio, rotating speed, size of ball, and operation time

Not required

Laboratory and industrial production

Width of gap and rollers speed

Not required

Laboratory and industrial production

Rotating speed, screw configuration, and processing temperature

location is a high-density catalytic deposition, which provides a high growth yield necessary for this alignment. Furthermore, these depositions need to be as closely packed as possible for successful alignment. The Van der Waals forces between adjacent CNTs bundle are key to this alignment method (Fan et al. 1999). The CVDs reaction time controls the nanotube length, and these bundles can grow up to few millimeters in the absence of any obstacles. Water-assisted etching can be continuously conducted to remove the end cap of CNTs for getting stacks of VACNTs (Zhu et al. 2005). Also, alignment is highly dependent on the thickness of the catalytic deposition. A thin film will produce VACNT, whereas a thicker film will entangle CNT (Nessim et al. 2010). Zeolites, aluminophosphate (AlPO-5), liquid-crystal matrices, aluminum membrane channel, or mesoporous silica are some of the materials used as substrates VACNTs formation.

Noncovalent functionalization

Dispersion technique Covalent functionalization

Surfactant adsorption Polymer wrapping Endohedral method

Sidewall

Defect

No No No

π stacking

Capillary effect

Yes

sp2 to sp3 hybridization of carbon atoms Physical adsorption

Principle Transformation of defect

Damage to CNTs Yes

Variable as per the miscibility of polymer wrapping in matrix Weak

Weak

Strong

Interaction with polymer matrix Strong

Table 4 Comparison of different functionalization methods for dispersion of CNTs in the polymer matrix

Yes

No

No

Yes

CNT reagglomeration in matrix Yes

Difficult

Easy

Easy

Easy

Practicality Difficult

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Fig. 15 VACNTs created by (a, b) CVD (Ahn et al. 2019) and (c) SAM technique

In another approach, FCNTs are used to fabricate thin film of aligned CNTs(Goh et al. 2019). Contrary to catalytic deposition, the functional group on the CNTs provides strong interaction with the substrate in this method (Fig. 15c). This strong interaction prevents the CNTs from collapsing. Same in the CVD process, Van der Waals interaction at the tail of CNTs will result in self-assembled monolayers (SAM) formation. Electrostatic interaction, surface condensation, and Au-S bonding are strategies that can be used to create SAM. For more details on the SAM method for VACNTs, please refer review article by Diao and Liu (2010).

CNTs Alignment Using Magnetic Field The magnetic field can be used for the in situ and ex situ alignment of CNTs by placing the substrate within the magnetic field range. This method allows contactless alignment in polymer and FRP composites. During the CVD process, a magnetic field is applied to manipulate the orientation of the catalytic nanoparticles (Ren et al. 2013). By doing so, nanoparticle crystalline facets with the highest carbon diffusion rate can be oriented in a particular direction to facilitate the creation of aligned CNTs. The type of catalytic nanoparticles will decide CNT alignment in one or more directions. Generally, ferromagnetic nanoparticles of cobalt, nickel, and iron are used as a catalytic deposition due to their ability to align parallel to the magnetic field. In ex situ alignment using a magnetic field, polymer with dispersed CNTs is placed in a strong magnetic field during fabrication of nanocomposites and FRP

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Fig. 16 (a) Schematic and (b) actual setup for CNT alignment using a magnetic field in polymer composite (Sharma et al. 2010)

composite as shown in Fig. 16a, b. Liquid polymer medium allows CNTs to rotate and align along the magnetic field freely. The driving torque for this rotation and alignment is produced due to CNT’s anisotropic magnetic properties. The alignment’s quality is decided by the polymer’s hydrodynamic drag to the driving torque in CNTs (Sharma et al. 2010). As per Korneva et al. (2005) study, CNTs filled with iron oxide particles during the fabrication (CVD) process improve their response to the magnetic field. The amount of magnetic field strength required for satisfactory alignment is reduced to a few Tesla from 10–30 T. In another article, procured carboxylated MWCNTs was deposited with magnetic particles of iron oxide using the coprecipitation method to control MWCNTs orientation effectively (Yavari et al. 2019). To create such a strong magnetic field, MRI magnet bore, superconducting magnet, and DC resistive solenoid are used as sources. The application of a magnetic field is a promising approach for achieving alignment in a larger composite area. It can also guarantee better CNT dispersion than the electric field method, as no CNTs are moved from their location except for a change in their orientation.

CNTs Alignment Using Electric Field The electric field can also be used for CNT alignment during growth and post-growth in polymer and FRP composites, like the magnetic field. This method utilizes the anisotropic polarizability of CNTs to generate electrical torque and orient it parallel to the electric field. In the in situ alignment process, the substrate is connected to an external electric field to produce aligned CNTs. Depending on the type of electric field, i.e., AC and DC, the strength of current required for aligning SWCNTs along its field direction

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will vary; for example, the AC electrical field would require double the DC electrical field’s strength. However, applying an external electric field will require many contact pads to the target electrodes during large-scale CNT arrangement. Hence, plasma-enhanced CVD (PECVD), which requires no external biasing, is utilized for aligning CNTs in horizontal or vertical directions (Cassell et al. 2004; Law et al. 2007). For this purpose, different types of PECVD are developed, such as electron cyclotron PECVD (Minea et al. 2005), hot-filament PECVD (Hayashi et al. 2001), microwave PECVD (Bower et al. 2000), and radiofrequency PECVD (Law et al. 2007). For the ex situ alignment process, two electrodes attached to electrical potential can be placed at the opposite terminal of the bulk composite, as depicted in Fig. 17. For a thin film of CNT-embedded polymer composites, these electrodes are interdigitated (Liu et al. 2004), as shown in Fig. 18. Here, the quality of alignment depends on the polymer viscosity, applied electric field strength (voltage), and frequency. Using high electrical voltage and frequency with a polymer/material with low viscosity could deliver maximum alignment efficiency (Fig. 19a, b). Both AC and DC electric field is observed to be effective in aligning CNTs. However, with DC electric field, CNTs get accumulated at the electrode’s surface, forming a nonuniform dispersion. Hence, AC electric field is generally used for improved CNT dispersion and alignment. With the AC electric field, the required voltage is also lower than the DC electric field. Furthermore, its frequency is one of the significant factors which can improve the degree of alignment. At a high AC frequency of 5 MHz, Chen et al. (2001) observed efficient alignment with straightened SWCNTs compared to aggregated and tangled SWCNTs at a low frequency of 500 Hz (Fig. 19b, c). Ma et al. (2008a) studied the effect of CNTs functionalization on its alignment and reagglomeration in poly(methylmethacrylate) (PMMA) matrix under voltage 300 Vp-p and 500 Hz frequency and ultrasonication. They observed

Fig. 17 Schematics showing application of electric field for ex situ CNT alignment in (a) polymer and (b) FRP composite

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Fig. 18 (a) Schematic (Rivadeneyra et al. 2014) and (b) actual interdigitated electrode setup with aligned CNTs for thin polymer film (Liu et al. 2004)

Fig. 19 SEM images of SWCNTs aligned using voltage and frequency of (a) 10 Vp-p and 5 MHz, (b) 6 Vp-p and 5 MHz, and (c) 10 Vp-p and 500 Hz, respectively (Chen et al. 2001)

ultra-fine alignment and no reagglomeration of FCNTs than thick bundles of pristine CNTs even after 180 min, as shown in Fig. 20.

CNTs Alignment Using Shear Force The sheer force method can only be used for ex situ alignment of CNTs in polymer and FRP composites. The elastic force induced during peeling, pushing, and cutting is responsible for the alignment of CNTs. As shown in Fig. 21a, the peeling-off method aligns CNTs in a vertical direction by removing a polymer membrane layer with CNTs packed between them (Zhao et al. 2013). The CNTs should be oriented adequately before initiating the peeling process to get effective alignment. Mechanical stretch force and shear force generated during peeling action are behind the out-of-plane alignment of CNTs. When substrate with vertically aligned CNTs is pushed in one direction with a plastic surface, CNTs tend to align in that direction (Fig. 21b). Usually, the CVD or

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Fig. 20 Digital camera images of (a) pristine CNTs and (b) oxidized CNTs degree of alignment in PMMA polymer at applied voltage and frequency of 300 Vp-p and 500 Hz, respectively (Ma et al. 2008a)

Fig. 21 Schematics of CNT alignment using (a) peeling method (Zhao et al. 2013), (b) pushing method (Wang et al. 2008), and (c) cutting method (Ajayan et al. 1994)

CNT suspension is used to obtain this thin film of CNTs on a substrate. This method is suitable to produce densely packed CNT alignment in thin films. Wang et al. (2008) used a similar domino pushing method and made buckypaper with tightly aligned CNTs in a larger area. Miansari et al. (2015) applied a variation of this method. He first deagglomerated CNT bundles with surface acoustic waves in a piezoelectric substrate and later used glass slides for shearing and aligning them. However, this method can damage thin films due to the shearing of the polymer as well. Another method that uses shearing is the cutting method, as represented in Fig. 21c. This method is commonly used for bulk nanocomposites with low nanotube density. As the name indicates, the nanocomposites are sliced in a particular direction with a sharp knife; this action aligns CNTs in the cutting path on the sliced surface (Ajayan et al. 1994). This method can achieve effective alignment in a film

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thickness of 50 nm to 0.5 μm; beyond 0.5 μm the alignment efficiency decreases. This method showed good alignment for CNTs with a length and diameter of several hundred nanometers and < 10 nm, respectively.

CNTs Alignment Using Extrusion The extrusion technique can also be used to produce aligned CNT/polymer rope or yarn. This technique will align CNTs along the drawing direction. From the extrusion-based method, melt fiber spinning and electrospinning can be used for nanocomposites. In the melt fiber spinning technique, primarily CNTs are dispersed in the polymer and followed by fiber extrusion to fabricate nanocomposites fiber with aligned CNTs. This technique can employ various polymer materials such as polycarbonate, PMMA, and more (Vigolo et al. 2000) for matrix materials. These polymers will be in a melted form before drawing into a fiber. In this method, fiber drawing speed is a crucial factor in controlling the degree of CNT alignment (Du et al. 2005). Lewicki et al. (2017) used a direct ink writing process to produce an effectively aligned CNT/epoxy composite. They observed a 37% increment in tensile strength due to the CNT alignment. This increment was highly dependent on the nozzle diameter, feed rate, and loading of CNTs. In the electrospinning technique, a polymer with dispersed CNTs is extruded through a nozzle to a metal collector to fabricate fine polymer fibers (Fig. 22). CNTs are premixed in the polymer and align along the extrusion direction due to a strong electric field in the range of 0.3–200 kV cm1 applied between nozzle and collector. This method is equally useful for the alignment of both SWCNTs and MWCNTs. It was observed that the type of polymer used for nanofiber controls the degree of CNT alignment.

Fig. 22 Schematic of the electrospinning alignment process (Sen et al. 2004)

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CNTs Alignment Using Pulling Method During the pulling method, mechanical forces are applied to the CNT/polymer film or pure CNT film and align parallel to the stretching direction (Frogley et al. 2002). The required aligning torque for CNT alignment can be used by stretching of CNT/polymer film, which induces shear forces at the nanotube/polymer interface. Whereas, in the case of pure CNT film, aligning torque can be applied at the CNTs end with entanglement. To allow alignment of CNTs in the polymer film, the polymer’s softening is done by dipping it in an alcohol solution or melting it. The stretching method is useful for CNT alignment in different polymers such as polyvinyl alcohol, polypropylene, polyurethane, and polyhydroxyaminoether. In this technique, the strain is observed to control the degree of alignment; a higher strain in the range of 40–80% will produce better alignment. Even though this method is simple, industrial usage is not favored due to skilled labor requirements and time consumption. One more disadvantage of this method is the difficulty in achieving aligned CNTs at the film’s edge. Another variation of the pulling process is the fiber drawing method, which produces yarns of highly aligned CNTs from the VACNT forest, as shown in Fig. 23a (Nam et al. 2016). This method can be used to form a CNT sheet, as shown in Fig. 23b. During fiber drawing, CNTs end to end interaction due to Van der Waals forces help produce continuous CNT fiber. Pulling tension and CNTs loading will decide the alignment degree in drawn fiber or sheet; an increase in factors above will improve CNT alignment. In Table 5, all the reported alignment techniques advantage and disadvantage are summarized.

Mechanical Properties of CNTs-Embedded Polymer Composite The Role of Dispersion Technique on Mechanical Properties Hong et al. (2015) reported a substantial increment in young modulus of FCNT (-COOH)/epoxy composite than pristine CNT/epoxy composite, whereas this increment was double compared to neat epoxy. For uniform dispersion of CNTs/FCNTs

Fig. 23 (a) CNT yarn production from vertically aligned CNT forest with SEM image showing continuous CNTs aligned horizontally, and (b) schematic showing aligned CNT film formation from VACNT array (Nam et al. 2016)

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Table 5 Advantages and disadvantages of various alignment techniques (Goh et al. 2019) CNT Alignment techniques Van der Waals interaction

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Advantages Covers large area Scalable CNTs can be patterned by arranging the SAM or catalyst Contactless approach Remote action Suitable for viscous polymer/ liquid Enhance CNTs dispersion Reduce agglomeration Less time consuming Suitable to viscous polymer/liquid

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Disadvantage Only applicable to high-density CNTs Not applicable to fiber-reinforced composite material

In situ/Ex situ alignment Both in situ and ex situ alignment

Alignment type Vertical alignment of CNTs

Strong magnetic source/field is required Paramagnetic particles are filled inside CNTs Difficulty in separating CNTs

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3D alignment of CNTs

Contact between electrode and material is required CNTs may accumulate at the electrodes CNTs may migrate Frequency of electric field can control the quality of alignment Can damage material during the process Film thickness decides the effectiveness of alignment Only suitable for low CNTs density May induce waviness in CNTs May produce CNT agglomeration Fiber formation is possible No alignment at the film edge due to inhomogeneous stretching Limited sample size Time consuming Require skilled labor

Both in situ and ex situ alignment

Vertical and 3D alignment of CNTs

Only ex situ alignment

In-plane alignment of CNTs

Both in situ and ex situ alignment

Aligned fiber thread and aligned fiber-reinforced matrix

Only ex situ alignment

In-plane alignment of CNTs and yarns of CNTs

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in an epoxy matrix, they conducted three roll milling processing for several min and gradually reduced the gap between rollers. We studied the effect of chemical functionalization of CNTs on the mechanical and creep performance of the polymer composite by introducing CNTs functionalized with a carboxyl group (FCNTs) and unmodified CNTs (UCNTs) to the epoxy (EP) composite using ultrasonication and magnetic stirring dispersion technique (Rathore et al. 2017). As illustrated in Fig. 24, the FCNT-EP composite showed improved flexural strength and modulus compared to the UCNT-EP composite at all studied in situ temperatures. Furthermore, a similar improvement in creep performance was observed by FCNT-EP composite at 50  C, 70  C, and 90  C in situ temperatures. In the UCNT-EP and FCNT-EP composite, the enormous surface area of CNTs was converted to a large CNT/epoxy interfacial area with stronger interfacial bonding due to chemical interaction between epoxy and FCNTs, facilitated effective stress transfer between soft polymer and stiff CNTs. Also, added CNTs may have reduced free volume in polymer and polymer chain slippage. Well, dispersion of FCNTs due to functional group improved significantly with noticeable CNT pullouts and cracked bridging in Fig. 25. We studied the effect of in situ temperature variation on the mechanical properties of GE and CNT-GE composite; CNTs dispersed by ultrasonication and magnetic stirring (Prusty et al. 2015). As represented in Fig. 26a, b, at a testing temperature of 80  C, impressive reinforcement efficiency of 30% by CNTs was observed. This improvement could result from enhanced matrix hardening and interfacial interlocking by the CNTs at such low temperatures. However, increasing testing temperature to 70  C, reinforcement efficiency dropped to 23%, which indicates the impact of testing temperature on the CNT/polymer interface. With further increase in testing temperature to 110  C, higher degradation in properties was reported in CNT-GE composite than GE composite. Higher interfacial area of CNT resulted in multiple debonding sites at elevated temperature, resulting in the CNT-GE composite’s early gross failure. We used similar dispersion techniques to disperse CNTs in the epoxy matrix in another article (Rathore et al. 2016). We performed optimization of CNT content in GE composite, and its response to different in situ

Fig. 24 (a) Flexural strength and (b) modulus variation for neat EP, UCNT-EP, and FCNT-EP with in situ temperature (Rathore et al. 2017)

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Fig. 25 SEM images of (a) FCNT dispersion, (b) pullouts, and (c) crack bridging (Rathore et al. 2017)

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temperatures was reported (Fig. 27). A 0.1 wt. % of CNT addition improved the flexural strength and modulus of GE composite by 32.8% and 11.5%, respectively, at 20  C in situ temperature. The addition of CNTs improved epoxy adhesion with the fiber and resulted in enhanced fiber/epoxy interfacial strength, which is evident by the deformed matrix in Fig. 28b; however, for higher CNT content of 0.5 wt. %, the flexural strength reduced due to CNT agglomerates, which can be a site for stress

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concentration and crack can generate as seen in Fig. 28d. With the increase in temperature, the properties degraded, resulting from variation in coefficient of thermal expansion (CTE) of composites constituent materials. Variation in CTE induces residual thermal stresses, which could have to generate microcracks and resulted in its failure. Shock conditioning of CNT-GE composite in liquid nitrogen can incite interfacial debonding at the CNT/polymer interface in addition to matrix embrittlement and development of residual stresses, exponentially degrading the CNT-GE composite’s mechanical properties compared to neat GE composite (Shukla et al. 2016). MWCNTs addition also helped reduce GE composite’s water absorption at lower condition temperature (25  C), with a diffusivity of 2.7  107 for GE composite and 2.2  107 for MWCNT-GE composite (Prusty et al. 2018). MWCNTs introduced a tortuous path for water absorption, which resulted in reduced water absorption and degradation in mechanical properties. Meanwhile, at elevated condition temperature (90  C), the MWCNT-GE composite showed more water absorption than the GE (2.3  106) composite with higher diffusivity of 2.7  106. An explanation for such variation in water uptake with condition temperature was the differential coefficient of thermal expansion of MWCNTs (0.73–1.49  105 K1) and epoxy (6.2  105 K1), which led to residual stresses and microcracks, facilitating more water absorption by the composite. Still, the CNT-GE composite produced better flexural strength and modulus than neat GE composite till saturation and regained properties to some extent after desorption. Yao et al. (2015) engineered carbon fiber/epoxy composite interface by introducing CNTs sizing on carbon fiber surfaces using a repetitious sizing treatment to improve its flexural interlaminar shear properties. During this process, ultrasonication was used to disperse and entangle CNTs in the suspension. Later carbon fiber was passed through this suspension to obtain CNT sizing over it, followed by various sizing treatments (zero, three, five, and seven) that controlled the CNT content. For five times sized carbon fiber/epoxy composite, 20.31% and 13.45% increment in flexural and interlaminar shear strength were obtained due to improved wetting and strong interfacial interaction between carbon fiber and epoxy. Excess sizing had a detrimental effect on the aforesaid mechanical properties of carbon fiber/

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Fig. 28 Fractured surface of (a) GE, (b) 0.1 wt. %, (c) 0.3 wt. %, and (d) 0.5 wt. % CNT-GE composite tested at 20  C temperature (Rathore et al. 2016)

CNTs/epoxy composites. In one of our studies, the electrophoretic deposition technique was used to alter carbon fiber surfaces with CNTs and fabricated CNT-modified carbon fiber/epoxy (CE) composite (De et al. 2020). During this work, ultrasonication was used to disperse and entangle CNTs in distilled water to prepare the electrolytic cell suspension. CNT-modified CE composite performed considerably well in terms of flexural and interlaminar shear properties at cryogenic (196  C), room (30  C), and elevated (120  C) testing temperature than unmodified CE composite. This increment was asserted to enhanced interaction at the interface due to the presence of CNTs and better stress transfer between fiber and matrix. Many such articles are available where a combination of multiple dispersion techniques with functionalization of CNTs is carried out to maximize the mechanical output of these nanotubes.

The Role of Alignment Technique on Mechanical Properties Using high shear stirring and alignment of CVD-grown MWCNT-reinforced epoxy composites, Sandler et al. were able to achieve a lesser percolation threshold than

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0.0025 wt. %, which was 0.07 wt. % with entangled MWCNTs (Sandler et al. 2003). Ürk et al. (2016) infiltrated polymer into the forest of VACNTs grown by the CVD process to fabricate epoxy (RTM6) nanocomposites. They improved storage modulus by 11% for 1% volume fraction (Vf) VACNT-RTM6 1% Vf randomly oriented CNTs (RCNT)-RTM6 composite. In comparison, this increment increased to 25% for 10% Vf VACNT-RTM6 than 10% Vf RCNT-RTM6 composite. Garcia et al. (2008) exponentially improved the interlaminar shear strength of FRP composite to 69% by incorporating woven fabric with aligned CNTs grown in situ on its surface by the CVD process, as shown in Fig. 29. Sharma et al. (2010) studied the effect of SWCNT/MWCNT alignment on the elastic constants and tensile strength of CNT/polycarbonate (PC) nanocomposites using a magnetic field (Fig. 16b). They observed a considerable increment in both the mechanical properties compared to randomly oriented CNT/PC nanocomposites. They reported an increase in the nanocomposites stiffness due to a higher number of

Fig. 29 Woven alumina cloth (a) without CNTs and (b) with CNTs grown on fiber surface using CVD process (Garcia et al. 2008)

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aligned CNTs restricting the polymer chain movement than randomly oriented CNTs. In another study, Ma et al. (2015) experimented with Ni-embedded CNTs dispersed in epoxy resin (EP). They studied its fracture toughness before and after CNT alignment using a magnetic field of 0.4 T. About 51% increment in fracture toughness of CNT/EP composites achieved after the orientation of CNTs transverse to crack growth. Jangam et al. (2018) were able to improve the fatigue life of epoxy nanocomposites by incorporating and aligning FCNTs (-COOH) under the application of 200 V/cm of DC electric field. They reported a 15% and 25% increment in the fatigue life of 0.2 wt. % aligned FCNT network over random oriented and neat epoxy. Pothos et al. (2021) aligned CNTs in a through-thickness direction around an open hole in GE composite by applying an AC electric field of 500 Vrms and 50 Hz for 30 min to improve its tensile properties. They achieved the highest increment of 25% in aligned CNTs, followed by randomly oriented CNTs with 12%, compared to neat GE composite. They asserted this increment due to delayed longitudinal splitting of glass fibers and reduced strain fields in the hole’s vicinity because of aligned CNTs. Gupta et al. (2016) aligned 0.5 wt. % functionalized MWCNTs in polyvinylidene fluoride (PVDF) using AC electric field at 220 V and 500 V. Before alignment, probe sonication and magnetic stirring were carried out for different duration to produce fine dispersion of MWCNTs in the PVDF polymer. Alignment of functionalized MWCNTs in parallel and perpendicular direction showed improvement in ultimate tensile strength and young’s modulus than neat PVDF and randomly oriented MWCNTs-PVDF thin film (Fig. 30). Perpendicular alignment of MWCNTs produced the highest increment of 28% in modulus and 46% in strength than random MWCNTs. During tensile loading of parallel MWCNTs-PVDF thin film, the MWCNTs and PVDF chain were parallel to the loading direction. During loading, these highly stiff MWCNTs deformed negligible than polymer chain. Furthermore, strong interfacial bonding due to functionalization helped effective stress transfer between PVDF polymer and MWCNTs. These mechanisms helped parallel aligned MWCNT-PVDF film to withstand higher load and produce better strength and modulus than neat and random MWCNT-PVDF composite. Further increment in properties for perpendicular aligned MWCNT-PVDF thin film was explained by the bridging mechanism of perpendicular MWCNTs to stretched polymer chains and its restriction to polymer chain movement. Lionetto et al. (2014) used a rotating cylindrical collector during electrospinning technique to gather continuous fiber with highly aligned CNTs. They reported an exponential increment in tensile modulus, i.e., 50% in aligned CNT/vinyl ester composite than a neat vinyl ester. Abbasi et al. (2010) employed a twin-screw extruder to disperse and entangle MWCNTs in the PC matrix finely. Later they used a shearing method called microinjection molding to align MWCNTs and evaluated its young’s modulus for different MWCNT content and injection speed. A monotonical increase in young’s modulus was reported with a rise in MWCNT content and injection speed. Maximum young’s modulus of ~1500 MPa at an injection speed of 800 mm/s for 10% aligned MWCNT/PC composite was noted. As per their observation, an increase in injection speed is responsible for the higher

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orientation of MWCNTs. During the electrospinning process, Song et al. (2017) placed a metal circular ring with a positive charge between the orifice and collector to produce a highly oriented CNTs/polyacrylonitrile (PAN) composite. This procedure of placing a third charged electrode is called as modified parallel electrode method (MPEM). By doing so, the tensile strength of 1% CNT/PAN composite increased by ~30% than fabricated without MPEM. Ogasawara et al. (2011) used the pulling (stretching) method to produce a sheet of aligned CNTs from a CVD-grown VACNT array and impregnated epoxy resin into this sheet to form aligned CNT/epoxy film using the hot-melting method. They conducted tensile testing on neat epoxy and aligned CNT/epoxy film with a thickness of 24–33 μm. Aligned CNT/epoxy film produced superior ultimate tensile strength and young modulus than conventional random CNT/epoxy film from different literatures and neat epoxy films. The authors asserted the explanation for this increment in performance to be a higher aspect ratio of aligned CNTs, which is even greater than 10,000. They also referred hot-melt prepreg method to be effective in maintaining CNT alignment during the epoxy impregnation process. Furthermore,

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they explained the reason for aligned CNT/epoxy film’s inferior performance than those described in other literature: entanglement, waviness, scattering of alignment, weak CNT/epoxy interface, and sword-in-sheath fracture of CNTs. In another literature, Nam et al. (2016) improved the mechanical properties of aligned MWCNT/epoxy composite by employing stretching and pressing techniques. These techniques reduced the entanglement and waviness of CNTs and produced highly dense-packed CNT sheets. Compared to neat epoxy composite, aligned MWCNT/epoxy composite showed a 32% and 27% increment in the tensile strength and elastic modulus, respectively.

Future Perspective Many different fundamental issues are yet to be understood and addressed, even with well-established knowledge about CNT’s behavior and properties of CNT-embedded polymer and fiber-reinforced polymer composite under various dispersion, functionalization, and alignment technique. They include but not restricted to the following areas: (i) Dispersion techniques showed uniform dispersion of CNTs in nanocomposites where prior dispersion of CNTs was carried out in a solvent. However, assessing the quality of CNTs dispersion in the solvent is not a simple task due to the nontransparency of solvent, even with a low concentration of CNTs. Also, no proven method is available to rate the quality of dispersion. Hence, techniques and tools need to be developed to access the grade of CNT agglomeration or dispersion in polymer or solvent. Furthermore, these tools should be efficient enough to correlate with different material parameters, such as aspect ratio or size of the particle, surface energy, and surface functionalities of the CNTs. (ii) Although several experiments have been conducted to change the surface characteristics of CNTs, many process variables are yet to be optimized to produce the best results. Furthermore, there are concerns about the detrimental response of dispersion and functionalization on CNTs structure and desirable properties. Previous research has yet to agree on the conditions regulating the dispersion and functionalization of CNTs, which dictate nanocomposites’ mechanical and functional properties. For that reason, detailed studies are needed to determine the relationships between different surface treatments, quality of CNT functionalization, fabrication, and processing parameters that affect the mechanical and functional properties of CNT/polymer nanocomposites. (iii) Understanding nanocomposites’ mechanical and transportation properties require an understanding of stress transfer between the CNTs and polymer matrix. Since interfacial adhesion and bond strength determine the stress transfer between the CNTs and matrix, detailed studies are required to understand the mechanism of stress transfer and evaluate the interfacial bond

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strength. In this field, fundamental studies such as simulation, estimation, and calculation of interfacial adhesion will be beneficial to widespread CNT/polymer nanocomposites applications. (iv) One of the significant challenges with the CNT-based nanocomposites is their high material cost, which can be reduced considerably using hybrid nanofillers; CNTs combined with low-cost fillers such as carbon nanofiber, carbon black, exfoliated graphite, and clay. However, to get this synergistic result, these hybrid nanofillers need to be finely dispersed. As these hybrid fillers have unique shapes and sizes, they will have different dispersion characteristics, which need to be addressed in detail. (v) Despite significant advances in CNT alignment methods, there are still several issues that must be resolved. For example, it is still tricky to get aligned CNTs with controlled diameter and chirality since metallic CNTs must be removed in a separate process. In contrast, the latest techniques used for metal removals, such as laser irradiation and plasma etching, can introduce defects to existing semiconducting CNTs. Furthermore, most alignment techniques cause CNTs misalignment due to the extreme randomizing effect between adjacent CNTs with high packing density. In short, future work in this field should be focused on the scalability of these techniques and easy incorporation into existing fabrication processes, for example, 3D printing.

Conclusion CNTs have a tremendous opportunity for tuning the functional and structural performance of polymer and fiber-reinforced polymer composite due to their outstanding mechanical, transport, and multifunctional properties. Even though several studies have been devoted to produce CNT-reinforced polymer nanocomposites for different applications, their uses in finished products are still in their beginning phases of recognition. Before this advanced material can be broadly used in actual structures and products, three main interrelated challenges must be addressed: i) insolubility and dispersion of CNTs in a polymer matrix, (ii) proclivity of CNTs to agglomerate, and (iii) weak interfacial bonding between CNTs and different polymers. This chapter presents an overview of CNT-embedded polymer and fiberreinforced polymer composites, emphasizing the concepts of CNT dispersion and alignment techniques and their subsequent consequences on the mechanical properties of the resulted nanocomposite. It is proven that regulating these two components, among many other material and processing factors, is critical because they strongly control the resulting properties of CNT-embedded polymer and fiberreinforced polymer composite. Also, variations in dispersion and functionalization are primarily troublesome at various stages of nanocomposite processing. Different mechanical techniques such as high sheer stirring, ultrasonication, ball milling, calendaring, and extrusion can be used to achieve uniform dispersion. Since such techniques can induce surface defects to CNTs and fracture it into smaller parts,

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proper selection of a technique or a combination of different techniques, depending on the desired properties of the end product, must be made. Like the advancement of dispersion techniques, numerous attempts have been made, with differing degrees of effectiveness, to change the intrinsically inert nature of nanotubes surfaces using functionalization. There are two main methods of functionalization: covalent and noncovalent. CNT surface can be decorated with covalent functional groups using chemical methods such as silanization, hydrogenation, bromination, chlorination, carbene and nitrene incorporation, cycloaddition, and fluorination. Using different ionic nature surfactants, wrapping CNTs with polymer, and the endohedral process are physical approaches of noncovalent functionalization. Also, CNTs are safe from structural damage from these physical processes. There are twofold benefits of functionalization: In addition to improved dispersion, it also strengthens the interfacial bonding and adhesion between CNTs and matrix. Both effects are highly demanding for optimized mechanical and physical properties of the nanocomposites. However, chemical functionalization can cause damage to the CNTs structure due to the uncontrolled use of high concentration acids and chemicals. Apart from that, existing CNT alignment techniques were reviewed, and emphasis was given on its working principles and its underlying mechanism. Furthermore, these techniques were categorized into six different groups: CNT alignment using Van der Waals forces, magnetic field, electric field, shear, extrusion, and pulling. This classification underlines the CNTs alignment technique’s key drivers. It is the most simple and effective way to improve CNTs functionalities by aligning them during their growth period. However, the end product in this approach is limited to standing or lying CNTs on a particular material as a substrate or containing metal crystal particles. Whereas alignment technique during post-growth of CNTs is time consuming and labor-intensive but delivers a much wider variety of CNT products. These clean and finely dispersed CNTs are a sound reinforcement for polymers or fiber-reinforced polymer composite. Furthermore, the post-growth alignment technique allows users for the selection of CNTs with a specific chirality.

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Carbon Nanotubes: Dispersion Challenge and How to Overcome It

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Mohsen Mohammad Raei Nayini and Zahra Ranjbar

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNT Properties and Its Potential to Be Commercially Utilized . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dispersibility: The Major Shortcoming of CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hansen Solubility Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theory of Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Modification of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Covalent Functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Noncovalent Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical Processes for Dispersion of Carbon Nanotube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dispersion by Cavitation: Ultrasonication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dispersion by Mechanical Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dispersion by Turbulent Flow: Jet Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Owing to their exceptional combination of electrical, thermal, mechanical, and optical properties, carbon nanotubes (CNT) have been recognized as favorable materials to be used in various applications, for example, nanocomposites, biotechnology, electronics, and energy related devices. Exploiting their remarkable properties in solutions and composites depends on their state of dispersion. M. M. R. Nayini Department of Printing Inks Science & Technology, Institute for color science and technology, Tehran, Iran Z. Ranjbar (*) Department of Surface Coatings and Novel Technologies, Institute for Color Science and Technology, Tehran, Iran Center of Excellence for Color Science Technology, Tehran, Iran e-mail: [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_64

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However, their great potential has been hindered by their intrinsic tendency to undergo bundling and lack of dispersibility in aqueous and nonaqueous media. It encourages several researches to improve the dispersibility of CNT in various environments. CNT dispersion commonly deals with surface modification which can be carried out either covalently or noncovalently. Covalent approach enables one to produce versatile functional groups on the CNT surface, which are covalently attached to the sidewall. It is desirable for improving CNT dispersibility in various environments. However, these methods disrupt the π-electron system of the CNT which is in charge for its remarkable electrical properties. It also imparts some structural damage and deteriorates the mechanical properties too. Noncovalent approaches however involve no chemical reaction and associate with physical adsorption of stabilizing agents on the CNT. Beside surface modification, CNT exfoliation by introducing energy to its bundles is also essential for CNT dispersion. Various procedures for surface modification as well as different methods for CNT exfoliation are briefly overviewed in this chapter. Keywords

Dispersion · Surface modification · Stabilization · Exfoliation · Nanocomposite · Interface

Introduction CNT Properties and Its Potential to Be Commercially Utilized In last decades, enormous efforts have been dedicated to carbon nanotubes (CNTs) and nanocomposites based on them. The chemical structure of this new carbon allotrope can be simply described as a cylinder, constructed by rolling up a graphene sheet. Such nanocylinder may either consist of a single tube which is called single walled carbon nanotube (SWCNT) or comprises coaxial multiple tubes which is called multiwalled carbon nanotube (MWCNT). The schematic theoretical structure of SWCNT and MWCNT is illustrated in Fig. 1. CNTs have outstanding electrical properties. Depending on their diameter, length, and chirality vector which is dependent on the rolling up angle of the graphene sheet relative to the tube axis, CNTs exhibit semiconducting or semi-metallic conducting behavior. Their electrical conductivity, beside their high aspect ratio, makes them ideal choice to impart conductivity to polymer composites. Such conductive polymer composites are widely applicable in electronics packaging, electrostatic discharge materials, and electromagnetic interference shielding. They are also good electron field emitters which make them suitable for being utilized in touch screens and flat displays. Owing to the high surface area and high charge-carrying capacity of CNT, they can also be utilized successfully in solar cells, capacitors, and batteries. Mechanical properties of CNTs are also astonishing. CNTs have very high tensile modulus (ca. 640 GPa), which in addition to their high aspect ratio (typically

Fig. 1 Schematic demonstration of (a) theoretically constructing CNT by rolling up graphene nanosheets (b) structure of (left) SWCNT and (right) MWCNT. (Reproduced with permission from Springer) (Backes 2012)

b

a

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ca. 100–300) makes them good candidate to be utilized as reinforcing filler in various matrices (Suzuki 2013). They can greatly improve the tensile strength and stiffness of polymers. Incorporating CNT into the polymer matrices is also proved to be a successful approach to improve the viscoelastic properties and vibration damping ability. It has been found that embedding CNT into polymer composites improves their damping ability by dominating interfacial slip and slip-stick mechanism over other energy dissipating mechanisms such as matrix tearing and plasticity (Zhao et al. 2018). CNTs are highly thermally stable and also have high thermal conductivity. Although the thermal stability of polymer composites is mainly depending on the thermal stability of polymer matrices, incorporating more thermally stable fillers such as CNT into the polymer matrices enhances their heat resistance. The high thermal conductivity of CNT further improves the thermal stability of the composite by dissipating heat faster. CNT has also reported to play the role of antioxidant (Mensah et al. 2015). The aforementioned properties of CNT, alongside with appropriate chemical stability, high antimicrobial activity and exceptional thermal properties make them a favorable choice to be applied in various applications. It makes CNT commercially highly interesting. The global market of CNT has been predicted to grow at the average compound annual growth rate (CAGR) of more than 10% from 2020 to 2027 and projected to reach $5.8 billion by 2027.

Dispersibility: The Major Shortcoming of CNTs In spite of their magnificent properties, a main hindrance remains to be overcome to get the most out of CNTs in the nanocomposite, uniform dispersibility. Besides nanocomposites and nanoelectronics, CNTs have a great potential to be used in coating and polymer industry as well as biological applications and drug delivery (Kang et al. 2008; Tripathi et al. 2015). However, CNT’s lack of dispersibility in most of common environments is a major obstacle for using them in this area (Kharissova and Kharisov 2017a, b). High surface energy and numerous π-π electron interaction of CNT build up high inter-tube interaction energy (500 ev/μm). It causes CNT to form tight, long, and difficult to be exfoliated bundles. Therefore, the CNT surface has to be altered in a way to obtain a uniform dispersion of it in the matrix and also improving CNT-matrix interfacial interaction (Suzuki 2013). Furthermore, compatibility extent of polymer matrix with functionalized CNT, greatly affects the mechanical properties of nanocomposites such as increases elastic modulus, failure strain, and loss factor. Better dispersion of CNT in polymer matrix increases the contact surface area between CNT and polymer matrix. Additionally, the improved surface interaction between functionalized CNT and the polymer matrix leads to more effective strain transfer which grows up the tensile modulus and toughness of the afforded nanocomposites. Similar to mechanical forces, hear transfer can also be modified by improving the CNT-matrix compatibility (Ma and Larsen 2013). From another point of view, the CNT matrix interface can be assumed as lower dimensional or nanodimensional system. All the forces that are dominant in

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nanodimension at the interface affect the macroscopic properties of the produced nanocomposites. The forces at this dimension can be originated from weak Van der Waals forces, dipole interactions, strong covalent bonds, molecular entanglement, etc. (Chen et al. 2018). Effective dispersion of CNT associates with surface modification of CNT in such a way to improve the interaction between CNT and matrix. Surface modification carries out by either covalent or noncovalent approach that will be discussed in detail. Overcoming the dispersion difficulties of CNTs is also a cornerstone in the field of nanoelectronics. Actually, CNTs can be regarded as protype of one-dimensional quantum wire. An increasing number of researchers have been attracted by conducting polymer nanocomposites. Conducting polymer nanocomposites which consist of dispersed conducting nanofillers and insulating polymer matrix can merge electrical conductivity of conducting nanoparticles and interesting properties of polymers, for example, low density, processability, flexibility, and low cost. Increasing the load of conductive fillers like CNTs into polymer matrix improves the conductivity of polymer composites slowly. At a certain concentration known as percolation threshold, a jump in conductivity occurs. At this concentration, conducting particles either touch each other or get close enough to create conducting pathways through the composite which causes the sharp insulating-conducting transition. Incorporating high amount of nanofillers may cause some drawbacks such as high cost, processing difficulties, and reduced flexibility. Therefore, it is favorable to reduce the percolation threshold to obtain conductivity with low concentration of conducting fillers. Beside shape and size of nanofillers, percolation threshold is greatly influenced by the state of dispersion. Thus, for obtaining highly conductive CNT containing nanocomposites, CNT should be effectively dispersed throughout the polymer matrix in uniform fashion (Soares 2018). In this chapter, the CNT dispersion will be overviewed. Fundamental aspect of CNT dispersion will be discussed first. It will be followed by review of surface modification processes in order to improve the dispersion state of nano composites. The modification methods by both covalent and noncovalent approach are considered. Eventually, the physical processes that have been utilized for CNT dispersion will be briefly presented.

Fundamentals Hansen Solubility Parameters Hansen theory is based on the idea that like dissolves like, which means that molecules that bond each other in same way would be miscible. It divides the intermolecular interaction forces into three parts, δD, δP, and δH, which represent the dispersion, polar, and hydrogen bonding force parameters, respectively, that are called HSP solubility parameters. For each solute and solvent, these three parameters can be projected in a three-dimensional coordinate that locate the center of HSP sphere. Ro is the radius of the sphere which is called interaction radius. The solvents that are located inside the HSP sphere of a solute are usually assumed as good

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Fig. 2 HSP sphere of a solute with dots that represent different solvents. Bright dots are good solvents, while the dark ones are bad solvents. (Reproduced with permission of American chemical society) (Ma and Larsen 2013)

GH

Ro

Ra

GP 2GD

solvents while the solvents out of the sphere are bad ones. In another word, Ro gives the boundary of good solvents. The HSP distance (Ra) is the distance between the specified solvent and the center of the solute sphere (Fig. 2). Relative energy difference (RED) is defined as: RED ¼ Ra =Ro

ð1Þ

Therefore, the solvents with RED values less than 1 indicate strong interaction. However, it is noteworthy that the HSP prediction is influenced by solvent molecular size. Smaller molecules tend to dissolve more easily. Thus, the large molecules have relatively small Ro. HSP approaches have also widely used to predict polymer miscibility. Because of the large molar volume of the polymers, they have very low interaction radius. Therefore, very small Ra of two polymers is a perquisite for their miscibility. HSP has been successfully utilized to predict the solubility of nanoparticles in various medium. Solubilization of nanoparticles means totally separate them from the aggregates and agglomerates and individualize them in a medium. In this approach, nanoparticles like CNT have been assumed as macromolecules with specific HSP. HSP of CNT can be improved to a large extent by its surface modification. It will be discussed later (Detriche et al. 2009; Ma and Larsen 2014).

Theory of Dispersion Solid particles such as CNT are usually supplied as agglomerates to be incorporated into the organic or inorganic medium. Reducing the size of agglomerates to obtain

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primary particles or small aggregates and evenly distribute them throughout the matrix is known as dispersion process. Breaking down the agglomerates to smaller particles is prerequisite to get the most out of the CNT, as well as the fillers and pigments. Actually, the performance of all pigments and fillers in the composites are dominantly controlled by their size distribution (Goldschmidt and Streitberger 2018; Hunger et al. 2019). Solid small particles tend to combine each other to form large agglomerates. The attraction forces among the solid particles can be assumed as the combination of Coulomb forces, Van der Waals’ forces, and hydrogen bonds. The strength of these forces depends on the particle distance on different manner. So, the extent of particles attraction force is a function of particles shape and inherent chemical properties. During dispersion process mechanical forces are applied to the agglomerates to overcome the attraction force between the primary particles and separate them. However, in majority of cases it is not adequate and specific considerations have to be given to the process. Even though agglomerate separation be achieved by simple applying mechanical force, the collision between separated particles occurs because of Brownian motion that in presence of active attraction forces results in reagglomeration. It therefore necessitates to stabilize the particle dispersion that is associated with covering the entire surface of the particles with a stabilizing skin. The dispersion process is therefore comprised of four major stages: wetting the surface of the particles and penetration of the medium into the bundle’s cavities; separating the agglomerates and aggregates into primary particles by applying mechanical force; evenly distributing the primary particles throughout the medium; stabilizing the primary particles and preventing them from reagglomeration and flocculation. These steps are called wetting, dispersion, distribution, and stabilization, respectively (Goldschmidt and Streitberger 2018; Hunger et al. 2019).

Wetting Wetting of particles can be described as the spreading of medium over the surface of the particles and its diffusion into the porosity of the aggregate and agglomerate cavities. It is decisively influenced by surface energetic aspects which are demonstrated by surface tension. Matching the surface tension of the particle and the medium which is intended to interact with it determines the wettability and the overall dispersibility of the particle. Surface tension is the amount of work that has to be done in order to form new surface that is created by pigment wetting. Complete wetting is carried out if the amount of this work remains smaller than the amount of energy that is generated from interaction of newly developed surface and the medium (work of adhesion). It can be quantitatively determined by Young’s equation. Young’s Eq. (2) correlates the surface tension of the solid-liquid interface γsl to the surface tension of the solid phase γs and liquid phase γl and the wetting contact angle of the intended liquid over the solid phase α (Goldschmidt and Streitberger 2018; Hunger et al. 2019). cosα ¼

γ s  γ sl γl

ð2Þ

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The wetting tension βsl and work of adhesion WA is therefore defined as below: βsl ¼ γ sl  γ s ¼ γ l ∙cosα

ð3Þ

W A ¼ γ l þ γ s  γ ls ¼ γ l ð1 þ cos αÞ

ð4Þ

The greater the value of wetting tension is, the easier the wetting process will be. The optimum wetting happens when the cosine of the contact angle approaches 1 and γ12 moves toward zero. It occurs when the surface tension of the medium stays less than the surface tension of the solid. Since the polar surfaces usually have higher surface tension, they usually undergo wetting more easily in comparison with nonpolar surfaces. However, having polar surface can be double-edged since the attraction between the primary particles of such surfaces are stronger and it is more difficult to separate them (Goldschmidt and Streitberger 2018; Hunger et al. 2019). Although it is thermodynamic aspects that determine whether a particle gets wet by a medium or not, kinetics of penetration of liquid into the agglomerates cavities is also of great concern in applied fields. The penetration rate of the medium into the cavities within the particle agglomerates that assumed as the wetting rate is determined by Washburn Eq. (5) (Goldschmidt and Streitberger 2018; Hunger et al. 2019). π∙r 3 ∙γ l ∙ cos α V_ ¼ k∙η∙l

ð5Þ

In this equation, V_ is the penetration rate, r and l are the average radius and length of the capillary tubes in the agglomerates, respectively. α represents the contact angle of liquid over the particle surface and γl and η are the surface tension and viscosity of the liquid, respectively. It has to be taken into account that the thermodynamically allowed wetting is a prerequisite for using the Washburn equation. As it can be found from the equation, as long as the surface tension of the liquid remains below the surface tension of the particle, higher γl promotes wetting of cavity walls. In another word, while the low surface tension of the liquid is more favorable for wetting the outer surface of the particles, high surface tension of the medium improves the penetration rate of the mixture into the cavities. It means that the optimization of the medium has to be carried out to find the best medium composition for dispersion (Goldschmidt and Streitberger 2018; Hunger et al. 2019). Apart from the surface tension of the two phases to get into contact, the viscosity of the fluid and capillarity of the solid particle, the nature of the intermolecular interactions of each phase has great impact on the wetting. Wetting is carried out when two phases get into contact and previously existed surfaces disappear. In another world, in the newly generated surface, interaction of the molecules with the molecules of their own kind, replaces with their interaction with the molecules of the other phase. It results in the release of energy equal to work of adhesion. The higher the amount of the energy releases, the more favorable the wetting will be. Hence, the intrinsic molecular interactions within each phase decisively affect the state of wetting. Analogous to the HSP approach, surface tension has two main

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components, polar part (γP) and dispersive part (γD). Work of adhesion depends on whether the molecules of two phases can interact in a same way or not. Therefore, particles with polar nature tend to be wet by polar solvents while nonpolar solvent wet nonpolar particles better. Surface modification of CNT takes place aiming manipulation of surface chemistry of the CNT in a way to change the interaction of its surface molecules that can improve their wetting in some liquids while deteriorating it in others.

Desagglomeration of Particles Even with the best wetting situation, only a small portion of the agglomerates is broken down by physiochemical interactions. To separate the entire particles, applying mechanical force to the agglomerates and aggregates is unavoidable. The mechanical force is applied to the particles in different ways: • Applying pressure and shear force between two solid surfaces like three roll mills and disc mills • Impacting the particles with hard substances like what happens in jet mill • Transmitting shear stress to the particles by fluids (Goldschmidt and Streitberger 2018; Hunger et al. 2019) Various methods have been utilized to disperse CNT in organic and inorganic matrices. Ultrasonication, extrusion, ball milling, and agitator milling have been reported to successfully govern break down of CNT coarse. It will be discussed in more detail in section “Physical Processes for Dispersion of Carbon Nanotube.”

Distribution of the Dispersed Particles Distribution of the dispersed particles can be achieved simultaneously during dispersion process by mixing the total volume of the medium. Thus, the uniform distribution depends mainly on the equipment and procedure condition under which the dispersion process has been carried out. Stabilization Stabilization can be defined as protecting the dispersed particles against reagglomeration or flocculation. The wetting and dispersing process can be reversed if the stabilization does not take place appropriately. Proper stabilization is attained if the surface of the dispersed particles is modified during previous steps in such a way that do not adhere to each other on collision. Particle stabilization can be achieved by two different mechanisms. In polar environment, inducing electric charge to the particles surface which causes electric repulsion between the similarly charged particles is the main mechanism of stabilization, while in nonpolar media, steric or entropic phenomenon is the main stabilization mechanism that is employed. Derjaguin–Landau–Verwey–Overbeek (DLVO) theory is one of the first and still current theories that describes the stabilization of dispersed particles by means of electrical repulsion. It is also widely utilized to predict the effectiveness of ionic surfactants to stabilize the CNT in aqueous media. Electrostatic stabilization

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approach explains the stabilization of electrically charged particles in polar, ion-forming environments. The mathematical model that is known as DLVO expresses that particle dispersion remains stable as long as the columbic repulsion force between the similarly charged particles exceeds the Van der Waals’ attraction force between them. This theory describes the colloidal particle stabilization based on the surface charge of the particles. The surface charge is stemmed from either deprotonation of particle surface molecules or ion adsorption from the media. The main attraction force between particles is weak Van der Waals forces interaction. Ionic surfactants induce charge on the particles surface. It results in Coulomb repulsion between similarly charged particles. The electrostatic repulsion is usually quantified as the electric charge close to the interface, for example, zeta potential (ζ). The zeta potential is the electrical potential of an interface at the slipping plane. When the interface moves in the fluid, the shear plane is an imaginary plane that separates the fluid that is attached to the surface and moves with it, from the bulk of the fluid. When the interface gets electrically charged, one or more layers of counter ions and solvent bind to it tightly (Fig. 3). It creates an electrical double layer which contains strongly bound ions and solvent molecules which is called Stern layer. This layer is surrounded with loosely attached ions and solvent molecules that can be slipped off when the plane displaces in the solution. Zeta potential is the electric potential at the hydrodynamic slipping plane (Backes 2012; Nazari et al. 2019; Tkalya et al. 2012). Fig. 3 Schematic illustration of the definition of the zeta potential as the potential at the slipping. (Reproduced with permission of Springer) (Backes 2012)

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While DLVO theory provides a powerful insight to explain and predict the particle stabilization in polar and ionic medias like aqueous environments, it is not applicable for particle stabilization in nonpolar environments. In such environments, no ionization takes place and no surface electric charge therefore generates which is prerequisite for particle stabilization by electrostatic method. However, in practice, even in this condition, stable dispersion can be acquired. In non-ion forming media, stabilization is mainly based on entropic (steric) mechanism. Steric stabilization associates with adsorption of long polymer chains on the particles surface. While certain parts of the polymeric stabilizing agents anchor firmly to the particle surface, the other parts should be able to freely move in the media. When particles with such surrounding polymeric stabilizing agent approach each other, the mobility of the freely dangling parts of the polymer chains will be restricted which can be translated as scaling down of entropy. Reduction in the entropic term of Gibbs-Helmholtz equation pushes the total free energy toward the positive value. Therefore, further approach would not take place. Based on the stabilization mechanism, this approach is called entropic stabilization or steric stabilization. It can be also explained by osmotic pressure. When the particles with adsorbed polymer layer get close, the concentration of the adsorbed molecules grow up in the overlapping area which increases the osmotic pressure. The generated osmotic pressure inhibits further approach (Goldschmidt and Streitberger 2018). Whether the osmotic pressure or entropic term be taken into account to explain the stabilization, a dense surface covering with long polymer chains that be firmly attached to the particle and, at the same time, well solvated in the media to remain uncoiled and freely movable affords optimum stabilization. So, the polymeric stabilizing agent should contain groups that tend to adsorb effectively on the particle surface that are called anchor groups. At the same time, other parts of the polymer should be adequately long and soluble in the media to generate enough repulsion. It should be considered that, since the entropic stabilizing molecules are relatively large in comparison with solvent molecules, the particle surface firstly gets wet with solvent molecules. The preliminary wetting has to be followed with substitution of the adsorbed solvent molecules with polymeric stabilizing molecules that can be thermodynamically favored only by gain in enthalpy. Thus, the affinity of the stabilizing agent to the particle surface and also the solubility of the polymer chain in the media predominantly influence the extent of stabilization. It has to be also noticed that good solubility of steric dispersants in the media can be a double-edge sword. Despite it improves the dispersion stability by uncoiling the polymer chains, but it can also prevent the dispersant to effectively adsorb onto the particle surface (Goldschmidt and Streitberger 2018).

Surface Modification of Carbon Nanotubes Dispersing CNTs is carried out by surface modification. Different methods of modifying CNTs surface can be divided into two main categories: covalent functionalization and noncovalent functionalization (Rahman and Mieno 2016).

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Covalent functionalization takes place by covalently attaching functional groups to the CNT main structure. Covalent functionalization might perform at sidewall (direct) or on defect sites (indirect). Direct functionalization causes the hybridization of CNT carbon atoms to be changed from sp2 to sp3 which is accompanied with loss of conjugation. In contrary, indirect functionalization takes advantage of defect sites that are already present on CNT scaffold and have sp3 hybridization. Such defect might be located at open ends, sidewall holes, and pentagon and heptagon irregularities in graphene structure of CNT sites and usually experience higher reactivity. Therefore, indirect functionalization does not disturb conjugation and delocalized π-electron cloud which is in charge of CNT conductivity. Noncovalent functionalization methods mainly take advantage of supramolecular complexation. Accordingly, various organic and inorganic compounds absorb on the outer surface of CNTs via noncovalent forces such as π-π stacking and Van der Waals interactions. All the mentioned approaches deal with outer surface of the tubes and categorized as “exohedral” approaches. Opposed to exohedral functionalization, endohedral functionalization deals with the inner space of the tubes and confining different structures like fullerenes and proteins inside them. This approach only slightly affects the surface properties of CNT. Thus, it can be considered as a special case in CNT functionalization and will not be discussed in this section (2010; Rahman and Mieno 2016). In this section, various approaches in surface functionalization of CNTs in polymer matrix and their advantages and drawbacks will be overviewed. The physical procedure for functionalization and dispersion will briefly be discussed, and finally, the characterization methods to take an insight into the state of dispersion would be discussed.

Covalent Functionalization As it was mentioned, covalent functionalization attaches chemical moieties to CNTs with strong covalent bonds. Therefore, such methods provide very strong linkages. Wide variety of chemical structures can be attached to CNT by covalent functionalization which can be assumed as one of its positive aspects. However, these methods are usually based on severe chemical reactions that can lead to CNT cut-off and reduce their length and also partial destruction of the CNT scaffold. The main structure of CNT is composed of six member rings with sp2 hybridized carbon atoms. These sp2 carbon bonds in main graphene structure of CNTs are even stronger than sp3 carbon bonds in diamond and are responsible for interesting mechanical strength and thermal conductivity of CNTs (Graham et al. 2005). All carbon atoms in CNT have sp2 hybridization, except at the defect sites, end caps, open ends, and vacancies which contain sp3 carbon atoms. The end caps of CNTs that are not closed with catalyst particles are composed of highly curved fullerene like structures. In contrast to the sidewall that is folded in one dimension, the caps that are hemispheres are folded in two dimensions. Because of high curvature at these points, the C-C bonds experience high strain and are prone to undergo

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chemical reaction. Defect sites on the CNT’s sidewall are also composed of fivemember and seven-member rings that make the CNT to be bent at the points. These pentagon and heptagon pairs are more reactive in comparison with hexagonal main structure of the CNT scaffold. As it was mentioned before, covalent functionalization can be performed either at these sites (indirect approach) or directly on the main structure of side walls. Direct functionalization changes the hybridization of carbon atoms from sp2 to sp3 which simultaneously disrupt π-conjugation system.

Direct Functionalization Fluorination and Derivatization of Fluorinated Carbon Nanotubes One of the first experiences in direct functionalization of CNTs has been reported by Mickelson and coworkers. According to their experiments, purified SWCNTs were fluorinated via reaction with elemental fluorine at different temperatures. Mixture of fluorine and helium was passed through prebaked, bucky paper at different temperatures. It was shown that at appropriate temperatures, the SWCNT’s structure was retained, and simultaneously, significant amount of C-F bonds was formed on the SWCNT’s. Such direct functionalization drastically reduced electrical conductivity of the SWCNTs and made them nonconductive (Mickelson et al. 1998). Further investigations revealed that fluorine atoms more favorably tend to attach around the circumference of the CNTs (Kelly et al. 1999) and next to each other (Bauschlicher 2000). Prior to these efforts, semiempirical studies had justified that C-F bonds in fluorofullerenes are weaker than C-F bonds in alkyl fluorides which brings them the potential to be substituted with various organic groups (Cahill 1995; Kelly et al. 1999). Therefore, sidewall fluorination was suggested as a starting point for direct functionalization of CNTs and several reports have published, describing replacement of fluorine groups with different chemical moieties (Khabashesku et al. 2011). The dangling fluorine groups on CNT sidewall can also play the role of radical generator with relatively high half-life to be undergone radical polymerization. It therefore facilitates grafting wide range of polymers on the CNT sidewall (Zha et al. 2016). Figure 4 schematically represents some direct functionalization methods on SWCNTs sidewall, through fluorination (Banerjee et al. 2005). Sidewall fluorinated CNTs can undergo Grignard synthesis with alkyl magnesium bromide or undergo reaction with alkyl lithium compounds to acquire sidewall alkylated CNTs (Boul et al. 1999). This reaction can also be facilitated by the higher electron accepting properties of fluorinated CNT in comparison with pristine CNT (Stevens et al. 2003). The alkyl groups can further be removed by oxidizing the alkylated CNTs in air to recover the pristine CNTs. Likewise sidewall fluorinated CNTs can be treated with hydrazine to regenerate the starting CNTs (Kelly et al. 1999). This method can also be used to tune the fluorine functionalization density by initially functionalizing the CNTs sidewall heterogeneously and then removing some of the functional groups by treating them with sub-stoichiometric amount of hydrazine (Mickelson et al. 1999). Attachment of organic substituent with reactive end groups like amines provides even more potential for chemical modification. Providing amine groups on the CNT

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N2H4

F2

150-325ºC

Pristine SWNTs air

Oxidation

[NH-(R)-NH-(CO)-(R)-(CO)-X]n

[F]n Fluorination

[NH-(R)-NH2]x ‘Amine terminated nanotube’

MO C M= H2 CH( Li, OH Na, )CH K

H ) nO H2 e )(C (R ridin py

[F]n-x

HN

X-(CO)-R-(CO)-X X= C1, Br

H2 )-N -(R H 2N idine pyr

R-Li or R-Mg-X

[F]n-x [R]x Alkylation

2 OH

[F]n-x [O-(R)-OH]x

[F]n-x [N(R)(CH2)nOH]x

‘Hydroxyl nanotubes’

Fig. 4 Schematic description of direct sidewall functionalization methods, started from fluorination. (Reproduced with permission of John Wiley and Sons) (Banerjee et al. 2005)

sidewall is highly favorable since amine groups can strongly interact with various biological and organic medias such as epoxy, polyamide, polyurethane, and peptides. Amine functionalization also reveals the ability to covalently bind DNA and polymer matrix to CNT sidewall. Amine-functionalized CNT has also gained high interest in the field of solid CO2 absorbents since the porous amine-solid absorbents are proved to serve very well for this application. High surface area and robustness of CNT alongside with high affinity of amine groups to CO2 makes the amino CNT as a favorable candidate to be governed as solid CO2 absorbent (Gelles et al. 2020). CNT functionalization with N-alkylidene amino groups was very first performed by condensation reaction between fluorinated CNT and alkylidene diamine in the presence of pyridine as catalyst (Stevens et al. 2003). Direct substitution of fluorine groups with primary amine groups has been recently reported at low temperature and in the presence of ammonia gas. The reaction was followed with hydrazine treatment to remove remained fluorine groups (Yokoyama et al. 2021). Hydroxylation of CNT is also of high interest since it can greatly improve the surface interaction between CNT and polar media such as physiological solutions. Providing hydroxyl groups on the CNT side wall has also been performed through fluorine substitution. It can be conducted by condensation of fluorinated CNT with either alkyl diols in the presence of alkali metals or amino alcohols in the presence of pyridine (Zhang et al. 2004).

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Cycloaddition New cyclic moieties from two reactants are generated by cycloaddition reactions. Cycloaddition reaction follows concerted mechanism, dealing with transition state, and no intermediates are created during the reaction. The most wellknown cycloaddition reactions that occur via concerted mechanism are [1,3] dipolar cycloaddition and Diels–Alder cycloaddition. The understanding of the reaction mechanism is obtained by considering molecular orbitals. Accordingly, the same orbital symmetry relationship determines the regiochemistry and stereochemistry of these reactions. During these reactions, two covalent bonds are formed in a single step which greatly improves the efficiency of synthesis (Carey and Sundberg 2007). The double bonds that are located on the CNT’s sidewall are appropriate dienophiles to undergo [1,3] dipolar cycloaddition with dipoles such as ozone, azomethine ylides, and nitrile imines. Five-member rings are appended on the CNT’s sidewall during this reaction (Sgobba et al. 2007). Successful ozone reaction with double bonds of olefins and fullerenes motivated the researchers to react CNT with ozone. Side wall functionalization of CNT through ozone reaction has been carried out in methanol at 78  C. This reaction results in unstable ozonide intermediates that can readily be dissociated. The dissociation process can be performed in the presence of suitable oxidizing and reducing agents to cleave the ozonide group and replace it with desirable oxygenated functional groups such as ketone/aldehydes, carboxylic acids, and alcohols (Banerjee and Wong 2002). Creation of such oxygen-bearing groups on CNT sidewall remarkably improves its dispersibility and processability in various matrices such as epoxy (Singh et al. 2019) (Fig. 5). [1,3] dipolar cycloaddition between CNT sidewall and in situ synthesized azomethine ylides (by condensation of α-amino acids and aldehydes) is known as Prato reaction. Depending on the reactivity of the diene, heating the reaction under reflux in a high boiling solvent may be required. Various organic groups have been

Fig. 5 Schematic description of direct sidewall functionalization methods, based on [1,3] dipolar cycloaddition. (Reproduced with permission of Royal Society of Chemistry) (Sgobba et al. 2007)

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attached to CNT’s sidewall by Prato approach thanks to the large diversity of α-amino acids structures which also enable the production of multifunctional CNTs. Therefore, Prato reaction has played a dominant role in enhancing CNT dispersibility in different medias (Sgobba et al. 2007). This reaction can also be followed up with other chemical processes to further modify the functional groups to more complex structures such as polymers, organometallic catalysts, antibodies, and proteins (Mohammad Raei Nayini et al. 2014; Skwarecki et al. 2020). It deserves to be mentioned that organic moieties that are attached to CNT sidewall via Prato reaction can be easily removed by simply heating to recover the main structure of CNT with almost the same electrical conductivity. Considering the greatly improved solubility, this reaction can be employed as a method to purify CNT (Sgobba et al. 2007). Similarly, Diels–Alder cycloaddition has also reported to covalently functionalize the CNT sidewall. However, this reaction suffers from the limitation of functionalization because of the possibility of happening retro Diels–Alder reaction. Accordingly, there is an equilibrium between addition and elimination reactions and this could explain the low yields obtained in some cases. [2 + 1] cycloaddition of carbenes and nitrenes to CNT sidewall have also been reported. 1,1-Dichlorocarbene can readily react with sp2 carbon atoms of CNT sidewall to yield a corresponding 1,1-dichlorocyclopropane structure on the CNT sidewalls. This approach has utilized to uniformly disperse and covalently attach metal nanoparticles on the sidewall of CNTs (Ismaili et al. 2011). Nitrenes can also undergo [2 + 1] cycloaddition with the sidewalls of CNT (Shojaei and Azhari 2018). According to this approach, azide compounds can be decomposed either thermally or photolytically and provide nitrene intermediates by nitrogen elimination. The afforded alkoxycarbonylnitrenes are reactive enough to give rise to cycloaddition with CNT sidewall. The solubility of CNT in organic solvents such as DMSO is greatly improved by this approach (Shojaei and Azhari 2018). It can also be utilized to purify the CNT from insoluble contaminants (Sgobba et al. 2007). Reductive Hydrogenation, Alkylation, and Arylation These processes mostly rely on Birch reduction of SWCNT and MWCNT. Birch process is conducted by CNT sidewall reduction using Li metal and methanol dissolved in liquid ammonia which results in sidewall hydrogenation of CNT. The hydrogenated derivatives have a stoichiometry of C11H and can withstand high temperatures up to 400  C. Sidewall hydrogenation can also be carried out by atomic hydrogen that can be generated using cold plasma. Reductive functionalization of CNT sidewall can also be carried out using lithium, sodium, or potassium in liquid ammonia and further treatment of the obtained CNT salts with alkyl/aryl halides to afford alkyl or aryl functionalized CNTs, for instance, dodecylated CNT, which is quite soluble in organic solvents. Providing alkyl and aryl groups that are covalently bonded to the sidewall of the CNTs greatly improves their dispersibility in organic medias and also modifies the properties of their composites (Fig. 6) (Sgobba et al. 2007). Several

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a

b

Fig. 6 Reductive functionalization of CNT: (a) Brich reduction (b) nucleophilic alkylation. (Reproduced with permission of Royal Society of Chemistry) (Sgobba et al. 2007)

reports announced that alkyl/aryl functionalization of CNTs sidewall improves both the mechanical and thermal properties of their corresponding polymer composites. Radical Addition Prior to experimental efforts, it had been predicted by classical molecular dynamics simulations that the sidewall of carbon nanotubes can probably undergo reaction with organic radicals. This prediction was fulfilled with different experiments. Organic radicals that have been generated thermally, photochemically, or electrochemically are the cornerstone of straightforward alkylation of sidewall of CNT which greatly improves the CNT dispersion in polymer matrices. As an instance, in situ polymerization of propylene in the presence of sidewall alkylated CNT (Fig. 11) resulted in nanocomposites with improved mechanical properties in comparison with the corresponding nanocomposites with pristine CNT. However, it was confirmed that, expectedly, the electrical properties of the composites that contained functionalized CNT were deteriorated because of π-conjugate distortion of functionalized CNT (Koval’chuk et al. 2008) (Fig. 7).

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The main strategies for covalent functionalize the CNT sidewall are briefly described in Fig. 8. There are also other strategies for obtaining sidewall functionalized CNT but they have not utilized extensively, and thus have not been discussed here.

Fig. 7 Reaction scheme of undecyl radical addition to CNT sidewall. (Reprinted with permission of American Chemical Society) (Koval’chuk et al. 2008)

Fig. 8 Brief schematic summery of different strategies for sidewall functionalization of CNT. (Reproduced with permission of Elsevier) (Ma et al. 2010)

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Indirect Functionalization Defect functionalization is another method for covalent functionalization of CNTs. This process takes advantage of chemical transformation of defect sites on CNTs. Defect sites can be the open ends and/or holes in the sidewalls, pentagon or heptagon irregularities in the hexagon graphene framework (Ma et al. 2010). Frequently, these intrinsic defects are supplemented by oxidative damage to the nanotube framework by strong acids which leave hole functionalized with oxygenated functional groups such as carboxylic acid, ketone, alcohol, and ester groups. In particular, treatment of CNT with strong acids tends to open these tubes and to subsequently generate oxygenated functional groups that serve to tether many different types of chemical moieties onto to the end and defect sites of these tubes (Banerjee et al. 2005). Oxidation Oxidation of CNT is one of the first processes that has been reported for functionalizing CNT. It was first carried out at elevated temperature in air, during 1993. Oxidation of CNT in gas or liquid phase is widely used as a post-processing method to purify the CNT. Removing amorphous carbon impurities and metal residues like Fe, Co, or Ni can be successfully conducted using oxidative procedures. Nevertheless, oxidative purification is not suited for SWCNT and leads to significant CNT etching. The oxidation of CNT is usually performed by heating the CNT in highly acidic environment like H2SO4, HNO3, H2O2, KMnO4, oxygen, and chlorine. This process has been carried out in different conditions and with various oxidants, most of them modify mainly the defect sites and end tips. Different procedures for CNT oxidation are briefly represented in Fig. 9 (Naqvi et al. 2020). CNT oxidation may accompany with some damage in the honeycomb structure of the CNT sidewall. Depending on the oxidation conditions, the CNT structure can be altered differently. It has shown that after a critical treatment duration, both ordered and disordered parts of the CNTs would be destroyed, and exfoliation of layers, unzipping of the tube structure or destruction into polyenes, polyphenylenes, and aromatic clusters would happen. Controlling the reaction condition would remove disordered parts of the tubes, curvatures and irregularities (Chernyak et al. 2017).

KMnO4 H2O2/HNO3 H2SO4/HNO3 Ozone HCIO4 K2S2O8/KOH

HO OH O CH CH HO

O CH

O

OH C O

O

OH

HO

OH

O

Fig. 9 Different methods for oxidation of carbon nanotubes. (Reproduced with permission of Elsevier) (Naqvi et al. 2020)

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The blocked disordered parts would be opened up in oxidation conditions and oxygen-bearing groups like carboxylic, epoxide, hydroxyl, ester, and formyl groups are created in the opened sites. These groups greatly improve the dispersibility of CNT in large variety of matrices and solvents, especially the aqueous solutions. Water-soluble polymer matrices such as polyvinyl alcohol (PVA) might be prone to microbial activity that can be a drawback in some applications. Uniform dispersion of CNT into such polymers can exert antimicrobial activity into them. CNT oxidation which greatly improves its hydrophilicity is an appropriate solution for lack of dispersibility of CNT in the matrix. Dispersibility and antimicrobial activity of oxidized CNT in aqueous solutions can even be modified by treating the oxidized CNT with quaternized polyimines (Naqvi et al. 2020). Electroactive shape memory nanocomposites have also obtained using oxidized CNT. Uniform dispersion of CNT throughout the polymer matrix increases its electric conductivity which enables it to get heated by passing electric current through it. Oxidized CNT greatly improves its dispersibility in hydrophilic polymers which reduces the percolation threshold, which is essential for electroactive nanocomposites (Du et al. 2015). Water dispersible oxidized CNT can also be used in inkjet printing inks that is of great importance in printed electronics (Kholghi Eshkalak et al. 2017). However, it happens at the expense of CNT regular structure. CNT structure simultaneously encounters some damages on the outer layers. Heavy oxidation of CNT creates some pores on their sidewalls and even makes them permeable to the fluids (Naqvi et al. 2020). Considering the importance of oxidation as a major approach for CNT surface modification, it has been widely investigated. The Hansen solubility parameter (HSP) has been effectively used to predict the effect of oxidation on the dispersibility and CNT-matrix interfacial interactions. Jing Ma et al. utilized this approach to predict the dispersibility of CNT in various solvents and polymer. It has found that the HSP of CNT can be significantly altered by oxidation. The hydrogen binding and, more significantly, polar parameters grow up by oxidation. Increasing the oxidation duration changes these parameters even more by increasing the amount of oxygen-bearing groups on the CNT structure. Binding such polar groups on the CNT structure improves its interaction with different solvents, especially the polar ones. CNT oxidation not only moves the solubility sphere of toward the higher δP and δH but also enlarges the its HSP solubility sphere radius. It means that the oxidized CNT can provide stable suspension in a greater number of solvents. It has been also shown that the extent of HSP improvement which relies on functional groups density is greatly affected by the raw CNT structure. CNTs with more irregularities have larger areas to be oxidized during oxidation. Hence, their solubility parameters improve to a greater extent by oxidation (Detriche et al. 2009; Ma and Larsen 2014). Derivatization of the Oxidized CNT The functional groups that are generated on the CNT structure through oxidation can also be additionally treated to further modify the surface properties. Hydroxyl groups can be appropriate choices to undergo salinization with different silane monomers.

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It causes a thin layer (2–3 nm thick) of silane monomers, coated on the CNT structure (Das et al. 2009). Considering the wide variety of silane monomers, surface properties of CNT can be well manipulated via this approach. As an instance, the HSP and solubility of oxidized CNT that has been treated with two different silane monomers were investigated. It was found that the fluorinated alkyl silane and cyanoalkyl silane were used for salinization. The results revealed that both polar and hydrogen bonding parameters of cyanoalkyl functionalized CNT is significantly higher in comparison with fluorinated alkyl silane treated samples. Therefore, cyanoalkyl silane treated sample can produce stable dispersion in highly polar solvents like dimethyl formamide (DMF) and N-methyl pyrrolidone (NMP) while the fluoroalkyl functionalized one cannot be dispersed in such medias (Detriche et al. 2009). Carboxylic acid groups that are generated on CNT structure during oxidation are suitable starting points for further derivatization via amide and ester linkages. Amide functionalization can be performed both on CNT sidewall via 1,3-dipolarcycloaddition and on defect sites through oxidation and then condensation with amines and maleimides (Naqvi et al. 2020). The later however is less complicated and can be simply carried out by heating the oxidized CNT with the amine compound for prolonged time (Fig. 10). Various amine compounds can be used to tailor CNT surface properties. While small amine molecules like ammonia and polyamines increase the surface polarity and hydrophilicity of CNT and its dispersibility in polar matrices and aqueous solutions, the long chain alkyl amines like dodecyl amine greatly improves its hydrophobicity and interaction with nonpolar matrices which might improve the thermal and mechanical properties of nanocomposites based on polyethylene (Ferreira et al. 2017). Polyamines can also be attached to CNT defect sites via amide linkage. It provides free primary amine groups on the CNT structure that greatly improves the affinity with epoxy and polyurethane networks and improves dispersibility and mechanical properties of the resulted nanocomposites (Cividanes et al. 2017). The pendant amine groups can be also neutralized in acidic environments which might stabilizes them in aqueous solutions (Naqvi et al. 2020). Esterification, as well as amination, is an applicable tool for manipulating surface functionality of the oxidized CNT. A wide range of hydroxyl bearing compounds have been attached to carboxyl functionalized CNT via ester linkage to alter its

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interaction with a solution or polymer matrix. The esterification takes place at temperatures above 100  C, accompanied by white extraction of condensed water. The reaction is usually catalyzed with acids like para toluene sulfonic acid (PTSA) and sulfuric acid. Abuilaiwi et al. have reported ester functionalization of oxidized CNT with three different alcohols: phenol, polyethylene glycol (PEG), and octadecanol (Abuilaiwi et al. 2010). Octadecanol grafted CNT shows good mechanical integrity with nonpolar polymers like natural rubber (NR) and considerably increases its modulus (Selvin Thomas et al. 2012). Grafting PEG on the CNT via esterification enhances its hydrophilicity. The aqueous dispersion of the PEG grafted CNT has been reported as a modified heat exchanging nanofluid. PEG grafted CNT rises up the specific heat capacity of water by 45% while keeping the pressure drop unchanged which means avoiding extra pumping energy (Manasrah et al. 2017). In contrast to PEG functionalization, grafting phenol onto the oxidized CNT improves its affinity to polypropylene (PP). Incorporating phenol functionalized in the PP matrix by extrusion process increases both its crystallinity and storage modulus. The nanocomposite filled with phenol grafted CNT showed higher storage modulus in comparison with the corresponding composite with nonfunctionalized CNT, while maintaining the elongation at break higher. Therefore, the phenol functionalization of CNT results in tougher PP nanocomposites (Girei et al. 2012). Polymers can be covalently attached to the CNT structure in two approaches: “grafting from” and “grafting to.” In the first approach, in situ polymerization of monomers takes place in the presence of CNT-supported initiator or reactive functionalized CNT, while in the latter approach, previously synthesized polymer is covalently bonded to the CNT structure. PVA has been successfully grafted to CNT with ester linkage, along with PEG. A 1.5 nm layer of grafted polymer was created on the resulted functionalized CNT, which is converted into bioactive material and granted dispersibility in aqueous solutions (Malikov et al. 2014). Biomolecules usually contain hydroxyl and primary amine groups that readily react with carboxylated CNT. Ascorbic acid and vitamin B1 have been successfully attached to carboxylated CNT via ester linkage. It has been announced that vitamin C and vitamin B1 grafted CNT have good compatibility with poly(ester-imid) and PVA (Fig. 11). The resulting nanocomposites represent improved thermal and mechanical properties and are expected to show biocompatibility (Mallakpour and Azimi 2019; Mallakpour and Soltanian 2016).

Fig. 11 Schematic representation of attaching vitamin B1 to carboxylated CNT via esterification. (Reprinted with permission of Royal Society of Chemistry) (Mallakpour and Azimi 2019)

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Acid groups can form salt in reaction with alkyl and aryl amines to form organic salts. The HSP of the functionalized CNT is manipulated by the counterion. Decorating the surface of oxidized CNT with long chain alkyl ammonium salts considerably reduces Ra of CNT in polar polymers like epoxy polycarbonate and polyurethane matrix which results in stable dispersion and good compatibility. While the raw CNT has very poor compatibility with such matrices, improving the CNT-matrix compatibility also dominantly influences the thermal and mechanical properties of the nanocomposites. The lower the Ra, the better the strain and heat transfer. It means that CNT oxidation and further functionalization affect the mechanical and thermal properties of nanocomposites, through HSP alteration. As an instance, the oxidized CNT that has been neutralized with dodecyl amine and that has very low Ra in epoxy resin have good dispersibility in it. It also brings higher mechanical strength to epoxy nanocomposites in comparison with relevant nanocomposites that are loaded with oxidized and unfunctionalized CNT (Ma and Larsen 2014). Carboxylic groups on the CNT structure might also undergo reaction with thionyl chloride which affords acyl chloride, one of the most reactive and versatile groups in organic chemistry. The acyl functionalized CNT can be taken advantage to attach various organic functionalities to the CNT (Naqvi et al. 2020). It reacts with amines to afford amine grafted CNT that can improve dispersibility and interfacial forces of the CNT in epoxy matrix (Roy et al. 2018). Acyl chloride also reacts with hydroxyl group to generate ester linkage. Hence, natural polysaccharides like starch and chitosan readily graft to acyl chloride functionalized CNT, thanks to their large number of hydroxyl functionalities. The starch grafted CNT can produce stable dispersion in water with concentration up to 12.5% wt. without obvious aggregation. Starch functionalized CNT have incorporated into chitosan electrode and exhibited higher electrical conductivity alongside with pristine CNT loaded electrodes at a same concentration because of its improved dispersibility that generates conductive network in the polymer matrix at lower concentration (Yan et al. 2011). Another functional group that can be produced on the carboxylated CNT is thiol (–SH). It can be generated on the CNT sidewall by reacting carboxylated CNT with mercapto-alkylhalides or SOCl2 and subsequently thiol-amine compounds. Thiol group is reactive and can play role as an anchor to attach other substances to the CNT, especially noble metal nanoparticles. Nanoparticles can also be directly synthesized from metal ion precursors on the previously thiol functionalized CNT. The CNT supported metal nanoparticles have high catalytic activity and interesting electrical and optical properties (Hu et al. 2005). Some of the most important approaches for further derivatization of carboxylic acid functionalized CNT have been briefly illustrated in Fig. 12.

Noncovalent Approach Covalent functionalization techniques provide versatile functional groups on the CNT sidewall that are attached by strong, covalent bonds. Beside their

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Fig. 12 Brief schematic summery of different strategies for indirect covalent functionalization of CNT, started from CNT oxidation. (Reproduced with permission of Elsevier) (Ma et al. 2010)

advantageous, they have two main drawbacks: firstly, the chemical reactions that are carried out in these methods, especially when accompanied with destructive mechanical processes like milling and ultrasonication, damage the CNT structure to some extent. The damages might be extensive enough to break it down to smaller tubes, unzip it, or even completely convert it to amorphous carbon. Because of the created defects, the mechanical properties of the CNT abate, as well as the electrical and thermal conductivity. The π electron system that wraps up the external surface of the CNT would be disrupted by the created defect sites. Electrons and phonons that are responsible for the electrical and thermal conductions of CNTs would be scattered as a result of π electron system disruption. Secondly, the strong reactants and reaction conditions that are used for chemically modification of CNT structure covalently are environmentally disruptive. Therefore, noncovalent surface modifications have also developed. Noncovalent modification is carried out either by absorption of surfactant or polymer on the CNT surface. In both cases the moiety has to be absorbed onto the CNT surface effectively which is thermodynamically

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motivated by weak, Van der Waals and nonpolar hydrophobic interactions and π-π stacking (Ma et al. 2010).

Surfactant Modification Surfactant modification involves the absorption of surfactant micelles to the surface of CNTs, which finally separates them apart and stabilizes them in the solution. Surfactants are molecules with distinct parts that can be described as hydrophilic polar heads and hydrophobic nonpolar tails. They tend to absorb at the interface and accumulate into supramolecular structure thanks to their intrinsic amphiphilic structure. Surfactants are generally classified according to their polar head structure. Surfactants with electrically charged head groups are called ionic surfactants while nonionic surfactants have heads with zero net charge. The stabilization mechanisms of ionic and nonionic surfactants are different according to their nonidentical structure. While ionic surfactants stabilization is based on electrostatic repulsion, the nonionic surfactants stabilization relies on steric hindrance. Surfactant modification of CNT is widely utilized in aqueous solutions to obtain stabilized colloidal particles. Dispersion of CNT in aqueous media associates with adsorption of surfactants hydrophobic tail to the CNT sidewall. Simultaneously, the ionic head groups dissociate and induce charge to the interface and create counterion. Since the ζ represents the electrostatic potential at the vicinity of CNT, it grows up presumably and results in more stable dispersion. The higher the electrostatic repulsion is induced, the more stabilized dispersion is obtained. Therefore, the surfactants that pack tightly on the CNTs surface are favorable for obtaining aqueous stable dispersion of CNT. Colloidal particles with ζ more than 30 mV or less than 30 mV are usually expected to be stable. According to an investigation, the dispersion quality of CNT in aqueous media using ionic surfactants is reported to be corresponded with the zeta potential. Five different surfactants, sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), lithium dodecyl sulfate (LDS), sodium cholate (SC) and tetradecyltrimethylammonium bromide (TTAB), were investigated that are all anionic surfactants, except the last one which is cationic. The dispersion quality was found in the following order: SDS > LDS > SDBS > TTAB > SC This finding correlates well with the zeta potential of the colloidal dispersion which shows the largest number for SDS (70.2 mV) and the lowest for SC (20 mV). It has also been reported that increasing the alkyl chain length of analogous anionic surfactants has inverse effect on the dispersion quality of the CNT. It corroborates the claim that higher packing density of the surfactant on the CNT surface provides more stable dispersion (Backes 2012). In some other comparative studies however, the dispersion properties of these surfactants stayed in different order. Structure of some of the most investigated surfactants for CNT dispersion is represented in Figs. 13 and 18. Although the stabilization mechanism of nonionic surfactants such as Triton ® X-100 (a polyethylene glycol alkyl phenyl compound) is based on steric hindrance,

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Fig. 13 Chemical structure of some of the most investigated surfactants for CNT dispersion. (Reproduced with permission of John Wiley and Sons) (Claudia Backes 2010)

it is noteworthy to mention that their stabilization performance augers with minor electrostatic stabilization. It is based on the presence of acid groups and ether linkages interacting with water and on the presence of a negative ζ values due to adsorbed impurities (Tkalya et al. 2012). Accordingly, some investigations revealed that the higher the molecular weight of the nonionic surfactants, the better they disperse the CNT in water (Claudia Backes 2010). In contradiction to these results, in another investigation, various Triton ® X-series of surfactants have been investigated and finally it has been claimed that the surfactants with shorter hydrophilic part provide better dispersibility owing to better adsorption and formation of smaller micelles (Al-Hamadani et al. 2015). Similarly, the effectiveness of Triton ® X-100 in dispersing CNT has been compared with SDS and SDBS. In contrast to the

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aforementioned reports, it was found that both SDBS and Triton ® X-100 exhibit better performance in comparison with SDS. It has been proposed to be due to the more effective adsorption of the SDBS and Triton ® X-100 on the CNT surface owing to the π-stacking of their phenylene group with the graphemic structure of CNT sidewall. However, the proposed reason has been failed by further investigations. Stabilization of CNT with SDBS and Triton ® X-100 is reported to cause no shift in its absorption spectra which would be red-shifted, if the π-stacking of phenylene ring in surfactant with CNT sidewall occurred. On the other hand, phenylene group in the SDBS structure is located in its polar part that is not expected to interact effectively with CNT sidewall. As it has been depicted, various reports claimed contradictory results about the dispersibility of same surfactants on the CNT. It might be stemmed from the fact that the effectiveness of CNT dispersion by noncovalent approach is highly influenced by media and dispersion conditions, for example, time, temperature, and power of dispersing facility. Additionally, the structure and properties of the pristine CNT significantly affect the dispersibility of CNT. Synthesis and further purification procedure that is governed to obtain the pristine CNT resulted in different structures and surface functional groups and therefore different point of zero charge (PZC). Variation in PZC of different CNT samples causes a same surfactant to adsorb on their surface non-similarly even in a same pH. The size of CNT particles is another effective factor. Smaller curvature radius of CNT favors some nonaromatic surfactant like SDS over the surfactants with aromatic constituents such as SDBS. Therefore, it is revealed that while SDBS shows no significant radius preference, SDS and SC preferentially adsorb on the surface of the CNT particles with smaller diameter (Claudia Backes 2010). Finally, various surfactants show their best CNT dispersion performance at a certain concentration. While SDBS optimum concentration is about 0.5%, Pluronic ® F-127 (Pluronic surfactants are nonionic triblock copolymers, composed of central hydrophobic polyoxypropylene block which is flanked with two hydrophilic polyoxyethylene blocks) exhibits best performance at about 5% of the overall solution (Al-Hamadani et al. 2015). Surfactants organization on the CNT surface has also been a concern. Different interaction causes different surfactant organization on the CNT surface. Surfactants that are adsorbed on the CNT surface can be organized differently. Some of the surfactants are proposed to encapsulate the entire surface of the CNT and the surfactant molecules get aligned perpendicular to the CNT sidewall, while some other are found to be aligned in hemimicellar structures like half-cylinders with axis either in parallel with the tube axis or perpendicular to it. While the anionic and cationic are often proposed to be adsorbed in hemimicellar formation, but the nonionic surfactants with long polyoxyethylene chain adsorb in random structure. It is assumed to be arisen from the negligible if any, electrostatic repulsion between polar head groups in nonionic surfactants while the electrostatic repulsion between ionic surfactant head groups is resulted to oriented micellar absorption of ionic surfactants (Claudia Backes 2010; Tkalya et al. 2012) (Figs. 14 and 15). As it is revealed in comparison between SDS and SDBS, the affinity of surfactant anchor group to the CNT side wall dominantly affects the dispersibility. The binding

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Fig. 14 Different organizations of surfactants on the CNT surface: (a) CNT encapsulated in cylindrical micelle with surfactant molecules that are aligned perpendicular to the CNT surface, (b) surfactant organization in hemimicellar structure, (c) surfactant organization in random formation. (Reproduced with permission of Springer) (Backes 2012) Fig. 15 Schematic description of dispersing mechanism of CNT using surfactants with polycyclic aromatic moieties. (Reproduced with permission of John Wiley and Sons) (Backes 2012)

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energy between the surfactant molecule and the CNT surface should be high enough to provide sufficient surface coverage of CNT with surfactant molecules to disaggregate the CNT. It is also favorable if the surfactant adsorbs preferentially on the CNT structure instead of impurities like amorphous carbon particles and metallic catalysts. In spite of some contradictory reports, it is widely admitted that the presence of aromatic moieties in the surfactants structure can provide strong interaction between surfactant and CNT surface via π-π stacking. This interaction is favorable over nonspecific hydrophobic interaction of aliphatic and cycloaliphatic hydrophobic interaction of nonaromatic hydrocarbon chains in the surfactants structure. Accordingly, polycyclic aromatic moieties are widely reported as a very effective anchoring group for dispersion and solubilization of CNT. Ionic derivatives of pyrene, perylene, and porphyrin are reported to efficiently disperse CNT in aqueous solutions. Chemical structure of some surfactants with polycyclic aromatic

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groups that are utilized to disperse CNT is represented in Fig. 21. Photoluminescence measurements indicate considerable red-shift in CNT spectra in the presence of these polycyclic aromatic surfactants that verify the successful occurrence of π-π stacking (Claudia Backes 2010). The π-π stacking in aqueous media however can be bolstered up by hydrophobic and charge transfer interactions (Kharissova and Kharisov 2017a, b). It has been also confirmed by Raman spectroscopy and UV/Vis adsorption spectra that the dispersion of CNT in aqueous media using pyrene surfactants is nondestructive process and induce no structural damage to CNT (Claudia Backes 2010). It has been also proved that pyrene ligand interaction with CNT is stronger than phenyl and naphthyl (Fujigaya and Nakashima 2015). Various investigations also announced that concentration of metallic catalyst in aqueous dispersion of CNT in the presence of ionic derivatives of surfactants that contain polycyclic aromatic groups is significantly lower in comparison with pristine CNT. It can be therefore concluded that these surfactants are preferentially adsorbed on CNT surface. Comparison between such derivatives and SDBS revealed that not only the individualization with polycyclic aromatic surfactants occurs in much higher degree (Fig. 22), but also the preferential adsorption on CNT is more significant for them (Claudia Backes 2010). The effectiveness of surfactants with polycyclic aromatic anchor groups motivated the researchers to investigate the dispersibility of structures with tailored more complicated structures like the one represented in Fig. 16b. C1s core level shifts to higher binding energy that is proved to take place in X-ray photoelectron spectrum (XPS) indicates charge transfer from surfactant to CNT. It is also known that the presence of amino groups in anchor group improves the interaction. Therefore, it has been concluded that in complement to π-π stacking, charge transfer and cation-π interaction improve the affinity of anchor group to the CNT structure (Claudia Backes 2010) (Fig. 17). As described so far, CNT dispersion in aqueous solutions mainly involves electrostatic stabilization. Since in most nonaqueous media, ionization of the functional groups on the surfactant structure cannot be effectively achieved, electrostatic stabilization of CNT in organic solvents does not exploit. Steric stabilization of particles requires adsorption of a dense layer of polymeric moieties with freely mobile parts that can be extended in the solution, far from the particle surface. It is usually assumed that wrapping up of the particle surface with a layer of steric stabilizing agents with somewhat high thickness is a prerequisite to obtain stable dispersion by entropic approach. Therefore, stabilization of CNT in organic environments cannot be efficiently achieved by non-polymeric surfactants. CNT dispersion in organic solvents requires polymeric compounds, with exception of solvents with the ability to dissolve CNT. As it has been mentioned in section “Hansen Solubility Parameters,” individualization of CNT particles in solvent can be assumed similar to polymer dissolution. According to thermodynamics of solutions, spontaneous dissolution occurs if the Gibbs free energy of mixing remains negative. Because of the large size and rigidity of CNT structure, entropy of mixing is very low. Therefore, spontaneous dissolution of CNT is only granted when the enthalpy of mixing is very low and close to zero. Therefore, analogous to polymers and

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Fig. 16 Molecular structure of some surfactants with polycyclic aromatic anchor groups: (a) pyrene, (b) perylene, (c) porphyrin that are governed for CNT dispersion. (Reproduced with permission of John Wiley and Sons) (Claudia Backes 2010)

according to HSP theory, very low Ro is required to solubilize the CNT in the solvent. It means that only the solvents with very good interaction with CNT surface can solubilize it. Because of CNT intrinsic hydrophobicity, its surface may be expected to be wetted conveniently by organic solvent. CNT dispersion, however, has been obtained only in a limited number of solvents. Polar aprotic solvents, for example, DMF and NMP, and aprotic chlorinated solvents, for example, o-dichlorobenzene (ODCB), chloroform, and methylene chloride are the most widely reported appropriate solvents. Nonetheless, gradual reagglomeration and lack of stability has been reported for them. Additionally, dispersion in these solvents, especially the chlorinated ones, may disrupt the electrical structure of CNT. It has been revealed that chlorinated solvents can be decomposed to generate hydrochloric gas and chlorine gas, during ultrasonication which is required for CNT dispersion. It is then followed by reaction between the produced gasses and iron catalyst particles that usually exist

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Fig. 17 Cryo-TEM image of CNT dispersion in aqueous solution using (a) SDBS and (b) a polycyclic aromatic surfactant with hydrophilic ionic groups, represented in Fig. 16c. (Reproduced with permission of John Wiley and Sons) (Claudia Backes 2010)

in pristine CNT to afford iron chlorides which perform as p-dopant for CNT. Furthermore, in CNT dispersion in these solvents, an equilibrium between individualized and bundles of CNT has been observed. So, the concentration of solubilized CNT in appropriate organic solvents is limited solely to dilute and semi-dilute solutions. Despite the aforementioned shortcomings of CNT solubilization in organic solvents, it can be altered in the presence of surfactants. In this case, surfactants are adsorbed to the CNT surface and improve its interaction with solvent. Enthalpy of mixing will be reduced this way and the free energy of mixing thus reduces to thermodynamically favor the solubility. Similar to surfactant-aided aqueous dispersion of CNT, in organic environments also the surfactant has to be adsorbed effectively on the surface. Although in contrast to aqueous media, the adsorption of surfactants in organic solvents is not stimulated by hydrophobic interactions, but in analogous to them, π-π stacking is the most effective interaction between the surfactant anchor group and CNT surface. Polycyclic aromatic compounds are therefore identified as good surfactants for CNT dispersion in organic environments. Small dye molecules based on terphenyl and anthracene are reported to effectively solubilize CNT in organic solvents like DMF and toluene. The occurrence of π-π stacking between these aromatic dye molecules and CNT has been revealed from UV/Vis spectra. Porphyrin derivatives alter CNT dispersibility in organic solvents similar to aqueous solutions. Complex mono and tripodal porphyrin structures have been proved both experimentally and by density functional theory (DFT) as efficient dispersant for CNT in organic solvents. Occurrence of charge transfer between these hosts and CNT guests has also been observed (Claudia Backes 2010; Garrido et al. 2020). In another investigation, a series of oligomers based on thiophene derivatives

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have been designed to improve the CNT dispersibility in NMP. 3-hexylthiophene was utilized as main monomer for producing the oligomeric surfactants and the effect of proportion between thiophene head groups to hexyl tail groups of the oligomers on the surfactant performance has been investigated. Strong adsorption of surfactant to CNT has been detected which is revealed by XPS as well as Raman spectroscopy that is stemmed from charge transfer from CNT side wall to electronegative thiophene group. It has been also verified by the fact that increasing the ratio of thiophene head groups to hexyl tail groups in the oligomer structure further improves the performance of the surfactant (Claudia Backes 2010; Kim et al. 2007). So, beside π-π stacking, charge transfer can improve the surfactant adsorption onto the CNT structure in organic environments. Surfactant-aided dispersion of CNT in organic solvents can be granted by surfactants with anchor groups with either polycyclic aromatic moieties or the groups that can involve charge transfer with CNT. However, polymeric substances can substitute the small molecule surfactants and be exploited to obtain stabilized dispersion. Polymeric dispersants usually afford more stable dispersion since their larger molecules provide more anchoring groups and therefore multipoint interaction with CNT. Figure 18 schematically illustrates the difference between the performance of low molecular weight and high molecular weight dispersants (Claudia Backes 2010; Fujigaya and Nakashima 2015). Additionally, combination of low molecular weight surfactants and high molecular weight dispersants usually is considered to represent synergism. In this approach, the small molecule dispersants in spite of their thermodynamically less favorable adsorption diffuse into the CNT bundles because of their high mobility and debundle them. The adsorbed low molecular weight surfactants then substitute with thermodynamically more favorable polymeric dispersants that grant stability to the dispersion (Fig. 19) (Kharissova and Kharisov 2017a, b). Fig. 18 Schematic description of the dispersibility aspects of CNT, using (top) low molecular weight dispersants and (bottom) high molecular weight dispersants. (Reprinted with permission of IOP science) (Fujigaya and Nakashima 2015)

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Fig. 19 Schematic illustration of the performance of (top) low molecular weight surfactants, (middle) high molecular weight surfactants, and (bottom) combination of both low and high molecular weight surfactants for CNT dispersion (Kharissova and Kharisov 2017a, b)

Polymer Modification Surface of the CNT can be noncovalently modified by polymeric substances. Since macromolecular dispersants contain large number of anchor groups in their structure, they bind to the particle more firmly in comparison with low molecular weight surfactants. Therefore, appropriately designed polymers are assumed to be able to irreversibly wrap around the CNT structure which provides the possibility to remove the unbounded dispersant molecules from the media by various methods such as simple filtration. It has to be considered that in some applications such as electronics and biology, the unbounded surfactant molecules cause some undesirable side effects and it is essential to remove them. Polymer binding to CNT surface is granted via several groups on the polymer chain that are able to involve π-π stacking, ion (cation/anion)-π stacking, hydrophobic and Van der Waals interaction with CNT sidewall (Fujigaya and Nakashima 2015). Usually, aromatic conjugated moieties in the polymer structure are in charge of binding the polymer to the CNT via π-π interaction (Fig. 26) (Claudia Backes 2010). π-conjugated polymers not only perform well on dispersing CNT by wrapping it up according to their π electron system, but also improves its electrical properties according to their intrinsic electric conductivity which is highly interested in microelectronic applications such as fabricating transistors with high charge carrier mobility. Poly(p-phenylenevinylene) derivatives (PPVs) (Fig. 20a)

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Fig. 20 Chemical structure of (a) PPV, (b) PFO, and (c) PFO-carbazole

conjugated conducting polymers have been widely used for CNT dispersion in organic solvents. PPV dispersing agents preferentially adsorb onto the surface of CNT particles with specific chirality and diameters. It is assumed to be arisen from the fact that their rigid backbone causes them to align in a preferential angle around the CNT with specific chirality that matches this angle to maximize the number of interaction point with the surface (Fujigaya and Nakashima 2015). Occasionally, the rigidity of the polymer backbone is as high as it prevents the polymer to wrap the CNT. It is the case for conjugated poly(aryleneethynylene) (PPE) (Claudia Backes 2010). Polyfluorene (PFO) (Fig. 20b) and its versatile derivatives are similarly reported as structures with the ability to preferentially disperse CNT particles with specific diameter and chirality. π-π Interaction of PFO with CNT can be supplemented by incorporating co-monomers like anthracene and porphyrin. It has been also reported to incorporate thiol containing carbazole-based comonomer into the PFO-based dispersant (Fig. 20c). The thiol functional groups in the dispersant molecule that wrap around the CNT can be further utilized to decorate CNT structure with nanometallic particles. As it was previously mentioned for short thiophen oligomers, thiophene polymers also tend to noncovalently attach to CNT via charge transfer interaction as well as π-π interaction. Poly(alkylthiophens) produce stable CNT dispersion in organic solvents and exhibit stability against UV degradation which is the drawback of poly(alkylthiophens) in the absence of CNT. Other π-conjugated polymers such as polypyrrole and polyaniline also represent good dispersibility properties for CNT. Aromatic polymers like polyimides can also perform CNT wrapping via π-π stacking. On the other hand, they usually withstand

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high temperatures which suite them as CNT dispersant in high temperature environments such as in aerospace applications (Fujigaya and Nakashima 2015). An efficient method to obtain polyimide composite with individually distributed CNT particles is in situ polycondensation of sulfonic acid containing amines in the presence of CNT. Successfully solubilization of CNT in organic solvents even at high concentrations has been reported (Claudia Backes 2010). Inserting pendant groups with the ability to firmly attach onto the CNT structure can greatly enhance the performance of a high molecular weight dispersant. Pyrene and porphyrin ligands are good choices for this purpose since they can undergo strong π-π interaction with CNT. it is noteworthy to mention that while CNT dispersions that are produced using non-polymeric pyrene derivatives are heat immune and can be only obtained in low concentrations, polymeric dispersants with pyrene pendant groups produce highly stable CNT dispersion in much higher CNT concentrations (Fujigaya and Nakashima 2015). CH-π interactions have also been reported to be exploited to organize synthetic polymer chains on the CNT sidewall. Various Poly(dialkylsilane) (PSi) with different alkyl groups have been investigated for CNT dispersion. It has been found that PSi samples with more flexible structure successfully wrap the CNT via CH-π while stiffer poly(n-decyl-i-butylsilane) cannot form helical conformation around the CNT (Fujigaya and Nakashima 2015). Block copolymers are another group of polymers that provide CNT dispersions with stability. Different block can either expose to environment to grant steric hindrance or physically attach to CNT as anchor groups. The schematic illustration of organization of block copolymers on the CNT surface is represented in Fig. 21. Pendant anchor groups such as pyrene derivatives perform more efficiently if they insert in the polymer chain in block sequence instead of random. It has been also revealed that such blocks behave much more efficiently in the case of small multiblock instead of fewer but larger blocks (Fujigaya and Nakashima 2015).

Fig. 21 Schematic description of CNT dispersion using aromatic polymers. (Reprinted with permission of John Wiley and Sons) (Claudia Backes 2010)

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Bioinspired Modification Since the discovery of CNT, combination of their extraordinary properties with biomaterials has been interested. They can be utilized in drug delivery by encapsulating drug and carry it into the cytoplasm and cell nucleus through cell membrane. Nano biosensors can be obtained by immobilizing bioactive materials on CNT surface. On the other hand, special electrical and optical properties of CNT make it suitable to be used in biosensors. The interesting mechanical properties of CNT can be taken advantage in tissue engineering. CNT can improve the mechanical stability of hydroxy appetite for bone implantation. In all these applications, good dispersion and integrity of CNT with the biological environment is intended while no decrease in biocompatibility is acceptable (Naqvi et al. 2020; Shojaei and Azhari 2018). Several researches have been conducted to disperse them in biological environments, with the aid of biocompatible molecules as dispersants. Nucleic acids are widely studied in this regard. Single and double strands of deoxyribonucleic acid (DNA), both in synthetic and natural form, have represented good potential to produce stable CNT dispersion in aqueous media (Naqvi et al. 2020). DNA is known to have high capability to wrap the CNT sidewall. The nucleobases in nucleic acid structure undergo strong interaction with CNT sidewall via both π-π and cationπ stacking. Meanwhile, the hydrophilic backbone which is composed of hydrophilic sugar-phosphate aligns outward to be exposed to aqueous environment. Stable dispersion of CNT can be attained by this conformation. The effect of sequence order of DNA on its dispersion performance of CNT has been also investigated. Experimental observations have revealed that polyadenine- and polycytosine-based synthetic strands that are well known for their high tendency to undergo self-stack in solutions show less dispersion ability in comparison with polyguanines and thymines. It confirms the proposed conformation of nucleic acids on CNT. Figure 22 schematically demonstrates the conformation of nucleic acids on CNT structure (Claudia Backes 2010; Fujigaya and Nakashima 2015; Naqvi et al. 2020). The wrapping stability of single strand DNA (ssDNA) on the CNT structure has been also investigated. Since no unbound DNA has been detected in DNA dispersed CNT solution by gel permeation chromatography (GPC) even after a month it has

Fig. 22 Schematic illustration of ssDNA conformation single while wrapping the CNT. Thymine and adenine nucleobases are represented in orange and green, respectively. Hydrophilic backbone is shown in yellow. (Reproduced with permission of springer nature) (Tu et al. 2009)

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been concluded that DNA wrapping on the CNT in aqueous solution is thermodynamically stable (Fujigaya and Nakashima 2015). It has to be noticed that omitting the complementary strand in DNA is essential to attain ssDNA wrapped CNT (Claudia Backes 2010) (Fig. 23). Polysaccharides are another group of biological molecules that are widely reported as efficient dispersant for CNT in aqueous environments. These natural polymers can wrap up the CNT. The mechanism of CNT covering with polysaccharides is not well understood. However, it is usually assumed to arise from multipoint interaction of amylose hydrophobic parts of polysaccharides with CNT that conforms helically around the CNT, while the hydrophilic hydroxyl groups tend to extend outward to expose to aqueous solution. The effectiveness of polysaccharides as CNT dispersants have been confirmed extensively. As an instance, starch stabilized aqueous dispersion of CNT undergo precipitation by addition of amyloglucosidase, an enzyme with the ability to hydrolyze the starch into glucose (Fujigaya and Nakashima 2015; Star et al. 2002). Carboxymethyl cellulose (CMC) is another biopolymer that effectively disperses CNT in aqueous media (Fujigaya and Nakashima 2015). In addition to aqueous media, polysaccharides also utilized to disperse CNT in organic solvents. Chitosan is an amino bearing saccharide with well-known

a

b

Fig. 23 Schematic demonstration of (a) the helical structure of in complexes with small molecules. (b) Result of molecular dynamics simulation of the conformation of maltooctaose around an SWCNT. (Reprinted with permission of John Wiley and Sons) (Star et al. 2002)

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bioactive properties. Acidic pretreated chitosan has been successfully exploited to produce oxidized CNT filled composites based on silicon rubber. Ammonium moieties on chitosan interact with acidic groups on oxidized CNT. Meanwhile, the hydroxyl groups of chitosan can be aligned in a way to expose to environments with the ability to undergo hydrogen bond interaction such as protic solvents and silicon rubber. Hence, acidified chitosan behaves as a coupling agent between silicon rubber and CNT. This function has been verified both with improved dispersibility of CNT in silicon rubber and enhanced mechanical properties of the final composite. It is of great importance since silicon rubber is biocompatible but its mechanical properties are shortcoming for being governed in tissue engineering (Fujigaya and Nakashima 2015; Shang et al. 2013).

Ionic Liquids Ionic liquids (IL) can be simply defined as salts with melting point below 100  C. They can be even liquid at room temperature. ILs consist of large organic cations which commonly have imidazolium, phosphonium, or pyridinium structure and relatively small anions such as tetrafluoroborate and Cl that are poorly coordinated. These liquid salts have great advantages over conventional solvents such as high ion conductivity, low volatility, low toxicity, and good solvation properties (Abo-Hamad et al. 2016). Fukushima and coworkers were the first researchers who found that ILs can disperse CNTs. They obtained well-dispersed SWNT in imidazoline-based ILs by agitating them. Eventually, a thick paste was acquired which was called “bucky gel.” It is thought that noncovalent functionalization of CNTs conducts through either the “cation-π” interaction of organic cations of ILs and carbon nanotube π electrons (Fukushima et al. 2003) or weak Van der Waals interactions (Wang et al. 2008). Since then, several researchers studied the CNT-ILs hybrids. It is shown that in most of the efforts, the outer surface of CNTs is covered with ILs. In such hybrids ILs can enact either as main binder or as a dispersing agent to stabilize the CNTs in a polymer or matrix (Tunckol et al. 2012). They show good dispersion stability and incorporating such hybrids into the polymer matrices improves both their thermal stability and mechanical properties (Das et al. 2009). Additionally, owing to their good electrical and ionic conductivity, they show good potential to be used in wide variety of electrochemical devices such as sensors, electrodes, and actuators (Soares 2018). Various surface modification methods of CNT and their main properties are summarized in Table 1.

Physical Processes for Dispersion of Carbon Nanotube As mentioned in section “Desagglomeration of Particles” desagglomeration of CNT bundles requires the mechanical force to be transmitted to the agglomerates. Different procedures have been reported as effective methods for physical debundling are discussed.

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Table 1 Surface modification methods of CNT and their main characteristics covalent

Direct sidewall functionalization

Fluorination and derivatization

Cycloaddition

Reductive hydrogenation, alkylation, and arylation

Radical addition

Indirect functionalization

Oxidation

Advantages: Possibility to substitute the fluorine atoms with other desirable groups. Good diversity Directly decorate the sidewall Strong covalent attachment of functional group to CNT Disadvantages: Uncomfortable reaction condition Nonuniform distribution of functional groups on the sidewall Sidewall distortion and great reduction in electrical conductivity Advantages: One-pot, more comfortable reaction condition (for Prato reaction) Good diversity Possibility to recover the electrical conductivity by heating Strong covalent attachment of functional group to CNT Disadvantages: Sidewall distortion Advantages: Possibility to attach alkyl and aryl chains which greatly improves interaction with organic media Strong covalent attachment of functional group to CNT Disadvantages: Deals with difficult reaction condition and difficult to obtain reactants Low diversity Advantages: Ability to directly attach polymers to CNT sidewall Strong covalent attachment of functional group and polymer chain to CNT Disadvantages: Sidewall distortion and great reduction in electrical conductivity Advantages: Simplicity of reaction condition Common reactants Strong covalent attachment of functional group to CNT Disadvantages: Causing damage to CNT structure. Reducing CNT conductivity (continued)

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Table 1 (continued)

Derivatization of the oxidized CNT

Noncovalent modification

Surfactant

Polymer modification

Ionic liquid

Bioinspired

Only the irregularity in CNT structure can be functionalized Advantages: Simplicity of reaction condition High diversity; amidation, esterification, acylation, thiolation, polymerization, etc. can be carried out on the oxidized CNT The diversity can even get improved by acylation Strong covalent attachment of functional group to CNT Disadvantages: Causing damage to CNT structure. Reducing CNT conductivity Only the irregularity in CNT structure can be functionalized Advantages: Causes no damage to CNT structure No chemical reaction No reduction in electrical and thermal conductivity Disadvantages: Low bonding strength with CNT Lower stability in comparison with polymers Advantages: Causes no damage to CNT structure No chemical reaction No reduction in electrical and thermal conductivity Higher stability in comparison with surfactants Possibility to be improved with the aid of surfactants Advantages: Causes no damage to CNT structure High electrical and ionic conductivity Good potential to be used and electrochemical devices No chemical reaction No reduction in electrical and thermal conductivity Disadvantages: Limited to polar matrices Advantages: Causes no damage to CNT structure High electrical and ionic conductivity Good potential to be used in (continued)

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Table 1 (continued) biological matrices Environmentally friendly Sustainability No chemical reaction No reduction in electrical and thermal conductivity

Dispersion by Cavitation: Ultrasonication Ultrasonication in the most frequently used method to disperse CNT. It is based on the application of high intensity ultrasonic energy to agitate particles in a liquid. The high intensity mechanical waves propagate through the liquid both in longitudinal and shear mode producing consecutive high pressure and low-pressure cycles. Frequency of the ultrasonic wave determines the duration of each cycle. During the low-pressure cycle, bubbles generate and grow up locally which then collapse vigorously during high pressure cycle. This process is called cavitation. The explosion of bubbles in this process causes a very high temperature and pressure to be reached and a very intense high energy jet of liquid to be generated locally that can overcome the attraction force between particles and exfoliate them. It has to be considered that low viscosity of dispersion media is required for obtaining ultrasonically dispersed CNT. Therefore, polymeric solutions have to be diluted prior to ultrasonication (Kharissova and Kharisov 2017a, b; Ma et al. 2010). The amount of energy which is exerted to the bundles by ultrasonication is high enough to partially or even completely damage the CNT structure. This is the main drawback of this method. The amount of damage that is induced to CNT structure depends on the sonication energy. Thus, the process has to be conducted in a way that the desired state of dispersion to be achieved with as low sonication energy as possible. Accordingly, long time and high intensity ultrasonication have to be avoided (Kharissova and Kharisov 2017a, b; Ma et al. 2010) (Fig. 24). Incorporation of surfactants can facilitate the ultrasonically dispersion of CNT. The most accepted mechanism for exfoliation of CNT in presence of surfactant with ultrasonication aid is called unzipping mechanism. As schematically illustrated in Fig. 21, it is carried out in three steps. During the first step, the ultrasonic energy induces high shear stress in the CNT bundles, which makes their ends to start dangling. These dangling ends become absorption sites for surfactant molecules that prevent the partly slackened tubes to reaggregate. It is accompanied with ongoing ultrasonication, induces relative movement of the loosened tubes that allows the surfactant molecules to progress along the nanotube length, and cover larger part of it. This carries on and finally results in individualization of CNT from the bundles (Backes 2012). It has to be considered that eventually, an equilibrium is stabilized between individualized CNT and aggregated bundles that limits the individual CNT concentration (Claudia Backes 2010).

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a

b

c

d

Fig. 24 CNT exfoliation from its bundles in the presence of surfactants and with the aid of ultrasonication according to unzipping mechanism. (Reprinted with permission of Springer) (Backes 2012)

Dispersion by Mechanical Force Calendering In contrast with ultrasonication, for calendering, the mixture has to be highly viscous. In this method, the shear force that is applied to the mixture by rotating rolls is in charge for particle exfoliation. The calender that is commonly noted as three roll mill is composed of three parallel rolls with same diameter that rotate adjacent to each other. The rolls rotate pairwise in opposite directions. The first roller is called feeding roller. The second and third rolls are called center and apron, respectively. The rotating rate of the rollers varies in way, the feeding roll has the lowest speed, and the apron rotates at lowest rate. The initial mixture feeds to

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Fig. 25 Schematic representation of three roll mill structure. (Reproduced with permission of Elsevier) (Ma et al. 2010)

the feeding roller. The mixture passes from feeding roller to the center roll and to the apron roll afterward. Finally, the processed paste is collected with a knife blade from the apron roll. The general procedure of calendering is represented in Fig. 25 (Goldschmidt and Streitberger 2018; Kolosov 2016; Ma et al. 2010). When the paste passes through the tiny nip between two adjacent rollers, it receives very high amount of shear because of the different speed of the rollers. The shear stress is transmitted to the agglomerates to break them up. Typically, the gap sizes between the rolls and the rotating speed of the rollers are adjustable (while keeping the speed ration constant) that dominantly influence the efficiency of the calendering process. The milling process can be repeated several times, until the desired state of dispersion is obtained. Relatively short retention time of CNT bundles in the high shear nip zone brings this method as the advantage of low CNT breakage. This feature along with the suitability of this method for highly viscous mixtures makes it an attractive choice for producing CNT filled polymer composites. However, there are some obstacles. The main drawback of this method is that the size of CNT diameter is too low in comparison with the distance between the rollers. Therefore, the break of large bundles to smaller ones effectively occurs by three roll mill, while the individualization is hardly attainable (Ma et al. 2010).

Ball/Bead Milling Various types of ball and bead mills consist of concealed cylindrical vessels that might operate either horizontally or vertically. The vessel can be non-rotating and contain an agitator propeller inside it, or otherwise be rotating without any agitator inside. Several tiny hard spheres, commonly made of stabilized ZrO2, are placed in the vessels that is called grinding media. Under the operational condition grinding media spheres collide on each other and generate high pressure which is in charge for debundling CNT agglomerates. This method can be used for milling the materials in both solid or liquid form and also in continuous or batch mode (Kharissova and Kharisov 2017a, b; Ma et al. 2010).

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The high shear force that applies to the agglomerates upon collisions in grinding media is high enough to not only desagglomerate them, but also to break down the CNT and reduce their length. In spite of this fact, in several researches, ball and bead mills have been successfully exploited to obtain CNT dispersion in various media such as ceramics, metals, and polymers. Sometimes ball milling is utilized for predispersion process which is then followed by extrusion and other methods to produce highly dispersed CNT filled composites (Kharissova and Kharisov 2017a, b; Ma et al. 2010). Ball/bead milling can be accompanied by chemical reaction by introducing chemical reactants into the grinding vessel. Amine functionalization of CNT has been successfully carried out during ball milling process in the presence of ammonium bicarbonate. It generates amine functional groups on the CNT structure that improves the dispersibility and disentanglement of CNT. It has been also reported that introducing electron donor nitrogen groups to the CNT structure also changes the semiconducting behavior of CNT from p-type to n-type (Ma et al. 2010). The operational varieties such as milling duration, size of grinding media, and its proportion to the mixture volume, as well as grinding mixture properties such as its viscosity, are determinant factors that have to be optimized for obtaining appropriate state of CNT dispersion (Kharissova and Kharisov 2017a, b; Ma et al. 2010).

High Shear Mixing Mixing the media with higher shear mixers is widely used to exfoliate the nanoparticle agglomerates. Although the effectiveness of this method depends on the amount of applied shear as well as the mixture properties, dispersion of MWCNT is generally more feasible by this procedure in comparison with SWCNT. As illustrated in Fig. 26, the mixer that is sometimes referred to as homogenizer contains a stator part and a high-speed rotor that rotates so fast within it. The mixture circulates and experiences very high shear (of the order of 105 s1) while passing the gap between rotor and stator and also the openings of stator. The described amount of shear is sufficient to exfoliate CNT bundles (Ma et al. 2010; Patel et al. 2017). High shear mixing is applicable for particle dispersion both in melt and solution circumstance. Using this method has been reported for CNT dispersion in various polymer matrix like epoxy resin. Similarly, friction stir process has been utilized to obtain CNT reinforced aluminum alloy with twofold increase in hardness. However, friction stir process highly damage CNT structure (Izadi and Gerlich 2012; Ma et al. 2010) (Fig. 26). Extrusion Extrusion is one of the most common methods for producing composites. Extruder generally contains one or two parallel long screws based on its type. For dispersion of nanoparticles, commonly twin-screw extruder is governed. Screws rotate at high speed, either in same or opposite directions in the extruder barrel which is equipped with heaters that are arranged along it. The initial mixture feeds into the barrel and pushes forward by screw rotation. The binder gradually melts down and desagglomeration takes place by very high shear force applies to the mixture by twin screws. Finally, the melted composite is forced into a die which shapes the

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a

385

b Stator

Stator

Rotor Stator openings Flow pattern

Rotor

Crucible Melt

Fig. 26 A schematic demonstration of (a) dispersion process in a melt mixture using high shear mixer and (b) bottom view of the rotor stator. (Reprinted with permission of springer) (Patel et al. 2017)

composite where it cools down and becomes solid. Although extrusion is usually applied to disperse CNT in thermoplastic polymers, performing CNT dispersion in thermoset polymers such as epoxy by reactive extrusion has also been reported. In addition to polymer nanocomposites, extensive reports have also been published on extrusion production of CNT filled metal matrix composites (MMC) as well as ceramic matrix composites. The state of dispersion of CNT by extrusion depends on various factors such as matrix and CNT surface properties as well as processing condition. It has been revealed that the amount of mechanical energy input which depends on various factors, such as screw speed and torque, greatly affects the state of dispersion. Another influencing factor is the residence time of the mixture in the extruder. Commonly, the longer the mixture stays in the extruder, the better the dispersion becomes. The screw configuration is another processing factor that affects the dispersion of CNT in the matrix (Kharissova and Kharisov 2017a, b; Ma et al. 2010).

Dispersion by Turbulent Flow: Jet Milling Jet milling process is principally based on high velocity impact, in between the particles and milling chamber. In various jet milling configurations, high velocity fluids, for example, air, inert gasses, and water, are used to impart energy to the particles that undergo desagglomeration under collision with other particles and mill chamber. The most common type of jet mill is called spiral or pancake jet mill. This kind of jet mill is composed of flat disk jet chamber with inclined nozzles that are located at its perimeter. The product outlet is also located at the disc center. These nozzles feed the chamber with high-speed fluid jet which produces a circular highspeed vortex in the chamber called grinding circle. Since the flow regime in the grinding circle is turbulent flow, particles deal multiple numbers of impacts with other particles in the grinding circle. Shear force that is exerted to the particles in this procedure is high enough to break down the coarse particles. Vortex of grinding

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Fig. 27 Schematic structure of spiral jet mill. (Reprinted with permission of Springer) (Morales et al. 2016)

Fig. 28 Schematic illustration of super growth carbon nanotube bundles exfoliation under (top) turbulent flow and (bottom) mechanical force mechanism. (Reproduced with permission of Springer Nature) (Yoon et al. 2014)

circle also behaves as integrated centrifugal classifier. It means that while the desagglomeration takes place in the grinding circle, the vortex separates the particles that have already been broken down to the desired size and let them leave through the outlet. Meanwhile, the larger particles are pushed outward by centrifugal force

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Fig. 29 Various dispersion methods of CNT, classified based on the desagglomeration mechanisms. (Reproduced with permission of Springer Nature) (Yoon et al. 2014)

and remain in the chamber for further grinding. The schematic structure of spiral jet mill is illustrated in Fig. 27 (Chamayou and Dodds 2007; Kharissova and Kharisov 2017a, b; Morales et al. 2016). Using turbulent flow mechanism for milling process is going to be extensively employed in various industrial sectors tanks due to their advantages, for example, ease of use and the ability to reach low particle size with narrow particle size distribution. Additionally, as it has been described, jet mills contain no rotating part which immune the product from contaminations that are resulted from erosion and mechanical wear of the moving parts of the mill. The efficiency of jet mill for CNT dispersion has also been admitted in some reports. The effectiveness of different milling processes for dispersing CNT has been compared. It was found that jet milling is the most effective technique for exfoliation of CNT in composites (Yoon et al. 2014). Using jet mill for CNT dispersion can afford hierarchical dendritic CNT network which can be suitable for improving the mechanical properties of the composites. Figure 28 schematically illustrates the difference between CNT dispersion with mechanical force and turbulent flow. Figure 29 briefly exhibits various methods that have been utilized for CNT dispersion (Yoon et al. 2014).

Conclusion Carbon nanotubes are cylindrical carbon allotropes, consisting of one or more graphene sheets. Their remarkable properties such as high mechanical modulus, exceptional electrical properties, low density, and astonishing thermal conductivity bring them good potential to be utilized in a wide range of commercial applications. Composite materials, microelectronics, energy storage, and biotechnology are main

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fields in which CNTs have been commercially used. Nevertheless, carbon nanotubes suffer from lack of dispersibility in most of common media. It arises from the π electron system that surrounds the CNT particle which induces high tendency in the particles to undergo stacking. Additionally, inappropriate interaction between the CNT and matrix deteriorates the properties of CNT filled composites. Since obtaining good dispersibility and strong interaction with environment are prerequisites for exploiting their potential features, several methods have been developed to overcome these drawbacks. Dispersion process consists of four steps: wetting, dispersion, distribution, and stabilization. Dispersion is usually accompanied with application of mechanical force to the agglomerates which break them down. Various physical procedures have been announced to efficiently debundle CNT, which are based on one of the three mechanisms: using mechanical force, utilizing turbulent flow, or governing cavitation approach. These processes can effectively exfoliate the CNT; however, the deagglomerated particles would restack simultaneously if no stabilization is carried out. Stabilization associates with manipulating the interface properties of CNT in a way to enhance its interaction with environment. It is commonly conducted by surface modification that can be carried out either covalently or noncovalently. Covalent approach provides good versatility for surface modification corresponding to the environment, intended for CNT dispersion. It also provides functional groups that are attached to the sidewall by strong covalent bonds. However, severe conditions that are required to incorporate such moieties on the CNT structure also damage and shorten the CNT particles. Therefore, the enhanced mechanical and electrical properties of the composites trade off partially with the deterioration of CNT main structure. Recently more interest is shown on noncovalent approaches since they impart no damage to the CNT graphenic structure. Different covalent and noncovalent approaches for CNT surface modification as well as the physical methods to break down the agglomerates have been overviewed in this chapter to provide an insight into CNT dispersion.

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Covalent Functionalization of Carbon Nanotube

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Ritu Yadav, Krishan Kumar, and Pannuru Venkatesu

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Overview of the Structure and Importance of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Defect Group Functionalization of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Covalently Functionalized Carbon Nanotubes in Polymer Science . . . . . . Application of Covalently Functionalized CNTs in Enzyme Immobilization . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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The exceptionally high mechanical, chemical, and thermal stability of carbon nanotubes (CNTs) make them undisputed protagonists in wide applications ranging from biomaterial science to pharmaceutical industries. CNTs intrinsically tend to hold together owing to strong van der Waals forces of attraction and result in a poor dispersion in various organic solvents as well as in an aqueous medium. For this purpose, a variety of surface modifications of carbon nanotubes have been investigated. Functionalization of carbon nanotubes is one of the proposed mechanisms to prevent this agglomeration behavior of CNTs. Covalent and noncovalent functionalization are the major accepted surface modifications of CNTs. The covalent functionalization of CNTs has significantly extended the utilization in various scientific fields. This method is designed to alter the surface properties of CNTs by making strong covalent bonds between the functional moieties and carbon skeleton, which results in improved adhesion characteristics and dispersion stability. As a result, the physicochemical properties of CNTs can be fine-tuned for intended applications, thus opening a new way to the assembly of functional materials. The enhanced properties of covalently functionalized R. Yadav · K. Kumar · P. Venkatesu (*) Department of Chemistry, University of Delhi, Delhi, India e-mail: [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_65

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CNTs enable the facile production of nanodevices and make them promising candidates in the field of biomedical science. The various functional groups attached to the functionalized carbon nanotubes (f-CNTs) promote further interactions with numerous biological macromolecules such as polymers and proteins. Herein, in this chapter, we present an overview of covalently f-CNTs and the subsequent advances made by f-CNTs in various biological media and polymeric nanoscaffolds. Keywords

Nanomaterials · Functionalized Carbon Nanotubes · Covalent Functionalization · Biomolecules · and Enzyme Immobilization

Introduction Nanoparticles (NPs) have created a wide and immense impact in various fields of science. The fascinating characteristics of NPs lead to their vast utility in various multidisciplinary research areas. These are also named ultrafine particles referring to their small size in nanometers (nm). Particularly, the particles having a size between 1 to 100 nm in diameter are classified as NPs (Christian et al. 2008). On the other hand, NPs have a specific size, morphology, and physical and chemical properties. Much effort has been aimed to develop extraordinary nanomaterials composed of nanoparticles possessing exotic electrical conductivity, tensile strength, mechanical strength, and surface area. Apparently, carbon nanotubes (CNTs) are one such class of NPs that exhibits abovementioned characteristics. The astonishing structures proposed by CNTs have provided new class of nanomaterials to the scientists and engineers which exhibit good electrical conductivity and have high surface area (Dyke and Tour 2004). CNTs are hollow cylindrical tubes of sp2 carbon formed by rolled-up graphene sheets and closed at their ends by fullerene caps. CNTs are the strongest material known due to high -C-C bond strength. Various types of CNTs have been characterized depending upon the fashion in which graphene sheets are arranged. One graphene sheet rolled up in a cylindrical manner leads to the formation of single-walled carbon nanotubes (SWCNTs). On the other hand, doublewalled carbon nanotubes (DWCNTs) are made up of two concentric graphene cylinders. Furthermore, numerous graphene sheets aligned in the concentric cylindrical manner give rise to multiwalled carbon nanotubes (MWCNTs) (Nayak et al. 2021). In the last decade, CNTs have been proved as an ideal reinforcing agent in various fields spanning from nanoelectronics to active drug delivery vehicles (Sahoo Cheng et al. 2011). The capability of CNTs to penetrate the cells makes them a potential candidate in offering drug delivery applications. CNTs scaffolds have been widely explored in various applications due to their diameter in the nanoscale range and lengths up to few micrometers (Crescenzo et al. 2014). The available literature suggests that CNTs have a high surface to the mass ratio which is responsible for

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better binding interactions between CNTs and various organic/inorganic moieties (Ihsanullah 2019). However, the strong van der Waals forces of attraction within the CNTs render their utility in various biomedical applications and make CNTs a propensity to form stable bundles resulting in extremely poor solubility in the aqueous medium. The vast applications of CNTs can be explored by unfolding their particular graphitic structure and making them evenly dispersible in the substrate. In this context, it is highly desirable to modulate these carbon nanostructures with some functional moieties to further extend their applications in drug delivery and biosciences. To enhance the solubility of CNTs and make them more dispersible, many approaches have been pursued. Covalent and noncovalent functionalization of CNTs are the major modifications in circumventing this issue. Noncovalent functionalization of CNTs includes different type of weak interactions such as л-л/ CH-л interactions between the binding moieties and CNTs, which may help the CNTs to disperse in organic solvents and water up to some extent (Bilalis et al. 2014). The noncovalent functionalization is mainly ascribed to the adsorption ability of any organic/inorganic species to the surface of CNTs (Guo et al. 2017). So far, numerous inorganic materials such as metal nanoparticles, oxides have been employed to modify the CNTs. A rather significant approach to make CNTs more susceptible towards the solvents is to do the covalent functionalization of CNTs. Yadav and Venkatesu (Yadav and Venkatesu 2019) recently reported a study on the covalent functionalization of CNTs thereby providing –COOH group on their outer surface facilitating better dispersion in the aqueous medium. CNTs functionalized with –COOH groups provide better binding interactions with the amino terminated sites of biomolecules such as proteins/enzymes and antibodies (Jha and Venkatesu 2016). The covalently modified CNTs can provide homogeneity of the CNTs within the solvents and efficiently enhance the interfacial interactions between CNTs and the other moieties under consideration (Sahoo Cheng et al. 2011). Covalent functionalization of CNTs leads to the attachment of various functional groups onto the sidewalls of CNTs and does not alter the bulk properties of nanostructures (Khabashesku 2011). The bond formation on the surface of carbon nanotubes results in covalency properties and reduced toxicity of CNTs. Nitric acid (HNO3), sulfuric acid (H2SO4) are some oxidizing agents that prompt the CNTs to undergo covalent functionalization (Ngoy et al. 2011). Guo et al. (2017) reported a study on the chemical functionalization of CNTs by spiropyran-40 , 60 -dicarbonylazide molecules and demonstrated the enhanced solubility in water and in a series of common nonaqueous organic solvents. Huang et al. (2013) functionalized the MWCNTs with cationic polyelectrolytes and observed the stable dispersion of MWCNTs in deionized water. The attached groups on the surface of carbon nanotubes act as precursors for the subsequent attachment of other functional groups. Zhang and Xu (2015) performed the nondestructive covalent functionalization of CNTs, that is, selective oxidation of sp3 carbon present on the surface of CNTs instead of -C¼C- carbon present in the intrinsic layers of cylindrical graphene sheets. CNTs contain many vacancies, dislocation, holes, and

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Scheme 1 Schematic illustration of various types of covalent functionalization of CNTs and their applications in various fields.

cracks which are energetically unstable (Guo et al. 2017). Notably, the oxidation of CNTs occurs on these defected sites and does not alter the hexagonal geometry of graphene sheets (Zhang and Xu 2015). Evidently, literature reveals that the covalently functionalized carbon nanotubes (f-CNTs) have gained widespread interest in this advanced field of nanomaterial science which is schematically shown in Scheme 1. Herein, in this chapter, we will exclusively deal with the covalent functionalization of carbon nanotubes and their influence on biomolecules.

Overview of the Structure and Importance of Carbon Nanotubes Graphene sheets are rolled up in a cylindrical manner and result in the formation of CNTs. Carbon nanotubes are large cylindrical molecules which are densely organized in sp2 hybridization of hexagonal sheets. The tubular structures exhibit unique properties and diameter in the nanoscale range. More significantly, the fashion in which graphene sheets are rolled up predicts the diameter and the carbon-carbon bonding orientation. Strong intertube van der Waals forces of attraction among the CNTs promote them towards higher tensile strength. The high C-C bond strength makes the CNTs respond at very high temperatures. The sp2 hybridized nature of carbon atoms in the nanotube structure is accountable for their extraordinary

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mechanical properties (Guo et al. 2017). Multilayers of carbon nanotubes produce thermal vibrations resulting in higher thermal conductivity. CNTs play a vital role as an anodic nanomaterial for lithium-ion batteries (Raval et al. 2018). The larger surface area of CNTs facilitates effective binding with target molecules. Moreover, the materials based on CNTs can deliver superior adsorption characteristics to the metal surface thereby acting as a corrosion protection barrier in various environments (Xia et al. 2020). The incorporation of biomolecules within MWCNTs enables them to develop biosensors of high quality (Lahiff et al. 2010). DNA has been shown to strongly interact with CNTs resulting in uniform coatings (Guo et al. 1998). On the other hand, wrapping of CNTs to other biopolymers including proteins, chitosan, chondroitin sulfate, etc., is also explained in the literature in detail (Bianco et al. 2008).

Classification of Carbon Nanotubes Depending upon the number of graphene sheets, carbon nanotubes are categorized as single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multiwalled carbon nanotubes (MWCNTs). The various types of carbon nanotubes are displayed in Fig. 1. SWCNTs are hollow, the long cylindrical arrangement of graphene sheet with extremely large aspect ratios (Sebastien et al. 2009). SWCNTs are reported to have a diameter of ~1 nm. These are one dimensional analog of fullerene moiety possessing unique physicochemical properties. In particular, their band gap is 0–2 eV which accounts for the metallic and semiconducting behavior of SWCNTs. This conducting behavior of SWCNTs is highly dependent on the choice of the rolling axis relative to the graphitic network. The orientation of the rolling axis and radius of graphene cylinders allow the different types of SWCNTs which can be named armchair, zigzag and chiral. Generally, chiral is the most twisted form of the SWCNTs and armchair and zigzag are the two achiral forms of SWCNTs. The two deciding factors in the generation of different types of SWCNTs

Fig. 1 (a) represents single-walled CNTs, (b) represents double-walled CNTs, and (c) represents multiwalled CNTs

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are chiral vector (Ch) and chiral angle (ɵ). The degree of twist is mainly determined by the chiral angle (ɵ). Armchair SWCNTs are shown to possess a chiral angle of 30 . On the other hand, zigzag SWCNTs are reported to have a chiral angle of 0 . Figure 2 shows the schematic description of the chiral vector and chiral angle along with the graphene sheets of CNTs. Compared with other potential materials envisioned for nanomaterials applications, SWCNTs possess extraordinary electronic and mechanical strength. In-depth study reveals that DWCNTs form when two graphene layers are combined in a cylindrical manner. In comparison to SWCNTs, DWCNTs have higher mechanical strength, thermal stability, and optical properties. It is worth commenting that the interlayer forces of attraction between the two graphene layers present in DWCNTs are primarily van der Waals forces. MWCNTs were formed when numerous graphene sheets rolled up in a cylindrical manner. The formation of MWCNTs was first discovered by Iijima and coworkers (Eatemadi et al. 2014). The name MWCNTs is restricted to nanostructures with an outer diameter of less than 15 nm, beyond which the structures are called carbon nanofibers (Saifuddin et al. 2013). The DWCNTs are more or less treated as MWCNTs. The walls of each layer of MWCNTs lie parallel to their central axis (Madani et al. 2012). The interlayer distance in MWCNT is close to the distance between graphene layers in graphite

Fig. 2 Schematic depiction of chiral vector and chiral angle along with the graphene sheets of carbon nanotubes

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(Raval et al. 2018). The ends of the graphene sheets are capped by domed shaped half fullerene molecules. The multilayer nature of MWCNTs shields the inner walls of nanotubes from chemical reactions resulting in very few deformations in the hexagonal structures of CNTs.

Synthesis of Carbon Nanotubes The commonly accomplished methods for CNTs generation include arc-discharge method, laser ablation, and chemical vapor deposition (CVD). Evaporation of graphite electrodes in electric arcs give rise to the formation of CNTs at very high temperature which was first time synthesized the carbon nanotube. CNTs produced from this method were highly impure and contain nontubular fullerene, various catalyst, and metal particles. Laser ablation method employs vaporization of graphite in high temperature furnace along with high power lasers. CNTs grown from this method are of high purity yet contained very low yield. Henceforth, these two methods account their low efficiency for the use in energy storage and other fields of applications. The most employed method for producing CNTs of high purity and higher yield as well as low set up cost is CVD which incorporates catalyst-mediated thermal decomposition of hydrocarbon chains. The vapor pressure, temperature, concentration of hydrocarbon, and catalyst affect the parameter of CNTs such as length, diameter, etc. Moreover, production of SWCNTs and MWCNTs is highly dependent on the reaction conditions provided during the CVD setup. The catalyst reduces the decomposition temperature of hydrocarbon and promotes faster production of carbon nanotubes. Generally, the catalysts used in CVD method contain Fe, Co, and Ni metal because of stronger adhesion characteristics and higher solubility of carbon in these metals. Decomposition of hydrocarbon is governed by liberation of high amount of heat energy and CNTs production majorly takes place on catalyst surface followed by the removal of H2 gas. Afterwards, the temperature of system is reduced to room temperature thereby providing the CNTs in crystallized form. MWCNTs usually generate in a low temperature range in comparison to SWCNTs which require high temperature for their production. Therefore, MWCNTs grow more easily than SWCNTs. More significantly, the selection of hydrocarbons is performed to obtain CNTs of specific type. Methane and carbon monoxide account higher thermal stability which indirectly favors the formation of SWCNTs. On the other hand, acetylene and benzene are not stable at high temperature and significantly yields MWCNTs at lower temperature. Henceforth, CVD is the most accepted method for mass production of carbon nanotubes on industrial scale. The schematic representation of CVD setup performed for CNTs production is shown in Fig. 3. Toxicity in Carbon Nanotubes CNTs have been proved as potential candidates in various aspects of science due to their diverse application areas in biomedical science, nanoscience, and nanoelectronics. However, the other properties of CNTs are reported to have a harmful impact on the human body. Exposure to CNTs directly affects the transformation and proliferation of normal epithelial cells to cancer cells. CNTs being the fibrous

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Fig. 3 Schematic depiction of CNTs generation by chemical vapor deposition method (Eatemadi et al. 2014)

materials which are reported to have high bio-persistence which is how long the materials can remain inside the human body (Kobayash et al. 2017). Graphene, the building block of CNTs, is mainly responsible for the high bio-persistence behavior of nanostructures. Introduction of CNTs into the abdominal cavity had asbestos-like pathogenicity due to their needle-like shape, including inflammation and the formation of granulomas (Wang et al. 2016). Studies on the toxicity of CNTs are mainly focused on the pulmonary effects after intratracheal administration, and only a few studies are reported about the toxicity of CNTs via other routes of exposure (Francis and Devasena 2018). CNTs toxicity is manifested as oxidative stress, inflammatory responses, malignant transformation, DNA damage and interstitial fibrosis (Liu et al. 2013). The functionalization of carbon nanotubes degrades the graphitic structure of CNTs resulting in low toxicity. Osmond-Mcleod et al. (2011) reported that f-CNTs having high solubility induced low pathogenic potential.

Functionalization of Carbon Nanotubes Aggregation formation in aqueous and organic solvents potentially limits the utility of CNTs in various fields such as biosciences, biosensors and nanodevices (Khabashesku and Pulikkathara 2006). The chemical inertness of CNTs slows down their processability and causes significant aggregation (Saifuddin et al. 2013). Consistent with these observations, the lack of biocompatibility of CNTs has also remained an issue. Especially stable solutions of CNTs can be obtained by

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performing low-cost and industrially feasible approaches of covalent and noncovalent functionalization of CNTs. Wrapping of polymers with the sidewalls of CNTs by means of л-л stacking interactions opens a new way of noncovalent functionalization of MWCNTs. In spite of л-л stacking interactions, the noncovalent functionalization of MWCNTs suffers from poor stability and longevity (Balakrishna et al. 2020). The major drawback associated with noncovalent attachment is that the forces between the binding moieties and the nanotube might be weak that ultimately results in poor efficiency of the load transfer (Liu 2005). Covalent chemistries offer several advantages over noncovalent chemistries because of the formation of strong covalent bonds between the carbon nanotubes surface and the targeting moiety and preserve the targeting moiety’s structure through the availability of biocompatible conjugation attachments (Chio et al. 2020). Functionalization of CNTs onto their sidewalls with various functional groups such as –OH, -COOH leads to the covalent bond formation between CNTs and functional groups thereby resulting in covalent functionalization of CNTs. The blossoming of these methods promises a new hybrid of CNTs which is essential in the development of carbon chemistry that interfaces with materials, biology and medical science (Zhao and Stoddart 2009). The structure of SWCNTs and MWCNTs functionalized with different groups named as R is shown in Fig. 4.

Covalent Functionalization of Carbon Nanotubes Covalent functionalization of carbon nanotubes includes the chemical bond formation at more reactive sites of the CNTs, in defect sites of the sidewall, or the tips of CNTs (Sharma et al. 2017). In covalent functionalization, oxidation of CNTs results in the incorporation of various functional groups (Quintero-Jaime et al. 2020). The most studied functional groups that have been covalently attached to CNTs include hydroxyl, amino, alkyl, carboxyl and halogenates (Mananghaya 2015). The presence of these functional groups, especially -COOH, elevated the importance of CNTs

Fig. 4 Structure of (a) f-SWCNTs and (b) f-MWCNTs which were downloaded from Nanotube Modeler and processed with PyMOL software in which functionalization group is represented by R

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since further functionalization can create a wide range of application-driven nanomaterials (Adhikari et al. 2014). Treatment with strong acids, such as HNO3 and H2SO4, or strong oxidizing agents, for example, KMnO4 and O3, results in oxidation of CNTs thereby providing the sidewalls of CNTs with –COOH, -OH, -C¼O groups (Quintero-Jaime et al. 2020). The functional groups attached to the sidewalls of CNTs act as an anchor group for further chemical reactions between CNTs and macromolecules (e.g., proteins, polymers) (Bianco et al. 2005). The surface functionalization of CNTs by covalent attachment of any organic moiety will aid the carbon nanotubes in becoming biocompatible and selective binding to bio-targets (Saifuddin et al. 2013). Covalent functionalization of CNTs destroys the carbon nanotubes sp2 network and results in poor electronic and optoelectronic properties (Setaro et al. 2017). However, Setaro and coworkers performed a nondestructive, covalent, gram-scale functionalization of carbon nanotubes by [2 + 1] cycloaddition reaction (Setaro et al. 2017). Chio et al. ( 2020) generated defect-free covalently functionalized SWCNTs by performing a chemical re-aromatization reaction and generated fibrinogen, insulin nanosensors with functionalized carbon nanotubes. The covalent functionalization of MWCNTs by performing Bingel reaction is also a key step to preserve the л-extended conjugation of carbon nanotubes (Stasyuk et al. 2020). The covalently functionalized MWCNTs enable the facile production of novel nanomaterials and nanodevices (Mallakpour and Soltanian 2016). The chemical modification of CNTs with functional groups allows broad tuning of electronic properties as well as conductivity of CNTs. Depending on the type of functionality provided onto the surface of MWCNTs, they can exhibit the properties of p-type/n-type semiconductors. If CNTs are covalently bonded to electronaccepting oxygen-containing groups, it can exhibit p-type semiconductor properties. On the contrary, if CNTs are modified with electron donor nitrogen-containing groups, it can behave as n-type semiconductors (Kulakovaa and Lisichkina 2020). The impact of covalent functionalization on electrical properties is very much enhanced since each covalently functionalized site scatters electrons (Mallakpour and Soltanian 2016). Oxygen, nitrogen, and phosphorus are promising functionalities that increase the active site for different reactions (Quintero-Jaime et al. 2020). Sahoo Cheng et al. (2011) introduced three types of chemical functional groups, aminophenyl (C6H4NH2), nitrophenyl (C6H4NO2), and benzoic acid (C6H4COOH), on the sidewalls of MWCNTs to find the optimal functionalization of MWCNTs making it more compatible with the liquid crystalline polymer (LCP). Ravichandran and coworkers (2014) reported a green chemistry approach for the covalent functionalization of SWCNTs with anthracene in molten urea. Inspired by the influence of covalent functionalization of CNTs, Ruan et al. (2021) illustrated the catalytic performance of nitrogen-doped carbon nanotubes supported PdNiCo nanoparticles for selected hydrogenation of furfural and demonstrated that the covalently functionalized CNTs by nitrogen functionality act as a better catalyst in comparison to nonfunctionalized carbon nanotubes. f-CNTs are best considered as robust and multifunctional support material for enzyme immobilization owing to their large surface area and high mechanical strength. The enzyme-CNTs hybrid act as a biocatalyst in various fields such as

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electrochemical bio-sensing, biodiesel production, environmental remediation, and lactose bioconversion (Bilal et al. 2020). Various organic and inorganic molecules can involve in the covalent functionalization of CNTs. The fluorinated f-CNTs can constitute materials for synthetic and biomedical applications (Gajewska et al. 2020). f-CNTs have been proved as a potential tool in developing biosensors of improved sensitivity. Henceforth, the covalently f-CNTs are having widespread applications in various fields of science. The functionality of CNTs enables interactions with a large number of target molecules. On taking a cursory look at the scientific literature, f-CNTs are emerging new candidates for advanced functional materials. Various functional groups may be attached to the carbon nanotubes to increase their processibility and compatibility. Below is the detailed description of the literature provided on the covalent functionalization of carbon nanotubes via different functional groups. A schematic representation of general modifications that have been widely accepted in the covalent functionalization of CNTs is depicted in Scheme 2.

Scheme 2 Schematic depiction of routes followed for covalent functionalization of carbon nanotubes.

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Covalent Functionalization of CNTs by Incorporating OxygenContaining Functionalities CNTs functionalized with oxygen-containing functional groups exhibit pronounced solubility and stability in polar organic solvents (Wang et al. 2006). Introducing carboxylic acid functionalities is the primary objective to overcome the intrinsic insolubility of CNTs. In an extensive study on the covalent functionalization of CNTs, Zulikifli and coworkers (2017) reported the –COOH group functionalized onto the surface of MWCNTs. Initially, MWCNTs were treated with the mixture of concentrated sulfuric acid and nitric acid followed by sonication to improve its dispersibility and to introduce carboxylic acid functional groups. The MWCNTs thus obtained showed enhanced stability and dispersibility in a range of organic solvents as well as in an aqueous solution. Nitrate and sulfate ions are of key importance in the intertube and intratube intercalation during the pretreatment of MWCNTs. Figure 5 depicts the key steps followed for –COOH functionalized carbon nanotubes. Once the CNTs are functionalized with the –COOH group, one can derive the numerous derivatives of carboxylic acid-multiwalled carbon nanotubes (CA-MWCNTs). In 1998, Liu group carried out the interconversion of CA-MWCNTs into the corresponding acid chloride by treating it with thionyl chloride (SOCl2) (Liu et al. 1998). Figure 6 shows the interconversion of CA-MWCNTs to the corresponding acid chloride derivate. A subsequent reaction of corresponding thionyl chloride derivative of MWCNTs was performed with NH2-(CH2)11-SH yielding a covalently coupled amide derivative of MWCNTs. Ramanathan et al. (2005) have confirmed the possibility of functional group interconversion by reducing the carboxylic acid functionality to

Fig. 5 Surface modification of CNTs to introduce –COOH group onto their sidewalls

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Fig. 6 Interconversion of CA-MWCNTs to the corresponding acid chloride derivative

the corresponding hydroxymethyl group by lithium aluminum hydride. This reaction protocol forms covalently f-CNTs which were soluble in organic media. Owing to its versatility, the carboxylic acid-based amidation has become a widely used tool for the construction of soluble carbon nanotube derivatives.

Covalent Functionalization of Carbon Nanotubes by Incorporating Nitrogen-Containing Functionalities Electron-donating substituents results in formation of higher functionalization degrees when compared with electron-withdrawing entities (Chao et al. 2019). In order to install surface-bound nitrogen groups on carbon nanotubes, the direct solvent-free amination of MWCNTs with octadecylamine was carried out and revealed that a large octadecylamine fraction is reacted over MWCNTs sidewalls through the chemical bond formation. Figure 7 depicts the covalent attachment of the amine group onto the sidewalls of carbon nanotubes. The insertion of nitrogen-containing functionalities in CNTs varies the band gap of CNTs in such a way that they can be exploited to make various nanoelectronic devices. The amino termination allows further possible covalent interactions with the biological systems such as DNA and proteins. Andreas and coworkers (2002) employed the highly beneficial electrochemical methods for surface functionalization of MWCNTs by electrochemical oxidation in presence of 4-aminophenyl phosphonic acid (4-APPA) in an aqueous solution to incorporate N and P

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Fig. 7 Covalent functionalization of amine group on carbon nanotube framework

heteroatoms onto the sidewalls of carbon nanotubes. Presence of nitrogencontaining functionalities could facilitate diazonium reaction between amino acid molecules and MWCNTs. The amino acid-modified CNTs promise multipurpose applications in broad areas such as biomedical industries and biosensors. Nucleophilic sidewall addition reaction can also be extended to incorporate nitrogen-containing functionality on the sidewalls of carbon nanotubes. The reaction of n-propyl amine with n-butyl lithium yields the corresponding amide, which attacks the SWCNTs generating negatively charged SWCNT derivatives that are subsequently reoxidized by air into the corresponding neutral amino-functionalized SWCNT derivatives.

Covalent Functionalization of Carbon Nanotubes by Incorporating Halogen-Containing Functionalities The direct fluorination of SWCNTs with elementary fluorine results in the sidewall functionalization of SWCNTs (Quintero-Jaime et al. 2020). The fluorinated CNTs can be extensively used as an intermediate for further functionalization. The fluorinated CNTs are depicted in Fig. 8a. For the large-scale production of halogenated CNTs, Lin et al. (2012) reported one step and facile synthesis to incorporate halogen atoms on the surface of CNTs. In the later stages, fluorination of CNTs was substituted by other halogen atoms such as Cl, Br, and I because of very dangerous reactive nature of fluorine. Electrochemical methods were employed for the chlorination and bromination of MWCNTs. Iodine and bromine-incorporated CNTs were found to enhance the electrical properties by increasing the density of free charge carriers. Incorporation of Cl, Br, and I was carried out by thermal treatment with SOCl2, Br2, and I2 respectively. Figure 8b shows the covalent attachment of various halogen atoms with CNTs surface. The halogenated multiwalled carbon nanotubes impart strong fluorescence properties which are more likely to originate from isolated sp2 carbon clusters. Prolonging the reaction time can enhance the fluorescence as well as the quantum yield of halogenated carbon nanotubes (QuinteroJaime et al. 2020; Lin et al. 2012).

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Fig. 8 Covalent functionalization of carbon nanotubes by incorporating halogen atoms

Various Functional Groups Incorporation Via Chemical Bond Formation in Carbon Nanotubes Various oxidants can be used during the oxidation of CNTs. When potassium permanganate (KMnO4) in alkali was used as the oxidizing agent, different amounts of -OH, -C¼O, and -COOH groups were introduced onto the sidewalls of CNTs. Kuznetsova et al. (2001) have reported the presence of oxygen-containing functional groups on SWNTs which further indicates that both carbonyl (-C¼O) and ether R-OR functionalities are present when the tube is purified with either H2O2/H2SO4 mixtures. The treatment of SWCNTs with sec-butyl lithium leads to the nucleophilic addition of the alkyl chain. Diazonium salts have been proven to be particularly useful precursors for free radicals. Sulfur and oxygen-centered radical species have also been used for the functionalization of CNTs. Photolysis thiols, disulfides, or peroxides gives rise to the formation of sulfur/oxygen radical species that can readily be specified on the CNTs sidewalls. Amide groups on carbon nanotubes surface can

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be used as useful precursors for additional functionalization with biologically active target species such as DNA, proteins, enzymes, and antibodies.

Defect Group Functionalization of Carbon Nanotubes Defects in CNTs are of great importance for generating CNT-derivatives since they can serve as anchor groups for further functionalization. The chemical functionalization of CNTs creates the defects in carbon backbone and the defect functionalization of CNTs has led to the development of a widely used versatile tool that enables the combination of material properties of CNTs with other substance classes benefiting from the highly functional nanotube derivatives. Also, the reported literature predicts that during the production of CNTs, a variety of defects and dislocated carbon atoms are generated in the CNTs structure. During the functionalization of CNTs, it is the most accepted strategy that the functionalization first occurs on the defected sites of the carbon backbone. Zhang and Xu (2015) reported a systematic investigation of the effect of chemical functionalization on CNTs framework. Chemical modification of CNTs was performed by incorporating various functional groups such as –OH, -COOH by treating the CNTs with different oxidants and concludes that mild oxidation of CNTs occurred only at the initial defected sites of carbon nanostructures by reacting the CNTs with HNO3/H2SO4. Specifically, the oxidation process mainly starts with the oxidation of defects created during the production of carbon nanotubes.

Applications of Covalently Functionalized Carbon Nanotubes in Polymer Science The covalently f-CNTs are undeniably promising candidates in varying the lower critical solution temperature (LCTS) in thermoresponsive polymers (TRPs). LCST temperature is the temperature above which the polymeric solution becomes turbid and polymer attains the compact globule conformation. At a temperature above the LCST, shrinkage of microstructure occurs due to dehydration of water molecules. Poly-(N-isopropyl acrylamide) (PNIPAM) and poly (N-vinyl caprolactam) (PVCL) are the widely accepted TRPs that account for their LCST near to physiological temperature and have promising applications as drug carrier vehicle. The f-CNTs impart the hydrophilic character and disturb hydrogen bonding network around polymer chains. The hydrophilic microenvironment provided by f-CNTs tends to stabilize the coil conformation of TRPs and make them resist the temperature response up to some higher degrees. Yadav and Venkatesu (2019) recently reported the LCST variation of PNIPAM by incorporating COOH group on CNT surface. The COOH-CNTs proved to be very prolific in studying the transition temperature of PNIPAM by providing hydrophilic character to the polymeric solution. Different biophysical studies such as dynamic light scattering (DLS), differential scanning calorimeter (DSC), thermal fluorescence

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Fig. 9 Schematic description of probable interactions between PNIPAM and COOH-CNTs

spectroscopy reveal that COOH-CNTs indirectly favors the binding interactions of surrounded water molecules with PNIPAM and eventually stabilizes the coil conformation of TRP up to human body temperature. This study may provide a better insight into the field of biomedical science, biosensors, and tissue engineering. Figure 9 depicts the probable interactions between the COOH-CNTs and PNIPAM. COOH-CNTs were also used for studying conformational transition behavior of PVCL. The cyclic structure of PVCL restricts the polymer to show interactions with COOH-CNTs and the polymer experiences an overall hydrophobic environment. Increase in hydrophobicity of systems results in early collapse of polymeric chains which is reflected in decrease in LCST of PVCL in presence of COOH-CNTs. The strong van der Waals forces of attraction among the different layers of CNTs make them interact with the amide bond of PVCL with a very little extent and results in a decrement in the phase transition temperature. The results explicitly elucidate that the presence of COOH-CNTs results in opposing thermal behavior for two TRPs (PNIPAM and PVCL). Interestingly, the amide groups present in the PVCL may directly interact with the hydrophobic C-C chain present on CNTs surface. Moreover, these two TRPs differ in their toxicity behavior. Under the same experimental conditions, PNIPAM generates toxic amine group containing moieties, whereas carboxylic acid is generated by PVCL and considered advantages because of lower toxicity (Binti et al. 2018). Fascinatingly, PNIPAM contains flexible amide bond which can interacts with COOH-CNTs up to higher temperature values and hence increase in LCST has been observed for PNIPAM with less agglomeration in comparison to PVCL. This

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Fig. 10 Schematic depiction of complex formation between PVCL and COOH-CNTs. (Reprinted from ref. (Yadav et al. 2020), Copyright 2014, with permission from Elsevier)

concept paves the way to design advanced nanodevices in the fields of drug delivery applications. Figure 10 systematically elucidates the binding of PVCL with the COOH-CNTs. Various carbon-based nanostructures such as CNTs, carbon nanofibres, and fullerenes offer unique adsorption characteristics for polymers because of higher surface area. Poly(ethylene imine) functionalized carbon nanofibers having the coating of cellulose paper are proved to be effective platform for the extraction of parahydroxybenzoates (parabens) from environmental water samples. The high efficiency of CNTs towards polymer reinforces the efforts to develop CNTs-based polymer composites. In this context, Li et al. (2021) recently designed a novel aptasensor for the electrochemical detection of carcinoembryonic antigen (CEA) by utilizing polymer-functionalized CNTs. The featured characteristic of aptasensors is of key importance in the early diagnosis and monitoring of cancer. Therefore, CNTs-polymer-based nanocomposites are more competent owing to their high sensitivity, high biocompatibility, fast response and low cost production. So far, studies based on carbon nanotubes have proved vast applications of these nanostructures in biomedical fields as an active drug carrier and therapeutic agent. Moreover, lower cytotoxicity and higher biocompatibility of fCNTs make them potential candidate in crossing the cellular membranes. In particular, these

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nanostructures along-with polymers facilitate their contribution in biological media. Celluzzi et al. (2018) investigated that polymer exploits the hydrophobic nature of CNTs thereby providing good biocompatibility and reduced toxicity of CNTs. The study extensively elucidates the significance of polyamine-coated CNTs in the delivery of miRNAs to human cells. The transfection efficiency of these nanocomposites was optimized to be employed as efficient drug delivery vectors in biomedical applications Covalently functionalized CNTs doped with polymer are emerging new candidates in information storage technology. Gu et al. (2021) designed covalently functionalized MWCNTs poly[(1,4-diethynyl-benzene)-alt-9,9-bis(4-diphenylaminophenyl)fluorene] (PDDF-g-MWCNTs) and fabricated a memory device with the features of PDDF-g-MWCNTs coated polydimethylsiloxane (PDMS). The study illustrates that prepared PDDF-g-MWCNTs-PDMS composite can be effectively used as a potential candidate for constructing high-performance flexible memory devices. Henceforth, the polymer grafting to functionalized carbon nanotubes continuous the advancement of these advanced materials in photonics and optoelectronics devices. The rapid development of geopolymers-based CNTs composites is gaining popularity in various industrial areas such as cement industry. Geopolymers, a type of highly durable polymers, act as an eco-friendly alternative to traditional Portland cement binder. Based on the high mechanical, tensile strength of multiwalled CNTs, the selection of high performance MWCNTs functionalized with –COOH and –OH group is made to incorporate the geopolymer paste. The enhanced mechanical behavior, lower setting time, and lower water absorption capacity of geopolymeric paste are observed in corporation with MWCNTs. The functional groups, that is, –OH and –COOH, on the multiwalled CNTs act as nucleation site and offer various adsorbed sites for free water resulting in accumulation of geopolymerized product.

Application of Covalently Functionalized CNTs in Enzyme Immobilization Covalent functionalization of CNTs with different functional groups has been discussed earlier in this chapter. Herein, we will discuss applications of such covalently functionalized CNTs in biological media. Conventional CNTs have limited applications in the biological system due to problems related to solubility. However, this drawback related to dissolution in the aqueous solution has been conquered by functionalization of the outer surface of CNTs with different types of moieties such as carboxylic acid, hydroxide, and amines. In addition to increased solubility of CNTs, functionalization can also increase the electrical or thermal conductivity and also reduces toxicity profile which further widens the application of functionalized CNTs in the biological benign realm. Generally, the protein interacts with the outer surface of the nanomaterial resulting in protein corona formation. Protein corona formation has attracted scientific interest due to its potential application in biotechnology, diagnostics, biomolecular assembly, biomedicine

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Fig. 11 Schematic illustration of enzyme immobilization on covalently functionalized CNTs

and pharmaceutical industries (Tegegn et al. 2014). Moreover, the general approach used for enhancing protein stability is immobilization on the solid support and these nanomaterials act as excellent matrices for immobilization. This corona formation also results in influencing the toxicological properties of CNTs. Figure 11 elucidates the immobilization of enzyme on the functionalized CNTs. Proteins are biological macromolecules which are essential for all life process. These biomacromolecules catalyze various reactions inside the human body. Significant attention has been given to these proteins by the scientific community due to their biological functions in the body. Whenever any foreign material such as nanoparticles (CNTs) enters the human body, they are immediately covered by these biomacromolecules which provide a basic key idea behind bionanoscience. Adsorption of biomolecules on NPs surface induces conformational changes in proteins and alters biological applications. Therefore, it is crucial to study interactions of f-CNTs with proteins for providing a clear picture and detailed depiction representing the effect of such interactions on the conformational stability of proteins. The adsorption of proteins over f-CNTs surface depends on the number of factors such as protein concentration, pH of the system, temperature, surface charge of the protein, type of protein, and functionalized group present on the surface of CNTs. Protein immobilization over f-CNTs surface can be achieved either by noncovalent attachment (physio sorption) and covalent conjugation. Both methods have certain advantages and disadvantages as noncovalent adsorption preserves properties of proteins but the attachment of protein is not durable and proteins can be detached with time, whereas in covalent conjugation attachment is durable; however, conjugation may disrupt enzyme/protein structure. These studies related to protein-f-CNTs interactions are significant for evaluating the biosafety profile of nanomaterials for in vivo applications.

Effect of f-MWCNTs on the Structural and Thermal Stability of Various Proteins Sekar et al. (2015) demonstrated binding interactions of three proteins hemoglobin (Hb), gamma globulin, and transferrin with hydroxylated functionalized multiwalled

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Fig. 12 Percentage secondary structure changes for three different proteins hemoglobin, gamma globulin, and transferrin which occur after interaction with OH-MWCNTs. (Reprinted from Sekar et al. (2015),Copyright 2015, with permission from Elsevier)

carbon nanotubes (OHMWCNTs). The obtained results show a hyperchromic shift in UV-visible spectra and quenching of fluorescence implies protein-nanotube complex formation. Loss of α-helical structure as represented in Fig. 12 was also observed in CD spectroscopy due to interaction between amino acid residues of proteins and OHMWCNTs surface. Patila et al. (2013) investigated the effect of different functionalized (carboxyl, alkyl, and amine group) MWCNTs on peroxidase activity and structure of cytochrome c (Cyt c). Enhanced peroxidase activity was observed for four different functionalized MWCNTs, that is, COOH-CNT, COOH-C10-CNT, NH2-C6-CNT, and CH3-C11-CNT. Results imply that interaction with nanomaterials induces microenvironment changes around heme moiety due to which active site around Cyt c becomes more available resulting in increased peroxidase activity. Moreover, the secondary structure of the protein is not altered in presence of different functionalized CNTs. Jha and Venkatesu (2016) also studied the interaction of COOH-MWCNTs with globular protein bromelain (BM). UV and fluorescence spectroscopy results imply complex formation between BM and COOH-MCNTs. Decreased thermal stability of BM was observed in presence of nanomaterial. Moreover, a slight secondary structure variation of BM was also depicted in CD spectroscopy. Enayatpour et al. (2018) investigated adsorption kinetics of lysozyme (Lys) on amino-functionalized MWCNTs and observed that the optimum time for the adsorption process of Lys on nanomaterial was 12 min.

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Paoli et al. (2014) reported the influence of different protein corona formation on the interaction of COOH-MWCNTs with blood platelets (PLT). The study shows the corona formation of four different proteins as human serum albumin (HSA), fibrinogen (FG), immunoglobulin (IgG), and histone H1 (H1) on the surface of COOHMWCNTs. This corona formation greatly affects the outcome of nanomaterial interactions with PLT. Noordadi et al. (2018) studied binding interactions of porcine trypsin (pTry) and COOH-MWCNTs via experimental and molecular dynamics (MD) simulations. Enzymatic activity and thermal stability were decreased due to the presence of COOH-MWCNTs. Static quenching in fluorescence was also observed in the presence of nanomaterial indicating the formation of pTry-COOHMWCNTs complex. Conformational stability of native protein was also disturbed (decrease in α-helix content) in the presence of nanomaterial as revealed from CD spectroscopy. MD simulation results show that COOH-MWCNTs interact with the S1 pocket of pTry which hinders the active site of protein resulting decrease in enzymatic activity. The main driving forces for protein binding on nanomaterial surfaces are electrostatic, Π-Π stacking, and Π-cation interactions.

Effect of f-SWCNTs on the Structural and Thermal Stability of Various Proteins Wang et al. (2014) studied the binding interaction of hemoglobin (Hb) and myoglobin (Mb) with hydroxylated functionalized single-walled carbon nanotubes (OHSWCNTs). Results imply that soret/heme bands of Hb/Mb were affected due to these interactions. Fluorescence spectroscopy results show protein-OHSWCNTs complex formation via several weak interactions such as hydrophobic, Π-Π stacking, and hydrogen bonding interactions. Complex formation as shown in Fig. 13 results in partial destabilization of the secondary structure of heme proteins. Li et al. (2014) investigated the interaction of bovine serum albumin (BSA) with amidated single-walled carbon nanotubes (e-SWCNTs). These nanotubes were prepared by successful functionalization of SWCNTs with ethylene diamine (EDA) via acylation. Functionalization with EDA improves dispersion and also helps in introducing active groups on the surface of CNTs. Static quenching in fluorescence was observed for BSA due to interaction with e-SWCNTs surface. Positive enthalpy and entropy changes imply hydrophobic force as the dominating factor for binding of BSA over e-SWCNTs. Moreover, it has also been observed that the secondary structure of BSA (α-helix) has been disturbed due to the presence of e-SWCNTs. Lu et al. (2018) studied binding interactions of three plasma protein human immunoglobulin G (IgG), fibrinogen (FG), and human serum albumin (HSA) to COOH-SWCNTs. Adsorption capacities of these proteins on nanomaterial surface follow order FG > HSA > IgG and amino acid residues which plays important role in binding are basic residues, that is, arginine (Arg), lysine (Lys), and histidine (His). The same research group has also studied binding interactions with HSA and enzyme-induced degradation of SWCNTs (Lu et al. 2014). Experimental and computational results show that binding of HSA on nanomaterial is only due to electrostatic interactions between carboxyl groups present on the surface with positively charged Arg residues of the protein. Moreover, several Π-Π staking interactions

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Fig. 13 Binding conformation of OH-SWCNTs with (a) Hb and (b) Mb as obtained from molecular docking analysis where (a1), (b1) shows lowest docking energy conformation with Hb and Mb, respectively. (a2), (b2) shows probable amino acid residues involved in binding with OH-SWCNTs for Hb and Mb, respectively. (Reprinted from Wang et al. (2014), Copyright 2014, with permission from Elsevier)

were also available between nanomaterial and aromatic tyrosine (Tyr) residue of protein. A functionally modified CNTs surface can stimulate the substrate approachability towards the enzyme molecule. –COOH-functionalized CNTs act as efficient binding support to the enzymes and produce enzyme-based nanobiosensors of high quality. The higher surface area of carbon nanotubes can lead to effective loading of enzyme and a strong peptide bond formation between the amine groups of enzyme and –COOH group functionalized carbon nanotubes. Efficient immobilization of the enzyme onto the carbon nanotubes while maintaining its native catalytic activity is the key consideration to develop ultrasensitive enzyme-based biosensors. In this regard, covalently functionalized CNTs are proved as an efficient tool in generating enzyme-based nanobiosensors.

Predominant Interaction Involved in Protein Immobilization on f-CNTs The main driving forces for protein immobilization on f-CNTs surface from above studies are noncovalent interactions which results in physical adsorption. Generally,

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in physical adsorption protein is incubated with f-CNTs in buffer solution which results in noncovalent immobilization after an optimum time (Enayatpour et al. 2018). Major interactions which are involved in forming protein-fCNTs complex are hydrophobic interactions, hydrogen bonding, electrostatic interactions, van der Waals forces, and π-π stacking interactions (Enayatpour et al. 2018; Noordadi et al. 2018; Wang et al. 2014; Li et al. 2014; Lu et al. 2018). These are highly dependent on available functional group on CNT surface and amino acid residues present inside proteins. Several computation and simulation studies (Gu et al. 2021; Tegegn et al. 2014; Sekar et al. 2015; Patila et al. 2013; Enayatpour et al. 2018) show that generally functional groups present on the surface of nanomaterials are negatively charged which can favorably interact with positively charged basic residues, that is, Arg, Lys, and His present in the proteins. Moreover, Π-Π stacking interactions were also available between nanomaterial and aromatic amino acid residues of protein. Scheme 3 shows complex formation between protein and fCNTs.

Scheme 3 Probable interactions involved in protein-fCNTs complex formation. CNTs were downloaded from Nanotube Modeler and processed with PyMOL software.

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Conclusion The unique transport properties combined with outstanding mechanical and thermal properties of carbon nanotubes provide wide potential applications of CNTs. The cluster/bundle formation of CNTs is reinforced by strong intertube van der Waals forces of attraction that account for their slow processibility. The unprecedented ultrahigh performance of CNTs was investigated by performing covalent functionalization of CNTs. Covalent functionalization involves stronger binding interactions between carbon substrate and incoming functional groups in comparison to noncovalent functionalization of CNTs which further manipulates the CNTs towards higher dispersibility and stability in typical organic solvents. Different organic and inorganic moieties can be decorated onto the surface of CNTs via the formation of chemical bonds. This chapter emphasizes the incorporation of various functional groups such as –OH, -COOH, -NH2 to greatly enhance the binding affinity of carbon nanotubes with biological substrates and other molecules. These CNTs offer remarkable characteristics as solid support for heterogeneous catalysis. Substantial development of covalently functionalized carbon nanotubes is mainly seen in the area of biomedical science. In this regard, covalently functionalized CNTs are considered noteworthy vehicles for the delivery of drugs.

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Setaro A, Adeli M, Glaeske M et al (2017) Preserving π-conjugation in covalently functionalized carbon nanotubes for optoelectronic applications. Nat Commun 8(1):1–7 Sharma A, Gifford BJ, Kilina S et al (2017) Tip functionalization of finite single-walled carbon nanotubes and its impact on the ground and excited state electronic structure. J Phys Chem C 121(15):8601–8612 Stasyuk OA, Stasyuk AJ, Voityuk AA et al (2020) Covalent functionalization of single-walled carbon nanotubes by the Bingel reaction for building charge-transfer complexes. J Org Chem 85(18):11721–11731 Tegegn D, Paoli SH, Diduch LL et al (2014) The effect of protein corona composition on the interaction of carbon nanotubes with human blood platelets. Biomaterials 35(24):6182–6194 Wang Y, Iqbal Z, Mitra S et al (2006) Rapidly functionalized, water-dispersed carbon nanotubes at high concentration. J Am Chem Soc 128(1):95–99 Wang YQ, Zhang HM, Cao J (2014) Binding of hydroxylated single-walled carbon nanotubes to two hemoproteins, hemoglobin and myoglobin. J Photochem Photobiol B Biol 141:26–35 Wang X, Mansukhani ND, Guiney LM et al (2016) Toxicological profiling of highly purified metallic and semiconducting single-walled carbon nanotubes in the rodent lung and E. coli. ACS Nano 10(6):6008–6019 Xia DH, Song S, Tao L et al (2020) Material degradation assessed by digital image processing: fundamentals, progresses, and challenges. J Mater Sci 53:146–162 Yadav R, Venkatesu P (2019) Functionalized carbon nanotubes modulate the phase transition behavior of thermoresponsive polymer via hydrophilic-hydrophobic balance. Polymer 178: 121573 Yadav R, Kumar K, Venkatesu P (2020) Monitoring phase transition behavior of poly (N-vinylcaprolactam) via nanostructure-based functionalized carbon nanotubes. J Mol Liq 318:114062 Zhang ZY, Xu XC (2015) Nondestructive covalent functionalization of carbon nanotubes by selective oxidation of the original defects with K2FeO4. Appl Surf Sci 346:520–527 Zhao YL, Stoddart JF (2009) Noncovalent functionalization of single-walled carbon nanotubes. Acc Chem Res 42(8):1161–1171 Zulikifli FWA, Yazid H, Halim MZBA et al (2017) Synthesis of gold nanoparticles on multi-walled carbon nanotubes (Au-MWCNTs) via deposition precipitation method. AIP Conf Proc 1877: 070003

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Monika Matiyani, Mayank Pathak, Bhashkar Singh Bohra, and Nanda Gopal Sahoo

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Noncovalent Surface Chemistry of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Driving Forces for Noncovalent Functionalization of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . Van der Waals Interactions/Hydrophobic Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . π-π Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Different Approaches for Noncovalent Functionalization of Carbon Nanotubes . . . . . . . . . . . . . . Endohedral Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Exohedral Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of Noncovalently Functionalized Carbon Nanotubes (CNTs) . . . . . . . . . . . . . . . . . . . Noncovalently Functionalized CNTs for Energy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Noncovalently Functionalized CNTs for Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . Noncovalently Functionalized CNTs for Electrochemical Biosensor Development . . . . . . . Noncovalently Functionalized CNTs for Bioimaging Applications . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Carbon nanotube (CNT), an allotrope of carbon atom, has been proposed as a promising and innovative candidate owing to its distinct and extraordinary properties. Because of its immense potential, CNTs have been used for broad range of applications in different fields such as energy, medical sensing, etc. However, some difficulties are found in handling and processing of CNT because of its hydrophobicity and highly complex structure which have imposed great restriction in its applicability in various fields. To overcome these problems, functionalization of CNT is a technique to modify its surface with multifunctional Monika Matiyani and Mayank Pathak contributed equally with all other contributors. M. Matiyani · M. Pathak · B. S. Bohra · N. G. Sahoo (*) Prof. Rajendra Singh Nanoscience and Nanotechnology Centre, Department of Chemistry, Kumaun University, Nainital, Uttarakhand, India © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_66

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characteristics which make it suitable for many applications. CNT can be functionalized via different approaches and among them noncovalent functionalization has gained enormous interest because the native structural and functional properties of CNT have been preserved in the aforementioned approach. This chapter focuses on the noncovalent surface chemistry of CNT and different approaches for noncovalent functionalization. In addition, this chapter also includes various practical applications of noncovalently functionalized CNT. Keywords

Carbon nanotube · Noncovalent functionalization · Hydrophobicity

Introduction In the past several years, the field of material science has been escalating at an accelerating rate for the development of various carbon nanomaterials having fascinating properties and diverse applications. These carbon nanomaterials include graphene, carbon nanotubes (CNTs), fullerenes, etc. Among them, CNTs have gained tremendous attention because of their distinct and appealing properties like large surface area, high aspect ratio, excellent thermal and electronic properties, great mechanical strength, high chemical stability, and ultralightweight (Dai 2002). CNTs, the allotropic form of carbon, are elongated, hollow, and cylindrical-shaped molecules. The wall of the carbon tube comprises of hexagonal rings of carbon atoms arranged in a concentric pattern and both ends of the tube contain pentagons and are usually caped by fullerene-like structures (Liu et al. 2014; Karakoti et al. 2018). Typical CNTs diameters range from 1 nm to 100 nm, although their length might be up to several centimeters (Zhu et al. 2002). CNTs are classified as single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). A SWCNT possesses simplest mono-layered geometry that can be viewed as the wrapped sheet of graphene into a tubular structure with a diameter of 0.4–2 nm. On the other hand, MWCNT consists of an array of coaxial cylindrical shells around the core of SWCNT with diameters of 2–100 nm (Tan et al. 2014) and structure of MWCNT is defined by two models: Parchment model and Russian doll model. According to the Parchment model, the nanotube is wrapped like a rolled paper around itself, whereas the outer nanotube has large diameter in comparison to the inner nanotube in the case of Russian doll model. CNT is composed of hexagonal rings of carbon which define their curvature, diameter, and electronic characteristics. Also, the arrangement of these conjugated carbon hexagons rings along the surface of CNT is known as the chirality of CNT which is responsible for its metallic or semiconducting behavior (Tasis et al. 2006). Based on its chirality and diameter, SWCNT may be either semi-metallic or semiconducting. SWCNT can show three different chiralities, based on its rolling direction, such as zigzag, armchair, and chiral (Tîlmaciu and Morris 2015) (Fig. 1). The chirality as well as the tube diameter of SWCNT is defined by the chiral vector C, C ¼ nα1 + mα2, where, α1 and α2 represent the lattice vectors of graphene and n, m

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Fig. 1 Different chiralities of SWCNT. (Reprinted with permission from Tîlmaciu and Morris 2015, Copyright 2015, Frontiers)

represents integers. In general, when m ¼ 0, then nanotubes possess zigzag chirality; and when n ¼ m, then nanotubes possess armchair chirality and in the other conditions, the nanotubes are called chiral. The nanotube is highly conductive or metallic in nature, when the difference of n and m is a multiple of 3 and if not, then nanotube is semiconductor or semi-metallic. The armchair structure of SWCNT is always metallic while the other forms can be semiconductor. In case of MWCNTs, the electron transport is equivalent to that in the case of SWCNT the maximum electric current passing across is confined to the outermost sheet of MWCNTs (Eatemadi et al. 2014; Chen et al. 2003). Elasticity is another impressive characteristic of carbon nanotube. When nanotube is subjected to the strong axial compressive forces, it can be twist, bend, buckle, and also return back to its original configuration without deformation in its native structure. But the elasticity has a limit; the shape of the nanotube can be distorted momentarily under high physical forces. The elasticity of SWCNT and MWCNTs can be measured with the help of elastic modulus. The elasticity modulus of MWCNTs is measured with the help of transmission electron microscopy (TEM). These one-dimensional and highly ordered nanostructures exhibit exceptional electronic, structural, and mechanical properties; therefore, CNTs are considered as one of the promising candidates for variety of applications viz. sensors, composite

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material, organic photovoltaic cells, supercapacitor, hydrogen storage systems, and biological and biomedical research. However, the versatile physiochemical characteristics of CNTs are restricted by several factors (Scheme 1). These factors can be illustrated as follows: (i) Nonfunctionalized CNTs are usually found to be inactive under various reaction conditions and also contain impurities of amorphous carbon and catalytic nanoparticles; (ii) CNTs form highly complex structure as a consequence of van der Waals interactions, which can further be dispersed in the solvent through sonication but they get instantly precipitated when the process is interrupted; and (iii) in both aqueous solution and organic solvents, CNTs have very poor solubility. The intrinsic hydrophobicity and lack of solubility have imposed huge constraint on any practical application of CNTs (Wu et al. 2010). To overcome these aforementioned shortcomings is a big challenge and thus it has become an attractive and challenging target for material scientists and synthetic chemists. In less than two decades, continuous efforts have been made toward significant advancements in this new area. Functionalization is a way which can alter the surface properties of CNTs and allow them for combination with other types of materials such as polymers, biomolecules, etc. as well as improve its solubility and processability. The electrical or mechanical properties of functionalized CNTs may be different from those of pristine or nonfunctionalized CNTs which may be utilized for producing modified CNTs for specific applications (Bahr et al. 2001; Hirsch and Vostrowsky 2005). Functionalization of carbon nanotubes can be broadly

Scheme 1 Limitations of CNTs

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categorized into two classes, (a) exohedral functionalization and (b) endohedral functionalization. In exohedral functionalization, the outer surface of nanotube modified with functional moieties whereas another category of functionalization is endohedral functionalization, in which the inner empty space of CNTs is filled with small molecules and atoms. Exohedral as well as endohedral functionalizations are further categorized into two types: covalent and noncovalent functionalization as in Scheme 2. The attachment of functional moieties which are covalently linked through making the permanent bond to the sidewalls or terminals of CNTs is known as covalent functionalization of carbon nanotubes. As a consequence of this, there is a change in hybridizations from Csp2 to Csp3 which causes destruction in the conjugation and somewhat affect the electron transport and/or electron acceptor properties simultaneously (Zhao and Stoddart 2009). On the other hand, functionalization through noncovalent approach is based upon the wrapping or adsorption of various chemical groups which are employing weak noncovalent interactions with CNTs. Noncovalent functionalization is a nondestructive functionalization because it preserves the native extended network as well as the functional characteristics of CNTs. The abovementioned functionalization approaches help to enhance both the solubility and dispersibility of nanotubes in organic as well as in aqueous solvents. Further, these functionalized CNTs with multifunctional properties can be explored for diverse applications. Among the abovementioned functionalization approaches, noncovalent functionalization has attracted much attention because it is remarkably improving the solubility as well as preserving the inherent properties of CNTs. Thus, as per the above description, in this chapter, we will discuss in detail about noncovalent surface chemistry of CNTs and different approaches for the synthesis of

Scheme 2 Classification of CNTs

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noncovalently functionalized CNTs as well as utility of these functionalized nanotubes for further application in different fields.

Noncovalent Surface Chemistry of Carbon Nanotubes The basic principle of noncovalent chemistry of carbon nanotubes is the selfassembling of atoms or molecules to the surface of CNTs. These self-assembled moieties are grouped together by various weak noncovalent interactions which results in the formation of supramolecular structures. Due to the noncovalent character, these supramolecular structures undergo persistent re- and de-assembly process spontaneously under specific conditions. Thus, self-assembling procedures are very quick due to the involvement of rapid dynamics and very precise noncovalent interaction. Noncovalent interactions between nanotubes and functional moieties are quite fascinating because during the interaction, the intrinsic characteristics of CNTs are preserved and also noncovalent modifications retain the structural conformation of the immobilized moieties after the functionalization onto CNT surface; on the other hand, π-π conjugative structure of CNT could affect in case of covalent methods, as a result negatively affecting their electronic and mechanical properties. Generally, CNTs can be considered as the wrapped form of graphene sheet and the curvature of sidewall is responsible for the pyramidalization of carbon atoms in CNT. Several chemical modifications onto the surface of nanotubes could alter its chemical properties reversibly or permanently and impart new functionalities. The surface reactivity mainly depends upon the curvature of CNTs and orientation of π-orbitals. It has also been reported that CNT curvature induces distorted alignment of π-orbitals within the graphene nanosheet. When the CNT’s diameter increases, i.e., in the case of CNTs having large diameter, both π-orbital misalignment as well as pyramidalization of carbon atom decreases, resulting in decreased carbon bond reactivity and gradually proceeding toward the two-dimensional graphite characteristics. Furthermore, wrapping the graphene sheet increases the convex surface’s reactivity while decreasing the concave surface’s reactivity relative to the planar graphene sheet. The functional moieties which are attached on the surface of CNTs exhibit higher activity on outer surface than inner surface (Britz and Khlobystov 2006). This implies that the CNTs having small diameters are more reactive than bigger CNTs and also the exterior surface of CNTs is realized to be more reactive than the interior one.

Driving Forces for Noncovalent Functionalization of Carbon Nanotubes The fundamental research on CNT focuses on its inherent properties and applications profoundly depend upon bonding interactions of nanotube with its surroundings. These bonding interactions may be expressed in a number of noncovalent

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forces that are existing between CNTs and functional moieties including ionic and molecular or macromolecular species. CNTs form van der Waals interactions (~500 eV/1μm length of nanotube) with each other and aggregate into ropes and bundles. As a result of this, multiple nanotubes are orientated parallel to one another and resulting in the formation of a complex network (Girifalco et al. 2000). In order to split this complex network of CNTs for the analysis of individual nanotube, extensive effort is needed. Although, the packing of bundles easily interrupted if some functional moieties are present on the exterior sidewall of CNT. Those species that have the noncovalent functionality and interact via dispersive bonding are considered as a good candidate for the modification of CNTs. Functionalization of CNT is a process of immobilization of atoms or molecules over surfaces so as to modify surfaces with specific properties. The noncovalent functionalization involves the wrapping/adsorption over the CNTs of organic molecules, surfactants, or polymers mainly through π-π interactions or van der Waals forces. Generally, π-π interactions are originates due to the presence of distributed π network of sp2 hybridized carbon atoms, while van der Waals forces between nanotubes and polymers or organic molecules are developed because of the hydrophobicity. For many practical applications, noncovalent interactions are very crucial since the expanded π network is not disrupted, which means that there is no effect on important characteristics such as electrical conductivity and mechanical strength of CNTs. Furthermore, these interactions provide high flexibility due to their reversibility and weak character. These driving forces which are involved in the noncovalent functionalization of CNTs are discussed in detail below.

Van der Waals Interactions/Hydrophobic Interaction Van der Waals forces involve the interactions of CNT with hydrophobic molecules like as polymers, surfactants, and macromolecules. Such types of interactions are typically utilized for the dispersion and solubilization of CNTs in organic as well as in aqueous medium. To solubilize the nanotubes, ambiphilic polymers and also some conjugated polymers are used instead of small surfactants molecule because the entropic penalty of micelle development is reduced in the case of polymers. The main disadvantage associated with polymers is that they form energetically unfavorable conformation by the interaction with mechanically rigid CNTs. In order to overcome this limitation, some polymers are wrapped in a helical pattern around the nanotube to minimize strain. Polyvinylpyrrolidone (PVP) is a polymer having hydrophilic pendant groups and hydrophobic alkyl group chain which can arrange itself around the nanotube in such a manner that the alkyl group chain exposed to the nanotube surface and pyrrolidone groups are in contact with water (O’Connell et al. 2001, 2002). On the other hand, polynucleotides have hydrophobic aromatic nucleotide bases as pendant groups and hydrophilic sugar–phosphate long chain so that their wrapping order is opposite to that of PVP. It is also reported that polynucleotides are wrapped helically around CNT, and due to this aromatic pendant groups are in intimate contact with the CNT (Zheng et al. 2003a). Conjugated polymers like

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poly(m-phenylenevinylene) (PmPV) also allow the development of effective van der Waals interactions between the sidewalls of nanotube and conjugated chain which results in the enhancement in the solubility of nanotubes.

π-π Interactions Apart from the hydrophobicity involves in the van der Waals interactions, CNT is also characterized by its extended π-system which promotes the interaction of CNT with aromatic functional groups. There are two general conditions for π-π interactions: (i) existence of π-conjugated system and (ii) the geometry of interacting moiety. In order to have a significant interaction, an overlap should be occurred between two interacting species. Ambiphilic molecules (surfactants) are often used for solubilization of CNTs and therefore, restricting them from agglomeration into ropes and bundles. For this purpose, aromatic groups containing surfactant molecules are more effective due to the formation of more specific and directional π-π interactions with graphenic surface of the CNTs. For example, two charged surfactants, namely sodium dodecylsulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS) have similar alkyl group chain but SDBS also has a phenyl ring in between the alkyl group chain and hydrophilic group. This phenyl ring of SDBS forms π-π stacking with CNT, therefore the presence of the aromatic ring makes SDBS more effective surfactant for dissolution of nanotubes than SDS (Islam et al. 2003). Similarly, polyaromatic and polyaromatic heterocyclic molecules such as anthracenes, porphyrins, and phthalocyanins can also form potent π-π interaction with CNTs in order to enhance the dispersibility of nanotubes in organic solvents. Therefore, it can be concluded that there are numerous approaches in which the CNTs can be noncovalently functionalized for different applications, which are briefly described in the upcoming section of this chapter.

Different Approaches for Noncovalent Functionalization of Carbon Nanotubes The surface modifications of CNTs through covalent and noncovalent techniques are generally achieved for various applications viz. chemical compatibility, solubilization, and etc. Covalent functionalization sometimes leads to several cracks in steady π-network and due to use of harsh chemicals in the process, the optical, mechanical, and electronical properties also get distorted. This is not in the case of noncovalent functionalization as there are no permanent chemical bonds, instead they held together by different adsorption forces like π-π stacking, weak van der Waals forces, electrostatic forces, hydrogen bonds, etc. So, this process does not affect the electronic network of the CNTs and leading to the final electrical and structural properties of synthesized material. The supremacy of noncovalent functionalization over covalent functionalization is also that this technique is in good correspondence with the green chemistry principles according to which more emphasis is given to the

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use of renewable resources, less hazardous chemicals, and moderate reactions conditions. So, they are supposed to be compatible with ecology, cost, and energy effectively. The 1D material such as carbon nanotubes is unique in its mechanical, physical, chemical, and electronic properties. Yet, the CNT-based technology is not developed to such a good extent that it can be used at industrial scale in various applications, due to their poor solubility in the already known solvents. CNTs form big bundles of aggregates together when it is dispersed in the solution and inhibiting the homogeneous solution formation. To separate these bundles and to develop the individual CNT, one of the most widely used methods is functionalization of CNT through noncovalent approach. The solubilization of nanotubes can be achieved by various techniques viz. adsorbing of small aromatic molecules upon the surface of CNTs or wrapping of polymers around the surface of nanotubes. These techniques to functionalize the CNTs noncovalently come under two broad categories. One is endohedral noncovalent functionalization and another is exohedral noncovalent functionalization.

Endohedral Approach CNT is such nanomaterial with the presence of inner cavity which offers enough vacant space for the residence of guest molecule. Commonly, water molecule can be encapsulated during the solubilization of CNT with the help of surfactant in aqueous environment. This is due to CNT caps break and water molecule penetrates into the tube inner cavity as the robust sonication powers engaged in the surfactant-based functionalization procedure. But the research says that it needs the highly pressurized liquid to enter inside the cavity of CNT so any liquid with the surface tension below ~180 mN.m1 could wet the internal cavity of CNT in atmospheric pressure via an open end (Dujardin et al. 1994). In some experiments, fullerene such as C60 was encapsulated into the CNT cavity through the defect site present at the ends or in the sidewalls. The so-called “bucky peapods” were formed, as the digested fullerenes lead to make chains that are attached by weak van der Waals attraction forces. Along with C60, few higherorder carbon spheres such as C70, C78, C80, and C82 were also encapsulated into the provided cavity of CNTs. Metallofullerenes such as Sm@C82 were also encapsulated inside the cavity of CNT. Apart from these, different elements like gold, silver, and their compounds were filled in the cavity of CNT. Also, small proteins like lactamase digested in provided inner space of CNT through the process of adsorption. Researchers demonstrated that DNA could also entrap into the CNT’s inner hollow space. RNA was also trapped in the CNT membrane having 1.5 nm wide pores, hydrophobically despite large energetic and entropic penalties. Various examples of endohedral functionalization are shown in the Fig. 2 with the help of graphical representation. Though there are many advantages of endohedral functionalization but there is a limitation that the size of the functionalities which are getting fitted inside the cavity of the tube should be less than the inner tube’s diameter; this is not in the case of exohedral functionalization.

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Fig. 2 Graphical representation of endohedral functionalization of CNT with various species

Exohedral Approach Attachment of numerous molecules/functional groups around the outer surface of nanotubes through many ways, to modify the solvation properties of CNT, comes under the exohedral functionalization category. Exohedral surface functionalization does not suffer with the limitation of the size of the functionalities, so functional groups of any shape and size can virtually attach to it. Nature of bond which immobilizes the functional groups onto the CNTs surface is the main factor for further classification of exohedral approach into two foremost classes: (i) covalent functionalization and (ii) noncovalent functionalization. Here, in this chapter we are emphasizing on the noncovalent functionalization. The exterior surface of the sidewalls of the CNTs can be noncovalently functionalized by physical adsorption of aromatic small molecule, wrapping of polymer, surfactants, biological compounds, etc. to alter the surface properties, which is shown in the Fig. 3.

Aromatic Small Molecule–Based Noncovalent Functionalization Some small aromatic molecules like porphyrin, pyrene, and their derivatives can interrelate with outer surface of CNTs by introduction of π-stacking, leading to the

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Fig. 3 Graphical representation of exohedral adsorptions of various species on CNTs

noncovalent functionalization. In this regard, researchers used N-succinimidyl-1pyrenebutanoate which is a bifunctional molecule and irreversibly adsorbed onto outer surface of CNTs in either methanol or N, N-dimethylformamide (DMF). This aforementioned approach has allowed the immobilization of variety of biologically available molecules onto the outer surface sidewalls of CNTs with good efficiencies and specificities. The solubilization of CNTs with the help of π-π stacking forces in aqueous solution was done through pyrene and porphyrin derivatives–based noncovalent association of CNTs by using a charged pyrene derivative, namely 1-(trimethylammonium acetyl) pyrene, (pyrene+). Hecht et al. used zinc porphyrin derivative to functionalize the CNTs noncovalently leading to the creation of CNTs/ FET devices, which was then used for the direct detection of a photoinstigated electron, relaying within the zinc porphyrin derivative CNT structure (Hecht et al. 2006). The CdSe-CNT hybrids were prepared by Hu et al. and showed that the CdSe nanoparticles functionalized with the help of pyrene(pyrene/CdSe), self-assembled on the outer sidewalls of nanotubes (Hu et al. 2008). Noncovalent surface functionalization with the help of 1-pyrene butanoic acid succinimidyl ester was done to hold together proteins effectively (Teker et al. 2005). A pyrene-modified β-cyclodextrin (pyrenecyclodextrin) derivative was prepared and utilized to develop pyrenecyclodextrin-decorated CNT/FET devices (Zhao et al. 2008). Then, the nonfluorescent organic molecules can be selectively detected by thus prepared devices as they serve as chemical sensors.

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Polymer-Based Noncovalent Functionalization Functionalization of carbon nanotubes with polymers is generally used to upgrade their dispersibility to develop CNT-based new composite material so that the new properties can be investigated. The polymers in noncovalent functionalization of CNTs are either wrapped around the CNT or adsorbed onto the CNT surface. Conjugated polymer is referred to as a good candidate for wrapping of CNTs due to π-π stacking interactions and weak van der Waals interactions between aromatic rings of conjugated polymer chains and the surfaces of nanotubes (Star et al. 2001). Researchers reported that it is thermodynamically favorable to wrap the CNTs for numerous polymers in aqueous media. The researchers indicated that the polymer interpolated between the CNTs at low loading fractions. This becomes a reason to untangling of ropes and a falloff in the interactions between the individual CNTs was observed due to it. It was also observed that polymer interrelate especially with nanotubes of particular diameters or a span of diameters (Dalton et al. 2000). Noncovalent surface functionalization of nanotubes with the help of macromergrafted polymers (MGPs) was achieved by Morishita et al. which revealed strong physisorption of polymer onto the CNT exteriors (Morishita et al. 2009). However, chloroform and hexane are usually poor solvent for CNTs but in the presence of MGPs acting as solubilizing agent, upgrading the dispersibility of CNTs in hexane and chloroform. The researchers synthesized PmPV/CNT composite by introducing the CNT into the poly(m-phenylenevinylene)-co-(2,5-dioctoxy-pphenylene) vinylene (PmPV) and observed that this polymer is in direct electrical contact with CNTs. They showed that the polymer enfolds itself throughout the CNTs’ stacks, instead around isolated tubes, which then manifest ropes by aggregation (Star et al. 2001). They also modified the PmPV polymer by functionalizing the meta-disubstituted phenylene ring at synthetically accessible C-5 position, managingpoly-(5-alkoxy-mphenylenevinylene)-co-(2,5-dioctoxy-p-phenylene)-vinylene (PAmPV) derivatives, and they demonstrated solubilization of CNT bundles with the help of PAmPV derivative in organic solvents by wrapping themselves throughout the CNT rolls (Star et al. 2003). The interactions between CNTs and stilbene-like dendrimers were also investigated by them for functionalization of CNTs via noncovalent approach and they observed that stilbene-like dendrimers which act as hyperbranched polymers are more efficiently breaking up the CNT bundles (Star and Stoddart 2002). The wrapping of CNTs with polymers-like polyvinylpyrrolidone (PVP) or polystyrenesulfonate (PSS) which carry polar side chains formed firm aqueous solutions of corresponding CNT/polymer composites. This is because the unfriendly interactions between the polar nanotubes walls and water solvents were avoided. Remarkable enhanced reaction rates were observed when grafting, and shortened CNTs were functionalized with polyethylene glycol (PEG) by microwave-assisted heating instead of conventional heating to produce a solubilized CNT derivative (Negra et al. 2003). The first technique for solubilization of CNTs through π-π stacking without polymer wrapping was reported through tiny, functional conjugated polymers, namely poly(arylene ethynylene), which allow solubilization of many types of CNTs in organic solvents and could also bring in various ionic and neutral functional groups onto the surface of nanotubes (Chen et al. 2002). The researchers prepared

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CNT/poly(phenylacetylene) composite for the very first time by in situ polymerization of phenylacetylene in the existence of the CNTs. They observed that polymer chain wraps around the nanotubes in a helical manner and this gives rise to the solubilization of the amalgam in commonly known organic solvents (Tang and Xu 1999). However, their range is limited to biocompatible polymers for exploiting in living organisms.

Surfactants-Based Noncovalent Functionalization Surfactants are the amphiphilic molecules with large hydrophobic tails and hydrophilic heads. After exceeding a certain concentration, generally known as CMC (critical micellar concentration), the surfactant molecules assemble together to develop a complex structure called micelles. Thus, generated micelles contain the hydrophobic inner cores and hydrophilic outer shells. Stabilization of CNTs can take place within the micelles inner cores as they are hydrophobic in nature. Hydrophobic tail adsorbs onto the hydrophobic surface of CNT and hydrophilic head stabilizes the formed conjugates in the aqueous environment. The existence of an aromatic group in hydrophobic tail of amphiphile surfactant molecule leads to a strong interaction, due to the formation of effective π-interactions with graphitic surface sidewalls of CNTs. In addition to this, various combinations of hydrophilic tails and hydrophobic heads available are combined in the form of many surfactants. Also, utilization of vigorous ultrasonic agitation or severe chemical treatments take place in most of the solubilization method which are used for CNTs. But the CNTs get damaged or cut into small pieces during these processes, resulting in loss of the advantageous properties. To overcome aforementioned drawbacks, solubilization with the help of surfactants is more feasible than others. Also, the physisorption of surfactant molecules onto the nanotube’s surface lowers the surface tension that inhibits the agglomeration of CNTs leading to the formation of bundles of CNT. On the basis of their relation with CNTs, surfactants could be studied as anionic, cationic, and nonionic surfactants. Ionic surfactants stabilize the CNT dispersion in aqueous solution by means of electrostatic repulsion. Among the synthetic anionic surfactants, the most common and majorly used surfactant in many studies is sodium dodecyl sulphate (SDS). Lately it was demonstrated by Islam et al. that SDBS (sodium dodecylbenzene sulfonate) is impressively efficient than SDS in the solubilization of CNTs (Islam et al. 2003). But most of the times SDBS is available with small impurities instead in the pure form and the presence of impurities alter the detergent properties. Also, the spectroscopic investigations get toughened due to the existence of aromatic ring in the surfactant. So, another anionic surfactant having the branched hydrophobic moiety, namely DIOCT (sodium dioctyl sulfosuccinate), was also used. Researchers observed that this surfactant is far more effective than SDS. Bile salt–like sodium salts of deoxycholic acid (DOC) and its taurine-substituted congener, taurodeoxycholic acid (TDOC), were also used as anionic surfactants and supposed to be extremely efficient in solubilizing and stabilizing the CNTs in an aqueous solution (Wenseleers et al. 2004). Cationic surfactants also stabilize the nanotubes in an aqueous solution through the formation of micelles. Some examples of cationic

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surfactants are cetylpyridinium chloride, CTAB (cetyltrimethylammonium bromide), imidazolinium halides, etc. which were utilized to disperse and stabilize CNTs in an aqueous solution noncovalently. Absence of coulomb repulsions is the disadvantage to keep the CNTs containing micelles from agglomerating, in case of nonionic surfactants. Hence, long or branched disordered polar chains in the nonionic surfactants are more preferable, to provide a huge solvation shell so that the CNTs can be suspended for a long enough time. Moore et al. observed that efficiency increases with increasing molecular weight of the surfactants and they used Tween 20, Tween 40, Tween 60, and Tween 80 (length of alkyl chains is the only difference between them) as nonionic surfactants to disperse and solubilize the CNTs in an aqueous solution (Moore et al. 2003). Triton X-100 (polyoxyethylene octylphenylether) used to change the surfaces of CNTs from hydrophobic to hydrophilic. In this way the hydrophilic surface of CNT-Triton conjugate interacted with hydrophilic surface of biliverdin reductase leading to the formation of a water-soluble compound of immobilized enzyme (Salvador-Morales et al. 2003). The adsorption of various surfactant molecules onto CNT surface is shown in Fig. 4. Although CNTs noncovalently functionalized with surfactants can remain stable in an aqueous solution for many months, yet they are potentially limited for biomedical applications as the surfactants are toxic molecules and therefore raise concerns for the human health. Also, the removal of excess complimentary surfactant molecules that are not physiosorbed on the CNTs surface is quite difficult which leads to the inhibition for further conjugation of the stabilized CNTs.

Biological Compound–Based Noncovalent Functionalization Many biological compounds can interact with CNTs without making covalent conjugate. But, because of the poor dispersibility of nanotubes in aqueous and organic solvents, many applications in biological fields have hindered. Also, if pristine CNT aggregates and comes in direct contact with cell membrane, it tears down the cell membrane which definitely leads to cell death. Therefore, to integrate such type of new composite materials with living network for application in nanotechnology, bioengineering, and in improved antimicrobial activity of CNTs,

Fig. 4 Schematic representation of adsorption of various surfactants onto CNT surface. (Reprinted with permission from Islam et al. 2003, Copyright 2003, American Chemical Society)

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dissolution of nanotubes with biological compounds is necessarily more relevant. Most of the natural biological compounds contain number of aromatic regions together with both hydrophilic and hydrophobic properties. Therefore, they mediate the effective solubilization and dispersion of nanotubes in the aqueous environment by introduction of coating around the surface of CNTs. The coating should possess the following features for noncovalent surface functionalization of CNTs with biological compounds: (i) the biocompatibility and nontoxic nature is must for the coating substance; (ii) coating should not detach itself in the biological environment from CNT surface, so the stability of the coated substance should be adequate; and (iii) to avoid the vacant surface of CNT, accessible for binding molecular cargoes, functional groups should remain available for further bioconjugation of the coating molecules (Ezzati Nazhad Dolatabadi et al. 2011). Generally, the interactions of CNTs with biological compounds such as nucleic acids, proteins, polypeptides, polysaccharides, enzymes, etc. are focused, to disperse the CNTs in aqueous system through noncovalent surface functionalization. Nucleic acids, collective name for DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are the large biomolecules which are composed of nucleotides. The monomer of nucleotide is made up of three components: carbon sugar, a phosphate group, and a nitrogenous base. During the interaction of nucleic acids and their complexes with CNTs, the nitrogenous bases interact with the exterior of nanotubes through π-stacking interaction, while sugar and phosphate groups behave as hydrophilic groups which flip toward the water environment. DNA chains surround, wrap, and disperse the CNTs in aqueous medium providing repulsion forces, namely electrostatic repulsion forces. It was observed that DNA molecules adsorbed on CNTs surface via nonspecific interaction. It was firstly demonstrated by Zheng et al. that in existence of single-stranded DNA (ssDNA), bundled CNTs could be successfully dispersed in the water by the process of sonication. They observed that ssDNA with the repeating arrangement of guanine and thymine (dGdT)n (n ¼ 10–45) could wrap around the CNT in helical and sequence-dependent technique with regular pitch which opens up the gateway to CNTs-based application in biotechnology (Zheng et al. 2003b). But it needs the certain repeating pattern which restricts the use of synthetically manufactured DNA with less than 150 bases. So, the long genomic single-stranded (>>100 base) DNA with thoroughly arbitrary pattern of bases were utilized to solubilize the nanotubes by Gigliotti et al. (2006). In addition, proteins are also used for the surface modification of CNTs via noncovalent manner. Proteins are the macromolecule which are formed by structural units called amino acids. The adsorption of proteins takes place individually and noncovalently along the nanotube lengths through the driving forces like van der Waals forces, π-π stacking interactions, and hydrophobic forces. The hydrophobic parts of the proteins are likely foremost for adsorption, while the hydrophobicity of the proteins mostly depends upon the amino acid composition and pattern. Accordingly, some proteins like lysozyme, bovine serum albumin, etc. were suitable for protein-moderate noncovalent surface functionalization of CNTs for water solubilization while others like pepsin etc. were not suitable. The bifunctional N-succinimidyl-1-pyrenebutanoate (Chen et al. 2001) irrevocably adsorbed onto

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the intrinsically hydrophobic surface of the CNT in an organic solvent (dimethylformamide (DMF) or methanol), which is the result of very effective π-π stacking of aromatic pyrenyl group with the CNTs graphitic side walls. In the above conjugates, the primary or secondary amino groups from proteins like streptavidin or ferritin could be added in the place of the succinimidyl group which caused immobilization of the biomacromolecules on the CNTs. Two powerful metalloproteins, ferritin and cytochrome C, were noncovalently adsorbed onto the CNT’s surface. Various functional groups like hydroxyl and amino are already present on the adsorbed proteins which might be used for supplementary functionalization of nanotubes. Liu et al. used 17-(1-pyrenyl)-13-oxa-heptadecanethiol (PHT) for noncovalent surface functionalization of nanotubes by π-π stacking and afterward treated it with a gold solution. They found that there is a substantial coverage of the CNTs’ surface by self-assembling of subsidiary gold nanoparticles (Liu et al. 2003). To produce individual nanotube solution through noncovalent surface functionalization, various proteins with different size, shape, and structure were used by Karajanagi et al. (2006). Polypeptides are one of the major biological compounds which are also used in functionalization of CNTs via noncovalent method. Polypeptides are made up of several peptide units. While short chains of amino acids are called peptides, which are connected through peptide bonds. The hydrophobic parts act as symmetric detergents in the peptide chain which contains such flexibility that it can wrap around the tubes by appropriate folding. Peptides disperse individual CNT more effectively as the electron density of the aromatic surplus gets increased on the hydrophobic part and also by turning up into an amphiphilic α-helix. Also, the aromatic surface of CNT could interact with the Phe (phenylalanine) residues on the hydrophobic part of the helix through π-π stacking interaction. Carbohydrate is one of the widespread biological substances in nature which is made up of molecules of carbon, hydrogen, and oxygen, and is also used for the surface modification of CNTs. The most abundant carbohydrates are polysaccharide which is made up of monosaccharide units bound together by glycosidic linkages. Even though nanotubes are insoluble in an aqueous solution of polymeric carbohydrate, i.e., in starch, they get solubilize in an aqueous solution of the starch-iodine complex due to the formation of hydrophobic cavity which is accessible to CNT by preorganizing the mainstay of amylose in starch into a helical array. However, interaction of amylose with CNTs in aqueous solution of DMSO in the absence of iodine was investigated by Kim et al. (2003). Numata et al. dissolved nanotubes in an aqueous solution with use of s-SPG (single-chain schizophyllan) and s-curdlan which wrap CNTs and form a twined helical structure. Specific designed peptides may be utilized to bring in the desired properties to CNTs for biological application (Numata et al. 2004). Bandyopadhyaya et al. dissolved and stabilize the CNT in an aqueous solution by using gum Arabic (Bandyopadhyaya et al. 2002). Also, various cyclodextrins such as γ-cyclodextrin, η-cyclodextrin, chitosan, etc. were used for the noncovalent surface functionalization of CNTs. These CNT composites may be further characterized by photochemical investigations and as aforementioned polysaccharides have nearly no light adsorption in UV-Vis region. Advantage of

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polysaccharide-coated CNTs is that these are generally biocompatible and can be used in therapeutic applications. CNTs can be also noncovalently functionalized with cationic and anionic lipids which then showed stable water dispersion and biocompatibility (Ciofani et al. 2009). In this order, cellulose is also an interesting material which is the generously renewable and cheap material and can also be used to acquire biocompatible CNTs. Thus, functionalized CNTs by aforementioned various noncovalent approaches can further be used in many applications such as energy, biomedical, biosensing, etc.

Applications of Noncovalently Functionalized Carbon Nanotubes (CNTs) CNTs are functionalized noncovalently by polymer, aromatic compound, biomacromolecule, and surfactant through π-π, OH-π, CH-π, van der Waals, hydrophobic, ionic, and other interactions, which are effective techniques for enhancing the physicochemical and other more important properties of nanofillers to improve their dispersion as well as interfacial interactions with different materials and their matrix (Bahr et al. 2001; Star et al. 2001; Hirsch and Vostrowsky 2005; Zhao and Stoddart 2009; Wu et al. 2010; Ezzati Nazhad Dolatabadi et al. 2011; Bilalis et al. 2014; Karakoti et al. 2018). Due to their unique properties, CNTs have been useful in different applications in the fields of optoelectronic, biomedical, energy materials, solar cell, fuel cell, supercapacitor, drug delivery system, biosensors, bioimaging, and much more applications (Fig. 5) under following subsections, and different fields of CNTs applications have been outlined.

Noncovalently Functionalized CNTs for Energy Applications The high aspect ratio and excellent physicochemical properties like electrical properties, mechanical properties, and thermal properties of CNTs made them very attractive materials for energy applications (Dai 2002). Pristine CNTs have electrical conductivity of about 107 to 108 S m1, that is similar to metal (Ebbesen et al. 1996). CNTs have an efficient property, i.e., high electrical conductivity, which is responsible for enhancement of their electrical conductivity in insulating material such as polymer epoxy material via making CNTs-based nanocomposites. Ma et al. showed the significant transformation of electrical conductivity in insulating polymer to conducting composites by very small amount of CNT (0.5 wt %) loading in the polymer matrix (Ma et al. 2010). Therefore, at present scientific community attracts much attention toward CNTs-based polymer nanocomposites which is very useful in flexible electronics, fuel cell, electromagnetic interference shielding, superconductor, solar cell, and optoelectronic applications (Cesano et al. 2013). Functionalization of CNTs have played an important role in enhancing the electrical conductivity of the polymer nanocomposites. The dispersion of CNT in the various type of polymer facilitates the construction of electrical conducting

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Fig. 5 Applications of CNT

networks in nanocomposites. As a result of this, the electrical conductivity of polymer nanocomposites increases. On the other hand, functionalization of CNTs by using concentrate acids can strictly breaks the nanotube walls, resulting decrease to aspect ratio. Above two facts are detrimental to the development of conducting network in polymer nanocomposites (Ma et al. 2010). However, the conjugated polymers act as an outstanding wrapping material for the functionalization of SWCNTs through noncovalent π-π and CH interactions between the aromatic conjugated polymers chain and the surfaces of MWCNTs. Poly(3- hexylthiophene) (P3HT) is a broadly used conjugated polymer and its morphology, structure, and conductivity with multiwall carbon nanotube (MWCNT) has been already reported by Kuila et al. They have described the variation of molecular weight with different weight of MWCNTs doping, which affects the regioregularity and polydispersity, as well as thermal, mechanical, and optical properties of P3HT/MWCNT nanocomposites (PCNC). The PCNC composite is prepared through in situ dispersion of MWCNTs (at different concentrations [w/w%] 8%, 2.5%, and 1% these are respectively named as PCNC-8, PCNC-2.5, and PCNC-1) onto 3-hexylthiophene in chloroform. They have concluded that the increment in mechanical properties as well as increased PCNCs conductivity with increasing of MWCNTs concentration, 105 to 103 S cm1 and storage modulus increased dramatically 158% at 50  C for the PCNC-8 sample. Thus, the enhanced mechanical and conductivity behavior of MWCNT-P3HT nanocomposite is attractive for its suitable use in various optoelectronic applications (Kuila et al. 2007). Functionalization of MWCNTs by acids and amines with polyimide polymer shows quite remarkable enhancement from polyimide nanocomposites. Yuen et al.

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reported functionalization of MWCNTs through amine which resulted enhancement in the electrical properties of the MWCNTs/polyimide composites, which unfortunately resulted in decreasing mechanical strength due to interaction of the aminofunctionalized MWCNTs and polymic acid (Yuen et al. 2007). Singh et al. reported the enhancement of electrical conductivity as well as mechanical and thermal stability of modified MWCNTs-based polyimide nanocomposities. They have functionalized MWCNTs via chemical reaction of nitric acid and octadecylamine (C18H39N), respectively. Thus, the resultant composites, amide-MWCNT/polyimide composites, have higher mechanical properties at low loading and other composites, amino-MWCNT/polyimide composites, showed better at higher loading as compared to that for pure polyimide (Singh et al. 2008). These MWCNT-based composites showed excellent improvement in thermal stability over the neat polyimide film. The electrical conductivity of acid-functioned MWCNTs-reinforced polyimide nanocomposites was 0.938 S cm1 and the amide-functioned MWCNTs-incorporated polyimide nanocomposites was 1.032 S cm1 (6.67  108 S cm1 for neat polyimide), which is much more achieved so far from MWCNT/polyimide nanocomposites. These type of nanocomposites with high electrical conductivity as well as high mechanical properties and improved thermal stability makes them perfect candidate for high-temperature EMI shielding materials. However, beside above discussion, the scientific community is also focused on finding some new routes for energy protection, and is giving more attention to different natural resource of electricity generation. The solar energy is one of the best natural resource of renewable energy. However, the photovoltaic solar cell is mostly used for converting solar energy into other energy sources. Use of CNTs in solar cell plays an important role for giving a pathway for electron transport in which charges are transferred rapidly between the interfaces of the electrodes without a loss that results in highpower conversion efficiency (PCE). Cakmak et al. synthesized bulk heterojunction (BHJ) of organic solar cell with octadecylamine-functionalized SWCNT (o-SWCNT). They have prepared three solar cells sample with different amount of o-SWCNTs such as 0.0001, 0.001, and 0.01 wt% by the same method. Then, they found out optical absorption of the active layer of the solar cells slightly increased with incorporation of different concentration of o-SWCNT, and does not affect in open-circuit voltage. However, the I–V measurement (or power conversion efficiency) of organic solar cell with 0.001 wt% of o-SWCNT was increased to about 31.8% which is more efficient than that of reference solar cell without o-SWCNTs. The enhancement in efficiency is due to electrons nanosized pathways of CNT in bulk heterojunction of solar cell, which enhances the electron mobility (Cakmak et al. 2015). Thus, the noncovalent surface modifications of CNTs can improve the compatibility of CNTs into the polymer nanocomposites as well as improves its optical and electrical properties.

Noncovalently Functionalized CNTs for Biomedical Applications From past few years, CNTs have attracted the attention of scientist from biomedical field due to their unique features such as high cargo loading, high surface area,

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Fig. 6 Mechanism of CNT-based drug or gene delivery: Loading of therapeutic agents (drug or gene) on functionalized CNT, cellular entry through endocytosis, releasing the therapeutic agents at the targeted site. (Reprinted with permission from Kiran et al. 2020, Copyright 2020, Elsevier)

ultrahigh aspect ratio, and excellent physicochemical properties and intracellular bioavailability (Kiran et al. 2020). CNT has been extensively used as a potential nanocarrier for drug delivery applications. The detailed mechanism of CNT based drug delivery is shown in Fig. 6. Although, CNTs along with biomolecules such as proteins, enzymes, peptides, plasmids, and DNAs and siRNAs can also be used in different applications such as chemical sensing, biological sensing, and cancer therapy. However, when pristine CNT is directly in contact with cell membranes, it will destroy the cell membrane leading to cell death (Ezzati Nazhad Dolatabadi et al. 2011). Also, the poor aqueous solubility of CNT limits its applicability for various drug delivery applications. The abovementioned barriers can be resolved by the surface modification of CNTs. Huang et al. describe the combination of doxorubicin (DOX)-loaded CNT-chitosan-folate-decorated nanocarrier for targeted delivery of anticancer drug DOX and reported that the noncovalent π-π interaction is responsible for the attachment of DOX onto the CNT surface (Huang et al. 2011). The noncovalent surface functionalization of CNT allows both the controlled drug release and targeted drug delivery. Functionalized CNTs as nanocarrier are very crucial for gene delivery, which eliminates the genetic defects via precise delivery of therapeutics inside the cells. However, for much authentic delivery of nucleic acid into the targeted cells, viral and nonviral genetic system are required. Thus, CNTs, liposomes, and polymeric-based efficient nanocarriers are studied extensively for the intracellular gene transportation. Liu et al. reported the attachment of DOX onto branched polyethylene glycol (PEG)functionalized SWCNT by supramolecular π-π stacking interaction. They found that new drug formulations of PEG-SWCNT/DOX afforded a promising enhancement in therapeutic efficacy and remarkably reduced in toxicity compared to pure DOX (Liu et al. 2009). Thus, the surface modification of CNTs with various functional moieties

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reduces the toxicity of pristine CNT, which can further lead the successful development of advanced and promising CNT-based nanocarriers for controlled and trageted drug delivery applications (Mehra et al. 2014).

Noncovalently Functionalized CNTs for Electrochemical Biosensor Development The recent advances in CNTs play an important role for the production of electrochemical biosensors because of its unique chemical and physical properties like excellent conductivity, biocompatibility, small size, large surface volume ratio, and hollow cylindrical shape and also having the ability to enhance the electrochemical reactivity of biomolecules such as nucleic acid, proteins, polypeptide, and enzymes. In addition to the above-mentioned properties of CNT, surface-modified CNTsbased electrodes have also showed remarkable potential for accumulation of many useful biomolecules as well as improve surface fouling effect (Wang 2005). For development of electrochemical biosensors, the CNTs were used for following two purposes: first as immobilization support for biomolecules and another to provide required conductivity for transduction (Zhou et al. 2019). Basically, CNTs are used for highly sensitive nanoscale sensors due to its extraordinary sensitivity of CNTs conductivity to the surface adsorbates. These all above properties make CNTs tremendously attractive for a broad range of CNTs-based electrochemical biosensor production as well as used for designing of renewable amperometry enzyme electrodes and detection of DNA hybridization biosensors (Wang 2005). The surface modification of CNTs through immobilization of biomolecules onto the surface of CNT, is known as biofunctionalization of CNT. This technique of immobilizing is mostly applicable in bioelectrodes development for biofuel cell, and electrochemical biosensor (Zhou et al. 2019). The biofunctionalization is of two types, the first one is covalent and another is noncovalent functionalization. For superior electrochemical sensing application, CNT needs to be properly modified and immobilized. The functionalization of CNTs through noncovalent approach is comparatively prominent for the appropriate immobilization of biomolecule on CNT because these noncovalent methods retain the inherent characterstics of CNT while introducing the novel functionalites into it. The covalent tailoring of CNTs can affect the respective inherent properties, like as decreasing its electrochemical properties basically by changing its hybridization from unsaturated to saturated carbon moieties. In biosensor platform, the noncovalent tailoring of CNTs enhances the bioaffinity as well as the electrochemical properties by establishing good electrical communication between the bioreceptors and the transducers. This leads to good supply of amplified signal, and the transduction in biosensor (Zhou et al. 2019). Thus, the functionalization approaches especially noncovalent tailoring through different biomolecules including enzyme, protein, nucleic acid, aromatic compounds, and polymers onto the CNTs shows remarkable potential for the development of electrochemical biosensor as noncovalent tailoring of CNTs retains their biological compatibility without altering its electrochemical properties.

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Functionalization of CNTs is broadly used to enhance the solubility of CNTs in various solvent through increasing the adsorption of biomolecules onto CNTs surface. Molecules containing aromatic ring compounds are capable to forming π-π stacking interactions with sidewall of CNTs. Woods et al. investigate the adsorption of benzene derivatives, compounds of a benzene ring with CH3, NO2, and NH3 on SWCNT using density functional theory (DFT) (Woods et al. 2007). DFT calculation was used to determine the physical characteristic in the adsorption process of benzene derivatives on SWCNTs and they obtained that the adsorption was achieved through π-π stacking interactions between aromatic ring and sidewall of SWCNTs. These results suggested that these aromatic rings are wieldy used for noncovalent functionalization of CNT and also these noncovalent interactions increases the dispersion of SWCNTs in the various solvents. Certain aromatic compounds and derivatives such as ferrocene, anthracene, and pyrene are mostly used as the agent for successive immobilization of biomolecules onto the surface of CNTs. Similar studies have been reported by Holzinger et al. They introduced different functional group onto CNT through π-π stacking interaction with aromatic rings by using dip coating method (Holzinger et al. 2011). Multiple functionalization of SWCNT was done by the attachment of three different types of pyrene derivatives (including biotin–pyrene, Adamantane–pyrene, and nitrilotriacetic acid (NTA)–pyrene) to the sidewall of CNT through π-π interactions for the construction of biosensor device. Li et al. modified SWCNTs using naphthalen-1-ylmethylphosphonic acid (NYPA) via noncovalent π-π interaction for the development of biosensor. It is found that due to the introduction of phosphonate groups onto the surface of SWCNTs shows good solubility in polar solvent through electrostatic repulsions and also improve the biocompatibility of SWCNTs. The manufactured biosensor revealed exceptional bio-electro-catalytic potential for the reduction of H2O2 and with the detection limit of 1.5  107 M. The developed NYPA–SWCNT hybrids are very useful for the development of protein immobilization and the fabrication of electrochemical biosensors (Li et al. 2012). Porphyrins are valuable conjugates of organic molecules that are also useful in noncovalent tailoring of CNT that facilitate good electronic transfer, resulting to superior activity of biosensors. Organohalide pollutant, trichloroacetic acid (TCA), is present in environment due to excessive amount used in agriculture, cork, and wine industry and its detection is major concern for preparing TCA biosensor. Tu et al. studied a mimic hemoprotein, i.e., hydroxyferriprotoporphyrin (hematin), which is water-insoluble porphyrin and can also be used for the functionalization of SWCNTs. The porphyrin dissolved ionic liquid, and 1-butyl-3-methylimidazolium hexafluoro-phosphate ([BMIM] [PF6]) was used for the modification of SWCNTs through noncovalent π-π stacking interaction. Thus, ionic liquid self-reinforced onto SWCNTs surface, which gives better dispersion of SWCNTs in the ionic liquid that results direct electrical and chemical response. Novel composite reported by authors, porphyrin/SWCNTs– [BMIM][PF6], produced synergic effect that causes electron transfer between the porphyrin and electrode surface, good electro-catalytic potential for the reduction of TCA, and leading highly sensitive and stable amperometric sensing of TCA

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biosensor. Thus, the functionalized composite is easily used for the preparation of TCA biosensor and water-insoluble hematin responsible for better stability of biosensor (Tu et al. 2009). These CNT-based biosensors show better analytical achievement in neutral media, like as low detection limit, rapid electro-catalytic response, wide liner range, good fabrication reproducibility, detection stability, better specificity, and precision and accuracy. Biosensors are wieldy used for the detection of DNA through quick production of the substitute for distinctive genetic assays. However, for the discovery of targeted DNA succession, the biosensors based on DNA are composed with a transducer, also known as probe and perform as recognized element. The excellent and remarkable properties of CNTs such as good biocompatibility, low cytotoxicity, and their ability to cross the cell membrane used in DNA biosensor, and functionalization of CNT make it compatible with aqueous media and also binds the genetic material strongly. The noncovalent tailoring of CNT through DNA arrays approach renders the cationic groups that are accessible for the DNA binding having negative charge, especially through ionic relations between them. Sanz et al. have reported a bilayer method for the noncovalent tailoring of SWCNTs with cationic surfactants polymerlike polyethyleneimine (PEI), polylysine, and poly (Lys:Phe, 1:1) by RNA wrapping. Although, the RNA wrapping confers negative charge on the CNT, which made it inappropriate for the binding of DNA. To overcome this problem, they investigated the use of a cationic ion, which acts as a bridge between the CNT and the negatively charged plasmid DNA and is ready for DNA binding as shown in Fig. 7 (Sanz et al. 2011).

Noncovalently Functionalized CNTs for Bioimaging Applications In recent years, the carbon nanotubes (CNTs) with various sole optical properties and the capability to cross cell membranes are acting as very interesting nanoprobes and are widely used in bioimaging and phototherapies. The SWCNTs owing their unique Fig. 7 Plasmid DNA binding by RNA-wrapped CNTs using Bilayer approach. (Reprinted with permission from Sanz et al. 2011, Copyright 2011, Elsevier)

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one-dimensional (1D) semiconducting structure exhibit a thin band gap of 1 eV (Gong et al. 2013), which reveals fluorescence emission near infrared (NIR) region, and also showed strong resonance Raman scattering. Such Raman probes are very useful in biological imaging. CNTs as a darkest material showed good absorbance in the NIR area, and thus can also be used as imaging disparity agents for phototherapy. Poor solubility of CNTs in many organic solvent and lower quantum yield (QY) of SWCNTs in NIR imaging were major drawbacks for their further utilization into bioimaging applications (Gong et al. 2013). To overcome these problems, the covalent and noncovalent tailoring of CNTs, have turned out to be very decent solution. Covalent functionalization can damage the configuration of SWCNTs that can radically decrease Raman spectrum basically altering the magnitude. However, noncovalent functionalization of SWCNTs can be used to manufacture the watersoluble surface that enhances Raman scattering (SERS) nanocomposites for Raman imaging of organic samples. In situ solution stage synthesis approach for obtaining noble metal-incorporated nanoparticle with noncovalent functionalization of SWCNTs was reported by Wang et al. They have used single-strand DNA (ssDNA) for functionalization of pristine SWCNTs (Wang et al. 2012). Further, poly(allylamine hydrochloride) (PAH) was used for the coating of DNA-functionalized SWCNT through electrostatic interface leading to SWCNT positive charges. Here, the CNTs were found of with negative charged gold metal seeds that generate growth of gold or silver nanoshells onto the surface of SWCNT, which is further stabilized by the polyethylene glycol (PEG). The novel metal-coated SWCNT gained good SERS property, with utmost enhancement of over 20 unit. The conjugation of SWCNT-Au-PEG nanocomposites with folic acid (FA) for specific cancer cell labeling as well as fast Raman imaging was realized, which can also act as a superb photothermal agent for cancer cell ablation. Furthermore, the SWNT-Au-PEG-FA nanocomposite exhibited the significantly enhanced photothermal cancer cell killing effectiveness, because of strong surface plasmon resonance absorption. Photothermal constancy of SWNT-Au-PEG is obtained significantly higher than that of AuNR-PEG. Thus, noncovalent functionalized SWNT-Au nanocomposite is very interesting platform and quite promising in optical imaging along with phototherapy and biosensing. Noncovalent PEGylation of SWNTs offers CNTs having excellent water solubility and fairly enhanced biological compatibility typically decrease the quantum yield of SWNTs. Welsher et al. developed antibody-conjugated SWCNTs as NIR fluorescent labels for selective probing cell surface receptors through better sensitivities and better specificity of cell imaging (Welsher et al. 2008). They have reported noncovalent functionalization of biologically inert SWCNTs with amine-terminated surfactant PL-PEG-NH2 along with long PEG chains and the resulting CNTs attached with residual amine groups, further conjugated them via antibodies such as Rituxan (CD20 cells are recognized as the surface receptor). Herceptin is well recognized as the HER2/neu receptor on convinced breast cancer cells. The high degree of PEGylation of SWCNT showed excellent hydrophilicity and water solubility to carbon nanotube. In solution, NIR photoluminescence emission spectra of such SWNTs conjugated with antibodies were in the spectral range of

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1000–1600 nm at the laser of 785 nm excitation. Result showed the expected NIR photoluminescence of semiconducting SWNTs unaffected by noncovalent antibody conjugation. The measured fluorescence quantum yield (QY) was relatively low by solving binding problems and such QY is sufficient for the imaging applications. Thus, the above literature survey concludes excellent connections between CNTs and living cells (or antibodies and biological ligands), specially the dependency of such properties on different kinds of surface functionalization onto CNTs.

Conclusion Functionalization of CNTs plays a crucial role in the modification of their physiochemical properties and also impart new functionalities in order to enhance its applicability for a variety of applications. For the functionalization of CNTs, noncovalent approach has received much attraction than covalent approach because noncovalent functionalization of CNTs does not affect its extended π-network and therefore preserves its electrical and optical properties. Generally, noncovalent functionalization can be done by the adsorbing onto or wrapping the surface of CNTs by functional entities with the help of weak van der Waals forces, hydrophobic interactions, electrostatic interactions, and π-π interactions. Aromatic small molecules, polymers, surfactants, biomolecules, etc. are also used to functionalize CNTs noncovalently, in order to modify the surface properties as well as to improve its solubility in aqueous and organic solvents. These noncovalently functionalized CNTs can be a potential candidate for the various applications in the fields of energy, environmental science, catalysis, green chemistry, photocatalytics, nanodevices, biomedicine, bioimaging, and biosensors.

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Double-Walled Carbon Nanotubes: Synthesis, Sorting, and Applications

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Anchu Ashok, Afdhal Yuda, Ibrahim M. Abu-Reesh, and Anand Kumar

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Techniques for DWCNTs Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catalytic Chemical Vapor Deposition (CCVD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arc Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Peapod Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sorting Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reversible Covalent Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biofunctionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular Nanocalipers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aqueous Two-Phase Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application of DWCNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Capacitors, Batteries, and Fuel Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nano Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nano-Motors and Nano-Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Field Effect Transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Double walled carbon nanotubes (DWCNTs) comprise two coaxially nested single walled carbon nanotubes (SWCNTs) and the difference between the diameter of two tubes in DWCNTs provides variation on the interaction between the two nanotubes. In that context, the modification is made on the outer layer nanotube by shielding the inner tube from the chemical reactions. This exceptional structure of DWCNTs provides unique optical, electrical, and mechanical A. Ashok · A. Yuda · I. M. Abu-Reesh · A. Kumar (*) Department of Chemical Engineering, College of Engineering, Qatar University, Doha, Qatar e-mail: [email protected]; [email protected]; [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_67

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properties that extend their application in various fields of research and industry. Owing to the presence of two nanotube walls and its complex inter wall interactions opens a new window to utilize the properties in various fields including batteries, capacitors, solar cells, microelectronics, drug delivery, cancer treatment, neuronal interfaces, and sensing applications. This chapter provides an insight on the current technologies for DWCNT synthesis and sorting methods and discusses the different potential application. Keyword

Double Walled Carbon Nanotubes (DWCNTs) · Sorting · Nanosensors · Nanomotors

Introduction Allotropes of carbon in the form carbon nanotubes are those characterized by hollow cylinders that consist of rolled-up graphene sheets. The graphene sheets itself are an extended hexagonal lattice of entirely sp2-bonded carbons. Generally, carbon nanotubes could either be single-walled or multi-walled and a class of carbon nanotubes called double-walled carbon nanotubes (or DWCNTs) is an emerging material that contains precisely two concentric carbon nanotubes. The double-wall arrangement makes them suitable for analyzing the effects of inter-wall coupling on the physical characteristic of carbon nanotubes (CNTs). Relative to single-walled carbon nanotubes (SWCNTs), DWCNTs have relatively greater thermal stability and mechanical strength as well as possessing appealing optical and electronic properties. Its discovery was in 1991 and the synthesis was first reported in 1998. It did not, however, receive much attention until recent production (Endo et al. 2005) and segregation (Green and Hersam 2008) of samples with high purity. Relative to single-walled carbon nanotubes (or SWCNTs), which has been studied more extensively and implemented for a wide range of applications (e.g., sensors, energy storage, solar cells, drug delivery, cancer treatment, neuronal interfaces), DWCNTs have remained under-explored. Although DWCNTs possess many similar properties displayed by their single-walled counterparts, it has other advantageous characteristics that result from the existence of the second wall and the resulting inter-wall interactions. For instance, the outer wall is proposed to be able to be functionalized selectively while still keeping the inner wall in its pristine form and accessible for signal transduction. A comparable phenomenon also occurs in DWCNT field effect transistors (FETs), in which a good chemical shielding of the inner wall from the external surroundings is offered by the outer wall. This further permits exploitation of the pristine nanotubes’ fine transconductance characteristics. In addition, DWCNTs provide the chance of exploring the interwall interactions between and within the carbon nanotubes. There is still, however, a challenge for realization that was also faced by SWCNT field until recently, and this includes, in particular, the inherent heterogeneity of as-synthesized DWCNT material. Currently, many research studies have been directed toward the

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processing of DWCNT in the area of material preparation with a defined length, electronic type, and diameter, which will add up to the information already obtained from the extensive study of SWCNT. This handbook will provide information collected from various studies on the recent development of DWCNTs in relation to the possible types of material synthesis technique, sorting method, and its possible applications.

Techniques for DWCNTs Synthesis The synthesis of DWCNTs can be accomplished by methods such as catalytic chemical vapor deposition (CCVD), arc-discharge, and “peapod” growth. In comparison to techniques that are used to prepare SWCNTs, there are not a lot of differences to the techniques previously mentioned. Slight variations in factors such as pressure, temperature, catalytic composition, and carbon feedstock can, however, selectively produce DWCNTs instead of SWCNTs and MWCNTs. An important point to remember is that the mechanical and thermal properties of CNTs are highly influenced by the techniques that are used to synthesize them. The reliance could also be related to the variations in diameter, length, and defects, of the CNTs being produced using the various methods. The three types of synthesis techniques will be discussed in the following sections as well as the factors that determine the growth of DWCNTs.

Catalytic Chemical Vapor Deposition (CCVD) This method is widely used to produce DWCNTs as it is cost effective, offers high yields, controllable, and it is suitable for industrial-scale production. The growth of nanotubes using this method involves decomposing a volatile gaseous carbon source, such as CO, CH4, or C2H2, at high temperature over metallic nanoparticles. Diffusion of the decomposed carbon atoms into the metal nanoparticles then takes place, and during saturation, their precipitation at the surface and the initiation of the growth of nanotube from nanoparticle occur. Therefore, the particles of the catalyst behave as both nucleation center for nanotube growth and the provider of catalytic enhancement. Oberlin et al. made the earliest report in 1976 on the growth of CCVD (Oberlin et al. 1976). Their study involved the decomposition of benzene in an atmosphere of hydrogen at 1100  C to synthesize nanotubes that had diameters of between 2 to 50 nm. A factor that is important for the growth of DWCNTs is the catalyst shape and its influence was demonstrated by Liu et al. (Liu et al. 2008). DWCNTs were grown from the decomposition of CH3 over porous Fe/MgO. The pore size was varied by applying an external pressure and they found that DWCNTs’ growth was very dependent on the catalyst’s pore size (pores less than 30 to 50 nm forms only MWCNTs). It was determined that compression and small pore size caused deformation and agglomeration of metal nanoparticles that forms MWCNTs and carbon

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capsules. In addition, the size of pore can physically inhibit the growth of DWCNTs when DWCNTs’ length reaches pore depth. In this moment, the growth can be terminated, extend into the structure of the catalyst, or buckle and modify direction. Hence, large size of catalyst pores or a loose stacked structure is preferred when utilizing porous catalyst. It must also be noted that the growth of high-purity DWCNTs is not as easy as solely controlling the size or shape of catalyst. Hafner and team analyzed the influence of catalyst composition on the synthesis of SWCNTs and DWCNTs through the catalytic decomposition of both C2H4 and CO using alumina of nanometer size and molybdenum as the catalyst (Hafner et al. 1998). It was found that when iron was inserted into the catalyst mixture, DWCNTs were produced during the reaction, and this underlines the effectiveness of iron in catalyzing the growth of DWCNTs. Iron is often used as a catalyst due to its activity toward the decomposition and generation of metastable carbides and also due to carbon being able to quickly diffuse through and over iron surface (Sinnott et al. 1999). Catalysts based on other transition metals have also been used to produce DWCNTs, such MgCoO catalyst, and its usage was performed by Flahaut et al. (Flahaut et al. 2003). In their study, gram-scale growth of DWCNTs (up to 1.3 g from 10 g of catalyst) was accomplished using methane as the feedstock with the involvement of molybdenum in the catalyst mixture, that had already been observed to enhance the formation of nanotube production (Flahaut et al. 2000). Later, it was determined that the catalyst synthesis route was as important as the catalyst composition (Flahaut et al. 2005). Modifying the route of synthesizing MgCoMoO from urea-based combustion to citric acid-based combustion, which is a milder combustion process, relatively greater specific surface area and homogeneity was achieved that could lead to 80% DWCNT growth. Adding sulfur as a growth promoter could also direct the reaction toward the production of DWCNTs rather than SWCNTs. This finding was studied by Ci and group for CCVD growth from the decomposition of C2H2 at a temperature of 900 to 1100  C (Ci et al. 2002). From the study it was determined that DWCNTs growth was highly influenced by sulfur addition and, excluding it in the catalyst material (ferrocene) could only lead to the production of SWCNTs. This analysis was also the first to report the CCVD synthesis of DWCNTs that utilized a “floating catalyst” in which the catalyst is sublimed into the gas phase instead of using substrate-bound form and it also gives the advantage of forming very long nanotube ropes (Zhu et al. 2002). The control of temperature could also help achieve improvements in DWCNT purity (Hafner et al. 1998). It was found that adjusting the growth temperature from 700  C to 850  C could increase the composition of DWCNTs during production from 30% to 70%. Using CCVD, DWCNTs could be grown in vertically aligned forests, which permit their direct implementation in devices such as sensors (Modi et al. 2003) and field emitters (Seung Il Jung et al. 2007). Yamada et al. performed the first demonstration of vertically aligned DWCNTs grown on flat substrates using water-assisted CCVD (T et al. 2006). The process was accomplished using Fe-Al2O3 (30 nm)/SiO2 (600 nm)/Si, with C2H4 serving as the carbon feedstock and it resulted in a DWCNT purity of 85%. Such high purity was ascribed to the precise control over the iron catalyst film thickness, which mostly dictates the catalyst particles’ size generated

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upon heating. Higher purity of DWCNT of 88% was achieved by Ci and team, who implemented iron films on an aluminum support layer with a feedstock of C2H4 at temperatures ranging from 700  C to 850  C (Lijie Ci et al. 2007). The study reported that iron film with a thickness of 1.5 nm that is deposited onto 10 nm of aluminum enhanced the selectivity to grow DWCNT. The growth conditions optimized lead to ultra-low-density arrangement because of the very large diameter of the produced DWCNTs (7.9 nm) and the low density of catalyst particle. HRTEM images of individual DWCNTs having different diameters of ultralow density are shown in Fig. 1a–d. Kim et al. reported the formation of ultra-long, super-aligned DWCNT forests that possessed lengths of 9 mm within a growth time of 10 hours (Kim et al. 2011). The synthesis was accomplished by initially reducing a catalyst of Fe film (1 nm)/Al2O3 (30 nm) in an atmosphere of He/H2 at a temperature of 750  C.

Fig. 1 (a–d) Images obtained from HRTEM analysis of individual DWCNTs of different diameters with ultralow density (0.015  0.002 g cm 3) synthesized by a water-assisted chemical vapor deposition process (image b also displays a cross-section view of a DWCNT) (Lijie Ci et al. 2007)

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The size and density of the catalyst particles was determined to be significantly affected by the reduction time that eventually altered the alignment and quality of nanotubes forest. With a reduction time of 5 minutes a catalyst with the smallest grain size could be formed that further lead to the best-quality growth. CCVD’s compatibility with a range of substrate has motivated the formation of aligned DWCNT arrangements on substrates that are relatively easier to be integrated into electronics (e.g., gold or silicon). Chen and team synthesized arrays of vertically aligned DWCNT through point-arc microwave plasma CCVD to permit the direct measurement of field-emission characteristics (Chen et al. 2008). A catalyst of three layers of Al2O3/Fe/Al2O3 (0.5 nm/0.8–1 nm/5 nm) was loaded on both SiO2 (200 nm)/Si and Cr (100 nm)/Si. The catalyst being deposited provided two functions in which the thicker, bottom layer of aluminum oxide serves as a barrier that prevents iron catalyst from reacting with the underlying substrate. The thin aluminum oxide on the top layer provides the possibility to increase the surface diffusion barrier of the iron atoms in order to limit aggregation. DWCNTs grow through methane decomposition in an atmosphere of H2 at a temperature of 600  C that possessed outer diameters of 2.5 to 3.8 nm. During the study it was established that the substrate of Cr/Si achieved higher density of nanotube; however, it had a lower growth rate when compared to the SiO2/Si substrate. Such observation could be explained by the heat accumulation by SiO2 during plasma radiation exposure. It must be noted that it is necessary for good contact to be present between the DWCNTs and the underlying substrate when the CNTs are grown for the purpose of direct integration into electronic applications. It was also determined that there is comparatively less resistance being provided by chromium than SiO2 and the DWCNT/Cr/Si surface displayed better field emission. Liu and group demonstrated the growth of DWCNT by using SiOx (30 nm) loaded onto SiO2 (100 nm)/Si surfaces (Liu et al. 2013b). They established important factors that were necessary for the growth which included deposited SiO2 layer thickness and pregrowth heat treatment. The two factors influence the size of the SiOx catalyst particles generated and, in turn, the number of walls. It was also found that DWCNT of 70% purity could be effectively grown with nanoparticles in the range of 3 to 5 nm, that are annealed at 850  C for 10 minutes in argon atmosphere. Iijima et al. performed direct growth on conducting metal foils and they determined that nickel-based alloys consisting of chromium or iron are the best for the growth of DWCNT and SWCNT (Tatsuki Hiraoka et al. 2006). The study involved depositing a catalyst layer of Al2O3 (30 nm)/Fe (1 nm) onto different metal foils and then exposing them to C2H4 in an atmosphere of H2O/ He at a temperature of 750  C. Selectively tuning the catalyst thickness to 1.8 nm permitted the production of DWCNTs and field-emission properties measurement showed the existence of good electrical contact. Furthermore, growing DWCNTs on catalyst (Fe/Al2O3)-coated gold films was performed by Fu and coworkers for application as field-effect emitters (Fu et al. 2012). In this work, reduction of the catalyst by hydrogen was first done before the introduction of C2H4 at 700  C. Figure 2a displays the CNT bundles that grew on silicon supported gold films and Fig. 2b is a TEM image of as-grown DWCNTs.

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Fig. 2 (a) Bundles of CNT grown on silicon supported gold films by thermal chemical vapor deposition (TCVD). (b) TEM image of the as-grown DWCNTs on gold films (Fu et al. 2012)

Although CCVD method has been the most commonly used method to grow DWCNTs, it still cannot yet be tuned to produce a specific type of DWCNT. Instead, a heterogeneous mixture of DWCNTs together with large- and small-diameter SWCNTs, triple- and multi-walled contaminants, amorphous carbon, and metal catalyst residues are produced. There is also the problem with slow development that is partly because the experiment consumes time and because of the large parameter space of the procedure itself. Overall, it can be said that a proper selectivity to grow DWCNTs using CCVD is difficult to attain as the chances of growing either SWCNTs or MWCNTs are always present when the variables have been optimized to maximize the growth of the other. Hence, the product of the process is commonly a mixture of various types of nanotubes.

Arc Discharge In this technique, a voltage and current are applied between two highly pure graphite electrodes with a spacing of typically 1 to 2 mm under inert environment. Normally, a partially hollowed anode is used and it is filled with carbon feedstock (e.g., graphitic powder) and catalyst/promoter. When current is applied, either steadily or as pulsations, a plasma is created between the electrodes. The random collisions between catalyst particles, carbon atoms, and gas result in the formation of carbonaceous material as a macroscopic deposit on the cathode and the reactor walls. The carbonaceous soot could consist of fullerenes and different types of carbon nanotubes (Gamaly and Ebbesen 1995). With the precise control of processing atmosphere, catalyst composition, carbon feedstock, and current/voltage conditions, the type of carbon nanotube formed could be directed toward the different possible variations (single-, double-, or multi-walled nanotube). The selective synthesis of DWCNTs using steady-state arc-discharge was performed by Hutchison and coworkers for the first time in 2001 (Hutchison et al. 2001). A mixture of cobalt, nickel, and iron was utilized in the graphite anode in an atmosphere of Ar/H2 with different quantities of sulfur being included as a growth

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promoter. The purity of the DWCNTs produced ranged from approximately 10% to 70% and it was determined that the use of sulfur in the catalyst was unnecessary to generate DWCNTs. Figure 3a and b shows the straight DWCNTs bundles that were formed at 350 Torr in the presence of Ar and H2 mixture of 1:1/v:v. The study performed by Saito et al., however, showed that sulfur is essential as a growth promoter for the DWCNTs’ production (Yahachi Saito et al. 2003). They also determined that iron must be present in the catalyst and that hydrogen was critical for DWCNT formation. Optimization of the catalyst (with ratio FeS:NiS:CoS of 1: 1:1) and the atmospheric content made it possible to achieve around 90% purity of DWCNTs that has an average outer diameter ranging from 2 to 5 nm. With the existence of halides the yield of DWCNTs can also be improved and this was analyzed by Qiu and team through performing steady-state arc discharge in the presence of hydrogen (Qiu et al. 2006). Involving chloride, specifically potassium chloride, to the mixture of catalyst, the team managed to attain an enhanced

Fig. 3 (a–b) Images (400 kV) displaying bundles (labeled as B) of straight DWCNTs synthesized through arc discharge method in the presence of argon and hydrogen mixture (1,1/v:v) at 350 Torr (catalytic particle labeled as CP) (Hutchison et al. 2001)

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DWCNTs yield (from 10% without KCl to 50 wt% with KCl) that has a purity of 90%. The growth of DWCNT is also possible without the use of reactive gases, and this was demonstrated by Huang and team in which no hydrogen was used in DWCNTs’ synthesis through modifying the cathode shape to a bowl-like structure (Houjin Huang et al. 2003). During the study, catalysts of nickel, iron, cobalt, and sulfur were utilized and DWCNTs with approximately 80% of purity were grown. The resulting DWCNTs could display better oxidation resistance relative to DWCNTs that are conventionally prepared by arc-discharge or those formed by CCVD. The improvement is because of the presence of large hot region within the bowl-like cathode that permitted in situ annealing or “defect-healing” of DWCNTs. Sugai et al. performed a DWCNT synthesis without the use of hydrogen gas using high-temperature pulsed arc discharge (Sugai et al. 2021a). In comparison to the steady-state methods, the type of technique the team implemented allowed the production of DWCNT without the presence of Fe and S. A catalyst of Y/Ni is often used in such a case to form SWCNTs (Sugai et al. 2000), but DWCNTs could be selectively produced by increasing the temperature during synthesis. Rising the temperature to 1200  C increased the diameter of the SWCNT formed after which a critical diameter is reached and DWCNTs production becomes more favorable. The possibility of having high control of arc conditions allowed the formation of small-diameter DWCNTs of 1.6 to 2 nm (pulse of 600 μs at 50 Hz). A steady-state arc-discharge technique was utilized by Zhao et al. to grow DWCNTs without the use of hydrogen or any expensive high-purity gases (Zhao et al. 2013). Iron-containing catalyst was utilized along with sulfur as a promoter for the synthesis and dry air with reduced pressure served as the atmosphere for the reaction. It was demonstrated that DWCNTs were mostly produced, other than SWCNTs and triple-walled CNTs, which had diameters ranging between 3 and 7 nm along with a significant quantity of the carbonaceous species. Another factor that has been studied that could influence the growth of DWCNT is the carbon feedstock composition. Li and coworkers used MWCNTs/CNF (carbon nanofibers) to grow DWCNTs of high purity by steady-state arch discharge in the presence of hydrogen (Li et al. 2005). MWCNTs/CNF was used to replace the anodic carbon feedstock (normally graphite of high purity with 99%) and it has a diameter ranging between 40 and 220 nm. The group discovered that involving MWCNTs/CNF permitted the formation of high quality DWCNTs than those synthesized using graphite powder (purity of 83% and outer diameter of between 1.75 to 4.87 nm). Xu et al. utilized a carbon feedstock of petroleum coke (Xu et al. 2014) and asphalt (Xu et al. 2013b) in place of high-quality graphitic powders that are very costly. Iron was being used as the catalyst in both the studies and it was found that using an argon atmosphere with the petroleum coke resulted solely in the production of DWCNTs that had diameters ranging between 3 and 4.4 nm. With arc-discharge method there is, however, a limitation of low yield and the formation of large diameters (greater than 2 nm) even though high-purity DWCNTs (reaching 90%) could be produced with little structure defects. In addition, large diameters cannot easily be characterized by Raman or absorption spectroscopy

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because of restrictions in the spectral windows, and with diameters of greater than 4 nm there is the possibility of easy breakage (Baughman 2006). With pulsed arc-discharge technique, the average diameter distribution could be minimized to form nanotubes with 1.6 to 2 nm diameters. The yield will, however, reduce dramatically to 10% (Yoshida et al. 2008). In comparison to the steady-state version of the technique that produces large diameter DWCNTs of up to 50% (Qiu et al. 2006), which implements iron, hydrogen, sulfur, and halides, the yield is quite low. Further investigation of the arc-discharge method in the area of thermodynamics, growth kinetics, and growth promoters’ function may possibly allow greater control over the diameter.

Peapod Growth This technique involves the encapsulation of a precursor material within an SWCNT that is followed by treatment to initiate coalescence to result in the formation of an inner wall. The first implementation of this technique was done by Smith et al. and the team established that during the pulsed evaporation of graphite C60 and C70 could get trapped within properly sized SWCNTs. The fullerenes were determined to be deposited on nanotubes surfaces (via in situ TEM) from the gas phase and were observed to enter the nanotubes through either uncapped ends or the defect sites. Within the nanotubes, the fullerenes began to assemble by itself into chains that are termed as “bucky-peapods” that possess uniform center-to-center distances. Once an extended exposure to a 100 kV electron beam or temperatures higher than 1100  C (Smith and Luzzi 2000) has taken place, the fullerenes merged to become DWCNT inner wall with an almost uniform inter-wall spacing of 0.3 nm. Further analysis performed by Bandow et al. related to dependence of the formation of DWCNT on temperature showed that coalescence process only takes place at temperature values above approximately 800  C, improves with rising temperature, and completes at around 1200  C (Bandow et al. 2001). The team has also reported that C76, C78, and C80 could be utilized as a precursor material and that the sole determinant of the final inner tube diameter is the parent wall diameter, regardless of the size of the fullerene. Hence, to form DWCNT other types of carbon-containing aromatic precursor materials could be used as long as they can be enclosed by the parent nanotube. Many reports have also been made on ferrocene encapsulation by SWCNT (Shiozawa et al. 2008) that further allows metal catalyst introduction (Amelia Barreiro et al. 2006). Berd and coworkers irradiated C60 peapods with UV laser of photons with energy greater than the bandgap of fullerene (C60 is 1.7 eV) that caused C60 fragmentation (Suzuki et al. 2000). Berd et al. also established an optimum laser excitation of 3.7 eV that effectively coalesced enclosed C60 molecules while preserving the parent nanotube’s structural integrity. For nanoelectronic applications the method could possibly be advantageous as it permits the in situ DWCNTs growth in ambient environment at room temperature (Kalbáč et al. 2005). However, most feasible nanoelectronic applications will surely be sensitive on the variant of inner wall

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being grown. In other words, the real-world application of this method is questionable unless the nanotube variant being grown can be properly tuned. An issue also arises from the unfilled SWCNT contamination that must be conveyed after synthesis. The expensive, long duration, and wasteful procedures remarkably contributes to production complexity which further limits the possibility for commercial application. This also means that peapod growth method will probably be restricted to small-scale research applications for a while. In addition, filling the parent outer wall poorly with the coalesced inner wall persists as a problem in that, predicted by theoretical calculations, when the entirely packed C60 molecules inside the parent nanotube has fully changed, only about two-thirds of the parent nanotube have been occupied, leaving empty space (Bandow et al. 2001). However, it is possible that this could be a unique benefit in some applications that demands nonstandard on-tube modifications in electron properties, which could theoretically be adjusted back and forth along the outer wall via control of inner wall position, or likewise in nano-electromechanical systems (or NEMS).

Sorting Techniques The features, properties, and its potential application of DWCNTs remain unexplored and still under the shadow of SWCNT community. In that context, the sorting methodologies of DWCNTs are the follow up studies of the well-known and established SWCNTs. Some technique that followed for SWCNTs sorting in DWCNTs shows significant results, but new techniques that are exclusively for deriving DWCNTs still need improvement in order to achieve the expected result for the potential applications.

Purification The purity of raw DWCNT could be improved post-synthesis by various methods to selectively remove contaminants. In the study performed by Sugai et al., the DWCNT purity was only 20% with pulsed arc-discharge method as a result of SWCNTs, fullerenes, and amorphous carbon being present (Sugai et al. 2021b). The team was able to significantly increase the percentage of purity to about 90% by initially washing the as-synthesized material with a solvent of CS2 (that selectively solubilize fullerenes) to remove fullerenes using standard laboratory separation processes (Ando 1998). The purification technique is then followed by the sonication of catalyst residues in concentrated acid of HCl to dissolve them. Different types of acids have been widely utilized to effectively get rid of metal particles during SWCNTs purification (Jin Zhang et al. 2003). To a certain degree the procedure could functionalize nanotubes depending on duration of exposure and temperature. Subsequent removal of amorphous carbon and SWCNT occurs through air oxidation at a high temperature of 500  C for 1 hour. The research team also established that DWCNTs’ relative higher thermal stability arises from their greater graphitization

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degree, with fewer defects, as well as from the interaction of the inner wall with the outer wall. Furthermore, the team assessed the effect by reporting the oxidation rate of DWCNTs being half that of SWCTs of similar diameter (1.6 nm) under optimized state; therefore, SWCNTs have higher potential of being oxidized and removed (Yoshida et al. 2008). Although high purity can be attained that could reach 90% (from 10%), certain hollow and metal-filled carbon capsules could still exist that could not be removed by HCl treatment. Hot air oxidation has also been revealed to demonstrate metallic SWCNTs selectivity and semiconducting species are left after a duration of 4 hours at 420  C, indicating that the suitable oxidation temperature should be attained to purify DWCNT (Yasumitsu Miyata et al. 2005). Despite the possibility of effective removal of SWCNTs and amorphous carbon through air oxidation under optimized states and the benefit of oxidation without introducing sidewall defects, the process could cause nanotubes destruction at temperatures greater than 750  C (Wiltshire et al. 2004). An alternative method of oxidizing SWCNTs could be performed through refluxing in H2O2 for 12 hours and this was shown in a study by Yoshida and coworkers in which a significant enhancement of purity of DWCNTs up to 95% (initially at 10%) could be accomplished (Yoshida et al. 2008). Similar to acid treatment, defect sites in the lattice are presented with certain oxygen-containing moieties (and Hewen Liu* 2006), but to a smaller degree, and analysis by Raman displayed a significant reduction in amorphous carbon quantity that existed (Feng et al. 2008). Furthermore, H2O2 favors the attack of nanotubes with particular electronic properties (semiconducting character in this case), a similar pattern observed in hot air oxidation. This was proven through treatment of SWCNTs with H2O2 that results in approximately 80% metallic composition after an hour of treatment (Yasumitsu Miyata et al. 2005). For DWCNT, H2O2 refluxing is for a longer duration, and it is not sure whether metallic SWCNTs are still present or not after such longer periods. However, it can be deduced that because H2O2 refluxing and air oxidation are selective toward semiconducting and metallic SWCNTs, respectively, combining the two processes could possibly result in an efficient purification technique.

Suspension The need to produce suspensions of individualized CNTs in either organic solvent (Jeong-Yuan Hwang et al. 2008) or water (Ghosh et al. 2010) is a generality being shared by all types of sorting techniques. Such requirement could be attained either by covalent or noncovalent techniques (Blanch et al. 2010). Using the covalent chemistry approach functional groups are introduced to the ends of nanotube and sidewalls that make them soluble (Woo-Jae Kim et al. 2008). With this approach, the production of suspensions of nanotubes that are well-dispersed and individualized is possible and the method could demonstrate certain preferability toward some diameters (and Stanislaus S. Wong* 2004) or electronic types (Blanch et al. 2012), which offers the possibility of separation according to these characteristics. Functionalization through covalent technique, however, is generally disadvantageous due to it

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being capable of disrupting the nanotube’s conjugated π system through the introduction of sp3 hybridization into the pure sp2 network (Jijun Zhao et al. 2004). Nanotubes distribution within these stabilizing agents could be accomplished by ultrasonication and subsequent centrifugation to get rid of bundles and residual catalyst particles that remain (Blanch et al. 2010). In dispersions that are surfactant-stabilized, the surface concentration and surfactant molecules’ orientation on the nanotube sidewalls influenced heavily by the surfactant type, solution concentration, the nanotube’s diameter and electronic type (Flavel et al. 2013). In the general case, the surfactant would initially produce a random layer and then with increasing amount of surfactant molecules binding to the surface hemimicelles start to form. Any additional surfactant would cause the generation of cylindrical micelle that are high packed (Collins et al. 2001). There are, however, differences as the diameter varies, with nanotubes of smaller diameters displaying less-organized surfactant molecules. For instance, SDS wrapping of small-diameter nanotubes of less than 1 nm would likely result in the creation of highly disordered, random configurations at low concentration of SDS (packing densities of approximately 1 molecule per nm2) (Tummala 2009), and highly ordered, cylindrical wrapping at high concentration of SDS (2.8 molecules per nm2). When large-diameter nanotubes of greater than 1 nm are wrapped at low concentration, disorder occurs; however, hemimicelles are produced at high SDS concentration (Tummala 2009). It was proposed through experiments that a relationship exists between wrapping and the nanotube structure as a result of differences in the surface π-electron states of the different SWCNT curvatures, which influence the SDS/nanotube interaction. The wrapping of SDS molecule around a small-diameter nanotube possessing a large bond curvature causes it to be confronted by a greater energetic barrier as a result of bending (Tummala 2009). Hence, it can be said that the adsorption of SDS to nanotubes with larger diameter and smaller curvatures is energetically more preferable. The degree of encapsulation of SDS is influenced by the electronic nature, in which there is greater extent of SDS wrapping with metallic nanotubes than with semiconducting nanotubes due to their higher polarizability. Co-surfactant wrapping is that which depends on competitive nonspecific binding between various surfactants such as SDS and SC and it could serve to provide other parameters that could be used to separate nanotubes according to electronic nature. Nanotubes suspension using DNA and proteins (Calvaresi et al. 2012) mainly depend on the strong π-stacking onto the sidewalls of nanotube. Through this the nanotubes could be effectively individualized and the opportunity to provide a negative surface charge density because of closeness of the phosphate backbone to the nanotube (Zheng et al. 2003). Fundamentally, DNA provides the potential to solubilize DWCNTs and this has been demonstrated in past studies (Li et al. 2010). It is possible to sort SWCNTs according to the (n,m) species and electronic type using DNA through implementing ion-exchange gels. Because they consist of biological elements, such suspensions rely heavily on the solution pH that requires optimization to attain well-dispersed nanotube suspension that has high concentration. Various techniques have been developed to sort purified nanotubes after a good

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suspension has been obtained through different techniques that show different specificity, complexity, and chances of success.

Reversible Covalent Chemistry Covalent modification processes tend to be selective to the diameter of nanotubes and for this reason they are very attractive for use in the purification and separation of DWCNTs. Deng and coworkers performed a study involving such processes in which Billups-Birch reductive alkylcarboxylation reaction was carried out (Deng et al. 2010). In the study, CNTs were covalently functionalized with alklycarboxylic acid groups progressively from nanotubes of smaller diameter to those with larger diameters. When these groups are added the nanotubes could become water soluble and the reaction’s diameter-dependent permitted standard phase separation in waterhexane. This causes the separation of functionalized samples into several aqueous extract of reducing solubility and functionalization, corresponding to increasing diameter. Another advantage of such approach is the reactions’ reversible nature. With alkylcarboxylation, it is possible to return sorted nanotubes to their initial form via simple annealing. Due to the correlation of wall number with diameter, reductive alkylcarboxylation could be applied to selectively remove nanotubes possessing single and multiple walls in addition to carbonaceous by-products, from as-synthesized DWCNT material (Deng et al. 2011a). The reductive-alkylcarboxylation method involves adding raw DWCNT material to liquid ammonia that consists of sodium to separate the nanotubes and to achieve homogeneity of the following reaction. The sodium dissolves completely when it is added to ammonia, with the lone pairs of nitrogen coordinating to the metal, that results in the formation of an electronic liquid consisting of [Na(NH3)x]+ complexes and free electrons. The introduction of the nanotubes to the solution causes the solvated electrons to be transferred to different nanotubes and carbonaceous material of aromatic nature that provides a negative surface charge and forming “nanotubide” radical anions. Through this, the van der Waals forces that keep the nanotubes in bundles are lightly overwhelmed by strong Coulombic repulsion, hence individualizing the nanotubes (Deng et al. 2010). With the addition of 6-bromohexanoix acid or any molecule that has a halogen-terminated alkyl chain a reduction reaction takes place that introduces aklylcarboxylic acid group to the sidewalls of the nanotube. The selectivity in diameter is the result of the variation in the nanotubes’ Fermi levels, in which nanotubes of smaller diameter (and higher Fermi levels) (O’Connell et al. 2005) display higher reduction potential than those with bigger diameter. Therefore, there is selectiveness toward smaller-diameter nanotubes during reduction by solvated electrons and with proper control of the reaction stoichiometry there is the chance of selectively adding alkylcarboxylic acid groups to smaller nanotubes. In the study performed by Deng and team (Deng et al. 2010) three cycles were accomplished, and in each, sodium and alkylcarboxylate source was further added initially before phase separation in a mixture of water/hexane. Distribution occurs in which the more easily functionalized SWCNTs and amorphous carbon were

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segregated into the aqueous portion, and the less-functionalized DWCNTs and MWCNTs were enriched in the insoluble solid. A study which demonstrated reduction occurring through defect activation and propagating particularly from sp3hybridized sites was demonstrated by Deng et al. that lead to functional groups’ clusters, and hence providing the reason amorphous carbon (mainly possessing sp3hybridized carbon atom) could be easily removed (Deng et al. 2011b). Performing a 3-cycle alkylcarboxylation further on the insoluble solid makes it possible for the MWCNTs and DWCNTs to be selectively functionalized as well as H2O-solubilized and this leaves MWCNTs remaining in the insoluble solid. Once DWCNTs are enriched, thermal annealing of the filtered and washed product in the presence of H2/ Ar can help in removing the alkyl functional groups. It was determined from highresolution TEM that even when the final sorted product still consisted of some MWCNTs, the SWCNTs and carbonaceous material was all eliminated. From the study it can be determined that the covalent chemistry not only assist in enriching DWCNTs, but also permits its implementation using a simple and very scalable technique to further reclaim the pristine form of the nanotubes. Even if it was observed that there was no enrichment by electronic character, a reliance on electronic character was displayed by the reduction potential of nanotubes in addition to on chirality. Therefore, it should theoretically be feasible to utilize sodium’s stoichiometric deficiency to selectively functionalize nanotubes that have the smallest reduction potential, and hence permits sorting to a certain extent.

Biofunctionalization The utilization of biological elements for suspending and sorting DWCNTs was performed by Nie et al. who saw the noncovalent biofunctionalization of DWCNTs with lysozyme (a protein) (Haiyu Nie et al. 2011). Using lysozyme is preferable as it mainly contains amine groups that could provide the nanotube with sufficient H2O solubility, (Calvaresi et al. 2012) and it also has hydrophobic portions in its core that could easily interact with nanotubes via π-π stacking. Protonated amines also interact with the nanotubes’ defect sites at low values of pH and through amine adsorption at high pH values. An observation was made that DWCNTs with large diameter could be selectively suspended by lysozyme (Haiyu Nie et al. 2011) and this matches with results from computational analysis in which larger-diameter MWCNTs (of 40 nm) frequently displayed greater protein binding than those with a smaller diameter (of 10 nm). Furthermore, the finding is also in agreement with analysis of interactions of protein with nanoparticles (Alexey A. Vertegel et al. 2004). Nie and team reported that the relatively stronger bundles modeled by nanotubes of smaller diameter may also play a role in reducing their solubilization through protein binding (Haiyu Nie et al. 2011). Due to the selective functionalization of DWCNTs of large diameter, with centrifugation the separation of smaller- and larger-diameter DWCNTs into pellet and supernatant, respectively, is possible. Analysis using TEM of the individual DWCNT samples showed two different diameter ranges of 1 to 4 nm (Gaussian average of 2.7 nm) and 3 to 5 nm (Gaussian average of 4 nm).

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Because there is a strong correlation between wall number and diameter, it is feasible to also separate DWCNTs from SWCNTs of smaller diameter and this offers a simple technique for enriching DWCNTs of such samples. This was demonstrated by the addition of a 4:1 mixture of pure DWCNTs and SWCNTs to lysozyme solution. Sonication of the mixture was then performed to assist with solubilization, followed by centrifugation to produce sediment and a supernatant that is lysozymebiofunctionalized. Within the supernatant, only large diameter DWCNTs were observed (established from TEM) and this underlines this method’s relevancy to selectively generate portions of DWCNTs with large diameters from such solutions, the suspended DWCNTs of small diameter by centrifugation process. This is, however, until now found solely on PL and optical absorption, which offers uncertainty for DWCNTs without verification using TEM studies.

Molecular Nanocalipers Liu and Peng use chiral diporphyrin nanotweezers and nanocalipers to process the SWCNTs in the form of nanotube diameter, handedness, and metallicity (Liu et al. 2013a). The sorting using diporphyrin involves its sterically compatibility with nanotubes of specific chiral angle and diameter. Such bonding forms a host-guest reaction with the diporphyrins and the target nanotube though CH-π and π-π bonding and the remaining unreacted nanotubes can be separated through some mechanical way of centrifugation. Spacing within the calipers is an important factor in this sorting mechanism which tunes the width of the molecule and the depth allowed to place the nanotubes in the host molecule. This means that larger nanotubes need deeper host position to be placed. Liu and team utilized chiral diporphyrins for the successful sorting of DWCNTs from MWCNTs and to an extend from SWCNTs (Liu et al. 2014). The reaction mechanism was made by tailoring the diporphyrin with chiral structure into nanocapilers with spacing of 1.9 nm between porphyrins. The sorting yield of DWCNTs in presence of nanocalipers increases from 77% to 90% within the narrow diameter distribution from 1.2–3.23 nm to 1.25–2.75 nm. Nanocalipers were washed repeatedly with tetrahydrofuran and pyrine followed by the circular dichroism measurement to confirm the presence of DWCNTs. They proved that the chiral diporphyrin nanocapilers are the most effective way for sorting of DWCNTs using molecular recognition.

Aqueous Two-Phase Extraction Standard aqueous phase separation method is another promising technique for the separation of the DWCNTs. Zheng et al. tried this method for the first time by dispersing the nanotubes in two water soluble polymer mix of dextran and polyethylene glycol (PEG) (Khripin et al. 2013). There occurs a spontaneous separation of hydrophilic dextran phases on the bottom and hydrophobic PEG rich phase on the top. This clears the existence of two distinct regime of separation using the

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two-polymer phase. Smaller diameter nanotubes (6,4) are attracted to the smaller diameter regime of 0.6–10 nm in the bottom hydrophilic dextran phase and the larger diameter CNTs (7,5) and (8,4) belongs to the hydrophobic PEG phase in top. Here, the degree of nanotube polarizability can be categorized as metallic/semiconducting separation where the metallic CNTs were more found in hydrophilic dextran-rich phase and semi-conducting CNTs were present in hydrophobic PEG phase. A more detailed study conducted by Subbaiyan shows that the sorting technique using liquid phase separation is more relied on the hydrophobicity of the surfactant on the CNT surface rather than the inherent hydrophobicity of the NCT itself (Subbaiyan et al. 2014). In that case, the type of surfactant and its ratio is critical parameter in sorting.

Application of DWCNTs Due to the smaller dimension and exceptional physio-chemical characteristics, DWCNTs have wide range of application in various field including capacitors and batteries, nano-sized semiconductor devices, field emitters, drug-delivery, biomedical applications, and so on.

Capacitors, Batteries, and Fuel Cells Supercapacitors have superior performance than batteries that include the higher efficiency and higher power density that achieved from the simple ion adsorption at the electrode. The behavior of the supercapacitors is governed by the texture, morphology, stability, and composition of the electrode materials used. DWCNT is considered as a promising candidate for electrochemical applications of batteries and capacitors due to the high-power density, high stability, large storage capacity, excellent cycling, and fast charging-discharging capability. Jang and coworkers studied the effect of surface modification using nitric acid (HNO3) on the capacitance response in both aqueous and nonaqueous medium. The capacitive nature of DWCNTs were tested before and after oxidation using nitric acid (HNO3) and found that DWCNTs treated with HNO3 have higher capacitance property owing to the modification in the interstitial pore structure of the CNT bundles and change in surface wettability after the origination of polar oxygen functional group on the surface of the DWCNT after HNO3 oxidation (Jang et al. 2009). In 2009, Cooper and coworkers developed a novel free standing, bendable, and thin electrode (10–30 μm) for supercapacitors by converting DNA-dispersed DWCNT (DWCNT/DNA) into thin film followed by thermal treatment with Argon at 600  C that subsequently convert the insulating DNA into a carbon material. It was found that DNA has high dispersion ability on the highly pure and crystalline strongly bundled DWCNTs that are packed in hexagonal arrays with an outer diameter of 1.6 nm. The high electrical conductivity of DWCNT and the larger capacitive nature of DNA derived carbon make DWCNT/DNA as a promising electrode material for supercapacitors. The porosity of electrode material can be

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thereby controlled by tuning the thermal treatment temperature or changing the amount of DNA in the mixture or changing the both. The capacitance values obtained at the rate of 1 mV/s for DWCNT/DNA is 2.3 times higher than pristine DWCNT (Cooper et al. 2009). Kang and coworkers fabricated flexible supercapacitors from a free-standing hybridized electrode comprised of DWCNT and activated carbon where xanthan gum as an additive. The use of xanthan gum in the chemical activation provide high dispersion of DWCNT that efficiently enhances the surface area with improved electrical and mechanical property and higher specific capacitance of 141.5 F/g at 100 mV/s (Kang et al. 2020). Iwasaki and team selectively synthesize vertically aligned DWCNT by controlling heating method that shows a promising specific capacitance of 83 Fg 1 in the nonaqueous solution and the superior structural properties are more promising than SWCNTs at the same capacitance. While changing the preheating temperature from 600 to 640  C, the selectivity of DWCNT increases from 3% to 90% and that for SWCNT decreases from 97% to 9% (Iwasakr et al. 2008). Due to the high surface area and crystallinity of DWCNT, it tends to aggregate into bundles due to Van der Waals attraction that act as an ideal support for nanoparticles. Song and coworkers synthesized Mn3O4/DWCNT nanocomposite using facile chemical precipitation route and shows excellent capacitive behavior in 1 M Na2SO4 aqueous solution. The specific capacitance of Mn3O4/DWCNT was found to be 1080 Fg 1 at the current density of 15 Ag 1 with excellent stability. The performance of Mn3O4/DWCNT is due to the synergetic effect of the higher power density of Mn3O4 and supreme conductivity and stability of the DWCNTs (Song et al. 2017). Thirumal et al. followed thermal chemical vapor deposition technique for the synthesis of helical/coil shaped nitrogen doped DWCNTs on metal foils and compare the performance for the supercapacitor applications with the pure CNTs in 0.5 M H2SO4 electrolyte. The measured specific capacitance of N-DWCTs (51.29 F/ g) are about two times higher than pure CNTs (23.40 F/g) at 0.125 A/g and the charge transfer resistance of N-DWCNT (7.731 Ω) is much smaller than pure CNTs (7.731 Ω) (Thirumal et al. 2016). DWCNTs are considered as an excellent additive for batteries not only due to high conductivity that enhances the electrical property of the electrode but also provide mechanical stability of the bundle network in the CNTs. Also, the most appropriate way to mitigate the volume expansion in the alloy material of Li-ion batteries (LIB), Na-ion batteries (NIB), and K-ion batteries (KIB) is the incorporation of carbon additives and binders in the electrode. Gabaudan and coworkers studied the effect of carbon additives on the bulky antimony electrode for KIB. Different types of carbon including carbon black, graphite, and double-walled CNTs as a conductive agent. DWCNTs provide nonreversible K+ storage, capacity retention, and long-term cycling when compared to other carbon materials. The benefit of using DWCNTs is the capability to withstand the large volume change from the mechanical reinforcement during the potassiation/depotassiation cycles. In 2006, Kim and coworkers conducted an in situ Raman study on the Lithium ion insertion/ desertion behavior of the SWCNT and DWCNT. A highly pure thin paper like foil (~30 μm) DWCNT that is mechanically rigid and exhibits high electrical

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conductivity due to the large interwoven long-bundles was successfully prepared using the conventional catalytic chemical vapor deposition technique followed by the two-step purification process. The evaluation of the insertion/desertion behavior from the spectroscopic studies shows that DWCNTs could be an outstanding material to fabricate the thin polymer anodic material for the LIB owing to the reduced Li ion path length that provide high capacitance rate at different voltages (Kim et al. 2006). Omar et al. developed a nanocomposite of DWCNT/Cobalt oxide as a high capacity anode material for LIB. The galvanostatic analysis shows that the synthesized nanocomposite gas the lithium storage capacity of 1575 mAh/g over 100 cycle in the first discharge (Al-Hartomy 2014). Lithium-sulfur (Li-S) batteries are considered as the energy storage device with high energy density for next generation. However, the shuttle effect of polysulfide that may migrate to the negative electrode further reacts with the lithium to form insoluble sulfide that consequently reduces the cycling and storage capacity that hinders the practical application. Sun and coworkers developed ultra-dense DWCNT membrane as an effective design to overcome the shuttle effect. DWCNTs are composed of two single-walled CNTs that are aligned coaxially with interwall distance of ~0.34 nm. The Raman spectroscopy shows that DWCNT has much less defects than MWCNTs, which helps for stacking and forming dense membrane, where the outer wall protects the inner wall from integrity and conductivity. The shuttle effect of polysulfide ion can be successfully blocked by minimizing the pores of the interlayers and the compactness achieved governs the passage of ions through the interlayers. The conductive electrolyte between the sulfur electrode and DWCNT membrane permits the absorption of membrane on the cathode area, which further reduces the formation of insoluble Li2S and Li2S2 (Sun et al. 2020). Moreover, to the electrochemical performance of DWCNT, it can also be used as a unit for the leakage detection on the lithium-ion batteries. The electrolyte vapor leakage from LIB may affect the safety of the user and may lead to heavy consequences. Somehow, it is very difficult to detect the leakage of the redox-neutral carbonate electrolytes such as dimethyl carbonates (DMC). Du and team, took the benifit of using functionalized, DWCNTs as a miniaturized sensor to detect the electrolyte vapor and detect the leakage from LIB. The sensor response shows rapid response even at low level of DMC leakage with high stability and sensitivity (Du et al. 2021). Direct methanol fuel cells (DMFC) and proton exchange membrane fuel cells (PEMFCs) are the best-known low temperature polymer electrolyte-based fuel cells that are used in automobiles and portable electronics owing to its high energy density, low noise, high energy conversion, low operating temperature, ease of handling, and minimal pollution. The commercialization of polymer fuel cell technology is more dependent on the activity, durability, and stability of the electrocatalysts. Most of the low temperature fuel cells in market depend on the supported Pt or its alloy. The critical properties of the electrocatalyst support are the electrical conductivity, surface area, cost, and corrosion resistance. Carbon blacks were the most widely used carbon support due to its high surface area, conductivity, and low cost. However, the poor crystallinity and low corrosion resistance limits the

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performance as the support for Pt. Later, many other carbon supports are also developed that include carbon nanofibers (CNF), carbon nanotubes (CNT), and carbon ribbons, foils, and arrays. Out of these CNTs gained much attention due to the unique electronic and structural characteristics. During fuel cell operation, CNTs are more corrosion resistive than carbon black. Li and coworkers studied the use of DWCNTs as an effective electrocatalytic support for low temperature fuel cells and compare its activity with SWCNT and MWCNTs (Li et al. 2006a). Pt-Ru nanoparticles with a diameter of 2–3 nm as shown in Fig. 4b were uniformly deposited on the three different CNTs prepared using ethylene glycol reduction method. A conventional, fast filtration method followed by a hot press film transfer is used to prepare the anode catalyst layer with the Nafion membrane as shown in Fig. 4c. MOR activity in a half-cell RDE experimental set up is used to measure the specific activity of the electrocatalyst in a well-controlled way without mass transportation effect. The oxidation peak of Pt-Ru/DWCNT is 0.037 mA/cm2 is much higher than Pt-Ru/SWCNT (0.012 mA/cm2), Pt-Ru/MWCNT (0.019 mA/cm2), and Pt-Ru/C (0.017 mA/cm2) as shown in Fig. 4d. The I-V characteristics of Pt-Ru with different carbon supports in a single cell DMFC in Fig. 4e show that open circuit voltage (OCV) for 30 wt%- Pt-Ru/DWNTs (0.627 V) is 50 mV higher than 30 wt % Pt-Ru/C and the increasing order of the OCV for the measured electrocatalyst is: Pt-Ru/ SWNTs > B) resulted in the conversion of carbon atoms surrounding born into

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negatively charged centers. Li et al. showed that the charge transfer characteristics of the B-doped CNTs structure are coming from the shift of Fermi energy level near to the valence-band region (Li et al. 2013a, b). This unique electronic behavioral changes after the B-doping transforms CNTs into a catalytically active material for ORR process. During ORR electrocatalysis, the electron deficient B atom along with the adjacent carbons acts as active sites to adsorb and reduce oxygen molecules and the B electronic structure establishes a bridge between the graphitic carbon active centers and analyte molecular oxygen for the electrons transfer process (Yang et al. 2011). Electrocatalytic ORR performance of B-doped CNTs is higher than that of pristine CNTs and the ORR activity of B-doped CNTs increases with the increase of the doped B content. The enhanced ORR activity of B-doped CNTs comes from the enhanced adsorption of molecular oxygen and successful utilization of neighboring carbon π electrons after the B doping as mentioned earlier (Lu et al. 2020). However, the ORR activity of B-doped CNTs is much lower than that of the commercial Pt/C ORR catalysts. On the other hand, it has been observed that when CNTs are co-doped with boron “B” and nitrogen “N,” the synergistic effect originating from the electron deficient B and electron-rich N induces both p-type and n-type conducting properties in the dual B-N-doped CNTs structure simultaneously (Blase et al. 1997). These bifunctionally doped CNTs (B-N-CNTs) displayed higher ORR performance than the mono-doped CNTs and exhibited performance comparable to the commercial Pt/C.

Oxygen- and Sulfur-Doped CNTs for ORR Due to the two lone pairs of electrons on the oxygen “O” atom, incorporation of oxygen into the CNTs structure creates several active sites to perform ORR electrocatalysis. Chen et al. suggested that the defects created due to the oxygen doping increase surface energy and hydrophilicity nature of the CNTs which in turn increase their ORR activity (Chen et al. 2011; Mohan et al. 2019). O-doped single walled (SW) and multiwalled (MW) CNTs were prepared by a post-synthesis treatment in which CNTs were plasma treated in presence of O2 gas, Fig. 7a (Chen et al. 2011; Wirth et al. 2011; Mohan et al. 2019). The oxygen plasma treatment created multiple suitable defect sites and increased the oxygenated surface functionalities (-OH, -COOH, -COH) which enhanced the ORR activity. Evaluation of the ORR electrocatalytic performance of O-doped CNTs indicated that single walled O-doped CNTs have higher ORR performance, significantly higher ORR onset potential, very low peroxide yield, and higher electrocatalytic stability than the O-doped multiwalled CNTs as depicted in Fig. 7b, c (Mohan et al. 2019; Lu et al. 2019). The enhanced ORR performance of O-doped SWCNTs over O-doped MWCNTs was attributed to the inherent structure of the SWCNTs which allowed more oxygen molecules to access, get adsorbed and eventually reduced on the defect sites. Sulfur “S” contains two lone pairs of electrons like oxygen. However, due to the narrow electronegativity difference between sulfur and carbon atoms, it (S atom) does not change the electronic structure of the doped CNTs much and hence the

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Fig. 7 (a) Representative TEM image of O-doped SWCNT (b, c) stability test of O-doped SWCNT and MWCNT respectively in 0.1 M KOH at a scan rate of 20 mV/s. Insets showing polarization curves before and after 1000 potential cycles. (Reprinted with permission (Mohan et al. 2019) Copyright © 2019 American Chemical Society). (d) TEM image of N, S-doped CNTs. (e) LSV curves for different doped CNTs and Pt/C. (f) Stability comparison of Pt/C with N, S-doped CNTs. (Reprinted with permission (Liu et al. 2015) Copyright © 2015 The Royal Society of Chemistry)

S-doped CNTs are displaying weak ORR electrocatalytic performance. However, it has been found when sulfur is co-doped with nitrogen, the ORR electrocatalytic performance increases significantly. Liu et al. synthesized N,S-codoped CNTs from coal tar by using CoCl2 with the aid of an evocating agent of dicyandiamide, Fig. 7d. Depending on the loading of N and S dopants, the ORR electrocatalytic activity changes and the highest ORR performance showed in Fig. 7e was achieved when the N and S loading was 6.07 wt % and 0.21 wt %, respectively (Liu et al. 2015).

Phosphorus-Doped CNTs for ORR Phosphorus “P” doping of CNTs creates effective defects, and catalytically active electronic structure enabling ORR electrocatalysis. Liu et al. studied the ORR electrocatalytic performance of P-doped multiwalled carbon nanotubes (MCNTs) synthesized by thermolytic treatment of toluene and triphenylphosphine precursors in presence of metal catalysts. In alkaline media, the ORR performance of P-doped CNTs materials was independent of the P content but rather on the morphology and size of P nanoparticles as well as the synthesis reaction temperature. P-doped CNTs with relatively low P content, that is, 5 wt% of the P precursor, and prepared at lower temperature displayed an ORR performance which outperformed that of P-doped CNTs synthesized at higher temperatures and commercial Pt/C catalyst in terms of electrocatalytic activity, and durability (Liu et al. 2011). Wu et al. have synthesized

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metal free P, O, N-doped CNTs using a unique synthesis process (Dar et al. 2018). At first, they have synthesized a noble type of poly [cyclotriphosphazene-co-1,3,5-triol nitrobenzene] (PCTNB) microspheres (precursors of P, N, and O for doping) which was wrapped with pre-synthesized CNTs. The PCTNB wrapped CNTs were pyrolyzed at 600  C under N2 atmosphere which resulted in P, O, N-codoped CNTs. The codoped CNTs catalyst exhibited high electrocatalytic performance, high methanol tolerance, and electrocatalytic stability for ORR in acidic PEM cells compared to Pt/C catalyst and metal-free ORR catalysts, respectively. For example, during the ORR process, it displayed higher half wave potential (0.85 V) and onset potential (0.94 V) than many other nonmetallic electrocatalysts.

Heteroatom-Doped CNTs Supported Metallic Sites for ORR Nonmetal-doped CNTs supported metallic sites experience a significant increase in the electrocatalytic activity because of the synergistic effect of physical and chemical properties of the added metals and nonmetal dopants. Chemical inertness of carbon in the CNTs makes adsorption and dispersion of metals on CNTs surface unfeasible. Surface functionalization of the carbon layers of CNTs by nonmetal/ heteroatom doping facilitates deposition and dispersion of the metal atoms and ions on the CNTs surface. Additionally, the large surface area and porous structure of CNTs inhibit the metal particles agglomeration on the surface and enable exposure of more active catalytic sites (Kongkanand et al. 2006; Demir et al. 2018a, b; Farghaly and Collinson 2016; Van Cleve et al. 2021). Li et al. suggested that metal deposition and dispersion on the surface of heteroatom-doped CNTs varies with the dopant nature. N-doping activates the neighboring carbon atoms due to the high electron affinity of N atom and so facilitates the metal atoms deposition on surface of the CNTs (Li et al. 2009). While B-doping of CNTs facilitates the metal deposition, that is, platinum through the strong hybridization between the B atom p orbital and d orbital of the platinum atom (Li et al. 2009). Higgins et al. showed that when N-doping concentration increased in N-doped CNTs, Pt deposition on the CNTs surface increased as well, the degree of platinum particles agglomeration decreased, and so their dispersion was increased as show in Fig. 8a, b (Higgins et al. 2010). The ORR electrocatalytic activity of Pt-N-CNTs catalysts changes with the change of N dopant precursors and it was positively correlated with the N content in the precursors. Pt supported on N-doped CNTs synthesized from ethylenediamine (Pt/NCNT(ED)) showed higher ORR activity and stability than commercial Pt/C and Pt/NCNT as showed in Fig. 8c due to the lower N content in pyridine with respect to the N-rich ethylenediamine precursor. Other than the active carbon atoms, the pyridinic-N and pyrrolic-N also played role in the adsorption Pt and Ru metals (Chetty et al. 2009). Surface defects created due to the N-doping enhanced the metal particles deposition and dispersion (Lin et al. 2009). Garcia-Garcia et al. reported that N-doped CNTs acted as a very strong support for noble metal particles and the synthesized metal-N-CNTs catalysts were sintering resistant and stable at very high temperatures (García-García et al. 2010). Engineering of noble metals supported on heteroatom-doped CNTs increases the electrocatalytic activity and stability of metal

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Fig. 8 (a–c) Illustration and TEM image of Pt deposited on N-doped CNTs and ORR polarization curve obtained for Pt supported on different N-doped CNTs synthesized using different precursors in an O2 saturated 0.1 M HClO4 electrolyte at 10 mV s1 scan rate at 900 rpm. Here ED is ethylenediamine, and -Py is pyridine. (Reprinted with permission (Higgins et al. 2010) Copyright © 2010 American Chemical Society). (d–f) graphic and TEM image of Co deposited on N, S-doped CNTs grafted multichannel fibers and LSV curve showing the comparison of the ORR activity by different catalysts, respectively. (Reprinted with permission (Wang et al. 2017) Copyright © 2017 The Royal Society of Chemistry). (g–i) illustration and TEM image of M2P supported on N, P-doped CNTs and LSV polarization curves of N, P-doped CNTs–Mn2P (1), N, P-doped CNTs– Co2P (2), N, P-doped CNTs–Ni2P (3), N, P-doped CNTs (4) and MWCNTs (5) in an O2-saturated 0.1 M KOH solution at 1800 rpm. (Reprinted with permission (Lu et al. 2020) Copyright © 2015 The Royal Society of Chemistry)

particles significantly and hence have the potential to minimize the capital cost of many heterogeneous catalytic processes. The scarcity and high cost of noble metals based electrocatalysts are the motifs behind the use of earth abundant and low-cost metals supported on doped CNTs materials. The catalytic activity of the non-noble-metal doped CNTs, high electrical conductivity, high surface area, and high porosity of CNTs collectively participate in improving the electrocatalytic performance efficacy for ORR electrocatalysis. Transition metals due to their unoccupied d orbitals display an optimum binding energy

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to adsorb oxygen molecule and so have the potential to exhibit an enhanced electrocatalytic properties. For ORR electrocatalysis, several earth abundant transition metals have been deposited on pristine or heteroatom-doped CNTs. Li et al. synthesized mesoporous carbon containing a network of Fe-N-doped CNTs catalyst for ORR process. Cluster of iron atoms deposited on mesoporous silica nanoparticles and aniline were used as catalyst for CNTs growth and N source, respectively to synthesize C/Fe-N-doped CNTs in a two-step process (Li et al. 2015). The developed catalyst at 900  C displayed higher ORR performance than C/Fe-N-doped CNTs prepared at 700 and 800  C with 59 mV and 30 mV more positive onset potential and positive half-cell potential, respectively. In addition, it displayed high durability, and tolerance to the methanol crossover than commercial Pt/C electrocatalyst (Li et al. 2015). Cobalt nanoparticles deposited on N,S-codoped CNTs grafted multichannel carbon fibers (MCF), Fig. 8d, and ORR electrocatalysts were prepared by electrospinning, heat treatment at 280  C, and pyrolysis in the presence melamine at high temperatures (700–1000  C) under N2 atmosphere (Wang et al. 2017). The synthesized materials displayed excellent electrocatalytic activity and stability performance for the ORR process comparable to that of commercial Pt/C in alkaline solution, Fig. 8f (Wang et al. 2017). Liu et al. deposited metal phosphide (Ni2P, Co2P and Mn2P) nanoparticles on heteroatom-doped CNTs by carbonization of polyphosphazene (precursor for P, N doping of CNTs and transition metal phosphide), p-phenylenediamine (N precursor), and metal acetate, Fig. 8g, h. Mn2P and Co2P decorated P-N-CNTs electrocatalysts displayed comparable ORR onset potential and high stability to those of commercial Pt/C electrocatalyst with four electrons dominant ORR process while Ni2P-P-N-CNTs catalyst displayed the lowest ORR performance, Fig. 8i (Chen et al. 2015). The observed changes in the ORR performance of transition metal phosphide-P-N-CNTs catalysts were attributed to the difference in electronegativity and binding energy of the Ni, Mn, and Co transition metals. These differences promoted the electron donating ability of metal phosphides and eventually led to an enhanced ORR catalytic performance (Chen et al. 2015). The ORR performance of Mn3O4-N-doped CNTs in alkaline media outperformed that of commercial Pt/C in terms of electrocatalytic performance and durability. The change in the electronic structure, improved oxygen adsorption, and charge transfer kinetics of CNTs upon Mn3O4 deposition and heteroatom doping accounted for their enhanced ORR electrocatalytic performance (Lu et al. 2020; Liu et al. 2014a).

Heteroatoms-Doped CNTs for Electrocatalytic Water Splitting Due to the growing energy demand and limitation of the fossil fuel resources alternative renewable energy sources are getting more and more attention in the scientific communities. Electrochemically water dissociates into hydrogen “H2” gas at the cathodic half-cell reaction while the anodic half-cell reaction involves oxygen “O2” gas production. The electrochemical water splitting reaction can be expressed as follows (Khalafallah et al. 2019a, b):

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2H+ (aq) + 2e ! H2 (g);

E ¼ 0.0 V

2H2O (l) ! 4e + 4H+ (aq) + O2 (g);

E ¼ 1.23 V

H2O (l) ! H2 (g) + 1/2 O2 (g);

E ¼ 1.23 V

Hydrogen evolution reaction (HER) is complementary to the oxygen evolution reaction (OER) and depends on the sluggish kinetics of the 4-electron OER process. Platinum group metals, alloys and oxides, for example, Pt, Pt3Co, Pd, RuO2, and IrO2, are known to be state-of-the-art catalysts for HER and OER processes due to their low overpotential, small Tafel slope value, and high exchange current density. However, the low abundance and high cost of these materials make the water splitting process commercially less feasible. Therefore, there is a demand for developing new catalysts from earth abundant and low-cost elements. As we discussed earlier, doping of CNTs changes their electronic structure significantly and enables the material to exhibit appealing electrocatalytic properties. Several reports have been found where doped CNTs were used as electrocatalysts for the electrochemical water splitting reaction. The following subsections discuss the utilization of heteroatom-doped CNTs and/or active metals supported on heteroatom-doped CNTs for electrochemical water splitting application in light of HER and OER electrocatalysis.

Heteroatoms-Doped CNTs for HER Among the different types of doped CNTs, N-doped CNTs have been studied extensively for HER process. In the N-doped CNTs, both nitrogen and neighboring carbon atoms due to their charge work as catalytic active sites for the water splitting reaction. Davodi et al. utilized N-doped MWCNTs as catalysts for HER process (Davodi et al. 2017). Different N-moieties exhibited different electrocatalytic performance toward electrochemical water splitting reaction. N-doped CNTs with high content of pyridinic-N displayed decent HER catalytic activity, high catalytic stability, and low overpotential in alkaline environment as shown in Fig. 9a–c (Davodi et al. 2017). In another study, Singh et al. reported that not only the pyridinic-N but also the graphitic-N are important for the HER performance (Singh et al. 2017). In N-doped CNTs, the pyridinic-N and graphitic-N atoms are responsible for the asymmetric distribution of charge on the surrounding carbon atoms and hence fine tuning of their ratio is crucial to improve the HER performance through reaching the most appropriate value of hydrogen adsorption Gibbs free-energy. Such balance between the content of pyridinic-N and graphitic-N in doped CNTs has been achieved by fine tuning the reaction temperature and resulted in superior HER performance with ~100% Faradaic efficiency compared to many carbon-based counterparts, Volmer–Heyrovsky mechanism with proton adsorption being the rate limiting step, high exchange current density, small Tafel slop, and low onset potential (Singh et al. 2017).

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Fig. 9 (a–c) Illustration of N-doped MWCNTs, polarization curve for HER to show the HER activity by MWCNTs, N-doped MWCNTs and Pt/C and chronoamperometry study to show the catalytic stability of the catalysts. (Reprinted with permission (Davodi et al. 2017) Copyright © 2017 American Chemical Society). (d–f) TEM image of FeP supported on N-doped CNTs, polarization curve showing different catalytic activity of NCNT, FeP NPs, FeP/NCNT, and Pt/C and catalytic stability of FeP/NCNT even after 1000 cycles. (Reprinted with permission (Liu et al. 2014a, b) Copyright © 2010 Elsevier)

When CNTs are doped with multiple dopants such as N, O, B, N, P, S, and transition metals better and efficient electrocatalysis performance becomes feasible. Qu et al. reported the in situ synthesis of N and S codoped CNTs by using a two-step route called “graft and pyrolyze” process. At first, the N precursor polydopamine was deposited on the surface of MWCNTs and later was grafted with thiol group from the S-precursor 2-mercaptoethanol (Qu et al. 2017). After this engineering, the resulting hybrid material was pyrolyzed to produce N, S-codoped MWCNTs where the S doping content can be increased up to 5.6 at%. The dual doped CNTs showed an improved HER performance in 1.0 M KOH compared to the mono-doped CNTs and other carbon-based electrocatalysts. The boosted HER performance of N,S-codoped CNTs cannot be explained based on the charge transfer principle because the S incorporation does not alter the electronic structure of the carbon system considerably due to the narrow electronegativity difference between S (2.58) and C (2.55) atoms. However, the S-doping provides more electrocatalytically active sites in the N, S-codoped CNTs via changing the spin density around the adjacent carbon atoms which in turn enhances the adsorption of H* (a key reaction intermediate in the HER process) and hence improves the HER electrocatalytic performance. Additionally, the evolved hierarchical pore structure in the codoped CNTs provides a facile pathway for the mass transport of reactants and products (Qu et al. 2017). Besides the nonmetal-doped CNTs, electrocatalytic HER performance of metals (active for HER) supported on heteroatom-doped CNTs has been investigated.

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The enhanced HER electrocatalytic activity of metal-heteroatom-doped CNTs is offered by the synergy between the metals and the doped CNTs along with the physicochemical properties of CNTs and is comparable to the electrocatalytic performance of the platinum group materials. Earth abundant transition metals are not known to be a good catalyst for electrochemical water splitting due to their very high or low binding energies toward hydrogen and oxygen but when they are transformed to their corresponding phosphide, their binding energies change to the optimum values required to bind H2 and O2 (Eladgham et al. 2020). When these transition metal phosphides are combined with heteroatom-doped CNTs, their electrocatalytic performance improves remarkably with respect to their nanoparticle counterparts. For instance, Liu et al. synthesized FeP/N-doped CNTs nanocomposites and investigated their HER performance. First Fe2O3/N-doped CNTs composite was prepared from N-doped CNTs and FeCl3.6H2O by hydrothermal treatment followed by calcination at 350  C in the presence of sodium hypophospite to produce FeP/N-doped CNTs, Fig. 9d (Liu et al. 2014a, b; Dubal et al. 2016) The nanocomposites exhibited a high HER activity (low overpotential and small Tafel slope value) with stability closer to that of Pt/C and surpassing that of N-doped CNTs and FeP, Fig. 9e, f (Liu et al. 2014a, b; Dubal et al. 2016). Liu et al. synthesized CoP/CNTs from Co3O4/CNT and sodium hypophosphite precursors by using phosphidation method. The phosphide nanocomposite catalyst displayed a low onset overpotential (40 mV), small Tafel slope (54 mV dec1), and near overpotential of the commercial Pt/C. Additionally, it displayed ~100% Faradaic efficiency and high stability where the catalytic activi over a course of 18 h (Liu et al. 2014a, b). Pan et al. studied the doping effect of transition metals (M: Fe, Ni, or Cu) on Co2P/N-doped CNTs for HER electrocatalysis (Pan et al. 2016). Thermal decomposition of the metal precursor in the presence of triphenyl phosphine and N-doped CNTs produced M-Co2P/N-doped CNTs nanocomposites. Different metal doping created different order of physical, chemical, and structural properties and hence resulted in different electrocatalytic performance for HER electrocatalysis. The order of the HER electrocatalytic performance of the nanocomposites was: Fe–Co2P/ NCNTs > Ni–Co2P/NCNTs > Cu–Co2P/NCNTs. The high electrocatalytic activity of Fe nanocomposite was assigned to its high positive charge compared to others studied transition metals which produced more asymmetry in the electronic structure of the hybrid material. Moreover, iron nanocomposite displayed high specific area due to the growth of the [111] crystal plane which enabled an access to a larger number of the catalytically active sites (Pan et al. 2016).

Heteroatoms-Doped CNTs for OER Oxygen evolution reaction (OER) is the counter half-cell reaction in the electrochemical water splitting reaction, crucial for energy storage technologies, regenerative fuel cells, and rechargeable metal-air batteries. IrO2 and RuO2 are known to be the best electrocatalysts for OER process but their high cost and scarcity limits their widespread commercial applications. Heteroatom-doped CNTs and metals

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(active for OER) supported on doped CNTs are potential alternatives to address the issues of the platinum group metals OER catalysts. Davodi et al. suggested that the electrocatalytic activity exhibited by N-doped CNTs depends on several factors such as the physical structure of CNTs, types, and amount of N functionality in the doped CNTs materials (Davodi et al. 2017). Pyridinic-N due to its high electronegativity induces positive charge on the neighboring carbon atoms which enables key intermediates OH* and OOH* of the water oxidation reaction to get adsorbed during the OER process. The adsorption of these oxygenated species represents the ratelimiting step for OER. Hence, controlling the amount of N-doping consequently could enhance OER performance of N-doped CNTs electrocatalysts. OER polarization curve, Tafel plots produced in 0.1 M KOH, and 1 M KOH and chronoamperometry study are presented in Fig. 10a–c (Davodi et al. 2017). Codoping of heteroatoms, for example, N, O, S, and P into the CNTs structure also improves their catalytic performance for OER (Davodi et al. 2017). N,S-codoped CNTs demonstrated excellent electrocatalytic activity and stability for OER process in the alkaline medium (Qu et al. 2017). The enhanced OER activity of the codoped CNTs was attributed to the improved mass and charge transfer kinetics which originated from the synergy between the N and S dopants and the hierarchical nature of the electrocatalyst porous structure. The heteroatom-doped CNTs were also used as a support for metals to investigate their OER activity. Tavakkoli et al. have developed a hybrid mesoporous material of graphitized graphene nanoflake-CNTs to simultaneously support single atoms of Co,

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Fig. 10 (a–c) Polarization curves for OER from N-doped MWCNTs, MWCNTs, Pt/C and IrO2, Tafel plot for N-doped MWCNTs in 0.1 M NaOH and 1 M NaOH and chronoamperometry study by N-doped MWCNTs for OER. (Reprinted with permission (Davodi et al. 2017) Copyright © 2017 American Chemical Society). (d–f) Polarization curves and Tafel plots shown for OER by Co, Mo deposited on NCNTs/ graphitized graphene nanoflake (GF) supported on GC, Au, Ni, RuO2 on GC and Ni; (i) catalytic study of Co, Mo deposited on NCNTs/GF on Ni for OER. (Reprinted with permission (Tavakkoli et al. 2020) Copyright © 2020 American Chemical Society)

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Mo, and N (N-Co-Mo-GF/CNT) (Tavakkoli et al. 2020). The hybrid nanocomposite material was synthesized by catalytic chemical vapor deposition (CCVD) method in which Mg0.99(Co1-xMox)0.01O (x  0.25) was applied as a catalyst to produce both graphene flakes and CNTs hybrid composite. The N-Co-Mo-GF/CNT nanocomposite was deposited on different substrates, for example, Ni, Au, or glassy carbon to investigate its electrocatalytic activity for OER, Fig. 10g–i. The different substrates displayed different catalytic performance and the best OER performance turned out when Ni was used as a substrate. The improved OER performance over the Ni substrate was attributed to the enhanced mass transfer through its mesoporous structure and generation of Ni interfacial catalytically active sites that were accessible to the analyte molecules (Tavakkoli et al. 2020). Chen et al. fabricated triphasic interfacial structure of NiCo2O4/NCNTs/NiCo where N-doped CNTs were derived using NiCo alloy and spinel interface of NiCo2O4 deposited on the CNTs surface to provide highly active catalytic sites (Chen et al. 2021a, b). Mixture of Ni(NO3)2‧ 6H2O, Co(NO3)2‧6H2O, glucose, and melamine were pyrolyzed at 800  C for 2 h with varying amounts of the Ni and Mo precursors. The performance of the developed electrocatalyst NiCo2O4/NCNTs/NiCo outperformed the commercial OER electrocatalyst IrO2. Theoretical calculations showed that the evolved heterogeneous interface in NiCo2O4/NCNTs/NiCo reduced the energy barrier, elevated the electronic state to the Fermi level and thus increased the catalytic activity for OER. Moreover, the synergetic interaction between the NiCo2O4/NCNTs/NiCo heterogeneous interface and ultrathin carbon layer significantly improved the electron transfer rate and consequently the reaction kinetics for OER (Chen et al. 2021a, b). Wang et al. synthesized Co nanoparticles encapsulated in N, S-codoped CNTs grafted multichannel carbon fibers by electrospinning followed by thermal annealing techniques (Wang et al. 2017). Interconnected network and multichannel structure provided facile mass transport during the OER electrocatalysis process. The encapsulated Co nanoparticles and N, S dopants in CNTs all acted as catalytically active sites for OER. The synergistic impact from the porous networking structure, active metallic sites, and heteroatom dual doping leads to better electrocatalytic performance for OER compared to the commercial RuO2 catalyst (Wang et al. 2017). To this end, the application of carbon-based OER catalysts in water electrolysis is still questionable due to the corrosion of carbon under the anion exchange membrane (AEM) water electrolyzer harsh operating conditions.

Heteroatoms-Doped CNTs for Zn-Air Batteries Metal-air batteries represent the bridge between the energy storage and conversion applications of heteroatoms-doped carbon nanotubes and so we conclude this chapter with the discussion of Zn-air batteries. Portable energy devices based on green energy sources, for example, water splitting, HER, OER, and ORR, are getting the top priority due to the decline of fossil fuel reserve and environmental issue. There is a demand of efficient and reasonably priced energy storage for portable energy devices. Lithium-ion batteries

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are the most common energy storage technology applied in the portable electrical devices. However, the scarcity of lithium source, issues with its safety, and low yield of energy density limit their widespread commercial application. Metal-air batteries are promising alternatives to address the demand supply of energy storage devices. In the metal-air batteries oxygen reacts to produce high energy density. Lithium, sodium, aluminum, potassium, iron, and zinc are the most investigated metals to construct metal air batteries. Among them zinc provides most stability during the charging and discharging processes and produces the highest theoretical volumetric energy density which makes zinc air “Zn-air” batteries more desirable portable energy storage devices. Moreover, the high abundance and low cost of zinc makes its battery affordable. In the Zn-air batteries, the ORR process occurs during the discharging (at cathode) and OER during the charging process. The reactions happening in the Zn-air batteries are as follows (Fu et al. 2017): Charging ZnO+H2O+2OH ! ZnðOHÞ2 4 (Cathode)  +2e ! Zn + 4OH ZnðOHÞ2 4 4 OH ! O2 + 2H2O + 4e (Anode) 2ZnO ! 2Zn + O2 (Overall reaction) Discharging Negative electrode: Zn + 4OH ! ZnðOHÞ2 4 + 2e  ZnðOHÞ2 ! ZnO + H 2O + 2OH 4  O2 + 2H2O + 4e ! 4OH 2Zn + O2 ! 2ZnO (Overall reaction) There are four main components in the Zn-air batteries: (i) air electrode constructed with catalyst, (ii) alkaline electrolyte (solid or aqueous), (iii) separator, and (iv) zinc electrode (Fu et al. 2017). In a typical working process, Zn electrode releases electrons during the discharging process. These electrons are transferred to the air electrode to reduce the atmospheric oxygen and forms hydroxide ions. The hydroxide ions travel to the Zn electrode and react to form zincate ions, which again dissociate to form ZnO. On the other hand, during the charging process, OER reaction occurs and the produced electrons are stored in form of electric energy. In the overall charging process, ZnO breaks apart and evolves O2 molecule, four moles of electrons and Zn2+ ions which get deposited on the Zn-electrode. However, the high overpotential gap arising from the charging and discharging processes often hinders the performance of Zn-air batteries. Noble metal-based air electrode produces lower overpotential gap, but their cost and scarcity increase the expenses of their batteries remarkably. Hence, there is an urge to develop catalysts for Zn-air batteries made with cost effective materials.

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CNTs doped with heteroatoms such as N, S, P, and B exhibit enhanced electrochemical properties and excellent ORR, OER, and HER performance. The intrinsic high surface area and porosity of heteroatoms-doped CNTs facilitate the metals’ adsorption and dispersion throughout the CNTs surface. Several studies reported the application of metal supported on doped CNTs as air electrode catalysts for Zn-air batteries. Lai et al. constructed bifunctional oxygen electrode electrocatalysts of Ni3Fe(nanoparticles)/N, S-codoped CNTs/graphene nanosheet composites electrodes with improved OER and ORR electrocatalytic performance compared to commercial Ir/C and Pt/C catalysts, respectively, and used them as oxygen electrode catalysts for Zn-air battery (Lai et al. 2018). A schematic diagram of the developed Zn-air battery is shown in the Fig. 11a. At first, CNT/graphene nanosheet composite was prepared by partial exfoliation of MWCNT which produced graphene nanoribbons attached to the inner walls of the MWCNT. Then pyrolyzed at 900  C to produce N,S-codoped CNTs in the presence of thiourea which acted as dual precursor for both N and S. Dispersed Ni3Fe alloy nanoparticles were adsorbed on the N, S-codoped CNTs by impregnation and hydrogen reduction. The developed catalysts showed excellent catalytic properties for both OER and ORR by having significantly lower overpotential of 215 mV at 10 mA cm2 for OER and half-wave potential of 0.877 V for ORR. In rechargeable Zn-air battery, the Ni3Fe/ N, S-codoped

a

b

c

d

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Fig. 11 (a) Schematic diagram of Zn–air battery where air electrode is made with Ni3Fe/ N, S-doped CNTs/graphene nanosheet composite, (b) Ni3Fe/N–S–CNTs, Ni3Fe/CNTs, Ir/C and Pt/C catalysts anodic/cathodic polarization curves of Zn-air battery and (c) galvanostatic charge/discharge cycling from 450th to 500th of Zn-air batteries using Ni3Fe/N–S–CNTs as the catalyst. (Reprinted with permission (Lai et al. 2018) Copyright © 2018 American Chemical Society). (d) Schematic illustration of three electrode battery made with CoO/N-CNT as ORR catalyst and NiFe LDH as OER catalyst, (e) charge/discharge polarization curve showing the comparison of three electrode batteries with Pt/C and Ir/C developed Zn-air batteries, and (f) cycling stability study for the batteries made with three electrode system, Pt/C and Ir/C. (Reprinted with permission (Li et al. 2013b) Copyright © 2013 Nature Publishing Group)

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CNTs/graphene nanosheet composite bifunctional oxygen electrode displayed a superior bifunctional performance with (i) high power density (180.0 mW cm2) > > Pt/C–Ir/C > N–S–CNTs > Ni/N–S–CNTs > Fe/N–S–CNTs > Ni3Fe/CNTs (77.9 mW cm2), (ii) lowest voltage gaps as indicated in the discharge/ charge curves, Fig. 11b, (iii) large specific capacity (718 mA h gZn1), and (iv) high cycling stability and durability (500 discharge/charge cycles over 500 h period with a slight change in the discharge/ charge overpotential), Fig. 11c (Lai et al. 2018). Jin et al. have synthesized Fe2P/N, P-codoped CNTs derived from ZIF-8/LiFePO4 and investigated their performance in Zn-air battery as oxygen electrode (Jin et al. 2020). To synthesize the Fe2P/N, P-codoped CNTs catalyst ZIF-8 and LiFePO4 were mixed and calcined at different temperatures in inert atmosphere. The MOF ZIF-8 served as precursor for N and LiFePO4 as the Fe and P precursor. The doped CNTs cathode electrode in Zn-air battery exhibited (i) higher ORR performance than commercial Pt/C (ii) produced a high peak power density of 140 mWcm2, 30 mWcm2 higher than commercial Pt/C electrocatalyst, and (iii) excellent stability and durability for 10 h. Fu et al. developed an interesting human hair like 3D nanostructured air electrode for rechargeable Zn-air where Co3O4 nanoparticles supported on N-doped CNTs (Co3O4-N-doped CNTs/SS) was used as a bifunctional oxygen electrode catalyst (Fu et al. 2016). To synthesize this unique catalyst, at first, iron oxide a catalyst for the N-doped CNTs growth was electrodeposited on the surface of stainless-steel (SS) mesh. The N-doped CNTs were grown directly and vertically by CVD process. To the vertically aligned N-doped CNTs, Co3O4 nanopetals were dispersed by electrodeposition to produce Co3O4-N-doped CNTs/SS. This doped CNTs electrocatalyst performed better as bifunctional catalyst for ORR and OER than commercial Pt/C and Ir/C catalysts. The authors developed a solidstate thin film Zn-air battery where Co3O4-N-doped CNTs/SS was used as air electrode and cellulose film gelled with 6.0 M KOH as the solid electrolyte. This battery showed lower overpotential in the discharge/charge polarization curves than the battery prepared with Pt/C and Ir/C. Moreover, this Co3O4-N-doped CNTs/SS Zn-air battery produced high power density (160.7 mWcm2), specific capacity (652.6 mAh g-1-), and large gravimetric energy density (847.6 Wh kg1) which are significantly higher than the commercial catalysts modified batteries. Besides that, the Co3O4-N-doped CNTs/SS Zn-air battery also showed excellent cycling stability and durability over 600 h at 25 mA cm2 current density. The high performance shown by the Co3O4-N-doped CNTs/SS Zn-air battery was attributed to the synergistic effect of Co3O4 and N-doping of the CNTs and the large number of electrocatalytic active sites provided by the vertically aligned CNTs structure which assures higher power generation (Fu et al. 2016). Li et al. constructed three-electrode system Zn-air batteries using a bifunctional oxygen cathode catalyst consisting of CoO nanoparticles deposited on N-doped CNTs to catalyze the ORR process and NiFe-layered double hydroxide (DLH) for the OER process (Li et al. 2013a, b; Khalafallah et al. 2022). These catalysts showed excellent catalytic activity and cycling stability in primary Zn-air battery with discharge peak power density of 265 mWcm2, current density of 200 mAcm2 at 1 V, and energy density of 700 Wh kg1. The performance of CoO/N-CNTs-NiFe DLH Zn-air battery was

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significantly higher than the Zn-air battery constructed with commercial Pt/C and Ir/C. A schematic of the primary/three electrodes Zn-air battery, charging/ discharging polarization curves, and cycling performance are presented in the Fig. 11d–f, respectively.

Conclusions Exploring nonprecious metal-free electroactive materials with reasonable cost, high activity, and exceptional durability for energy storage and conversion applications has been considered as one of the most competitive aims within today’s scientific work. The challenges are not only associated with the increasing demands to replace conventional fossil fuels-based energy resources, meanwhile, a wide range of sustainable prospects already exist, these necessitate require the design of high performance compounds that are capable of coping with specific challenges. Researchers have put a robust interest in heteroatom-doped carbon materials. Heteroatom doping can significantly alter the physicochemical properties compared with those of crude carbon materials. The by far most extensively used alternatives with various heteroatoms are CNTs. Heteroatom-doped CNTs have tremendously advanced in the past few years, especially by revealing their usefulness as electrode materials in supercapacitors and batteries or electrocatalysts for ORR in fuel cell cathodes. The inserted heteroatoms can yield synergistic effects and cause more structural defect sites or preferable spin densities inside the carbon framework because the electronegativities of carbon and dopants are closer to each other. Alongside, the spectrum of doping has been extended and novel doped CNTs have been introduced, proposing the attractive potential of these materials in the section of sustainable forms of energy recovery. This chapter addresses the recent updates on the emerging growth of studies of heteroatoms doped CNTs-based noble-metal-free electrocatalysts/electrodes for energy storage and conversion systems in recent years with a strong emphasis on phenomenological knowledge. This may also appeal to the scientific communities who are focusing on nanomaterials and nanotechnology and electrochemistry. Therefore, we look forward to seeing breakthroughs in the understanding of heteroatoms mechanism and their fundamental roles in contributing to electrocatalysis and energy storage. We hope that future advancements can provide a conceptually different scope to this topic.

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Vindhyasarumi, Akhila Raman, A. S. Sethulekshmi, Saritha Appukuttan, and Kuruvilla Joseph

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of CNT Hybrid Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrothermal Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Vapor Deposition Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sol-Gel Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hybrid Materials Based on CNT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNT–Inorganic Material Hybrids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNT-Two Dimensional (2D) Material Hybrids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CNT-Biomaterial Hybrids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other CNT Hybrids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electronic Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy Storage Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sensing Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biological Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The distinctive structure and properties of carbon nanotubes (CNTs) have resulted in enormous experimental and theoretical studies based on CNTs and its associated materials. The discovery of exceptional physical and chemical properties of CNTs has made them appropriate for many applications such as energy storage, thermal materials, fabrics and fibers, and catalyst support. One of the most Vindhyasarumi · A. Raman · A. S. Sethulekshmi · S. Appukuttan (*) Department of Chemistry, Amrita Vishwa Vidyapeetham, Amritapuri, Kerala, India K. Joseph Department of Chemistry, Indian Institute of Space Science and Technology, Valiyamala, Kerala, India © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_69

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attractive features of CNTs is their ability to encase atoms or molecules in their hollow space in the center, thus creating CNT-based hybrid materials with novel structures and properties. Nowadays, CNT-based hybrid materials are gaining significant interest in the research field due to the production of advanced materials with a combination of properties such as mechanical, electrical, and thermal. This chapter envisages a comprehensive discussion about the synthesis, properties, and applications of various carbon nanotube-based hybrid materials. Keywords

Carbon nanomaterials · Carbon nanotubes · CNT hybrids · Synthesis · Applications

Introduction Nanomaterials are attractive species for numerous applications such as biomedical, energy production, electronic devices, environmental protection, and sensing. Great efforts have been done for the fabrication of structurally uniform nanomaterials with well-controlled interfaces and surfaces and their use in various devices. Carbon is a very special element among numerous chemical elements due to its large number of bonds of various strengths and type (Silva 2008). It exhibits various allotropic forms and has the ability to produce many nanostructures such as graphene, CNT, fullerenes, nanodiamonds, carbon nano onions, carbon nanocones, and carbon nanohorns. Carbon nanomaterials have gained significant research interest in the past few years (fullerenes (1985), CNT (1991), and graphene (2004)) due to their excellent mechanical, thermal, electrical, optical, etc. properties. Carbon nanomaterials such as two-dimensional (graphene), one-dimensional (1D) (CNT), and zero-dimensional (fullerenes) are composed of sp2 bonded graphitic carbon. Usually, carbon nanomaterials possess size ranging from 1 to 100 nm. The properties of all these carbon nanomaterials are fully associated with their interactions with other materials as well as the atomic structures. The outstanding properties of carbon nanomaterials makes them excellent candidates in many fields such as material science, medicine, environmental science, biology, and energy production and storage. Figure 1 represents the various carbon nanomaterials. Among various carbon nanomaterials, CNTs are considered to be an eminent material in most of the fields in science as well as engineering due to its unique one-dimensional structure, excellent mechanical, electronic and magnetic characteristics, ultralight weight, and good thermal and chemical stability (Eatemadi et al. 2014). It was discovered by S. Iijima, a Japanese researcher in 1991. CNTs are cylindrical in shape with diameters ranging from 1 to 100 nm and which contains rolled graphene sheets. It holds greater aspect ratio, low mass density, and high flexibility. CNTs may vary in chirality, length, number of layers, and diameter. CNT is a strongest material and its tensile strength is greater than that of steel. As a result of these exciting properties, CNTs are promising candidates in numerous fields:

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Fig. 1 Carbon nanomaterials: (a) graphene, (b) CNT, (c) nanodiamonds, (d) fullerenes, (e) carbon nanoonions, (f) nanocones, and (g) nanohorns

sensors, electronics, energy storage and conversion devices, conductive composites, biocatalysts, biomedical, etc. Functionalization of CNT surface greatly enhances its solubility and thereby broadens its applications. CNT is excellent nanofiller for the preparation of nanocomposite with improved electrical, thermal, and mechanical properties. Moreover, CNT can disperse in various polymers and impart reinforcing effects on the polymer matrices. The rising demand of novel substances with tailored properties has forced hybrid systems to a place of fame in material science by advantage of their incredible multifunctionalites and new properties. Hybrid materials are obtained by the combination of two or more compounds. As a result, the intrinsic properties of the individual components are combined to give additional properties because of the synergistic effects among the components. Properties of hybrid materials can be tuned by varying their morphology and composition, thereby obtaining an advanced material with improved mechanical properties, thermal stability, gas permeability, light emission, and electron conductivity (Soliveri et al. 2012). A number of studies have been published on the synthesis of hybrid materials. Among them, CNT hybrid materials are one of the well-known hybrid materials that display not only the properties of CNT and other components but also additional novel chemical and physical properties owing to the interactions between CNT and other attached particles. This chapter begins by outlining the various synthesis methods of CNT hybrid materials. Then it discusses about different types of CNT hybrid materials in a

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comprehensive way. This chapter ends with the discussion of potential applications of CNT hybrid nanomaterials in electronics, energy storage, sensors, biological, and some other novel applications.

Synthesis of CNT Hybrid Materials Hybrid material can exhibit various chemical and physical properties from its individual components in a uniform way. Hybrid materials based on CNT is a well-studied system and it can be prepared by various methods such as hydrothermal, sol-gel, and chemical vapor deposition (CVD) methods. Geometry of substrates is not much significant in the case of CNT preparation. But while considering the development of CNT hybrids, the substrates will remain connected to the CNTs. Thus, in this case the geometry and type of substrate chosen is very important as it influences the properties of developed hybrids. This section discusses the various methods employed for the synthesis of CNT hybrids.

Hydrothermal Method Hydrothermal method or solvo-thermal method refers to the synthesis of substances through chemical reactions at above ambient temperature and pressure (Li and Liu 2010). Here, stainless steel autoclave is used for the heating of aqueous solution of precursors above the boiling point of water, thus the pressure within the autoclave vividly raised above the atmospheric pressure. The synergistic effect of higher pressure and temperature delivers a one-step method for the synthesis of highly crystalline compounds without the necessity of post-annealing method. This method is also useful in nanomaterial synthesis. Solvent, precursor’s type and concentration, reaction time and temperature, solvent and stabilizing agents are the reaction parameters which influence the product formation. As a result of the high pressure and temperature conditions, this method gives highly crystalline magnetic nanomaterials, but the product yield is very less compared to co-precipitation methods. Dai et al. (2013) adopted hydrothermal method for fabricating single-walled CNT (SWCNT)-titanium dioxide (TiO2) hybrid in which TiO2 nanoparticles (NPs) were grown on the surface of functionalized SWCNTs. The characteristic X-ray diffraction (XRD) peaks of SWCNTs were absent in SWCNT-TiO2 hybrids, which indicated the better dispersion of SWCNT in the hybrid system. Intimate contact between the TiO2 NPs and well-dispersed SWCNT helped to prevent the TiO2 grain growth and to enhance the hybrid thermal stability. In another research work, hydrothermal method was used for the synthesis of TiO2/multi-walled CNT (MWCNT) catalyst composite and evaluated the composites performance in acetaldehyde degradation (Everhart et al. 2020). In another work, CNT/iron oxide (Fe3O4) hybrids were fabricated through a simple hydrothermal method after the treatment of CNT with nitric acid (Guan et al. 2013). Large surface area of the CNTs was completely utilized through the in situ anchoring of Fe3O4 NPs on the CNTs surface.

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Scheme 1 The formation mechanism of CNT/Fe3O4 composites (Guan et al. 2013). (Reproduced with permission from Elsevier)

Poorer cycling stability and electrical conductivity of the Fe3O4 NPs was improved by combination of Fe3O4 NPs with high conductivity and chemical stability of CNT, which lead to the effective utilization of redox characteristics of Fe3O4. Scheme 1 illustrates the CNT/Fe3O4 hybrid. CNT/cobalt oxide (CNTs/Co3O4) core-shell one-dimensional (1D) nanocomposites were hydrothermally synthesized by Wang and coworkers (2009). A layer of cobalt oxide NPs was uniformly coated on the CNT surface (Fig. 2) and thus a core/shell 1D nanostructure was obtained. Pretreatment of CNT with concentrated nitric acid helped to functionalize the CNT surface with carboxylic acid (-COOH) groups and then Co2+/3+ ions were reacted with the COOH groups. Co3O4 nanocrystals were grown on the CNTs surface under alkaline and hydrothermal conditions. A hybrid system consisting of 1 T phase tungsten disulfide (1 T-WS2) and SWCNT (1 T-WS2/SWCNT) was prepared through hydrothermal method by Piao et al. (2018). Here, they mixed SWCNT water dispersion into a water solution containing ammonium tungsten oxide hydrate; thiourea and oxalic acid dehydrate (catalyst). After that, they transferred this mixture into an autoclave for 48 h at 220
C. Finally, the obtained black precipitate was centrifuged, washed, and dried at 60
C. A ternary composite consisting of MWCNTs, graphene oxide (GO), and Fe3O4 (GCF) was developed through one-pot hydrothermal method (Long et al. 2017). In this work, Fe3O4 NPs were performed as spacers to inhibit the GO sheet stacking. Carboxylated MWCNTs were mixed with GO dispersion and they obtained a GO/MWCNT framework. To this, they added ferrous sulfate heptahydrate followed by sodium hydroxide solution. After 1 h room temperature stirring, they added 3.2 g urea to the reaction mixture and then allowed for hydrothermal reaction in an oven for 12 h at 180
C. In another study three dimensional (3D) reduced grapheme oxide (rGO)/MWCNT hybrid (3DrGO-MWCNTs) was synthesized by hydrothermal method (Rajesh et al. 2019). Wet impregnation method was adopted for modifying the hybrid surface by cerium oxide (CeO2) and obtained CeO2/3DrGOMWCNTs nanocomposites. Into these nanocomposites, palladium NPs was dispersed by chemical reduction method. Under hydrothermal conditions, graphene nanosheets were inserted by MWCNTs through weak Van der Waals interactions which helped to

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Fig. 2 (a) FEG-SEM image of pristine MWCNTs; (b) FEG-SEM image of CNTs/ Co3O4 core-shell nanocomposite (Wang et al. 2009). (Reproduced with permission from Elsevier)

prevent the grapheme re-stacking and led to increment in specific surface area and conductivity. Chen et al. (2013) introduced an eco-friendly and simple one-step approach for the fabrication of reduced grapheme oxide/CNT/α-Nickel hydroxide (α-Ni(OH)2) composites through hydrothermal method.

Chemical Vapor Deposition Method Chemical vapor deposition is one of the well-known chemical methods for producing high-performance, high-quality solid materials. This method is widely used in many industries. In this method, the precursors undergo reaction or decomposition onto substrate surfaces and the desired product is deposited on the substrate surfaces. Graphene-CNT hybrids were developed by one-step CVD method using ethanol as a precursor (Dong et al. 2011). Here, graphene-CNT hybrids were grown on copper foil which was decorated by silica (Si) NPs. Graphene film uniformly grows on the substrate; however, CNTs germinate from the silica nanoparticles to develop a network on the top (Fig. 3). In a different study, graphene-CNT (G-CNT) hybrids were developed through CVD method with the help of a mixed catalyst (magnesium oxide (MgO) layer and iron (Fe)/MgO catalyst) (Zhu et al. 2012). MgO layers performed as a template for graphene growth whereas CNT growth was catalyzed

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Fig. 3 Schematic of one-step growth of graphene–CNT hybrid materials by CVD on Si NPs pre-coated copper foil (Dong et al. 2011). (Reproduced with permission from Elsevier)

by the iron particles on the magnesium oxide layers. Because of the composition of the CNT and graphene network, the obtained G-CNT hybrids exhibited porous structures containing hierarchical pore distributions. With the help of a two-step CVD method, Zhang et al. (2015) introduced the synthesis of 3D CNT/graphene hybrid. Here, they initially synthesized graphene on copper foil through CVD approach, and the obtained graphene was shifted onto Fe/aluminum oxide/silica dioxide (SiO2)/Si substrates which led to the development of CNT/graphene hybrid materials. Graphene films were strongly bonded with CNT arrays and thereby formed a jellyfish structure. Furthermore, their results confirmed that, existence of graphene affected the precipitation and growth rate of CNTs. CNT precipitation was postponed because of the presence of graphene. Interestingly, CNTs average growth rate for the entire process was faster in graphene region when compared to region without graphene.

Sol-Gel Method Sol-gel method is one of the most favorable techniques for nanomaterial synthesis such as nanocomposites, nano-powders, and nanocoatings with numerous functionalities. Metal organic compounds or inorganic metal salts are used as a starting material in sol-gel process. Sol or colloidal suspension is formed due to the series of polymerization and hydrolysis reactions of precursors. The obtained sol can be processed into various forms like fibers, monolith, monosized powders, and films. This method acquires great interest due to its mild processing conditions like low pressure and temperature. Hence, sol-gel method is an innovative path for the preparation of CNT-based hybrid materials. In order to improve the thermal stability of MWCNTs, TiO2 was coated on the MWCNT surface through sol-gel method (Rodríguez et al. 2017). Titanium tetra isopropoxide (TTIP) served as a precursor in this sol-gel treatment. Hsu et al. (2017) prepared poly(lactic acid) nanocomposites in which MWCNT-grafted silica nanohybrids were used as reinforcing agents. Here, Si NPs were formed on the surface of MWCNT via sol-gel method, thereby improving the compatibility between inorganic Si particle and MWCNT. Here, MWCNTs were functionalized using different groups such as acyl chloride, hydroxyl groups, -COOH and amine and dispersed in tetraethoxysilane (TEOS) via ultrasonication. MWCNT-grafted silica hybrid was obtained after gelation in which silica particles are well-dispersed on the modified

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MWCNTs. The obtained hybrid systems have many unique features such as controllable and specific morphologies and the ability to host molecules with different shapes, sizes, and functionalities. Montakhab et al. (2012) selected inexpensive precursors like sodium silicate and MWCNT for the production of SiO2-CNT hybrid via sol-gel method. In this method, MWCNT were uniformly distributed in the silicone matrix through the mixing of functionalized CNT with silicic acid followed by the titration with sodium silicate solution at a temperature of 800
C. They obtained a mesoporous structure hybrid with high surface area and high thermal conductivity when compared with MWCNTs. They used various ratios of MWCNTs and different concentrations of colloidal silica for the production of hybrid material. The SiO2-CNT hybrid having different morphologies with various precursor ratio was studied through scanning electron microscopy (SEM) and X-Ray diffraction (XRD).

Hybrid Materials Based on CNT It is evident from the above passage that CNTs are an emerging class of technological materials with unique intrinsic properties modulated by chirality along with the graphene sheet and either semiconducting or metallic electronic states are created. These nanomaterials can be functionalized with organic or inorganic molecules using covalent and noncovalent interconnections known as hydrogen bonding, π-π stacking, electrostatic interactions, and van der Waals forces, thereby providing more selective interaction with analytes. Some remarkable mechanical, thermal, electrical, and optical properties support CNTs as perfect candidates for hybrid formation with multifunctional applications in nanotechnology, electronics, optical, biomedical, environmental remediation, and sensing field.

CNT–Inorganic Material Hybrids CNTs have attained great importance on account of their mechanical, chemical, thermal, and electronic properties. Multifaceted nanohybrids have initiated various opportunities for a vast range of biotechnology and catalysis applications. Uniting inorganic nanoforms with CNTs is considered as a potential tool in developing new inorganic-organic hybrid structures having advantageous properties and applications in various fields like catalysis, energy storage, biomedical field, and sensing. The inert chemical surface of CNTs paves ways to develop different methods including noncovalent and covalent assembling mechanisms to build inorganic-CNT hybrid materials. By studying the chemistry of CNT hybrids, their properties can be operated according to the applications. In the past few years, the researches based on CNT are more concentrated on CNT–inorganic hybrid materials because of their tunable properties and attractive applications. The tunable properties are accredited to the charge transfer process through the interface of CNT-inorganic materials and their combined effects.

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Michele Melchionna et al. (2018) developed Fe-filled CNT hybrids, which can be potentially used as a nanocatalyst support for water-gas shift reaction and the photocatalytic production of hydrogen. Magnetic separation method was employed in the mechanism to produce effective homogeneous CNT/nanocatalyst (Fe@CNTs/ Pd@MO2-magn) materials. Palladium incorporated metal oxide (Pd@MO2-mag) was chosen as the nanocatalyst. A combined effect of Fe-filled CNTs (FE@CNTs) and the active phase (nanocatalyts) arising due to the homogeneity in carbon/ inorganic interface results in the high functionality of Fe-filled CNT hybrids as nanocatalyst support. Furthermore, the magnetic Fe core present in the CNT hybrid allows efficient separation of nanocatalyst and can be reused without any activity loss. The work reveals the potentiality of magnetic CNTs as a new age feasible catalyst support which can enhance the generation of hydrogen and lower the use of valuable metal catalysts. A graphical illustration of the synthesis of Fe@CNTs/ Pd@MO2-magn is shown in Scheme 2. Alicia Moya et al. (2015) developed a distinctive way to produce hierarchical gathering of mesoporous CNT/TiO2 photocatalysts by uniting sol-gel and electrospinning methods. The fabricated samples of TiO2 have shown mesoporous network structure with uniform connections and excellent crystalline nature. They have exhibited excellent photocatalytic activity compared to individual TiO2 nanoparticles. Combining TiO2 with Multi-walled Carbon nanotubes internally to develop CNT/TiO2 hybrids improved the photoactivity to a great extent. Xing Zhang et al. (2018) synthesized molybdenum phosphide (MoP)/carbon nanotube CNTs hybrid in which MWCNTs are coated evenly with excellently crystallized and small-sized MoP nanoparticles. In this work, they have prepared MoOx(OH)y/CNT precursor from which the MoP/CNT hybrids were synthesized. The hybrid material

Scheme 2 Graphical representation of the synthesis of Fe@CNTs/Pd@MO2-magn. (a) Dispersion of the already prepared hybrid and (b) liquid phase containing non-magnetic impurities after the separation of magnetic solid using magnet (Melchionna et al. 2018). (Reproduced with permission from Elsevier)

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Fig. 4 Diagrammatic illustration of the development of the MoP/CNT hybrid (Zhang et al. 2018). (Reproduced with permission from Wiley)

has shown outstanding efficiency in catalysis for hydrogen evolution reaction (HER). From the studies, it was proved that the developed hybrid is one of the best catalysts for HER. The outstanding catalytic activity on HER is mainly due to MoP crystallinity. Additionally, the inclusion of CNT remarkably lessens the sintering and agglomeration of nano MoP particles because of the good interconnections between CNT and metal ion. Diagrammatic illustration for the development of the MoP/CNT hybrid is shown in Fig. 4. Highly potential three-dimensional ZnO-CNT hybrids were synthesized by Xingbin Yan et al. (2007). The fabricated hybrids were expected to be a propitious material for various applications including liquid transfer, surface engineering, etc. as it exhibits revisable surface wetting properties in between superhydrophobicity and superhydrophilicity. The CNTs were first coated with poly (sodium 4-styrenesulfonate) (PSS), which is a polyelectrolyte. The PSS coating on CNT imparts negative charges on each tube making it capable of stabilizing the capillary force, thus forming unvarying pyramidic structure. These uniform CNT patterns were used to develop a highly functional complex structure containing ZnO nanorods in a framework/template design. Syed Rahin Ahmed et al. (2016) fabricated CNTs/gold nanoparticles (Au NPs) nanohybrids. From ultraviolet-visible (UV-vis) spectra and transmission electron microscopy (TEM) images, it was confirmed that the CNT surfaces are bonded with Au NPs. These nanohybrids (Au/CNT) exhibit excellent catalytic activity for oxidizing 3, 30 , 5, 50 -tetramethyl-benzidine (TMB) using hydrogen peroxide (H2O2) and generated a novel blue color solution in water. Its catalytic activity employed them in a sensitive calorimetry test for detecting influenza A virus (H3N2). The presence of H3N2 virus in the specific antibodyconjugated Au/CNT nanohybrids-TMB-H2O2 system develops a color of deep blue. The optical density of the system depends on the virus concentration. The detection limit of virus via this method (3.4 PFU/mL) was found to be 385 times lesser than the commonly used enzyme-linked immunoassay (ELISA) test (1312 PFU/ml). Moreover, this system is 500 times more sensitive than the commercially available immunochromatographic systems. Thus, CNTs/ Au NPs nanohybrid synthesis and

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detection of virus through calorimetry can have potential applications in nanobiotechnology and catalysis. In composites, CNTs have been employed as potential multifunctional fillers because of its excellent electrical and mechanical properties, which opens a new point of view toward the structural reinforcement and damage sensing in fibrous composites at nano level. Erik T. Thostenson et al. (Thostenson and Chou 2006) reported damage sensing and self-healing properties of CNTs in epoxy. Forming CNT networks in polymer matrix provide remarkable capability to the composites to monitor the composite’s health and detect damages. The electric percolation due to the presence of MCNTs at very low volume percentage permits the development of internal sensors which are not interfering and are not able to considerably change the mechanical properties of the composite. But the process of addition of CNT to the surface of the fiber is very complicated. In order to get a low viscous matrix, the CNT content should be always maintained minimum. Moreover, dispersion of CNT in the matrix should be uniform, otherwise aggregation of CNT will minimize the domination of the matrix’s mechanical properties in the composites. Excellent interfacial adhesion is also needed between CNT/matrix to attain productive load transfer to the matrix from CNT. To meet these specifications, the nanohybrids of CNTs can be added to the matrix instead of pure CNTs, which helps to gain enhanced interfacial properties and uniform dispersion of CNT in the matrix. In addition, the hybrids can also act as multifaceted fillers in the fibrous composites (Ci and Bai 2004). Weikang Li et al. (2014) developed CNT-Al2O3 hybrids consolidated glass-fiber fortified epoxy composites. The presence of a small quantity of CNT-Al2O3 hybrid in the composite acts as internal sensor to observe initiation and propagation of damages while applying mechanical loads by measuring the electrical resistance of the composite. CVD method is used for the fabrication of CNT-Al2O3 hybrids by depositing CNTs on the microspheres of Al2O3. The quasi-static tensile testing was used to compute the composite’s electrical resistance. The internal electrical resistance increases with the increase in the strain till the ultimate failure occurs.

CNT-Two Dimensional (2D) Material Hybrids Nowadays, the nanomaterials based on carbon achieve a significant attraction in the field of biomedicine as they have peculiar biocompatible and physicochemical properties. Xinxin Yan et al. (2016) have studied the interaction of graphene/ single-walled carbon nanotubes (G/SWCNT) with the mesenchymal stem cells of rat (rMSCs). The cell behavior and processes such as adipogenic and osteogenic differentiation was considerably influenced by the hybrid presence. But it did not adversely affect the cell viability of rMSCs. G/SWCNT hybrids helped in inhibiting the adipocytic differentiation, while improving the osteogenic differentiation of rMSCs. Therefore, this work acted a basis for employing G/SWCNT hybrids as functional tissue engineering scaffolds and nanodrug delivery carriers, which would help to improve the formation of bones.

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Graphene and CNTs are excellent conducting fillers because of their beneficial high aspect ratios and electrical properties. Jae Young Oh et al. (2016) have explored the application of these properties of graphene and CNTs to fabricate flexible composite conductors. The developed nanocomposites possessed improved electrical networks as it contained CNT/graphene hybrid. Moreover, the accumulation of graphene and CNTs were inhibited by the usage of this hybrid system as it exhibits synergistic effect. Jeng-Yu Lin et al. (2015) developed a hybrid material (MoS2/ RGO-CNTs) by integrating CNTs with nanocomposites of MoS2/reduced graphene oxide (RGO). The material was synthesized by the method of electrophoretic deposition. The developed materials exhibited attractive application as counter electrode (CE) in dye-sensitized solar cells (DSCs). Various electrochemical tests have led to the conclusion that the consolidation conductive CNT network gives extra electron transport pathway in MoS2/RGO-CNT hybrid material that help to increase the rate of charge-transfer at electrolyte/CE interface. Thus, the fabricated hybrid material has shown enhanced electrocatalytic activity in comparison to MoS2/ RGO nanocomposite. The DSC with the CE of MoS2/RGO-CNTs exhibited attractive cell performance (7.46%) that is greater than DSCs with CE of MoS2/RGO and Pt CE with cell performances of 6.82% and 7.23%, respectively. The photovoltaic properties of the DSCs developed with the MoS2/RGO, MoS2/RGO-CNTs, and Pt CEs are demonstrated in Fig. 5. Gui-Li Tian and his coworkers (2014) have developed a novel N-doped graphene/ single-walled carbon nanotube hybrid (NGSH). The developed hybrid material is an effective bifunctional electrocatalyst, which is free from noble metals for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The synthesis of

Fig. 5 Photovoltaic behavior of the DSC devices consolidated with MoS2/RGO, MoS2/RGOCNTs, and Pt CEs (Lin et al. 2015). (Reproduced with permission from Wiley)

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NGSHs were done via in situ doping at the time of CVD growth on a substrate of bifunctional catalyst derived from layered double hydroxide. This one-step approach helped for the parallel growth of SWCNTs and graphene and thus formed a 3D interlinked network. It also led to the inherent dispersion of CNTs, graphene, and the functional groups containing nitrogen. NGSHs exhibited wide surface area and good electrical conductivity of 812.9 m2g1 and 53.8 S cm1, respectively. The developed hybrid material can be considered as a cost-effective, highly functional catalyst for ORR and OER. This work also paves a new path for energy conversion associated with metal-free, copious, and adaptable catalysts. Highly deformable and receptive chemical sensors synthesized via an economical method will be a rising material because of their degradability and application on complicated and curved structures (Lee and Choi 2019). Even though numerous adaptable sensors having required properties were discovered, challenges were caused by their limitations in mechanical flexibility and chemical sensitivity. Woo Sung Lee et al. (Lee and Choi 2019) have developed a highly flexible chemical sensor with enhanced sensitivity by combining nanolayered transition metal dichalcogenides (TMDCs) with MWCNTs on cellulose paper. An inexpensive, simple, and easy process was chosen to synthesis this sensor. TMDCs and MWCNTs in the form of liquid dispersions got absorbed on the cellulose paper and dried. The substrate, cellulose paper, allows bending and twisting up to 0.25 mm and 1800
, respectively. It also helps to fold and unfold the material three dimensionally and reversibly. The presence of CNTs keeps a network of percolation and gas reactivity. The sensing of NO2 gas was more than 150% after the functionalization using TMDCs (WS2 or MoS2) and this developed material could sense NO2 selectively from a mixture of various gases. This sensor can detect the gas sensitively and constantly even if it undergoes drastic collapsing or folding. Incorporation of TMDCs and CNTs on to the cellulose paper may also help in the detection of various dangerous gases and can have applications in inexpensive transportable instruments which need valid deformability. Hui Zhang et al. (2019) have developed carbon nanotube hybrid materials densely packed with ZIF-8, a metal-organic framework (MOF) possessing a complex structure with pores. The hybrid material was fabricated via in situ growth and drying at room temperature. The porosity and bulk density of the hybrid was optimized by varying the amount of MOF, which resulted in synchronized large volumetric and areal capacities like lithium-sulfur battery electrodes. High volumetric and areal capacities are important factors for fabricating compact energy storage devices with elevated energydensity. The fabricated hybrid (CNTs@ZIF-8) also exhibited outstanding cycling stability and capacity, making them suitable for various applications in supercapacitors and Li-S, Sodium ion, and Lithium-ion batteries.

CNT-Biomaterial Hybrids In biomedical field, especially for fabricating tissue engineering scaffolds, in addition to cell affinity, biodegradation and structural support, beneficial mechanical

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properties, and sufficient electrical conductivity are also required. Scaffolds made from biomaterials like chitin, collagen, fibrin, etc. provide the required biological properties but are not able to offer adequate reproducibility and mechanical properties. By consolidating CNTs with these biomaterials to fabricate scaffolds, the lack of the mechanical characteristics can be resolved to a great extent. Thus, the hybrid materials incorporating CNTs and biomaterials together have gained magnificent attraction. Seokwon Pok et al. (2014) have fabricated CNT-Chitosan Scaffold which is biocompatible and have equivalent conductivity as that of heart. A minute amount of SWCNTs (order of tens of parts per million) which is below the toxic level was consolidated in to gelatin chitosan hydrogel. SWCNTs function as nanobridge for electrical conduction linking cardiomyocytes, which results in improved synchronous beating, cardiomyocyte functioning, and electrical coupling. The fabricated tissues exhibited matching excitation conduction velocities as that of the original myocardial tissues and also act as perfect patch for various cardiovascular defect repairing processes. Subeom Park et al. (Li et al. 2008) have biosynthesized CNT-based 3D scaffold via in situ hybridization of CNTs together with bacterial cellulose (BC), which resulted in an ideal design for tissue-engineering scaffolds. The process was done by culturing a BC-synthesizing bacteria, Gluconacetobacter xylinus, in a CNTs contained medium. The agglomeration of pure CNTs in the medium prevents the uniform hybridization of BC with CNTs. The hydrophilic BC and hydrophobic CNTs have very less binding energy between them; this also reduces the chance of hybridization. In order to resolve these difficulties, CNTs were treated with an amphiphilic comb-like polymer (APCLP). APCLP adsorbed CNT-BC hybrid scaffold (CNT-BC-Syn) exhibited uniformly distributed CNTs all over the microporous 3D structure of BC scaffolds. Comparing to pristine CNT-based BC scaffolds, CNT-BC-Syn scaffolds evinced outstanding osteoinductivity and osteoconductivity which resulted in high bone regeneration efficiency. Dilsat Ozkan-Ariksoysal et al. (2017) engineered a biosensor via wrapping DNA over MWCNT for sensing the presence of Escherichia coli in polymerase chain reaction (PCR) samples and the Staphylococcus aureus (S. aureus) was employed to study the biosensor’s selectivity. The capturing probes selectively detect E. coli DNA and are subjected to the interaction with MWCNTs resulting in single-stranded DNA (ssDNA) wrapping over the nanomaterial. Then, the DNA-wrapped MWCNTs were attached over the disposable pencil graphite electrode (PGE) surface for detecting hybridization of DNA. Differential pulse voltammetry (DPV) and Raman spectroscopy were used to study the electrochemical characteristics of modified PGEs. The results revealed that the developed genosensor could excellently sense DNA of E. coli within the detection time of 20 min with a detection limit of 0.5 pmol per 30 μL volume of sample. The applications of CNT-Biomaterial hybrids are not limited to the field of biomedicine. Jae Ok Ko et al. (2019) have synthesized a paper-type electrode via the interrelated hybridization of CNTs and methyl cellulose. The massive fabrication of this hybrid paper was done through a very cost-effective and simple solution process. The various characteristics of the developed hybrid, such as hydrophobicity of surface, chemical interaction involved between cellulose and CNTs, and structural

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features, were investigated by altering CNT concentration. The hybrid material revealed outstanding mechanical durability under very high stressed conditions also. Crumpling, re-flattening, folding, and refolding of the hybrid paper can be done easily even without considering CNT concentration. Their electrical properties were conserved when it was immersed in water and folded. The fabricated CNT/methyl cellulose hybrid material can be employed as a potential electrode for wearable next generation devices.

Other CNT Hybrids Apart from formally mentioned CNT hybrids, researchers have made various efforts to develop novel CNT hybrid materials with fillers such as dendrimers, polymers, and nanoclay. Nanopeapods are significant hybrid materials of CNT with increasing importance in the current scenario. Nanopeapods were synthesized by consolidating C60 molecules in the core of SWCNTs. The formed structure resembles peas in a pod and thus has several names such as “nanopeapod,” “fullerene peapod,” and “nano tube peapod.” Nanopeapods were discovered accidentally during the synthesis of SWCNTs. C60 molecules were produced as a by-product of SWCNTs production. It got fortuitously encased in the interior of SWNTs while the purification (Smith et al. 1998). Intrinsically conducting polymers got much attention due to the low band gap energy, environmental and thermal stability, lightweight and strong backbone, and easy processability at low cost. Polyaniline (PANI), as an ion conducting polymer (ICP), got much attention owing to the cost-effective monomer and versatile properties such as excellent blending capacity with thermoplastics, semi-flexibility, unique electrical properties, and environmental and thermal stability. Hybrid formation with CNT offers an opportunity for improving the mechanical properties of the polymer, along with introduction of new electronic properties based on interactions between the two materials. Common methods used to generate CNT-PANI hybrids are in situ polymerization, melt intercalation, and polymer intercalation from solution (Yazdi et al. 2019). Jeong-Gil Kim et al. (2021) developed an ideal flexible and conductive electrode by synthesizing a hybrid of polyaniline (PANI)/CNT liquid crystal fibers (CNTF). Sonochemical polymerization method was used to fabricate the hybrid material using electrically conductive CNTFs and pseudo capacitive PANI. This study reveals a novel technique for the development of devices with high capacitance energy.

Applications Hybrid materials perform much better than their components by adopting some individual properties. By efficient maneuvering of the inner structure of a hybrid, researchers can control the physical and chemical properties of the hybrid material. Electrical percolation has significant changes following the morphology and

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dispersion state of the filler particle. CNTs have a large aspect ratio, provide more excellent connectivity with filler particle, and form percolation networks. CNTs with polymers and inorganic nanoparticles as filler components magnify the mechanical, electrical, and thermal properties and form transparent conductive films. Owing to these advantages, CNTs-based hybrid materials are potential candidates in solar cells, FETs, flat-panel displays, switches, supercapacitors, batteries, transparent conductive electrodes, flash memories, spintronics, etc.

Electronic Applications Carbon nanomaterials are perfect materials for electronic applications due to its thermal and chemical stability, high transparency and conductivity, stretchability, and very small contact resistance with organic materials. For fabricating efficient electronic devices, the advanced performance of electrodes is quintessential and can be achieved by tuning morphology, transparency, energy absorption capability, and conductivity, especially for solar cells applications. Indium tin oxide (ITO) was the most popularized transparent electrode lacking brittleness and rupture at hightemperature scenarios, even though the above drawbacks are rectified by CNT-graphene oxide-based hybrids but have high sheet resistance due to the charge percolation process taking place in between CNT and graphene flakes (Shearer et al. 2014). And researches showed that the incorporation of metal nanoparticles improvises the sheet conductance in to next level. In comparison with conventional electrode indium tin oxide (ITO)/glass, Kyu Yang et al. (Yang and Lee 2020) got 94% transparency with shunt resistance (Rsh) of 50 Ω/square for silver nanowire (AgNW)/barrier/polyethylene teraphthalate (PET) and CNT/AgNW/barrier/PET. The performance of this system excels over the other one due to better interconnectivity provided by CNT bundles; here, the spray-coating of carbon nanotube accelerates the formation of well-linked wire with junctions and smooth surface it covers most of the intervening space and reduces charge recombination possibility. Performance of transparent electrode further improved by gold nanoparticle (Au NP)enriched nitrogen (N) (Au:NCNT)- or boron (B)-doped carbon nanotubes (Au: BCNT), respectively, as demonstrated by Lee et al. (2015). For fabricating the bulk heterojunction solar cell, they used poly({4,8-bis[(2-ethylhexyl)oxy]benzo [1,2-b,4,5-b0 ]dithiophene-2,6-diyl}{3-fluoro-2-[(2 ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl}) PTB7:PC70BM donor-acceptor layer and achieved a power conversion efficiency of 9.98% (Fig. 6). This value of power conversion efficiency is the top one ever published for single-junction solar cells based on PTB7. Here Au NPs promote localized surface plasmon resonance that prompt charge generation and dissociation, heteroatom incorporated CNTs promote charge selective transport and efficient local arrangement of organic semiconductors. Small-molecule photovoltaic cells are another emerging class of solar cells materials. Upon going through the literature, the device’s performance connected mainly with the performance and transparency of the electrode that is selected is essential. Incorporating metal nanoparticles with localized surface plasmons enhance its light absorption spectrum in the

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Fig. 6 (a) Schematic representation of chemical interactions for Au:NCNT (left) and Au:BCNT (right) hybrids. (b) Nanometer-scale morphology of Au:BCNTs by SEM. (c) Simulated absorption spectra of PTB7/PC 70 BM layers with Au NPs, BCNTs, and Au:BCNT hybrids. (d) Absorption spectra of above hybrids and e) electric energy density distribution around a polyethylene glycol (PEG)-capped Au NP on a PTB7/PC70 BM/BCNTs layer (Lee et al. 2015). (Reproduced with permission from Wiley online library)

near-infrared range, most of the time; it is very challenging to achieve a material with a broad spectrum of absorption. CNT, a good candidate with tiny size, can pack efficiently in the film with better interconnectivity and morphology decreasing the energy loss. Nanoscale patterning of the hybrid film furnishes high transparency comparable with ITO. Further extension of hybrids in dye-sensitized solar cell (DSSC) as the counter electrode was vital because it made the devices coast effective than conventional platinum-coated ITO/FTO electrodes. Graphene-CNT hybrid electrodes are the most researched area in DSSC with high performance. In DSSC, it can act as both photoanode and counter electrode and are transparent conductive film in the photoanode. CNT-graphene hybrid electrodes have large surface area with conductive networks in 3D space contributing better performance over other electrode and it contributes high stability to the cell. Graphene-nanoribbon-bridged CNTs was used as counter electrodes (Fig. 7) and obtained power conversion efficiency (PCE) of 8.23% instead of conventional platinum electrode, which shows PCE 7.61% for counter electrode under similar conditions (Yang et al. 2013b). Moreover, molybdenum disulfide/reduced graphene oxide (rGO) hybrids with CNTs were fabricated by Jeng-Yu Lin et al. (2015). Here MoS2/rGO and CNTs blend as a counter electrode (CE) for dye-sensitized solar cells. It forms a system with best electron transport and an efficient charge transport rate. The device accrued solar

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Fig. 7 (a) The structure of dye-sensitized solar cells based on R-GONR-bridged CNTs as counter electrode. (b) The mechanism of rapid electron transport in the counter electrode (Yang et al. 2013b). (Reproduced with permission from Wiley online library)

cells performance of 7.46%, which is higher value than DSSCs incorporating MoS2/ reduced graphene oxide counter electrode (6.82%) and platinum counter electrode (7.23%). Instead of using graphene-CNT hybrids as a transparent conductive electrode, its hole transporting property was also exploited for better organic photovoltaic device construction. Perovskite solar cells (PSCs) are another high-performing device made by Xuanhua Li et al. (2018) with a CNT/graphene hybrid. These hybrids coated as hole transport layer (HTL) showed significant thermal stability was measured for 50 h of heat-aging test at 100
C and excellent water stability under atmospheric humidity (30–50% relative humidity). Another work from Lee et al. (Youn et al. 2013) decorated titanium nitride (TiN) nanoparticles on a carbon nanotube-graphene hybrid perform expeditiously as a noble-metal-free counter electrode for QDSSCs employing a polysulfide electrolyte. While constructing organic photovoltaic devices, the electron-hole re-joining process that usually challenges the efficiency of the produced devices was one of the major concerns. This process largely deactivates the conversion of the absorbed photon into energy. Here, the CNT-Graphene hybrid can act as an electron absorber and charge transporter and adjust their film thickness accordingly to better charge conversion without recombination problems (Shearer et al. 2014). Field-effect transistors (FET) operate using an electric field effect and contain a semiconductor channel with electrodes at either end known as the drain and the source. FET is popularly used in sensors for biological, environmental remediation, agriculture, and national security applications because it seeks lower power levels than integrated chips (ICs) (Park et al. 2021). CNT-based materials are widely applied in FET devices, especially with variant organic or inorganic components. These advantages include fast and accurate sensing, low energy consumption, noise reduction, high conductivity, and flexibility. CNT-metal or metal oxide hybrids are effectively used to detect gases due to conductivity fluctuation when gas molecules interact with these nanomaterials. FET sensors with hybrids are effective for detecting chemicals, biologically relevant materials with sensitivity. Work from Seyoung Oh et al. (2018) demonstrated the low power switching effect of

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field-effect transistor device’s integration based on aluminum-doped zirconium hafnium oxide (ZrHfO2) with CNT. Switching was mainly due to the oxygen vacancy defect created charge trapping–detrapping process. Large surface area provided by CNT molecule escalates the adsorption and desorption of oxygen molecule on network of CNTs. The main obstacle to commercialize FET related to flexibility, reproducibility, and cost graphene-based systems crossed this barrio to some extent. In order to use OLEDs inside the houses and buildings, flexible or elastic displays and light emitting diodes (or electroluminescent devices (ELDs)) are particularly interesting; here, CNTs are promising nanomaterials to employ in these devices. The applicability of pristine CNTs as electrodes is limited. The major challenges faced by SWCNT devices include poor adhesion, direct of CNT emitters on a polyimide substrate helps to solve this problem to a greater extent (Park et al. 2013). Selfassembling of nanomaterials in a desired fashion to get maximum output got pivotal importance in a device aspect. Bonifazi et al. (Maggini et al. 2013) reported hydrogel with luminescent nature in which magnetically oriented europium (Eu (III))multiwalled CNTs hybrids exhibit anisotropic emission characteristics. Poly (N-isopropyl acryl amide) hydrogel was mainly selected because of its thermoresponsive, temperature dependent transparency that gives license for stimuli responsible “switch-on/switch-off” control of the composite emission properties. Supramolecular assembly of CNTs with organic or inorganic nanoparticles and polymer materials provides a wide selection of hybrid materials with inordinate structural, electrical, and device characteristics.

Energy Storage Application Supercapacitor Supercapacitors have an efficient hand in solving energy crises due to excellent energy conversion and storage efficiency. High-energy storage capacity, relatively short time for delivery, and longer lifetime are the benefits offered by them in advanced energy applications. Depending on the mechanism of energy storage, they can be classified as electrochemical-double layer capacitors (EDLCs), which follow the adsorption of ions to store energy, and pseudo-capacitors, which use fast surface redox reaction to store energy. The capacitance of supercapacitors is related to the performance of electrode materials; advancement in those materials is necessary to shoot up the amount of gathered energy without compromising the power density, which have energy density greater than the conventional dielectric capacitor (Shearer et al. 2014). Here, CNT-based materials have a prime hand because of their nanometer size and large surface area. They impart numerous structures, differing in thickness, length, numbers of layers, and types of helicity, that help to improve interaction with inorganic or organic materials, which improvise its inherent electrical, thermal, mechanical, and optical properties into next level (Zhang et al. 2017). Crossing of CNTs and graphene (G) or graphene oxide sheets exhibits better performances than individual CNTs, G, and GO. Therefore, they are extensively

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used for developing supercapacitors. Fan et al. (2015) studied and developed hybrid paper on supple freestanding 3D graphene-carbon nanotube doped with porous nitrogen paper (p-N-GC)-based supercapacitor with high specific capacitance value. The elevated electrochemical performance is imputed by the synergistic effect of the three-dimensional porous nanostructure. Moreover, effective CNT intercrossing and N-doping propound that p-N-GC has tremendous potential in electrode materials. Another pioneering work from the Ke-Ning Sun et al. (Yang et al. 2014b) introduced three dimensional architecture of graphene nanosheet (GN)-CNT hybrid supercapacitor electrode with a maximum specific capacitance of 413 F g1). The electrical conductivity of graphene and CNTs are almost comparable. The benefit obtained from the compiling of CNTs is that it contributes high diffusion than graphene and high surface area; doping with semiconductor material or nanoparticle further increases its morphological features, enhancing charge density value. Here resultant hybrid has open porosity than other systems and crafted the growth of CNT vertically by adopting the one-pot pyrolysis synthetic method, which largely contributed to the highest performance provided by surface area of 903 m2 g1 with significant durability. Other promising system include polymer-CNT composite by Yang et al. (2014a) in which polyaniline (PANI) was deposited on a single graphitized multiwalled carbon nanotube (GMWCNT) by in situ chemical polymerization reaction. The performance of the above system was compared with other hybrid systems containing GMWCNTs grafted poly(4-vinyl pyridine). Close observation of this type of system reveals the truth that the morphology of the obtained thin film of hybrid plays a crucial role in the performance of the devices. After selecting polymer as matrix or substrate, it was found to plasticize the material so that the material could be used as wearable or mobile devices without losing mechanical robustness and conductivity. Metal oxides or hydroxides are excellent pseudocapacitive materials and have great potential in energy storage applications like those that supercapacitors (Shearer et al. 2014). Mixing graphene-CNT or CNT directly with other materials helps to form hybrid materials with unique electrical and mechanical properties. Among these, manganese dioxide coated carbon nanotubes received attention, which was used for high geared super capacitor electrode (Liu et al. 2014). Moreover, layered double hydroxides (LDHs) are a potential candidate for future-generation supercapacitors, in which electrochemical-double layer capacitors and pseudocapacitance are achieved due to their rich electrochemically active sites and slabs (Yang et al. 2013a). Examples of such system includes anionic clays or layered double hydroxides (LDHs)/hydrotalcite-like materials are layered solid materials, intrinsically positive changed layers with ions that balance the interlayer (Yang et al. 2013a). The upgrade in device fabrication methodology is essential to get electrodes with excellent performance, so nanomaterials with controlled porosity and interconnectivity are essential for electrodes with a low volumetric area and high conductivity. CNT-graphene systems are well in this by growing CNTs in the middle of graphene layers with chemical bonds connects the carbons.

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Batteries Batteries are the coherent energy source that assists in solving our energy needs by storing energy and make available for use when there has a shortage. Modern technology promotes the utility of flexible battery devices; hence, it can be confidently used in small portable devices without compromising performance. Lithiumion batteries (LIBs) have supremacy in this category owing to their high power and energy density (Shearer et al. 2014). The device architecture of LIBs contains mainly three parts: lithium (Li+) intercalation cathode and anode and electrolytes. Excursion of Li+ions from the cathode to anode occurs during charging, and opposite movement occurs when discharging. Graphite materials are generally used in anodes has excellent recyclability, good capacity, and low redox potential. Using them in high power devices requires even more capacity and charge/discharge properties; here, nanocarbon materials like SWCNTs have 1000 mAh g1 capacity. But the releasing of intercalated ions is not following a uniform rate, and it opens some voids in CNTs lacks its efficiency; here, CNT-graphene hybrid materials are a good choice (Shearer et al. 2014). A direct growth of MnO2 onto fabrics of carbon nanotube fibers shows aligned nanocrystals with their central axis at right angle to the bundles of CNT and assembled into semi-spherical structures showing remarkable specific capacity over 1100 and 500 mAh/g at a discharge current density of 25 mA/g and 5 A/g, respectively (Rana et al. 2019). The flexibility of LIBs enhanced by screwing the advantage of polypyrrole probed by Sun et al. (Fan et al. 2014). Here, they developed self-weaving CNT–lithium niobate (LiNbO3) nanoplate– polypyrrole (PPy) hybrid for LIBs anode. In CNT–LiNbO3 nanoplate–PPy electrode, the charge conducting polymer can bind the CNT networks, which strengthen the flexible electrode and provide electrical connection between them. Moreover, CNTs are provided with enlarged surface area to help anchor polymers or metal over it, and high mechanical stability and conductivities are unavoidable.

Sensing Applications Sensors are significant in the biological field, food industry, environmental remediation, agriculture, and defense application. They have a prominent hand in detecting different components. At the same time, the highest goal and the challenge in sensing field is to manufacture sensors that can detect biomaterials in the presence of a variety of substances with accuracy and precision at micro levels with sufficient sensitivity and selectivity. CNT-based nanoscale hybrids have been intensively studied for biological interface applications. These materials have magnificent potential as sensors owing to their high surface area to volume ratio that allows high susceptibility and high temperature withstanding capability, which makes them applicable in various environments. Nevertheless, they can be easily modified to inculcate selectivity.

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Sensors for Environmental Monitoring Spotting of chemicals that are extremely damaging to environmental and human health in the form of both liquid and gas media poses attention. Pristine CNTs have the potential for effectively interact with much gaseous material, and there are countless continuing efforts for modifying of CNTs by functionalization in order to expand the analyte spectrum and its performance. The nature and mode of the connectivity between the analytes and the CNTs are essential for designing new synthetic selectors integrated with CNTs. Metaloxide-CNT hybrid materials detect wide spectrum of gases with high sensitivity. Multiple groups are developed on CNT-Hybrid sensors for detecting ammonia (NH3) and nitrogen dioxide NO2, both having similar mechanisms for sensing (Speranza 2021). Espinosa et al. (2007) developed MWCNT hybrid materials as sensors for detecting gases at low temperature. Numerous works are reported based on the hybridization of CNTs using different types of metal oxides such as Indium tin oxide, tin (IV)oxide (SnO2), tungsten trioxide, or titanium dioxide. In particular, sensors based on hybrid SnO2/ MWCNTs exhibit excellent sensitivity toward NO2, when operated at room temperature. Sensitivity toward sensing depends on the diameter and the environment of oxidation of CNT materials. Above material exhibited high porosity, contribute effective adsorption of gases was the prime factor for good sensitivity. Organic molecules such as poly (m-aminobenzenesulfonic acid) (PABS), a sulfonated poly (aniline), onto SWCNTs to create chemiresistors capable of detecting 20 ppm of ammonia (Schroeder et al. 2019). Porphyrins are an innovative material class, which can be used for noncovalent functionalization of CNTs. The aromatic porphyrinic cores are highly polarizable that interact strongly with π-conjugated graphenic sidewalls. T. M. Yafei Zhang et al. (Wang et al. 2011) developed single-walled carbon nanotube/cobalt phthalocyanine derivative (CoPc) material-based gas sensors, and cobalt phthalocyanine derivative had been successfully anchored on the surface of nanotubes through π–π stacking. Metal or metal oxide nanoparticles, graphene, organic molecule, and polymer functionalized SWCNT systems are also well established for the detection of hydrogen, methane, carbon monoxide, and hydrogen sulfide. Plenty of works related to pH-based sensors and metal ion sensors are also used for environmental applications. It is noted that structure, energy distribution, and alignment on film can be varied easily on the addition of a filler system, which is quintessential for detecting a large spectrum of materials. Sensors for Food and Agriculture Sensing pathogens and other chemicals have an inevitable role in the fast-growing food and agriculture industries for better production and damage-free function. Sensors play a crucial part in managing both quality production and the removal of harmful substances. CNT-based sensors are relevant here because when incorporated with various circuitries that provide instant status information of food using smartphones or other devices that are connected to the circuit. It allows smart packaging by detection of fruit ripening, pesticide, and spoilage detection in food. Esser et al. (2012) introduced a CNT-based sensor for ethylene gas in which copper (I) complexes are selected for the specific recognition of ethylene. In food safety

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field, food additives, such as, illegal additives, pesticide residues pathogens, allergens, and other unsafe factors, are habituating not only people’s health but also slowing down the rapid development of the food industry to a certain extent. Here also, sensors help to detect them in micro scale level (Schroeder et al. 2019). Food spoilage in the package can be monitored by sensing the presence of oxygen in the package (Schroeder et al. 2019). Sensors detect volatile components which cause food spoilage such as biomarkers including biogenic amines and ammonia spoilage bacteria and 1-octen-3-ol. Scheme 3 represents the CNT-based sensors for ethylene gas.

Sensors for Biological Field SWCNT as fluorescence quencher is mainly used for biological sensing purposes. Moreover, electrochemical sensors are also well-established entities for biological detection purposes as it offers added advantages of CNTs, including relatively inert electrochemistry, broad potential window and electrocatalytic activity toward a multiple redox reactions, and the capability to operate in turbid samples. Semiconducting properties of SWCNTs have unique NIR emission and tissue-penetrating properties. However, it can diminish the auto fluorescent background of tissue and fluid. Coming to an electrochemical biosensor, it is a freestanding device, which is

Scheme 3 Carbon nanotube–based sensor for ethylene gas (Esser et al. 2012). (Reproduced with permission from Wiley)

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functioned using an electrochemical transducer. When a specific biological recognition unit (bio-receptor) is combined within or closely related with an electrode (transducer), it transforms the small changes to electrical signals for differentiating or detecting the target compound (analyte) in the sample (Dong et al. 2012). Nevertheless, the significant challenge in engineering nanotube interface for selective detection is rectified with the help of functionalization via forming hybrid materials. All these methods are efficient in biomedical fields for the sensing of a wide range of biologically relevant elements and other harmful components in human body.

Sensors for Glucose Sensing Multiple works are brought out to detect glucose, including a) protein-based SWCNT fluorescent sensors. Yoon et al. (2011) produced glucose sensors, where glucose-binding protein (GPB), which is a periplasmic-binding protein having high affinity toward glucose, is covalently functionalized with carboxylated poly (vinyl alcohol)-wrapped SWNTs (cPVA/SWNTs). The functionalization leads to allosterically controlled optical transduction in response to glucose. (b) Boronic acid (BAs)SWCNT florescence sensors work on the reversible complexation of glucose with aromatic BAs to produce a stable boronate anion, changing the electronic properties of the Bas (Yum et al. 2012). Shahriary et al. (2014) developed an amperometric glucose sensor based on the Graphene-Multiwalled Carbon Nanotube hybrid, which is an example of an electrochemical sensor to detect glucose in alkaline solution with a sensitivity of 12.5 μAmM1 cm2. Moreover metal, metal oxide nanoparticles, and polymer-CNT hybrids are also used for electrochemical sensing of glucose. Sensors for DNA Sensing Detection of DNA is critical to diagnose and treat diseases related to a genetic disorder and helps in drug discovery in this area. Here CNT-based electrodes strengthen their position because it is a classic electrode for developing advanced biosensors, especially due to practical attachment of DNA or protein sequences over aligned or nonaligned CNT via noncovalent interactions or by simple wrapping owing to the aromatic interaction of sidewalls of SWCNT and base pairs of DNA (He and Dai 2004; Star et al. 2006). In the early stage, the DNA sensor works were based on immobilization technique that is ssDNA was immobilized on the electrodes, which lead to a change in electrical current stimulated by hybridization of the complementary sequence. Moreover, it workout through SWCNT as fluorescence quencher (Star et al. 2006) and signal transduction based on solvatochromism (Jeng et al. 2006) as a part of change in hybridization and adsorption of DNA over CNTs. Other CNT-Hybrid Sensors The importance of CNT-hybrid sensors is unavoidable in the national security field because of sensitivity and least power conceptions with high accuracy. It can be widely employed as chemical warfare agents (CWA) and electrochemical, biochemical sensors, etc. Researchers also utilized strain and pH responses of hybrid for developing sensors. Shi et al. effectively reinforced graphene in CNT using a chemical hybridization technique to form wearable sensors that detect human motion

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rapidly (Shi et al. 2016). Weak Van der Waals force interconnecting the CNTs institute a twisted network that can respond to the tensile loads. The interfacial sliding between nearby CNT makes this happen, and it causes a decrease in density and conductivity. Moreover, influenza virus detection is vital to protect living beings from virus outbreaks. Syed Rahin Ahmed et al. (2016) developed gold nanoparticle-carbon nanotube hybrids for low-cost colorimetric point-of-care diagnosis techniques which is a significant and sensitive colorimetric detection strategy for influenza virus A (H3N2).

Biological Applications Nowadays, hybrid materials are gaining tremendous attention in the biological field because of their inherent individual properties, solubility enhancement, biocompatibility, and control over design. Especially CNT has excellent biocompatibility compared to others, which boosts when they combine with special moiety and encase unique properties (Rajakumar et al. 2020). Apart from this, CNT possesses small size, rapid electron transfer kinetics, passive chemical actions, ultralightweight, high tensile strength, exhibit antifungal and antibacterial properties, acts as protein carriers, contains exposed functional groups, etc. Detailed sensing applications in the biological field are already discussed above, and remaining applications such as drug targeting, cancer diagnosis, and treatment, regeneration of tissue and bone, antibacterial and antifungal applications are discussed below.

Drug Delivery and Targeting Selectivity for delivering therapeutic agents into the targeted site is a complex mechanism. Smart material selection with pilot action is necessary for the proper functioning of the system. Dendrimers, metal and metal oxide nanoparticles, graphene, hydrogels, polymers, and proteins can form efficient targeting and delivering agents when they form hybrids with CNTs, the mode of interaction includes π-π interaction, hydrogen bonding, hydrophobic interaction, and covalent interactions (Singh et al. 2014). Dendrimers are leading nanocarriers with huge functional groups as well as derivatisable, monodisperse, well-defined, compartmentalized polymers of different size which can catalytically grow on MWCNT was shown by E. Kesharwani et al. (Kesharwani et al. 2018). Further, control over the growth of dendrimer-CNT nanoparticles is effectively put up by initial formation of dendrimer Co32 cluster with uniform morphology and then immobilization on CNT (Geng et al. 2006). CNT-dendrimer nanohybrids could be a better option for treating fungal infections and sexually transmitted diseases. Plethora of works showing the efficacy of these delivery systems has been demonstrated. Pilan et al. (Pilan and Raicopol 2014) probed the efficiency of poly(amidoamine) PAMAM dendrimer-functionalized CNTs (PAMM-Den-CNTs) for gene delivery. On compilation with dendrimer, there is a decrease in cytotoxicity of SWCNT which effectively enters in to breast cancer cells MCF-7 and successfully deliver anti-survivin oligonucleotides. SWCNTs-PAMAM complexes (Mei et al. 2010) were also

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explored for the morphological changes on human pancreatic cancer cell lines (BXPC-3 cell) and cytotoxicity (Mehra and Jain 2016). Quantum dots–CNTs hybrids are unique due to high signal detection provided by reduced photo-bleaching and high optical conductivity. Chen et al. (2012) investigated the effect of QDs that bind with Iron (II,III) oxide (Fe3O4)-filled carbon nanotubes for cancer-targeted imaging and magnetically controlled drug delivery. Work based on gold-CNT hybrids and their drug targeting and delivery application are probed by Minati et al. (2012) However, carbon nanotube/gold hybrids were used to deliver the anticancer drug doxorubicin hydrochloride into the adenocarcinomic human alveolar basal epithelial cells (A549) which is a lung cancer cell line. Advantage of metal nanoparticle-CNT hybrids are mainly bound on broad absorption extended to NIR region magisterial to the imaging application. Hydrogels are porous structured materials that can be engineered by keeping a tight rein on the cross-linking, and the engineer has complete control over their enlargement and diffusion features. According to this, selected drugs are released at the target cell. Nano fillers such as CNTs addition with hydrogel gave an added advantage over spatial and temporal control for the release of the cargos.

Cancer Diagnosis and Treatment Classical treatment methodology like chemotherapy, radiation, and surgery can damage healthy cells. CNTs excel over this because of drug delivery with dormant capability to pick out specific cancer cells with a smaller drug dosage than conventional drugs used. Hybrid-based drug delivery vehicles, lipid, antibody, other protein or peptide, polysaccharide and sensors are widely used for treating cancer cells. Iron nanoparticles encapsulated CNT peapods are also used in cancer thermotherapy (Kitaura and Shinohara 2006). Cancer hypothermia can be tested by utilizing the electromagnet radiation absorbing property of CNTs and microwave absorption characteristic utilized for microwave thermotherapy (Saito et al. 2014). Antibacterial and Antifungal Activity Antibacterial and antifungal properties of MWCNTs are related to factors such as surface structure, including surface-functionalized molecules, length, density, diameter, and purity of carbon nanotubes. On action with cell membrane, it first changes its current status and then reactive oxygen species generation occur leading to cell death and DNA damage. CNT/Chitosan (Mohamed and Abd El-Ghany 2018) derivatives are excellent due to their antibacterial activity and effective against gram-positive bacteria (E. faecalis and S. epidermidis) and gram-negative bacteria (E. coli). Chitosan stimulates the morphogenesis of the cell wall, which limits the growth of fungi, and the same system works well against fungi, namely A. niger, C. trobicalis, and C. neoforman (El Ghaouth et al. 1992). Engel et al. (2018) developed SWCNT/iron oxides system against E. coli bacteria. Other system such as MWCNT-DNA hybrids are also intelligent against E. coli selectively over S. aureus bacteria (Ozkan-Ariksoysal et al. 2017). Benincasa et al. (2011) probed Amphotericin B conjugated to carbon nanotube blends for antifungal treatment. Amphotericin B (AMB) is considered the most effective drug for antifungal

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treatment but limits the usage due to side effects; however, the f-CNT-AMB hybrids are less toxic to Jurkat cells at antifungal concentrations.

Other Applications Employment of CNTs in environmental remediation is pivotal. Multiple works on Graphene-CNTs and their sensing application is widely used for the detection and removal of heavy metal ions like lead (II), Copper(II), Arsenic (III), and Uranium (VI). They are also used for removal of oil and other organic specious and desalination of water (Fan et al. 2017). Graphene/CNT-based aerogels have outstanding properties like easy separation and sorption properties owing to the large specific surface area of these metals. 3D graphene-CNT-iron oxide nanostructures show an efficient removal of heavy metal arsenic from polluted water (Vadahanambi et al. 2013). Moreover, reduced graphene oxide-CNT hybrid is used to build nanofiltration membranes for drinking water purification to isolate the dyes, nanoparticles, sugars, proteins, and organophosphates (Fan et al. 2017). Aerogels of graphene-CNT hybrid are employed as capacitive deionization (CDI) electrode device for desalination of salt water (Fan et al. 2017). Remarkable progress in isolating and controlling quantum coherence using charges and spins in semiconductors is seen. Carbon nanotubes bridge the molecular and crystalline quantum worlds and these systems gain attraction due to their extraordinary electronic, mechanical, and optical properties. CNT-QDs hybrids are widely used as spintronics because confinement of carriers in a quantum dot material and to spin may shape the design of future quantum technologies (Urdampilleta et al. 2011). Urdampilleta et al. (2011) developed carbon nanotube (CNT) transistors, decorated with rare-earth metal complex known as terbium(III) bis(phthalocyaninato)(TbPc2)-based single-molecule magnets for molecular quantum spintronics. Work by Yan et al. (2007) to make functional materials for liquid transfer and surface wettable characteristics is achieved by producing aligned CNT-ZnO based hybrids.

Conclusion and Future Perspectives In the present scenario, the scientific community tries to absorb all possible methods to build new technological devices with outstanding performance, durability, and energy saving to solve the needs of the modern world. Compilation of CNTs and filler materials impute pleasant surprise to researchers because of their excellent morphological features, including improved light absorption, scattering, large surface area, fast and long-distance electron transport, etc. The hybrid material provides additional degree of freedom to develop smart materials that can lead to the emergence of new or upgraded properties such as conductivity, sorption, catalytic, mechanical, optical, and magnetic properties. Current developments also expel the harsh synthesis conditions; covalent and noncovalent methods are widely used to construct a new hybrid system. Control over functionalization offers control over

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self-assembly, which enhances their material properties. Solubility enhancement due to surface functionalization offers the possibility of CNTs to plasticize solution processability of hybrids and helps to decrease the efforts to make devices by following coherent methods like spin coating, spray coating method to reduce cost. The relatively small size and non-toxicity of CNTs with electrical and fluorescent properties open an era of sensor material in biological fields. Drug targeting and delivery, cancer diagnosis, and treatment possibility are also enhanced because of the compatibility of CNTs in making new hybrids with extraordinary performance. Strong footprints of CNTs have been seen in energy conversion and storage devices, and further advancements will surely occur due to its tunability.

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Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theoretical Investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Kinetic Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular Dynamics Simulation of CNT Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Molecular Dynamics Simulation: Ongoing Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A Brief Introduction: Molecular Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Kinetic Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discrete Computational Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influence of Catalyst Film Thickness on the Cluster Diameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influence of the Catalyst Film Thickness on the Growth Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influence of Temperature on the Growth Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

558 563 563 565 569 570 573 575 579 580 581 583 584

Abstract

The major theoretical and experimental investigations of carbon nanotube growth in the literature are discussed, and some of the recent studies by the authors on the topic are elaborated in the following. In the present investigation, molecular dynamics simulations are used to investigate the occurrence of the base and tip growth modes in carbon nanotubes during its chemical vapor deposition synthesis. The process using iron, cobalt, and nickel catalysts is studied, with different film thicknesses of the catalyst, at different deposition temperatures. The energy gain of the catalyst-assisted growth is obtained using MD simulations and compared with the binding energy of the catalyst and the substrate to understand K. Raji Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, VIC, Australia C. B. Sobhan (*) School of Materials Science and Engineering, National Institute of Technology, Calicut, India e-mail: [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_70

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the growth mode of the carbon nanotubes. In the molecular simulation, the deposition temperature obtained through a chemical kinetic model has been utilized as the initial system condition. It was observed that the switching of the growth mode of the carbon nanotubes between the base and tip modes is influenced by the film thickness of the catalyst. The system using iron as the catalyst is found to give base growth whereas nickel and cobalt catalysts provide tip growth at the lower range of thicknesses. It was inferred that the temperature does not have a significant role on deciding the growth mode. LAMMPS software was employed for the molecular simulation. The results obtained suggest tip or base growth of CNTs depending upon the process conditions and the type of catalysts used, and it is significant in various applications. Keywords

Chemical vapor deposition · Vapor-liquid-solid growth · Base and tip growth modes · Molecular simulation · Chemical kinetic model Abbreviations

E Fa Fi h k Qext Qr Ra Rg T

Potential energy (eV) Molar flow rate of component A (mol/s) Embedding energy of atom i (eV) Heat transfer coefficient (W/m2K) Rate constant (mol/m3)1-ns
1 Heat available due to external heating (W/m3) Heat associated due to the reaction (W/m3) Reaction rate of component A (mol/L.s) Universal gas constant (J/mol.K) Final temperature of the reaction (K)

Greek Symbols

εij ρi σ Φ(r)

Characteristic energy parameter (eV) Electron density of atom i (1/Ao2) Characteristic length parameter (Ao) Lennard-Jones potential energy (eV)

Introduction The carbon nanotube (CNT) has gained importance as an excellent new generation engineering material, due to its special properties suitable for various technological applications (Baughman et al. 2002; Endo et al. 2007). Of the different methods adopted to produce carbon nanotubes (Meyyappan 2004; Yu et al. 2005), the chemical vapor deposition (CVD) technique is a cheap and simple process, and hence has been investigated through experimental studies and theoretical modeling,

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with a view to optimize it (Khedr et al. 2008; Pirard et al. 2007; Kwok et al. 2010). The method involves a two-step procedure, consisting of a catalyst preparation step (coating of iron catalyst on an appropriate substrate) followed by deposition of the nanotubes on the catalyst-coated substrate placed inside a furnace, due to the chemical reactions, starting from a gaseous precursor such as acetylene or methane. The growth mechanisms of carbon nanotubes are specific to the preparation methods. Several investigators have proposed different hypotheses for the growth of CNTs. These are based on various mechanisms, such as defective deposition growth, lip-lip interaction growth, scooter mechanism, step flow growth, extrusion mode growth, and vapor-liquid-solid (VLS) growth (Bolton et al. 2006; Gohier et al. 2008; Zhang et al. 2007; Baker et al. 1973; Kumar and Ando 2010). The most widely accepted model of carbon nanotube growth in the CVD process is the VLS growth. Under this, the tip growth or the base growth modes have been proposed, depending on the position of the catalyst during the growth of CNTs (Bolton et al. 2006; Gohier et al. 2008; Zhang et al. 2007; Baker et al. 1973; Kumar and Ando 2010). The VLS growth mechanism is illustrated in Fig. 1. In this model, the carbon feedstock is initially in the vapor phase before dissolving into the metal catalyst to form a liquid metal-carbide particle (Bolton et al. 2006). When this particle is carbon saturated, the solid phase carbon nanotubes begin to grow. Although the VLS model does not offer a detailed description of carbon nanotube nucleation, it is believed that carbon nanotubes grow from “graphitic islands” that form on the saturated cluster surface. It is unclear if these islands have the same diameter as the metal particle. It is also not clear whether the islands are formed when the particle is supersaturated with carbon or at the iron carbide eutectic point. However, according to the model, as more carbon atoms deposit on the surface and add to the graphitic islands, the islands lift off from the cluster surface and produce carbon nanotube growth. Two physical models have been described for the VLS

Fig. 1 Vapor-liquid-solid (VLS) growth of carbon nanotubes (Bolton et al. 2006). (Reproduced with permission from American Scientific Publishers)

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growth mechanism (Kumar and Ando 2010). When the substrate-catalyst interaction is weak, the carbon precursor decomposes on the top surface of the metal and carbon diffuses down through the metal. This precipitates out CNT across the metal bottom, pushing the whole metal particle off the substrate. This growth, known as as the tip growth model, is shown in Fig. 2a. In the other case, when the substrate-catalyst interaction is strong, initial hydrocarbon decomposition and carbon diffusion occur similar to that in the tip growth, but the CNT precipitation fails to push the metal particles up; so, the precipitation is forced to emerge out from the apex of the metal region. This is called base growth model. This is shown in Fig. 2b. The mode of growth has a significant role in the efficient synthesis of carbon nanotubes. Experimental and theoretical investigations have been reported, aimed at the determination of the growth mode and its influence in the synthesis. The major investigations are described in the sections to follow. In a pioneering article, Lee and Park (2001) discussed the synthesis of vertically aligned CNTs on large areas of cobalt-deposited silicon substrates by thermal CVD using acetylene at 950  C. The results showed that the CNTs obtained were bamboolike structures with diameters distributed in the range of 80–120 nm and length around 20 μm. Wang et al. (2004) investigated the role of the layer thickness of the catalyst, catalyst dot size, deposition temperature, and source pressure on the growth process of straight, single-walled carbon nanotubes. Good-quality and high-yield nanotubes were obtained from thinner and smaller dot patterns, and at a lower gas

Fig. 2 VLS growth mechanism for CNTs. (a) Tip growth model. (b) Base growth model (Kumar and Ando 2010). (Reproduced with permission from American Scientific Publishers)

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pressure. It was observed that thermal decomposition has a clear advantage over the plasma technique for growing nanotubes with a uniform diameter. Okamoto and Shinohara (2004) synthesized single-walled carbon nanotubes on zeolite powder with Fe/Co catalysts using catalytic chemical vapor deposition (CCVD) with radio-frequency heating. They obtained better yield with an increase in the synthesis temperature, as shown in Fig. 3a. It was reported that the average single-walled carbon nanotube diameter is proportional to the size of the catalyst particle. The catalytic activity was found to be inversely proportional to the size of the catalyst particles, and directly proportional to the temperature. Terrado et al. (2006a) used spray-coated cobalt catalyst for the production of CNTs and produced uniform multiwalled CNTs using a mixture of acetylene and ammonia. The effects of catalyst pretreatment and growth temperature on the diameter of CNTs were discussed. It was found that thin, straight, and aligned multiwalled CNTs (MWCNTs) were formed at 750  C, while larger nanotube diameters and more defective structures were obtained at higher growth temperatures. Terrado et al. (2006b) deposited iron salt and iron solgel solutions on quartz and alumina substrates and observed that straight and aligned MWNTs were produced at just 850–900  C on quartz substrates, while curly, randomly oriented, and even coiled CNTs were grown at 800–900  C on alumina substrates. SampedroTejedor et al. (2007) investigated the synthesis of CNTs using acetylene along with iron-organometallic compounds at a temperature range of 650–850  C. They revealed the effect of the growth temperature on the purity of CNTs, and obtained CNTs with greater yield and purity at 750  C. They also found that a decrease in temperature or the iron concentration and an increase in the reaction flow produced a decrease in size of the nanotube, as shown in Fig. 3b. Devaux and Vergnat (2008) synthesized single-walled carbon nanotubes on ironcoated (0.5 nm) silicon substrate at a low temperature range of 400–800  C using methane or ethylene precursor. Rizzo et al. (2008) found that the thermal CVD process produces growth of vertically aligned carbon nanostructures for thinner catalyst films (i k>j>i

þ .........

ð3Þ

where the first term represents the effect of external field such as gravitational or electrostatic, and the remaining terms represent particle interactions, i.e., U2 is the potential between pairs of particles and U3 is the potential between particle triplets, etc. Normally, the first term is neglected. Including three- or more body interactions impose a very large increase in computing time compared with two-body calculations. The simplest form of U is to write it as sum of pair-wise interactions: U ðr 1 , . . . :, r n Þ ¼

XX   φr i
r j  i

ð4Þ

j>1

The most commonly used good approximation of pair-wise interaction is the Lennard-Jones (L-J) potential. It is given by:    12  σ 6 σ Φij ¼ U ðr Þ ¼ 4ε
r r

ð5Þ

The first term of the L-J potential represents the atomic repulsion whereas the second term is derived from the attraction or bonding between atoms. The boundary conditions applied in MD simulations can be periodic, damping, fixed, and bounce-back boundary conditions, according to the physics of the problem. The commonly used one is the periodic boundary condition. Statistical thermodynamics rules are the basics of MD, and hence they provide direct route from microscopic details of a system to macroscopic properties of interest. To serve this purpose, it is crucial to imitate the dynamics similar to the experimental conditions using the corresponding correct ensemble for the property of interest. In classical mechanics, Newton’s equation of motion conserves energy and hence conventional MD describes a system with constant number of particles (N), volume (V), and energy (E) (i.e., NVE or microcanonical ensemble). However, this ensemble does not resemble standard experimental conditions where a system is generally exposed to external pressure and/or exchanges heat with the environment, and therefore several other algorithms are available to sample in different ensembles. In most cases, temperature (T) and pressure (P) are most crucial properties which one would like to control. Most common thermodynamic ensembles relevant to experimental conditions are canonical (NVT) or isothermal-isobaric (NPT) ensembles. In some other cases where the number of particles also changes the grand canonical ensemble (μVT) is relevant.

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In MD simulations, the concepts of the thermostat and the barostat are used to bring the system into real physics. Most of the thermostats essentially belong to one of the three major classes: (a) scaling velocities (e.g., velocity-scaling and Berendsen thermostat); (b) adding stochastic forces and/or velocities (e.g., Andersen, Langevin, and dissipative particle dynamics thermostats); and (c) using formalisms (e.g., Nosé-Hoover thermostat). Each of these classes of schemes has its advantages and disadvantages, depending on its application. Practically, pressure can only be defined when the system is placed in a container having a definite volume. So, in a computer simulation, the unit cell subjected to boundary conditions is viewed as a container. To control the pressure in MD simulation, the volume of the simulation box is considered as a dynamical variable. The most commonly used barostats in MD are Anderson barostat and Berendsen barostat. Integration in MD simulation is performed using standard finite difference methods such as Verlet, Gear algorithm, and leap-frog algorithm. Here the time domain is discretized with finite time step, with Δt as the step size. The choice of a particular time step depends on the integrators as well as the system. Knowing the positions and some of their time derivatives at a time t, the integration scheme gives the same quantities at a later time t + Δt. By iterating the procedure, the time evolution of the system can be obtained. Once converged atomic positions are reached, standard methods of statistical mechanics are used to obtain the various properties of the material. As discussed in the previous section, the growth mode of carbon nanotubes plays a significant role in the performance of CNTs formed, and their suitability in several applications (Banerjee et al. 2008). It is understood that nucleation of catalyst particles at the end caps of CNTs can assist alignment of these nanostructures in the presence of magnetic or electrical fields. Hence it can impart directional properties to the materials on which these are embedded (Park et al. 2008). Such functionalized CNT tips can also be used to attract other molecules. An example is the functionalization of CNTs with protonated (
NH3+Cl) amines that add positive charges to CNTs, which allow binding to some biomolecules like RNA molecules. The tip growth mode is found to be useful in super capacitors (Shaijumon et al. 2008) or semiconductor industries as it allows the growth of CNTs on a conductive substrate directly, without developing any contact resistance. While developing continuum-based growth models for CNTs, it would be required to assume an appropriate growth mode. The switching of the growth mode from base to tip growth is expected to be dependent on the deposition temperature, catalyst type, and the catalyst film thickness. An appropriate prediction of the growth mode will help to develop an accurate continuum growth model. It will also provide guidelines in the selection of the appropriate process condition and catalyst layer thickness. In the present investigation, an attempt has been made to predict the growth and its switching between the tip and base growth modes, based on a molecular dynamics (MD) simulation. The deposition temperature for the growth of CNTs, which is used as the input parameter for molecular dynamic simulation, is obtained

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by modeling the chemical reactions inside the CVD reactor during the formation of CNTs using a continuum approach. The modeling methodology is explained below. The equilibrium molecular dynamics simulation method (Sadus 1999) has been utilized to predict the growth mode (tip or base growth) of carbon nanotubes formed by chemical vapor deposition, and also to study the influence of the temperature and thickness of the catalyst film on the growth mode. The growth mode depends on the energy gain ΔEnp ! CNT due to the addition of carbon and catalyst atoms from the metal-catalyst-carbon solution to the graphene sheet, to form carbon nanotubes (Banerjee et al. 2008). In tip growth, the catalyst particles will be lifted from the substrate. This happens only if the energy gain is sufficient to overcome the surface binding energy or interface energy between nanoparticles and the substrate, i.e., ΔEnp!CNT > γR2c where Υ and Rc denote the surface energy per unit area and the radius of meta+l catalyst particles, respectively. If the energy gain is not sufficient, it leads to base growth. Thus, understanding the dominant growth mode for specific conditions requires the calculation of ΔEnp ! CNT using molecular dynamics simulation. The MD simulation in the present study has been performed using LAMMPS software and the visualization of the MD trajectory has been done using the VMD open-source software. In the chemical vapor deposition process, the actual deposition conditions at which the carbon nanotube growth takes place will be different from the preset values of the initial controllable parameters, due to the chemical reactions occurring inside the furnace. The molecular dynamics simulations, therefore, will be accurate only if the true deposition conditions are utilized. Hence, in the present work, the molecular dynamics simulations are performed at the deposition temperature predicted using a chemical kinetics model explained below. The concept and the formulation of the mathematical expressions for developing a chemical kinetic model of CNT formation inside the CVD reactor are extensively explained in our previous publication (Raji et al. 2011). The design equation and energy equation have been solved using COMSOL Multiphysics software, to obtain the local variations of the parameters along the reactor length, by discretizing the domain in the axial direction.

Chemical Kinetic Model A series arrangement of plug flow and batch reactors is assumed to model the reaction phase in the chemical vapor deposition process (Raji et al. 2011). This brings into the model both the space and time dependency of the formation of carbon nanotube. It is assumed that the nanotube formation follows the VLS (vapor-liquidsolid) growth mechanism. The acetylene decomposition, iron carbide formation, and iron carbide decomposition reactions are given by Eqs. (6), (7), and (8): C2 H2 ðgÞ ! 2CðgÞ þ H2 ðgÞ

ð6Þ

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2CðgÞ þ 6FeðsÞ ! 2Fe3 CðlÞ

ð7Þ

2Fe3 CðlÞ ! 3Fe2 ðsÞ þ 2Cnanosized ðsÞ

ð8Þ

For the first two reactions (6 and 7), that is, the acetylene decomposition and iron carbide formation reactions, the reactor is considered as a plug flow reactor since feed is continuous. For the iron carbide decomposition reaction (8) it is considered as a batch reactor due to the time lag in the process because of the super-saturation of carbon vapor into the catalyst. The acetylene decomposition is taken as the major reaction which occurs inside the CVD reactor and formulated including the heat effects due to the other two reactions. The reactions inside the CVD reactor are solved using COMSOL with appropriate boundary conditions. The inside wall of the reactor tube is assumed to be at the furnace temperature. The design equation [Eq. (9)] and the energy balance equation [Eq. (10)] are solved to get the sectional average temperature and conversion along the length of the reactor: dF a ¼ Ra dV X

F C a a p,a

ð9Þ

dT ¼ Qr þ Qext dV

ð10Þ

The following relations are required to solve the Eqs. (9) and (10) Ra ¼ ka cA ¼ Aa exp


Ea c Rg T A

  Cp,i ¼ Rg a1 þ a2 T þ a3 T 2 þ a4 T 3 þ a5 T 4 X Qr ¼
H j r j

ð11Þ ð12Þ ð13Þ

j

Qext ¼ haðT amb
T Þ

ð14Þ

The format of Eq. (12) is known as the CHEMKIN or NASA format, which is a well-established format to obtain the database for the polynomial coefficients (of Eq. (12)) at different temperature intervals. The polynomial coefficients of acetylene, carbon, and hydrogen in CHEMKIN format for different temperature intervals are given in Tables 1 and 2. The activation energies and pre-exponential factors for the reactions are obtained from the literature (Towell and Martin 1961; Lee et al. 2002). The heat transfer coefficient (h) is calculated using Nusselt number correlations for fully developed laminar flow (Eq. (14)). The boundary conditions for obtaining the solutions are the following: At V ¼ 0 ði:e:, at x ¼ 0Þ,

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Table 1 The polynomial coefficients of acetylene from CHEMKIN format Acetylene

polynomial coefficients a1 a2 a3 a4 a5 a6 a7

Low temp (K) 300 alow 3.04452750E+00 1.18029962E
02
1.15997931E
05 6.20586714E
09
1.29415556E
12 2.59307684E+04 4.08219936E+00

Middle temp (K) 1405 ahigh 5.25518328E+00 4.11268316E
03
1.34210636E
06 2.01446821E
10
1.14017238E
14 2.53768442E+04
6.95830426E+00

High temp (K) 5000

The entering fluid boundary conditions are: u ¼ u0 , T ¼ T amb P ¼ Patm At the reactor walls, T wall ¼ T furn

Discrete Computational Simulations Molecular dynamics was used to simulate the growth of CNTs using iron, nickel, and cobalt catalysts, with varying thicknesses of the catalyst film or varying the corresponding mean diameters of the catalyst-nanoparticle cluster formed. The calculations were performed for different temperatures in the range 700–1200 K. The typical (baseline) simulation domain for growth of CNT with iron catalyst was chosen as a system consisting of 2748 carbon atoms, 6825 iron atoms (corresponding to 10 nm diameter particles), and 44 argon atoms (Banerjee et al. 2008). For the simulation of nickel and cobalt – assisted processes, the typical simulation (baseline) domains consisted of 6825 (3 nm) and 5118 (3 nm) catalyst atoms, respectively, in the same simulation box. The systems are shown in Figs. 10a–c. The number of atoms and corresponding film thickness of each element were calculated based on literature (Gohier et al. 2008; Banerjee et al. 2008) and are shown in Table 3. In further investigations, the parameters have been changed and the influences of these studied.

Polynomial coefficients a1 a2 a3 a4 a5 a6 a7

Carbon

Low temp (K) 300 alow 0.02498584E+02 0.08085776E
03
0.02697697E
05 0.03040729E
08
0.11066518E
12 0.08545878E+06 0.04753459E+02

Middle temp High temp (K) (K) 1000 5000 ahigh 0.02602087E+02
0.01787081E
02 0.09087041E
06
0.11499333E
10 0.03310844E
14 0.08542154E+06 0.04195177E+02

Table 2 The polynomial coefficients of carbon and hydrogen from CHEMKIN format Hydrogen Low temp (K) 300 alow 0.03298124E+02 0.08249441E
02
0.08143015E
05
0.09475434E
09 0.04134872E
11
0.10125209E+04 0.03294094E+02

Middle temp High temp (K) (K) 1000 5000 ahigh 0.02991423E+02 0.07000644E
02
0.05633828E
06
0.09231578E
10 0.15827519E
14
0.08350340E+04
0.13551101E+01

576 K. Raji and C. B. Sobhan

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Fig. 10 MD simulation domains with (a) iron, (b) nickel, and (c) cobalt catalysts. CNT and dissolved carbon (black), argon (blue), iron (ochre), nickel (green), and cobalt (yellow)

Table 3 Film thickness, mean diameter, and corresponding number atoms of catalyst particles (iron, nickel, and cobalt) in the simulation domain Iron Ft (nm) 0.6 1.2 1.8 2.4

Dm (nm) 2.5 5 7.5 10

N 1706 3412 5118 6825

Nickel Ft (nm) 0.3 0.6 0.9 1.2

Dm (nm) 1.5 3 4.5 6

N 3412 6825 8531 13,650

Cobalt Ft (nm) 0.405 0.81 1.215 1.62

Dm (nm) 1.5 3 4.5 6

N 2559 5118 6825 8531

The adaptive intermolecular reactive empirical bond order (AIREBO) potential was chosen as the interaction potential for carbon-metal-CNT system as suggested in the literature (Banerjee et al. 2008; Lee et al. 2002; Stuart et al. 2000; Vijayaraghavan and Wong 2013). It is given by: " # XX 1 XX REBO LJ TORSION E¼ Eij þ Eij þ Eijl 2 i j6¼i

ð15Þ

k6¼i, jl6¼i, j, k

EREBO term has the same functional form of the reactive empirical bond order (REBO) potential. The ELJ term accounts the longer-range interactions. The ETORSION term describes various dihedral angle preferences in hydrocarbon configurations. The Fe-Fe, Ni-Ni, and Co-Co interactions are modeled using embedded atom model (EAM) potential (Daw and Baskes 1984) (Eq. (16)): E¼F

X   1X   ρi r ij þ φij r ij 2 j6¼i

ð16Þ

j6¼i

where F denotes the embedding energy, which is a function of the atomic electron density, and ɸ is a pair potential interaction. The interactions between Ar-Ar, Ar-Fe, Ar-C, Ar-Ni, Ar-Co, C-Fe, C-Ni, and C-Co are modeled using the L-J potential, given by,

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"

6 # σ ij 12 σ ij Eij ¼ 4εij
r ij r ij

ð17Þ

Here, rij indicates the scalar distance between atoms i and j, and σ ij and εij are the LJ parameters (Verlet 1967; Mierzwa 2005; Böyükata et al. 2005; Cheng and Lee 2009) as shown in Table 4. Chemical kinetic calculations carried out using COMSOL Multiphysics software provided the cross-sectional average temperature and quantity of products formed during carbon nanotubes synthesis by acetylene CVD. The average temperature inside the acetylene reactor is shown in Fig. 11 as a function of the reactor volume (which represents the reactor length also, as the reactor is of constant cross-sectional area) at different values of the furnace temperature. At all the temperatures, the

Table 4 LJ interaction parameters for Fe, C, Ar, Ni, and Co

Interaction parameters Ar-Ar Ar-Fe Ar-C Ar-Ni Ar-Co C-Fe C-Ni C-Co

ε(eV) 0.0103 0.0516 0.00497 0.1426 0.0126 0.02495 0.0688 0.0075

σ(A)o 3.4 3.7 3.4 3.57 3.465 3.7 3.57 3.465

Fig. 11 Influence of furnace temperature on inside temperature of acetylene CVD reactor for 0.075 Lpm

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579

deposition temperature is found to be lower than the furnace temperatures. This is inferred as the cumulative effect of heat of reaction of two endothermic reactions which are higher than the exothermic reaction, which also substantiates our results of in-house coding reported before (Raji et al. 2011). Molecular dynamics simulations were used to explore the growth behavior of carbon nanotubes during gas-based chemical vapor deposition. The simulations were performed at the deposition temperatures obtained through the chemical kinetics solution. The growth mode of CNTs can be base growth or tip growth, based on the magnitude of the adhesion energy of catalyst to carbon nanotubes, in comparison with that between the catalyst and the substrate. When the adhesion energy of the catalyst to nanotubes is more than the catalyst- substrate adhesion energy, tip growth takes place, otherwise base growth occurs. This concept is used to determine the growth mode, by molecular dynamics simulation leading to the calculation of adhesion energies. LAMMPS software was employed for the simulation. The interatomic potentials utilized to simulate the interactions are the AIREBO, Lennard-Jones, and EAM potentials, as explained before. The energy gain of the system during carbon nanotube growth was investigated with different catalysts, namely iron, cobalt, and nickel, with different sizes, and the growth mode of CNTs explored. The dependence of the growth on the temperature was also studied. The results of the simulations are discussed below.

Influence of Catalyst Film Thickness on the Cluster Diameter The different catalysts used in the present investigations are iron, cobalt, and nickel. The relationship between the film thicknesses of these catalysts and the diameter of the cluster formed during the CVD process are available in the literature. These relationships are shown in Fig. 12. A linear relationship is seen in all the three transition metal catalysts used.

Fig. 12 Mean cluster diameter-film thickness relationships for catalysts

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Influence of the Catalyst Film Thickness on the Growth Mode Figure 13 shows the influence of film thickness of iron catalyst on the growth mode (base or tip) of CNTs, obtained by molecular dynamics simulation. From the result, it is understood that the energy gain during iron-assisted growth of CNTs is lesser than the surface interaction energy of iron, up to 1.8 nm film thickness of catalyst, and beyond that, it is higher. This result indicates that the iron catalyst will produce base growth of CNTs up to 1.8 nm thickness of the film, and beyond that, it will produce tip growth. Figure 14 shows the effect of the film thickness of the cobalt catalyst on the growth mode. Up to 0.4 nm thickness of film, the energy gain of the cobaltassisted CNT growth is found to be lesser than the surface energy, indicating that cobalt catalyst provides base growth. Beyond this film thickness, tip growth occurs. The influence of the film thickness of nickel catalyst is shown in Fig. 15. It can be inferred that the energy gain of nickel-assisted CNT growth is higher than the surface energy beyond 0.8 nm thickness of nickel film, which means that the nickel catalyst switches from base to tip growth beyond 0.8 nm film thickness. From these results, it can also be concluded that the iron catalyst provides base growth of carbon nanotubes at lower mean diameters of the cluster or film thickness, compared to cobalt and nickel catalysts.

Fig. 13 Influence of film thickness of iron catalyst on energy gain during CNT growth

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Fig. 14 Influence of film thickness of cobalt catalyst on energy gain during CNT growth

Fig. 15 Influence of film thickness of nickel catalyst on energy gain during CNT growth

Influence of Temperature on the Growth Mode Figures 13, 14, and 15 show the influence of temperature on the energy gain of the system for iron, cobalt, and nickel-assisted growth of CNTs. The surface energy of 10 nm diameter cluster of Fe and 1.5 nm clusters of Co and Ni are 1700 eV, 155.7 eV,

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Fig. 16 Influence of temperature on the energy gain during the growth of CNTs with iron catalyst

Fig. 17 Influence of temperature on the energy gain during growth of CNTs with nickel catalyst

and 113 eV, respectively (Banerjee et al. 2008; Vitos et al. 1998). Figure 16 depicts the influence of temperature on the energy gain of the system with iron catalyst. The surface interaction energy of iron catalyst which forms a 10 nm diameter cluster during CVD is 1700 eV (Banerjee et al. 2008). It is found that the energy gain calculated through the MD simulation is higher than this value of surface energy at all temperatures, which indicates that iron provides tip growth mode at all temperatures. Hence, it can be concluded that temperature has no significant effect on deciding the growth mode of CNTs with iron catalyst. A similar observation has been reported by Banerjee et al. for 10 nm cluster size of iron catalyst. The temperature dependence of the growth of CNTs using nickel and cobalt also has been studied, and similar observations have been made in the case of

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Fig. 18 Influence of temperature on the energy gain during the growth of CNTs with cobalt catalyst

these catalysts also. For nickel catalyst of 1.5 nm cluster diameter, the energy gain obtained through MD simulations is lesser than the surface interaction energy (155.7 eV) as shown in Fig. 17, which indicates that nickel catalyst gives base growth mechanism at all temperatures. Similarly, cobalt catalyst of 1.5 nm cluster diameter provides a base growth mechanism at all temperatures, as the energy gain is lesser than the surface interaction energy (113 eV) as shown in Fig. 18. The results of the MD simulations are indicative of the finding that the temperature has no significant effect on deciding the growth mode of CNTs while using iron, nickel, and cobalt catalysts for its synthesis.

Conclusion The growth mode of CNTs during the CVD process influences the performance of the systems to which nanotubes are embedded and hence an understanding of this is important. In the first part of the chapter, a review of major experimental investigations has been presented, which depicts a summary of the influence of processing parameters on the growth mode of CNTs. Furthermore, extensive theoretical investigations including chemical kinetics and molecular dynamics model on the growth mechanism of CNTs have been reviewed. Besides, a theoretical model based on a molecular simulation that developed in authors’ laboratory to study the growth modes involved in the formation of CNTs, with different types and film thicknesses of catalysts, at various temperature levels has also been discussed. The deposition temperatures have been obtained by a chemical kinetic model and are utilized as the initial temperature for the system in molecular dynamics simulations to explore the growth modes

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of CNTs. Carbon nanotube growth using iron, cobalt, and nickel at different temperatures in the range of 700–1200  C was investigated. Switching of the growth mode from base to tip growth was studied. It was found that iron is more likely to give base growth at lower film thickness ranges than nickel and cobalt. Based on surface energy calculations, it was understood that temperature does not have a considerable influence on the switching of growth from base to tip growth mode.

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Experimental and Theoretical Aspects of the Fragmentation of Carbon’s Single- and Multiwalled Nanotubes

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Sumera Javeed and Shoaib Ahmad

Contents Introduction to the Fragmentation of sp2-Bonded Carbon Structures . . . . . . . . . . . . . . . . . . . . . . . . . . The Fragmenting Multiwalled CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cumulative Cs+-Induced Damage in MWCNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fragmentation Profiles of the Irradiated SWCNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normalized Yields of the Sputtered Cx from Irradiated SWCNTs . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Origin of the Emitted C Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Localized Thermal Spike (LTS) Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LTS Temperature Ts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CCs and LTSs as Information-Generating Dynamical Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Probability Distribution Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Information Theoretic Entropy and Fractal Dimension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kullback-Leibler Divergence or Relative Entropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Spatially Coherent and Temporally Divergent CCs and LTSs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Irradiation with energetic ions is demonstrated as a powerful tool to investigate the fragmentation mechanisms of carbon nanotubes. Energetic ions generate linear and nonlinear dynamical processes in nanotubes. Atoms and clusters emitted from the irradiated nanotubes are directly related with the energydissipative and the entropy-generating mechanisms. Experimental results from the Cs-irradiated nanotubes are explained using the atomic collision cascade and thermal spike models. An information-theoretic model is described that comprehensively explains both of the linear and nonlinear fragmentation mechanisms. From the normalized experimental sputtering yields of the emitted species, S. Javeed Pakistan Institute of Nuclear Science and Technology (PINSTECH), Islamabad, Pakistan S. Ahmad (*) National Center for Physics, Islamabad, Pakistan © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_71

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probability distributions are constructed that yield the respective Shannon entropies. Fractal dimensions, derived from Shannon entropy of the emergent atoms and clusters, determine the spatial dimension of binary atomic cascades and the thermal spikes. Kullback-Leibler distance evaluated from the probabilities of emission of atoms and clusters identifies and characterizes the relative entropy of the two mechanisms. The characterization and the relative operational efficiency of the linear atomic collision cascades and the nonlinear thermal spikes in the irradiated, fragmenting single- and multiwalled carbon nanotubes can be comprehensively described by information-theoretic tools. Keywords

Ion irradiation · Radiation damage · Collision cascades · Thermal spikes · Atomic and cluster sputtering · Dynamical systems · Shannon entropy · Fractal dimension · Relative entropy

Introduction to the Fragmentation of sp2-Bonded Carbon Structures Carbon nanotubes were discovered by Iijima while observing the transformation of the graphitic sheets under energetic electron irradiation in TEM (Iijima 1991). The cylindrical hexagonal networks of C atoms curled in the form of nanometer-sized tubules were appropriately designated as carbon nanotubes CNTs. The single- and multiwalled carbon nanotubes, SWCNTs and MWCNTs, were capped with the half spherical shells of fullerenes with the matching diameter. Fullerenes had already been independently discovered by Kroto et al. few years earlier (Kroto et al. 1985). Pentagons were shown to induce spherical curvature in the spherical fullerenes. The newly discovered allotropes of carbon shared the ubiquitous sp2 bonded C atoms. These C atoms are responsible for the formation of hexagons and pentagons during the synthesis of the fullerenes and carbon nanotubes (Ajayan et al. 1993). The structural connectivity of carbon’s allotropes was evidenced in the mechanisms of their formation and fragmentation. Thus, the fragmentation of the sp2-bonded graphitic sheets can be described as the essential, preformation stage. In Iijima’s experiment, that was conducted with the energetic electrons having the energies E
100 keV in the transmission electron microscope, created the environment of the fragmentation followed by the topological rearrangements of the sheets into rolled nanotubes. CNTs were later formed by other techniques including the arc discharge between graphite electrodes (Ebbesen and Ajayan 1992; Farhat et al. 2001). In these techniques, the fragmentation mechanisms precede the formation of CNTs in carbonaceous discharges. Techniques for the formation of CNTs have been discussed in the preceding chapters of this Handbook and in various reviews and books; only a selected few of these are referenced as Endo et al. (1993, 1995); Thess et al. (1996); Yudasaka et al. (1997). Prior to the discovery of CNTs, experimental results had shown that the mass spectrometric profiles of the subliming and evaporating graphite

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were dominated by the C clusters containing up to thirty (30) C atoms among the fragments (Honig 1954; Berkowitz and Chupka 1964; Whittaker and Kintner 1969; Akhtar et al. 1997; Qayyum et al. 1998; Ahmad 1999). A persistently observed feature emerging out of the mass spectra was the presence of the diatomic carbon molecule C2. It was the most intense among the atomic and molecular fragments, especially the anions. Monatomic carbon C1 was also present but had lesser intense peaks. The even numbered Carbon clusters were seen with comparatively higher intensities than the odd numbered clusters. These experimental observations and the respective interpretations on the structural form and nature of graphite’s fragments existed prior to the discovery of CNT in 1991 (Iijima 1991). When similar experiments were conducted with CNTs in place of graphite as the target, the experimental results of the sputtered clusters or the fragmented species showed similarities. Such a comparative mass spectrum is shown in Fig. 1 where SWCNTs of 2 nm diameter and 10–13 μm length compressed in 2 mm diameter x 5 mm thick Cu targets of the Source of Negative Ions by Cesium Sputtering (SNICS) were irradiated with Cs+ ions at 5 keV (Javeed et al. 2013; Ahmad et al. 2012).

Fig. 1 The mass spectrum of the negative sputtered atoms and clusters C x ; x
1 from SWCNT irradiated with Cs+ at 5 keV energy in SNICS is shown. The inset has the sputtered species from graphite under identical conditions. The anionic currents are an order of magnitude higher for graphite as compared with those from SWCNTs. C2 displays the highest intensity from both of the allotropes of carbon. (Data from Fig. 1 of ref. Javeed et al. 2013; inset from Fig. 3 of Ahmad et al. 2012)

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The discovery of CNTs and the continuation of the earlier sequence of experimental investigations of the fragmentation patterns and profiles have clarified various aspects of the underlying mechanisms. In this chapter, we will highlight the processes and mechanisms of the fragmentation of SWCNTs and MWCNTs. Experimental and theoretic aspects will be presented and discussed. The scheme of the chapter is the following: (1) Presentation of the results from the mass spectrometry of the fragments emitted from the irradiated SWCNTs and MWCNTs. (b) Discussion of the irradiation-induced damage’s theoretical framework that separates the linear binary atomic collision cascades and the localized thermal spikes. (c) Presentation of the paradigm that the thermal, statistical mechanical and the information-theoretic models can be employed on the same data to illustrate the fundamental physical mechanisms. It will be demonstrated that the model developed by using the information-theoretic tools of Shannon entropy, the information-dependent fractal dimension, and the relative entropy can provide a comprehensive description of the collision cascades (CC) and localized thermal spikes (LTS) as the emerging dynamical systems that are induced by the externally induced ions in CNTs.

The Fragmenting Multiwalled CNTs MWCNTs have similarities with graphite for having the cylindrically bent graphene as coaxial cylindrical tubes with the intertube spacing almost similar to the intersheet spacing in graphite ~0.334 nm. The main difference appears in the dimensional characteristics of MWCNTs as compared to graphite where MWCNTs have one dimension (diameter) in the nanometer range while the lengths are in micrometers. Typically, a 10 nm diameter x 1 μm long MWCNT contains ~108 C atoms. The individual MWCNTs retain their structural identity even in a sample of the compressed MWCNTs and exhibit relatively poor conductivity as compared with the graphitic conductivity. The similarities and differences in their respective structural properties make MWCNTs and graphite as distinct and at the same time, comparable carbon structures for the study of irradiation-induced structural defects in the form of sputtered species that are emitted from the targets. It is assumed that the penetration ranges of the Cs+ ions are less than the CNT diameter d while the length of nanotube L > > d as shown in Fig. 2. The typical irradiation-induced damage profile is shown in the figure with the initiation of collision cascades, creation of vacancies, generation of recoils, and sputtering of the mobile atoms and clusters from the outer shell. Most sputtered species are neutral which acquire electron from the Cs-atomic layer on the nanotube. These are later extracted and mass analyzed as shown in Fig. 3 (Ahmad et al. 2012). The spectra from MWCNTs shown in Fig. 3 contain C1, C2, C3, C4, and low intensities of higher clusters as anions. The dominant emitted species is C2 at all energies. C1 acquires competitive intensities in the higher energy ranges E(Cs+)
3.0 keV. In addition, O and OH were sputtered from the outer cylindrical surfaces. In Fig. 4a, the experimental data on the left shows the sputtering yields Yx of C1-C4 are plotted against Cs+ energy E(Cs+). C2 is the most intense sputtered species

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Fig. 2 The schematics of Cs+ irradiation of a MWCNT in SNICS

followed by C3, C1, and C4 for the entire energy range. Figure 4b, on the right, plots the data of the normalized yields against E(Cs+) for all carbon clusters. C2’s relative yield decreases from ~100% to ~50% between low energy range 40 and 400 eV and then becomes stable for the higher energy range. The C3 yield decreases from ~50% to ~20% up to 3.5 keV. The C1 yield shows a gradual increase from ~5% to 20%, while C4’s contribution is stable around ~10% for entire range of E(Cs+).

Cumulative Cs+-Induced Damage in MWCNTs MWCNTs and graphite powder samples were irradiated for extended periods at high dose of Cs+ at constant value of E(Cs+) ¼ 5 keV (Ahmad et al. 2012). The same E

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+ Fig. 3 Three overlapping, mass spectra of the sputtered species C x at E(Cs ) ¼1.0, 2.0, and 3.0 keV. Data from ref. (Ahmad et al. 2012)

(Cs+) range was selected to study the comparative, postirradiation effects in the two allotropes of carbon. The adsorption of neutral Cs on the surface and the implantation of the energetic Cs+ play an important role in the formation and the emission of the carbon clusters (Cx; x
1) and the cesium-substituted clusters (Cs-Cx) as anions. The associated structural changes are observed by using scanning electron microscopy. Figure 5 shows three mass spectra from the Cs+-bombarded, compressed MWCNTs sample in the SNICS source (Ahmad et al. 2012). The first spectrum 5 (a) is from a pristine sample that was bombarded by Cs+ beam for 5 minutes at 5.0 keV. The intensities are plotted on logarithmic scale to highlight the emergence of higher clusters (Cx
C2) among the sputtered species as a function of ion dose. The spectrum shows monatomic, diatomic, and higher C clusters up to C7. At this energy, the cluster intensities are low, that is, in the range of ~ nA. The sputtering of the adsorbed water-related peak of OH is also present with intensities higher than some of the Cx anions. Here, it must be pointed out that the adsorbed water-related peaks of O and OH are a persistent feature of the sputtered species’ spectra due to the omnipresence of water in nanotubes. Figure 5b shows the mass spectrum collected after 20 min of irradiation at 5.0 keV. It can be noticed that the intensities of the carbon clusters Cx are two to three orders of magnitude higher than those in the pristine sample. This spectrum has the first, clear evidence of the sputtered Cs that had been implanted during earlier rounds of irradiation. The cesium-mediated cluster

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Fig. 4 (a) The sputtering yield Yxand (b) normalized Nx yields for C1–C4 are plotted for the pristine MWCNTs as a function of E(Cs+). Data from ref. (Ahmad et al. 2012)

CsC4 is also visible. As pointed out earlier, water is absorbed during MWCNTs sample preparations and is very difficult to remove. However, it can be removed more effectively from graphite target than MWCNT depending on the nature and size of the two very different surfaces for the two carbon allotropes. Figure 5c presents the spectra from a highly damaged sample of MWCNTs after 200 min of continuous Cs+ bombardments with the cumulative dose of 1020 ions m2. Higher intensities ~10–100 nA of the C anions can be seen. C1 to C6 have well-defined structures while the peaks of C7 to C10 are overlapped with the edge of C6. Sputtered Cs can still be traced by the presence of Cs mediated clusters such as CsC2 and CsC4–6. Peaks from all three of the spectra are broad and overlapping. These are the hard-to-resolve high masses with our analyzer. Irradiation of CNTs leads to the formation of both the pure carbon clusters Cx and the cesium-substituted carbon clusters CsCx on the surfaces of MWCNTs and SWCNTs. At low doses, C1s are more pronounced than C2s from MWCNTs. Majority of sputtered species is neutral when leaving the nanotubes. The formation of anionic clusters occurs on the cesium covered surfaces by electron exchange between sputtered atoms and clusters with Cs atoms. MWCNTs provide larger areas as compared to SWCNTs due to the size variations. Experiments show that MWCNTs demonstrate pronounced structural transformation under comparative irradiation with Cs+ ions. These may be due to the fact that those sputtered carbon clusters that may not escape outwards into the experimental chamber may be

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Fig. 5 (a) The mass spectrum from pristine MWCNTs. (b) Mass spectrum after 20 min irradiation of MWCNTs. (c) The spectrum from a heavily irradiated MWNTs after 200 min of continuous Cs+ irradiation (Fig. 1 of Ahmad et al. 2012)

deposited as surface adsorbents to act as fillers among the adjacent nanotubes in the case of MWCNTs. The re-deposition of the sputtered carbon clusters may participate in the localized melting and the formation of merged structures in MWCNTs. Localized melting due to thermal spikes will be discussed later in Section “Thermal Origin of the Emitted C Clusters.” Cs+-induced irradiation eventually may destroy the hexagonal structure of the surfaces of both types of CNTs

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leading to amorphous graphite. XRD spectra will be shown in the next section from the heavily irradiated SWCNTs and MWCNTs that provide the evidence of structural modifications.

Fragmentation Profiles of the Irradiated SWCNTs Experiments with the SWCNTs irradiated with Cs+ ions as a function of energy and subjected to increasing Cs+ dose have confirmed similar results as were obtained from MWCNTs and identified a number of new aspects of the mechanisms of fragmentation of CNTs. It is because of the mono-shelled nature of SWCNTs that one can understand and visualize the operation of the mechanisms of CC and LTS by analyzing the mass spectra of the sputtered carbon atoms and clusters emanating from the irradiated surfaces. These spectra show the relative number densities of the fragmenting species and evolution of the accumulating damage due to the creation of vacancies. Irradiation-induced fragmentation provides clues to the local structural transitions that occur. C2, C3, and C4 are the most persistent sputtered species from the irradiated SWCNTs. C1 has the least of the number densities among all species. It may be interpreted as the ease with which di-vacancies can be created as opposed to the single vacancies. The accumulation of the sputtered species in the inter-SWCNT space may be responsible for the increased connectivity among the nanotubes and results in the consequent increase in the electrical conductivity that was observed. The energetic ion irradiation effects have a destructive aspect that locally destroys the nanotube structure by introducing vacant spaces and at the same time the initiation of a reconstructive process that may occur due to the accumulation of the sputtered clusters. Ion irradiations of SWCNTs, the consequent defect production mechanisms, and the defect-related property changes have been investigated (Amorim et al. 2007; Skakalov et al. 2008; Krasheninnikov and Nordlund 2010). The relative ratios of the ion-induced defects for example, the single versus di-vacancy and the role that displaced and knocked-off atoms and diatoms may play in the formation of new structures within and between the nanotubes, has been simulated and experimentally studied (Krasheninnikov et al. 2004; Jin et al. 2008; Javeed 2014). Similar results from the irradiated C60-fullerite have also identified the structural transitions that occur due to fragmentation of the C60 cages as a function of Cs+ energy E(Cs+) (Zeeshan et al. 2012). Prolonged Cs+ irradiation led to the spherical fullerene cages’ destruction. Similarly, sputtering profile of the clusters from the irradiated MWCNTs studies as a function of the ion dose had indicated the vacancies-related structural transformation (Ahmad et al. 2012). Energy and dose of the irradiating ion acts as a crucial factor that determines the nature and extent of the damage to carbon nanostructures. High doses of the heavy energetic ions may induce structural changes in nanotubes that vary from the fusion and welding of nanotubes to amorphous graphite (Krasheninnikov et al. 2004; Skakalov et al. 2008; Krasheninnikov and Nordlund 2010; Javeed 2014). Large-scale sputtering from a collection of compressed carbon nanotubes may have twin effects: (1) single and di-vacancies in the nanotubes might

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open up the cylinders of tubes in localized regions and (2) the consequent re-deposition of these clusters on the neighboring nanotubes could initiate new nanostructures in and around the nanotubes. Two sets of the mass/charge (m/z) spectra of the Cx anions from SWCNTs were obtained as a function of E(Cs+). The spectra from the pristine sample are shown in Fig. 6a–c at E(Cs+) energy 0.2, 0.8, and 2.0 keV, respectively. The others were repeated in the same energy range on the Cs-rich sample of SWCNTs. These results were chosen from the two sequences of 19 m/z spectra of Cx that were obtained. Figure 6a is the first mass spectrum from the pristine SWCNTs showing only C2, C3,

Fig. 6 Two sets of mass spectra of Cx sputtered from the irradiated SWCNTs are shown. The first set (a–c) shows Cx emitted from the pristine sample while Fig. 6d–f is from the Cs rich, restructured SWCNTs at E(Cs+) ¼ 0.2, 0.6, and 2.0 keV (Fig. 1 Javeed et al. 2013)

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and C4. These three species retain the bulk of Cx output for Cs+ irradiations at all energies with relative variations in the intensities. C2 is present in all the spectra as the most prominent fragment. C1 makes its first appearance at E(Cs+) ¼ 0.4 keV and steadily rises to about 10%.

Normalized Yields of the Sputtered Cx from Irradiated SWCNTs Figure 7 shows the normalized yields designated as Cx/ΣCx for x ¼ 1 to 8 of the sputtered Cx from the pristine SWCNTs. The normalized plots present the landscape of the comparative number densities. C2, C3, and C4 are the main species emitted from the irradiated SWCNTs with the subtle differences between their number densities from the pristine set of SWCNTs in Fig. 7 and also those from the heavily irradiated ones in Fig. 8. These differences illustrate the nature of the damage to SWCNTs and the consequent emergence of the sputtered Cx-initiated structures. Figure 7 shows the C2’s relative yield decreasing from 64% at E(Cs+) ¼ 0.2 keV to ~50% between 1 and 2 keV. C3’s yield increases from 20% to a broad peak of ~25% at 0.6 keV and stabilizes around its initial yield. C4 appears as a relatively stable species with ~10% relative yield for the entire E(Cs+) range. The relative population of C1 increases from 0 to ~10% indicating a uniform increase in the production of

Fig. 7 δE(Cs+) ¼ 0.1 keV. The normalized yield Cx/ΣCx of C1, C2, C3, C4, C5 . . .C8 are plotted as a function of E(Cs+) from 0.2 to 2.0 keV for the pristine SWCNTs sample (Data adapted from Fig. 3a, Javeed et al. 2013)

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Fig. 8 δE(Cs+) ¼ 0.1 keV. The normalized yield Cx/ΣCx of C1, C2, C3, C4, C5 . . .C8 are plotted as a function of E(Cs+) from 0.2 to 2.0 keV for heavily damaged samples after Cs – irradiation (Fig. 3b, Javeed et al. 2013)

single vacancies with E(Cs+). The large clusters C5 and C6 are not detectable at E (Cs+) < 0.5 keV but once formed, these retain their steady share of ~3–5%. Figure 8 shows similar broad feature of cluster emission profile that is obtained after 100 minutes’ irradiation of the SWCNTs. The same ion energy range is chosen as in Fig. 7. In the figure, the normalized yield graphs are shown for C2, C3, C4, and higher clusters as the main constituents emitted from the Cs+ sputtered nanotubes. The profile of C3’s yield is similar to that in Fig. 7. C4 also stays around a steady 10%. Monatomic C1 can be seen not being emitted from the heavily irradiated ensemble of SWCNTs up to E(Cs+) ¼ 1.5 keV, after which its share is steady around 10%. This may indicate C1 that as its yield is not proportional to the ion energy, as it was in Fig. 7, it could have been emitted as a by-product of the larger Cx fragmentation. Energy dissipating collisions of Cs+ with SWCNTs have certain similarities with those in MWCNTs and also with those of the irradiated C60-fullerite (Zeeshan et al. 2012). The mechanisms of the generation of primary recoils are similar in fullerite and nanotubes, and the relative yields of the collision products differ in each case. Cs+-C60 collisions in fullerite occur in different energy regimes; below 400 eV only C2s are emitted while for the higher E(Cs+) all species are emitted (Zeeshan et al. 2012). This effect is unique to the irradiated fullerite and has not been observed in graphite, single- or the multiwalled carbon nanotubes. At the low irradiation regime,

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the irradiated SWCNTs throw out C2 as the most intense species with C1 being virtually absent; C3 is seen as the next competing in intensity cluster with C4 emitted having the third, stable cluster density. Collision cascades are likely to occur in MWCNTs with higher 3-D efficiencies of spreading as recoils among the overlapping nanotubes due to the availability of the nearest neighbors of a primary knock-on atom (Lindhard et al. 1963; Robinson 1965; Thompson 1968; Sigmund 1969). The spreading of the binary collision cascades does not seem likely to occur in SWCNTs, with the same efficiency as these may occur in MWCNTs. It is due to the lack of the adjacent shells which provide 3-D spreading of the cascades. The relative absence of C1s in the mass spectra of irradiated SWCNTs and the associated single vacancies that ought to have been produced provide the evidence. The observed predominance of the sputtered C2s from all the C allotropes demonstrates that the generation of di-vacancies is not unique to the irradiated SWCNTs and MWCNTs, and these are also produced in C60 and graphite, as discussed earlier. The diatomic C2 emerges as the most significant fragmentation species from the irradiated CNTs. The physical mechanisms will be discussed and described in the next sections. Figure 9 shows the experimental evidence of the atomic and cluster emissions from the two types of nanotubes with a coarse-grained measure of δE(Cs+) ¼ 0.5 keV (Javeed 2014). It may be pointed out that earlier Figs. 7 and 8 were obtained with δE(Cs+) ¼ 0.1 keV. The range of E(Cs+) is chosen from 0.5 to 3.5 keV. With this comparatively coarse-grained measure of δE(Cs+), one can expand the E(Cs+) range to higher irradiation energies without excessive Cs+-induced damage and avoid the effects of the Cs-implantation in nanotubes (Ahmad et al. 2012; Javeed et al. 2013). Figure 9a and c show two representative mass spectra of anions emitted from SWCNTs and MWCNTs at Cs+ energy E(Cs+) ¼ 1.5 keV. In Fig. 9b and d, the probabilities p(Cx) of the four sputtered species C1, C2, C3, and C4 as anions are obtained from the respective normalized emission densities and plotted against E (Cs+). The figure clearly demonstrates the predominant emission of C2, C3, and C4 while there is relatively lower probability for the emission of C1. The cluster emission does not explicitly depend on ion energy while for C1 the probability p(C1) / E(Cs+), as one would expect from linear atomic collision cascade theories (Lindhard et al. 1963; Robinson 1965; Thompson 1968; Sigmund 1969). The nondependence of p(Cx); x
2 on Cs+ energy variations has been discussed elsewhere as space-filling, multifractal, thermal spikes (Javeed and Ahmad 2017; Ahmad et al. 2018). The thermal model for the generation of LTS will be discussed in the next section. The accumulation of the sputtered species in the inter-CNT space may be responsible for the increased connectivity among the nanotubes and result in the consequent increase in the electrical conductivity (Javeed et al. 2013). Fragmentation of the SWNTs leads to the destruction of the nanotube structure locally leading to the opening of the tubes. The nature and extent of the cumulative irradiationinduced damage is to fragment the individual nanotube structure while simultaneously bonding or welding these with each other. The energetic ion irradiation effects are destructive for the nanotube structure and at the same time a

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Fig. 9 δE(Cs+) ¼ 0.5 keV. (a) Mass spectrum of anions sputtered from Cs+-irradiated SWCNTs at E(Cs+) ¼ 1.5 keV. Inset: shows the log-plot of the anion currents to demonstrate the presence and dominance of larger carbon clusters. (b) The probability of sputtering p(Cx) of the four anionic +    species C 1 , C2 , C3 , and C4 are plotted as a function of E(Cs ). (c) Mass spectrum of sputtered + anions from MWCNTs at energy E(Cs ) ¼ 1.5 keV. Inset: shows the log-plot of anion peaks. (d) The probability p(Cx) for the four C species is shown for the same energy range as that of (b). (Data plotted from Fig. 4 Javeed et al. 2013 and Fig. 4.19 Javeed 2014)

reconstructive process occurs due to the accumulation of the sputtered clusters. The irradiation-induced structural transitions occur with these two simultaneously operative processes. The route of the aforementioned structural transformation that occurs while the nanotubes are losing their constituents can be worked out from two experimental observations; one is the m/z plots for the first and the last spectra as Fig. 10a and b, respectively. The inset shows the increase in the target current as a function of the E(Cs+) and the ion dose. The mass spectrum from the heavily irradiated SWCNT is plotted in Fig. 10b. The anion current on logarithmic scale is used to enhance and clearly identify the low intensity large Cx in the 38th, consecutive spectrum where fragmentation processes of the nanotubes are accompanied by the implantation of Cs into the CNTs. At low E(Cs+) and dose, only C2, C3, and C4 are emitted, with C2 being the dominant species, while from a heavily irradiated sample of SWCNTs at high E(Cs+), all Cx in the range C1 to C10 are sputtered, as shown in Fig. 10b. Cs as anion and Cs-substituted carbon clusters CsCx; x ¼ 1, 4 are also knocked off by the energetic Cs+ ions.

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Fig. 10 From pristine SWCNTs in (a) to the heavily irradiated SWCNTs (b). The implanted Cs peak is clearly marked along with CsC4. (a) shows the first m/z spectrum with Cx density on logarithmic scale to highlight the differences with those i.n (b) as the 38th spectrum at E(Cs+) ¼ 2.0 keV, showing the entire array of C clusters Cx (X ¼ 1 to 10), and broad peaks of Cs and CsCx (x ¼ 1, 4). Inset shows the increase in the target current as a function of the E(Cs+) and dose. The initial sequence on the pristine sample is shown with open and the one on the Cs-rich is shown with filled circles. (Data replotted from refs. Javeed et al. 2013, Javeed 2014)

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Fig. 11 XRD spectra for (a) SWCNT and (b) MWCNT before and after irradiation with Cs+ (Fig. 9, Javeed et al. 2013)

Figure 11 shows the XRD spectra from the pristine and the heavily irradiated samples of SWCNTs and MWCNTs. In both cases, the intensity of the peak corresponding to the main graphitic planes (002) at 26.2o decreased after irradiation and became broader, signifying the partial loss of the crystal structure. The substrate peaks are not shown in the irradiated samples because several layers of the irradiated carbon nanotubes were taken out from the holder and tapped on glass slide; XRD was then performed to avoid substrate peaks. In MWCNT spectra, the peak at 22.5o is from tape on the glass slide.

Thermal Origin of the Emitted C Clusters Two fundamental questions arise when analyzing the mass spectra of carbon anions sputtered from Cs+-irradiated single-walled carbon nanotubes: 1. Why is C2 the main sputtered species rather than C1? 2. Which mechanism explains the dominance of the multi-atomic carbon clusters’ emission from the Cs+-irradiated single-walled carbon nanotubes? Collision cascade (CC) – based on binary atomic collision – was introduced in the ion-induced sputtering of the solid targets’ theoretical models (Lindhard et al. 1963;

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Robinson 1965; Thompson 1968; Sigmund 1969). These models explained atomic sputtering but did not include or explain the sputtering of clusters. The irradiationinduced binary atomic CC in bulk solids evolve on and under the irradiated surface resulting in the sputtering of the atomic constituents. The primary events of the energetic projectile ion-target atom interactions initiate the cascades of recoiling atoms and generation of mono-valent vacancies where the energy is shared with the neighbors of the recoiling atoms. Sputtering yields are the ratio of atoms emitted per ion when the cascades intersect the surface. The sputtering models utilize the ion to target mass ratio, angle of ion incidence, energy of the incident ion as the input parameters. Ion energy is shared among the target recoils in binary collisions through appropriately chosen interatomic potentials (Sigmund 1969). Sputtering yield is the total flux of the recoiling target atoms that leave the outer surface per ion. Clusters are neither considered nor sputtered from the cascades. There is a direct dependence of the atomic sputtering yield to the deposited energy. However, the mono-shelled structure of the SWCNTs ensures that all outward expanding collision sequences are restricted to the same layer due to the monatomic layered nature of the nanotubes. The confinement of the spreading cascades to planar geometries implies the removal of the mobile atoms or clusters, as will be discussed next. Unlike the MWCNTs, where the mobile atoms or the clusters may reside in interstitial positions or within shells, SWCNTs ensure the removal of the sputtered atoms and clusters from the nanotube shell. Therefore, one observes rather well-defined mass spectra of the sputtered species from SWCNTs. A thermal model was developed by considering and utilizing the earlier mentioned characteristics of the cluster emissions from irradiated SWCNTs (Javeed and Ahmad 2017) and was subsequently extended to the irradiated bulk solids like Si, Ge, and ZnO (Ahmad et al. 2018).

Localized Thermal Spike (LTS) Model A thermal model can be constructed when the emission of clusters could be assumed to be the net outcome of the emergence of a locally subliming region as a localized thermal spike (LTS). Such a model was built by utilizing the probabilities of emission of clusters that may be related with the formation energies of the respective multivacancies and the sublimation temperature TS (Javeed and Ahmad 2017; Ahmad et al. 2018). The model considers a monolayer of N carbon atoms either of a planar graphene sheet or in the cylindrical form as a SWCNT. We assume that thermal spikes are initiated in localized regions that are at elevated temperatures TS. It is further assumed that the monatomic or multiatomic vacancies can be created if one, two, three, four, or x- numbers of carbon atoms are removed in the form of clusters Cx. Single carbon atoms or clusters Cx are bonded to the matrix of the surrounding C atoms with their respective binding energies Ex. To create n vacancies with x C atoms in a target with N carbon atoms, the number of ways this can be done N! is W ¼ ðNn Þ!n! . The associated entropy is Sx ¼ klnW. The internal energy is U ¼ nEx and temperature Ts is related to Ex and leading to entropy Sx is 1=T S ¼ Ekx ð@W=@nÞ. This leads to the ratio

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px ¼


nx ¼ f exp ðEx =kT S Þ þ 1g1 ¼ probability of emission of Cx NS

ð1Þ

Here px denotes the probability of creation of a vacancy of x-C atoms. The normalized yields of the sputtered clusters can be obtained from the consecutive experimental mass spectra of clusters Cx. The experimentally measured density for the emission of Cx is directly proportional to the probability of the thermally created vacancies and is proportional to px. Probability of the emission of any of the cluster species, for example, C2, C3, C4, and the higher ones, is px ¼ nx/NS, where NS is the total number of C atoms in the spike region. Since NS is an unknown quantity, we utilize the ratio of the probabilities of two clusters to eliminate NS. Ratio of the probabilities allows calculation of physical quantities and variables like energies of formation of respective vacancies Ex,v and the spike temperature TS. These ratios are px =py ¼


  nx  ¼ exp Ey =kT S þ 1 =f exp ðEx =kT S Þ þ 1g ny

ð2Þ

In the first step of calculations, C1 will be included in the application of LTS model along with the multiatomic clusters. As the origin of monatomic C is not thermal and the density profile as a function of E(Cs+) is also different from those of the clusters (C2, C3 and C4), therefore, such a calculation will only demonstrate the inapplicability of the thermal model to the emission of C1. On the other hand, in Fig. 12, the ratios of the sputtering yields C2/C3 and C2/C4 are plotted against E (Cs+). These ratios clearly indicate the thermal nature of the emission profiles of the sputtered cluster species. The ratios of number densities of C2–C3 remain stable

Fig. 12 The ratios n(C2)/n(C3) and n(C2)/n(C4) are plotted as function of E(Cs+). (Fig. 2b, Javeed and Ahmad 2017)

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around 2.5  0.25 and 4.5  0.5 for C2/C4, for the entire range of Cs + energies. These ratios of sputtering yields will be further used in calculating the spike temperature TS.

LTS Temperature Ts The experimentally determined ratios for the probabilities of formation of vacancies ( px/py) along with the energies of formation of these vacancies in SWCNTs can be used to calculate TS by employing Eq. (2). Energies of formation of single and di-vacancies are available in literature while the tri- and quarto-vacancies can later be evaluated by using the calculated values of TS. The energies of formation of single and double vacancies ESV and EDV are obtained from DFT and DFT-based TB calculations for SWCNTs (Lu and Pan 2004; Krasheninnikov and Banhart 2007; Berber and Oshiyama 2008). These are ESV ¼ 6.8  1.0 eV and EDV ¼ 4.7  0.5 eV. The first step is to use the computationally determined values of ESV and EDV. Spike temperature TS is evaluated for each value of E(Cs+) by utilizing the proportionality px / nx in Eq. (2) and the relation px/py ¼ nx/ny. As exp(Ex/kTS)  1, the expression for LTS temperature TS ffi




 
1 Exv  Eyv =k  ln py =px

ð3Þ

The calculated values of TS for the ratio ( p2/p1) for C2/C1 ratios are shown as crossed open circles in Fig. 13 as a function of E(Cs+). The calculated values of TS based on the ratio on C2/C1 vary between 3000 K at E(Cs +) ¼0.4 keV to 5500 K at E(Cs +) ¼ 1.0 keV. Even higher values of TS ~ 9000 K are obtained for increasing E(Cs +). This line of argument demonstrates an energy-dependent TS that produces increasingly larger values that are even higher than the boiling point of graphite. Such anomalously high temperatures are due to the use of the ratio of the probabilities of emission of two sputtered species C2 and C1 that have different origins and produced by two different physical mechanisms. C1 in this case is the outcome of the binary atomic collision cascades-a nonthermal process. C2, on the other hand, is the outcome of thermal spikes in a localized region. The average values of tri- and quarto-vacancies obtained at the temperatures obtained from C2/C1 are ETV ¼ 5.13  0.5 eV and EQV ¼ 5.73  0.5 eV for E(Cs+) ¼ 0.4 to 1.0 keV. From the formation energies of C2, C3, C4 and their experimental probabilities p2, p3, p4, two sets of temperatures are calculated. These calculated values are shown in Fig. 13 as the filled red triangles and blue squares. A starred, black curve of the average of the two temperatures is shown ¼ 4016 K. This is the average spike temperature for Cs+irradiated SWCNTs in the energy range from 0.2 to 2.0 keV. The model developed using the experimental results of clusters emissions from Cs+-irradiated SWCNTs can explain localized thermal spikes. Sputtering of clusters implied a thermal origin, and we have modeled it. The normalized number densities

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Fig. 13 The spike temperature TS calculated from the normalized densities of C2/C1, C2/C3, and C2/C4 is plotted against E(Cs+). Average of TS (C2/C3) and TS (C2/C4) is shown as the thick black-starred line. (Fig. 3, Javeed and Ahmad 2017)

of clusters (C2, C3, and C4) and their mutual ratios show constancy against the variations of the energy of Cs+ as shown in Fig. 7, while C1, in the same figure, shows an energy E(Cs+)-dependent behavior.

CCs and LTSs as Information-Generating Dynamical Systems Solids irradiated by external energetic ions can also be treated as dynamical systems (Nicolis and Prigogine 1977; Berge et al. 1984; Alligood et al. 1997) where bifurcating recoil atomic trajectories may lead to collision cascades (CC) or localized thermal spikes (LTS). The two dynamical events are related yet occur in different time zones, as will be elucidated later. In this section, the dynamics of the emergence of these defects that have specific temporal and spatial profiles will be used to characterize these as the entropy-generating dissipative structures. The dynamical system will be described by evaluating information or Shannon entropy (Shannon 1948) of all the dynamical processes. Experimental results from the irradiated carbon nanostructures (Akhtar et al. 1997; Qayyum et al. 1998; Ahmad 1999; Zeeshan et al. 2012; Javeed et al. 2013; Javeed 2014) have demonstrated the need for revisiting the theoretical framework to explain the irradiation-induced damage on the 2-D surfaces of the SWCNTs. To identify the relative contributions of the mechanisms of CC and LTS, the normalized probabilities of emission of sputtered species are calculated from the experimental data

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of SWCNTs with 2 nm diameters, irradiated with Cs + with E(Cs+) ¼ 0.2 to 2.0 keV. From the experimentally determined probabilities in Section “Normalized Yields of the Sputtered Cx from Irradiated SWCNTs” (Fig. 7), the instantaneous and cumulative entropies (Shannon 1948; Cover and Thomas 1991; Ashraf et al. 2018) of all sputtered species can be obtained. We show that the Renyi’s definition of the fractal dimension (Renyi 1959) based on the cumulative information (Mandelbrot 1977; Falconer 1990; Cherepanov et al. 1995) provides the dimensional evidence about the simultaneous existence and the relative contributions of the twin-mechanisms of the binary atomic collisions cascades (CCs) and the thermal nature of the cumulative motion of multiple atoms as LTS. The fractal dimension of the four characteristic sputtered species    (C 1 , C2 , C3 , C4 ) is used as a diagnostic tool that identifies CCs and LTSs as the inter-dependent mechanisms of energy transfer by Cs+ ions to atoms of SWCNTs. It will be shown that the information-theoretic analysis is a powerful tool to probe the ion-solid interactions.

The Probability Distribution Function In this section, as in the earlier ones, the normalized probability mass distributions for each emitted species p(Cx) have the same meaning and definition and are evaluated at each increasing step of the irradiating Cs ion energy. Irradiating ion energy is treated as the variable parameter. The probability of generation of a lattice vacancy as a function of energy E(Cs+) had been defined earlier and once again for the formulation of Shannon entropy as pðCx Þ  pCx ðEðCsþ ÞÞ

ð4Þ

The advantages of employing this probability distribution function in the context of radiation damage theory, the information theoretic entropy, and fractal dimensions are to develop a comprehensive model that helps us to understand and explain the four experimental observations from the mass spectra of sputtered atoms and clusters Cx
1. These are (i) the relative absence of the monatomic C1 at very low E(Cs+), (ii) the persistent cluster emissions with the higher, relative densities of C2, C3, and C4 clusters at all irradiation energies, (iii) the energydependent probability of emission of p(C1) and consequently of the single vacancy generation on the energy of the irradiating ions E(Cs+), and (iv) the observation that probabilities of the generation of multiple vacancies with the emission of C2, C3, and C4 do not have explicit dependence on E(Cs+). These experimental observations were vividly demonstrated by the data in Fig. 6a that has shown the mass spectra dominated by clusters Cx
1. Figure 7 shows the cumulative data of emission probabilities p(Cx) of the sputtered species of the entire energy range of Cs+. The data presented in Fig. 7 is for the emission of anions from the irradiated surfaces. The structural stability of the emitted anions as opposed to cations, is well established (Middleton 1977).

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Information Theoretic Entropy and Fractal Dimension The information theoretic entropy Ixcan be evaluated from the experimentally determined probability distribution p(Cx) of Eq. (4) (Shannon 1948; Cover and Thomas 1991; Ashraf et al. 2018) I x ¼ H ðp ðCx ÞÞ ¼ 

X

pðCx ÞlnpðCx Þ ¼ ϵ

X

 pðCx Þ ln ϵ

1 pðCx Þ

ð5Þ

The summation is over all energy steps ϵ of the probability distribution for each sputtered species, from the minimum to the maximum. Two equivalent formulae are shown in Eq. (5), the first with the negative sign to make sure that the negative

values  of lnp(Cx) do not make entropy negative. The second formula with ln pðC1 x Þ not only removes the need of the

negative  sign, it also introduces the increasingly important role of the function ln pðC1 x Þ for low values of p(Cx). The second notation for Ix is the preferred one as it demonstrates the importance of the very low probability events. The information theoretic analysis can highlight all of the processes where the external energetic ions induce radiation damage in the irradiated nanotubes. Information on Shannon entropy Ix for each sputtered species provides a measure of information about the underlying mechanism that is responsible for the emission of atoms and clusters. The output from the irradiated nanotube is treated as the information given out from the SWCNT that received a well-defined ionic  1 energy input. The instantaneous values of information pðCx Þ ln pðCx Þ are plotted for the corresponding values of Cs energy  in the figures below. The sum of the instantaneous information pðCx Þ ln pðC1 x Þ yields Ix. In Fig. 14a and b, the radiation damage aspect of the model is demonstrated. The probabilities p(Cx) for the pristine and damaged SWCNTs are derived from the data of anion currents obtained from the two sets of the nineteen (19) mass spectra each, with ϵ ¼ 0.1 keV. Information-theoretic entropy Ix is the sum of p(Cx) ln (1/( p(Cx)) versus E(Cs+) shown in Fig. 14c and d. These are obtained from the experimental normalized emission densities as the probability data for the pristine and the damaged sets of SWCNTs. Information Ix alone cannot provide conclusive evidence about the detailed character of the energy dissipation mechanisms. We show that fractal dimension derived from information Ix emerges as an additional, analytical tool to unambiguously characterize the mechanisms responsible for atomic and cluster emissions. Fractal dimension can be calculated from the plots of p(Cx) ln (1/( p(Cx)) against E(Cs+) as defined by Renyi (Renyi 1959). d f ðCx Þ ¼

X ϵ

pðCx Þ ln ð1=pðCx ÞÞ= ln ð1=ϵ Þ

ð6Þ

Here ϵ refers to the number of energy steps. According to Renyi, it is the variable scale by which the dimension is defined (Renyi 1959). In Fig. 15, the fractal

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Fig. 14 (a) The probability distributions p(Cx), for the pristine sample of SWCNTs, of the 4 emitted species C1, C2, C3, and C4 are shown from the normalized anion currents at each step of E(Cs+). (b) For the highly irradiated and damaged set of SWCNTs, the probability distributions are plotted against E(Cs+). (c) The instantaneous information p(Cx) ln (1/( p(Cx)) for each of the species, at every energy step E(Cs+), is plotted for the pristine sample with p(Cx) obtained from (a). Figure (d) has the same for the highly irradiated SWCNTs as shown in (b). (Fig. 2, Ashraf et al. 2018)

dimensions df(Cx) are evaluated for the sputtered species from C1 to C4. The two parameters I(Cx) and df(Cx) are employed together to identify, distinguish, and quantify the information and the fractal dimensional space generated by the externally induced dynamic processes in irradiated carbon nanotubes. The abovementioned four (4) experimental observations from the mass spectrometric data and its information-theoretic description based on I(Cx) and df(Cx) configure the two dynamic processes: the linear, cascades of the outward expanding atomic collisions of primary and secondary collision partners as CC, and the nonlinear, localized hot spots of the LTS. Two different energy scales are associated with these processes that may occur in the similar spatial environments that occur with different time scales, as will be discussed later. The qualitative picture of C1’s dependence on Cs+ energy and the nondependence of the emission of C2, C3, and C4 on E(Cs+) is obvious from the p(Cx) versus E(Cs+) plots in Fig. 14a and b. Similar message is conveyed by the entropy plots in 14c and d. The quantitative analysis is presented in Fig. 15. For C1 the fractal

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Fig. 15 The fractal dimensions of the four emitted species C1, C2, C3, and C4 are plotted for the two sets of the pristine and damaged SWCNTs. (Fig. 3a, Ashraf et al. 2018)

dimension df(C1)~1 for the pristine and < 1 for the damaged SWCNTs. Fractal dimension is derived in Eq. (6) from the information I1  I(C1) that emerged from the energy dissipation mechanisms with E(Cs+) dependence. The pattern of I1 as a function of E(Cs+) and df(C1) ≲ 1 is indicative of linear, nonspace-filling, collision cascades. A recent experimental investigation (Wallace et al. 2017) that utilized and compared their data with the Monte Carlo simulations of SRIM (Ziegler et al. 1985) has confirmed the fractal dimension of monovalent vacancies df(single  vacancies) ~1. Our experimentally determined fractal dimension for C1 is shown in figure 15 df(C1)~1. For C2 and higher clusters, the fractal dimension df(Cx
2)~2 is shown in Fig. 15. The fractal dimension ~2 implies a space-filling, localized region that emits diatomic and larger clusters. Its space-filling character contrasts the linearly spreading, nonspace-filling character of CCs. It can happen only if the local temperature is high ~Ts and the energies of formation of multivacancies are less than the energy required for a monovalent vacancy Exv[x
2] < E1v. This is the essential requirement for p(C2) > p(C1). Only a space-filling, multifractal, localized thermal spike can describe df(C2)~df(C3)~2. The conditions that generate such a spike in irradiated SWCNTs have been discussed in the preceding section. The probability of emission of a cluster Cx with energy of formation Exv for an x-valent vacancy formation at temperature Ts has the dependence of the form p(Cx) / (exp(Exv/Ts) + 1)1. Here, p(Cx) for a particular cluster Cx is not dependent on Cs+ energy but on Ts.

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The p(Cx) vs E(Cs+) spectra in Fig. 14a and b confirm this conclusion. For the highly irradiated and damaged SWCNTs, df(C1) < 1. This may be the result of disruption of the C-C bonding due to the creation of a significant number of 2-, 3-, and 4-atom vacancies on the hexagonal network. On the other hand, the spike-related multiatomic cluster emissions are equally proficient in damaged SWCNTs as can be seen in Fig. 14b and d.

Kullback-Leibler Divergence or Relative Entropy Kullback-Leibler divergence (Kullback and Leibler 1951) or the relative entropy D( p(Cx) k p(Cy)) can be defined by using the probability distributions of any of the two emitted species Cx and Cy to estimate the information-theoretic divergence between the underlying physical processes (Kullback and Leibler 1951). Relative probability is a measure of the divergence or distance between the probability distributions p(Cx) and p(Cy). In physical terms, it implies that the two sets of information generating dynamical systems may have different origins yet may share some physical connection (Kullback and Leibler 1951). The CC and LTs are an example of such dynamical systems. Relative entropy is defined as



 D pðCx Þ k p Cy Þ ¼ pðCx Þ ln pðCx Þ=p Cy

ð7Þ

In Fig. 16, the relative probability of C1 versus C2 reduces by 50% for the damaged SWCNTs as opposed to the value for the pristine nanotubes. It implies that the distance between the two probability distributions p(C1) and p(C2) and reduces in the damaged SWCNTs. Relative probability is generally asymmetric between the two different probability distributions implying D( px k py) 6¼ D( py k px). Figure 16 shows that this is true for D( p(C1) k p(C2)) and D( p(C2) k p(C1)). This is because the origin of the emitted species are two different physical processes that generate probability distributions which do not relate to the identical mechanisms of energy dissipation. This asymmetric relationship can be employed for identifying the underlying dissipative structures. It can be shown that whenever the origin of the two emitted species is same then the relative entropies are approximately symmetric, for  example, D( p(C2) k p(C3)) ≈ D( p(C3) k p(C2)). It is due that C 2 and C3 are being generated by the same thermal mechanism of LTS in the irradiated CNTs, single- or multiwalled. It needs to be emphasized that the information theoretic entropy Ix is a measure of ignorance; therefore, the relative entropy D( px k py) may provide relative information for the experimental outputs of the two mechanisms, which in the case of the linear cascades and nonlinear spikes varies in magnitude. Their representative probability distributions have significant divergence from each other as compared with those of the set of two clusters, like C2 and C3 that are emitted from LTS. This relative divergence is shown to be farther in pristine SWCNTs as opposed to that in the damaged ones.

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Fig. 16 Relative entropy D( p(C1) k p(C2)) as a measure of the distance between the two probability distributions of C1 and C2 is calculated and shown for the pristine and damaged SWCNTs. (Fig. 3b, Ashraf et al. 2018)

Spatially Coherent and Temporally Divergent CCs and LTSs Figure 17a highlights the essential features of energy sharing by the sp2-bonded atoms of the hexagons. The fundamental difference between the energy dissipation mechanisms in the typical metallic bulk solids and the sp2-bonded CNTs is due to the covalently bonded carbon atoms in hexagons on a mono-shelled surface. The figure shows the hexagonal network of atoms. The first atom receives energy E1  1. If E1  Edis– the energy that is required to break all bonds and to force the struck atom to leave its lattice site, then a binary collision cascade is generated with the energetic primary knock-on atom. The collisions proceed until the recoiling energy Edisp cascades are likely to start, creating single vacancies, sputtering atoms C1. Probability of emission depends on linear energy dissipation (dE/dx)γ where γ identifies the atomic collision mechanisms. The collision times τC  C~1014 s; df(C1)~1 (ii) Thermal spikes generated by recycling of energy by atoms of the adjacent hexagons for ~ 1 eV, leading to localized spikes. Probabilities of cluster emission p(Cx) / (exp(Exv/Ts) + 1)1 depend upon spike temperature Ts. Spikes occur for longer times τC  C~1012 s; fractal dimension of the representative cluster C2 is df(C2)~2

on the primary knock-ons. Thermal spikes are initiated with energies Edisp where the sharing and recycling of energy in the localized hexagonal patches as shown in Fig. 17b. It starts around 1013s and subsides into collective atomic vibration time scale ~1012 s. The two processes occur in the same spatial regions but happen at different time scales. Emission of C1 from the collision cascades in Fig. 17a represents a nonequilibrium, linear process with df(C1) 1 that occurs at T~300 K. It is the by-product of a high energy (
Edisp), low information-theoretic entropy, dissipative structure with durations ~1014 s.

Conclusions The experimental observations from the mass spectra of the sputtered atoms and clusters from SWCNTs and MWCNTs for the different ranges of Cs+ energies had shown that

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(i) The domination of C2, C3, C4, and higher clusters is a persistent feature of the mass spectra over the monatomic emissions. (ii) The absence of C1 in the mass spectra at very low Cs+ energies and its consistent, low relative intensities as compared with those of the clusters, is indicative of the low efficiency of CC as opposed to the LTS. (iii) There is an energy dependence of the probability p(C1) for the emission of C1 on Cs+ energy p(C1) / E(Cs+), while the normalized probabilities of emission of C2, C3, and C4 show an energy-independence. By evaluating Shannon’s information-theoretic entropies of all emitted species Cx by utilizing their experimentally evaluated probability distributions p(Cx) as a function of E(Cs+), their respective fractal dimensions df(Cx) can be evaluated. The fractal dimensions of the individual species df(Cx) can be employed to identify, distinguish, and specify the externally induced dynamic processes and the associated spatial dimensions in the irradiated CNTs. The experimental observations of the mass spectrometric data and the information-theoretic fractal dimensions df(Cx) configure the emergence of the dynamic processes: (a) the linear CC and (b) the nonlinear LTS. Figure 18 graphically demonstrates the initiation of energy sharing processes in the irradiated sp2-bonded atoms in hexagonal networks of single- and multiwalled carbon nanotubes and graphene sheets of nanoscale graphite powder. Two different energy scales are associated with the two processes. The first is started with energies received by the atoms of the hexagons Ecascades
Edisp, where Edisp is the energy required to displace an atom from its site. In SWCNTs and MWCNTs, Edisp~30  40 eV. The energy needed to generate thermal spikes Espike Edisp. The two processes occur in the same spatial regions but happen at different time scales. Binary atomic collision cascades are initiated with energies
Edisp and occur at time scales ~1015  1014 s. Localized thermal spikes are initiated with energies

Edisp where the sharing and recycling of energy in the hexagonal patches. It starts around 1013s and subsides into the collective atomic vibration time scale ~1012  1011 s. Shannon entropy and the relative entropy of the probability distributions belonging to the two different physical processes and the data on fractal dimensions of the emitted species have been used here to determine the physically distinguishable features of the cascades and spikes. In Fig. 18, the graphical representation of the spatial and temporal route is demonstrated for the generation of localized hot subliming patches that emerge due to the energy recycling hexagons. These lead to the localized thermal spikes where Ts~3500  4000K (Javeed and Ahmad 2017; Ahmad et al. 2018). In conclusion, an information-theoretic model for the fragmenting, irradiated CNTs can describe two different yet related dissipative structures that had a common origin in the irradiating Cs ion. It can be presented with the Cs+ energy as the input parameter or the agent for the dynamic events. The energy is consumed and dissipated in linear and nonlinear processes that are initiated in the mono- and multishelled carbon nanotubes. The output signal is in the form of sputtered atoms and clusters, emitted from the surfaces of the irradiated nanostructures. From the

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normalized number densities of the sputtered species, the probability distributions p(Cx) are constructed for every species that is emitted from the irradiated hexagonal surfaces. This information is compiled for each of the emitted constituent for the three C allotropes in the form of information or Shannon entropy. Information is further employed to calculate the fractal dimension of all sputtered species. Relative entropy is calculated for pairs of the emitted species. We have shown and emphasized that fractal dimension, on its own, is a powerful analytical tool to determine the spatial nature of the physical dimension of a dissipative structure. However, under certain circumstances, it may need relative entropy to clarify the nature of the external ion-induced dynamical processes in carbon’s nanostructures. Together, these information-theoretic parameters are shown to identify, distinguish, and characterize the existence and operational efficiency of the linear cascades(CCs) and nonlinear thermal spikes (LTSs) in irradiated carbon nanotubes. The model can be extended to the emergence of dissipative structures in various physical and chemical environments. This model has been employed to diagnose and ascertain the nature of the self-organizing, dynamical systems as demonstrated in the case of the emergence of Buckyball from the fragmenting fullerenes in the hot carbon soot (Ahmad 2018a, b).

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Cover TM, Thomas JA (1991) Elements of Information Theory. Wiley, New York Ebbesen TW, Ajayan PM (1992) Large-scale synthesis of carbon nanotubes. Nature 358:220–222 Endo M, Takeuchi K, Igarashi S, Kobori K, Shiraishi M, Kroto HW (1993) The production and structure of pyrolytic carbon nanotubes (PCNTs). J Phys Chem Solids 54(12):1841–1848 Endo M, Takeuchi K, Kobori K, Takahashi K, Kroto HW, Sarkar A (1995) Pyrolytic carbon nanotubes from vapor-grown carbon fibers. Carbon 33(7):873–881 Falconer KJ (1990) Fractal geometry. Wiley, Chichester Farhat S, Chapelle ML, Loiseau A, Scott CD, Lefrant S, Journet C, Bernier P (2001) Diameter control of single-walled carbon nanotubes using argon–helium mixture gases. J Chem Phys 115 (14):6752–6759 Honig RE (1954) Mass spectrometric study of the molecular sublimation of graphite. J Chem Phys 22(1):126–131 Iijima S (1991) Helical microtubules of graphitic carbon. Nature 354:56–58 Javeed S (2014) Synthesis and fragmentation of carbon clusters and nanotubes. PhD Thesis, PIEAS, Islamabad Javeed S, Ahmad S (2017) Thermal spike-induced cluster sublimation from carbon nanotubes. Philos Mag 97:1436–1444 Javeed S, Zeeshan S, Ahmad S (2013) Dynamics of fragmentation and multiple vacancy generation in irradiated single-walled carbon nanotubes. Nucl Inst Methods Phys Res B 295:22–29 Jin C, Suenaga K, Iijima S (2008) Vacancy migrations in carbon nanotubes. Nano Lett 8:1127–1130 Krasheninnikov AV, Banhart F (2007) Engineering of nanostructured carbon materials with electron or ion beams. Nat Mater 6:723–733 Krasheninnikov AV, Nordlund K (2010) Ion and electron irradiation induced effects in nanostructured materials. J Appl Phys 107:071301–071370 Krasheninnikov AV, Nordlund K, Lehtinen PO, Foster AS, Ayuela A, Nieminen RM (2004) Adsorption and migration of carbon adatoms on zigzag carbon nanotubes. Carbon 42:1021– 1025 Kroto HW, Heath JR, O’Brien SC, Curl RF, Smalley RE (1985) C60: Buckminsterfullerene. Nature 318:162–163 Kullback S, Leibler RA (1951) On information and sufficiency. Ann Math Stat 22:79–86 Lindhard J, Nielsen V, Scharff M, Thomsen PV (1963) Integral equations governing radiation effects. Mat Fys Medd Dan Vid Selsk 33(10):1–42 Lu AJ, Pan BC (2004) Nature of single vacancy in achiral carbon nanotubes. Phys Rev Lett 92: 105504–105508 Mandelbrot BB (1977) The fractal geometry of nature. W. H. Freeman & Co., New York Middleton RA (1977) A survey of negative ions from a cesium sputter source. Nucl Inst Methods 144(3):373–399 Nicolis G, Prigogine I (1977) Self-organization in non-equilibrium systems. Wiley, New York Qayyum A, Ahmad B, Akhtar MN, Ahmad S (1998) Heavy ion induced cluster formation and fragmentation phenomena in amorphous graphite. Eur Phys J D 3:267–274 Renyi A (1959) On the dimension and entropy of probability distribution. Acta Math Acad Sci Hungaria 10:193–215 Robinson MT (1965) The energy spectra of atoms slowing down in structureless media. Philos Mag 12:145–156; Influence of the scattering law on radiation damage displacement cascades, Philosophical Magazine, 741–765 Shannon CE (1948) A mathematical theory of communication. Bell Syst Tech J 27:379–424 Sigmund P (1969) Theory of sputtering: sputtering yield of amorphous and polycrystalline targets. Phys Rev 184:383–416 Skakalov V, Kaiser AB, Osvath Z, Vertesy G, Biro LP, Roth S (2008) Ion irradiation effects on conduction in single-wall carbon nanotube networks. Appl Phys A 90:597–602 Thess A, Lee R, Nikolaev P, Dai H, Petit P, Robert J, Xu C, Lee YH, Kim SG, Rinzle AG, Colbert DT, Scuseria DT, Tomanek D, Fischer JE, Smalley RE (1996) Crystalline ropes of metallic carbon nanotubes. Science 273:483–487 Thompson MW (1968) The energy spectrum of ejected atoms during the high energy sputtering of gold. Philos Mag 18:377–414

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Wallace JB, Aji BB, Shao L, Kucheyev SO (2017) Fractal analysis of collision cascades in pulsedion-beam irradiated solids. Sci Rep 7:17574–17584 Whittaker AG, Kintner PL (1969) Particle emission during the sublimation of graphite. Carbon 7: 414–416 Yudasaka M, Komatsu T, Ichihashi T, Iijima S (1997) Single-wall carbon nanotube formation by laser ablation using double-targets of carbon and metal. Chem Phys Lett 278:102–106 Zeeshan S, Javeed S, Ahmad S (2012) Mass spectrometric identification of C60’s fragmentation regimes under energetic Cs+ bombardment. Int J Mass Spectrom 311:1–6 Ziegler JF, Biersack JP, Littmack U (1985) The stopping and ranges of ions in solids, vol I. Pergamon Press, New York

The Current Market for Carbon Nanotube Materials and Products

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Helen Uchenna Modekwe, Olusola Olaitan Ayeleru, Matthew Adah Onu, Nyam Tarhemba Tobias, Messai Adenew Mamo, Kapil Moothi, Michael Olawale Daramola, and Peter Apata Olubambi

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon Nanotubes Current Markets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Production Volumes of Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Characteristics of CNT Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Global Market for Carbon Nanotubes Products Based on Type, Application, and Regional Market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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H. U. Modekwe · K. Moothi Department of Chemical Engineering, Faculty of Engineering and the Built Environment, University of Johannesburg, Johannesburg, South Africa e-mail: [email protected]; [email protected] O. Olaitan Ayeleru (*) · M. A. Onu · N. T. Tobias · P. A. Olubambi Centre for Nanoengineering and Tribocorrosion (CNT), University of Johannesburg, Johannesburg, South Africa e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected] M. A. Mamo Research Centre for Synthesis and Catalysis, Department of Chemical Science, Doornfontein Campus, Faculty of Science, University of Johannesburg, Johannesburg, South Africa e-mail: [email protected] M. O. Daramola Department of Chemical Engineering, Faculty of Engineering, Built Environment and Information Technology, University of Pretoria, Pretoria, South Africa e-mail: [email protected] © Springer Nature Switzerland AG 2022 J. Abraham et al. (eds.), Handbook of Carbon Nanotubes, https://doi.org/10.1007/978-3-030-91346-5_73

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Abstract

In recent years, market for carbon nanotubes (CNTs) and its products have experienced huge expansion owing to availability and large-scale production of CNTs and their products. This obtainability is leading to average price reduction thereby opening-up endless opportunities for possible new applications and several end users as compared to several years back. Today, there are several driving factors that are increasing the global demand for CNTs. These factors include, increasing need for light weight structural polymer composites and low carbon emitting vehicles; superior physical, chemical and mechanical properties; technological advancement in electronics and electricals, automobiles, and the need for renewable energies. In this chapter, the present-day markets for CNTs materials and products are extensively addressed with respect to the type, their application sectors, available and projected end users market, and their market in different regions across the globe. It was concluded that growing market for MWCNTs would continue to lead while applications in structural polymer composites with electronics and automobile end users’ market would boost the CNTs market. Soon, extensive research and development (R&D) geared toward discovering new application areas for CNTs and development in CNTs manufacturing process such as growing zero defect CNTs and low-cost material design are expected to increase the demand for CNTs materials and products. Keywords

Global market · Multi-walled carbon nanotubes · Single-walled carbon nanotubes · Double-walled carbon nano tubes · Applications

Introduction Carbon nanotubes (CNTs) are allotropic form of carbon comprising sheets of graphene, which are rolled up into seamless cylinder. They can be categorized according to the number of rolled up graphene sheets as single-walled carbon nanotubes (SWCNTs) comprising a single graphene sheet with diameter in the range of 0.4–3 nm or multi-walled carbon nanotubes (MWCNTs) comprising multilayers of concentric graphene sheets with diameter up to 100 nm (Belin and Epron 2005; Fu et al. 2013; Manawi et al. 2018). CNTs comprise carbon atom of sp2 hybridized with the carbon–carbon σ bonds forming a hexagonal honeycomb-like structure and a pair of π electrons not taking part in the process. The bonding in the nanotubes is entirely of sp2 bonds, which gave them unique strength of 10 times higher than any industrial fiber. The extreme length-to-diameter ratio (aspect ratio) of about 1.32
108 is higher than other known existing materials. Interestingly, due to the unique hexagonal structure of CNTs, they have excellent physical and chemical, mechanical, electrical, and thermal properties, which singled them out as advanced modern-day material for many applications (Fu et al. 2013; Manawi et al. 2018).

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Carbon nanotubes are derivatives of fullerene and carbon fibers rolled into cylindrical shapes, whose molecules are made of 60 carbon atoms (Polizu et al. 2006). There is the convergence of views on the unique attributes of CNTs in terms of electrical, thermal, optical, and mechanical, as they are described simply as excellent and distinctive (Anzar et al. 2020; Jagadeesan et al. 2020). CNTs are qualified as possessing high chemical and thermal stability, ultra-lightweight, high tensile strength, and special electronic structure (Anzar et al. 2020).

Carbon Nanotubes Current Markets From 2004 to 2008, the CNT annual growth rate was around 30%, but later crashed between 2009 and 2010 because of market crisis globally, however, the annual growth has been predicted around 34% (Jensen et al. 2015). In 2009, the global CNT market was ~$100 million and rose to ~$159 million in 2014. Nevertheless, between 2004 and 2008, the global CNTs market has continued to grow and is expected to reach over ~$9.84 billion by 2023 from ~$3.95 billion in 2017 projecting the compound annual growth rate (CAGR) between 2018 and 2023 to be around 16.7% (Carbon Nanotubes (CNT) Market 2021). The increasing demand for CNTs materials and products is propelled by the growing need for lightweight structural polymer composites and materials that can withstand external wear and tear since CNTs are 100 times tougher and 5 times lighter than steel. CNTs products and materials early markets are dominated in the categories of CNT-structural polymer composites, conductive polymer composites, electron field emitters by the end userindustries in automotive and aerospace components, industrial construction and sporting goods, electronics housing, and packaging, electrostatic discharge, and electromagnetic interference (EMI) shielding, flat display panel, touch screens, and various sensors and capacitors and batteries (Beigbeder et al. 2008; Kingston et al. 2014; Manawi et al. 2018). Figure 1 shows the global CNTs market demand by end user, with electronics and data storage occupying the highest. Leveraging on the extraordinary properties of CNTs, the current consumer products and applications that use CNTs products are almost infinite (Kingston et al. 2014). Currently, there are few available market data that quantify the exact commercial applications of CNTs, this technical information is usually withheld by companies over a long period of time after product commercialization, and this makes it quite challenging to predict the real market progress and achievements in CNTs product and materials (Kingston et al. 2014; De Volder et al. 2013). However, it is known that there is an increase in the production capacity and sales of CNTs, which could be used as a measure for emerging CNTs applications and market. Furthermore, global production has been estimated to exceed the forecasted kiloton level per year as new manufacturers emerge while the existing manufacturers are also increasing their production capacities. Over 100 companies are major players in the manufacturing and processing of CNTs. These include: Nanocyl SA, Hyperion Catalysis International, Arkema Group, OCSiAl, Cheap Tubes Inc., Nanothinx SA, Continental Carbon Nanotechnologies, etc., of which the major driving

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Fig. 1 Global CNTs market size (%) by end users. (Source: Kingston et al. 2014 with permission from Elsevier)

factor is the increasing demands in various applications of CNTs products and materials from end user industries (Choudhary and Prasad 2020). CNTs market is highly commodity price dependent and has experienced considerable cost reduction especially for MWCNTs prices in recent years due to the emergence of several low-cost feedstocks, the use of metal catalysts to enhance yield, and reduction in energy consumption through the continuous development in chemical vapor deposition (CVD) method, which permits scaling-up and better control over CNTs morphology and properties (Thayer 2007; Aitken et al. 2006). Demand for CNTs has experienced rapid increase in applications such as lithium-ion batteries, field emission displays, integrated circuits, solar photovoltaic cells, fuel cells, hydrogen storage, aerospace and automotive, flame retardants, etc. Huge business potential for CNTs materials and products is envisaged in some commercial applications especially in the biotechnology and biomedical industries such as dental filling materials, cancer therapy, body implants, tissue engineering, medical devices and sensor probes, textile, agriculture, and food processing (Master 2019). For example, in the delivery of cancer drug doxorubicin to the receptor cell membrane, up to 60% could be loaded on CNT compared to 8–10% on liposomes (Choudhary and Prasad 2020). The likelihood of increase in more potential applications of CNTs will keep the demand and market for CNTs expanding while prices are expected to drop even further as production capacity continues to rise. However, the emergence of other substitutes such as the inorganic nanotubes; silicon carbide nanotubes (SiCNTs), boron nitride nanotubes (BNNTs), nanoribbons, etc., which are considered low-priced and less carcinogenic may negatively impact the global CNTs market soon (Rao et al. 2018).

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Production Volumes of Carbon Nanotubes In 2004, a global survey was conducted for the first time and about 54 manufacturers were interviewed and based on their responses, it was established that the annual production capacity of CNTs and nanofibers was less than 70 tons. A recent study has shown that the production of CNTs was around 600 to about 3500 metric tons in 2014 with about 1800 to around 2100 tons as the average guesstimate. This capacity is very high and is predicted to be in the range of around 2
104 tons in 2014. Besides, the quantity of MWCNTs produced from the overall batch is usually far higher than that of the SWCNT and other CNT materials. In addition, MWCNT is the most applied of all the CNT products and this could be attributed to its availability and affordability. However, in the year 2012, the production capacity did not correspond to the market demand, which stood at around 3250 to about 3750 tons. This clearly shows that the production capacity far exceeds yearly production. CNT products have greater market potential and in 2015; it was predicted to be around $ 20 billion. The highest possible manufacturers of CNTs globally are the firms owned by the Chinese (Jensen et al. 2015).

General Characteristics of CNT Products For the SWCNTs, prices often fall between £ 22 and £ 62 per kilogram and these prices vary and can be up to £ 265 per kilogram according to the prices of 2015. In most cases, the prices of SWCNT are usually above 25% for functionalized qualities. The majority of the functionalization, which enable SWCNT to be capable of being dispersed in water, include the acidic (-COOH), the hydroxyl (-OH), and the amidogen (-NH2). Usually, the composition of the functionalization within the SWCNT is relatively 1 and 40 nm (Jensen et al. 2015). DWCNT has a purity of >50% and less than