Nanomaterials Synthesis: Design, Fabrication and Applications 0128157518, 9780128157510

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Nanomaterials Synthesis

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Nanomaterials Synthesis Design, Fabrication, and Applications Edited by YASIR BEERAN POTTATHARA University of Maribor, Faculty of Mechanical Engineering, Slovenia

SABU THOMAS The Vice Chancellor of Mahatma Gandhi University, Kottayam and Founder Director of International and Inter University Centre for Nanoscience and Nanotechnology, and Professor at School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India

NANDAKUMAR KALARIKKAL Director and an Associate Professor of International and Inter University Centre for Nanoscience and Nanotechnology, and Director and Chair of School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, India

YVES GROHENS Director of LIMATB Laboratory, University of South Brittany (UBS), France

VANJA KOKOL Associate Professor at University of Maribor, Faculty of Mechanical Engineering, Slovenia

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2019 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-815751-0 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Simon Holt Editorial Project Manager: Andrae Akeh Production Project Manager: R. Vijay Bharath Cover Designer: Greg Harris Typeset by MPS Limited, Chennai, India

CONTENTS List of Contributors

xi

1. Synthesis and Processing of Emerging Two-Dimensional Nanomaterials

1

Yasir Beeran Pottathara, Yves Grohens, Vanja Kokol, Nandakumar Kalarikkal and Sabu Thomas 1.1 Introduction 1.2 Emerging 2D Nanomaterials: Uniqueness and Advances 1.3 Synthesis Approaches 1.4 Summary and Outlook References

2. Nanomaterial Synthesis: Chemical and Biological Route and Applications

1 3 5 16 17

27

Santanu Sasidharan, Shweta Raj, Shirish Sonawane, Shriram Sonawane, Dipak Pinjari, A.B. Pandit and Prakash Saudagar 2.1 Introduction and Background: Nanoparticle Synthesis Approaches 2.2 Different Chemical Routes for Nanomaterial Synthesis 2.3 Nanoparticle Synthesis Using the Biological Route 2.4 Application of Nanomaterials 2.5 Recent Advances in the Chemical and Biological Synthesis Routes 2.6 Scale-Up Issues of Nanoparticle Production and Challenges 2.7 Summary Acknowledgment References

3. Chemical Approaches for 1D Oxide Nanostructures

27 31 37 42 44 46 46 47 47

53

F.A. Taher and E. Abdeltwab 3.1 Introduction 3.2 1D Nanostructure Synthesis Techniques 3.3 1D ZnO Nanostructures 3.4 1D TiO2 Nanostructure 3.5 Conclusion References

53 55 57 67 79 79

v

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4. One- and Two-Dimensional Nanostructures Prepared by Combustion Synthesis

85

A.S. Mukasyan and K.V. Manukyan 4.1 Introduction 4.2 CS Fundamentals 4.3 Microstructural Characteristics of Combustion-Derived Nanomaterials 4.4 Conclusions References

5. Microwave-Assisted Synthesis for Carbon Nanomaterials

85 88 95 112 114

121

Sabzoi Nizamuddin, Sadaf Aftab Abbasi, Abdul Sattar Jatoi, M.T.H. Siddiqui, Humair Ahmed Baloch, N.M. Mubarak, G.J. Griffin, E.C. Abdullah, Khadija Qureshi and Rama Rao Karri 5.1 5.2 5.3 5.4

Introduction Methods of Synthesis of Carbon Nanomaterials Chemical Vapor Deposition Plasma-Enhanced Chemical Vapor Deposition-Based Carbon Nanomaterials 5.5 Microwave-Enhanced Chemical Vapor Deposition 5.6 Fluidized Bed Chemical Vapor Deposition 5.7 Vapor Phase Growth Chemical Vapor Deposition 5.8 Microwave-Assisted Synthesis of Graphene 5.9 Future Prospects for Carbon Nanomaterial Synthesis and Challenges 5.10 Conclusion References

6. Strategies in Laser-Induced Synthesis of Nanomaterials

121 122 124 127 130 134 137 138 139 140 141

149

V. Saikiran, Mudasir H. Dar, R. Kuladeep, L. Jyothi and D. Narayana Rao 6.1 Introduction 6.2 Experimental Fabrication 6.3 Results and Discussion 6.4 Conclusions References

7. Flame Synthesis of Nanostructured Transition Metal Oxides: Trends, Developments, and Recent Advances

150 158 162 190 191

201

Wilson Merchan-Merchan, Walmy Cuello Jimenez, Octavio Rodriguez Coria and Chad Wallis 7.1 Introduction

202

Contents

7.2 Properties and Applications of 1D and 3D Transition Metal Oxide Nanostructures 7.3 Fabrication Techniques to Synthesize TMO Nanostructures 7.4 Flames as a Unique Fabrication Tool to Produce TMO Nanoparticles 7.5 Flame Synthesis of Multidimensional TMOs Using the “Solid Support” Method 7.6 Volumetric Flame Synthesis of 1D and 3D TMOs 7.7 Core
Shell and Mixed Transition Metal Oxide Nanostructures 7.8 Conclusions Acknowledgments References

8. Design and Fabrication of Porous Nanostructures and Their Applications

vii

206 214 218 226 235 240 252 253 253

265

Arpita Hazra Chowdhury, Noor Salam, Rinku Debnath, Sk. Manirul Islam and Tanima Saha 8.1 Introduction 8.2 Classification of Porous Nanostructures 8.3 Synthesis of Porous Materials 8.4 New Synthesis Approaches and Challenges of Porous Nanostructures 8.5 Applications of Porous Materials 8.6 Conclusion References

9. Synthesis and Processing of Thermoelectric Nanomaterials, Nanocomposites, and Devices

266 267 267 278 279 288 288

295

Lazaros Tzounis 9.1 9.2 9.3 9.4 9.5

Introduction to Energy Needs and Wasted Thermal Energy Fundamentals of the Thermoelectric Effect and Thermoelectric Materials Inorganic Thermoelectric Nanomaterials Organic Thermoelectrics: Polymer and Nanocomposite Systems Working Principle and Specific Architectures of Thermoelectric Generators 9.6 Application of Thermoelectric Generators 9.7 Recent Trends and Challenges 9.8 Future Perspectives 9.9 Summary and Conclusions Acknowledgment References

295 297 302 309 319 321 322 326 326 327 327

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Contents

10. Fabrication Techniques of Group 15 Ternary Chalcohalide Nanomaterials

337

Marian Nowak, Marcin Jesionek, and Krystian Mistewicz 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 10.12 10.13

Introduction Fabrication of Composite Materials Ball Milling of Bulk Crystals Vapor-Phase Growth of SbSI Nanorods Graphoepitaxy Sonochemical Synthesis of SbSI-Type Nanowires Ultrasonic Spray Pyrolysis Filling of Carbon Nanotubes Solution Processing Microwave-Assisted Aqueous Synthesis Hydrothermal Growth Conversion of Sb2S3 Into SbSI Heat and Laser Formation of SbSI Nano-Objects in Chalcohalide Glasses 10.14 New Trends in Fabrication Techniques 10.15 Future Perspectives 10.16 Summary References

338 339 346 347 349 349 356 359 363 365 366 369

11. Advanced Carbon Materials for Electrochemical Energy Storage

385

370 377 378 378 378

Rohit Ranganathan Gaddam, Nanjundan Ashok Kumar, Ramanuj Narayan, K.V.S.N. Raju and X.S. Zhao 11.1 Introduction 11.2 Carbon Materials: Types and Sources 11.3 Carbon Materials for Energy Storage 11.4 Challenges and Future Perspectives Acknowledgments References

12. Organic
Inorganic Hybrid Nanomaterials: Synthesis, Characterization, and Application

385 388 397 413 414 414

419

Vesna Lazi´c and Jovan M. Nedeljkovi´c 12.1 Introduction 12.2 Interfacial Charge Transfer Complexes: Formation Mechanism and Optical Properties 12.3 Polymer Supports Decorated With Inorganic Nanoparticles 12.4 New Synthetic Approaches and Challenges

419 420 431 435

Contents

12.5 Potential Application of OrganicaInorganic Hybrids: Photo-Driven Processes and Antimicrobial Ability 12.6 Summary and Outlook Acknowledgments References

13. Fabrication, Characterization, and Optimization of MnxOy Nanofibers for Improved Supercapacitive Properties

ix

437 443 444 444

451

Jai Bhagwan, Nagesh Kumar and Yogesh Sharma 13.1 Introduction 13.2 Synthesis of 1D Nanofibers 13.3 Utilization of Binary MnxOy Nanofibers for Energy-Storage Applications 13.4 Future Aspects, Challenges, and Summary Acknowledgments References

14. Fabrication of Micro/Nano-Miniaturized Platforms for Nanotheranostics and Regenerative Medicine Applications

451 454 469 476 478 478

483

G. Praveen, Nandakumar Kalarikkal and Sabu Thomas 14.1 Nanotheranostics and Regenerative Medicine: Introduction 14.2 Bioartificial Organs: Introduction 14.3 Micro- and Nanofluidic Devices: Introduction 14.4 Biomimetics: Introduction 14.5 Biopatterning the Complexities of Life: Introduction 14.6 Bioprinting of Organs and Tissues 14.7 Conclusions References

484 487 490 494 497 501 514 515

15. Recent Trends in the Synthesis of Carbon Nanomaterials

519

María M. Afonso and José Antonio Palenzuela 15.1 Fullerenes 15.2 Carbon Nanotubes 15.3 Graphene 15.4 Graphene Nanoribbons 15.5 Carbon Dots 15.6 Challenges and Future Perspectives References Index

520 525 529 539 540 543 544 557

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LIST OF CONTRIBUTORS Sadaf Aftab Abbasi

School of Engineering, RMIT University, Melbourne, VIC, Australia E. Abdeltwab

Chemistry Department, Faculty of Science (Girls Branch), Al-Azhar University, Cairo, Egypt; Al-Azhar Technology Incubator (ATI), Cairo, Egypt E.C. Abdullah

Department of Chemical Process Engineering, Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia (UTM), Jalan Sultan Yahya Petra, Kuala Lumpur, Malaysia María M. Afonso

Department of Organic Chemistry, Universitary Institute of Bio-Organic Chemistry, University of La Laguna, La Laguna, Tenerife, Spain Humair Ahmed Baloch

School of Engineering, RMIT University, Melbourne, VIC, Australia Jai Bhagwan

Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee, India Arpita Hazra Chowdhury

Department of Chemistry, University of Kalyani, Kalyani, India Octavio Rodriguez Coria

School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK, United States Mudasir H. Dar

Department of Physics, Govt. Degree College, Anantnag, India; School of Physics, University of Hyderabad, Hyderabad, India Rinku Debnath

Department of Molecular Biology & Biotechnology, University of Kalyani, Kalyani, India Rohit Ranganathan Gaddam

School of Chemical Engineering, The University of Queensland, Brisbane, QLD, Australia G.J. Griffin

School of Engineering, RMIT University, Melbourne, VIC, Australia Yves Grohens

Director of LIMATB Laboratory, University of South Brittany (UBS), France

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List of Contributors

Sk. Manirul Islam

Department of Chemistry, University of Kalyani, Kalyani, India Abdul Sattar Jatoi

Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi, Pakistan Marcin Jesionek

Institute of Physics, Silesian University of Technology, Katowice, Poland Walmy Cuello Jimenez

School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK, United States L. Jyothi

School of Physics, University of Hyderabad, Hyderabad, India Nandakumar Kalarikkal

Director and an Associate Professor of International and Inter University Centre for Nanoscience and Nanotechnology, Kottayam, India; Director and Chair of School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, India Rama Rao Karri

Petroleum anc Chemical Engineering, Universiti Teknologi Brunei, Brunei Darussalam Vanja Kokol

Associate Professor at University of Maribor, Faculty of Mechanical Engineering, Slovenia R. Kuladeep

School of Physics, University of Hyderabad, Hyderabad, India Nagesh Kumar

Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee, India Nanjundan Ashok Kumar

School of Chemical Engineering, The University of Queensland, Brisbane, QLD, Australia Vesna Lazi´c

Vinˇca Institute of Nuclear Sciences, University of Belgrade, Belgrade, Serbia K.V. Manukyan

Nuclear Science Laboratory, Department of Physics, University of Notre, Notre Dame, IN, United States Wilson Merchan-Merchan

School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK, United States

List of Contributors

xiii

Krystian Mistewicz

Institute of Physics, Silesian University of Technology, Katowice, Poland N.M. Mubarak

Department of Chemical Engineering, Faculty of Engineering and Science, Curtin University, Sarawak, Malaysia A.S. Mukasyan

Department of Chemical & Biomolecular Engineering, University of Notre, Notre Dame, IN, United States Ramanuj Narayan

Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India Jovan M. Nedeljkovi´c

Vinˇca Institute of Nuclear Sciences, University of Belgrade, Belgrade, Serbia Sabzoi Nizamuddin

School of Engineering, RMIT University, Melbourne, VIC, Australia Marian Nowak

Institute of Physics, Silesian University of Technology, Katowice, Poland José Antonio Palenzuela

Department of Organic Chemistry, Universitary Institute of Bio-Organic Chemistry, University of La Laguna, La Laguna, Tenerife, Spain A.B. Pandit

Institute of Chemical Technology, Matunga, India Dipak Pinjari

Institute of Chemical Technology, Matunga, India Yasir Beeran Pottathara

University of Maribor, Faculty of Mechanical Engineering, Slovenia G. Praveen

International and Interuniversity Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India Khadija Qureshi

Department of Chemical Engineering, Mehran University of Engineering and Technology, Jamshoro, Pakistan Shweta Raj

Department of Biotechnology, National Institute of Technology, Warangal, India K.V.S.N. Raju

Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India D. Narayana Rao

School of Physics, University of Hyderabad, Hyderabad, India

xiv

List of Contributors

Tanima Saha

Department of Molecular Biology & Biotechnology, University of Kalyani, Kalyani, India V. Saikiran

Department of Electronics and Physics, Institute of Science, GITAM, Visakhapatnam, India; School of Physics, University of Hyderabad, Hyderabad, India Noor Salam

Department of Chemistry, University of Kalyani, Kalyani, India Santanu Sasidharan

Department of Biotechnology, National Institute of Technology, Warangal, India Prakash Saudagar

Department of Biotechnology, National Institute of Technology, Warangal, India Yogesh Sharma

Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee, India; Department of Physics, Indian Institute of Technology Roorkee, Roorkee, India M.T.H. Siddiqui

School of Engineering, RMIT University, Melbourne, VIC, Australia Shirish Sonawane

Chemical Engineering Department, National Institute of Technology, Warangal, India Shriram Sonawane

Chemical Engineering Department, Visvesvaraya National Institute of Technology, Nagpur, India F.A. Taher

Chemistry Department, Faculty of Science (Girls Branch), Al-Azhar University, Cairo, Egypt; Al-Azhar Technology Incubator (ATI), Cairo, Egypt; Physics Department, Faculty of Science, (Girls), Al-Azhar University, Cairo, Egypt Sabu Thomas

The Vice Chancellor of Mahatma Gandhi University, Kottayam, India; Founder Director of International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India; Professor at School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India Lazaros Tzounis

Department of Materials Science & Engineering, University of Ioannina, Ioannina, Greece

List of Contributors

Chad Wallis

School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK, United States X.S. Zhao

School of Chemical Engineering, The University of Queensland, Brisbane, QLD, Australia

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CHAPTER 1

Synthesis and Processing of Emerging Two-Dimensional Nanomaterials Yasir Beeran Pottathara1, Yves Grohens2, Vanja Kokol3, Nandakumar Kalarikkal4,5 and Sabu Thomas6,7,8, 1

University of Maribor, Faculty of Mechanical Engineering, Slovenia Director of LIMATB Laboratory, University of South Brittany (UBS), France Associate Professor at University of Maribor, Faculty of Mechanical Engineering, Slovenia 4 Director and an Associate Professor of International and Inter University Centre for Nanoscience and Nanotechnology, Kottayam, India 5 Director and Chair of School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, India 6 The Vice Chancellor of Mahatma Gandhi University, Kottayam, India 7 Founder Director of International and Inter University Centre for Nanoscience and Nanotechnology, Kottayam, India 8 Professor at School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India 2 3

Contents 1.1 Introduction 1.2 Emerging 2D Nanomaterials: Uniqueness and Advances 1.3 Synthesis Approaches 1.3.1 Top-Down Approaches 1.3.2 Bottom-Up Approaches 1.4 Summary and Outlook References

1 3 5 5 13 16 17

1.1 INTRODUCTION Nanoscience and nanotechnology represent the manipulation of matter on an atomic and molecular scale, which holds enormous economic potential for the present and future markets. The production of ever smaller, faster, and more efficient products with acceptable price-toperformance ratios has become an increasingly important success factor in the international competition for many industries. One of the first insights into the potential benefits of making devices at the nanoscale was by 

Corresponding Author: [email protected]

Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00001-8

© 2019 Elsevier Inc. All rights reserved.

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Nanomaterials Synthesis

Richard Feynman in his famous speech in 1959 entitled, “There’s Plenty of Room at the Bottom.” Recent advances in nanotechnology show that most of the novel devices of the future will be based on the properties of nanomaterials. It is these tools that have taken atomic manipulation out of the chemistry breaker and into the realm of engineering. Nanomaterials can be generally categorized into three types: zero-dimensional (0D), one-dimensional (1D), and two dimensional (2D), and each of these materials exhibits unique properties based on its particular characteristics. The discovery of exfoliated graphene by Novoselov, Geim, and coworkers in 2004 [1] has generated extensive research on ultrathin twodimensional (2D) nanomaterials in the fields of physics, chemistry, material science, and nanotechnology, with great economic and sustainable impacts. Two-dimensional (2D) nanomaterials possess sheet-like structures with single or a few atoms thickness (typically less than 5 nm) and above 100 nm, or up to a few micrometers length, which displays unique physical, chemical, and electronic properties due to electron confinement [2]. One atom thick graphene represents an archetypal model on the basis of its tremendous and surprising properties of ultrahigh specific surface area [3] and room-temperature carrier mobility [1], quantum Hall effect [4], high Young’s modulus [5] and optical transparency [6], and outstanding electrical [1] and thermal [7] conductivities. The large surface-to-volume ratio of 2D nanomaterials supplies more active sites, which makes them highly favorable for surface-active applications [8]. The wide interlayer spacing between nanosheets, including their natural electronic properties such as fast electron and ion transfer, makes 2D nanomaterials attractive for electronic device applications, especially fast-charging devices [2]. Furthermore, the atomic thickness offers them good mechanical flexibility, which makes them appropriate for the development of flexible and stretchable batteries [8]. Graphene-like ultrathin 2D nanomaterials, such as graphitic carbon nitride (g-C3N4) [9,10], hexagonal boron nitride (h-BN) [11,12], transition metal dichalcogenides (TMDs) [13,14], layered metal oxides [15], and layered double hydroxides (LDHs) [16,17], etc., also attract enormous interest because of their versatile properties due to their similar structural features of graphene. Massive research interest on 2D nanomaterials enriched the investigation of other 2D ultrathin materials, such as MXenes [18
20], metals [21], metal 2 organic frameworks (MOFs) [22,23], covalent 2 organic frameworks (COFs) [23,24], polymers [25,26], antimonene [27], silicene [28], black phosphorus [29], etc. This chapter gives an overview of the synthesis and processing of emerging two-dimensional nanomaterials, hybrid structures, and

Synthesis and Processing of Emerging Two-Dimensional Nanomaterials

3

Figure 1.1 Schematic illustration of some of the emerging 2D nanomaterials. Reproduced with permission from A.H. Khan, S. Ghosh, B. Pradhan, A. Dalui, L.K. Shrestha, S. Acharya, et al., Bull. Chem. Soc. Jpn. (2017) [38]. Copyright 2017 The Bulletin of Chemical Society of Japan.

composites potentially applied for various applications. The impact of synthesis routes, processing details, and merits and demerits are discussed in detail. Our aim is to summarize the state-of-art evolution on emerging 2D nanomaterials with a particular emphasis on recent advances and challenges in synthesis and processing.

1.2 EMERGING 2D NANOMATERIALS: UNIQUENESS AND ADVANCES To date, large numbers of 2D nanomaterials have been synthesized by various routes. Some of the emerging two-dimensional nanomaterials are

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schematically illustrated in Fig. 1.1. In this section, we describe a brief overview of 2D nanomaterials with their structure, uniqueness properties, and advances. We start with graphene, a single atom thick sheet of graphite, which possesses a hexagonally packed carbon structure. Each of the atoms is covalently bonded with another three neighboring atoms through the σ-bond [30]. Graphitic carbon nitride (g-C3N4) exhibits a layered structure by van der Waals bonding with the crystal structure of N-atom substituted graphite structure by sp2 hybridization [31]. Hexagonal boron nitride (h-BN) also has a similar layered structure to that of graphite. In this case, equal numbers of boron and nitrogen atoms are arranged in a hexagonal structure by covalent bonds [32]. TMDs have a layered structure with the general chemical formula of MX2, where M is a transition metal element and X represents a chalcogen such as S, Se, or Te [33]. In TMDs, the monolayers are stacked together by van der Waals forces of attraction similar to graphite. Metal oxides and double hydroxides also have a layered network in addition to black phosphorous and semiconductors. MXenes are another emerging class of 2D transition metal carbides and/or nitride formed by selective etching of the raw MAX phases having general formula of Mn11 AXn (n 5 1, 2, or 3). Here M represents transition metals such as Ti, V, Cr, Nb, etc., A represents an element from group IIIA or IVA, such as Al, Si, Sn, In, etc., and X stands for carbon and/or nitrogen [34]. In MXenes, the MAX phases have a layered structure in which M layers are hexagonally packed together and X atoms fill the octahedral sites, whereas A layers can be selectively etched using strong etching solutions. In the case of MOFs, the metal ions or clusters are linked by coordinating organic ligands to form bulk crystals [35], whereas in COFs, the organic units are covalently connected to form a porous crystalline framework [36]. On the other hand, 2D polymer nanosheets are obtained from layered bulk polymers [37]. Generally, 2D nanomaterials exhibit various unprecedented properties which alter their physical, chemical, optical, and morphological characteristics. These unique advances of 2D nanomaterials make them suitable candidates for multipurpose applications. The compelling electronic properties due to the electron confinement in a single-layer 2D nanomaterial make them ideal in numerous applications in condensed matter physics and electronic/optoelectronic devices [2]. The excellent optical transparency, mechanical strength, and flexibility of 2D materials are the

Synthesis and Processing of Emerging Two-Dimensional Nanomaterials

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Figure 1.2 Schematic illustration of graphene production by micromechanical cleavage technique. Reproduced with permission from F. Bonaccorso, A. Lombardo, T. Hasan, Z. Sun, L. Colombo, A.C. Ferrari, Mater. Today (2012) [55]. Copyright 2012 Elsevier.

result of their strong in-plane covalent bond and atomic thickness [2,39], while the large lateral size reflects their ultrahigh specific surface area. This uniqueness is utilized by many next-generation devices and energy storage applications [40,41]. Moreover, the flexibility in solution-based processability and high exposure of surface atoms allows the fabrication of freestanding thin films and easy regulation of properties and functionalities.

1.3 SYNTHESIS APPROACHES There have been enormous research efforts reported exploring a diversity of reliable synthesis methods for 2D nanomaterials and their exploration for potential applications. In general, 2D nanomaterials have been mainly obtained by top-down and bottom-up approaches. In this section, we highlight the advances and limitations of each of the synthesis methods of 2D materials for a wide range of applications.

1.3.1 Top-Down Approaches In top-down approaches, single- or few-layer 2D nanomaterials can be developed by removing the van der Waals interaction between the stacked layers of layered bulk crystals. The top-down approaches include mechanical cleavage [42], liquid exfoliation by mechanical force [43], liquid exfoliation by ion intercalation [44] and ion exchange [45], liquid exfoliation by oxidation [46], selective etching [47], laser thinning [48], etc. 1.3.1.1 Mechanical Cleavage Mechanical cleavage, usually referred to as the Scotch-tape method, is a conventional way to produce thin 2D nanosheets by peeling off from their layered bulk crystals by weakening the van der Waals interaction

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Nanomaterials Synthesis

Figure 1.3 Schematic illustration of liquid exfoliation of graphite by sonication. Reproduced with permission from A. Ciesielski, P. Samorì, Chem. Soc. Rev. (2014). Copyright 2014 The Royal Society of Chemistry.

between the layers without breaking the in-plane covalent bonds. This type of exfoliation method was first established by Novoselov, Geim, and coworkers in 2004 by cleaving a single layer of graphene sheet from graphite [2] and was then followed by a number of 2D nanomaterials, such as h-BN, TMDs including MoS2, NbSe2, TiS2, TaS2, TaSe2, MoSe2, WS2, WSe2, TaSe2, MoTe2, ReS2, etc. [49
51], antimonene [52], CuInP2S6 [53], and BP [54] from their layered bulk crystals. A typical micromechanical cleavage technique for graphene production is schematically illustrated in Fig. 1.2. Here, the fresh surface of a bulk crystal is attached on Scotch tape and then peeled to thin flakes by using another piece of Scotch tape, and this process is repeated to obtain a proper thin flake [42]. This cleaved thin flake is then transferred to a clean target surface of SiO2 or Si and mono- or few-layered nanosheets are obtained by peeling off the Scotch tape. These mechanically cleaved 2D nanosheets can be studied by optical microscopy, atomic force microscopy (AFM),

Synthesis and Processing of Emerging Two-Dimensional Nanomaterials

7

scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), etc. This nondestructive technique gives clean surfaces and excellent crystal quality, with minimum defects of exfoliated single- or few-layer nanosheets and having size up to a few to tens of micrometers. The mechanically cleaved 2D nanomaterials can be used for the fundamental study of their intrinsic physical, electronic, and optical properties in addition to the high-performing electronic and/or optoelectronic devices. Since the mechanical cleavage technique is free from chemicals, the nanosheets from this process provide a clean surface and high crystal quality in addition to their wide applicability. However, some of the limitations include the very low production yield and production rate. Moreover, the production parameters are difficult to control because of the manual operation. 1.3.1.2 Liquid Exfoliation by Mechanical Force Production of 2D nanomaterials by mechanical force-assisted liquid exfoliation is an active way to exfoliate bulk-layered crystals. Numerous liquid exfoliation methods have been reported by the effective application of mechanical force in liquid on the basis of sonication and shear force. In a typical sonication process, as shown in Fig. 1.3, the bulk crystals were dispersed in a solvent before sonication and, after the sonication, the suspension was purified via centrifugation. As a simple method, sonication imparts mechanical forces in the liquid phase, which ultimately break the interlayer van der Waals interaction, retaining the covalent bonding in the layers. As a result of this, the efficiency of exfoliation can be increased by matching the surface parameters of layered bulk crystal and the solvent. Liquid exfoliation by sonication was first established by Coleman’s group in 2008 for the graphene exfoliation [56] without using expensive equipment and chemicals. Later, a modified nonpolar solvent-aided liquid exfoliation of graphene was reported [57] and, after this alteration, a series of volatile solvents with various boiling points was used for the sonicationassisted exfoliation of graphene [58]. This method was extended for the exfoliation of other two-dimensional nanostructures such as MoSe2, MoS2, MoTe2, NbSe2, WS2, TaSe2, NiTe2, Bi2Te3, and h-BN [59]. Various amendments were later demonstrated for the solvents for the effective exfoliations; a recent study proves the potential of pure water at an elevated temperature to be a promising solvent for the sonication method [60]. This makes water an effective agent for sonication-assisted

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Nanomaterials Synthesis

liquid exfoliation of 2D nanosheets and this method is more promising for future applications. Currently, the sonication-assisted liquid exfoliation method is commonly employed because one can easily tune the nanosheet parameters by controlling the solvent system, shape of the vessel, sonication time and temperature, ultrasonic power, polymer additives, etc. [49]. Though this method offers low cost and high yield for the exfoliation process, there are some disadvantages present, such as the smaller lateral size of the nanosheets, low yield of single-layer nanosheets, and the defects on the exfoliated nanosheets. On the basis of production rate, the liquid exfoliation by sonication is much better than micromechanical cleavage, but still cannot meet the necessity for industrial production. Shear force-assisted liquid exfoliation was demonstrated to overcome this drawback by generating high shear rates in the liquid of bulk crystals [61]. The exfoliation of graphite into graphene with 300 2 800 nm lateral size and bulk black phosphorus into few-layer nanosheets by this simple shear force set-up with a mixing head and a rotor has been reported [61,62]. The proper selection of solvent and polymer further makes this process more effective and it was later realized that the shear rate is the key factor for exfoliating layered materials. In the case of graphene, the exfoliation efficiency is poor under a shear rate of 104 S21, while the efficiency is high if the shear rate is higher than 104 S21. Shear force-assisted liquid exfoliation using a kitchen blender was reported for the exfoliation of graphene [63,64] in order to make a high shear rate in all regions. Later, thin layers of MoS2, h-BN, and WS2 nanosheets were produced using a kitchen blender [65]. This shear exfoliation method is a promising procedure for mass production of graphene using rotating blade-stirred tank reactors. 1.3.1.3 Liquid Exfoliation by Chemical and Electrochemical Ion Intercalation The liquid exfoliation of two-dimensional nanomaterials by the ion intercalation method is based on the principle of intercalation of cation ions, such as Li1, K1, Na1, etc., in the interlayers of bulk crystals. This intercalation process weakens the van der Waals interaction between adjacent layers in bulk crystals. These intercalated compounds could further exfoliate to single- or few-layer sheets by a mild sonication process in water. In most of the cases, intercalated ions can react with water and generate hydrogen gas, which can also help for separating the adjacent layers during the sonication process. High-yield nanosheets can be obtained after the

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centrifugation process. The obtained single-layer 2D nanomaterials through the ion-intercalation method provide smaller lateral size. This disadvantage is overcome by the combined hydrothermal and chemical ion intercalation-assisted liquid exfoliation method [44]. Also, the long reaction time and higher temperature needed for the intercalation process were addressed by the pretreatment of bulk crystals with n-butyllithium within 1 h at room temperature [66]. Up to now, various ultrathin 2D nanomaterials have been prepared by the intercalation-assisted liquid exfoliation method. Parameters such as lateral size, layer number, and concentration, and number of defects of produced nanosheets can be altered by tuning the experimental conditions, such as the reaction time, temperature, particle size and initial concentration of bulk crystals, sonication time, intercalating agents, etc. Furthermore, ion intercalation into layered bulk crystals of MoS2 and WS2 can induce the phase transformation from the semiconducting hexagonal (2H) and metallic octahedral (1T) phase, as shown in Fig. 1.4, offering a powerful way for the phase engineering of 2D TMDs [67,68]. Also, the produced nanosheets by this method are positively charged with

Figure 1.4 Schematic illustration of the transition of the hexagonal (2H) phase to the octahedral (1T) phase in MoS2 upon Li1 intercalation. Reproduced with permission from M.A. Lukowski, A.S. Daniel, F. Meng, A. Forticaux, L. Li, S. Jin, J. Am. Chem. Soc. (2013) [68]. Copyright 2013 American Chemical Society.

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a clean surface, making them promising for electrocatalysis and energy storage applications [67]. The electrochemical ion intercalation method was also successfully reported for the exfoliation of many-layered bulk crystals into 2D nanosheets such as h-BN, graphene, MoS2, WSe2, WS2, TiS2, TaS2, ZrS2, Bi2Te3, NbSe2, Sb2Se3, Ta2NiS5, and Ta2NiSe5 [69
71], where the process was driven by electrochemical force and ion (Li) foil. In this process, metal foils with coated bulk crystals were used as cathodes, and Li foils as anodes. Upon the discharge process, Li ions intercalate into layered bulk crystals and forms Li intercalated compounds on the electrodes. These electrodes were washed and sonicated to obtain nanosheet suspensions. After purification through centrifugation, a high yield of single- or few-layer nanosheets can be obtained. The electrochemical ion intercalation method has several advantages, including high production yield, flexibility with different conditions, and environmentally friendly nature. One of the noted disadvantages of this method is its complex nature and irreversibility compared to the chemical ion intercalation method because of the inclusion of battery cells. Another disadvantage is the presence of additional additives such as activated carbon and polyvinylidenefluoride for the electrode fabrication process for improving the conductivity and quality. These additives may absorb on the exfoliated nanosheets, which makes them undesirable for exact applications. Due to the sensitive nature of oxygen and moisture, the intercalators such as n-butyllithium, Li foil, and LiBH4 have been recently replaced by common inorganic salts, such as CuCl2 and NaCl, for the exfoliation process in the ion intercalation method [72,73]. The inclusion of inorganic salts makes this method much safer, cheap, and more suitable for everyday applications. 1.3.1.4 Liquid Exfoliation by Ion Exchange Liquid exfoliation through cation or anion exchange was reported widely for the preparation of 2D nanosheets from bulk crystals. The cation exchange process is established mainly for the exfoliation of layered metal oxides and metal phosphorus trichalcogenides [74,75]. While immersing layered metal oxides in an acid-based aqueous solution, hydrated protonic compounds were formed by H1 cation exchange. This interlayer proton can be further replaced by organoammonium ions and thus the interlayer spacing of metal oxide-layered bulk crystals will expand due to large radius of organoammonium ions. This expansion leads to the exfoliation

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of metal oxide nanosheets having positive charge on their surface. The bulk crystals of metal phosphorus trichalcogenides were stirred in an alkali-based aqueous solution, where the metal cations can be somewhat replaced by K1 ion and form intermediate compounds. After this primary exchange, the interlayer spacing of bulk crystals was expanded and later the K1 ions were further exchanged by Li ions. Exfoliation of bulk crystals of layered double hydroxides has been established by anion exchange [76]. Anion interlayers can be exchanged by other anions and this exchange tends to expand the interlayer spacing of bulk crystals of layered double hydroxides. The exchanged layered double hydroxides were then exfoliated to thin layers of nanosheets by sonication or heating in organic solvents [77]. Generally, ion exchange is a potential method to attain high yield and large-scale production of thin 2D nanomaterials. 1.3.1.5 Liquid Exfoliation by Oxidation and Reduction Liquid exfoliation by the oxidation process is widely reported to separate graphitic layers by the modified Hummers’ method [78,79]. This method uses strong oxidizing agents, mainly a mixture of KMnO4 and H2SO4, to oxidize graphite to form graphite oxide. This oxidation process produces plentiful oxygen-containing functional groups, such as hydroxyl, carboxyl, and epoxy groups on the surface of graphene layers, which can expand the interlayer spacing and thus weaken the van der Waals interaction between adjacent layers of bulk graphite layers [78,79]. After successive sonication, the expanded graphite oxide can be exfoliated into singlelayer graphene oxide (GO) nanosheets. Single-layer GO nanosheets with high yield and a large amount in the solution phase can be achieved by this process but this method is not very safe because of the usage of strong oxidizing agents. Also, successful extension of this process to other layered materials remains difficult. The functional groups on the surfaces GO layers can be fully or partially removed by the reduction process, which produces reduced GO (RGO) layers. The reduction can be achieved by several approaches, such as thermal annealing, chemical reduction via reducing agents, electrochemical reduction, photochemical reduction, etc. [80
82]. The physical, chemical, and electronic properties of GO, RGO, and pristine graphene are entirely different. For example, GO is an electrical insulator, whereas RGO is an electrical conductor but its conductivity is not good compared to the excellent electrical conductivity of pristine graphene [83]. As GO contains many oxygen-involved functionalities, it has a hydrophilic

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nature, while graphene is extremely hydrophobic. The functional groups on the GO surface permit them to modify with other molecules through covalent bonding. The functional moieties on the GO surface act as nucleation sites for the growth of various nanocrystals, such as metal oxides, noble metals, metal chalcogenides, etc., on the surface for multifunctional applications [84
87]. 1.3.1.6 Liquid Exfoliation by Selective Etching Liquid exfoliation of 2D nanomaterials by the selective etching method was applied for the fabrication of MXenes from bulk MAX phases as shown in Fig. 1.5 [34]. MXenes are a special class of 2D transition metal carbides and/or nitride of the raw MAX phases having the general formula of Mn11 AXn (n 5 1, 2, or 3). Here M represents transition metals such as Ti, V, Cr, Nb, etc., A represents an element from group IIIA or IVA, such as Al, Si, Sn, In, etc., and X stands for carbon and/or nitrogen [34]. In MXenes, the MAX phases have a layered structure in which M layers are hexagonally packed together and X atoms fill the octahedral sites, whereas A layers can be selectively etched using strong etching

Figure 1.5 Schematic diagram of the synthesis of MXenes from MAX phases. Reproduced with permission from M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman, et al., ACS Nano (2012) [88]. Copyright 2012 American Chemical Society.

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Figure 1.6 Schematic illustration of a typical CVD set up for 2D transition metal dichalcogenides. Reproduced with permission from S.L. Wong, H. Liu, D. Chi, Prog. Cryst. Growth Charact. Mater. (2016) [101]. Copyright 2016 Elsevier.

solutions. Since the metallic bonding between Mn11 Xn layers in Mn11 AXn phases is more robust than the van der Waals bonding in layered compounds like graphite, an effective method of selective etching is required for the exfoliation of MXenes [34]. In a typical process, the bulk crystal powders of the MAX phase were dispersed in HF solution (50% concentrated) for etching the A layer. The etched product was then washed and sonicated with water to obtain the 2D nanosheet. In this method, several parameters, such as concentration of HF solution and reaction temperature and time, can be tuned for optimization. Gogotsi and his group reported that the obtained nanosheets from the selective etching technique have a different chemical composition than the original bulk form. The same group also reported this method for the Ti3C2 nanosheet from their bulk crystal [89]. The selective etching method is simple and offers massive production of MXenes with a high yield. This method has functional groups such as O, F, H, OH, etc., on the surface, which makes them suitable for some specific applications. One of the disadvantages of this method is the usage of strong corrosive chemicals in the preparation process and this was later replaced by some fluoride salt mixtures. The selective etching method is not applicable to many of the MXenes from MAX phase and is also not possible for the preparation of other 2D nanomaterials such as graphene and metal oxides.

1.3.2 Bottom-Up Approaches The formation of 2D nanomaterials in bottom-up approaches is mainly obtained by direct chemical synthesis, such as chemical vapor deposition (CVD) and wet-chemical synthesis methods.

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1.3.2.1 Chemical Vapor Deposition CVD is a widely used technique to make 2D nanomaterials and thin films on solid substrates. In this technique, the precursors, gas or vapor, can react or decompose on the preselected substrate at high temperature and vacuum in a chamber, as shown in Fig. 1.6. In this process, 2D nanosheets grow on the substrate with or without the help of catalysts [90]. CVD techniques were developed for the growth of various 2D nanosheets, such as graphene [91], h-BN nanosheets [92], TMDs [93], metal carbides [94], borophenes [95], antimonene [96], and silicene [97]. The first demonstration of graphene by the CVD technique was reported in 2006 by Somani et al. [91], whereas a CVD making single-layer graphene was reported in 2009 by Beton et al. [98]. By controlling the CVD parameters, such as type of substrate and precursors, catalysts, temperature, the growth of graphene layer can be tuned. Li et al. demonstrated the growth of few-layer as well as uniform large area MoS2 nanosheets and WS2 nanosheets by CVD techniques [99,100]. In comparison with other synthesis techniques, CVD has the highest level of control for the fabrication of 2D nanomaterials. The CVD technique offers massive production of 2D nanomaterials with high crystal quality, purity, and limited defects on the substrates. The 2D nanomaterial prepared by the CVD technique is used in a variety of practical applications such as electronics, optoelectronics, and solar cell devices. But in the case of CVD technique, it is always demanding the transfer of nanosheets from deposited substrates for further investigation. Besides, higher production cost is another drawback. 1.3.2.2 Wet-Chemical Synthesis Wet-chemical synthesis routes deal with chemical reactions in the solution phase using precursors at proper experimental conditions. Each wet-chemical synthesis method differs from the others, meaning that one cannot find a general rule for these kinds of synthesis approaches. These synthesis strategies have been used for the preparation of 2D nanomaterials which are unable to be prepared by top-down approaches. Wet-chemical synthesis routes offer a high degree of controllability and reproducibility for 2D nanomaterial fabrication. Solvothermal synthesis, template synthesis, self-assembly, oriented attachment, hot-injection, and interfacemediated synthesis are the main wet-chemical synthesis routes for 2D nanomaterials.

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The solvothermal/hydrothermal approach is a simple and scalable method which has been widely used for the preparation of 2D inorganic nanomaterials [102]. In a typical solvothermal process, water or organic solvent acts as the reaction medium in a closed vessel. In this process, the reaction temperature would be higher than the boiling point of solvent and, as a result, the solvent will be autogenerated at high pressure, which improves the crystallinity of the as-synthesized nanocrystals [103]. Two-dimensional nanosheets of metals [104], metal oxides [105], and metal chalcogenides [106,107] have been reported by this method. The wet-chemical synthesis of two-dimensional single- and multi layer transition metal dichalcogenides are schematically illustrated by Fig. 1.7. However, the solvothermal method offers low cost and high yield for 2D nanomaterial synthesis, it is difficult to figure out the growth mechanism by this method since the reactions occur in a closed vessel. Also, the 2D nanosheets synthesized by this method are few-layer rather than single-layer. Template synthesis is another wet-chemical approach which utilizes bulk or presynthesized nanomaterials as templates for the growth of anisotropic nanostructures [108]. Many reports have been published for the template-based synthesis of 2D nanomaterials. Jeong et al. [109] reported

Figure 1.7 Schematic illustration of wet-chemical synthesis of two-dimensional single- and multilayer transition metal dichalcogenides. Reproduced with permission from D. Yoo, M. Kim, S. Jeong, J. Han, J. Cheon, J. Am. Chem. Soc. (2014) [107]. Copyright 2014 American Chemical Society.

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the growth of hexagonal close-packed (hcp) gold nanosheets using GO nanosheets as a template. Wei et al. established a freestanding half-unit-cell α-Fe2O3 nanosheet with CuO nanoplate as the template [110]. Cu-based chalcogenide nanoplates via the cation exchange reaction were reported using presynthesized CuS nanoplates as the template [111]. In a typical self-assembly process, presynthesized low-dimensional nanocrystals, such as nanoparticles and nanowires, were spontaneously organized with each other by noncovalent interactions, such as van der Waals/electrostatic/hydrogen bonds for the synthesis of 2D nanomaterials [112]. This assembly mainly involves the combination of low-dimensional nanocrystals to form larger crystals in two dimensions. Many 2D nanomaterials, such as nanosheets of Au [113], Cu [114], PbS [115], etc., were reported by the self-assembly approaches of wet-chemical synthesis. Twodimensional-oriented attachment is another wet-chemical synthesis method which offers the fabrication of 2D nanocrystals with nonlayered structures having well-defined morphology. In this process, adjacent nanocrystals are associated and bonded together to form 2D nanosheets with a single-crystalline nature. Many 2D nanosheets were prepared by this technique, such as PbS nanosheets [116], Bi2Se3 nanosheets [117], etc. The hot-injection method is another wet-chemical synthesis method which is usually used to prepare monodispersed colloidal nanocrystals with uniform shape and size [118]. This method involves the rapid injection of highly reactive reactants into a surfactant, usually oleyl amine or oleyl acid, contained hot solution. The prepared 2D nanomaterials by this method offer high purity and uniform size and shape. Some of the disadvantages of this method include its relatively high reaction temperature, use of surfactants, and difficulties in large-scale production. For the synthesis of 2D metal coordination polymers, the main wet-chemical synthesis strategy is an interface-mediated approach [119]. In a typical process, organic ligands confined to the water
air interface reacted with metal ions dissolved in water and formed single-layer dense nanosheets. This process is also extended to polymers and inorganic nanosheets in addition to metal coordination polymers for the preparation of 2D nanomaterials.

1.4 SUMMARY AND OUTLOOK The research area of 2D nanomaterials, especially their advanced synthesis strategies, have been widely studied in recent decades for the development of next-generation technologies. In this chapter, we discuss the various

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synthesis approaches to 2D nanomaterials from diverse aspects. Starting with graphene, 2D nanomaterials have now become a new class of nanomaterials, having various structural features favorable for numerous potential applications and also capable of replacing current technologies. However, there are numerous challenges to the synthesis and processing of 2D nanomaterials yet to be overcome. The current production yield, production rate, and quantity of 2D nanomaterials have still not attained the industrial requirements. Generally, the structural features of a material determine their physical and chemical properties and thus the applicability for a specific requirement, so that a controlled synthesis protocol had to be developed for 2D nanomaterials. The development of effective characterization tools for understanding the growth mechanisms of 2D nanomaterials is another challenge. In addition, research has to be focused to improve the stability and durability of 2D nanomaterials, which has a great impact on potential applications. The current synthesis approaches and techniques are the main reason for this instability. Instabilities such as structural degradation, structural change, irreversible aggregation, etc., need to be overcome by incorporating new technologies for the controlled synthesis strategies for stabilizing 2D nanomaterials. These incorporations could also help the researchers to develop any type of 2D nanomaterial, the only concern is their dimensionality with single or few layers, with exciting functionalities and properties. Another way to overcome the disadvantages of a material is to make composites by hybridizing one material with another. This hybridization process results in a synergistic effect which enhances the overall properties of composite and may also bring new functionalities. In addition, exploration for the potential applications of advanced 2D nanomaterials and their hybrids needs to be studied in the near future.

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CHAPTER 2

Nanomaterial Synthesis: Chemical and Biological Route and Applications Santanu Sasidharan1, Shweta Raj1, Shirish Sonawane2, Shriram Sonawane4, Dipak Pinjari3, A.B. Pandit3 and Prakash Saudagar1 1 Department of Biotechnology, National Institute of Technology, Warangal, India Chemical Engineering Department, National Institute of Technology, Warangal, India Institute of Chemical Technology, Matunga, India 4 Chemical Engineering Department, Visvesvaraya National Institute of Technology, Nagpur, India 2 3

Contents 2.1 Introduction and Background: Nanoparticle Synthesis Approaches 2.2 Different Chemical Routes for Nanomaterial Synthesis 2.2.1 Ultrasound-Assisted Synthesis 2.2.2 Colloid Nanoparticle Synthesis 2.2.3 Nanoparticle Synthesis Using Microreactors 2.3 Nanoparticle Synthesis Using the Biological Route 2.3.1 Plant Extract-Based Nanoparticle Synthesis 2.3.2 Microbial Synthesis of Nanoparticles 2.4 Application of Nanomaterials 2.4.1 Nanoclay and Polymer Nanocomposite and Polymer Functional Nanolatex 2.4.2 Photocatalyst for Degradation of Organic Pollutants 2.5 Recent Advances in the Chemical and Biological Synthesis Routes 2.6 Scale-Up Issues of Nanoparticle Production and Challenges 2.7 Summary Acknowledgment References

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2.1 INTRODUCTION AND BACKGROUND: NANOPARTICLE SYNTHESIS APPROACHES Nanoparticles are ultrafine particles with dimensions in nanometers (nm). One nanometer is one billionth of a meter (1 nm 5 1029 m). They exist in natural forms as well as in manmade forms. They are smaller than solid Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00002-X

© 2019 Elsevier Inc. All rights reserved.

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particles but larger than atoms. Therefore, they do not follow quantum theory but show Brownian motion. Due to their small size and large surface area, they have found applications in various areas, such as pharmaceuticals, industrial manufacturing, catalysis, environmental remediation, engineering, etc. Nanoparticles are complex in nature and are mainly composed of three layers: 1. The surface layer: This is the outermost layer and can be active in the presence of small molecules, metal ions, and polymers. 2. The shell layer: This is the sandwiched layer and is made up of different chemical materials than the core. 3. The core layer: This is the most crucial and core part of the nanoparticle and shows all its remarkable properties. The method of synthesis is the key which decides the physical properties and applications of nanomaterials. Therefore, choosing the right and appropriate synthetic route for nanomaterial design is the driving force for many newly emerging methodologies. This had led to the development of different versatile and productive methods for the preparation of nanomaterials. There are two main approaches employed in various methods used for the synthesis of nanoparticles. 1. Top-down synthesis This method is also known as the mechanical-physical particle production process. In this method, a heavy force is applied for the crushing of a large particle into smaller particles which can be used as nanoparticles. This can be attained by using crushing, grinding, milling, lithographic cutting techniques, chemical vapor deposition, and physical vapor deposition. This approach is used for the production of metal-based nanoparticles and ceramic nanoparticles. 2. Bottom-up approach This method is also known as the building up process. In this method, small particles like atoms, molecules, and nanoparticles themselves are used for the synthesis of larger particles. This is based on the assembly of several small particles for the production of a complex particle. This can be done by spinning, green synthesis, biochemical synthesis, and sol
gel methods. Green and biochemically synthesized nanoparticles are costeffective and eco-friendly. This approach is used for the production of metal oxide nanoparticles and metal nanospheres. Fig. 2.1 explains the two methods of synthesis.

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It can be seen in the literature that the various morphological properties like size, shape, and stability are strongly influenced by metal ion interaction kinetics and adsorption processes. Therefore, designing or scaling-up a new synthesis method by controlling the physicochemical properties has become a new field of interest. There are several methods of nanoparticle synthesis available. 1. Mechanical grinding/milling The mechanical milling method is a top-down approach in which the decomposition of large structural particles into coarse particles is attained using grinders. This process involves the employment of mechanical, thermal, and centrifugal forces. In this method, broad size nanoparticles are synthesized ranging from 10 to 1000 nm in size. Their applications are in the production of nanocomposites and nanograined bulk materials. There are many types of mills available for this process, such as planetary ball mill, attrition ball mill, high-energy ball mill, low-energy tumbling mill, and vibrating ball mill. 2. Laser ablation Laser ablation is a process in which layers of solid metals are removed using a high beam laser. The laser beam irradiates the metal surface by exposing it to radiation. It also works on the top-down approach, that is, to disintegrate the metal surface using a laser beam. The breaking down process depends on the intensity, wavelength, and pulse length of the laser beam. It is used in the production of

Figure 2.1 Approaches for nanoparticle synthesis.

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4.

5.

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semiconductor nanoparticles, quantum dot nanoparticles, and fabricated nanoparticles. Electro-explosion Electro-explosion is also a top-down approach in which a highpower pulsed current passes through a thin metal wire, which leads to the explosion of metal ions present in the wire. A high amount of heat is dissipated into the wire using a pulsed discharge system, which results in melting and ionizing of the metal ions. This method is basically used for the production of metallic nanopowders. Chemical vapor deposition Chemical vapor deposition (CVD) is a bottom-down approach in which a solid metal is deposited on another heated metal surface using a different chemical reaction in the vapor or gas phase. This method requires additional activation energy to trigger the chemical reaction which can be provided by using a high temperature (1000°C). The CVD apparatus mainly comprises three components, a gas supply system, a deposition chamber, and an exhaust system. CVD has applications in the production of nanocomposites. Sol
gel process The sol
gel process is a method used for the preparation of colloidal nanoparticles by using a bottom-up approach. In this procedure, colloid particles are synthesized by mixing colloidal suspension and gel-like material (silica or gelatin) in a continuous liquid phase. This requires the use of some metal ion precursors like alkoxides and alkoxysilanes. The most commonly used precursors are tetramethoxysilane (TMOS) and tetraethoxysilanes (TEOS). It has a wide range of applications in the manufacture of sintered ceramic nanomaterials, protective nanoparticle coatings, optical and refractory ceramic fiber production, nanoscale powders, and injectable nanocomposites such as plasminogen activator entrapment in alumina. Green and biological synthesis The emerging technologies for the better production of nanoparticles should be cost-effective, simpler to synthesize, and eco-friendly. Therefore, the better route for their production is via biosynthesis and green synthesis. Among all the biological alternatives available, plants and plant extracts seem to be a good option because of their availability, lower maintenance, and low production cost. Other biological agents can also be used in the biosynthesis of nanoparticles such as microbes, protists, and algae. In this method, a metal salt is mixed with

Nanomaterial Synthesis: Chemical and Biological Route and Applications

31

Figure 2.2 Green synthesis of nanoparticles by different biological sources.

plant extract and kept for incubation for a few hours at room temperature, which reduces the metal salt into the respective nanoparticles. Nanoparticles synthesized using these techniques have a wide variety of applications such as in antimicrobial applications, production of scaffolds, in biosensors, in burn dressings, degradation of toxic compounds and insecticides, and in wastewater plants. Fig. 2.2 shows a green synthesis route for the production of nanoparticles from bacteria, fungi, yeast, and plants.

2.2 DIFFERENT CHEMICAL ROUTES FOR NANOMATERIAL SYNTHESIS 2.2.1 Ultrasound-Assisted Synthesis In the last few decades ultrasonication and sonochemical-assisted synthesis have become a promising technology for the production of various nanomaterials. Ultrasound or sonication is the most common tool used in laboratories to dispense a liquid into a fine mist and emulsify mixtures. This process is based on the passing of high-energy ultrasonic waves by the acoustic cavitation process. It is used for the initiation or amplification of catalytic reactions in both homogeneous and heterogeneous systems. In this process, a high-energy acoustic pressure wave from an electrical transducer device is passed through the liquefied medium and the change in pressure waves leads to the formation of cavitation bubbles. The collapse

32

Nanomaterials Synthesis

and collisions of these cavitation bubbles generate a high amount of energy, which causes the disintegration and dissociation of reactive chemical species into highly reactive radicals. The energy generated during the reaction depends upon the frequency and amplitude of the acoustic waves used for the cavitation. This method is very useful in reaction chemistry as it enhances the mass transfer with the reaction. Fig. 2.3 shows a typical setup of an ultrasonicator and the mechanism of ultrasound-assisted nanoparticle synthesis. There are a variety of nanoparticles that have been synthesized using the ultrasound-assisted method. In a study, a combination of Calothrix algae with ultrasound irradiation is used efficiently for the rapid synthesis of truncated shape Au nanoparticles with an average size in the range of 30
120 nm, which has potential usefulness in remediation of toxic chemicals and other catalyst-based industrial applications [1]. Ultrasonication synthesis can be applied for the synthesis of polymer nanocomposites which can improve the dispersion of nanofillers into the polymer base. High-intensity ultrasonic waves result in the uniform mixing of nanofiller in the base matrix. Among the nanocomposite polymers, PMMA [poly (methyl methacrylate)] is a multifaceted polymer because of its high strength, stability, and transmittance but also it has many disadvantages,

Figure 2.3 (A) The setup of an ultrasonicator and (B) nanoparticle synthesis using ultrasonication-assisted synthesis.

Nanomaterial Synthesis: Chemical and Biological Route and Applications

33

such as poor electrical conductivity, ductility, and thermal stability. PMMA properties can be improved by the addition of appropriate nanofillers. One such attempt was made using an ultrasonic two-step method for the synthesis of magnetic PMMA nanocomposites using nanoparticles of Fe3O4 as nanofiller [2]. A sonochemical process is also developed for the synthesis of modified graphene oxide (GO) nanodiscs by a metal stabilization procedure. In this study, scientists synthesized reduced graphene oxide (RGO) nanodiscs using GO sheets by stabilizing Sn nanoparticles on its edges using an ultrasonication method [3]. RGO nanodiscs have potential applications in the optoelectronics field. In another study, Mn3O4 nanoparticles anchored graphene nanosheets (MG) synthesized by a simple ultrasound-assisted synthesis at room temperature without using any templates or surfactants, and since MG composite has very effective and enhanced properties such as high capacitance, good rate capability, and capacitance retention, it is believed to be a promising candidate for supercapacitor applications [4]. Ultrasound-assisted synthesis can be paired with green synthesis for the betterment of the environment. One such attempt was made to demonstrate the formation of pure nanocrystallinenanostructured hydroxyapatite using eggshell waste which can be used for orthopedic tissue regeneration [5]. Ultrasound-assisted synthesis is also used for the production of nanoflakes. An efficient ultrasound-based synthesis approach has been demonstrated for the production of aluminum nanoflakes from aluminum isopropoxide and lithium aluminum hydride and subsequent good-quality dispersion in di-octyl adipate and hence showed greater promise for use in the application of energetic materials [6]. This method has many advantages over the conventional method of synthesis such as an improvement in the crystalline nature of metals, enhancement of thermal stability, and shape uniformity because of the cavitation effects during ultrasonication.

2.2.2 Colloid Nanoparticle Synthesis Liquid-based synthesis methods are generally based on the precipitation of nanoparticles from a chemical solution. It is usually performed as a batch process so that uniformity and characteristics of end product readily vary. It can be divided into five main classes: (1) colloidal method; (2) sol
gel process; (3) water
oil emulsion method; (4) polyol method; and (5) hydrothermal synthesis. This method is totally based on the precipitation of nano-sized particles within a continuous fluid system.

34

Nanomaterials Synthesis

The colloidal method is a liquid-based precipitation process in which different ionic solutions under controlled pressure and temperature are mixed together for the formation of insoluble precipitates. The colloidal metal nanoparticle preparation has been known since antiquity for the formation of gold nanoparticles to produce red- and purple-stained glass. The first scientific colloidal nanoparticle was produced by Faraday in 1857 using gold as the metal. In recent years, the colloidal process has gained more attention to produce metal and metal oxide nanoparticles which can be used in organics, optical physics, and the pharmaceutical field. A colloid is a mixture of two phases, that is, liquid in liquid or solid in liquid and all of these particles are dispersed in the aqueous medium to various degrees. The properties of a colloidal mixture totally rely on the particle size. Therefore, the term colloid is specific for an individual particle size. These particles are larger than an atom but small enough to show Brownian motion. The dynamic motion of larger particles in an aqueous medium is totally based on gravitation force but in the case of smaller colloidal particles irregular Brownian motion results in the bombardment and collision of particles. Several metal colloidal nanoparticles have been synthesized to date, and they have a wide range of applications. Silver has been known for its broad-spectrum antimicrobial activity since ancient times. Silver nanoparticles of average diameter 5 nm were synthesized by silver nitrate reduction with sodium citrate and stabilized with ammonia and they were used to evaluate their effect against Candida albicans and Candida glabrata adhered cells and biofilms [7]. In another study, highly stable concentrated aqueous dispersions of silver nanoparticles of narrow size distribution were prepared by reducing silver nitrate solutions with ascorbic acid in the presence of Daxad 19 (sodium salt of a highmolecular-weight naphthalene sulfonate formaldehyde condensate) as a stabilizing agent. This shows the excellent ability to prevent the aggregation of nano-sized silver at high ionic strength and high metal concentration [8]. There are many strategies proposed for the synthesis of colloidal nanoparticles. However, the extraction of synthesized nanoparticles from the solvent is very difficult, and is a big hurdle in their application. One such attempt was made for the synthesis of amphiphilic colloids of CdS and noble metal nanoparticles, which can be dispersed both in water and organic solvents such as ethanol, N, N-dimethylformamide, chloroform, and toluene by grafting the amphiphilic and thermos responsive polymer of thiol-terminated poly(N-isopropylacrylamide) to CdS and noble metal nanoparticles [9]. Size-controlled silver colloid nanoparticles were also

Nanomaterial Synthesis: Chemical and Biological Route and Applications

35

generated using a one-step modified Tollens process. In this process, the pH and type of reducing saccharide of the reaction system were found to influence the size of the particles and their bactericidal properties to many Gram-positive and Gram-negative bacteria, including multiresistant strains, were demonstrated [10]. Also, silver colloids were produced by chemical reduction of silver salt (silver nitrate, AgNO3) solution and using trisodium citrate, sodium borohydride, ascorbic acid, PVP (polyvinylpyrrolidone), and glucose as reducing agents and used in surface-enhanced Raman spectroscopy measurements of pyridine [11]. Due to their varied morphological properties, the hunt for morphology-dependent synthesis has gained momentum. Unique features of nonspherical nanoparticles have proved them to be an ideal source in various applications like Raman spectroscopy, fluorescence enhancement, analytics and sensing, photothermal therapy, (bio-)diagnostics, and imaging [12]. The formation of emulsion nanoparticles via colloid emulsion is diagrammatically represented in Fig. 2.4. As shown in Fig. 2.4, initially the water and oil phases are separated. Using the conventional agitation method, it is not possible to form a stable emulsion, as well as the droplet size of the emulsion not being uniform and the droplets are larger and may be in the micrometer range. While using ultrasound cavitation it is possible to make the droplets of emulsion minute in size. As shown in Fig. 2.4, a small quantity of surfactant is added in order to reduce the surface tension between the two phases. Furthermore, on application of high-power ultrasound above 22 kHz, there will be a large shearing effect due to the cavitation microstreaming. This phenomenon leads to formation of the homogeneous

Figure 2.4 Process for the formation of a stable emulsion via colloid emulsion.

36

Nanomaterials Synthesis

phases. Usually, a probe ultrasound is used to make the stable emulsion. This kind of emulsion process is quite helpful in food and pharmaceutical emulsion preparation. Usually the droplet size due to the ultrasound emulsion process is very small, and may be in range of 50
100 nm. The droplet size is also uniform. This technique is usually used to produce a number of emulsion polymerization products such as polybutyl methacrylates. As shown in Fig. 2.4, the critical micelle concentration of the surfactant in the colloid structures is usually in the nm range. Due to the ultrasound emulsion preparation method, formation of agglomerates, as well as sticking of the emulsion droplet, is avoided.

2.2.3 Nanoparticle Synthesis Using Microreactors A number of attempts have been made for the synthesis of nanoparticles using a microreactor. A microreactor offers a number of advantages for nanoparticle synthesis, such as homogeneous mixing, rapid heat, and mass transfer. The major advantage is that the microreactor offers a higher aspect ratio. For nanoparticle production, it is possible to manipulate restriction of the growth period in the microreactor by controlling the nucleation and growth by controlling the space
time of the reactor. The following is an example of platinum nanoparticle production using microreactor as shown in Fig. 2.5. The platinum precursor is taken into one syringe and the reducing agent is taken into another syringe. The ratio of the platinum precursor to the reducing agent is maintained at a 1:4 ratio. The reactor temperature is maintained at 25°C. The particle size of the platinum nanoparticle is in the range of 10
15 nm as shown in TEM image (Fig. 2.6).

Figure 2.5 Experimental set-up for the synthesis of Pt nanoparticles in a continuousflow microreactor. Reprinted with permission from Elseiver from P.L. Suryawanshi, S.P. Gumfekar, P.R. Kumar, B.B. Kale, S.H. Sonawane. Synthesis of ultra-small platinum nanoparticles in a continuous flow microreactor. Colloid Interface Sci. Commun. 13 (2016) 6
9 [13].

Nanomaterial Synthesis: Chemical and Biological Route and Applications

37

Figure 2.6 Effect of flow rate in a continuous-flow microreactor on nanoparticle size and crystallinity as observed by TEM and SAED. (A) Flow rate 5 14 μL/s and (B) flow rate 5 28 μL/s (image scale: both TEM 20 nm, both SAED 5 nm). The inset histogram shows the size distribution of particles synthesized at a flow rate of 14 μL/s. Reprinted with permission from P.L. Suryawanshi, S.P. Gumfekar, P.R. Kumar, B.B. Kale, S.H. Sonawane. Synthesis of ultra-small platinum nanoparticles in a continuous flow microreactor. Colloid Interface Sci. Commun. 13 (2016) 6
9.

2.3 NANOPARTICLE SYNTHESIS USING THE BIOLOGICAL ROUTE The synthesis of nanoparticles can be brought about by pure chemical compounds but the method has numerous limitations as well as being costly and injurious to health. The biological production of nanoparticles using environmentally benign materials like plant extracts (seed, leaf, flower, root, peels, and bark), fungal and bacterial enzymes renders them eco-friendly and compatible for pharmaceutical and biomedical purposes. Green synthesis is a bottom-up approach where the chemical reducing agents that are used are replaced by biological entities. The production of nanoparticles using bacteria, yeasts, fungi, algae, and plants in vivo has been studied, with the latter proving to be the best agent owing to their availability, ease of scale-up, and the nontoxic waste products produced. However, the mechanism of reduction is vaguely understood in most of the biological process. The basic idea of green synthesis is a bottom-up approach. The “bottom-up” synthesis involves the process of nanoparticle synthesis from small entity molecules like atoms and molecules by reduction or oxidation reaction, with the former playing a major role. The biochemical pathways, phytochemical contents, and activity of the enzymes are to be considered when selecting the organism or plant extracts for the production

38

Nanomaterials Synthesis

of nanoparticles. The size and shape of the nanoparticles are usually controlled by the nature of the biological entities and organic reducing agents also play a major role [14]. The other factors that influence the structural characteristics of the nanoparticles are pH, temperature (either room temperature or requires external heat sources), varying time, agitation conditions, the concentration of the target salt, and biological reducing agent. The production of nanoparticles can take place either within the cell or outside the cell, depending on the location of the reducing agent involved, and other extracts like flavonoids, phenolics, terpenoids, and cofactors support the nanoparticles by acting either as stabilizing agents or capping agents [15]. The biosynthesis of nanoparticles is increasingly gaining attention due to the environmentally proven technologies in material synthesis. The green nanoparticles provide advantages over the physiochemical method, including low capital investment [16], exclusion of toxic solvents and harmful byproducts [17,18], better structural properties, and various other values like stabilization and modification as discussed above [19]. They have also been found to have remarkable applications in the biological and chemical fields of drug testing and drug delivery. With the ease of production armed with modification and shape distribution, different green methods of biosynthesis of zinc oxide, selenium, and silver are discussed below.

2.3.1 Plant Extract-Based Nanoparticle Synthesis Plant-based reduction of ZnO has gained a lot of attention in recent years. Reduction of aqueous Zn1 to ZnO nanoparticles of size 24
40 nm have been performed using aloe vera extract [20]. The flower extract of Cassia auriculata was also used to reduce Zn(NO3)2 to ZnO nanoparticles of the size 110
280 nm [21]. The variation in the size of the nanoparticles was found to be dependent on the concentration of the leaf or flower extract that was used. The production of ZnO nanoparticles using green tea (Camellia sinensis) extract of size 16 nm has also been reported [22]. The manufacture of spongy ZnO nanoparticles of 30
35 nm size using Hibiscus rosa-sinensis at 100°C has also been studied [23]. Ag1 nanoparticles have been synthesized by plant extracts widely. Croton sparsiflorus has been used previously to synthesize Ag nanoparticles of size 22
52 nm and spherical shape [24]. Volvariella volvscea, an edible mushroom, was also used to synthesize triangular-shaped Ag nanoparticles

Nanomaterial Synthesis: Chemical and Biological Route and Applications

39

of size 20
150 nm [25]. The production of Ag nanoparticles of 20
25 nm size using hot water extract of olive leaves was synthesized [26]. The synthesis of Ag nanoparticles using Dioscorea batatas when maintained at 80°C rather than at room temperature was reported by Nagajyothi et al. [27]. Citrus peel extracts were also used as both reducing and capping agents to produce oval Ag nanoparticles and spherical Ag nanoparticles of size 62
74 nm [28]. Green synthesis of selenium nanoparticles from fenugreek seed extract was conducted and the nanoparticles were found to be of size 50
150 nm. These nanoparticles were found to be cytotoxic in nature towards breast cancer cell line (MCF7) [29]. Green Se nanoparticles were also found to be produced with raisins (Vitis vinifera). The Se nanoballs produced by this method were characterized by FTIR and were of sizes 3 and 18 nm [30]. Se nanoparticles of 35 nm size have been found to be stable for 30 days when produced by gum arabic. The gum arabic was found to be a better stabilizer than hydrolyzed gum arabic [31]. The synthesis of Se nanoparticles using Citrus reticulata peel extract was performed earlier and was characterized to be 70 nm in size and spherical in shape [32]. Stable Se colloidal nanoparticles were synthesized from Terminalia arjuna extract and characterized to have an absorption maxima of 390 nm [33]. The extract proved to be both a stabilizing and capping agent. Lemon leaf extract exhibited an absorption maximum at 395 nm when used to produce Se nanoparticles and TEM characterization results showed particle sizes of 60
80 nm [34]. The plant extractbased syntheses of Zn, Ag, and Se nanoparticles are tabulated in Table 2.1.

2.3.2 Microbial Synthesis of Nanoparticles Microbial extracts of Aeromonas hydrophila were found to synthesize ZnO nanoparticles of 57.7 nm size [35]. The nanoparticles also exhibited antimicrobial activity. Zinc nanoparticles of 10
20 nm using Streptomyces sp. (HBUN 17119) and synthesis of Mn21 nanoparticles using the same strain were reportedly synthesized [36]. A fungal culture, such as Aspergillus flavus (NCIM 650), was used to produce Ag nanoparticles of 8
10 nm size and they were found to be stable for 3 months [37]. The synthesis of Ag nanoparticles less than 10 nm using filamentous cyanobacteria Plectonema boryanum UTEX 485 at 25°C for up to 28 days has also been reported [38]. Basavaraja et al. reported the production of Ag nanoparticles using Fusarium semitectum at 27°C for 72 h [39]. The nanoparticles were found

40

Nanomaterials Synthesis

Table 2.1 Plant extract-based syntheses of zinc, silver, and selenium nanoparticles S. no.

Natural resource

1

Zinc nanoparticles Aloe vera leaf extract

5 6 7

Cassia auriculata flower extract Camellia sinensis Hibiscus rosa-sinensis Silver nanoparticles Croton sparsiflorus Olive (plant) Dioscorea batatas

8

Citrus (plant)

9 10 11 12 13 14

Selenium nanoparticles Fenugreek seed extract Vitis vinifera Gum arabic Citrus reticulata peel extract Terminalia arjuna Lemon leaf extract

2 3 4

Conditions

NP size (nm)

References

5
6 h at 150° C 60
80°C

24
40

[20]

110
280

[21]

12 h at 60°C 100°C

16 30
35

[22] [23]

29°C 24 h 25°C and 80° C 20°C and 60° C

22
52 20
25


[24] [26] [27]

35 and10

[28]

100°C






3 and 18 35 70 60
80

[29] [30] [31] [32] [33] [34]

to be of size ranging from 10 to 60 nm in size and mostly spherical. Ag nanoparticles of 2
5 nm size were synthesized from silver-tolerant yeast strain MKY3 at 30°C for 24 h in dark [40]. Bacterial reduction by silver of Vibrio alginolyticus, Pseudomonas aeruginosa KUPSB12, Rhodopseudomonas sp., Halococcus salifodinae BK6, Bacillus strain CS 11, Lactobacillus crispatus, Exiguobacterium mexicanum PR 10.6, and Alteromonas macleodii of size varying between 6 and 100 nm has been reported [41
48]. Microbial reduction by selenium to its elemental nanoparticles has been reported using many bacterial cultures. Synthesis of Se nanoparticles using Lactobacillus acidophilus, Lactobacillus casei, Bifidobacterium sp., and Klebsiella pneumonia has been reported and their sizes were found to range from 50 to 550 nm [49,50]. Amorphous Se nanoparticles in the presence of Shewanella sp. HN-41 have been synthesized under precise conditions [51]. Use of Rhizobium selenireducens sp., Dechlorosoma sp., Pseudomonas sp., Paracoccus sp., Enterobacter sp., Thaurea sp., Sulfurospirillium sp., Desulfovibrio sp., Pseudomonas alcaliphila, and Shewanella sp. to produce nanowire and nanorods has also been reported [52
55]. The average size of all nanoparticles

Nanomaterial Synthesis: Chemical and Biological Route and Applications

41

synthesized above is 103 6 5.1 nm. Reduction to elemental Se using Escherichia coli proteins of size 10
90 nm has been done and its amorphous nature has been studied in detail [56]. Metabolism of elemental Se inside the cell is represented as a pathway in Fig. 2.7. The microbial syntheses of Zn, Ag, and Se nanoparticles are tabulated in Table 2.2.

Figure 2.7 Exfoliation of nanoclay platelets into the polymer to form a nanocomposite. Table 2.2 Microbial-based syntheses of zinc, silver, and selenium nanoparticles S. no.

Natural resource

1

Zinc nanoparticles Aeromonas hydrophila

2

Streptomyces sp. (HBUN 17119)

3 4

Aspergillus flavus (NCIM 650) Plectonema boryanum UTEX 485

5

Fusarium semitectum

6

Yeast strain MKY3

7 8

Vibrio alginolyticus Pseudomonas aeruginosa KUPSB12 Rhodopseudomonas sp. Halococcus salifodinae BK6 Bacillus strain CS 11 Lactobacillus crispatus Exiguobacterium mexicanum PR 10.6 Alteromonas macleodii Lactobacillus acidophilus, Lactobacillus casei, Bifidobacterium sp., and Klebsiella pneumonia

9 10 11 12 13 14 15

Conditions

Size (nm)

References

24 h at 30°C 72 h at 35°C
25°C for 28 days 27°C for 72 h 30°C for 24 h



57.7

[35]

10
20

[36]

8
10 10

[37] [38]

10
60

[39]

2
5

[40]

50
100 50
85

[41] [42]








6
10 42
92 70
98 5
40

[43] [44] [45] [46] [47]





70 50
550

[48] [49,50]

(Continued)

42

Nanomaterials Synthesis

Table 2.2 (Continued) S. no.

Natural resource

Conditions

Size (nm)

References

16 17

Shewanella sp. HN-41 Rhizobium selenireducens sp., Dechlorosoma sp., Pseudomonas sp., Paracoccus sp., Enterobactor sp., Thaurea sp., Sulfurospirillium sp., Desulfovibrio sp., Pseudomonas alcaliphila, and Shewanella sp. Escherichia coli





Amorphous 103 6 5.1

[51] [52
55]




10
90 and amorphous

[56]

18

2.4 APPLICATION OF NANOMATERIALS There are a number of applications available for nanoparticles on a commercial scale, for example, nanocontainers for the release of the drug; nanoclay particle applications in polymer nanocomposites for improvement of the mechanical properties; nano calcium carbonate used for the filler in PVC composites; and titanium dioxide used for the degradation of the organic pollutants and used as a photocatalyst. Some of these applications are discussed below.

2.4.1 Nanoclay and Polymer Nanocomposite and Polymer Functional Nanolatex Clay is basically aluminosilicates which are exfoliated into smaller platelets of thickness of one nanometer using shear application. There can be more than 1000 platelets. Shearing can be brought about using ultrasound or a high-pressure homogenizer. The clay platelets were intercalated first using long-chain surfactants, such as cetyl trimethyl ammonium bromide, by sonication which can bring brought about intercalation. The addition of nanoclay into the polymer at melt conditions, for example, polypropylene nanocomposite is prepared by the addition of the clay at 165°C at melt condition. The shearing is brought about by extrusion due to which the clay platelets exfoliate into the polymer, forming the nanocomposites. Due to this exfoliation, there is a drastic improvement in the mechanical properties such as the tensile strength will increase 30% by the addition of nanoclay at less than 5%. The exfoliation method of formation of nanocomposites is represented in Fig. 2.7. Functional nanolatex can be produced using the miniemulsion

Nanomaterial Synthesis: Chemical and Biological Route and Applications

43

process in which the latex particle size is not uniform. There will be distribution of the nanolatex. This can be achieved using the ultrasound technique. For example, the polybutyl methacrylate (PBMA) can be produced using an ultrasound-assisted method in which the water and butyl methacrylate are taken and these are basically two different phases which can be disbursed into each other by addition of the surfactant above the critical micelle concentration. It is also possible to prepare the inorganic attachment of the nanoparticles onto the polymer latex. Core
shell morphology can also be possible. Due to the ultrasound technique, because of cavity collapse, there is a generation of radicals which will initiate the polymerization into the reactor. Due to ultrasound shearing, the existing particles will reduce in there size. The addition of the surfactant above CMC makes a stable micellar formation which leads to the formation of polymer latex of the same size in the nanometer range. The process is diagrammatically shown in Fig. 2.8.

2.4.2 Photocatalyst for Degradation of Organic Pollutants The photocatalytic degradation is usually carried out by a photocatalyst, such as TiO2, ZnO, or combinations of the photocatalyst, such as CeTiO2, Fe-TiO2, etc. The nano-size TiO2 with different phases has an impact on the degradation. The complete mineralization of the organic components is always issued so that the photocatalyst is usually used by using a UV light and with the addition of hybrid systems such as a photo Fenton process or combination of ultrasound and hydrodynamic cavitation. A number of attempts are currently being made to develop a photocatalyst which can cover the UV and visible range. A number of organic pollutants, such as phenol dyes and pharmaceutical components were degraded using a combination of the photocatalyst and ultrasound process. Inorganic core Surface reaction

M

H 2O (Vap)

. H + OH–

M M Impulsion

Cavitation bubble

+

Monomer droplet size reduction

Polymer Precursor

Polymer particle

+ Functionalized Inorganic core

Inorganic core

Polymer

Figure 2.8 Polymer functional nanolatex using the ultrasound-assisted technique.

44

Nanomaterials Synthesis

2.5 RECENT ADVANCES IN THE CHEMICAL AND BIOLOGICAL SYNTHESIS ROUTES Over the last few years, there have been a number of advancements in research for the production of nanoparticles. Despite their nanosize, their immense applications have always been fascinating. Various chemical methods have evolved with their production. The sonochemical- or ultrasound-assisted methods have been used for the production of various noble metal nanoparticles such as gold, silver, and platinum for medicinal uses. There is a simple, one-step and economical protocol for the synthesis of gold nanoparticles (Au nanoparticles) that has been developed via an ultrasound probe sonicator with dimethyl sulfoxide (DMSO) as the reducing agent as well as the capping agent with the elimination of additional surfactants or stabilizers [57]. Likewise, in a breakthrough study, a nanocomposite of the iron-based metal-organic framework [MIL-53(Fe)] and graphene was prepared by the one-pot solvothermal method and was applied for the selective oxidation of alcohols to aldehydes and ketones [58]. Clays have been found to be one of the most important industrial materials and with recent advancements in nanotechnology, researchers have developed nanoclay which has various applications and can also be used as a nanofiller. In one study, polyurethane (PUR)
nanoclay composites were synthesized using methylene diphenyl diisocyanate, polyol, and hectorite clay, which showed a significant increase in its tensile and flexural strengths, abrasion resistance, and thermal properties [59]. Synthesis of superparamagnetic iron oxide nanoparticles (SPIONs) and their surface modification for various biomedical and instrumentation applications have become a new interest in the field of nanoparticles. Due to their superparamagnetic properties, these nanoparticles possess high magnetic susceptibility and controlled magnetic behavior. Chemical methods such as coprecipitation and thermal decomposition techniques are the most adopted methods for synthesizing SPIONs [60]. Similarly, several chemical methods have been designed for the enhanced production of nanoparticles. There is an increasing demand for biological synthesis of nanoparticles over conventional chemistry approaches owing to their eco-friendly, costeffective, scalability and multifunctional stability. The last two decades have seen the development of nanoparticles for biomedical applications and toward cancer in specific. Several researchers have also developed rapid synthetic methodologies with high yields using plant sources

Nanomaterial Synthesis: Chemical and Biological Route and Applications

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wherein silver nanoparticles were synthesized using various plant extracts in 2 min [61], 5 min [62], 45 min [63], 1 h [64], and 2 h [65]. Gold nanoparticles were also developed in 3 min [63], 5 min [65], and 10 min [64] time intervals, thus corroborating the simple and fast synthesis methods. The major advantage in the biological synthesis of nanoparticles is the reduction of a number of steps required to attach and functionalize the nanoparticles for various biological activities, which is in addition to the chemical synthesis [66]. In the field of antimicrobial activity, biological nanoparticles from Desmodium gangeticum have higher antioxidant, antibacterial activity and biocompatibility compared to their chemical counterpart [67]. Zinc biological nanoparticles have greater antimicrobial potential against Salmonella typhimurium ATCC 14028, Bacillus subtilis ATCC 6633, and Micrococcus luteus ATCC 9341 when compared with chemically synthesized zinc nanoparticles [68]. The mechanism of antimicrobial activity still remains elusive with many hypotheses like cell membrane disruption and cellular DNA alteration. Biologically synthesized nanoparticles have made huge advancements in biomedical applications. Silver nanoparticles derived from Olax scandens leaf displayed efficacy towards cancerous cell lines A549 (human lung cancer) and MCF7 (human breast cancer), biocompatibility for drug delivery and imaging facilitator activity. These biological nanoparticles showed bright-red fluorescence inside cells which could help detect drug localization inside cells. The biocompatibility of these nanoparticles on normal cell lines like rat cardiomyoblast normal cell line (H9C2), human umbilical vein endothelial cells (HUVEC), and Chinese hamster ovary cells (CHO) was found to be more than for chemically synthesized nanoparticles [69]. Gold nanoparticles synthesized with extract of seaweed Corallina officinalis showed cytotoxic activity against the human breast cancer cell line MCF7 [70]. Novel proangiogenic biosynthesized gold nanoconjugates were also developed to accelerate the growth of new blood vessels through redox signaling [71]. The anticancer activities are related to their size and shape, which in turn are related to the reactive oxygen species (ROS), thereby causing damage to cellular components [72]. They may also find their way to apoptosis via mitochondrial-dependent and caspasedependent pathways [62]. Biological nanoparticles have added applications in addition to anticancer and antimicrobial activities like sensor designing. Silver nanoparticles synthesized biologically were successfully used in the development of

46

Nanomaterials Synthesis

optical fiber sensors for the detection of H2O2, making them portable and compatible with various industrial purposes [73]. The main problems raised against biologically synthesized nanoparticles are the degradability, clearance, toxicity, immunogenicity, and pharmacokinetics of these nanoparticles. The toxicity concerns over biological nanoparticles have been observed to be low in vitro and in vivo in recent studies [69,74,75]. The adsorption of biological nanoparticles into serum proteins in the blood was found to be low, suggesting their stability in vivo [76].

2.6 SCALE-UP ISSUES OF NANOPARTICLE PRODUCTION AND CHALLENGES In nanoparticle production, there are a number of challenges. First, the important challenge is maintaining the particle size with narrow particle distribution. Hence, controlling the growth of the particles by Oswald ripening and agglomeration is an important task. This could be achieved by the addition of capping agents during the production of nanoparticles. The second important issue of scale-up is drying of the particles, during the drying the particles come together forming larger lumps and hence it is difficult to separate the particles which can be done by addition of surfactants to keep the particles apart. Generally, this is a major issue in the case of colloid particles which are in high concentrations. In this case, freeze-drying is carried out. Maintaining the consistent particle size distribution is difficult, hence methods such as microreactor technology could be a better option for scale-up of production of these nanoparticles.

2.7 SUMMARY Nanoparticle production has a number of methodologies. Each method has its limitations and advantages. There is a large impact from the concentration of the precursors, dilution ratio, capping agent concentration, and surfactant concentration. The top-down approach has issues such as loss of energy; and the distribution of the particles can be wide. There are a number of methods for scaling up, such as nanoclay production which is successfully used as a nanocomposite for automotives. There are certain challenges, such as maintaining narrow particle size distribution, which could be overcome by controlling the addition of the surfactant and capping agents.

Nanomaterial Synthesis: Chemical and Biological Route and Applications

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ACKNOWLEDGMENT Dr. Shirish Sonawane acknowledges the Science and Engineering Research Board (SERB) DST Govt of India for financial support through grant No: SERB/ EMR/2016/007585.

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CHAPTER 3

Chemical Approaches for 1D Oxide Nanostructures F.A. Taher1,2,3 and E. Abdeltwab1,2 1

Chemistry Department, Faculty of Science (Girls Branch), Al-Azhar University, Cairo, Egypt Al-Azhar Technology Incubator (ATI), Cairo, Egypt 3 Physics Department, Faculty of Science, (Girls), Al-Azhar University, Cairo, Egypt 2

Contents 3.1 Introduction 3.2 1D Nanostructure Synthesis Techniques 3.3 1D ZnO Nanostructures 3.3.1 Crystal Structure of ZnO 3.3.2 Zinc Oxide Nanostructures: Synthesis Methods 3.3.3 ZnO Nanostructures Through Hydrothermal Growth 3.3.4 Effects of the Hydrothermal Temperature, Growth Solution Concentration, Reaction Strength, and Reaction Duration 3.3.5 Doping of ZnO nanostructures Through Hydrothermal Routes 3.3.6 Recent Challenges in Nanotechnology Based on 1D ZnO Nanostructures 3.4 1D TiO2 Nanostructure 3.4.1 Crystal Structure of TiO2 3.4.2 1D TiO2 Nanostructure Synthesis Techniques From Chemical Solution 3.4.3 Growth Mechanism of 1D TiO2 Nanostructures From Chemical Solution 3.4.4 Recent Synthesis Trends and Challenges in Nanotechnology Based on 1D Nanostructures 3.5 Conclusion References

53 55 57 58 59 59 62 64 65 67 67 68 77 78 79 79

3.1 INTRODUCTION In the last three decades, there has been a shift towards shape-dependent strategies for producing one-dimensional (1D) oxide nanostructures that have outstanding properties as evidenced by abundant magnificent reviews. In particular, 1D oxide nanostructures serve as an intensely valuable class of nanomaterials due to their diverse morphologies. One-dimensional nanostructures are specifically nanomaterials with one dimension outside the nanometer range (from several hundreds of Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00003-1

© 2019 Elsevier Inc. All rights reserved.

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nanometers up to a few centimeters in length) and diameters of only a few nanometers (1
100 nm). Nanowires, nanotubes, nanoribbons, nanobelts, nanowhiskers, nanoneedles, nanocages, nanorings, nanocombs, etc. are a few nanometers in diameter and several micrometers in length [1]. In addition, these anisotropic shapes determine the way in which the electron density can be polarized [2]. Typically, nanowires are structures with cross-sections of 2
200 nm and length upwards of several micrometers [3]. These structures are confined only in two dimensions, thus allowing electrons, holes, or photons to propagate freely in the third dimension [4]. Vertically aligned nanowires are used in many applications, such as vertical field-effect transistors [5], thermoelectric coolers [6], solar conversion systems [7] and optoelectronic devices [4]. Mohaddes-Ardabili et al. [8] reported a simple approach through the decomposition of La0:5 Sr0:5 FeO3 to create self-assembled α 2 Fe nanowires that have uniaxial anisotropy normal to the film plan (Fig. 3.1A). Jamshidi et al. [9] also reported the large-scale assembly of individual semiconducting single silver nanowires with diameters below 20 nm by optoelectronic tweezers. Zhu et al. [10]

Figure 3.1 (A) TEM of self-assembled nanostructures in La0.5Sr0.5FeO3 thin films. (C) [001] dark-field cross-section image of a film showing α-Fe nanowires embedded in LaSrFeO4 matrix. Reprinted with permission from L. Mohaddes-Ardabili, et al. Selfassembled single-crystal ferromagnetic iron nanowires formed by decomposition. Nat. Mater. 3 (2004) 533
538. Copyright 2004, Springer Nature. (B, C) Branched ZnO nanostructures. Regrowth of ZnO nanowires on already grown ZnO nanowires (grown at 650°C for 10 h) on an SLG substrate yield branched structures [12]. (D) SEM of ZnO 3D nanotubes. (E) SEM of nano TiO2 3D nanotubes [13].

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Figure 3.2 (A) The cross-sectional (A) and plan (B) SEM images of 1D/3D TiO2 nanorods, (B) FESEM images of the cross-sectional (C)and top (D) views of 3D TiO2 nanotubes. Reprinted with permission from H. Wang, et al. Significant enhancement of power conversion efficiency for dye sensitized solar cell using 1D/3D network nanostructures as photoanodes. Sci. Rep. 5, 9305 (2015). Copyright 2015, Springer Nature.

prepared high-purity, monocrystalline CuO nanowires with a monoclinic structure and large aspect ratio via a thermal oxidation method [11]. Sugavaneshwar and Nanda [12] developed a simple noncatalytic synthesis of ultralong ZnO nanowires in a large area of soda lime glass with controllable aspect ratio (kinetically or thermodynamically) and branched structures (Fig. 3.1B and C). Filippin et al. [13] presented a three-step vacuum procedure for the fabrication of an ample variety of ZnO and TiO2 nanotubes with tunable length, hole dimensions and shapes, and tailored wall composition, microstructure, and porosity (Fig. 3.1D and E). Wang et al. [14] synthesized 1D nanorods/3D TiO2 nanotubes that exhibited fast electron transport and high surface area using a hydrothermal growth process followed by a postetching treatment (Fig. 3.2).

3.2 1D NANOSTRUCTURE SYNTHESIS TECHNIQUES Owing to the fascinating shape-dependent properties of 1D nanostructures such as being uniform, high-yield, low-cost, homogeneous, and monodisperse, definite morphologies have been expanded toward

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synthetic strategies for 1D oxide nanostructures. Compared with the bulk materials, the nanosize of these nanostructures enables (1) the higher density of state which could lead to more advanced electronics, (2) higher aspect ratio (length to width) which could promote the individual and collective properties of the nanostructures, and (3) quantum confinement effect which also could afford more desirable properties for semiconductor photonics [1]. Different approaches have been suggested to grow a 1D nanostructure by numerous methods (Table 3.1). These approaches [15
20] can be broken down into six basic categories of principles, including: 1. An inherent emphasis favoring the inherent anisotropic crystallographic structure of a specific solid; 2. Kinetic control provided by the presence of a directing, capping agent; 3. Self-assembly and aggregation of precursor 0D nanostructures; 4. Confinement by a liquid droplet as in a vapor
liquid
solid process; 5. Directional growth through the use of the spatially confining pores of a template; 6. Size reduction of a larger 1D microstructure.

Table 3.1 Summary of synthetic methods and strategy principle for the synthesis of 1D oxide nanostructures by solution-based methods Methods

Strategy principle

Chemical and physical vapor deposition methods (CVD and PVD, respectively) Chemical solution growth method (sol
gel, precipitation, spray pyrolysis, hydrothermal and molten salt; with and without template)

Confinement by a liquid droplet as in a vapor
liquid
solid process

Electrochemical methods (anodic oxidation and electrodeposition) Electrospinning Lithographic methods (photolithography, deep UV lithography)

Directional growth through the use of the spatially confining pores of a template, kinetic control provided by the presence of a growth directing or capping agent Oriented attachment, self-assembly and aggregation of nonspherical nanocrystals to form 1D nanostructures Oriented attachment, self-assembly and aggregation of nonspherical nanocrystals to form 1D nanostructures Kinetic control provided by the presence of a directing, capping agent Directional growth through the use of the spatially confining pores of a template

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57

Here, we put the spotlight on approaches 1, 2, and 3, which all can be taken advantage of in the solution-based methods of 1D nanostructures. The synthetic strategies for 1D oxide nanostructures can primarily be classified into four methods: the chemical and physical vapor deposition methods (CVD and PVD, respectively), chemical solution growth method (sol
gel, precipitation, spray pyrolysis, hydrothermal and molten salt; with and without template), electrochemical anodization and electrodeposition and electrospinning. Sometimes, lithographic methods (photolithography, deep UV lithography and laser-based micromachining) can be designed as 1D oxide nanostructure methods [2]. However, vapor deposition methods need complex technical conditions, high temperatures, and even corrosive and noxious precursor gases. Also, these deposition methods cannot be applicable for more complex oxides, such as perovskite [21]. Moreover, these lithographic methods tend to be rather expensive, laborious, consume too much time, not fitting for practical and universal applications, energy consuming, and relatively complicated. By comparison, chemical solution-based methods are relatively simple, less hazardous, economical, lower temperature, high yielding, and largescale production. In this chapter, we only discuss the chemical solution approach of 1D oxide nanostructure fabrication.

3.3 1D ZNO NANOSTRUCTURES ZnO is a significant technological material with a wide band-gap (3.37 eV) and high exciton binding energy (60 meV). Structures containing 1D nanoscale exhibit interesting chemistry as well as size, shape, and material-dependent properties that lead to enhanced properties. Their enhanced structural, optical, electronic, and magnetic properties, along with nanosized morphology, which are all characterized by the surface to volume ratio, and chemical reactivity, have led to a wide range of applications in nanoelectronics, optoelectronics, medical diagnostics, catalysis, drug delivery, therapeutics, and chemical sensing. There are a variety of methods for fabricating one-dimensional nanostructures. The most common 1D ZnO nanostructure shapes are nanorods, nanowires, tubes, ribbons, belts, whiskers, and needles. When nanostructures are called “nanowires,” this means that the nanostructure has specific growth, but its cross-section may not be uniform or specific in shape. On the other hand,

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a nanobelt is a nanowire that has well-defined side surfaces. In this section, we summarize the conditions leading to the growth of different 1D ZnO nanostructures via a hydrothermal technique, which is important for bottom-up strategies in self-assembly of 1D nanostructured building blocks into large ordered hierarchical heterostructures. Doping of ZnO nanostructures through a hydrothermal method is also highlighted.

3.3.1 Crystal Structure of ZnO ZnO has a wurtzite hexagonal structure (space group C6mc) with lattice parameters a 5 0.3296 and c 5 0.52065 nm. The structure of ZnO, for example, can be described as a number of alternating planes composed of fourfold tetrahedrally coordinated O22 and Zn21 ions, stacked alternately along the c-axis, as shown in Fig. 3.3. The two important characteristics of the wurtzite-structure are polar surfaces and the noncentral symmetry. There are two primary polar planes in the ZnO wurtzite crystal structure, where the top is a Zn-terminated (0001) positively charged plane and exhibits high surface energy. The bottom polar plane is an O-terminated (000-1) negatively charged basal plane. The oppositely charged polar surfaces result in a normal dipole moment and spontaneous polarization along the c-axis as well as a variance in surface energy. The most common polar surface is the basal plane (0001). The tetrahedral coordination in ZnO results in piezoelectric and pyroelectric properties due to the absence of inversion symmetry. In addition to these two

Figure 3.3 Hexagonal ZnO crystal structure with an indication of corresponding facets and growth directions.

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primary polar planes, other secondary nonpolar planes can be distinguished in the wurtzite crystal structure. The most commonly nonpolar planes in ZnO are the a-plane (2-1-10) and 6 (01-10) side planes which have lower energy than the top/bottom planes 6 (0001) [22].

3.3.2 Zinc Oxide Nanostructures: Synthesis Methods One-dimensional oxide nanostructures can be grown by either chemical synthesis or physical methods, such as physical deposition methods or lithographic methods. The later methods are expensive and complicated. The chemical synthesis approach has several advantages over physical deposition methods or lithographic methods including that it is very simple, less costly, can easily be scaled-up and the synthesis parameters like temperature, precursor concentration, and solution strength can easily be controlled. The requirements for fabrication of 1D nanostructures via the chemical synthesis approach are less restrictive than those for physical methods because it is more structurally controlled than physical deposition methods. Here we separate details of hydrothermal synthesis 1D ZnO nanostructures that are considered as a simpler, faster, and less expensive method for large-scale production 1D ZnO nanostructures and the capability to synthesize large quantities of uniform 1D ZnO nanostructures that has been reported by different groups.

3.3.3 ZnO Nanostructures Through Hydrothermal Growth Andres-Vergés et al. [23] first reported the hydrothermal method of growing ZnO nanostructures. There was not much interest until Vayssieres et al. [24] successfully used the thermal decomposition method for controlling fabrication of ZnO nanowires on glass and Si using hexamethylenetetramine (HMT), C6H12N4, and zinc nitrate to initiate the growth of a thin layer of ZnO on the substrate. The role of HMT is to supply the hydroxyl ions to drive the precipitation reaction. Hexamethylenetetramine is one of the most popular nonionic, cyclic tertiary amine organic compounds that is widely used to support the hydrothermal growth of ZnO nanorods. The proposed reaction supposes that the thermal degradation of HMT releases hydroxyl ions which react with Zn21 ions to form ZnO [25,26]. Depending on several hydrothermal reaction factors, such as reactant concentrations, pH, duration time, and temperature of the reaction, different morphologies such as prisms,

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needles, and nanowires are formed. In the absence of any surfactants, the HMTA-assisted in situ hydrolysis resulted in microtube and multipod morphologies [27]. The formation of these morphologies is mainly determined by kinetic conditions, where faces can be developed from the solution complexes [28]. Vayssieres et al. [29] used the hydrothermal method to grow ZnO nanorods from an equimolar (0.1 M) aqueous solution of zinc nitrate, Zn (NO3)2.4H2O, and HMT at 95°C for 1
10 h on different substrates. This growth approach takes place through a two-step chemical method. First, a ZnO seed layer is needed to initialize the uniform growth of oriented nanorods [25]. Then, the immersion of seeded ZnO layer in an aqueous solution of Zn(NO3)2 and hexamethylenetetramine at low temperature yields one-dimensional ZnO nanorods on the substrate surface. The most effective way to obtain ZnO nanorods is to lower the overall concentration of precursors, while keeping the same 1:1 ratio [24]. The zinc precursors that are usually used in the hydrothermal synthesis of 1D ZnO nanostructures are zinc acetate, zinc nitrate, and zinc chloride. However, well-developed ZnO nanorods with a hexagonal cross-section were obtained from zinc nitrate:HTMA with 2:1 molar ratio at 90°C for 2 h. The transformation from nanorods to nanotubes was realized in similar experimental conditions, but at a different time. The transformation from nanorods to nanotubes is attributed to the Kirkendall effect, where the inner core continuously reacts with OH2 as the reaction time increases, thus resulting in hollow tubular structures [30
32], as shown in Fig. 3.4. However, the mechanism which controls the growth process in the presence of HMT is still not clear. It was believed that in acidic aqueous solution at elevated temperature, HMT hydrolyzes to formaldehyde

Figure 3.4 SEM of nanorods and nanotubes at (A) 2 h and (B) 4 h [35].

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ammonia, which undergoes acid
base equilibrium, according to the reaction illustrated below [33]: C6 H12 N4 1 6H2 O-6HCHO 1 4NH3 2 NH3 1 H2 O-NH1 4 1 OH

ZnðNO3 Þ2 :6H2 O-Zn21 1 2NO2 3 1 6H2 O Zn21 1 2OH2 - 1 ZnðOHÞ2 In this reaction, Zn21 ions react with OH— groups to form zinc hydroxide and subsequently tetrahydroxozincate growth units are formed in the presence of an excess of OH—. Therefore, the OH— group plays a vital role in controlling the morphology of ZnO. In other words, the higher the concentration of the HMT, the more the evolved ammonia gas and the more the provided continuous OH— groups that is a key factor for controlling the growth rate and thus leads to the formation of 1D ZnO nanostructures [33]. Sugunan et al. [34] discussed the contribution of HMT in the growth process of ZnO nanowires. It was proposed that HMT has a long-chain polymer and a nonpolar chelating agent, which attach to the nonpolar facets of the zincite crystal, leaving only the polar 6 (0001) planes for epitaxial growth. Therefore, HMT therefore acts more like a shape-inducing polymer surfactant rather than as a buffer [35]. According to the literature, the fastest growth rate under hydrothermal conditions is in direction [0001], which leads to an increase in the surface area of the (2-1-10) or (01-10) facets. However, in fact, the grown crystal face is strongly dependent on the growth conditions, such as hydrothermal temperatures, growth reaction strength, and reaction duration [36,37]. In the case of hydrothermal growth, the main parameters affecting n the shape, morphology, and orientation of ZnO 1D nanostructures are: (1) concentrations of ZnO precursors; (2) pretreatment of the substrate as well as the presence or absence of a seed layer; and (3) hydrothermal parameters of growth (growth temperatures, reaction strength, and growth time). By adjusting these parameters, different morphologies of 1D, 2D, and even 3D ZnO nanostructures can be obtained.

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3.3.4 Effects of the Hydrothermal Temperature, Growth Solution Concentration, Reaction Strength, and Reaction Duration The temperature of the hydrothermal treatment is an important parameter in ZnO nanostructure formation, taking into consideration that the duration of time leads to an increasing degree of crystallinity, but for a long reaction time morphological evolution to 3D could occur. Concerning the temperature of the hydrothermal process, it promotes the nucleation and growth processes. The influence of the reaction strength on the morphological change of the products depending on change in growth solution concentration has been studied [33]. It was confirmed that increasing the reaction strength assisted in the transformation from 1D nanorods to 2D nanosheets. The authors urged this morphological change to slower crystal growth kinetics at high reactant concentration. The SEM investigation shows obtaining of high-density nanorods, as shown in Fig. 3.5, from 0.005 M at 90°C for 3 h, by increasing the concentration to 0.025 M, 2D nanosheet morphology had been obtained at the same temperature and reaction time, as represented in Fig. 3.6. The difference in growth mechanism are attributed to changes in reaction conditions, where the growth of the c-plane polar surface or m-plane nonpolar surface could be achieved by controlling the reaction strength. The increasing Zn:HMT concentration facilitates the growth of nanosheets along the [10] direction to form ZnO with dendritic morphology. In addition, the growth process is related to the surface energy that determines the preferential growing surfaces, and the growth kinetics that determine the final structure. In the case of a low concentration, ZnO has a preference to grow along the polar (0001) c-plane in 1D nanorods since the (0001) surface is thermodynamically unstable and has higher growth rates to reduce their higher surface energy. This means, at higher concentrations in the

Figure 3.5 SEM images of the Co-doped ZnO dendrite-like structures grown with 0.005 M at 90°C for 3 h. Reprinted with permission from E. Abdeltwab, F.A. Taher, Polar and nonpolar self-assembled Co-doped ZnO thin films: structural and magnetic study. Thin Solid Films 636, (2017). Copyright 2018, Elsevier.

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Figure 3.6 SEM images of the Co-doped ZnO dendrite-like structures grown with 0.05 M at 90°C for 3 h. Reprinted with permission from E. Abdeltwab, F.A. Taher, Polar and nonpolar self-assembled Co-doped ZnO thin films: structural and magnetic study. Thin Solid Films 636, (2017). Copyright 2018, Elsevier.

Figure 3.7 Morphological evolution sketch of ZnO nanostructures.

hydrothermal process, that the interacted or coordinated OH
functional group on the surface of ZnO will prevent the dissociated Zn(OH)422 ions from growing along the polar (0001) direction; but still grow sideways along the nonpolar (10
10) direction of 2D nanosheets due to its lower surface energy and slower growth velocity, as shown in Fig. 3.7. An equimolar 0.1 M aqueous solution of zinc nitrate, Zn(NO3)3.4H2O, and HMT at 90°C for 2 days is used to obtain highly oriented ZnO microtubes with well-defined crystallographic faces along the [0001] direction [38]. It was also observed that there was a change in the

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morphology of aqueous solution-grown ZnO microrods on glass substrates with a change in the concentration of HMT by Mridha and Basak [39].

3.3.5 Doping of ZnO nanostructures Through Hydrothermal Routes Doping of nanostructured materials is the most effective way to control properties such as optical, electrical conductivity, and ferromagnetism. Transition metal and/or rare earth doping of nanostructured ZnO semiconductors have had great interest from researchers for possible applications in spintronics and visible light photocatalysis [40
44]. ZnO nanorods doped with Mn, Cr, and Co was hydrothermally synthesized using precursor nitrate and hexamethylenetetramine [45]. For the first time, ZnO doped with transition metals was reported to be nonferromagnetic [43,46]. It is noted that various transition metals can be easily doped. Among them, Co-doped and Mn-doped ZnO have been frequently reported to be ferromagnetic with high-temperature ferromagnetism, however several groups claimed an extrinsic mechanism for the ferromagnetism such as metal precipitations. As for the structural properties, some have reported the appearance of an impurity phase or precipitation, but others have not. The preferred crystallographic orientation of the Co-doped ZnO nanostructures is determined by analysis of its GIXRD pattern, where the GIXRD patterns, as represented in Fig. 3.8, of the nanorod-like structure Co-doped ZnO films show that the nanorods grow in the polar (0002) c-plane orientation with a peak position at around 34.43 degrees. Meanwhile, sheet-like structure Co-doped ZnO films grow in the nonpolar (10
10) m-plane orientation with peak positions at around 31.72 and 31.64 degrees, respectively [33]. Although there no diffraction peaks attributed to the Co-related secondary phases or precipitation were detected, their existence cannot be completely excluded due to the sensitivity limits of XRD methods. If there is any precipitation, it is necessary to investigate the origin of the magnetism. However, spectroscopies can probe the chemical state, thus ruling out possible metal precipitation. However, they may not separate the magnetic signal of the bulk from that of nonmetal precipitation having similar chemical states, such as oxide. In N-co-doped ZnO nanorods films are successfully deposited under hydrothermal conditions, where the substrates covered with the seed layer were introduced in a Teflon autoclave containing an aqueous solution of precursor nitrate, and HMT with a ratio of 2:1. The reagents used

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Figure 3.8 XRD patterns of (A) ZnO powder, (B) Co-doped ZnO powder, (C
E) GIXRD patterns of Co-doped ZnO films; (C) nanorods and (D, E) nanosheets.

contained nitrogen in high amounts. In, N-co-doped ZnO nanorod films a flat hexagonal cross-section was visualized by SEM (Fig. 3.9). Chemical characterization of ZnO films could be examined by XPS even at high temperature, as shown in Fig. 3.8. The nature of the In, N-co-doped ZnO nanorods films was proved by XPS [47]. Our recent experiment found that the nanostructure morphology and the growth direction of the grown Co-doped ZnO films could be tuned by the growth reaction rate by controlling the growth solution concentration. These results indicate that the polarization of the Co-doped ZnO films can be controlled through the preparation condition.

3.3.6 Recent Challenges in Nanotechnology Based on 1D ZnO Nanostructures The ZnO nanostructures are a unique group that is likely to have important applications in nanosize electronic, optical, sensor, and optoelectronic devices [48]. A spontaneous polarization is induced across structurally controlled ZnO nanobelts due to the noncentral symmetry of the (ZnO4)62 tetrahedron unit in the ZnO structure, which results in its anisotropic piezoelectric properties [4]. Structurally, the ZnO Wurtzite-structure can be described schematically as a number of alternating planes composed of

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Figure 3.9 SEM images of In, N-co-doped ZnO films annealed at 100°C (A, D), 300°C (B, E), and 500°C (C, F). (G) XPS survey of In, N-co-doped ZnO films annealed at 100° C, 300°C, and 500°C. Reprinted with permission from M. Duta, et al. Properties of In
N codoped p-type ZnO nanorods grown through a two-step chemical route. Appl. Surf. Sci. 344 (2015) 196
204. Copyright 2015, Elsevier.

fourfold coordinated O22 and Zn21 ions, stacked alternately along the c-axis. The presence of two oppositely charged ions result in Zn positively charged (0001) polar surfaces and O negatively charged (0001) polar surfaces, producing a normal dipole moment and spontaneous polarization, as well as a divergence in surface energy. As a result of the presence of positive and negative ionic charges on the zinc- and oxygen-terminated (0001) surfaces, helical nanostructures and nanorings are formed by rolling up nanobelts. Generally, the polar surfaces exhibit massive surface reconstructions, but both positively and negatively charged polar surfaces of ZnO are an exception, and are atomically flat, stable, and without

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reconstruction [49,50]. These polar surface-dominated ZnO nanobelts are considered to be an ideal system for one-dimensional nano-scale sensors, transducers, and resonators [4]. Transition metal- and rare-earth-doped ZnO are attracting significant attention as diluted magnetic semiconductors for spintronics due to a measured Curie temperature above room temperature [51]. A proposed spintronics technology, that uses the electron spin rather than the electron charge for reading and writing information [52,53], will require very low dimensions in order to make real use of the advantage offered by spin. Furthermore, it has found that the doping technique of ZnO nanosized structures can control the growth behavior, possibly resulting in tunable structural and magnetic properties [33]. The key characteristics of transition metal- or rare-earth-doped ZnO nanostructures are cations with mixed valence and oxygen vacancies. The latter are responsible for the observed high-temperature ferromagnetism. Development of techniques for integration of nanostructures with other microstructures, such as nanoelectromechanical and biosensing systems, is needed. Simple techniques to grown ZnO nanostructures and in self-assembly structures with complex functionality are also required. This is a key step toward nanosystem integration. By using a hydrothermal simple approach, well-aligned doped ZnO nanorods with identical crystallographic orientation have been synthesized [33]. Through controlling the growth solution concentration, the growth reaction rate and the growth direction can be tuned. Therefore, the polarization of ZnO nanostructures can be controlled through the preparation condition.

3.4 1D TIO2 NANOSTRUCTURE 3.4.1 Crystal Structure of TiO2 TiO2 naturally exhibits four different types of polymorphs, that is, anatase, rutile, brookite, and TiO2(B), as shown in Fig. 3.10. In all polymorphs, titanium cations are sixfold coordinated to oxygen anions forming a distorted TiO6 octahedra unit, but joining in different ways by sharing the octahedral edges and corners [54
56]. Generally, anatase is the most active phase and therefore has the highest catalytic activity and is always found in solution-based systems. Rutile is a more thermodynamically stable phase of bulk titania than the other phases that transform to rutile under pressure or high temperature. Typically, rutile has the lowest bulk

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Figure 3.10 Crystal structures of TiO2 polymorphs: (A) rutile; (B) anatase; (C) brookite; and (D) TiO2(B). Purple spheres represent Ti atoms, and the blue octahedra represent TiO6 blocks. Oxygen atoms at the corner of the octahedra are omitted for clarity. Reprinted with permission from Y. Zhang, et al. Titanate and titania nanostructured materials for environmental and energy applications: a review. RSC Adv. 5 (2015) 79479
79510. Copyright 2015, Royal Society of Chemistry.

Gibbs free energy and the lowest molecular volume. Brookite and TiO2(B) were less common and also obtained from a solution-based system. As can be seen in Table 3.2, these four phases [55,57
60] have different symmetry, morphology, and growth behaviors.

3.4.2 1D TiO2 Nanostructure Synthesis Techniques From Chemical Solution Both bottom-up and top-down techniques can be used to synthesize 1D TiO2 nanostructures. Bottom-up techniques cover a large diversity of solution- and vapor-based growth methods. Hydrothermal/solvothermal, sol
gel, surfactant-assisted, microwave-assisted, sonochemical, electrospinning, and high-temperature pyrolysis methods are essential approaches for solution-based bottom-up techniques. Vapor-based bottom-up techniques are conducted through chemical/physical vapor deposition, atomic layer deposition, and pulsed layer deposition. On the other hand, direct oxidation, electrochemical etching, and photo-electrochemical etching are regarded as top-down techniques. Herein, we will represent only solution-based bottom-up methods. 3.4.2.1 Hydrothermal/Solvothermal Method Commonly, a desired high temperature is adjusted to heat a solution of metal ions and an oxidizing agent in an autoclave with or without pressure and in the presence or absence of a template. Therefore, the

Table 3.2 Space group and crystal structure for four different types of TiO2 polymorphs Phases Anatase Rutile Brookite

Space group

Tetragonal I41/amd (141) a 5 0.459 nm and c 5 0.296 nm

Tetragonal P42/mnm (136) a 5 0.379 nm and c 5 0.951 nm

Crystal structure

Four edges are shared per octahedron, but there is no corner sharing

Two opposite edges of octahedron are shared in order to form a linear chain along the [001] direction and chains are joined through corner connections

Orthorhombic Pbca (61) large unit cell consisting of eight TiO2 groups Octahedrons share three edges as well as corners

TiO2(B)

Monoclinic C2/m (14) a 5 1.216 nm large unit cell with a more open crystal structure Two TiO6 octahedra, with adjacent sheets joined together by edge-sharing

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precursor solubility is increased by the formed supercritical fluid and the nanostructures are precipitated. Hierarchical rutile TiO2 mesocrystals (RTMs), of a sword-like end branched structures with exposed {101} facets, were synthesized on a large scale by a one-step hydrothermal method under a weak acid assistant (oxalic acid). Additionally, another {101} twinned plane is formed at the interface of the “trunk” and “branch” at 65 degrees, as shown in Fig. 3.11A. Interestingly, it has been found that homogeneous nanocrystals with diameters below 14 nm are epitaxially attached on the surface of the {110} planes with a lower surface energy at an angle of about either 113 or 67 degrees, as in Fig. 3.11B and C. Accordingly, these rutile TiO2 crystal planes with the twinned {101} and {200} planes sharing the same zone axis [010] were modeled as building blocks of RTMS (Fig. 3.11D). By performing a series of time-dependent experiments (Fig. 3.11), the growth mechanism of 3D RTMs is ascribed to the nanocrystal-oriented attachment and the formation of rutile {101} twinned structures. As a result of these experiments, (1) after 4 h in the reaction solution, nanoparticles are aggregated from some rutile nanocrystal nuclei (Fig. 3.11A). (2) By duplication of the system time, after 8 h, oriented attachment mechanism became predominant for the nanorods into central stems and hence some nanoparticles were well-organized (Fig. 3.11B). (3) After 12 h, the RTMs were gradually formed (Fig. 3.11C; 12 h). Due to the spatial freedom, nanocrystals underwent oriented attachment on the nanorods with sword-like ends, along the twinned {110} plane surface and twinned {200} side surfaces so as to minimize their surface energy and the unique growth processes. (4) Finally, after 14 h, the 3D hierarchical RTMs with more branch-like nanorods were obtained (Fig. 3.11D) [61]. Li et al. [60] uncovered the growth mechanism of the branching of TiO2 nanowires by performing a series of time-dependent (for 6 h) and titanium precursor concentration-dependent experiments. Based on SEM analysis, increasing the titanium precursor concentration increases the rutile branching density with a decreased distance between branches. At a higher titanium precursor concentration, more nuclei consisting of anatase particles were produced that correspondingly increase the branching. Also, based on HRTEM analysis, by increasing the reaction progress, anatase TiO2 nanoparticles were solid-state transformed to growing rutile nanowires (by coinciding with nanowire tips via particle-oriented attachment) with branches that are thicker in the center and became narrower toward both ends. This means the main nanowire formation and branching occurred simultaneously.

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Figure 3.11 (A) Typical TEM image of an individual RTM. (B) Typical HRTEM image of an individual nanorod tip taken from a nanorod viewed along the [010] direction. (C) TEM image of the same nanorod tip viewed along the [111] direction. (D) Crystal lattice model for the rutile TiO2 mesocrystal with the twinned {101} and {200} planes sharing the same zone axis [010]: red circle, O2, blue circle, Ti41. To clarify the relationship between the twinned {101} and {200} planes, other parts of the rutile TiO2 mesocrystal structure are not shown. (E
H) The formation process of the RTMs at different reaction times. Reprinted with permission from H. Wang, et al. Hierarchical rutile TiO2 mesocrystals assembled by nanocrystals-oriented attachment mechanism. Cryst. Eng. Comm. 14 (2012) 2278
2282. Copyright 2012, Royal Society of Chemistry.

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To explain the formation of branched nanowires with higher concentrations, a preferential binding energy was explained. Since the anatase has the lowest binding of ð103Þ facet and also its attraction strengths
energy  with both rutile 101 and (101) facets are in the order (100) , (103) , (221) , ð103Þ, the anatase deforms its higher energy surface (103)
crystal  to bind with the rutile 101 facet confirming phase transformation to rutile by ion-by-ion attachment [60] (Fig. 3.12). By particle-oriented attachment and an ion-by-ion attachment mechanism, self-assembled harvest nanoflowers of Co-doped ZnO DMS thin films have been recently grown by a hydrothermal method, as shown in Fig. 3.13. 3.4.2.2 Sol
Gel Method The sol is a colloidal suspension of the stirred mixture of metal organic species or inorganic metal salts and water. The gel is a three-dimensional oxidic network obtained through hydrolysis, condensation, and sometimes aggregation reaction of the sol. The gel has a continuous solid matrix surrounded by a continuous liquid phase. In aqueous systems, the oxygen for the formation of the oxidic compound is supplied by water molecules, while in the case of nonaqueous systems, the source of oxygen is the solvent of the organic constituent of the precursor [34]. 3.4.2.3 Surfactant-Assisted Method The use of a surface-selective surfactant is the key parameter for the shape control that selectively binds to specific crystalline facets based on different surface energy-producing anisotropic shapes. The crystal growth rate is exponentially proportional to the crystal surface energy that is reduced or increased during surface adhesion. By the surface-assisted elimination of a high-energy facet, the progressive addition of a selective surface-active agent should yield a variety of shapes [62]. 3.4.2.4 Microwave-Assisted Method Microwave flash heating can dramatically reduce the reaction time for synthesizing organic and inorganic materials by kinetic control. The microwave-assisted method is widely used to synthesis 1D TiO2 nanostructures. Based on the simulation of ionic motion and molecular dipolar polarization of microwave irradiation, rapid precursor dissolution and accelerated reaction kinetics are induced, resulting in high fluxes of nuclei

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Figure 3.12 Illustration of the preferred attachment of anatase crystals to rutile rods depicting the mechanism of transformation from anatase to rutile, nanowire growth along the [001] direction, and the branch (101) twin formation. Reprinted with permission from D. Li, et al. Growth mechanism of highly branched titanium dioxide nanowires via oriented attachment. Cryst. Growth Des. 13 (2013) 422
428. Copyright 2013, ACS Publications.

with a large concentration gradient. In addition, the microwave-induced dipole moment in primary nuclei building blocks facilitates interparticle collision and anisotropic attachment along the polar direction [59].

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Figure 3.13 Self-assembled harvest nanoflowers of Co-doped ZnO DMS thin films.

Integration of microwave with hydrothermal methods to synthesize mesoporous anatase TiO2 nanorods is described by Jia et al. [63]. Typically, acidolysis of TiCl4 forms the TiO2 nanocrystal building blocks that, by increasing temperature, aggregate into large particles to lower the overall system energy. Then, the TiO2 nanorod grows along the [001] direction forming long necklace-shaped nanorods [64]. In fact, microwave heating rapidly raised the solution system thermal energy, resulting in a homogeneous nucleation and certainly accelerated growth rates that might introduce more difficulties in morphology manipulation [65,66]. 3.4.2.5 Sonochemical Method Sonocation, a high-energy technique in condensed phases at room temperature [67], has been used as a green- technique for synthesizing 1D nanostructures [66]. The idea is that when sound passes through a liquid, acoustic cavitation (the formation of gas bubbles in a liquid) occurs by expansion (negative pressure) waves and compression (positive pressure) waves. This significant pressure drop generates gas bubbles that grow and recompress. These expansions and recompressions among gas bubbles can generate high-energy shock waves of approximately 5200 K with hundreds of atmospheres and more than 1010 K/s heating and cooling rates. Mesoporous TiO2 nanorods were successfully developed using a template-free sonochemical method from bulk Ti powder [68] in mild conditions (atmospheric pressure and relatively low temperature). Furthermore, the crystal growth mechanism was proposed as an

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Figure 3.14 Schematic diagrams for the oxidation-shaking off-rolling up-shrinkage (OSRS) formation mechanism of UA-TiO2. Reprinted with permission from S. Guo, Z. Wu, H. Wang, F. Dong. Synthesis of mesoporous TiO2 nanorods via a mild template-free sonochemical route and their photocatalytic performances. Catal. Commun. 10 (2009) 1766
1770. Copyright 2009, Elsevier.

oxidation-shaking off-rolling up-shrinkage mechanism [68] (Fig. 3.14). Briefly, NaOH oxidizes the bulk Ti powder layer by layer and then is recrystallized to sodium titnate according to the following equation: Ti 1 NaOH 1 H2 O-Na2 Tix O2x11 1 H2 Next, by ultrasonic waves the oxidized layers are shaken off and split into nanosheets of different shapes and sizes. After dipping in acid, these nanosheets are rolled up to mae nanotubes by replacing Na ions with H ions according to the following equation. Later, after the annealing process, H2Ti3O7 nanotubes shrink to obtain TiO2 nanorods. Na2 Tix O2x11 1 HNO3 1 H2 O-Na2 Tix O2x11 1 NaNO3

3.4.2.6 Electrospinning Method Electrospinning is another simple chemical solution-based process for synthesis of 1D nanostructures, specifically nanofibers with a very high surface area-to-volume ratio. This technique mainly depends on using a highvoltage power supply to eject the melt precursor through a syringe with a metal needle (electrode). This results in a jet (Tylor cone) of the melt droplets toward the collector (counter electrode), forming solid nanofibers on the collection plate [11]. As can be seen in Fig. 3.15, Kim and his co-workers [69] reported the directly electrospinning of TiO2 nanofiber mats (diameter 200
500 nm)

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Figure 3.15 Schematic diagram of the processing steps used to fabricate TiO2 nanofiber mats on Al2O3 substrates with interdigitated Pt electrode arrays. Inset: Scanning confocal laser micrograph of a calcined TiO2 nanofiber mat on top of the Al2O3 substrate (dark region) and Pt electrode (bright region). Reprinted with permission from I.D. Kim, et al. Ultrasensitive chemiresistors based on electrospun TiO2 nanofibers. Nano Lett. 6 (2006) 2009
2013. Copyright 2006, American Chemical Society.

at 15 kV onto interdigitated Pt electrode arrays, hot pressed at 120°C and calcined at 450°C. 3.4.2.7 High-Temperature Pyrolysis Method Spray pyrolysis is another simple, reproducible, and inexpensive chemical solution-based process for synthesis of 1D nanostructures specifically for uniform large-area applications [17]. High-temperature pyrolysis usually combines a hydrothermal process followed by high-temperature sintering. This technique is not often used for 1D TiO2 nanostructure fabrication. However, several successfully attempts have been demonstrated to produce 1D TiO2 using modified pyrolysis approaches. TiO2 nanowires were synthesized by a molten salt-assisted pyrolysis process [70] from TiCl4ethyl acetate and Na2S-ethyl acetate mixture. Anatase TiO2 nanowires were obtained at a relatively low calcination temperature (820°C), with rutile TiO2 nanowires at a higher temperature (970°C).

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3.4.3 Growth Mechanism of 1D TiO2 Nanostructures From Chemical Solution This section discusses the fundamental crystal nucleation and growth mechanism of 1D TiO2 nanostructures. Since TiO2 crystals have lower anisotropic property, the crystals typically necessitate further kinetic approaches to evolve the nanocrystals in one growth direction, achieving 1D nanostructure. Based on the well-documented previous reviews, these approaches involve (1) using catalyst, (2) introducing dislocations along the propagation direction, (3) surface functionalization among the different crystal facets, and (4) increasing the building block concentrations along the different crystal facets [71]. Our discussion will highlight only the growth mechanism of 1D nanostructures from chemical solutions. Hence, in the solution system, the surfactant-controlled growth and the oriented attachment (surfactant-free) and dissolve and grow mechanism are reported. 3.4.3.1 Surfactant-Controlled Growth By using selective surfactant coverage, nanocrystal anisotropic growth can be achieved and the 1D morphology of the TiO2 nanostructure can be manipulated. The change in surfactant chemical properties can control the hydrolysis rate and tune the different shapes and sizes of TiO2 nanostructures [56]. For example, Cozzoli et al. [72] reported simple wet-chemical synthesis using different surfactants in the hydrolysis of titanium tetraisopropoxide to grow anatase TiO2 squares, rods, and rounded rhombicshaped nanoparticles. Mainly, TiO2 truncated octahedrons enclosed by {101} and {001} facets were obtained by {010} growing. They showed the dependence of controllable growth of TiO2 nanorods on the anisotropic reactivity of the TiO2 precursor, the surfactant nature that might suppress the directional crystal growth by an absorbing-chelating agent and proper control of the hydrolysis process [59,72]. 3.4.3.2 Oriented Attachment (Surfactant-Free) Generally, Ostwald ripening was proposed to be a major mechanism for the growth of large crystals from small crystals or sol particles in TiO2 nutrient solution. Oriented attachment growth is the organization of TiO2 nanocrystals based on the surface energy reduction and registration of these nanocrystals into a single crystalline 1D nanostructure. To explain in detail the oriented attachment, a simple molecular dimer formation model was proposed by Penn and Soltis [73], where the primary crystals were treated as molecules that form dimers (oriented aggregation of two

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primary TiO2 crystals). As this model was based only on two oriented primary particles, a stepwise polymerization kinetic model was suggested by Ribeiro et al. [74], where primary particles (monomer) form a multimer (oriented aggregate). By applying this model, the kinetics of the oriented attachment growth were found to be directly dependent on the solution viscosity and temperature [75]. 3.4.3.3 Dissolve and Grow Mechanism Guo et al.’s [76] work grew rectangular bunched TiO2 nanorod arrays on carbon fibers from titanium by a “dissolve and grow” method. At the very beginning of the hydrothermal process, in the presence of HCl, Ti foil reacts with H1 at high temperature and pressure and gradually dissolves, continually releasing the unstable Ti(III) precursors into the reaction solution according to the following equations: 2Ti 1 6HCl-2TiCl3 1 3H2ðgÞ Ti31 1 H2 O-TiOH21 1 H1 2 TiOH21 1 O2 2 -TiðIVÞ 2 oxo species 1 O2 -TiO2

After dissolving the Ti precursor, the Ti(IV) complex ions act as the growth for rutile TiO2 nanorods, where a Ti atom bonds to six oxygen atoms, forming a TiO6 octahedron. This octahedron shares a pair of opposite edges with the next octahedron, forming a chain-like structure along the [001] direction [76].

3.4.4 Recent Synthesis Trends and Challenges in Nanotechnology Based on 1D Nanostructures Based on morphology-dependent device performances, numerous efforts have been devoted to the synthesis of nanostructures with 1D morphologies, such as nanowires, nanobelts, nanorings, nanohelices, etc. [77
79]. It has been suggested that the technological limitations of the top-down processes, used for fabricating integrated circuits, can be overcome by using bottom-up processes instead. Hence, the synthesis of 1D nanostructures by controlling the crystal growth rate is essential for improving the physical and chemical properties of 1D nanostructures [80]. Despite great success that has been made on the controllable synthesis of 1D nanostructures, several challenges still remain. There is still room for improvement

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in terms of quality and scale of the nanostructure products. Moreover, to understand the formation mechanism of the nanostructures with the ability of control over geometry and chemistry we also need to further optimize the chemical and physical properties. Study of the device performance and morphological evolution of 1D nanostructures during the practical applications is of significance to develop strategies to improve the stability.

3.5 CONCLUSION One-dimensional nanostructures are being increasingly used in applications for a host of devices such as solar cells, sensors, detectors, energy generators, as well as artificial structures for tissue engineering. The unique properties and versatility of 1D nanostructures pave the way to use various methods to synthesize 1D nanostructures. One-dimensional nanostructure morphology appears to be a unique physical and structural characteristic of this family of semiconducting oxides with materials of distinct crystallographic structures. Each nanostructure has specific structural, optical, electrical, and physicochemical properties, permitting remarkable applications that have been fabricated based on individual morphology. Hydrothermal synthesis of 1D nanostructures is the mostly popular simple and efficient way to synthesize 1D nanostructures. Various precursors and additives are used in aqueous medium to successfully synthesize nanostructures of different morphologies, with a mixture of zinc nitrate and hexamine being the most popular.

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CHAPTER 4

One- and Two-Dimensional Nanostructures Prepared by Combustion Synthesis A.S. Mukasyan1 and K.V. Manukyan2 1

Department of Chemical & Biomolecular Engineering, University of Notre, Notre Dame, IN, United States 2 Nuclear Science Laboratory, Department of Physics, University of Notre, Notre Dame, IN, United States

Contents 4.1 Introduction 4.2 CS Fundamentals 4.2.1 Thermodynamics 4.2.2 Kinetics 4.3 Microstructural Characteristics of Combustion-Derived Nanomaterials 4.3.1 Solid
Gas and Solid
Solid CS Systems 4.3.2 Solution Combustion Synthesis 4.4 Conclusions References

85 88 89 92 95 96 105 112 114

4.1 INTRODUCTION Combustion synthesis (CS) is a specific approach for fabrication of a variety of materials [1
6]. To understand its fundamentals, one may want to consider the following definition of combustion: combustion is a complex phenomenon with the self-sustained propagation of chemical reactions accompanied by a rapid release of heat. Based on this definition, the main feature of the combustion process is an involvement of the self-sustained exothermic chemical reactions. Taking into account the above features, we may define CS as a method for production of valuable solid products through self-sustained noncatalytic chemical reactions. There are different classifications of CS. One of them accounts for the combustion nature of the process. There are two fundamentally different types of combustion processes, that is, deflagration and detonation. Deflagration (de 1 flagrare, “to burn down”) is a subsonic reaction Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00004-3

© 2019 Elsevier Inc. All rights reserved.

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propagating along the media. Different mechanisms of heat conduction and mass transfer from the “hot” burning part of the media are responsible for initiation of the reactions in the subsequent layers of the “cold” reactants. In turn, detonation is a supersonic wave, which propagates through shock compression of the explosive. The shockwave heats the explosive, ignites the chemical reactions with the release of considerable energy, which pushes the shock wave into next layer of the explosive. While a majority of CS-based approaches are based on deflagration combustion, some publications report the detonation-like conditions for materials synthesis [7
9]. In this chapter, we consider only deflagration type of CS. The deflagration CS can be accomplished in two different modes. First is a selfpropagating high-temperature synthesis (SHS) mode. In this case, the reactive media is locally (B1 mm3) preheated by an external source to the ignition temperature, at which point reaction is initiated in this layer. Next, the “hot” reacted layer preheats and ignites the next “cold” layer and thus combustion front self-propagates along the reactive mixture resulting in the formation of the desired solid product. Second is the volume (or thermal explosion) combustion synthesis mode. In this case, the entire reactive media are uniformly heated by some external source to ignition temperature, and reaction starts at each point of the media essentially uniformly, again leading to the production of valuable materials. Note that, for both cases, the characteristic reaction time is about milliseconds and the duration of the cooling stage primarily defines the duration of the synthesis process. The latter is one of the main parameters responsible for the microstructure and thus the properties of the produced materials. The maximum synthesis temperature is limited by the thermodynamics of the considered systems, and is in the range 1500K
4000K. It is important that after “ignition” no external heat sources are required. Hence, CS is an energy-efficient method. The rate of temperature change at the self-ignition stage is very high (103
106 K/s), which defines unusual extremely nonequilibrium conditions for material fabrication. Another classification is based on the state of the initial precursors, that is, gas phase, gasless systems, solid
gas, solution CS. In this chapter, we discuss the last three types of CS. The gasless CS involves self-sustained reactions between elements, that is, initially solid reagent metals (such as Al, Ti, Zr, Ni, Hf) and nonmetals (C, B, Si). Among them, one may highlight, so-called, solid flame systems. For gasless systems, the maximum

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combustion temperature is below the melting points of any intermediate and final products. The classical example is a reaction between tantalum and carbon to synthesize refractory carbide: Ta 1 C 5 TaC 1 145 kJ=mol21

(4.1)

The maximum (adiabatic) temperature for reaction (4.1) is relatively high (2743K). However, the lowest possible eutectic in this system exists at 3110K (Ta melts at 3293K; C sublimes at 3915K; TaC melts at 4153K). Thus, even without heat losses, the combustion temperature is B400K below any possible phase transformation. Also, the reduction type CS is considered, where metal (TiO2, Nb2O3, etc.) or nonmetal (B2O3, SiO2) oxides react with a reducing metal (Al, Mg, etc.). An example is shown in reaction (4.2) between silica and magnesium to produce pure silicon: SiO2 1 Mg 5 Si 1 MgO 1 310 kJ=mol21

(4.2)

The solid
gas CS involves reactions between any solid metal (Al, Ti, Nb, etc.) or nonmetal (Si, B) with gases (nitrogen, oxygen, hydrogen, and others) leading to the production of a variety of nitrides, oxides, and hydrates. An example of the solid
gas process is a synthesis of hexagonal boron nitride through direct interaction of boron and nitrogen through reaction (4.3) at high gas pressure: 2B 1 N2 5 2BN 1 250 kJ=mol21

(4.3)

It is worth noting that the CS system may comprise both gasless and solid
gas reactions leading to the synthesis of complex compounds and composite materials. Solution combustion synthesis (SCS) involves self-sustained chemical reactions in homogeneous aqueous solutions of different oxidizers (e.g., metal nitrates) and fuels (e.g., urea, glycine, hydrazides) [10]. An example, using glycine as fuel, is as follows:
 Mev ðNO3 Þv  mH2 O15=9ϕv NH5 C2 O2 1 5=4 v½ϕ21O2
-  -MeOv=2 1ð25=18  ϕv 1mÞH2 O1vð5ϕ=1811=2ÞN21 10=9  vϕCO2 (4.4) In reaction (4.4) ϕ 5 1 means that the initial mixture does not require external (atmospheric) oxygen for complete oxidation of fuel, while ϕ. 1 (, 1) implies fuel-rich (lean) conditions.

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Several features distinguish the SCS possesses from all other varieties of CS discussed above. First, the initial reactants for SCS are mixed in an aqueous solution at the molecular level, while in SHS powdered reactants are typically mixed on the micrometer level. Indeed, the sizes of ions and ligands in the solution are commonly in the range 0.1
1 nm, while the sizes of solid powder particles are B102
105 nm. Second, the reaction that is responsible for SCS-solid product formation can be different from those that control the self-sustained combustion process. In the SCS processes, the most substantial part of the heat evolves due to burning (oxidation) of components of organic fuel (e.g., carbon and hydrogen), while the target products are metal oxides or metals. Third, the SCS processes generate a significant amount of gaseous byproducts. Such gasification leads to a significant expansion of the solid product, and a rapid decrease of temperature after the reaction, which makes the solid product porous and finely dispersed. These features play a critical role for SCS of nano-powders. Thousands of compounds in the form of powders, bulk materials, and net-shape articles have been produced by the CS method [5,6]. However, two significant questions are typically under debate: (1) can one control the CS process, which is so rapid and “nonequilibrium”?; (2) can one produce nanomaterial by using such a high-temperature approach? We attempt to address the first question in Section 4.2 of this chapter, while the second question, along with various examples of the CS nanomaterials, is addressed in Section 4.3. It is shown that being based on self-sustained reactions, which are defined by a variety of tightly “crossedbounded” parameters (one cannot change one of them without affecting the others), CS can be precisely controlled based on the knowledge (thermodynamics, kinetics, structure formation mechanism) of the combustion process [11
13]. It is also important that many structure formation processes taking place in the CS wave appear to be similar to those observed in conventional conditions used in the powder metallurgy, which also help to understand and factors that control the CS [5,14]. It is also demonstrated that based on the fundamental understanding of the CS process one may suggest different routes to control the structure of the synthesized products, which leads to a variety of approaches for fabrication of nanomaterials.

4.2 CS FUNDAMENTALS The strength of the CS method is that it was established based on the fundamental theoretical and experimental studies in the field of the

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heterogeneous combustion [11
13]. This knowledge allows one to understand the processes of heat and mass transfer, which occur in the combustion wave, define main controlling parameters and thus synthesize materials with desired microstructure and properties. Several aspects should be accounted for controlling the CS processes. One is related to the thermodynamics of the process. This issue we briefly discuss in Section 4.2.1. The other important factor is the kinetics of hightemperature reactions that we overview in Section 4.2.2.

4.2.1 Thermodynamics At room temperature, the mixture of the reactive precursors can exist for an indefinitely long time, without noticeable changes in composition, temperature, or pressure. However, this initial stationary state of the system is not stable. After the reaction initiation, the CS proceeds in a selfsustained manner, without any external energy heat sources. The driving force of the process is the reduction of the internal energy of the system by conversion of its chemical potential into heat and formation of new compounds, which leads to a new stable stationary state. In general, thermodynamics allows us to calculate the maximum temperature of a reaction, as well as the composition of equilibrium products formed. Such calculations can be performed assuming that there is no exchange of heat between the considered system and the environment, so-called adiabatic conditions. Correspondingly, the maximum reaction temperature is called the adiabatic combustion temperature (Tad). It is also assumed that the amount of matter in the system remains constant during CS, that is, the system is isolated. Since any exchange with the environment is excluded, the total energy of the system remains unchanged. At the same time, the reaction has been completed, and the system does not spontaneously return to its original state thus we have irreversible reactions. By definition, the stable equilibrium state of the system is one in which the thermodynamic potential is minimum. Thus the thermodynamics of irreversible reactions are based on the minimization of the thermodynamic potential (F), which for a system with N(g) gas and N(c) condensed solid number of components, for example, at constant pressure (P), can be expressed as follows:   X N ðgÞ N ðcÞ X Pk nk ln 1 Gk 1 nGi (4.5) F ðfnk gfni gÞ 5 P k51 l51

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where Pk is the partial pressure of the “k” gas-phase component, ni and Gi are the numbers of moles and molar Gibbs free energy of the i-th component. At present, many software packages exist for the calculation of the simultaneous calculation of the equilibrium combustion products and adiabatic combustion temperature (see Ref. [5] for an overview). Developed specifically for CS processes, the program THERMO is noteworthy [15,16]. To determine the equilibrium state, this program searches for a minimum value of the thermodynamic potentials (Eq. (4.5)) for the reacting system accounting for the contributions of all compounds. The calculation of the equilibrium combustion products can be carried out in two regimes: constant pressure (P 5 const) or constant volume (V 5 const). In the former case, the program searches for the minimum of Gibbs free energy, while in the latter case, for the minimum of the Helmholtz free energy. From a materials science standpoint, thermodynamic calculations are a powerful tool to control the composition and microstructure of the synthesized materials. Indeed, the experiments showed that in the majority of cases the phase compositions of the combustion products fit well with those predicted by the thermodynamics. This means that preliminary thermodynamic calculation allows for optimization of the composition of the initial reactive mixture, as well as synthesis parameters (such as initial temperature, gas pressure), which lead to the production of a material with the desired phase composition. Equally important is that for many systems such calculation allow to formulate conditions for synthesis of the materials of the same phase composition but different maximum temperatures and lower or higher amounts of gas phase products. The latter permits to control the microstructure of the materials. Indeed, typically the low synthesis temperature and high amount of gas phase products during CS lead to smaller particle size and higher specific surface area of the synthesized powder, which favor many applications. Let us consider an example, which illustrates the above statements. To find out the optimum system to synthesize graphene by the combustion method several systems have been investigated [17,18]. The adiabatic combustion temperatures and equilibrium product compositions were calculated as functions of the initial mixture composition and inert gas (argon) pressure for reactions between polytetrafluoroethylene (PTFE) and refractory material carbides (TaC, ZrC, TiC, SiC), nitrides (TaN, ZrN, TiN), and (TaB2, ZrB2, TiB2) using the THERMO software. It was

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shown that all systems have adiabatic combustion temperatures in the range 1900K
3500K (Fig. 4.1A), which suggests that self-sustained combustion reactions can be accomplished for all compositions [2]. It is essential that for optimized initial reactive mixtures elemental carbon is the only solid-state product that can be formed in the combustion wave. The theoretical yields of carbon (the ratio of the amount of carbon to the amount of initial reactive mixture) for different systems are presented in Fig. 4.1B. It can be seen that the highest yield shows SiC 1 PTFE. Moreover, from the standpoint of CS of graphene the SiC 1 PTFE system has more potential, as SiC was already proven to form “native” surface graphene layers upon vacuum annealing [19]. Therefore, the SiC 1 PTFE system was selected for further detailed investigations.

Figure 4.1 Calculated adiabatic temperatures (A) and yields of carbon (B) for reactions of different ceramic compounds with PTFE. Reprinted with permission from K.V. Manukyan, S. Rouvimov, E.E. Wolf, A.S. Mukasyan. Combustion synthesis of graphene materials. Carbon 2013;62:302
331.

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Figure 4.2 Adiabatic temperature (A), equilibrium composition of solid (B) and gas (C) products for SiC 1 PTFE system depending on PTFE quantity and inert gas pressure. Reprinted with permission from K.V. Manukyan, S. Rouvimov, E.E. Wolf, A.S. Mukasyan. Combustion synthesis of graphene materials. Carbon 2013;62:302
331.

To optimize combustion conditions in the SiC 1 PTFE system, further thermodynamic studies were performed. It was shown (Fig. 4.2) that Tad continuously increases up to B3800K with increases in both the amount of PTFE and argon gas pressure in the reactor (Fig. 4.2A). The quantity of SiC in the product decreases gradually with the increase of PTFE content. Meanwhile, the carbon quantity rises, and it becomes the only solid product at B65 wt.% of PTFE content in the initial mixture (Fig. 4.2B). The composition of the gas product depends on both calculated parameters. For example, tetrafluorosilane (SiF4) is the primary gas phase product at the optimal PTFE content (Fig. 4.2C), under which the fabrication of oxygen-free graphene has been performed [17]. This example illustrates that thermodynamic analysis, using advanced software packages, is a powerful tool to control the CS conditions and allows effective optimization of the synthesis parameters.

4.2.2 Kinetics To tailor the final product microstructure and composition, two features that strongly depend on the characteristic reaction time and temperature, it is imperative to control the reaction conditions that occur in the CS wave. Thus, intimate knowledge of the kinetics for the specific chemical reactions that occur is vital to fabricate materials with desired properties [5,20,21]. The rate of the chemical reaction in the condensed state is generally a function of the temperature (T) and degree of conversion (η): dη 5 FðT ; ηÞ dt

(4.6)

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The single-step approximation employs the assumption that the function in Eq. (4.6) can be expressed as a product of two separable functions that are independent of each other; the first one, K(T), depends solely on the temperature, and the other, Φ(η), depends solely on the degree of conversion, η. In this case, Eq. (4.6) can be rewritten as follows: dη 5 KðT ÞΦðηÞ dt

(4.7)

The temperature function is typically expressed by the Arrhenius equation: K ðT Þ 5 Aexpð2 Ea =RT Þ

(4.8)

where A and Ea are considered to be the preexponential factor and the activation energy, respectively, T is the absolute temperature, and R is the gas constant. The presence of condensed phases makes it difficult to transport and mix the reactants and products. Therefore, in general, the kinetics of such reactions are determined both by the intrinsic rate of the chemical reaction and by the mass transport (e.g., diffusion) and thus can be defined as activation energy [12,21]. It is important to note that for materials science applications it is sufficient to know effective (apparent) kinetics for precise prediction of the temperature
time schedule of the synthesis process. Typically, the activation parameters are obtained from a set of kinetic runs from the dependences of time versus temperature (for isothermal measurements), temperature versus heating rate (for integral and incremental methods with linear heating rates), or from reaction rate versus temperature. A variety of experimental techniques has been developed to accurately determine the kinetics of CS reactions accounting for the extremely high temperatures of the processes ( . 1800K) and rapid heating rates (103
105 K/s). While standard nonisothermal TGA/DTA-based approaches [22] are still used to evaluate the kinetics of such reactions, several unique methods such as electrothermal explosion (ETE) [23] and electrothermography (ET) [24] were specifically designed to fit the experimental conditions of CS reactions. Moreover, recently a variety of advanced in situ diagnostics, including timeresolved X-ray diffraction (TRXRD) [25], high-speed X-ray phasecontrast imaging [26], and high-speed transmission electron microscopy (HSTEM) [27] were modified to obtain the kinetics of phase transformations under unique CS.

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(A)

(B) 150 Activation energy (kJ/mol)

Activation energy (kJ/mol)

150

120

90

120

90

60

60 0

10

20 30 40 Milling time (min)

0

0.005 0.01 0.015 Surface area/volume (nm–1)

Figure 4.3 Dependence of effective activation energy of the reaction as a function of milling time (A) and specific contact surface area (B) between Ni and Al phases. Reprinted with permission from C.E. Chuck, A.S. Mukasyan. Reactive Ni/Al nanocomposites: structural characteristics and activation energy. J. Phys. Chem. A 2017;121 (6):1175
1181.

For example, the ETE method was used to define the influence of mechanical activation on the effective activation energy in an Ni-Al system used to synthesize nanostructured intermetallics [28]. It was shown that increasing the milling time leads to an increase in the contact surface area between precursors, which results in a significant decrease in the apparent activation energy (Fig. 4.3). Another example is related to SCS in a nickel nitrate
glycine system, which was used to produce high surface area supported catalysts [29]. Applying the Kissinger
Akahira 2 Sunose (KAS) method (Fig. 4.4) it was shown that the apparent activation energy of this reaction is 175 6 kJ/mol, which is slightly higher than for the thermal decomposition of anhydrous nickel nitrate (153 kJ/mol). Thus, by using different approaches one may define the effective kinetics parameters of the reactions and predict the temperature
time schedule of the synthesis process. It is important to note that kinetics describes system behavior in the combustion wave up until the system reached complete conversion. The time required for complete reaction in CS systems ranges from seconds to several minutes. This stage is followed by the cooling stage, and the cooling rate is a critical parameter, which, in many cases, defines the microstructure of the synthesized materials. As experiments have shown, one could precisely change the cooling rate

One- and Two-Dimensional Nanostructures Prepared by Combustion Synthesis

(A)

1/Tmax(1/K )

(B) –4.0

440

1.64E-03

400 340ºC

360

1.66E-03 1.68E-03

1.70E-03 1.72E-03

–6.0

333ºC 326ºC

320

305ºC Tmax

280

Tign

240

In (β/T2max)

Catalyst bed temperature (ºC)

95

–8.0 –10.0

Ea = (176±5)kJ.mol–1

–12.0

200 160 120

1ºC/min

2.5ºC/min

5ºC/min

10ºC/min

80

–14.0 –16.0

0

20 40 60 80 100 120 140 160 180 200 220 Time (min)

Figure 4.4 Temperature 2 time profiles for a combustion of nickel nitrate 2 urea 2 alumina mixture at different heating rates (A) and Arrhenius plot according to the KAS method (B). Reprinted with permission from L.S. González-Cortés, F.E. Imbert. Fundamentals, properties and applications of solid catalysts prepared by solution combustion synthesis (SCS). Appl. Catal. A 2013;452:117 2 131.

within the range 10 K/min (low heat losses) to 103 K/s (quenching) [3]. The latter in turn allow us to control the material structure and thus properties.

4.3 MICROSTRUCTURAL CHARACTERISTICS OF COMBUSTION-DERIVED NANOMATERIALS By definition, a 1D structure is a structure having two dimensions (x,y) at the nanoscale and the last dimension (L) not at the nanoscale. Typically, nanowire, nanorod, and nanotube morphologies represent 1D nanostructures. A more diverse classification is also available [30], which includes mixing of 0, 1, and 2D structures. For example, a 1D0 structure is a molecular chain polymer; 1D1—bundles, ropes, cables, corals; 1D11—hetero chains, combs; 1D10—fullerene-containing fibers. In turn, by definition, a 2D structure is a structure having one dimension (t) at the nanoscale, and the other two dimensions (Lx, Ly) not nanoscale. Typically, nanofilms and nanocoatings represent 2D nanostructures. However, 2D0, 2D1, and 2D2 structures also exist, for example, fullerene films, fiber-film, MOS-structure. Recently several extensive reviews on the CS, including fabrication of 1D and 2D structures, have been published, which provides a detailed overview of the subject [10,31
33]. Here, we briefly summarize the capabilities of the combustion-based approaches, both heterogeneous solid
solid and solid
gas CS and SCS for fabrication of such materials.

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Figure 4.5 Schematic representation of a CS reactor.

4.3.1 Solid
Gas and Solid
Solid CS Systems These types of synthesis processes are typically conducted in stainless steel chemical reactor (Fig. 4.5), under gaseous atmosphere or vacuum. Two different kinds of reactors are used depending on the product synthesized. The first type can maintain pressures up to 15 MPa and is widely used for the production of powders in gasless and gas
solid systems. Carbides, borides, silicides, intermetallics, chalcogenides, phosphides, and nitrides are usually produced in this type of reactor. The second type, a high-pressure reactor (up to 500 MPa), is used for the production of nitride-based articles and materials since higher initial sample densities require elevated reactant gas pressures for full conversion. The green mixture of the desired composition in loose form or as the pressed compacts is loaded inside the vessel, which is then sealed and evacuated by a vacuum pump. After this, the reactor may be filled with inert or reactive gas (Ar, He, N2, H2, CO, etc.). The reaction is locally initiated using an igniter (typically hot tungsten wire) and propagates in the form of a combustion wave along the reactive media. 4.3.1.1 1D Nanostructures Formation of 1D structures during CS process is typically related to the vapor
liquid
solid (VLS) mechanism of growth taking place in the combustion wave. This mechanism may occur just through evaporation dissolution and condensation of precursors in “pure” reactive system, that is,

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without special addition of volatile compounds. Alternatively, specific readily volatile compounds, such as ammonium salts (NH4Cl, NH4F), metal azides, and organic polymers (such as PTFE) can be added to enhance gas phase mass transport during CS. Let us start with gas
solid CS, where the VLS mechanism occurs in a more obvious way. The silicon
nitrogen system is highly exothermic: 3SiðsÞ 1 2N2 ðgÞ 5 Si3 N4 ðsÞ 1 180 kcal=mol ðΔH 5 2 750 kJ=molÞ (4.9) CS of Si3N4 occurs under unique conditions. The maximum combustion temperature in the reaction front equals the dissociation temperature of silicon nitride under considered nitrogen pressure, for example, B2400K at P(N2) 5 12 MPa, which is also much above the melting point of silicon (1683K) [34]. It is worth noting that the characteristic reaction time in CS wave is on the order of seconds. It is essentially impossible to conduct a synthesis of Si3N4 according to reaction (4.9) at such high temperatures by a conventional long-term (hours) reaction sintering method owing to the dissociation limitations. Investigation of the morphology of as produced β-Si3N4 powder showed that it consists of long fibers with an aspect ratio well above 10 and the diameters of the crystals depend on the synthesis conditions [34]. A theoretical model was developed, which proved that under CS conditions such crystals could grow by the VLS mechanism in the time span of several seconds [35]. It was also shown that the addition of ammonium salts to the initial mixture allows the production of millimeter-long fibers of α-Si3N4 phase with a thickness less than 50 nm [36]. These pioneering works were followed by many reports on the synthesis of 1D silicon nitride with fiber, whisker, and rod type morphologies. For example, synthesis of nano (diameter less than 100 nm) single-crystal β-Si3N4 fibers was accomplished by adding a tungsten catalyst [37]. The typical microstructure is shown in Fig. 4.6A. The long ( . 100 μm) β 2 Si3N4 nanofibers with diameters in the range 60
400 nm were fabricated through a CS route with small amounts of additives, Al and NH4F [41]. Rod-like crystals of β-Si3N4 with aspect ratio B10 were fabricated by CS in the Si-N2 system with a small (less than 5 wt.%) addition of rare-earth oxides [42,43]. One-dimensional structures were also obtained during CS of SiAlONs [44,45]. For example, α-SiAlON and β-SiAlONs were synthesized through combustion in the Si-Al-Si3N4-AlN system in nitrogen at B 2 MPa gas pressure. The morphology of the obtained α-SiAlON is

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Figure 4.6 Different compounds with 1D nanostructures synthesized by the combustion method in solid
gas systems. Reprinted with permission from (A) M. Xia, C. Ge, H. Guo. Aligned single-crystalline β- Si3N4 whiskers prepared with SHS process. Adv. Eng. Mater. 2012;14 (3):166
169; (B) C.L. Yeh, F.S. Wu, Y.L. Chen. Effects of α and β-Si3N4 as precursors on combustion synthesis of (α 1 β)-SiAlON composites. J Alloys Compd. 2014;604:260
265; (C,D) G. Liu, K. Chenb, J. Lia. Growth mechanism of crystalline SiAlON microtubes prepared by combustion synthesis. Cryst Eng. Comm. 2012;14:5585
5588; (E) A.S. Mukasyan. Combustion synthesis of nitrides: mechanistic studies. Proceed Combust Inst 2005;30 (2):2529 2 2535; (F, Insert) Z. Shi, M. Radwan, S. Kirihara, Y. Miyamoto, Z. Jin. Morphology-controlled synthesis of quasi-aligned AlN nanowhiskers by combustion method: effect of NH4Cl additive. Ceram. Int. 2009;35 (7):2727
2733.

shown in Fig. 4.6B. It can be seen that it consists of elongated crystals and fine fibers with diameter B 200 nm. The rod-like microstructure of β-SiAlON fabricated using the CS method in the Si-Al-Si3N4-Al2O3 system under 6 MPa nitrogen pressure [38] is presented in Fig. 4.6C. In this work, it was also demonstrated that SiAlON microtubes could be produced by CS (Fig. 4.6D). Aluminum nitride (AlN) powders fabricated by CS often possess the 1D type morphology [39]. Fig. 4.6E shows long (more than 50 μm) AlN whiskers synthesized by combustion in a “pure” (no additive) Al-N2 system. It was shown that whiskers are formed in the so-called “leading” combustion zone. Thus by optimization, the postcombustion conditions, that is, quenching, may produce this type of morphology. Synthesis of AlN nanofibers with the addition of 5 wt.% of ammonia (NH3) to nitrogen gas was reported by Bradshaw and Spicer [46]. It was also reported that addition of 3 wt.% of MgCl2 to Al powders favors the formation of

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AlN whiskers in the combustion wave [46]. High aspect ratio (B2000) AlN nanofibers (diameter B50 nm) were prepared by adding 5 wt.% of PTFE to Al-nitrogen system [47]. AlN whiskers were also synthesized by so-called salt-assisted combustion in Al-AlN-MeCl (Me 5 K, Na, Mg) systems [48]. The whiskers 60
150 nm in diameter uniformly aligned along the [001] crystal direction were grown by CS in the Si-N2 system by adding 5 wt.% of NH4Cl [49]. The SEM image of AlN nanowires produced by adding NH4Cl to the initial Al-AlN reactive mixture is shown in Fig. 4.6F [40]. Recently it was demonstrated that low gas pressure during CS of AlN favors the formation of 1D morphology [50]. Boron nitride nanotubes were prepared [51,52] by using the precursor containing B, Mg, Fe, and O (B31Fe17(MgO)27) and prepared by selfpropagation high-temperature synthesis through the following reaction: 8B2 O3 1 27 Mg 1 Fe2 O3 1 15 FeB 5 B31 Fe17 ðMgOÞ27

(4.10)

The compound obtained through reaction (4.10) was annealed in the NH3/H2 atmosphere at a temperature in the range 1300K
1600K. The four types of BN nanotubes, that is, cylindrical, wave-like, bamboo-like, and bubble chain were fabricated (see Fig. 4.7). Another approach for SHS synthesis of BN nanotubes was demonstrated by Qiongli et al. [53] and involved the combination of high-energy ball milling and a self-sustained reaction in an FeB1.3
NH4Cl
Mg
FeO3

Figure 4.7 Typical SEM and TEM images of the BN nanotubes with different morphologies. Reprinted with permission from W. Jilin, Z. Laiping, Z. Guowei, G. Yunle, Z. Zhanhui, Z. Fang, et al. Selective synthesis of boron nitride nanotubes by selfpropagation high-temperature synthesis and annealing process. J. Solid State Chem. 2011;184(9):2478
2484 [52].

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system. This approach allows fabrication of Fe-filled BN nanotubes with an outer diameter of 20
150 nm and a wall thickness of about 20 nm and a length of more than 5 μm. Among solid
solid systems, only silicon carbide and carbonaceous types of materials produced by CS method have a 1D type of morphology [31,54]. First, we have to outline that the silicon
carbon reaction has a moderate enthalpy of formation: Si 1 C 5 SiC 1 73 kJ=mol

(4.11)

Thus has a relatively low adiabatic combustion temperature B 1860K (compare with the Si
N2 system). Therefore, it is not easy to accomplish a self-sustained SHS process according to reaction (4.11). Several approaches have been developed to enhance the reactivity of this system [55]. One of the activation approaches is to use (
CF2
CF2
)n polytetrafluoroethylene (PTFE) as an additive for the Si 1 C powder mixture [56]. The following set of equations represents the main chemical reactions that take place in the combustion front: Low temperature reaction SiðsÞ 1 ð2CF2 2CF2 2Þn -SiF2 ðgmÞ 1 C ðsÞ 1 Q1 (4.12) Intermediate reaction SiF2 ðgÞ 1 CðsÞ-SiF4 ðgmÞ 1 SiCðsÞ 1 Q2

(4.13)

High-temperature reaction Siðs; l Þ 1 CðsÞ-SiCðsÞ 1 Q3

(4.14)

It can be seen in the Si 1 C 1 PTFE system that the combustion wave consists of two main zones: (1) involves mainly reaction (4.12) and results in the preheating of the Si 1 C reaction media; and (2) the carbidization stage proceeds owing to reactions (4.13) and (4.14). Note that the gas phase reaction (4.13) and condensed phase reaction (4.14) should lead to different morphologies of the SiC product. Indeed, it was shown that two different types of particles could be synthesized. With a certain amount of PTFE additive, the cube-shaped particles with size on the order of 10 mm can be produced, while for the other composition the formation of the long B1 mm thin (less than 500 nm) fibers was observed (Fig. 4.8A). In many other publications, the formation of SiC nanowires during CS was reported in different systems, including SiC/Al2O3, SiC/ ZrO2 with a variety of organic polymers [58
61]. It was stated that the formation of SiC nanowhiskers (diameter 50
500 nm) with aspect ratio

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Figure 4.8 Different morphologies of 1D structures of SiC. Reprinted with permission from H.H. Nersisyan, J.H. Lee, J.R. Ding, K.S. Kim, K. Manukyan, A.S. Mukasyan. Combustion synthesis of zero-, one-, two- and three-dimensional nanostructures: current trends and future perspectives. Prog. Energy Combust. Sci. 2017;63:79
118; A. Huczko, M. Kurcz, A. Da˛browska, P. Baranowski, A. Bhattarai, S. Gierlotka. Self-propagating high-temperature synthesis (SHS) of crystalline nanomaterial. J. Cryst. Growth 2014;401:469
473.

more than 103, takes place owing to reactions between silicon fluorides and the gaseous hydrocarbons, which appeared after decomposition of polymers. Also, a set of works was published on CS of 1D structures in Si-(C2F4)n systems [62
65]. Typical microstructures of as-produced nanowires, branch and comb-like morphologies are shown in Fig. 4.8B
D. It is worth noting that this approach was extended to synthesize 1D structures of silicides and fluorides including CrSi2, FeSi2, and NbSi2 [57]. 4.3.1.2 2D Nanostructures Over the past decade, significant progress has been achieved in the synthesis of nanomaterials using low-cost combustion methods, which are based on self-sustained exothermic reactions [32,33,54]. A variety of 2D nanostructures, including graphene, hexagonal boron nitride (h-BN), molybdenum sulfide (MoS2), and tungsten oxide (WO3) have been synthesized using different CS-based approaches.

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Single-layer and few-layer graphene sheets were prepared by CS in the ceramic (SiC)
polymer (PTFE) system, that is, both precursors are in the solid state, under the argon environment [17,18]. It was shown that under optimum synthesis conditions (see also Section 4.2.1) that PTFE completely decomposes in the preheating zone with the formation of C2F4 gaseous species, which react with SiC to produce carbon, as the only solid phase product: SiCðsÞ 1 C2 F4ðgÞ -SiF4ðgÞ 1 CðsÞ

(4.15)

The microstructural characterization of the combustion product obtained through reaction (4.15) indicates that the combustion product consists of mesoporous carbon nanoparticles and graphene sheets (Fig. 4.9A,B). It is more important that 70% of observed carbon sheets were single or bilayer graphene and the rest were multilayer graphene. It is worth noting that the synthesized graphene contains less than 1 wt.% of oxygen. Graphene was also produced by CS in gas
solid systems. This approach includes the combustion of metals (Me 5 Mg, Li, Ca, Ti, Al, Zr) in carbon dioxide, followed by the chemical leaching of metal oxides [68,69]. It was demonstrated that Li, Mg, and Ca facilitate the formation of ordered carbon sheets, while other metals predominantly form mixtures of corresponding oxides and carbides. The experiments show that asprepared graphene exhibits good gas absorption properties. For example, the graphene materials (Fig. 4.9C,D) produced by magnesium combustion in CO2 adsorb 0.85 wt.% of H2 at 6.5 MPa and 77K [66]. High surface area (1270 m2/g) graphene foams (Fig. 4.9D,F) prepared by the combustion of sodium ethoxide in air showed even higher hydrogen storage capacity (2.1 wt.%) at 1 MPa and 77K [67], which is twice larger than that for commercially produced graphene (SA: 650 m2/g). The fabrication of 2D porous carbon nano-sheets by CS in an Me2CO3 (Me 5 K, Na)
silicon system was also reported [70]. It is shown that the number of layers in such nanosheets can vary from a few to 20, and thus their thickness ranges between 1 and 6 nm (Fig. 4.9G,H). Two-dimensional nanosheets of h-BN were also synthesized by different CS approaches. For example, 2D h-BN crystals were prepared by combustion of gels containing boric acid (H3BO3), urea ((NH2)2CO), sodium azide (NaN3), and ammonium chloride (NH4Cl) [71]. A two-step approach was used including CS followed by postsynthesis annealing in nitrogen at 1300K
1700K. The size of as-produced h-BN nanoplates is in the range of 300
500 nm (Fig. 4.10A,B), and the thickness could be

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Figure 4.9 Typical morphologies of 2D structures for carbon-based materials. Reprinted with permission from (A,B) K.V. Manukyan, S. Rouvimov, E.E. Wolf, A.S. Mukasyan. Combustion synthesis of graphene materials. Carbon 2013;62:302
31; A.S. Mukasyan, A.S. Rogachev. Discrete reaction waves: gasless combustion of solid powder mixtures. Prog. Energy Combust. Sci. 2008;34(3):377
416; (C,D) B.V. Cunning, D.S. Pyle, C.R. Merritt, C.L. Brown, C.J. Webb, E.M.A. Gray. Hydrogen adsorption characteristics of magnesium combustion derived graphene at 77 and 293 K. Int. J. Hydrogen Energy 2014;39(12):6783
6788; (E,F) S.M. Lyth, H. Shao, J. Liu, K. Sasaki, E. Akiba. Hydrogen adsorption on graphene foam synthesized by combustion of sodium ethoxide. Int. J. Hydrogen Energy 2014;39(1):376
380; (G,H) H.H. Nersisyan, J.H. Lee, J.R. Ding, K.S. Kim, K. Manukyan, A.S. Mukasyan. Combustion synthesis of zero-, one-, two- and three-dimensional nanostructures: current trends and future perspectives. Prog. Energy Combust. Sci. 2017;63:79
118 [17].

roughly measured to be below 30 nm. Another approach involves CS in a B2O3
Mg
NH4Cl system [72,73,75]. The as-synthesized product consists of three phases, that is, BN, MgO, and MgCl2. However, after chemical leaching, a single-phase (h-BN) powder was obtained. The

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Figure 4.10 Two-dimensional structures for different compounds produced by the CS method. Reprinted with permission from (A,B) Z. Zhao, Z. Yang, Y. Wen, Y. Wang. Facile synthesis and characterization of hexagonal boron nitride nanoplates by two-step route. J. Am. Ceram. Soc. 2011;94 (12):4496
4501; (C) H.H. Nersisyan, T.H. Lee, K.H. Lee, S.U. Jeong, K.S. Kang, K.K. Bae, et al. Thermally induced formation of 2D hexagonal BN nanoplates with tunable characteristics. J. Solid State Chem. 2015;225:13
18; (D) H.H. Nersisyan, T.H. Lee, K.H. Lee, Y.S. An, J.S. Lee, J.H. Lee. Few-atomic-layer boron nitride nanosheets synthesized in solid thermal waves. RSC Adv 2015;5:8579
8584 [74]; (E) A.S. Mukasyan, K.V. Manukyan. Combustion/micropyretic synthesis of atomically thin twodimensional materials for energy applications. Curr. Opin. Chem. Eng. 2015;7:16
22; (F) H.H. Nersisyan, B.U. Yoo, S.H. Joo, T.H. Lee, K.H. Lee, J.H. Lee. Polymer assisted approach to two-dimensional (2D) nanosheets of B4C. Chem. Eng. J. 2015;281:218
226.

microstructural analysis reveals that BN 2D nanosheets have a thickness less than 10 nm and size on the order of a few microns (Fig. 4.10C,D). The 2D molybdenum sulfate (MoS2) crystals were also fabricated by a combustion reaction, which takes place during high-energy ball milling of the MoCl2
Na2S powder mixture system [33]. The following exothermic metathesis reaction leads to the formation of the desired phase: 2MoCl5 1 5Na2 S 5 2MoS2 1 10NaCl 1 S

(4.16)

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The results of electron-energy-loss spectroscopy revealed that the thickness of the sheets changed in the range 3
5 nm, which corresponds to 4
7 S
Mo
S layers and high-resolution TEM analysis indicates the crystalline nature of these sheets (Fig. 4.10E). Finally, it was demonstrated that the CS method could be used for fabrication of 2D nanosheets of boron carbide (B4C) [74]. The synthesis was conducted in a B2O3
Mg
polyvinylchloride (C2H3Cl)n system. Analysis of the obtained product indicates that B4C nanosheets have a thickness in the range 10
40 nm and size B 100 nm (Fig. 4.10F).

4.3.2 Solution Combustion Synthesis As mentioned in Section 4.1, SCS is a rapid and energy-efficient preparation of a large variety of materials. The synthesis of powdered materials by this method involves heating of homogeneous solutions containing metal nitrates and organic compounds on a hot plate (or in a furnace). Rapid heating leads to evaporation of solvent and formation of a viscous gel. Further heating initiates a high-temperature self-sustaining reaction leading to the creation of nanostructured simple and complex oxides. SCS provides simple formation of high-quality multielement compounds with complex crystal structures (garnets, perovskites, spinels, silicates, phosphates, etc.). Although thousands of publications on SCS of materials have been published, the microstructure formation mechanisms of products are mostly unknown [10,31,33,76]. We can postulate that heating of reactive solutions continuously increases the viscosity, resulting in the formation of a gel-like matter. At a critical temperature, often coinciding with the reactant’s decomposition temperature, a highly exothermic reaction triggers spontaneous nucleation of the solid product. The increase in reaction temperature facilitates rapid growth of the primary nuclei, their agglomeration, and sintering processes. The final morphology of the product also depends on the gaseous product amount and gas release rate. The temperature and gas release features are likely to have the most critical role in determining the morphology of the final products. Other factors that influence the morphology of final products are the fuel to oxidizer ratio, the chemical composition of fuels, pH of solutions, and heating rates. It should be noted that the synthesis temperatures depend on the fuel to oxidizer ratio, and changing this ratio allows control of the temperature. The gas product quantity also depends on the

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Figure 4.11 Typical morphology of the SCS product. Reprinted with permission from T.S. Nguyen, G. Postole, S. Loridant, F. Bosselet, L. Burel, M. Aouine, et al. Ultrastable iridium
ceria nanopowders synthesized in one step by solution combustion for catalytic hydrogen production. J. Mater. Chem. A 2014;2:19822
19832.

fuel to oxidizer ratio. We can assume that initial solutions with increased pH could cause limited hydrolysis of metal nitrates during the heating stage and could lead to the formation of ultra-small colloidal particles, which can serve as nucleation centers during the rapid combustion stage. The heating rate of the solution could also influence the nucleation process. Slow heating would allow more time for the formation of precombustion colloidal particles in the solutions, while precipitation of such particles may be delayed due to the more rapid heating. All these factors result in porous products consisting of randomly assembled irregular or near-spherical particles with sizes ranging between 20 and 200 nm. A typical example of such a product is the microstructure of the doped-CeO2 porous material (Fig. 4.11) reported by Nguyen and coauthors [77]. This type of microstructure prevails in most SCS-derived materials. SCS has a relatively low level of control over the morphological uniformity of resulting products [10,31]. The rapid and high-temperature nature of the process makes difficulties for preparation of regular, uniform nonagglomerated 1D and 2D nanoscale materials. Below we present the recent works on the development of one- or two-dimensional nanoscale materials and discuss the conditions that permit their formation. 4.3.2.1 1D Nanostructures There are a good number of investigations on the preparation of onedimensional nanostructures by SCS, some of which we discuss below [78
85]. According to these researchers, several factors could influence the dimensionality of SCS product. One of these factors is the type of fuel used. For example, Raja and co-workers synthesized Co3O4

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nanopowders using starch, urea, or glycine as fuel under microwave irradiation conditions [86]. Microscopic investigations of the resulting powder showed that the initial solutions containing starch as fuel consist of nanorods. The product of urea-containing solutions was a powder with ultra-small (10
20 nm) near-spherical nanoparticles. The product for the glycine-containing solution consisted of larger (100 nm) spherical particles. However, the formation mechanism of nanorods during the combustion of the starch-containing solution is not known. Kaur et al. also reported significant differences in the morphology of SCS-derived GdFe2O4 when urea, oxalyldihydrazide, glycine, or ethyleneglycol were used as fuels [85]. In this case, solutions containing only urea resulted in nanorods, while products formed with other fuels appeared as nanoscale particles. Nagabhushana and Chandrappa prepared monoclinic vanadium dioxide nanorods using combustion of ammonium meta-vanadate and malic acid solutions with a molar ratio of 1:3 [78]. The immersion of these solutions into a muffle furnace maintained at 470°C resulted in the initiation of smoldering combustion. Then the reacting system was taken from the furnace and allowed to cool at different air partial pressures. The reaction resulted in the formation of a VO2
C composite when airflow was limited. The pure VO2 product was obtained when smoldering was allowed in air excess. When the smoldering combustion was allowed to proceed in the hot furnace for 30 min, it led to the formation of V2O5. The microscopic examination indicated that pure VO2 consisted of single crystalline nanorods with an approximate length of 650 nm and diameter of B50 nm, which were assembled in larger bundles. The authors did not discuss the growth mechanism of these crystals, but they emphasized that the time of exposure to heat and temperature at which the reaction was carried out determined the formation of VO2. One can assume that slower smoldering combustion occurring outside the furnace allowed sufficient time for nanorods to grow. A similar strategy was also reported by Wen and co-authors, who prepared nanostructured anatase nanobelts [87]. The initial solution containing titanium oxysulfate, an excessive amount of glycine, and nitric acid was immersed in a furnace maintained at 400°C. After low-temperature combustion (rapid pyrolysis), the products were treated with hydrogen peroxide solutions to remove excess organic compounds and produce a hydrogen titanate hydrate (H2TiO5  H2O). TEM analysis suggested that as-prepared nanobelts have 1
2 nm thickness. Low-temperature thermal

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treatment of hydrogen titanate hydrate allowed preparation of anatase with a similar structure. Bai et al. used this approach for the fabrication of Ni-doped anatase nanobelts [88]. They added nickel nitrate hexahydrate in reactive solutions for the incorporation of Ni into anatase nanobelts. TEM analysis revealed that the thickness of the belts is in the range of 1
2 nm, and the Ni was uniformly distributed in them (Fig. 4.12). It is worth noting that anatase nanobelts are self-assembly polycrystalline nanostructures, while the VO2 nanorods prepared by Nagabhushana and Chandrappa [78] are nanoscale monocrystals. Chen et al. prepare pure and Fe-doped W18O49 single crystalline nanorods using solutions of ammonium paratungstate and ammonium

Figure 4.12 TEM image (A) and high-resolution TEM image (B) of the Ni-doped TiO2 nanobelts. EDS mapping of (D) Ti, (E) O, and (F) Ni for part of a nanobelt (inset in C). Reprinted with permission from J.-Q. Bai, W. Wen, J.-M. Wu. Facile synthesis of Ni-doped TiO2 ultrathin nanobelt arrays with enhanced photocatalytic performance. Cryst. Eng. Comm. 2016;18:1847
1853.

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nitrate with different fuels (glycine, urea, citric acid) [80
82]. These works suggested that the type and amounts of fuel were the most critical parameters that govern the morphology of the materials. The authors indicated that glycine supports the formation of single crystalline nanorods (Fig. 4.13A). The glycine quantity, however, should be significantly higher than the stoichiometric fuel to oxidizer ratio. This excessive glycine could form a crosslinked polymeric network by dehydration between amino and carboxyl groups. The authors suggest that this network facilitates crystal growth along the [010] direction. The combustion product obtained from urea-containing solutions formed near-spherical nanoparticles of W18O49 (Fig. 4.13B). This fact allowed the authors to propose that urea is unable to create polymeric compounds due to the lack of a carboxyl group in its molecule. The authors showed that the application of urea mixed with citric acid (which contains three carboxyl groups) allowed production of 1D W18O49 nanorods, thus supporting the hypothesis of the importance of polymerization for the formation of 1D structures.

Figure 4.13 TEM images W18O49 formed by combustion of solutions containing glycine (A) or urea (B) as fuels, as well as TEM image (C) of Sn0.8Y0.2O1.9 nanorods obtained from KCl-assisted solution combustion process after annealing at 400°C for 30 min, high-resolution TEM image (D) of a Sn0.8Y0.2O1.9 nanorod. Reprinted with permission from P. Chen, M. Qin, Z. Chen, B. Jia, X. Qu. Solution combustion synthesis of nanosized WOx: characterization, mechanism and excellent photocatalytic properties. RSC Adv. 2016;6:83101
83109; W. Chen, M. Liu, Y. Lin, Y. Liu, L. Yu, T. Li, et al. A novel synthesis route to Sn1-xRExO2-x/2 nanorods via microwave-induced salt-assisted solution combustion process. Ceram. Int. 2013;39 (7):7545
7549.

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The morphology of the SCS product can be tuned also by introducing dopant elements. For example, Babu et al. [90] showed that incorporating Co is capable of significantly modifying the dimensionality of SCSderived SnO2. Particles of pure SnO2 appeared as nanoflakes, while incorporation of a small amount of Co favors nanorod formation. Kavitha et al. [91] demonstrated that adding Sr modifies the morphology of hydroxyapatite. An increase in the Sr quantity led to a significant expansion of the aspect ratio of the nanoparticles. An efficient approach for the preparation of 1D nanostructures is a combination of reactive solutions, molten salts, and microwave heating [92
94]. This approach was used for the development of phosphate bioceramics including hydroxyapatite, fluorapatite, and chlorapatite. In a typical synthesis process, a significant amount of NaNO3 was added to the reactive solutions; then the solutions were irradiated for 5 min. The microwave irradiation helped in uniform molecular-level heating of solutions. It was proposed that the molten salt dissolved the primary precipitate product particles, which form during the evaporation of water. This dissolved material could serve as nucleation centers in the molten salt, and during the cooling stage, rapid crystallization of the final product occurs along the preferred growth axes resulting in nanorods or nanotubes by a sequential dissolution 2 crystallization 2 growth mechanism. Chen et al. [89] also reported the synthesis of several rare-earth elements doped tin oxide by a combination of SCS under microwave irradiation in the presence of KCl flowed by subsequent annealing of products for 30 min at 400°C. The electron microscopy investigations showed that each Sn0.8RE0.2O1.9 nanorod is a single crystal with rutile structure and has a preferred [001] growth direction. The microstructural analysis suggested that combusted products consisted of nanorods, which form only after the annealing step (Fig. 4.13C). The authors attempted to describe the formation mechanism of these nanorods. They indicated that ultrasmall KCl nanoparticles precipitated on the Sn0.8RE0.2O1.9 product’s nanoscale spherical particles. Melting of KCl particles could then create conditions for recrystallization of product and their preferential growth along the [001] direction (Fig. 4.13D). However, given the fact that the annealing temperature (400°C) was significantly lower than the melting point (770°C) of the KCl, such a mechanism seems unrealistic. The authors suggested that ultra-small KCl particles could melt at considerably lower temperatures, thus creating conditions necessary for recrystallization.

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4.3.2.2 2D Nanostructures SCS was also used for the preparation of 2D nanomaterials including graphene-based hybrids and composite materials [95
103]. For example, Mayavan and co-authors reported on a one-step synthesis of silver nanoparticle-decorated nitrogen-doped graphene materials by a solution combustion route [98]. In those cases, graphene oxide (GO) was dispersed in solutions of silver nitrate and glycine, which were then uniformly heated to initiate the combustion reaction. The presence of nitrogencontaining gases (such as NH3 and NO2) during the combustion facilitated simultaneous nitrogen doping of reduced GO. Gao and coauthors also reported this strategy for the preparation of reduced-GO/ TiO2 materials [99]. Chen et al. treated the graphite using a solution of H2SO4 and KMnO4, followed by microwave irradiation to induce a combustion reaction [101]. They demonstrated that this approach could produce sulfur-doped graphene (Fig. 4.14). Wan and co-workers used microwave energy to initiate exothermic thermal decomposition of GO in the presence of metal nitrate [103]. This approach was shown to be a viable strategy for preparation of porous graphene with controlled pore sizes. An alternative “bottom-up” combustion strategy was suggested to generate oxide nanoparticle-decorated graphene composites in which glucose was added to reactive solutions containing metal nitrate—a fuel

Figure 4.14 TEM image with electron diffraction pattern (inset) of a sulfur-containing graphene nanosheet produced by a microwave irradiation-assisted combustion process. Reprinted with permission from K. Chen, H. Yang, F. Liang, D. Xue. Microwaveirradiation-assisted combustion toward modified graphite as lithium ion battery anode. ACS Appl. Mater. Interfaces 2018;10 (1):909
914.

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(glycine) [104,105]. It should be noted that glucose alone can also serve as a fuel source. In such cases, excessive amounts of glucose undergo thermal decomposition leading to the formation of 2D carbon-based nanostructures. This process could provide a method for the preparation of powders with an ultra-high specific surface area. For example, Dong et al. used combustion of zinc nitrate hydrate and glucose solution to produce ZnO/ carbon composites, which were then subjected to annealing at 800°C in an inert atmosphere, followed by leaching of ZnO from the product [104]. The surface area of as-prepared 2D carbon nanomaterials can be controlled from 700
1700 m2/g by changing the fuel to oxidizer ratio in the initial reactive solutions. Another interesting strategy was suggested by Xu et al. [106]. They showed that hydrous hydrazine solution chemically absorbed CO2 gas. The sorption of CO2 with hydrazine resulted in the formation of a hydrazinoformic acid solution. Excessive Mg powders can be dissolved in this solution, and the obtained homogeneous gel-like phase can be initiated by local preheating. Purification of combustion products from MgO resulted in the porous assembly of 2D carbon sheets, with a surface area of B650 m2/g.

4.4 CONCLUSIONS As can be concluded from the above, the combustion-based methods for synthesis of 1D and 2D crystals do not require any external energy source, since they occur in a self-sustained synergetic manner. Thus, CS is an energy-saving process, which requires simple equipment without heating sources. It is also worth noting the unique conditions of the considered approach (e.g., high temperatures, rapid self-heating, and cooling) that facilitate the formation of crystalline nanomaterials. These conditions allow for a short synthesis time, which is in the minute range. Indeed, the time-limiting process stage is the cooling one, which can and should be controlled. Other advantages of CS are its ability to synthesized complex multielement compounds with complex crystal structures, including perovskite, garnet, spinel, silicate, and phosphate nanostructures, as well complex structures, such as nanoparticle-decorated nitrogen-doped graphene sheets. Being a combustion-based method, CS permits easy scale-up of material production. Indeed, increasing the amount of reactive mixture leads to closer adiabatic conditions and a more steady-state combustion regimen.

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Finally, it is important to note that the CS of nanomaterials can be performed by continuous schemes. However, several limitations of CS need to be resolved before this method can be implemented in the large-scale production of advanced nanoscale materials. The first issue is related to the uniformity of the morphology for the produced materials. The preparation of regular, nonagglomerated nanostructures has been a challenging task. However, many recent works showed that by investigating the mechanisms of the combustion reactions and structural transformations, which occur during CS, one might establish new effective ways to control the morphology of the products. Using a self-template (excessive amounts of fuels in SCS), new approaches based on the reactive spray pyrolysis, use of microwave ignition, and molten salt-assisted methods have significantly enhanced the structural diversity of combustion products. The controllability of the process is the critical point for addressing most of the method limitations. Can it be fully accomplished for combustion? Examples from other engineering fields suggest a positive answer. More and more studies currently are dedicated to the kinetics of CS reactions by using vast numbers of state-of-the-art in situ and operando diagnostics. It is crucial to outline first the theoretical findings, which suggest computational models predict the CS conditions. We believe that modern experimental and computational methods can provide a fundamental basis for the development of CS-based technologies with a sufficient level of control, which is required for their industrial application. The drawback of the conventional SCS-based routes is a necessity of using an additional calcination step, which makes it a two-step method. For many systems, SCS takes place in relatively low temperatures, which results in the formation of amorphous products. In these cases, a prolonged calcination step is required to improve the crystallinity of the materials. Recent works have demonstrated different approaches, which allow us to avoid this additional time- and energy-consuming procedure. It can be accomplished by better control over the combustion temperature and a width of the reaction front by using a gasified oxidizer, such as ammonium nitrate. Environmental impacts of hazardous gases (NO2, NO, N2O, N2O5) that may be released during large-scale applications of SCS processes is another issue which should be addressed. Indeed, in recent works, the nitrous oxides (NO2, NO, N2O) were detected by using massspectroscopic analysis. Depending on their quantities, several possible

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solutions may be developed to prevent their formation. The fuel-rich reactive solution or usage of mixed fuel formulations with different quantities of reducing (C, H) elements may lower the harmful gases due to in situ reduction by fuel fragments. During the last decade, the CS direction has made several significant steps forward in finding new routes of material synthesis, from entirely uncontrolled thermal explosion by heating in a furnace to precisely controlled steady-state self-propagating mode. From agglomerates with nonuniform microstructures to super fine (less than 10 nm) nanoparticles. From powders to thin films and 1D and 2D crystals. The full capabilities of all these novel approaches are required for in-depth exploration to conclude their applicability on an industrial scale. We have to continue our work on fundamentals of CS: kinetics and mechanisms of structure formation under unique CS conditions, while being difficult tasks, should be under the continuous attention of the research community. Otherwise, we wil need to forget about the controllability of the process. Finally, in our opinion, the most promising future directions in the CS field are related to: 1. A significant broadening of the families of materials that can be synthesized by the SCS method, which is capable of producing not only oxides, metals, and alloys, but also nitrides, carbides, intermetallics, etc.; 2. Combinations of SCS and solid-state heterogeneous CS approach, which will allow the quality of the products to be significantly enhanced. All the above allows us to expect that, in the near future, CS products will find broad applications in different industrial fields such as energy conversion, storage and optical devices, catalysis, electronics, and biomedicine.

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3366.

CHAPTER 5

Microwave-Assisted Synthesis for Carbon Nanomaterials Sabzoi Nizamuddin1, Sadaf Aftab Abbasi1, Abdul Sattar Jatoi2, M.T.H. Siddiqui1, Humair Ahmed Baloch1, N.M. Mubarak3, G.J. Griffin1, E.C. Abdullah4, Khadija Qureshi5 and Rama Rao Karri6 1 School of Engineering, RMIT University, Melbourne, VIC, Australia Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi, Pakistan 3 Department of Chemical Engineering, Faculty of Engineering and Science, Curtin University, Sarawak, Malaysia 4 Department of Chemical Process Engineering, Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia (UTM), Jalan Sultan Yahya Petra, Kuala Lumpur, Malaysia 5 Department of Chemical Engineering, Mehran University of Engineering and Technology, Jamshoro, Pakistan 6 Petroleum anc Chemical Engineering, Universiti Teknologi Brunei, Brunei Darussalam 2

Contents 5.1 Introduction 5.2 Methods of Synthesis of Carbon Nanomaterials 5.2.1 Arc Discharge Method 5.2.2 Laser Ablation 5.3 Chemical Vapor Deposition 5.3.1 Chemical Vapor Deposition Substrate Catalyst Method 5.3.2 Metal Catalyst Influence in the Chemical Vapor Deposition Technique 5.4 Plasma-Enhanced Chemical Vapor Deposition-Based Carbon Nanomaterials 5.5 Microwave-Enhanced Chemical Vapor Deposition 5.6 Fluidized Bed Chemical Vapor Deposition 5.7 Vapor Phase Growth Chemical Vapor Deposition 5.8 Microwave-Assisted Synthesis of Graphene 5.9 Future Prospects for Carbon Nanomaterial Synthesis and Challenges 5.10 Conclusion References

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5.1 INTRODUCTION Carbon nanomaterials possess a unique place in nanoscience due to their outstanding mechanical, chemical, electrical, and thermal properties. This material has found applications in many disciplines, such as energy storage, Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00005-5

© 2019 Elsevier Inc. All rights reserved.

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sensors, drug delivery, nanoscale electronic components, and composite materials. The carbon nanomaterials include carbon nanotubes (CNTs), fullerenes, and graphene, etc. The CNTs are allotropes of carbon with a cylindrical (tube) shape, having a nanoscale diameter ranging from less than 1 to 50 nm. The CNTs possess several structures, and vary in thickness, length (several microns to centimeters), and number of layers. The carbon can be made in closed and open atmosphere with honeycomb structure. The C60 molecule was the first such structure, and was discovered by Kroto et al. in 1985 [1]. In 1991, Ijima studied the tubular arrangement of carbon [2]. CNTs are produced from a graphene sheet by rolling it in more than one direction [3]. As a result, CNTs were classified by their structures, for example, single-walled CNTs, multiple-walled CNTs, nanotubes, nanobuds, and nanohorns. Researchers have developed different manufacturing techniques for producing nanomaterials. Some of these techniques require a long reaction time and temperature [4,5] and some require highly reactive species [6]. The microwave-assisted synthesis technique for producing nanomaterials has attracted much attention over the conventional techniques such as refluxing. Recently, microwave-assisted synthesis has emerged as a widely used technique in the production of nanomaterials [7
9]. The microwave-assisted technique is a fast, economic, environmentally friendly, clean, and simple method. The basic principle of the microwave technique is to convert the electromagnetic energy to thermal energy within the material used, due to the molecular interface with the electromagnetic field. Carbon materials have the capability to engage microwave energy and change it into thermal energy (dielectric tangent loss) at 2.45 GHz [10], which makes this process cost-effective and rapid. Recently, the literature has shown a great deal of success toward the making of high-purity nanoparticles with narrow particle size distribution by using microwave-assisted techniques [11,12].

5.2 METHODS OF SYNTHESIS OF CARBON NANOMATERIALS 5.2.1 Arc Discharge Method The arc discharge method is the oldest and the easiest method to manufacture CNTs through plasma-based synthesis. Using this method, a current (50 amps) is passed between two graphite electrodes (separated by approximately 1 mm) to vaporize the graphite. Some of graphite condenses on the wall of the reaction vessel and some on the cathode. The

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deposition on the cathode contains CNTs. The single-walled CNTs are manufactured with the addition of Co and Ni or some other metals to the anode. The studies [13,14] have shown that CNTs can be manufactured by gas (including carbon). The catalyst usually contains Fe, Co, or Ni nanosized particles to catalyze the separation of the gaseous molecule into carbon and then the tube starts to grow with a metal particle at the tip. In 1991, the preparation of CNTs with a needle structure was reported using the arc discharge evaporation method [2]. The needle CNTs were made using a direct current (dc) to evaporate the carbon in the vessel (filled with argon). The CNTs produced in this way were poorer quality than with the arc evaporation method. However, great improvements have been made in recent years. Choi et al. [15] reported an economical technique to manufacture high-purity single-walled carbon nanotubes (SWCNTs) with a diameter of 1.2 nm. They used FeS (20 wt.%) as a catalyst and an argon DC arc discharge from the carbon source—charcoal. The third useful method of making CNTs is vaporizing metal graphite (target) by a powerful laser. The SWCNTs with high yield may be manufactured with this method [16]. In 1992, Ebbesen and Ajayan reported the mass production of multiwalled carbon nanotubes (MWCNTs) by a standard variant [17].

5.2.2 Laser Ablation For the first time, in 1996, Smalley’s group at Rice University produced SWCNTs on a large scale [17,18]. In this synthesis method, carbon atoms were evaporated from heated graphite and deposited on a cooled substrate. A continuous or pulse laser is made to vaporize the composite target (1.2% of cobalt/nickel and 98.8% of graphite) in a hightemperature (1200°C) reactor. The nanosized metal particles were formed in the feather of vaporized graphite to catalyze the growth of SWCNTs. As soon as vaporized species cool down, the molecules and atoms of carbon immediately condense to build the fullerenes (larger clusters). During the condensation process, the catalysts join carbon clusters so that they may avoid closing into the cage structure. The SWCNTs were grown in tube shapes from these early clusters until the conditions were cooled down. The SWCNTs formed are gathered by short-range electrostatic attractive forces. The nanotubes (including byproducts) are gathered from the

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composite target on a cold finger downstream through condensation. Technically, both methods (arc discharge and laser ablation) are comparable methods, as both use a graphite (impregnated with metal) target to manufacture SWCNTs. These methods also produce MWCNTs and fullerenes by using pure graphite but the length of MWCNTs produced by the arc discharge method is much longer as compared with laser ablation. Therefore, laser ablation is not a suitable method for production of MWCNTs. In addition, the laser ablation process is an expensive method due to high energy consumption and is not likely to produce CNTs with bulk production. However, this method is still used due to its yield capability.

5.3 CHEMICAL VAPOR DEPOSITION Due to the involvement of temperature and limitation of volume ratio between sample and carbon source, gas-phase techniques such as chemical vapor deposition (CVD) have replaced the laser ablation and arc discharge methods. CVD is a governable process to manufacture nanotubes with pre-et properties [19]. The CVD method is the outcome of a significant effort made to obtain a controllable route to synthesize nanotubes by breaking the gaseous molecules. However, the arc discharge method has a higher level of production but with unpurified nanotubes. The CVD process is carried out in two steps: • The deposition of catalyst (metal catalysts, i.e., Ni, Fe, or CO) on substrate and nucleation through thermal annealing or chemical (ammonia) etching; • The gas phase in the chamber where carbon sources such as methane, carbon monoxide, or acetylene is placed. The carbon molecule is then changed into atoms by a plasma process or hot coil. The carbon is diffused toward the substrate coated with catalyst and nanotubes grow over the metal catalyst with a typical yield of 30% [20,21]. There are several structures such as amorphous carbon layers on the catalyst, graphite layers with metal particles, SWCNTs and MWCNTs produced by the CVD method. The characteristics of nanotubes produced by the CVD method depend on the temperature, pressure, volume, and concentration of hydrocarbon, and the nature, size, and pretreatment of metallic catalysts.

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5.3.1 Chemical Vapor Deposition Substrate Catalyst Method In CNT growth, the catalysts need an appropriate substrate material for quality and yield of CNTs. The substrate material, its surface morphology, and texture properties affect the yield and quality of the grown CNTs. It not only acts as a medium for support but also interacts with the catalyst and growth environment, so the catalyst
substrate interaction should be studied with great attention. Either physical or chemical interactions may take place between the substrate and the catalyst material. Physical interactions, for example, van der Waals and electrostatic forces, prevent catalyst particle movement on the support material, reducing thermally driven diffusion and sintering of metal particles on the substrate material. This physical interaction results in stabilization of the catalyst particle size distribution during synthesis of CNTs. Chemical interactions between the catalyst particles and surface groups of the substrate can also aid in maintaining the size distribution of the catalyst particles during growth of CNTs. Furthermore, in 2007 Noda et al. [22] reported that the oxide substrate basically used as a physical support for metal catalyst might play some role in CNT growth. In a significant development, in 2015, Ghosh et al. [23] chemically treated graphite substrate to obtain a graphene layer on it and then deposited a Pt layer of thickness 0.2 nm. They found that the grown SWCNTs by ethanol CVD on the treated substrate are semiconducting in nature with a narrow diameter distribution. Various substrates used in CVD for the growth of CNT are silicon [24,25], silicon carbide [26] graphite [27], quartz [28], silica [29,30], alumina [31], magnesium oxide [32], calcium carbonate (CaCO3) [33], zeolite [34], and NaCl [35], etc. Chai et al. [36] investigated the effects of different support materials on a CoO catalyst.

5.3.2 Metal Catalyst Influence in the Chemical Vapor Deposition Technique The thermal dehydrogenation reaction is an essential part in the CVD process that leads to cracking the gaseous hydrocarbon precursor into various carbonaceous and hydrogen products, and this is mainly done in the presence of precisely heterogeneous catalysts [37]. In this aspect, the catalyst plays a significant part in the CVD process for the production of carbon nanomaterials. The specific character of the catalyst not only affects the production of carbon nanomaterials quantitatively but also has a large impact on product quality. Transition metal catalysts, specifically iron, cobalt, and nickel, have been found to be efficient catalysts for CNT

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growth, as well as being capable of decomposing various feedstocks of hydrocarbon effectively [38]. The reasons for this behavior are that these metals support the catalytic degradation of volatile carbon substances, are able to form metastable carbides, and are also able to diffuse carbon [39,40]. One of the most important steps is the selection of the catalyst and its support which ultimately optimizes and enhances the overall production of carbon nanomaterial. Su et al. [41] studied the role of novel aerogel supported by iron and molybdenum and found it effective in improving 200% of the actual capacity of single-layered CNT production by the CVD process. Chemical or physical interaction, porous structure, crystallographic orientations, and surface roughness also enhance the effectiveness as a catalyst [38]. However, the catalyst size also has an important role in the synthesis of carbon nanostructures as it was reported by various researchers that the type of CNT produced has an impact on the catalyst particle size distribution [42,43]. Cheung et al. [30] utilized various particle sizes of iron nanoparticles with diameters from 3 to 13 nm, which were found to be effective in producing highly controlled CNTs with specific diameters from 3 to 12 nm. In a similar type of study, several catalysts were used for fluidized bed CVD including cobalt, iron, and nickel. It was found that nickel showed the growing of graphite nanofibers (GNFs) specifically, however using a mixture of catalysts based on Fe and Co in support of molybdenum, or tungsten, was found to be effective for growth of SWCNTs [44]. Another study produced carbon nanofibers using a CVD reactor at a low temperature of 545°C in a nickel catalyzed system as it showed zero defects in the carbon nanofibers [16]. Apart from the catalyst, the carrier or support is also a significant factor in dispersing in the active site, stopping sintering of catalyst, and enhancing the physical strength. To analyze the importance of support of the catalyst a number of studies have been carried out for synthesis of CNTs using the CVD process by keeping the same catalyst. However, these studies changed the support material and it was found from these studies that the carrier having higher surface characteristics including silica and alumina showed outstanding CNT nucleation and growth [45
48]. A larger surface area is responsible for the diffusion of carbon atoms with the catalyst nanoparticles. The ideal metal ratio between catalyst and support promotes CNT synthesis with the specific characteristics at the highest yield. However, numerous studies have reported that increasing the metal ratio is productive only if it promotes more active sites for CNT nucleation, as compared with the average particle size of the catalyst [49
52]. However, the real test is to find an ideal metal that can be utilized to produce

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high-quality carbon nanostructures using the CVD technique. However, the selection of catalyst and its preparation step are the most important features during the CVD process in order to boost the overall efficiency of the process for the production of carbon nanostructures.

5.4 PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITIONBASED CARBON NANOMATERIALS One of the great achievements of science is the development of techniques which enable us to understand matter and allow us to modify atomic structures. The modification of different materials and their surfaces at the nanoscale has been exploited in recent decades. The synthesis of carbon nonmaterials (CNMs) and alteration of their surfaces provide an opportunity to bolster scientific efforts in order to create a more resourceful world community capable of confronting its challenges. Functionalized CNMs have unlocked an array of applications across a wide spectrum of fields. Among CNMs, CNTs and graphene have many superior properties, such as low-weight, high aspect ratio, high electrical conductivity, and extraordinary mechanical, optical, and thermal properties [53
56]. The application of carbon nanomaterials to various fields has been assisted by functionalization of their surfaces and the importance of modified nanosurfaces is well reflected in scientific work over the last decade. The physiochemical features of these functionalized nanomaterials have been exploited for energy [56], cancer treatment [57
59], antiviral drug development [60], drug transportation in biological systems [61
63], biotechnological applications [64,65], and aerospace [66,67]. In addition, theoretical efforts have been made to analyze and optimize functionalization [68,69]. CNMs are produced through different techniques by which analysis and work are done through the utilization of a modified technique. In addition, plasma-enhanced CVD is a modified technique which works to recover nanomaterials based on plasma-enhanced CVD. Different nanomaterials are generated through this technique with the help of plasma such as CNTs and nanofibers, etc. Plasma-enhanced CVD is also known as glow discharge CVD. It uses electron energy (plasma) as the activation method to enable deposition to occur at a low temperature and at a reasonable rate, by supplying electrical power at a sufficiently high voltage to a gas at reduced pressures [70]. Plasma-enhanced CVD of CNTs and CNFs is an extremely complex process with numerous coupled phenomena: plasma chemistry, neutral and ion reactions, surface chemistry, catalyst growth, catalyst particle,

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aggregation/segregation/migration of plasma heating, electric field effects, ion bombardment, heat and mass transfer, chemically reacting flow, etc. Parameters dictating the growth characteristics include nature of feedstock, diluents such as H2, Ar, N2, etc., feed gas composition and flow rate, nature of the plasma power source, plasma input power, pressure, substrate temperature, nature of the catalyst and how it is applied, metal underlayer or diffusion barrier (if any), between the catalyst and the substrate, and nature of catalyst pretreatment (if any). A review of plasmaenhanced CVD status in CNT growth was presented in 2003 at the early stages of the adoption of this process by the CNT community [71]. Plasma-enhanced CVD reactors are mainly classified by the type of plasma source used to generate the gas discharge of the feedstock [72,73]. The most common in CNT growth is a dc plasma source, as shown in Fig. 5.1 [74
76].

Figure 5.1 Schematic of a plasma-enhanced CVD set-up [71].

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Fig. 5.1 shows a typically set up of plasma-enhanced CVD for synthesis of nanomaterials, which consists of accessories for controlling temperature, pressure, etc. Fig. 5.2A and B demonstrates the difference between continuous catalyst films resulting in dense, about 171 m high CNT cones (Fig. 5.2B) after identical processing on the same sample. Both the continuous film

Figure 5.2 Results of plasma-enhanced CVD growth of CNTs: (A) tall CNT “forests”; (B) small CNT cones; (C) CNT structures of the same height from both individual dots; (D) magnified view of the individual CNT in (C); (E) AFM image of the lines formed by individual catalyst dots before growth; (G) whereas starting growth when the final temperature is already reached may result in growth only from the larger catalyst areas (F) [77].

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and the small dots consist of a 7-nm nickel film that is deposited simultaneously on the whole sample. The difference in CNT heights is more than an order of magnitude and is solely due to the different sizes of the catalyst patterns. As a consequence, if all CNTs on the substrate are to be equally tall, catalyst lines and areas can also be patterned as individual dots, as shown for lines in Fig. 5.2C and D. In this case, the rectangle around the single CNT is e-beam patterned as a string of individual dots. An AFM image of the surface and the individually patterned catalyst dots before CNT growth can be seen in Fig. 5.2E
G, further demonstrating the necessity of careful parameter adjustment for individual CNTs. When plasma growth was started at the final high temperature, typically no growth was observed from the small catalyst dots, whereas starting growth already at a lower temperature (500°C) resulted in reproducible growth on both continuous catalyst layers (L-shaped) and arrays of individual dots. This indicates that, due to their high surface to volume ratio, small particles are especially sensitive to passivation at high temperatures without plasma [77].

5.5 MICROWAVE-ENHANCED CHEMICAL VAPOR DEPOSITION There are a number of known methods for producing CNTs. It is possible to produce nanotubes with differing properties and in different forms. The most common methods used for the production of nanotubes are arc discharge [78,79], laser vaporization [79,80], and CVD. Our interest lies mainly in the area of electronic device applications, for which CVD is particularly suitable as it allows the location of nanotubes to be precisely controlled [79]. Microwave-enhanced CVD is a well-known technique for the synthesis of CNTs [81] and carbon nanowalls [82]. Similar foil-like carbon nanostructures were synthesized by plasma-enhanced chemical vapor deposition (PECVD) [83]. Microwave-enhanced CVD growth has received significant attention mainly because of the potential for low-temperature CNT synthesis required for compatibility with standard nanofabrication and CMOS processes and the ability to produce highly graphitized, vertically aligned CNTs [71,84]. A distinguishing feature of the microwave-enhanced CVD process is the presence of a highly reactive microwave environment, which enhances the decomposition of the hydrocarbon feedstock during CNT growth. The generation of highly energetic ions and their

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subsequent transport to the growth surface are two critical factors that influence the growth properties [85]. Using the wide parameter space of the microwave-enhanced CVD, a key advantage over other CVD processes, low temperature growth [86,87], CNT alignment [88], chiral [89] and diameter control [90] have been demonstrated. In many microwave-enhanced CVD studies, the plasma source used is microwave energy which is characterized by high plasma density with a resonant field that is able to concentrate the plasma, ensuring that significant electron loss to the surrounding area does not occur [85]. The plasma intensity is controlled by the microwave power while the ion flux directed at the substrate may be controlled by a dc bias voltage applied to the growth substrate. These parameters operate independently in microwaveenhanced CVD and are capable of substantially altering the properties of CNTs [86]. Recently, microwave-assisted synthesis has extensively been used in organic synthesis [6,7,9]. Microwave-assisted modification of CNTs is a noninvasive, simple, fast, environmentally friendly, and clean method as compared to traditional methods. Usually, the use of microwave facilitates and accelerates reactions, often improving relative yields. In the case of microwave-assisted functionalization of CNTs, microwave irradiation of CNTs and CNFs reduces the reaction time and gives rise to products with higher degrees of functionalization than those obtained by the conventional thermal methods [91]. Interestingly, a competitive effect of microwave irradiation that both promotes functionalization and removes some functional groups that are initially present has been suggested [92] On the other hand, Vazquez and co-workers showed that a solvent-free technique combined with microwave irradiation produces functionalized nanotubes in just 1 h of reaction, paving the way for large-scale production of functionalization [9]. Although materials are heated differently by microwaves, the maximum temperature is determined primarily by the dielectric properties of the receptor. The microwave heating of carbon-based materials gives rise to hotspots that appear as small sparks or electric arcs, with local temperatures higher than 1100°C. These hotspots have been well established as the thermal sensitizer upon microwave irradiation in the fields of organic synthesis, environmental remediation, preparation of catalysts, and carbon nanostructures [93]. According to the study by Mendez et al. [94], microwave heating was used to produce CNTs from graphite by applying microwave power of 800 W, with oven temperature at

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approximately 1200°C for 60 min. This study also showed that no catalyst was employed and graphite was directly targeted for microwave heating as graphite does not require preliminary heating. Along with graphite, a few other samples such as sucrose
graphite and boric acid with graphite were also tested with this microwave heating method. By doing this, the formation of nanotubes and nanofibers for a sucrose
graphite sample was observed [94]. The same yield for boric acid and graphite with an additional yield of microparticles was also observed. Hong et al. [95] reported a method for the synthesis of CNTs by microwave irradiation. CNTs were successfully synthesized by microwave heating of the catalyst loaded on various supports such as carbon black, silica powder, or organic polymer substrates (Teflon and polycarbonate). Microwave (2.45 GHz, 800 W) irradiation used acetylene as a hydrocarbon source, and 3D transition metals and metal sulfides were used as the catalysts. Kharissova et al. [93] developed a study for obtaining long and aligned CNTs with or without Fe filling by a highly efficient one-step technique. Ferrocene was utilized as a catalyst to synthesize the aligned CNTs by heating them through microwave irradiation. However, in this study, a catalyst (Fe) and a silica fused target were used. Lee [96] reported that synthesis of CNTs on various supports, even on organic polymer substrates, by microwave heating of the catalysts under atmospheric pressure was conducted in order to establish a better understanding of the influence of chemical structures and processing conditions on the properties of CNTs which are directly synthesized on polymeric and organic materials through microwave irradiation. Benito et al. [97] reported that the microwave-hydrothermal treatment determines the chemicophysical properties of Co
Zn
Al catalysts obtained by calcination of layered double hydroxide at 500°C. The treatment affects the distribution of the cations within the layers of the precursors because of an improved order. This effect is also observed in the catalysts. The kinetic study of carbon growth from the catalytic decomposition of methane indicates that the treatment improves the activity and the stability of the Co
Zn
Al catalyst. However, remarkable differences between the different aged catalysts are not found. This observation is in agreement with the results obtained by characterization, the greatest changes in the properties are observed in the sample aged for the shortest period of time. On the other hand, the transmission electron microscopy results show that the carbon products obtained also depend on the duration of microwavehydrothermal treatment, producing a change of the type of nanofilament

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formed (herringbone carbon nanofibers or multiwalled CNTs) and of the quantity of amorphous carbon produced. In addition, Zajickova et al. [98] reported that the microwave operating in the atmospheric pressure was successfully applied to the fast deposition of MWCNTs on the substrate without the necessity for any vacuum or heating equipment. Different ways to generate in situ and ex situ catalytic nanoparticles for CNT growth were tested. Dense straight standing nanotubes were successfully prepared on silicon substrates with or even without a barrier SiOx layer and oxidized Al layer. Therefore, it was possible to produce CNTs directly on conductive Si and use them as electron-emitting electrodes of the gas pressure sensor. The CNTs grown in a microwave torch were also intended to create a gas sensor based on the changes to electrical resistance measured between two planar electrodes connected by CNTs. Zeng et al. [99] reported that microwave-assisted pyrolysis of methane on the growth and morphology of carbon nanostructures (carbon nanospheres and CNTs) in the absence of a catalyst was studied. A vertical quartz tube was placed in a 2.45-GHz domestic microwave oven to carry out the experiment. A microwave absorber material carbon/carbon composite without mineral content was placed in the tube, which acted like a catalyst for nanostructure growth. Flushing the nitrogen before performing the experiments eliminated the oxygen from the cavity. The reaction was performed in a mixed gas flow CH4/N2 (ratio of 1:4) for 60 min and then the substance deposited on the quartz wall was used for further analysis. The results demonstrated that microwave-assisted pyrolysis is a novel and promising approach for synthesizing carbon nanostructures. In addition, the experimental conditions, such as methane to nitrogen ratio, reaction temperature, and total gas flow rate, affected the nature of the produced nanostructures. Vivas-Castro et al. [100] reported that the synthesis of CNTs and other carbon nanostructures by microwave irradiation is simple and lowcost. With this technique and under various preparation conditions we obtained nanostructured material from a graphite/iron acetate powder mixture using a commercial microwave oven as an energy source. Microwave absorption by the powder mixture results in pyrolysis of iron acetate. Decomposition of the acetate provides metallic iron nanoparticles that act as catalysts in the synthesis of nanotubes and other carbon nanostructures. Different types of nanostructured carbon can be obtained by variation of the preparation conditions. Direct irradiation of (vacuumsealed) quartz ampoules, and attenuated irradiation by partially submerging

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the ampoules in water provide examples of the variety of nanostructures that can be obtained at different stages of microwave exposure. Reactions are very fast and may become so violent that they can cause explosion of the ampoules. To avoid any accidents, the sites inside the microwave oven with maximum radiation intensity should be determined beforehand. Exposure to microwaves and temperature gradients are static, resulting in the growth of well-oriented and aligned MWCNT arrays. Bekarevich et al. [101] have successfully grown MWCNTs and few-layer graphene sheets on Si and polyimide substrates at relatively low temperatures of 230°C
260°C by the microwave-excited surface wave plasma technique. Graphite-encapsulated Ni nanoparticles have been used as the catalyst for growing CNMs in NH3/CH4 plasma. It has been found that bias voltage and the nature of the substrate can greatly influence the CNM structure. Konno et al. [102] reported that CNTs can be directly prepared by microwave plasma decomposition of methane over Fe/Si activated by biased hydrogen plasma. In this research, using Fe/Si catalyst, methane was decomposed to H2 and CNTs by microwave. After the hydrogen plasma treatment, both methane and hydrogen gas mixtures (ration 1:4) were flowing into the reactor, exposed to power of 500 W for 30 min reaction time at a temperature of 600°C. The resulting CNT diameter reached a maximum value of 22.5 nm at a treatment time of 20 min and a minimum value of 9.8 nm at a treatment time of 30 min. The maximum height of the MWCNTs increased with increasing plasma treatment time, and reached 30.7 nm at 10 min.

5.6 FLUIDIZED BED CHEMICAL VAPOR DEPOSITION CNTs are crystalline, tubular, carbon structures with extraordinary mechanical, chemical, optical, and electrical properties. These unique properties make CNTs potentially valuable in a wide range of end-use applications. Currently, research into nanotubes and their applications is hampered by the lack of a suitable technique for manufacturing them in large quantities, which we define here as 10,000 tons per plant per year. Consequently, research into large-scale manufacturing techniques is on-going. There are three established methods of CNT synthesis: (1) arc discharge, (2) laser ablation, and (3) CVD. Among these, CVD techniques show the greatest promise for economically viable, large-scale synthesis, based upon yields reported in the literature and the inherent scalability of

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similar technologies, for example, fluidized CVD. In particular, the fluidized bed CVD technique (where the CVD reaction occurs within a fluidized bed of catalyst particles) has the potential to produce highquality CNTs, inexpensively, and in large quantities [37]. Fluidized bed technology has been recognized as an efficient technique to perform a gas
solid reaction and it has been employed in a wide range of industrial applications. Coupled with CVD, gas
solid fluidization has great potential to modify the surface properties of particles or to create new materials [103
105]. A schematic of a fluidized bed CVD is shown in Fig. 5.3. Many powder-based applications, for example, electronics, semiconductors, and drug delivery, have investigated the use of nanosized powders to improve overall reaction efficiency [106]. However, the fluidization of ultrafine particles (,1 μm) is considerably more complicated than for larger ( . 100 μm) particles, as particles in the nanometer range exhibit remarkably different characteristics to their bulk counterparts. Their high surface area and mobility lead to significantly increased cohesive forces between particles. As a result, nanoparticles tend to form agglomerates in order to minimize their overall surface energy. Generally, nanoparticle fluidization is achieved through the formation of such agglomerates. Nanoparticle agglomerates are fractal structures with a complex formation mechanism and have the characteristic of decreasing bulk density with increasing agglomerate size [107]. Observed agglomerates with an equilibrium hydrodynamic diameter between 230 and 331 μm during fluidization of 7
16 nm silica powders. The fluidization of these agglomerates occurred essentially without bubbles but with very large bed expansion [108]. In

Figure 5.3 Schematic diagram of the fluidized bed CVD reactor [105].

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many circumstances, these complex agglomerates may be considered as discrete fluidized agglomerates, although the fluidization mechanism is more complex than that for discrete particles. Thus, classical fluidization theory holds to an extent in nanoparticle fluidization. In addition, external forces, for example, vibration, acoustic waves, or direct stirring, may be employed to break up nanoparticle agglomerates to assist in fluidization [109]. The formation of nanoparticle agglomerates was reported during CNT synthesis using fluidized bed CVD. Yao et al. [107] reported that the structures of the CNT agglomerates were in the form of “loose” agglomerates, while Hao et al. [110] reported an increase in agglomerate size with time during CNT synthesis, from 125 μm at 9 min to 497 μm at 195 min. The reported behaviors are consistent with the observations of Yao et al. [107] when researching silica nanoparticle fluidized beds. The formation of large and light porous nanoparticle agglomerates is fundamental to nanoparticle fluidization and, thus, to the scale-up process and synthesis of CNTs in fluidized beds [37]. An overview of different parameters to synthesize CNTs using fluidized bed CVD is listed in Table 5.1.

Table 5.1 Summary of parameters for synthesis of CNTs using fluidized bed CVD Catalyst type

Catalyst loading

Substrate type

Carbon source

CNT type

References

Fe Fe Fe Fe

N/A 1 1 0.5

Al2O3 Al2O3 Al2O3 Al2O3

Ethylene Acetylene Acetylene Acetylene

[111] [112] [112] [112]

Ni Co Fe Ni Ni Fe
Mo Fe Fe Fe Co
Mo Ni
Cu Fe

1.2 1.2 N/A 60 60 N/A 2.5 2 2.5 1/55:4/55 15/20:3/20 5

Al2O3 Al2O3 Al2O3 Silica gel Silica gel Al2O3 Al2O3 Silica Silica Al2O3 Al2O3 mgo

Acetylene Acetylene Ethylene Methane Methane Propylene Ethylene Acetylene Acetylene Methane Methane Acetylene

Co

N/A

La-2o3

Acetylene Methane

MWCNT MWCNT MWCNT SWCNT/ MWCNT MWCNT MWCNT MWCNT MWCNT MWCNT MWCNT MWCNT MWCNT MWCNT MWCNT MWCNT SWCNT/ MWCNT MWCNT

[112] [112] [42] [113] [113] [110] [114] [43] [115] [116] [116] [117] [118]

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5.7 VAPOR PHASE GROWTH CHEMICAL VAPOR DEPOSITION In order to enhance the production of CNTs, the growth mechanisms involved in their formation must be completely understood, there are a number of theories that have been suggested to describe this growth. There are two mechanisms of CNTs using the CVD technique: tip growth and base growth. In the base growth mechanism, the catalysts stay on the substrate throughout the growth process [119]. Two possible reasons limit the growth of nanotubes. One is the termination of nanotube growth because of the strong van der Waals interaction between the nanotubes and the substrate surface when the nanotubes reach a certain length. For the base growth mechanism, since the whole nanotube needs to slide on the surface, once they rest on the surface, the nanotube
substrate interaction would increase as a function of the length. The growth would eventually stop when the force needed to move the whole nanotube becomes energetically unfavorable. For the tip-growth mechanism, this would not present a problem since the catalysts were on the tip of the nanotubes. The other reason for the length difference between the two growth methods may be diffusion of the feeding gas to the surface of the catalysts. The flow rate of feeding gas on the substrate surface is much lower than above the surface [119]. One mechanism has been proposed by Sinnott et al. [120], who describe the development of CNTs in the presence of a metal catalyst, where carbon diffuses into nanometer-scale catalytic particles. When the solubility limit within the metal is reached, this carbon precipitates outwith the graphitic structure. Depending on the size of the catalyst particle, graphite, carbon filaments, or CNTs can form (in order of decreasing particle size and increasing particle curvature). Other studies [121,122] have described that, as more carbon is deposited or diffused over the surface of the particle and becomes incorporated into the graphitic lattice, the tube’s length is increased. During the formation of carbon fibers or nanotubes, the original catalyst particles will either remain fixed to the substrate (root growth) or detach from the surface and remain encapsulated within the opposite end (tip growth). The vapor phase growth of CNTs in the presence of metal catalysts has been reported by Zhang et al. [123], who described that catalyst particles initially rise with the growth of the CNTs, however friction between the tube walls and particles causes the particles to become fixed in place. Growth of CNTs slowly occurs until another catalyst particle is deposited on the

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Figure 5.4 Proposed continuous rapid growth model of aligned CNTs. (A) Slow growth state. The tip of the nanotube is open. (B) Start of the rapid growth stage. An Fe catalyst particle falls on the tip. (C) Rapid growth state. The particle is deformed due to squeezing of the tip, and a new particle may fall on it. (D) End of the rapid growth stage (that is, the start of the slow growth stage).

surface; they also proposed rapid growth as shown in Fig. 5.4, due to opening of nanotube tips. Deck and Vecchio [124] studied the growth mechanism of vapor phase CVD through a spray pyrolysis process in the presence of metal catalysts and found that CNTs grow by condensation of particles in both the middle and end of tubes, with enormous diameter to length ratios.

5.8 MICROWAVE-ASSISTED SYNTHESIS OF GRAPHENE Carbon has a remarkably unique electronic structure, and has a number of forms. Due to the availability of different hybridization forms of atoms, there are many allotropies of carbon present in simple substances like graphite, carbon fiber, carbon aerogel, etc. Carbon allotropies possess extraordinary chemical and physical strength due to diverse crystal structures. The 3D crystal structures of carbon, graphite, and diamond (natural ores) are well-known [125]. In 1947, Philip Wallace, in a research paper,

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aroused the interest of researchers in studying in-depth the strength of graphene [126]. However, Kostya Novoselvo and Andre Geim in 2004 were the first to prepare graphene via mechanical cleavage at the University of Manchester [127]. Graphene was well described by the International Union of Purity and Applied Chemistry as a “mono-carbon layer of graphite structure, expressing its nature from analogy into polycyclic aromatic hydrocarbon (holds more than two un-substituted fused rings of benzene) of quasi endless size” [128]. Graphene molecules are referred to as outsized polycyclic aromatic hydrocarbons, ranging from 1 to 5 nm in size, and nanographene with 1
100 nm range. In reality, graphene is referred to as a 2D, sp2 carbon atoms and hexagonal honeycomb structure. Carbon (sp2) hexagonal network with a size greater than 100 nm is categorized as graphene. Graphene nanoribbons, a type of graphene, have less than 100 nm width, whereas graphene quantum dots are close to 100 nm in size [129]. Despite varying in size, all graphenes hold similar composition and structure. Like CNTs, graphene is also categorized as mono-, double- and multilayer with the forces like van der Waals among the layers attached. Similar to CNTs, graphene is also gaining attention for various applications including treatment of wastewater due to its excellent chemical durability, significant specific surface area, attachment of functional groups on its surface, and improved active sites [130].

5.9 FUTURE PROSPECTS FOR CARBON NANOMATERIAL SYNTHESIS AND CHALLENGES The motivation for the synthesis of carbon nanomaterials with distinguished characteristics and outstanding features is drawing great attentions from scholars due to their remarkable applications in various research fields including drug delivery, energy storage, composite designing, optical application, carbon dioxide capture, nanoelectronics, biosensing, environmental remediation, hydrogen storage, and catalytic activities. Utilization of microwave technology in various production techniques to synthesize exclusive carbon nanostructures including graphene, MWCNTs, SWCNTs, fullerene, and other controlled morphologies has made the process more convenient, cost-effective, and with fast kinetics that was not evident earlier. Microwave-based methods have boosted the overall performance in the synthesis phase which can be an integral part of the heat transfer system in other thermochemical processes. Furthermore, it also opens new prospects to fabricate novel carbon nanomaterials of

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higher specific properties that could be utilized in various new areas in the future. However, there are still many barriers and trials that need to be overcome for their successful implementation. There is always a question as to know the ideal feedstock which is economical, flexible, and environmentally friendly in nature. One interesting approach may be utilizing the waste gases from the environment to produce green sustainable products. One challenge may be to deal with the catalyst utilization and providing the highest possible recovery of the costly catalyst or maybe looking for a more economical and high-performance catalyst to improve the overall efficiency. Presently, these carbon nanomaterials are solely limited to the laboratory scale or batch scale. Therefore, there is a higher requirement at the moment to set up industrial-scale production and in this case many issues can arise, including scale up complications, cost effectiveness, maintaining efficiency, and introducing new products to the market. In this way, both technical and managerial issues can be faced by the industrialists to promote these new products in substitution of conventional materials. On the other hand, for the production of unique carbon nanostructures, specific improvement in efficiency in the field of application may be responsible for its adoption and acceptability by consumers. One more issue that is relevant with nanolevel products is their safety and proper handling, and on this specific issue the awareness level is quite low and more efforts may be required in this area. Specifically, the effects of disposal of carbon nanomaterials in the environment should be discussed in detail. Also, it lacks a proper testing standard system which requires more in-depth work in order to characterize various carbon nanomaterials in the lab. Producing unique carbon nanomaterials of high stability, unique morphology, controlled structure, higher surface area, and multifunctional characteristics can play an important role and prove to be an innovation for different fields.

5.10 CONCLUSION Microwave-assisted growth and ambient reaction conditions lower the cost, and simplify the procedure leading to a high yield of high-quality carbon nanomaterials with minimal impurities. Unlike conventional heating, microwave heating has a higher heating rate which results from the intrinsic transition of electromagnetic energy to thermal energy by a

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molecular interaction with the electromagnetic field, rather than heat transfer by conduction or convection. The development of new techniques for the efficient and selective synthesis of carbon nanomaterials via microwave technology at the cheapest possible cost is of current general interest. Carbon nanomaterials are in the limelight globally as a new dream material for the 21st century and are broadening their applications to almost all scientific areas, such as wastewater treatment, materials industry, etc.

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CHAPTER 6

Strategies in Laser-Induced Synthesis of Nanomaterials V. Saikiran1,3, Mudasir H. Dar2,3, R. Kuladeep3, L. Jyothi3 and D. Narayana Rao3 1 Department of Electronics and Physics, Institute of Science, GITAM, Visakhapatnam, India Department of Physics, Govt. Degree College, Anantnag, India School of Physics, University of Hyderabad, Hyderabad, India

2 3

Contents 6.1 Introduction 6.1.1 Long- and Short-Pulse Interaction 6.1.2 Femtosecond Laser
Solid Interactions: Fundamental Processes 6.1.3 Structural Changes Induced by Ultrashort Laser Pulses 6.2 Experimental Fabrication 6.2.1 Synthesis Methods 6.2.2 Characterization Methods 6.2.3 Nonlinear Optical Properties (Z-Scan Studies) 6.3 Results and Discussion 6.3.1 Surface Nanostructuring on Graphite 6.3.2 Formation of Graphene Quantum Dots by Laser Irradiation of Graphite in Water 6.3.3 Ge Nanoparticles by Pulsed Laser Ablation of Ge in Different Liquids 6.3.4 Surface Nanostructuring on Au Film-Coated Si Substrates for Surface-Enhanced Raman Scattering Studies 6.3.5 Blue Luminescent Si Nanoparticle Synthesis by Laser Ablation of Si in Water 6.3.6 Nanostructuring of Titanium Metal Towards Fabrication of Low-Reflective Surfaces 6.3.7 Formation of Metal Nanostructures in a Polymer Matrix by Using Femtosecond Laser Irradiation 6.3.8 Aluminum Nanoparticles Prepared by Laser Ablation in Different Liquids for Application as a Potential Optical Limiter 6.3.9 Metal Nanoparticles in Liquids (Mo, Ti, In, Au
Ag Alloy, and Al) 6.4 Conclusions References

Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00006-7

© 2019 Elsevier Inc. All rights reserved.

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6.1 INTRODUCTION Since the invention of the laser in 1960 [1], it has attracted enormous research interests, whereas initially it was called “a solution looking for a problem,” because of its uniqueness and many possible applications such as spectroscopy, medicine, biology, and material processing, etc. With the further discovery of pulsed lasers, the applications of lasers haves gained more attention in many different fields. Nanomaterial synthesis, which involves the use of lasers, is one such important field where more advances have been achieved recently. Many researchers have focused on nanomaterial synthesis with the use of lasers by using laser ablation method in different environments such as gas and liquid [2
6]. This laser ablation in gaseous medium involves the synthesis of thin-film deposition in vacuum chambers by laser ablation of target materials, whereas laser ablation in liquids is a simple, single-step, and direct method of synthesis of nanoparticles. Laser ablation in liquids also involves the difficulty of focusing the laser on the solid target because focusing of the laser exactly onto the material surface in a confined liquid is more difficult than that in a vacuum or gaseous medium [7,8]. During the last decade there have been increasing efforts in the synthesis of nanomaterials by laser ablation in liquids because of the availability of high-end cameras to observe the focusing of the laser on the sample surface [9
15]. The nanomaterials are very important as they differ from their bulk counterparts in exhibiting different novel characteristics. The nanomaterials are involved in various fields at various levels of applications such as in biomedicine, sensors, drug delivery, solar cells, photonic devices, etc. [16
24]. In this chapter a detailed description of nanomaterial synthesis and processing using lasers, which covers the basic mechanism of synthesis of various nanostructures and nanoparticles by laser writing/ablation method has been presented along with their applications in surfaceenhanced Raman scattering (SERS), optical limiting, and photonic devices. The synthesis of different nanomaterials involves dependence on different laser parameters such as laser pulse energy, pulse duration, wavelength, laser fluence, pulse repetition rate, etc. [25
30]. Apart from these parameters, the surrounding dielectric medium (air or liquid) where the laser interaction with matter is taking place also affects the synthesis of nanomaterials by lasers [31
34]. The ultrashort-pulse lasers opened new frontiers in ultrafast optics and motivated several research groups across the globe [35
37]. Ultrashort pulse lasers have the following advantages compared to

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conventional lasers [38
40]. First, the ultrashort-pulse duration enables a measurement with extremely short temporal resolution on a femtosecond (fs) scale. Dynamic studies of chemical reactions in gases and charge carriers in semiconductors have been carried out using ultrashort pulse lasers [41,42]. Ultrafast laser pulses can create high nonequilibrium conditions. Because the response of ionic cores is much slower than valence electrons, a fs laser pulse can excite valence electrons to a very high temperature while the ions are still cold. Ultrashort pulses, when focused, can produce extremely high intensity, which enables new frontiers in science and technology of light
matter interactions such as ultrafast X-ray generation and filament propagation in air to supercontinuum generation and ultrafast laser material processing [43
47]. For most of the available commercial fs laser systems, the intensity at the focus can reach more than 1013 W/cm2 when the laser is tightly focused inside a material. As a result, nearly all the materials, including wide band-gap materials, can be easily ionized through nonlinear absorption, resulting in optically induced breakdown. Ultrafast lasers with extremely short pulse duration (and extremely wide spectral bandwidth) are used in the fabrication of integrated photonic communications and signal processing systems inside transparent materials. A new approach for local modification of transparent materials through nonlinear optical processes has been investigated due to the extraordinarily high peak powers of short-duration laser pulses. Recent demonstrations of three-dimensional (3D) micromachining of glass and polymers using ultrafast laser pulses include the fabrication of waveguides, couplers, gratings, binary data storage devices, lenses, and channels [48
55]. The most important feature of this microfabrication technique is its ability to integrate 3D optical or photonic devices inside transparent materials by sequential direct-writing of individual devices. Although such a sequential approach is slow in comparison with conventional lithography, the new capability for 3D integration is priceless, in that it is quite difficult to achieve by other methods. Another important technique that utilizes ultrashort laser pulses is the two-photon polymerization technique [38]. In this process, fs laser pulses initiate two photon absorption (TPA) and subsequent polymerization in the localized focal volume, thereby resulting in the formation of 3D nanoscale structures. Through nonlinear field ionization, the intensity in the focal volume can become high to initiate absorption, when an intense fs laser pulse is tightly focused inside the bulk of a transparent material. This nonlinear absorption results in the creation of electron-ion plasma that is localized

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to the focal volume. As the plasma recombines and its energy is dissipated, permanent structural changes can be induced in the material. Because the nonlinear absorption allows energy to be deposited into the bulk of a transparent material, these structural changes can be produced inside the sample without affecting the surface, allowing 3D structures to be fabricated by translating the laser focus through the sample. Fig. 6.1 shows various mechanisms that occur when an intense fs laser pulse interacts with a transparent dielectric [56]. Fig. 6.1A shows the interaction of a fs pulse inside a transparent dielectric. As the fs laser pulse is focused we have nonlinear absorption of laser energy playing a major role. There are two types of interactions that take place as shown in Fig. 6.1B. The first is field ionization which can be either tunneling ionization or multiphoton ionization, where seed electrons are produced in the conduction band which undergo avalanche ionization. Thus, the generated hot electron-ion plasma transfers the energy to the lattice as shown in Fig. 6.1C. There are three major phenomena reported in the literature when focused intense fs pulses interact with transparent dielectrics, which are summarized in

Figure 6.1 (A) Focusing a fs laser pulse inside a transparent dielectric with microscope objective. (B) Nonlinear absorption of laser energy which is the cause of modification inside the material. (C) The resultant hot electron-ion plasma transfers the energy to the lattice after nonlinear absorption of laser energy. (D) Three basic permanent material changes reported in the literature after the modification. Reproduced with permission from K. Itoh, W. Watanabe, S. Nolte, C.B. Schaffer, Ultrafast processes for bulk modification of transparent materials, MRS Bull. 31 (2006) 620
625. Copyright (2006), Cambridge University Press. All rights reserved.

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Fig. 6.1D. When the energy of fs laser pulses is low, it results in refractive index change of the material which is called the isotropic refractive index change (Δn) mechanism. At moderate energies, it leads to formation of a nanograting, called birefringent refractive index change mechanism. Still at higher energies, it results in void formation. Due to higher intensities associated with these ultrashort laser pulses, they find applications in material processing. The laser material processing is based on the local thermal action and subsequent ablation that is the removal of the material caused by its melting, evaporation, and shearing as resultant phenomena of the interaction between the fs pulse and the material. UV excimer lasers are successfully used for micromachining as they have lower wavelengths, which lead to minimum obtainable focal spots. However, the disadvantage is that most of the transparent materials absorb this radiation and hence the surface of the materials is modified.

6.1.1 Long- and Short-Pulse Interaction If the pulse duration is longer, typically of the order of ns, compared to the relaxation processes, then the whole system will be in equilibrium during the interaction process. For shorter pulses, pulse width is much shorter than the energy relaxation time. Absorption occurs on a time scale that is short compared to the time scale for energy transfer to the lattice, decoupling the absorption and lattice heating process. At the end of the laser pulse, we are essentially left with hot electrons and a cold lattice. Moreover, during the laser material processing, the absorbed pulse energy can transfer only to the lattice on the order of 10 ps. As a result, in contrast to the material modification using nanosecond or longer laser pulses where the processing is dominated by the thermal effect, for the fs laser interactions with the materials, a very clean modified region with minimum collateral damage and heat-affected zone can be reduced, making it a promising method.

6.1.2 Femtosecond Laser
Solid Interactions: Fundamental Processes The primary interaction of laser pulses with a solid is the excitation of electrons. By the absorption of photons, electrons are excited into higherlying unoccupied states. Upon laser irradiation, three basic processes of optical excitation in a solid are shown in Fig. 6.2. The electrons can undergo transitions from valance band to conduction band (interband

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Figure 6.2 Schematic of electronic excitation under irradiation of fs laser pulses: (A) single photon excitation, (B) multiphoton excitation, (C) free carrier-absorption, (D) impact ionization.

transitions) through (1) single-photon excitation (linear absorption), (2) multiple-photon excitation (nonlinear absorption), or (3) intraband transitions (transitions within the conduction band) by free-carrier excitation. Single-photon valance band to conduction band excitation is the primary process, whereas the other two excitations become increasingly significant with increasing laser intensities. If the photon energy is more than the bandgap energy, laser light can be absorbed by the solid directly and the electrons are excited from the valance band to the conduction band. Fig. 6.2A illustrates the excitation of the valance band electron to the conduction band through single-photon absorption. Material ablation can take place under the deposition of sufficient energy into the surface through linear absorption. Absorption of a single photon cannot excite an electron from the valence band to the conduction, in the materials which are transparent to the laser wavelength. Only through nonlinear absorption of photons is it possible to modify the material. Multiphoton ionization and avalanche ionization are two classes of nonlinear excitation mechanisms that play a role in nonlinear absorption. Under higher laser frequencies multiphoton ionization takes place by simultaneous absorption of multiple photons by an electron, as illustrated in Fig. 6.2B. Avalanche ionization involves free-carrier absorption followed by impact ionization. An electron already in the conduction band can absorb a photon to reach higher excited states in the conduction band and while coming down can transfer the energy to two or more electrons in the valence band through collisions, thereby leading to a large number of

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electrons in the conduction band, as illustrated in Fig. 6.2C. After the sequential absorption of n photons, where n is the smallest number that satisfies the relation nħω . Eg, the energy of the electron exceeds the conduction band minimum by more than the band-gap energy. The electron can then collisionally ionize another electron from the valence band, as illustrated in the right-hand panel of Fig. 6.2. As a result of collisional ionization two electrons present near the conduction band minimum, each of which can absorb energy by free-carrier absorption and subsequently impact, ionize additional valence band electrons. This process increases the number of free carriers in the conduction band. Multiphoton ionization provides the seed electrons essential for avalanche ionization to take place. For fs laser pulses, photoionization takes place during the leading edge of the pulse and avalanche ionization during the trailing edge of the pulse [57]. If the laser intensity is high enough, multiphoton absorption and impact ionization can lead to optical breakdown, which produces plasma [58]. Mechanisms leading to surface and structural modifications through longer duration laser pulses, such as a few tens of picoseconds, are thermal in nature because of the subpicosecond nonradiative relaxations. In contrast, high-intensity ultrashort laser pulses give rise to physical and chemical processes which are considerably different from thermal processes. Nonthermal pathways can be accessed that occur on a timescale shorter than a picosecond, much before thermal processes kick in. The interaction of fs laser pulses with solids differs from the interaction of continuouswave light or longer pulses in two ways. First, the energy deposition occurs on a timescale that is short compared to any relaxation process. The laser energy is absorbed by the electrons, leaving the ions cold, and only after the laser pulse is gone does thermalization take place. Second, the intensity of a fs pulse, even with very moderate energy, is high enough to drive highly nonlinear absorption processes in solids that do not normally absorb at the laser wavelength. There are two types of photoionization, namely multiphoton and tunneling ionization regimes explained by the Keldysh parameter [59]. The Keldysh parameter (γ) which provides information about the dominant mechanism, is defined as   ω mcnε0 Eg 1=2 γ5 ; e I where m and e are reduced mass and charge of the electron, respectively, I is the intensity, ω is the laser frequency, ε0 is the permittivity of

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free space, Eg is the band gap of the material, n is the refractive index of the material, and c is the velocity of light. Multiphoton ionization is dominant in the case of γ . 1.5, and tunneling ionization is favored in the case of γ , 1.5. If γ 5 1.5, then both ionizations take place [60
62].

6.1.3 Structural Changes Induced by Ultrashort Laser Pulses In the laser micromachining experiments inside the transparent material, nonlinear absorption takes place at the focal position owing to the high intensities resulting in the structural modifications without affecting the surface of the material. Hence, by translating the sample in desired directions across the laser focus one can fabricate 3D structures. Three different structural changes under laser material processing reported are birefringent refractive index change, isotropic refractive index change, and void formation. Voids are formed under the irradiation of laser pulses with higher pulse energies, typically .500 nJ, and have been attributed due to explosive expansion of material out of the focal position, called microexplosion. Birefringent refractive index changes occur by periodic nanometer-sized ripples with varying material composition and thereby density is formed in the laser irradiated volume. These nanogratings are formed as a result of interference phenomena that lead to periodic modulations in the electron plasma density. The formation of periodic surface nanostructures or nanogratings has been observed on a variety of materials including semiconductors [63
68], metals [69
76], and dielectrics [77
80]. Laserinduced micro/nanostructuring is recognized as an interesting tool to modify the properties of the material surfaces, providing a wide range of applications such as enhanced optical absorption, SERS substrates, hydrophobic, and biocompatible materials. In this chapter, studies of the laser irradiation effects on semiconductors and metals where the laser beam is focused on the surface of the material in the surrounding of different media such as air, liquid, etc. have been discussed. Laser-induced periodic surface structures (LIPSS) were first observed by Birnbaum on semiconductor surfaces [81]. With the potential applications of fs laser micro/nanoscale material processing, these studies of the formation of periodic nanostructures and nanomaterials by laser irradiation has received much attention since 2000. The formation of two types of LIPSS is observed: high spatial frequency LIPSS (HSFL) and low spatial frequency LIPSS (LSFL), in which the fundamental physics of formation mechanism are under discussion [82
87].

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The mechanism of the HSFL formation is not yet well understood, while the formation of LSFL is explained by the interference of the incident laser radiation and the surface electromagnetic wave originating from surface roughness on the substrate surfaces for semiconductors and metals with high linear and/or nonlinear absorption coefficients at the incident wavelength [88]. The formation of LSFL is explained as follows: long-range surface plasmon polaritons (SPPs) are excited on the surface roughness generated by the irradiation of fs laser pulses, leading to the interference of the SPPs and irradiated fs laser pulses. Surface plasmons are collective longitudinal oscillations of electrons propagating along a metal
dielectric interface at optical frequencies and it is well known that a surface plasmon is necessarily excited by a transverse magnetic (TM) polarized wave [89
92]. Due to the effective coupling of the incident electromagnetic radiation with the plasmon oscillation, a significant enhancement of the field in the vicinity of the structure can be produced, explaining the polarization dependence of ripples. The spatial periodicity of LSFL is determined by the wavelength of SPPs and irradiated laser wavelength, depending on both the dielectric permittivity of the substrates material and the wavelength of the incident laser. The wavelength of SPPs is the same as or slightly less than the incident laser wavelength. Theoretically and experimentally spatial periodicity of LSFL features are studied by considering the change in free electron density Ne on the fs laser-irradiated surface [93]. It is with this background that our investigations in the field of fs laser interaction with various materials began. The main objective of our work is to explore novel applications using a fs laser-direct writing technique and to gain further knowledge of the fundamental laser matter interactions. In this chapter the formation of subwavelength surface structures and nanoparticle formation of different materials like silicon, germanium, graphite, and metals like titanium, aluminium and indium using laser irradiation/ablation are presented in detail [94
100]. The laser beam was made to incident normal to the material surface in air and immersed in various solvents by focusing with a microscopic objective or focusing lens and scanning of the sample was done both along and normal to the laser polarization direction in transverse geometry. The morphology, orientation, and spatial periodicity of the subwavelength structures depend upon various parameters like laser pulse energy, number of pulses per focal spot, laser polarization, surrounding dielectric medium, and nature of the material. We also discuss about the synthesis of nanoparticles by laser ablation and the study of the optical properties of the nanoparticles.

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6.2 EXPERIMENTAL FABRICATION 6.2.1 Synthesis Methods A Ti:sapphire fs oscillator-amplifier system operating at a central wave length of 800 nm and delivering B110 fs pulse duration with a maximum energy of B1 mJ at a repetition rate of 1 kHz is used for the laser irradiation of the material surface. The laser energy has been controlled by a combination of a half-wave plate and a prism polarizer. The schematic of the experimental setup of fs laser writing is given in Fig. 6.3. The laser irradiation induced fabrication of periodic structures has been carried out in scanning mode by focusing fs laser pulses on the material surface in air and water environments. The experimental setup used for the fs laser machining consists of a light source, beam-delivering system, and the three-stage nanopositioner. The laser beam used was extracted from a Spectra Physics laser system comprising of a mode-locked Ti:sapphire oscillator (Mai Tai) and regenerative amplifier (Spitfire) seeded by the output of Mai Tai. The surface fabrication experiments are carried out using three nanopositioner stages purchased from Newport, USA, which are of 15 nm resolution. The laser polarization is controlled by using a half-wave plate and neutral density filters along with a combination of a half-wave

Figure 6.3 The experimental setup for femtosecond laser direct writing.

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plate and a polarizer which were utilized to adjust the irradiation fluence on the sample surface. The laser beam was incident normal to the sample surface and focused by a microscopic objective lens. In order to account for the reflection losses of the lens all the energy/fluence measurements were performed after the lens. A CCD camera with high magnification is used to control the focal depth on the sample surface and to monitor the writing process. Two different writing geometries are classified depending upon the translation direction of the nanopositioner stage, with respect to the propagation direction of the laser beam. In longitudinal writing geometry the stage is translated along the laser beam propagation direction, and in transverse writing geometry the stage is translated perpendicular to the laser beam propagation direction. Q-switched Nd:YAG pulsed laser delivering 10 Hz repetition rate pulses with 6 ns pulse width at 1064 nm fundamental wavelength was used for the laser ablation of Ge wafer. The cleaned germanium wafer was placed at the bottom of the beaker and filled with 10 mL of the respective liquid in which the Ge is ablated. After keeping the substrates perfectly parallel to the optical bench, the laser pulses were allowed to focus on the substrates with a plano-convex lens (focal length f 5 10 cm) for different times. Different pulse energies were used for the generation of NPs to see the effect of pulse energy on the formation of NPs. During the laser ablation, the target was moved using a rotation system to achieve more and uniform irradiation of the germanium surface. The laser was focused normally on the target for ablation. The ablated solution was then used for all the characterizations. Fig. 6.4 shows a schematic diagram of a typical experimental setup for pulsed laser ablation in liquids using a nanosecond laser. The setup basically consists of a pulsed laser, beam delivery optics, and a container to hold the target and liquid (acetone or water or others). The setup may be modified to control the ablation process, but the common features still exist, that is, a laser beam is focused onto a target immersed in liquid, and the ablated materials are dispersed into the liquid.

6.2.2 Characterization Methods The fabricated nanostructures on the surface of the bulk material and the nanoparticles in liquids have to be characterized by different methods to understand the formation kinetics and the morphological behavior of the nanostructures. Field emission scanning electron microscopy (FESEM)

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Figure 6.4 Experimental setup used for laser ablation in liquids using nanosecond laser.

and transmission electron microscopy (TEM) are important tools to understand the structural and morphological nature of the fabricated nanostructures. Raman scattering is another important tool to understand the nature of the nanostructures and the important application of the fabricated nanostructures related to SERS studies, so Raman scattering experiments have been performed to understand the formation of nanostructures and nanoparticles, the modified regions of the irradiated surfaces and the creation of defects due to irradiation. Apart from these important characterization tools the results are discussed based on UV visible absorption spectroscopy, photoluminescence (PL) emission studies, etc. for the different synthesized nanomaterials by laser-induced methods.

6.2.3 Nonlinear Optical Properties (Z-Scan Studies) Nonlinear absorption and scattering studies are another important study method which are carried out using the open aperture Z-scan technique [101]. A modified Z-scan experimental setup, which our group proposed earlier [102,103], was used to collect both nonlinear absorption and nonlinear scattering at different forward angles simultaneously, as presented in Fig. 6.5. In a typical Z-scan experimental setup, a laser beam with a transverse Gaussian profile is focused using a lens. The sample (colloidal solutions were taken in a 1-mm cuvette) is then moved along the propagation direction of the focused beam. At the focal point, the sample experiences maximum pump intensity, which gradually decreases in either direction from the focus. An f/24 configuration is used for the present studies. A frequency-doubled Nd:YAG (yttrium aluminum garnet) laser (SpectraPhysics, INDI 40, 532 nm, 6 ns, 10 Hz) is used as the excitation source.

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Figure 6.5 Schematic of the Z scan set up for recording the nonlinear absorption and scattering. A, aperture; S, sample; F1, F2, F3, F4, neutral density filters; D1, D2, D3, detectors; BS, beam splitter; L1, L2, L3, L4, lens. Reprinted from R. Kuladeep, L. Jyothi, P. Prakash, S. Mayank Shekhar, M. Durga Prasad, D. Narayana Rao, Investigation of optical limiting properties of aluminium nanoparticles prepared by pulsed laser ablation in different carrier media, J. Appl. Phys. 114 (2013) 243101, doi:10.1063/1.4852976, with permission of AIP Publishing. All rights reserved.

Apertures are introduced in the path for beam shaping and calibrated neutral density filters are used to vary the laser intensity. The values of beam waist at focus are B20
30 μm and the corresponding peak intensities are B108
109 W/cm2. A 50
50 beamsplitter introduced immediately after the sample collects the transmitted light that includes scattered light. This reflected beam is focused onto detector 1 using a large-area lens. Detector 1, therefore, sees only the losses due to linear and nonlinear absorptions of the sample. The other half of the transmitted beam after the beamsplitter is collected with a small area lens at the far field to reduce the scattered light falling on detector 2. Hence, detector 2 accounts for the absorptive as well as scattering losses. Fig. 6.5 shows the experimental setup for the Z-scan experiment. The sample cell, beamsplitter, and detector 1, along with the collection lens L3 are mounted on a translation stage. The scattering at different forward-scattering angles with beam propagation direction is collected using detector 3. The data are recorded by scanning the cell across the focus, and the transmitted beam is focused onto the photodiode (FND-100) with a lens. A boxcar averager (model SR250) is used for signal averaging, the output of which is given to a computer with an analog-to-digital converter card. The cell is translated along the beam propagation direction using a computer-controlled stepper motor and the data are collected at steps of 0.5 mm.

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6.3 RESULTS AND DISCUSSION 6.3.1 Surface Nanostructuring on Graphite Fig. 6.6 shows the morphology of graphite surface, evolved after nanostructuring at 800 nm wavelength, N 5 3000, and F 5 1.33 J/cm2. In Fig. 6.6A, incident laser polarization is parallel to the writing direction, Fig. 6.6B shows the incident polarization perpendicular to the writing direction. One interesting feature we observed on the graphite surface with fs laser irradiation under high pulse number is the formation of quasiperiodic nanogratings with a variable grating period as indicated in Fig. 6.6C. The development of subwavelength periodic surface structures on different types of materials under fs laser irradiation occurs primarily due to the interference between incident laser light with that of the excited SPP waves from the material surface. Under the influence of fs laser irradiation, the dispersion relation that represents the SPP wavenumber Kspp is modified due to the presence of the temporary electronic states which possess huge electron density and a high temperature [72,90,100], which is given as,  rffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 εm εd Kspp 5 Re λ εm 1 εd where εm and εd , which are given as εm 5 ε1 1 iε2 and εd 5 ɳ1 1 iɳ2 , respectively, denote the complex dielectric constant values of the material and the surrounding dielectric medium (air or water), wherein E1 and E2 are the real and imaginary parts of the material, and ɳ1 and ɳ2 are the real and imaginary parts of the surrounding dielectric medium (air and water).

Figure 6.6 FESEM image showing the polarization dependence of nanograting formation on the irradiated graphite surface, F 5 1.33 J/cm2 and N 5 3000. Incident laser polarization: (A) along the writing direction, (B) perpendicular to the writing direction, (C) shows the formation of quasi-periodic nanogratings with variable periodicities within the irradiated region. Reprinted from V. Saikiran, M.H. Dar, D.N. Rao, Femtosecond laser induced nanostructuring of graphite for the fabrication of quasiperiodic nanogratings and novel carbon nanostructures, Appl. Surf. Sci. 428 (2018) 177
185, doi:10.1016/j.apsusc.2017.09.126, Copyright (2017), with permission from Elsevier. All rights reserved.

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After the incorporation of these terms the modified real part of Kspp is given by sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  h pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffii 1 1 Kspp 5 A 1 A2 1 B2 λ 2D where A, B, and D are given as

 A 5 ɳ 1 ε21 1 ε22 1 ε1 ɳ 21 1 ɳ 22 ;

 B 5 ɳ 2 ε21 1 ε22 1 ε2 ɳ 21 1 ɳ 22 ; and
2
2 D 5 ɳ 1 1ε1 1 ɳ 2 1ε2 The observed grating periods on the graphite surface vary from 130 to 230 nm. The structural periodicity of the grating periods has been studied with respect to the incident laser fluence, scanning speed, polarization, and surrounding dielectric medium (air and water).

6.3.2 Formation of Graphene Quantum Dots by Laser Irradiation of Graphite in Water When the fs laser irradiation has been done in water medium we have observed the formation of novel carbon nanostructures in the ablated solution in the form of colloids. The observed novel compounds include graphene quantum dots (GQDs) and other graphene-related nanocarbon products such as few-layered graphene, etc. depending on the incident laser fluence. During the laser irradiation process in water, the surface of graphite was evaporated by the incident laser and in turn produces a plasma plume on the surface of the graphite. The plasma plume expands and then condenses, thereby generating nanoparticles of graphite, nanographene, and GQDs. The properties of the generated nanostructures depend on laser irradiation parameters and the nature of the dielectric liquid medium surrounding graphite. Fig. 6.7A shows the absorption spectra of the colloidal solution of GQDs formed at 0.93 J/cm2 fluence. It shows two absorption bands at 260 and 400 nm. The peak at 260 nm is generally observed for smaller-sized GQDs observed in the solution, which is generally due to π
π transition of electrons of carbon domains in the sp2 molecular orbital and it matches well with the reported values [104]. The peak present at 400 nm may be due to other larger-sized structures

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Figure 6.7 Absorption spectrum of the GQDs and the strong blue luminescence from the GQDs in water. Reprinted from V. Saikiran, M.H. Dar, D.N. Rao, Femtosecond laser induced nanostructuring of graphite for the fabrication of quasi-periodic nanogratings and novel carbon nanostructures, Appl. Surf. Sci. 428 (2018) 177
185, doi:10.1016/ j.apsusc.2017.09.126, Copyright (2017), with permission from Elsevier. All rights reserved.

Figure 6.8 TEM micrograph shows the formation of graphene quantum dots and graphitic nanoflakes due to fs laser irradiation in water at a laser fluence of 1.8 J/ cm2. Reprinted from V. Saikiran, M.H. Dar, D.N. Rao, Femtosecond laser induced nanostructuring of graphite for the fabrication of quasi-periodic nanogratings and novel carbon nanostructures, Appl. Surf. Sci. 428 (2018) 177
185, doi:10.1016/j. apsusc.2017.09.126, Copyright (2017), with permission from Elsevier. All rights reserved.

formed in the ablated solution. The GQDs are reported to show strong blue luminescence properties and the PL emission spectra of the GQDs is shown in Fig. 6.7B. Here we have observed the strong blue luminescence from the GQDs and the PL emission can be tailored with the size of GQDs [105]. The reason for this emission from nano-sized carbon materials, which include GQDs and other nanocarbon structures, may be due to the optical selection of quantum sizes and defects present in the GQDs [106]. Apart from the blue luminescence centered at 412 nm, we have also observed a slightly weaker luminescence band at around 660 nm

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(red), which may be due to the larger-sized NPs formed in the irradiation process. In the TEM images also we have observed such larger grain-sized particles, which had an absorption band at 400 nm. TEM has been used to study the size and distribution of GQDs and to observe directly the formation of different nanocarbon products due to laser ablation of graphite in water. The colloidal solution was drop casted onto a copper grid to record the TEM images and the results are discussed below. Fig. 6.8 shows the TEM images of the GQDs and graphene flakes, fewlayered graphene along, with the formation of porous graphene. The average size was observed to be around 2
4 nm. These GQDs are observed to form over a graphitic layer and at some places they are embedded within it. So it is observed that the formation of GQDs and other graphene nanostructures clearly depend on the fs laser irradiation energy.

6.3.3 Ge Nanoparticles by Pulsed Laser Ablation of Ge in Different Liquids The optical and structural properties of the Ge nanoparticles synthesized by laser ablation in different liquids are discussed in this section [107]. Micro-Raman spectroscopy measurements were performed on all the samples synthesized in different liquids by evaporating NP solution on a cover glassslide and the observed spectra are shown in Fig. 6.9A. When we compare the spectra of Ge NPs formed in different liquids with that

Figure 6.9 (A) Micro-Raman spectra of Ge NPs formed in different liquids. (B) MicroRaman spectra of Ge NPs formed in toluene showing the graphitic carbon peaks. Reprinted with permission from S. Vadavalli, R. Kuladeep, M.H. Dar, D.N. Rao, Influence of solvent on the optical and structural properties of Germanium nanoparticles synthesized by nanosecond laser ablation in liquids, in: 12th International Conference on Fiber Optics and Photonics, OSA Technical Digest (online), 2014, paper T3A.64, Optical Society of America. ,https://doi.org/10.1364/PHOTONICS.2014.T3A.64..

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of the bulk crystalline Ge
Ge vibrational peak at 300 cm21 (symmetrical at the position), we observed that the Ge NP peak shows an increase in FWHM and a shift toward lower wavenumber side along with asymmetry. This is a consequence of the formation of various sizes of Ge NPs, leading to quantum confinement effects and changes in the structural characteristics of the Ge NPs which include strain and the amorphous nature of Ge NPs. The broadening and shift toward the lower wavenumber side can be related to the nanoparticle size using the phonon confinement model [108]. No peak is observed around 440 cm21 in any of the samples, which indicates that the GeO/GeO2 phase has not been formed in the NPs. Only the major peak corresponding to Ge
Ge optical phonon mode at 300 cm21 with lower shift and broadening is observed in the Raman spectra of the NPs in acetone and water, whereas for the NPs in toluene and chloroform we have seen a broad peak at 260 cm21, which indicates the amorphous nature of the NPs. Apart from the Ge
Ge vibrational peak we have also seen graphitic carbon related peaks at 1350 and 1485 cm21 (Fig. 6.9B) for the NPs in toluene. These peaks are due to the C
C bond vibrations which generally occur in graphiterelated compounds [109
111]. This indicates the formation of graphitic nanostructures due to laser ablation in toluene. Thus pure crystalline Ge NPs have been observed in acetone and water and amorphous and partially crystalline NPs with a graphitic carbon network are observed in toluene and chloroform. Fig. 6.10A and B shows the TEM images of Ge NPs formed in acetone and water. We have observed that the nanoparticles are well dispersed in acetone and water without any agglomeration. The number density of the NPs in these liquids is also greater in comparison to toluene and chloroform. These NPs are observed to be crystalline in nature, whereas the NPs formed in toluene and chloroform solvents show different results. Fig. 6.10C and D presents the NPs formed in toluene and chloroform. These NPs were observed to be formed over the edges of a carbon sheet/network. We have also observed the formation of amorphous graphitic sheets in the solvent. Fig. 6.11 shows high-resolution images of the NPs in acetone and the image of the graphitic carbon sheet formed in toluene. The NPs formed in toluene and chloroform show an amorphous nature. The crystalline planes of the Ge NPs in acetone can be clearly seen from the images, which indicate the crystalline nature of the NPs. In Raman spectra we have also observed the peaks due to graphitic carbon in the case of toluene and the same was confirmed from the

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Figure 6.10 TEM images of Ge NPs obtained by laser ablation of Ge wafer in: (A) acetone, (B) water, (C) toluene, and (D) chloroform. Reprinted with permission from S. Vadavalli, R. Kuladeep, M.H. Dar, D.N. Rao, Influence of solvent on the optical and structural properties of Germanium nanoparticles synthesized by nanosecond laser ablation in liquids, in: 12th International Conference on Fiber Optics and Photonics, OSA Technical Digest (online), 2014, paper T3A.64, Optical Society of America. ,https://doi. org/10.1364/PHOTONICS.2014.T3A.64..

HRTEM image of NPs in toluene. It is observed that the NPs formed in acetone are smaller in comparison to the other liquids and the number density of the NPs formed in acetone and water are greater in comparison with organic solvents, toluene and ChCl3. The optical absorption and emission properties of the Ge NPs in different liquids have been investigated by optical absorption (UV
Vis) and PL emission spectroscopy studies. Fig. 6.12A shows the optical absorption spectra of the Ge NPs in different liquids. It is observed that the spectra show an absorption band at around 330 nm, which is common for all the samples and which shifts with the surrounding environment, and additionally for the NPs in toluene we see an absorption band at 560 nm. This shift is due to the changes in the NP size and also the number density of NPs.

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Figure 6.11 High-resolution TEM image of Ge nanoparticles in acetone and highresolution image of the graphitic layer formed in toluene. Reprinted with permission from S. Vadavalli, R. Kuladeep, M.H. Dar, D.N. Rao, Influence of solvent on the optical and structural properties of Germanium nanoparticles synthesized by nanosecond laser ablation in liquids, in: 12th International Conference on Fiber Optics and Photonics, OSA Technical Digest (online), 2014, paper T3A.64, Optical Society of America. ,https://doi. org/10.1364/PHOTONICS.2014.T3A.64..

On the other hand, Fig. 6.12B presents the PL emission spectra of the Ge NP solutions in different liquids. The PL spectra appear broad and range from 380 to 650 nm, with different peak positions for different NP colloids. The NPs formed in acetone display a strong blue emission with peak emission at 420 nm, whereas the NPs formed in water also show a strong blue emission (peak at 420 nm) as well as green emission with a peak around 510 nm. The larger average size of the NPs formed in water may be the reason for the blue and green emission. The NPs formed in acetone show only blue emission related to their average particle size. This indicates that the PL peak emission is found to shift toward the lower wavelengths with the decrease in NP size, which is in relation to the quantum confinement effects. The colloidal Ge NPs prepared in toluene show blue emission with two peaks around 420 and 436 nm. This may be due to the strong contribution from Ge NPs formed as part of laser ablation and also due to the carbon/graphitic structures that formed during the ablation. Also, the NPs in toluene show strong green emission centered at 512 nm. The Ge NPs formed in ChCl3 show broad emission with different peak emission centers, which indicates that other carbon nanostructures which cover the formed Ge NPs also contribute to the broad emission with different central emission wavelengths. Therefore Ge NPs prepared by laser ablation in different liquids show different optical and structural properties the solvent has a lot of influence on the optical

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Figure 6.12 (A) UV
Visible absorption spectra and (B) photoluminescence spectra of the various Ge NPs formed in different liquids.

and structural properties. The surrounding liquid environment influences the optical emission form the Ge NPs, thereby we can tune the emission from NPs.

6.3.4 Surface Nanostructuring on Au Film-Coated Si Substrates for Surface-Enhanced Raman Scattering Studies Fig. 6.13 shows the FESEM images of the gold-coated Si surface in air after fs laser irradiation at a laser fluence of 0.27 J/cm2. The structures are observed to be perpendicular to the incident laser polarization and more uniform with fewer defects. The presence of gold on the Si before the formation of surface structures enhances the electron
phonon coupling during the fs laser interaction due to the diffusion of hot electrons in the gold film [112]. Thus, the electron density decreases the effect of defects on the formation of structures on Si, thereby forming the large area surface structures without defects and with uniform periodic structures. Three different thicknesses of gold thin films are used in the study: 5, 10, and 20 nm. The structures formed on 10 nm gold-coated Si are shown in Fig. 6.13C and D. It is observed that the gold films are forming a chaintype structure on the periodic surface structures formed on Si after laser irradiation. These crosslinking chains, which are on the structures, connect the adjacent surface structures. This has been increased with the gold film thickness and can be clearly seen in the fs LIPSS formed on 20 nm goldcoated Si, which are shown in Fig. 6.13E and F. For all the different thickness gold-coated samples the laser parameters such as fluence, incident polarization, and number of pulses were kept constant. The formation of LIPSS on gold-coated silicon has been generally explained by the

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Figure 6.13 FESEM images of (A) 5 nm gold-coated Si surface in air after irradiation with fs laser pulses with a laser fluence of 0.27 J/cm2. (B) Magnified image of (A). (C) and (D) 10 nm gold-coated Si, which represent perpendicular and parallel polarization to the writing direction. (E) Image of 20 nm gold-coated Si surface in air after irradiation with fs laser pulses with laser fluence of 0.27 J/cm2 and (F) enlarged image of (E), which shows the Au chain-type nanostructures. Laser polarization is indicated by the solid double-headed arrow and the direction is indicated with a dotted single-headed arrow. Reproduced from V. Saikiran, M.H. Dar, R. Kuladeep, N.R. Desai, Ultrafast laser induced subwavelength periodic surface structures on semiconductors/metals and application to SERS studies, MRS Adv. 1 (2016) 3317
3327, doi:10.1557/ adv.2016.468, with permission from Cambridge University Press. r2016 All rights reserved.

interference between the incident laser pulse and SPPs, and then the mechanism of grating-assisted SPP-laser coupling dominates the evolution of LIPSS. The fabricated structures have good applications in SERS. Si is not an active SERS substrate, but here, because of the gold film deposited on the Si, the structures are formed with a chain kind of linking and Au NPs on the fabricated surface structures. The presence of gold will enable these

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substrates to be suitable for use in SERS studies. The laser-induced periodic structures have more active sites for enhancement in the SERS signal of the attached molecule in that particular site. The formation of chain structures on the periodic structures with a combination of Au NPs will be more useful for SERS studies because the periodic structures with metallic NPs act as active sites for SERS. Aqueous solutions of crystal violet (CV) dye of different molar concentrations were prepared by a sequential dilution method in water. The substrates with periodic surface structures were incubated in 1 μM CV dye solution and then air dried for 1 hour at room temperature, which provides a uniform distribution of the CV molecules on the entire surface. Fig. 6.14A shows the SERS spectra recorded with a laser excitation wavelength of 633 nm and 1 μM concentration of CV dye on the fabricated surface structures. The 20 nm golddeposited substrates show better enhancement than the 5 and 10 nm gold-deposited substrates. The 20 nm gold substrates after the laser irradiation show a chain kind of Au/Si-linked nanostructure formation on the LIPSS formed due to laser irradiation, whereas these chain structures are not observed as often in the case of 5 and 10 nm gold-deposited substrates. The 5 nm gold film gets ablated and removed from the structured Si during the initial few pulses only, whereas in the 20 nm case it remains and forms some NPs of gold on the periodic structures. For comparison we have also recorded the SERS spectra on the annealed 20 nm Au on Si sample. This sample was prepared by thermal annealing of the as-deposited sample at a normal temperature of 400°C for the synthesis of Au NPs. We have observed a good distribution of Au NPs over the Si substrate due to annealing. The 1 μM CV dye is added to this substrate containing Au NPs and recorded the observed SERS spectrum is shown in Fig. 6.14C along with the spectrum (presented in Fig. 6.14B) of 1024 M CV dye on 20 nm Au-coated bare Si substrate (asdeposited). It is observed that the annealed sample gives an enhancement in comparison with the as-deposited sample. The analytical enhancement factor (AEF) was calculated using the following formula [113,114], AEF 5

ISERS =CSERS IRS =CRS

where ISERS represents the Raman intensity obtained for the SERS substrate under a certain concentration of CSERS, and IRS corresponds to the Raman intensity obtained under non-SERS conditions at a concentration

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Figure 6.14 (A) SERS spectra recorded with a laser excitation wavelength of 633 nm for 1 μM CV dye on the periodic surface structures formed with laser irradiation on 5, 10, and 20 nm Au-coated Si substrates. (B) SERS spectra of 1024 M CV dye on 20 nm Au-coated bare Si substrate and 1 μM CV dye on the annealed Au-coated Si. (C) Comparative SERS spectra of the 1 μM CV on the annealed 20 nm Au on Si substrate and LIPSS formed on 20 nm Au-coated Si. Reproduced from V. Saikiran, M.H. Dar, R. Kuladeep, N.R. Desai, Ultrafast laser induced subwavelength periodic surface structures on semiconductors/metals and application to SERS studies, MRS Adv. 1 (2016) 3317
3327, doi:10.1557/adv.2016.468, with permission from Cambridge University Press. r2016 All rights reserved.

of CRS. The C
H in-plane ring bending vibrations of the CV molecule present at B1170 cm21 have been taken as reference and the respective Raman intensities are used for calculating AEF. We have observed a high enhancement factor for fs laser-irradiated samples prepared with 20 nm Au film on Si. All the AEFs are the averages of the measurements at four or five different locations. The estimated AEF values are of the order B106.

6.3.5 Blue Luminescent Si Nanoparticle Synthesis by Laser Ablation of Si in Water Fig. 6.15 shows the FESEM image of the Si surface after laser irradiation in the presence of water at a laser fluence of 0.21 J/cm2. It is evident from the

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Figure 6.15 FESEM image of the Si surface after laser irradiation in the presence water at a laser fluence of 0.21 J/cm2.

Figure 6.16 TEM images of the Si NPs formed in water after laser irradiation of Si in the presence of water at a laser fluence of 0.21 J/cm2.

image that there is a formation of Si NPs at the irradiated position of the Si wafer. As observed from the image, discontinuous deep subwavelength surfaces also known as HSFL are formed on Si with linearly polarized fs pulses with a laser fluence of 0.21 J/cm2. The average period and width of these ripples along the laser polarization direction are B120 and B80 nm, respectively, which are about 6.7 and 10 times smaller, respectively, than the freespace wavelength of the incident laser wavelength. In this case, formation of ripples is observed to be always perpendicular to the laser polarization, irrespective of the sample scanning direction. These deep subwavelength structures appear like nanoparticles and the density of these particles is very high on the irradiated region of the surface. Fig. 6.16 confirms the presence of ultra-small spherical Si NPs that formed in water after laser irradiation of Si

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Figure 6.17 (A) Absorption spectra and (B) PL emission spectra of Si NPs formed in water after laser irradiation of Si in the presence of water with 0.21 J/cm2 fluence.

in water. The NPs are highly luminescent and they show a strong absorption around 270 nm, which is due to the great density of the Si NPs. The absorption spectra and the PL emission spectra of the Si NPs are presented in Fig. 6.17A and B, respectively. The Si NPs show a strong blue luminescence which ranges from 350 to 520 nm, with a peak maximum centered around 400 nm. With the shift in the excitation wavelength the PL peak maximum slightly shifts toward the higher wavelength side, which is due to quantum confinement-related effects. The blue luminescence from Si NPs is mainly due to the presence of ultra-small NPs [115].

6.3.6 Nanostructuring of Titanium Metal Towards Fabrication of Low-Reflective Surfaces The effect of surface nanostructuring on the reflectance of the Ti surface has been studied using fs laser-induced processing of the Ti surface under different conditions. To perform the reflection measurements, laser surface processing was carried out on a large area of 15 mm2. The reflectance of the processed area was measured by a UV
Vis
IR spectrometer. In order to understand the reduction in the reflective properties of the nanostructured surface, we have used FESEM to understand the morphology of the irradiated samples. Nanostructural features affect the reflectance of the surface since the optical properties of the nanostructured material are quite different from those of the bulk material [116]. The reduction in reflectance is related to the nanostructuring of the surface rather than the chemical modifications of the surface layer. We have observed the formation of periodic surface structures, nanoparticles, nanogrooves, and

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Figure 6.18 Reflection studies on the nanostructured surface at 5° incidence. By controlling the laser parameters, the titanium surface becomes absorbing over a wavelength range of 250
2000 nm. Images (B)
(E) are the respective FESEM images of nanostructured Ti surface relating to the conditions mentioned in the reflection spectra. Reprinted from M.H. Dar, R. Kuladeep, V. Saikiran, D. Narayana Rao, Femtosecond laser nanostructuring of titanium metal towards fabrication of low-reflective surfaces over broad wavelength range, Appl. Surf. Sci. 371 (2016) 479
487, doi:10.1016/j. apsusc.2016.03.008, r2016, with permission from Elsevier. All rights reserved.

nanocavities with different laser parameters. All these laser-induced structures contribute to the decrease in the reflectance. Surface roughness [117] can enhance the absorption of light by multiple scattering. Vorobyev and Guo [118] attributed the enhanced absorption to the combined effect of nanostructural and microstructural surface modifications. LIPSS also affects the reflectance of the surface. When we write the grating with p polarized fs laser pulses and measure the reflectance of both s and p polarized light, we observe that the reflectance of p polarized light is more than the reflectance of s polarized light, which implies that the reflectivity of the surface is modified due to the grating. Further, when the nanostructuring is carried out in the air environment, the ablated fragments are redeposited and solidify on the laser-processed region, which may also contribute to the decrease in reflectivity. Fig. 6.18 presents the percentage reflectance of the nanostructured Ti surface with different processing parameters such as fluence, scanning speed (pulse number), and surrounding environment. Curve A represents the reflectance of the mechanically polished sample. Curve B represents the reflectance of the sample irradiated with fluence of 0.9 J/cm2 at a scanning speed of 0.2 mm/s in a water environment. The surface morphology of this sample is shown in the respective FESEM image (Fig. 6.18B). Curve C

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shows the reflectance of the sample processed at the laser parameters of fluence of 1.2 J/cm2 and at a scanning speed of 0.05 mm/s in a water environment. The morphology of this sample is shown in the FESEM image (Fig. 6.18C). Curve D shows the reflectance of the sample processed at the laser parameters of fluence of 1.2 J/cm2 and scanning speed of 0.005 mm/s in water environment. The morphology of this sample is shown in the FESEM image (Fig. 6.18D). Curve E shows the reflectance of the sample processed at the laser fluence of 1.6 J/cm2 and scanning speed of 0.2 mm/s in an air environment. The morphology of this sample is shown in the respective FESEM images (Fig. 6.18E).

6.3.7 Formation of Metal Nanostructures in a Polymer Matrix by Using Femtosecond Laser Irradiation The synthesis of silver nanostructures fabricated in polymer matrix containing metal ions, upon fs laser irradiation, leads to the growth of nanoparticles into defined continuous metal particle structures. Thin films of PVA 1 AgNO3 were prepared by mixing the solutions of 50 mg of AgNO3 dissolved in 10 mL of water and 225 mg of PVA (MW 5 88,000 g/mole) dissolved in 10 mL of water and stirring for 8 hours for complete miscibility in a dark room to avoid photo-dissociation of silver nitrate. Next, a 1 mL aliquot solution was uniformly distributed on a 1 cm 3 1 cm glass plate using the spin-coating technique. These homogeneous films were dried in an oven for 2 hours at 30°C. The thickness of the film was found to be 2 μm. Absorption spectra of these films were taken immediately to confirm that there was no absorption peak in the visible region due to surface plasmon resonance of Ag nanoparticles. This confirms that the film does not contain any species of Ag nanoparticles before laser irradiation. Fig. 6.19A shows an FESEM image of fs laser-written microstructures on PVA 1 AgNO3 thin film obtained by spin-coating technique on Si substrate, with pulse energies ranging between 5 and 1 μJ; below the energy threshold of 1 μJ we did not observe any microstructure formation. The patterns were created using horizontal polarized laser pulses, by translating the sample perpendicular to the laser propagation direction at 100 μm/s scanning speed. Fig. 6.19B and D shows the formation of spherical-shaped Ag nanoparticles inside the microstructures that were fabricated with pulse energies of 5 and 2 μJ, respectively. Fig. 6.19C shows an image of a pristine (nonirradiated) region of AgNO3 1 PVA thin film. The nanoparticles inside the fabricated microstructures with both the pulse energies are found to have polydispersity in size distribution; the particle density is greater inside

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Figure 6.19 (A) FESEM images of representative photo-deposited silver microstructures on AgNO3 1 PVA thin films with pulse energy ranging between 5 and 1 μJ. (B) and (D) Ag NPs inside the microstructures fabricated with 5 and 2 μJ, respectively. (C) FESEM image of pristine (nonirradiated) region of AgNO3 1 PVA thin film. (E) Particle size distribution of Ag NPs formed inside the microstructure [shown in (B)] and (F) Energy Dispersive X-Ray Analysis ( EDAX) of elemental mapping on the fabricated microstructure showing the presence of silver. The observed silicon signal is from the underlying silicon substrate.

the microstructure fabricated with 5 μJ pulse energy compared with 2 μJ. Fig. 6.19E shows particle size distribution of Ag NPs formed inside the microstructure fabricated with 5 μJ, having an average particle size of 18 nm. Here, PVA acts simultaneously as the reducing agent [119], stabilizer for the Ag nanoparticles, and the matrix for homogeneous distribution and

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immobilization. Fig. 6.19F shows an energy dispersion X-ray spectroscopy (EDS) elemental map, confirming that the fabricated structures contain silver. The presence of silver is corroborated by the appearance of silver signal in the EDS spectrum. The observed silicon signal in the EDS spectrum is from the underlying silicon substrate. In order to distinguish the composition variation of the irradiated area, we measured the UV
Vis absorption spectra. For UV
Vis absorption measurement, several structures, such as a grating structure, were created in PVA 1 AgNO3 thin film on glass substrate by focusing laser pulses with 10 3 objective. The spacing between adjacent lines in this pattern is 20 μm, and each structure is of the order of 30 μm in width, as shown in Fig. 6.20A. These structures were created by translating the sample at 100 μm/s scanning speed and with pulse energy of 3 μJ. Unirradiated PVA 1 AgNO3 thin film is transparent; the absorption spectra of these films show that there is no absorption peak in the visible region due to the surface plasmon resonance of Ag nanoparticles. This confirms that the film does not contain Ag nanoparticles before laser irradiation. Fig. 6.20B shows the extinction spectrum of fs laser-inscribed grating film and the inset shows an extinction spectrum of the freshly prepared film. After irradiation, the formation of Ag nanoparticles was confirmed by the appearance of surface plasmon peak centered at 448 nm in the UV
Vis absorption spectrum as shown in Fig. 6.20B. Colloidal Ag nanoparticles have a similar absorption band, which is due to the surface plasmon

Figure 6.20 (A) FESEM image showing closely fabricated microstructures on AgNO3 1 PVA thin film having a microstructure width of 30 and 20 μm spacing between adjacent microstructures for UV
Vis extinction measurement. (B) Extinction spectrum of fs laser inscribed grating on AgNO3 1 PVA film and inset shows extinction spectrum of the freshly prepared thin film.

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absorption; the collective oscillation of free conduction electrons induced by the detecting light field in a metal particle [120]. In the results we present here, PVA was dissolved in water and cast as a thin film containing silver ions. By combining PVA and water, we obtain both a support matrix and controlled growth, where PVA acts as both a reducing and stabilizing agent. It is now well known that silver ions from the precursor, AgNO3, get reduced by the hydroxyl groups of PVA when the temperature of the mixture bath is raised [121]. The mechanism that takes place is as follows: R2 CHOH ðof the polymerÞ 1 2AgNO3 - R2 C 5 O 1 2HNO3 1 2Ag;

where R indicates the alkyl group of the polymer [122]. Absorption of light complements the heating process as the absorbed photons raise the temperature of the sample irradiated through nonradiative processes [123]. In the present studies, though there is no absorption at a laser wavelength of 796 nm, we see a strong absorption through TPA [124]. This TPA becomes stronger with the formation of the nanoparticles of Ag as Ag nanoparticles have an absorption cross-section at around 450 nm. Due to the diffusion process, the Ag ions aggregate, forming Ag nanoparticles inside the fabricated microstructures. Such a study helps in the development of new devices that may find applications in microphotonics and electronics.

6.3.8 Aluminum Nanoparticles Prepared by Laser Ablation in Different Liquids for Application as a Potential Optical Limiter In this work, different oxygen free organic solvents like carbon tetrachloride (CCl4), chloroform (CHCl3), benzene (C6H6), toluene (C6H5CH3), and chlorobenzene (C6H5Cl) were used as the liquid environment in laser ablation of the Al target. A pulsed Nd:YAG laser with a fundamental wavelength of 1064 nm and a duration of 6 ns was used for ablating the target. Al foil was irradiated with 300 pulses at a fluence of 4 J/cm2 by focusing the laser beam with a 10 cm focal length lens to 100 μm diameter normal to the Al surface. Fig. 6.21 shows the optical extinction spectra of Al NPs prepared in different ablation environments in the range from 200 to 600 nm. The spectra show absorption peaks at UV wavelengths positioned between 240 and 290 nm. Optical extinction spectra of samples prepared in carbon tetrachloride, chloroform, benzene, toluene, and chlorobenzene show extinction peaks at 261, 244, 279, 285, and 288 nm, respectively.

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Figure 6.21 UV
Vis extinction spectrum of corresponding Al NPs dispersions. Reprinted from R. Kuladeep, L. Jyothi, P. Prakash, S. Mayank Shekhar, M. Durga Prasad, D. Narayana Rao, Investigation of optical limiting properties of aluminium nanoparticles prepared by pulsed laser ablation in different carrier media, J. Appl. Phys. 114 (2013) 243101, doi:10.1063/1.4852976, with permission of AIP Publishing. All rights reserved.

Fig. 6.22 shows TEM images of the Al NPs produced by ablation in a different liquid environment that are spherical in shape. These NPs possess a well-distinguished core
shell structure. Except in carbon tetrachloride, particles ablated in remaining solvents have a bright region of cavity (core), which suggests the porosity of Al NPs, surrounded by a dark region of shell. As shown in Fig. 6.22, cavities are more frequently observed in Al NPs ablated in benzene, toluene when compared to chloroform and chlorobenzene, whereas in case of carbontetrachloride we did not observe any cavities. The HRTEM studies shown in Fig. 6.23 suggest that the central bright portion of the particles could be either due to an empty region or a region with lower density (referred to as a cavity) surrounded by a dark region. Here, both the cavity and the dark region exhibit a crystalline nature surrounded by an amorphous metal Al shell. HRTEM studies in the case of carbontetrachloride (Fig. 6.23C) reveal that the particles are made of a crystallized Al inner dark zone surrounded by an amorphous metal Al shell. Fig. 6.23D shows an HRTEM image of a crystalline cavity inside Al NPs obtained by ablating in chloroform. Insets in Fig. 6.23 shows the corresponding electron diffraction pattern of Al NPs. Stratakis et al. and Viau et al. [124
127] reported a similar kind of porosity in Al NPs obtained by ablating bulk Al target in ethanol. Their studies suggest that the pores inside the Al NPs are due to the dissolution of the surrounding gas in molten nanoparticles during laser ablation. Light gases like hydrogen are highly soluble in metals, especially

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Figure 6.22 TEM view of nanoparticles generated via ablation of a bulk Al target in (A) benzene, (B) toluene, (C) carbon tetrachloride, (D) chloroform, and (E) chlorobenzene using ns laser radiation. Reprinted from R. Kuladeep, L. Jyothi, P. Prakash, S. Mayank Shekhar, M. Durga Prasad, D. Narayana Rao, Investigation of optical limiting properties of aluminium nanoparticles prepared by pulsed laser ablation in different carrier media, J. Appl. Phys. 114 (2013) 243101, doi:10.1063/1.4852976, with permission of AIP Publishing. All rights reserved.

in their liquid state. Due to the sharp temperature dependence of H2 solubility in Al, there is a possible formation of Al hydrides inside the particles. The hydrogen released during the solidification may provide additional pressure, leading to complete crystallization of the metal inside it. In the

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Figure 6.23 High-resolution TEM images of Al NPs generated by laser ablation of Al target in (A) benzene, (B) toluene, (C), carbon tetrachloride, and (D) chloroform using a Nd:YAG 6 ns laser. Insets show the corresponding electron diffraction pattern of Al NPs. Reprinted from R. Kuladeep, L. Jyothi, P. Prakash, S. Mayank Shekhar, M. Durga Prasad, D. Narayana Rao, Investigation of optical limiting properties of aluminium nanoparticles prepared by pulsed laser ablation in different carrier media, J. Appl. Phys. 114 (2013) 243101, doi:10.1063/1.4852976, with permission of AIP Publishing. All rights reserved.

case of Al NPs ablated in carbon tetrachloride, the lack of cavities could be due to the absence of hydrogen in the surrounding environment. Nonlinear optical properties of Al NPs were measured by the standard Z-scan technique [101], using a frequency-doubled, Q-switched Nd: YAG (Spectra Physics, INDI-40) laser, delivering 6 ns laser pulses at 532 nm wavelength at a repetition rate of 10 Hz. To compare the nonlinear optical behavior of Al NPs in various solvents, Z-scan and optical limiting studies were performed with Al NP dispersions having 70% linear transmission at 532 nm and at the same intensity levels ensuring identical experimental conditions. The contribution from the pure nonlinear absorption and that from both nonlinear absorption as well as scattering, are estimated by recording Z-scan curves with detector 1 and detector 2

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placed at their respective positions. Detector 1, looking at the "whole transmitted light," is taken as that due to the two-photon absorption alone. Detector 2, kept at the far field and looking at the transmitted beam minus scattered beam, is taken as that due to both two-photon absorption and nonlinear scattering. The open-aperture Z-scan studies of Al colloidal solutions of chlorobenzene, chloroform, toluene, carbon tetrachloride, and benzene revealed reverse saturable absorption (RSA) behavior at low as well as at high intensities. Nonlinear scattering was observed at higher intensities in all the solvents. The open aperture Zscan curves of Al colloidal solutions of various solvents recorded with both detectors are shown in Fig. 6.24. An enhanced depletion in the transmitted beam collected with detector 2 is shown in Fig. 6.24. It goes to ,0.3 in the case of Al NPs in chlorobenzene (Fig. 6.24A), whereas in chloroform it is B0.3 (Fig. 6.24B), when the losses due to nonlinear scattering are taken into account. To account for the scattering losses as seen in detector 2, we have introduced the scattering losses, αs, as derived by Joudrier et al. [128]. The Z-scan results can be fitted well (shown by the solid line in Fig. 6.24) by considering linear absorption, two-photon absorption, and scattering by using the following equation [129]: dI 5 α0 I 2 βI 2 2 αs I dZ where αs 5 gs ðΔnÞ2 5 gs ðΔn0 1Δn2 I Þ2 "   #1=2 2

ωðzÞ 5 ω0 11 πω20 Z0 5 λ I 5 I00

Z Z0

(6.1)

! ! ! ω20 2 t2 2 2r 2 Uexp Uexp 2 ω2 ðZ Þ τ 2p ω ðZ Þ

αs is the effective scattering coefficient, α0 is the ground-state absorption coefficient, β is the two-photon absorption coefficient, gs is a parameter which is independent of intensities but depends only on the size, shape, concentration of particles, and wavelength of light, Δñ is the refractive index mismatch between Al NPs and solvents under study, the linear mismatch Δn0 is the difference in the linear refractive indices of Al NPs and solvents under study. The nonlinear refractive index of solvents can be

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Figure 6.24 Open aperture Z-scan curves of Al NP dispersions in (A) chlorobenzene, (B) chloroform, (C) toluene, (D) benzene, and (E) carbon tetrachloride collected with both detectors; detector 1 gives the transmitted light and scattering, whereas detector 2 gives the transmitted light without scattering, with input intensity of 0.68 GW/cm2 at 532 nm, 6 ns laser pulses, and the solid lines are the curves obtained by theoretical fitting. Reprinted from R. Kuladeep, L. Jyothi, P. Prakash, S. Mayank Shekhar, M. Durga Prasad, D. Narayana Rao, Investigation of optical limiting properties of aluminium nanoparticles prepared by pulsed laser ablation in different carrier media, J. Appl. Phys. 114 (2013) 243101, doi:10.1063/1.4852976, with permission of AIP Publishing. All rights reserved.

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neglected, and the nonlinear mismatch Δn2 can be written as n2, where n2 is the nonlinear refractive index of Al NPs, ω0 is the beam waist at focus, Z0 is the Rayleigh range, I is intensity as a function of r, t, and z, I00 is peak intensity at the focus of the Gaussian beam, and τ p is the input pulse width used. The differential equations are solved numerically using the Runge
Kutta fourth-order method. The differential equations are first decoupled and then integrated over time, length, and along the radial direction. Assuming the input beam to be Gaussian, the limits of integration for r, t, and z are varied from 0 to N,
N to N, and 0 to L (length of the sample), respectively. The typical number of slices used for r, t, and z are 30, 20, and 5 respectively. β and αs are then estimated through least square fit of the experimental data. Initially, the two-photon absorption coefficient β using αs as zero by fitting the Z-scan curve, which was obtained from detector 1 was estimated. Using this value, nonlinear scattering coefficient αs was estimated, from the curve obtained from detector 2. The refractive index of Al, chlorobenzene, chloroform, toluene, benzene, and carbon tetrachloride are taken as 0.93, 1.52, 1.45, 1.5, 1.51, and 1.46, respectively. Al NPs exhibit negative nonlinearity under 6 ns excitation at 532 nm and the calculated value of n2 is 3.49 3 10213 cm2/W. Scattering is a fundamental manifestation of the interaction between matter and radiation, resulting from the inhomogeneities in the refractive index [130]. As the Al NPs show strong two-photon absorption, the scattering observed with the nanosecond pulses is attributed to the local heating of Al NPs, which gives rise to bubble formation. At high pump fluence, nonlinear scattering usually arises from the formation of two types of scattering centers. The energy absorbed by the nanoparticle at high pump fluence creates rapid expansion of the metal nanoparticle, which acts as a scattering center. This expansion is due to the melting and vaporization of surface atoms. The energy is then dissipated through transfer to the solvent, leading to nanoparticle cooling and at the same time solvent is heated up and the bubbles are formed, which acts as the secondary scattering centers [128]. The heating of the metal particle and the transfer of the energy to the solvent through the metal
solvent interface are faster. Once the bubble formation starts, scattering from bubbles contributes to a decrease in the transmission, which leads to the optical limiting action. On the other hand, at low fluences, the energy absorbed by the nanoparticles is not sufficient to heat the solvent and to produce bubbles as scattering centers, because of which, at intensities lower than 75 MW/cm2, no scattering was observed. The solvent in which NPs are dispersed plays

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an important role in enhancing the optical-limiting activity of the NPs through microbubble formation. The larger magnitudes of effective scattering coefficient of Al NPs in polar solvents, like chlorobenzene and chloroform, is due to larger nonlinear absorption of NPs in these solvents due to which a large amount of heat is transferred to the surrounding solvent resulting in bubble formation. The nonlinear scattering behavior of the Al NPs ablated in chlorobenzene as a function of the input intensity (Z position) is as shown in Fig. 6.25A, in which the curves shown are of those obtained for three different forward-scattering angles with beam propagation direction using detector 3. Fig. 6.26 shows the far-field scattering distribution of the transmitted light through the Al NP dispersions in chlorobenzene and chloroform observed at higher intensities. The optical limiting curves of Al NPs in different carrier media are plotted in Fig. 6.25B. In comparison, Al NP colloidal solutions of chlorobenzene and chloroform exhibit lower optical limiting threshold values (I1/2) of 0.41 and 0.46 J/cm2, respectively, due to large linear, nonlinear absorption, and nonlinear scattering. I1/2 is defined [131] as the input intensity at which the transmittance reduces to half of the linear transmittance. Nonlinear absorption coefficients, nonlinear scattering coefficients, and optical limiting threshold values of different Al NP colloidal solutions were estimated and are presented in Table 6.1.

Figure 6.25 (A) Scattering of Al NPs in chlorobenzene at three different forward angles with intensity (z-position) collected through detector 3. (B) Optical limiting behavior of Al NPs dispersions in chlorobenzene, chloroform, toluene, benzene, and carbon tetrachloride. Reprinted from R. Kuladeep, L. Jyothi, P. Prakash, S. Mayank Shekhar, M. Durga Prasad, D. Narayana Rao, Investigation of optical limiting properties of aluminium nanoparticles prepared by pulsed laser ablation in different carrier media, J. Appl. Phys. 114 (2013) 243101, doi:10.1063/1.4852976, with permission of AIP Publishing. All rights reserved.

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Figure 6.26 Snapshot of the far-field scattering distribution of the transmitted light through the Al NPs dispersions of (A) chlorobenzene and (B) chloroform at input intensity of 0.68 GW/cm2 at 532 nm, 6 ns laser pulse excitation. The central bright portion is mainly due to the reflection from the white paper.

Table 6.1 Two-photon absorption coefficient (β), nonlinear scattering coefficient (αs), and optical limiting threshold (I1/2) values of Al NPs in different liquid environment having 70% linear transmission at 532 nm wavelength Al NPs colloidal solution β (cm/GW) αs (cm21) Limiting threshold I1/2 (J/cm2)

C6H5Cl CHCl3 C6H5CH3 C6H6 CCl4

80.7 82.9 38.9 19.7 0.82

184 172 114 67 11

0.41 0.46 0.68 1.09 1.38

6.3.9 Metal Nanoparticles in Liquids (Mo, Ti, In, Au
Ag Alloy, and Al) Fig. 6.27A
C shows TEM images of the Mo, Ti, and In metal nanoparticles that are formed due to fs laser ablation of the respective bulk metal targets in water. This method of laser ablation in liquids has been discussed above. It is observed that the nanoparticles are formed without defects and they show a crystalline nature. By varying different laser irradiation parameters the synthesis of NPs can be controlled and thereby the properties of the NPs will be tuned. Fig. 6.27D shows the Au
Ag alloy NPs synthesized by another method of laser irradiation of liquids. Here, instead of ablation of the bulk targets in water, a method of synthesis of NPs by laser ablation of the precursor solutions of the sol
gel processing method has been used for the NP synthesis. The alloy NPs are synthesized by first preparing a

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Figure 6.27 TEM images of metal nanoparticles formed after laser irradiation in water for (A) Mo, (B) Ti, (C) In, and (D) Au
Ag alloy.

mixture of different combinations of the precursor solutions of HAuCl4, AgNO3, and PVA in different concentration ratios. After mixing these solutions they have been stirred for an hour and then ablated using a pulsed laser for the synthesis of alloy NPs. In this way different combinations of alloy NPs can be prepared by the laser ablation method. Fig. 6.28 presents the morphology and size distribution of fabricated Al NPs by fs laser ablation in different liquids investigated using TEM measurements. From the TEM images the shape of the particles seems mostly spherical and a significant number of NPs exhibit a welldistinguished core
shell structure in both ethanol and water. As shown in Fig. 6.28C and D, HRTEM images show that the center bright portion of the particles could be either due to an empty region or a region with lower density (referred to as a cavity), which suggests the porosity of Al NPs, surrounded by a dark region. Both the cavity and the dark region have crystalline inclusions as shown in Fig. 6.28D, surrounded by an amorphous metal Al shell (Fig. 6.28C). The insets in Fig. 6.28D and F

Figure 6.28 Plane view TEM images of Al NPs generated (A)
(D) in water, (E) and (F) in ethanol. (B) and (F) shows single particle images with the formation of internal cavities in both water and ethanol. Insets in (A), (E), (D), and (F) show the particle size distribution and electron diffraction patterns of Al NPs in both solvents, respectively. Reprinted from R. Kuladeep, M.H. Dar, K.L.N. Deepak, D.N. Rao, Ultrafast laser induced periodic sub-wavelength aluminum surface structures and nanoparticles in air and liquids, J. Appl. Phys. 116 (11) (2014) 113107, doi:10.1063/1.4896190, with permission of AIP Publishing. All rights reserved.

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show the corresponding electron diffraction patterns of Al NPs in both water and ethanol, respectively. Kuzmin et al. described the formation of porous NPs of both Al and Ti generated by laser ablation in ethanol [132]. It has been argued that the formation of pores inside Al NPs is due to dissolution of the surrounding gas in molten NPs during laser irradiation. Light gases like hydrogen are highly soluble in metals, especially in their liquid state. Due to the sharp temperature dependence of H2 solubility in Al, there is a possibility of formation of Al hydrides inside the particles. Hydrogen released during the solidification may provide an additional pressure that may lead to the formation of crystalline inclusions inside the metal nanoparticle. The Al NPs synthesized in various organic solvents, such as carbon tetrachloride, chloroform, benzene, toluene, and chlorobenzene, conveyed that the formation of cavities in nanoparticles is possible except when ablated in carbon tetrachloride [101]. The reason for the lack of cavities in Al NPs ablated in carbon tetrachloride was interpreted as being due to the absence of hydrogen in the surrounding environment. Size distributions of Al NPs are shown in the insets of Fig. 6.28A and E, with an average particle size of 39 and 48 nm in water and ethanol, respectively.

6.4 CONCLUSIONS In conclusion, laser micro/nanomachining on semiconductors and metals for the synthesis of different nanostructures and nanoparticles for SERS, optical limiting and low reflective surfaces and the advantages of the laserinduced synthesis method over other methods have been discussed in this chapter. The dependence of laser parameters like laser fluence, number of pulses irradiating the sample, and laser polarization on the formation of fs laser-induced surface nanostructures is revealed in material processing. It is observed that the formation of subwavelength structures is only possible within a rather narrow range of laser fluences while their surface morphology depends on processing parameters like laser fluence and number of applied pulses. This technique is efficient, universal, cost-effective, and environmentally friendly, which has potential applications in the fabrication of micro/nanostructures on a variety of materials for microelectromechanical systems, nanoelectronics, and nanophotonics. Therefore, by tuning the incident laser parameters and using them in the processing of different nanomaterial syntheses there is large scope for various applications in different fields such as photonics, optoelectronics, etc.

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CHAPTER 7

Flame Synthesis of Nanostructured Transition Metal Oxides: Trends, Developments, and Recent Advances Wilson Merchan-Merchan, Walmy Cuello Jimenez, Octavio Rodriguez Coria and Chad Wallis School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK, United States

Contents 7.1 Introduction 7.1.1 Motivation 7.2 Properties and Applications of 1D and 3D Transition Metal Oxide Nanostructures 7.2.1 Solar Panels 7.2.2 Electrochromic Devices 7.2.3 Lithium-Ion Batteries 7.2.4 Capacitors 7.2.5 Gas Sensors 7.2.6 Light-Emitting Diodes 7.2.7 Catalysis 7.3 Fabrication Techniques to Synthesize TMO Nanostructures 7.3.1 Chemical Vapor Deposition 7.3.2 Sol
Gel 7.3.3 Plasma 7.3.4 Flames 7.4 Flames as a Unique Fabrication Tool to Produce TMO Nanoparticles 7.4.1 Flame Synthesis of Metal-Oxide Nanoparticles 7.4.2 The “Aerosol” and “Spray Pyrolysis” Methods 7.4.3 Approaches Used for Control Synthesis in “Aerosol” and “FSP” 7.5 Flame Synthesis of Multidimensional TMOs Using the “Solid Support” Method 7.5.1 Parameters Affecting the Flame Synthesis of Multidimensional TMOs 7.5.2 Effect of Probe Diameter and Oxygen Content in the Oxidizer 7.5.3 Effect of Thermal Property of the Source Material—Iron Oxide Structures 7.5.4 Effect of Flame Positon 7.5.5 Hybrid Approach to Form Core
Shell TMOs Through “Solid Support” and Electrodeposition Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00007-9

© 2019 Elsevier Inc. All rights reserved.

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7.6 Volumetric Flame Synthesis of 1D and 3D TMOs 7.6.1 TMOs (1D) Formed Using “Aerosol” and “Spray Pyrolysis” 7.6.2 “Gas Phase” Flame Synthesis of 1D and 3D TMOs 7.7 Core
Shell and Mixed Transition Metal Oxide Nanostructures 7.7.1 Flame Synthesis of Core
Shell and MTMOs on Solid Substrates 7.7.2 Synthesis of MTMOs and Related Core
Shell Nanostructures Using “Flame Spray Pyrolysis” 7.7.3 Gas-Phase Synthesis of 1D and 3D Core
Shell and MTMOs 7.8 Conclusions Acknowledgments References

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7.1 INTRODUCTION This chapter presents an overview of the flame synthesis of transition metal oxide (TMO) structures of various chemical and physical morphologies (i.e., composition, size, shape). It presents trends, developments, and recent advances in the application of flames for the synthesis of TMOs and related nanostructures. Section 7.1.1 starts by providing a brief description of the nature of transition metals (TMs) and TMOs and why it is important to study them. The intercalation of a base material (i.e., TM) with oxygen forms a new material with superior properties called TMOs. The degree of improved properties can vary depending on the shape (spherical and nonspherical), size (nano- and microscaled), and composition (single and multiphase) of the TMOs. In order to better understand TMOs, it is necessary to understand their base or bulk materials (TMs). Hence that section starts with a brief description of TMs. Section 7.2 discusses some of the properties and different applications of some types of nanostructured TMOs. The unique properties and niche applications in diverse fields underscore the importance of these nanomaterials (NMs). Nano-sized TMO structures with unique morphology and dimensionality [i.e., one- (1D), two- (2D), and three-dimensional (3D)] are among the most highly sought-after NMs. The unique morphologies of TMOs (size and shape) combined with their molecular structure yield NMs with unmatched properties and a broad array of applications. Owing to the unique properties of these NMs, they have become important components in various sectors including health care (i.e., in drug delivery for tumor therapies), renewable and clean energy (i.e., increasing power output in solar panels and/or increasing the

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performance in Li-ion batteries), and as catalytic material, among many others. Section 7.3 presents an overview of current fabrication methods (nonflames and flames) and techniques being employed for the synthesis of multidimensional TMOs. Robust, scalable, energy-efficient, and inexpensive methods for the synthesis of these novel TMOs are needed to meet the demands of current and future applications. Methods such as chemical vapor deposition (CVD), laser deposition, plasma, sol
gel, and a variety of their combinations have been employed for the synthesis of multidimensional nanostructured TMOs. Most of these methods require pre- and/or postprocessing for their synthesis, resulting in a batch-tobatch limited scale production of the TMO nanoforms. This is discussed in detail. Section 7.4 highlights the capability of flames for the synthesis of TMOs (nanoparticles). It contains a historical review of the flame synthesis of TMOs with an emphasis on the flame type (burner design), precursor delivery (or source material), scalability, and product morphology (aggregated spheroidal particles and powders). It has been shown that the manner in which the raw (source) material is introduced into the flame medium can influence the shape (elongated vs spherical) and size of the synthesized TMOs. Pioneers in the application of flames for synthesis of TMOs have introduced the source material using the “aerosol” and “flame spray pyrolysis” (FSP) methods. These methodologies have resulted in the synthesis of TMOs displaying chain-like spheroidal primary particles that are fused together to form aggregates and powders. Section 7.5 deals with the application of flames for the controlled synthesis of multidimensional TMOs (nonspherical). Among these flame methods, a variety of techniques have been proposed and tested for controlling the product characteristics. Notably, it has been shown that the introduction of the source material in the form of a solid support (wires and/or meshes) in a flame medium can result in the synthesis of highly ordered and complex TMO nanostructures (i.e., multidimensional and of multiple chemical structures). Through this method, the bulk source is introduced in or near a region of high temperature and a highly reactive flame environment where it melts and sublimates the converted metal oxides to form vapors. This represents a simpler process compared to the “aerosol” and the “FSP” methods where the source material is gaseous or in the form of a fine spray, respectively, as it is introduced into the flame. Various types of flames including normal and inverse diffusion, premixed, and partially premixed flames have been employed for the synthesis of these complex TMO nanostructures. A variety of

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well-defined multidimensional TMO structures have been generated directly in flame environments using the “solid support” method. In the “solid support” method the wire acts as a source and as a surface for recrystallization of the newly formed vapors. The NMs can be formed directly on certain locations of the metal probe serving as the source or they can be deposited on specially designed supports. Hybrid methods that first include flame synthesis followed by an electrodeposition have been tested. In Section 7.6 it is shown that flame-formed multifaceted TMOs can be achieved by introducing new parameters into the “aerosol” and “spray pyrolysis” precursor delivery technique. Moreover, it has been shown that the solid-fed-precursor method can be used for the volumetric flame synthesis of well-defined TMOs (Section 7.6). That is, the introduction of the source in the form of a “solid support” or meshes has extended to the gas-phase of multidimensional TMOs directly in the flame volume. This is very unique as it provides a continuous method of growth and allows scalability of the process for forming nonspherical TMOs. This review ends by introducing new developments in the application of flames for the synthesis of core
shell (spherical and nonspherical) and mixed transition metal oxide (MTMO) nanomaterials (Section 7.7). It has been shown that the simple crosslinking of two transition metals with oxygen atoms or the combination of two TMOs into a single compound allows for the formation of an entirely new type of material. This new type of material, which is a mixed transition-metal oxide of a “spinel-like structure,” has received significant attention as a result of the superior properties that it exhibits over its nonmixed counterparts. TMOs with a core
shell morphology have also received much attention. TMOs/MTMOs with core
shell morphology have been synthesized on solid supports and directly in the gas phase (volumetric).

7.1.1 Motivation The largest group of elements of the periodic table is the so-called transition metals (TMs). This includes groups 3
12 (d-block) and the f-block. According to the International Union of Pure and Applied Chemistry (IUPAC), a TM is any element with a partially filled d electron subshell. In TMs the valence electrons are present in more than one shell and often exhibit several common oxidation states [1,2]. The f-block elements

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(lanthanide and actinide) are referred to as inner transition metals. Despite many similarities, the TMs do vary considerably in certain properties. There is a general decrease in size of atomic radii from left to right for each of the series in the periodic table. There is a significant increase in atomic radius from 3d to 4d metals, although the 4d and 5d metals have remarkably similar atomic radii. Within the 3d, 4d, and 5d block of the transition series all of these elements contain a cubic and hexagonal crystal structure with the exception of Hg, which contains a rhombohedral structure. Some of these metals have very different melting points, such as W with 3410°C and Hg as a liquid at 25°C. TMs such as Mo, Fe, and Ti present high level of hardness, great strength, and make for very useful structural materials; others such as Cu, Ag, and Au, are relatively soft and highly conductive. The simple intercalation of a TM with oxygen to form a new material (TMO) can significantly improve the properties of the base metal. Experimental and theoretical studies have shown that by converting bulk material into minute structures (at the nano- and micron-sized range) a remarkable modification of their properties (i.e., chemical, catalytic, magnetic, electronic properties) relative to their bulk can take place. Additionally, the alteration of the elemental composition of the nanoand micron-sized structures (i.e., 1D and 3D) from its bulk material can significantly enhance the material properties, providing many useful applications, and this has generated a significant interest within the scientific and industrial communities. Materials with such improved properties due to their transformation from bulk to nano-/micron-size and the modification of the elemental composition are TMOs. More recently, it has also been shown that the mixing of two transitional metals to form a MTMO structure can result in a structure that has superior properties. Different methods and techniques have been developed for the synthesis of various types of TMOs. The flame method represents a single-step process where the structures can be formed in a few minutes process. The underlying hypothesis of the flame synthesis is that coherent variations in temperature and chemical species can produce controlled growth conditions leading to controlled and highly selective formation of TMO nanostructures. The flexibility to introduce various synthesis parameters has yielded a wide-spectrum collection of TMO nanostructures with varying shape (i.e., spherical vs 1D and 3D), structure (crystalline vs amorphous), elemental composition (stoichiometric vs nonstoichiometric), and mixing

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type (uniform vs nonuniform or core
shell). Therefore, the focus of this contribution is to provide an insight into the many studies done in the field of the flame synthesis of TMOs. We present approaches taken by several experts/researchers in the field to control the variety of chemical and physical morphologies in the flame-generated TMOs.

7.2 PROPERTIES AND APPLICATIONS OF 1D AND 3D TRANSITION METAL OXIDE NANOSTRUCTURES Owing to the unique properties of TMO NMs, we have seen a significant increase in their application as important components in various sectors including in the fields of photocatalysts [3,4], luminescence [5,6], piezoelectric transducers and actuators [7
15], electrochromic (EC) displays [16,17], high-performance anodes in Li-ion batteries [18
21], gas-sensing components [22,23], data storage media [24], optical absorption and emission [25], biosafety and biocompatibility [26], among others. Popular applications of TMOs in selected technologies are briefly described below (Fig. 7.1).

Figure 7.1 Some typical applications of nanostructured multidimensional transition metal oxides. (A) Solar panels; (B) Electrochromic; (C) Rechargeable batteries; (D) Capacitors; (E) Gas sensors; (F) Light emitting diodes; and (G) Catalyst.

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7.2.1 Solar Panels In the solar panel industry, the use of photovoltaics allows for the conversion of sunlight (photons) into electricity (Fig. 7.1A). Si-based solar cells have been in development for over 50 years [27], however, efficiencies of 5%
15% are typically attained in commercially available cells as a result of the reduced utilization of photons by the Si layer of the cell [28,29]. The uses of different types of photovoltaic materials, of unique morphological characteristics, and of nanostructured scales, have become topics of study in recent years for increasing the power output of solar panels [30]. That is, electron transport in photoactive materials (regardless of its type) can be influenced depending on whether the material is in: (1) bulk or micronsize compared to nanosize scales. Nanosized photoactive materials are reported to offer advantages in enhancing the efficiency of the cells due to the increased surface-to-volume ratio. That is, nanosized NMs allow for a shorter diffusion path for charge carriers to the surface, additional adsorption sites for reactants, and more active sites for the catalytic reactions, in addition to the localized electronic states favoring charge transport and trapping near the surface. (2) Physical morphological properties of the photoactive materials (spherical vs non-spherical, regardless of the type of material). Spherical particles have been employed for solar panel applications; however, they possess a key problem: coalescence of the fabricated atomic clusters, affecting the original physical and chemical interactions. 1D nanostructures (nano-wires, -rods, -tubes, -cones, etc.) applied to photovoltaics favor the influence of both absorption and reflection due to light scattering between the nanostructures which increases the travel path for the photons. Moreover, 1D structures orthogonalize the direction of electron transport, which facilitates the absorption of light and the collection of charge carrier; whereas, 3D structures result in improved surface reaction rates, electrical transport, chemical stability, and specific surface area, which causes a dramatic enhancement in storage ability. The exceptional review of Anta shows the intensive efforts by various research groups to study the efficiency of light-harvesting devices based on nanostructured metal oxides [30]. One application of the metal oxide for solar cells is by creating nanostructured films [31,32]. It has also been shown that thermal treatment (annealing) of the films can result in significantly higher external quantum efficiencies compared to the non-annealed solar cells [33,34]. For instance, Dittrich et al. showed that annealed thin absorber layers based on ZnO nanowires with length of 1.5 μm coated

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with absorber materials of 25 μm of In2S3 and CuSCN resulted in higher efficiencies compared to those non-annealed layers [34]. Kuznetsov et al. [35] reported the use of TMO nanorods to coat Si-based solar cells to increase their power output to levels up to 5%. Thus, two different mechanisms can be proposed to enhance solar cell power output/efficiency: (1) replacing Si films with other materials that require thin films, or (2) coating the Si films with nanostructures to increase the film surfaceto-volume ratio, and thus, their efficiency. Hybrid components can also result in enhancement of power conversion efficiency. For instance, ZnO nanorod structures combined with poly-3-hexylthiophene polymer results in a conversion efficiency over four times greater than that of similar devices based on single nanoparticles [36].

7.2.2 Electrochromic Devices An application of EC properties is shown in Fig. 7.1B using a “smart window” as a typical application of this property. A smart window typically employs two EC films; one a cathodic oxide and the other an anodic oxide. Upon an electrical stimulus, ion insertion/extraction will cause both films to undergo color-bleach cycles. Briefly, a generic EC device consists of several superimposed layers placed in between two transparent substrates (i.e., glass, polycarbonate, polyethylene terephthalate, etc.) [37]. The variation of the optical properties results from ion insertion/extraction via centrally positioned electrolytes from the EC films. Small ions are typically favored for such purposes (i.e., H1, Li1, etc.; Fig. 7.1B). Transparent liquid and/or polymeric-based electrolytes and ions carrying thin oxide films are typically employed [37]. The ion transfer can be triggered by applying an electrical field between the two EC films, a voltage of 1
2 V DC is typically required, which suggests that powering the systems is easily possible [38]. The movement of electrons can result in intervalence transition (i.e., yield polaron absorption), which is the main reason for optical absorption [38]. Films are commonly fabricated based on NMs with tailored nanoporosity and made of TMOs based on two types of oxides: cathodic EC film (coloring under ion insertion, i.e., Ti, Nb, Mo, W, Ta) and an anodic EC film (coloring under ion extraction, i.e., Cr, Mn, Fe, Co, Ni, Rh, Ir). A significant amount of effort has also been devoted to the development of superior EC films. That is, the formation of of EC films composed of V-based oxides, which are hybrid in nature (anodic/cathodic), and the use of MTMOs is being

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explored. Mixed TMOs can yield optimized properties with superior performance when compared to the individual metal oxides. For instance, the addition of a small quantity of Ti to W (parent material) to form mixed nanostructured W-Ti oxide films results in a new material with improved electrochemical cycling durability compared to its counterpart [39]. A variety of other elements (e.g. including Ag [40], Pt [41], Au [42], Ta [43], Mo [44], Ru [45], Ni [46], V [47], etc.) have been introduced to form W-based hybrid oxide films to further enhance their durability and performance.

7.2.3 Lithium-Ion Batteries Commercially available lithium-ion batteries (LIBs) (Fig. 7.1C) employ films of spheroidal-shaped TMO particles as cathodes. These are the most advanced among the rechargeable batteries available to date [48]. Recently, in order to improve the performance of rechargeable batteries, the use of a cathode having one of its surfaces coated with a layer of 1D and 3D TMO nanostructures instead of conventional bulk microcrystalline films has become a very attractive field of study [49
51]. Cathodes coated with bulk microcrystalline metal oxides generally suffer from poor kinetics and/or capacity fades with cycling. On the other hand, cathodes composed of 1D and 3D TMO nanostructures have recently been shown to improve the rate capabilities of solid-state electrodes because of the small diffusion lengths [52]. Typically, nanoparticles are not very stable and difficult to fabricate on large scales, which has driven research to focus on 1D nanostructures that can render electrochemical LIBs more versatile. Nonetheless, in order to obtain higher energy and charge densities, 3D nanostructures may be required, due to electrodes having higher surface areas, large surface to volume ratio, and mass transport abilities along with favorable structure stability over 1D structures and nanoparticles. There are significant efforts aimed at developing novel electrodes to eliminate the dependence on carbon materials (carbon-based electrode) and to further optimize their performance [53]. Much attention has been devoted to TMO nanostructures as candidates for LIB electrodes since they can offer advantages of large and reversible capacities of two to three times greater than graphite, along with corrosion resistance, ecofriendliness, less volumetric expansion upon lithiation, better lithium intercalation process, and optimum cost
performance ratio [54].

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7.2.4 Capacitors Nanoscopic scale TMO structures have attracted much attention in energy storage applications not only due to their outstanding mechanical, electrical, and structural properties, but also because of their high pseudocapacitance (capacitors) as a result to their multiple valence state changes that are typically not possible with carbon-based materials [55]. Moreover, their large specific surface area and suitable pore size distribution are also features that favor their use for capacitor applications [56] that result in high specific capacities due to effective contact between electrodes and electrolyte along with enhanced transportation of ions/ electrons in both the electrode bulk and the electrode
electrolyte interface (Fig. 7.1D). RuO2 (720 F/g) and IrO2 (550 F/g) nanoparticles are recognized as the most promising candidates for capacitor applications due to their large specific capacitance values. However, low specific surface area is one of their disadvantages. This shortcoming is addressed by employing 1D nanostructures that are capable of solving the issue of specific surface area, and thereby enhancing the electrode’s active material utilization. 3D structures, however, improve the surface reaction rates, electrical transport, and chemical stability, along with optimized surface area, and facilitation of ion transfer in the system with improved storage capacity. Hybrid structures are reported to further improve the capacitance performance, NiCo2O4 nanoneedles grown on 3D phraphen-Ni foam resulted in capacitance of up to 1588 F/g.

7.2.5 Gas Sensors It is now well-known that the electrical conductivity of metal-oxide semiconductors is very sensitive to the composition of the surrounding gases. This property can be utilized in the production of gas sensors. Oxygen vacancies on the surface of metal oxide materials cause changes in electrical conductivity of the materials as a result of molecular adsorption. Consequent to the adsorption of some molecules, such as NO2 or O2 at the vacancy sites, conductivity is reduced; however, the conductivity increases when molecules, such as CO and H2, are adsorbed at the vacancy sites on the surface. Most metal-oxide gas sensors operate based on this principle [57]. Similar to the above-described applications, the sensing properties of NMs can be affected by several factors including their: (1) chemical composition, (2) surface modification with other metals, and (3) morphology,

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among others. A gas sensor typically consists of a coated ceramic tube (i.e., metal oxide), where the coating corresponds to the sensing film with predefined thickness (Fig. 7.1E). The actual sensing mechanism consists of the nanostructures (i.e., TMOs) exposed to a targeted environment (i.e., gas mixed air). This results in O22 ions (formed on the surface) by obtaining an electron from the conduction band, which results in electron depletion at the surface (Fig. 7.1E). Upon interactions with volatile compounds and/or chemical species through adsorption, chemical reactions, or charge transfer, the physicochemcial properties of the sensing film can be modified, thus generating an electrical signal [58]. 7.2.5.1 Chemical Composition One of the key advantages of TMO gas sensors refers to their ability to have reversible interactions between the sensing film and the gas, which is characteristic of conductometric semiconducting metal oxides [59]. Only TMOs with d0 and d10 electronic configurations are ideal for gas-sensing applications. Moreover, the sensitivity to gases in a gas sensor can be optimized when using core
shell TMOs of various elemental compositions [60]. Similarly, sensors formed from a combination of metal oxides have been shown to exhibit significantly higher sensitivity than sensors constructed solely from a binary composition [61,62]. A literature survey of different metal oxides employed for gas-sensing applications suggests that ZnO and SnO2 are the most extensively researched for sensing films in chemoresistive gas sensors [63]. However, it has been shown that a binary composite ZnO-SnO2 sensor exhibits superior sensitivity to sensors constructed solely from zinc oxide or tin oxide under identical experimental conditions [60]. 7.2.5.2 Physical Morphological Properties The specific shape of employed sensing materials can play a critical role in their performance owing to the fact that higher active surface areas result in higher sensitivity between the sensing materials and the targeted gas. For instance, Geng et al. [64] indicated that 14-faceted polyhedral ZnSnO3 microcrystals resulted in higher sensitivity than octahedral components. This is mainly due to the increased active surface for interaction when exposed to multiple gases including H2S, C2H5OH, and HCHO.

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Moreover, gas sensors are frequently required to operate in very challenging environments (i.e., temperatures up to 600°C [65]), which results in significant operational limitations as a result of the sensor’s poor sensitivity at such conditions. However, metal oxides with nanoscale dimensions have been found to be ideal for such extreme environments. The nanoscaled TMOs forming the sensing film enhance the adsorption of gases due to the significant increase in the surface area compared to their micro or bulk counterparts. The use of nanoscaled TMOs increases the sensor’s sensitivity and allows for expanding the operational regimen of the devices. In addition to the employment of metal oxide components with nanoscale size to enhance the performance and sensitivity of gas-sensors; another technique to increase their performance consists of carefully aligning the nanostructures. The combination of nanoscaled (smallest size 1D) TMO materials and functionalization (orientation) is making the fabrication of nano-devices a reality. Binary TMOs such as TiO2 and V2O5 represent the d0 configuration; whereas, post-TMOs such as ZnO and SnO2, represent the d10 configuration.

7.2.6 Light-Emitting Diodes Recent advances suggest the use of nanoengineering for fabricating flexible and high-temperature resistance LEDs (Fig. 7.1F). Typically, TMOs and hybrids thereof are employed for these applications, especially those composed of 1D heterostructures grown directly on top of flexible films which are ideal for optoelectronic devices’ flexible, transferable, and stretchable features [66]. In particular, hybrid nanorods synthesized on graphene-based flexible films with very small contact areas favor great endurance under deformation [67]. Nanostructure morphology also plays a critical role in light output power. For instance, 3D cone-shaped nanostructures with side-wall angles of B24.1 degrees resulted in light out power enhancement of 300% higher than flat-surface conventional LEDs [68]. The ability of TMO nanostructures and/or nanoparticles to participate in redox and acid-based reactions makes them suitable for applications as catalysts for industrial applications [69,70].

7.2.7 Catalysis In recent years, much effort has been devoted to the field of nanoscience regarding TMOs due to their relatively high chemical activity along

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213

with defined interactions [71
75]. TMOs with increased surface area to volume ratios are highly efficient considering the oxidation reactions that can take place at the surface (Fig. 7.1G). Their performance as catalytic NMs can be further optimized by placing the TMO particles on a magnetic core, and thus allowing for collection and separation of the catalyst from the resultant mixture when exposed to a magnetic field. Furthermore, employing magnetic nanoparticles allows for recycling of the catalyst as a result of the ease of its recovery capabilities at the end of the reaction, along with the ability to turn off a reaction by applying a magnetic field [58]. Generally, catalytic activity increases with reduced particle size; however, decreasing particle sizes can also result in nanoparticle agglomeration as a result of lack of stability [76]. Nonetheless, this is typically resolved by immobilizing active species on solid supports. Although, Mn-oxide nanoparticles are known as efficient catalysts in the oxidation of olefins with epoxides as products [77], Rahaman et al. reported the preparation of Mn2O3 1D structures (i.e., nanorods) with enhanced catalytic activity and high selectivity for the direct synthesis of multiple aldehydes vial oxidation of alcohols [78]. Furthermore, it is important to highlight the fact that the catalytic activity of nanoscale structures can result in a 75% improvement in catalytic activity when compared to bulk components [79]. 1D structures have gained increased focus in photocatalyst applications as a result of their large surface-to-volume ratio and lower number of grain boundaries [80]. 3D mesoporous structures have been reported to yield high catalytic activity due to their ability to generate high light-harvesting ability along with the reduction of the recombination rate between photoelectrons and holes. The capacity of some TMOs to exist in various oxidation states and with a large variety of crystal structures, and others with an ability to undergo phase transition configurations (physical and chemical), makes them unique. Depending on the type, some TMOs are essential in catalysis (large surface area), photoelectrochemical (i.e., photochromism), electroluminescence (i.e., light-emitting diode), electrochemical (i.e., Li-ion batteries), optical properties (i.e., in solar panels), among others. Some of these technologies will have far-reaching practical application as TMOs with superior properties are being exploited for the development of many new technological applications as discussed above.

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7.3 FABRICATION TECHNIQUES TO SYNTHESIZE TMO NANOSTRUCTURES A variety of techniques have been proposed and developed for the synthesis of TMOs, including aqueous chemistry [81
83], CVD [84
86], pulsed laser deposition [87
89], high-temperature heating process in a restricted vacuum chamber [90], metalorganic vapor-phase epitaxy [91], combustion synthesis methods or self-propagating high-temperature synthesis [92], and flames, among others. All of these methods are based on the assembly of molecules through evaporation and deposition on a substrate through vapor-phase transport or chemical reactions. Through this method, highly pure crystalline and complex nanostructures (physically and/or chemically) can be produced. Table 7.1 lists and highlights specific methods and their combinations that have been employed to synthesize TMO nano/microstructures. Each method is briefly discussed below.

7.3.1 Chemical Vapor Deposition In the CVD method, thin films of TMOs can be formed on a substrate through chemical reactions. Oxygen and/or nonreactive gases are fed into a furnace chamber and the gases react with the metal (source) and deposit on the substrate to form a thin film of metal oxide. A variety of CVD configurations have been established and examined for the synthesis of TMOs (Table 7.1). The CVD configurations include plasma-enhanced CVD, vapor-trapping CVD, vertical furnace, and plasma-enhanced CVD; among others. The work of Teo et al. provides an excellent review of the various types of CVD and plasma-enhanced CVD systems [118].

7.3.2 Sol
Gel The sol
gel method is based on the formation of an oxide network through reactions of a metal (source) in a solution. A sol is a stable dispersion of amorphous or crystalline particles in a solvent, and a gel encompasses a solution. In the sol
gel method, the film of material is formed from agglomeration of the particles (Table 7.1).

7.3.3 Plasma Current plasma reactors have been redesigned for the synthesis of TMOs. In the plasma method, two graphitic probes perform as negative and positive electrodes. An electrical discharge is created as a result of a conducting

Table 7.1 A list of classical/conventional methods used for the growth of TMO NMs Method Experimental configuration

Illustrative works (resembled with flame-made TMOs)

CVD and related processes

Nb oxide [93,94] Zn oxide [95
97] W oxide [98,99] W oxide [100]

Sol
gel and related processes

Nb oxide [101,102] Zn oxide [103] W oxide [104,105] Fe oxide [106
109] Mo oxide [110]

(Continued)

Table 7.1 (Continued) Method

Plasma and related processes

Flames

Experimental configuration

Illustrative works (resembled with flame-made TMOs)

Nb oxide [111,112] Zn oxide [113,114] Mo and W oxide [83,115
117] Fe oxide [89]

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217

path between the two electrodes. Similar to the CVD combinations, a variety of plasma configurations (sputtering, cathodic electrodeposition, and anodization, etc.) have been designed and tested for the formation of TMOs (Table 7.1).

7.3.4 Flames Although the above-described classic conventional (nonflame) methods are capable of producing TMOs, some are generally limited by the complexity of the process (multistep), scalability, and selectivity of their products. Most of these methods require hours for the synthesis process, whereas flame synthesis can be achieved in a matter of minutes or seconds. Owing to its simplicity, flame technology represents a method that is capable of producing nanoparticles and nanofibers at the scale of several million metric tons annually. For instance, flame-generated materials include carbon blacks [produced by Cabot, Columbia, Evonik (formerly Degussa, Evonik Industries, Essen, Germany)] and the highly ordered nanostructured carbon materials such as carbon nanotubes, fullerenes and fullerene derivatives (at Nano-C), fumed silica (Cabot, Evonik), titanium dioxide or titania (Evonik, Dupont, Ishibara, Millenium, Kerr-McGee) with and without pigmentary properties and optical fibers (Corning, Heraeus, Lucent, Sumitomo) [119,120]. Flames are routinely used for synthesis of various single oxide nanoparticles such as TiO2, Al2O3, GeO2, PbO, V2O5, Fe2O3, SnO2, ZrO2, and ZnO [121
127]. Millions of tons of SiO2, TiO2, Al2O3 [122], and ZnO [128] are annually produced through this considerably rapid and inexpensive method. Various combustion-based processes and unique catalytic-fed methods have been employed for the synthesis of TMO nanopowders. Typical methods of precursor delivery into the flame medium for synthesis of TMOs include “aerosol” and “FSP” (Fig. 7.2A). It has been shown that reactant mixing, additives, and introduction of electric fields, among others can be used to control morphology (primary particle size, agglomeration, shape, particle crystallinity, etc.) and composition in the flame synthesis of TMOs. For the most part these methods of precursor delivery have resulted in the synthesis of metal-oxide products composed of aggregates formed of tightly bonded primary particles (mostly spheroidal). Powders composed of agglomerates are usually formed from the fumed aggregates during their final production.

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Figure 7.2 Schematics showing different approaches utilized in the flame synthesis of TMOs. (A) Evolution of metal oxide nanoparticles as the base material is introduced into a reaction zone using the “aerosol method” and “flame spray pyrolisis” technique; (B) “solid support” synthesis of 1D/3D TMOs; (C) gas-phase synthesis of TMOs and hybrid nanomaterials (B1
B2); (C) synthesis of TMOs by the direct oxidation of a transition metal in a postflame region.

It is anticipated that flames can also be used as an inexpensive viable alternative to produce complex nanostructured TMOs that are essential for the development of new technologies (i.e., 1D, 2D, and 3D and of unique chemical morphology). It has been shown that the “solid support” can be used to directly generate multidimensional TMOs in flames (Fig. 7.2B to C).

7.4 FLAMES AS A UNIQUE FABRICATION TOOL TO PRODUCE TMO NANOPARTICLES What makes flames attractive for the synthesis of NMs is the relatively inexpensive and single-step process. The rapid succession of heating and cooling during combustion creates materials with rich physical morphologies and composition.

7.4.1 Flame Synthesis of Metal-Oxide Nanoparticles An important controlling parameter in the flame synthesis of TMOs is the manner in which a raw material (source or precursors) can be delivered into a flame environment. The flame-synthesized products can have very different shapes and at times varying chemical composition from that of the originally introduced source or precursors.

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219

7.4.2 The “Aerosol” and “Spray Pyrolysis” Methods Table 7.2 highlights some works for the synthesis of TMOs in flames by introducing the source material into the reaction zone using “aerosol” and “FSP”. The “aerosol method” is a gaseous delivery process where the precursor is vaporized in a bubbler or evaporator before being introduced into a flame medium. In the “FSP” the raw material is introduced into the flame as a form of fine spray. Burner configuration and corresponding flame structure, type of fuel used, type of catalytic/source materials, and method of delivery of the source material are evident in Table 7.2. A schematic of the representative “morphology” of the formed powders is also presented. It is interesting to note that these methods of precursor/source delivery have resulted in TMO powders composed mostly of spherical-like primary particles and their aggregates. The general conclusion is that some of the important parameters for determining the morphological structure of the produced powders are: (1) the concentration of precursor introduced into the flame. Not only can this parameter affect the flame structure itself, but the frequency of particle collisions during synthesis can increase with an overloading of precursor vapor leading to a higher rate of agglomeration (usually undesirable); (2) the combined influence of residence time of the particles and the flame temperature. Flame temperature (directly influenced by fuel type and oxygen content) can affect the flame synthesis of the TMOs since fuel type and oxygen concentration can affect flame structure as well [119]. For instance, Jensen et al. [129] reported on the flame synthesis of both ZnO and Al2O3 powders composed of nanoparticles using a premixed methane
air flame at atmospheric pressure. High-purity zinc and aluminum precursors were introduced to the flame environment by employing an “aerosol” technique with nitrogen flow. The synthesized ZnO and Al2O3 particles had diameters in the range of 25
40 and 7.1
8.8 nm, producing surface areas of 27
43 and 184
229 m2/g, respectively. The particle diameter range was achieved by adjusting the flame temperature and precursor vapor pressure parameters. Stark et al. [130] also used the “aerosol method” in a methane-fueled coflow diffusion flame for the synthesis of vanadia-titania oxide nanoparticles. The assynthesized particles had diameters ranging from 10 to 50 nm and specific areas of 23
120 m2/g. Stark and coauthors observed that an increase of the oxygen flow rate resulted in larger diameters and smaller specific areas. During the last decade extensive research efforts have been

Table 7.2 Flame configurations and generated TMO nanoparticles Author Burner Flame type Fuel/oxidizer configuration

Jensen et al. [129]

Stark et al. [130]

Premixed

Coflow diffusion

CH4

CH4

Catalytic material and delivery

Zinc and aluminum acetyl-acetonates Aerosol—nitrogen gas

TMO structures

ZnO and Al2O3 nanoparticles

Vanadium and titanium alkoxides Aerosol—argon V2O5-TiO2 nanoparticles

Tani et al. [128]

Qin et al. [131]

FSP premixed

Coflow nanopremixed

CH4

CH4

Zinc acrylate FSP—94% methanol 1 6% acetic acid

ZnO nanoparticles

Tetraethyl orthosilicate, yttrium, and europium nitrates FSP—ethyl alcohol and nitrogen gas

Y2SiO5:Eu31 nanophosphors

Fennell et al. [132]

Flat laminar premixed

CH4

MgCl2 aqueous solution FSP—nitrogen gas MgO nanoparticles

Wang et al. [133]

Premixed

Chiang et al. [134]

Premixed

Ng et al. [135]

Premixed

CH4/O2 1 N2

Titanium tetraisopropoxide (TTIP) TiO2 nanopowders

CH4/O2 1 N2

CH4/O2

Copper nitrate, aqueous solution FSP method

CuO nanoparticles

Vanadium oxytripropoxide FSP method V2O5 Nanoparticles

(Continued)

Table 7.2 (Continued) Author Burner configuration

Pratsinis et al. [136]

Flame type

Fuel/oxidizer

Catalytic material and delivery

Premixed

CH4/O2

Titanium tetraisopropoxide (TTIP) FSP/aerosol

TMO structures

Ti oxide Nanopowders Kumfer et al. [137]

Inverse coflow diffusion

C2H4 or CH4/Ar Iron pentacarbonyl 1 O2 Aerosol FeO nanoparticles

Nasir K, Memon et al. [138]

Coflow diffusion

H2 or C2H4/Ar 1 O2

Titanium tetraisopropoxide (TTIP) Aerosol TiO oanoparticles

Height et al. [139]

Premixed

CH4/O2

Zinc naphthenate FSP method Zn oxide nanoparticles

Zhao et al. [140]

Strobel et al. [141]

Axisymmetric H2 flat premixed

Flame-spray pyrolysis

CH4—O2

Titanium tetra-iso-propoxide and aluminum tri-sec-butoxide FSP - fuel/oxidizer gas mixture

TiO2 nanoparticles

Premixed coflow flames

Pt/Ba/Al2O3

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devoted to the synthesis of TMOs in flames via the FSP method. Typical nanopowders created with this method are titania, MgAl2O4, gamma-Fe2O3, alumina, manganese oxide, and zirconia. For instance, Tani et al. [128] reported on the use of the FSP method for synthesis of ZnO nanoparticles using a coflow premixed flame. The average particle diameter was controlled between 10 and 20 nm by the solution feed rate, and the particles exhibited a highly crystalline wurtzite (hexagonal) structure. Zinc acrylate was dissolved in methanol to create the precursor solution, and methane and oxygen were used as the fuel and oxidizer, respectively. By increasing the precursor feed rate, the flame height was increased, resulting in greater surface growth and larger-diameter particles. Similarly, Qin et al. [131] reported the production of europium-doped yttrium silicate (Y2SiO5:Eu31) nanophosphors. The experimental combustion system consisted of a coflow methane/oxygen diffusion flame and an ultrasonic spray generator. As with other metaloxide flame synthesis studies, the particle diameter increased as the precursor loading increased. Fennell et al. [132] utilized a fuel-lean, premixed, laminar flat-flame burner to synthesize highly crystalline MgO nanoparticles with narrow-diameter size distributions. The flame was formed by acetylene and nitrogen/oxygen compositions. An aqueous solution of MgCl2 was nebulized as a fine spray and mixed with the fuel/oxidizer stream before combustion with a magnesium concentration of approximately 13 ppm. Memon et al. [138] studied the growth of titanium oxide nanoparticles that are synthesized using a multielement diffusion flame burner (MEDB). Argon and hydrogen or ethylene are used as the precursor carrier gas and oxygen and argon are used as the oxidizer in the MEDB. The TTIP precursor, injected through a syringe pump, is heated to nearly 180°C to prevent condensation of the TTIP. The nanomaterials were collected on an aluminum plate, maintained at 100°C, downstream of the burner. When using the hydrogen/argon mix and the ethylene/argon mix as the carrier gas, the team reported the growth of pure anatase TiO2 nanoparticles and carbon-coated TiO2 nanoparticles, respectively. Height et al. [139] synthesized zinc oxide (ZnO) nanorods using a premixed flame formed with methane and oxygen, and by inserting the precursors using the “FSP method”. The solvent precursor was fed through the nozzle using a syringe pump as oxygen sheath gas was concentrically introduced around the nozzle. Zinc naphthenate and toluene were combined to form the liquid precursor solution. With the help of a vacuum pump, the nanomaterials were

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225

collected on a water-cooled glass fiber filter. The group reported that ZnO displayed single-phase wurtzite structure when doped with indium, tin, and lithium.

7.4.3 Approaches Used for Control Synthesis in “Aerosol” and “FSP” As discussed earlier, conventional methods (i.e., CVD and plasma) can be combined to form a hybrid or combined system (plasma-enhanced CVD) to design or form novel types of TMOs. Similarly, in the application of flames for the synthesis of TMO nanoparticles, some innovative processes (besides the popular precursor feed rate parameter) have been introduced as a means to control their formation. The ability to easily introduce many variables to fine tune/manipulate controlled synthesis makes flames a powerful method compared to the conventional methods. Some introduced variables include unique burner configurations, the simultaneous combination of two flame volumes, and the introduction of external forces such as electric fields, among others (Table 7.2). One interesting and frequently explored variable is the introduction of electric fields (EFs). This may be due to the successful early works of pioneers in this field revealing the effect of the change in flame geometry, velocity of flame gases, heat transfer rate, and control of the particulate matter (flame luminosity) upon the introduction of EFs [142
146]. Therefore, those early EF flame works have provided unique tools for researchers in flame synthesis. The very recent work of Li et al. presents an excellent review on the various techniques employed for controlling the flame synthesis of metal-oxide nanopowders to form structured films [147]. Li et al.’s work presents a review of the flame aerosol deposition to form the TMO films and discusses the various deposition mechanisms for their formation. The introduction of external EFs in the flame synthesis of ceramics and TMOs has allowed for high-precision control of the primary particle size, degree of the particle agglomeration (powder morphology), and crystallinity [148,149]. EFs formed using electrodes of various geometrical shapes (i.e., plate vs needle shapes placed across the flame) and of various strengths have been tested in flames (diffusion and premixed) and have been revealed to be an excellent control variable in the manufacture of ceramics and TMOs [150,151]. EFs can be used to either increase or decrease the size of the primary particle [152,153]. As the particles are forming in the flame, they are charged by the EF and repel each other to prevent coagulation. Zhao et al. [140] studied the effect of EFs on controlling the

226

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nanoparticle size and level of agglomeration on titania nanoparticles. A flat-flame burner geometry was used with the fuel source (hydrogen), the oxidizer (oxygen), and the precursor vapor and carrier gas all being delivered as a premixed blend. The nanoparticles were collected thermophoretically on a cooled substrate downstream from the flame. The EF was established between the burner itself and the cooled substrate by a dual-polarity, high-voltage power source. Applying the EF allowed for a decrease of particle size from 40 to 18 nm without affecting any other material properties (e.g., crystallinity). Another innovative and creative approach is to introduce “multinozzle flames” that deal with the merging of flame volumes which has been proposed and developed in the flame synthesis of TMOs using the “aerosol” and “FSP” methods. For instance, Strobel et al. used a two-nozzle FSP for the single-step preparation of Pt/Ba/Al2O3 NMs [141]. In one of the flames, Al precursors were introduced, while in the other barium/platinum was introduced, resulting in individual crystalline Al2O3 and BaCO3 nanoparticles. It is reported that the internozzle distance can be used as a variable to control the products in the two-nozzle synthesis. It is concluded that if the two flames come into contact later (tips region only) mixing of the two types of particles only agglomerate and do not sinter into a single particle as lower temperature prevails late in the flame [141]. The introduction of a swirler device inside a burner to tune/manipulate the flow dynamics and deposition of TMOs on a substrate is another interesting approach that was introduced by Wang et al. [133].

7.5 FLAME SYNTHESIS OF MULTIDIMENSIONAL TMOs USING THE “SOLID SUPPORT” METHOD The introduction of the raw material in the highly oxidative environment of a flame in the form of high-purity wires or meshes can yield TMOs of higher dimensional shapes. This method does not involve the introduction of catalysts or dopant vapors, but instead the raw material is introduced in the form of a solid substrate and we refer to it here as the “solid-support”. The wire/meshes can also serve as supporting surfaces, where structures are grown or recrystallized. The “solid support” method has been applied to the synthesis of various TMOs including Mo, W, Fe, Zn, and others as the base metals (Table 7.3). This method works very well for the synthesis of multidimensional shaped TMOs on solid supports or directly in the flame medium [119,155,156,163,166
172]. It is well

Table 7.3 Flame configurations and generated 1D and 3D TMO nanostructures Author Burner configuration Flame type Fuel/oxidizer

Catalytic material

Structure

Rao et al. [154]

Coflow diffusion

CH4 1 H2/air

Mesh (W)

Nanowires

Rao et al. [155]

Coflow diffusion

CH4 1 H2/air

Mesh (Fe)

Nanowires

Xu et al. [156]

Counterflow diffusion

CH4/air

Probe (W)

Nanowires

MerchanMerchan et al. [157]

Counterflow diffusion

CH4 1 C2H2/ O 2 1 N2

Probe (W)

Hollow and semihollow large 3D structures

(Continued)

Table 7.3 (Continued) Author Burner configuration

Flame type

Fuel/oxidizer

Catalytic material

Rao et al. [158]

Premixed

CH4/air

Mesh (W)

Cai et al. [159]

Coflow diffusion

CH4 1 H2/air

Mesh (Mo)

Single (i), branched (ii), flower-like (iii)

MerchanMerchan et al. [160]

Counter-flow diffusion

CH4 1 C2H2/ O 2 1 N2

Probe (Mo)

Solid channels (i), hollow channels (ii), dendrite formation (iii)

MerchanMerchan et al. [161]

Counterflow diffusion

CH4 1 C2H2/ O 2 1 N2

Probe (Nb)

Kathirvel et al. [162]

Premixed

C2H2/O2

Metallic zinc (Zn)

Structure

Nanorods

Xu et al. [163]

Inverse coflow diffusion

CH4/air

Probe (Zn)

Hexagonal nanowires

Xu et al. [164]

Counterflow diffusion

CH4/air

Probe (Zn)

Tower-like structure

Dong et al. [165]

Hybrid method (“solid support” and electrodeposition)

CH4/air

Probe (W)

WO2.9@Al (core
shell) Counterflow diffusion

Probe (Al)

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Nanomaterials Synthesis

understood now that the interaction of a transition metal wire (source material) with a highly oxidative environment containing O and OH radicals results in rapid oxidation of the wire with the formation of metal oxides in high oxidation states in the form of layers on the surface of the probe (Fig. 7.2B). The layers are typically formed on one side of the probe, most likely the side exposed to the highly oxidative region of the flame. This is followed by the formation of metal oxide precursors originating from the oxide layers upon their vaporization or sublimation. The metal oxide precursors are transferred by the gas flow along the wire surface for their crystallization in colder regions in and/or out of the flame (on the same probe or other designed surfaces). Some of the most popular parameters for selective design of the TMO morphology with multidimensional shapes in the “solid support” method are the geometry of the reactor used to form the flame medium, diameter size of probe, the ratio of introduced fuel to oxidizer, oxygen content in the oxidizer stream (O2/N2), among others. Table 7.3 presents a list of works on the flame synthesis of multidimensional TMOs using the “solid support” in a variety of flame configurations. The underlying growth mechanism of the TMO structures using the “solid support” in flames appears to be composed of multi- and interrelated submechanisms that occur continuously and nearly simultaneously. The first submechanism is responsible for the conversion of the metal to metal oxide layers on the surface of the “solid support” upon its exposure to the flame volume. The second submechanism is responsible for the melting/sublimation of the newly formed metal oxide layers into vapors or precursors. The third submechanism is of a molecular level that deals with the nucleation and crystallization for the growth of the TMOs.

7.5.1 Parameters Affecting the Flame Synthesis of Multidimensional TMOs In this section, as an illustration of the “solid support”, we show the effect of selected parameters on the synthesis of multidimensional TMOs. The tested probes/wires were made of high-purity Mo, W, and Fe and inserted in a counterflow diffusion flame. The studied parameters included: (1) wires (raw material) of 1.0 and 0.75 and 0.5 mm diameters; (2) time of probe/flame interaction; (3) flame position; and (4) effect of oxygen concentration on the oxidizer stream. Fig. 7.3 represents typical morphological characteristics of TMOs synthesized using the “solid support” upon the various tested parameters which are discussed below.

Flame Synthesis of Nanostructured Transition Metal Oxides

231

Figure 7.3 The flame synthesis using the “solid-support” method resulted in TMO structures with a variety of shapes and sizes depending on elemental composition of the introduced material source, probe size diameter, and flame characteristics. (A) Schematic illustrating deposit or crystallization locations on the surface of a utilized probe; (B) representative of W oxide rods, channels, and sheet like structures; (C) 1D Fe oxide nanostructures of different morphologies including sudden bending, zigzag, and branching; (D) typical 3D Mo oxide (channels with nanoscale wall thickness) and conical shaped structures. B1
B6: Adapted from W. Merchan-Merchan, A.V. Saveliev, W. Cuello Jimenez, Solid support flame synthesis of 1-D and 3-D tungsten-oxide nanostructures, Proc. Combust. Inst. 33 (2010) 1899
1908; C1
C6: Adapted from W. Merchan-Merchan, A.V. Saveliev, W. Cuello-Jimenez. Novel flame-gradient method for synthesis of metal-oxide channels, nanowires and nanorods. Journal of Experimental Nanoscience, 2010.

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7.5.2 Effect of Probe Diameter and Oxygen Content in the Oxidizer The introduction of a W wire (B1 mm in diameter) resulted in deposits on the top of the probe (Fig. 7.3A2). The SEM image in Fig. 7.3B1 was collected on the surface of the 1-mm W probe inserted at a flame height of Z 5 B13 mm for a deposition time of 2 min. HR-SEM imaging of an area in Fig. 7.3B1 reveals that the layer is composed of numerous elongated structures randomly oriented with very high aspect (length-todiameter) ratios (Fig. 7.3B2). Nanorods with lengths of more than 50 μm and with diameters of less than 100 nm are observed. To test the effect of probe diameter on the synthesis of the oxides, a W probe with a diameter of 0.5 mm was used; the fuel (96%CH4 1 4%C2H2) and oxidizer were kept the same (air). The reduction of the probe diameter resulted in the synthesis of a different type of structure from those present in the 1-mm diameter probe. The introduction of a 0.5-mm W diameter probe at the flame height of Z 5 11 mm for a deposition time of 2 min resulted in the formation of high-density materials composed of large 3D structures (Fig. 7.3B3). Fig. 7.3B4 represents a HR-SEM view of a selected area in Fig. 7.3B3 showing that the cluster is composed of a high density of 3D structures of square, rectangular, and triangular shapes as illustrated by the arrows. To test the effect of oxygen content in the oxidizer stream (keeping the fuel content and probe diameter B1 mm the same) the oxygen was changed to 50%O2 1 50%N2. The insertion of 1-mm diameter W probes in the flame medium at flame heights of Z 5 8 and 9 mm for 2 min resulted in deposits formed of micron-sized flat ribbon-like structures of nanoscale thickness (Fig. 7.3B5 and B6). The morphologies are quite different to those formed using the oxidizer as air only. Arrows 1 and 2 in Fig. 7.3B5 are pointing to both the flat ribbon-like and the elongated nanorod structures, respectively. High-resolution SEM on a selected area between the boundary of the ribbon-like and nanorod structure layers shows that nanorods extrude from the edges of the ribbon-like structures as pointed out by the arrows in Fig. 7.3B6. The interpretation of the synthesis mechanism of the TMO structures is based on the formation of metaloxide layers on the surface of the probe/source and their subsequent vaporization or sublimation to form the metal-oxide precursors/vapors. It is important to note that the flame is rich in oxygen species. The vapors are transferred by the gas flow along the wire surface and their

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233

condensation/deposition occurs on the upper sides of the probe in the reduced-oxygen, lower-temperature environment (Fig. 7.3A). The insertion of the probe with a reduced diameter (in this case a 0.5-mm W) resulted in the formation of high-density materials composed of large 3D structures. The synthesis results are affected by the size of the probe due to several factors: (1) the size of the probe limits the variation of chemical environment and temperature over its surface, (2) the temperature of the probe is defined by the balance of convective heating and radiant losses, and thus the smaller-diameter probes have higher temperatures approaching the local flame temperature at the limit of extremely small probe diameters, and hence the evaporation or sublimation rates of the oxide layers are higher. The amount of oxygen concentration in the oxidizer stream can affect flame temperature, chemistry, and importantly the overall flame structure. Depending on the flame type, the oxygen enrichment can significantly change flame structure. For instance, in a counterflow reactor (as in the present case) the flame’s thickness is compressed, reducing the overall flame volume but yet increasing the high-temperature blue zone (oxidative region rich in OH and O radicals) [173]. This mechanism was also tested by the introduction of a different type of transition metal. The introduction of 1-mm diameter Mo probes at the flame height of Z 5 B11 for a period of 2 minutes resulted in the formation of striking rectangular micron-sized hollowed channels (Fig. 7.3D1). The thin, prismatic, four-faced structures are entirely hollow with very large inside cavities devoid of any other materials. The SEM images in Fig. 7.3D4
D8 are characteristics of the deposits formed in the Mo probe with a diameter of 0.75 mm for a deposition time of 30 seconds. Interestingly, the formed structures exhibit completely different morphology from those synthesized on the 1-mm diameter Mo probe at the same flame position (Fig. 7.3D2). Close inspection shows that a few rectangular structures, although physically disturbed, are present at the base as highlighted by white arrows in Fig. 7.3D8.

7.5.3 Effect of Thermal Property of the Source Material—Iron Oxide Structures The introduction of an iron wire resulted in the formation of iron oxide nanorods (Fig. 7.3C). Experiments with Fe probes showed that probes positioned closer to the flame front where the temperature is high (BZ 5 11 and 12 mm) resulted in melting (almost instantaneously)

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of the substrates [146]. At the Z 5 11 mm the flame temperature is approximately 2500K, therefore the probe temperature easily reached the melting point of iron (1811K). In order to prevent melting of the probes, the probes were inserted in colder flame zones (i.e., BZ 5 8.5, 9.0, 9.5, and 10.0 mm). SEM images collected of an iron probe reveal a high density of the synthesized nanostructures protruding from the probe surface (Fig. 7.3C). HR-SEM imaging analysis reveals that the materials formed are composed of 1D elongated structures characterized by high length to diameter ratios (Fig. 7.3C). The diameters of the iron oxide rods vary from approximately 10 to 100 nm with a typical length of a few microns. Close inspection of the as-grown products by HR-SEM/TEM analysis reveals that among the straight and uniform iron-oxide structures, nanostructures with different morphologies are also synthesized, specifically structural geometries that contain bent, branched, and zigzagged formations. The branched structures have T- and Y-branched shapes, as highlighted by the arrows in Fig. 7.3C. In the area of nanomaterials, the modified “branched” and “bent” structures are excellent candidates for fabricating composite materials with enhanced mechanical or electronic properties [174
176]. The multibranched structures have great potential in the development of CNT-based circuits in nanoscale electronics devices [177].

7.5.4 Effect of Flame Positon Fig. 7.3D1 represents SEM images collected from the 1-mm diameter Mo probe exposed to the flame at the height of BZ 5 11 mm. The repositioning of the Mo probe to the flame height of Z 5 B12 mm (high flame temperature), resulted in the formation of channels with square and rectangular as well as circular cross-sections Fig. 7.3D2. The repositioning of the 1-mm diameter Mo probe to the flame height of Z 5 13 mm (where the flame temperature and oxygen concentration are substantially higher) resulted in structures that were much less organized and uniform than those formed at the flame heights of Z 5 11 or 12 mm (Fig. 7.3D3). The progression of the flame synthesis of TMOs at the flame heights of Z 5 11, 12, and 13 mm shows the morphological change of the structures as a function of the flame environment. As the probe is relocated closer to the flame front, the probe oxidation rate increases, providing a high rate of Mo oxide and affecting the morphology of the synthesized structures.

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7.5.5 Hybrid Approach to Form Core
Shell TMOs Through “Solid Support” and Electrodeposition Similarly, some innovative approaches/processes that combine flames with sequential nonrelated flame processes have been developed for the formation of unique/modified TMOs. For instance, an innovative approach for the synthesis of TMOs in the form of composites has been developed by Tse’s group [165]. In that contribution, a two-step synthesis process (flame and electrodeposition) for the synthesis of tungsten-oxide/ Al nanowires (WO2.9/Al NWs) was employed. In that sequential process the first array composed of W oxide nanowires (NWs) is formed using the “solid support” in a W probe inserted in a counterflow flame. The probe with newly formed W oxide NWs is exposed to an ionic-liquid electrodepostion method containing an Al wire. It is reported that the electrodeposition allows for a uniform Al film of nanoscale thickness deposited on the surface of the W oxide NWs as a result forming TMOs of core
shell morphology. In a second contribution, the flame-formed WO2.9 NWs were exposed to an aqueous ethylenediamine solution where the Sn21:Zn21 molar ratio was used a control parameter. This resulted in the WO2.9 NWs decorated with hexagonal ZnO nanoplates, Zn2SnO4 nanocubes, and SnO2 nanoparticles. The morphology of the Zn-based materials depends on the molar ratio [178]. Zheng’s group synthesized a variety of TMOs by employing high-purity transition metals in the form of meshes as the source material. The meshes are not necessarily introduced in the flame reaction zone but are rather placed just above a flame zone [154,155,158,159] (Table 7.3). This method requires the introduction of a substrate placed further down the flame front for material deposition. This process has resulted in the flame synthesis of TMOs of higher-dimensional shapes.

7.6 VOLUMETRIC FLAME SYNTHESIS OF 1D AND 3D TMOs “Gas phase” combustion synthesis has proved to be one of the most versatile and promising techniques employed for scale-up production of nanoscale materials [179,180]. Modern applications of TMOs require them to be of multidimensional (i.e., 1D, 2D, and 3D) shapes. Whether this becomes a reality will depend on the ability to design controllable synthesis methods for the generation of elongated objects with a high aspect ratio and at large scales. The ability to synthesize the structures directly in the gas-phase makes the flame a continuous single-step process ideal for large-scale production of the nanostructures. Table 7.4 represents

Table 7.4 Gas-phase burner configurations for flame synthesis of 1D and 3D TMO nanostructures Author Burner Flame type Fuel/oxidizer Catalytic material configuration

Height et al. [139]

Tani et al. [128]

Memon et al. [138]

Coflow premixed

Premixed

Coflow diffusion

CH4/O2

CH4

Toulene/zinc naphthenate solution, indium and tin dopants FSP

Zinc acrylate FSP—94% methanol 1 6% acetic acid FSP

Structure

ZnO nanorods

ZnO nanoparticles

H2 or C2H4/ Titanium tetraisopropoxide (TTIP) Ar 1 O2 Aerosol TiO nanoplatelets

MerchanMerchan et al. [174]

Counterflow CH4 1 diffusion C2H2/ O 2 1 N2

Probe (Mo)

Cubical nanorods

MerchanMerchan et al. [176]

Counterflow CH4 1 diffusion C2H2/ O 2 1 N2

Probe (W) Nanorods, nanoplatelets, hybrid nanomaterials

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a list of works on the “gas phase” synthesis of 1D and 3D TMO nanoforms using “aerosol”, “FSP”, and “solid-support” feed; these methods are discussed below.

7.6.1 TMOs (1D) Formed Using “Aerosol” and “Spray Pyrolysis” In some cases under certain conditions the “aerosol method” and “FSP” have resulted in the synthesis of elongated TMOs. Height et al. reported that the introduction of tin and indium dopants in their “FSP” experiments altered the structures to form rod-like shapes (Fig. 7.4A1
A4) (with closely controlled aspect ratio) as the concentration was elevated while the lithium dopant did not have an effect on the shape of the ZnO [139]. Furthermore, they reported that the specific surface area for the indium- and tin-doped ZnO materials increased as the dopant concentration was elevated. These materials are promising in sensing, electronics, and optical displays. Tani et al. [128] reported on the use of the FSP method for the synthesis of ZnO nanoparticles using a coflow premixed flame. By increasing the precursor feed rate, the flame height was increased, resulting in greater surface growth and larger-diameter particles (Fig. 7.4B1
B2). The introduction of a higher precursor feed rate resulted in elongated TMOs made of Zn (Fig. 7.4B3
B4). Memon et al. reported that the introduction of metal vapor precursors (aerosol) using hydrogen and ethylene as the carrier can result in the synthesis of nonspherical particles [138]. For the hydrogen mix, the group reported nanoparticles exhibiting a polyhedral shape with diameters ranging from 50 to 100 nm (Fig. 7.4C1
C2). For the ethylene mix, the group reported a carbon-coating thickness of approximately 3
5 nm [138].

7.6.2 “Gas Phase” Flame Synthesis of 1D and 3D TMOs The “solid support” studies also lead to the flame gas-phase synthesis of 1D and 3D TMO nanostructures by the authors and others. During the “solid support” synthesis of TMOs it was noticed that the flame medium slightly changed color as the result of the insertion of the wire (source). One of the hypotheses was that the change in flame color was due to the rapid oxidation of the inserted probe at its surface. To prove this hypothesis TEM grids were inserted (thermophoretic sampling technique) in the flame volume at various distances from a W probe for potential sample collection. Indeed, these experiments showed that when a Mo probe was

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239

Figure 7.4 Evolution of the volumetric flame synthesis of 1D and 3D various TMO structures (Zn oxide A1
A4 and B1
B4; Ti oxide C1
C2; Mo oxide nanostructures D
F). A1
A4: Adapted from M.J. Height, L. Mädler, S.E. Pratsinis, Nanorods of ZnO made by flame spray pyrolysis, Chem. Mater. 18 (2006) 572
578; B1
B4: Adapted from T. Tani, L. Madler, S.E. Pratsinis, Homogeneous ZnO nanoparticles by flame spray pyrolysis, J. Nanopart. Res. 4 (2002) 337
343; C1
C2: Reprinted from N.K. Memon, D.H. Anjum, S.H. Chung, Multiple-diffusion flame synthesis of pure anatase and carboncoated titanium dioxide nanoparticles, Combust. Flame 160 (2013) 9; D
F: Adapted from W. Merchan-Merchan, A.V. Saveliev, M. Desai. Volumetric flame synthesis of welldefined molybdenum oxide nanocrystals. Nanotechnology 20 (2009) 475601 (6pp).

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inserted in the flame, Mo oxide structures of polygonal shape were formed directly in the gas phase (Fig. 7.4D
F). The presence of ultrasmall crystals is observed close to the surface of the solid probe (Fig. 7.4D1
D2). Particle transformation occurs as the small crystals are pushed upstream in the direction of the fuel nozzle by the gas flow. Sampled material collected farther away from the probe’s surface shows the presence of larger nanocrystals with the shape of cubes and rods (Fig. 7.4F1 and F2). The arrows in Fig. 7.4F2 highlight nanorods. The direct synthesis of the TMOs in the flame volume involves oxidation of the solid wire to form various oxide layers on the surface of the probe exposed to the hightemperature oxygen-rich region of the flame (Fig. 7.4D). Evaporation and/ or sublimation of the oxides and their further transfer from the hightemperature oxygen-rich region to the lower-temperature fuel zone creates an ideal environment for metal oxide synthesis. As the metal oxide vapor precursors travel in the gas flow, structures are crystallized in the form of elongated 1D nanorods and 3D nanorod octahedron nanoplatelets (Fig. 7.4D
F). It appears that the manner in which the source material is introduced into the flame medium continues to be one of the key parameters for the synthesis control of TMOs. The arrow in Fig. 7.4F3 points to an intermediate structure attached to the surface of a large cube supporting our hypothesis that the ultra-small crystal structures present at the lower part of the flame are clearly the building blocks for the well-faceted Mo oxide cubic and nanorod crystals formed in the upper flame zone. It can be suggested that these small crystal structures are formed by the initial vapor condensation at the high-temperature zone of the flame.

7.7 CORE
SHELL AND MIXED TRANSITION METAL OXIDE NANOSTRUCTURES The simple crosslinking of two transition metals with oxygen atoms or the combination of two TMOs into a single compound (or a core
shell TMOs X@Y) allows for the formation of an entirely new type of material. A core-shell structure is defined as comprised of a core (inner material) and a shell (outer layer material). For instance, a core-shell structure composed of an inner metal with an outer oxide layer is referred as metal@oxide. In recent years, extensive research has been devoted to the study of these NMs revealing the unique properties and the potential applications in a broad-spectrum of sciences and

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241

engineering. MTMO and core
shell TMOs have been shown to have superior chemical, piezoelectric, optical, EC, and photochromic properties and are essential components for the development of new state-of-the-art technologies (ranging from catalysis, LIBs, smart windows, medical and biomedical applications, among others) that stem from the superior properties that they exhibit [181
186]. However, the development of these new technologies is greatly hampered by the high cost of production of the NMs. Therefore, a continuous and facile method for the production of these novel nanomaterials is needed. The ideal method should be able to produce the NMs at a high rate with controllable composition, nanosizes, crystallinity, shape, structure, and morphological characteristics. Therefore, the development of a method to generate such structures that is facile, inexpensive, and has a fast growth rate, will impact whether these new technologies become commercially viable in the future. Various synthesis methods including CVD [187,188], sol
gel method [189], and plasma [190,191] have been used for the synthesis of mixed Mo-W oxides. CVD and the sol
gel methods are currently considered the “ideal methods” for the synthesis of MTMOs. Both CVD and sol
gel are composed of multistep processes. Both methods initially require the pyrolytic decomposition of the metal/alkoxide precursors. The sol
gel method requires a timeintensive ultrasonic bath to achieve homogeneous mixing of the solution containing the pyrolytic decomposed precursors. In the same process, an extensive “aging process” is required (B24 h). Both methods require extensive deposition time and annealing in the final stage.

7.7.1 Flame Synthesis of Core
Shell and MTMOs on Solid Substrates Table 7.5 lists important works on the flame synthesis of MTMOs and core
shell TMOs using different types of flame configuration and techniques. Some of these methods are hybrid, which allows for the combination of different deposition processes of the vapors [163,198]. The process includes “flame vapor deposition”, “solid diffusion growth”, or a combination of the two to synthesize 1D binary metal oxide and complex core
shell TMO nanostructures [192]. In the “solid diffusion growth” process some areas of the substrate serve as the metal source. This process requires the flame to rapidly heat the metal. This causes the metal to diffuse to the surface of the source material and become oxidized by the flame, forming metal oxide nanostructures on the original substrate. The

Table 7.5 Flame-generated core
shell and mixed metal oxide nanostructures Authors Burner configuration Method of growth Fuel—oxidizer

Cai et al. [192]

Solid diffusion

CH4 1 H2—air

Flame type

Structure

Coflow diffusion Cu oxide nanowires

Sequential solid
vapor CH4 1 H2—air

Coflow diffusion

(1) Core
shell NWs; (2) NWs with branches

Cai et al. [192]

Vapor
vapor

CH4 1 H2—air

Coflow diffusion W-doped MoO3: (1) nanoflowers and (2) platelet-like structures

Farmahani et al. [193,194]

Gas phase

CH4 1 C2H2—O2 Counterflow 1 N2 diffusion W-doped MoO3 nanocubes

Stark et al. (Maddler) [195]

Ismail et al. (Memon) [196]

Flame-spray pyrolysis

Flame-spray pyrolysis

CH4—O2 1 N2

H2—O2 1 Ar

Air-assisted nozzle with flamelets

Ce0.5Zr0.5O2

Diffusion flame and flamelets C-TiO2, Fe/C-TiO2, V-TiO2

(Continued)

Table 7.5 (Continued) Authors Burner configuration

Method of growth

Fuel—oxidizer

Zhang et al. [197] (Axelbaum)

Flame-assisted spray technology (FAST)

MeOH 1 H2—Air Coflow diffusion

Flame type

Structure

LiMn0.5Ni1.5O4

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245

“flame vapor deposition” process for the synthesis of MTMOs as described by Cai et al. encompasses: (1) the flame oxidizes the surface of the metal (e.g., Mo mesh); (2) the newly formed metal oxide sublimates to form vapors; and (3) the metal oxide vapors condense onto colder growth substrates, placed further down the flame front, in the form of 1D NMs. A multistep process called “sequential solid-vapor growth” was used to generate nano TMOs of a core
shell structure. The sequential solid-vapor growth method combines both the flame vapor deposition and solid diffusion growth process to form the complex metal oxide nanostructures. In the Cai et al. contribution, copper oxide (CuO) nanowires were formed first by the process of solid diffusion growth on the surface of a Cu substrate. The newly grown CuO nanowires serve as nanosubstrates to be used in the flame vapor deposition of a 4 3 4 cm, 99% purity Mo wire mesh (0.318 cm wire spacing and 0.064 cm wire diameter). MoO vapors are deposited on the already-formed CuO nanowires, coating them with MoO3 in the form of solid shells or branches. It was reported that the morphology of the metal oxide nanostructures was dependent on the MoOx vapor concentration, growth substrate temperature, and the time of growth. When TMo and TCu are maintained at 480°C and 420°C, respectively, for 10 minutes with a 0.5 fuel to oxygen ratio, MoO3 is deposited evenly around the already-grown CuO nanowires. For complex metal oxide nanostructures to occur, the growth rate for the creation of both oxides (e.g., MoO and CuO) needs to be similar. This can be difficult since the flame vapor deposition method has a much faster growth rate than the solid diffusion growth method. Therefore, the Cu substrate is kept at a temperature at which the nanostructure growth is fastest (B500°C) and the Mo mesh wire was maintained at a temperature resulting in low MoOx vapor concentration, where the fuel (CH4 and H2) to oxidizer (air) equivalence ratio was 0.53 and the Mo source mesh was B550°C. The temperatures of the meshes are achieved by the use of heat sinks in the form of plain steel cooling meshes. The cooling meshes are placed between the flame and the source mesh as well as between the source mesh and the growth substrate as needed for temperature control. Fig. 7.5A1 and B1, and C1 and D1 contains SEM images of the as-grown structures as the result of the synthesis process via the sequential solid
vapor growth and the simultaneous vapor
vapor growth methods, respectively. The CuO distribution for the complex metal oxide (Fig. 7.5A2 and A3) is highly concentrated at the center of the nanostructure, while the MoO is more densely dispersed on the outside of the structure (Fig. 7.5A4), forming a core
shell morphology. It is reported that the effect of increasing the Mo mesh temperature

Figure 7.5 TEM and SEM images of various flame-formed MTMOs, both with corresponding elemental mapping (where appropriate). Structures in (A
D) are all synthesized on solid supports while the structures shown in (E1
E4) are via flame spray pyrolysis or related technology. (A and B) Via “sequential solid
vapor growth” (A1
A4) TMOs of CuO/MoO3 core
shell with corresponding elemental mapping; (B1
B4) TMOs with branched structures; (C and D) TMOs formed via “vapor
vapor growth” (C1
C4) and (D1
D4) MTMOs in the shape of platelets and nanoflower-like structures, respectively. (E1 and E2) Ceria/zirconia mixed nanocrystals and (E3 and E4) nanostructured LiNi0.5Mn1.5O4. A1
A4; B1
B4, C1
C4, and D1
D3: Adapted from L. Cai, P.M. Rao, Y. Feng, X. Zheng, Flame synthesis of 1-D complex metal oxide nanomaterials, Proc. Combust. Inst. 34 (2013) 2229
2236; E2: W.J. Stark, M. Maciejewski, L. Mädler, S.E. Pratsinis, A. Baiker. Flame-made nanocrystalline ceria/zirconia: structural properties and dynamic oxygen exchange capacity, J. Catalysis 220 (2003) 35
43; E3
E4: X. Zhang, H. Zheng, V. Battaglia, R.L. Axelbaum. Flame synthesis of 5 V spinel-LiNi0.5Mn1.5O4 cathode-materials for lithium-ion rechargeablebatteries, Proc. Combust. Inst. 33 (2011) 1867
1874.

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247

(TMo 5 B510°C) and growth time (t 5 B30 min) while simultaneously decreasing the substrate temperature (TCu 5 B350°C) results in an MoOx vapor concentration increase. This increase results in the growth of triangular-shaped nanoplates (average side length 200 nm) forming on the Cu substrate nanorods (Fig. 7.5B1
B4). As the MoOx vapor concentration was increased even more, the formation of more densely packed rectangular nanoplates occurred on the CuO nanowires. The dispersion of the CuO and MoO for the nanostructures did not change, as the MoO3 still formed around the CuO core. Changes in substrate or growth metal temperature did not result in any changes to the diameter of the CuO core already preformed by the flame through the solid diffusion growth; only variances in the MoO shell are observed using sequential solid
vapor growth. Cai et al. [192] (Table 7.5) also used the simultaneous vapor
vapor deposition method to synthesize 1D complex W-doped MoO3 nanoflower-like structures. This flame synthesis process requires the simultaneous oxidation of two source materials and their deposition on the surface of a substrate placed further downstream. The simultaneous vapor
vapor deposition process involves the use of two different transition metals (i.e., Mo and W), which are placed in the postflame region. The metals are then oxidized by the flame, producing metal oxide vapors by means of sublimation. The sublimated vapors formed, WOx and MoOx, then move with the gas flow toward a silicon (Si) substrate placed further downstream. Complex W-Mo-O nanostructures are formed when the vapors condense and deposit on the colder Si substrate. In order to successfully form mixed metal oxides, rather than separate binary oxides, the nucleation and growth rates of both metals need to be similar. The structure of the complex metal oxides are found to be dependent on the temperature of the growth substrate. W and Mo mesh wire temperatures were kept relatively constant, using the steel cooling meshes, at around B670°C and B560°C, respectively. When the vapor concentrations and substrate temperature (B351°C) were comparatively low, square and hexagonal nanoplates of average area 1 μm were formed on the Si substrate after 20 minutes (Fig. 7.5C1
C4). To obtain a low vapor concentration the fuel to oxidizer equivalence ratio was set at 0.8 and the temperature of the metals was B667°C for the W mesh and B558°C for the Mo mesh. These nanoplates had layer structures indicating the nanostructures contained evenly dispersed Mo and W (Fig. 7.5C3 and C4). The atomic ratios of Mo to W were estimated to be 22:1. Most of the complex metal oxides formed have a crystal structure

248

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similar to that found in α-MoO3. Additional nanostructures were observed after increasing the vapor concentrations and substrate temperature (415°C). This was done by setting the fuel oxidizer ratio to 0.5; the precursor temperatures were Tw 5 B673°C and TMo 5 B563°C. The change in parameters resulted in the formation of WO3 nanowires as well as MoO3 nanobelts (Fig. 7.5D1
D3). Gold (Au) nanoparticles (150 nm diameter) were then seeded onto the Si growth substrate as catalysts. Au catalysts can absorb metal oxide vapors while lowering the nucleation barrier of complex metal oxides; thus, promoting the formation of Cu oxide@Mo oxide NWs [192] (Table 7.5). The average length and width of the nanoribbons were 3 and 0.2 μm, respectively (Fig. 7.5D). A higher amount of W doping occurs with increased substrate temperature, as the Mo to W atomic ratio was estimated to be 16:1 for the nanoflowers. The crystal structure for most of the nanoribbons maintained the α-MoO3 crystal structure, while a small population are transformed to the β- MoO3 phase.

7.7.2 Synthesis of MTMOs and Related Core
Shell Nanostructures Using “Flame Spray Pyrolysis” As noted in the previous sections of this chapter, flame aerosol synthesized products are strongly dependent on the state in which the precursor is fed into the flame medium. This basic principle was employed by Stark et al. in FSP to selectively synthesize ceria-oxide and ceria/zirconia mixed nanocrystals of a high specific surface area and improved thermal stability [195]. It was reported that the introduction of Ce (“FSP”) using an isooctane/acetic acid/2-butanol carrier resulted in large CeO2 structures of a polydisperse size distribution [199]. Wellstructured nanocrystals of ceria-zirconia oxides with high temperature stability and of large surface areas have been synthesized using a unique approach (Fig. 7.5E1) [200]. In that approach, solutions or precursors formed of cerium acetate hydrate and zirconium tetra acetylacetonate, and dissolved in a lauric-acetic acid were sprayed into a methane oxygen flame to form the structures. On the other hand, structures (i.e., with platelet-like morphology) of single-phase mixed ceria/zirconia oxide (Ce0.5Zr0.5O2) with sizes of a few nanometers (less than 10 nm) were formed by introducing the precursors using an acetic/lauric acid carrier instead. The formed objects have advanced physical morphologies and improved thermal properties. They have sharp edges, with striking thermal stability and improved high specific surface area (Fig. 7.5E2). It was shown by Ismail et al. that the multi-element

Flame Synthesis of Nanostructured Transition Metal Oxides

249

diffusion flame burner (MEDB) with the spray pyrolysis delivery can be used to synthesize TMOs, MTMOs, and core
shell structures with unique morphologies [196]. The controlling parameter in these experiments was the type of source of material itself (Si/Ti, V/Ti, and Fe/Ti). The shell
core structures included TiO2@C, MTMOs of V-TiO2, and Ti oxide composites formed with carbon, iron, and silicon. The MTMO powders were formed of spherical-like particles with diameter size that has a polydispersity size distribution [196]. Zhang et al. employed the flame-assisted spray technology (FAST) that allowed for the continuous production of nanostructured LiNi0.5Mn1.5O4 with an outstanding average particle of 16 nm [197]. The MTMOs appear to consist of powders composed of spherical-like primary particles. Of interest are the wide ranges of particle size that exist in the powder; that is, the particle diameter ranges from the micro- to the nanoscale (Fig. 7.5E3 and E4). It is reported that the spinel structures are valuable materials for energy applications.

7.7.3 Gas-Phase Synthesis of 1D and 3D Core
Shell and MTMOs Recently, using a counterflow diffusion flame we showed that MTMO nanopolyhedrals made of W-doped MoO3 can be synthesized directly in the gas phase. High-purity molybdenum and tungsten probes, 1 mm in diameter, are introduced simultaneously into the oxygen-rich flame zone as the material source as shown in Fig. 7.6 (i.e., Z 5 14 mm and Z 5 12 mm). The process starts with the formation of Mo oxide and W oxide vapors in a region of the flame, initial crystallization of the vapors into shapeless particles in an intermediate flame zone, and ending with the transformation/combination into well-developed cubes (Wdoped MoO3) in another region of the flame. The process takes place as flame gases move in the direction of the stagnation plane. The underlying mechanism is as follows: (1) the high-temperature flame causes fast formation and sublimation of molybdenum oxides from the surface of the Mo wire, (2) molybdenum oxide vapors cool down and nucleate to form shapeless particles that are transformed to nanocubes as they travel inside the flame volume; (3) tungsten oxides evaporated from the tungsten probe deposit on the surface of existing MoO3 nanocubes and form chain-like tails attached to the nanocubes; (4) chain-like tails diffuse into the nanocubes and elemental tungsten intercalate into layers of nanocubes; (5) as nanocubes are transported inside the flame, W atoms distribute evenly

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Nanomaterials Synthesis

Figure 7.6 Morphological evolution of nanostructures sampled thermophoretically from the flame volume; (A1, A2), (B1, B2), (C1, C2), and (D1, D2) at flame heights of Z 5 12.0, 11.5, 10.0, and 9.2 mm, respectively. (E1
E4) TEM EDS elemental mapping analysis of the 1D mixed metal oxide nanostructures (W-doped MoO3 structures). (F
H) The core
shell directly formed in the corresponding positions of the flame volume. A
E: Adapted from M. Farmahini-Farmahni, A.V. Saveliev, W. Merchan-Merchan, Volumetric flame synthesis of mixed tungsten
molybdenum oxide nanostructures, Proc. Combustion Institute 36 (2017) 1055
1063; F
H: Adapted from W. Merchan-Merchan, A.V. Saveliev, S.G. Sanmiguel, M. Farmahini-Farahani, Flame volume synthesis of carbon-coated WO3 nanoplatelets and nanorods, J. Nanopart. Res. 14 (2012) 1276.

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inside the structures and increase the lattice spacing of the existing nanocubes. However, the nanocubes maintain their crystalline structure. Fig. 7.6 shows low- and high-resolution TEM images of nanomaterials collected thermophoretically from the flame at various axial positions. The Mo and W wires positioned at Z 5 14 mm and 12 mm, respectively, generated oxide vapors transported by the gas flow in the direction of the fuel nozzle toward the stagnation plane. An evolution of the nanostructures from early amorphous particles near the surface of the probes to intermediate agglomerates of W and Mo oxides and a final state of well-defined cubical structures is clearly observed in Fig. 7.6. TEM images of particles collected at Z 5 12 mm show the presence of large “mature” and small amorphous “young” particles (Fig. 7.6A1 and A2). Some of the large particles are surrounded by the small particles while others are attached to each other. The large particles resemble spherical shapes with irregular boundaries. The large particles are most likely formed from molybdenum oxide vapors. The morphology of these particles resembles structures grown in the gas-phase synthesis of molybdenum oxides [202]. The small particles are most likely from W oxide vapors formed near the surface of the probe in the gas phase and are characterized by neither regular shapes (meaning no well-defined edges) nor sharp corners. It is also clear from the low-resolution TEM image in Fig. 7.6B1 and B2 that at Z 5 11.5 mm the morphology of the structures is quite different. The density of the small particles at this flame position significantly increased compared to the previous sampled position (Fig. 7.6B). Small discrete shapeless particles (arrows) are accompanied by larger crystalline structures (Fig. 7.6B2). The TEM image in Fig. 7.6E1 shows typical 1D cubic well-mixed structures formed in the flame volume similar to those presented in Fig. 7.6D1. These structures are developed under constant flux of MoOx and WOx in the form of vapor. EDS elemental mapping reveals that the nanostructures contain evenly distributed W, Mo, and oxygen (Fig. 7.6E2
E4). The detailed structural characteristics of the deposited material is obtained from high-resolution electron microscopy studies performed with a JEOL 2010-F HR-TEM/STEM. The introduction of a W probe in a high-temperature region of the flame resulted in direct formation of W oxide and core
shell WO3@C nanostructures with shapes of platelets and rods in the flame volume (Fig. 7.6D
F). After the 1-mm W probe was introduced into the flame, samples of materials were collected at the various flame heights using TEM grids.

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The W wire was introduced in the oxygen-rich zone of the flame where layers of W oxides were formed and evaporated/sublimated to form the vapors (Fig. 7.6F). As the vaporized precursors are carried by the gas flow toward the stagnation plane of the flame where cooled regions of the flame are present, they are crystallized into octagonal nanoplatelets and rods (Fig. 7.6G). Samples collected at flame heights equal to those of the solid W probe showed the presence of ultra-small particles possessing irregular shapes along with some early formed polygonal-shaped structures (Fig. 7.6F). Materials sampled farther away from the W probe displayed the presence of elongated crystals with rod-like shapes. As the formed structures continued to travel in the gas flow, they encountered the hydrocarbon-rich zone of the flame, where the formed structures were coated with several layers of graphite forming the core
shell WO3@C. These findings show the significant potential of the flame method to synthesize simple or complex TMOs, or hybrid materials, and in a few minutes process.

7.8 CONCLUSIONS In this contribution an overview of the various trends, developments, and recent advances in the flame synthesis of TMO nanostructures is presented. Historically, flames have been used for the synthesis of TMOs at quantities that are needed to satisfy the industrial demand. Most of these industrial-scale TMOs are powders composed of chain-like spheroidal particles that are fused together to form aggregates. These flame-generated nanomaterials (NMs) at the large scale are formed by introducing the precursors or source material in the form of “aerosol” or “FSP”. Some of these techniques can result in a high degree of agglomeration of the primary particles, chunks, or particles with a polydisperse diameter size distribution. In order to be more selective, innovative techniques have been proposed and tested for synthesis control of the TMOs. This includes the introduction of electric fields (EFs) in the flame medium, redesign of the burner configurations, and the addition of dopants, among others. More recently, in the flame synthesis the application of the “solid support” for introducing the source material has resulted in the formation of 1D and 3D TMOs. Properties (i.e., electronic, chemical, etc.) are significantly improved by the simple transformation of TMOs to 1D and 3D shapes (compared to spheroidal primary particles and their aggregates). The superior properties that multidimensional TMO nanostructures possess

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position them squarely within the spectrum of highly sought-after NMs. Similar to the earlier works using the “aerosol” and “FSP”, innovative methods have also been employed for the synthesis of new types of novel TMOs. More recently, research has shown that the properties of multidimensional TMOs can be further improved by the simple crosslinking of two transition metals with oxygen atoms or the combination of two TMOs into a single compound. This has led to extensive research of these MTMO materials revealing the unique properties and the potential applications in a broad spectrum of science and engineering. MTMO NMs have been shown to have superior chemical, piezoelectric, optical, EC, and photochromic properties and are essential components for the development of new state-of-the-art technologies (ranging from catalysis, LIBs, smart windows, medical and biomedical applications, among others) that stem from the superior properties they exhibit. Researchers in this area have developed and tested unique approaches for the flame synthesis of MTMOs as discussed in this chapter.

ACKNOWLEDGMENTS The authors would like to express their sincere appreciation to Dr. Alan Nicholls from the Electron Microscopy Service (EMS) at the University of Illinois at Chicago Research Resource Center (UIC-RRC). The sharing of his great expertise of electron microscopy, material characterization, and providing a welcoming and friendly environment at the EMS during our visits is greatly appreciated. We also thank Dr. Preston Larson from the Samuel Roberts Noble Electron Microscopy Laboratory at the University of Oklahoma for sharing his great expertise with SEM and for the very helpful discussions. Many thanks to Dr. Ke-Bin Low from the UIC-RRC for assistance with TEM studies using the JEOL JEM- ARM200CF and helpful discussions. We would also like to thank Mr. Weston Hamric for his great contributions to this project.

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CHAPTER 8

Design and Fabrication of Porous Nanostructures and Their Applications Arpita Hazra Chowdhury1,#, Noor Salam1,#, Rinku Debnath2,#, Sk. Manirul Islam1,* and Tanima Saha2,* 1 Department of Chemistry, University of Kalyani, Kalyani, India Department of Molecular Biology & Biotechnology, University of Kalyani, Kalyani, India

2

Contents 8.1 Introduction 8.2 Classification of Porous Nanostructures 8.3 Synthesis of Porous Materials 8.3.1 Synthesis of Microporous Materials 8.3.2 Synthesis of Mesoporous Materials 8.3.3 Synthesis of Macroporous Materials 8.3.4 Synthesis of Purely Organic Porous Materials 8.3.5 Synthesis of Inorganic Nanoporous Materials 8.3.6 Synthesis of Organic
Inorganic Hybrid Polymeric Material 8.4 New Synthesis Approaches and Challenges of Porous Nanostructures 8.5 Applications of Porous Materials 8.5.1 Biomedical Use 8.5.2 Catalysis 8.5.3 Sensors and Supercapacitors 8.5.4 Adsorption, Separation, and Catalytic Conversion of CO2 8.5.5 Food Industry 8.5.6 Water Treatment 8.5.7 Gas Separation, Purification, and Storage 8.5.8 Photocatalyst 8.5.9 Agriculture 8.6 Conclusion References

#

*

266 267 267 267 269 272 273 274 277 278 279 280 282 283 283 284 285 286 287 287 288 288

These authors contributed equally. Corresponding author.

Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00008-0

© 2019 Elsevier Inc. All rights reserved.

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8.1 INTRODUCTION Nowadays, porous materials with high surface area have attracted considerable attention from researchers in every field as they have the ability to interact with molecules, ions, and atoms at the external surface, as well as within the interior surface of the material. Porosity can be viewed as a thoughtful concept which helps us to understand nature and generate advanced structures. There are various interesting examples of porous structures present in nature, for example, honeycomb with hexagonal cells, hollow bamboo, and alveoli in the lungs. The design and fabrication of porous architectures have long been an important research topic. Porous polymers have various important structural characteristics which should be described, including pore size, pore geometry, pore surface functionality, and polymeric framework structure, topology, and functionality (Fig. 8.1) [1]. Porous materials have a high surface area and well-defined porosity, which are advantageous for different applications [2,3]. Porous polymers have easy processability. For example, they can be prepared in a molded monolithic form [4] or in thin films [5], which are advantageous for many applications. On the other hand, the past decade has provided substantial advances in the synthesis of new porous metal oxides, metal sulfides, metal phosphates, etc., with ordered structures, which are potentially applicable in a wide range of applications. Porous materials can be used in different fields, such as separation materials [6],

Figure 8.1 Illustration of pore size, pore surface, pore geometry, and framework structure of porous polymers.

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Classification of nanoporous materials

Depending upon pore size

Depending upon framework building blocks

Microporous materials Mesoporous materials Macroporous materials Pore diameter > 50 nm Pore diameter < 2 nm Pore diameter 2–50 nm e.g., sponge, cotton, e.g., MCM-41, SBA-15, e.g., zeolites, metal maximum reported porous recently reported some organic framework silica and metal oxides, etc. metal oxides, etc. (MOF), etc.

Purely inorganic Organic–inorganic hybrid e.g., pure silica, metal e.g., periodic mesoporous silica (PMO), organosilica, doped silica, metal metal oxophenylphosphate, oxide, mixed oxide, MOF metal phosphate

Purely organic e.g., organic porous polymer, porous carbon

Figure 8.2 Flowchart of classification of nanoporous materials.

gas storage, as encapsulation agents for controlled release of drugs [7], sensors [8], as catalysts [9,10], as supports for catalysts [11] and as precursors of nanostructured carbon materials [12], and as supports for biomolecular immobilization. These high-value applications attract researchers in the development of facile methods for preparation of porous nanomaterials, with well-designed pore architectures in addition to the customized framework as well as pore surface functionalities.

8.2 CLASSIFICATION OF POROUS NANOSTRUCTURES Porous material can be divided into three categories, depending on their porosity and the framework of their building blocks (Fig. 8.2).

8.3 SYNTHESIS OF POROUS MATERIALS 8.3.1 Synthesis of Microporous Materials Microporous materials (Fig. 8.3) can be defined as solids, containing interconnected pores of less than 2 nm in size. Thus, they possess large surface areas, typically 300
2000 m2/g as measured by gas adsorption [13].

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Figure 8.3 Structure of microporous material.

Example include zeolites, AlPO4, metal organic frameworks (MOFs), clays, carbon, etc. Zeolites are the most well-known group of microporous materials. Zeolites are the aluminosilicates commonly known as “molecular sieves.” A zeolite framework is a neutral compound, comprising exclusively oxygen-sharing SiO442 tetrahedra. Although there are natural zeolites, most of the zeolites known are synthetic. Barrer first synthesized zeolite Y in the mid-1950s. It was an attempt to imitate the conditions under which natural zeolites were supposed to have formed on the Earth [14,15]. Zeolites are prepared in the laboratory by crystallization of gels containing alumina and silica in an aqueous medium at temperatures in the range of 100°C
190°C for several days or weeks [16]. The gel can be prepared from other sources of Al, Si, and some other metals other than silica and alumina. Deville first reported the laboratory-synthesized zeolite levyne (levynite) Ca9 [Al18Si36O108], H2O in 1862 [17]. The synthetic process required heating potassium silicate and sodium aluminate in a glass ampule. Since 1950, a wide range of zeolites has been synthesized by simple isomorphous substitution of not only aluminum but also several other elements due to their excellent properties and larger pores than their counterparts. For example, the family of ZSM such as ZSM-5 [18], ZSM-12 [19] ZSM-22 [20], ZSM-23 [21], and ZSM-48 [22] have been obtained by using various templates. A germanosilicate zeolite [23] (ITQ-15) was first reported in the patent literature, which has a large pore volume with a channel system formed by 14 X12R pores [24] and has been assigned as zeotype UTL.

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8.3.2 Synthesis of Mesoporous Materials The mesoporous materials (Fig. 8.4) can be defined as the materials with monodispersed mesosized (2
50 nm) pore space, arranged in a longrange ordered array. Examples include MgO, SiO2, TiO2, ZnO, SnO2, TiPO4, AlPO4, carbon nanotubes, etc. Mesoporous materials have many attractive properties such as high surface areas, periodically arranged mesopore space, tunable pore sizes, alternative pore shapes, and large open active sites. Due to these properties, mesoporous materials are of high interest in technological applications in diverse fields such as catalysis, adsorption, drug delivery, and so on. In the 20th century materials like MCM (Mobil Composition of Matter) [25] were successfully synthesized by Mobil scientists. The discovery of an M41S family of ordered mesoporous materials with pore dimensions of 2
10 nm using quaternary alkyl ammonium surfactants (e.g., cetyltrimethylammonium bromide, CTAB) as the template is one of the most important discoveries in the history of the porous world. It suggested the huge expectations toward their applications as heterogeneous catalysts [26,27]. Mesoporous silica for example, MCM-41, MCM-48, MCM-50, FSM-16 [28], SBA-15 [29], etc. are well-known among the ordered mesoporous materials discovered initially. Although there are some differences in the synthesis conditions and structural properties of these silicas, the basic strategy for the synthesis of the materials is similar in all cases, which is based on the supramolecular self-assembly of the surfactants (or templates) [30]. Fig. 8.5 depicts a typical synthetic pathway for the formation of highly ordered 2D hexagonal mesoporous silica MCM-41 mediated by the surfactant. The structure and pore size of the silica can be modified from

Figure 8.4 Different types of 3D structures of mesoporous materials.

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Figure 8.5 Surfactant-assisted synthesis of mesoporous silica with 2D ordered hexagonal pore arrangement.

hexagonal (e.g., MCM-41), cubic (e.g., MCM-48), to lamellar (e.g., MCM-50) by varying the type of surfactant used, the surfactant
silica ratio, and the pH of the solution. Mesoporous SBA-15-type materials are another family of silica materials. They can be synthesized in the highly acidic conditions in the presence of nonionic block copolymer surfactants [29]. In addition to the cooperative pathways, nanocasting using already formed ordered mesoporous materials as hard templates has been developed to synthesize mesoporous materials [31]. This method of nanocasting is highly effective for the synthesis of other nonsiliceous porous oxide and carbon materials with ordered pore arrangements which are difficult to prepare directly by the surfactant-assisted route [32]. Nonsiliceous mesoporous materials like oxides, mixed oxides [33], metal sulfides [34], metal phosphates [35], polymers [36], carbons [37], and carbon nitrides [38] have also been synthesized successfully using surfactant templating routes. Mesoporous materials can also be synthesized without using any organic templates. Hydrothermal synthesis is one of the commonly used methods to form mesoporous materials with unique morphologies. For example, Chowdhury et al. synthesized mesoporous magnesia (MgO) with a grainy rod-like microstructure by the simple hydrothermal process at 180°C/5 h in the presence of urea, where urea has a significant role in the formation of the grainy rod with porous structure [39]. Organic surfactant molecules play an important role in generating porosity in the mesoporous material blocks and act as templates or

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structure-directing agents (SDA). This soft templating method is the most successful path for the synthesis of ordered and disordered mesoporous matrices. 8.3.2.1 Role of Template in the Formation of Porosity Template means pattern or overlay used in graphic arts (drawing, painting, etc.). In nanoporous systems template molecules help to create or design porosity in the matrix. Therefore, a template acts as a SDA in the formation of porous materials [40]. There are several kinds of SDAs. (1) Surfactants can be used as SDAs. SDA should have coexistence of a chemically bonded hydrophobic (nonpolar) hydrocarbon “tail” and a hydrophilic (polar) “head” group. These molecules possess high molecular weight and they aggregate in the solvent to form a self-assembled micelle [41,42]. (2) Some SDAs bear hydrophobic
hydrophilic groups in a single molecule. They are not surfactants but they play the role of the template in fabricating mesopores in a material. These templates may or may not form self-assembly [43]. (3) Another type of SDA is a dendrimer or polymer. They can be the macromolecular single molecule which has high molecular weight [44]. On the other hand, porous silica or colloidal silica spheres, polystyrene, etc., act as hard templates that are also used to generate porosity within the matrix [45]. The soft template SDAs can be classified as follows shown in Fig. 8.6. Template or structure directing agents

Depending upon the charge

Depending upon functionality

Cationic

Anionic

e.g., CTAB, CPC, etc.

Surfactants Large molecule, high molecular wt, form micelle, e.g., CTAB, SDS, etc.

Nonionic

e.g., SDS, lauric acid, etc.

e.g., P123, F127, etc.

Nonsurfactants

Single molecule template, no selfassembly e.g., TPA, etc.

Small molecule, low molecular wt, selfassembly, e.g., urea, sodium saliculate, etc.

Figure 8.6 Flowchart of classification of templates.

Single macromolecule template, high molecular wt, do not form micelle, e.g., dendrimers.

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Figure 8.7 A general route for the formation of a mesoporous solid [47].

8.3.2.2 Role of Surfactants as Structure-Directing Agents Surfactants possess both hydrophobic and hydrophilic groups in the molecules, which can behave specifically in polar and nonpolar solvents. These molecules form aggregates where the hydrophobic parts are oriented within the cluster and the hydrophilic parts are exposed to the solvent [46]. Such kinds of aggregates are called micelles. As the surface becomes crowded with the surfactant, more molecules will arrange into micelles. Significantly, the surface becomes completely loaded with surfactant molecules at a certain concentration and any further additions must arrange as micelles. This certain concentration is known as the critical micelle concentration (CMC). Beyond the CMC value with further increasing concentration, the self-assembly of the micelle occurs to generate a 3D spherical or 2D rod-like array, and this self-assembly helps in the pore generation. These SDA molecules act as the “placeholder,” which becomes the void space to create nanoporous material. They not only control the variation in pore size but also the shape of the pores. Therefore, the total structural design of the template molecule, its size, and shape are stamped in the porous solid (Fig. 8.7). Mesoporous materials can be classified into three categories: purely inorganic, organic
inorganic hybrid materials, and completely organic, as summarized in Table 8.1.

8.3.3 Synthesis of Macroporous Materials Macroporous materials have a pore diameter greater than 50 nm, which is the largest pore dimensions in the family of porous materials [48]. The most facile and extensively used route to prepare macroporous materials is the colloidal templating route. Macroporous metal oxides such as silica, titania, and zirconia and polymers like polyacrylamide and polyurethane with well-defined pore sizes in the submicrometer regimen have been

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Table 8.1 Possible types of mesoporous materials with examples Framework Type of materials Examples composition

Purely inorganic

Mesoporous silicas Metal-containing mesoporous silicas Mesoporous metal oxides and mixed metal oxides Mesoporous metallophosphates

Organic
inorganic hybrid

Periodic mesoporous organosilicas (PMOs) Metal oxophenylphosphates

Purely organic

Mesoporous carbons Mesoporous polymers

MCM-41/MCM-48, SBA15, etc. Ti-MCM-41, Zn-silica, etc. TiO2, AlO2, ZrO2, ZnTiO3, etc. Silicotitanium phosphate, silicoalumino phosphate, etc. Various metal-containing PMOs, etc. Iron phosphonate, chromium phosphonate, etc. CMK-3 Triazine-based polymer, triallylamine-based polymer

successfully synthesized by employing the self-assembled templates of colloidal spheres [49,50].

8.3.4 Synthesis of Purely Organic Porous Materials High surface area porous organic polymers have been attracting increasing interest over the years [51
54]. Designing chemical reactions that will facilitate the creation of pores of desired dimensions in the mesoporous organic materials is a challenging task to researchers. Such a goal can be achieved by covalent organic frameworks (COFs) because COFs are porous crystalline materials with predesigned 2D and 3D polymer structures produced by covalently linked functionalities [55,56]. There are a few reports on the soft templating strategy for the synthesis of ordered mesoporous polymers [57
59]. The choice of polymer precursor is the key to the successful organization of organic
organic mesostructures. The key conditions for the formation of polymer, which have to be fulfilled are (1) dissolution of the material in the same medium as the surfactants, (2) interaction with the template molecules, (3) organizing

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itself precisely around the template, (4) further polymerization, without losing the interaction with the template, and (5) lastly the removal of template without destroying the polymer mesostructure. Mesoporous polymers were mainly synthesized by a hard-templating method before the generation of the soft-templating approach. In this approach, a monomer is infiltrated into a hard template (e.g., an opal-like colloidal assembly of silica), which is removed after polymerization to give mesoporous polymer networks [60]. The most used method for the preparation of mesoporous polymers is the EISA (evaporation-induced self-assembly) method, usually using ethanol as solvent. On the other hand, the same kind of materials can also be prepared by a liquid crystal templating or cooperative assembling method [61].

8.3.5 Synthesis of Inorganic Nanoporous Materials Syntheses of inorganic nanoporous materials are illustrated below. 8.3.5.1 Silica-Based Mesoporous Materials Since the discovery of the M41S family of mesoporous silicas, extensive work has been on going over silica-based mesoporous materials due to their several advantages, such as a great variety of possible structures, enhanced thermal stability, as well as for the applications in various promising fields. Mesoporous silica and silica-based materials are usually prepared via the endotemplate method under hydrothermal conditions using acidic or basic media. The hydrothermal condition is actually a sol
gel process with a number of steps. The steps are: (1) formation of surfactant self-assembly to form a homogeneous surfactant solution in common solvent media (usually aqueous), (2) addition of silicate precursor, such as tetraethyl or tetramethyl orthosilicate or inorganic sodium silicate to the surfactant solution, (3) formation of silicate oligomer sol, (4) condensation of oligomers with surfactant micelle via cooperative assembly and aggregation to form an inorganic
organic hybrid, which finally precipitates in the form of a gel, and (5) hydrothermal treatment of the gel for further condensation, solidification, and reorganization of the material to an ordered arrangement [62,63]. Finally, the resultant product is cooled, filtered, washed, and dried (Fig. 8.8). Calcination or solvent extraction of the assynthesized solid leads to the formation of ordered mesostructured silica material [64,65]. This is the most well-known and convenient method of silica synthesis. Generally, CTAB and SDS are used as surfactants [66] for

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Figure 8.8 Stepwise formation of mesoporous silica material [64].

the synthesis. Mesoporous MCM-41, MCM-48 [67], etc., and different transition as well as nontransition metal-doped silica [68] are synthesized using this strategy. 8.3.5.2 Nonsilica-Based Mesoporous Materials Since 1993, the surfactant templating strategy has been commenced for the synthesis of nonsilica-based mesostructure, mainly metal oxides [69]. Nonsiliceous mesostructured materials like phosphate, sulfide materials, as well as mesoporous metals are also well developed [70,71]. Various mesoporous metal oxides of Nb, Ta, V, W, Zr, Sn, Cu, Ni, Hf, Al, Zn, Mg, etc. have been synthesized after the first successful approach toward the synthesis of mesostructured titania [72]. In the soft-templating route, mesoporous oxide materials are generally synthesized in the hydrothermal method [73], at low temperature (freezing) [74] or at room temperature [75]. In all the methods, the inorganic metal precursor forms a hydroxo species and electrostatically interacts with template molecules to form a metal
template composite in aqueous sol
gel process [76]. In the end, we get the desired solid porous metal oxide after removal of the template by calcination or solvent-extraction method. Recently, Chowdhury et al. synthesized mesoporous sheetlike MgO by a simple ammonia precipitation method maintaining the NH4OH to Mg mole ratio at 6:1, followed by calcination at 450°C [10]. Chowdhury

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et al. synthesized cube-shaped mesoporous anatase TiO2 by a simple hydrothermal technique at 180°C for 24 h in the presence of glucose followed by calcination at 600°C for 2 h. In this method, the dehydrated species of glucose was adsorbed on some specific facets of TiO2 particles and induced cube-shaped morphology to the sample [77]. Recently, Wang et al. reviewed recent advances in ordered meso/macroporous metal oxides and their applications in heterogeneous catalysis supplying clear information about the synthesis and modifications of the morphology and surface chemistry of metal oxides to get an ordered meso/macroporous structure [78] (Fig. 8.9).

Figure 8.9 (A) Stabilized highly reactive metal precursor for metal oxide synthesis and (B) interaction of metal-surfactant to form a mesostructure.

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8.3.6 Synthesis of Organic
Inorganic Hybrid Polymeric Material Organic
inorganic hybrid polymers are the combination of both organic and inorganic units. Organic functionalization of the inorganic nanoporous materials tunes the surface properties (e.g., hydrophobicity, hydrophilicity, binding to guest molecules), alters the surface reactivity, along with modifying bulk properties as well as stabilizing the materials toward hydrolysis [79]. Stein et al. reported the generalized method of preparing organic
inorganic hybrid mesoporous silicates with uniform channel structures bearing both reactive and passive organic groups in the porous solids by grafting methods or by cocondensation under surfactant control [80]. Organic
inorganic hybrid materials involve mainly the silica-based materials, though there are some reports on microporous MOFs, hybrid phosphates, phosphonates, and polymers. Few organic
inorganic hybrid mesoporous aluminophosphates have been synthesized via surfactant templating route [81]. Ghosh et al. prepared porous iron-phosphonate nanoparticles HPFP-1(NP) through a hydrothermal method via simple chemical reaction between hexamethylenediamine-N,N,N0 ,N0 -tetrakis(methylphosphonic acid) and FeCl3 (Fig. 8.10) [9]. Organic
inorganic hybrid polymers can be prepared through: (1) sol
gel process; (2) self-assembly process; (3) assembling or dispersion of nanobuilding blocks; and (4) hierarchical structures [79] and interpenetrating networks [82]. Depending upon the supramolecular templating mechanism, organic functionalized silica molecules can be prepared by three routes as shown in Fig. 8.11.

Figure 8.10 Synthetic pathway for the preparation of HPFP-1(NP) nanomaterial [9].

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Figure 8.11 Different methods for the synthesis of organic
inorganic hybrid mesoporous silica: (1) grafting, (2) cocondensation or in situ grafting, and (3) organic bridged periodic mesoporous silica.

8.4 NEW SYNTHESIS APPROACHES AND CHALLENGES OF POROUS NANOSTRUCTURES The most efficient method to synthesize porous nanostructures is the softtemplating method. In this method, generally surfactants or amphiphilic block copolymers act as a template, and are used to coassemble with organic (or inorganic) framework precursors [83]. In the past two decades, commercially available soft templates including surfactants (e.g., CTAB) and amphiphilic block copolymers (e.g., poly(ethylene oxide)-b-poly (propylene oxide)-b-poly-(ethylene oxide), PEO-b-PPO-b-PEO, such as Pluronic P123 and F127) have been intensively used to synthesize porous nanostructures with variable morphologies. The major challenge of this soft-templating approach is limited accessibility of the commercially available soft templates, which causes small pore size and amorphous (or semicrystalline) frameworks of the common porous nanostructures. It limits

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Figure 8.12 Synthetic scheme of porous metal oxide (Al2O3, MgO) layers [88].

their applications in many fields. In recent days, tailormade amphiphilic block copolymers have emerged as suitable alternative soft templates for the synthesis of new porous nanostructures with controllable molecular weights and compositions. These nanostructures have exhibited many unique features like adjustable mesostructures and framework compositions [84,85], ultra-large pores, thick pore walls, high thermal stability, and crystalline frameworks. Highly ordered mesoporous carbon has been synthesized by the selfassembly of resol (a low-molecular-weight phenol-formaldehyde resin) and commercial Pluronic block copolymers through the EISA process [86,87]. However, the pore size is below 5.0 nm. Recently, Chen et al. reported a new synthetic approach (Fig. 8.12) to prepare mesoporous Al2O3 and MgO layers with high specific surface areas up to 558 m2/g on silicon wafer substrates [88]. They have used poly(dimethylacrylamide) hydrogels as porogenic matrices. They followed the following synthetic process: (1) anchoring adhesion promoter on the Si wafer substrate, (2) spreading the polymer through spin-coating, (3) preparation of hydrogel films by photo-crosslinking and anchoring to the substrate surface, (4) swelling the hydrogels in the respective metal nitrate solutions, and (5) combustion of the hydrogel and formation of porous metal oxides by subsequent thermal conversion.

8.5 APPLICATIONS OF POROUS MATERIALS Nanoporous materials have numerous applications (Fig. 8.13) depending on their pore size, structure, type of material (organic, inorganic, or

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Biomedical Catalysis

Agriculture

Gas separation, purification and storage

Adsorption and conversion of CO2

Application of porous nanomaterials

Water treatment

Sensors, supercapacitors

Food industry

Figure 8.13 Applications of porous nanomaterials.

organic
inorganic hybrid) as well as chemical and physical properties. Nowadays the nanoporous materials have been extensively used in drug delivery and biosensing, agriculture, food industry, wastewater treatment and purification, sensors and supercapacitors, gas adsorption, separation and storage, CO2 capture, and photocatalysis, etc.

8.5.1 Biomedical Use Recent progress in biomedical sciences with development of advanced materials and technologies have rapidly expanded controlled drug-delivery applications [89,90]. Safe delivery of the drug in specific sites of the human body with their regulation for maximum therapeutic benefits is the aim of controlled drug delivery. Nanoporous particles are used in the storage and delivery of molecular therapeutics due to their large surface area and porous interior. Nanoporous anodic alumina has been used widely in electronic, optoelectronic, sensing devices, dental, and orthopedic implants due to their properties such as electrical insulation, optical transparency, chemical stability, bioinertness, and biocompatibility. Porous silica is also a biocompatible material which has optical properties, it has been used in drug-delivery applications and implantable devices. Highly porous nanostructured titania
silica ceramic has been widely used as a biomaterial by replacing commercial titanium implants [91,92]. Biocompatibility, the capacity of

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self-setting within the bone cavity, moldable ,and osteoconductive nature are some unique properties of nanostructured biomaterials, hydroxyapatite, and other calcium phosphates. These properties make them popular in drug-delivery applications and as implantable bone ceramic [93
95]. As mesoporous silica protects the molecular cargo from premature release and degradation, it is used for drug delivery. Sustained release of ibuprofen from mesoporous particles in simulated body fluid, one of the first drug deliveries using nanoporous particles, has been studied [96]. Porous nanoparticles with 1.8 nm pore-diameter release 55% of the adsorbed drug in 24 h, whereas 2.5 nm pore-diameter containing porous particles release 68% in the same time. For cellular delivery and release of camptothecin, a hydrophobic anticancer drug, nanoporous particles have been used [97]. For gene therapy, potentially dangerous and nonefficacious delivery vehicles, like viral capsids, have been used previously to deliver DNA in cells. The use of mesoporous particles may circumvent these vehicles. Lin and coworkers have shown that a plasmid DNA vector electrostatically binds and successfully transfects a number of mammalian cells by tethering second-generation poly(amidoamine) dendrimers to the surface of mesoporous particles [98]. Nanoporous membranes can act as support for kidney cells in kidney applications as well as a blood filter which retains serum proteins but flows out the smaller waste substances [99]. Nanoporous membranes used in implantable devices function as a semipermeable compartment to hold the implant or drug during the passage of the desired molecule. Nanoporous membranes are also used in diagnosis and protein separation. Many microfabricated devices have been developed which perform separation, mixing, reaction, detection, or preconcentration to automate biological analyses and reduce sample consumption and cost. In the pharmaceutical industry, food industry, and biotechnology, many techniques, including size exclusion chromatography and gel electrophoresis of biopolymers are used for isolation and purification of molecules [100,101]. Due to the biosensing property, gold nanoporous membranes with pore radius , 1 nm are used for detection of molecules and are important in the pharmaceutical industry, medical diagnosis, and detection of hazardous biomolecules [102]. A biological component with a physiochemical detection component which detects analytes is combined in the majority of biosensing devices in biological feed streams. For example, glucose oxidase immobilized in the porous nanocrystalline TiO2 film is capable of sensing the blood glucose level [103]. Similarly, cholesterol biosensors have been developed by immobilizing

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cholesterol oxidase enzyme in the ZnO nanoporous thin films [104]. Recently, a glucose-sensing system has been developed that contains a nanoporous platinum electrode embedded in a microfluidic chip comprising a microfluidic transport channel network and a miniaturized electrochemical cell [105]. It is possible to access information on the concentration, structure, size, and sequence of single- and double-stranded DNA or RNA by measuring the frequency, magnitude, and duration of blockage in the ion current of an electrolyte when biomolecules are drawn through nanopores embedded in insulating membranes [106]. For detection of biomolecules, this technique has been used by embedding membrane-bound receptors like α-hemolysin (α-HL) protein pores in a lipid membrane [107]. Micron-sized apertures in polymeric film-incorporated lipid membranes were used much earlier by researchers for analysis of single molecules. To expand the functionality of single-molecule detectors, synthetic nanopores like glass, polymers, and solid-state membranes are now used [108]. Protection of implanted cells or drug-release systems from the immune reaction is referred to as immunoisolation. Encapsulated nanoporous semipermeable membranes isolate the transplanted cells from the body’s immune system by allowing small molecules such as oxygen, glucose, and insulin but impeding the passage of much larger immune system molecules such as immunoglobulin. Nanoporous silicon interfaces prepared by microfabrication techniques have been used in the implantable artificial pancreas to treat diabetes by Desai et al. [109]. For controlled release of pharmacological agents, nanoporous membranes with suitable pore size, porosity, and membrane thickness make them an attractive route for making capsules [110]. For the sustained release of ophthalmic drugs, nanoporous inorganic membranes have been tested [111].

8.5.2 Catalysis Unique pore structure, large surface area, excellent structural stability, and high electrical conductivity of porous Pt-based nanostructured materials make them important catalysts in electrochemical reactions like oxidation of hydrogen and small organic molecules in the anode and reduction of oxygen in the cathode in fuel cells. This can eliminate the use of a carbon support in fuel cells and address the disadvantages of traditional carbonsupported catalysts [112].

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Zeolites are crystalline microporous materials that are the most widely used catalysts in different industries such as oil refining, petrochemistry, and organic synthesis in the production of fine chemicals [113
115].

8.5.3 Sensors and Supercapacitors To prepare the multifunctional materials for sensors containing uninfluenced photoluminescence and emission lifetimes, in the presence of water at ambient conditions, porous lanthanide-based metal
organic frameworks (Ln-MOFs) are appropriate materials [116
118]. Available LnMOF materials have very small pores through which the molecules of interest are not allowed and these materials are stable only up to 673K in air, so it is impossible to use them in luminescence sensors working in moisture at ambient temperature [119,120]. New possibilities will be opened for the production of low-cost sensors by MOFs that combine the magnetic and anisotropic properties with high-emission quantum yields under ambient conditions [121]. In clinical diagnoses, bioprocessing, environmental and food industries, enzyme-free amperometric detection of glucose and hydrogen peroxide is an important application. Porous Pt-based electrocatalysts are widely used to detect glucose and hydrogen peroxide [122,123]. In the fabrication of pH sensors, porous Pt-based nanomaterials are also used. From investigations, it has been revealed that nanoporous Pt material fabricated pH-sensitive electrodes exhibit near-Nernstian behavior with ignorable hysteresis, a short response time, and high precision. For example, fabrication of a solid-state reference electrode has been successfully done by combining nanoporous Pt with a polyelectrolyte (PE) junction [124]. Recently, activated carbons have become one of the most suitable electrode materials for supercapacitor preparation. Supercapacitors have a reversible electrical energy storage system with high power-energy capability and long life. All these properties make the supercapacitorcontaining devices suitable for various applications such as power electronics, backup power systems, digital electronic devices, wind turbines, electric vehicles, etc. [125].

8.5.4 Adsorption, Separation, and Catalytic Conversion of CO2 An important factor responsible for global warming is the emission of CO2 from industry and power plants. To avert the rise in CO2 levels, its

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capture, storage, and utilization (CCU) technique is one of the obvious solutions. However, during this process, further CO2 is emitted due to a certain amount of energy consumption [126,127]. Therefore, CCU techniques should be low-energy regeneration techniques with the net reduction in CO2 emission. Nanoporous materials have a high surface area and a high pore volume which make them the most suitable solid adsorbents for CO2. Zeolites are microporous crystalline materials, and have been widely used as adsorbents of CO2 due to their high surface area, specific porous structures, and availability. Other porous nanomaterials such as MOFs, mesoporous silicas, carbon nanotubes, organic cage frameworks, and COFs have also been examined for this technique [128
133]. Photocatalytic conversion of CO2 is a type of CO2 reduction reaction (CRR) using solar energy for the production of chemical fuels is considered as one of the most economical conversion. Porous carbon materials adsorb CO2 on the surface of the photocatalyst and enhance the CRR efficiency remarkably. For example, Wang et al. have shown carbon@ TiO2 hollow spheres exhibited enhanced photocatalytic conversion of CO2 compared with commercial TiO2 (P25) [134].

8.5.5 Food Industry Nanoporous materials are used in food safety for the detection of pathogenic microorganisms like Salmonella enteritidis, bacteriophage virus MS2, Escherichia coli O157:H7, Staphylococcus aureus, etc. and small organic molecules like food allergens in peanut. Nanoporous silicon has been synthesized electrochemically and functionalized with DNA probes for their utilization in biosensors capable of selective detection of S. enteritidis, which exhibit promising results in screening applications [135]. Nanoporous silicon films conjugated with antibodies detect bacteriophage virus MS2 and remove the contaminant from drinking water. Nanoporous silicon films have a detection level at 1 mg/mL and they outperform nonporous silicon-based biosensors due to their high surface area [136]. For the detection of E. coli O157:H7 and S. aureus, a polydimethylsiloxane microfluidic sensor has been developed with antibodies immobilized on an alumina nanoporous membrane [137]. Gibberellic acid is a plant growth hormone and, as a natural bioactive product, it is of great interest. However, using traditional methods, isolation and determination of gibberellic acid are difficult due to its low concentrations in plants and lack of stability during isolation. Novel sorbents have been

Design and Fabrication of Porous Nanostructures and Their Applications

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developed for nondestructive isolation of small organic molecules from plant matrix, such as a nanoporous silica
sucrose material with the tunable pore structure that binds to gibberellic acid for its isolation [138]. Mesoporous silicas have been developed to remove a broad range of metal contaminants from water and beverages, and also fortified calcium in water samples during removal of uranium [139,140].

8.5.6 Water Treatment Industrial wastewater treatment and drinking water purification by the adsorption process using activated carbon is an important process. Their microporous structure, high porosities, large surface area, and chemical nature have made them efficient adsorbents for the removal of heavy metals and low-molecular-weight chemicals such as metal ions, dyes, and organic compounds from industrial wastewater. Malathion is a broadspectrum organophosphate insecticide and miticide that has various agricultural, industrial, and governmental uses. Activated carbon can be used generally for removal of malathion from water which is responsible for taste, odor, and color problems [141]. In air pollution control, pharmaceutical and chemical industries, wastewater treatment, and sugar syrup purification, activated carbon is also used as catalyst support. The mesoporous activated carbons are generally used for the separation and adsorption of bulky organic materials such as dyes and humic substances [142
145]. Silica templates generated nanoporous carbons (SMC1) with pore sizes 10
100 nm, very high pore volumes, and high surface areas, exhibited excellent adsorption capacities for bulky dyes like Acid green 20, Acid violet 17, and Direct blue 78. The adsorption capacity of these nanoporous carbons is sometimes over 10 times higher than that of commercially activated carbons [146]. Zeolites are crystalline minerals, which are formed by tetrahedral units of SiO4 and AlO4. This type of structure and higher aluminum content of zeolites, make them an effective agent for the removal of specific pollutants, catalysts, and molecular sieves used in water treatment and other applications [147]. Among the natural zeolites, clinoptilolite, mordenite, scolecite, chabazite, and phillipsite have been studied for wastewater treatment applications [148
150]. ZSM-5 and MCM-22 are synthetic zeolites used for the removal of inks and common dyes, respectively [151,152]. Zeolite mixtures of kaolin and mordenite have shown enhanced uptake of chromium compared with natural zeolites [153]. The uniform pores

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and large surface areas of mesoporous silicates make them suitable for adsorption and ion exchange. Due to these properties, they are used for removal of dyes and as the catalyst in many applications. Titaniumsubstituted mesoporous silicates have shown around 700 mg/g adsorption capacity of ionic dyes, which is much higher than any other material [154]. Nanofiber membranes such as chitosan nanofiber membranes, chloridized polyvinyl chloride nanofiber membranes, wool keratose, silk fibroin nanofiber membranes, and polyacrylonitrile nanofiber membranes have high surface areas that make them efficient candidates for removal of heavy metal ions from an aqueous solution [155
159]. Mesoporous poly (vinyl alcohol)/SiO2 composite nanofiber membranes functionalized with mercapto groups with diameters of 300
500 nm have shown high efficiency in absorbing Cu(II) ions from waste water [160].

8.5.7 Gas Separation, Purification, and Storage In cryogenic air separation units, mixtures of rare gases are usually found. The mixtures of rare gases have been separated by adsorption on MOFs (MOF-5), which is a far simpler process and can replace the cryogenic distillation. After separation of the mixture, xenon and krypton can be marketed separately, for example, in the lamp industry krypton is used as a filler and xenon is used as a narcotic medical gas [161]. Cu-BTC-MOF is used to remove sulfur odorant components from natural gas. It has the special arrangement of channels with open metalligand sites which allows a dual-type sorption behavior. For the separation of polar components from nonpolar gases, it is a powerful material [162
164]. It successfully removes amines and ammonia, water traces, alcohols, and oxygenates, etc. In MOF-filled canisters, storage of a gas can be used either to transport an equivalent amount of gas at a far lower pressure or to enhance the capacity of the gas in a given volume. Mueller et al. have shown that the volume specific uptake is higher for the rare gases, argon, krypton, and xenon in the case of a gas cylinder filled with MOF-5 [161]. Some other gases, like methane and hydrocarbons, can also be stored in the same manner [165,166]. Similarly, MOF-5-, IRMOF-8-, and Cu-BTC-MOF-filled cylinders can take up higher amounts of hydrogen compared to the pressurizing of an empty container with hydrogen [161]. For many volume-limited fuel-cell applications such as the mobile and portable cases, volume-specific data

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storage will be industrially much more relevant in comparison to weightspecific storage capacity. The high specific surface area and well-developed porous structures of activated carbon, carbon fiber, and carbon nanotubes make them excellent adsorbent materials for gas adsorption/separation and CO2 capture. Commonly used mesoporous silica for CO2 adsorption are MCM-41 and SBA-15 [167,168]. MOFs are a class of porous materials with a high specific surface area and a large amount of space both on the inside and outside. Due to these properties, they are also used in gas separation, storage, and catalysis [169,170]. Because of the better chemical and thermal stability of modified microporous organic polymers (MOPs) compared to MOFs and inorganic porous materials, they are used in gas storage, adsorption, separation, and heterogeneous catalysis [171,172].

8.5.8 Photocatalyst For the supply of clean and recyclable hydrogen energy by splitting of water, semiconductor photocatalysis is an environmentally friendly process. In this technique, solar energy decomposes harmful organic and inorganic pollutants present in the air and aqueous systems [173
176]. TiO2 is stable, cheap, and currently the most widely used highly efficient photocatalytic material. Nitrogen-doped TiO2 has recently been reported for visible-light photocatalysis [177
179]. Tungsten trioxide (WO3) also has many advantages for visible-light-driven photocatalysis, like strong adsorption within the solar spectrum, stable physicochemical properties, and resistance to photocorrosion effects [180]. Abe et al. have shown that loaded Pt in a Pt-loaded WO3 nanotubular structure (Pt/WO3) can trap photogenerated electrons from WO3 to reduce O2 to H2O2 and enhance the photocatalytic properties [181]. Metal nanoparticles loaded on nanoporous TiO2 supports are used in catalyzing reactions like photocatalytic generation of hydrogen from water, carbon monoxide oxidation, and organic pollutant photodegradation [182
184].

8.5.9 Agriculture In agriculture, nanoporous materials are used for the detection, separation, catalysis, and controlled release of materials and as sorbents, binding toxicants. Activated carbons have affinity for organic molecules, whereas zeolites have affinities for gases, ions, metals, and small organic molecules, based on this property they are used as sorbents. Zeolites are used as the

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catalyst in biofuel production [185]. Microporous clays have been used as sorbents and as additives to sequester contaminants through in-feed applications, and also reduce human exposure to the contaminants [186,187]. Aluminosilicate zeolites selectively adsorb guest molecules through their channels, cavities of a size, shape, dimension, and charge balancing cations [188,189]. Divalent cation exchanged and surfactantmodified synthetic zeolites such as zeolite X, Y, ZSM-5, and Beta have been studied as pheromone dispensers for insect attractants. They disperse the female sexual pheromone n-decanol of Agrotis segetum and Cydia pomonella, and the male synthetic attractant trimedlure for Ceratitis capitata [190]. The pheromone-loaded surfactant-modified zeolite A has dispersed pheromone for Riptortus pedestris and trapped them [191].

8.6 CONCLUSION In this chapter, we have briefly classified the porous materials as well as vividly discussed various methods to prepare a different kind of porous nanomaterial. We also discuss the possibilities for intentionally modifying the surface chemistry and morphology of the materials and their applications in various fields. After analyzing all the prospects, we can conclude that there is a broad window for future research on developing a facile, environmentally friendly green synthetic way to synthesize porous materials at an industrial scale. The green synthesis of porous materials minimizes the use of hazardous chemical reagents, making them potentially useful in commercial fields.

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7781. [182] M. Gratzel, Nature 414 (2001) 338. [183] T. Sato, M. Koizumi, T. Miyao, S. Naito, Catal. Today 111 (2006) 164. [184] M. Srinivasan, T. White, Environ. Sci. Technol. 41 (2007) 4405. [185] P.M. Mortensen, J.-D. Grunwaldt, P.A. Jenson, K.G. Knudsen, A.D. Jensen, Appl. Catal. A 407 (2011) 1
19. [186] R. Slamova, M. Trckova, H. Vondruskova, Z. Zraly, I. Pavlik, Appl. Clay Sci. 51 (2011) 395
398. [187] T.D. Phillips, E. Afriyie-Gyawu, J. Williams, H. Huebner, N.-A. Ankrah, D. Ofori-Adjei, et al., Food Addit. Contam., Part A 25 (2008) 134
145. [188] D.W. Breck, Zeolite Molecular Sieves, Wiley, 1974. [189] S.M. Seo, W.T. Lim, K. Seff, J. Phys. Chem. C 116 (2012) 13985. [190] J. Muñoz-Pallares, A. Corma, J. Primo, E. Primo, J. Agric. Food Chem. 49 (2001) 4801. [191] S.M. Seo, J.M. Lee, H.Y. Lee, J. An, S.J. Choi, W.T. Lim, J Porous Mater. 23 (2016) 557
562.

CHAPTER 9

Synthesis and Processing of Thermoelectric Nanomaterials, Nanocomposites, and Devices Lazaros Tzounis Department of Materials Science & Engineering, University of Ioannina, Ioannina, Greece

Contents 9.1 Introduction to Energy Needs and Wasted Thermal Energy 9.2 Fundamentals of the Thermoelectric Effect and Thermoelectric Materials 9.3 Inorganic Thermoelectric Nanomaterials 9.3.1 Metal Chalcogenides 9.3.2 Metal Oxides 9.3.3 Superionic Conductors 9.3.4 Silicon-Based Materials 9.3.5 Skutterudites, Clathrates, and Half-Heusler Alloys 9.4 Organic Thermoelectrics: Polymer and Nanocomposite Systems 9.4.1 Conjugated Polymer Thermoelectric Materials 9.4.2 Nanocomposite Polymer Thermoelectric Materials 9.5 Working Principle and Specific Architectures of Thermoelectric Generators 9.6 Application of Thermoelectric Generators 9.7 Recent Trends and Challenges 9.7.1 Market for Thermoelectric Generators and Recent Technologies in Thermoelectric Materials and Devices 9.7.2 Challenges in Potential Thermoelectric Generator Applications 9.8 Future Perspectives 9.9 Summary and Conclusions Acknowledgment References

295 297 302 304 306 306 307 308 309 309 314 319 321 322 322 325 326 326 327 327

9.1 INTRODUCTION TO ENERGY NEEDS AND WASTED THERMAL ENERGY Due to the finite supply of fossil fuels and human-induced global climate change, an emerging energy crisis has been realized in the 21st century

Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00009-2

© 2019 Elsevier Inc. All rights reserved.

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giving rise to the exploitation of “green” energy and alternative energy resources [1
3]. There has been a substantial increase in the consumption of energy resources and especially that of petroleum feedstocks mainly due to (1) industrial development and (2) population growth [4]. In industrial environments and our daily life, large amounts of the generated heat energy cannot be effectively used, and are inevitably wasted in the environment, for example, emissions of factory boilers, car exhausts, friction, etc. A major contributor to waste heat is the transport sector, where only 20% of the fuel’s energy ends up as useful energy. Relatively, aeronautics and automotives are examples of high energy usage with low efficiency, where roughly 75% of the energy produced during combustion is lost in the turbine/exhaust or engine coolant in the form of heat. In relation to this, more than 60% of the energy produced in the United States is never utilized, as most of it is dissipated in the form of waste heat [5]. Fig. 9.1 shows the average effectively used energy from fossil fuels, while it represents schematically the different sectors that large amounts of waste heat are generated and could be transformed to electrical energy via the deployment of thermoelectric (TE) materials. Thus, a key solution for the huge amounts of wasted thermal energy ( . 60%) will be energyharvesting technologies to effectively recycle and partially or fully harvest this inevitably generated and regrettably wasted thermal energy. Besides searching for alternative energy sources, such as solar energy, thermal

Figure 9.1 Schematic representation of different sectors contributing to large amounts of wasted thermal energy.

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energy, wind energy, hydrogen energy, and biomass energy to replace the conventional fossil fuels, improving the current efficiency of energy use is undoubtedly an expedient and viable solution.

9.2 FUNDAMENTALS OF THE THERMOELECTRIC EFFECT AND THERMOELECTRIC MATERIALS The TE effect, otherwise known as the “Seebeck effect” is the direct conversion of a temperature difference between two dissimilar electrical conductors or semiconductors to an electrical voltage. When the sides of TE materials are exposed to different temperatures, then a voltage is created across the two sides of the material. Conversely, when a voltage is applied, a temperature difference can be created, known as the “Peltier effect.” At the atomic scale, when a temperature gradient is applied at the two end sides of a thermocouple, the electrons and holes move faster and have a lower density at the hot side, resulting in diffusion of electrons/ holes toward the cold side as schematically demonstrated in Fig. 9.2. This movement of carriers (electrons for n-type and holes for p-type materials) is translated into the generation of an electric field across the thermocouple. This is called as the “Seebeck effect” and the voltage created for a temperature difference, ΔT, under thermodynamic equilibrium is S 3 ΔT, where S is the Seebeck coefficient. TE materials are therefore one potential candidate for harvesting waste thermal energy, due to their ability to convert it into electricity, even under very low-temperature Electric field

Low density of holes

High density of holes

Electric field

Cold side

Hot side

P-type

N-type Low density of electrons

High density of electrons

Figure 9.2 A schematic representation of the TE effect showing the charge carriers of a p- or an n-type material to diffuse from the hot side to the cold, when a temperature gradient is applied.

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gradients relative to the environmental temperature. This technology exhibits distinct advantages over other energy-harvesting technologies: (1) TE conversion is reliable and operates in silence as it works without mechanical movement, (2) it is an environmentally friendly green technology, since no heat and no gaseous or chemical wastes are produced during operation, and (3) it can be widely used in places where other energy-conversion technologies are unavailable, such as in the remote outer space, etc. [6,7]. The “thermoelectric effect” encompasses three distinct effects: (1) the Seebeck effect, (2) the Peltier effect, and (3) the Thomson effect. The Seebeck and Peltier effects are different manifestations of the same physical process, often referred as the Peltier
Seebeck effect. The Thomson effect is an extension of the Peltier
Seebeck model and is credited to Lord Kelvin. Historically, Seebeck in 1821 observed first that when a temperature gradient was applied between two ends of copper (Cu) wires and bismuth (Bi) wires this can generate a voltage, which is commonly referred to as the Seebeck effect [8]. Later, in 1834, Peltier observed that when an electric current flows through Cu and Bi wires at room temperature, a temperature difference could be created, and this is known as the Peltier effect, which is widely deployed in refrigeration processes [9]. The Seebeck effect provides a theoretical basis for the applications of TE energy converters (energy harvesting), while the Peltier effect is applied in cooling devices (refrigeration). To date, TE materials have been widely used in several high-tech applications, such as aerospace, military, medical thermostats, microsensors, wearables, etc. TE materials obey the TE or Seebeck effect described by the thermoelectric power (TEP), or thermopower, or Seebeck coefficient (S) [10]. The Seebeck coefficient is defined as shown in Eq. (9.1): S5

ΔV ΔT

(9.1)

where ΔV is the electric potential difference or the generated thermovoltage created by a temperature gradient, ΔT. It is an intrinsic material property related to the electronic properties, and it is positive for p-type and negative for n-type semiconductors [11]. The Seebeck coefficient is used for the calculation of the power factor (PF 5 σ 3 S2) (σ is the electrical conductivity), a well-known entity for comparing the voltage output (Vout) of different TE materials. The efficiency of TE materials is characterized by a dimensionless figure of merit (ZT); ZT 5 (σ 3 S2/κ) 3 T,

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where σ represents the electric conductivity (S/m), S is the Seebeck coefficient (μV/K), κ is the material’s thermal conductivity (W/mK), and T is the absolute temperature (K) [12]. In recent years, great progress has been made in improving the ZT. It can be seen that an efficient TE material should exhibit high electrical conductivity combined with a high Seebeck coefficient and low thermal conductivity. The Seebeck coefficient Ѕ, characteristic of the average entropy per charge transport, should be large in order to create a high voltage induced by a temperature gradient. The Seebeck coefficient, however, is not the only parameter to be optimized in order to maximize ZT. The electrical conductivity (σ) must be large to minimize the Joule heating during charge transport. Apart from the two parameters mentioned, a good TE material should also exhibit low thermal conductivity (κ), to prevent heat flow through the material. These three factors are interdependent in bulk TE materials, and altering one changes the other two. The difficulty in simultaneously optimizing them causes TE research to decay, until great reduction of thermal conductivity was both theoretically and experimentally proven in nanomaterials in 1993 [13]. The difficulty of designing high-performance TE materials arises from the fact that both electrical and thermal conductivity are related via the carrier concentration, and thus, optimizing one parameter will negatively affect the other. This interdependence has delayed the development of TE materials for many years. Although TE technology possesses many merits and has been known for two centuries, it has only been applied in narrow fields because of its low conversion efficiency (typically less than 6%) [14]. The conversion efficiency strongly depends on the figure of merit (ZT) of TE material, as described by Eq. (9.2), pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 ZT 2 1 (9.2) n 5 nc pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Tc 1 1 ZT 1 Th where n is the conversion efficiency of heat to electricity, Tc and Th are the temperatures of the cold and hot sides of a TE material, respectively, nc represents the Carnot efficiency expressed as nc 5 1 2 Tc/Th or (Th
Tc)/Th 3 100%. It can be clearly seen that ZT should be at least above 3 in order to ensure that the TE material/device conversion efficiency is competitive with that of traditional power generators, which can reach 40% of Carnot efficiency [15]. The electrical conductivity (σ),

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being an important factor in the ZT equation, can be expressed by Eq. (9.3): σ 5 neμ

(9.3)

where n is the charge carrier density or concentration, μ is the mobility of charge carriers, and e is the charge of unit carrier (electron or hole). The electrical conductivity can be improved by chemical doping in which each dopant atom can have one more valence electron than its host atoms, and can therefore facilitate the increase of charge carrier density. Meanwhile, the dopants can reduce the mobility of charge carriers due to the enhanced scattering between dopants and carriers. The ideal density of charge carriers has been reported to be in the range of 1019 to 1021 cm23 [16]. Doping improves also the Seebeck coefficient by changing the electron density of states (DOS) [17]. As shown in Eq. (9.4), the Seebeck coefficient is mainly affected by the charge carrier concentration, as well as the effective mass of the charge carriers (m ), which usually decreases with increasing carrier mobility (kB and h are the Boltzmann constant and Planck constant, respectively).  23 8π2 k2B  π S5 m T (9.4) 2 3n 3eh The thermal conductivity is decreased by phonon scattering (e.g., phonon
boundary scattering, phonon
defect scattering, and phonon
phonon scattering). The thermal conductivity is the summation of two components: (1) the electron thermal conductivity (ke ) and (2) the lattice thermal conductivity (kc ), as is expressed in Eq. (9.5), k 5 ke 1 kc

(9.5)

ke is proportional to the electrical conductivity according to the Wiedemann
Franz law [18]. In semiconductors, usually .90% of thermal conductivity arises from the lattice thermal conductivity (kc ), which is independent of the electrical conductivity. Hence, reducing the lattice thermal conductivity will lead to a pronounced enhancement of the TE performance. Then, the dimensionless “figure of merit,” ZT could be rewritten as shown in Eq. (9.6): ($  23 %2 )

8π2 k2B  π μ ZT 5 m T qT (9.6) 3n k 3qh2

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where S, σ, k, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively; kB is the Boltzmann constant, h is Planck’s constant, q is charge, n is charge carrier concentration, μ is mobility, and m is effective mass of the charge carrier. From the above, it can be seen that the Seebeck coefficient, thermal conductivity, and electrical conductivity are strongly dependent, and it is therefore very challenging to improve them simultaneously in bulk TE materials. This is the main reason that so little research exists on TE materials from the 1960s to the 1990s, until the article published in 1993 by Dresselhaus and coworkers showing theoretically that low-dimensional materials can have higher ZT than their bulk analogs, due to both their lower thermal conductivity and quantum confinement effects [19,20]. This work triggered the scientific community’s research interest into TE materials and provided a mainstream approach and strategy for the enhancement of TE material’s performance and ZT values. This can be achieved more precisely by creating proper material “nanostructuring,” inducing a large number of interfaces and thus facilitating phonon scattering resulting in low thermal conductivity values. The TE material’s research and the promising nanostructuration approaches are part of the wider field of nanotechnology, where key elements are the long-range ordering with controlled nanostructures for enhanced optical [21
29], electrical [30
39], mechanical [40
42], and magnetic [43,44] properties. Fig. 9.3 demonstrates the number of publications on TE materials from 1965 till today, highlighting the slight increase in the TE research rate from 1965 to 1993, and the rapid increase from that year till now, due to the flagship stimulating work of Dresselhaus (data collected from Science Direct data library using “thermoelectric” as the key word). The extreme interest in TE materials during the last decades and mainly during the last decade has been the driving force for many review articles that have appeared focusing on the different types of TE materials [45
51] including oxide [45] and organic based [47]. For example, Sootsman et al. summarized new and old concepts in inorganic TE materials in 2009 [52]. M. S. Dresselhaus and coworkers reviewed in 2007 new directions for low-dimensional inorganic TE materials [13]. Li et al. highlighted progress in TE materials with high ZT and the related fabrication processes for producing nanostructured materials including Bi
Te alloys, skutterudite compounds, Ag
Pb
Sb
Te quaternary systems, half-Heusler (HH) compounds, and high ZT oxides [53]. Over the last decade, reviews on organic-based TE materials and devices have appeared. Du et al. reviewed

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25,000

Number

20,000 15,000 10,000 1993* 5000 0

5 0 8 0 0 5 0 5 5 0 97 197 198 198 199 199 200 200 201 201 1 – – – – – – – – – – 65 70 975 980 985 990 995 000 005 010 1 2 2 1 1 2 19 19 1 1

Year

Figure 9.3 Number of publications in the area of TE materials and related technology since 1965. The data were collected from Science Direct data library using “thermoelectric” as the key word.

in 2012 the research progress on polymer
inorganic TE nanocomposite materials [54]. Gao et al. summarized in 2016 the conducting polymer/ carbon particle TE nanocomposites as emerging green energy materials [55]. Chen et al. reviewed in 2017 advances in polymer TE composites [56]. A very important review by He et al. in 2017 has summarized all the state-of-the-art TE devices: architectures, geometries, device interconnections, contact optimization, etc., based on inorganic and organic (small molecule, polymer, micro-, and nanocomposites) TE materials [57]. Finally, it is worth mentioning that, currently, the highest value of ZT (ZT 5 3.6 at 580K) has been reported by Harman et al. for PbSe0.98Te0.02/PbTe quantum-dot superlattices (QDSLs) grown by molecular beam epitaxy [58].

9.3 INORGANIC THERMOELECTRIC NANOMATERIALS Traditional TE materials and thermoelectric generator (TEG) devices are generally based on low bandgap inorganic semiconductors, for example, Bi2Te3 [59], Bi2S3 [60], PbTe [61], etc. There are many kinds of inorganic TE materials, for example, semiconductors, ceramics, oxides from bulk to superlattice, and with different geometries ranging from nanoparticles to nanowires [45,46,48
50,62]. Fig. 9.4A shows the temperature

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(A) Waste heat harvest

Refrigeration

Automobile vehicles

Micro-and opto-electronics

0

Boilers and furnaces

300

600

900

1500

Zn2Sb3

Bi, Sb

Bi2Te3 Thermoelectric materials

1200

CsBi2Te3

CoSb 3 PbTe

Toxic, rare

Ag-Sb-Ge-Te(TAGS) Heavy-element based Ag-Sb-Pb-Te(LAST), half-Heusler (Si, Ge) Mg2(Si, Sn) MnSi2.75 β-FeSi2

Silicides

Eco-benign, abundant

Bi2Sr2Co2Oy γ-NoCoO2, Ca3Co4O3

Oxides

SrTiO3, CaMnO3 ZnO

(B)

Si Fe Mg Na Ca Ti

10000

Abundance (ppm)

Mn

Sr Zn

Pb Co

100 Cs Ge Sn

1.0 Bi In

Ag

Sb

0.01

Te

Elements Figure 9.4 (A) Application temperature ranges for different TE materials and (B) abundance of elements in the Earth’s crust used in TE materials.

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ranges of application for different TE materials and Fig. 9.4B the natural abundance of the constitutive elements. It can be seen that most of the high-performance TE materials contain highly toxic or low abundance elements [63,64].

9.3.1 Metal Chalcogenides Metal chalcogenides have been extensively used for energy-conversion and energy storage devices, for example, TE generators and coolers, photovoltaics, lithium ion batteries, fuel cells, and supercapacitors [65
68]. The most commonly used metal chalcogenides are the lead and bismuth selenides and tellurides, with the latter being the main candidates for TE applications. Recently, the utilization of nanotechnology principles has resulted in dramatically improved records in the laboratory, via chemical doping, and engineering of their electronic structure, reaching in some cases ZT values above 2 or even 3 [69
72]. 9.3.1.1 Lead Chalcogenides Lead is one of the most useful elements for the developed TE materials, as it has great natural abundance in the Earth’s crust and can be easily processed. The main bottleneck, however, for its wide use towards largescale TE devices is its high toxicity. Lead possesses a heavy atomic weight, which has benefits for the reduction of lattice vibration and thermal conductivity. PbTe/PbTeSe QDSLs have been reported by Harman et al. with a ZT of 3.0 at 550K [58]. The use of nanoinclusions represents an important advance for Pb-based TEs in order to reduce the thermal conductivity as has been reported in various studies [73
80]. The nanoinclusions usually arise from a well-dispersed secondary phase and can be divided into “coherent” precipitates, which are slightly mismatched with the matrix, and “incoherent,” which show a clear boundary with the host matrix material. The coherent nanoinclusions act as point defects and scatter short-wavelength phonons. The incoherent ones possess a large mismatch with the matrix and therefore function as nanoparticles selectively scattering mid- to long-wavelength phonons. A ZT of 1.7
1.8 for n-type and p-type PbTe has been reported through the use of coherent nanoinclusions for AgSbTe2 [81], NaSbTe2 [75], and SrTe [76]. On the other hand, a ZT of 1.4
1.5 has been achieved in n-type PbTe by incoherent nanoinclusions of Sb [77] and Ag2Te [82], while this slightly lower performance has been attributed to the relative reduction of the charge carrier mobility. By facilitating phonon scattering on all length scales in a

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hierarchical multiscale system (i.e., from atomic-scale lattice disorder and nanoscale-endotaxial precipitates to mesoscale grain boundaries), making use thus of a “multiscale phonon scattering,” Kanishka et al. [83] achieved the maximum reduction in the lattice thermal conductivity and a ZT of 2.2 at 915K in p-type PbTe doped with 4% SrTe (molar concentration). Chemical doping is another approach for the enhancement of the power factor. Biswas et al. [76] found that doping with Se, a p-type PbTe can increase carrier mobility without decreasing carrier concentration, due to the convergence of electronic bands, reaching a ZT of 1.8 in Na0.02Pb0.98Te0.85Se0.15. Reports exist also on nanostructured TEs made from wet-chemical approaches. For instance, Ibanez et al. [84] synthesized core
shell PbTe@PbS nanoparticles and after consolidating them into pellets obtained a ZT of 1.07 at 700K. Se and S are cheaper and less toxic than Te, and therefore PbSe and PbS as alternatives could have a bright future as TEs which should be further explored. 9.3.1.2 Bismuth Chalcogenides Bi2Te3 and its alloys with Sb and Se, with a ZT of 0.6 at 300K, have been commercialized and used for TE refrigerator applications since their discovery in the 1950s. The advances in nanotechnology have recently boosted the ZT to a new record of 2.4 at 300K for a superlattice of Bi2Te3-Sb2Te3 films [69]. In the same approach as for the lead chalcogenides, the introduction of nanostructures can improve the TE performance of bismuth chalcogenides. Poudel et al. [85] and Xie et al. [86] increased the ZT of p-type BixSb2
xTe3 from 1 to 1.4
1.5 by employing nanoinclusions during powder metallurgy synthesis. Liu et al. [87] achieved a ZT of 0.94
0.99 in an n-type Cu0.01Bi2Te2.7Se0.3, which is higher than that of Bi2Te2.7Se0.3 (ZT 5 0.85). In 2010, Zahid and Lake [88] theoretically calculated the ZT of Bi2Te3 thin films with five atomic layers thickness to be 7.2 at room temperature. This high ZT results from the change in the distribution of the valence band electron density due to the quantum confinement effect realized in the thin film. In addition to thin films, the ZT of Bi2Te3 nanowires has also been theoretically calculated, and the results show a strong diameter dependence, for example, the ZT value can be over 6 if the nanowire diameter is smaller than 5 nm. Zhang et al. [89] synthesized uniform 8-nm Bi2Te3 nanowires and achieved a ZT of 0.96 at 380K. Besides the use of zero-dimensional nanoparticles or onedimensional (1D) nanowires to improve the TE performance, the introduction of two-dimensional (2D) nanosheets into bismuth

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chalcogenides can also lead to a high ZT. Li et al. [90] mixed graphene with Bi2Te3 to improve its conductivity, resulting in a 1.5 times higher ZT (0.3 at 350K) compared to that of the single-crystal Bi2Te3.

9.3.2 Metal Oxides Metal oxide-based materials have also been reported as TEs and have shown several advantages over metal chalcogenides. However, oxidebased TEs exhibit lower ZT values compared to conventional TE materials, for example, Bi2Te3, etc., due to their inherent higher thermal conductivity. However, their good chemical and thermal stability combined with their low-cost renders them a promising candidate utilized over a wide range of temperature gradients and under ambient conditions. There are several types of metal oxides, including cobalt oxides (p-type), manganese oxides (n-type), and zinc oxides (n-type). Research interest in oxide-based TE materials was ignited when a p-type NaxCoO2 single crystal with a ZT around 1 was reported first by Terasaki in 1997 [91]. Since then, the highest ZT obtained in oxides was around 2.4 at room temperature, which was realized for a SrTiO3 superlattice [92]. In 2011, Jood et al. successfully doped Al into ZnO nanoparticle crystals using a microwave solvothermal approach [93]. The resultant doped ZnO nanoparticles showed a ZT of 0.44 at 1000K, 50% higher than the best non-nanostructured counterpart at the same temperature. A higher ZT of 1.1 at 923K for Bi0.875Ba0.125CuSeO was reported by Li et al. [94]. This high ZT value was achieved by the combined effect of heavy doping with Ba and refinement of grain size (200
400 nm).

9.3.3 Superionic Conductors An “ideal” TE material should be a phonon glass and an electron crystal (PGEC), exhibiting thus simultaneously high electrical conductivity and low thermal conductivity. In contrast to the structure of lead and bismuth chalcogenides, cuprous and silver chalcogenides have a special structure, where the chalcogen anions form a crystalline lattice and provide a pathway for charge carriers, while the cuprous or silver cations (Cu1, Ag1) are highly disordered around the chalcogen sublattice. The “liquid-like” behavior of Cu1 and Ag1 makes their chalcogenides phonon liquid and electron crystal, similarly to the PGEC concept. Therefore, outstanding performance is foreseen and it has been reported for p-type Cu2
xSe ZT 5 1.6 at 1000K; the highest value amongst superionic conductor TE

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materials [95]. In 2013, Liu et al. achieved an even higher ZT (2.3 at 400K) in n-type Cu2Se [96]. The temperature fluctuation near phase transition temperatures can lead to “critical scattering,” which can selectively scatter phonons without affecting the electron transport. There are only few reports on the TE properties of copper and silver chalcogenides, due to difficulties in large-scale preparation of their nanostructures with stoichiometrically proper compositions, as well as uniform crystal size and morphology. Xiao et al. synthesized monodispersed Ag2S, Ag2Se, and Ag4SeS nanocrystals via a solvothermal method and investigated the TE properties at the phase transition temperature [97]. Their ZT at the corresponding transition temperature (0.33) is within the same order of magnitude as the maximum ZT for Ag4SeS. Compared to silver chalcogenide nanostructures, only a few articles exist on copper chalcogenides. Cu2
xSe nanowires have been reported by Zhang et al., synthesized by a hydrothermal process with a Seebeck coefficient of 180 μ/K without any ZT values reported [98].

9.3.4 Silicon-Based Materials Silicon is a cheap, abundant, and environmentally friendly chemical element widely used in semiconductor electronic devices, for example, transistors, solar cells, etc. Bulk silicon exhibits poor TE performance, as it has inherently extremely high thermal conductivity (B150 W/mK). However, this has been successfully overcome by reducing its grain size down to nanoscale dimensions. A ZT of 0.6 at room temperature was achieved by B-doped Si nanowires with 50 nm in diameter, fabricated by silicon wafer etching [99]. Sabah et al. employed a versatile high-energy ball-milling method to synthesize doped Si nanoparticles on a large scale and consolidated them into pellets [100]. A ZT of 0.7 at 1275K was measured for this n-type bulk nanostructured Si-based TE material. Silicon TE performance can be significantly improved also by doping with germanium, although it is an expensive element. The SiGe alloys are currently the best TE materials at high temperatures (B1000°C) and are therefore applied in radioisotope TE generators (RTG) in aerospace. In specific, SiGe TE modules with ZT of 0.5 for p-type and 0.9 for n-type have been used in RTGs by NASA in the United States since 1976 [101]. In 2012, a ZT of 2 at 800K was reported by Lee et al. who simultaneously tested the thermal conductivity, electrical conductivity, and power factor of a single SiGe nanowire [102]. The high ZT arises from

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the huge reduction in thermal conductivity due to the quantum confinement effect. Alternative SiGe TE materials with a reduced amount of Ge element could be Mg2BIV (BIV 5 Si, Ge, and Sn) compounds and their alloys, among which, Mg2Si-based materials have been extensively investigated as they can be used in the engine operating temperature range (i.e., 500
800K). Mg2Si TEs have lower density, higher resistance to oxygen, nontoxicity, and environmental friendliness. The huge abundance of Mg and Si leads to low fabrication cost, which would enable their wide application. However, the low ZT is due to their high thermal conductivity due to the lightweight of the Si atoms [103
105]. Therefore, Zaitsev et al. doped Mg2Si0.4Sn0.6 with Sb and got a ZT of 1.1 at 780K [106].

9.3.5 Skutterudites, Clathrates, and Half-Heusler Alloys Skutterudites (SKUs), clathrates, and HH alloys are PGEC TE materials prepared mainly by solution casting or powder metallurgic synthetic processes. SKUs are bulk TE materials with the structure of (Co, Ni, Fe) (P, Sb, As)3 and cubic space group Im3. They contain vacancies that lowcoordination ions (usually rare earth elements) can be inserted to decrease the thermal conductivity without reducing electrical conductivity. Such a structure makes them “PGEC” materials. The chemical formula of SKUs can be expressed as ReM4X12, where Re is a rare earth element, M is a transition metal element, and X is a nonmetal element from Group V, such as phosphorus, antimony, or arsenic. ZT of SKUs can be significantly improved by double, triple, and multiple filling of elements into their structural vacancies. Usually, alkali metals, alkaline-earth metals, lanthanides, and similar elements are selected as “dopants” for SKUs because of their moderate atom size. The advantages of SKUs are the following: (1) the ZT is relatively high (up to ZT 5 1) and can be easily tuned by controlling their composition and structures; (2) components are relatively cheap but nontoxic; and (3) the application temperature can be up to 600°C. The sublimation of Group V elements (e.g., antimony) and poor resistance to oxidation limit their wide application .600°C, as oxidation and volatilization take place in harsh environments such as in air at high temperatures leading to decreased lifetime of the respective TE generators. In 2011, CoSb3 with multiple fillers of Ba, La, and Yb were synthesized, and a high ZT of 1.7 at 850K was reported [107]. Clathrates compared to SKUs possess a more complex cage-like structure and a wider variation in

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composition. They have a general formula of AxByC46
y (type I) and AxByC136
y (type II), in which B and C are elements from Groups III and IV and can form a framework serving as a host for “guest” atoms of element A (alkali or alkaline-earth metal), which are encapsulated between two different polyhedra. The main differences between types I and II clathrates are the number and size of the vacancies in their unit cells. Their complex unit cells allow a significant reduction in the thermal conductivity and lead to a high ZT. In addition, clathrates are highly chemically and thermally stable at high temperatures. Powder metallurgical and crystal growth techniques have been deployed to synthesize clathrates. The highest ZT of 1.35 at 900K was reported in 2006 for the ntype Ba8Ga16Ge30 clathrate [108]. HH alloys (ABX) have potential in high-temperature TEP generation, especially as n-type material. These alloys have three components selected from different element groups or from the same group. A and X are transition metals, and B may be a metal or nonmetal. The crystal structure of a HH alloy could be considered as a simple rock salt structure formed by A and X, which is filled with B at one of the two body diagonal positions (1/4, 1/4, 1/4) in the unit cell, leaving the other one (3/4, 3/4, 3/4) unoccupied [109]. These alloys are relatively cheap and have a flexible composition. In 2010, Yan et al. reported a p-type Zr0.5Hf0.5CoSb0.8Sn0.2 with a ZT of 0.8 at 973K [110]. The ZT enhancement comes from a simultaneous increase in the Seebeck coefficient and a decrease in the thermal conductivity due to nanostructuring. Progress in nanostructured HH materials has been summarized in Reference [111]. Table 9.1 summarizes the ZT values of high-performance inorganic TE materials together with the plausible mechanisms governing their TE performance.

9.4 ORGANIC THERMOELECTRICS: POLYMER AND NANOCOMPOSITE SYSTEMS 9.4.1 Conjugated Polymer Thermoelectric Materials Organic thermoelectric (OTE) materials or “organic thermoelectrics” have attracted increased scientific interest as an alternative approach to conventional inorganic TEs. Conducting polymers have been suggested as TE materials for potential large-area TE applications. This is because OTEs are compatible with inexpensive, large-scale processing methods and often possess unique mechanical flexibility, which makes them

Table 9.1 A summary of representative high-performance inorganic TE materials together with the respective ZT values and the plausible mechanism of their TE behavior Type Material Temperature (K) ZT References Mechanism

Metal chalcogenides Superionic conductor Oxides Silicon based

PGEC

PbTe(SrTe)4Na2 Bi2Te3-Sb2Te3 p-type Cu2
xSe n-type Cu2
xSe SrTiO3 Si SiGe Mg2Si0.4Sn0.6 (Sr, Ba, Yb)0.07Co4Sb12 Ba8Ga16Ge30 Zr0.5Hf0.5CoSb0.8Sn0.2

915 300 1000 400 B300 1275 800 800 800 900 973

2.2 2.4 1.6 2.3 2.4 0.7 2 1.1 1.8 1.35 0.8

[83] [69] [95] [96] [92] [100] [102] [106] [112] [108] [110]

Multiscale phonon scattering Superlattice Structure, doping, phase transition Structure, doping, phase transition Superlattice Nanowire Nanowire Solid solution Dope, nanostructuring Extrinsic to intrinsic transition Nano-inclusion

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311

geometrically versatile to be integrated in complex three-dimensional (3D) shaped objects, for example, wearables, car roofs, etc. Till now, PANI has been the most studied conductive polymer due to its high electrical conductivity, good chemical and thermal stability, and ease of preparation and processing/casting from solution. PEDOT:PSS is the second most-used polymer with potential processability from water dispersions that are commercially available, good charge transport properties (both of electronic carriers, as well as ions) resulting in relatively high electrical conductivities. However, PEDOT:PSS exhibits low resistance to humid environments due to its hygroscopic behavior arising from the PSS phase. In 2002, the first studies appeared utilizing electrically conductive polyaniline films as OTE materials [113]. OTEs are particularly attractive for low-quality waste heat harvesting, such as waste heat at low temperatures. Till then, high ZT values ( . 0.1) had been achieved for both p-type and n-type OTE materials [114,115]. Similarly to inorganic TEs, organic ones should also be optimally “electronic crystals and phonon glasses.” The main crucial parameter to tune and optimize for OTEs is to achieve “electronic crystals,” since their thermal conductivities are intrinsically relatively low. Therefore, the TE parameter to put effort on and optimize is the power factor, by simultaneously increasing the electrical conductivity and Seebeck coefficient. The Seebeck coefficient is directly related to the density of states (DOS) as stated several times in this chapter, reflecting the average entropy transported per charge carrier. As such, it decreases with increasing carrier concentration since the mobility is decreased. On the contrary, the electrical conductivity increases with carrier concentration, n, and carrier mobility μ (σ 5 neμ). Organic semiconductors have been largely neglected as TE materials, despite their inherent low thermal conductivities (  0.3 W/mK) and high electrical conductivities ( . 1000 S/cm) [47]. This is due to the fact that conducting polymers are not stable at high temperatures with their maximum operational temperatures limited in the range of 200°C
250°C. Hence, the benchmark for polymer-based TEs is Bi2Te3 alloys that exhibit a ZT of 1.2 at room temperature [85]. Conjugated polymers and mainly polyaniline (PANI) [116
118] and polythiophenes [119
123] have been investigated as OTE materials. Fig. 9.5 depicts the molecular structures of representative p- and n-type semiconducting polymers and dopants, associated with the power factor values that have been experimentally determined.

C8H17

p-type

n-type

C6H13 N

O O

S

n

O S

n

Fe(NTf2)3

C6H13

n SO3H

PDPP3T+iron triflimide PF~12 μW/m/K2

O

SO3Na

NaO3S

S N S

S

xCu• N

S

S

S n N

O

N

N

O

C8H17

P(NDIOD-T2) with N-DMBI PF~0.6 μW/m/K2

C10H21

CPE-Na PF~0.84 μW/m/K2

C18H37 C18H37

C14H29 S S

PDI-OH PF~1.4 μW/m/K2

N H

poly[Cux(Cu-ett)] PF~6.5 μW/m/K2

n

OH N

O

O

S

S Cu S

N

N

C8H17 O

N

O

O

N

C10H21

C8H17

PEDOT-PSS PF~469 μW/m/K2

OH

poly[Kx(Ni-ett)] PF~66 μW/m/K2

O

N

S Ni S

S

n

S

S xK•

S

S

F n

F FF F F

F

S C14H29

Cl Cl Si Cl

F

O

N

O

O

FF F F F F

n O

PBTTT+FTS from vapor phase PF~100 μW/m/K2

O

O

N

F C18H37

FBDPPV with N-DMBI PF~28 μW/m/K2

C18H37

Figure 9.5 Representative p-type and n-type semiconducting polymers and dopant materials together with the respective power factors.

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The most used p-type polymeric material investigated till now is the blend of poly(3,4-ethylenedioxy thiophene) polystyrene sulfonate (PEDOT:PSS), which has been utilized in various organic electronic devices, for example, OPVs, OTFTs, OTEs, OLEDs, etc. PEDOT:PSS is a polyelectrolyte consisting of positively charged PEDOT and negatively charged PSS. Crispin et al. reported power factors of 300 μW/m/K2 and ZT values of 0.25, after de-doping highly conductive PEDOT:tosylate with tetrakis(dimethylamino)ethylene [47]. Also, the addition of carbon nanotubes or graphene is a popular approach to modulate the TE properties of PEDOT:PSS [124]. By carefully structuring the composite film, conductivities of 105 S/m and Seebeck coefficients of 120 μV/K were measured, leading to power factors of 2710 μW/m/K2, one of the highest values reported for OTE materials. While the p-doping of organic semiconductors can be readily achieved, n-doping is more challenging. Organic electron-deficient semiconductors are associated with the high electron affinities (
3 to
4 eV), making the negatively charged molecules prone to reactions with environmental moisture or oxygen [125]. The charge transfer cocrystal salt of tetrathiafulvalene and tetracyanoquinodimethane is probably the most studied charge transfer salt showing promising n-type TE properties. Electrical conductivities of 500 S/cm and power factors of up to 40 μW/m/K2 have been reported. However, several drawbacks significantly limit the applicability of charge transfer crystals as TE materials, for example, modulating the carrier densities is difficult since the stoichiometry of the cocrystals must be accurately respected, and the physical properties of the cocrystals are not isotropic. Alternative n-type conductors for TE applications have mainly focused on perylenediimide- and naphtalenediimide-containing organic semiconductors. Segalman et al. synthesized a series of perylene diimide (PDI)-based molecular semiconductors functionalized with tertiary amine-containing side chains [126]. Upon thermal annealing, the functionalized PDI moieties self-dope via a dehydration reaction of the tethered tertiary ammonium hydroxide. By carefully designing the side chains, the self-doped PDI moieties achieve conductivities of 0.5 S/cm and power factors of 1.4 μW/m/K2. Chabinyc et al. extrinsically doped the high-performing n-type polymer poly([N,N 0 -bis(2-octyldodecyl)1,4,5,8-napthalenedicarboximide-2,6-diyl]-alt-5,50 -(2,20 -bithiophene)) (P(NDIOD-T2)) with the molecular dopant (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl) (N-DMBI) [127]. While the conductivity initially increases as a function of dopant loading, a sharp drop in

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conductivity was observed at higher loadings. The miscibility of the NDMBI dopant in the polymer phase is limited, which is why at higher dopant loadings, the dopant crystallizes and the phase separates from the polymer matrix, thus reducing the doping efficiency. Despite the morphological instabilities, Seebeck coefficients of
850 μV/K have been achieved with power factors of 0.6 μW/m/K2. In a recent report, Pei et al. showed that BDOPV-based FBDPPV polymers have reached a record power factor of 28 μW/m/K2 [128]. Huang et al. reported that thiophene-diketopyrrolopyrrole-based quinoidal (TDPPQ) can exhibit a high power factor of 113 μW/m/K2, when the material is interfacially doped by the bismuth. The performance is the best value for all reported n-type small molecules [129].

9.4.2 Nanocomposite Polymer Thermoelectric Materials Nanocomposite polymer TEs using a conducting and nonconducting matrix and organic (e.g., CNTs, graphene oxide, fullerenes, etc.) or inorganic nanoinclusions (e.g., Bi2Te3, PbTe, Te nanowires, etc.) have also been extensively studied. PEDOT:PSS (CLEVIOS PH1000) mixed with Bi2Te3 particles have reached power factors (PF 5 σ 3 S2, σ is the electric conductivity and S the Seebeck coefficient) in the range of B130 μW/m K2 [130]. The incorporation of carbon nanotubes (CNTs) may enhance their performance via increased conductivity or molecular orientation effects of the polymer chains. Thereby, high filler loadings ( . . 50 wt.%) can be realized [131], resulting in high electrical conductivities [132
135]. Namely, electrical conductivities up to 4 3 105 S/m and power factors in the range of B140 μW/m/K2 have been reported by Moriarty et al. for single-walled carbon nanotubes (SWCNTs) in a PEDOT:PSS matrix [122]. However, the low thermal and moisture stability of these materials is an impediment to engineering and structural applications. CNTs have been introduced also via solution and melt mixing methods in engineering nonconductive thermoplastic polymer matrices resulting in nanocomposites with thermal energy harvesting properties [136
140]. Polymer nanocomposites are attractive organic materials due to their ease of production, relatively low cost, flexibility, and high specific properties [30,32,33,141,142]. SWCNT polycarbonate (PC)/ SWCNT nanocomposites prepared by solvent mixing showed that by increasing the SWCNT content (up to 30 wt.%), the electrical

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conductivity increases to approximately 1000 S/m and the Seebeck coefficient reached 60 μV/K with only a slight dependence on the SWCNT content [143]. CNT composites with polymers having electron-rich functional groups, like PVA and polyethyleneimine, have been found to act as n-doping to the incorporated SWCNTs, and resulted in coefficients up to
21.5 μV/K [144]. Antar et al. reported on melt-mixed composites of polylactide (PLA) with multiwalled carbon nanotubes (MWCNTs) and expanded graphite with high filling levels (up to 30 wt.%), resulting in electrical conductivities of B4000 S/m [145]. The Seebeck coefficient reached a maximum of 17 μV/K for the composites with expanded graphite and B9 μV/K for MWCNT ones. Research from our group using a series of melt-mixed polycarbonate
MWCNT nanocomposites has shown that an increasing filler content results in an increase in the power factor due to an increase in the electrical conductivity [36,146,147]. The TE properties of melt-mixed conductive nanocomposites of polypropylene (PP) filled with single-walled carbon nanotubes (2 wt.%) and copper oxide (5 wt.%) showed that by adding polyethylene glycol (PEG) during melt mixing p-type composites switched into n-type with Seebeck coefficient up to 145 μV/K and
56 μV/K, respectively [140]. Hierarchical CNT-coated fibrous reinforcement structures have also been reported as TE reinforcements upon their incorporation in polymer matrices for potential large-scale thermal energy harvesting by structural composites, for example, in aerospace and automotives [34,42,148,149]. Most of the polymeric matrices studied so far are based on aliphatic or semiaromatic backbones. This severely limits their applicability as engineering materials capable of operating in high-temperature environments. High-performance engineering polymers such as all-aromatic polyimides and poly(ether-imide)s (PEIs) are capable of withstanding high temperatures ( . 200°C) and exhibit glass-transition temperatures (Tg) above 200°C with superlative mechanical properties. Recently, Tzounis et al. demonstrated for the first time the synthesis of all-aromatic PEI-SWCNT nanocomposite films as TEs. Semicrystalline nanocomposites of PEI/SWCNT (10 vol.%) reached a maximum power factor of B1.8 μW/m/K2. In a polymer (insulating or conjugated) matrix, the nanoinclusions have the ability to allow electron (n-doping) or hole (p-doping) transport by a plausible tunneling or hoping mechanism, while at the same time, phonon scattering occurs at the nanoparticle
polymer
nanoparticle interfaces, preventing their effective transmission and resulting in low

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Nanomaterials Synthesis

Figure 9.6 (A) Schematic representation of the mechanism based on electronic charge transport that governs the TE effect upon exposure to a temperature gradient, (B) a TEM image of a polymer/CNT nanocomposite.

thermal conductivities. Therefore, nanocomposites are considered as promising TE materials for TEP generation (used for the fabrication of TEGs) and a continuous demand remains for an increase in their electrical conductivity, Seebeck coefficient, and power factor values compared to existing reported values. Fig. 9.6A shows the mechanism that governs the TE mechanism of a polymer nanocomposite and Fig. 9.6B depicts a representative transmission electron microscopy (TEM) image of a polymer/CNT TE nanocomposite showing the dispersed 1D nanoparticulates within the polymer matrix (from the author’s personal microscopy investigation results data). Besides, carbon nanomaterials as thin films and thick self-standing films (buckypapers) have been reported also in TEs. Carbon nanotubes (CNTs), for instance, are well-known for their semiconductor electronic properties and have thus shown promising TE performance. Their TE efficiency has been found to be enhanced by the level of doping [150,151], as well as the dopant nature [34,36,39,146,152
155]. Doping of the SWCNTs by using, for example, polyethyleneimine (PEI) or hexafluoroacetone [143,144] resulted in Seebeck coefficients up to
50 μV/K. Hewitt et al. reported Seebeck coefficients of CNT buckypapers between 11 and 19 μV/K, and also discussed the dependence of the Seebeck coefficient on the CNT acidic treatment protocol [150]. Recently, Dörling et al. demonstrated that nitrogen-doping of the CNT graphitic lattice results in n-type TE behavior [156]. Table 9.2 summarizes the electrical conductivity, Seebeck coefficient, thermal conductivity, power factor, and ZT of different representative conjugated polymer-based TE materials with or without nanoinclusions.

Table 9.2 A summary of the TE properties for eight representative conjugated polymer-based thermoelectrics [157] Polymer

Nanofiller

Preparation method

σ (S/cm)

S (µV/K)

PF ( μW/ m/K2)

κ (W/m/K)

ZT (T)

References

Polyacetylene




Casting

4.990
11.560

11.4
28.4




0.7

[157]

Polypyrrole Polypyrrole

Casting In situ chemical polymerization Casting In situ chemical polymerization Solution mixing

100 41.6

12 26.9

7000 38

7 26 110

Polythiophene




Casting

100

21

Poly(para-phenylene)




Casting

1025

12




0.1
0.2

Poly(p-phenylene vinylene) Poly (carbazolenevinylene) PEDOT:PSS




Casting

1025

7




0.1
0.2




Casting

5 3 1023

230




0.1
0.2




Casting

55

13




0.1
0.2


rGO (21 wt.%) Graphene (2 wt.%)

Casting Mixing

900 715.03

75 22.9

0.24 B0.2

Solution spin casting

32.13

58.77


32.4 (300K) 11.09 (300K)

0.051 (300K) 1.95 3 1023 (453K) 0.18 (350K) estimated 0.0066 (300K) 2.1 3 10210 (300K) 7.2 3 10211 (300K) 8 3 1025 (300K) 0.0014 (300K) 0.42 (300K) 0.067 (300K)

[157] [159]

60


3.01 (300K)
2.6 (453K) 5.1 (350K)


0.1
0.2


Polyaniline (PANI)


rGO (67 wt.%)
GNs (30 wt. %), HCl Bi2Te3

0.0047
0.38 (300K) 0.002 (300K)


0.14

0.021 (300K)

[162]

Polyaniline (PANI) Polyaniline (PANI)

PEDOT:PSS PEDOT:PSS PEDOT:PSS

0.1
0.2 0.6
0.1
0.2

[157] [158]

[160] [157] [157] [157] [157] [157] [157] [161]

(Continued)

Table 9.2 (Continued) Polymer

Nanofiller

Preparation method

σ (S/cm)

S (µV/K)

PF ( μW/ m/K2)

κ (W/m/K)

ZT (T)

References

PEDOT:PSS

Fullerene (9%) rGO (16%) SWCNT (DMSO)

In situ chemical polymerization

50.8 6 5.9

31.8 6 2.3

5.2 6 0.9 (300K)







[163]

Film casting

B4000 (95 wt.%)

14
26

0.4
0.7

0.03 (40 wt. %) (300K)

[122]

PEDOT:PSS

Tos

Film casting

6 3 1024
300

40
780

B140 (85 wt.%)


0.37

[164]

PEDOT:PSS

Te

Films

19.3 6 2.3

163 6 4

70.9

0.22
0.30

0
0.25 (300K) 0.10 (300K)

PEDOT:PSS

[165]

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319

9.5 WORKING PRINCIPLE AND SPECIFIC ARCHITECTURES OF THERMOELECTRIC GENERATORS TEGs convert heat into electricity through the Seebeck effect. An applied temperature gradient across the generator will force heat to flow from the hot to the cold side by thermal conduction, while some of this heat is converted to electricity. The possibility of converting a heat flux into an electrical current or vice versa is realized by a TEG or thermoelectric cooler that employs the coupled transport between electrons and phonons. Fig. 9.7 illustrates schematically the operation principle of (A) a TEG and (B) a Peltier device, respectively. Regarding the Seebeck effect, when the junctions at the top are heated and those at the bottom are cooled, a temperature difference will occur. The electron/hole pairs are created at the hot end by absorbing heat, then they recombine and liberate heat at the cold end. Driven by the mobility of the hole/electron, the Seebeck voltage generates between the two ends, resulting in a current flow. As for the process of TE cooling or the “Peltier effect,” when a voltage is applied across a p/n junction, electron/hole pairs are generated in the vicinity of the junction and flow away, leading to cooling of the junction on one end and heating on the other end. For an ideal TE device with constant TE properties, the maximum heat to electrical power conversion efficiency (ηmax) and the output power density (Pmax) are expressed as: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi TH 2 TC 1 1 ZT 2 1  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi T (9.7) nmax 5 TH 1 1 ZT 1 c Th

Figure 9.7 Operation principle of (A) a TEG and (B) a Peltier device. A TE device generally consists of p- and n-type TE materials connected in series through conducting plates.

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Nanomaterials Synthesis

Pmax 5

ðTH 2TC Þ2 2 S σ 4L

(9.8)

where L is the length of the TE leg, and TC and TH are the cold-side and hot-side temperatures, respectively. The cooling efficiency of a TE cooling device is characterized by the coefficient of performance (COP). pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi T 1 1 ZT 2 H TC pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi TC COP 5 (9.9) TH2TC 1 1 ZT 1 1 The term ZT [otherwise reported as ZTM with TM the average temperature, TM 5 (TH 1 TC)/2] is the average value of ZT, the TE device figure of merit, between the hot and cold sides and is defined by Eq. (9.10). ZT 5

S2 T RK

(9.10)

where S, R, K, T are Seebeck coefficient, electrical resistance, thermal conductance, and absolute temperature, respectively. Another expression of COP is the following, described by Eq. (9.11): COP 5

STC I 2

I 2R 2

2 KΔT

SIΔT 1

I 2R 2

; ΔTmax 5

ZTH2 2

(9.11)

where I is the current, R is the resistance, and K is the thermal conductance. If 20°C of cooling is required, the COP would typically be in the region of 2. For comparison, a conventional refrigerator under the same circumstances has a COP around 14. The maximum temperature difference possible for a TE cooler, ΔTmax, is often around 50K. If the module is used as a heat pump, the COPmax of the TE heat pump is given by Eq. (9.12) COP 5 SIΔT 1 STC I 1

I 2R 2 KΔT 2

(9.12)

As an example, for a temperature difference of 20°C, the COP for a TE heat pump would typically be 3, which is comparable to the COP of conventional heat pumps.

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Besides, the Peltier effect can be used to create a refrigerator that is compact and has no circulating fluid or moving parts. Such refrigerators are useful in applications where their advantages outweigh the disadvantage of their very low efficiency. The Peltier effect is also used by many thermal cyclers, laboratory devices used to amplify DNA by the polymerase chain reaction (PCR). Finally, thermocouples and thermopiles are devices that use the Seebeck effect to measure the temperature difference between two objects. Thermocouples are often used to measure high temperatures, holding the temperature of one junction constant or measuring it independently (cold junction compensation). Thermopiles use many thermocouples electrically connected in series, for sensitive measurements of very small temperature differences.

9.6 APPLICATION OF THERMOELECTRIC GENERATORS A TEG is typically used for energy transduction through the Seebeck effect. TEG devices display a variety of advantages compared to other common energy technologies. TEGs function like heat engines, but are less bulky, have no moving parts, no noise, and a long operating lifetime; however, they are typically more expensive and less efficient. TEGs have wide applications in military, aerospace, cogeneration, medical thermostat, microsensors, etc. They have use in power plants for converting waste heat into additional electrical power (a form of energy recycling) and in automobiles as automotive TE generators (ATGs) for increasing fuel efficiency. Space probes often use radioisotope thermoelectric generators (R-TEGs) with the same mechanism but using radioisotopes to generate the required heat difference. Recent uses include body-heat—powered lighting and a smartwatch powered by body heat. As examples of TEG applications in the transport sector, for example, in the aircraft environment, temperature differences can be found in various locations (i.e., between the interior and exterior during flights, near turbines), and in automotives in structural components (e.g., bonnet, between the interior and exterior of the cabin, chassis, exhaust system, etc.). Recently, flexible thermoelectric generators (f-TEG) have been developed for human body applications to power wearable electronic devices with the highest power of 2.28 μW/cm2 [166].

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Nanomaterials Synthesis

9.7 RECENT TRENDS AND CHALLENGES 9.7.1 Market for Thermoelectric Generators and Recent Technologies in Thermoelectric Materials and Devices TEGs for energy harvesting to enable (1) self-powered wireless sensors and (2) wearable devices have shown exponential growth in the last decade. Namely, the annual market for subwatt TEGs, shown in Fig. 9.8, is expected to grow at a compound annual growth rate of more than 110% over the period 2014
20 (according to market research firm Infinergia LLC, Grenoble, France). In other words the market for these low-power TEGs will on average more than double each year from about 100,000 units shipped in 2014 to about 9 million shipped in 2020. The market size for TEGs considering low-power units is estimated to move from $26 million in 2014 to $77 million in 2020 (thus reaching a CAGR of about 20%). TEGs are moving into second- and third-generation technologies which are opening up new opportunities, according to Infinergia. Products using subwatt thermal energy harvesting are being commercialized in several applications across two main segments: (1) infrastructure and buildings, and (2) industrial and professional. The most recent established technology on TE materials and TEGs is related to OTEs and organic TEGs (OTEGs). The OTEGs can be deposited by vacuum technologies or facile and scalable solution deposition 80 70 60 50

3rd Gen. 2nd Gen.

Technology market share details in the report

1st Gen.

40 77 M$

30 20 10 0

26 M$ 2014

2015

2016

2017

2018

2019

2020

Figure 9.8 Annual market size for subwatt TEGs will grow at a compound annual growth rate of more than 110% over the period 2014
20 (according to market research firm Infinergia LLC, Grenoble, France).

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techniques on rigid or flexible substrates (e.g., flexible glass, PET, PEN, etc.). Organic semiconductor materials have advantages of low cost, lightweight, mechanical flexibility, and low-temperature solution processability over large areas, enabling the development of personal, portable, and flexible thermal modules. Polymer-based (conducting and nonconducting), nanocarbon-based (CNTs, graphene, fullerenes, etc.), and nanocomposite systems have been reported as TE materials, and in many cases TEG prototypes have been fabricated to demonstrate their potential for power generation. Critical challenges of organic TE materials include lifetime stability in operation and the relatively low temperatures that can withstand as TE materials. However, the continuous increase of OTEG power output, together with a major recent trend of combining TE and photovoltaic devices to maximize the electric energy output [167], comprise a viable avenue for the future market of organic TE materials and OTEGs. Fig. 9.9 demonstrates a fully printed SWCNT f-TEG (or OTEG) fabricated onto a flexible Kapton polymeric film substrate. Kapton polyimide-based polymer is a high-temperature engineering thermoplastic that exhibits a thermal stability of .500°C [168]. The f-TEG, upon being exposed to a temperature gradient of B110K, creates a TE voltage output of 41.1 mV. The TEG device demonstrated has been recently fabricated by the author of this chapter (Dr. Lazaros Tzounis), while more experimental details will be included in a scientific publication which is under preparation. f-TEGs have been reported also based on conventional inorganic low bandgap semiconductor nanomaterials (Bi2Te3, PbTe, Ag2Te3, etc.), that in the form of colloidal nanocrystal inks (or pastes) can be printed on various flexible substrates enabling highly efficient TEGs. Printing techniques that have been utilized range from slot-die printing [169], ink-jet printing, screen printing [170], to aerosol jet printing [171]. The screen printing technique has been deployed by Varghese et. al. [172] to fabricate an f-TEG with high figure of merit onto flexible polyimide substrates. First, bismuth telluride-based nanocrystals have been synthesized using a microwave-stimulated wet-chemical method, and formulated further as inks. N-type printed films demonstrated a peak ZT of 0.43 along with superior flexibility, which is among the highest reported ZT values in flexible TE materials. A flexible TEG-fabricated device using the printed films exhibited a high power density of 4.1 mW/cm2 at 60°C temperature difference. The additive printing can enable a highly scalable and low-cost

324

Nanomaterials Synthesis

Figure 9.9 (A) A carton of a fully printed flexible OTEG with 14 p-type serially interconnected SWCNT thermocouples with Ag junctions, (B) the demonstration of the real device by a digital photo, and (C) the f-TEG in operation yielding 41.1 mV upon being exposed to a ΔT 5 B110K.

roll-to-roll (R2R) manufacturing process to transform high-efficiency colloidal nanocrystals into high-performance and flexible TEG devices for various applications. Fig. 9.10 demonstrates another potential application of printed f-TEGs by their application underneath flexible photovoltaics (e.g., organic and/ or perovskite photovoltaics) in order to increase the total overall efficiency of the hybrid PV-TEG resulting devices. In that case, the PVs upon operation and due to light absorbance increase dramatically their temperature at the back contact with the temperature values rising possibly up to 100° C
120°C. Therefore, the TEG can utilize this temperature increase as a heat sink and can drive through its other surface-side (“cold side”) carriers generating Seebeck voltage.

Synthesis and Processing of Thermoelectric Nanomaterials, Nanocomposites, and Devices

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Figure 9.10 Demonstration of a hybrid PV-TEG device with an f-TEG coupled with a perovskite or organic photovoltaic toward the increase in the total efficiency.

9.7.2 Challenges in Potential Thermoelectric Generator Applications In order to realize an f-TEG (and any kind of TEG technology) that can power practical devices or for energy harvesting, at least 20 mV should be generated. This value arises from the fact that the voltage output should be generally enhanced via a voltage step-up converter for practical applications [powering of a light-emitting diode (LED), storage of the harvested energy in a capacitor, etc.]. Currently, commercial step-up converters, which are highly integrated DC\DC converters with no additional power needed, for example, the LTC3108 (Linear Tech.), operate at inputs of at least 20 mV to give an output voltage of 2.2 V or even higher. Utilizing the LTC3108, Wei et al. [173] powered a LED via a polymer-based f-TEG containing 300 pieces of parallel connected thermocouples (10 in parallel, 30 in series) generating a power output of B50 μW with an open circuit voltage higher than 40 mV. Utilizing TEGs with a voltage output of 20
50 mV, with or without a step-up converter driving the DC voltage produced in a capacitor, could enable the powering of low-consumption electronic devices integrated in buildings, for example, ultra-low-power microcontrollers, wireless sensor networks, etc. [174]. When the TEG produces voltage outputs greater than 100 mV, more sophisticated devices in applications including smarthomes, Internet-of-Things, to name a few, could be powered [175].

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9.8 FUTURE PERSPECTIVES There is no doubt that sheet-to-sheet (S2S) printing additive manufacturing technology on planar rigid or flexible substrates will be the most promising technology for TEG fabrication in the future. Printing technology for TEGs is the most rational choice for the fast preparation of TE films that can be in principle applied, for example, with ease to hot tubes whereby one can extract energy from waste heat. The printing techniques can be easily transferred to a high-yield and high-throughput R2R process for scalable and large-scale production of TEGs on flexible substrates. It is clear that the carrier substrate, which is required for the S2S or R2R processing, leads to a thermal gradient loss as a significant part of the volume is occupied by the carrier. This should be counteracted by employing thinner carrier substrates. A significant improvement should be also considered regarding the maximum possible area usage of the carrier substrate [known also as the filling factor (FF)]. Related to OTE materials and OTEGs, significant improvements by three to four orders of magnitude are required in terms of ZT. The sustainable use of OTEs from devices with energy payback times comparable with the lifetime of devices is unlikely unless higher thermal gradients can be employed (larger than the temperature range that organic materials can generally endure) [169]. In an attempt to improve the overall energy-conversion efficiency, TE devices could be combined with other devices, such as solar cells. Many TE materials can also be used in solar cells, such as Si and metal chalcogenides. The wide application of clean solar energy would significantly reduce fossil fuel consumption, our CO2 footprint, and environmental deterioration, so the use of multiple energy-conversion devices to yield maximal output would be an important direction in this area. It could be envisaged that TEG technology could contribute to (1) the protection of the environment due to the reduction of conventional fuels usage, (2) strengthening the interfaces between the energy and transport, transmission and distribution systems, and (3) promoting synergies with the energy/ICT sectors.

9.9 SUMMARY AND CONCLUSIONS In this chapter, recent advances in both inorganic and organic TE materials from experimental and theoretical perspectives have been summarized.

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Detailed ZT values for inorganic and organic TE materials were listed in Tables 9.1 and 9.2, respectively. In most cases, the enhancement of TE performance is attributed to the introduction of nanostructures into the host matrix, including nanoinclusions, nanocomposites, nanowires, nanograins, point defects, etc. Therefore, the development of TE performance depends on advances in nanoscience and nanotechnology. Nanostructures can significantly reduce thermal conductivity by enhancing phonon scattering. The inorganic TE nanomaterials show high performance, however, the following issues have to be solved: (1) large-scale preparation of size, shape, and composition; (2) poor stability of nanostructures, as they can be destroyed during compression at high temperature and/or high pressure into pellets; (3) 1D nanowires and 2D nanosheets are limited for large-scale fabrication of TE devices without destroying their nanostructure, and (4) ordered nanostructures are desired, however, they are not easily achieved in bulk TEs. Most inorganic TE materials also have issues of high cost or environmental unfriendliness, and cannot be processed in large-scale surfaces for TEG fabrication and large-scale thermal energy harvesting (e.g., geothermal energy harvesting, heat exchangers in industrial pipe systems, aeronautics fuselage, etc.). Great effort has been given in order to highlight the unique potential of organic materials as TEs and to fabricate accordingly OTEG devices. Carbon materials, such as graphene and carbon nanotubes, may be relatively promising candidates for further development of lightweight and low-cost polymer composites for TE applications. The generation of thermoelectricity using organic-based structural, engineering polymeric materials (elastomers, thermoplastics, thermosets) that are routinely exposed to high temperatures could represent a breakthrough in high-performance multifunctional material development.

ACKNOWLEDGMENT L.T. gratefully acknowledges the Bodossaki Foundation for financial support.

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CHAPTER 10

Fabrication Techniques of Group 15 Ternary Chalcohalide Nanomaterials Marian Nowak, Marcin Jesionek, and Krystian Mistewicz Institute of Physics, Silesian University of Technology, Katowice, Poland

Contents 10.1 Introduction 10.2 Fabrication of Composite Materials 10.2.1 Nanocomposites of SbSI Dots in Sodium Borosilicate Glass 10.2.2 Nanocomposites of SbSI Dots in Organically Modified Titanium Dioxide Glass 10.2.3 Electrospinning of Fibers and Mats 10.2.4 Fabrication of Piezoelectric Paper 10.3 Ball Milling of Bulk Crystals 10.4 Vapor-Phase Growth of SbSI Nanorods 10.5 Graphoepitaxy 10.6 Sonochemical Synthesis of SbSI-Type Nanowires 10.7 Ultrasonic Spray Pyrolysis 10.8 Filling of Carbon Nanotubes 10.9 Solution Processing 10.10 Microwave-Assisted Aqueous Synthesis 10.11 Hydrothermal Growth 10.12 Conversion of Sb2S3 Into SbSI 10.12.1 Conversion by Physical Vapor Method 10.12.2 Conversion by Spinning Coating SbI3 Solution 10.13 Heat and Laser Formation of SbSI Nano-Objects in Chalcohalide Glasses 10.13.1 Growth Activated by Heat Treatment 10.13.2 Laser-Induced Growth 10.14 New Trends in Fabrication Techniques 10.15 Future Perspectives 10.16 Summary References

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10.1 INTRODUCTION Group 15 ternary chalcohalides (AVBVICVII, where AV 5 Bi, Sb, As; BVI 5 S, Se, Te, O; CVII 5 I, Br, Cl, F) have been known since the beginning of the 19th century, after the first description of the synthesis of antimony sulfoiodide (SbSI) given by Henry and Garot [1] in 1824. However, the intensive investigation of SbSI and other group 15 ternary chalcohalides started after the discovery of its photoconductivity (in 1960) and ferroelectric properties (in 1962). The SbSI being a ferroelectric semiconductor [2], known as a photoferroelectric semiconductor [3], has an unusually large number of interesting properties, for example, pyroelectric [4], piezoelectric [5], electromechanical [6], and electrocaloric [7]. They are influenced by light leading to photoferroelectricity [8], photostriction [9], photoconductivity [10], and to pyro-optic [11], electro-optic [12], photorefractive [13], and other nonlinear optical effects [14]. The main properties of chalcohalides have been reviewed in a few monographs [2,3,15
17]. Chalcohalides are attractive and suitable materials for many applications, for example, thermal imaging [18], light modulation [19], ferroelectric field effect transistors [20], gas sensors [21], piezoelectric actuators [22], and photonic crystals [23]. Being promising materials with potential applications, group 15 ternary chalcohalides were synthesized in a variety of ways. It should be noted that the structure of SbSI-type materials belongs to the orthorhombic system where double chains (Sb2S2I2)n are oriented in the c-direction of the cell (Fig. 10.1). There are strong covalent-ionic bonds within the double chains and a weak van der Waals interaction between them. This extremely large anisotropy in bonding forces predetermines a number of physical properties. For example, the growth rate along the c-axis is two orders of magnitude larger than along the a- or b-axis [24]. As a result, relatively thin needle-like crystals or polycrystalline bulk forms could be obtained in spite of various attempts. However, after the first description of the synthesis of SbSI quantum dots by Yuhuan Xu et al. [25] in 1999, special attention has been given to the nanoscale forms of group 15 ternary chalcohalides. This is due to their potential applications in nanodevices [26] and in catalysis [27]. As it is impossible to cover the whole field of this subject in a short form, especially where the physical properties and applications are concerned, in this review only the fabrication techniques of group 15 ternary chalcohalide nanomaterials are summarized.

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Figure 10.1 Typical HRTEM image of an individual SbSI nanowire. The fringe spacings of 0.642(2) nm correspond to the interplanar distances between the (110) planes of SbSI crystal. Diagrams show double chains (Sb2S2I2)n oriented in the cdirection of the cell (parallel to the nanowire) as well as perspective view of SbSI crystalline structure (from plane normal to the nanowire).

10.2 FABRICATION OF COMPOSITE MATERIALS Applications of individual nanowire are restricted due to nanoscale manipulations. One should design materials with new properties by fabricating composite consisting of a bulk matrix and the nano-objects embedded within. The use of SbSI-type nanomaterials in composites is a promising approach for obtaining volume-sensitive bulk pieces useful in many applications (e.g., second harmonic generators and piezoelectric generators).

10.2.1 Nanocomposites of SbSI Dots in Sodium Borosilicate Glass The processing for fabrication of SbSl quantum dot-doped sodium borosilicate glass by the sol
gel method includes [25]: (1) preparation of the Na2O-B2O3-SiO2 gel matrix; (2) mixing the Na2O-B2O3-SiO2 wet gel with the solution of antimony tri-iodide (SbI3) in solvent (e.g., CS2) and stirring the mixture at 323K for few hours; (3) casting or dipping the SbI3-doped Na2O-B2O3-SiO2 wet gels and aging the samples at room temperature for 1 week, and then drying them at 333K for 3 weeks; (4)

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Figure 10.2 (A) HRTEM image of SbSl dot-doped glass. Crystallites with size around 6 nm are distributed in the glass matrix, and some large particles present lattice fringes (B). Reprinted from Y. Xu. et al., Nanocomposite of semiconducting ferroelectric antimony sulphoiodide dot-doped glasses, Ferroelectrics 230 (1999) 11
20. Copyright (1999), with permission from Taylor & Francis.

sulfidation of the dried gel in H2S at 453K by 3 h; and (5) heat treatment at 623K under O2 flow for 13 h, and then at 653K in air for 12 h. Therefore, by the chemical reactions: SbI3 1 H2 S-SbSI 1 H2 1 I2 ðgasÞ

(10.1)

H2 1 Oðin gel Þ-H2 OðgasÞ

(10.2)

the SbSI dot-doped Na2O-B2O3-SiO2 glasses (bulk or thin film) are finally obtained (Fig. 10.2).

10.2.2 Nanocomposites of SbSI Dots in Organically Modified Titanium Dioxide Glass The (3-glycidoxypropyl)trimethoxysilane (GLYMO)-modified TiO2 hybrid material was chosen as the SbSI doping matrix due to its good mechanical, optical, and thermal properties [28
30]. GLYMO consists of three hydrolyzable groups along with a polar group capable of coordinating the metal ion. Therefore it was used to anchor Sb31 to the gel matrix upon hydrolysis and polycondensation with the hydrolyzable groups. As a result, it significantly suppressed the diffusion and precipitation of SbI3 in the gel network [28]. Moreover, GLYMO is helpful to control the final SbSI dot size distribution in the matrix [28].

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The sol
gel procedure for the SbSI quantum dot composites in the form of thin films and bulk solids can be summarized as follows [28,29]. GLYMO diluted with ethanol was first hydrolyzed with H2O and acetic acid. After stirring for 1 h at room temperature, titanium n-butoxide (Ti (OBu)4) was added into the solution to form the GLYMO-modified TiO2 hybrid matrix. Meanwhile, proper amounts of SbI3 in carbon disulfide (CS2) and thiourea (SC(NH2)2) in ethanol were dissolved. The above two solutions were then added to the matrix solution successively and slowly at 333K. After mixing for about 1
2 h to evaporate the majority of the solvent, the solution was covered for aging at room temperature. In Ref. [30] the CH3OC2H5OH was used in place of ethanol to dilute GLYMO. Also, SbI3 and SC(NH2)2 were diluted in CH3OC2H5OH. The SbI3-SC(NH2)2-doped inorganic
organic hybrid sol was either left for 1 day to be coated on the slide glass by the dipcoating process [28
30], or was left for 30
60 days at 333K to form a stiff bulk solid [28,29]. The coatings were heated at 473K for 10 h in a nitrogen atmosphere and then at 453K for 2 h in a H2S atmosphere. The bulk solids were heated at 423
473K for 0.5
2 h in air and at 393K for 2 h in H2S successively. The film and bulk solid samples were colored dark yellow to dark red, depending on the dot size controlled by the heat treatment. The bulk solid samples were crack-free, and easily polished for the optical measurements [28]. According to the X-ray line broading (Fig. 10.3), the average dot size of SbSI in bulk solid GLYMO-TiO2 was calculated as 16 nm for the 453K-heated sample and 25 nm for the 473K-heated sample [28]. From the HRTEM image, the crystallites with sizes less than 20 nm were distributed in the matrix. Some particles were found with clear diffraction fringes in the matrix [28]. The light absorption edge was shifted to red with the higher annealing temperature of the SbSI-doped bulk solids [28]. It was attributed to the quantum confinement effect in the SbSI dots. In addition, a typical exciton absorption peak at 420 nm was observed in the case of a 373K-heated sample [28]. The advantages of SbSI/GLYMO-TiO2 glass nanocomposite are [28,29]: (1) potentially large electrooptical (Pockels’) effect, which exists only in those materials with an anisotropy structure; (2) good transparency for the wavelength in the 600
2000 nm range; (3) good homogeneity of optical quality because of liquid chemical reaction processing; (4) the possibility of preparing particles with high concentrations and uniform size

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2000.00

B

1000.00

121 220 211 310 040 320 330 150 241 411

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Count number

120 200

3000.00

0.00 0.00

20.00 40.00 Diffraction angle (deg.)

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Figure 10.3 X-ray-diffraction patterns of SbSI dot-doped GLYMO-TiO2 bulk solids with a heat treatment at 473K for 2 h in the air. Reprinted from H. Ye, et al., Semiconducting ferroelectric SbSI quantum dots in organically modified TiO2 matrix, Proceedings of SPIE 3943 (2000) 95
101. Copyright (2000), with permission from SPIE.

distribution; (5) relatively low temperature of heat treatment; (6) reliability of mass production of the materials, especially, for large-area films coated on different substrates; (7) good mechanical property and chemical stability for fabrication of devices; and (8) relatively low manufacturing cost due to simple facilities and processing.

10.2.3 Electrospinning of Fibers and Mats When polymeric nanofibers are electrospinned [31] it is convenient to add into them components in the form of ferroelectric nanoparticles with high values of piezoelectric coefficients along their length (Fig. 10.4). In Ref. [32] fabrication of polymeric, polyacrylonitrile nanofibers containing ferroelectric and semiconducting antimony sulfoiodide (PAN/SbSI) was presented. The well-crystallized SbSI nanowires of high purity, with lateral dimensions in the range from 10 to 50 nm and average lengths reaching up to several micrometers, used as the filler, have been prepared sonochemically from Sb2S3 and SbI3 (see Section 10.6). The typical route of preparing PAN/SbSI nanofibers is summarized as follows. A measured amount of SbSI (0.75 g) was added to 8.95 mL of N, N-dimethylformamide (DMF) solvent and the prepared solution was sonicated for 60 min, to shatter the agglomerates and to ensure its uniform

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Figure 10.4 Schematic of the electrospinning and in situ poling process. Reprinted from M. Nowak, et al, Using sonochemically prepared SbSI for electrospun nanofibers, Ultrasonics Sonochemistry 38 (2017) 544
552. Copyright (2017), with permission from Elsevier.

Figure 10.5 Photo of PAN/SbSI nanofibers mat.

dispersion inside nanofibers. Then, 0.75 g of PAN was added to the sol of DMF/SbSI, and this mixture was stirred for 24 h at room temperature. The PAN/SbSI composite nanofibers were obtained from this mixture using the electrospinning method with constant process parameters (see Ref, [32]). The PAN/SbSI nanowires were deposited on different substrate forming mats (Fig. 10.5). Fig. 10.6A and B shows the SEM images of fibrous surface of a PAN/ SbSI composite mat with the 50% nanowires mass concentration. Application of the backscattered electrons detector, due to the much greater atomic weight in comparison to the atomic weight of the

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(A)

(B)

1

2 3 SbSI nanowires 1 μm

200 nm

EHT = 15.00 kV Signal A = QBSD WD = 10 mm Mag = 20.00 K X

1.4

597

(C) 1.1

0.9 KCnt 0.6

(D) 478

S

Sb Sb I I

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EDS point analysis from the place indicated by the number “2” on the SEM image

739

(E) EDS point analysis from the place indicated by the number “3” on the SEM image

443

295 C

119 S 0 0.00

Signal A = QBSD Mag = 75.00 K X

591

EDS point analysis from the 358 place indicated by the number “1” on the SEM image 239 C

0.3 C

0.0

EHT = 15.00 kV WD = 10 mm

2.00

147

Sb I I Sb 4.00 6.00 8.00 Energy-keV

10.00

0 0.00

I S Sb I Sb 2.00

4.00 6.00 8.00 10.00 Energy-keV

Figure 10.6 (A, B) SEM surface topography images of the PAN/SbSI nanofibers; (C
E) EDS spectra for the locations designated on the SEM image (peak with highest intensity comes from the aluminum substrate). Reprinted from M. Nowak, et al. Using sonochemically prepared SbSI for electrospun nanofibers, Ultrasonics Sonochemistry 38 (2017) 544
552. Copyright (2017), with permission from Elsevier.

polymer, allowed for the observation of SbSI nanowires dispersed inside the PAN nanofibers (Fig. 10.6A and B). These images and EDS spectra (Fig. 10.6C
E) attested the presence of individual nanowires SbSI disposed with the fiber direction. The diameter of the nanofiber did not exceed 200 nm.

10.2.4 Fabrication of Piezoelectric Paper According to Ref. [33], fabrication of the piezoelectric paper based on SbSI nanowires can be summarized as follows. At first, SbSI nanowires (with lateral dimensions of 10
100 nm and length up to several micrometers) were sonochemically synthesized from pure elements (see Section 10.6). Cellulose fibers with lateral dimensions of 10
25 μm and length up to a few millimeters were sonically dispersed in water. In the next step SbSI xerogel was added to them in the mass ratio 1:4. Ultrasound irradiation was used again for 2 h in order to ensure homogeneous mixture of cellulose fibers and SbSI nanowires. The dilute

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Figure 10.7 Photo of cellulose/SbSI sheet. Reprinted from B. Toro´n et al. Novel piezoelectric paper based on SbSI nanowires, Cellulose (2017) 7
15. Copyright (2017), with permission from Springer Nature.

Figure 10.8 (A) SEM micrograph of cellulose/SbSI nanocomposite; (B) magnification of SbSI nanowires filling the free space between cellulose fibers. Reprinted from B. Toro´n, et al. Novel piezoelectric paper based on SbSI nanowires, Cellulose (2017) 7
15. Copyright (2017), with permission from Springer Nature.

suspension of cellulose fibers/SbSI nanowires was deposited on blotting paper and pressed to make sheets of cellulose/SbSI nanocomposite (Fig. 10.7). The typical thickness of these sheets was 0.05 mm. Fig. 10.8 shows SbSI nanowires dispersed between cellulose fibers all connected together. The composite of tough sonochemically produced SbSI nanowires with very flexible cellulose leads to an applicable, elastic

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material suitable to use in fabrication of, for example, piezoelectric nanogenerators [33]. Cellulose/SbSI nanocomposite may also be used for gas nanosensors and actuators.

10.3 BALL MILLING OF BULK CRYSTALS Nanoparticles can be obtained by mechanically milling bulk crystals in a cylindrical vial of stainless steel with hardened steel ball. The vial and the ball were kept in motion by a vibrating frame. Prior to milling, the vial was evacuated and sealed. The milling process could be carried out for a long time. This simple method is one of the top-down methods to produce materials in nanoscale. This technique was applied by A.V. Gomonnai et al. to obtain the SbSI nanocrystals [34
36]. The single crystals of SbSI which were grown by chemical transport were used as batch material. The milling process was carried out for 50 h and as a result SbSI nanocrystals were obtained (Fig. 10.9A). Based on the TEM image (Fig. 10.9A), it was estimated that the length of the rod-shaped SbSI

Figure 10.9 TEM image of SbSI nanocrystals (A) and histogram of their thickness distribution (B) after 50 h milling. Reprinted from A.V. Gomonnai, et al. X-ray diffraction and Raman scattering in SbSI nanocrystals, Materials Research Bulletin 38 (2003) 1767
1772. Copyright (2003), with permission from Elsevier (A). I.M. Voynarovych, et al. Characterization of SbSI nanocrystals by electron microscopy, X-ray diffraction and Raman scattering, Journal of Optoelectronics and Advanced Materials 5 no 3 (2003) 713
718. Copyright (2003), with permission from JOAM (B).

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crystallites varied from 0.5 to 1 mm, while their thickness was mostly in the range 50
100 nm. The presented histogram shows that the average rod thickness is about 70 nm (Fig. 10.9B). Bo Peng et al. [37] presented a theoretical possibility to obtain by mechanical exfoliation SbSI-type nanowires thinner than 7 Å from bulk crystals of group 15 ternary chalcohalides. This is due to the van der Waals forces between the atomic chains in these materials.

10.4 VAPOR-PHASE GROWTH OF SbSI NANORODS In Ref. [38], arrays of c-axis-oriented SbSI nanorods were synthesized on anodic aluminum oxide/titanium/silicon (AAO/Ti/Si) substrates by vapor-phase deposition of a mixture of Sb2S3 and SbI3 powders. AAO/ Ti/Si substrates were prepared by the two-step anodization of Al/Ti/Si substrates at 40 V in oxalic acid, at 277K. The deposition was carried out in a two-zone tube furnace with the source placed in the middle of a quartz tube, while the substrate was placed downstream. The quartz tube was evacuated (about 1024 Torr) for 1 h and then sealed. The source and substrate zone temperature were then raised to 673K and 523K, respectively, at a heating ramp rate of 1 K/min and the deposition was carried out for 1 h. Fig. 10.10 shows the changes in the morphology of the synthesized SbSI nanostructures as a function of temperature. The surface morphology of a bare AAO rough surface on a Ti/Si substrate (Fig. 10.10A) shows pores with diameters ranging between 20 and 50 nm, including grain boundaries. SbSI forms by a gaseous phase reaction between Sb2S3 and SbI3 at a source temperature of 673K to form (SbI3)x(Sb2S3)12x, which is then transported across a temperature gradient onto an AAO/Ti/Si substrate. The onset of nucleation of SbSI starts close to 433K, under conditions close to the critical saturation for the condensation of (SbI3)x(Sb2S3)12x. Fig. 10.10B shows tiny islands of (SbI3)x(Sb2S3)12x condensate nucleating at a substrate temperature of 443K with a mean lateral size from 200 to 300 nm. As the substrate temperature is increased to 523K, vertically oriented SbSI nanorods with a mean length of about 3 μm and diameter range between 150 and 300 nm were formed on the surface of the AAO/Ti/Si substrate (Fig. 10.10C). Upon increasing the substrate temperature to 548K, SbSI nanorods tend to coalesce to form vertically oriented clusters with a mean length of about 3 μm and diameter of 600 nm (Fig. 10.10D). The coverage and density of the vertical

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Figure 10.10 SEM images showing (A) plan-view image of an AAO/Ti/Si substrate, (B) islands of (SbI3)x(Sb2S3)12x formed at 443K on an AAO/Ti/Si substrate, (C) vertical SbSI nanorods formed at 523K (insets show magnified plan-view and side-view SEM image of SbSI nanorods), (D) vertical SbSI nanorod clusters formed at 548K, and (E) side-view SEM images of SbSI nanorods formed on an AAO/Ti/Si substrate at 523K. Reprinted from J. Varghese, et al. Surface Roughness Assisted Growth of Vertically Oriented Ferroelectric SbSI Nanorods, Chem. Mater. 24 (2012), 3279 2 3284. Copyright (2012), with permission from American Chemical Society.

SbSI crystallites on the AAO/Ti/Si substrates increase as the substrate temperature is increased from 523K to 548K, as a higher temperature could increase the migration rate of SbSI species, promoting a high uniform distribution of SbSI on the surface. Fig. 10.10E shows a side-view of SbSI nanorods, clearly indicating that nucleation starts from the AAO surface. The optimum temperature to obtain phase pure SbSI was found to be 523K [38]. Deposition temperatures lower and higher than 523K resulted in SbI3-rich SbSI and nonstoichiometric SbSI, respectively. Because no catalytic particle was observed on the tips of the SbSI nanorods (see inset in Fig. 10.10C), the nanorod formation occurred via a self-catalyzed vapor
solid growth mechanism.

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10.5 GRAPHOEPITAXY In graphoepitaxy, known also as “artificial epitaxy,” an artificial lattice is operating instead of a crystallographic one, as it does in classical epitaxy [39]. In this approach a crystallographically symmetric micro-relief is created on a flat substrate, a better amorphous one, and ensures orientation of deposited crystallites. Graphoepitaxy is easy to realize in growth from gas or solution. The principal idea consists of the fact that, during the crystallization, micro- or nanocrystallites are formed on the patterned substrate with their flat faces attached to the constituents of the microrelief. In graphoepitaxy, microcrystallites are attached to steps or kinks with micrometer or nanometer sizes. The “artificial lattice” necessary for graphoepitaxy can be prepared using photolithography and anisotropic etching [39]. Growing needs in single-crystalline nanowires dictate the application of graphoepitaxy. Some results of the graphoepitaxial growth of SbSI nanowires from a vapor phase on a substrate with striated micro-relief are illustrated in Fig. 10.11. This figure shows an initial stage of the growth. Due to the quasi-one-dimensional internal structure of SbSI, its microcrystallites have a needle shape. They are attached to constituents of the relief, that is, along the striations. If the substrate has the most-closely packed orientation for a given material, practically all the nanowires grow perpendicularly to such a substrate, and hence they are mutually parallel [39]. Oriented arrays of single-crystalline nanowires, rather than nanowire “wool” consisting of disordered fibers, are necessary for most of the applications.

10.6 SONOCHEMICAL SYNTHESIS OF SbSI-TYPE NANOWIRES When a large negative pressure of ultrasound is applied to a liquid, intermolecular van der Waals forces are not strong enough to maintain cohesion and small cavities or gas-filled microbubbles are formed. The rapid nucleation, growth, and implosive collapse of these micrometerscale bubbles constitutes the cavitation. According to the thermal “hot spot” theory, extreme local temperatures (in the range of 5200K 6 650K [40]) and pressures (up to 1700 atm) are produced inside the cavitating bubbles and at their interfaces when they collapse. Therefore, local turbulent flow associated with cavitation, and ultrasound streaming not only

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Figure 10.11 Elongated SbSI crystallites deposited on a patterned substrate by evaporation of the compound. Initial stage of graphoepitaxial growth; the crystallites are shown by single arrows, whereas projected striations of the relief are by double ones. Reprinted from E.I. Givargizov, Graphoepitaxy as an approach to oriented crystallization on amorphous substrates, Journal of Crystal Growth 310 (2008) 1686
1690. Copyright (2008), with permission from American Chemical Society.

greatly accelerate mass transport but also can stimulate heterogeneous chemical reactions and physical changes in liquids at room temperature and ambient pressure that normally occur only under extreme conditions of hundreds of atmospheres and degrees. These exceptional local conditions can be used to generate nanostructured materials. In 2008, sonochemical synthesis of SbSI nanowires was reported [41]. The SbSI-type nanomaterials can be fabricated sonically from the stoichiometric mixture of elements (e.g., Sb, S, and I2 [42]) or from compounds (e.g., Sb2S3 and SbI3 [43]; SbCl3, Na2S  H2O, and KI [42]). The growth of SbSI nanowires is faster in the latter cases than in the first one. In the usually applied procedure [42], the mixture of 0.250 g S, 0.949 g Sb, and 0.990 g I2 was placed in 4 mL of ethanol in a polypropylene container closed by a polyethylene plug in order to avoid outflow of volatile synthesis products. The container was submerged in water in the cup-horn of a 20-kHz and 750-W ultrasonic processor. The ultrasound power density was 565 W/cm2. The water was kept at a constant temperature of 293K by a refrigerated circulating bath.

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During the sonication a sol is formed in which the color changes gradually from red (before sonification) into olive, green, yellow, and then red-orange (Fig. 10.12), indicating the growth process of SbSI nanorods. The sonochemical reaction can be continued so as to complete the gelation of SbSI (Fig. 10.12). The whole synthesis process can be finished within 2 h. It seems that SbSI-type materials naturally grow into 1D nanostructures, and this habit is determined by the highly anisotropic bonding in

Figure 10.12 Change of color and consistency during the sonication of Sb, S, and I2 in ethanol: (A) dry elements before the process; (B) testtube with the elements in ethanol at the beginning of the process; (C) after 20 s of sonication, T 5 323K; (D) 3 min; (E) 6 min; (F) 26 min; (G) 48 min.; (H) 75 min; (I) SbSI ethanogel solidified after 110 min. Reprinted from M. Nowak, Photoferroelectric nanowires, in: Nanowires Science and Technology, N. Lupu (Ed.), (2010) pp. 269
308. Copyright (2010), with permission from INTECH.

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the crystallographic structure. The probable reaction route of SbSI-type synthesis and the mechanism of formation of its nanowires in the presence of ethanol under ultrasonic irradiation can be summarized as follows [17]: 1. I2 dissolved in ethanol reacts with Sb and forms the SbI3, also dissolved in ethanol 2Sb 1 3I2 -2SbI3

(10.3)

2. Dehydrogenation, dehydration, as well as decomposition of ethanol in or close to the cavitation bubbles leads to the formation of hydrogen and water as the main products C2 H5 OH2C2 H4 O 1 H2 ; C2 H5 OH2C2 H4 1 H2 O;

(10.4)

C2 H5 OH2C2 H5 O 1 H 3. The sonolysis of water yields the H∙ and OH∙ radicals H2 O ÞÞÞ H∙ 1 OH∙

(10.5)

4. The ultrasonic irradiation facilitates the reduction of chalcogens (S and Se) to the active forms of S22 and Se22 that react with the in situ generated H∙ radicals forming H2S and H2Se S 1 2H∙ -H2 S;

Se 1 2H∙ -H2 Se

(10.6)

5. The released H2S and H2Se react with SbI3 to yield SbSI and SbSeI molecules SbI3 1 H2 S-SbSI 1 H2 1 I2

(10.7a)

SbI3 1 H2 Se-SbSeI 1 H2 1 I2

(10.7b)

6. The created SbSI and SbSeI molecules, under the microjets and shockwaves formed at the collapse of the bubbles, are pushed toward each other and are held by chemical forces. Therefore, the nuclei of SbSI and SbSeI are formed as a result of the interparticle collisions; 7. The freshly formed nuclei in the solution are unstable and have the tendency to connect with each other and self-assemble to form double chain-type structures. These [(SbSI)N]2 or [(SbSeI)N]2 structures consist of two chains related by a two-fold screw axis and linked together by short and strong Sb
S or Sb
Se bonds. High temperature, local turbulent flow associated with cavitation, and acoustic streaming

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greatly accelerate mass transport in the liquid phase and are favorable for the self-assembly of the SbSI and SbSeI nuclei; 8. The SbSI or SbSeI chains can be readily crystallized into a 3D lattice of nanowhiskers through van der Waals interactions. Induced by this structure, crystallization tends to occur along the c-axis, favoring the stronger covalent bonds over the relatively weak, interchain van der Waals forces. Thus, this solid material has a tendency to form highly anisotropic, 1D structures; 9. The aggregated SbSI or SbSeI nanowires produce larger species. Ultrasound can also promote chemical reaction and crystal growth by mixing heterogeneous phases involving the dispersion of an insoluble solid reactant, for example, SbSI, in a liquid medium. The surface state of the nanowires might change during sonication: the dangling bonds, defects, or traps decrease gradually, and the species grow until the surface state becomes stable; surface corrosion and fragmentation by ultrasound irradiation affect the formation of regular nanowires. In a typical procedure [32], the fabricated gel was 10 times rinsed with pure ethanol to remove the remaining substrates and centrifuged to extract the product. SbSI gel was dried under 60 Pa pressure at room temperature for 72 h. The obtained xerogel (Fig. 10.13) consisted of SbSI nanowires with lengths up to several micrometers and lateral dimensions in the range of 10
50 nm. Microstructural analysis revealed that SbSI nanorods crystallize in an orthorhombic structure and predominantly grow along the [001] direction. The XRD, SAED (Fig. 10.14), and

Figure 10.13 Image (A) and typical SEM micrograph (B) of the sonochemically prepared SbSI xerogel. Reprinted from A. Starczewska, et al. Influence of humidity on impedance of SbSI gel, Sensors and Actuators A: Physical, 183 (2012) 34
42. Copyright (2012), with permission from Elsevier.

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Figure 10.14 Typical TEM micrograph (A), electron diffraction pattern (B), and simulated diagram (C) of sonochemically prepared SbSI gel. Reprinted from A. Starczewska, et al. Influence of humidity on impedance of SbSI gel, Sensors and Actuators A: Physical, 183 (2012) 34
42. Copyright (2012), with permission from Elsevier.

Figure 10.15 Typical HRTEM image of aggregated SbSI nanowires from the sonochemically prepared gel. Reprinted from A. Starczewska, et al. Influence of humidity on impedance of SbSI gel, Sensors and Actuators A: Physical, 183 (2012) 34
42. Copyright (2012), with permission from Elsevier.

HRTEM (Figs. 10.15 and 10.16) patterns show that the as-prepared particles are well crystallized. The liquid, used in the sonochemical preparation of nanomaterials, strongly affects the yield of the sonochemical process and the properties of the produced material [43]. The sonication depends on such properties of the liquid as, for example, the decomposition rate, viscosity, surface tension, vapor pressure, and sound speed. Likewise, the mechanisms of interaction with crystal surface of the ions (e.g., HS2) solvated, for example, by alcohol molecules and water, are fundamentally different, because

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Figure 10.16 (A) HRTEM micrograph of SbSI nanowires showing clear Moir’ fringes; (B) fast Fourier transform (FFT) image of single SbSI nanowire marked in Fig. 10.16A. Reprinted from B. Toro´n, et al. Novel piezoelectric paper based on SbSI nanowires, Cellulose (2017) 7
15. Copyright (2017), with permission from Springer Nature.

the chemical properties of the ions in solution can be strongly modified by the surrounding solvation shells. The solute reactivity depends on temperature and the following solvent properties: dielectric constant, dipole moment, molecule polarizability, etc. The variation in the dielectric constant leads to a change in the nucleophilic properties of the reacting ions during bond formation. Such a variation can affect the charge transfer between bonding atoms, the bond length, and the covalent character of bonding [17]. The time of sonochemical synthesis of SbSI in methanol is shorter than the time needed for sonification in ethanol for bath temperatures greater than 314K [43]. At lower temperatures the sonification in ethanol is faster [43]. Probably, these differences are due to different temperature dependences of solubilities of the components in ethanol and methanol. The sonochemical synthesis of SbSI-type nanomaterials has been performed in different liquids: ethanol [41,44
52], methanol [27,43], isopropyl alcohol [53], and water [54
56]. The authors of Ref. [56] attempted to control SbSI particle growth and dimensions (shape/size) by adding different surfactants and fillers to the reaction system. The sonochemical synthesis was also performed in order to fabricate SbSeI nanowires [46]. The temperature dependence of the dielectric constant of sonochemically prepared SbSI gel (Fig. 10.17) proves that this material is a ferroelectric with the Curie temperature (TC) near 293K (Fig. 10.17). Such a

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Figure 10.17 Temperature dependence of the averaged dielectric constant of SbSI gel (E 5 4.15 kV/m; f 5 1 kHz; p 5 105 Pa; RH 5 73%). The solid curve represents the Curie
Weiss relation calculated for the best fitted C 5 1.41(14)  104 K and TC 5 293.0(2) K. Reprinted from P. Szperlich, et al. Desorption of Gasses Induced by Ferroelectric Transition in SbSI Nanowires, Acta Physica Polonica A 126 (2014) 1110
1112. Copyright (2014), with permission from Polish Academy of Sciences.

Curie temperature is characteristic for bulk SbSI single crystals. The determined [55] maximum value 1.46  104 of dielectric constant is less than the ε(TC 5 291K) 5 6.2  104 measured along the polar axis of the best SbSI single crystals [57]. One should remember that SbSI is a highly anisotropic material and the permittivity of the SbSI single crystal of the polar direction is about 2000 times greater than that of the perpendicular direction. The chaotically oriented SbSI nanocrystals amounts to only 4.7% of the xerogel volume [41]. Therefore, the determined averaged value of SbSI xerogel dielectric constant does not represent the exact value for the SbSI nanowires.

10.7 ULTRASONIC SPRAY PYROLYSIS In Ref. [58] the films of bismuth sulfide iodide (BiSI) rods were synthesized by traditional ultrasonic spray pyrolysis (TUSP) as well as by asynchronous pulse ultrasonic spray pyrolysis (APUSP) [59]. In TUSP the

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mixed solution of BiCl3, I2 (dissolved in ethanol), and thiourea (SC (NH2)2) were nebulized and sprayed continuously onto the hot substrates with just one nozzle. In APUSP the two solutions, the BiCl3 and I2 (dissolved in ethanol) mixture (denoted as solution A) on one hand and the thiourea solution (denoted as solution B) on the other, were nebulized and pulse-sprayed onto the hot substrates through two separate nozzles (Fig. 10.18). Typically, BiCl3 and [CS(NH2)2] were dissolved in distilled water at 0.05 and 0.1 M concentrations, respectively, while iodine was dissolved in ethanol at 0.04 M concentration. Prior to the film deposition N2 gas was first introduced to the reaction chamber at a relatively low and steady flow rate for about 30 min to drive the air out. The solutions A and B were nebulized by two commercial ultrasonic humidifiers, and introduced into the reactor by N2 gas through two separate nozzles. As for APUSP, the nebulized solutions were delivered to the substrates in spray pulses that began with thiourea solution.

Tu 4

T (s) N2

Bi+

5 7

2 6

Pulse control 1

3

4

60° N2

5 7 6

N2

Pulse control

Figure 10.18 Scheme of the asynchronous-pulse ultrasonic spray pyrolysis (APUSP) method used to obtain BiSI rod-like particle thin films, with (1) substrate, (2) furnane, (3) spray nozzles, (4) carrier gas, (5) solution, (6) membrane, and (7) ultrasonicator. Reprinted from S.-Y. Wang and Y.-W. Du. Preparation of nanocrystalline bismuth sulfide thin films by asynchronous-pulse ultrasonic spray pyrolysis technique, Journal of Crystal Growth 236 (2002) 627
634. Copyright (2002), with permission from Elsevier.

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There were two routes adopted in the APUSP. For route 1 in the APUSP: each spray pulse time lasted 5 s for A and B solutions. This spray process was continuous. For route 2 in the APUSP: after the pulse spray of B was conducted, a delay of 4 s was employed to ensure that the introduced thiourea was completely decomposed before conveying a pulse spray of A. The deposition was carried out by repeatedly performing these spray processes. In the APUSP experiments, the cycle times for routes 1 and 2 were 10 and 14 s, respectively. In both cases, the total deposition time was about 15 min. The substrate was at 593K during the deposition. The reaction process was expressed in detail as follows [58]: I2 1 H2 O-HI 1 HOI

(10.8)

BiCl3 1 3HI-BI3 1 3HCl

(10.9)

CSðNH2 Þ2 1H2 O-NH3 1 H2 S 1 CO2

(10.10)

2BiCl3 1 3H2 S-Bi2 S3 1 6HCl

(10.11)

Bi2 S3 1BiI3 -3BiSI

(10.12)

The films grown by TUSP consisted of congregated rod-like and abnormal grains, while only rod-like particles (Fig. 10.19) were found in the films grown by APUSP along routes 1 and 2 [58]. XRD patterns of the synthesized samples were in agreement with the reported data for orthorhombic BiSI. No other characteristic peaks of impurities, such as

Figure 10.19 Typical SEM of morphology of BiSI films deposited on glasses at 593K by: (A) route 1 in the APUSP and (B) route 2 in the APUSP. Reprinted from W. Wang, et al. Growth of rod-like crystal BiSI films by ultrasonic spray pyrolysis, Materials Research Bulletin 40 (2005) 1781
1786. Copyright (2005), with permission from Elsevier.

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Bi2S3, BiI3, BiOI, or S and I2 were observed [58]. The mean size of rodlike particles was 100
200 nm in diameter and 2 mm in length. The appropriate ratio of pulse time to interval time between pulses played an important role in crystal growth direction. BiSI rods were strongly elongated in a direction almost perpendicular to the substrate. This result was attributed [58] to the fact that the number of the nuclei changes with the ratio of pulse time to interval time. In the initial stage of deposition, the nucleation and growth of nuclei compete with each other. In the succeeding reaction of route 1, the nuclei generation rate is high, exceeding that of vertical nuclei growth, whereas in the case of route 2, the growth rate predominates. This latter process results in the minimum surface mobility of the nuclei and consequently the formation of elongated particles perpendicular to the substrate. The TUSP was used to deposit polycrystalline BiSI thin films from a solution containing Bi(NO3)3  5H2O, thiourea, and NH4I in ethylene glycol [60]. The precursor solution was pumped through an ultrasonic spray nozzle (130 kHz) positioned above a hot plate in a ventilated enclosure under normal atmospheric conditions. The spray parameters were controlled by a syringe pump, which was programmed to spray intermittently (a spray pulse, followed by a rest period) for a set number of deposition cycles. A nozzle height of 12 cm, pulse volume of 0.3 mL, and rest times of 25 s were selected as optimal. The mean size of the crystallites increased when the deposition temperature was increased from 498K to 523K, but remained relatively constant thereafter at about 26 nm as calculated by the Scherrer equation. Deposition temperatures between 523K and 548K appeared to optimize the films’ performance due to their formation of well-crystallized microrods without their subsequent surface conversion to BiOI. In Ref. [61], tuning of the BiS12xSexI band gap, by replacing selenium for sulfur, was accomplished by substituting various amounts of SeO2 in ethanol for thiourea in the BiSI spray pyrolysis precursor solutions. General film synthesis procedures were performed in a similar manner to that used in Ref. [60]. The average crystallite size decreased from 24 to 19 nm as Se-doping increased.

10.8 FILLING OF CARBON NANOTUBES There are a few methods known for filling carbon nanotubes (CNTs) with different substances: catalytic synthesis of nanotubes using the metals

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as catalysts, capillary drawing-in of molten materials, or materials dissolved in solvents having a low surface tension, saturation with metal vapor, as well as electrochemical methods or using sonochemistry. It is well known that powerful ultrasound evoking cavitation can induce new reactivity leading to the formation of unexpected chemical species. The application of sonochemistry to fill CNTs is also justified by the fact that ultrasonication is often used to cut the outer caps of CNTs. Fabrication and properties of CNTs filled sonochemically with SbSI and SbSeI were presented in Refs. [27,62
69]. SbSI [62] and SbSeI [63] were prepared in CNTs ultrasonically from elements (Sb, I2, and S or Se). Alcohols (methanol or ethanol) served as solvents. In a typical procedure of SbSI@CNT fabrication [62], the mixture with a stoichiometric ratio of, for example, 0.380 g Sb, 0.099 g S, and 0.394 g I2, was immersed with 0.282 g of CNTs in 40 mL of alcohol. The fabrication of SbSeI@CNTs was performed for a four times greater amount of Se in comparison with stoichiometric ratio [63]. It allowed excess formation of SbI3 to be avoided. The reagents were contained in closed Pyrex glass cylinders. The cylinders were partly submerged in water in the cup-horn of an ultrasonic reactor biased with an ultrasonic processor. The used ultrasounds had a frequency of 20 kHz and 565 W/cm2 power density. The cup-horn was filled with water continuously pumped through a refrigerated circulating bath. The sonolysis was carried out for 3 h at 323K. The color of the suspension changed gradually, indicating the growth process of SbSI and SbSeI. To control this process, measurements of optical diffusive reflectance Rd(λ) were performed ex situ using a spectrophotometer equipped with an integrating sphere. It was assumed that the sonochemical process was finished when the spectral characteristics of Rd(λ) did not change with time. At the end, brown-purple SbSI@CNTs or dark brown-purple SbSeI@CNT sols were obtained. The products were extracted using a centrifuge. Liquids above sediments were replaced with pure alcohol to wash the precipitates. Finally, alcohol was evaporated from the samples in air at room temperature. It is known that the mode of insertion dictates the nature and morphology of the obtained filling of CNTs. When the filling is induced via solution
deposition, small discrete encapsulates are formed, whereas when it is obtained via capillarity, continuously filled CNTs are observed. Probably, the latter happens when CNTs are filled sonochemically by SbSI or SbSeI. The transient high-temperature and high-pressure field produced during ultrasound irradiation provide a favorable environment

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for the 1D growth of SbSI and SbSeI nanocrystals from elements inside CNTs in alcohol, though the surrounding solution is at a relatively low temperature (T 5 323K) and atmospheric pressure. Simplified growth of SbSI-type materials in CNT is shown in Fig. 10.20. Fig. 10.21 presents an HRTEM image of an individual multiwalled CNT filled with SbSeI. This image exhibits clear (200) lattice fringes of SbSeI parallel to the nanocable axis and indicates the growth of SbSeI inside the CNT in [001] direction. An HRTEM image of an individual CNT sonochemically filled with SbSI (Fig. 10.22B) exhibits its clear (220) lattice fringes parallel to the nanocable axis and indicates the growth of SbSI inside the CNT in [001]

Figure 10.20 Simplified growth of SbSI-type materials in CNT. Reprinted from M. Jesionek et al. Sonochemical growth of nanomaterials in carbon nanotube, Ultrasonics 83 (2018) 179
187. Copyright (2018), with permission from Elsevier.

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Figure 10.21 Typical HRTEM image of an individual multiwalled CNT filled with SbSeI ultrasonically in ethanol. The fringe spacings of 0.332(5) nm (sign 1) and 0.432 (13) nm (sign 2) correspond to the interplanar distances between the (002) planes of carbon nanotube and (200) planes of orthorhombic SbSeI crystal, respectively. Reprinted from M. Jesionek et al. Sonochemical growth of antimony selenoiodide in multiwalled carbon nanotube, Ultrasonics Sonochemistry 19 (2012) 179
185. Copyright (2012), with permission from Elsevier.

Figure 10.22 Typical TEM (A) and HRTEM (B) Images, as well as electron diffraction pattern (C) of an individual SbSI@CNT (1—0.319(2) nm (220) planes of SbSI crystal (B); 2—0.209(2) nm (101) planes of carbon nanotube (B); for description of SAED reflexes see Table 1 in Ref. [69]. Reprinted from M. Jesionek et al. Sonochemical growth of nanomaterials in carbon nanotube, Ultrasonics 83 (2018) 179
187. Copyright (2018), with permission from Elsevier.

direction. All these results corresponded well with the XRD patterns of CNTs sonochemically filled with SbSI. The SAED pattern (Fig. 10.22C) recorded at the end of the CNT filled with SbSI (Fig. 10.22B) indicated interplanar spacings typical for CNTs as well as SbSI crystals (see Table 1 in Ref. [69]).

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Electron microscopy images (Figs. 10.21 and 10.22) of individual CNTs sonochemically filled with SbSeI and SbSI revealed that the products consist of coaxial nanocables. The lateral dimensions of SbSI@CNTs and SbSeI@CNTs were in the ranges 30
200 nm and 20
170 nm, respectively. Lengths of these nanocables reached up to several micrometers in both cases.

10.9 SOLUTION PROCESSING Solution processing, also known as the liquid reaction method, is an appealing alternative to other procedures due to its ease of fabrication, scalability, and potential to lower device manufacturing costs. In Ref. [70] SbSI was synthesized using a modified solution synthesis process, based on the method reported in Ref. [71]. For this process 1 mmol of SbCl3, 1 mmol of KI, and 1 mmol of thioacetamide were taken in a three-neck round-bottomed flask along with 30 mL of glacial acetic acid. The solution was heated to 323K for a sufficient period of time until a clear yellow-colored solution was formed. After this, the temperature of the solution was raised to 383K and the solution was refluxed with constant stirring for 2 h. A wine-red compound was formed which settled at the bottom of the flask. The reaction mixture was then cooled down to room temperature and acetic acid and unreacted chemicals were removed by centrifugation. Finally, the wine-red sediment was washed with absolute ethanol (five times) and then dried at 343K under vacuum for 12 h. SEM images (Fig. 10.23A, B, and F) revealed a spherical morphology with a large concentration of rods on the surface, closely resembling a sea “urchin.” The particle growth mechanism involves various intermediate stages, starting from needles/rods (1D) and then passing through a formation of bundles, dumbbells, and finally leading to urchin-like structures (3D). Each of the urchin-like structures was about 10
20 μm in diameter and consists of one-dimensional SbSI rods. The HRTEM image (Fig. 10.23H) clearly shows lattice fringes with a spacing of 0.39 nm, which is in good agreement with the distances between (210) planes of orthorhombic SbSI [70]. Thus, the electron microscopy and PXRD confirm that the evolved structure of SbSI is highly crystalline. The elemental mapping images reveal a uniform distribution of the elements, namely, Sb, S, I elements in the individual SbSI urchin (Fig. 10.23C
E). The SbSI grows in a one-dimensional rod shape (Fig. 10.23F and G) with (100), (010) planes expected to be side surface of the rods.

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Figure 10.23 (A) Low magnification FESEM image of “urchin”-shaped SbSI. (B) Highmagnification FESEM image of a single unit. (C
E) Elemental mapping using FESEM (C) Sb (red), (D) sulfur (green), (E) iodine (white) in single unit of SbSI. (F
G) FESEM and TEM image of rods of SbSI. (H) HRTEM of single rod SbSI. Reprinted from T. Muthusamy and A. J. Bhattacharyya, Antimony Sulphoiodide (SbSI), a Narrow Band-Gap Non-Oxide Ternary Semiconductor with Efficient Photocatalytic Activity, RSC Adv. 6 (2016) 105980
105987. Copyright (2016), with permission from Royal Society of Chemistry.

D. V. Chirkova et al. [72] developed a method for SbSI anion doping during synthesis in an aqueous solution, which could lead to a shift of the Curie point. Using this method they have obtained the bromide-doped SbSI without contaminants. Another solution process was used in Ref. [28] to synthetize SbSI in situ by using the SbI3 carbon disulfide solution. This solution was mixed with the SC(NH2)2 ethanol solution and dark-red precipitates were formed. The precipitates were heated at 473K for 2 h in air. The X-ray diffraction patterns of the synthesized material were identified as those of SbSI crystal [28]. Other solution-processing methods of obtaining 15 group chalcohalide nanomaterials are presented in Sections 10.2.1, 10.2.2, 10.6, 10.8, 10.10
10.12.2.

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10.10 MICROWAVE-ASSISTED AQUEOUS SYNTHESIS Recently, a green microwave-assisted chemical technique for the fabrication of nanostructured semiconductor has attracted significant attention. In Ref. [73] hierarchical Bi19S27Br3 superstructures were synthesized on a large scale by a facile microwave-assisted aqueous chemical process using bismuth nitrate as the metal precursor, thiourea as the source of S22, and hexadecyl trimethyl ammonium bromide as the bromine source. In a typical synthesis, 0.194 g Bi(NO3)3•5H2O was dissolved in 10 mL distilled water containing 1 mL HNO3 in a 100-mL conical flask to form a colorless transparent solution. Then, 0.456 g thiourea (TU) solution (20 mL) and 0.2 g hexadecyl trimethyl ammonium bromide (CTAB) solution (30 mL) were added with continuous stirring to obtain yellow precursor emulsion. Subsequently, the precursor was transferred into an MAS-II apparatus with reflux equipment, and heated with microwave irradiation (power 600 W) at 353K for 30 min with refluxing. After this, the reaction mixture was cooled naturally. The synthesized precipitates were filtered out by a microporous membrane with pores of 0.45 μm in diameter, washed by acetone, anhydrous ethanol, and distilled water several times. The gray-black products were obtained finally after being dried in a vacuum at 333K for 6 h. The reaction process was expressed in detail as follows [73]:  31 Bi31 1 nTu2 BiðTuÞn (10.13) 22 Tu 1 2H2 O-NH1 4 1 CO2 m 1 S

(10.14)

2Bi31 1 3S22 -Bi2 S3

(10.15)

Bi31 1 3Br2 -BiBr3

(10.16)

9Bi2 S3 1 BiBr3 -Bi19 Br3 S27

(10.17)

The self-supported fabric-like Bi19S27Br3 superstructures (Fig. 10.24A) possessed a hexagonal phase with diameters of 4
5 μm, constructed by cross-bedded nanofibers of a few micrometers length. Furthermore, the nanofibers were aggregated by ultrafine nanosilks containing stacking faults [73]. The magnified image in Fig. 10.24B clearly displays that the

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(A)

(C)

(B)

(D)

(E)

Figure 10.24 (A, B) FESEM images, (C) TEM image, (D) HRTEM image, and (E) SAED pattern of as-prepared Bi19S27Br3 superstructures. Reprinted from C. Deng, et al. Novel Bi19S27Br3 superstructures: facile microwave-assisted aqueous synthesis and their visible light photocatalytic performance, Materials Letters 108 (2013) 17
20. Copyright (2013), with permission from Elsevier.

nanofibers are perpendicularly assembled with needle-like tips, being of about 80 nm in average diameter. Upon closer examination, some of nanofibers extended outward from the body (indicated by the white arrow). The zoomed image (inset of Fig. 10.24B) indicates that the nanofibers with a rough surface are actually aggregated by the ultrafine nanosilks. Fig. 10.24C shows a typical TEM image of the part of the microfabric. It can be observed clearly that the dedicated microfabrics are constructed by the nanofibers, wherein the nanofibers are melted by the ultrafine nanosilks. The HRTEM image (Fig. 10.24D) shows the regular lattice spacing of 0.373 nm, corresponding to an interlayer spacing of the (310) plane of hexagonal Bi19S27Br3 [73]. Additionally, the SAED pattern (Fig. 10.24E) from a branch identifies a single crystalline nature of Bi19S27Br3 micropatterns.

10.11 HYDROTHERMAL GROWTH Hydrothermal synthesis is one of the most important and frequently applied methods for growth of ternary chalcohalides nanomaterials. In 2001, Wang et al. used this method for the first time for fabrication of

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SbSI nanorods [74]. Their preparation was carried out in an autoclave at 453K 4 463K for 8
10 h. Wang et al. proposed the following mechanism of formation of SbSI ðNH2 Þ2 CS 1 H2 O-H2 S 1 CO2 1 2NH3

(10.18)

2SbCl3 1 3H2 S-Sb2 S3 ðnanorodsÞ 1 6HCl

(10.19)

I2 1 H2 O-HI 1 HOI

(10.20)

1 2HOI-I2 1 H2 O 1 O2 2

(10.21)

Sb2 S3 ðnanorodsÞ 1 2HI-H2 S 1 2SbSIðnanorodsÞ

(10.22)

Synthesized SbSI displayed a rod-like morphology with diameters in the range of 20
50 nm and lengths up to several micormeters [74]. The absorption edge of SbSI nanorods was shifted to a shorter wavelength in comparison with that of bulk SbSI crystals. A similar approach to hydrothermal growth of SbSI was reported in Ref. [75], where antimony trichloride, iodine, and thiourea were also used as the starting materials. The synthesis of one-dimensional (1D) SbSI submicron rods was completed after a very long time (over 24 h) and required an elevated temperature of 473K. Chen et al. successfully reduced the time and temperature of the hydrothermal preparation of SbSI to 4 h and 433K, respectively [71]. In this case, SbCl3, (NH2)2CS, NH4I, and HCl aqueous solution were chosen as reagents. Reported in Ref. [71] an alternative hydrothermal synthetic route for the growth of 1D crystalline SbSI was highly efficient and low-cost for large-scale synthesis of this material in a facile and easily conducted way. When the concentration of hydrochloric acid solution was 2.4 mol/L, the obtained sample presented the irregular macron-scale bulks assembled by smaller 1D SbSI nanobelts with a thickness of about 40
300 nm and width of about 100
300 nm [71]. Hydrothermal synthesis was reported as the fabrication method of the ternary chalcohalide nanomaterials containing bismuth (e.g., porous BiOI nanoparticles [76], BiOI 2D nanowalls [77], BiSI nanorods [77,78], and BiTeI submicrometer hollow spheres [79]).

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Nanomaterials Synthesis

Aguiar et al. [78] synthesized BiSI nanorods from Bi2S3 and I2 in a homemade Teflon-lined autoclave heated at 453K during 20 h. Different solvents were used, that is, distilled water (sample S1), monoethyleneglycol (MEG) (sample S2) and a mixture of MEG:H2O (sample S3). Fig. 10.25 presents results of TEM investigations of three representative samples. In all cases the nanostructures exhibited a rod morphology, which is characteristic for bulk chalcohalides. It was found that the type of solvent had no noticeable influence on the size of the obtained rods.

Figure 10.25 BiSI nanorods hydrothermally grown in different liquids (description in the text). (A) TEM image of sample S1. (B) TEM image of sample S2 (above), HR-TEM and the respective FFT (below). (C) TEM image of sample S3. Reprinted from I. Aguiar, et al. Influence of solvothermal synthesis conditions in BiSI nanostructures for application in ionizing radiation detectors, Mater. Res. Express 3 (2016) 025012. Copyright (2016), with permission from IOP Publishing.

Fabrication Techniques of Group 15 Ternary Chalcohalide Nanomaterials

369

10.12 CONVERSION OF Sb2S3 INTO SbSI 10.12.1 Conversion by Physical Vapor Method Gödel and Steiner [80] developed a new route for the growth of SbSI microcrystals. This method comprised two steps, where the first one involved the deposition of a thin film of Sb2S3 in a low-temperature chemical bath. In the second step, the substrate with predeposited Sb2S3 was mounted facing the SbI3 target, as presented in Fig. 10.26. Afterwards, the target was heated to 523K. An excess of SbI3 sublimed and reacted with Sb2S3 leading to conversion of amorphous Sb2S3 to crystalline SbSI according to chemical reaction Sb2 S3 1 SbI3 -3SbSI:

(10.23)

The fabricated SbSI microcrystals were used to prepare the photodetector with a sandwich-type architecture (Fig. 10.27). This SbSI-based device seems to be promising for application as an efficient and low-cost light detector [80].

10.12.2 Conversion by Spinning Coating SbI3 Solution Another approach to conversion of Sb2S3 into SbSI was proposed by R. Nie et al. [26]. A thin layer of SbSI was prepared in three steps. At first, a blocking layer TiO2 (BL-TiO2) and mesoporous TiO2 (mp-TiO2) were deposited onto fluorine-doped tin oxide (FTO) substrates (Fig. 10.28A). Then, the chemical bath deposition (CBD) method was applied to overlay the mp-TiO2 electrode with Sb2S3 (step 1 depicted in Fig. 10.28B). A

Figure 10.26 Schematic diagram of deposition of SbSI needle-shaped microcrystals via the evaporation of SbI3 onto amorphous Sb2S3. Reprinted from K.C. Gödel and U. Steiner. Thin film synthesis of SbSI micro-crystals for self-powered photodetectors with rapid time response, Nanoscale (2016) 15920
15925. Copyright (2016), with permission from The Royal Society of Chemistry.

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Nanomaterials Synthesis

Figure 10.27 Fabrication process (A), cross-sectional SEM micrograph (B), and working principle (C) of SbSI photodetector. Reprinted from K.C. Gödel and U. Steiner. Thin film synthesis of SbSI micro-crystals for self-powered photodetectors with rapid time response, Nanoscale (2016) 15920
15925. Copyright (2016), with permission from The Royal Society of Chemistry.

solution of SbI3 in N,N-dimethylformamide (DMF) was spin-coated onto the Sb2S3 layer in the second step (Fig. 10.28C). Finally, the SbSI layer was obtained on the mp-TiO2/BL/FTO by thermal annealing in Ar or N2 gas (step 3 shown in Fig. 10.28D). A reaction leading to formation of SbSI was expected to follow Eq. (10.23).

10.13 HEAT AND LASER FORMATION OF SbSI NANO-OBJECTS IN CHALCOHALIDE GLASSES The phase relationships in the Sb-S-I system were analyzed in Ref. [81], while the glass-forming region in this ternary system was presented, for example, in Refs. [82,83]. Sb-S-I glasses of stoichiometric composition [24] and of the Sb2S3-SbSI [84] system were obtained using fast cooling regimes of solidification and were built through a weak interaction between trigonal pyramids SbS3/2 and SbI3 molecules [24,85]. Also, glasses of As2S3-SbSI [85
87], As2Se3-SbSI [88], and GeS2-SbSI [82,87] systems contain these binary groups that are “frozen” at temperatures lower than the temperature of glass transition (Тg).

Fabrication Techniques of Group 15 Ternary Chalcohalide Nanomaterials

371

Figure 10.28 Preparation of SbSI layers through the reaction between Sb2S3 deposited by the CBD process and SbI3 onto mp-TiO2 electrode. Reprinted from R. Nie, et al. Efficient Solar Cells Based on Light-Harvesting Antimony Sulfoiodide, Adv. Energy Mater. (2017) 1701901. Copyright (2017), with permission from WILEY-VCH.

372

Nanomaterials Synthesis

10.13.1 Growth Activated by Heat Treatment The thermal annealing of glasses in the temperature range from Tg to crystallization temperature Tсr, leads to irreversible structural transformations. The structural clusters forming the Sb-S-I, As2S3-SbSI, As2Se3SbSI, and GeS2-SbSI glass matrices get some freedom of rotation, thus allowing the atoms to move small distances. This facilitates the possible breaking and alteration of the chemical bonds Sb-S, Sb-I, S-S, As-І, As-S, Ge-S, and Ge-I. This process is accompanied by the diffusion of atoms at distances of the interatomic order. As a result, the highly polarized SbS2/2I structural chains typical for crystalline SbSI are formed [87,88]. Hence, nanocrystalline SbSI, possessing ferroelectric properties, was obtained in the matrices of (As2S3)1
x(SbSI)x [85,86,88,89], (As2Se3)1
x(SbSI)x [88], and (GeS2)1
x(SbSI)x [87] glasses at the certain thermal treatments in the temperature range between Tg and Tсr. An increase in the temperature and the annealing time results in growing sizes of the crystalline inclusions. Then the intensities of the reflexes on the diffractograms (Fig. 10.29) grow, whereas their half-widths decrease. Moreover, the growth of the crystalline inclusions is accompanied by growing dielectric permittivity [86]. Two SbSI crystallization processes were observed upon heating Sb-S-I glass [24,84]. At temperatures just above Tg 5 400K, crystallization of the SbSI phase starts from the sample surface [24]. However, maximum crystallization of SbSI was observed in two different temperature ranges around 413K and 463K for an Sb-S-I glass of stoichiometric composition [24]. These two crystallization processes shift to higher temperatures and the temperature interval between them decreases with increasing Sb2S3 concentration [84]. The low-temperature crystallization process corresponds to the formation of SbSI at the surface and mainly along the c-direction parallel to the (Sb2S2I2)n double chains [24,84]. For such crystallization to occur, some bonds of SbS3/2 and SbI3 units must switch or establish long-range ordering in line with the already-existing SbSI chains in the glass [24]. The three-dimensional bulk crystallization and the growth of the SbSI crystal in a direction normal to chains requires more complicated shifting, switching, and diffusion of SbS3/2 and SbI3 units, or reorientation of existing (Sb2S2I2)n double chains. Such a complex process of crystallization needs higher activation energy and hence becomes active only at higher temperatures [24,84].

Fabrication Techniques of Group 15 Ternary Chalcohalide Nanomaterials

373

Figure 10.29 X-ray diffractograms of as-prepared (1) and crystallized (2, 3) at T 5 423K for 1 h (2) and 5 h (3) (As2S3)0.2(SbSI)0.8 glass and polycrystalline SbSI (4). Reprinted from D.I. Kaynts, et al. Formation of Ferroelectric Nanostructures in (As2S3)1002x(SbSI)x Glassy Matrix, Ferroelectrics 371 (2008) 28
33. Copyright (2008), with permission from Taylor & Francis.

10.13.2 Laser-Induced Growth Laser irradiation of glass can induce oriented crystal growth in spatially selected regions ([24] and references within) due to the absorption of radiation, which leads to selective heating of the laser-irradiated and close-surrounding regions. This facilitates processes basically similar to the growth of nanocrystals activated by heat treatment of glass matrices but restricted to the area of the laser spot size (Table 10.1). The movement of a sample relative to the laser beam makes the direct-write method [82] creating tailored architectures of single crystals on the glass (Figs. 10.30 and 10.31). For forming stoichiometric SbSI, it is important to choose a composition of the glass that satisfies the following conditions: (1) forms

Table 10.1 Examples of laser-induced growth of SbSI crystals on chalcohalide glasses Type of glass Type of laser “Written” SbSI features Comments 1

(GeS2)1
x(SbSI)x x 5 0.9

CW Ar laser, λ 5 488 nm

(As2S3)1
x(SbSI)x (0.2 , x , 0.7)

CW Kr1 laser, 647.1 nm, 3
9 mW CW Kr1 laser, 647.1 nm, 3
9 mW CW Ar1 laser, λ 5 488 nm

(As2S3)1
x(SbSI)x (0.2 # x # 0.7)

Sb-S-I glass of stoichiometric composition CW Ar1 laser, Bulk (Sb2S3)1
x(SbSI)x λ 5 488 nm x 5 0.82 Sb-S-I glass of CW diode laser, stoichiometric λ 5 520 nm composition CW Kr1 laser, Bulk and 1.5 μm thick films of λ 5 647.1 nm, (As2S3)1
x(SbSI)x P 5 0.3 4 6 mW x 5 0.55

Millimeter long polycrystalline lines; up to 15 μm long single crystal lines Nanocrystals

References

Power of 1 4 10 mW and a laser scan speed [82] of 10 4 100 μm/s For power P 5 9 mW process is finished within 17 min

[90]

Nanocrystals

No laser-induced crystallization of As2S3 due [91] to short duration of the treatment

Single-crystal

Power densities from 0.25 to 0.32 mW/μm2 [92] and long exposure times ( . 1 min)

Polycrystalline lines; up to 30 μm long single-crystal lines Crystalline lines

Optimal power density of 0.3 mW/μm2 and [83] a laser scan speed of 5 μm/s

Nanocrystals

Optimal power density of 0.05 mW/μm2 and a laser scan speed of 0.1 μm/s

[24]

Threshold power density depends on thermal history of the film; mechanical strain facilitates the phase separation

[93]

375

Fabrication Techniques of Group 15 Ternary Chalcohalide Nanomaterials

1000 SbSI powder diffraction file 900

Laser surface crystallized Ge10 121

200

800

310

100

411 420 241

200

330

031

300

320

400

201 211

220 130

500

210

120

600

020

Intensity (a.u.)

700

0 15

20

25

30

35

40

45

50

2θ (deg.) Figure 10.30 Comparison of X-ray diffraction pattern from laser crystallized surface of (GeS2)0.1(SbSI)0.9 sample (image in inset) and standard powder diffraction file of SbSI crystalline phase. Reprinted from P. Gupta, et al. Laser fabrication of semiconducting ferroelectric single crystal SbSI features on chalcohalide glass, Optical Materials Express 1 (2011) 652
657. Copyright (2011), with permission from OSA.

Figure 10.31 SEM micrograph of the crystal line created using 1 mW CW Ar 1 laser (λ 5 488 nm) and scanning speed of 10 μm/s. The inset shows Kikuchi diffraction patterns from a number of spots on the line and (GeS2)0.1(SbSI)0.9. Reprinted from P. Gupta, et al. Laser fabrication of semiconducting ferroelectric single crystal SbSI features on chalcohalide glass, Optical Materials Express 1 (2011) 652
657. Copyright (2011), with permission from OSA.

376

Nanomaterials Synthesis

into glass easily, (2) crystallizes only into SbSI phase, and (3) has sufficient strength for normal handling. D. Savytskii et al. [83] presented improvements in SbSI crystal size and quality stemming from a refinement of the experimental techniques reported by P. Gupta et al. [82]. Fig. 10.32A shows a diagram of the used experimental setup. First, the sample was irradiated and scanned in one direction (horizontal in Fig. 10.32B). During this process, crystal nuclei were produced. Next, the sample was scanned in a direction perpendicular to the initial line and a single crystal grew from one of the seed crystals formed earlier [83]. The reason for using seed lines instead of spots is that after a certain distance the process reaches equilibrium and the remainder of the line becomes homogeneous in composition and morphology. Therefore, reproducible nucleation sites are provided for subsequent growth of multiple crystal lines consistently [83]. Unfortunately, the setups used in Refs. [82,83] could not ascertain the beginning of crystallization that would allow to determine the size of the smallest crystal made using the technique. The complicated morphology of laser-modified spots created with CW 488 nm Ar1 laser (with power densities from 0.25 to 0.32 mW/μm2) is a result of competing processes (A)

(B) Diode laser

Pinhole

Power meter

Ar+ laser

Camera ½ Waveplate Glan-laser polarizer PC

Microscope objective

Micron stage (x,y,z, roll, pitch)

Dichroic beam splitters

785 nm laser line filter

SE

2 μm

Figure 10.32 (A) Diagram of laser crystallization setup. (B) SEM image of SbSI single crystal line laser “written” on (GeS2)0.1(SbSI)0.9 glass. Reprinted from D. Savytskii, et al. Formation of laser-induced SbSI single crystal architecture in Sb
S
I glasses, Journal of Non-Crystalline Solids 377 (2013) 245
249. Copyright (2013), with permission from Elsevier.

Fabrication Techniques of Group 15 Ternary Chalcohalide Nanomaterials

377

including contraction, expansion, and crystallization of SbSI [92]. The proximity of the temperatures of decomposition and surface crystallization complicates the crystallization of stoichiometric SbSI glass by CW laser irradiation [92]. The chemical composition of spots fabricated by the laser beam after 1 min exposure indicated the evaporation of SbI3 as well as depletion of S and an increase of Sb [92]. Surface oxidation (if performed in air) was observed in crystalline grains at low power densities (0.25
0.32 mW/μm2) and long exposure times (2
3 min) [92]. Such changes in chemical composition were larger in spots fabricated with higher power densities [92]. In an attempt to avoid selective evaporation of SbI3 from the SbSI surface under irradiation and the undesirable change of composition [83,92], the 520-nm CW diode laser was used for writing crystal lines [24]. This laser provided the ability to precisely control the irradiation intensity. Surface heating of the Sb-S-I glass induced by a 520-nm CW laser showed two regions [24]: needle-like crystalline formations and bulk crystallization. This was explained by the two different types of crystallization activated by heat treatment: at low-temperature and high-temperature regions of the laser-treated Sb-S-I glass (see section 10.13.1).

10.14 NEW TRENDS IN FABRICATION TECHNIQUES Reviewing the methods of fabrication of 15 group chalcohalide nanomaterials one can recognize the special potential in the already-known microwave-assisted sonochemical method that has not been used for the preparation of presented materials so far. Also, the asynchronous pulse ultrasonic spray pyrolysis should be useful in fabrication of the known as well as novel nanostructures. These methods are easy, low-cost, efficient, and they do not require expensive equipment. It may be predicted that up scaling of these methods will lead to large quantities of nanomaterials with uniform morphology and high purity. Besides the nanomaterials of ternary chalcohalides, also multiple compounds containing atoms from the 15, 16, and 17 groups are produced in nanosize forms, for example, quaternary SbS1
xSexI nanowires, BiOBrxCl1
x nanostructures, and Cu0.507(5)Pb8.73(9)Sb8.15(8)I1.6S20.0(2) nanowires. Novel materials, for example, quaternary chalcohalide compounds CdSbS2X (X 5 Cl, Br), CdBiS2X (X 5 Cl, Br), CdBiSe2X (X 5 Br, I), XSbS2Br2 (X 5 La, Ce), MnSbS2Cl, MnSbSe2I, Cu3Bi2S4Br, Cu3Bi2S4Cl, Ag1.2Bi17.6S23Cll8, Ag3xBi5
3xS8
6xCl6x
1, and Pb12.65Sb11.35S28.35Cl2.65,

378

Nanomaterials Synthesis

have received a lot of attention because of their enormous potential for many useful technological applications.

10.15 FUTURE PERSPECTIVES Probably, the sonochemical synthesis of SbSI-type nanowires is the most promising among fabrication techniques of nanosized ternary chalcochalides. This easily applicable method will allow the tuning of physical properties of AVBVICVII nanostructures by simply changing used substrates, type of solvent, and time of sonication. In the future, special attention should be paid to the synthesis of SbSClxI1
x nanostructures because the ferroelectric phase-transition temperature was estimated as TC 5 330K for SbSCl0.1I0.9 bulk crystal. This could be an essential advantage in many applications in comparison with the TC 5 293K of pure SbSI.

10.16 SUMMARY Fabrication of group 15 nanostructured ternary chalcohalides represents a relatively new field of materials science. Practically all of the methods presented in this chapter could be used not only for preparation of the exemplified materials but can be extended to the fabrication of some other group 15 ternary chalcohalide nanomaterials. Due to strongly coupled semiconductive and ferroelectric properties of such materials, they should be attractive for many useful applications like photodetectors, solar cells, gas sensors, energy harvesters, actuators, and ionizing radiation detectors.

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CHAPTER 11

Advanced Carbon Materials for Electrochemical Energy Storage Rohit Ranganathan Gaddam1, Nanjundan Ashok Kumar1, Ramanuj Narayan2, K.V.S.N. Raju2 and X.S. Zhao1 1

School of Chemical Engineering, The University of Queensland, Brisbane, QLD, Australia Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India

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Contents 11.1 Introduction 11.2 Carbon Materials: Types and Sources 11.2.1 Graphene 11.2.2 Fullerene 11.2.3 Carbon Nanotubes 11.2.4 Biomass-Derived Carbon Materials 11.2.5 Heteroatom-Doped Carbon Materials 11.3 Carbon Materials for Energy Storage 11.3.1 Lithium-Ion Batteries 11.3.2 Sodium-Ion Batteries 11.3.3 Other Battery Systems 11.3.4 Supercapacitors 11.4 Challenges and Future Perspectives Acknowledgments References

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11.1 INTRODUCTION Energy management and climate change are one of the greatest challenges faced in this millennium. Sustainable routes to generate energy (via wind, water, and solar), though abundant are intermittent, and require proper storage for efficient management. Hence, affordable and sustainable energy storage technologies are quintessential to cater to future societal energy needs. It is estimated that around 2 billion people in the world do not have access to electricity and might not be able to procure power supply through grids [1]. Hence, for both on-grid and off-grid electricity Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00011-0

© 2019 Elsevier Inc. All rights reserved.

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supply, energy storage seems to be essential. It is not feasible to develop a single energy storage system to cater to today’s technological needs. Therefore, the integration of various technologies, like batteries, supercapacitors, and magnetic and kinetic energy storage systems is being considered [1]. The energy storage technologies used in large-scale storage are subdivided into electrical, mechanical, chemical, and electrochemical (Fig. 11.1) [3]. Amongst them, electrochemical energy storage, in particular, has captured more interest due to its low carbon footprint, high efficiency, flexible power-energy regime for grid operations, high shelflife, and low costs associated with upkeep. The principles of electrochemical energy storage were known in the early 1700s. Such electrochemical systems convert the electrical energy into chemical energy (and vice versa) via a redox reaction at the interface of the active electrode mass and electrolyte [4]. In general, an electrochemical cell is made of a positive electrode, a negative electrode, and an electrolyte (which is electronically insulating and conductive to ions). Batteries and supercapacitors are at the lead of these electrochemical energy storage systems for portable electronics as well as for grid-level energy storage.

Figure 11.1 A general comparative chart of discharge time and system power ratings for different energy storage technologies [2]. Copyright 2013. Electric Power Research Institute.

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Though supercapacitors and batteries are both electrochemical devices, the mechanism of their energy storage varies significantly, affecting their energy and power density (Fig. 11.2) [5]. Supercapacitors, on one hand, have a long cycle life along with short charging time, on the other hand, batteries provide high energy density [6]. For instance, in alkali-ion batteries, the insertion of alkali-ion facilitates a diffusioncontrolled redox reaction in the electrode, which can be slow. In contrast, supercapacitors do not involve redox reactions and store charge by adsorption of ions without any diffusion limitations, making it easy to charge and thus providing high power density. Nevertheless, the ion adsorption is surface-confined, making it have lower energy density than that of batteries. It was also identified that some materials had an electrical double like a capacitor (EDLC)-like behavior along with a surface redox process that can lead to a much better charge storage. Such a storage mechanism, called pseudocapacitance, is exhibited by some metal oxides, nitrides, and carbides [7].

Figure 11.2 Ragone plot for electrochemical energy storage devices showcasing specific power vs. energy [8]. Reprinted with permission from Springer Nature. Copyright 2008.

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Further improvement in the performance of these energy storage devices is invested in the design and development of novel electrode materials with a focus on understanding the behaviou of ions. Since the early 19th century, many materials have been investigated as active materials for both batteries and supercapacitors. Amongst them, carbon materials, in particular, have attracted more interest due to their superior electrical conductivity, easy synthesis strategy, low cost, and availability of various allotropes. Carbon is a unique element in the periodic table that can exist in a variety of allotropic forms. Its microtexture, unique aspect ratio, tunable physicochemical properties, and superior thermomechanical properties make carbon a promising material for electrochemical energy storage [9
13].

11.2 CARBON MATERIALS: TYPES AND SOURCES Carbon has a prominent role to play in the advancement of sustainable clean-energy technologies. Carbon naturally assumes various allotropic forms like graphene, fullerenes, carbon nanotubes, etc. The forms of carbon depending on their degree of graphitization can be classified into two categories: (1) “hard carbons,” that cannot be easily graphitized and contain turbostratic nanodomains and (2) easily graphitizable “soft carbons.” This section describes important varieties of carbon allotropes like graphene, carbon nanotubes, and fullerene, along with recently developed carbon materials using bio-derived precursors. Hard carbons prepared from biomass have recently gained significant interest due to their superior ion-storage capability and cheaper mass production costs. The type of carbon and their microstructure mainly depends on the precursor used and treatment conditions (Fig. 11.3) [14], which dictate their electrochemical performance. Further improvement in the performance of such carbon materials can be made by introducing heteroatom dopants like nitrogen, oxygen, boron, or sulfur.

11.2.1 Graphene Graphene is a monolayered carpet of sp2 hybridized carbon network packed into a honeycomb-like lattice, that provides tremendous opportunities for surface design. It was initially thought to be nonexistent, until the first discovery of graphene made by mechanical peeling of graphite galleries using Scotch tape till a single layer of graphite was obtained. This earned Geim and Novoselov a Nobel prize in 2004 [15]. Since then,

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Figure 11.3 Illustration of soft carbon and hard carbon production via pyrolysis of thermoplastic and thermosetting precursors [14]. Reprinted with permission from John Wiley & Sons, Inc. Copyright 2018.

there has been a significant amount of research interest invested in the production of graphene. The method used for producing graphene determines the properties of the final material. Generally, graphene is prepared either by top-down or bottom-up approaches. A bottom-up approach to graphene synthesis involves chemical vapor deposition or other chemical synthesis techniques [16]. Exfoliation of graphite using chemical, thermal, and electrical methods to form a graphene-oxide is a typical top-down approach (Fig. 11.4). Reduction of graphene oxide and liquid-phase exfoliation are the most common methods to generate graphene in bulk. In a liquid-phase exfoliation, an expanded graphite (usually by thermal means) is dispersed into a solvent. This eventually helps in the reduction of van der Waals forces between the graphene layers followed by application of an external stimulus (ultrasonication, electric field, etc.) to exfoliate graphite into individual sheets [17]. However, this method leaves behind some unexfoliated graphite which needs to be isolated. Nevertheless, the ease of synthesis makes the present method most suitable for the bulk production of graphene. In the other method, graphite is strongly oxidized to produce a highly defective graphene called graphene oxide. Graphene oxide offers a wide variety of carbonyl and epoxy groups, which can be selectively transformed into other functionalities depending on its application.

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Figure 11.4 Graphene preparation methods in terms of quality (G), cost (C), largescale production capabilities (S), yield (Y), and purity (P) [17]. Reprinted with permission from Springer Nature. Copyright 2014.

The alternation of carboxyl groups into other functionalities requires activation, which can then form covalent linkages with nucleophiles. In general, carboxyl groups are transformed into amide or ester groups by reaction with an amine or hydroxyl containing nucleophiles. Similarly, alteration of GO through epoxy is believed to happen via a ring-opening reaction [16]. Such alternations restore the π-conjugations of such reduced graphene oxide with properties similar to those of graphene. Graphene and related materials have been widely investigated for use in state-of-art energy storage devices owing to their unique properties. They are promising to improve the energy density and power density of the existing energy storage systems.

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11.2.2 Fullerene Fullerene discovery was initially made amongst the gas-phase carbon ions produced by the laser ablation of graphite [18], and macroscopic quantities of fullerenes were obtained from graphite using the arcdischarge technique. The first fullerene discovered was buckminsterfullerene (C60) in 1985 [18], which is a zero-dimensional carbon with 60 carbon atoms forming a spherical configuration (called truncated icosahedron). These carbons are made up of both pentagonal and hexagonal carbon atoms, where the pentagonal carbon atoms provide curvature to the material. Smalley et al. [19] suggested that, during C60 synthesis, the carbon atoms come close to each other to form a linear carbon species which would add carbon atoms until it reaches a few dozen carbons. They postulated that a more thermodynamically favorable open graphene sheet-like structure is formed as they are more higher reactive sites than the cyclic or linear counterparts given their dangling bonds. These graphene sheets gather enough pentagons and finally form fullerene [19]. Several types of fullerenes can be realized mathematically; with the increasing number of carbon atoms, the curvature is small because of a lower strain. The discovery of fullerene paved the way for the synthesis of many advanced carbon materials like graphene and carbon nanotubes. Their unique carbon arrangement leads to good electronic conductivity, large specific surface area, and superior absorption capacities. They mainly enhance the conductivity and do not provide good mechanical properties owing to their aspect ratio. Alteration of fullerene chemistry via functionalization allows easy tuning of properties via addition, polymerization, and substitution reactions (Fig. 11.5) [20]. The functionalized fullerene could be of two types, namely (1) exohedral fullerene (where the functional moieties are attached to the exterior of the cage) and (2) endohedral fullerene (moieties is within the cage). Another interesting arrangement of the fullerene is as a peapod, where the fullerene is encapsulated within a nanotube [21]. Modified fullerenes have been used as potential electrode materials in lithium-ion batteries (LIB) [22] and magnesium batteries [23].

11.2.3 Carbon Nanotubes Carbon nanotubes (CNTs) can be visualized as the wrapping of graphene sheet into the form of a cylinder. These nanotubes are essentially made of

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Figure 11.5 Some general reactions that occur with buckminsterfullerene [20]. Reprinted with permission from Springer Nature. Copyright 1993.

sp2 carbon atoms which are many times stronger than the sp3 hybridized carbons in diamond. CNTs exhibit excellent stability against chemicals, possess a unique aspect ratio, high surface area (B1500 m2/g), superior tensile strength, and high electrical and thermal conductivity [24]. CNTs can be metallic or semiconducting depending on their arrangement and how the graphene gets rolled [24]. The rolling of graphene sheets can be carried out in many ways which break the symmetry of the graphene plane and create a distinct direction along the hexagonal lattice. For a single-walled nanotube formation (Fig. 11.6), the rolling of the graphene sheet is carried out along a lattice vector (m, n), which dominates the chirality of nanotube. An armchair-type carbon nanotube is formed when “n” and “m” are equal. If either “m” or “n” equals zero a zigzag-type

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Figure 11.6 (A) Chiral vectors defining the SWNT unit cell; (B) armchair, zigzag, and chiral SWCNTs [26]. Reprinted with permission from Elsevier Ltd. Copyright 2004.

nanotube is formed. When an inequality exists between “n” and “m” a “chiral” tube formation takes place [25]. The parameters of (m, n) are quite important in determining the chirality and in turn the optical, mechanical, and electronic properties of CNTs. Ballistic transport of electrons could be observed in defect-free singlewalled carbon nanotubes, where no scattering or migration of electrons could be observed. Both single-walled (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) can be prepared by (1) laser ablation, (2) arc discharge, and (3) chemical vapor deposition. Most of the major synthesis methods used in the preparation of SWCNTs introduce some impurities that could be eradicated by treatment with acids. However, such treatments reduce the length of the nanotubes, create imperfections, and add to the cost. In addition, such synthesis methods produce a mixture of semiconductor and metallic nanotube, which could be an important aspect to consider for an electronic device. Although the metallic nanotubes can be selectively removed by electrical heating, no large-scale synthesis of ultra-high purity SWCNTs exists. In general, carbon nanotubes find their commercial application as a composite. The low density of functional groups available on the surface makes it difficult for the CNTs to disperse in the matrix. Therefore, functionalization by covalent (chemical) and noncovalent (physical) means is carried out for CNTs (Fig. 11.7). CNTs, in general, possess a high surface area of porous nanotube arrays which makes it electrochemically active for applications involving supercapacitors and batteries. CNT-based supercapacitors have higher power densities and storage capabilities as compared to ordinary capacitors. Even in the case of lithium-ion batteries, high discharge capacities at larger current densities were observed [27]. However, certain limitations with respect to the absence of a voltage

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Figure 11.7 Schematic representation of CNT functionalization. (A) Functionalization at the defect site, (B) attaching moieties onto the sidewall, (C) exohedral functionalization using surfactants, (D) attaching polymer moieties using noncovalent means, and (E) pea-pod-like CNT
fullerene structure [28]. Reprinted with permission from John Wiley & Sons, Inc. Copyright 2002.

plateau and voltage hysteresis exist which could be overcome by making composite materials. There are further discussions on the electrochemical performance of CNTs below.

11.2.4 Biomass-Derived Carbon Materials Several synthesis strategies have been employed for the preparation of carbon materials with tailored physicochemical properties. However, scalability issues and inherent toxicity involved in the production methods using fossil-fuel-derived precursors make the production costly. Biomass compounds have the potential to be a sustainable source for producing several carbon allotropes (Fig. 11.8). Precursors like carbohydrates, cellulose, protein, amino acids, etc., have been widely used to synthesize carbon materials. To improve the performance of biomass-derived carbon materials such as porosity, electrolyte wettability, conductivity, and

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Figure 11.8 Methods commonly used for obtaining carbon materials from biomass [30]. Reprinted with permission from the Royal Society of Chemistry. Copyright 2017.

strength, several activation processes are carried out using physical and chemical means [29]. Procuring carbon materials from natural sources sometimes can involve pyrolysis at temperatures ranging from B400°C to 1300°C and activated in situ during the process by introduction of gases like oxygen, carbon dioxide, water vapor, or other gas mixtures. Pretreatment of biomass or post-treatment of obtained carbon material with agents like KOH, NaOH, H3PO4, and H2SO4 could also be carried out so as to activate the surface. A combination of both ex situ and in situ activation methods is quite possible to obtain carbon materials from biomass [29]. In some cases, hydrothermal carbonization is also employed to obtain carbon materials. This thermochemical treatment initially yields a hydrochar that has a high density of oxygen-containing functional groups which sometimes are directly used or further processed to serve as electrode materials in batteries. The hydrochar could be subjected to further pyrolysis with or without chemical or physical activation. The process utilized for the generation of carbon materials significantly affects its physicochemical and thermomechanical properties [30]. Apart from pyrolysis and hydrothermal processes, several unconventional methods have also been employed to generate carbon materials, like carbon nanoparticles from biomass. Gaddam et al. reported carbon nanoparticles from the flame deposition of coconut oil and camphor for

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use in energy applications [31,32]. The size of the nanoparticles ranged from 40 to 50 nm with tuneable surface chemistries (Fig. 11.9). In another work [33], highly fluorescent carbon nanodots were also prepared from such a flame deposition route where the obtained carbon nanoparticles were disintegrated into smaller particles by using strong oxidizing agents. The carbon nanodots showed a blue-green fluorescence and were used for sensing the presence of heavy metal-ions in water.

11.2.5 Heteroatom-Doped Carbon Materials Doping is a method of replacing a carbon atom with a heteroatom in the graphitic plane (Fig. 11.10). Doping of carbon materials enables alterations to their electrochemical and thermomechanical properties. The doping of heteroatoms in carbon materials can be done either during the synthesis or even after the synthesis has been carried out. Doping after synthesizing carbon will help maintain the bulk properties. However, the even distribution of functional groups can be achieved when the carbon material is

Figure 11.9 FESEM and TEM images of carbon nanoparticles prepared by flamedeposition method [31]. Reprinted with permission from Elsevier Ltd. Copyright 2016.

Figure 11.10 (A) Heteroatom dopants for graphite [36]. Reprinted with permission from American Chemical Society. Copyright 2015. (B) Post-treatment doping of heteroatom [37]. Reprinted with permission from American Chemical Society. Copyright 2013.

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doped with heteroatom in situ. Although structural deformations take place while doping, heteroatoms like sulfur, nitrogen, boron, or dualdoping offer properties beneficial for the state-of-the-art energy applications. Such a doping might help enhance the energy state at the Fermi level and therefore overcome capacitance limitations of carbons and improve the specific capacity leading to superior device performance [34]. Recent investigations have shown that the electrochemical performance arises from a change in the charge distribution caused by the differences in the electron negativities between carbon and heteroatoms [35].

11.3 CARBON MATERIALS FOR ENERGY STORAGE 11.3.1 Lithium-Ion Batteries Lithium-ion batteries have conquered the market of portable energy storage technology owing to their high energy storage capabilities, endorsed by the small size of lithium-ion, which can readily afford an efficient intercalation and deintercalation profile. A typical LIB consists of a cathode, an anode, a separator, and an ion-conducting electrolyte. The electrolyte should be conducting ions and inert to the electron transport. The anode and cathode materials are generally isolated onto the current collector that helps in transport of electrons originating from the redox reactions to the external load. When a battery is discharged the lithium-ions from the anode get inserted into the cathode and the opposite occurs during charge. The discharge process in the battery reduces the cathode as it accepts electrons and oxidizes the anode. This lithiation and delithation process indicates the reversibility of the battery. Superior conduction of electrons, low cost, stability during prolonged cycling, and reversible insertion and deinsertion of ions are some of the characteristics required for an active material. In addition, the electrolyte chosen should be nonaqueous, as lithium undergoes an exothermic reaction with water and might raise safety concerns. In general, organic liquids like ethylene carbonate, dimethyl carbonate, or diethyl carbonate, which are compatible with lithium salts, are used as electrolytes. The most commonly used state-of-the-art cathodes in LIBs are LiCoO2, LiMn2O4, LiFePO4, etc., while graphite is the most commonly used anode material [38]. In general, carbon materials are used in the anode component of the LIB. Fig. 11.11 represents the operation of a lithium-ion-based battery system.

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Figure 11.11 Schematic illustration of the working principle of an LIB system [39]. Reprinted with permission from Taylor & Francis Ltd. Copyright 2016.

Anode materials for LIBs have received much attention in the past decade. The selection of anode materials is generally based on the material’s working potential, electrical conductivity, cost, and stability. Pristine lithium metal was considered a suitable candidate as an anode owing to its redox potential and very high theoretical capacity of B3860 mAh/g [38]. Nevertheless, upon repeated cycling, the lithium metal forms dendrites, causing thermal runaway and thus hindering the practical application of lithium metal as an anode in LIB [40]. Hence, researchers have focused on other carbonaceous materials broadly classified into (1) graphitizable carbons (soft carbons) where an orderly arrangement of graphitic crystallites is present and (2) nongraphitizable carbons (hard carbons) where a disordered arrangement of crystallites is present [38]. Soft carbons are quite often used in commercial batteries owing to their cycling stability, significantly reversible specific capacity, cycle life, and coulombic efficiency ( . 90%) [41]. The mechanism of lithium interaction with such carbon systems, especially graphite, has been subject to extensive study. Graphite is amongst the most commonly used anode materials in LIB, with a theoretical capacity of 372 mAh/g [42]. This is established on the consideration that a lithium atom reacts

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with six carbon atoms in a completely reversible intercalation and deintercalation process [42]. Despite the immense production of graphite, it shows low specific capacity, especially for applications involving hybrid electric vehicles, making its use confined to low-power portable electronic devices like laptops and mobile phones. Hence, hard carbons that have high capacity (. 500 mAh/g) were researched as an alternative anode to soft carbons [38]. Hard carbons are made up of carbons with a high level of disorders arising from the random arrangement of graphene sheets making lithium insertion more feasible but with dawdling lithium diffusion. Their high specific capacity has attracted industries to target such carbons for use in electric vehicles. Hard carbons have poor rate capability, high loss in initial capacity, and low tap density. As a result, many methods like surface oxidation, fluorination, or alloying have been used to overcome this problem [43,44]. It is interesting to note that such treatments have resulted in higher coulombic efficiencies and specific capacity. Hu et al. [45] observed that porous hard carbons generated a capacity of more than 400 mAh/g. In another work (Fig. 11.12), sucrose-derived hard carbons with nanoscale porosity show a good cycling stability, rate capability, and reversible specific capacity of 503 mAh/g [46]. Carbon nanotubes are amongst the most promising materials for use as anodes in LIBs owing to their having the highest specific theoretical capacity (1116 mAh/g for single-walled nanotubes) achievable for any carbon material [47,48]. Such capacities are achieved by lithium intercalation with pseudo-graphitic layers and carbons present inside the hollow

Figure 11.12. (A) First cycle charge
discharge curve (at 0.2C) and (B) rate capability studies of sucrose derived carbon as anodes in LIB [46]. Reprinted with permission from Elsevier Ltd. Copyright 2012.

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tubes [38]. DiLeo et al. [49] reported single-walled carbon nanotube electrodes with titanium contacts as anodes in LIBs with an exceptionally high specific capacity of 1050 mAh/g, representing a dramatic improvement in capacity over the conventional graphite electrode. Nevertheless, the electrodes had a relatively low coulombic efficiency owing to its architecture and high-voltage hysteresis [38]. Hence, overcoming such issues Oktaviano et al. [50] proposed an effective strategy for energy nanoscale porosity (4 nm sized holes) onto carbon nanotubes by anchoring cobalt-oxide nanoparticles and etching them out using an acid wash (Fig. 11.13). A superior-performing anode with improved cycling stability, rate capability, and efficiency was obtained. In addition, other strategies like carbon-alloy composites are used to further enhance the capacity of carbon. For instance, carbon nanotubes and few-layered graphene combined with a variety of metal oxides or transition metals have been reported. In a study by Vinayan and Ramaprabhu [51], SnO2 nanoparticles dispersed in nitrogen-doped graphene anode material showed a very good rate capability and reversible capacity of 1220 mAh/g after 100

Figure 11.13. Schematic representation of the strategy for nanopore creation on carbon nanotubes [50]. Reprinted with permission from the Royal Society of Chemistry. Copyright 2012.

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cycles in LIBs. Similarly, silicon nanowire, graphene sheaths, and reduced graphene oxide-derived free-standing electrode showed an excellent performance with a specific capacity retention of 1600 mAh/g at 2.1 A/g after 100 cycles (B80% capacity retention) [52]. Our group has recently developed a high-performance anode from a biomass-derived carbon nanoparticle that delivered a specific discharge capacity of 741 mAh/g at a current density of 100 mA/g in the second cycle [31]. The electrodes showed superior cycling stability and rate capabilities. These carbon nanoparticles, when treated with piranha solution, were decorated with carboxyl groups, which when used as an anode delivered a superior performance as compared to the pristine electrode. In another work [32], we reported binder- and additive-free three-dimensional carbon anodes using a simple flame deposition onto a nickel template (Fig. 11.14). The electrode, when tested against lithium, showed a superior cycling stability up to 500 cycles with a specific discharge capacity of 664 mAh/g at 1 A/g current density.

11.3.2 Sodium-Ion Batteries Although LIBs are presently used in portable electronics and electric vehicles, it is necessary to consider the availability of lithium precursors on the Earth’s crust [53]. Only 20 ppm of lithium is present in the Earth’s crust, which is geographically limited and might cause political fluxes in the

Figure 11.14. (A) Schematic representation of the three-dimensional binder-free carbon anode, (B) digital image of the electrode (top) and nickel foam (bottom), and (C) mechanical flexible electrode [32]. Reprinted with permission from the Elsevier Ltd. Copyright 2018.

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future [54]. Hence, the sodium-ion battery (NIB) is an effective technology not only because of its unlimited presence on the Earth’s crust but also because of its similarities with lithium in terms of chemical interactions. The NIB design is similar to that of LIB, comprising an anode, cathode, a separator, and an ion-conducting electrolyte (Fig. 11.15). The commercialized sodium-based technologies like Na/S and Na/ NiCl2 are only operable at a temperature of B300°C for maintaining the electrodes in a liquid state [55,56]. Such systems cause safety hazards. In contrast, NIBs use insertion materials, making it free from metallic sodium. Room-temperature operable NIBs can find potential applications for electrical grid storage, where specific volumetric and gravimetric energy density are not stringent [53]. Using such renewable resource-derived NIBs can significantly reduce the cost involved and can penetrate the energy market as a rival to LIB technology. During the process of discharge, sodium ions from the anode get inserted into the cathode and vice versa during charge. The reversible insertion and deinsertion of sodium ion indicate reversible charging/discharging of the battery. Although NIBs cannot be compared with their LIB counterpart as a leading technology, they should not be marginalized. In fact, NIBs were researched on a par with LIBs in the late 1970s [57]. However, the successful application of LIBs diverted the research focus from NIBs [57,58]. An important aspect of a battery is to enhance the energy density. In the case of commercial LIBs, the energy density is largely dependent on graphite as the anode and LiCoO2 as a cathode material. As a result, the research on NIBs is focused on increasing the energy density of secondary batteries by finding suitable electrode material. It is important to underline

Figure 11.15. (A) Schematic for the working principle of sodium-ion battery and (B) resource availability of lithium and sodium in the Earth’s crust [53]. Reprinted with permission from American Chemical Society. Copyright 2014.

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the fact that sodium ions have a relatively large ionic radius (0.102 nm) as compared to that of lithium (0.076 nm), and preferably coordinate in the octahedral or prismatic sites [59]. One particular disadvantage of NIBs is that sodium has a higher ionic radius than lithium (1.02 Å of Na1 vs. 0.76 Å of Li 1 ), thus demanding larger channels and interstitial sites for sodium-ion intercalation [60]. Consequently, the important factor is to identify and develop suitable electrode materials with high interlayer d-spacing for easy transport of sodium ions. There has been significant research progress in cathode materials for NIBs [61,62], whilst only a few anode materials are found to be suitable for NIBs. Though a specific capacity of B 1165 mAh/g can be obtained while using pristine sodium metal as anode it will eventually lead to the formation of dendrites causing catastrophic failure of the battery [63]. Thus other anodes like carbon materials [57], metal oxides [64
66], metal nitrides [67], and alloyed materials [68,69] were investigated. Amongst the limited number of anode materials for SIBs, carbon nanomaterials are promising due to their abundance, ease of production, conductivity, corrosion resistance, and low cost [57,70]. Nevertheless, sodium insertion into the commonly used commercial anode for LIBs, that is, graphite shows a low reversible capacity of 35 mAh/g because of its interlayer spacing of B3.4 Å [71]. Therefore, it was calculated that a d-spacing of B 0.37 nm for carbon materials provides better sodium-ion transport [72]. Therefore, the key factor to store sodium ions is to increase the interlayer spacing in graphite/graphene lattice, introduce turbostratic disorders, or generate vacancies. 11.3.2.1 Carbon-Based Electrode Materials for NIBs The main reason for choosing carbon as a potential anode material is due to its cost-effectiveness, high abundance, excellent corrosion resistance, conductivity, and high surface area. However, graphitic carbons seldom show good performance in NIBs, unlike LIB. This is because of the larger ionic radius of sodium rendering such insertions thermodynamically unfavorable. Hence, hard carbons (carbons with turbostratic disorders) have been studied as anode materials for NIB. Such carbon materials possess edge/defect sites at vacancies, the enhanced interlayer spacing in the turbostratic domains, and empty pores for sodium interaction. Such a structure achieves a reversible capacity up to 300 mA /h/g for a stoichiometry of NaC7.4 [73]. The morphology of carbon material also seems to

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affect the performance. Doeff et al. [74] used petroleum coke of different sizes and inferred that the reversibility of hard carbon is size-dependent. Hence, size and morphology seem to play a significant role in the electrochemical performance of sodium. Materials with nanodomains like nanofibers, nanosheets, mesoporous carbon, carbon nanotubes, nanospheres, etc. have been reported as negative electrode materials in NIB [57]. Tang et al. [75] showed a superior rate capability in hollow carbon nanospheres. A specific discharge capacity of 100 mAh/g was obtained at a current density of 2000 mA/g, which was much higher than those previously reported for NIBs. Few-layered graphene has a large surface area with superior conductivity and chemical inertness. Therefore, it holds great potential as an electrode material for electrochemical energy storage. Though such graphene sheets have been used in the past as anodes for LIBs, they have recently emerged as potential anode materials for NIBs [77
82]. As seen earlier, the larger ionic radius of sodium mandates larger interlayer d-spacing for reversible ion insertion. Reduced graphene oxide (RGO) has shown promise in this regard with superior sodium-ion storage properties. A recent study on RGO by Dou and coworkers [78] has shown that an interlayer spacing of 3.7 Å could deliver a capacity of 174 mAh/g at a current density of 40 mA/g. In order to further increase the specific capacity, the interlayer spacing was increased to 4.3 Å, which delivered a discharge capacity of 280 mAh/g at a current density of 20 mA/g (Figs. 11.16 and 11.17). In another work, a reduced graphene oxide prepared by an environmentally friendly metal-based reduction of graphene oxide delivered a capacity of 272 mAh/g at a current density of 50 mA/g, respectively, with an excellent cycling stability of more than 300 cycles [83]. Density functional theory calculations were carried

Figure 11.16. Schematic for sodium insertion in expanded graphite [76]. Reprinted with permission from Springer Nature. Copyright 2014.

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Figure 11.17. (A) Second cycle charge/discharge curves and (B) short-term stability of graphite (PG), graphene oxide (GO), and expanded graphite (EG) at a current density of 20 mA/g. (C) Stability of EG for 2000 cycles. (D) Rate capability test for EG [76]. Reprinted with permission from Springer Nature. Copyright 2014.

out in order to investigate the superior performance of the anode, which revealed that the defects in the graphene aided better sodium-ion storage. Also, the work calculated the capacity obtained in the presence of a small amount of Stone-Wales defects that showed a reasonable estimate of the obtained capacity. Further improving the capacity, recent works on hard carbons from sustainable biomass resources have shown superior performances with capacities reaching that of graphite-based electrodes in LIBs. In a recent work by Gaddam et al. [84], spinifex nanocellulose-derived hard carbons were employed as potential anode material that delivered a specific capacity of 386 mAh/g at 20 mA/g current density. In another work, Wang and coworkers reported a hierarchically porous carbon from peanut shell as an anode material for NIBs [85]. A high initial charge capacity of 431 mAh/g at 100 mA/g was observed. Excellent cycling stability was

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also reported, where 83%
86% of the capacity was retained after 200 cycles. The surface area, accessible surface pores, increased graphite interlayer spacing, and the overall geometry were responsible for the battery stability. In a recent report by Yang et al. [86] okra-derived nitrogendoped carbon sheets were tested as anodes in NIBs. The carbon sheets possess a high specific surface area and show a reversible capacity of 292 mAh/g, good cycling stability (about 2000 cycles), and near 100% coulombic efficiency was obtained. The capacities of the anode materials can further be improved by introducing heteroatom dopants, with the most common one being nitrogen. Such doped carbons can enhance redox reactions, create defects, and lead to the formation of disordered structures, thereby boosting the sodium-ion storage [87]. Various nitrogen species like quaternary, pyridinic, and pyrrolic nitrogen [88
90] have been identified to influence the sodium-ion storage capabilities [91]. In a work by Yan and coworkers [86], hard carbons derived from biomass okara, that were enriched pyrrolic and graphitic nitrogen functional groups, showed a longer cycle life and rate performance in NIBs. Similar results were also observed by our group where the contribution of amide groups in nitrogen-rich hard carbons was evaluated [92]. A specific discharge capacity of B520 mAh/g with an excellent rate tolerance and superior cycling stability up to 1000 were obtained. Density functional theory studies confirmed that doping the carbon with nitrogen increases the sodium-ion interaction which could lead to enhanced specific capacity. 11.3.2.2 Carbon-Based Composite Electrode Materials for NIBs Alloy materials like tin, antimony, germanium, and selenium can chemically interact with sodium to produce a theoretical specific capacity of 847 (Na3.75 Sn), 660 (Na3 Sb), and 678 (Na2 Se), respectively [57]. However, large-volume expansions have made pristine alloy materials unsuitable as anode materials for NIB. In order to solve this problem, carbon-based composites have been used. It is well known that carbon addition enhances the electrical conductivity, improves the chemical stability of the samples, cushions the stress generated in the electrode, and prevents agglomeration of material during cycling [57]. In this regard, Datta et al. [93] investigated the composite of Sn and graphite that showed the first cycle discharge capacity of 584 mAh/g and a charge capacity of B410 mAh/g. Only a 0.7% fade in capacity could be seen in each cycle unlike pure microcrystalline Sn (B400% volume expansion was observed)

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[93]. In another study, highly stable Sb/C nanofibers were used as anodes for NIBs. Here, a scalable electrospinning method was used to encapsulate Sb nanoparticles (B30 nm in size) onto 1D carbon fibers. Overall, the composite delivered a good stability on prolonged cycling with a reversible capacity of 350 mAh/g (Fig. 11.18) [94]. Phosphorous is a potential candidate as an anode for NIBs owing to its high theoretical specific capacity of 2595 mAh/g [59,95
97]. In addition to the capacity, the low price of phosphorous makes it a prospective anode material for NIBs. Nevertheless, the large-volume expansion of phosphorous (B300%) makes it unsuitable for practical applications, leading to loss of electrical contact, pulverization of electrode surface, delamination of active material, and formation of an unstable SEI. As a result, a rapid fading of specific capacity during cycling, low coulombic efficiency, and degradation of the electrode could be observed [98]. As a 0.4

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discharge profiles (current density 5 0.1 A/g), and (C) cycling stability at a current density of 100 mA/g [94]. Reprinted with permission from American Chemical Society. Copyright 2013.

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result, carbon black and carbon nanotube-based phosphorous composites have been developed. Wang and coworkers [99] reported a composite of red phosphorous and carbon nanotube through a vaporization condensation approach that provided a specific capacity of 550 mAh/g after 200 cycles at a current density of 0.5 A/g. The work suggested that a strong bonding between phosphorous and carbon can endure the volume changes in phosphorous occurring during cycling. In a recent work by Song et al. [98], chemically bonded phosphorous and carbon nanotubes produced by a simple ball milling technique were reported as an anode in NIBs. The anode showed a specific discharge capacity of 1586.2 mAh/g after 100 cycles with a cycling efficiency of B99%. The better stability of the electrode was credited to the effective crosslinking binder with that of the active material, which symbiotically helps in the maintenance of the electrode intact during cycling (Fig. 11.19).

11.3.3 Other Battery Systems Apart from LIBs and NIBs, carbon has been exploited as a potential electrode material in many other monovalent and multivalent cationbased ion-storage systems. A lithium-sulfur battery that possesses a large

Figure 11.19. Diagrammatic representation of sodium interaction with phosphorusbased anodes during cycling [98]. Reprinted with permission from American Chemical Society. Copyright 2015.

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Li-ion Discharge

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Figure 11.20. Diagrammatic representation of Li-ion-based battery systems [102]. Reprinted with permission from Springer Nature. Copyright 2011.

theoretical capacity of B 1675 mAh/g has problems associated with the nonconducting nature of sulfur and solubility issues of lithium polysulfides in electrolytes (Fig. 11.20) [100]. Therefore, such batteries possess poor cycle life, polysulfide migration and dissolution issues that need to be overcome. Though polymers, silica and metal-organic frameworks were used to overcome this problem, carbon materials with pores and large surface areas have attracted significant interest. In a work by Cheetam and coworkers [101], carbon with hierarchical pores was utilized as a potential cathode scaffold for lithium-sulfur batteries. Another battery technology based on potassium-ion storage can achieve a better rate tolerance due to smaller Stoke’s radius of potassium ions when solvated in a liquid electrolyte. Also, the higher redox potential of K/K1 (
2.92 V) compared to that of Na/Na1 (
2.71 V) makes it an attractive alternative to NIBs [103]. In addition, similar to that observed for LIBs, graphite intercalation-based anodes for KIBs seem to be quite promising as a negative electrode delivering a specific capacity of 200 mAh/g [104,105]. These features make the development of KIB more important than any other battery technology. As such, carbonaceous materials have attracted considerable attention in storing potassium ions. However, the energy and power density of such carbonaceous materials seem to be lagging behind when compared to commercial LIBs. In this regard, hard carbon materials could be employed as potential anodes for KIBs. Recently surface-driven capacitive storage behaviors have captured interest for battery applications. Contrasting the intercalation mechanism such capacitive storage behavior takes place at the surface of the active material and the near surface to the electrode that can provide faster kinetics of ion storage. Given the disordered structure and the presence of

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turbostratic nanodomains, hard carbon could be employed for such surface-driven potassium-ion storage. The absence of obvious intercalation could help maintain the structure of the electrode material, thereby increasing the cycling stability and rate capacity. There is only a small amount of research available exploring potassium-ion storage in hard carbons [103,106
113]. Further research needs to be invested in developing carbonaceous anode materials for potassium-ion batteries. In a recent work [114], hard carbons prepared from sugar using a hydrothermal approach were tested as anodes for KIBs as well as for NIBs. These carbons showed a specific capacity of 262 mAh/g with 83% capacity retention, even after 100 cycles in KIBs. These anodes showed a superior rate tolerance in KIBs than that for NIBs. Multivalent cation-based battery technologies like aluminum-ion and magnesium-ion batteries also utilize carbon as a potential electrode material. One of the first reports by Dai and coworkers [115] on aluminum-ion batteries involved the use of graphite foam as cathodes that delivered a capacity of 70 mAh/g with a highly stable cycling performance up to 7000 cycles. The battery functioned due to the movement of aluminum ions across the electrode and the reversible intercalation of chloroaluminate anions in the graphite. The same group further improved the battery performance using natural graphite as cathode material [116]. The cathode could achieve a capacity as high as 110 mAh/g at a current density of 99 mA/g, with superior cycling stabilities up to 6000 cycles. Other reports [117
119] have also showcased the potential of using carbon materials as anodes for aluminum-ion batteries. Similarly, for a magnesium-ion battery when fluorinated graphene sheets were used as cathodes a reversible capacity of 100 mAh/g was obtained [120]. The present battery system overcomes the sluggish diffusions involved with multivalent cations making use of the kinetically favorable redox processes on the surface functional groups and electrolyte.

11.3.4 Supercapacitors Electrochemical supercapacitors represent an important family of the electrochemical energy storage device. The energy store in such systems mainly stems from pseudocapacitive or electrical double-layer mechanisms. In the electrical double-layer storage mechanism, the ion gets adsorbed on the surface of the electrode, which implies that the electrical conductivity and the surface area of the material have a major role to

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play. Hence, materials with a superior surface area, like that of activated carbons, are preferred for electrical double-layer capacitors (EDLCs) [121]. On the other hand, for a pseudocapacitive energy storage, Faradic charge transfer occurs that involve redox reactions, intercalation processes, and/or adsorption. Materials such as transition metal oxides, functionalized carbon materials, and polymeric materials exhibit a pseudocapacitive charge storage, which showcase a higher energy density than EDLCs [121]. There has been a significant expansion in the area of carbon-based electrochemical supercapacitors [122,123]. In particular, graphene-based materials have been found to be quite suitable for such electrochemical energy storage applications. Graphene has a superior electrical conductivity and a theoretical specific surface area of B2630 m2/g [124]. Though predictions for EDLCs utilizing graphene indicate a possible B550 F/g capacitance for graphene, experimentally this is quite difficult to achieve [125]. This is mainly because graphene tends to restack and agglomerate, given the π
π interactions which restrict the surface area that could be exploited for ion adsorption. Several researchers have worked in this area to enhance the capacitive performance of graphene-based materials. The creation of pores was sought to enable the electrode material to achieve higher capacities through an increase in surface area. For instance, Chen et al. [126] prepared a three-dimensional porous graphene with an exceptional surface area (3523 m2/g) and electrical conductivity. The presence of micro-, meso-, and macropores enabled the electrode materials to exhibit capacitance as high as 202 and 231 F/g in 1 M TEABF4/AN and EMIMBF4 electrolyte, respectively. The electrodes possessed a high energy density of 98 W/h/kg. Electrochemical activation could also help achieve higher surface area through pore generation. Such an activation in graphene could help in overcoming the van der Waals energy to increase the d-spacing between the graphene layers. In a recent report, a significant increase in surface area from 5 to 2687 m2/g was achieved which lead to a specific capacitance of 220 F/g [127]. Such an activation enables easy movement of ions through the electrode and allows access of ions to areas that were unavailable prior to activation. Preparation of hydrogels/aerogels of graphene has undergone extensive investigations. This is because the formation of aerogels due to selfassembly of graphene sheets inhibits restacking and enables easy access of the electrode surface to the electrolyte. Graphene aerogel can possess capacities ranging from 128 to 308 F/g, which is mainly dependent on the specific surface area, functional groups present on the surface, and the

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treatment method used [121]. In a study by Shi and coworkers [128], graphene hydrogels were directly deposited on nickel foam yielding a composite electrode. Here, the nickel interpenetrated the aerogel (B1 mm thickness), which enabled better contact of the electrode with the current collector. With the improved conductivity and ion transport, the electrode exhibited an aerial specific capacitance of 45.6 mF/cm2, a stable cycling performance up to 10,000 cycles (with 90% capacity retention) and an exceptional rate tolerance at higher current densities. The interconnected 3D porous networks, superior electrical conductivity, and high surface area help the aerogels to be used directly as electrodes in flexible energy storage devices. Recently a flexible solid-state supercapacitor was manufactured by using graphene aerogel as an electrode and a solid-state electrolyte made up of sulfuric acid and polyvinyl alcohol (Fig. 11.21) [129]. Given the effect of infiltration of the electrolyte into the electrode, the flexible supercapacitor provided a capacitance of 372 mF/cm2 (film thickness of B120 μm). No obvious loss in capacitance was noticeable when the electrodes were mechanically bent. In another work [130], a nitrogen-doped graphene derived from a hydrothermal treatment of graphene oxide with pyrrole followed by pyrolysis (1050°C) delivered a specific capacitance of 484 F/g in an LiClO4 electrolyte. Negligible change in specific

Figure flexible D) and Society.

11.21. Schematic illustration (A) and photographs (B) of the solid-state supercapacitor. Scanning electron microscope images of hydrogel before (C, after (E,F) pressing [129]. Reprinted with permission from American Chemical Copyright 2013.

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capacitance was observed even when subjected to a compression of 50%. Though such aerogels provide superior performance in capacitors, their poor elasticity owing to their low densities and low elastic bending stiffness limit their commercial uptake.

11.4 CHALLENGES AND FUTURE PERSPECTIVES The global energy demand of an ever-increasing population fueled by technological advancements that produce energy-hungry devices has raised serious concerns regarding future energy storage. Hence, innovative, efficient, eco-friendly, and green solutions for energy storage are a major challenge in today’s world. Carbon has demonstrated itself as a sustainable material to store and generate energy. Researchers have put great efforts into developing high-performance carbon materials for use in advanced storage systems. Using carbon in energy storage has its own advantages including, but not limited to, low cost, a wide range of potential working window, tuneable physicochemical, electrical, and thermal properties [57]. Carbon also offers easy alteration to its surface chemistry by covalent or noncovalent modifications, which significantly affect the end use of the material. The synthesis involving carbon material has a common issue of having toxic and/or costly starting materials which in turn hinders the industrial0scale development of such materials. Also, in some cases, a bottom-up approach for realizing an overall uniform structure at a microscale is not feasible owing to the technological and economic considerations. Therefore, bio-derived materials can be sought for a real-time green solution to meet the industrial demand for carbon materials. Biomass is a competent carbon precursor for synthesizing valuable carbon materials with high quality and huge yield in an eco-friendly way from a renewable resource. Biomass mainly represents herbaceous source material preliminarily consisting of carbon, hydrogen, and oxygen. The total wild plant growth is estimated at around 146 billion tons a year [131]. Conversion of this waste to wealth utilized in energy production would be highly advantageous. These carbon materials, easily acquirable from a natural source, possess exceptional electrochemical performance, and could be a solution to the need for economic and eco-friendly carbon-based energy material. Devices like supercapacitors and batteries serve the purpose of continued and balanced energy storage. However, each of these devices suffers either from low energy density or power density. This goal of

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enhancing energy storage and conservation constantly fuels researchers to engineer efficient materials that can cater to today’s needs. Also, biomass-derived carbon materials have not yet been able to reach the industrial standards, therefore further developments should be devoted to developing biomass-derived carbon materials of industrial standards. Therefore, for carbon materials, some principal challenges of controlling the microstructure, enhancing the energy density by using dopants, largescale chemical/physical manipulation of biomass, and developed nanofabrication techniques need to be overcome to address the present-day energy challenges.

ACKNOWLEDGMENTS This work was supported by the Australian Research Council (ARC) under the Laureate Fellowship Program (FL170100101). RRG acknowledges support from the Australian Government through an Australian Government Research Training Program Scholarship for his PhD.

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CHAPTER 12

Organic
Inorganic Hybrid Nanomaterials: Synthesis, Characterization, and Application Vesna Lazi´c and Jovan M. Nedeljkovi´c Vinˇca Institute of Nuclear Sciences, University of Belgrade, Belgrade, Serbia

Contents 12.1 Introduction 12.2 Interfacial Charge Transfer Complexes: Formation Mechanism and Optical Properties 12.3 Polymer Supports Decorated With Inorganic Nanoparticles 12.4 New Synthetic Approaches and Challenges 12.5 Potential Application of OrganicaInorganic Hybrids: Photo-Driven Processes and Antimicrobial Ability 12.6 Summary and Outlook Acknowledgments References

419 420 431 435 437 443 444 444

12.1 INTRODUCTION The increasing interest in hybrid organic
inorganic nanomaterials has been facilitated by the input from molecular chemistry in the field of nanomaterials science. Having in mind the importance of the hybrid interface, two distinct classes of these materials can be recognized. The first class is characterized by the weak interaction between organic and inorganic components (hydrogen, van der Waals, or ionic bonds), while, in the second class, the two phases are totally or partly linked together through strong chemical bonds, characterized by a strong orbital overlap (covalent or iono-covalent bonds). For the sake of clarity, this review will be limited to hybrid materials where the formation of strong chemical bonds between organic and inorganic components takes place.

Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00012-2

© 2019 Elsevier Inc. All rights reserved.

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Another important feature in the tailoring of hybrid organic
inorganic nanomaterials concerns the chemical pathways that are used to prepare the desired hybrid material. Based on the types of material and synthetic approach, the additional restriction of the subject of this review was made. Special attention will be paid to surface modification of wide band-gap metal oxides (TiO2, Mg2TiO4, Al2O3, etc.) with small colorless organic molecules that lead to the formation of visible-light responsive hybrid materials. The formation of interfacial charge transfer (ICT) complexes is facilitated by polycondensation reaction between hydroxyl groups originated from the surface of metal oxide and the organic moiety. Secondly, we will focus on in situ preparation of silver and titanium-dioxide nanoparticles supported by functionalized epoxy-resins. Epoxy-resin has a dual function serving, at the same time, as reactant and support. Some potential applications of multifunctional organic
inorganic hybrid nanomaterials will be addressed in this review, such as the photocatalytic performance of visible-light responsive wide band-gap oxide materials and antibacterial performance of silver-containing hybrids. On the other hand, selective adsorption of ionic species from wastewater to surface-modified inorganic materials will be omitted. Of course, an exhaustive description of all the synthetic approaches involved in the preparation of organic
inorganic hybrid materials, as well as their properties and potential applications, is beyond the scope of this chapter. The reader is referred to some excellent reviews [1
7].

12.2 INTERFACIAL CHARGE TRANSFER COMPLEXES: FORMATION MECHANISM AND OPTICAL PROPERTIES Wide band-gap metal-oxides (TiO2, ZnO2, CeO2, and SnO2) have been extensively studied for a range of diverse applications because they are abundant, chemically stable, biocompatible, and readily affordable. In particular, numerous photo-driven processes-heterogeneous photocatalysis for removal of inorganic and organic pollutants, water-splitting reaction, solar cells, etc. -using metal oxides have been extensively investigated in order to achieve the desired level of practical efficiency [6,8
13]. However, the efficient solar light utilization of oxide materials is limited by their large band-gap. For example, the most studied photocatalyst, TiO2, due to its large band gap (3.2 eV), absorbs less than 5% of the available solar light photons, allowing only UV photons to produce electron
hole pairs and stimulate redox processes on the catalyst surface. There

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Inorganic Hybrid Nanomaterials: Synthesis, Characterization, and Application

421

has been great interest in recent years to increase visible-light absorption of TiO2, including dye sensitization [14,15], doping [16,17], and the use of plasmonic noble metal nanoparticles [18,19]. Another emerging approach to extend absorption of TiO2 into a more practical range of solar spectrum involves an interfacial charge transfer (ICT) from a surface modifier into the conduction band of TiO2 particles. It should be emphasized that there is a fundamental difference in photo-generation of charge carriers between this approach and sensitization with dye molecules. In the former case, electrons are in a single step directly injected from the ground state of the ICT complex, located in the semiconductor band-gap, into the semiconductor conduction band, while the latter case involves two steps: first, excitation of the dye molecules, and subsequent electron transfer from the excited state into the semiconductor conduction band. A graphical presentation of energydiagrams for organic-to-inorganic ICT transition and photoexcitation of a dye-sensitized semiconductor is shown in Fig. 12.1. The use of ligand molecules, mainly benzene derivatives, leads to the formation of ICT complexes followed with the red-shift of absorption onset. Until recently, the strong ICT transitions have been exclusively reported between surface Ti atoms (Tisurf) and aromatic compounds with either two adjacent hydroxyl groups (catecholate type of ligands) or adjacent hydroxyl and carboxyl groups (salicylate type of ligands) [20
34]. In these complexes, ligands are chemisorbed onto TiO2 surfaces by a double TiaOaC linkage formed in the condensation reaction between surface hydroxyl groups and benzene derivatives (see Fig. 12.2).

Figure 12.1 Energy-diagram of organic-to-inorganic ICT transition (A), and photoexcitation of dye-sensitized semiconductor (B).

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Nanomaterials Synthesis

Figure 12.2 Schematic presentation of ICT transitions.

Recently, the ICT complex formation between Tisurf and organic monohydroxy compounds was reported [35
37]. Since the ICT transition can also be induced with a single TiaOaC linkage, the range of ligand molecules available for ICT absorption is considerably expanded. Of course, there are a few reports concerning the formation of the ICT complexes with ligands that do not belong to phenolate-, catecholate- and salicylate-types, such as ascorbic acid [38
41], thiosalicylic acid [42], hydrazine [43], and organic dyes [44
46]. Until recently, the ICT complex formation has been mostly studied using various morphological forms of TiO2. During the last couple of years successful surface modification of different wide-band-gap oxides, such as commercial Na2Ti3O7 nanotubes [47], Mg2TiO4 [48], BaTiO3 [49], ZnO [30], Fe3O4 [50], Al2O3 [51], as well as biogenic (B
Hap) and synthetic hydroxyapatite (S
Hap) [52,53], has been achieved. A comprehensive list of literature concerning the ICT complex formation, including the morphology of metal-oxides, ligands, and positions of absorption onset, is presented in Table 12.1. So far, almost four times more reports concerning the ICT complex formation have been published with TiO2 than all other wide-band-gap oxides. Among them, in particular, at the early stages of development of this field, most of the work has been done with extremely small colloidal TiO2 nanoparticles due to their unique surface structure. It is well-known that the coordination of the surface Ti atoms changes from octahedral (six-coordinate)

Table 12.1 Literature overview—the ICT complexes between various wide-band-gap oxides and different types of ligands Oxide Size Ligand λg (nm)

TiO2

3 nm 45 Å

Lauryl gallate 6-Palmitate ascorbic acid Ascorbic acid 1-Hydroxy-2-naphthoic acid 2-Hydroxybenzoic acid 2,5-Dihydroxybenzoic acid 2,3-Dihydroxybenzoic acid 3,4-Dihydroxybenzoic acid Catechol Catechol Pyrogallol Gallic acid Catechol 2,3-Dihydroxynaphthalene Anthrarobin 2-Hydroxybenzoic acid 3-Hydroxy-2-naphthoic acid; 3,5-Dihydroxy-2-naphthoic acid 3,7-Dihydroxy-2-naphthoic acid Catechol 3-Methylcatechol 4-Methylcatechol 3-Methoxycatechol 3,4-Dihydroxybenzaldehyde 4-Nitrocatechol

675 650 775 540 500 580 620 620 630 630 655 645 600 590 670 475 530 580 600 590 590 590 590 540 540

References

[31] [39] [38,40,41] [28] [20]

[21]

[22]

[24]

[26]

(Continued)

Table 12.1 (Continued) Oxide Size

4.6 6 0.1 nm 10 nm 4
15 nm 320 m2/g (ST-01, Ishihara Co., Ltd.)

7.3 6 2.7 nm 13.6 6 2.3 nm 20 nm (P90, Aerosil) Degussa P25

, 50 nm [Neutrino Co (Iran)] 300
350 nm

Ligand

λg (nm)

References

Catechol Hydrazine Thiosalicylic acid Catechol 4-t-Butyl catechol 3-Methoxy catechol 3,4-Dihydroxy benzonitrile Tiron Catechol 5-Amino salicylic acid Catechol 5-Amino salicylic acid Phenol Phenol 4-Nitrophenol 4-Bromophenol 4-tert-Butylphenol Hydroquinone Catechol 5-Amino salicylic acid Thiamine hydrochloride

600 800 560 520 540 530 490 480 650 650 650 650 550 685 475 540 555 790 650 650 650

[32] [43] [42] [29]

Tiron

600

[33]

[27]

[35] [37]

[27] [36]

370
500 nm

420
470 nm 10 3 40 nm NRs; rutile rhombus shape; (26
70) 3 (8
12) nm Nanoporous film TiO2 electrode

Na2Ti3O7 Mg2TiO4

TiO2 NPs supported by epoxy resin (5
8) 3 (50
500) nm NTs (Nanobakt) 10 nm

Fe3O4

100 nm (Sigma-Aldrich) (10
30) nm 3 (0.2
1) μm NRs 5 nm

Al2O3

0.1
0.3 μm

B-HAP S-HAP

(5
10) 3 (30
50) nm NRs (10
20) 3 (40
60) nm NRs

BaTiO3 ZnO

Dopamine Catechol 2,3-Dihydroxynaphthalene Anthrarobine Ascorbic acid Dopamine Caffeic acid Ascorbic acid

680 665 700 715 670 650 Not reported 600

[23]

2-Anthroic acid Organic dye LEG4 3,4-Dihydroxybenzoic acid Dopamine

600 790 550 650

[44] [45,46] [34] [31]

Caffeic acid Gallic acid Catechol 5-Amino salicylic acid Catechol Caffeic acid

800 550 . 800 . 800 620 Not reported

[47]

Salicylic acid 3,4-Dihydoxyphenylacetic acid Dopamine Catechol 5-Amino salicylic acid 5-Amino salicylic acid 5-Amino salicylic acid

Not reported

[50]

980 750 700 750

[51]

[25] [30] [40]

[48] [49] [30]

[52] [53]

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to square-pyramidal (penta-coordinate) when the size of TiO2 particles is smaller than a certain critical diameter (d , 20 nm) [54]. Later, it was shown that the ICT complex formation is not exclusive of extremely small in size TiO2 nanoparticles, but rather a general phenomenon that can be observed even with the use of commercial TiO2 particles, such as the commercial photocatalyst Degussa P25 [37]. The most striking feature of the ICT complex formation is the redshift of absorption onset, and, consequently, the appearance of the absorption in more practical, visible or near-infrared spectral range. An instructive example of how extensive optical changes can be upon the ICT complex formation is surface-modified Al2O3 particles (the size range 0.1
0.3 μm) with catechol and 5-aminosalicylic acid [51]. The Al2O3 is an insulator with the band gap of about 8.7 eV [55], and, of course, it does not absorb solar light at all. However, its hybrid with catechol has absorption onset in the near-infrared spectral region at 1.26 eV (see Fig. 12.3). It transpires that the surface modification is a simple way to transform the insulator into a hybrid semiconductor-like material capable of harvesting a large portion of the solar spectrum.

Figure 12.3 Kubelka
Munk transformations of UV
VisaNIR diffuse reflection data of surface- modified Al2O3 powders with catechol (blue) and 5-aminosalicylic acid (red). Taken from V. Ðordevi´c, J. Dostani´c, D. Lonˇcarevi´c, S.P. Ahrenkiel, D.N. Sredojevi´c, ˇ N. Svraki´ c, et al., Hybrid visible-light responsive Al2O3 particles, Chem. Phys. Lett. 685 (2017) 416
421.

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Several attempts, based either on extended conjugation of ligands [22,28] or introduction of an electron-withdrawing and electrondonating aliphatic functional group to the benzene ring [29,37], have been made to achieve fine tuning of the optical properties of organic
inorganic hybrids. The use of ligands with fused benzene rings can induce only slight additional red-shift of the absorption onset of surfacemodified TiO2 nanoparticles in comparison to catechol and salicylic acid [22,28]. The effect of aliphatic functional groups is a little more pronounced. The presence of an electron-withdrawing functional group leads to an increase in the energy of the ICT transition (blue-shift), while the presence of electron-donating groups leads to a decrease in the energy of the ICT transition (red-shift) compared to the primary ligand molecule. Data concerning surface modification of TiO2 with catechol and its derivative, presented by Higashimoto and coworkers [29], can serve as a textbook example. Briefly, the energy gaps with ligand molecules having electron-donating functional groups (4-tert-butylcatechol and 3-methoxycatechol) are smaller compared to catechol (2.28, 2.32, and 2.40 eV, respectively). On the other hand, hybrids with ligands having electron-withdrawing functional group (3,4-dihydroxybenzonitrile and Tiron) have larger energy gaps compared to catechol (2.52, 2.57, and 2.40 eV, respectively). So far, it seems that infrared spectroscopy is a method of choice to study the coordination of ligand molecules to the surface of wide-bandgap oxides, mainly TiO2 [20
30,35,44]. In addition, there is only one study where solid-state NMR combined with density functional theory (DFT) was employed to understand the chelated geometry of catechol on TiO2 [32]. Generally, the differences in infrared spectra (appearance/ disappearance and shift of vibrational peaks) between free and bound ligands are used to understand the surface structure of organic
inorganic hybrids. As an example, FTIR spectra of catechol, free and adsorbed on TiO2 nanoparticles are shown in Fig. 12.4 (A and B, respectively), but their detailed analysis is beyond the scope of this review. It is important to point out that this methodology does not provide an opportunity to discriminate if either the formation of bidentate mononuclear chelating or bidentate binuclear bridging complexes take place. Proposed coordination structures for catecholate- and salicylate-type binding are presented in Fig. 12.5. While the usage of materials in their powder form is advantageous from the technological point of view, the use of colloids consisting of

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Nanomaterials Synthesis

Figure 12.4 FTIR spectra of catechol: experimental spectra of free catechol (A) and adsorbed on TiO2 nanoparticles (B); scaled predicted spectra at the B3LYP/6-31G level of theory for bridging (C) and chelating (D) bidentate binding structure. Taken ˇ ˇ Veljkovi´c, S.D. Zari´c, V.M. Raki´c, et al., from T.D. Savi´c, M.I. Comor, J.M. Nedeljkovi´c, D.Z. The effect of substituents on the surface modification of anatase nanoparticles with catecholate-type ligands: a combined DFT and experimental study, Phys. Chem. Chem. Phys. 16 (2014) 20796
20805.

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Figure 12.5 Proposed coordination structures (chelating and bridging coordination) for catecholate- and salicylate-type of ligands.

nanometer-sized particles sometimes has an advantage from the fundamental point of view. Basically, simple spectrophotometric methods for determination of composition and stability constants of complex species in homogeneous media-Job’s method [56] and Benesi
Hildebrand analysis [57], respectively—turned out to be applicable for heterogeneous colloidal systems when the size of organic
inorganic hybrids is sufficiently small, and the ICT complex exhibits optical properties distinct from its constituents. Job’s method of continuous variation is based on assumption that only a single-type complex is present in the solution. For example, in the case of TiO2 nanoparticles, the stoichiometric ratio (n) between surface Ti atoms (Tisurf) and the ligand is determined from the plot of absorbance versus the mole fraction (x) of metal or ligand. The ratio x 5 [Tisurf]/([Tisurf] 1 L), where xmax corresponds to mole fraction in the absorbance maximum equals stoichiometric ratio (n). The molar concentration of surface Ti atoms [Tisurf] can be determined from the following equation [58]: ½Tisurf  5 12:5 3 ½TiO2 =D where [TiO2] is the molar concentration of TiO2, and D is the diameter of the particle in angstroms. Appling this methodology for surface-modified 45 Å TiO2 colloids, the stoichiometric ratio [Tisurf]:[L] 5 2:1 was found, indicating bridging coordination of various

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Nanomaterials Synthesis

catecholate- and salicylate-type ligands to the surface Ti atoms [20
22,24,26,28,41]. Values of the stability constants of the ICT complexes for the same set of ligands were found to be similar, of the order 1023 M21 [20
22,24,26,28,41]. A slightly smaller stability constant was found when thiosalicylic acid was used as a ligand [42]. It should be emphasized that the ability to extend the usage of typical solution chemistry methods to heterogeneous systems is additional proof of the enormous growth of nanoscience, followed with the development of sophisticated synthetic approaches for the preparation of high-quality nanoparticles with desired morphology (shape and size) and surface properties. On the theoretical side, the quantum chemical calculations based on density functional theory (DFT) have been used to predict optical properties of organic
inorganic hybrids [22,24,26,28
30,32,35,37,41, 42,44
46,49,51,59
61]. The agreement between calculated highest occupied molecular orbital
lowest unoccupied molecular orbital (HOMO-LUMO) gap values, as well as calculated electronic excitation and vibrational spectra, and experimental data depends on how well the chosen model mimics the corresponding system, and, of course, on the level of theory. Even with initial simple attempts to obtain deeper insight into properties of the ICT complexes combining an experimental and theoretical approach, the same trends between measured and calculated data were found, besides the fact that DFT calculations were performed using molecular complex species as a simple model of the ICT complexes [22,24,26]. For example, the calculated infrared spectrum of catechol bound to the TiO2 surface, assuming bridging coordination, is quite similar to the measured one [26] (see Fig. 12.4) supporting the fact that stoichiometric ratio between Tisurf and catecholate-type of ligands is 2:1 [20
22,26,41]. However, the DFT data have a predictive character when the constructed model is proper and the level of theory sufficient. For example, the remarkable agreement between Kubelka
Munk transformation of reflection data obtained for surface-modified TiO2 powder with phenol and calculated electron excitation spectrum of corresponding [Ti8O14(OH)3-O-benzene] cluster can be observed [35]. It should be emphasized that like in the case just mentioned, the extensive periodic calculations might be replaced with the modeling on computationally less demanding molecular systems (Fig. 12.6).

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Inorganic Hybrid Nanomaterials: Synthesis, Characterization, and Application

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Figure 12.6 (A) Kubelka
Munk function spectra of TiO2 (blank) and the TiO2-phenol sample together with photographs of these samples. (B) DFT-optimized structure (gray: carbon; white: hydrogen; red: oxygen; large white: titanium), and (C) electronic excitation spectrum of [Ti8O14(OH)3-O-benzene]. Taken from J. Fujisawa, S. Matsumura, M. Hanaya, A single Ti-O-C linkage induces interfacial charge-transfer transitions between TiO2 and a p-conjugated molecule, Chem. Phys. Lett. 657 (2016) 172
176.

12.3 POLYMER SUPPORTS DECORATED WITH INORGANIC NANOPARTICLES In Section 12.2 some specific properties of organic
inorganic hybrids consisting of inorganic material coated with small organic molecules were presented, while, in this section, we will focus on nanocomposites where the organic component is a polymer network. Obviously, three distinct components of nanocomposites can be recognized: (1) inorganic, (2) organic, and (3) hybrid interface. Both, inorganic and organic components can either be presynthesized or in situ generated during the course of preparation of nanocomposites. Of course, inorganic phasenanoparticles-can either be embedded or attached to the polymer network. Various synthetic approaches for the preparation of polymerbased hybrids are generalized and schematically presented in Fig. 12.7. The chemical nature of the interface, that is, bonding between organic and inorganic components of the hybrids, is the most important parameter that crucially determines hybrids’ properties. Having that in mind, only a synthetic pathway, that includes usage of polymers and inorganic precursor, for in situ preparation of supported inorganic nanoparticles is described. More specifically, the desired functional groups are introduced to the polymer network, and they do not just serve to link inorganic and organic components of hybrids, but also have a dual function serving, at

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Nanomaterials Synthesis

Figure 12.7 Schematic presentation of different routes for the preparation of organic
inorganic hybrid materials.

the same time, as reactants. Basically, the additional reactant/catalyst that would drive the formation of nanoparticles from chosen inorganic precursor is not necessary. Crosslinked macroporous poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate) (poly(GMA-co-EGDMA)) resin was chosen to be a model system of polymer support due to the presence of reactive epoxy groups and macroporous structure. In this particular case, the average particle size and average pore size of the poly(GMA-co-EGDMA) copolymer were around 30 μm and 130 nm, respectively [62]. In addition, due to the macroporous nature of the poly(GMA-co-EGDMA) resin, the specific surface area is reasonably large (36 m2/g) [62]. The amino functional group can easily open the epoxy ring, providing a simple pathway for attachment of desired molecules to epoxy-resin. Functionalization of the poly(GMA-co-EGDMA) copolymer is schematically presented in Fig. 12.8. In the case of multifunctional molecules, all other functional groups remain free to participate in successive synthetic steps that lead to the fabrication of organic
inorganic hybrids [31,62
64]. The amino-functionalized poly(GMA-co-EGDMA) copolymer will be formed when ammonia, or any compound with two amino-groups, is attached to the epoxy-resin by opening the epoxy ring. It is well-known

Organic
Inorganic Hybrid Nanomaterials: Synthesis, Characterization, and Application

433

Figure 12.8 Schematic presentation of functionalization of the poly(GMA-co-EGDMA) copolymer.

that the amino group has a strong reducing ability, and it has been broadly used for reduction of silver ions to metallic silver nanoparticles [65
67]. The morphological and optical properties of in situ synthesized silver nanoparticles attached on poly(GMA-co-EGDMA) copolymer functionalized with ammonia are shown in Fig. 12.9. The formation of organic
inorganic hybrid is accompanied by the appearance of a characteristic yellow color. The distinct surface plasmon resonance bands in the wavelength range 400
450 nm can be observed, indicating that silver particles attached to polymer support are nanometer sized. The morphology of composites, thoroughly investigated by transmission electron microscopy (TEM), revealed that the poly(GMA-co-EGDMA) support is decorated with many well-separated, nanometer-sized, nearly spherical silver particles (see Fig. 12.9A). The size distribution of silver particles is narrow (12.1 6 2.2 nm), and is shown in Fig. 12.9B. Thus, the optical property determined by reflection spectroscopy is in agreement with the TEM analysis of an organic
inorganic hybrid. It is important to emphasize that the morphology and content of the inorganic phase, that is, shape, size, and concentration of silver particles supported by epoxy-resin, can be tuned by proper choice of molecules used to open the epoxy ring in order to obtain various amino-functionalized poly(GMA-co-EGDMA) copolymers [62,64].

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Nanomaterials Synthesis

Figure 12.9 (A) TEM image of silver nanoparticles supported by epoxy-resin, and (B) corresponding particle size distribution. (C) Reflection spectrum with photo-image of synthesized organic-inorganic hybrid. (D) FTIR spectra of (a) poly(GMA-co-EGDMA) copolymer, (b) functionalized poly(GMA-co-EGDMA) copolymer with ammonia, and (c) and silver nanoparticles supported by functionalized poly(GMA-co-EGDMA) copolymer with ammonia.

The two-step synthetic pathway in preparation of amino-functionalized polymer support and consequent reduction of silver ions to metallic silver by amino groups can be followed by infrared spectroscopy. The FTIR spectra of poly(GMA-co-EGDMA), before and after functionalization with ammonia, as well as of polymer support decorated with silver nanoparticles are shown in Fig. 12.9D (curves A, B, and C, respectively). After functionalization of poly(GMA-co-EGDMA) copolymer with ammonia, complete disappearance of peaks that belong to the epoxy ring (vibrations centered at 845 and 907 cm21) can be observed (compare FTIR spectra a and b in Fig. 12.9D). Also, the appearance of the new vibrational band at 1575 cm21 that belongs to the N
H bending vibration, and broadband in the region 3600
3100 cm21 originating from the stretching vibrations of NaH and OaH bonds, can be clearly observed. These results indicated that the amino groups are attached to poly(GMA-co-EGDMA) support,

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Inorganic Hybrid Nanomaterials: Synthesis, Characterization, and Application

435

Figure 12.10 TEM image (A), as well as Kubelka
Munk transformation of diffuse reflection data with photo-image (B) of TiO2 nanoparticles supported by the poly (GMA-co-EGDMA), functionalized with dopamine. Taken from I. Vukoje, T. Kovaˇc, J. Dˇzunuzovi´c, E. Dˇzunuzovi´c, D. Lonˇcarevi´c, S.P. Ahrenkiel, et al., Photocatalytic ability of visible-light-responsive TiO2 nanoparticles, J. Phys. Chem. C 120 (2016) 18560
18569.

and this finding is in agreement with data concerning functionalization of the poly(GMA-co-EGDMA) with the amino group in the reaction with ethylenediamine [64] and amino acid arginine [62]. We emphasized that this ship-in-a-bottle synthetic approach has a general character and is not exclusively for in situ preparation of metallic particles attached to the polymer supports. For example, the hydrolysis of titanium(IV) isopropoxide in organic aprotic solvents in the presence of the poly(GMA-co-EGDMA) functionalized with dopamine leads to the formation of TiO2 nanoparticles attached to polymer support with extended absorption in the visible spectral range due to the ICT transitions [31]. The TEM image and Kubelka
Munk transformation of diffuse reflection data with photo-image of the obtained hybrid are shown in Fig. 12.10.

12.4 NEW SYNTHETIC APPROACHES AND CHALLENGES The wide range of potential applications of organic
inorganic hybrid nanomaterials facilitated the development of novel synthetic approaches. Here, special attention will be paid only to the description of new synthetic routes that provide formation of hybrids consisting of organic and inorganic phases strongly linked together. Also, shortcomings of the

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Nanomaterials Synthesis

existing synthetic approaches and the challenges that have to be overcome in order to obtain organic
inorganic hybrid nanomaterials with desired properties are addressed. The ICT complex formation is based, as previously mentioned, on the condensation reaction between two hydroxyl groups, the first one originating from the oxides’ surface, and, the second, from the organic molecule (see Fig. 12.2). The knowledge concerning various methods for synthesis of highly uniform metal-oxide nanoparticles with desired morphology (shape and size) by far exceeds their usage for preparation of the ICT complexes (see Table 12.1), and, consequently, understanding of the ICT complex properties. However, the expertise in nanomaterial synthesis is not mandatory for the preparation of hybrids with extended absorption in the visible spectral range since recent studies have indicated that the formation of the ICT complex can take place by dispersing commercial oxide powders in a solution of suitably chosen ligands [35,37,47,49]. Therefore, the completely simplified way for the preparation of this type of organic
inorganic hybrids opens up the possibility to a nonexpert in materials synthesis to study the properties of the ICT complexes. Bearing in mind the shortage of information in this field, it is straightforward that in the first step various combinations of oxides and organic molecules should be examined in order to determine those suitable to provide formation of the ICT complexes with desired properties. Meanwhile, on one side, the number of metal-oxides is limited, the choice of ligand molecules is practically limitless. So far, the influence of catechol and salicylic acid, as well as their derivatives on the ICT transition, have been examined in a more systematic manner [20
27,29
31,33,34,44,47
53], but studies concerning the influence of another type of ligand are at the embryonic stage [42,43]. Of course, the increase in efficient harvesting of solar light in the ICT complexes, where electron injection occurs in one step from the ground state of organic ligand to the conduction band of metal-oxide, is the main goal. Thus far, the extinction coefficients of the ICT transitions are small and estimated to be of the order of 103 cm21 L mol21 [35,37,60], which is considerably smaller than typical values for organic dyes (about 105 cm21 L mol21). Because of that, the preparation of the ICT complexes with high absorptivity in the visible spectral range remains the greatest challenge in this area. Based on their morphology, two classes of hybrids consisting of the polymer network and nanoparticles can be recognized: the first, with

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Inorganic Hybrid Nanomaterials: Synthesis, Characterization, and Application

437

nanoparticles embedded in the polymer matrix, and the second, when nanoparticles are attached to the polymer matrix and fully exposed to the surrounding media. In the first case, regardless of the method used for preparing nanocomposites (blending or in situ polymerization), different amphiphilic agents have to be utilized to modify the surface of nanoparticles and to improve the interfacial interactions necessary for successful incorporation of hydrophilic nanoparticles into hydrophobic polymer matrices [67
71]. In contrast, functionalization/activation of the polymer surface plays a key role for successful attachment of inorganic nanoparticles. Functionalization of polymers can be achieved using either physical or chemical methods. Plasma treatment can increase the hydrophilicity of polymer fibers due to the formation of new polar functional groups (hydroxyl, carbonyl, carboxyl, etc.) and improve loading of colloidal nanoparticles [72
74]. This method is suitable for large-scale, industrial preparation of natural and synthetic fibers decorated with inorganic nanoparticles which have antimicrobial and/or self-cleaning ability. While plasma treatment leads to nonselective functionalization of polymers, the chemical methods provide an opportunity to introduce desired functionality onto the polymer surface, as described in the previous section for the ship-in-a-bottle synthetic approach [31,62,64]. For example, it is common knowledge that the toxicity of silver is strongly dependent on the size and shape of the particles, oxygen availability, surface charge, as well as the type of coating. Because of that, proper choice of both the polymer and the attached molecule is the main challenge that must be overcome for the successful in situ synthesis of silver or any other inorganic nanoparticles with desired morphology onto polymer support. The homogeneous distribution of silver particles through the entire polymer is related to the distribution of amino-functional groups, that is, distribution of epoxy groups, while the size of silver particles depends on the separation and reducing capability of amino-functional groups.

12.5 POTENTIAL APPLICATION OF ORGANICaINORGANIC HYBRIDS: PHOTO-DRIVEN PROCESSES AND ANTIMICROBIAL ABILITY The main incentive to study the ICT complex formation is a possibility to use this simple methodology to tune optical properties of chemically stable nontoxic wide-band-gap oxides and to increase their photoresponse in the visible spectral range. Presently, however, the number of

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Nanomaterials Synthesis

reports concerning potential application of this type of hybrid in photodriven processes (photocatalysis and solar cells) is relatively small. On the other hand, the possibility to take advantage of the introduction of desired functional groups to inorganic particles with a large specific surface area, or to use them as carriers of biologically important molecules (vitamins, drugs, antioxidants, etc.) is at the embryonic stage [41,75]. Because of that, we will focus on the ability of this type of organic
inorganic hybrid to harvest a large portion of solar light and serve as an absorber in photovoltaic cells [39,44
46], as well as photocatalyst for hydrogen production in a water-splitting reaction and degradation of various organic polluters [27,29,31,33,48,51]. An important and instructive example is the study carried out by Fujisawa and Nagata, reported in Reference [44]. These authors prepared the photovoltaic cell based on the ICT complex between TiO2 and 2anthroic acid and compared its performance with the blank TiO2 cell. The absorption spectrum of the thin nanoporous TiO2 film before and after immersion in the 2-anthroic acid solution is shown in Fig. 12.11A. Clearly, a red absorption shift can be observed due to the ICT complex formation between TiO2 and 2-anthroic acid. The comparison of the J
V curves of the photovoltaic cell based on the ICT complex between TiO2 and 2-anthroic acid with the blank TiO2 photovoltaic cell is presented in Fig. 12.11B. From the J
V curves, the power conversion

Figure 12.11 (A) UV-vis absorption spectra of 2-anthroic acid solution and thin nanoporous TiO2 films before and after immersion in the 2-anthroic acid solution. (B) J
V curves of the photovoltaic cells based on TiO2 and 2-anthroic acid (solid curve) and TiO2 (blank, dashed curve) under simulated solar illumination (AM1.5 G, 100 mW/cm2). Taken from J. Fujisawa, M. Nagata, Efficient light-to-current conversion by organic
inorganic interfacial charge-transfer transitions in TiO2 chemically adsorbed with 2-anthroic acid, Chem. Phys. Lett. 619 (2015) 180
184.

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efficiency under simulated solar illumination for the photovoltaic cell based on the ICT complex was found to be 2.2%, while for the untreated TiO2 photocatalytic cell it was 0.17%. Also, the maximal incident photon-to-current efficiency (IPCE) value of 86% was found in the visible spectral range at 440 nm. This result indicates that efficient light-tocurrent conversion is possible for ICT transitions. The photo-oxidative ability of surface-modified wide-band-gap oxides was studied using degradation of organic dyes (TiO2 [27,29,31,33], Mg2TiO4 [48] and Al2O3 [51]), while hydrogen production was used to demonstrate the ability of surface-modified TiO2 particles to induce photoreduction processes [29,31]. The energy conversion efficiency from solar to hydrogen by TiO2 water-splitting is still low, although the photocatalytic mechanism with pristine TiO2 is well understood [9
12]. However, there is a lack of information concerning the photocatalytic performance of surface-modified TiO2 particles. Thus far, only Higashimoto et al. [29] tried to correlate the electronic structure of the ICT complexes and their photocatalytic activity, and, in addition, it was shown that the steady-state photocatalytic hydrogen production rate over a organic
inorganic hybrid consisting of surface-modified TiO2 nanoparticles with dopamine, attached to polymer support, is two times larger compared to a commercial TiO2 photocatalyst (Degussa P25) [31]. The best example for enhanced photocatalytic ability due to the ICT complex formation is photocatalytic oxidation of organic dye methylene blue over mesoporous Al2O3 particles modified with 5-aminosalicylic acid [51]. It has already been mentioned that pristine Al2O3 is a nonabsorbing material, that is, an insulator with large band-gap of about 8.7 eV [55]. However, when mesoporous Al2O3 particles are coated with the organic moiety (either catecholate- or salicylate-type ligands) the obtained hybrids absorb in the visible spectral range (see Fig. 12.3). The photo-degradation kinetic data for different initial concentrations of methylene blue (MB) are presented in Fig. 12.12. Based on kinetic measurements, some general features can be readily discerned. First, surface-modified Al2O3 powder with 5-aminosalicylic acid has the ability and complete decolorization of MB was observed after 3 h of illumination. Second, unmodified Al2O3 powder, as expected, does not display photocatalytic activity (see inset in Fig. 12.12). Third, comparison of the photocatalytic performance of an organic
inorganic hybrid and the most studied commercial Degussa P25 TiO2 photocatalyst was performed either under illumination that simulates solar light or under

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Figure 12.12 Kinetics of photocatalytic degradation of MB (initial concentrations of MB: 1, 2, and 5 ppm) under simulated sunlight over surface-modified Al2O3 powder with 5-aminosalicylic acid (0.5 mg/mL). Inset: comparison of photocatalytic performances of unmodified and surface-modified Al2O3 with 5-aminosalicylic acid, as well as TiO2 (Degussa P25) under identical experimental conditions (2 ppm MB and 0.5 mg/mL of photocatalyst) under simulated sunlight and with reduced UV light by optical filter based on the NaNO2 solution. Taken from V. Ðordevi´c, J. Dostani´c, D. ˇ Lonˇcarevi´c, S.P. Ahrenkiel, D.N. Sredojevi´c, N. Svraki´ c, et al., Hybrid visible-light responsive Al2O3 particles, Chem. Phys. Lett. 685 (2017) 416
421.

visible light illumination conditions (inset to Fig. 12.12). The optical filter, based on the NaNO2 solution, was used to reduce the UV part of the emitted light from the light source. The obtained results clearly indicate that the photocatalytic performance of modified Al2O3 powder with 5-aminosalicylic acid powder is almost the same after reducing UV light. On the other hand, the photocatalytic performance of Degussa P25 is significantly diminished when the NaNO2 optical cut-off filter was used. Also, it was established that red-shifted TiO2 [27,31] and Mg2TiO4 [48] powders are able to induce photo-oxidation of organic dyes under crystal violet and methylene blue under visible light illumination, that is, without the use of high-energy photons for excitation of organic
inorganic hybrid photocatalysts. While photocatalytic properties of surface-modified wide-band-gap oxides have been studied in recent years, silver and silver compounds have been known for decades as powerful biocides. Soluble silver compounds, despite their excellent antimicrobial activity, are not suitable for the application due to uncontrolled reduction processes when exposed to

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441

air and light. On the other hand, the development of a variety of synthetic procedures that provide the desired morphology and surface properties of silver nanoparticles enabled their efficient application as an antimicrobial agent [76
78]. For many applications, it is more suitable to use immobilized instead of free-standing silver particles. So far, silver nanoparticles have been either embedded or attached to various inorganic and organic supports, such as zeolite [79], silica or fiberglass [80], hydroxyapatite [53], natural macroporous materials [81], paper [82], and various polymers [83
85] including textile fibers [86
92]. However, the mechanism of toxic action of silver and silver compounds is still not fully understood, since it is difficult to discriminate the contribution of cooccurring free silver ions and silver containing solid particulates [93
97]. Some studies suggest that both silver nanoparticles and silver ions contribute to the antibacterial activity and toxicity, although their apparent relative importance varies considerably depending on the system under study [85
87,91]. Other studies showed that dissolved silver ions are responsible for most, if not all, silver nanoparticle toxicity, indicating that metallic silver nanoparticles mostly serve as a source of silver ions [98
100]. At least, it is now generally accepted that the antimicrobial activity of silver is highly influenced by the size of the particles-the smaller the particles, the higher the antimicrobial efficiency. Of course, released ionic silver retains its cytotoxicity and ecotoxicity even at concentrations as low as 1 mg/L, and proper adjustment of release rate (not too high—not too low) is necessary for the usage of silver nanoparticles as disinfection agents in various practical applications such as filters for wastewater, cosmetics, medical supplies, etc. The in situ synthesis of silver nanoparticles supported by aminofunctionalized poly(GMA-co-EGDMA) copolymer provides the opportunity to control the content and morphology of inorganic phase. The organic
inorganic hybrid, consisting of nanometer-sized silver particles (,10 nm) attached to the arginine-functionalized epoxy resin, is chosen to demonstrate the antimicrobial action of this type of material since this particular hybrid has the low content of inorganic phase (1.0 wt.% Ag) [62]. The percentage of microbial reduction (R, %) can be determined as follows: R 5 100 3 ðC0
C Þ=C0 where C0 (CFU - colony-forming units) is the number of microbial colonies in the control sample (copolymer without silver nanoparticles), and C

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Nanomaterials Synthesis

(CFU) is the number of microbial colonies in the organic
inorganic hybrid. The concentration- and time-dependent percentages of microbial reduction against E. coli, S. aureus, and C. albicans are shown in Fig. 12.13 (A and B, respectively). Data concerning concentration-dependent

Figure 12.13 The percentage of microbial reduction (R, %) against E. coli, S. aureus, and C. albicans as a function of the concentration of hybrid (A) and contact time between microbial cells and hybrid (B). Organic
inorganic hybrid consists of Ag nanoparticles supported by the poly(GMA-co-EGDMA) copolymer functionalized with arginine; contact time between microbial cells and hybrid was 1 h (A); concentration of hybrid was 0.1 mg/mL (B). Taken from I.D. Vukoje, E.S. Dˇzunuzovi´c, D.R. Lonˇcarevi´c, S. Dimitrijevi´c, S.P. Ahrenkiel, J.M. Nedeljkovi´c, Synthesis, characterization, and antimicrobial activity of silver nanoparticles on poly(GMA-co-EGDMA) polymer support, Polym. Composite 38 (2017) 1206
1214.

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antimicrobial efficiency (the contact time between microbial cells and hybrid was 1 h) indicated that microbial reduction varies in ascending order from C. albicans to S. aureus to E. coli in the entire concentration range (from 50 to 2000 μg/mL of hybrid, i.e., 0.5
20.0 μg/mL of silver). It should be noted that differences in antimicrobial activity against various microbial strains are more pronounced at the lower concentrations of hybrid. The time-dependent experiments indicated that after 24 h of contact, the concentration of 100 μg/mL of hybrid ensured a 100% reduction of both types of bacterial cells (Gram-negative E. coli and Grampositive S. aureus), while the reduction of fungi cells reached a satisfactory level of 96.8%. Obviously, different biological responses of various microbial species exposed to epoxy resin decorated with silver nanoparticles can be noticed. Reduction of E. coli and S. aureus cells reached almost 100% after 4 h of contact. On the other hand, a similar level of reduction of C. albicans was achieved after 24 h of contact. In order to mimic devices for wastewater treatment, the most sensitive bacterial species (E. coli) was exposed to a silver-containing hybrid in a homemade flow setup. Preliminary experiments indicated the high percentage of the E. coli cell reduction (99.3%), while the concentration of released silver ions (0.8 ppm) was below the cytotoxicity and ecotoxicity level of 1 ppm. To briefly conclude, the advantage of using this type of organic
inorganic hybrid as a disinfectant agent lies in the fact that polymer support cannot be dissolved in water, and after complete dissolution of silver, its further handling is harmless to the environment.

12.6 SUMMARY AND OUTLOOK The key factor to create organic
inorganic hybrid nanomaterials with desired properties is to understand their interface chemistry. Current knowledge concerning surface-modified wide-band-gap metal-oxides is still at the fundamental level, while, in the case of inorganic nanoparticles supported with functionalized epoxy-resins, investigations are directed to their potential applications. The fundamental behavior of photoresponsive ligand-bound wide-band-gap oxides is not fully understood, and investigation of potential applications based on photo-driven processes (solar cells, degradation of organic polluters, water-splitting reaction) is at the embryonic stage. Studies concerning other potential applications of surface-modified wide-band-gap metal-oxides, such as selective adsorption of ionic species from wastewater and usage as carriers for humanly

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consumed compounds, are practically nonexistent. On the other hand, the antimicrobial ability of silver has been known for decades. The main challenge in this field on the fundamental level is to understand the toxic mechanism of metallic silver and/or silver ions, while from a technological point of view the main focus is the development of a synthetic procedure that should provide the desired rate of oxidation of metallic silver and consequent controlled release of the silver ions into surrounding media. Knowing that many issues are still being debated, the general interest in organic
inorganic hybrids is expected to grow in the coming years.

ACKNOWLEDGMENTS Financial support for this study was provided by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project III 45020).

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4275.

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CHAPTER 13

Fabrication, Characterization, and Optimization of MnxOy Nanofibers for Improved Supercapacitive Properties Jai Bhagwan1, , Nagesh Kumar1, and Yogesh Sharma1,2 1 Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee, India Department of Physics, Indian Institute of Technology Roorkee, Roorkee, India

2

Contents 13.1 Introduction 13.2 Synthesis of 1D Nanofibers 13.2.1 Drawing Method 13.2.2 Template Method 13.2.3 Phase Separation 13.2.4 Self-Assembly 13.2.5 Electrospinning 13.3 Utilization of Binary MnxOy Nanofibers for Energy-Storage Applications 13.4 Future Aspects, Challenges, and Summary Acknowledgments References

451 454 454 455 456 456 457 469 476 478 478

13.1 INTRODUCTION The world of nanomaterials comprises a wide range of intriguing materials with unique characteristics, and outstanding physical and chemical properties. On the basis of their physical dimensions, these nanomaterials have been classified mainly into four categories, namely: zero-dimensional (0D) such as nanoparticles or quantum dots; one-dimensional (1D) such as nanowires, nanorods, nanofibers, and nanotubes; two-dimensional (2D) such as nanosheets, nanoribbons, graphene; and three-dimensional (3D) such as bundles/array of nanowires, and nanotubes as well as 

These authors contributed equally.

Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00013-4

© 2019 Elsevier Inc. All rights reserved.

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multinanolayers [1
5]. One-dimensional nanofibers of transition metal oxides stand out among the rest of the nanomaterials and have gained tremendous attention in nanotechnology research owing to their unique optical, structural, and electronic properties and exhibit potential technological applications in several fields such as biomedical [6,7], sensors [8,9], filtration [10,11], tissue engineering [12], scaffold constructions [13], wound dressings [14], catalysts and enzyme carriers [15,16], protective clothing [17], cosmetics [18], electronic/semiconductive materials [19,20], and energy-storage devices [21
23]. Nanofibers of a variety of materials, such as synthetic polymers, natural polymers, semiconducting nanomaterials, carbon-based nanomaterials, and composite nanomaterials have been prepared in the last decade. Recently, mesoporous 1D high aspect ratio nanofibers have gained huge consideration from energy researchers to be used as a potential electrode material for energy-storage devices (batteries, supercapacitors, fuel cells). In particular, supercapacitors have increasingly attracted intense research attention due to their fast charge/discharge time (few seconds), high power density ( . 10 kW/kg), excellent reversibility, and long cycle lifetime ( . 105 cycles) [24
27]. Moreover, supercapacitors can complement or even replace batteries/fuel cells in certain electrical energystorage and harvesting applications, where faster high power delivery or high power energy storage is required. The unique charge-storage mechanism of supercapacitors enables them to store and deliver a large amount of charge in a short time period and hence deliver higher power than batteries. However, these can be fully discharged in seconds, and as a consequence exhibit lower energy density (5
10 W/h/kg) than batteries. This is the main drawback which hinders the potential commercial utilization of supercapacitors [28]. On the basis of a charge-storage mechanism, supercapacitors have been classified into two main categories, namely (1) electrochemical double-layer capacitors (EDLC), where capacitance arises from the pure electrostatic charge accumulation at the electrode
electrolyte interface, for example, carbon material-based supercapacitors, (2) pseudocapacitors or Faradic supercapacitors (FSCs) in which application of a potential (V) induces faradaic current from fast and reversible oxidation/reduction (redox) reactions on the surface of the material, for example, metal oxides and conductive polymer-based SCs [29]. Generally, in supercapacitors both of the two charge-storage mechanisms occur simultaneously, but at a particular moment one of the mechanisms occupies the leading position and the other is relatively weak. In

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comparison to EDLC, FSCs show higher specific capacitance (Cs) and hence better energy density (E 5 1/2CV2) but poor long-term cyclability. In view of this, recently, FSCs have gained considerable attention, and continuous research efforts are being made by researchers worldwide to improve the energy density and cyclability of FSCs while lowering the cost of existing devices. In FSCs, among metal oxides, manganese-based oxides (spinel-Mn3O4) are of special interest owing to their lower synthesis cost and structural benefits (tetrahedral and octahedral voids). The foreign electrolytic ions may intercalate inside these voids and thereby improved energy-storage performance is obtained. However, poor cyclability, poor rate capability, and low energy density are issues still associated with spinel-Mn3O4. It has been well documented that the electrochemical performance of a material depends not only on high surface area but also some other parameters such as the pore size distribution (mesopores) of the material, pore shape and structure, accessibility of the electrolytic ions, and electrical conductivity. High surface area and porosity affect the specific capacitance and rate capability. High electronic conductivity affects the rate capability and power density. Furthermore, desirable electroactive sites in material enable pseudocapacitance performance. High thermal stability and chemical stability play a crucial role in cyclic stability. All these properties of a material depend on its morphology. In view of this, 1D nanofibers with high surface area and porosity enable the high specific capacitance. Optimized nanofibers would enable the reduced diffusion path of ions and increase the contact area of active material to the aqueous electrolyte. This will lead to increased mobility of foreign ions and a reduction in the charge transfer resistance at the electrode
electrolyte interface and hence improved supercapacitive properties of the electrode material may be obtained. These voids/gaps in nanofibers (structural as well as morphological) act as intercalation/deintercalation sites for extra storage performance, and also work as buffering space to accommodate stress/strain produced with long-term cyclings. Therefore mass production of high surface area, mesoporous nanofibers with high reproducibility is highly desirable. In this chapter, we discuss how nanotechnology can be employed for material engineering by creating a specific morphology in which high specific surface area, single dimension, and good porosity are obtained. Such material engineering not only improves the charge-storage performance (specific capacitance) of the material but also the long-term cyclability and rate capability. However, the difficulty in mass production of

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nanostructure/nano-phase, poor control over size and morphology, poor reproducibility, and prohibitively high fabrication cost due to involvement of sophisticated fabrication techniques are some of the hindrances, which prevent the full implementation and utilization of nanotechnology. There are a number of synthesis techniques commonly used to fabricate nanofibers, such as the drawing method [30], template method [31], phase separation [32], self-assembly [33], and electrospinning [34
36]. Among these, electrospinning is found to be a simple, versatile, cost-effective, and unique technique to produce 1D, porous, and high aspect ratio nanofibers with a good degree of reproducibility. This nonmechanical and electrostatic technique involves the use of a high-voltage electrostatic field to charge the surface of a polymer solution droplet, which induces the ejection of a liquid jet through a spinneret and provides exceptionally long nanofibers with uniform diameter and a solid and hollow interior. It is also advantageous to control the nanofiber composition to achieve the desired property or functionality, which offers more flexibility in surface functionalities. Furthermore, by controlling various parameters, such as process parameters and system parameters, the diameter of nanofibers can be tuned. Here, we also discuss the supercapacitive performance of different nano-morphologies of manganese-based materials fabricated by the electrospinning process.

13.2 SYNTHESIS OF 1D NANOFIBERS One-dimensional nanofibers of metal oxides are subjected to research due to their potential applications in several fields. Over the last decade, various novel techniques have been reexplored to generate 1D nanofibers. There are a few very common nanofiber synthesis methods: the drawing method, template method, phase separation, and electrospinning method. A brief description of each of these methods is given below.

13.2.1 Drawing Method Here, a drawing process is usually used to produce nanofibers with high aspect ratio (aspect ratio . 20) [37]. In this method, a micropipette is used to pull out a nanofiber from the surface of a polymer solution, as shown in Fig. 13.1. The drawing is accompanied by evaporation of the solvent, leading to solidification of the fiber and the length of drawn

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Figure 13.1 Illustration of the basic production process of nanofibers by drawing from a droplet of a polymer.

nanofiber is of the order of microns. The yield of this method is quite small as only one fiber can be made at a time. However, the drawing method offers increased flexibility in the control of key parameters of drawing, for example, waiting time before drawing, that is, viscosity, drawing speed, and thus repeatability, and controlled dimensions of the fibers can be obtained. This process permitted the formation of thin suspended nanofibers connecting droplet-shaped dots on the substrate. This technique has been utilized to fabricate polystyrene nanofibers having diameters ranging from 10 nm to numerous microns in highly ordered patterns. However, this method is not suitable to produce metal oxide nanofibers when required.

13.2.2 Template Method In this method, templates of porous anodized aluminum oxide (AAO) membranes are used to fabricate the nanofibers [38]. The dimensions of the nanofiber depend on the size of the pores of the template. Alumina network templates having pore diameters of 25
400 nm with pore depths ranging from 100 nm to several 100 μm can be synthesized easily by an electrochemical method [39]. AAO templates are fully infiltrated with homogeneous solution and the nanofibers are obtained when the polymer is extruded through the nanopores of the template, as shown in Fig. 13.2. The length of nanofibers fabricated from alumina templates can be tuned by changing the various parameters, including melting time and temperature. The nanofibers containing a linear mesocages array represent an interesting platform for different types of applications, such as sensor technology, drug delivery,

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Figure 13.2 One-dimensional nanofiber from template method.

and waveguiding devices. Although nanofibers of any shape and size can be fabricated by this method, the synthesis of template and reproducibility are major drawbacks. Furthermore, it cannot produce continuous nanofibers at one time. In addition, the template synthesis is expensive too.

13.2.3 Phase Separation In this procedure, polymer is mixed with the solvent before undergoing gelation. First, one phase, such as solvent, was extracted from the solution through a filter and the rest then transferred to a freezer at 220°C and kept for 2
6 h [40]. In this method, we get a matrix form of nanofibers instead of a single fiber. The nanofibric morphology obtained from this method can be tuned by changing different parameters including the polymer concentration and gelatinization temperature. Fig. 13.3 represents a phase separation procedure in which a suitable solvent is added to a polymer to form a porous nanofibric network that contains fibers of diameter ranging from 50 to 500 nm. Nanofibric networks with porosities up to 98.5% have been synthesized using polymers such as poly-lactic-coglycolic acid, poly-L-lactide acid, and poly-DL-lactic acid. Although this method can be utilized to form nanofibers in a large quantity, reproducibility and dimensional control of fibers are some of its drawbacks.

13.2.4 Self-Assembly The main mechanism for a generic self-assembly is the intermolecular forces that bring the smaller units together. The shape of the smaller units of molecules determines the overall shape of the macromolecular nanofiber. Molecular self-assembly provides an innovative way to design and

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Figure 13.3 Schematic representation of the phase separation method to fabricate nanofiber. Nanofibric morphology adapted from P.X. Ma, R. Zhang, Synthetic nanoscale fibrous extracellular matrix, J. Biomed. Mater. Res. 46 (1999) 60
72 with permission. Copyright John Wiley 1999.

produce novel materials at multiple levels (nano
micro
macro). It has been reported that the self-assembly process of peptide nanofibers involves various driving forces (e.g., hydrophobic interactions, electrostatic force, hydrogen bonding, π
π interactions, van der Waals force, etc.) [41]. However different conditions such as pH, ionic strength, and assembling rate can also affect the self-assembly process [42].

13.2.5 Electrospinning Electrospinning is a simple, versatile, adaptable, and fast-emerging technique to fabricate 1D morphology. Nanofibers from the electrospinning process can be fabricated in both academic research as well as industrial areas owing to their low cost and high yield. A comparison of different methods to fabricate nanofibers is given in Table 13.1. As can be seen from the table, electrospinning is a versatile technique, which enables nanofibers to have all the required important properties. Electrospinning with good reproducibility involves uniaxial stretching of viscoelastic solution by applying an electric field that results in nonwoven nanofiber layers. Although the electrospinning technique seems simple, fabricating aligned, size-tunable 1D nanofibers is still a challenging task. However, by changing the system parameters (molecular weight of polymer, viscosity,

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Table 13.1 Compression of different methods to fabricate nanofibers Process

Technological advantage

Can the process be scaled

Repeatability

Convenient to process

Control on fiber dimension

Drawing Template synthesis Phase separation Electrospinning

Laboratory Laboratory

X X

O O

O O

X O

Laboratory

X

O

O

X

Laboratory (with potential for industrial processing)

O

O

O

O

conductivity, dielectric constant, and surface tension) and/or process parameters (flow rate, electric potential, concentration of polymer solution, and distance between the needle and collector) as well as ambient parameters (humidity, temperature, and velocity of air in the chamber) the abovementioned properties of nanofibers can be achieved [43]. The controlled, elongated, high aspect ratio with extremely small diameter (#50 nm) fiber might bring a change in the behavior of various nanofiber systems including energy-storage devices [44,45]. With electrospinning being the most useful technique, in this chapter we have focused only on the electrospun Mn-based nanofibers and investigated their energy-storage properties. The construction of electrospinning setup and working principle of electrospinning techniques are discussed in the next section in detail. 13.2.5.1 Setup and Working Principle Fig. 13.4 shows a schematic diagram of the electrospinning setup. It consists of three major parts: (1) syringe pump, (2) high-voltage source, and (3) collector. In this system, a hypodermic syringe filled with metal precursor and appropriate polymer solution to be electrospun is loaded along with the syringe pump, and a stainless steel spinneret is connected with it. The separation between the syringe and collector is kept fixed. With the help of a syringe pump, solution is pumped through the syringe at a constant flow rate. When a high electric field is applied between the spinneret and collector (covered with aluminum foil), the pendant drop at the tip of the spinneret becomes charged and experiences two types of electrostatic forces: electrostatic repulsion between the surface charges distributed over the

Fabrication, Characterization, and Optimization

459

Figure 13.4 Schematic diagram of an electrospinning setup.

polymer solution and a columbic force of attraction between the collector and pendant drop. With the action of these electrostatic forces, the pendant drop converts into a conical shape called a Taylor cone with a half angle of 49.3 degrees. As the electric field increases and attains critical value, these forces overcome the surface tension of the solution and an electrified jet comes out from the tip of the pendant drop, which subsequently exhibits bending instabilities caused by repulsive forces between the charges carried with the jet. The jet extends through spiraling loops, that is, as the loops increase in diameter the jet grows longer and thinner until it solidifies or collects on the target. As discussed Section 13.2.5, different process and system parameters influence the electrospinning process to yield nanofibers from polymer mixed-metal precursor solutions, and

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Figure 13.5 Schematic illustratution of: (A) splaying of the fibers and (B) a single fiber that contained bending instability.

several studies have focused on analyzing the effects of these parameters. Son et al. investigated the effect of viscosity and water content on chitosan/polyvinyl alcohol (PVA) nanofiber synthesis [46]. Park et al. optimized the electrospinning conditions for preparation of nanofibers from polyvinylacetate (PVAc) in ethanol solvent [47]. Although the setup of electrospinning seems to be simple, the spinning mechanism is rather complicated. Before 1999, the formation of fiber by an electrospinning process was often ascribed to the splitting or splaying of an electrified jet as a result of repulsion of the surface charge (Fig. 13.5A) [29]. However, later, Ranker et al. demonstrated that thinning of the jet during electrospinning is mainly caused by the bending instability associated with the electrified jet [48]. It has also been observed that the jet is initially a straight line which then becomes unstable toward the collector. It seems that the cone-shaped instability region is composed of multiple jets. However, closer examination using high-speed photography established that the conical envelope contains only a single, rapidly bending or whipping thread (Fig. 13.5B). Thus thinning as well as elongation of the nanofiber strongly depend upon the bending instability, which leads to the formation of a series of spiraling loops with increasing diameters. As the loops increase in diameter, the jet grows longer, and thinner nanofibers are collected on the collector. The electrospun jets usually bend owing to mutual repulsive forces between the electric charges transported by the jets. To explain the mechanism of bending instability, Reneker et al. reported some great findings through the modeling of a jet instability mechanism [48]. According to them, the jet of the solution to be electrospun is accelerated in the direction of the collector screen by electrostatic forces. These forces give a longitudinal stress to stabilize the jet of the

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461

electrospun solution, keeping it initially straight. After covering some distance from the initial point, the jet started to experience stress relaxation at some midway point and the position of this point depends on the applied electric field intensity. By increasing the value of the electric field intensity, the length of the stable jet can be increased. Once stress relaxation begins, the electrostatic interaction between the charged elements of the jet starts to govern the ensuing motion, initiating and perpetuating the chaotic movement of the jet. A mathematical model has been projected to explain the chaotic motion of the jet of the electrospun solution, which has been referred to as the “bending instability” or “whipping” of the jet [48]. The bending instabilities in jets have been modeled by generating a system having viscoelastic dumbbells connected to each other. Two dumbbells “A” and “B” having appropriate mass and charge are shown in Fig. 13.6. These dumbbells are positioned in the electrical field created between the target and pendant droplet by the imposed potential difference, thereby both experience a Columbic force between each other. This viscoelastic jet is modeled by Maxwellian springs [48]. The stress, σ, pulling B back to A is given by Eq. (13.1) dσ dl σ 5G 2G dt ldt μ

(13.1)

Figure 13.6 Viscoelastic dumbbells representing a segment of the rectilinear part of the jet. From D.H. Reneker, A.L. Yarin, H. Fong, S. Koombhongse, Bending instability of electrically charged liquid jets of polymer solutions in electrospinning, J. Appl. Phys. 87 (2000) 4531
4547 with permission. Copyright AIP, 2000.

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where l is the filament length, t represents the time (s), G is the elastic modulus, and μ denotes the Newtonian viscosity. The length l is taken to be larger than the radius of filament. The spinneret is represented by a single massless point of charge qo fixed at x 5 0, which we call the nozzle bead. The momentum balance for bead B is given by Eq. (13.2): m

dv e2 eV o 52 2 2 1 πa2 σ dt l h

(13.2)

where m and e represent the mass and charge on the bead, respectively. Vo is the electric potential that is applied between the spinneret and the conducting collector, h is the distance of the collector from the injection point, and a is the cross-sectional radius of the filament which does not change for a small perturbation, and v is the velocity of bead B which satisfies Eq. (13.3): dl 52v dt

(13.3)

In addition to the viscoelastic forces, electrostatic forces, surface tension, air friction, and gravity have also been considered. The compressive stress along the jet axis of the air drag can be deserted in comparison to the stretching owing to gravity and electrical forces. For better understanding, the Earnshaw instability applicable to the electrospinning process is considered in which three charges points such as A, B, and C, originally in a straight line, have been considered by Reneker and coworkers as shown in Fig. 13.7 [48]. Two Coulomb forces having magnitudes F 5 e2 =r 2 are pushed against charge B from opposite directions. If a perturbation causes point B to move off the line by a distance δ to 0 B , the net force acted on charge B in a direction perpendicular to the line, and tended to cause B to move further in the direction of the perturbation away from the line between the fixed charges, A and C. 2e2

F1 5 2Fcosθ 5 3 δ (13.4) r The growth of a small bending perturbation, characterized by δ, is governed by the linear approximation as given below: m

dδ2 e2 5 2 δ l13 dt 2

(13.5)

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Figure 13.7 Schematic presentation of the Earnshaw instability leads to bending of an electrified jet. From D.H. Reneker, A.L. Yarin, H. Fong, S. Koombhongse, Bending instability of electrically charged liquid jets of polymer solutions in electrospinning, J. Appl. Phys. 87 (2000) 4531
4547 with permission. Copyright AIP, 2000.

where m is the mass and l1 is the distance between charges A and B (Fig. 13.7). Eq. (13.6) represents the solution to this equation: " 1=2 # 2e2 δ 5 δo exp t (13.6) ml13 The above expression shows that the small perturbation increases exponentially. The increment is sustained because the electrostatic potential energy of the system decreases by e2 =r 2 as the perturbations grow. This mechanism is believed to be responsible for the observed bending instability of jets in electrospinning and is widely accepted in general. 13.2.5.2 Effects of System/Process Parameters on the Nanofibric Morphology 13.2.5.2.1 Effect of Applied Voltage The applied voltage controls the fiber formation during the electrospinning process. As the voltage is increased, the tip of the pendant drop

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converts its shape into a conical shape, which is called a Taylor cone. However, as the voltage is increased the volume of the pendant drop starts to decrease until the Taylor cone is formed. Furthermore, the fiber diameter decreases with increasing voltage up to the desired level of significance [49]. However, the affect of the applied voltages on the diameter of electrospun fibers is a little controversial. For example, Reneker and Chun [50] have demonstrated that there is not much of an effect of the electric field on the diameter of electrospun polyethylene oxide nanofibers. 13.2.5.2.2 Effect of Solution Flow Rate The rate of ejection of the electrospun solution from the tip of the needle has a significant effect on the smooth fabrication of nanofibers via the electrospinning process. Such an effect of the flow rate of the electrospun solution is mainly attributed to the formation of the Taylor’s cone [51]. Furthermore, if the flow rate is high, beaded fibers will form owing to the incomplete stretching forces and short drying time prior to reaching the collector surface. 13.2.5.2.3 Effect of Needle Diameter The inner diameter of the needle plays an important role in the fabrication of as-spun nanofibers. In Fig. 13.8A, the inner diameter of the needle is 0.603 mm and most of the fibers have collapsed with each other due to incomplete solidification of the nanofiber. However, for needle diameters of 0.337 and 0.260 mm, individual fibers can be obtained as shown in Fig. 13.8B and C, respectively. In Fig. 13.8C, the fabrication of nanofibers having different diameters can be ascribed to the inhomogeneous ejection of the solution from the needle. Therefore for a specific flow rate of the solution, optimization of the inner diameter of the needle is an important feature. 13.2.5.2.4 Effect of Distance Between Needle and Collector The strength of the applied electric field during formation of nanofibers is a crucial factor. Keeping all other parameters invariant, the effect of the electric field intensity on the diameter of nanofibers is shown in Fig. 13.9. As the electric field intensity is decreased (by increasing the distance between the tip of the needle and the collector), the diameter of nanofibers increases due to a reduced interaction between a charged jet with an electric field, and as a result less bending instability occurs. The electric

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Figure 13.8 As-spun nanofibers using (A) needle of inner diameter 0.603 mm, (B) needle of inner diameter 0.337, and (C) needle of inner diameter 0.26 mm.

Figure 13.9 FESEM images of as-spun nanofibers with distances between the collector and needle of (A) 7 cm, (B) 13 cm, and (C) 19 cm.

field intensity can be changed in two ways: changing the applied voltage and changing the distance between the needle and collector. However, it has been proven that changing the electric field intensity by varying applied voltage does not influence the diameter of the nanofiber

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significantly. Nevertheless, if the electric field intensity is changed just by varying the distance between the collector and needle, the diameter of the nanofibers changes considerably. When the distance between the collector and needle is kept to 7, 13, and 19 cm the average diameters of the obtained nanofibers are found to be 237, 444, and 488 nm, respectively. 13.2.5.2.5 Effect of Polymer Concentration It has been observed that the diameter of the nanofiber increases as the concentration of the polymer in the precursor is increased. The higher concentration of the polymer causes higher viscosity, resulting in a higher viscous force that opposes the stretching of fiber. Furthermore, on increasing polymer concentration, the splitting degree of the charged jet becomes a little slower, which leads to increase the diameter of the fiber [52]. FESEM micrographs in Fig. 13.10 represent the variation of diameter of nanofiber with polymer concentration. It reveals that for polyvinylpyrolidone (PVP) concentrations 5%, 7%, and 10% the diameters of nanofiber are 205, 285, and 517 nm, respectively. It has been noted that for a lower concentration of PVP, fiber fuses on the collector due to an excess amount of solvent, while at higher concentration the ejection of the solution through the needle becomes difficult. Therefore an optimum

Figure 13.10 For polymer concentrations of (A) 5%, (B) 7%, and (C) 10%, the diameter of the fiber is 205, 285, and 517 nm, respectively.

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concentration of PVP is required for the formation of smooth, long, and continuous nanofibers. 13.2.5.2.6 Effect of the Molecular Weight of Polymer Used The viscosity of precursor solution is directly proportional to the molecular weight of the polymer used and the concentration of the solution. It is a crucial factor that greatly influences the morphology of as-spun nanofiber. Polymer of higher molecular weight possesses a longer polymer chain with more entanglement, which provides higher viscosity to the precursor solution and prevents the breakage of the fibers to obtain an aligned, high aspect ratio, nanofiber of required characteristic. The solvent molecules distribute over the entangled polymer molecules and lead to the formation of a long nanofiber without beads in it. However, on using polymer of lower molecular weight, such as 40,000, with an 18% concentration solution of PVP, the formation of several beads in the as-spun nanofiber occurs (Fig. 13.11A). Moreover, if the concentration of polymer solution is increased from 18% to 25%, only a few beads appear in the as-spun nanofiber (Fig. 13.11B). Furthermore, if polymer (PVP) of higher molecular weight, such as 360,000 is used, the smooth fiber can be obtained even at lower polymer concentration (7%) (Fig. 13.11C). Therefore on using PVP of higher molecular weight, smooth fiber can be obtained

Figure 13.11 Nanofibric morphology (A) with molecular weight (Mw) 5 40,000 and 18% PVP solution, (B) with molecular weight (Mw) 5 40,000 and 25% PVP solution, and (C) with Mw 5 360,000 and 7% PVP solution.

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using a lower polymer concentration, whereas in the case of lower molecular weight of the polymer, a higher concentration solution of polymer is required to get smooth and long continuous nanofiber. 13.2.5.2.7 Effect of Surface Tension The surface tension of a precursor solution is an important parameter in the electrospinning process. In the absence of an applied electric field the shape of the drop of the precursor solution at the tip of the needle is controlled by surface tension. The applied electric field induces a charge at the surface of the drop and deforms it into a conical shape (Taylor cone). A minimum value of electric field, known as the threshold voltage, is required to exceed the surface tension of the precursor solution to pull the fluid out from the apex of the cone in the form of a jet. The precursor solutions having smaller surface tension possess a smaller threshold voltage. The surface tension effect becomes prominent with reducing polymer concentration or solution viscosity and the formation of a beaded fiber occurs. Generally, precursor solutions with low surface tension and high viscosity offer low resistance to forming continuous orderly fibers. Yang and Wang investigated the effect of surface tension on the nanofibric morphology of electrospun nanofibers by preparing PVP solution in different solvents such as ethanol, N,N-dimethylformamide (DMF), and dichloromethane (MC) [53]. They observed that different solvents may contribute different surface tensions. With the concentration fixed, reducing the surface tension of the solution, beaded fibers can be converted into smooth fibers. 13.2.5.2.8 Effect of Conductivity/Surface Charge Density The conductivity of precursor solution plays a vital role in the electrospinning process. Generally, electric conductivity of solvents is very low (typically between 1023 and 1029 ohm21 m21) as they contain very few free ions, if any, which are responsible for electric conductivity of the solution [54]. The electrical conductivity of a precursor solution is a representative of the charge density on the jet and thus determines the degree of elongation of the jet by the applied electric field intensity. Therefore higher electrical conductivity may result in higher elongation of a jet by the electrostatic force to form uniform fibers of small diameter [55]. The conductivity of the solution can be increased significantly by adding mineral salts, mineral acids, carboxylic acids, some complexes of acids with amines, stannous chloride, and some tetra alkyl ammonium salts. With the aid of ionic salts like KH2PO4 and NaCl, nanofibers with small diameters can be

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obtained. Sometimes high solution conductivity can also be achieved by using organic acid as the solvent. 13.2.5.2.9 Effect of Surrounding Temperature and Humidity Ambient parameters, such as temperature and humidity, can also affect the morphology and average diameter of the electrospun nanofiber [56]. At relatively higher temperature the solvent evaporation rate will increase and the viscosity of the precursor solution to be electrospun will decrease, and as a result thinner nanofibers would be obtained. At higher humidity, the rate of solidification of the fibers becomes slower, resulting in a decrease in the average fiber diameter [57,58].

13.3 UTILIZATION OF BINARY MNXOY NANOFIBERS FOR ENERGY-STORAGE APPLICATIONS The nanofibers of transition metal oxides have shown potential application in numerous fields such as energy, medicine, electronics, environmental engineering, and security. The basic properties which make nanofibers useful in various applications are their 1D porous structure, high surface area with homogeneous pore size distribution, and alignment with a high aspect ratio. The energy and electrical applications of metal oxide nanofibers as supercapacitor electrode materials have been reported by various research groups [59]. Lee et. al. worked on MnOx nanofibers and showed a specific capacitance of 360 F/g at 1 A/g [60]. They suggested that electrospinning permits the creation of 3D electrodes that are characterized by both increased surface area, and efficient and fast ion transport between the electrode and the electrolyte, therefore, enhancing the electrochemical properties of the supercapacitors. Kolathodi et al. synthesized beaded manganese oxide (Mn2O3) nanofibers by an electrospinning process in which PVA was used as a binder [61]. Uniform and smooth as-spun nanofibers of Mn2O3 were obtained (Fig. 13.12A). The as-spun composite nanofibers were calcinated in air at 700°C for 1 h with a heating rate of 2°C/min. Fig. 13.12B shows an FESEM image of sintered Mn2O3 nanofibers and the inset shows the magnified image. The rectangular cyclic voltammetry (CV) plots (Fig. 13.12C) and symmetrical galvanostatic charging discharging (GCD) curves (Fig. 13.12D) recorded at different scan rates and current densities, respectively, in 0.5 M Na2SO4 indicate appreciable supercapacitive performance of the beaded nanofibers.

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(A)

Current density (A/g)

50 40

(C)

30

100 mV/s 50 mV/s 20 mV/s

20 10 0 –10 –20 –30 –40 0.0

0.2

0.4

0.6

10 mV/s 5 mV/s 2 mV/s 0.8 1.0

Potential (V) vs Ag/AgCl

(B)

1.0

(D)

0.8 0.6 0.4 0.2 0.0

0

Capacitance retention (%)

Specific capacitance (F/g)

Potential (V) vs Ag/AgCl 400 (E)

300

200

100

0

0

5

10

15

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20

1 A/g 2 A/g 5 A/g 10 A/g 20 A/g

120

200

400 Time (s)

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(F)

100 80 60 40 20 0

0

500

1000

1500

2000

2500 3000

Cycle number

Figure 13.12 (A) FESEM images of as-spun MnAc
PVA fibers. (B) Mn2O3 nanofibers and inset shows the magnified FESEM image. (C) CVs plots collected at various scan rates. (D) GCD plots recorded at various current densities. (E) Specific capacitance versus current density. (F) Cyclic stability test conducted for 3000 continuous GCD cycles at a current density of 10 A/g. From M.S. Kolathodi, S.N. Hanumantha Rao, T.S. Natarajan, G. Singh, Beaded manganese oxide (Mn2O3) nanofibers: preparation and application for capacitive energy storage, J. Mater. Chem. A. 4 (2016) 7883
7891 with permission. Copyright RSC 2016.

A maximum Cs of 379 F/g was observed at a current density of 1 A/g, and the variation in Cs value with current densities is shown in Fig. 13.12E. The active material exhibited 82.6% retention even after 3000 cycles at a current density of 10 A/g (Fig. 13.12F).

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Table 13.2 Compression of different methods to fabricate nanofibers Material MN0.33:1 MN0.5:1 MN1:1

MN2:1

Viscosity (cP)

235

172

188

220

Figure 13.13 FESEM images of as-spun nanofibers: (A) MN0.33:1, (B) MN0.5:1, (C) MN1:1, and (D) MN2:1 show the high aspect ratio with a diameter in the range of 200
300 nm. From J. Bhagwan, A. Sahoo, K.L. Yadav, Y. Sharma, Porous, one dimensional and high aspect ratio Mn3O4 nanofibers: fabrication and optimization for enhanced supercapacitive properties, Electrochim. Acta. 174 (2015) 992
1001 with permission. Copyright Elsevier Science 2015.

Compared to the as-spun nanofibers, calcined nanofibers were found to be rough and thin due to the decomposition of polymer templates and other organic components during calcination. Bhagwan et al. prepared Mn3O4 nanofiber and observed the effect of metal precursor to polymer ratio on nanofibric morphology [43]. They prepared four electrospun solutions having metal precursor to polymer ratios of 0.33:1 (MN0.33:1), 0.5:1 (MN0.5:1), 1:1 (MN1:1), and 2:1 (MN2:1) in a 7% polymer (PVP) solution. By increasing the weight of the metallic precursor over polymer from 0.33:1 to 2:1, a steady increase in the viscosity was obtained (Table 13.2). This variation in viscosity and dispersion of metal precursor into the PVP did not affect the shape and size of as-spun nanofiber significantly (Fig. 13.13). However, a drastic change in the morphology was noticed after a sintering process at 350°C (Fig. 13.14).

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Figure 13.14 FESEM images of sintered Mn3O4 nanofiber for polymer to metal precursor ratio: (A) MN0.33:1, (B) MN0.5:1, (C) MN1:1, and (D) MN2:1. From J. Bhagwan, A. Sahoo, K.L. Yadav, Y. Sharma, Porous, one dimensional and high aspect ratio Mn3O4 nanofibers: fabrication and optimization for enhanced supercapacitive properties, Electrochim. Acta. 174 (2015) 992
1001 with permission. Copyright Elsevier Science 2015.

Results show if the metal precursor is taken one third of PVP, sample MN0.33:1, severe destruction of fibrous morphology can be observed that leads to the formation of individual nanoparticles (Fig. 13.14A). This can be ascribed to the wider dispersion of metallic precursor into the polymer solution and thereby the larger separation between individual metal precursor particles occurs. Furthermore, if the weight of the metallic precursor slightly increases to half of that of the polymer (MN0.5:1), nanofibers are broken down into shorter segments and form nanorods (Fig. 13.14B). A further increase of metal precursor equal to polymer (MN1:1) (viscosity 220 cP) provides suitable dispersion between two individual metal particles mediated by the polymer and then slow removal of PVP assisted by a precisely controlled heating rate creates gaps or pores between two individual Mn3O4 nanoparticles and facilitates unidirectional growth of Mn3O4 nanofibers (Fig. 13.14C). If the metal precursor is taken to be twice that of the polymer (MN2:1), a closer dispersion of metal particles into polymer is expected which leads to forming a dense nanofiber (Fig. 13.14D), as depicted in the schematic diagram in Fig. 13.15.

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Figure 13.15 Schematic scheme for the fabrication of nanoparticles, nanofibers, and nanorods. From J. Bhagwan, A. Sahoo, K.L. Yadav, Y. Sharma, Porous, one dimensional and high aspect ratio Mn3O4 nanofibers: fabrication and optimization for enhanced supercapacitive properties, Electrochim. Acta. 174 (2015) 992
1001 with permission. Copyright Elsevier Science 2015.

Hence, an equal weight ratio of metallic precursor to PVP is found to be appropriate to fabricate the hollow and high aspect ratio nanofiber. The effect of nanofibric morphology on energy-storage devices has been investigated. The supercapacitive properties of Mn3O4 nanoparticles (MN0.33:1), nanorods (MN0.5:1), nanofiber (MN1:1), and dense nanofiber (MN2:1) have been evaluated by CV (Fig. 13.16A) and GCD (Fig. 13.16B) analysis in a three-electrode setup with 1 M KCl electrolyte. Nanoparticles and nanorods show a Cs of 58 and 65 F/g, respectively, at a current density of 0.3 A/g using GCD analysis. However, Cs of 210 and 86 F/g were obtained from nanofiber and dense nanofiber, respectively, at the same current density. Electrochemical impedance spectroscopy (EIS) has been used to complement the findings of CV and GCD studies, and to find the reason for improved performance of MN1:1 (nanofiber) as compared to other samples/morphologies. The Nyquist plots of the MN0.33:1, MN0.5:1, MN1:1, and MN2:1 electrodes in 1 M KCl are shown in Fig. 13.16C, which indicates a small semicircle in the higher to medium frequency region (1 MHz to 50 kHz) followed by a sloping line making an angle of B45 degrees with the x-axis. To distinguish the individual process and corresponding impedance parameters, an equivalent circuit comprising a series and parallel combination of equivalent series resistance (Rs), electric double-layer capacitance (Cdl), pseudocapacitance (Cp), charge transfer resistance (Rct), and frequency-dependent Warburg impedance (W) is fitted [62,63]. The fitted values of Rs and Rct of MN1:1 are found to be lowest at 0.2 and 1.0 Ω, respectively. The value of Rs is found to be independent of morphology. However, the diameter of the

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Figure 13.16 Comparison of (A) CV curves of MN0.33:1, MN0.5:1, MN1:1, and MN2:1 at the scan rate of 3 mV/s, (B) GCD curves of MN0.33:1, MN0.5:1, MN1:1, and MN2:1 at the current density of 0.3 A/g, (C) Nyquist plots of MN0.33:1, MN0.5:1, MN1:1, and MN2:1, and (D) shows the cyclic stability up to 500 cycles. From J. Bhagwan, A. Sahoo, K.L. Yadav, Y. Sharma, Porous, one dimensional and high aspect ratio Mn3O4 nanofibers: fabrication and optimization for enhanced supercapacitive properties, Electrochim. Acta. 174 (2015) 992
1001 with permission. Copyright Elsevier Science 2015.

semicircle along the x-axis increases, indicating the variation in charge transfer resistance. In the case of dense nanofiber (MN2:1), nanorods (MN0.5:1), and nanoparticles (MN0.33:1), the values of Rct increase to 3, 5, and 6 Ω, respectively. Furthermore, the sloping region (B45 degrees) in the low-frequency regime represents the frequency dependence ion diffusion/transportation of the electrolyte [64
66]. A higher value of Rct and significant Warburg region are found to be detrimental to good supercapacitive performance in these samples. However, in the case of MN1:1, the lowest value of Rct (1.0 Ω) and unnoticeable Warburg impedance facilitates the smooth and facile movement of electrolytic ions into electrode material and vice versa. Lower Rs and unnoticeable Warburg impedance are considered to be good for improving the energy

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475

density and power density of supercapacitor material, which further justifies the suitability of aligned and high aspect ratio nanofiber of Mn3O4. Furthermore, the cycling stability of MN1:1 nanofiber is also examined and the results are shown in Fig. 13.16D, where a stable Cs value of 210 ( 6 5) F/g with almost 100% efficiency, at least up to 500 cycles, is obtained. Hence, tuning and optimization of morphology in terms of pores, aspect ratio, and dimensionality by a simple and cost-effective electrospinning method for Mn3O4 opens a new opportunity not only to tune the supercapacitive properties but also to the other allied applications. This indicates that 1D nanofiber can enhance both the ionic transport pathways and the surface area by increasing the contact area between the electrolytic ions and the surface of the electrodes. Liang et al. fabricated coaxial cable, like Mn2O3 nanofibers and nanoparticles, using a facile and cost-effective single-nozzle electrospinning technique and subsequent calcination. These nanoparticles and nanofibers were utilized as electrode material for supercapacitor applications [67]. The electrochemical performance of coaxial cable, like Mn2O3 nanofibers, as evaluated via GCD and CV analysis is shown in Fig. 13.17A and B, respectively. The specific capacitances of these nanofibers and nanoparticles were found to be 216 and 70 F/g at a current density of 0.5 A/g, respectively. The coaxial cable like Mn2O3 electrode retains 52% of its initial capacitance value as the current density increases from 0.5 to 5 A/g (Fig. 13.17C). The higher specific capacitance of coaxial cable like Mn2O3 nanofibers in comparison to Mn2O3 nanoparticles can be ascribed to the regular hollow structure with larger specific surface area (16 and 3 m2/g for nanofibers and nanoparticles, respectively) and pore volume, which provide short and effective diffusion channels for the electrolytic ions in hallow nanofibers. Furthermore, there are more electrolytes in the inner void space of Mn2O3 nanofibers, which guarantees a steady supply of electrolytic ions. The capacity retentions of Mn2O3 nanofibers and nanoparticle electrodes were reported to be 93%, and only 60% over 1000 cycles, respectively, which further signifies the importance of the nanofibric morphologies over the nanoparticles (Fig. 13.17). Mondal et al. fabricated MnOx nanofiber by an electrospinning technique and subsequently calcined at 350°C and reported the specific capacitance of 166 F/g at 10 mV/s, whereas MnOx hollow nanofiber calcined at 700°C showed specific capacitance of 361 F/g at 10 mV/s [68]. Hence, it is clear that the most striking features of nanofibers are their exceptionally high surface area-to-volume ratio, hollow structure, and high porosity with

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(A)

(B) 0.5 A/g 1.0 A/g 2.0 A/g 5.0 A/g

0.3 0.2 0.1

0.5 A/g 1.0 A/g 2.0 A/g 5.0 A/g

0.4

Potential (V)

Potential (V)

0.4

0.3 0.2 0.1 0.0

0.0 0

50 100 150 200 250 300 350 400

0

20

Time (s)

150 100 50 0 1

2

3

4

Current density (A/g)

5

6

Specific capacitance (F/g)

Specific capacitance (F/g)

(D) Coaxial cable like Mn2O3 nanofibers Mn2O3 particles

0

60

80

100

120

Time (s)

(C) 200

40

250 200 Coaxial cable like Mn2O3 nanofibers Mn2O3 particles

150 100 50 0 0

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400

600

800

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Cycle number

Figure 13.17 (A) Chronopotentiometry curves of coaxial cable like Mn2O3 nanofibers, (B) Mn2O3 particles, (C) dependence of specific capacitances on the different current density, and (D) cycling stability tested at 0.5 A/g. From J. Liang, L.-T. Bu, W.-G. Cao, T. Chen, Y.-C. Cao, Facile fabrication of coaxial-cable like Mn2O3 nanofiber by electrospinning: Application as electrode material for supercapacitor, J Taiwan Inst. Chem. Eng. 65 (2016) 584
590 with permission. Copyright Elsevier Science 2016.

excellent pore interconnectivity, which play a critical role in improving the electrochemical performance of the material.

13.4 FUTURE ASPECTS, CHALLENGES, AND SUMMARY Nanofibers have gained tremendous attention because of their numerous applications in energy storage and generation, chemical and biological sensors, pharmaceutical and textile industries, water purification, and environmental remediation. Although a lot of work has been done regarding the fabrication of transition metal oxide nanofibers, but their integration at specific positions into nano-matric requires nanofibres to be synthesized with good reproducibility, well-controlled orientation, tunable size, and high aspect ratio. The large-scale production of nanofibers with such

Fabrication, Characterization, and Optimization

477

characteristics is still a challenging task as the widely used electrospinning techniques have some drawbacks, namely, low yield, high operating voltage, and difficulty in attaining in situ deposition of nanofibers on different substrates. The yield of nanofiber synthesis can be increased using electrospinning setup with multineedles or needleless electrospinning. Moreover, electrospun nanofibers exhibit poor mechanical strength due to their poor crystallinity, random alignment, and disordered orientations. To improve the physical and mechanical properties of nanofibers, their fibric structure needs to be manipulated in terms of fibric dimensions, surface functionalization, and interfiber adhesion using suitable materials or via synthesizing nanofiber composites adopting advanced synthesis methods such as coaxial electrospinning. However, there is less information available on the mechanical properties of nanofibers and nanofiber composites. Therefore further studies are required to overcome critical issues with nanofiber embedment in nanocomposites during the processing and large-scale manufacturing. It has been demonstrated that the energy-storage capacity of nanofibers strongly depends on the porosity of the fibers, and the energy-storage performance can be improved by optimizing porosity and pore size distribution in the nanofibers. Finally, the design and construction of process equipment for controllable, reproducible, continuous, and mass electrospinning production would represent the most efficient translation of the properties of nanofibers and could act as a stimulus for the manufacture of new products. In summary, the optimization of various parameters is extremely important to synthesize long, continuous, and smooth nanofibers by adopting a facile, versatile, and scalable electrospinning process. The synthesis procedure of as-spun nanofibers of binary MnxOy/polymer via the electrospinning process has been reported. Furthermore, the effect of process and system parameters on the morphology of synthesized nanofibers have been discussed. It has been shown how the metal to polymer ratio in the electrospinning solution tunes the morphology of sintered Mn3O4, that is, nanoparticles/nanorods/nanofibers. The best optimized nanofiber of Mn3O4 in terms of surface area, pore size distribution, and high aspect ratio can be obtained, when equal amounts of metal precursor and polymer (MN1:1) are used in the precursor solution, and the obtained as-spun nanofibers are sintered at 350°C at a ramping rate of 1°C/min. Mn2O3 nanofiber was obtained after sintering at 700°C for 1 h with a heating rate of 2°C/min. The supercapacitor performances of MnxOy nanofibers have

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been evaluated via CV, GCD, and EIS techniques. The better electrochemical performance of MnxOy nanofibers over nanoparticles and nanorods can be ascribed to the smooth, long, continuous nanofibric morphology, which provides a short diffusion path and low interparticle resistance.

ACKNOWLEDGMENTS We are thankful to DAE-BRNS, Govt. of India, for providing a research grant to support our research through Grant No. 2012/34/44/BRNS. We also acknowledge the partial support received from DST-SERB through grant No. SR/FTP/PS-137/2011.

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CHAPTER 14

Fabrication of Micro/ Nano-Miniaturized Platforms for Nanotheranostics and Regenerative Medicine Applications G. Praveen1, Nandakumar Kalarikkal2,3 and Sabu Thomas4,5,6 1

International and Interuniversity Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India 2 Director and an Associate Professor of International and Inter University Centre for Nanoscience and Nanotechnology, Kottayam, India 3 Director and Chair of School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, India 4 The Vice Chancellor of Mahatma Gandhi University, Kottayam, India 5 Founder Director of International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India 6 Professor at School of Chemical Sciences, Mahatma Gandhi University, Kottayam, India

Contents 14.1 Nanotheranostics and Regenerative Medicine: Introduction 14.1.1 Regenerative Medicine and Tissue Engineering 14.1.2 Organ-Specific Bioengineering 14.2 Bioartificial Organs: Introduction 14.2.1 Types of Bioartificial Organs 14.2.2 Scope of Research Into Bioartificial Organs 14.2.3 Bioartificial Organs and Theragnostics 14.3 Micro- and Nanofluidic Devices: Introduction 14.3.1 Micro- and Nanofluidic Cell Culture Devices 14.3.2 Micro- and Nanofluidic Drug-Screening Devices 14.3.3 Micro- and Nanofluidic Biosensing Devices 14.4 Biomimetics: Introduction 14.4.1 Nanostructured Biomimetics 14.4.2 Scaffolding for Biomimetics 14.5 Biopatterning the Complexities of Life: Introduction 14.5.1 Biopatterning the Microstructures 14.5.2 Biopatterning the Nanostructures 14.5.3 Fluidic Simulations on Patterned Surfaces 14.5.4 Bioreactors for In Situ Simulations 14.5.5 Complexities in Biopatterning

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14.6 Bioprinting of Organs and Tissues 14.6.1 History of Bioprinting 14.6.2 Techniques Used in Bioprinting 14.6.3 Applications of Bioprinting 14.6.4 Organ Specific Bioprinting 14.6.5 Complexities in Bioprinting 14.7 Conclusions References

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14.1 NANOTHERANOSTICS AND REGENERATIVE MEDICINE: INTRODUCTION Nanotheranostics is the integration of diagnostic and therapeutic function in one system using the benefits of nanotechnology, and is extremely attractive for personalized medicine. Personalized and precision medicine (PM) is an envisaged arena that identifies biomarkers for understanding and treating specific disorders based on their precise diagnosis. By predominantly utilizing the unique properties of nanoparticles to achieve biomarker identification and drug delivery, nanotheranostics can be exploited to noninvasively discover and target image biomarkers and further deliver treatment based on the biomarker distribution. Regenerative medicine is a broad field that includes tissue engineering but also incorporates research into self-healing—where the body uses its own systems, sometimes with help from foreign biological material, to recreate cells and rebuild tissues and organs. The terms “tissue engineering” and “regenerative medicine” have become largely interchangeable, as the field hopes to focus on cures rather than treatments for complex, often chronic, diseases. However, as described in this chapter, current nanotechnology-based theranostics systems engineered for PM applications are not yet sufficient. PM is an ever-growing field that will be a driving force for future discoveries in biomedicine, especially cancer theranostics. In this chapter, the authors discuss the current advancements and research progress in these fields toward the development of nanotheranostics-based PM.

14.1.1 Regenerative Medicine and Tissue Engineering The aim of regenerative medicine is to regenerate the soft and hard tissues, partial organoids, or even biologically active organ support systems that can be readily available for patients with critical disabilities. Tissue engineering is a science that aims to grow replacement tissues and organs

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in the laboratory to help solve the shortage of donated tissue available for transplants. Tissue engineering has been used to develop scaffolds to regenerate various tissues such as skin, nerve, bone, blood vessels, etc. Though the concept of regenerative medicine initially emerged as an extension of research progress into tissue engineering, it requires integration of emerging knowledge in the physical and life sciences with bioengineering and clinical medicine so as to understand how to trigger the failed human tissues and organs. In the last two decades, regenerative medicine has shown potential for “bench-to-bedside” translational research in specific clinical settings. The success of engineered matrices for tissue regeneration and organ development mainly depends upon the development of degradable extracellular matrix (ECM) analogs that provide a temporary support for cell adhesion, proliferation, maturation, and differentiation. In the case of liver tissue engineering, the scaffold needs to be degradable, possess unique properties like a growth-permissive environment for better cell adhesion, three-dimensionality for cell interactions, and desirable porosity to facilitate the diffusion of nutrients and gaseous exchange. In addition, the scaffolds should be able to induce vascularization, exhibit sufficient mechanical strength for transplantation, and should be biocompatible to eliminate the inflammatory response, in addition to possessing appropriate cues to maintain hepatocyte characteristics without the loss of its phenotype and functions [1]. Progress made in cell and stem cell biology, material sciences, and tissue engineering has enabled researchers to develop cutting-edge technology, which has lead to the creation of nonmodular tissue constructs such as skin, bladders, vessels, and upper airways. In all cases, autologous cells were seeded on either artificial or natural supporting scaffolds. However, such constructs were implanted without reconstruction of the vascular supply, and the nutrients and oxygen were supplied by diffusion from adjacent tissues [2]. Engineering of modular organs (organs organized as functioning units referred to as modules and requiring the reconstruction of the vascular supply) is more complex and challenging. Models of functioning hearts and livers have been engineered using “natural tissue” scaffolds and efforts are underway to produce kidneys, pancreas, and small intestine. Creation of custom-made bioengineered organs, where the cellular component is exquisitely autologous and that have an internal vascular network will theoretically overcome the two major hurdles in transplantation, namely the shortage of organs and the toxicity deriving from life-long immunosuppression.

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14.1.2 Organ-Specific Bioengineering At the tissue level, cells are systematically organized in their direct environment, which is composed of a complex 3D network of fibrillar proteins, proteoglycans, and glycosaminoglycans (GAGs) collectively termed as the ECM. The ECM plays crucial roles not only in supporting the cells by providing them with a tissue-specific environment and architecture, but also in cell signaling and maintaining tissue homeostasis. Cells sense and respond to various stimuli from the ECM by integrins and/or mechanosensitive ion channels. Furthermore, it serves as a reservoir of water, nutrients, cytokines, and growth factors. In tissue engineering, scaffolds are 3D structures from synthetic, natural or semisynthetic materials with ECM-mimicking properties [3]. They are engineered as temporary matrices for cell proliferation, differentiation, and ECM deposition and vascularization, and are expected to degrade in concert with bone tissue development and in growth. The literature indicates that the success of a 3D tissue engineering scaffold depends on the extent of similitude in its structural and functional properties with the natural ECM. Therefore, different materials and engineering strategies have been employed to make the scaffolds more biocompatible, biofunctional, and resorbable [4]. Scaffold properties such as compatibility, architecture, bioactivity, mechanical property, and degradability have been accepted among the tissue engineering community as key properties for their successful use in bone tissue engineering. Biomaterials which lack cytotoxicity and support cell biomaterial interaction according to the local and organ-specific situation where the biomaterial is applied have been generally defined as biocompatible. However, in tissue engineering, biocompatibility indicates that the scaffold material or its degradable products would not elicit toxicity or provoke any rejection, inflammation, or immune response, along with the above-mentioned features. Therefore, there has been an increasing interest in the use of biocompatible materials and less toxic fabrication processes and surface modifications in order to enhance the biocompatibility of the scaffolds [5]. The porosity, pore size, and pore interconnectivity and the structural dimensions in the scaffold architecture are crucial scaffold properties. Porosity and pore interconnectivity allow the infiltration/distribution of cells throughout the 3D structure with seeding and migration of the seeded cells thereafter. Moreover, it is essential for the migration of endothelial cell vascularization [6]. The size of the pores is equally important, as this ultimately decides the

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distribution/infiltration and migration of the cells throughout the scaffolds. Capillary ingrowth from the surrounding tissue and vascularization of the scaffolds is another major aspect which will be controlled by the interconnected porosity and pore size of the scaffolds. Porosity and pore sizes have a direct relation with the mechanical properties of the scaffolds. Therefore, a balance between the interconnected porosity/pore size and the mechanical properties is essential for tissue engineering purposes. The bioactivity of a scaffold in bone tissue engineering is defined as the capability of the scaffold material to elicit a specific biological response at the interface of the material, which results in the formation of a bond between the tissue and that material [7]. Recently, bioactive scaffolds and various methods of incorporating bioactivity have gained considerable interest in the field of bone tissue engineering. The review articles by Zhang et al. and Rezwan et al. give an overview on the current development and application status of polymer inorganic composite scaffolds and electrospun composite nanofibers for constructing biomimetic and bioactive scaffolds. In vitro, mechanical properties which are sufficient to withstand the hydrostatic pressures and to maintain the spaces required for cell in-growth and matrix production are essential. Moreover, in vivo, the tissue engineering scaffolds should have mechanical properties matching those of the tissue at the implantation site or mechanical properties that are sufficient to shield cells from damaging compressive or tensile forces without inhibiting appropriate biomechanical cues. Several investigators are exploring the feasibility of using Micro-Electro-Mechanical Systems (MEMS) for recreating biomimetic miniaturized platforms (Fig. 14.1).

14.2 BIOARTIFICIAL ORGANS: INTRODUCTION 14.2.1 Types of Bioartificial Organs Over the last decade, significant progress has been made in the field of organ engineering, mostly related to the replacement of hollow tissue structures, that is, trachea, urinary bladder, and blood vessels [8]. A valuable technology for parenchymal organs has been developed to produce “organ-shaped” scaffolds with preserved ECMs and vascular networks. This strategy is based on perfusing the organ with specific detergents under pressure-controlled conditions and is aimed at creating decellularized templates ready to be seeded with suitable cell lines (Fig. 14.2).

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Figure 14.1 The schematic representation of implantable artificial kidney (IAK) when introduced into the vasculature with blood pumped using the patient's blood pressure into the HemoCartridge with membranes that mimic the slit-shaped pores of podocytes and then through the BioCartridge that contains living tubular cells, thus mimicking the glomerulus-tubule arrangement of the kidney. Adapted with permission from Salani, Roy, Fissell, Innovations in Wearable and Implantable Artificial Kidneys. Am J Kidney Dis. 72(5) (2018): 745
751. http://dx.doi.org/10.1053/j.ajkd.2018.06.005.

14.2.2 Scope of Research Into Bioartificial Organs Recent advances in the fields of artificial organs and regenerative medicine are now joining forces in the areas of organ transplantation and bioengineering to solve continued challenges for patients with end-stage renal disease. The waiting lists for those needing a transplant continue to exceed demand. Dialysis, while effective, brings different challenges, including quality of life and susceptibility to infection [9]. Unfortunately, the majority of research outputs are far from delivering satisfactory solutions. Current efforts are focused on providing a self-standing device able to recapitulate kidney function. In this review, we focus on two remarkable innovations that may offer a significant clinical impact in the field of renal replacement therapy: the implantable artificial renal assist device (RAD) and the transplantable bioengineered kidney. The artificial RAD strategy utilizes micromachining techniques to fabricate a biohybrid system able to mimic renal morphology and function. The current trend in kidney bioengineering exploits the structure of the native organ to produce a kidney

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Figure 14.2 The creation of a bioengineered kidney (A) Bottom-up method to create bioactive UPy-U membranes. (B) Fourfold hydrogen bonding ureido-pyrimidinone (UPy) moieties form dimers and stack in lateral way via extra hydrogen bonding in the middle of urea (U) functionalities, thus becoming nanofiber constructions. (C) Diagram of a kidney, one of its million nephrons, and a live membrane that can be a well-thought-out mimic of a part of the renal tubular system. Adapted with permission from Dankers et al., Bioengineering of living renal membranes consisting of hierarchical, bioactive supramolecular meshes and human tubular cells. Biomaterials 32 (3) (2011) 723
733. http://dx.doi.org/10.1016/j.biomaterials.2010.09.020.

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that is ready to be transplanted. Although these two systems stem from different technological approaches, they are both designed to be implantable, long-lasting, and free-standing to allow patients with kidney failure to be autonomous. However, for both of them, there are relevant issues that must be addressed before translation into clinical use.

14.2.3 Bioartificial Organs and Theragnostics Bioartificial organs involve the design, modification, growth, and maintenance of living tissues embedded in natural or synthetic scaffolds to enable them to perform complex biochemical functions, including adaptive control and the replacement of normal living tissues. Future directions in this area will lead to an abandonment of the trial-and-error implant optimization approach and a switch to the rational production of precisely formulated nanobiological devices. This will be accomplished with the help of three major thrusts: (1) use of molecularly manipulated nanostructured biomimetic materials; (2) application of microelectronic and nanoelectronic interfacing for sensing and control; and (3) application of drug delivery and medical nanosystems to induce, maintain, and replace a missing function that cannot be readily substituted with a living cell and to accelerate tissue regeneration. Biomimetics involves employment of microstructures and functional domains of organismal tissue function, correlation of processes and structures with physical and chemical processes, and use of this knowledge base to design and synthesize new materials for health applications. Nanostructured materials should involve biological materials (rather than synthetic ones) because their prefabricated structure is suitable for modular control of devices from existing materials.

14.3 MICRO- AND NANOFLUIDIC DEVICES: INTRODUCTION Fabrication of micro- and nanofluidic devices is possible by incorporating unique features at the micro- or nanoscale, the précised study and application of fluid flow in micro- or even nanochannels/nanopores with at least one characteristic size smaller than 100 nm, has enabled the occurrence of many interesting transport phenomena and has shown great potential in both bio- and energy-related fields. The unprecedented growth of this research field is apparently attributed to the rapid development of micro/ nanofabrication techniques [10]. Three major nanofabrication strategies, including nanolithography, microelectromechanical system-based techniques, and methods using various nanomaterials, are introduced with

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specific fabrication approaches. Other unconventional fabrication attempts utilize special polymer properties, various microfabrication failure mechanisms, and macro/microscale machining techniques.

14.3.1 Micro- and Nanofluidic Cell Culture Devices The use of hydrogels in microfluidics is a simple platform to provide relevant 3D matrices to vascular cells, while maintaining the ability to apply shear stress. Kim et al. used the flexibility of PDMS to generate a microfluidic device capable of containing fibrin matrices. The multichannel device enabled the study of vasculogenesis and angiogenesis, and the fibrin matrix was able to support cell growth. Kim and colleagues were able to show the formation of a perfusable vascular network, and the maintenance of barrier function [11]. Additionally, the vascular networks under flow exhibited increased nitric oxide production compared with static conditions (Fig. 14.3). It is challenging to mimic the vascular environment of the human body in vitro. Arteries, arterioles, veins, venules, and capillaries are all part of the vascular system, but differ in the structural and cellular compositions. In addition, blood vessels are subject to a range of biophysical stimuli because of the pulsatile nature of blood flow. Endothelial Cells (ECs) lining the lumen of vessels experience flow-induced pulsatile wall

Figure 14.3 Schematic of nanochannel fabrication based on sacrificial layer releasing method. Step 1: Deposition of the bottom layer. Step 2: Deposition of the sacrificial layer. Step 3: Pattern sacrificial layer to create the male form of the nanochannel. Step 4: Deposition of the capping layer. Step 5: Formation of access reservoirs. Step 6: Nanochannel releasing.

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Figure 14.4 Typical SEM photos of various nanoporous materials, top: top-view; bottom: cross-sectional view. (A) AAO membrane. Reprinted with permission from Vajandar et al., Nanotechnology 18(27), 275705 (2007). Copyright 2007 Institute of Physics. (B) Track-etched nanoporous membrane. Reprinted with permission from Ali et al., ACS Nano 33, 603
608 (2009). Copyright 2009 American Chemical Society. (C) BCP nanoporous matrix. Reprinted with permission from Uehara et al., ACS Nano 34, 924
932 (2009). Copyright 2009 American Chemical Society.

shear stress and transmural pressure. ECs and vascular smooth muscle cells both experience cyclic mechanical stretching, which causes the vessels to increase in diameter in response to blood flow [12]. Hemodynamic parameters contribute to the maintenance of homeostasis in the vessel wall, with several microfluidic models studying the effects of hemodynamics in vitro (Fig. 14.4).

14.3.2 Micro- and Nanofluidic Drug-Screening Devices Several microfluidic platforms have been used for in vitro drug screening and for the development of drug-delivery systems. Microfluidic models can be applied to study thrombosis, occlusion, and stenotic regions. Li et al. designed a microfluidic system to study stenotic regions and thrombus formation under different shear rates. Low shear rates led to longer occlusion times [13]. However, the administration of increasing concentrations of the antiplatelet eptifibatide in high shear stress did not reduce the occlusion times compared to no drug. This highlights the need for studying drug effects, in vitro, together with relevant biophysical stimuli such as shear rates.

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Another system that mimics a stenotic region was used to design “smart” drugs. Blood vessels are often narrowed in thrombotic regions. At this site, wall shear stress can rapidly increase by two orders of magnitude. Orin et al. engineered microparticles that were responsive to shear stress that would thus breakup into smaller nanoparticles when exposed to the shear stresses observed in stenotic regions [14]. By incorporating tissue plasminogen activator (tPA) in the nanoparticles, Korin et al. were able to show local delivery and rapid thrombolysis. This innovative platform illustrates how microfluidic biomimetic vascular systems can be used in the context of drug development. In this field, researchers have used the physical properties of the stenotic regions to target drugs, thus avoiding the need for systemic delivery of tPA and the resultant adverse side effects. Other vascular models have been used to study drug carriers and physical characteristics. Thrombotic and stenotic models are closely related; Muthard et al. engineered a microfluidic system to probe the effects of wall shear stress and trans-thrombus pressure gradients in the thrombogenesis [15]. In this system, a side flow region was filled with collagen gel with the pressure across it controlled by computer. The collagen area was exposed to the main fluidic channel and visualized directly on the device. As expected, the perfusion of whole blood induced thrombus formation at the collagen site. Interestingly, by varying the pressure gradient in the trans-thrombus collagen area, the authors observed a decrease in thrombin with higher pressure-gradients. This device can act as a useful tool to assess thrombotic areas and the hemodynamics of pressure gradients in the vessel wall.

14.3.3 Micro- and Nanofluidic Biosensing Devices Droplet microfluidics has been proposed as a screening platform for a plethora of applications such as in chemical synthesis, and screening for small-molecule drugs, cells, or proteins. While all of these applications are of tremendous interest, the droplet microfluidics technology platform with the poignant characteristic that the encapsulation reagents and surfactants used must be compatible with the droplet contents, the limited chemical variations and generally large size of biopolymers such as proteins make these a particularly interesting group of molecules to apply this set of screening techniques to have more diverse chemical traits [16]. Droplet microfluidics involves the controlled formation and manipulation of nanoto femtoliter droplets of one fluid phase in another fluid immiscible with

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the first. Earlier systems exploiting two immiscible fluids for biological experimentation have been utilized for protein analysis prior to the advent of microfluidic circuits for droplet generation, notably, in 1961 the Nobel Prize winner Joshua Lederberg used aqueous droplets in oil to study the characteristics of β-galactosidase in what has to be one of the earliest examples of a single biomolecule study. Generation of droplets in microfluidic circuits has however enabled the formation of droplets with an extreme monodispersity of size, enabling quantitative assays in droplets [3]. In many cases aqueous droplets are generated in a hydrocarbon or fluorocarbon oil continuous phase containing surfactants to stabilize the interface, allowing the droplets to remain stable over long periods of time. The development of a multitude of functional manipulations of droplets, for example, splitting, fusion, incubation, and most notably active sorting, capable of processing thousands of droplets per second, provides the basis for automated high-throughput handling of fluid packets. Combined with biological assays these manipulations have the potential to serve as a highthroughput screening and experimentation platform. Several companies have been started around the use of droplet microfluidic technologies (or have come to use them), for example, RainDance Technologies, QuantaLife, Sphere Fluidics, Emerald Biosciences, and GnuBIO. These companies have thus far mainly focused on products and solutions related to DNA sequencing. The first such product brought to market was introduced by RainDance Technologies in 2008. Emerald Biosciences, which markets a protein crystallization screening product using a microfluidic formulation and screening in a plug format, is the only company to provide products relating to protein screening or analysis [17]. While commercial efforts have focused around genomics and sequencing, the tools and automation developed are in many respects very well suited for screening and analysis of engineered proteins. Proteins, such as green fluorescent protein, have, for example, been expressed in vitro in droplet microfluidic devices (Fig. 14.5).

14.4 BIOMIMETICS: INTRODUCTION Biomimetics is a newly emerging interdisciplinary field in materials science and engineering and biology in which lessons learned from biology form the basis for novel technological materials. It involves investigation of both structures and physical functions of biological composites of engineering interest

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Figure 14.5 Artist’s view of biological assays in droplet microfluidics. Adapted from L. Shang, Y. Cheng, Y. Zhao. Emerging Droplet Microfluidics, Chem. Rev. 2017, 117, 7964
8040.

with the goal of designing and synthesizing new and improved materials [18]. It is now evident that hierarchical organization play an important role in the amazing mechanical properties of natural biocomposites. The more important structural characteristics and mechanical properties of natural biocomposites are exposed as a base that has inspired scientists and engineers to develop biomimetic strategies that could be useful in areas such as materials science, biomaterials development, and nanotechnology.

14.4.1 Nanostructured Biomimetics Biomimetics is the extraction of good design from nature. Usually it involves the use of conventional engineering methods to make direct analogs of the reflectors and antireflectors found in nature. However, recent collaborations between biologists, physicists, engineers, chemists, and materials scientists have ventured beyond experiments that merely mimic what happens in nature, leading to a thriving new area of research involving biomimetics through cell culture [19]. In this new approach, the nanoengineering efficiency of living cells is harnessed and natural organisms such as diatoms and viruses are used to make nanostructures that could have commercial applications (Fig. 14.6).

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Figure 14.6 Examples of molecular biomimetics. Proteins can be used, for example, to bind nanoparticles to a functionalized substrate, to create linkers onto a specific substrate, or to connect different nanoparticles with each other. Adapted from M. Sarikaya, C. Tamerler, A.-Y. Jen, K. Schulten, F. Baneyx, Molecular biomimetics: nanotechnology through biology, Nat. Mater. 2 (2003) 577
585.

14.4.2 Scaffolding for Biomimetics There are numerous approaches for producing natural and synthetic 3D scaffolds that support the proliferation of mammalian cells. However, many of these products are proprietary, expensive, or require chemical synthesis. Three-dimensional scaffolds better represent the natural cellular microenvironment and have many potential applications in vitro and in vivo. The research group of Andrew Pelling at University of Ottawa, Canada, have demonstrated the suitability of apple-derived cellulose scaffolds in supporting the in vitro culture of mammalian cells. Plant-derived cellulose scaffolds offer an alternative approach for 3D culture, offering the advantage of ease of production and modification, reduced cost, and the ability to fabricate the cellulose into shapes specific to the user. Native ECM provides structural support to the multicellular organism on a macroscopic scale and establishes a unique microenvironment (niche) to tissue- and organ-specific cell types. Both these functions are critical for optimal function of the organism. These natural ECMs comprise predominantly fibrillar proteins, collagen, and elastin, and are synthesized as monomers but undergo hierarchical organization into well-defined nanoscaled structural units [20]. The interaction between the cells and ECM is dynamic, reciprocal, and essential for tissue development, maintenance of function, repair, and regeneration processes. Tissue-engineering scaffolds are synthetic, biomimetic ECM analogs that have great promise in regenerative medicine. Ongoing efforts in mimicking the native ECM in terms of composition and dimensions have resulted in three strategies that permit the generation of scaffolds in nanometer dimensions (Fig. 14.7).

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Figure 14.7 Schematic representation depicting decellularization of apple tissue for proliferation of mammalian cells. (A) McIntosh Red apples were exposed to 2 20°C temperatures for a max duration of 5 minutes, to increase the firmness of the outer apple hypanthium tissue. (B) Uniform 1.2 6 0.1 mm thick slices of the apples were obtained using a mandolin slicer. (C) The apple slices were cut into uniform 2.0 by 0.5 cm segments. (D) A 0.5% SDS solution was added to the microcentrifuge tubes and placed on a shaker for 12 hours at room temperature. (E) The scaffolds were then coated with Type 1 collagen, chemically cross linked with glutaraldehyde or incubated in PBS. (F) All the samples were then incubated in mammalian cell culture medium (DMEM) for 12 hours in a standard tissue culture incubator maintained at 37°C and 5% CO2. (G) The scaffolds were placed in PDMS coated 24 well plates and a 40 μL cell suspension was placed on each. Adapted from D.J. Modulevsky, C. Lefebvre, K. Haase, Z. Al-Rekabi, A.E. Pelling. Apple Derived Cellulose Scaffolds for 3D Mammalian Cell Culture. PLoS ONE 2014, 9(5): e97835.

14.5 BIOPATTERNING THE COMPLEXITIES OF LIFE: INTRODUCTION Biopatterning the complex living systems is an interdisciplinary research approach in which principles from engineering, chemistry, and biology

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are applied to the synthesis of materials, synthetic systems, or machines that have functions that recreate the biological architectures. Biomaterials are the precisely patterning substrates that interact with a complex biological system and could be used in regenerative medicine, tissue engineering, and drug delivery.

14.5.1 Biopatterning the Microstructures The patterning of biomolecules in well-defined microstructures is a critical issue for the development of biosensors and biochips. However, the fabrication of microstructures with well-ordered and spatially discrete forms to provide the patterned surface for the immobilization of biomolecules is difficult because of the lack of distinct physical and chemical barriers separating patterns. Patterning biomolecules in well-defined microstructures is critical for developing biosensors, high-throughput screening apparatuses, biochips, labs on a chip, and tissue engineering [21]. However, creating microstructures capable of immobilizing biomolecules is difficult because the physical and chemical barriers separating the patterns are indistinct. One of the most frequently used methods of patterning is microcontact printing.

14.5.2 Biopatterning the Nanostructures Research and development into the synthesis of nanowires and nanoparticles, nanopatterning, as well as nanoscale phenomena and properties, literally exploded in the past several years, developing a large variety of nanostructures extending to a myriad of other physical or chemical components. Some of the biological applications of the nanostructures include nanopatterns, nanochannels, nanotubes, and nanoparticles, synthesized for fluorescent biological labeling and tagging, drug and gene delivery, biodetection of pathogens and proteins, nanotubes and nanochannels for probing the structure of DNA, tissue engineering, magnetic nanoparticles for tumor destruction via heating (hyperthermia), and magnetic resonance imaging contrast enhancement, separation and purification of biological molecules and cells [21].

14.5.3 Fluidic Simulations on Patterned Surfaces Organs-on-chips are engineered devices that combine cells, biomaterials, and microfabrication to simulate the activity and function of tissues and organ subunits. They are often in multichannel three-dimensional

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microfluidic formats that mimic cell responses more accurately than regular in vitro cell cultures in two dimensions [22]. The devices integrate microengineered three-dimensional tissue with microfluidic network systems, allowing living cells to be cultured in micrometer-sized chambers that are continuously perfused, thereby modeling essential functions of living organs or tissues at a small scale. The microenvironment of the cell culture can be tailored to mimic an organ very realistically; for example, biophysical constraints such as mechanical strain associated with breathing in the lung can be included, or perfusion with blood or blood substitutes at rates equivalent to the true shear stress on blood vessel walls [23]. Cells can be grown as monocultures using just one cell type or as co-cultures of two or more cell types in two dimensions on, for example, a permeable membrane, or in three dimensions (Fig. 14.8).

Figure 14.8 Microfluidic lung-on-a-chip: The chip consists of two channels, which represent the alveolar air compartment and the alveolar capillary. Top right illustrates the standard setup of the chip. The upper channel represents the alveolar air compartment through which air can flow, whereas the lower channel represents the alveolar capillary through which a “blood-like” substance can be pumped. The channels are separated by a porous membrane on which lung epithelium (air compartment) and endothelium (capillary compartment) are seeded (see bottom left). The vacuum channels on each side of the air and capillary channels can be used to stretch the porous membrane to which the cells are attached to mimic the breathing motion of the human lung (bottom right). Figure has been reproduced with permission from Huh et al. [20], Copyright: Nature Protocols.

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Similar methodologies have been used with different hydrogels with the end goal of creating vascular networks in vitro to study different aspects of vascular biology. Boland et al. used type I collagen gels with embedded cells and hollow channels that were later seeded with ECs [24]. This simple system allows for the creation and design of defined microvascular endothelialized geometries that are useful for the study of permeability and blood
vasculature interactions in a tissue-engineered construct.

14.5.4 Bioreactors for In Situ Simulations Initially, tissue engineers treated bioreactors as “black boxes,” where tissue engineering constructs (TECs) are cultured. Quickly, they realized that an adequate mathematical description of the controlled environment is necessary, in order to optimize the operating conditions together with the bioreactor topology, to ensure proper cellular responses such as attachment, migration, and proliferation. Nowadays, computational fluid dynamics is used to get a more detailed description of fluid mechanics and nutrient transport within bioreactors, including here the impact of fluidic forces and stresses on cells and TECs. A biomedical engineering bioreactor is a confined volume, where a biological process develops, under tight control of temporal and spatial state variables. The bioreactor exchanges mass and energy with the surrounding world through controlled interfaces. The biological processes imply living mammalian cells, which can be suspended as free cells or small agglomerates, or adhered to the surface of a solid phase (scaffold), where they multiply and differentiate according to environmental cues. Though many complex loading patterns can exist within the body, most loading patterns used in bioreactors can be described simply through some combination of stretch, shear, pressure, compression, bending, and torsion. The kinetics of the living cells population is extremely complex, and should take into account the segregated nature of the population— cells are not clones, but clusters of individuals in different cell-cycle stages [25]. The classification of the bioreactors can be done according to several criteria, like the internal flow field, the operating regimen, the number of phases present in the system, the thermal regimen, etc. All the bioreactors used in biomedical engineering are heterogeneous due to the presence of at least the suspended microorganisms free or agglomerated in the liquid phase, if not the scaffold on which the microorganisms adhere. The design principles behind bioreactors used for tissue engineering can be

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categorized as elements which promote tissue health and elements which guide tissue function. A number of different mixing bioreactor designs can be utilized to ensure sufficient nutrition delivery and cell survival. Tissue structure, organization, and function can be formed through a variety of bioreactors that utilize appropriate mechanical loading. Proper implementation of these bioreactor design principles can lead to better tissue-engineered products like artificial organs for transplantation.

14.5.5 Complexities in Biopatterning Coupling optical behavior to biodegradable materials can provide the next generation of soft biophotonic devices. For instance, periodically structured materials are commonly found in natural systems to form coloration and remarkable optical behavior that are purely physical in nature. Simultaneously, a big challenge in the fabrication of high-resolution soft micro-optical systems is the adaptation of fabrication strategies to form architectures of high complexity and spatial resolution, while being scalable and low cost [26]. Photolithography is a widely used and well-developed tool in the semiconductor industry that is also scalable (micro- and nanostructures can be repeatably, rapidly, and reproducibly formed over large areas). The protocols are highly developed and a wide diversity of shapes and sizes can be patterned. Among the various biopolymers reported, silk proteins have stood out as particularly attractive building blocks for optical components at the micro- and nanoscales

14.6 BIOPRINTING OF ORGANS AND TISSUES Bioprinting is generally considered to be the application of additive manufacturing techniques to create cell-based scaffolds. Many of these techniques can be adapted to print with cells as long as the material, deposition method, and processing minimally impact cell viability and function. Biological materials used for printing need to match the native environment of the host to support the function of those cells. In addition, the cells must be able to overcome the shear stress during the printing process and survive the nonphysiological conditions of the printing regimen [27]. A wide variety of available bioprinting techniques have shown promise in creating complex architectures by using a “bioink” that is printed onto a substrate in a layer-by-layer process to create 3D constructs that mimic native tissue and organs.

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The field of 3D printing continues to make advances in leaps and bounds, with successive implementations pushing the frontiers of resolution and widening the diversity of printing media. While initially working with starches and polymers, 3D printing technology has expanded to incorporate metals, sintered ceramics by extrusion, selective laser sintering, and a number of other materials. Modern devices, such as the Stratasys Dimension Elite, reach resolutions of 250 μm, while those that use stereolithography rather than fused media reach into the hundreds of microns, recently achieving layers that are only tens of microns thick [28]. Twophoton polymerization techniques can achieve some of the best resolutions available at present, with 200 nm repeatable features achievable. However, such techniques are fundamentally limited by the effects of surface tension of the fusing medium, an ability to remove heat from the sintering volume, or the quality of the optical mask for stereolithography. In addition, all the above methods have the fundamental limitation of homogeneous media, meaning that one type of material and composition must be the medium for construction. The bioprinting process is not all that different from an inkjet printer except a mixture of hydrogel and living cells are used as the “ink” to incrementally build a defined biological structure. The precision of biologically inspired 3D printing makes it a promising method for replicating the body’s complex tissues and organs. However, current technologies used for printing organ and tissue miniatures based on jetting, extrusion, and laser-induced forward transfer cannot produce structures with sufficient size or strength to implant in the body. Not all 3D printing methods are additive in nature. Subtractive methods have shown more promise at increasing resolution of 3D printing. Recent advances in two-photon lithography have produced wafer-sized 3D periodic structures with micron resolution. Ion beam lithography, by avoiding photons entirely, can extend the resolution below the 100 nm range. The structures formed can be heterogeneous if layers of different compositions are deposited in sequence before lithographic steps [29]. A combination of holographic lithography and two-photon laser writing has been demonstrated to create 3D photonic crystals by selective creation of localized structural defects. Here the throughput is greatly enhanced by using holographic lithography to determine an overall structure, and two-photon writing to make smaller, localized modifications. Soft lithography, meanwhile, has been used to create 3D, multilayer microfluidic devices, while a similar method

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combined with a 3D sugar printer is used to create sugar-filled channels that can be dissolved after fabrication to create microfluidic channels. Most recently, IBM has developed a scanning probe lithography system, which can exceed the electron beam lithography resolution limit, subtractively creating 3D replicas of objects with single nanometer height resolution. Subtractive methods, however, cannot create 3D structures of arbitrary complexity with respect to both shape and material heterogeneity.

14.6.1 History of Bioprinting Traditional techniques for fabricating tissue engineering scaffolds such as gas foaming, solvent casting, fiber bonding, phase separation, particulate leaching, and freeze drying provide macroscale scaffold features but often lack the complexity of native tissue. Many tissues, such as the lobules of the liver or nephrons of the kidney, have complex structural units that coordinate multiple types of specialized cells and are critical for tissue function [30]. Fabrication methods that can produce complex geometries have a distinct advantage in their ability to fit an irregular defect site but are also capable of mimicking tissue complexity through the precise positioning of multiple materials and cell types (Table 14.1).

14.6.2 Techniques Used in Bioprinting Techniques with even higher precision are currently being investigated to enable reproduction of smaller tissue features such as hepatic lobules and kidney nephrons. Despite the expanding number of rapid prototyping techniques and variants that have emerged, categories can be used to group these techniques based on the material type and method used to combine each layer. In inkjet-based bioprinting, bioink droplets are deposited onto a substrate that gels to form polymeric structures [31]. Microextrusion bioprinting, however, uses a mechanical extruder to deposit the bioink as the extruder is moved. Extrusion-based bioprinting allows for the use of high cell density with easier processing, but occurs at a slower speed than drop-based bioprinting. Laser-assisted bioprinting (LAB) has a picoliter (pL) resolution through which cells and liquid materials can be printed. This method of printing is rapidly growing and shows promise to fabricate tissue-like constructs that mimic the physiogical behavior of their host counterpart [32]. Each of these bioprinting methods

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Table 14.1 The history and key developmental stages in the progress of 3D bioprinting Year Key developments

1984 1986 1989 1993 1996 1999

2002 2005 2007 2007 2009 2009 2010 2012 2014

Charles Hull invents “Apparatus for making 3D objects by stereolithography” Carl Deckard invents “Method and apparatus for producing parts by selective sintering” Scott Crump, cofounder of Stratasys, patents Fused Deposition Modeling MIT patents “3 Dimensional Printing techniques” and licenses to six companies Innovations started on clinical application of biomaterials for tissue regeneration Luke Massella receives the first 3D-printed bladders using a combination of 3D-printed biomaterials and his own cells at Wake Forest Institute Early-stage kidney prototype manufactured at Wake Forest Institute Adrian Bowyer (Rep Rap)—3D printer that can print most of its own components Selective laser sintering machine creates 3D-printed parts from fused metal/plastic. RepRap releases Darwin, the first self-replicating printer Fused deposition modeling patent expires, boost for innovations in industry Maker Bot starts selling customized kits to make a 3D printer Organovo, Inc., announced the first fully bioprinted blood vessels Extrusion-based (syringe) bioprinting for an artificial liver Multiarm bioprinter to integrate tissue fabrication with printed vasculature

is discussed coupled with a focus on their respective print mechanics, applications, and drawbacks. Recently, a research group lead by Prof. Michael P. Short (Massachusetts Institute of Technology) designed and nearly completed building the first prototype of our 3D micromaterial printer. The printer functions at the one-micron resolution level, close to the limit of optical microscopy. It consists of a custom-designed optical microscope, with a 400-mW ultraviolet sintering diode laser in focus with the microscope’s optical plane, and a Sutter Instruments nanomanipulator with 40-nm resolution to pick-and-place starting materials from an “artist’s palette” of nonclose-packed microspheres (Fig. 14.9). There are three broad categories of bioprinting, namely microextrusion, LAB, and inkjet-based bioprinting (Fig. 14.10).

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Figure 14.9 Extrusion based bio-printing has great versatility in printing various biologics, including cells, tissues, tissue constructs, organ modules and microfluidic devices, in applications from basic research and pharmaceutics to clinics. (A) pneumatic micro-extrusion including (A1) valve-free and (A2) valve-based, (B) mechanical micro-extrusion including (B1) piston- or (B2) screw-driven and (C) solenoid microextrusion. Adapted with permission from Ozbolat, Hospodiuk. Current advances and future perspectives in extrusion-based bioprinting. Biomaterials 76 (2016) 321
343 http://dx.doi.org/10.1016/j.biomaterials.2015.10.076.

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Figure 14.10 Schematic comparison of commonly used 3D bioprinting techniques: (A) microextrusion bioprinting, (B) laser-assisted bioprinting, and (C) inkjet bioprinting. (1) Different bioprinting approaches. (2) Exemplary scaffolds composed of 10% w/v gelatin methacrylamide illustrating the resolution and detail of microextrusion bioprinting. (3) Exemplary patterns consisting of a high density of cells in culture medium illustrating the resolution and detail possible with laser-assisted bioprinting. (4) Exemplary scaffolds composed of alginate and multiple cell types, as indicated by different colors, illustrating the resolution and detail possible with inkjet-based bioprinting. Adapted from S. Derakhshanfar, R. Mbeleck, K. Xu, X. Zhang, W. Zhong, M. Xing. 3D bioprinting for biomedical devices and tissue engineering: A review of recent trends and advances (2018), 3 (2), pp: 144
156.

14.6.2.1 Extrusion Bioprinting Due to the popularity of open-source projects, such as RepRap and Fab@home, extrusion-based printing methods have become one of the most economical techniques for rapid prototyping. Extrusion bioprinting is a type of Solid Freeform Fabrication (SFF) that typically involves pressure or screw/plunger-actuated dispensing of a fluid containing cells and/or biomaterials. An ideal bioink for extrusion-based bioprinting should be shear thinning to allow for minimal resistance under flow but must also chemically or physically crosslink relatively quickly after extrusion to support successive layers. Furthermore, possible detrimental

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effects of polymerization and shear forces on cell viability and function must be considered [33]. The ability of extrusion-based bioprinters to accurately deposit material allows for the fabrication of complex patterned structures, including the use of multiple cell types. Compared with the other methods discussed, extrusion-based bioprinting is capable of depositing materials with a high concentration of cells to accelerate growth and neotissue formation. Increasing print resolution and increasing print speed are challenges in extrusion-based bioprinting. Using biocompatible materials that have improved mechanical properties during the printing process will improve cell viability. Furthermore, modification of print mechanics might decrease print times and permit coextrusion of multiple materials. Although fabrication time is relatively long to achieve high resolution in complex structures, extrusion-based bioprinting has successfully demonstrated the fabrication of clinically relevant scaffolds for tissue engineering. Similar to other SFF techniques, extrusion bioprinting is ideally suited for biological materials because of its ability to deposit multiple materials with wide-ranging properties. Extrusion-bioprinted scaffolds are typically soft, because of their high water content, and the deposited material must undergo some form of gelation to support each layer. Therefore, without some kind of mechanical reinforcement, these scaffolds are typically limited to soft-tissue applications.

14.6.2.2 Laser-Assisted Bioprinting LAB, also known as biological laser printing, is a group of techniques that use laser energy to facilitate transfer or coordination of scaffold materials. One type of LAB is laser-based direct writing (LDW) that uses a laser pulse to locally heat a slide consisting of an energy-absorbing layer and solution of cells. Laser patterning of biological scaffolds was first demonstrated by Odde and Renn. The laser pulse causes sublimation or evaporation of material, expelling the solution of cells on the opposite side and precisely depositing them on the substrate. LDW methods can be further subdivided into laser-induced forward transfer and matrix-assisted pulsed laser evaporation direct writing, which have been used to deposit fibroblasts, keratinocytes, and human mesenchymal stem cells (hMSCs), various cancer cell lines, and a range of biopolymers. LDW is nozzle-free, thereby permitting the use of high-viscosity bioink unlike that of drop-based bioprinting or extrusion-based bioprinting. In

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addition, lasers allow for high precision, thus making this method ideal for bioprinting the smallest details of native tissues and organs. LDW printing has been successfully demonstrated with various cells and biomaterials. Gaebel et al. used LAB printing to pattern human umbilical vein endothelial cells and hMSCs onto a polyester urethane cardiac patch that showed improved cardiac function up to 8 weeks after myocardial infarction. Although this technique allows for direct printing of cells, there remain several limitations that should be considered. The heat and damaging forces resulting from the laser pulse can have a detrimental effect on cell survival and long-term behavior. Additional challenges to LAB printing include increased build time, difficulty building scaffold height, and need for new biomaterials that can be crosslinked after deposition. Gudapati et al. reported that cell encapsulation in crosslinked hydrogels was critical for cell survival in laser-based bioprinting techniques. LAB methods offer the most precise positioning of cells and cellular material, but are the most limited in their ability to build constructs vertically [34]. Laser-based methods are most applicable in conjunction with other techniques or methods to create 3D scaffolds. 14.6.2.3 Inkjet Bioprinting Inkjet bioprinting is a powerful method of precisely depositing cells and biomaterials that leverages sophisticated advances in 2D inkjet printing to create 3D scaffolds. In inkjet bioprinting, a fixed volume of fluid is jetted into a precise pattern specified by the software. Inkjet bioprinting has become a popular method in fabricating cell-laden constructs that can mimic the complexity of native tissue or organs. One key advantage of this technique is the speed at which it can construct scaffolds while maintaining a complex 3D architecture. This speed also poses challenges as it severely limits the number of polymeric materials that can be used to bioprint as it requires the gelation time to be greater than or equal to the drop deposition time. Inkjet bioprinters can be adjusted and specifically tailored to allow for printing materials at increasing resolutions and speeds. Inkjet bioprinting uses thermal or piezoelectric energy to deposit droplets of solution into a predefined pattern. Inkjet bioprinters typically consist of one or many ink chambers with multiple nozzles corresponding to piezoelectric or heating components. To eject a droplet of ink, a short pulse of current is applied to actuate the component. In thermal bioprinters, the sudden increase in local temperature causes vapor bubbles to form and collapse, ejecting ink droplets onto the substrate [35].

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In piezoelectric inkjet printing, piezocrystals actuate the chamber itself, causing an increase in pressure, resulting in droplet ejection. Deposition from the nozzle onto the print bed results when an electric charge induces vibration in the crystals. Heat and mechanical stresses generated during thermal inkjet bioprinting have been shown to adversely affect cell viability. The largest detrimental effect occurs in the nozzle orifice where the temperature is greatest. There is a need to mitigate and alleviate this issue. Lorber et al. were able to successfully print retinal ganglion and glia cells harvested from the adult central nervous system without causing an adverse effect on cell viability. From this study, researchers were able to show that piezoelectric printing did not compromise the phenotype or activity of these cells. In an effort to increase throughput and accessibility to this technology, Boland et al. reported the printing of thermosensitive gels by using a modified cartridge from a commercially available inkjet printer to create multilayer scaffolds. In addition, researchers have successfully demonstrated a multihead inkjet-based approach for bioprinting multiple cell lines into heterogeneous scaffolds for tissue engineering. A key disadvantage of inkjet printing is that the biological agents need to be in a liquid state to permit deposition. The deposited droplets must then solidify into the required geometry. To address this requirement, commonly used materials are crosslinked using physical, chemical, pH, or ultraviolet methods. However, chemical crosslinking of many natural materials, such as those derived from ECM, modifies both the chemical and material properties, and the use of some crosslinking mechanisms is known to pose a detriment to cells, thus decreasing cell viability and functionality. Although inkjet bioprinting allows for encapsulation of live cells, relatively low concentrations are required to form cohesive droplets and prevent clogging of the nozzle. Despite the addressed disadvantages, inkjetbased bioprinters continue to have great potential because of their low cost, high resolution, and high compatibility with many biomaterials. Because commercially available 2D printers harness this technology, researchers can easily adapt components for research applications. The versatility of inkjetbased technology has refined the capabilities of these printers to accurately deposit fine droplets with precise volume to create high-resolution scaffolds with cells intact. Droplet size can be modulated from 1 to 300 pL with deposition rates from 1 to 10,000 droplets per second. Future work will continue to grow this technology to print more biologically relevant materials and to further retain the functionality and

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bioactivity of cells and biomaterials. Multimaterial printing using inkjet technology is a developing adaptation that needs to be further developed to print multiple cell types in complex constructs. Inkjet bioprinting is capable of creating scaffolds with accuracy within 100 μm, which makes it very useful for creating complex tissue-engineered scaffolds. Although it is limited in its ability to produce tall structures because of the typical mechanical properties of the gel inks, the ability to print multiple materials and cell types makes it a useful method to create complex tissue with great accuracy.

14.6.3 Applications of Bioprinting Three-dimensional (3D) bioprinting has been a powerful tool in patterning and precisely placing biologics, including living cells, nucleic acids, drug particles, proteins, and growth factors, to recapitulate tissue anatomy, biology, and physiology. Since the first cytoscribing of cells was demonstrated in 1986, bioprinting has made a substantial leap forward, particularly in the past 10 years, and it has been widely used in the fabrication of living tissues for various application areas [36]. The technology has been recently commercialized by several emerging businesses, and bioprinters and bioprinted tissues have gained significant interest in medicine and pharmaceutics. Scientists from the University of Illinois have made what they called “bio-bots” or tiny machines “powered by biological components.” They printed muscle cells onto flexible skeletons in the shape of rings. The muscle cells are engineered to have light-sensitive switches, so when they are exposed to light, they contract like normal muscles do. The beauty of bio-bots is that they “can sense, process, and respond to dynamic environmental signals in real time, enabling a variety of applications.” Some of these applications could include bio-bots made up of other types of tissue (brain, heart, etc.) and general use for disease research (Fig. 14.11).

14.6.4 Organ Specific Bioprinting The emergence of hybrid multicomponent gels that integrate desirable physical properties from each constituent component represents an exciting new direction in bioink development. For instance, biodegradable polymers are commonly strengthened with osteoinductive ceramics, such as calcium phosphate, nanofibrous cellulose has been used to increase the shear thinning of alginate gels, while a mixture of Pluronic and acrylated Pluronic has been

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Figure 14.11 The design of a biobot inspired by the muscle
tendon
bone complex found in nature. There is a backbone of 3D printed hydrogel, strong enough to give the bio-bot structure but flexible enough to bend like a joint. Two posts serve to anchor a strip of muscle to the backbone, like tendons attach muscle to bone, but the posts also act as feet for the bio-bot. A bot’s speed can be controlled by adjusting the frequency of the electric pulses. A higher frequency causes the muscle to contract faster, thus speeding up the bio-bot’s progress. Graphic by Janet SinnHanlon, Design Group@VetMed. From https://news.illinois.edu/view/6367/204565.

used to generate a synthetic gel that can be crosslinked using both temperature and ultraviolet irradiation [37]. While these hybrid systems report printability and short-term cytocompatibility (4
14 days), they have not demonstrated practical applicability over a long-term, tissue engineering course. Here, we report on the rational design of a novel Pluronic
alginate multicomponent bioink with complex phase behavior, which was used in a two-step 3D printing process to engineer bone and cartilage architectures. Specifically, 3D structures containing hMSCs were printed by extruding the shear-thinning, cell-laden gel onto a heated stage, resulting in instantaneous solidification via the sol
gel transition of the Pluronic, and the structures were then stabilized through alginate crosslinking using CaCl2 immersion. The Pluronic constituent also served as a sacrificial template, being completely expelled during crosslinking, which drove the formation of micron-sized pores or anisotropic microchannels [38]. Moreover, the Pluronic-templated alginate gel exhibited favorable biomaterial properties, including increased shear thinning, compressive modulus, and shear modulus.

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This provided a platform for printing macroscopic structures (ear, nose, tracheal cartilage ring), as well as fine fibers and meshes. Significantly, hMSCladen 3D architectures showed no significant loss in cell viability over 10 days, and the encapsulated cells could be differentiated into osteoblasts and chondrocytes to engineer printed tissue constructs over 5 weeks, including a full-size tracheal cartilage ring. Recently, scientists from Wake Forest University have developed technology to make custom-made living body parts by 3D-printing stem cells onto biodegradable scaffolds [39]. The stem cells are printed in a hydrogel solution using a special 3D printer they call, Integrated Tissue-Organ Printer (ITOP). This printer makes it possible for the printed stem cells to develop into life-sized tissues and organs that have built-in microchannels that allow blood, oxygen, and other nutrients to flow through. Using the ITOP technology, the team was able to generate segments of jawbone, an ear, and muscle tissue. To demonstrate that ITOP can generate organized soft-tissue structures, printed muscle tissue was implanted in rats. After 2 weeks, tests confirmed that the muscle was robust enough to maintain its structural characteristics, become vascularized, and induce nerve formation [40] (Fig. 14.12). After several weeks of incubation in liquid nutrients, a matrix of cartilage had grown throughout the ear. And, to look at tissue growth in an animal, the ear was implanted under the skin of the mice. A couple of months after implantation, even more cartilage had formed and the shape of the ear was intact (Fig. 14.13).

Figure 14.12 Three-dimensional printing external ear structures, consisting of the pinna and ear lobe, that benefit hearing and are important cosmetically. The pinna or ear shell is the shell-like part of the external ear and is made of cartilage and skin. The pinna directs sound waves from the outside into the external auditory canal (ear canal). Adapted from H.W. Kang, S.J. Lee, I.K.Ko, C. Kengla, J.J. Yoo, A. Atala. A 3D bioprinting system to produce human-scale tissue constructs with structural integrity,Nat Biotechnol. 2016 Mar;34(3):312
9.

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Figure 14.13 Three-dimensional printed pinna (external ear) made of cartilage and skin. The pinna directs sound waves from the outside into the external auditory canal (ear canal). Used with permission from Wake Forest Institute for Regenerative Medicine.

Muscle-forming cells, or myoblasts, were printed to mimic the muscle fiber bundles seen in native skeletal muscle. After growing a week in the lab under conditions that stimulate muscle cell formation, the muscle-like fibers were implanted into rats. Two weeks after implantation, the bioprinted muscle had not only grown into well-organized muscle fibers, they also were functional in that they were responsive to electrical stimulation. For reconstruction of the jawbone, a 3D computer model was generated from actual CT scan data of a human jaw with a missing piece of bone—as in the case of a traumatic injury—the precise printing pattern necessary to rebuild the shape of the jaw fragment. In a case study, bioprinting was carried out using human amniotic fluid stem cells. With the right cues, these stem cells readily specialize into osteogenic (bone-forming) cells. After 28 days being cultured in liquid nutrients containing bone-promoting factors, the surface of the bioprinted human jaw showed calcium deposits, with plenty of blood vessels and no necrosis, or cell death, inside the bone.

14.6.5 Complexities in Bioprinting The technology has advanced considerably so that it can enable us to print any organ we need, although a lot more testing is needed to safely bring this technology into a clinical setting for human use. Compared with nonbiological printing, 3D bioprinting involves

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additional complexities, such as the choice of materials, cell types, growth and differentiation factors, and technical challenges related to the sensitivities of living cells and the construction of tissues [41]. Addressing these complexities requires the integration of technologies from the fields of engineering, biomaterials science, cell biology, physics, and medicine. Three-dimensional bioprinting has already been used for the generation and transplantation of several tissues, including multilayered skin, bone, vascular grafts, tracheal splints, heart tissue, and cartilaginous structures. Other applications include developing highthroughput 3D-bioprinted tissue models for research, drug discovery, and toxicology.

14.7 CONCLUSIONS The practical application of micro- and nano-miniaturized platforms for nanotheranostics and regenerative medicine is still in an early phase of development, but the first proof-of-concept systems already demonstrate how organs-on-chips could be effectively implemented in the drug development process and complement current high-throughput screening methods for identifying drug candidates and targets. Alternatively, at the preclinical stage of drug development, organs-on-chips could provide additional information on human relevance, which may eventually lead to partial replacement of animal testing. There are, however, challenges to organ-on-chip development, particularly in the design choices. These design choices could be the following: what are the costs, which materials should be used, is there a preferred cell source, and what on-chip sensors and analytical measures are available and should be used. A combination of these choices could also be preferred; however, not all design choices can be implemented, and therefore tradeoffs have to be made. Which choices have a higher weight according to stakeholders? There are many ways of using organ-on-chip systems in drug development, and therefore the dialog between developers (or researchers) and other stakeholders at an early stage is important to decide the exact steps that should be taken in the development. On the one hand, the developers are important stakeholders with respect to their knowledge of the possibilities and limitations of the technology. For example, there are still several challenges to overcome before multiple organs can be connected. On the other hand, future end-users should also be included to give their view on how organ-on-chip technology could have an impact on drug development.

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The exact purpose of the model is an important factor that can be defined by asking questions such as, “Do the users need a model for general toxicity screening or for a specific target organ?” and “What are the most valuable read-outs for toxicity?” Furthermore, the specific pharmacological information that the model should yield is an important factor. Examples of questions related to this factor are as follows: “Do the users need a system to model the effects of different drug dosages or the properties of the drug at certain dosages?,” “How long does the model need to be viable?,” and “How can a drug best be delivered?” Another factor that can be surveyed is how different end-users think about concrete design choices, for example, by asking questions such as, “Does this model need to be organor tissue-specific?,” “Would end-users prefer a more specific disease model?,” “How many target cell types should be included?,” “Is it easier to use than the current golden standard models?,” and “Is it important to include multiple connected organs on one chip?” Finally, an essential consideration is how to factor in the cost and economic impact of organs-on-chips in drug development, for example, by asking: “What would be the acceptable cost per data point?” Different users could have different opinions. The users could be academic researchers in a university (biomedical research) or pharmaceutical companies. These two groups can differ significantly in opinions because of the different purposes of the systems.

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CHAPTER 15

Recent Trends in the Synthesis of Carbon Nanomaterials María M. Afonso and José Antonio Palenzuela Department of Organic Chemistry, Universitary Institute of Bio-Organic Chemistry, University of La Laguna, La Laguna, Tenerife, Spain

Contents 15.1 Fullerenes 15.1.1 Pristine Fullerenes 15.1.2 Endohedral Fullerenes From Carbon Sources 15.1.3 Endohedral Fullerenes From Pristine Fullerenes 15.1.4 Exohedral Fullerenes 15.2 Carbon Nanotubes 15.2.1 Carbon Nanotube Synthesis and Production 15.2.2 Functionalization of Carbon Nanotubes 15.2.3 Biobased Synthetic Routes 15.3 Graphene 15.3.1 Graphene Synthesis and Production 15.3.2 Biobased Synthetic Routes 15.3.3 Chemical Synthesis 15.3.4 Graphene Heterostructures 15.3.5 Graphene Nanocomposites 15.3.6 Heteroatom-Doped Graphene 15.3.7 Halogenated Graphenes 15.3.8 Hydrogenated Graphenes 15.3.9 Chemical Functionalization of Graphene 15.4 Graphene Nanoribbons 15.5 Carbon Dots 15.5.1 Carbon Dot Synthesis 15.5.2 Biobased Synthesis Routes 15.6 Challenges and Future Perspectives References

Nanomaterials Synthesis DOI: https://doi.org/10.1016/B978-0-12-815751-0.00015-8

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15.1 FULLERENES After the discovery of the fullerenes in 1985 [1], many laboratories around the world started to work on the synthesis, modification, and applications of these fascinating compounds. As a result, in a short time, the most common fullerenes, C60 and C70, have become commercially available, due especially to the introduction of the vaporization of graphite by arch discharge, a method presented by Krätschmer and coworkers [2]. Since then, the research in this field has grown continuously, has reached the market in several industrial applications, and has diversified into various branches. Many review articles and books on the different areas have become available. In this section, we review the currently used synthetic methods for the most relevant areas of research on fullerenes and discuss the expected evolution of the field following the general scheme shown in Fig. 15.1.

Figure 15.1 Synthetic pathways used in the synthesis of fullerene derivatives.

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15.1.1 Pristine Fullerenes Several methods used for the vaporization of graphite such as electron beam evaporation [3], heat resistivity [4], diffusion flame [5], ion beam sputtering [6], or the use of solar furnaces [7] were also used to prepare fullerenes following path a in Fig. 15.1. Fullerenes in large amounts are also prepared by pyrolysis of polycyclic aromatic hydrocarbon [8], combustion of hydrocarbons [9], or by using coal as the source of carbon [10]. Today, the worldwide production of fullerenes is estimated at several tons/year using either the combustion of hydrocarbon method [11], which provides fullerene-rich carbon shot in a continuous manner or scaled-up versions of the arc discharge method [12]. The most important production is of C60 and C70, both as a mixture and as pure compounds. The chemical synthesis of fullerenes has also been accomplished using lengthy synthetic procedures, resulting in the production of specific isomers in pure form [13]. To study the formation process, less harsh conditions are needed, and, for instance, it has been determined that the on-surface synthesis of fullerenes can be accomplished by placing an appropriate aromatic hydrocarbon on a Pt(111) surface and applying heat. This way, C60 [14] and C84 [15] have been synthesized. Also, the electron beam irradiation of graphene under transmission electron microscopy (TEM) has been used to study the formation of fullerenes [16]. Today, the efficient synthesis of fullerenes with sizes different from C60 or C70 is still a challenge, although progress has been made in the preparation of fullerenes of different sizes, such as giant fullerenes [17]. Although fullerenes are interesting by themselves, research quickly focused on the modification of their structure, seeking a change in properties. The modifications included the introduction of atoms or molecules inside the cage following path b or c in Fig. 15.1 (endohedral fullerenes) or functionalization of the surface of the fullerene as in path d in Fig. 15.1, (exohedral fullerenes) either in a covalent or noncovalent fashion. Also, hybrids of endohedral fullerenes with exohedral modifications (path e in Fig. 15.1) have been prepared. In this part of the chapter, we present the most recent synthesis of the different types of fullerene derivatives.

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15.1.2 Endohedral Fullerenes From Carbon Sources Endohedral fullerenes can be obtained directly from a carbon source (path b in Fig. 15.1) or from pristine fullerenes (path c in Fig. 15.1). Path b is the main route for the preparation of endohedral metallofullerenes (EMFs). As soon as 1985, mass spectrum peaks assigned to La-containing fullerenes were observed [18], although the isolation and characterization of the La-containing fullerenes, especially large ones, were reported in 1991 [19]. The technique used to synthesize those EMFs was the same as that used for pristine compounds, vaporization of graphite by one of the techniques available, but mixing the graphite rod with a metal oxide. This resulted in mixtures of empty fullerenes of different sizes and fullerenes containing the metal coming from the corresponding oxide. The yield and selectivity depend on the technique used [20]. The main method used today for the formation of EMFs is the arc discharge method using the Krätschmer
Huffman system, usually with some modifications [20]. Many of the metals introduced in the fullerene cage correspond to the clusterfullerene family. Since 1999, different modifications to the arch discharge technique by the introduction of different gases such as nitrogen, ammonia, and others have resulted in the production of fullerenes with metallic clusters inside the cage [21]. Today, different families of clusterfullerenes are known, such as metal nitride, metal carbide, metal oxide, metal sulfide, metal cyanide, and metal carbonitride [22]. These methods have been used to prepare fullerenes mainly with metals of groups IIIB and IVB. Metals from other groups are still a challenge, although some examples, such as V [23] or Pt [24] among others, have been reported. The preparation of EMFs in large quantities for practical use is still an unresolved challenge. Other techniques such as laser ablation, although giving lower yields and being more costly, are also used when more controlled conditions are needed, especially when the mechanism of formation is being studied. An interesting result is that the introduction of metals inside the cage of a fullerene allows the formation of isomers with two adjacent pentagons, breaking the isolated pentagon rule, which indicates that those compounds are not stable.

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The largest cluster encapsulated inside a fullerene by the methods mentioned contained seven atoms [25]. This was the record until the use of electron irradiation, in aberration-corrected high-resolution transmission electron spectroscopy of a metal cluster surrounded by amorphous carbon inside a carbon nanotube (CNT) serving as a nanoreactor, produced fullerenes enclosing up to 55 atoms of Ni [26].

15.1.3 Endohedral Fullerenes From Pristine Fullerenes The introduction of atoms inside the fullerene cage has been achieved by different methods. Using high temperature and pressure, noble gases He and Ne were introduced into the C60 cage as early as 1993 [27]. More recently, Ar [28], Kr [29], and Xe [30] have been encapsulated into the C60 fullerene following path c in Fig. 15.1. Nitrogen atom-containing C60 was reported in 1996 using ion implantation, by the bombardment of C60 with nitrogen ions from a plasma discharge ion source [31]. This technique has been used to introduce other atoms inside the fullerene cage, including metals [32]. Another method developed to encapsulate nonmetal-containing molecules is so-called molecular surgery, consisting of the chemical opening of the fullerene cage, creating a hole big enough to let the desired molecule enter, and chemically closing the hole [33]. This way dihydrogen was first introduced into C60. Later, the same team successfully encapsulated He into C60 and C70 cages. In 2011, H2O was introduced into a C60 cage by the same method, thus providing an environment to study an isolated water molecule [34]. Two recent uses of the molecular surgery methodology are the encapsulation of HF [35] and of two molecules of water in the same cage, this time in a larger C70 fullerene [36]. Research into new and efficient methods for the opening and closing of the fullerene cage is currently being conducted [37].

15.1.4 Exohedral Fullerenes Modifications at the surface of the fullerenes have received a great deal of attention, since for many practical uses it is necessary to modify the physicochemical properties. For instance, in biological applications water solubility is an important factor, and for other uses lipophilicity is needed. This chemistry has been used in both empty and EMFs (paths d and e in Fig. 15.1).

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Fullerenes behave as electron-poor polyenes, and so chemical reactions working on those systems apply to fullerenes. Although many reactions have been used to modify fullerenes, most of the chemistry performed on those substrates belongs to a short number of reactions. The more commonly used reactions are described below. 15.1.4.1 Carbene Addition Carbene addition to fullerenes has been reported in both empty and EMFs. The carbenes used range from simple dichlorocarbenes to more complex ones carrying functional groups, which can serve for further transformations [38]. Recent publications on carbene addition to fullerenes indicate that this is a method of interest [39]. 15.1.4.2 Bingel
Hirsch Reaction The cyclopropanation of fullerenes using malonates is one of the most commonly used reactions on fullerenes. The possibility of modifying malonate to introduce other functional groups makes this reaction an excellent entry point to prepare larger compounds [40]. Depending on the reaction conditions, only one malonate unit is inserted on the fullerene or up to six are symmetrically distributed on the surface [41]. Recently this approach has been used for the preparation of large molecules with high symmetry for different uses, such as antivirus agent [42] or for the creation of metal-organic frameworks [43]. 15.1.4.3 Prato Reaction The most commonly used cycloaddition reaction on fullerenes is the 1,3-dipolar cycloaddition of azomethine ylides, known as the Prato reaction. This reaction introduces a pyrrolidine moiety on the surface of the fullerene that can be used for further transformations and is in use today [44,45]. This reaction has been used to prepare enantiopure fullerenes [46] 15.1.4.4 Other Reactions Used Many other reactions such as Diels
Alder reactions, [2 1 2] cycloadditions, oxidation and epoxidation, hydrogenation, or perfluoroalkylation have been used to functionalize fullerenes. Those are covered in several reviews and books [47,48].

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15.2 CARBON NANOTUBES CNTs can be described as rolled-up graphene sheets with nanometer diameter and length ranging from a few microns to several millimeters. Because the circular curvature will cause quantum confinement and σ
π rehybridization, CNTs display extraordinary properties such as high tensile strength, and excellent electrical and thermal conductivities. They are classified into single-walled CNTs (SWCNTs), double-walled CNTs (DWCNTs), and multiwalled CNTs (MWCNTs), etc. based on the number of layers [49,50]. CNTs are usually obtained as arrays, and based on morphology differences, CNT arrays can be divided into three types: randomly stacked, vertically aligned (VACNT) arrays, and horizontally aligned (HACNT) arrays [51]. For SWCNTs, the way of rolling up the graphene sheet can be defined by a vector called the “chiral vector.” The vector is determined by two integers (n,m). Three types of SWCNTs can be constructed, achiral armchair (n,m; n 5 m), achiral zigzag (n,0), and chiral SWCNTs (n,m; n . m and m6¼0). This is relevant from a synthetic point of view since the electronic and optical properties of the three types of SWCNTs are different. For instance, some show metallic behavior (m-SWCNTs), whereas others are semiconductors (s-SWCNTs) and thus, a selective synthesis of each type is highly desirable.

15.2.1 Carbon Nanotube Synthesis and Production CNTs came to the attention of the scientific community when Ijiima and coworkers [52,53] reported the structural determination of MWCNTs by TEM in 1991. The CNTs were isolated from carbon shot produced by the arc discharge method. Thereafter, thousands of publications and patents have been published on the synthesis and applications of CNTs. Nowadays, CNTs are available for industrial applications in metric ton quantities and several commercial products containing CNTs have reached the market, mostly using MWCNTs. CNTs can be produced by arc discharge, laser ablation, and chemical vapor deposition (CVD). Several general reviews have been published on this topic [54
57]. Arc discharge and laser ablation were the first methods used to produce CNTs in large amounts (grams). These methods consist of the vaporization of a carbon source and posterior deposition onto a substrate. The arc discharge synthesis of CNTs generally involves arc-vaporization of two graphite water-cooled electrodes, separated by approximately

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1 mm, in a chamber filled with inert gas at low pressure. The use of hydrogen or methane atmospheres has also been reported. The yields of CNTs depend on the uniformity of the plasma arc and the temperature. Although the most commonly used carbon source for arc discharge is graphite, the use of carbon black or coal has been reported. Usually MWCNTs are produced with no catalyst and SWCNTs are produced when transition metal catalyst is present, habitually as a composite anode of graphite and a metal, such as Ni, Fe, and Co, along with Y, S, Mo, and Cr added as a promoter. The catalyst nature controls the number of walls of the synthesized CNTs and the size of nanotubes. The major advantage of the arc discharge technique is the production of a large quantity of CNTs with few structural defects in a simple experimental setup. However, its major drawback is the difficulty of controlling chirality, essential for many electronics applications. Different approaches have been published to circumvent this problem, one of these is tuning arc parameters. For example, applying a magnetic field perpendicular to the electric field in the arc plasma SWCNTs with selected diameter distributions has been synthesized [58]. Due to the higher growth temperature, laser ablation produces CNTs with fewer structural defects than arc discharge. In the laser ablation method, graphite rods are vaporized by laser irradiation in an inert atmosphere, with a catalyst mixture, usually Co and Ni, at high temperature followed by heat treatment in a vacuum to remove fullerenes and other carbon nanomaterials. The typical lasers used for the ablation are Nd: YAG and CO2. The yields and purity of CNTs increase with the use of a dual-pulsed laser. By using the laser ablation technique, long bundles of preferentially m-SWCNTs with good quality and purity can be produced [59,60]. Although the yield of this process is high, this method is not widely used due to the expensive laser set-up and the cost of the highly pure graphite required. On the other hand, CNT synthesis by direct heating by microwave irradiation would result in a process more rapid and economical than conventional heating. Different microwave techniques have been proposed, a “poptube” approach with a conductive polymer and ferrocene in a microwave oven at room temperature [61] and a mixture of graphite, ferrocene, and a commercial carbon fiber [62], among others. Nowadays, the production of CNTs lies mainly on the thermal or plasma-enhanced catalytic chemical vapor deposition (CCVD) process,

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which allows CNT synthesis at lower temperatures than the arc discharge and laser ablation methods [63]. The CVD technique to produce CNTs involves the pyrolysis of a carbon precursor that is carried in a stream of an inert gas through a furnace at a temperature high enough to decompose the vapor (500°C
1200°C), and in the presence of a transition metal catalyst. This method is a scalable process, and can be used for both MWCNTs and SWCNTs. To control the growth mechanism of CNTs, different parameters have to be considered, such as precursor composition, catalyst, temperature, pressure, gasflow rate, time, and reactor type. Different alternatives based on CVD techniques have been developed over the years for scalable, economical, efficient, and large-area CNT production; examples are the CoMoCAT process [64], the HiPCO process [65], or the water-assisted CVD process [66]. At the present, many CVD options are available to synthesize CNTs, such as fixed bed reactors [67], fluidized bed reactors (FBCVD) [68], laser-assisted [69], hot filament [70], dc-glow discharges [71], radio frequency [72], aerosol-assisted [73], and floating catalysts [74]. FBCVD is the most commonly used technique for the synthesis of bulk CNTs [54,56,68,75]. Microwave-assisted chemical vapor deposition is considered as a cost-effective method to synthesize large-scale CNTs due to the fast heating and cooling processes [76]. Long and defect-free CNTs have been produced by CVD by optimizing the growing parameters and applying a synthesis method where the catalyst nanoparticle is at the floating end of the CNT and keeps moving forward with its growth [77]. The most common metals used in CNT production by the CVD process are Fe, Ni, Co and their alloys, and as supports materials Al2O3, SiO2, MgO, CaO, ZrO2, or TiO2 are used. The composition, density, size, and other parameters, such as intermediate layers of substrate, are crucial for CNT growth. Supported and free catalyst have been used in the synthesis of bulk CNTs. Catalysts can be preloaded onto a porous or flat substrate to grow CNTs. The organization of CNTs on surfaces is an important issue for many scientific and commercial applications. VACNTs are bundles of CNTs oriented perpendicular to a substrate, and HACNTs are parallel to the substrate. Vertically aligned arrays of CNTs are produced principally by CCVD. Horizontally aligned arrays of CNTs are produced by direct growth on surfaces by CVD under the influence of aligning forces, electric or magnetic fields, by using the feeding gas as flow director, or by surface-directed growth [78
80].

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A catalyst size of around a few nanometers favors formation of SWCNTs, while MWCNTs are more likely to form for catalyst sizes above 10 nm. The introduction of a small and controlled amount of water into the growth ambient of standard CVD increases the efficiency of the synthesis of SWCNTs [66]. As-prepared SWCNTs usually have diverse structures and properties, however high-quality s-SWCNTs with a narrow band-gap distribution are crucial for the fabrication of highperformance electronic devices. Therefore, the diameter, chirality, and electronic properties of SWCNTs must be well controlled to be useful for SWCNT applications. Nowadays, at a research scale, it is possible to prepare SWCNTs with highly enriched single-chirality species, by direct controlled growth and postsynthesis separation approaches [81
85]. Selective synthesis of s-SWCNTs has been intensively investigated. As a result of these efforts, effective approaches to growing high-enriched s-SWCNTs species have been reported by either selectively etching of metallic nanotube with an in situ etchant or by using a specific nanoparticle catalyst which inhibits the formation of m-SWCNTs. Less attention has been paid to selective synthesis of m-SWCNTs, although some reports have been published. Although controlled growth of pure SWCNTs with specific chirality is still a difficult task, many and substantial signs of progress have been already made. The direct synthesis of chirality-controlled SWCNTs has been accomplished by catalyst engineering, by using bottom-up synthetic strategies from carbonaceous molecular end-cap precursors, and by seeded growth using CNT segments as templates [86]. As the complementary method, different postsynthesis separation techniques have been developed as an effective route to obtaining highly pure SWCNTs. CNT types can be sorted using strategies based on the differences in their physical and chemical properties. High-purity s- and mSWCNTs or even SWCNTs with specific chirality and purity higher than 90% can be obtained. Sorting CNT strategies, such as electrical breakdown, dynamic supramolecular coordination chemistry, H-bonded supramolecular polymer, selective etching by gas-phase reaction, light irradiation, dielectrophoresis, DNA-assisted dispersion and separation, ultra-centrifugation-based separation, selective chemical functionalization, and thermocapillary flows have been developed [86
89]. Although the applied techniques so far are very effective, they are also rather expensive, therefore developing an inexpensive, easy-to-handle method is desirable.

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15.2.2 Functionalization of Carbon Nanotubes For many applications, functionalization of CNTs is necessary, for instance to enhance solubility or to change other properties such as the absorption of specific analytes. Those modifications often have the drawback of modifying the π network of the CNTs, changing the electronic or optical properties. The methods of functionalization are usually divided into covalent and noncovalent methods [90
92]. The most commonly used covalent method is oxidation using mineral acids, usually employed for purification. This oxidation introduces carboxylic acid and hydroxyl groups on the CNTs, mostly at the tip and on the defects that may occur at the outer wall. Those functional groups can then be used to introduce other fragments into the CNT to achieve the change in properties. Other oxidants, such as hydrogen peroxide or ozone, have been used. Many other reactions have been used for covalent functionalization, such as nucleophilic, electrophilic and radical additions, cycloadditions or halogenations. The noncovalent functionalization makes use of the π
π stacking interaction between the CNT outer wall and appropriate aromatic compounds, usually carrying other functional groups for further transformation. Wrapping the CNTs on large polymers, such as biomolecules, has been used for biomedical applications. Surfactants are also employed to facilitate the dispersion of CNTs in water.

15.2.3 Biobased Synthetic Routes The utilization of waste material as carbon precursor to produce CNTs is economically and environmentally desirable [93]. Some studies have reported the use of industrial waste for the production of CNTs. Among these, the use of plastic polymers as the carbonaceous feed of CNT production may be of interest. The production of MWCNTs from plastic polymers is seen to be viable, but much work needs to be done in adjusting all the operating parameters [94,95].

15.3 GRAPHENE Graphene is a single layer of carbon atoms derived from the graphite structure [96]. It is a 2D allotrope of carbon, in which one atom with sp2 hybridization forms each vertex of the hexagonal lattice. Graphene represents the base structure for graphitic materials such as fullerenes, CNTs, graphite, and other related materials [97].

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Although it has been known of since 1960, it was in 2004 that Andre Geim and Konstantin Novoselov obtained graphene sheets isolated by the Scotch tape method [98]. These studies set the focus on the first 2D material ever obtained, and its unique properties. These remarkable properties of graphene included optical and thermal properties, large surface area, high modulus of elasticity, and good electrical conductivity [99]. For this reason, large-scale production of graphene became a need due its wide variety of potential applications [100]. However, it is well known that these graphene properties are adversely affected due to the presence of defects, impurities, structural disorders, and wrinkles in the graphene material. The fabrication of graphene is still a challenge and the most important problem lies in the preparation of highquality and well-defined graphene in bulk quantities, both for industry and for research [101]. Several preparation strategies have been proposed, and companies supplying graphene already employ a number of them [102,103].

15.3.1 Graphene Synthesis and Production The fabrication methods can be categorized into top-down and bottomup approaches. Top-down methods include exfoliation of graphite and graphite intercalation compounds (GICs) in solid or liquid phase, electrochemical exfoliation of graphite, chemical, thermal, or electrochemical reduction of exfoliated graphite oxide, and more unusual methods, such as CNT unzipping. Bottom-up approaches involve epitaxial growth, CVD, and chemical synthesis. All of them have advantages as well as limitations, in terms of material quality, accessibility, and scalability, but to date, CVD is the most commonly used for large-area high-quality scale production of graphene films. 15.3.1.1 Micromechanical Cleavage Micromechanical cleavage of graphite or the Scotch tape technique was the first example of the top-down approach for the preparation of graphene flakes. The Nobel Prize in Physics 2010 was awarded to Andre Geim and Konstantin Novoselov for these groundbreaking experiments. Continuous mechanical delamination by a three-roll mill machine with a polymer adhesive has been reported, which can be useful in situ for fabrication of polymer/graphene nanocomposites [104]. To remove the tape residues of the exfoliated graphene films and flattening the edges, a thermal annealing

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process is applied [105]. Although the Scotch tape method provides crystals of high quality, it is limited to laboratory research. 15.3.1.2 Liquid-Phase Exfoliation Liquid-phase exfoliation (LPE) of graphite is extremely versatile and can be used to deposit graphene in a variety of environments and on different substrates not available using mechanical cleavage. Sonication, shear mixing, or ball milling, followed by ultracentrifugation, assists LPE. The initial methods involved sonication of graphite in organic solvents or in water with surfactants. In recent years, however, significant advances in the use of hydrodynamic forces for economical and size-controlled production of high-quality graphene sheets have been reported [106]. For instance, large-scale graphene production by ultrasound-assisted exfoliation of natural graphite in supercritical CO2/H2O medium has been described [107]. The graphene yield was more than 50%, with 93% of products being # 3 layers. This method has many advantages over other methods, such as simplicity, cost-efficiency, no post-treatment, and the quality of the graphene is high. Although LPE allows the production of high-quality graphene sheet dispersion, the yield of single-layer graphene sheets is still relatively low. LPE is a potentially up-scalable approach to produce graphene, however, reproducible process conditions are needed. 15.3.1.3 The Graphite Oxide Route Different chemical species can be inserted between the graphite interlayer to produce GICs. Of these, graphite oxide has attracted enormous interest as a route for the large-scale production of graphene. Graphite can be oxidized to graphite oxide in various ways, such as the methods developed by Brodie, Staudenmaier, Hofmann, and Hummers and their modified and improved forms. The choice of oxidation method greatly influences the structural and electromechanical properties of the graphite oxide obtained. The polar character of the oxygenated groups confers to graphite oxide a strongly hydrophilic behavior, and that allows the intercalation of water molecules between the layers. Graphite oxide is easily exfoliated by LPE techniques, and so separates into individual graphene oxide (GO) sheets. Subsequently, reduction of GO by chemical or thermal reduction recovers many properties of graphene-like materials. Various conventional reducing agents have been used, such as hydrazine hydrate, hydroquinone, sodium

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borohydride, hydroxylamine, strongly alkaline solutions, metal-alkaline, and others [108,109]. Looking for green alternatives, different “ecofriendly” reducing agents have been reported, such as amino acids, vitamins, organic acids, sugars, plant extracts, microorganisms, and proteins [110]. The reduced graphene oxide (rGO) derived by GO reduction still contains a significant amount of oxygen and other heteroatoms. The GO reduction by thermal treatment is achieved at high temperature ( . 1000°C) in the presence of inert or reducing gases or in a vacuum. Rapid heating exfoliates and reduces graphite oxide, yielding a black powder, which can be dispersed in several organic solvents [111]. In addition, GO reductions have been achieved by microwave reduction [112]. By this microwave-heating, dry graphite oxide, suspension of graphite oxide, or graphite-intercalated compounds can be used. Photoreduction [113] and electrochemical reduction [114] also have been successfully used as alternative reducing processes of GO. This method can be potentially scaled-up and avoids the use of hazardous chemicals. The complete reduction of GO into graphene has not been achieved yet, and the lack of chemical homogeneity of GO, together with the inevitable generation of defects during the oxidation process, need to be addressed. Even so, large quantities of rGO are produced annually, finding applications in many fields. 15.3.1.4 Electrochemical Exfoliation Electrochemical exfoliation of graphite has been receiving increased attention over recent years as a scalable fabrication route to graphene. It is a facile, low-cost, ecofriendly, and efficient process [115]. These processes are based on the intercalation of ions between the layers of a graphite electrode due to the flow of electrical current in an electrolytic cell. A wide range of graphite precursors, such as highly ordered pyrolytic graphite, expanded graphite, natural graphite flakes, graphite powder, graphite foil, and GICs, have been explored. Both anodic and cathodic electrochemical exfoliation processes are currently widely used to produce graphene [116]. Cathodic exfoliation takes place in organic solvents, while anodic exfoliation is typically carried out in aqueous electrolytes. A problem derived from aqueous anodic exfoliation is the formation of highly reactive oxygen species, which produce oxidation and structural degradation of the graphene layers. Different alternatives have been proposed to prevent graphene oxidation during anodic exfoliation. Recently, a method on the addition of scavengers or specific electrolytes, as sodium

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halides, to reduce the oxidation caused by water electrolysis has been published [117,118]. The success of the electrochemical exfoliation relies largely on the selection of the electrolyte, but the graphite sources also show great influence on the oxygen content and defect density of exfoliated graphene [119]. This process enables the fabrication of relatively high-quality graphene flakes with useful production rates. 15.3.1.5 Chemical Vapor Deposition The CVD method is, to date, the optimal method used for large-scale high-quality graphene film production. In the CVD process, the carbon source reacts at high temperatures in the presence of a metal, which serves both as a catalyst for the decomposition of the carbon species and as a surface for the nucleation and growth of the graphene lattice. There are many factors involved in this process: the surface, morphology, and properties of the substrate catalyst [120], the carbon precursor [121], and the temperature and pressure of the CVD [122,123]. Graphene can be catalytically grown on many metallic substrates, such as Cu, Ni, Pt, Ru, Ir, etc., and also can grow directly on insulating substrates, like silicon carbide (SiC), SiO2, sapphire, and h-BN, or metal- or semiconducting-assisted growth on dielectric substrates [120,124,125]. Although the graphene grown on metallic substrates is generally a highquality graphene thin film, the graphene grown on insulating substrates still presents some drawbacks, such as low growth rate, low catalytic power, and small domain size. Today, for scale-up, large-area graphene film production, CVD growth of graphene on a Cu substrate has become an important approach due to its low price, mild catalytic activity, and low carbon solubility. However, the as-synthesized graphene films are polycrystalline, consisting of many single-crystalline grains separated by defective grain boundaries that degrade their electrical and mechanical properties [120]. Different parameters affect the CVD growth of large-area single-layer graphene. Many of these factors are related to the physical and chemical properties of the substrate, the crystallographic orientation of the metal, the surface morphology, surface diffusion, the hydrogen flow in the furnace, the concentration of carbon, carbon solubility, and the presence of oxygens on the copper substrate, etc. [126
128]. Despite these points, the growth of meter-sized single-crystal graphene with ultra-highly oriented grains has been reported. It was achieved using

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a temperature-gradient-driven annealing technique to produce a singlecrystal Cu(111), under a continuous oxygen supply which was used as substrate for graphene growth [129]. In other important work, by locally feeding carbon precursors to the desired position of a substrate, allowing the nucleation of a single nucleus on optimized Cu-Ni alloy, an B1.5-in.-large graphene monolayer was formed in 2.5 h [130]. In a novel approach, through “self-selection” of the fastest-growing domain orientation, a foot-long single-crystal graphene film with growth at rates up to 2.5 cm/h has been prepared [131]. A scalable synthesis of high-quality graphene-based, nanoporous atomically thin membranes has been recently reported [132]. It was achieved by a roll-to-roll CVD process, producing the material at a speed of B5 cm/min. CVD is also a promising approach to grow large-area bilayer and trilayer graphene. A high-yield synthesis of crystalline bilayer graphene, in Bernal-stacked form, has been published [133]. An 80-μm uniform singlecrystalline trilayer graphene with ABA stacking has been achieved on a premelting copper layer [134]. Many theoretical studies of graphene films grown with CVD have been carried out, and probably more are still needed to better understand the nature of graphene
substrate interfaces [135]. The production of graphene materials still needs greater control over some aspects, layers numbers, crystallinity, size, edge structure, and spatial orientation, and a better understanding of the mechanisms of the global process. 15.3.1.6 Epitaxial Growth of Graphene on Silicon Carbide Substrate The growth of epitaxial graphene on SiC by thermal decomposition of the SiC substrates produce large areas with uniform thickness and highquality graphene. That is possible due to the difference in the vapor pressures of silicon and carbon, and so, when SiC substrates are heated in an argon atmosphere, only the silicon leaves the surface, and the remaining carbon rebonds to form one or more layers of graphene on the SiC surface. Many and essential research studies have been done over the last three decades on this process [136]. These studies have shown that the thermal decomposition of SiC in an argon environment results in better morphology and size graphene compared to ultra-high-vacuum graphitization. Likewise, other improved techniques, such as the confinement controlled sublimation method [137], and the presence of extra Si flux, have enhanced the quality of epitaxial

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graphene [138]. Also, it is known that the initial surface conditions are very important, and for that reason an etching process is commonly used to treat the SiC samples before graphitization [139]. Graphene grown on the C-face of hexagonal SiC single crystal is significantly different from Si-face graphene [136]. Graphene growth is much faster on the C- than on the Si-face. On the Si-face the number of graphene layers can be controlled by optimizing the growth temperature and Ar pressure. However, on the C-face multilayer graphene is typically grown. In addition, Si-face epitaxial graphene grows Bernal stacked, while C-face graphene grows in an ordered set of relative rotational angles. Another issue with graphene grown on the Si-face is the presence of a buffer layer. It is a graphene-like honeycomb structure with covalent bonds to the Si atoms of SiC. Elimination of this layer is necessary for growing graphene for device application. This buffer layer can be converted to a referred as quasi-free-standing monolayer graphene (QFSMLG) by atom intercalation, with hydrogen being the most popular. Recently, a rapid-cooling technique has been described to convert the buffer layer into QFSMLG [140]. The use of SiC wafers has some disadvantages including wafer sizes, cost, and micromachining processes. Alternative processes, such as epitaxial graphene, fabricated on 3C-SiC/Si(111) by thermal decomposition, are currently being explored [141]. A drawback to be addressed is the transfer of single-layer graphene directly from a SiC surface to a target substrate. To address this issue a method is used in which a single- or bilayer graphene grown on SiC is exfoliated via the stress induced with a Ni film and transferred to another substrate [142]. Recently, a novel approach to that method, using a Ni/Cu catalytic alloy, provides a transfer-free bilayer graphene at temperatures potentially compatible with conventional semiconductor processing [143].

15.3.2 Biobased Synthetic Routes Biomass and waste materials have been investigated as eco-friendly sources of graphene as for other carbon nanomaterials. Many alternative carbon sources have been used as peanut shells, rice husk, honey, sugar, chicken fat, insects, and even solid plastic waste. In some cases, the synthesis of few-layered graphene from no-value biomass, waste peanut shell, without using any graphitizing agents has been reported. Although the control of

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parameters in this type of material can cause some difficulties, for some applications bulk production of graphene by this way is an important advance [144].

15.3.3 Chemical Synthesis Varieties of nanographenes (NGs) from bottom-up organic synthesis have been obtained over the past two decades [145,146]. The main approaches lie in the Scholl reaction, an intramolecular oxidative cyclodehydrogenation of designed oligophenylene precursors [145], and the single-step annulative π-extension reaction [147]. These tools have been used to expand the scope of available NGs, including embedded seven- or eightmembered rings, heteroatom doping of graphene molecules at defined positions, and chemical modification of the edges of graphene molecules without prior functionalization [145,146]. As an example of this synthesis, a helical bilayer nanographene has been synthesized in three steps by combining the advantages of nanographenes and helicenes [148]. Also, a water-soluble warped nanographene prepared this way was successfully internalized into HeLa cells and promoted photo-induced cell death [149]. New advances in the synthesis of graphene molecules should be basic for future applications on nanoelectronic and optoelectronic devices.

15.3.4 Graphene Heterostructures Two-dimensional heterostructures have been an attractive new topic for both fundamental research and applied physics. Graphene’s contributions to these atomically thin 2D materials are its high conductivity and carrier mobility, and its transparency and mechanical flexibility. Vertically stacked 2D heterostructures of graphene with other materials can be prepared by mechanical stacking. In addition, the direct growth of graphene’s heterostructures using CVD or related methods make possible a more efficient fabrication. The 2D heterostructures synthesized by CVD have a much cleaner interface and possibility for the synthesis of lateral 2D heterostructures [150]. Different 2D heterostructures of graphene with other materials have been reported with a broad range of applications, such as field effect/ tunneling transistors, biosensors, light-emitting diodes, light detectors, photovoltaic, and energy-storage devices [151].

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In a different way, thin films of oriented 2D covalent organic frameworks grown on single-layer graphene, under operationally simple solvothermal conditions, have been described. This technique allows the preparation of organic subunits into 2D predictable structures with longrange order [152].

15.3.5 Graphene Nanocomposites Graphene nanocomposites have become an area of interest as the properties of graphene can enhance the characteristics of polymeric materials [153]. The nanocomposites have been prepared by wet chemical processes, such as liquid phase ultrasonication, electrochemical intercalation, or solvothermal methods. Another strategy is the in situ functionalization by electrodeposition. The relative ratio of each component tunes the properties of the laminar structure [154].

15.3.6 Heteroatom-Doped Graphene Heteroatom-doped graphene has shown great potential as an active catalyst for a wide range of reactions [155]. The common methods for heteroatom doping are in situ doping (CVD) and postsynthesis treatment of GO or rGO with heteroatom precursors. For N-doped graphene, the first method mainly forms pyridinic- and/or pyrrolic-N-species, while hightemperature post-treatment doping methods normally forming graphiticN species. Double- and triple-doped graphene have also been prepared, with the most studied being B,N-graphene. A two-step procedure by separating the doping steps, which prevents the formation of h-BN in the synthesis of B,N-doped graphene, has been described [155]. On the other hand, graphene with topological defects, prepared by nitrogen atom subtraction from N-doped graphene, has been reported both experimentally and theoretically, which improves its catalytic activity in electrochemical reactions [156].

15.3.7 Halogenated Graphenes Fluorinated graphene, experimentally prepared in 2010 [157], was the first halogenated graphene described. Later, partially brominated or chlorinated examples were reported. Due to their interesting properties, various synthetic routes have been developed for the synthesis of halogenated graphenes, and these include bottom-up and top-down approaches, such as

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mechanical or chemical exfoliation of fluorographite, CVD graphene fluorination with XeF2, and thermal treatment of graphene in N2/F2 mixture [158].

15.3.8 Hydrogenated Graphenes Theoretically, it has have been predicted that a fully hydrogenated graphene monolayer (graphane) should be stable, with a binding energy comparable to other hydrocarbons, and with potential applications [159]. Partially hydrogenated graphene also displays remarkable properties, as tunable band gaps and ferromagnetism. The synthesis of partial hydrogenated graphene have been performed by Birch reduction of single-layer CVD graphene, as well as hydrogenation of sp2 carbon materials in a hydrogen gas/plasma atmosphere and by using electrochemical methods. Hydrogenation of graphene is thermally, chemically, and mechanically reversible [160,161]. Recently, it has been shown that hydrogen adsorption induced phase transition of a few layer graphene to a diamond-like structure on Pt(111) [162]. In addition, it has been reported that mechanochemical synthesis, at high pressure, can convert polycrystalline or single-crystal benzene monomer into single-crystalline packings of carbon nanothreads [163].

15.3.9 Chemical Functionalization of Graphene The chemical functionalization of graphene has been a necessary tool to modify graphene chemical and physic properties, and thus tune its electronic and surface properties. A wide range of chemical functionalizations of graphene have been done in recent years [164
166]. Among these, atomic chemical doping by nitrogen or boron atoms has been proposed as a way to open the band-gap and form p-type or n-type graphene, but frequently introduces defects. Another approach for graphene structural modifications has been achieved through covalent and noncovalent functionalization. Additionally, the chemical functionalization of graphene can be realized in solution or on the substrate. It has been shown that graphene reactivity is strongly affected by the supporting substrate and the number and relative orientation of graphene layers, as well as the presence of defects and grain boundaries or local curvature [165]. Nowadays, the focus on graphene’s functionalization points to the development of strategies that can be scaled-up for graphene functionalization under controlled conditions. For example, patterning the surface using an atomic force

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microscope, supramolecular functionalization of epitaxial graphene, or site-selective covalent functionalization of graphene [167
170]. Noncovalent functionalization of graphene is mostly based on van der Waals forces, electrostatic interactions, or π
π interactions with polycyclic molecules, biomolecules, polymers, and other molecules. The works on noncovalent functionalization of graphene are focused on the fabrication of self-assembly supramolecular architectures on graphene surfaces [171,172].

15.4 GRAPHENE NANORIBBONS Graphene nanoribbons (GNRs) are narrow strips of graphene with a very high length-to-width ratio. GNRs have been extensively studied both theoretically and experimentally [173]. Due to their promising properties, like band-gap, charge carrier mobility, or energy level alignment [174], new studies, especially those directed toward the synthesis and characterization of new atomically precise GNR structures, are continuously being published. GNR’s properties are dependent on width, edge orientation or termination, or presence of heteroatoms on the GNR structure. In addition, a few structural changes greatly influence the nanoribbon's electronic properties [175]. GNRs have been synthesized by top-down or bottom-up approaches. Top-down approaches to GNRs have been done by nanolithography patterning methods or unzipping of CNTs via an oxidative process or plasma etching approach [176,177]. Nevertheless, these techniques lack reliable control over the width and edge structure. To date, bottom-up fabrication through solution-mediated or surface-assisted protocols, has several advantages in terms of atomically precise GNR fabrication, and the success of this synthetic approach relies on the rational design of suitable reactants [178]. GNRs with a variety of widths, edge structures, and heterojunctions have been synthesized on metal surfaces (Au, Ag, and Cu) in ultra-high-vacuum, or in solution, through a two-step sequence of polymerization followed by cyclodehydrogenation from molecular precursors [176
180]. Although direct growth on insulating substrates remains a challenge, promising strategies are being developed [181,182]. On the other hand, full characterization of atomically precise chiral GNRs remains needed [183].

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15.5 CARBON DOTS After the discovery of fluorescent nanocarbon structures during the synthesis of CNTs in 2004 [184], many research groups started to study those structures, their properties, and possible applications [185]. The structure of the carbon dots (CDs), as they were soon called, consists of an inner core of graphitic or, in some cases, amorphous carbon with a surface passivated with different functional groups. In most cases, the size of the CDs is usually below 10 nm, although some authors include larger structures. Other similar structures are the graphene quantum dots (GQDs), composed of a few layers of graphene with a small lateral size. The properties and synthetic methods used for GQDs are similar to those of CDs [186]. Different studies have shown that the luminescence of CDs depends on various factors, such as size or localized units within the structure, but the modifications on the surface also play an important role [186]. Thus, many CDs show low quantum yields (QYs) when synthesized, but after treatment with passivating agents such as organic molecules or polymers, the QY increases noticeably [187]. This seems to occur because the fluorescence in these particles comes in part from the surface states, and the defects on the surface are enhanced by passivating agents. The exact nature of the photoluminescence of the CDs is still open to debate [188]. After the initial discovery, many different techniques were used for the preparation of CDs. The earliest works used the same methods as for other carbon nanomaterials, but other approaches were later introduced. CDs are composed mainly of carbon, show low toxicity, high biocompatibility, and good chemical inertness and solubility [189]. However, due to concerns over the large-scale use of CDs on industrial products, studies on the effects of CDs on living organisms are still been carried out [190,191]. The synthesis of CDs is usually divided between top-down and bottom-up methods (Fig. 15.2).

15.5.1 Carbon Dot Synthesis 15.5.1.1 Top-Down Methods The top-down methods are those in which large, organized structures, such as graphite, graphene, graphene oxide, CNTs, or other sp2-based structures are broken into smaller pieces. Laser ablation of graphite rods can be used for the synthesis of CDs, but usually without fluorescence. Oxidation of the obtained structures with nitric acid and passivation with

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Figure 15.2 Scheme of the synthetic routes used for the preparation of carbon dots.

polymers is often needed to obtain luminescence [192]. The arch discharge method was used when CDs were first identified. As with laser ablation, oxidation and passivation are usually needed to obtain photoluminescent CDs [193]. Electrochemical oxidation has been used with high-purity graphite as the carbon source in ionic liquids/water solutions [194] or pure water together with sonication [195]. Fluorescent CDs with only moderate QYs were obtained. 15.5.1.2 Bottom-Up Methods In this case, the source of carbon consists of small molecules or other nonorganized carbon sources, such as carbon shoots from combustion, plants, or almost any carbon-rich structures. Pyrolysis has been extensively used for the preparation of CDs [196]. An important process is microwaveassisted synthesis, since CDs can be obtained in a few minutes [197]. Chemical oxidation is another simple method that is commonly used [198]. Hydrothermal or solvothermal treatments have been used to produce CDs from a large number of sources, many of natural origin [199]. In most of these methods, fluorescent CDs are obtained directly without the need for passivation. Another interesting method is the use of templates for the synthesis of CDs. In this approach, a porous compound such as silica spheres is used as a template in which the CDs are formed. Then etching of silica particles liberates CDs with a narrow size distribution [200].

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The bottom-up methods are the most frequently used in the preparation of CDs. 15.5.1.3 Surface Passivation As indicated before, the number and type of the groups on the surface of the CD are important not only to avoid aggregation of the particles but also to increase the fluorescence of the CDs. The strategies followed for the passivation can be divided into one-pot or multistep processes. In the first case, the one-pot approach, passivation is achieved by mixing two different compounds, one acting as a carbon source and the other as a passivating agent. For example, carbonization of citric acid with branched polyethylenimine gave CDs with high QY [201]. In the multistep approach, the CDs are first prepared from the carbon source and then, if needed, an oxidation step is used to introduce oxygen-containing functional groups on the surface. Those groups are then chemically modified, for instance, forming amides with adequate amino-containing molecules [202]. This way, more control over the properties of CDs is obtained. 15.5.1.4 Doping The introduction of atoms other than carbon produces changes in the properties of CDs. The examples in the literature using metals indicate the feasibility of the process, but it has not been pursued as much as with nonmetallic dopants because it goes against the idea of biocompatibility. For that reason, the doping of CDs has been done mostly with elements such as N or S and to a lesser extent Si, B, or P [203]. However, interesting examples of doping with other elements such as Cu, Zn, or Gd have been reported [203]. The methods used to introduce dopants in CDs can be the same as those used for nondoped ones. Either the source used also contains the doping element or a mixture of both the carbon source and dopant source, which are treated together. A commonly employed method is hydrothermal treatment. For instance, treatment of milk, which has many N-containing species, gave small (B3 nm) N-doped CDs [204]. An example of a mixture employed for doping is the solvothermal treatment of citric acid as the carbon source, and urea as the dopant source [205]. Microwave heating also has been used to produce doped CDs. Using glucose as the carbon source and ammonia as the nitrogen

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source, CDs were obtained by microwave heating [206]. CDs co-doped with N and S have been prepared by hydrothermal treatment of citric acid and L-cysteine. The resulting CDs showed high QY [207].

15.5.2 Biobased Synthesis Routes As indicated above, the bottom-up methods are very flexible in the type of carbon source used. Different optical properties of CDs are found, due to the doping caused by the elements being different from the carbon that exists in those materials. This flexibility of the bottom-up methods regarding the carbon source has resulted in the possibility of using renewable resources for the preparation of CDs [208]. Examples include the use of plants, fruits, eggs, waste products, or even human hair as the carbon source. Large-scale preparation of CDs has been achieved using bee pollen as the precursor [209]. This approach is expected to grow in the near future as more knowledge is gained in the preparation processes.

15.6 CHALLENGES AND FUTURE PERSPECTIVES As a conclusion to the synthetic routes described in this chapter, we present the expected direction of research on each family of nanomaterials for the coming years. For instance, in the fullerenes family of nanomaterials, research is still needed for the large-scale production of both smaller and larger than the more common C60 and C70 with high selectivity. Other areas needing more synthetic efforts are EMFs, since only a few can be prepared in sufficient amounts for research and better methods are needed for practical applications. As for the exohedral modifications, the most interesting are those of metal-containing fullerenes, although more efforts are also needed in the selectivity of the reactions commonly used. As more consumer products use fullerenes, it is expected that the current methods will be improved to produce the quantities needed by the industry. For CNTs, the synthesis in bulk quantities, especially of MWCNTs, is a reality. Although improvement in production processes is expected, future development in this field must be in the direction to scale-up the production of pure SWCNTs of a specific type. For bioapplications a full chirality map of SWCNTs as a part of the structure control of SWCNTs [81] is desirable. The surface functionalization seeking increased solubility in water for medical applications or for selective detection of specific

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compounds will be continuously improved [210
213]. The application of CNTs to an ever-growing number of processes makes this an active area of research in both laboratories and industrial settings. For the graphene family, the fabrication of high-quality and welldefined graphene sheets in bulk quantities is still a challenge. Greater control is still needed over some aspects including layer numbers, crystallinity, size, edge structure, and spatial orientation, and a better understanding of the mechanisms of the global process. The known procedures, such as reduction of graphene oxide, need to be improved to achieve the properties of pristine graphene. The LPE of graphite is another process to be considered in the future if reproducibility is achieved. Being the most recently discovered carbon nanostructures, research on CDs has been very active in the last few years. It is expected that it will continue to develop in the near future as, for instance, there is a need to determine the exact mechanism for the optical properties of CDs as a means to plan controlled synthesis of CDs with specific characteristics. Improved methods to achieve the preparation of CDs with specific and controlled sizes are also needed. Procedures to improve the efficiency of doping are important to control the characteristics of CDs. Since many uses of carbon nanomaterials are related to biological applications, a complete assessment of their safety is important and should be pursued, especially when large-scale production is intended. Finally, applications of the principles of green chemistry to the synthesis and purification of carbon nanomaterials are already emerging, with carbon sources coming from renewable feedstocks and fewer contaminant solvents and catalysts. This trend will continue to grow as more industrial applications for carbon nanomaterials become available.

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196.

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INDEX Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A AAO membranes. See Anodized aluminum oxide membranes (AAO membranes) AAO/Ti/Si substrates. See Anodic aluminum oxide/titanium/silicon substrates (AAO/Ti/Si substrates) Absorption, 153 free-carrier, 154
155 linear, 153
154, 183
185 nonlinear, 151
154 radiation, 373
376 two photon, 151, 183
185 Acid green 20 dye, 285 Acid violet 17 dye, 285 Activated carbons, 283, 285, 287
288 Activation energy, 93
94, 94f Active sorting, 493
494 Adiabatic combustion temperature (Tad), 89 AEF. See Analytical enhancement factor (AEF) Aeromonas hydrophila microbial extraction, 39
41 Aerosol method, 202
204, 219
226, 238 AFM. See Atomic force microscopy (AFM) Agglomerates, 135
136 Aging process, 240
241 Ag
Pb
Sb
Te quaternary systems, 301
302 Agriculture, nanoporous materials in, 287
288 Al NPs. See Aluminum nanoparticles (Al NPs) Aliphatic functional groups, 427 Alloy materials, 406
407 α-hemolysin protein (α-HL protein), 282 AlN powders. See Aluminum nitride powders (AlN powders) Alumina, 126 Aluminosilicate zeolites, 287
288

Aluminum (Al), 187
190 Aluminum nanoparticles (Al NPs), 180
182, 189f prepared by laser ablation in liquids, 179
186, 184f Aluminum nitride powders (AlN powders), 98
99 Aluminum oxide (Al2O3), 439, 527 Ambient parameters, 469 Amino functional group, 432 Amino-functionalized poly(GMA-coEGDMA), 441
443 5-Aminosalicylic acid, 426 Ammonia (NH3), 98
99, 522 Amorphous carbon layers, 124 Analytical enhancement factor (AEF), 171
172 Anatase, 67
68, 68f crystals to rutile rods, 73f space group and crystal structure, 69t Andrew Pelling at University of Ottawa, Canada, 496 Angiogenesis, 491 Anodic aluminum oxide/titanium/silicon substrates (AAO/Ti/Si substrates), 347 Anodized aluminum oxide membranes (AAO membranes), 455 Antimicrobial ability of silver, 437
443 Antimicrobial activity, 45 nanoparticles exhibits, 39
41 of silver, 440
441 Antimonene, 2, 14 Antimony sulfoiodide (SbSI), 338, 360, 363 anion doping, 364 conversion of Sb2S3 into, 369
370 heat and laser formation of SbSI nanoobjects, 370
377 growth activated by heat treatment, 372

557

558

Index

Antimony sulfoiodide (SbSI) (Continued) laser-induced growth, 373
377 HRTEM image of individual SbSI nanowire, 339f laser-induced growth of SbSI crystals on chalcohalide glasses, 374t nanocomposites of SbSI dots in organically modified titanium dioxide glass, 340
342 in sodium borosilicate glass, 339
340 SbSI-type material growth in CNT, 361f SbSI/GLYMO-TiO2 glass nanocomposite, 341
342 sonochemical synthesis of SbSI-type nanowires, 349
356 synthesized, 367 vapor-phase growth of SbSI nanorods, 347
348 Antimony tri-iodide (SbI3), 339
340 Antioxidants, 437
438 Apple-derived cellulose scaffolds, suitability of, 496 Applied electric field, 468 Applied voltage, system/process parameters effect of, 463
464 APUSP. See Asynchronous pulse ultrasonic spray pyrolysis (APUSP) Aqueous droplets, 493
494 Arc discharge method, 122
123, 130, 134
135, 522, 525
526, 540
541 Arc-vaporization of two graphite watercooled electrodes, 525
526 Argon (Ar), 523 Artificial epitaxy. See Graphoepitaxy Artificial lattice, 349 Artist’s palette of nonclose-packed microspheres, 503
504 As-spun nanofibers, 464, 469
470 FESEM images, 465f, 471f effect of inner diameter of needle, 465f Ascorbic acid, 34
36 Asynchronous pulse ultrasonic spray pyrolysis (APUSP), 356
357, 357f ATGs. See Automotive TE generators (ATGs) Atomic force microscopy (AFM), 5
7 Automotive TE generators (ATGs), 321 Avalanche ionization, 151
155

B Bacillus subtilis ATCC 6633, 45 Backscattered electron detector, 343
344 Bacterial cells, 441
443 Bacteriophage virus MS2, 284 Ball milling of bulk crystals, 346
347 Basal plane, 58
59 Batteries, 386
387 lithium-ion, 209, 391, 397
401 lithium-sulfur, 408
409 potassium-ion, 409
410 sodium-ion, 401
408 “Bench-to-bedside” translational research, 484
485 Bending instability of jet, 460
462 β-galactosidase, 493
494 Binary MnxOy nanofibers for energystorage applications, 469
476 compression of methods to fabricating nanofibers, 471t fabrication of nanoparticles, nanofibers, and nanorods, 473f FESEM images of as-spun nanofibers, 471f of sintered Mn3O4 nanofiber, 472f Bingel
Hirsch reaction, 524 Bio-bots, 510, 511f Bioartificial organs, 487
490 acellular bioscaffolds to bioengineered kidneys, 489f scope of research into, 488
490 and theragnostics, 490 Biobased synthetic routes, 529, 535
536, 543 Biochemically synthesized nanoparticles, 28 Biodegradable polymers, 510
512 Bioengineering, organ-specific, 486
487 Bioink, 501, 503
504, 506
507 Biological laser printing. See Laser-assisted bioprinting (LAB) Biological nanoparticles, 45
46 Biological processes, 500
501 Biological route, nanoparticle synthesis using, 37
41 microbial synthesis of nanoparticles, 39
41

Index

plant extract-based nanoparticle synthesis, 38
39 Biological synthesis of nanoparticles, 30
31, 45 advances in, 44
46 Biomass, 413
414, 535
536 biomass-derived carbon materials, 394
396 Biomaterials, 486
487, 497
498 Biomedical engineering bioreactor, 500
501 Biomedical uses of nanoporous particles, 280
282 Biomimetics, 490, 494
496 molecular, 496f nanostructured, 495 scaffolding for, 496 Biopatterning bioreactors for in situ simulations, 500
501 complexities of life, 497
501 fluidic simulations on patterned surfaces, 498
500 microstructures, 498 nanostructures, 498 Bioprinting of organs and tissues, 501
514 applications, 510 complexities in, 513
514 history, 503, 504t organ specific, 510
513 techniques using in, 503
510 three-dimensional printing external ear structures, 512f pinna, 513f Bioreactors for in situ simulations, 500
501 Biosensing devices, micro-and nanofluidic, 493
494 Birefringent refractive index change mechanism, 151
153 Bismuth (Bi) bismuth telluride alloys, 301
302 bismuth telluride-based nanocrystals, 323
324 chalcogenides, 305
306 wires, 298 Bismuth sulfide iodide (BiSI), 356
357, 359

559

Black phosphorus, 2 Blue luminescent Si nanoparticle synthesis, 172
174 BN nanotubes. See Boron nitride nanotubes (BN nanotubes) Bone tissue engineering, bioactivity of scaffold in, 486
487 Boric acid, 131
132 Boron atoms, 538
539 Boron carbide (B4C), 105 Boron nitride nanotubes (BN nanotubes), 99, 99f Borophenes, 14 Bottom-up approaches, 13
16, 28, 37
38, 68, 530, 539. See also Topdown approaches CDs, 541
542 chemical vapor deposition, 13f, 14, 30 green and biological synthesis, 30
31 sol
gel process, 30 wet-chemical synthesis, 14
16, 15f Brookite, 67
68, 68f space group and crystal structure, 69t Buckminsterfullerene (C60), 391, 392f Building up process. See Bottom-up approaches

C C60 fullerene, 521, 523 C70 fullerene, 521, 523 Calcined nanofibers, 471 Calcium carbonate (CaCO3), 125 Calcium phosphate, 510
512 Calothrix algae combination with ultrasound irradiation, 32
33 Candida albicans, 34
36, 441
443 Candida glabrata, 34
36 Capacitors, 210 Capacity retentions of Mn2O3 nanofibers and nanoparticle electrodes, 473
476 Carbene addition, 524 Carbides, 96 Carbidization, 100
101 Carbon, 86
87, 138
139, 388 carbon-based composite electrode materials, 406
408

560

Index

Carbon (Continued) electrode materials, 403
406 materials, 210 endohedral fullerenes from, 522
523 materials, 388
397 biomass-derived, 394
396 carbon nanotubes, 391
394 fullerene, 391 graphene, 388
390 heteroatom-doped, 396
397 obtaining methods from biomass, 395f Carbon dioxide (CO2), 526 adsorption, separation, and catalytic conversion of, 283
284 photocatalytic conversion of, 284 Carbon disulfide (CS2), 341 Carbon dots (CDs), 540
543 biobased synthesis routes, 543 synthesis, 540
543 synthetic routes, 541f Carbon nanomaterials (CNMs), 121
122, 127, 316 for energy storage li-ion-based battery systems, 409f lithium-ion batteries, 397
401 lithium-sulfur battery, 408
409 multivalent cation-based battery technologies, 410 potassium-ion batteries, 409
410 sodium-ion batteries, 401
408 supercapacitors, 410
413 methods of synthesis arc discharge method, 122
123 laser ablation, 123
124 trends in synthesis CDs, 540
543 challenges and future perspectives, 543
544 CNTs, 525
529 fullerenes, 520
524 GNRs, 539 graphene, 529
539 synthesis and challenges, 139
140 Carbon nanotubes (CNTs), 121
122, 313
314, 316, 391
394, 399
401, 523, 525
529 biobased synthetic routes, 529

chiral vectors defining SWNT unit cell, 393f filling of, 359
363 functionalization, 394f, 529 preparation by flame-deposition method, 396f strategy for nanopore creation, 400f synthesis and production, 525
528 unzipping method, 530 Carnot efficiency, 299
300 Cassia auriculata flower extract, 38
39 Catalysis, 212
213, 282
283 Catalyst
substrate interaction, 125 Catalytic chemical vapor deposition process (CCVD process), 526
527 Catechol, 426 Catecholate-type binding, 427 Catecholate-type of ligands, 421
422, 429f Cathodes, 209 Cathodic exfoliation, 532
533 CCU technique. See CO2 capture, storage, and utilization technique (CCU technique) CCVD process. See Catalytic chemical vapor deposition process (CCVD process) CDs. See Carbon dots (CDs) Cell culture devices, micro-and nanofluidic, 491
492 Cellulose/SbSI nanocomposite, 345
346 Ceria-zirconia oxides, 248
249 Cetyltrimethylammonium bromide (CTAB), 269 CFU. See Colony-forming units (CFU) Chalcogenides bismuth, 305
306 lead, 304
305 Chalcohalides, 338 glasses heat and laser formation of SbSI nanoobjects, 370
377 laser-induced growth of SbSI crystals, 374t group 15 ternary chalcohalides, 338 Charge-storage mechanism, 452
453 Chemical and electrochemical ion intercalation, liquid exfoliation by, 8
10

Index

Chemical doping method, 304
305 Chemical oxidation method, 541 Chemical solution-based methods, 57 Chemical vapor deposition (CVD), 13f, 14, 30, 57, 124
127, 134
135, 202
204, 214, 525, 527, 533
534 metal catalyst influence in, 125
127 substrate catalyst method, 125 Chinese hamster ovary cells (CHO), 45 Chiral vector, 393f, 525 Chirality-controlled SWCNTs, direct synthesis of, 528 Chlorobenzene, 185
186 Chloroform, 34
36 CHO. See Chinese hamster ovary cells (CHO) Cholesterol biosensors, 280
282 Citrus reticulata peel extract, 38
39 Clathrates, 308
309, 310t Clay, 42, 44 Clean solar energy, 326 Climate change, 385
386 Clusterfullerenes, 522 CMC. See Critical micelle concentration (CMC) CNMs. See Carbon nanomaterials (CNMs) CNTs. See Carbon nanotubes (CNTs) CO2 capture, storage, and utilization technique (CCU technique), 283
284 CO2 reduction reaction (CRR), 284 Cobalt oxides, 306 Coefficient of performance (COP), 320 COFs. See Covalent
organic frameworks (COFs) Coherent nanoinclusions, 304
305 Collector, distance effect between needle and, 464
466 Colloid, 34
36 emulsion, 34
36, 35f nanoparticle synthesis, 33
36 Colloidal method, 33
34 Colony-forming units (CFU), 441
443 Combustion process, 85, 217 combustion-derived nanomaterials, 95
112 of hydrocarbon method, 521 Combustion synthesis (CS), 85

561

fundamentals, 88
95 kinetics, 92
95 thermodynamics, 89
92 reactor, 96f Combustion-derived nanomaterials microstructural characteristics, 95
112 SCS, 105
112 solid
gas and solid
solid CS systems, 96
105 Commercialized sodium-based technologies, 402 CoMoCAT process, 527 Composite material fabrication, 339
346 electrospinning of fibers and mats, 342
344 piezoelectric paper fabrication, 344
346 SbSI dot nanocomposites in organically modified titanium dioxide glass, 340
342 in sodium borosilicate glass, 339
340 Compressive stress, 462 Computational fluid dynamics, 500
501 Condensation process, 123 Conductivity/surface charge density, effect of, 468
469 Confinement controlled sublimation method, 534
535 Conjugated polymer thermoelectric materials, 309
314, 312f Constant pressure regime, 89
90 Constant volume regime, 89
90 Control synthesis in “aerosol” and “FSP”, 225
226 Conventional agitation method, 34
36 Conventional engineering methods, 495 Conventional reducing agents, 531
532 Conventional thermal methods, 131
132 COP. See Coefficient of performance (COP) Copper (Cu), 298, 533, 542 Cu-BTC-MOF, 286 wires, 298 Copper oxide (CuO), 241
247 Core
shell TMO nanostructure, 240
252 through solid support, 235 Coulomb forces, 462 Covalent functionalization, 529

562

Index

Covalent
organic frameworks (COFs), 2, 4, 272
273 Critical micelle concentration (CMC), 272 “Critical scattering”, 306
307 “Crossed-bounded” parameters, 88 Crosslinking mechanisms, 509
512 Croton sparsiflorus, 38
39 CRR. See CO2 reduction reaction (CRR) Crystal violet (CV), 170
171 CS. See Combustion synthesis (CS) CTAB. See Cetyltrimethylammonium bromide (CTAB) Curie temperature (TC), 355
356 Cutting-edge technology, 484
485 CV. See Crystal violet (CV)Cyclic voltammetry (CV) CVD. See Chemical vapor deposition (CVD) Cyclic mechanical stretching, 491
492 Cyclic voltammetry (CV), 469
470 Cycling stability of MN1:1 nanofiber, 473
476 Cyclopropanation of fullerenes, 524

D Daxad 19, 34
36 Decellularization of apple tissue, 496, 497f Deflagration, 85
86 Degussa P25 TiO2 photocatalyst, 439
440 Density functional theory (DFT), 404
405, 427, 430 Density of states (DOS), 300, 311 Desmodium gangeticum, 45 Detonation, 85
86 DFT. See Density functional theory (DFT) Dialysis, 488
490 Diatoms, 495 Dielectrophoresis, 528 Diels
Alder reactions, 524 Dihydrogen, 523 Dimethyl sulfoxide (DMSO), 44 (4-(1,3-Dimethyl-2,3-dihydro-1Hbenzoimidazol-2-yl)phenyl) (NDMBI), 313
314 N,N-Dimethylformamide (DMF), 34
36, 342
343, 369
370 Dioscorea batatas, 38
39

1,3-Dipolar cycloaddition of azomethine ylides. See Prato reaction Direct blue 78, 285 Dissolve and grow mechanism, 78 DMF. See N,N-Dimethylformamide (DMF) DMSO. See Dimethyl sulfoxide (DMSO) DNA-assisted dispersion and separation, 528 Doped-CeO2 porous material, 106 Doping of carbon materials, 396
397 of CDs, 542
543 of ZnO nanostructures through hydrothermal routes, 64
65 DOS. See Density of states (DOS) Double-doped graphene, 537 Double-walled carbon nanotubes (DWCNTs), 525 Drawing method, 454
455, 455f Droplet microfluidics, 493
494 artist’s view of biological assays in, 495f Drugs, 437
438 Dual-pulsed laser, 526 DWCNTs. See Double-walled carbon nanotubes (DWCNTs) Dynamic supramolecular coordination chemistry, 528

E Earnshaw instability, 462, 463f ECM. See Extracellular matrix (ECM) ”Ecofriendly” reducing agents, 531
532 ECs, 491
492 EDLCs. See Electrical double-layer capacitors (EDLCs) EDS. See Energy dispersion X-ray spectroscopy (EDS) EFs. See Electric fields (EFs) EIS. See Electrochemical impedance spectroscopy (EIS) EISA method. See Evaporation-induced self-assembly method (EISA method) Electric fields (EFs), 225
226 intensity, 464
466 Electrical breakdown, 528 Electrical conductivity (σ), 299
300 of precursor solution, 468
469

Index

Electrical double-layer capacitors (EDLCs), 387, 410
411, 452
453 Electro-explosion, 30 Electrochemical exfoliation of graphite, 532
533 intercalation, 537 ion intercalation method, 10 oxidation, 540
541 reduction, 532 supercapacitors, 410
411 systems, 386 Electrochemical energy storage, 386 carbon materials, 388
397 nanomaterial, 397
413 challenges and future perspectives, 413
414 devices showcasing specific power vs. energy, 387f Electrochemical impedance spectroscopy (EIS), 473
476 Electrochromic (EC) devices, 208
209 displays, 206 Electrode materials carbon-based, 403
406 carbon-based composite, 406
408 Electrodeposition, 235 Electrodes, 401 Electrolytes, 473
476 Electron(s), 153
154 beam irradiation of graphene, 521 diffraction pattern of Al NPs, 180
182, 182f electron-donating functional groups, 427 electron-energy-loss spectroscopy, 105 electron-ion plasma, 151
153 electron-poor polyenes, 524 electron
phonon coupling, 169
170 irradiation, 523 Electronic crystals, 311 Electrospinning, 453
454, 457
469 effects of system/process parameters on nanofibric morphology, 463
469 of fibers and mats, 342
344 method, 75
76 setup and working principle, 458
463 splaying of fibers and single fiber, 460f

563

Electrospun composite nanofibers, 486
487 Electrospun nanofibers, 476
477 Electrostatic forces, 458
460 Electrothermal explosion (ETE), 93 Electrothermography (ET), 93 Emerald Biosciences, 493
494 EMFs. See Endohedral metallofullerenes (EMFs) Endohedral fullerene, 391, 521 from carbon sources, 522
523 from pristine fullerenes, 523 Endohedral metallofullerenes (EMFs), 522 Energy crisis, 295
297 management, 385
386 needs, 295
297 sources, 295
297 storage energy-storage capacity of nanofibers, 477 technologies, 386, 386f Energy dispersion X-ray spectroscopy (EDS), 176
178 Engineered microparticles, 492
493 Enhanced photocatalytic ability, 439
440 Entanglement, 467
468 Epitaxial growth of graphene on SiC substrate, 534
535 Epoxidation, 524 Epoxy-resin, 420 Escherichia coli, 441
443, 442f O157:H7, 284 ET. See Electrothermography (ET) ETE. See Electrothermal explosion (ETE) Ethanol, 34
36 Europium-doped yttrium silicate (Y2SiO5: Eu31), 219
225 Evaporation-induced self-assembly method (EISA method), 273
274 Exfoliated graphene, 2 Exfoliation of graphite, 388
389, 530 method of formation of nanocomposites, 42
43 Exohedral fullerene, 391, 521, 523
524 Bingel
Hirsch reaction, 524

564

Index

Exohedral fullerene (Continued) carbene addition, 524 Prato reaction, 524 reactions, 524 Experimental fabrication characterization methods, 159
160 nonlinear optical properties, 160
161 synthesis methods, 158
159 Extra Si flux, 534
535 Extracellular matrix (ECM), 484
487, 496 Extrusion bioprinting, 506
507 extrusion-based printing methods, 506
507 extrusion-bioprinted scaffolds, 507

F f-TEG. See Flexible thermoelectric generators (f-TEG) Fab@home, open-source projects, 506
507 Fabrication of group 15 ternary chalcohalides ball milling of bulk crystals, 346
347 composite material fabrication, 339
346 conversion of Sb2S3 into SbSI, 369
370 filling of carbon nanotubes, 359
363 future perspectives, 378 graphoepitaxy, 349 heat and laser formation of SbSI nano-objects, 370
377 hydrothermal growth, 366
368 microwave-assisted aqueous synthesis, 365
366 new trends, 377
378 solution processing, 363
364 sonochemical synthesis of SbSI-type nanowires, 349
356 ultrasonic spray pyrolysis, 356
359 vapor-phase growth of SbSI nanorods, 347
348 methods, 503, 530 techniques to synthesizing TMO nanostructures, 214
218 CVD, 214

flames, 217
218 plasma, 214
217 sol
gel, 214 Far-field scattering distribution, 185
186, 187f Faradic supercapacitors (FSCs), 452
453 FAST. See Flame-assisted spray technology (FAST) FBCVD. See Fluidized bed CVD (FBCVD) Femtosecond (fs), 158
159 experimental setup for fs laser direct writing, 158f laser irradiation, 176
179 laser
solid interactions, 153
156, 154f FESEM. See Field emission scanning electron microscopy (FESEM) Few-layer graphene sheet, 102 FF. See Filling factor (FF) Fiber(s) bonding, 503 electrospinning, 342
344 Field emission scanning electron microscopy (FESEM), 159
160, 162f, 170f, 173f, 177f, 178f Filament propagation, 150
151 Filling factor (FF), 326 Flame-assisted spray technology (FAST), 249 Flame spray pyrolysis (FSP), 202
204, 225
226, 248
249 Flame(s), 217
218, 218f synthesis of nanostructured TMOs core
shell and mixed TMO nanostructures, 240
252 fabrication techniques to synthesize, 214
218 motivation, 204
206 of multidimensional TMOS using “solid support” method, 226
235 as unique fabrication tool to produce, 218
226 volumetric flame synthesis of 1D and 3D TMOs, 235
240 vapor deposition, 241
247 Flexibility of PDMS, 491 Flexible thermoelectric generators (f-TEG), 321, 323
324

Index

Fluidic simulations on patterned surfaces, 498
500 Fluidized bed CVD (FBCVD), 134
136, 135f, 136t Fluidized bed reactors, 527 Fluorescent nanocarbon structures, 540 Fluorinated graphene, 537
538 Food industry, nanoporous materials in, 284
285 Free-carrier absorption, 154
155 Freeze drying, 503 fs. See Femtosecond (fs) FSCs. See Faradic supercapacitors (FSCs) FSP. See Flame spray pyrolysis (FSP) FTIR spectra of catechol, 427, 428f Fullerenes, 121
122, 391, 520
524 endohedral from carbon sources, 522
523 from pristine fullerenes, 523 exohedral, 523
524 pristine, 521 synthetic pathways, 520f Functional groups, 524 Fusion, 493
494

G g-C3N4. See Graphitic carbon nitride (gC3N4) Gadolinium (Gd), 542 GAGs. See Glycosaminoglycans (GAGs) Galvanostatic charging discharging curves (GCD curves), 469
470 Gas(es), 522 foaming, 503 gas-phase combustion synthesis, 235
238 flame synthesis of 1D and 3D TMOs, 238
240 synthesis of 1D and 3D core
shell and MTMOs, 249
252, 250f techniques, 124 hydrocarbons, 100
101 sensors, 210
212 chemical composition, 211 physical morphological properties, 211
212

565

separation, purification, and storage, 286
287 GCD curves. See Galvanostatic charging discharging curves (GCD curves) Gene therapy, 280
282 Germanium (Ge), 307
308 nanoparticles by pulsed laser ablation, 165
169, 165f Germanosilicate zeolite, 267
268 Gibberellic acid, 284
285 GICs. See Graphite intercalation compounds (GICs) GIXRD patterns, 64 Global warming, 283
284 Glow discharge CVD, 127
128 Glucose, 34
36 glucose-sensing system, 280
282 (3-Glycidoxypropyl)trimethoxysilane (GLYMO), 340 Glycine, 87, 108
109 quantity, 108
109 Glycosaminoglycans (GAGs), 486
487 GLYMO. See (3-Glycidoxypropyl) trimethoxysilane (GLYMO) GNRs. See Graphene nanoribbons (GNRs) GnuBIO, 493
494 GO. See Graphene oxide (GO) Gold (Au), 204
205 Au
Ag alloy, 187
190, 188f film-coated Si substrates, 169
172 nanoparticles, 44
45 GQDs. See Graphene quantum dots (GQDs) Gram-negative bacteria, 34
36 Gram-positive bacteria, 34
36 Graphene, 3
4, 3f, 14, 44, 102, 121
122, 313, 388
390, 411, 529
539 biobased synthetic routes, 535
536 chemical functionalization, 538
539 chemical synthesis, 536 electron beam irradiation, 521 exfoliated, 2 few-layered, 404
405 graphene-like ultrathin 2D nanomaterials, 2 halogenated, 537
538 heteroatom-doped, 537

566

Index

Graphene (Continued) heterostructures, 536
537 hydrogels/aerogels, 411
412 hydrogenated, 538 microwave-assisted synthesis, 138
139 molecules, 138
139 nanocomposites, 537 preparation methods, 388
389, 390f production by micromechanical cleavage technique, 5f synthesis and production, 530
535 Graphene nanoribbons (GNRs), 539 Graphene oxide (GO), 32
33, 111 nanosheets, 11 reduction, 389
390 reductions, 532 sheets, 531
532 Graphene quantum dots (GQDs), 163
165, 164f, 540 by laser irradiation of graphite in water, 163
165 Graphite, 397
399, 525
526 electrochemical exfoliation, 532
533 exfoliation, 530 heteroatom dopants for, 396f LPE, 531 oxide route, 531
532 vaporization, 521 Graphite intercalation compounds (GICs), 530 Graphitic carbon nitride (g-C3N4), 2
4, 3f Graphitic carbons, 403
404 Graphitizable carbons, 398
399 Graphoepitaxy, 349 Green energy, 295
297 Green fluorescent protein, 493
494 Green microwave-assisted chemical technique, 365 Green nanoparticles, 28 Green synthesis, 37
38 of nanoparticles, 30
31, 31f of selenium nanoparticles, 38
39 Green tea (Camellia sinensis) extract, 38
39 Group 15 ternary chalcohalides, 338 Growth of small bending perturbation, 462
463

H h-BN. See Hexagonal boron nitride (hBN) HACNT arrays. See Horizontally aligned carbon nanotube arrays (HACNT arrays) Half-Heusler alloys (HH alloys), 301
302, 308
309, 310t Halogenated graphenes, 537
538 Hard carbons, 388, 389f, 398
399, 403
404, 409
410 Heat formation of SbSI nanoobjects, 370
377 process, 88
89 Helical bilayer nanographene, 536 Helium (He), 523 Hemodynamic parameters, 491
492 Heteroatom-doped carbon materials, 396
397 Heteroatom-doped graphene, 537 Heterogeneous photocatalysis, 420
421 Hexadecyl trimethyl ammonium bromide, 365 Hexagonal boron nitride (h-BN), 2
4, 3f, 101 nanosheets, 14 Hexamethylenetetramine (HMT), 59
61 HH alloys. See Half-Heusler alloys (HH alloys) Hibiscus rosa-sinensis, 38
39 Hierarchical CNT-coated fibrous reinforcement structures, 315 High spatial frequency LIPSS (HSFL), 156
157 High-energy acoustic pressure wave, 31
32 High-intensity ultrashort laser pulses, 155 High-performance engineering polymers, 315 High-quality graphene sheets, 531 High-resolution images of NPs in acetone, 166
167, 168f soft micro-optical systems, 501 High-resolution transmission electron microscopy (HRTEM), 5
7, 70, 71f

Index

High-speed transmission electron microscopy (HSTEM), 93 High-temperature pyrolysis method, 76 High-voltage electrostatic field, 453
454 High-voltage source, 458 High-yield nanosheets, 8
9 Highest occupied molecular orbital
lowest unoccupied molecular orbital (HOMO-LUMO), 430 HiPCO process, 527 hMSCs. See Human mesenchymal stem cells (hMSCs) HMT. See Hexamethylenetetramine (HMT) Holographic lithography, 502 HOMO-LUMO. See Highest occupied molecular orbital
lowest unoccupied molecular orbital (HOMO-LUMO) Homogeneous aqueous solutions, 87 Horizontally aligned carbon nanotube arrays (HACNT arrays), 525, 527 Hot-injection method, 16 HRTEM. See High-resolution transmission electron microscopy (HRTEM) HSFL. See High spatial frequency LIPSS (HSFL) HSTEM. See High-speed transmission electron microscopy (HSTEM) Human mesenchymal stem cells (hMSCs), 507 hMSC-laden 3D architectures, 510
512 Human umbilical vein endothelial cells (HUVEC), 45 Humidity effect on electrospun nanofiber, 469 Hybrid approach to form core
shell TMOs, 235 Hydrazine hydrate, 531
532 Hydrogels, 500 in microfluidics, 491 Hydrogen (H), 13 H-bonded supramolecular polymer, 528 Hydrogenated graphenes, 538 Hydrogenation, 524 Hydroquinone, 531
532 Hydrothermal

567

carbonization, 394
395 growth, 366
368 hydrothermal/solvothermal method, 68
72 synthesis, 33, 269
270, 366
367 treatments, 541
542 Hydroxylamine, 531
532 Hypodermic syringe, 458

I ICT. See Interfacial charge transfer (ICT) Ideal bioink for extrusion-based bioprinting, 506
507 Immunoglobulin, 282 Immunoisolation, 282 Implantable artificial kidney, 486
487, 488f Implantable artificial renal assist device, 488
490 In situ functionalization by electrodeposition, 537 In situ simulations, bioreactors for, 500
501 In situ synthesis of silver nanoparticles, 441
443 In vitro in droplet microfluidic devices, 493
494 Incident laser light, 162
163 Incident photon-to-current efficiency (IPCE), 438
439 Incoherent nanoinclusions, 304
305 Incubation, 493
494 Indium (In), 187
190, 188f Infrared spectroscopy, 427 Inkjet bioprinters, 508 bioprinting, 508
510 Inner transition metals, 204
205 Inorganic nanoinclusions, 314 Inorganic nanoparticles, 431
435, 432f Inorganic nanoporous material synthesis, 274
276 nonsilica-based mesoporous materials, 275
276 silica-based mesoporous materials, 274
275, 275f

568

Index

Inorganic thermoelectric nanomaterials, 302
309. See also Organic thermoelectric material (OTE material) metal chalcogenides, 304
306 metal oxides, 306 silicon-based materials, 307
308 SKUs, clathrates and HH alloys, 308
309, 310t superionic conductors, 306
307 Integrated Tissue-Organ Printer (ITOP), 510
512 Interfacial charge transfer (ICT), 420
421, 422f, 423t complex formation, 437
438 complexes, 420
430 International Union of Pure and Applied Chemistry (IUPAC), 204
205 Intraband transitions, 153
154 Ion beam lithography, 502 Ion exchange, liquid exfoliation by, 10
11 Ionic salts, 468
469 IPCE. See Incident photon-to-current efficiency (IPCE) Iron (Fe), 527, 533 iron-based metal-organic framework [MIL-53(Fe)[, 44 oxide structures, 233
234 Isolated pentagon rule, 522 Isotropic refractive index change mechanism, 151
153 ITOP. See Integrated Tissue-Organ Printer (ITOP) IUPAC. See International Union of Pure and Applied Chemistry (IUPAC)

K Kapton polyimide-based polymer, 323 Keldysh parameter, 155 KH2PO4, 468
469 KIBs. See Potassium-ion Batteries (KIBs) Kinetics, 92
95 Kirkendall effect, 60
61 Kissinger
Akahira
Sunose method (KAS method), 94 Krätschmer
Huffman system, 522

Krypton (Kr), 523 Kubelka
Munk function spectra, 430, 431f

L LAB. See Laser-assisted bioprinting (LAB) Lanthanide-based metal
organic frameworks (Ln-MOFs), 283 Lanthanum (La), 308
309 La-containing fullerenes, 522 Laser crystallization, 376f formation of SbSI nanoobjects, 370
377 irradiation of graphite in water, 163
165 laser-by-laser ablation, 150 laser-induced forward transfer LDW, 507 laser-induced growth of glass, 373
377, 376f vaporization, 130 Laser ablation, 29
30, 123
124, 134
135, 150, 522, 525
526 of graphite rods, 540
541 in liquids for application as potential optical limiter, 179
186 of Si in water, 172
174 Laser-assisted bioprinting (LAB), 503
504, 507
508 Laser-based direct writing (LDW), 507 Laser-induced periodical surface structures (LIPSS), 156
157 Laser-induced synthesis of nanomaterials Al NPs preparation by laser ablation, 179
186 blue luminescent Si nanoparticle synthesis, 172
174 experimental fabrication, 158
161 formation of metal nanostructures in polymer matrix, 176
179 fs laser pulse inside transparent dielectric, 152f fundamental processes, 153
156 Ge nanoparticles by pulsed laser ablation, 165
169 GQDs by laser irradiation of graphite in water, 163
165

Index

long-and short-pulse interaction, 153 metal nanoparticles in liquids, 187
190 nanostructuring of titanium metal toward fabrication, 174
176 structural changes induced by ultrashort laser pulses, 156
157 surface nanostructuring on Au film-coated Si substrates, 169
172 on graphite, 162
163 Layered double hydroxides (LDHs), 2 Layered metal oxides, 2 LDHs. See Layered double hydroxides (LDHs) LDW. See Laser-based direct writing (LDW) Lead chalcogenides, 304
305 “Leading” combustion zone, 98
99 LEDs. See Light-emitting diodes (LEDs) LIBs. See Lithium-ion batteries (LIBs) Light irradiation, 528 Light-emitting diodes (LEDs), 212, 325 Linear absorption. See Single-photon excitation Linear mesocages, 455
456 LIPSS. See Laser-induced periodical surface structures (LIPSS) Liquid exfoliation by chemical and electrochemical ion intercalation, 8
10 by ion exchange, 10
11 by mechanical force, 6f, 7
8 by oxidation and reduction, 11
12 by selective etching, 12
13 Liquid reaction method. See Solution processing Liquid-phase exfoliation (LPE), 531 of graphite, 531 reduction, 389
390 Liquid(s) liquid-based synthesis methods, 33 metal nanoparticles in, 187
190 phase ultrasonication, 537 Lithium ions, 10
11 Lithium-ion batteries (LIBs), 209, 391, 397
401. See also Sodium-ion batteries (NIBs)

569

Li-ion-based battery systems, 409f working principle, 398f Lithium-sulfur battery, 408
409 Living mammalian cells, 500
501 Ln-MOFs. See Lanthanide-based metal
organic frameworks (LnMOFs) Long and defect-free CNTs, 527 Long-pulse interaction, 153 Low spatial frequency LIPSS (LSFL), 156
157 Low-reflective surfaces, 174
176 LPE. See Liquid-phase exfoliation (LPE) LSFL. See Low spatial frequency LIPSS (LSFL) Luminescence of CDs, 540

M m-SWCNTs. See Metallic behavior singlewalled carbon nanotubes (mSWCNTs) Macroporous material synthesis, 272
273 metal oxides, 272
273 Magnesium batteries, 391 Magnesium oxide (MgO), 125, 527 Malathion, 285 Malonates, 524 Manganese oxide (Mn2O3), 306, 469
470 Manganese-based oxides (Spinel-Mn3O4), 452
453 Mass transfer process, 88
89 Matrix-assisted pulsed laser evaporation direct writing, 507 Mats, electrospinning of, 342
344 MCM. See Mobil Composition of Matter (MCM) Mechanical cleavage, 5
7, 530
531 Mechanical force, LIQUID exfoliation by, 7
8 Mechanical grinding/milling, 29 Mechanical-physical particle production process. See Top-down approaches MEDB. See Multielement diffusion flame burner (MEDB) MEG. See Monoethyleneglycol (MEG) Mesoporous

570

Index

Mesoporous (Continued) magnesia, 269
270 material synthesis, 267
268 3D structures, 269f surfactants role as SDA, 272 template role in porosity formation, 271 1D high aspect ratio nanofibers, 452
453 SBA-15-type materials, 269
270 silica, 269, 280
282, 284
285 TiO2 nanorods, 74
75 Metal organic frameworks (MOFs), 267
268 MOF-5, 286 Metal-oxide nanoparticles, flame synthesis of, 218 Metal(s), 2, 3f carbides, 14, 522 carbonitride, 522 catalyst influence in CVD technique, 125
127 chalcogenides, 304
306 bismuth chalcogenides, 305
306 lead chalcogenides, 304
305 cyanide, 522 of groups IIIB and IVB, 522 metal-alkaline, 531
532 metal
dielectric interface, 156
157 nanoparticle, 185
186 in liquids, 187
190 nanostructures formation in polymer matrix, 176
179 nitride, 522 oxides, 306, 522 precursor, 472
473 sulfide, 522 Metallic behavior single-walled carbon nanotubes (m-SWCNTs), 525 Metallic substrates, 533 Metal
organic frameworks (MOFs), 2, 3f, 4 Methane-fueled coflow diffusion flame, 219
225 MG. See Mn3O4 nanoparticles anchored graphene nanosheets (MG) Mg2TiO4, 422
426, 439
440

Micelles, 272 Micro-and nanofluidic drug-screening devices, 492
493 Micro/nano-miniaturized platform fabrication bioartificial organs, 487
490 biomimetics, 494
496 biopatterning complexities of life, 497
501 bioprinting of organs and tissues, 501
514 micro-and nanofluidic devices, 490
494 nanotheranostics and regenerative medicine, 484
487 Microbial synthesis of nanoparticles, 39
41 exfoliation of nanoclay platelets, 41f of zinc, silver, and selenium nanoparticles, 41t Micrococcus luteus ATCC 9341, 45 Microexplosion, 156 Microextrusion bioprinting, 503
504 Microfluidic biomimetic vascular systems, 492
493 devices, 490
494 biosensing devices, 493
494 cell culture devices, 491
492 drug-screening devices, 492
493 lung-on-a-chip, 498
499, 499f Micromechanical cleavage of graphite, 530
531 Microporous clays, 287
288 material synthesis, 267
268 Microporous organic polymers (MOPs), 286
287 Microreactors, nanoparticle synthesis using, 36 Microstructural analysis, 353
354 Microstructure biopatterning, 498 formation mechanisms, 105 Microwave flash heating, 72
73 heating, 542
543 techniques, 526 Microwave-assisted aqueous synthesis, 365
366

Index

chemical vapor deposition, 527 method, 72
74 pyrolysis, 133 synthesis, 122 CNM synthesis and challenges, 139
140 CVD, 124
127 f graphene, 138
139 fluidized bed CVD, 134
136 methods of CNMs, 122
124 microwave-enhanced CVD, 130
134 plasma-enhanced CVD-based CNMs, 127
130 vapor phase growth CVD, 137
138 Mixed transition metal oxide (MTMO), 204 nanostructure, 240
252 flame synthesis of core
shell, 241
248 gas-phase synthesis of 1D and 3D core
shell, 249
252 and related core
shell nanostructures, 248
249 Mn-oxide nanoparticles, 212
213 Mn3O4 nanoparticles anchored graphene nanosheets (MG), 32
33 MnxOy nanofibers future aspects and challenges, 476
478 synthesis of 1D nanofibers, 454
469 utilization of binary MnxOy nanofibers for energy-storage applications, 469
476 Mobil Composition of Matter (MCM), 269 MCM-22, 285
286 Modified Hummers’ method, 11 Modules, 484
485 MOFs. See Metal organic frameworks (MOFs)Metal
organic frameworks (MOFs) Molecular self-assembly, 456
457 sieves, 267
268 surgery, 523 weight effect of polymer, 467
468, 467f Molybdenum (Mo), 187
190, 188f Molybdenum sulfide (MoS2), 101, 104
105

571

Momentum balance for bead, 462 Monoclinic vanadium dioxide nanorods, 107 Monoethyleneglycol (MEG), 368 MOPs. See Microporous organic polymers (MOPs) MTMO. See Mixed transition metal oxide (MTMO) Multidimensional TMOs, 226
235, 227t flame positon effect, 234 hybrid approach to form core
shell TMOs, 235 parameters affecting flame synthesis, 230
231 probe diameter and oxygen content effect in oxidizer, 232
233 thermal property effect of source material, 233
234 Multielement diffusion flame burner (MEDB), 219
225, 248
249 Multifunctional organic
inorganic hybrid nanomaterials, 420 Multimaterial printing using inkjet technology, 509
510 Multiphoton ionization, 151
153 Multiple-photon excitation, 151
154 Multiscale phonon scattering, 304
305 Multitude of functional manipulations of droplets, 493
494 Multivalent cation-based battery technologies, 410 Multiwalled carbon nanotubes (MWCNTs), 123, 314
315, 393, 525, 528 Muscle-forming cells, 513 MWCNTs. See Multiwalled carbon nanotubes (MWCNTs) MXenes, 2, 3f, 4, 12
13, 12f Myoblasts, 513

N N-co-doped ZnO nanorod films, 64
65, 66f N-DMBI. See (4-(1,3-Dimethyl-2,3dihydro-1H-benzoimidazol-2-yl) phenyl) (N-DMBI) N-doped graphene, 537

572

Index

Nanobelts, 53
54, 57
58 Nanocages, 53
54 Nanocarbon-based nanocomposite, 322
323 Nanochannel fabrication, 491, 491f Nanoclay nanocomposite, 42
43 Nanocombs, 53
54 Nanocomposite graphene, 537 polymer thermoelectric materials, 314
318, 317t systems, 309
318 Nanofibers, 453
454, 476
477 of materials, 451
452 membranes, 286 of transition metal oxides, 469
470 Nanofibric networks with porosities, 456 Nanofibrous cellulose, 510
512 Nanofluidic devices, 490
494 biosensing devices, 493
494 cell culture devices, 491
492 drug-screening devices, 492
493 Nanographenes (NGs), 536 Nanoinclusions, 304
305 Nanomaterials (NMs), 150, 202
204, 451
452 applications, 42
43 nanoclay and polymer nanocomposite, 42
43 photocatalyst for degradation of organic pollutants, 43 polymer functional nanolatex, 42
43 synthesis. See Nanoparticle synthesis Nanoneedles, 53
54 Nanoparticle synthesis, 150 advances in chemical and biological synthesis routes, 44
46 approaches, 27
31, 29f using biological route, 37
41 bottom-up approach, 28 chemical vapor deposition, 30 green and biological synthesis, 30
31 sol
gel process, 30 chemical routes colloid nanoparticle synthesis, 33
36 ultrasound-assisted synthesis, 31
33, 32f

using microreactors, 36 scale-up issues of nanoparticle production and challenges, 46 top-down approach, 28 electro-explosion, 30 laser ablation, 29
30 mechanical grinding/milling, 29 Nanoparticles (NP), 27
28, 277, 346
347 agglomerates, 135
136 fluidization, 135
136 Nanoporous anodic alumina, 280
282 materials, 491
492, 492f membranes, 280
282 particles, 280 silicon films, 284
285 Nanoribbons, 53
54 Nanorings, 53
54 Nanoscience, 1
2 Nanosized photoactive materials, 207
208 Nanostructured/nanostructures biomimetics, 495 biopatterning, 498 materials, 301
302, 490 of titanium metal toward fabrication, 174
176, 175f Nanotechnology, 1
2, 453
454, 484 challenges in nanotechnology based on 1D ZnO nanostructures, 65
67 synthesis trends and challenges based on 1D nanostructures, 78
79 Nanotheranostics, 484
487 Nanotubes, 53
54 Nanotube
substrate interaction, 137 Nanowhiskers, 53
54 Nanowires, 53
55, 57
58 Nd:YAG, 526 Needle diameter effect, 464, 465f distance effect between collector and, 464
466 Neon (Ne), 523 NGs. See Nanographenes (NGs) NIBs. See Sodium-ion batteries (NIBs) Nickel (Ni), 527, 533 Ni-Al system, 94 Ni/Cu catalytic alloy, 535

Index

nickel nitrate
glycine system, 94 Nitride-based articles, 96 Nitrogen (N), 522 atom-containing C60, 523 atoms, 538
539 nitrogen-doped graphene, 412
413 nitrogen-doped TiO2, 287 NMs. See Nanomaterials (NMs) Noble gases, 523 Noncovalent functionalization, 529 Nonequilibrium, 88 Nongraphitizable carbons, 398
399 Nonlinear absorption. See Multiple-photon excitation Nonlinear field ionization, 151
153 Nonlinear optical properties of Al NPs, 182
183 of laser-induced synthesis, 160
161 Nonmetal-containing molecules, 523 Nonsilica-based mesoporous materials, 275
276 Nonsiliceous mesoporous materials, 269
270 NP. See Nanoparticles (NP)

O One-dimension (1D) nanomaterials, 1
2 nanostructures SCS, 106
110 solid
gas and solid
solid CS systems, 96
101, 98f nanowires, 451
452 SbSI, 367 structure, 95, 101f, 202
204 One-dimensional nanofibers (1D nanofibers) with high surface area and porosity, 453 of metal oxides, 454 synthesis, 454
469 compression of methods to fabricating nanofibers, 458t drawing method, 454
455, 455f electrospinning, 457
469 phase separation, 456, 457f self-assembly, 456
457 template method, 455
456, 456f

573

One-dimensional oxide nanostructures (1D oxide nanostructures), 53
54 1D TiO2 nanostructure growth mechanism from chemical solution, 77
78 synthesis techniques from chemical solution, 68
76 synthesis trends and challenges in nanotechnology, 78
79 1D ZnO nanostructures, 57
67 challenges in nanotechnology, 65
67 synthesis techniques, 55
57 and strategy principle, 56t Organ-specific bioengineering, 486
487 bioprinting, 510
513 Organic dyes, 439 nanoinclusions, 314 photocatalyst for degradation of organic pollutants, 43 porous material synthesis, 273
274 semiconductors, 311, 322
323 surfactant molecules, 269
270 thermoelectrics. See Organic thermoelectric material (OTE material) Organic thermoelectric generators (OTEGs), 322
323 Organic thermoelectric material (OTE material), 309
311. See also Inorganic thermoelectric nanomaterials conjugated polymer TE materials, 309
314 nanocomposite polymer TE materials, 314
318 Organic-to-inorganic ICT transition, 421, 421f Organically modified titanium dioxide glass, 340
342 Organic
inorganic hybrid nanomaterials, 420 application, 437
443 formation mechanism and optical properties, 420
430 new synthetic approaches and challenges, 435
437

574

Index

Organic
inorganic hybrid nanomaterials (Continued) polymer supports decorated with inorganic nanoparticles, 431
435 Organic
inorganic hybrid polymeric material synthesis, 277, 278f Organs-on-chips, 498
499 Oriented attachment (surfactant-free), 77
78 Oscillator-amplifier system, 158
159 OSRS formation. See Oxidation-shaking off-rolling up-shrinkage formation (OSRS formation) Osteoinductive ceramics, 510
512 Ostwald ripening, 77
78 OTE material. See Organic thermoelectric material (OTE material) OTEGs. See Organic thermoelectric generators (OTEGs) Oxidation, 524 liquid exfoliation by, 11
12 Oxidation-shaking off-rolling up-shrinkage formation (OSRS formation), 74
75, 75f Oxides, 3f Oxygen content in oxidizer, 232
233 oxygen-free graphene, 92

P PAN/SbSI. See Polyacrylonitrile nanofibers containing ferroelectric and semiconducting antimony sulfoiodide (PAN/SbSI) PANI. See Polyaniline (PANI) Parenchymal organs, valuable technology for, 487 Partially hydrogenated graphene, 538 Particulate leaching, 503 Pathogenic microorganisms, 284 PBMA. See Polybutyl methacrylate (PBMA) PC. See Polycarbonate (PC) PCR. See Polymerase chain reaction (PCR) PDI. See Perylenediimide (PDI) PE. See Polyelectrolyte (PE)

PEDOT:PSS. See Poly(3,4-ethylenedioxy thiophene) polystyrene sulfonate (PEDOT:PSS) PEI. See Polyethyleneimine (PEI) PEIs. See Poly(ether-imide)s (PEIs) Peltier effect, 297
298, 319, 321 Peltier
Seebeck effect, 298 Percentage of microbial reduction (R, %), 441
443, 442f Perfluoroalkylation, 524 Perfusable vascular network formation, 491 Periodically structured materials, 501 Perturbation, 462 Perylenediimide (PDI), 313
314 Phase separation, 456, 457f, 503 Phonon glass and an electron crystal (PGEC), 306
309 Phosphorous, 407
408 Photo-degradation, 439
440 Photo-driven processes, 420
421, 437
443 Photocatalytic/photocatalysis, 437
438 conversion of CO2, 284 degradation, 43 for degradation of organic pollutants, 43 Photocatalyst, 287 Photoexcitation, 421 Photoferroelectric semiconductor, 338 Photolithography, 501 Photoluminescence (PL), 159
160, 169f, 174f Photons, 153
154 Photoreduction, 532 Physical morphological properties, 211
212 Physical vapor deposition methods (PVD), 57 Sb2S3 conversion by, 369 π-conjugations, 389
390 Piezocrystals, 509 Piezoelectric paper fabrication, 344
346 PL. See Photoluminescence (PL) PLA. See Polylactide (PLA) Plant extract-based nanoparticle synthesis, 38
39 of zinc, silver, and selenium nanoparticles, 40t

Index

Plant-derived cellulose scaffolds, 496 Plasma, 214
217 plasma-based synthesis, 122
123 plasma-enhanced CVD-based CNMs, 127
130, 128f, 129f Platinum (Pt), 533 nanoparticle production, 36, 36f Pluronic, 510
512 block copolymers, 279 Pluronic
alginate multicomponent bioink, 510
512 PM. See Precision medicine (PM) PMMA. See Poly(methyl methacrylate) (PMMA) Poly-DL-lactic acid, 456 Poly-L-lactide acid, 456 Poly-lactic-co-glycolic acid, 456 Poly(3,4-ethylenedioxy thiophene) polystyrene sulfonate (PEDOT: PSS), 311, 313
314 Poly(ether-imide)s (PEIs), 315 Poly(methyl methacrylate) (PMMA), 32
33 Polyacrylonitrile nanofibers containing ferroelectric and semiconducting antimony sulfoiodide (PAN/SbSI), 342, 343f Polyaniline (PANI), 311 Polybutyl methacrylate (PBMA), 42
43 Polycarbonate (PC), 314
315 Polycrystalline, 533 Polyelectrolyte (PE), 283 Polyethyleneimine (PEI), 316 Polylactide (PLA), 314
315 Polymer, 2, 309
318 adhesive, 530
531 concentration effect, 466
467, 466f functional nanolatex, 42
43 using ultrasound-assisted technique, 43f inorganic composite scaffolds, 486
487 metal nanostructures formation, 176
179 molecular weight effect of, 467
468 nanocomposite, 42
43 polymer-based nanocomposite, 322
323

575

precursor, 273
274 solution droplet, 453
454 supports decorated with inorganic nanoparticles, 431
435 surface, 436
437 Polymerase chain reaction (PCR), 321 Polyol method, 33 Polypropylene (PP), 314
315 Polytetrafluoroethylene (PTFE), 90
91, 100 Polyurethane (PUR), 44 PUR
nanoclay composites, 44 Polyvinyl alcohol (PVA), 458
460 Polyvinylacetate (PVAc), 458
460 Polyvinylpyrrolidone (PVP), 34
36, 466
467 “Poptube” approach, 526 Pore interconnectivity, 486
487 Porosity, 266
267, 486
487 Porous nanostructures applications of porous materials, 279
288, 280f adsorption, separation, and catalytic conversion of CO2, 283
284 agriculture, 287
288 biomedical use, 280
282 catalysis, 282
283 food industry, 284
285 gas separation, purification, and storage, 286
287 photocatalyst, 287 sensors and supercapacitors, 283 water treatment, 285
286 classification, 267, 267f synthesis approaches and challenges, 278
279 synthetic scheme of porous metal oxide layers, 279f synthesis of porous materials inorganic nanoporous materials, 274
276 macroporous materials, 272
273 mesoporous materials, 267
268 microporous materials, 267
268 organic
inorganic hybrid polymeric material, 277

576

Index

Porous nanostructures (Continued) purely organic porous materials, 273
274 Porous polymers, 266
267, 266f Porous Pt-based electrocatalysts, 283 Porous silica, 280
282 Postsynthesis separation techniques, 528 Potassium-ion Batteries (KIBs), 409
410 PP. See Polypropylene (PP) Prato reaction, 524 Precision medicine (PM), 484 Presynthesized low-dimensional nanocrystals, 16 Printing techniques, 323
324 Pristine Al2O3, 439
440 Pristine fullerenes, 521 endohedral fullerenes from, 523 Probe ultrasound, 34
36 Process and system parameter, 458
460, 477
478 Proteins, 493
494 Pseudocapacitance, 387 Pseudocapacitors, 452
453 PTFE. See Polytetrafluoroethylene (PTFE) Pulsed laser ablation in different liquids, 165
169 in liquids using nanosecond laser, 159, 160f Pulsed Nd:YAG laser, 179 PUR. See Polyurethane (PUR) Pure and Fe-doped single crystalline nanorods (W18O49), 108
109, 109f PVA. See Polyvinyl alcohol (PVA) PVAc. See Polyvinylacetate (PVAc) PVD. See Physical vapor deposition methods (PVD) PVP. See Polyvinylpyrrolidone (PVP) Pyrolysis, 394
395, 541

Q QuantaLife, 493
494 Quantum yields (QYs), 540 Quantum-dot superlattices (QDSLs), 301
302 Quasi-free-standing monolayer graphene (QFSMLG), 535 Quaternary chalcohalide compounds, 377
378

R R-TEGs. See Radioisotope thermoelectric generators (R-TEGs) R2R manufacturing process. See Roll-toroll manufacturing process (R2R manufacturing process) RAD. See Renal assist device (RAD) Radioisotope thermoelectric generators (R-TEGs), 307
308, 321. See also Thermoelectric generator (TEG) RainDance Technologies, 493
494 Raisins (Vitis vinifera), 38
39 Raman spectroscopy, 34
36 Randomly stacked arrays, 525 Reactive oxygen species (ROS), 45 Red absorption shift, 438
439 Redox processes, 420
421 Reduced graphene oxide (rGO), 11
12, 32
33, 404
405, 531
532 Regenerative medicine, 484
487 Renal assist device (RAD), 488
490 RepRap, 506
507 Resol, 279 rGO. See Reduced graphene oxide (rGO) Roll-to-roll manufacturing process (R2R manufacturing process), 323
324 ROS. See Reactive oxygen species (ROS) RTMs. See Rutile TiO2 mesocrystals (RTMs) Runge
Kutta fourth-order method, 183
185 Ruthenium (Ru), 533 Rutile, 67
68, 68f space group and crystal structure, 69t Rutile TiO2 mesocrystals (RTMs), 70, 71f

S s-SWCNTs. See Semiconductor-singlewalled carbon nanotubes (sSWCNTs) SAED pattern, 365
366 Salicylate-type binding, 427 of ligands, 429f Salmonella enteritidis, 284 Salmonella typhimurium ATCC 14028, 45 Sb2S3 conversion into SbSI

Index

conversion by physical vapor method, 369 conversion by spinning coating SbI3 solution, 369 SbSeI, 352, 360
361 Scaffolds/scaffolding, 486
487 for biomimetics, 496 Scanning electron microscopy (SEM), 5
7 Scanning probe lithography system, 502 Scattering, 185
186, 186f Scholl reaction, 536 Scotch-tape method. See Mechanical cleavage SCS. See Solution combustion synthesis (SCS) SDA. See Structure-directing agents (SDA) Seaweed (Corallina officinalis), 45 Seebeck coefficient, 298
299, 311 Seebeck effect, 297
298, 319 Selective chemical functionalization, 528 Selective etching by gas-phase reaction, 528 liquid exfoliation by, 12
13 Selenium nanoparticle microbial synthesis, 41t plant extract-based nanoparticle synthesis of, 40t Self-assembly, 456
457 α
Fe nanowires, 54
55, 54f Self-propagating high-temperature synthesis (SHS), 86 Self-sustaining combustion process, 88 exothermic chemical reactions, 85 propagation, 85 reactions, 88 SEM. See Scanning electron microscopy (SEM) Semiconductor photocatalysis, 287 Semiconductor-single-walled carbon nanotubes (s-SWCNTs), 525 Sensors, 283 Sequential solid-vapor growth, 241
247 SERS. See Surface-enhanced Raman scattering (SERS) Shear force-assisted liquid exfoliation, 8 Shearing, 42

577

Sheet-to-sheet printing additive manufacturing technology (S2S printing additive manufacturing technology), 326 Ship-in-a-bottle synthetic approach, 435 Short-pulse interaction, 153 SHS. See Self-propagating hightemperature synthesis (SHS) SiC. See Silicon carbide (SiC) Silica, 126 silica-based mesoporous materials, 274
275, 275f spheres, 541 Silicene, 14 Silicon (Si) Si-based solar cells, 207
208 Si-face graphene, 535 silicon-based materials, 307
308 silicon
nitrogen system, 97 Silicon carbide (SiC), 100, 533 epitaxial growth of graphene on, 534
535 Silicon dioxide (SiO2), 527 Silicone, 2 Silver nanoparticle(s), 34
36, 440
441 derived from Olax scandens leaf, 45 microbial synthesis, 41t plant extract-based nanoparticle synthesis, 40t Silver nitrate (AgNO3), 34
36 Simple noncatalytic synthesis of ultralong ZnO nanowires, 54
55 Simultaneous vapour
vapour growth method, 241
247 Single-layer GO nanosheets, 11 graphene sheet, 102 2D nanomaterials, 8
9 Single-nozzle electrospinning technique, 473
476 Single-photon excitation, 153
154, 183
185 Single-walled carbon nanotubes (SWCNTs), 122
123, 314, 393, 525, 528 Size-controlled silver colloid nanoparticles, 34
36

578

Index

Skutterudites (SKUs), 301
302, 308
309, 310t Sodium borohydride, 34
36, 531
532 Sodium borosilicate glass, SbSI dots nanocomposite in, 339
340 Sodium chloride (NaCl), 468
469 Sodium-ion batteries (NIBs), 401
408. See also Lithium-ion batteries (LIBs) carbon-based composite electrode materials, 406
408 carbon-based electrode materials, 403
406 sodium insertion in expanded graphite, 404f sodium interaction with phosphorusbased anodes, 408f working principle, 402f Soft biophotonic devices, 501 Soft carbons, 388, 389f, 398
399 Soft lithography, 502 Soft templating approach, 273
274, 278
279 Solar cells, 437
438 Solar panels, 207
208 Sol
gel process, 30, 33, 72, 214, 215t Solid diffusion growth, 241
247 Solid flame systems, 86
87 Solid substrates, 241
248, 242t Solid support method, 226
235, 231f Solid-state supercapacitor, 412
413, 412f Solid
gas CS system, 87, 96
105 1D nanostructures, 96
101 2D nanostructures, 101
105 Solid
gas process, 87 Solid
solid CS system, 96
105 1D nanostructures, 96
101 2D nanostructures, 101
105 Soluble silver compounds, 440
441 Solution combustion synthesis (SCS), 87, 105
112, 106f 1D nanostructures, 106
110 2D nanostructures, 111
112 Solution flow rate, effect of, 464 Solution processing, 363
364 Solvent casting, 503 Solvothermal methods, 15, 537 treatments, 541 of citric acid, 542
543

Sonication, 6f, 7
8, 11, 31
32, 354
355, 364 liquid exfoliation by, 8 sol formation, 351 sonication-assisted liquid exfoliation method, 7
8 Sonocation, 74 Sonochemical process, 32
33, 74
75 of SbSI-type nanowires, 349
356, 378 Sphere Fluidics, 493
494 Spinifex nanocellulose-derived hard carbons, 405
406 Spinneret, 460
462 SPIONs. See Superparamagnetic iron oxide nanoparticles (SPIONs) Splitting, 493
494 degree of charged jet, 466
467 SPPs. See Surface plasmon polaritons (SPPs) “Spray pyrolysis” method, 76, 202
204, 219
225, 238 Stable equilibrium state, 89
90 Staphylococcus aureus, 284, 441
443 Stem cell biology, 484
485 Stenotic models, 492
493 Stratasys Dimension Elite, 502 Strongly alkaline solutions, 531
532 Structure-directing agents (SDA), 270
271 surfactants role as, 272 Subtractive methods, 502 Sulfur-doped graphene, 111, 111f Supercapacitive properties of Mn3O4 nanoparticles, 473
476 Supercapacitors, 283, 386
387, 410
413, 452
453 Superionic conductors, 306
307 Superparamagnetic iron oxide nanoparticles (SPIONs), 44 Supersonic wave, 85
86 Surface modification, 426 Surface nanostructuring on Au film-coated Si substrates, 169
172, 172f on graphite, 162
163 Surface passivation, 542 Surface plasmon polaritons (SPPs), 156
157 Surface tension effect, 468

Index

Surface-enhanced Raman scattering (SERS), 150, 169
172 Surface-modified wide-band-gap oxides photo-oxidative ability, 439 photocatalytic properties, 440
441 Surfactants, 529 role as structure-directing agents, 272 general route for mesoporous solid formation, 272f types of mesoporous materials, 273t surfactant-assisted method, 72 surfactant-controlled growth, 77 SWCNTs. See Single-walled carbon nanotubes (SWCNTs) Synthetic nanopores, 282 Syringe pump, 458

T Tantalum, 86
87 Taylor cone, 458
460, 463
464 TDPPQ. See Thiophenediketopyrrolopyrrole-based quinoidal (TDPPQ) TE nanomaterials. See Thermoelectric nanomaterials (TE nanomaterials) TECs. See Tissue engineering constructs (TECs) Teflon autoclave, 64
65 TEG. See Thermoelectric generator (TEG) TEM. See Transmission electron microscopy (TEM) Temperature effect on electrospun nanofiber, 469 Temperature function (T function), 92
93 Temperature
time schedule, 93
95 Template method, 455
456, 456f role in porosity formation, 271 synthesis, 15
16 TEOS. See Tetraethoxysilanes (TEOS) TEP. See Thermoelectric power (TEP) Terminalia arjuna extract, 38
39 Tetraethoxysilanes (TEOS), 30 Tetrafluorosilane (SiF4), 92 Tetramethoxysilane (TMOS), 30 Theragnostics, 490 Thermal “hot spot” theory, 349
350

579

Thermal conductivity, 300 Thermal dehydrogenation reaction, 125
126 Thermocapillary flows, 528 Thermocouples, 321 Thermodynamics, 89
92 adiabatic temperatures, 91f, 92f Thermoelectric effect, 297
302, 297f Thermoelectric generator (TEG), 302
304 application, 321 challenges in potential TEG applications, 325 market for, 322
324 working principle and specific architectures, 319
321 Thermoelectric materials, 297
302 Thermoelectric nanomaterials (TE nanomaterials), 295
297. See also Organic
inorganic hybrid nanomaterials future perspectives, 326 inorganic, 302
309 organic, 309
318 recent technologies in TE materials and devices, 322
324 TEG application, 321 challenges in potential TEG applications, 325 market for, 322
324 working principle and specific architectures, 319
321 Thermoelectric power (TEP), 298
299 Thermopiles, 321 Thermosensitive gels, printing of, 509 Thiol-terminated poly(Nisopropylacrylamide), 34
36 Thiophene-diketopyrrolopyrrole-based quinoidal (TDPPQ), 313
314 Thiourea (TU), 341, 365 Thomson effect, 298 Three-dimension (3D), 151, 202
204 bioprinting, 504, 506f, 510, 513
514 computer model, 513 micromaterial printer, 503
504, 505f nanowires, 451
452 printing, 502

580

Index

Three-dimension (3D) (Continued) properties and applications of 3D TMO nanostructures, 206
213 scaffolds, 496 Threshold voltage, 468 Thrombotic models, 492
493 Thrusts, 490 Time-resolved X-ray diffraction (TRXRD), 93 Tissue engineering constructs (TECs), 500
501 Tissue engineering, 484
485 scaffolds, 496 Tissue plasminogen activator (tPA), 492
493 Titanium (Ti), 187
190, 188f toward fabrication nanostructuring, 174
176 Titanium dioxide (TiO2), 217, 439, 527 crystal structure, 67
68, 68f TiO2(B), 67
68, 68f space group and crystal structure, 69t TMDs. See Transition metal dichalcogenides (TMDs) TMO. See Transition metal oxide (TMO) TMOS. See Tetramethoxysilane (TMOS) TMs. See Transition metals (TMs) Tollens process, 34
36 Toluene, 34
36 Top-down approaches, 5
13, 28, 68, 530. See also Bottom-up approaches CDs, 540
541 electro-explosion, 30 to GNRs, 539 laser ablation, 29
30 liquid exfoliation by chemical and electrochemical ion intercalation, 8
10 by ion exchange, 10
11 by mechanical force, 7
8 by oxidation and reduction, 11
12 by selective etching, 12
13 mechanical cleavage, 5
7 mechanical grinding/milling, 29 tPA. See Tissue plasminogen activator (tPA) TPA. See Two photon absorption (TPA) Traditional TE materials, 302
304

Traditional ultrasonic spray pyrolysis (TUSP), 356
357, 359 Transition metal dichalcogenides (TMDs), 2
4, 3f, 14 2D, 9
10, 13f Transition metal oxide (TMO), 202
204 fabrication techniques to synthesizing TMO nanostructures, 214
218 flames as unique fabrication tool, 218
226, 220t “aerosol” and “spray pyrolysis” methods, 219
225 control synthesis in “aerosol” and “FSP”, 225
226 synthesis of metal-oxide nanoparticles, 218 nanofibers, 469
470 properties and applications of 1D and 3D TMO nanostructures, 206
213, 206f capacitors, 210 catalysis, 212
213 electrochromic devices, 208
209 gas sensors, 210
212 LEDs, 212 LIBs, 209 solar panels, 207
208 volumetric flame synthesis of 1D and 3D TMOs, 235
240, 236t Transition metals (TMs), 4, 202
205 catalysts, 125
126 transition metal-and rare-earth-doped ZnO, 67 Transitions within conduction band. See Intraband transitions Transmission electron microscopy (TEM), 107
108, 108f, 159
160, 164f, 167f, 173f, 181f, 316, 432
433, 434f, 435f, 521 Transparent dielectrics, 151
153 Transplantable bioengineered kidney, 488
490 Triple-doped graphene, 537 Trisodium citrate, 34
36 Truncated icosahedron, 391 TRXRD. See Time-resolved X-ray diffraction (TRXRD)

Index

TU. See Thiourea (TU) Tungsten trioxide (WO3), 101, 287 Tunneling ionization, 151
153 TUSP. See Traditional ultrasonic spray pyrolysis (TUSP) Two photon absorption (TPA), 151, 183
185 coefficient, 187t Two-dimension (2D), 202
204 graphene-like ultrathin, 2 heterostructures, 536 nanomaterials, 1
2, 3f nanosheets, 305
306, 451
452 nanostructures, 7
8 SCS, 111
112 solid
gas and solid
solid CS systems, 101
105 structure, 95, 103f, 104f synthesis approaches, 5
16 bottom-up approaches, 13
16 top-down approaches, 5
13 uniqueness and advances, 3
5 Two-photon laser writing, 502 Two-photon lithography, 502 Two-photon polymerization techniques, 502 (2 1 2) cycloadditions, 524 Type I collagen gels, 500

U Ultra-centrifugation-based separation, 528 Ultrafast lasers, 151 X-ray generation, 150
151 Ultrashort laser pulses, structural changes inducing by, 150
151, 156
157 Ultrasonic spray pyrolysis, 356
359 Ultrasonicator setup, 31
32, 32f Ultrasound, 31
32, 353 ultrasound-assisted synthesis, 31
33, 32f, 43, 43f Ultrathin 2D nanomaterials, 8
9 Ultraviolet (UV) excimer lasers, 153 UV
Vis absorption spectra, 178
179

581

V VACNT arrays. See Vertically aligned carbon nanotube arrays (VACNT arrays) Van der Waals electrostatic/hydrogen bonds, 16 Van der Waals interaction, 7
8 Vapor deposition methods, 57 phase growth CVD, 137
138, 138f growth of SbSI nanorods, 347
348 vapor-based bottom-up techniques, 68 vapor
vapor deposition process, 247
248 Vaporization of graphite, 521 Vapor
liquid
solid (VLS), 96
97 Vascular models, 492
493 Vascular smooth muscle cells, 491
492 Vasculogenesis, 491 Veins, 491
492 Venules, 491
492 Vertically aligned carbon nanotube arrays (VACNT arrays), 525, 527 Vertically stacked 2D heterostructures of graphene, 536 Viruses, 495 Viscoelastic dumbbells, 460
462, 461f Viscosity of precursor solution, 467
468 Vitamins, 437
438 VLS. See Vapor
liquid
solid (VLS) Volumetric flame synthesis of 1D and 3D TMOs, 235
240, 239f gas-phase flame synthesis of 1D and 3D TMOs, 238
240 TMOs (1D) formed using “aerosol” and “spray pyrolysis”, 238 Volvariella volvscea, 38
39

W Wake Forest University, 510
512 Warburg impedance, 473
476 Wasted thermal energy, 295
297, 296f Water (H2O), 523 nanoporous materials in water treatment, 285
286 solubility, 523

582

Index

Water (H2O) (Continued) water-assisted CVD process, 527 water-soluble warped nanographene, 536 water
oil emulsion method, 33 Wet-chemical synthesis, 14
16, 15f, 537 “Whipping” of jet, 460
462 Wide band-gap metal-oxides, 420
421 Wiedemann
Franz law, 300

X Xenon (Xe), 523

Y Y2SiO5:Eu31. See Europium-doped yttrium silicate (Y2SiO5:Eu31) Yield polaron absorption, 208
209 Yttrium aluminum garnet (YAG), 160
161

Z Z-scan studies, 160
161, 161f Zeolites, 267
268, 283
286 Zero-dimensional nanomaterials (0D nanomaterials), 1
2, 451
452

Zinc (Zn), 542 nanoparticle microbial synthesis, 41t plant extract-based nanoparticle synthesis of, 40t Zinc nitrate (Zn(NO3)2.4H2O), 60
61, 63
64 Zinc oxide (ZnO), 57
58, 219
225, 306 crystal structure, 58
59, 58f nanostructures Co-doped ZnO dendrite-like structures, 62f, 63f doping through hydrothermal routes, 64
65 growth solution concentration effect, 62
64 through hydrothermal growth, 59
61 hydrothermal temperature effect, 62
64 morphological evolution sketch, 63f reaction duration effect, 62
64 reaction strength effect, 62
64 synthesis methods, 59 Zirconium dioxide (ZrO2), 527 ZSM, 267
268, 285
288