Carbon-Based Nanomaterials and Nanocomposites for Gas Sensing 9780128213452

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CARBON-BASED N A N O M AT E R I A L S A N D NANOCOMPOSITES FOR GAS SENSING

CARBON-BASED N A N O M AT E R I A L S A N D NANOCOMPOSITES FOR GAS SENSING Edited by NAVINCHANDRA GOPAL SHIMPI Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India

SHILPA JAIN Department of Chemistry, Jai Hind College, Churchgate, Mumbai, Maharashtra, 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: 978-0-12-821345-2 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

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List of contributors Shilpa Jain Department of Chemistry, Jai Hind College, Mumbai, Maharashtra, India Mohammad Khalid Graphene and Advanced 2D Materials Research Group, School of Engineering and Technology, Sunway University, Jalan University, Subang Jaya, Malaysia Jolina Rodrigues Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India Tanushree Sen Department of Chemistry, University Mumbai, Santacruz (East), Mumbai, Maharashtra, India

of

Akshara Paresh Shah Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India Navinchandra Gopal Shimpi Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India Golnoush Zamiri Centre of Advanced Materials, Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia

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Contents List of contributors ................................................. xi About the editors ................................................. xiii Preface ................................................................... xv

Part 1 Introduction to carbon based nanomaterials .......................................1 1 Carbon-based smart nanomaterials.......................3 Shilpa Jain and Navinchandra Gopal Shimpi 1.1 Introduction to carbon-based nanomaterials................................................. 3 1.2 Types of carbon nanomaterials..................... 6 1.2.1 Carbon nanotubes ................................. 6 1.2.2 Fullerenes............................................... 8 1.2.3 Graphene ............................................... 9 1.2.4 Carbon nanofibers............................... 11 1.3 Synthesis methodologies and variations...... 12 1.4 Gas sensors and their comparison ............. 16 References ........................................................... 19

2 Carbon nanomaterials-based gas sensors.........25 Shilpa Jain, Akshara Paresh Shah and Navinchandra Gopal Shimpi 2.1 Types of gas sensors based on carbon-based nanomaterials ....................... 26 2.1.1 Electrochemical sensors ..................... 27 2.1.2 Electrical/chemiresistive sensors ....... 27 2.1.3 Mass-sensitive gas sensors ................ 29 2.1.4 Thermometric (calorimetric) gas sensors .......................................... 30 2.2 Parameters of gas sensor ............................ 31 2.2.1 Sensitivity ............................................ 31 2.2.2 Selectivity............................................. 32 v

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2.2.3 Stability ................................................ 32 2.2.4 Response time ..................................... 33 2.2.5 Recovery time...................................... 33 2.3 Functionalization of carbon-based nanomaterials............................................... 33 2.4 Sensing mechanism..................................... 37 2.4.1 Sorption gas sensors .......................... 37 2.4.2 Ionization gas sensors ........................ 39 2.4.3 Capacitive gas sensors........................ 39 2.4.4 Resonance frequency shift gas sensors ................................................. 39 2.5 Fabrication of sensors.................................. 40 References ........................................................... 42

3 Carbon-based gas sensing materials..................51 Golnoush Zamiri and Mohammad Khalid 3.1 Introduction to carbon-based gas sensing materials ......................................... 51 3.1.1 Gas sensors ......................................... 52 3.2 Detection mechanism of gas sensors......... 53 3.3 Carbon nanomaterials and nanocomposites for sensing ....................... 55 3.3.1 Carbon black ........................................ 56 3.3.2 Carbon nanofibers............................... 57 3.3.3 Carbon nanotubes............................... 57 3.3.4 Graphene ............................................. 58 3.4 Carbon black materials and composites for gas sensors......................... 59 3.5 Carbon nanofibers and composites for gas sensors ............................................. 60 3.6 Carbon nanotubes and composites for gas sensor ............................................... 62 3.6.1 Carbon nanotubes and metal or metal oxide composites for gas sensor ...................................... 64 3.6.2 Carbon nanotubes and polymer composites for gas sensor ................. 65 3.7 Graphene materials and composites for gas sensor ............................................... 66

Contents

3.7.1 Graphene and metal or metal oxide nanocomposite for gas sensor........... 68 3.7.2 Graphene and polymers nanocomposites for gas sensor ......... 72 3.8 Conclusion .................................................... 73 Acknowledgment ................................................ 74 References ........................................................... 74

Part 2 Application of carbon nanomaterials in gas sensing ........81 4 Carbon nanotube-based gas sensors ..................83 Tanushree Sen and Navinchandra Gopal Shimpi 4.1 Introduction .................................................. 83 4.2 Sensing mechanism..................................... 85 4.3 Carbon nanotube/metal nanocomposite-based gas sensors ............ 87 4.4 Carbon nanotube/semiconducting metal oxide nanocomposite-based gas sensors ................................................... 89 4.5 Carbon nanotube/conducting polymer nanocomposites for gas sensors ................ 93 4.6 Functionalized carbon nanotubes as gas sensors ................................................... 97 4.7 Conclusions and outlook ............................. 98 References ........................................................... 99

5 Carbon nanofiber-based gas sensors................105 Jolina Rodrigues, Shilpa Jain, Navinchandra Gopal Shimpi and Akshara Paresh Shah 5.1 Introduction .................................................105 5.2 Methods of carbon nanofiber preparation ..................................................106 5.2.1 Electrospinning.................................. 106

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5.2.2 Catalytic thermal chemical vapor deposition growth ............................. 107 5.2.3 Substrate method.............................. 109 5.3 Fabrication/construction of carbon nanofibers ....................................................110 5.3.1 Carbon nanofibers modified with metal oxides .............................. 110 5.4 Carbon nanofibers as gas sensors.............111 5.4.1 ZnO/CNFs ......................................... 111 5.4.2 Sn SnO2/CNFs ................................ 111 5.4.3 CNFs/polystyrene ............................ 113 5.4.4 SnO2/CNFs ....................................... 114 5.4.5 V2O5/CNFs ........................................ 114 5.4.6 Au-Pt/CNFs....................................... 115 5.4.7 Multifunctional carbon nanofibers ........................................ 116 5.4.8 Mesoporous carbon nanofibers ..... 117 5.4.9 WO3/CNFs ........................................ 117 5.4.10 Ni/CNFs ............................................ 118 5.4.11 CNFs/PPy.......................................... 119 5.4.12 WS2/CNFs......................................... 119 5.4.13 Ni-CNF .............................................. 119 5.4.14 Graphitic carbon nanofibers........... 121 5.4.15 Graphitic-carbon nanofibers/ polyacrylate ..................................... 122 5.4.16 PAN/(PAN-b-PMMA)........................ 122 5.4.17 5,6;11,12-di-o-phenlyenetetracene/ carbon nanofibers ........................... 123 References ..........................................................125

6 Graphene-based gas sensors..............................127 Akshara Paresh Shah, Shilpa Jain and Navinchandra Gopal Shimpi 6.1 Gas sensor mechanism ..............................129 6.2 Graphene and its derivative/metalbased gas sensor.........................................131 6.3 Graphene and its derivative/ metal oxide-based gas sensor ..............................134

Contents

6.4 Graphene and its derivative/polymer based gas sensor.........................................140 References ..........................................................142

7 3D Hierarchical carbon-based gas sensors ....149 Jolina Rodrigues, Shilpa Jain and Navinchandra Gopal Shimpi 7.1 Introduction .................................................149 7.2 Importance of 3D nanomaterial .................150 7.3 Construction/fabrication of 3D architectures ................................................151 7.4 3-D metal oxide/graphene nanocomposite as gas sensors ..................153 7.5 3-D functionalized graphene nanocomposite as gas sensors ..................163 7.6 3-D metal doped graphene nanocomposite as gas sensors ..................166 7.7 3-D metal oxide/carbon nanotube and metal oxide/graphene oxide/carbon nanotube nanocomposite as gas sensors .............................................167 7.7.1 Sensing mechanisms of 3D TiO2/ graphene-carbon nanotubes gas sensors ........................................ 169 7.8 3D metal oxide/carbon nanocomposite as gas sensors .............................................171 7.9 3D graphene-based gas sensors ................174 References ..........................................................178

8 Conducting polymer-based gas sensors...........181 Jolina Rodrigues, Shilpa Jain and Navinchandra Gopal Shimpi 8.1 Introduction .................................................181 8.2 Conducting polymers-based gas sensors ... 182 8.3 Polyaniline as a gas sensing material .......183 8.4 Polypyrrole as gas sensing material..........190 8.5 Polythiophene as gas sensing material.....207 References ..........................................................228

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9 Future prospects: carbon-based nanomaterials and nanocomposites..................233 Shilpa Jain, Navinchandra Gopal Shimpi and Akshara Paresh Shah References ..........................................................237 Index..................................................................... 239

About the editors Dr. Navinchandra Gopal Shimpi has been working as an Professor at the Department of Chemistry, University of Mumbai, Mumbai, since April 2014. Previously, he was associated with the University Institute of Chemical Technology, Jalgaon. He completed PhD from North Maharashtra University, Jalgaon, in 2006. He was the recipient of Young Scientist Award from Asian Polymer Association in 2014 and Dnyanjoti Puraskar in 2008 from Shirsathe Foundation, Jalgaon. So far, he has published more than 100 papers in international journals of good impact factor and delivered more than 40 lectures as an invited speaker. He has generated Rs 1.65 crores for outstanding research from various funding agencies. He is at present handling one research project from UGC, New Delhi, and one consultancy project from Indofil Chemicals Ltd, Thane. So far, fourteen students have completed their PhD, and eight are doing their PhD under his guidance. He is having two granted patents, and four are under examination. Moreover, he has guided 15 students for their MTech dissertation. Besides this, he has organized five national and international conferences with five staff development programs and four professional certificate courses. He is an associate editor of the International Journal of Chemical Studies and worked as a lead guest editor for Advancement in Polymeric Nanomaterials and Nanocomposites, a special issue of International Journal of Polymer Science. Dr. Shilpa Jain has been working as an assistant professor at the Department of Chemistry, Jai Hind College, University of Mumbai, Mumbai, since November 2016. She has done her PhD. under the guidance of Dr. Navinchandra Shimpi from the Department of Chemistry, University of Mumbai. Her PhD. topic was “Technique development in synthesis of smart nanomaterials and their application in sensing.” Her research areas include nanomaterials, nanotechnology, gas sensors, and polymer nanocomposites. She has worked on hybrid nanomaterials and nanocomposites and their application in highly efficient gas sensors. Her other areas of research interest are morphology-dependent sensing, carbon-based nanostructures, and nano catalysts. She has published 10 research papers in various international journals with high impact factor having more than 200 citations.

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Preface Carbon-based Nanomaterials and Nanocomposites for Gas Sensing mainly deals with the application of carbon-based nanomaterials (CBNs) such as carbon nanotubes, fullerenes, graphene, and carbon nanofibers and their nanocomposites in gas sensing. Various nanomaterials such as metal oxides, semiconducting metal oxides, polymers, nanocomposites, and CBNs have been studied extensively for the detection of various gases. CBNs are considered as a substitute of expensive electronic grade semiconductors because of their extraordinary properties and easy manufacturing. They are considered as one of the prime members of smart nanomaterials and advanced nanotechnology. CBNs with high aspect ratio, high carrier mobility, unique structure, and properties are excellent chemical and biological sensors. Carbon nanostructures with inherent properties become an ideal sensing material for the next generation of sensor technology. Gas sensors have become essential component in several industries, process control unit, and environmental monitoring. This book begins with the description of CBNs, their types, synthesis methodologies, properties, and their application in gas sensing. In addition, it provides an overview of various CBNs and their application, challenges, and opportunities for highly efficient gas sensors. This book focuses on the unique characteristic of these CBNs, which enhances the selectivity toward a specific gas and can be easily tailored with functionalization and doping. It also describes various types of gas sensors based on CBNs and their fabrication and sensing mechanism. It further discusses the modifications in microstructure, doping, functionalization, and hybrid nanocomposites with conducting polymers to attain selectivity and sensitivity toward particular gas with maximum gas response. This book gives a broad idea about various types of CBNs, their properties, synthesis methodology, and application in gas sensing. It introduces the readers with the recent developments, technology, and importance of CBNs in highly efficient and smart gas sensors. Also, it covers several aspects in terms of societal, academic, industrial, and research benefits. This book will be of great importance not only to the learners but also to the more experienced researchers, research scholars, and students of postgraduate and graduate levels. The intention behind

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Preface

editing this book is to introduce the readers to the fascinating world of carbon-based nanostructures and their application in gas sensing. Based on the theme, this book is divided into two parts: Part one: Introduction to Carbon based Nanomaterials, which contains three chapters and Part two: Application of carbon Nanomaterials in gas sensing, which contains five chapters. Chapter 1 presents a brief introduction on CBNs and their types, properties, and synthesis techniques. It gives a brief comparison on various types of gas sensors. Chapter 2 presents in detail the parameters of gas sensors, various types of gas sensors, their sensing mechanism, fabrication, and functionalization of CBNs. Chapter 3 presents in brief the detection mechanism, carbon nanomaterials, and nanocomposites for sensing such as carbon black, carbon nanofibers, carbon nanotubes, and graphene. The nanocomposites with polymers are discussed for gas sensing. Chapter 4 deals with carbon nanotube-based gas sensors in detail. Their sensing mechanism, synthesis methodologies, characterization, and functionalization are discussed thoroughly. In addition, this chapter focuses on CNT-based nanocomposites with metals, semiconducting metal oxides, and conducting polymers such as polyaniline and polypyrrole. All these CBNs and nanocomposites are discussed for being used as highly efficient gas sensors. Chapter 5 focuses on carbon nanofiber-based gas sensors. Methods of nanofiber preparation such as electrospinning, vapor deposition and substrate method, characterization, and functionalization are discussed. Nanocomposites of carbon nanofibers with metals, semiconducting metal oxides, and conducting polymers are discussed with their gas sensing capabilities. This chapter also covers mesoporous and graphitic carbon nanofibers and their composites with polymers. Chapter 6 focuses on graphene-based gas sensors. Sensing mechanism, fabrication, characterization, and extraordinary sensing capabilities of graphene and its derivatives are indicated. Nanocomposites of graphene and its derivatives with metals, metal oxides, and conducting polymers are discussed with leads to highly selective and sensitive gas sensors with maximum response. Chapter 7 deals with 3D hierarchical carbon-based gas sensors. Their importance, synthesis methodologies, and fabrication of 3D architectures are discussed. Furthermore, nanocomposites with metals, semiconducting metal oxides, graphene, and its derivatives are discussed as effective gas sensing materials. 3D hierarchical carbon with CNT, metal-doped

Preface

graphene, metal oxide/graphene oxide, metal oxide/CNT are discussed in this chapter. Such functionalized nanocomposites are promising candidates for smart sensors. Chapter 8 covers conducting polymers and their application in gas sensing. Conducting polymers such as polyaniline, polypyrrole, and polythiophene are studied in detail. Their synthesis technique, thin films, characterization, doping, and functionalization are discussed. Their advantage in room temperature sensing is explored with highly selective and sensitive gas sensor with maximum response. Chapter 9 presents the advantages, challenges, and opportunities associated with the use of carbon-based nanostructures and their application in gas sensing. It deals with the future prospects of carbon-based nanostructures as smart gas sensors.

Navinchandra Gopal Shimpi Shilpa Jain

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Part 1

Introduction to carbon based nanomaterials

1 Carbon-based smart nanomaterials Shilpa Jain1 and Navinchandra Gopal Shimpi2 1

Department of Chemistry, Jai Hind College, Mumbai, Maharashtra, India Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India 2

Nanoscience and nanotechnology aid in controlling matter and building its units atom by atom, layer by layer into more advanced systems. It involves subsequent findings of properties and phenomena at nanoscale (1029 m) and manipulation of materials at nanometer sizes. Nanotechnology enables working at molecular levels and creating the larger structure with fundamentally new molecular organization and properties [1 4]. Rapidly growing technology not only requires miniaturization of devices but also an ultimate performance with specific functionality and selectivity. The recent trend in technology requires smart nanomaterials and systems with specific functionality and thorough understanding of their properties. Recent advancement in analysis techniques has enabled examination and probing of atoms and molecules with great precision, leading to comprehensive understanding, expansion, and development of nanoscience and nanotechnology. With nanodimensions, material surface to volume ratio increases manifold, leading to an emergence of quantum size effects and the surface atom effects. This can change or enhance chemical reactivity, electronic, optical, mechanical, magnetic and transport characteristics of nanomaterial as compared to their bulk analogs [5,6]. Owing to their advanced properties and specific functionalities, nanomaterials have shown vast use in several fields ranging from energy storage devices to biomedical fields [7 9].

1.1

Introduction to carbon-based nanomaterials

Currently, carbon-based nanomaterials (CBNs) are vastly studied and explored because of their exceptional electronic, optical, thermal, and mechanical properties. These CBNs have shown vast applications in material science, energy storage Carbon-Based Nanomaterials and Nanocomposites for Gas Sensing. DOI: https://doi.org/10.1016/B978-0-12-821345-2.00007-3 © 2023 Elsevier Inc. All rights reserved.

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Chapter 1 Carbon-based smart nanomaterials

Figure 1.1 Properties and applications of carbon-based nanomaterials in various fields [16].

devices, aerospace, optoelectronics, catalysis, light emitters, biotechnology and sensors etc. [10 15]. Fig. 1.1 illustrates the application of CBNs in various fields based on their properties [16]. Owing to their exceptionally high mechanical strength, optical properties, electrical, magnetic and thermal conductivity, CBNs are considered as one of the prime members of smart nanomaterials and advanced nanodevices. Carbon itself is an inimitable unique element with the relative abundance of 180 270 ppm [17]. Carbon is the most significant element (after oxygen) in the human body [18], earth crust (17th in relative abundance), and the entire universe (6th most common) [19]. Carbon has a unique property of catenation and it can form several metastable phases at ambient conditions. Conventionally, carbon has two allotropes known as graphite and diamond (crystalline form), which are entirely different from each other in crystal structure and properties. Diamond is an electrically insulating and hardest natural substance known, while graphite is soft and conducting [20 22]. The various nanocrystalline forms of carbon were discovered at the end of 20th century with

Chapter 1 Carbon-based smart nanomaterials

progress in nanotechnology. Fullerenes (C60) were discovered by Harold Kroto, Richard Smalley and Robert Curl in 1985 in a sooty residue of vaporized graphite in the helium atmosphere. Fullerenes can be described as cage like molecules with 60 carbon atoms (held by single and double bonds). Fullerenes can be imagined as hollow sphere resembling football with 12 pentagonal and 20 hexagonal faces. Considering the properties and application in various fields, the trio was awarded with the “Nobel prize in Chemistry” in 1996 for their discovery and synthesis of fullerenes. Their discovery fueled intense research of CBNs and further in 1991, a Japanese physicist Sumio Iijima discovered carbon nanotubes (CNTs) using an arc discharge method and started a new era of carbon-based smart nanomaterials. Discovery of CNT and its extraordinary properties accelerated the growth of CBNs. Further in 2004, Andre Geim and Konstanin Novoselov extracted single atom thick carbon termed as “graphene” from graphite using micromechanical cleavage or scotch-tape method at the University of Manchester. They were awarded knighthood and 2010 Nobel prize in physics for their discovery of the wonder material graphenes. The various nanocrystalline forms of carbon consist of carbon nanotubes, fullerenes, carbon fibers, nondiamond, and graphene, which are illustrated in Fig. 1.2.

Figure 1.2 Different types of carbon nanostructures.

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Table 1.1 Comparison of some properties of carbon-based nanomaterials [30 32]. Carbon Hybridization Experimental nanoparticles specific surface area (m2 =g) Graphite

sp 2

B10 20

Graphene

sp 2

Carbon nanotube Fullerene

Thermal Electrical Tenacity Hardness conductivity conductivity (W /mK) (S=cm) Anisotropic 2 3 3 104

B1500

Anisotropic: 1500 2000, 5 10 4840 5300

Mostly sp 2

B1300

3500

Structure dependent

Mostly sp 2

80 90

0.4

10210

1.2

22000

Flexible, nonelastic Flexible, elastic Flexible, elastic Elastic

High

(for single layer) High High

Types of carbon nanomaterials

CBNs can be broadly classified as CNTs, fullerenes [23,24], carbon nanofibers (CNFs), graphene [25,26], and its derivatives such as graphene oxide, reduced graphene oxide, and quantum dots [27 29]. Properties of CBNs compared with graphite are shown in Table 1.1.

1.2.1

Carbon nanotubes

Carbon nanotubes are defined as hollow cylinders of carbon with the hexagonal lattice of single graphitic sheets rolled up within the diameter range of few nanometers. They are further classified as single walled carbon nanotube (SWCNT, diameter ,1 nm) or multi walled carbon nanotube (MWCNT, diameter .100 nm). The discovery of CNT is credited to Prof. Sumio Iijima with a revolutionary paper “Helical microtubes of graphitic carbon” reporting MWCNTs from arc discharge method [33]. Carbon source was co-vaporized in the presence of transition metal catalyst iron in an Ar/CH4 atmosphere and the CNTs were found in the deposited soot. Further in 1993, Bethune et al. synthesized CNT by vaporizing carbon monoxide and graphite under helium atmosphere and accelerated research on CNTs [34]. MWCNTs contains many tubes of graphene which ˚. are rolled multiple times with an interlayer spacing of 3.4 A Two models “Russian Doll model and Swiss roll model” are used to describe MWCNTs [35 37] as illustrated in Fig. 1.3.

Chapter 1 Carbon-based smart nanomaterials

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Figure 1.3 (A) Swiss roll model, (B) Russian doll model of carbon nanotubes. [Ref-nanotechweb.org, nanotech-now.com].

Figure 1.4 Graphene sheet lattice with the lattice vector “a” and “b” and the angles θ and ϕ, which determine the type of nanotube.

According to the Swiss roll model, a single sheet of graphite is rolled in around itself resembling a scroll of parchment or a rolled newspaper. In the Russian Doll model, sheets of graphite are arranged in concentric cylinders like smaller diameter SWCNT within a larger diameter SWCNT [38 40]. On the basis of structure, CNTs can be further classified as armchair, zigzag, and chiral depending on the rolling graphitic sheets as shown in Fig. 1.4. The nanotubes are generally described as (n,m), where “n” indicates the carbon atoms around the tube, and “m” determines the offset of NTs wrapping around. During the formation of cylindrical part of CNT,

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Table 1.2 Comparison of some properties of carbon nanotubes [41 43]. Material Young’s modulus (GPa)

Tensile strength (GPa)

Density (g/cm3 )

SWCNT MWCNT

150.000 150.000

2.600

1054.0 1200.0

Thermal conductivity (W/m K)

Phonon mean free path (Nm)

B20,000

B100

ends of chiral vector meet each other during rolling of the graphene sheet. This chiral vector and different values of n and m determines the chirality or “twist” of the CNT. It is possible to recognize zigzag (n or m 5 0, θ 5 0 degrees), armchair (n 5 m, θ 5 30 degrees), and chiral CNTs (0 degrees , θ , 30 degrees) by their cross-sectional structure and pattern across diameter (Fig. 1.4). Depending on length, diameter, chirality, and functionalization, CNTs can be metallic or semiconductive and possess intrinsic superconductivity, high thermal conductivity, and field emission properties [41,42]. Owing to high aspect ratio and ballistic one-dimensional electronic transport, CNTs are conducting with minimal heating loss (current densities upto 109 to 1010 A/cm2) [43]. Furthermore, reactions within CNTs interwall can redistribute current non-uniformly over the individual tubes. Owing to exceptionally high electronic and thermal properties, SWCNTs are used in smart miniaturized electronics. CNTs have extraordinary mechanical properties and their measured tensile strength, rigidity and elasticity are more than some industrial high-strength materials such as kevlar, high tensile steel and carbon fibers. Several composites of CNTs are also studied with metal oxides, ceramics and clay which enhance its multifunctional nature. Attributable to its unique mechanical, thermal, optical and electrical properties, CNTs show interesting potential in the field of electronics, optics, field emission, flat panel displays, and reinforcing materials [42,44]. Table 1.2 shows various physical and electrical properties of CNTs.

1.2.2

Fullerenes

Fullerenes are the man-made third allotrope of carbon with structure similar to football and named after architect Richard Buckminster Fuller. They are caged molecules with the sheet

Chapter 1 Carbon-based smart nanomaterials

Figure 1.5 Different types of fullerenes.

like hexagonal and pentagonal (or sometimes heptagonal) rings, which makes it non-planar and impart closed shell structure [45]. The structure of fullerenes comprises of truncated icosahedron but without hexagonal packing. The structure of fullerenes comprises of hexagons, pentagons (or even heptagons) which bends the sheet into spheres, ellipses or cylinders. Fullerenes consist of about 20 hexagonal and B12 pentagonal rings (icosahedral symmetry) with closed cage structure and this structural and electronic bonding leads to high stability [46]. Depending on the different number of pentagonal and hexagonal rings, an infinite number of fullerenes can exist. Fig. 1.5 shows the structures of various fullerenes with size variation. The fullerenes show high affinity for organic molecules such as C60 with 60 π electrons are used as good adsorbent. Although, it can’t adsorb metal ions, anions and polar molecules which imparts selectivity and therefore fullerenes can be used as electrochemical sensors. Fullerene-based nanostructures are found to be potential sensing material for the surface acoustic wave- and quartz microbalances-based gas sensors.

1.2.3

Graphene

Graphene known as “Wonder material” is single atom thick carbon monolayer (sp2-bonded) with honeycomb crystal lattice ˚ and bond length of 1.42 A ˚ and interlayer spacing of 3.4 A [25,26]. Graphene is stronger than diamond with extremely high tensile strength (130 GPa). Unique covalently bonded and planar two-dimensional structure with the van der Waal’s forces makes it harder than diamond, tougher than steel yet lighter than aluminum. Graphene is most attractive and studied material of the present era owing to its unique structure and extraordinary properties. The edge and basal planes of graphene shows

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Chapter 1 Carbon-based smart nanomaterials

variant electrochemical properties such as edge has higher electron motion, greater specific capacitance and catalytic power [47]. Graphene has some peculiar properties such as optically transparent with high electron mobility, impermeability, high electrical and thermal conductivity and extremely high surface area etc. [48 51]. It is expected that defect free single sheet of graphene should possess excellent transport properties and experimentally it has remarkably high electron mobility (200,000 cm2/Vs, RT), low resistivity (1026 Ω cm), lower than metallic silver and unexpectedly high transparency (only absorbs B2.3% of white light). Owing to these extraordinary electrical and optical properties, extremely high surface area and free π electrons, it is termed as a wonder material and ideal candidate for several applications [26,51]. Various derivatives of graphene such as graphene oxide (GO) and reduced graphene oxide (rGO) have different potential applications due to the presence of free π π electron, aromatic ring and reactive functional groups. GO has oxygenous functional groups with hydroxyl and epoxide groups on its basal plane and carboxylic, ketone and aldehyde groups on its layer edge [52]. There are several methods purposed for the structure of GO namely the Hofmann, Ruess, Scholz-Boehm, Nakajima-Matsuo, Lerf-Klinowski and Szabo models [53] as shown in Fig. 1.6. After detailed and comprehensive analysis by solid-state NMR and X-ray diffraction, the “LerfKlinowski” molecular structure is widely accepted as the role model of GO [54]. During an oxidation process, various defects, impurities, structural disorders, fragmentation, and other structural attributes are generated which influences electronic, optical and adsorption properties. Further, GO can be converted into reduced graphene oxide (rGO, with carbon to oxygen ratio of 8:1 246:1) by chemical or physical reduction methods [20]. Another derivative of graphene with micro, meso- and microporous structure known as “three-Dimensional hierarchal graphene” possess unique functionalities. Controllable synthesis which determines the porosity, surface area, high electron transport and superior electrochemical performances makes it a versatile material and imparts high structural and mechanical properties over CNTs and graphene. Another attractive nanomaterial is graphene quantum dots (GQDs). They can be described as a zerodimensional graphene sheet with size less than 100 nm in one to a few layers (3 10) [51]. The GQDs exhibits high surface area, high transparency, high photoluminescence, excellent hole transporting properties, and chemical stability due to quantum confinement [55,56].

Chapter 1 Carbon-based smart nanomaterials

Figure 1.6 Structures of pristine graphene (A) and its graphene oxide derivatives based on Hofmann (B), Ruess (C), Scholz-Boehm (D), Nakajima-Matsuo (E), Lerf-Klinowski (F), and Szabo (G) models [54].

1.2.4

Carbon nanofibers

Carbon nanofibers are the ultralong, thin strand of carbon (diameter B10 1000 nm) with atoms bonded together in crystallite and aligned parallel to the longer axis of the fiber. Fibers having diameter less than 100 nm are generally considered as NFs (as per National Science Foundation) [57]. CNFs are widely studied owing to their exceptional physical and chemical properties which is similar to fullerenes and CNTs [58,59]. CNFs inherits high aspect ratio, high thermal and electrical conductivity, low density, smaller number of defects, high specific

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Chapter 1 Carbon-based smart nanomaterials

modulus and strength, which makes them potential candidate for various applications. The unique crystal alignment imparts high mechanical strength to CNFs. As the diameter decreases, surface functionalities such as surface area, mechanical and thermal properties enhances manifold [30,60]. In CNFs, the graphite layers form an angle with inner tube axis, which may be hollow or solid like interior. The range of CNFs diameter is B10 to 500 nm with length B10 μm. Due to basal and edge planes, CNFs can be easily functionalized and their surface can be modified accordingly forming functional hybrid CNF-based nanomaterials. These functionalized CNFs have potential application in the various fields of energy, sensors, biomedicine, nanoelectronics, environmental science and tissue engineering [31,32,61,62]. Polyacrylonitrile (PAN) is one of the most common polymer for the synthesis of CNFs. PAN has several advantages over other precursors such as stability, spinnability and ability for large scale production. PAN is extensively explored due to its high carbon content and ability to tailor CNFs structure with various functionalities, doping and surface modifications. Various PAN based precursors, composites and blends are synthesized from PAN homopolymer for functionalized CNFs. Nanocomposites of these CNFs with metals, SMOs, polymers, CNTs etc. are studied for various applications. These nanocomposites have extremely high surface area and excellent thermal, mechanical, electrical and optical properties. They have potential application in gas sensors, biosensors, tissue engineering and energy storage devices.

1.3

Synthesis methodologies and variations

There are several methods for the synthesis of CBNs, but these are broadly classified as the top-down and bottom-up approach. In top-down approach, carbon nanomaterials like CNTs, graphene and CNFs etc. are generated from bulk carbon material such as graphite. Several methods such as laser ablation, chemical exfoliation and arc-discharge method etc. are used as top-down approach. While bottom-up approach includes chemical vapor deposition and pyrolysis techniques, where CNTs, graphene, and CNFs etc. are synthesized from simple hydrocarbons. In top-down approach, the arc-discharge method is one of the primary methods used for the synthesis of CNTs and fullerenes [33,63]. In this method, high temperature is generated

Chapter 1 Carbon-based smart nanomaterials

using an elevated current (B20 25 V) between the two carbon electrodes which instigates vaporization of one of the carbon electrodes. These carbon vapors get deposited on the surface of the other electrode (rod-shaped carbon growth) leading to CNTs and fullerenes according to reaction conditions. The carbon rods in an arc-vaporization chamber are placed end to end separated by a distance of 1 mm in an inert gas atmosphere at low pressure. From arc-discharge technique, although CNTs are obtained in higher yield but the mixture of components are obtained which entails further purification techniques and removal of residual catalytic metals from crude product [6,64]. Another famous top-down technique “Laser ablation” involves removal of matter from the surface (evaporation or sublimation) using a laser beam at the lower flux in a closed chamber at high vacuum or an inert atmosphere. Normally laser is used in continuous pulses, so the laser intensity is usually kept high to carry out laser ablation of material. In 1995 Guo et al. studied laser ablation technique using a block of graphite and further with catalytic metal in it [65,66]. Different catalytic metals such as cobalt, niobium, platinum, nickel and copper etc. were used for the growth of CNTs from carbon plasma state. In bottom up approach, chemical vapor deposition (CVD) is an important technique for the synthesis of various CBNs. In CVD, carbon in its gaseous form is deposited on a substrate at ambient temperature. Several factors such as nature of gas, catalyst, pressure and temperature of the reaction chamber influences the growth of CBNs. Hydrocarbons such as methane, acetylene and xylene etc. are used as precursor gas for the synthesis of CNTs and graphene along with carrier gases [67,68]. These gases at high temperature reacts and decomposes, when into contact with a heated substrate resulting in solid growth onto the substrate. Although the history of CVD technique dates back to the 19th century, but in 1890 some French researchers studied the growth of carbon filaments using cyanogen (over red-hot porcelain) which accelerated the interest in CVD technique [69]. In 1993 Yacaman et al. used CVD technique to grow MWCNTs from acetylene gas using iron particles as catalyst at 600 C 800 C [70]. Since SWCNTs have higher energy of formation, the temperature of the CVD chamber should be notably higher (900 C 1200 C) and precursor gases like carbon monoxide or methane are used owing to their superior stability (compared to acetylene) at higher temperatures. In CVD technique, properties of CNTs such as length, shape, chirality, defects, diameter and graphitization etc. can be tailored and controlled using several factors such as precursor gas, temperature and

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reaction time, catalyst and gas flow rate etc. Apart from carbon fibers and CNTs, graphene is also synthesized from this technique [68,69]. The growth of the graphene is accomplished on the copper surface which acts as a sacrificial catalyst. Other than copper, nickel can also be served as a catalyst for the growth of graphene. The Geim group at the Manchester university discovered the famous “Scotch tape method” for the synthesis of grapheme [25]. Micro-mechanical cleavage of graphite was done using adhesive tape and graphitic crystals were repeatedly spliced into thinner layers until optically transparent flake was obtained. These smaller flakes were dispersed in non-polar solvent such as acetone and monolayers of graphite were sedimented on a silicon wafer resulting high quality graphene crystallites. Chemical exfoliation is another successful technique for the synthesis of graphene and its derivatives. The hummer’s method is another famous exfoliation technique used for the synthesis of GO. In the hummer’s method, graphite is oxidized initially using sodium nitrate in sulfuric acid followed by addition of potassium permanganate and peroxide with vigorous agitation. The bright yellow suspension is filtered, washed repeatedly several times to obtain GO residue. This residue is dispersed in deionized water and reduced using hydrazine hydrate at about 100 C resulting in graphene or RGOs [67]. Several other modifications of Hummer’s method has been investigated using various oxidizing and reducing agents (Fig. 1.7). The top-down approaches used for the synthesis of graphene are mechanical exfoliation, oxidative exfoliation-reduction, liquid-phase exfoliation, arc discharge and unzipping of CNT [73 75]. These methods yields good quality graphene with minimal impurities and defects but suffers from disadvantages such as low yield, inconsistency and dependency on graphite as the precursor. The bottom-up approaches which are generally used for the synthesis of graphene are CVD [76,77], epitaxial growth [78,79], substrate free gas-phase synthesis [80], template route [81] and total organic synthesis [82]. These methods leads to graphene with large surface area and almost defect-free (from atomic-sized precursors) but requires costly instrumentation and high operational setup for large scale synthesis. The various methods used for the synthesis of graphene are summarized in Fig. 1.8. Pyrolysis is another interesting technique for the synthesis of CBNs. In pyrolysis, organic material is decomposition or carbonized in the absence of oxygen thermochemically resulting in

Chapter 1 Carbon-based smart nanomaterials

Figure 1.7 Graphite oxidation route schemes [71,72].

several products and a solid residue known as char (high carbon content). In this carbonization process, CBNs formed depends on the starting precursors, reaction conditions and methodology. Pyrolysis technique is used to synthesize mesoporous carbon, activated carbon and CNFs. For the synthesis of CNFs, PAN and rayon are commonly used as carbon source along with other precursors such as pitch, phenolic resins, poly vinylidene fluoride, poly (styrene sulfonate-co-maleic acid) etc. [83 86]. With the advanced electrospinning technique, polymer fibers with diameter ranging from 50 to 500 nm can be fabricated from PAN and some other polymers which then successively converted to carbon fibers through the carbonization process [87].

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Chapter 1 Carbon-based smart nanomaterials

Mechanical exfoliation

Top Down

Arc Discharge method

Chemical reduction

Oxidative exfoliation-reduction

thermal reduction

Liquid phase exfoliation

Electrochemical

Unzipping of CNT Graphene CVD Epitaxial growth on SiC Bottom up

Substrate free gas phase synthesis Template route Total organic synthesis

Figure 1.8 Various synthesis methodologies used for the synthesis of graphene.

1.4

Gas sensors and their comparison

With global industrialization and increased pollution levels, there has been high demand of strict regulations and automated process monitoring and control systems with green technology and minimal pollutants. Several industries manufacture and utilize various toxic and combustible gases. Continuous monitoring and detection are important to prevent industrial leakages, asphyxiation, explosion, fire and other casualties. Gas sensors have potential application in various industries such as chemicals, petrochemicals, food and processing, agriculture and power plants etc. Gas sensors are used for environmental monitoring to regulate air pollution from industrial exhaust and automobiles. These industrial and automobile effluents lead to acid rain, ozone depletion, photochemical smog and other environmental issues. Gases which are major source of pollution are nitrogen oxides (NO2, NO), sulfur oxides (SO2, SO3), carbon dioxide and carbon monoxide. Gas sensors are

Chapter 1 Carbon-based smart nanomaterials

17

Table 1.3 Role of gas sensors in various fields. Field of application

Function

Examples of detected gases

Environment

Monitoring toxic gases from industrial exhaust and automobiles Diagnostics, drug and disease monitoring, breath analysis, artificial organs, and prostheses Soil and water testing, Plant/animal diagnostics, waste/sewage monitoring, and metal/poultry inspection Detection of particular molecules (formed during deterioration) Process monitoring and quality control, workplace monitoring, waste stream monitoring, and leakage alarms

CO, CO2, CH4, humidity, O2, NOx, VOCs, SO2, HCs, NH3, H2S O2, NH3, NOx, CO2, H2S, H2, Cl2, anesthesia gases NH3, amines, humidity, CO2

Medical/Clinical Agriculture

Food quality control Industry: Petrochemicals Steel Water treatment Semiconductor Oil and gas Automotive/power plants Traffic/tunnels/car parks Defense/military Aerospace

Humidity, CO2 etc. HCs, conventional pollutants O2, H2, CO, conventional pollutants Cl2, CO2, O2, O3, H2S, CH4 H2, CH4, HCl, AsH3, BCl3, PH3, HF, O3, H2Cl2Si, TEOS, C4F6, C5F8, GeH4, NH3, NO2, O2 HCs, H2S, and CO O2, CO, HCs, NOx, SOx, CO2, H2, and HCs

Control of the gas concentration in the gas boiler and engine and control combustion process for higher efficency Traffic control and management, air quality CO, O3, NOx, SO2, CH4, and LPG monitoring Detection of chemical, biological, and toxin Agents, explosives, and propellants warfare agents Monitoring of oxygen, toxic, and flammable gases H2, O2, CO2, and humidity

highly used in quality control of manufactured products, food packaging, cosmetics, perfumes, biological pathogen detection and biochemical analysis. Table 1.3 represents the application of gas sensors in various fields. Sensors generates a measurable response to physical or chemical conditions such as temperature, pressure and resistance etc. and converts its stimuli into an electrical signal. Sensors are critical for in-situ measurements in process control units, environmental monitoring and healthcare [88,89]. Sensors can be categorized according to their functionality (physical or chemical effects on the basis of which they operate) as physical and chemical sensors. Among various chemical sensors, gas sensors are extensively studied and used in several industries for

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Chapter 1 Carbon-based smart nanomaterials

monitoring and control purposes. Especially for environmental monitoring, gas sensors are used in detecting the pollutants and their concentration at ppm or ppb level. Effluent gases produced from industrial processes and automobiles contains several other pollutants and gases which makes their detection extremely difficult. From past few decades demand of gas sensors with high selectivity and sensitivity has increased manifold leading to study of new advanced materials with superior functionality. Several gas sensors are developed for hazardous gases such as CO2, CO, NO2, NO, SOx, H2S and NH3 etc. Gas sensors based on toxic volatiles such as volatile organic compounds, toxic industrial chemicals and chemical warfare agents have been developed [90 92]. There are several factors which determines the efficiency of sensor such as high selectivity, stability, response and recovery time and low working temperature. Different materials have been explored for efficient sensing such as metal oxides, semiconducting metal oxides, polymers, carbon-based nanostructures and porous structured materials. The mechanism of sensing involves interaction of gas and solids such as adsorption, chemical and electrochemical reactions. Different types of gas sensors based on different mechanisms are shown in Table 1.4. Various semiconducting metal oxides (SMOs)-based gas sensors were developed with the detection principle based on the resistance change of sensing material on the approach of analyte gas. In case of dielectric materials, capacitance studies are done. Similarly, different types of sensor materials are developed based on different operating principles such as electrochemical (current, voltage, impedance), optical, magnetic, and thermometric etc. [93,94]. Among various types of gas sensors, solid state gas Table 1.4 The classification of gas sensors on the basis of different operating principle (suggested by the analytical chemistry division of IUPAC in 1991). Class of gas sensors Operating principle Electrochemical Magnetic or optical devices Electrical Thermometric Mass sensitive

Changes in current, voltage, capacitance: voltammetry, potentiometry Changes of paramagnetic g properties, changes in light intensity, color or emission spectra, absorbance, luminescence, light scattering, reflectance, and refractive index Metal oxide conductivity, electrolytic conductivity, and electric permittivity Changes in heat flow, temperature, heat content, pyrroelectric, thermal conductivity, and thermoelectric Changes in the weight, amplitude, shape, size or position: surface acoustic wave propagation, quartz crystal microbalance, cantilever

Chapter 1 Carbon-based smart nanomaterials

sensors are likely the most successful in terms of efficiency, cost, sustainability, stability, and large scale production. These solid state gas sensors have enormous advantages such as high sensitivity, lower detection limit, stability, small size, online operation and lower batch production cost. The detection principle of these sensors are based on reversible interaction of analyte gas with the surface of sensing material either by physisorption or chemisorption. The gas solid interaction leads to either change in conductivity, capacitance, work function and optical properties which can be measured in terms of sensing signal. With recent advances in nanotechnology and synthesis of smart nanomaterials with novel properties and high surface area, various nanomaterials with gas sensing capabilities are explored [95 98]. Several CBNs are studied for gas sensing applications such as CNTs, graphene, porous carbon and CNFs etc. After the discovery and realizing the amazing properties of CNTs, several CNT-based devices for sensing of different gases has been studied. Various techniques such as doping, functionalization and deposition are used to enhance the sensing capabilities of CNTs. Initially, Dai et al. studied CNT based gas sensors for detection of NO2 and NH3 with high sensitivity [99]. Their research work opened the possibility of CNT based gas sensors with low response time, high sensitivity and stability. Recently, graphene-based sensors were also studied and it was found that graphene can detect gases at very low concentration and temperature. Owing to the layered structure of graphene with single atom thickness, the individual gas molecule can be adsorbed and detected imparting high sensitivity and gas response [100]. CNFs and other 3D hierarchal carbon nanostructures are also studied extensively for gas sensing. These CBNs have several advantages over other nanostructures such as high aspect ratio, control over porosity, size and structure, which makes them perfect material for gas sensors. These CBNs can be easily functionalized and doped to enhance selectivity and sensitivity. These CBNs have lower response and recovery time due to their high surface area and unique structure (layered, hollow, and porous). The CBNs are studied for the variety of gas sensors along with sensing of organic molecules, biomolecules, drugs, and metal ions [99,101 103].

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[89] T. Moseley, B.C. Tofield (Eds.), Solid State Gas Sensors, Adam Hilger Bristol and Philadelphia, 1987. [90] G.C. Franco, A.T. Silver, J.M. Domı´nguez, A.S. Jua´rez, Thin film tin oxidebased propane gas sensors, Thin Solid Films 373 (2000) 141 144. [91] S.C. Gadkari, K. Manmeet, V.R. Katti, V.B. Bhandarkar, K.P. Muthe, S.K. Gupta, Encyclopedia of Sensors, American Scientific Publishers, 2006. [92] D.S. Lee, H.Y. Jung, J.W. Lim, M. Lee, S.W. Ban, J.S. Huh, et al., Explosive gas recognition system using thick film sensor array and neural network, Sens. Actuators B 71 (2000) 90 98. [93] H. Meixner, J. Gerblinger, U. Lampe, M. Fleischer, Thin-film gas sensors based on semiconducting metal oxides, Sens. Actuators B 23 (1995) 119 125. [94] T. Takeuchi, Oxygen sensors, Sens. Actuators 14 (2) (1988) 109 124. [95] M.J. Madou, S.R. Morrison (Eds.), Chemical Sensing with Solid State Devices Academic, Press New York, 1989. [96] A. Mandelis, C. Christofides (Eds.), Physics, Chemistry and Technology of Solid State Gas Sensor Devices, Wiley, 1993. [97] P.T. Moseley, Solid state gas sensors, Meas. Sci. Technol. 8 (1997) 223. [98] I. Lundstro¨m, Approaches and mechanisms to solid state based sensing, Sens. Actuators B 35 (1996) 11 19. [99] J. Kong, N. Franklin, C. Zhou, M. Chapline, S. Peng, K. Cho, et al., Nanotube molecular wires as chemical sensors, Science 287 (2000) 622 625. [100] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, et al., Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652 655. [101] A. Ghosh, K.S. Subrahmanyam, K.S. Krishna, S. Datta, A. Govindaraj, S.K. Pati, et al., Uptake of H2 and CO2 by graphene, J. Phys. Chem. 112 (2008) 15704 15707. [102] K. Srinivasu, K.R.S. Chandrakumar, S.K. Ghosh, Quantum chemical studies on hydrogen adsorption in carbon-based model systems: role of charged surface and the electronic induction effect, Chem. Phys. 10 (2008) 5832 5839. [103] C.B. Jacobs, M.J. Peairs, B.J. Venton, Carbon nanotube based electrochemical sensors for biomolecules, Anal. Chem. 662 (2010) 105 127.

2 Carbon nanomaterials-based gas sensors Shilpa Jain1, Akshara Paresh Shah2 and Navinchandra Gopal Shimpi2 1

Department of Chemistry, Jai Hind College, Mumbai, Maharashtra, India Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India 2

Sensors are an essential component of processing and control units as they provide measurable response to various stimuli (physical, chemical, magnetic, or biological) [1]. There are two major components of a sensor, i.e. detector and transducer. Detector comprises of an active element, which is generally metal oxides, composites, organic polymers, and hybrid/functional nanomaterials. While transducer converts the obtained response from various stimuli into measurable electrical signals [2]. Sensors can be broadly categorized as physical and chemical sensors according to their principle of conversion (physical or chemical effects on the basis of which they operate). Physical sensors employ physical effects such as piezoelectric, magnetostriction, ionization, thermoelectric, photoelectric and magnetoelectric, etc. [3
5]. Chemical sensor works on the principle of using receptor as a chemical interface which interacts with analyte, leading to a detectable change in chemical properties such as concentration, composition, etc. This chemical change is converted into analytical signal using a transducer [6,7]. Among different types of chemical sensors, gas sensors are predominately important in several industries, for environmental control and combat pollution levels. The principle behind functioning of gas sensor is electrical resistance change while interacting with gas molecules. There are several characteristics that determine the capability of sensor such as response time, recovery time, detection limit, selectivity, sensitivity, stability, and working temperature [8
10]. A sensor should be environmental friendly, stable, efficient, durable, and feasible for large-scale manufacturing. Various nanomaterials such as metal oxides, semiconducting metal oxides (SMOs), conducting polymers, nanocomposites, and Carbon-Based Nanomaterials and Nanocomposites for Gas Sensing. DOI: https://doi.org/10.1016/B978-0-12-821345-2.00001-2 © 2023 Elsevier Inc. All rights reserved.

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Chapter 2 Carbon nanomaterials-based gas sensors

carbon-based nanomaterials (CBN) have been studied for various gases [11,12]. The CBN are considered as a substitute of expensive electronic grade semiconductors. Among several nanostructures, CBNs such as CNTs, graphene, nanofibers etc., has several advantages such as extra-ordinary electrical, magnetic and mechanical properties, large expanse of surface area and remarkable chemical properties [13
16]. The unique characteristic of these CBNs enhances the selectivity toward a specific gas and they can be easily tailored with functionalization and doping. Such as CNTs with high aspect ratio, high carrier mobility and unique structure are excellent chemical and biological sensors. CBNs are mainly used in detection and quantification of toxic gases such as CO2, CO, NO2, NO, H2S, CH4, SO2, LPG, H2, NH3, Cl2, etc. These CBNs works on different operating principle such as capacitance, resistance, ionization, sorption or resonance frequency shift [17,18]. These different operating principles are based on variant current, voltage, impedance, and quantity of adsorbed gas molecules on the surface. The signal is produced electrically with interaction of gas molecules with CBNs. CBNs are mainly used in the detection and quantification of toxic gases such as CO2, CO, NO2, NO, H2S, CH2, SO2, LPG, H2, NH2, Cl2, etc. In 1998 Tans et al. developed field-effect transistor (FETs) based on CNT, which had superior properties than traditional FETs based on metal oxides and SMOs [19]. Similarly, graphene and its derivatives are widely used in sensors because of its excellent thermal and electrical conductivity, mechanical strength, large expanse of surface area and faster electron transfer rates that are heterogeneous [20
22] in nature. Seekaew et al. used graphene bilayers as sensing material at room temperature for NO2 sensing. The sensor has gas sensibility of 1409/ppm with concentration of 1
25 ppm [23]. Similarly, Wu et al. synthesized NO2 sensor based on superhydrophobic reduced GO (rGO) with sensitivity of 25.5/ppm and detection limit of 9.1 ppb. This high sensing capability was due to the high surface area (850 m2/g), defects, high adsorption, and hydrophobicity. Graphene decorated with Ag nanoparticles were used to develop H2S sensor with high selectivity, lower detection limit (100 ppm), and response time of B1 second and recovery time of B20 seconds [24].

2.1

Types of gas sensors based on carbonbased nanomaterials

There are different operating principles such as capacitance, resistance, ionization, absorbance, reflectance, conductivity, or

Chapter 2 Carbon nanomaterials-based gas sensors

resonance frequency shift on which gas sensor works. These different operating principles are based on variant current, voltage, impedance, and number of gas molecules adsorbed on surface and other factor accordingly. The different type of gas sensors based on various transduction mechanism are as follows.

2.1.1

Electrochemical sensors

Electrochemical sensors work on the principle of charge transfer from analyte to an electrode (or vice-versa) and detection of electroactive species amperometrically, potentiometrically, or conductometrically [25,26]. An analyte gas is usually reduced or oxidized at the electrode’s surface (sensing material) and this reaction varies the potential with respect to reference electrode. Several techniques such as cyclic voltammetry, differential pulse voltammetry, chronoamperometry, linear sweep voltammetry and stripping voltammetry are used to determine electroactive species in solution [27]. This electrochemical process responds according to the desired gas’s composition type of sensing material (electrode), applied potential, rate of adsorption and electrolyte. Morphology, size and surface area of electrode influences the electrochemical response [27,28]. CBNs are highly used due to their low background current, large potential and aptness. CNTs and graphene are electrochemically active due to its unique cylindrical structure and high π-electron conjugation. They are used in various electrochemical gas sensors and biosensors [29,30]. Kim et al. studied various CNT-based devices for NH3 and NO2 sensing [31]. CNTs have adsorbed on their surfaces. These gases and small current changes leads to electron transfer and increase in sensitivity. Similarly, Dhall et al. synthesized CNT-hybrid composites for H2 gas sensing. Their studies indicate that Ni functionalized MWCNTs were more sensitive than Cu functionalized MWCNTs with sensor response of 0.05% at room temperature [32]. CNTSnO2 graphene oxide-based NO2 sensor were synthesized by Liu and coworkers. It showed response time as low as 8 second and recovery time of 77 seconds with detection limit of 5 ppm [33]. Humayun et al. fabricated SnO2-MWCNTs for methane gas with detection limit of 10 ppm. These CBNs shows better selectivity and sensitivity than traditional systems along with challenges of large-scale production and reproducibility [34].

2.1.2

Electrical/chemiresistive sensors

Electrical sensors work on the basis of operating principle of change in electrical properties such as resistance, conductance,

27

28

Chapter 2 Carbon nanomaterials-based gas sensors

or work function. These sensors generally work on the principle of adsorption and surface interaction with analyte gas. Metals, metal oxides, SMOs and polymers work as sensing layers for resistive, capacitance, work-function, FET-based sensors and schottky barrier sensors [35]. Some of the most commonly studied sensing materials are Ag, Au, Pt, SnO2, ZnO, V2O5, NiO, CuO, polyaniline, polypyrrole and mixed oxides such as ZnFe2O4 and BaTiO3, etc. [36
40]. Both n and p-type SMOs are used as gas sensor material. When target gases interact with sensing layer, various surface phenomena along with catalytic effects takes place leading to reduction/oxidation processes and change in resistance and surface potential. This leads to delocalized electrons from a conduction band to localized surface states (vice versa) and measurable change in electrical resistance or potential in ambient conditions. These gas sensors have several advantages for example high sensor sensitivity, lower detection limit, easy functionality and scalability. There are some limitations such as effect of humidity, high temperature operation, poor reproducibility and selectivity [41
43]. Various gas sensors made of SMOs and metal oxides have been commercially viable such as Figaro, FIS, UST, City Tech, MICS, Applied-sensors and New Cosmos, etc. Chemiresistor gas sensors based on CBNs showed promising results over SMOs because of robustness, operating at room temperature, higher sensitivity, and selectivity toward several gases such as CO, NH3, NO2, CO, and VOCs, etc. [18,44]. Several nanocomposites based on CBNs have been explored such as Cu2O/graphene hybrid synthesized by Zhou et al, which showed high sensitivity toward H2S with as low as possible detection limit 5 ppb at room temperature. The high surface area and synergistic effect leads to higher gas absorptivity and higher sensor response. Cu2O nanocrystals acts as reaction centers for analyte and graphene with extraordinary 2-D structure and conductivity leads to higher electron transfer efficiency and maximum sensor response [45,46]. Lundstro¨m and coworkers categorized kind that is nonresistive gas sensors as Schottky, MOs diode, FET, and MOSFET [47
49]. Their principle is based on change in schottky barrier height or band potential at Pd/Pt electrode with the approach of analyte. Gases adsorb atop of Pd/Pt electrode resulting in dissociation and diffusion through metal surfaces and gets absorb on the gate insulator junction’s inner surface, they become polarized at this point. The dipole layer increases the surface potential at interface and the response is proportional to the magnitude of the voltage shift at the threshold and generally measured as drain-source current (IDC) or gate-source voltage (VGS) as shown in Fig. 2.1.

Chapter 2 Carbon nanomaterials-based gas sensors

29

Figure 2.1 Types of nonresistive gas sensors: (A) Schottky diode (B) Capacitance-based (C) field-effect transistorbased devices and effect of H2 adsorption on interface having catalytic metal (Pd) gates. Reproduced from G. Korotcenkov, Handbook of Gas Sensor Materials: Properties, Advantages and Shortcomings for Applications Volume 1: Conventional Approaches, Integrated Analytical Systems.

2.1.3

Mass-sensitive gas sensors

Mass-sensitive gas sensors work on the principle of change in the surface mass of the sensor while interacting with a substance that has been identified as analyte. During physical/chemical interaction, change in mass is due the sensing layer deposited on mechanical resonator. These are monitored by deflecting micromechanical structure due to various factors such as change in stress, mass loading, acoustic wave or a resonant structure’s frequency characteristics. Different types of mass sensitive sensor are surface acoustic wave (SAW), quartz crystal microbalance (QCM), and microcantilever-based sensors [50]. Among mass sensitive sensors, QCM sensors are particularly notable due to their extensive applications. The QCM sensor contains a piezoelectric quartz crystal which act as electromechanical oscillators. The quartz oscillator’s resonance frequency changes are directly related to surface mass. This QCM microgravimetry measure the shift in frequency according to the mass loading, this is closely linked to concentration of gas measured. In 1964 King et al. detected some analytes using QCM type sensors. Marian Varga et al. synthesized nanocrystalline diamond (NCD) using microwave plasma system on quartz substrate to develop NCD-coated QCM sensors for detection of NH3 [51]. SAW type sensor generates acoustic waves using piezoelectric crystal. A mechanical wave is a SAW when acoustic energy is used which is limited to an isotropic single crystal’s surface. An electric field is generated as voltage is applied

30

Chapter 2 Carbon nanomaterials-based gas sensors

between two polarized transducers of a transmitter IDT (interdigital transducers) and mechanical deformation in the form of a wave takes place on a surface. This acoustic surface waves exit the transmitter IDT on both sides and traverses the surface in a straight line over a delay line to the receiver IDT, it is retransformed into an electric signal here [50,52]. Yao et al. developed SAW gas sensor using poly-N-vinylpyrrolidone (PNVP) film for detection of NH3. Polymers such as PNVP improves the sensitivity (6.91 Hz/ppm) and repeatability with detection limit of 8 ppm [53]. These QCM- and SAW-based sensors depends on temperature and humidity. Microcantilever-based sensors works on the principle of change in surface stress or vibrational frequency with adsorption of analyte and thus nonmechanical bending of microcantilever [54]. The change in the resonant frequency and the microcantilever’s bending detection techniques can be used to keep track of it. Such techniques are as optical, piezo resistive, capacitive, and interferometric. Typically, the microcantilever is composed of silicon, silicon nitride, and polymers which are all silicon derivatives.

2.1.4

Thermometric (calorimetric) gas sensors

Thermometric sensor is founded on the idea of transforming temperature conversion into electrical signals as a result of chemical interactions such as current, voltage, and resistance [55,56]. Flammable gases are used for oxidation process which leads to catalytic reaction and increase in temperature and thus sensor signals. Resistance change or conductance are directly related to concentration of gas. These sensors are stable, reliable and has long operating life. Different type of thermometric sensors includes thermoelectric, pyroelectric, and thermoconductivitybased sensors. Thermoconductivity sensors detect thermally conductible gases higher than air such as methane and hydrogen, etc. Thermoelectric and pyroelectric sensors are two types of sensors that detect the change in temperature due to heat of adsorption, oxidation or chemical reactions of hydrocarbons, CO and H2 gas [57]. Colorimetric gas sensors work on the principle of change in color of a porous matrix contains a chemo chromic reagent. while interacting with analyte. Schmitt and coworkers studied several gas sensors (NH3, NO2, CO) based on colorimetric principle and found high sensitivity and lower detection limit as compared to other sensors. It was discovered that the detection limit was 5 ppm for ammonia, 30 ppm for CO, and 0.5 ppm for NO2 [58].

Chapter 2 Carbon nanomaterials-based gas sensors

31

Table 2.1 Gas sensors are classified in several ways according to the detection principle. Detection principle

Examples

Sensors based on reactivity of gas

• • • • • •

Sensors based on the physical properties of gas

• • • • • • • • • • •

Sensors based on gas sorption

Chemiresistive Electrochemical sensors Microcalorimetric gas sensor Colorimetric Chemiluminescence Nonresistive type (Schottky barrier, heterocontact sensor, FET-based sensors) Nondispersive infrared UV absorption Photo acoustic sensor Thermal conductive sensor Gas ionization Polymer sensors (swelling) Fiber-optics sensors Mass sensitive (quartz crystal microbalance) Surface acoustic wave Bulk acoustic wave Microcantilevers

Sensors can also be classified further on the basis of detection principle such as reactivity of gas, physical properties and gas sorption as shown in Table 2.1.

2.2

Parameters of gas sensor

There are some parameters, which determine the sensing capabilities of sensing material. There are chemical/physical changes while interacting with target gas, which leads to sensor response. This change depends on sensing material, reaction conditions, humidity and gaseous nature (reducing or oxidizing). The sensor response is characterized by parameters such as sensitivity, selectivity, response time, recovery time, detection limit and stability.

2.2.1

Sensitivity

Sensitivity can be defined as variations in the sensing material’s physical/chemical properties while interacting with the

32

Chapter 2 Carbon nanomaterials-based gas sensors

analyte. Such as in chemiresistive sensors, the ratio of the two variables is known as sensitivity where electrical property (resistance/conductance/potential) of the comparing the target gas’s detecting element to that in the air. The sensitivity “S” is defined by the equation S 5 ðRo 2 Rg Þ=Rg

ð2:1Þ

where Ro is the initial resistance of the sensing material, and Rg is the resistance after exposed to target gas. Sensitivity depends on several factors such as crystallite size, porosity, thickness, operating temperature, and presence of additives [5,7]. Surface characteristics such as porosity and surface area are the most crucial factors and therefore CBNs has high sensing capabilities owing to their extraordinary structure.

2.2.2

Selectivity

Selectivity is another important parameter, which determines the ability of sensing material to identify a specific gas among the mixture of gases specifically. The ability to respond toward a specific gas is known as selectivity. There are several factors which controls selectivity such as morphology, senor architecture and operating temperature. Since each gas and/or sensing layer has a different operating temperature at which it gives maximum sensing response. This property can be used to distinguish gases at selected temperature. This parameter is important particularly in process control units, where a particular gas or VOCs needs to be monitored among various other gases. Some studies indicate the use of catalyst, which reduces the operating temperature toward a particular gas and enhances sensitivity and selectivity [8,59].

2.2.3

Stability

Stability of a sensor can be determined in terms of repeatability of sensing capability over a period of time. Sensor should be able to sense repeatably and accurately after many cycles in ambient conditions. There are some factors which effects the stability of sensor such as surface contamination, humidity, thermal expansions, interfacial reactions at interface and change in morphology over the time due to temperature. Decomposition products (C, CO2, H2O) gets deposited on the surface of sensing layer and reduces its sensitivity after a certain period of time. Various thermal treatments are done periodically to remove these decomposition products from surface. CBNs has

Chapter 2 Carbon nanomaterials-based gas sensors

advantage over other materials in terms of stability because of their robustness and mechanical and thermal qualities that are remarkable. CNTs and graphene are extremely stable structure and morphology doesn’t change at higher temperature.

2.2.4

Response time

The term “response time” refers to the amount of time required by sensing layer to reach to its maximum sensor response ( . 90%) such as resistance, potential, etc., when exposed to analyte. Shorter response time indicates the fast-sensing ability. Response times depends on crystallite size, sensor geometry, electrode position and diffusion rate.

2.2.5

Recovery time

The definition of recovery time is the time required by sensing layer to reduce to its 10% of saturation value after exposure of analyte and then placed in clean air. Lower recovery time indicates better sensor that can be used repeatably and requires less time for a next cycle.

2.3

Functionalization of carbon-based nanomaterials

Several novel nanomaterials with high sensing properties are discovered and studied but CBNs are extensively explored due to its robustness, extraordinary properties and high sensing capabilities [60
63]. Several modifications in microstructure, doping and functionalization have been done to attain selectivity and high sensitivity toward particular gas with maximum gas response. There are several techniques for surface functionalization of CBNs, which can be broadly classified as chemical (covalent) and physical (noncovalent) functionalization. Functional groups include the following:
COOH,
OH, and
NH2, etc., leads to covalent interactions, while π
π stacking, π
cation/anion interaction and hydrophobic interaction are some of the noncovalent interactions [64,65]. Generally, free-radical chemistry [66], amidation [67], esterification [46], carboxylation [68], fluorination [69], Bingel reaction [70] and composite formation [71,72] are some of the common techniques for surface functionalization of CBNs. Surface functionalization of CBNs specially CNTs and graphene are done for the fabrication of high response gas sensors. Pristine CNT has low sensitivity for various gases such as NO2, CO or NH3

33

34

Chapter 2 Carbon nanomaterials-based gas sensors

[73]. Doping with metal nanoparticles increases sensor response, reduces working temperature and enhances response and recovery time [74]. Adjizian et al. doped MWCNTs with boron and nitrogen, which leads to NO2 that can be detected at concentrations as low as 50 ppb at room temperature [75]. Doping results in charge transfer between NO2 gas molecule and MWCNTs which enhances gas response. Several metal NPs such as Ag, Au, Pt, Pd etc., have already been introduced into CNTs to form metal-CNTs nanocomposites. These nanocomposites have better recovery time than pristine CNTs [76,77]. Dilonardo et al. synthesized Au/Pd decorated MWCNT nanocomposite using electrophoretic process for detection of NO2 [78]. These nanocomposites lowered the working temperature to 150 C (as compared to .400 C) with lower response/recovery time and high sensitivity at 10 ppm. Similarly, SnO2-CNT nanocomposite films were utilized to find out ethanol, methanol and with great sensitivity to H2S at ambient temperature [79]. Similarly, NiFe2O4-MWCNTs was synthesized by sol-gel method which shows the response toward H2S at 100 ppm [80]. Acid functionalized MWCNTs faster recovery time toward H2 gas(100 seconds) as compared to pristine MWCNTs (190 seconds) [80]. Reduced-graphene oxide nanosheets decorated with gold nanoparticles was used to detect toxic arsine gas with a detection limit of 0.01 ppm [81]. Kaniyoor and coworkers studied gas sensing capabilities of MWCNTs and graphene with a platinum coating-based nanocomposites toward H2 at room temperature [82]. Gautam et al. synthesized graphene/Pt nanocomposites using CVD technique for detection of NH3. With Pt decoration, sensitivity increased to B80% with detection limit of 50 ppm [83]. Zhang et al. synthesized 3-D array structure using aligned SnO2 nanorods on graphene nanosheets using hydrothermal method and it shows high sensor response toward H2S with a sensitivity of 2.1 and response and recovery times of 5 seconds and 10 seconds respectively [84]. Jain et al. fabricated NH3 gas sensor based on polypyrrole-ZnO nanocomposite doped with camphor sulfonic acid (CSA) [85]. With 15% CSA doping, PPy-ZnO nanocomposites had sensor response of 79% toward 120 ppm of NH3 with faster response time (24 seconds) and recovery time (34 seconds). As a result of CSA doping, delocalization of π-electrons is accelerated, the conjugated bonds are stabilized by this substance where counter anions are implanted by implanting counter anions leading into the polymeric chains which are to higher sensitivity and faster response. Similarly various metal nanoparticles such as Au, Ag and Cu can also enhance the gas-sensing performance of these nanocomposites, which is understandable by schottky contact or spill over effect of metal [86,87]. Jang et al. [88] studied sensing

Chapter 2 Carbon nanomaterials-based gas sensors

behavior of polypyrrole with GO and RGO which are two graphene derivatives. GO hybrid showed lower sensitivity due to low conductivity of GO and fewer electron-charge transfer. While RGO/PPy composite has more conductivity and better sensitivity. Similarly, NO2 sensor based on Ni doped ZnO/PANi nanocomposite exhibited sensor response of 75% toward 100 ppm with response time of 82 seconds, recovery time of 399 seconds and detection limit of 5 ppm [89]. Table 2.2 represents gas sensing characteristics of some of the functionalized CNTs. Functionalized CNTs are also used for the detection of volatile organic compounds (VOCs) such as hydrocarbons, benzene, toluene, ethanol and acetone, etc. Wang et al. synthesized MWCNTs with functionalization of propargyl, allyl, alkyltriazole, thiochain, thioacid and hexafluoroisopropanol (HFIP) group [100]. Similarly, Shrisat, et al. functionalized SWCNTs with different porphyrins like octaethyl porphyrin (OEP), ruthenium OEP (RuOEP), iron OEP (FeOEP) and tetraphenylporphyrin (TPP) [101]. Wei et al. synthesized functionalized SWCNTs with tetrafluorohydroquinone [TFQ for detection of dimethyl methylphosponate (DMMP) at ppm levels] [102]. TFQ creates additional binding sites between hydroxyl groups in TQF and DMMP through hydrogen bonding leading to high conductance. The sensor exhibits faster sensor response and ultrasensitivity up to 20 ppt. MWCNTs was impregnated with poly (methyl methacrylate) (PMMA) for detecting ethanol with the detection limit of 0.5 ppm. Polymer nanocomposites are another class of advanced materials, where polymers are incorporated as nanofillers in CBNs. The polymer nanocomposites have unusual large expanse of surface and high thermal and mechanical properties. Li et al. coated chlorosulfonated polyethylene polymer on CNTs and studied for Cl2 sensing. Similarly, hydroxypropyl cellulose-CNT composite was studied for HCl sensing [103]. Lee and co-workers fabricated polyacrylonitrile/carbon nanofiber composites using electrospinning technique [104]. The surface of nanocomposite was activated using fluorination technique and conductivity was increased using carbon black. Additionally, activated samples’ surfaces was altered by a fluorination process. This nanocomposite showed high sensor response toward NOx and CO gases. Graphene/ polyaniline nanofibers composite was studied for H2 detection by Wlodarski and co-workers. These nanocomposites showed 16.57% sensor response toward 1% H2 gas as compared to pristine graphene (0.83%) and PANi (9.38%) [105]. Several metal oxides such as ZnO and SnO2 were impregnated on carbon nanofibers. These nanocomposite shows high sensing capabilities

35

Table 2.2 Gas sensing characteristics of functionalized CNTs. Sr. Carbon no. nanotube (CNT) type

Detected Functionalization Response gas ways

1.

NO2

Chemical solution deposition (CSD)

NO2

Spray deposition

NO2

NO2 NO2 (250 ppb) NO2 (250 ppb) NO2 (0.75 ppm to 5 ppm) NH3 NH3 (100, 250, 500 ppm) CO and NH3 CO and NH3

2. 3.

Multiwalled carbon nanotubes (MWCNTs) m-SWCNTs

4. 5.

Single walled carbon nanotubes (SWCNTS) MWCNTs MWCNTs/SnO2

6.

SWCNTs/SnO2

7.

SWCNTs

8. 9.

ZnO-T-CNT MWCNTs

10.

CNTs

11.

CNTs

Response time (s)

Recovery time (s)

Operating References temperature

5.5 3 103

2.3 min

6.8 min

50 C

[90]

120 s




RT

[91]

Sputtering

ZnO/m-SWCNT showed better response then ZnO/Au B63%

180 s

1200 s

100 C

[92]

Thermal evaporation Spray deposition

1.023 17.9%

93.1 s 125 s

285.2 s 65 s


100 C

[93] [94]

Spray deposition

13.3%

115 s

75 s

100 C

[95]

Spray coating

21.58% to 167.7%

B240 to 300 s 780 s to 900 s

RT

[95]

Dripping procedure Plasma treatment

B330 22.5%, 27.9%, and 31.4%, respectively

35 s


RT RT

[96] [97]

Mixing

Detected 7 ppm of CO and 20 ppm of NH3 38.4%

18.4 s 260, 312, and 330 s, respectively 16 s




RT

[98]

4 and 4.3 s




RT

[99]

Dispersion

Chapter 2 Carbon nanomaterials-based gas sensors

for DMMP at having a detection limit which is as low as 0.1ppb, at room temperature owing to their high surface area [104].

2.4

Sensing mechanism

The chemiresistive sensors show response toward analyte with change in electrical properties such as conductivity or change in charge carrier concentration. The CBNs has high surface area and high rate of molecular adsorption of gas on the surface. Gas molecules interact with particular surface functional groups and donate or accept electrons from sensing layer according to the nature of gas (oxidizing/reducing). This interaction alters the concentration of charge carriers which can be measured in terms of gas response. This sensing materials can be classified as n-type (where major charge carriers are electrons) or p-type (where major charge carriers are holes). Functionalization, doping and reaction conditions can lead to n-type or p-type semiconductors. For example, doped graphene acts as n-type semiconductor and NH3 as a reducing gas donates its lone pair of electrons to graphene and increase the concentration of electrons (reduction in resistance). Similarly, when there is an oxidizing gas like NO2 present, sensing substrate donates electron to gas and concentration of electron decreases (increases for holes) and increase in resistance is observed which is related to gas concentration [106
110]. Fig. 2.2 illustrate the chemiresistive-type sensors’ sensing mechanism. Based on different sensing mechanism, gas sensors come in a variety of forms based on structure and properties of CBNs such as gas sensors for sorption, capacitance gas sensors, ionization gas sensors and gas sensors with a resonant frequency shift.

2.4.1

Sorption gas sensors

Gas sensors based on sorption, gas molecules are adsorbed on the surface, which is the rate determining step and there is an electron transfer. The electron transfer leads the alteration of electrical characteristics, which is linked to gas concentration. Several studies indicates that MWCNTs, SWCNTs and functionalized CNTs works on sorption principle and are highly sensitive for NO2, NH3 and other VOCs [18,111]. CNT based sensor based on principle of conduction threshold shift due to adsorption of gases and charge transfer between them. This charge transfer leads to change in Schottky barrier and generation of spatial charge regions on the surface which increases the

37

38

Chapter 2 Carbon nanomaterials-based gas sensors

Figure 2.2 Schematic of chemiresistive-type sensors’ sensing mechanism [106].

resistivity of CNT [112,113]. These sensors were fabricated by template printing technique and shows response toward NH3 [114]. Similarly modified MWCNT was found to be selective toward Cl2 with recovery time of 150 seconds [115]. Functionalization generally increases sensor response and selectivity of CBNs. This generates reactive sites at sidewalls and terminals of nanostructures and leads to enhanced adsorption and charge transfer. Carboxylated-CNTs were found to be sensitive for CO gas with a detection limit of 1 ppm while pristine CNT didn’t respond to gas at all [116]. Although sorptionbased gas sensors show high sensitivity, but suffers from several disadvantages for example, a lack of selectivity, long exposure time, long recovery time and dependence on ambient

Chapter 2 Carbon nanomaterials-based gas sensors

conditions (humidity, temperature and gas flow rate). These gas sensors can’t detect gases with low adsorption energy [117].

2.4.2

Ionization gas sensors

The concept of operation for ionization gas sensors is accelerating ion collisions at high voltage (102
103 V) with target gas molecules and determining gas ionization parameters. Gases with low adsorption energies are detected using principle of ionization. CNTs are mainly used as sensor electrode (anode) along with aluminum cathode and glass insulation layer between them. At high voltage, an electric field is induced at the electrodes and condition tends to self-sustained electrode discharge. In presence of gas molecules, small change in breakdown voltage is observed while discharge current changes manifold linearly with concentration of gas. Several results are reported for CO2, N2, O2, He and Ar gas [118
120].

2.4.3

Capacitive gas sensors

In capacitive type sensors, CNTs are produced in an array atop of a SiO2 layer. When external voltage is applied, it produces a strong electric field at CNT terminals which leads to adsorbed gas molecule polarization and a boost in sensor capability [121]. These capacitive gas sensors are highly sensitive toward VOCs such as ethanol, methanol, chlorobenzene, hexane, benzene, acetone and di-nitrotoluene [122]. There major disadvantage being CNT alterations that are irreversible as a result of chemisorption and poor performance at high humidity.

2.4.4

Resonance frequency shift gas sensors

These sensors operate on the basis of the alterations in the electrical characteristics in presence of analyte gas. The sensitive layer is generally disk resonators with CBNs on the outer surface of disks. When sensitive layer interacts with gas, dielectric permeability changes which results in a shift in resonance frequency [123,124]. Since different gases show different frequency shifts, these sensors are highly selective and exhibits high sensitivity at lower concentration. Gases such as NH3, CO, N2, H2, O2 and Ar are detected using this principle. These Chemoreceptive type of gas sensors are commonly used to detect a variety of gases/ vapors. Advantages over other sensors are room temperature operations, low cost and easy fabrication. When voltage is applied across electrodes, the sensors can detect the change in current as composition/concentration of gas changes with time.

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2.5

Fabrication of sensors

There are many techniques to integrate CBNs into gas sensor systems such as screen printing, casting and dielectrophoresis (DEP), etc. Li et al. fabricated gas sensor that is resistive where SWNTs are cast on electrodes with interdigitated digits [63]. Photolithography technique was used to deposit metals such as Au and Ti on Si electrode and CBNs with B99.6% purity were deposited in an electrode area using several techniques. In screen printing technique, CNTs paste are mixed with organic binders such as terpineol, ethyl-cellulose, etc., and Electrode-coated glass with glass frits as the element that detects gas for the detection of NO2 gas [125]. Fig. 2.3 depicts the structure of FET type sensor using CNT or graphene as conducting channels. This sensor has a source and drain electrodes connected by conducting channels along with gate insulator to induce the movements of charge carriers [126
128]. The Silicon back gate amplifies the charge carrier density and threshold voltage, while SiO2 is used as a dielectric barrier layer between channel and gate. In presence of target gas molecules, the charge carrier concentration changes due to the difference in current density of source and drain electrodes, which can be measured analytically [129
131]. Dielectrophoresis is another technique used, where dielectrically polarized materials are used for fabrication. The electrokinetic motion in nonuniform electric fields is used for orientation and positioning of CBNs into arrays [132,133]. In DEP technique, a glass substrate was printed on interdigitated

Figure 2.3 Schematic of field-effect transistor type gas sensor using (A) carbon nanotubes (B) graphene as conducting channels [126].

Chapter 2 Carbon nanomaterials-based gas sensors

Figure 2.4 Schematic of multiwalled carbon nanotube-based gas sensor fabrication using dielectrophoresis technique on microelectrode [135].

microelectrode with a castle-wall pattern in a sealed chamber. These castle-wall electrode forms periodic high and low electric field and DEP trapping was done using AC voltage. The CNTs or other nanomaterials suspension were continuously fed from a peristaltic pump from a reservoir. The number of trapped nanostructures is generally regulated using electrical impedance [134,135]. Fig. 2.4 depicts the fabrication of CNT-based sensor using DEP technique. Spray coating and ink-jet printing are other successful techniques used for CBNs sensors. They are simple, easily scalable, and have several advantages for mass manufacturing of flexible gas sensors [106,136]. Similarly, ethanol sensor was fabricated using CNT which was printed on flexible substrate [137]. Initially, CNTs as an active layer and electrode on which PET substrate was used for printing and then they were functionalized with PEDOT: PSS and COOH groups. Seekaew et al. synthesized graphene-PEDOT: PSS composite films and fabricated flexible gas sensors using ink-jet printing technique (Fig. 2.5) [138]. This sensor has high selectivity and sensor response toward ammonia gas with a detection limit of 25 ppm at ambient temperature.

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Figure 2.5 Schematic of sensor fabrication using ink-jet printing technique [106,138].

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5448. [122] E.S. Snow, F.K. Perkins, E.J. Houser, S.C. Badescu, T.L. Reinecke, Chemical detection with a single-walled carbon nanotube capacitor, Science 307 (2005) 1942
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2773. [126] E. Akbari, V.K. Arora, A. Enzevaee, M.T. Ahmadi, M. Saeidmanesh, M. Khaledian, et al., An analytical approach to evaluate the performance of graphene and carbon nanotubes for NH3 gas sensor applications, Beilstein J. Nanotechnol. 5 (2014) 726
734.

Chapter 2 Carbon nanomaterials-based gas sensors

[127] I. Heller, A.M. Janssens, J. Ma¨nnik, E.D. Minot, S.G. Lemay, C. Dekker, Identifying the mechanism of biosensing with carbon nanotube transistors, Nano Lett. 8 (2008) 591
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2981.

49

3 Carbon-based gas sensing materials Golnoush Zamiri1 and Mohammad Khalid2 1

Centre of Advanced Materials, Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia 2Graphene and Advanced 2D Materials Research Group, School of Engineering and Technology, Sunway University, Jalan University, Subang Jaya, Malaysia

3.1

Introduction to carbon-based gas sensing materials

Gas sensors have been used to detect and monitor flammable and toxic gases to safeguard the environment and conserve energy [1
3]. A chemical sensor is made up of a transducer and an active layer that converts chemical information into electronic signals such as frequency, current, and voltage changes [4]. For good and efficient gas detecting systems, there are a few basic factors to consider: 1. 2. 3. 4. 5.

Sensitivity and selectivity are high; Response and recovery time are quick; The consumption of analysts is low; Operating at a low temperature; Performances that are stable.

Polymers with a high vapor sensitivity, metal oxides semiconductors, and additional porous cellular structures like porous silicon are all common gas sensing materials [5
7]. Optical fibers, inorganic semiconductors, conductive polymers, and carbon nanostructures are among the materials which have been explored to fabricate extremely sensitive, a low price and power consumption and gas sensors that may be carried around [8
10]. Nanomaterials have a high surface-to-volume ratio and hollow construction, making them an ideal component for gas molecules’ adsorption and storage. In recent years, carbon nanomaterials-based gas sensors, such as carbon

Carbon-Based Nanomaterials and Nanocomposites for Gas Sensing. DOI: https://doi.org/10.1016/B978-0-12-821345-2.00002-4 © 2023 Elsevier Inc. All rights reserved.

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Chapter 3 Carbon-based gas sensing materials

nanoparticles, nanotubes, graphene and its derivatives have been widely studied [11]. Nowadays, carbon nanomaterials and their nanocomposites are well-known materials that have sparked significant curiosity because of their incredible electrical, electronic, and mechanical capabilities [12]. Carbon nanostructures with inherent properties become an ideal sensing material for the next generation of sensor technology. Carbon nanomaterials’ sensitivity and selectivity may be altered by utilizing a variety of ways to produce functional groups for deformities and grafts to their surface in a controlled manner [13,14]. As a result, researchers have devised a number of innovative approaches in recent years and have emerged gas sensors that come in a variety of forms based on carbon nanomaterials and their nanocomposites.

3.1.1

Gas sensors

Gas sensors, also known as chemical sensors, are attracting enormous attention because of their wide range of uses. Space exploration, biomedicine, environmental monitoring, industry, and pharmaceutics are just a few of the fields that are being explored. Gas sensors are devices that can be used to detect flammable, gases that are harmful, and the lack of oxygen [15]. A gas sensor is made up of a sensing substance that either connects two electrodes or coatings a pair of interdigitated electrodes for converting the gas volume fraction into a corresponding electrical signal [16]. However, traditional detection methods are unreliable due to its requirement to get precise real-time concentration measurements of the target gas [4]. Nowadays, the search for gassensing materials that are novel and efficient has become a prime topic for developing gas sensors with excellent properties included high sensitivity and selectivity, quick reaction, stability and mobility, cheap cost and low power consumption [17]. Gas sensors are classified as chemiresistor, silicon-based field-effect transistor, capacitance sensor, surface work function change transistor, surface acoustic wave change transistor, optical fiber sensor, and others depending on their reactions with external atmospheres [18]. The gas sensors’ performance is evaluated by several characteristics that include the following aspects: 1. Sensitivity The lowest concentration of the target gas is sensed at the time of detection, referred to as sensitivity. Sensitivity for reducing and oxiding gases can be defined as Ra/Rg and Rg/

Chapter 3 Carbon-based gas sensing materials

2.

3.

4.

5.

6.

Ra, respectively. Where Ra is the gas sensor’s resistance in the air and Rg is the resistance of the sensor under target gas exposure [19]. Selectivity The factors influencing a sensor that can respond to an analyte selectively or a set of analytes are referred to as selectivity. Response time expressed as tres is the time when concentration of gas gets to a certain value to that when a sensor sends a signal to a computer. Recovery time expressed as trec which is after a step concentration shift from a specific concentration value to zero, the time it takes for a sensor signal to recover to its initial value. Stability A gas sensor’s capability to duplicate results in a certain amount of time is known as stability. If the fluctuation is fierce, it is impossible to reflect correct information about the detected gas. Repeatability Repeatability is the measure of consistency in the obtained test results when gas sensors are constantly tested in a similar environment. Repeated testing over time can impact the sensor’s life span. Limit of detection The limit of detection is based on the signal-to-noise ratio (S/N) and is the lowest analyte concentration that the sensor is capable of detecting under certain circumstances. Working temperature Working temperature is defined as the temperature at which the gas sensor has the highest sensitivity. The rate of gas adsorption and desorption is affected by the reaction temperature as well as numerous sensing parameters.

3.2

Detection mechanism of gas sensors

The detection mechanism of the most gas sensors depends on two following parts: molecular recognition and recognition event transduction [20]. Gas molecules interact with the sensing materials in the recognition step, which alters the chemical structure of sensing materials and composition [21]. The interaction between gas molecules and sensing materials are divided into (1) coordination, hydrogen bonding, van der Waals forces, and π 2 π interactions between the analyte and the sensing material. The analyte and the sensing material have noncovalent interactions; and (2) covalent bonding resulting

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from a sensing material’s reaction with gas molecules. The reversible response can be provided by noncovalent interactions. On the other hand, high selectivity and sensitivity can be provided by covalent bonding which leads to an irreversible response. Therefore, it is very important to be aware of the sensing mechanism of gas sensors to improve selectivity and sensitivity. The conductivity of n-type semiconductors is widely known (e.g., ZnO, SnO2) to increases in the presence of a reducing analyte and decreases in the presence of an oxidizing analyte [22]. On the other hand, p-type semiconductors (e.g., NiO, CuO) exhibit the opposite response. When the semiconductor material is annealed in air atmosphere, adsorption of oxygen on the surface extracts the conduction electrons. As a consequence, the electron concentration and the conduction channel decreases, as a consequence the conductivity is reduced. The adsorbed oxygen is removed when a reducing gas reacts with it, and thereby, increasing the conductance. As the majority charge carriers are electrons, an oxidizing gas depletes the chargecarrying electrons in the sensor layer, resulting in a reduction in conductivity. The conductivity of an n-type semiconductor, on the other hand, increases when it comes into contact with a reducing gas. The opposite effects are observed in a p-type semiconductor where holes are the predominant charge carriers. Conversely, Fig. 3.1 depicts the usual sensing method of a p-type semiconducting sensor. When an oxidizing gas is present, conductivity increases in proportion to the number of holes grows, and when a reducing gas is present, conductivity falls as the concentration of hole charge carriers drops [23]. The gas sensing mechanism of ZnO/S, N: GQDs/PANI nanocomposite is based on the resistance change caused by

Figure 3.1 p-n heterojunction in the ZnO/S, N: GQDs/PANI nanocomposite [24].

Chapter 3 Carbon-based gas sensing materials

adsorption, charge transfer, and desorption of target gas molecules on the surface of the sensor. Therefore reactions at roomtemperature have been suggested to be responsible for changes in resistance upon the adsorption of acetone gas. In the ZnO/S, N: GQDs/PANI heterostructures, two different types of depletion layers would exist. The sensing properties of n-type MOS-based sensors are affected by the surface reaction between chemisorbed oxygen species. The chemisorption of oxygen creates the electron depletion layer on the surface of n-type MOS. When the sensor is exposed to air, oxygen molecules in atmospheric air traps free electrons from ZnO/S, N: GQDs/PANI to form oxygen ion as expressed by the following reaction: e2 1 O2ðgÞ 5 O2 2ðadsÞ

ð3:1Þ

The acetone molecules react with the former oxygen species to generate CO2 and H2O when the sensor is exposed to acetone gas. The trapped electrons are reinjected back to the conduction band of n-type MOS. The reaction is simplified as below [24]: 1 2 2 CH3 COCH3ðgÞ 1 O2 2ðadsÞ -CH3 C O 1 CH3 O 1 e

3.3

ð3:2Þ

CH3 C1 O-CH1 3 1 COðgÞ

ð3:3Þ

2 COðgÞ 1 O2 2ðadsÞ -CO2 1 e

ð3:4Þ

Carbon nanomaterials and nanocomposites for sensing

Sensing materials play a significant influence in the detection of various target gases. The interaction between sensing materials and gas molecules can cause changes in internal physical parameters of sensing materials [25]. A diverse set of materials, such as conducting polymers, are used to detect the target gases [26,27], carbon nanotubes (CNTs) [28] and metals or metal oxides, in many forms, for example, nanorods, nanowires, thick or thin films, and so forth [29], have been widely used. Although metal oxide semiconductor gas sensors are extremely sensitive, they have poor selectivity, a limited lifetime, and a high working temperature [8,30]. These disadvantages limit the use of metal oxide as a detecting layer in gas sensors, necessitating the use of other materials such as carbon based materials to the center of

55

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Chapter 3 Carbon-based gas sensing materials

Figure 3.2 Carbon nanomaterials.

the researcher’s attention [31
33]. Nowadays, CNTs and graphene have become the most investigated carbon materials for gas sensors application. However, carbon nanomaterials include more than just CNTs and graphene. Carbon nanomaterials also exist in the form of nanoparticles, nanoporous, nanofibers, and carbon-based nanocomposites. Recently, carbon nanomaterials drew significant interest in scientific community due to their fascinating optical, electrical, and mechanical properties which is what makes them suitable for the new generation of gas sensors. The properties and fabrication methods for some of carbon materials such as carbon black, carbon nanofibers, CNTs and graphene and its derivatives (Fig. 3.2) are discussed in the following section.

3.3.1

Carbon black

Under regulated settings, carbon black is colloidal carbon which is nearly pure elemental carbon. Combustion that is not complete or decomposition by heat of gaseous or liquid hydrocarbons produces carbon black. Amorphous carbon, often known as carbon black, is a kind of amorphous carbon, which has a large surface-to-volume ratio. The particle size of carbon black is almost lower than 50 nm and it has a high surface area around 1000 m2/g. There are two manufacturing methods such

Chapter 3 Carbon-based gas sensing materials

as furnace black and thermal black to produce carbon black materials which the most commonly used process is furnace black. Typically, heavy aromatic oils as feedstock are utilized in the furnace black process and the feedstock oil is atomized by utilizing a closed reactor in a controlled environment. In most furnace reactors, steam or water sprays control the reaction rate. In a continuous process, the carbon black created is carried through the reactor, cooled, and collected in bag filters [34]. On the other hand, natural gases, including methane or heavy aromatic oils are used as feedstock materials in the thermal black process. In thermal black process, a pair of furnaces are used which alternate between preheating and carbon black synthesis every 5 minutes. After that, the stream material is filtered in a baghouse and quenched with water sprays [34].

3.3.2

Carbon nanofibers

Scientists and engineers have become interested in fibrous carbon materials. Catalytic chemical vapor deposition can be used to make carbon nanofibers (CVD) with a structure and properties that are different from carbon nanofibers produced by vapor grown carbon fiber (VGCF) [35,36]. Carbon nanofiber prepared by VGCF have a variety of diameters ranging from 60 to 200 nm and lengths ranging from 1
100 mm. Further, these fibers have a substantial surface area as well as a well-defined structure and shape. Therefore, it is feasible to create carbon nanofibers in a variety of conformations while also controlling the degree of their crystalline organization by carefully manipulating various factors. Another technique for creating carbon nanofibers is electrospinning, which often uses a polymer like a polyacrylonitrile as the carbon source [37]. The carbon nanofibers obtained by electrospinning process has diameter between 40
400 nm and the length of 70 μm [38
40].

3.3.3

Carbon nanotubes

CNTs are seamless cylinders made by rolling graphene sheets in one direction alone. Fullerene-like hemispherical molecules are used to seal the ends of CNTs. Single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) are the two types of CNTs, which depend on the number of graphene sheets that have been wrapped [41]. SWCNTs are manufactured from a one-atom thick coating of graphite that has been shaped into a cylinder with no seams a nanometer-sized particle with a diameter of several nanometers

57

58

Chapter 3 Carbon-based gas sensing materials

and a length of 1
100 m. Graphene layers rolled in many layers are wrapped together to form a tube shape in MWCNT [42]. CVD method, laser ablation technique and arc-discharge method are the main techniques for preparation of CNTs [43]. During the arc-discharge process, two carbon electrodes serve as a carbon source in a vacuum chamber. A plasma of the inert gas (typically helium gas) is formed when a high dc voltage is introduced between the carbon anode and cathode to evaporate the carbon atoms. This method is known as the first method to produce both SWCNTs and MWCNTs with lengths of around 50 μm with few structural defects. In laser ablation technique, a pulsed laser vaporizes a carbon target, which is subsequently ablated in a furnace in the presence of a catalyst and inert gas, with lengths of roughly 50 μm and has minimal structural flaws. Smalley and his colleagues investigated at this method to manufacture MWCNTs [44]. To make SWCNTs, the same researchers employed a graphite and metal catalyst particle composite [45]. In a CVD system, metal catalyst particles in a layer, such as nickel, cobalt, iron, or a mixture of these elements, are used as substrate [46]. CNT diameters are proportional to the size of metal particles and can be adjusted through metal layer patterned metal deposition, annealing, or plasma etching. CNTs can be prepared at a lower temperature by CVD method compared with two other techniques. Therefore the CVD process is more productive and allows SWCNTs to be scaled up. High quality SWCNTs and MWCNTs can be produced by CVD methods by modifying and controlling the growth factors in a calculated way. One of the most serious drawbacks of the CVD procedure is that MWCNTs have a relatively high defect density, which can be explained by a lack of thermal energy.

3.3.4

Graphene

Graphene is a honeycomb-like two-dimensional carbon sheet made up of covalently linked carbon atoms arranged in a two-dimensional array by sp2 bonds [47]. The large surface area and excellent electrical and thermal conductivities make graphene and its derivatives very interesting for fabricating gas sensors. Various methods have been employed to prepare graphene materials. CVD, epitaxial growth, chemical cleaving or exfoliation of graphite, and micromechanical exfoliation of graphite are some of these processes. The earliest method for producing graphene was micromechanical exfoliation. For the first time, graphene with a single layer was created by

Chapter 3 Carbon-based gas sensing materials

constantly peeling away graphite and sticking it to a silicon dioxide/silicon (SiO2/Si) sticky adhesives on the substrate [47]. To prepare reduced graphene oxide from graphene oxide, chemical exfoliation of graphite utilized. In this process, a strong acidic solution is introduced oxygen containing functional group into graphene sheets [48]. Later, graphene oxide is reduced chemically or thermally. But, reduced graphene oxide differs from pure graphene in that it still contains oxygencontaining groups and structural flaws [49]. In CVD method, graphene grows on the copper or nickel as a metal substrate at temperatures between 700 C to 1000 C. This method can produce high-purity graphene on a big scale with great quality. However, the slow growth rate of the standard CVD process has halted graphene from expanding [50].

3.3.4.1

Gas sensors based on carbon nanomaterials and nanocomposites

Carbon nanoparticles’ characteristics have been used to produce gas sensors that use various transduction concepts. This section is limited to the performance of gas sensors based on carbon nanomaterials and their nanocomposite as sensing materials under different target gases exposure.

3.4

Carbon black materials and composites for gas sensors

Carbon black is a member of carbon in a variety of forms that is amorphous and has a high ratio of surface area to volume, which is distributed in organic polymers that act as insulators. However, because of carbon black’s particular conductivity and mechanical characteristics, it can not be used as a sensing material in gas sensor applications. The possibility of using carbon black in gas sensors is making composite with another which can provide gas sensing properties. Normally, in polymer-based composites, carbon black is employed. In particular, the carbon black gives the films electrical conductivity, while the different organic polymers give the sensing materials chemical variety [51,52]. Many factors have been found to alter the response of electrical resistance of gas sensors based on carbon black and polymer composite. For example, the response, reproducibility and stability of composite can be changed by modification of carbon black surface, crystallinity, molecular weight of polymer materials and dispersivity of carbon black in the nanocomposite.

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Figure 3.3 The operating mechanism of gas sensors based on a combination of carbon black and a polymer [34].

The composites are insulators at lower carbon black loadings, due to the absence of a continuous channel of conductive particles across the substance. Therefore, the resistivity of the composite can be decreased by increasing carbon black amount in the composite. The operating mechanism of gas sensors based on carbon black and polymer composite is indicated in Fig. 3.3. The resistance of composites increases during organic vapor absorption and then returns to its original value once the vapor has entirely desorbed. Lewis and coworkers investigated carbon black and different polymer composite to respond to different vapor or gases. Standard chemometrics were used to create the seventeen-element array’s has output electrical resistance signals. The identification of the presence of various vapors and gases was the objective of their research. This strategy is simple to implement into software or hardware-based design recognition processors, enabling for the sensing integration and analysis functionalities into a small, electricity consumption is minimal, low-cost, and a simple vapor analyzer [34].

3.5

Carbon nanofibers and composites for gas sensors

The sensor’s self-sensing function is possible due to the making of carbon nanofibers and their composites by looking at how electrical characteristics change that has taken place because external variables are changed, such as stress and

Chapter 3 Carbon-based gas sensing materials

strain, as well as the gaseous environment. With a reversible change in the external circumstances, the conductivity of electricity carbon nanofibers can be modified by several orders of magnitude. Gas sensors based on NO and CO sensors made of carbon nanofibers were looked into several stages of preparation and alteration of the surface and fluorination of carbon nanofibers and coelectrospinning of PAN and carbon black as well as incorporating carbon black into carbon nanofibers, PAN and carbon black are co-electrospun and employing a hightemperature KOH procedure to activate carbon nanofibers. By following as-mentioned steps, it shows that the overall electrical conductivity is increased when carbon black added to the carbon nanofibers. On the contrary, by inducing surface functional groups of the gas sensor, chemical activation of KOH produced a porous fiber structure with a greater region of particular surface, which dramatically improved target gas adsorption. The target gas was attracted by the surface change affixed to the gas sensor’s surface [53]. The NO and CO gas sensitivity was fivefold increased by using these methods of manufacturing. To get around the problem, carbon black and polymer composites have a high degree of instability, the carbon nanofiber dispersion through the use of a polymer has been proposed by Fu and coworkers [54]. When the composites absorb vapor molecules, the instability is occurred as carbon black particles tend to reaggregate due to their nanoscale size. In contrast, fabrication of carbon nanofibers and polymer composites offers an improvement in the vapor sensing stability. Lee and colleagues looked at how to make a polyacrylonitrile and carbon black complex as sensing materials utilizing electrospinning method [53]. Fong and colleagues reported electrospinning and a wet chemistry approach in their work to produce carbon nanofiber mats [55]. The mats’ top surface was made functional after soaking the amidoxime-functionalized mats in aqueous solution containing NH2OH, it was permissible for the amidoxime groupings to exist in an aqueous solution of Pd(NO3)2, using Pd21 ions to coordinate/chelate. The same methods can be used to fabricate carbon nanofibers decorated with various metals nanoparticles. Sensors had a reaction to 100% hydrogen is moderate, with response and recovery periods that were almost as lengthy. Metal element dimensions connected to the fibers surface affects reactivity, and the approach used here results in a large dispersion of Pd cluster size. Jang and his group investigated a technique to make electrospun carbon nanofibers with metal oxide decorations [9]. Hybrids of carbon nanofibers with metal oxide embellishments were dissolved in a solution of ethanol

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Chapter 3 Carbon-based gas sensing materials

Figure 3.4 SEM photos of carbon nanofibers with different ZnAc2 concentrations, and SnCl4 in the PVP solution (A) 0.5wt.%, (B) 0.75wt.%, (C) 1wt.%, (D) 1.25wt.%, (E) 1.5wt.%, and (F) 2wt.% [9].

using ultrasonic technology and the interdigitated electrodes (IDE) were covered using spin-coating so that routine resistance tests can be carried out. The sensors were tested at ambient temperature when dimethyl methylphosphonate is present (DMMP). The detection limit of hybrid carbon nanofibers is 0.1 parts per billion, and this high degree of sensitivity may be owing to the presence of metal oxides on the surface of the carbon nanofibers, which increases the surface area and improves the attraction for DMMP vapors. Fig. 3.4 shows scanning electron microscope of carbon nanofibers at low and high resolution with various metal oxide loadings.

3.6

Carbon nanotubes and composites for gas sensor

CNTs can be used as an effective sensing material, offering a large number of gas molecule binding sites since they have defect sites, a porous structure, and a high surface area to

Chapter 3 Carbon-based gas sensing materials

volume ratio. Wang et al. reported p-type semiconductor behavior is demonstrated by carbon nanotubes (CNTs), and the sensitivity of SWCNTs was tested for NO2 and NH3 gases [56]. The oxidizing gas NO2 absorbs electrons from SWCNTs and produces holes that can cause lower resistance after the interaction. Alternatively, the reducing gas NH3 gives electrons. Following the encounter, they join with a hole to form a new entity, enhancing SWCNTs’ resistance. Kong et al. also studied the response of gas sensors based on SWCNT to NO2 and NH3 target gases [57]. The response is good at room temperature but a recuperation time is really long, as observed from the results. Theoretical research by Zhao et al. on gas molecules that are adsorbed on CNTs revealed that CNTs only serve as adsorbents for high-adsorption-energy reactive gases, like NH3 and NO2 [58]. According to their findings, NH3 and NO2 gases exhibit strong interactions, whereas other gases with low adsorption energies can only interact via van der Waals forces or weak bonds. Woods [59] also recorded that the gas molecules of volatile organic compounds (VOC) a weak interaction with SWCNTs. The interactions between MWCNTs and gas molecules are more complicated compare to SWCNTs because of the effect of multiple tubes. Gas molecules and CNTs have weak interactions, that can be the reason why decreases in the response of gas sensors. Nowadays, many researchers are working to improve the reactivity of CNTs. Some of the investigations offer functionalization of CNTs as an effective way to increase the performance of gas sensors based on CNTs [60]. Molecules have functional groups containing specific atoms that are in charge of the molecules’ unique chemical reactions. During chemical functionalization of CNTs when high-intensity conditions are there, additional flaws appears. The carboxyl group, which is very flexible group fixed to CNTs, is acidic, forcing it to shed its proton and create COO2 ion. The resulting electrophile quickly interacts with electrophilic reagents to create ionic salts. Some substances have the ability to bind to the group of hydroxyl while hydrogen bonding with the
OH group’s H atom. As a result of charge carrier transfer inside or outside the CNTs during the adsorption of target gas molecules, the output resistance of f-CNTs varies. Additionally, functionalization allows for gaseous molecule binding to be selective [61]. Modifying CNTs by noble metals, metal oxide and polymers can improve performance of gas sensors in terms of sensing based on CNTs under other gases exposure besides NO2 and NH3. For example, the composite can detect CO, H2, and SO2 gases with lower energy adsorption compared to NO2 and NH3 gases.

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3.6.1

Carbon nanotubes and metal or metal oxide composites for gas sensor

When compared to the usage of metals, using metal oxides to modify CNTs improves gas sensor performance. The gas sensing properties of tin oxide–decorated, oxygen plasma–treated multiwall carbon nanotube hybrids were investigated by Leghrib et al. [62] and compared against the sensing properties of pure tin oxide nanoparticles or pure oxygen plasma–treated multiwall carbon nanotubes. Figure 3.5A and B shows the response toward increasing nitrogen dioxide concentrations for two sensors based on carbon nanotube and tin oxide hybrid nanomaterials. It was found that at low operating temperatures (room temperature and 150˚C), hybrid nanomaterials were significantly more responsive to nitrogen dioxide and carbon monoxide than pure nanomaterials (pure tin oxide or pure MWCNTs) [62]. Jesus et al. [62] reported that a gas sensor based on Pt/MWCNT shows an inadequate response under CH4. It means that the composites of CNTs with metals do not increase the response of gas sensors. However, it could be the reason for increased selectivity and reduced response time, or it could be the cause of the gas and the surface of the gas sensor’s faster interface interaction. It is challenging to detect CH4 gas by pure MWCNTs as it exhibits low energy adsorption to MWCNTs. However, by adding Pt to MWCNTs, CH4 detection sensitivity could be increased [63]. The presence of noble metals may aid in dissociating gases such as H2 with a spillover mechanism, hence improving sensing performance. Metal oxide gas sensors made of CNTs have a high response rate for various reasons that can help them respond more quickly, including changes to the reaction synergy (A) 38000 Dry air

Dry air

(B) 432000

37000

430000 Resistance (ohm)

Resistance (ohm)

1ppm

431000

36000 50ppb 35000 100ppb 34000

500ppb

33000

1ppm

Dry air &heating

32000 50

100

150

200 250 Time (min)

300 450

500

500ppb

429000 428000 427000 426000

Dry air & heating

100ppb

425000 50ppb 424000 423000 422000 421000 420000

Dry air 100

200

300 Time (min)

400

500

Figure 3.5 Response of tin oxide–decorated oxygen plasma treated carbon nanotube hybrids towards increasing concentrations of nitrogen dioxide. Sensors were operated at room temperature (Fig. 11.5A) and at 150 ˚C (Fig. 11.5B) [62].

Chapter 3 Carbon-based gas sensing materials

65

and depletion layer [64]. Iqbal and his group investigated a gas sensor based on MWCNTs-MeOx, which recorded a high response and fast response time under SO2 and CO gases [65]. When two or more composite materials have various mechanisms, synergy reactions can occur. Manipulation of the depletion layer and synergy reaction is thought to improve gas sensor response considerably. Gas sensors based on SnO2 as a famous material have the potential to detect NH3, CO, and NO2 gases at room temperature when combined with CNTs. The advantage of gas sensors operating at room temperature is lower energy consumption. On the contrary, ZnO gas sensors also can operate at room temperature by combining with CNTs. However, CNT-based gas sensors are not as common as SnO2-based gas sensors.

3.6.2

Carbon nanotubes and polymer composites for gas sensor

The use of polymers as a gas sensor renders them effective when coupled with CNTs. On the another hand, composites of CNTs and polymers have been developed extensively investigated for gas sensors. An et al. [68]. created a nanocomposite based on SWNT/polypyrrole utilizing an in-situ polymerization technique. They generated (Ppy) nanocomposite by pyrrole and SWMTs combination in situ chemical polymerization. The composite was then coated by spin casting on IDEs to test resistance. Fig. 3.6 depicts resistance reactions of as-prepared materials to the gas like NO2.

Figure 3.6 For NO2 gas, sensitivity changes as a function of gas exposure time [68].

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Chapter 3 Carbon-based gas sensing materials

As can be seen in Fig. 3.6, all of the materials show deterioration after an extended period of exposure; however, the nanocomposite indicated improved sensitivity compared with the pristine materials, which could be due to distributed CNTs increasing the material’s conductivity. Philip et al. [69] used MWCNTs to construct gas sensors or oxidation-modified MWCNTs (f-CNTs) in a polymethylmethacrylate (PMMA) composite thin film. Both the composites change the resistance, which are measured under various target gases exposure such as dichloromethane, chloroform and acetone. The CNT/PMMA and the f-CNT/PMMA composites show increased resistance under these vapors at room temperature. The volume expansion of the CNT surface and the vapor molecules’ polar interactions were used to explain this behavior. The gas sensor behaviors of f-CNT/PMMA, such as the response time, sensitivity, and recovery, have all been enhanced. Furthermore, CNTs with a polymer coating sensing characteristic to a number of gases were also studied. Gas sensors based on AC dielectrophoresis assembly of SWNT networks followed by electropolymerization of PEDOT:PSS on SWNTs surface were developed by Badhulika, et al. [68]. Cyclic voltammograms on both bare and coated devices were performed (Fig. 3.7A), revealing that electropolymerization increases the PEDOT:PSS–coated current density sensor. As shown in Fig. 3.7B–D, exposure to the VOCs of interest increased the sensors’ resistance. The hybrid sensors responded linearly to a wide range of saturated vapors of methanol, that is, from 2.5% to 75% concentrations with a limit of detection of 1.3% of saturated vapors considering a signal-tonoise ratio of 3. While the bare SWNT devices did not respond to ethanol and methyl ethyl ketone, the hybrid devices responded to both the analytes. The limit of detection was calculated to be 5.95% and 3% for ethanol and methyl ethyl ketone vapors, respectively, with a signal-to-noise ratio of 3 [68]. CNTs often only respond to gases and vapors with high adsorption energies; however, doping or coating CNTs with an enhancing element can broaden their range of applications.

3.7

Graphene materials and composites for gas sensor

Over the last decades, graphene and its derivatives were the most popular investigated carbon materials due to their suitability for gas sensors application [18,70
75]. Based on graphene the first gas detector was investigated by Schedin et al.

Chapter 3 Carbon-based gas sensing materials

67

Figure 3.7 Cyclic voltammograms of bare SWNTs and PEDOT: PSS coated SWNTs device with scan rate of 50 mV/s and Response curves of the PEDOT:PSS coated SWNTs sensor towards different concentrations of (b) methanol, (c) ethanol, and (d) methyl ethyl ketone [68].

[70]. Later, Yavari et al. [76]. reported graphene films-based NO2 and NH3 gas sensors which was prepared using CVD methods. According to Schedin and his coworkers, graphene-based gas sensors have the best sensitivity, enabling the identification of single gas molecules [70]. The high sensitivity of graphenebased sensors for a wide range of gases, up to ppm levels, is their main advantage. However, employing graphene in gas sensor applications has numerous drawbacks, including a slow recovery time, a general lack of specificity, and potentially high cost. Consequently, materials made of graphene don’t appear to be appropriate for gas detection at ambient temperature. To

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enhance the sensing performance of graphene-based gas sensors Chung et al. (2012) used ozone treatment which is an economical and effective technique to improve graphene’s sensing properties by adding the right quantity of functional oxygen groups. As a result, the percentage response time, detection limit, and response are all factors to consider of the ozonetreated graphene sensor were all significantly improved [77]. Nantao Hu and his team developed the method based on chemical reduced graphene oxide (rGO) for DMMP gas, in which p-phenylenediamine (PPD) was used to reduce graphene oxide. According to them, CRG decreased by PPD exhibited a more positive response to DMMP than CRG that has happened by reducing with hydrazine. As compared to CRG, the DMMP shows response which is 5.7 times greater at 30 ppm [78]. An examination of the literature illustrates that graphene-based gas sensors composites decorated with metals, polymer and metal oxides show better response compare to pure graphene at room temperature.

3.7.1

Graphene and metal or metal oxide nanocomposite for gas sensor

The capacity of noble metals to improve sensor performance has been established, but they are costly [79
81]. Nanowires or nanorods made of metal oxide, such as ZnO, SnO2, and Cu2O, have a large region of particular surface, aspect ratio is big, and significant adaptability and are intensively studied in the field of sensing [82]. As a result of the low electrical conductivity, of mentioned nanostructures, metal oxide and graphene in combination can improve electrical conductivity while also improving gas sensor response. To study the performance of gas sensors according to graphene composite with other materials, Huang et al. [83] developed ZnO quantum dots (QDs) decorated graphene nanocomposites for formaldehyde gas, which worked at room temperature. ZnO QD decorated graphene nanocomposites, were created at low temperatures utilizing a solution-based approach. For the detection of liquefied petroleum gas (LPG) at low temperatures, Nemade and Waghuley reported chemiresistors made of graphene/SnO2 quantum dots composites with high sensing response and selectivity, low working temperature, quick response times, and quick recovery speeds [84]. They also explored graphene/Al2O3 QDs composites as a CO2 sensing material for the first time. Graphene in various weight

Chapter 3 Carbon-based gas sensing materials

1000

RGO

Response, ['R/R]x100

RGO/SnO2-2 RGO/SnO2-3 100

10

1 50

100

250 150 200 Temperature (°C)

300

350

Figure 3.8 Response to NO2 (8 ppm) of the reduced graphene oxide (black), RGO/SnO2-2 (red) and RGO/SnO2-3 (blue) sensors [86].

percentages (20–80 wt%) was blended in a consistent manner of 1g Al2O3 composite samples, which Nemade and Waghuley carried out. It was found that increasing the weight percent of graphene improves the sensing response, and graphene composition with 80 wt% graphene/Al2O3 has the maximum sensing response [85]. Neri et al. [86] synthesized SnO2 and reduced graphene oxide nanocomposites by a one-pot microwave-assisted nonaqueous sol-gel method, and the synthesized samples were used as sensing layers. Based on the SnO2 and reduced graphene oxide nanocomposites, the fabricated resistive sensors showed good sensing characteristics to NO2. The sensor response was found to be dependent on the SnO2 and reduced graphene oxide nanocomposites ratio. Increased SnO2 content on reduced graphene oxide enhanced the response to NO2, as shown in Fig. 3.8. To enhance H2S sensing properties of WO3 gas sensors, Jinjin Shi and the team synthesized rGO/hexagonal WO3 (rGO/ h-WO3) nanosheets composites as a result of the hydrothermal technique and subsequent postcalcination treatment. The reason for the improved performance of the rGO/h-WO3 H2S sensor is due to its 3D hybrid nanostructure, which creates a large number of pathways for diffusion of H2S gas into the sensor material. During the response procedure, which results in good interaction between H2S and interior WO3 grains, in addition to quick absorption in the recovery process. In addition, charge carriers are transported was made less difficult by rGO’s

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Chapter 3 Carbon-based gas sensing materials

Figure 3.9 Sensing mechanism of H2S gas (A) pure WO3 nanosheets and (B) rGO/h-WO3 composites [85].

electrical qualities that are superior. As additional oxygen molecules on materials’ surfaces snatched electrons from the WO3 conduction bands, the amount of chemisorbed oxygen has increased. (as seen in Fig. 3.9B) [85]. For ammonia gas detection (NH3), a two-dimensional (2D) based on gas sensing material with different metal (Ag, Au, and Pt) nanoparticles-rGO nanocomposites was developed. In comparison to the sensors decorated with Au and Pt NPs, the sensor with AgNPs shows improved sensitivity, selectivity, response/ recovery times, and stability to NH3 gas. The performance of AgNPs-rGO was the best, indicating a substantial dependence on the metal type [86]. Oleksandr Ovsianytskyi and his team treated single layer wet chemical method graphene with a result of AgNO3 and Fe (NO3)3 for the selective and fast H2S gas sensing. The sensing mechanism can be described in the following steps:
O2 gas -O2 ðadsÞ ð3:5Þ

Chapter 3 Carbon-based gas sensing materials

Figure 3.10 Schematic representation of H2S sensing by graphene decorated with AgNO3 [87].

O2 ðadsÞ 1 e2 2 -O22 ðadsÞ

ð3:6Þ

2H2 S 1 3O22 ðadsÞ-2H2 Om 1 2SO2 m 1 3e2

ð3:7Þ

The sensor’s exposure to an oxidizing gas (e.g., O2) triggers a reaction (2). As mentioned earlier, O2 molecule act as an acceptor of charges, the resistance is decreasing by removing electrons from the graphene surface. The adsorption of the gas likely initiates, when the surface is exposed to H2S gas. Owing to its interaction with Ag rather than carbon with adsorbed oxygen species. As Ag is less electronegative. The adsorption of H2S causes it to dissociate, and SO2 and H2O are produced as electrons are released. These free electrons are returned to graphene and recombine with core holes. The charge carrier concentration is reduced as a result of this occurrence and increases the resistance of Ag-doped graphene (Fig. 3.10) [87]. Shafie et al. demonstrated the performance based on a gas sensor of the hybrid SnS2-rGO film toward different gases including NO2, CH4, NH3, C2H5OH (ethanol), (CH3)2CO (acetone) at various temperatures up to 100 C. They reported that, in comparison to the responses to NH3, CH4, acetone, ethanol, and humidity, the sensor revealed a high response to NO2 [88].

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35

RGO/PANI

ΔR/R0 (%)

30 25 20 15 10 5

RGO

0 0

200

400

600

800

1000

1200

1400

Time (sec) Figure 3.11 Response curve when NH3 gas was exposed to pristine graphene and graphene/polyaniline hybrid materials [93].

3.7.2

Graphene and polymers nanocomposites for gas sensor

The composite of the graphene and polymers not only has the suitable properties of both materials but also increases the gas sensors sensitivity, selectivity, and stability [89
91]. According to Dan and Coworkers, photolithography preparation of graphene devices will invariably leave polymer photoresist with graphene surface which has a thickness of 1 nm [92]. It is possible to boost the gas sensor performance by combining the advantages of graphene and polyaniline. First, graphene/polyaniline hybrid material was dispersed in organic solvents to fabricate the gas sensor. Then the solution was coated dropwise on the surface of a Pt electrode which was dried in the oven. It can be seen in Fig. 3.11 that, when compared to pure, this composite’s response to NH3 is substantially enhanced, and the change in resistance rate can exceed 30%. For the first time, Al-Mashat et al. showed a gas sensor for H2 based on graphene/PANI nanocomposite. They found that due to the integrated graphene nanosheets’ high surface-tovolume ratio, the developed sensor indicated higher sensitivity (16.57%) toward 1% of H2 gas, compared to sensors using solely PANI nanofibers have higher sensitivity (9.38%) and only graphene (0.83%) [89]. Li et al. synthesized a graphene oxide/ polypyrene nanocomposite for the detection of various gases [94]. Because of its unusual structure, the composite showed good sensing properties. With a sensitivity of 9.871024, toluene

Chapter 3 Carbon-based gas sensing materials

gas demonstrated a quick, linear, and reversible reaction. Yang et al. used LB deposition and an in-situ polymerization approach to RGO nanocomposite with a single layer and porous PEDOT nanocomposite synthesis, and the RGO/porous PEDOT composite’s gas sensing performance suggested improved as compared to a sensor based on bare RGO, sensing performance to NO2 gas [95]. Tien and Hur [96] developed NO2 gas sensors made of RGO
PPy composites. When 50 ppm of NO2 gas is present at normal temperature, the sensitivity is significant (32%). The RGO-PPy composite gas sensor with TiO2 decoration showed a response of 102.2% to 50 ppm of NH3 [97]. Avossa et al. [98]. developed a chemical sensor, which was nanofibrous and conductive. These nanofibrous conductive sensor was made up of two insulating polymers (PS and PHB, dubbed PsB) doped with 5,10,15,20-tetraphenylporphyrin (H2TPP), which were chosen because they are ecocompatible, versatile, recyclable (PS), biodegradable [99,100], and thermally resistant. The conductivity and temperature have nonlinear connections, according to the sensor. It means that as the temperature rises, so does the electrical conductivity. Preparation of graphene/polymer composite materials have significant effects for detecting certain gases due to their unique selectivity, sensitivity and electrical properties. In comparison with pristine graphene, graphene/polymer nanocomposites have excellent gas sensing characteristics, with a sensitivity of up to 30%.

3.8

Conclusion

Sensing materials play a significant influence in the detection of various target gases. The interaction between sensing materials and gas molecules can cause changes in the internal physical parameters of sensing materials. Carbon nanomaterials, including carbon black, carbon nanofibers, carbon nanotubes, and graphene, drew a great deal of interest in the scientific community due to their fascinating optical, electrical, and mechanical properties, making them suitable for the new generation of gas sensors. However, carbon black cannot be used as a sensing material due to its particular conductivity and mechanical characteristics. The possibility of using carbon black in gas sensors is making a composite with other materials which can provide gas sensing properties. CNTs can be used as an effective sensing material compared to carbon black and carbon nanofibers due to the presence of defect sites, porous

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structure, and a high surface area to volume ratio that can offer a large number of gas molecule binding sites. On the other hand, incorporating CNTs with noble metals, metal oxide, and polymers can improve the performance of gas sensors under various gas exposure. CNTs typically only react to gases and vapors with high adsorption energies; however, by doping or coating CNTs with an enhancing element, the application range may be expanded. Recently, graphene materials were the most popular investigated carbon materials due to their suitability for gas sensors application. In addition to carrying the desirable qualities of both materials, the combination of graphene with metal/metal oxide and polymers improves the stability, sensitivity, and selectivity of gas sensors. The composite of metal oxide and graphene materials can improve electrical conductivity and gas sensor response. Preparation of graphene and polymer composite materials has significant effects on detecting certain gases due to their unique selectivity, sensitivity, and electrical properties. According to the literature, graphene and polymer nanocomposites exhibit outstanding gas sensing properties with a sensitivity of up to 30% more than pure graphene.

Acknowledgment The authors acknowledge the financial support provided by the University of Malaya grant through the Post-Doctoral Research Fellowship (GZ) and Impact-Oriented Interdisciplinary Research Programme (IIRG) (No.: IIRG018C-2019).

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17365. [83] K. Nemade, S. Waghuley, In situ synthesis of graphene/SnO2 quantum dots composites for chemiresistive gas sensing, Mater. Sci. Semiconductor Process. 24 (2014) 126
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583. [85] J.J. Shi, Z.X. Cheng, L.P. Gao, Y. Zhang, et al., Facile synthesis of reduced graphene oxide/hexagonal WO3 nanosheets composites with enhanced H2S sensing properties, Sens. Actuators B-Chem. 230 (2016) 736
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348.

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Part 2

Application of carbon nanomaterials in gas sensing

4 Carbon nanotube-based gas sensors Tanushree Sen and Navinchandra Gopal Shimpi Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India

4.1

Introduction

With his famous lecture at the 1959 American Physical Society meeting at Caltech titled “There’s Plenty of Room at the Bottom” [1], where he introduced the concept of manipulating matter at the atomic level, Nobel Laureate Richard Feynman opened the door to the field of nanotechnology. Fifteen years later, in 1974, Norio Taniguchi coined the term “nanotechnology” to describe semiconductor processes with nanometer-scale control such as thin film deposition and ion beam milling [2]. However, the real impetus to modern nanotechnology was provided by two main discoveries: the discovery of fullerenes by Kroto, Smalley, and Curl in 1985 [3] and that of carbon nanotubes (CNTs) by Sumio Iijima in 1991 [4]. In particular, the discovery of CNTs by Iijima is an important landmark in the history of modern nanotechnology, as it attracted the attention of the research community and generated great interest in CNTs. In 1991, Iijima discovered multi-walled carbon nanotubes (MWCNTs) in the carbon residue obtained during the preparation of fullerene by the electric arc discharge method [4]. In 1993, Iijima and Ichihashi synthesized single-walled carbon nanotubes (SWCNTs) by arc discharge using carbon electrodes and iron as a catalyst in the presence of a mixture of CH4 and Ar gases [5]. In 1996, Smalley synthesized bundles of SWCNTs for the first time [6]. These works ushered in a new era in modern nanotechnology. CNTs are allotropes of carbon. They are one-dimensional tubular structures consisting of concentric graphite layers (sp2-hybridized carbon atoms connected to each other to form a hexagonal lattice) with diameters in the nanometer range [7]. CNTs can be classified into MWCNTs and SWCNTs. A MWCNT consists of graphene sheets stacked into concentric cylinders. Each nanotube Carbon-Based Nanomaterials and Nanocomposites for Gas Sensing. DOI: https://doi.org/10.1016/B978-0-12-821345-2.00005-X © 2023 Elsevier Inc. All rights reserved.

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has a diameter of several nanometers and a length of tens of micrometers. On the other hand, a SWCNT contain only about 10 atoms around the circumference, with a typical diameter of 1 2 nm. In addition to these basic structures, CNTs can differ depending on the rolling of the graphene sheet. The chirality of a CNT, which is determined by the mutual orientation of the hexagonal network and longitudinal axis of the nanotube, is an important property that governs the electrical, optical, and mechanical properties of CNTs [8]. Chiral nanotubes have a screw symmetry, whereas achiral nanotubes have a cylindrical symmetry and can be further classified into zigzag or armchair nanotubes depending on the chiral vector. Fig. 4.1 shows the models of a MWCNT and SWCNT and their different configurations based on chirality. The chirality of a nanotube is also defined by the chiral angle, which is the angle between the nanotube rolling direction and the zigzag motif in the honeycomb lattice. Although most nanotubes are semiconductors with a band gap of 0.1 0.2 eV, they can show metallic conductivity as well. Generally, chiral or zigzag CNTs are semiconducting in nature, whereas armchair CNTs exhibit metallic conductivity. CNTs are commonly synthesized by arch discharge, laser vaporization, and chemical vapor deposition (CVD), among which CVD is the most widely used method. CNTs are typically a surface structure with a large unit surface of nanotubes, which determines their electrochemical and adsorption properties. The electronic properties of CNTs are highly sensitive to the molecules adsorbed on their surface. This makes CNTs a promising material for chemical and biological sensors. CNTs are hydrophobic in nature and form insoluble aggregates. Moreover,

Figure 4.1 Models of single-walled and multi-walled carbon nanotubes and their different configurations based on chirality [9]. Reproduced with permission from “S.Y. Madani, A. Mandel, A.M. Seifalian, A concise review of carbon nanotube’s toxicology, Nano Rev. 4 (1) (2013) 1 14.” “Article published by Taylor & Francis in NANO REVIEWS on December 3, 2013, available online at https://doi.org/10.3402/nano.v4i0.21521.”

Chapter 4 Carbon nanotube-based gas sensors

they are nonreactive owing to the absence of dangling bonds on their surface. Therefore for many applications, surface functionalization is necessary. The ends of CNTs are usually capped with fullerene hemispheres. Since the curvatures of the ends are larger than that of the sidewalls, the chemical reactivities of the CNT ends and sidewalls are different. This enables the selective functionalization of the ends of CNTs for specific applications. In recent times, CNTs have steered many technological innovations and advancements, central to which are sensors. Gas sensors are used to monitor toxic, combustible, and flammable gases in both domestic and industrial environments. The broad range of applications has necessitated the development of inexpensive, reliable, small, and low-power consumption sensors with high sensitivity and selectivity. Semiconducting metal oxides (SMOs) have been traditionally used as gas sensors [10]. However, one of their major disadvantages is high operating temperatures, which is required to achieve sufficient sensitivity to analytes. This restricts their application in areas where detection in ambient conditions or room temperature (RT) is required. Moreover, the high operating temperature may result in issues related to sensor instability. This led to the search for alternative materials with suitable surface and bulk properties that can overcome the limitations of SMO-based sensors. In 2000, Kong and coworkers [11] demonstrated that the electrical resistance of individual SWCNTs changes upon exposure to NO2 or NH3 gases. In the same year, Collins and coworkers [12] reported the sensitivity of SWCNTs to O2 gas. This formed the basis for CNT sensors. In 2005, Huang and coworkers [13] reported the use of MWCNTs as a sensing layer for N2 gas. These works were followed by numerous studies on CNTs for sensing applications. In this chapter, we discuss the different types of gas sensors that are currently in use and the various CNT-based hybrid materials that are used as sensing elements in these gas sensors.

4.2

Sensing mechanism

The electrochemical and adsorption properties of CNTs are governed by their large surface area. Hence, CNTs are promising materials for chemical and biological sensors. However, in this chapter, we discuss only CNT-based gas sensors. Based on the working principle, CNT-based gas sensors can be classified as sorption, ionization, resonance frequency shift, capacitive, and electrochemical gas sensors [8]. Sorption gas sensors constitute

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the major part of CNT-based gas sensors. These sensors operate on the principle of adsorption, that is, the electrical property of the nanotubes changes due to charge transfer between the adsorbed gas molecules and the nanotubes. Adsorption can occur via chemisorption or physisorption. Chemisorption is characterized by bond formation between the adsorbate and the sensing element, whereas physisorption occurs via van der Waal’s and dipole interactions between the adsorbate and the sensing element. In both the cases, the resistivity of the senor changes; however, the energy and desorption time scales of the two processes are different (chemidesorption is slower than physidesorption). Functionalized CNTs or CNTs decorated with metal nanoparticles form good sorption sensors. These sensors are used to detect NO2, CO, CO2, CH4, H2, and NH3 gases with good sensitivity and selectivity [14 16]. Depending on the measured parameter, sorption sensors can be classified as amperometric, conductometric, and potentiometric sensors. Despite being the most widely researched and used sensor, sorption sensors have several disadvantages such as poor selectivity, long exposure and recovery times, irreversible changes in electrical property due to chemisorption, and poor detection of gases with low adsorption energies. Another type of gas sensors is ionization gas sensors, in which accelerated ions are collided with gas molecules and the sensor response is determined by measuring the gas ionization parameters. As there is no chemical interaction between the sensing layer and the analyte gas, gases with low adsorption energies can be detected. Ionization gas sensors have been used for CO2, H2, N2, NH3, and O2 detection [17]. However, high operating voltages and unsuitably large dimensions and weight hinder their wide application. Resonance frequency shift gas sensors are used to detect dangerous and harmful gases. In these sensors, the sensing element is a disk resonator coated with a CNT layer on its outer surface. When exposed to gases, the dielectric property of the CNT-bearing disk resonator changes, which results in a shift in its resonance frequency. Because different gases bring about different changes in the resonance frequency, theses sensors exhibit good sensitivity and selectivity. However, these sensors require additional equipment to monitor the dielectric permeability and resonance frequency. Capacitive gas sensors are used to detect volatile organic compounds (VOCs) such as benzene, chlorobenzene, ethanol, toluene, and methanol [18]. In these sensors, an external voltage is applied between two plates, one of which is the sensing

Chapter 4 Carbon nanotube-based gas sensors

layer comprising an array of misoriented CNTs, while the other is silicon. This generates an electric field at the CNT ends, which leads to the polarization of adsorbate molecules, resulting in a change in capacitance. A drawback of capacitive gas sensors is that the sensing element requires frequent replacement because of the irreversible changes in the CNT properties due to chemisorption. The operation of electrochemical sensors is based on the reduction and oxidation reactions that occur during their interaction with analytes. CNT-based electrochemical sensors are commonly used in biomedical research for the detection of biomolecules such as glucose and nucleic acids [19,20]. For this purpose, CNTs are functionalized with redox polymers, as in the case of glucose detection, or with oligonucleotides, as in the case of nucleic acid detection. Zaporotskova et al. [8] published an excellent comprehensive review of the different types of sensors based on CNTs. In the following sections, we discuss various CNT-based hybrid materials used as sensing elements in gas sensors.

4.3

Carbon nanotube/metal nanocompositebased gas sensors

CNT are combined with metals such as gold, silver, platinum, palladium, copper, and nickel for gas sensing applications [21 24]. However, most of the studies pertain to the functionalization of the CNT surface with metal nanoparticles. In this section, we discuss only CNT/metal nanocomposites used as gas sensors. Lin et al. [21] fabricated a Pd/CNT/Ni hybrid film for H2 gas sensing application. They synthesized a CNT/Ni composite film on a glass substrate by the nanocomposite plating technique and then decorated the film with Pd nanoparticles. The hybrid film exhibited excellent H2 gas-sensing properties compared with the film without Pd nanoparticles. The authors proposed that the sensing mechanism is based on the interaction between H2, Pd, and CNT/Ni. The exposure to H2 gas leads to the formation of palladium hydride, which has a lower work function than Pd. Thus H2 dissociates and reaches the interface between Pd and CNT/Ni, thus lowering its work function and facilitating the transfer of electron from Pd to CNT/Ni, which decreased the conductance of the p-type CNT. In this case, however, the hybrid sensor surface-functionalized with Pd nanoparticles proved superior to the CNT/Ni sensor.

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Czerwosz et al. [22] prepared CNT Ni Pd nanocomposite films for an optical gas sensor. They employed a multistep method in which the films were first prepared on quartz substrates by physical vapor deposition (PVD) and then CNTs were grown by CVD. The hybrid film comprised a CNT/Ni composite film coated with a CNT/Pd composite film. They evaluated the gas sensing properties of the films by measuring the optical absorbance in the wavelength range of 250 2500 nm from RT to 80 C. The authors proposed that compared with the CNT/Ni/Pd nanocomposite, the CNT/Ni nanocomposite exhibited good responses toward CO and NO2 gases because of the high number of defects and dangling bonds, which facilitated the binding of gas molecules. An interesting work in this field is that of Shen and coworkers [25], who developed a miniature optical emission spectrometer based on a CNT/Pt nanocomposite-decorated fluorine-doped tin oxide (FTO) electrode and used the system for H2 sensing at RT. Compared with the CNT/FTO electrode, the Pt-CNT/FTO electrode exhibited remarkable gas sensing performance as the Pt nanoparticles facilitated the adsorption of a large amount of H2 molecules and lowered their ionization energy, which led to the ionization of more number of H2 molecules and increased the light emission. The optical emission spectrometer could also be applied for the detection of O2 gas and solution of metal ions at RT. Fig. 4.2 shows the schematic of the miniature optical emission spectrometer and the H2 emission intensity for 0.0167% 0.33% H2 at different electrodes.

Figure 4.2 (A) Schematic of the miniature optical emission spectrometer. (B) Hydrogen emission intensity for 0.0167% 0.33% H2 at different electrodes: (A) bare fluorine-doped tin oxide (FTO), (B) carbon nanotube (CNT) FTO, and (C) Pt/CNT FTO. Each measurement was made from 650 to 660 nm; discharge voltage: 1.32 kV [25]. Reproduced with permission from “L. Shen, P. Chen, B. Yan, C. Zhang, A miniaturized optical emission spectrometry based on a needle-plate electrode discharge as a light source for room temperature hydrogen sensing with Pt/CNT nanocomposites, Sens. Actuators B: Chem. 215 (2015) 9 14.”

Chapter 4 Carbon nanotube-based gas sensors

Although CNT/metal nanocomposites show considerable gas sensing properties, these hybrid systems are more effective when the metal nanoparticles are decorated/functionalized on the CNT surface. We will discuss the gas sensing properties of surface-functionalized CNTs later. In this section, we also consider the nanocomposites of CNTs with metal organic frameworks (MOFs). MOFs are a class of compounds that consist of metal ions or clusters coordinated to organic ligands forming one-, two-, or three-dimensional structures. These compounds are often porous, and extraneous metals and gas molecules can be incorporated into the pores. Because of this feature, MOFs are potential candidates for a variety of applications such as gas purification, gas separation, catalysis, and sensors. Chappanda et al. [26] developed a highly sensitive quartz crystal microbalance (QCM) humidity sensor based on a CNT/MOF nanocomposite. They used HKUST-1 MOF in which Cu Cu dimers are connected to 1,3,5-tricarboxylate linkers to form a 3D network. They prepared a CNT/HKUST-1 nanocomposite film on a QCM by spin coating and measured the variation in its resonance frequency upon exposure to water vapor. The CNT/HKUST-1 nanocomposite film exhibited remarkable responses toward humidity. The authors argued that the reduced crystal size due to the presence of CNTs increased the water vapor sorption kinetics, and thus the sensitivity of the nanocomposite films. A recent work in this field is that of Ghanbarian and coworkers [27] who prepared a ternary nanocomposite using CNTs, Ag nanoparticles, and bimetallic MIL-53(Cr Fe) (a binary MOF) for the detection of VOCs. The authors proposed that upon exposure to VOC vapors, the MOF expanded readily and reversibly due to the adsorption of gas molecules into the pores, which increased the resistance of the sensor. The application of MOFs in gas sensors is limited by the insulating behavior of MOFs, which makes signal transduction difficult. However, the scenario is changing, with recent works focusing on the synthesis of electrically conductive MOFs. This research direction appears promising for the development of MOF-based gas sensors for real-world applications.

4.4

Carbon nanotube/semiconducting metal oxide nanocomposite-based gas sensors

Semiconducting metal oxides have been extensively investigated for gas sensing applications because of their sizedependent properties and cost-effectiveness. In particular, SMO

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nanoparticles, which have a very high surface area per unit mass, are extensively used for monitoring toxic, inflammable, and exhaust gases in both industrial and domestic settings. Some of the most significant SMOs that are used for gas sensing are SnO2, In2O3, TiO2, WO3, ZnO, Cr2O3, CuO, and Co2O3 [28 35]. Based on the operating temperature, which dictates the sensing mechanism, SMOs can be classified as surface conductance materials (e.g., SnO2 and ZnO) and bulk conductance materials (e.g., TiO2) [36]. The former operates at low temperatures of 400 C 600 C, while the latter operates at high temperatures of .700 C. This is a major drawback of SMO gas sensors, as such high operating temperatures (400 C to over 1000 C) render them unsuitable for many applications that require operation at ambient or RT. In addition, high operating temperatures may affect the stability of the sensors, resulting in poor reproducibility of the sensor performance. Although much effort has been devoted to fabricating gas sensors based on SMO nanostructures that operate at a low or RT [37 39], the combination of SMOs with CNTs is a simple strategy to achieve the above-mentioned goal. The porous structure and high surface-to-volume ratio of CNT-based nanocomposites provide a large area for the adsorption of gas molecules, thereby enhancing the sensitivity and selectivity of the sensor. Li et al. [40] prepared a Co3O4/SWCNT nanocomposite that exhibited significant responses toward H2 gas compared with pure Co3O4 and SWCNT films. The authors attributed the enhanced response to the reaction of Co3O4 with the gas molecules and the formation of an electrical continuum between Co3O4 and the SWCNTs. Zhang et al. [41] prepared a CNT/CuO nanocomposite by a hydrothermal reaction for the detection of organic volatiles at RT. The considerable response of the CNT/CuO nanocomposite to NH3 vapor at RT was attributed to the catalytic oxidation of NH3 by CuO, which changed the sensor resistivity. Majumdar et al. [42] prepared a SnO2/CNT nanocomposite thick film sensor for H2 sensing. They prepared the SnO2/CNT nanocomposite by a wet chemical method and investigated its sensing performance at 200 C. The sensing mechanism was attributed to the surface reactions involving the O and O2 species that were formed by consumption of free electrons from the conduction band of SnO2 by the atmospheric O2 chemisorbed on the SnO2 surface, which increased the resistance. However, upon exposure to H2 gas, the oxygen species reduced the H2 gas, resulting in the release of electrons back into the conduction band of SnO2, which decreased the resistance. Fig. 4.3 shows the

Figure 4.3 TEM images of (A) pure SnO2 nanoparticles and (B) SnO2 1 0.1% carbon nanotube (CNT) composite powder, (C) selected area electron diffraction pattern of SnO2 1 0.1% CNT composite powder, and (D) highresolution transmission electron microscopy image of SnO2 1 0.1% CNT composite powder with respective plane indexing. (E) Dynamic response curves of 0.1% CNT/SnO2 composite sensor measured against four different concentrations of H2 at 200˚C operating temperature [42]. Reproduced with permission from “S. Majumdar, P. Nag, P.S. Devi, Enhanced performance of CNT/SnO2 thick film gas sensors towards hydrogen, Mater. Chem. Phys. 147 (1 2) (2014) 79 85.”

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transmission electron microscopy (TEM) images of pure SnO2 powder and 0.1% CNT/SnO2 composite and the dynamic response curves of 0.1% CNT/SnO2 composite at different H2 gas concentrations measured at 200 C. Navazani et al. [43] synthesized a reversible CH4 sensor based on a hybrid system containing SnO2 and Pt-doped MWCNTs. CNTs are typically insensitive to nonpolar gases, such as CH4, as they do not undergo a charge-transfer reaction with CNTs. The presence of SnO2, however, rendered the MWCNTs sensitive to CH4 through the surface reactions involving oxygen species. The high response of the SnO2-Pt/MWCNTs hybrid system is because of the formation of two depletion layers, one at the interface between Pt/MWCNTs and SnO2, and the other at the surface of SnO2 particles, which increased the adsorption sites for gas molecules. Fan et al. [44] prepared a nanocomposite of CNTs with CuO/SnO2 nanofibers for the detection of H2S gas at low concentrations. The hybrid sensor exhibited excellent sensitivity toward H2S at a gas concentration as low as 0.1 ppm. In these CNTbased nanocomposites, the CNTs form nanochannels, which facilitate gas diffusion and provide more adsorption sites for the gas molecules, thereby increasing the sensor response. Barthwal et al. [45] fabricated a gas sensor chip in which the sensing layer was a ZnO CNT nanocomposite for NO2 gas detection. The sensor showed the maximum response at 150 C. Recently, Lupan et al. [46] integrated individual ZnO CNT nanowires on a nanosensor device for the highly selective detection of NH3 gas. They functionalized the surface of ZnO nanowires with CNTs and attached the nanowire on prepatterned Au/Cr contacts by Pt complex. The finest nanowire (100 nm diameter) exhibited the highest sensing performance toward NH3 gas. Sayago et al. [47] prepared SWCNT/TiO 2 nanotubes for the sensing of pollutant gases. Seekaew et al. [48] prepared a TiO2/graphene-CNT hybrid that exhibited a high response toward toluene. In another work, Seekaew and coworkers [49] prepared a hybrid Sn TiO2/reduced graphene/CNT nanocomposite that exhibited a highly selective response toward NH3 gas in the presence of several interfering gases. The highest sensor response was obtained for a Sn/Ti molar ratio of 1:10, and the sensing mechanism was explained by the formation of p n heterojunctions between the p-type reduced graphene/CNT and the n-type Sn TiO2 nanoparticles via a low-temperature oxidizing reaction process. Table 4.1 summarizes the sensitivity and response and recovery times of various CNT/SMO-based gas sensors.

Chapter 4 Carbon nanotube-based gas sensors

93

Table 4.1 Sensitivity and response and recovery times of carbon nanotube/semiconducting metal oxide-based gas sensors. Sensor

Analyte/ Operating concentration temperature (˚C)

SWCNT/Co3O4 MWCNT/CuO CNT/SnO2 SnO2-Pt/MWCNT CNT-CuO/SnO2 ZnO/SWCNT ZnO-CNT SWCNT/TiO2

H2/4% NH3/10 mL H2/4% CH4/100 ppm H2S/0.1 ppm NO2/1000 ppm NH3/50 ppm NH3/80 ppm; NO2 /0.9 ppm; H2/1500 ppm Toluene/ 500 ppm NH3/250 ppm

TiO2/grapheneCNT Sn-TiO2/rGb /CNT

Sensitivity Response Recovery References time time

RTa RT 200 RT 40 150 RT 200

200% 100% 84% 28.25% 4.441 B9 430 4.85; 32.62; 12.63

RT RT

176 s 8.3 s 70 s 25 s

120 s 763 s 11.5 s 100 s 18 s 1h

[40] [41] [42] [43] [44] [45] [46] [47]

42%

9s

11 s

[48]

85.9%

99 s

66 s

[49]

40 50 s

RT, room temperature. rG, reduced graphene.

a b

4.5

Carbon nanotube/conducting polymer nanocomposites for gas sensors

Another important class of materials that are used alone or in combination with CNTs and other materials for gas sensing are conducting polymers (CPs). More correctly known as intrinsically conducting polymers (ICPs), they are organic polymers with conjugated chain structures. The π-electrons of the conjugated macromolecular chains are delocalized over large segments, thereby imparting electronic properties to the conjugated polymers. CPs typically include straight chain units, five- or six-membered rings, heteroatoms, side chains, and a combination of these. Some of the prominent ICPs are polyacetylene (PA), polyaniline (PANI), polypyrrole (PPY), polythiphene (PTH), and poly(3,4-ethylenedioxythiophene) (PEDOT). Fig. 4.4 shows the molecular structures of CPs. The discovery of highly conducting (doped) PA by Hideki Shirakawa, Alan Heeger, and Alan MacDiarmid, who won the Nobel Prize in chemistry in 2000 for the discovery, led to the launch of the field of ICPs. However, PA has extremely poor

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Chapter 4 Carbon nanotube-based gas sensors

Figure 4.4 Structures of typical conducting polymers.

environmental stability, which is the major barrier to its application. The discovery of PA was soon followed by other ICPs such as PANI and PPY, which have a good environmental stability and tunable electronic properties, and thus greater applicability than PA. CPs are insulators in their neutral state with an electrical conductivity of 10 10 10 5 S/cm and can be converted into conductive states with an electrical conductivity of 1 104 S/cm through redox reactions by chemical or electrochemical means [50]. CPs are characterized by the formation of polarons, bipolarons, and solitons upon doping via redox reactions or protonation. When an electron is added to or removed from the CP, lattice deformation occurs and the charge is inserted in a lower energy level. This leads to the formation of a localized electronic state in the gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital. The localized charge particle with the local lattice distortion is called a polaron. Chemically, a polaron is a radical (1/2 spin) associated with lattice distortion. The removal of a second electron leads to the formation of a bipolaron, that is a pair of like charges bound with a strong local lattice distortion. In CPs with nondegenerate ground states, polarons and bipolarons are the charge carriers, whereas in CPs with degenerate ground states, solitons are the charge carriers [51]. There are many good reviews on ICPs that deal with their synthesis, properties, and applications [52 54].

Chapter 4 Carbon nanotube-based gas sensors

The nanocomposites of CNTs with CPs have been extensively used for gas sensing. PANI is often used for NH3 gas detection, as it is highly sensitive to NH3, which easily deprotonates PANI, leading to a measurable change in its electrical conductivity. Kim et al. [55] fabricated a PANI/SWCNT nanocomposite for sensing CO and NH3 gases at RT. The gas sensor was composed of a PANI/SWCNT film and a pair of Ti/Au interdigitated electrodes, and efficiently detected both CO and NH3. The sensing mechanisms of the PANI/SWCNT nanocomposite were attributed to the charge transfer from the nanocomposite to CO and from NH3 to the nanocomposite, which decreased and increased the resistance of the nanocomposite, respectively. Roy et al. [56] prepared a PANI/MWCNT nanocomposite as a sensing layer for CO gas, which exhibited good sensitivity at RT. They reported that the adsorption-desorption behavior of CO on the nanocomposite fits the simplest first order adsorption-desorption kinetic model. An interesting work was reported by Shen and coworkers [57], who developed an interdigital wireless passive gas sensor based on the principle of LC mutual coupling. First, they screen-printed the inductor coil using a lead-free aluminum paste on an alumina ceramic substrate and sintered it at a high temperature. Next, they coated the inductor with an insulating protective SiO2 layer followed by coating with the PANI/CNT nanocomposite to obtain a wireless sensor, which transmits signal through electromagnetic coupling. The sensor exhibited good specificity for NH3 gas with a sensitivity of 0.4 MHz/ppm. PPY is also widely used with CNTs for gas sensing [58 61]. However, some recent works are notable in their approach. Liu et al. [62] prepared a flexible PPY/N-doped MWCNT gas sensor for NO2 detection. The PPY/N-doped MWCNT nanocomposite films were fabricated by an in situ self-assembly process followed by an annealing treatment. When the sensor was exposed to NO2, an oxidizing gas, the gas captured electrons from the p-type PPY nanocomposite sensor to form NO2- ion, which decreasing the resistance of the polymer. When exposed to air, the captured electrons were released into the polymer and the resistance increased. The sensor exhibited a high sensitivity after high-temperature annealing. The authors attributed this to the reduction in the stacking of PPY on the surface, which afforded more chemisorption sites to the gas molecules. Hamouma et al. [63] prepared a chemiresistive sensor based on PPY/CNT nanocomposite coated on cellulosic paper. The CNTs were functionalized with aminophenyl groups and coated on cellulosic paper. The functionalized CNT layer was then coated

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with PPY. The resultant sensor exhibited excellent response to NH3 gas at a low gas concentration of 0.1 ppm. The limit of detection (LOD) was as low as 0.04 ppb. The synthetic approach is noteworthy because of its potential in providing low-cost functional materials for electronic applications. One of the greatest advantages of PEDOT is its optical transparency in its conductive state. However, the polymer has poor solubility, which is often resolved by mixing it with sodium polystyrene sulfonate (PEDOT:PSS). PEDOT:PSS has been used in combination with CNTs as gas sensors [64 66]. In a recent work, George and coworkers [67] prepared a CNT-based inkjet-printed radiofrequency gas sensor for the detection of VOCs, in which the sensing layer, fabricated on a paper substrate, was composed of MWCNTs embedded in PEDOT:PSS. The sensitivity of the sensor was measured based on the differential measurement of the frequency responses of the reference transducer (without sensitive layer) and the gas-sensitive transducer (with sensitive layer). The sensitivity and response and recovery times of various CNT/CP-based gas sensors are summarized in Table 4.2.

Table 4.2 Sensitivity and response and recovery times of carbon nanotube/conducting polymer-based gas sensors. Sensor

Analyte/ Operating Sensitivity concentration temperature (˚C)

Response Recovery References time time

MWCNT/PANI CNT/PANI PPY/MWCNT PPY/MWCNT PPY/NMWCNT PPY/MWCNTcoated cellulosic paper PEDOT:PSS/ MWCNT SWCNT/ PEDOT:PSS

CO/100 ppm NH3/300 ppm NH3/33.2 ppm NH3/2000 ppm NO2/5 ppm

RT RT RT RT RT

26.7% 0.04 MHz/ppm 65% 17.11% 24.82%

76 s 375 s , 10 min 34 s 65 s

210 s 84 s

668 s

[56] [57] [58] [59] [60]

NH3/0.1 ppm

RT

525%

138 s

465 s

[61]

NH3/30 ppm

RT

16%

B15 s

NH3/200 ppb (LODa)

RT

LOD, limit of detection.

a

12 s

[65] 18 s

[66]

Chapter 4 Carbon nanotube-based gas sensors

4.6

Functionalized carbon nanotubes as gas sensors

Functionalized CNTs constitute a large section of CNT-based gas sensors. CNTs can be surface-functionalized with functional moieties or even decorated with metal, metal oxide, or metal sulfide nanoparticles. Dhall et al. [68] prepared acid-functionalized CNTs for H2 detection, wherein the functional groups such as carboxyl groups on the CNT surface acted as catalytic sites for the dissociation of H2 gas. Kang et al. [69] prepared a highly sensitive NO gas sensor by functionalizing CNTs with an aminebased polymer, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane. The sensor exhibited a high selectivity and an excellent response to sub-ppm NO gas, which was attributed to the selective reaction of the polymer with the NO gas molecules. In a novel work, Kim et al. [70] prepared a flexible gas sensor based on CNTs functionalized with a dye-functionalized sol-gel matrix. The CNT-dye sensor with different dye species showed high sensitivity and selectivity toward different gases. The reversible binding of the analyte molecules on the dye in the sol-gel matrix changed the ion density on the layer, which was measured by the underlying CNT-based transistor. Star et al. [71] prepared a CNT-based gas sensor array through site-selective electroplating of PD, Pt, Rh, and Au metals on isolated SWCNT networks assembled on a single chip for various toxic and combustible gases. Compared with other fabrication methods, the electrodeposition method allows device miniaturization. Recently, Cui et al. [72] prepared a Pt and Pd codecorated CNT sensor for the detection of SF6, which is a potent greenhouse gas, and performed DFT studies to understand the sensing mechanism. They reported that the interaction of the PtPd-CNT sensor was stronger with SOF2 than with SO2F2, the SF6 decomposition species. Savu et al. [73] prepared Ti and Cu nanoparticle-decorated CNT-based gas sensors for O2 sensing. The gas sensing mechanism was based on two processes: surface oxidation, and oxygen diffusion followed by bulk Ti oxidation. Au-decorated CNTs have also been reported for gas sensing [74 76]. Metal oxide nanoparticles have also been used to functionalize CNTs for gas sensing applications [77,78]. Recently, Zhao et al. [79] prepared a Co3O4-decorated reduced graphene oxide and CNT nanocomposite for nitrite detection. The Co3O4 nanoparticles on the surface acted as catalytic sites for the oxidation

97

98

Chapter 4 Carbon nanotube-based gas sensors

Table 4.3 Sensitivity and response and recovery times of functionalized carbon nanotube-based gas sensors. Sensor

Analyte/ Operating Sensitivity concentration temperature (˚C)

Acidfunctionalized MWCNT en-APTASaSWCNT Au-VAb-CNT IrOx-MWCNT

H2/0.05%

RT

0.8%

NO/100 ppb

RT

16%

[79]

NO2/1 ppm NH3/100 ppm; NO2/1000 ppb

RT 100 150

6.2% 1.71 (10 2 2%/ppm) 3.2 (10 2 3%/ppm)

[75] [77]

a b

Response Recovery References time time 100 s

[68]

en-APTAS, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane. VA, vertically aligned.

of nitrite. Table 4.3 summarizes the sensitivity and response and recovery times of various functionalized CNT-based gas sensors.

4.7

Conclusions and outlook

CNT are at the forefront of technological innovations and advancements for real-world technologies. CNTs are particularly attractive as chemical sensors for domestic and industrial applications in a wide range of fields. Although CNT-based gas sensors overcome the limitations of traditional SMO-based gas sensors, their selectivities are often insufficient. Therefore, CNTs have been combined with metal and metal oxide nanoparticles as well as CP and even surface-functionalized with functional polymers and metal/metal oxide nanoparticles. Over the years, gas sensors based on CNT nanocomposites or functionalized CNTs having different sensor architectures have been developed, and the sensing mechanisms have been elucidated for a better understanding of the working of CNT-based sensors to guide the design of efficient sensors for real-world applications. Future works are required to elucidate the structure property relationship in CNTs to gain insights into the sensing mechanism and to identify the technological barriers that hinder the commercialization of CNT-based gas sensors.

Chapter 4 Carbon nanotube-based gas sensors

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103

5 Carbon nanofiber-based gas sensors Jolina Rodrigues1, Shilpa Jain2, Navinchandra Gopal Shimpi1 and Akshara Paresh Shah1 1

Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India 2Department of Chemistry, Jai Hind College, Mumbai, Maharashtra, India

5.1

Introduction

In 1879 Thomas Edison prepared first carbon fibers (CFs) by carbonizing bamboo and cotton. CNFs were used as filament of light bulb synthesized from bamboo and cotton. In scientific research and applications, carbon fiber has been developed tremendously [1 5]. Carbon nanofibers (CNFs) are an important member of carbon fiber family with several applications such as energy conversion and storage, reinforcement of composites, and self-sensing devices [5 10]. The conventional CF and CNFs have difference in their diameters. The diameter of conventional CF is in micrometers, while that of CNFs is approximately 50 200 nm. Various zero-dimensional (0D) to three-dimensional (3D) nanomaterials are synthesized in nanotechnology and material science, which are used as gas sensors. The one-dimensional (1D) nanomaterials show better gas sensing properties as compared to other dimensional nanomaterials [11]. The 1 D architectures shows improved gas sensing performance due to short path for electron transfer, which leads to penetration of electrolyte along the longitudinal axis of nanofiber. The most widely used 1D nanomaterials are CNFs, which have been used for the fabrication of biosensors and high-performance gas sensors, as CNFs has unique electrical, mechanical, and magnetic properties [12 15]. The high surface area and high adsorption ability of CNFs toward various molecules and biomolecule make CNFs very good candidates to fabricate chemical and biological sensors with high sensitivity and selectivity. CNFs have unique chemical and physical properties, which are of similar structure to CNTs and fullerenes; hence, CNFs are widely studied. The structure of Carbon-Based Nanomaterials and Nanocomposites for Gas Sensing. DOI: https://doi.org/10.1016/B978-0-12-821345-2.00008-5 © 2023 Elsevier Inc. All rights reserved.

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106

Chapter 5 Carbon nanofiber-based gas sensors

MWCNTs consists of hollow tubes with graphitic layers which is parallel to the axis of the inner tube. CNFs also contain graphite layers, which form an angle with the axis of the inner tube and the interior. CNFs interior can be hollow or solid. The diameter of CNFs is in the range of 10 500 nm, and the length can reach up to 10 μm, while the diameters of CNTs are usually less than 100 nm. For the production of hybrid CNFs-based nanomaterials, surface modification or functionalization are possible due to CNFs having high potential basal graphite planes and edge planes. These hybrid CNFs-based nanomaterials have several applications in the fields of biomedicine [16], tissue engineering [17], nanodevices [18], sensors [19,20], energy [21], and environmental science [22]. Zhang et al. synthesized CNFs and explained in detail its procedure by electrospinning method as well as its variety of applications in the field of biomedicine, sensor, energy, and environmental fields. Feng et al. worked on the synthesis, properties, and applications of CNFs and CNFs-based composites, with detailed explanation of CNFs applications in electrical devices, batteries, and supercapacitors. Zhang and coworkers worked on advances in the synthesis of electrospun and application of CNFs in electrochemical energy storage [23 25].

5.2

Methods of carbon nanofiber preparation

Carbon nanofibers can be synthesized by mainly three techniques: electrospinning, catalytic thermal chemical vapor deposition growth, and substrate method.

5.2.1

Electrospinning

In 1930 electrospinning was developed to produce continuous CNFs. The polymer nanofiber is the most commonly used precursor for the synthesis of CNFs by electrospinning method. The type of polymer solution and processing parameters are important factors in deciding the properties of final CNFs which is synthesized. In electrospinning method, the most commonly used polymer for synthesis of CNFs is PAN and pitches. The polymer used in electrospinning process are polyimides, polybenzimidazole, polyvinyl alcohol, phenolic resin, polyvinylidene fluoride and lignin etc. This polymer solution is charged with thousands to tens of volts of static electricity. Under the influence of electric field, spinning forms the taylor cone from charged polymer solution. The taylor cone is drafted and accelerated, when the electric force exceeds the internal

Chapter 5 Carbon nanofiber-based gas sensors

tension of the spinning solution. Owing to extreme rate of motion, the moving jet it grafted and thinned which leads to deposition of fiber on collection plate. And hence the fibrous mat similar to non-woven fabric is formed which is further preoxidized in air and carbonized in nitrogen environment. For the formation of CNFs, the heat treatment is used which carbonize polymer nanofiber to CNFs. Carbonization is a process which is done above 1000 C in specific environment, where the nanofiber volume and weight changes leading to decrease in diameter of CNFs. The heat treatment processes such as atmosphere and temperature play an important role in deciding the morphology, purity, crystallinity, diameters, and porosity of CNFs being synthesized. By electrospinning process mostly commonly web or mat structured CNFs are formed. As compared to other synthesizing CNFs methods, electrospinning is mostly preferred as it has many advantages. Electrospinning requires voltage of thousand volt or more but energy consumption is small. A nanofiber non-woven fabric in 2D form can be directly prepared with no need of further processing. At room temperature, -spun is possible-, so material having poor thermal stability can be spun. The wide range of material can be used to prepare nanofiber by electrospinning. The nanofibers can also be synthesized by electrospinning processes from synthetic polymers such as polyester, -polyamide-, and naturally occurring high molecular substances such as collagen, silk, and DNA [24,26] (Fig. 5.1).

5.2.2

Catalytic thermal chemical vapor deposition growth

By catalytic thermal chemical vapor deposition method, two different types of CNFs can be synthesized, which are the cup-stacked CNFs and the platelet CNFs. The cup-stacked CNFs are also called conical CNFs. In catalytic thermal chemical vapor deposition growth method, several different types of metals and alloys are used for synthesis of CNFs. These metals and alloys are used to dissolve carbon to form metal carbide. The iron, cobalt, nickel, chromium, and vanadium are used as catalyst. In temperature range from 700K to 1200K, the carbon sources are used such as molybdenum, methane, carbon monoxide, synthesis gas (H2/CO), ethyne or ethene etc. The shape of catalytic nanosized metal particles plays an important role in determining the structure of CNFs. Hydrocarbons, which are dissolved with metal particles, precipitates on metal surface as graphitic carbon [24].

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Figure 5.1 Schematic demonstration of (A) vertical, (B) horizontal electrospinning setup for carbon nanofibers fabrication [27].

Chapter 5 Carbon nanofiber-based gas sensors

5.2.3

Substrate method

The substrates used are ceramic or SiO2 fibers. The Fe, Co, Ni, and other transition metals are used to uniformly disperse nanosized catalyst particles in and on the surface of ceramic or SiO2 fibers. On the surface of catalyst, the hydrocarbon gas is pyrolyzed and carbon is deposited, which lead to growth of carbon nanofibers. Enrique et al. synthesized high purity CNFs by using carbon source as CH4/H2, ceramic substrate and Ni as catalyst at 873K. The different carbon sources and temperature influence the formation of CNFs like its layered thickness, porosity and uniformity. High purity CNFs can be synthesized by substrate method but the CNFs can only grow on substrate dispersed with catalyst particles. It is difficult to achieve large scale continuous production of CNFs by substrate method, as nanoscale catalyst preparation is difficult and the product and catalyst cannot be separated in time. The substrate method contains further three sub-parts, which are spray method, gas phase flow catalytic method, and plasma-enhanced chemical vapor deposition method [28].

5.2.3.1

The spray method

The number of CNFs prepared by spray method is very less. The liquid organic substances like benzene are mixed with catalyst in this spray method technique. Later to obtain CNFs, spraying of this mixed solution with catalyst is done at high temperature. The spray method is favorable for continuous production of CNFs on industrial scale. The major drawback of these method is that the catalyst particles are not even distributed during spraying process which leads to small production of CNFs and large amount of carbon black being formed [29,30].

5.2.3.2

The gas-phase flow catalytic method

In gas-phase flow catalytic method, the catalyst precursor is heated and introduced to the reaction chamber with hydrocarbon gas. The decomposition of both catalyst and hydrocarbon takes place at different temperatures leading to formation of decomposed catalyst atoms that are graduated toward nanosize. Hence, the CNFs are produced on these nanoscale catalyst particles. As the amount of volatilization of catalyst can be controlled and the catalyst compound decomposed from organic compound can be distributed in 3D space, the amount of CNTs produced is large and continuous [31,32].

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5.2.3.3 Plasma-enhanced chemical vapor deposition In chemical vapor deposition process, the required activation energy is provided by plasma, as plasma contains a large number of high-energy electrons. The various chemical groups with high activity are generated with collision of electrons with gas phase which results in decomposition, compounding, excitation, and ionization processes of gas molecule. Although plasma-enhanced chemical vapor deposition method can produce aligned CNFs, it has the major drawbacks of difficulty to control the process, low production efficiency, and high cost [33 35].

5.3

Fabrication/construction of carbon nanofibers

With the rapid development in nanofabrication technology, more carbon-based nanomaterials have been recently used as gas sensors for detecting different toxic gases. Classification of the carbon-based nanofibers used as sensors is done into five types, depending on the type of material being loaded on them which is pure CNFs, metal nanoparticles loaded on CNFs, metal oxides loaded on CNFs, metal alloys loaded on CNFs and others. The CNFs used as gas sensors are mostly CNFs modified with metal oxides.

5.3.1

Carbon nanofibers modified with metal oxides

Some toxic gases cause change in electrical resistance of metal oxide that are decorated on CNFs. For the detection of specific gas and organic gas, this metal oxide decorated CNFs are used as gas sensors. Lee and coworkers synthesized ZnO/SnO2 CNFs by electrospinning method using two phase-separated polymer solutions. The ZnO/SnO2 nanonodules were decorated on CNFs by single nozzle co-electrospinning for detection of dimethyl methyl phosphonate gas [36]. Later, this group modified CNFs by decorating WO3 nanonodule on CNFs, which was used to detect NO2 gas. The sensor shows increased sensitivity due to WO3 nanonodule-decorated CNFs which increased the amount of the sensor surface [37]. Xia and coworkers synthesized CNFs by electrospinning method which were further calcinated. The porous CNFs contained ultrafine transition metal oxide like ZnO, MnO, and CoO nanoparticles. The hybrid metal oxide like ZnO, MnO and CoO with CNFs sensor materials contain lots of abundant interconnected pores. The porous CNFs contains homogeneously distributed transition elements like Zn, Mn, and Co elements [38].

Chapter 5 Carbon nanofiber-based gas sensors

5.4 5.4.1

Carbon nanofibers as gas sensors ZnO/CNFs

Zhang and coworkers synthesized ZnO/CNFs by electrospinning method, which was further used to sense H2S gas. The resistance of ZnO/CNFs to H2S gas varied from 62.9 to 0.616 M Ω while that of pure ZnO varied from 144 to 1.988 M Ω. As the concentration of H2S gas increases, the sensor response also increases. ZnO/CNFs show enhanced response than pure ZnO. When ZnO/CNFs nanofiber sensors are exposed to 1 ppm of H2S gas, the sensor response was found to be 2.55 which was higher than that of 1.98 for pure ZnO nanofiber. The recovery time depends on the factors like porosity, grain size and surface area. Since pure ZnO are porous, recovery time required is less while ZnO/CNFs has higher recovery time due to its dense and sold structure. The ZnO/CNFs sensor shows maximum response time at 250 C for H2S gas. ZnO/CNFs sensor shows high selectivity to H2S gas as compared to other gases like hydrogen, water, nitrous oxide, methane and SO2 at 250 C. High selectivity of H2S gas toward ZnO/CNFs sensor is mainly due to two factorsFirst, H-SH bond energy is very less hence can be broken easily and hence participate in chemical adsorption at low temperature. Second, the electron transport and diffusion both are possible in ZnO/CNFs sensor. ZnO/CNFs sensor shows a long-term stability up to 60 days for 20 ppm of H2S gas while pure ZnO shows a falling trend. When ZnO/CNFs sensor are exposed to air, it adsorbs oxygen molecules and these oxygen molecules trap the electrons from the sensor surface and leads to formation of depletion layer. Further when sensor is exposed to H2S gas, the H2S that are close to sensor surface reacts with oxygen species. During the reaction process oxygen is removed from the surface and electron transfer is chaotic. ZnO has band gap of 3.2 eV and ZnS has band gap of 3.6 eV. Although ZnO can react with H2S to form ZnS but ZnS not contribute to the decrease of resistance of air and improves the response of sensor. In both ZnO/CNFs and pure ZnO sensor the resistance gradually increases from 150 C to 250 C and decreases gradually from 250 C to 400 C. The activation of oxygen species like O2, O2, and O22, increase the resistance of sensor due to which there is enlargement of depletion layer leading to enhancement of resistance [39].

5.4.2

Sn SnO2/CNFs

Yanand and Wu synthesized micro-pored Sn SnO2/carbon heterostructure nanofibers. The amorphous carbon and homogeneously distributed Sn SnO2 nanoparticles lead to the

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composition of hetero-nanofibers. As temperature increases, Sn SnO2/CNFs sensor response increases till optimum temperature and then decreases with the increase in temperature. Sn SnO2/CNFs sensor shows maximum response at 220 C which is 46.15 seconds, while SnO2 NFs shows maximum response at 280 C which is 15.16 seconds for ethanol gas. There is twofold enhancement in response value toward ethanol gas by Sn SnO2/ CNFs sensor. The response of Sn SnO2/CNFs to different concentration of ethanol varied from 10 to 2500 ppm at 240 C, which showed that as concentration of ethanol gas increases the response also increases till 2000 ppm after that gets more or less saturated. The Sn SnO2/CNFs sensor show higher selectivity toward ethanol gas as compared to other gases like methanol, methane, carbon monoxide and ammonia toward 500 ppm at 240 C. The Sn SnO2/CNFs sensor when compared with SnO2 nanofibers give high selectivity to ethanol gas whereas for other gases SnO2 nanofiber shows better response than Sn SnO2/CNFs sensor (Fig. 5.2). The enhanced response of Sn SnO2/CNFs sensor is due to the adsorbed reactive oxygen species (O22 or O2) that traps conduction electrons and thus leads to decrease in electrical conductivity. The ethanol gas being reducing gas reacts with oxygen adsorbates and releases electron back to the conduction band of SnO2 and increases conductivity. At low temperature, the SnO2 nanofibers sensor surface has relatively small coverage of chemisorbed oxygen and hence small change of resistance when exposed to target gas. SnO2 nanofibers has higher resistance at this temperature and its response to ethanol is low. Whereas, Sn SnO2/CNFs hybrid system containing Sn-core and SnO2-shell

Figure 5.2 Response of Sn SnO2/CNFs and SnO2 NFs to 500 ppm of different gases at 240˚C [40].

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nanoparticles which are distributed throughout carbon nanofibers. The rapid electron transfer throughout the Sn SnO2/CNFs sensor material is due to high conductivity of metallic Sn and CNFs. The reduction of resistance leads to high response at lower temperature. With increase in temperature, more oxygen adsorbates are formed on the surface of the sensor leading to the decrease of resistance of SnO2. Hence pure SnO2 nanofiber shows higher response at high operating temperature while Sn SnO2/CNFs shows higher response at low operating temperature making it more energy efficient. The resistance of Sn SnO2/CNFs sensor decreases rapidly when exposed to ethanol gas and recovers back to initial value after gas is released (Fig. 5.3). The recovery time of Sn SnO2/CNFs sensor is short compared to SnO2 nanofiber due to presence of carbon as support system [40].

5.4.3

CNFs/polystyrene

Zhang and team synthesized conductive composite CNFs/ polystyrene (CNFs/PS) by solution mixing method. First, the CNFs and carbon black were synthesized by vapor grown method. The filling of polystyrene was done by fillers where hybrid CNFs and carbon black was filled in polystyrene. For sensing, gases like ethyl acetate, acetone, ethanol, tetrahydrofuran (THF), benzene, toluene, chloroform, n-hexane, methanol, cyclohexane; and carbon tetrachloride; were studied for their response. The THF shows maximum response to the hybrid CNFs/PS sensor. The CNFs/PS sensor resistance remains almost same, which shows good repeatability and stability for THF gas. The CNFs/PS sensor shows high selectivity toward THF and less polar gases while shows poor selectivity toward polar gases like ethanol, methanol and n-propanol as they are highly polar and forms hydrogen bonding compounds. At different temperatures

Figure 5.3 Gas sensing mechanism of Sn SnO2/CNFs heterostructure including target gas reacting with reactive oxygen and electron transfer [40].

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like 25 C, 35 C, 45 C the gas sensitivity varies. The sensor shows low resistance response at higher temperature. The sensor shows maximum electrical response about 2 3 105 times at 15 C to about 3.4 3 104 times at 55 C [41].

5.4.4

SnO2/CNFs

Wang and team synthesized SnO2/CNFs sensor by electrospun and hydrothermal method. The SnO2/CNFs sensor shows high selectivity to H2 gas as compared to other gases like CH4, butane, toluene ethanol, acetone, and CO at 200 C for 100 ppm. As SnO2 nanosheets are on surface of CNFs and doesn’t gather into microsphere, leading to more surface for sensing. Oxygen adsorbs on surface of SnO2 conduction band, when sensor is exposed to air. Further, oxygen ionizes to O2 and O22 and hence leads to formation of depletion layer. When reducing gas like H2 reacts with SnO2/CNFs sensor, there is reaction between adsorbed oxygen and H2 gas. These reaction releases trapped electrons back to conduction band. The p-type CNFs and ntype SnO2 leads to formation of p n heterojunction, which results in improved gas selectivity. In SnO2/CNFs sensor two depletion layer exist. First is on the surface of the SnO2 nanosheet and second in the interface between CNFs and SnO2 nanosheet [42].

5.4.5

V2O5/CNFs

Santangelo and coworkers synthesized V2O5/CNFs as gas sensors. By modified atomic layer deposition method tubular CNFs were coated with vanadium oxide. The thickness of vanadium oxide layer is 5 nm. There is formation of p n heterojunction between the sensor material, as CNFs acts as p-type and V2O5 as n-type material. The V2O5/CNFs sensor when exposed to NO2 gas show sensitivity due to morphology of adsorptive layer and heating of senor material. The electronic properties of sensor film are affected by formation of nanocrystalline domains and oxidation to V2O5 which give rise to changes in sensor response upon thermal treatment. The optimum temperature for this sensor is 220 C. Above this temperature the response is lowered and leads to degradation of film. Owing to increase in temperature of 225 C, the film deposition densification increases and also structural order in oxide layer increases. Further, the relative width of depletion layer inside the oxide film and at the interface between oxide and CNFs changes giving rise to improved senor response to NO2 gas [43].

Chapter 5 Carbon nanofiber-based gas sensors

5.4.6

115

Au-Pt/CNFs

Nair and team synthesized bimetallic Au Pt nanoisland functionalized CNFs by electrospinning method and then by chemical reduction method (Fig. 5.4). The resistance of Au Pt/ CNFs sensor decreases when exposed to H2 gas. When exposed to H2 gas, the CNFs/Pt nanoislands sensor shows response in range of 0.01% 4% and sensitivity in range of 1.9% to 36.9%. When the surface of CNFs is anchored with platinum nanoislands, due to increased surface area the adsorption capacity of H2 is high. The CNFs/Pt nanoislands sensor requires more recovery time to attain base resistance, as there is formation of platinum hydride. The CNFs/Au Pt nanoislands sensor shows sensitivity in range of 2.6% 47% toward H2 gas from concentration of 0.01% 4%. When CNFs/Au Pt nanoislands sensor is in contact with reductive gas like H2, the H2 molecules get absorb on Au Pt nanoislands on CNFs and dissociates into hydrogen atoms leading to electron accumulation at interface. The formation of platinum hydride is not possible in CNFs/Au Pt nanoislands sensor due to the integration of Au Pt bimetallic nanoislands. Also, owing to bimetallic Au Pt over CNFs the catalytic sites are enhanced to H2 gas. When compared to other gases like hydrogen sulfide, ethanol ammonia, acetone, and nitrogen dioxide the CNFs/Au Pt nanoislands sensor shows

Figure 5.4 Synthesis process for fabrication of CNFs/Au Pt nanoislands [44]. r 2020 American Chemical Society.

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high selectivity toward H2 gas. Owing to decoration of Au Pt nanoislands over CNFs, the response time and recovery time is greatly enhanced. The CNFs/Au Pt nanoislands sensor shows a short response time of 6.6 seconds with a recovery time of 18 seconds toward 0.5% of H2 gas (Fig. 5.5). As compared to other sensors, the CNFs/Au Pt nanoislands sensor shows rapid response and recovery time at room temperature for wide range of H2 gas [44].

5.4.7

Multifunctional carbon nanofibers

Monereo and team synthesized flexible gas sensor based on CNFs. These CNFs were deposited in interdigital silver electrode via spray technique. The sensor shows low response to CO gas but shows high response for NH3 and H2O. With increase in concentration of NH3 and H2O, the response also increases. The response and recovery time for NH3 gas was 35 and 45 minutes respectively. The response time and recovery time for H2O was same as that for NH3, only the baseline value was different. When relative humidity is 0%, NH3 sensing response is low but

Figure 5.5 Gas sensing mechanism of CNFs/Pt nanoislands and CNFs/Au Pt nanoislands toward H2 gas [44]. r 2020 American Chemical Society.

Chapter 5 Carbon nanofiber-based gas sensors

when relative humidity is 50%, the response time increases. The gas detection properties at room temperature showed good selectivity and requires moderate power consumption [45]. CNFs was dispersed in solution and electrospray over interdigital electrode (IDE). IDE was held over ceramic substrate with an integrated heater and a thermo-resistance. When sensor is exposed to H2O molecules, the H2O gets adsorb on defective site of active sensor surface. H2O acts as electron donor in p-type semiconductor like CNFs and hence resistance increases. Self-heating is an effective way to modulate the detection of gases. As temperature increases, resistance rapidly decreases and lowering response signal in case of NH3 gas. But in case of NO2 gas, as temperature increases response also increases. Self-heating in CNFs is effective as it contains low power consumption and enhanced sensing. Self-heating is possible with temperature up to 225 C and power consumption in the range of tens of mW. Also, self-heating activates the response of sensor toward gases like NH3, NO2 and H2O. The main limitation of self-heating is that at high temperature the carbonous material degrade easily forming carbon, CO and CO2 etc. [46].

5.4.8

Mesoporous carbon nanofibers

Liao and workers synthesized mesoporous CNFs network as gas sensors. The gas sensing performance of MCNFs is better than that of pure CNFs, as MCNFs has large surface area. The electronic properties of MCNFs are altered by direct charge transfer between the gas and nanofiber surface, during gas sensing process. When MCNFs sensor is in contact with electron acceptor gas like NO2, then NO2 accepts electron from valance band of MCNFs which leads to increase in hole density and decrease in resistance. There is change in electronic and transport properties of CNFs due to charge transfer and fluctuation, when gas gets adsorbed on carbon materials. As the concentration of gas increases from 5 to 40 ppm, the response of sensor also increases. The 5 ppm of NO2 gas shows shorter response time of 10 seconds. The MCNFs sensor shows better selectivity and fast response to NO2 gas but not stable for a long run time [47].

5.4.9

WO3/CNFs

Lee and team synthesized WO3 nanonodule decorated on CNFs by electrospinning method. When NO2 gas is adsorbed on CNFs surface, there is a formation of continuous pathway for change carriers. NO2 gas acts as strong electron acceptor. When

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NO2 gas is in contact with CNFs, attracts electrons from CNFs and forms NO gas and O22. Which further leads to formation of holes in CNFs. The p-type CNFs and n-type WO3 leads to formation of p n heterojunction. Hence, more electrons are flowed from CNFs toward NO2 gas. When sensor was tested for stability, the response and sensitivity do not change for ten cycles and the result is almost similar. As thermal energy at room temperature is not enough to overcome the activation energy of desorption, so the WO3/CNFs sensor requires few days for full recovery at room temperature for NO2 gas [37].

5.4.10

Ni/CNFs

Lii and coworkers synthesized porous hollow CNFs with functionalized nickel nanoparticles by anodic aluminum oxide template. The porous hollow CNFs/Ni sensor have good adsorption and better performance for NOx gas. The porous hollow CNFs/Ni sensor shows fast response time to NOx gas and good selectivity in air at room temperature. As the concentration of NOx gas increases from 0.97 to 97 ppm, the response also increases. The porous hollow CNFs/Ni sensor shows selectivity of 65.4% and response time of 9 seconds to 97 ppm of NOx gas. The lowest detection limit of porous hollow CNFs/Ni sensor was 0.97 ppm for which selectivity was 27.8%. The porous hollow CNFs/Ni sensor when exposed to NOx gas, the resistance decreases as NOx molecules acts as electron acceptors and electron moves from porous hollow CNFs/Ni sensor to NOx gas. Hence porous hollow CNFs/Ni sensor has p-type conductivity as CNFs sensor. The response of porous hollow CNFs/Ni sensor is better than that of porous hollow CNFs sensor toward NOx gas, as nickel nanoparticles acts as active centers of chemical adsorption. Hence, porous hollow CNFs/Ni sensor shows high selectivity and good stability at room temperature for NOx gas. As concentration of NOx gas increases, the sensitivity and response time both increases. When porous hollow CNFs/Ni sensor is in contact with electron withdrawing group like NOx, the nitrogen of NOx is adsorbed on the metal nickel. First, the electrons transfer from outer carbon layer of porous hollow CNFs to nickel atoms and then after that move to NOx molecules. Hence, p-type conductivity due to which number of holes increases, conductivity increases and resistance decreases. The strongly bound NOx Ni CNFs leads to slow recovery of sensor at room temperature. This method is inexpensive, pollution free, direct fabrication and gives good gas sensing properties [48].

Chapter 5 Carbon nanofiber-based gas sensors

5.4.11

CNFs/PPy

Jang and team synthesized CNFs/PPy coaxial nanocables by one step vapor deposition polymerization method. The CNFs/PPy sensor has enhanced sensitivity compared to pristine CNFs and PPy. There are more active sites for PPy for gas adsorption in CNFs/PPy sensor than pristine PPy. The CNFs/PPy sensor has surface area about three times greater than that of pristine PPy. The sensing mechanism of CNFs/PPy sensor depends on oxidation level of PPy layer. The response of CNFs/PPy sensor is extremely high than that of PPy, when exposed to NH3 and HCl gas of 20 ppm. The CNFs/PPy sensor when exposed to target gases like NH3 and HCl, a de-doping reaction rapidly occurs. When NH3 gas interacts with CNFs/PPy sensor the electrical conductivity decreases but when exposed to HCl gas electrical conductivity increases. As PPy is in doped state in CNFs/PPy sensor, the response of HCl was smaller than that of NH3. Also, the CNFs/PPy sensor shows longer recovery time for NH3 than HCl gas. These is due to dedoping reaction associated with NH3 shows higher reaction potential and a short reaction time than interaction with HCl gas. The response of PPy coated CNFs were reversible and reproducible after interaction with NH3 and HCl gas [49].

5.4.12

WS2/CNFs

Lee and coworkers synthesized WS2 functionalized multichannel carbon nanofibers by electrospinning method. The response of WS2/CNFs sensor is much higher than that compared to NH3 and C7H8 gas. The pure CNFs shows response of 0.8% and 0.45% for 5 ppm of NH3 and C7H8 gas respectively. The WS2/CNFs sensor shows response of 1.8% toward NO2 gas at very low concentration of 20 ppb. Humidity affects the sensor response to NO2 gas. There was 70% decrease in response of NO2 gas at 1 ppm. When NO2 gas is exposed to CNFs sensor, it attracts electrons from CNFs. Hence increase in number of holes which leads to decrease in resistance. WS2/CNFs sensor when exposed to NO2 gas molecules are attracted to WS2 far more strongly than to CNFs. Hence superior sensing is observed compared to pure CNFs. A high response of 15% was observed for 1 ppm of NO2 gas [50].

5.4.13

Ni-CNF

Alali and coworkers synthesized 3D Ni-MWCNT/CNFs hybrid gas sensor. MWCNT were grown on CNFs via one step electrospinning followed by CVD treatment (Fig. 5.6). The growth of

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Figure 5.6 Synthesizing processes of carbon nanofibers by electrospinning method [51].

CNTs on CNFs has greatly enhanced the sensitivity toward Dimethyl methyl phosphonate (DMMP) vapor as compared to pure CNTs, pure CNFs and Ni/CNFs composite. The 3D NiMWCNT/CNFs sensor shows rapid response time and recovery time of 11 and 13 seconds respectively for 1 ppm of DMMP at 25 C. As compared to other VOCs, the 3D Ni-MWCNT/CNFs sensor shows more selectivity toward DMMMP as it has polarity of 3.62 D and acceptor group like OCH3 which generates strong hydrogen bonding. Sensor shows excellent repeatability after 6 test cycles at 1 ppm concentration of DMMP, which means that the adsorption-desorption process of DMMP molecule on sensing surface is not affected after 6 cycles. When 3D Ni-MWCNT/CNFs sensor is exposed to air, as sensing surface is p-type containing large number of holes on surface with high conductivity. When 3D Ni-MWCNT/CNFs sensor are exposed to DMMP, the sensor absorbs DMMP molecules on its surface and electron are transferred from DMMP to surface of sensor material. DMMP being electron donor; will donate electrons to senor and form a continuous pathway for charge carrier. Hence,

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number of holes will decrease, conductivity will be low and resistance increases (Fig. 5.7). The enhanced 3D Ni-MWCNT/CNFs sensor response is due to enlarge surface area of sensor, hybrid material having high charge carrier density and high ratio of defect sites in carbon structure which helps to absorb many functional groups such as COOH, OH on side walls and defects of CNTs and CNFs. The 3D Ni-MWCNT/CNFs sensor shows high selectivity toward DMMP vapor at 25 C with response of 18.8 and 15 seconds at 1 ppm and 100 ppb respectively [51].

5.4.14

Graphitic carbon nanofibers

Li and team synthesized graphitic CNFs as gas sensor. While synthesis of CNFs, the CNFs heated to 800 C were labeled as

Figure 5.7 Sensing mechanism of hybrid 3D Ni-CNTs/CNFs with the change in the energy band gap in the air and dimethyl methyl phosphonate ambient [51].

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CNFs-800 and CNFs heated to 1000 C are labeled as CNFs-1000. The CNFs show n-type response against H2, CO, CH4 gas as the resistance decreases on exposure to these gases. Although CNFs is normally p-type in air, but many gas sensors based on CNFs and graphene exhibit n-type response against reducing gases. Even though ethanol gas is reducing gas, CNFs shows p-type behavior. The CNFs sensors can be fully recovered in air within range of tens of seconds to 3 minutes. During sensing process, the gas molecules gets adsorbed on the defect surface of CNFs with oxygenated groups, which further results in electron transfer to CNFs and change of resistance. The unique structure and defect sites play an important role in increasing conductance response toward H2, CO, and CH4 gas. The CNFs-800 sensor shows response time of 151, 144, 247, 40 seconds to H2, CO and CH4 and ethanol gases respectively. The CNFs-800 sensor shows recovery time of 65, 53, 526, 135 seconds towards H2, CO and CH4 and ethanol gases respectively. The CNFs-1000 sensor shows response time of 193, 97, 97, 75 seconds to H2, CO and CH4 and ethanol gases respectively. The CNFs-1000 sensor shows recovery time of 30, 50, 42, 195 seconds to H2, CO and CH4 and ethanol gases respectively [52].

5.4.15

Graphitic-carbon nanofibers/polyacrylate

Li and coworkers synthesized graphitic CNFs/polyacrylate polymer (GCNFs/PAA). The GCNFs/PAA sensor shows very high response to acetone gas than methanol gas and toluene gas and very low response to chloroform gas. The functionalized CNFs shows better response than bare CNFs. The complete desorption of acetone is not possible as there is strong adsorption acetone vapor during detection cycles. The GCNFs-PBA functionalized by butyl acrylate polymer show response ashexane , chloroform , methanol , toluene , acetone , THF. The GCNFs/PAA sensor shows very high response to methanol vapor than THF, due to strong hydrogen bonding of methanol and surface carboxylic acid groups. When strong hydrogen bonding interaction occurs between analyte gas and GCNFsPAA polymer sensor, the response is greatly enhanced [53].

5.4.16

PAN/(PAN-b-PMMA)

Bhati and team synthesized nanoporous carbon nanofibers loaded on ZnO nanostructures. Porous CNFs with high surface area were obtained from polymer blend of PAN and PAN-bPMMA. Further these CNFs is loaded on Au pattern inter-digital electrodes over ZnO surface. The formation of p n heterojunction

Chapter 5 Carbon nanofiber-based gas sensors

between p-type CNFs and n-type ZnO leads to increase of adsorption. When PAN/(PAN-b-PMMA) sensor is exposed to air, oxygen molecules interact with electrons from ZnO surface. As oxygen is more electronegative than ZnO, oxygen gas extract electrons and forms oxygen ions leading to the formation of depletion layer on ZnO surface. Further as resistance increase, the depletion layer and barrier height increases. When PAN/(PAN-b-PMMA) sensor are exposed to hydrogen gas, the oxygen ions interact with hydrogen gas and electrons are liberated from oxygen species to conduction band. Hence, there is reduction of depletion layer. The PAN/(PANb-PMMA) sensor response is high compared to that of pure CNFs and pure ZnO, which is due to mainly two reasons. First, CNFs contains large number of nanoporous structures with high surface area through which the hydrogen gas can easily diffuse. Second, there is creations of more depletion and increased increases number of oxygen ions due to the formation of p n heterojunction (Fig. 5.8). The maximum response is obtained for 0.2 wt.% CNFs/ ZnO nanostructures for 100 ppm of hydrogen gas at 150 C [54].

5.4.17

5,6;11,12-di-o-phenlyenetetracene/carbon nanofibers

Wombacher and team synthesized 5,6;11,12-di-o-phenlyenetetracene/CNFs at 1000 C on various substrates such as Cu foil, SiO2/Si, and SiO2/Si coated with Pt/Pd layer. The thickness of Pt/Pd layer was about 20 nm. In case of hybrid sensing material, two sensing mechanisms are possible. First, gas molecules adsorb on Pt/Pd nanoparticle, and there is charge transfer between nanoparticle and CNFs. This charge transfer leads to change in resistance. Second, there is formation of continuous pathway for charge carriers between two electrodes as gas molecules adsorb on to the surface of CNFs network. Depending on the analyte gas, electrons are either transferred from CNFs to the gases like NO2 and SO2, which act as electron acceptor or from gas molecules like NH3, which acts as an electron donor to CNFs. The sensor when exposed to NO2 or SO2 gas, number of holes increases in CNFs and hence resistance decreases. CNFs acts as p-type material. When sensor is exposed to NH3 gas, the number of electrons increases and hence resistance decreases. Limit of detection for time exposure of 20 minutes at 30 C is 4 ppm for NO2, 16 ppm for NH3, and 4 ppm for SO2. These allfabricated sensors like 5,6;11,12-di-o-phenlyenetetracene/CNFs show better response to low concentration of NO2 and SO2 gas, whereas change in resistance is negligibly low for an exposure of 10 50 ppm of NH3 gas [55].

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Figure 5.8 Band diagram demonstration of CNFs/ZnO nanostructures (A) before contact and (B C) after contact (presence of air and hydrogen) and (D E) schematic diagram for hydrogen sensing mechanism [54].

Chapter 5 Carbon nanofiber-based gas sensors

References [1] X. Huang, Fabrication and properties of carbon fibers, Materials 2 (2009) 2369 2403. [2] C. Xiang, N. Behabtu, Y. Liu, H.G. Chae, C.C. Young, B. Genorio, et al., ACS Nano 7 (2013) 1628 1637. [3] W. Lu, M. Zu, J.H. Byun, B.S. Kim, T.W. Chou, Adv. Mater. 24 (2012) 1805 1833. [4] S. Chand, Review carbon fibers for composites, J. Mater. Sci. 35 (2000) 1303 1313. [5] Z. Wangxi, L. Jie, W. Gang, Carbon 41 (2003) 2805 2812. [6] J.L. Vilaplana, F.J. Baeza, O. Galao, E. Zornoza, P. Garce´s, Materials 6 (2013) 4776 4786. [7] F.J. Baeza, O. Galao, E. Zornoza, P. Garce´s, Materials 6 (2013) 841 855. [8] M. Endo, Y.A. Kim, T. Hayashi, K. Nishimura, T. Matusita, K. Miyashita, et al., Carbon 39 (2001) 1287 1297. [9] J.H. Du, C. Sun, S. Bai, G. Su, Z. Ying, H.M. Cheng, J. Mater. Res. 17 (2002) 1232 1236. [10] F.Y. Wu, J.H. Du, C.G. Liu, L.X. Li, H.M. Cheng, N. Carbon Mater. 19 (2004) 81 86. [11] J.I. Hahm, Annu. Rev. Phys. Chem. 67 (2016) 691 717. [12] Q.X. Zhao, M.M. Zhao, J.Q. Qiu, W.Y. Lai, H. Pang, W. Huang, Small 13 (2017) 1701091. [13] W. Li, F. Zhang, Y.Q. Dou, Z.X. Wu, H.J. Liu, X.F. Qian, et al., Adv. Energy Mater. 1 (2011) 382 386. [14] M.M. Barsan, M.E. Ghica, C.M.A. Brett, Anal. Chim. Acta 881 (2015) 1 23. [15] M. Meyyappan, Carbon nanotube-based chemical sensors, Small 12 (2016) 2118 2129. [16] N.L. Teradal, R. Jelinek, Carbon nanomaterials in biological studies and biomedicine, Adv. Healthc. Mater. 6 (2017) 1700574. [17] A.M. AbdEl-Aziz, R.M. ElBackly, N.A. Taha, A. ElMaghraby, S.H. Kandil, Mater. Sci. Eng. C 76 (2017) 1188 1195. [18] L.F. Chen, Y. Lu, L. Yu, X.W. Lou, Energy Environ. Sci. 10 (2017) 1777 1783. [19] Y. Li, M.F. Zhang, X.P. Zhang, G.C. Xie, Z.Q. Su, G. Wei, Nanomaterials 5 (2015) 1891 1905. [20] J.R. Magana, Y.V. Kolen’ko, F.L. Deepak, C. Solans, R.G. Shrestha, J.P. Hill, et al., ACSAppl Mater. Interfaces 8 (2016) 31231 31238. [21] L.F. Chen, Y. Feng, H.W. Liang, Z.Y. Wu, S.H. Yu, Adv. Energy Mater. 7 (2017) 1700826. [22] Y. Shen, L. Li, K.J. Xiao, J.Y. Xi, ACS Sustain. Chem. Eng. 4 (2016) 2351 2358. [23] L.F. Zhang, A. Aboagye, A. Kelkar, C.L. Lai, H. Fong, J. Mater. Sci. 49 (2014) 463 480. [24] L.C. Feng, N. Xie, J. Zhong, Materials 7 (2014) 3919 3945. Available from: https://doi.org/10.3390/ma7053919. [25] B.A. Zhang, F.Y. Kang, J.M. Tarascon, J.K. Kim, Prog. Mater. Sci. 76 (2016) 319 380. [26] Z. Wang, S. Wu, J. Wang, A. Yu, Nanomaterials 9 (7) (2019) 1045. Available from: https://doi.org/10.3390/nano9071045. [27] A. Rogina, Appl. Surf. Sci. 296 (2014) 221 230. [28] E. Garcia-Bordeje, I. Kvande, D. Chen, M. Ronning, Carbon 45 (2007) 1828 1838.

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[29] M. Endo, K. Takeuchi, T. Hiraoka, T. Furuta, T. Kasai, X. Sun, et al., Chem. Solids 58 (1997) 1707 1712. [30] H.J. Guan, J. Zhang, Y. Liu, Y.F. Zhao, B. Zhang, Electrochim. Acta 295 (2019) 997 1005. [31] L.J. Ci, J.Q. Wei, B.Q. Wei, J. Liang, C.L. Xu, D.H. Wu, Carbon 39 (2001) 329 335. [32] Y.I. Bauman, I.V. Mishakov, Y.V. Rudneva, P.E. Plyusnin, Y.V. Shubin, D.V. Korneev, et al., Ind. Eng. Chem. Res. 58 (2019) 685 694. [33] M.A.R. Saidin, A.F. Ismail, S.M. Sanip, P.S. Goh, M. Aziz, M. Tanemura, Thin Solid Films 520 (2012) 2575 2581. [34] R. Shoukat, M.I. Khan, Electr. Eng. 100 (2018) 997 1002. [35] R. Gupta, S.C. Sharma, Contrib. Plasma Phys. 59 (2019) 72 85. [36] J.S. Lee, O.S. Kwon, S.J. Park, E.Y. Park, S.A. You, H. Yoon, et al., ACS Nano 5 (2011) 7992 8001. [37] J.S. Lee, O.S. Kwon, D.H. Shin, J. Jang, J. Mater. Chem. A 1 (2013) 9099 9106. [38] G.L. Xia, L.J. Zhang, F. Fang, D.L. Sun, Z.P. Guo, H.K. Liu, et al., Adv. Funct. Mater. 26 (2016) 6188 6196. [39] J. Zhang, Z. Zhu, C. Chen, Z. Chen, M. Cai, B. Qu, et al., Nanotechnology (2018). [40] S. Yan, Q. Wu, Sens. Actuators B 205 (2014) 329 337. [41] B. Zhang, R. Fu, M. Zhang, X. Dong, L. Wang, C.U. Pittman, Jr. Composites: Part A 37 (2006) 1884 1889. [42] Z. Wang, S. Liu, T. Jiang, X. Xu, J. Zhang, C. An, et al., RSC Adv. 5 (2015) 64582. [43] S. Santangelo, G. Messina, G. Faggio, M.-G. Willinger, N. Pinna, A. Donato, et al., Diam. Relat. Mater. 19 (2010) 590 594. [44] K.G. Nair, V. Ramakrishnan, R. Unnathpadi, K.K. Karuppanan, B. Pullithadathil, J. Phys. Chem. C 124 (2020) 7144 7155. [45] O. Monereo, S. Claramunt, M. Martı´nez de Marigorta, M. Boix, R. Leghrib, J.D. Prades, et al., Talanta 107 (2013) 239 247. [46] O. Monereo, J.D. Prades, A. Cirera, Sens. Actuators B 211 (2015) 489 497. [47] L. Liao, M. Zheng, Z. Zhang, B. Yan, X. Chang, G. Ji, et al., Carbon 47 (2009) 1841 1845. [48] R. Lu, K. Shi, W. Zhou, L. Wang, C. Tian, K. Pan, et al., J. Mater. Chem. 22 (2012) 24814. [49] J. Jang, J. Bae, Sens. Actuators B 122 (2007) 7 13. [50] J.-H. Cha, S.-J. Choi, S. Yu, I.-D. Kim, J. Mater. Chem. A 5 (2017) 8725. [51] K.T. Alali, J. Liu, K. Aljebawi, Q. Liu, R. Chen, J. Yu, et al. J. Alloys Comp. [52] W. Li, L.-S. Zhang, Q. Wang, Y. Yu, Z. Chen, C.-Y. Cao, et al., J. Mater. Chem. 22 (2012) 15342. [53] L. Li, J. Li, C.M. Lukehart, Sens. Actuators B 130 (2008) 783 788. [54] V.S. Bhati, A. Nathani, A. Nigam, C.S. Sharma, M. Kumar, Sens. Actuators: B Chem. 299 (2019) 126980. [55] P. Krauß, T. Wombacher, J.J. Schneider, RSC Adv. 7 (2017) 45185.

6 Graphene-based gas sensors Akshara Paresh Shah1, Shilpa Jain2 and Navinchandra Gopal Shimpi1 1

Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India 2Department of Chemistry, Jai Hind College, Mumbai, Maharashtra, India

With the increasing growth of current industry, detection of hazardous gases has become challenging for living species and the emphasis on the environment. Sensors play a significant role in the sensing of the surrounding gas environment. Gas sensor uses specific effects (physical & chemical) to convert components and concentrations of several gases into standard electrical signals [1] and to identify explosive, flammable toxic gases, and/or oxygen depletion [2
4]. The development of gas sensor technology has found wide applications in environmental monitoring, industrial plants, automotive technology, protection and medicine [5]. For the development of gas sensors with superior sensitivity, promptness of response, great selectivity, robustness, minimal power consumption, reproducibility, stability, and reversibility, novel and superior materials have been explored [6]. Various materials like carbon-based nanomaterials, noble metals, inorganic semiconductors, and conjugated polymers have been discovered for the development of superior gas sensors [7
11]. The progress of electronic tool is like miniaturization, combination and micronotarization lately. Nanomaterial is a key factor for gas sensor enhancement. Nonetheless, 0-dimensional and 1-dimensional nanomaterials, with a partial surface and electronic confinement effect, can’t extend the terms for the development of gas sensors. 2dimensional nanomaterial has a more specific surface area as compared to 0-D and 1-D nanomaterials according to several studies. Special membranous or lamellar structure has powerful potentiality for gas molecule absorption. Graphene-based gas sensors having excellent properties and two-dimensional atom-thick structure are excellent toward sensing [12
20]. Graphene with honeycomb lattice configuration and single layer carbon atomic structure have shown tremendous electronic, chemical, optical, mechanical, and thermal Carbon-Based Nanomaterials and Nanocomposites for Gas Sensing. DOI: https://doi.org/10.1016/B978-0-12-821345-2.00004-8 © 2023 Elsevier Inc. All rights reserved.

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properties. Gas molecule adsorption enhances detection sensitivity because of nanomaterials’ enormous surface area. The relationship between adsorbates and sheets of graphene could differ from van der Waals (weak) interactions to covalent bonding (strong). This relationship will disturb the graphene electronic system, examined by appropriate electronic methods [21]. Experimental and theoretical results indicated that reduced graphene oxide (rGO) and graphene oxide (GO) (derivatives of graphene) show more specific surface area (2630 m2/g), tremendous conductivity, high electron mobility (15,000 cm2/Vs), low electrical noise, and easy gas molecule adsorption (Fig. 6.1). Surface can be simply modified by functional groups, which makes them potential candidate for gas sensing through increasing its properties [13]. For modulate interactions with gaseous analytes and electronic structures, graphene derivatives can functionalize by chemically grafting or integrating with other sensing material [22,23]. Graphene-based materials have been extensively utilized for sensing toxic and volatile gases [24]. Several techniques for preparing graphene are described in the literatures. Among these, mechanical and chemical cleaving methods, epitaxial growth and chemical vapor deposition (CVD) are extensively used methodology. CVD method offers many advantages like large-scale, high-quality graphene synthesis on several substrates which is shown in Table 6.1. One of the authors have developed [26,27] an eco-friendly method for graphene synthesis (flower petals as carbon feedstock) to overcome the problem of utilizing hazardous chemicals like hydrazine, hydrocarbon gases, etc. Superior quality of graphene material can be developed over 800 C with nickel nanoparticles. This material eliminates the impurities of oxygen having good latency in graphene-based utilizations such as sensors.

Figure 6.1 Graphene properties.

Chapter 6 Graphene-based gas sensors

129

Table 6.1 Methods for graphene synthesis and its advantages and limitations [25]. Methods

Advantages

Micromechanical method High-quality material, simple and lowcost method, and poor control Chemical vapor High-quality material, large scale deposition (CVD) production that can be transferred relatively easily to any substrate, and good control Epitaxial growth High quality, and good control

Chemical exfoliation (reduction of GO)

Other methods (thermal exfoliation, unzipped Carbon nanotubes (CNTs etc)

6.1

Relatively cheap, large-scale synthesis possible, no need of substrate, presence of functional groups makes these reactive, and moderate control Mixed quality, and high cost

Limitations Production of only tens of microns in size. Not suitable for mass production Relatively costly, hazardous, and only grown on special substrates from which it needs to be transferred. Very inert due to the absence of functional groups Difficult to control surface morphology, high temperature process, interface role not well understood, and not for mass production Several process steps, use of hazardous chemicals, presence of residual oxygen and moisture, and poor quality with defects and functional groups Not suitable for mass production

Gas sensor mechanism

Chemiresistive type gas sensors are established on the principle of electrical property changes such as conductance, resistance, or work function. Graphene being a p-type semiconductor enhances its conductance in the presence of electron withdrawing gases such as NO2, NO, CO etc. While electron donating gases such as ammonia, methane etc. de-dopes the graphene and reduces its conductance [28]. In graphene-based gas sensors, sensing mechanism are based on conductance changes due to adsorption of analyte. Graphene and its derivatives interacts with gaseous analytes according to their structure and composition both physically and chemically. With adsorption of gas molecules, charge carrier concentration changes between graphene and analyte changes [29,30]. Such as NO2, halogens etc. (open-shell adsorbates) acts as temporary dopants and generates electrons or holes in graphene, which influences the charge carrier density [31,32]. They are bounded weakly to graphene layers by Van der Wal’s interaction. While H2O (closed-cell adsorbates) doesn’t induce localized impurity states however it redistributes

130

Chapter 6 Graphene-based gas sensors

the electron density inside or between the graphene sheets [33]. In pristine graphene with no functional groups, interaction is mainly due to defects. In case of graphene, line defects and edges are more dominant as compared to CNTs, where point defects play a major role [34]. Khojin et al. fabricated CVDgrown graphene nanoribbons (GNRs) gas sensors and found that these sensors had high sensitivity and better gas response as compared to defect-free graphene (Scotch-tape method) [35]. Graphene-polymer nanocomposites exhibit enhanced gas sensing because of more surface area and porous form, which lead to higher rate of gas diffusion [36
38]. In Fig. 6.2 both the polymer and graphene can adsorb the gas molecule and influence the charge carrier density and hence conductance of sensing layer (Fig. 6.2). In case of graphene-metal nanocomposites, nanoparticles such as Pt, Pd, Au etc. get impregnated on graphene sheets and catalyzes the gas adsorption process [39,40]. The adsorbed oxygen and water molecules also influence the sensing response. The adsorbed oxygen on graphene surface causes the generation

Figure 6.2 Catalytic and electronic mechanism general steps active in additives (metal oxide) and gas sensing layers (graphene).

Chapter 6 Graphene-based gas sensors

131

of various oxygen species such as oxide, peroxide and superoxide by catching e2s from metal oxide [41] as shown in Eqs. (6.1) and (6.2). In presence of analyte, conductance changes due to change in concentration of electron on metal oxide suface and sensing layer (Eqs. 6.3 and 6.4) [34,42], which influences the sensing. O2 ðgasÞ 1 e2 $O2 2 ðadsÞ

ð6:1Þ

22 2 2 O2 2 ðadsÞ 1 e $O2 ðadsÞ$2O ðadsÞ

ð6:2Þ

Reactions for reducing gas exposure such as CO are as follows:

6.2

CO1O2 ðadsÞ-CO2 1 e2

ð6:3Þ

2 2CO1O2 2 ðadsÞ-CO2 1 e

ð6:4Þ

Graphene and its derivative/metal-based gas sensor

For the fabrication of catalytic combustion sensors, noble metals have been selected due to their excessive catalytic properties to combustible gases [43]. Scientists have attached hybrid materials like noble metal nanostructures with graphene for sensing flammable gas. Table 6.2 the literature of noble metal graphene hybrids sensing performance in which topmost Table 6.2 Comparison of gas sensing performance of noble-metal/graphene hybrids [44]. Materials

Target gases OT (˚C) Detection range Lowest Detectable Concentration (LDC)

References

Pt/grapheme Pd/graphene Pt/graphene Pd/rGO Pd nanocube/rGO Pt/holey rGO Ag/rGO Pd/graphene Ag/rGO

H2 H2 H2 H2 H2 H2 NH3 H2 NO2

[45] [39] [46] [47] [48] [49] [50] [51] [52]

100 RT RT RT RT RT RT RT RT

0.06%
1% 30
1000 ppm 50 ppm
20% 200
3300 ppm 6
1000 ppm 60
40,000 ppm 15
100 ppm 20
1000 ppm 0.5
50 ppm

0.06% 30 ppm 50 ppm 50 ppm 6 ppm 60 ppm 15 ppm 20 ppm 0.5 ppm

132

Chapter 6 Graphene-based gas sensors

sensors worked at ambient condition. Nevertheless, topmost noble metal-based sensors control at extreme temperature [45,53] but due to synergistic effect of nanostructures of noble metal with graphene creates the hybrids performance at ambient temperature. The response of sensor was depending on interaction of surface effect among gases and metal nanoparticle (NP), then graphene conductivity changes close to the reaction interface. Accordingly, due to the balance in noble metals thickness, conduction paths will be produced to surface layer parallelly, which is not affected by gas concentration changes [49]. Noble metals have been expanded as carrier-transfer pathways along with their mechanism of sensing like catalysis sites. Chen et al. have synthesized Ag NP-decorated rGO hybrid nanostructure for ammonia sensing in a simple manner. The hybrid sensors (rGO/Ag) show a superior sensitivity than rGO and shows quick response (6 seconds) and recovery speed (10 seconds) toward NH3. Sensing sensitivity is affected by Ag NPs density with maximum loading [36]. Pak et al. have synthesized polymer residue-free GNRs via laser interference lithography. For hydrogen gas sensing, palladium nanoparticles as catalysts were deposited on the GNRs, and the GNR array was used as an electrically conductive path having less noise. The palladium-decorated GNR array showed a rectangular curve with fast response (90% within 60 seconds at 1000 ppm) and recovery (80% within 90 seconds) in nitrogen ambient. Moreover, repeatable and reliable behaviors of sensing were shown even at 30 ppm when the array was exposed to several gas concentrations (Fig. 6.3) [47]. Ghosh and his coworkers have fabricated highly selective and sensitive H2 gas sensors by using rGO
Pt composite at

Figure 6.3 Arbitrary concentrations of graphene nanoribbon sensor with cycles of response (5 min) and recovery (5 min).

Chapter 6 Graphene-based gas sensors

Figure 6.4 Mechanism of H2 sensing by reduced graphene oxide
Pt in N2 and air ambience.

ambient temperature. By changing the proportion of Pt and rGO, four samples were synthesized for their sensing performance, which was executed in air as well as inert (N2) atmosphere. Schematic representation of H2 sensing in N2 and air ambience of rGO:Pt is shown in Fig. 6.4. rGO:Pt ratio of 1 and 3, 1 hour showed the excellent H2 sensing behavior toward response time (diversed from 19% at 200 ppm to 57% at 5000 ppm), sensitivity and recovery time at ambient temperature in air ambience. Further, the response time (65 seconds) and recovery time (230 seconds) of the rGO:Pt ratio of 1 and 3 were found at 5000 ppm, respectively, in air atmosphere. While in N2 atmosphere, the rGO:Pt ratio of 1 and 3 gave the finest response (97%) at 500 ppm having poor recovery [54]. Huang and its coworkers fabricated Ag NP-decorated sulfonated rGO (Ag
S
rGO) for the detection of nitrogen dioxide at room temperature. Sensing response was found to be 74.6% within 12 seconds against 50 ppm and recovery was attained within 20 seconds at room temperature [55]. The chemical alteration of rGO with Ag NPs and sulfonic acid groups showed greater performance of this hybrid sensor. Even after 100 bending cycles, Ag
S
rGO based sensor resulted in excellent flexibility. This hybrid sensor could be utilized for real-time nitrogen dioxide detection because of its several benefits such as light weight, mechanical robustness, and easy handling. It is relatively simple to prepare gas sensor using a polymer substrate with noble metal graphene hybrids. Hydrogen gas sensor was prepared by utilizing single-layer graphene decorated with PdNPs on polyethylene terephthalate (PET) substrate, which showed a gas response (33% at 1000 ppm H2) and

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was able to find as low as 20 ppm hydrogen at room temperature [56]. The Pd/graphene sensor was so adjustable that any powerful degradation could not be detected when it was bent into a curve even though of 3 mm radius.

6.3

Graphene and its derivative/ metal oxidebased gas sensor

In current scenario, developing reliable gas sensor is in demand having vast applications in agriculture, ecological control, contaminated materials, industrial wastes, and medical diagnosis [57,58]. Metal oxide semiconductor (MOS) gas sensors are extensively used because of its quick response time and superior sensitivity [59]. For the detection of gases, semiconductor metal oxide-based gas sensor require high temperature, which limit their application in sensing at room temperature. Specific surface area is a key factor for material gas sensitivity, which is entire surface area of a material per mass unit [9]. Nanocomposites based on graphene have been proven as potential candidate for production of room temperature owing to flexible structure, more specific surface area, exclusive electronic property and tremendous adsorptivity [60]. Furthermore, the nanocomposites excellently counteract the cluster of metal oxide and irreversible restacking of graphene. Graphene
metal oxide nanocomposites have extensively utilized in solar cell, photocatalysis, lithium battery, drug delivery and gas sensor [61
65]. Table 6.3 exhibits the literature of metal oxide graphene hybrids Table 6.3 Comparison of gas sensing performance of metal oxide/graphene hybrids [7]. Materials

Target gases

Operating Detection temperature (˚C) range

Lowest detection concentration

References

ZnO/rGO Co3O4/rGO α-Fe2O3 /rGO WO3/rGO SnO2/rGO Cu2O/GO Flower-like SnO2/ rGO Nanoflower-like CuxO/graphene

NO2 NO2 Ethanol NO2 Benzene H2S NH3

RT RT 280 C 250 C 210 C RT RT

1
25 ppm 300
1000 ppm 1
1000 ppm 1
20 ppm 5
100 ppb 5
100 ppb 10
50 ppm

1 ppm 300 ppm 1 ppm 1 ppm 5 ppb 5 ppb 10 ppm

[66] [12] [67] [68] [69] [70] [71]

NOx

RT

97 ppb
97 ppm 97 ppb

[72]

Chapter 6 Graphene-based gas sensors

sensing performance in which topmost sensors worked at various temperature. The synergistic effect among metal oxide and graphene are the following: (1) during the synthesis, graphene controls morphology and the size of metal oxides; (2) conductivity of metal oxides increases due to graphene electrons transfer obtained from the metal oxide and surface reaction of gas molecules to electrodes; (3) p-n junctions, which modulate the space-charged layers among metal oxides and graphene at the interfaces; and aggregation of graphene is prevented by metal-oxide nanostructures [44]. Numerous researches on graphene
metal oxide materials have been described lately. Graphene shows great carrier mobility and measureable change in electrical resistance at room temperature after gas adsorption [73,74]. Metal oxides, such as WO3 [75], NiO [76], SnO2 [77], Fe2O3 [78] have been decorated on graphene nanosheets which is used in gas sensors. There is no unified description of MOS gas sensors mechanism till now. Oxygen-adsorption theory is described as in [79]: semiconductor electrons was catched by oxygen molecules for oxygen anions development. Operating temperature affects oxygen anions and below 147 C (operating temperature), the oxygen anions becomes O22. While increasing the temperature it gets converted into O2. The temperature over 397 C, O2 are transformed into O22. The reaction equations are as follow: O2 ðgasÞ-O2 ðadsÞ;

ð6:5Þ

O2 ðadsÞ 1 e2 -O2 2;

ð6:6Þ

O2 ðadsÞ 1 2e2 -2O2 ;

ð6:7Þ

O2 ðadsÞ 1 4e2 -2O22 :

ð6:8Þ

In case of graphene-metal oxide nanocomposites, gas sensing capability depends on several factors such as electron mobility of metal oxide, temperature, and surface area [80,81]. In case of n-type semiconductors where electrons are major charge carriers, electrons will be donated from the surface of semiconductor. In presence of oxidizing gas, electron depletion layer increases and hence electrical conductivity decreases. While in the presence of reducing gas electron depletion layer decreases and conductivity increases. For p-type semiconductors (holes are major charge carriers), hole accumulation layer enhances by catching electrons in presence of oxidizing gases and thus conductivity increases. While in presence of reducing

135

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Chapter 6 Graphene-based gas sensors

gases, electrons are free into the semiconductor which reduces the width of hole accumulation and sensor conductivity. These sensors work at higher temperature (above 200 C) [82
84]. There are many types of toxic gases like carbon monoxide, ammonia, nitrogen oxide, and most volatile organic compounds from industrial route and emissions of car around us. According to World Health Organization, people should not be exposed above 35 ppm NH3 to an environment for beyond 15 minutes or with over 10 ppm CO to an environment for above 10 minutes. [85]. Gas sensor having outstanding sensitivity, minimal cost and excellent selectivity is urgent needed for detecting poisonous gases. [25] graphene-metal oxide increases the sensing performance such as response time, sensitivity, operating temperature and recovery time as compared with MOS sensors. According to different opinion of researchers, there are several mechanisms for the gas sensing performances enhancement. The extensively accepted mechanisms are: synergetic effect among two components, semiconductor interfaces formation and improves structure and morphology due to graphene introduction. Sberveglieri and his coworkers have synthesized composite material of TiO2 NTs and rGO to study its sensing properties having structural effects of each material. For the enhancement of conductance, rGO platelets enhance charge transport through TiO2. Depletion layer among rGO plates and TiO2 plays important role for sensing response improvement. The rGO concentration and ideal value for functionalization of TiO2 should be considered. Variation of concentration and its reduction conditions should be considered to develop the detection of metal oxide and functional properties [86]. Wang et al. fabricated NO2 sensor using In2O3
rGO nanocomposite which showed high response (8.25), tremendous selectivity, short response (4 minutes) and recovery time (24 minutes) at 30 ppm nitrogen dioxide at room temperature. It shows gas sensor having In2O3
rGO nanocomposite would be potential candidate for examining NO2 gas in environment [87]. Now a days, there are several work on gas sensor having SnO2
graphene nanohybrid received much interests by the researchers [34,75,77,88,89]. Mao et al. [75] fabricated SnO2 nanocrystals (NCs) with rGO and gold (Au) as inter digitated electrodes (Fig. 6.5). SnO2 NCs were prepared (mini-arc reactor) then placed on rGO sheets [75]. This sensor exhibited tremendous response toward target gases at room temperature. SnO2 NPs prevent agglomeration of graphene, which leads to high specific area. Conductivity of material was increased by graphene deposition, which allowed the sensors to attain a

Chapter 6 Graphene-based gas sensors

137

Figure 6.5 (A) Representative presentation of the reduced graphene oxide sheet decorated with metal
oxide NCs gas sensor and (B) Representative presentation of the sensor testing system. Reproduced from X.Q. An, J.C. Yu, Y. Wang, Y.M. Hu, X.L. Yu, G.J. Zhang, WO3 nanorods/graphene nanocomposites for high-efficiency visible-light-driven photocatalysis and NO2 gas sensing, J. Mater. Chem. 22 (2012) 8525
8531 [75] with permission of The Royal Society of Chemistry.

superior response at low operating temperatures. Sensors response was observed to be 171 minutes, and response time was found to be 7 minutes against 5 ppm of NO2 at 25 C. More adsorption sites are available due to graphene introduction at the surface of the material, which improves significant response [90]. For unloaded n-type SnO2 NPs, oxygen species will extract e2 from conduction band to a depletion region on the surface of SnO2 NPs having larger width and barrier height, represented the physical model in Fig. 6.6A. Several e2s can transport through the NPs, which increases the resistivity. Hence the extent of increases, enhances with smaller NPs size because density of adsorption sites (NO2 distributed evenly) per particle volume will enhances as decrease in particle size shown in Fig. 6.6A. The slighter barrier height and depletion width will ease for electron transport, which results to low resistance in contrast to unloaded n-type SnO2 NPs (Fig. 6.6B and C). Zhang et al. prepared hybrid material rGO/SnO2/Au (rGO/ SnO2/Au) by hydrothermal process with gas sensing property at low operating temperature. This hybrid material was characterized by various techniques such as X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. The sizes of Au NPs of rGO/SnO2/Au-1 to rGO/SnO2/Au-4 range from about 12 to 90 nm, respectively. Chemical gas sensors have been fabricated for sensing NO2 gas using such hybrid nanomaterials. rGO/SnO2/Au sensor shows excellent response (19 seconds) as

138

Chapter 6 Graphene-based gas sensors

Figure 6.6 Representative physical models for nitrogen dioxide sensing mechanisms of (A) SnO2 nanoparticles and SnO2 nanoparticles loaded with graphene at (B) moderately low and (C) high graphene concentrations. Reproduced with permission from N. Tammanoon, A. Wisitsoraat, C. Sriprachuabwong, D. Phokharatkul, A. Tuantranont, S. Phanichphant, et al., Ultrasensitive NO2 sensor based on ohmic metal 2 semiconductor interfaces of electrolytically exfoliated graphene/flamespray-made SnO2 nanoparticles composite operating at low temperatures, ACS Appl. Mater. Interfaces 7 (2015) 24338
24352 [90], copyright 2015 American Chemical Society.

Chapter 6 Graphene-based gas sensors

compared to rGO/SnO2 (427 seconds) and pure rGO (798 seconds). Recovery time of rGO/SnO2/Au was observed much better (20 seconds) than rGO/SnO2 (908 seconds) and pure rGO (8319 s) at operating temperature (50 C) against 5 ppm NO2. The superior features of sensing can be attributed to heterojunction with extremely conductive graphene, Au NPs and SnO2 film [91]. rGO
Cu2O nanowire mesocrystals were prepared by nonclassical crystallization using hydrothermal conditions in the presence of o-anisidine and GO. rGO
Cu2O consist of extremely anisotropic nanowires as building blocks, which possess an octahedral morphology with eight {111} equivalent crystal faces. The mechanism of the rGO
Cu2O are as follow: (1) At the initial stage, agglomeration of Cu2O NPs is promoted by GO which leads to transition growth mechanism to particle mediated crystallization from ion-by-ion growth. (2) The growth of microspheres into hierarchical structure and then to mesocrystal nanowire through transformation of mesoscale. (3) Reduction of GO and mesocrystals occur simultaneously, which results in integrated hybrid architecture where 3D framework structures scattered between 2D rGO sheets. Fig. 6.7A shows the dynamic response of Cu2O, Cu2O
rGO and rGO sensor for room temperature detection of increasing concentration of NO2 gas (0.4
2.0 ppm). rGO
Cu2O attained a higher sensitivity due to better conductivity and more specific surface area toward NO2 gas at room temperature compared to Cu2O and rGO shown in Fig. 6.7B. Owing to interplay and synergy of 3D mesocrystals and high conductivity of rGO, provides composite with enhanced NO2 sensing, having detection limit of 64 ppb at room temperature. The sensitivities of rGO
Cu2O composite, Cu2O and rGO were found to be 67.8%, 44.5% and 22.5% respectively. The sensing mechanism of rGO
Cu2O is shown in Fig. 6.7C as the charge doping. The gas molecule obtains an electron from surface oxygen ion and developes the hole conductivity in the Cu2O device when it comes to contact with NO2 gas [92]. Zhang et al. have prepared core-shell 3D Fe3O4@rGO heterostrutures for the fabrication of NO2 gas sensor at room temperature. Effect of various ratios of Fe3O4@rGO and humidity was investigated. The sensors show a high sensitivity (183.1%) at 50 ppm NO2, which is 8.17 times more than pure 2D rGO sensor at room temperature. Larger surface area of rGO with 3D structure and as-formed heterojunctions plays significant role for improvement of sensing performance. The strategy to transform 2D to 3D core-shell, increases the sensing performance of rGO based gas sensor at ambient temperature [93].

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Chapter 6 Graphene-based gas sensors

Figure 6.7 (A) Dynamic response of Cu2O NW, reduced graphene oxide (rGO)
Cu2O, and rGO devices under increasing NO2 exposure. (B) The sensitivities of NO2 sensor for the three devices. (C) Schematic for mechanism of NO2 sensing of rGO
Cu2O.

6.4

Graphene and its derivative/polymer based gas sensor

Carbon-polymer composite materials have gained significant attention due to fast response time, flexibility, high sensitivity, and easy operation at room temperature [94
96]. Electronic structures of these composite material is consisting of conjugated

Chapter 6 Graphene-based gas sensors

π-bonds which changes due to chemical species adsorbed on their surface. This is because of acid base or redox type interaction between the chemical species and polymer. Incorporating graphene into polymer matrices is new in trend to produce novel nanocomposite materials, which is reported by several researchers recently [97,98]. Graphene nanofiller is dispersed in a polymer matrix to provide nanocomposite which enhances thermal, electrical, mechanical and other properties having large surface to volume ratios. More active nucleation sites of graphene are used for PANI and electron transfer pathways [98]. By combining inorganic counterparts with organic materials, it shows effective method for the enhancement of the sensors characteristics and mechanical strength [99]. Inorganic nanofiller like graphene is added into PANI by using chemical oxidative polymerization, enhances the properties of NH3 sensor based on graphene/PANI composite. Wlodarski et al. have reported the development of hydrogen (H2) gas sensor using Graphene/PANI nanocomposite. Assynthesized nanocomposite shows higher sensitivity (16.57%) and sensitivity toward 1% of H2 gas compared to PANI nanofibers and only graphene sheets. The response of graphene- and PANIbased gas sensors was found to be 0.83% and 9.38%, respectively. In nanocomposites, both PANI and graphene components interact with gas (H2) with opposite effects. Specific surface area was increased due to graphene and large porosity of PANI nanofibers [100]. Chen have synthesized Graphene/PANI nanocomposite for the fabrication of NH3 gas sensing. Graphene/PANI sensor shows 5 times more sensitivity in contrast to PANI, and demonstrates linearity of concentration from 1 to 6400 ppm. The detection limit of graphene/PANI found lower (1 ppm) than PANI (10 ppm) for ammonia sensor. It demonstrates that, by adding graphene into PANI, sensitivity of synthesized material is increased for detection of NH3. This result is supported that the adsorption of NH3 gas of quartz crystal microbalance coated graphene/PANI is larger than that of PANI [101]. Johnson and coworkers have synthesized GO/polypyrrole (GO/PPy) composite arogel by using hydrogel for the fabrication of NH3 gas sensor. PPy aerogel shows extensive application in sensing due to their high conductivity and large specific area. Owing to the insolubility of PPy, in situ polymerization of pyrrole monomer into GO dispersion was carried out to produce GO/PPy composite. In Fig. 6.8, the resistance of GO/PPy aerogel is enhanced by 40% within 600 seconds at 800 ppm NH3, which is higher than pure PPy (7%) due to dedoping PPy by NH3 [66] (Fig. 6.8).

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Figure 6.8 (A) Tree devices toluene vapor sensing performance with sensing elements of graphene oxide(GO)/PPr (black), PPr (red) and GO (green). (B) Sensing response of the devices based on lyophilized GO/PPy hydrogel (blue line) electrochemically deposited PPy film (red line), and GO/PPy hydrogel dried in air (black line) to 800 ppm NH3. Inset is a schematic of the gas sensor device. Reproduced with permission from H.J. Song, L.C. Zhang, C.L. He, Y. Qu, Y.F. Tian, Y. Lv, Graphene sheets decorated with SnO2 nanoparticles: in situ synthesis and highly efficient materials for cataluminescence gas sensors, J. Mater. Chem. 21 (2011) 5972
5977 [66]. Copyright Royal Society Chemistry (2011).

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7 3D Hierarchical carbon-based gas sensors Jolina Rodrigues1, Shilpa Jain2 and Navinchandra Gopal Shimpi1 1

Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India 2Department of Chemistry, Jai Hind College, Mumbai, Maharashtra, India

7.1

Introduction

Industries and vehicles are emitting enormous amount of harmful gases, which have bad impact on human health and environment. Therefore, there is an urgent need for monitoring these gases. Inhalation of these toxic gases such as H2S, CO2, NH3, NO2, NO, CO, etc can lead to worst effect on human health and also on the ecosystem [1
4]. Gas sensors are used to detect and identify gases mainly toxic gases and measure its concentration. These gas sensors transduce the signal in the presence of these toxic gases, recognize, and send an alert signal to desired system. These gas sensors have received much attention because of their application in various areas such as industry and domestic area [1], biomedicals [2], agriculture [3], and industrial wastes [4] etc. For environmental safety and supervision of human health, detection of volatile organic compounds is of great importance [1
3]. Gas sensors based on nanomaterials have gained much attention in recent years due to their superior properties like electric, thermal, mechanical, and optical properties along with higher surface area, high selectivity, high sensitivity, fast response and recovery, better reproducibility, and stability [5] etc. Carbon sensors like graphene, graphene oxide, carbon nanotubes, etc. show enhanced response behavior with simple modification in chemical treatment [6
8]. Hence such carbon-based gas sensors are able to detect harmful toxic gases. Also, carbon nanocomposites are derived and used as gas sensors, which include graphene and metal oxide, graphene and metal, graphene and carbon nanotube, carbon nanotubes, metal oxides, Carbon-Based Nanomaterials and Nanocomposites for Gas Sensing. DOI: https://doi.org/10.1016/B978-0-12-821345-2.00003-6 © 2023 Elsevier Inc. All rights reserved.

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Chapter 7 3D Hierarchical carbon-based gas sensors

etc. Functionalization of carbon nanostructures shows better sensing behavior with increased sensitivity and improved gas sensing performance. Carbon-based nanomaterials are used as gas sensors due to properties like high stability at various temperature, a strong covalent bond structure, unique electronic properties, ease of composite formation and low cost. Carbonbased nanomaterials can be developed in all dimensions like 0-D (fullerene), 1-D (carbon nanotubes), 2-D (graphene), 3-D (nanocarbon network) [9]. In terms of material science, nanostructures are defined as materials having at least one dimension smaller than 100 nm. They are nanostructures such as atomic clusters, core shell quantum dots, and quantum dots [10]. 1-D nanostructures are systems with lateral dimension in nanometer scale and one dimension outside the nanometer range. Such systems include nanowires, nanoribbons, nanosheets, nanobelts, nanorods and nanotubes. These material exhibit very large surface to volume ratio [11]. 2-D nanostructures are systems in which two dimensions are outside the nanometer range. The simplest form of 2-D nanostructure is a plane or thin film with a depth less than 100 nm and two other dimensions larger than nanometric dimensions. These nanostructure include multilayer thin film and coating [12]. 3-D nanostructures are bulk systems with three dimensions are outside the nanometric range. Such as fullerenes, nanoflowers, nanocrystalline, nanocomposites materials, etc. The 3-D nanostructures a have high surface area and supplies enough adsorption site for all involved molecule in small space. Their 3-D porosity provides improved transportation of molecules [13]. Some carbon-based nanostructures are graphene, carbon nanotubes, carbon nanofibers, or a combination of graphene and metal oxide or combination of graphene and carbon nanotube, etc. In recent years, 3-D hierarchical gas sensors have gained much attention due to their large surface area and porous structure that lead to enhanced adsorption and desorption of gas [14
17].

7.2

Importance of 3D nanomaterial

Behavior of 3-D nanomaterials depends on size, shape, dimensionality, and morphology. These are important factors to their performances in gas sensing application. Hence 3-D nanomaterials have attracted significant research interest and have been synthesized in the past 10 years. 3-D nanomaterials have large surface area and other superior properties due to which

Chapter 7 3D Hierarchical carbon-based gas sensors

151

they are being used in the wide range of applications such as a catalyst, gas sensors, magnetic material and electrode material for batteries [13]. 3-D nanostructured materials have high surface area and enough absorption sites for interaction of molecules in small space. Also porosity of such 3-D materials leads to better gas sensing, porous nanostructured films are most commonly used as they have high specific surface area and are low power consuming. Layers of porous thin film of different materials are combined to form a single 3-D nanostructured sensor, which detects the number of gases [18].

7.3

Construction/fabrication of 3D architectures

1-D metal oxide when combined with 2-D material like graphene oxide (GO) can be used for the construction of 3-D material with the synthesis process like template assisted or multistep sequential growth. Template assisted is mostly used for 3-D material synthesis due to the variety of morphology obtained, and synthesis is done on a large scale. Choi et al. developed a process where 3-D tungsten trioxide (WO3) hemitubes functionalized by graphene was synthesized. WO3 hemitubes was synthesized by RF sputtering WO3 film on to O2 treated with polyvinylpyrrolidone and polymethylmethacrylate composite nanofibers (Fig. 7.1). Then

Figure 7.1 (A
F) Schematic illustration of the formation of 3D Graphene-WO3 hemitube architectures [19], 2012 American Chemical Society.

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Chapter 7 3D Hierarchical carbon-based gas sensors

high-temperature calcination was done to remove the polymeric template. Graphene was homogeneously mixed with WO3 hemitubes leading to the formation of heterojunction 3-D nanocomposite [19]. Multistep sequential growth of 3-D nanostructured material is most preferred due to low cost, no need of template, and more conventional and preferred. Abideen et al. developed a 3-D rGO/Zinc Oxide (ZnO) core-shell nanofibers by electrospinning method. Deng et al. developed 3-D rGO conjugated cuprous oxide nanowire as mesoporous hybrid which was synthesized by one pot hydrothermal method. GO is induced in agglomeration of amorphous spherical cuprous oxide nanoparticles. The amorphous microsphere are developed into hierarchical mesoporous nanowire assemblies through mesoscale transformation by Ostwald ripening, which leads to the formation of 3-D mesoporous hybrid architecture [20,21]. 2-D metal oxide nanosheets are used for the synthesis of 3-D rGO/ZnO hybrid as gas sensor as they have a large surface area. Hoa et al. have developed novel 3-D porous nanocomposites containing 2-D GO and 2-D Nickel Oxide (NiO) nanosheets. First, graphene oxide was synthesized and coated on the electrode and then reduced by heating treatment to form. NiO seeds were coated and annealed on these rGO film [22]. As 3-D structures have larger surface area, so gas adsorption and desorption is enhanced. Hence 3-D metal oxide structure have been combined with 2-D material like GO nanosheets to form 3-D architecture. Zhang et al synthesized rGO/alpha-Fe2O3 composite by the hydrothermal method which is low cost and environmental friendly. Also rGO/In2O3 cube nanocomposite was synthesized by facile one step microwave assisted hydrothermal method. Liu et al fabricated 3-D rGO/In2O3 composite by one step hydrothermal method in which flexible and transparent rGO sheets were placed among the flower like hierarchical In2O3 [23,24]. Ternary 3-D hybrid are also used as gas sensors. Uddin et al developed Ag loaded ZnO/rGO nanostructure. ZnO/rGO was synthesized by the facile hydrothermal method and later Ag was deposited on to ZnO/rGO by photochemical route. Esfandiar et al prepared 3-D rGO/WO3
Palladium (WO3
Pd) nanostructure using hydrothermal process. In this process, WO3
Pd nanostructures was incorporated in partially rGO which resulted in ternary hybrid with a large surface area [25,26]. The multilayer layer network of GO with metal oxide is developed like 3-D rGO aerogel/ZnO sphere composite by the facile solvothermal method. ZnO nanospheres were synthesized by solvothermal method while graphene oxide was reduced to rGO by insitu reduction. To maintain the 3-D monolithic architecture,

Chapter 7 3D Hierarchical carbon-based gas sensors

freeze drying process was carried out. These 3-D architecture have been widely used as gas sensors to detect harmful gases like H2S, CO2, NH3, NO2, NO, CO and; SO2 etc. [27].

7.4

3-D metal oxide/graphene nanocomposite as gas sensors

1. Zinc oxide/reduced graphene oxide Ha and team synthesized highly porous ZnO/3-D rGO by using the hydrothermal method, in which graphene oxide was prepared by hydrothermal method and ZnO was loaded on reduced graphene oxide hydrogel (RGOH). The sensor showed response time of 7 seconds and recovery time of 9 seconds to 1000 ppm of the CO gas at 200 C (Fig. 7.2A). The fast response and recovery time were obtained for CO gas sensor which was less than 10 seconds in the concentration range of 1
1000 ppm and also the sensor showed long-term stability up to two months (Fig. 7.2C). When compared to other gases like CO2, H2, NO2, O2, N2 the response is high for CO gas (Fig. 7.2D). CO behaves as reducing gas with n-type ZnO material due to which these reductive gas reacts with chemisorbed oxygen on the ZnO surface and releases electron in the conduction band leading to increase in conductivity. Also the rGO traps the released electron leading to the high response of the sensor. The fast response time, recovery time and high sensitivity is due to the high porosity of the scaffold 3-D rGO and ZnO/3-D rGO. Also the 3-D structure provides increased porosity which results in a larger active surface area of ZnO/3-D rGO. As the concentration of CO gas increases, the response of sensor also increases (Fig. 7.2B) [28]. 2. Tin dioxide/reduced graphene oxide Li and team synthesized 3-D SnO2/rGO composite by hydrothermal and lithography methods. When the temperature increases, the response of 3-D SnO2/rGO sensor increases till optimum temperature then gradually decreases. But in case of recovery time, as temperature increases recovery time decreases. With increase in concentration of NO2 gas increases the sensor response also increases. 3-D SnO2/rGO sensor shows higher selectivity to NO2 gas as compared to other gases like CO, ethanol, ethylene glycol, acetone, toluene, ammonia, trihalomethanes, formaldehyde and phenylcarbinol etc. The formation of p
n heterojunction between SnO2 and rGO, leads to easily electron transfer and lowers working temperature. Also NO2 shows high response because

153

Figure 7.2 Sensing performance of ZnO/3-D reduced graphene oxide sensor (A) at various working temperature, (B) response with different concentrations of CO gas, (C) stability of sensor, and (D) the selectivity of sensor [28].

Chapter 7 3D Hierarchical carbon-based gas sensors

it react with oxygen ions on the surface of a sensor and easily gets adsorbed in to the sensor at that temperature. Whereas for larger gas molecules the diffusion is much difficult and hence they do not enter 3-D SnO2/rGO sensor, which results in less chances of these gases to react with oxygen ions and low response [29]. 3. Molybdenum disulfide/reduced graphene oxide Chen and coworkers developed novel 3-D MoS2/rGO composites, which were prepared by hydrothermal and selfassembly method at low temperature (Fig. 7.3). The 3-D MoS2/ rGO composite sensor shows extremely high sensor response of 2483% [30] for 10 ppm of NO2 gas at 80 C. Also, the sensor shows response at very low concentration of about 27 ppb. When sensor is exposed to NO2 gas, the electrons from MoS2/ rGO are transferred to NO2 due to strong electronegativity leading to reduction of heterojunction between them. When NO2 ions are desorbed from sensor surface the electrons return back to MoS2/rGO sensor, which results in high resistance change. 3-D MoS2/rGO composite shows good selectivity to NO2 while for other gases like NH3, CO, C2H5OH, H2 and HCHO low response is observed. MoS2 has higher gas adsorption energy for NO2 gas compared to other gases. Sensing response of NO2 (oxidizing gas) is positive as it shows electron withdrawing properties while that of other gases like NH3, H2, CO, C2H5OH and HCHO are negative as it shows electron donating properties. Enhanced response of sensor is due to

Figure 7.3 Schematic diagram of fabricating 3-D MoS2/rGO composites as gas sensor [30].

155

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Chapter 7 3D Hierarchical carbon-based gas sensors

p
n heterojunction, MoS2 increases chemical active sites and rGO nanosheets are p-type semiconductor having large surface area, better conductivity and also adsorbs gas molecules from ambient due to Van-der Waal’s forces [30]. 4. Fe3O4/reduced graphene oxide Zou and team synthesized 3-D Fe3O4/rGO sensor. First, Fe3O4 nanospheres were synthesized by hydrothermal method, and then 3-D Fe3O4/rGO heterojunction core shell material was developed by electrostatic self-assembly method. As GO is negatively charged and Fe3O4 is positively charged. Compared to 2-D rGO sensor, 3-D Fe3O4/rGO sensor shows better selectivity of 183.1% for 50 ppm of NO2 gas which is around 8.17 times greater. The 3-D Fe3O4/rGO sensor show stronger selectivity to NO2 as compared to other gases like NH3 and SO2. Sensor showed excellent stability up to one month and response time of sensor did not change much but recovery time of sensor was slightly affected. Enhanced response of 3-D Fe3O4/rGO sensor is due to p
n heterojunction formed due to p-type rGO and n-type Fe3O4, which leads to electron depletion layer when exposed to gas. When sensor is in contact with air, oxygen molecules capture electron from Fe3O4 surface and forms oxygen ions (Eqs. 7.1 and 7.2). Electron transfer from ntype Fe3O4 to p-type rGO till fermi level is in equilibrium. When sensor exposed to NO2 gas, it reacts with oxygen ion on sensor surface, leading to formation of NO2 2 ions (Eq. 7.3). As electron in Fe3O4 decreases, number of holes increases and there is rapid transfer of holes to rGO, hence resistance increases (Eq. 7.4). NO2 is small in size, polar in nature and also has lone pair of electrons. Due to all these properties, NO2 can easily capture electrons. The 3-D Fe3O4/rGO sensor has core-shell like structure due which there is less agglomeration, much larger surface area and more adsorption sites available as that compared to 2-D rGO sensors [31]. O2 ðadsÞ 1 e2 - O2 2 nðadsÞ

ð7:1Þ

2 2 O2 2 ðadsÞ 1 e -2O ðadsÞ

ð7:2Þ


NO2 gas 1 e2 -NO2 2 ðadsÞ

ð7:3Þ


2 2 2 NO2 gas 1 O2 2 ðadsÞ 1 2e -NO2 ðadsÞ 1 2O ðadsÞ

ð7:4Þ

5. Zinc oxide/reduced graphene oxide aerogel Liua and coworkers developed a novel 3-D graphene aerogel
ZnO spheres composite, by simple solvothermal

Chapter 7 3D Hierarchical carbon-based gas sensors

157

Figure 7.4 Fabrication process for the 3-D Graphene aerogel
Zinc Oxide [32].

method, which has graphene interconnected sheets and ZnO nanospheres spread on surface (Fig. 7.4). ZnO/graphene aerogel composite to 50 ppm NO2 at room temperature. 3-D ZnO/ graphene aerogel sensor shows response and recovery time of about 132 and 164 seconds for 50 ppm of NO2 gas at room temperature, while pure graphene shows lower response (149 seconds) and longer recovery time (243 seconds) as compared to those of 3-D ZnO/graphene aerogel composite. Heterojunction formed between ZnO spheres and graphene attracts more electrons and more adsorption sites for NO2 which enhances response than pure graphene. When 3-D ZnO/graphene aerogel composite is exposed to air, oxygen molecules captures electron from ZnO and forms oxygen ion which leads to depletion of ZnO surface (Eqs. 7.5 to 7.7). When sensor is exposed to NO2 gas, it reacts with oxygen ions which leads to the formation of the depletion layer at ZnO/graphene aerogel interface and traps electrons from the graphene toward NO2 through the bonded ZnO pathway. Due to this process the electron density on sensor surface decreases and hole as carriers increases. As graphene has p-type properties which dominates and the 3-D ZnO/graphene aerogel sensor shows decrease in resistance when exposed to NO2 gas (Eqs. 7.8 and 7.9). 3-D graphene has interconnected porous networks which leads to easy gas diffusion resulting in enhanced sensor response. Also the continuous porosity of the sensor behaves as a multidimensional transport pathway due to which gas gets

158

Chapter 7 3D Hierarchical carbon-based gas sensors

adsorb easily and hence there are more active sites available for the reaction of NO2 and surface adsorbed oxygen ions [32].
ð7:5Þ O2 gas -O2 ðadsÞ O2 ðadsÞ 1 e2 - O2 2 ðadsÞ

ð7:6Þ

2 2 O2 2 ðadsÞ 1 e -2O2 ðadsÞ

ð7:7Þ


NO2 gas 1 e2 -NO2 2 ðadsÞ

ð7:8Þ


NO2 gas 1 O2 ðadsÞ- NO2 3 ðadsÞ

ð7:9Þ

6. CuxO/Reduced Graphene Oxide Yang and team synthesized novel 3-D nanoflower-like CuxO/multilayer graphene composites (CuMGCs) by the vacuum-assisted reflux method. The novel 3-D CuMGCs sensor shows a low detection limit of NO2 with gas response of 27.1% at concentration as low as 97 ppb at room temperature. The gas response for 97 ppm NO2 (95.1%) was higher than NH3 gas (3.87%), but insensitive to other gases like O2, H2, CO, and C2H2 at the same concentration of 97 ppm. CuMGCs sensor in air reacts with oxygen molecules, these oxygen molecules capture the electrons from a conduction band of CuMGCs sensor and leads to the formation of oxygen ions at the sensor’s surface. When sensor is exposed to oxidizing NO2 gas, the NO2 gas due to high electron affinity attracts the electrons from the surface of CuMGC sensor and decreases electron density leading to increase in number of holes which results in resistance decreases of CuMGC sensor 2 2 and leads to formation of NO2 2 , NO , NO3 ions (Eqs. 7.10 to 7.13). The enhanced sensing properties of 3-D CuMGC sensor is due to the superior conductivity of multilayer graphene and the high surface area of CuxO nanoparticles [33].
ð7:10Þ NO2 gas 1 e2 -NO2 2 ðadsÞ
NO gas 1 e2 -NO2 ðadsÞ

ð7:11Þ


NO2 gas 1 O2 -NO2 3 ðadsÞ

ð7:12Þ


NO gas 1 O2 -NO2 2 ðadsÞ

ð7:13Þ

Chapter 7 3D Hierarchical carbon-based gas sensors

Figure 7.5 Schematic diagram of the synthesis of 3-D structured SnS2/rGO [34].

7. SnS2/reduced graphene oxide Wu and coworkers developed a method to synthesize 3D structured SnS2/rGO heterojunction by the facile hydrothermal method (Fig. 7.5). 3-D SnS2/rGO sensor shows the positive conductance shift when it is in contact with an electron donating NO2 gas molecule. The rGO sensor and SnS2 sensor were not able to detect gas of 20
500 ppb, whereas 3-D SnS2/rGO sensor detects NO2 gas at such low concentration level. The sensor shows repeatability up to 2 ppm NO2 for consecutive three cycles. In 3-D SnS2/rGO sensor shows temperature variance in both response and recovery time. With increase in temperature from 26 C to 156 C, the response for 8 ppm of NO2 gas declines from 49.8% to 14.7%, but the recovery rate increase from 37.4% to 52.4%. Therefore, at low temperature adsorption occurs, while with increase in temperature desorption occurs. NO2 shows high selectivity among various gases, including NH3, CO2, CH4, H2S, and H2 which is due to high adsorption energy on SnS2/rGO sensor and also NO2 molecules not only get adsorbed on the sensor surface but also diffused into sensor interlayers (Fig. 7.6A). Also the formation of hydrogen bonding between NO2 molecules and oxygenated groups present on rGO increases rate of adsorption. The 3D SnS2/rGO sensor show superior response for NO2 gas as compared to pure rGO and SnS2 due to a larger surface area, interlayer spacing, oxygenated groups, and micro/ nanoscale pores (Fig. 7.6B) [34].

159

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Chapter 7 3D Hierarchical carbon-based gas sensors

Figure 7.6 (A) Sensing responses of SnS2/reduced graphene oxide (rGO) and rGO to different gases, (B) Schematic illustrating the NO2 sensing mechanism of SnS2/rGO heterojunction [34].

Figure 7.7 Schematic illustration of the synthesis of 3-D α-Fe2O3 nanorods array/ GO [35].

8. Alpha-Fe3O4/reduced graphene oxide Song and coworkers synthesized 3-D GO/α- Fe2O3 nanorods array by the one-step forced hydrolysis method (Fig. 7.7). The response time of 3-D GO/α-Fe2O3 nanorods array was 7 seconds and the recovery time was 8 seconds for 50 ppm acetone gas at 220 C (Fig. 7.8B). The 3-D GO/ α-Fe2O3 nanorods array can detect acetone gas at low concentration of about 5 ppm and at the high concentration of about 800 ppm. For 50 ppm of acetone gas, 3D GO/α- Fe2O3

Chapter 7 3D Hierarchical carbon-based gas sensors

161

Figure 7.8 (A) Response of α-Fe2O3 Nanorods array/GO and α-Fe2O3 nanoparticle based sensors toward 50 ppm of different gases. (B) Response and recovery curve of gas sensors based on α-Fe2O3 and α-Fe2O3 Nanorods array/ GO toward 50 ppm acetone gas [35].

nanorods array sensor shows stability upto seven weeks. 3-D GO/α-Fe2O3 nanorods array shows high selectivity to acetone gas as compared to NH3 and ethanol and very low response is observed for methanol and toluene gas (Fig. 7.8A). α-Fe2O3 shows n-type and GO sheets shows ptype behavior forming a p
n heterojunction in 3-D GO/ α-Fe2O3 nanorods array sensor. When the sensor is in contact with air, the oxygen molecule capture electrons from the surface of α-Fe2O3 sensor and forms oxygen ions. When sensor is in contact with acetone gas which acts as reducing gas, it reacts with oxygen ions and releases electrons to alpha-Fe2O3 conduction band. The unique 3-D GO/α-Fe2O3 nanorods array sensor has superior response due to combination of α-Fe2O3 nanorods array and GO sheets which leads to efficient space, adsorpsion capacity, and formation of p
n heterojunction between α-Fe2O3 and GO sheets leading to the formation of depletion layer between them (Fig. 7.9) [35]. 9. GdInO3/reduced graphene oxide Balamurugan and team synthesized 3-D rGO/GdInO3 sensor. First, GO was prepared by modified hummer’s method, then rGO/GdInO3 composite was synthesized by one step hydrothermal method. The pure GdInO3 material shows high response at operating temperatures higher than 150 C. While the rGO/GdInO3 composite shows response at 90 C which is due to strong interaction between the rGO and GdInO3 material resulting in electrical resistance of the material and creation of vacancies or small holes. Sensor

162

Chapter 7 3D Hierarchical carbon-based gas sensors

Figure 7.9 Schematic illustration of the gas sensing mechanism. (A) α-Fe2O3 nanorods array/GO in air. (B) α-Fe2O3 nanorods array/GO in the presence of acetone vapor [35].

Figure 7.10 (A) CO response of the reduced graphene oxide(RGO)/GdInO3 sensor, (B) selectivity of the rGO/GdInO3 sensors to different concentration of CO [36].

shows response time of 14 seconds and recovery time of 15 seconds to 100 ppm of CO gas at 90 C as compared to that of pure GdInO3 sensor, that shows response time of approximately 70 seconds at 100 ppm of CO gas at 150 C

Chapter 7 3D Hierarchical carbon-based gas sensors

(Fig. 7.10A). The 3-D rGO/GdInO3 nanocomposite sensor shows high sensitivity towards 100 ppm of CO gas than as compared to 100 ppm NH3, 100 ppm ethanol, 100 ppm H2S at same operating temperature (Fig. 7.10B). Both the material in sensor shows p-type behavior. When exposed to air, oxygen gets chemisorbed on GdInO3 and when exposed to CO gas resistance increases. As CO being strong reducing and electron donating gas, it donates electrons to sensor surface and number of holes are reduced. As the concentration of CO gas increases, the response of sensor also increases [36].

7.5

3-D functionalized graphene nanocomposite as gas sensors

The 3-D reduced graphene oxide hydrogel (RGOH) gas sensor performance is enhanced by chemical functionalization by introducing numerous doping and defect sites in RGOH. The 3-D RGOH was chemically modified by hydroquinone molecules. GO was first synthesized by modified Hummer’s method, further functionalizations were carried out by simple one step hydrothermal method. Performance of FRGOH sensor was evaluated by resistance change on exposure to NO2 gas molecule, which showed fast response at concentration ranging from 10
0.2 ppm. The response of RGOH sensor and FRGOH sensor was 7.4% and 3.7%, respectively. On exposure to 10 ppm of NO2 gas, response of FRGOH sensor is two times greater than response of RGOH sensor. The enhanced performance of FRGOH sensor is not only due to 3-D porous nanostructure, but also due to hydroxyl groups on the sensor that contains oxygen atoms, which are introduced by hydroquinone molecules. The hydroxyl molecules on the surface of the FRGOH sensor acts as an active site for adsorption of NO2 gas molecules. The formation of hydrogen bonding between NO2 and hydroxyl group leads to higher binding energy. The FRGOH sensor contains microheater imbedded in it which enhances selectivity of NO2 sensing and also leads to fast recovery process. As temperature increases from 22 C to 38 C, the recovery process also increases from 57% to 73% for 8 ppm of NO2 gas. Capability of FRGOH sensor to absorb NO2 gas decreases gradually from 10 ppm to 1 ppm as most of the sensor sites are occupied by 10 ppm NO2 molecules. The FRGOH sensor not only has twofolds high response, but it also increases recovery percentage of CO2 gas as compared to RGOH sensor.

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Chapter 7 3D Hierarchical carbon-based gas sensors

The adsorption energy of doped GO and NO2 is greater than doped GO and CO2 gas which results in weak interaction between doped GO and CO2 molecule.,also leading to fast and easy desorption of CO2 from FRGOH surface. The FRGOH sensor shows 1.65% response and 49.5% recovery whereas RGOH shows 0.95% response and 33.3% recovery. With increase in temperature the response decreases due to weak interaction between RGOH and CO2 gas molecules, which leads to fast desorption of CO2 gas from sensor surface. Seebect coefficient of RGOH and FRGOH were positive indicating p-type semiconducting behavior. NO2 gas is oxidizing gas, it withdraws electron from FRGOH surface and leads to formation of holes. Since, the resistance change is same in NO2 and CO2 gas, the imbedded micro-electrode is used to distinguish between two gases. Elevated temperature does and does not significantly improve the selectivity of detecting CO2 and NO2 respectively within an approximate temperature range. Not only to CO2 gas but also gases like acetone and methanol, FRGOH sensor gives low response. High response of NO2 gas on FRGOH is not only due to interaction with surface layer but also penetration into internal stack of 3-D FRGOH due to high adsorption energy. Compared to 2-D rGO sheets, 3-D FRGOH sensor shows stronger response to NO2 and CO2 gas and enrollment of microheater boosts gas sensing performance [37]. Ding et al. developed a method for the synthesis of 3-D Co3O4/functionalized graphene hydrogel (FRGH) composites. First, GO was synthesized by oxidation and exfoliation of natural graphite powder with modified Hummer’s method and then functionalized by hydroquinone to obtain 3-D FRGH powder. Further, granular Co3O4/FRGH composites were obtained by pyrolysis process. Co3O4/FRGH composite was used as sensing layer on to the Pt electrode and Al2O3 as substrate. FRGH shows no response to acetone gas but Co3O4 shows some response to acetone gas. When FRGH and Co3O4 combined show high response to acetone gas, Co3O4 can easily adsorb acetone molecule through FRGH framework and interact with acetone gas to produce large amount of electrons. Compared to pristine Co3O4 or FRGH framework, the response of Co3O4/FRGH composite to acetone gas is higher. Response time of Co3O4/FRGH framework is 81.2 to 50 ppm acetone gas, while that of pristine Co3O4 is 3.9 to 50 ppm (Fig. 7.11B and C). Co3O4 shows high response to acetone (74.5 at 25 ppm), ethanol (18.5 to 100 ppm), NH3 (3.34 to 100 ppm) and very low response to CO2, O2, CH4 gas. Co3O4/ FRGH shows stability for five months, having slight fluctuations but with approximately 5% variation in concentration range of

Chapter 7 3D Hierarchical carbon-based gas sensors

Figure 7.11 (A) Schematic illustration of acetone sensing mechanism for Co3O4/FRGH, (B) and (C) Dynamic response of Co3O4 and Co3O4/FRGH toward 25 ppm acetone, respectively [38].

1 to 50 ppm. Co3O4 acts as p-type semiconductor where holes are the major carriers. When Co3O4 is in contact with air, oxygen molecules are adsorbed on Co3O4 surface and get ionized. When acetone (reducing gas) comes in contact with Co3O4 surface, the O22 ion interacts with acetone to produce CO2, H2O gas and electrons. And hence, the electrons return back to conduction band of p-type Co3O4 and resistance increases on exposure to acetone gas. These electrons will now transfer from Co3O4 to FRGH framework through Co3O4/FRGH junction and neutralize p-type FRGH and resistance of p-type FRGH will

165

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Chapter 7 3D Hierarchical carbon-based gas sensors

increase. The enhanced sensing performance of Co3O4/FRGH composite as compared to that of pure Co3O4 is due to unique 3-D porous structure and modulation of electrical transport properties of Co3O4/FRGH junction (Fig. 7.11A). The porous structure of 3-D Co3O4/FRGH leads to great amount of penetration and interaction between acetone gas and sensor [38].

7.6

3-D metal doped graphene nanocomposite as gas sensors

Phan and coworkers synthesized platinum (Pt)-doped 3-D graphene by polymer-assisted hydrothermal method. GO was prepared by Hummer’s method, and Pt and GO solution were used for synthesis of Pt doped 3D graphene. Pt doped 3-D graphene when exposed to H2 gas led to increase in resistance. Also the response and recovery time increased as temperature increased. The response of Pt/3-D graphene at 10,000 ppm of H2 gas was 6.1%, 9.2%, 15.1% at temperature 25 C, 100 C, 200 C, respectively. As 3-D graphene has high porosity, H2 gas sensor shows better response in concentration range of 10
10,000 ppm. The response of H2 sensor based on Pt doped 3-D graphene composite was 16% at 10,000 ppm. Compared to other Pt/Pd based graphene sensors, the Pt doped 3-D graphene shows better response due to increased surface area of sensor (Table 7.1) [39]. Shao and team developed a 3-D novel sensor based on nitrogen-doped graphene quantum dots/TiO2 (NGQD/TiO2) nanospheres (Fig. 7.12). Sensor shows response time of 9.1 seconds to 100 ppb concentration of formaldehyde (HCHO) gas at 150 C., and recovery time of 18- 20 seconds to 100 ppb concentration of Table 7.1 Sensor response of different Pt/Pd based graphene sensors to H2 gas. Sensor

Gas

Concentration (ppm)

Sensor response (%)

References

Pd/Gr Pt/Gr Pt/Gr Pd/Gr Pd/Gr composite Pd/polyaniline composite Pt/Pd-Gr composite Pt/3-D graphene composite

H2 H2 H2 H2 H2 H2 H2 H2

1000 10,000 40,000 1000 5000 1000 20,000 10,000

6 3 16 33 32 7 4 16

[29] [30] [31] [32] [33] [34] [23] [39]

Chapter 7 3D Hierarchical carbon-based gas sensors

Figure 7.12 Schematic illustration of the fabrication for sensing materials [40].

HCHO gas at 150 C. The creation of p
n heterojunctions between NGQDs and TiO2 leads to reduction of resistance when in contact with formaldehyde gas, but comes back to original resistance when exposed to air. When air is in contact with sensor, a large number of oxygen molecules get adsorbed in the interface of NGQD/TiO2 nanospheres heterojunction and capture electrons from TiO2 conduction band, leading to formation of chemisorbed oxygen species (O22). When sensor is exposed to HCHO gas, HCHO molecules reacts with the adsorbed oxygen species, resulting in release of electron in heterojunction region which leads to electron transfer to conduction band of TiO2. A lot of electrons are present on sensor surface which leads to lowering of surface resistance and hence, the electron depletion layer thin’s (Fig. 7.13). Sensor gives better response to HCHO gas than compared to other gases [40].

7.7

3-D metal oxide/carbon nanotube and metal oxide/graphene oxide/carbon nanotube nanocomposite as gas sensors

Jo and coworkers fabricated gas sensor based on 3-D network of SWNT on quartz substrate by thermal Chemical Vapor

167

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Chapter 7 3D Hierarchical carbon-based gas sensors

Figure 7.13 Sensing mechanism of NGQD/TiO2 sensor (A) in air and (B) in gas, and the proposed energy band diagram of nanocomposite (C) before and (D) after contact at equilibrium [40].

Deposition method. Different sizes of TiO2 nanoparticles was deposited on SWNT by ALD (Atomic-layer Deposition). TiO2 grown on SWNT were exposed to NH3 gas, where NH3 molecules physically attracted to surface of SWNT, where electrons are transferred from the NH3 molecules to the SWNTs. Sensitivity of the 3-D SWNT gas sensor was as high as 4.8%. The undesired resistance drift was observed for sensors due to weak interaction between SWNT and substrate in SWNT network, as gases could quickly diffuse through structure due to large gap between SWNT and substrate. Incorporation of TiO2 nanoparticles lead to increase in sensitivity of TiO2 based gas sensor, which improved from 4.82% to 5.97% without significant resistance drift. Enhancement of sensor response was due to increase in total surface area of gas sensor, asTiO2 nanoparticles and SWNTs are n-type and p-type semiconductors which formed p
n heterojunction at the interface between the TiO2 and SWNTs. 3-D TiO2/SWNTs sensor when exposed to NH3 gas, it was observed that as the concentration of NH3 decreases the sensor response decreases [41]. Seekaew and team synthesized 3-D TiO2-decorated graphenecarbon nanotubes (G-CNT) material as gas sensor. First, graphene was grown on nickel foam and then carbon nanotubes (CNT) were grown on them by using chemical vapor deposition technique. Then TiO2 nanoparticles were decorated on G-CNT by

Chapter 7 3D Hierarchical carbon-based gas sensors

sparking method. The 3-D TiO2/G-CNT sensor shows enhanced response to toluene gas at room temperature compared to other sensors. The 3-D TiO2/G-CNT sensor shows enhanced response to toluene gas compared to other sensors. When TiO2 nanoparticles coated on to the CNT do not show good response whereas TiO2 nanoparticles on 3-D G-CNT shows enhanced response. The response obtained for TiO2/CNT, CNT and graphene were very low while pure TiO2 nanoparticle gave no response to toluene gas at room temperature. After loading of TiO2 nanoparticles on the 3-D G-CNT sensor the baseline of sensor increases by almost two orders of magnitude. Due to the formation of M-S Schottky junction, there is decrease in electron concentration which leads to increase in base line resistance. The 3-D TiO2/G-CNT sensor shows 42.9% response which is seven times greater than that of sensor response (6.3%) for 3-D Graphene-Carbon nanotubes sensor. At high concentration to toluene gas the response and recovery time of 3D TiO2/G-CNT sensor is greater than that of 3-D GCNT. When toluene concentration is lower at 100
200 ppm, the response and recovery of 3-D TiO2/G-CNT decreases as compared to 3-D G-CNT. The decorations of TiO2 nanoparticles leads to decrease in response and recovery time at low concentration of toluene gas. The high sensitivity, fast response and high selectivity to toluene gas at room temperature was observed for 3-D TiO2/G-CNT sensor. The 3-D G-CNT gas sensor exhibits better toluene selectivity, while other gases like toluene, diethyl amine, acetone, dimethylformamide, NH3, ethanol, methanol, isopropanol, formalin, H2 and CO2 shows low response as compared to toluene gas at room temperature. When the TiO2 nanoparticles is decorated on the 3-D G-CNT sensor the enhancement in sensor response is observed for toluene gas. Therefore, the 3-D TiO2/GCNT sensor offers not only high sensitivity and fast response but also high selectivity towards toluene at room temperature.

7.7.1

Sensing mechanisms of 3D TiO2/graphenecarbon nanotubes gas sensors

The formation of M-S Schottky junction between 3-D G-CNT and TiO2 nanoparticles where electrons from 3-D G-CNT transfer to n-type TiO2, which leads to formation of holes on the surface of 3-D G-CNT. As a result there is upward energy band bending and resistance increases. In air, O22 is chemisorbed on surface of 3-D TiO2/G-CNT and captures free electron from conduction band of TiO2 and fermi level of 3-D G-CNT (Eq. 7.14) and (Eq. 7.15) which lead to formation of surface

169

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Chapter 7 3D Hierarchical carbon-based gas sensors

Figure 7.14 Schematic representation of energy band diagrams of toluene-sensing mechanism of 3-D TiO2/G-CNT sensor [42].

depletion layer on 3-D TiO2/G-CNT. When toluene gas interacts, toluene being reducing gas reacts with oxygen species leading to discharge of electrons on the surface of the n-type TiO2 sensor. Due to transfer of electron from 3-D G-CNT to TiO2, there is enlargement of energy barrier height. Resistance changes due to change of effective energy barrier height of Schottky junction in air and toluene conditions (Fig. 7.14). With increase in Ti content the gas response of 3-D G-CNT increases due to increase in number of Schottky M-S junction on 3-D GCNT. One toluene gas molecule generates nine electrons, these nine electrons are discharged on the 3-D TiO2/G-CNT interface, leading to high barrier height and resistance change as compared to other gases (Eq. 7.16) [42].

Chapter 7 3D Hierarchical carbon-based gas sensors


O2 gas -O2 ðadsÞ

ð7:14Þ

O2 ðadsÞ 1 e2 -O2 2 ðadsÞ

ð7:15Þ




2 C7 H8 gas 1 9O2 2 ðadsÞ-7CO2 gas 1 4H2 O gas 1 9e

7.8

ð7:16Þ

3D metal oxide/carbon nanocomposite as gas sensors

Carbon materials have attracted most attention in gas sensing due to their large surface area and better electronic attributes and high carrier mobility. Combination of carbon lowers the working temperature and enhances the selectivity of gas sensor. Zhang and team synthesized SnO2/carbon core shell by facile chemical hydrothermal method. Then these homogeneous paste was spread on ceramic tube that was printed on Au electrode. Diameter of SnO2 core and thickness of carbon shell was measured as 60 and 20 nm. When temperature is about 700 C, carbon is released as carbon dioxide. The response of SnO2/Carbon nanoparticle increases with increase in temperature to 140 C. At 140 C it reaches maximum, then the response decreases. The SnO2 and SnO2/carbon sensor response time was obtained, which showed that SnO2 has optimum temperature at 350 C and SnO2 has optimum temperature at 140 C. Introducing carbon to the SnO2 decreases the operating temperature of sensor leading to better selectivity of NO2 gas. The SnO2/carbon sensor shows detection limit up to 2 ppm of NO2 gas at 140 C. The response time and recovery time of 5 ppm NO2 gas was 30 and 189 seconds. Recovery time of sensor was long indicating that sensor needed long time to come back to original resistance. Sensor showed long term stability up to 25 days. Also the response of sensor material remains almost same, hence the sensor were highly reproducible. In SnO2/carbon case the operating temperature is reduced due to presence of carbon used as p-type and SnO2 used as n-type, which leads to formation of p
n heterojunction as shown in (Fig. 7.15). Incorporation of metal oxide in carbon affect the electronic density state of carbon. The synergistic effect of p
n heterojunction and carbonyl and carboxylic groups present on surface of carbon leads to enhanced sensing performance (Fig. 7.16). NO2 shows high electron donating ability and carbon shows strong electron withdrawing ability due to which NO2 interacts on active site of carbon surface while other gases like C2H6O, C3H6O cannot interact as they are weak electron

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Chapter 7 3D Hierarchical carbon-based gas sensors

Figure 7.15 (A) TEM image of SnO2/Carbon core-shell nanospheres; (B) Sensing mechanism diagram of SnO2/Carbon core-shell nanospheres-based sensor towards NO2 gas [43].

Figure 7.16 Schematic illustration of integrating SnO2 nanoparticles with carbon [43].

donating molecules. Hence these gases require high temperature for sensing [43]. Dankeaw and coworkers fabricated 3-D carbon doped zirconium oxide (ZrO) film directly on interdigital electrode via one step electrostatic spray deposition using zirconate precursor without any carbon source or template. The morphology and structure of metal oxide have large influence on its physical and chemical properties. Carbon doped ZrO gives enhanced response compared to ZrO when exposed to acetone gas. Sensor material when exposed to air, oxygen molecules get adsorb on surface and chemisorbed oxygen molecules in the atmosphere are ionized to oxygen ions through the capture of free electrons from the conduction band, as shown in (Fig. 7.17). These results in

Chapter 7 3D Hierarchical carbon-based gas sensors

173

Figure 7.17 (A) Energy band diagrams and (B) schematic representation of reaction mechanism of carbon doped ZrO2 sensor to acetone gas [44].

formation of thick space charge layer which increases potential barrier and leads to increase in resistance. When sensor is exposed to target gas (i.e acetone reducing gas) the gas reacts with O2- ions on surface of sensing film to form CO2 and H2O, which further leads to recovery of electrons back to conduction band from sensing film and hence resistance increases. The response and recovery time of carbon doped ZrO is 6.5 and 21.5 seconds respectively, while that of pure ZrO is 19.5 and 46.5 seconds respectively. Carbon doped ZrO has short response and recovery time due to enhanced response rate due to carbon doping which increases rate of adsorption and desorption (Fig. 7.17). The sensor was exposed to four gases and the response for acetone (63.5), methanol (30.9), ethanol (19.7), npropanol (3.4) was observed. Response for acetone gas is high due to its potential and it gets adsorbed on metal surface strongly due to large dielectric coefficient, dipole moment, high reaction between acetone and adsorbed oxygen, high vapor pressure (24.7 Kpa at 20 C) and low boiling point (56.2 C) indicating oxygen can easily adsorb and desorb from metal oxide surface. The repeatability of sensor was checked at different concentration of acetone from 10 to 500 ppm for five successive cycles at 25 C which showed no significant distortion. Sensor showed long term stability at 500 ppm acetone gas at 25 C for 30 days. Therefore, the carbon doped ZrO has great stability and can be continuously reused. The excellent gas sensing performance is due to carbon doping, geometry and unique 3-D morphology that leads to high specific surface area, low resistance, reduced band gap and increased oxygen vacancy at room temperature [44] (Fig. 7.17).

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Chapter 7 3D Hierarchical carbon-based gas sensors

7.9

3D graphene-based gas sensors

Duy and team developed simple 3-D chemiresistor-based gas sensor based on SU-8 micro-pillar arrays. 3-D rGO networked layer was synthesized by the reduction of GO nanosheets which were self-assembled on the surfaces of an atomic-layer deposited of Al2O3 ultra-thin layer deposited on SU-8 micro-pillars (3-D sensor). Electron transfer between 3-D rGO sensor and adsorbed gas molecules leads to change in resistance. As NO2 is electronwithdrawing gas, the resistance decreases while NH3 being electron-donating gas the resistance increases. 3-D rGO gas sensors had response time about 2% per second towards NH3 at room temperature. The 3-D sensor has slower recovery time which is approximately 90
100 minutes than that of 2-D sensor which is approximately 60 minutes. The slow recovery time is due to large surface area, more defects and high energy binding sites. Also these sites on sensor reacts slowly but make stronger bonds with reactive gas molecules such as NO2 and NH3. Low response is obtained for NO2 gas as compared to NH3 gas. Both 2-D and 3D sensor shows high selectivity to NH3 gas at room temperature but with combination of SU-8 micro-pillar, the 3-D sensor shows extremely high sensor response. The mechanism of the enhancement of sensor response is not only due to increased space with interaction-site density but also due to increased surface area by 3-D geometry of micro-pillar arrays. Within short exposure time, the 3D sensor allows interaction with more gas molecules leading to higher sensitivity and fast response at room temperature [45]. Yavari and coworkers synthesized foam-like 3-D network macro graphene, in which foam consist of some layers of graphene. A template of porous nickel (Ni) foam was used on which graphene was deposited due to which the material obtained is interconnected and there is no interference and physical breaks in network. Graphene foam shows p-type semiconducting behavior containing holes as carriers, due to its electron withdrawing nature. NH3 gas has a lone pair and acts as reducing agent due to which it donates the electron to p-type graphene foam resulting in reduction of conductance. On Ni foam surface there is formation of graphene film, which is well separated, these results in increase in adsorption site for gas molecules, high porosity, flexible nature and light weight. The adsorption process is comparatively much fast then desorption process, hence to speed up the desorption process a microheater was placed under the foam, which also measure the temperature of foam. The 3-D network of graphene foam shows high sensitivity to NO2 and NH3 gas at room temperature and atmospheric pressure. The high enhanced response of 3-D graphene foam is due to large

Chapter 7 3D Hierarchical carbon-based gas sensors

Figure 7.18 Schematic for mechanism of NO2 sensing of 3-D Graphene-based scaffold. The inset shows the periodic structure of graphene-based scaffold [47].

porosity of foam in which gas molecules can uniformly and easily enter, few layers of graphene foam leading to charge carriers throughout the foam and also operation of sensor is stable [46]. You and coworkers synthesized 3-D graphene scaffolds which contained regular aligned graphene network. Graphene is deposited by flexible substrate which leads to formation of few layer stacked graphene having interconnected conductive pathways which not only allows gas molecule to diffuse in inner layers of graphene but also enhances the transmission efficiency of carriers (Fig. 7.18) The response of 3-D graphene scaffold sensor to H2O and NO2 gas for 100 ppm concentration was 2% and 2.5% respectively. When NO2 is in contact with graphene, it captures electron from graphene and creates holes in graphene scaffold which leads to reduction in resistance. The excellent sensor response is due to large surface and highly interconnected conductivity network of sensor as shown in (Fig. 7.18) [47]. Wu and coworkers developed a method to fabricate nanoscale porous steamed graphene hydrogel (S-GH). First, graphene oxide (GO) was synthesized by modified hummers method, then aqueous. The GO which acts as hydrophilic was reduced from rGO, which is hydrophobic in nature. GO was converted to 3-D graphene hydrogel by one step hydrothermal process. To etch graphene oxide, facial water vapor steaming method was used resulting in formation of porous GO structure with defect sites to obtain better sensing response. The sensor response of 3-D S-GH was three times more than that of unmodified graphene hydrogel when exposed to NO2 gas. The limit of detection of 3-D S-GH sensor is as low as 57.5 ppb whereas that of graphene hydrogel is 178 ppb. The 3-D S-GH sensor shows response to NO2 and NH3 gas that is almost three times and ten times larger than that of

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Figure 7.19 (A) Alignment of the 3-D Porous S-GH on the Au strips of the interdigital electrodes (B) Schematic representation of NO2 gas molecules that not only adsorb on the surface of S-GH, but also diffuse into the micro/ nanoscale pores [48].

unmodified graphene hydrogel. The sensor shows good repeatability for several cycles when exposed to NO2 gas. The response variations obtained for 500 ppb NO2 gas was 5.56% for three continuous cycles and 9.1% for five different cycles. Steam etching activates the sensing properties of sensor than that of pristine graphene, which results in enhanced sensitivity towards NO2 and NH3 gas. Steam etching also increases As NO2 and NH3 are small in size they not only interact on surface layer of sensor but also penetrate in to the porous materials due to its high binding energy leading to better sensitivity, defect sites on surface due to which there is easy interaction between surface and gas molecules as shown in (Fig. 7.19). Large molecules like alcohol are not able to diffuse into the pores of 3-D sensor and hence interaction is limited only to few upper layer of sensor resulting in low response [48]. Zhu et al. developed a 3-D synergistical rGO/Eu(TPyP)(Pc) hybrid aerogel, which is a sandwich type sensor impregnated on the surface of rGO by in-situ self-assembly method as shown in (Fig. 7.20). The sensor shows response time of 172 seconds and recovery time of 828 seconds to 20 ppm of NO2 gas. At ambient temperature the limit of detection of NO2 gas of 100 ppm concentration is 80 ppb. Even after 120 days the response of rGO/Eu(TPyP)(Pc) sensor shows nearly the same response as original response, which shows it has good stability. The sensor shows high response to NO2 gas as compared to other gases like ethanol, acetone, NH3, and CO by tens to hundreds of times for 100 ppm concentration. The superior response for NO2 gas is due to donor-acceptor interaction between NO2 and Eu(TPyP)(Pc) molecule. Sensor showed recovery rate in the range of 98.3% to 103.1%. The p-type rGO and n-type Eu(TPyP)(Pc) combined together to form of p
n heterojunction which leads to the transfer of electron from rGO

Chapter 7 3D Hierarchical carbon-based gas sensors

177

Figure 7.20 Overall architecture of 3-D reduced graphene oxide(rGO)/Eu(TPyP)(Pc) hybrid aerogel as sensing device [49].

Figure 7.21 Energy band structure diagrams of rGO/Eu(TPyP)(Pc) hybrid aerogel based sensor before contacting with each other, in air and in NO2 vapors.

to Eu(TPyP)(Pc) till fermi level of both material are not same. Further leads to increased in resistance as compared to that of pure rGO. When 3-D rGO/Eu(TPyP)(Pc) sensor exposed to air the oxygen molecules get trapped and take electrons from Eu (TPyP)(Pc) and forms O22 ions on the surface of sensor. Later

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Chapter 7 3D Hierarchical carbon-based gas sensors

when NO2 gas comes in contact with sensor, NO2 gas extracts electron from Eu(TPyP)(Pc) and also reacts with O22 ions, which leads to increase in sensor resistance. The enhancement in sensor response is due to Eu(TPyP)(Pc) which acts as large surface sensor leading to adsorption of NO2 molecules and rGO acts as conductive channel resulting in electron carrier [49] (Fig. 7.21).

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179

8 Conducting polymer-based gas sensors Jolina Rodrigues1, Shilpa Jain2 and Navinchandra Gopal Shimpi1 1

Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India 2Department of Chemistry, Jai Hind College, Mumbai, Maharashtra, India

8.1

Introduction

Detection of pollutants, toxics, inflammables, and pollutant gases are important for system and process management, safety monitoring, and environmental conservation. Increase in air pollution due to the industrialization and a rise in emissions from automobile exhaust is a serious concern. There has been a dramatic increase in the number of gases that are needed to be sensed by gas sensors. Gases such as H2S and NH3 are poisonous and harmful gases used in manufacturing processes and frequently encountered in living circumstance, so gas sensors are used for detecting such harmful gases [1 4]. These gaseous elements are often found in trace amounts and are combined with a variety of poisonous gases, making their detection increasingly difficult. The recent global increase in consumption of energy and related environmental issues have increased the necessity of the sensors, which can detect air-pollutants in environment such as SOX, COX and NOX or can be integrated in control systems of combustion exhausts from factories and automobiles [5 7]. Highly efficient and compact gas sensors are needed for monitoring of useful as well as inflammable/hazardous gases. Conducting polymers in different transduction modes have been used to create a variety of gas sensors, which can be loosely divided into five distinct groups, based on different operating modes [8,9]. They are (1) conductometric mode, (2) potentiometric mode, (3) amperometric mode, (4) colorimetric mode and (5) gravimetric mode.

Carbon-Based Nanomaterials and Nanocomposites for Gas Sensing. DOI: https://doi.org/10.1016/B978-0-12-821345-2.00006-1 © 2023 Elsevier Inc. All rights reserved.

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8.2

Conducting polymers-based gas sensors

Conducting polymers (CPs) combine the electrical, magnetic, and optical properties of metals and semiconductors with the mechanical properties and processing benefits of polymers. Hence CPs are very attractive sensing materials [10,11]. As the monomeric chemical structure alone offers a veritable database for tailoring chemical affinity interaction, CPs with conjugated backbones are particularly well suited for advanced sensor systems [12 14]. For better selectivity and sensitivity, not only the secondary and tertiary structures are used but also the dopant and concentration are varied [15]. For example, metallic to near insulating is the range where conductivity of CPs lies, leading to the implementation of various conduction mechanisms solely based on conductivity. The modulation of their doping level and interaction with certain gases, which results in an instantaneous controlled conductivity response, is the basis for their ability as sensors [16]. The ability of electronic delocalization of π-electrons of conjugated polymers provides them a highway for the charge mobility along the polymer chain. The degree to which -electrons delocalize is determined by the degree of disorderness (interchain and intrachain) [17,18]. Because of their design versatility, polymers are used in a large number of chemical sensors. By doping and introducing additional functional additives, the flexibility of polymer properties is enhanced. Doping in conjugated polymers is a charge-transfer reaction that causes partial oxidation (or, less commonly, reduction) of the polymer. Furthermore, the structure of the poly-conjugated chain, interchain/interaction disorder, and doping stage decide the stability of charge-carriers such as solitons, polarons, and bipolarons, as well as free chargecarriers in CPs. During chemical or electrochemical polymerization, primary dopants (anions) are added to improve electrical conductivity while preserving charge neutrality [19 21]. Charge carriers in the polymer chain induce charge exchange between the polymer and the dopant species due to chemical changes in the polymer structure. The nature of the anion has a major impact on the polymer’s morphology. In the interaction of the CPs with the analyte gas anions serves as specific binding sites. The most important advantage of CPs is its tailorable electrical, electronic and magnetic properties. The other advantages may include (1) ease of preparation, (2) low temperature fabrication, (3) tailorable mechanical properties, (4) good environmental stability and (5) low capital investment. The conducting polymer has a number of advantages over metal oxides, including a quick response time, high sensitivity, room temperature activity, and using various

Chapter 8 Conducting polymer-based gas sensors

substituents to tune both chemical and physical properties [22,23]. The porous and sorption process of CPs is similar to absorption rather than adsorption, leading to enhanced sensing response and sensitivity of gas sensors.

8.3

Polyaniline as a gas sensing material

PANi is distinguished from other CPs by its simple and reversible doping dedoping process, stable electrical conduction mechanism, ease of synthesis, and high environmental stability. It depicts a wide range of chemical structures and reaction mechanisms for various gases. The electrical conductivity of PANi is determined by the polymer backbone’s ability to carry charge carriers and hope between neighboring polymer chains [24,25]. Polyaniline’s electrical conductivity increases from the undoped insulating emeraldine base form to the fully doped emeraldine salt by modulating its oxidative state by changing counter ions and degree of doping [26]. PANi is a type of conducting polymer that comes in a variety of shapes and sizes. Chemical and physical properties vary between these types. In the base and salt types, the average oxidation state of pernigraniline (completely oxidized), emeraldine (half oxidized), and leucoemeraldine (fully reduced). The base form of emeraldine PANi, as shown in Fig. 8.1, comprises alternate reduced and oxidized groups that can be protonated by acid doping, resulting in a tenfold increase in conductivity. The degree of protonation is determined by the acid’s pH and the polymer’s oxidation state. The use of HCl acid to fully proton the imine groups in emeraldine base, resulting in a delocalized polysemiquinone radical cation, could greatly increase conductivity. The emeraldine salt of PANi can be easily synthesized using chemical or electrochemical polymerization methods by oxidizing aniline, which can then be converted to emeraldine base using aqueous ammonium hydroxide. To obtain an electronic conducting form, PANi must be doped. PANi, which is formed by the oxidative polymerization of aniline, is made up of both reduced (B NH B NH) and oxidized (B N 5 Q 5 N ) repeat units, with B denoting a benzenoid ring and Q denoting a quinoid ring. Thus, the ratio of amine to imine units yields various structures: the fully reduced form “leucoemeraldine” polymer, the half-oxidized form “emeraldine,” and the fully oxidized form “pernigraniline” as shown in Fig. 8.1. The ability of CPs as sensors is dependent on doping level modulation and interaction with certain gases. This has the

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Figure 8.1 Different forms of polyaniline.

effect of causing a balanced conductivity response right away. PANi conductivity can be manipulated in a reversible manner using electrochemical or chemical oxidation/reduction methods, as well as protonation/deprotonation in a simple acid base chemical reaction. The electrical conductivity of this polymer is determined by the polymer backbone’s ability to transport charge carriers and carriers jumping between neighboring polymer chains. The conduction mechanism relies on delocalized electron clouds and/or non-bonding electrons to provide a conduction path through the polymer chains when no additives are present. The low conductivity of these polymers in their pure state, on the order of 10 100 mho/cm, through a doping procedure, can easily be increased to values on the order of 103 105 mho/cm. Polyaniline’s electrical conductivity increases with doping from the undoped insulating emeraldine base form (10210 mho/cm) to the completely doped conducting emeraldine salt form (.1 mho/cm) by changing the counter ions (dopant) and the degree of doping. Fig. 8.2 shows the repeat unit of polyaniline’s emeraldine oxidation state in the

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Figure 8.2 Emeraldine oxidation state of polyaniline in the undoped, base form, and the fully doped, acid form.

undoped, base form and the fully doped, acid form. Any strong acid, HX, can be used to dope, with X acting as a counter ion to preserve charge balance. Any solid foundation can be used to perform dedoping, OH. By regulating the pH of the dopant acid solution, dopants can be added in any desired quantity until all imine nitrogens (half of the total nitrogens) are doped. The emeraldine salt form can also be combined with common bases including ammonium hydroxide to extract dopants. PANi polymer is prone to acid/base and reducing/oxidizing compounds such as ammonia (NH3), nitrogen dioxide (NO2), hydrogen (H2), and certain volatile organic compounds (VOCs) due to its ability to move between conducting and insulating modes [27,28]. PANi has sensing capabilities at room temperature and is easy to use, making it a promising candidate for the production of a variety of gas sensors. The sensing characteristics of PANi properties are altered by dopants or interface interaction in a composite [29 31]. The various acidic and basic gases can be detected by PANI, as conductivity of doped PANI can be controlled by acidic and basic reactions. For example, when PANI comes in contact with ammonia gas, there is large decrease in conductivity of PANI. Due to the shift of proton from NH of PANI to ammonia gas, forming ammonium ion. When PANI exposed to normal air, the ammonium ion gets converted back

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to ammonia gas. Hence, the process is reversible. PANi’s response mechanism to a variety of gases and vapors was investigated by Hussein et al. [32], who used a four-probe process, elemental analysis, and the X-ray fluorescence (XRF) technique to measure its conductance and mass changes. Films were exposed to dilute ammonia solution and the conductance changes were found to be partly due to two-stage sorption, possibly involving polymer swelling and gas diffusion. PANi sensing of different gases and vapors results in a mixed response involving both electronic and physical effects, according to the researchers. Sadek et al. [33] developed surface acoustic wave (SAW) gas sensors based on PANi nanofibers generated by a template-free aniline rapid mixed polymerization. At room temperature, PANi nanofiber-based SAW gas sensors demonstrated quick response and recovery times, as well as good repeatability and baseline stability for various H2 gas concentrations. Such intriguing sensing properties at room temperature pave the way for low-power sensors made from low-cost PANi-based nanofibers. Krutovertsev et al. [34] studied the modified PANi films doped with 2:18 series hetero-polycompounds with respect to ammonia. It demonstrated a high sensitivity of PANi-based sensors toward ammonia. PANi-based ultra-thin films can be used as sensitive elements in gas sensors, according to Xie et al. [35], who concluded that PANi-based ultra-thin films can be used as sensitive elements in gas sensors, with promising applications particularly for microsensors. The oxidative doping of NO2, which results in a shift in conductivity of PANi thin film, is explained as the mechanism of sensitivity of PANi dependent thin film. Deshpande et al. investigated the gas sensing properties of PANi thin films on glass and Si substrates [36]. Fibrous PANi thin films had better gas sensing properties than dendritic PANi thin films. PANi thin film obtained by anodic polymerization on Au and ITO substrates using a variety of counter electrodes. Preliminary sensing tests proved that PANi might be used for humidity and acidity detection [37]. The effect of the dopant’s existence on the response of a sensor array based on PANi films under the influence of organic solvent vapor was investigated. The embracing capacity of the analyte molecules and the morphology of the films, as well as the likelihood of additional donor acceptor interaction between the analyte molecules and the dopant, were discovered to be the main factors determining the magnitude of the PANi films’ response. It was demonstrated that using hetero-polyacids as PANi dopants would significantly improve the selectivity of the sensor array’s response [38]. According to Virji et al., when exposed to 100 ppm HCl acid, the resistance of a de-doped PANi film changes by orders of

Chapter 8 Conducting polymer-based gas sensors

magnitude, whereas when exposed to 50 ppm ammonia, the resistance of a doped PANi film changes by a factor of two. Since the addition of even a few charge carriers quickly increases the conductivity of the films, the doping (i.e., acid) response is greater than the dedoping (i.e., base) response [39,40]. When protonic acid (such as HCl, H2SO4, HClO4) is present in PANi, the resistance of the sensor increases due to the formation of a co-ordinate bond between the nitrogen atoms (having lone pair) of ammonia molecules and the free atomic orbital of the proton on doped PANi, according to Li et al. [41]. As shown in Fig. 1.5, the increase in resistance is induced by the decrease in charge carries caused by deprotonation of doped PANi. There are several exceptions, such as when doped PANi is exposed to ammonia gas, and the electrical resistance of the doped PANi decreases. As acrylic acid doped PANi (AA-PANi) is exposed to ammonia vapor, for example, its resistance decreases [42]. In this case, NH3 molecules remove a proton from AA-PANi in one of two ways: from the -NH group of doped PANi, which increases resistance due to deprotonation, or from the proton of the acrylic acid molecule trapped in PANi chains, which decreases resistance due to an increase in conduction sites. Some dangerous gases, such as CO, responded as well, despite the fact that they are inert at room temperature. When exposed to CO gas, the sensor resistance decreased [43]. The CO sensing process was defined as the removal of lone pair electrons from amine nitrogen by CO. The positive charge carriers in PANi chains increased as a result, and the sensor resistance decreased. The grain model was used to create another CO response mechanism. The reduction in grain height between polymer grains was due to the decrease in sensor resistance of PANi films, which is close to the gas sensing mechanism of metal oxide-based sensors. Due to the presence of CO gas, oxidation at grain surfaces decreased the barrier height and depletion time. Composites of CPs with metal or metal oxide [44 47] nanoparticles, carbon nanotubes [48 50], organic and metal organic [51] compounds and insulating polymers [52,53] provide a new analytical possibility. With the CP with a depletion field, metal oxide particles are thought to form n p heterojunctions. The depletion region is changed and the conductivity of the junction is modulated by adsorbed gases [54 56]. PANi nanocomposites are studied extensively for NH3 detection at room temperature. Kumar et al. [57] fabricated PANi thin films by the use of gold nanostars (AuNS) that have been chemically synthesized as catalysts in the AuNS-PANi (AuNS170 nm) composites sensitivity improved by up to 52%. Similarly, Tai et al. investigated the NH3 gas-sensing

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properties of a PANi/TiO2 nanocomposite fabricated using an insitu chemical oxidation polymerization process and discovered that it had higher conductivity than pristine PANi [33]. Using electro-polymerization, Zhang et al. [58] created a gas sensor based on CSA-doped PANi SWCNT nanocomposite (diameter 17 25 nm) for the selective and sensitive detection of NH3 (10 ppb 400 ppm). At 0% relative humidity (RH), the sensor response for 400 ppm of NH3 gas was 50 (RH) [37]. Using the in situ self-assembly method, PANi nanocomposites with TiO2, SnO2, and In2O3 were fabricated by Tai et al. [59] for NH3 sensing (23 141 ppm). All PANi-based nanocomposite systems were found to have a faster response time (2 3 seconds) and recovery time (23 50 seconds), as well as better reproducibility (4 cycles) and long-term stability (30 days). P-type PANi and n-type oxide semiconductor can form a hetero junction and a positively charged depletion layer on the surface of inorganic nanoparticles, lowering the activation energy and enthalpy of physisorption for NH3, resulting in better gas sensing properties than pure PANi thin film. Gong et al. [60] electrospun p-type conductive PANi nanograin onto an n-type semiconductive TiO2 fiber surface for NH3 detection as low as 10 ppt [40]. Pawar et al. [61] developed a PANi/TiO2 nanocomposite with strong gas response up to 20 ppm for selective NH3 detection. With a response time of 41 seconds and a recovery time of 520 seconds, the sensor response for NH3 (100 ppm) was found to be 48% [61]. Using the cyclic voltammetry technique, Shirsat et al. developed a sensitive and selective chemiresistive sensor for H2S detection at room temperature using gold nanoparticles electrochemically functionalized polyaniline nanowires [62]. When PANI is exposed to H2S gas, PANI does not react with H2S gas. Hence, there is no interaction and does not show any electrical conductivity, as H2S gas is weak acid. But when PANI nanowire network was functionalised with gold nanoparticles by electrodeposition method which were of the size of 70 120 nm, this sensor shows excellent response to H2S gas. Even at very low concentration of 0.1 ppb, the sensor shows very good selectivity and reproducibility. There is drop in resistance of sensor material, due to transfer of electron from PANI to Au. For trace level H2S gas detection the PANI/CuCl2 sensor was screen-printed on interdigital electrode by Crowley and co-workers [63]. For selective detection of H2S gas (10 100 ppm), a CSA doped PANI CDS nanocomposite was synthesized via chemical polymerization method by Raut and his co-workers [64]. The maximum response of 76% was observed at 100 ppm for 40% doped CSA in PANI CDS nanocomposite, with 97.34% stability after 10 days. The

Chapter 8 Conducting polymer-based gas sensors

gases like NO2, CH3OH, C2H5OH, NH3 shows negligible response which is about 225%, for CSA doped PANI CDS sensor. Chaudhary and Kaur developed a PANI/WO3 hybrid nanocomposite with a honeycomb morphology for SO2 gas sensing [65]. At room temperature, when sensor was exposed to SO2 gas of 5, 10, 25, 40, 60 and 80 ppm and it shows response of B4.3%, B10.6%, B24%, B36%, B51.5% and B69.4% respectively. PANi/TiO2 nanocomposite thin film based chemirestive sensor for CO2 gas of 1000 ppm detection was developed by Nimkar et al. [66]. with sensor response of 5% at 35 C along with response and recovery time of 70 seconds and 80 seconds respectively. Sen et al. fabricated PANi/Co3O4 nanocomposites and observed their susceptibility to CO gas at room temperature [67]. Co3O4 NPs were synthesized using an ultrasound aided coprecipitation method and then integrated into the PANi matrix. The highest response was obtained for 75 ppm CO gas concentration, which was 0.81 for 40 seconds [67]. Sen et al. reported the detection of LPG gas at room temperature using a PANi/-Fe2O3 nanocomposite sensor [68]. For LPG gas, the sensor showed response of 1.3 at concentration of 200 ppm. Bhanvase et al. [69] used PANi and PANi/ZnMoO4 nanocomposite thin films with different ZnMoO4 (ZM) NP loadings to fabricate LPG sensors. With response and recovery times of 600 seconds and 840 seconds, the sensor response for LPG concentration (1800 ppm) was found to be 20.6% and 45.8% for PANi and PANi/ZM nanocomposite sensors, respectively. Shinde et al. announced the fabrication of a PANi/ Cu2ZnSnS4 (CZTS) thin film based heterostructure for a room temperature-based LPG sensor [70]. At 0.06 vol.% LPG gas, the sensor displays a maximum gas response of 44%. The LPG response decreased from 44% to 12% when the relative humidity was 90%. The LPG response decreased from 44% to 12% when the relative humidity was 90%. Xu et al. [71] demonstrated NO2 sensing using a SnO2 ZnO/PANi composite thick film and a solvothermal hot-press fabrication process. At 180 C, the sensor showed a high sensor response (368.9%). The selectivity of various analytes such as NO2, NH3, H2, C2H5OH, and CO was less than 3%. Sharma et al. investigated the gas detecting properties of (0.5% 3% PANi)-SnO2 sensors for trace NO2 gas detection [72]. The high sensitivity toward NO2 gas of 301% at 40 C for 10 ppm of gas, was shown by (1% PANi)-SnO2 sensor film. Xu et al. [73] reported SnO2/PANi double-layered film sensor synthesized with nanoporous SnO2 and PANi layers and detection of NO2 gas. The double layered film sensor exhibits high selectivity and reaction even at very low NO2 gas concentrations. To 37 ppm NO2 at 140 C, the sensor response, response time, and recovery time

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were found to be 4%, 17 seconds, and 25 seconds, respectively. A depletion layer forms between the junction interface in a SnO2/PANi double-layered film sensor, causing a significant resistivity difference between air and NO2. As a result, the sensor has a better response for NO2 gas sensing.

8.4

Polypyrrole as gas sensing material

Oh and team synthesized nanocomposite which contains Ru nanoclusters (Ru/CPPy) that are decorated on carboxyl polypyrrole nanoparticles. Spin-coating process was used to immobilized Ru/CPPy on the interdigitated array electrode. On the surface of Ru/CPPy sensor, the H2 gas molecules gets adsorbed and forms H2 atoms as Ru has reducing properties. Further, these electrons get transferred from the Ru to the PPy and hence, the charge (hole) transfer in the polymer chain reduced. The CPPy NPs-based electrode shows no signal, since there are no active sites against H2 gas molecules, there is no signal change. The Ru-particle-based electrode demonstrated a reaction to high concentrations of H2 gas due to the limited active surface area of H2 gas molecules. The carboxyl PPy and Ru nanoclusters served as both charge (hole) transfer pathways and active sites for the target analyte during sensing. As the temperature increases, response and recovery times reduces. As the H2 gas desorbs faster, the response time decreased faster than the recovery time, with increase in working temperature. The morphology disintegration of the Ru component during H2 exposure, was prevented as the Ru nanostructures were bonded on the CPPy NP surface without aggregation. For about 15 days, the sensor showed sensing ability to 1 ppm of H2 gas. Even after repeated exposure to H2 gas, there were no morphological transitions in the nanoparticles. Sensor shows enhanced selectivity toward H2 gas at low concentration of 1 ppm, as compared to other VOCs of 100 ppm. The sensor response was 5 times greater to H2 gas than that of other gases [74]. Majumdar and team developed a simple vapor phase polymerization process for producing polypyrrole coated filter papers (PPy-FP) (Fig. 8.3). The polymer-coated filter papers were used to make HCl-PPy-FP, CSA-PPy-FP, and PTSA-PPy-FP composites which were doped with hydrochloric acid (HCl), camphor sulfonic acid (CSA), and p-toluene sulfonic acid (PTSA). Polarons and bipolarons are generated which serve as charge carriers in conducting polymers PPy, and their movement is regulated by doping which leads to development of

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Figure 8.3 (A) Schematic representation of the synthesis of PPy-FP; the inset shows the photograph of the BlankFP and PPy-FP. (B) Photographs showing the flexibility of the PPy-FP [75], 2012 American Chemical Society.

conductivity in PPy chain. The conductivity order for sensors is HCl-PPy-FP . CSA-PPy-FP . PTSA-PPy-FP . PPy-FP. Due to addition of HCl the doping level increases and also there is generation of some additional bands which leads to improvement in mobility of charge carriers. The conjugation in chain is decreased with CSA and PTSA, since they are large molecules that cause adjacent ring distortion in the PPy chain. Since HCl doping aids in more compact PPy chain alignment, more localized polaron and bipolaron sites are developed, and charge mobility is improved. The PPy-FP sensor shows enhanced response to NH3 gas as compared to that of acetone and ethanol. As NH3 is reducing gas containing free lone pair of electrons, these electrons are donated to PPy and hence reduction of PPY chain occurs (Fig. 8.4). The sensing performance of the sensor increases with doping. As PPy-FP was exposed to 100 ppm NH3 vapor, the resistance of the compound improved by 43.5%, increasing to 62.1%, 70.7%, and 84.5% for PTSA-PPy-FP, CSA-PPy-FP, and HCl-PPy-FP, respectively. After doping, there is increase in maximum limit of detectable NH3 vapor. The response of PTSA-PPy-FP, CSA-PPy-FP, and HCl-PPy-FP to NH3 vapor concentration is up to 200, 230, and 300 ppm, respectively. The PTSA-PPy-FP, CSA-PPy-FP, and HCl-PPy-FP sensor calculated LOD values were 10, 7.9, and 5.2 ppm, respectively. The average response and recovery times for PPy-FP are 53 and 79 seconds, respectively, for the three measurements. The response time improves and recovery time slightly increases due to doping. For these PTSA-PPy-FP, CSA-PPy-FP, and HCl-PPy-FP three different types of PPy-FP, the response times were 24 seconds, 23 seconds, and 20 seconds, respectively, and the recovery times were 88 seconds, 90 seconds, and 110 seconds [75]. Tiwari and co-workers synthesized reduced graphene oxide/ polypyrrole nanocomposites (PPy/rGO) by drop cast in-situ

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Figure 8.4 Schematic representation of the mechanism involved in NH3 gas sensing (A) electron transfer and (B) proton transfer [75], 2012 American Chemical Society.

Figure 8.5 (A) Schematic representation of deposition of thin film by drop cast in-situ method. (B) Photograph of prepared thin film of the PPy/rGO composite [76].

oxidative polymerization method (Fig. 8.5). When exposed to NH3 steam, the sensing film’s resistance increases at first. Due to conducting nature of graphene, PPy/rGO composites have a lower resistance than PPy thin film sensors. When sensor is

Chapter 8 Conducting polymer-based gas sensors

exposed to NH3 gas, the resistance of PPy/rGO composites increases. When PPy/rGO sensor is exposed to NH3 gas, the NH3 gas molecule transfers electron to PPy/rGO composite and resistance increases as the positive hole-density decreases. The high sensitivity of the PPy/rGO sensor is not only to the higher surface area and porous nature of the PPy/rGO, but also due to the existence of certain defect sites in the rGO caused by the chemical reduction of GO. Because of high-energy binding sites including vacancies, structural flaws, and oxygen functional classes, the main drawback of rGO-based gas sensing devices is that they have low recovery. As the NH3 gas is adsorbed onto the nanocomposite, the current drops from its initial value, lowering conductivity. As graphene and PPy are p-type materials, NH3 serves as an n-type dopant as a result of the lower hole density, the current is decreased. But after heating the sensor at approximately 50 C it attains the initial values. After the gas sensing experiment, the recovery is achieved by heating the sensors [76]. Mekki and team developed by facile direct electrochemical oxidation of pyrrole in an aqueous solution of dodecylbenzene sulfonic acid, polypyrrole-dodecylbenzene sulfonic acid (PPy/DBSA) films is deposited on N- (3-trimethoxysilylpropyl) pyrrole modified ITO coated polyethylene terephthalate (PET) flexible substrate (DBSA). The PPy/DBSA sensor shows high selectivity to NH3 gas as compared to that of carbon dioxide, ethanol, methanol, hydrogen sulfide, acetone, and nitrogen dioxide at room temperature. As the concentration of NH3 gas rises, so does the sensor’s response. PPy-DBSA film showed high response toward NH3. The sensor had a lower response to the other target, which was made up of reducing gases like H2S, C2H5OH, CH3OH, and oxidizing gas such as NO2, CO2, and C3H6O. It’s clear that the film’s high rate of response is geared toward minimizing NH3 vapors. Due to the electrostatic interaction between the PPyDBSA structure and the NH3 vapor and the presence of sulfonyl groups in the DBSA dopant structure, it causes the PPy to build more hydrogen bonds [77]. Yague and Borro´s synthesized polypyrrole (PPy) thin films, which have been obtained using plasma enhanced chemical vapor deposition (PECVD) as the deposition technique and iodine vapor as the doping agent. CO2 gas exposure causes an increase in sensor resistance. Since each CQO bond in the carbon dioxide molecule has two lone pairs of electrons. The polymer chain gets reduced by formation of reversible bonds, which is due to interaction of positive charge on the PPy and lone pair of electrons. Therefore, due to this interaction, as the number

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of effective positive charges in the polymer decreases, so does its conductivity. High sensitivity is shown by thicker films, leading to more interaction of CO2 molecule with the polymer. Hence, resistance change observed is large. The various responses shown for the same experimental conditions provide extremely useful information since they can be used to establish an analyte’s fingerprint, which can then be used for further identification and quantification. Three cycles were carried out to check the reproducibility of the PPy sensor. The first cycle was found to have a higher sensitivity than the other two cycles, which were almost constant. The adsorption time is approximately 10 minutes, and the desorption time is approximately 20 minutes in all three cycles. Also, the recovery process shows almost similar values and often comes close to achieving complete recovery. The electrical conductivity of the polymer returns to its original state, when the gas molecules are displaced. Hence, after a period of time CO2 molecules get desorbed as, they are reversibly bounded to polymer [78]. Yang and co-workers synthesized polypyrrole/silver composite nanotubes (PPy/Ag). The response of PPy nanotubes is improved by introduction of metal nanoparticles in it. The collection of several charges is affected by each trapped Ag site. The larger Ag nanoparticles present in PPy/Ag composite nanotubes, provides larger electron trap sites due to which when in contact with NH3 gas, large resistance change is observed. After removal of NH3 gas, the conductivity is lost. The size and distribution of modifier sites changes as the organic thin matrix is modified, due to which optimization of response is achieved. The resistance of the composite nanotubes steadily improved as the NH3 gas concentration increased. At a low concentration of 10 ppm of NH3 gas, the resistance of the composite nanotubes changes. At room temperature, the PPy/Ag composite nanotubes sensor has excellent reversibility and reproducibility. Even after removing NH3 gas and purging N2 gas for more than 600 seconds, the PPy/Ag composite nanotubes sensor synthesized without PVP does not return to its original conductive state. PVP could not return to its conductive state after removal of NH3 in the case of PPy/Ag composite nanotubes synthesized, but the response in the first and third circles almost returned to the same baseline, indicating that the sensor has fair reproducibility. PPy/Ag composite nanotubes synthesized without PVP shows full recovery to their original state with high-purity N2 within 500 seconds due to the nanocomposites smaller size and uniform distribution of Ag nanoparticles. Because of the uniform distribution of Ag nanoparticles in the PPy/Ag composite

Chapter 8 Conducting polymer-based gas sensors

Figure 8.6 Preparation procedure of SnO2/PPy nanocomposite [80].

nanotubes, NH3 molecules diffuse uniformly in and out. The use of a sensor can be achieved in a reversible and repeatable manner. As compared to that of pure PPy, PPy/Ag composite nanotubes are more effective in detecting NH3. Sensors based on PPy nanotubes packed with uniform and tiny Ag nanoparticles have excellent reversibility and reproducibility [79]. Li and team synthesized novel polypyrrole-coated SnO2 (PPy/SnO2) nanosheet nanocomposites by vapor phase polymerization method (Fig. 8.6). The PPy/SnO2 nanocomposite with a polymerization time of 1 hour displays the highest Sensitivity of 15% and 75% when exposed to NH3 at 1 and 10.7 ppm, respectively, which is much more than pure PPy alone. With the threshold limit of 25 ppm for NH3, the PPy/SnO2 sensor shows limit of detection to be B257 ppb. As compared to propylamine and butylamine, the sensor shows much higher response magnitude (S) toward NH3 gas (Fig. 8.7). As compared to the organic vapors of 5000 ppm, the PPy/SnO2 nanocomposite sensor shows much higher S higher response magnitude to NH3 of 10.7 ppm. When PPy/SnO2 nanocomposite sensor, is exposed to NH3 gas, the resistance of composite increases but when sensor is exposed to organic vapors, the resistance of the composite decreases. The excellent selectivity to NH3 gas on PPy/SnO2 sensor is observed due to combination of two-dimensional p-type PPy and n-type SnO2 nanosheets. At the interface of SnO2 and PPy, the p/n junction is formed and hence, depleting zone is established. The NH3 molecules’ diffusion through the PPy overlayer which is facilitated by the nanostructure of the composite. In PPy/SnO2

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Figure 8.7 Response magnitude of SnO2/PPy nanocomposite to different vapors at room temperature. Concentration of the vapors: [NH3] 5 10.7 ppm; [Propyl amine] 5 [butyl amine] 5 10 ppm; 5000 ppm for other organic solvents. (doping acid: PSSA, polymerization time: 1 h) [80].

nanocomposite, the NH3 molecules get adsorbed and the protons in PPy were withdrawn by the nanocomposite. Hence, the concentration of holes in PPy chain decreases. The resistance increases as the PPy was deprotonated. The resistance increases as the depletion layer is expanding (i.e., increment in the depletion barrier height), which due to interaction of nanocomposite with NH3. The nanocomposite shows enhanced sensitivity to NH3 at room temperature as compared to pure PPy alone, due to formation of p-n heterojunction and hence leads to a greater change in resistance and a greater magnitude of response [80]. Zhang and co-workers synthesized Polypyrrole-coated SnO2 (PPy/SnO2) hollow spheres hybrid material by an in-situ polymerization method. PPy is p-type and SnO2 is n-type semiconductors. Between p-type PPy, an electron donor, and n-type SnO2, an electron acceptor, p-n heterojunctions are formed. The improved response of the sensor is due to the increase in depletion barrier height formed between donor-acceptor system. The PPy/SnO2 sensor shows recovery times of several minutes and quite quick response times of 15 seconds, 12 seconds, and 9 seconds was observed towards 3, 10, and 20 ppm NH3 [81]. Cho and team synthesized polypyrrole (PPy) by chemical oxidative method. The sensitivity of PPy sensors is higher in dry conditions than in humid conditions, and it decreases as

Chapter 8 Conducting polymer-based gas sensors

humidity increases. Since water vapor adsorbs faster than CH3OH, the more water vapor applied, the less CH3OH vapor was adsorbed on the surface of the PPy sensor. The sensitivity of sensor shows excellent reproducibility for about 10 cycles. As the CH3OH gas was switched off, there was no complete recovery of baseline in the first test. For further cycles, this phenomenon did not occur any longer. The rise in gap was observed between recovery curve and initial baseline, which was due to chemically absorbed CH3OH molecules on the sensor surface that is because of the heavy chemical bonding with the polymer chain, it could not be desorbed during the recovery process. CH3OH molecules adsorb and desorb at biologically active sites on the polymer surface, resulting in perfect recovery. To measure response and recovery times, the duration of a 90% total resistance change was used. The sensor’s response and recovery time were observed to be stable for more than 200 seconds and 500 seconds, respectively, at temperatures about 0 C and 40 C. The sensor’s response and recovery times were found to be less than 150 seconds and 400 seconds, respectively, at a temperature of 25 C. The polymer sensors with operating temperature 0 C and 40 C shows slower response as compared to that sensor operating at 25 C. The sensitivity variance of the PPy sensor increased as humidity increased. At high humidity, the resistance shift caused by CH3OH gas was very small, so the effect of temperature was magnified. At 25 C, the sensitivity variation of sensors under RH 5 and 10% showed the most variation, while the sensitivity variation of sensors under RH 20% shows a small increase with increasing temperature [82]. Pirsa and Lotfi synthesized polypyrrole (PPy) on the polyester and teflon fibers by solution phase and vapor phase polymerization method. Electron transfer causes changes in the sensing material’s resistance and work function. This process occurred when PPy film exposed in DMF, methylamine and other redoxactive gasses. Electron donating gas, such as methylamine can enhance the electron density at the aromatic rings of PPy. As this happens to a p-type PPy, the doping level as well as the PPy’s electric conductance are reduced. However, the selectivity of this sensor is poor, and water has significant interference in the different gas sample analysis by the PPy gas sensor, a fact that could limit the widespread use of PPy in sample analysis in atmospheric conditions. As a result, we agreed to increase the PPy sensor’s selectivity and reduce water interference. A hydrophobic teflon fiber was used as a sensor substrate to decrease water interference. When fabricated PPy and PPy-m sensors are exposed to vapors of some VOCs. The relative resistance

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difference (RRD) of the sensor is used to quantify the signal obtained from the interaction of the sensor and the volatile compound. PPy-polyester-solution polymerization-teflon membrane has the best responses (selectivity) to methylamine. Results show that using of hydrophobic substrate and hydrophobic membrane in gas sensor based on the PPy result in decreases water (humidity) interference in the other gas sample detection, so it is possible to the determination of gas samples in atmospheric condition. The sensor when exposed to sample gas shows complete recovery, as it returns back to its original value in all cycles. Also, all the subsequent cycle tests are reversible. It was observed that when sensor was exposed to analyte gas, there was an increase in electrical resistance. When sensor was exposed to air, the sensor showed complete recovery. The PPy-polyester-solution polymerization-teflon membrane sensor exhibits a high sensitivity to the methylamine and PPy-polyester-solution polymerizationpolypropylene membrane sensor exhibits a high sensitivity to the DMF. As a result, it could be used as a highly selective sensor for methylamine (DL 5 100 ng) and DMF (DL 5 0.6 g) detection [83]. Chitte and co-workers modified polypyrrole (PPy). Polypyrrole (PPy) was synthesized using ferric chloride (FeCl3) as an oxidant and dopants such as lithium per chlorate (LiClO4), para-toluene sulfonate (p-TS), and naphthalene sulfonic acid as dopants (NSA)When PPy doped with p-TS and NSA were exposed to NH3 gas, the current decreases. PPy doped with p-TS and NSA has higher electrical conductivity than pure PPy, indicating that a large number of charge carriers are generated and the doping level is high. The pyrrole ring has more charge on it because of the sulfonic acid group’s existence. Due to the presence of NH3 gas molecule, no extra charge is produced but leads to decrease in effective charge. The PPy samples doped with p-TS and NSA show a slight rise in electrical current (1 to 2 a) when exposed to LPG gas. The difference in electrical resistance, is due to the type of dopant present in PPy. When compared to NH3 steam, the range of effective current change was found to be very small. The response time of NH3 gas was just few seconds while that of LPG was of few minutes. Since LPG gas is primarily composed of Butane (C4H10) and Propane (C3H8), it has little electronic contact with the PPy. Hence, the PPy materials synthesized is not as effective at detecting LPG gas as it is at detecting NH3 gas [84]. Mane and team synthesized dodecyl benzene sulfonic acid (DBSA) doped PPy WO3 hybrid nanocomposites (Fig. 8.8). In gas sensing study of chemiresistive gas sensors, doping plays an important role. The formation of p n heterojunctions is induced by p-type PPy and n-type WO3 with uniform DBSA

Chapter 8 Conducting polymer-based gas sensors

Figure 8.8 Schematic formation of DBSA doped PPy WO3 hybrid nanocomposite film [85].

distribution over the sensor surface in a 20% DBSA doped PPy WO3 hybrid nanocomposite sensor. The 20% DBSA doped PPy WO3 hybrid nanocomposite sensor shows a decrease in resistance when exposed to oxidizing NO2 steam (electronaccepting). The CSA doped PPy Fe2O3 based sensor shows same behavior as these sensors. Due to the electron charge transfer process, the adsorption of NO2 gas by the DBSA doped PPy WO3 hybrid nanocomposite sensor results in a shift in resistance. Since the WO3 nanoparticles are thoroughly integrated in the PPy matrix, the electron charge transfer occurs between NO2 and PPy, when exposed to NO2 gas. NO2 gas molecules interact with PPy’s p electron networks, reducing sensor resistance with embedded WO3 nanoparticles obtained from the PPy chain. The p-type semiconductor behavior is confirmed by decrease in resistance of DBSA doped PPy WO3 hybrid nanocomposite sensor film and PPy leads to charge transfer. In the PPy WO3 hybrid nanocomposite, to increase the rate of a reaction by adding more active sites, DBSA is added. The addition of dopant causes the development of distinct donor levels (ED) below the bottom edge of the conduction band in n-type WO3. The impurity atoms in the donor level are spaced far apart at low doping concentrations. Hence, no interaction with one another. As the doping concentration

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increases, there is decrease in separation between doping atoms and hence, interaction between both material increases. In case of hybrid sensors greater number of electrons are available, which are accepted by NO2 gas. Hence, resistance decreases and sensor response increases. The construction of energy band diagram was done by experimentally measured values of band gap energies (Fig. 8.9). Between the p-type PPy and the n-type WO3, a p-n heterojunction is formed. Due to which the depletion layer is formed and donor energy level splits. The width of the depletion layer narrows as the surface of a DBSA doped PPy WO3 hybrid nanocomposite sensor is exposed to NO2, reducing the sensor’s resistance. As a result, charge carriers are quickly transported through the sensor’s respective energy band. Further, as the DBSA doped PPy WO3 sensor material’s resistance decreases, the gas sensing response is enhanced. The stability of about 90% was observed by 20% DBSA doped PPy WO3 hybrid nanocomposite sensor for NO2 gas. At first, the response of sensor is 72% and then after 20 days it drops to 65% and later almost remains constant till 40 days. Because of the ageing factor in polymer materials i.e., humidity effects there is decrease in response of sensor. The 20% DBSA doped PPy WO3 hybrid nanocomposite sensor has a higher level of reliability than the 50% DBSA doped PPy WO3 hybrid nanocomposite sensor. The reaction time and recovery time were found to be inversely proportional to the NO2 gas concentration. When the concentration was increased from 5 to 100 ppm of NO2 gas, the decrease in response time was observed from 371 seconds to 288 seconds and the increase in response time was observed from 1094 seconds to 5990 seconds. Response time is reduced since higher concentrations of NO2 gas have a faster diffusion rate than lower concentrations. The recovery period that is left behind after gas contact increases as a result of gas reaction species, resulting in a decrease in desorption rate [85]. Thangamani and co-workers synthesized polyvinyl alcohol (PVA)/polypyrrole (WPPy)/vanadium pentoxide (V2O5) nanocomposite by the solution casting method (Fig. 8.10). 15 wt.% V2O5 loaded sensor shows high selectivity toward LPG gas, as compared to other gases like benzene, xylene, toluene, acetone, butane, propane, pentane, ethane and ammonia (Fig. 8.11). At room temperature, the PVA/WPPy/V2O5 nanocomposite films outperform the PVA/WPPy blend in terms of sensitivity, efficient reaction, and recovery time for LPG gas. At 50 C, both the PVA/ WPPy blend and the PVA/WPPy/V2O5 nanocomposite have a lower sensitivity response to LPG gas. The charge carrier

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Figure 8.9 Schematic view of (A) energy band diagram of DBSA doped PPy WO3 hybrid nanocomposite in presence of air and (B) energy band diagram of DBSA doped PPy WO3 hybrid nanocomposite in presence of NO2 gas [85].

concentration rises as the amount of V2O5 nanorods in the PVA/WPPy blend matrix increases leading to enhanced sensitivity response. With increasing temperature, the sensitivity of blend and nanocomposite films increases with temperature, suggesting that LPG molecules can be detected. As compared to other temperature, the sensor shows excellent selectivity response at room temperature for LPG gas. The sensor shows maximum selectivity at 600 ppm and at 1000 ppm of LPG gas saturation was observed at room temperature. As the LPG gas concentration reaches above 1000 ppm, saturation is seen in sensing response. Saturation occurs as a multilayer of LPG molecules forms on the surface of nanocomposite films, further blocking the sensing sites. The response and recovery time shown by PVA/WPPy blend sensor is 29 seconds and 31 seconds respectively. With a 15% increase in V2O5 material in the PVA/WPPy blend matrix, response and recovery times are

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Figure 8.10 Synthesis of PVA/WPPy/V2O5 nanocomposite films [86].

Figure 8.11 Selectivity of chemiresistive sensor based on PVA/WPPy/V2O5 nanocomposite film with 15 wt.% V2O5 loading at Room Temperature [86].

reduced to 10 seconds and 8 seconds, respectively. When the filler loading is increased to 20 wt.%, there was an increase in response and recovery time of the sensor which was 10 seconds and 9 seconds, respectively. It was observed that even after 3 months, the PVA/WPPy/V2O5 nanocomposite shows no noticeable deviation in the behavior of sensor response. For 600 ppm LPG gas concentration at room temperature, the PVA/WPPy/V2O5 nanocomposite-based chemiresistive gas sensor demonstrates longterm stability of about 90 days [86].

Chapter 8 Conducting polymer-based gas sensors

Patois and team developed a sulfonated cobalt phthalocyanine (sCoPc) which was electrochemically incorporated into conducting polypyrrole (PPy/sCoPc). Polypyrrole is made of both neutral and oxidized monomer units and is p-type semi-conducting material. As polypyrrole is exposed to ammonia gas, an electron transfer occurs between ammonia molecules, resulting in oxidized monomer units. The oxidized monomer unit receives one electron from Ammonia and hence, the monomer unit becomes neutral. As the ammonia gets adsorbed on sensor, the number of oxidized units in the polymer chain is decreasing. The ammonia gas cause reduction of polymer, due to which conductance of the film decreases. When ammonia gas gets desorbed from the sensor film, the conductance of the sensor increases in air. Very less studies are done on the mechanism of polypyrrole/ phthalocyanine films and ammonia molecules. The phthalocyanines when incorporated in PPy leads to electron transfer and lengthening of the conjugation. As phthalocyanines have many conjugated bonds and are semiconducting materials. The central metal ion and the conjugated -electron structure are two examples of gas adsorption sites in metal phthalocyanines. Also, highly conjugated π π systems is observed in metal phthalocyanines. The interaction mechanism of PPy/sCoPc and ammonia is same as PPy and ammonia gas. The PPy/sCoPc hybrid polymer films act as p-type and ammonia acts as electron donor. The PPy/ sCoPc hybrid polymer films show enhanced response as compared to pure PPy, as central cobalt in PPy/sCoPc hybrid polymer films also adsorbs some ammonia molecules [87]. Hernandez and co-workers synthesized single nanowire of conducting polypyrrole (PPy) by chemical polymerization method. The excellent selectivity toward ammonia gas is observed by sensor and even at concentrations as high as 100 ppm, shows no detectable response to nitric dioxide (NO2). When PPy is in highly doped state, it does not show any response to NO2 gas. As NO2 gas being electron acceptor, it cannot extract electrons from the backbone. The sensors based on PPy, are sensitive to very humidity. Due to humidity, the response decreases to almost 75% and 5% increase in relative humidity. Until analysis, the sample is passed through a desiccant, or the chloride is substituted with a more hydrophobic dopant, such as p-toluene sulfonate, to solve the humidity issue. The good limit of detection, sensitivity and excellent selectivity for gaseous ammonia is observed for single nanowires-based sensors [88]. Tai and team developed the in situ self-assembly technique was used to create TiO2/polypyrrole (TiO2/PPy) nanocomposite ultrathin films. When exposed to NH3 levels in the air and N2 in

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the atmosphere, the TiO2/PPy ultrathin film gas sensor was tested for reaction and recovery properties. The TiO2/PPy ultrathin film gas sensor shows increase in resistance, as NH3 gas concentration increases and decrease in resistance as NH3 is replace by air or N2. When the concentration of NH3 increases, the sensor’s sensitivity improves. Sensitivity in N2 is lower than that of sensitivity in air. Due to the moisture and O2 in air, the response time or the recovery time in air is faster than in N2. When a TiO2/PPy ultrathin film sensor is exposed to NH3 gas, the NH3 molecules absorb protons from the PPy molecules, leading to formation of ammonium-NH41. In ambient air, the ammonia and protons are produced due to decomposition of ammonium. As ammonia and protons are applied to PPy, the original level of doping is restored. The variation in sensitivity and response/recovery rate is due to the potential of the CO atom to donate electrons differs from that of the NH3 atom. The TiO2/PPy sensor shows response time and recovery time, which are very shorter as compared to pure PPy sensor. The TiO2/PPy and PPy ultrathin-film sensors shows increase in sensitivity, the relative humidity rises from 5.9% to 92.8% as the temperature rises. The increase in sensitivity of sensor is due to combination of the PPy layer’s conductivity transition or as the surface of nanocomposite adsorbs water. The reconfiguration of nanocomposite film, shows that TiO2/PPy films also increased their resistance. The interaction between PPy and TiO2 particles is decreased at high temperatures. With increase in temperature, the PPy film’s resistance is decreasing due to thermal activated behavior [89]. Scindia and team fabricated polypyrrole (PPy) based sensors prepared by surfactant assisted doping. When PPy sensor was exposed to NH3 gas, the NH3 gets adsorbed on PPy surface. The nitrogen atom of NH3 gas has lone pair of electrons and PPy has holes which are charge carriers. The nitrogen atom containing lone pair of electrons are donated to PPy backbone, the formation of the NH1 moiety is the result of this process. When the electron transfers from NH3 molecule to PPy, the charge density on PPy increases and hence, resistance of sensor surface decreases. As NH3 gas gets adsorb on PPy, the PPy polymer becomes more conducting. Interaction of PPy with ammonia gas, leads to reduction in PPy. The LPG gas contains 60 mol.% of propane (C3H8) and 40 mol.% of butane (C4H10). Both types saturated hydrocarbons and unsaturated hydrocarbons are present in LPG gas. The electronic interaction of LPG gas with the PPy chain is restricted. As LPG gas has faint smell, some amount of odorant is added to detect trace amount of escaping gas. The stanching agent used in LPG gas is ethyl mercaptan

Chapter 8 Conducting polymer-based gas sensors

(C2H5SH), which is an organo-sulfur compound. The ethyl mercaptan is made up of two groups: an ethyl group (Et) and a thiol group (SH). Its structure is same as that of ethanol, only difference is that in place of oxygen there is sulfur atom. The EtSH is highly flammable gas and its odor is infamous. Due to very less ability ability to engage in hydrogen bonding, ethane thiol is more volatile as compared to ethanol. C2H5SH reacts with PPy and is attracted to the H1 ions in the PPy sensor as it comes into contact with it. Due to which the polymer becomes less conducting in nature and hence, resistance of sensor increases. The pure and doped PPy are oxidized due to C2H5SH group. The polymer surface can adsorb NO2 gas quickly as NO2 gas is a good oxidizing agent with a lot of nucleophilic properties. Due to these properties of NO2 gas, the NO2 gas is easily adsorbed on the PPy polymer surface. When NO2 gas is exposed to PPy sensor surface, the NO2 gas reacts with an electron of the sensor surface. The PPy sensor gets oxidized and there is formation of nitrite (NO22) nucleophile. The PPy has holes as charge carriers and NO22 nucleophile contain electron. These electron on NO22 nucleophile are transferred to PPy backbone chain. The charge density of PPy polymer matrix decreases as PPy gets oxidized. The gases like SO2, methanol, Cl2, CO, benzene, LPG, aniline shows same redox mechanism, when interacts with PPy or doped PPy sensor surface. When gases like SO2 or C6H6 are exposed to PPy or doped PPy sensor surface, PPy gets highly reduced as Fe21 ions are presenst in some amount in PPy matrix. The partial reduced state is observed in PPy and hence, PPy acts as an n-type semiconductor. The PPy based sensors shows excellent reproducibility. For about 4 cycles, the response time and recovery time of the sensor remains almost constant. As PPy interacts with molecules like CH3COCH3, C2H5OH, C6H5NH2, C6H6, CO, LPG, SO2 and Cl2, its resistance increases whereas, it decreases for NH3 vapor. For gases like aniline, NH3, Cl2, alcohol, CO vapors, the sensor shows fast response and recovery time, which is below the time limit of 150 seconds. The sensor shows quick response to LPG and SO2 gas which is just about few seconds. As the sensor has high capacity of holding gas vapor, the recovery observed is almost average. The sensor material is partially desorbed, after purging with air. The sensor shows slight variation in response time and recovery time, when exposed to gases. The Pure PPy shows high potential to be a good sensor, as high % response is observed for NH3 and Cl2 gas, which is about 93% and 70% respectively. CSA PPy demonstrated a quick response and recovery time of less than 60 seconds for NH3, C6H6, NO2, SO2 and Cl2. For

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CH3COCH3, CO, LPG and SO2, it shows response within 100 seconds and recovery for C2H5OH and C6H5NH2 within 150 seconds. Thus CSA PPy found to be very sensitive for CH3COCH3, C2H5OH, Cl2 and NH3, CO, LPG, NO2 and SO2. It shows high % response for NH3 (70%), C2H5OH (35%), LPG (18%) and NO2 (34%) exposures. The CSA PPy sensor shows high response for most of the gases, so can be efficiently used as gas sensors. When DBSA PPy sensor is exposed to different gases like Cl2, CO, CH3COCH3, it shows excellent response time and recovery time. CSA PPy demonstrated a fast response time and recovery time of less than 60 seconds, there is decrease in the resistance of the sensor due to reduction. The DBSA PPy sensor when exposed to gases like Cl2, CO, CH3COCH3, there is increase in resistance of the sensor due to oxidation. The DBSA PPy sensor shows fast response to Cl2 gas, as Cl2 gas gets adsorb quickly and due to the sensor’s high gas holding capability, recovery is slow. The DBSA PPy sensor shows high % response for Cl2 gas which is 88% as compared to that of CH3COCH3, which is about 45%. Hence, it can be used as a Cl2 gas sensor. When NSA-PPy sensor is exposed to gases like Cl2 and CH3COCH3, sensor shows increase in resistance due to oxidation. When NSA-PPy sensor is exposed to gases like NH3 and C6H6, sensor shows decrease in resistance due to reduction. The DBSA PPy sensor shows fast response and good recovery for most of the gases. This sensor shows very high % response of 82% for CH3COCH3, good response for NH3 (30%) and Cl2 (29%). Owing to the oxidation reaction, PTSA PPy shows sensing efficiency for CH3COCH3, C2H5OH, C6H6, NO2, and Cl2, while it shows sensing for NH3 due to the reduction reaction. The PTSA PPy sensor shows fast response time and recovery time for CH3COCH3 and NH3 vapor. When PTSA PPy sensor is exposed to Cl2 gas, it shows quick response which is about 12 seconds as compared to other gases. Also, good recovery is observed by these PTSA PPy sensor. It shows high % response for CH3COCH3 (90%), NH3 (90%), C2H5OH (70%) and Cl2 (78%) exposures. As the surface area of nanoparticle is large, higher response is obtained when exposed to different gases. The interaction of sensor material is more with gas molecules due to the large surface area of nanoparticles. Hence, the rate of adsorption and desorption of gas vapors is high. The active surface area of sensor materials is increased due to surfactant doping. The doped PPy has higher % response than pristine PPy. For VOCs and toxic gases, doped PPy sensors display improved response, recovery, high % response, and noiseless stable sensing curves [90].

Chapter 8 Conducting polymer-based gas sensors

Pirsa and Alizadeh synthesized centerd on nanostructured conducting polypyrrole (PPy) doped with sulfonate anion developed a selective dimethyl sulfoxide (DMSO) gas sensor. The PPy film was basically doped with five type of sulfonate anions. The five sulfonate anions are dodecyl sulfonate (DS), dodecyl benzene sulfonate (DBS), 5-sulfo salicylate (SS), para toluene sulfonate (PTS) and HSO32. Redox reactions can change the doping level of conducting polymers by moving electrons from/to the analytes. The electron transferring mechanism is responsible for the change in resistance and the sensing material’s work purpose. The work function of a conducting polymer is the minimum energy required to extract an electron from the bulk to the vacuum energy level. The process mentioned above occurs when conducting polymers such as PPy, PTh, and in some cases PANi films are exposed to electron donating (DMSO, DMF, NH3, and H2S) and electron accepting (NO2 and I2) gases. Since DMSO functions as an electron donor, PPy’s electric conductance drops dramatically when exposed to it. Following carrier gas washing, the sensing layer’s resistance can be restored completely or partially. The electron donor molecules like DMSO, pyridine and DMF show lower detection limits. As reported in the literature [46,47], demonstrates the PP-S sensors’ selective response to DMSO. The reaction between DMSO and sulfate is extremely quick. Benzene, chloroform, and acetone are examples of gas molecules that lack electron donation groups. Hence, these gas molecules do not interact with polymer matrix. Their non-polar nature is also a factor, these gas molecules act as barrier in polymer chain. The PPy-S sensor shows detection limit which decreases in order of PPy-DS, PPyDBS, PPy-PTS . PPy-SS . PPy-HSO3. When PPy-S sensors is exposed to DMSO, sensor shows lower detection limit as PPy-S sensor has multi charge anion (PPy-HSO3 and PPy-SS). When PPy-HSO3 sensor is exposed to DMSO gas, the sensor shows complete recovery for about all cycles. When exposed to analyte gas, the electrical resistance increases but later recovers back when flushed with nitrogen flow [91].

8.5

Polythiophene as gas sensing material

Xu and co-workers developed an in situ chemical oxidative polymerization method was used to create SnO2 hollow spheres/polythiophene (hs-SnO2/PTH) hybrid materials. When exposed to NO2 gas, the hybrid PTH sensor shows shorter recovery time than that of pure PTH. The hs-SnO2/PTH hybrids

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sensor shows enhanced response to NO2 gas as compared to other gases like formaldehyde, H2S, CHCl3, ethanol, acetone and methanol for 100 ppm of gas concentration. The maximum response of the hybrids hs-SnO2/(10%)PTH, hs-SnO2/(20%) PTH, and hs-SnO2/(40%)PTH sensors is 3.69, 5.16, and 5.67, respectively, which is higher than that of pure PTH (3.16), for about 200 ppm of NO2 gas concentration. By constructing hybrid materials, the response time of the gas sensing system has been greatly improved. Hence these hybrid materials are used as gas sensors for the detection of low NO2 concentrations at low temperatures. The gas diffusion process is enhanced by the porous and loose structure of hs-SnO2, and the excellent sensing efficiency of hs-SnO2/PTH hybrids has a higher surface to volume ratio, which benefits the gas sensor response and especially shortens the recovery response time. The electron donor acceptor system is generated between p-type PTH and n-type hs-SnO2 due to the formation of p n heterojunctions. The response and recovery of the sensor was improved by increasing the height of the depletion barrier. By altering the depletion zone, the conductivity of the junction can be adjusted using test gas. As the gas NO2 was added, the polythiophene channel’s conductivity increased as the width of the depletion region shrank. In contrast to pure PTH, the hs-SnO2/PTH has a much higher response and a much faster response recovery time. However, the hybrids’ response is lower when compared to pure n-type SnO2. When 10 ppm concentration of NO2 gas, is exposed to hs-SnO2/(20%) PTH hybrids sensor at 90 C, the gas response is 2.07. Between p-type PTH and n-type hs-SnO2, a p n heterojunction is formed. The hs-SnO2/PTH sensor, the SnO2 surface is covered with PTH, due to which the active sites and oxygen species are potentially covered. In surface sensing reactions, for n-type semiconducting materials like hs-SnO2, the important role is played by surface oxygen species. As a result, pure SnO2 has a higher response than hybrids. When compared to PTH in its purest form, the hs-SnO2/PTH sensor has a faster response time and recovery time [92]. Bai and co-workers synthesized graphene modified polythiophene (RGO-PTH) hybrids by a chemical polymerization in situ method. Firstly, by using Hummer’s method GO was synthesized then GO was reduced to form RGO. Using anhydrous FeCl3 as the oxidant, in the presence of prepared RGO, in-situ chemical polymerization of thiophene monomers produced RGO-PTH hybrids (Fig. 8.12). The 5% RGO-PTH hybrid-based sensor has a response of 26.36 for a 10 ppm NO2 gas concentration, which is nearly 4 times higher than pure PTH. Owing to

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Figure 8.12 Schematic diagram of preparation process for rGO-PTh hybrid [93], 2012 American Chemical Society.

the increase in the content of RGO, sensor responses increase and then decrease. The amount of RGO in the RGO-PTH hybrid-based sensor plays an important role in gas sensing efficiency, as too much RGO reduces gas sensing performance. When compared to other gases such as NH3, Cl2, NO2, C2H5OH, and CH2O with a fixed concentration of 10 ppm of each gas, the RGO-PTH sensor has a far higher response to NO2 than the 5% RGO-PTH sensor, but also has a much higher selectivity to NO2 gas (Fig. 8.13). The NO2 gas is electron-accepting gas, hence extracts electrons from PTH. When NO2 gas comes in contact with RGO-PTH hybrid-based sensor, the hole density increases and resistance decreases. PTH hybridizing with modified graphene significantly enhanced the sensor response to NO2. The RGO-PTH hybrid’s excellent sensing efficiency is attributed to the hybrid’s greater surface area, which is due to incorporating RGO, which eventually increases adsorption/desorption on surface of sensor material. The interaction of PTH and RGO results in between the conjugated PTH and RGO sheets, an increase in charge carriers and electron transfer, lowering the resistance of the hybrid sensing material. Not only does the sensor have a high sensitivity and selectivity for NO2, but it also has versatile,

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Figure 8.13 Response of sensors based on PTh and 5% rGO-PTh to different tested gases at room temperature [93], 2012 American Chemical Society.

clear, inexpensive, and portable electronic devices for detecting hazardous gases in the atmosphere [93]. Husain and co-workers synthesized polythiophene/graphene/ zinc tungstate (PTH/G/ZT) nanocomposite by chemical oxidative polymerization in situ method with CHCl3 as a solvent and anhydrous FeCl3 as an oxidant. Initially when PTH/G/ZT sensor is exposed to cigarette smoke, the conductivity of sensor decreases. The presence of PTH polarons and lone pairs of electrons or p-electrons in the different constituents of cigarette smoke decreased the mobility of polarons, resulting in a decrease in conductivity. The sensor pellet was then exposed to the ambient air for an additional 60 seconds. To test the sensors’ reversibility (i.e., recovery of initial conductivity), they were first exposed to cigarette smoke for 20 seconds and then to ambient air for another 20 seconds for a total of 360 seconds, consisting of nine consecutive cycles. As shown in the findings, a small amount of graphene can significantly increase reversibility, which is useful for long-term sensor use. The PTH/G/ZT sensor was exposed to gases like ethanol, benzene, formaldehyde, ammonia, acetone, carbon dioxide, toluene and phenol for about 60 seconds, to know which component of cigarette smoke leads to change in conductivity of the sensor. Gases are adsorbed on the surface of the PTH/G/ZT sensor pellet as it is exposed to these gases/vapors in the atmosphere, resulting in a decrease in charge carrier mobility (polarons). The reduction in the mobility of charge carriers is as PTH polarons interact with lone pairs on oxygen atoms

Chapter 8 Conducting polymer-based gas sensors

in carbon dioxide, formaldehyde, ethanol, acetone, and phenol, or p-electrons in benzene, toluene, and phenol, or lone pairs on nitrogen atoms in ammonia (NH3). The electrical conductivity of a device decreases as the movement of charge carriers decreases as a result of these electronic interactions. The weekly bounded lone pair of electrons interacts strongly with PTH polarons in the PTH/G/ZT hybrid sensor. Hence, leading to higher change in the DC electrical conductivity. Ammonia and ethanol may be the most responsible constituents for sensing reaction to cigarette smoke, as revealed. When the PTH/G/ZT hybrid sensor is exposed to cigarette smoke, the sensor has pores on which the cigarette smoke gets adsorb easily. As a result, since the movement of polarons/bipolarons was obstructed, there was a significant decrease in conductivity. When the sensor surface was exposed to ambient air, the cigarette smokes began to desorb from the pores, and the electrical conductivity reverts toward the initial value because of this. Owing to adsorption desorption process of cigarette smoke on PTH/G/ZT-3 sensor having large surface area, it significantly leads to altering the motion of polarons/bipolarons. Hence, the electrical conductivity rises and drops, when in contact with ambient air and cigarette smoke respectively [94]. Chandra and co-workers modified PTH through oxidative polymerization of thiophene, polythiophene supported tin doped titanium nanocomposites (PTH/Sn-TiO2) were created using a modified sol-gel process. The experiment was carried out at temperatures ranging from 50 degrees to 400 degrees Celsius because PTH composite materials are both photo and thermally stable. When PTH/Sn-TiO2 sensor exposed to LPG gas, with increase in temperature the sensitivity increases and then further decreases. As the content of polythiophene changes, the sensing behavior of composite materials changes with respect to temperature. The adsorption potential of metal oxide increases when PTH is present on the surface, allowing it to adsorb a greater number of molecules due to electrostatic forces. The sensor has a large surface area and a small crystallite size, which enhances composite material sensing. When LPG gas interacts with composite sensor material, the chemisorbed oxygen reacts with LPG gas and a surface charge layer is created when electrons are exchanged between the LPG and the oxide surface. The conductivity of sensing material decreases as the freed electrons recombine with the bulk of the sensor’s carriers (holes) (Fig. 8.14). PTH is a strong sensitizer and supporter for tin doped titania, allowing it to develop its LPG sensing properties [95].

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Figure 8.14 Sensing mechanism of PTh/Sn-TiO2 composite material with liquid petroleum gas [95].

Moghaddam and Malkeshi synthesized ZnO/Polythiophene (Zn-PTH) nanocomposite nanofibers, CTAB as a surfactant and anhydrous FeCl3 as an oxidant were used to chemically polymerize thiophene by in situ method. The formation of p-n heterojunctions creates a special electron donor-acceptor structure between p-type PTH and n-type ZnO. When a sensor is exposed to ammonia gas, the resistance decreases as the heterojunction’s charier concentration decreases and the heterojunction’s potential barrier height rises. The adsorption and desorption rates of ammonia leads to change in response time of sensors. The ability of ammonia molecules to penetrate, is much deeper into sensing layers at higher concentrations of ammonia gas. Hence, for interaction, more sites are provided, which further leads to an increase in rate of physisorption and chemisorption. Therefore, the sensitivity of the sensor increases, whereas response time of sensor decreases. The reproducibility and stability of sensing materials are critical factors in their reliability. After 5 days, the initial sensitivity of hybrid sensor film was observed, which was found to be 22.8%. After 30 and 60 days, the initial sensitivity of hybrid sensor film was reduced to 21.04% and 20.94%, respectively. As the concentration of ammonia gas increases from 0.0308 vol.% to 0.185 vol.% and the sensitivity of sensor film increases from 22.95 to 222.5, the reaction time decreases from 97 seconds to 55 seconds [96]. Husain and co-workers developed an in-situ chemical oxidative polymerization method was used to create polythiophene/ tin oxide (PTH/SnO2) nanocomposites. When the sensor was placed in an alcohol vapor area, there was increase in resistance. When PTH/SnO2 sensor was exposed to ambient air, the decrease in resistance was observed. The response behavior of different alcohols to PTH/SnO2 sensor was different. It was observed that, primary alcohol shows maximal change in DC electrical conductivity. And that for secondary alcohol shows minimal change in DC electrical conductivity. The DC electrical conductivity shift observed in the presence of primary alcohol, secondary alcohol, and tertiary alcohol was 7.7 3 1023 S/cm,

Chapter 8 Conducting polymer-based gas sensors

5.6 3 1023 S/cm, and 2.5 3 1023 S/cm, respectively. As alcohol molecules contain lone pairs of electrons, there is a decrease in electrical conductivity due to the charge transfer between alcohol and the polarons of PTH. Some polarons (charge carriers) become neutralized as the polarons’ (charge carriers’) mobility decreases, resulting in a decrease in DC electrical conductivity. As the sensor pellet is exposed to ambient air, the electrical conductivity begins to rise as the alcohol molecules on the sensor surface are desorbed. Since the sensor contains more SnO2 material in PTH than pure PTH, the sensor has a higher sensing efficacy. The increased response of the PTH/SnO2 sensor is due to its highly porous and wide surface area, which provides several active sites for analyte vapor adsorption [97]. Huang and team synthesized polythiophene/WO3 (PTH/WO3) organic-inorganic hybrids by a process of chemical oxidative polymerization in situ. As compared to pure WO3 and PTH, the response of PTH/WO3 sensor is enhanced. WO3 is an n-type semiconductor, while PTH is a p-type semiconductor. As a result, the PTH/WO3 hybrids have p-n junction properties. The depletion region changed when the PTH/WO3 hybrids were exposed to NO2, which served as an electron dopant, and the resistance of the conducting polymer decreased continuously. As a result, the depletion region’s width narrows and the polythiophene channel’s conductivity rises. The resistance of PTH/WO3 hybrids varies so dramatically that a small amount of NO2 can be detected with high sensitivity. The hybrid sensors exhibit high sensor response to NO2. However, at the same concentration of 100 ppm, H2S, ethanol, methanol, and acetone produced less or no gas response [98]. Ma and team developed a method by adding a small amount of boron trifluoride etherate to the monomer oxidant containing mixture, a polythiophene (PTH) composite film was formed through in situ polymerization at room temperature onto interdigital carbon electrodes immersed in the mixture. The sensor is reversible at room temperature. The gas sensitivity of chemical sensors is observed as a result of interactions between the sensing film and the adsorbed gas molecules. The chemical bond is responsible for the strong interaction, while hydrogen bonding and the van der Waals force are responsible for the weak interaction. Film recovery is normally difficult for strong interacting systems, while film recovery is relatively quick for weak interacting systems at room temperature with high-purity N2. At room temperature, the absorbed vapors are completely recovered with high-purity N2, allowing them to be reused, meaning that the sensing film’s interactions with the adsorbed

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gas molecules are weak interaction systems. The polarity of vapors influenced gas exposure and response time significantly. The sensitivity increases as the polarity of the vapor increases. As a result, toluene and n-hexane have very poor responses. The moisture effect on toluene and n-hexane cannot be overlooked. As the sensor shows different response to different gases trimethylamine, ammonia, alcohol, acetone, and toluene are some examples, the sensor film can be used as electronic nose sensor arrays [99]. Navale and team synthesized polythiophene (PTH) films using the spin coating method on glass substrates. At room temperature, PTH films demonstrate high selectivity for NO2 gas, in comparison to other gases including C2H5OH, CH3OH, H2S, NO2, and Cl2 (each with a fixed concentration of 100 ppm). As the NO2 gas concentration rises from 10 to 100 ppm, the sensitivity also increases and response of PTH sensor was found to be 9%, 14%, 16%, 22%, 25% and 33%, respectively. When the concentration of gas is higher, the coverage of gas molecules on the surface is greater and as a result, there is more surface interaction between the film’s surface and the gas molecules. When the gas concentration is lower; however, the surface coverage of gas molecules is reduced. As a result, there is less surface interaction between the film’s surface and the gas molecules. The surface reaction gets saturated, with further increase in gas concentration, further leading to slow increase in response. The increase in the charge carrier density is observed when p-type PTH interacts with oxidizing gases like NO2 and Cl2. As the NO2 and Cl2 gas shows electron accepting nature, the density of holes increases. Further leading to increase in decrease in film resistance and increase in the conductivity of PTH. The NH3, C2H5OH, CH3OH, and H2S are examples of reducing gases, when such reducing gases interact with sensor, the charge carrier concentration decreases. The PTH conductivity decreases, when electron donating gases like NH3, C2H5OH, CH3OH and H2S interact with sensor. When the concentration of water vapor increase, the NO2 sensitivity decreases. This is due to the fact that, the active sites of PTH film gets adsorbed by water vapor, and hence NO2 gas cannot get adsorbed on PTH films. The PTH sensor has excellent gas sensing properties at room temperature, allowing it to be used as a gas sensor [100]. Gonsalves and co-workers developed a technique to synthesize poly (3-hexylthiophene) (P3HT) via using ferric trichloride for oxidative polymerization, which was then fabricated on an interdigitated gold electrode. P3HT when in contact with methanol vapor shows the minimum response and with THF vapor

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shows maximum response. Toluene and n-hexane are nonpolar and weak solvents for P3HT. As a consequence, it exhibits a range of responses (between methanol and THF). Owing to poor physical interactions between analytes and polymers, the polymer films swell, and some VOC responses are observed. Owing to the absorption of organic vapors, the matrix of the polymer swells, which leads to the gap between polymer chains longer, thereby hopping conduction decreases. The swelling effect is primarily caused by interactions between polymer and vapor, as well as polymer solubility. The aggregation of polymer chains occurs as a result of this operation, which causes them to change conformation and can be reversed by passing a nitrogen flow in some cases. THF is a polar analyte that can be used to dissolve P3HT since it is a polar analyte, highest response is observed for THF, by the swelling effect. As the analyte molecules join the polymer film, the conductivity decreases and the distance between the polymeric chains widens. Since methanol is not a solvent for P3HT, there is low sensitivity due to the lack of the swelling effect [101]. Husain and team synthesized polythiophene/zinc oxide (PTH/ZnO) nanocomposite by chemical oxidative polymerization via in situ method. The sensing response observed when a PTH/ZnO nanocomposites-based sensor is exposed to 2400 ppm of LPG gas is 1.58 times greater than that of a pristine PTH-based sensor. For LPG gas at 600, 1200, 1800, and 2400 ppm, the sensing response of the PTH/ZnO sensor was found to be 43.22%, 55.69%, 66.14%, and 71.98%, respectively. The number of molecules of ethyl mercaptan increases, as the concentration of LPG rises, it has a wide number of polaron interactions, causing them to become less mobile. At higher concentrations, there is a more important shift in conductivity. When compared to PTH in its purest form, the PTH/ZnO sensor shows excellent response by DC electrical conductivity as a function. As compared to pristine PTH, the PTH/ ZnO sensor had a 4.9 times greater reversibility response. For long term use, a very critical parameter is the stability of a sensor operating at room temperature. The sensor shows stability for 15 days at lowest concentration of 600 ppm and highest concentration of 2400 ppm. In terms of the number of days, the sensing response decreases gradually. The decrease in sensor response was observed after 15 days, at 2400 ppm and 600 ppm, respectively, by 19.20% and 15.61%. The sensor shows better selectivity for LPG gas as compared to other gases like benzene, toluene, chlorobenzene, and hexane are examples of VOCs (Fig. 8.15). LPG contains propane, butane and ethyl mercaptan in trace amounts. There is no interaction between hydrocarbon molecules and

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Figure 8.15 Selectivity of PTh/ ZnO-3 based sensor at 600 and 2400 ppm [102].

polarons of PTH/ZnO sensor, since at room temperature the hydrocarbon molecules are neutral. When a PTH/ZnO nanocomposite is exposed to LPG gas, the only constituent that causes a change in conductivity is ethyl mercaptan. As LPG is adsorbed on the sensor surface, the lone pairs of electrons on the sulfur atom interact electronically with the polarons of the PTH/ZnO sensor (Fig. 8.16). Hence, the speed of polarons slows down and results in reduction of conductivity. The Ethyl mercaptan molecules are desorbed from the pores of the PTH/ZnO dependent sensor when exposed to ambient air, and conductivity returns to its original value [102]. Xian-zhi and team synthesized polythiophene/WO3 (PTH/WO3) organic-inorganic hybrids by a simple mechanically mixing. The gas response increases to some degree as the concentration of NO2 rises from 0.01% to 0.15% (volume fraction). When the NO2 gas concentration is 0.15%, the best response is observed (volume fraction). The PTH mass fraction was increased from 5% to 20%, and a 20% PTH/WO3 hybrid was developed, response of PTH/WO3 hybrids sensor gets enhanced for NO2 gas. Further the response of gas sensor decreases as the PTH mass fraction increases. The PTH/WO3 hybrid sensor shows lower response at 60 C or 90 C and high response at room temperature for NO2 gas. Since PTH is a p-type semiconductor and WO3 is an n-type semiconductor, p n heterojunctions form between PTH/WO3 hybrids, resulting in a special electron donor acceptor configuration. Which further leads to increasing the sensor response by increasing the depletion barrier height. As the sensor is exposed to NO2, the depletion region narrows and the conductivity of the PTH channel increases, resulting in a higher response than pure PTH or WO3 at low operating temperatures. The PTH/WO3

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Figure 8.16 Sensing mechanism of PTh/ZnO nanocomposites involving the interaction of polarons of PTh with LPG [102].

combination has a strong response to NO2, but a weaker response to H2S, and no gas response to CO, H2, NH3, ethanol, methanol and acetone gas [103]. Jang and team developed surface-mediated vertical phase separation process for metal organic frameworks in a blended polythiophene hybrid film. With a response time of 0.23 seconds and a recovery time of 2.91 seconds, the sensor responds rapidly to the sensor. Owing to the slow desorbtion of water molecules from MOFs, the sensor had a tenfold slower recovery time than the response time. Unlike other gas sensors, which take hundreds of seconds to recover, this one takes just a few seconds, the recovery time is fast. As compared to conventional polymerbased films in terms of sensitivity, MOF/polymer hybrid performs exceptionally well and MOFs have excellent gas adsorption properties which contribute to their stability [104]. Husain and co-workers synthesized polythiophene/ graphene (PTH/G) nanocomposite by chemical oxidative polymerization technique via in situ method. The PTh/G sensor shows enhanced response for ethanol gas, which is around 6500 times greater, as compared to that of pure PTH. The surface area of nanocomposites increases, PTh’s graphene nanosheet loading increases as the volume of graphene nanosheets increases. The PTh/G sensor has larger surface area, which leads to more active sites for the adsorption and large number of charge carriers. The sensing response of PTh/G sensor increases, as ethanol gas concentration increases from 400 to 2000 ppm. The number of ethanol molecules and lone pair of electrons increases as concentration rises. As a result, there are more PTh polarons that can be neutralized by a single pair of ethanol

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molecules. As the electrical conductivity decreases, the polarons of PTh gets neutralized. As a result, at 2000 and 400 ppm, respectively, the maximum and minimum sensing responses, that is changes in electrical conductivity of PTh/G, were observed. After 360 seconds, PTh’s electrical conductivity did not return to its previous level, while the electrical conductivity of PTh/G reverted to its original value. PTh/G nanocomposites have a far higher reversibility than PTh nanocomposites, indicating that they could be used to make a completely recoverable ethanol sensor. The PTh/G sensor shows high selectivity toward ethanol gas, as compared to methanol, sec-butanol, isopropanol, tert-butanol, toluene, acetone, formaldehyde, benzene, acetaldehyde, and chlorobenzene are examples of VOCs. at concentration 2000 ppm (Fig. 8.17). High response to ethanol gas is because among the VOCs, ethanol has the highest potential to donate electrons. When exposed to ethanol gas, PTh and PTh/G have lower DC electrical conductivity, which returns to its original value, and when exposed to the atmosphere, it returns to its original value (Fig. 8.18). In a PTh/G nanocomposite, the electrons of graphene nanosheets interact with the lone pair on the sulfur atoms of PTh. The molecules of ethanol are adsorbed on the surface of the PTh/G nanocomposites when the sensor is exposed to ethanol gas at room temperature (27 C). The polarons of PTH interact with lone pairs of electrons on the oxygen atom of the ethanol molecule, resulting in polaron mobility and a decrease in the electrical conductivity of the sensor content (Fig. 8.19). The ethanol molecules on the surface of the PTh/G nanocomposites are desorbed when the PTh/G sensor is exposed to ambient air, and the electrical conductivity returns to its original value. The simple adsorptiondesorption process of ethanol on the broad surface area of PTh/ G nanocomposites induced the decrease and increase in electrical conductivity [105]. Bai and team synthesized polythiophene/WO3 (PTH/WO3). WO3 architectures were first constructed from nanosheets using a simple hydrothermal process, and then updated with polythiophene using an in-situ polymerization method (Fig. 8.20). When exposed to 100 ppm H2S, the PTH/WO3 hybrid exhibits n-type gas sensing behavior, meaning that the sensor’s resistance decreases (reducing gas). As the operating temperature rises from 20 to 70 degrees Celsius, the PTH/WO3 sensor response rises and then falls. Between p-type PTH and n-type WO3, a p-n heterojunction is formed. Formation of these p n junction, leads to generation of electron donor acceptor system. The conductivity of the PTH channel increases, when electron donating H2S gas

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Figure 8.17 Selectivity of the PTh/G-3 nanocomposite toward ethanol and different VOCs on exposure to 2000 ppm for 60 seconds [105].

Figure 8.18 Steady-state response of DC electrical conductivity of PTh/G-3 on exposure to 2000 ppm ethanol followed by ambient air with respect to time [105].

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Figure 8.19 Proposed mechanism of interaction of ethanol with PTh/G-3 nanocomposite [105].

Figure 8.20 Schematic drawing of the sensor element and corresponding hybrid sensing layer [106].

interacts with sensor. As a result, more electrons migrate from PTH to WO3 at p n junctions, speeding up the density shift in the electron depletion field. Further the response decreases, as the PTH amount increases because, the PTH layer on theWO3 surface would be too thick for the gas to contact with the WO3, and the p n junction’s reverse-bias resistance will be too high for electron migration. The response of the PTH gas sensor increases as the H2S gas concentration rises. As compared to other gases such as ammonia, ethanol, methanol, and acetone, the PTH sensor has a high selectivity for H2S gas (Fig. 8.21) [106]. Huang and team synthesized polythiophene/WO3 (PTH/WO3) organic-inorganic by a process of chemical oxidative polymerization via in situ method. In comparison to pure WO3 and PTH, the PTH/WO3 sensor shows enhanced response. The high gas sensitivity is observed at low temperature. Hence, PTH/WO3 is an excellent sensor. Between the p-type PTH and the n-type WO3, a p-n

Chapter 8 Conducting polymer-based gas sensors

Figure 8.21 Response of 10%PT-WO3 hybrid to various gases [106].

heterojunction is formed. So, the PTH/WO3 hybrids contain the properties of p-n junctions. The PTH/WO3 hybrid sensor material has low content of PTH. Hence, PTH/WO3 materials’ electronic properties are primarily dominated by WO3, not PTH. In PTH/ WO3 hybrid sensor material, the electron migrates from WO3 to PTH at p-n heterojunctions, this causes a positively charged depletion layer to form on the WO3 surface. Owing to these, when PTH/WO3 hybrid sensor is in contact with electron-donating gas, there is decrease in activation energy and enthalpy of physisorption. When NO2 comes in contact with PTH/WO3 hybrid sensor, NO2 acts as an electron dopant, which leads to change in depletion region. And hence, there is decrease in resistance of conducting polymer. The conductivity of the polythiophene channel increases as the width of the depletion area decreases. Even at low concentration, NO2 gas detection is possible with high sensitivity, as the PTH/WO3 hybrids resistance changes dramatically. At all the three different operating temperatures (40 C, 70 C and 90 C), it has been discovered that as the NO2 concentration rises, so does the sensor response. With the same concentration of 100 ppm, the PTH/WO3 hybrid has a high sensor response to NO2, while that to H2S, ethanol, methanol, and acetone, on the other hand, produce little or no gas response, implying that the PTH/WO3 hybrid sensor has a high selectivity for NO2 [98]. Husain and team synthesized polythiophene/SWCNT (PTH/ SWCNT) nanocomposite. As the ammonia concentration rises from 5 ppm to 2000 ppm, the signal intensity rises because the polarons of PTH become more neutralized, causing the DC electrical conductivity to fall. The PTH/SWCNT-3 sensor shows lowest detection

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limit at 5 ppm of ammonia concentration. The PTH/SWCNT sensor shows high selectivity to ammonia gas as compared to toluene, acetone, ethanol, benzene, acetaldehyde, chlorobenzene, isopropanol, and n-hexane are examples of VOCs, at a concentration of 1000 ppm (Fig. 8.22). As the concentration of NH3 gas increases the DC electrical conductivity of the PTH/SWCNT also increases. In the PTH/SWCNT sensor, the lone pairs of PTH communicate with the -electrons of SWCNT. At room temperature (27 C), DC electrical conductivity fluctuates between low and high as adsorption and desorption of ammonia vapors is easy. The lone pairs of ammonia hinder the mobility of the polarons as the PTH/SWCNT nanocomposite is exposed to ammonia, resulting in a decrease in DC electrical conductivity (Fig. 8.23). As the sensor was exposed to the ambient air, ammonia molecules desorbed from the PTH/SWCNT nanocomposite. The enhanced response for ammonia gas molecules is due to ammonia molecule adsorption desorption on the vast surface of the PTH/SWCNT sensor [107]. Kamble and co-workers developed a simple chemical bath deposition method was used to deposit an interconnected nanofibrous polythiophene (INPTH) film on the glass substrate. The resistance of an INPTH thin film sensor increased when it was exposed to reducing gases such as H2S, NH3, CO, and LPG, whereas it decreased when exposed to oxidizing gases such as NO2 and SO2. The higher response of 47.58% was observed for NO2 as compared to other gases like H2S, NH3, CO, SO2, and LPG by INPTH thin film sensor (Fig. 8.24). When INPTH thin films are exposed to low concentration of gas, there is less contact between the surface of nanofibers and gas molecules. As the gas

Figure 8.22 Selectivity of PTh/ SWCNT toward ammonia and different volatile organic compounds of 1000 ppm [107].

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Figure 8.23 Proposed mechanism of sensing showing the interaction of ammonia with PTh/SWCNT [107].

concentration increases, the interaction of gas and surface increases leading to further saturation of sensor surface. The INPTH thin film sensor shows same response after four consecutives 25 ppm NO2 exposures. Sensor shows no significant variation, leading to better reproducibility. The sensor shows longterm stability for 10 days intervals for NO2 gas. But after 50 days, the sensor showed slight variation in response performance of INPTH thin film sensor to NO2 gas (Fig. 8.25) [108]. Husain and team developed a method using an in-situ chemical oxidation technique, researchers created polythiophene (PTH) and a series of PTH nanocomposites with multiwalled carbon nanotubes (MWCNTs). As the amount of MWCNTs in a PTH/MWCNTs nanocomposites sensor increases, the surface area of the PTH/MWCNTs nanocomposites increases as well. As a result, when compared to other sensors, the sensor with the highest MWCNT content has the highest sensing response. The conductivity decreases when a large number of ammonia molecules interact with a large number of polarons. The % sensing response (i.e., change in conductivity) is greater for high

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Figure 8.24 Selectivity of INPTh thin films sensor for 100 ppm of various gases operating at room temperature [108].

Figure 8.25 Stability study of INPTh thin film sensor for NO2 gas at room temperature for fixed concentration 100 ppm [108].

concentration of gases and lesser for low concentration of gases. The highest sensing response was found to be 2000 ppm while that of lowest sensing response was 0.1 ppm. When PTH/ MWCNTs nanocomposites was exposed to dry ammonia

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environment, for about 140 seconds the sharp decrease in electrical conductivity of the sensor pellet was observed after that it became saturated. In comparison to dry ammonia, the conductivity of sensor pellets dropped slowly when exposed to an atmosphere of pure water vapors. The PTH/MWCNTs sensor shows high selectivity to ammonia gas as compared to that of toluene, acetone, ethanol, benzene, chlorobenzene, acetaldehyde, and isopropanol are some examples of VOCs, at concentration of 2000 ppm and 0.1 ppm (Fig. 8.26). Ammonia gas has the highest sensing response as compared to other gases because it easily neutralizes a large number of polarons. In case other gases like isopropanol, ethanol, acetaldehyde and acetone, a marginal change in conductivity resulted from the neutralisation of the lesser polarons, since oxygen atoms have a lower electrondonating affinity than nitrogen atoms. Because of the weak interaction between the bond and the polarons, the shift in conductivity of aromatic hydrocarbons like toluene, chlorobenzene, and benzene is minimal. Hence, ammonia vapors show superior selectivity for PTH/MWCNTs sensor as compared to other VOCs. At both the concentrations (0.1 ppm and 2000 ppm), even after 20 days, the sensor pellet showed improved stability. The ammonia molecules are first adsorbed on the sensor surface, and then the polarons of PTH interact with the lone pairs on the nitrogen atoms of ammonia (Fig. 8.27). This neutralizes the polarons, resulting in a sharp decrease in conductivity. As ammonia molecules begin to desorb, conductivity rises in the surrounding air. The initial conductivity is retrieved, after full ammonia desorption from sensor pellet [109].

Figure 8.26 Selectivity of the sensor based on PTh/ MWCNTs-3 toward ammonia as well as different volatile organic compound at (A) 2000 ppm and (B) 0.1 ppm on exposure for 120 s [109].

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Figure 8.27 Sensing mechanism showing the interaction of ammonia with PTh/MWCNTs nanocomposite [109].

Barkade and team synthesized polythiophene/SnO2 (PTH/SnO2) hybrid using in situ method for oxidative polymerization of thiophene monomers using ultrasound. The charge transfer between PTH and SnO2 leads to increase in conductivity and in PTH chains, there is an increase in conjugation length. Hence, as compared to pure PTH film, the PTH/SnO2 hybrid nanocomposite shows an increased conductivity. The p n hetero-junction formed between n-type SnO2 and p-type PTH enhances the response of the PTH/SnO2 hybrid nanocomposite. As a result, LPG’s activation energy and physisorption enthalpy are lower. When the LPG concentration was raised from 0.5 to 2.5 vol.%, the response time decreased from 196 to 94 seconds, while the recovery time increased from 182 seconds to 466 seconds. The recovery time was long for higher concentrations of LPG gas, this is because LPG has a higher density and after the reaction, the reaction products do not immediately leave the interface. As PTH/SnO2 is exposed to LPG gas, PTH adsorbs the LPG gas molecules, and LPG molecules interact at the p-PTH/n-SnO2 heterojunction interface. On PTH surface, the LPG gas gets adsorbed and, on the p-side, more negative acceptor ions result from the donation of electrons. On the n-side

Chapter 8 Conducting polymer-based gas sensors

of the junction, an electron depletion layer is formed because negatively charged adsorbed O2 ions cover the surface of SnO2 nanoparticles in most cases. As a consequence, the carrier concentration in the heterojunction decreases, while the potential barrier height in the heterojunction increases. As a result, the hybrid’s resistance increases (Fig. 8.28). The synergistic effect is observed between conducting polymer (PTH) and SnO2, resulting in enhanced sensor properties. The conducting polymer (PTH) and SnO2 when combined together show fast response and recovery, lower operating temperature, and thermal stability [110].

Figure 8.28 Schematic representation of LPG sensing model for PTP/SnO2 nano hybrid [110].

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9 Future prospects: carbon-based nanomaterials and nanocomposites Shilpa Jain1, Navinchandra Gopal Shimpi2 and Akshara Paresh Shah2 1

Department of Chemistry, Jai Hind College, Mumbai, Maharashtra, India Department of Chemistry, University of Mumbai, Santacruz (East), Mumbai, Maharashtra, India 2

Nanoscience and nanotechnology have been emerged as inimitable and powerful interdisciplinary research areas, which have generated several futuristic ideas that are finding important applications in the present day world. Gas sensing has become a requisite in several high-technology industries, process control unit, and environmental monitoring. Especially, solid-state gas sensors have gained tremendous interest because of their robustness, properties, and wide applications in several industries such as environmental monitoring, healthcare, food processing, biomedicine, pharmaceutics, and space exploration etc (Fig. 9.1). Important aspects, which determine the properties of viable gas-sensing devices, are low operating temperature, high sensitivity and selectivity, quick response and recovery times, lower power consumption, temperature independence, and good stability. All these properties can not be inherited by a single sensing material. Several materials such as semiconducting metal oxides, conducting polymers, and their composites have been explored, and each has their own advantages and disadvantages. For practical and industrial applications, a gas sensor must have high response, strong selectivity, rapid response & recovery time, high stability, and good repeatability. Along with these, sensor should be operating at room-temperature with minimal cost and ease of fabrication [1 3]. Based on these requirements, several types of gas sensors have been explored and developed as an active sensing layer bridging a pair of electrodes. Surface acoustic wave, chemiresistor, field-effect transistor, solid-state electrochemical sensor, quartz-crystal microbalance and Carbon-Based Nanomaterials and Nanocomposites for Gas Sensing. DOI: https://doi.org/10.1016/B978-0-12-821345-2.00009-7 © 2023 Elsevier Inc. All rights reserved.

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Figure 9.1 Schematic representation of Gas sensors used in different fields.

gas capacitor etc. are few examples of various types of sensors that have been explored. Chemiresistor sensor which are based on change in resistivity are widely used sensors. They emerged as a potential candidate due to advantages such as ease of fabrication, wide adoption of sensitive materials and simple sensing data. These advantages ensure its adoption for large scale synthesis and commercialization success in a variety of applications [4 6]. However, finding a single chemiresistor sensor that can match all of these conditions is highly unlikely. Hybrid functional materials are interesting choices for chemiresistive sensors because of their high sensitivity, selectivity, response/recovery time, stability, repeatability, room temperature functioning, and ease of fabrication. Normally, an array of sensors known as E-Nose (artificial olfactory) are used to meet all expectations. Each sensor has a unique response to the target gases (to a subset), which are recognized using sensor signal pattern recognition. Gas sensors of this type are usually bulky devices while future technologies will require compact, integrated, powerful and smart devices. Upcoming sensor technology focuses on more precise, compact and reliable E-Noses with high sensitivity, selectivity and stability. Traditional sensors based on thick films have lower sensitivity, slow response, and

Chapter 9 Future prospects: carbon-based nanomaterials and nanocomposites

high power consumption than these integrated gas sensors with a micro-hotplate. Several researchers are working on advanced functional nanomaterials with high sensing capabilities. With advances in technology and need for economically viable sensors with ability to large scale production, there is a high demand for sensing materials and efficient fabrication techniques. The sensing layer and signal conditioning circuitry should be into a single integrated device. These smart, compact and highly efficient sensors are need of current technologies and lead to development of advanced functional materials. Silicon processing technology has been used commercially to achieve full integration of microelectronic and micromechanical components on a single wafer [1]. These integrated electronics can easily detect and digitally process any change in the surface of the gas-sensitive layer. Recently to build a smart sensor that can sense ambient gas efficiently, they are fabricated on a thin, flexible plastic stripes or woven into a cotton fabric using a weaving machine. These fabrics act as wearable sensors and are highly efficient and show promising applications. The fabrication techniques used to prepare wearable sensors are simple and suitable for large-scale roll-to-roll production. Another intriguing technology is inkjet printing, which can be utilized as a cost-effective option for fabricating long sensor stripes. Another advantage of these type of sensors are that they uses only 15 mW of power due to the smaller size of the sensing chips which makes them an ideal candidate for low-power equipment and battery-operated devices. Potential areas such as transportation, buildings and structures, healthcare, medical monitoring and robotics etc. will be beneficial from these small integrated sensors with advanced functional materials. The market of gas sensors is rapidly changing owing to increasing demands of smart and highly efficient sensors with more technical advancements. New opportunities for efficient and smart gas sensors arise with their integration into devices such as cell phones and wearable devices. These devices are becoming smaller in size, requires less support infrastructure than currently employed equipment, and are capable of running independently. Chemical sensors embedded in smartphones and other wireless and wearable devices may do everything from alerting people about hazardous gases in environment, air quality and diagnosing health concerns via breath analysis etc [7]. Interdisciplinary perspectives of materials science, microelectronics, nanotechnology, and signal processing/ pattern recognition have sparked recent progress in gas sensor

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devices in an attempt to overcome the limitations of early systems with the advent of the latest technological breakthroughs. The effect of microstructure, size, shape, doping and functionalization etc. have been studied extensively to understand their effect on sensing capabilities, and the advantages of innovative nanostructured materials have been explored in order to increase sensor performance and stability. Various functional nanomaterials have been explored for gas sensing capabilities. Among them, carbon-based nanostructures have shown promising results. Various carbon nanostructures such as CNT, graphene, graphene oxide, nanofibers and there nanocomposites have been studied extensively by several researchers. These advanced functional nanomaterials are believed to be future of sensing material due to their unique and extra-ordinary properties. There is huge scope in developing synthesis techniques for cost effective, pure and monodispersed nanomaterials. Morphology plays an important role in sensing and thus more intense studies are required on morphology dependent sensing. In future, flexible gas sensors with good physical properties at lower cost needs to be developed. Use of carbon-based nanomaterials such as graphene, carbon nanotubes, etc in organic hybrids such as polymers will add unique properties even at low loading of reinforcements. Conducting polymers such as polyaniline, polypyrrole etc. are used as an organic component in inorganic-organic hybrid nanocomposite and carbon-based nanomaterials along with semiconducting metal oxides are used as inorganic component. This new family of hybrid nanostructures has lot of potential for ultra-small sensor applications since the conductivity of these materials changes dramatically when gas or liquid molecules adhere to their surfaces. These nanocomposites have high specific area, extraordinary thermal and mechanical properties with excellent electrical conductivity. Similarly, functionalization and doping can be explored for high performance and stability [8]. Future technology demands smart, reliable, and efficient miniaturized gas sensors, and carbon-based nanostructures are one of the most promising candidates for sensing layer. Sensors are now required to be integrated into smart phones for a variety of purposes such as healthcare, public safety, environmental and food monitoring [7]. Owing to the high operating temperatures (above 150 C), currently commercially available chemiresistive sensors are not appropriate for low power portable operations. Considering that, the development of low power gas sensors are necessary for wireless and portable devices [9,10]. Various functionalized materials and

Chapter 9 Future prospects: carbon-based nanomaterials and nanocomposites

hybrid nanostructures have been investigated to lower the optimum sensing temperature and power consumption. Selfheating mode [11] and the incorporation of gas sensors into MEMS platforms have recently decreased power usage considerably [12]. Future belongs to smart gas sensors with low power consumption with rapid development in optoelectronic technology. These sensors will be integrated into popular electronic devices such as smartphones, tablets, and smart watches. Smart, portable, low power use, stable, and flexible gas sensors with fast response and recovery times will be in high demand for these applications. Carbon-based hybrid nanostructures are promising candidates in self-heated portable gas sensors. With substantial breakthroughs in the development of low power gas sensors, several challenges and issues need to be addressed for obtaining highly sensitive, selective, and stable products with low response/recovery time. With advances in synthesis and fabrication techniques, several new materials specially carbonbased nanostructures are explored as sensing materials for high-performance smart gas sensors with room temperature operation.

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Chapter 9 Future prospects: carbon-based nanomaterials and nanocomposites

[10] O. Monereo, O. Casals, J.D. Prades, A. Cirera, A low-cost approach to lowpower gas sensors based on self-heating effects in large arrays of nanostructures, Procedia Eng. 120 (2015) 787 790. [11] H.M. Tan, C.M. Hung, T.M. Ngoc, H. Nguyen, N.D. Hoa, N.V. Duy, et al., Novel self-heated gas sensors using on-chip networked nanowires with ultralow power consumption, ACS Appl. Mater. Interfaces 9 (2017) 6153 6162. [12] A. Singh, A. Sharma, N. Dhul, A. Arora, M. Tomar, V. Gupta, MEMS based microheaters integrated gas sensors, Integr. Ferroelectr. 193 (2018) 72 78.

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

A Acetone, 14, 35 37, 39, 66 Acetylene, 12 14 Acrylic acid doped PANi (AA-PANi), 187 Adsorption process, 85 86, 174 175 Adsorption-desorption process, 119 121 Agriculture, 149 Air pollution, 16 17, 181 Alpha-Fe3O4/reduced graphene oxide, 160 161 Ammonia (NH3), 33 35, 129 130, 185, 193 195, 203 204, 210 211, 221 225 Amorphous carbon, 56 57, 111 113 Amperometric mode, 181 Analyte gas, 27 Applied-sensors, 27 28 Arc-discharge method, 12 14, 58 Arch discharge, 84 Architecture construction/fabrication of 3D, 151 153 WO3, 218 220 Atomic clusters, 150 Atomic-layer Deposition (ALD), 167 168 Au-Pt/CNFs, 115 116

B Benzene, 35 37, 39, 86 87 Bimetallic Au Pt nanoisland functionalized CNFs, 115 116, 122 Bipolarons, 190 191

Bottom-up approach, 12 Butane (C4H10), 198, 204 206

C Camphor sulfonic acid (CSA), 33 35, 190 191 Capacitive gas sensors, 39, 86 87 Carbon, 3 4 carbon based gas sensors, 149 150 carbon based materials, 55 56 composites for gas sensor, 60 66 materials, 171 172 nanoparticles, 55 nanostructures, 55, 236 sensors, 149 150 Carbon black, 56 57 materials for gas sensors, 59 60 Carbon fibers (CFs), 8, 105 106 Carbon nanocomposites, 149 150 gas sensors based on, 59 for sensing, 55 59 Carbon nanofibers (CNFs), 6, 11 12, 19, 56 57, 105 106 CNFs/PPy, 119 CNFs/PS, 113 114 fabrication/construction of, 110 as gas sensors, 60 62, 111 124 5,6;11,12-di-ophenlyenetetracene/ carbon nanofibers, 123 124

Au-Pt/CNFs, 115 116 CNFs/polystyrene, 113 114 CNFs/PPy, 119 graphitic carbon nanofibers, 121 122 graphitic-carbon nanofibers/polyacrylate, 122 mesoporous carbon nanofibers, 117 multifunctional carbon nanofibers, 116 117 Ni-CNF, 119 121 Ni/CNFs, 118 PAN/(PAN-b-PMMA), 122 123 SnO2/CNFs, 114 Sn SnO2/CNFs, 111 113 V2O5/CNFs, 114 WO3/CNFs, 117 118 WS2/CNFs, 119 ZnO/CNFs, 111 methods of preparation, 106 110 catalytic thermal chemical vapor deposition growth, 107 108 electrospinning, 106 107 substrate method, 109 110 modified with metal oxides, 110 sensors, 121 122 Carbon nanomaterials, 55 56 carbon nanomaterials-based gas sensors, 55 fabrication of sensors, 40 41 functionalization of carbonbased nanomaterials, 33 37 gas sensors based on, 59

239

240

Index

Carbon nanomaterials (Continued) parameters of gas sensor, 31 33 for sensing, 55 59, 56f sensing mechanism, 37 39 types of, 6 12 carbon nanofibers, 11 12 carbon nanotubes, 6 8 fullerenes, 8 9 graphene, 9 10 Carbon nanotubes (CNTs), 3 4, 6 8, 19, 25 26, 55 58, 83 84, 168 169, 236 carbon nanotube-based gas sensors models of single-walled and multi-walled carbon nanotubes, 84f sensing mechanism, 85 87 carbon nanotube/conducting polymer nanocomposites for gas sensors, 93 96 sensitivity and response and recovery times of, 96t structures of typical conducting polymers, 94f carbon nanotube/metal nanocomposite based gas sensors, 87 89 carbon nanotube/ semiconducting metal oxide nanocompositebased gas sensors, 89 92 sensitivity and response and recovery times of, 93t CNT-based gas sensors, 85 86 comparison of some properties of, 8t functionalized carbon nanotubes as gas sensors, 97 98 for gas sensor, 62 66 graphene sheet lattice, 7f Carbon-based gas sensing materials, 51 53

carbon black materials and composites for gas sensors, 59 60 operating mechanism of gas sensors, 60f carbon nanofibers and composites for gas sensors, 60 66 carbon nanotubes and metal or metal oxide composites for gas sensor, 64 65 carbon nanotubes and polymer composites for gas sensor, 65 66 carbon nanomaterials and nanocomposites for sensing, 55 59 carbon black, 56 57 carbon nanofibers, 57 carbon nanotubes, 57 58 graphene, 58 59 detection mechanism of gas sensors, 53 55 gas sensors, 52 53 graphene materials and composites for gas sensor, 66 73 graphene and metal or metal oxide nanocomposite for gas sensor, 68 71 graphene and polymers nanocomposites for gas sensor, 72 73 Carbon-based hybrid nanostructures, 236 237 Carbon-based nanomaterials (CBN), 3 4, 25 26, 149 150, 236 functionalization of, 33 37 gas sensors types based on, 26 31 Carbon-based nanostructures, 18, 150, 236 Carbon-based smart nanomaterials, 3 5 comparison of some properties of, 6t

different types of carbon nanostructures, 5f gas sensors and comparison, 16 19 properties and applications in various fields, 4f synthesis methodologies and variations, 12 15 graphite oxidation route schemes, 15f various synthesis methodologies used for synthesis of graphene, 16f types of, 6 12 Carbon-polymer composite materials, 140 141 Carbonization process, 14 15, 106 107 Carboxyl groups, 97 Carboxylated-CNTs, 37 39 Casting method, 40 Catalytic metals, 12 14 Catalytic thermal chemical vapor deposition growth method, 106 108 Char (high carbon content), 14 15 Chemical exfoliation, 12, 14 Chemical oxidative method, 196 197 Chemical oxidative polymerization, 140 141 via in situ method, 220 221 Chemical polymerization method, 183, 203 Chemical sensor, 25, 51, 235 236 Chemical vapor deposition (CVD), 12 14, 58, 84, 110, 128 CVD-grown graphene nanoribbons, 129 130 Chemical warfare agents, 17 18 Chemiresistive sensors, 31 32, 37 Chemiresistor sensor, 233 234 Chemisorption, 85 86 Chlorobenzene, 39, 86 87 Chloroform, 66

Index

Chromium, 107 Chronoamperometry, 27 City Tech (gas sensors), 27 28 Cobalt, 12 14, 58, 107 Collagen, 106 107 Colorimetric gas sensors, 30 Colorimetric mode, 181 Composites for gas sensors, 59 60 Conducting polymers (CPs), 25 26, 55 56, 93 94, 181 183 conducting polymer-based gas sensors, 181 183 polyaniline as gas sensing material, 183 190 polypyrrole as gas sensing material, 190 207 polythiophene as gas sensing material, 207 227 Conductivity order for sensors, 190 191 Conductometric mode, 181 Conical CNFs, 107 Conjugated polymers, 182 183 Copper, 12 14, 87 Core shell quantum dots, 150 Cup-stacked CNFs, 107 CuxO/multilayer graphene composites (CuMGCs), 158 CuxO/Reduced Graphene Oxide, 158 Cyclic voltammetry technique, 27, 188 189

D Decomposition products, 32 33 Deposition, 19 Desorption process, 174 175 Detection mechanism of gas sensors, 53 55 Di-nitrotoluene, 39 5,6;11,12-di-ophenlyenetetracene/ carbon nanofibers, 123 124 Diamond, 3 4

Dichloromethane, 66 Dielectrophoresis (DEP), 40 41 Differential pulse voltammetry, 27 Dimethyl methylphosponate (DMMP), 35 37, 60 62, 119 121 Dimethyl sulfoxide (DMSO), 207 Dodecyl benzene sulfonate (DBS), 207 Dodecyl benzene sulfonic acid (DBSA), 198 200 Dodecyl sulfonate (DS), 207 Dodecylbenzene sulfonic acid, 193 Doping, 19

E E-Nose, 234 235 Electric arc discharge method, 83 Electrical sensors, 27 28 Electrical/chemiresistive sensors, 27 28 types of nonresistive gas sensors, 29f Electrochemical polymerization methods, 183 Electrochemical sensors, 27 Electron donating gas, 197 198 Electron transfer, 197 198 π-electrons, 182 183 Electrophoretic process, 33 35 Electrospinning method, 57, 106 107, 111 Electrospun method, 114 11-D metal oxide, 151 152 Emeraldine, 183 Ethanol, 35 37, 39, 86 87, 217 218 Ethyl group (Et), 204 206 Ethyl mercaptan (C2H5SH), 204 206 Ethyl-cellulose, 40

F Fabricated flexible gas sensors, 41

241

Fabrication of sensors, 40 41 schematic of field-effect transistor type gas sensor, 40f schematic of multiwalled carbon nanotube-based gas sensor fabrication, 41f schematic of sensor fabrication using ink-jet printing technique, 42f Fabrication techniques, 235 Fe3O4/reduced graphene oxide, 156 Ferric chloride (FeCl3), 198 Field-effect transistor (FET), 25 26 Figaro (gas sensors), 27 28 FIS (gas sensors), 27 28 Flammable gases, 30 Flexible substrate (DBSA), 193 Fluoride, 106 107 Fluorination process, 35 37 Fluorine-doped tin oxide (FTO), 88 Freeze drying process, 152 153 Fullerenes, 3 4, 8 9, 150 fullerene-like hemispherical molecules, 57 58 Functionalization, 19 Functionalized carbon nanotubes as gas sensors, 97 98 sensitivity and response and recovery times of, 98t Functionalized CNFs, 11 12, 115 116, 122 Functionalized graphene hydrogel composites (FRGH composites), 164 166 Furnace black process, 56 57

G Gas adsorption process, 130 131 Gas molecules, 53 Gas phase flow catalytic method, 109

242

Index

Gas sensing material polyaniline as, 183 190 polypyrrole as, 190 207 polythiophene as, 207 227 mechanism, 97 process, 117, 233 Gas sensor(s), 16 19, 52 53, 136, 149, 181, 217, 233 235, 234f 3D hierarchical carbon-based gas sensors 3-D functionalized graphene nanocomposite as gas sensors, 163 166 3D graphene-based gas sensors, 174 178 3-D metal doped graphene nanocomposite as gas sensors, 166 167, 166t 3D metal oxide/carbon nanocomposite as gas sensors, 171 173 3-D metal oxide/carbon nanotube as gas sensors, 167 171 3D metal oxide/graphene nanocomposite as gas sensors, 153 163 3-D metal oxide/graphene oxide/carbon nanotube nanocomposite as gas sensors, 167 171 construction/fabrication of 3D architectures, 151 153 importance of 3D nanomaterial, 150 151 sensing mechanisms of 3D TiO2/graphene-carbon nanotubes gas sensors, 169 171 based on carbon nanomaterials and nanocomposites, 59 carbon black materials and composites for, 59 60 carbon nanofibers as, 111 124 and composites for, 60 62

carbon nanotubes for, 64 65 and composites for, 62 66 conducting polymer nanocomposites for, 93 96 and polymer composites for, 65 66 classification of gas sensors on basis of different operating principle, 18t functionalized carbon nanotubes as, 97 98 grapheme materials and composites for, 66 73 and metal or metal oxide nanocomposite for, 68 71 and polymers nanocomposites for, 72 73 market of, 235 236 mechanism, 129 131 metal composites for, 64 65 metal oxide composites for, 64 65 parameters of, 31 33 recovery time, 33 response time, 33 selectivity, 32 sensitivity, 31 32 stability, 32 33 role of gas sensors in various fields, 17t systems, 40 types based on carbon-based nanomaterials, 26 31 electrical/chemiresistive sensors, 27 28 electrochemical sensors, 27 mass-sensitive gas sensors, 29 30 thermometric gas sensors, 30 31 Gas-phase flow catalytic method, 109 Gases, 39, 181 GdInO3/reduced graphene oxide, 161 163 Glucose, 87

Gold (Au), 87, 136 Au-Pt/CNFs, 115 116 Gold nanostars (AuNS), 187 188 Graphene, 3 4, 9 10, 12 14, 19, 25 26, 55 56, 58 59, 129 130, 140 141, 151 152, 236 composites for gas sensor, 66 73 and derivative/metal oxidebased gas sensor, 134 139 and derivative/metal-based gas sensor, 131 134 and derivative/polymer based gas sensor, 140 141 for gas sensor, 68 73 gas sensors based on carbon nanomaterials and nanocomposites, 59 graphene-based gas sensors, 127 128 and derivative/metal oxidebased gas sensor, 134 139, 134t and derivative/metal-based gas sensor, 131 134, 131t gas sensor mechanism, 129 131 methods for graphene synthesis and advantages and limitations, 129t properties, 128f graphene-based utilizations, 128 graphene-metal nanocomposites, 129 130 graphene-polymer nanocomposites, 129 130 graphene metal oxide nanocomposites, 134 135 materials for gas sensor, 66 73 structures of pristine graphene, 11f synthesis, 128, 129t

Index

Graphene modified polythiophene sensor (RGO-PTH sensor), 208 210 Graphene nanoribbons (GNR), 129 130 Graphene oxide (GO), 9 10, 127 128, 151 153, 175 176, 236 GO/PPy composite, 141 Graphene-PEDOT, 41 Graphene quantum dots (GQD), 10 Graphite, 3 4, 12 monolayers of, 14 Graphitic carbon nanofibers (GCNFs), 121 122 GCNFs/PAA, 122 Graphitic-carbon nanofibers/ polyacrylate, 122 Gravimetric mode, 181

H Heat treatment process, 106 107 “Helical microtubes of graphitic carbon”, 6 7 Heterojunction, 156 3-D nanocomposite, 151 152 p-n heterojunctions, 198, 220 221 Hexafluoroisopropanol (HFIP), 35 37 Hexane, 39 High tensile steel, 8 High-quality graphene synthesis, 128 High-resolution transmission electron microscopy (HRTEM), 137 139 Hummer’s method, 14, 164 166 Hybrid(s) carbon nanofibers, 60 62 functional materials, 234 235 nanostructures, 236 sensor, 92, 133, 198 200 Hydrocarbons, 12 14, 35 37, 107, 204 206

Hydrochloric acid (HCl), 190 191 Hydrogen (H2), 30, 185 gas sensor, 133 134, 141 H2 gas molecules, 190 Hydrothermal method, 114, 137 139, 152 153

I In situ polymerization of pyrrole monomer, 141 In situ self-assembly technique, 95, 203 204 In-situ chemical oxidation technique, 223 225 In-situ polymerization method, 218 220 Industrial wastes, 149 Ink-jet printing technique, 41 Inorganic nanofiller, 140 141 Inorganic semiconductors, 55 Interconnected nanofibrous polythiophene (INPTH), 222 223 Interdigital electrode (IDE), 60 62, 116 117 Interdigital transducers (IDT), 29 30 Intrinsically conducting polymers (ICPs), 93 94 Ion beam milling, 83 Ionization gas sensors, 39, 86 Iron, 58, 107 FeOEP, 35 37

K Kevlar material, 8

L Laser ablation, 12 14 Laser vaporization, 84 “Lerf-Klinowski” molecular structure of GO, 9 10 Leucoemeraldine, 183 Lignin, 106 107 Limit of detection (LOD), 95 96 Linear sweep voltammetry, 27

243

Liquefied petroleum gas (LPG), 68 69 Lithium per chlorate (LiClO4), 198 Local lattice distortion, 94 Low power gas sensors, 236 237 Low-temperature oxidizing reaction process, 92

M Mass-sensitive gas sensors, 29 30 Materials science, 235 236 Mechanical wave, 29 30 Mesoporous carbon nanofibers, 117 Metal composites for gas sensor, 64 65 Metal nanocomposite for gas sensor, 68 71 Metal nanoparticles, 33 35 Metal oxide, 18, 25 26, 35 37, 135 carbon nanofibers modified with, 110 composites for gas sensor, 64 65 graphene hybrids, 134 135 nanocomposite for gas sensor, 68 71 nanoparticles, 97 98 Metal oxide semiconductor (MOS), 134 135 Metal organic frameworks (MOFs), 89 Methane, 12 14, 30, 129 130 Methanol, 39, 86 87 Methylamine, 197 198 Micro Electro Mechanical Systems platforms (MEMS platforms), 236 237 Microcantilever-based sensors, 29 30 Microelectronics, 235 236 MICS (gas sensors), 27 28 Molybdenum disulfide/reduced graphene oxide, 155 156

244

Index

Multi-walled carbon nanotubes (MWCNT), 6 7, 57 58, 83, 223 225 Multifunctional carbon nanofibers, 116 117

N N-(3-trimethoxysilylpropyl) pyrrole, 193 N-type oxide semiconductor, 187 188 Nanocomposites, 25 26, 217 218, 236 3-D functionalized graphene nanocomposite as gas sensors, 163 166 3-D metal doped graphene nanocomposite as gas sensors, 166 167, 166t 3-D metal oxide/carbon nanotube as gas sensors, 167 171 3D metal oxide/graphene nanocomposite as gas sensors, 153 163 3-D metal oxide/graphene oxide/carbon nanotube nanocomposite as gas sensors, 167 171 of CNTs, 95 films, 200 202 materials, 150 Nanocrystalline, 150 Nanocrystalline diamond (NCD), 29 30 Nanocrystals (NCs), 136 Nanofibers, 25 26, 236 Nanoflowers, 150 Nanomaterials, 55, 127, 236 Nanoparticle (NP), 130 132 Nanoscale porous steamed graphene hydrogel (S-GH), 175 176 Nanoscience, 3, 233 Nanostructures, 150 “Nanotechnology”, 3, 83, 233, 235 236 Nanotubes, 55

Naphthalene sulfonic acid (NSA), 198 NSA-PPy sensor, 204 206 New Cosmos, 27 28 Nickel (Ni), 12 14, 58, 87, 107, 174 175 Ni-CNF, 118 121 Niobium, 12 14 Nitrogen dioxide (NO2), 185, 203 Nitrogen-doped graphene quantum dots/TiO2 (NGQD/TiO2), 166 167 Noble metals, 132 graphene hybrids, 131 132 Non-polar solvent, 14 Nonpolar gases, 92 Nucleic acids, 87

O Octaethyl porphyrin (OEP), 35 37 One pot hydrothermal method, 152 One step electrostatic spray deposition, 172 173 One step hydrothermal method, 152 153, 175 176 One-dimension(1D) nanomaterials, 105 106 nanostructures, 150 One-step forced hydrolysis method, 160 161 Optical fibers, 55 Organic hybrids, 236 Oxidation process, 10, 30 Oxidation-modified MWCNTs (f-CNTs), 66 Oxidizing gases, 222 223 Oxygen diffusion process, 97 Oxygen-adsorption theory, 135

P p-phenylenediamine (PPD), 68 p-toluene sulfonic acid (PTSA), 190 191 P-type PANi, 187 188

P-type semiconductors, 53 54, 135 136, 198 200 Palladium, 87 hydride, 87 Para-toluene sulfonate (p-TS), 198, 207 Pernigraniline, 183 Peroxide, 130 131 Phenolic resin, 106 107 Photolithography technique, 40, 72 Physical vapor deposition (PVD), 88 Plasma enhanced chemical vapor deposition method (PECVD method), 109 110, 193 194 Platinum (Pt), 12 14, 87 platinum-doped 3-D graphene, 166 Polaron, 94, 190 191 Poly (3-hexylthiophene) (P3HT), 214 215 Poly(3,4ethylenedioxythiophene) (PEDOT), 93 94 Poly (methyl methacrylate) (PMMA), 35 37, 66 Poly-N-vinylpyrrolidone (PNVP), 29 30 Polyacetylene (PA), 93 94 Polyacrylate polymer (PAA), 122 Polyacrylonitrile (PAN), 12, 57 PAN/(PAN-b-PMMA) sensor, 122 123 Polyaniline (PANI), 27 28, 93 94, 183, 236 as gas sensing material, 183 190 emeraldine oxidation state of polyaniline, 185f forms of polyaniline, 184f nanocomposites, 187 188 PANi-based nanocomposite systems, 187 188 PANi-based ultra-thin films, 185 187

Index

PANi/TiO2 nanocomposite thin film, 189 190 PANi/ZnMoO4 nanocomposite thin films, 189 190 polyaniline’s electrical conductivity, 183 185 polymer, 185 Polybenzimidazole, 106 107 Polyester, 106 107 Polyethylene terephthalate (PET), 133 134, 193 Polyimides, 106 107 Polymer(s), 18, 29 30, 55, 57, 236 composites for gas sensor, 65 66 nanocomposites, 35 37 for gas sensor, 72 73 nanofiber, 106 107 sensors, 196 197 Polypyrrole (PPy), 27 28, 93 95, 196 198, 200 206, 236 as gas sensing material, 190 207 DBSA, 199f deposition of thin film by drop cast in-situ method, 192f energy band diagram of DBSA, 201f mechanism involved in NH3 gas, 192f PVA/WPPy/V2O5 nanocomposite films, 202f SnO2/PPy, 196f synthesis of PPy-FP, 191f nanocomposites, 191 193 nanostructured conducting PPy, 207 on polyester and teflon fibers, 197 198 PPy-FP, 190 191, 191f PPy-polyester-solution polymerizationpolypropylene

membrane sensor, 197 198 PPy-polyester-solution polymerization-teflon membrane, 197 198 PPy/Ag, 194 195 PPy/DBSA, 193 PPy/sCoPc, 203 PPy/SnO2, 195 196 PPy Fe2O3 based sensor, 198 200 PPy WO3 hybrid nanocomposite sensor, 198 200 thin films, 193 Polythiophene (PTH) composite film, 213 214 as gas sensing material, 207 227 mass fraction, 216 217 PTH/G, 217 218 PTh/G nanocomposites, 217 PTH/G/ZT sensor, 210 211 PTH/SnO2, 212 213, 226 227 PTH/SWCNT, 221 222 PTH/WO3, 213, 216 221 PTH/ZnO nanocomposite, 200 202 PTH/ZnO nanocompositesbased sensor, 215 216 Polythiophene supported tin doped titanium nanocomposites (PTH/ Sn-TiO2), 211 Polythiphene (PTH), 93 94 Polyvinyl alcohol (PVA), 106 107, 200 202 Polyvinylidene, 106 107 Porous carbon, 19 Porous structured materials, 18 Potentiometric mode, 181 Propane (C3H8), 198, 204 206 Protonic acid, 185 187 Protons, 203 204 Pyrolysis techniques, 12, 14 15, 164 166 Pyrrole, 193

245

Q Quantum dots (QDs), 68 69, 150 Quartz crystal microbalance (QCM), 29, 89 QCM-based sensors, 29 30

R Raman spectroscopy, 137 139 Redox reaction, 207 Reduced graphene oxide (rGO), 9 10, 25 26, 68, 127 128 PPy/rGO, 191 193 rGO/Eu(TPyP)(Pc) sensor, 175 176 rGO/hexagonal WO3 (rGO/hWO3), 69 70 rGO Cu2O nanowire mesocrystals, 139 Reduced graphene oxide hydrogel (RGOH), 153 Relative humidity (RH), 187 188 Relative resistance difference (RRD), 197 198 Resonance frequency shift gas sensors, 39, 86 Room temperature (RT), 85 Ru nanoclusters, 190 Russian Doll model, 6 7 and Swiss roll model, 6 7 Ruthenium OEP (RuOEP), 35 37

S “Scotch tape method”, 14 Screen printing technique, 40 Self-heating mode, 116 117, 236 237 Semiconducting metal oxides (SMOs), 25 26, 85 SMOs-based gas sensors, 18 19 Semiconducting metal oxides, 18, 89 90 Senor architecture, 32 Sensing materials, 55 56

246

Index

Sensing mechanism, 70 71 of 3D TiO2/graphene-carbon nanotubes gas sensors, 169 171 of carbon nanomaterialsbased gas sensors, 37 39 capacitive gas sensors, 39 ionization gas sensors, 39 resonance frequency shift gas sensors, 39 schematic of chemiresistive-type sensors’ sensing mechanism, 38f sorption gas sensors, 37 39 of carbon nanotube-based gas sensors, 85 87 of PANI/SWCNT nanocomposite, 95 Sensing process, 121 122 Sensors, 25, 127 128, 181, 195 196, 222 223, 235 237 material, 204 206 technology focuses, 234 235 Signal processing/pattern recognition progress, 235 236 Silicon dioxide/silicon (SiO2/Si), 58 59 Silicon processing technology, 235 Silk, 106 107 Silver, 87 Single walled carbon nanotube (SWCNT), 6 7, 57 58, 83 SnO2 hollow spheres/ polythiophene sensor (hs-SnO2/PTH sensor), 207 208 SnO2/CNFs, 114 SnS2/reduced graphene oxide, 159 Solid-state gas sensors, 233 Sorption gas sensors, 37 39, 85 86 Spray coating technique, 41, 190

Spray method, 109 Stripping voltammetry, 27 Substrate method, 106, 109 110 gas-phase flow catalytic method, 109 plasma-enhanced chemical vapor deposition, 110 spray method, 109 5-sulfo salicylate (SS), 207 Sulfonate anions, 207 Sulfonated cobalt phthalocyanine (sCoPc), 203 Superoxide, 130 131 Surface acoustic wave (SAW), 29, 185 187 SAW-based sensors, 29 30 Surface functionalization, 84 85 Surface oxidation process, 97 Surface-mediated vertical phase separation process, 217 Swiss roll model, 6 7 Synthetic polymers, 106 107

T Ternary 3-D hybrid, 152 153 Terpineol, 40 Tetrahydrofuran (THF), 113 114 Tetraphenylporphyrin (TPP), 35 37 Thermal black process, 56 57 Thermoconductivity sensors, 30 Thermometric gas sensors, 30 31 gas sensors are classified in several ways according to detection principle, 31t Thin film deposition, 83 Thiol group (SH), 204 206 Three-dimension (3D) 3-D reduced graphene oxide hydrogel, 163 carbon doped ZrO, 172 173 chemiresistor-based gas sensor, 174 Co3O4/FRGH composites, 164 166

GO/α-Fe2O3 nanorods, 160 161 graphene scaffolds, 175 graphene-based gas sensors, 174 178 “hierarchal graphene”, 10 hierarchical carbon-based gas sensors 3-D functionalized graphene nanocomposite as gas sensors, 163 166 3-D metal doped graphene nanocomposite as gas sensors, 166 167, 166t 3D metal oxide/carbon nanocomposite as gas sensors, 171 173 3-D metal oxide/carbon nanotube as gas sensors, 167 171 3D metal oxide/graphene nanocomposite as gas sensors, 153 163 3-D metal oxide/graphene oxide/carbon nanotube nanocomposite as gas sensors, 167 171 3D nanomaterial, 150 151 construction/fabrication of 3D architectures, 151 153 sensing mechanisms of 3D TiO2/graphene-carbon nanotubes gas sensors, 169 171 monolithic architecture, 152 153 nanomaterials, 105 106 nanostructured materials, 150 151 nanostructures, 150 rGO/ZnO, 152 synergistical rGO/Eu(TPyP) (Pc) hybrid aerogel, 175 176 TiO2-decorated graphenecarbon nanotubes (G-CNT), 168 169 WO3, 151 152

Index

Tin (Sn) Sn SnO2/CNFs sensor, 111 113 hybrid Sn TiO2/reduced graphene/CNT nanocomposite, 92 Tin dioxide/reduced graphene oxide, 153 155 Toluene, 35 37, 86 87 Top-down approach, 12 Toxic gases, 136, 149 Toxic industrial chemicals, 17 18 Toxic volatiles, 17 18 Traditional detection methods, 52 Transmission electron microscopy (TEM), 90 92 Tungsten trioxide (WO3), 151 152 WO3/CNFs, 117 118 WO3 Pd nanostructures, 152 153

Two-dimension (2D) metal oxide nanosheets, 152 153 nanostructures, 150 nickel oxide nanosheets, 152 153

V Vanadium, 107 Vanadium pentoxide (V2O5), 200 202 V2O5/CNFs, 114 Vapor grown carbon fiber (VGCF), 57 Vapor phase polymerization process, 190 191 Viable gas-sensing devices, 233 234 Volatile organic compounds (VOCs), 17 18, 35 37, 39, 86 87, 185

W World Health Organization, 136 WS2/CNFs, 119

247

X X-ray diffraction (XRD), 137 139 X-ray fluorescence technique (XRF technique), 185 187 X-ray photoelectron spectroscopy (XPS), 137 139 Xylene, 12 14

Z Zero-dimensional nanomaterials (0D nanomaterials), 105 106 Zinc oxide (ZnO), 152 ZnO/CNFs, 111 ZnO/reduced graphene oxide, 153 ZnO/reduced graphene oxide aerogel, 156 158 Zinc oxide/Polythiophene (Zn-PTH), 212 Zirconium oxide (ZrO), 172 173