Layered 2D Materials And Their Allied Application 9781119655206, 111965520X

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Layered 2D Advanced Materials and Their Allied Applications

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Layered 2D Advanced Materials and Their Allied Applications

Edited by

Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri

This edition first published 2020 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2020 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no rep­ resentations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchant-­ ability or fitness for a particular purpose. No warranty may be created or extended by sales representa­ tives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further informa­ tion does not mean that the publisher and authors endorse the information or services the organiza­ tion, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Library of Congress Cataloging-in-Publication Data ISBN 978-1-119-65496-4 Cover image: Pixabay.Com Cover design by Russell Richardson Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines Printed in the USA 10 9 8 7 6 5 4 3 2 1

Contents Preface xv 1 2D Metal-Organic Frameworks Fengxian Cao, Jian Chen, Qixun Xia and Xinglai Zhang 1.1 Introduction 1.2 Synthesis Approaches 1.2.1 Selection of Synthetic Raw Materials 1.2.2 Solvent Volatility Method 1.2.3 Diffusion Method 1.2.3.1 Gas Phase Diffusion 1.2.3.2 Liquid Phase Diffusion 1.2.4 Sol-Gel Method 1.2.5 Hydrothermal/Solvothermal Synthesis Method 1.2.6 Stripping Method 1.2.7 Microwave Synthesis Method 1.2.8 Self-Assembly 1.2.9 Special Interface Synthesis Method 1.2.10 Surfactant-Assisted Synthesis Method 1.2.11 Ultrasonic Synthesis 1.3 Structures, Properties, and Applications 1.3.1 Structure and Properties of MOFs 1.3.2 Application in Biomedicine 1.3.3 Application in Gas Storage 1.3.4 Application in Sensors 1.3.5 Application in Chemical Separation 1.3.6 Application in Catalysis 1.3.7 Application in Gas Adsorption 1.4 Summary and Outlook Acknowledgements References

1 1 2 3 4 4 4 4 5 6 6 8 9 9 10 10 11 11 12 12 13 13 14 14 15 16 16

v

vi  Contents 2 2D Black Phosphorus Chenguang Duan, Hui Qiao, Zongyut Huang and Xiang Qi 2.1 Introduction 2.2 The Research on Black Phosphorus 2.2.1 The Structure and Properties 2.2.1.1 The Structure of Black Phosphorus 2.2.1.2 The Properties of Black Phosphorus 2.2.2 Preparation Methods 2.2.2.1 Mechanical Exfoliation 2.2.2.2 Liquid-Phase Exfoliation 2.2.3 Antioxidant 2.2.3.1 Degradation Mechanism 2.2.3.2 Adding Protective Layer 2.2.3.3 Chemical Modification 2.2.3.4 Doping 2.3 Applications of Black Phosphorus 2.3.1 Electronic and Optoelectronic 2.3.1.1 Field-Effect Transistors 2.3.1.2 Photodetector 2.3.2 Energy Storage and Conversion 2.3.2.1 Catalysis 2.3.2.2 Batteries 2.3.2.3 Supercapacitor 2.3.3 Biomedical 2.4 Conclusion and Outlook Acknowledgements References

21 22 23 23 25 25 26 28 28 30 30 31 31 33 33 34 34 35 36 36 37 38 39 40 41 41

3 2D Metal Carbides 47 Peiran Hou, Xinxin Fu, Qixun Xia and Zhengpeng Yang 47 3.1 Introduction 3.2 Synthesis Approaches 48 48 3.2.1 Ti3C2 Synthesis 50 3.2.2 V2C Synthesis 50 3.2.3 Ti2C Synthesis 51 3.2.4 Mo2C Synthesis 3.3 Structures, Properties, and Applications 52 3.3.1 Structures and Properties of 2D Metal Carbides 52 52 3.3.1.1 Structures and Properties of Ti3C2 3.3.1.2 Structural Properties of Ti2C 53 3.3.1.3 Structural Properties of Mo2C 53

Contents  vii 3.3.1.4 Structural Properties of V2C 54 3.3.2 Carbide Materials in Energy Storage Applications 55 3.3.2.1 Ti3C2 56 57 3.3.2.2 Ti2C 58 3.3.2.3 V2C 58 3.3.2.4 Mo2C 3.3.3 Metal Carbide Materials in Catalysis Applications 60 60 3.3.3.1 Ti3C2 3.3.3.2 V2C 61 3.3.3.3 Mo2C 62 3.3.4 Metal Carbide Materials in Environmental Management Applications 63 3.3.4.1 Ti3C2 in Environmental Management Applications 63 3.3.4.2 Ti2C in Environmental Management Applications 64 3.3.4.3 V2C in Environmental Management Applications 64 3.3.4.4 Mo2C in Environmental Management Applications 65 3.3.5 Carbide Materials in Biomedicine Applications 66 66 3.3.5.1 Ti3C2 in Biomedicine Applications 66 3.3.5.2 Ti2C in Biomedicine Applications 68 3.3.5.3 V2C in Biomedicine Applications 68 3.3.5.4 Mo2C in Biomedicine Applications 3.3.6 Carbide Materials in Gas Sensing Applications 69 69 3.3.6.1 Ti3C2 in Gas Sensing Applications 69 3.3.6.2 Ti2C in Gas Sensing Applications 70 3.3.6.3 V2C in Gas Sensing Applications 71 3.3.6.4 Mo2C in Gas Sensing Applications 3.4 Summary and Outlook 72 Acknowledgements 72 References 73 4 2D Carbon Materials as Photocatalysts Amel Boudjemaa 4.1 Introduction 4.2 Carbon Nanostructured-Based Materials 4.2.1 Forms of Carbon 4.2.2 Synthesis of Carbon Nanostructured-Based Materials 4.3 Photo-Degradation of Organic Pollutants

79 79 80 80 80 81

viii  Contents 4.3.1 Graphene, Graphene Oxide, Graphene Nitride (g-C3N4) 81 4.3.1.1 Graphene-Based Materials 82 4.3.1.2 Graphene Nitride (g-C3N4) 84 4.3.2 Carbon Dots (CDs) 87 4.3.3 Carbon Spheres (CSs) 87 4.4 Carbon-Based Materials for Hydrogen Production 88 90 4.5 Carbon-Based Materials for CO2 Reduction References 90 5 Sensitivity Analysis of Surface Plasmon Resonance Biosensor 103 Based on Heterostructure of 2D BlueP/MoS2 and MXene Sarika Pal, Narendra Pal, Y.K. Prajapati and J.P. Saini 5.1 Introduction 104 5.2 Proposed SPR Sensor, Design Considerations, and Modeling 107 5.2.1 SPR Sensor and Its Sensing Principle 107 108 5.2.2 Design Consideration 5.2.2.1 Layer 1: Prism for Light Coupling 108 5.2.2.2 Layer 2: Metal Layer 109 110 5.2.2.3 Layer 3: BlueP/MoS2 Layer 5.2.2.4 Layer 4: MXene (Ti3C2Tx) Layer as BRE for Biosensing 110 5.2.2.5 Layer 5: Sensing Medium (RI-1.33-1.335) 110 110 5.2.3 Proposed Sensor Modeling 5.3 Results Discussion 112 5.3.1 Role of Monolayer BlueP/MoS2 and MXene (Ti3C2Tx) and Its Comparison With Conventional SPR 112 5.3.2 Influence of Varying Heterostructure Layers for Proposed Design 114 5.3.3 Effect of Changing Prism Material and Metal on Performance of Proposed Design 115 5.4 Conclusion 125 References 125 6 2D Perovskite Materials and Their Device Applications B. Venkata Shiva Reddy, K. Srinivas, N. Suresh Kumar, S. Ramesh, K. Chandra Babu Naidu, Prasun Banerjee, Ramyakrishna Pothu and Rajender Boddula 6.1 Introduction 6.2 Structure 6.2.1 Crystal Structure 6.2.2 Electronic Structure of 2D Perovskites

131

131 134 134 134

Contents  ix 6.2.3 Structure of Photovoltaic Cell 6.3 Discussion and Applications 6.4 Conclusion References 7 Introduction and Significant Parameters for Layered Materials Umbreen Rasheed, Fayyaz Hussain, Muhammad Imran, R.M. Arif Khalil and Sungjun Kim 7.1 Graphene 7.2 Phosphorene 7.3 Silicene 7.4 ZnO 7.5 Transition Metal Dichalcogenides (TMDCs) 7.6 Germanene and Stanene 7.7 Heterostructures References

135 136 139 139 141 143 147 148 150 151 152 153 156

8 Increment in Photocatalytic Activity of g-C3N4 Coupled Sulphides and Oxides for Environmental Remediation 159 Pankaj Raizada, Abhinadan Kumar and Pardeep Singh 8.1 Introduction 160 8.2 GCN Coupled Metal Sulphide Heterojunctions for Environment Remediation 163 163 8.2.1 GCN and MoS2-Based Photocatalysts 8.2.2 GCN and CdS-Based Heterojunctions 168 8.2.3 Some Other GCN Coupled Metal Sulphide Photocatalysts 171 8.3 GCN Coupled Metal Oxide Heterojunctions for Environment Remediation 173 177 8.3.1 GCN and MoO3-Based Heterojunctions 179 8.3.2 GCN and Fe2O3-Based Heterojunctions 8.3.3 Some Other GCN Coupled Metal Oxide Photocatalysts 180 8.4 Conclusions and Outlook 181 References 181 9 2D Zeolites Moumita Sardar, Manisha Maharana, Madhumita Manna and Sujit Sen 9.1 Introduction 9.1.1 What is 2D Zeolite? 9.1.2 Advancement in Zeolites to 2D Zeolite

193 193 195 196

x  Contents 9.2 Synthetic Method 9.2.1 Bottom-Up Method 9.2.2 Top-Down Method 9.2.3 Support-Assisted Method 9.2.4 Post-Synthesis Modification of 2D Zeolites 9.3 Properties 9.4 Applications 9.4.1 Petro-Chemistry 9.4.2 Biomass Conversion 9.4.2.1 Pyrolysis of Solid Biomass 9.4.2.2 Condensation Reactions 9.4.2.3 Isomerization 9.4.2.4 Dehydration Reactions 9.4.3 Oxidation Reactions 9.4.4 Fine Chemical Synthesis 9.4.5 Organometallics 9.5 Conclusion References 10 2D Hollow Nanomaterials S.S. Athira, V. Akhil, X. Joseph , J. Ashtami and P.V. Mohanan 10.1 Introduction 10.2 Structural Aspects of HNMs 10.3 Synthetic Approaches  10.3.1 Template-Based Strategies 10.3.1.1 Hard Templating 10.3.1.2 Soft Templating 10.3.2 Self-Templating Strategies 10.3.2.1 Surface Protected Etching 10.3.2.2 Ostwald Ripening 10.3.2.3 Kirkendall Effect 10.3.2.4 Galvanic Replacement 10.4 Medical Applications of HNMs 10.4.1 Imaging and Diagnosis Applications 10.4.2 Applications of Nanotube Arrays 10.4.2.1 Pharmacy and Medicine 10.4.2.2 Cancer Therapy 10.4.2.3 Immuno and Hyperthermia Therapy 10.4.2.4 Infection Therapy and Gene Therapy 10.4.3 Hollow Nanomaterials in Diagnostics and Therapeutics

197 197 198 199 200 200 203 203 203 203 204 204 204 205 206 206 206 207 211 212 213 214 215 215 217 218 219 219 219 220 220 221 222 224 224 226 226 227

Contents  xi 10.4.4 Applications in Regenerative Medicine 10.4.5 Anti-Neurodegenerative Applications 10.4.6 Photothermal Therapy 10.4.7 Biosensors 10.5 Non-Medical Applications of HNMs 10.5.1 Catalytic Micro or Nanoreactors 10.5.2 Energy Storage 10.5.2.1 Lithium Ion Battery 10.5.2.2 Supercapacitor 10.5.3 Nanosensors 10.5.4 Wastewater Treatment 10.6 Toxicity of 2D HNMs 10.7 Future Challenges 10.8 Conclusion Acknowledgement References 11 2D Layered Double Hydroxides J. Ashtami, X. Joseph, V. Akhil , S.S. Athira and P.V. Mohanan 11.1 Introduction 11.2 Structural Aspects 11.3 Synthesis of LDHs 11.3.1 Co-Precipitation Method 11.3.2 Urea Hydrolysis 11.3.3 Ion-Exchange Method 11.3.4 Reconstruction Method 11.3.5 Hydrothermal Method 11.3.6 Sol-Gel Method 11.4 Nonmedical Applications of LDH 11.4.1 Adsorbent 11.4.2 Catalyst 11.4.3 Sensors 11.4.4 Electrode 11.4.5 Polymer Additive 11.4.6 Anion Scavenger 11.4.7 Flame Retardant 11.5 Biomedical Applications 11.5.1 Biosensors 11.5.2 Scaffolds 11.5.3 Anti-Microbial Agents 11.5.4 Drug Delivery

227 228 229 230 231 231 232 232 232 233 234 234 237 239 240 240 249 250 251 252 253 254 254 254 255 255 255 255 257 260 261 261 262 263 263 263 265 266 267

xii  Contents 11.5.5 Imaging 11.5.6 Protein Purification 11.5.7 Gene Delivery 11.6 Toxicity 11.7 Conclusion Acknowledgement References

269 269 270 272 273 274 274

12 Experimental Techniques for Layered Materials 283 Tariq Munir, Arslan Mahmood, Muhammad Imran, Muhammad Kashif, Amjad Sohail, Zeeshan Yaqoob, Aleena Manzoor and Fahad Shafiq 284 12.1 Introduction 12.2 Methods for Synthesis of Graphene Layered Materials 285 12.3 Selection of a Suitable Metallic Substrate 287 12.4 Graphene Synthesis by HFTCVD 287 12.5 Graphene Transfer 289 291 12.6 Characterization Techniques 12.6.1 X-Ray Diffraction Technique 291 12.6.2 Field Emission Scanning Electron Microscopy (FESEM) 292 12.6.3 Transmission Electron Microscopy (TEM) 293 294 12.6.4 Fourier Transform Infrared Radiation (FTIR) 12.6.5 UV-Visible Spectroscopy 295 12.6.6 Raman Spectroscopy 295 12.6.7 Low Energy Electron Microscopy (LEEM) 296 12.7 Potential Applications of Graphene and Derived Materials 297 298 12.8 Conclusion Acknowledgement 298 References 299 13 Two-Dimensional Hexagonal Boron Nitride and Borophenes 303 Atif Suhail and Indranil Lahiri 13.1 Two-Dimensional Hexagonal Boron Nitride (2D h-BN): An Introduction 304 305 13.2 Properties of 2D h-BN 13.2.1 Structural Properties 305 306 13.2.2 Electronic and Dielectric Properties 13.2.3 Optical Properties 307 13.3 Synthesis Methods of 2D h-BN 308 13.3.1 Mechanical Exfoliation 309

Contents  xiii 310 13.3.2 Liquid Exfoliation 13.3.3 Chemical Vapor Deposition (CVD) 310 13.3.3.1 Synthesis Parameters 312 13.3.3.2 Growth Mechanism 313 13.3.3.3 Transfer of 2D h-BN Onto 314 Other Substrates 13.3.4 Physical Vapor Deposition Method (PVD) 315 316 13.3.5 Surface Segregation Method 13.4 Application of 2D h-BN 317 13.4.1 2D h-BN in Electronic Manufacturing 318 13.4.2 2D h-BN as a Filler in Polymer Composites 319 13.4.3 2D h-BN as a Protective Barrier 320 321 13.4.4 2D h-BN in Optoelectronics 13.5 Borophene 323 13.5.1 Theoretical Investigation and Experimental Synthesis 324 326 13.5.2 Properties and Application of Borophene 13.5.2.1 Electronic Properties of Borophene 326 13.5.2.2 Chemical Properties 326 13.5.3 Potential Applications of Borophene 328 References 328 14 Transition-Metal Dichalcogenides for Photoelectrochemical Hydrogen Evolution Reaction 337 Rozan Mohamad Yunus, Mohd Nur Ikhmal Salehmin and Nurul Nabila Rosman 14.1 Introduction 337 339 14.2 TMDC-Based Photoactive Materials for HER 14.2.1 MoS2 339 14.2.2 MoSe2 341 14.2.3 WS2 341 14.2.4 CoSe2 342 14.2.5 FeS2 343 14.2.6 NiSe2 344 14.3 TMDCs Fabrication Methods 345 14.3.1 Hydrothermal 345 14.3.2 Chemical Vapor Deposition/Vapor Phase Growth Process 346 14.3.3 Metal-Organic Chemical Vapor Deposition (MOCVD) 347 14.3.4 Atomic Layer Deposition (ALD) 348

xiv  Contents 14.4 Current Photocatalytic Activity Performance 14.5 Summary and Perspective References

350 351 352

15 State-of-the-Art and Perspective of Layered Materials Tariq Munir, Muhammad Kashif, Aamir Shahzad, Nadeem Nasir, Muhammad Imran, Nabeel Anjum and Arslan Mahmood 15.1 Introduction 15.2 State-of-the-Art and Future Perspective 15.2.1 Electronic Devices 15.2.2 Optoelectronic Devices 15.2.3 Energy Storage Devices 15.3 Conclusion References

363

363 364 365 369 372 374 374

Index 379

Preface Ever since the discovery of graphene, two-dimensional layered materials (2DLMs) have been the central tool of the materials research community. The reason behind their importance is their superlative and unique electronic, optical, physical, chemical, and mechanical properties in layered form rather than in bulk form. The 2DLMs have been applied to electronics, catalysis, energy, environment, and biomedical applications. Layered Advanced Materials and Their Allied Applications is an in-depth exploration of 2DLMs and their applications, including fabrication and characterization methods. It also provides the fundamentals, challenges, as well as perspectives on their practical applications. The comprehensive chapters herein are written by various materials science experts from all over the world. Therefore, this book is an essential reference guide for junior research scholars, faculty members, engineers, and professionals interested in materials science applications. The following topics are discussed in the book’s 15 chapters: Chapter 1 discusses the research status and development prospects for 2D metal-organic frameworks and the different techniques used to synthesize them. The advantages and limitations of these methods are summarized. Also, the structure, characteristics, and various applications of 2D ­metal-organic frameworks are mentioned. Chapter 2 mainly discusses the research on 2D black phosphorus (BP) and its application in various fields. Several studies on 2D BP are introduced, including its properties and structures, preparation methods, and antioxidants. The major focus is given to communicating the advantages of 2D BP in practical applications.

xv

xvi  Preface Chapter 3 reviews the synthesis methods of MXenes and provides a detailed discussion of their structural characterization and physical, electrochemical, and optical properties. The major focus is given to introducing the applications of MXenes in catalysis, energy storage, environmental management, biomedicine, and gas sensing. Chapter 4 describes the carbon-based materials and their potential applications via the photocatalytic process using visible light irradiation. Furthermore, 2D carbon-based materials are described for most largescale photocatalytic applications mentioned in the literature for addressing environmental issues such as pollutant degradation, heavy metal elimination, hydrogen (H2) generation, and CO2 reduction. Chapter 5 discusses the importance of 2D materials like graphene, TMDCs, few-layer phosphorene, MXene in layered form, and their heterostructures. It analyzes the sensitivity of surface plasmon resonance (SPR) biosensor based on heterostructure of 2D blueP/MoS2 and MXene (Ti3C2Tx). Their performance is analyzed for the different number of heterostructure layers and different prisms in the visible region. Chapter 6 summarizes the structure and applications of 2D perovskites. Chapter 7 details the exotic properties of layered materials. Physical parameters of pristine layered materials, ZnO, transition metal dichalcogenides, and heterostructures of layered materials are discussed. All parameters are calculated using density functional theory employing Vienna ab initio simulation package. The major focus of this chapter is on the significant parameters and intriguing applications of layered materials. Chapter 8 describes the coupling of graphitic carbon nitride with various metal sulfides and oxides to form efficient heterojunction for water purification. The optical band edge alignments and mechanistic viewpoint of charge migration and space separation are also explored. Finally, challenges in the proposed field are also discussed. Chapter 9 details the structural features, synthetic methods, properties, and different applications of 2D zeolites. It gives a brief account of advancements in 2D zeolites. Different synthetic methods of 2D zeolites,

Preface  xvii their properties, and various applications especially as a catalyst in different types of reactions are also elaborated in the chapter. Chapter 10 discusses the importance and scope of 2D hollow nanomaterials. The methods for synthesizing hollow nanostructures are featured and their structural aspects and potential in medical and nonmedical applications are highlighted. Furthermore, the challenges and futuristic perspective of these nanomaterials are mentioned. Chapter 11 features the characteristics and structural aspects of 2D layered double hydroxides (LDHs). The various synthesis methods and role of LDH in nonmedical applications as adsorbent, sensor, catalyst, etc., are discussed. Besides which, the application scope and biocompatibility of LDH in various biomedical applications are focused on in detail. Chapter 12 primarily focuses on the synthesis of graphene-based 2D layered materials. Such materials can be synthesized using top-down and bottom-up approaches where the main emphasis is on the hot-filament thermal chemical vapor deposition (HFTCVD) method. Moreover, the characterization techniques, including X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), UV-Vis spectroscopy, Raman spectroscopy, and low-energy electron microscopy (LEEM), are discussed. Chapter 13 discusses the different properties of 2D h-BN and borophene in detail. The chapter also includes various methods being used for the synthesis of 2D h-BN, along with their growth mechanism and transfer techniques. Applications like electronics, fillers in polymer composite, and protective barrier are also discussed in detail. Chapter 14 discusses the physical properties and current progress of various transition metal dichalcogenides (TMDC) based on photoactive materials for photoelectrochemical (PEC) hydrogen evolution reaction. Besides which, an overview of TMDC fabrication methods is presented and mitigation of an issue related to TMDC as a photocatalyst for PEC hydrogen evolution reaction is addressed.

xviii  Preface Chapter 15 focuses on the state of the art and perspective of 2D layered materials and associated devices, such as electronic, biosensing, optoelectronic, and energy storage applications, due to their excellent properties. Moreover, recent developments in this area are discussed and perspectives on future developments are offered. Editors Inamuddin Rajender Boddula Mohd Imran Ahamed Abdullah M. Asiri February 2020

1 2D Metal-Organic Frameworks Fengxian Cao1‡, Jian Chen1‡, Qixun Xia1* and Xinglai Zhang2† Henan Key Laboratory of Materials on Deep-Earth Engineering, School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, China 2 Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), Shenyang, China 1

Abstract

The metal organic framework (MOF) is a crystalline porous material formed of an inorganic metal ion or cluster and an organic ligand. The invention has the characteristics of large pore volume, high specific surface area, variable structures, and multiple functions. It was widely applied in the fields of gas storage, separation, catalysis, sensing, and biomedicine. The emergence of this kind of material, to a large extent, has provided opportunities for the common development of other disciplines. In this chapter, the recent research and development of MOFs materials, including the synthesis methods (sol-gel method, hydrothermal solvothermal method, and microwave synthesis, etc.), the development status, the applications, i.e., hydrogen storage, energy storage, gas adsorption, catalytic reaction, sensors, biomedical applications, and so on, and the research hotspots of MOFs will be addressed. Keywords:  MOF, biomedicine, gas storage, sensors, catalysis

1.1 Introduction Amidst the highly porous materials, metal organic frameworks (MOFs) exhibited incomparable tunable and structural diversity. Furthermore, MOFs *Corresponding author: [email protected] † Corresponding author: [email protected] ‡ The authors contributed equally Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (1–20) © 2020 Scrivener Publishing LLC

1

2  Layered 2D Advanced Materials and Their Allied Applications synchronously demonstrate porosity and excellent electrical conductivity, which are a burgeoning group of materials and provide a wide range of applications, for instance, energy storages, electrocatalytic oxidation, gas adsorption, biomedical [1–6]. The atomic-level control over molecular and supramolecular structure provided by MOFs gives the chance for exploiting some new materials for a variety of applications [7]. As a new type of porous inorganic-organic hybrid crystal material, MOFs materials have attracted extensive attention in chemistry, material, physics, and other fields. It combines the characteristics of inorganic and organic materials. It has a wide range of potential values in gas storage and separation, luminous, sensing, catalysis, magnetism, and other fields. When MOFS was made into membrane, the application of MOFs material in gas phase field was expanded. The gas separation application of MOFs extends from adsorption separation to membrane separation. By using the adjustable or modified characteristics of pore size, shape, and surface chemical properties of MOFs, MOFs material is endowed with better membrane separation performance for some light gas molecules. In addition, MOFs film extends the detection range of MOFs to gas, which can realize humidity detection and fluorescence detection of other gases or vapors. In these cases, the MOFs will play an important role in the generation, transmission, adsorption, and storage. The objective of this chapter is to summary recent literature describing the progress of MOFs. We first review the technology about how to grow MOFs thin films, including sol-gel method, hydrothermal solvothermal method, and microwave synthesis, etc. Whereafter, we summarized the structural feature and physicochemical properties description of MOFs. Subsequent sections discuss the MOF films in various applications, including hydrogen storage, energy storage, gas adsorption, catalytic reaction, sensors, biomedical applications, and the like. Finally, we discuss some limitations of MOFs in practical application.

1.2 Synthesis Approaches The synthesis of two-dimensional (2D) MOFs compounds materials is generally carried out by cultivating single crystals. X-ray single crystal structure analysis is the most important method to determine the structure of metallic organic skeleton materials [8]. The accurate molecular structure of organometallic skeleton materials can be obtained by analysis. At present, the methods of synthesizing organometallic skeleton materials reported in the literature mainly include solution volatilization method, diffusion

2D Metal-Organic Frameworks  3 method, and hydrothermal/solvothermal synthesis route. These methods complement each other and sometimes use different synthesis methods or the same method and different conditions to obtain materials with different structures and functions [9]. With the development of collocation chemistry and material chemistry, ultrasonic synthesis, ion-liquid method, solid phase reaction method, sublimation method, microwave synthesis, method and two-phase synthesis method have also been applied to the synthesis of MOFs materials. Various synthesis ways have their own advantages and disadvantages. Therefore, the choice of synthesis methods is very important for the synthesis of MOFs, and even affects its structure and properties.

1.2.1 Selection of Synthetic Raw Materials When the synthesis of MOFs is started, it is important to maintain the integrity of skeleton looseness in addition to geometric factors. Therefore, it is necessary to find sufficient mild conditions to maintain the function and structure of the organic ligand, while having sufficient reactivity to establish the coordination bond between the metal and the organic [10]. First of all, the metal components are mainly transition metal ions, and most of the valence states used by Zn2+, Cu2+, Ni2+, Pd2+, Pt2+, Ru, and Co2+. Secondly, organic ligands should contain at least one multi-dentate functional group, such as CO2H, CS2H, NO2, SO3H, and PO3H. CO2H was more commonly used in multi-dentate functional groups, such as erephthalic acid (BDC), tribenzoic acid (BTC), oxalic acid, succinic acid, etc. The selection of suitable organic ligands can not only form MOFs with novel structure, but also produce special physical properties. In addition, solvents can dissolve and protonize ligands in the process of synthesis. Metal salt and most ligands are solid as solvent is needed to dissolve it. Before metal ions and ligands are coordinated, ligands (such as carboxylic acids) need to be deprotonized, so alkaline solvents are often used. At present, many deprotonated alkaloids are used as organic amines, such as triethylamine (TEA), N, N2 dimethyl formamide (DMF), N, N2 diethylamide (DEF), N2 methyl pyrrolidone. At the same time, they are good solvents. In recent years, there are gradually examples of deprotonation with strong bases such as sodium hydroxide. Sometimes, solvents can also coordinate with metal ions as ligands or form weak interactions with other ligands, such as hydrogen bonds, which can be excluded by heating and vacuum. Finally, in order to make the synthesized organometallic skeleton have ideal pores, it is necessary to select the appropriate template reagent. Template reagents are sometimes separate substances, sometimes the solvents used.

4  Layered 2D Advanced Materials and Their Allied Applications

1.2.2 Solvent Volatility Method Solvent volatility method is suitable for the metal salt and ligands with good solubility and the obtained products that have a poor solubility in the used solvent. If the solubility of the ligands is poor, the dissolution of the ligands can be promoted by proper heating, and the coordination reaction can also be accelerated. The crystallization of the obtained coordination products is precipitated in the process of cooling [10, 11]. Solvent volatilization method is the most traditional method to synthesize MOFs materials and the principle of this method is that the crystal precipitates from saturated solution by solvent volatilization or decreasing temperature, and slowing down the volatilization rate or cooling is beneficial to the cultivation of perfect crystal form [12]. Specifically, by dissolving the selected organic ligands and metal salts in the appropriate solvent and placing them at rest, waiting for their slow self-assembly to form complex crystals.

1.2.3 Diffusion Method Diffusion method means that the metal salt organic ligands and solvents are mixed into solution in a certain proportion, put into a small glass bottle that is placed in a large bottle with deproteinized solvent, seal the bottle mouth of the large bottle, and then the crystal can be formed after a period of static setting. Diffusion methods can be divided into gas phase diffusion, liquid layer diffusion, and gel diffusion.

1.2.3.1 Gas Phase Diffusion The gas phase diffusion method is to dissolve the selected organic ligands and metal salt in the appropriate solvent, and then cause the lazy volatile solvent or volatile alkaline substance (for the carboxylic acid ligand containing hydrogen protons) to diffuse into the solution to reduce the solubility of the obtained complex product or speed up the coordination reaction, so that the complex precipitates in the form of crystallization. The volatilization rate of volatile solvents or alkaline substances in gas phase diffusion method will affect the nucleation speed of the complexes, and then affect the quality of precipitated crystals.

1.2.3.2 Liquid Phase Diffusion The liquid phase diffusion method is to dissolve the selected organic ligands and metal salt in different solvents, and then put the seed solution

2D Metal-Organic Frameworks  5 on top of the other solution, or add another solvent to the interface of the two layers of solution that can slow down the diffusion rate. The reactant diffuses slowly and reacts in the solvent, and the reaction product precipitates in the form of crystal. The diffusion rate of reactants in liquid phase diffusion method will affect the morphology of the precipitated crystals. In general, the diffusion method is mild and it is easy to obtain high-quality single crystal, but it is time-consuming and the solubility of reactants is required to be better and can be dissolved at room temperature.

1.2.4 Sol-Gel Method The sol-gel method is to use the compounds containing high chemical active components as precursors, which are uniformly mixed in liquid phase, hydrolyzed and condensed, and form a steady transparent sol system in the solution. In this process, the sol was slowly polymerized between aging colloidal particles to form a three-dimensional (3D) network structure gel before the network was filled with illiquid solvents to form a gel. After drying, sintering, and curing, the gel prepared molecular and even nano-substructure materials [13]. In 2017, Tian et al. [14] synthesized a porous monolithic metal-organic framework monoHKUST-1 (Cu3(BTC)2(H2O)3, BTC = 1,3,5-benzenetricarboxylate) by a sol-gel process. In the reaction process, the crystal primary MOFs particles were first formed, then the mother liquor was centrifuged, and the dense solid (gel) was washed for removing the unreacted precursors. In summary, sol-gel method demonstrated following advantages: 1) The reactants may be uniformly mixed at the molecular level when the gel was formed as the primary materials utilized in the sol-gel method were first disseminate to the solvent for forming a lower viscosity solution, the uniformity at the molecular level can be obtained in a very short period of time. 2) In the step of solution reaction, add a small amount of elements, what is needed to achieve uniform doping of 2D metal-organic skeleton at the molecular level. 3) The reaction temperature required for sol-gel synthesis is lower, so it is easier to carry out the reaction than the solid state reaction [15]. What’s more, the components in the sol-gel system were diffused in the nanometer range, while the components in the solid state reaction were diffused in the micron range, so the reaction of the sol-gel system is easy and the reaction temperature is low. 4) Various new 2D metal-organic frameworks materials can be prepared by selecting suitable conditions. On the other hand, sol-gel method’s disadvantages are described as follows: 1) the used raw materials are more expensive, some organic materials are harmful to health; 2) the whole sol-gel process usually takes a long time,

6  Layered 2D Advanced Materials and Their Allied Applications often taking a few days or weeks; 3) there are a number of micropores in the gel, which will escape a lot of gases and organic matter and produce shrinkage in the drying process of 2D metal-organic frameworks [16].

1.2.5 Hydrothermal/Solvothermal Synthesis Method Hydrothermal/solvothermal synthesis is the most effective way to synthesize MOFs that refers to the fact that ligands, metal salt, and reaction solvent are put into the reaction vessel together. At high temperature and high pressure (generally below 3,000 C) [12], the difference of solubility of each component is minimized and the viscosity of solvent decreases and the diffusion effect is strengthened which makes the complex tend to crystallization and precipitate. Large skeleton organic ligands with low solubility at room temperature and pressure are very suitable for hydrothermal/ solvothermal method. In general, the crystals synthesized by this method are easier to generate high-dimensional frame structures than the reactions at room temperature. According to the different reaction vessels used in the synthesis process of hydrothermal/solvothermal method, they can be divided into two common methods: reaction kettle and pipe sealing [17, 18]. Zheng et al. [19] synthesized a series of novel POMMOFs from {Ni6PW9} cluster with 2D structure under a hydrothermal route. Li et al. [20] synthesized flake MOF-2, with zinc ion and terephthalic acid in different solvents. This kind of flake material with 1.5–6 nm thickness, and they found that, if different solvents were used, the thickness of the prepared nanoparticles was different. After comparing methanol, ethanol, acetone, and DMF as solvents, it was found that the nanoparticles were the thinnest when acetone was used as solvent, and the monodisperse nanoparticles prepared would not regroup. This method has a short reaction time and solves the problem that the reactants cannot be dissolved at room temperature. The solvents used in the synthesis, especially the organic solvents, have different functional groups. Different polarity, different dielectric constant, different boiling point and viscosity can greatly increase the diversity of synthesis route and product structure. Solvothermal growth technology has perfect crystal growth. Equipment simply saves energy and other advantages, so it has become a hot spot in recent years.

1.2.6 Stripping Method Peeling 3D layered organometallic skeleton (MOFs) from top to bottom is one of the effective ways to control the preparation of ultra-thin

2D Metal-Organic Frameworks  7 organometallic nanoparticles on a large scale, because the interlaminar interaction force is weak van der Waals force or hydrogen bond, and peeling can be realized by simple mechanical grinding or ultrasonic method. Junggeburth et al. [21] use CTAB as surfactants, 1-hexyl alcohol and water as mixed phase microemulsion method. ZnBIM 2D organic complexes with lamellar accumulation were prepared from zinc acetate and benzimidazole. The single layer of the coordination polymer is only 2.6 nm, and the monolayer polymer plus surfactants layer is only 5.2 nm. For the intercalation/chemical stripping method, the organometallic nanoparticles obtained by mechanical peeling are usually small in size, larger in thickness, and less efficient (1,600 A/cm2) as well as current on/off ratios up to 800 at low temperature. In addition, experiments showed that the short-circuit current-gain cutoff frequency and the maximum oscillation frequency of the FET based on BP are 12 and 20 GHz, respectively, which indicated the BP FET can work in high frequency range. Surprisingly, it was also found that BP FET exhibit bipolar behavior in experiments. Du et al. [89] assembled a BP-based FET and found bipolar behavior of BP, the p-type can be converted to n-type by some special operations such as channel length, gate bias, and so on.

2.3.1.2 Photodetector Photodetector is a common sensor which can convert light signals into electrical signals in daily life [90]. It is widely used in many fields, including imaging, telecommunications, environment monitoring, security checking, etc. BP is a new member of 2D materials with broad application prospects in the field of photodetectors because it has high carrier mobility, tunable direct bandgap, and strong light absorption efficiency. Huang et al. [26] fabricated a BP Photodetector which can effectively work in the wavelength range from 400 to 900 nm (Figure 2.8d) and the photoresponsivity of the BP Photodetector are 7×106 AW−1 at 20 K and 4.3×106 AW−1 at 300 K. This indicates that BP is one potential 2D material for broadband photodetection at a wide temperature range. Doping and building heterojunction can improve the performance of BP photodetectors. Xu et al. [24] reported selenium-doped BP applied to the photodetectors and the performance of device can be changed by controlling the concentration of Se doping Figure 2.8f. The EQE of 1.6 wt% Se dope BP photodetector increased from 149% to 2,993% and the responsivity changed from 0.765 to 15.33AW−1. Ye et al. [91] tried to apply a 2D BP and MoS2 heterojunction to a photodetector Figure 2.8e and the photodetector based on 2D BP and MoS2 heterojunction have a photoresponse of 15 μs and photoresponsivity of 153.4 mAW−1 at near-infrared (λ = 1.55 μm). The performance of heterojunction-type BP photodetector is greatly better than the only BP. Besides, there are many BP photodetectors with different structures and different principles. Miao et al. [92] used a novel Au-BP-Al structure to construct a photodetector

36  Layered 2D Advanced Materials and Their Allied Applications which had a photoresponse of 2 ms, a responsiveness of ≈3.5 mAW−1, and an external quantum efficiency of 0.65% at an incident light intensity of 38 Wcm−2. Ren et al. [93] applied 2D BP to photoelectrochemical (PEC)type photodetector which has self-powered features. Surprisingly, it exhibits preferable photo responsive activity and environmental robustness.

2.3.2 Energy Storage and Conversion 2.3.2.1 Catalysis With human demand for energy growing, developing new energy sources is becoming more and more urgent. Hydrogen (H2) is considered to be an ideal energy carrier because of its high energy density and no pollution. The most ideal way to obtain hydrogen is to make the water splitting by catalytic. The oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) play an important role in the process of catalytic. Finding efficient, inexpensive, and easily accessible OER and HER catalysts is an important step toward success. BP as a fresh 2D material has been applied in catalytic field because 2D BP has many advantages, such as large specific surface area, amounts of catalytically active sites, high carrier mobility, excellent photoelectrochemical performance, and so on. A great number of studies about the application of BP in OER and HER had been done. Wang et al. [56] applied BP to the OER for the first time and found the BP had excellent electrocatalytic activity (Figure 2.9a). Further, they prepared BP (a)

3O 2

visible light

1O 2

(b)

(c)

PET

(d) Pt film

Magnetron sputtering

(g)

(f)

Cu BP nanoflakes

Drop casting dispersion of BP nanoflakes

(e)

PVA/H3PO4 gel electrolyte

BP nanoflakes PVA/H3PO4

•OH (h)

ultraviolet light

H 2O

Assemble BP-ASSP

PET

Graphene

Graphene Sodiation Desodiation Phosphorene

Na3P

Figure 2.9  (a) Schematic diagram of photocatalysis based on black phosphorus [56]. (b–g) The fabricated process of supercapacitors based on BP nanoflakes [101]. (h) The sodiation and desodiation processes for BP-graphene [99].

2D Black Phosphorus  37 on the carbon nanotubes (CNTs) as an OER catalyst whose performance is almost comparable to commercial RuO2 electrocatalysts. Ren et  al. [94] used the LPE method to get the BP nanosheets which was applied in electrocatalytic OER. The result shown the catalytic performance of BP increased with the thickness of the nanosheets decreasing and the exfoliated BP nanosheets were long-term stable. Lin et al. [95] dispersed the Ni2P nanoparticles uniformly on the surface of the layered BP to form a HER catalyst and the Ni2P/BP hybrid catalyst shown high HER electrocatalytic performance (The overpotential and Tafel slope were 70 mV and 81 mV dec−1, respectively). Zhu et al. [96] used mechanochemical method to get BP nanosheets which were applied in HER photocatalysts and the photocatalytic hydrogen evolution rate (512 µmol h−1g−1) is much higher than the bulk BP under visible light.

2.3.2.2 Batteries Energy drives the modern society and energy storage devices play an important role in the process. Developing a safe, high energy density, and fast charge and discharge energy storage device has become the target of the explorers. Due to its unique layered structure, high electron mobility, and excellent electrochemical performance, BP has become a candidate electrode material for lithium ion batteries, sodium ion batteries, and supercapacitor. Amazingly, the theory predicts that the capacity of BP can reach 2,596 mAh g−1 [97]. The large space between layers and huge surface area of BP are conducive to ion insertion and extraction and provide a large number of active sites for lithium ions. The process of lithium ion intercalation can be described as: xLi++xe−1+yP Lix Py Li3P. Li+ combined with P to form an intermediate LixPy, then it is converted into Li3P [98]. The discharge process is converting Li3P to Li+ and P. Unfortunately, although BP has a higher theoretical capacity as an anode material for lithium ion batteries, the lattice structure is destroyed easily during its charge and discharge process. This limits the development of BP in lithium-ion battery applications. There are two ways to solve the problem: (1) Convert multiple layers of BP into a monolayer to reduce volume change during charge and discharge process. (2) Combine BP and other more stable materials to form a more stable BP composite material. Park et al. [18] reported the charge capacities of BP is 1279 mAh·g−1, but the first cycle efficiency is only 57%. Sun et al.[15] studied the electrochemical activity of BP synthesized at different temperatures and pressures. The results showed the first discharge and charge capacities of batteries based on BP prepared at 4 GPa and 400°C are lower than the BP at 4.5 GPa and 800°C, proving synthesis conditions have an important

38  Layered 2D Advanced Materials and Their Allied Applications influence on the properties of BP. Liu et al. [99] built a sandwich structure about BP which was used as anode of LIBs and specific capacity was 1,633 mAhg−1 after 10 cycles and kept at 1,401 mAhg−1 after 200 cycles. Sodium has a large reserve on the earth and is easily accessible compared to lithium. The chemical property of sodium is similar to lithium, so sodium is seemed as the next generation batteries material to replace the lithium. The researchers found that NaMnO2, NaFePO4, and Na3V2(PO4)3 as cathode materials for sodium ion batteries showed the best performance. The key is to find a suitable anode material to achieve high energy density, high safety sodium ion battery. The highest theoretical capacity of NIBs based on phosphorous is 2,786 mAhg−1. Explorers are trying to apply BP to sodium-ion batteries just as doing on LIBs [100] (Figure 2.9h). However, the same problem will be faced in the application of BP in sodium ion batteries. Insertion and deintercalation of sodium ions during charge and discharge will destroy the crystal structure of the material and significantly degrade the performance of the batteries. Many methods to improve the long-term cyclic stability have been developed. Liu et al. [101] took the BP/ graphene composite as anode material for SIBs to enhance cycle stability as well as the result shown it had a high reversible capacity of 1,472 mAhg−1 at current density of 0.1 Ag−1 after the 50 cycles and 650 mAhg−1 at current density of 1 Ag−1 after the 200 cycles. Chowdhury et al. [102] explored the possibility of h-BN/BP as an anode material for SIBs and had a very high theoretical capacity (445 mAhg−1). h-BN as a capping agent can effectively avoid the change of BP volume during charge and discharge, which is beneficial for a long-term cycling stability of BP.

2.3.2.3 Supercapacitor The fast charge and discharge and long cycle life are the main characters of supercapacitors, which requires the electrode to adsorb more ions and electrode material to have a large specific surface area. As a layered material, BP can provide a sufficiently large adsorption surface area and charged ions can be rapidly transported between layers of BP. This makes it a potential electrode material for supercapacitors. Hao et al. [103] manufactured a novel supercapacitor based on BP nanoflakes which were got by LPE method (Figures 2.9b–e). The researches indicated the high capacitance of BP supercapacitor is 45.8 Fg−1at the scan rate of 0.01 Vs−1 as well as 84.5% and 71.8% capacitance can be retained after 10,000 and 30,000 cycles, respectively. Luo et al. [104] applied a laminated polypyrrole (PPy)/BP self-standing film on capacitance which have a high capacitance of 497.5 Fg−1 and can keep performance almost unchanged after 10,000 charging/discharging cycles.

2D Black Phosphorus  39 (a)

(b)

100 µm

(d)

(c)

100 µm

(e)

200 µm

100 µm

(f)

200 µm

200 µm

Figure 2.10  The BP is used to in bioimaging. (a, d) The bright field images of live HeLa cells with BP [105]. (b, e) The fluorescence images of live HeLa cells with BP [105]. (c, f) The merged images of live HeLa cells with BP [105].

2.3.3 Biomedical BP is a nano-structured material with biocompatibility, biodegradability, and photothermal properties and has potential application prospects in cancer treatment, diagnosis, and theranostics. Photothermal therapy (PTT) is the material that generates heat through light to raise the temperature and kill cancer cells. As a photothermal agent, BP has less damage to the human body and high therapeutic efficiency in the treatment of cancer. Sun et al. [105] found taking advantage of the photothermal property of BPQDs which were obtained by LPE method can effectively kill C6 and MCF7 cancer cells under NIR. BP is also used in biological imaging due to the excellent optoelectronic properties and unique nanostructure. Lee et al. [106] found BP-nanodots (average diameter of ≈10 nm) maybe a promising agent for diagnosis because they had little effect on cell viability in the process of blue- and green-fluorescence cell imaging (Figure 2.10). In addition, some researcher discovered that BP can work in imaging and therapeutic. Li et al. [107] developed the synergistic cancer therapy based on BPQDs. PEGylated BPQDs can promote the therapeutic efficacy of cancer treatment, enable cancer cell imaging and is low toxicity.

40  Layered 2D Advanced Materials and Their Allied Applications

2.4 Conclusion and Outlook Phosphorus is abundant in the earth and plays an important role in daily life. For example, white phosphorus is often used in the military and red phosphorus is a raw material for the production of matches and pesticides. In addition, phosphorus is involved in most important physiological activities of the human body. Unlike white phosphorus and red phosphorus, BP with unique structure and performances is a new member of 2D materials. The characteristics, such as directly adjustable bandgap, high electron mobility, excellent photoelectron properties, large surface area, and anisotropy, give BP a broad range of applications. In this chapter, we have detailed the application of BP in electronics and optoelectronics (FET, photodetector), energy storage, and conversion (OER, HER, LIBs, NIBs, and supercapacitor), as well as biomedical (cancer treatment, diagnosis, and theranostics). Although BP is a 2D material with great potential for application, the preparation method of 2D BP still stays in the laboratory. People can obtain bulk BP from RP or WP under high pressure environment, and then, 2D BP is obtained by mechanical exfoliation or liquid-phase exfoliation. During the research of BP, it was found that 2D BP is easily degraded when exposed in air. The electronic properties, chemical properties, and mechanical properties of degraded BP will be worse than undegraded BP. Al2O3 and methyl methacrylate as protective layer can isolate air to prevent BP from being oxidized. Chemical modification and doping can change the surface activity of BP so that it cannot easily combine with oxygen or water molecules. There are many opportunities for 2D BP, but there are some challenges at the same time. Although 2D BP shows potential application value in many fields, the way to practical application is still long. There are many antioxidant methods, but they do not completely prevent the degradation of BP after a long time. At present, the preparation of 2D BP is very expensive and limited to laboratory research as well as layer-control, large-area, and high-quality 2D BP cannot be prepared for industry. These problems which are not solved limits the industrial application of BP. Finding better ways to protect BP and preparing layer-control, large-area, and high-­ quality 2D BP will be an important part in the next study of BP. As people have a better understanding for BP and many problems that plague applications are resolved, we see that the BP will have a bright future. In a word, we hope that the chapter can give the readers comprehensive information about the 2D material and provide some positive guidance for next research on BP.

2D Black Phosphorus  41

Acknowledgements This work was supported by the Provincial Natural Science Foundation of Hunan (No. 2016JJ2132), Science and Technology Program of Xiangtan (No. CXY-ZD20172002), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT_17R91).

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3 2D Metal Carbides Peiran Hou‡, Xinxin Fu‡, Qixun Xia* and Zhengpeng Yang† *

†

‡

Henan Key Laboratory of Materials on Deep-Earth Engineering, School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, China

Abstract

A new two-dimensional (2D) metal carbides material (known as MXene) has expanded rapidly since the discovery of titanium carbide in 2011. Its chemical formula can be represented by Mn+1XnTx, where M represent the transition metal (such as Ti, Zr, Hf, V, Nb, Ta, Cr, Sc, etc.), and X means C or/and N, n is generally 1–3, Tx refers to surface groups (such as O2−, OH−, F−, etc.). Due to its high specific surface area and high conductivity, MXenes has shown great potential in the fields of energy storage, catalysis, adsorption, sensors, and biomedicine. In this chapter, we have comprehensively summarized the latest progress in MXene research. Firstly, we review the synthesis methods and then discuss the structure characterizations, mechanical, electrical, and optical properties of MXenes. Finally, the application of MXenes in catalysis, energy storage, environmental management, biomedical, gas sensing, and other fields were described. Keywords:  MXenes, energy storages, gas sensor, catalysis, biomedicine

3.1 Introduction In the last few years, effected by the breakthrough of graphene, twodimensional (2D) atomic layered materials have received stupendous attention owing to their inimitable structural, optical, mechanical, and electronic properties [1]. So far, several 2D materials, such as grapheme, *Corresponding author: [email protected] † Corresponding author: [email protected] ‡ These authors have contributed equally to this work Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (47–78) © 2020 Scrivener Publishing LLC

47

48  Layered 2D Advanced Materials and Their Allied Applications silane, decane, and phosphine have been prepared, most materials contain two or more elements [2]. MXenes is a new class of graphene-like 2D transition metal carbides, carbonitrides, and nitrides [3]. Their general formula is Mn+1XnTx (n = 1~3), where M is an early transition metal (such as Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, etc.), X is on behalf of carbon or nitrogen and Tx stands for surface termination (eg, hydroxyl, oxygen, or fluorine) [2–5]. Through a large number of theoretical and experimental researches, more than 60 different MAX phases, which is the precursor of MXene, have been discovered [6]. MXenes have been reported to have many special properties, such as excellent energy storage performance and outstanding mechanical properties. These remarkable properties may stir up important applications and have attracted extensive attention from researchers in many domains. Xie et al. reported that MXenes can be used as supporting materials for Pt nanoparticles, realizing a superiorly stable catalyst in fuel-cell applications [7]. Wang et al. confirmed that MXenes displayed remarkable enzyme fixation capacities and excellent biocompatibility for redox proteins, which indicated that they were expected for electrochemical biosensors applications [8]. We primarily present the synthesis methods, structures, properties, and applications in the energy storage, biomedical, catalyst, environmental management, and gas sensing of Ti3C2, Ti2C, Mo2C, and V2C MXenes in this chapter. Any metal carbide has a unique structure and properties that give it an excellent application prospect for solving problems in science and life.

3.2 Synthesis Approaches 3.2.1 Ti3C2 Synthesis MXenes were obtained by selective removal of the A atomic layers from their laminar MAX phases precursors. Etching process is needed, because the strong metallic bonds between A and M elements make it almost impossible to separate the M-X layer using mechanical methods. At present, MXenes were mainly obtained by removing weakly combined A-site elements in the MAX phase by HF acid or a mixed solution of hydrochloric acid and fluoride. In 2011, Naguib et al. successfully synthesized the first multilayered MXene, i.e., Ti3C2Tx, which was obtained by selectively etching Al from Ti3AlC2 using HF acid via the following process, 1) The Ti3AlC2 powder was prepared by ball milling Ti2AlC and TiC powder in a 1:1 molar ratio using zirconia balls for 24 h. 2) The mixture was heated to 1,350°C under argon atmosphere and held for 2 h, and then grinded to

2D Metal Carbides  49 obtain powder. 3) Approximately, 10 g of powder was then immersed in approximately 100 ml of 50% concentrated HF solution and kept at room temperature for 2 h. 4) The obtained suspension was then washed several times with deionized water and centrifuged to separate the powder. The etching process of Ti3AlC2 as shown as Figure 3.1 [9]. This exfoliation process can be concluded as follows:

Ti3AlC2 + 3HF = AlF3 + 3/2H2 + Ti3C2

(3.1)

Ti3AlC2 + 2H2O = Ti3C2(OH)2 + H2

(3.2)

Ti3C2 + 2HF = Ti3C2F2 + H2

(3.3)

Reaction (3.1) is essential in the etching process. Reaction (3.2) and (3.3) form the −OH and −F terminations which usually occur simultaneously. In 2013, the single-layer MXene flakes were exfoliated by inserting large organic molecules and dividing the sheets from each other, developing a new method for the MXene synthesis. In 2014, Halim et al. produced Ti3C2Tx MXene using an HF-containing etchant such as ammonium difluoride salt. Its milder nature and accompanying cation intercalation during etching process, make it more

Ti

C

O

H

(c)

Sonication

(b)

HF Treatment

(a)

AI

Figure 3.1  (a) Ti3AlC2 structure. (b) Al atoms replaced by OH after reaction with HF. (c) Breakage of the hydrogen bonds and separation of nanosheets after sonication in methanol [reprinted with permission from [9]. Copyright 2011 Wiley].

50  Layered 2D Advanced Materials and Their Allied Applications suitable for the preparation of few/single layers MXene. The atomic layers in Ti3C2Tx are more evenly spaced and less agglomeration due to the reaction process is slower and less active [10]. Meanwhile, Ghidiu et al. demonstrated a new and efficient method for the preparation of new MXene-based electrode. In this work, Ti3C2Tx “clay” was synthesized by in situ HF using lithium fluoride (LiF) salt and hydrochloric acid (HCl) as the etchant. This method significantly simplifies the synthesis process and improves the performance of MXene in energy storage applications [11].

3.2.2 V2C Synthesis There are few reports about V2CTx MXene due to it is difficult to produce high purity V2CTx MXene. The V2CTx MXene was typically fabricated by selectively etching the aluminum layer in the V2AlC MAX phase for 8 h at room temperature in a 50% concentrated HF solution. However, the V2CTx MXene prepared by the above method contains a large amount of vanadium aluminum carbide (V2AlC) precursor. In 2017, Liu et al. discovered a new method, sodium fluoride/hydrochloric acid (NaF+HCl) was etched at 90°C for 72 h to synthesize high purity V2CTx MXene with good Li storage properties [12].

3.2.3 Ti2C Synthesis In 2011, after the first MXene, i.e., Ti3C2Tx was synthesized, Naguib et al. used the same method to immerse the Ti2AlC MAX phase powder in HF to synthesize the new Ti2CTx MXene. This process can be represented by the following reactions [13]:

Ti2Al + 3HF = AlF3 + 3/2H2 + Ti2C

(3.4)

Ti2C + 2H2O = Ti2C(OH)2 + H2

(3.5)

Ti2C + 2HF = Ti2CF2 + H2

(3.6)

2D Metal Carbides  51 Since then, Shao et al. exhibited the steps of Ti2CTx MXene preparation: 1) 1.5 g of Ti2AlC MAX phase powder was immersed etching agent (HF: 50 ml + ammonium fluoride: 16 g) and kept at room temperature for 24 h. 2) After the resulting slurry was washed several times with deionized water and centrifuged to obtain a precipitate, 1 g of Ti2C was added to 12 ml of dimethyl sulfoxide (DMSO) and stirred at room temperature in 20  h for further exfoliation. 3) The suspension was centrifuged to separate the inserted Ti2C from the mixture. Then, 100 ml of deionized water was added to the residue. The Ti2C nanoflakes were separated by sonication for 2 h. 4) Finally, the mixture was centrifuged, washed, and dried under vacuum at 60°C for 18 h to obtain the Ti2CTx MXene product in a yield of 31% [14].

3.2.4 Mo2C Synthesis As a new member of the MXene family, MO2CTx has attracted attention due to its special properties and wide potential applications. The MO2GaC as the precursor of MO2CTx has two “A” layers between the MO2C layers, by selectively etching the Ga layer using HF or LiF and HCl solutions, the MO2GaC film cannot be completely converted into MO2CTx. In addition, the prepared MO2CTx nanosheets are small, and the crystal size and thickness are not uniform. In 2015, Xu et al. introduced a large surface area, high quality MO2C crystals with a thickness of 3.4 nm by chemical vapor deposition route [15]. In 2016, Halim et al. reported that a 2D Mo2CTx MXene (T represent surface terminations) was synthesized by selective etching of Ga from the lately explored Mo2Ga2C, as shown as Figure 3.2. This exploration achieved large-scale synthesis and stratification [16]. a

C

Delamination

acid etching

Mo2Ga2C

Removal of Ga

Mo2CTX

d-Mo2CTX

Figure 3.2  Schematic showing synthesis and delamination of Mo2CTx [reprinted with permission from [16]. Copyright 2016 Wiley].

52  Layered 2D Advanced Materials and Their Allied Applications

3.3 Structures, Properties, and Applications 3.3.1 Structures and Properties of 2D Metal Carbides 3.3.1.1 Structures and Properties of Ti3C2 The 2D transition metal carbide Ti3C2Tx was widely studied, which has been found to have excellent electrochemical properties and expected to be used in various fields [17–20]. However, its quantitative structure is unknown. Density functional theory (DFT) calculations can be able to anticipate its structure. The previously DFT investigations obtained two different configurations on energy-friendly tropism of OH− or F− in Ti3C2Tx, whereas these discoveries didn’t verified by experiments. PDF studies can generate quantitative structural information of nanostructures on a nanometer scale. Chen et al. successfully determine the structure of the original Ti3C2Tx by using this technique [20]. This research group modeled the structure using PDFGUI [21] and SRFIT programs [22]. The structural factors and the experimental PDF of the original Ti3C2Tx are presented in Figures  3.3a and b, severally. F(Q) and PDF demonstrated the compossibility of aiguilles and broad peaks, manifesting a legitimately well ordered but less well-ordered structure.

(a)

F(Q)

F(Å−1)

15 10 0

15 10 Q(Å−1)

(b)

20

25 G(r)

G(Å−2)

5

5

0 0

5

10 15 20 25 r(Å)

30 35 40

Figure 3.3  (a) The structure factors. (b) The measured PDF of pristine Ti3C2Tx [reprinted with permission from [20]. Copyright 2014 American Physical Society].

2D Metal Carbides  53 rutile-TiO2 anatase-TiO2 MXene

(a)

(b)

Intensity (a.u.)

d

c b a 10

20

30 2θ (degrees)

40

50

det 11/7/2012 dwell HV HPW WD mag 3:25:59: PM 10 µs 5.00 kV 17.3 µm 4.3 mm 12 007 x TLD

5µm Nanjing University Helios

Figure 3.4  (a) XRD patterns of Ti2C MXene. (b) The SEM image of Ti2C MXenes [reprinted with permission from [3]. Copyright 2015 Elsevier].

3.3.1.2 Structural Properties of Ti2C The most common preparation method for Ti2C is synthesized by stripping the Ti2AlC powder in HF solution [3]. The phase structures and morphology of Ti2AlC powder were studied, respectively, as shown as Figure 3.4. As can be seen from the XRD patterns, it is evident that the intensity of the (000l) peaks is lowed after HF-treatment. Furthermore, the XRD peaks of the obtained Ti2C nanoflakes obviously become broader and toward to lower angles compared with the untreated Ti2AlC samples, indicated that Ti2C was well exfoliated. From the SEM image, the treated Ti2C exhibited a typical layered structure.

3.3.1.3 Structural Properties of Mo2C Liu et al. reported the unique regional structure of 2D α-Mo2C superconductive materials by CVD method. The α-Mo2C has an orthotropic structure constituted of HCP Mo atoms (Figures 3.5a, b). The first-principles calculation results showed the interstitial carbon atoms in the octahedron were appreciably offset from the center in the b direction of the α-Mo2C lattice (Figure 3.5b), constituting a zigzag structure in the c direction [23]. Figure 3.5c shows a HAADF-STEM image of an α-Mo2C, which is known as a Z-contrast image. The atomic model shown in Figure 3.5b, it is exhibited that the carbon element is lighter than the Mo. It is also necessary to obtain a BF-STEM image on the identical regional axis, which is show the position of the carbon atom (Figure 3.5d). In order to further determine the direction deviating from the positive center, such structural information

54  Layered 2D Advanced Materials and Their Allied Applications (a)

(c)

(d)

(e)

b

c

a b

(b)

c

5nm

(f)

020

002 b c Mo 3/4 Mo 1/4

C0

1 nm

C 1/2

1 nm

[100]

Figure 3.5  Crystalline structure of 2D α-Mo2C crystals. (a) Atomic model of a unit cell of α-Mo2C crystal. (b) Atomic model of an α-Mo2C crystal projected in the [100] direction. The purple arrows show the off-center directions of carbon atoms in Mo octahedra. Yellow and blue lines show the zigzag configuration of carbon atoms in each layer. (c, d) Atomic-resolution HAADF-STEM image (c) and BF-STEM image (d) of an ultrathin 2D α-Mo2C crystal with superimposed atomic models shown in the top-right insets (blue solid circles represent Mo atoms, and red solid circles represent carbon atoms). (e, f) Large-area HAADF-HRSTEM image (e) and the corresponding FFT pattern (f) of a 2D α-Mo2C crystal [reprinted with permission from [23]. Copyright 2016 American Chemical Society].

can be obtained from the reciprocal space via fast Fourier transform analysis. A large area HAADF-HRSTEM image can get a clearer FFT pattern (Figure 3.5e). It can be seen in Figure 3.5f that the orthogonal characteristic of 2D α-Mo2C crystals, which indicates that eccentric replacing of the carbon atoms in the Mo octahedron is in the inverse spatial phasor [020] direction [23].

3.3.1.4 Structural Properties of V2C In the MXenes series, V2C has received much attention because it is one of the lightest and the application of its electrical characteristics. It has been demonstrated in many documents that V2C can be embedded in Na ions application for Li and Na ion batteries [24, 25]. Shan et al. [26] introduced the electrical behavior of V2C under different electrolytes. Figure  3.6a shows a schematic representation of the synthesis and stratification of V2C. Under the etching of the HF solution, a plurality of layers of V2C was formed, and then, the multilayer powder was layered by TMAOH. Under the influence of HF, the (002) peak in the XRD pattern of V2AlC moved to 8.6° (Figure 3.6b) that relevant C has a lattice parameter of 20.5 Å. Since the layers of the V2C powder of the multilayer are bonded by van der Waals

2D Metal Carbides  55 (a)

TMA+

TMAOH Intercalation

Etching

V

Intensity (a.u.)

(b)

AI

Delamination

C

(d)

(c)

(002)

Delaminated V2C film (004)

(006) (008)

(002)

Multilayered V2C

V2AIC 10

20

30 40 2 Theta (º)

50

5 µm

60

Figure 3.6  (a) Schematic representation of synthesis and intercalation-assisted delimitation of V2C. (b) Powder XRD patterns of V2AlC precursor before (black line) and after (blue line) exposure to HF as well as of the rolled V2C film (red line). (c) A cross-section SEM image of the rolled V2C film. (d) An optical image of flexible V2C film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) [reprinted with permission from [26]. Copyright 2018 Elsevier].

force or hydrogen bonding, and the embedding of TMAOH easily weakens the bonds, so the purpose of delamination can be achieved. Consequently, the lattice parameter of C after embedding is further extended to 24.2 Å, and the corresponding peaks of (004), (006), and (008) can be seen, which indicates that the V2C layer has been reorganized during the preparation process. An SEM image of a cross section of the V2C film is shown in Figure 3.6c, which can be seen as a layer stack. The rolled V2C showed good flexibility (Figure 3.6d). These electronic and mechanical properties indicate that V2C is promising for using in flexible and wearable electronic devices.

3.3.2 Carbide Materials in Energy Storage Applications MXenes has been widely concerned with energy storage technologies, including lithium ion batteries (LIBs) and supercapacitors (SCs) because of their outstanding physicochemical properties such as low resistivity, fast ion and molecular transportation, and high theoretical specific capacitance. The following sections describe the Ti3C2, Ti2C, V2C, and Mo2C materials for energy storage and energy transformation.

56  Layered 2D Advanced Materials and Their Allied Applications

3.3.2.1 Ti3C2 Ti3C2Tx MXene is most widely studied in electrochemical capacitors. Xia et  al. introduced Ti3C2Tx electrode in seawater electrolyte with a specific capacitance of 67.7 F g−1 (1 A g−1), corresponding to the volumetric capacitance of 121.8 F cm−3 [27], indicating the abundant and cost-­effective seawater shows the potential as a natural electrolyte application in supercapacitors. The independent Ti3C2Tx paper electrode in neutral and alkaline electrolytes has a capacitance of 380 F g−1 and a volume capacitance of 1,500 F cm−3 at a scan rate of 10 Vs−1. These outstanding values e​​ xceed the most full carbon-based electric double layer capacitors (EDLCs). The charge storage mechanism in Ti3C2Tx was generally considered to be an highly reversible redox reaction [28], which depending on the change in the oxidation state of titanium [Reaction (3.7)]:

Ti3C2(OH)y + σe- + σH+ = Ti3C2(OH)x-σ(OH)y + σF2

(3.7)

Ghidiue et al. [11] demonstrated a MXene clay as the electrode achieved a high volumetric capacitance of more than 900 F cm−3 in 1 M H2SO4 electrolyte, probably because the proton is the smallest cation and the corners enter the maximum amount of electrochemical activity sites. However, the mechanism of MXene’s high volumetric capacitance is unclear. There are some parameters that influence the volumetric capacitance of MXenes. Firstly, the mass density of the electrodes was used as a transformation factor for weight and volume performance. Second, MXene surface chemistry structure is also an important factor affecting the electrochemical performance. For example, replacing a fluorine-containing functional group with an oxygen-containing group results in a significant increase in capacitance [29]. In addition, Ti3C2-based nanocomposite also widely used as electrode in energy storage device with enhanced electrochemical performance. Kim’s group reported a Ti3C2/NiO nanocomposite exhibited a high specific capacity of 92.0 mA h cm−3 at current density of 1 A g−1 and 53.9 mA h cm−3 at current density of 10 A g−1 [30]. Moreover, Xia et al. [31] synthesized a Ti3C2/BiOCl nanocomposite as electrode for supercapacitor with a volumetric specific capacitance of 396.5 and 228.0 F cm−3 at 1 and 15 A g−1, respectively. A symmetric supercapacitor fabricated using Ti3C2/BiOCl nanocomposite demonstrated a high energy density of 15.2 Wh kg−1 at a power density of 567.4Wkg−1, and with cycle life retention of 85.0% after 5,000 cycles.

2D Metal Carbides  57

Sunlight

Water

H2O

Ti C Ti C Ti

Figure 3.7  Schematic diagram of Ti3C2 MXene photothermal conversion [reprinted with permission from [32]. Copyright 2017 ACS].

Photothermal conversion is an original means of using solar energy, and the emergence of MXenes nanomaterials has led to the re-consideration of solar thermal conversion. MXenes have high visible light absorption and exceptional solar-thermal conversion efficiency. Such as, the Ti3C2 film achieves 100% photothermal conversion efficiency as measured during droplet laser heating. In fact, this extraordinary energy conversion efficiency reflects 84%. This value indicates that the Ti3C2 as a photothermal conversion materials has potential application in energy conversion areas [32], as shown in Figure 3.7.

3.3.2.2 Ti2C Two-dimensional transition metal carbides are promising anode materials for LIBs. By using the first principle calculation, Wan et al. investigated the impact of vacancies on the adsorption and diffusion of Li in Ti2C nanosheets. Importantly, they found that carbon vacancies (CVs) are inclined to strengthen the adsorption of Li in the Ti2C nanoflakes. Ti vacancies also take the similar effect in Ti2CT2 when terminations are appeared. It can be seen that the existence of vacancies further affects the diffusion behavior of Li. In this case, they have proposed the idea of weakening the adverse effect on Li diffusion ability by adjusting functional group. In the existence of CVs, when Li diffuses around the CVs, it is suggested to modify the surface of the Ti2C monolayer with an OH-group due to its properly low diffusion barrier within the scope of 0.025 to 0.037 eV. While VTi is present, it is recommended that the surface be removed from the functional group such that when the Li atoms diffuse around the VTi, the energy barrier can be reduced by about 1 eV [33].

58  Layered 2D Advanced Materials and Their Allied Applications

3.3.2.3 V2C V2C MXene as the new member of MXene family has the potential application in energy storage. Agnese et al. [24] studied the electrochemical behavior of V2C MXene as a sodium ion capacitor is very similar to that of Ti2C MXene. In this research, V2CTx was used as the cathode for the sodium ion capacitor, and the device was fabricated using hard carbon as the anode with a capacity of 50 mA h g−1 and achieved to high voltage, high to 3.5 V. This result shows that V2C material can be used as a attracting electrode material with a wide operating potential window by inserting Na ions between the V2CTx layers. He et al. [34] studied the electrochemical properties of 2D V2C. In this work, V2C synthesized by selectively etching Al atoms from V2AlC MAX phase by NaF+HCl etching agent at 90°C for 72 h and seawater as the electrolyte. V2C electrode materials showed a capacitance of 181.1 F g−1.

3.3.2.4 Mo2C Electrode materials based on Mo2Cx have attracted attention on account of their outstanding electrical conductivity. Mo2Cx-based materials have shown the potential applications in the field of LIBs. Chen et al. used Mo2C to qualify the electrochemical performance of MoO2 [35]. In this work, Mo2C insures the structural stability of the MoO2 particles by reducing the resistance of charge transport. This property provided a multiaperture heterojunction electrode with excellent stability and high rate performance. Such electrodes demonstrated specific capacities of 623 and 510 mA h g−1 at current density of 500 and 1,000 mA g−1 after 140 cycles, respectively. Later, Cao et al. provided a round of researches in lithium energy reserve property. Mo2C exhibits better performance through the combined effects of nanohybridization and porous structure (Figures 3.8a, b). The Mo2C/C composite was obtained by freeze drying and heat treatment in a protective atmosphere, exhibited a three-dimensional poriferous structure with a specific surface area of 200.6 m2 g−1. Even at current density of 1,000 mA g−1, the capacity of 777.7 mA h g−1 can be delivered after 1,000 cycles (Figure 3.8c) [36]. Xia et al. synthesized a Mo2C/RGO hybrid which has a sheet-on-sheet structure employed as a electrode for LIB with a particular capacity of 456.4 mA h g−1 at 1,000 mA g−1 after 400 cycles [37]. Wang et al. testified Mo2C nanoparticles (diameter: 10–40 nm) that were growing on a graphene by a straightforward heat treatment route [38]. The optimized Mo2C/GR showed a capcity of 813 mAh g−1 at 100 mA g−1 after 100 cycles, which

2D Metal Carbides  59

100 mA g

600 400

0

60

–1

40

2000 mA g

200

80

300 mA g–1 500 mA g–1 1000 mA g–1

100 mA g bulk Mo2C 20 –1

0

10

20 30 40 Cycle number (n)

0

200 nm

20

40 60 Cycle number

2000

0

100

200

300

400

Specific capacity (mAh gtotal–1)

500

40 20

0 100

80

Discharge Charge

1600 1200

52.6 % 38.7 %

800

62.4 % 400

GR Bulk Mo2C 0

20

40

60

80

100

Cycle number 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

Mo2C/CNT

Specific capacity (mAh gtotal–1)

CNT only Mo2C only Mo2C/CNT

207.6

Bare Mo2C(0.1 Ag–1)

0

Voltage (V)

Voltage (V)

Li2O2 (E° = 2.96 V)

310.2

(0.1 Ag–1)

Pure C

300

(f)

2Li+ +O2+2e–

60

600

0

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

1196.8 80 873.6

Ag–1)

(0.3 Ag–1)

0

(d)

(e)

HP-Mo2C-C(0.1

900

50

(c)

100

Coulombic efficiency (%)

1200

Coulombic efficiency (%)

–1

(b) 1500

Specific capacity (mAh g–1)

800

100

Capacity (mAh g–1)

Coulombic efficiency,100 mA g–1

Coulombic efficiency (%)

Specific capacity (mAh g–1)

(a) 1000

500 400 300 200 100 0

0

2Li+ +O2+2e–

Li2O2 (E° = 2.96 V)

discharge charge

0

30 60 90 120 150 Number of cycles (#)

100

200

1st cycle 30th cycle 70th cycle

300

10th cycle 50th cycle 100th cycle

400

Specific capacity (mAh gtotal–1)

500

Figure 3.8  (a) Cycling performance of mesoporous Mo2C spheres. (b) Cycling performance of hierarchically porous Mo2C-C hybrid at different current densities compared with pure C and bulk Mo2C. (c) TEM image and (d) electrochemical performance of the Mo2C/GR composites at 200 mAg−1. (e) The first discharge/charge profiles and (f) cycling performance of the pristine carbon nanotube, Mo2C, and Mo2C/ CNTs at 100 and 500 mAhg−1 in Li-O2 batteries [reprinted with permission from [37]. Copyright 2016 The Royal Society of Chemistry].

is superior than raw GR (443 mA h g−1) and bulk Mo2C (110 mA h g−1) (Figures 3.8c, d) [36]. Fellinger and his colleagues introduced a series of metal carbides@ CNS synthesized by the salt template method [39]. MoC0.654@CNS used as anode for LIB exhibits splendid cycling performance and produces a capacity of 815 mA h g−1 after 680 cycles. Mo2C is also able to be applied

60  Layered 2D Advanced Materials and Their Allied Applications as an electrode in Li-O2 battery. Sun et al. introduced a cathode based on Mo2C nanoparticles. The Mo2C/CNTs composite exhibits higher energy efficiency (88%), very low polarization (0.47 V), and low overpotential (3.25~3.4 V), compared to the bulk Mo2C source and the original CNT. (Figures 3.8e, f). Furthermore, the constructed battery maintains excellent cycle characteristics up to 100 cycles at current density of 500 mA g−1. Nazar et al. researched the interface reaction and cathodic stabilization of metal carbides [40]. After surface inactivation, the complex no carbon required lacunaris fiber-based Mo2C cathode exhibited a low potential of 3.2 V for Li2O2, making it promising for use in Li-O2 batteries.

3.3.3 Metal Carbide Materials in Catalysis Applications Electrochemical water decomposition can produce H2 in a sustainable and carbon neutral method. However, there are many limitations owing to the high cost and low availability of catalyst precious metals currently used for electrochemical water decomposition [37]. Therefore, it is necessary to find an alternative electrocatalyst for water decomposition. First-principles calculation studies have shown that MXenes may be the promising electrocatalyst application in water decomposition [9, 38].

3.3.3.1 Ti3C2 Ma et al. [41] synthesized graphite carbon nitride (g-C3N4)/Ti3C2 nanosheets composite film, which was as electrode materials for oxygen evolution reaction (OER), as shown in Figure 3.9. The Ti3C2 MXenes was synthesized by selective etching of Al species from the Ti3AlC2 MAX, then mixed with g-C3N4 by an ultrasonic assisted lift-off process to obtain flexibility film. Compared with Ti3C2 and g-C3N4, the mixed Ti3C2/ g-C3N4 film exhibited obviously enhanced OER capability in alkaline medium [42]. Zinc air batteries are expected to achieve low cost and large capacity storage due to the high theoretical specific energy density, rich Zn, and low pollution. They are relatively stable as aqueous electrolytes in alkaline solutions. The MXene heterostructure exhibited excellent performance as an electrocatalyst for OER in an aqueous alkaline solution. Ti3C2 interacts with graphite carbonitride (g-C3N4) to form a porous matrix, which leads to a tight coupling between the nitrogen-rich network and the transition metal center, where the Ti-Nx motif provides an electroactive site to enhance oxygenation, chemical activity, and stability.

2D Metal Carbides  61

n tio ica n Son tio olia Exf

g-C3N4

HF

etc

Porous TCCN film

Exfoliation

Sonication

Filt ra Film tion cas ting

hin

g Ti AI N C

Ti3AIC2 (MAX)

Ti3C2 (MXene)

Figure 3.9  Schematic illustration of the fabrication of porous TCCN film [reprinted with permission from [41]. Copyright 2016 Wiley].

Guo et al. introduced that 2D MXenes familiarly terminated with a mixture of −O and −OH exhibited excellent hydrogen evolution reaction (HER) performance. The results indicated that O termination can be used as an effective catalytic active site for HER [43].

3.3.3.2 V2C The extremely high catalytic activity of the transition metals (Fe, Co and Ni) promotes 2D MXenes, fully oxidized vanadium carbides (V2CO2) for HERs. Ling et al. [44] reported the HER performance of vanadium oxide carbide V2CO2 with and without transition metal promoted in a firstprinciples calculation framework was investigated. Their research shows that pure V2CO2 is not an ideal catalyst for HER, but it can be designed as an excellent HER catalyst by introducing transition metal (TM) atoms into the surface. TMs such as iron, cobalt, and nickel atoms were adsorbed on V2CO2 to decline the interplay between hydrogen and oxygen atoms, as shown as Figure 3.10. The TM adsorbed on the surface of V2CO2 forms TM-O and TM-V bonds, which led to improves the HER performance. Ling et al. found that among all TM-V2CO2 materials, the H-O bond in Fe-V2CO2 was the weakest, and the best HER performance was obtained by Fe-V2CO2. This research opens a new door to the application of 2D MXenes [38].

62  Layered 2D Advanced Materials and Their Allied Applications Low Occupancy

Partially Filled

σ* H1s

σ Fully Filled

O 2pz

Strong H-O bonding Too Negative Δ GH

Increase of Energy

(a) Occupancy Increase

Partially Filled

e

TM

σ* H1s

σ Fully Filled

O 2pz

Weakened H-O bond Optimal Δ GH

(b) Metal promotion

Figure 3.10  The scheme of modulating the HER performance of V2CO2 by introducing transition metal onto the surface. (a) The combination of H 1s orbital and O2pz orbital forms a fully filled bonding orbital and a partially filled anti-bonding orbital, in which the occupancy of anti-bonding orbital will determine the strength of HO bond. (b) Charge transfer from H to O will occur when H adsorbs on O; by introducing a TM atom onto the surface, O will gain extra electrons from TM, leading to less charge transfer from H to O and a higher occupancy of anti-bonding orbital when forming the HO bond [reprinted with permission from [44]. Copyright 2016 Wiley].

3.3.3.3 Mo2C The Mo2C catalyst has high activity against water gas shift (WGS) reaction with respect to the commercially available Cu/ZnO/Al2O3 catalyst [45, 46] and can maintain high stability in oxidation under the reaction conditions. Boudart et al. firstly studied Mo2C [47], they proposed that Mo2C can be reduced by direct carburization and temperature programming of bulk MoO3 [47]. However, the surface area of Mo2C produced by this method is relatively small, which limits the activity of the catalyst. In principle, high surface area Mo2C can be prepared by hot isostatic pressing (HIP), chemical vapor deposition (CVD), microwave assisted synthesis, and molten salt method [48], among them, the electrochemical synthesis method of molten salt has considerable advantages. Integrated

2D Metal Carbides  63 reactions and heat exchange in a piece of equipment have more advantages than traditional unit operation designs. Delsman et al. introduced a design for a micro-device that can preferentially oxidize, which can be used for preferential oxidation of CO in hydrogen-rich in portable fuel processors [49]. This micro-device achieves a recovery efficiency of 90% by the configuration of a constant temperature microreactor and two counterflow heat exchangers. Rebrov et al. [45] researched a new generation of highly active and steady catalytic coatings for fuel processors in water gas shift reactions. The coating was prepared by melt salt synthesis in a solution of a mixture of equimolar NaCl-KCl of 5 wt% Li2CO3 at 1,123 K for 7 h. The results exhibited that molybdenum carbide exists as a thin layer (Mo2C/Mo) on the Mo substrate, and the catalytic activity was improved.

3.3.4 Metal Carbide Materials in Environmental Management Applications With the development of industrialization, the world’s pollution is becoming more and more serious, including heavy metal ions, organic matter, biotoxins, and toxic gases, which have serious impacts on human health and ecosystems. Therefore, it is very necessary to develop chemical and biological technologies to remedy pollutants. 2D MXenes materials can effectively adsorb many molecules and ions due to their hydrophilicity and abundant high-activity functional sites on the surface, which has attracted great attention in the purification of environmental pollutants.

3.3.4.1 Ti3C2 in Environmental Management Applications Adsorption is seen as the most effective way to remove heavy metal ions from contaminated water or soil. MXenes have a rich active site on the surface, so heavy metal ions can be adsorbed by electrostatic and chemical interactions. In 2014, for the first time, Peng et al. prepared the (Ti3C2(OH/Ona)XF2-x) by a chemical stripping and then alkalized intercalation method, the obtained (Ti3C2(OH/Ona)XF2-x) materials exhibited preferential Pb(II) adsorption behavior. The Pb(II) ion was trapped in the hydroxyl well and forms a strong bond between Pb and the oxygen atom (the hydroxyl group loses the H atom). Experimental and computational studies have shown that the adsorption behavior is related to the hydroxyl groups in the activated Ti site. The alk-MXene exhibits efficient Pb(II) adsorbing performance with the applied adsorption capacities of 4,500 kg water per

64  Layered 2D Advanced Materials and Their Allied Applications alk-MXene, and the effluent Pb(II) contents are under the drinking water standard suggested by the World Health Organization (10 μg/L) [50]. In addition, the Pb(II) and Cr(VI) are also important heavy metal contaminants in water. Ying et al. reported that the Ti3C2Tx MXene can reducing the Cr(VI) to less toxic Cr(III) and also adsorbs reduced Cr(III) to remove Cr(VI) from sewage. Then, the generated Cr(III) ions interaction with the [Ti-O] bond on MXene and form a Ti-O-Cr(III) structure. Other oxidants such as K3[Fe(CN)6], KMnO4, and NaAuCl4 have also been shown to be removed by Ti3C2Tx MXene [51]. Organic dyestuff wastewater is also one of the major contaminants. Ti3C2Tx MXene materials can be integrated with ultraviolet catalytic technology to further degrade organic dye contaminants [49]. In 2018, Pandey et al. produced Ag@MXene composite as a novel membrane. The composite membrane exhibited high mechanical stability and outstanding water flux for practical water treatment [50].

3.3.4.2 Ti2C in Environmental Management Applications Guo et al. [52] calculated the adsorption performances of Pb and Cu on Ti2C by DFT. They also researched the effects of surface adornment with functional groups such as H, OH, and F [52, 53]. The results demonstrated that the lattice parameters of the decorated Ti2C will decrease slightly, which indicated that Ti2C is more jarless after being modified by terminals. By calculating the binding energy of a simplex adsorbed atom on pristine and decorative specimens, it was found that the surface decoration significantly reduced the binding energy of adsorbing plumbum and copper. Particularly, the binding energy of copper to H-attached samples was lower than 0.3353 eV, while the binding energy of Pb was higher than 2 eV. Therefore, it was obvious that, in an acidic environment, Ti2C can only adsorb Pb atoms. On the contrast, in the case of OH decoration, Ti2C prefers to assimilate copper instead of plumbum in alkalescent surroundings. For F decoration, Ti2CF2 was not suitable for the adsorption of metal ions because the interactions were too weak. Their results also indicated that the theoretical assimilation ability of plumbum on Ti2C can be achieved to 2,560 mg g−1 on bare MXenes and as high as 1,280 mg g−1 on H-linked MXenes. The excellent metal sorption capacity indicates Ti2C has favorable capacity in the water treatment.

3.3.4.3 V2C in Environmental Management Applications Recently, nuclear energy has been greatly developed, resulting in a serious problem of nuclear waste treatment. It is well known that, due to the

2D Metal Carbides  65 long-term radioactivity and chemical toxicity of act elements, even in trace amounts, it can cause significant harm to living things and the environment. The unique properties of MXenes, such as high radiation resistance and good compatibility with molten salt, make it possible to clean up nuclear waste-treated radionuclides. V2CTx MXene has been shown to adsorb U(VI) with a high absorption capacity of 174 mg g−1, as well as rapid adsorption kinetics and ideal selectivity. DFT calculation combined with X-ray absorption fine structure characterization indicates that uranyl ion prefers to coordinate with the hydroxyl group bonded to the V site of MXene by forming a bidentate inner sphere complex. In order to illustrate the sorption feature of U ions on metal carbides, the sorption performance of hydroxylated V2C nanosheets on uranyl ions of different ligands is generally [UO2(L1)x(L2)y (L3)z]n (wherein, L1, L2, and L3 represent H2O, OH, and CO3) [54], respectively. All uranyl species can strongly bind to V2C(OH)2 nanoflakes with an adsorption energy more than 3 eV. In this work, neo-uranyl [UO2(H2O)5]2+ bonds the electrons to the hydroxylated V2C nanosheets. It has also been found that the terminal-F group on the V2C nanosheet weakens the ability to adsorb uranyl ions.

3.3.4.4 Mo2C in Environmental Management Applications The increasing concentration of carbon dioxide (CO2) in the environment is one of the main reasons of the greenhouse effect, global warming, and ocean acidification. Therefore, it is necessary to find a way to reduce the concentration of CO2. There are three main ways to reduce the concentration of carbon dioxide in the atmosphere; (i) access to energy conservation and protection measures that promote carbon dioxide reduction; (ii) use of sustainable energy sources include hydrogen, solar and wind, geothermal, or biomass energy to reduce carbon emissions; (iii) capture of carbon dioxide by active solid substrate [55]. Several materials have been showed clearly for CO2 capture or activation including metals, metal oxides, graphene-based materials, and ­metal-organic frameworks (MOFs), and so on. Transition metal carbides have recently attracted much attention for CO2 capture, storage, and activation. Garcia et al. [55] have researched the metal carbides capabilities for CO2 sequestration. Through DFT, this group found that even at low CO2 partial pressure and high temperature, bare Mo2C without surface functional groups can also effectively adsorb CO2, so Mo2C is promising materials in the capture, storage, and activation of CO2.

66  Layered 2D Advanced Materials and Their Allied Applications

3.3.5 Carbide Materials in Biomedicine Applications Metal carbide MXenes as a novel 2D functional material have been widely studied in medicine areas because of their unique super nanostructures and corresponding ideal physiochemical/biological properties. The field involves drug/gene transfer, photodynamic therapy, molecular imaging, biosensing, antibacterial activity, and even tissue engineering.

3.3.5.1 Ti3C2 in Biomedicine Applications In terms of drug carriers, Ti3C2 MXenes has a high drug-loading capacity of up to 211.8% and also exhibits an on-demand drug release in response to pH and near-infrared ignition. Han et al. [56] successfully developed 2D Ti3C2 MXenes as the drug delivery nano-platform for effective chemotherapy for the first time. Moreover, Ti3C2 has high biosafety due to its high histocompatibility in vivo and their ease of excretion. In the treatment and diagnosis of cancer, Liu et al. [57] first reported the construction of superparamagnetic 2D Ti3C2 MXenes to achieve effective cancer treatment diagnosis by directly decorating superparamagnetic iron. The etching process and surface engineering of the magnetic 2D Ti3C2-IONPs-SPs nanocomposites and their schematics for the diagnosis of ­multifunctional tumors are shown in Figure 3.11. In biosensing, Xu et al. demonstrated an efficient technique for assembling FET transistors based on ultra-thin conductive Ti3C2 MXene micropatterns for performing highly sensitive, unlabeled detection of dopamine, and Monitor spike activity in hippocampal neurons. MXene-FET biosensors were well compatible with long-term neuronal culture. A schematic diagram of a biosensor device is shown in Figure 3.12. The transparent ultra-thin (≈5 nm) Ti3C2-Meene micro-pattern layer does not interfere with conventional microscopy, so the electrical measurement using MXene-FET can simultaneously perform calcium imaging of neural activity with a time resolution of ≈50 ms [58].

3.3.5.2 Ti2C in Biomedicine Applications Cancer is still an urgent medical problem in modern medicine. Routine chemotherapy can cause serious side effects. Photo thermal therapy (PTT) has known stupendous potential for anticancer therapy [59]. Its main advantage is to destroy cancer cells without damaging the normal cells of surrounding tissues. Szuplewska et al. used Ti2C-based MXene as a new and high-efficiency selective agent for PTT. In the exfoliation process using

2D Metal Carbides  67

TI3AIC2 ceramics

TI3C2 layer

AI layer SP

Centrifugation Ti3C2 nanosheets

TPAOH insertion

HF etched

TPAOH IONP TPAOH Soybean phospholipid (SP) AI Ti O

IONP

Blo

od

sse

NI

er

as

Rl

ve

l

Tumor tissue

II: PTT-tumor ablation

II: T2-weighted MRI

Figure 3.11  Schematic illustration of exfoliation process and surface engineering of magnetic 2D Ti3C2-IONPs-SPs nanocomposites and their multifunctionalities for tumor theranostics, i.e., efficient T2-weighted MRI-guided PTT against tumor [reprinted with permission from [57]. Copyright 2018 The Royal Society of Chemistry].

C

Target Biomolecules

Cell Culture Media MXene strips

S

D

Figure 3.12  Schematic of a biosensing device based on MXene field-effect transistors [reprinted with permission from [58] Copyright 2016 Wiley].

68  Layered 2D Advanced Materials and Their Allied Applications high concentration HF, Ti2C surface-modified with polyethylene  glycol was gained from a laminated commercially Ti2AlC MAX phase. The PEG coated Ti2C film exhibited higher photothermal conversion efficiency and excellent biocompatibility. According to extracorporeal studies, Ti2C with a concentration of 37.5 μg ml–1 modified with PEG showed excellent NIR-induced cancer cell ablation ability, which had the least effect on non-malignant cells. Compared to other MXene-based photothermal agents, Ti2C-PEG at a dose that was even 24 times lower. To developing new reagent for photothermal therapy, Ti2C-PEG is hopeful to exploit the practicability of 2D MXenes for biomedical applications [60].

3.3.5.3 V2C in Biomedicine Applications Two-dimensional vanadium carbide quantum dots (V2C QDs) exhibit good photothermal properties on PTT under the NIR-II bio-light source. Cao et al. used it for nuclear target cryothermal treatment [61]. The obtained V2C-TAT@Ex-RGD demonstrated long cycle life and complete biocompatibility. The RGD modification confers good cancer targeting ability to V2C-TAT@Ex-RGD, and the small-sized V2C-TAT QD can easily enter the nucleus and directly destroy the genetic material of low temperature PTT. At the same time, the experiment also confirmed the in vivo imaging ability of V2C-TAT QD with multimodal tumors. Therefore, the application of V2C in the field of biomedicine has certain potential [62].

3.3.5.4 Mo2C in Biomedicine Applications At present, nanotechnology-based methods for treating cancer received widespread attentions [63]. Among them, near-infrared (NIR) induction and nanoparticle photothermotherapy (PTT) have become examples of non-invasive topical treatment [64]. Two-dimensional materials have experienced explosive growth in energy storage and nanomedicine applications in recent years because of their ultra-thin nanostructures and unique properties. However, the explosive growth of the 2D MXenes in nanomedicine is still in its infancy. Feng et al. [65] reported the configuration of a new 2D Mo2C MXenes for photon tumor hyperthermia. They prepared Mo2C by eliminating Ga from the non-MAX phase precursor Mo2Ga2C, which is a typical example. The results in the experiment showed that the ultra-thin Mo2C MXene nanofilm has high photothermal properties and rapid biodegradability under physiological conditions. To ensure ideal biocompatibility and physiological stability, PVA was used to modify the surface of Mo2C nanofilms by

2D Metal Carbides  69 hydrogen bonding and van der Waals interactions. Mo2C-PVA nanosheets represent high biocompatibility and rapid degradability after further surface engineering with PVA. Among them, PVA engineering systematically studied Mo2C MXenes (denoted as Mo2C-PVA). It is important to confirm the concept of Mo2C-PVA nanoparticles by photo-thermal ablation of tumor allografts in NIR I and II windows. Therefore, the ultra-thin Mo2C MXene not only effectively provides a promising nano-platform for photon cancer treatment but also broadens the metal carbides application in the biomedical field.

3.3.6 Carbide Materials in Gas Sensing Applications Gas sensing technology plays a vital role in environmental monitoring and other aspects. Two-dimensional materials are considered to be one of the most desirable materials for future sensing applications due to their high specific surface area, excellent thermal stability, and strong selective adsorption of gas molecules.

3.3.6.1 Ti3C2 in Gas Sensing Applications Lee et al. introduced a gas sensor capable of detecting various gases based on Ti3C2Tx. They used a flexible polyimide film as the substrate to synthesize Ti3C2 by removing aluminum layers from Ti3AlC2 precursor and integrated them using a simple solution-based method. On the substrate, a gas sensing device was fabricated. They proved that the manufactured sensors can smoothly detect various 100 ppm polar gases. In addition, the sensor shows the highest response to ammonia and the lowest response to acetone. Gas absorption can occur at the active defect sites on the Ti3C2 surface and also can be produced by interaction with surface functional groups. At the defect site, gas absorption was caused by a dispersing force such as an electrostatic force, and a comparatively small resistance change because of its weak intermolecular force. For another, strong hydrogen bonds between surface functional groups and gas molecules may also cause gas absorption. The charge is transferred between the adsorbent and the adsorbed gas, causing a great resistance change in the Ti3C2. The new Ti3C2 gas sensor is expected to be a versatile sensor for future wearable devices [66].

3.3.6.2 Ti2C in Gas Sensing Applications Ammonia (NH3) has been diffusely used in refrigeration systems, fertilizer parturition, and reducing the amount of NOx in diesel powered car [67].

70  Layered 2D Advanced Materials and Their Allied Applications Nevertheless, NH3 is a noxious chemical compound with volatility that can cause significant damage to the human because it can greatly restrain oxygen uptake of human tissue [67, 68]. Therefore, it is indispensable to search an good route for sensing and capturing NH3 for atmospheric and industrial applications [67, 68]. The first materials studied were semiconducting metal oxides and low-dimensional materials because they have the advantages of low cost and small size. However, these sensors lack selectivity for specific gases, and most of them need longer recovery time at room temperature [68]. So far, researchers have found that metal halide ammines Sr(NH3)8Cl2, Mg(NH3)6Cl2, and mixed metal halide amines can store NH3 very efficiently [67, 69, 70]. However, the releaser multi-step reaction of NH3 from these materials requires high temperature (>450 K) tions, which limits their use in the automotive field [67, 68]. Therefore, to design an optimal NH3 sensor, the prime challenges are to achieve effective release of NH3 from these materials and guarantee its effect at room temperature [67]. Lately, a lot of work on MXene has been reported due to its similar structure and properties as graphene [67, 68]. Related research has showed that the Ti2C MXene nanolayer is a good sensor and capture agent for NH3 because the strength of the interaction between NH3 and Ti2C can be enhanced by the applied strain [68, 71]. This particular structure exhibits the strongest binding energy compared with H2, CH4, CO2, N2, and NO2, because the nitrogen atom of NH3 is situated exactly above the Ti atom of Ti2C [71]. The adsorption of NH3 on a single layer of Ti2C can be readily taken place by reducing the applied twin-shaft strain, which is suitable for fuelcell applications. In addition, studies have revealed the mechanism of reversible adsorption of NH3 on O-terminated Ti2C, that is, the adsorption behavior of NH3 can change from chemical adsorption to physical adsorption. And the chemical adsorption of NH3 has a significant charge transfer. This result highlights a truth that MXenes semiconductor O-terminated are potential candidates for NH3 sensors and capture agents [71].

3.3.6.3 V2C in Gas Sensing Applications The 2D V2C Tx shows excellent performance in detecting non-polar gases. Lee et al. introduced the RT gas response and ultra-high selectivity of layered monolayer/small layer d-V2CTx MXene for non-polar analytes. They produced a 2D V2CTx gas sensor by dropping a single layer/small layer of V2CTx MXene sheet synthesized by selective etching and intercalation on a polyimide sensor platform. The manufactured 2D V2CTx gas sensor is capable of detecting various gases including non-polar gases. At room

2D Metal Carbides  71 temperature, the theoretical low detection limit (LOD) of hydrogen is 1 ppm and the theoretical LOD of methane is 9 ppm [72].

3.3.6.4 Mo2C in Gas Sensing Applications There are still many energy and environmental technology challenges such as gas purification, gas sensors, etc., to improve the selectivity, stability, and durability of catalytic and adsorbent materials under suitable operating conditions [73]. In addition to traditional metal and metal oxide based materials, there are many other 2D materials other than graphene, such as transition metal carbide (TMC), silicon carbide, silane, etc., which have been widely used in this field. Many examinations have shown that the terminated MXenes exhibit good selectivity for specific gases. Due to the

400 nm

CH3COCH3

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100 105 110 115 Time [min]

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Figure 3.13  Gas sensing characteristics of molybdenum carbide (α-MoC1-x and β-Mo2C) gas sensors. (a) SEM images of integrated Au electrodes and drop-cast β-Mo2C nanoparticles. (b) Channel resistance variations of the α-MoC1-x and β-Mo2C sensors. (c) The maximal resistance change ((DR/Rb)max (%)) of the α-MoC1-x and β-Mo2C gas sensors for 5 ppm NO2, SO2, and NH3 and 1,000 ppm CH3COCH3 (acetone), CH3CH2CHO (propanal), C2H5OH (ethanol), C6H14 (hexane), and C6H5CH3 (toluene). (d) SNR comparison of the α-MoC1-x and β-Mo2C gas sensors for 1000 ppm ethanol. (e) Real-time sensing response of the α-MoC1-x and β-Mo2C gas sensors to various analytes. (f) Total and projected DOS for α-MoC1-x (upper panel) and β-Mo2C (lower panel) bulk structures [reprinted with permission from [74]. Copyright 2018 The Royal Society of Chemistry].

72  Layered 2D Advanced Materials and Their Allied Applications unique properties of molybdenum carbon compounds, it can be applied to develop the solid state gas sensors with high sensibility, high stability, and good adaptability. Cho et al. explored that the α-MoC1-x and β-Mo2C application in gas sensors [74]. Figure 3.13a showed that the β-Mo2C nanoparticles are linked with the 100 mm Au electrode to form a sensing channel. Figure 3.13b demonstrates the resistance ofα-MoC1-x and β-Mo2C sensing channels. It signifies that metal properties of β-Mo2C are higher. Figure 3.13c displays their dynamical of sensing response. The α-MoC1-x sensor has an active gas response to most analytes, especially for NO2 ((DR/Rb)max = 15%) and ethanol ((DR/Rb)max = + 6%). The α-MoC1-x sensor exhibits a brisk gas response to the majority analytes, especially ethanol ((DR/Rb)max ¼ +6%) and NO2 ((DR/Rb)max ¼ 15%). On the contrary, the gas response amplitude of the β-Mo2C sensor was reduced by two to three times relative to α-MoC1-x. The SNR of two samples was computed as shown as Figure 3.13. It has proved that molybdenum carbon compounds are able to be employed as gloriously susceptive sensing channels. Figure 3.13e displays the real-time answer data of the sensors. It manifests that β-Mo2C can give vigorous respond to all the analytes. Figure 3.13f can further prove that β-Mo2C has stronger metal properties than α-MoC1-x [74, 75].

3.4 Summary and Outlook The purpose of this chapter is to introduce the synthesis method, structure performance, and application of MXenes, reflecting the rapid development of MXene research community. The application section describes the latest developments in MXenes, including electrochemical capacitors, photodegradation of organic contaminants, photocatalysis and electrocatalysis, photothermal therapy for cancer therapy, drug carriers, and gas sensors. These applications reflect the great value of MXene. As the research progresses, researcher will continue to explore the potential applications of MXene, improve existing applications, and extend the MXene family through different combinations of elements. We look forward to Mxene’s exciting development in the future.

Acknowledgements We gratefully acknowledge the support of this work by the China Postdoctoral Science Foundation (2019M652537), the program for Science

2D Metal Carbides  73 & Technology Innovation Talents in the Universities of Henan Province (18HASTIT007), Henan Postdoctoral Foundation (19030065), the Natural Science Foundation of Henan Province of China (182300410202), and the Foundation of He’nan Educational Committee (20B430006).

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78  Layered 2D Advanced Materials and Their Allied Applications 67. Xiao, B., Li, Y.-c., Yu, X.-f., Cheng, J.-b., MXenes: Reusable materials for NH3 sensor or capturer by controlling the charge injection. Sens. Actuatuators B Chem., 235, 103–109, 2016. 68. Yu, X.-f., Li, Y.-c., Cheng, J.-b., Liu, Z.-b., Li, Q.-z., Li, W.-z., Yang, X., Xiao, B., Monolayer Ti2CO2: A promising candidate for NH3 sensor or capturer with high sensitivity and selectivity. ACS Appl. Mater. Interfaces, 7, 13707– 13713, 2015. 69. Sørensen, R.Z., Hummelshøj, J.S., Klerke, A., Reves, J.B., Vegge, T., Nørskov, J.K., Christensen, C.H., Indirect, reversible high-density hydrogen storage in compact metal ammine salts. J. Am. Chem. Soc., 130, 8660–8668, 2008. 70. Lysgaard, S., Ammitzbøll, A.L., Johnsen, R.E., Norby, P., Quaade, U.J., Vegge, T., Resolving the stability and structure of strontium chloride amines from equilibrium pressures, XRD and DFT. Int. J. Hydrogen Energy, 37, 18927– 18936, 2012. 71. Zhu, J., Ha, E., Zhao, G., Zhou, Y., Huang, D., Yue, G., Hu, L., Sun, N., Wang, Y., Lee, L.Y.S., Recent advance in MXenes: A promising 2D material for catalysis, sensor and chemical adsorption. Coord. Chem. Rev., 352, 306–327, 2017. 72. Lee, E., VahidMohammadi, A., Yoon, Y.S., Beidaghi, M., Kim, D.-J., Twodimensional vanadium carbide MXene for gas sensors with ultrahigh sensitivity toward non-polar gases. ACS Sens., 6, 1603–1611, 2019. 73. Rezaei, F., Rownaghi, A.A., Monjezi, S., Lively, R.P., Jones, C.W., SO x/NO x removal from flue gas streams by solid adsorbents: A review of current challenges and future directions. Energy Fuels, 29, 5467–5486, 2015. 74. Cho, S.-Y., Kim, J.Y., Kwon, O., Kim, J., Jung, H.-T., Molybdenum carbide chemical sensors with ultrahigh signal-to-noise ratios and ambient stability. J. Mater. Chem. A, 6, 23408–23416, 2018. 75. dos Santos Politi, J.R., Viñes, F., Rodriguez, J.A., Illas, F., Atomic and electronic structure of molybdenum carbide phases: Bulk and low Miller-index surfaces. Phys. Chem. Chem. Phys., 15, 12617–12625, 2013.

4 2D Carbon Materials as Photocatalysts Amel Boudjemaa

*

Centre de Recherche Scientifique et Technique en Analyses Physico-Chimiques (CRAPC), Bou-Ismail, Algérie

Abstract

This chapter describes the carbon based materials, their potential applications through photo-catalytic process using visible light irradiation. The reaction mechanisms were also proposed. Carbon-based materials can be applied for the various fields in the environment- such as pollutant degradation, heavy metal elimination, hydrogen (H2) generation, and CO2 reduction. Regarding the 2D carbon materials, most large-scale photo-catalytic applications mentioned in the bibliography is based on the use of carbon-based materials. However, many efforts have been consecrated for the utilization of carbon materials as electron donors coupling or modified with transition metals or others materials used as hetero-systems to improve their reactivity under visible light irradiation. The present chapter supplied an overview on the 2D carbon nanostructured materials to improve their catalytic performance as photo-catalyst for different applications. Keywords:  Carbon nanostructured, elaboration, applications, energy, environment

4.1 Introduction With the discoveries of fullerene (C60) and carbon nanotubes, carbon material has become the building block in the field of nanotechnology [1]. Carbon is a very useful element and forms a basis of organic chemistry due to its ability to exist in many electronic configurations, i.e., sp, sp2, and sp3 hybridization. So, the carbon can exist in many allotropic forms include

Email: [email protected]; [email protected] Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (79–102) © 2020 Scrivener Publishing LLC

79

80  Layered 2D Advanced Materials and Their Allied Applications carbon nanotubes (CNT) and nanofibers (CNF) [2], onions [3], horns [4], calabashes [5], flasks [6], and carbon spheres (CS) [7, 8]. Graphite, diamond, and amorphous carbon are the natural allotropes of carbon [9]. The physical properties of carbon such as form, color, durability, and conductivity, etc., are varied with the allotropic form. Inagaki and Serp have gone a step further by classifying them in two different ways [10, 11]. Inagaki classified these materials through the arrangement of the carbon layers while Serp et al. [12] regrouped them by size, i.e., fullerenes (1 μm) [17, 18]. For these properties, carbon materials are a subject of intense research, with strong relevance to both science and technology.

4.2 Carbon Nanostructured-Based Materials 4.2.1 Forms of Carbon With four valence electrons, carbon is not only able to form a vast number of different bond types with other elements but can also bond with itself in different and complex ways. Today, the carbon can exist in many allotropic forms, each with extreme variations in physical and chemical behavior. These allotropes include graphene (G), fullerene, CNT and CNF, onions, horns, calabashes, flasks, and more relevant to the current study CSs [3–7]. The exceptional subtlety in bonding configurations resulting from carbons ability to use almost any conceivable combination of sp, sp2, and sp3 hybridization is the main reason for the divers’ application of ­carbon-based materials [8].

4.2.2 Synthesis of Carbon Nanostructured-Based Materials Carbon materials can be classified through their dimension includes fullerenes, CNT, G, and graphite as 0D, 1D, 2D, and 3D structures, respectively. These materials are prepared mainly by pyrolysis [19], hydrothermal [20], mechanical or chemical exfoliation [21], and the chemical vapor deposition (CVD) methods [22]. Among these different synthesis routes, the CVD technique offers the advantage and ability to modulate the carbon morphology and the product yield. Ever since the discovery of fullerenes and CNTs [23, 24] which have been the key events that have promoted the study of carbon materials especially nanostuctured materials which have been develop prevalent application

2D Carbon Materials as Photocatalysts  81

Carbon chains

C60

Polycyclic aromatic hydrocarbons

C70

Graphite

Carbon nanotubes

Amorhous Carbon

Nanodiamond

Schema 4.1  The different forms of carbon [25].

in various fields. These discoveries coincided with attempts to miniaturize devices and the development of new characterization equipment (scanning probe microscopes) to visualize these new structures (Schema 4.1) [25].

4.3 Photo-Degradation of Organic Pollutants 4.3.1 Graphene, Graphene Oxide, Graphene Nitride (g-C3N4) With its unique properties, graphene has demonstrated not only high potential for various applications, but also the subject for studying various fundamental physics. Graphene is largely used because it can be elaborated at large scale using the chemical technique. Also, it is useful to understand the relationship between its structure and its properties. In photo-­catalysis, graphene and graphene oxide and graphene nitride are preferred to be used due to their high surface area [26], which facilitate the adsorption of pollutants the first step of photo-catalysis process before the photo-­ degradation of the molecule. These properties expedite the electrons hopping helping the rapid separation of charge carriers lead to reducing their (e−/h+) recombination [27]. Graphene oxide and graphene quantum as precursor material have been used for the preparation of graphene and also tested in many reactions [28–32]. Markad and co-authors studied the photocatalytic properties of TiO2/graphene composite for methylene bleu (MB) degradation [33]. The good performances are due to the both transport and the efficient separation of (e−/h+) pair.

82  Layered 2D Advanced Materials and Their Allied Applications

4.3.1.1 Graphene-Based Materials Several challenges for utilizing graphene oxide (GO) or reduced graphene oxide (rG) modified with TiO2 in photo-catalysis have been investigated [34]. So, different methods are investigated to synthesized TiO2-graphene nanocomposite using commercial TiO2 (P25) [35, 36]. Hybrid materials based on metals nanoparticles and rGO revealed advanced features, such as high adsorption of target molecule, and low charge recombination [37]. It is reported that the G/TiO2 nanocrystal hybrid fabricated by a direct growing of TiO2 nanocrystals onto GO sheets has a superior photo-­ catalytic activity compared to other TiO2-based materials [38]. Therefore, graphene can be proceed as an acceptor of the photo-generated electrons from TiO2 and assure rapid charge carriers due to the high conductivity [35, 39]. Faster charge transportation supplied more photo-induced carrier charges giving a higher photo-catalytic activity. Thus, the combination of microsized ZnO with G is estimated to demonstrate an enhancement photo-catalytic reactivity. Xu et al. reported that ZnO/rGO demonstrated an enhanced photo-­catalytic efficiency compared to ZnO flowers maybe due to the synergistic effect between ZnO and rGO [40]. Therefore, the incorporation of ZnO into G-TiO2 composites to form ternary composites is a promising method to design advanced photo-catalyst materials for the dye degradation [41, 42]. The degradation yield over rGO/TiO2/ZnO, rGO/TiO2, and TiO2 is around 92%, 68%, and 47%, respectively, after 2 hrs of irradiation [43]. Otherwise, the enhancement photo-reactivity of G-metal composites is also investigated [32]. Composite TiO2/G can provide reduced (e−/h+) recombination and acted as a visible photocatalyst [44]. Li and Cao reported that improvement photoactivity is due to the both enhancement photosensitized electron injection and the low electron recombination [45]. Liang and co-­workers investigated the effects of TiO2 on rGO for RhB’s degradation [46]. Besides Ag, other noble metals, such as Au and Pt, combined into composites form with G have been reported for the photo-catalytic removal of a pollutant [47]. Many studies over the nanocomposites such as TiO2/ rGO have been investigated [48], and also Ag/rGO [49, 50], Ag/TiO2/GO [51] and Ag/TiO2/rGO [52] are a subject of many studies. A good photodegradation efficiency of Ag/TiO2/rGO compared to TiO2/rGO, Ag/TiO2, and TiO2 is due to the incorporation of the nanoparticles. The effect of Ag nanoparticles owes boost by rGO through the extension of TiO2 response toward visible-light range and the acceleration of (e−/h+) separation [53]. The rGO surface can rapidly transport the e− from Ag’s via extended π-conjugation structure [54].

2D Carbon Materials as Photocatalysts  83 Graphene

C-Dot

TiO2 e– e–e– e– e–

e– e– e– e–

O2

d rte ve n o L pc P

O2– H2O

e– e –

le sib

ht

lig

Vi

h+ h+

U

OH•

Mineralized Product

OH•

C-Dots

h+ h+ h+h+ H2O/OH–

TiO2

MB

Schema 4.2  Mechanism for electron and hole transfer for the photo-catalytic removal of MB on G-CD-TiO2 semiconductor [55].

Novel ternary composite G-carbon nanodots-TiO2 (G-CD-TiO2) is used as efficient photo-catalyst. The enhanced photo-reactivity is due to e− hopping from CD to TiO2 (Schema 4.2) [55]). AuNPs-GO hybrid nanostructure used as photo-catalyst demonstrated an excellent performance through the photo-degradation of RB [56]. The rate is obviously improved after the introduction of AuNPs. In addition, this incorporation is important to avoid the aggregation of NPs and show robust catalytic performance [57]. New photo-catalyst; CdSe/CdS, quantum dot QDs/AuNP, and QDs/Au-Gr are also used for the photodegradation [58]. Therefore, under irradiation, the e− is excited from valance band to conduction band of QD and GO semiconductors. The efficient e− is transferred from QDs to AuNPs revealed by an obvious decrease in PL after introduction of AuNPs into QDs. Compared to QD-G, QD/Au-G reveals an efficient photo-catalytic performance. The photo-catalytic performances of G-based material have been regrouped in Table 4.1. G-TiO2 demonstrated higher photo-catalytic efficiency via the removal of sodium pentachlorophenol (PCP-Na) with the reaction efficiency ~82.9% [65]. So, the lifetime of (e−/h+) pair can prolonged for the removal of PCP-Na. Thus, G-TiO2 has high stability and important photo-catalytic activity due to the improved quantum efficiency and narrow Eg [66, 67]. Most of studies are used MB [35], benzene [35], RB [68], bropanol [69], and acid orange [70]. Benjwal et al. successfully elaborated ternary composite as rGO-TiO2/rGO-Fe3O4 and rGO-Fe3O4-TiO2 [71]. Elaboration of nanomaterial is used for the degradation of MB. So, compared to binary, ternary

84  Layered 2D Advanced Materials and Their Allied Applications Table 4.1  Photo-catalytic performance of different G-based hybrid materials. Photocatalysts

Degradation rate efficiency (min−1)

Reaction

Light source

Reference

G-TiO2

Methylene blue

Solar light

G-TiO2: 0.0195 Pure TiO2: 0.0152

[59]

G-BiOBr

Rhodamine B

Visible-light

0.042

[60]

G-ZnS

Methyl orange

300 W mercury

0.02558

[61]

G-CdS

Methyl orange

300 W mercury

0.01362

[61]

G-Bi2S3

Methyl orange

300 W mercury

0.01204

[61]

ZnO1-x/G

Methylene blue

Visible and UV

Visible: 0.60 h−1 UV light: 0.19

[62]

rGO-ZnO

Methylene blue

Visibleirradiation

0.0335

[63]

rGO-Au

Rhodamine B

UV

0.0087

[56]

rGO-Cu

Rhodamine B

UV

0.015

[64]

one exhibited a highest efficiency due to both the synergetic interaction and a higher surface area.

4.3.1.2 Graphene Nitride (g-C3N4) Graphene nitride or graphitic carbon nitride (g-C3N4) is an organic photo­ sensitive semi-conductor characteristic by strong covalent between carbon and nitrogen in each layer by van der Waals forces. Recently, it is appeared as a promising visible light responsive polymeric photo-catalyst due to 2D structures, the chemical stability and the tunable electronic structure [72]. The exceptional properties of this materials attracted scientist attentions [73]. Compared with other carbon materials, g-C3N4 presented e− rich properties basic surface functionalities and H-bonding motifs due to the presence of nitrogen and hydrogen atoms [74]. The Eg of ~2.7 eV is making it good photo-catalyst in visible region for many reactions [75]. Photo-catalytic reactions are still hindered by several obstacles, such as the high recombination of (e−/h+) pair, low electronic

2D Carbon Materials as Photocatalysts  85 conductivity, and a small specific surface. Consequently, an improvement of g-C3N4-based visible photo-catalysts with enhanced activities for environmental applications has drawn increasing research attentions during the recent years. Many approaches take place according to increase the photo-reactivity by doping with non-metal element or used as heterojunction [38, 76]. Cited for example S-doped g-C3N4 which improved the visible light absorption, increasing the carrier mobility and enable the separation of (e−/h+) [77]. Recently, many works examined the removal of dyes using g-C3N4based materials. Hetero-junction semiconductors are widely used to elaborated a hybrid semi-conductors with C3N4, such as TiO2 [78, 79], CdS [80], BiOBr [81], and MoS2 [82]. Photo-reactivity of g-C3N4/CDs estimated via removal of Methyl Orange shown that CDs increased the ­photo-catalytic reactivity of g-C3N4/CDs [34]. Also, the reaction efficiency of sulfamethazine is 70% after 90 min of irradiation [83]. The sulfur-doped g-C3N4 and zinc phthalo-cyanine (ZnTNPc) has been investigated to remove MB. ZnTNPc/g-C3N4 demonstrated a good performance compared to ZnTNPc/g-C3N4 [84]. In this case, the sulfur limited the aggregation of ZnTNPc particles. The diagram energetic of g-C3N4, g-CNS, and ZnTNPc photo-catalysts (Figure  4.1) shown that S-doped g-C3N4 not only narrows Eg but also downshift CB [85]. Same performances have been mentioned for I, P, and N hetero-atom [86, 87]. LUMO level of ZnTNPc is higher than CB of g-CNS/g-C3N4. So, the excited e− on ZnTNPc is easily injected into g-CNS/g-C3N4, making charge separation more efficient (Figure 4.2) [88]. In other side, CB of g-CNS is lower than that of g-C3N4, and the photo-generated e- are more easily transferred into ZnTNPc/g-CNS lead to a better photo-catalytic reactivity of ZnTNPc/g-CNS compared to ZnTNPc/g-C3N4 [85]. −3

Potential (V) vs. SCE

−2 −1 0 1 2

g-CNS e– –0.84

e–

CB

e–

2.0 eV HOMO h+ h+

2.53 eV 1.69 h+

ZnTNPc e– e– LUMO

VB

h+

h+

g-C3N4 e– e– e– –1.03

CB

2.65 eV 1.62 h+

VB

h+

h+

3

Figure 4.1  Diagram energetic of g-C3N4, g-CNS, and ZnTNPc samples [85].

86  Layered 2D Advanced Materials and Their Allied Applications MB visible light

O2

•O2–

e− e− e− CB 2.53 eV

Degradation products

e−

e−

LUMO

2.0 eV ZnTNPc HOMO

g-CNS

VB h+ h+ h+

e− e−

h+ h+

MB

h+ h+

Degradation products

Figure 4.2  Schematic of photo-catalytic removal of MB over the ZnTNPc/g-CNS photocatalysts [88].

BiOBr/g-C3N4 used for the removal RhB has been investigated. The optimum photo-activity of 0.5BiOBr/g-C3N4 hetero-junction increases and reaches values greater than 5 and 17 times than both BiOBr and g-C3N4 ­ hoto-catalytic [87]. Many g-C3N4-based hetero-junctions showed a good p reactivity for RB degradation. Cited for example the case of MoS2, Ag3P 4, In2S3, BiIO4, InVO4, ZrO2, SmVO4, V2O5, SrTiO3, and CdS [82, 90–97, 98]. In addition, g-C3N4-based photo-catalysts have been found to decompose MB. TiO2/g-C3N4 nanosheet demonstrated an efficient photo-­catalytic performance under UV and visible light [99]. Besides, many other g-C3N4-based photo-catalysts are used for MB degradation [100], such as Bi2O3/g-C3N4 [101], Ag3VO4/g-C3N4 [102]. Thus, g-C3N4based photo-­catalysts have been studied to remove Methyl orange (MO) from waste­water. AgBr/g-C3N4 and AgI/g-C3N4 hetero-junction improved their reactivity through the removal of MO [58]. Cu2O and g-C3N4 in the hetero-junction could be raised the reactivity under visible light spectral response compared to g-C3N4 [103]. MO is decomposed under visible irradiation using CdS/g-C3N4 [81]. The activities are greater compared to TiO2/g-C3N4 and CdS/TiO2. Wang et al. elaborated Ag3VO4 anchored on g-C3N4 sheets for the degradation of different kind of dyes under the visible irradiation [104]. Ag/AgVO3/g-C3N4 photo-catalysts are fabricated using one-step in situ hydrothermal method [105]. The material demonstrated highest apparent rate constant (0.0701 min−1) with best performances photo-degradation. The study about Acid Orange-II photo-degradation over CuO/g-C3N4 hetero-junction composite is exhibited by [106]. Lei and coauthors demon­ strated that the TiO2/g-C3N4 photo-catalyst can decompose Acid Orange 7 under UV and visible irradiation. So, g-C3N4 enlarged both the light absorption of ZnO-Ag2O and the reaction rate constant (0.057 min−1),

2D Carbon Materials as Photocatalysts  87 which is much higher than ZnO, g-C3N4, and Ag2O alone [107]. Au/Pt/­ g-C3N4 has been successfully removed tetracycline from water [40].

4.3.2 Carbon Dots (CDs) Recently, carbon dots (CDs) have appeared as a hopeful new class of ­photo-catalyst, exhibited a semiconductor properties and shown an excellent performance for photo-electrochemical and photo-catalytic applications [108]. Li and coauthors exploit their photoreactivity in 2010 [109]. The photo-catalytic performance of CDs is related to their optical and structural properties. These are matched well with the necessary conditions for photo-catalytic reactions, indicated that CDs-based materials as promising photo-catalysts [110]. Mirtchev and coauthors mentioned that CDs can be used for solar cells [111]. Various references are introduced the utilization of CDs for visible light sensitization [112]. Because CDs does not contain any toxic metals, therefore, viewed as most environmental friendly sensitizers [113]. Several nanocomposites have been achieved, containing inorganic and organic hetero-system, such as ZnO/CDs for photo-catalytic dye degradation [114]. Furthermore, the mixed of CDs with TiO2 or SiO2 shown efficient visible light response toward dye’s degradation. So, CDs/TiO2 and CDs/SiO2 photo-catalysts are shown completely degradation of MB after 20 min under halogen irradiation. While without CDs, TiO2, and SiO2 represented a poor photo-­ degradation. These results confirmed that the catalytic activity depend on the interaction between CDs and the metal oxides semi-conductors [115]. CDs-based photo-catalysts coupled metal oxides semiconductors as Fe2O3, ZnO, Ag3PO4, and Cu2O have been evaluated their performance for visible light photo-catalytic degradation reactions [86, 116]. G-CDs-TiO2 novel ternary composite demonstrated an efficient photo-reactivity for MB’s elimination. The enhanced photo-reactivity is due to the photo-injection of e− from CDs to TiO2 and the excellent ability of G to adsorb target molecules as well e− sink from TiO2 [117].

4.3.3 Carbon Spheres (CSs) Others carbon-based materials are carbon spheres (CSs) with a similar properties to graphite or fullerene and those to CNTs such as high strength, good thermal resistance, high oxidative inertness, weight, and sp2 hybridized carbon atoms [8]. CSs are relatively cheaper to produce than CNTs. CSs have also been used as support to deposit metal or metal oxides nanoparticles [56, 118], as templates to synthesize hollow metal or

88  Layered 2D Advanced Materials and Their Allied Applications metal oxides nanoparticles [119]. CSs have two forms; solid and hollow one. In particular, solid CSs attract an increasing attention due to their many potential applications [120]. Hollow CSs are also of great interest due to their physical-chemical properties such as chemical stability, surface, thermal insulation, etc. [121]. Very few studies are focused on the use of CSs in water treatment. CdS nanoparticles coated CSs shown to have high photo-catalytic activity via the degradation of RB [20]. CSs are also employed as template for the elaboration of porous TiO2 nanospheres. They are very effective for the elimination of MO [122]. B-doped TiO2 hollow spheres are an active photo catalyst versus Reactive Brilliant’s removal [123].

4.4 Carbon-Based Materials for Hydrogen Production Hydrogen generation from water employed semiconductors as photo-­ catalysts supplies a mean method to generate H2, due to its clean, low-cost, and environmentally friendly process by using solar energy. The solar to fuel conversion has been considered as one of the most promising methods to solve some problems [59, 124]. Recently, carbon-based materials have been demonstrated a high ability for develop photo-catalytic H2 generation performance over photo-catalysts. Particularly, photo-catalytic hydrogen evolution via water splitting offers a potentially sustainable method to achieve the future energy demand without environmental interruption. As well, many strategies to increase the photo-catalytic H2 generation efficiency are investigated in terms of surface chemical functionalization. GO is used for water splitting [125]. The H2 yield is 5,666 µmol.h−1g−1 under mercury lamp (UV) illumination with a similar yield is found for a 5 wt% Pt/GO composite. Thus, g-C3N4 with Eg = 2.7 eV has a sufficient absorption and shows photo-catalytic activity for water photo-reduction to H2 under visible irradiation [126]. The photo-catalytic efficiency of g-C3N4 is limited by high recombination of (e−/h+) pair. Therefore, the hetero-­junctions of g-C3N4 with other semi-conductors provide a feasible method to enhance photo-catalytic performance. g-C3N4 modified TiO2 nanorod arrayed shown high photo-electrochemical activity [127]. The materials based on Fe2O3 are examined with g-C3N4 and CDs for water splitting and degradation reactions, respectively [84, 128]. It is estimated that the combination of g-C3N4 with Fe2O3 would be an ideal in inhibiting the recombination of (e−/h+) pair and enhancing photo-activity compared to individual g-C3N4

2D Carbon Materials as Photocatalysts  89 and Fe2O3. However, g-C3N4 is used as a support and the composites are constructed by decorating the nanoparticles onto g-C3N4 [129]. The photo-­ electrochemical performance of the hetero-junction is owing to the efficient separation of (e−/h+) pair [130]. Due to the beneficial e− transfer property of CDs, H2 production rates increased to ~75 and 246 μmolg−1h−1 for TiO2NP/CDs and NT/CD, respectively [131]. The modification of photocatalyst with CSs ameliorates the separation efficiency of photo-generated (e−/h+) pair. Numerous photo-catalysts have been connected onto CSs, such as Bi2WO6 [132], CuO [133], and TiO2 [122]. Fe2O3-CSs showed H2 amount ~370 mol/g.s compared to Fe2O3/CSs (179 mol/g.s), Fe2O3 (147 mol/g.s) and CSs (281 mol/g.s) [118]. When hv ≥ Eg is absorbed, e− from − and h +VB able to VB is promoted to CB leaving a hole in the VB. The eCB decompose water into hydrogen:



+ Fe 2O3 + hv → e − BC(Fe2 O3 ) + t BV (Fe2 O3 )

(4.1)



− − eBC ( Fe2 O3 ) + CSs → e CSs

(4.2)

In electrolyte, when the material irradiated, H2 generated with simultaneous oxidation of sulfite to sulfate ions [120].



+ SO32− + 2h VB + 2OH − → SO 24− + H 2O



2SO32− + 2h +VB → S 2O62−



E° = −1.17 V (4.3)

E° = − 0.49 V

− 2H + + 2eCB → H2

(4.4) (4.5)

Zhang et al. established a new flame assisted hydrolysis methods to elaborate microspherical carbon-incorporated TiO2 photo-catalysts [122]. The highest amount of hydrogen produced is 8.1 µmol/h, 1.8 times greater than TiO2 with a similar result compared to G/TiO2 [48]. The enhanced ­photo-catalytic reactivity is attributed to carbon properties, and high photon absorption [134]. CNTs-based semiconductors have been enhanced the separation and the recombination of (e−/h+) pair efficiency. We cited the case of CdS [135], CdSe [136], and TiO2 [137].

90  Layered 2D Advanced Materials and Their Allied Applications

4.5 Carbon-Based Materials for CO2 Reduction Numerous studies investigated GO for the photo-reduction of carbon dioxide (CO2) to fuels as methane (CH4), carbon monoxide (CO), H2, etc. Photo-reduction of CO2 into fuels is interesting strategy for simultaneously harvesting solar energy and capturing the greenhouse gas. rGO-TiO2 hybrid nanocrystals elaborated via solvothermal method exhibited an efficient photo-catalytic performances [138]. Whereas CO2 can be converted into CH4 using rGO-TiO2 photo-catalyst and into CO over rGO-Cu2O nanocomposites [139]. Liang and coauthors shown that G-TiO2 nanocomposites with low defect densities in G are significantly superior compared to TiO2 [139]. Another study used 2D sandwich G-TiO2 investigated the synthesis of CH4 and C2H6 in homogenous media [140]. G-WO3 synthesized by hydrothermal technique are also investigated for the photo-­reduction of CO2 to CH4 [141]. So, the production rate of CH4 is around 0.11 μ mol/h. Hollow spheres with alternating titania and G-nanosheets are elaborated and tested in the photo-reduction of CO2. It is shown that the rates of CO and CH4 are 8.91 μmol/g.h and 1.14 μmol/g.h, respectively [142]. Yu and coauthors reported that CdS nanorods and rGO is used as photo-catalyst for the transformation of CO2 into CH4 and into a trace amount of CH3OH [142]. The modification of g-C3N4 with Pt improved photo-catalytic performances [143]. Also, Au and Ag nanoparticles exhibited enhanced performance through CO2 photo-reduction to CH3OH, HCOOH, and CH4 [144]. The CH4 and CO efficiency is 1.53 and 10.05 μmol/g.h over TiO2/ g-C3N4 photo-catalyst [145].

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5 Sensitivity Analysis of Surface Plasmon Resonance Biosensor Based on Heterostructure of 2D BlueP/MoS2 and MXene Sarika Pal1*, Narendra Pal2, Y.K. Prajapati3 and J.P. Saini4 Department of Electronics and Communication Engineering, National Institute of Technology, Uttarakhand, India 2 Care Manager, Nokia Services Networks, Dehradun, India 3 Department of ECE, Motilal Nehru National Institute of Technology, Allahabad, India 4 Department of ECE, Netaji Subhas University of Technology (NSUT), New Delhi, India 1

Abstract

This work presents the brief discussion about the unique properties of 2D materials in layered form and their ability to exfoliate or restack them for making van der Waals heterostructure. Recently, MXene-based heterostructures are appealing to design electronic and optoelectronic. So, we proposed surface plasmon resonance sensor based on heterostructure of BlueP/MoS2 and MXene (Ti3C2Tx) for biosensing. Unique properties of MXene like its layered architecture, larger surface area, highly accessible hydrophilic surface; chemical stability, smaller work function, and strong light matter interaction are utilized to enhance the sensitivity of proposed sensor. Highest sensitivity of 203ο/RIU is obtained for monolayer MXene (Ti3C2Tx) and three layers of BlueP/MoS2 for Ag metal layer and CaF2 prism. Sensitivity variation for the proposed sensor on varying number of BlueP/ MoS2 and MXene (Ti3C2Tx) is also observed. The performance of proposed sensor is checked for different prism material and metal layer at 633-nm operating wavelength. These results will open an innovative route to design and develop

*Corresponding author: [email protected]; [email protected] Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (103–130) © 2020 Scrivener Publishing LLC

103

104  Layered 2D Advanced Materials and Their Allied Applications SPR biosensor practically with fabrication possibilities of MXene (Ti3C2Tx) with BlueP/MoS2. Keywords:  Two-dimensional (2D) materials, MXene (Ti3C2Tx), Blue Phosphorene (BlueP), Transition Metal Dichalcogenides (TMDCs), Surface plasmon resonance (SPR), Sensitivity (S), Heterostructures

5.1 Introduction Since the discovery of graphene, two-dimensional layered materials (2DLMs) have been central tool of research and investigation for their applications in electronics and optoelectronics [1–3]. The reason behind their importance is their unique electronic, optical, physical, and mechanical properties in layered form rather than in bulk form [4, 5]. Graphene in layered form offers superior physical property (high surface to volume ratio), exceptional transport of charge carriers (high mobility), zero band gap, and partial optical and electrostatic transparency with tunable work function making it suitable for its use in van der Waals (vdW) heterostructure [4–8]. Transition metal di-chalcogenides (TMDCs) of from MX2 where M stands for metal (Molybednum or Tungsten) and X refers chalcogenide (Sulphur, Selenium, or Tellurium) like MoS2, MoSe2, WS2, and WSe2 offer tunable optical and electronic properties in layered from [9–12]. Black phosphorus in layered form has found profound application in sensing, vdW heterostructures, etc., due to its high carrier mobility, tunable band gap lying in between graphene and TMDCs, tunable work function and good optical absorption [13–15]. A new class of 2DLMs known as MXenes synthesized from MAX phases have found insightful applications in FET, transparent conductor, electromagnetic interference, energy storage, water purification, thermoelectric, gas and pressure sensor, p ­ hoto-electro catalysis, and plasmonics [16–33]. MXene are two-dimensional transition metal carbide, nitride, or carbonitride with chemical formula Mn+1XnTx, M refers transition metal (Ti, Sc, V, Zr, Cr, Nb, Mo, Hf, Ta), X refers (C or/and N), and Tx refers surface termination (such as O, OH, or F group) [34]. They are synthesized by removing “A” element from MAX phase compounds where A refers group 13–16 elements (Si, Ge, Al, P, Ga, S, As, In, Sn) from periodic table. MXenes show excellent conductivity due to free electrons of transition metal carbide or nitride and hydrophilic nature due to surface termination groups 4. Experimental and theoretical studies on MXene’s suggest for its profound use as 2DLMs due to attractive unique physical, chemical, mechanical, electrical, electronic, optical, and plasmonic

Surface Plasmon Resonance Biosensor  105 properties [17, 24, 34–36]. Now, we will discuss above mentioned properties of MXene in layered form along with applications in different field. Physical, chemical, and mechanical properties: MXenes are chemically, physically, and mechanically stable due its ceramic nature. MXenes are hexagonal lattices where inner Ti-C is bonded with outer Ti- O/OH/F (surface termination groups) ones [37]. Interlayer coupling is significantly stronger than vdW forces though surface termination groups weaken interlayer coupling. Interlayer coupling is stronger in OH terminated surfaces due to formation of hydrogen bond. Electronic Functionality: Pristine MXenes are metallic, but on surface functionalization, it becomes semiconducting. Surface functionalization tunes bandgap, work function, transport of charge carriers, and electronic conductivity of MXenes. On termination of metal surface Fermi level shifts to lower value due to reduced density of state of surface transition metal when electrons are transferred from transition metal to electronegative surface terminations [38, 39]. MXenes found applications as switching device, transparent conductors, and conductive filters due to its high electronic conductivity and as field emitter cathodes in FETs due to ultra low work function of OH terminated surfaces [17–18, 37–40]. Enhanced electronic functionality is achievable due to tuning of number of valence electrons and relativistic spin orbit coupling for complex material MXenes. Optical Properties: Some of theoretical and experimental studies suggests: MXene (Ti3C2Tx) show 77% transmittance in visible range, intercalated MXene (Ti3C2Tx) with NH4HF4 show 99% transmittance and absorbance depends linearly on thickness of MXene and intercalated film [41]. Plasmonic Properties: MXene (Ti3C2Tx) found its promising use in surface enhanced Raman spectroscopy, metamaterial absorber, energy harvesting, sensing and biomedical imaging applications due to attractive plasmonic properties [32–36]. Its plasmonic behavior can be tuned via its surface terminations or changing number of layer for multilayered MXene (Ti3C2Tx). Exploiting these novel properties of MXene, exploration of vdW heterostructure is possible to design new electronic and optoelectronic devices for future application. Recent studies show that MXene a 2DLM can be assembled layer by layer with MXene itself or other 2DLMs and resulting in vdW heterostructures [42]. Monolayer of 2DLMs consisting of single or few atom thickness is covalently bonded dangling bond free lattice and each layer is bonded to its neighbouring layer by weak vdW forces [43, 44]. This leads us to integrate different incompatible 2DLMs without constraint of lattice matching for creating vdW heterostructures. Vertical stacking of

106  Layered 2D Advanced Materials and Their Allied Applications different 2DLMs facilitates formation of vdW heterostructure which has given us intriguing possibilities of controlling and manipulating the unique properties obtained that are different from constituent layered material composition [44]. There are two primary strategies: Top down and bottom up approaches for preparation of single or multi-layer of 2DLM. Top down synthesis is direct exfoliation of bulk crystal through mechanical or chemical exfoliation. The bottom up synthesis gives atomic level control of their composition and morphology to create vdW heterostructure of 2DLMs. Bottom up synthesis have got success in producing 2DLMs like graphene, boron nitride and various TMDCs and their alloys with high quality and uniformity using chemical vapor deposition (CVD) processes [45, 46]. But this approach becomes secondary strategy for creating complex vdW heterostructures due to highly sensitive growth condition for producing each 2DLM which may damage prior layer [45, 46]. However, mechanical stacking approach with relative alignment error less than 1% between layers or restacking offers a new degree of freedom to design complex vdW heterostructures of 2DLMs produced through either of top down or bottom up synthesis [44]. But mechanical stacking of 2DLMs does not necessarily give strong interlayer coupling. However, interlayer distance can be reduced through proper thermal annealing which induces interlayer coupling. Layer coupling can also be induced between layers of different 2DLM or multilayers of same 2DLM through introduction of dielectric between them [47]. Combination of different 2DLMs give rise to formation of vdW heterostructure with improved electronic and optoelectronic properties due to mutual interaction between different layers [48]. On investigating graphene-based heterostructures, it was found that improved electronic properties were obtained in comparison to individual graphene [5]. Use of Black phosphorus/graphene heterostructure in sodium ion batteries shows much higher capacity [8]. TMDC-based wdW heterostructure creates increasing research applications due to extraordinary electronic and optoelectronic properties [9–12]. Peng et al. investigated stacking characteristics, charge density distributions, and optical properties of Blue phosphorene (BlueP)/TMDC vdW heterostructure using density functional theory with perfect lattice match between BlueP and TMDCs [49]. Experimental and theoretical studies suggest that MXene/TMDC heterostructure has been analyzed frequently due to close lattice matching of MXene and TMDCs [50]. Recently, heterostructure of MXene and Blue phosphorene (BlueP) possessing small lattice mismatch and negative binding energy indicates strong interaction with metallic induced behavior in band diagram of Blue P [42, 43, 49]. MXene have established its intense use in biomedical applications like diagnostic imaging, biosafety evaluation,

Surface Plasmon Resonance Biosensor  107 antimicrobial, and biosensing with grand interdisciplinary research [50– 57]. Fascinating electronic, physiochemical, hydrophilic nature, and plasmonic properties of layered MXenes engrossed researchers for its immense applications in biosensor, label-free analysis of biological events, surface plasmon resonance, and localized surface plasmon resonance [32, 35, 50, 58, 59]. 2DLMs like graphene, TMDCs, few layer phosphorene, metal oxide, and their heterostructures have found appealing use in surface plasmon resonance biosensor with day-by-day progression in technology [60]. Here, we investigate vdW heterostructures of BlueP/MoS2 and MXene (Ti3C2Tx) in surface plasmon resonance biosensor.

5.2 Proposed SPR Sensor, Design Considerations, and Modeling This section describes about SPR sensor and it sensing principle, sensor design, and modeling of the proposed SPR biosensor. At the end of this section, different formula used for calculating performance parameters for the proposed sensor will be discussed.

5.2.1 SPR Sensor and Its Sensing Principle SPR sensors are preferred kind of optical sensor for chemical, biochemical, gas and biosensing due to its real-time, label-free, reliable and fast detection capabilities [60–64]. For biosensing SPR signal detects change in biological activity near sensing region as these signals are much sensitive to change in surrounding environment. Surface plasmons are TM or p-polarized charge density oscillations propagating alongside metal/dielectric interface with electric field which decay exponentially in both medium. Surface plasmon wave (SPW) can be generated by coupling of p-polarized incident light with SPs generated at metal/dielectric interface. Various methods of light coupling like prism coupling, grating coupling, waveguide coupling can be used in SPR sensor to achieve resonance condition [65]. Prism coupling along with total attenuated reflection as per Kretschmann configuration is mostly preferred in SPR sensor. In this configuration, a thin metal layer is laid on flat face of prism over which dielectric layer to be sensed get in touch with it. On impinging p-polarized light on one of the side face of prism at an angle more than critical angle, it penetrates metal layer evanescently and SPW is excited resulting in transfer of energy from incident light to SPW. This is known as surface plasmon resonance condition, which results in zero or minimum reflection intensity when measured through

108  Layered 2D Advanced Materials and Their Allied Applications photodetector from other side of prism. The angle at which SPR condition is achieved is resonance angle and its value depends on refractive index of dielectric placed over metal layer. When liquid sample containing biomolecule make contact with metal surface, it changes the refractive index of sample due to adsorption of biomolecules on metal surface resulting in shifting of resonance angle. To check the performance of SPR, resonance curve (refection intensity vs. incident angle) is plotted.

5.2.2 Design Consideration The proposed sensor is based on Kretschmann configuration as shown in Figure 5.1. The operating wavelength of the proposed design is 633 nm to obtain the highest sensitivity with insignificant Kerr effect [61]. Layerwise description of the proposed design is as follows.

5.2.2.1 Layer 1: Prism for Light Coupling To analyze its performance resonance curve are plotted analytically, ATR geometry is used. Abdelghani et al. (1997) analyzed that sensitivity of SPR sensor can be varied on changing prism refractive index without changing other parameters [66]. This work considers three different prisms: SF-10, BK-7, and CaF2 prism having RI 1.723, 1.515, and 1.4329, respectively. Prism thickness used here for the proposed design is zero while the practical thickness of the substrate can be chosen around 25 nm.

Sensing region (nc = 1.33-1.335) MXene (Ti3C2Tx) layer BlueP/MoS2layer Metal layer

θ Prism

Figure 5.1  Schematic of proposed SPR biosensor.

Surface Plasmon Resonance Biosensor  109

5.2.2.2 Layer 2: Metal Layer In SPR sensor, different metals (Au, Ag, Al, Cu, In, Na, etc.) can be deposited on hypotenuse of the prism for SPs generation but every metal affects SPR sensor performance differently. Gold (Au) is the most suitable metal for SPs generation as it is biocompatible with high stability and durability and can avoid oxidation also. Silver (Ag) is also preferable to get sharp resonance curves if it is coated with some dielectric layer to prevent oxidation [62]. Aluminum (Al) exhibits narrower reflectance in comparison to Au and Ag but susceptible to oxidation which can be prevented by placing another dielectric layer on it. So other metals are rarely used as indium (In) is expensive, sodium (Na) is reactive, and silver (Ag), aluminum (Al), and copper (Cu) are prone to oxidation. The proposed sensor performance is analyzed for four: metals Silver, Gold, Aluminum, and Copper. The dielectric constant of these four metal layers is calculated from Drude Model given as:

  λ 2 λc ε metal = 1 −  2   λ p (λc + i × λ ) 



(5.1)

where λp and λc are plasma wavelength and collision wavelength, respectively, and their values attained for metals (Ag, Au, Al, and Cu) are shown in Table 5.1. The optimized thickness for each metal is calculated after plotting a curve between change in resonance angle, minimum reflectance (Rmin) as a function metal thickness for proposed design as shown in Figures 5.5 and 5.6. Table 5.1  Plasma and collision wavelength for Au, Ag, Al, and Cu metal as per Drude Model. Different metals

Au metal

Ag metal

Al metal

Cu metal

Plasma wavelength (λp)

1.6826 × 10−7 m

1.4541 × 10−7 m

1.0657 × 10−7 m

1.3617 × 10−7 m

Collision wavelength (λc)

8.9342 × 10−6 m

1.7614 × 10−5 m

2.4511 × 10−5 m

4.0852 × 10−5 m

110  Layered 2D Advanced Materials and Their Allied Applications

5.2.2.3 Layer 3: BlueP/MoS2 Layer BlueP and MoS2 are 2DLM having lattice constant of 3.268 Å and 3.164 Å, respectively, used for formation of vdW heterostructures with interlayer mismatch of 3.168% [49]. Both BlueP and MoS2 are monolayer having hexagonal lattice structure and each layer is bonded through vdW forces. MoS2 is placed over top of BlueP to prevent it for oxidation. The use of BlueP/ MoS2 heterostructure in SPR enhances the adsorption of biomolecules due to stronger vdW forces and thus increasing the field at interface. In this way, this heterostructure improves the sensor performance of proposed design. The refractive index and monolayer thickness for BlueP/MoS2 is 2.7915+i*0.335 and 0.75 nm thickness, respectively, at 633 nm operation wavelength [64].

5.2.2.4 Layer 4: MXene (Ti3C2Tx) Layer as BRE for Biosensing MXene (Ti3C2Tx) exhibits plasmonic property, a useful material as 2DLM for SPR sensors and surface enhanced Raman spectroscopy applications [32, 58]. There are intense surface plasmons in multilayered MXene (Ti3C2Tx) possessing energy from 0.3–1 eV in comparison to bulk form. So it is new 2D layered biosensing material with extraordinary properties of tunable bandgap, tunable work function which can be accustomed as per surface terminations, chemical, and physical stability and larger specific area to attach biomolecules. Larger surface area and its hydrophilic nature enhances the adsorption of biomolecules on it, and due to this, it can be used as bio recognition element (BRE) layer for biosensing. The refractive index and monolayer thickness for MXene (Ti3C2Tx) is 2.39+i*1.33 and 0.993 nm in visible range at 633 nm operating wavelength [58].

5.2.2.5 Layer 5: Sensing Medium (RI-1.33-1.335) For better adsorption of biomolecules, aqueous solution with RI 1.33 to 1.335 is chosen as the sensing medium. Adsorption of biomolecules on sensing surface changes carrier concentration of the aqueous solution resulting in modification of the RI of the sensing medium.

5.2.3 Proposed Sensor Modeling Proposed SPR sensor is modeled through N-layer modeling by means of transfer matrix method [63]. The kth layers with dielectric constant (εk), thicknesses (dk), and RI (nk) are stacked along the z-axis, respectively.

Surface Plasmon Resonance Biosensor  111 On  applying boundary condition, the tangential fields at Z = Z1 = 0 are related with tangential field at Z = ZN−1 as follows:

 U N −1  = M  VN −1 

 U1   V1



  

(5.2)

where U1, UN−1, and V1, VN−1 are the tangential components of the electric and magnetic fields, respectively, at the boundary of the 1st layer and at the Nth layer. The characteristic matrix (Mij) can be given as:

 Mij = 



∏

 M11  Mk  =  k=2 ij  M 21 N −1

M12   M 22 

(5.3)

with,

 cos β k Mk =   −iqk sin β k



(−i sin β k )/qk   cos β k 

(5.4)

where k lies from 1 to N.

µ  qk =  k   εk 



1/ 2

cosθ k

(ε =

k

− n12 sin 2 θ1 εk

)

1/ 2

(5.5)



and



βk =

2π 2π dk nk cosθ k (z k − z k−1 ) = ε k − n12 sin 2 θ1 λ λ

(

)

1/ 2



(5.6)

Reflection coefficient for TM or P-polarized light is as given below:



rp =

( M11 + M12qN )q1 − ( M 21 + M 22qN ) ( M11 + M12qN )q1 + ( M 21 + M 22qN )

(5.7)

112  Layered 2D Advanced Materials and Their Allied Applications where q1 and qN are the relative components of the 1st and the Nth layer, respectively, and can be calculated from Eq. (5.3). The reflectivity Rp for this N-layer structure is given as:

Rp = |rp|2

(5.8)

The performance of proposed SPR biosensor is calculated from SPR curves by calculating sensitivity (S), detection accuracy (DA), and Figure of Merit (FoM). Sensitivity (S) is ratio of shift in resonance angle (∂θr = θ2 − θ1) to difference in refractive index (RI) of sensing medium (∂ns = n2 − n1), i.e., aqueous solution after DNA hybridization, given as S = ∂θr/∂ns (ο/RIU) [63]. Here, (θ1, θ2) and (n1, n2) are resonance angle and RI of aqueous solution before and after adsorption of biomolecules, respectively. Detection Accuracy (DA) is the ratio of shift in resonance angle (∂θr) to the Full width at half maximum (FWHM) of SPR curve, which is given as DA = 1/FWHM (unit less quantity) [64]. The FWHM refers the spectral width of the resonance curve at 50% reflectivity. Figure of Merit (FoM) also gives quality factor which is the product of sensitivity to DA and is given as: FoM = S*DA (RIU−1).

5.3 Results Discussion This section is divided into three subsections for proper explanation of results obtained for proposed design. Its first subsection, i.e., 5.3.1, will analyze and compare the role of BlueP/MoS2 and MXene (Ti3C2Tx) monolayer for proposed SPR design by calculating its performance parameters. Second subsection, i.e., 5.3.2, will analyze influence of changing number of BlueP/MoS2 and MXene (Ti3C2Tx) layers for proposed sensor design. Its third subsection, i.e., 5.3.3, will describe effect different prism material and metal layer on for different number of hetreojunction layer on performance of proposed SPR biosensor design.

5.3.1 Role of Monolayer BlueP/MoS2 and MXene (Ti3C2Tx) and Its Comparison With Conventional SPR In this section, resonance curve as per Figure 5.1 (i.e., proposed SPR biosensor design) are plotted based on the modeling discussed in subsection 5.2.3. First of all, we have plotted resonance curve for conventional SPR sensor which consists of prism, metal layer and dielectric or sensing medium with

Surface Plasmon Resonance Biosensor  113 Conventional SPR sensor

Reflection Intensity (a.u.)

1

0.8 0.6 nc = 1.33 0.4 0.2

Reflection Intensity (a.u.)

0 65

nc = 1.335 S = 217 (°/RIU) DA = 0.478 (1/°) FoM = 104.2 (1/RIU) 70

75

85

90

Ag-MXene (Ti3C2Tx) based SPR sensor

1 0.8 0.6 0.4 0.2 0 65

S = 240 (°/RIU) DA = 0.313 (1/°) FoM = 75 (1/RIU) 70

nc = 1.33 nc = 1.335 75

1

Reflection Intensity (a.u.)

80

Angle (°) (a)

80

Angle (°) (b)

85

90

Proposed SPR sensor

0.8 0.6 0.4 0.2 0 65

S = 264.6 (°/RIU) DA = 0.279 (1/°) FoM = 73 (1/RIU) 70

nc = 1.33 nc = 1.335 75

80

Angle (°) (c)

85

90

Figure 5.2  Reflectance curve for (a) Conventional SPR biosensor, (b) Ag- MXene (Ti3C2Tx) SPR biosensor, (c) Proposed SPR biosensor.

114  Layered 2D Advanced Materials and Their Allied Applications refractive index 1.33. For this, we have used CaF2 prism and silver metal at 633 nm operating wavelength to get highest sensitivity with reduced FWHM. Resonance curve obtained for conventional SPR sensor with sensing medium refractive index variation of 1.33–1.335 on adsorption of biomolecules is shown in Figure 5.2a. The resonance angle and minimum reflectance (Rmin.) obtained for sensing medium RI 1.33 and 1.335 are [77.608ο, 0.0013 a.u.] and [78.698ο, 0.0002 a.u.], respectively. Resonance angle shift and FWHM obtained is 1.089ο and 2.09, respectively. Sensitivity, DA and FoM calculated from resonance angle shift and FWHM for conventional SPR are 217ο/RIU, 0.478 Degree−1, and 104.2 RIU−1, respectively which is also marked in Figure 5.2a. Thereafter, monolayer MXene (Ti3C2Tx) is laid over silver metal of conventional SPR to check its role and resonance curve is plotted as shown in Figure 5.2b. Resonance angle and minimum reflectance obtained at sensing medium RI 1.33 and 1.335 is [78.746ο, 0.1375 a.u.] and [79.947ο, 0.1592 a.u.], respectively. The resonance angle shift and FWHM obtained are 1.201ο and 3.2, respectively. It is clearly indicated from Figure 5.2b that sensitivity is increased up to 240ο/RIU while DA and FoM decreases to 0.313 Degree−1, and 75 RIU−1, respectively. The decrease in DA and FoM is due to lossy nature of MXene (Ti3C2Tx) because of larger value of imaginary part of its refractive index. Introduction of MXene over conventional SPR enhances sensitivity due to better binding of biomolecules at larger surface are of MXene (Ti3C2Tx). Now, reflectance curve is plotted for proposed sensor design (Figure 5.1) which consists of CaF2 Prism, Silver layer, BlueP/MoS2, MXene (Ti3C2Tx), and sensing medium as shown in Figure 5.2c. The resonance angle and minimum reflectance obtained is [79.845ο, 0.1984 a.u.] and [81.168ο, 0.2324 a.u.], respectively. The resonance angle shift and FWHM is 1.323ο and 3.58, respectively. Sensitivity (264.6ο/RIU), DA (0.279 Degree−1) and FoM (73 RIU−1) are calculated from above values for Figure 5.2c. It can be concluded from above results much higher sensitivity is achieved for proposed SPR biosensor in comparison of conventional SPR but DA and FoM is reduced. Sensitivity improvement of proposed SPR biosensor is due to use of heterostructure of BlueP/MoS2 and MXene (Ti3C2Tx) which enhance the field at interface due to better confinement of charge carriers and shifts the SPR resonance angle.

5.3.2 Influence of Varying Heterostructure Layers for Proposed Design In this subsection, the performance of proposed SPR design is checked by changing number of BlueP/MoS2 and MXene (Ti3C2Tx) layers keeping one of them fixed as monolayer. Figure 5.3 presents reflectance curve for monolayer MXene (Ti3C2Tx) and varying number of BlueP/MoS2 (L)

Surface Plasmon Resonance Biosensor  115 Varying BlueP/MoS2 layers (L)

1

Reflection Intensity (a.u.)

for Proposed SPR sensor 0.8

0.6

0.4

0.2

0 70

S = 264.6 °/RIU (L= 1) S = 292 °/RIU (L = 2) S = 302 °/RIU (L = 3) S = 225 °/RIU (L = 4) 75

L=1 L=2 L=3 L=4 80 Angle (°)

85

90

Figure 5.3  Reflectance curve for proposed SPR biosensor on varying layers of BlueP/MoS2 keeping monolayer of MXene (Ti3C2Tx).

layers upto 4. It clearly reflects that resonance angle increases on increasing trend for number of BlueP/MoS2 layers. On calculating sensitivity from Figure 5.3 in similar way as we have calculated it in subsection 5.3.1, we get sensitivity of 264.6ο/RIU (L = 1), 292ο/RIU (L = 2), 302ο/RIU (L = 3) and 225ο/RIU (L = 4). So, sensitivity first increases on increasing number of BlueP/MoS2, thereafter decreases due to limitation of angular range. Highest sensitivity of 302ο/RIU is obtained for three layers of BlueP/MoS2. Now, Figure 5.4 shows reflectance curve for proposed design on varying MXene (Ti3C2Tx) layers and keeping monolayer BlueP/MoS2. It is indicated that resonance angle goes to higher value on increasing number of MXene (Ti3C2Tx) layers (M). We get sensitivity of 264.6ο/RIU (M = 1), 274ο/RIU (M = 2), 222.4ο/RIU (M = 3), and 131.4ο/RIU (M = 4). Sensitivity first increases on increasing number of MXene (Ti3C2Tx) layers, thereafter, it decreases for higher number of MXene layers. Highest sensitivity of 274ο/RIU is achieved for two layers of MXene. The resonance angles at ns = 1.33–1.335, resonance angle shift and sensitivities obtained for both Figures 5.3 and 5.4 are summarized in Table 5.2.

5.3.3 Effect of Changing Prism Material and Metal on Performance of Proposed Design As we know that sensing capability of SPR sensor is dependent on thickness and refractive indices of various layers used, so in this subsection,

116  Layered 2D Advanced Materials and Their Allied Applications

Reflection Intensity (a.u.)

1

Varying MXene Ti3C2Tx layers (M) for Proposed SPR sensor

0.8 0.6 0.4 0.2 0 70

M=1 M=2 M=3 M=4

S = 264.6 °/RIU (M = 1) S = 274 °/RIU (M = 2) S = 222.4 °/RIU (M = 3) S = 131.4 °/RIU (M = 4) 75

80 Angle (°)

85

90

Figure 5.4  Reflectance curve for proposed SPR biosensor on varying layers of MXene (Ti3C2Tx) keeping monolayer of BlueP/MoS2.

we investigated the effect of using three different prisms BK-7, SF-10, CaF2, and four metal (Au, Ag, Al, Cu) layers on performance of proposed design. The refractive indices used for BK-7, SF-10, CaF2 prism is 1.515, 1.723, and 1.4329, respectively. First of all metal layer thickness is optimized for each of the prism by plotting curves (i) minimum reflectance (Rmin.) as a function of metal layer thickness (ii) resonance angle shift corresponding to metal layer thickness as shown in Figure 5.5 and Figure 5.6, respectively. Figures 5.5a–c indicates variation of minimum reflectance (Rmin.) for Au, Ag, Al, and Cu layer thickness variation from 25 nm to 60 nm for BK-7, SF-10, and CaF2 Prism respectively. Figures 5.5a–c explains that in general minimum reflectance (Rmin.) first decrease on increasing metal layer thickness becomes minimum and then increases again. Similarly, Figures 5.6a–c indicate resonance angle shift for Au, Ag, Al, and Cu layer thickness variation from 25 nm to 60 nm for BK-7, SF-10, and CaF2 Prism, respectively. Generally, it is seen that change in resonance angle increases for higher thickness of metal layer for most of the metal layers for BK-7 and SF-10 prism, while for CaF2 prism, resonance angle increases become maximum and then starts decreasing for gold layer. Maximum resonance angle shift indicates higher sensitivity and decreasing trend in resonance angle is obtained due to limitation of angular range. The optimized thickness of metal layer is chosen at which Rmin is at minimum value as it gives narrower FWHM and optimum sensitivity. So, optimized thickness of Au, Ag, Al, Cu metal layer for BK-7, SF-10, and CaF2 prism is (42 nm, 41 nm,

θr1 (ο) at ns = 1.33

79.845

81.096

82.498

83.897

θr1 (ο) at ns = 1.33

79.845

81.189

82.347

82.954

Number of BlueP/MoS2 layers (L)

L=1

L=2

L=3

L=4

Number of MXene (Ti3C2Tx) layers (M)

M=1

M=2

M=3

M=4

83.611

83.459

82.562

81.168

θr2 (ο) at ns = 1.335

85.022

84.012

82.559

81.168

θr2 (ο) at ns = 1.335

0.657

1.112

1.373

131.4

222.4

274

264.6

Sensitivity (ο/RIU)

δθr = (θr2-θr1) (Resonance angle shift) 1.323

225

302

292

264.6

Sensitivity (ο/RIU)

1.125

1.514

1.463

1.323

δθr = (θr2-θr1) (Resonance angle shift)

Table 5.2  Resonance angle shift and sensitivities obtained for different number of BlueP/MoS2 and MXene (Ti3C2Tx) layer keeping one of them monolayer for proposed SPR design.

Surface Plasmon Resonance Biosensor  117

118  Layered 2D Advanced Materials and Their Allied Applications

Minimum Reflectance (a.u.)

0.9 (a) For BK-7 prism

0.8 0.7

Gold (Au) layer Silver (Ag) layer Aluminum (Al) layer Copper (Cu) layer

0.6 0.5 0.4 0.3 0.2 0.1 0

25

35 45 Metal layer thickness (nm)

55

Minimum Reflectance (a.u)

1 (b) For SF-10 prism

0.9 0.8

Gold (Au) layer

0.7

Silver (Ag) layer

0.6 0.5

Aluminum (Al) layer

0.4

Copper (Cu) layer

0.3 0.2 0.1 0 25

35 45 Metal layer thickness (nm)

Minimum Reflectance (a.u.)

0.9

55

(c) For CaF2 prism

0.8 0.7

Gold (Au) layer Silver (Ag) layer Aluminum (Al) layer Copper (Cu) layer

0.6 0.5 0.4 0.3 0.2 0.1 0 25

30

35 40 45 50 Metal layer thickness (nm)

55

60

Figure 5.5  Variation of minimum reflectance (Rmin.) with respect to Au, Ag, Al, Cu metal layer thickness for (a) BK-7 prism, (b) SF-10 prism, (c) CaF2 prism for proposed SPR biosensor.

Resonance angle shift (Degree)

Surface Plasmon Resonance Biosensor  119 0.85 0.75 0.7

Resonance angle shift (Degree)

(a) For BK-7 prism

0.65 0.6 0.55 0.5 0.45 0.4

25

0.45

30

35 40 45 50 Metal layer thickness (nm)

Gold (Au) layer Silver (Ag) layer Aluminum (Al) layer Copper (Cu) layer

0.43 0.41 0.39

55

60

(b) For SF-10 prism

0.37 0.35 0.33 0.31 0.29 0.27 0.25

Resonance angle shift (Degree)

Gold (Au) layer Silver (Ag) layer Aluminum (Al) layer Copper (Cu) layer

0.8

25

30

35 40 45 50 Metal layer thickness (nm)

55

60

2 1.8 1.6 1.4

Gold (Au) layer Silver (Ag) layer Aluminum (Al) layer Copper (Cu) layer

(c) For CaF2 prism

1.2 1 0.8 0.6 0.4 25

30

35 40 45 50 Metal layer thickness (nm)

55

60

Figure 5.6  Variation of resonance angle shift with respect to Au, Ag, Al, Cu metal layer thickness for (a) BK-7 prism, (b) SF-10 prism, (c) CaF2 prism for proposed SPR biosensor.

120  Layered 2D Advanced Materials and Their Allied Applications 31 nm, 42 nm), (43 nm, 41 nm, 30 nm, 41 nm) and (36 nm, 40 nm, 32 nm, 41 nm), respectively. These values are also indicated in Table 5.3. Now, sensitivity variation with respect to number of BlueP/MoS2 for monolayer MXene (Ti3C2Tx) is plotted for proposed SPR sensor design in Figures 5.7a–c for BK-7, SF-10, and CaF2 prism. It is clearly observed from Figure 5.7a that for BK-7 prism, highest sensitivities of 184.7ο/RIU, 184.5ο/ RIU, 130.9ο/RIU, 185.1ο/RIU is observed for Au, Ag, Al, Cu metal layer for 7, 11, 12, and 12 layers of BlueP/MoS2, respectively. Similarly, from Figure 5.7b, we get highest sensitivities for SF-10 prism of 116.9ο/RIU, 116.4ο/RIU, 85.6ο/RIU, 112.5ο/RIU for Au, Ag, Al, Cu metal layer for 15, 20, 20, and 20 layers of BlueP/MoS2, respectively. Similarly, it is can be clearly observed from Figure 5.7c that for CaF2 prism, highest sensitivities of 275.4ο/RIU, 284.4ο/RIU, 170.6ο/RIU, 272.9ο/RIU are observed for Au, Ag, Al, Cu metal layer for 1, 3, 4, and 4 layers of BlueP/MoS2, respectively. For easier analysis, all this interpretation from Figure 5.7 is shown in Table 5.4. Lastly, sensitivity variation with respect to number of MXene (Ti3C2Tx) for monolayer BlueP/MoS2 is plotted for proposed design in Figures 5.8a–c for BK-7, SF-10, and CaF2 prism. In Figure 5.8a for BK-7 prism, we get highest sensitivities of 157ο/RIU, 147.3ο/RIU, 138.4ο/RIU, 146.5ο/RIU for (Au, Ag, Al, Cu) metal layers for (4, 7, 12, 8) layers of MXene (Ti3C2Tx). In Figure 5.8b for SF-10 prism, we get highest sensitivities of 94.5ο/RIU, 93.7ο/RIU, 100.7ο/RIU, 94.7ο/RIU for (Au, Ag, Al, Cu) metal layers for (10, 14, 20, 16) layers of MXene (Ti3C2Tx). Similarly, in Figures 5.8c for CaF2 prism, we get highest sensitivities of 275.4ο/RIU, 253.9ο/RIU, 185.3ο/RIU, 241.1ο/RIU for (Au, Ag, Al, Cu) metal layers for (1, 2, 4, 3) layers of MXene (Ti3C2Tx). All results obtained of highest sensitivities for different number of MXene (Ti3C2Tx) layers are summarized in Table 5.5 for easier analysis. Tables 5.4 and 5.5 tell that higher number of BlueP/MoS2 and MXene (Ti3C2Tx) layers are required to get higher sensitivities for Al layer due to reduced optimized thickness of Al layer. These results are well justified as per the results produced by S. Pal et al. [15] and Ouyang et al. [12] which show that Table 5.3  Optimized metal layer thicknesses for different prism. Type of prism

Au thickness (nm)

Ag thickness (nm)

Al thickness (nm)

Cu thickness (nm)

BK-7 prism

42

41

31

42

SF-10 prism

43

41

30

41

CaF2 prism

36

40

32

41

Surface Plasmon Resonance Biosensor  121 200 (a) For BK-7 prism

Sensitivity (Degree/RIU)

180 160 140 120 100 80

S-Au layer S-Ag layer S-Al layer S-Cu layer

60 40 20 0

0

5 Number of BlueP/MoS2 layer

10

Sensitivity (Degree/RIU)

140 (b) For SF-10 prism

120 100 80 60

S-Au layer S-Ag layer S-Al layer S-Cu layer

40 20 0

0

Sensitivity (Degree/RIU)

300

5 10 15 Number of BlueP/MoS2 layer

20

(c) For CaF2 prism

250 200 150 S-Au layer S-Ag layer S-Al layer S-Cu layer

100 50 0

0

1 2 3 Number of BlueP/MoS2 layer

4

Figure 5.7  Sensitivity variation for proposed SPR biosensor with respect to number of BlueP/MoS2 layers with single layer MXene (Ti3C2Tx) for (a) BK-7 prism, (b) SF-10 prism, (c) CaF2 prism.

Sensitivity (ο/RIU)

184.7

184.5

130.9

185.1

Type of metal layer

Au layer

Ag layer

Al layer

Cu layer

BK-7 prism

12

12

11

7

No. of BlueP/ MoS2 layers

112.5

85.6

116.4

116.9

Sensitivity (ο/RIU)

SF-10 prism

20

20

20

15

No. of BlueP/ MoS2 layers

272.9

170.6

284.4

275.4

Sensitivity (ο/RIU)

CaF2 prism

Table 5.4  Number of BlueP/MoS2 layers needed to get maximum sensitivity for proposed design.

4

4

3

1

No. of BlueP/ MoS2 layers

122  Layered 2D Advanced Materials and Their Allied Applications

Surface Plasmon Resonance Biosensor  123

Sensitivity (Degree/RIU)

180 (a) For BK-7 prism

160 140 120 100 80 60

S-Au layer S-Ag layer S-Al layer S-Cu layer

40 20 0

0

5 10 15 Number of MXene (Ti3C2Tx) layer

20

Sensitivity (Degree/RIU)

110 (b) For SF-10 prism

100 90 80 70

S-Au layer S-Ag layer S-Al layer S-Cu layer

60 50 40

0

5 10 15 Number of MXene (Ti3C2Tx) layers

20

Sensitivity (Degree/RIU)

350 (c) For CaF2 prism

300 250 200 150

S-Au layer S-Ag layer S-Al layer S-Cu layer

100 50 0

0

1 2 3 Number of MXene (Ti3C2Tx) layer

4

Figure 5.8  Sensitivity variation for proposed SPR biosensor with respect to number of MXene (Ti3C2Tx) layers with single BlueP/MoS2 layer for (a) BK-7 prism, (b) SF-10 prism, (c) CaF2 prism.

Sensitivity (ο/RIU)

157

147.3

138.4

146.5

Type of metal layer

Au layer

Ag layer

Al layer

Cu layer

BK-7 prism

8

12

7

4

No. of MXene (Ti3C2Tx) layers

94.7

100.7

93.7

94.5

Sensitivity (ο/RIU)

SF-10 prism

16

20

14

10

No. of MXene (Ti3C2Tx) layers

241.1

185.3

253.9

275.4

Sensitivity (ο/RIU)

CaF2 prism

Table 5.5  Number of MXene (Ti3C2Tx) layers needed to get maximum sensitivity for proposed design.

3

4

2

1

No. of MXene (Ti3C2Tx) layers

124  Layered 2D Advanced Materials and Their Allied Applications

Surface Plasmon Resonance Biosensor  125 decreasing trend for metal thickness can be compensated by increasing number of TMDC layers to obtain higher sensitivities.

5.4 Conclusion It shows remarkable sensitivity of 302°/RIU for monolayer MXene (Ti3C2Tx) and three layers of BlueP/MoS2, 50-nm silver thickness, and CaF2 prism.

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6 2D Perovskite Materials and Their Device Applications B. Venkata Shiva Reddy1, K. Srinivas1, N. Suresh Kumar2, S. Ramesh1, K. Chandra Babu Naidu1*, Prasun Banerjee1, Ramyakrishna Pothu3 and Rajender Boddula4 Department of Physics, GITAM Deemed to be University, Bangalore, India 2 Department of Physics, JNTUA, Anantapuramu, India 3 College of Chemistry and Chemical Engineering, Hunan University, Changsha, China 4 CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing, China 1

Abstract

This chapter explains the normal structure, electronic structure, and crystal structures of 2D perovskite materials. The power conversion efficiency (PCE), humidity, ultraviolet light, chemical versatility, and working at high temperature of different 2D perovskites are reviewed. In addition, a comparison is made on how the 2D perovskites are better than 3D perovskites in terms of durability tenure. Besides, the structure and applications of 2D perovskite photovoltaic (solar cell) device is highlighted. In case of this perovskite solar cell, the contribution of organometal halides is also elucidated. In addition, the requirement of research towards the further development of solar cell based photovoltaic devices to the global need is addressed. Keywords:  Perovskites, humidity, PCE value, science and technology

6.1 Introduction Two-dimensional (2D) perovskites have more advantages over the threedimensional (3D) perovskites in majority of properties such as resistance *Corresponding author: [email protected] Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (131–140) © 2020 Scrivener Publishing LLC

131

132  Layered 2D Advanced Materials and Their Allied Applications to humidity, withstand to light, high temperature, and long durability. The gap is there between 2D perovskites and 3D perovskites in PCE values, in 2D perovskites, the PCE value has raised from 4.7% to 18.20 % due to the result of more research on it. 3D perovskites possess more PCE value about 22.2% [1]. The most common 3D perovskites used in solar cell is CH3NH3PbI3 but it can hydrolyze in presence of humidity and form PbI2 and CH3NH3I [2]. We generally use (C4H9NH3)2(CH3NH3)2Pb3I10 as a solar cell absorber in mesoscopic solar cells [2]. So, in order to fill the gap between these two PCE values, research is taking place across the world research centers. The main applications of this 2D perovskites are in solar cells, light-emitting diodes, transistors, and optoelectronic devices. The main reasons to prefer 2D perovskites are less volatile and hydrophobic due to organic cation attached to inorganic anion; the hydrophobic nature improves chemical and thermal stability of material [1]. Two-dimensional perovskites comprising variety of cations and anions of metal and organic materials, the general formula of them is ABX3 where A is metal or organic cation; B is bigger metal cation, and X indicates anion halides, sulfides and oxides [1]. The recent research is going on RuddlesdenPopper phase 2D perovskites which is in forefront instability, its general formula is (A)2(B)n−1PbnX3n+1, where X is halide ion, B is mamidinium (FA) or methylammonium (MA), and A is long-chain cation used as a spacer to convert 3D perovskite into 2D perovskite. If 5-ammonium valeric acid iodide (5-AVAI) is used then nit can withstand to 1-year moisture, which is not possible in 3D perovskites [3]. With this combination, we can achieve PCE value up to 18.2% and with 2,400-h stability and 40% relative humidity. The first combination was (C6H5(CH2)2NH3ý)2(CH3NH3)2Pb3I10 with PCE 4.7%, 1,100-h stability and 52% relative humidity. In light-emitting applications, we use this 2D perovskites, in single-layered perovskites, luminescence is very dull so metal doping is done to improve its luminescence the dopants are generally Eu3+ and Mn2+ [4]. To study the electronic structure and optical properties of 2D perovskites, we consider Density functional theory and Green’s function formalism, here, we have taken two compounds such as butylammonium (BA) lead halide and ([CH3(CH2)3NH3]2PbX4 ((BA)2PbX4)) and phenylethylammonium (PEA)-lead-halide ([C6H5(CH2)2NH3]2PbX4(PEA)2PbX4 (X = I and Br)). These PEA molecules improve electronic transition along and across the lead halide layers of 2D structures irrespective of halide composition [5]. To improve PCE value, we have developed new 2D perovskites called quasi-2D perovskites prepared by two-step solution deposition method example is (CH5C2H4NH3)2FAn-1PbnI3n+1 its open-circuit voltage is 0.826 V and power conversion efficiency (PCE) is 11.46 % [6].

2D Perovskite Materials and Applications  133 The fully in organic 2D perovskites without organic elements have developed for some specific purpose in optoelectronic field; here, Cl-based fully in organic halide perovskites have been designed and ideal for UV radiation range for photodetection devices. From facile solution method, we prepared CsPbCl3 with efficient 2D structure morphology for UV good photodetection [7]. To improve electronic structure, we can introduce pseudo halide; here, we can introduce out of phase halide with pseudo halide, some of them are 2D-BA2MX4 and 2D-BA2MX2Ps2 (M = Ge2+, Sn2+, and Pb2+; X = I–; Ps = NCO–, NCS–, OCN–, SCN–, SeCN–). Here, we improve electronic structure along with bandgap, work function, effective mass, exciton binding energy [8]. Here, we classify the 2D perovskites into two major groups such as Ruddlesden-Popper (RP) phase 2D perovskites and Dion-Jacobson (DJ) phase 2D perovskites; in RP phase, 2D perovskites monoammonium cation is present which develop more Van der Waals force and hinders the PCE value. But we can convert as RP phase 2D perovskites into the DJ phase 2D perovskites with introducing diammonium cation which reduces the Van der Waals gap [9]. Figure 6.1 indicates the schematic representation of DJ and RP phase of 2D layered perovskites. The hole and electron transport phenomenon are important concept in 2D perovskites; here, TiO2 and ZnO films are act as an electron transport material. Besides, organic molecule acts as a hole transport material which is 2,2 ,7,7 -tetrakis-(N, N-di-4-methoxyphenylamino)9,9 -spirobifluorene(spiro-OMeTAD) which is most common transport material (HTM). 2D perovskites are also called low-dimensional perovskites, might be attained from its parental 3D perovskites through cut Ruddlesden-Popper (PA)2(MA)3Pb4l13

NH3+ C I Pb MA

Dion-Jacobson (PDA)(MA)3Pb4l13

Hydrogen bonding

Figure 6.1  Schematic illustration of RP and DJ phase 2D layered perovskites [9].

134  Layered 2D Advanced Materials and Their Allied Applications up through one of the crystallographic-planes with introducing elongated organic-cation [10]. The butylammonium (BA+) and phenylethylammonium (PEA+) are used as a spacer cation to cut the crystallographic planes [11]. We prefer (001) crystallographic plane in optoelectronic devices [12].

6.2 Structure The structure of perovskites decides all physical, chemical, electrical and dielectric properties, which effects on PCE of a solar cell [13]. In 2D perovskites, generally, two structures such as Ruddlesden 2D (RD) and DJ perovskites are being in practice in research and technology. These two structures are different from conventional 3D perovskite structures in some characteristics such as resistant to humidity, withstand to UV light, and high temperature, and durability of 2D is more comparatively 3D perovskites, but lack in PCE in 2D structures than 3D perovskites.

6.2.1 Crystal Structure In 2D-perovskites R2(A)n-1MnX3n+1, where R can take varied values and R should have at least one group of cations that could form H (hydrogen) bond together with anions which are inorganic and the molecule shape and size determines the establishment of layers and its structure. The molecular cross-section should be equal to more or less area between the in organic terminal halides. Here, aromatic-aromatic π-interactions and Van der Waals force is existed between cations that able to destabilize or stabilize the structure and hydrophobic force. The length of the Alkyl group is important in this context, which affects on both lattice parameters, stoichiometry, octahedral structure, and direction of planes, and the most common type of planes is octahedral which are located along plane and flat [10].

6.2.2 Electronic Structure of 2D Perovskites Electronic structure of 2D perovskites will have both valance band and conduction band as if in the case of 3D structures, valance band contains predominantly halide p orbital hybridized with some metal s orbital and conduction band predominantly metal p orbitals. 2D perovskites will have more excitation binding energies than 3D perovskites due to the presence of dielectric confinement effect in the layers. So, excited electrons are attracted strongly to the holes that means higher excitation binding energy. The excitation binding energy (Eb) is proportional to effective mass (m)

2D Perovskite Materials and Applications  135 C

B

A

Є1

Distance (a.u.)

d

Є2

L

d

Є2 < Є1

L

EHOMO-LUMO

Eg

VB

Eexc

CB

Є2 Energy (a.u.)

Figure 6.2  Energy diagram corresponding to the 2D structures. Valence band, conduction band, electronic bandgap (Eg), and optical bandgap Eexc. The organic framework has a dielectric constant (ε2) while the inorganic slabs have a (ε1) [1].

and dielectric constant (ε). Thus, in solar cells, small excitation binding energy is preferable [1]. The energy diagram is shown in Figure 6.2.

6.2.3 Structure of Photovoltaic Cell Figure 6.3 shows simple structure of perovskite solar cell, in photovoltaic devices, organometal halides play a very important role in producing next-generation low-cost photovoltaic devices because of its long exaction diffusion length (100 to 1,000 nm), broad absorption spectrum, and low-temperature solution process. In conventional n-i-p structured devices, BiO2 and ZnO films are used as an electron transport materials and organic small molecule, 2,2 ,7,7 -tetrakis-(N,N-di-4methoxyphenylamino)-9,9 -spirobifluorene(spiro-OMeTAD), is the most

Au HTM

CH3NH3PblxCI3-x Compact TiO2 FTO

Figure 6.3  Structure of perovskite solar cell.

136  Layered 2D Advanced Materials and Their Allied Applications common hole transport material (HTM). But synthesis of spiro-OMeTAD is complex process and its thickness is about 350 to 500 nm which requires more manufacture cost [14]. Polymeric materials possess more advantages over small molecules due to highly entangled structure that helps to charge recombination at interface in HTM. Such polymers are being used in recent days such as polytriarylamine, Poly-p-phenylenevinylene, and polyfluorene.

6.3 Discussion and Applications Since 1839 to 2019, lot of transition took place in perovskite structures and its efficiencies; here, we have taken specifically 2D perovskites than 3D structure because of shortcomings in 3D structures, mainly short durability and prone to humidity. In 2D structures, we prefer DJ perovskites than RD and easily we convert 3D structure in to 2D structures by cutting across certain planes with spacers, among Dion-Jacobson and Ruddlesden, we prefer Dion-Jacobson because it gives more PCE value so we convert RD to DJ by incorporating diammonium as a spacer. Riddlesden-Popper (RP) phase halide perovskite can form by proper crystallization over a quantum well which play a vital role in photovoltaic devices and their functioning can regulate in 2D(BA2MA3Pb4I13)-perovskites thru managing the staking at molecular level. In addition, the BA2MA3Pb4I13 polycrystalline films which are oriented in vertical direction can enhance the PCE ratio of around 14.3% along with almost negligible hysteresis. With the help of hot casting method, we can prepare (BA)nMAn−1PbnI3n+1 (where n is equal to 4) films which gives vertically aligned BA (butylamine)-spacers which gives 12.51% PCE value in p-i-p solar cells [12]. However, the photovoltaic properties are restricted or controlled by wide bandgap and carrier mobility, results low PCE value than 3D perovskites. Synthesized 2D (CH3(CH2)3NH3)2(CH3NH3)n−1PbnI3n+1 (n = 1, 2, 3, and 4) perovskite thin films are remain unchanged or un affect about two months in 40% humidity, Van der Waals heterostructures are the highperformance electronic devices and also creates new functions. The heterostructures improve efficiency and stability of devices; heterostructure can be constructed via BA2XBr4 (X = Pb, Sn, and Ge) perovskite and black phosphorus (BP), in which the BA2SnBr4-BP and BA2GeBr4-BPheterostructures are type-II band arrangement, but the BA2PbBr4-BP heterostructure is type-I band arrangement [15].

2D Perovskite Materials and Applications  137 Here, another class of material is called quasi-2D perovskites which gives more PCE value than normal 2D perovskites and more stable than 3D perovskites. In this quasi-2D perovskite, the exciton binding energy is reduced so that it improves PCE value. We can prepare high performance crystalline 2D perovskites by aromatic phenylethyl ammonium by introducing SiO2 layer coated with alloy made up of Au Hg(AuAgNPrisms@SiO2)nano prisms, then the current density is enhanced by 32.79% and PCE value is increased by 33.9%. Here, the investigations of morphology reveal that the injected compound (AuAgNPrisms@SiO2) acts as inducer to form smooth quasi2D perovskite films. The collective studies of AuAgNPrisms@SiO2 resulted the reduction in resistance, increment in absorption of light, recycling of photon can take place, and assist exciton dissociation at the interface. The figure 1a and 1b of reference [16] gives the information about 2D quasi-PSC combined with AuAgNPrisms@SiO2 along with comparative energy band structure. By taking ethanol as a solvent absorption spectrum is taken for AuAg-NPrism and AuAg-NPrism@SiO2. In ethanol, AuAgNPrisms display broad absorption spectrum over the wavelength range 300–800 nm, the peak is observed at wavelength 590 nm; further, the shift is taken place in resonant wavelength about 6 nm after coating SiO2 shell, which reveals SiO2 is very thin, this is shown in figure 2a of [16] and figure 2b of reference [16] shows NP shape and thickness is not uniform so wide absorption takes place. Anyway, solar cells should be exposed to sunlight in open field to absorb the light, because we know that in solar cell, the conversion of solar energy into electrical energy can takes place. While exposing, we are not able to resist nature’s parameters such as humidity, rain, high temperature, and etc. If  humidity is more or rainwater has entered into cell, efficiency gets low and decomposition takes place. In the ABX3, structures where A is organic cation such as MA+ and FA+ are extremely hydroscopic which releases MAI and FAI leaving PbI2 remain. Due to high adsorption energy nearly 3eV in CH3NH3PbI3 with active plane (001) definitely water enters into cell, such entered water molecule affects optical property and electronic structure properties of 2D perovskites that subsequently degrades the performance of cell. So, to avoid moisture penetration into cell, the PSCs fabrication process is usually done in the harsh environment, e.g., in the nitrogen-filled glovebox with relative humidity 7 Cd2++S2– CdS

O2– CdS

g-C3N4

ys is

pH=10

ca tal

pH=7

Methyl orange dye

Ph o

to

pH=5

S2–+h+

OX

g-C3N4

Heterojunction

Cd2+

–0.34

2.04 eV

e–

(e)

(c)

Cl C N

Recombination

OH–

VB +

h

VB h+

·OH +

h

CdS/g-C3N4

Figure 8.4  Schematic representation of (a) TEM image of GCN/CdS (1:3) composite, (b) FT-IR spectra of GCN, CdS, and GCN/CdS (1:3) photocatalyst. (c) Schematic diagram of the separation and transfer of photocarriers over GCN-CdS composite under visible-light exposure. Reproduced with permission from Elsevier (Licence no. 4636910135129) [95]. (d) Diagrammatic representation of the combination process between GCN sheets and CdS NPs. (e) Schematic representation of the photocatalytic mechanism of CSCN heterojunctions. Reproduced with permission from Elsevier (Licence No. 4636451405595) [97].

170  Layered 2D Advanced Materials and Their Allied Applications excellent photodegradation efficiency by degrading 90.45% of MB within 180 min of visible-light exposure. Li et al. fabricated GCN/CdS (CNCS) nanohybrid with different mass ratios of GCN and CdS in which a core-shell structure was formed by wrapping of GCN NSs over CdS nanorods (NRs) [96]. The CNCS nanohybrid with 3:1 mass ratio exhibited enhanced photocatalytic efficacy for the degradation of TC and erythromycin (Ery) antibiotics. The improvement in photocatalytic ability was credited to the apt matching of band edge potential, tightly adjoined surface contact, and effectual spatial separation of photogenerated excitons through Z-scheme charge transfer pathways. Moreover, atomic absorption spectroscopy results exposed that the Cd2+ ions concentration in the CNCS-3:1 sample solution was significantly lower than that of pristine CdS. In another work, Xu et al. reported CdS/GCN (CSCN) nanocomposites which were synthesized through an in situ process and the amount of CdS in CSCN composite was estimated by a newly developed wet chemical process [97]. It was observed that the as-prepared photocatalyst exhibited much better photodegradation efficacy for the degradation of an azo dye methyl orange (MO) than that of pristine CdS and GCN photocatalysts. Moreover, with pH = 7 and beyond, the surface of GCN became negatively charged, and as a result, the positively charged Cd(II) ions were easily attached onto its surface due to electrostatic interaction. Besides, GCN NSs offered much larger surface sites for the deposition of Cd(II) as compared to BGCN. The process of anchoring of CdS NPs onto the surface of GCN NSs is depicted in Figure 8.4d. Due to effectual interactions between GCN NSs and CdS NPs, the CSCN composite exhibited excellent photocatalytic firmness even after 5 catalytic cycles with 93.8% degradation efficiency of MO. The experimental studies revealed that based on band edge positioning of CdS and GCN, the charge migration pathway followed double charge transfer mechanism (Figure 8.4e) which was responsible for effective separation of photocarriers. Other than GCN NSs, BGCN coupled with CdS also displayed improved photocatalytic efficiency, for instance, Cui et al. synthesized GCN/CdS composite utilizing ammonium thiocyanate and cadmium chloride via template free one-step calcination method [98]. The study inferred that enhanced photocatalytic activity was observed for RhB as target pollutant under visible light exposure. Moreover, through experimental data, it was found that nanocrystals of CdS (hexagonal) were firmly dispersed on BGCN sample and due to significantly matched band structures of both the photocatalysts, the GCN/CdS composite provided efficient migration of photocarriers.

g-C3N4 Coupled Sulphides and Oxides  171

8.2.3 Some Other GCN Coupled Metal Sulphide Photocatalysts In order to attain the best out of photocatalysis, many other photocatalytic materials involving metal sulphides have been employed for environmental remediation [99]. For example, Ma et al. reported a novel CuS/GCN nanocomposite in which CuS NPs were successfully decorated over GCN sheets tailored simply through solvothermal method [100]. The detailed investigation involving morphological studies, optical absorption, charge migration/separation pathways, and photodegradation performance revealed that as-synthesized composite displayed improved Fenton-like catalytic, CuS/GCN photocatalytic, and direct H2O2 photocatalytic abilities. And the results further explored that the as-fabricated CuS/GCN nanocomposite can work efficiently even in the absence of sun light due to the combined action of photocatalytic activity with Fenton-like catalytic ability along with direct H2O2 photocatalysis which can be utilized as a new strategy for effectual applications. In another work, Khan and co-­workers fabricated 3D flower-like CuS/GCN nanohybrid by varying the wt% of CuS utilizing hydrothermal reaction [101]. By dispersing CuS NPs onto GCN surface, the photocatalytic activity of the aforementioned nanocomposite was significantly enhanced for the photo-discoloration of MB under visible light exposure. The results further exploited that the composite with 10 wt% of CuS with GCN exhibited excellent photocatalytic activity which was attributed to Z-scheme charge transfer mechanism in which h+ and e− are present at VB and CB of CuS and GCN, respectively. Furthermore, from radical trapping experiments •O−2 and •OH radicals were found to be the main radical species responsible for MB discoloration. Other than binary metal sulphides, some ternary mixed metal sulphides are also explored for various photocatalytic applications. For instance, Yao et al. fabricated Cu2MoS4/GCN photocatalyst by anchoring of ­flower-like Cu2MoS4 onto GCN NSs [102]. The flower like Cu2MoS4/GCN nanocomposite displayed excellent adsorption of RhB in an aqueous sample. Nevertheless, ZnIn2S4 also displayed vital applications in visible light driven photocatalysis. Qiu et al. synthesized ZnIn2S4/GCN nanocomposite through hydrothermal process and utilized it for the photodegradation of 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous phase [103]. The morphological studies inferred that the GCN NSs were decorated properly with flower-like ZnIn2S4 due to which an intact surface junction was formed between them. From experimental data, it was found that 20% ZnIn2S4/ GCN nanohybrid exhibited highest photocatalytic degradation of 2,4-D [3.69 mmol/(gcat h)] than pristine ZnIn2S4 and GCN. Depending upon the

172  Layered 2D Advanced Materials and Their Allied Applications experimental results, possible mechanism of interface migration of photocarriers and the degradation process of 2,4-D is depicted in Figure 8.5. The process of photodegradation was initiated with adsorption of 2,4-D which was in accordance with Langmuir-Hinshelwood model. The photocatalytic reaction was carried out by photogenerated h+ and •O−2 radicals which were the main reactive radical species for 2,4-D degradation. Yang and co-workers synthesized 2D/2D ultra-thin ZnIn2S4/protonated g-C3N4 (ZIS/pCN) heterojunction by electrostatic interaction between negatively charged ZIS NSs and protonated CN NSs [104]. The as-fabricated ZIS/pCN (5:3) photo­ catalyst exhibited superior photocatalytic efficacy for H2 production and TC degradation and the photodegradation activity was 2.28 and 4.13 times higher than that of pristine ultrathin ZIS sheets and pCN, respectively. The electrostatic self-combination ZIS and pCN facilitated formation of intact heterojunction which offered large adjoining area, improved separation of photocarriers and superior photocatalytic activity as well as stability.

TAA

Zn2++In3+ EG+DMF

nucleation Zn2+ In3+ ZnIn2S4 nuclei ZnIn2S4 nanosheet

V/NHE

O2•–

e– e– e– e–

e–

e–

1 2

2,4-D

O2

ZnIn2S4

g-C3N4

0

Cl–,CO2,H2O

e–

–2 –1

nuclei growth

h+ h+ h+

h+ h+ h+ h+

2,4-D Cl–,CO2,H2O

Figure 8.5  Mechanistic insight for the formation of ZnIn2S4/GCN heterojunction system along with charge separation route. Reprinted with permission from Elsevier (Licence No. 4640050230425) [103].

g-C3N4 Coupled Sulphides and Oxides  173

8.3 GCN Coupled Metal Oxide Heterojunctions for Environment Remediation Over the past few years, most of the work has been focused on developing visibly driven photocatalysts especially, metal oxides-based semiconductor photocatalysts are in mainstream interest in order to utilize their potential for various photocatalytic applications [105–113]. The proper designing and engineering of such photocatalytic materials is necessary in order to enhance their optical as well as electrical properties. By improving the optoelectronic features of photocatalysts, they enable wide optical absorption, spatial separation of photoinduced excitons, rapid interfacial migration of charge carriers, and ultimately, enhanced photocatalytic abilities. The binary metal oxides such as TiO2 and ZnO are widely explored photocatalysts which displayed improved photocatalytic efficiencies under visible light exposure. After absorbing the photo-irradiation, the metal oxides NPs produce photogenerated EHP, which exhibited great potential for oxidation and reduction of organic as well as inorganic molecules adsorbed at their surfaces [114, 115]. These photoinduced EHP participate in surface reactions and generate reactive oxygen species (ROS) as depicted in Figure 8.6a. These ROS possessing high redox potential ability react with organic molecules and degrade them into CO2 and H2O [116]. But the main concerns of working with metal oxide photocatalysts are rapid reassembly of photocarriers, wide band gap energies, insufficient surface sites, and photocorrosion. To overcome these bottlenecks, fabrication of multicomponent assemblies comprising metal oxides and functional carbon nitride photocatalysts is an optimal strategy. For example, Lei et al. rationally tailored TiO2/GCN nanocatalyst possessing inverse opal structure through an easy technique [117]. The as-fabricated TiO2/ GCN photocatalyst was employed for the photodegradation of three major organic pollutants, namely, RhB, levofloxacin, and phenol. It was observed that the GCN with inverse opal structure displayed effective interactions with TiO2 and it facilitated the matching of band edge potentials of both GCN and TiO2. Moreover, this effective interaction enhanced the interfacial contact along with active surface sites which stimulate the migration of photoinduced excitons. The heterojunction formation accompanying inverse opal structure greatly boosted the photocatalytic performance of TiO2/GCN photocatalyst for organic pollutant degradation. Specifically, the rate constant for RhB degradation was 0.184 min−1 which was found to be 4.2 times more than inverse opal TiO2 whereas 2.7 times more than usual TiO2/GCN composite. The plausible mechanism of charge migration and organic pollutant degradation by inverse opal TiO2/GCN nanocomposite is

174  Layered 2D Advanced Materials and Their Allied Applications Reduction

O2

(a) Oxidation •OH OH– + h+ •OH + •OH

UV

e–

MO

H2O2

Valence Band h+

h+

Degradation (CO2 + H2O

e–

Conduction Band

•OH

Degradation (CO2 + H2O

e–

•O – 2

O2 + e– •O – + H+ 2 •HO + H+ + e– 2

•O2– •HO 2

H2O2

h+

H2O

(b) Organic pollutants Degraded products

e–

•O – 2

CB CB

IO g-C3N4

2.7 eV

O2

TiO2 3.2 eV

VB Organic h+ pollutants VB Degraded products

Figure 8.6  Schematic illustration of (a) the photocatalytic action of metal oxide (MO) nanomaterials under UV irradiation. Reproduced with permission from RSC (Licence No. 4640041233569) [116]. (b) Representation of proposed mechanism of organic pollutant degradation over inverse opal TiO2/GCN nanohybrid. Reproduced with permission from RSC (Licence No. 4640190990558) [117].

represented in Figure 8.6b. In another study, Jiang and co-­workers reported a novel work in which GCN was hybridized with hierarchical TiO2 yolkshell spheres through simple solvothermal process in which GCN powder was mixed with the suitable feedstock of TiO2 yolk-shell in dimetbylformamide (DMF) [118]. The presence of DMF facilitated the effectual decoration of TiO2 yolk-shell spheres onto GCN surface which in turn provided intact surface junction as well as efficacious surface sites for various photocatalytic reactions. The incorporation of yolk-shell structure aided as an effective tool for harnessing of solar light due to multiple reflections within the core voids of titania micro-spheres. The experimental observations revealed that the aforementioned TiO2/GCN (TCN) nanohybrid displayed greatly improved photocatalytic abilities for the removal of MB, RhB, and ciprofloxacin along with H2 production. Thereby, it is evident that with suitable modifications either in band potentials or in surface morphologies TiO2 can bring about efficacious photocatalytic activities with pilot scale applications [119, 120].

g-C3N4 Coupled Sulphides and Oxides  175 Other than TiO2, ZnO is another binary photocatalyst which has been explored widely in photocatalysis and is still been utilized by optimum modifications in various photocatalytic applications [121]. For instance, Zhang et al. smartly utilized adsorption and photocatalytic abilities for the photocatalytic degradation of organic pollutants [122]. They employed self-assembly along with freeze drying technique to fabricate GCN-ZnO@ graphene aerogel (GCN-ZnO@GA) a direct Z-scheme photocatalytic system which not only showed excellent adsorption ability but also displayed outstanding photoactivity for pollutant mitigation. The morphological analysis exposed that the hierarchical porous structure as well as the synergic effect between the different components were the main reason behind firmness in the heterojunction. Moreover, 82.7% and 81.0% of RhB was successfully removed by GCN-ZnO@GA (30%) composite under visible light and UV irradiation, respectively. Along with photoactivity, the GCN-ZnO@GA (30%) sample showed excellent reusability, and from cyclic experimental analysis, it was observed that an initial activity of 87.1% was preserved after four catalytic cycles. Kumar et al. synthesized N-doped ZnO/GCN nanohybrid core-shell nanoplates by economic and green ultrasonic dispersion technique in which the average diameter of core-shell nanoplates was found to be 50 nm while the thickness of GCN shell relied on the amount of GCN added [123]. Due to formation of intact surface junction between N-doped ZnO and GCN shell the band gap energy was decreased by the introduction of mid-gap carbon energy levels. The band gap engineering facilitated the separation/migration of photocarriers, exposed surface sites for catalytic reactions, and boosted the redox ability of the as-fabricated N-doped ZnO/GCN nanocatalyst. The photocatalyst was used for the treatment of RhB present in aqueous solution. The synergistic effect induced due to tight interfacial contact provided enhanced photocatalytic activity as well as photostability. Nowadays, combining two components having different dimensionality in order to attain a heterojunction system with apt redox potential and morphology is in trend. Fang et al. constructed 0D metal-free GCN quantum dots (CNQDs) which avoided the use of toxic and photocorrosive metals and combined them with 2D ZnO NSs possessing oxygen vacancies (OV-ZnO) [124]. The as-synthesized 0D/2D CNQDs/OV-ZnO composite displayed excellent photocatalytic activity for the degradation of MB and bisphenol A. It was observed that the unique features like intact coupling of OV-ZnO and CNQDs, firmly decorated 0D CNQDs, highly exposed active surface sites on 2D OV-ZnO NSs and effectual migration and space separation of photocarriers through Z-scheme mechanism were responsible for efficacious photocatalytic activity. Besides, 3D flower-like structure aids as

176  Layered 2D Advanced Materials and Their Allied Applications an effective strategy for harnessing of light through multiple reflections. In order to identify the main reactive radical species involved in the photodegradation process, the radical trapping experiments were performed by using different radical scavengers as shown in Figure 8.7a. From tapping experiments •OH and •O−2 radicals were the main species involved in the photodegradation process. The unique features of 3D flower-like structure of CNQDs/OV-ZnO are depicted in Figure 8.7b, which enlighten the typical significance of the synergistic improvement in photocatalytic activity. Moreover, based on different experimental results the two plausible charge migration and photodegradation mechanisms are depicted in Figure 8.7c. Based on the band edge potentials and the ability of generating •OH and radicals, Z-scheme charge transfer mechanism was found to be an •O−2 (a)

(b)

Uniquie nanosheet structure

Degradation rate (%)

0.8

0D/2D Z-scheme heterojunctions

0.6

Efficient Z-scheme charge separatiom

0.4 Enhanced visible-light harvesting

0.2

Improved charge transport

Q

Synergistic enhancement of photocatalytic performance

PB

IPA

EA

No

ne

0.0

CNQDs

hv

Z-scheme heterojuction O2

type-II heterojuction

(c)

Potential Vs. NHE (eV)

–2 –1

Re

e– e– e–

du

e– e– e–

e– e– e–

0 1 2 3

Oxidation

ct

io

OV-ZnO

·OH

n

E(O2/·O2–) e– e– e– e–

CNQDs

h+h+h+

·O2–

e–e– e–

Vo

CNQDs h+ h+h+

OV-ZnO nanosheet

Vo

= –0.33 eV

e– e–

E(·OH/OH–)

h+h+h+

OV-ZnO

=1.99 eV

h+h+h+

Figure 8.7  Schematic representation of (a) the trapping experiments of reactive species of 0D/2D CNQDs/OV-ZnO utilizing different radical quenchers under room temperature and (b) the mechanism of synergistic effect of CNQDs over OV-ZnO to improve the photocatalytic performance. (c) Schematic illustration of spatial separation and transport of photocarriers along with the photocatalytic activity of CNQDs/OV-ZnO nanocomposite under visible light illumination. Reproduced with permission from Elsevier (Licence No. 4640321200385) [124].

g-C3N4 Coupled Sulphides and Oxides  177 optimal route which not only rendered the spatial separation of photocarriers but also maintained the high redox ability in CNQDs/OV-ZnO composite.

8.3.1 GCN and MoO3-Based Heterojunctions The metal oxide MoO3 has three typical polymorphic structures that are orthorhombic (α-MoO3) which is highly stable and the two metastable phases, namely, hexagonal (h-MoO3) and monoclinic (β-MoO3) [125, 126]. By utilizing their unique morphology, MoO3 and its derivatives have various applications in industry as catalysts, display devices, sensors battery electrodes, and certain other nanomaterial uses [127]. In order to control their structural properties and morphology, various techniques have been employed like hydrothermal route, chemical processes, template synthesis, thermal evaporation, and sputtering [128, 129]. The literally data confirmed that by utilizing the solution phase hydrothermal route, it is feasible to tailor metastable structures of MoO3 endowing controlled dimensions and size [130]. Coupling MoO3 with GCN can result in the formation of a useful heterojunction system due to their matching band alignments. For instance, Xie et al. rationally fabricated carbon dots (CDs) modified GCN/MoO3 (CCM) Z-scheme heterojunction system with varying amount of CDs and utilized its potential for TC degradation [131]. The incorporation of CDs helped to boost the photo­ catalytic performance by providing superior charge transfer through Z-scheme mechanism and the wide optical absorption via upconversion photoluminescence (UPPL) feature. It was observed that by doping 0.5% CDs into the GCN/MoO3 heterojunction, the photocatalytic ability was utmost and was 46.2 and 3.5 times more than that of pristine GCN and individual MoO3/GCN composite. Moreover, photogenerated h+ were the main reactive species involved in the photodegradation of TC. The proposed Z-scheme mechanism of charge migration and photocatalytic degradation of TC by 0.5CCM3 composite is shown in Figure 8.8a. It can be seen that after exposure of visible light, the photoilluminated CB e− in MoO3 get attracted by VB h+ of GCN, and further, the photoexcited CB e− in GCN were captured by the CDs which acted as the reservoir. By this way, the photoinduced EHP were spatially separated through Z-scheme charge transfer stimulated by CDs. In another work, MoO3/GCN nanohybrid was easily tailored by a facile mixing-calcination technique and its photocatalytic performance was evaluated for the degradation of organic dyes under the exposure of visible light [132]. The report highlighted that with slight variation in the parameters like initial concentration of

178  Layered 2D Advanced Materials and Their Allied Applications

Autoclave

•O2

lig ht

O2 – –

CDs e e e UP -co PL O2/•O2– g-C3N4 – e– e– pr nver e op te er d ty h+ h+ –

HO

CH3

OH O

–2 –1 0 1 2 3 4

–1.16 eV –0.05 eV 0.54 eV

1.53 eV MoO3 2.27 eV

•OH/OH–

H3C

N

h+ h+

CH3

3.54 eV

OH

OH OH O

1st step : Growth of nanorods

vs SHE

Crucible

3rd step : g-C3N4 coating 3.5 cm

NH2 O

Exact Mass: 444.15

Furnace

2nd step : Remove residue

FTO substrate Photoanode film

400°C, 30 min

ble

–

s rod no na n ow Gr

O3 Fe 2

520°C, 4 hr

Vis i

100°C, 24 hr

(b)

(a)

CO2+H2O g-C –3

Degradation g-C3N4 products

V vs. VNHE (eV)

–2 –1 •OH

(c)

Solar energy

0

2

0

s

N4

3

(d)

Degradation products

1.66 eV

•OH

Degradation products H2

g-C3N4

–2 –1

rod no na

Fe2O3

OH–

–3

V vs. VNHE (eV)

O2

3 e 2O @F

2.68 eV

1

3

Fe2O3 g-C3N4

O2

H2O2

Degradation products

•O2–

s rod no na

H+

(e)

Pt

Fe2O3

2.68 eV

1 2 3

1.86 eV

TEOA+ TEOA

Figure 8.8  (a) Proposed photodegradation mechanisms of TC under visible light exposure with 0.5CCM3 composites. Reproduced with permission from Elsevier (Licence No. 4641370231245) [131]. (b) Tailoring of facile and simple heterostructures of GCN@ Fe2O3. The direct Z-scheme mechanism of 0.75g GCN@Fe2O3 heterostructures depicting (c) a graphical representation of coated GCN@Fe2O3 composite, band gap energy diagrams for (d) photocatalytic degradation and (E) H2 evolution. Reproduced from RSC [136] (licenced under CC BY-NC 3.0).

MO, pH of the solution containing MO and the amount of MoO3/GCN composite, can effectively increment the photocatalytic efficacy. Similar to the previous report, Huang et al. synthesized GCN/MoO3 nanohybrid via mixing-calcination route and by varying the amount of GCN to control the morphology of the composite [133]. The report illustrated that the GCN/MoO3 nanohybrid with 7% GCN content, displayed improved photocatalytic activity for the discoloration of MB which reached about 93% after 3 h of visible light exposure. The matched band-edge alignments and intact surface junction facilitated the photoactivity by providing efficacious migration of photocarriers. Thereby, hybridizing GCN with MoO3 in order to form a visibly driven nanohybrid can be a promising approach to attain efficient photoactivity and photostability.

g-C3N4 Coupled Sulphides and Oxides  179

8.3.2 GCN and Fe2O3-Based Heterojunctions The oxide of transition metal Fe2O3 is capable of utilizing solar energy up to wavelength of 600 nm and possesses a band gap energy of about 2.1 eV. Due to some inherent features like cost-effectiveness, magnetically recoverable, abundant, facile synthesis, and good chemical stability, Fe2O3 can serve as an effectual photocatalytic material [134]. Howsoever, like other single component systems, Fe2O3 bear some bottlenecks like short hole diffusion length along with fast reassembly of photocarriers which drastically restricts its photocatalytic performance [135]. Thereby, hybridizing Fe2O3 with other semiconductor photocatalyst can be of great deal to overcome these pitfalls. For example, Kang and co-workers tailored Fe2O3 NRs (possessing as aspect ratio of 17:1) wrapped by GCN through a sequential solvothermal process (at low temperature) trailed by thermal evaporation route [136]. Through the facile coating of GCN over Fe2O3 NRs, an intact heterojunction is formed in which photoinduced excitons were migrated through Z-scheme mechanism providing optimal redox ability for the surface reactions. The 0.75GCN@Fe2O3 composite showed highest photocatalytic ability for the photodegradation of 4-nitrophenol (4-NP) which was found to be 15 times higher than pristine Fe2O3 with the rate constant of 12.71 × 10−3. Along with photodegradation, the composite also displayed enhanced H2 production ability. The overall procedure of tailoring of simple heterojunctions of GCN@Fe2O3 is depicted in Figure 8.8b. The report illustrated that the Z-scheme charge transfer mechanism (Figures 8.8c–e) not only facilitated the spatial separation of photocarriers but also rendered the high redox ability to the system. And to support the Z-scheme mechanism for the photodegradation of 4-NP and for H2 evolution, the schematics of band alignments of heterojunction can be seen in Figures 8.8d and e, respectively. In another work, Xiao et al. utilized hydrothermal method to design α-Fe2O3/GCN nanohybrids for the reduction of heavy-metal ion Cr(VI) [137]. The intimate contact between α-Fe2O3 and GCN along with their similar band structure helped to improve the photocatalytic ability. Furthermore, the anchoring of α-Fe2O3 onto GCN did not hinder the size and morphology of GCN in fact, it aided as and effective strategy which suppressed the reassembly of photocarriers. Along with photocatalytic performance, the Fe2O3/GCN nanohybrids displayed excellent stability and recyclability during the photocatalytic reduction of Cr(VI). In spite of great potential of iron oxide, it is very difficult to work with it at various pH ranges and different chemical environments. However, hematite (α-Fe2O3) phase is found to possess high chemical stability and

180  Layered 2D Advanced Materials and Their Allied Applications corrosion resistivity in acidic as well as alkaline media [138]. Kang et al. used facile one-step carbonization process to fabricate Fe2O3/C-GCN heterojunction system in which a carbon layer is used to join α-Fe2O3 and GCN [139]. The photocatalytic study revealed a comparative outlook of Z-scheme charge transfer mechanism in Fe2O3/C-GCN system and double charge transfer mechanism in simple Fe2O3/GCN nanohybrid for the photocatalytic degradation of RhB dye. And it was found that due to intimate contact between Fe2O3 and GCN though an amorphous carbon layer as mediator, the Fe2O3/C-GCN system displayed improved photoactivity which was two times higher than Fe2O3/GCN system. Thus, by engaging effective strategies, the photocatalytic abilities of the metal oxide heterojunctions can be significantly enhanced.

8.3.3 Some Other GCN Coupled Metal Oxide Photocatalysts Extensive research has been done on the metal oxide photocatalysts due to their vital applications in photocatalysis. Metal vanadates like BiVO4, Ag3VO4 and so on are being utilized as an effectual photocatalysts in waste water treatment due to their layered structure, facile synthesis, and apt morphology [140, 141]. Other than vanadium oxides, molybdenum oxide, tungsten oxide, indium oxide, and cerium oxide have been widely explored for their unique photocatalytic abilities [142–145]. For example, Li et al. constructed GCN/Bi2MoO6 heterojunction system by using solvothermal process for the photocatalytic degradation of RhB [146]. The photocatalytic performance of the photocatalyst strictly depends upon the content of Bi2MoO6 in the composite. Report illustrated that that the GCN/Bi2MoO6 heterojunction with 16.1 wt% of Bi2MoO6 exhibited the highest photocatalytic activity for RhB degradation. The incremented photoactivity of the composite was attributed to the effectual spatial separation of photoilluminated EHP which was caused by the transfer of photoinduced h+ from GCN to Bi2MoO6. And from radical trapping experiments •O−2 radicals were the dominant species involved in photodegradation of RhB. Recently, Jiang et al. fabricated visibly driven direct dual Z-scheme WO3/GCN/Bi2O3 photocatalyst by utilizing one-step co-calcination process and evaluated its potential for the photocatalytic degradation of TC [147]. The dual direct Z-scheme in the ternary heterojunction endowed optimal space separation of photocarriers along with broad optical absorption. The intimate contact of components not only resulted in the incremented photocatalytic ability but also rendered the high photocatalytic stability. Thus, the formation of heterojunction with metal free conjugated

g-C3N4 Coupled Sulphides and Oxides  181 GCN is an effectual strategy to overcome the typical pitfalls of metal oxides as well as metal sulphides and to attain their usefulness in photocatalysis.

8.4 Conclusions and Outlook In summary, we have enlightened the polymeric graphitic carbon nitride, a metal free conjugated semiconductor material, and its outshined properties for the photocatalysis of waste water. Moreover, the general strategy of heterojunction formation in order to avoid the pitfalls associated with GCN as single component system has also been discussed. Typically, the heterojunction of GCN involving GCN coupled metal oxides and sulphides to satisfy (i) broad visible light absorption, (ii) effectual migration and separation of photoinduced EHP, (iii) improved redox ability for various surface reactions, and (iv) apt morphology for efficaciously enhanced surface active sites for adsorption has been precisely highlighted and discussed. The ever growing progress in the field of photocatalysis implies that fabricating GCN-based heterojunctions affords a favorable technique to increment the photocatalytic efficacies of semiconductor photocatalysts. Till date, various reports have been published which highlighted the use of GCN as semiconductor photocatalyst to improve the overall photocatalytic performances by facilitating the use of solar energy or by providing effective migration/separation of photocarriers, inspired from that some of the hot researches are discussed in this report. However, the unsystematic studies, more optimal characterization techniques, the effective mechanism of charge transfer, as well as photodegradation reactions and highly efficacious photocatalytic systems with photostability along with facile recovery need further exploration. The incessant and collective efforts of researchers from all over the world are highly desirable to make photocatalysis adaptable to overcome various environmental and energy problems.

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9 2D Zeolites Moumita Sardar, Manisha Maharana, Madhumita Manna and Sujit Sen* Department of Chemical Engineering, National Institute of Technology Rourkela, Rourkela, India

Abstract

The development of new zeolites in the area of zeolite science is a continuous interest due to their worldwide implementation in adsorption, separation, and catalysis. Two-dimensional (2D) zeolite accounted as a promising material because of their potential application in various field. 2D zeolite is a porous material prepared from lamellar precursors, in which the crystalline nano layers are joined weakly in one specific direction. Due to the absence of covalent bond, the stacking sequence can be manipulated and the structural diversity can be controlled. Different types of synthetic strategies have been developed for 2D zeolite. The synthetic procedures are based on by adopting bifunctional surfactants or by using Assembly-Disassembly-OrganizationReassembly method. At first, traditional hydrothermal synthesis was discovered followed by post-synthetic modifications. In this book chapter, the synthetic methods are mainly classified as top-down methods and bottom-up methods. Depending on the various synthesis procedures, different properties like structural, acidic and catalytic, and adsorption have been discussed. Finally, their potential applications in various sectors such as petro-chemistry, biomass conversion, various reactions, and the synthesis of fine chemical have been given. Keywords:  Zeolites, 3D, 2D, microporous, mesoporous

9.1 Introduction At present, the porous substances are more promising materials in catalysis, adsorption, purification, gas separation, and energy storage system rather *Corresponding author: [email protected] Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (193–210) © 2020 Scrivener Publishing LLC

193

194  Layered 2D Advanced Materials and Their Allied Applications than traditional dense substances like TiO2, CdS, and Au. Depending on the pores size, the porous substance has been classified into three categories such as (1) microporous (pore size 50 nm). In porous materials, there are certain parameters which are very important and widely used in industrial applications such as properties, selectivity, kinetics, and stability [1]. Due to such properties, zeolite becomes an interesting research topic to the researchers in these respects. The growing demands for fuels and olefins fascinating in zeolites synthetization with higher selectivity, catalytic activity, and lifespan [2]. Zeolites also can be used as catalysts, adsorbents, ion exchangers, and membranes in a huge number of production applications [2–4]. Zeolite is an important class of inorganic microporous crystalline alumino-silicate substance that is used as molecular sieve due to their microporous structure as well as catalyst. This group of substances is provided with different size of pore diameter as well as pore system such as 1D, 2D, and 3D [5, 6]. Zeolites are also considered as most significant solid diversified catalysts in the chemical and petrochemical industries because of their pore configuration, strong intrinsic acidity, composition, unique shape selectivity, and large stability [5, 7]. The porous structure gives large surface area with more thermal and mechanical constancy. Also, the presence of aluminum molecules causes the acidic property in zeolite structures [2]. Despite all advantages, zeolite nanoparticles suffer from certain limitations such as intra-crystalline diffusion due to their pores size (0.3–0.8 nm) that impact on catalytic performance for large molecules transformation and unavoidable agglomeration [2, 8]. This disadvantage can be overcome by either increasing the internal surface with larger pores size or decreasing the size of crystal by thin zeolites layers formation [9, 10]. So, the current challenges in zeolite chemistry are (i) introducing mesoporosity for improving the active sites accessibility and (ii) ultrathin zeolite layers modified into expanded assemblies [6]. Two-dimensional (2D) zeolites are very important class of mesophase materials that constructed with very thin crystalline components [8]. The 2D zeolite membranes formation relies on: 1) the suspensions that contain zeolite nanosheets with high aspect ratio and a uniform thickness, colloidal stability, and free of non-exfoliated and amorphous contaminants; and 2) the developed techniques for the formation of porous substrates with ultrathin membranes from suspended zeolite nanosheets [11].

2D Zeolites  195 In conventional zeolites, the catalysis and adsorption occur on the surface of the zeolites, but in 2D zeolites, it occurs inside the pores also. Therefore, the ultrathin 2D zeolite layers are found very effective in this regard [12]. Different methods have been established to enhance the external surface of zeolites such as i) micro-mesoporous materials fabrication by developing mesoporosity, ii) zeolite crystals size reduction to nanoscale, iii) thin zeolite layers formation with larger external surface areas [8]. This book chapter presents a brief introduction on 2D zeolite and comprises with different synthesis techniques and properties. At the end, the applications of 2D zeolites are summarized.

9.1.1 What is 2D Zeolite? Zeolites are naturally occurring or artificially created highly crystalline microporous alumino-silicate substances having pores of small, medium, large, and extra-large sizes containing rings of 8, 10, 12 with more than 12 tetrahedral atoms (T = Si, Al, Ge, and P) linked by two-coordinated oxygen atoms [12–14]. Almost 200 types of zeolites are found based on different shape, size, and connectivity of channels between 8- and 30-rings [15]. The variation of zeolite formation with different structural features like odd-ring numbers (11-, 15-rings), extra-large pores (16-, 18-, 20-, 28-, and 30-rings), chiral pores, and complex framework topologies are mainly discovered due to the different synthesis strategies [11]. The structure of 2D zeolite is given in Figure 9.1. Due to the open structure and surface exposed active sites of zeolites, 2D zeolites become very popular and promising to overcome the limitation of mass transport and active site accessibility [7]. A zeolite can be considered as 2D zeolites when one dimension of the zeolite crystal is very less than several nanometers [16]. The precursors of 2D zeolites contain stacked sheets of 1–2 unit cells or very lower thickness with very weak forces through the 3D

Figure 9.1  Structure of 2D zeolite.

2D

196  Layered 2D Advanced Materials and Their Allied Applications interactions between organic-inorganic ions or molecules that is located in the “interlayer” spaces [7]. The sequence of stacking can be manipulated and controlled as no covalent bonds are present between the layers [17]. So, that increases the external surface per mass/volume, characteristic of 2D zeolites, and post-synthesis structural modifications like swelling, pillaring, delamination, and exfoliation. Generally, three approaches are there for synthesizing 2D zeolites such as (1) zeolites (like MWW, FER, SOD, NSI) are prepared from lamellar precursors for preservation of 2D character, (2) 2D zeolites can be synthesized by using some structure directing agents, restricting the mechanism of crystal growth that blocks the growth of crystal in one direction forming nanosheet single crystals, and (3) 2D zeolites are obtained by top-down post-synthesis modification [16].

9.1.2 Advancement in Zeolites to 2D Zeolite The first natural zeolite, “stilbite” was found almost 260 years ago by a Swedish mineralogist, Axel Fredrik Cronstedt, and in 1862, the first synthetic zeolite “levynite” was developed by St. Claire Deville. In 1948, the first synthesis of zeolites “ZK-5” systematically was obtained by Barrer [1]. The further development in the field of zeolite science was discovered in 1964 by developing zeolite Y and in 1969 for ZSM-5 [18]. The layered 2D character in zeolite was established in the 1980s and early 1990s for “ERB-1”. Other synthesized zeolites are MCM-22, SSZ-25, PSH-3 [19]. Roth et al. (2011) converted germanosilicate UTL zeolite with or without the presence of boron into 2D zeolite material “IPC-1P” which can be further modified to other layered materials in a similar way [20]. Chlubna et al. (2013) developed 2D lamellae from 3D UTL framework by structure modification with pillaring [6]. Kubu et al. (2013) introduced a 2D zeolite “TUN” from the intermediate layered precursor IPC-3P [9]. Hermann et al. (2014) investigate the 2D zeolites properties which is obtained from ITH and UTL zeolite by interacting several molecules such as CH4, CO2, H2O, H2, N2 [21]. Eliášova et al. (2015) developed new zeolites from zeolite UTL using ADOR (Assembly-Disassembly-Organization-Reassembly) protocol [22]. Kumar et al. (2015) demonstrate a method for determining the wrinkles and thickness of 2D zeolite by using comprehensive 3D mapping of its reciprocal lattice [23]. Zukal et al. (2017) used magnesium oxide into zeolites MCM-36, MCM-22, IPC-3PI, IPC-3, and ITQ-2 by using 2D layered precursors ICP-3P and MCM-22P which is further tested as adsorbents for CO2 adsorption at a temperature of 0°C to 60°C [24]. Jayaramulu et al. (2018) prepared 2D zeolite framework for oxygen evolution reaction using cobalt. The exfoliated 2D Co-ZIF-9(III) has an advantage of more

2D Zeolites  197 accessible active sites than 3D MOF crystals [25]. Wei et al. (2018) explore a mesoporous 2D MFI zeolites synthesis procedure by changing the type of precursor such as cations or anions for regulating the nanosheet size [26].

9.2 Synthetic Method Two-dimensional zeolites are synthesized mainly using bottom-up method and top-down method. Bottom-up method comprises with hydrothermal synthesis at certain circumferences or synthesis by using structure-­ directing agents. But, top-down method involves dismantle of constructed zeolites. The 2D zeolites are exfoliated and used as membranes or catalysts for producing pillared materials of different structure.

9.2.1 Bottom-Up Method In this process, the 2D zeolites are recognized as intermediate product which is called as layered precursors at the time of syntheses using structuredirecting agents. Here, the molecules in structure-directing agents organized the layers in such a way that the opposite silanol groups can condense into oxygen bridges on the surface of two adjacent layers, forming a four-connected zeolite upon calcination [16]. The first layered precursor during the hydrothermal synthesis was MCM-22P. Here, the 2D zeolites can be easily formed by swollen, pillared, and delaminated in the lacking of covalent bond linking the layers. Other precursors for 2D zeolites are CDO and FER consisting with nonporous ­ferrierite-type silicate layers in 3D CDO and FER zeolites. RUB materials like RUB-20, RUB-40, and RUB-48 are also related to CDO or FER precursors depending on different order of layers and nature of structure-­directing agents such as, di-ethyl-di-methylammonium, tetramethylammonium, methyl-triethylammonium, tri-methyl-iso-­propylammonium, and ­tetramethyl-phosphonium [10]. Ryoo et al. (2006) established a new method for developing 2D zeolites by using multifunctional cationic structure-­ directing agents with two parts. One part is responsible for the zeolite phase development and another part prevents the formation of crystallization in one-way. Such types of bifunctional structure-directing agents comprise with quaternary ammonium groups (−N+(CH3)2−) and hydrophobic tail like long aliphatic chains (−C22H45) prevents the 3D zeolite crystal formations. This method is very effective for the limitation of zeolite accessibility for transformations of bulky molecule [8]. For multifunctional SDA, both the hydrophobic and hydrophilic parts can be adjusted for the arrangement

198  Layered 2D Advanced Materials and Their Allied Applications of final formation of lamellar material where SDA located in the pores and the hydrophobic aliphatic sequence length governs the distance between the adjacent layers. The thickness can be controlled by the different number of quaternary ammonium groups which are connected by aliphatic bridges. At last, the crystallization process can be tuned by changing the nitrogen atom surrounding for the zeolite with mesostructured or layer assemblies. The 2D zeolite layer arrangement depends on the variation of synthesis process at different conditions and the design of SDA molecules. MWW zeolite was also developed as 2D zeolite by using multifunctional SDAs. MIT-1 zeolite is found as superior textural properties than MWW, e.g., MCM-56 using hydrothermal process without the presence of multifunctional SDAs [10].

9.2.2 Top-Down Method In this method, the 3D zeolite can be converted into 2D zeolite by chemically selective transformation like utilizing the anisotropy in the zeolite structure of crystals for structure unreliability. It is the opposite way, in which the respective layers should be as intact in the whole procedure and the disassembly of condensed zeolite participates for the formation of layered derivative [16]. The position of changed units in the adjacent layers is the most important requirement in the transformation of top-down approach. The higher ratio of Si/Ge in the 2D layers confirms the higher constancy at the time of treatment but low ratio of Si/Ge in d ­ ouble-four rings gives necessary changes and responsible for reorganization of units. In germane-silicate zeolites, the chemically selective disassembly is possible using different topologies such as ITH, UTL, ITR, and IWR. In top-down approach, the fine layers formation with narrow thickness from original zeolite is observed as favorable and highly desirable [10]. The thin layer of 2D zeolite thickness leads to a very lower stability which restricted the post-synthesis transformation like exfoliation or swelling. The hydroxyl molecule with large surface density also limited the swelling or exfoliation after the disassembly of layers. So, the 2D zeolite materials are used as augmented building units for the layer assembly configuration known as Assembly-Disassembly-Organization-Reassembly (ADOR) procedure [16]. The ADOR procedure comprises with four steps: (1) assembly (the initial zeolite preparation); (2) disassembly (degrades into 2D building-blocks/layers); (3) organization (arrangement of layers in a particular orientation); (4) reassembly (condensation to a new structure). This procedure was first applied for the developing of OKO and PCR type zeolites from UTL as parent framework [22]. This is still very rare due to

2D Zeolites  199

D4R

ssembly

isassembly

rganization

eassembly

S4R

Figure 9.2  Schematic diagram of the ADOR process (Reprinted with permission Prech et al., 2018 (copyright 2018) The Royal Society of Chemistry Inc.)).

the theoretical calculations and “unfeasible” synthesis [16]. The schematic diagram of the ADOR process is given in Figure 9.2. In this method, the growth of 2D zeolite crystal can be controlled by non-templating methods in the absence of structure-directing agents as porogens. The thickness of the crystal is controlled by organic and inorganic morphology modifiers which interconnect some crystal facets that causes in-commensurate growth rate in various directions. The major disadvantage of this process is less than several nanometers thickness rarely achieved. For synthesize FAU zeolite, different organo-silanes can be used which varying the pore system of zeolite crystals: 1) the mesopores or macro pores linking the adjacent layers; 2) intracrystalline mesopores; 3) zeolitic microspores. But, the organo-silane molecules are very tough to make ready and also very difficult the unreacted molecules removal by post-­ synthetic treatment. So, different type of inorganic salts is used as morphology modifiers. The assemblies of FAU zeolite nanosheet can be obtained in the absence of morphology modifiers. For developing plate-like crystal of FAU or EMT aluminum concentration is very important. Such inorganic modifier is desirable and cost effective for the formation of 2D zeolite [10].

9.2.3 Support-Assisted Method In this approach, some 2D facilitating agents are used with good mechanical and thermal stability for 2D zeolite synthesis. Graphene oxide is used for the preparation of hybrid nanocomposite as well as soft material. Multilamellar MFI can be obtained and also the 2D silica on metals like Pd (111) and Ru (0001) can be produced by this process. In this process, at first, the

200  Layered 2D Advanced Materials and Their Allied Applications metal film is coated on support then silica/alumina is deposited. When the materials deposited to metal by weakly physisorption, then the crystal layers incommensurately grow and thus the crystal of 2D zeolite is formed [10].

9.2.4 Post-Synthesis Modification of 2D Zeolites The post synthesis modification of 2D zeolite is a technique in which the structure of nano-sheet assemblies can be changed but the interlayer of 2D zeolite structure cannot be changed. The process includes swelling, pillaring, exfoliation, delamination, and interlayer expansion [10]. In swelling, the modification is done by the rupture of hydrogen bonds in the interlayer and enhancement in the space of interlayer. First swelling was accomplished for 2D zeolites using quaternary ammonium surfactant at high pH medium. After swelling, when the layers get separated by using organic surfactants, the other materials also prepared and when the surfactants are removed the layers get collapsed. The interlayer distance can be preserved by using supplemental support that can be transforms into amorphous mesoporous silica pillars. MCM-36 in MWW family was the first reported zeolite pillared with silica. In the modification of 2D zeolites, the pillars structure was uncertain due to the lack of regularity [10, 16]. A delaminated substance is a collection of randomly placed lamellae which contain non-aggregate or discrete zeolites. So, the morphological damage or deterioration can be possible during delamination or exfoliation. The main feature of the material is open structure and the whole external surface is accessible from the inter-particle space or mesopores [10]. Exfoliation with efficient purification makes very thinner membrane from 2D zeolite. To optimize the segregation, different organic solvents can be used for complete dispersion of MFI nano-sheets. The swollen materials are distributed into the single lamellae colloidal solution. MWW and MFI can be formed by exfoliation into single layers [16]. Non-functionalized polymers and polybutadiene that possess carboxyl or hydroxyl groups play an important role in the exfoliation process of 2D MWW formation. The best option for dispersion was n-Octanol and stabilization of the suspension [10].

9.3 Properties When one dimension of a zeolite is very less below several nanometers (1–2 nm) consist with one or two unit-cells those are accounted as 2D zeolites. In such type of zeolite, external surface is increased and the modification of post-synthesis is possible [16]. The determination of adsorption,

2D Zeolites  201 transport, and catalytic properties of such zeolites are very essential [23]. Also, for zeolitic performance, the wrinkling and layer thickness determination is highly preferable [10]. Various types of zeolite are available like AFO, MTF, RRO, AST, MWW, and SOD. Synthesized zeolites with differently arranged from 2D precursors are NSI, CDO, CAS, and FER [22]. The properties of different zeolites framework depend on various synthesis procedures. Here, the different properties of 2D zeolites has been discussed like structural, acidic, catalytic, and adsorption properties [10]. The properties of 2D zeolite are given in Figure 9.3. The thickness of 2D zeolites can be measured by atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray reflectometry (XRR). The structure of 2D zeolite evaluation is important for internal and external composition of the materials. Quantitative mapping of reciprocal lattice and STEM are used for the determination of ­atomic-scale wrinkles [23]. The changes in intensity of the diffraction spot for the tilt angle determines the characteristic of thickness. The change in shape of diffraction spot identify the extent of wrinkling [10]. The enhanced accessibility of active sites is the most important advantage of 2D zeolites over 3D analogues while other characteristics like coordination, geometry, and strength of acidity remain the same. The active sites of 2D and 3D zeolites are represented by the substitution of isomorphously silicon atoms in the crystalline structure of silica. The heteroatoms such as Ti, Al, Zr, Sn, Ga, Fe, B, and Ge are incorporated in the zeolite structure by replacing silicon atoms for introducing the catalytic activity. Various atoms as silanol groups in extra-framework positions can also be active Non toxic 10

Fine tuning of bulk properties

Adjustable surface properties

1

8

3

Nanozeolite

7

Bi-modal porosity

Diverse morphology

2

9

Short diffusion pathway

Tunable size

4 6

5

Large surface area

High stability in suspensions

Easy shaping

Figure 9.3  Properties of 2D zeolites (Reprinted with permission Zaarour et al., 2013 (copyright 2013) Microporous and Mesoporous Materials Inc.)).

202  Layered 2D Advanced Materials and Their Allied Applications catalytically. The above mention heteroatoms incorporated in the zeolite structure represent the acid sites of Brønsted or Lewis nature [16]. Some 2D exfoliated zeolites can be used as molecular sieves. The thickness and wrinkling determination are very crucial for their performance. Those can be determined by comprehensive 3D mapping of its reciprocal lattice [23]. The properties of surface of 2D zeolites, obtained from ITH and UTL zeolites, can be improved by incorporating several molecules such as CO2, CH4, H2O, N2, and H2. The changes in binding energies is done by various distinctive effects: (i) hydrogen-bond with electrostatic interactions with the surface silanols, (ii) the confined space increases the dispersion interactions for 3D zeolites, and (iii) structural flexibility of 2D zeolites [21]. Wu et al. (2016) studied the modification of surface of the PMFI and PMWW zeolites employing atomic layer deposition by using silicon (ALD-Si) and aluminum (ALD-Al). By keeping the micropore almost intact, they found that the modifications reduces the mesopores volume and significant changes in the composition of external surface [7]. Novel organic-inorganic material was developed by using IPC-1P precursor from zeolite UTL. By varying the connector or process conditions, the structural properties can be managed in a wide range. Surface areas and the volumes of pores are enhanced than parent zeolite UTL and the micropores has been increased with enlarging the size of organic connector and rigidity [8]. Due to the large electrostatic interaction with surface silanols, the molecule interconnection with dipole and quadrupole is more strong for 2D zeolites derived from UTL and ITH than 3D zeolites [10]. Using the adsorption measurements and molecular simulations, the properties of adsorption of 2D and 3D zeolites have been examined. For longer n-nonane and n-hexane adsorbates because of the “edge effects”, the adsorption in 2D zeolites is more complex. Only a fragment of adsorbate is restricted into the micropores, when the remaining particles are placed in the mesopores. It has been studied that for 2D MFI, adsorption in the micropores has been unaffected due to the existence of mesopores [10]. The acidic property of nanolayered ZSM-5 zeolite which is synthesized with the multi-quaternary ammonium surfactants has been observed. Here, the Brønsted acid site has same strength as bulk HZSM-5. 2D zeolite consists with Brønsted acid sites at the external surface. Uni-lamellar zeolite contains higher masses of Brønsted acid and silanol sites than multilamellar ones. The Brønsted acid sites in the 2D zeolites are comparatively less than the tetrahedral aluminum, that increases the zeolite thickness [27]. Acidity of PMWW and PMFI zeolite was studied and found for ALD-Si the acidity from external surface moderately decreased, while for ALD-Al, acidity at the surface has been increased and expanded acid sites [7]. The strong

2D Zeolites  203 Brønsted acid sites are developed by connecting [Si(OH)Al] and trivalent ions of metals. The other tetravalent hetero-elements develops different strength of Lewis acid [16]. Strong acidity and stability is highly preferable for catalytic applications at elevated reaction temperatures [27]. Ultrathin zeolite can increase accessibility to reactive sites as well as catalytic applications. The zeolite UTL can be formed by using any number of aliovalent dopants which provide active sites for catalysis [6]. Wu et al. (2016) found that the ALD-Al and ALD-Si cannot effect the inherent catalytic performance in PMWW and PMFI zeolites of Brønsted acid sites, but they modulate the selectivity of acid sites in benzyl alcohol alkylation with reactions [7].

9.4 Applications Zeolites can be used in various fields such as catalysts, adsorbents, ion exchangers, and membranes in a huge number of production applications. But, 2D zeolite improves its performance because of its higher selectivity and activity. So, 2D zeolites effectively are used as catalysts in the various reactions. The reactions rate can be intensified due to its surface-active sites that enhance the mass transfer.

9.4.1 Petro-Chemistry The advantage of 2D zeolite is easy diffusion of reactants and the products due to the open structures of 2D zeolites. Ethylbenzene is produced from ethylene by liquid-phase benzene alkylation process using a delaminated zeolite (MCM-56) which has greater activity than conventional zeolites such as, MCM-49 and MCM-22 [19]. In 2D zeolite, the smaller channels length and higher acid sites concentration on the outside surface remarkably increases the activity of the zeolites. Martinez et al. (2007) found cobalt-supported ITQ-2 which was better than SBA-15 catalysts. It is very effective in bioethanol steam reforming [28]. With high concentration of silanol groups on the external surface makes the 2D zeolites very popular in ε-caprolactam production from cyclohexanone oxime [19].

9.4.2 Biomass Conversion 9.4.2.1 Pyrolysis of Solid Biomass In this process, zeolites perform very well due to their potential of aromatic productions and thus produce lower oxygen oils with higher grade

204  Layered 2D Advanced Materials and Their Allied Applications as biofuels. 2D zeolite has the advantage of larger outside surface areas and pore size that effectively diminish the diffusion effect [16]. Lee et al. (2014) studied uni-lamellar mesoporous MFI nanosheet (UMNs) that upgrades the pyrolysis vapors from the separate fragment of lignocellulose such as hemicellulose, cellulose, and lignin. UMN has larger activity for deoxygenation and cracking with greater quality [29]. Naqvi et al. (2017) found that medium pore size 2D zeolite like ITQ-2 and ZSM-5 give best performance than MCM-22 for obtaining aromatics for better pore dimension with more acidity [30].

9.4.2.2 Condensation Reactions For biofuel and others valuable chemicals production from biomass, condensation reactions are found very interesting. In this reaction, the small, oxygenated, and reactive compounds are converted to more substantial and larger particles. 2D zeolites are accounted as catalysts for aldol condensation in large composite obtained from biomass [16]. Kikhtyanin et al. (2014) developed different types of layered zeolites from MWW used for aldol condensation for acetone [31].

9.4.2.3 Isomerization Due to the high versatility of zeolites, they can be used in isomerization reactions. The 2D zeolite in isomerization reaction is very effective such as Sn-SPP zeolites effectively converted glucose to fructose when glucose is converted to ethyl-fructoside and after hydrolysis of the fructoside with water at the reaction end, fructose is obtained [16]. Xylose also converted to xylulose using acidic zeolites by isomerization reaction. Paniagua et al. (2015) studied that maximum xylulose is formed up to 47% by using zeolites FAU (Y) compared to other zeolites like MFI (ZSM-5) and BEA (Beta) [32]. Li et al. (2014) observed the mechanism for the production of fructose from glucose by different types of Sn-zeolites (MFI, BEA, MOR, and MWW) using periodic DFT calculations. It is found that very small amount of Sn-MWW gives higher conversions and larger reactive sites [33].

9.4.2.4 Dehydration Reactions Organic matters are formed by using dehydration reactions obtained from biomass. Sugars has been transformed into 5-hydroxymethyl by zeolites. The hexoses dehydration and into 5-HMF completely depends on the

2D Zeolites  205

Lignocellulose

Catalytic cracking and pyrolysis Hydrotreatments Biofuels Condensations

Sugars

Isomerization Biochemicals

Lipids

Dehydration Alkylation

Figure 9.4  Biomass conversion using 2D zeolites (Reprinted with permission Prech et al., 2018 (copyright 2018) The Royal Society of Chemistry Inc.)).

properties of zeolites, various situations, and sugar types. Glycerol also converted to acrolein by dehydration which is the middle product for acrylic acid production, mainly used for paints, plastics, and adhesives [16]. Abdelrahman et al. (2017) also studied conversion of butadiene from biomass by dehydration followed by different reactions. Among them, P-SPP zeolites were found highly efficient by the conversions up to 83% than other catalysts such as ZSM-5, Sn-Beta, ZrO2 [34]. The biomass conversion using 2D zeolites is given in Figure 9.4.

9.4.3 Oxidation Reactions Oxidation occurs by the substitution of titanium or iron with 2D MFI in addition to aluminum for oxidation-reduction properties. Iron-modified MFI has higher catalytic exploiting in oxidation of benzene in converting phenol from nitrous oxide because of the presence of iron and deactivation rate decreases with the reducing of iron content. In epoxidation reaction, Titanium modified 2D zeolite (Ti-ITQ-2) exhibits higher selectivity and activity in olefins epoxidation. Similarly, Titanium modified 2D zeolite ITQ-6 also a highly active epoxidation catalyst [19]. Corma et al. (2000) studied that Ti-ITQ-6 is better than Ti-BEA or Ti-FER in norbornene and hexene epoxidation. Using hydrogen peroxide as oxidation agent,

206  Layered 2D Advanced Materials and Their Allied Applications delaminated titano-silicates provide higher catalytic performance [35]. Wu and Tatsumi (2004) also found the performance of delaminated Ti-zeolites is excellent in cyclo-dodecene and cyclooctene epoxidations [36].

9.4.4 Fine Chemical Synthesis MWW and BEA are found to have similar results with ITQ-2 in di-­ methylacetal origination and tetra-hydropyranylation of phenols and alcohols. Here, ITQ-2 performance was superior from other zeolites. It has been found that the acetalization of 2-acetyl-naphthalene mixed with propylene glycol gives higher yield for ITQ-2 about 63% [19]. Lima et al. (2010) experimented on delaminated zeolite from xylose to furfural by liquid phase cyclodehydration. This delaminated zeolite enhanced the xylose production around 80%−85% [37].

9.4.5 Organometallics Two-dimensional zeolites with large exterior surfaces can be modified with organic or organometallic moieties for better performance. ITQ-6 can be modified with organic amines that provides excellent adsorption capacity than similarly modified SBA-15 materials for CO2 capture [19]. The exterior surface of delaminated zeolite like ITQ-2 and ITQ-6 has been modified with organometallic by using different reactions. It provides higher activity and recyclability without leaching by addition with palladium acetate. Corma et al. (2006) experimented on delaminated ITQ-2 by used N-heterocyclic carbene-gold composite for better performance [38]. ITQ-2 and ITQ-6 also can be immobilized by β-galactosidase enzymes which are highly stable due to its electrostatic and covalent interactions. Delaminated ITQ-2 can also be modified by different amines accommodated with Pd. These types of metals are catalytically very active for reaction of butyl acrylate [19].

9.5 Conclusion In the field of research and development, 2D zeolites have gained the growing interest due to their flexible nature and better performances. Although the 2D zeolites chemistry is more complex and their preparation is quite challenging, still they are obtained with improved properties. But, optimize

2D Zeolites  207 synthesis procedure makes them very cost effective and susceptible in industrial applications. Two-dimensional zeolites are synthesized mainly using bottom-up method and top-down method. Bottom-up method comprises with hydrothermal synthesis at certain circumferences or synthesis by using structure-directing agents. But, top-down method involves dismantle of constructed zeolites. The 2D zeolites are exfoliated and used as membranes or catalysts for producing pillared materials of different structure. The structure analysis of 2D zeolite is very essential for conglomerate and inherent applications. 2D zeolites permit the managing of diffusion length in nm scale. The different properties of 2D zeolites are structural, acidic and catalytic, and adsorption properties mainly depend on different synthesis procedures. But, the challenges that limit the application are cost, length, low yield, and purification process. Two-dimensional zeolites can be used in wide area such as adsorbents, catalysts, ion exchangers, and membranes. But, the performance of 2D zeolite is improved because of its higher selectivity and activity. So, 2D zeolites effectively can be used as catalysts as well as membranes. The performance can be intensified due to its surface-active site that enhances the mass transfer.

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208  Layered 2D Advanced Materials and Their Allied Applications 8. Opanasenko, M.V., Shamzhy, M.V., Jo, C., Ryoo, R. et al., Annulation of phenols: Catalytic behavior of conventional and 2d zeolites. ChemCatChem, 6, 1919–1927, 2014. 9. Kubů, M., Roth, W.J., Greer, H.F., Zhou, W. et al., A new family of two-­ dimensional zeolites prepared from the intermediate layered precursor IPC-3P obtained during the synthesis of TUN zeolite. Chem. - Eur. J., 19, 13937–13945, 2013. 10. Heard, C.J., Čejka, J., Opanasenko, M., Nachtigall, P. et al., 2D Oxide Nanomaterials to Address the Energy Transition and Catalysis. Adv. Mater., 31, 1–33, 2019. 11. Liu, G., Jin, W., Xu, N., Two-Dimensional-Material Membranes: A New Family of High-Performance Separation Membranes. Angew. Chem. - Int. Ed., 55, 13384–13397, 2016. 12. Roth, W.J. and Ejka, J., Two-dimensional zeolites: Dream or reality? Catal. Sci. Technol., 1, 43–53, 2011. 13. Jiang, J., Jorda, J.L., Diaz-Cabanas, M.J., Yu, J. et al., The synthesis of an extralarge-pore zeolite with double three-ring building units and a low framework density. Angew. Chem. - Int. Ed., 49, 4986–4988, 2010. 14. Jo, D., Park, G.T., Shin, J., Hong, S.B., A Zeolite Family Nonjointly Built from the 1,3-Stellated Cubic Building Unit. Angew. Chem. - Int. Ed., 57, 2199– 2203, 2018. 15. Čejka, J., Centi, G., Perez-Pariente, J., Roth, W.J., Zeolite-based materials for novel catalytic applications: Opportunities, perspectives and open problems. Catal. Today, 179, 2–15, 2012. 16. Přech, J., Pizarro, P., Serrano, D.P., Áejka, J., From 3D to 2D zeolite catalytic materials. Chem. Soc. Rev., 47, 8263–8306, 2018. 17. Xu, L. and Sun, J., Recent Advances in the Synthesis and Application of TwoDimensional Zeolites. Adv. Energy Mater., 6, 1–18, 2016. 18. Abdullahi, T., Harun, Z., Othman, M.H.D., A review on sustainable synthesis of zeolite from kaolinite resources via hydrothermal process. Adv. Powder Technol., 28, 1827–1840, 2017. 19. Roth, W.J., Nachtigall, P., Morris, R.E., Čejka, J., Two-dimensional zeolites: Current status and perspectives. Chem. Rev., 114, 4807–4837, 2014. 20. Roth, W.J., Shvets, O.V., Shamzhy, M., Chlubná, P. et al., Postsynthesis transformation of three-dimensional framework into a lamellar zeolite with modifiable architecture. J. Am. Chem. Soc., 133, 6130–6133, 2011. 21. Hermann, J., Trachta, M., Nachtigall, P., Bludský, O., Theoretical investigation of layered zeolite frameworks: Surface properties of 2D zeolites. Catal. Today, 227, 2–8, 2014. 22. Eliášová, P., Opanasenko, M., Wheatley, P.S., Shamzhy, M. et al., The ADOR mechanism for the synthesis of new zeolites. Chem. Soc. Rev., 44, 7177–7206, 2015.

2D Zeolites  209 23. Kumar, P., Agrawal, K.V., Tsapatsis, M., Mkhoyan, K.A., Quantification of thickness and wrinkling of exfoliated two-dimensional zeolite nanosheets. Nat. Commun., 6, 1–7, 2015. 24. Zukal, A., Kubů, M., Pastva, J., Two-dimensional zeolites: Adsorption of carbon dioxide on pristine materials and on materials modified by magnesium oxide. J. CO2 Util., 21, 9–16, 2017. 25. Jayaramulu, K., Masa, J., Morales, D.M., Tomanec, O. et al., Ultrathin 2D Cobalt Zeolite-Imidazole Framework Nanosheets for Electrocatalytic Oxygen Evolution. Adv. Sci., 5, 1–9, 2018. 26. Wei, R., Yang, H., Scott, J.A., Aguey-Zinsou, K.F. et al., Synthesis of 2D MFI zeolites in the form of self-interlocked nanosheet stacks with tuneable structural and chemical properties for catalysis. Appl. Mater. Today, 11, 22–33, 2018. 27. Wu, L., Magusin, P.C.M.M., Degirmenci, V., Li, M. et al., Acidic properties of nanolayered ZSM-5 zeolites. Microporous Mesoporous Mater., 189, 144–157, 2014. 28. Martínez, A. and Prieto, G., Breaking the dispersion-reducibility dependence in oxide-supported cobalt nanoparticles. J. Catal., 245, 470–476, 2007. 29. Lee, H.W., Park, S.H., Jeon, J.K., Ryoo, R. et al., Upgrading of bio-oil derived from biomass constituents over hierarchical unilamellar mesoporous MFI nanosheets. Catal. Today, 232, 119–126, 2014. 30. Naqvi, S.R. and Naqvi, M., Catalytic fast pyrolysis of rice husk: Influence of commercial and synthesized microporous zeolites on deoxygenation of biomass pyrolysis vapors. Int. J. Energy Res., 42, 1352–1362, 2018. 31. Kikhtyanin, O., Chlubná, P., Jindrová, T., Kubička, D., Peculiar behavior of MWW materials in aldol condensation of furfural and acetone. Dalton Trans., 43, 10628–10641, 2014. 32. Paniagua, M., Saravanamurugan, S., Melian-Rodriguez, M., Melero, J.A. et al., Xylose isomerization with zeolites in a two-step alcohol-water process. ChemSusChem, 8, 1088–1094, 2015. 33. Li, G., Pidko, E.A., Hensen, E.J.M., Synergy between Lewis acid sites and hydroxyl groups for the isomerization of glucose to fructose over Sn-containing zeolites: A theoretical perspective. Catal. Sci. Technol., 4, 2241–2250, 2014. 34. Abdelrahman, O.A., Park, D.S., Vinter, K.P., Spanjers, C.S. et al., BiomassDerived Butadiene by Dehydra-Decyclization of Tetrahydrofuran. ACS Sustainable Chem. Eng., 5, 3732–3736, 2017. 35. Corma, A., Diaz, U., Domine, M.E., Fornés, V., Ti-ferrierite and TiITQ-6: Synthesis and catalytic activity for the epoxidation of olefins with H2O2. Chem. Commun., 2, 137–138, 2000. 36. Wu, P. and Tatsumi, T., A new generation of titanosilicate catalyst: Preparation and application to liquid-phase epoxidation of alkenes. Catal. Surv. Asia, 8, 137–148, 2004.

210  Layered 2D Advanced Materials and Their Allied Applications 37. Lima, S., Antunes, M.M., Fernandes, A., Pillinger, M. et al., Catalytic cyclodehydration of xylose to furfural in the presence of zeolite H-Beta and a micro/mesoporous Beta/TUD-1 composite material. Appl. Catal. A Gen., 388, 141–148, 2010. 38. Corma, A., Gutiérrez-Puebla, E., Iglesias, M., Monge, A. et al., New heterogenized gold(I)-heterocyclic carbene complexes as reusable catalysts in hydrogenation and cross-coupling reactions. Adv. Synth. Catal., 348, 1899–1907, 2006. 39. Ryoo, R., Cho, K., Mota, F.M., Mesostructured Zeolites, Xiao F.S., Meng X. (eds), pp. 101–148, Springer, Berlin, Heidelberg, 2016.

10 2D Hollow Nanomaterials S.S. Athira†, V. Akhil†, X. Joseph†, J. Ashtami† and P.V. Mohanan* *

†

Toxicology Division, Biomedical Technology Wing, Sree Chitra Tirunal Institute for Medical Sciences and Technology (Govt. of India), Thiruvananthapuram, India

Abstract

Discovery of engineering nanomaterials can be considered as a milestone in nanoscience owing to the appearance of an array of nanostructures which could resolve many flaws of traditional technologies in industrial, commercial, as well as medical sectors. Regarding family of such engineered nanomaterials, 2D hollow nanomaterials (HNMs) embrace a bunch of unique and attractive characteristics. Multidisciplinary research has put loads of attention on these materials as it ensure high surface area, aspect ratio, well-defined active sites, and void inner core which offers room for different cargoes. Present chapter addresses a comprehensive execution on account of HNMs which covers: a brief introduction, structural aspects and properties, various synthesis strategies, medical and non-medical applications, challenges and toxicity profiling, and future perspectives. Major non-medical applications considered here are: HNMs as nanosensors, in batteries, as catalysts, etc. Relatively, more emphasis has devoted for medical applications including in diagnostics and therapeutics, in cancer treatment, photothermal therapy (PTT), in anti-neurodegeneration, regenerative medicine, and as biosensors. Being a major challenge faced by scientists nowadays, toxicity assessment of nanomaterials is a prerequisite in nanotechnology. Hence, this would also be discussed regarding HNMs. A future outlook has provided which focuses on the exciting possibilities on the topic in future. Keywords:  2D hollow nanomaterials, imaging and diagnostics, cancer therapy, regenerative medicine, nanosensors, energy storage, cytotoxicity

*Corresponding author: [email protected]; [email protected] † All authors contributed equally Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (211–248) © 2020 Scrivener Publishing LLC

211

212  Layered 2D Advanced Materials and Their Allied Applications

10.1 Introduction Nanotechnology has witnessed an implausible rise in its aptitude all along the track it travelled from the period of emergence. The ultimate reason behind this exciting progression includes development of countless nanostructures which could rectify or outweigh the traditional technologies for reasonable applications. Among the well-known engineered nanomaterials, HNMs encompass relatively a newer category which offers immense level of unique characteristics to be explored in a useful way [1]. They have captured immense interdisciplinary scientific interest primarily due to varying structural and functional properties with tremendous application potentials. Structurally, they possess well-defined boundaries and internal void space. These materials are provided with high aspect ratio, surface area, lower density, and, moreover, certain desirable features like magnetic properties, catalytic properties, and high volumetric drug loading capability [2]. HNMs have generated an inimitable place in various application scenarios like photo catalysis, electro catalysis, homo and heterogeneous catalyses. One of the major drawbacks of nanocatalysts is the heterogeneous distribution of surface atoms and thereby the overall nanosurface energy level required for effective functioning [3]. There exist a plethora of studies focusing on facet controlled synthesis of nanoparticles for bypassing this obstacle, which could generate different atomic energy states. Past few years have witnessed the fabrication of hollow nanostructures which ensure the homogenous distribution of surface atoms with high level of surface area, energy, as well as catalytic properties [4]. The major attractive characteristic of such hollow nanomotifs is the presence of facets with edges possessing large cavities. This unique cavity further presents elevated frequencies of collision between reactants and surface atoms during catalytic reactions. HNMs can adopt various shapes and morphologies including: nanospheres, nanotubes, nanocages, nanorods, etc. Certain polymeric structures like dendrimers and micelles can also be included in this category as it contains a hollow interior space for loading of cargoes [5]. Conventionally, HNMs can be developed by incorporating a void space inside a solid shell material, probably with a core material inside. However, fabrication of such a material in nanoscale range is undoubtedly a difficult to mission to perform. In order to turn an intended application to fulfil its aim, HNMs should be generated with precise homogeneity, uniformity, and reliability. Functionalization strategies using suitable agents have provided enormous advantages in the expected results [6]. For instance, nanotubes can be functionalized with biomolecules like proteins, nucleic

2D Hollow Nanomaterials  213 acids, amino acids, and antibodies for specifically targeting sites of interest in body. The cargo incorporated earlier can be delivered securely without any non-specific leakage or degradation. This exciting discovery has laid a milestone in the field of nanotechnology which offers advantages unbeatable by conventional technologies [7]. Experts of nanotechnology have incorporated anti-neoplastic, anti-inflammatory, and anti-biotic drugs to HNMs with astonishing outcomes against cancers and infectious diseases, respectively. Also, these structures have find their applicability in regenerative medicine wherein different tissue scaffolds have generated which allowed tissue regeneration without any side effects nearly similar consistency to that of normal living tissue. Because of the occurrence of manifold parameters in HNM organization, fine tuning of these parameters allows scrutinization of these nano entities developed to meet a particular function. Outer surface, inner surface, pores in the shell, core material, etc., can act as perfect sites for functionalization in accordance with our intention [8]. During the middle of past century, the stage of HNM synthesis remained as a difficult to meet procedure and travelled up to now with great story of victory in terms of more accuracy, reliability, and ensured multidisciplinary application probabilities. Since there is plenty of rooms to accommodate even millions of atoms, catalytic reaction can be made faster compared to convention rationale [9]. As the applicability of such nanostructures increases day by day, there occurs a parallel hike in demand for toxicity assessment and compatibility evaluations. On account of this, scientists have devoted plentiful efforts on toxicity analysis of different types of HNMs, especially nanotubes, and spheres. In view of all these facts, the present chapter narrates different aspects of HNMs including structural details, categorization, various synthesis procedures, applications, as well as toxicity profiling done so far. For the acquaintance of readers, authors would like to make a declaration that intensions alike reviewing of such emerging topics is required all time in order to widen the application possibilities associated with it.

10.2 Structural Aspects of HNMs By the virtue of their novel morphological characteristics like high surface area, high porosity HNMs have emerged as an important class of functional materials. Mainly, HNMs exist in the form of spheres, tubes, fibers, and boxes [10]. Depending on the structural complexity, these materials are classified into simple hollow materials and complex hollow materials. Simple hollow materials have only a single layer around the cavity whereas

214  Layered 2D Advanced Materials and Their Allied Applications complex materials will have multiple cavities leading to multiple boundaries and interfaces. The fabrication of the latter will be more challenging due to their complex structure when compared to the simple ones. The multiple boundaries and interfaces can lead to additional features than can attribute to various applications. According to their number of shells and type of arrangement different names can be found in the literature. The simplest one is a single shell hollow nanoparticle consisting of one shell. Multi-shell hollow nanoparticles are complex with 2 or 3 shells around the core. Hollow materials with more than 3 shells are generally termed as Onion like nanomaterials. It includes yolk-shell nanomaterials and rattle type HNMs. Yolk-shell hollow materials have a rigid solid particle in the inner cavity. These materials can be of either a single shell or multi shell. Rattle-type hollow materials have a core@void@shell structure, with a moving core inside the shell [11]. Multi-shelled structures are better in terms of performance when compared to single shelled materials.

10.3 Synthetic Approaches  After the emergence of hollow 2D materials, there have been several ­bottom-up approaches proposed by various research groups for the synthesis of these nanomaterials [12]. Based on the types of templates utilized for the fabrication of hollow cavity, these strategies can be classified into two: template-based strategies and self-template-based strategies  [13]. Synthesis of Hallow Nanomaterials

Template Mediated

Self Templated Surface Protected Etching

Hard Templating

Ostwald Ripening

Soft Templating

Polymer Based Templates

Emulsion Based Templates

Kirkendall effect

Silica Based Templates

Micelle/Vesicle Based Templates

Galvanic replacement

Carbon based Templates

Gas Bubble Based Templates Electrospray

Figure 10.1  Flowchart showing various synthetic strategies for HNMs.

2D Hollow Nanomaterials  215 The strategies that require a template for the synthesis of hollow materials are classified as template-based strategies. Soft templating and hard templating are the two main methods of template-based strategies [1]. The soft templating strategy does not use templates directly. Self-templating strategy widely uses Etching, Ostwald ripening, Kirkendall effect, and galvanic replacement for the synthesis of the hollow inner cavity. The methods so far utilized for the synthesis of HNMs are summarized in Figure 10.1 [14–17].

10.3.1 Template-Based Strategies The general synthetic strategy involves coating a layer of shell material over the synthesized templates followed by the selective etching of template. The size and shape of the template material will determine the size and shape of the cavity whereas the thickness of the shell is determined by the extent of the coating. Hard templating and soft templating methods of template-based strategies are described below.

10.3.1.1 Hard Templating Out of all other strategies, hard templating is considered to be the simple, most efficient, and straightforward strategy for synthesizing HNMs [1]. The process begins with the synthesis of hard templates using a rigid inorganic material or a polymer with a definite shape. The outer surface of the template is then chemically modified for the attachment of the desired nanomaterial. The chemical modification is achieved by the change in the surface charge or polarity of the template. The desired core material is then coated over the treated template surface and the selective etching of the template will yield HNMs. The material is deposited to the template surface by either sol-gel process or by hydrothermal reactions. Depending on the composition of the hard template, the method of etching is selected. Chemical etching, thermal treatment, or using a proper solvent which can dissolve the template are the various commonly employed strategies to remove the template.

10.3.1.1.1 Polymer

Poly(methyl)methacrylate (PMMA) and Polystyrene (PS) are the two common candidates used for polymer-based hard templating [18]. The polymeric template can be easily removed by using a solvent where the polymer dissolves. In 1998, Donath et al. initially reported the synthesis of hollow spheres through electrostatic layer by layer self-assembly (LbL) of

216  Layered 2D Advanced Materials and Their Allied Applications silica nanoparticles on polystyrene colloidal template. LbL technique relies on the electrostatic attraction between the deposited charged species [19]. Calcination process was used to remove the colloidal templates. Recently, another study revealed the fabrication of hollow G-titania nanocomposite spheres by the assembly of titania and graphene nanosheets on PMMA beads via LbL assembly technique [20]. In the etching step, the PMMA templates were removed by decomposing into the exhaust gas and THF washing.

10.3.1.1.2 Silica

Silica-based templates use micrometer sized monodispersed SiO2 spheres as hard templates [1]. High uniformity, size tunability, as well as low cost are some of its unique features that made the silica spheres to be one of the widely used hard templates. These microspheres are prepared by the typical gel-sol method which involves the hydrolysis and condensation of silicon alkoxides in water/alcohol mixture and ammonia as a catalyst [21]. For example, Hwang et al. fabricated multi shell TiO2 hollow nanoparticles (MS-TiO2-HNPs) in which the SiO2 spheres were prepared via solgel method [22]. Recently, Wu et al. utilized uniform SiO2 spheres as hard templates for the preparation of hollow graphene-based spheres (HSG). The authors utilized the electrostatic assembly process for the formation of graphene oxide coated SiO2 spheres and then removed SiO2 templates using Conc. HCl [23].

10.3.1.1.3 Carbon

Carbon-based materials are widely used as hard templates due to their unique porous nature. Porous nature of these materials will facilitate the formation of shell structure by absorbing the precursors. Moreover, these materials will result in the desired hollow structures due to their easy removal after shell formation. Mesoporous carbon and solid carbon are the most widely used materials for carbon-based hard templating methods. By using mesoporous carbon spheres as templates, Dong et al. reported the synthesis of mesoporous spheres of metal oxides and phosphates [24]. Thomas and co-workers proposed a generalized strategy for the synthesis of metal oxide hollow spheres through hydrothermal approach by using solid carbon as a template [25]. Even though hard templating is considered as the most efficient strategy for the hollow material synthesis, the scarcity in finding cheap rigid materials with desired physico-chemical properties and lack of scale-up

2D Hollow Nanomaterials  217 techniques are considered as their main drawbacks. Sometimes, the etching of the hard template can cause physical as well as chemical changes in the shell structure. In 1995, Ajayan et al. observed a change in morphology of the shell while removing the carbon nanotube (CNT) template through heat treatment [26].

10.3.1.2 Soft Templating This method utilizes liquid or gas materials such as surfactants, polymers, emulsions, and bubbles, etc., as templates for the synthesis of HNMs. Depending on the type of templates, soft templating can be broadly classified into 3 categories, (1) emulsion based, (2) vesicle/micelle based, and (3) gas bubbles based, and are discussed below.

10.3.1.2.1 Emulsion

Oil in water and water in oil emulsion are the widely used emulsion-based soft templates for the synthesis [27]. It is important to note that the influence of pH, temperature, and ionic concentration on emulsion-based synthesis will not only limit the harsh reaction conditions but also will help in making complex hollow materials. McDonald et al. in 2000 initially proposed the preparation of hollow emulsion-based nanomaterials by using a carboxylated core [28]. The hollow structures were formed due to the ionization of the carboxylated core in basic conditions. Following this seminal work, many reports came for the preparation of hollow structures using polymers [29]. One interesting report among them is the fabrication of hollow latex particles with double-layered shell through seeded emulsion polymerization of methyl methacrylate/divinylbenzene/acrylic acid (MMA/DVB/AA) on natural rubber latex seed [30].

10.3.1.2.2 Vesicle/Micelle

Amphiphilic molecules tend to self-assemble in a single-phase solvent system [31]. This self-assembly occurring above its critical micellar concentration (CMC) often leads to micellar or vesicular structures. So, micelles/vesicle with different size and properties can be easily synthesized by varying the parameters like concentration, pH, temperature, and ionic concentration. One of the initial studies revealed the use of surfactant cetyl-trimethylammonium bromide (CTAB) as the vesicle template for the synthesis of uniform multi-layered hollow Cu2O spheres [32]. In a recent study, Qiu et al. demonstrated the synthesis of well-defined hollow

218  Layered 2D Advanced Materials and Their Allied Applications rectangular platelet micelles through the seeded growth method [33]. They added the blends of the crystalline block copolymer and their crystalline homo-polymer to a cylindrical micelle seed.

10.3.1.2.3 Gas Bubble

Gas bubbles dispersed in liquid media are an efficient soft template for the synthesis of hollow nanostructures. The method involves the formation of gas bubble emulsions followed by the attachment of particles onto the surface of gas bubbles. The further growth of the particles over the surface leads to the shell morphology. Gas bubble emulsions are prepared by various methods such as sonication, chemical reactions, or by blowing gas stream into the reaction system. Recently, Zhuo et al. reported the synthesis of IF-MoS2 nanocages by utilizing an ammonium cation bubble as a template for energy storage application [34]. Another study described the construction of a honeycomb-like hollow microsphere for a Li1.2Mn0.52Ni0.2Co0.08O2 cathode material using bubble template method [35].

10.3.1.2.4 Electrospray Method

The electrospray method is widely used for the synthesis of a wide variety of HNMs [36]. The method utilizes an electric field (using an electrospray atomizer) to make charged liquid droplets by injecting the proper liquid through a stainless-steel capillary. The evaporation of the solvent leads to the formation of the hollow structure. In the initial report, Rulison et al. described the synthesis of hollow Yttria nanoparticles by injecting hydrated yttrium nitrate in n-propanol into electron spray atomizer [37]. The particle was pyrolyzed at high temperature for obtaining the final hollow nanomaterial. Recently, this approach has been utilized for the fabrication of hybrid microcapsules with high encapsulation capacity using chitosan and calcium phosphate [38]. Soft templating methods always offer structural and morphological control. The size and shape of the hollow materials can be easily tuned by using soft templating methods. Along with these features, the easy removal of template materials when compared to hard templating projects out them as a promising strategy for the synthesis of hollow materials.

10.3.2 Self-Templating Strategies In comparison to the template-based strategies, self-templating strategies are highly preferred in the industry due to their reduced synthetic steps as well as ease of scaling up. A typical self-templating method has two steps:

2D Hollow Nanomaterials  219 1. Template synthesis, and 2. Conversion of templates into a hollow structure. The templates used in this method are not just templates but they are an integral part of the outer shells. The widely used self-templating strategies are listed below.

10.3.2.1 Surface Protected Etching Surface protected etching is the widely used etching technique among the several other etching methods [14]. It involves the coating of particles using a protective layer and subsequent etching of interior for producing hollow structures. The size of the particles will be retained by the protecting agent whereas the etching agent creates the void. The most widely used particles are sol-gel colloids. The porous structure of sol-gel colloids allowed the efficient migration of the etching agent. The dissolution of colloid particles by etching agent is prevented by using a proper polymeric ligand. Hong et al. used polyacrylic acid as a polymeric ligand, PAA in the synthesis of TiO2 microcapsules from TiO2 microspheres. They synthesized hollow microcapsules by the etching out of the core using DEG as etching agent [39].

10.3.2.2 Ostwald Ripening Ostwald ripening is defined as the deposition of smaller sol particles on the surface of larger particles [15]. It is a thermodynamic process where larger particles are energetically more favoured than the smaller ones leading to the higher solubility of the smaller particles. In 2004, Yang et al. demonstrated the synthesis of hollow TiO2 spheres through Ostwald ripening by the hydrolysis of TiF4 in water [40]. Hollow cavities are formed by the dissolution and re-deposition of the less dense crystallites in the central region to the outer region. Time-dependent experiments revealed the formation of solid TiO2 spheres in shorter time, whereas formation of inner cavities with longer time which is consistent with the dissolution of smaller crystallites in the center. In another work, Peng et al. synthesized core-void-shell NiCo2O4 spheres from porous nanosheets through inside-out Ostwald ripening [41]. Ostwald ripening has become a rapidly growing research area in a short time for the synthesis of multi-shell hollow materials.

10.3.2.3 Kirkendall Effect Kirkendall effect refers to the movement of boundary layers of two metal atoms due to the difference in their diffusion rate [16]. The substitution reactions which involve different diffusion rates such as oxidation, sulfidation,

220  Layered 2D Advanced Materials and Their Allied Applications phosphination, or nitridation reactions of metal with binary compounds or the conversion of the binary compound into a ternary metal oxide or sulfide are the two important methods utilized for the fabrication of hollow materials through Kirkendall effect [42]. Even though it is a classical metallurgical phenomenon, the Kirkendall effect is widely exploited for the design and synthesis of complex hollow nanostructures. In 2004, Yin et al. reported the synthesis of hollow cobalt sulphide nanocrystals by reacting metallic cobalt nanocrystals and sulfur in 1,2-dichlorobenzene at 180oC [43]. As a result of the outward flow of cobalt through the sulfide, shell vacancies are generated. The supersaturation of these vacancies thereby resulted in the cavity formation. Kirkendall effect extended its scope by fabricating hollow materials using a wide variety of materials such as selenides, sulfides, tellurides, etc. [16]. Recently, Zhang et al. reported the synthesis of hollow MoS2@C nanotube through hydrothermal sulfuration of Mo3O10(C2H10N2) nanowire method in which MoS2 nanotubes formation and glucose carbonization happened co-operatively [44]. Kirkendall effect explains the formation of the tubular structure by using Mo3O10(C2H10N2) as a sacrificing template in the sulphuration process.

10.3.2.4 Galvanic Replacement As the name suggests, this is a replacement process arising from the electrochemical potential difference between two metals [17]. Like in Galvanic cell, one of the metals here behave as cathode whereas the salt of other metal as the anode. The process starts with the synthesis of anode nanostructures followed by the oxidation of them. The other metal ions are reduced and coated over the anode outer surface. The initial anode metal will determine the shape and size of the final hollow structure. For example, hollow gold nanostructures were developed using the galvanic replacement reaction between AuCl −4 and Ag nanoparticles [45]. AgNPs are oxidized to Ag+ ions by AuCl −4 due to its high reduction potential resulting in the formation of cavities as well as the reduction of AuCl −4 leads to the deposition of Au atoms on the anode. Inspired from this seminal work reporting the galvanic replacement approach, various hollow nanostructures have been synthesized utilizing this method [46].

10.4 Medical Applications of HNMs Inorganic 2D HNMs including CNT, gold nanoshells, etc., exhibit tremendous unique properties to be applied in various medical applications.

2D Hollow Nanomaterials  221 This could be attributed to the versatile interior as well as outer surfaces which can accommodate large pay loads for an array of medical applications including targeted drug delivery, induced photo thermal therapy, catalytic reactions, etc. Moreover, some more supplementary advantages over other routine strategies include: easier and cost-effective processing steps, precise control over size and composition, tunable structural and functional properties, and also the exciting multifaceted therapeutic approaches possible through a single window [47]. The following section elaborates certain medical applications of HNMs.

10.4.1 Imaging and Diagnosis Applications Hollow nanospheres due to its specialized characteristics, like specific surface area, resonating internal cavity, and attractive optical properties, have been employed widely in bio-imaging applications, photo thermal therapy, as well as drug delivery. As the fact says, Magnetic Resonance Imaging (MRI) is a non-invasive diagnostic and imaging modality which offers high throughput screening of functional and anatomical features of living body [48]. In this high resolution, three-dimensional imaging technique, contrast agents are indispensable as it provide a contrast between damaged and healthy tissue parts. Nanospheres comprise an appropriate option for this; because of their suitability in terms of electronic distribution. MRI measures the hydrogen bonding features of water molecules and spatial distribution of the water molecules. One of the major roles of these contrasting agents is to increase the rate of relaxation of water molecules in the target area so that a contrast can be created indeed for obtaining magnified images of the tissues of interest. The effectiveness of such contrast agents is defined in terms of an ability to accelerate spin-lattice relaxation (T1) or to shorten spin-spin relaxation (T2). If the agent has a high T1 value, it is said to be positive contrast images (Paramagnetic), and if it has a lower T2 value (Super paramagnetic), it is termed as negative contrast images [49]. Among the various ion complexes used as contrasting agents, Gadolinium ion complexes (Gd3+) are being used widely. Especially, it is applied for positive contrast effect for T1-weighted imaging. Huang et al. in 2008 have developed super paramagnetic hollow Gd2O3 spheres using biological gelatin as template. This was introduced as novel study in an aspect of using gelatin as the central core of the hollow structure which determines the overall shape and structure director. The structure was developed via sol-gel method or precursor deposition method using solvent evaporation technique. The resulted hollow spheres of Gd2O3 exhibited

222  Layered 2D Advanced Materials and Their Allied Applications super paramagnetic behavior in which it magnetizes strongly by the induction of an applied magnetic field and the property ceases once the field is removed. According to the authors, this occurred primarily due to presence of the organic layer at the core region. The aforementioned magnetic property got vanished and changed to paramagnetic state thereafter when the residual carbon from the surface of the nanostructure was eliminated or cleaved out after calcination at 800°C. The methodology resulted in a largely brightened T1 images which relies on the higher surface area of the structure and aids proton coordination and also the interaction between water and Gd3+ ions. Authors recommend the use of such newly fabricated nanostructure for a multifunctional therapeutic approach, combining magnetism-based targeted drug delivery and imaging of target tissues [50]. Nevertheless, most of the paramagnetic T1 contrasting agents in MRI are inherently shown to be toxic (e.g., Gd3+). As a result, it cannot be administered directly into patient’s body. Instead, they are being applied as chelates with other agents like diethyltriaminepenta acetic acid (DTPA), ethylene diaminetetra acetic acid (EDTA), etc., in order to assure patient safety and intended bio distribution potential. On account of the observed toxic response of available paramagnetic contrast agents, a new approach have been put forward which includes a new class of MR contrast agent Au3Cu1 [51]. Infrequently high oxidation state of copper ion provides elevated signal contrast both in T1-weighted and T2-weighted imaging even at lower concentrations applied. This porous hollow nanostructure was then arisen in the field of MR angiography because of the uniquely high brightened images. Moreover, surface functionalized polyethylenimine (PEI) moieties on Au3Cu1 nanosphere offers platforms for the attachment of biological functional groups, hence mediating therapeutic purposes.

10.4.2 Applications of Nanotube Arrays The term nanotube is conventionally used to refer CNTs which has obtained immense scientific interest over the last few years and are the commonly known building blocks of nanotechnology. These are generally allotropes of carbon derived from graphite in the form of nanoscaled cylindrical structure with several millimeters in length [52]. They have been introduced in drug delivery and other medicinal applications since the period of 21st century itself. The captivating properties of CNTs include: high surface area, excellent chemical, and physical stability, rich electronic distribution, etc. Also they can be conjugated to other biologically relevant moieties like proteins, nucleic acids, drugs, antibodies, to name a few. The internal hollow space allows the structure to act like an excellent vehicle to carry drugs

2D Hollow Nanomaterials  223 to targeted site. The system neither creates any drug leakage nor turning it to be metabolized during penetration into cells. A plethora of reports are there proving the safety loading of different drugs into nanotubes and their delivery into target sites thereafter [53, 54]. Paramount utilization of this structure comprises conjugation of antineoplastic drug on CNTs for cancer treatment along with antibiotic compound on the same in order to compact inflammatory response. Afterwards, a long array of applications came into mainstay assigned for gene therapy, immunotherapy, tissue regeneration, and disease diagnosis. As a matter of fact, all these strategies employ various bioactive molecules like genes, proteins, nucleic acids, etc. [55]. Different application potentials of functionalized nanotubes are illustrated in Figure 10.2. Away from these foremost applications, CNTs have already been introduced in the field of chemical science in which separation of enantiomers for chiral drugs and some chemicals in pharmaceutical industry. CNTs are also used widely in extraction of drugs and impurities from crude media by means of solid phase extraction [56]. Poor solubility and stability of nanotubes in aqueous solutions further necessitates it to get functionalized via proposed methods. It is inevitable for reducing inherent toxic response in in living cells and to obtain improved biocompatibility. Multi-step chemical processing stages are sometimes required for the introduction of biomolecules. For example, conjugation of amino acids demands carboxyl group at the end of nanotubes for peptide bond formation. Conventionally, oxidation with strong acids [e.g., Nitric acid (HNO3)] brings out this step,

Cancer therapy

Biosensor vehicles Therapeutics

Disease diagnosis

Infection therapy

COOM H

G

R NH2

COOM G

R

Gene therapy

Carbon nanotubes

H

NH2

Enantioseparation of chiral drugs

Extraction of drugs & biochemicals

Antioxidants

Figure 10.2  Different biomedical applications of CNTs.

Tissue Anti-neuro regeneration degeneration

224  Layered 2D Advanced Materials and Their Allied Applications which improves the hydrophilic nature of the structure [57]. Some of the major application fields of nanotubes are narrated in following sections.

10.4.2.1 Pharmacy and Medicine In biomedical scenario, nanotubes find their major presentation in targeted drug delivery. In nutshell, the drug of interest s introduced onto or at the interior space via non-covalent interaction and introduced into body via routes like intravenous, intraperitoneal, or ingestion routes or by the aid of magnetic conjugates using assistance of external magnetic fields to organs like lymph nodes. The cell then ingests the drug incorporated nanotube capsule followed by disintegration of biodegradable caps to release the free drug into the site [58]. Factually, nanotubes interact with or cross biological barriers by means of: high level hydrophobicity, π-π specific interaction, covalent and non-covalent bonding, and hydrogen bonding as well. More advantageous feature of using these nanotubes is the low required dose of drugs and thereby lower toxicity and subsequent side effects in tissues; especially in the context of anti-cancer drugs [59].

10.4.2.2 Cancer Therapy Distinct from bare drug moieties used for cancer therapy, hollow nanostructures bearing drugs incorporated within, it can efficiently accomplish drug release without any undesirable drop off in the pathway to target site. Because of the capability to penetrate cell and nuclear membrane effortlessly, the drug can be administered in the exact concentration initially provided. Another advantage in using nanotubes is the availability of multiple sites owing to the high available surface for drug incorporation in contrast to traditional therapeutic means [60]. A collection of reports are available on various anti-tumor drugs administered via this way for cancer management including: doxorubicin, cisplatin, methotrexate, quercetin, paclitaxel, etc. In different situations, magnetic-based targeted drug delivery schemes also have been proposed with excellent outcomes. For example, Xiao et al. in 2012 have proposed a novel synthesis strategy for the development of magnetic-based drug delivery vehicle using magnetic multi-walled carbon nanotubes (MWCNTs) which incorporated Fe3O4 beads for the more easier delivery of a cytostatic antibiotic drug (Epirubicin hydrochloride). Advanced external and internal area of the nanostructure allowed an ordered and arrangement of Fe3O4 beads onto the nanotube surface; providing a network like organization which also offered enough space for conjugation of drug. By the aid of an externally regulated magnetic field,

2D Hollow Nanomaterials  225 the transport of the drug got tuned well without any detrimental side effect to normal tissue. The method provides a new insight to the fabrication of MWCNTs with more flexible advantageous possessions [61]. Considerable focus has been put forward in the field of drug delivery mediated by various nanocarriers as it promises excellent biocompatibility, stability, and safe delivery with minimum toxic impacts. Among the long list of nanomaterials employed for the process, hollow nano silica materials have grasped more attention in this regard, undoubtedly because of the characteristic silane chemistry. In addition, more number of nano­ materials can also be incorporated into these silica-based structures which further offer multifunctional benefits [62]. Unfortunately, this attempt may increase the size of silica nanocarriers, exceeding 200 nm sometimes. One can improve the drug loading capacity and tissue penetration of silica if more porosity is being included. Drugs can enter and retain for longer period until delivery. Occasionally, hydroxyl groups on silica surface hinder normal delivery, which could be avoided if amine groups are conjugated which will provide positive surface charge to the structure. Afterwards the carrier must be covered with some biocompatible polymers like poly-­ ethylene glycol (PEG) or Dextran [63]. Amine modification also enhances controlled delivery of drugs in target sites compared to carboxylic groups. Precise monitoring and regulation of nanocarrier drug delivery remained as an obstacle until the discovery of fluorescing compound-based drug release. Scientists initiated the usage of fluorescing molecules integrated with nanocarriers which can be released at the target site along with the drug thereby tracing the path via fluorescence reading or imaging. Wu et al. in 2007 have demonstrated the synthesis of porous iron oxide nanorods which carried Fluorescein isothiocyanate (FITC) for the monitoring of drug release using in vitro culture of HeLa cells. Controlled or sustained delivery of the drugs made possible by the aid of electrolytes like polyacrylic acid (PAA) and polyethylenimine (PEI); based on the relative compactness of the electrolytes [64]. However, there exists a prospect of fluorescent leakage before the nano vehicle reaches target site. Stimuli-based drug delivery presents an appropriate remedy for this difficulty. Zhu et al. (2018) has proposed a methodology in which the team have fabricated core/shell formulation containing silica as core particle and Fe3O4 as shell. Poly Venyl Pyrrolidone (PVP) was used for the functionalization of the construction to enhance stability and transportation through circulation. The effort fulfilled its objective in which the drug got released at the target without disintegration of the fluorescent molecule anywhere in the circulation. When the drug loaded nanoshell reaches target site, an external magnetic field was applied (performed like stimuli) which triggered rupturing of the shell

226  Layered 2D Advanced Materials and Their Allied Applications thereby releasing drug alone. Fluorescence was not observed to lost by any means, hence ensured a trustful monitoring [65].

10.4.2.3 Immuno and Hyperthermia Therapy CNT-based vaccine development has gained extensive consideration since several years. To mention one, Pantarotto et al. (2003) have developed soluble CNTs bearing biologically active peptides for the elicitation of an immunogenic response in order to analyze the vaccination potential of the preparation. The group has introduced viral protein 1 (VP1) responsible for foot mouth disease into CNTs and introduced into living body. Via gene expression evaluation, it was obvious that the immunogenic response was not due to CNT alone, but a combination effect of CNT-VP1. This confirmed the non-immunogenicity of CNTs indirectly and the validity of using CNTs for the preparation of potent vaccines [66]. CNTs possess strong absorption capabilities at near infrared region (NIR) which can productively be used for ablation of cancer cells. Upon absorption of NIR at 700–1,100 nm, MWCTs generate immense heat which is capable of disintegrating cancer cells with minimum damage to normal cells. Such photothermal approach has shown significant prominence for the eradication of various types of cancers [67].

10.4.2.4 Infection Therapy and Gene Therapy Pathogenic microorganisms including some bacteria and fungus develop resistance against their static agents probably because of the inability or inaccuracy of their vaccines to exhibit their intended action in the body. Nanotube-based delivery of anti-microbial agents has already came into play with exciting outcomes. Vaccine compounds can be conjugated to nanotubes and administered into body through suitable routes. Rosen et al. (2011) have determined that strong anti-fungal agent ­amphotericin-B during combination with CNTs could compact microbial infection far better than (~40% more) bare drug [68]. CNT-assisted vaccine delivery safeguards antigen specific protein confirmation until its delivery which is a crucial factor in determining juvenile antibodies to identify and to establish a proper binding. Gene therapy strategies require governing of nucleic acid strands to cell nucleus; probably for the repairing of any damaged gene sequence or to mimic certain deformed nuclear functions. Due to the high vulnerability of DNA strands, a stable carrier system is very much necessary since it may get subjected to enzymatic degradation or irreversible binding with

2D Hollow Nanomaterials  227 nuclear binding protein or by self-defense systems. MWCNTs can rectify this impediment to a large extend by fighting against these situations. The hollow space can shelter the nuclear material without imparting any damage by external stimuli until delivery which guarantee good therapeutic efficiency [69].

10.4.3 Hollow Nanomaterials in Diagnostics and Therapeutics Apart from being utilized for drug delivery, hollow nanomaterials have long been employed for disease diagnosis and therapeutics as well. Gao et al. (2007) have synthesized another type of core/shell nanoparticles in which FePt/CoS2 served as the base material. Preliminary cytotoxicity evaluation demonstrated that the core moiety Pt exhibited extremely higher level of cytotoxic response in HeLa cells when administered at a concentration of 5 µg/ml. Further tracing of the response has found that upon entering the cells, Fe/Pt core undergoes oxidative disintegration, thereby releasing Fe3+ and Pt2+ species into cell cytoplasm. The upper level permeability of CoS2 shell allowed the passage of Pt2+ ions to assess deeper regions of cell; especially into mitochondria and nucleus. Pt2+ has shown multiple times toxicity levels than traditionalcytotoxic agent, cisplatin. Inside the nuclear compartment, Pt2+ induced apoptotic pathway which ultimately ended up in cell death. This substantiates the applicability of Fe/Pt-CoS2

FePt core CoS2 shell Blood vessel

Cell membrane

Pt2+

FePt destruction Pt2+ release

DNA binding & breakage

Pt2+ Nucleus

Pt2+ Apoptosis Mitochondrial damage

Figure 10.3  Immediate lethal response of Fe/Pt-CoS2 core-shell nanoparticleas a potent anti-tumor agent.

228  Layered 2D Advanced Materials and Their Allied Applications as a potentanti-tumor agent with immediate lethal response (Figure 10.3). The crew has also developed another multifunctional nanoshell which simultaneously acts like a cytotoxic agent and as a contrast agent for MRI. Here, instead of CoS2, they have utilized Fe2O3 as shell part. By conjugating drugs of interest, the shell can be guided safely towards the target site via exposure of a magnetic field [70].

10.4.4 Applications in Regenerative Medicine Hike in advancement of research on structural and functional aspects of hollow nanostructures and its possibility of applying to apply in field of regenerative medicine have contributed great enough for a continual progress in the development of regenerative medicine. Natural as well as synthetic polymers have long been considered foremost because of its higher biocompatibility, degradation potential, and stability in circulation. However, chronic effects of these polymers remain questionable even in this century. In such a scenario, hollow nanomaterials find a non-replaceable role as it could rectify majority of drawbacks existing for aforementioned polymers and other conventional tissue engineering materials [71]. Hollow structures like nanotubes, rods, shells, spheres, etc., have found to assure desirable scaffold characteristics alike mechanical stability and conductivity. Utility of collagen-based CNT scaffolds in smooth muscle cell proliferation has been demonstrated by MacDonald et al. in 2005. Carboxylated MWCNTs in definite concentrations were mixed with aqueous solution of collagen Type-1 along with smooth muscle cells for the development of a cell incorporated CNT-collagen composite material. Authors suggest the applicability of the preparation for faultless tissue regeneration applications [72]. Other mode of applications in tissue engineering field applies for cell tracking, sensing of cell behavior, and proliferation pattern, identifying presence of biological and chemical agents in tissues and accelerating scaffold matrices [52].

10.4.5 Anti-Neurodegenerative Applications Owing to the highly versatile surface characteristics, hollow nanostructures can be easily functionalized so as to turn them permeable to blood brain barrier (BBB). After internalization, these formulations can assess the profounder regions of neuronal tissue in order to deliver the anti-­ neurodegenerative drugs incorporated formerly. Similar to other bodily tissue, in brain also there exists an array of targeting methods for the effective execution of drug delivery [73]. A group of researchers have

2D Hollow Nanomaterials  229 evaluated the target organelles of MWCNTs and the safer routes for the drug delivery in neuronal tissue. The methodology they adopted was mainly focused against one of the dreadful diseases of brain; Alzheimer’s disease (AD) using external administration of the neurotransmitter—­ acetyl choline into the degenerating neurons. In this particular study, it was perceived that MWCNTs induce toxic impacts in mitochondria whereas it exerts pharmacological effect in lysosome; exclusively depending on the dose being administered. A precise knowledge on the safe dose of a drug is very essential so as to profile the vulnerability of sub-cellular organelles. The study emphasized the fact that MWCNTs induce excellent pharmacological effect in lysosomal compartment; by simultaneously causing structural and functional damage to mitochondria as it was evident from the altered membrane potential. Moreover, pharmacologically safer dose of the nanotube was detected as maximum of 300 mg/kg. According to this study, MWCNTs could be considered as an admirable drug delivery vehicle in neuronal tissue at a concentration below mentioned value [74].

10.4.6 Photothermal Therapy Most advantageous feature of nanomaterials in the field of photothermal therapy is their ability to discriminate light absorption in photothermal therapeutic procedures. There exists a wide range of nanomaterials capable to meet the purpose whose size varies widely from colloidal consistency to longer polymers. Gold nanocarriers are the first type of materials used in this category due to their unique property of surface plasmon resonance. Highly versatile nature of gold nanostructure fabrication allows researchers to form varying range of morphologies like nanorods, nanoshells, nanocages, as well as nanospheres [75]. Huang et al. in 2006 evaluated a combinatorial procedure for both diagnosis and photothermal ablation of malignant cells in vitro. The team have utilized the selective overexpression of epidermal growth factor receptors (EGFR) on cytoplasmic membrane of malignant cells (oral epithelial cell lines; HOC313 clone 8 and HSC3) for specific targeting. Antiepidermal growth factor receptor (anti-EGFR) was conjugated on the surface of gold nanorods for enhancing discrimination form non-­malignant epithelial cell line; HaCat. Upon incubation of cells with nanorods, the brightened red light scattered from nanorods was occurred in prominence in malignant cells under dark field microscopy. Prolonged exposure of the laser light at 800 nm caused ablation of malignant cells with lower doses than required for HaCat cells. This further substantiated selective PTT induced tumor ablation in HOC313 and HSC3 cell lines. The particular study

230  Layered 2D Advanced Materials and Their Allied Applications

Non-malignant cell

Malignant cells

Gold nanorods pre-coated with anti-EGFR Cell death at lower doses of laser light

HOC313 clone 8

Cell death at higher doses of laser light

(Red laser light at 800 nm)

HSC3

HaCat cells

Cell incubation with solution of Au nanorods

Au nanorods

Anti-EGFR

Specific binding between EGFR on malignant cells & Anti-EGFR on nanorods

Poor binding of nanorods with non-malignant cells

EGFR on malignant cells

Figure 10.4  Applicability of gold nanorods in combined photodiagnostics and therapy using non-malignant and malignant cell types.

suggests the use of gold nanorods as suitable candidate for combinatorial ­application of photodiagnostics and therapy (Figure 10.4) [76].

10.4.7 Biosensors Nanosensors have emerged as an innovative discovery in the branch of nanoscience which allows detection and administration of agents to inaccessible loci or else by means of conventional methods. Among the different categories, biosensors need prime importance as it allows early detection of lethal diseases, determines the presence of foreign entities during immunological elicitation, recognizes foreign DNA strands to name a few [77]. Functionalized dendrimers have long been applied for the eradication of allergic asthma; because of its tiny size range and reliability to bypass lung airways and even reach the alveolar regions to identify the allergen. Branched pattern of this dendrimers offers rooms for drug incorporation and also targeting ligands can be included [78]. Starkey and Halvorson have reported the presence of iron reducing bacteria during their analysis of role of microbes in natural precipitation on iron. This microbe has been identified to possess significant role in conversion of iron minerals. The undesirable occurrence of such microbes can successfully be eliminated via using iron containing hollow nanostructures. For instance, silver nanoparticles (AgNPs) in caged or sphered structure have been used widely for anti-bacterial activity against E. coli and S. aureus [79].

2D Hollow Nanomaterials  231

10.5 Non-Medical Applications of HNMs The exquisite properties of hollow nanostructures such as the presence of interior void, large surface area, low density, as well as the multiple shell morphology make them an ideal candidate for various practical applications. The interior space of HNMs is widely exploited for various non-medical applications such as catalytic micro or nanoreactors, water treatment materials, sensors, and energy storage materials as shown in Figure 10.5 [8].

10.5.1 Catalytic Micro or Nanoreactors Yolk-shell or rattle-type hollow structures have been utilized as a catalyst for various coupling reactions such as Suzuki-Mayura coupling, Heck-, and Ullmann-type reactions, etc. [80]. Kim et al. demonstrated the synthesis and application of Pd hollow nanospheres in Suzuki coupling reaction [81]. In another work, authors synthesized hollow Pd-Fe nanospheres and investigated its catalytic activity in Sonogashira-, Heck-, and Ullmanntype coupling reactions in water [82]. In both the reports, the catalysts exhibited high catalytic efficiency and excellent recycling capacity. Other than the coupling reactions various yolk-shell structures were identified and investigated as catalysts for reduction as well as oxidation reactions. Hollow nanostructures such as nanocages and nanoframes had emerged

Catalysis

Sensors

Figure 10.5  Various non-medical applications of HNMs.

Energy Storage

Water treatment

232  Layered 2D Advanced Materials and Their Allied Applications as promising electrocatalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), owing to their increased amount of active sites when compared to their solid counterparts [83].

10.5.2 Energy Storage In the 21st century, energy storage devices such as battery and supercapacitors are getting huge attention due to their intrinsic applications in portable electronic materials. Due to their unique morphology, HNMs are now becoming an important class of materials for the fabrication of energy storage devices. Recent advancements in the field of energy storage devices such as lithium ion battery and supercapacitors are listed below.

10.5.2.1 Lithium Ion Battery Lithium ion batteries are the widely used energy storage device in the modern-day portable electronics such as mobile phones, portable chargers, etc. The urge towards the fabrication of new generation lithium-ion batteries having higher density, higher efficiency, and higher rate capability ended up in the synthesis and usage of hollow materials [84]. Hu et al. synthesized a branched graphene nanocapsule that can be used as an anode in the Li-ion battery with high reversible capacity [85]. Peng and co-­workers reported the synthesis of hollow graphene spheres and investigated their role as a supporting matrix in lithium sulphide battery. The material showed high initial discharge capacity (1,098 mAh g−1) and discharge capacity of 419 mAh g−1 after 1,000 cycles at a current density of 1C [86]. Zuo et al. synthesized Inorganic Fullerene-MoS2 (IF-MoS2) nanocages by utilizing bubble template method. The hollow IF-MoS2 nanocage exhibited excellent reversible capacity and improved rate capability [34].

10.5.2.2 Supercapacitor Intense research in the field of HNMs has fortified their role in the fabrication of supercapacitors. HNMs with their high surface properties can result in the development of supercapacitor electrode materials with improved energy storage mechanism. Carbon-based hollow nanostructures are extensively studied due to their high surface area and short mass diffusion. You et al. synthesized mesoporous hollow carbon spheres by

2D Hollow Nanomaterials  233 using silica as a template. These hollow nanospheres exhibited excellent stability and high specific capacity of 251 F/g at 50 mV/s in H2SO4 solution [87]. Recently, Moon et al. showed that layered assembly of nitrated TiO2 hollow shells can be an ideal candidate for making supercapacitors [88]. These hollow shells exhibited a high specific capacitance of 743.9 F/g at 10 mV/s scan rate.

10.5.3 Nanosensors Sensing application of HNMs arises from their unique cavity structure and shell morphology. The change in resistance of these materials upon exposure of various gases is the working principle behind the use of these materials as sensors. They are extensively studied and utilized for sensing various hazardous gases and chemicals such as CO, H2, ethanol, acetone, butanol, and formaldehyde [89]. A sensor based on hollow Cu2-xTe nanomaterial has been developed for the detection of toxic CO gas [90]. Hollow Cu nanomaterial was synthesized from solid copper nanoparticles and trioctylphosphine-tellurium (TOPTe) through Kirkendall effect. The resistance of synthesized p-type semiconductor Cu2-xTe hollow material will decrease when exposed to air and increases in the presence of CO gas. The sensitivity (S) of the sensor is defined as the ratio of resistance of the sensor in test gas (Rgas) to that of dry air (Rair) ie, S = Rgas/Rair. The as-prepared sensor exhibited an excellent sensitivity of S = 1.11 for 5 ppm CO gas as well as a response time of 21 s for the detection of 100 ppm CO gas, at 286⁰C operating temperature. A hollow SnO2-based material was synthesized and used for the detection of highly dangerous hydrogen gas [91]. Hollow SnO2 nanofibers were synthesized through an electrospinning method. The fabricated hollow fibers exhibited its H2 sensing potential at a low temperature of 150°C. The material exhibited improved H2 sensing property due to the presence of oxygen vacancies and rough surface. In addition to this, hollow materials were used for the detection of metal ions with the help of various fluorescent probes. Wang et al. synthesized Rhodamine B integrated hollow Fe3O4@SiO2 nanospheres for the detection of mercury ions (Hg2+) [92]. The Fe3O4@SiO2 spheres exhibited a core-shell structure with two shells. The inner silica shell was non-porous whereas the outer shell was a mesoporous one with rhodamine encapsulated. The increase in the fluorescence intensity at 584 nm provided the evidence of Hg-Rhodamine complex. The results showed that this material is an excellent Hg2+ ion sensor with a detection limit of 10 ppb. Along with the detection of gases, chemicals and

234  Layered 2D Advanced Materials and Their Allied Applications metal ions, hollow materials were synthesized for the detection of physical quantities like humidity, pressure, etc. [93, 94].

10.5.4 Wastewater Treatment HNMs are used for wastewater treatment and environmental remediation owing to their high surface area and porous nature. The environmental remediation involves the removal of toxic pollutants such as heavy metal ions, dyes, and other organic impurities. Carbon-based hollow materials are widely employed for the removal of toxic pollutants due to their high adsorbing property [95]. For example, hollow CNTs and carbon hollow fibers have been synthesized and utilized for the removal of toxic chemical dichlorobenzene [96]. Recently, Cui et al. reported the synthesis of urchinlike Fe3O4@polydopamine (PDA)-Ag hollow microspheres by growing Ag nanoparticles on the top of PDA coated Fe3O4 hollow structures [97]. The hollow microspheres showed excellent activity in removing toxic organic dyes like methylene blue and rhodamine B from water through its unique mussel-inspired PDA surface coatings. In another work, TiO2-coated pyrex hollow glass beads were synthesized by sol-gel method and used for the control of Anabaena, Microcystis, and Melosira algae growth (commonly known as cyanobacteria) in water. The algal growth was prevented using the photocatalytic activity of TiO2. Due to the semiconductor nature of TiO2, irradiation of UV light leads to the formation of electron-hole pair on the TiO2 surface. The ROS produced by these electron-hole pairs can inactivate the algae [98].

10.6 Toxicity of 2D HNMs Toxicity of nanomaterials is purely depended on their size, base material, morphology, surface charge, composition of core and shell, materials used for functionalization, coating materials, dose, and also synthesis procedure and the chemicals used. Defined toxicity profiling is essential for every engineered nanomaterials before getting applied for various applications [99]. One of the major challenges faced by scientists in 21st century is the effortless transport potential of nanoparticles to bypass various physiological barriers. Major mechanisms by which nanomaterials exerts their toxic response is via generation of oxidative stress, organelle damage, abolition of cellular and nuclear membrane integrity, protein dysfunction, and perturbation of phagocytic activity.

2D Hollow Nanomaterials  235 Among various factor determining nanomaterial toxicity, shape plays a remarkable role. Due to the peculiarity of morphology, spherical nanomaterials are more susceptible to cellular internalization, and hence, it can reach inaccessible sub-cellular loci. In contrast, nanotubes possess the morphology which is not compatible in view of internalization. Furthermore, they tend to form blockage in ion channels (Ca+, K+, Fe2+, and Na+ channels) thereby creating ionic imbalance in cells [100]. This circumstance points out to the usage of CNTs as a potent ion channel inhibitor in cancer treatment and docking studies. Certain other reports also emphasize the fact that sharp ended nanomaterials retain more toxicity than spherical ones owing to their cell permeation properties which damage organelles [101]. Considering HNMs in bulk, major portion of the toxicity reports have been focused on nanotubes in large extend. Available toxicity reports on an average demonstrate a caution that, bare nanotubes are more toxic than those experienced suitable functionalization. Shi Kam et al. in 2003 reported that SWCNTs in their functionalized state did not cause any toxic impacts in HL60 and Jurkat cells, while showed extensive cytotoxicity within 48 h when functionalization was not present. When conjugated with anti-cancerous drug streptavidin, the particles showed toxic response. For all cases, endocytic route of internalization was observed [102]. Immunological influence of hollow structures was analyzed by Dumortier et al. in 2006 using T and B lymphocytes and macrophages. Two types of functionalization procedures were carried out for this purpose which varied widely in their addition of functional moieties. It was found that type-1 functionalization (1,3-dipolar cycloaddition reaction) resulted in negligible level of cytotoxicity in immunogenic cells. Second type functionalization (via oxidation/amidation reaction) imparted some cytotoxic impression in macrophages alone for which gene expression analysis proved some immunogenic cytokine release. Altered solubility could be the underlying reason for this drastic change in toxicity magnitude [103]. Applicability of bare hollow nanotubes in pulmonary tissue may occasionally leads to serious health hazards including granuloma formation and whole tissue necrosis. For instance, an acute toxicity study was performed by Lam et al. in (2004) using intratracheal administration of nanotubes in mice. On 7th-day euthanization, mice developed tracheo-epithelial granulomas in a dose dependent manner. On the other hand, negative control (carbon black) did not elicit any pathological lesions whereas even positive control (quartz) failed to outweigh nanotubes except some minute anomalies in alveolar premises. Continued incubation with nanotubes for 90 days imparted severe inflammatory abrasions and tissue necrosis in the alveolar vicinity.

236  Layered 2D Advanced Materials and Their Allied Applications This observation raises a thought provoking concern over usage of such engineered nanostructures in the context of medical application [104]. Being a suitable candidate to be used in PTT experiments, gold nanoshells were studied in terms of biocompatibility and toxicity levels. The study was planned for a chronic period of 404 days both via in vitro and in vivo systems. In vitro evaluation was carried out in Chinese hamster ovary cells (CHO) and no observable toxicity was seen even after prolonged exposure. In vivo toxicity evaluation was carried using intravenous injection of nanoshells prepared in trehalose-water mixture in specific ratio through a series of analysis. The experiment was performed in different testing animals including mice, Sprague-dawley rats, and Beagle dogs. Certain toxic tissue lesions were observed during histopathological evaluation. Nanoshells got eliminated from the living system completely through proper degradation pattern without causing any lethal side effects to the organism [105]. Dendrimers are well organized polymeric nanomaterials which possess hollow cage like internal cavity for the incorporation of various drugs, hence increasing its applicability in biological imaging procedures, tissue engineering, and gene therapy. Higher level applicability in turn necessitates its toxicity profiling also. Pryor et al. (2014) have reported that among the different tested dendrimers, cationic higher generation dendrimers (polyamidoamine/PAMAM G6-amine) possessed greater toxicity in embryonic zebra fish than lower generation anionic dendrimers (PAMAM G6-succenamic acid), and neutral dendrimers. This denotes that surface charge is the crucial factor in determining biocompatibility of dendrimer molecules in living system. As the magnitude of surface charge increases, toxicity will simultaneously increase [106]. Controversial reports also exist which claim that PAMAM dendrimers exert dose dependent cytotoxicity primarily through generation of mitochondrial dysfunction and subsequent apoptosis. They hinder the transportation potential of mitochondrial membrane and consequently disrupt calcium channels, hence contribute to abolition of electron transport chain [107]. In another report, dendrimers induce dose and generation dependent cytotoxicity in mouse macrophage cells in accordance with the number of amine groups present per generation. Detailed analysis confirmed that cell death occurred mainly due to the generation of reactive oxygen species (ROS) and ultimately leading to expression of pro-inflammatory cytokines for immune response [108]. Recent studies have focused on a different context where genotoxic potential of nanocarriers was analyzed. Genotoxicity of silver and metal oxide nanocarriers indicated that these are capable of inducing significant DNA damages via generation of ROS

2D Hollow Nanomaterials  237 induced lipid peroxidation, depletion of antioxidant system, and depolarization of mitochondrial membrane potential [109]. Reconsidering CNT toxicity analyses, there occur both cytotoxic and non-toxic reports available. Cui et al. in 2005 have analyzed the biocompatibility of non-functionalized CNTs in human embryonic kidney (HEK293) cells. According to the results, CNTs markedly ceased cell proliferation capacity and bonding capacity of cells in a concentration reliant manner. Cell cycle analysis indicated that CNTs blocked cell cycle at G1 phase; leading to induction of cellular apoptotic pathway. Many of the cell adhesive proteins in HEK293 cells were de-regulated; whereas apoptotic gene expression was elevated tremendously [110]. Similar cytotoxicity was also observed with human skin fibroblastic cells in which MWCNTs provided at a cytotoxic concentration induced cell cycle arrest and accelerated apoptosis/necrotic pathways of cell death. Nanotubes devoid of any functionalization are susceptible to more aggregation in aqueous solutions and in cell culture media because of the hydrophobicity on nanotube surface. A group of scientists have assessed the biocompatibility of oxidized MWCNTs using T-lymphocytes; wherein apoptosis was quite noticeable. This highlights that simple oxidation is not satisfactory to mask the toxicity of MWCNTs. Hence, oxidation alone is not at all a reliable functionalization strategy for nanomaterials [111]. Although studies concentrate very widely on the context of toxicity aspects of hollow nanostructures, more deepened research is required, especially using more number of animal models.

10.7 Future Challenges Emergences of HNMs have made a breakthrough in different aspects of nanotechnology. Abundance of synthetic approaches and increasing knowledge on possibilities of employing these structures prompted a rapid advancement in this field. Over the last 20 years, engineered nanomaterials have popularized nanoscience and related areas to a great outspread. Along with the potentially high application possibilities of nanomaterials in future, these may extensively be used in medical as well as industrial scenario which will subsequently carry the economic balance to an unconceivable range all around the world [112]. Considering synthesis approaches for HNMs, hard template method using hard or rigid template materials encompass certain drawbacks including heterogeneous distribution of material on the template surface before selective removal.

238  Layered 2D Advanced Materials and Their Allied Applications Advanced research is recommended for overcoming this drawback including suitable functionalization of the template surface or the raw material of interest. If a perpetual resolution for this is available, then we can fabricate HNMs with innumerable shapes and morphology with spacious interior. In most circumstances, the selective removal of template core is a difficult task because of the inherent toxicity potential of the conventionally used solvents. Moreover, there is still a wide gap exist for fabricating nanostructures with desired composition, stability and uniformity for several applications [8]. Hence, for fulfilling the prerequisites of industrial applications, future research must be tuned in such a way to develop cost effective, easy to scale up, safe, and reliable synthesis procedures. Ever synthesis practice requires safety validity before the synthesized material is being applied for any practical expenditure. More sophisticated toxicity studies are indispensable as it may harm every living creature or even environment either directly or indirectly. Use of eco-friendly reagents (For example, biomaterials as soft templates in synthesis of hollow nanoforms) and lenient steps are strongly recommended for futuristic applications. There should be a trustworthy approach for exclusion of reaction by-products as well [113]. Future application opportunities lies in HNMs comprise: earlier disease diagnosis in far more advanced form wherein precise nursing of the whole body tissues can made possible based on coding of individual’s genetic material. Composition of the nanomaterials can then be designed accordingly which is a prerequisite for nullifying immune response otherwise. Since the susceptibility of individuals vary in accordance with genetic constitution, clear-cut control over selection of raw materials is essential during nanomaterial fabrication and drug loading. It is greatly expected that current shortcomings of diagnostics and therapeutics, drug delivery, imaging, as well as tumor therapy can be rectified using these materials because of the possibilities of multifunctional dimensions it possess [114]. However, there are still many challenges in application of nanomaterials in different fields especially in medical ground. This occurs predominantly because of the toxic impacts caused by nanostructures in tissues and organs. For instance, studies have not yet been concentrated on drawing an accurate picture on consequences occurring during chronic exposure of anti-tumor drugs in vivo [115]. There are controversies still going on regarding this context which all need to be debated in future. Most of the biocompatibility studies claim the fact that functionalization is an inevitable necessity in order to minimize cytotoxicity in cells. Without functionalization, most of the nanoforms significantly compromise cellular morphology and functionalities, which ultimately leads to cell death.

2D Hollow Nanomaterials  239 This should be a matter of consideration in nearby future as it can turn into a major hindrance in medicinal and therapeutic applications. An in-depth research and detailed awareness should be evolved in the field of hollow nanostructures as it offers fascinating opportunities to be spread over uncountable applications.

10.8 Conclusion Among the family of such new generation nanostructures, 2D HNMs require foremost consideration. According to the morphological features, they can be categorized as hollow spheres, nanotubes, polyhedral nanostructures, nanocages, polymeric nanohollow structures, fibers, etc. Subdivisions can also be assigned based on complexity and number of shells present. Such structures can include single or multi-layers of shell material around the core part. As the name implies, HNMs consist of an empty or hollow (void) space inside whose dimension falls in the nano range. Based on the composition, these hollow structures can be divided into organic or inorganic and more accurately as polymers, metals, metal oxides, ceramics, and as composite materials. The overall performance of the material is dependent on their physico-chemical properties, size, morphology, surface charge, nature of agents used for functionalization (if there is) and indeed their structural organization. Scientists have realized the exciting inherent peculiarities of hollow nanoforms as large surface area and high aspect ratio, high loading capacity, stability, and biocompatibility. Although the surface area of these hollow structures is remarkably high, the density is observed to be lower compared to their bulk counterpart with similar composition, with contrasting properties. Synthesis methods of HNMs can be of several types: hard templating method, soft templating method, and self-templating method. Among the three, hard templating method is widely employed as it is confirmed to be trustworthy in a practical sense. Simply, the shell material is coated on a hard material template followed by selective removal of the template, giving the nanocarrier its characteristic consistency. Nevertheless, there exist some drawbacks associated with the heterogeneous distribution of the shell material coated on template surface. Scientists suggest suitable functionalization strategies to bypass the issue. Intrinsic properties of the hollow nanocarriers have been characterized very well and confirmed it to be an appropriate choice for the accommodation and transport of different cargoes for various applications. Because of reliability of the void space, these structures can be used in lithium ion batteries as cathodes and anodes.

240  Layered 2D Advanced Materials and Their Allied Applications Catalytic properties of these materials turn them into suitable agents for several photo and chemical catalytic reactions. A combinatorial effect of magnetic and catalytic properties made them ideal candidates for water purification and environmental remediation purposes. They have already been proved as an ideal option for an array of biomedical applications such as: targeted drug delivery, biosensing, magnetism-based photo­thermal therapy, anti-neuro degeneration, tissue engineering and regenerative medicine, therapeutics and diagnostics, as contrasting agents in imaging techniques; to name a few. However, despite of the explored application potentials, deepened research is recommended to extend the usage in various fields and to rectify the side effects associated with. Eradication of toxicity associated with such useful nanostructures is an indispensable matter to be resolved. Long-term effects of the materials should also be studied in detail for the validation of usage in therapeutics. Precise control over the selection of reagents in synthesis procedures and adoption of an ecofriendly approach of synthesis would significantly minimize the existing troubles associated with applicability of hollow nanostructures.

Acknowledgement The authors wish to express their sincere thanks to Director and Head, Biomedical Technology Wing, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram for their encouragement and support for conducting this study. Athira thank CSIR, New Delhi. Ashtami thank SCTIMST, Trivandrum. Akhil and Joseph thank Department of Science and Technology, New Delhi for financial support of Research Fellowships.

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246  Layered 2D Advanced Materials and Their Allied Applications 81. Kim, S.-W., Kim, M., Lee, W.Y., Hyeon, T., Fabrication of hollow Palladium spheres and their successful application to the recyclable heterogeneous catalyst for Suzuki coupling reactions. J. Am. Chem. Soc., 124, 7642, 2002. 82. Li, H., Zhu, Z., Li, H., Li, P., Zhou, X., Recyclable hollow Pd–Fe nanospheric catalyst for Sonogashira-, Heck-, and Ullmann-type coupling reactions of aryl halide in aqueous media. J. Colloid Interface Sci., 349, 613, 2010. 83. Park, J., Kwon, T., Kim, J., Jin, H., Kim, H.Y., Kim, B., Joo, S.H., Lee, K., Hollow nanoparticles as emerging electrocatalysts for renewable energy conversion reactions. Chem. Soc. Rev., 47, 8173, 2018. 84. Lai, X.Y., Halpert, J.E., Wang, D., Recent advances in micro-/nano-­structured hollow spheres for energy applications: From simple to complex systems. Energy Environ. Sci., 5, 5604, 2012. 85. Hu, C., Lv, L., Xue, J., Ye, M., Wang, L., Qu, L., Branched Graphene Nano­ capsules for Anode Material of Lithium-Ion Batteries. Chem. Mater., 27, 5253, 2015. 86. Peng, H.J., Liang, J., Zhu, L., Huang, J.Q., Cheng, X.B., Guo, X., Ding, W., Zhu, W., Zhang, Q., Catalytic self-limited assembly at hard templates: Amesoscale approach to graphene nanoshells for Lithium-Sulfur batteries. ACS Nano, 8, 11280, 2014. 87. You, B., Yang, J., Sun, Y.Q., Su, Q.D., Easy synthesis of hollow core, bimodal mesoporous shell carbon nanospheres and their application in supercapacitor. Chem. Commun., 47, 12364, 2011. 88. Moon, G.D., Joo, J.B., Dahl, M., Jung, H., Yin, Y., Nitridation and layered assembly of hollow TiO2s for electrochemical energy storage. Adv. Funct. Mater., 24, 848, 2014. 89. Lee, J.-H., Gas sensors using hierarchical and hollow oxide nanostructures: Overview. Sens. Actuators B: Chemical, 140, 319, 2009. 90. Xiao, G.J., Zeng, Y., Jiang, Y.Y., Ning, J.J., Zheng, W.T., Liu, B.B., Chen, X.D., Zou, G.T., Zou, B., Controlled synthesis of hollow Cu2-xTe nanocrystalsbased on the Kirkendall effect and their enhanced CO gas-sensing properties. Small, 9, 793, 2013. 91. AbKadir, R., Li, Z.Y., Sadek, A., Rani, R.A., Zoolfakar, A.S., Field, M.R., Ou, J.Z., Chrimes, A.F., Kalantar-zadeh, K., Electrospun granular hollow SnO2 nanofibers hydrogen gas sensors operating at low temperatures. J. Phys. Chem. C, 118, 3129, 2014. 92. Wang, C., Tao, S., Wei, W., Meng, C., Liu, F., Han, M., Multifunctional mesoporous material for detection, adsorption and removal of Hg2+ in aqueous solution. J. Mater. Chem., 20, 4635, 2010. 93. Zhuo, M., Yang, T., Fu, T., Li, Q.H., High-performance humidity sensors based on electrospinning ZnFe2O4 nanotubes. RSC Adv., 5, 68299, 2015. 94. Chun, J., Lee, K.Y., Kang, C.-Y., Kim, M.W., Kim, S.-W., Baik, J.M., Embossed hollow hemisphere-based piezoelectric nanogenerator and highly responsive pressure sensor. Adv. Funct. Mater., 24, 2038, 2014.

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11 2D Layered Double Hydroxides J. Ashtami†, X. Joseph†, V. Akhil†, S.S. Athira† and P.V. Mohanan* Toxicology Division, Biomedical Technology Wing, Sree Chitra Tirunal Institute for Medical Sciences and Technology (Govt. of India), Thiruvananthapuram, India

Abstract

The pioneering advancement in 2D materials and its gripping properties have brought them to the focal point of current research interests. A lot of attention is been devoted to 2D material research and layered double hydroxide (LDH) ascend to be a reliable and potential multifunctional material. The brucite-like layered structure with high charge density and modifiable inter-layer domain chemistry makes LDH highly appreciable for a wide array of applications. This 2D clay exhibits exceptional high surface area and absorption that contribute towards their role adsorbent and in catalysis. The diverse chemical composition, ion-exchange perspective, and chances of intercalation of active molecules further improve its potential as a sensor. Apart from the scope of LDH as nanosheets, it also exhibits synergic effect in hybrid structures. LDH functions as nanofillers in polymer composite and augments mechanical and thermal qualities of the polymer. The tunable functional properties make LDH highly recommendable for various non-­medical applications like anion scavenger, flame retardant, electrode material, etc., and as a biocompatible choice for delivery vector, tissue engineering, anti-microbial activity, protein purification, etc. The possibility to exfoliate thin sheets of LDH and fine-tune the LDH chemistry has successively unlocked the restrictions in outreaching as a versatile material. Keywords:  Layered double hydroxides, 2D clay, ion-exchange, nanocomposite, gene delivery, biomedical applications, toxicity

*Corresponding author: [email protected]; [email protected] † These authors contributed equally Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (249–282) © 2020 Scrivener Publishing LLC

249

250  Layered 2D Advanced Materials and Their Allied Applications

11.1 Introduction Two-dimensional (2D) clay materials have provoked interest in research world notably for the reason of its interesting properties like layered structure with high surface area, anion- exchange properties, anisotropic nature, proficiency to act as an excellent adsorbent, biocompatibility etc. Layered double hydroxides (LDHs) are 2D clay materials with cationic metallic layer balanced by inter-layer anions and water molecules. LDHs possess high charge density that makes them excellent vectors for biomolecules and drug moieties. Negatively charged drug molecules can be intercalated into the interlayer space between positively charged LDH layers imparting appreciably high drug loading capacity. LDH also possesses an ioninterchangeable property by which anions of interest can be intercalated in-­between layers by replacing anions with lower affinity [1]. The first LDH identified was that of mineral hydrotalcite with the chemical formula [Mg6Al2(OH)16](CO3)·4(H2O). The name hydrotalcite was given on account of its water content and similarities to talc. LDHs possess a structure similar to that hydrotalcite and therefore are often quoted as hydrotalcite-like materials. The basic structural formula of LDH is [MII 1− xMIIIx (OH)2]x+[An− x/n·yH2O]x− [2]. LDH of various combinations of metals in +2 and +3 oxidation state as well the inter anions can be synthesized. This gives way for a wide range of chemical composition for LDH. LDH has been in long used as an antacid and anti-pepsinic agent for its ability to adjust stomach pH. LDH is an active component in many commercialized products like Talcid and Almax [3]. Currently, LDH has potential applications in many high priority environmental or industrial issues. The high adsorption capacity of LDH holds the solution in amending the environmental pollution hazards by acting as a pollutant adsorbent. There are various reports on LDH’s use in wastewater treatment [4]. The chemical characteristics of LDH also facilitate its use in catalysis. LDH serves as an ideal catalyst with high surface area and interesting surface properties [5]. In fact, LDH has the potential to serve as a multi-functional material. LDH is extensively used as a polymer additive. The addition of LDH is reported to cause improvement in mechanical properties of the polymer [6] and thereby inflate the application scope of polymer in various fields [7]. Other remarkable applications of LDH are as anion scavenger, electrode material, and flame retardant [8]. The 2D layered property of LDH imparts high functionalities as a biomaterial. The biomedical applications of LDH are far-reaching with drug delivery, biosensors, tissue engineering, and anti-microbial activity being the major domains. The layered structure with appreciably high surface

2D Layered Double Hydroxides  251 area makes LDH ideal option as delivery vector for various drugs and biomolecules. The high surface charge on LDH layers makes possible loading of charged drugs or biomolecules. The positively charged LDH layers can conjugate DNA, gene, or other biomolecules of counter-charge. The pH-sensitive release capability of LDH further favors targeted delivery of drugs or bioactive molecules avoiding pre-mature release [9].

11.2 Structural Aspects LDH has got a brucite-like structure with the metal ions occupying the central position of octahedron joined by hydroxyl groups at the edges. The metal ions in +2 oxidation states are replaced occasionally by metal ions in +3 oxidation states in regular patterns. This partial substitution imparts excess positive charge which is balanced by the anions and water molecules occupying the inter-layer space (Figure 11.1). The structural formula of LDH is specified x+ as  M12−+x M 3x+ (OH)2  (A m− )x/m ⋅ nH 2O. M2+and M3+ in the formula denote metal ions in +2 and +3 oxidation states. Common M2+ found in LDH structures are magnesium (II), Zinc(II), Nickel(II), and calcium(II). Iron(III), Aluminium (III), and Chromium (III) are most common +3 oxidation state metals found in LDH. The synthesis methods of LDH like co-precipitation also open up the possibility of wide M2+ and M3+ combinations [10].

Brucite layer

OH–

Inter layer space

M2+/M3+ H2O Anion

Figure 11.1  Structure of LDH.

252  Layered 2D Advanced Materials and Their Allied Applications The charge density of the brucite-layer is a key functionality responsible for most of the applications of LDH. The charge density is influenced by the chemical composition ratio (M2+/M3+ ratio), mode of synthesis, and synthetic parameters like temperature, time, etc. The common M2+/M3+ ratio for LDH are 2:1, 3:1, and 4:1. The different types of anions such as inorganic, organic, complex clusters like EDTA and biomolecules can be incorporated at the inter-layer domain. Furthermore, anions of interest can be intercalated into the inter-layer space. As discussed previously, the structural chemistry of LDH also facilitates ion exchange in which anion of lesser affinity can be replaced with any anions of interest having a higher affinity towards positively charged LDH layers [1]. This provides a wide range of tunable chemical composition for LDH. The property of ion-exchange has indeed improved the application mode of LDH. The interlayer anions and space between layers depends on the anions at the inter-layer space. Basal spacing between layers is controlled by the nature of anions and their affinity towards LDH layers. When carbonate, nitrate, and chloride intercalated LDH was analyzed, it was found that carbonate intercalated LDH had lower basal spacing when compared to the latter. At the same time, the particle size and surface area examination showed the following trend: LDH-Chloride > LDH-Nitrate > LDH-Carbonate [11]. Also, the intercalation of larger anions increases the gap between layers and reduces the interaction amidst layers. The high charge of LDH makes the exfoliation technique a bit tiresome. LDH is usually exfoliated by chemical amendment or by intercalation of larger molecules like surfactants on to the interlayer space replacing smaller anions already present [12].

11.3 Synthesis of LDHs Synthesis of LDH is generally classified mainly into three routes such as co-precipitation method, ion exchange method, and finally reconstruction method. Apart from these three synthesis methods, hydrothermal treatment, urea hydrolysis, and sol-gel method are other preferred path for LDH preparation. The physico-chemical properties of LDH are strongly influenced by the synthesis condition, and therefore, the synthesis parameters are vital in determining the quality and performance of LDH. Interestingly, the synthesis also assures as an opportunity to fine-tune the properties of LDH by optimizing the synthesis parameters. Recent studies show that the application of LDH in the various fields requires modified synthesis

2D Layered Double Hydroxides  253

Ion-exchange

Co-precipitation

H

Hydrothermal synthesis

O

H

O

C

O O

H

O

H

Layered double Hydroxides (LDH)

Urea hydrolysis

Sol-gel method Re-construction

Figure 11.2  Synthesis methods for layered double hydroxides (LDH).

according to the requirements of the particular application. Different types of methods are available for LDH synthesis (Figure 11.2).

11.3.1 Co-Precipitation Method Among the three routes, the most sort after method for LDH synthesis is by the co-precipitation method. Co-precipitation gives the opportunity to synthesis LDH of varying metal ion combinations as well as provides a chance for intercalating a wide option of anions depending upon the affinity [9]. During co-precipitation, the cation with lesser solubility will get precipitated first while other cation with higher solubility will stay in the solution. But the subsequent addition of alkali to the hydroxyl group will lead to the formation of an anionic complex by taking other cation (lesser solubility) inside the solution. LDH precipitate out as solid having positively charged layers held together by anions. Co-precipitation works by the

254  Layered 2D Advanced Materials and Their Allied Applications mechanism of condensation of hexahydrate salts of metal leading to brucite layer formation. The formed layers have uniformly distributed divalent metal ions, consistently substituted by trivalent cations and balanced by anions and water molecules [13]. The precipitation can be achieved either at constant pH or varying pH. LDH is precipitated out at basic pH of range 8–12. The particle size and properties of LDH are greatly influenced by the reaction pH, the type of base used, time, and temperature conditions at different stages of synthesis like nucleation, aging and drying, precursor addition, and solvent type [14].

11.3.2 Urea Hydrolysis Urea is widely known as a precipitating agent for LDH synthesis. The hydrolysis of urea provides a pH of about 9 which is optimal for precipitating out LDH. LDH with high charge density is produced from the urea method. Also, the urea hydrolysis method provides more control over particle size and LDH of better crystalline nature is formed from urea hydrolysis. When the urea hydrolysis rate is slow, then LDH of higher particle size is obtained. For instance, few nuclei are formed during slow hydrolysis and therefore formed nuclei grow to a larger size over time [15].

11.3.3 Ion-Exchange Method The ion-exchange method also known as the indirect method is employed primarily to incorporate the anion of interest to the inter-layer space replacing the existing anion of lower affinity. The ion-exchange depends on the affinity of anions towards the brucite layers. The order of anion exchange capability could be observed as NO3− >Br − > OH− >SO24− , >CO3− . The ion exchange reaction largely depends on the electrostatic interaction between the exchangeable anions and hydroxylated cation sheets [16]. Surfactants can be intercalated into LDH by ion-exchange. Zhang et al. decarbonated LDH and then incorporated zwitterionic surfactant to LDH by ion-exchange [12].

11.3.4 Reconstruction Method The reconstruction method is based on the most peculiar and interesting characteristics of LDH known as “memory effect”. LDH when calcinated at the temperature range of 400°C–500°C results in the elimination of all the interlayer water molecules and anions. The escape of anions like carbonate, nitrate ions, etc., as carbon dioxide, nitrate, etc., and water molecules as

2D Layered Double Hydroxides  255 vapors leaves behind mixed metal oxides (MMO). Interestingly, the MMO upon hydration goes back to the original LDH structure. This effect is known by the name memory effect of LDH [17].

11.3.5 Hydrothermal Method Hydrothermal mode of synthesis protocols is followed usually to get better control over size as well as the distribution of size. In hydrothermal synthesis metal precursor, for example, MgO and Al2O3 are added on to Teflon autoclave and heated to a temperature of 383K for a specific time period to yield Mg-Al LDH. The formation during hydrothermal treatment is proposed to occur by a mechanism involving dissolution-deposition and diffusion. According to the findings, during the hydrothermal treatment hydration of MgO and Al2O3 results in the formation of Mg(OH)2 and Al(OH)3. The dissociation products of Mg(OH)2, namely, Mg2+ and OH− get deposited on Al(OH)3/Al2O3 while Al(OH)3 ionizes to form Al(OH)4 that eventually get deposited on Mg(OH)2 forming pre-LDH structures. On further heating, the pre-LDH structure acquires better crystalline nature through metal ion diffusion [18]. Also, in some cases, hydrothermal treatment is given after co-precipitation to improve the crystalline nature of LDH particles produced during co-precipitation. The particle size of LDH increases with hydrothermal treatment in comparison to normal temperature treatment [19]. The temperature and time of hydrothermal treatment influence LDH particle size greatly.

11.3.6 Sol-Gel Method The hydrolysis and incomplete condensation of metal precursors lead to the formation of sol followed by gel formation. The LDH formed from the sol-gel process has a better surface area in comparison to the co-precipitation method. Prinetto et al. synthesized Mg-Al LDH by hydrolyzing alkoxide or acetylacetonate with HNO3 /HCl. In the study, the properties of LDH synthesized via the sol-gel method were compared with the LDH prepared by the co-precipitation method. It was found that the sol-gel method produced LDH of about 10% greater surface area than LDH synthesized via co-precipitation method [20].

11.4 Nonmedical Applications of LDH 11.4.1 Adsorbent Environmental pollution is a major concern seeking world-wide attention for amendable solutions. Wastewater treatment remains a high priority

256  Layered 2D Advanced Materials and Their Allied Applications objective and requires highly efficient purification materials. Ideal materials with the capability to remove nitrogen/phosphorous content are eagerly looked upon. In this context, LDH is been thoroughly investigated in the limelight of its potentially high adsorption capacity. The exceptionally high surface area added with its peculiar surface properties makes LDH a reliable adsorbent material. The ion-exchange capacity and economically feasible production rates further make LDH meritorious candidate as adsorbent. There are several reports on waste-water treatment using LDH as the adsorbent for toxic pollutants. Recently, rosette-shaped LDH with synthesized using magnesium and aluminium nitrates salts with hexamethylenetetramine was found to exhibit − and nitrate NO3 anions. The high selectivity for phosphate HPO2− 4 newly developed rosette developed LDH crystals of the size 1–10 µm did not cause any clogging issues. Commercially available LDH having platelike morphology usually show clogging issues. The presence of heavy metals in water does raise concern due to their latent chance of environmental harm [21]. Various methods are been explored to remove heavy metals such as lead (Pb2+) and nickel (Ni2+) from water [22]. In a study by Rahmanian et al., Ca-Fe LDH intercalated with citrate anions effectively removed Pb2+ and Ni2+ anions from water [23]. The intercalated citrate ions also enhanced the rate of heavy metal removal by acting as a chelating agent. The Ca-Fecitrate LDH achieved Pb2+ and Ni2+ removal within 30 min at neutral pH. Another major concern is the presence of organic dyes in water. The organic dyes can be of the form anionic, cationic, or neutral. The alarming factor is that most of the organic dyes possess toxicity and may cause mutagenicity or carcinogenicity. Among various traditional methods of dye exclusion techniques, adsorption is highly sought after one in consideration of its relatively cheap cost, simple functioning, and wide choice of adsorbent materials with reusable option [24]. In recent times, NiFeTi LDH exhibited remarkable adsorption potential for anionic dyes. The synthesized NiFeTi removed anionic dyes such as methyl orange, methyl blue, orange G, and Congo red with an efficiency of above 96%. The high surface area as well as the higher concentration of carbonate and hydroxyl groups are been attributed as the key factors in achieving fast dye removal. The biocompatibility of NiFeTi LDH was evaluated by the response of mouse macrophagic cell lines on exposure to the synthesized material. The results revealed that the NiFeTi is non-toxic and hence can be used as the effective material for synthetic waste water treatment. Besides that, one of the alarming contents in water is environmental estrogen. The synthetic or biological compounds that are skilled to mimic natural hormones are termed as environmental estrogen. The latent risk

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2D Layered Double Hydroxides  257 factor resides with the ability of these compounds to behave like natural hormones and engage in dangerous interactions causing unfavorable responses [25]. For instance, the presence of environmental estrogen potent of endocrine- disruption in water raises hazard to aquatic life as well as other living beings. A study focused on examining the efficiency of LDH on adsorbing 17β-estradiol, a compound reported to be present in various water sources and most importantly an endocrine and reproductive problematic compound. For the study, Mg-Al LDH intercalated with sodium dodecyl sulfate (SDS) was synthesized and analyzed for adsorption efficiency. The results demonstrate that the synthesized Mg-Al-SDS LDH performed well with a 17β-estradiol removal efficiency of 96%. Furthermore, the synthesized LDH materials exhibited reusability upto 5 cycles on alkali treatment without much compromise on performance. This validates the fact that the developed LDH could be used as an efficient and economical adsorbent [26]. Additionally, gas adsorption on to LDH is yet another novel explored application as adsorbent. Notably, the greenhouse effect and the subsequent climate changes are matters of severe anguish worldwide. The anxiety has triggered concern to reduce the amount of the major causative for the change which is carbon dioxide. Various materials are tested for its capacity to absorb CO2 and reduce its amount in the atmosphere. LDH is highly recommended as CO2 adsorbent primarily because of its high affinity for carbonate ions and changeable semiconductor characteristics. LDH is been widely investigated and innovative changes have been adopted to improve its efficiency. As per literature updates, the CO2 adsorption efficiency of LDH can be enhanced by fine-tuning its properties such as size, shape, calcination temperature, or by either adding dopants. The LDH intercalated with (3-aminopropyl)triethoxysilane (APS) showed improved stability as well as adsorption capacity. The so developed APS-LDH showed appreciable performance until fifth recycle [27].

11.4.2 Catalyst The peculiar characteristics of LDH notably high surface area, surface properties, high charge density, uniform distribution of cations, and possibility to intercalate active catalytic anions formulate its efficiency as a catalyst. For instance, the possibilities of forming extensive combinations of metal brucite layer with ion-exchange ability are advantageous in catalytic point of view. Furthermore, LDH upon calcination forms mixed metal oxides (MMOs) with higher catalytic properties. The surface chemistry of both normal LDH and MMOs opens up the possibility

258  Layered 2D Advanced Materials and Their Allied Applications to adsorb active catalytic species. The feasibility of forming hybrid structures of LDH with other suitable material further expands the scope of achieving the more pronounced catalytic activity. LDH serves as a catalyst, catalytic precursors, or catalyst support for a wide span of reactions covering organic reactions, photoreductions, and decomposition reactions, to name a few [5]. The abundant presence of basic sites in brucite layers along with the even allocation of cations decisively allows LDH nanosheets to act as a catalyst. The cations present in LDH can act as catalytic active sites. Alternatively, catalytically active species can be immobilized on to the LDH sheets [28]. The high charge density of the brucite layer and the scope for electrostatic and hydrogen bonding further facilitate the fabrication of LDH as a nanocatalyst. For instance, the exfoliated Ni-Fe and Ni-Co LDH exhibited better catalytic efficiency for oxygen evolution reaction compared to Iridum oxide catalyst available in the market. The study put forward the feasibility of the synthesized LDHs as a catalyst with enhanced stability, performance, and cost-effectiveness compared to highly expensive IrO2 catalysts [29]. An example of the immobilization of active catalytic species on LDH nanolayers was shown by Wang et al. and co-workers in 2011. The group reported the development of a chiral catalyst using LDH immobilized with amino acids for the enantioselective asymmetric allylic alcohol­ epoxidation. In the study, the catalytic activity of amino acids in solution as a homogeneous catalyst, LDH intercalated with amino acids acting as a heterogeneous catalyst and LDH layers immobilized with amino acids serving as pseudo-homocatalyst were compared. The comparative study pointed out that highest catalytic activity was shown at pseudo-homogeneous catalysis condition. The delamination of layers in a pseudo-homogeneous condition further increased the reaction rate. The Zn-Al LDH immobilized with amino acids showed good reusability without compromising enantioselectivity or catalytic activity [30]. Apart from the performance as a direct catalyst, another mode of the contribution of LDH as a catalyst is by acting as a catalyst precursor. As discussed earlier, LDH upon calcination yield MMOs which have active catalytic sites in abundance. Recently, CuZnGa LDH precursor acted as an effective catalyst for methanol generation by carbon dioxide hydrogenation. The CuZnGa LDH ultrathin layers synthesized were treated by aqueous miscible organic solvent method and later used as catalyst precursor. The treatment resulted in the formation of active Cu(Zn) atoms that improved the catalytic activity. The newly developed catalytic precursor showed better performance compared to commercially available copper

2D Layered Double Hydroxides  259 catalysts. The improvement in catalytic activity is attributed to the increase in active Cu surface [31]. MMOs generated upon calcination of LDH have a huge number of Mn+ and O2− pairs that provide acidic and basic sites that can potentially act as active sites for catalysis. The amount, as well as its kind, depends upon a variety of factors such as the synthesis parameters, type and ratio of metal ions, anions in-between layers, calcination temperature, etc. The LithiumAluminium metal oxide obtained from LDH precursor accomplished catalytic action for Knoevenagel condensation reaction. The increase in surface area and a decrease in crystalline size were found to induce increment in surface basicity. The possibility of ion-exchange has opened up the feasibility of intercalating any active catalytic species of interest into the inter-layer space. Interestingly, catalytic active anions or biomolecules can be intercalated into LDH and thereby augment in the catalytic performance. For example, Mn(TPP)OAc (TPP = tetraphenylporphyrin dianion) was intercalated on to ZnAl-LDHs-intercalated with dodecyl sulfonate (DDS) to form heterogeneous catalyst with superior performance in boosting alkene epoxidation. The LDH intercalated with DDS forms 2D hydrophobic assembly with the resemblance to natural biological phospholipids for smooth capturing of Mn complex on to the interlayer space. The newly prepared heterogeneous catalyst showed outperformed homogeneous Mn complex catalyst [32]. While intercalation of active catalytic species on the interlayer space has been stated to enrich the catalytic activity, there resides constraining factors. The geometrical limitations and the hindrance in reaching the inter-layer space limit performance success. In the context, developing composite catalyst using LDH comes as a resolution. Assembling LDH with other suitable material in turn helps to act as bi-functional heterogeneous catalyst. LDH acts as support, it induces the formation of the metal catalyst by exterior confinement and prevents aggregation thereby assuring uniform size, shape, and distribution [33]. LDH and MMOs owing to their high surface area hold the option to act as supporting catalyst since various metallic nanoparticles can be immobilized on to the surface. In a study, LDH supported palladium catalyst was used for selective acetylene hydrogenation. The synthesized Pd/MgO-Al2O3 had a large surface area, palladium of homogeneous size and shape, less acidity sites when compared to Pd-Al2O3 catalyst. The well-distributed Pd with definite shape led to a net increase in active sites and the lower acidic sites at the surface. This helped to enhance the selectivity of product formation [34].

260  Layered 2D Advanced Materials and Their Allied Applications

11.4.3 Sensors Yet another major efficacy of LDH lies in its possibility as a sensor. The layered structural arrangement, inter-layer anion incorporation ability, option for functionalization, etc., add merits to its choice as a sensor. LDH can be used as gas sensors. Zn-Al LDH with chloride as counter anion was used for the detection of CO, CO2, NO, NO2, CH4 at room temperature with appreciable sensitivity. The results are worthwhile for considering LDH-based sensors for detecting harmful pollutant gases at room temperature avoiding high-temperature treatments usually associated with gas sensing [35]. An optical sensor for pH sensing using thin layer of magnesiumaluminium LDH incorporated with fluorescein (FLU) and 1-heptanesulfonic acid sodium (HES) was developed by Shi et al. The sensor developed worked well within the range of 5.02 to 8.54 with excellent reproducibility and appreciable stability. The work shed light towards the scope of LDH in chromophore-based optical sensors [36]. Recently, Lithium-Aluminium (Li-Al) LDH was reported to show highly efficient CO2 sensing property. For the study, Li-Al-CO3 LDH was calcinated at high temperature yielding amorphous MMOs that emitted blue luminescence under excitation. After calcination, the blue emission was found probably due to the oxygen defects sites developed on LDH. The calcinated LDH was used as a luminescence sensor for detecting carbonate (Figure 11.3). The sensor works on the principle that on the introduction of the carbonate-containing solution, the carbonate ions get re-intercalated on to the LDH structure

UV lamp excitation

Calcination at 300°C

Calcination

Li-Al LDH

Blue photoluminescence emission UV lamp excitation

Presence of CO2

Mixed metal oxide (MMO)

Luminescence quenching On CO2 sensing

Calcination at 900°C Red photoluminescence emission

Figure 11.3  Application of Li-Al LDH as CO2 sensor.

2D Layered Double Hydroxides  261 causing luminescence quenching. So, quenching of blue emission can be speculated to the presence of carbonate ions [37].

11.4.4 Electrode LDH containing transition metal atoms are being investigated for their feasibility as an electrode. The economically low production rate, eco-friendly nature, and appreciable redox potential of LDH promote the exploration. The capacitance of LDH depends upon the ration of divalent to trivalent metal atoms. Tuning the cationic composition of LDH can influence the capacitance output of LDH. Lesser the presence of trivalent metal atom, the higher will be the capacitance. Besides, the anions present in the interlayer space also have an influential role in determining the capacitance of LDH. Introducing macromolecules on to the inter-lattice space actually increases the distance between layers thereby preventing agglomeration and contribute positively to increase stability. Also, doping metals ions on to LDH further helps to improve the capacitance [38]. Also, Cobalt-Nickel (Co-Ni) LDH acted as an efficient electrode for supercapacitor. The Co-Ni LDH synthesized via co-precipitation route with the aid of poly-ethylene glycol with a notable capacitance value of 1809 F g−1 [39]. The poor conductivity of LDH limits its use as an electrode. Also, the agglomeration of layers hinders its performance over successive cycles. But composite formation aids in overcoming these hurdles. Preparation of composite electrode of LDH with carbon helps to increase electron conductivity and at the same time prevent LDH layers from agglomerating. The LDH layers can be assembled either parallel or vertically on conducting substrate materials [38]. The composite electrode with Ni-Al LDH platelets incorporated on to graphene exhibited good performance as electrode for supercapacitor [40]. Nickel-Iron (Ni-Fe) LDH/reduced graphene oxide/carbon nanofiber composite material was developed as an electrode material and executed great specific capacitance [41].

11.4.5 Polymer Additive The addition of LDH to polymers is reported to contribute positively towards improving the mechanical properties of polymer and stability. A lot of researchers are actively engaged in developing polymer-LDH nanocomposites to serve different purposes. The charge associated with brucite layers and the electrostatic interactions between layers makes the exfoliation of LDH tiresome. LDH-polymer composite preparation demands an organic amendment to LDH. Incorporating organic moieties increase

262  Layered 2D Advanced Materials and Their Allied Applications the inter-layer space and making space for polymeric chain interaction. Secondly, the organic modification also contributes in strengthening the LDH interactions with hydrophobic polymers. Besides that, the addition of organic moieties also helps to bring about the desired functional property requirement for various applications. The inorganic brucite layers improve the physico-chemical factors of the polymer-like thermal, mechanical, and stability properties [42]. A polymer composite of polystyrene incorporated with sodium dodecyl sulfate as filler showed improved mechanical strength and thermal stability. The nanocomposite exhibited an increase in tensile strength by 34.5% on a filler concentration of 1 wt%. The introduction of LDH as a nanofiller illustrated better-withstanding tendency against degradation at high temperatures [43]. There are also reports on LDH acting as nanofillers for polymers without any organic functionalization. For example, Nagendra and co-workers worked on analyzing the effect of different LDH, namely, Zn-Al LDH, Co-Al LDH, and Co-Zn-Al LDH on the thermal stability of polypropylene. The results revealed that the LDH with tri-metallic cations improved the thermal stability of the polymer as evidently detectable from the 39% decrease in the rate of heat release during combustion [44].

11.4.6 Anion Scavenger The notable characteristic property of LDH is its ion-exchange capacity. The positively charged brucite layers leave the opportunity for large range anions to be incorporated into the interlayer space. In fact, LDH can act as an immobilization site for anions and can be potentially exploited for scavenging toxic anions present as pollutants. Nickel-Iron (Ni-Fe) LDH was reported to acts as a scavenger against a major hazardous contaminant in water-chromate ions. The incorporation of chromate ion at the interlayer space occurred by ion-exchange with chloride ion/nitrate ion instead of adsorption. The chromate ions can be successfully leached out using sodium carbonate solution since LDH has more affinity towards carbonate ions than with chromate ion [45]. Zinc-Aluminium (Zn-Al) LDH intercalated with ethylenediamine­ tetraacetate (EDTA) showed scavenger property against copper ions. The copper ions were taken up ion exchange with EDTA ions. The results validate the scope of using Zn-Al LDH-EDTA as a scavenger for various heavy metals. But the LDH-EDTA partially gets into an aqueous solution that limits its application range [46]. Also, another limitation for fully exploiting the scope of LDH as an anion scavenger is the affinity of LDH towards different anions. LDH has a high affinity for carbonate and therefore

2D Layered Double Hydroxides  263 carbonate contamination can hinder the ion-exchange/scavenging property of LDH. Usually, nitrate/chloride intercalated LDH as used as anion scavenger. The release of these ions to aqueous solutions while carrying out the scavenger action [45].

11.4.7 Flame Retardant The interesting properties of LDH like chemical composition, 2D layered structure and inter-layer composition chemistry contribute to the flame retardancy properties of LDH. Organic moieties can be intercalated into the inter-lattice space between individual LDH layers. The organic intercalated LDH can be used successfully in polymer composite as nanofillers. On high-temperature treatment, decomposition of LDH lead to the eventual breakdown of the water and anions encapsulated in-between layers. These cause the release of gas moieties like water vapor, carbondioxide, etc., utilized energy and result in the unavailability of fuels for further combustion. At this condition, combustion stops, leaving behind polymer with char coating. Finally, heat release is reduced leading to subduing the smoke formation. To put it brief, the flame retardancy mechanism of LDH works on heat adsorption, gas dilution and formation of char [47]. Recently, the efficiency of LDH and zinc borate as nanofillers in polypropylene polymer matrix as an option for flame retardancy was evaluated. In the study, utilizing 20% LDH modified with ammonium polyphosphate and 2% zinc borate in polymer composite yielded a reduction in heat release by 58% [48]. Also, while analyzing the effect of metallic composition on flame retardancy efficiency of LDH, it was found that the LDH with tri-cations (Co-Zn-Al LDH) showed better competence than bi-metallic LDH. The higher performance may be attributed to better char formation in the case of tri-metallic LDH [44].

11.5 Biomedical Applications 11.5.1 Biosensors LDH has received significant consideration in sensor development in past years owing to their quick charge transfer, large surface area, elevated charge density, layered structure, facile fabrication, and abundant surfaceactive sites. The biosensor can be combined with cyclic voltammetry [49], fluorescence [50], and impedance [51, 52] for effective reading output. LDH-based sensors are used for the detection of biologically

264  Layered 2D Advanced Materials and Their Allied Applications relevant molecules including ions, glucose [53], cholesterol [54], hydrogen peroxide [49], dopamine [55], urea [52]. Glucose biosensors without glucose oxidase enzyme have been developed by using LDH, for example, Ni/AL-LDH nanoflakes and chitosan-based biosensor [53]. The detection is taking place by electro-oxidation of Ni ion in the Ni/Al-LDH and it is effectively detected as amperometric measurements. Hydrogen peroxide efflux in the biological system is considered as a biomarker in oxidative stress, tumor growth, and cancer diagnostics. Knowing the importance of hydrogen peroxide in the biological system, many scientists have worked in precise detection of its level. In one example of hydrogen peroxide biosensor, MnO2 nanoparticles were intercalated to MgAl LDH [49]. The LDH not only offers outstanding intercalation but also helps in significant entrapment of nanoparticle in the host layer. This will enhance the redox reaction site for hydrogen peroxide. Moreover, MgAl LDH acts as a P-type semiconductive channel, due to the combined effect of catalytic activity of MnO2 and semiconductive nature of LDH. This system provides low detection limits and high sensitivity. It can also be used for real-time monitoring of hydrogen peroxide levels in the live-cell system. In another work, core-shell iron oxide-CuAl LDH [56] electrochemical biosensor was used for the detection of hydrogen peroxide. LDH-based biosensor has been reported for the detection of neurotransmitters such as dopamine [55]. For instance, the biosensor is prepared by stacking nanosheets of catalytically active NiAl-LDH and conductive graphene layer by layer (LBL) (Figure 11.4). This can enable real-time monitoring of dopamine from live cells. Researchers also developed LDH based chemosensor for the detection of iron ions from serum using fluorescence measurements [50]. The LDH matrix provides a stable environment for the immobilization of fluorophore and thereby increasing its optical stability

LDH nanosheets Ion exchange LDH–CO32–

Exfoliation LDH–NO–3

LBL assembly Graphene

NiAl-LDH/G LBL composite

Reduction

NiAl-LDH/GO LBL composite

Figure 11.4  Steps involved in the fabrication of NiAl-LDH/G LBL composite.

2D Layered Double Hydroxides  265 and efficiency. LDH can be used for the fabrication of DNA biosensors [51, 57]. One of the examples for LDH-based DNA biosensors is the detection of etopside (ETO) [57], an anticancer agent, for the effective monitoring of drug level in the biological sample. Etopside will lead to DNA damage in cancer cells due to the formation of free radicals or by effectively inhibiting DNA topoisomerase II. The electrode is prepared by depositing graphene oxide-cobalt ferrite—ZnAl LDH nanocomposites onto the FTO coated glass. Subsequently, DNA is immobilized to the surface through electrostatic interaction between the positive layer of the LDH and the negatively charged phosphate backbone of DNA. The electrode has shown outstanding sensitivity and low detection limit due to the high surface area, exceptional electrical conductivity, and electron transfer capability. In another example, DNA biosensor has been employed for the detection of long ssDNA sequences [51]. The electrode was prepared by anchoring DNA to the LDH and then allowed for the hybridization with the target sequence. The hybridization reaction then detected through electrochemical impedance spectroscopy. LDH-based biosensor has also been reported in the detection of urea [52]. The biosensor is fabricated through the immobilization of the urease enzyme in ZnAl-Cl LDH followed by coating on the transducer. The response is measured by using impedance. In the case of cholesterol [54], cholesterol oxidase enzyme is immobilized onto the LDH-biopolymer composite. It is found that the biopolymer layer helps in the increased adhesive capacity as well as biocompatibility of the LDH resulting in improved performance of the electrode.

11.5.2 Scaffolds In tissue engineering, scaffolding plays a key role. Many researchers have paid attention in recent years to the design and preparation of new material for the applications in tissue engineering. The LDH has been found as a promising agent in tissue engineering applications as scaffolds due to biocompatibility, low toxicity, and stability [58]. It supports adhesion, growth, and proliferation of cells. The drug delivery capability of LDH also enhances tissue regeneration. Ramanathan et al. [59] described a method for graft fabrication using LDH. The hydroxyapatite (HA) is a major component in the bone along with collagen. The nanocomposite was prepared by using a combination of polymers, LDH and HA. The dual-layered nanofibrous bone graft was fabricated by electrospinning the polymer solution (PHB, PVP)-LDH mixture over the prepared HA pellet. The HAP pellet was made by a simple hydraulic press method. The LDH in the nanofibrous

266  Layered 2D Advanced Materials and Their Allied Applications matrix provides excellent cell adhesion capacity. Bone tissue engineering scaffold by using LDH and HA but, gelatine (GEL) as the matrix for the scaffold was prepared [60]. The bone tissue engineering (BTE) scaffold fabricated by using co-precipitation followed by solvent casting. The scaffolds showed interconnected non-uniform pores with a porosity of 92+/−0.15% and young’s modulus of 12.5 GPa. It has also been showed good biocompatibility and a drastic increase in the new bone formation. LDH-based nanocomposite film for guided tissue regeneration was prepared by intercalating alendronate anions in Mg/Al-LDH followed by dispersing in plasticized poly(lactic-co-glycolic acid) film matrix. The LDH acts as a reservoir for alendronate ions and regulates its release. They also found that a long-term elevated osteoblast activity through new bone nodule formation as well as increased alkaline phosphatase level in cell culture while using this nanocomposite [7]. Chen et al. [61] utilized the synergetic effect of bivalent Mg ions in the bone metabolism and regeneration, sustained drug release nature of LDH, biocompatibility and anti-microbial properties of chitosan, cementing ability of Al element in implant surgery, and prepared Mg/Al LDH-chitosan-PFTα scaffolds. This hybrid has been used to study the cytocompatibility and osteogenesis by using human bone marrow stromal osteoprogenitor cells as a cell model. It showed better osteoinductivity suggesting better stem cell osteogenic differentiation as well as local bone regeneration. The macropores in the scaffold help in enhanced adhesion and distribution of human bone marrow stromal osteoprogenitor cells.

11.5.3 Anti-Microbial Agents A number of research articles have been published on the anti-microbial properties of LDH due to the cations present in the LDH will help in the elimination of microorganisms. The nonphysiological metal cations interact with the phosphate groups present in the cell causing a change in permeability which in turn leads to leakage of intracellular material from microbes. Other mechanisms include the metabolism of lipoproteins present in the membrane of gram-positive bacteria, DNA synthesis inhibition through complex formation with DNA polyanions, cellular damage due to super­ oxide ions production, and cytoplasmic membrane breakage due to hydroxide groups in LDH. We can also use this LDH for the treatment of allergies, skin infection, odor prevention. The effect of various components of LDH against Corynebacterium ammoniagenes was studied [62]. Both nitrate and chloride layered LDH was prepared through coprecipitation. Antibiogram assays including Kirby-Bauer disc diffusion and broth dilution turbidometric analysis were carried out to find the bacteriocidal activity. It is found that

2D Layered Double Hydroxides  267 the anti-microbial property is largely depending on the divalent cation present in the LDH. Out of the LDHs tested for bactericidal activity, ZnLDH showed inhibition of 99.9% with 50 mg/ml. LDH-based nanocomposite can be used for the carrier for antibiotics and antifungal agents. Oxytetracycline and tetracycline immobilized on LDH using coprecipitation and anionic exchange was checked for the antibiotic activity against Escherichia coli and Staphylococcus epidermis [63]. It is evident from the result that, coprecipitation favors antibiotic diffusion without degradation. In another example, LDH-Doxycycline nanohybrid was synthesized through co-precipitation and compared the antibacterial activity along with LDH alone [64]. The LDH did not show any antibacterial activity whereas LDH-Doxycycline showed a substantial antibacterial activity. This may be due to the compacted layers of synthesized LDH. Since they acting together as a single layer and shunts the interaction between cell wall. A carboxymethylcellulose -CuAl-LDH hydrogel (CMC/LDH) was synthesized and used it as a carrier for antibiotic loading [65]. The presence of carboxymethyl cellulose provides pH-sensitive drug release. The increase in the pH causing the swelling by virtue of deprotonation of the carboxyl group and a further increase in electrostatic repulsion. The LDH enters into the polymer structure through either auxillary cross linking or by increasing the hydrophilicity of the polymer. The biocompatibility and cytotoxic studies revealed that the CMC/LDH hydrogel as a suitable agent for treating colonic bacterial infection. Furthermore, Kojic acid-Zn/Ti layered double hydroxide was prepared through co-precipitation and anion exchange process [66]. Further studied the anti-microbial properties against Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus. Being a metabolic product of Penicillium and Aspergillus moulds, Kojic acid (KA) is used as an anti-microbial and whitening agent. The intercalation of KA into the galleries of LDH enables the controlled delivery of intercalated KA ions over a prolonged period in a sustained manner and also increased the thermal stability significantly. The results showed ZnTi-KA-LDH inhibiting microbial growth in an acidic environment and the KA ion diffusion into the medium is due to proton attack to the KA. The rate of diffusion depends on the medium as well as the layer matrix. The hybrid also showed anti-microbial activity towards Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus.

11.5.4 Drug Delivery The LDH is found to be a promising tool in drug delivery. It can act as the reservoir for drugs and hence enables the slow and sustained release of drugs.

268  Layered 2D Advanced Materials and Their Allied Applications Vaccines [67], antibiotics [63], and anti-cancer drugs [58] can be intercalated into LDH and it also provides better stability for the drugs. The LDH can also alter the thermal stability of the guest molecule which is intercalated to the lamellae. LDH offers protection for intercalated compound by protecting them from enzymatic degradation and other harsh biological conditions. It has been found that LDH also increases the dissolution rate of poorly soluble therapeutic molecules. Additionally, a drug delivery system for praziquantel (PZQ) by using calcium and aluminium LDH (CaAl-LDH) was developed by Timóteo et al. [68]. The CaAl -LDH prepared by using aluminium nitrate and calcium nitrate tetrahydrate in sodium hydroxide solution through the co-­ precipitation method at a pH of 10. The PZQ-CaAl-LDH blends then prepared by co-solubilization followed by the homogenization method. Subsequently, solvent evaporation was done to obtain the dried residue of PZQ-CaAl-LDH. The thermal degradation and in vitro drug release studies proved an increase in thermal stability and better bioavailability respectively. Interestingly, in another work, Glibenclamide (GLIB) was used as the guest molecule in the LDH hybrid system for the treatment of diabetes mellitus II [69]. The low solubility of GLIB leads to poor bioavailability in the biological system. But LDH-GLIB showed an increased dissolution rate and thus substantiating the improved bioavailability. Antioxidant activity has also been observed in the zinc-based LDH system suggesting that the potential agent for reducing the effect of free radicals in the nervous system and cardiovascular system. The LDH can also be used for pH-responsive drug release. For instance, a pH-sensitive system by using NaCa-LDH for the release of dacarbazine (DAC) was reported [70]. The NaCa-LDH is prepared through coprecipitation and DAC intercalated to LDH through simultaneous nucleation and aging method. The result from the drug release study showed that sustained release of drugs and enhanced cell death. Kamyar et al. [71] intercalated dexamethanose in ZnAl-NO3 layered double hydroxide. Afterward, the nanocomposite is developed by depositing drug-LDH hybrid into anodized titanium nanotubes (ATS) for studying the possible application of drug-loaded LDH in implant application. Surprisingly, the nanohybrid showed good thermal stability and reduced but consistent drug release. The cytotoxicity and cell proliferation assays exhibited that the nanohybrids are not only toxic but also inducing cell adhesion and proliferation. The consistent delivery of drugs also favors a reduction in inflammation. LDH nanocomposite has also been studied in the delivery of vaccines, for example, alginate-chitosan-coated LDH as a carrier for oral vaccine delivery [67]. The LDH prepared through the coprecipitation followed by

2D Layered Double Hydroxides  269 the hydrothermal treatment. Subsequently, antigen containing LDH was synthesized through nanoparticle formation by the dropwise addition of LDH into the protein/antigen solution with simultaneous stirring. Finally, the polymer coating was done using the ionic gelation method. The mucoadhesive nature of chitosan (CHT) and pH-responsive nature of alginate (ALG) synergetically improve the surface properties of the nanocomposite. Furthermore, the exceptional colloidal and surface characteristics protect the drug from acidic degradation and help in the binding and penetration across small intestinal mucous layers. The ALG-CHT-LDH showed improved immune response due to many reasons including generous cellular uptake through antigen-presenting cells, inherent high protein/drug loading capacity, and controlled delivery of vaccine.

11.5.5 Imaging LDH also employed for magnetic resonance (MR) imaging [72]. As an illustration, nanohybrids were prepared by immobilizing a diagnostic contrast agent such as gadolinium (GA) in ZnAl-LDH [72] followed by a coating of gold nanoparticles onto the surface of nanohybrid. Gold nanoparticle will help in the improvement of the contrast of the MR image. The GA loaded nanohybrid showed improved signal in comparison with GA alone. Cellular uptake of LDH can be studied using fluorescencelabeled molecules. For example, LDH nanoparticle labeled with fluorescein isothiocyanate (FITC) was studied for cellular localization using confocal microscopy [73]. The confocal images have proved the intrusion of 20-nm NO3-LDH-FITC and CO3-LDH-FITC into the nucleus while 180-nm NO3-LDH-FITC and CO3-LDH-FITC stayed inside the cytoplasm. The intercalation of different fluorescent dyes into LDH, cellular uptake, and localization was investigated [74]. Out of different fluorescent dyes [fluorescein, 8-aminonaphthelene-1,3,6 trisulfonic acid (ANTS), fluorescein isothiocyanate (FITC) and 8-aminopyrene-1,3,6-trisulfonic acid (APTS)] studied, fluorescein-LDH showed better intensity and stability and ANTS-LDH exhibited the least intensity. Analysis on the photoluminesence property of 1-anilinonaphthalene-8-sulfonate (ANS) intercalated LDHs proved that the luminescence behavior of ANS largely depends on the type and ratio of cation groups in the LDH matrix.

11.5.6 Protein Purification The metal hydroxide layer in the LDH possesses a positive surplus charge. This enables the intercalation of the anionic molecule to the

270  Layered 2D Advanced Materials and Their Allied Applications lamellar space through electrostatic interaction. But it is limited by the size of the guest molecule. Since the size of biomolecules is comparatively large, often a modification of LDH employed. Nanosheets, nanofibers, or hierarchically arranged LDH with well-defined pores are constructed for biomolecule separation, carrier applications, and scaffold preparation. Hierarchically porous Ni-Al LDH/Al2O3 for bovine serum albumin (BSA) separation was fabricated using the biotemplate method with celluloses as the template. The fabrication process involved the direct growth of LDH nanosheets on the surface of Al2O3 fibers [75]. The combined effect of mesoporous structure and electrostatic charges results in the excellent BSA adsorption. It also found that maximum adsorption occurring at neutral pH and the adsorption mechanism is established by electrostatic interaction. Desorption was carried out by the introduction of highly negative charged anions. In another study, the magnetic separation of proteins using a microsphere consisting of NiAl-LDH nanoplatelet shell and silica-coated magnetite core was done [76]. The Ni2+ cation in the shell act as a binding site for histidine. The microsphere exhibited superparamagnetism. This can be separated from the cell lysate by using an external magnetic field.

11.5.7 Gene Delivery Thanks to the biocompatibility and unique properties, the LDH has been proven as a successful candidate for drug delivery. The LDH has also been got wide attention in the gene delivery landscape also. Nevertheless, the size of the nucleic acid sequence determines the efficiency of gene delivery. Most of the researchers have focused on nucleic acid limited size such as polymerase chain reaction (PCR) fragments, small interfering (siRNA) [63, 73, 77–79], sheared genomic DNA, plasmid DNA [73, 80, 81], etc. The LDH-nucleic acid system should be transported across the cellular membrane for the adequate delivery of genes, followed by the release of exogenous genetic material without any degradation, and finally be transported to the function site. It is also important to activate it at a time when it should be released from its complex form and interact with the host gene. The delivery of the genetic material and further transfection through LDH considered as a non-viral approach for gene delivery and chemical transfection, respectively. The LDH is providing a net surplus positive surface charge when the nucleic acid is immobilized in their interlayer galleries. Through the electrostatic interaction between positively charged fusogenic LDH complex and negatively charged cell membrane, it enables transport across the cell membrane (Figure 11.5).

2D Layered Double Hydroxides  271

Endocytosis

LDH nanoparticle

Gene of interest

Endosome

LDH-DNA complex

Nucleus

Nucleic acid releasing into cytoplasm

Figure 11.5  Pictorial representation of LDH mediated gene delivery.

The DNA that is deposited just below the cellular membrane by the fusogenic particle then goes to the nucleus through the intrinsic transport pathway or through the help of peptide which is carrying nuclear localization sequences. The activation of the genetic material is taken place through either a simple diffusion process or through the endosome mediated dissociation of fusogenic complex or through the neutralization of charge. The zeta potential also controls the cellular uptake of the nanoparticle. A net positive zeta potential allows the electrostatic interaction with the negatively charged cell wall and aid in the cellular uptake. MgAl-LDH nanoparticle synthesized through a combined co-precipitation and hydrothermal method and wrapped with plasmid DNA was reported to act as a gene vector [81]. Subsequently, the developed LDH system was used for transfection with a GFP-reporter for both adherent cell lines (HEK293T, NIH 3T3, COS-7, and CHO-K1) and suspension cell lines (CHO-S). Significant transfection efficiencies were observed when transfecting adherent cell culture but not in suspension culture. This is due to the possible aggregate formation as a result of DNA-LDH complex and settles on the top of adherent cells but not on the surface of suspension cultures by virtue of continuous agitation. HEK 293T was transfected with pEF-eGFP plasmid by using MgAl-DNA nanohybrid and observed GFP expression from the first day onwards and it increases as the DNA load in nanohybrids increases [80]. A novel disulfide-linked polycation conjugated functionalized LDH nanoparticle was developed for gene delivery and for increased transfection efficiency by modifying LDH surface through atom transfer radical polymerization (ARTP) of 2-dimethylamino ethyl methacrylate (DMAEMA) [82]. It has already been established that DMAEMA can

272  Layered 2D Advanced Materials and Their Allied Applications condense negative charged DNA into compact nanocomplexes spontaneously and thereby protect plasmid DNA from enzymatic nuclease degradation and promote cellular transfection [83]. The LDH coated with amine-functionalized SiO2 nanodot through the condensation of 3-aminopropyl triethoxysilane (APTES) also been used for gene delivery [77]. The amine functionalization offers versatile dsDNA loading including DNA-LDH, DNA-NH₃+-R, or LDH-DNA-NH₃+-R. During the cell uptake study using osteosarcoma cancer cell line (U2OS), it was found that NH3-SiO2-LDH has delivered siRNA in a uniform distribution fashion and it inhibited cancer growth compared to LDH nanoparticle alone [77]. LDH for-delivery of anticancer drug (5-Fluro uracil) and cell death siRNA (CD-SiRNA) was developed by Li et al. [84]. On checking the cell uptake in cancerous cell line MCF-6, U2OD, and HCT-116, it is found that the internalization is handled by clathrin-mediated-endocytosis [84]. In another study, siRNA was intercalated on two different sized (45 nm, 114 nm) LDH nanoparticle and used for cellular uptake and subsequent gene-splicing application [85]. The 45-nm-sized nanoparticles have offered enhanced loading capacity due to the high specific surface area, more efficient internalization, and extensive gene splicing than 114-nm-sized nanoparticle.

11.6 Toxicity Increasing focus on exploiting the application of LDH and an increase in chances for exposure also creates an equally important need to evaluate the biocompatibility of LDH. The toxicity evaluation of Mg-Al LDH on different cells including HOS, HeLa, A549, and normal lung epithelial L-132 cell lines were done. The results showed that at a concentration range of 200– 500 µg/ml, LDH evoked a toxic response to cancerous cell lines. Exposed LDH caused an inflammatory response, oxidative stress, and membrane rupture. The effects were more pronounced in the case of A549 compared to HOS and HeLa cell lines. Interestingly, LDH treatment did not produce any noticeable toxicity in normal L-132 cells. Notably, as per the study, the synthesized LDH has potential application as a targeted drug carrier for cancer treatment without affecting normal cells [86]. The 2D layered structure and ion-interchange capability of LDH make them a reliable choice as a drug/gene delivery vector. With the increasing interest in developing LDH-based delivery vectors, it is equally important to scrutinize the material for toxicity. In these backgrounds, the compatibility of folic acid functionalized Zn-Al LDH as a delivery vector in HeLa cells was analyzed. The results validate the use of LDH as a compatible drug delivery vector.

2D Layered Double Hydroxides  273 But at the same time, the study also highlights the importance to focus on the optimal safe concentration of LDH. At a high concentration of 400– 800 µg/ml, LDH was found to induce DNA damage [87]. As discussed earlier, an important healthcare application of LDH of is in the field of tissue engineering as a scaffold material. The chance of direct exposure of LDH demands a thorough evaluation of its tissue response before commercialization. In a recent study, Mg-Al LDH and Zn-Al LDH were synthesized and implanted at the intramuscular site for tissue repair. The implant with appreciable bioactivity was found to be compatible. No unnecessary toxic response was triggered at the implant site nor was any detectable inflammation [88]. Meanwhile, in an effort to know the effect of LDH on aquatic life forms, it was found that LDH exerted toxic response on green algae Scenedesmus quadricaud. It was reported that the trimetallic LDH, namely, Cu-Mg-Fe influenced the photosynthetic behavior of the algae. The chlorophyll content was found to diminish after LDH treatment. Also, elevation in reactive oxygen species (ROS) production together with lipid peroxidation was evident in S. quadricaud upon exposure to LDH. The mechanism involved in the detected toxicity was speculated to be caused by oxidative stress along with particle interface and particle agglomeration. The result brings out the importance to have more concise attention on the chances of exposure of LDH on aquatic life and concerns evoking from the interactions [89]. Hybrid material composed of LDH with silver nanoparticles was reported to exhibit anti-microbial activity. The cytotoxicity evaluation of the hybrid revealed that the hybrid material exerted only non-toxic response when exposed to V79 lung fibroblast cells. The results shed promising light on the potential of LDH hybrid for biomedical and cosmetic fields [90]. The knowledge that can be gathered about the toxicity of LDH does not assure a conclusive point. The literature data currently available contains contradicting reports and claims which demands a more apprehend evaluation. Biocompatibility evaluation is vital since the harmful response any biomaterial can evoke maybe sometimes much more severe than its positive impact it holds. Therefore, the toxicity evaluation of LDH is as important as developing/exploring its materialistic or application scope.

11.7 Conclusion LDHs with its 2D layered structure, diverse chemical composition, and ion-exchange capacity transpires to be a suitable choice for various application domains. The large surface area and high adsorption ability of LDH put them in demand as adsorbent, catalyst, and anion scavenger.

274  Layered 2D Advanced Materials and Their Allied Applications Proper focus and selection of synthesis mode based on the application can help a lot to achieve the desired characteristics of LDH and attain the target requirements. The memory effect characteristic of LDH needs to be exploited more, since this, a very peculiar property of LDH, can contributes positively towards widening its application scenario. Furthermore, the investigations on the sensing potential of LDH provide promising scope. The proper regulation and control on the metallic composition and ratio can improve the capacitance of LDH and thereby increase its potential as an electrode. The possibility to fine-tune the inter-layer domain further helps to improve the functionality of LDH. The brucite layer arrangement and the feasibility to load a molecule of interest on to inter-layer domain indeed make LDH as prospective material as delivery vehicle notably in drug/gene transport, imaging as well as protein purification. The anti-microbial activity of LDH further facilitates the leeway for a biocompatible and bioactive coating material for medical devices. The LDH is found to promote better cell-adhesion, proliferation, and growth of cells with an added advantage of biocompatibility. Considering all these factors LDH holds credibility as a tissue regenerative scaffold. The option for intercalation of contrast agents in LDH indeed creates leeway for imaging purposes. A better understanding of the influence of parameters like size, shape, metal ion ratio, type of anions, basicity, etc., on toxicity can help improve the feasibility of LDH as a biomaterial. With more focussed and explicit research, LDH is anticipated to make a positive contribution as futuristic multi-functional material.

Acknowledgement The authors wish to express their sincere thanks to Director and Head, Biomedical Technology Wing, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India for their encouragement and support for conducting this study. Ashtami thanks SCTIMST, Trivandrum, Athira thanks CSIR, New Delhi, Joseph and Akil thanks DST, New Delhi for financial support of Research Fellowships.

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2D Layered Double Hydroxides  281 80. Xu, Z.P., Walker, T.L., Liu, K.L., Cooper, H.M., Lu, G.M., Bartlett, P.F., Layered double hydroxide nanoparticles as cellular delivery vectors of supercoiled plasmid DNA. Int. J. Nanomed., 2, 163, 2007. 81. Ladewig, K., Niebert, M., Xu, Z.P., Gray, P.P., Lu, G.Q., Controlled preparation of layered double hydroxide nanoparticles and their application as gene delivery vehicles. Appl. Clay Sci., 48, 280, 2010. 82. Hu, H., Xiu, K.M., Xu, S.L., Yang, W.T., Xu, F.J., Functionalized Layered Double Hydroxide Nanoparticles Conjugated with Disulfide-Linked Polycation Brushes for Advanced Gene Delivery. Bioconjug. Chem., 24, 968, 2013. 83. Xu, F.J. and Yang, W.T., Polymer vectors via controlled/living radical poly­ merization for gene delivery. Prog. Polym. Sci., 36, 1099, 2011. 84. Li, L., Gu, W., Chen, J., Chen, W., Xu, Z.P., Co-delivery of siRNAs and anticancer drugs using layered double hydroxide nanoparticles. Biomaterials, 35, 3331, 2014. 85. Chen, M., Cooper, H.M., Zhou, J.Z., Bartlett, P.F., Xu, Z.P., Reduction in the size of layered double hydroxide nanoparticles enhances the efficiency of siRNA delivery. J. Colloid Interface Sci., 390, 275, 2013. 86. Choi, S.J., Oh, J.M., Choy, J.H., Toxicological effects of inorganic nanoparticles on human lung cancer A549 cells. J. Inorg. Biochem., 103, 463, 2009. 87. Li, Y., Liu, D., Ai, H., Chang, Q., Liu, D., Xia, Y., Liu, S., Peng, N., Xi, Z., Yang, X., Biological evaluation of layered double hydroxides as efficient drug vehicles. Nanotechnol., 21, 105101, 2010. 88. Cunha, V.R.R., De Souza, R.B., Koh, I.H.J., Constantino, V.R.L., Accessing the biocompatibility of layered double hydroxide by intramuscular implantation: Histological and microcirculation evaluation. Sci. Rep., 6, 30547, 2016. 89. Ding, T., Lin, K., Chen, J., Hu, Q., Yang, B., Li, J., Gan, J., Causes and mechanisms on the toxicity of layered double hydroxide (LDH) to green algae Scenedesmus quadricauda. Sci. Total Environ., 635, 1004, 2018. 90. Marcato, P.D., Parizotto, N.V., Martinez, D.S.T., Paula, A.J., Ferreira, I.R., Melo, P.S., Durán, N., Alves, O.L., New hybrid material based on layered double hydroxides and biogenic silver nanoparticles: Antimicrobial activity and cytotoxic effect. J. Braz. Chem. Soc., 24, 266, 2013.

12 Experimental Techniques for Layered Materials Tariq Munir1*, Arslan Mahmood1, Muhammad Imran1, Muhammad Kashif1, Amjad Sohail1, Zeeshan Yaqoob1, Aleena Manzoor1 and Fahad Shafiq2 Department of Physics, Government College University Faisalabad (GCUF), Faisalabad, Pakistan 2 Department of Botany, Government College University Faisalabad (GCUF), Faisalabad, Pakistan

1

Abstract

Due to the excellent thermal and electrical properties, graphene-based 2D layered materials offer greater promise for various fields, for instance, sensing devices, photovoltaic device, field effect transistor, super capacitor, and medical physics. Also, 2D graphene-based layered materials can be suitable for flexible devices like solar-panel, etc. In this chapter, synthesis and characterization techniques of various graphene derived 2D layered material for advanced application is focused via bottom up and top down techniques. Generally, the carbon source is used to produce the monolayer of graphene which is used to control the different factor of substrate like pressure, temperature, and deposition time to enhance the quality of monolayer graphene-based 2D layered materials. For characterization, the most suitable techniques include X-ray Diffraction Technique (XRD), Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM), Fourier Transform Infrared radiation (FTIR), UV- visible spectroscopy, Raman analysis, and Low Energy Electron Microscopy (LEEM). Above all, the significance of using these techniques and further practical applicability of 2D graphene-based layered materials is provided. Keywords:  Graphene, HFTCVD, FESEM, raman spectroscopy, TEM, LEEM techniques

*Corresponding author: [email protected] Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (283–302) © 2020 Scrivener Publishing LLC

283

284  Layered 2D Advanced Materials and Their Allied Applications

12.1 Introduction In present days, carbon-based 2D layered materials are utilized in almost every field including electrical devices, energy generation, storage, biosensing, bio-imaging, and drugs delivery [1]. In addition, different types of 2D layered materials contributed to multiple applications such as hexagonal boron nitride (h-BN), MoS2, WS2, MoSe2, and Mo2C but the major focus here is on graphene-based 2D layered materials [2–4]. The discovery of graphene paved the way for new area in material science and condensed matter physics. Graphene exhibits unique properties like physical, chemical, thermal, and electrical properties which enable its use and enhance efficiency for multiple potential applications (Piezoelectric materials, photo­voltaic cell, multifunctional nanocomposites, flexible wearable electronics, thermoelectricity, and transparent materials). Graphene is basically a monolayer of graphite material in which there is extensive carbon-carbon covalent bonding and the single layer exhibits extraordinary properties compared to graphite. More specifically, graphite contains multilayer graphene sheets having inter-planner space 3.5 A and bond length of each carbon to carbon atom is 0.142 A. Furthermore, the defect-free graphene-based 2D layered materials have outstanding physical properties like high intrinsic mobility, high thermal conductivity, high Young’s modulus, optical transmittance (about 98%), and high specific area [4–7]. The Sp2 hybridization and sigma bonds exist between carbon atoms of 2D layered graphene materials which is densely packed in a honey­comb crystal lattice [8]. Multiple novel approaches have been developed to synthesize high quality monolayer graphene-based 2D layered materials such as CVD, plasma-enhanced CVD, mechanical exfoliation, liquid-phase exfoliation, Hummers’ method, and molecular beam epitaxy that are most common. Among the methods, the exfoliation method is the most suitable technique to produce monolayer graphene. However, this method produces graphene which is scattered on substrate due to this condition, and therefore, it is not preferred for commercial and research laboratory purposes. On the other hand, the Hummers’ method involves the reduction of oxide group and other constituent is very difficult task to synthesize the monolayer graphene. Another method is CVD, also called chemical vapor deposition technique which is used to produce the high quality graphene sheet by using the different substrates like Ni and Cu [7]. The use of Cu as a substrate is mostly practiced to produce uniform and high

Experimental Techniques for Layered Materials  285 quality graphene compared to the use of Ni as substrate [8, 9]. Other techniques related to CVD include HFTCVD, T-CVD, and LPCVD which consist only four steps like initially heating, annealing, and then deposition on the different substrates followed by cooling. Different gases that contribute in every step such as hydrogen, argon, and methane are very important to complete this task. Furthermore, we need to develop nanocomposite of graphene-based 2D layered materials, because these composites produced to overcome the defects and enhance the properties (electrical, thermal, optical, and mechanical) of materials. The Van der Waals forces play a central role in order to produce nanocomposites of graphene-based 2D layered materials. Overall, in this chapter, we will discuss synthesis of graphene-based 2D layered materials by using hot filament chemical vapor deposition (HFTCVD) technique and characterized this material with the help of XRD, FESM, TEM, FTIR, UV-visible spectroscopy, and Raman spectroscopy and LEEM.

12.2 Methods for Synthesis of Graphene Layered Materials Different techniques have been used to prepare the high quality mono and multilayer of graphene. Two different processes are bottom-up CVD growth and top down exfoliation technique. During top down exfoliation process, the transparent/adhesive tape can be used to produce the high quality graphene. But the major disadvantage of this process produce low yield of graphene which makes this process as unsuitable for commercial scale applications. Another technique uses bottom up CVD approach to synthesize the high quality and single layer of 2D layered material by using Cu substrate [10]. The CVD bottom up technique is a multistage process used to transfer the graphene on substrate and high temperature is generally required to complete this task. A comparison of single-layer graphene on Cu substrate by using different techniques is presented (Table 12.1). Few other methods used to synthesized the graphene by reducing graphene oxide such as plasma-enhance CVD and molecular beam epitaxy. In addition, hot wire chemical vapor deposition (HWCVD) [11] and hot filament chemical vapor deposition (HFTCVD) offer multiple advantages and are low cost process [12]. The HWCVD and HFTCVD are preferred due to following advantages:

Methodology

HWCVD

HFCVD

HFTCVD

HWCVD

CVD

HFCVD

CVD

CVD

HFTCVD

CVD

LPCVD

CVD

LPCVD

T-CVD

LPCVD & APCVD

Sr. no.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

730

900

1,100

---------------

--------------

720

1,000

1,000

1,000

1,000

1,045

800–900

1,000

1,000

1,000

Substrate Temperature (°C)

~ 0.94 ----------------

Monolayer

------------------

------------------

------------------

~ 0.2

Single-layer

Monolayer

Bilayer

Monolayer

Monolayer

0.1

0.1

Monolayer Controllable layer

0.1

0.2-0.3

0.1

~ 0.7

Multilayer

Bilayer

Monolayer

Multilayer

~0.5

0.1

Monolayer Monolayer

~1.00

ID/IG ratio

Multilayer

Number of layered

Table 12.1  Comparison of single-layer graphene on Cu substrate by using different techniques.

[21]

[20]

[19]

[18]

[17]

[16]

[9]

[48]

[47]

[46]

[45]

[40]

[39]

[38]

[37]

Reference

286  Layered 2D Advanced Materials and Their Allied Applications

Experimental Techniques for Layered Materials  287 1. Most suitable to produce high quality graphene on commercial level and research laboratory and much suitable for high temperature inclined [13]. 2. HWCVD and HFCVD in both processes to produce plasmafree thin film by bombarding energetic ions or slowly reaction take place [14]. 3. The flexible materials substrate were most suitable because it has no effect on reaction and produce uniform, high quality, and large surface area of graphene-based 2D layered material [15].

12.3 Selection of a Suitable Metallic Substrate Metals have low carbon solubility and are preferred for substrate like Fe, Co, Ir, Cu, Ru, and Ni. In case of industrial scale, the most suitable metals are Fe, Co, Cu, and Ni due to large surface area and low cost materials. While other factors that play a key role to grow these layered materials include hydrogen gas or methane, high temperature, and binding energy that is equal to 440 kJ mol−1. In addition, different metal substrate used to reduce the temperature of substrate, but among them, few substrate (Co, Fe, and Ni) were not suitable for this purpose. The metal material like Ni was most suitable to control the thickness of film but Cu has great ability to produce high quality thin film because it has least solubility of carbon. Finally, it was concluded that the Cu base substrate was most suitable to grow the 2D graphene-based layered materials [22].

12.4 Graphene Synthesis by HFTCVD The hot filament chemical vapor deposition (HFTCVD) technique was used to synthesize the graphene-based 2D layer material on Cu substrate. Here, the experimental set up (Figure 12.1) and the schematic representation of graphene growth (Figure 12.2) deposited through HFTCVD technique is presented. The Al tube fitted inside the HFTCVD apparatus is used to monitor the substrate temperature place inside Cu foil during fabrication process and both end preserved with tungsten filament. The external power supply to the filament is fixed in two copper rods. Before the fabrication of graphene-based 2D layered material the base pressure is adjusted 5 × 10–5 mbar. Moreover, hot tungsten filament adjusted

288  Layered 2D Advanced Materials and Their Allied Applications Gas Inlet Air Admittance Valve

Thermocouple Feedthrough

Copper Rod

7 cm View Port

Alumina Tube

Pressure Gauge

To Vacuum Pump

Figure 12.1  The Experimental set-up [9]..

with aluminum tube is used to enhance the temperature of Cu foil up to 1,000°C. The optical pyrometer was used to measure the temperature of filament up to 1,750°C. To remove the oxide in graphene-based 2D layered materials, hydrogen gas is used at a flow rate 50 sccm on copper foil for 20 min at 3.3 × 10−1 mbar pressure. During growth process the mixture of methane/hydrogen gas (10 sccm/50 sccm) is used to control the temperature (1,000°C) of substrate at 3.7 × 10−1 mbar pressure for 5 to 40 min but in case to maintained the temperature by varying the flow of hydrogen gas 100 sccm for 10 to 40 min by using different value of pressure (1.9, 2.4, 3.7, 5.2, 6.3 × 10−1 mbar). The uniform methane and hydrogen gas flow up to 10 sccm was used to remove the oxides by annealing at 1000°C. At this stage to grain growth the most defect produce in surface of sheet. After that, the graphene surface again treated to produce the uniform of graphene in Figure 12.3b. The  production monolayer

Experimental Techniques for Layered Materials  289 Pre-cracking of the Precursor Gases at Hot-filament CH3 + H*

CH4 H2

C*, CHx*, H* etc.

H* + H*

Localized Plume of Active Radicals

Alumina Tube

Circumfluence flow

70 mm

Tungsten Wire

0

50

100

150

200 X (mm)

250

300

350

Figure 12.2  Schematic of graphene growth deposition in HFTCVD technique [9]. (a)

Copper Oxide

(b)

1000 C, CH4/H2

(c)

Copper

Figure 12.3  Synthesis of graphene by using CVD growth on copper substrate (a) copper foil with local oxide; (b) Copper foil by using treatment of CH4/H2 atmosphere at 1,000°C leading to the nucleation of graphene; (c) The graphene with different lattice orientations depends on the substrate grain orientation.

graphene have different orientation due to the crystallographic orientations of copper grain Figure 12.3c. If we increase the growth time, then grain size of Cu substrate is also increased combined graphene 2D layer material. Finally, the prepared graphene was cooled on the surface of Cu substrate at room temperature by using 100 sccm pressure of N2 gas [9].

12.5 Graphene Transfer The flow chart for synthesis of high quality graphene by using HFTCVD technique is presented in Figure 12.4. Presently, transfer of the as-grown graphene from metallic surfaces onto desired insulating substrates is performed using various methods. The straight forward method for

290  Layered 2D Advanced Materials and Their Allied Applications HFTCVD–Graphene synthesis method Precipitation of Graphene by Hot-filament Thermal Chemical Vapor Deposition Deposition of Graphene

1

Any deposition?

Bottom-up method

Optimization of quality of graphene

2

High quality?

Pick the highest quality graphene Tuning the number of graphene layers

3

Tunable graphene layer? Graphene transfer process Successful transfer

Figure 12.4  Flow chart for synthesis of high quality graphene by using HFTCVD technique.

transferring graphene grown on metals is to chemically etch the metal away to obtain free floating graphene membranes that can be scooped onto desired Si substrates. Wet etching of substrates such as Ni and Cu are feasible but is challenging for metals such as Ru, Ir, Pd, and Pt. The exposure to various chemicals during transfer is a cause for concern in terms of introducing defects or undesirable impurities in the transferred graphene. The key advantage of graphene growth is the ability to transfer the graphene to an arbitrary substrate (Figure 12.5). The graphene removed from copper foil in furnace and cooled. The transfer is typically done by depositing a protective polymeric [Polydimethylsiloxane (PDMS) or ploy (methylmethacrylate) (PMMA)] coating on top of the graphene based 2D layered material. The organic ligands also used to remove the graphene layered material on the surface of Cu foil with slightly effort of ferric chloride (FeCl3). The graphene layer attached with organic ligands is used to adjust any other substrate like solar cell,

Experimental Techniques for Layered Materials  291 Graphene Copper

Drop Cast

Polymer (PMMA) Graphene Copper Ferric Chloride

Polymer (PMMA) Graphene Target Substrate

Acetone

Polymer (PMMA) H2O Surface Tension

Graphene

Graphene Target Substrate

Figure 12.5  Graphene transfer methodology.

etc. After some interval of time, the organic ligands dissolved and leave the graphene layer on given substrate [23, 24].

12.6 Characterization Techniques The quality and purity of grown graphene are characterize by using different techniques like X-ray Diffraction Technique (XRD), Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Micros­ copy (TEM), Fourier Transform Infrared radiation FTIR, UV-visible spectroscopy, Raman analysis, and Low Energy Electron Microscopy (LEEM).

12.6.1 X-Ray Diffraction Technique It has been reported that the XRD characterization technique can be used to calculate the miller indices, structural identification, and crystalline size of the graphene-based 2D layered materials [25]. The XRD spectra of graphite, graphene oxide, and graphene [26, 27] are presented (Figure 12.6). In case of graphite, graphene oxide, and graphene, only one dominant crystalline peak appeared at different angles θ 11o, 25o, and 26o [28]. The miller indices at different angles can be calculated by using Bragg’s law and d-spacing. Furthermore, the hexagonal structure was most dominated in all types of 2D layered material. The single peak of graphene oxide, graphene, and graphite has huge variation of angle in these 2D layered materials. Therefore, the d-spacing of graphene oxide

Intensity (a.u.)

292  Layered 2D Advanced Materials and Their Allied Applications

Graphite

GO

Graphene 0

10

20

2θ

30

40

50

Figure 12.6  XRD spectra of graphite, graphene oxide, and graphene [28].

is 8.33 A, in case of pure graphene is 3.70 A, and graphite 3.36 A. This value was indicated that the d-spacing increases in case of graphene oxide due to the presence of oxygen functional groups. After the hydrothermal treatment, the graphene oxide was converted in to graphene and d spacing slightly increase due to graphite because the presence of oxide functional group. Finally, the crystalline size in the range of 15 to 38 nm was calculated by using the Scherrer’s equation.



dhkl =

λ ..............(12.1) 2 sin θ

D=

kλ ..............(12.2) β cosθ



Here, some parameters used in these equations are described. D = grain size of the material, k = constant or the factor of the dimension shape, λ = wavelength of X-ray radiation, β intensity of the full width half maxima, θ indicates the Bragg’s angle.

12.6.2 Field Emission Scanning Electron Microscopy (FESEM) It is reported that FESEM is preferred in order to study high quality surface morphology of graphene-based 2D layered material [27]. The FESEM technique yields better results compared to scanning electron microscopy. Therefore, FESEM is generally preferred to study the surface morphology of 2D layered materials of graphene oxide and graphene. An image of FESEM of graphene oxide 2D layered material and the average grain size in

Experimental Techniques for Layered Materials  293 (a)

(b)

100 nm CMPS

SEI

10.0kV X20.000

1µm

WD 6.0mm

100 nm CMPS

SEI

10.0kV X20.000

1µm

WD 5.9mm

Figure 12.7  (a) FESEM of Graphene Oxide (GO) (b) FESEM of Graphene [25].

the range of 500 nm (Figure 12.7a). Also, Figure 12.7b indicated that pure graphene 2D layered sheet in which benzene ring overlapping with each other and the size of the sheet in the range of 15–38 nm [25]. The size of pure graphene decreased as compare to graphene oxide due to thermal dissociation of oxygen group. Moreover, the image clarification of graphene is usually better compared with graphene oxide [29].

12.6.3 Transmission Electron Microscopy (TEM) The TEM is always an important tool to measure the size of 2D layered materials and is preferred to investigate the size of graphene-based nanomaterials. Figure 12.8 shows the pure graphene sheet of 2D layered

100 nm

Figure 12.8  Shows the TEM image of pure graphene 2D layered material [28].

294  Layered 2D Advanced Materials and Their Allied Applications material [30]. The image shows the very thin film of graphene 2D layered materials dispersed in the form of holy carbon grid [28]. The average size of the sheet can be calculated by using image j software in the rage of 15 to 38 nm.

12.6.4 Fourier Transform Infrared Radiation (FTIR) The FT-IR is generally used to study about the function groups attached on the surface of graphene sheet. Figure 12.9 shows FT-IR image of broad and few spectral line of graphene-based 2D layered materials [25]. Two strong absorption bands are 3,431 and 1,630 of graphene oxide and 3,430 and 1,631 of pure graphene. The stretched vibration of hydroxyl group (−OH) shows the absorption at 3,431 cm−1 and water molecule (H2O) at 1,722 cm−1. Furthermore, after the oxidation of graphene oxide into graphene 2D layered sheet, the starched vibration of two different function groups (−COOH and C = O) are very closed to each other at band 1,630 cm−1 because the function groups are oxidizes but not effect on the benzene ring of the sheet [29]. The C = C was showed the stretched vibration at 1,400 cm−1 and few other epoxy function groups attached on the peaks like C2O, C-OH at 1,264 cm−1 and 1,068 cm−1. Moreover, few bending vibrational bands due to hydroxyl group are attached at 1,400 cm−1. Collectively, we can change the graphene oxide into graphene 2D layered graphene

1633 3430

1722 1630 3431

4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm–1)

Transmittance (%)

1264 1068 1400

2928 2850 Transmittance (%)

1066

graphene oxide

4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (cm–1)

Figure 12.9  FTIR image of graphene oxide and graphene [25].

Experimental Techniques for Layered Materials  295 materials by removing the oxygen of the following function groups such as –OH, C = O, −COOH, and CO2 [31].

12.6.5 UV-Visible Spectroscopy The UV visible spectroscopy is also preferred to determine the absorption spectra of graphene and graphene oxide of 2D layered materials. Figure 12.10 shows the UV spectrum of graphene oxide and graphene [31]. The absorption peak of graphene oxide is indicated at 235 nm. Basically, sp2 hybridization takes place in graphene oxide due the π and π* transition [32]. After that, the graphene oxide reduced into graphene, then the wavelength of the peak shifted at 265 nm. Moreover, to increase the value of p-conjugation due to the reduction of graphene oxide to graphene, because the small quantity of energy used for this type of transition which corresponds to the observed shift of the absorption to the longer wavelength region [33].

12.6.6 Raman Spectroscopy The Raman spectroscopy is used to investigate the thickness and crystalline quality of the graphene-based 2D layers materials [34–36]. Figure 12.11 shows some spectra of graphene layered materials but the peak D and G indicate the high defect density at wave number 1,354, 1,625, and 2,957 cm−1 due to the inhalation of sp2 mode. TRT and

Intensity (a.u.)

235 nm 265 nm

Graphene

GO 200

300

400

500 600 Wavelength (nm)

700

Figure 12.10  UV spectrum of Graphene Oxide and Graphene [28].

800

296  Layered 2D Advanced Materials and Their Allied Applications D

G

2D

Normalized Intensity (arb. unit)

Multi-layer

ID/IG = 0.62

Thermal Release Tape

Multi-layer

ID/IG = 0.12

PMMA Support

Mono-layer

ID/IG = 0.16

Thermal Release Tape

Mono-layer

ID/IG = 0.20

PMMA Support

1200

1500

1800

2100

2400

2700

3000

Wavenumber (cm–1)

Figure 12.11  Raman spectra to transfer graphene on substrate by using the PMMA and TRT method [9].

PMMA techniques can be used to check the crystallinity of mono and multilayered graphene. Therefore, the peak D represents the spectra at 1,337 cm−1 and it was indicated multilayer graphene sheet [28]. After that, the ratio of ID/IG by using both methods was observed 0.62 and 0.12. In case of single layered, the ratio of ID/IG slightly varies 0.16 and 0.20. The PMMA technique was most suitable to transfer the mono-graphene on Cu substrate and TRT for multilayer purpose [29]. Furthermore, the PMMA was most suitable for monolayer graphene of 2D layered materials [30, 31].

12.6.7 Low Energy Electron Microscopy (LEEM) The LEEM can be used to investigate the monolayer, bilayer, and trilayer graphene chemical attached copper substrate. But, the resolution in micro-level is expressed (Figure 12.12). The major advantage of LEEM is that there is no need to transform the graphene monolayer, bilayer, and trilayer and easily scan the large area of the sample [41]. Furthermore, the image was collected at different sccm like 10, 25, and 50 by using the concentration of graphene sample 7.5, 15, and 50 μm, respectively. All images indicate the homogeneous concentration of graphene in graphene monolayer, bilayer, and trilayer was expressed in characterized samples by using the incident electron beam energy of 3 eV. Therefore,

Experimental Techniques for Layered Materials  297 Variation of H2 Flow Rate 10 sccm

25 sccm

50 sccm

Field of View 50 µm

15 µm

7.5 µm

Figure 12.12  LEEM images collected at 50, 15, and 7.5 μm at 10, 25, and 50 sccm sample. These images represent the homogeneous composition of graphene layered material on copper substrate by using HFTCVD synthesis technique [9, 41].

some different types of spot appeared in these images due to the roughness of substrate was used in handling process. Finally, more zoom image 7.5 μm field of view (FOV) at this stage, the graphene monolayer was cleared seen as shown in Figure 12.12. The different color arrow head was indicated that the different layers of graphene such as red color represent the trilayer, blue color express the bilayer and last green color shows the mono-layer of graphene [9]. The required mono-layer of graphene was appeared only in 7.5 μm as compared to 15 and 50 μm at high resolution level [41].

12.7 Potential Applications of Graphene and Derived Materials After the discovery of graphene-based 2D layered materials, such materials are central to enhance the efficiency of different devices like electronic (flexible and wearable), energy storage, and composite of graphene-based material for next generation and even applications in agriculture [42–44]. It can potentially play the significant role to remove the barrier in new technology for the fabrication of low cost material and devices in future. Figure 12.13 shows the contribution of graphene and graphene oxide in

298  Layered 2D Advanced Materials and Their Allied Applications Research

Graphene films

Applied research and development Water membrane OLED/LED lighting

Demonstration

Electron microscopy Photodetectors Flexible transparent conductors

Semiconductor growth Sensors Optoelectronics DNA sequencing Printed electronics

Filtration systems

Graphene oxide flakes

Metals alloy

Commercial

Oil and functional fluids

Humidity sensors

Ceramics composites Li-ion batteries Supercapacitors

Polymer composites

Multifunctional coatings Thermal interface and heat spreaders

Figure 12.13  Potential applications of graphene-based 2D layered materials.

smart potential applications especially in electronics, sensing devices, and medical physics.

12.8 Conclusion The present work focuses on synthesis and characterization of high quality graphene and its derivative by using bottom up techniques. Among various techniques, HFTCVD technique can be preferred due to low cost and controlled synthesis. Moreover, XRD analyses confirmed hexagonal structure is dominant in all types of graphene-based 2D layered material. In addition, FESEM and TEM image revealed benzene ring overlapping with each other and the size of the sheet in the range of 15–38 nm. By using FTIR, different functional groups attached on graphene-based 2D layered material can be identified and absorption spectra via UV-visible spectroscopy. For calculations, 235 to 265 nm can be used. Finally, the Raman spectroscopy confirmed reduce defect density in mono and multilayer of graphenebased 2D layered materials and LEEM show mono-layer of graphene was appeared at 7.5 μm as compared to 15 and 50 μm at high resolution level. Due to these versatile properties, graphene-based 2D layered materials offer potential to be used in different applications.

Acknowledgement The authors wish to express their sincere thanks to all the cited journals for providing permissions to reuse research data (figures and Tables). We are also grateful to Higher education commission Pakistan (HEC) for research grant 8466/Punjab/NRPU/R&D/HEC/2017.

Experimental Techniques for Layered Materials  299

References 1. Coles, S., Sensors in Industrial Applications. Johnson Matthey Technol. Rev., 63, 2, 74–75, 2019. 2. Novoselov, K.S., Geim, A.K., Morozov, S.V., Jiang, D., Zhang, Y., Dubonos, S.V., Grigorieva, I.V., Firsov, A.A., Electric Field Effect in Atomically Thin Carbon Films. Science, 306, 666–669, 2004. 3. Stankovich, S., Dikin, D.A., Dommett, G.H.B., Kohlhaas, K.M., Zimney, E.J., Stach, E.A., Piner, R.D., Nguyen, S.T., Ruoff, R.S., Graphene-based Composite Materials. Nature, 442, 282–286, 2006. 4. Geim, A.K. and Novoselov, K.S., The Rise of Graphene. Nat. Mater., 6, 183– 191, 2007. 5. Rao, C., Biswas, K., Subrahmanyam, K., Govindaraj, A., Graphene, the new nanocarbon. J. Mater. Chem., 19, 17, 2457–2469, 2009. 6. Balandin, A.A., Thermal properties of graphene and nanostructured carbon materials. Nat. Mater., 10, 8, 569–581, 2011. 7. Soldano, C., Mahmood, A., Dujardine, E., Production, properties and potential of graphene. Carbon, 48, 8, 2127–2150, 2010. 8. Voiry, D., Yang, J., Kupferberg, J., Fullon, R., Lee, C., Jeong, H.Y., Chhowalla, M., High-quality graphene via microwave reduction of solution-exfoliated graphene oxide. Science, 353, 6306, 1413–1416, 2016. 9. Hafiz, S.M., Chong, S.K., Huang, N.M., Rahman, S.A., Fabrication of high-quality graphene by hot-filament thermal chemical vapor deposition. Carbon, 86, 1–11, 2015. 10. Saqib, S.S., Ruoyu, Z., Zhu, J.I., Graphene synthesis: A Review. Mater. Sci.Poland, 33, 3, 566–578, 2015. 11. Stojanović, D., Woehrl, N., Buck, V., Synthesis and characterization of graphene films by hot filament chemical vapor deposition. Phys. Scr., 2012, T149, 014068, 2012. 12. Wang, B.B., Zheng, K., Cheng, Q.J., Wang, L., Zheng, M.P., Ostrikov, K., Formation and electron field emission of graphene films grown by hot filament chemical vapor deposition. Mater. Chem. Phys., 144, 1–2, 66–74, 2014. 13. Piner, R., Li, H., Kong, X., Tao, L., Kholmanov, I.N., Ji, H., Graphene synthesis via magnetic inductive heating of copper substrates. ACS Nano, 7, 9, 7495–9, 2013. 14. Schropp, R., Frontiers in HWCVD. Thin Solid Films, 517, 12, 3415–9, 2009. 15. Hawaldar, R., Merino, P., Correia, M.R., Bdikin, I., Grácio, J., Méndez, J., Large-area high-throughput synthesis of monolayer graphene sheet by hot filament thermal chemical vapor deposition. Sci. Rep., 2012, 2, 2012. 16. Bointon, T.H., Barnes, M.D., Russo, S., Craciun, M.F., High quality monolayer graphene synthesized by resistive heating cold wall chemical vapor deposition. Adv. Mater., 27, 28, 4200–4206, 2015.

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Experimental Techniques for Layered Materials  301 32. Zhou, Y., Bao, Q., Tang, L.A., L. Zhong, Y., Loh, K.P., Hydrothermal dehydration for the “green” reduction of exfoliated graphene oxide to graphene and demonstration of tunable optical limiting properties. Chem. Mater., 21, 13, 2950–2956, 2009. 33. Park, S., An, J., Jung, I., Piner, R.D., An, S.J., Li, X., Ruoff, R.S., Colloidal suspensions of highly reduced graphene oxide in a wide variety of organic solvents. Nano Lett., 9, 4, 1593–1597, 2009. 34. Liu, Y., Liu, Z., Lew, W.S., Wang, Q.J., Temperature dependence of the electrical transport properties in few-layer graphene interconnects. Nanoscale Res. Lett., 8, 1, 335, 2013. 35. Ling, X., Fang, W., Lee, Y.H., Araujo, P.T., Zhang, X., Rodriguez-Nieva, J.F., Dresselhaus, M.S., Raman enhancement effect on two-dimensional layered materials: Graphene, h-BN and MoS2. Nano Lett., 14, 6, 3033– 3040, 2014. 36. Zhan, H., Guo, D., Xie, G., Two-dimensional layered materials: From mechanical and coupling properties towards applications in electronics. Nanoscale, 11, 28, 13181–13212, 2019. 37. Stojanović, D., Woehrl, N., Buck, V., Synthesis and characterization of graphene films by hot filament chemical vapor deposition. Physica Scripta, 2012, T149, 14068–14071, 2012. 38. Kataria, S., Patsha, A., Dhara, S., Tyagi, A.K., Barshilia, H.C., Raman imaging on high-quality graphene grown by hot-filament chemical vapor deposition. J. Raman Spectrosc., 43,12, 1864–1867, 2012. 39. Hawaldar, R., Merino, P., Correia, M.R., Bdikin, I., Grácio, J., Méndez, J., Singh, M.K., Large-area high-throughput synthesis of monolayer graphene sheet by hot filament thermal chemical vapor deposition. Sci. Rep. 2, 682, 1–9, 2012. 40. Soler, V.M.F., Badia-Canal, J., Roca, C.C., Miralles, E.P., Serra, E.B., Bella, J.L.A. Hot-wire chemical vapor deposition of few-layer graphene on copper substrates. Jpn. J. Appl. Phys., 52, 1S, 1AK02–1AK08, 2013. 41. Hafiz, S.M. and Jaafar, S.M., Growth and characterization of graphene and graphene/copper oxide nanocomposites by hot-filament thermal chemical vapor deposition for flexible pressure sensor application Doctor dissertation, University of Malaya, Malaysia, 2017. http://studentsrepo.um.edu.my/ 42. Singh, V., Joung, D., Zhai, L., Das, S., Khondaker, S.I., Seal, S., Graphene based materials: Past, present and future. Prog. Mater. Sci., 56, 8, 1178–271, 2011. 43. Bich, H.N. and Van, H.N., Promising applications of graphene and graphenebased nanostructures. Adv. Nat. Sci.: Nanosci. Nanotechnol., 7, 023002(15pp), 2016. 44. Shafiq, F., Iqbal, M., Ali, M., Ashraf, M.A., Seed pre-treatment with polyhydroxy fullerene nanoparticles confer salt tolerance in wheat through up-­ regulation of H2O2 neutralizing enzymes and phosphorus uptake. J. Soil Sci. Plant Nutr., 19, 4, 734–742, 2019.

302  Layered 2D Advanced Materials and Their Allied Applications 45. Shi, Y.G., Wang, D., Zhang, J.C., Zhang, P., Shi, X.F., Hao, Y., Fabrication of single-crystal few-layer graphene domains on copper by modified low-­ pressure chemical vapor deposition. Crystal Engr. Comm., 16, 32, 7558–7563, 2014. 46. Mendoza, F., Limbu, T.B., Weiner, B.R., Morell, G., Large-area bilayer graphene synthesis in the hot filament chemical vapor deposition reactor. Diamond Relat. Mater., 51, 34–38, 2015. 47. Kasap, S., Khaksaran, H., Celik, S., Ozkaya, H., Yank, C., & Kaya, I.I., Controlled growth of large area multilayer graphene on copper by chemical vapour deposition. Phys. Chem. Chem. Phys., 17, 35, 23081–23087, 2015. 48. Lin, H.C., Chen, Y.Z., Wang, Y.C., Chueh, Y.L., The essential role of cu vapor for the self-limit graphene via the Cu catalytic CVD method. J. Phys. Chem. C, 119, 12, 6835–6842, 2015.

13 Two-Dimensional Hexagonal Boron Nitride and Borophenes Atif Suhail1 and Indranil Lahiri1,2* Nanomaterials and Applications Lab., Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee, India 2 Centre of Excellence: Nanotechnology, Indian Institute of Technology Roorkee, Roorkee, India

1

Abstract

Hexagonal boron nitride (h-BN) is one of the promising 2D materials with excellent mechanical properties, good thermal conductivity and excellent chemical as well as thermal stability. These properties of h-BN are very much useful for various high temperature applications such as insulators for furnaces, crucibles for melting glass and metals, deep ultraviolet light sources, insulating and thermally conductive fillers, dielectric layers, cosmetic products, anti-oxidation lubricants, and protective coatings. In this chapter, different properties and synthesis methods of 2D h-BN are discussed. The synthesis methods are basically divided as bottom up and top down approaches that are similar to the well-known graphene sheet growing methods, with required changes in input materials and processing parameters. Most common synthesis methods include mechanical exfoliation, liquid exfoliation, chemical vapor deposition (CVD) and physical vapor deposition (PVD). Also, theoretical investigation and experimental synthesis of newly developed material, borophene, is discussed in detail. Its outstanding properties can be very useful in electronics, optoelectronics and mechanical applications. Keywords:  2D h-BN, synthesis, CVD, PVD, properties, borophene

*Corresponding author: [email protected] Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (303–336) © 2020 Scrivener Publishing LLC

303

304  Layered 2D Advanced Materials and Their Allied Applications

13.1 Two-Dimensional Hexagonal Boron Nitride (2D h-BN): An Introduction Research in two-dimensional (2D) materials has seen a huge boost in last decade owing to strong expectations of attractive properties and exciting applications [1]. Since the discovery of graphene in 2004 [2], its fascinating properties attracted researchers worldwide toward other 2D nanomaterials for various potential applications. Similar to graphene, other 2D materials are also projected for next-generation electronic and optical devices. Hexagonal boron nitride (h-BN) is one of the promising 2D materials with superb mechanical properties, good thermal conductivity, and excellent chemical, as well as thermal stability [3, 4]. These properties of h-BN are very useful in various high temperature applications such as insulators for furnaces, crucibles for melting glass and metals, deep ultraviolet light sources, insulating and thermally conductive fillers, dielectric layers, cosmetic products, anti-oxidation lubricants, and protective coatings [5–7]. Two-dimensional h-BN, a structural analogue of graphite, in which the same number of atoms of boron (B) and nitrogen (N), is alternatively placed. The alternate B and N atoms are connected by strong B-N bonds and 2D layers are held together by weak van der Waals forces. The B-N bond length is 1.44 Å which is very similar to the graphene C-C bond of 1.42 Å [8]. The hexagonal crystal structure of 2D h-BN consists of lattice parameter of a = 0.25 nm and c = 0.66 nm, and interlayer spacing of 0.33 nm which is, again, very close to graphite [9]. h-BN is an insulating material with band gap of ~5.9 eV [10]. The insulating properties of h-BN can be tuned by various strategies such as doping, substitution, functionalization, and hybridization for different functionalities. A schematic model of planer structure of 2D h-BN and corresponding SEM micrograph are shown in Figure 13.1. The structure of h-BN can be synthesized into zero-dimensional (0D) fullerene like structure, one-dimensional (1D) nanotubes, 2D nanosheets, and three-dimensional (3D) bulk crystal structures. Synthesis techniques, different properties, and promising applications of these BN nanostructures were thoroughly investigated [11, 12]. After the emergence of graphene, the 2D h-BN nanosheet has attracted more attention due to its extraordinary properties and tremendous potential applications. Atomically thin h-BN nanosheets have white color appearance, sometimes called “white graphene” because of similar lattice constant and structural characteristics to that of graphene [13, 14].

Two-Dimensional Hexagonal Boron Nitride  305 (a)

(b)

1 µm

Figure 13.1  (a) Structural model of 2D h-BN (reprinted with permission from ref. [27]). (b) SEM micrograph of 2D h-BN sheet (reprinted with permission from ref. [26]. Copyright © 2009, American Chemical Society).

Like well-known graphene growth synthesis method, 2D h-BN is also s­ynthesized by bottom-up and top-down approaches. The top-down approach to the 2D h-BN production primarily involves liquid exfoliation and mechanical exfoliation. In the case of bottom-up approaches, the techniques for synthesizing 2D h-BN mainly comprise chemical vapor deposition method (CVD), physical vapor deposition (PVD), and surface segregation method.

13.2 Properties of 2D h-BN 13.2.1 Structural Properties The single layer of 2D h-BN has a similar structure of graphene, one of sublattices has boron atoms and the other one has nitrogen atoms. The hexagonal structure of 2D h-BN and primitive unit cell of the honeycomb structure with Bravais lattice vectors are shown in Figure 13.2. Usually, the combination of sp2 orbitals of B and N atoms forms the bond between these two atoms. However, owing to electronegativity difference between B and N atoms, electrons are transferred from B to N atoms. To modify its structure, it is very essential to understand the band structure as well as h-BN layers interaction. In a very close comparison, Robertson conducted tight binding calculation on graphitic interaction whose electronic structure is much better known to understand the electronic band structure [97]. He reported that the overlap of π bond in the pz orbitals, that leads to large π bandwidths, is significantly higher than the

306  Layered 2D Advanced Materials and Their Allied Applications (a)

h-BN

2D BN

(b)

a1 a2

B

c

N a

b

Figure 13.2  (a) Structure of h-BN. (b) Unit cell of honeycomb structure of h-BN with Bravais lattice (Reprinted with permission from ref. [15]).

px,y orbitals. Liu et al. reported the structural stability and electronic properties of h-BN by first-principle calculations based on the local-­density approximation (LDA) of density functional theory (DFT) [98]. They studied five different possible crystal structure of h-BN (namely A, B, C, D, and E) in which structure A, B, and C have space group P63/mmc and structure D and E have space group P3m1 and P6¯m2, respectively. They found that the A and B structures with P63/mmc symmetry and structure D with symmetry P3m1 have “good” stacking. These three out of first five possible structures were may be stable or unstable structure. Structure C with symmetry P63/mmc and structure E with P6¯m2 symmetry were unstable. These structures were referred to as a “bad” stacking of h-BN layers and have a large lattice constant c compared to “good” stacking sequence structures. The electronic characteristics of multilayer h-BN are dependent on layer thickness of stacking modes. The band gap can efficiently modulate with an external electric field or by changing the interlayer distance between the layered h-BN [99, 100]. External fields can redistribute free electron states sensitively, quickly decreasing the band gap of 2D h-BN. Also, the band gap is reduced by decreasing interlayer distance which can be achieved through hydrostatic pressure [101].

13.2.2 Electronic and Dielectric Properties Two-dimensional h-BN is generally recognized to have a wide band gap of 5.9 eV [10] and hence is considered as an insulating material. Its surfaces are generally extremely smooth, with no dangling bond and have outstanding physical characteristics which make the 2D h-BN a distinctive

Two-Dimensional Hexagonal Boron Nitride  307 material for the design and production of high-quality devices. Due to outstanding electrical insulation property of h-BN, it is used in electronic devices as charging leakage barrier [16]. It is also used for graphene devices as an improved dielectric [17]. Atomically thin multilayer BN nanosheetgraphene heterostructure configuration (BN-C-BN) have shown exceptional charge mobility which is approximately up to ~500,000 cm2 V−1 s−1 [18]. Various kinds of 2D heterostructures are built not only for enhanced efficiency, but also for the high functionality and new physical characteristics that can be used in optoelectronic devices. Another heterostructure pattern, C-BN-C is used in the field-effect transistor system where a thin BN nanosheet separates two graphene layers [19].

13.2.3 Optical Properties In visible region (380–740 nm), 2D h-BN has no optical absorption, whereas a sharp absorption peak is observed in the deep ultraviolet (DUV) range (200–220 nm) [20]. 2D h-BN nanosheet may be an excellent candidate for use in ultraviolet laser, photon emission, and DUV detector due to the direct wide band gap as well as the ultraviolet luminescence property [21– 23]. Fourier transform infrared (FTIR) spectroscopy is most widely used for the structural phase composition determination of 2D h-BN [24–26]. BN vibration mode of h-BN is at 1,365 cm−1 [24] when it is in single crystal state while in multilayer condition peak value changes to 800 and 1372 cm−1 [26]. The FTIR spectrum of 2D h-BN nanosheet shown in Figure 13.3. The peak around 1,374 cm−1 is attributed to the in-plane B-N of the transverse optical modes of the sp2 bonded h-BN, while peak around 816 cm−1 is B-N-B out of plane which belongs to the bending vibration modes of BN [26]. Optical absorption property of h-BN was revealed by Wang et al. using DFT calculations, after calculating the single layer absorption of 2D h-BN through simulation [27]. In Figure 13.4a, three strong absorption peaks near 197, 203, and 208 nm, respectively, can be seen in the spectrum. These absorption peaks results from a high transfer of charge and the vibration of BN. After theoretical calculation, Jin et al. reported the absorption of deep UV spectrum of 2D h-BN experimentally [28]. They found a high absorption peak at 234.80 nm, with corresponding energy band gap of 5.28 eV. In 2009, Gao et al. reported UV-vis optical spectra of 2D h-BN by examining a synthesized h-BN nanosheet. The absorption peaks of 251, 307, and 365 nm and corresponding energy gaps of 4.94, 4.04, and 3.40 eV, respectively, as presented in Figure 13.4b. These two absorption peaks of 307 and 365 nm are caused by optical transition and the redistribution of 2D h-BN

1374

818

Intensity (a.u.)

308  Layered 2D Advanced Materials and Their Allied Applications

4000

3500

3000

2500

2000

1500

1000

500

Wavelength (cm–1)

Figure 13.3  FTIR spectra of h-BN (reprinted with permission from ref. [26]. Copyright © 2009, American Chemical Society).

(a)

h-BN

Absorption (f )

0.5 0.4 0.3 0.2

(b)

251

Absorbance (a.u.)

0.6

307 365

0.1 0.0 175 180 185 190 195 200 205 210 215 220 Wavelength (nm)

225 250 275 300 325 350 375 400 425 450 475 500 Wavelength (nm)

Figure 13.4  (a) Simulation model of UV-visible absorption spectra of h-BN (Reprinted with permission from ref. [27]). (b) Experimental UV-vis absorption spectra of h-BN (Reprinted with permission from ref. [26]. Copyright © 2009, American Chemical Society).

electron-hole density between the excited states. Both the simulation and experimental finding demonstrate high optical response of 2D h-BN in the deep UV region.

13.3 Synthesis Methods of 2D h-BN Synthesis and processing play a crucial role in controlling the composition, crystallinity, and characteristics of 2D h-BN and its different applications.

Two-Dimensional Hexagonal Boron Nitride  309 Typical h-BN nanosheet synthesis process is categorized into two main approaches: (i) top-down and (ii) bottom-up approaches. Top-down methods mainly include mechanical exfoliation and liquid exfoliation, while bottom-up approaches include CVD, PVD, etc. These methods are most common methods that are widely reported. Most of them are similar to the well-known graphene sheet growing methods with slight changes in input materials and processing parameters. These methods are discussed here in details.

13.3.1 Mechanical Exfoliation The method of mechanical exfoliation, also referred to as mechanical peeling, was the ground-breaking technique for obtaining h-BN atomic sheets. This technique was originally used in 2004 to isolate monolayer of graphene [29] and later used for other 2D materials such as h-BN, NbSe2, and MoS2 [30]. In this technique, BN layers are removed by adhesive tape and then fixed to a substrate that can be observed by a simple optical microscope. 2D h-BN produced by this technique has fewer defects than those developed by chemical methods and hence is a candidate material to explore its inherent characteristics and fundamental research in optical and electronics applications [31]. Due to low yield produced by mechanical cleavage by adhesive tape, Li et al. reported an effective and efficient way through ball milling process to get a high quality and large yield 2D h-BN under appropriate milling condition [32]. In this process, small shear forces were introduced rather than direct pulling forces through peeling under N2 atmosphere. Benzyl benzoate (C14H12O2) has been used as a milling agent for reducing ball impact and milling contamination. Figure 13.5 shows the SEM images of 2D h-BN produced by this method and their corresponding mechanism. (a)

(c)

500 nm (b) hBN

500 nm (d) hBN

Figure 13.5  (a and c) SEM micrograph of 2D h-BN synthesized by wet ball milling method. (b and d) Corresponding diagram illustrating exfoliating mechanism (Reprinted with permission from ref. [32]).

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13.3.2 Liquid Exfoliation In 2008, Han et al. initially reported single- and few-layer 2D h-BN synthesis using a derived chemical solution technique [33]. In this process, the sample was put in poly(m-phenyl-enevinylene-co-2,5-dictoxy-p-phenylenevinylene) (1.2 mg/10 ml) with 5 ml of 1,2-dichloethane solution and sonicated for few hours, so that h-BN crystal would be dispersed and broken down into few-layered 2D h-BN. The h-BN is dispersible in some organic solvents (e.g., chloroform and DMF) and its dispersion is created by means of chemical exfoliation with the help of sonication [34]. Because of the greater interlayer strength of h-BN, the solvents used should be strong polar solvents, like N-dimethylformamide (DMF), polyethylene glycol (PEG), and methane sulphonic acid [35, 36]. Solvent choice could be further optimized to match the surface energy of h-BN to overcome the van der Waals forces between layers. Warner et al. reported that 1,2-dichloroethane is used as a solvent with a reduced boiling point which guarantees that solvent is removed easily and quickly [36]. In order to obtain water-soluble h-BN, BN powder precursor is functionalized with Lewis base. The base-like amine molecules with hydrophilic and lipophilic chains are attached to the boron atoms on the surface of h-BN particles, as could be explained in terms of Lewis acid-base interactions. This Lewis base complexation probably facilitates the interaction of the functional molecules into the h-BN layered structures, which are therefore exfoliated into mono- and few-layered 2D h-BN [37]. In another approach, Li et al. reported a molten hydroxide used to exfoliate h-BN [38]. In this process, synthesized products may be transferred readily to any type of substrate by re-dispersing in prevalent solvent, like ethanol, and water. The mechanism involves in this exfoliation process is in the following sequence: (i) deposition of hydroxide on h-BN, (ii) peripheral self-curling of sheets, (iii) insertion of hydroxides, (iv) cut by the reaction of the reactants, and (v) exfoliation of sheets. Figure 13.6 demonstrates the mechanism involved and corresponding SEM images showing curved and flat 2D h-BN nanosheet in several hundred nanometers in size. 2D h-BN is produced from liquid exfoliation [39].

13.3.3 Chemical Vapor Deposition (CVD) CVD is the most common mass production technique for 2D layered materials, as that is a key manufacturing method for use in industrial requirements. 2D h-BN synthesis on metal substrate by CVD is a heterogeneous catalytic chemical reaction process in which metal plays dual roles to serve

Two-Dimensional Hexagonal Boron Nitride  311 (a)

curling, cutting

molten hydroxides deposition bulk h-BN

BN + 3NaOH Na2BO2 + NH2+

(i)

(iii)

(ii) Na

n

na

peeling off oll n anosh eet

cr os

O H (v) (b)

(iv) (c)

100 nm

100 nm

Figure 13.6  (a) Representation of exfoliation mechanism, (b) SEM images of curved 2D h-BN nanosheet with nanometers in size, and (c) A flat nanosheet of h-BN (Reprinted with permission from ref. [38]).

as a substrate and catalyst. The CVD method has key advantages including low cost, high crystallinity, low defect density, high throughput, and high productivity and scalability [40]. The method of growth process in CVD comprises of four different stages: (i) adsorption of precursor gas molecule on the surface; (ii) precursor decomposition and mobile surface species formation; (iii) diffusion of species formed on the surface; and (iv) nucleation and growth through further incorporation [41]. Figure 13.7 shows the schematic of low-pressure CVD scheme for 2D h-BN growth [42]. The epitaxial growth of 2D h-BN thin film through CVD technique has been applied for many years. The h-BN monolayer was first produced in 1990 with adsorption of borazine (B3N3H6) on transition metals like Pt(111) and Ru(0001) [43]. The control of morphology, growth parameter, growth mechanism, and transfer of h-BN to other substrate will be discussed systematically, and also, type of precursors, substrates, as well as ambient gas condition used in the 2D h-BN growth will be discussed in details in the following sub-sections.

312  Layered 2D Advanced Materials and Their Allied Applications Ammonia Borane (BH3NH3)

Hydrogen (H2)

130 °C Polyiminoborane -[BH=NH]H N

Borazine H

N H

n

H

N N

Ammonia borane (BH3NH3)

H N N H

H

N

H N

N H

H N N N H H

Hydrogen (H2)

1100 °C

Boron nitrite

Borazine

N H N

Pt foil

H

Borazine, Hydrogen

Pt foil

H2 Furnace (1100 °C)

Hot plate (130 °C)

Figure 13.7  Schematic representation of a low-pressure CVD system for the growth of 2D h-BN (Reprinted with permission from ref. [42]).

13.3.3.1 Synthesis Parameters The growth of 2D h-BN monolayer or multilayer by CVD depends on various factors that include type of precursors, different substrate, and ambient gas condition. The precursor for the CVD growth of h-BN may be gas (diborane and ammonia), liquid (borazine), and solid (ammonia borane) [44, 45]. Chatterjee et al. produced 2D h-BN nanosheet by CVD technique using decaborane precursors on polycrystalline metal substrate [46]. For gaseous precursors, boron/ammonia ratio is a vital component in preparing stoichiometric 2D h-BN. Liquid precursors used for the 2D h-BN growth are borazine (B3N3H6), trichloborazine (B3N3H3Cl3), and hexachloborazine (B3N3Cl6) [47, 48]. Kim et al. demonstrated growth of large area h-BN on Cu foil using atmospheric pressure CVD with borazine as a precursor [49]. In general, a higher rate of growth is obtained with high concentration of borazine and growth temperature. Apart from precursors, different types of substrates have also been used for the synthesis of 2D h-BN. Cu and Ni have been the most widely used substrates for the synthesis of 2D h-BN by CVD. Other metal substrates, also used for the h-BN growth by CVD, are Pt, Co, and Fe [50–52]. Gao et al. synthesized 2D h-BN on Pt foils as a substrate by atmospheric pressure CVD [50]. They found that monolayer, bilayer, and few-layer h-BN films can be obtained by just changing the concentration of ammonia borane

Two-Dimensional Hexagonal Boron Nitride  313 used in the reaction system. In another approach, Kim et al. reported detailed study of h-BN synthesis on Cu foil substrate using low pressure CVD [53]. They observed nucleation and growth process on Cu surface and also found that the morphology of the Cu surface affected the location and density of the h-BN flakes.

13.3.3.2 Growth Mechanism Control of the h-BN layers during growth is very essential for practical application. An in-depth knowledge of growth mechanism is very essential for effective control on the layer thickness. In CVD method, the main steps are: (1) pre-annealing phases during which samples in H2 or NH3 are slowly heated, (2) the stage of vacuum that precedes precursor exposure, (3) period of exposure of precursors, and (4) stage of vacuum cooling [54]. These steps are schematically illustrated in Figure 13.8. The initial growth stage involves the nucleation or seed formation, which is usually pretreated. With the control of flow rate of precursors and optimization of heating temperature, nucleation can be controlled, which is followed by growth and domain formation. These domains can be combined by increasing growth time and temperature to form a single h-BN layer. Kidambi et al. performed in situ investigation of growth mechanism of 2D h-BN by CVD on Cu substrate which is shown in Figure 13.9 [55].

Temperature

Annealing (H2 or NH3) Vacuum

1

2

Growth (HBNH)3

3

Cooling (vacuum) Air

4

Time

Figure 13.8  Schematic diagram of the stages of the CVD: (1) pre-annealing stage, (2) the vacuum stage, (3) exposure period, and (4) vacuum cooling stage (Reprinted with permission from ref. [54]).

314  Layered 2D Advanced Materials and Their Allied Applications Multi-layer h-BN

JN

Ji

JB’=JN’ Rd

JB

Monolayer h-BN Cu + B Cu (fcc) Time

Figure 13.9  Schematic representation of 2D h-BN growth mechanism on Cu substrate, where Ji is impingement flux of borazine precursor, Rd (rate of dissociation), JB is diffusion flux of B atoms, JN is the diffusion flux of N atoms, and “JB=JN” means the B and N flux needed to form Cu sublimation under low pressure condition of CVD (Reprinted with permission from ref. [55]).

Borazine precursors were first created and then adsorbed on catalytic surface, where they polymerize and eventually form 2D h-BN on the surface of the substrate through dehydrogenation reaction at elevated temperatures [49]. When the substrate comes in contact with the borazine precursors, it can decompose into boron and nitrogen [54]. At high temperature, boron has adequate solubility in Cu (0.29% at 1013°C [56]), while N atoms has low solubility, due to which it is expelled from the surface. From Figure 13.9, it can be observed that monolayer h-BN formed on the substrate at initial stage. When B atoms are diffused more towards the substrate surface, it can form multilayer islands of 2D h-BN with N atoms. Three well-established thin film growth models often explain the growth of multilayered 2D h-BN. The monolayer of 2D h-BN growth at a relatively low precursors concentration follow a “Frank van der Merwe” (also known as layer by layer growth) model in which the adatom is preferably attached to the surface sites, rather than to the “Volmer-Weber” (Island growth) model in which the adatom-adatoms interaction are stronger than those between the adatom and the surface of the substrate [57, 58]. At very high concentration of precursor, the growth process follows a “StranskiKrastanov” model in which the extra layers grow on ongoing h-BN film with growth time expansion [53].

13.3.3.3 Transfer of 2D h-BN Onto Other Substrates The h-BN nanosheet materials may be transferred to different substrates for various applications. One of the most commonly used method for the transfer of h-BN nanosheet is the polymer supported electrochemical bubbling transfer technique [42, 59]. The transfer method generally comprises

Two-Dimensional Hexagonal Boron Nitride  315 of three important steps: (i) polymer supporting film coating on a substrate with h-BN, (ii) etching for the removal of substrate layers, and (iii) release and transfer onto a target substrate of h-BN. CVD grown 2D h-BN has been transferred onto different metal substrates with supports of various polymers like polydimethylsiloxane (PDMS), poly(methyl methacrylate) (PMMA), and many water soluble polymers [20, 60–62], out of which the most commonly employed polymer for h-BN nanosheet transfer is PMMA [61]. Figure 13.10 schematically describes the detail transfer method of h-BN grown on Pt foil [42]. After h-BN growth on Pt foil, the h-BN/Pt foil structure was spin coated with PMMA. Then, PMMA/h-BN/Pt foil structure was submerged into 1M aqueous NaOH solution, where a pristine Pt foil was used as anode and PMMA/h-BN/Pt foil used as a cathode. A continuous current is applied for a few minutes and due to H2 bubble formation the PMMA/h-BN layer is separated from the Pt foil [63]. In another approach, Wen et al. have reported the electrochemical bubbling treatment after the spin coating without baking [62]. They found that the PMMA was entirely dissolved by methylene chloride after h-BN was separated non-destructively without thermal annealing.

13.3.4 Physical Vapor Deposition Method (PVD) PVD is also one of the bottom-up synthesis approaches in which the materials from a solid or liquid source vaporizes into a low-pressure gas (plasma) environment or ultra-high vacuum (UHV) and transport them OFF

ON –

+

–

+

PMMA coated h-BN on Pt foll

Cathode

Bubbling

1M NaOH

H2 bubble

Cleaning

Dl water

Transfer

Remove PMMA

Figure 13.10  Schematic representation of stepwise transfer of h-BN from substrate by electrochemical bubbling method after growth (Reprinted with permission from ref. [42]).

316  Layered 2D Advanced Materials and Their Allied Applications to a substrate for deposition in atomic or molecular form [64]. Two most popular methods used to generate the vapor from the source materials are (i) sputtering deposition and (ii) thermal evaporation. Sputtering is generally done by accelerated gas ions (usually Ar+) by bombarding the source materials. In the process of thermal evaporation, a concentrated high electron beam from the heated source materials is used to vaporize the sample. A very simple procedure, avoiding unusual precursors, to produce high quality 2D h-BN on Cu foil by ion beam sputtering deposition (IBSD) is reported by Wang et al. [65] A benign and non-toxic h-BN target with Ar ion beam was sputtered to achieve triangular and polygonal domains of h-BN on Cu foils, which is shown in Figures 13.11a and b. By introduction of H2 into the chamber, the domain density of h-BN can be reduced, and therefore, size of h-BN domain can also be increased. In another approach, Sutter et al. also reported the growth of mono- and few-layer 2D h-BN on Ru(0001) substrate in N2/Ar atmosphere by radio frequency (RF) magnetron sputtering of a B target at high substrate temperature [66]. The use of Ru(0001) substrate provides a preferred orientation for B and N species that assemble into an ordered h-BN films.

13.3.5 Surface Segregation Method Xu et al. first reported the surface segregation method to grow few-layer 2D h-BN nanosheet [67]. This technique provides an easy and inexpensive way for mass manufacturing of mono-layer or few-layer 2D h-BN nanosheet on metal or alloys containing trace amounts of B and N atoms. (a)

nm

(b)

Cu foil

B N

10 8 6

Ion so

Ar’io urce

n bea

m

BN h- get r ta

4 1 µm

2 0

Figure 13.11  (a) Schematic representation of the IBSD process for synthesis of 2D h-BN. (b) AFM images of h-BN domains structure grown on Cu foils transferred to SiO2 substrate (Reprinted with permission from ref. [65]).

Two-Dimensional Hexagonal Boron Nitride  317 Electropolished iron-chloride-nickel alloy doped with B and N was used for synthesizing few-layer 2D h-BN nanosheet by heat treatment in UHV condition. Triangular h-BN domains with layer by layer pattern were observed on the alloy surface, which shows the controllability of layer numbers, grain morphologies, and structural orientation. Zhang et al. investigated a controlled co-segregation process for growth of wafer-scale 2D h-BN film [68]. Boron and nitrogen atoms dissolve in the metals and after that co-segregate on to growth substrates by vacuum annealing. A schematic of sandwiched Fe/(B, N)/Ni substrate is shown in Figure 13.12a and high resolution TEM image of as-grown 2D h-BN films nanosheet, grown on this substrate, is presented in Figure 13.12b. On both sides of the metal foils or films, the 2D h-BN layers were formed from the solid sources. In another approach, Suzuki et al. investigated a very easy growth of 2D h-BN nanosheet on polycrystalline Ni/Co films by annealing in vacuum based framework of Ni(Co)/amorphous BN/SiO2 structure [69].

13.4 Application of 2D h-BN Two-dimensional h-BN has different properties from bulk h-BN that include excellent strength, strong electrical insulation, large elastic modulus, high thermal conductivity, and low friction coefficient, which promises a wide array of applications. Their implementation in electronics equipment is one of the most promising areas [70]. In addition, the h-BN nanosheet can be used as a multi-functional composite filler, highly sustainable field emitters, chemically inert hydrophobic films, gas sensing, and thin films for flexible resistive memory applications. These applications are discussed in following section. (a)

Fe film B, N source Ni film or foll

(b)

0.33 nm N B

2 nm

Figure 13.12  (a) Scheme of h-BN synthesis by the vacuum annealing of sandwiched substrates Fe/(BN)/Ni. (b) TEM image of 2D h-BN nanosheet grown on substrate (Reprinted with permission from ref. [68]).

318  Layered 2D Advanced Materials and Their Allied Applications

13.4.1 2D h-BN in Electronic Manufacturing The lattice constant and crystal structure of 2D h-BN is similar to that of graphene, which makes it very promising material for electronic application, along with graphene. Furthermore, it is almost without dangling bonds and charge traps with atomically smooth surface, large electrical band gap, and optical phonon modes [71, 72]. The van der Waals heterostructure is one of the most attractive application of 2D h-BN in which different 2D layers are stacked up, as shown in Figure 13.13. While, strong covalent bonds provide in-plane stability, van der Waals forces are relatively strong enough to hold the stack together [73]. This kind of heterostructures gives superior mobility and carrier inhomogeneity in lateral graphene devices and allows new conceptual flexibility in the design of electronic, optoelectronic, micromechanical, and other devices [74, 75]. Dean et al. reported fabrication of exfoliated mono- and bi-layer graphene devices of high quality on h-BN substrate using the mechanical transfer method, which is shown in Figure 13.14 [17]. The mobility and carrier inhomogeneity of graphene devices on the h-BN substrate are better than those on SiO2. These devices generally show less roughness, chemical responsiveness, and intrinsic doping. Liu et al. showed direct growth

Graphene hBN MoS2 WSe2 Fluorographene

Figure 13.13  Schematic of construction of van der Waals heterostructures of 2D materials (Reprinted with permission from ref. [73]).

Two-Dimensional Hexagonal Boron Nitride  319 Water-soluble layer

PMMA Graphene

Glass slide

Si/SiO2 DI water

(i)

(iii)

Floating PMMA BN

Graphene

(iv)

(ii)

Figure 13.14  Schematic illustration of the transfer method for fabrication of grapheneon-BN devices (Reprinted with permission from ref. [17]).

of h-BN by CVD on pyrolytic graphite and graphene, which are highly oriented [76]. They also reported two-stage CVD method to undergo extensive growth of graphene/h-BN stacks. The graphene/h-BN film is continuous and can be transferred to any substrate for further characterization and device fabrication. Garcia et al. demonstrated synthesis of graphene domains of nanometer-size on boron nitride nanosheet by molecular beam epitaxy (MBE) [77]. The effective growth of graphene layer relies on the substrate temperature, but is not contingent on the incident flow of carbon atoms. In addition, functionalized 2D h-BN may be used to prepare materials for electronics production in relation to the previously mentioned h-BN/ graphene heterostructures. The layers of boron-carbon-nitrogen (B-C-N) structures have band gaps, which demonstrate electronics properties almost halfway between pure graphene and h-BN [78]. Ci et al. also reported atomic layers of very large area of h-BNC materials synthesized by thermal catalytic CVD method [79]. The structure of h-BN and graphene includes hybridized randomly scattered domains of BN and carbon phases. This would allow the production of devices for electronic and optical applications, using band gap-engineered materials with distinct properties from graphene and h-BN.

13.4.2 2D h-BN as a Filler in Polymer Composites The high strength, high elastic modulus, and very low density of 2D h-BN contribute to the improvement of mechanical behavior of polymer composites. Polymers, such as poly(methyl methacrylate) (PMMA),

320  Layered 2D Advanced Materials and Their Allied Applications polybenzimidazole, and polyvinyl acetate (PVA) were investigated separately as matrices using exfoliated h-BN nanosheet as a filler to enhance the mechanical properties of the composites [35, 80, 81]. The reason for improving mechanical properties can be better dispersion, high strength, and partial polarity bonds of B-N [35]. The thermal conduction of the polymer matrix composites, having metal, carbon, and ceramic fillers, can be enhanced further by adding highly thermally conductive fillers to polymers [82]. The 2D h-BN nanosheet with a very high thermal conductivity of up to 400 W/mK is highly promising material for polymer composites. Theoretical investigation shows that higher conductivity can be achieved if a decrease in phonon-phonon scattering can be overcome in layers of 2D materials [83]. It should be emphasized that 2D h-BN is a very interesting material for battery application with outstanding electrical insulation and thermal conductivity properties.

13.4.3 2D h-BN as a Protective Barrier The superb impermeability, excellent mechanical strength, good thermal conductivity, and non-reactive nature to most chemicals make 2D h-BN a potential material to protect metals from oxidation and corrosion. More importantly, h-BN is electrically insulating and therefore does not cause galvanic corrosion. Moreover, h-BN nanosheets are more thermally stable than graphene and therefore can function at higher temperature [84]. Husain et al. reported enhanced corrosion resistance of stainless steel under simulated marine environment after coating with h-BN/­polyvinyl alcohol [85]. 2D h-BN incorporated with polymer also exhibits an increased corrosion resistance to oxygen atoms, needed in low-earth orbit spacecraft [86]. The oxygen-atom protective efficiency of 2D h-BN is due to barrier and bonding effect in BN layers [87]. In addition, as Boron is an effective neutron absorber with an extremely high neutron-capture segment, 2D h-BN filled composites are anticipated to show a better shielding effect against space radiation which exhibits promising potential spacecraft application [88]. Furthermore, Li et al. shown that no oxidation of the monolayer h-BN covered Cu is observed at 200°C in air in sharp comparison with the pristine Cu surface [89]. However, due to defects and grain boundaries, including vacancies, the surface of the coated Cu is oxidized gradually at higher temperatures. Thicker film of h-BN can be used to fix this problem. Liu et al. investigated growth of 2D h-BN nanosheet on Ni foil with controlled thickness and subsequently transferred to Cu foil [90]. They observed that Cu surface can be protected by 5nm h-BN coating at 500°C for 30 min,

Two-Dimensional Hexagonal Boron Nitride  321 (a)

(c)

(b)

(d)

Figure 13.15  (a) SEM images of pure Ni after oxidization at 1,100°C and (b) 5 nm h-BNcoated Ni foils at 1,100° C for 30 min after oxidization. Scale bars, 100 μm, and (c and d) SEM images of Cu foils after oxidization with and without h-BN coatings at 500°C for 30 min. Scale bars, 5 nm (Reprinted with permission from ref. [90]).

while for nickel, a temperature up to 1,100°C can be used for protection by h-BN coating, as shown in Figure 13.15.

13.4.4 2D h-BN in Optoelectronics Two-dimensional h-BN nanosheet can be a very promising material for application in ultraviolet lasing, photon emission, and deep ultraviolet (DUV) detectors because of its direct wide band gap and ultraviolet luminescence property [91–93]. When h-BN photodetector was lighted by a DUV deuterium lamp, the charging current is considerably enhanced by three orders of magnitude. Properties and functioning of single-layer h-BN-based DUV detector are presented in Figures 13.16a and b [94]. Aldalbahi et al. demonstrated a 2D h-BN DUV detector with a fast response time (0.6 ms) and a recovery time of 1.8 ms [93]. The detector is more susceptible to DUV than visible light at room temperature with a stable sensitivity of 0.05 nA. The thinner 2D h-BN is comparatively less

322  Layered 2D Advanced Materials and Their Allied Applications 2.5

(a)

UV Lamp

Au

0.5

h-BN SiO2

Au

Si A

0.0 Under UV Lamp

6

–1

UV Lamp ON

1.5 1.0 0.5 0.0 UV Lamp OFF

Dark –0.5 –3

(b)

2.0 Current (µA)

Current (µA)

1.0

0 1 Voltage (V)

2

Light off Light off Light off

3

–0.5 0

40

80 120 160 200 240 280 Time (s)

(c)

(d)

R (kΩ)

Light on

4

Sweep direction Light on Light on

0.8 µm 0.0 0.4 0.8 0

–40 Vg (V)

Figure 13.16  (a) I–V characteristics of 2D h-BN thin sheet DUV detector in the dark and under a DUV deuterium lamp illumination. (The inset shows the schematic view of the DUV detector operation). (b) Time-dependent photocurrent of the detector at a bias voltage of 5 V (Reprinted with permission from ref. [94]). (c) h-BN/graphene heterostructures photoinduced doping. Graphene resistance R(Vg) is traced by alternating switched on and off (Reprinted with permission from ref. [95]). (d) Scanning confocal map of 2D h-BN multilayer sample that shows bright luminescent spots (Reprinted with permission from ref. [96]).

sensitive than thicker 2D h-BN because of the profound penetration of the DUV light. A defect transformation in 2D h-BN can lead to photo-induced doping of h-BN/graphene heterostructure that allows flexible and reproducible writing and erasing of charge doping by visible light, as presented in Figure 13.16c [95]. In addition, at room temperature, the ultra-bright single-photon emission was measured at around 623 nm from localized defects in monolayers and multi-layer h-BN.

Two-Dimensional Hexagonal Boron Nitride  323

13.5 Borophene Two-dimensional nanosheet of boron, which is also called “borophene”, has been predicted theoretically decade back, but could be synthesized experimentally very recently in 2015 [102]. Borophene, a single atomic boron layer, was first synthesized under ultrahigh-vacuum on a silver substrate. However, electron deficiency makes the boron sheet energetically unstable, as boron has only three valence electrons. Thus, it was expected that the borophene would have a triangular lattice with regular arrangement of holes to form a most stable structure [103]. Different stable structures of borophene, with distinct triangular lattice and periodic holes, have been predicted and some of them were synthesized successfully on silver (Ag) or aluminum (Al) surface [104]. A structural model of borophene sheet on Ag surface is shown in Figures 13.17a and b and corresponding highresolution scanning tunneling microscope (STM) image of borophene on Al surface is shown in Figure 13.17c. The boron element, situated in the periodic table on immediate left of carbon atom, has an impressive carbon similarity, ranging from planar structure and cages to 1D nanotubes, that motivates the study of borophene. Boron has the ground state configuration of 2s22p1 with three valence electrons, while four existing valence orbitals can be found with an electron promoting from the 2s to the 2p orbital, which indicates that the half-filled atomic orbitals are more than the number of available electrons. This prevents the fulfillment of octet rule leading to electron deficiency of the boron atoms. This means that the boron atom doesn’t have sufficient electrons to fill all the electron orbits in a chemical bond based on the classic 2c-2e bonds in carbon system. Hence, localized 2c-2e and delocalized (a)

(b)

(c)

Figure 13.17  (a and b) Structural model of honeycomb borophene on Ag (111). (c) A high resolution STM image of borophene on Al (111) (Reprinted with permission from ref. [104]).

324  Layered 2D Advanced Materials and Their Allied Applications two-electron (nc-2e) bond must coexist to remove an electron deficiency and to stabilize boron crystal [105]. Borophene has certain distinctive physical and chemical properties and different phases of it have been synthesized till now. These phases include triangular lattice (2-Pmmn), β12, χ3, and graphene-like honeycomb phases. All of these phases show metallic behavior [102–104]. The structural model of these four phases is shown in Figure 13.18 [113]. Out of these four phases, graphene-like honeycomb structure of borophene is most important due to its two main characteristics: (i) it contains Dirac fermions and therefore shows fascinating electronic characteristics similar to other elementary 2D materials of group IV; [106] (ii) a well-known superconductor MgB2 has the honeycomb crystal structure of boron similar to graphene [107]. Therefore, boron-based 2D materials is most important for the application of high Tc superconductors.

13.5.1 Theoretical Investigation and Experimental Synthesis A lot of theoretical studies have been undertaken to explore borophene. In 1997, Boustani predicted by using density functional method to a fundamental unit of a B7 puckered cluster that it is possible to make a ­quasi-planar 2D boron sheet [108]. This research led to greater interest in the development of boron’s free-standing monolayer structure. Yang et al. reported formation of 2D boron sheets, extracted from the unzipping of B80 buckyball onto a plane [103]. The α-sheet consists of triangular grid designed by isolated HHs, of a D6h symmetry. It is more stable by 37 meV per atom a2 a2

a2 a2

a1

a1

a1

(a) 2-pmmn

(b) β12

a1

(c)

3

(d) graphene-like

Figure 13.18  Atomic structure model of four different phases of borophene (a) triangular lattice (2-pmmn), (b) β12 phase, (c) χ3 phase, and (d) graphene-like honeycomb structure (Reprinted with permission from ref. [113]. Copyright © 2016, American Chemical Society).

Two-Dimensional Hexagonal Boron Nitride  325 compared to buckled triangular sheet because the 2D sheet is electronically equivalent to graphene. The ideal fill-up of the in-plane electronic orbitals gives its high stability and also accounts for its flat conformation. In a similar way, Tang et al. proposed various boron sheets in order to arrange the HHs in different patterns and concentrations, showing that many of them possess cohesive energies as useful as α-sheet [109]. This outcome opens a larger curiosity regarding the possibility of 2D boron polymorphism. After the theoretical prediction of borophene, the first experimental synthesis of 2D boron nanofilm was reported by Tai et al. by using the conventional CVD method [110]. They synthesized the film from a mixture of boron and boron oxide powders. The mixture was heated to form diborane dioxide vapor at ~1,400 K and finally deposited on Cu foil at ~1,300 K for the growth. Mannix et al. reported deposition of monolayer boron sheet on a single crystal Ag (111) surface by the molecular beam epitaxy (MBE) method using direct evaporation of a boron source [102]. The synthesized 2D boron sheets were characterized by STM which disclosed that mono­ atomic boron sheet layers displayed distinct phases based on the deposition rate. In a similar approach, Feng et al. demonstrated synthesis of single-layer boron islands on Ag (111) surface by using same method of MBE [111]. They found intrinsic polymorphism of 2D boron sheet grown on Ag surface. The STM topographic image of borophene grown at 570 K is shown in Figure 13.19a in which boron islands are labeled as S1 phase. After increasing the temperature to 650 K, a new S2 phase of parallel chains has been developed, which is shown in Figure 13.19b. This phase shows parallel atomic ranges and formed alternate brighter and darker segments and a (a)

S2

(b) S1 S1

Ag(111)

(c)

1.5 nm

1.5 nm

3.0 Å 5.0 Å

Ag(111)

[110] 20 nm

20 nm

[112]

1 nm

Figure 13.19  (a) STM image of boron structures on Ag (111), with a growth temperature of approximately 570 K. (b) STM image of boron sheets after annealing to 650 K. The two different phases are marked as “S1” and “S2”, (c) High-resolution STM image of phase S1. A black rectangle indicates S1 unit cell, and the 1.5 nm stripes are marked by solid lines (Reprinted with permission from ref. [111]).

326  Layered 2D Advanced Materials and Their Allied Applications shorter spacing than the S1 phase. In Figure 13.19c, high resolution STM image of sample grown at 570 K is displayed, in which a rectangular lattice with parallel atomic rows exhibiting the S1 phase can be observed.

13.5.2 Properties and Application of Borophene 13.5.2.1 Electronic Properties of Borophene Unlike the insulating and semiconducting characteristics of its 3D structures, majority of borophenes show metallic behavior. The electronic structure of buckled borophene was found from the precise DFT calculations in which the px, py, and pz states of boron orbits cross the fermi level in the direction of Y-S and Γ-X, that are parallel to the noncorrugated direction. On the other hand, out-of-plane corrugation in other direction opens a band gap along the Γ-Y and S-X direction [102]. Hence, borophene is therefore a strongly anisotropic metal in which electrical conductivity is restricted to the chains. This anisotropic conductivity, proved by direct transmission calculations, in which directional dependence of transport characteristics was discovered in the borophene where the current is two times greater along the chain path than the out-of-plane corrugation direction [112]. The band structure of the β12 and χ3 phases, calculated by LDA, is shown in Figures 13.20a and b, respectively [113]. The polymorphs of borophene show metallic behavior and the metallic states mainly come from the 2pz state, that is extremely delocalized through a high energy window around Fermi level. The metallicity of the χ3 phase is also partially supported by the 2px and 2py states that otherwise form a band gap near the fermi level [114]. The theoretical prediction of metallicity was later confirmed by experimental measurements [111]. Feng et al. conducted a thorough assessment with angle-resolved photoemission spectroscopy (ARPES) of the electronic properties of the β12 phase on Ag (111) [115]. They measured the metallic bands from boron and found that the fermi surface is made up of single electron pocket (B1) on Ag (111), which is shown in Figure 13.20c.

13.5.2.2 Chemical Properties In order to prevent environmental contamination and degradation, most of the experimentally produced borophenes were grown chemically in ultrahigh-vacuum circumstances. However, borophene was reported to be inert towards oxidation, where the area of perfect sheets was found to remain intact under ambient conditions for a sufficiently long time period  [116].

Two-Dimensional Hexagonal Boron Nitride  327 (a)

4

(b)

Pz Pz Py s

2

4 2

–2

–2

ε-εF (eV)

0

ε-εF (eV)

0 –4

–4

–6

–6

–8

–8

–10

–10

–12

Pz Pz Py s

–12 Γ

X

M

(c)

Γ

Y

Γ

X’

Y

0.0

B1

sp –0.5

X

Γ

X’

Cut 2

Cut 1 0.0

E-EF (eV)

M

B4

B2

sp –0.5

B5

B6 –1.0

–1.0

B3

–1.5

–1.5 1.0

1.2 kø (1/Å)

1.4

1.0

1.2 kN (1/Å)

1.4

Figure 13.20  Band structures of (a) the β12 phase and (b) χ3 phase calculated using LDA in a vacuum. Black solid lines in panel (a) are tight-binding bands of single orbital pz (Reprinted with permission from ref. [113]. Copyright © 2016, American Chemical Society). (c) ARPES intensity plots of the borophene in the β12 phase on Ag (111) at E-EF = −100 meV, where the the Brillouin zone of Ag(111) indicated by green lines and B1–B6 denote the bands of 2D boron and sp denotes the bulk sp bands of Ag(111). (Reprinted with permission from ref. [115]).

Deficiency of electron in boron atoms and the comparatively strong chemical activity may still contaminate after a long-lasting exposure to air [102]. Theoretical studies have shown that oxygen atom may be adsorbed by chemical adsorption on χ3 phase spontaneously after dissociation by overcoming the energy barrier of ~0.35 eV [117]. However, because of the high B-O bonding and very low mobility at room temperature, the dissociated oxygen can hardly be moved on the surface which eventually results in boron oxides. On the other hand, it also emphasizes the need to develop packaging technologies for manufacturing of air-stable devices based on

328  Layered 2D Advanced Materials and Their Allied Applications borophene. For instance, a capping layer can be used to seal the boron sheet from the air as shown by Guisinger et al., in which a capping layer of silicon/silicon oxide could significantly prevent the oxidation of borophene [102].

13.5.3 Potential Applications of Borophene Two-dimensional boron can be utilized for the design of sophisticated composites because of its outstanding mechanical characteristics and very low density. For example, 2D boron can withstand a heavy load before failure, similar to graphene, allowing effective load transmission due to its covalent bonding between host matrix. The abundance of 2D boron structural data has also led to the study of its electronic transport characteristics [118]. Borophenes also seem to be appealing as energy storage materials. Their hydrogen storage capabilities were likened to graphene capacity [119]. Because of its more reactive basal plane, hydrogen binding molecule of the borophene is greater than that of graphene. Also, hydrogen binding energy and storage capacity were found significantly enhanced by the spread of alkali metals, such as Li, Na, and K, onto the 2D boron sheet. On the other hand, 2D boron offers a model to establish a stable grid of highly bound metal atoms with a closest range. 2D boron sheets are found to be outstanding adsorbents to capture ambient CO2 with extremely large adsorption energies of up to 3.0 eV [120]. Other theoretical investigation also showed that 2D boron as a good electrode material with an electrochemical efficiency, both for Na-ion and Li-ion batteries [121, 122]. It may be recalled that borophene is in its nascent stage of development now. A strong research focus on this newly developed material is expected to open up new exciting areas of theoretical understanding and practical applications. The upcoming decade is going to be crucial period for development of borophene-based materials and their applications.

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14 Transition-Metal Dichalcogenides for Photoelectrochemical Hydrogen Evolution Reaction Rozan Mohamad Yunus*, Mohd Nur Ikhmal Salehmin and Nurul Nabila Rosman Fuel Cell Institute, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia

Abstract

Hydrogen generation via photoelectrochemical (PEC) hydrogen evolution reaction process is attracted a significant research interest due to its capability to produce clean source energy for the future. Transition metal dichalcogenide (TMDC) is a versatile material for the development of photocatalyst because of the capability to absorb light energy efficiently and promising optical absorption range. Hence, comprehensive discussion on the physical properties and current progress of TMDC-based photoactive materials are presented. The overview of TMDC fabrication methods and its mitigation addressing issue related to TMDCs as photocatalyst for PEC hydrogen evolution reaction are also discussed. Keywords:  Transition metal dichalcogenides, photoelectrocatalyst, water splitting, hydrogen evolution reaction, hydrogen generation

14.1 Introduction Likewise graphene, TMDCs have drawn an intense research progress due to inherent atomically thin 2D materials, transparent, flexible, and most importantly excellent electron mobility [1, 2]. Unlike graphene, most of 2D TMDCs are characterized as a semiconductor. The chemical formula of TMDCs compounds is defined by MX2, where M is transition metal (Ti, Hf, Mo, Zr, Ta, Nb, W, Re, V, etc.) that is chemically structured between X; *Corresponding author: [email protected] Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (337–362) © 2020 Scrivener Publishing LLC

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338  Layered 2D Advanced Materials and Their Allied Applications the chalcogen (S, Se, and Te) [3–5]. Several layers of TMDCs that have a weak van der Waals (vdW) bonds, which enables the physical exfoliation of bulk TMDCs to acquire monolayer sheets [6]. The TMDCs monolayer can be either configured in two different polymorphs, namely, trigonal prismatic or octahedral which referred as 2H and 1T, respectively [3, 6, 7]. The TMDCs band gap is highly dependent on the thickness layer. While the bulk phase of 2D TMDCs exhibits an indirect band gap with relatively smaller energies, the monolayer counterpart shows a direct and wider band gap such as MoS2 (1.9 eV), MoSe2 (1.5 eV), WS2 (2.1 eV), and WSe2 (1.7 eV). Despite a tunable band gap, a strong photoluminescence (PL) and large exciton binding energy are the other characteristic of TMDCs which substantial for various opto-electronic devices such as, photo detector, phototransistor, light-emitting diodes, and solar cells [8–10]. Among the potential application aforementioned, solar cells application especially the photo­electrochemical hydrogen evolution reaction (PEC-HER) has gained considerable research interest. They are at least six most investigated TMDCs semiconductors devoted for PEC-HER, namely, MoS2 [1–3], MoSe2 [11, 12], FeS2 [4–6], CoSe [7–9], WS2 [10–12], and NiSe2 [13–15]. General feature that enabled TMDCs monolayer worked as photocatalyst is their wide surface area of 2D planar, which could provide more active sites for photoelectrocatalytic reaction. 2D planar configuration was also proven effective for the separation of photogenerated charges and photon conversion efficiency which ascribed to the exposure of certain facet with distinct atomic arrangement [13, 14]. In addition, TMDCs monolayer also provides many interfaces that is promising for semiconductor-­semiconductor intersection [15, 16]. The most active catalytic reaction occur at the active sites of the small coordinated stages, terraces, edges, and curves atoms, which exclusive for TMDCs monolayer [17]. Furthermore, 2D monolayer TMDCs also can be a good candidate to promote plasmonic effect that allows extended range of absorption of solar spectrum [18, 19]. With regard to band gap energies and band edges position of certain TMDCs, they are mostly dependent on their lateral dimension and thickness [20–24]. Regularly, either the conduction band (CB) will be shifted towards HER potential or the valence band (VB) will be moved towards oxygen evolution reaction (OER) potential [24]. In certain cases, both phenomenon can occur simultaneously, straddling the HER and OER potential [24]. Most importantly, for an efficient PEC reaction, the band edges of TMDCs semiconductors not only have

Transition-Metal Dichalcogenides  339 high kinetic potential, it also must straddle the theoretical water redox potential.

14.2 TMDC-Based Photoactive Materials for HER 14.2.1 MoS2 Among reported, the TMDCs for PEC-HER application, MoS2 has been chosen as photocatalyst due to the following reason; i) thickness dependent band gap (1.8 eV, direct band gap for single layer, and 1.2 eV, indirect band gap for bulk layer of MoS2) [25–27], 2) a good protective photocatalyst as manifested by a stable HER activity in acidic condition and low penetrability [28, 29], 3) chemically active over wide range of pH electrolyte solution and low overpotential requirement [30], and 4) facile exfoliation, the weakly-interaction-adjacent layers of MoS2 to obtain a single layer sheets [23]. The most energetic active site of MoS2 for HER is at its edges but suffers from its deficiency which limits its practical purpose. Therefore, progress has now geared towards the development of nanostructured MoS2. For example, using chemical vapor deposition (CVD) techniques, a vertically aligned layer of MoS2 was fabricated, a dimensional structure which exposed more active sites that appreciably improved HER catalytic activity [Figure 14.1a(i, ii)] [31]. HER activity can also be enhanced by tuning the polymorphs phase and electronic structure approach of MoS2 as demonstrated by Lukowski et al. [Figure 14.1a(iii, iv)] [32]. The study has proven that 1T-MoS2 phase is catalytically more active, stable, and lower charge transfer resistance than 2H-MoS2 phase for HER as indicated by electrochemical impedance spectroscopy studies [32]. Another drawbacks is associated with high recombination rate of photoinduced charge and low surface area [33]. Therefore, the incorporation of MoS2 with conductive scaffolds such as graphene could create synergistic effect upon HER activity since graphene is characterized not only as an excellent charge collectors, it also provides high surface area for a well-dispersed MoS2 nanoparticles on it [33]. For PEC application, the optical transparency of catalyst plays critical role in overall PEC efficiency. However, MoS2 is hindered with high light absorption or reflection which limits the light from reaching the absorber films underneath. Therefore, amount of catalyst loading and degree of optical transparency should come in balance, which apparently remained a challenging effort [34–36]. Concerning on this matter, a modified MoS2

340  Layered 2D Advanced Materials and Their Allied Applications (a) MoS2 (i)

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Figure 14.1  Physical properties of layered TMDCs (center image). (i) Chemical structure layered TMDC where M is transition metal and X is a chalcogen, and (ii) two types of polymorphs of monolayer TMDCs: the trigonal prismatic (1H) and octahedral (1T) (Reprinted with permission from [93]. Copyright 2013 American Chemical Society). a(i) TEM image of a MoS2 prepared via rapid sulfurization. a(ii) Modelled structure of edge-terminated MoS2 (Reprinted with permission from [31]. Copyright 2013 American Chemical Society). a(iii) Aerial view and its magnification (inset) SEM images and a(iv) HRTEM image of nanostructured 2H-MoS2 (Reprinted with permission from [32]. Copyright 2013 American Chemical Society). a(i) SEM image of MoSe2 particle and b(ii) its corresponding TEM image (Reprinted with permission from [11]. Copyright 2013 Royal Society of Chemistry). c(i) Schematic illustration of WS2 nanosheets synthesis, c(ii) Typical SEM images of WS2 nanosheets (Reprinted with permission from [60]. Copyright 2012 Elsevier). d(i) proposed molecular structure of embedded CoSe2 in C3N4, and d(ii) cross-sectional SEM images of C3N4-CoSe2 (Reprinted with permission from [66]. Copyright 2016 American Chemical Society). e(i) FESEM images and e(ii) the illustration of core-shell heterojunction of FeS2/TiO2 composite (Reprinted with permission from [73]. Copyright 2013 Elsevier), e(iii) SEM image of FeS2 nanoparticle prepared via solvothermal technique (Reprinted with permission from [82]. Copyright 2013 Elsevier). f(i) SEM images of as-prepared NiSe2/Carbon cloth (Reprinted with permission from [91]. Copyright 2016 Springer). f(ii) The illustration of charge transportation across Si NW and NiSe2 nanocrystal, f(iii) SEM images of NiSe2 nanocrystal/Si NW (Reprinted with permission from [92]. Copyright 2018 American Chemical Society).

with Cl, namely, amorphous MoSxCly showed a semi-transparent catalyst with a large band gap that allows little light absorption at visible and nearinfrared region [36]. The deposition of amorphous MoSxCly on p-type Si produced slightly improved Voc and Jsc than that 1T-MoS2. The study managed to achieve an excellent photocurrent density of 43 mA/cm2 mA cm−2,

Transition-Metal Dichalcogenides  341 a highly generated photovoltage, and approximately 6% solar-to-hydrogen efficiency [36].

14.2.2 MoSe2 Replacing the sulfide with selenide, MoSe2 can be prepared with indirect band gap ranging from 1.7–1.9 eV, approximately within the band gap owned by MoS2 [37, 38]. Categorized under the family of TMDCs, it also structured in sandwiched forms where the layers of Se-Mo-Se are held together with vdW binding, which easily exfoliated to generate nanosheets film. Previous reports on the fabrication of MoSe2 employed laborious and energy intensive technique such as chemical or physical vapor deposition, and spray pyrolysis. To alleviate the problem, two different studies were carried out with simpler fabrication method, namely, electrodeposition [12] and hydrothermal [11], without neglecting the PEC performance. The employment of electrodeposition method allowed control over the film thickness, homogeneity, and purity which workable under low temperature [39–41]. While, the hydrothermal method offers green and low-cost process which can be implemented at low temperature [42–44]. Although MoSe2 has been well investigated for physicochemical and electronic properties, but the study on photo-responsive properties is lacking and need further research. For this purpose, flower-like nanostructured MoSe2 was fabricated and utilized for PEC reaction [Figure 14.1b(i, ii)] [11]. The study revealed that the nanostructured film is photo-responsive material, which translated to a large photocurrent. In another effort, MoSe2 nanoparticle was loaded on hybrid reduced graphene oxide (rGO) and polyimide (PI) forming nanocomposite of MoSe2 NP-rGO/PI [12]. The nanocomposite exhibits excellent stability in acidic solution under dark and light conditions, where the generated cathodic current appears at more positive potential in comparison to other reported MoSe2 [31, 45] and MoS2 [46, 47]. Based on the promising PEC characterization laid out above, the development of nanostructured MoSe2 has a great value for PEC-HER.

14.2.3 WS2 WS2 is another 2D TMDCs that has been actively pursued due to abundant source of material, thus inexpensive [48], excellent chemical stability, and highly photoactive semiconductor for HER catalytic activity [49, 50]. Like any other TMDCs, the molecular structure is appeared as “S-W-S” where the adjacent trilayers are interconnected with vdW forces [50]. In addition, WS2 nanosheet is endowed with extended range of solar absorption

342  Layered 2D Advanced Materials and Their Allied Applications spectrum from UV to infrared region (i.e., 910 nm, which correspond to 1.35 eV) [51, 52]. However, the usability of WS2 as HER photoelectrode are still limited due to the deficiency and reactivity of active edge sites, and poor ohmic contact onto conductive substrates [53–55]. In addition, the HER performance of WS2 that is still incomparable with MoS2 can performed. Employing a facile and short microwave-assisted intercalation reaction, the solution containing exfoliated sheets can be easily drop casted onto conductive substrates [56]. Such a simple preparation technique still can deliver the highest HER performance ever reported for WS2-based photocatalyst. Likewise graphene, the resulted WS2 nanosheets layer have the tendency to stack, forming a bulk microplates or closed dimensional nanostructure resembling quasi-0D or nanotube (NTs), consequently diminish the hanging bonds and reducing the total energy of the system [57–59]. In effort to alleviate the issue, the study utilized a robust ball milling approach followed by annealing to prepare a nanostructured WS2 [Figure 14.1c(i, ii)] [60]. It is worth mentioning that an ultrathin WS2 nanosheets layer ( 1.0 × 105 cm) [67], which also potential as a nonnoble metals photocatalyst owing to its ideal activity [68]. Nevertheless, low catalytic activity of pristine CoSe2 is extremely lower than the reported theoretically estimation. In addressing this issue, many studies employing nanostructural modulation technique as a powerful approach to enhance the photoelectrocatalytic activity of CoSe2. For example, Basu et al. have fabricated marcasite CoSe2 nanorods via a facile and low temperature technique of hydrothermal followed by spin coating step of CoSe2 on silicone

Transition-Metal Dichalcogenides  343 microwire (MWs) [64]. The study suggested that the heterojunction between CoSe2 and Si MWs provides rapid photoexcited electron transport from Si to CoSe2/electrolyte interface and eventually to bulk electrolyte. In addition, there exists a linear relationship between the amount of deposited CoSe2 nanorod on Si MWS and HER/OER activity with a high catalytic efficiency [69, 70]. However, another problem arises where a high charge accumulation which normally happened during PEC water-splitting reaction, consequently limits its durability [66]. The problem was encountered by grafting the CoSe2 on the C3N4 nanosheets which function to reduce the charge accumulation by increasing the firmness of C3N4, as demonstrated by Basu et al. [Figure 14.1d(i, ii)] [66]. In different approach, Kong et al. reported that CoSe2 nanoparticle grafted on carbon fiber has resulted with an excellent current generation of 100 mA cm−2, which ascribed to the highly conductive and porous, thus large surface area of carbon fibers [68].

14.2.5 FeS2 FeS2 (pyrite) has received great interest in photovoltaic PV [71, 72] and PEC [73–76], and photodetector application [77] due to low toxicity, earth-abundant elements, significantly small band gap energy (0.95 eV) [76] and great optical absorption coefficient of ~105 cm−1 in visible range of the solar spectrum [76, 78, 79]. Meanwhile, with adequate minority carrier diffusion length ranging from 100 to 1,000 nm, high carrier mobility (360 cm2V−1S−1) and power conversion efficiency about 28% was reported from the theoretical calculations [80–82]. However, bare FeS2 photocatalyst shows low PEC activity since it only yields small open circuit potential (OCP < 0.2V) [80]. Therefore, FeS2 should be designed in heterojunction form of high OCP material such as TiO2, purposely to prolong the light absorption capacity range (UV to infrared) [80]. For instance, TiO2 was layered on top of FeS2 to form FeS2/TiO2 composites in methanol solution [Figure 14.1e(i, ii)] [73]. Result indicated that the HER activity of FeS2/ TiO2 composites was dependent on the amount of FeS2. In addition, FeS2/ TiO2 composites have generated H2 with 20 and 4 times higher than that of pristine TiO2 and bare FeS2, respectively [83]. Using solvothermal method, Guo et al. have synthesized FeS2 nanoparticles for PEC hydrogen evolution reaction [Figure 14.1e(iii)] [82]. The employment of solvothermal method does not need inert environment, specialized precursor, and post-annealing step for the crystallization of FeS2 NPs [84]. The nanoscale designs of FeS2 provide large specific surface area that improve light scattering and exposed significant active sites for photocatalytic reaction. Both effects were translated to a significantly improved photocurrent generation [82].

344  Layered 2D Advanced Materials and Their Allied Applications

14.2.6 NiSe2 Experimental and theoretical reports have shown that the most of TMDCs are suffers from low HER catalytic activity and poor stability under PEC condition as compared to platinum-based composite [85–87]. However, among TMDCs, NiSe2 exhibits stable catalyst in both acidic and alkaline condition [65, 88, 89] and highly conductive [88] which beneficial for an efficient HER activity. Meanwhile, Ni is abundance on earth, thus inexpensive, which is likely suitable to be applied as non-noble-­metal-based photocatalyst. Literature ascertained that NiSe2 has been well investigated for electrochemical HER application but the feasible study for PEC-HER is still lacking. In addition, although transparent conductive substrates such as FTO and ITO are frequently utilized as support for photoelectrode, but previous reports for the preparation of NiSe2 photocatalyst onto glass substrate have employed a complex and strenuous synthesis process which restricts the scaling up effort [90]. In effort to address the problem, Shen et al. used a simple low-temperature approach of hydrothermal to synthesize NiSe2 nanosheets (NC) which grafted on carbon cloth (CC) and carried out PEC characterization and measurement [Figure 14.1f(i)] [91]. The study revealed that the utilization of CC facilitates the photoinduced-charge separation from the surface of NiSe2 to CC substrate, consequently enhancing the electron transport efficiency and PEC-HER activity. Notably, the results also showed an excellent stability of the photocatalyst even after 50 cycles of PEC-HER reaction [91]. It is well known that there are three polymorphs of NiSe2 as bifunctional photocatalysts, namely, cubic, hexagonal, and orthorhombic phase of NiSe2. However, no comparison study was carried out to evaluate the PEC activity of these NiSe2 type. Not until, Lee at al. have fabricated NiSe2 nanocrystal (NCs) on top of Si NW arrays to study the effect of different polymorphs on PEC-HER and OER activity [Figure 14.1f (ii, iii)] [92]. Based on the structural analysis and PEC characterization, the orthorhombic NiSe2 stands out as an excellent catalyst to conduct HER and OER activities due to positive and negative shifting of their flat-band potentials, respectively. Both consequences have resulted with anodic and cathodic shifting of the onset potential which corresponds to HER and OER, respectively. Moreover, the appearance of metallic orthorhombic NiSe2 as evident through XPS analysis has contributed to a large downward band bending that facilitates the transportation of photoinduced electrons to the semiconductor’s surface and channels them to conduct HER and OER, respectively.

Transition-Metal Dichalcogenides  345

14.3 TMDCs Fabrication Methods 14.3.1 Hydrothermal The hydrothermal is most reported technique and effective ways to produce the nanomaterial with a variation of morphologies and thickness. The reactants are put into an autoclave and fill up with water or organic compound to perform the reaction under high pressure and temperature conditions [94]. Hydrothermal methods are an efficient process to produce inorganic nanomaterials with good crystallinity, morphological control, and high yields [95]. It also has important economic and environmental advantages, and most common “bottom-up” route [96]. Senthilkumar et al. have demonstrated one step hydrothermal synthesis of MoS2 on FTO substrate. The hydrothermal synthesis was performed without the use of any binder, surfactant, or surface modification. The reactant C2H5NS acted as the S-source and a reducing agent in the hydrothermal process, which reduces Mo6+ to Mo4+ (MoS2). Moreover, the as-prepared FTO-supported MoS2 thin films exhibited marigold flower-like nanostructures, in the size range of 100–300 nm [Figure 14.2(i)]. They indicated high carrier concentration, high charge mobility, high electrocatalytic activity, and low charge transfer resistance [95]. A monolayer MoS2 quantum dots using facile method of one-step hydrothermal have been studied by Ren et al., where the MoS2 QDs are shown in TEM image [Figure 14.2(ii)]. The MoS2 QDs has shown outstanding HER activity with low overpotential and considerable high current densities, due to the large number of exposed active (i) Number of particles

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20 nm

Figure 14.2  (i) SEM images of flower-like MoS2 thin films synthesized for 15 hours at 200°C (Reprinted with permission from [95]. Copyright 2018 Journal of Solid State Elecrochemistry). (ii) TEM image of the MoS2 QDs with the size distribution (Reprinted with permission from [96]. Copyright 2015 Journal of Materials Chemistry A).

346  Layered 2D Advanced Materials and Their Allied Applications edges and the excellent intrinsic conductivity of monolayer MoS2 [96]. In other works, pyrite FeS2 nanorods have been synthesized by Morales et al. by simple hydrothermal method due to the simplicity, low cost, and involve moderate synthesis conditions [97]. The bunch of pyrite FeS2 nanorods determined excellent photocataytic activity against methylene blue under UV light. Meanwhile, Jia et al. have been successfully synthesized MoSe2 flower-like nanostructures by using hydrothermal method. The morphology and crystal structure of the as-prepared products have been determined by SEM [Figure 14.1b(i)]. The as-prepared flower-like MoSe2 nanostructures have excellent conductivity and their characteristic photoresponse to red light was then studied by fabricating a solid devices. The device containing the as-prepared nanostructures exhibited a large photocurrent, sensitivity to red illumination, and stable and revisable photo­responsive properties [11].

14.3.2 Chemical Vapor Deposition/Vapor Phase Growth Process TMDCs vapor production by CVD is commonly related to the thermolysis and volatilization of precursors that consist of the required elements. The reactive metal and chalcogen ions produced in the gas phase can then react to form the TMDC monomer in the gas phase. A standard example in the growth of TMDCs is the reaction of a sulfur vapor (produced directly from elemental sulfur) with metal oxides. Particularly, Group VI oxides (for instance, MoO3 and WO3) are usually preferred as low-cost, simply controlled precursors with comparatively low melting, and evaporation temperatures [98]. Stepwise sulfurization of MoO3 to MoS2 via CVD has been synthesized by Pondick et al. (2018) [99]. By controlling the local and global S:Mo molar ratio present in the gas-phase at the growth substrate surface, they have verified that MoO3 can be sulfurized to form MoO2, then MoOS2, and finally MoS2 as a function of enhancing sulfur vapor concentration. Figure 14.3(i) shows the triangular crystals of MoS2 which is the complete stepwise sulfurization of MoO3 through the reaction [99]. Özden et al. have thoroughly studied the synthesis of single layer MoS2 via CVD by changing the zone configuration and controlling the precursors’ ratio. They have compared the horizontal growth and face-down growth with identical growth conditions. Face-down approach is discovered to be more promising to obtain larger flakes presumably due to the higher Mo confinement in the growth zone. For the horizontal growth, either these formations are too small (1 µm in size (Reprinted with permission from [101]. Copyright 2015 Chemistry of Materials).

much larger flakes (~90 μm) and larger continuous films (850 μm × 1 cm) [100]. Samad et al. have been reported the direct CVD method of phase-pure pyrite (FeS2) using FeCl3 and di-tert butyl disulfide (TBDS) precursors under suitable reaction conditions and atmospheric pressure [101]. The films were verified to be pure iron pyrite by using both XRD and Raman spectroscopy. The phase-pure FeS2 thin films with crystal size is less than 1µm [Figure 14.3(ii)] in a conductive substrate that represents conceivably the best platform for future pyrite-based solar cells [101].

14.3.3 Metal-Organic Chemical Vapor Deposition (MOCVD) The use of MOCVD technique has received much consideration because of its excellent potential application to produce high quality films [102]. This MOCVD method allows consistently setting the concentration of precursor gases within the gaseous mixture that is carried to the substrate by controlling the evaporation rates of the solid precursor and mass flow rates [103]. Moreover, this technique using metal-organic sources (i.e., Mo(CO)6, W(CO)6, (CH3)2Se, (C2H5)2S) is commonly necessary from the standpoint of economical and a higher amount for production with accurate control of the chemical potential [104]. An extensive study of 2D MoS2 layers by metal-organic chemical vapor deposition is performed by Kim et al. and the result reported that low growth temperature (400°C) can produce highly uniform and accurate layer-controlled of 2D MoS2 film over entire 8-inch SiO2/Si substrates [104]. Cwik et al. have demonstrated that the direct method of MOCVD to the MoS2 and WS2 materials makes this method highly appealing compared to other known CVD processes. In particular, great surface coverage could be attained in the broad

348  Layered 2D Advanced Materials and Their Allied Applications (ii)

(i)

5 µm

(iii)

Increasing H2 flow

5 µm

500 nm

Figure 14.4  Grain size variation of monolayer MoS2 depending on the hydrogen flow rate; (i) 5 sccm (SEM image shown), (ii) 20 sccm (SEM), and (iii) 200 sccm (TEM) (Reprinted with permission from [105]. Copyright 2015 Nature).

temperature range (600°C –800°C). The use of toxic H2S can be avoided by replacing with the elemental sulfur in the high reactivity of the Mo and W precursors. Moreover, the suitability of the MOCVD-grown MoS2 films as electrocatalysts for HER was studied. Kang et al. reported the preparation of high coverage area of MoS2 and WS2, which directly grown on insulating SiO2 substrates using MOCVD method [105]. The results show outstanding electrical properties for both samples, where the reported MoS2 electron mobility are 30 cm2V−1s−1 and 114 cm2V−1s−1 at room temperature and 90 K, respectively. This process only utilizes gas-phase precursors such as Mo(CO)6, W(CO)6, (C2H5)2S, and H2, all diluted in argon as a carrier gas. Figure 14.4 shows the two major effects of H2, which are essential for removing carbonaceous species produced during the MOCVD growth and the average grain size rises from hundreds of nanometers to more than 10 mm with reducing H2 flow. The MoS2 grains grown under higher H2 flow [Figure 14.4(iii)] have mostly perfect triangular shapes without merging with neighboring grains. In contrast, a trend has been disappeared with lower H2 flow [Figure 14.4 (i), (ii)].

14.3.4 Atomic Layer Deposition (ALD) ALD is one of thin film growth methods in vacuum, in which a thin film can be deposited by a self-limiting growth mechanism via chemisorption of precursor molecules. Owing to the characteristic conformal growth behavior, ALD can be utilized to prepare catalysts on a highly porous substrate [106]. ALD can provide excellent conformality, digital controllability of the film thickness, and perfect large-area uniformity due to its inherent surface-saturated and self-limited reaction mechanisms [107]. ALD is a monolayer stepwise growth process where the reactants are alternately injected into the growth area, and following each reaction, excess species and by-products are purged out. As a result, high quality films are grown

Transition-Metal Dichalcogenides  349 over large areas by sequential surface reactions. This enables precise control of the film composition and thickness as the growth proceeds layer by layer; hence, this approach is ideal for growth of layered materials [108]. In addition, the amorphous MoS2 thin film is grown directly by ALD method as reported by Shin et al. and they revealed a novel chemical route for ALD of MoS2 using Mo(CO)6 and dimethyldisulfide (CH3S2CH3, DMDS) through which the amorphous MoS2 film could be grown at 100°C. The great turnover frequency about 3 H2/s at 0.215 V versus the reversible hydrogen electrode (RHE) has been reported where each of the amorphous MoS2 active site film successfully catalyzes for HER. Hydrogen evolution catalysis on amorphous MoS2 thin film with different thicknesses (2.0 nm and 9.4 nm) show much higher current densities at lower overpotential compared to a bare Au specimen as a control sample [106]. The amorphous thin films of MoS2 at 300°C were successfully grown by Mattinen et al. via ALD method using tris(2,2,6,6-tetramethylheptane3,5-dionato)molybdenum(III), Mo(thd)3 and H2S as Mo and sulfur precursors, respectively. Meanwhile, the growth of MoS2 films using ALD method demonstrates typical characteristics, which is self-limiting growth with linear thickness control and great film uniformity. The films were found to be rather rough, consisting of flake-like grains with size of ≈10–30 nm [109]. In addition, MoSe2 structures can be synthesized by the ALD method using ((CH3)3Si)2Se andMo(CO)6 or MoCl5 precursors [110]. Figure 14.5 (i, ii) showed the morphology of as-deposited Mo-Se films which was observed by SEM. The SEM image of MoSe2 thin film deposited from Mo(CO)6 revealed that the surface of the fused silica substrate is uniformly covered. In contrast, MoSe2 grown from MoCl5 at 300°

Mo(CO)6

(i)

100 nm

MoCl5

(ii)

100 nm

Figure 14.5  SEM images of as-deposited MoSe2 thin films or flakes, prepared by ALD using (i) Mo(CO)6 and (ii) MoCl5, respectively, as Mo precursors and ((CH3)3Si)2Se as the Se precursor (Reprinted with permission from [110]. Copyright 2018 physica status solidi (RRL)).

350  Layered 2D Advanced Materials and Their Allied Applications is formed from well grown standing flakes with an average size of 100 nm attached to a thin underneath base layer. Upon on characterization, they found that high quality crystalline MoSe2 flakes can be obtained from the MoCl5 precursor, in contrast to MoSe2 films with MoOx content obtained using Mo(CO)6 precursor.

14.4 Current Photocatalytic Activity Performance The photocatalytic activity performances together with its fabrication methods are summarized in Table 14.1. Generally, it is known that the higher current density leads to the higher production of PEC hydrogen Table 14.1  Performance and fabrication methods of current TMDCs photocatalyst. Materials

Fabrication method

Light source

Photocurrent

References

MoS2

Lithium exfoliation

150 W Halogen lamp light source

100 µAcm−2 at 0.8 V

[111]

MoS2/Al2O3/ n+p-Si

Magnetron Sputtering Deposition

100 mA cm−2 Xe lamp

35.6 mAcm−2 at 0.4 V RHE

[112]

MoS2/Au

Lithium exfoliation

150 W Halogen lamp light source

370 µAcm−2 at 0.8 V

[111]

g-C3N4/Ngraphene/ MoS2

Hydrothermal

–

27.76 µAcm−2

[113]

WS2

CVD thermolysis

–

23 mAcm−2 at −300 mV vs. RHE

[114]

WS2@P, N, O-graphene

Vacuum-filter

532 nm solid laser

0.131 mAcm−2

[115]

Graphene/WS2

Electrodeposition

–

10 mAcm−2 at 306 mV

[116] (Continued)

Transition-Metal Dichalcogenides  351 Table 14.1  Performance and fabrication methods of current TMDCs photocatalyst. (Continued) Materials

Fabrication method

Light source

Photocurrent

References

NiSe2/carbon cloth

Hydrothermal synthesis

350 W Xenon arc lamp

1 mAcm at 20 mV s−1

[117]

NiSe2/Si

Novel photoinduced cationexchange reaction

Xe lamp illumination

5.8 mAcm−2 at 1.23 V

[65]

FeS2

Solvothermal method

300 W Xe lamp

3.30 mAcm−2 at −0.80 V

[82]

FeS2 mixed with pyritemarcasite phase

Sulphurization

Xenon lamp

4.30 mAcm−2 at 0.5 V vs. Hg/Hg2SO4

[81]

FeS2/TiO2

Sol-gel method

300 W Xe lamp

0.917 mAcm−2 at 1.23 V vs. RHE

[118]

N-Graphene/ CoSe2

Hydrothermal reduction

–

10 mAcm−2 at 0.366 V

[119]

RGO@ CoSe2-SnSe2

Facile aqueous reaction, spray drying and sele nization

Laser light scattering

34.7 mAcm−2 at ~-250 mV (vs RHE)

[120]

−2

generation. There are current trend shows the incorporation of the TMDCs with graphene in the designing of novel materials for photocatalysis. The combination of TMDCs and graphene was reported to enhance the properties of the TMDCs such as improved charge separation and transport properties, as well as high stability, which can enhance the overall photocatalytic PEC hydrogen generation.

14.5 Summary and Perspective General feature that enabled TMDCs monolayer worked as photocatalyst is the surface area of 2D planar which could provide more active sites for

352  Layered 2D Advanced Materials and Their Allied Applications photoelectrocatalytic reaction. 2D planar configuration has proven effective for the separation of photogenerated charges and photon conversion efficiency, which attributed to the exposure of certain facet with distinct atomic arrangement. TMDCs monolayers show a remarkable performance for HER in PEC water splitting and electrocatalysis. However, there are still remain challenges in producing the TMDCs monolayers due to its low yield production. Meanwhile, the microstructures of TMDCs also give significant role towards properties and functions of photocatalyst. Thus, a cautious design of photocatalyst is needed by considering its conductivity and morphology and understanding the photo-induced current mechanism in order to obtain high current density for further improvement of the PEC hydrogen generation, which will pave a way to the era of photocatalysis with atomically thin TMDCs.

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358  Layered 2D Advanced Materials and Their Allied Applications 80. Xin, Y., Li, Z., Wu, W., Fu, B., Zhang, Z., Pyrite FeS2 Sensitized TiO2 Nanotube Photoanode for Boosting Near-Infrared Light Photoelectrochemical Water Splitting. ACS Sustainable Chem. Eng., 4, 6659–6667, 2016. 81. Wu, L., Dzade, N.Y., Gao, L., Scanlon, D.O., Öztürk, Z., Hollingsworth, N., Weckhuysen, B.M., Hensen, E.J., De Leeuw, N.H., Hofmann, J.P., Enhanced Photoresponse of FeS2 Films: The Role of Marcasite–Pyrite Phase Junctions. Adv. Mater., 28, 9602–9607, 2016. 82. Guo, C., Tong, X., Guo, X.-y., Solvothermal synthesis of FeS2 nanoparticles for photoelectrochemical hydrogen generation in neutral water. Mater. Lett., 161, 220–223, 2015. 83. Barawi, M., Ferrer, I.J., Flores, E., Yoda, S., Ares, J.R., Sanchez, C., Hydrogen photoassisted generation by visible light and an earth abundant photocatalyst: Pyrite (FeS2). J. Phys. Chem. C, 120, 9547–9552, 2016. 84. Liu, W., Rui, X., Tan, H., Xu, C., Yan, Q., Hng, H., Solvothermal synthesis of pyrite FeS2 nanocubes and their superior high rate lithium storage properties. RSC Adv., 4, 48770–48776, 2014. 85. Tang, C., Pu, Z., Liu, Q., Asiri, A.M., Sun, X., NiS2 nanosheets array grown on carbon cloth as an efficient 3D hydrogen evolution cathode. Electrochim. Acta, 153, 508–514, 2015. 86. Tang, C., Xie, L., Sun, X., Asiri, A.M., He, Y., Highly efficient electrochemical hydrogen evolution based on nickel diselenide nanowall film. Nanotechnology, 27, 20LT02, 2016. 87. Zhang, Z., Liu, Y., Ren, L., Zhang, H., Huang, Z., Qi, X., Wei, X., Zhong, J., Three-dimensional-networked Ni-Co-Se nanosheet/nanowire arrays on carbon cloth: A flexible electrode for efficient hydrogen evolution. Electrochim. Acta, 200, 142–151, 2016. 88. Zhou, H., Wang, Y., He, R., Yu, F., Sun, J., Wang, F., Lan, Y., Ren, Z., Chen, S., One-step synthesis of self-supported porous NiSe2/Ni hybrid foam: An efficient 3D electrode for hydrogen evolution reaction. Nano Energy, 20, 29–36, 2016. 89. Liu, T., Asiri, A.M., Sun, X., Electrodeposited Co-doped NiSe2 nanoparticles film: A good electrocatalyst for efficient water splitting. Nanoscale, 8, 3911–3915, 2016. 90. Wang, D., Li, X., Chen, J., Tao, X., Enhanced photoelectrocatalytic activity of reduced graphene oxide/TiO2 composite films for dye degradation. Chem. Eng. J., 198, 547–554, 2012. 91. Shen, Y., Ren, X., Qi, X., Zhou, J., Xu, G., Huang, Z., Zhong, J., Hydrothermal synthesis of NiSe2 nanosheets on carbon cloths for photoelectrochemical hydrogen generation. J. Mater. Sci.: Mater. Electron., 28, 768–772, 2017. 92. Lee, S., Cha, S., Myung, Y., Park, K., Kwak, I.H., Kwon, I.S., Seo, J., Lim, S.A., Cha, E.H., Park, J., Orthorhombic NiSe2 Nanocrystals on Si Nanowires for Efficient Photoelectrochemical Water Splitting. ACS Appl. Mater. Interfaces, 10, 33198–33204, 2018.

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15 State-of-the-Art and Perspective of Layered Materials Tariq Munir1, Muhammad Kashif1*, Aamir Shahzad1, Nadeem Nasir2, Muhammad Imran1, Nabeel Anjum1 and Arslan Mahmood1 Department of Physics, Government College University Faisalabad (GCUF), Faisalabad, Pakistan 2 Department of Applied Sciences, Faculty of Science, National Textile University, Faisalabad, Pakistan 1

Abstract

In this chapter, we review the state-of-the-art applications of layered materials and the associated device applications. Layered materials specifically two-dimensional (2D) materials have got considerable attention due to their wide applications in electronic, optoelectronic, and energy storage devices. These materials preferred due to these versatile properties such as tunable band gap, versatility of heterostructure construction, light interaction, and mechanically flexibility for next generation devices. Graphene-based 2D layered materials also play the central role to enhance the efficiency of electronic and optoelectronic devices. These advanced 2D layered materials bring more opportunities for the wide band gap coverage, higher mechanical flexibility, and excellent photoresponse. Finally, we conclude with the recent development in this area and offer our perspectives on future developments. Keywords:  Layered materials, electronic devices, optoelectronic devices, energy storage devices

15.1 Introduction Layered materials particularly two-dimensional (2D), such as graphene, boron-nitride, transition metal dichalcogenides, and phosphorene, have *Corresponding author: [email protected] Inamuddin, Rajender Boddula, Mohd Imran Ahamed and Abdullah M. Asiri (eds.) Layered 2D Advanced Materials and Their Allied Applications, (363–378) © 2020 Scrivener Publishing LLC

363

364  Layered 2D Advanced Materials and Their Allied Applications attracted huge interests due to their outstanding properties which cannot be observe in bulk materials. Nowadays, it is active area of research to search new 2D layer materials which show the desired direct band gap and high carrier mobility. Two-dimensional graphene extensively studies due to excellent properties such as ultrahigh conductivity ( 5,300 W/m·K) [1, 2]. Twodimensional layered transition metal dichalcogenides (TMDs) such as WS2, MoSe2, WSe2, and MoS2 [3–6] have aroused extensive research interest due to potential applications in the field of nanoelectronic devices, spintronics, catalysis, and biosensors [7–9]. These materials show exceptional physical and chemical properties, such as tunable band gaps, high absorption of light, and high reduction sites. These 2D materials construct with two-layer arrangement of two layers of chalcogen (S, Se, Te) atoms in which one layer of metal (Mo, W) atoms sandwiched between these two layers, and van der Waals interactions exist in the neighboring layers. Due to this weak van der Waals interaction, it is possible to create single- or few-layer nanosheets through exfoliation. Boron-nitride (BN) sheet shows a wide band gap of 4.71 eV and have many fascinating properties such as outstanding mechanical properties, chemical stabilities, and thermal conductivities. Recently, h-BN has been used to enhance the performance of heterostructures 2D layered materials [10–12]. Monolayer phosphorene [13] shows unique electronic properties with theoretically predicted direct band gap of 1.51 eV and hole mobility in the range of 10,000–26,000 cm2 V−1 s−1 [14]. Experimentally, it is observed that when black phosphorus field-effect transistors were fabricated, it shows hole mobility up to 5,200 cm2 V−1 s−1 and drain current modulation up to 105. Two-dimensional phosphorene derivatives such as GeP3, InP3 exfoliated atomic layers from their bulk material shows high carrier mobility. GeP3 with electron and hole mobility of 8,840 and 8,480 cm2 V−1 s−1, and InP3 with electron mobility of 1,919 cm2 V−1 s−1 [15−17]. In this chapter, we review 2D layered materials for electronic, optoelectronic, and energy storage devices. Finally, we conclude with the recent development in this area and offer our perspectives on future developments.

15.2 State-of-the-Art and Future Perspective Layered materials such as metal dichaloginides MoSe2,WS2, MoS2, WSe2, TiS2, HfS2, SnS2, and ZrS2 are used in many electronic, optical, and energy storage devices due to their excellent properties. These layered materials

State-of-the-Art of Layered Materials  365 are also used in many applications such as FETs, biosensor, photodetectors, phototransistors, and energy storage devices.

15.2.1 Electronic Devices Layered materials due to their excellent electronic properties such as tunable band gap, high electron/hole mobility, and electrostatic integrity are used in developing new generation transistors, digital circuits, optoelectronic systems, and chemical sensors. In this section, we present latest development on layered materials and their heterostructures as they relate to their role as electronic devices. Monolayer WS2 is used for developing of Wafer-scale field-effect transistor arrays [18]. It is observed that mobility of WS2-based field-effect transistor reaching from 0.2 to 4.0 cm2 V−1s−1, which is higher than other TMDC devices reported so far. Schematic diagram of monolayer WS2based Back-gate FETs is shown in Figure 15.1a and the optical image of this device is shown in Figure 15.1b. N-type behavior of the FET device is observed with transfer curve (Figure 15.1c) and the carrier mobility is found 8.1 cm2 V−1s−1 (Figure 15.1d). WS2-based field-effect transistor unit with dimension of 300 μm × 300 μm were fabricated in Wafer-scale as shown in Figure 15.1e. From the transfer curve and output characteristics, it was observed that mobility of Wafer-scale WS2 FET is in the range of 0.2 to 4.0 cm2 V−1s−1 (Figures 15.1f, g). MoS2/BP/MoS2 heterostructure is used for developing of tunnel fieldeffect transistor (TFET) device which shows higher switching performance

Ids (µA)

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Figure 15.1  Schematic diagram, optical image, and electrical measurement of WS2-based field-effect transistor (Adapted from reference [18] with the permission).

366  Layered 2D Advanced Materials and Their Allied Applications and lower off-state current [19]. Due to 2D structure, this device shows bidirectional current flow as compare to conventional TFET. These devices have the potential in the low power and fast speed applications. The schematic diagram and optical microscope images of MoS2/BP/ MoS2 heterostructure are shown in Figures 15.2a and b, respectively. Transfer characteristic and output characteristics of device are shown in Figures 15.2c and d, respectively. Two-dimensional MoS2-based FET biosensors fabricated for DNA detection in noninvasive prenatal testing (NIPT) for trisomy 21 syndrome [20]. With the high response up to 240% of biosensor Down syndrome screening was conducted with the detection of DNA fragments in limit below 100 aM. 2D layered-based FET sensors can be used in the disease diagnostics and direct gene profiling due to low cost and high sensitivity. Figure 15.2a shows the schematic diagram of the device in which gold nanoparticles (Au NPs) were deposited on the MoS2channel, and for the detection of specific detection of target DNA, immobilized probe DNA was used on the Au NPs. Figures 15.3b and c represent the CVD-based

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Id (µA)

Id (µA)

Id (A)

Vtg(V)

–50

10–8 10–9

160

100

10–7

–100 SS=58.4 mV dec

10–10 –0.8

–0.4

–1

Gate

(MoS2 FET)

(c)

10–6

(TFET)

(MoS2 FET)

TG SiO2 + P Si

10–5

Drain

Source

PEO/LiCIO4

S

Source

0.4 0.6 0.8 1 1.2

80 40

–0.4

0.0

120

Vtg (V)

–0.2

0.4

Vtg (V)

–0.0

Vd (V)

0.8

–0.2

–0.4

1.2

0 0.0

0.1

0.2

0.3

0.4

0.5

Vd (V)

Figure 15.2  (a) Schematic diagram, (b) optical microscopy image, (c) transfer curves, (d) output characteristics curves (Adapted from reference [19] with the permission).

0.6

State-of-the-Art of Layered Materials  367 Probe DNA

(a)

Ti/Au

Target DNA Au NP

MoS2

MoS2 SiO2/Si

(b)

Ti/Au deposition

Probe immobilization

SiO2 O2 plasma etching Au NP PMMA functionalization passivation

(c) MoS2 Au NPs 10µm

Figure 15.3  (a) Schematic diagram of MoS2FET-based biosensor, (b) construction process diagram, (c) optical image of biosensor (Adapted from reference [20] with the permission).

construction process of the MoS2-based FET biosensors and fabricated FET arrays, respectively. It is reported that gate-tunable properties can be obtained by a MoTe2/ BP heterostructure in which electrostatic gating use to change the junction either p−p or p−n due to gate-tunable band alignment of MoTe2 and BP [21]. In MoTe2/BP heterostructure, it is possible to change the photovoltaic and direction of rectification. Due to strong electric field between BP and MoTe2, it enables the high responsivity (0.2 A/W), EQE (48.1%), and with a fast response time (2 ms) which gives exceptional optical properties. Schematic diagram of the MoTe2/BP heterostructure is shown in Figure  15.4a which was fabricated by in order piling of BP, MoTe2, and boron nitride (BN) on the silicon-based substrate and SEM image of the device is shown in Figure 15.4b. Figure 15.4c shows the band arrangements of MoTe2 and BP. Molybdenum ditelluride (MoTe2)-based FET reported for the detection of O2 during dry-air contact and electrothermal treatment [22].

368  Layered 2D Advanced Materials and Their Allied Applications (a)

BP

MoTe2

Si/SiO2

BN

(b)

(c)

Vaccum

BP S1

D2 MoTe2

D1

–3.8 eV

–4.1 eV

0.9 eV

S2

0.4 eV MoTe2

BP

Figure 15.4  (a) Schematic diagram of MoTe2/BP heterostructure device, (b) optical microscopy image, (c) energy band alignment of MoTe2 and BP (Adapted from reference [21] with the permission).

Schematic diagram of MoTe2 FET is shown in the Figure 15.5a, and Figure 15.5b represents the optical microscopy image which shows the MoTe2 and electrodes on silicon-based substrate. Height profile of the MoTe2 FET is shown in Figure 15.5c. MoTe2 FET is placed in vacuum, Ar, and N2, and resistance behavior is measured. Resistance change in the MoTe2 channels (a)

(b)

Vds

(c) 10 Au/Ti electrode

MoTe2

8 Height (nm)

Au/Ti electrode

MoTe2 SiO2/Si substrate

(d)

10 µm

Vbg (e)

6

~ 7 nm

4 2 0 0

1 2 Distance (µm)

(f)

3

1011 1010

Vacuum exposure

Ar exposure

109

N2 exposure

R (ohm)

1010 109

108

108 Vbg=10 V Vbg=20 V 0

Vbg=30 V Vbg=40 V

10 20 30 40 50 Electrothermal voltage (V)

107

109 Vbg=10 V Vbg=20 V 0

Vbg=10 V Vbg=20 V

Vbg=30 V Vbg=40 V

10 20 30 40 50 Electrothermal voltage (V)

0

Vbg=30 V Vbg=40 V

10 20 30 40 50 Electrothermal voltage (V)

Figure 15.5  (a) Schematic diagram, (b) optical image, (c) height profile, and (d, e, f) shows resistance (R) behavior of the MoTe2 FET under vacuum, Ar, and N2 (Adapted from reference [22] with the permission).

State-of-the-Art of Layered Materials  369 is measured during the O2 detection and it is observed that this resistance change is related to O2 content. During back gate modulation, the O2 index decreases 100% to 12.1% due to Joule heating. MoTe2-based FET has the promising application for sensing O2.

15.2.2 Optoelectronic Devices Recent progress of 2D layered materials use in the optoelectronic devices is review in this section. Layered materials have attracted recent interest as channel materials in optoelectronic devices due to their excellent tunable band gap, electrical properties, and extraordinary light absorption. Optoelectronic devices working depend on the light absorption and ability to transform light into electrical signal on this basis, there are two types of optoelectronic devices such as photodetector and photovoltaic devices. A large number of layered materials due to their 2D structure are used for the light absorption in the all wavelength range such as visible, uv, infrared region. By changing the thickness of the layered makes it possible to tune the band gap of the materials for different optoelectronic applications. For the next-generation optoelectronic devices, flexibility and transparency are nowadays more demanding. Recently, photodetector fabricated using heterostructure of n-type MoS2 and p-type Si for low-light detection [23] as shown in Figure 15.6. With MoS2 layer this photodiode shows outstanding performance with a responsivity, detectivity, and low noise equivalent power (NEP) as compare to conventional photodiode. A self-rolled-up technique is use to develop for changing 2D b ­ uriedgate graphene field-effect transistors (GFETs) into 3D tubular GFETs [24]. Photoresponsivity of this 3D GFETs increased due to increased light Multilayer MoS2 Laser

IDS Ti / Au

Ti / Au SiO2 Si

Figure 15.6  Schematic diagram of device (Adapted from reference [23] with the permission).

370  Layered 2D Advanced Materials and Their Allied Applications interaction with graphene. These devices show high photoresponsivity, wide spectral range, and quick response; therefore, these devices have potential applications in optoelectronic devices and systems. Two-dimensional Tin (II) SnS nanoflakes-based field-effect transistors (FET) are reported for photodetector application [25]. These SnS-based FET shows high photoresponsivity, fast switching time, and excellent stability. Layered GeSe-based FETs [26] as shown in Figure 15.7 is reported which provide wide fluorescence spectrum, three order on/off ratio, and mobility of 4 cm2/(Vs). Very broadband and fast photodetections are possible with these devices as measurement depicts a fall (rise) time of 19 μs (13 μs) when expose to visible to 1,400-nm light. Layered GeSe-based FETs can be use in photoresponse devices which show high switching speed and broadband range. PdSe2-MoS2-based infrared photodetector is reported which show the high sensitivity and air-stability [27]. Photodetector shows significant photoresponsivity of ~42.1 AW−1, suppression of dark current, and noise power density. Schematic diagram of the PdSe2-MoS2 photodetector use in infrared region is shown in Figure 15.8. High photoresponse and stability are obtained with MoSe2/WSe2/4HSiC photodetector [28]. The schematic diagram of MoSe2/WSe2/4H-SiC photodetector and band gap arrangement is shown in Figure 15.9. It is observed that type II MoSe2/WSe2 heterostructure and TMDCs/SiC junctions show increase and separation of the charge carriers due to photoresponse. These devices show EQE and maximum Rλ of up to 37% and 160 mA·W−1 when use as photovoltaic device. As a photodetector maximum Rλ is obtained 7.17 A·W−1, EQE, and detectivity 1.67 × 103 % and 5.51 × 1011 Jones, respectively, with the largest Ilight/Idark ratio order of three, the Vg changes from 0 to 10 V.

Vds A AugGe1 100nm 300nm

Few layer GeSe

AugGe1

SiO2 P-Si

Vg

Au

Figure 15.7  Schematic diagram of GeSe FET (Adapted from reference [26] with the permission).

State-of-the-Art of Layered Materials  371 MoS2

SiO2 Si

PdSe2

MoS2

PdSe2

Figure 15.8  Schematic diagram of the PdSe2-MoS2 (top) and optical microscopy image (bottom) (Adapted from reference [27] with the permission).

Type II

Vds

e Sounr+c

Se

2

e– e–

ain

yp

p +W

Se

2

eS N+ iC (11 Ba type µm ck SiC ) ga (3 te 60 (N µ m i/A ) g)

(a)

Dr

Nt

Mo

e– e– e– e–

E2g E1

g

Vg

h+

h+

h+ h+

e– e– e– e–

e–

h+ h+ h+ h+

h+ h+

n-SiC

e–

E3g

h+ h +

n-MoSe2 p-WSe2

n-SiC

(b)

Figure 15.9  (a) Schematic diagram from the MoSe2/ WSe2 heterostructure. (b) Band alignment (Adapted from reference [28] with the permission).

Graphene Schottky varactor diode which shows high photoresponses is compared to pure graphene device of 160 mA/W in presence of low dark current [29]. The schematic of the device is shown in Figure 15.10. These devices give higher external quantum efficiency (EQE) of 37% and remove the high dark currents as compare to pure graphene-based varactor diode. Boron nitride (h-BN) used as surface passivation layer on the MoS2/ WSe2 solar cell [30] as shown in Figure 15.11 which significantly reduce the electrical loss of 2D width dW and increase the photovoltaic performance. It is observed that power conversion efficiency increases up to 74%. The increase in efficiency is attributed to low recombination rate at the junction and surface of the non-overlapping semiconductor regions as well as

372  Layered 2D Advanced Materials and Their Allied Applications (a)

(b)

AI\Pd Ge SiO2 P++Si

Figure 15.10  (a) Optical image and (b) schematic diagram of graphene Schottky varactor diode with Pd/Al/Ge contacts (Adapted from reference [29] with the permission).

(a)

(b)

h-BN ITO

ITO

ITO

MoS2

h-BN

WSe2 MoS2 Glass substrate

ITO WSe2

(c)

h-BN

10µm

(d)

MoS2

h-BN

Height profile

WSe2

WSe2 Height (nm)

15 Thickness of h-BN 10 11.4 nm 5 0 0.0 0.4 0.8 Position (µm)

MoS2

1.2

10 nm

Glass

Figure 15.11  (a) Schematic diagram of device, (b) optical microscopy image, (c) AFM image (top) and the height profile of h-BN (bottom). (d) HRTEM image (Adapted from reference [30] with the permission).

open-circuit voltage and short circuit current. The h-BN surface passivation increases the stability of solar cell under ambient conditions.

15.2.3 Energy Storage Devices Due to the increase demand of energy storage devices, the advanced electrode materials must develop to increase high energy density and power density. There is increase interest in the 2D layer materials such as graphene, transition metal oxides, car­bides, nitrides, or chalcogenides for the electrode materials due to their layer structures. This layer structure helps to manipulate their interlayer spacing for the desired applications.

State-of-the-Art of Layered Materials  373 Using 2D layer materials, it is possible to increase fast ion transport and decrease the diffusion path. There is substantial potential for the engineering of these 2D layer materials structurally to increase their electrochemical performances. Toward this end, an overview of recent research layered materials for the electrode materials is presented. In lithium–sulfur (Li–S) batteries, a 2D material consist of g-C3N4/ graphene sheet composite can be used as a interlayer for sulfur/carbon (S/KB) cathode [31]. Substantial enhancement in the electrochemical performance was observed due to increased Li+ diffusion and charge transfer. With the help of this interlayer sufficient suppression of diffusion of the dissolved polysulfide species take place from cathode to the anode. Discharge capacity of S/KB cathode with g-C3N4/GS interlayer (1,191.7 mAhg−1) increases 90% as compared to S/KB cathode without interlayer (625.8 mAhg−1) at 0.1 C after 100 cycles. S/KB cathode with g-C3N4/GS interlayer shows a discharge capacity of 612.4 mAhg−1 for 1 C after 1,000 cycles. The ultrathin TiS2 nanosheets as anode material in the sodium ion battery and observed that TiS2 anode shows exceptional cycling performance 220 mAh g−1 at the 2nd cycle and 386 mAh g−1 at the 200 cycles at 0.2 A g−1 [32]. For flexible lithium-sulfur batteries, S2-4/carbon composite with 2D-MXene nanosheets-based electrode was used [33]. 2D-MXene nanosheets provide the path for fast charge transfer due to its excellent conductivity and flexibility to the electrode. This S2-4 electrode based on 2D MXene nanosheets shows discharge capacity of 1,029.7 mAh g−1 at 0.1 C and 946.7 mAh g−1 after 200 cycles. MoS2 nanosheet/Co anode material used for lithium-ion batteries shows initial discharge specific capacity of 1,130 mAh g−1 at 100 mA g−1 and maintains 766 mAh g−1 at 2 A g−1. After 50 cycles, capacity increases to 1,340 mAh g−1 at 100 mA g−1 [34]. Layered WS2 anode along with conventional graphite cathode is used in the WS2-graphite dual-ion battery (DIB) as shown in Figure 15.12 [35]. high capacity

Dual-ion battery

WS2

:W

:S

:Li

:PF6–

:C

high voltage

Figure 15.12  Schematic diagram of the WS2-graphite dual-ion battery (DIB) (Adapted from reference [35] with the permission).

374  Layered 2D Advanced Materials and Their Allied Applications With this WS2-graphite dual-ion battery (DIB), higher charge specific capacities and current rates can be obtain as compare to conventional dualion batteries (DIB). The high capacity and high operating voltage make this WS2 base DIB attractive for future energy storage devices.

15.3 Conclusion In this review, 2D layered materials preferred for advanced applications due to the following versatile properties like electrical, optical, thermal, and biosensing. High response FET biosensors based on MoS2 detect target DNA fragments with a detection limit below 100 aM. Moreover, 2D layered nanostructure TFET-based MoS2/BP/MoS2 shows bidirectional current flow as compared to conventional TFET. n-type MoS2 and p-type Si-based heterostructure used as a photodetector for low-light detection and SnS-based detector shows high responsivity. Transition metal chalcogenides layered materials are excellent candidate for the rechargeable battery materials due to tunable composition, fast ion transport, and structural stability. A new route for the transition metals dichalcogenides (TMDs) and graphite for the dual-ion battery (DIB) enhanced high charge capacity and current rate for the advancement of next-generation energy storage devices.

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Index 2D perovskite materials, 131 2D zeolite, 194-198 Active radicals (°OH), 87 ADOR, 196, 198, 199 Adsorbent, 194, 196, 203, 207, 255 Adsorption energy, 137 Alumino-silicate substance, 194, 195 Angle resolved photoemission spectroscopy, 326 Anion scavenger, 262 Anti-microbial agents, 266 Anti-neurodegenerative applications, 228 Antioxidant, 30 Atomic layer deposition (ALD), 348, 349 Band gap, 85, 306 Battery, 37 Binary and ternary mixed metal sulfides, 171 Binary metal oxide, 175 Binary metal oxide photocatalysts, 175 Biomedical, 39, 263 Biomedicine, 12, 66 Biosensors, 230, 263 BlueP/MoS2, 106 Borophene, 323 Butylammonium (BA+), 134 Cancer therapy, 224 Carbon, 216 Carbon dots (CDs), 83

Carbon nanofibers (CNF), 79 Carbon nanotubes (CNTs), 79 Carbon spheres (CSs), 79 Catalysis, 14, 36, 60 Catalyst, 194, 197, 203, 204, 205, 257 Catalytic micro or nanoreactors, 231 Characteristic properties, 163 Characterization techniques, 291 Chemical separation, 13 Chemical vapor deposition (CVD), 80, 310, 339, 346, 347, 350 Co-precipitation, 253 Commercial TiO2 (P25), 82 Conduction band (CB), 85 Conventional double charge transfer, 162 CoSe2, 340, 342, 343, 351, 356, 357, 361 Coupled metal oxides, 173 Crystal structure, 304 Deep ultraviolet, 307 Detection accuracy, 112 Diffusion method, 4 Dion-Jacobson (DJ) phase, 133 Distinct dimensionality, 166 Drug delivery, 267 Edge potentials, 166 Effective decoration, 165 Electrode, 261 Electron/hole (e−/h+), 81 Electronic and dielectric properties, 306

379

380  Index Electronic and optoelectronics, 34 Electronic manufacturing devices, 318 Electrospray method, 218 Emulsion, 217 Energy storage, 55, 232 Environmental management, 63 Environmental recovery, 163 FeS2, 338, 340, 343, 346, 347, 351, 357, 358 Field emission scanning electron microscopy (FESEM), 292 Figure of merit, 112 Flame retardant, 263 Flower-like structure, 176 Fourier transform infrared radiation (FTIR), 294 Fullerene (C60), 79 g-C3N4/graphene, 373 Galvanic replacement, 220 Gas adsorption, 14 Gas bubble, 218 Gas sensing, 69 Gas storage, 12 Gene delivery, 270 Gene therapy, 226 Germanene and stanene, 152 GeSe-based FETs, 370 Global issues, 160 Graphene, 80, 143, 287 Graphene field-effect transistors, 369 Graphitic carbon nitride (g-C3N4), 81, 161 Graphene oxide (GO), 82 Graphene transfer, 289 Growth mechanism, 313 Hard templating, 215 Heterostructures, 153, 163 Hexagonal boron nitride, 304 Hole transport material (HTM), 136 Hollow nanomaterials in diagnostics and therapeutics, 227

Hydrogen evolution reaction (HER), 337, 338, 343 Hydrothermal method, 6, 193, 197, 207, 255, 341, 342, 344-346, 350 Imaging and diagnosis applications, 221, 269 Immuno and hyperthermia therapy, 226 In-situ route, 167 Infection therapy, 226 Ion-exchange method, 254 Kirkendall effect, 219 Langmuir-Hinshelwood model, 169 Lattice parameters, 168 Light harvesting, 176 Liquid exfoliation, 310 Lithium ion battery, 232 Low energy electron microscopy (LEEM), 296 Mechanical exfoliation, 309 Medical applications of HNMs, 220 Membrane, 194, 197, 200, 203, 207 Metal carbides, 47 Metal organic frameworks, 1 Metal sulphides, 163 Metal sulphides photocatalysts, 163 Metal vanadates, 180 Metal-organic chemical vapor deposition (MOCVD), 347, 348 Methods for synthesis of graphene layered materials, 285 Methyl blue (MB), 81, 83 Methyl orange (MO), 84 Microporous, 193-195, 201 Microstructures, 164 Microwave synthesis, 8 Mo2C, 51 Molecular beam epitaxy, 319 MoS2, 338-342, 345-350, 352 MoS2-based FET biosensors, 366

Index  381 MoS2/BP/MoS2 heterostructure, 365 MoSe2, 338, 340, 341, 346, 349, 350 MoSe2/WSe2 heterostructure, 370 MoTe2 FET, 368 MoTe2/BP heterostructure, 367 MXene, 104 Nanocomposites, 167 Nanotube arrays, 222 Nanosensors, 233 NiSe2, 338, 340, 344, 351 Non-medical applications, 231, 255 Optoelectronics, 321 Organic dyes, 175 Ostwald ripening, 219 Perovskite solar cell, 131 Pharmacy and medicine, 224 Phenylethylammo­nium, 134 Phosphorene, 147 Photo-irradiation, 174 Photocarriers, 168, 178 Photocarriers migration, 178 Photocarriers transportations, 168 Photocurrent, 340, 341, 346, 350, 351 Photodegradation efficiency, 170 Photodetection devices, 133 Photodetector, 369 Photoelectrochemical (PEC), 337-339, 341, 343, 344, 350, 352 Photothermal therapy, 229 Photovoltaic cell, 135 Physical vapor deposition, 315 Pollutant degradation, 164 Polymer, 215 Polymer additive, 261 Polymeric graphitic carbon nitride, 161 Polymeric structures of MoO3, 177 Postsynthesis modification, 196, 200 Power conversion efficiency (PCE), 131 Preparation methods, 26

Protein purification, 269 Protonation, 172 Quantum dot (QDs), 87, 167 Raman spectroscopy, 295 Reconstruction method, 254 Reduced graphene, 82 Reflectance curve, 113 Regenerative medicine, 228 Rhodamine B (RB), 83 Ruddlesden-popper (RP), 133 Scaffolds, 265 Schottky varactor diode, 371 Self-templating, 218 Self-assembly, 9 Sensitivity, 112 Sensors, 13, 260 Silica, 216 Silicene, 148 Sodium pentachlorophenol, 83 Soft-templating, 217 Sol-gel method, 5, 255 Solvent volatility method, 4 Special interface synthesis method, 9 SPR sensor, 107 Stripping method, 6 Structural aspects, 213, 251 Structural properties, 305 Structural unit, 161 Structure-directing agents, 197, 199, 207 Supercapacitor, 38, 232 Surface protected etching, 219 Surface segregation, 316 Surfactant assisted synthesis method, 10 Synthetic approaches, 214 Template-based strategies, 215 Ti2C, 50 Ti3C2, 48 TiS2 nanosheets, 373 TMDCs, 106 Toxicity, 234, 272

382  Index Transition metal dichalcogenides (TMDCs), 151 Transition metal oxides, 179 Transmission electron microscopy (TEM), 293 Two-step solution deposition method, 132 Ultrasonic synthesis, 10 Urea hydrolysis, 254 UV-visible spectroscopy, 295 V2C, 50 Valance band (VB), 85

Van der Waals heterostruture, 106 Vesicle, 217 Wastewater treatment, 234 WS2, 338, 340-342, 347, 348, 350, 373 WS2 FET, 365 X-ray diffraction technique, 291 Yolk-shell structure, 174 Z-scheme, 162 Zinc phthalo-cyanine (ZnTNPc), 85 ZnO, 150