Carbon Nanomaterials and their Nanocomposite-Based Chemiresistive Gas Sensors: Applications, Fabrication and Commercialization 9780128228371

Table of contents :
Cover
Half Title
Carbon Nanomaterials and their Nanocomposite-Based Chemiresistive Gas Sensors: Applications, Fabrication and Commercialization
Copyright
Contents
Introductuin
List of contributors
Preface
Part I: Introduction to sensing materials
1. Carbon-based nanomaterials
1.1 Introduction carbon nanomaterials
1.2 Types of carbon nanomaterials
1.2.1 Diamond
1.2.2 Graphite
1.2.3 Carbon nanotubes
1.2.4 Fullerene
1.2.5 Carbon nanomaterials
1.3 Quantum confinement in carbon nanomaterials
1.4 Properties of carbon nanomaterials
1.4.1 Properties of carbon nanotubes
1.4.2 Properties of graphene
1.4.3 Properties of activated carbons (nanocarbons)
1.4.4 Properties of template carbons
1.4.5 Properties of carbon fibers
1.4.6 Properties of carbon nanomaterials
1.5 Synthesis methodologies and variations
1.5.1 Synthesis of carbon nanotubes
1.5.2 Synthesis of pure graphene
1.5.3 Synthesis of activated carbon
1.5.4 Synthesis of other carbon nanostructures
1.6 Acid simulation of carbon materials and their importance
1.7 Acid functionalization and their importance
1.7.1 End defect functionalization
1.7.2 Side wall and surface functionalization
1.7.3 Wet oxidation of carbon nanomaterial
1.7.4 Wet oxidation of graphene
1.8 Applications
1.8.1 Carbon nanotubes
1.8.2 Graphene
1.9 Conclusion
References
2. Semiconductor oxide nanomaterial
2.1 Introduction of semiconductor oxide nanomaterials
2.2 Synthesis of oxide nanomaterials and variations
2.2.1 Physical vapor deposition
2.2.2 Thermal evaporation or resistive heating technique
2.2.3 Electron beam evaporation
2.2.4 Radio frequency heating
2.2.5 Flash evaporation
2.2.6 Laser ablation technique/pulsed laser deposition
2.2.6.1 Factors responsible for PLD deposition
2.2.6.1.1 Deposition conditions
2.2.6.1.2 Laser beam parameters
2.2.7 Sputtering technique
2.2.7.1 Working of sputtering system
2.2.8 Molecular beam epitaxy
2.2.8.1 Various components of MBE
2.3 Chemical/solution method for the growth of metal oxide nanoparticles
2.3.1 Chemical vapor deposition
2.3.2 Photochemical vapor deposition
2.3.3 Plasma-enhanced CVD
2.3.4 Chemical methods
2.4 Types of oxide materials and their importance
2.5 Need of functionalization of oxide materials
2.6 Future aspects for MOS
References
3. Conducting polymers as gas sensing material
3.1 Introduction of conducting polymers and their role as gas sensing material
3.1.1 Theory of conductivity
3.1.2 Band theory
3.1.2.1 Gas sensing mechanism of conducting polymers
3.1.2.2 Amperometric gas sensors
3.1.2.3 Potentiometric sensors
3.1.2.4 Electrical device sensors
3.2 Synthesis of conductive polymers and their importance
3.2.1 Synthetic preparation methods of conducting polymers
3.2.1.1 Chemical method
3.2.1.2 Electrochemical method
3.2.1.3 Photochemical method
3.2.1.4 Metathesis method
3.2.1.5 Concentrated emulsion method
3.2.1.6 Inclusion method
3.2.1.7 Solid-state method
3.2.1.8 Plasma polymerization
3.2.1.9 Pyrolysis method
3.2.2 Some examples
3.2.2.1 Polyacetylene
3.2.3 Polyaniline
3.2.3.1 Polypyrrole
3.3 Need of functionalization of conducting polymers with carbon materials
3.4 Applications
References
Part II: Application of carbon nanomaterials in gas sensing
4. Carbon nanomaterial-based chemiresistive sensors
4.1 Introduction to sensor and its types
4.2 Importance of chemiresistive gas sensors
4.2.1 Semiconductor metal-oxide gas sensors
4.2.2 Conductive-polymer gas sensors
4.3 Fabrication of carbon nanomaterials-based sensors
4.3.1 Hydrogen gas detection
4.3.2 Volatile organic compound detection
4.3.3 Fossil fuel emissions detection
4.3.4 Military and defense explosives detection
4.3.5 Greenhouse gases
4.3.6 Biological contaminants
4.4 Sensor comparison at lab/industrial level
4.5 Sensing mechanism of chemiresistive sensors
References
5. Semiconductor oxide based chemiresistive gas sensors
5.1 Introduction
5.2 Fabrication and designing of C-SMO gas sensors
5.2.1 Growth techniques of sensing material for C-SMO gas sensors
5.2.1.1 Traditional technology
5.2.1.2 Thick film technology
5.2.1.3 Thin film technology
5.3 Working principle of a C-SMO-based chemiresistors
5.4 Performance parameters for C-SMO gas sensor
5.5 Sensing mechanism in C-SMO gas sensor
5.5.1 Pristine oxides
5.5.2 Metal/metal oxide functionalized metal oxides
5.6 Factors influencing sensing characteristics of C-SMO gas sensor
5.7 Semiconductor oxide sensor outcomes
5.7.1 At lab level
5.7.2 At industrial level
5.8 Challenges and future prospect
References
6. Synthesis and application of carbon-based nanocomposite
6.1 Introduction
6.2 Synthesis of carbon materials/SMO nanocomposites
6.2.1 Ex-situ techniques
6.2.1.1 Covalent interactions
6.2.1.2 π-π stacking
6.2.1.3 Electrostatic interactions
6.2.2 In-situ techniques
6.2.2.1 Electrochemical techniques
6.2.2.2 Chemical reduction and oxidation
6.2.2.3 Electrodeposition
6.2.2.4 Sol-gel process
6.2.3 Hydrothermal and aerosol techniques
6.2.3.1 Vapor-assisted, polyol-assisted process
6.2.3.2 Supercritical solvent
6.2.4 Gas-phase deposition
6.2.4.1 Evaporation and sputtering
6.2.4.2 Pulsed laser deposition
6.2.4.3 Chemical vapor deposition
6.2.4.4 Atomic layer deposition
6.3 Synthesis of graphene/SMO-based nanocomposites
6.3.1 Common synthesis methods of the G-SMO nanocomposites
6.3.2 Hydrothermal method
6.3.3 Self-assembly method
6.3.4 In situ method
6.3.5 Solution mixing method
6.3.6 Spin coating
6.4 Conclusion
6.5 Synthesis of CNTs/conducting polymers-based nanocomposites
6.6 Functionalization of carbon nanotubes with covalent and noncovalent
6.6.1 Synthesis techniques
6.6.1.1 Arc discharge method
6.6.1.2 Laser ablation method
6.6.1.3 Solution mixing
6.6.1.4 Melt mixing
6.6.1.5 In situ polymerization
6.7 Applications of nanocomposites
6.7.1 Graphene/SMOs nanocomposites and CNTs/SMOs nanocomposites
6.7.1.1 Sensors
6.7.1.2 Energy storage and conversion
6.7.1.3 Lithium-ion batteries/sodium-ion batteries/zinc-ion batteries
6.7.1.4 Supercapacitors
6.7.1.5 Solar cells
6.7.1.6 Photodetector
6.7.1.7 Photocatalysts
6.7.1.8 Hydrogen storage
6.7.2 CNTs/CPs nanocomposites
6.7.2.1 Sensors
6.7.2.2 Supercapacitors
6.7.2.3 Lithium-ion batteries
6.7.2.4 Fuel cell
6.7.2.5 Solar cell
6.7.2.6 Electromagnetic interference shielding
References
7. Fabrication of chemiresistive gas sensor with carbon materials/polymers nanocomposites
7.1 Introduction
7.2 Fabrication of CNTs/SMOs-based sensors
7.3 Fabrication of graphene/SMOs-based sensors
7.4 Fabrication of CNTs/conducting polymers-based sensors
7.5 Fabrication of wireless-based networks sensors
7.5.1 Network architecture
7.5.2 Materials used in fabrication of devices for WSN
7.5.3 Fabrication of FBT
7.5.3.1 Bipolar field effect transistor
7.6 Sensing mechanisms
7.6.1 Sensor outcomes at laboratories/industrial level
7.7 Conclusions and future prospects
Acknowledgement
References
8. Potential applications of chemiresistive gas sensors
8.1 Introduction
8.1.1 Sensor response (S)
8.1.2 Sensitivity (S0)
8.1.3 Response time
8.1.4 Recovery time
8.1.5 Selectivity
8.1.6 Limit of detection
8.1.7 Stability
8.1.8 Linearity
8.2 Environmental monitoring
8.2.1 Carbon monoxide (CO) gas sensor
8.2.2 Hydrogen sulfide (H2S) gas sensor
8.2.3 Ammonia (NH3) and nitrogen dioxide (NO2) gas sensors
8.2.4 Chlorine (Cl2) gas sensor
8.3 Medical diagnosis
8.4 Food and agriculture applications
8.5 Detection of explosives and military applications
References
Index

Citation preview

Carbon Nanomaterials and Their Nanocomposite-Based Chemiresistive Gas Sensors Applications, Fabrication, and Commercialization

Carbon Nanomaterials and Their Nanocomposite-Based Chemiresistive Gas Sensors Applications, Fabrication, and Commercialization Edited by

Shivani Dhall Department of Physics, DAV College, Jalandhar, Punjab, India

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2023 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-12-822837-1 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Ana Claudia A. Garcia Editorial Project Manager: Joshua Mearns Production Project Manager: Surya Narayanan Jayachandran Cover Designer: Greg Harris Typeset by MPS Limited, Chennai, India

Contents List of contributors ...................................................................................................xi Preface ................................................................................................................... xiii Introduction..............................................................................................................xv

Part I Introduction to sensing materials CHAPTER 1 Carbon-based nanomaterials........................................ 3 Shivani Dhall, Rashi Nathawat and Kapil Sood 1.1 Introduction carbon nanomaterials ................................................4 1.2 Types of carbon nanomaterials ......................................................5 1.2.1 Diamond .............................................................................. 5 1.2.2 Graphite............................................................................... 7 1.2.3 Carbon nanotubes................................................................ 8 1.2.4 Fullerene............................................................................ 10 1.2.5 Carbon nanomaterials ....................................................... 11 1.3 Quantum confinement in carbon nanomaterials..........................12 1.4 Properties of carbon nanomaterials..............................................13 1.4.1 Properties of carbon nanotubes......................................... 13 1.4.2 Properties of graphene ...................................................... 14 1.4.3 Properties of activated carbons (nanocarbons)................. 14 1.4.4 Properties of template carbons ......................................... 16 1.4.5 Properties of carbon fibers................................................ 16 1.4.6 Properties of carbon nanomaterials .................................. 16 1.5 Synthesis methodologies and variations ......................................18 1.5.1 Synthesis of carbon nanotubes ......................................... 18 1.5.2 Synthesis of pure graphene............................................... 19 1.5.3 Synthesis of activated carbon ........................................... 21 1.5.4 Synthesis of other carbon nanostructures ......................... 22 1.6 Acid simulation of carbon materials and their importance.........23 1.7 Acid functionalization and their importance ...............................25 1.7.1 End defect functionalization ............................................. 26 1.7.2 Side wall and surface functionalization ........................... 27 1.7.3 Wet oxidation of carbon nanomaterial ............................. 27 1.7.4 Wet oxidation of graphene ............................................... 28 1.8 Applications..................................................................................32 1.8.1 Carbon nanotubes.............................................................. 33 1.8.2 Graphene ........................................................................... 33

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1.9 Conclusion ....................................................................................34 References.................................................................................... 34

CHAPTER 2 Semiconductor oxide nanomaterial ........................... 41 S.K. Tripathi and R. Ridhi 2.1 Introduction of semiconductor oxide nanomaterials ...................41 2.2 Synthesis of oxide nanomaterials and variations ........................43 2.2.1 Physical vapor deposition ................................................. 43 2.2.2 Thermal evaporation or resistive heating technique ........ 46 2.2.3 Electron beam evaporation ............................................... 48 2.2.4 Radio frequency heating ................................................... 48 2.2.5 Flash evaporation .............................................................. 49 2.2.6 Laser ablation technique/pulsed laser deposition............. 49 2.2.7 Sputtering technique ......................................................... 52 2.2.8 Molecular beam epitaxy ................................................... 55 2.3 Chemical/solution method for the growth of metal oxide nanoparticles.................................................................................56 2.3.1 Chemical vapor deposition ............................................... 56 2.3.2 Photochemical vapor deposition....................................... 58 2.3.3 Plasma-enhanced CVD ..................................................... 58 2.3.4 Chemical methods............................................................. 58 2.4 Types of oxide materials and their importance ...........................61 2.5 Need of functionalization of oxide materials ..............................65 2.6 Future aspects for MOS ...............................................................67 References.................................................................................... 67

CHAPTER 3 Conducting polymers as gas sensing material.......... 75 3.1

3.2

3.3 3.4

Boyina Rupini Introduction of conducting polymers and their role as gas sensing material............................................................................75 3.1.1 Theory of conductivity...................................................... 77 3.1.2 Band theory ....................................................................... 77 Synthesis of conductive polymers and their importance.............83 3.2.1 Synthetic preparation methods of conducting polymers ...... 84 3.2.2 Some examples ................................................................. 86 3.2.3 Polyaniline......................................................................... 86 Need of functionalization of conducting polymers with carbon materials ...........................................................................87 Applications..................................................................................90 References.................................................................................... 94

Contents

Part II Application of carbon nanomaterials in gas sensing CHAPTER 4 Carbon nanomaterial-based chemiresistive sensors ...................................................................... 107 4.1 4.2

4.3

4.4 4.5 4.6

Sandeep Kumar, Arshdeep Singh and Anil Kumar Astakala Introduction to sensor and its types ...........................................107 Importance of chemiresistive gas sensors .................................109 4.2.1 Semiconductor metal-oxide gas sensors......................... 110 4.2.2 Conductive-polymer gas sensors .................................... 111 Fabrication of carbon nanomaterials-based sensors ..................111 4.3.1 Hydrogen gas detection .................................................. 113 4.3.2 Volatile organic compound detection............................. 113 4.3.3 Fossil fuel emissions detection ....................................... 114 4.3.4 Military and defense explosives detection ..................... 115 4.3.5 Greenhouse gases ............................................................ 116 4.3.6 Biological contaminants.................................................. 117 Sensor comparison at lab/industrial level..................................118 Sensing mechanism of chemiresistive sensors ..........................121 Conclusion ..................................................................................123 References.................................................................................. 123

CHAPTER 5 Semiconductor oxide based chemiresistive gas sensors ............................................................... 133 5.1 5.2

5.3 5.4 5.5

5.6 5.7

Vishal Baloria, Aditya Yadav, Preetam Singh and Govind Gupta Introduction ................................................................................133 Fabrication and designing of C-SMO gas sensors ....................135 5.2.1 Growth techniques of sensing material for C-SMO gas sensors....................................................................... 135 Working principle of a C-SMO-based chemiresistors ..............139 Performance parameters for C-SMO gas sensor .......................141 Sensing mechanism in C-SMO gas sensor ................................145 5.5.1 Pristine oxides ................................................................. 146 5.5.2 Metal/metal oxide functionalized metal oxides ............. 146 Factors influencing sensing characteristics of C-SMO gas sensor ...................................................................................148 Semiconductor oxide sensor outcomes......................................157 5.7.1 At lab level...................................................................... 157

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5.7.2 At industrial level............................................................ 159 5.8 Challenges and future prospect..................................................159 References.................................................................................. 163

CHAPTER 6 Synthesis and application of carbon-based nanocomposite .......................................................... 169 6.1 6.2

6.3

6.4 6.5 6.6

6.7

Rashi Nathawat, Satyapal S. Rathore, Poonam R. Kharangarh, Reena Devi and Anita Kumari Introduction ................................................................................169 Synthesis of carbon materials/SMO nanocomposites ...............169 6.2.1 Ex-situ techniques ........................................................... 170 6.2.2 In-situ techniques ............................................................ 171 6.2.3 Hydrothermal and aerosol techniques ............................ 173 6.2.4 Gas-phase deposition ...................................................... 173 Synthesis of graphene/SMO-based nanocomposites .................174 6.3.1 Common synthesis methods of the G-SMO nanocomposites ............................................................... 175 6.3.2 Hydrothermal method ..................................................... 176 6.3.3 Self-assembly method ..................................................... 178 6.3.4 In situ method ................................................................. 178 6.3.5 Solution mixing method.................................................. 179 6.3.6 Spin coating..................................................................... 179 Conclusion ..................................................................................180 Synthesis of CNTs/conducting polymers-based nanocomposites ..........................................................................180 Functionalization of carbon nanotubes with covalent and noncovalent.................................................................................181 6.6.1 Synthesis techniques ....................................................... 183 Applications of nanocomposites ................................................186 6.7.1 Graphene/SMOs nanocomposites and CNTs/SMOs nanocomposites ............................................................... 187 6.7.2 CNTs/CPs nanocomposites............................................. 190 References.................................................................................. 193

CHAPTER 7 Fabrication of chemiresistive gas sensor with carbon materials/polymers nanocomposites ........... 205 Sarath Chandra Veerla, N.V.S.S. Seshagiri Rao and Anil Kumar Astakala 7.1 Introduction ................................................................................205 7.2 Fabrication of CNTs/SMOs-based sensors ................................207

Contents

7.3 Fabrication of graphene/SMOs-based sensors...........................209 7.4 Fabrication of CNTs/conducting polymers-based sensors ........211 7.5 Fabrication of wireless-based networks sensors........................212 7.5.1 Network architecture....................................................... 213 7.5.2 Materials used in fabrication of devices for WSN......... 215 7.5.3 Fabrication of FBT.......................................................... 215 7.6 Sensing mechanisms ..................................................................217 7.6.1 Sensor outcomes at laboratories/industrial level............ 217 7.7 Conclusions and future prospects ..............................................218 Acknowledgement ..................................................................... 218 References.................................................................................. 218

CHAPTER 8 Potential applications of chemiresistive gas sensors ............................................................... 223 Anshul Kumar Sharma and Aman Mahajan 8.1 Introduction ................................................................................223 8.1.1 Sensor response (S) ......................................................... 224 8.1.2 Sensitivity (S0 ) ................................................................. 225 8.1.3 Response time ................................................................. 225 8.1.4 Recovery time ................................................................. 225 8.1.5 Selectivity........................................................................ 225 8.1.6 Limit of detection ........................................................... 225 8.1.7 Stability ........................................................................... 225 8.1.8 Linearity .......................................................................... 226 8.2 Environmental monitoring .........................................................227 8.2.1 Carbon monoxide (CO) gas sensor................................. 227 8.2.2 Hydrogen sulfide (H2S) gas sensor ................................ 229 8.2.3 Ammonia (NH3) and nitrogen dioxide (NO2) gas sensors....................................................................... 229 8.2.4 Chlorine (Cl2) gas sensor................................................ 231 8.3 Medical diagnosis.......................................................................231 8.4 Food and agriculture applications..............................................236 8.5 Detection of explosives and military applications ....................237 8.6 Conclusion ..................................................................................241 References.................................................................................. 241 Index ......................................................................................................................247

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Introduction In ancient times, carbon and its various forms such as charcoal, graphite, and carbon black have been used in art and technology. Also photocopy toner is mainly composed of carbon black. With the advancements in technology, carbon appears in different sizes from 0D to 3D depending on their properties and applications. Carbon materials have played significant roles in human lives since prehistoric times. It is widely used in the household to the industrial applications. Carbonbased materials, such as graphite, charcoal, and carbon black, have been used as writing and drawing materials. In the last two decades, conjugated carbon nanomaterials, particularly carbon nanotubes, fullerenes, activated carbon, and graphite, have been used as energy materials due to their exclusive properties. Several carbon materials have been developed for various energy applications and further development in progress for the betterment of mankind. These materials have outstanding chemical, mechanical, electrical, and thermal properties, which led their applicability in various fields, including drug delivery, electronics, composite materials, sensors, field emission devices, and energy storage and conversion. Following the global energy outlook, it is predicted that the world energy demand will be double by 2050. This calls for a new and effective means to double the energy supply in order to meet the challenges that forge ahead. These materials are pretending to be appropriate and promising (when used as energy materials) for the situation. The amazing properties of such materials and extreme potentials toward a greener and environment-friendly synthesis techniques and industrialscale production of nanostructured materials are certainly necessary for the coming century. This is grounded on the incredible future that lies ahead with these smart carbon-based materials. This proposal introduces the various forms of carbon nanomaterials and their composites for gas-sensing applications. The fabrication and commercialization of gas sensor at broad level are also covered.

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List of contributors Anil Kumar Astakala Nanomaterials for Photovoltaics and Biomaterials Laboratory, Godavari Institute of Engineering and Technology (Autonomous), Rajahmundry, Andhra Pradesh, India; Department of Humanities and Basic Sciences, Godavari Institute of Engineering and Technology, Rajamahendravaram, Andhra Pradesh, India Vishal Baloria Sensor Device and Metrology, CSIR-National Physical Laboratory (NPL), New Delhi, India; Centre for Advanced Materials and Devices, BML Munjal University, Gurugram, Haryana, India Reena Devi Department of Physics, DAV College, Jalandhar, Punjab, India Shivani Dhall Department of Physics, DAV College, Jalandhar, Punjab, India Govind Gupta Sensor Device and Metrology, CSIR-National Physical Laboratory (NPL), New Delhi, India; Academy of Scientific and Innovative Research, CSIR-HRDC Campus, Ghaziabad, Uttar Pradesh, India Poonam R. Kharangarh Department of Physics and Astrophysics, University of Delhi, New Delhi, India Sandeep Kumar Department of Physics, DAV University, Jalandhar, Punjab, India Anita Kumari Department of Electronics, Sri Aurobindo College, University of Delhi, New Delhi, India Aman Mahajan Material Science Laboratory, Department of Physics, Guru Nanak Dev University, Amritsar, Punjab, India Rashi Nathawat Department of Physics, School of Basic Science, Manipal University Jaipur, Jaipur, Rajasthan, India Satyapal S. Rathore Department of Physics, Cluster University Jammu, Jammu, India R. Ridhi Department of Physics, Panjab University, Chandigarh, Punjab, India; DAV College, Chandigarh, Punjab, India Boyina Rupini SOITS, Indira Gandhi National Open University, New Delhi, India

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List of contributors

N.V.S.S. Seshagiri Rao Department of Physics, Institute of Aeronautical Engineering, Dundigal, Hyderabad, India Anshul Kumar Sharma Material Science Laboratory, Department of Physics, Guru Nanak Dev University, Amritsar, Punjab, India; Centre for Sustainable Habitat, Guru Nanak Dev University, Amritsar, Punjab, India Arshdeep Singh Department of Physics, DAV University, Jalandhar, Punjab, India Preetam Singh Sensor Device and Metrology, CSIR-National Physical Laboratory (NPL), New Delhi, India; Academy of Scientific and Innovative Research, CSIR-HRDC Campus, Ghaziabad, Uttar Pradesh, India Kapil Sood Department of Physics, Government Degree College, Dhaliara, Himachal Pradesh, India S.K. Tripathi Department of Physics, Panjab University, Chandigarh, Punjab, India Sarath Chandra Veerla Nanomaterials for Photovoltaics and Biomaterials Laboratory, Godavari Institute of Engineering and Technology (Autonomous), Rajahmundry, Andhra Pradesh, India Aditya Yadav Sensor Device and Metrology, CSIR-National Physical Laboratory (NPL), New Delhi, India; Academy of Scientific and Innovative Research, CSIR-HRDC Campus, Ghaziabad, Uttar Pradesh, India

Preface This book is based on the carbon nanomaterials and their nanocomposite for the application in chemiresistive gas sensor. The carbon nanomaterials contain extraordinary physical, chemical, and sensing properties that make them promising materials in future electronics and sensing technology. The sensing devices on carbon materials and their nanocomposites is the most growing field of research. Therefore this book presents the status of aforesaid field and encourages the reader to follow up on particular topic through ongoing discussions in scientific literature. This book provides the idea of sensor fabrication and their status in lab/ industry level. In addition, the synthesis of carbon nanomaterials and their composites with nanoparticles which further used as base material in the fabrication of chemiresistive gas sensors are the topic of interest for the reader. It offers a onestep resource, bringing together information currently scattered over journal papers, industrial/lab outcomes, and project reports. This book is designed for the scientific community, materials researchers, and chemiresistive sensor engineers who are looking for the understanding of chemiresistive gas sensor and their fabrication at lab level. The book also provides detailed knowledge about the application of chemiresistive gas sensor in different areas. It should be useful for the graduate and postgraduate students of different institutes. The book begins with Chapter 1, which introduces the carbon nanomaterials with types and their different properties. It also explains the diverse synthesis method for carbon nanomaterials. In this chapter, the importance of functionalization of carbon materials are discussed. Chapter 2 discusses the synthesis and properties of semiconductor oxide materials. The concept of functionalization of oxide materials required for chemiresistive gas sensor is also included. Chapter 3 explains the importance and properties of conducting polymer. This chapter also addresses the synthesis of carbon materials and conducting polymer based composites which has wide application in gas-sensing field. Chapter 4 focuses on carbon nanomaterial based chemiresistive gas sensor. It covers the fabrication of chemiresistive gas sensor at lab levels and explains the possible sensing mechanism. Chapter 5 incorporates the fabrication of chemiresistive gas sensor using semiconductor oxide as sensing layer and their results at lab/industrial levels. Chapter 6 explains the synthesis method for carbon-based nanocomposites with semiconductor oxide materials or polymers and summarizes their applications. Chapter 7 examines the sensing properties of carbon-based nanocomposites layer at lab/industry level. It covers the fabrication steps involved in designing of gas sensor and their possible mechanisms behind sensing. The book concluded with Chapter 8, which reviews the application of aforesaid sensors in different areas.

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Preface

I would like to thank Elsevier for their encouragement and patience. I would also like to thank to all authors for their contribution and hard work. Most importantly, I want to thank my parents, husband, and loving daughters, Shritika and Harshika, for their continuing love and support. Shivani Dhall

Part I Introduction to sensing materials

CHAPTER

1

Carbon-based nanomaterials

Shivani Dhall1, Rashi Nathawat2 and Kapil Sood3 1

Department of Physics, DAV College, Jalandhar, Punjab, India Department of Physics, School of Basic Science, Manipal University Jaipur, Jaipur, Rajasthan, India 3 Department of Physics, Government Degree College, Dhaliara, Himachal Pradesh, India

2

Carbon is one of the basic elements of organic compounds that form many kinds of creatures with other elements to establish lives on the earth. Carbon has the ability to polymerize at the atomic level and form the long carbon chains. It is widely used in the household and in industrial applications. The discovery of buckyballs in 1985 has created an entirely new field of carbon nanotechnology. However, the consequent discovery of carbon nanotubes in 1991 has opened up a new era in nanomaterial technology. The wonder of the carbon world remains with the successful isolation of monolayer graphene from graphite simply using adhesive tape in 2004. Due to their outstanding chemical, mechanical, electrical, and thermal properties, carbon-based materials have applications in various fields, including drug delivery, electronics, sensors, field emission devices, and energy storage and conversion. The amazing properties of such materials and extreme potentials toward a greener and environment-friendly synthesis techniques and industrial-scale production of nanostructured materials are certainly necessary for the coming century. This is grounded on the incredible future that lies ahead with these smart carbon-based materials. Carbon materials have played significant role in human lives since prehistoric times. It is widely used in the household and in industrial applications. Carbon-based materials, for example graphite, charcoal, and carbon black, have been used as writing and drawing materials. Due to their outstanding chemical, mechanical, electrical, and thermal properties, carbon-based materials have recently found application in various fields, including drug delivery, electronics, composite materials, sensors, field emission devices, energy storage and conversion, etc. Following the global energy outlook, it is predicted that the world energy demand will be double by 2050. This calls for a new and effective means to double the energy supply in order to meet the challenges that forge ahead. These materials are believed to be appropriate and promising (when used as energy materials) to cushion the threat. The amazing properties of such materials and extreme potentials toward a greener and environment-friendly synthesis techniques and Carbon Nanomaterials and their Nanocomposite-Based Chemiresistive Gas Sensors. DOI: https://doi.org/10.1016/B978-0-12-822837-1.00008-3 © 2023 Elsevier Inc. All rights reserved.

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CHAPTER 1 Carbon-based nanomaterials

industrial-scale production of nanostructured materials are certainly necessary for the coming century. This is grounded on the incredible future that lies ahead with these smart carbon-based materials.

1.1 Introduction carbon nanomaterials Carbon is a group IV element in the periodic table with atomic number 6. Carbon materials have diverse structures and properties. Nanomaterials composed of carbon atoms are termed as carbon nanomaterials. In general, nanomaterials are defined as materials containing particle size between 1 and 100 nm [1]. Carbon is an element that has the ability to polymerize at the atomic level and can form long carbon chains. Carbon has four electrons in its valence shell, which can be linked via single, double, or triple covalent bonds. Carbon atoms can be attributed to their special electron structure and smaller size compared with other group IV elements. It is one of the elements with a wide range of structures and properties. Carbon can assume diverse structures such as three-dimensional diamond (3D), two-dimensional (2D) graphene, one-dimensional (1D) carbon nanotube (CNT), and zero-dimensional (0D) fullerenes. Such materials are frequently termed as “wonder materials,” because of their outstanding and novel features, which makes them promising materials in numerous applications. In nanometerscale dimensions, the properties of carbon nanomaterials are strongly dependent on their atomic structures and interactions with supplementary materials. The cylindrical morphology of carbon comprises carbon fibers, whiskers, CNT, and carbon nanofiber (CNF). All carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. In reality, all the allotropes of amorphous carbons are made of microcrystals of graphite arranged in an irregular fashion. In graphite, only three of the four valence electrons of each carbon atom are used in bonding, leaving the fourth valence electron as free. Therefore, graphite is a good conductor of electricity. Graphite is a good electrical conductor and its conductivity is strongly enhanced by AsF5 intercalation, becoming almost comparable to that of metallic copper, whereas diamond is completely insulating. The carbon atoms in diamonds have a three-dimensional (3D) tetrahedral network of covalent bonds, which causes the electrons to be held tightly. Therefore, diamonds are very hard and have high melting and boiling points. The structure is closely packed and causes a diamond to be denser than graphite. Since all its electrons end up in forming the covalent bonds, it does not conduct electricity. Carbon materials have shown great versatility because they can also be chemically combined with other carbon-based materials and with a range of different elements to form strong covalent bonds. As a result, they exhibit excellent characteristics such as high strength, high density, and high hardness. Apart from this, fullerenes behave as molecules, in comparison to other carbon materials. Fig. 1.1 represents the classifications of carbon nanomaterials.

1.2 Types of carbon nanomaterials

Carbon Material allotropes

Onion like carbon Carbon dot fullerene Graphene quantum dot

CNTs SWCNTs MWCNTs Carbon nanohorns

0D

1D

Carbon Graphite Graphene Multilayer graphitic sheet Nanoribbons

2D

Diamond

3D

FIGURE 1.1 Classification of carbon nanomaterials in different dimensions.

Research, development, and innovation in carbon materials are taking place in various fields such as medicine and industries. In the meantime, carbon nanomaterials have numerous technical applications in micro- and nanoelectronics, gas storage, production of conductive plastics, composites, displays, antifouling paints, textiles, batteries with improved durability, gas biosensors, and others [2
4]. This chapter will address the synthesis, characterization, properties, and applications of carbon-based nanomaterials, giving great emphasis to materials that have proven promising applications aimed at solving current environmental and energy problems.

1.2 Types of carbon nanomaterials 1.2.1 Diamond Diamond has been known to us since prehistoric times. The ordered linking of Csp3 generates diamond, which has been investigated in hundreds of papers from solid-state physics to applied industrial technology. Diamond is an indirect bandgap material with an energy gap of 5.49 eV. In diamond, C-sp3 atoms are joined by single σ bonds. The 3D lattice structure of diamond consists of two interpenetrating face-centered cubic lattices (as shown in the Fig. 1.2), displaced one from the other by 1/4 of the principal diagonal of the cube. Each atom of one lattice is at the center of a tetrahedron formed by its four neighbors of the other lattice. The smallest translational primitive cell contains only two atoms. These two atoms generate six branches in the phonon dispersion relation. The phonons that propagate along the symmetry directions of the first Brillion one belong to welldefined symmetry species and determine the number of phonon branches experimentally observed by neutron scattering, along a chosen symmetry direction.

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CHAPTER 1 Carbon-based nanomaterials

FIGURE 1.2 Structural illustration of diamond [5].

Diamond has the highest thermal conductivity when compared to any other materials. This is conclusively accredited to low phonon scattering and strong covalent bonding that holds its atoms. The thermal conductivity of the diamond was reported to be 2200 W/(mK), which is five times greater than copper [5
7]. Because of the high thermal conductivity of diamond, it is extensively used in the semiconductor industry to prevent silicon and other semiconducting materials from overheating [8,9]. It is well recognized that the diamond has outstanding carrier mobility, saturated carrier velocities, and electric field breakdown strength [10]. However, it has a poor dielectric constant and may possibly show “negative electron affinity.” According to its physicochemical properties, diamond is chemically and physically robust, and radiation “hard.” It is supposed that diamonds can be used to develop electronics devices, that perform at the highest levels but should also be capable of operating in extreme environments [7,9,11]. Its optical properties are considered uncommon and been inherently linked with the carbon family [5,12,13]. Diamond is also considered to be biocompatible inside a living body. Thus, it is not prone to unwanted cell adhesion or particulate generation. In relation to thermal stability, diamonds can easily be oxidized in the air when heated beyond 7000 C. However, in a flow of high purity argon gas and in the absence of oxygen, it can be heated up to 17,000 C. Shatskiy et al. [14] also reported that the material can bear a temperature of 30,000 C or even higher. In general, these combined properties paved the way for industrial applications such as windows, cutting and polishing tools, heat spreaders, and the scientific applications as an optical detector material, diamond anvil cells, diamond knives, etc. Diamond is very high in elastic modulus because of its covalent network in the crystal structure. Although diamond is not thermodynamically stable at ordinary temperatures and pressures, it exists as a metastable phase at ordinary temperatures and pressures. At high temperature, diamond inclines to change to graphite, because the thermal energy would allow the carbon atoms in diamond to move and the movement would cause the conversion of the metastable phase

1.2 Types of carbon nanomaterials

to the stable phase. The diamond family denotes carbon materials that are similar to diamond in the sp3 hybridization and the resulting tendency for tetrahedral coordination of the carbon atoms. It includes diamond, diamond-like carbon, and graphene.

1.2.2 Graphite The word graphite is derived from the primordial Greek word “graphein” [15]. It was named after Abraham Gottlob Werner in 1789 and consists of carbon atoms connected together in huge flat networks that are piled on top of each other. Graphite has the sp2 hybridization in the carbon atom, in which each carbon has three covalent bonds (σ bonds) directed to three other carbons in the same plane. The fourth valence electron is delocalized through π
π interaction, resulting in in-plane metallic bonding. Therefore, inside the layer, bonding is a combination of covalent bonding and metallic bonding. The graphite layers are weakly bonded through van der Waals forces; thus, it is highly anisotropic, with mechanical, electrical, and thermal properties that vary in-plane direction. The elastic modulus is high and the coefficient of thermal expansion is low, due to the strong in-plane covalent bonding. Graphite is thermodynamically stable phase of carbon in ambient conditions, and used by humankind for centuries. It is used for a very miscellaneous range of applications such as nuclear reactor moderators, pencils, electric motor brushes, and addition of carbon to steel. A single atomic layer of graphite is called graphene. The smallest building block (primitive cell) can construct the graphene lattice. The primitive cell consists of two atoms that can be labeled as A and B. The primitive cell has a hexagonal structure. The size of the primitive cell depends on each graphene layer stake to arrange the graphite crystal. Graphite originates in nature with numerous stacking arrangements; however, here we will discuss the most common and thermodynamically stable stacking: Bernal (or ABAB) stacking. In this stacking, the B atom in the second layer is directly above the A atom in the first layer, and then in the third layer, there is an A atom at this location, just as in the first layer. The primitive unit cell of Bernal-stacked graphite, therefore, consists of four atoms in two adjacent layers. The graphite crystal is shown in Fig. 1.3, which exhibits Bernal stacking. Graphene is an excellent electrical conductor making it a very good material for electrode [16]. The delocalization of electrons in carbon atoms in graphite conduct electricity, which is not possible in diamonds. Because of the restricted movement of the electron in the lattice diamond cannot conduct electricity. Graphite is the stable form of carbon under ambient conditions. An American scientist Edward Acheson in 1896 developed first synthetic graphite. It is characterized with a marked lustrous black sheen feature and experimentally tested to be very flexible but nonelastic. Graphite also possess both metal and nonmetal properties [17
19].

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FIGURE 1.3 Schematic representation of the structure of the bulk hexagonal graphite crystal. The dashed lines show the axes of bulk unit cell. Side insets: top view of the basal plane of graphite and schematic representation of the surface structure (carbon atom) of graphite most viewed by SPM, where every other atom is enhanced (right side inset) and viewed under ideal conditions, where every single atom is seen (left-side inset) [20].

1.2.3 Carbon nanotubes CNTs are exceptional nanostructures that can be considered theoretically as a prototype one-dimensional (1D) quantum wire. CNTs are incredible objects that are set to transform the technological landscape in the nearby future. Future civilization will be shaped by nanotube applications, just like silicon technologies dominating society today. In the 1980s, a very small diameter (,10 nm) carbon filament was developed by the synthesis of vapor grew carbon fiber by the decomposition of hydrocarbon at high temperature in the presence of transition metal nanoparticles, but it was not reported in early years. In 1991, the observation of CNTs was reported by Iijima from NEC laboratory, Tsukuba, Japan using high-resolution transmission electron microscopy [21].

1.2 Types of carbon nanomaterials

In less than 2 year period, Iijima and his group also experimentally discovered multiwalled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs). The discovery of SWCNTs was significant because it is more fundamental and they had been the basis for a large body of predictions and theoretical studies for experimental observations. CNTs are allotropes of carbon family with a cylindrical nanostructure. The name of CNTs is derived from its structure with the walls formed by one atom thick sheet of carbon and tens of atoms around the circumference. CNTs could be either semiconducting or metallic depending on their geometrical characteristics, specifically their diameter, and orientation of their hexagons with respect to the nanotube axis (chiral angle) [22]. MWCNTs and SWCNTs are the two basic forms of CNTs that can be constructed. Whereas SWCNTs consist of a single tube of graphene, MWCNTs contain several concentric tubes of graphene fitted one inside the other as shown in Fig. 1.4. The diameter and length of CNTs are of the order of nanometer and micrometer, respectively [23]. Generally, SWCNTs have a diameter of around 1
3 nm and a length of a few micrometers. MWCNTs have a diameter of 5
40 nm and a length of around 10 μm. CNTs exhibit high thermal and electrical conductivity. SWCNTs are mostly classified into three subclasses: (1) arm-chair (electrical conductivity . copper), (2) Zig-zag (semiconductive), and (3) chiral (semi-conductive). On the other hand, MWCNTs consist of multiple carbon layers with variable chirality. CNTs stimulated great interest in the research community because of their outstanding electronic properties, and this interest continues as other remarkable properties are discovered and promises for practical applications develop. Space elevators tethered by the strongest of cables, hydrogen-powered vehicles, artificial muscles: these are just a few of the technological marvels that may be made possible by the emerging science of CNTs.

FIGURE 1.4 Graphene and carbon nanotubes: (A) SWCNTs and (B) MWCNTs [24].

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1.2.4 Fullerene Fullerene is a class of carbon allotropes in the form of spherical, cage molecules with carbon atoms located at the corner of a polyhedral structure consisting of pentagons and hexagons [25]. There is a carbon atom at each corner of the 20 hexagons and 12 pentagons that build the surface of the ball. The soccer ball-like structure of fullerene is called buckyball after the architect Buckminster Fuller, who designed geodesic domes that resemble spherical fullerene in appearance. Carbon atoms are present in the sp2 hybridization form in fullerene and are linked by covalent bonds. C60 is the best-investigated fullerene. Fullerene was discovered by Robert F. Curl, Jr., Richard E. Smalley, and Sir Harold W. Kroto in 1985 at Rice University during laser spectroscopy experiment. They jointly received the 1996 Nobel Prize in chemistry for their discovery of fullerenes. Microscopic amounts of fullerene were produced by Kroto et al. in laser vaporization of carbon in an inert atmosphere [25]. Kra¨tschmer et al. developed isolable quantities of fullerene C60 by an arc to vaporize graphite in 1990 [26]. In fullerene, each carbon atom has three bonds connected to three other carbon atoms. The 6
6 bonds connect atoms common to two adjacent hexagons. These 6
6 bonds have ˚ . The 5
6 bonds (1.45 A ˚ ) connect atoms common to a pentaa length of 1.38 A gon hexagon pair as shown in Fig. 1.5. The pentagonal rings allow for the outline of the curvature, which is essential for the closing of the carbon cage. The curvature of the fullerene structure has the effect that the three bonds of any carbon atom with its three neighbor atoms are not in a plane [27]. The pyramidalization modifies the hybridization from the pure sp2 hybridization characteristic of planar graphene to an intermediate between sp2 and sp3. That is, it results in a gain in p character, in which the p lobes extend further outside the surface than they do into the interior of the sphere; in addition, p orbitals gain some s character [28]. These changes exert an important effect on many properties, for example, contributing to the high electron affinity of the molecule. The pyramidalization is also responsible for the increase of the chemical reactivity of the fullerene toward addition reactions compared with that of a planar graphene layer [29]. Fullerenes

FIGURE 1.5 (A) The two type of bonds 6
6 and 5
6 in C60 and (B) structure of corannulene [35].

1.2 Types of carbon nanomaterials

have been employed as drug delivery systems [30], nanosensors [31], antioxidants [32], solar cell construction materials [33], and in many other applications due to their excellent electronic properties [34] and chemical reactivity [26].

1.2.5 Carbon nanomaterials However, carbon has been known for thousands of years, during the last three decades new forms of carbons such as C60 and graphene have been discovered and the inventors also won the Nobel Prize. The novel nanostructures are demonstrated in Fig. 1.6. The stabilization of C60 is achieved by the formation of closed-shell structures that avoid the need for surface heteroatoms to stabilize the dangling bonds, like in the case of bulk crystals diamond and graphite. Nanocarbons have been invented by controlling the structure in the nanometer scale as well as the binding nature of carbon atoms. They can provide highly functional advanced performances, which are difficult to obtain from conventional carbons. Fullerene, CNTs, and graphene are exceptional in the larger family of nanomaterial as interrelated prototypes for 0D quantum dots (fullerenes), 1D quantum wires (CNT), and 2D quantum wells (graphene). In 1960, Bacon synthesized graphite whiskers as scrolls, by principally the same conditions as for the synthesis of CNTs except for the use of helium pressures higher by an order of magnitude to synthesize the scrolls. The crosssectional morphology of MWCNTs and carbon whisker scrolls are different. Carbon fiber was found accidentally inside the furnace containing hydrocarbon gases and CO in the presence of melting metals catalysts. Their dimensions are several nanometers in diameter and micron in length with a tubular microstructure. The development of these fibers at low temperatures ,1000 C can be a source of cheap high modulus carbon fibers for reinforcement in composites. Carbon nano-onions (CNOs) contain the concentric graphitic shells, which represents another new allotropic nanophase of carbon nanomaterials. CNOs show a variety of plausible applications such as solid lubrication, electromagnetic shielding, fuel cells, heterogeneous catalysis, gas and energy storage, and electrooptical devices due to their outstanding chemical and physical properties. Ultrahigh-power

FIGURE 1.6 Nanomaterials of carbon (A) 0D Fullerene, (B) 1D carbon nanotube, (C) 2D graphene, (D) carbon dot, and (E) nanodiamond [36].

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micrometer-sized supercapacitors are also produced by using CNOs owing to their accessible external surface area for ion adsorption. A variety of carbon nanomaterials such as carbon soot, carbon nanoshells, metal encapsulated carbon shells, sea urchins, bucky onions, helical coils, spinning cones, and nanohorns can be produced by direct current arc discharge between graphite electrodes. Development of carbon nanoshells during the discharge technique demonstrated that it is possible to encapsulate metal nanoparticles inside the shells for potential applications in magnetic nanoparticles as data storage, ferrofluids, confinement of radioactive waste, and targeted drug delivery nanocapsules.

1.3 Quantum confinement in carbon nanomaterials Wave vector k has large number of values in bulk semiconductors. Thus, density of states is very large in the kx, ky, and kz directions. There will be quantization of energy level if dimensions become in the range of de Broglie wavelength of thermal electron and the energy levels transform in discrete. The wavelength λB is given by λB 5

h h 5 pffiffiffiffiffiffiffiffiffiffiffiffi p 2mTE

where h is Planck’s constant, p is the electron momentum, m is the effective mass of electron, and E is the energy. The wavelength λB is in the order of nanometers. The energy
momentum relations in the direction of confinement is intensely different for lower and bulk dimensions, because of the spatial confinement of the charge carriers, which result a new density of state in both the semiconducting materials (shown in Fig. 1.7). The density of state becomes quantized from

FIGURE 1.7 Representation of electronics density of states in conduction band depending on dimensionality (A) 3D graphite, (B) graphene, (C) CNTs, and (D) carbon nanodots.

1.4 Properties of carbon nanomaterials

quasi-continuous (Fig. 1.7A) and energy
momentum relation convert in to subbands. Consequently, if semiconductor dimensions is less than λB or comparable, then electron charge carrier is confined in 2D and behaves as quantum well structure. The confinement of electron is in z-direction and allowed k values are highly discrete in case of thin sheet of graphene (Fig. 1.7B). The density of state remains constant for certain values of k and follows conduction staircase. In case of CNTs, charge carrier movement is confined in 1D; therefore, they are treated as quantum wire (Fig. 1.7C). In CNTs, each peak corresponds to a single quantum subband because of number of singularities. Lastly, in quantum dot, charge carrier movement is confined in 3D of only a few nm. The valence and conduction bands have sharp energy levels in quantum dots because of limited set of energy levels (Fig. 1.7D). Therefore, quantum dots are occasionally mentioned as “artificial atoms,” but it actually contain thousands of atoms. Such lower dimensional structural materials have new optoelectronics properties in comparison with bulk graphite.

1.4 Properties of carbon nanomaterials 1.4.1 Properties of carbon nanotubes CNTs have high thermal conductivity, high electrical conductivity, high aspect ratio, high elasticity (B18% elongation to failure), moderate to high surface area (depends upon type and orientation), very high tensile strength, high flexibility (can be bent considerably without damage), low thermal expansion coefficient, and are good electron field emitters. Due to the strong in-plane graphitic C
C bonds, SWCNTs are so stiff that it is suitable to use them as probes. With application of physical forces, the tip of the tube bends and regains its position with removal of the applied force. It shows its mechanical strength at its best level till now. The current measured value of Young’s modulus value of SWCNTs is around 1 TPa. However, such measurements are not so easy to record the correct value; therefore, reported value of Young’s modulus of SWCNTs is found to vary in literature. This variation may be due to varying chirality and different sizes of SWCNTs. The amount of disorder and the diameter of the CNTs also affect this measurement value. The extremely high surface area of CNTs (1000 m2/g) makes its surface accessible to electrolyte, which is an important factor for many applications. Electrical properties of CNTs vary with diameter and combinations of structural parameters M and N. These M and N decide “chirality,” indicating how much the nanotube is twisted, i.e., degree of twist. CNTs can be semiconducting or metallic. Till now, the reported experimentally calculated maximum conductivity value for SWCNT ropes is in the order of 104 Siemen/cm at 27 C with current density of 107 A/cm2. The properties of CNTs calculated experimentally as well as theoretically are listed in Table 1.1 [37].

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Table 1.1 Properties of CNTs observed theoretically as well as experimentally [37]. Property

Value

Density

0.8 g/cm3 (SWCNT) 1.8 g/cm3 (MWCNT, theoretical) 1 TPa (SWCNT)) 0.3
1 TPa (MWCNT) 50
500 GPa (SWCNT)) 10
60 GPa (MWCNT) 3000 W m21K21 (theoretical) 22 3 106 EMU/g (perpendicular to plane)) 0.5 3 106 EMU/g (parallel to plane) 2 3 1025 K21 (SWCNT) 300 C
600 C (CVD MWNT)) 600 C
800 C (arc-grown MWNT) . 1000 m2/g (SWCNT)

Elastic modulus Strength Thermal conductivity Magnetic susceptibility Thermal expansion Thermal stability Specific surface area (BET)

1.4.2 Properties of graphene Graphene is the single material which is harder than diamond and can sustain up to double amount of heat passing thorough it than diamond. Single carbon atom in honeycomb structure makes a layer which is found more elastic than rubber. It is known for its toughness, which is better than steel. On the one hand, it is strongest material till now and is a tough material. However, on the other hand, it is known for its lightness. Its weight is compared with aluminum and found that graphene is lighter than it. Even pristine graphene sheet monolayer has the similar strength and is difficult to break easily until a heavy mass (B2 3 103 kg) is applied as force to break it. Graphene is the most applicable material in different technologies due to its extreme electronic conducting nature. Silicon, which is used in technology till now, may be replaced in near future for many application with graphene due to its higher electron mobility. Graphene electron mobility is 100 times more than electron mobility of silicon. Its electrical conductivity is much higher than that of copper. Perfect graphene layer is impervious even by smallest atom helium. Graphene has the maximum surface area of 2630 m2/gm accountable through both side of the layer. The detailed properties of graphene with different applications are given in Fig. 1.8 [38].

1.4.3 Properties of activated carbons (nanocarbons) Activated carbons are in the category of nanocarbons, which is synthesized in view to increase the porosity of material. Therefore, activated carbons have a porous structure. The biomass-derived microporous carbons have shown very high surface area (3100 m2/gm) with large pore volume (1.68 cm3/g). In general, disordered structure has been displayed by the activated carbons because of

1.4 Properties of carbon nanomaterials

FIGURE 1.8 Properties and application of graphene [38].

generation by itching processes. Their structure is observed with presence of wide pore size distribution [39
42]. Activated carbons are generally not pure carbon but have some heteroatoms attached to its surface with some amount of mineral matter too [43]. These heteroatoms are chemically bonded on the surface of activated carbons. These attached heroatoms make activated carbons as chemically reactive materials. Such nanocarbons are known for their good electrical conductivity. Nanocarbons are good conductors of heat and possess high mechanical strength. Pure nanocarbons are generally low toxic materials and therefore they are environment friendly candidates for suitable applications. Porous carbon surfaces are hydrophobic in nature. The properties of activated carbon prepared using precursors almond and apricot shells are given in Table 1.2 [44]. However, these properties of the prepared activated carbon vary from precursor to precursor due

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Table 1.2 Properties of almond and apricot shell derived activated carbon [44]. Activated carbon

Property

0.50
2.36 900
1100 0.48 0.53 0.73

Particle size (mm) Surface area (m2 g21) Solid density (g cm23) Packing density (g cm23) Pore volume (ml g21)

to varying properties of the used raw material, and also depend on used activating agent. Carbonization temperature is also an important factor which governs the properties of prepared activated carbon.

1.4.4 Properties of template carbons Carbons produced using templating method are ordered structured carbons and have narrow pore size distribution than activated carbons [45]. Templated carbons are of two types and accordingly, their properties also vary. Generally hard templated carbons produce microporous, mesoporous, and macroporous carbons depending on chosen template. However soft template carbons are generally mesoporous in structure.

1.4.5 Properties of carbon fibers Carbon fibers are much used in civil engineering, aerospace, military and motorsports due to their high stiffness, high chemical resistance, high temperature tolerance, low thermal expansion, high tensile strength, and low weight. Reinforced carbon-carbon composites or graphene-carbon fiber composites are known for their very high heat tolerance. However carbon fiber-reinforced polymers are very rigid and to some extent brittle too. These polymer carbon fiber composites have high strength to weight ratio. The properties of Pan-based carbon fibers are given in Table 1.3 [46].

1.4.6 Properties of carbon nanomaterials Nanomaterials have proven themselves in direct application in many technologies; therefore, these are most important category of smart materials. Crystal structure and properties of two naturally existing allotropes of carbon—graphite and diamond—are different from each other. Graphite is highly conducting due to access of movement of π electrons in the lattice arrangement. However, diamond is non conducting as electron movement is restricted due to its lattice arrangement. Carbon atoms are arranged with valance bond in different ways, which forms different allotropes. Activated carbons are those carbon nanomaterials, which are known for its high surface area than its volume. This property makes activated carbon to be useful in water purification. The properties of carbon nanomaterials are summarized in Fig. 1.9 [47].

1.4 Properties of carbon nanomaterials

Table 1.3 Properties of Pan-based carbon fibers [46]. Fiber type

Type I

Type II

Type III

Nominal Designation

High strength

High modulus

Ultrahigh modulus

Carbon content, wt% Specific gravity Filament diameter, μm Tensile modulus, GPa Tensile strength, GPa Tensile elongation, % Toughness, MPa Electrical resistivity, μΩ/m Longitudinal CTEa, 10 mm21 K21 Range of available to counts, 1000 fibers/bundle

92
94 1.7
1.8 7
8 220
250 2.5
3.5 1.2
1.4 20 15
18
0.5 1
320

. 99 1.8
1.9 7
8 340
380 2.2
2.4 0.6
0.7 7.5 9
10
0.7 1
12

. 99.9 1.9
2.1 8
9 520
550 1.8
1.9 0.3
0.4 3 6
7
0.9 (est.) ,1

a

CTE, coefficient of thermal expansion.

FIGURE 1.9 Properties and applications of nanocarbons [47].

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1.5 Synthesis methodologies and variations Few of carbon nanostructures came into existence by chance initially as the by-product in search of target materials and then lots of carbon nanostructures were synthesized to check other possibilities of carbon allotropes. Researchers tried to produce different allotropes of carbon in its nanoform because these were stable in nature, and therefore were application oriented. In last 15 years, many new approaches have been achieved by researchers to improve their performance for particular applications. In this section, we try to cover the recent synthesis processes of carbon allotropes.

1.5.1 Synthesis of carbon nanotubes CNTs exist in its three forms: zigzag, armchair, and chiral. CNTs can be constructed in its two basic forms: SWCNTs and MWCNTs. Researchers have prepared double-walled CNTs (DWCNTs) too. These different forms of CNTs can be produced by adopting a suitable synthesis process. The synthesis methods are mainly divided into two categories such as high and low temperature. 1. High temperature methods a. Arc discharge method: Arc discharge are the high temperature ( . 1700 C) techniques which produces more uniform structural CNTs than other methods. Researchers have prepared all kind of CNTs, SWCNTs, MWCNTs, and DWCNTs using arc discharge technique with varying synthesis conditions [48]. Due to high temperature, these techniques got discarded in present time. b. Laser ablation method: Another similar technique is laser ablation, where high energy is provided using laser source [49]. UV laser, infrared laser, XeCl excimer laser, Nd:YAG and CO2 lasers have been used to prepare CNTs using laser ablation method. In this technique, laser energy hits polystyrenenanotubes pellets on alumina substrates or graphite pellet doped with different catalyst materials: usually nickel and cobalt at different temperatures. Researchers tried to see the effect of laser power in the properties of prepared CNTs in detail. Common observation is that CO2 laser power is proportional to the diameter of the SWCNTs [50
52]. 2. Low temperature methods: a. Chemical vapor deposition: Chemical vapor deposition (CVD) technique is comparably a low temperature technique where the properties of CNTs can be monitored easily. Using this method, all three types of CNTs are possible to grow. Base growth process is used to grow SWCNTs; however, MWCNTs and DWCNTs are grown using tip growth process. SWCNTs can grow only if subsurface carbon atom layer is added on the formed base of catalyst nanoparticles to grow SWCNTs. Till date, there is no synthesis process which can produce pure

1.5 Synthesis methodologies and variations

CNTs. There is always some proportion of other allotropes of carbon as well as some metal particles presents with CNTs, which act as impurity while using the CNTs in a particular application. Researchers tried many possibilities to achieve the target properties of CNTs via varying the experimental conditions of CVD method. This gave rise to different CVD techniques like water-assisted CVD [53
55], oxygen-assisted CVD [56], hotfilament [57,58], microwave plasma [59,60], or radiofrequency CVD. In CVD method, one needs carbon precursor, catalyst, and substrates to grow CNTs. The composition and morphology of used catalyst decides the properties of synthesized CNTs. Catalysts are used to decompose the carbon precursor and to form CNTs via new nucleation. The decomposition of carbon source is done generally via either plasma irradiation (plasma-enhanced CVD) or heat (thermal CVD). The most commonly used catalyst, carbon precursor, and substrates are summarized in the following section. The most frequently used catalysts are transition metals, primarily Fe (ferrocene as source), Co, Al, Ni, MgO, or traditionally used catalysts doped with other metals, e.g., with Au, Ni(NO3)2, nickel oxide
silica binary aerogels, catalysts derived from Co/Fe/Al layered double hydroxides, Fe
Mo/Al2O3, and ferrocene. Till now, researchers have used many natural carbon precursors as hydrocarbons such as methane, ethane, ethylene, acetylene, xylene, eventually their mixture, isobutane or ethanol, acetonitrile, dimethyl sulfide. Substrates which are used frequently by researchers are Ni, Si, SiO2, Cu, Cu/Ti/Si, stainless steel or glass, rarely CaCO3, mesoporous silica, zeolite, graphite, and tungsten foil. There are many other synthesis methods such as flame pyrolysis, solid-state pyrolysis, and bottom-up organic approach, which are used to produce different diameter and length of CNTs successfully. In summary, no method is perfect to prepare all types of CNTs using different carbon precursors. The choice of method also depends on the choice of carbon precursor, catalyst, and substrate. One schematic diagram is shown in Fig. 1.10 which shows different methods to prepare CNTs.

1.5.2 Synthesis of pure graphene As graphene is a subunit of graphite, graphene has been synthesized using natural and synthetic graphite, along with nongraphitic origins. The main challenge is to extract long range crystalline order in extracted graphene. Natural source is always better to extract required product but, in this case, graphene prepared from natural source has out-of-plane as well as in-plane grain boundaries which affect its crystalline nature. Due to the presence of grain boundaries in out of plane, its crystalline nature exists only for less than 1 mm thickness, which produces very thin graphene sheets and is difficult to handle too. In-plane grain boundaries, free graphene sheets may be prepared using synthetic graphite source. Highly ordered pyrolytic graphite and kish graphite can easily be prepared.

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FIGURE 1.10 Schematic diagram to show different synthesis routes to prepare CNTs [61].

Micromechanical cleavage or the scotch tape method is one of the easy and straightforward method to extract graphene single layer or few layers of graphene from graphite without need of any apparatus. For achieving single layer, many steps are required which make this method time-consuming. Optical microscopy confirms the presence of single layer on used substrate with increase in optical contrast. Another method to achieve graphene is using graphite oxide. Here graphite oxide is first collected from graphite and then finally graphene is achieved from the extracted graphite oxide. Chemical cleavage is the extraction method of graphene in its oxide form. This method is known as Hummer’s method, and is very popular and safe method to synthesize graphite oxide. In this method, graphite, sulfuric acid, sodium nitrate, and potassium permanganate are used to synthesize graphite oxide. Once graphite oxide is achieved, two-step process is required to achieve graphene. Sonication followed by centrifugation is the first step which is called as exfoliation. After this, the graphene can be achieved via reducing the extra layers. This method has both advantages and disadvantage. During oxidation, lots of defects are created inside the matrix, which cannot be removed in reduction process. Therefore, even the obtained graphene is single or few layer only but it is different from prestine graphene and is therefore called as reduced graphene oxide. Researchers have tried graphite intercalation

1.5 Synthesis methodologies and variations

compounds too. Many different species can intercalate into graphene via introducing alkali ions into interplanar regions of graphene. It increases the interlayer spacing between the layers of graphene which makes it easy to extract single layer as Van der Waal’s forces reduce between two layers. Also, alkali ions donate electron to graphene to make it negatively charged. If any proton donor or other atom is quenched, it forms hydrogenated graphene or functionalized graphene. Graphene can also be produced using non graphite sources. Many reports exist on epitaxial graphene growth from silicon carbide, as this technique produces highquality graphene. Researchers have produced different graphene nanostructures using bottom-up synthesis. Even through much research has been produced in this direction, many more processes are being checked to further optimize the processes to obtain high-quality desired nanostructure graphene. Schematic diagram is shown in Fig. 1.11 which represents different methods to prepare graphene.

1.5.3 Synthesis of activated carbon Pure form of carbon material is not useful for all kinds of applications. Electrocatalysis in fuel cells and supercapacitor need modified form of pure carbon. For such applications, activated carbons, i.e., porous carbon is required to fulfill device requirements. Preparation of activated carbon is a recent trend for researchers who are working in supercapacitor, fuel cell, batteries, and water purification applications. Mesoporous carbon materials can be developed using few different techniques such as carbonization and activation of polymeric resins [63], direct self-assembly using block copolymers together with a carbon precursor

FIGURE 1.11 Schematic diagram to represent different stages involved in different routes to prepare graphene [62].

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[64], and metal etching of metal carbide precursors (carbide derived carbons) [65]. Carbon materials used for supercapacitor application as an electrode material need activated carbon. Activated carbons should have mesoporous as well as microporous structure to use them as electrode materials. One needs to control the micropores, which facilitates adsorption sites to adsorptive ions whereas macropores provides diffusion pathway for ions to reach inner sites. Initial synthesis of porous carbon was based on physical or chemical carbonization process. Physical carbonization process was generally done in presence of CO2 or H2O; however, chemical carbonization is done in the presence of some activation agent: KOH, H3PO4, ZnO, etc. This method was improved by researchers later by introducing some metal particles during synthesis which act as catalyst to broaden the pore size distribution in the material. Large mesoporous structure is excellent candidate for adsorption, separation, and catalytic applications dealing with large molecules. Further, to improve the performance of activated carbons, different heteroatoms can be doped in the host material. To prepare activated carbons many organic precursors have been used to prepare carbons. There are many reports where people have used biowaste materials to prepare activated carbons. Olive stones, natural graphite, melamine, glucose, collagen within fish scale, bamboo leaves, crab, prosopis, juliflora, waste tea, cucumber stem, pine sawdust, carchorus olitorius, oil palm kernel shell, banana-peel waste, soybean root, orange peels, cashew nut husk, lotus leaves, papaya leaves, and many more bio waste materials have already been used to synthesize activated carbon. Hetroatom doping has improved the performance of prepared carbons. Still this field has much scope for researchers to develop modified form of carbon structures for different applications. Hetroatom doping in different nanostructures carbon-based materials with carbonization process has also been reported and find promise for many applications, especially for supercapacitors and batteries. Apart from porous carbon, many researchers are trying to synthesize nonporous carbon also. Porous carbon has lots of advantages, but still suffers from low electrical conductivity because of its high surface porosity. Therefore, supercapacitor device made using porous carbon suffers from low gravimetric capacitance. To increase the gravitational capacitance value of the supercapacitor, one needs electrode material made of nonporous carbon. People also tried to attach heteroatoms to improve the performance of the material. Porous as well as nonporous carbon has been tested in supercapacitor applications as electrode material in detail [66]. Still a lot of research is required to improve the synthesis method, to choose new precursors and to adopt suitable environment to synthesize new carbon nanostructures in bulk which must be suitable for different technologies.

1.5.4 Synthesis of other carbon nanostructures Researchers have synthesized different forms of carbons in nanostructures. Carbon dots, carbon fibers, nano carbon spheres, and many more have been tried by researchers to use them in applications. Generally, these nanostructures are

1.6 Acid simulation of carbon materials and their importance

FIGURE 1.12 Schematic diagram of synthesis and some possible application of carbon dots [67].

more useful in medical applications, as fluorescence probes, and in heavy machineries. The green synthesis of carbon dots is clearly shown in Fig. 1.12. Fig. 1.12 also describes the possible applications of carbon dots as fluorescence probe and its fluorescence origin within its formation mechanism. The structure of carbon fiber can be understood as tow wound onto a reel. Thousands of individual carbon filaments held together is known as tow here. These carbon filaments have mostly carbon atoms and long cylinders of diameter 5
10 μm. Whenever such carbon fibers are needed for any application, these fibers are collected via unwinding the tow from reel where tow is mostly of carbon atoms. Carbon fibers are costly; therefore, more research is required to grow them using new cheaper methods to make them easily accessible. The synthesis of carbon fibers is schematically shown in Fig. 1.13.

1.6 Acid simulation of carbon materials and their importance Carbon-based materials, including graphene, SWCNTs, and MWCNTs, are very promising materials for developing future-generation electronic devices. Their efficient physical, chemical, and electrical properties, such as high electrical conductivity, efficient thermal and electrochemical stability, and high specific surface area enable them to fulfill the requirements of modern electronic industries [69]. At nanodimension range, the physical, electrical, and chemical properties of carbon nanomaterials are mainly dependent on their atomic structures and interactions with other materials. However, as-synthesized nanocarbon materials such as CNTs, nanohorn, and graphene are normally curled, twisted, and agglomerated, dispersion of these materials is difficult for different applications [69
72]. In

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FIGURE 1.13 Flow chart to synthesis carbon fibers [68].

addition, these carbon materials usually agglomerate and form bundle due to Van der Waals forces, which affect their dispersion stability in different solvents. This agglomeration tendency has been an obstacle for their application in different fields. Moreover, efforts have been made by various researchers to produce highquality carbon materials using methodologies such as arc discharge, CVD, and laser ablation method. However, the nanocarbon materials so obtained contain a large amount of impurities, such as growth of metal catalyst (Fe, Ni, Cu, etc.), particles soot, and amorphous carbon as a synthetic residue [71,72]. The removal of these impurities without altering their structure is a challenging job. In order to reduce this agglomeration tendency and to explore their application areas, different functionalization processes have been successfully developed by researchers. The functionalization of carbon nanomaterials is an essential task to prevent their aggregation, which further promote better dispersion and stabilization within various solvents. Chemical methods are designed to alter the surface energy of the carbon nanomaterials, improving their wetting or adhesion characteristics and their dispersion stability. These methods are aimed to modify their surface chemistry either noncovalently (adsorption) or covalently (functionalization). Functionalized carbon materials such as CNTs and carbon fiber might have electrical, optical, or mechanical properties that are different from those of the original nanotubes. Thus, it is an appealing area to functionalize carbon nanomaterials for all kinds of applications.

1.7 Acid functionalization and their importance

There are two major techniques used for the functionalization of carbon materials, such as covalent functionalization (chemical/acids functionalization) and noncovalent functionalization (physical functionalization) [73]. In covalent functionalization, various types of functional groups are attached on the surface of carbon materials using different acids. These functional groups act as anchoring seeds for the deposition of nanoparticles on surface of carbon materials by improving the bonding between them [74]. The good interaction between the carbon materials and nanoparticles enhances their electrical and mechanical properties significantly. In addition, functionalized carbon materials disperse very easily in different solvent/polar media, which enhance their properties in paint and ink. However, excessive chemical treatment affects the structure of carbon materials such as shortening their length and change of hybridization from sp2 to sp3 which significantly affect their overall electrical conductivity [74,75]. Therefore, there is huge interest in noncovalent functionalization in these areas. In noncovalent functionalization process, the surfactants, polymers and biomolecules are used to wrap the surface of carbon materials, which reduce their surface tension. In this noncovalent functionalization, different type of surfactants are used such as nonionic surfactants, anionic surfactants, and cationic surfactants. This process does not disturb their electronic structure and C
C bonding. However, this method has some limitations such as weak forces between wrapped molecules decrease the load transfer that may lower the load transfer in the composite [76,77].

1.7 Acid functionalization and their importance Covalent functionalization process introduces various functional/chemical bonds such as carboxylic and hydroxyl groups on the carbon materials using different acids that have rich chemistry. It also involves formation of covalent bond between functional groups and skeleton of carbon materials, which affect their dispersion stability in different solvents as well as in water. Moreover, it enhances the adhesion property and surface chemistry of the carbon materials [78]. The attached functional groups introduce additional partially occupied bands in the electronic band structure of carbon materials facilitating the charge transfer phenomena [79]. The covalent functionalized carbon materials can show improved electrical and chemical properties as compared to as-synthesized carbon materials. In addition, these carbon materials are used as building blocks in various applications such as nanostructure formation, bio, chemical, and gas sensors [78,79]. Covalent functionalization includes various techniques such as oxidation, defect functionalization, amidation, thiolation, halogenations, hydrogenation, addition of free radicals, cyclo-addition, and Diels-Alder reaction [74
79]. Among all these, oxidation of carbon materials is one of the useful and effective method for the functionalization. Oxidation is mainly accomplished with wet and dry methods. In wet oxidation, different acids such as sulfuric acid (H2SO4), nitric

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oxide (HNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), etc. are used for the functionalization of carbon materials [74]. On the other hand, dry oxidation includes plasma, air, and ozone atmosphere oxidation. This oxidation is useful for industrial scale functionalization because it is a simple process that reduces waste and drying steps. The oxidation method mainly depends on various conditions such as oxidation time, type of oxidizing agent, and temperature. The possible reactive sites in oxidation treatment of carbon materials are shown in Fig. 1.14 and discussed briefly here.

1.7.1 End defect functionalization In defect functionalization, defects are created on the carbon materials, which act as anchoring seeds for the deposition of nanoparticles. The presence of defects in carbon materials such as CNTs are promising and opening point for the progress of covalent chemistry. The strong acid treatment on carbon materials produces carboxylic groups at the “end,” on the surface of tips as shown in Fig. 2.5, after the ring opening due to 1,3-dipolar cycloaddition reaction. Therefore, defect functionalization is also named as carboxyl functionalization used for improving their dispersion stability in aqueous solution. These types of functionalization are mainly useful in drug delivery for therapeutic molecules, such as methotrexate, paclitaxel, and doxorubicin, for treatment of diverse types of carcinomas [80].

FIGURE 1.14 Different methods of functionalization.

1.7 Acid functionalization and their importance

1.7.2 Side wall and surface functionalization Side wall functionalization is mainly used for the dispersion of carbon materials in aqueous solvent. The sidewall functionalization can be accomplished by directly reacting CNTs with organic species such as nitrenes, carbenes, and other radicals to generate respective functional moieties. Therefore, SWCNTs are considerably susceptible material towards side wall functionalization as compared to MWCNTs [80]. In surface functionalization, there is attachment of oxygen-containing functional groups on the surface of carbon materials by functionalization with robust oxidizing agents in wet and dry oxidation. The most common oxidizing agents used in industrial level are air, ozone, nitric acid, and mixture of air/nitric oxide. In dry oxidation, plasma treatment is one of the efficient and fast methods for the surface oxidation of carbon materials [74]. The attachment of functional groups on graphene and CNTs after oxidation is shown in Fig. 1.15. Aforesaid functionalization is accomplished by wet and dry oxidation, which will be discussed in brief in the following section.

1.7.3 Wet oxidation of carbon nanomaterial Wet oxidation is a simple and economical method for the surface, side wall and defect functionalization of carbon materials. During wet oxidation, strong oxidizing agents HNO3 [81], HNO3 1 H2SO4 [82,83], H2SO4 1 KMnO4 [84], and

FIGURE 1.15 Role of oxidation in (A) Graphene and (B) CNTs.

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H2SO4 1 H2O2 [85] are used for functionalization process. These oxidizing agents introduce various functional groups such as carboxylic (-COOH), hydroxyl (-OH), aldehydic, ketonic, and esteric oxygenated on the surface as well on sides of carbon materials [86]. Moreover, larger the amount of functional groups on the carbon materials, better will be their hydrophilic property and hence high dispersion stability [74]. The acid treatment primarily affects the surface of the metal catalysis by oxidation and further donates their charge to the carbon materials. Acids such as H2SO4 and HNO3 introduce a strong p-type doping effect in carbon materials and hence some of sp2 hybridized carbon atoms are changed to sp3 hybridization, thus reducing the number of delocalized π electrons [86
88]. In addition, acid functionalization method opens up the band gap in case of metallic carbon materials, namely CNTs and graphene, resulting in destruction of the optical transitions between the nearest van Hove singularities [89,90]. In last few years, the carbon nanomaterials such as CNTs, graphene, carbon dots, CNFs, and graphite are emerging as promising materials in biomedical, sensing, electronics, supercapacitor, and electromagnetic interference shielding applications because of their extraordinary electrical, mechanical, and structural properties [73,74]. Also, these materials are light in weight and have high surface to volume ratio. However, their hydrophobic nature and robust van der Waal forces lead to poor dispersion and strong agglomeration of these materials in different media. To overcome these problems, wet oxidation of carbon materials plays important role to increase their hydrophilicity required for better dispersion and their strong interaction with different materials/solvent. It is reported that CNTs are much more reactive than flat graphene because of orbitals of sp2 hybridized C atoms, resulting in large tendency to react covalently with chemical species [11] In addition, the defects on CNTs/CNFs induced by wet oxidation are mainly stabilized with
COOH or
OH groups. The amount of attachment of functional groups on CNTs/CNF, graphene, and carbon dots depends on many factors like oxidation temperature, time, method, oxidant concentration, and so on. Among aforesaid materials, acid oxidation of CNTs/CNF is most common and famous method at lab scale level. With acid oxidation, the opening of closed end of CNTs is also possible. The acid functionalization of CNTs using the mixture of HNO3 and H2SO4 acids was first reported by Liu et al. [90] via sonication technique. Afterwards, researchers have reported several results on the covalent functionalization of carbon materials and its applications [91
98].

1.7.4 Wet oxidation of graphene The covalent functionalization is a most powerful approach for the creation of bandgap in graphene for applications in nanoelectronic devices. The dispersion of functionalized graphene sheets in organic solvents is a critical step toward the formation of their nanocomposite materials. However, Hummers method is one of

1.7 Acid functionalization and their importance

famous and easy method for the production of huge amount of graphite oxide through the addition of potassium permanganate to a suspension of graphite, sodium nitrate, and sulfuric acid [99]. The graphene oxide mainly undergoes two reactions: (1) reduction, removal of oxygen groups from graphene oxide, and (2) chemical functionalization, which is adding other functionalities to graphene oxide [99]. A huge number of studies have been reported on reduction and chemical functionalization of graphene oxide. He et al. [100] adopted nitrene cycloaddition method for the attachment of various functional groups and polymers chain on graphene sheets. They found that functionalized graphene shows better chemical, electrical, thermal, and excellent dispersion stability as compared to pristine graphene. Moreover, the dispersion of functionalized graphene in P3HT polymer forms active layer in heterojunction photovoltaic cells. The strong interaction and electron transfer from P3HT to graphene promotes the better power conversion efficiency of the cell [101]. In 2015, Orozco et al. [102] reported the synthesis of Pd nanoparticle anchored graphene nanostructures. They investigated that these nanostructures are useful for the detection of H2 gas at room temperature with reproducible results and fast response/recovery times. In 2018, Dhall et al. [103] reported dual sensing properties of graphene and nanoparticles-based sensor at ambient temperature conditions. They found that at room temperature, Pd-decorated graphene is most sensitive to H2 gas, whereas SnO2 attached graphene is highly sensitive to ethanol at high temperatures, which indicates the role of graphene interface with nanoparticles and testing gases. Rooyanian et al. [104] reported graphene oxide by Hummers’ method, and then reduced and turned into graphene nanosheets. The diazonium grafting route has been reported for the production of benzoic acid functionalized graphene for the sensing of Pb(II) ion. They reported linear response from 1.0 3 109 to 1.0 3 103 M Pb(II) and a detection limit of 1.5 3 1010 M Pb(II), which indicate the role of graphene nanosheets functionalized with benzoic acid. Graphene finds application in biotechnology as it is biofunctionalized with biomolecules such as proteins, peptides, and so on. The biofunctionalized graphene emerges as a promising material in the fabrication of fluorescence resonance energy transfer biosensors because of its quenching capability toward different organic dyes, quantum dots, and in fast DNA sequencing [105
107]. In the handling of CNT/CNF, one of major problem is their poor dispersions in solvents. However, in some applications such as electronic and gas sensing, there is need of well-dispersed solutions as well as a strong interfacial interaction of CNTs/ CNF with dispersion medium. It is well known that grown CNTs are not significantly active to the gas molecules; therefore, modification of these materials is required [97]. Researchers have adopted different chemical methods to increase their hydrophilicity for well dispersion in different media [74,79,83,97]. The covalent functionalization of CNTs using HNO3 and H2SO4 acids mixture was initially reported by Liu et al. [81] via sonication technique. Lau et al. [108] have functionalized MWCNTs by different methods and then studied the comparative electrical properties. They

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found that dry UV-ozonolysis proved as the best method for large increment in electrical conductivity of MWCNTs. Espino et al. [109] have demonstrated the effect of chemical functionalization on the electrical properties of SWCNTs networks formed by inkjet deposition technique. They investigated that carboxyl and sulfonic acids provide mobile carriers such as protons and hydronium ions for uniform electrical transport in the SWCNT films. In addition, they observed that small densities of SWCNTs in networks produce nonlinear I-V characteristics and vice versa. In 2013, Dhall et al. [96] reported the chemical functionalization of MWCNTs at room temperature for the application of H2 gas sensor. They investigated that functional groups such as
COOH and
OH play important role in detection of 0.05% H2 gas at ambient conditions. Chemicals such as boron and acids like HNO3 and H2SO4 are used for p-type doping in the CNTs/CNF and other carbon materials. Oxidation process significantly initiates defect sites to the sidewalls of CNTs and hence hole-hopping occur due to introduction of impurity states at the Fermi level of CNTs [96]. Generally, HNO3 oxidized CNTs/CNFs show the intercalant in form of neutral HNO3 molecules admixed with charged NO3 2ions and hence intercalated nanotubes act as ptype semiconductors. The reactions involved in this process are as follows: On the other hand, nitrogen atmosphere creates n-type doing in CNTs/CNFs [110]. The presence of functional groups on CNTs/CNFs creates new partially occupied bands in their electronic band structure and these behave as electron acceptor/donor, facilitating the charge transfer phenomena between them [73]. However, due to specific chemical groups, various localized kinds of perturbation changes occur in the local electronic structure of the CNTs. These changes are specific to type of functional group and are localized about the chemical groups. During functionalization, some sp2 hybridized carbon atoms are transformed to sp3 hybridized carbon atoms, and various defects such as irregular arrangements of pentagon/heptagon, creation of holes in the sidewalls, and oxidization of sites are possible [110,111]. This hybridization change has considerable effect on the charge transport properties of the CNTs. It is also well known that the sp3 bond formation facilitates intershell bridging, which proves helpful in charge carrier hopping to inner shells of multi-layers CNTs/CNFs. This intershell bridging provides an extra transport path for charge carriers in the multilayer CNTs/CNFs [111,112]. In literature, there are some simple techniques such as FTIR and Raman spectroscopy for the identification of attachment of functional groups on structure and surface of carbon materials. However, the effect of acid treatment on the interlayer spacing of CNTs can be identified with the help of XRD and HRTEM analysis. Furthermore, the shifting of D (disorder/defect) and G (graphitic) bands indicates the doping in carbon materials after the functionalization. In addition, the increment in ID/IG (intensity ratio) in Raman spectrum after acid treatment also indicates the breaking of bonds and introduction of functional groups on the side walls as well as on the ends of CNTs. The insertion of functional groups on CNT walls is interpreted as defects in the structure [96].

1.7 Acid functionalization and their importance

Fig. 1.16 shows the Raman, FTIR and HR-TEM analysis before and after acid treatment of CNTs. The large-scale oxidation of carbon materials at industrial level with wet oxidation is difficult and unappealing because of its health hazardous and environment issues. In wet oxidation, different types of chemicals are used, which are extremely corrosive and require special handling. Sometimes, chemically treated materials lose their structural and fluffy property required for various applications. To overcome this problem, industries mainly use dry oxidation, which is also named as gas phase oxidation. Dry oxidation involves different media such as ozone, steam, and mixture of air with some gases. It is a well-controlled method that does eliminate any waste to the environment. Recently, the oxygen and nitrogen plasma treatment were applied to produce graphene with abundant edges, oxygen functional groups, and nitrogen doping. The plasma-etched graphene was then used as a metal-free electrocatalyst in a solid fuel cell [113].

FIGURE 1.16 HR-TEM images (A) P-MWCNTs and (B) F-MWCNTs, Raman, and FTIR analysis of MWCNTs before and after acid functionalization [96].

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1.8 Applications The widespread applications of carbon nanomaterials are summarized in Fig. 1.17. The material science and nanotechnology field have witnessed a remarkable expansion in the recent years in all industrial sectors. The carbon nanomaterials applications in global energy division attracted countless attention. CNTs, graphene, and CNF find application in sensor, supercapacitors, field-effect transistors (FETs), thermoplastics, elastomers, thermosets, and widely use in existing high volume molding processes without any major new manufacturing development [73]. CNF has extraordinary intrinsic properties, such as its high surface to volume ratio, mechanical strength, and elastic modulus, and thus finds application in mechanical and gas sensors. In addition, CNTs, CNF, and graphene are equally important in electronics

Good absorption: Acoustic Sensor

Electronics Applications: Transistor, Transparent thin film, LSI writing etc.

Nanoscale structure, Ion absorption, catalysis support

Biotechnology applications such as; Drug delivery system, Biosensors, Cell cultivating etc.

Carbon Nanomaterials

Energy Storage application: Fuel cell, capacitor, Lithium ion batteries etc.

High surfce to voulme ration: Gas sensor

Nanotechnology e.g., Aerospcae, defence, automotiv etc.

High mechinacal strength: Force sensor

FIGURE 1.17 The relation of carbon nanomaterials with their properties. The properties were shown in yellow circle and corresponding application are in green circle [37].

1.8 Applications

by insertion of their lower loading as conductive filler in composites to get high electrical conductivity. The applications of carbon nanomaterials to fulfill global energy requirement has also attracted countless attention.

1.8.1 Carbon nanotubes CNTs are the prominent building blocks for the fabrication of gas sensor and open a new way for multi-transducer and multi sensors array as compared with other carbon forms, namely graphite, carbon blacks, and graphene [69,83]. The adsorption of the gas molecules on the walls of the CNTs and hence modification in their electrical conductivity either by donating/accepting charge carriers is the main mechanism of CNT-based gas sensor. Therefore, the alteration in resistance of CNTs depends on their gas adsorption kinetics and charge transfer capabilities. The introduction of defects like vacancies, functional groups, and stone wall defects on nanotubes can enhance their sensitivity toward different gases. These defect sites lower the activation energy barrier thus enabling chemisorption’s of analytes on the surface of CNTs and make room temperature measurements possible [96]. CNTs decorated with metal/metal oxide nanoparticles have been widely used to detect the different type of gas molecules. The nanoparticles surface and their interfaces with CNTs act as reactive sites for the adsorption of the gas molecules and also improve the charge transfer process into CNTs. CNTs have been extensively used as biosensors in a variety of ways. Before fabrication of biosensors, the immobilization of biomolecules with specific functionalities on the sensing material is very important. These biomolecules acts as anchoring groups to bind the particular species in the testing sample and catalyze the reaction of a specific analyte. The recognition of specific target molecules is the main feature of the biological sensing. CNTs can be used as force sensors in atomic force microscopy and scanning tunneling microscopy. In recent years, SWCNTs are used as molecular and microscopic pressure sensors [68,69]. The dependence of the SWCNTs Raman band structure on mechanical deformations acts as the origin for the development of nanotubes based strain sensors. It has been observed that when hydrostatic pressure is applied to SWCNTs, there is a shift of nanotube band in Raman spectroscopy. The CNTs are used as strain sensors for getting the strain profile in the matrix of polymer. To investigate the individual strain/stress component, the Raman band of CNTs in a specific direction must be selected. The Raman technique can also be used to measure the individual stress component [69,96]. In addition, CNTs find application in energy storages, FET, electronics, and CNTs as anode materials for lithium-ion batteries and in water purification [73].

1.8.2 Graphene Graphene is one of promising materials which is used in FETs, optoelectronic devices, memory storage, sensors, solar cells, and as transparent conducting films [73]. In graphene, the control on charge (electron or hole) density by using an

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electric field is possible, so it is widely used in electronic devices such as FET and sensors [73]. Graphene is also used as transparent conductive electrodes in solar cells, which is a substitute to metal oxides as reported by Wang et al. [81]. They reported the fabrication of graphene films from exfoliated graphite oxide using thermal reduction. The electronic property of graphene can be controlled by the chemical alteration without significant degradation in charge carrier mobility. The chemically altered graphene, whether p or n-typed, is widely useful as sensing layer for the detection of single molecules. The composites of graphene with metal/metal oxides find application in sensor technology [73,81].

1.9 Conclusion This chapter contains the introduction of carbon nanomaterial and their types such as graphite, graphene, CNTs, and carbon dot. We have discussed the different routes for the synthesis of carbon nanomaterials in brief. Also, we have explained the different properties of carbon nanomaterials including electrical, mechanical, and sensing properties. On the basis of applications in sensing area, the acid functionalization of carbon nanomaterials has also been discussed in this chapter. The combination dry and wet oxidation methodologies discussed in this chapter will provide extensive ideas to select the suitable method for industrial as well laboratory level. The different applications as sensing layer of carbon nanomaterials have also been incorporated in this chapter.

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2452.

39

CHAPTER

2

Semiconductor oxide nanomaterial

S.K. Tripathi1 and R. Ridhi1,2 1

Department of Physics, Panjab University, Chandigarh, Punjab, India 2 DAV College, Chandigarh, India

2.1 Introduction of semiconductor oxide nanomaterials The versatility of metal oxide semiconductors (MOS) in the fields of physics, chemistry, materials science, and nanotechnology with an aptitude to integrate inputs from electronic, optical, and lattice parameters makes them imperative study materials for practical implications [1]. The leeway behind utilizing MOS nanomaterials for semiconductor devices dwells over conventionally applied nanomaterials in their increased lattice parameters with diverse valencies of metals integrating MOS [2]. Structure, surface, and electronic properties tend to drive physical, chemical, optical, and electrical parameters of MOS nanomaterials [3]. Distinctive electronic structures allow their differentiation into semiconducting, insulating, and metallic metal oxide nanomaterials. Generally, metals residing at the corners of the periodic table constitute insulating metal oxides (e.g., Al2O3, ZrO2, MgO, SiO2, CaO) and those lodging at the middle comprises metal and semiconducting creations of oxide nanomaterials (e.g., CeO2, V2O5, SnO2, ITO, MnO2, TiO2, ZnO, NiO, Cr2O3, Fe2O3) [4,5]. From the technical viewpoint, MOS nanomaterials tend to solicit into photovoltaic, photocatalytic, fabricating microelectronic, and piezoelectric devices, fuel cells, Li-ion batteries, capacitors, and chemresistors [4,5]. Current pandemic scenario requires contributions in fraternities associated with industrial and technological values of materials. Therefore, it is imperative to understand the principle phenomena underlying these properties to inculcate them for technological and industrial applications. Identifying bottlenecks associated with each property, to resolve and amend it depending on pre-requisite of the particular application. Table 2.1 lists the various properties associated with MOS, guiding principles, and required amendments to enhance its contributory factor and efficiency. For example, tendency of TiO2 to absorb light in visible region and block UV radiation had motivated its utilization as photocatalyst, dye-sensitized solar cells, and sunscreens [6,7]. Similarly, the large exciton energy (60 meV), binding energy (25 meV), and large photoconducting properties coupled with its noncentrosymmetric nature of ZnO enable its implementation in light emitting diodes, photovoltaic devices, field effect transistors (FET), Carbon Nanomaterials and their Nanocomposite-Based Chemiresistive Gas Sensors. DOI: https://doi.org/10.1016/B978-0-12-822837-1.00007-1 © 2023 Elsevier Inc. All rights reserved.

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CHAPTER 2 Semiconductor oxide nanomaterial

Table 2.1 Metal oxides with their crystal states, magnetic state, band gaps, and hardness. S. no.

Metal oxide

Crystal state

Magnetic state

Band gap (eV)

Hardness (Moh)

1. 2. 3. 4.

ZnO MnO CuO CoO

Hexagonal Cubic Cubic Cubic (Rock salt)

n-type n-type p-type p-type

3.3 3.4 4.1 1.2 1.8 2.4

4.5 5 6

5. 6. 7. 8.

CdO FeO ITO TiO2

FCC (cubic rock salt) Cubic (Rock salt) Cubic, rhombohedral Tetragonal (rutile, anatase, orthorhombic, brookite)

n-type p-type n-type n-type

9. 10.

SnO2 MnO2

n-type n-type

11.

VO2

12.

CeO2

Rutile (tetragonal) Rutile (β tetragonal), Orthorhombic (γ) Tetragonal (above 68 C), monoclinic (below 58 C) Cubic

2.2 2.9 2.4 2.5 4.0 3.2 (anatase), 3.0 (rutile), 2.96 (brookite) 3.6 3.9 β: 0.26, γ: 0.58 0.7 0.5 0.7

13. 14.

RuO2 WO3

15. 16. 17.

Cu2O V2O5 Ga2O3

18.

Al2O3

19.

Fe2O3

20.

In2O3

21.

Cr2O3

Tertragonal (rutile) Triclinic (250 C to 17 C), monoclinic (170 C 330 C), tetragonal ( . 740 C) Cubic Orthorhombic Rhombohedral (α), monoclinic (β) Hexagonal (α-alumina), cubic (γ-alumina) Rhomohedral/ corundum (α), cubic (γ) Cubic, rhombohedral Corundum (rhombohedral)

n-type

n-type at low pressure and ptype at high pressure Amphoteric n-type

3.2

310 345 Knoop 3.0 5 5.5 6 6.5

6 7 6 6.5

6

2.2 2.5

p-type n-type n-type

2.1 2.2 2.2 2.3 β 4.7 4.9

3.5 4 3 3.5

Insulator

6.0 8.8

9.0

n-type

α 2.2, γ 2.0

5.5 6.5

n-type

3.6 (direct), 2.6 (indirect) 3.3 3.4

8 8.5

p-type

2.2 Synthesis of oxide nanomaterials and variations

switches, and piezoelectric devices [8,9]. The ability of VO2 to undergo typical metal-to-insulator transition allows its function as memresistors, and optical and electrical switches [10 12]. Owing to extraordinary hardness of Al2O3 (Moh’s hardness of 9.0) and Fe2O3, they have been successfully applied as abrasives and polishing agents [13,14]. Co3O4 constituted Li-ion batteries are strongly dependent on size and structure of Co3O4. Li et al. [15] reported the best discharging capacity of Co3O4 in nanotubes form (  1100 mAh/g) amongst nanoparticles and nanorods (  800 mAh/g) and concluded more enhancement with lowering particle sizes [16]. Cu2O nanowires have been utilized as conducting channels for FETs on account of its flexibility in altering conduction properties of oxide layers [17]. Similarly, variations in resistivity and color changes of MOS with adsorption of oxidizing and reducing gases enables their utilization for gas sensors [18 22]. Moreover, the tendency of metal oxides to exist in p-type and n-type semiconductors, ability of same material to exist in versatile crystal structures (Fig. 2.1), and varied band gaps allow their diverse applications. This makes them a valuable tool for industrial and technological applications. Tables 2.1 and 2.2 list the various metal oxides, their magnetic state, band gap, hardness, and crystal state (Table 2.1) with their varied properties and applications (Table 2.2). From Table 2.1, we find that the crystal structure and magnetic state of MOS are strongly dependent on temperature and pressure conditions. Same MOS can be converted from one phase to other at elevated temperatures and from n-type to ptype at higher pressures. Hence, optimized control in temperature and pressure conditions should be well maintained while synthesizing MOS materials. Therefore, suitable MOS preparation technique should be utilized as per the desired application.

2.2 Synthesis of oxide nanomaterials and variations Top down and bottom up are the two common approaches for synthesis of nanoparticles shown in Fig. 2.2. Bottom-up approach inculcates nanoparticle formation during atom-by-atom or molecule-by-molecule creation. In top-down approach, tearing down of bulk materials is conducted to gradually reduce them to nano regime materials. Nanoparticles exhibit fast diffusivities and unusual adsorption capacities as compared to their bulk counterparts [23,24]. Thermodynamic phenomena like surface diffusion, stability, adsorption, and solubility of MOS are strongly affected by temperature used during synthesis of nanoparticles.

2.2.1 Physical vapor deposition Physical vapor deposition (PVD) refers to a wide range of technologies in which a material is released from a source and deposited on a substrate using

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CHAPTER 2 Semiconductor oxide nanomaterial

FIGURE 2.1 Crystal structure of metal oxides.

mechanical, electrochemical, or thermodynamic process. The primary requirement of this method is to achieve a vacuum of 1025 torr or better. In this method, the material evaporates or sublimes in vacuum due to thermal energy (resistive heating) and then the vapor stream of atoms or molecules condenses on a cooler substrate so as to form continuous and adherent deposits of desired thickness. PVD can be achieved by a wide variety of methods such as thermal evaporation, electron beam evaporation, radio frequency (RF) induction heating,

Table 2.2 Different properties of MOS with governing principle and applications. S. no. 1.

2.

3.

Property

Principle

Materials

Application

Higher electron mobility, high photosensitivity, high solar to current conversion efficiency High aspect ratio coupled with constant surface density of catalysts

Photons with sufficient energy excite dye molecules followed by a rapid injection of electrons into the conduction band of MOS Upon adsorption of photons (with work function higher than catalytic material), electrons are excited from valence band to conduction band, resulting in migration and reaction of charge carriers at the catalyst adsorbed surface. This promotes decomposition phenomena Transition MOS (where M: Ni, Cu, Co or Fe) having electrochemical capacities of 700 mAh/g with 100% capacity retention acting as anode for fuel cells Higher capacitance of MOS (  1370 F/g) and electrodes developed with MOS nano-network tends to enhance charging capacities of devices Reactive Oxygen species (ROS) mechanism

TiO2, Cu2O, NiO, WO3, SnO2, ZnO

Photovoltaic applications: (DSSC)

ZnO, TiO2, WO3

Photocatalysts

CuO, Co3O4, VO2

Li-ion batteries

RuO2 (1300-2200 F/ g), MnO2 (1370 F/g) and Co3O4 (111 F/g) CuO and ZnO

Supercapacitors

6.

High electrochemical capacities with capacity retentions Enhanced electrochemical performance for fast charging and higher power density Durability, availability in different shapes and robust nature Piezoelectric effect

7. 8.

Luminescence Gas sensing

9.

Electrochromic materials

10.

Transparent conducting oxides

4.

5.

To measure pressure, acceleration, strain or force and converting them into electrical signal

ZnO, BaTiO3

Strong exciton binding energy coupled with Charge transfer phenomena between adsorbed gases and surface atoms of MOS Ability of MOS to undergo color change upon oxidation and reduction

ZnO, CdO CuO, TiO2, In2O3, VO2, WO3, ZrO2 TiO2

High electrical conductivity and optical transparency in visible region

CdO, ITO, ZnO

Antimicrobial agent Piezoelectric sensors, ultrasonic transducer, piezoelectric motor, piezoelectric nanogenerators LED and photonic materials Chemresistors to detect air pollutants Electrochromic devices such as electrochromic windows and displays Flat panel displays, solar cells

46

CHAPTER 2 Semiconductor oxide nanomaterial

FIGURE 2.2 Top down and bottom approaches for synthesizing metal oxide nanomaterials.

laser beam evaporation, molecular beam epitaxy (MBE), activated reactive evaporation, electron gun heating, etc (Fig. 2.2). Let us briefly discuss about these techniques:

2.2.2 Thermal evaporation or resistive heating technique Thermal evaporation of the material can be achieved by a variety of physical methods. The oldest process is evaporation from a boat or wire helix source, which consists of a refractory metal (tungsten, tantalum, or molybdenum) having high melting point and very low vapor pressure that are heated by an electric current (Fig. 2.3).

2.2 Synthesis of oxide nanomaterials and variations

FIGURE 2.3 Thermal evaporation system using resistive heating.

This technique involves three basic steps: (i) Generation of vapor from the condensed phase, solid or liquid. (ii) Transfer of vapor from the source to the substrate in the presence of low pressures. (iii) Condensation of vapor molecules on the surface of the substrate. There are three basic stages of this technique with their essential requirements: First stage: Transformation of solid material to be deposited into gaseous form. Essential requirements: In this stage, transformation of the material takes place from solid to gaseous state. For example, if this is to be accomplished by resistive heating, then it is always essential that the material of the evaporation source (i.e. boat) should withstand very high temperature (which are always greater than the melting point/ boiling point of the material to be evaporated). In addition, the

47

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CHAPTER 2 Semiconductor oxide nanomaterial

power supply used for such resistive heating should be capable of delivering enough current to the boat so that it reaches these high temperatures conveniently. Second stage: Transport of the created material (in vapor state) to the substrate from the evaporation source in the form of vapor stream. Essential requirements: This process can be better accomplished only if the mean free path of the evaporated gas molecules is greater than the distance between the evaporation source and the substrate. If this condition is not fulfilled, then the vapor gas molecules will be diverted from their normal path due to collisions with the ambient gas molecules or amongst themselves; therefore, it is always essential that the pressure in the chamber should be reduced substantially. Third stage: In this stage, the nucleation and condensation of the vapor molecules on the substrate takes place to form a continuous film. Essential requirements: This process can be optimized if the rate of incidence of the vapor molecules at the substrate is kept less than the rate of nucleation. Besides, it is always essential that the rate of impingement of the ambient gas molecules should be very small compared to that of evaporated molecules in order to minimize the incorporation of impurities in thin film.

2.2.3 Electron beam evaporation In this technique, the electrons emitted from a tungsten cathode are accelerated by high voltages. The emitted electrons are focused on the substance to be evaporated which is kept in a water cooled crucible. The kinetic energy of the electrons is transferred into thermal energy, so that the material in the crucible (boat) melts and evaporates into the vacuum maintained conditions. High purity films can be prepared by this method because crucible materials or reaction products are practically excluded from evaporation. We can evaporate any material using this method, and rate of evaporation is higher than that of resistive method. It works similar to that of thermal evaporation system, but here, the power supply connected to copper electrodes is replaced by hot electron gun. This hot electron gun produces hot electron beam, which impinges on the material and heats the material placed in crucible to its melting point. The obtained vapor beam of the material deposits on the substrate placed above the sample (Fig. 2.4). Hass et al. [25] and Dickey et al. [26] synthesized MOS using electron beam evaporation.

2.2.4 Radio frequency heating RF can be used to heat the evaporant while inducing eddy currents in the charge causing heating effect in the crucible. By suitable arrangement of RF coils, levitation and evaporation can be achieved thereby eliminating the possibility of contamination of the film by the support crucible. However, this heating method is seldom utilized in nanofabrication or microfabrication industries on account of its lesser efficiency in comparison to resistive heating method.

2.2 Synthesis of oxide nanomaterials and variations

FIGURE 2.4 Schematic representation of electron beam evaporation assembly.

2.2.5 Flash evaporation This method is useful in the preparation of multi-component alloys or compounds that tend to fractionate in the evaporation process. In this method, the selected material is prepared in powder form with as small a grain size as possible and its small quantity is dropped onto a boat that is hot enough to ensure that the material is evaporated instantaneously. The temperature of the boat should be high enough to evaporate the less volatile material fast. Various methods (ultrasonic, mechanical, electromagnetic, etc.) are used to drop the powder on boat. A drawback of this technique is the difficulty in pre-out gassing the evaporant powder. Degassing the powder can be accomplished to some extent by vacuum storage for around 24 36 hours prior to deposition.

2.2.6 Laser ablation technique/pulsed laser deposition A long wavelength of the laser beam and a very short pulse (nanosecond (ns) to femtosecond (fs)) duration induce instant local vaporization on the surface of a target material generating a plasma plume consisting of photons, electrons, ions, atoms, molecules, clusters, and liquids or solid particles. This phenomenon is

49

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known as “laser ablation,” a term derived from latin word ablation meaning to carry away. Laser ablation is the base principle of most applications involving laser processing of materials, precise cutting, hole drilling, laser cleaning of surfaces, compositional analysis, and thin film deposition. A plate/slide/wafer can be easily positioned in front of the plasma plume, acting as a collector for the hot ablated material that condenses in the form of a thin film. This deposition method is known as pulsed laser deposition (PLD). Historically, this method was known under several denominations: pulsed laser evaporation, laser induced flash evaporation, laser MBE, laser assisted deposition and annealing, and laser sputtering. Various parts of PLD are demonstrated in Fig. 2.5. The material that is irradiated by the laser beam is called the “target,” while the collector is commonly referred to as the “substrate.” They have to be placed plane-parallel in a deposition chamber, which is under vacuum conditions. A high intensity laser placed outside the deposition chamber is used as an energy source to ablate the target material and to deposit the thin film. The target vaporization is induced by photons, so no contamination/ impurification occurs during deposition process. PLD involves the following stages: 1. 2. 3. 4.

Coupling of the optical energy to the target material. Melting of the surface of the sample. Vaporization in the form of a plume of the thin upper layer of molten surface. Photon absorption by the vaporized species, which eventually limits the laser fluence at the target surface.

FIGURE 2.5 Schematic representation of laser ablation technique.

2.2 Synthesis of oxide nanomaterials and variations

5. Propagation of the plume in the direction normal to the target. 6. Return of the initial state after few nanoseconds from the end of the pulse, with a re-solidified surface. If the deposition is made in a reactive gas and the obtained film has a composition different from that of the target, the synthesis process is reactive pulsed laser deposition (RPLD).

2.2.6.1 Factors responsible for PLD deposition 2.2.6.1.1 Deposition conditions Conditions of deposition include nature of ambient-ultra high vacuum, reactive gas, target-substrate separation distance, and number of pulses. Depending on the structure and composition of the thin films that one desire to achieve by PLD, in the deposition chamber a gas, which can be active or passive can be introduced. In principle, passive influence of the gas is necessary because it helps to compensate the eventual losses of the constituent elements. The explanation is that in ambient oxygen, due to the intense collisions with the environmental atoms, the ejected matter is confined to elongated “cigar-shaped plasma.” A thermal equilibrium is reached as a result of collisions during transfer and the substrate condenses in large quantities forming compact thin films. In vacuum, at much lower collision rates, the matter is ejected in all directions with high energies and speed. These high energetic species bombard the layers previously deposited and cause damage (by sputtering off atoms from the outer layers) or defects (dislocations, cracks, holes) on the deposited films. These bombardments occur for each pulse, resulting in a very disordered thin film that is full of defects.

2.2.6.1.2 Laser beam parameters 2.2.6.1.2a Influence of number of pulses. A very low number of pulses (generally under 100) generate a deposition of nano/microparticles on the substrate surface. Slightly increasing the number of pulses produces islands of material. On increasing the number of pulses, the substrate is covered by a continuous thin film. 2.2.6.1.2b Influence of laser wavelength and pulse duration. The energy delivered by the ultra-short laser pulses is absorbed in a thinner layer as compared to nanosecond (ns) laser pulses, thus producing higher temperatures at the surface level and faster vaporization of the target material. Electron microscopy showed that the droplets of material spread and density decreased, when using laser pulses with shorter duration. Thin films deposited using an ns laser source had their surface covered by droplets. These droplets were spherical and had an average diameter of 10 μm. Their presence and morphology are indicative of expulsion of molten material from the surface of target. When using picoseconds (ps) or femtosecond (fs) pulses, the morphology of the films surface changed from droplets covered to smooth surfaces. When using ps laser pulses, there were still particulates on film surface but they completely disappeared when ablation was

51

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conducted with fs pulses. The smoothness of these film surfaces is a consequence of the removal of ablated material with reduced expulsion of melted particles. 2.2.6.1.2c Influence of laser fluence. The laser pulse fluence can be defined as the optical energy that is delivered to a selected area on the target. Therefore, the fluence can be varied by changing the laser energy or the dimension of the spot area on the target. 2.2.6.1.2d Influence of target-substrate separation distance. The best depositions are obtained when the plasma length is identical to the target-substrate separation distance. This is because for target-separation distances longer than plasma length, the species in plasma lost their kinetic energy by collisions with other species and gas molecules from the ambient and therefore, the ablation rate was significantly lower for larger and smaller lengths (than that of target-substrate distance). Irwin et al. [27] reported PLD of NiO using 248 nm KrF excimer laser with 25 ns duration and 2 5 Hz repetition rate.

2.2.7 Sputtering technique When a solid surface is bombarded with energetic particles such as accelerated ions (most commonly argon ions), surface atoms of the solid are scattered due to collisions with accelerated ions, and atoms are ejected out of the surface. These ejected atoms from the source (target) are then deposited on the substrate to form a uniform thin film. This phenomenon is called sputter deposition. Sputtering is a relatively high energy process, in which the sputtered atoms leave with several eV (much higher than evaporated atoms which have around 0.1 eV) [28]. This high energy is very good for the thin film growth process, which leads to hard dense films.

2.2.7.1 Working of sputtering system Sputtering is a process of depositing thin films where a controlled gas, usually chemically inert argon, is introduced into a vacuum chamber and then a cathode is electrically energized to generate a self-sustaining plasma. The exposed surface of the cathode, called the target, is a slab of the material to be coated onto the substrates. The gas atoms become positively charged ions by losing electrons inside the plasma, which are then accelerated into the target and strike with sufficient kinetic energy to dislodge atoms or molecules of the target material. This sputtered material now consists of a vapor system, which traverses the chamber and hits the substrate, sticking to it as a coating or “thin film” (Fig. 2.6). An important advantage of sputtering is that even materials with very high melting points are easily sputtered, while evaporation of these materials in a resistance evaporator or Knudsen cell is difficult and problematic. In order to get good film adhesion, it is, of course, necessary for the substrate surface to be clean. Appropriate cleaning and handling steps must be employed prior to placing

2.2 Synthesis of oxide nanomaterials and variations

FIGURE 2.6 Schematic diagram of sputtering system.

substrates onto the vacuum chamber. There are also considerations such as what type of power to be used on cathode. Depending on that, they are classified as DC sputtering, RF sputtering, and magnetron sputtering. DC sputtering: DC power is suitable for conductive materials. D.C. sputtering is the most basic and inexpensive technique for PVD metal deposition and electrically conductive target coating materials. Two major advantages of DC as a power source are that (i) it is easy to control and (ii) it is a low cost option. DC sputtering is used extensively in the semiconductor industry, creating microchip circuitry on the molecular level [28]. In the basic configuration of a DC sputtering coating system, the target material to be used as a coating is placed in a vacuum chamber parallel to the substrate to be coated. The vacuum chamber is evacuated to a base pressure removing H2O, air, H2, etc. and then backfilled with a high purity inert gas, usually argon. Argon is commonly used as ion beam in sputtering system due to its relative mass and ability to convey kinetic energy upon impact

53

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CHAPTER 2 Semiconductor oxide nanomaterial

during high energy molecular collisions in the plasma that creates the gas ions that are primary driving force of sputter thin film deposition. Typically sputter pressure ranges from 0.5 m torr to 100 m torr. A DC electrical current typically in the range of 2 to 5 kV range is applied to the target material that is cathode or point at which electrons enter the system known as the negative bias. A positive charge is also applied to the substrate to be coated which becomes the anode (Fig. 2.6). The electrically neutral argon gas atoms are first ionized as a result of the forceful collision of the gas atoms onto the surface of the negatively charged target which eject atoms off into the plasma (a hot gas like state consisting of roughly half gas ions and half electrons that emits the visible plasma glow). The ionized argon gas atoms are then driven to the target (sample) and strike it with sufficient kinetic energy, which dislodges atoms of the material and accelerates them towards the substrate. On striking the substrate, the materials atoms condense and form a thin film coating on the substrate. Ju et al. [28] reported deposition of InTaO thin films on glass substrates utilizing this technique. DC magnetron sputtering uses magnetic field behind the negative cathode to trap electrons over the negatively charged target material so they are not free to bombard the substrate, allowing for faster deposition rates (Fig. 2.6).

RF sputtering RF power can also sputter nonconducting materials. RF sputtering is the technique that involves alternating the electrical potential of the current in the vacuum environment at RFs to avoid charge building up on certain types of sputtering target materials. Examples of dielectric or nonconductive or insulating materials are aluminum oxide, silicon oxide, and tantalum oxide. By alternating the electric potential with RF sputtering, the surface of the target material can be cleaned of a charge build-up with each cycle. On the positive cycle, electrons are attracted to the target material or cathode, giving it a negative bias. On the negative portion of the cycle, which is occurring at the RF, ion bombardment of the target to be sputtered continues. A DC sputtering system requires voltages between 2 and 5 kV, whereas RF sputtering needs  1012 kV to sputter dielectric insulators. In RF sputtering, electrons present at the space between substrate and target (inter electrode space) do not get enough energy by the RF field to cause ionization. But, if we apply a magnetic field parallel to the RF field, it will constrain the electrons without being lost to the flow, thus improving the RF discharge efficiency. So, a magnetic field is more important for RF sputtering than in a DC sputtering. Gan et al. [29] reported the influence of target power in affecting the various parameters of CuxO deposition using RF magnetron sputtering technique.

Magnetron sputtering Sputtering sources often employ magnetrons that utilize strong magnetic fields to confine charged plasma particles close to the surface of the sputter target in order to increase the sputter yield. In a magnetic field, electrons follow helical paths around magnetic field lines. This causes an increase in the effective path length

2.2 Synthesis of oxide nanomaterials and variations

causing more ionizing collisions with gaseous neutrals near the target surface. The sputtered atoms are mostly neutral and also much heavier and are unaffected by the magnetic trap. In helical drift path, electrons are confined to increase the sputter yield in a toroidal path generated by the magnets placed behind the target. An electron launched by the target is affected by the component of magnetic field bending towards the target surface (magnetron component) and finally returns, completing the magnetic circuit. The confinement of plasma due to electrons trapped in cycloidal and the resultant intense plasma allow magnetron sputtering systems to operate at much lower pressures and lower target voltages than are possible for RF diode sputtering. Here the deposition rates are higher and cover large deposition areas.

2.2.8 Molecular beam epitaxy MBE can be considered as a more sophisticated version of evaporation technique. In MBE, the vacuum is very high such that the pressure inside the reactor is of the order of 1028 2 10 11 Pa. At this pressure, the mean free path of the gas molecules far exceeds the distance between the source and the target. Apart from ultra high vacuum system, MBE usually consists of real time structural and chemical characterization capability. Other analytical instruments may also be attached to the deposition chamber such that the grown films can be transferred from the growth chamber without exposing to the ambient. MBE set up is expensive and cost  1 million $. In MBE, the evaporated atoms or molecules from one or more sources do not interact with each other in the vapor phase under such a low pressure. Although some gaseous sources are used in MBE, most molecular beams are generated by heating the source material in effusion cells (known as Knudsen cells). The material is raised to the desired temperature by resistive heating. Suja et al. [30] reported MBE deposition of Cu-doped ZnO thin-film growth for MOS device fabrication.

2.2.8.1 Various components of MBE Its main elements are sources of molecular beams, a manipulator for heating, translating, and rotating the sample; a cryoshroud surrounding the growth region, shutters to occlude (stop) the molecular beams; a nude Bayard-Alpert gauge to measure chamber base pressure and molecular beam fluxes; a RHEED (reflection electron diffraction) gun and screen to monitor film surface structure and quadrupole mass analyzer to monitor specific background gas species or molecular beam flux composition. Pyrometer is used to measure and control the temperature. MBE growth process involves controlling, via shutters and source temperature, molecular and/or atomic beams directed at a single crystal sample (suitably heated) so as to achieve epitaxial growth. The gas background necessary to minimize unintentional contamination is predicted by the relatively slow film growth of approximately 1 μ/h and is commonly in 10211 torr range. The mean free path of gases at this pressure and in the beams themselves is several orders of

55

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CHAPTER 2 Semiconductor oxide nanomaterial

magnitude greater than the normal source to sample distance of about 20 cm. Hence, the beams impinge un-reacted on the sample with a cryoshroud cooled by liquid nitrogen. Reactions take place predominantly at the sample surface where the source beams are incorporated into the growing film. Proper initial preparation of the sample will present a clean, single crystal surface upon which the growing film can deposit epitaxially. Time actuation of the source shutters allows film growth to be controlled to the monolayer level. It is this ability to precisely control epitaxial film growth and composition that has attracted the attention of material and device scientists. In MBE, the molecules strike on the single-crystal substrate resulting in the formation of desired epitaxial layer film. (i) The extremely clean environment due to ultra-high vacuum ensures absence of impurity or contamination. Thus, a highly pure film can be readily obtained. (ii) Individually controlled evaporation of the sources permits the control of chemical composition of the deposit at any given time. (iii) The slow growth rate ensures sufficient surface diffusion and relaxation so that the formation of crystal defects is kept minimal.

2.3 Chemical/solution method for the growth of metal oxide nanoparticles 2.3.1 Chemical vapor deposition The deposition of thin films from gaseous phases by thermal decomposition or chemical reactions on substrates at high temperature is known as chemical vapor deposition (CVD) process. Sometimes, a carrier gas is also introduced either to control the rate of reaction or to prevent undesired reactions at the prevailing elevated temperature. Films of high purity and quality with required composition and doping levels can be prepared by this method. In this method, solid films are formed on the substrate maintained at a suitable temperature by the reaction of the constituents in their vapor phase. The chemical reaction itself is an important characteristic of all CVD processes. Several types of chemical reactions such as chemical transfer, thermal decomposition (pyrolysis), reduction, oxidation, nitride and carbide formation, etc. are available to carry out CVD processes. CVD is also known as vapor-phase epitaxy. CVD is a process whereby an epitaxial layer is formed by a chemical reaction between gaseous compounds. CVD can be performed at atmospheric pressure (APCVD) or at low pressure (LPCVD). Susceptors in the epitaxial reactors (Fig. 2.7) are analogous to the crucible in the crystal growing furnaces. Not only do they mechanically support the wafer, but in induction heated reactors, they also serve as the source of thermal energy for the reaction.

FIGURE 2.7 Schematic diagram of CVD system.

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CHAPTER 2 Semiconductor oxide nanomaterial

The mechanism of CVD involves a number of steps: 1. The reactants such as the gases and dopants are transported to the substrate region. 2. They are transferred to the substrate surface where they are adsorbed. 3. A chemical reaction occurs, catalyzed at the surface followed by growth of the epitaxial layer. 4. The gaseous products are desorbed into the main gas stream. 5. The reaction products are transported out of the reaction chamber. CVD process is versatile because for depositing a given film, many different reactants or precursors can be used. From the same precursors and reactants, different films can be obtained by varying the ratio of reactants and deposition conditions. One of the important applications of CVD is to make diamond films from gas phase deposition at low pressure.

2.3.2 Photochemical vapor deposition Photo CVD is a recently developed, low-temperature deposition technique for the preparation of high-quality films for technological applications. In this process, the photochemical deposition takes place when high energy photons selectively excite states in the surface absorbed or gas phase molecules, leading to bond rupture and the production of free chemical species to form the films on the adjacent substrate. Ultraviolet lamps or lasers are the sources used for the photochemical deposition.

2.3.3 Plasma-enhanced CVD It is a versatile technique for depositing a wide variety of film materials for microelectronic, photovoltaic, and many other applications. In this process, the plasma produced by RF, DC, or microwave fields are used to promote the chemical reactions.

2.3.4 Chemical methods In chemical deposition method, thin films are deposited on the substrate from aqueous solution either by passing a current or by chemical reaction under appropriate conditions. Large or small and even or uneven surfaces of all types—conducting or insulating—can be coated with relative ease by this cost-effective method. The electrodeposition, spray pyrolysis, closed space sublimation, anodization, solution growth, etc. are different chemical deposition techniques. These techniques use simpler equipments and are more economical than physical methods. Various chemical methods are as follows: (i) Electrolytic deposition: This is an electrochemical process in which the anode and cathode are immersed in a suitable electrolyte and the passage of electric current serves to deposit the material on the cathode. By this

2.3 Chemical/solution method for the growth

method, it is possible to deposit only on metallic substrates and the film may be contaminated by the electrolyte. (ii) Electroless deposition: It is simple and is possible for large area deposition from a solution, in which no electrode potentials is applied, unlike electrodeposition. The rate of deposition depends on the temperature of the bath and the chemical reduction in some cases needs to be stimulated by a catalyst. (iii) Anodization: It is used to deposit oxide films over certain metals. Here when the electric current is passed, the anode reacts with negative ions from the electrolyte and forms an oxide coating. (iv) Vapor-liquid-solid is commonly employed to synthesize MOS nanowires like SnO2, In2O3, ZnO, ZnSnO3, V2O5, Ga2O3, and TeO2 [31 33]. The growth of MOS nanowires is accomplished in low pressure-high temperature furnace (Fig. 2.8). The temperature of the source material is increased to its melting point to allow evaporation of its vapors. Sometimes an additive is used to reduce melting temperature of source material. For example, for growth of ZnO nanowires, graphite powder is mixed in same ratio to decrease melting temperature of ZnO (  1975 C) to 800 1000 C. In this case, graphite acts as a catalyst for vapor formation and deposition of ZnO nanowires. Carrier gas carries these evaporated vapors to deposit on catalysts (Au, Ni, Fe, Co) supported substrate. The evaporation sources commonly employed are generally laser ablation (enhance uniformity of the nanowire diameter), thermal evaporation, or inductive heating (effective heating and faster synthesis). The carrier gases utilized are argon or nitrogen, and oxygen is often mixed with the carrier gas to form MOS nanowires. Low temperature synthesis of nanowires can be conducted using sol-gel method, electrochemical deposition, template method, and polymer assisted methods (Fig. 2.9). Sol-gel method involves low temperature chemistry, simple experimental set up, better reproducibility as well as high surface to volume ratios of the obtained products [34]. Sol-gel prepared MOS nanoparticles possess excellent electrical

FIGURE 2.8 Schematic diagram of vapor-liquid-solid (VLS) synthesis method for nanowires.

59

FIGURE 2.9 Schematic diagram of sol-gel synthesis method for nanowires.

2.4 Types of oxide materials and their importance

and optical properties [34]. Solvents, additives, pH of the solutions, pre- post heat treatments, aging time, and type of polymer utilized for condensation play a vital role in affecting properties of MOS nanoparticles and nanofilms [34]. Ghosh et al. [35] reported versatility in ultraviolet photosensing and optical emission of sol-gel deposited ZnO films at different cooling rates. Kim et al. [36] reported low cost sol-gel fabricated ZnO/Al flexible memory devices. Template method is another powerful method that involves filling materials into nanoporous membrane forming nanostructures that can be released by dissolving template. Nanoporous anodic aluminum oxide and polycarbonate membranes are extensively utilized as template for nanowire synthesis [37,38]. Hyodo et al. [39] utilized polymethyl methacrylate (PMMA) template for synthesis of microstructured SnO2 sensors for detection of NO, NO2, and H2.

2.4 Types of oxide materials and their importance MOS have an advantage over commercially applied organic devices in terms of better stability and reproducibility [40]. The significance of MOS in gas sensors lies in their unique advantages such as high sensitivity, stability, low cost, ease of synthesis, low power consumption, resistance to high temperatures, are catalytically active, and have simple sensing mechanism [40]. Table 2.3 lists the versatile properties and enhancement parameters associated with MOS chemresistors. MOS chemresistors are utilized for detection of toxic gases such as H2S, NO2, and CO; ambient gases like O2, NH3, CO2, and O3; and combustible gases such as LPG, H2, and CH4 [40,41]. These sensors rely on the principle of band theory and their varying resistances with exposure and adsorption of target gases on account of different surface reactions and kinematics (Fig. 2.10) [41]. A conductometric MOS sensor comprises a receptor and a transducer governed by gassolid interactions and microstructure of the oxide, respectively (Fig. 2.10). The analyte target gas adsorbs on the receptor surface and undergoes charge transfer interaction. The transducer converts the observed signal into a detectable form and is strongly dependent on the MOS sensor device dimensions, design, form, and stability. The converted electrical signal is detected by multimeter or any other electrical measurement device to sense change in resistance or current. It constitutes a heating coil/wire to obtain optimum working temperature, sensing film which varies its resistance after exposure of gas and conducting electrodes for measuring resistance. Versatile sensing properties and responses are obtained while making alterations in the receptor and transducer by mixing oxides, tuning size effects, and adding suitable dopants. Restricted sizes of edges and corners on the surface of MOS nanoparticles coupled with their high density makes them suitable candidates for participation in surface-interface studies having unique physical and chemical properties [70]. These properties enable utilization of these materials in versatile applications such as light emitting diodes, solar

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Table 2.3 Classification of MOS chemresistors on the basis of versatile properties. S. no.

Property

MOS examples

Enhanced parameter

References

1.

Wide band gap

Electrical conductivity

[42 44]

2.

Size and structure

Electrical conductivity

[45 48]

3.

Porosity of polymer MOS nanocomposites Surface conduction effects

TiO2, ZnO, SnO2, WO3 WO3, SnO2, In2O3 PANI/SnO2

Catalytic activity of sensing matrix

[49]

ZnO, SnO2

Low temperature sensing efficiency (400 C 600 C) High temperature sensing efficiency (. 700 C) Sensing efficiency towards reducing gases

[48]

Sensing efficiency towards oxidizing gases Selectivity and sensitivity towards H2S and ethanol Faster responserecovery times

[53 59]

Better stability and reproducibility High temperature conductivity

[41,65]

4.

5.

Bulk conduction effects

CeO2, TiO2, Nb2O5

6.

Magnetic n-type

7.

Magnetic p-type

8.

p-n junctions

ZnO, TiO2, WO3, In2O3, MoO3, SnO2 NiO, Ag2O, CoO, CuO CuO-SnO2, Ag2O-In2O3

9.

Thin film (6 1000 nm) MOS chemresistors Thick film (10 300 μm) Lattice oxygen deficiency

10. 11.

In2O3

SnO2 Ga2O3

[50]

[51 53]

[60 64]

[65 67]

[68,69]

cells, laser technology, gas and explosive detectors, nanoscale electronic devices, etc [42,43,71]. The ability of wide band gap MOS such as TiO2, ZnO, SnO2, and WO3 to undergo change in their electrical conductivity proportional to concentration of exposed gas makes them promising inexpensive sensors [42 44]. Electrical conductivity of MOS is strongly dependent on size of MOS, especially for WO3, SnO2, and In2O3 gas sensors [45]. Additions of dopants and other catalytic impurities (Pt, Au, Pd, Fe2O3) further enhance the sensing efficiency of MOS chemresistors [40,41,70]. MOS polymer nanocomposites enable encapsulation of MOS nanoparticless into polymer matrix giving it porosity for efficient diffusion of analyte gases with better charge transfer interaction with surface atoms of MOS nanocomposite [51,72]. Polymer matrix also prevents aggregation of MOS nanoparticles to form MOS nanocomposites with high density of surface atoms [72]. Ram

FIGURE 2.10 Schematic diagram of chemresitor and charge transfer mechanism.

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et al. [49] showed that polymer nanocomposites provide better control over catalytic activity of sensing matrix while studying PANI/SnO2 nanocomposite sensor. It was established that CO could oxidize PANI due to assistance of SnO2. The origin of MOS chemresistor gas sensors arises from the works of Seiyama et al. [44] and Taguchi et al. [73] when they had reported zinc oxide (ZnO) thin films layers as first reported gas sensors in 1962 [44]. It was based on change in resistance/conductance of film upon adsorption of analyte species [44,73]. The high electron mobility, high surface-to-volume ratio coupled with good biocompatibility, and stability makes ZnO as promising gas sensor [44,54,73]. Recently, MOS with sizes in the range of 1 nm to 100 nm have gained attention as chemresistors on account of flexible size-dependent properties, lower power consumption (  100 mW), high sensitivity, selectivity, and reproducibility [74,75]. Yamazoe et al. [46] and Ansari et al. [47] reported increase in sensitivity of SnO2 towards H2 by an order of magnitude and 10-fold by decreasing size below 10 nm and 22 nm, respectively. Similarly, In2O3 tends to modify its sensing efficiency by an order of magnitude with drop in particle sizes to 20 nm to 30 nm from 50 nm [48]. MOS gas sensors have excellent sensitivity, response time, suitability to portable instruments, and low maintenance cost in comparison to electrochemical and thermal conductivity gas sensors [76]. MOS chemresistors are classified on the basis of their magnetic state: n-type MOS (e.g., ZnO, titanium dioxide (TiO2), tungsten oxide (WO3), indium trioxide (In2O3), molybdenum trioxide (MoO3), and tin oxide (SnO2)) having electrons as majority carriers and p-type MOS (NiO, Ag2O, CoO, and CuO) with holes as charge carriers [54]. In the former, conduction increases and in the latter it decreases when redox reaction takes place at the surface of MOS nanostructure (Fig. 2.10) [54]. Recently, Afzal et al. [76] reported gallium oxide (Ga2O3) sensing properties and its lattice oxygen deficiency leading to donor like doping phenomena make it suitable sensor on account of its high temperature conductivity [68,69]. The division of MOS chemresistors is also based on the materials following surface (ZnO and SnO2) and bulk conduction effects (CeO2, TiO2, and Nb2O5), respectively. Former division operates at lower temperatures (400 C 600 C), and conduction changes are attributed to removal and formation of surface oxygen. Materials following bulk conduction effects operate at higher temperatures ( . 700 C) and respond to variations in oxygen partial pressure of MOS at elevated temperatures [48,50]. Both p-type and n-type MOS have been utilized for detection of versatile gases such as CO [77], NH3, NO [52], NO2 [55 57], H2S [58], C2H5OH [53], and O3 [59]. The selectivity and sensitivities of MOS can be further enhanced by doping p-type with n-type and vice versa forming p-n junctions [78]. According to Sergeev et al. [79] and Bochenkov et al. [79], electronic mechanism and chemical scheme lead to amendment in sensing responses of MOS nanofilms. Katti et al. [60], Patil et al. [61], and Khanna et al. [62] reported enhanced selectivity and sensitivity of CuO (p-type)-SnO2 (n-type) composite towards H2S gas by forming p-n junction. H2S reacts with CuO to form CuS, resulting in breakdown of junction yielding large decrease in resistance of films [60 63]. Mashock et al. [63] also reported

2.5 Need of functionalization of oxide materials

room temperature CuO-SnO2 sensor for 1000 ppm NH3. Another p-n junction based composite Ag2O (p-type)-In2O3 (n-type) containing 15% Ag2O for enhancing ethanol sensitivity has been reported by Mehta et al. [64]. Its charge transfer mechanism involves transfer of conduction electrons from In2O3 towards Ag2O, resulting in electron depleted region in In2O3 and increasing its resistance. The sensing response towards ethanol is increased with incorporation of Ag2O. Xu et al. [80] reported threefold increase in sensitivity of In2O3 towards ethanol and H2S by incorporating p-type CeO2 nanoparticless. In2O3 is also utilized as a dopant in several sensors to modify selectivity of sensors [80]. Klein et al. [81] reported enhanced sensitivity of In2O3-SnO2 composite towards CO as pure to individual SnO2 nanocrystal chemresistor. Similar magnetic state MOS composite also tends to enhance the sensing properties while utilizing additive effect of its properties [82]. Mondal et al. [82] reported hydrogen gas sensor using ZnO-SnO2 composite with high selectivity (in the presence of interfering gases CO and CH4), excellent response-recovery times, and reproducibility at 150 C. MOS sensitivities can be further improved by incorporating impurities like noble metals Nb [83], Pt [84], Pd [85], Au [86], rare earth oxides [87], and metal oxide PdO [88]. Addition of noble metals to MOS not only amends its sensing ability but also reduces response and recovery times, which are characteristics of a good sensor [75,89]. Wan et al. [90] and Gao et al. [91] reported amendment in sensing efficiency of ZnO towards relative humidity (RH) and ethanol with doping of Cd and CdSe, respectively. Thickness of the MOS films deposited also categorize MOS nanofilms into two categories with versatile sensing properties [65]. Thin film MOS chemresistors with 6 1000 nm film surface show better sensitivity coupled with faster response-recovery times but are challenging to manufacture in the context of stability and reproducibility [41,65]. However, the second category of thick MOS sensors with thickness ranging from 10 to 300 μm is easily manufactured and commercially utilized [41,65]. Sysoev et al. [66] reported a comparative study on SnO2 nanoparticle film and nanowire network for 46 days of continuous sensor operation and found excellent stability and reproducibility for SnO2 nanowire network. In2O3 nanowires of 10 nm diameter were reported to have best sensing performance towards NH3 in terms of detection limit [67].

2.5 Need of functionalization of oxide materials Room temperature sensing with low power consumption is one of the key parameters for practical application of sensing devices [92,93]. MOS are low power devices, having longer lifetimes, faster response times, and size miniature characteristics, which have made them preferable over commercially utilized electrochemical and optical sensors [94,95]. However, they require high operating temperatures (usually . 200 C) and have poor selectivity [95]. Moreover, in order to enhance compatibility between inorganic MOS biosensors (ZnO, SnO2, WO3, TiO2) and organic materials for their effective utilization as

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biosensors for medical applications, they are functionalized with certain organic entities [96,97]. Therefore, there is a need to functionalize them suitably to exploit their sensing advantages under ambient conditions. Recently, Joshi et al. [98] reported UV activated Au modified ZnO nanorods to detect ozone (O3) gas at room temperature. UV activation has not only enhanced its stability and selectivity towards interfering gases (NH3, CO2, CO, NO2, CH2O) but also enabled faster recovery of ZnO sensor towards O3. Mashock et al. [63] also developed room temperature ammonia sensor while making p-n junction utilizing CuO nanowire networks and SnO2. Functionalization of In2O3 with Au [99] and Zn [100] has enhanced its selectivity towards CO under room temperature, making it a promising CO room temperature sensor. Wang et al. [101] reported comparative study on sensing efficiencies of binary (Ni-Zn, Ni-Co and Fe-Co), ternary (Ni-Zn-Co and Ni-Mn-Co), and quatarnary (Ni-Co-Fe-Cu-Zn) oxides and found much higher response (10.9) with faster response-recovery times (85/160 s) towards 50 ppm ethanol at 80 C as compared to individual MOS. Functionalization of Nb to WO3 nanowires has been reported to enhance its sensing response towards H2 upto one order of magnitude [102]. Selectivity is one of the key issue in current sensing technology. This is because it is very important for a sensor to be free from the effect of interfering gases such as CO, ethanol, NH3, NO2, NO, and CO2 present in ambient conditions to detect the target gas. In order to enhance the selectivity of MOS chemresistors to make them suitable candidates for practical sensor device, there is a need to functionalize them with appropriate additives. For instance, Au doping on In2O3 and WO3 has made these MOS selective towards O3 and NH3, respectively [103]. Similarly, Pd and Pt have been reported to enhance selectivity of MOS towards CO [104]. Table 2.4 lists the MOS sensors having targeted selectivity towards specific gas, and addition of certain additives enhances their selectivity towards target analyte gas. Table 2.4 MOS sensors having targeted selectivity towards analyte gas. S.no.

MOS chemresistor

Targeted gas

References

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

TiO2 Fe2O3 SnO2 In2O3 WO3 LaFeO3 CuO WO3-CuO WO3-Au ZnO-SnO2 Ga2O3 Sb-SnO2 Li-SnO2

O2, H2, C2H5OH CO H2S, CH4, CO, H2, NO2 NO2, O3 NH3, NO2 NOx NH3, NO2, H2S, C2H5OH H2S C2H5OH H2 C2H5OH Cl2 H2

[105] [106] [107] [103] [103] [108] [53,109 111] [112] [113] [82] [114] [115] [116,117]

References

2.6 Future aspects for MOS Innovative synthesizing strategies of MOS governing the shape, structure, and morphologies of MOS are needed to be considered from technological applications view point. This is on account of strong dependence of electrical, optical, and spectroscopic properties of MOS on structure of metal oxides. Although materials such as ZnO and TiO2 have become saturated in terms of available synthesis strategies, to exploit their properties from technical view point, they need to be given more detailed significance. Moreover, the current emerging technological applications of VO2, SnO2, MnO2, and perovskite materials makes an enthusiastic approach for studying their intriguing properties. The tendency of Ga2O3 to detect reducing (at low temperatures, , 700 C), oxidizing gases (at high temperatures . 900 C) and biosensors makes it a promising and emerging MOS sensor. Unique electronic structures coupled with complex interplay between their magnetic, electronic, optical, and structural properties poses a great challenge to the researchers of materials sciences. The current scenario in fusing MOS composites to apply the additive property effect of constituent MOS makes them interesting devices for technology and industrial implementations. The indispensible need of sensors for explosive detection, environmental monitoring, control of technical processes, and healthcare systems makes MOS chemresistors as requisite materials to study on. Their tendency to inculcate better selectivity, sensitivity, stability, and reproducibility while tuning their size, structure, electrical, optical, and magnetic properties allows their ample applications for versatile gas sensors. Ability of transparent conducting oxide to sustain heavy doping exhibits DC conductivity, which can easily be utilized as sensing transducer on account of varied charge transfer capability. The doping of MOS chemresistors to have significant amenability of selectivity towards a target gas under interference of ambient gases is the need of current approach of sensing materials. The selectivity of MOS under ambient conditions needs to be tapped further for utilizing them as promising commercialized sensors.

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[77] M. Hubner, C.E. Simion, A. Haensch, N. Barsan, U. Weimar, COsensingmechanism withWO3 based gas sensors, Sens. Actuators, B: Chem. 151 (2010) 103 106. [78] P. Kanitkar, M. Kaur, S. Sen, A. Joshi, V. Kumar, S.K. Gupta, et al., Growth and gas-sensing studies of metal oxidesemiconductor nanostructures, Int. J. Nanotechnol. 7 (2010) 883 906. [79] V.E. Bochenkov, G.B. Sergeev, Metal oxide nanostructures and their applications, in: A. Umar, Y.B. Hahn (Eds.), Metal Oxide Nanoparticles and their Applications, American Scientific Publication, 2010, pp. 31 52. [80] L. Xu, H. Song, B. Dong, Y. Wang, J. Chen, X. Bai, Preparation and Bifunctional Gas Sensing Properties of Porous In2O3-CeO2 Binary Oxide Nanotubes, Inorg. Chem. 49 (2010) 10590 10597. Available from: https://doi.org/10.1021/ic101602aPMid:20949903. [81] A. Klein, Electronic properties of In2O3 surfaces, Appl. Phys. Lett. 77 (2000) 2009 2015. Available from: https://doi.org/10.1063/1.1312199. [82] B. Mondal, B. Basumatari, J. Das, ZnO SnO2 based composite type gas sensor for selective hydrogen sensing, Sens. Actuator B 194 (2014) 389 396. Available from: https://doi.org/10.1016/j.snb.2013.12.093. [83] T. Anukunprasert, C. Saiwan, E. Traversa, The development of gas sensor for carbonmonoxide monitoring using nanostructure Nb-TiO2, Sci. Technol. Adv. Mater. 6 (2005) 359 363. [84] M. Zhang, Z. Yuan, J. Song, C. Zheng, Improvement and mechanism for the fastresponse of a Pt/TiO2 gas sensor, Sens. Actuators B 148 (2010) 87 92. [85] A. Ponzoni, C. Baratto, N. Cattabiani, M. Falasconi, V. Galstyan, E.N. Carmona, et al., Metal oxide gas sensors, a survey of selectivity issues addressed at the SENSOR lab, Brescia (Italy), Sens. (Basel, Switz.) 17 (2017) 714. Available from: https://doi.org/10.3390/s17040714. [86] N. Joshi, V. Saxena, A. Singh, S.P. Koiry, A.K. Debnath, M.M. Chehimi, et al., Flexible H2S sensor based on gold modified polycarbazole films, Sens. Actuators B Chem. 200 (2014) 227 234. Available from: https://doi.org/10.1016/j.snb.2014.04.041. [87] M.R. Mohammadi, D.J. Fray, Nanostructured TiO2 CeO2 mixed oxides by an aqueous sol gel process: Effect of Ce: Ti molar ratio on physical and sensing properties, Sens. Actuators B 150 (2010) 631 640. [88] M. Yuasa, T. Masaki, T. Kida, K. Shimanoe, N. Yamazoe, Nanosized PdO loaded SnO2 nanoparticles by reverse micelle method for highly sensitive CO gas sensor, Sens. Actuators B 136 (2009) 99 104. [89] G.F. Fine, L.M. Cavanagh, A. Afonja, R. Binions, Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring, Sensors 10 (2010) 5469 5502. [90] Q. Wan, C.L. Lin, X.B. Yu, T.H. Wang, Appl. Phys. Lett. 84 (2004) 124. [91] T. Gao, T.H. Wang, Synthesis and properties of multipod-shaped ZnO nanorods for gas-sensor applications, Appl. Phys. A 80 (2005) 1451 1454. [92] N. Joshi, T. Hayasaka, Y. Liu, H. Liu, O.N. Oliveira, L. Lin, A review on chemiresistive room temperature gas sensors based on metal oxide nanostructures, grapheme and 2D transition metal dichalcogenides, Microchim. Acta 185 (2018) 213. Available from: https://doi.org/10.1007/s00604-018-2750-5. [93] S.K. Tripathi, Metal Oxide Nanonostructured Films for photovoltaic Applications, Chapter 7, A.K. Srivastava, Book: Oxide Nanostructures, Growth, Microstructures and Properties (2014), https://doi.org/10.1201/b15633.

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[111] D. Zappa, E. Comini, R. Zamani, J. Arbiol, J.R. Morante, G. Sberveglieri, Preparation of copper oxide nanowire-based conductometric chemical sensors, Sens. Actuators B: Chem. 182 (2013) 7 15. [112] S. Park, S. Park, J. Jung, T. Hong, S. Lee, H.W. Kim, et al., H2S gas sensing properties of CuO-functionalized WO3 nanowires, Ceram. Int. 40 (2014) 11051 11056. [113] S. Vallejos, T. Stoycheva, P. Umek, C. Navio, R. Snyders, C. Bittencourt, et al., Au nanoparticle-functionalised WO3 nanoneedles and their application in high sensitivity gas sensor devices, Chem. Commun. 47 (2011) 565 567. [114] M.F. Yu, M.Z. Atashbar, Mechanical and Electrical Characterization of β-Ga2O3 Nanostructures for Sensing Applications, IEEE Sens. 5 (2005) 20. [115] A. Chaparadza, S.B. Rananavare, Room temperature Cl2 sensing using thick nanoporous films of Sb-doped SnO2, Nanotechnology 19 (2008) 245501. [116] M. Ramanathan, G. Skudlarek, H.H. Wang, S.B. Darling, Crossover behavior in the hydrogen sensing mechanism for palladium ultrathin films, Nanotechnology 21 (2010) 125501. [117] F. Favier, E.C. Walter, M.P. Zach, T. Benter, R.M. Penner, Hydrogen sensors and switches from electrodeposited palladium mesowire arrays, Science 293 (2001) 2227 2231.

CHAPTER

Conducting polymers as gas sensing material

3 Boyina Rupini

SOITS, Indira Gandhi National Open University, New Delhi, India

3.1 Introduction of conducting polymers and their role as gas sensing material A human nose can identify more than 400 varieties of aroma receptors, enabling us to smell trillions of various odors. Unfortunately, most of us are unable to recognize the different types of gases present in the atmosphere. This is where gas sensor works similar to our nose with more potential to identify and measure toxic gases in the atmosphere with different parameters in order to protect the biotic environment from unexpected threats. Usually, gas sensors are found in a device to detect gases like oxygen, carbon dioxide, nitrogen, methane, etc., and are used to detect leakage of toxic gases and harmful fumes and monitor the air quality in offices and industrial set up. A typical gas sensor is a device that not only detects the gaseous substance but also measures their concentration. The working principle is based on the gas concentration the sensor produces. Based on the concentration of various gases, the gas sensor will produce a signal as output voltage that corresponds to the potential difference by changing the resistance of the material of the sensor. On the basis of signal, the relevant gas concentration can be estimated. Basically, the gas sensing layer is a chemiresistor that changes its resistance value depending on the concentration of certain gas present in the environment (Fig. 3.1). Chemiresistive sensor interprets chemical information through the difference in electrical resistance [1]. The electrical resistance leads to electric signal that could be analyzed by simple methods. In the case of conducting polymer (CP), the working technique is hugely flexible by nanofabrication. In order to enhance the efficiency of these fabricated sensors, the configuration and polymer structure for transportation of charge and adsorption will be utilized [2]. In these sensors, an operational layer is accumulated over a range of electrodes to calculate the change in electrical resistance in the presence of target analytes that is a gas. In this mechanism, the basic charge carrier and the nature of gas contacting with the active layer generate the change in resistance at the time of exposure of gas [1,3]. The estimation of sensor performance is based on criteria like sensitivity, Carbon Nanomaterials and their Nanocomposite-Based Chemiresistive Gas Sensors. DOI: https://doi.org/10.1016/B978-0-12-822837-1.00006-X © 2023 Elsevier Inc. All rights reserved.

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Gas Molecules

Gas Sensor

Conducting Polymer film

FIGURE 3.1 Conducting polymer nanocomposite.

response time, operating temperature, recovery time, response, selectivity, and limit of detection [4,5]. So far, we understood broadly how the gas sensor works. Now we will discuss about how conductive polymers work as a gas sensors and their various applications. If we analyze commercially available gas sensors, they are usually metal oxides which works at high temperatures. The easy and effective alternatives to the metal oxide sensors are CPs. The active layers of CP sensors have ameliorated properties like easy synthesis, high sensitivity, good mechanical properties for facile fabrication, lower detection threshold value, reusability, and room temperature operation. The rapid development in polymer industry started when its conductivity was reported by research fraternity that fetched two Nobel prizes in polymer chemistry. The first one is Rudolph A. Marcus in 1992 for his research study on theory on electron transfer in redox processes in biopolymers science applications and the second is Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa in 2000 for development of intrinsic electroconductive polymers related to transpolyacetylene. Shirakawa, MacDiarmid, and Heeger manifested that transpolyacetylene shows conductivity of about 103 S/m or 1 S/cm after doping. The CPs in undoped phase are neither semiconductors nor electrical insulators. In order to become a gas sensor and to achieve highly conductive polymers, doping processes like redox doping or protonic acid doping is necessarily followed by removing electrons on the polymer backbone. Now we will discuss briefly and learn about the phenomenon of doping. The concept of doping is the unique and underlying feature which differentiates CPs from all other polymers. In doping process, an organic polymer, either

3.1 Introduction of conducting polymers

an insulator or semiconductor having a small conductivity, usually in the range 10210 to 101 S/cm, is converted to a polymer which is in the conducting regime of a metal (21 to 104 S/cm). The controlled addition of known, minute (#10%), nonstoichiometric quantities of chemical species results in changes in the electronic, electrical, magnetic, optical, and structural properties of the polymer. Doping is a reversible process to generate the original polymer with little or no degradation of the polymer backbone structure. Doping is a phenomenon involving deliberate introduction of impurities into an intrinsic semiconductor for the purpose of modification of its characteristic properties such as electrical, structural, and optical parameters. It is found that upon doping of trans-polyacetylene with Cl2, Br2, or iodine vapors, its conductivity enhances 109 times greater than nondoped trans-polyacetylene film. A polymer’s characteristic trait that causes it to become conductive is the presence of conjugated double bonds. A type of electrical structure known as conjugation involves the polymer chain having alternate single and double bonds. While in the undoped state, the polymer includes a conjugated backbone like trans-(CH)X, which will stay in an altered form after doping, or it may occasionally have a nonconjugated backbone, the doped polymer’s backbone is made up of a delocalized П electron system. CPs are classified based on the type of electric charge movement that depends on the chemical structure of polymer. They are redox polymers that possess redox potential within their structural groups based on reduction/oxidation capacity, and intrinsic electroconductive polymers in which polymers conjugated either with П
П or p
П systems. Now we will look into the basic theory of conductivity to understand how the conductive polymer works.

3.1.1 Theory of conductivity Any polymer must have conjugation in its structure to exhibit conductivity. The conjugation is made up of alternating single bonds with localized sigma bonds and less localized, weaker double bonds, despite the fact that their conductivity is rather low due to the electrons being locked in this strong sigma and directed bonds, which prevent electrons from moving when an electric field is applied and causes them to not exhibit a high conductivity. Having free electrons available is the only prerequisite for the concept of conductivity to exist. The band theory explains polymers’ weak conductivity.

3.1.2 Band theory This theory makes the assumption that the electrons flow inside distinct energy levels known as bands. Due to the good justification of atomic spectra, this can be well explained by the physical chemistry approach to correlate with quantum mechanics of atomic structures. The fundamental idea that atoms would adopt separate energy levels and that distinct atoms have distinct energy states is one that was established by quantum mechanics. The valence band is the highest

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occupied band, whereas the conduction band is the lowest unoccupied band. The allowed excitation of electrons from one energy level to another, as indicated in Fig. 3.2, leads to the emission lines of spectra. Atoms cannot be observed in isolation in a solid crystal due to their close proximity and chemical bonds. As a result, sharp energy states of each atom will manifest as energy bands due to the interchange of electrons with nearby atoms. According to the illustration above, 3s and 3p electrons in a single metallic atom will overlap with an equal probability across the crystal to generate the energy bands. The energy bands produced by a molecule’s bonding orbitals are known as valence bonds and are electrically inert. In contrast, the energy bands produced by an antibonding orbital that participates in electric conduction are known as conduction bands. The diameter of each independent band in the entire crystalline solid at all energy levels is known as bandwidth. The valence band corresponds to the highest occupied molecular orbital (HOMO) and the conduction band corresponds to the lowest unoccupied molecular orbital (LUMO). The energy gap between highest occupied and lowest unoccupied energy level is known as band gap This band gap corresponds to various energy levels that are not available to electrons and is also known as the fundamental energy gap or energy gap or forbidden gap. The magnitude of band gap counts on alternate single and double bonds and the extent of delocalization. The band gap provides the information about whether the nature of conductive polymer is metal or semiconductor or insulator (Fig. 3.3). The application of CPs as chemical sensors is based on electronic, optoelectronic, and electromechanical transduction mechanisms apart from others. The

3p

3p

3s

3s One atom

four atoms

3p

3s Close proximity of many atoms

FIGURE 3.2 Formation of bands in crystal solid. This figure explains that in a single metallic atom, 3s and 3p electrons with equal probability in the entire crystal overlap and form the energy bands. The energy band formed by the bonding orbitals of a molecule is known as valence bond, which is electrically inert and the energy band formed by the antibonding orbitals which partake in the electric conduction is known as conduction band.

3.1 Introduction of conducting polymers

Conduction Band Eg = 0 eV

Conduction Band Eg = 0.1-3.0 eV

Valence band

Valence Band

Semiconductor

Conduction Band

Eg > 3.0 eV Valence Band Insulator

FIGURE 3.3 The band gap of metal, semiconductor, and insulator. The band gap provides the information about whether the nature of conductive polymer is metal or semiconductor or insulator.

sensing property is based on the macroscopic and molecular structure of CPs because they are relatively open structures that allow access of gases into their core. Heeger, MacDiarmid and Shirakava [6,7] manifested that the molecular arrangement in CPs should contain alternative single and double bonds to permit the formation of delocalized electronic states. The resonance-stabilized structure of polymer is the driving force for the delocalization. The huge energy gap is formed because of limitation on the scope of delocalization and the bond alternation. Most CPs, such as polyaniline (PANI), polypyrrole (PPy) [8], and poly (phenylene sulfide-phenylene amine) [9], are p-type semiconductors, unstable in the undoped state. On the other hand, polythiophene (PTh) films are stable when undoped or very lightly doped. In order to maintain charge neutrality and increase the electrical conductivity, primary dopants like anions are added during the chemical or electrochemical polymerization process. The nature of the anionic dopant highly regulates the morphology of the polymer [10] and provides a specific binding site for interaction of the CP with the analyte gas [11]. Through such doping process, charge defects (polaron, bipolaron, and soliton) are established, which can proceed through the spine of the polymer, or through the conduction band. Arrangement of positive charge on the polymer skeleton requires the charge localization over a small section of the chain, resulting in formation of new electronic energy states within the bandgap. The localization of the charge across the chain and the lattice bond distortion causes the polymeric system to obtain elastic energy, reduces the ionization energy of the distorted chain and decreases its electron affinity, and converts the polymer into oxidizable form. The amalgamation of a charged site coupled to a free radical through local lattice distortion is called a polaron. It can take the form of either a radical cation or a radical anion. On further oxidation of the polaron, a dication is formed by the loss of an electron generated spinless defect that is known as a bipolaron. Both polaron and bipolaron

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defects are situated symmetrically within the bandgap. In the middle of the bandgap, there is one more energy level known as the Fermi energy level, which thermodynamically corresponds to the electrochemical potential of an electron. At a given temperature, it is the highest occupied energy level in the bandgap. The energy difference between the Fermi energy level and the vacuum level is known as the work function, which is denoted by work function (WF). The introduction of an electrically neutral gas into a conductive polymer in the form of “inert” secondary dopant, which, when applied to a primary-doped polymer, induces changes in its optical, electronic, and magnetic properties [12].

3.1.2.1 Gas sensing mechanism of conducting polymers So far, we have learnt that conductive polymers have conjugated chain [7,13,14]. In the neutral state, these polymers show weak conductivity. To enhance their conductivity or semi conductivity, they have to be subjected to either chemical or electrochemical doping process. The electrons in σ bonds of conjugated double bonds in conductive polymers provide required mechanical properties to the polymers, whereas the delocalized П electrons provide conducting and semiconducting properties [15]. Conductive polymers operate conducting mechanism by two methods: (i) intrachain transport method and (ii) interchain transport method. In interchain transport method, the electrons are localized and anisotropic in nature, where the transport of electrons occurs instantaneously along the conjugated backbone in one direction [2,16
18]. The performance of charge carrier is based on the nature of the polymer that consists of polarons, bipolaron, and soliton in which they bring in energy states among LUMO and HOMO [19]. These charge carriers will be generated by anionic chemical dopants and photons, and thereby the dopant charge is balanced by charge carrier that is administered to the adjacent monomers, causing minimum deformation of the polymer backbone. This mechanism can be introduced and physically converted through the polymer chain where doping promotes leaping to the adjacent chain [19,20]. Since the transport properties of CPs are susceptible to the chain length, they will depend on interchain charge transport to drift across a device [21
23]. In interchain charge transport, the mechanism operates through tunneling or leaping of the charge carrier which is thermally activated and controlled by the distance of interchain [16]. Along with these, the CP should possess tertiary structure, its conformation, and a polymer dopant. In general, the transportation mechanism is favored by interchain charge transport by leaping or hopping in the transverse direction, whereas intrachain transport maximizes in aligned parallel direction [24]. Gas sensing mechanism of CPs is mainly based on the doping and dedoping process. P-type CPs can be formed by doping with oxidizing agent that will introduce charge carriers into electronic structure of the polymer, whereas doping with reducing agent results in n-type CPs [19]. The working of p-type CP sensor begins with chemisorption of oxygen molecules from air on its surface resulting in removal of electrons from the conduction

3.1 Introduction of conducting polymers

band. These oxygen molecules are converted into ionosorbed single or double oxygen ion on the surface. The removal of electrons from the conduction band decreases the electron density and increases the concentration of hole, with reduction in resistance. For example, in p-type sensing material, in case of reducing gas like ammonia, when it reacts with ionosorbed single or double oxygen ions, electrons will be donated to the conduction band thereby reducing the hole concentration with increase in resistance. In n-type sensing material, in case of oxidizing gas like nitrogen dioxide, electrons are withdrawn from the valance band resulting in increase in hole concentration with decrease in resistance. The resistance and work function of the sensing materials are dependent on electron transportation process. Here the work function means the minimum energy required to remove an electron from majority to void level [25,26]. The electrochemical sensor devices are composed of a sensitive material to a specific analyte and a transducer that transforms the concentrations of an analyte into detectable signals like current or absorbance or mass. Many CPs are based on electrochemical techniques using either amperometric measurement in which current is measured at constant potential or potentiometric measurement in which current is measured during different potentials. In electrochemical sensors, the charge transport properties of CPs tend to get modified when exposed to an analyte, and the variance can be correlated quantitatively to the analyte concentration [27,28]. In both the cases, the optimum current, as the voltage is examined, is proportional to the concentration of the object. On the basis of the electrical transduction modes, conductive polymer-based electrochemical sensors are organized into the following ways.

3.1.2.2 Amperometric gas sensors This sensor works based on the principle to measure the current produced by the redox reaction of an analyte at a working electrode, where the current is subject to Faraday’s law of kinetics and a dynamic reaction, achieving steady-state conditions in the system under constant voltage on the chemically stable CP-modified electrode. The primary application of this sensor is for environmental monitoring and clinical analysis of electroactive species, in liquid or a gas phase [27].

3.1.2.3 Potentiometric sensors Potentiometric sensors are also known as ion-selective or ion-sensitive sensors (ISEs), which work on the potential difference between the cathodic and anodic potentials that are proportional to the logarithm of the gas analyte concentration, which can be estimated from Nernst equation (Eq. 3.1). E 5 Eo 1 RT=nFlnQ

(3.1)

where Eo is the standard electrode potential in volts, R is the universal gas constant (8.314472 J/K/mol), T is the absolute temperature in kelvin, F is Faraday’s

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constant (9.648 3 104 coulombs/mol), n is the number of electrons participating in the electrochemical reaction, and Q is the chemical activity of the gas analytes. The main application is to monitor the voltage released in specific electrochemical redox reaction for determination of the gas analyte concentration by measuring accumulation of a charge potential at the working electrode when zero or no current flow arises mainly from shifts in the “dopant” anion equilibrium within the polymer chain used as a sensing membrane [29]. According to Hyodo et al., CO, CO2, and H2 sensing properties of potentiometric gas sensor are achieved by using noble metals (Ag, Au, Ir, Ru, Rh, Pd, or Pt), loaded metal oxides (Bi2O3, CeO2, In2O3, SnO2, ZnO, or V2O5), or carbon black as sensing electrode materials and anion-CPs electrolyte to enhance the selectivity of chemical sensors [30]. The gas sensing mechanism is based on the overall potential emerging from the electrochemical reduction of oxygen and oxidation of CO, in the presence of wet synthetic air at 30 C. Among the sensors, Pt-loaded SnO2 exhibited the most excellent CO selectivity against H2. This sensor is based on thermodynamically precise signal for potentiometric sensors following Nernst’s law of thermodynamics.

3.1.2.4 Electrical device sensors Conductometric gas sensor is an electrical device-based measurement that measures the signal produced by the change of electrical properties of CP after interaction with the analyte, and without any electrochemical reactions [28]. This process causes changes in carrier density or mobility, resulting in a conductivity change (ρ) which is the reciprocal of resistivity (Eq. 3.3). ΔR 5 Ro
Rexposure =Ro

(3.2)

ρ 5 RA=L

(3.3)

where Ro is the resistance before exposure and R, A, and L are the resistance, sample area, and thickness, respectively. The conductivity of CPs increases with the donation of electrons by a p-type, and decreases by electron accepting. At the interface of electrode and conductive polymers, a space charge region is created, and the effective resistance greatly depends on the bias voltage applied during the measurement [31]. Conductometric sensor-based conductive polymers are classified into polymer-absorption sensors subgroup. These are also known as chemiresistors and common type of sensors that measure resistance difference of electrically active sensitive objects on interaction with a target gas analyte. These chemiresistor gas sensors use PANI thin films by different chemical techniques like spinning, evaporation for H2S, NOX, SO2, CO, and CH4, etc. [32]. Fabricated ammonia gas sensors with PANI/multiwalled carbon nanotubes (MWCNTs) [33] and S, N-doped graphene quantum dots (S, N: GQDs) by in situ chemical oxidative polymerization are flexible and display five times greater response than pure PANI due to the cavities provided by S, N: GQDs that promote high interaction cites for NH3 by the way of π-electron lattice. Another

3.2 Synthesis of conductive polymers and their importance

chemiresistor comprising PANI/MWCNTs works on the principle of physisorption or chemisorption of NH3 gas as the cooperation between the effect of acid-base doping/dedoping of PANI and the transfer of electron between NH3 and carbon nanotube/graphene quantum dots (CNT/GQDs). After complete adsorption of NH3 on PANI, it reacts with amine (N
H) groups of PANI and produces NH41 that leads to the localization of PANI polarons, thus increasing the sensor resistance. In addition to these electrical device sensors, optical devise sensors work on the basis of optical transductions in which the change in absorbance and luminescence result as an output of interaction between gas analyte and sensitive material. The recent interest on optical gas sensors for multi-analyte array-based gas sensing is on account of low cost, diminished optoelectronic light sources, and coherent detectors [34]. Fiber-optic sensors are a group of optical sensors which use optical fibers to identify gaseous analytes. Light is generated by a source of light and is sent through an optical fiber, then reflects the absorption property of the surface of CP when it returns through the optical fiber, and is finally captured by a photo detector [35]. A fiber-optic device based on PANI was used to identify HCl, NH3, hydrazine (H4N2), and dimethyl methyl phosphonate (DMMP), a nerve agent [36]. Muthusamy and coworkers developed gas sensors based on PPy and PPy/Prussian blue (PPy-PB) nanocomposite coating on fiber optic to observe NH3, acetone, and ethanol gases at room temperature [37]. Surface plasmon resonance (SPR) is another group of optical sensors that are referred to excitation of surface plasmonbased optical sensor for chemical sensing by using light. SPR optical sensor is a thin film refractometer sensing instrument that calculates the difference in refractive index at the surface of a plasmon-supported metal film. It is excited by the monochromatic light, resulting in variation of the refractive index of a dielectric material produced to a difference in propagation constant of the exterior plasmon [38]. The propagation constant of a radiation adjusts the light wave properties coupled to the surface plasmon [39]. After interacting with analytes, the minimum in the reflectance curve is shifted, suggesting the residence of analyte.

3.2 Synthesis of conductive polymers and their importance Polymers are classified into four main groups, such as conjugated CPs, charge transfer polymers, ionically CPs, and conductively filled polymers. Electrically CPs are the possible choice for various applications owing to their random-access memory such as corrosion, weight, matrix incongruity, ecological integrity, corrosion resistant, and light weight, in addition to customized for different applications. The achievement of Nobel Prize in Chemistry in 2000 to MacDiarmid, Shirakawa, and Heeger for the invention and development of CPs [14] got the significant attention in the scientific community of this field due to their exceptional features such as electrical characteristics, reversible doping-dedoping procedure, controllable chemical and electrochemical properties, and simple processibility.

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Synthesis of some of the CPs, e.g., polyacetylene (PA), PANI, PPy, poly(phenylene)s, poly(p-phenylene), poly(p-phenylenevinylene), poly(3,4-ethylenedioxythiophene) (PEDOT), polyfuran, and other PTh derivatives, have represented precise application in the domain of nanoscience and nanotechnology [40].

3.2.1 Synthetic preparation methods of conducting polymers 3.2.1.1 Chemical method Oxidation or reduction of monomers and subsequent polymerization of corresponding monomers are important synthetic chemical methods and are economically feasible. For example, poly (3-hexylthiophene) CP, PPy, and PANI can be prepared chemically; however, electrochemically prepared variants have frequently improved conductivity and mechanical properties. Chemical polymerization requires conjugation and stability. An oligomer precipitates out of solution and then the polymerization process should continue as a heterogeneous method, with a decrease in concentration of monomer and reactive polymer. In a suitably soluble system, the chemical polymerization allows for the choice of specific oxidant to selectively produce cation radicals at the appropriate position on the monomer.

3.2.1.2 Electrochemical method Electrochemical synthesis of CPs is very important among the other synthetic methods. CPs can be synthesized in a single section glass cell that are reproducible and can be fabricated in order to get required thickness and uniformity. Anodic oxidation of appropriate electroactive functional monomers is the most common electrochemical technique for synthesizing electrically conducting polymers (ECPs). In general, the potential of monomer oxidation directing to polymerization is greater than that of charging of polymer oligomeric intermediates. Basically electropolymerization of an electroactive monomer, like pyrrole or thiophene, is involved in substitute chemical and electrode reaction steps [41]. For example, in potentiodynamic electropolymerization of thiophene, a radical cation is formed in the beginning of electrode reaction step of electrooxidation, manifested by an anodic peak of high positive potential [42], and in the next step radical cation reacts with the monomer resulting in the generation of protonated dimer of a radical cation, subsequently undergoing electrooxidization to dication at the electrode reaction step.

3.2.1.3 Photochemical method This method is useful for the fabrication of some important CPs. For example, pyrrole is polymerized to PPy by visible light irradiation using either as the photosensitizer or a suitable electron acceptor. Polymerizations of aniline in the

3.2 Synthesis of conductive polymers and their importance

presence of hydrogen peroxide in oxidative free radical coupling reactions proceed through horseradish peroxidase, which is an environmentally benign method when compared to the chemical and electrochemical techniques.

3.2.1.4 Metathesis method Metathesis is a chemical reaction between two compounds, in which one part of each two compounds interchange to form two different compounds. There are three types of metathesis polymerization procedures: metathesis of ring opening of cycloolefins; metathesis of alkynes, acyclic or cyclic; and metathesis of diolefins.

3.2.1.5 Concentrated emulsion method This method follows heterophase polymerization in which three different phases are present. First is the water phase, second is the latex particle phase, and the third is the monomer droplet phase following a radical polymerization mechanism. This method works with one phase also with bulk solution polymerization. In this, both monomer and the initiator are present in the same bulk solution phase. In this method, micelle form of surfactant with water soluble initiator and water insoluble monomer are in the same phase. After polymerization reaction, the obtained polymer remains in the same phase till high modification made [43].

3.2.1.6 Inclusion method In this method, complex polymers are synthesized at the atomic or molecular level. Hence, it is a unique synthetic path to prepare potentially lowdimensionality composite materials.

3.2.1.7 Solid-state method In solid-state polymerization, the length of polymer chain can be enhanced on heating in the absence of oxygen and water. This can be performed either by vacuum or by eliminating an inert gas to avoid the reaction by-products. This method is mainly used in industrial manufacturing of bottle-grade poly ethylene terephthalate (PET), films, and advanced industrial fibers.

3.2.1.8 Plasma polymerization It is a new and innovative method to customize thin films from organic and organometallic starting materials. Plasma polymerized films are perforation-free and highly cross linked; hence, they are insoluble, thermally stable, chemically inert, and mechanically robust. These films are highly coherent and adherent to a diverse substrate containing conventional polymer, glass, and metal surfaces [44]. Because of these outstanding properties, they are extensively used in perm selective membranes, protective shells, biomedical materials, electronic, optical devices, and adhesion supporters.

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3.2.1.9 Pyrolysis method Pyrolysis is a process wherein organic materials are subjected to heating at elevated temperatures for chemical disintegration. This process provides accurate results for the investigation and detection of organic polymeric substances in the plastic and rubber production, dentistry, ecological shelter, and in the failure testing. Synthetic and natural polymers analysis is measured by pyrolysis gas chromatography [45]. The polymerization process proceeds with the reaction between conjugated monomers and excess quantity of an oxidant in suitable solvent, mostly an acid with constant stirring followed by electrochemical polymerization resulting in the polymer thin film, which instantly begins to deposit on the working electrode.

3.2.2 Some examples 3.2.2.1 Polyacetylene In 1970, PA, an organic polymer with a formula of (C2H2) n, was accidentally fabricated by Shirakawa and was also the first polymer to conduct electricity [46]. PA is one of the polymers in the study that resulted in the Chemistry Nobel prize in 2000. In 1976, Alan MacDiarmid et al. reported that conductivity of PA was amplified by six orders of magnitude when mixed with iodine as the result of charge carriers. PA comprises the heart of all conjugated CPs, which show superconductivity at ambient temperature when accurately substituted [47]. Polydiacetylenes are prepared by the topochemical polymerization reaction of the ethyne (as commercially known as acetylene) with Ziegler-Natta catalyst.

3.2.3 Polyaniline Polyaniline, symbolized as PANI, is one of the promising conductive polymers of the semi-flexible rod polymer class with aromatic rings bonded together by nitrogen atoms. The structure comprises x units of reduced species with 1
x alternative units of oxidized species. PANI was first explained in 19th century by Henry Lethe about the electrochemical and chemical oxidation products of aniline in the presence of acid [48]. Doping causes oxidation of PANI through the protonation of the nitrogen atoms in the amine groups in the presence of an acid. The positive charges on the polyaniline structure are stabilized by resonance. Lee et al. reported the effect of the electrode material on the electrical-switching type of a nonvolatile resistive memory apparatus based on poly (o-anthranilic acid) thin film responsible for the switching characteristics of the active polymer layer [49]. Athawale et al. reported that PANI and its substituted derivatives act as sensors for gaseous methanol, ethanol, propanol, butanol, and heptanol [50]. Misra et al. synthesized high-quality doped PANI thin films by using vacuum deposition technique for detection of CO [51]. Crawley reported the fabrication and functioning

3.3 Need of functionalization of conducting polymers

of a PANI/CuCl2 as a H2S sensor [52]. Banerjee reported the fabrication of PANI nanofiber reinforced nanocomposite crystal microbalance sensor as hydrogen chloride sensor [53].

3.2.3.1 Polypyrrole Polypyrrole is symbolized as PPy and is an organic polymer synthesized by polymerization of pyrrole using oxidation method and was discovered as CP in 1968. PPy is also one of the CPs extensively studied because of its simple preparation methods, superior redox properties [54], stabilized oxidized form, capacity to provide excellent conductivity [55], solubility in water, commercially availability, and profitable electrical and optical properties. Pyrrole blacks have been known for a century when they were initially obtained as powders by chemical polymerization of pyrrole. These mysterious polymers have not been characterized in huge detail though they are known to be polymers of pyrrole where the bonding is mainly via α, α0 carbons. Dall’Olio et al. synthesized PPy by oxidation of pyrrole in sulfuric acid as a black powder with room temperature conductivity of 8 S/cm [56]. The conductivity of these films, when cycled electrochemically, varies from 100 to 200 S/cm between a conducting state and an insulating state [57]. It is obtained by the oxidization of the pyrrole, resulting in the structure shown below.

3.3 Need of functionalization of conducting polymers with carbon materials In the past few decades, significant research efforts have been made in room temperature CP-based gas sensors. Despite their relatively low conductivity and more affinity with regard to volatile organic compounds and water vapor, CPs show lesser sensitivity, low stability, and gas selectivity, which hamper their gas sensing properties. To overcome some of these properties, there is a need to fabricate by functionalizing the CPs with carbon nanomaterials. Carbon nanomaterials have several sensing applications and have become the influential material in the nanosensing research [58]. Carbon exhibits sp2 hybridization and has the ability to show different bonding orientations, and because of this unique character it exists mainly in two different allotropic forms: carbon and diamond. Various carbon nanomaterials like graphene single sheets, mono and MWCNTs, nano onions, carbon fibers, carbon black, fullerenes, and nanodiamonds are leading materials in gas sensing applications [59]. Research has progressed to disseminate conduction in polymer skeleton by combining insulating polymers with conductive carbon materials mentioned above and conductive polymers itself [60]. This blending leads to conductive polymer composite with the resistivity between metallic conductor and insulating material [61,62]. These carbon materials like carbon black have a greater tendency to form conductive network because of their chain like cluster structure, whereas carbon fibers have chain like cluster of carbon particles with extended chain length.

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There are two types of CNTs have important sensing applications: single-walled CNTs (SWCNTs) and MWCNTs. SWCNTs have hexagonal honeycomb structure with sp2 hybridization with hollow tube morphology, whereas MWCNTs are multiple concentric tubes surrounded by one another [63]. One of the approaches to enhance conductivity is that increase in the concentration of CNT/loading leads to a good conducting state at a particular threshold volume fraction of the conductive filler. The volume fraction of CNT is also known as the percolation threshold since establishing the continuous conductive path is essential inside the polymer matrix [64]. In these nanostructures, the CNT will be utilized as an electrode and with its strong electrocatalytic activity have the ability to allow the electron transfer reactions [65,66]. CNTs have a tendency to agglomerate with each other to form bundles because of the Van der Waals forces and are difficult to disperse in polymer matrix. To overcome this problem, the structure of the CNTs has to be modified chemically at the side wall. In addition, this improves the surface of nanotubes by chemical functionalization that highly enhances the solubility properties, which can improve the process of fabrication of CNT sensors with ease [67]. The main use of chemical functionalization is to augment the nanotube
polymer interface, and increasing interfacial bonding between SWCNTs and the polymer improves the load transfer mechanism, thereby improving the macroand microscopic mechanical properties of the nanocomposite system [68]. Various chemical functionalization reactions involve chemical [69], electrochemical [70], mechano-chemical [71], and plasma treatment [72]. The functionalization of CNTs may be treated to functionalize their surfaces and side chain. In chemical functionalization, treatment with strong acids eliminates the end caps and compresses the length of the CNTs, and it also adds oxide groups, primarily carboxylic acids, carbonyl, and hydroxyl groups to the nanotube ends and defect sites of CNTs [73]. This will facilitate more chemical reactions on the oxide groups to functionalize with groups such as amides, thiols, or other groups [74
76]. Balasubramanian et al. used thermally activated chemistry, electrochemical modification, and photochemical approaches for covalent functionalization of SWCNTs [77]. There are two types of functionalization of CNT methods: covalent and noncovalent. In covalent functionalization method, addition of required functional groups directly or to the sidewalls of CNTs or surface bound functional group takes place after required alteration on the end of the CNT [78,79]. Most common method is by oxidation process resulting in the incorporation of carboxyl group that is helpful for further functional group derivatization [80
117]. Noncovalent functionalization of CNTs is obtained by interactions like π
π, CH
π, or NH
π between the CNT surface and the dispersants which are operated by enthalpy or interactions operated by entropy like hydrophobic interaction using surfactants [118]. In the case of the surfactant dispersion, sodium dodecyl sulfate [119], sodium dodecyl benzene sulfonate, sodium cholate (SC) [120], and cethyltrimethylammonium bromide [121] have been used. These functionalized CNTs are highly proactive in chemiresistive sensors. It is one of the potential transduction units, with characteristic features like simple to

3.3 Need of functionalization of conducting polymers

use, rapid response, and inexpensive in obtaining. On account of these features, many of other commercialized gas sensors are constructed into chemiresistive sensors [1]. As we have discussed in the sensing mechanism, a chemiresistive sensor translates chemical information along the alternating in two-point contact electrical resistance [122]. Electrical resistance is a simple electrical signal, which is to be analyzed. In case of CP, this working mode is hugely adjustable by converting into nanostructures. These derived sensors make use of the composition and structure-dependent charge transportation and adsorption, in order to enhance the gas sensing performance [123]. Hypothetically, the carbon atoms in a carbon nanotube are present on the surface, which facilitate them ideally as chemical sensors. However, it is necessary requirement for the analyte that the gas molecule should have either the electron accepting or donating ability, in case of NH3 as a donor and NO2 as an acceptor. Some more examples like the presence of O2, NH3, and NO2 gases and many other gaseous molecules can either donate or accept electrons, leading to modification of the inclusive conductivity [124,125]. The reason behind this behavior is that the adsorption of gaseous molecules on the nanotubes is analogous to the partial charge transfer, which modifies the adsorption pattern thereby changing the concentration of charge carrier, and resulting in change in the electrical resistance of each nanotube, which is harnessed as a sensor signal. Although for some gaseous analytes like CO and H the adsorption is poor, the change in resistance is also very less. To surpass this disadvantage, a possible route is functionalization of the side wall of CNT with a conductive polymer. The fact is that the basic gas sensing principle is based on the adsorption and desorption of gas molecules on sensing materials. By enhancing the contact junction between the gas analytes and sensing materials, the susceptibility can be significantly increased. Advanced research in nanotechnology has generated high opportunity to manufacture portable sensors with less expense, low power consumption, and high sensitivity. Therefore, for perfect adsorption and storage of gas molecules, the nanomaterial should possess extremely high surface-to-volume ratio and hollow structure. Hence, conductive polymers based on nanomaterials are potential gas sensors, such as CNTs, nanofibers, nanowires, and nanoparticles, that have been investigated [126
128]. Fig. 3.4 shows chemically conductive polymer-functionalized MWCNTs, which contain COOH groups attached along the sidewall of the MWCNTs and the ethanol vapor detection using MWCNTs
OOH sensors [129,130]. Organic CPs including PPy [131], PANI [132], PTh [133], PEDOT [134], and PA [135] are examples of materials for fabricating gas sensors that we have already discussed. It is observed that some CPs can operate like semiconductors because of the nature of heterocyclic compounds, which exhibit physicochemical features that influence the reversible alterations on the sensing layer; hence, conductivity can be noticed at room temperature at the time of adsorption of polar chemicals on the surfaces [136]. This impact is caused by the charge transfer between the polymer film and gas molecules [137]. The volumetric changes are also possible in

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δ+

δ+

δ-

δ-

Hydrogen bonding

δ+ δ-

FIGURE 3.4 Chemically conductive polymer-functionalized MWCNTs with
COOH in the detection of EtOH.

the polymer matrix based on the adsorption of concerned analytes, which gives rise to required change in percolation-type conductivity throughout a particular framework of the material, which is known as “percolation threshold” which is the structure of conducting particle. The thread like structured unidimensional CNTs exhibit good conductivity which are perfect for gas sensing system since insulating matrix composition has the dispersed particles as conducting particles. These flexible fabrication methods and their features like adjustable conductivity, excellent mechanical properties, and high environmental stability [138
143] of CNTs/conductive polymer as gas sensors have attracted the research fraternity globally [144
147].

3.4 Applications Conductivity, electronic, magnetic, wetting, mechanical, optical, and microwaveabsorbing properties have drawn attention of CP in various applications. The CPs due to their resonance stabilized conductivity properties act as a metal. Given their relatively inert and good mechanical properties, they are known to have many applications in electronics, such as batteries, sensors, and microelectronics devices. PPy and PANI are extensively used in protection of metals, as an anticorrosive coating, and in the medical field, they are used in the manufacturing of

3.4 Applications

artificial muscles, biosensors, and drugs controlled-release agents. Some important applications of CPs are discussed as follows. 1. Supercapacitors (SC): Electrodes in SCs are made of transition metal oxides, high surface carbons, and CPs. Super conductors of CPs are potential alternatives for conventional SCs due to their high capacitive energy density and affordable material cost. In addition, their remarkable electrical conductivity, potential pseudo-capacitance, and expeditious rate of doping/dedoping along the charge or discharge procedure make them significant materials. CPs have short span of life cycle and moderate ion transport system. Hence, electrodes based on CNTs coated with CPs have been used for SCs [148]. The substantial surface area and high conductivity of CNT enhance the redox property of CPs, and the structure and capacitance of the nanocomposite facilitate for easy synthesis. When compared to other class of polymers such as gel polymer electrolytes, the proton conducting membranes in CP-based SCs, their intensely hydrated form has higher mechanical strength, elevated ionic conductivity, and excellent dimensional stability. 2. Light emitting diodes: Ultrathin film of CP employed as a hole injection layer for organic optoelectronic apparatus like organic light emitting diode successfully makes holes from anode into the instrument [149]. All polymer light emitting diodes (PLEDs), producing over the entire visible wavelength region from blue to red, have been well documented [150], and the light emitting diode (LED) performance was significantly improved by means of additional charge-transport layers [151]. Besides PANI, PEDOT, mostly due to their exceptional electrical conductivity and optoelectronic properties, are initiated into organic light emitting diodes as hole injection layer. 3. Solar cells: Polymer solar cells (PSCs) have been developed as a potential and commercial alternative to solar cells based on silicon. Preference of PSCs over others is due to their simple processing, highly economical production, mechanical flexibility, and versatility of chemical structure. At present, various research studies have been reported on thin and flexible devices using a plastic film substrate over a brittle glass substrate. The synthesis is based on the application of transparent anode through organicbased materials to obtain fully plastic PSCs. Polydimethylsiloxane-based patternable solution of flexible PSCs using [152] PSCs is used as one of the renewable sources of electrical energy because of their low-cost production and simple processing on flexible substrates. The working efficiency of PSCs has been enhanced exponentially by using bulk heterojunction model as an active layer in which electron donor and acceptor materials are uniformly combined in a solution and fabricate a thin film sandwiched between two electrodes. The power conversion efficiency of PSCs has been reported to be more than 9% for single cells, 10% for tandem cells, and 11.0% for the PSC based on P3HT.

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4. Transistors and data storage: CPs have established applications in electronics as field effect transistors and for charge storage because of their outstanding properties. In order to get high-sensitivity character of CPs, they are operated as field effect transistors due to their ability to magnify in situ and to gate modulate channel conductance. Majumdara and coworkers have synthesized and characterized sandwich-type apparatus based on an oriented PTh derivative [153]. They displayed conductance switching by calculating capacitance of the apparatus and have explored the high and low conducting states of it. Hiraoka et al. studied the characteristics of polymer lightemitting transistors with Ag-nanowire source/drain electrodes manufactured on polymer substrate [154]. The maximum external quantum efficiency is 0.6%. They also demonstrated the possibility of producing flexible polymer light-emitting transistors using Ag nano wire electrodes. CPs have multiple technological applications in electronics and optoelectronics tools like energy storage applications as solid-state rechargeable batteries [155]. SnO2 nanoparticles evenly decorate PPy nanowires which are produced by electrochemical reaction method and show porous reticular morphology and homogenous allocations [156]. In lithium-ion batteries as anode materials, the distinct nanostructured hybrids have shown improved Li1 storage performance. 5. Sensors: This application includes the CPs as an electrode modification in order to enhance sensitivity, to impart selectivity, to suppress interference, and to give a support matrix for sensing materials. Some sensors employing CPs are as follows: Gas sensors: Gaseous pollutants like SOx, NOx, and toxic gases from different industries has become a serious concern of the general environment. Sensors are mandated to recognize and evaluate the concentration of these gaseous pollutants. As we studied, PANI, PPY, and PTh have generally been used in fabricated CPs of gas sensor apparatus. The fabrication, characterization, and multi frequency calculations of poly N-(2-pyridyl) pyrrole for sensing applications [157], and the synthesis, characterization, and subsequent assessment of PEDOT doped with poly (styrene sulfonic acid) coated SWCNT sensors for sensing analytes in industries are [158] well documented. Electrical characterization in terms of change in resistance, cyclic voltammetry, and field effect transistor (FET) measurements was performed to confirm the presence of PEDOT: poly styrene sulfonate (PSS) coating on SWCNTs. PEDOT nanowires were developed by wetting Al2O3 membrane template technique in order to find highly arranged structure of nanowires and the self-assembly film of nanowires at air/water boundary [159]. The results demonstrated that PEDOT nanowire surfactant complex at air/water interface had fine selfassembly capacity and the stable float sheet was produced with collapse pressure .50 mN/m. Waghuley et al. studied the synthesis of PPy and its application as a CO2 gas sensor [160]. The resistance increase of the

3.4 Applications

material in the presence of CO2 gas is due to the orbital overlap of neighboring molecules of the PPy structure, the π electrons delocalize along the complete chain, which offers semiconducting and conducting properties as CO2 molecules formed weak bonds with π electrons of PPy. PANI has extended -electron delocalization, which is responsible for fluorescence emission; hence, it is a fluorescent conjugated polymer. The photoluminescence of PANI emeraldine salt was used as selective fluorescent sensors for the detection of electron deficient nitroaromatics like PA, trinitrotoluene and 2,4-dinitrotoluene. Biosensors: CPs are used in chemical analysis for the detection of ions and molecules in the liquid phase at large scale. The utility of biosensors has been a significant area over the last two decades. Biosensors are also used in food investigation, environmental control, clinical detection, medicinal and farming industries, etc. Films synthesized by electrochemical codeposition of enzymes on CP or conductive substrates have been used to produce biosensors [161]. Humidity sensors (HSs): HSs have the potential for the detection of the relative humidity in different environments with respect to electrical, optical, and other physical parameters. In medical and industrial field, the calculation and control of humidity are crucial in several regions such as food and electronic industry, domestic atmosphere, pharmacy, medical treatment methods, etc. Hydrophilic properties of polymer, polymer composites, and modified polymers have been used in HS devices. 6. Corrosion protection: PANI and PPy coated with CP have been used in corrosion protection of ferrous and nonferrous metals. PANI-TiO2 composite by chemical polymerization method in the existence of aniline and TiO2 by ammonium persulfate (APS) oxidant has corrosion-resistant properties of CPs. Organic phosphorous acid doped PANI with acrylic binder on the corrosion protection of steel in 3.5% NaCl solution is used as cathode substance in rechargeable battery [162]. 7. Batteries: The commercial impact of CPs application are in batteries. Batteries have various parts: electrodes allow for compilation of current and diffusion of power; the cathode material get reduced as the anode material is oxidized and vice versa; the electrolyte offers a physical partition among the cathode and the anode and supplies a source of cations and anions to balance the redox reactions. Polymer battery arrangement based on PPy doped with dopants of p-toluene sulfonic acid and indigo carmine has significant work capacity. Unique single ion CP electrolyte, lithium polyvinyl alcohol oxalate borate, from the reaction of poly (vinyl alcohol) with different molar ratio of boric acid, oxalic acid, and lithium carbonate has been used, and its electrochemical window can be stable till 7 V which is of huge importance for high voltage lithium-ion batteries with high energy density. 8. Electrochromic device: Electrochromic devices are of significant industrial importance due to their convenient transmission, absorption, and/or

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reflectance. Electrochromic response of PPy/PB and PANI/PB composite films in various electrolytes, for example KCl, LiClO4, K2SO4, KNO3, etc., by putting the PB films on zenith of the conducting PPy and PANI films was synthesized by electrochemical techniques. A recent method for the electrochemical description of an apparatus comprised of two electroactive films of CPs, for example electrochromic windows, artificial muscles, polymeric batteries, or SCs is used [163]. Another device is also widely used in which solid-state electrochromic devices via polymeric ionic liquids as solid electrolytes and electrodes are formulated by vapor phase polymerization of EDOT [164]. 9. Radar application: Radio Detection and Ranging (RADAR) is basically an object detection method that utilizes radio waves to find the range, altitude, path, or speed of objects. It is used to identify aircraft, ships, spacecrafts, motor vehicles, weather formations, and topography. 10. Actuators: An actuator is a device that is used for controlling a system and is operated by a source of energy, usually electric current, hydraulic fluid pressure, and changes that energy into action. This actuator provides knowledge and information of its own location and hence can also be used as a position sensor. Temperature dependence of water vapor absorption and electro-active polymer actuating behavior of free-standing films made of PEDOT doped with poly (4-styrenesulfonate) was investigated. The bending actuators fabricated using the conventional double-layer beam bending theory executed by ignoring the thickness of the thin intermediate metal coatings for the sake of simplification are some notable actuators [165].

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1648. [163] J. Padilla, V. Seshadri, T.F. Otero, G.A. Sotzing, Electrochemical study of dual conjugated polymer electrochromic devices, J. Electroanal. Chem. 609 (2007) 75
84. [164] A.S. Shaplov, D.O. Ponkratov, P.H. Aubert, E.I. Lozinskaya, C. Plesse, A. Maziz, et al., Truly solid-state electrochromic devices constructed from polymeric ionic liquids as solid electrolytes and electrodes formulated by vapour phase polymerization of 3,4-ethylenedioxythiophene, Polymer 12 (2014) 1
12. [165] B. Gaihre, S. Ashraf, G.M. Spinks, P.C. Innis, G.G. Wallace, Comparative displacement study of bilayer actuators comprising of conducting polymers, fabricated from polypyrrole, poly(3,4-ethylenedioxythiophene) or poly(3,4-propylenedioxythiophene), Sens. Actuators A 193 (2013) 48
53.

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Part II Application of carbon nanomaterials in gas sensing

CHAPTER

Carbon nanomaterial-based chemiresistive sensors

4

Sandeep Kumar1, Arshdeep Singh1 and Anil Kumar Astakala2 1

Department of Physics, DAV University, Jalandhar, Punjab, India Department of Humanities and Basic Sciences, Godavari Institute of Engineering and Technology, Rajamahendravaram, Andhra Pradesh, India

2

4.1 Introduction to sensor and its types The industrial and technological revolution has proved to be very fruitful for human society. But this advancement has come at the expense of depletion of various natural resources. The quality of air and water has been a major concern in almost every part of the globe. Indoor and outdoor air quality has been affected by various unwanted and harmful gases and chemicals. For example, emissions from multiple industries and automobile exhausts are considered significant air pollutants. Further, certain gases and vapors accumulated inside building and office premises can cause sick building syndrome [1]. The detection of these toxic and combustible chemical and biological substances in various fields such as public security, medical diagnosis, agriculture, industrial mining, civil aviation, automotive, food quality monitoring, wine, and other alcohol quality monitoring is of great concern [2]. The development of small-sized, portable gas sensors is of much interest as they can detect these gas analytes at minimal concentration. Monitoring biomolecules in disease diagnosis and detection of explosives and nerve gases in defense and military are some major examples that intensify the applications of sensors [3
5]. In a simple context, sensors are devices that can provide a noticeable signal to an external stimulus such as a rise in temperature, the presence of chemical gases or vapors [1]. The change in these characteristics may cause a change in one or more properties, such as the conductivity of the sensing material. Despite the place and application, a sensor must meet some basic requirements such as ease of use, high sensitivity, high selectivity, good stability, low power consumption, low cost, and a long life [5]. The overall performance of sensors can be measured by their lower limit of detection (LOD). The detection of minimal amount of analytes in a mixture of several other gases can govern the lowest LOD of sensors [6]. The ability of a sensor to distinguish between several analytes is termed its selectivity [7]. Apart from these, response recovery times, power consumption, repeatability, stability, and drift are well-known characteristics considered for sensor performance [8]. Response time is the time taken by the sensor to generate a warning signal when the gas Carbon Nanomaterials and their Nanocomposite-Based Chemiresistive Gas Sensors. DOI: https://doi.org/10.1016/B978-0-12-822837-1.00001-0 © 2023 Elsevier Inc. All rights reserved.

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concentration reaches a specific value [9]. Time taken by the sensor to decline to 10% of its baseline after the analytes are removed is called recovery time. The amount of power consumed by a sensor during its operation is called power consumption. Repeatability and stability influence the lifetime and working of the sensor. Stability is the sensor’s capability to produce the same response signal over multiple runs for an identical stimulus. Drift occurs if the response of a sensor changes for fixed surroundings. Apart from these, Fennel et al. [10] have discussed some other parameters such as hysteresis, deadtime, risetime, and dynamic range. The above characteristics of sensors are greatly influenced by a large number of constraints as listed: 1. The material used for gas sensing element: the sensor’s response, selectivity, and stability are determined by the type of material employed for the fabrication of sensors [8]. 2. Doping concentration or composition of sensing elements: the concentration of dopants should be maintained below the solubility limit for improving sensing characteristics, or else it can result in the undesirable drift in the sensing response [7]. 3. Relative environmental humidity: the sensor’s stability can be affected by relative humidity in its surrounding environment. The presence of humidity results in the hindrance of oxygen ions by surface adsorbed OH2 ions thus affecting the sensor’s performance. At higher operating temperatures, humidity is reduced because the thermal energy nullifies the actions of surface adsorbed OH2 ions [11]. 4. Operating temperature: the sensor’s operating temperature also significantly impacts its performance. The sensor’s response is greatly enhanced at higher temperatures [12]. Moreover, the selectivity and stability at elevated temperatures are governed by alterations in structural properties such as phase separation, enhanced grain growth and grain boundaries, contact degradation, weaker adhesive properties at the surface, etc. [13]. One primary concern regarding operation at elevated operating temperatures is the selectivity issue due to interaction between gas molecules [9]. Thus, the operating temperature must be adjusted in such a way that different target gases can be detected simultaneously. At lower and higher operating temperatures, the restriction in the reaction rate and the rate of diffusion of target analytes are reported. The temperature at which both these rates become equal is considered the optimal operating temperature of the sensor [14]. Gas sensors are employed to sense the presence of different gases or chemicals during leaks, mining, confined space monitoring, etc. These sensors detect selective/multiple gases and chemicals in a particular environment. As a huge variety of sensors are used to detect many gas analytes in a chemical environment, it is difficult to classify different sensors based on single, convenient distinguishing property. Based on the mechanism [15], the classification of various gas sensors is presented in Fig. 4.1.

4.2 Importance of chemiresistive gas sensors

FIGURE 4.1 Different kinds of gas sensors based on their sensing mechanism.

The various sensors in Fig. 4.1 employ different sensing mechanisms to detect various analytes. Owing to their significant characteristics like high sensitivity, selectivity, simplicity in fabrication, compactness, high precision measurement, less power consuming nature, and low operating temperature, chemiresistive type sensors are one of the most widely used sensors in day-to-day life [6,16]. Furthermore, these sensors work on the principle that slight variations in the chemical environment of the sensing element may cause a change in its electrical resistance, which is further governed by the sensing material type and nature of the target gas [17]. The choice of the material for the sensing element is the crucial step in fabricating a sensor. Owing to their tunable transport properties, various materials such as ZnO [18], MgO [19], CuO [20], NiO [21], CdO [22], TiO2 [23], SnO2 [24], CeO2 [25], ZrO2 [26], WO3 [27], MoO3 [28], Fe2O3 [29], Ga2O3 [30], Co3O4 [31], and V2O5 [32], have been used as sensing elements. Apart from these materials, carbon nanotubes (CNTs) have gained much interest due to their high sensitivity, room temperature operation, compact size and excellent electronic and mechanical properties [5].

4.2 Importance of chemiresistive gas sensors Chemiresistive gas sensors consist of two or more electrodes across which an active layer is deposited. In the presence of a gas, this arrangement measures the electrical resistance variations of the sensing material. The deposition of the active sensing

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layer on the substrate by different methods such as physical vapor deposition, chemical vapor deposition, sol-gel method, hydrothermal, electrospinning, etc. with the thickness range 500 nm
30 μm was reported [33]. A good ohmic contact was required in order to notice any change in resistance with the exposure of analytes. The sensors having an electrical circuit were then placed inside a chamber filled with target gas and the resulting current was recorded by using a source meter [24]. The chemiresistive gas sensors can be classified into semiconductor metal oxide (SMO) gas sensors and conductive polymer gas sensors (Fig. 4.1). These sensors can form arrays consisting of a large number of sensing elements exhibiting different sensitivity and selectivity. In another arrangement, several microsensors get incorporated into a single small substrate, thus developing into a miniature sensor elements array [34], thereby reducing the size and power consumption of the sensing device.

4.2.1 Semiconductor metal-oxide gas sensors SMO gas sensors are generally of two types: p-type and n-type. With increased temperature, various oxygen species such as O22, O2 are chemisorbed on the metal oxide surface resulting in the generation of electron depletion regions and hole accumulation layers in n-type and p-type SMOs. Further, the surface reactions occur between chemisorbed oxygen species and target gas molecules, changing the electron depletion region or hole accumulation layer thickness, thus altering the sensor’s resistance [17]. Various properties like surface area, porosity, donor density, agglomeration, the presence of catalysts, acid-base property of the sensing material, and the sensing temperature affect the gas sensing characteristics such as high sensitivity, high selectivity, quick gas response, and high responding speed [35]. Sakai et al. [36] found that the sensor’s performance is greatly influenced by the porous structure of the sensing film, which further affects the rate of diffusion of gases. Furthermore, the response and selectivity of the sensors can be influenced by particle size as the grain boundaries can act as electron scattering centers [37]. Despite these advantages, certain drawbacks like elevated operating temperature, high power consumption, poor precision, humidity sensitiveness, and limited sensor coatings limit their operation [9]. The small size of monodisperse metal oxides nanoparticles is unstable which may easily form large clusters under heating conditions [38]. Sun [16] suggested that 1-dimensional (1D) or quasi-1D metal oxide nanostructures, nanoporous structures, nanoparticles, and CNTs have also been developed for gas-sensing applications. 1D or quasi-1D metal oxide nanostructures (such as nanowires [30], nanotubes [39], nanofibers [23], nanobelts [40], and nanorods [41]) have several advantages such as well-defined chemical composition, high surface-tovolume ratio, surface terminations, free from dislocation, and other extended defects, superior stability owing to their high crystallinity [42]. Further, the high specific surface area of 1D structures increases the surface adsorption of the analytes essential for high performance of the gas sensors [43]. The common methods utilized for the synthesis of 1D metal oxide semiconductor sensors are electrospinning [44], electrochemical deposition [45], hydrothermal [46], and chemical vapor deposition [47].

4.3 Fabrication of carbon nanomaterials-based sensors

4.2.2 Conductive-polymer gas sensors A conductive-polymer gas sensor incorporates intrinsically conductive polymers as the sensing active layer and is widely used for chemical vapor sensing. As soon as the sensing element starts interacting with chemical vapors (after exposure), variation in conductivity of conductive polymers occurs due to transfer of electrons to or from the analytes [34]. Since 1980s, the active layers of gas sensors have been developed from a variety of conducting polymers such as polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), and their derivatives [48]. The sensors made from conducting polymers show improved characteristics over metal oxide-based sensors in terms of high sensitivities and short response time, operation at an ambient temperature, and low power consumption [34,49]. Further, conducting polymers offer an advantage over small molecules since these are sensitive to minor perturbations due to the amplification by a total system response [50]. The molecular chain structures of conducting polymers can be modified by copolymerization or structural derivations. Various fabrication techniques have been developed to prepare polymer filmbased sensing materials along with different configurations. These techniques include electrochemical deposition [51], dip-coating [52], spin-coating [49], vapor deposition polymerization [53], drop-coating [54], Langmuir-Blodgett technique [55], layer-bylayer self-assembly technique [56], etc. One major disadvantage when forming a sensor array of conductive polymers is the difficulty of generating various sensors [57]. Conductive polymer composite sensors fabricated by coating or encapsulating a mix of conductive and nonconductive materials on an electrode surface can be a good substitute for eliminating this limitation [57]. In the early and late vapor-diffusion stages, the nonconductive polymer can absorb and desorb the target and the conductive materials can provide electrical conductivity to the sensing films. Further, when exposed to a vapor, the polymers swell to increase the resistance level [34]. Moreover, the conductive polymer composite sensors are highly sensitive to humidity which must be reduced before sensing measurement.

4.3 Fabrication of carbon nanomaterials-based sensors Carbon nanomaterials include different allotropes of carbon, such as amorphous carbon, graphite, diamond, carbon nanostructures, and fullerenes [58]. Among carbon nanostructures, graphene and CNTs are mainly used for producing sensors [59]. CNTs are hollow cylindrical graphene sheets (two-dimensional layers of graphite structure) of covalently bonded carbon molecules whose high surface area, excellent mechanical and electronic properties led to their applications in nanoelectronic devices, nanocomposites, chemical sensors, biosensors, etc. [60]. CNTs can be further divided into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). SWCNTs are cylindrical single sheets having a diameter between 1 and 5 nm and length extending to several micrometers, while MWCNTs are comprised of a coaxial arrangement of multiple

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layers of CNTs having a diameter of about 2
20 nm and an interlayer spacing of 0.34 nm [2,60]. CNTs have a tendency to aggregate together, forming tubular bundles, due to the presence of strong interaction between them [5]. Further, their ability to adsorb molecules on their unparalleled unit surface is responsible for their application as chemical and biological sensors [61]. The use of CNTs for sensing operations has been started after the findings of Kong et al. [62] for detection of NH3 and NO2 molecules. Since then, a large number of reports have been published remarking on the applications of CNTs as sensors in detection of NO2, HCl, O2, and other harmful and explosive substances [1]. CNT sensors possess compact size, portability, and low power consumption. In addition, high sensitivity, room temperature operation, and miniaturization of massive sensor arrays make them advantageous over conventional metal oxide-based sensor materials [5]. Based on CNTs, various sensing devices such as chemiresistive [5], chemicapacitive [63], and field effect transistors [64] have been developed. Among these, chemiresistive types have gained much attention due to simple structure, low cost, and high precision measurements. These sensors can be used for detection of various gaseous analytes such as hydrogen and other greenhouse gases, fossil fuel emissions, volatile organic compounds (VOCs), etc. [5] (Fig. 4.2).

FIGURE 4.2 Various analytes detected by chemiresistive CNT sensors.

4.3 Fabrication of carbon nanomaterials-based sensors

Various researchers have contributed to developing CNT-based chemiresistive sensors to detect different chemical and biological analytes in the surrounding environment. The following subsections explore these sensors based on the detected target analytes [5].

4.3.1 Hydrogen gas detection Hydrogen gas (H2) is a colorless, odorless, buoyant, flammable gas. It must be detected in case of any leakage for the sake of safety due to its low spark ignition energy (0.017 mJ), high heat of combustion (142 kJ/g), and wide flammable range (4%
75%) [65,66]. For these reasons, H2 sensors are required wherever hydrogen gas is used such as hydrogen fuel cells and H2 storage systems [67]. The United States Department of Energy (DOE) has set some standards for the use of H2 sensors such as operating temperature (230 C to 80 C), concentration range (0.1% 2 10%), response time (,1.0 s), ambient air environment with 10% 2 98% humidity, and lifetime .10 years [68,69]. The operation of H2 sensors is based upon the chemical reactions between hydrogen gas and the sensing materials, which are transduced to produce a sensing signal using mechanical, electrochemical, optical, or electrical measurements [66]. Compared to electrochemical sensors, the chemiresistive sensors detect changes in sensor’s resistance (or conductance) which can be interfaced easily with minimal processing. Srivastava et al. deposited PANI, SWCNT/PANI, and MWNT/PANI films on finger type aluminum interdigitated electrodes (IDE) coated glass substrate and indium tin oxide coated glass substrate by spin coating technique [70] for room temperature detection of 2% concentration of hydrogen gas at 1.3 atm pressure. The highest response was observed from IDE type sensor deposit coated by SWCNT/PANI composite films. Zhang et al. [65] fabricated H2 sensors using SWCNT transducers with Pd, Pt, Cr, and Au microelectrodes via lift-off photolithography. The sensors were exposed for 15 min toward 50
2000 ppm concentration of H2 gas followed by 20 min of drying. Pd 2 SWCNTs combination demonstrated highest sensitivity by lowering the work function of Pd due to formation of Pd 2 H at the Pd 2 SWCNT contacts, thereby modulating the Schottky barrier height. Dhall et al. [71] detected hydrogen gas at room temperature using Pristine-MWCNTs and acids functionalized-MWCNTs using photolithography method. The pristine-MWCNTs exhibited a 190 s recovery time for 0.05% H2 gas, which decreased to 100 s for functionalized-MWCNTs with increased sensitivity of 0.8%.

4.3.2 Volatile organic compound detection VOCs are organic compounds present in building materials, paints, furniture, cleaning products, and cosmetics. Prolonged exposure to these VOCs can lead to severe health problems, including respiratory problems, sensory irritation, and even cancer [72]. Therefore, several methods have been employed to detect the presence of

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VOCs in the vicinity. Shirsat et al. [73] used low-pressure chemical vapor deposition to deposit the SiO2 insulating layer on the substrate, followed by e-beam evaporation of 20 nm Cr and 180 nm Au layers. These SWCNT 2 porphyrin hybrid sensors were used to investigate the room temperature responses of acetone, ethanol, methanol, methyl ethyl ketone, and water vapors. The performance of the sensors depends on the types of central metal and the functional groups present. Free-base and metal (e.g., ruthenium and iron) substitution with octaethyl and tetraphenyl porphyrins (TPPs) showed good selectivity and sensitivity toward the VOCs. Sarkar et al. [74] fabricated SWCNT- poly(porphyrin) hybrid through an electrochemical route whose coating thickness was adjusted by charge density modulation. It used this sensor to detect acetone vapors. Compared to bare and TPP-functionalized SWCNTs, the hybrid device showed fourfold sensitivity toward acetone detection with 180 days of stable sensing performance. He et al. [75] fabricated portable covalent 4-pyridyl (7,6)-SWCNTs sensors functionalized by cobalt (III) TPP selector and were able to detect 1 ppb N-nitrosodimethylamine (NDMA), N-nitrosodiethylamine (NDEA), and N-nitrosodibutylamine (NDBA) in air. The variation in the conductance value was attributed to the binding of N-nitrosamines to cobalt (III) TPP. Ionic liquids functionalized chemiresistive CNT-based sensors were fabricated by Park et al. [76]. They interacted with different VOCs containing alkyl, aromatic, and hydrophilic substituents relevant to human diseases with nine different sensing channels. The sensors were able to differentiate different VOCs within 2 min successfully. A reversible response was observed with all channels of the sensor array over one month. Shi et al. [77] detected formaldhyde using tetrafluorohydroquinone (TFQ)/SWCNT at ppb level. A less than 1 min response time with high selectivity over other VOCs was observed. They attributed the increased response of the sensors to the change in carrier mobility and density of SWCNTs due to the interaction of TFQ with formaldehyde that produced weak and reversible charged intermediate complexes. Formaldehyde sensors with amino/MWCNTs were prepared by Xie et al. [78]. The sensors provide an excellent response of 7
10 s to 20 ppb of formaldehyde and good selectivity toward other organic and inorganic gases. He at al. [53] prepared ammonia gas sensors using polymerization method employing PANI-coated MWCNTs, and 0.2 ppm of ammonia gas was detected within response time of 10
120 s. The relationship between gas sensing properties of the sensor and the thickness of PANI coatings was also investigated.

4.3.3 Fossil fuel emissions detection Although fossil fuels such as coal, petroleum, and natural gas are present in a limited amount in the environment, they are capable of producing toxic and harmful emissions leading to air pollution. The inhalation of these harmful emissions may cause dysfunction of different organs of the body or even lead to death [79]. Li et al. [80] reported poly(ionic liquid) (PIL)-wrapped SWCNTs percolating network chemiresistive sensors with CO2 detection limit down to 500 parts per trillion (ppt). PIL/SWCNTs suspensions were drop-deposited on prefabricated IDE

4.3 Fabrication of carbon nanomaterials-based sensors

patterns on a Si/SiO2 substrate and highly selective, reproducible, and humidity resistant sensors were obtained. Nguyen et al. [81] had grown CNT-based sensors integrated onto silicon-based circuits at room temperature and conducted sensing experiments with air/CO2 and Ar/CO2 environments. CO2 in Ar environment shows a better response and could be used to monitor the expiration of perishable food products. Young and Lin [82] used a simple method to directly transfer CNTs grown on SiO2 substrate onto a flexible polyimide substrate coated with an acrylic adhesive. The room-temperature sensor produced 12 and 56 s of response and recovery time for 50 ppm CO2 vapors, respectively and good response in various bending tests. Song et al. [83] used 80 and 100 μm electrode separation ionization sensors to detect SO2 and NO, respectively. With rise in concentrations of both gases, decreased response due to ample consumption of metastable states N2 (a0 1Σu 1 ) and N2(A3Σu 1 ) was observed. Further, the sensors measured both SO2 and NO concentrations of mixed gases with a response time of 8 and 7 s, respectively. Recently, Thangamani and Pasha [84] prepared polyvinyl formal (PVF)/TiO2 nanocomposites film-based fabricated chemiresistive sensors to detect SO2 gas. The 25 wt.% PVF/TiO2 nanocomposite film sensors at an operating temperature of 150 C were able to detect 600 ppm SO2 gas with B83.75% sensitivity, 66 s of response time, 107 s of recovery time, and 60 days of stability.

4.3.4 Military and defense explosives detection The rise in terrorist activities has increased the threat of public and military safety and thus resulted in increased demand for specialized sensors capable of detecting various harmful explosives like nitroaromatic (NA) compounds, dimethyl methylphosphonate (DMMP), cyclohexanone, etc. [85]. Frazier and Swager [86] fabricated robust SWCNT chemiresistors employing bis(trifluoromethyl) aryl groups capable of withstanding high temperature and humidity. These sensors were able to produce reversible, reproducible, responses in less than 30 s with a recovery period of 60 s to detect 10 ppm of cyclohexanone and produced 5 ppm LOD due to selective hydrogen bonding. Zhang and coauthors [54] used the drop-casting method to fabricate noncovalently functionalized carbazolylethynylene oligomer Tg-Car/CNT composite sensor. The prepared suspension of Tg-Car/CNT in chloroform was stable for 3 years without precipitation. Further, the sensors showed high selectivity and sensitivity toward 1.5, 2.0, and 3.0 ppm of three NA explosive compounds, namely 4-nitrotoluene (NT), 2,4,6-trinitrotoluene (TNT), and 2,4-dinitrotoluene with analyte exposure time and recovery time of 20 and 40 s, respectively. Wei et al. [87] developed 1-pyrenemethylamine (PMA) functionalized SWCNT network to detect TNT in water at ppt concentration level in less than 60 s. It was proposed that the interaction between amino substituent of PMA and TNT produced negatively charged complexes on the SWCNT surface and acted as a molecular gate to change the electrical conductance, thus detecting TNT at ppt level. Kumar et al. [88] reported flexible 4-(hexafluoro-2-hydroxy isopropyl) aniline (HFiP-1) functionalized SWCNT sensors. DMMP vapors controlled by the mass flow controller

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were carried from the bubbler head to the sensor by nitrogen gas acting as a carrier gas. A constant response of 16.0% and 47.0% with 300 and 90 s of response and recovery time was observed for 24 and 1200 ppm of DMMP, respectively, for six cycles and was attributed to strong interactions between acidic OH group of HFiP-1 and DMMP. Similar phenomena was observed by Fennell et al. [89], who proposed noncovalent exohedral functionalization of derivatized poly(3,4-ethylenedioxythiophene) (PEDOT) polymer/SWCNTs composite. The sensor produced a solid response to 5 and 11 ppm DMMP, and detection limit of 2.7 and 6.5 ppm was reported in dry N2 and air (24% RH) and response and recovery time of 60 and 140 s, respectively. Wei et al. [90] developed the SWCNT-TFQ network using the lift-off technique in which the SWCNT network deposited between IDEs was further modified TFQ. The sensor was capable of detecting 20 ppt DMMP in less than 120 s and high sensitivity was ascribed to higher interaction between DMMP and SWCNTs and heavy hole doping of SWCNTs by TFQ. DMMP sensors were also developed by Wang et al. [91], employing the dip-dropping method to fabricate SWCNT-6FBPA hybrids that produced a response and recovery time 960 and 720 s, respectively, for 20 ppm concentration of DMMP vapors.

4.3.5 Greenhouse gases The increased concentrations of greenhouse gases like CO2, CH4, nitrous oxide, ozone (O3), fluorocarbons, etc. are directly responsible for global warming and changes in the climatic patterns across the world. About two-thirds of the total greenhouse effect originates from CO2 alone. The presence of colorless and odorless CH4 gas acts as an explosion hazard at distribution centers, mines, and petroleum fractional distillation plants. Ahmad et al. [92] developed a room temperature operated CO2 gas sensor fabricated from MWCNTs/alumina composite film using gel-cast method. The sensors were exposed to varying concentrations of CO22 (50
450 ppm). A rise in sensor’s response was observed with an increase in CO2 concentration due to resistance changes with the adsorption of CO2 molecules on the nanotubes. Further, UV and heat application exposure causes a decrease in the recovery time and was attributed to desorption of CO2 molecules in the presence of external energy. Bezdek et al. [93] incorporated a platinum-polyoxometalate-based CH4 oxidation precatalyst into SWCNT-P4VP composites to fabricate a CH4 sensor that can be operated at room temperature in air. The sensor displayed ppm-level sensitivity to CH4 and good selectivity over various hydrocarbons and other greenhouse gases. The chemiresistive response was attributed to the generation of a high-valent platinum intermediate during CH4 oxidation. Leghrib et al. [94] compared the effect of O2, N2, and B-doping of SnO2/CNTs hybrids for gas sensing properties considering NO2 as a model pollutant. Low response/recovery times and high sensitivity were shown by N2, and B-doped SnO2/CNTs hybrids attributed to width modulation between two depletion layers of the hybrids. Jeong et al. [95] prepared a flexible NO2 gas sensor by growing vertical CNTs/reduced graphene hybrid film on a polyimide substrate

4.3 Fabrication of carbon nanomaterials-based sensors

that produced a response/recovery time of 1 h for 5 ppm of NO2 gas at room temperature. They proposed that the vertical CNT arrays facilitate the attachment of NO2 gas molecules, which further diffused into the bottom perpendicular substrate. For 5 ppm gas, a rise in sensitivity was observed till 200 C which further decreased till 350 C due to desorption of the gas molecules from hybrid film surface, thereby producing enhanced recovery. The reduced graphene/ polyimide combination was considered responsible for the mechanical flexibility of the sensor that has stable results under bending stress. Flexible NO2 sensors from MWCNTs-WO3 hybrid on polyethylene terephthalate substrate were fabricated by Yaqoob et al. [96]. With a negligible drift of 0.2%
0.3%, the sensors exhibited mechanical flexibility and repeatability at different bending angles, 14% response, and 27 min of the recovery period. Muangrat et al. [97] synthesized CNTs on Nideposited SiO/Si substrates using thermal chemical vapor deposition at different temperatures. NO2 gas at room temperature with 600, 1800, and 3000 ppm concentrations was exposed to the sensor and produced maximum response at 950 C due to favorable electronic structure and high absorption area. Gaikwad et al. [98] used photolithographic technique and electron beam deposition techniques to prepare polythiophene-SWCNTs that produced a good response in the range of 10 ppb
10 ppm. Liu et al. [99] made rGO-CNT-SnO2 hybrids using hydrothermal/dip-coating to detect NO2. The gas sensors detected 5 ppm of NO2 with response and recovery time of 8 and 77 s, respectively.

4.3.6 Biological contaminants Garcia-Aljaro et al. [100] reported a CNT-based immunosensor based on parallel aligned SWCNTs network bridging two gold electrodes used to function as a transducer element and for detection of bacteria and viruses. SWCNTs were functionalized with specific antibodies via covalent immobilization to the noncovalently bound 1-pyrene butanoic acid succinimidyl ester for different microorganisms. A fast response time of 60 and 5 min for bacteria and virus, respectively, was observed. Wasik et al. [101] used heparin, instead of traditional antibody, as the biorecognition molecule to functionalize SWCNT network chemiresistive sensor to detect the dengue virus (DENV). A self-assembly approach was employed on lithographically patterned interdigitated gold electrodes to prepare the transducer. Heparin was side-on attached on SWCNT network in which the carboxyl groups of heparin were cross-linked to primary amine groups on the pyrene-linker, 1-PMA, previously adsorbed onto the SWCNTs. To evaluate the biosensor selectivity, influenza virus H1N1 was used as a negative control. DENV suspended in phosphate buffer was detected with LOD of 8.4 3 102 TCID50/mL (B8 DENV/chip) with 10 μL sample incubated only for 10 min. Rajesh et al. [102] used 3-mercaptopropionic acid (MPA) capped gold nanoparticles/SWCNT hybrid device to detect cardiac-specific biomarker troponin-I (cTnI). The MPA capping causes cTnI specific protein antibody (Ab-cTnI) covalent immobilization through carbodiimide linkage. A linear response to target cTnI

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was observed in 0.1
10 ng/mL concentrations with a sensitivity of about 20% per decade ng/mL cTnI. Puri et al. [103] used electrochemical deposition of conducting copolymer poly(pyrrole-co-pyrrolepropylic acid) (PPy-PPa) on p-type SWCNT, which was electrophoretically aligned between gold microelectrodes to detect human cardiac biomarker, myoglobin (Ag-cMb). A sensitivity of 118% per decade toward myoglobin was observed in the range of 1
1000 ng/mL. It was attributed to increased electron transfer to the SWCNT on immunoreaction due to high myoglobin antibody (Ab-Mb) probe loading of PPy-PPa copolymer. Nouri et al. [104] used pulsed alternative current arc discharge method to synthesize CNTs on a high-density polyethylene substrate and used them as conducting channels to detect double-stranded DNA electrochemically. I
V characteristics were investigated before and after the sensing process in different temperatures and the sensor’s response produced acceptable agreement when investigated experimentally and analytically. As discussed in above sections, a large number of CNT-based chemiresistive sensors have been developed to detect different types of chemical and biological analytes in the last decade. Therefore, a tabular representation of various sensing characteristics of the reported sensors and their synthesis process is given Table 4.1.

4.4 Sensor comparison at lab/industrial level In the modern era, sensors have gained importance in the normal activities. Sensors are the devices that perceive the changes in the physical parameter and gather and processe the signal. The normal day of a human being is made easier with the advent of the various sensor technologies in diverse fields such as manufacturing, biomedicals, and various industrial applications. The gathering of electrical signal from the source and processing the signal are the two important steps involved in the sensing technology [111,112]. In the recent past, various sensors including the radars, ultrasound, and laser technologies are gaining more importance in the industrial sector. In medicine, the targeted drug delivery and the location of tumors are become more effective with the various sensors [113]. Recently, in the COVID-19 pandemic, the usage of pulse oximeters gained importance in sensing the saturated oxygen levels in the body [114]. Further, sensors are implanted with everything required for the normal-day medical diagnosis. Further, to capture the real time data/analysis, the sensors that are often used for medical applications are to be designed to integrate with the smart devices with the recent advancements in communication technologies [115]. Now a days, various sensors such as temperature, gyroscopic, accelerometric, etc. are often used in the personal and industrial purposes so that the real time data of any physical parameter as a function of other parameters can be observed and analyzed. The automobile industries utilize various sensors such as ultrasonics, radars, etc. for the automatic control of devices [116].

Table 4.1 A brief summary of different characteristics of chemiresistive CNT sensors. Target analyte

Fabrication method

Sensing film/electrode

Analyte concentration

Response time

Recovery time

Reversibility

References

H2 gas

Lift-off photolithography Photolithography

2000 ppmv 0.05% 50 ppm 1 ppb

0.37 min 100 s 190 s — —

[65] [71]

Electrochemical route —

1.13 min — — 50 s 10 min

Not stated Not stated

Acetone NDMA, NDEA, and NDBA Breath related VOCs Formaldhyde

Pd Functionalized-MWCNTs Pristine-MWCNTs SWCNTs 2 poly(TPP) hybrid Cobalt(III) TPP/4-pyridyl (7,6)SWCNTs Ionic liquids (ILs)-CNT

Not stated Not stated

[74] [75]

1000 ppm

2 min

—

Reversible

[76]

TFQ/SWCNT Amino/MWCNTs PANI-coated MWCNT PIL/SWCNTs Polyimide/CNT MWCNTs/alumina composite film Polyethyleneimine-CNT CNT cathode/ionization

ppb level 20 ppb 0.2 ppm 500 ppt
50 ppm 50 ppm 450 ppm

,1 min 7
10 s 10
120 s 10 min 12 s 53.7 s

— — — 1 min 56 s 14.15 s

Not stated Reversible Not stated Not stated Not stated Not stated

[77] [78] [53] [80] [82] [92]

200
1000 ppm 0
1120 ppm

10 min 8s

10 min 7s

Reversible Not stated

[105] [83]

Solvothermal method

Ni3BTC2/OH-SWCNT composite

15 ppm

5s

10 s

Reversible

[106]

Solvothermal method

Ni-MOF/
OH
SWCNTs Ni-MOF/
OH-MWCNTs

0.5
15 ppm

10 s

30 s

Not stated

[107]

Cyclohexanone

—

10 ppm

30 s

60 s

Reversible

[86]

NT DNT TNT

Drop-casting

SWCNT/bis(trifluoromethyl) aryl group Tg-Car/CNT

1.5, 2.0, and 3.0 ppm

20 s

40 s

Not stated

[56]

TNT

—

PMA-SWCNT network

10 ppt

,1 min

—

Not stated

[87]

NH3 CO2

SO2

— — — Polymerization method — Direct transfer process Gel-cast method — —

(Continued)

Table 4.1 A brief summary of different characteristics of chemiresistive CNT sensors. Continued Target analyte

Fabrication method

Sensing film/electrode

Analyte concentration

Response time

Recovery time

Reversibility

References

DMMP

Vacuum filtration 1 drop cast

4-(Hexafluoro-2-hydroxy isopropyl)aniline SWCNTHFiP-1 Poly(3,4ethylenedioxythiophene) (PEDOT) polymer/SWCNTs composite SWCNT-TFQ network

24 ppm

300 s

90 s

Not stated

[88]

5 and 11 ppm

60 s

140 s

Reversible

[89]

20 ppt

, 2 min

—

Not stated

[90]

SWCNT-6FBPA hybrids SWCNT-P4VP-Pt-POM composite

20 ppm 5000 ppm

960 s 0.87% 6 0.16%

720 s 120 s

Not stated Not stated

[91] [93]

Drop-casting

CH4

Lift-off technique 1 dropcasting Dip-dropping —

NO2

—

O2-CNT/SnO2 N-CNT/SnO2 B-CNT/SnO2

100
1000 ppb

7 min 1 min 1 min

3h 2.5 h 2.5 h

Not stated

[94]

— Solution route —

CNTs/RGO/PI MWCNTs-WO3NPs hybrid Ni-deposited SiO/Si CNTs

5 ppm 0.1
10 ppm

60 min 10 min 30 min

.60 min 27 min -

Not stated Not stated Not stated

[95] [96] [97]

Photolithographic technique and electron beam deposition Dip-coating

Polythiophene-SWCNTs

10 ppb

20 s

—

Not stated

[98]

rGO-CNT-SnO2 ternary hybrids Polyethyleneimine-SWCNT CNT/ZnO composite

5 ppm

8s

77 s

Not stated

[99]

50 ppm 25 ppm —

3.15 s 3 min

4 min 3.45 s —

Irreversible Reversible Reversible

[108] [109] [110]

Methanol Gasoline

Thermal CVD — —

MWCNT-g-polyisoprene (PI) Si-MWCNT/natural rubber (NR)

600 ppm 1800 ppm 3000 ppm

4.5 Sensing mechanism of chemiresistive sensors

In industries, the chemical sensors are widely used for the purpose of detecting/analyzing the toxic chemicals released as a by-product/ industrial waste in the concerned industry. These chemical sensors play a vital role in calculating the percentage of harmful chemicals in the industry to safeguard the environmental aspects of the surroundings [117]. Similarly, the gaseous sensors are used to detect the various harmful gases such as CO2, SO2, methane, etc. liberated as a by-product in the chemical reactions of the concerned industry. The gaseous sensors are used in the industries to check the quality of air, the detection of various gases in the industries and coal mines, chemical research laboratory, etc. In gaseous sensors, the host materials such as CNTs, graphene and its derivatives, metal/metal oxide nanoparticles decorated graphene, twodimensional materials such as MoS2, MoSe2, and other transition metal dichalcogenides are widely explored as sensing materials in various gas sensors. Various distinctive home applications of the various sensors include the electronic applications such as remote, analyzer, thermal analyzers, breath analyzers, infrared devices, wearable electronics, optical devices, and ultrasonic devices for automotive applications [118
120]. The gaseous and smoke detector sensors are equipped in the smart homes that are useful to detect the emitted chemicals and the smoke liberated [121,122]. In industrial applications, the variations of physical parameters such as temperature, pressure, and flow rate are calculated accurately using various sensors. Further, the electrical parameters such as voltage, current, frequency, impedance, and the other environmental parameters such as humidity, velocity, wind direction, and trace gases can be measured using the sensors. In summary, the sensors work on the digitalization of the data/response from electrical signal to the readable digital format that can be integrated on the multimedia devices. Sensors can be used in monitoring the real-time metabolism changes in body and can be used for the early warning of illness.

4.5 Sensing mechanism of chemiresistive sensors Among various carbon nanomaterials, graphene and CNT are the most effective chemiresistive sensing materials due to their excellent electrical properties, mechanical properties, and sizeable active surface area possessing adoption sites to analyte [59]. Functional groups on the surface of carbon nanomaterials, especially CNTs, further enhance the gas adsorption sites [59]. In CNT-based chemiresistive sensors, the CNTs act as the electrode’s conducting channel. The sensing response is recorded with or without analyte by measuring the conductance between two electrodes. The presence of surface atoms in CNTs facilitates the observation of appreciable conductance change for even a minimal change in the sensor’s chemical environment. Fennell and the coworker [10] reported a review on the various nanowire structures, including CNTs, their surface functionalization, and sensing mechanism to detect analyte. Tang and co-workers reviewed the work of CNTs-based sensors and presented the sensing mechanism [5]. The change of conductance of chemiresistive sensors on analytes adsorption might be due to modulation in the Schottky barrier (when analytes are absorbed on CNT-metal interface), charge transfer between the analyte and sensing

121

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element, changed CNT
CNT junction distance, etc. [5]. Analyte introduces electron to the valence band of p-type CNT semiconductor resulting in reduced hole concentration and decreased conductance. On the other hand, the presence of electrons withdrawing SWCNTs increases hole concentration, thereby enhancing conductance. He et al. [53] proposed that PANI-coated MWCNTs, a p-type semiconductor, increased the electron delocalization, enhanced the charge transfer between PANI and MWCNTs, and improved the sensitivity toward NH3. Shirsat et al. [73] used Porphyrin 2 SWCNT hybrid to detect various VOCs. They ascribed the sensing response to reduced charge mobility, electrostatic gating, and Schottky barrier modification. Leghrib et al. [94] prepared n-SnO2/p-CNT hybrid forming interface between SnO2 and CNTs to detect NO2 molecules. The charge transfer between NO2 adsorbed on SnO2 matrix affected the conduction properties of CNTs and thus resulted in increased sensor response. Liu et al. [99] prepared rGO-CNT-SnO2 hybrids to detect NO2 sensors and proposed that the introduction of SnO2 nanoparticles into rGO alters the active surface sites for adsorption/desorption of NO2 molecules. Further, the addition of CNTs into hybrids improved the electron-transfer rate between the rGO/SnO2 heterojunction. Further, due to the individual small length of CNT, the conducting channels can be formed by connecting multiple CNTs. Henceforth, the conductance change is observed after the absorption of analytes on the surface of the interconnected nanomaterial tubes network, as presented in Fig. 4.3.

FIGURE 4.3 A schematic representation of nanowire arrays structured chemiresistive sensor.

References

4.6 Conclusion This chapter reviewed the carbon nanomaterial-based chemiresistive sensors and their applications in detecting multiple analytes. The types of chemiresistive sensors and their fabrication by various researchers are also presented. The sensor’s performance to detect a specific analyte was found to be dependent on the sensing element and fabrication procedure. It was analyzed that the specific sensing material enabled the sensor to detect selective target analytes in a mixture of multiple analytes. This review can help explore and better understand various fabrication procedures of different CNT-based chemiresistive sensors and improve their performance to detect multiple target analytes in the future.

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CHAPTER

Semiconductor oxide based chemiresistive gas sensors

5

Vishal Baloria1,2, Aditya Yadav1,3, Preetam Singh1,3 and Govind Gupta1,3 1

Sensor Device and Metrology, CSIR-National Physical Laboratory (NPL), New Delhi, India 2 Centre for Advanced Materials and Devices, BML Munjal University, Gurugram, Haryana, India 3 Academy of Scientific and Innovative Research, CSIR-HRDC Campus, Ghaziabad, Uttar Pradesh, India

5.1 Introduction The environment around us is getting polluted, and various natural and humanmade factors are responsible for it. The natural factors include climate change, global warming, volcanoes, wildfires, while human-made factors include vehicles, industrialization, power plants, stubble burning, etc. Both these factors have led to the emission of various gases, thereby polluting the environment [1]. This pollution is happening rapidly, and the days are not far when we will be gasping for fresh air to respire. For instance, the city population daily gets exposed to various toxic gases such as NO2, CO, SO2, CO2, NH3, and H2S which cause various diseases and disabilities. The exposure limit for different toxic gases is in ppm. There have been worldwide efforts to detect and monitor such gases below their threshold limits and hence the need for sensors to detect them [2,3]. Gas sensors are electronic devices that detect and identify different type of gases as well as their concentration. A gas sensor’s primary function is to detect toxic gases that pose a danger to human health even at low concentrations (ppm, ppb, or even ppt levels). Such sensors are also employed to monitor the concentration of toxic gases such as CO, SO2, H2S, NO2, and volatile organic compounds (VOCs) to safeguard the natural and working environment. Fig. 5.1 summarizes various sectors where gas sensors have been utilized along with detectable gas in that particular domain. Currently, global gas sensor market is B $2.5 billion, and is expected to grow with a compound annual growth rate of over 6.0% during 2020
25. Gas sensors vary widely in size (portable and fixed), range, and sensing ability [4]. We can classify these gas sensors based on their detection principles as solid-state, mass sensitive, and optical sensors [3]. Chemiresistivesemiconductor metal oxide (C-SMO)-based gas sensors belong to a class of solidstate gas sensors, which work on the principle of change in semiconducting oxide resistance on interaction with an analyte gas. Depending on the kind of SMO Carbon Nanomaterials and their Nanocomposite-Based Chemiresistive Gas Sensors. DOI: https://doi.org/10.1016/B978-0-12-822837-1.00004-6 © 2023 Elsevier Inc. All rights reserved.

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FIGURE 5.1 Various sectors along with detectable gas sensors.

employed (n- or p-type) and the nature of analyte gas (oxidizing or reducing), the resistance either increases or decreases. C-SMOs are widely investigated owing to their low cost, high sensitivity, and relative ease in operation. These sensors can detect toxic gases both in high and low concentrations. The requirement for detection of toxic gases in high concentration comes from various Industries which employ these toxic gases, commercial refrigerants, as well as residential use (LPG). On the other hand, the requirement for detection in low concentration comes from air purifier, exhaust market, automotive, and indoor air quality. Metal oxides belong to a class of materials which can be tailored to make them conducting, semiconducting, or insulating. For gas sensing applications, SMOs are mainly required. Various types of these SMOs have been employed due to their low cost, ease of fabrication, and long-term stability. Most of these

5.2 Fabrication and designing of C-SMO gas sensors

materials exhibit either n- or p-type semiconducting characters and are attractive for gas sensing application due to their specific properties [5]. These SMOs can be employed to detect various toxic reducing and oxidizing gases. Table 5.1 consists of SMOs (n- and p-type) and their characteristic behavior for specific gases. These SMOs can be utilized for the fabrication of chemiresistive gas sensors, and these sensors detect different gases depending upon their chemical interaction with the gas.

5.2 Fabrication and designing of C-SMO gas sensors The design and fabrication process plays a significant role to achieve highperformance gas sensing devices. The fabrication of a gas sensor requires a sensing material deposited on an insulating substrate, a heater to obtain the desired operating temperature of sensing material, and electrodes for measuring device resistance. A typical schematic of a C-SMO-based gas sensor structure is shown in Fig. 5.2 and consists of SMO as the sensing material which is fabricated by using various growth techniques described in Section 5.2.1. The sensing material is the heart of the device and it has to be optimized to achieve high selectivity, sensitivity, and stability also known as “3 S” for a gas sensor. Various designs and dimensions of electrodes such as two electrodes, interdigitated electrodes (IDEs), etc., have been used to measure sensor performance, and noble metals such as Au and Pt are employed as main materials for the fabrication of these electrodes [6]. Several types of substrates such as alumina, fused silica, sapphire, Si-SiO2, low temperature cofired ceramic, glass, etc., [7
9] are used for this application. The substrates used should be electrically insulating to eliminate its contribution in the measured resistance and must have good thermal conductivity. An external heater is used for the optimization of the sensing material performance and after optimization, heaters are generally integrated into the sensor device by either printing them on the backside of the substrate either by screen printing, electron beam deposition, sputtering and microfabrication as discussed elsewhere [10]. Pt, n1/p1, polysilicon, Ti, Mo, W, etc., are generally used for the fabrication of such heaters.

5.2.1 Growth techniques of sensing material for C-SMO gas sensors To make an efficient gas sensor, the sensing material structure needs to be optimized. For this purpose, various physical and chemical vapor deposition techniques have been utilized. The different growth techniques and process conditions may lead to different sensing performance of the fabricated gas sensor. Fig. 5.3 describes the journey of C-SMO-based gas sensors which can be classified as traditional, thick film, thin film, and MEMS along with their power consumption and the device dimension is discussed below.

135

Table 5.1 Semiconductor metal oxide materials and their attractive properties for gas sensing application. Electronic configuration

Band gap (eV)

Crystal structure

Target pollutant

B3.6

Tetragonal

ZnO

[Kr] 4d105s05p0 [Ar]3d10 4s0

B3.37

Wurtzite

TiO2

[Ar]3d04s0

3.2/ 3

Anatase/rutile

H2, CO, NO2, NH3, H2S, SO2 H2, CO, NO2, NH3, H2S CO, NO2

WO3

2.4
2.8

Monoclinic

H2, H2S, NO2

B 3.7

Cubic/ rhombohedral Monoclinic

CO, NO2, H2S

CuO

[Xe] 4f145d06s0 [Kr] 4d105s05p0 [Ar]3d94s0

NiO

[Ar]3d84s0

Co3O4

[Ar]3d6
74s0

Cr2O3

[Ar]3d34s0

Material n-type

SnO2

In2O3 p-type

B (1.2
1.9) B (3.6
4) 1.48 and 2.19 3.4

Rocksalt (NaCl) Spinel Corundum

Attractive property for gas sensing High chemical sensitivity, good stability, nontoxicity, good heat tolerance, corrosion resistant Good thermal and chemical stability, nontoxicity Chemical stability, nontoxicity, high-temperature stability, harsh environment tolerance Presence of W in multiple oxidation states (12 to 16), rich morphology, and high intrinsic surface area Good stability, good physio-chemical properties

CO, NO2, H2S, VOCs NH3, H2S

Nontoxicity High thermal and chemical stability

CO, NO2, NH3

Good conductivity and oxidative catalytic capacity

H2, NO2

Thermo dynamical stability and resistance to chemical attack, presence of Cr in multiple oxidation states (12 to 16)

5.2 Fabrication and designing of C-SMO gas sensors

FIGURE 5.2 A typical structure of C-SMO gas sensor.

FIGURE 5.3 C-SMO gas sensor fabricated with different technologies and their power consumption and device dimensions.

5.2.1.1 Traditional technology Traditionally, the sensing material is prepared in the form of a pellet from commercial or prepared metal oxide powder. The powder is then finely milled either using a pestle mortar or a ball mill and pressed to form a pellet by either adding a binder or cold pressing and sintering at a high temperature (500 C
1000 C) [11]. To record the sensing response, electrodes are first deposited on the pellet, and metal wires (such as Pt, Cu, etc.) are attached to these electrodes using silver paste. Thereafter, the pellet is placed on an external heater of a sensing system to record the gas sensing characteristics. A schematic along with a typical pressed SnO2 pellet used as a CSMO gas sensor is shown in Fig. 5.4A and B. Although this process of making the sensors is cheap, it has several disadvantages such as very high power consumption, poor selectivity, slow response and recovery times, and baseline resistance drift.

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FIGURE 5.4 (A) Schematic for a pellet-based C-SMO-based gas sensor; (B) a typical picture of SnO2 pellet.

5.2.1.2 Thick film technology Thick film technology has emerged as a workhorse in the area of gas sensors. The sensing layer thickness in the sensor devices prepared using this technology varies from a few µm to 100 µm. Electrical as well as mechanical properties of thick films are generally less sensitive to the type of substrate employed. Various pastes and inks for sensing material fabrication are either commercially available or can be formulated. In this technique, the sensing material is either painted on an alumina tube (Taguchi sensors) or deposited on an alumina substrate having predeposited IDEs by screen printing [12]. These are then sintered at a high temperature to form a thick sensing film. The desired operating temperature in the case of an alumina tube is achieved by using a metal coil heater within the tube while for alumina substrate metallic heater is patterned on the backside of the substrate. This is a versatile technology for material preparation and simple in film formation thereby lowering the manufacturing cost. Though thick film-based sensors still occupy a major market share, the power consumption in them is generally high (B 1 W) [13]. A schematic and an actual photograph of a typical sensor structure based on thick film is shown in Fig. 5.5A and B.

5.2.1.3 Thin film technology Thin film technology has received worldwide attention due to its wide applications. The fabrication of thin films involves creation, transportation, and condensation of the desired sensing layer with thickness varying from nm to a few µm. The resistance of films prepared using this technology is higher due to lesser thickness and they have low power consumption in comparison to films prepared by thick film technology [14]. The sensing electrodes and heaters are mainly deposited using physical deposition techniques such as magnetron sputtering and thermal and electron beam (E-beam) evaporation, while sensing material is deposited either using both physical and chemical deposition techniques such as thermal evaporation [15], magnetron sputtering [16], electron beam evaporation [17], pulsed laser deposition [18], and chemical vapor deposition [19], respectively. The source material is taken in the form of powder, chunks, pellets, etc., and is atomized using the above techniques to fabricate a thin film of the desired

5.3 Working principle of a C-SMO-based chemiresistors

FIGURE 5.5 (A) Typical schematic and (B) an actual photograph [14] of a thick film (Taguchi) sensor.

SMO-based sensor. A schematic of a thin film-based C-SMO gas sensor and image of an actual fabricated device using this technology are shown in Fig. 5.6A and B. MEMS (microelectromechanical systems) represents microscopic and highly functional electrical machine systems and devices applied and manufactured with micromachining technology. Microfabrication-based MEMS technology is the latest technology being employed in the realization of gas sensors [20
22]. MEMS microfabrication makes use of either bulk or surface Si micromachining or both for integration of the required sensor components such as heater and sensing material in a minimal area on a single chip. It thereby leads to low power consumption (BmW), fast sensor heating, and quick heater response time and correspondingly, reduced cost of the complete sensor device due to batch processing. Low power consumption is especially desired in applying these sensors in battery-powered portable, wireless, and wearable devices in the near future. Besides, it also provides the possibility of integrating the sensor electronics on the same chip and extension from an individual sensor to a sensor array, through which more than one gas can be detected simultaneously. Many commercial players have developed a wide range of sensor platforms with reduced size and power consumption. For example, Figaro, United States, has successfully exploited this technology for fabricating sensors with reduced power consumption (B15 mW) in comparison to their conventional thick film sensor (B 210 mW). A schematic of an actual sensor device [23] fabricated using this technology is depicted in Fig. 5.7A and B.

5.3 Working principle of a C-SMO-based chemiresistors In general, the sensing element in a C-SMO gas sensor is an SMO-based material with a high surface to volume ratio. In such sensors, reaction with the test gas alters the majority charge carrier density in the SMOs thus altering the resistance, and this is used as a probe for gas detection [5]. Adsorption and desorption of gas

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CHAPTER 5 Semiconductor oxide based chemiresistive gas sensors

FIGURE 5.6 (A) Schematic and (B) actual photograph of a thin-film based C-SMO gas sensors.

FIGURE 5.7 MEMS-based C-SMO gas sensor: (A) schematic diagram and (B) actual device [23].

is a widely accepted operating principle among such sensors [24]. In general, these sensors operate at a typical temperature (100oC
400oC) known as their operating temperature. At this temperature, atmospheric oxygen gets adsorbed onto the SMO grains capturing electrons from its surface and forming a depletion region. The formation of a depletion region results in a potential barrier for electron transfer from one grain to another. This further increases sensor resistance for n-type SMOs and decreases it for a p-type. Reducing gas removes the adsorbed oxygen and lowers the potential barrier, thereby returning captured electrons to the SMO, reducing the depletion region, and improving conduction. As a result, resistance of n-type SMOs decreases, and that of p-type SMOs decreases [25]. The physical and band model for a C-SMO gas sensor employing n-type SMO material is depicted in Fig. 5.8A and B. Similarly, in the case of oxidizing

5.4 Performance parameters for C-SMO gas sensor

FIGURE 5.8 Physical and band model for a C-SMO gas sensor employing n-type SMO material in (A) air and (B) reducing gas.

gases, opposite changes take place in these devices [17]. Based on the resistance change, the SMOs can be categorized as either n- or p-type [26], as specified in Table 5.2.

5.4 Performance parameters for C-SMO gas sensor The performance of C-SMO gas sensors is characterized by parameters such as response, sensitivity, selectivity, stability, response/recovery time, detection limit and range, sensor drift, etc. These performance parameters are used to study the characteristics of fabricated gas sensor and are described in detail below. A response curve (a plot of sensor resistance with time on introduction and removal of analyte gas) is used to extract some of these performance parameters and a typical response curve for n-type SMO in the presence of oxidizing and reducing gases is shown in Fig. 5.9A and B. Response: Sensor response represents the variation in resistance in presence of test gas. The response is a unitless parameter and is defined as the ratio of sensor resistance in target gas to resistance in air, mathematically expressed as: Response 5 5

Ra ðfor reducing gasesÞ Rg

(5.1)

Rg ðfor oxidizing gasesÞ Ra

(5.2)

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CHAPTER 5 Semiconductor oxide based chemiresistive gas sensors

Table 5.2 Resistance change based con lassification of sensing material. Gas exposed

Sensing material

n-type (e.g., SnO2, ZnO, etc.) p-type (e.g., Cr2O3, Co3O4, etc.)

Reducing (e.g., CO, H2S, etc.)

Oxidizing (e.g., NO2, SO2)

Resistance decreases Resistance increases

Resistance increases Resistance decreases

FIGURE 5.9 Typical response curve for n-type C-SMO gas sensor in the presence of (A) oxidizing and (B) reducing gas.

It can also be expressed in terms of relative change in resistance and percentage change in resistance as Response 5

5

Rg 2 Ra

(5.3)

Ra Rg 2 Ra Ra

3 100

(5.4)

where Ra, and Rg are the resistance of the sensor in reference gas (usually air) and target gas, respectively. Sensitivity: It is defined as the rate of change of sensor response per unit target gas concentration, that is, slope of the calibration curve obtained on plotting response as a function of gas concentration. Sensitivity can also be defined as the minimum concentration of target gas which the sensor can perceive, and thus lower is its value, the better is the sensitivity of the gas sensor. It is used to

5.4 Performance parameters for C-SMO gas sensor

evaluate the variation of target gas concentration in the ambient [27]. The sensitivity can be mathematically expressed as follows: Sensitivity

ðQÞ 5

Rg 2 Ra Ra 3 C

(5.5)

where Ra, Rg, and C are the resistance of the sensor in reference gas, target gas, and its concentration, respectively. It is also important to mention different levels of sensitivity under dissimilar situations; for example, ppm detection might be enough for industrial application, while low ppm or ppb or even ppt detection is desired for environmental or disease diagnostic application. In order to improve the sensitivity, many factors such as chemical components and physical structure of SMO, humidity, environmental condition, and temperature play a crucial role either individually or collectively. Selectivity: Selectivity is defined as a sensor’s ability to differentiate a specific gas from a mixture of gases. For example, detection of NH3 from a blend of NOx gases makes the sensor selective to NH3. The selectivity of a sensor to a specific gas is closely associated with its operating temperature. In general, C-SMO gas sensors endure from a lack of selectivity and drift [28]. It is generally influenced by water vapors; hence changes in the moisture content of the ambience considerably interfere with the gas to be detected. A sensor will detect a specific signal due to adsorption of the desired gas while it remains insensitive to others. The selectivity coefficient/factor (K) of target gas to an interfering gas is defined as follows: K5

Sensitivity for Interfering Gas Sensitivity for Target Gas

(5.6)

C-SMO-based gas sensors are usually nonselective and hence respond to multiple gases (cross-sensitivity). The response from a cross-sensitive sensor is vague and thus cannot serve as unique fingerprint for an unknown gas. Thus, there is a need to improve the selectivity in C-SMO-based gas sensors. Response time: Response time is defined as the time taken to achieve 90% of the ultimate change in resistance from its base resistance, that is, the total response of the signal in the presence of analyte gas of particular concentration. This is because the reaction is often very rapid initially but has a long-drawn-out tail before the steady value is reached. Typical response time is shown in Fig. 5.9. The response time is a vital parameter as it determines the applicability of the sensor. It is one of the trickiest of all the operating parameters of the sensor. Recovery time: Similar to the response time, recovery time is defined as the time required to recover 90% of the original resistance of the sensor upon removal of the analyte gas. Both response and recovery time depend upon the concentration of the gas exposed on the sensors, thickness of the sensor material, and the operating temperature of the testing sensor. Typical recovery time is shown in Fig. 5.9.

143

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CHAPTER 5 Semiconductor oxide based chemiresistive gas sensors

Stability: Stability is a characteristic that takes into account the reproducibility of device measurement after prolonged use. It is defined as a gas sensor’s ability to provide almost constant results when it is continuously used for a certain period [29]. To avoid nonrepeatability, several gas sensor manufacturers subject these materials to a thermal pretreatment, which decreases subsequent material instabilities. The sensor stability is accessed by obtaining the sensor response on repetitive exposure to several cycles of same concentration of the test gas as depicted in Fig. 5.10. Detection limit and range: Detection limit is defined as the smallest concentration of analyte that can be detected, that is, the least concentration which a sensor can detect by showing a measurable change in resistance. In comparison, the detection range is defined as the concentration range between the lowest and the highest value detected by the sensor with a minimum and maximum possible detectable change in resistance. C-SMO sensors have a wide detection range varying from 1 ppt to 1% concentration of analyzing gases as shown in Fig. 5.11. Sensor drift: The sensor drift (also called sensor response instability) consists of small and temporal variations of the sensor response when it is exposed to the same concentration of analytes under identical conditions. It can be divided into two parts: short-term and long-term drift. The sensor drift phenomenon has been

FIGURE 5.10 Representative response curve obtained upon exposure to consecutive cycles of same concentration of a reducing gas, indicating the stability and repeatability of n-type C-SMO gas sensor.

FIGURE 5.11 Typical range of C-SMO-based gas sensors.

5.5 Sensing mechanism in C-SMO gas sensor

recognized as one of the most significant hindrances to the performance of CSMO gas sensors. The major problem is an unavoidable modification of the SMO-sensitive layer caused by molecule adsorption during the target gas exposure. These modifications produce variabilities in the sensor response leading to short-term drifts. The sensor drift involves the sensing materials property, sensor structures, nature, concentration and exposure time of target gas, ambient temperature, and relative humidity. This problem causes inaccurate results, false alarms, and frequent replacement of sensors. The main solution to significantly reduce the drift is to plan an efficient cleaning process after each gas exposure. But in practice, the cleaning process is very time consuming. For practical and continuous measuring applications, sensor drift duration should be minimized [30,31]. A representative drift in sensor’s baseline resistance after each exposure in sensor response curve is presented in Fig. 5.12.

5.5 Sensing mechanism in C-SMO gas sensor The working principle of C-SMO gas sensors is relatively simple, but their sensing mechanism is equally complex [5]. Hence, for the development of gas sensors, understanding the sensing mechanism is essential. The sensing mechanism can be broadly defined for two types of SMOs. The first one explains it for nonsurface functionalized or pristine SMOs, and the second one discusses it for surface-functionalized ones.

FIGURE 5.12 Representative drift in baseline resistance observed after same exposure of a reducing gas for n-type C-SMO-based gas sensor.

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CHAPTER 5 Semiconductor oxide based chemiresistive gas sensors

5.5.1 Pristine oxides There are various SMOs such as SnO2, ZnO, TiO2, WO3, In2O3, CuO, NiO, Co3O4, Cr2O3, etc., which are used for gas sensing in their pristine form, that is, without any kind of surface functionalization. In this type of MOs mainly two kinds of mechanisms, reduction-oxidation and ionosorption, as discussed below, occur. Reduction-reoxidation mechanism: This mechanism can be explained based on oxygen vacancies present on the SMO surface. These oxygen vacancies are responsible for causing the resistance change, for example, SnO2 owing to its oxygen deficiency is an n-type semiconductor, and its oxygen vacancies act as an electron donor. This reduction-reoxidation of the oxygen vacancies results in changing the oxygen stoichiometry and is responsible for the sensing behavior as described below for a reaction with reducing gas CO [32]. 2Vo_ 1 O2ðgasÞ 1 2e2 ðCBÞ"2Oxo

(5.7)

COðgasÞ 1 Oxo "CO2ðgasÞ 1 VOx

(5.8)

VOx "Vo_ 1 e2 ðCBÞ

(5.9)

Vo_ "VO::

2

1 e ðCBÞ

(5.10)

Ionosorption mechanism: This mechanism can be explained based on adsorbed oxygen onto the SMO surface. The adsorption results in space charge effects such as depletion and accumulation by changing the surface potential. The reaction of the test gas with adsorbed oxygen is responsible for causing the resistance change in this case, for example, in ZnO at its operating temperature; oxygen is ionosorbed on its surface by capturing electrons from the conduction band of ZnO. Reducing CO gas reacts with this ionosorbed oxygen, releasing the captured electrons back into the conduction band of ZnO and decreasing the sensor resistance as described below [32] O2gas "O2ads

(5.11)

O2gas 1 e2 "O2 2 ads

(5.12)

2 2 O2 2 ads 1 e "2O ads

(5.13)

O2 ads 1 e2 "O2
ads

(5.14)

COads 1O2 ads "CO2 1 e2 ðCBÞ

(5.15)

5.5.2 Metal/metal oxide functionalized metal oxides A single SMO seldom has all the desired properties suitable for superior gas sensing performance and hence needs functionalization. This functionalization is achieved either by surface modification or surface doping using metals such as Pd, Pt, Au, Al, etc., or SMOs such as TiO2, Cr2O3, CuO, PdO, etc., to enhance

5.5 Sensing mechanism in C-SMO gas sensor

the sensor response. Surface modification results in improving not only the sensor sensitivity but also the sensor response. In several cases, it also leads to lowering the operating temperature of the sensor device. MO functionalization leads to heterojunction formation that results in improved catalytic activity, electron depletion, additional adsorption sites, and changes in the band structure and hence improved sensor response. Thus the effect of metal/metal oxide functionalization can be explained either based on chemical interaction, electronic interaction, or pn, n-n, p-p heterojunction formation mechanisms as discussed below. Chemical interaction mechanism: In this case, the SMO surface is functionalized with a small amount of noble metal. The noble metal activates the test gas by increasing the reaction cross-section via spillover effect by easing the reaction of activated fragments with adsorbed oxygen. Thus the spillover effect accelerates the reaction and thereby enhances sensitivity and decreases the response time. This is also termed as chemical sensitization, for example, surface functionalization with Pt results in lowering the activation energy and enhances the sensor response for H2 as shown in Fig. 5.13A [33]. Electronic interaction mechanism: In this case also, the SMO surface is functionalized with a small amount of noble metal, and direct electronic interaction between the noble metal and the SMO gives rise to electronic sensitization and alignment of Fermi level between them. Here, the noble metal plays the role of an acceptor or a donor. As its oxidation state changes with the nearby atmosphere, consequently, the electronic state of the SMO also changes. A typical example is Pd surface-functionalized SnO2, where Pd forms PdO under atmospheric conditions and owing to its higher work function than SnO2, it creates a vast depletion region in the supporting SnO2 surface and gives rise to reduced conduction. Exposure to H2, in this state, reduces PdO to Pd and gives rise to higher conduction and enhanced response shown in Fig. 5.13B [33].

FIGURE 5.13 Mechanism of sensitization by metal additive: (A) chemical and (B) electronic. Adapted after modification with permission from N. Yamazoe, et al., Oxide semiconductor gas sensors, Catal. Surv. Asia 7 (1) (2003) 63
75. Copyright (2020) Elsevier.

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CHAPTER 5 Semiconductor oxide based chemiresistive gas sensors

Heterojunction formation mechanism: In general, for gas sensing applications both n- and p-type SMO are individually employed. These materials can also be combined in different ways to form heterojunctions and improve the sensing properties. Depending on the type of SMO employed for this purpose they are classified as either p-n, n-n, or p-p heterojunctions, schematically depicted in Fig. 5.14A
C [34]. In the n-p heterojunction, the electrons move from n-type SMO to p-type SMO whereas holes move from p-type SMO to n-type SMO until the Fermi level is aligned. In air, the depletion region at the interface of the heterojunction expands and the C-SMO resistance increases. Now when this material is exposed to a reducing gas, for example, H2S, it reacts with the adsorbed oxygen giving back captured electrons to the material. As a result, the majority of electrons enter into the conduction band on the n-type SMO side while few electrons also enter into the conduction band of the p-type SMO side resulting in electronhole recombination. This increases the concentration of electrons and decreases the concentration of holes on the p-side. Consequently, carrier concentration on both sides of the p-n junction is reduced and due to limited carrier diffusion, the barrier at the interface is also reduced. Thus, the depletion region shrinks, and the C-SMO resistance decreases. In the case of n-n heterojunction, the majority carrier electrons move at the interface between the two n-type materials due to their different conduction band edges. As a result, a depletion region is formed on one n-side with higher conduction band energy due to the loss of electrons, whereas the accumulation region is formed on the other n-side with lower conduction band energy due to the accumulation of electrons. In the case of p-p heterojunction, majority carrier holes move at the interface between two p-type materials due to their different valance band energies. Consequently, a hole depletion region is formed on one p-side with higher valance band energy, and an accumulation region results on the other p-side with low valance band energy.

5.6 Factors influencing sensing characteristics of C-SMO gas sensor Gas sensing is a surface phenomenon, and many factors significantly influence the sensing characteristics in a C-SMO gas sensor. The effect of some of these influencing factors is summarized below: Crystallinity and microstructure: The sensing material in various C-SMO gas sensors can be employed in different states such as amorphous, glassy, polycrystalline, nanocrystalline, and single-crystalline with every state having their unique properties. Among these various states, nanocrystalline and polycrystalline are the most suitable for gas sensing applications. It is due to the ideal composition of some desired properties, such as enough surface, low-cost design technology, and the necessary stability of both structural and electrophysical properties [5]. For instance, the sensing response of sol-gel derived amorphous and polycrystalline

FIGURE 5.14 Schematic energy band structures for different type of heterojunctions: (A) p
n, (B) n
n, and (C) p
p. Adapted with permission from Z. Li, et al., Advances in designs and mechanisms of semiconducting metal oxide nanostructures for high-precision gas sensors operated at room temperature, Mater. Horizons 6 (3) (2019) 470
506. Copyright (2020) The Royal Society of Chemistry.

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CHAPTER 5 Semiconductor oxide based chemiresistive gas sensors

WO3 thin films has been studied, and it was observed that the amorphous WO3 thin films showed no response to ozone while the polycrystalline thin films showed a very stable response to oxygen and a promising one towards ozone [35]. In recent years, even low dimensional nanostructures have attracted much attention as they have a large surface to volume ratio, remarkable physical/chemical properties, and are highly crystalline and hence have improved thermal and, therefore, temporal stability [17]. Very recently, thin two-dimensional (2D) nanomaterials have emerged as a new dimension in gas sensing owing to their distinctive physical, chemical, and electronic properties related to their ultrathin thickness [36]. Such 2D layers with atomic or molecular thicknesses immensely affect the gas sensing performances. Apart from crystallinity, the crystal phase formed also affects the sensing characteristics. For example, gas sensing selectivity of hexagonal and monoclinic phase WO3 nanoparticles prepared using a chemical route has been found to be different. It has been observed that h-WO3 is highly selective towards 10 ppm H2S in comparison to m-WO3 at 200 C in the presence of interfering gases like CH4, CO, H2, and NO. Besides, the estimated response time was found to be significantly shorter for h-WO3 than that for mWO3 [37]. The microstructure, specifically the film thickness and its porosity, has a critical effect on the response time and sensitivity. Sensing layers are penetrated by oxygen and analyte molecules so that a concentration gradient is formed, which depends on the equilibrium between the diffusion rates of the reactants and their surface reaction. The rate leading to the equilibrium condition determines the response and recovery time. Therefore the analyte and oxygen’s fast diffusion rate into the sensing body is essential which depends on its mean pore size and the working temperature. Furthermore, maximum sensitivity will be attained if all percolation paths contribute to the total change of resistance, that is, they are all accessible to the analyte molecules in the ambient condition. Thus lower film thicknesses together with a higher porosity give rise to higher sensitivity and faster response time [38,39]. In addition, two functions mainly govern the gas sensor response: the receptor function, which recognizes a chemical, and the transducer function that transforms the chemical recognition into an electrical signal. Among these functions, the transducer function is of immense importance as SMOs with an optimal morphology and structure can only perform efficient transduction. It has been experimentally observed that for most SMOs, gas response is directly related to its grain size (D). It has been found in the case of SnO2 that for D. 20 nm, the gas response is almost independent of the grain size; for 10 nm ,D . 20 nm, the response increases with decreasing D while for D , 10 nm the response significantly increases as shown in Fig. 5.15 [40]. Role of vacancies: The carrier concentration in SMO materials can be altered either by modulating the concentration of defect or the defect type. Thus the sensing performance can be tuned by modulating the intrinsic defects and the concentration of these defects [41]. Oxygen vacancies and metal vacancies are the two main types of intrinsic defects which are frequently present on SMO surface. The

5.6 Factors influencing sensing characteristics of C-SMO gas sensor

FIGURE 5.15 Variation of sensitivity with crystallite size for SnO2-based C-SMO gas sensors for CO and H2. Adapted with permission from C. Xu, et al., Grain size effects on gas sensitivity of porous SnO2-based elements, Sens. Actuators B Chem. 3 (2) (1991) 147
155. Copyright (2020) Elsevier.

n-type semiconducting character is attributed to the presence of oxygen vacancies and is the main intrinsic defect in n-type whereas metal vacancies are the main ones in p-type SMOs. They both play a significant role in gas sensing as they act as the dominant adsorption sites for oxygen adsorption and hence for the improved reaction between adsorbed oxygen and test gas, thereby increasing sensitivity. It has been observed that the sensor response for materials with oxygen vacancies is higher in comparison to material without oxygen vacancies [42]. A typical example is ZnO nanorods with oxygen vacancies, which exhibit improved sensing properties compared to the untreated ZnO nanorods without oxygen vacancies, as depicted in Fig. 5.16 [42]. Besides, NiO nanosheets with different Ni vacancies are prepared by increasing the annealing temperature. The Ni vacancies were found to increase on increasing the annealing temperature from 400oC to 600oC, whereas the NO2 sensing response increases till 500oC and decreases thereafter, as depicted in Fig. 5.17 [41]. Based on the study, it is suggested that NiO with a higher concentration of Ni vacancies exhibits a higher sensitivity to NO2 at room temperature. With further increase in annealing temperature to 600 C, the NO2 response decreases as the specific surface area of the NiO decreases and only limited NO2 is physiosorbed. Role of chemical composition: Chemical composition also plays a key role in improving the sensing performance; a typical example is substoichiometric WO3-x

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CHAPTER 5 Semiconductor oxide based chemiresistive gas sensors

FIGURE 5.16 Sensing responses of the H2O2-treated/annealed ZnO and untreated ZnO (A) film and (B) nanorod array gas sensors at 400 C for 3 ppm ethanol. Adapted with permission from W. Kim, et al., Generation of oxygen vacancies in ZnO nanorods/films and their effects on gas sensing properties, Sens. Actuators B Chem. 209 (2015) 989
996. Copyright (2020) Elsevier.

FIGURE 5.17 (A) Relative amount of Ni vacancies obtained at different annealing temperatures, their specific surface area and response for various NO2 concentration; (B) hole hopping conduction model of NiO before and after exposure to NO2. Adapted with permission from J. Zhang, et al., Effect of nickel vacancies on the room-temperature NO2 sensing properties of mesoporous NiO nanosheets, J. Phys. Chem. C 120 (7) (2016) 3936
3945. Copyright (2020) American Chemical Society.

5.6 Factors influencing sensing characteristics of C-SMO gas sensor

(x B 0, 0.11, 0.2, 0.28) fabricated for H2S detection, where different oxides of W exhibit diverse H2S response at their different operating temperatures, as depicted in Fig. 5.18 [43]. Morphology: The morphology of the SMO materials critically influences the sensing performance of C-SMO-based gas sensors. The sensing performance can be defined as the reaction rate and amount of oxygen species in the redox reaction, which also depends on the shape and size of the employed SMO. Moreover, different morphologies, due to their higher surface to volume ratio, have several advantages, such as increased surface-active sites, fast response kinetics, and enhanced gas diffusion. Various morphologies of SMOs such as zero-dimensional (quantum dots); onedimensional (nanowires, nanofibers, nanorods); two-dimensional (nanosheets and thin films); three-dimensional (nanoflowers and urchin-like), etc., with each having their unique advantages have been fabricated using various top-down and bottom-up approaches. Moreover, several other morphologies such as hollow and core-shell structures also display high gas-sensing performances due to their large specific

FIGURE 5.18 Response curves for various sub-stoichiometric forms of WO32x on exposure to 1, 2, 3, 4, 10 and 50 ppm H2S. Adapted with permission from W. Yu, et al., Improving gas sensing performance by oxygen vacancies in sub-stoichiometric WO32x, RSC Adv. 9 (14) (2019) 7723
7728. Copyright (2020) The Royal Society of Chemistry.

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CHAPTER 5 Semiconductor oxide based chemiresistive gas sensors

surface areas by allowing both the inner and outer surface of the structure to adsorb more target gas species. A typical example of representative morphologies of SnO2 along with their sensing performance is depicted in Fig. 5.19 [44]. Temperature: Temperature is an important factor in the operation of C-SMO gas sensors as C-SMO gas sensors respond to target gas with temperature. In general, the sensor response increases, reach their maximums at a certain temperature, called operating temperature, and then decreased rapidly with increasing the temperature, called recovery temperature. This tendency is commonly observed in C-SMO-based gas sensors. Both operating and recovery temperature are dependent on the properties and nature of materials used for the fabrication of the CSMO gas sensor and on the target gas to be measured. Humidity: Humidity has an extreme effect on the performance of C-SMO-based gas sensors. The role of moisture is still debatable as, in some instances, it enhances, whereas, in others, it lowers the sensor response. The water vapors present in air when adsorbed on the surface of SMO react with surface oxygen resulting in decreasing the adsorption sites for the incoming gas and decreasing the baseline resistance of the sensor and thereby the sensor response [45]. On the other hand, the sensor response can be enhanced when the sensor is operating in an environment with a humidity level in a particular range. Besides, it also depends on the sensor operating temperature; at low temperatures, humidity significantly affects

FIGURE 5.19 SEM images of SnO2 samples with different morphologies: (A) solid spheres, (B) nanoneedle-assembled nanourchins, (C) nanosheet-assembled nanoflowers and their response on exposure to 400 ppm H2 at different temperatures in (D) atmosphere and (E) vacuum and response curve for the three sensors under 400 ppm H2 at 350 C in (F) atmosphere and (G) vacuum, respectively. Adapted with permission from L. Zhu, et al., A non-oxygen adsorption mechanism for hydrogen detection of nanostructured SnO2 based sensors, Mater. Res. Bull. 109 (2019) 108
116. Copyright (2020) Elsevier.

5.6 Factors influencing sensing characteristics of C-SMO gas sensor

the sensor response, but when the sensor operates at a high temperature, the effect of moisture can be reduced. Fig. 5.20 shows the effect of relative humidity on NH3 sensing performance of undoped and Co-doped ZnO at 100 ppm NH3 concentration. The plot indicates that the sensor response decreases with an increase in humidity [45]. It has been suggested that at low relative humidity level, the charge carrier transport is mainly governed by chemisorbed hydroxyl ions by hopping mechanism whereas at higher relative humidity level the charge carrier transport might be due to the physisorption of water molecules [45]. Ambient atmosphere: The general phenomenon in gas sensors is the reaction on SMO surface with the target gas in the ambient atmosphere. Humidity level is different in the atmosphere and can be divided into two parts: dry and wet atmosphere. It has a significant impact on the response of C-SMO gas sensors [46,47]. In a wet atmosphere, water molecules play a major role while in dry atmosphere oxygen plays a crucial role in influencing the sensing performance. The dense water molecules and high electronegativity of oxygen can make them easily adsorb on the SMO surface, as an electron acceptor and due to this interaction the sensor resistance is altered [48].

FIGURE 5.20 Room temperature response of undoped and Co-doped ZnO thin films for 100 ppm NH3 at different relative humidity levels. Adapted with permission from G.K. Mani, et al., A highly selective and wide range ammonia sensor—nanostructured ZnO:Co thin film, Mater. Sci. Eng. B, 191, 2015, pp. 41
50. Copyright (2020) Elsevier.

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In wet atmosphere, the water vapors dissociate and the generated hydroxyl ions [Eq. (5.16)] adsorb on the surface of C-SMO [49]. Metals in SMOs are active sites and the adsorption mechanism is decided by the interaction of these active sites with the water molecules. H2 O$OH2 1 H 1

(5.16)

The effect of humidity on SMO surfaces can be explained on the basis that whether one water molecule is interacting with two lattice active sites or with one lattice active site of SMO. For example, the interaction of water molecules with SnO2 is shown in Fig. 5.21A and B and explained by Eqs. (5.17) and (5.18).
H2 O 1 2ðSn 1 OÞ22 Sn1 2 OH2 1 Vo21 1 2e2
2 H2 O 1 ðSn 1 OÞ2 Sn1 2 OH2 1 ðOHÞ1 o 1e

(5.17) (5.18)

It is evident from the Eq. (5.17) that if one water molecule interacts with two tin oxides then two free electrons ð2e2 Þ are produced from two oxygen vacancies ðVo21 Þ due to the formation of two Sn-OH bonds as shown in Fig. 5.21A. On the other hand, if two water molecules interact with two tin oxides, then each generated hydroxyl ion can interact with single metal site as shown in Eq. (5.18). The lattice oxygen binds with hydrogen ion and per metal site two OH bonds are formed as shown in Fig. 5.21B. In both cases, the water molecules adsorbed on the surface of SMO has a significant impact on the sensing response of C-SMO sensor. Thus, humidity has a strong impact on the sensing response of SMOs and therefore it needs to be separately optimized for better performance of C-SMO sensors [46,50].

FIGURE 5.21 Mechanism of humidity adsorption on SnO2 surface with (A) one water molecule with two metal sites and (B) one water molecule per metal site.

5.7 Semiconductor oxide sensor outcomes

If # 20% RH level is present in the atmosphere, it is considered as dry and . 20% RH level is classified as wet atmosphere. In the dry atmosphere, the other gases such as N2 and O2 have a characteristic influence on the gas-solid interaction. Due to the nonreactive nature of molecular N2, it is often used as a carrier gas to study the chemical kinetics of the interaction between a target gas and the SMO surface. Oxygen has high electronegativity (B 3.65) and lone pair electrons, which makes it suitable to adsorb easily on SMO surface. Thus the presence of oxygen plays a crucial role in the sensing process of SMOs. When O2 interacts with SMO surface, it accepts the surface electron and ionized in O2 form [51] as shown in Eq. (5.19) [24]. An ionic layer is formed on the metal oxide surface from these ionized (O2) ions, which contributes in the adsorption process of target gas on the surface. Thus the adsorption and desorption rate in C-SMO gas sensors depend on the concentration of oxygen and are responsible for the resistance change of C-SMO sensor during the measurement of sensing performance. O2 ðgasÞ 1 e2 ðsurface electron of SMOÞ"O2 2 ðionized formÞ

(5.19)

It can be depicted that the concentration of electrons on the SMO surface acts as an active site for the O2 adsorption. The number of electrons gained by the oxygen (O2) through the adsorption decides the ionic strength (α) of the adsorbed species (O22 or O222). At higher temperatures, molecular species turn to atomic, either lattice (O22) or dissociative (O2) [24,33], and by using a sensitizing layer (catalysts or dopants) [52], it can be enhanced further. On SMO surface, the oxygen deficiency sites ðVo Þ are present and then the adsorption process can be described as 1 VO 1 2e2 1 O2 "Olattice 2

(5.20)

Ambient O2 diffuses on the oxygen deficiency sites on SMO surface by taking free electrons (one or two) and reduces the number of active sites for further adsorption process. As a result, the resistance of n-type SMO materials increases.

5.7 Semiconductor oxide sensor outcomes 5.7.1 At lab level The interest of industrial and scientific world on C-SMO-based gas sensors comes from their several advantages, namely small size, high sensitivity in detecting very low concentrations (at levels of ppm or even ppb), the possibility of online operation, and batch production at low cost. C-SMO-based solid-state gas sensors are the best candidates for the development of commercial gas sensors for the detection of a wide range of gaseous chemical compounds. In recent years, much effort has been put by researchers to detect low concentration of pollutant gases using different combinations of sensitizers with pristine SMO materials. Table 5.3

157

Table 5.3 Various outcomes at lab level for C-SMO-based gas sensors. SMOs n-type

SnO2

ZnO

TiO2

WO3

In2O3

p-type

CuO

NiO Co3O4

Cr2O3

Target gas

Detection limit (ppm)

Operating temperature ( C)

Response time (s)

H2 CO NO2 NH3 H 2S SO2 H2 CO NO2 NH3 H 2S CO NO2

200 10 2 50 5 50 0.1 0.1 1 50 100 50 97

200 400 200 RT 300 85 350 RT 100 RT 200 300 RT

50 6 30 175 58.1 3 74 210 35
48 4 6

H2

500

150




H 2S CO NO2 H 2S CO NO2 H 2S NH3 H 2S CO NO2 NH3 H2

5 1 0.25 0.2 10 1 0.2 50 50 50 5 100 10

350
150 RT 235 RT RT RT 215 100 RT RT 300


15 120

66 234 36 50 60 1.5 2 3
6

Technique

References

Hydrothermal Co-precipitation Hydrothermal Sol-gel Electrospinning Thermal evaporation Electrospinning VLS method Hydrothermal and dipping Flame transport Hydrothermal Sol-gel/ solvothermal Vacuum pressure induction technology Aerosol-assisted chemical vapor deposition Electrospinning Solvothermal Chemical synthesis Carbothermal Electrospinning Solution process Hydrothermal Chemical reduction Electrospinning Hydrothermal Chemical synthesis Template synthesis VLS method

[53] [54] [53] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [38] [39] [72] [73] [74] [75] [76]

5.8 Challenges and future prospect

summarizes the different SMO-based low detection limits of air pollutant gases at the laboratory level developed in thin film and nanostructure form by various techniques such as hydrothermal, coprecipitation, sol-gel, electrospinning, thermal evaporation, vapor liquid solid (VLS) growth, flame transport, solvothermal, vacuum pressure induction technology, aerosol-assisted chemical vapor deposition, chemical synthesis, carbothermal, solution process, etc.

5.7.2 At industrial level Research on C-SMO-based solid-state gas sensors is going on from long and various sensor manufacturers are involved in the fabrication of commercial gas sensors throughout the world, as summarized in Table 5.4. These sensors are available in different detection ranges and depending on the requirement, the users can order these sensors. The majority of these commercial gas sensors are using the latest MEMS technology and hence power consumption in these sensors is low (B mW). Although most of these commercial sensor manufacturers are developing a single sensor for a target gas, there are companies which apart from a single sensor are also integrating two and three different SMO materials for simultaneous detection of more than one gas like SGX Sensortech, and Unwelt Sensor Technik, etc.

5.8 Challenges and future prospect An ideal C-SMO-based gas sensor possess high response, sensitivity, selectivity and stability, dynamic range, low detection limit, good linearity, small response/ recovery time, and long-life cycle. Investigators usually make efforts to study only some of these performance parameters and disregard others. As fabrication of a perfect sensor for some gases is extremely difficult, so far real applications usually do not require sensors with all excellent characteristics at once. For example, a sensor device monitoring the concentration of a component in the industrial process does not need a detection limit at the ppb level. However, the response time at a range of seconds or less would be desirable. Additionally, when pollutant concentration usually changes slowly in environmental monitoring applications, the detection limit requirement can be much higher. Still, the response time of a few minutes can be acceptable. The performance of C-SMO gas sensors is subjected to degradation (drift) over a period of their operation for various reasons that cause variation in the characteristic parameters of the gas sensors and lead to uncertain results and false alarms. These gas sensors need frequent calibration in order to ensure accurate and precise measurements or sensor replacement, if required. The chemical stability of gas sensors is affected by their routine operation towards certain analytes under identical conditions causing temporal variation of sensor response, which is

159

Table 5.4 Various outcomes at the industrial level of C-SMO gas sensors. Company (Reference) Alphasense, United Kingdom [77]

SGX Sensortech, Switzerland [78]

Target gas

Detection Range (ppm)

Sensor resistance in air (kΩ)

Operating temperature range ( C)

Heater power consumption (mW)

H 2S

1
100

280 6 50


20 to 120

CO

5
500

220 6 50


20 to 120

VOCs (C4H8, C2H5OH, C6H6)

C4H8 (1
100); C2H5OH (0
100); C6H6 (0
0.5)

220 6 45


20 to 120

360 6 30 for 400 C 340 6 30 for 400 C 340 6 30 for 400 C

NO2 (MiCS2714) H2 (MiCS2714) NH3 (MiCS5914) CO (MiCS5524) CO, NO2 (MiCS 4514)

0.05
10

0.8
20


30 to 120

30
50

1
1000

0.8
20


30 to 120

30
50

1
500

10
1500


30 to 85

60
73

1
1000

100
1500


30 to 85

71
81

CO (1
1000)

100
1500


30 to 85

71
81

NO2 (0.05
10)

0.8
20

CO (1
1000)

100
1500

NO2 (0.05
10)

0.8
20

30
50

NH3 (1
300)

10
1500

60
73

CO NO2 NH3 (MiCS6814)

30
50
40 to 120

71
81

Response (Ra/Rg) (S) B3 at 8 ppm B1.1 at 8 ppm B3.5 at 20 ppm C4H8 B7 at 10 ppm C2H5OH B 1.1 at 0.5 ppm C6H6 B10 at 1 ppm NO2 B 1 at 1 ppm H2 B0.3 at 10ppm NH3 B0.01 at 1000ppm CO B0.01 at 1000ppm CO B 10 at 1 ppm NO2 B 0.01 at 1000 ppm CO B 10 at 1ppm NO2 B 0.5 at 10 ppm NH3

Figaro, United States [79]

Nissha, FIS, Japan [80]

Microsens SA, Switzerland [81]

NH3 (TGS 826) H2 (TGS 821) CO CH4 (TGS3870B00) H2S (SB51
00) CO (SB500
12) NH3 (SB53
01) H2 (SB19
00) CH4 (SB12C-00) VOC/Alcohol (SB-30
04)

CO (MSGS3000i) CO VOC (MSGS5000i)

30
300

B 115

30 to 50

833

B 0.75 at 100 ppm NH3 B 1 at 100 ppm H2 B1 at 100 ppm CO B 1 at 3000 ppm CH4

10
5000





10 to 40

660

CO (50
1000)

B 1000


10 to 50

11
120

CH4 (1%
25% LEL)

B 10

1
100





10 to 50

120

20
100





10 to 50




1
100

5
50


10 to 50

120

10
2000





10 to 50

120

300
10000





10 to 50

12

C2H5OH (1
100) C4H10 (20
100)





10 to 50

120

1
1000

20
400


40 to 120

85
120

B 1 at 60 ppm H2S B 1 at 100 ppm CO B 0.2 at 10ppm NH3 1 at 100 ppm H2 1 at 1000 ppm CH4 0.2 at 20 ppm C2H5OH 0.6 at 20 ppm C4H10


CO (1
1000)C2H5OH (10
500)

100
1500


30 to 85

71
81




(Continued)

Table 5.4 Various outcomes at the industrial level of C-SMO gas sensors. Continued Company (Reference) Umwelt Sensor Technik (UST), Germany [82]

Target gas NOx (GGS 7330 T) NH3 (GGS 4430 T) H2 (GGS 6530 T) C2H5OH (GGS 8530 T) VOC GGS 10530 T CO, NO2, CH4 (3A4P102T)

Detection Range (ppm)

Sensor resistance in air (kΩ)

Operating temperature range ( C)

Heater power consumption (mW)

Response (Ra/Rg) (S)

0.01
1

50 6 35


25 to 70

280

B 3 at 1ppm

1
1000

50 6 35




590

1
1000

20 6 35




B 460

0.6 at 10 ppm NH3 1 at 1 ppm H2

1
1000

30 6 20




B 425

0.8 at 1000 ppm C2H5OH

C4H10 (1
100)

200 6 150




B 340

B 0.6 at 10 ppm C4H10

CO (7.5
60)

50
3500




B 450

NO2 (0.075
0.6)

30
3000

CH4 (5
60)

30
3500

B0.4, 0.9, and 0.9 at 7.5 ppm CO B2, 14, and 2 at 0.6 ppm NO2 B0.85, 0.92, and 0.80 ppm CO

References

generally attributed to “sensor aging” or “thermomechanical degradation.” The environmental operating conditions also influence the chemical stability of sensing elements in a long run and the performance and life span maintenance is a challenge. Hence, the periodic testing and validation of gas sensors are of great requirement. The stability of sensing layer can be improved by proper optimization of sensor fabrication process and precise sensor quality control. Also, following rigorous measuring procedures, which includes sensor cleaning and regeneration of sensing surface conditions, would support to achieve good repeatability of gas sensor characteristics. These measures will help to prolong the validity of gas sensor performance, but as per the nature of individual gas sensors, recalibration is necessary at regular intervals. In C-SMO gas sensors, the different structural and morphological variations, chemical diffusion of oxygen vacancies, and degradation of the electric contacts lead to the generation of drift. Therefore, all the gas sensors need to be tested and recalibrated using calibrated gases in precise and accurate simulated atmosphere to ensure the characteristics of performance parameters for desired applications of C-SMO gas sensors.

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591. [49] O. Wurzinger, et al., CO-sensing properties of doped SnO2 sensors in H2-rich gases, Sens. Actuators B Chem. 103 (1) (2004) 104
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19. [53] B. Bhangare, et al., Role of sensitizers in imparting the selective response of SnO2/ RGO based nanohybrids towards H2S, NO2 and H2, Mater. Sci. Semicond. Process. 105 (2020) 104726. [54] C.-T. Wang, et al., Gold/vanadium
tin oxide nanocomposites prepared by coprecipitation method for carbon monoxide gas sensors, Sens. Actuators B Chem. 176 (2013) 945
951. [55] K. Khun Khun, et al., SnO2 thick films for room temperature gas sensing applications, J. Appl. Phys. 106 (12) (2009) 124509. [56] W.-T. Koo, et al., Metal
organic framework templated catalysts: dual sensitization of PdO
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[57] X. Zhong, et al., SO2 sensing properties of SnO2 nanowires grown on a novel diatomite-based porous substrate, Ceramics Int. 45 (2, Part A) (2019) 2556
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CHAPTER

Synthesis and application of carbon-based nanocomposite

6

Rashi Nathawat1, Satyapal S. Rathore2, Poonam R. Kharangarh3, Reena Devi4 and Anita Kumari5 1

Department of Physics, School of Basic Science, Manipal University Jaipur, Jaipur, Rajasthan, India 2 Department of Physics, Cluster University Jammu, Jammu, India 3 Department of Physics and Astrophysics, University of Delhi, New Delhi, India 4 Department of Physics, DAV College, Jalandhar, Punjab, India 5 Department of Electronics, Sri Aurobindo College, University of Delhi, New Delhi, India

6.1 Introduction Carbon nanotubes (CNTs) and their derivatives have opened up a broad range of technological applications due to their fascinating physical properties. In the last decade, the field of CNT semiconductor metal oxide (SMO) nanocomposites and inorganic hybrid carbon-based nanostructures have shown remarkable growth to achieve novel functional properties [1 3]. However, the product properties in composites were observed to be highly sensitive to the CNT parameters such as shape, size, interaction, dispersion, alignment, purity, aspect ratio, and the volume fraction of nanotubes. All these parameters can be controlled by choosing a suitable synthesis technique to achieve reproducible electrical, physical, and mechanical properties, essential for device applications. Thus, various synthesis techniques were employed to synthesize CNT SMO nanocomposites along with their advantages and drawbacks will be discussed in detail in this chapter.

6.2 Synthesis of carbon materials/SMO nanocomposites The methods developed to combine CNT with SMOs can be broadly divided into ex-situ and in-situ (Fig. 6.1): The first step, which is invariably utilized in both in-situ and ex-situ approaches, is the functionalization of the CNTs. The hollow and layered nanostructure of CNTs has a high aspect ratio, high surface to volume ratio, low mass density, controllable surfaces, and finely developed mesopores for supporting the SMOs like TiO2, SnO2, ZnO, In2O3. Due to the high aspect ratio, and the van der-Waal forces the CNTs prefer to stick together as bundles which in turn Carbon Nanomaterials and their Nanocomposite-Based Chemiresistive Gas Sensors. DOI: https://doi.org/10.1016/B978-0-12-822837-1.00005-8 © 2023 Elsevier Inc. All rights reserved.

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FIGURE 6.1 The methods developed to combine CNT with SMOs can be broadly divided into ex-situ and in-situ.

reduces their interaction with SMOs [1,4]. This challenge can be overcome through functionalization (introduction of chemical functional groups such as alcoholic, aldehydic, carboxylic, ketonic, and esteric oxygenated) of CNTs. Especially for CNT SMO nanocomposites, the alcoholic and carboxylic functional groups are utilized to facilitate the binding of the SMO nanostructure on the surface of CNTs by increasing the active sites.

6.2.1 Ex-situ techniques In this approach, the individual components, that is, functionalized CNTs, as well as the SMOs with required size and morphology, are first synthesized separately followed by decoration of CNTs with SMOs. The strength of interaction regulates the distribution of SMOs by functionalization on CNT surfaces. This technique can be further classified based on the interaction between the constituent phases namely covalent, π-π stacking, and electrostatic interaction. The main advantage of this approach is the flexibility of decorating with a variety of SMOs irrespective of their chemical and physical characteristics.

6.2 Synthesis of carbon materials/SMO nanocomposites

6.2.1.1 Covalent interactions SMOs can be attached to the carboxyl group because of their hydrophilic nature [5 7]. On the other hand, functionalized CNTs are easily attached to amineterminated metal oxide nanoparticles via amide bonds [8]. A weak interaction between oxides and acid-treated CNTs is observed. To improve the adhesion, capping agents are required.

6.2.1.2 π-π stacking The π-π stacking can be achieved by abstemiously strong interaction between π-electrons of CNTs and aromatic organic compounds such as pyrene [9 11], porphyrins [12 14], phthalocyanines [15], as well as benzyl alcohol or triphenylphosphine. The major advantage of this technique is that long alkyl chains of these molecules are terminated with thiol, amine, and acid groups, which can be easily connected to SMOs nanoparticles. The strong adsorption of aromatic organic compounds on the CNT surface provides improved solubility and also allows constant redispersion of CNTs in aqueous and organic solvents.

6.2.1.3 Electrostatic interactions The electrostatic interaction between SMOs and CNTs can be attained by deposition of ionic polyelectrolytes to attract nanoparticles like Al2O3, ZrO2, and TiO2 [16]. Polyelectrolytes such as polyethyleneimine covalently bond to the functional groups on the CNT by physisorption [17 22].

6.2.2 In-situ techniques The main advantage of in-situ synthesis is the direct deposition of metal oxides on pure or modified CNTs in the form of crystalline film or nanostructure such as nanoparticles, nanorods, nanobeads, etc. In this technique, the growth of both the constituent phases can be achieved simultaneously. Physical and chemical deposition techniques are employed for the said process, either in solution or in the gas phase. The SMOs deposition on CNTs is widely described in two phases: (1) solution phase and (2) gas phase. In solution-phase process, the following methods are used: (1) electrochemical reduction, (2) electrodeposition, (3) sol-gel process, and (4) hydrothermal technique; on the pther hand, in gas phase, chemical vapor deposition (CVD) and physical vapor deposition (thermal evaporation, and sputtering) are employed [23,24].

6.2.2.1 Electrochemical techniques The nucleation and growth of MO nanoparticles can be effectively controlled by electrochemical techniques such as chemical reduction and oxidation, electrodeposition, and sol-gel process.

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6.2.2.2 Chemical reduction and oxidation In such processes, metal oxides require either oxidizing or reducing agents to process as the name states. In addition to agents, these processes are carried out with heat, light, ultrasound, microwave, etc. As an example of the chemical reduction process, MnO2 is deposited on CNTs via the reduction of KMnO4 [25]. In this process, KMnO4 is used both as an oxidizer and reactant.

6.2.2.3 Electrodeposition In comparison to the chemical reduction and oxidation processes, the electrodeposition process is less time-consuming. In addition, metal/metal oxide nanoparticles deposited on CNTs surface exhibit higher purities and good adhesion. The principle of strong adhesion behind this is the van der Waals interaction between both the phases. The electrodeposition of TiO2 on CNTs is reported [26] by Frank et al. In this process, TiCl3 was used as a precursor and electrolyte which has been retained at pH 2.5 with HCl/NaCO3.

6.2.2.4 Sol-gel process The sol-gel process is a low-temperature and cost-effective technique. Furthermore, such techniques control the chemical composition and concentration of dispersed dopants. The sol-gel process was used to develop nanoparticles of ceramic material and also for thin-film coating. This is the elementary process of the transition of a liquid to a solid. The developed product is in the form of an amorphous phase instead of a crystalline phase, which is the limitation of this technique. The postannealing and crystallization processes are essential to opt for a good quality synthesized product. 1. Covalent: In this technique surface chemistry of CNTs has been modified by robust oxidizing acids like H2SO4 or HNO3. Such procedures include different variety of organic groups with partial control on the number, type, and location. The covalent technique has been reported on acid-treated CNTs for numerous MO deposition such as SnO2 [27 31], TiO2 [32 34], RuO2 [35,36], CeO2 [37], NiO [38], and mixed oxides [39]. 2. Noncovalent: The noncovalent technique is also employed to grow the SMOs on the CNT’s surface. For example, the coating of TiO2 on CNTs surface by using benzyl alcohol as a surfactant, nondestructive, and simple process explored by the researcher [40]. Surfactant adsorbs on the CNT’s surface by π-π interactions which is similar to ex-situ approach with the alcohol’s benzene ring, providing hydrophilic hydroxyl group for the hydrolysis of the Ti precursor. 3. Electrostatic: The electrostatic interaction for an in-situ approach in sol-gel route is successfully applicable for to a few metal oxide, Al2O3, TiO2, and ZnO. Researchers have developed a method to interact acid-treated CNTs with Zn12 ions to synthesize ZnO nanoparticles in the presence of a Li-OH [41].

6.2 Synthesis of carbon materials/SMO nanocomposites

6.2.3 Hydrothermal and aerosol techniques The hydrothermal method is one of the best techniques to synthesize crystalline particles or films without postannealing and calcinations. This technique helps researchers to achieve nanostructure at a low cost. Various types of organicinorganic hybrid structures can also be developed by hydrothermal techniques [42].

6.2.3.1 Vapor-assisted, polyol-assisted process In vapor-assisted hydrothermal synthesis process, either pristine or acid-treated CNTs are mixed with precursor and retained in an autoclave at moderate temperature to produce metal oxide films like ZnO, TiO2, and Fe2O3 [43 45].

6.2.3.2 Supercritical solvent Supercritical solvents, such as CO2, work as an antisolvent and reduce the strength of alcohol in an aqueous solution, resulting in the deposition of Eu2O3 [46], CeO2, La2O3, Al2O3 [47], SnO2 [48,49], and Fe2O3 [50] onto pristine CNTs.

6.2.4 Gas-phase deposition Physical and CVD is the utmost method to develop metal oxide nanostructure with size, shape, and uniformity control. These techniques are employed to achieve a uniform and continuous films on CNTs substrate without altering its 3D integrity.

6.2.4.1 Evaporation and sputtering Evaporation is one of the simplest methods of physical vapor deposition that uses a resistive heat source at a high temperature in a high vacuum chamber to evaporate a solid material to deposit a thin film in a vacuum environment. Another important technique is sputtering, which depends upon plasma to attack the target material at low temperatures. The ions from plasma strike the target and atoms are sputtered from the surface. The sputtering film is found to be better than the evaporated film in adhesion and composition. In contrast, better structural and morphological control of thin film can be obtained by evaporation.

6.2.4.2 Pulsed laser deposition Pulsed laser deposition (PLD) is a type of physical evaporation technique, where a high-power pulsed laser beam is used to strike the target material to deposit a film. When the laser beam is absorbed by the target material, it produces a plasma comprising numerous charged particles such as electrons, ions, atoms, etc. These particles are then deposited as a thin film on a typically hot substrate. PLD is commonly used for depositing oxide thin films. Such a technique is operated in ultrahigh vacuum and in the presence of a background gas as compared to others.

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6.2.4.3 Chemical vapor deposition The CVD technique is used to develop high-quality solid materials. It is also a vacuum deposition technique that is mainly used in the semiconductor industry. The process operates at moderate temperature and pressure in comparison to the high temperature and pressure techniques. By this technique, one can synthesize a high-quality crystal structure with utmost purity and composition. In addition, a high deposition rate and easy scalability can be achieved by CVD. In this method, the substrate is exposed to volatile precursors, which decompose on the substrate surface for required deposition. Researchers have used this technique to produce CNT-metal oxide composites SnO2 and RuO2 [51,52].

6.2.4.4 Atomic layer deposition Atomic layer deposition (ALD) is another kind of CVD technique, where two precursors are used as reactants, which sequentially react with the material. The thin film is deposited in repeated exposure of different precursors. By this process, an atomic-scale deposition can be controlled. An acid-treated CNT is coated with TiO2 and Al2O3 by this technique, using metal chloride as a precursor [53].

6.3 Synthesis of graphene/SMO-based nanocomposites When SMO is used as a gas sensor, it prevails to be a sensor with fast sensing response, ability to withstand abrasive ambiance, economical, and stable over a long period of time with easy fabrication steps. SMO has proved to be a great photocatalyst as well as a wonderful storage device and can be used in green energy systems. Graphene has received enormous research to produce graphenebased composites due to its peculiar characteristics, which include fascinating high conductivity, tunable electronic band structure, extremely-high specific surface area, and tailorable structure. Besides these unique properties, the main characteristic of graphene is that it expresses its presence where it is incorporated. Literature reveals the presence of graphene-enhanced efficiency for a large number of catalytic reactions and have attracted extensive attention due to their potential applications in photovoltaics, photocatalysis (including water splitting and CO2 reduction, degradation of pollutants, organic synthesis) [54], gas sensor, supercapacitors, lithium-ion batteries (LIBs), and photovoltaic areas [55 60]. A very few are listed here. Wang et al. displayed that a very small quantity of material is required for superior photocatalytic activity in Co3O4/rGO composite [61]. Jeevitha et al. showed that incorporation of graphene at an optimum value modifies the properties of SMO-based NH3 sensor by tuning the surface area and pore size [62]. The high surface area provided by graphene (intrinsic capacitance of 21 uF/cm2) in graphene-semiconductor metal oxide composites (G-SMO), with both the surfaces available for the electrolyte makes it the best potential candidate among all carbon-based material SMOs [63]. Researchers have utilized graphene

6.3 Synthesis of graphene/SMO-based nanocomposites

in SMO LIBs to create a barrier for oxygen diffusion to prevent corrosion which is one of the major issues in batteries. This also helps to make a better interface between an electroactive material and a current collector. These modified current collectors have been known to show an enhanced electrochemical performance which could be attributed to reduced internal resistance and improved charge transfer characteristics [64]. Fig. 6.2 presents various applications of G-SMOs. The large demand for graphene/SMO makes the synthesis of the nanocomposite one of the key steps to meet various research needs. Various techniques of synthesis of the G-SMO composite will be discussed later in this chapter.

6.3.1 Common synthesis methods of the G-SMO nanocomposites To date, various kinds of SMO have been synthesized which have been provided by a support with graphene. Literature survey showed that the graphene and family have been employed in a large number of semiconductor, which includes CdS, MoS2, Cu2O, ZnO, TiO2, GaP, ZnS, CdS, ZrO2, Ta3N5, LaTiO2N, Nb2O5, C3N4, TaON, NiO, BiVO4, Ce2O3, In2O3, InVO4, SnS2, CuInS2, CuO, ZnSe, In2S3, CuWO4, Ag3PO4, SnO2, MoO3, V2O5, WO3, Ag2O, Bi3S3, PbS, CdTe [63] and metal oxide such as TiO2, ZnO, SnO2, MnO2, Co3O4, Fe3O4, Fe2O3, NiO, and Cu2O [65,66] to improve the device quality. Koo et al. reported that graphene has the ability to improve the stability of composite (Cu(In, Ga)Se2/CdS/RGO/Pt) [67]. In graphene-based composites, the role of graphene can be understood in three broad categories: first, it acts as a functional component. Second, it may provide a percolation path for the other components in the composite owing to

FIGURE 6.2 Various applications of graphene-semiconductor metal oxide nanocomposites.

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conductive robust structure, thereby increasing the charge carrier transport and response. Third, graphene can be used as a substrate, providing space for anchoring the metal oxides owing to the large surface-to-volume ratio and enforcing the mechanical strengths of the outcome. In this section, we will mainly focus on recent achievements dealing with the development of effective strategies for synthesizing high-quality graphene SMOs. Fig. 6.3 depicts the various synthesis methods of G-SMOs.

6.3.2 Hydrothermal method Li et al. [68] prepared reduced graphene oxide (RGO)-WO3 composites using GO in water as a precursor for RGO and then dissolved 0.05 g NaCl, maintaining the pH to about 2 using HCl solution under hydrothermal conditions. After continuous stirring for half an hour, the suspension was transferred to a 50 mL Teflon-lined stainless steel autoclave. The whole process is simple, scalable, and industrially compatible with different weight ratios of GO and Na2WO4•2H2O. They have shown that the incorporation of graphene enhances the photocatalytic properties of SMO owing to the facilitation of electron transfer and thereby conductivity. An increment in the light absorption was also noticed by the researchers. In a similar typical synthesis of the Co3O4-graphene, Yao et al. dispersed 0.93 g of cobalt acetate tetrahydrate (Co(C2H3O2)2
4H2O) and 0.3 g of GO in 20 and 250 mL of distilled water (DW) followed by sonication, respectively. Subsequently, GO and then 10 mL of ammonia solution were gradually added to it, and the final result was centrifuged and washed thoroughly with water and ethanol [69]. They observed higher catalytic activity because of the incorporation of graphene which was fully

FIGURE 6.3 The various synthesis methods of G-SMOs.

6.3 Synthesis of graphene/SMO-based nanocomposites

exfoliated and decorated with Co3O4 NPs in SMO, proved by the characterization techniques Yao group performed. A similar approach was taken to synthesize the composite by Khadgi et al. through the coprecipitation of Zn (NO3)2
6H2O, Fe (NO3)30.9H2O, and AgNO3 in the presence of the GO powder. In a typical synthesis process, the suspension with 15 wt.% graphene was sonicated with a power of 100 W for a duration of 60 min at 40 kHz frequency. Consequently, 1 mM Zn (NO3)26H2O, 2 mM Fe (NO3)39H2O, and 0.05 mM AgNO3 were mixed in 20 mL of ethanol followed by stirring for 30 min. The solution was again stirred for 2 h once added to the GO suspension, followed by transfer into a Teflon-lined stainless steel autoclave. The authors depicted ZnFe2O4 and Ag NPs immobilized on graphene sheets, which not only generated the electron hole pair efficiently but also suppressed the probability of electron hole recombination and prolonged the lifetime of the charge carriers [60]. Liu et al. synthesized composite of GO with NiFe2O4 using the same process; where GO, NiSO4H2O and FeCl36H2O, which were separately dissolved in 10.0 and 15.0 mL of deionized water respectively, followed by stirring. The suspension solution was then transferred into an autoclave and kept under high pressure to obtain GO NiFe2O4 precipitates [70]. By the above-mentioned process, the authors obtained a catalyst that may have a significant function in photo-Fenton catalysis during organic pollutant removal. MnFe2O4-graphene nanocomposites were synthesized by selecting different graphene contents by Fu et al. In this process, graphene in 60 mL of ethanol was sonicated for 1 h and Mn(NO3)2 solution with Fe (NO3)3 9H2O were mixed together and transferred into a 100 mL Teflon-lined stainless-steel autoclave, where was heated to 180 C for 20 h under autogenous pressure [71]. They reported a significant enhancement in photoactivity which was accredited to the excellent conductivity of the RGO, which is favorable for the efficient separation of photogenerated carriers in MnFe2O4. Erping et al. also synthesized with similar r two-step route in which a mixture of Bi(NO3)3 5H2O dispersed into 5 mL of 4 M nitric acid solution and GO suspension was magnetically stirred (for 1 h) and then Na2WO4 (in deionized water) was added dropwise to the solution, adjusting pH of the final suspension to about 7. The suspension was centrifuged, washed with absolute ethanol five times, and dried in a vacuum oven at 60 C for 12 h [72]. The resultant composites can be employed as potential photocatalysts. A graphene-SMO nanocomposite was obtained by Zhang et al. with metal ions acting as interfacial mediators in the composite by a similar process. The chemical compound Cd(CH3COO)20.2H2O (0.4 mmol) was added to inhomogeneous graphene-metal-dimethyl sulfoxide dispersion. After stirring for 30 min, the mixture was transferred to a 50 mL Teflonlined stainless steel autoclave [73]. Rani et al. reported a faster degradation rate of rGO/ZnFe2O4 towards methylene blue dye as compared to pristine Fe3O4 and ZnFe2O4 nanostructures ascribed to the larger surface area and the synergic effects of graphene sheets and ZnFe2O4 nanoparticles. GO dispersion obtained using the modified Hummer’s method [74] by the authors was used to make a solution with 0.7 M C2H6O2, containing 2 M FeCl36H2O and 1 M ZnCl2. Subsequently, 1 M CH3COONa was added and stirred for 1 h. The solution was then transferred to a

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50-mL Teflon-lined stainless steel autoclave and heated at 200 C for 8 h. The centrifuged output, rGO/ZnFe2O4 composite, was then washed with deionized water and ethanol several times before drying [57].

6.3.3 Self-assembly method Shao et al. synthesized ZnO nanosheets amended by graphene quantum dots (GQDs) and SnO2 quantum nanoparticles for addressing the low selectivity issue of SMO-based gas sensors at room temperature. The Sn precursor solution was prepared by dissolving 1 g of SnCl4 in 5 mL ethanol in companion to 0.1 g GQD. At that time, 0.2 mL 12 M HCl (37 wt.%) was added dropwise with the aid of ultrasound for 5 min, followed by constant stirring. The final sensor device was achieved by spin-coating, and thus obtained product solution onto a substrate with ZnO nanorods at 4000 rpm for 60 s under controlled conditions of humidity [75]. The authors demonstrated that the presence of graphene modified the hierarchical structure of SMO, which has a high potential in the noninvasive exhaled diagnosis. They tested the performance of GQD-SnO2/ZnO sensors for various target gases.

6.3.4 In situ method An in-situ growth approach has been considered as one of the best methods to synthesize graphene-based nanocomposites [76]. Zhang et al. [77] demonstrated the synthesis process of a layered structure graphene metal oxide composite with well-dispersed nanoparticles (SnO2 and TiO2), decorated on the surface of the RGO. Graphene metal oxide composite was prepared by mixing the solutions of 4 mL of the GO in 50 mL of DI water and 2 mL of titanium trichloride (0.14 g of tin(II) chloride dihydrate in SnO2 case) in 50 mL of HCl solution followed by sonication for 5 min. RGO-metal oxide solids thus obtained were stirred at 90 C for 6 h and collected by centrifugation, washed several times with DI water, and dried in a vacuum oven at 120 C. According to the author’s observation, the existence of the graphene sheets dramatically improves the photocatalytic activity of the composite for the degradation of dyes under visible light irradiation. This is due to effective charge separation and rapid conductivity at the same time as effective retardation of charge recombination. Graphene-based nanocomposites were synthesized by a similar in situ approach by Gao et al. [78] for potential applications in catalysis and pollutant removal. Polydopamine-modified RGO (PDA/RGO) was used as a versatile platform for further functionalization of RGO by noble metal (Au, Ag, Pt) NPs, a metal oxide (Fe3O4), and a semiconductor (TiO2) as model nanoparticles. This was done by in situ nucleation and growth on the surfaces of RGO by simply mixing PDA/RGO and the corresponding precursors at room temperature for 30 min, under mild conditions which are free from hazardous chemicals, reducing agents as well as high temperature or pressure conditions. The polydopamine acts as a reducing agent to reduce metal/metal oxide/

6.3 Synthesis of graphene/SMO-based nanocomposites

semiconductor and grow the corresponding NPs on the surfaces of RGO. For example, chloroauric acid was used as a precursor to depositing Au NPs on the surfaces of RGO. They presented a facile and versatile approach toward designing graphene-based nanocomposites. Song et al. [79] reported functionalized graphene/ZnO (FGZnO) nanohybrids were synthesized through a modified in situ method for enhanced gas-sensing properties. In the typical process, zinc nitrate hexahydrate (Zn(NO3)26H2O, 1.8593 g) was added into an aqueous solution of GO with a concentration of 4 mg/mL and polyvinyl pyrrolidone (PVP, 80 mg) under strong stirring for 5 h under ambient conditions. Free Zn(OH)2 and PVP were washed off from the resultant FGO sheet mixture. To this, aqueous solutions of Zn(NO3)26H2O and hexamethylenetetramine were subsequently added with various concentrations. After being stirred for 30 min at room temperature, the mixture was left in an oven at 80 C for 12 h. Afterward, it was irradiated under UV light for 3 h and then washed and dried at 60 C for 12 h. Bai et al. [80] reported the photocatalytic efficiency of ZnWO4 nanocomposite under visiblelight and UV-light irradiation increased 2.3 times because of the employment of graphene synthesized by the method. This is attributed to the high separation efficiency of photoinduced electron hole pairs stimulated by graphene hybridization. To acquire ZnWO4/graphene composites, GO dispersed in 100 mL water was exfoliated and ZnWO4 synthesized by the hydrothermal process was added, followed by ultrasonication for 30 min and stirring for 48 h. The RGO is obtained with an appropriate amount of hydrazine solution and ammonia solution. After being vigorously stirred for a few minutes, the dispersion was put in a water bath for 3 h.

6.3.5 Solution mixing method Solution mixing is a direct and simple approach used to prepare graphene-based nanocomposites by solution mixing of graphene and SMOs. Mukherji et al. [81] demonstrated that the Pt-loaded graphene staging Sr2Ta2O7 xNx photocatalyst could be employed for achieving more efficient clean energy conversion systems. The deposition of Pt nanoparticles on graphene sheets was carried out by mixing GO (prepared by the Hummers’ method) with H2PtCl6 followed by drop mixing 50 mM of NaBH4 solution to reduce the platinum salt onto the GO layer. Sr2Ta2O7 xNx was synthesized by calcining stoichiometric mixtures of SrCO3 and Ta2O5 powders and doped with nitrogen in an ammonia atmosphere, and then added to this resultant material in varying ratios.

6.3.6 Spin coating Ni-doped flame-spray-made SnO2 nanoparticles loaded with graphene of different weights have been studied by Singkammo et al. [82] for acetone-sensing applications. The sensor film in their work was made by spin-coating a paste produced by thoroughly mixing and grounding Ni-doped SnO2 and graphene nanopowder (obtained by electrolytic exfoliation process) in the binder solution comprising

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ethyl cellulose. They concluded that acetone response was substantially enhanced owing to the resultant large surface area of the nanocomposite, which provides a stage for a high interaction rate between gas vapor and graphene 2 Ni-doped SnO2 nanoparticles. Zhang et al. proposed graphene fortified slag-based nanocomposite, which was incorporated as a photocatalyst for solar green hydrogen evolution. The nanocomposites were obtained by employing graphene into the alkali-activated blast-furnace slag-based cementitious material (1.25 wt.% TiO2 and 1.07 wt.% Fe2O3) by mixing graphene with slag and NaOH in water in certain weight ratios with stirring nearly 2 min. This was followed by curing in a box at 20 C for various curing ages. The authors concluded that the efficiency of hydrogen generation was substantially increased due to the large separation of photogenerated electron hole pairs in the environment-friendly nanocomposite [83].

6.4 Conclusion From the literature review on synthesis methods of graphene-based SMO, it may be concluded that many attempts have to be taken to control the quality and morphology of the composite material. In order to achieve it, a lot of development is required in the synthetic procedures with precise control in thickness, lateral dimensions, and defect levels of all the constituent materials. This may be done by targeting the unique properties of graphene and doing slight modifications to impart exotic functionalities. As we are aware that graphene has remarkable properties, it can bring in revolution in the field of SMO composites. Based on this, a more novel and efficient graphene-based SMO can be fabricated, which is presumed to play an important role in catering to various environmental-related issues.

6.5 Synthesis of CNTs/conducting polymers-based nanocomposites Carbon is considered to be the second most abundant and promising element on the earth. The carbon nanostructures that have versatile photoluminescence properties include CQDs [84], GO [74,85], nanodiamonds [86], GQDs [87 89], and CNT [90]. In 1991, Iijima discovered needle-like CNTs [91]. CNTs have been rolled-up forming tubes as a result in single-walled carbon nanotubes (SWCNTs) due to the presence of SWCNTs and multi-walled CNTs (MWCNTs) owing to multiple layers of graphene rolled up sheets. The single layer of graphene in SWCNTs is efficiently confined into a cylindrical tube. MWCNTs consist of an arrangement of concentric cylinders coaxially organized around a central hollow core between adjacent layers with van der Waals forces. The various geometries might be responsible due to the wrapping of graphene layers. The tubes are

6.6 Functionalization of carbon nanotubes

categorized in the form of “armchair, zigzag or chiral,” depending on the angle in terms of rolling of the graphene sheet. CNTs and conducting polymers (CPs), with their unique characteristics, are good materials. Mostly, CPs are classified as conjugated polymers with single and double bonds in their sp2 hybridized structure and well-thought-out as linear chains. Similarly, sp2 hybridized bonds in CNTs are brought into being throughout the structure. The coupling of CPs and CNTs furthermore explains significant effects due to their good mechanical, high thermal conductivity, electrical, and electrochemical features [92 94]. In general, for CPs/CNT scheme, there are two ways, either to functionalize CNTs with CPs via chemical polymerization or modification or through doping of CPs with CNTs. The fabrication of CNTs, SWCNTs, or MWCNTs with altered geometry is done by using different synthetic methods involving gas-phase processes in CVD, laser ablation, and arc discharge which are explained in brief.

6.6 Functionalization of carbon nanotubes with covalent and noncovalent Chemical methods are considered to be one of the best methods for modifying the energy of the surface for the CNTs by refining their dispersion stability and adhesion properties. These approaches are designed to alter the surface interaction by functionalization (covalently) or adsorption (noncovalently). The diverse physical properties of functionalized CNTs have been observed as compared to those of the original nanotubes. Thus, the main advantage of functionalized CNTs is that is attractive in all kinds of applications [90]. Dispersion of CNTs plays a crucial part during the fabrication. To improve the dispersion, CNTs are functionalized with polymer molecules. Thus, CNT-based composites were synthesized to explore novel belongings. The covalent and noncovalent attachments are the main tools to make improvement in CNTs with polymers [95]. However, the covalent attachment method denotes “grafting to” and “grafting from,” while noncovalent attachment is known as polymer wrapping and absorption. In the grafting technique, the surface of CNT was attached by polymers via in situ polymerizations of monomers in the occurrence of reactive groups of CNTs. The benefit of this method is the production of high grafting density of polymers [96]. Grafting method refers to the attached molecules of polymer on the CNT surface by chemical route for an instant reaction between functional groups on the surface of nanotube and polymers [95,96]. In 2007, Kitano et al. modified SWCNTs by using an azo-type radical initiator having “poly(2-methacryloyloxyethyl D-glucopyranoside) blocks (PMEGlcinitiator).” The oxidation of SWCNTs was done by incubation through mixing a solution of nitric acid and sulfuric acid to have carboxyl groups at their ends. As a result, the SWCNT covalently improved with glycopolymers owing to its high radical trapping

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activity consisting of “cloven macroinitiator (PMEGlc-SWNT)” [97]. Furthermore, SWCNT covalently functionalized with “terminal-aminated poly(N-isopropyl acrylamide) (PIPA)” between carboxylated SWCNT and PIPA through a condensation reaction. “Terminal-aminated PIPA, 1-hydroxybenzotriazole monohydrate (HOBt) and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide HCl (WSC)” were added to the dispersion of carboxylated SWCNT at pH level of 5.5 for 3 days. Finally, the SWCNT was completely washed with DW through centrifugation as well as by ultrafiltration. The functionalized SWCNT was found to be stable when dispersed in aqueous solutions while maintaining a definite temperature responsiveness. Hence, these are promising methods for the modification in SWCNTs which are functionalized with various polymers. After 3 years (in 2010), PTEG/MWCNT nanocomposites were fabricated by selecting precursors as “Poly(trimethylene terephthalate)-poly(ethylene glycol) segmented copolyester (PTEG) and MWCNT” with different compositions of “MWCNT and copolyester” [98]. MWCNTs were prepared by using CVD by having lengths (few microns) and diameters (in between 70 and 100 nm). The purification of MWCNTs was done to eliminate the present impurities related to iron and amorphous carbon. Hence, MWCNTs play an effective role in nucleating agents during crystallization for the preparation of composite by creating many more sites of nucleation towards the phase to speed up the route for the crystallization of the PTEG matrix for the creation of smaller spherulite. In 2011, the CNTs were functionalized covalently which was investigated by “surface-initiated ring-opening polymerization of epoxides” [99]. The functionalized morphology of CNTs revealed that the nanotubes were rolled with chains made of polymers forming core-shell structure. The extent of grafted polymer changed from 14 to 74 (wt.%) by the rising reaction temperature. After the surface modification of CNTs, the oxygen to carbon (O/C) ratio of CNTs increased significantly from 5.1% to 29.8% [99]. In 2013, the polystyrene (PS)/MWCNTs nanocomposites were prepared by using an in situ bulk polymerization a facile and simple method [100]. Firstly, for the preparation of PS/MWCNT nanocomposites, the researchers introduced polymerizable groups on the MWCNT surfaces through esterification which is based on the “carboxylate salt of carbon nanotubes and p-chloromethylstyrene in toluene” [100]. Then the modification in MWCNTs with styryl was prepared by “copolymerization with styrene and pchloromethylstyrene grafted MWNTs initiated by 2, 20-azobis(isobutyronitrile) (AIBN),” which was used as macrocomonomer. The solutions prepared for pMWNTs/PS show the influence of hyperchromic effect in chloroform. The optical characterization carried out by “Fourier transform infrared (FTIR),” “1 H nuclear magnetic resonance (NMR),” and “UV-visible spectroscopy” of prepared nanocomposites reveals that styryl group was covalently bonded of MWNT surface. To improve Young’s modulus and strength without any damage in distortion; Seligra et al. [101] described an approach by “biodegradable polylactic acid (PLA),” which is linked covalently to MWCNTs within a great demand to make sure that there will be decent transfer in stress results in a good response for mechanical purpose. During synthesis, the modification in PLA was done with benzoyl chloride and functionalization of MWCNTs through “Fenton reaction” and reformed materials were confined

6.6 Functionalization of carbon nanotubes

covalently via a reaction of esterification. The noncovalent functionalization of MWCNTs was achieved by Morishita et al. [102] by using macromer-grafted polymers (MGPs). For enhancement in physical adsorption of MGPs on MWCNT surfaces, MGP was planned having chains of polymer side to improve the MWCNT solubilization inappropriate moieties related to solvents and pyrene in comparison to both chloroform and hexane, which are not considered to be a good solvent of MWCNTs. In another study, Mallakpour and Soltanian reported the functionalization of surfaces aimed at MWCNTs for achieving high solubility to resolve dispersionrelated issues [103,104].

6.6.1 Synthesis techniques 6.6.1.1 Arc discharge method Arc discharge is the process that uses high temperatures above 1700 C, resulting in the development of CNTs with fewer structural defects in comparison with the other techniques. This method is relatively simple under the condition with all the possible circumstances of growth confirmed. The furthermost developed approaches use DC arc discharge between the two electrodes of graphite rods (size 5 6 12 mm in diameter) in a chamber filled with helium gas. In brief, the chamber consists of two electrodes: one is anode filled with graphite powder along with the catalysts, and the other one is cathode with a rod for pure graphite. The chamber is generally filled using a gas or sometimes with a liquid atmosphere. An electric field is applied between the two electrodes to bring closure for the generation of an arc that maintains the gap of 1 2 mm to achieve a fixed discharge. A very high temperature (B4000 C) plasma is generated by the arc current, which sublime the carbon precursor inside the arc forming the CNTs. Nevertheless, some researchers use hydrogen or methane atmosphere during the synthesis of MWCNT. For example, Ebbesen and Ajayan [105] used the same technique for the synthesis of fullerene under the helium atmosphere to obtain the high yield for large-scale manufacture for CNTs. Later, Wang et al. used CH4 gas to obtain very thin and extended MWCNTs by selecting a gas pressure of 50 Torr and an arc current of 20 Afor the anode with specific diameter(B6 mm) [106], while Zhao et al. used a different atmosphere of hydrogen gas for synthesis of adequate and extended MWCNTs [107]. In the comparison of He and methane, the obtained results are quite different. Explicitly, the observed carbon smoke for evaporation in CH4 and He gases was found to be more as compared to H2 gas [108]. After 1 year [109], they showed that the deposition of graphite sheets on the cathode was done throughout the evaporation of graphite electrodes in the presence of H2 gas. Later, Zhao et al. [110] revealed the fabrication of MWNTs via the same techniques with different atmospheres like helium, hexane, acetone, and ethanol atmospheres under several pressures (150 500 Torr). They found that arc discharges in the organic atmospheres like ethanol, acetone, and hexane are much more productive, that is, producing more MWCNTs at least two times

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larger as compared with that of He atmosphere. This could be due to the ionization of helium, acetone, ethanol, and hexane and the decomposition of molecules into hydrogen and carbon atoms. As a result, the production of a higher yield of CNTs gets higher due to the contribution of ionized species during the synthesis of MWCNTs. This showed that there is an increment in yield in MWCNTs with an increase in pressure up to 400 Torr in the case of organic molecular atmospheres. Jiang et al. [111] demonstrated that this technique in the ammonia (NH3) atmosphere is considered to be one of the very effective ways for the synthesis of CNTs. They found that there are no noteworthy changes in shapes and structures as obtained in other atmospheres such as He, H2, etc. This method also has an advantage that it is very expensive due to high power usage needs and costly lasers.

6.6.1.2 Laser ablation method This method is very analogous to the arc discharge technique due to many related reaction conditions which occur with identical mechanisms. It was used for the synthesis of fullerenes. In this experiment, a laser pulse was focused onto a target related to graphite holding a catalyst of metal. The target is reserved in inert gas flow and heated to the system around 1000 C using an electric furnace. The laser vaporizes the targeted object into plasma with the formation of CNTs. This method produces a very high purity of CNTs ( . 90%) and narrower size distribution in comparison to other techniques like arc discharge and CVD technique. The main disadvantage of this method is that it is not appropriate for bulk fabrication of CNTs. At first in 1995, Smalley and his coworkers synthesized SWCNTs by using this method [112]. In this process, the carbon precursor material, mainly graphite, is doped by few extents of a metallic catalyst like Co and Ni. Then this mixture is evaporated through the existence of inert gas (typically argon) through the pulsed laser beam at very high temperatures (T 5 1200 C, const. P 5 500 Torr). As the laser beam (Nd:YAG) fills the target material, vaporization gets started. On the other hand, it gets condensed comparatively at a lower temperature. The double pulsed laser was also used for increasing the evaporation. As a result, the production of SWCNTs increased up to 1 g/day [113]. The different synthesis methods of MWCNTs are described in Fig. 6.4.

6.6.1.3 Solution mixing This is one of the easiest and commonly used techniques to synthesize the nanocomposites of CNT/polymer. In this process, CNT and polymer are dissolved in an aqueous medium. A nanocomposite film is made on the substrate surface after evaporation in a well-ordered condition for the good fabrication of various polymers like PMMA, PVA, and PS [114 116]. Many discrepancies have been confirmed by researchers for getting an improved dispersion of CNTs. Safadi et al. demonstrated the dispersion of MWNTs into the “polystyrene matrix” by using the ultrasonic agitation method [117,118]. A lot of work showed that the surface of CNTs is modified through functionalization to make an appropriate dispersion

6.6 Functionalization of carbon nanotubes

FIGURE 6.4 Different synthesis methods for the synthesis of MWCNTs.

[119]. Nonetheless, a lot of critical issues related to the compatibility of the polymer matrix-related functionalized group arose. A method of dispersion of CNT by surfactants addition was used to resolve this problem [120,121]. The CNT structure gets completed by using this approach. Sometimes, agglomeration was seen during synthesis in some cases during evaporated solvent. Guo et al. [115] presented spin casting to avoid this problem. An electrospinning is also another method where polymer fibers are processed by applying an electric field [122 124]. The disadvantage of this method is that the removal of the whole solvents is a serious matter while preparing nanocomposites. A lot of issues including toxicity can also arise while using organic solvents. Also, the mechanical properties are altered due to the presence of a residual solvent which induces plasticization of the polymer matrix.

6.6.1.4 Melt mixing The solution mixing method is a limiting factor in terms of solubility for the polymers into the solvent. To resolve this issue, an alternate approach called as melt mixing method was used. In this method, the various “thermoplastic polymers such as polypropylene, polystyrene, polycarbonate, poly(ethylene-2,6-naphthalate),” etc. are used [125 128]. This process comprises the formation of viscous liquid through the melting of the polymer after merging together with CNTs. The extrusion and injection molding techniques help to attain an improvement in the

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dispersion of CNTs [129,130]. This technique is supposed to be not considered as a highly efficient tool in comparison to the solution mixing method because of thermoplastic polymers with high viscosity which interferes to attain a constant CNT dispersion.

6.6.1.5 In situ polymerization This technique concerns the dispersion of CNT into the “monomer matrix” with or without solvent followed by regular approaches of polymerization. In this method, “monomers” have been chosen as precursor material as compared to polymers. The main benefit of this method is that it supports the “grafting of polymer molecules” on CNT leading to better dispersion coefficients and interactions between CNT and the polymeric host matrix. This method deals with polymers that are insoluble and thermally unstable which cannot be synthesized by other techniques like “solution mixing and melts processing.” A lot of work has been reported on “MWNT/polystyrene, MWNT/polyurethane, MWNT/polyethylene, MWNT/polypyrrole, MWNT/nylon, etc.” [131 146]. All the techniques are summarized in Table 6.1.

6.7 Applications of nanocomposites In the past decade, there has been tremendous amount of research in the field of carbon materials such as graphene and CNTs-based nanocomposites, due to their extraordinary electrical, mechanical, optical, and chemical properties. These nanocomposites combine the characteristic properties of their components to achieve Table 6.1 Processing methods for polymer-CNT nanocomposite systems. Methods

System

Procedure

Applications

Solution mixing

Thermoplastic resins (PS/ epoxy)

Modification of polymer behavior; synergistic effect; shape memory nanocomposites

Direct mixing

Thermoset resins Polymers, N6

Dispersion of 0.2% 1% CNTs, (100 nm dia, 10 μm long); removal of solvent or precipitation of polymer; cure Dispersion of CNTs; cure Mechanical mixing of CNTs with prepolymer melt followed by extrusion, injection, or compression molding Use of ultrasonic for dispersion in monomer/ matrix; cure

Use of 0.2% 2.0% MWCNT, twin screw mixer

Melt mixing

In-situ polymerization

Polyaniline-CNT, MMA-CNT, epoxy-CNT, poly(ether-ester)

Preparation of the polymer with CNT, good chemical bonding

6.7 Applications of nanocomposites

multifunctionality and structural stability as per desired application. The field of carbon material-based nanocomposites has gained attention due to numerous applications such as energy storage, supercapacitors, chemical sensing, biosensing, solar cells, etc. [147,148]. Presently, we are discussing the applications of graphene/SMOs, CNTs/SMOs, and CNTs/CPs nanocomposites [63].

6.7.1 Graphene/SMOs nanocomposites and CNTs/SMOs nanocomposites In the last decade, lots of research work has been done in the field of fabrication of graphene and CNTs-based nanocomposites with SMOs, and these materials have been explored for various applications [63] (Fig. 6.5).

6.7.1.1 Sensors Researchers paid a lot of attention to graphene/SMOs and CNTs/SMOs nanocomposites for cost-effective, highly sensitive, and selective gas sensors (for detection and environmental protection) that can work at room temperature [149,150]. Among SMOs, SnO2, ZnO, WO3, Fe3O4, and TiO2 based graphene and CNTs nanocomposites are widely explored for outstanding gas-sensing properties in terms of sensitivity, selectivity, limits of detection, recovery time, and response towards specific gaseous targets (such as ethanol, ammonia (NH3), methane (CH4), SO2, NO2, H2, H2S, formaldehyde, and acetone) [151]. SnO2/graphene or RGO and CNTs-based nanocomposites showed an excellent response and improved sensitivity to NO2 at room temperature. It has been reported that binary SnO2/graphene nanocomposites are more sensitive to NO2 gas in comparison with NH3, SO2, and ethanol gases. Graphene-Pd/SnO2 shows gas-sensing properties for

FIGURE 6.5 Applications of graphene/SMOs and CNTs/SMOs.

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H2 and ethanol at two different temperatures. Pd-doped SnO2/RGO, SnO2/RGO/ polyaniline (PANI) and SnO2-Pt/MWCNTs exhibit improved gas-sensing properties for methane gas [152 154]. It was found that ZnO2/RGO nanocomposites have a good response for ethanol at high temperatures [155,156] and for NO2, NH3, and CO gas at room temperature [157,158]. A room temperature highperformance NH3 gas sensor is prepared using a metal-organic framework-derived ZnO/RGO nanocomposite, and copper-doped ZnO/RGO nanocomposites synthesized for room temperature H2S gas sensors [159,160]. ZnO/RGO nanocomposites and metal-doped ZnO/RGO nanocomposites show sensing properties for hydrogen gas [161 164]. ZnO functionalized MWCNT nanocomposites were synthesized for detection of NOx gas [165]. Fabrication of ZnO/GO nanosheet for acetone vapor sensing has been reported and it is found that the sensing performance of these nanocomposites is superior [166]. WO3/graphene nanocomposites show gassensing properties for NO2, alcohol, and H2. Cu2O/RGO nanocomposites exhibit gas-sensing properties for NO2 and NH3 at room temperature. Similarly, Co3O4, NiO, In2O3, and MoO2 show gas-sensing properties for NO2, acetone, and H2S. For ethanol detection some chemiresistive gas sensor materials are graphene/ α-Fe2O3@2%, α-Fe2O3/GO, SnO2/GO, ZnO/GO, CuO/RGO, Co3O4/RGO, Co3O4/ MWCNTs, CuO/MWCNTs, and Sn/SnO2/SWCNT nanocomposites. And for acetone detection, some of the reported promising materials are ZnO/graphene, 0.1% Ni-doped SnO2/graphene, Ag/SnO2/graphene, α-Fe2O3/GO, SnO2-TiO2/GO, ZnO/ GO, and α-Fe2O3/MWCNT nanocomposites. Efficient room temperature detection of H2 gas was obtained by a novel ZnFe2O4-Pd decorated RGO nanocomposite [167].

6.7.1.2 Energy storage and conversion Graphene and CNTs-based SMOs nanocomposites have gained immense popularity in the field of electrochemical energy storage and conversion, due to their excellent properties like high surface area, enhanced electrical conductivity, and most importantly high thermal, chemical, and mechanical stability. These nanocomposites are observed to have application in the field of energy storage and conversion devices such as supercapacitors, fuel cells, solar cells, and LIBs [168 171].

6.7.1.3 Lithium-ion batteries/sodium-ion batteries/zinc-ion batteries Graphene/TiO2 nanocomposites have improved lithium storage capabilities in comparison to pure TiO2 [172,173]. When vanadium pentoxide V2O5 is combined further with graphene and CNTs, it shows improved electrical conductivity. Graphene-based nanocomposites with ZrO2, MnO2, Mn3O4, Co3O4, CoO, Fe3O4, and CuO have also been reported to show high performance for lithiumion storage [174]. Recently, SnO2/CNT nanoparticles encapsulated in porous graphene suggest outstanding lithium (Li) and sodium (Na) storage with an excellent rate, high specific capacity, and remarkable cycle stability. These modified structure materials ensure SnO2/CNT/graphene as a promising material

6.7 Applications of nanocomposites

for LIBs and SIBs [175]. SnO2/graphene nanocomposites were reported as bifunctional anode materials for LIBs and SIBs [176]. A very recent study on SnO2-ZrO2/CNT nanocomposite reported a promising long lifetime anode for next-generation LIBs [177]. Recently, due to low cost, high safety, and environment-friendliness, zinc-ion batteries (ZIBs) have received great attention. For ZIBs, V2O5 has great potential as a cathode material but has poor stability at high current density. The stability of V2O5 improves when it is combined with highly conductive CNTs and it is reported as a promising material as the cathode in ZIBs [178].

6.7.1.4 Supercapacitors Supercapacitors or electrochemical capacitors are energy storage devices that have high power density, long life cycle, and excellent charge/discharge rates in comparison to common batteries. The two-dimensional structure of graphene provides highly reversible pseudocapacitance along with electrochemical double-layer capacitance. Vanadium pentoxide (V2O5)/graphene and V2O5/CNTs/graphene nanocomposites have reported high specific capacitance and high capacitance retention. It is observed that the assembled devices (based on V2O5/graphene and V2O5/CNTs/graphene nanocomposites) have high power density and energy density [179,180]. The blend of graphene/CNTs with layered MnO2 shows refined electrochemical performance which makes these nanocomposites a potential candidate for supercapacitor application [181 184]. In literature, Mn3O4, WO3 Fe3O4 Fe2O3, RuO2, Co3O4, and NiO-based graphene nanocomposites are some other materials that have been explored for supercapacitor application. In a very recent review, the ultra-light and flexible electrodes of MnO2/CNTs and ZnO/CNTs nanocomposite have been reported for highperformance wearable energy storage devices [185,186] (Fig. 6.2).

6.7.1.5 Solar cells Solar cells, also known as photovoltaic cells, are the electrical devices that convert solar energy into electrical energy due to the photovoltaic effect. Due to low cost, high conductivity, and improved electrochemical properties, the TiO2/graphene nanocomposite emerged as a promising electrode material for solar cell applications [187]. In a study, it was observed that graphene/Cu2O nanocomposites have enhanced optical properties, which make them a suitable material for application in photovoltaic devices [188]. Recently, the nanocomposite of RGO/ NiO was reported as an excellent photocathode material for DSSCs (dye-sensitized solar cells) due to its large optical band gap and its high ionization potential [189] (Fig. 6.3).

6.7.1.6 Photodetector TiO2-based graphene or RGO nanocomposites have great potential as photodetectors. For UV (ultraviolet) light TiO2/graphene derivative-based nanocomposites are widely explored. In a very recent study, TiO2/RGO nanocomposite was reported as high-performance UV photodetector material [190]. Also, it is

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reported that a photodetector developed by doping TiO2 on a p-n-p graphene monolayer has the ability to detect visible and near-infrared light (405 1310 nm) [191] (Fig. 6.4).

6.7.1.7 Photocatalysts The photocatalytic activity is the property of the material to convert toxic and nonbiodegradable organics into CO2, H2O, and inorganics. The photocatalytic properties of TiO2/CNTs@CoFe2O4 nanocomposites were investigated for degradation of methylene under UV light irradiation and it is found that the degradation rate of methylene blue for TiO2/CNTs@CoFe2O4 nanocomposites is 96%, while it is 71% for pure TiO2. Also, this nanocomposite is recyclable due to CoFe2O4 (by providing a magnetic field). SnO2/MWCNTs and WO3/CNTs nanocomposites are reported for sonocatalytic degradation of organic pollutants and pharmaceutical wastewater treatment respectively [192,193]. SnO2/CNTs hybrid nanocomposites are reported as a promising material for visible light photocatalytic activity. The SnO2/CNTs nanocomposites were observed to have higher photocatalytic activity in comparison to pure SnO2. This is due to the rapid transferring of electrons and the effective separation of holes and electrons on SnO2/CNTs [194].

6.7.1.8 Hydrogen storage SWCNT-based nanocomposites SnO2, WO3, and TiO2 nanocomposites show reversible and reproducible hydrogen storage capacity at room temperature [195]. Also, investigations suggest that ZrO2/RGO, SnO2/MWCNTs and nanocomposite have hydrogen storing capability [196 198]. The enhancement in hydrogen storage capacity of GO-based SMOs has been observed at liquid nitrogen temperature (77K) in comparison to pristine SMOs/GO [199].

6.7.2 CNTs/CPs nanocomposites Due to extraordinary properties such as tunable electrical conductivity, high mechanical stability, low cost, lightweight, facile production approach, and ease in material processing, CPs have been widely investigated. In comparison to bulk counterparts, the composites of CPs with CNTs possess superior electrochemical activity, large surface area, and high electrical conductivity. These properties make nanocomposites promising candidates for various applications, such as sensors, energy storage, energy harvesting, and protection applications [200] (Fig. 6.6).

6.7.2.1 Sensors Literature reported that the CNTs/CPs nanocomposites have great potential in the field of sensors [201 203]. In the field of gas sensing, some of the most popular CPs are PANI, polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT) [204 206]. It is found that MWCNT/PANI nanocomposites sensor shows superior sensitivity and excellent reversibility to NO2 gas [207]. MWCNT-based nanocomposites with PEDOT:PSS (polystyrene sulfonic acid) and PANI have been studied

6.7 Applications of nanocomposites

FIGURE 6.6 Various applications of CNTs-conducting polymer-based composite.

for high-temperature tolerant NH3 gas sensors. It has been observed that both the sensors have excellent sensitivity, but MWCNTs/PEDOT: PSS nanocomposites have better thermal stability, higher sensitivity, and low response time for trace level sensing of NH3 gas than MWCNTs/PANI nanocomposite [208]. Another study of MWCNTs/PPy nanocomposites sensors shows good sensitivity to NH3 gas at room temperature. These nanocomposites were prepared for different wt.% of MWCNTs and it is observed that nanocomposites with 4 wt.% of MWCNTs are most sensitive to NH3 gas [209]. It is observed that gold layers modified with MWCNTs/PPy nanocomposites layers have the potential to detect trace amounts of mercury (Hg), Iron (Fe), and lead (Pb) ions using the surface plasmon resonance technique [210]. CNT-based CPs have great potential for developing flexible sensors for various environmental polluting gases like CO2, H2S, and NH3 [211].

6.7.2.2 Supercapacitors CPs have been extensively studied as energy storage materials due to their large pseudocapacitance. CP-based capacitors have a higher specific charge density in comparison to carbon materials. CNT-based CP nanocomposites are of great importance in the field of fabrication of supercapacitors because of the increase in active surface area in contrast to other materials. Binary PANI/CNT nanocomposites are regarded as promising candidates for supercapacitors. In some studies, it is reported that addition of some electrochemically active inorganic materials in these CNT/CP nanocomposites results in an increase in high specific capacitance. Au-doped PANI/MWCNTs nanocomposites are highly stretchable supercapacitors. It has been reported that Ag-doped PANI/MWCNTs nanocomposite and PEDOT:PSS/CNTs/MnO2 nanocomposite supercapacitors have outstanding energy density and PEDOT/MWCNTs/Ni(OH)2 as pseudocapacitive material for

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supercapacitors [200]. In a recent study, binary nanocomposite of self-suspended polyaniline (S-PANI) and functionalized-MWCNTs has been synthesized for high-performance supercapacitor applications [212].

6.7.2.3 Lithium-ion batteries A simple battery has high energy capacity, less charging time, and a long cycle time and all these can be achieved by using CNTs/CP electrodes. It has been reported that the efficiency of such a battery is very high in the case of CNTs/CP electrodes due to good interaction between lithium and the CNTs/CP electrode. It has also been reported the reason for the highest efficiency is all the interstitial sites of CNTs are available for Li-intercalation. In recent work, the enhanced electrochemical performance of CNTs/PPy-based nanocomposite as anode material for LIBs has been reported [213]. In another study, nanocomposite consisting of gas-phased produced silicon nanoparticles, CNTs and PANI has been developed as anode material for LIB with improved cycling performance [214].

6.7.2.4 Fuel cell CPs supported with CNTs have high hydrophobic surfaces which makes CNTs/ CP nanocomposites a promising material for fuel cell application. Highly dispersed Pt Ru/PPy/CNTs and Pt-Ru/PTh (polythiophene)/CNT nanocomposites show high catalytic activity for methanol oxidation and ethylene glycol oxidation, respectively, which makes them suitable candidates for fuel cells. Co-PPy/ MWCNTs nanocomposite was reported as a catalyst for hydrogen and alcohol fuel cells. Superior electrocatalytic activity for methanol electrooxidation has been observed for highly dispersed Pt nanoparticles supported MnO2/PEDOT/ CNTs and for Pd nanoflower/PPy/MWCNT nanocomposites [215]. For environmental protection, microbial fuel cells are an emerging and promising technology device to treat waste matter (pollutants) in renewable energy sources through the action of electroactive microorganisms. In a recent review article, manganese oxide-based MWCNTs/PPy nanocomposite was reported to have catalyst property and can be used as anode in a microbial fuel cell for sewage wastewater treatment and energy generation [216].

6.7.2.5 Solar cell CNT-based CP nanocomposites are widely explored for every component of photovoltaic systems such as to support charge conduction, improved electrode flexibility, and active light-absorbing materials. The study of electrodeposited PANI/ MWNTs nanocomposite films indicates that their optimized conductivity are useful for low-cost hybrid solar cell applications [217,218]. In a recent study of PANI/MWCNTs, the results showed that by varying the amount of MWCNTs in the nanocomposite, cell efficiency can be improved. A significant increase in the photovoltaic profile was observed. Excellent synergy was observed between the active layer of PANI/MWCNTs and the dye cell components [219].

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6.7.2.6 Electromagnetic interference shielding CNTs/CP nanocomposites made up of PANI or PPy or polyacetylene (Pan) have great potential for electromagnetic shielding materials. These nanocomposites are lightweight as well as the density of these can be reduced significantly [220]. In addition, CNTs/CP-based nanocomposites have application as an antistatic agent and as anticorrosion materials.

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Fabrication of chemiresistive gas sensor with carbon materials/ polymers nanocomposites

7

Sarath Chandra Veerla1, N.V.S.S. Seshagiri Rao2 and Anil Kumar Astakala1 1

Nanomaterials for Photovoltaics and Biomaterials Laboratory, Godavari Institute of Engineering and Technology (Autonomous), Rajahmundry, Andhra Pradesh, India 2 Department of Physics, Institute of Aeronautical Engineering, Dundigal, Hyderabad, India

7.1 Introduction In the present era, the globalization and growing demand for energy with population results in the various environmental hazards, such as the discharge of various harmful pollutants like nitrous compounds, sulfur derivatives, and fluorocarbons into the environment [1,2]. Monitoring/sensing of these hazardous pollutants in the atmosphere is a vital task to preserve the environment. Increase in the level of pollutants in the atmosphere results in the various environmental effects such as acid rains, global warming, and causes the depletion of ozone layer [3]. The checking/sensing mechanism is essential to curb the presence of these hazardous pollutants in the atmosphere. Real-time capture of the traces of the gases is utmost important to safeguard the environment, and for the past two decades researchers are working on the various sensors that are to be deployed to monitor the real-time presence of the toxic gases/residuals in the ambience [4]. Chemiresistive sensors are the types of sensors that have a conducting interdigitated electrode (IDE) and a chemical composition residing on the top of the IDE which acts as a sensing layer. These types of sensors are used more often to sense the chemical changes in the environment. The chemiresistive sensor works on the principle of the interaction between the chemical species present in the solution/ concentrations with the sensing layer deposited on the top of the IDE resulting in the alteration of the resistance [5,6]. The variation of the resistance with the exposure of the chemicals/species can be easily monitored as the measurement is costeffective, sensitive, and reliable [7]. The schematic representation of the various constituents of a chemiresistive sensor is shown in Fig. 7.1. A typical sensor network consists of sensors, controller, and a communication system. If the communication system in a sensor network is implemented using a wireless protocol, then the networks are known as wireless sensor networks Carbon Nanomaterials and their Nanocomposite-Based Chemiresistive Gas Sensors. DOI: https://doi.org/10.1016/B978-0-12-822837-1.00003-4 © 2023 Elsevier Inc. All rights reserved.

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FIGURE 7.1 Schematic diagram of the chemiresistive sensor.

(WSN). According to technologists, WSN is an important technology for the 21st century. Recent developments in MEMS sensors (microelectromechanical system) and wireless communication has enabled cheap, low power, tiny and smart sensors, deployed in a wide area and interconnected through wireless links for various civilian and military applications. A WSN consists of sensor nodes deployed in large quantities and support sensing, data processing, embedded computing, and connectivity. With the advent of low-power embedded systems and wireless networking, new possibilities have emerged for distributed sensing applications. These technologies led to the implementation of WSNs, allowing easily configured, adaptable sensors to be placed almost anywhere, and their observations similarly transported over large distances via wireless networks. WSN consists of distributed, wirelessly enabled embedded devices capable of employing a variety of electronic sensors. Each node in a WSN is equipped with one or more sensors in addition to a microcontroller, wireless transceiver, and energy source. The microcontroller functions with the electronic sensors as well as the transceiver to form an efficient system for relaying small amounts of important data with minimal power consumption. The most attractive feature of WSN is their autonomy. When deployed in the field, the microprocessor automatically initializes communication with every other node in range, creating an ad hoc mesh network for relaying information to and from the gateway node. This negates the need for costly and ungainly wiring between nodes, instead relying on the flexibility of mesh networking algorithms to transport information from node to node. This allows nodes to be deployed in almost any location. Coupled with the almost limitless supply of available sensor modules, the flexibility offered by WSNs offers much potential for application-specific solutions. WSNs have many advantages over traditional sensing technology due to their

7.2 Fabrication of CNTs/SMOs-based sensors

embedded construction and distributed nature. The first, and for many the most notable, feature is their cost. Using low-power and relatively inexpensive microcontrollers and transceivers, the sensor nodes used in WSNs are often less than 100 dollars in cost. This opens the seven doors for many commercial or military applications, as the relatively diminutive cost of nodes allows for not only large numbers of sensors to be deployed, but also for large numbers of sensors to be lost. For example, sensor nodes can be dropped from a plane, allowing widespread coverage of an area with minimal effort involved in positioning the individual nodes. The relatively low cost of the sensors allows for some nodes to be damaged or lost without compromising the system, unlike larger, more centralized sensors [8,9]. Another advantage WSNs hold over traditional wireless sensing technology lies in the mesh networking scheme they employ. Due to the nature of radio frequency (RF) communication, transmitting data from one point to another using a mesh network takes less energy than transmitting directly between the two points. While embedded systems must respect their power envelope, the overall energy spent in RF communication is lower in a mesh networking scenario than using traditional point-to-point communication [10 12]. Sensor networks can also offer better coverage than more centralized sensing technology. Utilizing node cost advantage and mesh networking, organizations can deploy more sensors using a WSN than they could using more traditional technology. This decreases the overall signal-to-noise ratio of the system, increasing the amount of usable data. For all these reasons and more, WSNs offer many possibilities previously unavailable with traditional sensor technology [12]. In this chapter, we summarize the recent advancements in the field of semiconductor metal oxides (SMOs), and polymer-based composites as active material in the sensors for the sensing of various gases and the chemicals such as H2S. NO2, NH3, toluene, ethanol, and acetone existing in the environment/ industrial by-products. Furthermore, we discuss the present challenges in the application of the chemiresistive sensors along with the usage of wireless networking embedded with the sensors.

7.2 Fabrication of CNTs/SMOs-based sensors The composite of carbon nanotubes (CNTs) and SMOs as an active material in the chemiresistive sensors has attracted research interest over the past decade due to sensitivity and applicability of these devices in different environments. Nanostructured metal oxides are capable of detecting/sensing the hazardous gases/organic compounds present in a sample due to their high surface area and better electrical conductivity as compared with the bulk sample. Among the nanostructured materials, oxides such as titanium oxide, tin oxide, zin oxide, and indium oxide are the well-studied materials for sensing the hazardous compounds due to their stability and better electrical conductivity.

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Majumdar et al. [13] synthesized a nanocomposite of stannous oxide and CNTs as sensing material for the H2 gas sensing and achieved a highest of 84% sensitivity with H2 gas of about 4% and 120 s of recovery time. Further, the composite of SnO2 and CNT is highly suitable for sensing H2 gas, as it operates at the comparable lower temperature and also the composition can absorb moisture, which makes it a viable candidate for the H2 gas sensing. Zhao et al. [14] fabricated the spin-coated PEG assisted CNT/SnO2 nanocomposite film. The Cu coated on the surface of the nanostructured CNT/SnO2 composite acts as an active sensing material to detect H2S gas in about 19% for a concentration of 40 ppm, with sensor having a recovery time of 10 min and the same composition is reported with an sensitivity of 4.1%, 0.2%, and 0.1% for the gases NH3, CO, and SO2, respectively, at the concentration of 40 ppm. Inaba et al. [15] fabricated the sensors with the multijunction film with n-type SnO2 and p-type CNT for the detection of NO2 gas and achieved the normalized sensitivity of about 80% and 20% in the N2 and the normal air environments respectively at the concentration of 1 ppm. Navazani et al. [16] developed the chemiresistive sensor with hybrid nanocomposite of stannous oxide and pt/multiwalled carbon nanotubes (MWCNTs) for sensing of methane gas at the concentration of about 1000 ppm. The sensitivity of 1%, 20%, and 28% is achieved with the SnO2, platinum-coated MWCNT, and SnO2-platinum coated CNTs, respectively, at a concentration of 100 ppm for methane gas. Nguyet et al. [17] designed a sensor for the detection of NO2 gas with functioning of the active material comprising MWCNTs developed on the stannous oxide nanowires grown of platinum electrode. At 50 C, response of about 11,300 is achieved at the concentration of 1 ppm NO2. Sharma et al. [18] developed the stannous oxide incorporated boron nitride nanostructures for sensing NO2 gas at the concentration of 5 ppm showing a response of about 2600 at a temperature of 100 C. Sinha et al. [19] fabricated chemiresistive sensors to sense organic compounds using the CNT/ZnO based nanocomposites on the working principle of adsorption switching mechanism and found the sensitivity of sensing ethanol about 73% at an operating temperature of 150 C. Seekaew et al. [20] reported the fabrication of the 3D TiO2 and graphene-CNT composite using the chemical vapor deposition (CVD) technique for the detection of the organic material toluene, which is more hazardous to the environment and is the by-product of various industrial products. Sensitivity of about 42% is achieved for sensing toluene at the concentration of 500 ppm at room temperature, as the sensor operates at the room temperature, is cost-effective, and shows better sensitivity of toluene as compared with the previous reports. Kohli et al. [21] fabricated a sensor using the nanocomposite matrix of CNT/ In2O3 for sensing acetone traces in the given sample. The sensor is sensitive up to 10 ppm concentration of acetone and has a sensitivity response of about 4% at the operating temperature of 300 C. Similarly, Huang et al. developed the palladiumloaded indium oxide nanocomposite for the detection of NO2 present in the environment and the sensor showed better sensitivity as compared with the other reports and can sense NO2 in the sample with the traces of about 5 ppm. The sensitivity responses of various gases/chemicals of sensors based on the CNT/SMOs are shown in Fig. 7.2.

7.3 Fabrication of graphene/SMOs-based sensors

FIGURE 7.2 Sensitivity response of various gases/chemicals on CNT/SMO composites-based sensors.

7.3 Fabrication of graphene/SMOs-based sensors Graphene/SMO-based nanocomposites have been developed in different fields such as chemical sensor, gas and biosensors, electrochromic smart windows, photocatalytics, and energy storage applications. Recently, reduced graphene oxide (rGO) and metal oxide nanocomposites have received considerable attention in sensing toxic gas detection of various gas species, such as CO, CO2, NO2, NH3, H2S, CH2O, and C3H6O. Graphene has efficacy to detect the single gas molecule at room temperature, but the disadvantage is of long recovery time by using graphene gas sensor, due to strong attachment of gas molecules on the surface of graphene. To overcome these issues, graphene and metal oxides composites shorten the recovery time and enhance the response [22]. There is demand in analyzing the biomarkers by using various nanomaterials-based gas sensors in realtime, low cost, and portable breathalyzers. Sensors employed with synthesized graphene oxide/hexagonal tungsten oxide nanosheet composites have been fabricated for the detection of hydrogen sulfide (H2S) and showed the detection limit of 10 ppb temperature of about 330 C [23]. Kim et al. reported that they have obtained very high and selective selectivity to nitrogen dioxide (NO2) by fabricating graphene 2 SnO2 nanocomposites using a commercial microwave oven. NO2 sensing tests revealed that the graphene 2 SnO2 nanocomposites exhibited

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significantly high responses of 24.7, 52.0, and 72.6 at concentrations of 1, 3, and 5 ppm, respectively [24]. The detection of carbon monoxide (CO) gas at room temperature by using copper oxide (CuO) nanoflowers/rGO layered nanocomposites sensor were fabricated by layer-by-layer technique [25]. The porousstructured graphene oxide nanosheet-tungsten oxide (WO3) composite nanofibers with the highest response of 35.9 100 ppm acetone at 375 C, which is 4.3 times higher than that of the pristine WO3 nanofibers, were fabricated by the electrospinning method for acetone sensing property [26]. rGO was incorporated with tungsten oxide (WO3) nanocomposite using a simple ultrasonication method which exhibits porous structurefor effective sensing and selective trace level detection of NH3 at room temperature. The 5% rGO/WO3 nanocomposite shows the maximum response when compared with the other two percentages of the nanocomposites and pure WO3. The response values of 5% rGO/WO3 nanocomposite are 4.50, 5.22, 7.53, 9.69, 12.88, and 15.83 for different concentrations of NH3 such as 10, 20, 40, 60, 80, and 100 ppm, respectively [27]. Recently, Kalidoss et al. [28] reported that graphene and its derivatives/SMO-based binary nanocomposites have been used for sensitive and selective detection of biomarkers in portable breathalyzers. The oxygen vacancies in SMO can alter its conductivity further as it accelerates the carrier transport rate and has an impact on the adsorption behavior on the target analyze on the sensing materials [28]. The as-synthesized rGO-wrapped hollow tin oxide (SnO2) nanospheres (HSnO2@rGO) composites showed significant contributions to gas adsorption, electrons transportation, and reaction efficiency, resulting in enhanced sensing properties. The response for 10 ppm formaldehyde reached 435 at optimal temperature of 130 C [29]. The rGO decorated with 8:2 ratio of In2O3 (n-type) and NiO (ptype) sensing electrode showed high sensitivity of 40% towards CO2 at 50 ppm. Further, the sensor can detect upto 5 ppm concentration of CO2 with quick response time of 6 s and recovery of 5 s [30]. The real-time response towards 50 ppm NO2 for tin oxide nanoparticles incorporated with copper oxide nanopetals on reduced graphene (SnO2-CuO/rGO) sensor exhibits a higher response (B250%), which is 8 15 times greater than CuO/rGO (30%) and SnO2/rGO (17%) sensors within 90 s [31]. The ternary nanocomposite of rGO and metal (platinum or lead) coloaded tin oxide nanofibers (rGO/Pt-co-loaded SnO2 NFs or rGO/Pd-co-loaded SnO2 NFs) sensors showed highest response and good selectivity of the fabricated sensors towards either toluene (C7H8) or benzene (C6H6) and could be tuned by loading with either Pt or Pd. The rGO/Pd-coloaded SnO2 NF sensor to 1 and 5 ppm CO, C6H6, and C7H8 gases at 200 C, the highest response to C6H6 (12.3 at 5 ppm) compared with the other gases. The rGO/Pt-coloaded SnO2 NF sensor to 1 and 5 ppm CO, C6H6, and C7H8 gases at 200 C, the highest response to C7H8 (16.0 at 5 ppm) compared with the other gases [32]. The rGOloaded ZnO nanofibers-based gas sensor exhibits 2524 sensor response at 10 ppm of H2 gas. The rGO-loaded ZnO NFs exhibited excellent sensor responses of 865.9 to a very low concentration of H2 gas (100 ppb) [33]. The rGO/α-Fe2O3

7.4 Fabrication of CNTs/conducting polymers-based sensors

composite nanofibers showed 8.9 response to 100 ppm of acetone which is about 4.5 times higher than pure α-Fe2O3 [34]. The responses of various gases/chemicals of sensors based on the graphene/SMOs are shown in Fig. 7.3.

7.4 Fabrication of CNTs/conducting polymers-based sensors CNTs are composed of carbon atoms with fullerene structure, in which every carbon atom is covalently bonded to three adjacent carbon atoms, which could be suitable for chemical sensing. It has excellent structural and mechanical properties and also has unique widely adjustable conductivity, high chemical and thermal stability, stability, good heat conductance, and high elasticity. However, in gas sensing applications, the material shows poor sensitivity and selectivity, which usually needs surface modification. To overcome these issues, CNTs with polymer gas sensing materials exhibit relatively low chemical stability and conductivity but good sensitivity and selectivity [35,36]. The CNTs coated with poly (o-anisidine) nanocomposites showed higher gas sensor sensitivity of 28% upon exposure to small concentrations (100 ppm) of HCl in air. The increase in sensing capability for inorganic vapors is attributed to direct charge transfer with electron hopping effects on innertube conductivity through physically adsorbed poly(o-anisidine) between CNT [37]. Recently, Kim et al. has reported that p-MWCNT (plasma-functionalized multiwalled carbon nanotubes)/PANI (polyaniline—a

FIGURE 7.3 Response of various gases based on graphene/SMO composites-based sensors [22 34].

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conducting polymer) composite sensor could be used to sense the NH3 gas which exhibits a linear response of 3.34% per ppm ammonia (NH3) for concentrations ranging from 10 to 100 ppm and a sensitivity of about seven times higher than that from a corresponding pristine p-MWCNT sensor [38]. The greenhouse gas carbon dioxide gas was sensed at 22 C using a new composite of Baytron-P-CNT having a sensor response of 10 50 ppm concentration of carbon dioxide gas, revealing that the parallel configuration lowers the initial resistance and shows higher stability [39]. The conducting polymer of PANI/MWCNTs-based nanocomposites can detect the NH3 gas at trace level concentrations (2 10 ppm) showing a sensor response of 15.5% 32.0% with response time 6 24 s and the recovery time of 35 62 s [40]. Zhang et al. reported that a resistive flexible humidity sensor-based CNT was fabricated, and its capability of the relative humidity ranging from 30% to 60% observed that the resistance of the humidity sensor increased from 12 to 17K Ω during the humidification process and recovered back during the dehumidification process [41]. The fabrication of conducting polythiophene/single-walled CNT (PTh/SWCNT) nanocomposites with different SWCNT loading (5%, 10%, and 15% of SWCNT) has excellent ammonia gas sensing property for 15% SWCNT/polythiophene showing considerable response to NH3 [42]. The fabrication of piezoresistive strain sensor based on conductive poly(styrene-butadiene-styrene)/CNT fiber (SBS/CNT fiber) showed piezoresistive performance under the maximum workable applied strain range of 267% and its gauge factor (GF) is 2889 [43]. Akhtar et al. reported that a flexible and stretchable sensor fabricated by highly aligned CNTs embedded in polydimetylsiloxane (PDMS) has high-pressure sensitivity at 1.29 kPa 1; also at high strain of 60% the GF was achieved at 218, while at low strain (15%) the sensor showed GF of 780 [44]. The fabrication of room temperature resistive volatile organic compound with vertically aligned-CNTs (VA-CNT) coated with highly conductive polymer (poly(3,4-ethylenedioxythiophene); PEDOT) sensing materials has detected n-pentane with 50 ppm of limit of detection and also showed significant precision over methanol and toluene [45]. The fabrication of microstructured CNT/PDMS arrays by an ultraviolet/ozone (UV/O3) microengineered method could be used to detect the pressure/strain-based sensors. The pressure sensor has showed a broad sensing range of 7 Pa to 50 kPa with a sensitivity of around 20.101 6 0.005 kPa, fast relaxation speed of 10 ms, and a good cycling stability [46]. The response and recovery time of PANI/MWCNTs for NH3 gas, VACNT for n-pentane gas, and CNT-PDMS strain sensor are shown in Fig. 7.4.

7.5 Fabrication of wireless-based networks sensors A typical WSN can be divided into two elements: sensor node and network architecture. A sensor node in a WSN consists of four basic components: power supply, sensor, processing unit, and communication system. The sensor collects the

7.5 Fabrication of wireless-based networks sensors

FIGURE 7.4 Response time and recovery time of PANI/MWCNTs for NH3 gas, VACNT for n-pentane gas, and CNT-PDMS strain sensor [19,22,23].

analog data from the physical world and an ADC converts this data to digital data. The main processing unit, a microprocessor or a microcontroller, performs an intelligent data processing and manipulation. Communication system consists of radio system, a short-range radio for data transmission, and reception. As all the components are low-power devices, a small battery like CR-2032 is used to power the entire system. A sensor node consists of not only the sensing component but also other important features like processing, communication, and storage units. With all these features, components, and enhancements, a sensor node is responsible for physical world data collection, network analysis, data correlation, and fusion of data from other sensor with its own data. The basic components of WSN are shown in Fig. 7.5.

7.5.1 Network architecture When a large number of sensor nodes are deployed in a large area to monitor a physical environment, the networking of these sensor nodes is equally important. A sensor node in a WSN not only communicates with other sensor nodes but also with a base station (BS) using wireless communication. The BS sends commands to the sensor nodes and the sensor node performs the task by collaborating with each other. The sensor nodes in turn send the data back to the BS. A BS also acts as a gateway to other networks through the internet. After receiving the data from

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FIGURE 7.5 Basic components of WSN in block diagram.

FIGURE 7.6 The sensing network architecture.

the sensor nodes, a BS performs simple data processing and sends the updated information to the user using internet. If each sensor node is connected to the BS, it is known as single-hop network architecture. Although long distance transmission is possible, the energy consumption for communication will be significantly higher than data collection and computation. Hence, multihop network architecture is usually used. Instead of one single link between the sensor node and the BS, the data are transmitted through one or more intermediate node. Fig. 7.6 shows the sensing network architecture.

7.5 Fabrication of wireless-based networks sensors

7.5.2 Materials used in fabrication of devices for WSN In recent times, WSN has emerged as a promising technology providing opportunity to a variety of communication applications. They allow networking of communication devices to be assembled within the Internet of Things, mobile/ vehicular ad-hoc network, and many more potential applications. To fabricate these sensors, different varieties of material are used. However, it is seen that with silicon, sufficiently high power cannot be achieved to reach the desired frequencies. Hence, new materials are required to be searched which can cater to the demands of speedy information transfer, with less energy and bandwidth consumption. To fulfill these requirements, many new classes of materials are being used and being further investigated for improvement in future applications. As the wireless communication industry matures, the components for wireless networking sensors will undergo continuous research and development. These components include thin-film dense wavelength division multiplexer (DWDM) filters and waveguides for multiplexing/demultiplexing, vertical-cavity surface-emitting laser (VCSEL), edge emitting lasers, photodiode detectors, erbium-doped fiber amplifier (EDFA), Raman amplifiers, and many more. For all these purposes, piezoelectrics, metamaterials, and specialized semiconductors are used nowadays. Incorporating smart materials into material structures provides new opportunities to encode sensor information in various electromagnetic systems. These systems form the basis for a low-cost wireless sensor technology carrying vast potential applications in manufacturing, inventory control, security, surveillance, and new human computer interfaces. Gallium arsenide (GaAs)-based semiconductors have dominated wireless and high-speed applications such as power amplifiers and switches for cellular phones, smart and feature phones, WLAN enabled devices, and the infrastructure supporting these capabilities. These devices are also used for wireless broadband and Wi-Fi functionalities in PCs, notebooks, and tablets, for cable TV, direct broadcast satellite, telecommunication, data communication, social media, cloud, and other modern technologies. This chapter presents the trends of recent research of such materials for a wide range of communication devices. In the next section, we discuss about recent WSN devices and their usability as per the industrial requirement along with their fabrication mechanism. The properties of the materials and the devices fabricated using these materials include Hetero-junction Bi-polar Transistor (HBT), and Bipolar Field Effect Transistor (BiFET).

7.5.3 Fabrication of FBT HBTs are primarily used in the production of power amplifiers necessary for wireless communication and data transfer and can handle signals of very high frequencies up to several hundred GHz. High-speed operation of the device can be obtained by hetero-junctions in the device by creating changes in the structure design such as lower emitter doping and high base doping. They offer

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performance improvement over standard bipolar junction transistors due to the use of different materials in the device. With proper choice of materials, the barrier potential for this injection is much lower than that of the backward injection of holes at the base-emitter junction [47 49]. For many years, the wide gap emitter material of choice was AlGaAs, but most recent HBTs use InGaP. In present times, HBT structures are grown using metal organic chemical vapor phase deposition (MOCVD) technique. In an HBT, the electrons travel vertically through the device.

7.5.3.1 Bipolar field effect transistor In attempt to emphasize on smaller, lower cost circuitry in mobile phones, much effort has been focused on increasing the level of integration. By combining HBT functionality with field effect transistors (FET) switching capability in the same device, some of the biasing-related features can be integrated in the power amplifier. BiFET technology uniquely integrates InGaP-based HBTs with FETs on the same GaAs substrate. This technology opens up new avenues to embed analog signal processing in wireless applications. Existing research for WSN on energy efficiency focus on the design of energy efficient MAC protocol, or routing protocols. Many parameters are considered at the physical layer for WSN, viz. low power consumption, low complexity, low transmission, and reception range. However, energy efficiency at the physical layer is more important for WSN. Presently, we use battery- powered or wired sensors which provide us different information. In future, placing these sensors in remote places or incorporating them in structures may make them hard to maintain. Here lies the role of intelligent sensors [47]. In [48], the green communication in WSN and some energy efficiency metrics for the wireless sensors running at near ground level has been proposed. The proposed metrics introduce a useful guidance focusing mainly on mobile and cellular communication system that shows the relationship between the energy efficiency, the spacing between nodes, and antenna height for ground level wireless communication. Table 7.1 presents the power requirements of sensing devices for wireless communication.

Table 7.1 Power requirements of different WLAN requirements. Mode of operation

Bluetooth

Wi-Fi

Zigbee

RFID

Talking Reception Transmit Stand by

50 μW 29 mW 27 mW 8 μW

500 μW 90 mW 350 mW 10 μW

414 μW 84 mW 72 mW 4 μW

36 μW 19 mW 20 mW 6 μW

7.6 Sensing mechanisms

7.6 Sensing mechanisms Sensor is an electrical device that is used to sense/detect the presence of trace gases present in the mixture/sample (analyte gas). Typically, a sensor consists of a test chamber where the host materials/composition is placed on the conducting electrodes. The carrier gas of known composition is allowed to pass through the test chamber with the help of various flow meters and is allowed to mix with the gas whose resistance is found [49]. The carrier gas along with the trace gas under consideration is allowed to interact with the active material in the test chamber and the resistivity of the active materials is known with the help of the electrometer and from that the sensitivity and response can be calculated with the relation, sensitivity 5 (Ra Rc)/ΔC and the response 5 (Ra Rc)/Rc, where Rc is the resistance of the test chamber in the presence of carrier gas, typically the carrier gas will be N2 and other gases which are inert in nature, Ra is the resistance developed in the test chamber in the presence of the carrier gas and the gas under investigation, and ΔC is the change in the gas concentration [50,51]. Generally, the standard sensing of the gas is done in the presence of N2 gas (carrier gas) at room temperatures and at very low humidity conditions. Fig. 7.7 shows the typical instrumentation for the gas sensing. The block diagram representing the functioning of a gaseous sensor is shown in Fig. 7.7.

7.6.1 Sensor outcomes at laboratories/industrial level A sensor is a device that senses the gases/chemicals present in the environment and provides the presence/concentration of such traces in the form of electric pulses. Sensors paly a major role in industrial applications to monitor the various gases/ chemicals liberated as a by-product. In the normal day life, the various works of

FIGURE 7.7 Block diagram on functioning of a gaseous sensor.

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the humas become easier with the use of various sensors for diverse applications such as manufacturing, biomedicals, and various industrial applications [52]. There are several sensors that are very useful in the industrial applications such as humidity sensors, gaseous sensors, chemical sensors, flow sensors, temperature, and pressure sensors. In laboratories/industries, the chemical and gas sensors are very important to sense the outbreak /liberation of various gases/chemicals which are very hazardous for the mankind. The usage of the sensors in laboratories is gaining importance as the traces/concentration of the gases in the ambience can be estimated and the commensurable measures can be taken to curb the gases. In general, the carbon derivatives such as graphene, CNT, MWCNT, and the metal oxides such as SnO2, ZnO, TiO2, and In2O3 were extensively used as an active material in the sensor applications [53,54]. Further, investigations are being made to increase the sensitivity of the sensors to trace the gases which are in lower concentrations.

7.7 Conclusions and future prospects Sensors are the devices that convert the presence of a physical quantity into an electrical signal. Among sensors, the chemiresistive sensors are the versatile and the economically feasible, which are having an active medium that comprises semiconductor SMOs/carbon derivatives such as graphene, CNTs/composites of the SMOs, and the carbon derivatives. The sensing of various gases/chemicals such as NO2, H2S, NH3, toluene, acetone, and ethanol is reported in the recent literature, and the chemiresistive sensors have shown better sensitivity as compared with the previous reports. Further, other compounds such as 2D materials such as chalcogenides, dichalcogenides, metal organic frameworks (MOFs), and the nanocomposites of these materials with graphene need to be explored for better selectivity and sensitivity.

Acknowledgement S.C.V. and A.A.K gratefully acknowledge Godavari Institute of Engineering and Technology (Autonomous), Rajamahendravaram, Andhra Pradesh, India, for the start-up fund to initiate Nanomaterials for Photovoltaics & Biomaterials Laboratory (NPBL).

References [1] L. Wang, X. Su, J.-H. Xie, L.-J. Ming, Specific recognitions of multivalent cyclotriphosphazene derivatives in sensing, imaging, theranostics, and biomimetic catalysis, Coord. Chem. Rev. 454 (2022) 214326. [2] Y.K. Gautam, K. Sharma, S. Tyagi, A.K. Ambedkar, M. Chaudhary, B. Pal Singh, Nanostructured metal oxide semiconductor-based sensors for greenhouse gas detection: progress and challenges, Royal Soc. Open Sci. 8 (3) (2021) 201324.

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CHAPTER

Potential applications of chemiresistive gas sensors

8

Anshul Kumar Sharma1,2 and Aman Mahajan1 1

Material Science Laboratory, Department of Physics, Guru Nanak Dev University, Amritsar, Punjab, India 2 Centre for Sustainable Habitat, Guru Nanak Dev University, Amritsar, Punjab, India

8.1 Introduction Chemiresistors are chemical sensors that depend on the analyte’s chemical interaction with the detecting medium and their intrinsic resistance/conductance may alter in reaction to changes in the chemical properties of surrounding environment [1]. They work on the idea that for a known amount of exposed gas, an alteration in electrical resistance is measured as a function of time. A basic ohmmeter can be used to determine the resistance. Consequently, a steady potential is provided to the sensor, and the current change is observed. The electrical resistance of the sensor is extremely sensitive to changes in chemical environment. The sensing materials play an important role in chemical sensors. The most often utilized chemiresistive materials are metal-oxide semiconductors, metallic nanoparticles (NPs), conductive polymers, and carbon-based nanomaterials like graphene and carbon nanotubes (CNTs) [2]. A basic chemiresistor is made up of a layer of sensing material that fills the single gap between two electrodes or series of interdigitated electrodes (IDEs) coated on alumina, glass substrate, etc. [3]. To maintain proper ohmic contact with the sensing material, which regulates the charge flow between electrodes, the electrodes are frequently made of conductive metals such as platinum and gold. The amount of material in contact with the digits increases overall conductivity in an IDE [3]. Figs. 8.1 and 8.2 illustrate a schematic representation of a chemiresistive gas sensor and a typical response curve demonstrating the fluctuation of sensor resistance as a function of time. Furthermore, chemiresistive sensors are predominantly used because of their easier manufacturing, less power use, and due to its longer operational use. A set of parameters have been established to describe the sensor performance, and they are mentioned below.

Carbon Nanomaterials and their Nanocomposite-Based Chemiresistive Gas Sensors. DOI: https://doi.org/10.1016/B978-0-12-822837-1.00002-2 © 2023 Elsevier Inc. All rights reserved.

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FIGURE 8.1 Schematic of a chemiresistive gas sensor.

FIGURE 8.2 Schematic response-curve of a chemiresistive gas sensor.

8.1.1 Sensor response (S) A sensor response is defined as relative change in resistance and has been determined using Eq. (8.1): Sð%Þ 5 jðRg 2 Ra Þ=Ra j 3 100

(8.1)

8.1 Introduction

where Rg and Ra correspond to the sensor resistance in the presence of gas and in air atmosphere, respectively.

8.1.2 Sensitivity (S0 ) Sensitivity is defined as the rate of change in sensor response per unit change in gas concentration and is determined using Eq. (8.2): S0 ð%Þ 5 jðRg 2 Ra Þ=Ra 3 Cj 3 100

(8.2)

8.1.3 Response time Response time is defined as the time taken for sensor resistance to reach 90% of its equilibrium value after the gas is purged into the sensing chamber.

8.1.4 Recovery time Recovery time is the time required for the sensor resistance to regain 90% of its baseline value after elimination of the target gas from sensing chamber.

8.1.5 Selectivity The property that governs the gas sensor’s selective response to a specific target gas among a collection of target gases is called selectivity. Cross-sensitivity refers to the gas sensor’s selectivity to other pollutants present in the environment.

8.1.6 Limit of detection Limit of detection (LOD) of the sensor is the lowest concentration of the target gas that the sensor can detect at a certain operating temperature. Further, LOD of the sensor is determined using Eq. (8.3): LOD 5

3 3 concentration signal to noise ratio

(8.3)

The signal-to-noise ratio is defined as the ratio of the highest resistance change to the baseline resistance’s root mean square noise (rmsnoise).

8.1.7 Stability The capacity of a sensor to produce repeatable response performance over time is referred to as sensor stability. A good sensor has the property that it exhibits a stable and reproducible signal for a time of at least 23 years.

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8.1.8 Linearity It is the degree to which the response calibration vs target gas concentration resembles a prescribed straight line. The sensor’s linearity is measured by how closely it resembles a straight line. The linear zone is defined as the area where changes in response are proportionate to changes in concentration. All of these sensing characteristics are influenced by sensing material, morphology, gassensor interaction, operating circumstances, and so on. The fulfilment of the “4-S selection criterion” is the primary requirement for a sensor to be suitable for commercial deployment. Each “S” stands for sensitivity, selectivity, stability, and suitability. A perfect chemical sensor would have high sensitivity, selectivity for a specific gas, long-term stability, a low detection limit, fast response and recovery times, and a long shelf life. A good sensor should be impervious to external influences such as humidity, dust, and vibrations. It is worth noting that the microstructural features of chemiresistive materials have an impact on their sensing ability. Nanofibers (NFs) in particular are intriguing because of their high surface-to-volume ratio, high porosity, and easy-toadjust features including morphology, composition, structure, and diameter. Indeed, carbonaceous materials such as CNTs, fullerenes, and graphene have recently attracted a lot of interest in gas sensing technology because of their exceptional structural, optical, and electrical capabilities as well as their low operating temperatures. Among these, CNTs are considered to be ideal sensing materials for chemical sensors due to their high mobility, huge surface area, and amazing electronic properties, etc. Basically CNTs are divided into two types: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) as shown in Fig. 8.3 [4]. Single rolled layers of graphene with diameters ranging from 0.4 to 6 nm and lengths reaching a few microns constitute SWCNTs. MWCNTs are constituted of

FIGURE 8.3 The structures of (A) SWCNTs and (B) MWCNTs.

8.2 Environmental monitoring

many rolled graphene layers with sizes ranging from a few nanometers to tens of nanometers. CNTs tend to aggregate together to form tube bundles as a result of their strong interaction. They could be used for hydrogen storage, onedimensional quantum wires, field emitters in display technologies, chemical sensors, rechargeable battery electrodes, resistors, and interconnects, etc. It is worth noting that according to their chirality, diameter, and functionalization, CNTs exhibit metallic, semi-metallic, or semi-conducting properties [5,6]. Without any doping, CNTs behave like p-type semiconductors. Furthermore, interactions with atmospheric oxygen [7] or metal electrodes [8] as well as impurities and flaws introduced during synthesis or processing have all been suggested as possible causes for the p-type character [9]. The loss of electrons from the CNT by chemisorbed oxygen is thought to cause p-type behavior in the presence of oxygen. Furthermore, charge transfer and chemical doping by a variety of compounds have a major impact on CNT electrical properties. Whenever electron-withdrawing or electron-donating molecules interact with p-type semiconducting CNTs, the concentration of primary charge carriers in the “bulk” of the nanotube varies, affecting CNT conductance. CNTs can be used as electrical chemical gas sensors because of this property. Gas sensors made of pure CNTs, on the other hand, have a number of flaws, including low sensitivity to analytes with low adsorption energy or affinity, paucity of selectivity, irreversibility, and a prolonged recovery time [10]. To address these constraints, researchers are actively working on the functionalization of CNTs with various compounds in order to modify the chemical nature and enhance their sensing ability. In this chapter, a comprehensive review on the most essential aspects of chemiresistive sensor mechanism and its capability for number of applications such as environmental monitoring, medical diagnosis, food processing, human illness detection, agricultural industries, military applications, and explosive detection has been highlighted (Fig. 8.4).

8.2 Environmental monitoring 8.2.1 Carbon monoxide (CO) gas sensor The potential of low-power, low-cost sensors has sparked research into CNTassisted chemical sensors for a number of applications, including environmental sensing. Recognition of potentially harmful contaminants in liquid and gaseous mediums presents unending challenges in terms of selectivity and sensitivity. Pristine CNTs have been demonstrated to interact with a variety of gaseous contaminants. As a result, by altering the functionalization moieties of the CNTs, a continuous attempt has been made to develop the analytes selectivity scope. For example, carbon monoxide is a deadly gas that is colorless, odorless, and tasteless. Industrial processes, incomplete combustion of fuels, and vehicle exhaust are the main causes of its creation, posing a substantial hazard to human health and

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FIGURE 8.4 Presentation of different applications of chemiresistive sensor.

the environment. CO is a gas that is created when hydrocarbons are burned incompletely and is present in practically all combustion processes; hence accurate assessment of environmental CO levels is critical. Vehicles are a major source of CO emissions in the environment exhaust, which contributes to smog production. Bittencourt et al. [11] have reported oxygen plasma functionalized MWCNTs to WO3 for the detection of CO with lower concentration of 10 ppm operated at ambient temperature. Han et al. [12] have demonstrated a CO gas sensor based on Pt-loaded CNT sheets. They have used a simple and fast approach for developing a composite by depositing platinum (Pt) NPs on a CNT sheet, and demonstrated nanocomposite’s application as sensitive CO gas sensor that works at room temperature. Choi et al. [13] presented significant improvement of COsensing performances in SWCNTs when modified with Au NPs. The increased response to particular gases caused by the Au NPs can be explained by a synergistic interaction of catalytic and electronic sensitizations. At room temperature, the Au NP-functionalized SWCNT sensors are found to be very selective to low ppm CO. Kauffman et al. [14] have described room temperature response of metal NP decorated SWCNTs and have found that there is an electron concentration transfer from the SWCNT to the NP species, and CO molecule adsorption on the NP surface is accompanied by an electronic density transfer back into the SWCNT. Leghrib et al. [15] have studied gas sensors based on MWCNTs decorated with SnO2 nanoclusters. It has been observed that hybrids are highly

8.2 Environmental monitoring

sensitive to nitrogen dioxide and carbon monoxide than pure nanomaterials at low working temperatures. The hydrothermal approach was used to successfully synthesize new 1D Co3O4/polyethyleneimine-carbon nanotubes composites (CoPCNTs) at various temperatures [16]. The CoPCNT sensors fabricated at a temperature of 160 C have shown highest response to CO and NH3 gases with response time of 4 and 4.3 s, respectively.

8.2.2 Hydrogen sulfide (H2S) gas sensor Hydrogen sulfide is a common toxic gas that causes contamination in the environment. Natural gas, petroleum refineries, and coal are the main sources of H2S [17]. It is a deadly gas with a foul odor and no color. At a modest concentration of the gas, the main side effects are irritation of the eyes, nose, and ears. However, if present in excessive amounts, it can operate as a neurotoxic [18]. Long-term exposure can result in mortality and can de-sensitize a person’s ability to smell (OSHA). As a result, detecting H2S in the environment is critical in order to avert its harmful consequences. Dia et al. [19] have demonstrated the synthesis of SnO2/MWCNTs nanaocomposite and observed that LOD of the sensor towards H2S is about 20 ppm at room temperature. The transduction platform consists of a simple resistor constructed by dropping a random network of silver NP studded CNTs into a channel that links two electrodes, and a gas sensor composed of AgNP decorated SWCNTs that is selective to H2S at ambient temperature [20]. Asad et al. [21] have studied H2S gas sensors fabricated using CuO-SWCNT nanostructure with the LOD of sensor about 100 ppb at room temperature. Star et al. [22] have showed the testing of electronic sensor array based on SWCNTs decorated with metal NPs for detection of various test gases (H2, CH4, CO, and H2S).

8.2.3 Ammonia (NH3) and nitrogen dioxide (NO2) gas sensors The increased amount of NH3 in the environment causes major health risks to humans. A rapid exposure to ammonia fumes produces acute irritation of the throat and nose, resulting in diseases like pulmonary edema [23]. Moreover, the presence of ammoniacal at lesser levels irritates the skin and eyes. The total permitted weight-average (TWA) of ammonia exposure limit is between 25 and 50 ppm for 8 h, according to the Occupational Safety and Health Administration (OSHA) [18]. On the other hand, nitrogen dioxide (NO2) has similar negative impacts on the environment and human health. It pollutes the atmosphere with photochemical haze and acid rain. Exhaust from motor vehicles and combustion processes in various chemical facilities contribute significantly to the spread of harmful pollutants in the environment [23]. Rigoni et al. [24] have demonstrated SWCNT sensors that can operate at room temperature and exhibit an enhanced response towards ammonia with LOD as low as 20 ppb. A screen-printing technology is used to make sensors out of SWCNTs with diameters of 1.2 nm. At room temperature, the fabricated sensors are found to be selective to ammonia

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(NH3) gas with detection limit of about 5 ppm [25]. Salehi-Khojin and colleagues discovered that detection response is influenced by defect levels [26]. For NO2 and NH3 gas sensing applications, MWCNT films were produced utilizing a plasma-enhanced chemical vapor deposition method onto Cr-Au patterned alumina substrates with a 3 nm thick Fe growth catalyst, at sensor temperatures of 100 C250 C [27]. They observed that on sputtering noble metal surface catalysts on the surface of MWCNTs, sensitivity of functionalized MWCNT gas sensors increases towards NH3 and NO2 due to a spillover effect [27]. Low amounts of ammonia may be detected using gas sensing devices made from SWCNTs of four different types: pristine, boron doped, nitrogen doped, and semiconducting. In addition, chemoresistive experiments revealed that using nanotubes as conducting channels results in the maximum device performance when compared to other materials [28]. Furthermore, Liang et al. have demonstrated the effect of central metal of phthalocyanine molecule in TFPMPc/ MWCNT hybrid and observed that at room temperature and atmospheric conditions, the sensor is highly selective towards ammonia with LOD of the sensor of about 60 ppb [29] (Fig. 8.5).

FIGURE 8.5 Schematic of the sensor based on TFPMPc/MWCNT hybrids. Reproduced with permission from X. Liang, Z. Chen, H. Wu, L. Guo, C. He, B. Wang, et al., Enhanced NH3sensing behavior of 2,9,16,23-tetrakis(2,2,3,3-tetrafluoropropoxy) metal(II) phthalocyanine/multi-walled carbon nanotube hybrids: An investigation of the effects of central metals, Carbon, 80(2014) 268278. Copyright 2014, Elsevier.

8.3 Medical diagnosis

8.2.4 Chlorine (Cl2) gas sensor Chlorine (Cl2), a dangerous gas with a harsh odor (OEL 5 500 ppb), has been widely used in paper goods, plastics, medications, water disinfectants, household cleaning products, and so on. It quickly combines with water to produce hydrochloric acid and is one among the compounds that might deplete the ozone layer. Furthermore, inhaling Cl2 can injure the respiratory system and induce catastrophic climatic change. As a consequence, owing to the severe impacts on human life, a highly effective and reversible Cl2 sensor with a strong sensitivity response and ppb level detection limit should be developed. Li et al. [30] have reported chemiresistive sensors for the detection of Cl2 using coating of CNTs with polymers. A resistive gas sensor made of 2D mats of MWCNTs was produced by chemical vapor deposition with the help of aerosols [31]. By adjusting both the CNT network shape and the CNT electronic characteristics, the sensor sensitivity with chlorine as analyte was improved [31]. When operated at room temperature, optimized devices have been calibrated across a broad range of concentrations and shown to be sensitive down to 27 ppb of chlorine [31]. We recently discovered that attaching fluorine substituted MPcs on CNTs increases the hybrid sensor’s sensing properties for Cl2 detection [32,33]. Despite the fact that fluorine substituted metal phthalocyanines (MPcs) enhance sensor response to Cl2 detection down to ppb levels, they function at 150 oC [34,35]. Furthermore, substituting various functional groups might be useful strategy to increase the sensitivity of a CNT/phthalocyanine-based hybrid. In particular, CuPcOC8/SWCNTs-COOH hybrid sensors for Cl2 detection at room temperature have been developed [36]. The current findings show that attaching CuPcOC8 molecules to CNTs improves Cl2 sensitivity substantially. In comparison to other gases examined, the CuPcOC8/SWCNTs-COOH hybrid sensor had a response of 94% to 2 ppm Cl2 with a consistent sensing response throughout a broad range of relative humidity (10%85%) and a maximum detection of roughly 1.33 ppb [36] (Fig. 8.6). Table 8.1 lists different CNT-based chemiresistive gas sensors.

8.3 Medical diagnosis Early detection and treatment of diseases improves survival rates while cutting treatment costs. In comparison to traditional blood testing procedures, medical diagnostics aims to give a technology that can detect and, if required, noninvasively monitor illnesses such as lung cancer, diabetes, melanoma, and breast cancer at an early stage, when the chances of recovery are greatest [45]. Human breath analysis is now being considered as a possible solution to address this need. Several promising investigations have been undertaken on the construction of extremely sensitive chemiresistive sensors for illness diagnosis by human breath with high sensitivity employing simple and low-cost metal oxide semiconductors [46]. Diagnostic instruments are commonly used in the medical field.

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CHAPTER 8 Potential applications of chemiresistive gas sensors

FIGURE 8.6 (A) Selectivity histogram of hybrid sensors for 1000 ppb of various tested gases. (B) Response curve for various chlorine doses at room temperature. (C) Variation of response vs chlorine concentrations at room temperature. (D) Repeatability of the sensor to 500 ppb of chlorine. Reproduced with permission from A.K. Sharma, A.K. Debnath, D.K. Aswal, A. Mahajan, Room temperature ppb level detection of chlorine using peripherally alkoxy substituted phthalocyanine/SWCNTs based chemiresistive sensors, Sens. Actuators B: Chem., 350(2022) 130870. Copyright 2022, Elsevier.

Portable diagnostic gadgets, on the other hand, have a large potential. Although these medical sensors may not be as sensitive as GC-Ms or spectroscopy, they would provide a fast and accurate approach for first responders to assess a patient’s status. Because of their high environmental sensitivity and ability to be incorporated into extremely small, low-power electronic systems, CNTs are particularly well suited for the development of portable first-response medical diagnostic equipment. CNTs are widely used in medical research nowadays, and they have been studied extensively in the areas of effective medication delivery and biosensing technologies for health monitoring. Due to their hydrophobic surfaces, pristine CNTs are insoluble in aqueous solutions. Solubilization of CNTs, as well as biocompatibility and low toxicity for medicinal applications, necessitates surface functionalization. The functionalization process for CNTs may be divided into two basic ways, depending on the kind of biomolecule connected to the

8.3 Medical diagnosis

Table 8.1 Summary of CNT-based chemiresistive gas sensors. S. no.

Sensing material

Analyte

Detection limit

1. 2.

Response

References

H2S CO

500 ppb 1 ppm

3.6% N/S

[1] [37]

3.

(PANISWCNTs) Carboxylated SWCNT SWCNTs

NO2, nitrotoluene

N/S

[38]

4. 5.

MWCNTs PMMA/MWCNTs

NH3 Dichloromethane, chloroform, acetone

44 ppb (NO2), 262 ppb (Nitrotoluene) 10 ppm N/S

[39] [40]

6.

Polyaniline/ SWCNTs Pd/SWCNTs MWCNTs Au/SWCNTs

NH3

50 ppb

N/S 809 407 84 2.44%

CH4 NO2 NO2

6 ppm 510 ppb 4.6 ppb

N/S 66% N/S

[42] [43] [44]

7. 8. 9.

[41]

CNT: covalent attachment (chemical bond generation) and noncovalent attachment (physioadsorption). Also because of their large surface area, CNTs have been effectively used in medicine to adsorb a wide range of medicinal and diagnostic substances. They were initially shown to be an effective carrier for delivering drugs directly into cells without having to go through the body’s metabolic process. CNTs have now been widely employed in a variety of applications, including tissue regeneration, biosensor diagnostics, chiral drug enantiomer separation, and drug and pollutant extraction and analysis [47]. CNTs have been found to be a good vehicle for drug delivery, penetrating straight into cells and preserving the drug’s integrity during travel in the body. The general procedure for medication distribution utilizing CNTs is summarized in Fig. 8.7. The drug is applied to the surface or interior of functionalized CNTs. The conjugate is then injected into the animal’s body through typical techniques (oral, injection) or delivered directly to the target area using a magnetic conjugation that is guided to the target organ, such as lymphatic nodes, by an external magnet. The cell consumes the drug CNT capsule, and the nanotube then spills its contents into the cell, delivering the medicine [4850]. He et al. [51] established the adsorption of the anticancer medication epirubicin hydrochloride (EPI) on MMWCNTs, which may also be utilized to adsorb other pharmaceuticals on CNTs made of Fe3O4 spheres or other inorganic compounds like ferrite and metal oxide. They also demonstrated that MMWCNTs with good magnetic characteristics, large adsorption surfaces, and high adsorption capacities are suitable vehicles for loading EPI or other drugs

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CHAPTER 8 Potential applications of chemiresistive gas sensors

FIGURE 8.7 A diagram depicting the use of carbon nanotubes in treatments and biological diagnostics and analysis [47].

[51]. Mulchandani et al. [52] investigated a very sensitive, antibody-free chemiresistive biosensor for dengue viral detection. The biosensor is made up of a SWCNT network chemiresistor that detects the virus by employing heparin as the biorecognition molecule rather than antibody (Fig. 8.8). They have shown that when dengue virus (DENV) is bound to the heparin receptor, the electrical resistance is enhanced with each step of functionalization. For DENV suspended in phosphate buffer (PB), a LOD of 8.4 3 102 TCID50/mL has been achieved after only 10 min of incubation of a 10 μL sample. The selectivity experiments also revealed that a functionalized sensor could detect DENV but not influenza H1N1 [52]. Tianhong et al. [53] have reported CNTs as electric immunoassay for the detection of swine influenza virus (SIV) H1N1. To make thin film, Poly diallyldimethylammonium chloride (PDDA)/ poly(stylenesulfonate) (PSS) have been self-assembled as a charge enhancement precursor layer on the substrate, followed by the assembly of PDDA/SWCNT as

8.3 Medical diagnosis

FIGURE 8.8 A diagram depicting chemiresistor used to detect the dengue virus. Reproduced with permission from D. Wasik, A. Mulchandani, M.V. Yates, A heparin-functionalized carbon nanotube-based affinity biosensor for dengue virus, Biosens. Bioelectron., 91(2017) 811816. Copyright 2017, Elsevier.

an electrochemical transducing material. Multiple testing with roughly 10% background signal yielded a 103-fold dilution as the detection limit of the SWCNT network for SIVs. In addition, a 103-fold dilution of SIV could be detected against Feline calicivirus and transmissible gastroenteritis virus, according to the selectivity test. SIV has a detection limit of 180 TCID50/mL at a 103-fold dilution [53]. One of the easiest approaches for detecting early signs of diabetes is to monitor glucose levels in the blood. When a patient is diagnosed with this ailment, this type of procedure becomes routine and is performed several times each day. Invasive needle-pinning, on the other hand, is not only inconvenient for many people, but it also necessitates care. In addition, measurements taken less times a day yield discrete data points rather than continuous results. As a result, it is impossible to adequately track the influence of dietary habits. Constant monitoring of blood glucose levels, on the other hand, would allow for the early detection of a potentially life-threatening illness. High blood sugar, for example, might affect the body’s ability to regulate blood pressure,

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CHAPTER 8 Potential applications of chemiresistive gas sensors

Table 8.2 CNT-based sensors in medical application. S. no.

Sensing material

1.

SWCNT

2.

MWCNT

3.

SWCNT/ MWCNT Doublewalled carbon nanotube

4.

Analyte

Detection technique

References

Detection of the dengue Virus Glucose sensing

Chemiresistive biosensor Nanoelectromechanical systems Microfluidic electrochemical sensor Electrochemical immunosensing

[52]

Cholesterol detection Determination of two cytokines interleukin-1β and tumor necrosis factor α

[57] [58] [59]

leading to fainting [54]. Losing consciousness while driving can have serious/ fatal consequences. Excess hyperglycemia has also been linked to renal damage [55] and blindness [56]. Table 8.2 lists CNT-based sensors in medical application.

8.4 Food and agriculture applications The most important consumable is food along with water. Chemical sensing may be used to monitor and improve a wide range of food management and agricultural production activities. Food maturity monitoring, food spoilage prevention, food storage quality assurance, and pesticide detection are all instances of these activities [60]. It is worth mentioning that about one-third of all food produced for human consumption is wasted or lost owing to a lack of accurate and real-time monitoring of perishable commodity quality across the supply chain. Cost-effective and efficient sensing devices might educate both suppliers and consumers, improve harvest and transport coordination, monitor storage in both industrial and consumer contexts, and identify pesticide-contaminated food. Sensors made of CNTs provide a variety of advantages that are useful in food applications. They might be utilized to produce cost-effective sensors for product monitoring and supply chain management because of their small size, low power consumption, and simplicity, as well as their ability to detect complex analytes. CNT-based sensors can offer real-time information on the state of food via smartphones or other devices for applications such as smart packaging [61], fruit ripening detection [62], and other applications such as food spoilage [63]. CNT-based gas sensors have been used to characterize the flavor (taste) and odor (smell) of

8.5 Detection of explosives and military applications

items to authenticate and regulate the quality of food products, in addition to increasing food safety and decreasing food loss. Climacteric fruits (such as apples and bananas) release the fruit-ripening hormone ethylene when they begin to ripen. Ethylene is one of the smallest molecules having biological function, and it regulates changes in the color, aroma, texture, and flavor of fruit, whether it is produced naturally in the fruit or artificially to speed up the ripening process [60,64]. As a result, the concentration of ethylene in the environment may be used to influence the ripening and senescence of these fruits [60]. Monitoring and regulating ethylene concentrations is beneficial for determining the best harvesting time as well as preserving freshness throughout storage and transportation. At room temperature, pristine CNTs show little sensitivity to ethylene gas [62], necessitating the inclusion of selection moieties. Leghrib et al. [65] have reported MWCNTs coated with tin oxide (SnO2) NPs to detect ethylene at ambient temperature. Interference from NO2 was detected, despite the fact that the chemiresistors reacted to as little as 3 ppm of ethylene. Esser et al. [62] created an ethylene selective sensor by combining a fluorinated tris (pyrazolyl)borate copper (I) complex with SWCNTs that could produce air-stable ethylene complexes. CNT sensors are utilized to accurately evaluate the taste and smell of various foods in addition to monitoring fruit maturity. Electronic tongues and noses are common names for sensors used in this application (e-tongues and e-noses). The idea is to use a sensor readout to mimic the perceptions of taste and smell. Around 400 distinct olfactory receptors in humans work together to create the sensation of smell [66]. Researchers have included olfactory receptors in sensors to differentiate between distinct odorants differing by a single carbon center to mimic similar processes in sensors. Liu et al. [63] have reported chemiresistive detection of amines and meat spoilage using SWCNTs/Pc composites having sub-ppm sensitivity. Shaalan et al. [67] reported the potential of CNT-based sensor for fruit monitoring when they reported the response of fabricated sensor for monitoring the ethylene gas. They observed that sensor exhibits high response towards ethylene at low operating temperature in the concentration range of 300 ppb to 10 ppm with response of 2% up to 28%. Fig. 8.9A depicts the variations in response of a CNT-based sensor as a function of time after the sensor was subjected to five cycles of a series of air, comprising generated ethylene (response for 180 s) and dry air (recovery for 300 s), for three bananas with varied levels of ripening (Fig. 8.9B).

8.5 Detection of explosives and military applications Security is a critical component of international peace. Terrorists use IEDs (improvised explosive devices) to carry out their assaults. Chemical sensors are

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CHAPTER 8 Potential applications of chemiresistive gas sensors

FIGURE 8.9 (A) Repeatability of the sensor signal (B) for different bananas at different ripening levels. Reproduced with permission from N.M. Shaalan, O. Saber, F. Ahmed, A. Aljaafari, S. Kumar, Growth of Defect-Induced Carbon Nanotubes for Low-Temperature Fruit Monitoring Sensor, Chemosensors, 9(2021). Copyright 2021, Elsevier.

the sole way to detect IEDs, but because of their poor sensitivity and restricted potential, they are relatively hard to operate. At the single molecule level, using nanosensors is far more efficient and sensitive. Nanomaterials have a lot of potential for making explosive detection sensors. In general, trained canines can detect IED, but it is difficult to maintain them and at the same time have high environmental risk. Some of the current technologies for detecting chemical warfare agents (CWAs) include ion mobility spectroscopy, mass spectrometry, surface acoustic wave (SAW), electrochemical sensors, infrared spectroscopy, colorimetric sensors, etc. Despite the fact that all of these procedures have high sensitivity and specificity, each has its own set of limitations. Mass and infrared spectroscopies, for example, need large, sensitive apparatus that is inappropriate for field work. Colorimetric detectors are lightweight and portable, but they cannot monitor real-time data and are not easily integrated into more complex electronics.

8.5 Detection of explosives and military applications

CNTs open us the option of constructing portable sensing technologies for continuous environmental monitoring that naturally couple with electrical devices based on chemiresisitive or electrochemical sensors. Sensors made of CNTs have the potential to be used in defense and homeland security. Unmanned defense systems, such as unmanned aerial vehicles, can use them. The term “security” refers to protection from bombs and weapons at entry points. CNT-based chemical sensors, as previously indicated, can be employed as electronic noses to detect hidden explosives and chemical weapons in luggage, automobiles, and planes. However, exposing passengers to some types of explosive detection screening systems already in use poses certain health hazards. Certain types of explosive detection screening equipment might expose people to low levels of radiation. The usage of explosive detection systems based on CNT-based sensors will help to mitigate the risks caused by current scanning equipment. Several groups have worked on building CNT-based explosives and nerve agent sensors. Li et al. [38] have demonstrated sensor developed using casting of SWCNTs on an IDE for the detection of NO2 and nitrotoluene with detection limit of about 44 ppb and 262 ppb, respectively. Lee et al. [68] have reported arrays of metallic CNTs as sensors for the detection of dimethyl methylphosphonate (DMMP) which are a nerve agent precursor. Cattanach et al. [69] demonstrated that chemiresistor sensors developed using SWCNT for the detection of CWA. SWCNT bundles were placed over a polyethylene terephthalate polymer sheet to make the sensor devices. SWCNT bundles demonstrate an exceptionally substantial resistance shift and a strong sensor response to CWA simulants in the presence of vapors routinely seen in battle-space and urban air environments, even when subjected to CWA simulant vapors at ambient settings, according to the study. Nerve agents disrupt the neurological system by inhibiting the enzyme acetylcholinesterase (AChE), which breaks down the neurotransmitter acetylcholine, in a reversible manner (ACh). When AChE is blocked, a buildup of ACh in the cell results in overstimulation [70]. In chemiresistive devices, Zhang et al. [71] employed oligomer wrapped CNTs to selectively detect nitroaromatics. They observed an increase in conductance in devices containing pure CNTs in agreement with the charge accepting nature of nitroaromatics and also a drop in conductance was observed in devices comprising wrapped CNTs, which is attributed to oligomeric material swelling and the accompanying enhancement in spacing between CNTs (Fig. 8.10). Novak et al. [72] have demonstrated the use of CNTs as a sensor for chemical nerve agents. These sensors are reversible and capable of detecting DMMP at ppb levels, as well as being intrinsically selective to interferent signals like hydrocarbon vapors and humidity. Ishihara et al. [73] have reported a SWCNT-based chemical sensor that measures the conductivity of a SWCNT network that is individually encased in an insulating metallosupramolecular polymer. Table 8.3 lists CNT-based chemiresistive gas sensors for detection of explosives and military application.

239

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CHAPTER 8 Potential applications of chemiresistive gas sensors

FIGURE 8.10 (A) Sensor response to NT vapor at different concentrations. (B) Selectivity histogram for coated and the uncoated sensor. (C) Schematic of sensing mechanism. (D) Scatter plot of the conductance changes of the coated and CNT sensor. Reprinted with permission from Y. Zhang, M. Xu, B.R. Bunes, N. Wu, D.E. Gross, J.S. Moore, et al., OligomerCoated Carbon Nanotube Chemiresistive Sensors for Selective Detection of Nitroaromatic Explosives, ACS Appl. Mater. & Interfaces, 7(2015) 74717475. Coyright year (2015) American Chemical Society.

Table 8.3 CNT-based chemiresistive gas sensors detection of explosives and military application. S. No.

Sensing material

1. 2. 3.

Bare CNTs Metallic CNTs DNAfunctionalized CNTs Bare CNTs DNAfunctionalized CNTs

4. 5.

Analyte

Detection limit (ppb)

Method

References

CWAs CWAs CWAs

,1 700 ,1

Chemiresistor Chemiresistor Chemiresistor

[72] [68] [74]

Explosives

262 ,1

Chemiresistor Chemiresistor

[38] [74]

References

8.6 Conclusion CNTs are active ingredients in chemical sensors. The principle of these sensors has been originally discussed in this study, including sensing processes. The reader is then exposed to CNT-based sensors and specific analytes for a variety of applications, including environmental monitoring, food and agricultural applications, biological sensors, and national security. Chemiresistive gas sensors, due to their high sensitivity, ease of device manufacture, and downsizing potential, are capable of monitoring human breath for medical purposes. CNT-based chemiresistive sensors are showing early promise in the detection of a number of diseases, including breast cancer, diabetes, and asthma. The purpose of these conversations is to provide the reader with a working grasp of the present status of the discipline. CNT-based sensor technology has a bright future, and more research in this area will help to overcome current challenges, resulting in a class of sensor materials with better sensitivity, smaller size, and longer life spans for a variety of situations and applications.

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245

Index Note: Page numbers followed by “f” and “t” refer to figures and tables, respectively.

A Acetylcholinesterase (AChE), 237 239 Acetylene, 86 Activated carbons, 14 16 synthesis of, 21 22 Actuators, 94 Aerosol techniques, 173 Ammonia (NH3) and nitrogen dioxide (NO2) gas sensors, 229 230 Arc discharge method, 18, 183 184 Atomic layer deposition (ALD), 174

B Band gap, 78 Band theory, 77 83 amperometric gas sensors, 81 electrical device sensors, 82 83 gas sensing mechanism of conducting polymers, 80 81 potentiometric sensors, 81 82 Bandwidth, 78 Base station (BS), 213 214 Batteries, 93 Biological contaminants, 117 118 Biosensors, 93 Bipolar Field Effect Transistor (BiFET), 216 Bipolaron, 79 80

C Carbon Carbon Carbon Carbon

dots, 22 23, 23f fibers, 16 filament, 22 23 materials/SMO nanocomposites, synthesis of, 169 174 ex-situ techniques, 170 171 covalent interactions, 171 electrostatic interactions, 171 π-π stacking, 171 gas-phase deposition, 173 174 atomic layer deposition (ALD), 174 chemical vapor deposition (CVD), 174 evaporation and sputtering, 173 pulsed laser deposition (PLD), 173 hydrothermal and aerosol techniques, 173 supercritical solvent, 173 vapor-assisted, polyol-assisted process, 173 in-situ techniques, 171 172 chemical reduction and oxidation, 172

electrochemical techniques, 171 electrodeposition, 172 sol-gel process, 172 Carbon monoxide (CO) gas sensor, 227 229 Carbon nano-onions (CNOs), 11 12 Carbon nanofiber (CNF), 4 Carbon nanomaterial-based chemiresistive sensors fabrication of, 111 118 biological contaminants, 117 118 fossil fuel emissions detection, 114 115 greenhouse gases, 116 117 hydrogen gas detection, 113 military and defense explosives detection, 115 116 volatile organic compound detection, 113 114 sensing mechanism of, 121 122 sensor comparison at lab/industrial level, 118 121 Carbon nanomaterials, 4 5, 11 12, 16 17 acid functionalization, 25 31 end defect functionalization, 26 side wall and surface functionalization, 27 wet oxidation of carbon nanomaterial, 27 28 wet oxidation of graphene, 28 31 acid simulation of, 23 25 applications, 32 34 classifications of, 4, 5f properties of, 13, 14t quantum confinement in, 12 13 types of, 5 12 carbon nanotubes, 8 9 diamond, 5 7 fullerene, 10 11 graphite, 7 Carbon nanotubes (CNTs), 4, 8 9, 33, 109, 111 112, 121 122, 169, 223, 226 227 CNT-based chemiresistive gas sensors, 233t in detection of explosives and military application, 240t fabrication of CNTs/conducting polymers-based sensors, 211 212 fabrication of CNTs/SMOs-based sensors, 207 208 in food applications, 236 237 in medical research, 231 234, 236t portable sensing technologies, 237 239 synthesis of, 18 19

247

248

Index

Carbon nanotubes with covalent and noncovalent, functionalization of, 181 186 synthesis techniques, 183 186 arc discharge method, 183 184 laser ablation method, 184 melt mixing, 185 186 in situ polymerization, 186 solution mixing, 184 185 Carbon-based nanomaterials, 223 Chemical reduction and oxidation, 172 Chemical vapor deposition (CVD), 18 19, 56 58, 174 Chemical warfare agents (CWAs), 237 239 Chemiresistive gas sensors, 223 detection of explosives and military applications, 237 240 environmental monitoring ammonia (NH3) and nitrogen dioxide (NO2) gas sensors, 229 230 carbon monoxide (CO) gas sensor, 227 229 chlorine (Cl2) gas sensor, 231 hydrogen sulfide (H2S) gas sensor, 229 food and agriculture applications, 236 237 importance of, 109 111 limit of detection (LOD), 225 linearity, 226 227 medical diagnosis, 231 236 recovery time, 225 response time, 225 selectivity, 225 sensitivity, 225 sensor response, 223 225 stability, 225 Chemiresistive-semiconductor metal oxide (CSMO)-based gas sensors, 133 134 challenges and future prospect, 159 163 fabrication and designing of, 135 139 factors influencing sensing characteristics of ambient atmosphere, 155 157 crystallinity and microstructure, 148 150 humidity, 154 155 morphology, 153 154 role of chemical composition, 151 153 role of vacancies, 150 151 temperature, 154 growth techniques of sensing material thick film technology, 138 thin film technology, 138 139 traditional technology, 137 outcomes at industrial level, 159 at lab level, 157 159 performance parameters detection limit and range, 144

recovery time, 143 response, 141 142 response time, 143 selectivity, 143 sensitivity, 142 143 sensor drift, 144 145 stability, 144 sensing mechanism in, 145 148 metal/ metal oxide functionalized metal oxides, 146 148 pristine oxides, 146 working principle of, 139 141 Chemiresistor, 82 83 Chirality, 13 Chlorine (Cl2) gas sensor, 231 CNTs/conducting polymers-based nanocomposites, synthesis of, 180 181 CNTs/conducting polymers-based sensors, fabrication of, 211 212 CNTs/CPs nanocomposites, 190 193 electromagnetic interference shielding, 193 fuel cell, 192 lithium-ion batteries, 192 sensors, 190 191 solar cell, 192 supercapacitors, 191 192 CNTs/SMOs-based sensors, fabrication of, 207 208 Colorimetric sensors, 237 239 Conducting polymers (CPs), 75 83, 180 181, 190 192 applications, 90 94 actuators, 94 batteries, 93 corrosion protection, 93 electrochromic device, 93 94 light emitting diodes, 91 radar application, 94 sensors, 92 93 solar cells, 91 supercapacitors, 91 transistors and data storage, 92 band theory, 77 83 amperometric gas sensors, 81 electrical device sensors, 82 83 gas sensing mechanism of conducting polymers, 80 81 potentiometric sensors, 81 82 with carbon materials, 87 90 nanocomposite, 76f polyacetylene, 86 polyaniline, 86 87 synthesis of, 83 87 synthetic preparation methods of, 84 86

Index

chemical method, 84 concentrated emulsion method, 85 electrochemical method, 84 inclusion method, 85 metathesis method, 85 photochemical method, 84 85 plasma polymerization, 85 pyrolysis method, 86 solid-state method, 85 theory of conductivity, 77 Conduction bands, 78 Conductive polymers, 223 Conductive-polymer gas sensors, 111 Corannulene, 10f Corrosion protection, 93 Covalent functionalization, 25 26 Covalent interactions, 171

D Dengue virus (DENV) detection, 231 234 Diamond, 5 7 3D lattice structure of, 5 structural illustration of, 6f thermal conductivity of, 6 Dimethyl methylphosphonate (DMMP), 82 83 Doping, 76 77 Double-walled CNTs (DWCNTs), 18 19

E Electrochemical capacitors, 189 Electrochemical sensors, 237 239 Electrochemical techniques, 171 Electrochromic device, 93 94 Electrodeposition, 172 Electromagnetic interference shielding, 193 Electron beam evaporation, 48 Electronic noses, 236 237 Electronic tongues, 236 237 Electrostatic interactions, 171 Energy gap, 78 Environmental monitoring chemiresistive gas sensors in, 227 231 Ethylene, 236 237 Evaporation, 173 Explosives detection, 237 240

F Fermi energy level, 79 80 Field effect transistors (FET), 32 33, 41 43, 216 Flash evaporation, 49 Forbidden gap, 78 Fossil fuel emissions detection, 114 115 Fuel cell, 192

Fullerene, 10 11 Functionalized CNTs, 181 Functionalized graphene/ZnO (FGZnO) nanohybrids, 178 179 Fundamental energy gap, 78

G G-SMO nanocomposites, common synthesis methods of, 175 176 Gallium arsenide (GaAs)-based semiconductors, 215 Gas sensors, 92 93, 133 134 Grafting method, 181 Graphene, 7, 9f, 33 34, 121 122, 223 properties of, 14, 15f synthesis of pure, 19 21 wet oxidation of, 28 31 Graphene quantum dots (GQDs), 178 Graphene/SMO-based nanocomposites, synthesis of, 174 180 common synthesis methods of the G-SMO nanocomposites, 175 176 hydrothermal method, 176 178 self-assembly method, 178 in situ method, 178 179 solution mixing method, 179 spin coating, 179 180 Graphene/SMOs nanocomposites and CNTs/SMOs nanocomposites, 187 190 energy storage and conversion, 188 hydrogen storage, 190 lithium-ion batteries/sodium-ion batteries/zincion batteries, 188 189 photocatalysts, 190 photodetector, 189 190 sensors, 187 188 solar cells, 189 supercapacitors, 189 Graphene/SMOs-based sensors, fabrication of, 209 211 Graphite, 4, 7 Greenhouse gases, 116 117

H Hetero-junction Bi-polar Transistor (HBT), 215 216 Highest occupied molecular orbital (HOMO), 78 Human breath analysis, 231 234 Humidity sensors (HSs), 93 Hummer’s method, 20 21 Hydrogen gas detection, 113 Hydrogen sulfide (H2S) gas sensor, 229 Hydrothermal techniques, 173, 176 178

249

250

Index

I

Laser ablation method, 18, 49 52, 184 Light emitting diodes, 91 Limit of detection (LOD), 107 108, 225 Linear zone, 226 Linearity, 226 227 Lithium-ion batteries (LIBs), 188 189, 192 Lowest unoccupied molecular orbital (LUMO), 78

radio frequency heating, 48 resistive heating technique, 46 48 thermal evaporation, 46 48, 47f top down and bottom approaches for, 46f versatility of, 41 43 Metal oxides, 134 135 Metal-oxide semiconductors, 223 Metal/ metal oxide functionalized metal oxides chemical interaction mechanism, 147 electronic interaction mechanism, 147 heterojunction formation mechanism, 148 Metallic nanoparticles, 223 Microfabrication-based MEMS technology, 138 139 Military and defense explosives detection, 115 116 MnFe2O4-graphene nanocomposites, 176 178 Molecular beam epitaxy (MBE), 43 46, 54 55 Multi-walled carbon nanotubes (MWCNTs), 8 9, 82 83, 111 112, 180 183, 190 191, 226 227, 229 234

M

N

Macromer-grafted polymers (MGPs), 181 183 Mass spectrometry, 237 239 Medical diagnosis, chemiresistive gas sensors in, 231 236 Melt mixing, 185 186 MEMS (microelectromechanical systems), 138 139 3-Mercaptopropionic acid (MPA) capping, 117 118 Metal organic chemical vapor phase deposition (MOCVD) technique, 215 216 Metal oxide nanocomposites, 209 211 Metal oxide nanoparticles chemical methods, 58 61 anodization, 59 electroless deposition, 59 electrolytic deposition, 58 59 vapor-liquid-solid, 59 chemical vapor deposition, 56 58 photochemical vapor deposition, 58 plasma-enhanced CVD, 58 Metal oxide semiconductors (MOS) crystal structure of, 44f electrical conductivity of, 62 64 future aspects for, 67 synthesis of oxide nanomaterials, 43 56 electron beam evaporation, 48 essential requirements, 48 flash evaporation, 49 laser ablation technique, 49 52 physical vapor deposition, 43 46 pulsed laser deposition, 49 52

Nanocarbons, 14 16 Nanocomposites, applications of, 186 193 CNTs/CPs nanocomposites, 190 193 electromagnetic interference shielding, 193 fuel cell, 192 lithium-ion batteries, 192 sensors, 190 191 solar cell, 192 supercapacitors, 191 192 graphene/SMOs nanocomposites and CNTs/ SMOs nanocomposites, 187 190 energy storage and conversion, 188 hydrogen storage, 190 lithium-ion batteries/sodium-ion batteries/ zinc-ion batteries, 188 189 photocatalysts, 190 photodetector, 189 190 sensors, 187 188 solar cells, 189 supercapacitors, 189 Nanofibers (NFs), 226 Negative electron affinity, 6

Improvised explosive devices (IEDs), 237 239 In situ method, 178 179 In situ polymerization, 186 Infrared spectroscopy, 237 239 Interdigitated electrode (IDE), 205, 223 Ion mobility spectroscopy, 237 239 Ion-sensitive sensors (ISEs), 81 82

K Knudsen cell, 52 53, 55

L

O Operating temperature, 108

P p-MWCNT (plasma-functionalized multiwalled carbon nanotubes)/PANI (polyaniline-a conducting polymer) composite sensor, 211 212

Index

Percolation threshold, 87 90 Photocatalysts, 190 Photochemical vapor deposition, 58 Photodetectors, 189 190 Photovoltaic cells. See Solar cells Physical vapor deposition (PVD), 43 46 π-π stacking, 171 Planck’s constant, 12 Plasma polymerization, 85 Plasma-enhanced CVD, 58 Polaron, 79 80 Poly (3,4-ethylene dioxythiophene) (PEDOT), 84 Poly (vinyl alcohol), 93 Polyacetylene (PA), 84, 86 Polyaniline (PANI), 78 79, 86 87 PANI-coated MWCNTs, 122 PANI/MWCNTs-based nanocomposites, 211 212 Polydopamine-modified RGO (PDA/RGO), 178 179 Polymer-CNT nanocomposite systems, processing methods for, 186t Polyol-assisted process, 173 Polypyrrole (PPy), 78 79, 87 Polythiophene (PTh), 78 79 Porphyrin 2 SWCNT hybrid, 122 Portable diagnostic gadgets, 231 234 Potentiometric sensors, 81 82 Power consumption, 107 108 Pristine CNTs, 227 229 Pristine oxides ionosorption mechanism, 146 reduction-reoxidation mechanism, 146 Pulse oximeters, 118 Pulsed laser deposition (PLD), 49 52, 173 deposition conditions, 51 laser beam parameters, 51 52 influence of laser fluence, 52 influence of laser wavelength and pulse duration, 51 52 influence of number of pulses, 51 influence of target-substrate separation distance, 52 sputtering technique, 52 55 Pyramidalization, 10 11

R Radar application, 94 Radio Detection and Ranging (RADAR), 94 Radio frequency (RF), 43 46 RF communication, 206 207 Radio frequency heating, 48 Reactive pulsed laser deposition (RPLD), 51 Recovery time, 107 108, 225

Reduced graphene oxide (rGO), 209 211 Reduced graphene oxide (RGO)-WO3 composites, 176 178 Relative humidity (RH), 64 65 Resistive heating technique, 46 48 Response time, 107 108, 225 rGO-CNT-SnO2 hybrids, 122

S Scotch tape method, 20 Selectivity, 225 Self-assembly method, 178 Semiconductor metal oxides (SMOs), 169 171, 207 deposition on CNTs, 171 Semiconductor metal-oxide gas sensors, 110 Sensing mechanisms, 217 218 Sensitivity, 225 Sensor aging, 159 163 Sensor drift, 144 145 Sensor network, 205 207 Sensor response, 223 225 Sensor response instability, 144 145 Sensor stability, 225 Sensors, 92 93 biosensors, 93 gas sensors, 92 93 humidity sensors, 93 types, 107 109 Signal-to-noise ratio, 225 Single-hop network architecture, 213 214 Single-walled carbon nanotubes (SWCNTs), 8 9, 87 88, 111 112, 115 116, 180 181, 226 227, 229 230, 237 239 functionalized with specific antibodies, 117 118 SWCNT 2 porphyrin hybrid sensors, 113 114 SnO2/graphene nanocomposites, 187 188 Sodium-ion batteries (SIBs), 188 189 Sol-gel process, 59 61, 172 Solar cells, 91, 189, 192 Solution mixing, 179, 184 185 Spin coating, 179 180 Sputtering technique, 52, 173 magnetron sputtering, 54 molecular beam epitaxy, 54 55 RF sputtering, 52 55 working of, 52 55, 53f Stability, 107 108, 225 Stannous oxide, 208 Supercapacitors (SC), 91, 189, 191 192 Supercritical solvent, 173 Surface acoustic wave (SAW), 237 239 Surface plasmon resonance (SPR), 82 83

251

252

Index

T

W

Template carbons, 16 Thermal evaporation, 46 48 Thermomechanical degradation, 159 163 Thermoplastic polymers, 185 186 Thick film technology, 138 Thin film, 52 53 Thin film technology, 138 139 Toxic gases, 133 134 Transistors and data storage, 92

Wireless sensor networks (WSN), 205 207, 216 materials used in fabrication of devices for, 215 network architecture, 213 214 sensor node, 212 213 Wireless-based networks sensors, fabrication of, 212 216 FBT fabrication, 215 216 materials used in, 215 network architecture, 213 214 WO3/graphene nanocomposites, 187 188 Wonder materials, 4 Work function, 79 80

V Valence bonds, 78 Vapor-assisted hydrothermal synthesis process, 173 Vapor-liquid-solid (VLS) method, 59, 59f Volatile organic compound (VOC) detection, 113 114

Z Zinc oxide (ZnO), 64 65 Zinc-ion batteries (ZIBs), 188 189 ZnO/RGO nanocomposites, 187 188 ZnO2/RGO nanocomposites, 187 188