Functional Nanomaterials: Advances in Gas Sensing Technologies [1st ed.] 9789811548093, 9789811548109

Table of contents :
Front Matter ....Pages i-x
Tungsten Oxide Nanocomposites as High-Performance Gas Sensors: Factors Influencing the Sensor Performance (Digambar Y. Nadargi, Imtiaz S. Mulla, Sharad S. Suryavanshi)....Pages 1-18
High-Performance Gas Sensors Based on Nanostructured Metal Oxide Heterojunctions (Shulin Yang, Zhao Wang, Gui Lei, Huoxi Xu, Yongming Hu, Haoshuang Gu)....Pages 19-70
Chemiresistors and Their Microfabrication (Vishal Baloria, Chandra Shekhar Prajapati, Navakanta Bhat, Govind Gupta)....Pages 71-94
Novel CO and CO2 Sensor Based on Nanostructured Dy2O3 Microspheres Synthesized by the Coprecipitation Method (Carlos R. Michel, Alma H. Martínez-Preciado, Miguel A. Lopez-Alvarez, George P. Bernhardt, José A. Rivera-Mayorga)....Pages 95-116
Graphene-Metal-Organic Framework Modified Gas Sensor (Abdolhossein Sáaedi, Mahmood Moradi, Mohamed H. Alkordi, Mohammad Reza Mahmoudian, Gholam Hossein Bordbar, Ramin Yousefi)....Pages 117-142
Recent Advances on UV-Enhanced Oxide Nanostructures Gas Sensors (Nirav Joshi, Vijay K. Tomer, Ritu Malik, Jing Nie)....Pages 143-159
Hierarchical Oxide Nanostructures-Based Gas Sensor: Recent Advances (Sudip K. Sinha, Shashank Poddar, Subhas Ganguly)....Pages 161-188
Reduced Graphene Oxide (rGO)-Based Nanohybrids as Gas Sensors: State of the Art (Bhagyashri Bhangare, Niranjan S. Ramgir, K. R. Sinju, A. Pathak, S. Jagtap, A. K. Debnath et al.)....Pages 189-217
Graphene–Polymer-Modified Gas Sensors (Flavio M. Shimizu, Frank Davis, Osvaldo N. Oliveira Jr., Seamus P. J. Higson)....Pages 219-243
Functionalization of Graphene and Its Derivatives for Developing Efficient Solid-State Gas Sensors: Trends and Challenges (Debanjan Acharyya, Partha Bhattacharyya)....Pages 245-284
Hybridized Graphitic Carbon Nitride (g-CN) as High Performance VOCs Sensor (Prashant Kumar Mishra, Ritu Malik, Vijay K. Tomer, Nirav Joshi)....Pages 285-302
Graphene Oxide (GO) Nanocomposite Based Room Temperature Gas Sensor (Umesh T. Nakate, Sandip Paul Choudhury, Rafiq Ahmad, Pramila Patil, Yogesh T. Nakate, Yoon-Bong Hahn)....Pages 303-328
Carbon Nanotube Based Wearable Room Temperature Gas Sensors (Abhay Gusain)....Pages 329-348
Recent Advances in Functionalized Micro and Mesoporous Carbon Nanostructures for Humidity Sensors (J. Sharath Kumar, Naresh Chandra Murmu, Tapas Kuila)....Pages 349-381
Room Temperature Chemiresistive Gas Sensing Characteristics of Pristine Polyaniline and Polyaniline/TiO2 Nanocomposites (Arvind Kumar, M. Murali Krishnan, Vipul Singh, Soumen Samanta, Niranjan S. Ramgir)....Pages 383-397
Conducting Polymer Nanocomposite-Based Gas Sensors (Kalpana Madgula, L. N. Shubha)....Pages 399-431
Calixarene-Based Gas Sensors (Frank Davis, Seamus P. J. Higson, Osvaldo N. Oliveira Jr., Flavio M. Shimizu)....Pages 433-462

Citation preview

Materials Horizons: From Nature to Nanomaterials

Sabu Thomas Nirav Joshi Vijay K. Tomer   Editors

Functional Nanomaterials Advances in Gas Sensing Technologies

Materials Horizons: From Nature to Nanomaterials Series Editor Vijay Kumar Thakur, School of Aerospace, Transport and Manufacturing, Cranfield University, Cranfield, UK

Materials are an indispensable part of human civilization since the inception of life on earth. With the passage of time, innumerable new materials have been explored as well as developed and the search for new innovative materials continues briskly. Keeping in mind the immense perspectives of various classes of materials, this series aims at providing a comprehensive collection of works across the breadth of materials research at cutting-edge interface of materials science with physics, chemistry, biology and engineering. This series covers a galaxy of materials ranging from natural materials to nanomaterials. Some of the topics include but not limited to: biological materials, biomimetic materials, ceramics, composites, coatings, functional materials, glasses, inorganic materials, inorganic-organic hybrids, metals, membranes, magnetic materials, manufacturing of materials, nanomaterials, organic materials and pigments to name a few. The series provides most timely and comprehensive information on advanced synthesis, processing, characterization, manufacturing and applications in a broad range of interdisciplinary fields in science, engineering and technology. This series accepts both authored and edited works, including textbooks, monographs, reference works, and professional books. The books in this series will provide a deep insight into the state-of-art of Materials Horizons and serve students, academic, government and industrial scientists involved in all aspects of materials research.

More information about this series at http://www.springer.com/series/16122

Sabu Thomas Nirav Joshi Vijay K. Tomer •

•

Editors

Functional Nanomaterials Advances in Gas Sensing Technologies

123

Editors Sabu Thomas School of Chemical Sciences School of Energy Materials International and Inter University Centre for Nanoscience and Nanotechnology Mahatma Gandhi University Kottayam, Kerala, India

Nirav Joshi Institute of Physics University of Sao Paulo São Paulo, Brazil

Vijay K. Tomer Berkeley Sensor & Actuator Center University of California, Berkeley Berkeley, CA, USA

ISSN 2524-5384 ISSN 2524-5392 (electronic) Materials Horizons: From Nature to Nanomaterials ISBN 978-981-15-4809-3 ISBN 978-981-15-4810-9 (eBook) https://doi.org/10.1007/978-981-15-4810-9 © Springer Nature Singapore Pte Ltd. 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

In recent years, the progress in the gas sensing field has swiftly escalated for their exceeding utilization in the application having high technological throughputs. Due to the emerging electronic technologies backed up with novel and innovative ideas, avant-garde gas sensors are now commercially available, which not only offers superior advancements like increased portability and size miniaturization but also feeds-on minimized power yet offering prolonged durability. This book primarily focuses on the development of functional nanomaterials and their key aspects in synthesis strategies and tunable properties while addressing the critical gas sensing issues such as sensitivity, selectivity, and temperature dependency. Further, it highlights the applicability of the miniaturized sensors in a wide range of modern nano-tools and nano-devices deployed in electronics, healthcare, food, pharmaceutical, and medical industries, that have a significant impact on our society. The field of gas sensors is indeed following a fast and consistent growth. There is an incredibly wide range of sensing materials, and almost all known materials, including metal oxides, polymers, dichalcogenides, ferrites, etc., can be utilized in designing gas sensors; however, the desired application-oriented selection of these materials is a challenging and multivariate task. While taking this situation into account, a comprehensive analysis of the available sensing materials with respect to their design and development strategies, which are compatible with modern semiconductor fabrication technology, has been concluded in this book. Besides, the remarkable electronic, mechanical, electrical, and thermal properties of these functional materials are also discussed herein. Close attention is given to the problems associated with stability, selectivity, and functionalizing of these materials. This book consolidates up-to-date information on every aspect of these functional materials, including their sensing mechanisms, engineering, and state-of-the-art applications covering the entire spectrum of most recent literature citations, current market, and patents in a very detailed manner.

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In a nutshell, this book is an effort made in the quest to unravel the recent advances in the gas sensing world, whereupon simultaneously suggesting potential solutions to tackle the limitations of selectivity and stability in gas sensors. This book will be a valuable and accessible guide to the material-scientists and researchers from universities and national laboratories working in this phenomenal and exciting field of functional gas sensing materials.

Contents

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Tungsten Oxide Nanocomposites as High-Performance Gas Sensors: Factors Influencing the Sensor Performance . . . . . . . . . . . Digambar Y. Nadargi, Imtiaz S. Mulla, and Sharad S. Suryavanshi High-Performance Gas Sensors Based on Nanostructured Metal Oxide Heterojunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shulin Yang, Zhao Wang, Gui Lei, Huoxi Xu, Yongming Hu, and Haoshuang Gu

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Chemiresistors and Their Microfabrication . . . . . . . . . . . . . . . . . . Vishal Baloria, Chandra Shekhar Prajapati, Navakanta Bhat, and Govind Gupta

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Novel CO and CO2 Sensor Based on Nanostructured Dy2O3 Microspheres Synthesized by the Coprecipitation Method . . . . . . . Carlos R. Michel, Alma H. Martínez-Preciado, Miguel A. Lopez-Alvarez, George P. Bernhardt, and José A. Rivera-Mayorga

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Graphene-Metal-Organic Framework Modified Gas Sensor . . . . . . 117 Abdolhossein Sáaedi, Mahmood Moradi, Mohamed H. Alkordi, Mohammad Reza Mahmoudian, Gholam Hossein Bordbar, and Ramin Yousefi

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Recent Advances on UV-Enhanced Oxide Nanostructures Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Nirav Joshi, Vijay K. Tomer, Ritu Malik, and Jing Nie

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Hierarchical Oxide Nanostructures-Based Gas Sensor: Recent Advances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Sudip K. Sinha, Shashank Poddar, and Subhas Ganguly

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Reduced Graphene Oxide (rGO)-Based Nanohybrids as Gas Sensors: State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 Bhagyashri Bhangare, Niranjan S. Ramgir, K. R. Sinju, A. Pathak, S. Jagtap, A. K. Debnath, K. P. Muthe, and S. W. Gosavi

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Graphene–Polymer-Modified Gas Sensors . . . . . . . . . . . . . . . . . . . 219 Flavio M. Shimizu, Frank Davis, Osvaldo N. Oliveira Jr., and Seamus P. J. Higson

10 Functionalization of Graphene and Its Derivatives for Developing Efficient Solid-State Gas Sensors: Trends and Challenges . . . . . . . . 245 Debanjan Acharyya and Partha Bhattacharyya 11 Hybridized Graphitic Carbon Nitride (g-CN) as High Performance VOCs Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Prashant Kumar Mishra, Ritu Malik, Vijay K. Tomer, and Nirav Joshi 12 Graphene Oxide (GO) Nanocomposite Based Room Temperature Gas Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 Umesh T. Nakate, Sandip Paul Choudhury, Rafiq Ahmad, Pramila Patil, Yogesh T. Nakate, and Yoon-Bong Hahn 13 Carbon Nanotube Based Wearable Room Temperature Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329 Abhay Gusain 14 Recent Advances in Functionalized Micro and Mesoporous Carbon Nanostructures for Humidity Sensors . . . . . . . . . . . . . . . . 349 J. Sharath Kumar, Naresh Chandra Murmu, and Tapas Kuila 15 Room Temperature Chemiresistive Gas Sensing Characteristics of Pristine Polyaniline and Polyaniline/TiO2 Nanocomposites . . . . . 383 Arvind Kumar, M. Murali Krishnan, Vipul Singh, Soumen Samanta, and Niranjan S. Ramgir 16 Conducting Polymer Nanocomposite-Based Gas Sensors . . . . . . . . 399 Kalpana Madgula and L. N. Shubha 17 Calixarene-Based Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 Frank Davis, Seamus P. J. Higson, Osvaldo N. Oliveira Jr., and Flavio M. Shimizu

About the Editors

Sabu Thomas is currently Professor and Pro-Vice Chancellor at Mahatma Gandhi University, Kerala, India, in addition to being the founder director of the International and Inter University Centre for Nanoscience and Nanotechnology. After his B.Tech. in Polymer Science and Rubber Technology from the University of Cochin, he pursued his Ph.D. from Indian Institute of Technology, Kharagpur. Prof. Thomas has received many national and international awards, including Fellowship of the Royal Society of Chemistry, MRSI award, SESR award, Dr. APJ Abdul Kalam Award for Scientific Excellence, an honorary degree by Université de Lorraine, and multiple fellowships by prestigious societies and universities. Prof. Thomas’ research has spanned many areas of nanocomposite and polymer science and engineering, and he has edited more than 70 books, holds 5 patents and has authored over 750 research publications. Nirav Joshi is currently a FAPESP postdoctoral fellow at the University of Sao Paulo, Brazil. He completed his M.S. and Ph.D. in Applied Physics at Maharaja Sayajirao University of Baroda, after which he joined as a Research Associate at Bhabha Atomic Research Centre, India with joint collaboration with Paris University, France. He has postdoctoral experience from South Korea, Brazil and United States, having most recently completed his tenure at University of California Berkeley, USA where he developed selective and sensitive microsensor by MEMS techniques. His present research focuses on synthesis and characterization of oxide nanostructures and 2D material-based gas sensors. He has authored more than 20 international research papers and 3 book chapters. Vijay K. Tomer is currently working as Fulbright-Nehru Postdoctoral fellow in Berkeley Sensor & Actuator Center, University of California, Berkeley, USA, where he is exploring the insights of ordered mesoporous hybrid materials for novel humidity/gas sensing applications. He received his Ph.D. in Materials Science and

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Nanotechnology from D.C.R. University of Science & Technology, India, after which he worked with the Institute of Nano Science & Technology, Punjab, India and the University of Kiel, Germany. Dr. Kumar has authored more than 50 international research papers and 11 book chapters.

Chapter 1

Tungsten Oxide Nanocomposites as High-Performance Gas Sensors: Factors Influencing the Sensor Performance Digambar Y. Nadargi, Imtiaz S. Mulla, and Sharad S. Suryavanshi

1 Introduction In the present dynamic lifestyle, amenity-with-safety is of prime importance. Amongst many other safety possessions, gas sensors have a special place as inhouse safety utensils. The need and hence the demand for gas sensors in recent years have increased enormously. Going back in the history, the origin of gas sensors lies somewhere in coal mines, where the use of canary birds as an early warning mechanism was incepted in the form of the first gas alarm system. One of the first artificial gas detectors, Davy’s lamp (1815), was designed to detect the presence of methane and oxygen deficiency in coal mines (Simonin 1869). The first professional product designed for the consumer market was launched in 1970 (Aswal and Gupta 2007), based on the works of Seiyama (Seiyama et al. 1962) and Taguchi (Taguchi 1962) [1–3]. Figure 1 shows the glimpse of the early history of gas sensor development. Photographic image of mining foreman showing a small cage with a canary bird used for testing carbon monoxide gas (Fig. 1a). Davy’s lamp and Taguchi’s schematic of a gas sensor are highlighted in Fig. 1b, c. Prior to Industrial Revolution (year 1750), the concentration of harmful gases in the atmosphere was hardly 250–300 ppm. At present (year 2018), the value has gained a dramatic peak of 40%, which is further continuing to increase. Figure 2 illustrates the annual greenhouses gas emissions by various sectors of the society [4]. Certainly, D. Y. Nadargi · S. S. Suryavanshi (B) School of Physical Sciences, Punyashlok Ahilyadevi Holkar Solapur University, Solapur, Maharashtra 413255, India e-mail: [email protected] D. Y. Nadargi e-mail: [email protected] I. S. Mulla CSIR, New Delhi, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_1

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Fig. 1 Glimpse of early history of gas sensor development. a Photographic image of mining foreman with a canary bird for testing CO, b and c schematic of Davy’s lamp and Taguchi’s gas sensor, respectively

Fig. 2 Pollution scenario and annual greenhouses gas emissions by various sectors of the society

its potential consequences will not only disturb the ecosystem and biodiversity, but also the livelihood of people worldwide [5–7]. This gives an unprecedented call to monitor hazardous gases (CO2 , CH4 , H2 S), and thereby spreading the awareness to stop/minimize their liberation at domestic as well as industry sectors. Gas sensors play a vital role in monitoring and wrong alarming of such gases. IDTechEx analysts estimated the value of the sensor industry would be worth $400 million in 2020, $2.4 billion in 2025, and over $3 billion in 2030 [8]. In the state of the art, amongst the various gas sensors (electrochemical sensors, semiconducting gas sensors, and optical particle monitoring), semiconducting gas sensors have received a lot of attention due to their robustness, stability, and cost-effectiveness [9–12].

1 Tungsten Oxide Nanocomposites as High-Performance Gas Sensors …

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Fig. 3 Bibliometric data on the number of publications and number of citations per year. The results obtained for “wo3 AND gas sensor” from Scopus

2 WO3 and Its Development Tungsten oxide (WO3 ), as an n-type metal oxide semiconductor, has attracted considerable interest in a variety of applications such as electrochromic devices, gas sensors, photoelectrodes, and optoelectrical devices [13–18]. Moreover, it is highly stable against photocorrosion in a harsh acidic environment [19]. The past few years have witnessed several innovative strategies to address the challenges in developing WO3 gas sensors, and excellent breakthrough results were reported. The graphs shown in Fig. 3a, b depict the number of articles published thus far and the corresponding number of citations (source: Scopus). The remarkable increasing trend in the number of articles published per year together with the outstanding number of citations suggests the prominence of WO3 -based gas sensing material. Here, we give a brief account of WO3 synthesis through different synthetic parameters and approaches. A facile hydrothermal synthesis strategy is unitized in general. The obtained powder is treated for drying and thick film fabrication. The noble metal-ruthenium, structure-assisting agent-glycine, and graphene oxide are utilized to study their independent influence on the structure formation of WO3 and thereby gas sensing properties. Figure 4 shows the schematic illustration of development routes of WO3 and the overall synthesis strategy.

3 Effect of Various Synthetic Parameters on the Performance of WO3 Gas Sensor In the following section, we shall discuss the various parameters which affect/enhance the properties of WO3 in view of gas sensing application. In all total, three different angles are discussed over here (see Fig. 4, right part), which are (i) influence of noble metal (Ru) loading, (ii) morphology tuning using glycine as a structure-assisting agent, and (iii) influence of graphene loading.

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Fig. 4 schematic illustration of development routes of WO3 and overall synthesis strategy

3.1 Influence of Noble Metal Loading—Ru-Loaded Mesoporous WO3 Microflowers for Enhanced H2 S Sensing Before discussing the gas sensing properties, let us have a brief detail about the synthetic protocol. In a typical procedure, WO3 is developed at various hydrothermal reaction timings (i.e., 4, 12, 18, and 24 h) at 180 °C, followed by Ru loading from 0.25 to 1.00 wt% in the pristine WO3 . The samples are labelled as S-4 h, S-12 h, S-18 h, and S-24 h for the hydrolysis timing 4, 12, 18, and 24 h; and R-0.25, R-0.5, and R-1.0 for Ru loadings 0.25, 0.5, and 1.00 wt%, respectively [20, 21]. Developed WO3 material displays striking hierarchical/ordered marigold nanostructures irrespective of Ru loading (see Fig. 5). These ordered nanostructures are self-possessed numerous nanosheets, that aligned themselves in marigold form. The hierarchical erection of microflower using numerous individual nanosheets is evidently demonstrated in a higher magnification SEM image. The obtained microflower

Fig. 5 FESEM images of pristine and Ru-loaded WO3 samples

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Fig. 6 Elemental mapping of WO3 with various loading of Ru

is of ~5 µm averagely in diameter, while the nanosheets are of ~90–165 nm in length, and 80–95 nm in width (see TEM images). The elemental mapping of Ru-loaded WO3 samples showcase the uniform distribution of Ru in the WO3 matrix (Fig. 6). The mild dispersion of Ru is seen in Sample R-0.25, whereas a blotting effect is observed for higher loading of Ru (sample R-1.0). The optimal and uniform dispersion of Ru is well-maintained in sample R-0.5. The dispersion effect and uniformity has a major role in gas response properties, which is discussed later in the “spillover mechanism” section. The hierarchical base/foundation of the microflower, i.e., nanosheet, is analysed under TEM (see Fig. 7). With a high aspect ratio, they are found to be uniform and identical. The volume fraction of Ru insertion in WO3 is seen qualitatively in the TEM images. The higher contrast is seen due to heavier element Ru in the bright field image. The attached Ru nanoparticles are of the size 10–50 nm. These ultrathin nanosheets with large aspect ratio property make them encouraging propensity to get customized loosely and in a poriferous form. Therefore, it enables numerous channels for the efficient and rapid diffusion of gases, which certainly improves the potentiality of the parent material in view of gas sensing applications [22]. Furthermore, the HRTEM fringes and SAED patterns with bright rings (Fig. 7) match with WO3 planes, belonging to the orthorhombic single crystal phase with the interplanar distance of 0.37 nm. The aptness of the obtained 3D-microflowers (composed of 2D-ultrathin nanosheets) for gas sensing applications is visualized by testing towards various reducing gases (Diethanolamine: DEA, Trimethylamine: TMA, Xylene, Acetone, Propanol, Ammonia, Ethanol, and Hydrogen sulphide: H2 S). The following section describes the gas sensing proficiency of the developed material. The material displayed the best selectivity towards H2 S gas (Fig. 8a), with Ra /Rg explicitly 82 for

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Fig. 7 TEM images of low and high magnification with HRTEM and SAED pattern of pristine and 0.5% Ru-loaded WO3 samples

Fig. 8 a Selectivity test of developed samples towards various test gases at an operating temperature 200 ◯ C, gas response as a function of operating temperature for b pristine WO3 , and c for various Ru-loaded WO3 samples, d effect of Ru loading on response

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barely 1 ppm gas concentration at 200 ◯ C operating temperature. On the contrary, other remaining gases shows unsatisfactorily response even at 100 ppm target concentration at the equal working parameters. An Optimal Operating Temperature (OOT) of the developed pristine and Ru-loaded WO3 towards H2 S gas is described below (see Fig. 8b, c). The sensor material displays the mound, having an increment in the gas response, and reaching a certain peak value followed by decreased response as a function of increasing operating temperature. This behaviour of gas response is primarily due to an adsorption and desorption of oxygen molecules on the metal oxide surface (i.e., WO3 in the present case) and relative reaction with the target gas. Initially, the oxygen gets absorbed on the WO3 surface. Depending on the sensor temperature, the absorbed oxygen gets dissociated to its other forms, and combines with electrons from metal oxide. Therefore, the depletion layer is formed. When the target gas passes over through sensing material (WO3 ), the absorbed oxygen reacts with the target gas, releasing the electron back to the conduction band of WO3 . Finally, the diffusion of reaction products takes place to recover back the original state of the metal oxide (WO3 ) [23]. Markedly, the massive performance difference in gas response can be seen between the pristine and Ru-loaded WO3 . The pristine WO3 sample with 18 h hydrothermal treatment (S-18 h) displayed the maximum response (Ra/Rg = 3.2 for 100 ppm) in comparison to other hydrothermal variation samples (S-4 h, S-12 h, S-24 h). Therefore, sample S-18 h is treated as an optimized reference sample for studying the effect of Ru loadings. Upon loading Ru in the WO3 matrix, the response got improved enormously (Fig. 8c). The Ra/Rg value dramatically shoots up, and records the value 142 at 200 ◯ C for 10 ppm, from the previous value of just 3.2 at 275 ◯ C. Amongst the developed Ru-loaded WO3 , sample R-0.5 has the highest response for 10 ppm. The loading concentration greatly affects the size, porosity, and catalytic behaviour of the developed Ru/WO3 composites. With an increasing Ru loading percentage, the aggregation started to happen. This eventually hampered the spillover mechanism, and thereby a decrease in gas sensing performance. At 0.5 wt% of Ru loading, the exact amount of catalyst distribution occurred, which gave rise to the highest response. From the elemental mapping, it is clearly noticeable that the population density of Ru without any aggregation is higher in sample R-0.5. In Fig. 8d, the track of maximum response of all the developed samples is displayed. It obviously demonstrates the 0.50 wt% of Ru loading is well optimized for H2 S gas sensing in comparison to other Ru-loaded samples. This is due to the fact that the number of active surfaces in case of R-0.5 is more, compared to other loading values. For lesser Ru loading (R-0.25), there is still scope for response improvement, while for higher Ru loading (R-1.0) the blotting effect happens to occur. Therefore in both the cases, the response is seen to be low. Figure 9a shows the sensor response as a function of concentration of test gas (H2 S). All the developed sensors demonstrated a strident increase in the initial stage (1–50 ppm). With further advancement in the concentration, the gradual saturation is observed. This is due to the depleting active surface sites for the feasible gas interaction. The transient response property of sample R-0.5 is graphed in Fig. 9b. The profound basin observed in the transient response, that leads the developed sensor, is

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Fig. 9 a Sensor response as a function of H2 S concentration, b transient response curve of developed sensors at 1 ppm H2 S, c response/recovery behaviour of R-0.5, d stability parameter of developed H2 S sensor

the best candidate for H2 S sensing, with quick response of 48 s and relatively faster recovery 197 s for 1 ppm of H2 S. The flat posture or diminishing the basin format is observed in the other samples, which indicates the lesser response. Therefore these are the least preferred candidates for H2 S sensing. The response/recovery time of the samples are tabulated in Table 1. The response and recovery functionality of the best optimized sample (R-0.5) with the gas concentration is highlighted in Fig. 9c. In the fitness of natural behaviour, as the concentration of H2 S increased, the corresponding response and recovery time of the sensors got increased. Speaking about stability, the fine-tuned sensor with all Table 1 Variation of response/recovery time and sensitivity of H2 S with Ru concentration, operating temperature (200 ◯ C), and H2 S concentration Ru concentration (wt%)

H2 S concentration (ppm)

Response time (s)

Recovery time (s)

Sensitivity (Ra/Rg)

0

10

58

363

1.11

0.25

10

55

243

1.91

0.50

1

48

197

82

1.00

10

56

253

13.05

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Fig. 10 Spillover mechanism in Gas sensing and of Ru/WO3

the synthetic parameters (R-0.5) has ~96% of its initial performance over the period of 60 days, on barely 1 ppm H2 S concentration (Fig. 9d). This clearly confirms the stability and reproducibility of R-0.5 for commercial applications.

3.1.1

Role of Ru Loading in WO3 Matrix

Noble metals (Au, Ru, Pt, Pd, Ag, etc.) are the best additives to metal oxide matrix in improving the gas sensing characteristics of the parent material, in a dramatic way [24–26]. In particular, these metals play a key role in boosting the electronic and chemical processes responsible for the gas sensing property. By using these additives, the catalytic activity of the base oxide gets modified, favouring the generation of active sites/phases and thereby enhancing the electron exchange rate and as well as lowering the operating temperature. Figure 10 shows a schematic illustrating the role of Ru in boosting the sensing properties of WO3 . This dramatic improvement in H2 S sensing performance by the addition of Ru can be explained in three major steps as follows: Step 1: Oxygen molecules are adsorbed on the Ru surface, Step 2: Spilled over to WO3 , where dissociated oxygen species are ionized with electrons from WO3 , Step 3: Finally, in the presence of H2 S, the ionosorbed oxygen reacts with H2 S forming SO2 and injecting the electrons back into the conduction band of WO3 . As the concentration of ionosorbed oxygen species is larger on the Ru-loaded WO3 , a larger number of electrons are released to the conduction band under H2 S exposure causing a higher sensor signal. While dealing with the present scenario of Ru loading concentration, with increasing Ru dosing up to 0.50 wt%, the sensor material has good control of the resistance. Therefore, spillover phenomena have occurred in an efficient way. However, beyond the particular threshold value of Ru dosing (at 1.00 wt% and beyond), the nanoparticles began to agglomerate, which resulted in the sinking of the spillover effect.

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3.2 Influence of Structure-Assisting Agent—Tunable Microporous WO3 Nanostructures for Gas Sensing After discussing the influence of noble metal loading and spillover mechanism, we shall now move to the next spot (yellow circle—Fig. 4), i.e., influence of structureassisting agent to tune the microstructure using glycine. The influence of structureassisting agent (glycine) on morphology, and thereby gas sensing performance of WO3 is described in this section. One can see it as “Materials-by-Design”. This single and unique strategy of developing mesoporous WO3 with the morphologies ranging from nano to micro is promising to tune the sensing performance of WO3 as per the need. To brief, Na2 WO4 · 2H2 O (6.6 g, 0.2 mol) is dissolved in 100 ml of distilled water. The pH of the solution raised to 1–1.5 by drop-wise addition of HCl (3 M) at constant stirring. Separately maintained glycine (4, 6, 8, 10 mmol) is added to the precursor solution on constant stirring for 30 min, followed by the hydrothermal process at 180 °C for 12 h. The samples are labelled as WG1, WG2, WG3, WG4 for 4, 6, 8, and 10 mmol concentration of glycine, respectively [27]. The various forms of morphology obtained for WO3 are presented in Fig. 11. Flower-like spheres are obtained for samples WG1 and WG2. The typical diameter of the microflower averaged to ~5–6 µm in case of sample WG1, whereas comparatively smaller sized microflower is obtained for sample WG2, which is averaged to ~1 µm. These microflowers are again evolved from the hierarchical development of

Fig. 11 a SEM images of developed WO3 structures, b initial stages of glycine–metal complexation in an aqueous solution

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thin nanosheets (as described in Sect. 3.1). The WO3 nanosheets are of ~50 nm thickness, in general. While moving on to increasing concentration of glycine (beyond 6 mmol), it is noticed that flower-like assembly got diminished, due to increased density of the solution. At a higher concentration of glycine, the dissolution–nucleation and recrystallization process get forbidden. Therefore, the chances of getting the 3D micro-assembly structures are very uncommon. With the increased glycine concentration (8 mmol, WG3), the morphology of WO3 gets altered to entirely uniform 2D micro-assembly structures, i.e., nanorods of 1–2 µm in length and 100 nm in diameter. The increased concentration of glycine results in nanorods instead of nanosheets. Interestingly, unlike nanosheets, the nanorods are segregated. Their development is quite isolated and uniform with a high aspect ratio. This is due to the fact that the glycine at higher concentrations has remarkable coordination capacity with WO4 2− crystal facets [28]. The further increment in the glycine concentration up to 10 mmol reforms the nanorods into square-like nanoplates. The obtained nanoplates’ dimensions are found to be 100–200 nm in length and 10–20 nm in thickness. The morphological analysis of the developed tungsten oxide material clearly depicts the tunable morphology or the concept of material by design, using the simple trick varying the glycine concentration in the synthetic parameter. The notable modification/alteration in the morphology can be attributed to the zwitterion (German word “Zwitter” means dual) characteristics of glycine. Carboxylic (–COOH) and Amino (–NH2 ) groups as two different end groups can be utilized for complex formation [28, 29]. Mainly, it forms a complex with the metal ions involved by acting as a bidentate ligand. And formerly, it enhances the solubility of metal ions. Therefore the selective precipitation of the metal ion can be avoided. An aforementioned zwitterion characteristic of glycine in forming a stable metal complex is represented in Fig. 11 (lower part). Speaking about gas sensing performance, the variation in gas sensing is observed to be dependent on the morphology of the material. Different morphologies with varying surface area, and thereby surface re-activity, greatly influences the gas sensing performance. As a function of temperature, the developed sensors have shown similar curves (shape-wise) over the span of temperature (Fig. 12a). The optimum operating temperature of sample WG1 is obtained as 350 °C, whereas the rest of the samples (WG2, WG3, and WG4) have 300 °C of optimum operating temperature. The sensor having the microflower morphology (WG2) shows the maximum response of 83.87%, whereas other sensors show the response value as WG1 = 74.77%, WG3 = 35.66%, WG4 = 23.08%, respectively for 10 ppm concentration of acetone. The improvement in acetone gas sensing properties of the sensor WG2 is due to the hierarchical development of ultrathin nanosheets to form the microflower structure of WO3 . This gives rise to the high surface area, necessary for potential adsorption/desorption of oxygen molecules. Notably, on the one hand the response got increased, and on the other hand the operating temperature got decreased. The ultrathin WO3 nanosheets with larger aspect ratio (see TEM) favour the condition of micro/meso-porosity and thereby high surface area. As mentioned in the aforementioned para, the high surface area led to efficient diffusion and acceleration of the test gas [27]. Figure 12b displays the behaviour of WG2 sensor as a function of acetone gas concentration. Initially, the sensor displayed a sharp and steep increase in the

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Fig. 12 a OOT analysis, b effect of gas concentration, c transient response characteristics, d selectivity, and e acetone sensing mechanism of the sensor

response up to 40 ppm gas concentration. However, a steady increase and eventually saturation in the sensitivity are seen from 40 to 500 ppm of acetone concentration. Due to a large surface coverage, the surface reactions (adsorption/desorption) proceed faster with an increase in the gas concentration (steep part of the graph). After the percolation threshold of the gas adsorption, the chances of surface reactions get ceased. Therefore, the response remains unchanged. The transient gas response characteristics as a function of time for all the developed sensors is highlighted in Fig. 12c. The sensors are subjected to barely 10 ppm acetone concentration at 300 °C. The optimized sensor, WG2, displayed the quick response of 40 s, as compared to the remained sensors, WG1, WG3, and WG4. The perceived performance is due to the properties like morphology, porosity, and larger surface area of the said sensor. These properties expressively facilitate oxygen adsorption. In addition to fast response, WG2 shows quick recovery as well, just in 50 s to its initial value after the exclusion of acetone. For an ideal gas sensor, the selective response is the key parameter at a lower concentration of the gas. The selectivity of all the developed sensors where the nanostructures are tuned is highlighted in Fig. 12d. Notably, the optimized sensor WG2 has the response of 83.87% towards acetone at 10 ppm concentration only, in comparison to other test gases. This evidently illuminates the practical usability of the investigated material as an acetone gas sensor. The mechanism of acetone gas response of WO3 is highlighted in Fig. 12e.

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3.3 Influence of Graphene Oxide Loading—RGO/WO3 Nanocomposites In this section, we shall discuss the influence of RGO loading in the WO3 matrix for potential gas sensing material. The self-made RGO is utilized to develop the nanocomposites [30]. An appropriate amount of RGO and sodium tungstate (6.6 g, 0.2 mol) are ultrasonicated in 100 ml DW. The pH of the solution is raised to 1–1.5 by drop-wise addition of HCl (3 M) at constant stirring. This results in dark lemon coloured precipitate, to which 6.3 g of oxalic acid (6.3 g, 0.7 mol) and Rb2 SO4 (0.3 g, 0.01 M) are added. The mixture turned into a homogeneous, translucent, and stable sol of WO3 , which is treated to hydrothermal at 180 °C for 18 h. The obtained WO3 material is finally treated to 400 °C for 2 h in air. The samples are labelled as G0 (Pristine), G1 (0.15 wt% RGO), G2 (0.3 wt% RGO), and G3 (0.5 wt% RGO). The crystallographic identification confirms the monoclinic nature of the developed WO3 samples (Fig. 13a). As discussed in the earlier section, the microflower morphology is seen, with no effect on loading the RGO (Fig. 13b). TEM image of sample G2 (Fig. 13c) clearly shows the WO3 nanosheets are well loaded onto the graphene sheets. HRTEM indicates that the distance of lattice fringe is 0.37 nm, which is in agreement with the distance of the (100) lattice plane of WO3 [31]. It is can be seen that the incorporation of graphene shows an impact on the SAED pattern from single crystalline to polycrystalline nature. N2 sorption isotherms of the developed RGO/WO3 samples present the hysteresis loop at relative pressure (P/P0 ) of 0.1–0.9, similar to the characteristic feature of Type IV isotherm with H3 and H4 loop (Fig. 13d) [32]. Let us now discuss gas sensing performance of RGO-loaded WO3 sensor material. Pristine WO3 and RGO/WO3 sensors with hierarchical morphology are very

Fig. 13 XRD, SEM, TEM, HRTEM, SAED, and BET of the developed RGO/WO3 material

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promising due to their large effective surface area and great surface activity, which make them a suitable candidate for gas sensing applications. The variation in the gas sensing performance is observed to be dependent on the RGO loading. Varying surface area, and thereby surface activity, has a direct influence on the gas sensing performance. As expected, the sensors (G1 –G4 ) display an increase in the gas response with an increase in the temperature, up to certain maxima. Thereafter it goes on decreasing with further increase in the temperature (Fig. 14a). The respective OOT values are G1 : 325 °C, and G0 /G2 /G3 : 300 °C, respectively. At OOT, the response values are 43.96, 72.12, 95.35, and 50.60 for G0 , G1 , G2 , and G3 sensors

Fig. 14 a OOT analysis, b effect of gas concentration, c transient response characteristics, d selectivity, and e stability of the developed RGO/WO3 sensor

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Table 2 Variation of response/recovery time and sensitivity of H2 S as a function of RGO loading, operating temperature: 300 ◯ C, and H2 S concentration: 1 ppm RGO concentration (wt%)

Response time (s)

Recovery time (s)

Sensitivity (Ra/Rg)

0

33

76

11.71

0.15

38

79

34.36

0.3

25

67

67.84

0.5

34

71

10.38

to 100 ppm H2 S, respectively. The possible reasons for the enhancement in H2 S gas response properties of RGO/WO3 composites are (i) ultrathin two-dimensional sheet-like morphology, (ii) increased surface area due to RGO, (iii) loose and poriferous structured aggregates providing numerous channels for the efficient and rapid diffusion of H2 S gas. The mesoporous structure provides favourable precondition both for the rapid sorption–desorption of H2 S gas. Upon the variation of H2 S concentration, G2 shows the sharp and steep increase in the sensitivity at lower concentration (0–40 ppm) of H2 S vapours, while gradual increase and ultimately saturation in the sensitivity is seen from 40 to 500 ppm (Fig. 14b). As discussed in the above section, due to a large surface coverage, the adsorption and desorption proceed faster with the availability of gas molecules. At a threshold limit, populated surface coverage has no scope to adsorption and desorption of further available gas molecules, which ultimately shows a steady/saturated response. Figure 14c shows the transient gas response properties as a function of time for the best optimized sample (G2 ). This sensor is subjected to barely 1 ppm H2 S concentration at 300 °C. Fast response of 25 s is seen, with the sharp drop in the resistance values, upon injection of H2 S gas. Upon purging with the air, the sample shows an increase in the resistance, and attains its 90% of earlier resistance value within 67 s, indicating the good reversibility of the sensor responses towards H2 S gas. Table 2 shows the response/recovery time of WO3 and RGO/WO3 sensors. The selective response at a lower concentration of the gas is the key parameter for the ideal gas sensor. Figure 14d shows the selectivity of developed RGO/WO3 nanostructures towards various target gases. The selective response of the sensors is towards acetone, triethanolamine, xylene, propanol, ammonia, ethanol for 20 ppm and H2 S for 1 ppm. It is evident that sensor G2 exhibits a larger response to H2 S compared with other examined test gases. For an accurate comparison, the sensor responses at the same operating temperature of 300 °C are summarized. The response of sensor G2 for 1 ppm of H2 S is six times higher than other sensors tested for 20 ppm. The gas sensing behaviour of the material, i.e., selectivity is very important for real application. Higher selectivity values imply the more selective detection of H2 S in the presence of other gases. For the practical utility, the sensor should exhibit high selectivity with a good response for low concentration of target gas. Sample G2 fulfils these criteria due to the enhanced adsorption and mass transport of H2 S by the porous channels amongst the interlaced mesoporous nanosheets. The stability and reproducibility of the G2 sensor is depicted in Fig. 14e). The stability

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and reproducibility of the sensor has been tested by measuring the sensor response towards the 1 ppm concentration of H2 S operating at OOT (300 °C). The gas sensing measurements are repeated after every 10 days of initial measurement for a span of 2 months. The sensor response is ~85% of its initial measurement, confirming the reproducibility and stability of the developed material.

4 Conclusions From the old times where canary birds were treated as live sensors in coal mines, the development in the gas sensor field is dramatically improved. For the present environmental pollution scenario, semiconductor-based gas sensors (especially WO3 ) are having the versatile capability of being the potential candidate for wrong alarming of harmful gases. Within this chapter, we have elucidated synthesis and gas sensing analysis of WO3 material. This new and fascinating material is very sensitive to doping/loading with other materials. The chapter encompasses an influence of (i) noble metal loading (Ruthenium), (ii) structure-assisting agent (glycine), and (iii) graphene oxide loading (RGO) on gas sensing performance of WO3 material. The sensor with glycine-modified WO3 has decent selectivity towards acetone, whereas Ru and RGO loading have the best selectivity towards H2 S sensing. The following graph (Fig. 15) elucidates the comparison of the aforementioned intruders in the WO3 network matrix for gas sensing performance. The sensor developed with Ru-loaded WO3 nanocomposite showed a selective response of 83.87% for barely 1 ppm H2 S, whereas glycine-modified WO3 has 83.87% sensing towards acetone at 10 ppm concentration. Sensor derived from RGO-loaded WO3 nanocomposite showed a selective response of 64.2% for barely 1 ppm H2 S. All the developed sensor materials are robust and stable with ~95% of its initial performance over the period of 2 months, and thus showcase the ability of commercialization. Fig. 15 Overall comparison of WO3 material associated with various loadings/modifiers

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References 1. Seiyama T, Kato A (1962) A new detector for gaseous components using semiconductor thin film. Anal Chem 34:1502–1503 2. Taguchi N (1971) Gas detecting devices. U.S. Patent 3,631,436, 28 Dec 1971 3. Aswal DK, Gupta SK (2007) Science and technology of chemiresistor gas sensors. Nova Science Publishers 4. Victor DG, Zhou (2014) Climate change 2014, Ch. 1: Introductory chapter. Fifth assessment report of the intergovernmental panel on climate change. https://www.ipcc.ch/site/assets/ uploads/2018/02/ipcc_wg3_ar5_full.pdf 5. IPCC AR4 SYR Appendix Glossary (PDF). Retrieved 14 Dec 2008 6. NASA GISS: science briefs: greenhouse gases: refining the role of carbon dioxide. www.giss. nasa.gov. Retrieved 26 Apr 2016 7. Karl TR, Trenberth KE (2003) Modern global climate change. Science 302(5651):1719–1723 8. Chansin G, Pugh D (2017) Environmental gas sensors 2017–2027: technologies, manufacturers, forecasts. Scientific report. www.idtechex.com/research/reports/environmental-gassensors-2017-2027-000500.asp 9. Zhang J, Sokolovskij R, Chen G, Zhu Y, Qi Y, Li XLW, Zhang GQ, Jiang Y-L (2019) Impact of high temperature H2 pre-treatment on Pt-AlGaN/GaN HEMT sensor for H2 S detection. Sens Actuators B: Chem 280:138 10. Zhao Y, Song J-G, Ryu GH, Ko KY, Woo WJ, Kim Y, Kim D, Lim JH, Lee S, Lee Z, Park J, Kim H (2018) Low-temperature synthesis of 2D MoS2 on a plastic substrate for a flexible gas sensor. Nanoscale 10:9338–9345 11. Mehta SS, Nadargi DY, Tamboli MS, Chaudhary LS, Patil PS, Mulla IS, Suryavanshi SS (2018) Ru-loaded mesoporous WO3 microflowers for dual applications: enhanced H2 S sensing and sunlight driven photocatalysis. Dalton Trans 47:16840 12. Mehta S, Nadargi D, Tamboli M, Patil V, Mulla I, Suryavanshi S (2019) Macroporous WO3 : tunable morphology as a function of glycine concentration and its excellent acetone sensing performance. Ceram Int 45(1):409 13. Rajkumar C, Thirumalraj B, Chen SM, Veerakumar P, Liu SB, Appl ACS (2017) Mater Interfaces 37:31794 14. Mendieta-Reyes NE, Díaz-García AK, Gómez R (1990) ACS Catal. 2018:8 15. Wang Z, Fan X, Li C, Men G, Han D, Gu F, Appl ACS (2018) Mater Interfaces 10:3776 16. Cook B, Liu Q, Butler J, Smith K, Shi K, Ewing D, Casper M, Stramel A, Elliot A, Wu JZ, Appl ACS (2018) Mater Interfaces 10:873 17. Mehta SS, Tamboli MS, Mulla IS, Suryavanshi SS (2018) J Solid State Chem 258:256 18. Li S, Lin P, Zhao L, Wang C, Liu D, Liu F, Sun P, Liang X, Liu F, Yan X, Gao Y, Lu G (2018) Sens Actuators B 259:505 19. Berenguer AG, Celorrio V, Iniesta J, Fermin DJ, Ania CO (2016) Carbon 108:471 20. Mehta SS, Nadargi DY, Tamboli MS, Chaudhary LS, Patil PS, Mulla IS, Suryavanshi SS (2018) Dalton Trans 47:16840 21. Fujioka Y, Frantti J, Nieminen RM, Asiri AM (2013) J Phys Chem C 117:7506 22. Kim SJ, Choi SJ, Jang JS, Kim NH, Hakim M, Tuller HL, Kim ID (2016) ACS Nano 10:5891 23. Patil J, Nadargi D, Mulla IS, Suryavanshi SS (2018) Mater Lett 213:27 24. Chen D, Zhang H, Liu Y, Li J (2013) Energy Environ Sci 6:1362 25. Wang Q, Wen Z, Jeong Y, Choi J, Lee K, Li J (2006) Nanotechnology 17:3116 26. Wen Z, Wu W, Liu Z, Zhang H, Li J, Chen J (2013) Phys Chem Chem Phys 15:6773 27. Mehta SS, Nadargi DY, Tamboli MS, Mulla IS, Suryavanshi SS (2019) Ceramic Int 45(1):409 28. Lin S, Guo Y, Li X, Liu Y (2015) Mater Lett 152:102–104 29. Yin M, Yu L, Liu S (2017) J Alloy Compd 696:490–497

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30. Hummers WS, Offeman RE (1958) J Am Chem Soc 80:1339 31. Lu Y, Zhang J, Wang F, Chen X, Feng Z, Li C (2018) ACS Appl Energy Mater 15:2067–2077 32. Haiyun Xu, Gao Jie, Li Minhan, Zhao Yuye, Zhang Ming, Zhao Tao, Wang Lianjun, Jiang Wan, Zhu Guanjia, Qian Xiaoyong, Fan Yuchi, Yang Jianping, Luo Wei (2019) Front Chem 7:266

Chapter 2

High-Performance Gas Sensors Based on Nanostructured Metal Oxide Heterojunctions Shulin Yang, Zhao Wang, Gui Lei, Huoxi Xu, Yongming Hu, and Haoshuang Gu

1 Introduction The reliable and accurate detection of the gas species and the gas concentrations in the air atmosphere are of great significance to the people’s health and safety in the normal life or during industrial productions [1–3]. Generally, the effective gas sensors are used to achieve these detections. The gas-sensing performance of the applied gas sensor is extremely significant to successfully monitor the gas molecules and detect the gas concentrations. Up to now, lots of attention has been focused on the gas sensors based on the semiconductor metal oxides due to their advantages of easy fabrications, low prices, and a wide range of raw resources [4–6]. The sensors based on metal oxides are reported to effectively sense the reducing gases (H2 , H2 S, CO, CH4 , volatile organic compounds, etc.) and the oxidizing gases (O3 , NO2 , NH3 , etc.) [7–13]. At the first stage of the sensor based on the metal oxides, many of the reported sensors were mainly composed of the bulk materials prepared with the technology of mixture or calcination [14, 15]. The bulk metal oxides were reported to be working at high temperatures with the poor gas-sensing performances due to their low specific surface areas and the limited active sites in the sensing materials. The sensor based on the porous ZnO ceramic was found to exhibit a low sensitivity of 1.8–400 parts per million (ppm) CO at the high working temperature of 400 °C [16]. Then the thin film, synthesized with the methods of radio frequency magnetron sputtering, thermal evaporation, chemical deposition, or sol-coprecipitation were widely used to improve the gas-sensing properties of the metal oxides [17, 18]. The sensor based S. Yang (B) · G. Lei · H. Xu School of Physics and Electronic Information, Huanggang Normal University, Huanggang 438000, People’s Republic of China e-mail: [email protected] Z. Wang · G. Lei · Y. Hu · H. Gu Faculty of Physics and Electronic Sciences, Hubei University, Wuhan 430062, People’s Republic of China © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_2

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on the ZnO film with the thickness of 1.2 μm prepared by a low-cost chemical deposition was reported to show a sensor response of 2.33 toward the 15 ppm H2 S at 150 °C [19]. The low gas sensor responses with the high working temperatures make the sensors based on the bulk materials or the thin films not effective to be used in the practices. By now, much attention has been paid to the sensors based on the nanomaterials due to their advantages of easy fabrications, high specific surface areas and high active surfaces, which were the positive effectors to further enhance the gas-sensing performance of the metal oxides. With the developments in the methods to synthesize the nanostructured materials, various nanomaterials based on the semiconductor metal oxides have been successfully prepared, such as nanoparticles, nanowires, nanorods, nanoribbons, nanofibers, nanotubes, and nanosheets [20–22]. The sensor based on the obtained nanomaterial could be used to exhibit an enhanced sensing property to a given gas at a certain working temperature based on the oxidation or reduction of the targeted gas molecules on the surface of the sensing material. However, the pure nanostructured materials are reported to show relatively low gas-sensing performances because of the limited active sites in their surfaces and the weak interactions between the targeted gas molecules and the sensing materials [23, 24]. As reported, it could be an effective and successful strategy to improve the gas-sensing performance of the nanostructured metal oxide through establishing the heterojunctions in the sensing material. Many of the n-type metal oxides (ZnO, SnO2 , TiO2 , WO3 , In2 O3 , Fe2 O3 , MoO3 , Nb2 O5 , V2 O5 , etc.) and the p-type metal oxides (Co3 O4 , CuO, NiO, PdO, Cr2 O3 , TeO2 , Bi2 O3 , Ce2 O, Mn2 O3 , etc.) have been successfully assembled to be heterojunctions to enhance their gas-sensing properties [6, 25–27]. For example, the hierarchical CoO/SnO2 nanostructures were prepared by Lu et al. through a two-step hydrothermal process. The sensor based on the decorated SnO2 nanomaterial showed a higher gas sensor response of 145 toward 100 ppm ethanol at 250 °C compared with that of the pure SnO2 (~13.5) [28]. The Co3 O4 /In2 O3 hollow microtubes were also reported to be assembled by Zhang and his team workers [29]. Their results showed that the gas sensor response of the Co3 O4 /In2 O3 hollow microtubes was counted to be 786.8 at 250 °C when exposed to 50 ppm triethylamine (TEA), over two times higher than that of the pure In2 O3 . And the sensor based on the p-CuO/n-ZnO heterojunction prepared by Ramachandran et al. also presented an enhanced gas-sensing performance to the ethanol [30]. Moreover, more and more researchers have focused their attention on the sensors based on the heterojunctions composed of metal oxides in recent years, which could be confirmed by the increasing number of published articles shown in Fig. 1. Also, the sensors based on the heterojunctions have also been assembled to be field-effect transistor (FET) or the micro-electromechanical systems (MEMS) devices to improve the gas-sensing properties of the metal oxides [31, 32]. Therefore, the heterojunctions in the sensing material could effectively enhance the gas-sensing properties of the sensors based on the composites composed of different metal oxides. In this article, the types of the heterojunctions are firstly reviewed, namely, the n–p, n–n, p–n, and p–p heterojunctions. And the heterojunctions with the n–p–n structure or the p–n–p structure are also discussed; even there were fewer published

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Fig. 1 The number of the published articles on the gas sensors based on the nanostructure metal oxide heterojunctions for the period of 2009–2019 from Web of Science

articles reporting their gas-sensing performances compared with the four popular ones mentioned ahead. Then the construction strategies of the various heterojunctions were also discussed in this work based on the reported references to display the methods to establish the novel heterostructures. Also, the studies of the gas sensors based on the most investigated metal oxides, such as ZnO, SnO2 , TiO2 , WO3 , In2 O3 , Fe2 O3 , MoO3 , Co3 O4 , and CuO, were discussed in detail to show the enhancement in the gas-sensing performance of a given metal oxide and provided the methods to be referenced to synthesize the other potential heterojunctions. Furthermore, the enhanced gas-sensing mechanisms of a given type of heterojunction to the reducing gases or the oxidizing gases were reviewed, and the current challenges in the developments in the gas sensors based on the nanostructured heterojunctions were also discussed.

2 Types of the Metal Oxide Heterojunctions and Their Assembly Strategies There have been numerous types of heterojunctions based on the metal oxides assembled, including n–p, n–n, p–n, p–p, n–p–n, and p–n–p junctions. The sensors made of the heterojunctions showed promising gas-sensing performances with the help of the energy bands bend of the metal oxides in the sensing composites. In this part, the methods to prepare the metal oxide heterojunctions were summarized to review the strategies to assemble the heterojunctions.

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2.1 n–p Junctions In the case of the gas sensors based on heterojunctions, the n–p heterojunctions are one of the widely investigated heterostructures to improve the gas-sensing performances of the metal oxides. As reported, the n–p heterojunctions are composed of an n-type metal oxide and a p-type metal oxide. When the p-type metal oxide (usually in small sizes) is used to modify the n-type metal oxide (usually in large sizes), the n–p heterojunctions would be formed between their interfaces. It should be noted that the n-type metal oxides are the main phases in the n–p heterojunctions. And the type of the gas-sensing performance of the built n–p heterojunctions is always consistent with the sensing behavior of the n-type metal oxides in the heterojunctions [33, 34]. The sensors based on the n–p heterojunctions were reported to exhibit promising gas-sensing performances to a wide range of gases, such as H2 , NH3 , acetone, NO2 , ethanol, and H2 S. In the n–p heterojunctions, there will be diffusions of electrons and holes between the metal oxides in the heterojunctions due to their unique band structures and different Fermi levels. And the Fermi levels of the two materials reach an equilibrium state eventually, resulting in the band bending in the heterojunction and the formation of a potential barrier between the two materials. For example, in the Co3 O4 -decorated SnO2 system, the transfer of the electrons to the Co3 O4 and the diffusion of holes to the SnO2 would result in the formation of the potential barrier between their interfaces in the heterojunctions [35]. When the n–p heterojunctions are placed in the targeted gases, the highness of the built potential barrier in the heterojunctions will vary with the reactions between the targeted gases and the sensing materials. Therefore, the electrical properties of the sensors based on the n–p heterojunctions are modified greatly, leading to the enhanced gas-sensing performances of the n–p heterojunctions. The Co3 O4 -decorated WO3 based material was a typical composite composed of n–p heterojunctions. Ji et al. have successfully assembled the Co3 O4 –WO3 junction through a two-step method of a hydrothermal process followed by calcination [36]. In their research, the WO3 nanowires with the average diameters of 300–400 nm were firstly prepared via a hydrothermal method. The Na2 WO4 ·2H2 O was used as the raw material in the synthesized process, and the H2 SO4 was used to modify the pH of the precursor. The WO3 nanowires were obtained after the precursor and were kept at 180 °C for 5 h. In the next step, the prepared WO3 nanowires were dispersed in the absolute ethanol following by adding the cobalt acetate and the oxalate. The obtained suspension stayed at 180 °C for 2 h and the prepared products were further calcined in air at 400 °C to synthesize the Co3 O4 nanoparticlesdecorated WO3 nanowires. The gas sensor response of the sensor based on the decorated WO3 was counted to be 5.3–100 ppm acetone at the working temperature of 280 °C, over one time higher than that of the pure WO3 nanowires (2.45). The formation of the n–p heterojunctions in the composite was reported to be an important factor to enhance the gas-sensing performance of the Co3 O4 -decorated WO3 nanowires. The core-shell structured Co3 O4 -coated SnO2 nanowires were established by Kim et al. through a two-step method [37]. The author firstly synthesized the uniform SnO2

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nanowires through a typical thermal evaporation process with the source material of Sn nanopowders at the temperature of 800 °C for 1 h. Then the obtained SnO2 nanowires were coated by a thin layer of Co with the method of sputtering. The Co-coated SnO2 nanowires were further annealed in the air atmosphere at 450 °C for 1 h to synthesize the Co3 O4 -coated SnO2 nanowires. The author also pointed out that the SnO2 nanowires decorated by Co3 O4 nanoparticles might be obtained at a higher annealed temperature. The sensor based on the Co3 O4 -coated SnO2 nanowires was found to show an enhanced gas sensor response of 3.47–10 ppm NO2 at 25 °C, which was more than one time higher than that of pure SnO2 nanowires (1.51). A facile solvothermal method was used by Sun et al. to assemble the NiO nanoparticles-decorated ZnO hollow spheres [38]. In their study, the zinc acetate dehydrate and the trisodium citrate dehydrate were used as the raw materials to synthesize the pure ZnO hollow spheres via a simple liquid-phase reaction with the aid of microwave. The obtained ZnO hollow spheres were collected and then annealed at 500 °C for 2 h. In the following step, a certain amount of ZnO hollow spheres were added in a mixed solution of the nickel (II) nitrate hexahydrate, ethanol, and ethylene glycol to prepare the decorated ZnO hollow spheres via a hydrothermal process at 160 °C for 8 h. The collected samples were further calcined at 500 °C for 2 h in the air to obtain the NiO nanoparticles-decorated ZnO hollow spheres. The sensor based on the decorated ZnO exhibited an enhanced gas sensor response of ~30–100 ppm acetone compared with that of the pure ZnO (~5) at 275 °C. Meanwhile, the PdO decorated WO3 nanowires were reported to be prepared via a simple method of aerosol assisted chemical vapor deposition approach with the raw material of W(CO)6 and Pd(acac)2 [39].

2.2 n–n Junctions The n–n heterojunctions based on the metal oxides were also popularly established to study their enhanced gas-sensing performances. Similar to the n–p heterojunction discussed ahead, the n–n heterojunction was composed of two different n-type semiconductor materials. Generally, when one n-type metal oxide contacts or is loaded on another n-type metal oxide, the n–n heterojunction between the two metal oxides is naturally formed at the interfaces between the different metal oxides. In a typical way, the metal oxide acting as the substrate in the n–n heterojunctions were firstly synthesized and then decorated with the second metal oxide. As is known, the major carriers in the n-type semiconductors are the electrons, the concentration of which has a significant effect on the conductivity of the semiconductor. The difference in the Fermi levels of the n-type metal oxides would also induce the transfer of the electron between the metal oxides in the composites. This would result in the formation of a potential barrier in the sensing materials. The variation of the highness of the built potential barrier would then lead to the enhancement in the gas-sensing properties of the n–n heterojunctions. For example, the Fermi level of the TiO2 was reported to be higher than that of the SnO2 [40]. The electrons would

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diffuse from the TiO2 to the SnO2 , resulting in the formation of a depletion layer and a potential barrier between their interfaces. The variation in the highness of the built potential barrier in the targeted gas would then be responsible for the significantly enhanced gas-sensing performances of the sensors based on the n–n heterojunctions. The n–n heterojunctions have been reported to be sensitive to various gases, such as O3 , NO2 , CO, H2 , H2 S, NO2 , and other volatile organic compounds. The gas sensor response of the highly ordered TiO2 nanotubes was found to be significantly improved to be 1410–300 ppm H2 at 250 °C through decorating them with the SnO2 nanoparticles, which was over 30 times higher than that of the pure TiO2 nanotubes [41]. The sensor bases on the one-dimensional (1D) SnO2 -coated ZnO nanowires or the ZnO nanoplates surfaced-decorated by WO3 nanorods also showed improved gas-sensing performances to the n-butylamine or NH3 , respectively [42, 43]. The gas-sensing properties of the sensors based on the n–n heterojunctions were so promising that many researchers have paid their attention to the effective synthesization of the n–n heterojunctions. There have been some effective routes to prepare the n–n heterojunctions based on the metal oxides, such as the hydrothermal process, sol–gel, thermal oxidations, and electrospinning. As reported by Dang et al., the n–n composite composed of ZnO nanoplate decorated by the single-crystal WO3 nanorods could be successfully synthesized by a two-step hydrothermal method followed by a mixture process [43]. Typically, the uniformed ZnO nanoplates and the WO3 nanorods were prepared via hydrothermal methods at the temperatures of 180 °C and 120 °C with the raw materials of the zinc nitrate hexahydrate and the sodium tungstate dihydrate powder, respectively. Then the WO3 /ZnO composite was assembled by directly mixing the suspension liquids of the synthesized ZnO nanoplates and the WO3 nanorods. The authors have also successfully prepared the composites with different ratios in the weights of ZnO nanoplates and the WO3 nanorods in a similar way. The hydrothermal method also was reported by Wang et al. to be effective to prepare the ZnO nanowires decorated with the SnO2 particles [42]. The authors synthesized the ZnO nanowires via a hydrothermal process at 140 °C for 12 h with the raw material of Zn(NO3 )2 ·6H2 O. Then the obtained ZnO nanowires were immersed into a 30 mL of ethanol solution and decorated by the SnO2 nanoparticles with the method of a solvothermal process at 120 °C. The results showed that SnO2 /ZnO heterojunction nanostructures were successfully obtained followed by the final calcinations at 400 °C for 2 h. And Fan et al. have synthesized the ZnO-modified In2 O3 heterojunction through the hydrothermal process followed by the ultrasonic treatments. The In2 O3 nanoparticles were prepared via a hydrothermal method at 160 °C for 20 h. Then the obtained In2 O3 nanomaterials were further to be annealed at 550 °C for 2 h under the air atmosphere. The annealed In2 O3 were decorated by the ZnO nanoparticles with the method of ultrasonic reaction [44]. Besides the one-dimensional nanoparticles-decorated metal oxides heterostructures, the brush-like n–n heterojunctions were also constructed by the hydrothermal process and other methods to study their improved interaction with the targeted gases. For example, Zhang et al. have reported that they successfully assembled the brush-like ZnO–TiO2 hierarchical heterojunctions nanofibers with three-dimensional structures via a two-step method [45]. The TiO2 nanofibers were prepared in advance

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through a method of electrospinning at an applied electric voltage of 20 kV. The synthesized TiO2 nanofibers were calcimined at 500 °C for 2 h in the air. Then the ZnO nanorods were uniformly decorated on the surface of the annealed TiO2 nanofibers via a hydrothermal process at 100 °C. The results showed that the TiO2 nanofibers with the average diameter of ~100 nm were decorated by the ZnO nanorods with the diameters of 100–300 nm. And the α-Fe2 O3 nanorods/TiO2 nanofibers with branchlike nanostructures were also by the same team through a similar process [46]. Meanwhile, the hierarchical assembly of In2 O3 nanoparticles on the ZnO hollow nanotubes could also be obtained through a facile hydrothermal method [47]. The electrospinning was also widely used to establish the heterojunctions composed of different metal oxides. For example, the mesoporous ZnO–SnO2 nanofibers were fabricated by a typical electrospinning process at the working voltage of 10 kV with the raw materials being SnCl2 ·2H2 O and ZnCl2 [48]. Then the obtained nanofibers were further annealed in the air at 600 °C for 4 h. Sun et al. have also reported their work about the route to synthesize the In2 O3 /SnO2 nanofibers through electrospinning [49]. The nanoparticles in the composite were found to be only 16 nm, much smaller than those in the pure In2 O3 or SnO2 nanofibers (35–40 nm or 20 nm, respectively). The TiO2 –In2 O3 composite nanofibers and the In2 O3 /α-Fe2 O3 heterostructure nanotubes were also reported to be prepared by the electrospinning processes [50, 51].

2.3 p–n Junctions The p–n heterojunctions based on the metal oxides were also the widely and systematically studied metal oxide heterojunctions, which were normally composed of the p-type and n-type semiconductors. Similar to the n–p heterojunctions, the majority of the p–n heterojunctions based on the metal oxides were always assembled in a two-step process. The researchers firstly synthesized the metal oxides nanomaterials as the substrate and secondly decorated the obtained nanostructured materials with small nanoparticles or other semiconductors. When the p-type semiconductor material and the n-type semiconductor material contact with each other, a common heterojunction will be correspondingly built between their surfaces, namely, the p–n heterojunctions. In the typically p–n heterojunctions, there also does exist the bending in the energy bands of the two different semiconductors [52]. The difference in the Fermi levels of the metal oxides will result in the flow of free electrons and holes through the contacted interfaces between the two different materials to the unoccupied low-energy states. It has been reported that the high potential barrier between the heterostructure and the thick accumulation layer (usually being the conduction channel for the carriers in the p–n heterojunctions) in the sensing materials had a significant contribution to the gas-sensing performance toward oxidizing or reducing gases. The formation of the heterojunctions in the sensing materials could also increase the number of active sites for the redox reactions responsible to the gas-sensing properties of the

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p–n heterojunctions, making the stability of the gas-sensing performance be largely dependent on the p–n heterostructure [53]. As reported, when the gas sensor based on p–n heterojunctions is exposed to the atmospheric environment, the oxidizing gas molecule (such as O2 ) could be adsorbed by the n-type semiconductor and the p-type semiconductor could be an ideal substrate to absorb the reducing gas molecule (such as CO), which will result in the decrease in the semiconductor carrier in the sensing materials and the increase in the resistance of the sensors [54]. On the contrary, when the reducing gas is adsorbed on n-type semiconductor and the oxidizing gas is adsorbed on p-type semiconductor, the carrier will increase sharply and the resistance of the metal oxide will decrease. Therefore, we can see that the sensors based on the p–n heterojunctions could exhibit different responses to the different kinds of gases and change the resistances of the sensors with two integrated properties. By now, lots of works have been done to investigate the methods to assemble the composites composed of p–n heterojunctions. For example, Lee et al. have reported that the Fe2 O3 -functionalized CuO nanorods could be successfully synthesized by a solvothermal route [55]. The CuO nanorods were prepared directly on the Cu film through a facile thermal oxidation at 500 °C for 1 h in the air atmosphere. The results showed that the CuO nanorods exhibited typical diameters and lengths of 20–60 nm and 5–10 μm, respectively. Then the Fe2 O3 nanoparticles were synthesized with an ultrasonication of a solution of iron chloride and NaOH. Finally, the obtained Fe2 O3 nanoparticles were spin-coated on the prepared CuO nanorods followed by an anneal process at 500 °C for 1 h in the air to assemble the Fe2 O3 -functionalized CuO nanorods. The CuO nanorods decorated with Fe2 O3 nanoparticles were also prepared in a similar way by Chang et al. [56]. Meanwhile, Lee et al. have synthesized the Nb2 O5 nanoparticles-decorated CuO nanorods through a method of thermal evaporation and spin coating of NbCl5 solution [57]. The hierarchical α-Fe2 O3 /NiO composite with hollow structures was reported by Sun et al. to be synthesized with a facile hydrothermal process. In their study, the NiO microspheres were prepared via a hydrothermal process with the source materials of the NiCl2 ·6H2 O and the mixed solvent of hexamethylenetetramine, ethanol, and ethanolamine. The obtained precursor was then kept at 160 °C for 12 h to synthesize the flower-like samples. In the next step, a certain amount of the synthesized flowerlike NiO microspheres was added in a solution of FeCl3 ·6H2 O and Na2 SO4 ·10H2 O. Then the mixture was placed in an oven at 120 °C for 2.5 h. The final product was calcined at 450 °C for 2 h to obtain the α-Fe2 O3 -decorated flower-like NiO microspheres. The sensor based on the decorated NiO microspheres showed a gas sensor response of 18.68–100 ppm toluene at the working temperature of 200 °C, which was found to be approximately 13 times higher than that of the bare NiO (1.5) [58]. The modulation of the thickness of the accumulation layer in the α-Fe2 O3 /NiO composite was one of the important factors responsible for its enhanced gas-sensing property. The TiO2 decorated Co3 O4 acicular nanowire arrays synthesized by Li et al. were prepared via a two-step method [59]. The authors firstly synthesized the Co3 O4 acicular nanowires arrays through a hydrothermal method. The cobalt nitrate, ammonium

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fluoride, and urea were used as the raw materials in the preparation process. Then the obtained Co3 O4 acicular nanowires arrays were decorated with the TiO2 nanoparticles via a PLD method at 1300 °C. The sensor based on the TiO2 decorated Co3 O4 acicular nanowire arrays showed a gas sensor response of 65–100 ppm ethanol at 160 °C, which was over 3 times higher than that of the pure Co3 O4 (~20).

2.4 p–p Junctions In addition to the n–n, n–p or p–n heterojunctions discussed above, the p–p is also one of the important heterojunctions. Similar to the n–n heterojunctions, the p–p heterojunction is also composted by two different p-type metal oxides [60]. It is known that the holes are the major carriers in the p-type materials. Therefore, the variation in the concentration of the holes in the p–p heterojunctions is of importance to the sensors based on these heterojunctions. When one p-type metal oxides contacts with another p-type semiconductor, the p–p heterojunctions would be formed. Most of the reported p–p heterojunctions were successfully assembled in two steps, similar to the routs to prepare the n–n heterojunctions described in Sect. 3.2 [61]. In the p–p heterojunctions, the Fermi levels of the two metal oxides are always different, which will induce the electrons and the holes transferred to the metal oxide with lower Fermi level and the one with higher Fermi level, respectively. Accordingly, there would be an accumulation layer formed in the sensing materials, which was also reported to be the conduction channel for the holes in the p–p heterojunctions. The adsorption of the molecules on the active sites in the composite composed of p–p heterojunctions would make a more effective modulation of the thickness of the accumulation layer, leading to the remarkable improvements in the sensing performances of the p–p heterojunctions to the targeted gases. The sensors based on the p–p heterojunctions have been reported to be effective to detect numerous gases, such as CO, ethanol, NO2 , xylene, H2 S, and acetone. Moreover, the methods of the electrospinning, thermal evaporation, galvanic replacement, hydrothermal treatment, and reflux process were reported to be successful routes to assemble the p–p heterojunctions. The ring-like PdO-decorated NiO with a lamellar structure was presented to be a novel type junction with the p–p heterostructure [62]. The PdO-decorated NiO was synthesized by Zhang et al. through a hydrothermal process. As reported by the authors, the Ni(Ac)2 ·4H2 O was used as the raw material to prepare the ring-like NiO lamellar structures in the first step. It should be pointed out that the 1 mL of glycerol was also added into the precursor and then the obtained precursor was kept at 200 °C for 5 h to synthesize the lamellar NiO. In the next step, the prepared NiO lamellar structures and the PdCl2 were reported to be mixed in the methanol. And the mixture was further calcined at 350 °C for 1 h to obtain the PdO-decorated NiO. The sensor based on the novel ring-like PdO-decorated NiO showed an excellent gas-sensing performance at 180 °C with a high sensor response of 70.1–100 ppm CO.

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The metal-organic framework (MOF) templates could also be used to assist the assembly of the p–p heterojunctions [63]. For example, the Co3 O4 hollow nanocages functionalized by nanoscaled PdO were successfully synthesized with the help of the metal@MOF templates. In a typical process, the zeolitic imidazolate framework-67 (ZIF-67) was firstly prepared by the precipitation reaction of a solution composed of the Co(NO3 )2 ·6H2 O, 2MeIm, and methanol at room temperature for 5 h. The obtained ZIF-67 was dispersed in the deionized water followed by adding the K2 PdCl4 and NaBH4 solution. The Pd encapsulated ZIF-67 was calcined at 400 °C for 1 h to obtain the Co3 O4 hollow nanocages functionalized by nanoscaled PdO catalyst. The sensor based on the PdO-functionalized Co3 O4 hollow nanocages was found to exhibit an enhanced sensor response of 2.51–5 ppm acetone, almost on time higher than that of the pure Co3 O4 powders. The NiO/NiCr2 O4 nanocomposite was reported to be effectively prepared via a facile hydrothermal process [64]. The CrCl3 ·6H2 O were measured and then added in the deionized water followed by adding the NiCl2 ·6H2 O and the hexamethylenetetramine. The obtained solution was kept magnetic stirring for 10 min, during which the ethanolamine was also added in the solution. In the next step, the precursor was transferred into a Teflon-lined stainless steel autoclave and was kept at 180 °C for 8 h. The obtained product was then annealed in a muffle furnace at 500 °C for 3 h. The sensor based on the NiO/NiCr2 O4 nanocomposites exhibited an enhanced gas sensor response of 66.2–100 ppm xylene, which was found to be 37.2 times higher than that of the pure NiO.

2.5 n–p–n Junctions and p–n–p Junctions Different from the n–n, n–p, p–n, and p–p heterojunctions, the p–n–p or n–p–n heterojunctions are always composed of three different metal oxides or semiconductors. A p–n–p heterojunction could be seen as a p–n heterojunction combined with an n–p heterojunction. For example, Wei et al. have prepared the PANI-coated CuO–TiO2 heterostructure nanofibers, which was a typical p–n–p heterojunction. It was reported that there was a p–n heterojunction between the PANI and the TiO2 and an n–p heterojunction between the TiO2 and the CuO [65]. Therefore, the band bending would have occurred in the built separate heterojunctions and there were two potential barriers constructed in the p–n–p heterojunctions, which was also proved to be a positive factor to improve the gas-sensing performance of the metal oxides. Similarly, the n–p–n heterojunction could be seen as an n–p heterojunction combined with a p–n heterojunction. The variation of the highness of the built potential barriers would greatly enhance the gas-sensing performances of the p–n–p or n–p–n heterojunctions. Some of the reported studies were reviewed in the following text to show the methods to build the junctions with the p–n–p or n–p–n heterostructures, which could also be used as the promising gas-sensing materials. Kim et al. have successfully synthesized the n–p–n heterojunctions composed of the ZnO-branched SnO2 nanowires decorated with Cr2 O3 nanoparticles [66]. In their

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research, the ZnO nanowires-branched SnO2 nanowires were firstly assembled. The author used the Sn powders as the raw materials to prepare the SnO2 nanowires with a method of thermal evaporation at the temperature of 900 °C in the ambient gas of Ar and O2 . The obtained SnO2 nanowire was coated with a thin layer of Au with the help of a turbo sputter coater. The Au-coated SnO2 nanowires were used as the substrates to prepare and collect the further synthesized ZnO nanowires. The ZnO nanowires were also prepared with the thermal evaporation of the Zn powders at 500 °C in the air. The obtained ZnO nanowires-decorated SnO2 nanowires were then coated by a thin layer of Cr with thickness of approximately 6 nm. The Cr-coated ZnO/SnO2 nanowires were finally annealed for 1 h to prepare the ZnO nanowiresbranched SnO2 nanowires decorated with Cr2 O3 nanoparticles. The sensor based on the obtained n–p–n heterojunctions exhibited a sensor response of 26.6–2 ppm NO2 at the working temperature of 300 °C, which was much higher than that of the sensor based on SnO2 –ZnO (only 3.4). In addition, there were also some researchers reporting the gas-sensing performance of the p–n–p heterojunctions. For example, the PANI-coated CuO–TiO2 nanofibers mentioned ahead were reported to be synthesized by a method of electrospinning combined with the following calcination process [65]. In the first stage, the CuO–TiO2 /SiO2 was prepared by a typical electrospinning process. The HNO3 aqueous solution and the KH-560, TEOS, and the TBT were used to prepare the TiO2 /SiO2 sol. And the prepared homogeneous PVP solution composed of cupric acetate and 12 mL 5 wt% PVP absolute ethyl alcohol was also added in the TiO2 /SiO2 sol. Then the TiO2 /SiO2 sol was treated by an electrospinning process with the working voltage of 16 kV. The obtained nanofibers were further annealed at 800 °C for 3 h to obtain the CuO–TiO2 /SiO2 flexible composite nanofiber. The obtained nanofibers were immersed in the HCl solution containing aniline. Then the HCl solution containing APS was also added to start the polymerization. Finally, the CuO–TiO2 PANI membranes were thoroughly washed and dried at 40 °C for 8 h. The sensor based on the prepared CuO–TiO2 PANI membranes exhibited a high gas sensor response of 45.67–100 ppm ammonia. Based on the discussion of the n–n, n–p, p–n, p–p, p–n–p, or n–p–n heterojunctions above, it was clearly observed that the gas-sensing performances of the gas sensors could be significantly enhanced through establishing the heterojunctions in the sensing materials. And many researchers have studied the enhanced gas-sensing performances of the n-type or the p-type heterojunctions to the reducing or oxidizing gases. The sensors based on the n-type or the p-type metal oxides have showed enhanced gas-sensing properties to various gases with high gas sensor responses, fast response/recovery times and outstanding selectivity. Some of the widely studied metal oxides would be discussed and compared in detail to better understand the effects of the formed heterojunctions on the enhanced gas-sensing performances of the composites in the following Sect. 3.

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3 The Most-Studied Metal Oxide Heterojunctions for High-Performance Gas Sensors and Their Enhanced Gas-Sensing Mechanisms As reported, the heterojunction based on nanostructured metal oxides always exhibits enhanced gas-sensing properties with a high gas sensor response and fast response/recovery time to the target gases. The formation of the heterojunctions in the sensing materials is a positive factor which makes the metal oxides to be the promising materials to detect gases. Numerous metal oxides have been reported to be composited with other metal oxides to improve their gas-sensing performances.

3.1 ZnO-Based Heterojunctions ZnO is a typically n-type semiconductor metal oxide with a wide band gap of 3.37 eV [67]. In the recent decades, the ZnO-based materials have attracted lots of researchers’ attention due to their advantages of the low-cost prices, easy preparations, high electron mobility, and excellent chemical/thermal stabilities [68]. There has been a variety of ZnO nanomaterials successfully synthesized, such as nanoparticles, nanorods, nanowires, nanoplates, nanoflowers, and nanofibers, via the hydrothermal process, CVD, chemical precipitation, electrospinning, and vapor phase transport. By now, the nanostructured ZnO materials were reported to be applied in the areas of light-emitting diodes (LEDs), solar cells, thin-film transistors, memories, and gas sensors. ZnO is a promising resistive-type gas-sensing material, which has been reported to effectively detect H2 , ethanol, CO, CO, H2 S, HCHO, acetaldehyde, NH3, and NO2 . There were two commonly different sensing mechanisms for the gas sensors based on the metal oxides for the reducing gases (such as H2 ) or the oxidizing gases (such as NO2 ). In the case of the H2 , it is the reactions between the adsorbed oxygen ions on the metal oxides and the H2 molecules that release electrons back to the sensing materials and then lead to a decrease in the resistance of the sensor, which is responsible for the sensing performances of the metal oxides-based sensors. While in the case of the NO2 , the NO2 gas molecules will be adsorbed on the active sites in the metal oxide surface, acting as an acceptor of electrons and reducing the concentrations of free carriers in the sensing material. The sensing mechanisms discussed above are the basic process in the gas-sensing responses of almost all the reported metal oxide-based materials. Seiyama et al. have assembled a gas sensor based on ZnO film for the first time to study the gas-sensing performance of the metal oxides [69]. Up to now, the ZnO based gas sensors have been studied to detect several gases, such as trimethylamine (TMA), H2 , oxygen, H2 O, ethanol, and NH3 [68]. However, the ZnO-based films always exhibited limited sensor responses to the gases, and their relatively high operating temperatures made the sensors exhibited relatively poor selectivity to the

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targeted gases. The formation of the heterojunctions in the sensing materials has proved to be an effective and easy way to enhance the gas-sensing performance [70]. The n–p type heterojunctions-based ZnO composites have been studied to be potential materials to sense the toxic organic gases, such as acetone, formaldehyde, and TMA. Sun et al. reported that the well-dispersed ZnO hollow spheres with a uniform size of 1.5 μm were successfully decorated with the NiO nanoparticles with a diameter of 10 nm via a facile solvothermal followed by annealing [38]. Their results showed that the p-type NiO/n-type ZnO composites exhibited a sensor response of 29.8–100 ppm acetone at the optimized working temperature of 275 °C, which was 3.2 times higher than that of the pure ZnO (~9.2). Moreover, the response and recovery times of the NiO-decorated ZnO hollow spheres to 100 ppm acetone were found to be only 1 s and 20 s at 275 °C, respectively, much shorter than those of the pure ZnO (6 s and 130 s) under the same conditions, as shown in Fig. 2. Obviously, the gas-sensing performance of the ZnO hollow spheres could be significantly improved by decorating them with the NiO nanoparticles. In their work, the enhanced gas-sensing mechanism of the NiO decorated ZnO hollow spheres were attributed to three positive factors. The first one was the formation of the n–p heterojunction and the barrier between NiO and ZnO due to their different Fermi levels. When the sensor based on the composites was exposed to the acetone, the highness of the potential barrier decreased rapidly and greatly along with the reactions between the acetone molecules and the pre-absorbed oxygen ions on the

Fig. 2 Low-magnification and high-magnification scanning electron microscope (SEM) images of the pure ZnO hollow spheres (a, b) and the NiO nanoparticles-decorated ZnO hollow spheres (c, d), the gas-sensing properties of the pure ZnO hollow spheres (e) and the NiO nanoparticles-decorated ZnO hollow spheres (f) to 100 ppm acetone at 275 °C [38]. The structure of the assembled sensor composed of NiO nanoparticles-decorated ZnO hollow spheres was similar to that of the SnO2 based sensor shown in the inset in Fig. 3g. In this figure, the nm, μm, and ppm are the abbreviations of the nanometer, micrometer and parts per million, respectively

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sensing materials, resulting in a strong promotion of gas-sensing performance. The additional electron depletion expansion in the ZnO surface and the high oxidative catalytic activity of NiO toward the acetone molecules were the other two factors attributing to the higher gas-sensing performance of the NiO-decorated ZnO hollow spheres. Similar improved results were also found in the p-type NiO/n-type ZnO nanofibers and the NiO/ZnO microflowers, which were synthesized via the methods of an electrospinning technique and a one-step hydrothermal process, respectively. The NiO/ZnO heterojunction microflowers synthesized by Meng et al. exhibited a higher sensor response of 26.2–100 ppm formaldehyde at 200 °C than that of the pure ZnO microflowers (9.6). The composite was found to still show an obvious response to 1 ppm formaldehyde with a sensor response of 1.99, but no sensing performance of the pure ZnO microflower was observed toward the formaldehyde with the same concentration [71]. The nanofibers of p-type NiO/n-type ZnO heterojunction synthesized by Ruan et al. presented a sensor response of 892–100 ppm TMA at the working temperature of 260 °C, also extremely higher than that of the pure ZnO (~75 at 280 °C) [72]. A typical n–n heterojunction of SnO2 nanoparticles-coated ZnO nanowires was established by Wang et al. via an effective hydrothermal method followed by the solvothermal treatment and the calcination at 400 °C [42]. The synthesized composites showed an improved sensor response of approximately 7.0 toward 10 ppm n-butylamine at the working temperature of 240 °C, which was higher than that of the sensor based on the pure ZnO nanowires. The small size effect of SnO2 nanoparticles and the unique 1-D wire-like morphology of the ZnO support were the two possible reasons for the improvement in the gas-sensing performance of the SnO2 nanoparticles-coated ZnO nanowire. It should be noted that the modulation in the thickness of the depletion layer and in the highness of the potential barrier between the SnO2 and the ZnO was another factor responsible for the enhanced gas-sensing performance of the composite. Also, the SnO2 –ZnO nanofibers were also successfully constructed by Kim et al. with a method of electrospinning [73]. In the composited nanofibers, there were numerous heterojunctions formed between the nanostructured SnO2 and the ZnO, leading to the establishment of the potential barrier at their interface. In the study of the sensing performance to the 10 ppm H2 S at 300 °C, the sensor response of the SnO2 –ZnO nanofibers was reported to be 168.6, much higher than those of the pure SnO2 (4.2) or the pure ZnO (63.8). They pointed out that the modulation in the highness of the potential barrier at the interface between the SnO2 nanograins and the ZnO-to-Zn transformations of the ZnO surface in the sensing material were the two main reasons that should be responsible for the enhancement in the gas-sensing performance of the SnO2 –ZnO composite nanofibers.

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3.2 SnO2 -Based Heterojunctions SnO2 is also a widely researched n-type semiconductor material with a relatively wide band gap of 3.6 eV [74]. There have been a variety of nanostructured SnO2 based materials, such as nanowires, nanorods, nanobelts, nanotubes, nanoparticles, nanospheres, and nanodisks, which were successfully synthesized through the methods of a wet chemistry process, hydrothermal process, CVD, thermal oxidation, and electrospinning. The SnO2 nanomaterials have been applied in the fields of catalysts, lithium-ion batteries, transparent electrodes, and gas sensors. The properties of easy preparation, environmental friendliness, chemical stability, and large surface area of the synthesized SnO2 nanomaterials make them be greatly promising candidates for high-performance gas sensors [75, 76]. Based on the previous reports, SnO2 is widely used as a gas-sensing material and exhibits promising sensor responses toward numerous reducing gases and oxidizing gases, such as CO, CO2 , H2 S, CH4 , NH3 , H2 , ethanol, and NO2 . For example, the SnO2 nanorods were successfully synthesized by Huang et al. via an inductively coupled plasma-enhanced CVD [77]. There were no catalysts used during the preparation of the uniform SnO2 nanorods, The sensor based on the individual SnO2 nanorod, assembled through a focused ion beam, exhibited a gas-sensing performance to 100 ppm H2 at 250 °C with the sensor response of 12.5. Lee et al. also have fabricated a novel gas sensor based on the SnO2 nanowires, which were synthesized with a method of a simple thermal evaporation of Sn metal powders [78]. The sensor with the SnO2 nanowires bridging the gap between two pre-patterned Au catalysts showed the highest sensor response of 18–0.5 ppm NO2 at the optimized working temperature of 200 °C with a response time and a recovery time of 43 s and 18 s, respectively. Therefore, the SnO2 -based nanomaterials could be promising materials to sense the gases. And the formation of the heterojunctions in the SnO2 -based sensing materials was also an effective way to improve their gas-sensing performances. Many researchers have reported that the positive effects of n–p junctions on the gas-sensing performances of the SnO2 -based composites. For example, the p-type CuO/n-type SnO2 nanospheres were successfully synthesized by Sunkara et al. via a simple hydrothermal method and were assembled to be the chemo-resistive CO2 gas sensor [79]. The results showed that the CuO–SnO2 nanocomposite exhibited an obviously enhanced gas-sensing performance toward CO2 gas. The sensor based on the nanocomposite showed a higher sensor response with short response and recovery times of 5 s and 20 s, respectively, toward 1% CO2 than that of the pure SnO2 . The variation in the highness of the potential barrier between the SnO2 and the CuO played an important role in the sensing process of the nanocomposite and the construction of the n–p heterojunction would also enhance the adsorption of more oxygen species, which are two main factors responsible for the enhanced CO2 sensing performance of the CuO–SnO2 nanocomposite. Meanwhile, Hu et al. have prepared a typical p–n junction of NiO/SnO2 hierarchical structures through a hydrothermal process [80]. Their results showed that the sensor based on the as-fabricated NiO/SnO2

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composite exhibited a shorter recovery time and a higher sensor response of 6 s and 20.18–50 ppm acetone at 300 °C, respectively, than those of the pure SnO2 (20 s and 6.01). There were also two main aspects responsible for the improved acetone gas-sensing. The first one is the increased conduction channel width accompanying adsorption and desorption of the acetone molecules due to the presence of the p-type NiO. And the formation of the n–p heterojunction between the SnO2 and the NiO would widen the electron depletion layers between their interfaces, which led to a great decrease in the resistance of the gas sensor based on the composite, which was another factor responsible for the enhanced gas-sensing properties of the sensor based on the NiO/SnO2 hierarchical structures. In addition, the NiO/SnO2 hollow sphere or the NiO–SnO2 heterojunction microflowers were also synthesized via a template-assisted hydrothermal method or a one-step simple hydrothermal method to effectively detect the formaldehyde at 100 °C or 200 °C, respectively [76, 81]. The n-type TiO2 /n-type SnO2 heterogeneous junctions, prepared via a simple hydrothermal method followed by a pulsed laser deposition, were also reported by Xu et al. to be an excellent sensing material to detect the triethylamine [75]. As discussed in their paper, the formation of the n–n heterojunctions in the TiO2 /SnO2 composite led to the bending of their energy bands and the formation of a high potential barrier at the interfaces between the SnO2 and the TiO2 , which would result in the higher resistance of the composite than that of the pure SnO2 in air. Then the interaction between the triethylamine molecule and the adsorbed oxygen ions on the material surface would significantly reduce the resistance of the TiO2 /SnO2 composite and improved the gas-sensing performance of the composite. The sensor based on the TiO2 /SnO2 heterojunctions showed an extremely higher sensor response of 52.6 toward 50 ppm triethylamine than that of the pure SnO2 (~3), as shown in Fig. 3. The response/recovery time of the composite was also found to be 12 s/22 s, shorter than those of the bare SnO2 (16 s/35 s). And the Polyaniline/TiO2 :SnO2 nanocomposite sensor was also synthesized to exhibit a higher sensor response of 6.18–8000 ppm H2 compared with that of the pure TiO2 or the pure SnO2 [82]. The In2 O3 /SnO2 composite hetero-nanofibers were prepared by Sun et al. via an electrospinning method to study their gas-sensing performance [49]. Their research revealed that the In2 O3 /SnO2 composite hetero-nanofibers exhibited an outstanding sensing performance to the formaldehyde at 275 °C. The sensor based on the In2 O3 /SnO2 composite performs a superior sensing performance toward the formaldehyde than those of the pure SnO2 or the pure In2 O3 nanofibers. The sensor response of the In2 O3 /SnO2 composite to 10 ppm formaldehyde was calculated to be ~10, which was much higher than that of the pure SnO2 (~3). And the response time and the recovery time of the composite to the 100 ppm formaldehyde were found to be 37 s and 42 s, respectively. The enhanced formaldehyde sensing performance of the In2 O3 /SnO2 composite hetero-nanofibers was attributed to the increased surface adsorption oxygen and the more electrons transfer due to the formation of the potential barrier in the n–n heterojunctions between the SnO2 and the In2 O3 .

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Fig. 3 The transmission electron microscope (TEM) and the high-resolution transmission electron microscope (HRTEM) images of the SnO2 nanosheets (a, b) and the TiO2 /SnO2 nanosheets (c, d) prepared by a hydrothermal method followed by a pulsed laser deposition, the repeatability testing of SnO2 nanosheets and the TiO2 /SnO2 nanosheets to 50 ppm triethylamine (TEA) (e) and the dynamic gas-sensing properties of the synthesized gas sensors at 260 °C. The inset in Fig. 3g was the sensor assembled by TiO2 /SnO2 nanosheets [75]. In this figure, the Ra and the Rg were the resistances of the sensor in the air and the triethylamine, respectively. And the nm and ppm are the abbreviations of the nanometer and parts per million, respectively

3.3 TiO2 -Based Heterojunctions TiO2 is a popular n-type semiconductor material with the band gap of ~3.2 eV [83]. The advantages of the TiO2 -based materials in simple preparations, friendly to the environments, non-toxic products, and stable chemical properties make them be widely applied in various fields of transistors, solar cells, batteries, and electronic components. Up to now, there have been a wide range of TiO2 nanomaterials successfully synthesized, such as nanotubes, nanoparticles, nanowires, and nanofibers, via the routes of the sol–gel template, anode oxidation, electrostatic spinning, and hydrothermal method. The high specific surface areas of the obtained TiO2 nanomaterials and the fast charge transportation on their surfaces made them be potentially applied in the gas sensors [84]. As reported, the TiO2 -based gas sensors have been commonly used to detect NH3 , NO2 , H2 S, CO, and H2 . For example, the TiO2 nanostructured film has been prepared to study its gas-sensing performance [85]. The results showed that the film made of TiO2 nanotubes exhibited a sensor response of 25–50 ppm toluene, slightly lower than that of the commercial TiO2 nanoparticles (P-25) (~26). Jho et al. have synthesized the vertically aligned TiO2 nanotube array with the ALD combined with an anodic aluminum oxide template [86]. Their results showed that the average length of the prepared TiO2 nanotubes arrays was 250 nm, and the sensor based on the TiO2

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nanotube array exhibit a sensor response of 18.7–100 ppm H2 . The construction of the heterojunctions in the TiO2 -based sensing materials has been widely explored to be an effective method to improve their gas-sensing performances. One of the typical built TiO2 -based heterojunctions was the highly ordered TiO2 nanotube array decorated with the ZnO nanoparticles, synthesized by an immersioncalcination method [87]. The results showed that the ZnO nanoparticles with the size of 5–10 nm had no effects on the morphologies of the nanotubes, and were uniformly decorated on the surfaces of the prepared nanotubes. The sensor based on the ZnOdecorated TiO2 nanotubes showed a sensor response of ~520 toward 100 ppm H2 at the optimized working temperature of 350 °C, which was much higher than that of the pure TiO2 . The enhanced gas-sensing performance of the decorated TiO2 nanotubes array was attributed to the more adsorbed oxygen ions due to the formation of the n–n heterojunctions between the TiO2 and the ZnO, promoting the gas reactions and modulating the charge transportation channels in the sensing material. Similar improved gas-sensing performance was also noticed in the highly ordered SnO2 decorated TiO2 nanotubes. The ordered TiO2 nanotubes were prepared by an anode oxidation of a titanium foil, and further modified with nanostructured SnO2 via an impregnation roasting method [41]. The SnO2 -decorated TiO2 nanotubes exhibited a high sensor response of 725–300 ppm H2 at 275 °C. The unique morphology of the nanotubes and the formation of heterojunctions were the two reasons responsible for the improved gas-sensing performance of the composite. The p-Co3 O4 -decorated n-TiO2 nanotubes were synthesized by Öztürk et al. through a two-step electrochemical deposition of an anodization and a cathodic deposition [88]. There were numerous n–p heterojunctions formed between the Co3 O4 and the TiO2 , leading to a decrease in the cross-sectional area for charge carriers and greatly increasing the initial resistance of the composite. Then the resistance of the sensor based on the decorated TiO2 nanotubes dramatically decreased after being exposed to the H2 atmosphere due to the modification in the highness of the potential barrier in the heterojunction. And the Co3 O4 could also act as the catalyst material to the targeted gases, also contributing to the better gas-sensing performance of the decorated TiO2 nanotubes. As expected, the sensor based on the p-Co3 O4 -decorated n-TiO2 nanotubes exhibited a significantly improved H2 sensing response at 200 °C, which is 9 times higher than that of the pure TiO2 . The enhanced phenomena are attributed to the formation of n–p heterogeneous junctions at the interfaces between the TiO2 and the Co3 O4 in the composite. Also, the n–p heterojunction between the CuO and the TiO2 was reported to exhibit an improved sensor response [89]. The TiO2 nanotube was prepared by the anode oxidation of a titanium foil, and then the CuO film was covered on the nanotube by oxidation of the evaporated copper. The formation of n–p heterogeneous junctions was proved to be a positive factor enhancing the sensor response to H2 of the composite. The brush-like ZnO–TiO2 nanofibers were established by Zhang et al. via an electrospinning method followed by a hydrothermal process [45]. In the novel composite, the uniform ZnO nanorods with the diameters of 100–300 nm were observed to directly grow on the side surface of the core of TiO2 . Their results revealed that the gas sensor based on the brush-like composites showed a sensor response of 50.6–500 ppm ethanol at the optimized working temperature of 320 °C, which was

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extremely higher than that of the pure TiO2 nanofibers (~6) and the bare ZnO nanorods (~13.7), as shown in Fig. 4. And the ZnO–TiO2 nanofibers also presented an excellent selectivity to the ethanol compared with the H2 S, CH4 , C3 H6 O, CO, CH3 OH,

Fig. 4 The SEM images of the as-prepared pristine TiO2 nanofibers (a), pristine ZnO nanorods (b) and ZnO–TiO2 heterojunctions (c, d), the dynamic ethanol sensing performances (e) and gas selectivity (f) of the pristine ZnO nanorods, TiO2 nanofibers and ZnO–TiO2 heterojunctions [45]. The sensor based on the ZnO–TiO2 heterojunction was similar to that of the sensor based on SnO2 shown in Fig. 3g. In this figure, the Ra and the Rg were the resistances of the sensor in the air and in the ethanol, respectively. And the nm, μm, and ppm are the abbreviations of the nanometer, micrometer, and parts per million, respectively. The C2 H5 OH, H2 S, CH4 , C3 H6 O, CO, CH3 OH, and C2 H2 are the ethanol, hydrogen sulfide, methane, acetone, carbon monoxide, methanol, and acetylene, respectively

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and C2 H2 . The enhanced gas-sensing performance of the composite was attributed to the fact that the modified TiO2 nanofibers had a good structure with a high specific surface area. The formation of n–n heterogeneous junction between the ZnO nanorod and the TiO2 nanofiber would also make a great modulation in the resistance of the sensor and promote more oxygen molecules being more easily adsorbed on the TiO2 surface. The hierarchical heterostructure of α-Fe2 O3 nanorods/TiO2 nanofibers was also built through a similar method [46]. The branch-like α-Fe2 O3 /TiO2 heterostructure exhibited a significantly improved gas-sensing performance compared with that of the pure TiO2 nanofibers. The sensor response, the response time, and the recovery time of the composite to the 50 ppm triethylamine at the optimized temperature of 240 °C were calculated to be 13.9, 0.5 s, and 1.5 s, respectively.

3.4 WO3 -Based Heterojunctions WO3 , a popular n-type semiconductor with a relatively wide band gap of 2.7 eV, is found to be a potential sensing material for its sensitive variation in electrical properties after being exposed to targeted gases [90]. It is reported that a majority of WO3 -based nanostructured materials (nanoparticles, nanowires, nanorods, and nanoplates) could be synthesized with methods of the hydrothermal process, sol– gel, and the sputtering or thermal evaporation technology [91–94]. And WO3 -based materials have shown remarkable advantages in their application in gas sensors due to their interesting properties of outstandingly electrical or optical behaviors, nontoxicity to the environment and the presence of oxygen vacancies on the WO3 surface [95]. In the area of gas sensors, WO3 is reported as one of the widely studied gassensing materials to mainly detect H2 S, H2 , CH4 , CO, NO2 , O3 , and NH3 . Naik et al. have also researched the NO2 sensing performance of the WO3 –ZnO composite [96]. And they found that the sensors based on the 50 wt%:50 wt% WO3 :ZnO composite displayed the highest sensor response of 36–100 ppm NO2 at the working temperature of 350 °C among all the studied sensors in their research, which was 1.5 times higher than that of the sensor based on the pure WO3 and 6.5 times higher than that of pure ZnO. In addition, Yin et al. reported that the sensors based on the pure WO3 nanoplates presented a sensor response of ~25–5 ppm H2 S at 150 °C [97]. And their results also showed that the gas sensor response of the WO3 nanoplates could be further improved to be 225 through compositing them with the CuO nanoparticles. Therefore, the WO3 -based materials, especially the composited ones, have the great potentials to be high-performance gas sensors. Apart from the studies listed above, there were more researches focusing on the enhanced gas-sensing performances of the gas sensors based on the WO3 composites or their heterojunctions. For example, Lee et al. have synthesized the WO3 nanorods with a method of the thermal evaporation of the WO3 powders mixed with graphite powders in an oxidizing atmosphere at a high temperature of

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1100 °C and then the obtained WO3 nanorods were decorated with Cr2 O3 nanoparticles [98]. The Cr2 O3 -decorated WO3 nanorods presented an enhanced sensor response of 5.58 toward 200 ppm ethanol at 300 °C, 4.4 times higher than that of the pristine WO3 nanorods, which was attributed to the formation of n–p junctions, the catalytic effect of Cr2 O3 , the formation of structural defects, and the acid-based properties of the composites. And the hydrothermal method was also reported to be an effective way to modify the metal oxide nanomaterials. The WO3 nanowires decorated with Co3 O4 nanoparticles were synthesized by Ji et al. through a twostep method of a hydrothermal process combine with calcination [36]. The sensor based on the Co3 O4 nanoparticles-modified WO3 nanowires presented a gas sensor response of 5.3–100 ppm acetone at 280 °C, approximately two times higher than that of the obtained WO3 nanowires (only 2.45). Moreover, there has been a study reporting the H2 S sensing performance of the CuO nanoparticles-decorated WO3 microspheres through a hydrothermal method [99]. The CuO nanoparticles were uniformly distributed on the WO3 microspheres, and the sensor response of the CuO nanoparticles-decorated WO3 microspheres was found to be 105.2–5 ppm H2 S at 80 °C. The increased concentration of the active sites, more oxygen molecules adsorbed on the surface of the composite, and the formation of n–p heterojunction were reported to be the three main factors attributed to the enhanced H2 S sensing properties of the composites. The WO3 nanowires decorated with Cu2 O nanoparticles were also synthesized by Llobet et al. through an aerosol assisted CVD [100]. The results showed that the Cu2 O nanoparticles could be uniformly distributed on the surfaces of the WO3 nanowires. And the sensor based on the WO3 nanowires decorated with Cu2 O nanoparticles exhibited a shorter response/recovery time of 2 s/684 s than that of the pure WO3 nanowires (11 s/734 s), as shown in Fig. 5. The thin films composed of granular WO3 nanoparticles decorated with Ni2 O3 nanoparticles were reported by Kim et al. to be an outstanding material to detect the NH3 gas [101]. The method of decorating Ni2 O3 nanoparticles on the WO3 film is unique and facile. In a typical way, the sputter deposition of tungsten was firstly applied to prepare the WO3 granular film, following by oxidation. Then the Ni2 O3 nanoparticles were deposited onto the surface of the obtained WO3 film with a method of arc-discharge deposition of single-wall carbon nanotubes (SWCNTs) with Ni catalyst nanowires, followed by burning the carbon nanotubes. The Ni2 O3 nanoparticlesdecorated WO3 film exhibited a higher sensor response of ~13.5–200 ppm NH3 at the optimal working temperature of 250 °C than that of the pure WO3 film (~5). The enhanced gas-sensing performance of the decorated film was reported to be attributed to three possible reasons. Firstly, the p–n heterojunctions in sensing layers have a significantly important effect on the enhancement in the gas-sensing performance of the gas sensor. The other two reasons were the higher surface roughness and the high surface area of the sensing films based on the Ni2 O3 nanoparticles-decorated WO3 and the important role of the Ni2 O3 for the decomposition of NH3 .

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Fig. 5 SEM images of the pure WO3 nanowires (a, c) and the WO3 nanowires decorated with Cu2 O nanoparticles (b, d), TEM and HRTEM images of the WO3 nanowires decorated with Cu2 O nanoparticles (e, f, g, h), the dynamic gas-sensing properties of the WO3 nanowires decorated with Cu2 O nanoparticles (i) and the pure WO3 nanowires (j) to 50 ppm H2 S at 390 °C [100]. The inset in Fig. 5i was the schematic diagram of the sensor based on the WO3 nanowires decorated with Cu2 O nanoparticles. In this figure, the nm and the μm are the abbreviations of the nanometer and the micrometer, respectively. The Cu2 O/W and the W refer to the WO3 nanowires decorated with Cu2 O nanoparticles and the pure WO3 nanowires, respectively

3.5 MoO3 -Based Heterojunction As one of the important n-type semiconductors, MoO3 -based materials were widely explored in the fields of solar cells, photocatalysts, supercapacitors, lithium-ion batteries, UV photodetectors, organic light-emitting diodes, and gas sensors [102–106]. It has reported that there are three main crystalline forms of MoO3 , including the most stably orthorhombic α-MoO3 and the two metastable β-MoO3 and h-MoO3 [107]. Among the three forms of MoO3 , the α-MoO3 was the most widely studied one due to its unique layered microstructures, high chemical activities and the numerous vacancies on its surface. The band gap of MoO3 was reported to be ~2.39– 2.9 eV [106]. And the intrinsic structural anisotropy of the α-MoO3 makes it possible to synthesize abundant α-MoO3 -based materials, such as nanoparticles, nanoplates,

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nanorods, nanowires, nanoribbons, and hollow spheres. Meanwhile, the α-MoO3 based nanomaterials have the advantages of low costs, easy preparations, and low power consumptions, which also makes this material attract lots of interests in their gas-sensing performance [108, 109]. The gas sensors based on the various MoO3 nanomaterials exhibited promisingly gas-sensing performances in detecting a wide range of gases, such as H2 , CO, NH3 , H2 S, ethanol, formaldehyde, and triethylamine. For example, the MoO3 nanorods were successfully synthesized by Comini et al. through a method of infrared irradiation heating a Mo foil directly in air [110]. And their results showed that the sensor based on the MoO3 nanorods presented a sensor response of ~6 toward 100 ppm CO at 200 °C and 40% RH. Moreover, the purely crystalline characteristics of the prepared MoO3 nanorods were reported to be a positive factor guaranteeing their long-term stability. But the operating temperature of the MoO3 nanorods-based gas sensor needs to be reduced due to the low melting point of MoO3 . Several common strategies were mainly conducted to improve the gas-sensing performances of the MoO3 -based sensors, such as doping the nanostructured MoO3 with metal atoms and decorating the nanostructured MoO3 with semiconductor nanoparticles. However, it is still a challenge to develop the high-performance gas sensor based on the nanostructured MoO3 materials, especially the sensors working at the low temperatures. The novel α-MoO3 and Ni-doped α-MoO3 nanolamella were synthesized by Chen et al. through a facile solvothermal method and an annealing technique [111]. The sensor based on the Ni-doped nanolamella displayed a sensor response of approximately 42.5–100 ppm formaldehyde at the optimized working temperature of 250 °C, which was over 7 times higher than that of pure MoO3 -based sensor. They discussed that the improvement in the gas-sensing performance of the doped α-MoO3 nanolamella should be attributed to three positive factors. The first factor was the increased concentration of oxygen vacancies due to the presence of the low-valance Ni2+ ions. The second factor was the p–n junction in the sensing material because they found that some new compound NiMoO4 (p-type semiconductor) appeared. And they also found that the concentration of the doped Ni also had a significant effect on the gas-sensing performance of the Ni-doped α-MoO3 nanolamella. There was a decrease in the gas sensor response toward the formaldehyde when the concentration of the doped Ni was higher than 5 mol% due to the lattice deformation and the decrease in oxygen vacancies in the sensing materials. And the CoMoO4 nanoparticles-decorated MoO3 nanorods were synthesized by Song et al. via a hydrothermal method combined with a dipping-annealing process. The sensor based on the CoMoO4 -decorated MoO3 nanobelts exhibited a high sensor response of 104.8 toward 100 ppm TMA at 220 °C, which was much higher than that of the bare MoO3 nanobelts (~12), as shown in Fig. 6 [65]. Meanwhile, the cage-like MoO3 /ZnO composites were reported by Chen et al. to be synthesized via a hydrothermal method [112]. The specific surface area of the cage-like composite was found to be greatly increased to 58.22 m2 /g, much higher than that of the pure MoO3 (only 8.94 m2 /g), due to its porous characteristics. The higher the specific surface of the composite, the more micropores are present in the

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Fig. 6 a SEM images of bare MoO3 nanobelts, SEM image (b) and EDS analysis (c) of CoMoO4 /MoO3 nanocomposites, TEM images of the CoMoO4 /MoO3 nanocomposites (d, e) and the lattice fringes image of CoMoO4 nanoparticles (f), dynamic gas response of the sensor based on pure MoO3 and CoMoO4 /MoO3 nanocomposites to different-concentration trimethylamine (TMA) (g, h) and the repeatability of the CoMoO4 /MoO3 nanocomposites to 5 ppm TMA (i) [65]. The sensor based on the CoMoO4 /MoO3 nanocomposites was similar to that of the sensor based on SnO2 shown in Fig. 3g. In this figure, the Ra and the Rg are the resistances of the sensor in the air and in the TMA, respectively. And the nm and ppm are the abbreviations of the nanometer and parts per million, respectively

composite and the formation of the n–n heterojunctions led to the enhancement in the gas-sensing performance of the synthesized MoO3 /ZnO composite. The results showed that the synthesized composite exhibited a higher sensor response of ~30 toward 100 ppm H2 S at 270 °C than that of the pure MoO3 (~20) or the pure ZnO (~9). The 1-D MoO3 /CuO composite was also reported to be an outstanding material to detect H2 S [113]. Gao et al. have prepared the uniformly n-type MoO3 nanorods via a hydrothermal method and then the nanorods were decorated with p-type CuO nanoparticles through being irradiated with a high-intensity ultrasonication (150 W). The sensor based on this n–p composite showed an improved sensor response of ~272–10 ppm H2 S at the optimized working temperature of 270 °C. It was noted

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that the great enhancement in H2 S sensing properties of the composite should be attributed to the disappearance of p–n junctions in the MoO3 /CuO composites after being exposed to H2 S gas. In addition, the porous MoO3 /SnO2 nanoflakes with n–n junctions also presented an enhanced H2 S sensing performance due to the modulation in the highness of the built potential barrier in the composite [114]. Also, the SnO2 /α-MoO3 heterogeneous nanobelts are successfully synthesized by a wet chemical method [115]. The gas sensor based on this n–n heterojunction showed a high sensor response of 67.76 toward 500 ppm ethanol at 300 °C, which was much higher than that of the bare MoO3 nanorods (~30). Similar to the other reported MoO3 -based heterojunctions, there was a formation of a potential barrier at the interfaces between the MoO3 nanorod and the SnO2 nanoparticle, which played an important role in the domination of the electronic properties of the composite. The highness of the potential barrier would greatly decrease after the SnO2 /α-MoO3 heterogeneous nanobelts exposed to ethanol, resulting in the enhanced gas-sensing performance even at the low working temperature. The core-shell MoO3 -based materials were also prepared to study their gassensing performance [116]. For example, Elder et al. have synthesized the MoO3 /TiO2 core-shell nanorods via a hydrothermal method and subsequent annealing process. The results showed that the thickness of the outer TiO2 shell was 40– 70 nm, and the MoO3 nanorods were in a uniform size with the average length being 12 μm. And the MoO3 /ZnO core-shell nanorods were also prepared via a typical two-step method: the uniform MoO3 nanorods were synthesized through a common hydrothermal process and then the nanorods were coated by ZnO through an ALD process. In the core-shell nanorods, the thickness of the ZnO in the shell layer is found to be ~30 nm. Both the MoO3 /TiO2 core-shell nanorods and the MoO3 /ZnO core-shell nanorods exhibited enhanced gas-sensing performances to the targeted gases due to the potential barriers at the core-shell heterojunctions.

3.6 Fe2 O3 -Based Heterojunctions It has been reported that there are four common polymorphs of Fe2 O3 , namely, the naturally occurred α-Fe2 O3 and γ -Fe2 O3 and the generally synthesized ε-Fe2 O3 and β-Fe2 O3 [117]. The α-Fe2 O3 (band gap of ~2.1 eV) is the most stable iron oxide with normally n-type semiconducting properties, which has attracted lots of interests due to its specific advantages of easy fabrications, a wide range of sources, friendly to the environments, and outstanding adherence with substrates [118]. As reported, various nanostructured Fe2 O3 materials have been synthesized, such as nanospindles, nanorings, nanospheres, nanorods, nanobelts, and nanowires [119–122]. The promising Fe2 O3 has been applied in the photocatalysts, magnetic materials, supercapacitors, lithium battery negative materials, and gas sensors [118, 123–126]. Based on the recent reports, the Fe2 O3 -based materials were found to be sensitive to a branch of gases, such as methane, propane, H2 , CO, H2 S, NO, and NH3 . The α-Fe2 O3 with various geometries, including nanoparticle, nanorods, nanofibers,

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and nanowires, were successfully assembled to be the excellent gas sensors at the working temperatures of RT-300 °C [127–130]. The gas-sensing performance of the purely urchin-like α-Fe2 O3 synthesized by a hydrothermal method was systematically investigated by Yang et al. Their results showed that the urchin-like α-Fe2 O3 exhibited a sensing property to 56 ppm ammonia with the gas sensor response of 2.4 [131]. Meanwhile, the bundle-like α-Fe2 O3 nanorods prepared in a hydrothermal way by Lu et al. also presented a gas sensor response of ~5 toward 100 ppm ethanol at 175 °C [128]. As can be seen, the gas sensor responses of the pure α-Fe2 O3 to the small gas molecules were too low to be practically applied in the gas-contained environments, which should be due to the limited interaction between the gas molecules and the sensing materials. Recently, one of the methods to improve the gas-sensing performance of the αFe2 O3 was to composite it with the novel metal nanoparticles. For example, a novel gas sensor based on Ag/Fe2 O3 core-shell nanocomposites can be used to detect the ethanol. The results showed that the sensor based on the nanocomposites showed an enhanced gas sensor response to 12.5 ppm ethanol at 250 °C, which was over one time higher than that of the pure Fe2 O3 [132]. However, there are some drawbacks regarding decoration of the noble metals on the metal oxides, such as passivation of the effective surface area of metal oxides, loss of catalytic activity of the Ag nanoparticles after treatment in high temperature, and poisoning of Ag nanoparticles by many chemicals including sulfur or phosphorus. Meanwhile, the α-Fe2 O3 was also reported to be composited with the twodimensional materials, which was proved to be a more reasonable way to improve their gas-sensing performance. The ethanol sensing properties of α-Fe2 O3 have been successfully enhanced with the help of graphene or g-C3 N4 . For example, the αFe2 O3 /g-C3 N4 nanocomposites were synthesized by a hydrothermal method combined with a pyrolysis process. Due to the formation of heterojunctions between the Fe2 O3 and g-C3 N4 , the Fe2 O3 /g-C3 N4 nanocomposites demonstrated a better gas-sensing performance than those of the pure α-Fe2 O3 or the g-C3 N4 . The sensor response of the nanocomposites was found to be 7.76 toward 100 ppm ethanol at 340 °C, which was much higher than those of the pure α-Fe2 O3 or pure g-C3 N4 [133]. Moreover, Wang et al. have also synthesized the α-Fe2 O3 nanoparticles-decorated graphene nanomaterials through a hydrothermal method. Their results showed that the sensor based on the composites containing 2 wt% of graphene exhibited a sensor response of 30 toward 1000 ppm ethanol at 280 °C, higher than that of the pure α-Fe2 O3 nanoparticles (~10) [134]. Also, it was an effective strategy to improve the gas-sensing performances of the Fe2 O3 nanomaterials through compositing them with nanostructured metal oxides. The work of Lee et al. reported that the sensing performance of α-Fe2 O3 nanorods to H2 S could be significantly enhanced through decorating them with the TiO2 nanoparticles [135]. In their study, the Fe2 O3 nanorods were firstly synthesized on the Fe foil with the method of a thermal oxidation and then decorated with TiO2 nanoparticles via a solvothermal treatment. When the sensor based on the TiO2 -decorated Fe2 O3 nanorods was exposed to H2 S, the depletion layers at the interfaces between

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the Fe2 O3 and the TiO2 was built, which results in a decrease in the electron density and an increase in the resistance of the composite. Different from the pristine Fe2 O3 nanorods-based gas sensor, there were numerous Fe2 O3 /TiO2 n–n heterojunctions in the TiO2 -decorated Fe2 O3 nanorods-based sensor, directly contributing to a higher sensor response toward H2 S of the composite. The sensor based on the decorated Fe2 O3 nanorods showed a gas sensor response of 7.4–200 ppm H2 S at 300 °C (shown in Fig. 7), which was much higher than of the pure Fe2 O3 nanorods (2.6). And Huang et al. have assembled the Fe2 O3 -loaded NiO nanosheets through a microwave-assisted liquid-phase synthesis [136]. Their results showed that the sensor response of the Fe2 O3 -loaded NiO nanosheets toward 100 ppm ethanol was counted to be 170.7 with the response/recovery time of 0.5 s/14.6 s at the working temperature of 255 °C. The ZnO nanoparticles were also loaded onto the Fe2 O3 nanoplates to improve the gas-sensing performance of Fe2 O3 via a method of an ALD technique [137]. The sensor based on the ZnO-decorated Fe2 O3 nanoplates

Fig. 7 SEM image (a) and TEM image (b) of Fe2 O3 nanorods decorated with TiO2 nanoparticles, the dynamic gas-sensing performances of the pure Fe2 O3 nanorods (c) and the Fe2 O3 nanorods decorated with TiO2 nanoparticles (d) to H2 S with different concentrations at 300 °C [135]. The inset in Fig. 7c was the schematic diagram of the sensor based on the Fe2 O3 nanorods decorated with TiO2 nanoparticles. In this figure, the nm, μm, ppm, , and sec are the abbreviations of the nanometer, micrometer, parts per million, ohm, and second, respectively

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showed a higher gas sensor response of 133.1–100 ppm H2 S at 250 °C than that of the pure Fe2 O3 nanoplates (~8.7).

3.7 In2 O3 -Based Heterojunctions In2 O3 is one of the typically wide-band transparent semiconductors with its direct band gap being 3.55–3.75 eV [138]. The In2 O3 -based materials have exhibited excellent performances in various applications including solar cells, flat panel displays, and gas-sensing devices. By now, numerous In2 O3 nanomaterials, such as nanocubes, nanofibers, nanowires, and nanorods, have been successfully synthesized with wet chemical methods and electrospinning technologies. The nanostructured metal oxides with high specific surface areas are the key materials for the development of In2 O3 -based gas sensors with improved gas-sensing properties. The sensors based on the In2 O3 nanomaterials were reported to be sensitive to a wide range of gases, such as H2 S, ammonia, NO2 , H2 , CO, and triethylamine. In recent years, the In2 O3 modified with metal nanoparticles have shown improved gas performances compared with the bare one. Liu et al. have synthesized the Pd nanoparticles-functionalized In2 O3 composites via a hydrothermal method followed by a deposition–precipitation process [139]. The gas-sensing performance of the obtained composites showed that the composite exhibited a high sensor response of 47.56–50 ppm with the response and recovery times of 4 s and 17 s, respectively. Apart from decorated with metal nanoparticles, the In2 O3 nanomaterials compositing with other nanostructured metal oxides have also proved to be effective gas-sensing materials, which were widely studied by the researchers. Typically n–p heterojunctions based on the In2 O3 -based nanorods were synthesized by Park et al. by a thermal evaporation of n-type In2 O3 powders in an oxidizing atmosphere followed by a solvothermal deposition of p-type Cr2 O3 [140]. The results displayed that the Cr2 O3 nanoparticles were decorated on the surfaces of the In2 O3 nanorods with an average diameter of ~150 nm. The introduction of the p-type Cr2 O3 on the In2 O3 nanorods would induce the building of potential barrier between their interfaces and the structural defects in the sensing materials. The highness of the potential barrier could be modulated in the ethanol atmosphere accompanying the adsorption and desorption of ethanol gas molecules. And the structural defects were always regarded as a positive factor to increase the adsorption of the gas molecules on the surface of the composite. Therefore, an enhancement in the ethanol sensing performance of the Cr2 O3 -decorated In2 O3 nanorods was observed. The sensors based on the composites showed a higher sensor response of 16 and a shorter response/recovery time of ~16 s/50 s than those of the pure In2 O3 nanorods (~4, 18 s/60 s), as shown in Fig. 8. Meanwhile, the CuO-loaded In2 O3 nanofiber was also successfully prepared by Lee et al. via a method of an electrostatic spinning [141]. The sensor based on the CuOmodified nanofibers showed an ultrahigh sensor response of 11600–5 ppm H2 S at the 150 °C, which was extremely higher than that of the pure In2 O3 nanofibers (~515). The enhanced H2 S gas-sensing performance of the CuO-loaded In2 O3 nanofiber was

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Fig. 8 a SEM image of Cr2 O3 -decorated In2 O3 nanorods, b XRD patterns of pure and Cr2 O3 decorated In2 O3 nanorods, the dynamic gas-sensing performance of the pure (c) and Cr2 O3 decorated In2 O3 nanorods (d) to different-concentration ethanol [140]. The sensor based on the Cr2 O3 -decorated In2 O3 nanorods was similar to that of the sensor based on SnO2 shown in Fig. 10e. In this figure, μm, ppm,  and sec are the abbreviations of the micrometer, parts per million, ohm, and second, respectively. And the arb. units refer to the arbitrary units

reported to be mainly attributed to the numerous p–n junctions between the CuO and the In2 O3 and the creation and disruption of this heterojunctions in the presence and absence of H2 S, respectively. And the p-type Ni2 O3 particle-modified In2 O3 has been assembled to be the high-response CH4 gas sensor, which also exhibited an improved sensing performance to CH4 with a higher sensor response than that of the pure In2 O3 [142]. Lee et al. have also prepared the n-ZnO/n-In2 O3 nanofibers through an electrospinning process [143]. Their results showed that the sensor based on the composites exhibited a maximum sensor response of 133.9–5 ppm trimethylamine at 300 °C, which was much higher than that of the pure In2 O3 nanofiber or the pure ZnO nanofiber. And the response time of the n-ZnO/n-In2 O3 nanofibers was calculated to be only 1–2 s. One of the reported reasons for the enhanced gas-sensing performance of the ZnO–In2 O3 composite nanofibers is their smaller particle sizes leading to more effective electron depletion. Then there were lots of n–n heterojunctions formed in the sensing materials, which would enhance the adsorption and the oxidation of the

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analyte gas on the heterostructures. The In2 O3 –ZnO core-shell nanowires were also used to detect the CO, H2 , and ethanol, which were synthesized through a two-step process [144]. These unique structures with single-crystal cores and polycrystalline shells have some advantages in the chemical sensors. The roughly outer surfaces of the core-shell nanowires are rich in adsorption sites for the gas molecules, and the combination of homogeneous and heterogeneous interfaces at junctions would also increase their gas-sensing performances. The ZnO/In2 O3 belt-tooth shape were synthesized with a CVD process and also showed an enhanced C2 H2 gas-sensing properties at a relatively low working temperature of 90 °C [145]. The Bi2 O3 -decorated In2 O3 nanorods were also established by Lee et al. via a simple method to study their ethanol sensing performance. Their results showed that the Bi2 O3 nanoparticles were uniformly decorated on the surfaces of the In2 O3 nanorods. The Bi2 O3 nanoparticle-decorated In2 O3 nanorods exhibited an obviously enhanced ethanol sensing performance at the working temperature of 200 °C. The sensor response of the sensor based on the composites was found to be 17.74– 200 ppm ethanol with the recovery time of 180 s, while the pure In2 O3 only showed a lower sensor response and a longer recovery time of 3.6 and 360 s, respectively. The modulation of the depletion layer widths between the Bi2 O3 nanoparticles and the In2 O3 nanorods and the highness of the potential barrier between their interfaces were analyzed to be the two main positive effectors for the enhanced ethanol sensing performance of the composite. Moreover, the Bi2 O3 nanoparticles also was a possibly effective catalytic for the reactions between the adsorbed oxygen species and ethanol molecules. And the introduction of the Bi2 O3 nanoparticles on the surface of the In2 O3 nanorods would induce the structural defects, resulting in more gas molecules adsorbed on the sensing materials [146].

3.8 Co3 O4 -Based Heterojunctions In recent years, more and more attention has been focused on the development of the high catalysis activity and low cost of semiconductor metal oxides and their applications in the gas sensors. Among the studied metal oxides, the Co3 O4 is a typically p-type semiconductor with indirect band gaps of 1.5 eV [147]. There were numerous kinds of Co3 O4 -based nanomaterials successfully synthesized, such as nanoparticles, nanowires, nanorods, nanotubes, and nanofibers, through the methods of the hydrothermal process, thermal decomposition, microwave-assisted solvothermal route, and electrospinning. The advantages in the catalytic properties and simple preparation of the Co3 O4 -based materials make them widely applied in the fields of catalytic, lithium-ion batteries, supercapacitors, and gas sensors. In the case of gas sensors, the Co3 O4 -based nanomaterials have been reported to be sensitive to the various gases including C2 H5 OH, NH3 , NO2 , H2 S, acetone, and toluene [148–153]. Wang et al. have synthesized the uniform Co3 O4 nanocubes with an average size of 250 nm to study their acetone-sensing performance [152]. Their results showed that the response time and the recovery time of nanocubes to

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500 ppm acetone were calculated to be 2 s and 5 s, respectively, at 240 °C. But the sensor response of the sensor to 500 ppm acetone was found to be only 4.8. The Co3 O4 -crossed nanosheet arrays were also prepared to be the sensing materials, showing a gas sensor response of 16.5–100 ppm at the working temperature of 110 °C [154]. However, the high working temperature and the low gas sensor response of the pure Co3 O4 -based sensors were needed to be improved to promote their practical applications. There have been some strategies to effectively enhance the gas-sensing performances of the Co3 O4 nanomaterials. Among the used methods, the gas sensors based on the composites made of Co3 O4 heterojunctions have been widely studied in recent years by the researchers due to their promising and excellent gas-sensing performances. The n-SnO2 -p-Co3 O4 nanofibers were prepared by Kim et al. via a route of an electrospinning followed by an annealing process [54]. In the research, the effects of the ratio of SnO2 /Co3 O4 were systematically studied and the 0.5SnO2 -0.5Co3 O4 showed the most outstanding gas-sensing performance among all the studied composites. The results displayed that the sensor based on the 0.5SnO2 -0.5Co3 O4 exhibited a short response time and recovery time of 10.25 and 15.2 s to 10 ppm C6 H6 . And the gas sensor response to the same concentration of C6 H6 was found to be 22, much higher than that of the 0.1SnO2 -0.9Co3 O4 (~3). As reported, there were three different junctions, the homojunctions of SnO2 /SnO2 and Co3 O4 /Co3 O4 junctions and the heterojunctions of SnO2 /Co3 O4 junctions in the 0.5SnO2 -0.5Co3 O4 composite nanofibers. The heterojunctions were reported to play a more important role in the gas-sensing process because the potential barriers were constructed in these small regions. Then the modulation of the n–p heterojunctions by the C6 H6 would greatly make a variation in the resistance of the sensor based on the 0.5SnO2 -0.5Co3 O4 . The maximum number of n–p heterojunctions in the 0.5SnO2 -0.5Co3 O4 composite therefore led to their best gas-sensing performances. Similar enhanced gas-sensing performance was also observed in the Co3 O4 –SnO2 core-shell hollow spheres, synthesized by Lee et al. via a galvanic replacement followed by calcination. The gas sensor based on the Co3 O4 –SnO2 core-shell hollow spheres exhibited a higher sensor response of 18.6–10 ppm xylene at 275 °C than that of the pure Co3 O4 (~6) [155]. In addition, Xiang et al. have established the Co3 O4 hollow spheres functionalized with ZnO nanoparticles via a facile precipitation method [156]. The results showed that the specific surface area of the composite was calculated to be 73.3 m2 /g, which was much higher than that of the pure Co3 O4 hollow spheres (41.3 m2 /g). And the p–n junctions between the ZnO and the Co3 O4 led to the formation of the additional depletion layer between their interfaces. Therefore, the sensor based on the functionalized Co3 O4 hollow spheres exhibited a higher sensor response of 5.68–10 ppm HCHO at the optimized working temperature of 160 °C than that of the pure Co3 O4 hollow spheres (~1.72). Moreover, the mesoporous ZnO/Co3 O4 microspheres were prepared by Liu et al. through a route of a solvothermal method followed by an annealing process. The obtained ZnO/Co3 O4 composite showed an improved gas sensor response of 42–50 ppm ethanol at the optimized working temperature of 275 °C compared with that of the pure ZnO (~12.5). And the sensor based on the ZnO/Co3 O4 also showed a response time and a recovery time of 5.6 s and 29 s to 50 ppm ethanol,

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respectively, shorter than those of the pure ZnO (11.5 s and 48 s). The n-ZnO/pCo3 O4 nanoparticles-based network sensor has also been successfully assembled by Lee et al. to study its enhanced NO2 gas-sensing performance. The ZnO/Co3 O4 nanoparticles were prepared through a method of a facile solvothermal process. There were several junctions in the sensing materials, namely, the n–n junctions between the ZnO and ZnO, the p–p junctions between the Co3 O4 and Co3 O4 , and the p–n heterojunctions between the ZnO and Co3 O4 . It has been reported that the heterojunctions played a more important and effective role in controlling the resistance of the composites because of the great modulation of the highness of the potential barrier in the NO2 atmosphere. The research showed that the gas sensor response of the ZnO/Co3 O4 composite nanoparticles was highly elevated to be 133–500 ppm ethanol at 275 °C, over six-time higher than that of the pure metal oxide (~20) [157]. The ZIF-derived porous ZnO–Co3 O4 hollow polyhedrons were also prepared by Xue et al. through a method of thermal decomposition. The sensor based on the ZnO– Co3 O4 hollow polyhedrons showed an enhanced gas sensor of 106 toward 1000 ppm ethanol at 200 °C with a short response time of 7 s, as shown in Fig. 9 [158]. The core-shell Co3 O4 /α-Fe2 O3 heterostructure nanofibers, synthesized through the facile and template-free coaxial electrospinning method, were also designed by Zhang et al. to study their enhanced gas-sensing performance [159]. The formation of the heterojunctions between the Co3 O4 and the α-Fe2 O3 and the catalysis properties of the Co3 O4 in the sensing materials were reported to be the two positive factors responsible for the enhanced gas-sensing performance of the sensor based on the core-shell nanofibers. The sensor response of the core-shell nanofibers to 800 ppm acetone at 240 °C was calculated to be ~27.5, much higher than that of the pure Co3 O4 nanofibers (only ~10). And they also established the hybrid Co3 O4 /SnO2 core-shell nanospheres with a one-step hydrothermal method [160]. The hybrid nanospheres also showed a more outstanding gas-sensing performance at 200 °C. The improved sensor response of the hybrid Co3 O4 /SnO2 core-shell nanospheres to 100 ppm NH3 was found to be 13.6, 3 times higher than that of the solid Co3 O4 /SnO2 nanospheres. Ruan et al. have synthesized the hierarchical Fe3 O4 @Co3 O4 core-shell microspheres. And they found that the gas sensor of the hierarchical microspheres could be ~300– 5000 ppm acetone, much higher than that of the pure Co3 O4 (~75).

3.9 CuO-Based Heterojunctions Among the widely studied p-type sensing materials, the typical CuO-based materials have been proved to be the promising sensing candidates [161, 162]. Generally, CuO is one of the most important p-type semiconductors with a narrow band gap of 1.2– 1.9 eV [163]. Various CuO-based nanomaterials, such as nanoparticles, nanofibers, and nanowires, have been successfully synthesized via the hydrothermal process, electrospinning, and thermal oxidation. The CuO-based nanomaterials have been applied in photovoltaics, field-emitting devices, biosensors, and gas sensors.

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Fig. 9 SEM and TEM images of a, c the pure Co3 O4 (the inset is the SEM image of ZIF-67 polyhedrons) and b, d 2 mol% ZnO–Co3 O4 porous hollow polyhedrons, the dynamic sensing characteristics of the 2 mol% ZnO–Co3 O4 at different working temperatures and d the gas-sensing performance of the composites to ethanol gas with the gas concentrations of 1–1000 ppm [158]. The inset in Fig. 9f was the schematic diagram of the sensor based on the ZnO–Co3 O4 porous hollow polyhedrons. In this figure, the Ra and the Rg were the resistances of the sensor in the air and in the ethanol, respectively. And the nm, ppm, and  are the abbreviations of the nanometer, parts per million, and ohm, respectively

In the field of gas sensors, the CuO-based materials have been reported to monitor the NH3 , CO, H2 S, NO2 , H2 , acetone, and ethanol. Steinhauer et al. have assembled a novel gas sensor based on the CuO nanowires synthesized through a one-chip method of thermal oxidation of electroplated copper microstructures [164]. The novel gas sensor exhibited an enhanced sensor response because the nanowires were fully

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surrounded by the targeted gas atmosphere. And the gas sensor also showed a sensing property to the H2 S with the concentration as low as 10 ppb. The porous CuO nanowires also were synthesized by Kim et al. with a route of combined processes of a deposition of Cu on porous single-wall carbon nanotube substrate and a followed thermal oxidation [165]. The sensors based on the CuO nanowires prepared at 400 °C showed the highest gas sensor response to 6% H2 at the working temperature of 250 °C with the recovery time of ~2.5 min. The recovery time of the pure CuO nanowires to the targeted gas was so long that some measures must be taken to overcome this problem. It has been proved to be an effective way to improve the gas-sensing performances of the CuO-based materials through compositing them with the other sensing materials. The Fe2 O3 -functionalized CuO nanorods were fabricated by Park et al. through a thermal oxidation of a Cu film followed by a solvothermal process [55]. The results showed that the Fe2 O3 nanoparticles (~5–30 nm) were combined steady with the CuO nanorods showing the length and the diameter of 5–10 μm and 20–60 nm, respectively. It should be noted that the sensors based on the pure CuO and the CuO– ZnO nanoparticles showed typical p-type sensing performances to the acetone. The sensor based on the functionalized CuO nanorods exhibited an improved sensor response of ~11–1000 ppm acetone at the optimized working temperature of 240 °C, which was approximately one time higher than that of the pure CuO nanorods (~6). The enhanced acetone sensing performance of the functionalized nanorods was mainly attributed to the modulation of the potential barrier between the CuO and the Fe2 O3 , the more adsorbed sites induced by Fe2 O3 nanoparticles and the high catalytic activity of the Fe2 O3 for the oxidation of the acetone. The CuO nanorods decorated with Fe2 O3 nanoparticles were also constructed by Chang et al. via a thermal oxidation combined with a hydrothermal process [56]. The decorated CuO nanorods were reported to show a higher gas sensor response of 4–5 ppm H2 S at 150 °C compared with that of the pure CuO nanorods (~2), as shown in Fig. 10. In addition, they also prepared the CuO–ZnO nanoparticles through a facile solvothermal process [166]. The study showed that the sensor based on the CuO–ZnO composite exhibited a higher sensor response of ~10 to the 2 ppm H2 S at 225 °C compared with that of the pure CuO (~3). And the response time and the recovery time of the composite were calculated to be 30 s and 98 s, respectively, much shorter than those of the pure CuO (~122 s and 125 s, respectively). The authors discussed that the formation of the higher potential barriers between the CuO and the ZnO was an important factor for the enhanced gas-sensing properties. The core-shell nanostructures were also established to form the heterojunctions to improve the gas-sensing performances of the CuO-based materials. For example, Ruan et al. have synthesized the CuO–NiO core-shell microspheres by a two-step hydrothermal method and study their H2 S gas-sensing performance [167]. The results showed that the broken nanosheets were combined on the outer layer of the CuO– NiO microspheres with the average diameters of approximately 800 nm. And the p–p heterojunctions would form between the CuO and the NiO, resulting in the electrons and the holes transferring between the CuO to the NiO and the formation of a potential barrier or an accumulation layer between them. Meanwhile, more oxygen molecules

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Fig. 10 Typical SEM images of a the synthesized pure CuO nanorods and b the assembled CuO nanorods decorated with Fe2 O3 nanoparticles, c TEM image of the CuO nanorods decorated with Fe2 O3 nanoparticles, the dynamic gas-sensing performances of the pure CuO nanorods (d) and the CuO nanorods decorated with Fe2 O3 nanoparticles (e), the gas selectivity of the prepared samples (f) [56]. The sensor based on the CuO nanorods decorated with Fe2 O3 nanoparticles was similar to that of the sensor shown in the inset in Fig. 9f. In this figure, the Ra and the Rg were the resistances of the sensor in the air and in the H2 S, respectively. In this figure, the nm, μm, ppm, ppb, , and sec are the abbreviations of the nanometer, micrometer, parts per million, part per billion, ohm, and second, respectively. The H2 S, CH4 , NH3 , CO, and H2 are the hydrogen sulfide, methane, ammonia, carbon monoxide, and hydrogen, respectively

were confirmed to be adsorbed on the surface of the composite. Therefore, both the formation of the heterojunctions (or the accumulation layer) in the sensing material and the catalytic NiO in the outer surface of the CuO–NiO microspheres would promote the gas-sensing performance of the CuO–NiO core-shell microspheres. The sensor based on the composite exhibited a higher sensor response of ~80–1000 ppm H2 S compared with that of the pure CuO (~35). In addition, the CuO–ZnO core-shell nanowires and the CuO–In2 O3 core-shell nanowire with the p–n heterojunctions were also synthesized to improve their gas-sensing performances. As reported, the CuO– ZnO core-shell nanowires prepared by Kim et al. showed a high sensor response to CO and C6 H6 , superior to those of pristine p-CuO nanowires [168]. And the CuO– In2 O3 core-shell nanowires synthesized by Tang et al. exhibited a higher gas sensor response to H2 , C3 H8 , and CO at 300 °C than those of the pure CuO [53]. From the summary of the gas-sensing performances of the ZnO, SnO2 , TiO2 , WO3 , In2 O3 , Fe2 O3 , MoO3 , Co3 O4 , and CuO, we could infer that the establishment of the heterojunctions in the composites composed of different metal oxides was of greatly important and effective to improve their gas sensor responses and short their response/recovery times. The metal oxides could be used to form different types of

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heterojunctions through compositing them with n-type or p-type metal oxides. And the formation of the potential barrier and the accumulation layer in the composites could directly induce the enhancements in the gas-sensing properties of the sensor based on a certain heterojunction. Even the gas sensors based on the metal oxides composites always showed higher sensor responses and shorter response times to the targeted gases compared with the pure metal oxides. It should be noted that the sensing stability and the baseline shifts of the several sensors still needed to be investigated to further improve the gas-sensing performance of the composite, which would be discussed in detail in Sect. 4.

4 Current Challenges and Promising Outlook As can be indicated from the above discussions, the formation of metal oxide heterojunctions in the sensing materials is an effective and promising route to greatly improve the gas-sensing performances of the gas sensors. In recent years, lots of researchers have done a series of experiments in this field and many highperformance gas sensors have been explored, but there are still some important issues not completely solved.

4.1 Long-Term Stabilities of Gas Sensors The long-term stabilities of the gas sensors based on the metal oxides typically represent the repeatability or the stabilities of the gas-sensing properties after the gas sensors experience numerous response processes toward the targeted gases in a long period (several days or months). Many of the reported gas sensors based on the metal oxides have exhibited poor or unreliable long-term stabilities. As discussed above, one of the significant processes in the sensing response or recovery of the gas sensor to the targeted gas is the adsorption of the gas molecules on the active sites in the metal oxide surface. Lots of experimental and theoretical studies have investigated the important role of the active site, such as oxygen vacancies, in the enhancement of the gas-sensing performance of the metal oxide. In the case of the hydrogen-sensing performance of MoO3 nanoribbons, our previous study found that the oxygen vacancy in the surface of MoO3 was the key factor that affecting the adsorption of the oxygen molecules and the hydrogen molecules [107]. In our further study of the hydrogen gas-sensing performance of the signal MoO3 nanoribbon, the results showed that the MoO3 nanoribbon annealed in the reducing gas owned the highest-concentration oxygen vacancies, exhibiting the best hydrogen-sensing performance with the highest gas sensor response [109]. Similar results were also found in other studies on the gas-sensing performances of the ZnO, WO3 , TiO2, and SnO2 , in which certain concentrations of oxygen vacancies were mainly generated through the annealing process [169–172]. However, the oxygen vacancies on the

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metal oxide surface could be passivated due to the oxidation by the oxygen species during the repetitive response or recovery processes in a long time. And a part of the vacancies on the surface of the metal oxide might also be occupied by H2 O molecules in the atmosphere. This passivation of the oxygen vacancies in the surfaces of the metal oxides was a factor responsible for the attenuation of the long-term stability of the gas sensors. Meanwhile, many of the studied heterojunctions are almost based on the nanoscale metal oxides. It is generally believed that the small nanoparticles are more likely to become coarser over time at the high temperature, while the larger ones are likely to change in a contrary tendency. This is especially true to the films composed of nanostructured grains. Due to the small size, the molecular thermal motion in the grain is remarkably intense and the surface activity in the heterojunctions is extremely high, resulting in the poor chemical stability of the particle at the high-temperature environment [173]. In addition, there is always a small amount of diffusion in the region of heterojunctions, which can lead to the formation of the intermetallic compounds or mixed oxides at these interfaces, which will seriously affect the highness of the potential barriers and the electronic properties of heterojunctions [174]. Therefore, the long-term stability of nano-oxide heterojunction is not particularly ensured, which also results in the changes in the gas-sensing performances of the gas sensors based on the heterojunctions. Different kinds of efforts could be devoted to overcome the drawbacks in the long-term stabilities of the gas sensors. Among all the adopted treatments, it is an effective way to restrain the passivation of the oxygen vacancies in the metal oxide surfaces through doping them with other metal atoms. It has been reported that a number of oxygen vacancies would be produced through substitutional doping of low-value metal ions for high-value metal ions to maintain the charge balance of the metal oxide, such as the Zn-doped SnO2 , the Fe-doped MoO3, and the Ag-doped ZnO [175–177]. In the case of the Zn-doped SnO2 microsphere, the concentration of the oxygen vacancies can be adjusted by the Zn concentrations and the grain sizes in the sensing materials also decreased, leading to a higher specific area of the doped SnO2 microspheres, which are the two positive factors that increase the sensor response of the Zn-doped SnO2 microsphere. The similar enhancements were also found in the gas-sensing performances of other gas sensors based on doped metal oxides [176–178]. This effective method might be a possible way to improve the stabilities of the gas sensors based on the metal oxide heterojunctions, which should be further investigated. Apart from the method of doping the metal oxide with metal atom, a dry desiccant could be integrated into the gas-sensing devices to limit the effects of the H2 O molecules on the surface activities of the sensing materials. And another possible way to control the humidity is to use the air conditioner or the dehumidifier to remove the H2 O molecules in the environments where the gas sensor is placed. Moreover, the low working temperature should be a method to prevent the intermetallic compounds and mixed oxides in the region of heterojunctions. A promising way to restrain the diffusion of the element at the heterojunctions might be to assemble the heterojunctions with the metal oxides which exhibits a high gassensing performance at a low temperature or room temperature. Then the negative

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effects of the change in the grain sizes or the compositions at the heterojunction on the gas-sensing performances must be weakened in the sensors based on metal oxide heterojunctions.

4.2 Baseline Drift of the Gas Sensor Another main challenge in the high-performance gas sensors based on the heterojunctions is the baseline drift of the detected signals, such as the current, voltage, and resistance, during the sensing processes. Generally, the baseline drift of the detested signal is defined as the drift of the electronic properties of the gas sensor in the continuing sensing processes. As reported, the baseline of the sensor based on the SnO2 /SnS heterojunction showed an obvious increase in the resistance of the sensor after its response toward 1–8 ppm NO2 [179]. The original resistance of the sensor during the response to 1 ppm NO2 is ~60 M, but the original resistances of the sensor during the response to 5 ppm or 8 ppm NO2 are found to increase by ~75 M or ~100 M, respectively. In the case of the Fe2 O3 nanoparticle-coated SnO2 nanowire, the results also showed that the resistance of the sensor could not recover fully back to its original value after the ethanol was stopped [180]. A similar phenomenon was also found in the gas sensors based on the WO3 -decorated ZnO nanoplates, NiO/ZnO hollow spheres, and Cr2 O3 nanoparticle-functionalized WO3 nanorods [38, 43, 98]. Therefore, the baseline drift of the gas sensor still needs to be done more researches to improve the stabilities of the gas-sensing properties. As we discussed above, the redox reaction between the pre-adsorbed oxygen ions and the targeted gas molecules (such as hydrogen gas) is mainly responsible for the response process of the metal oxide-based gas sensor. For example, in the case of the hydrogen gas-sensing process, the hydrogen gas firstly adsorbed on the surface of the metal oxide, then diffused along the nanomaterial and finally diffused into the heterojunctions. Meanwhile, the hydrogen gas would also interact with the preadsorbed oxygen ions, resulting in the release of the electrons and the response of the metal oxide to the hydrogen gas. During the recovery process of the metal oxidebased gas sensor, the unreacted hydrogen molecules will be desorbed from the surface of the metal oxide or the region of the heterojunctions and the oxygen molecules will be adsorbed on the active site and diffuse to the heterojunctions. However, the unreacted gas molecules might not be desorbed completely and the adsorbed oxygen ions also need a long time to totally occupy the active sites especially the ones near the heterojunctions, which will have a significant effect on the resistance of the sensing material and result in the drift in the baseline of the detected electronic property of the gas sensor. Another important factor for the baseline drift is that the active sites in the metal oxide surface are always not triggered completely in the first several response circles, which will lead to the difference in the occupied concentration of the active sites and the variation in the original resistance of the gas sensor in every response process. To overcome this baseline drift of the gas sensor, the assembly of the gas sensor based on the paralleled ultra-long metal oxide nanoribbons or

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nanowires which were decorated with the uniform metal oxide nanoparticles might be an effective method. Meanwhile, the activation of the gas sensor with the highconcentration targeted gas is also a possible way to totally stimulate the active sites to interact with the gas molecules. But there were so few references reporting the method to steady the baseline of the electronic properties of the gas sensors that more investigations should be done to solve this issue.

4.3 Exploration of the Sensor Based on the Heterojunctions Composed of Three or More Nanostructured Metal Oxides Nowadays, most of the reported heterojunction-based gas sensors are mainly composed of two nanostructured metal oxides, whose gas-sensing performances have been really improved compared with the pure metal oxide-based gas sensors. The improvement in the gas-sensing performance of metal oxide heterojunction-based gas sensor is mainly owned by the excellent properties of the nanostructured metal oxides and the heterojunctions between them. Normally, at least one of two nanostructured metal oxides is sensitive to the detected gas. If more nanostructured metal oxides are combined to form a new type of heterojunctions composed of ternary composites, then the performance of gas sensor will be further improved. For example, Drmosh et al. have prepared a ternary rGO/ZnO/Pt system through a pulsed laser ablation in liquid (PLAL) and a direct current (DC) sputtering to detect the hydrogen gas [181]. Their results showed that the Pt-loaded rGO/ZnO gas sensor exhibited a higher hydrogen gas sensor response than those of the pure ZnO and the rGO/ZnO nanocomposite systems. Apart from the heterojunctions between the rGO and ZnO nanoparticles, the improved gas-sensing performance of the ternary rGO/ZnO/Pt system was also attributed to the high surface area of the rGO and the fast spill-over effect of the uniformly coated Pt nanoparticles. Therefore, the formation of the heterojunctions with three kinds of materials could be a helpful method to further improve the gas-sensing properties of the heterojunctions-based gas sensors. Similar results were also reported in the studies on the Pt–ZnO–In2 O3 nanofibers, Ptactivated TiO2 –MoS2 nanocomposites, and Pt-functionalized SnO2 –ZnO core-shell nanowires [182–184]. However, it is indeed a huge challenge for the combination of many kinds of nanostructured metal oxides, other semiconductors, and the novel nanoparticles. It should be noted that the entire component in the ternary composite should be kept its original properties. How to keep the balance of the three materials and how to develop the advantages of each materials together to improve the gas-sensing performance of the gas sensor are not completely clear, to which more attention should be paid. Adding some corresponding catalysts under these properties will also have a good improvement in its stability and the selectivity, then how to choose the catalysts is also a complex and unsolved problem. In addition, most of the published articles mainly

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focused on the noble metal nanoparticles-decorated metal oxide heterojunctions. There were few references that reported the gas-sensing properties of the small metal oxide nanoparticles-decorated heterojunctions, which might also be a possible way to improve the gas-sensing performance of the ternary composite.

4.4 The Combination of Gas Sensors and Intellectual IC Systems The excellent gas-sensing performances of the metal oxide heterojunctions make them be the great potentials to assemble the high-performance gas sensors. In a specific application region, there are sometimes existing more than one kind of gases. For example, the CO or the CH4 in the environment might have a negative effect on the hydrogen gas-sensing performance of the metal oxide [185]. Then the accurate detection of the targeted gas remains a challenge to the assembled gas sensors based on the single-type metal oxide heterojunctions. The assembly of the E-nose composed of an array of different sensors has been reported to be a promising route to solve this confusion and help the human being to estimate the concentration of the target gas as well as identify the gas in the real mixture. Each gas sensor in the electronic nose (E-nose) should exhibit outstanding gas-sensing properties to certain gas molecules and could main its stability in the integrated IC system. Gebicki et al. have assembled an E-nose prototype comprised of six TGS-type sensors and one PID-type sensor [186]. Their research indicated a level of 75–80% for odor intensity and 57–73% for hedonic tone. And a novel device with a gas-selective thermoelectric array sensor was also successfully assembled by Shin et al. to be the micro-machined sensor to detect the H2 , CO, and CH4 in breath. The authors assembled the device with three different combustion catalysts of Pd/Al2 O3 , Pt, Pd, Au/Co3 O4 , and Pt/Al2 O3 catalysts, which were heated to be 320 °C, 200 °C, and 125 °C, respectively. Due to the selective catalyst performances of the novel metals to different gases, the sensors array composed of Pt/Al2 O3 , Au/Co3 O4 , and Pd/Al2 O3 catalysts was effective to detect the H2 [187]. The researches listed above reveals the E-nose could be an effective device to monitor and detect the gases. Then the selection of the gas sensor and the integration of the selected sensors are the two important factors affecting the eventual gassensing performance of the E-nose. By now, we have not found the clear principles to assemble the E-nose. But what we should fully understand is that the arrangement of the separated gas sensor and the following data processing arithmetic are equally important to obtain the high-performance E-nose. Due to the existence of the array of gas sensors, the E-nose could record the multi-channel data at the same time. The data processing arithmetic should be able to identify the data from the certain sensor in the E-nose and present the distinct analysis results. In certain situations, the sensing data of the gas sensor should be further trained by the modern artificial intelligent (AI) algorithms. Therefore, the targeted gas could be effectively detected.

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Also, the training data is of great importance to the accuracy of the E-nose, on which more attention should also be focused.

5 Conclusion The formation of the heterojunctions in the sensing materials has a great influence on the gas-sensing performance of the metal oxides. Various methods have been employed to synthesize the n–p, n–n, p–n, p–p, n–p–n, and p–n–p heterojunctions, such as the common hydrothermal process, electrospinning, and thermal oxidation, which was summarized in our paper to show the strategies to establish the heterojunctions. And the gas-sensing performance of the widely investigated metal oxides, mainly including various nanomaterials based on ZnO, SnO2 , TiO2 , WO3 , In2 O3 , Fe2 O3 , MoO3 , Co3 O4 , and CuO, were also discussed in our paper. The bending in energy bands of the metal oxides and the formation of the potential barriers in the heterojunctions were found to be the essential factor that should be responsible for their enhanced gas-sensing performances. And the long-term stability shift to the baseline of the sensor and the methods to further modify the gas-sensing performances of the heterojunctions should be paid more attention to improve their application in the high-performance gas sensors. Acknowledgements Shulin Yang and Zhao Wang contributed equally to this work. This work was financially supported by the National Natural Science Foundation of China (Grant no. 51802109 and 51972102), the Science and Technology Research Project for Young Professionals of Education Department of Hubei Province (Grant no. Q20182903) and the ChuTian Scholars Program of Hubei Province.

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Chapter 3

Chemiresistors and Their Microfabrication Vishal Baloria, Chandra Shekhar Prajapati, Navakanta Bhat, and Govind Gupta

1 Introduction Rapid industrialization during the past few decades has resulted in the emission of gases that pollute the environment and pose a risk to humanity. Therefore, there is a significant concern for the protection of the situation, in particular, the air we breathe in. People, especially in urban areas, are exposed to a high quantity of toxic and harmful gases such as Cl2 , NO2 , NO, CO, CO2 , NH3 , H2 S, and SO2 [1]. The threshold limit for most of these gases is in ppm [2] and our target is to detect them below this limit as a safety concern. Thus, there is a massive demand for the monitoring of these hazardous gases, and hence, the need for gas sensors. This has provided a tremendous impetus to the development of gas sensors. In general, a gas sensor consists of two main functions [3]. The first one being Receptor function, which recognizes a chemical substance present at the surface of the semiconductor, and the second one being Transducer function, which converts the chemical signal on the semiconductor surface into an electric output signal. Based on their detection principle, gas sensors can be classified among various types such as electrical, optical, and mass sensitive. Among the electrical ones, Chemiresistive gas sensors (CGS) are widely investigated because of their simplicity, ease of production, low cost, and capability of detecting a large number of toxic and volatile gases under different conditions. Moreover, such sensors have both receptor V. Baloria (B) · G. Gupta Sensor Devices Metrology Section, Environmental Sciences and Biomedical Metrology Division, CSIR-National Physical Laboratory, New Delhi 110012, India e-mail: [email protected] C. S. Prajapati · N. Bhat Centre for Nano Science and Engineering, Indian Institute of Science, Bengaluru 560012, India N. Bhat Department of Electrical Communication Engineering, Indian Institute of Science, Bengaluru, India © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_3

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and transducer functions in the same device [4]. The power consumption in CGS is generally high due to their operation at higher temperatures and there have been continuous efforts to minimize it by microfabricating such sensors [5–8]. Microfabrication is the process of fabricating miniaturized structures having a scale in micrometers or lower. It can also be defined as a collection of technologies that are utilized to fabricate micron size devices. One of the most common micromachined components in CGS is the microheater [9]. Such microheaters consist of a thermally isolated support platform which is fabricated using a combination of microfabrication processes. Microfabrication results in low power consumption and rapid sensor heating to attain the desired operating temperature due to lower thermal mass and fast response time of microheater [10]. In addition, mass production owing to batch processing results in cost reduction.

2 Chemiresistive Gas Sensors A CGS is a transducer which detects gas molecules and converts this signal into a measurable change either in resistance or current whichever is recorded. In this chapter, for the sake of uniformity, we will be talking in terms of current. The measured current in between the sensor electrodes of a CGS changes drastically (either increases or decreases) when it is exposed to the molecules of analyte gas. An increase or decrease in current depends on the type of sensor material (either n- or p-type) and the nature of gas (reducing or oxidizing). During sensing material optimization process in CGS, electrode deposited thin films of a particular sensing material on a substrate are generally placed on an external heater in a static or a dynamic gas sensing setup to check their sensing response. A schematic of the same along with a typical response curve showing a variation of current with time on exposure to and withdrawal of analyte gas is shown in Figs. 1 and 2, respectively.

2.1 Working of a Chemiresistive Gas Sensors The most accepted working principle of CGS is adsorption and desorption of gas. In general, when such gas sensors operate at a particular temperature, oxygen from air gets adsorbed on the surface of metal oxides’ grains and results in the formation of surface states in the following manner (Eqs. 1–4) [11]: O2 gas → O2 ads

(1)

O2 gas + e− → O− 2 ads

(2)

− − O− 2 ads + e → 2Oads

(3)

3 Chemiresistors and Their Microfabrication

Fig. 1 Schematic of a chemiresistive gas sensing setup

Fig. 2 Typical response curve of a chemiresistive gas sensor

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(4)

Here subscript (ads) denotes adsorbed oxygen at the surface. The above mentioned three ionic forms of oxygen are a function of operating temperature [12]. For temperature 200°C), which is one of the biggest disadvantages for this type of gas sensing device and it limits their applications as a gas sensor because it can be led to poor selectivity and low stability [4, 5]. Although one of the biggest advantages of the conductive polymer (CP) based sensors is the low operating temperature close to room temperature (RT), the sensitivity of CP-based gas sensors is low and requires improvement [6]. In fact, other gas sensing parameters need to be taken into account (e.g., response/recovery time and even sensitivity). There have been serious efforts to overcome these problems [7–9]. CPs contain a smooth molecular surface with a lot of properties, two of which are useful measurement with improved permissibility and low response/recovery time. The surface area and morphology of CPs can be changed via the electrical conductivity by adding the doping in polymers [8]. Moreover, doping can affect the rate of protonation as it increases the concentration of active sites for the gas molecule, and also the gas sensitivity [7]. Another method to improve the gas sensing performance of CP is the formation of an inorganic semiconductor heterostructure [9]. Nevertheless, this method is a major challenge in industrialization, which is nothing more than a precise combination of two substances with different stages of organic and inorganic matter [10]. One of the key and important factors for gas sensitivity of any material is the specific surface area (SSA). This factor is defined as the total surface area of a material per unit of mass or volume [11]. The enhancement of this factor for any nanomaterials facilitates the adsorption of gas molecules in its structures, which resulted to enhancing the gas sensitivity [12]. In front of all these materials, MOFs can be introduced, which was formed by the coordination of metal ions and organic bridging ligands [13–15]. These novel classes of crystalline materials have a large surface area, ultrahigh porosity, structure tunability, and diverse structures [3, 16]. With these interesting features, MOFs have great potential in a plenty of various fields, such as gas storage and separation, batteries, supercapacitor, energy applications, catalysis, chemical sensors, and so on [17–20]. The different applications of MOF composites are summarized in Fig. 1.

2 Structure of MOFs The MOFs are low-density crystalline compounds that consist of two units of metal ion or cluster as a node and organic ligands as a linker. Synthesis of MOFs is usually carried out in the temperature range of 25–220 °C, pressure between atmospheric to 20 atmospheres and pH in the range of 1–10. The cavities formed in this class of

5 Graphene-Metal-Organic Framework Modified Gas Sensor Fig. 1 Presentation of different applications of MOFs

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Gas sensing Water treatment

Catalysis MOFs applications Gas storage

Supercap acitors Drug delivery

nanoporous materials have a certain size and shape distribution, and in this respect, they differ from other porous materials. The final structure and properties of MOFs are strongly dependent on their molecular building units as well as heavily affected by the synthesis condition. Most MOFs have nanometer cavities and are classified as microporous solids. A particular carboxylate-based MOF scaffold can rationally be decomposed into clusters of particular shape and composition of metal-craboxylates (secondary building units, SBUs) interlinked through organic linkers (spacers). Some SBUs, which are usually observed in metal carboxylate MOFs, are shown in Fig. 2. Although increasing the length of the organic binders increases the volume of the cavities formed, in some cases this increase improves the penetration process of the networks.

3 Graphene-MOFs (MOF@G) Today, many kinds of MOFs have been widely utilized in gas sensing areas. Although, the sensing system of MOF-based gas sensors is rather complicated, so further enhancement is required. However, many efforts have been made to improve the performance of these types of sensors so far [22]. For instance, one of the most important of these efforts was the use of graphene for modification of MOFs. Despite all the amazing properties of MOFs, they suffer from some defects such as poor stability, which leads to detraction on their gas sensing performance. The main idea to overcome these problems is to synthesize MOF with other materials to produce new composites, which have better properties than individual MOFs [23– 26]. Graphene-based materials have recently introduced new possible applications because of their unique structure and low toxicity, as well as their excellent electronic, thermal, electrochemical, and mechanical properties [27, 28]. Graphene is

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Fig. 2 Inorganic secondary building units (SBUs) commonly occurring in metal carboxylates include a the square “paddlewheel”, with two terminal ligand sites, b the octahedral “basic zinc acetate” cluster, and c the trigonal prismatic oxo-centered trimer, with three terminal ligand sites. The SBUs are reticulated into metal–organic frameworks by linking the carboxylate carbons with organic units, but may also be linked by replacement of the terminal ligands. Examples of organic SBUs include the conjugate bases of d square tetrakis (4-carboxyphenyl) porphyrin, e tetrahedral adamantane-1,3,5,7-tetracarboxylic acid, and f trigonal 1,3,5-tris(4-carboxyphenyl)benzene. Metals are shown as blue spheres, carbon as black spheres, oxygen as red spheres, and nitrogen as green spheres [21] (Reproduced with permission from J. L. C. Rowsell & O. M. Yaghi, Metal–organic frameworks: a new class of porous materials, Microporous and Mesoporous Materials 73 (2004) 3–14)

a two-dimensional (2D) sheet of carbon atoms in a hexagonal configuration (honeycomb) with unique properties such as a high surface area (~2630 m2 g−1 ), high Young’s modulus (~1 TPa), good thermal conductivity (~5000 W m−1 K−1 ), excellent charge carrier mobility (2 × 105 cm2 V−1 s−1 ) due to π-conjugation structure, and high optical transparency (~97.7%) [28, 29]. There are some reasons that graphene-based nanocomposites have the significant roles in different devices and applications. It could improve different applications of nanostructures such as gas sensing, photocatalytic, and solar cell performance [30–41]. There are several reasons that show the benefit of MOF@G nanocomposites-based gas sensors that can be listed as follow: (i) Large theoretical specific surface area In single-layer graphene sheet, all of the atoms can be considered as surface atoms. In this situation, all of them can be adsorbing gas molecules since this material can be providing the largest sensing area per unit volume [42, 43].

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(ii) Varying in interaction between graphene sheets and adsorbents The interaction between graphene sheets and adsorbates could be varied by different interactions such as a weak van der Waals interaction and a strong covalent bonding. The electronic system of graphene could be perturbed by such different interactions and this change in the electronic system can be readily monitored by convenient electronic methods [43]. (iii) High carrier mobility at room temperature Graphene exhibits remarkable high carrier mobility at room temperature (2 × 105 cm2 V−1 s−1 ) with a carrier density of near 1012 cm−2 , since it is more conductive than silver and it has the lowest resistivity at room temperature corresponding to a resistivity of 10−6  [2, 44, 45]. This is well known that carrier mobility plays an important role in gas sensors [46]. For this reason, the graphene has been introduced as a highly sensitive chemical sensor because of its remarkable carrier mobility, which has turned it as a unique and attractive sensing material for gas sensing application [46]. (iv) Low electrical noise Graphene has a high-quality crystal lattice along with its 2D structure that results in low electrical noise structures. Since this structure making it capable of screening more charge fluctuation, which a small number of extra electrons, as well as gas adsorption even a molecule level, can cause a noticeable change in the conductance or resistance of a graphene sheet [47, 48]. (v) Large scale synthesizing capability Chemically converted graphene materials like reduced rGO can be synthesized at relatively low costs on a large scale. Furthermore, rGO sheets are able to be processed or assembled into ultrathin sensing layers by a variety of deposition or printing techniques, which can simplify the gas sensors fabrication procedure [49]. More recently, many reports have confirmed that the introduction of graphenebased materials into other materials can lead to surprising improvements in electronic conductivity and stability [3]. According to all of the benefits mentioned above for graphene, it can be found that graphene derivatives are appropriate for hybridizing/composition with MOFs to the synthesis of MOF@Gs [50]. Additionally, graphene-based materials could enhance the properties of the composites in many ways such as [50] (i) build a conductive bridge for electron transfer, (ii) diminish aggregation, (iii) increase intermediate species, (iv) structure distortion, (v) enhance dispersion, (vi) synergistic effect, (vii) increase available active sites, (viii) enlarge specific surface area, (ix) modify structure, and (x) accelerate electron transfer. According to recent research, MOF@G composites bring unique performance in many applications due to enabling the composites to possess improved electrical conductivity, increase selectivity, and improve stability [25, 26, 51–53].

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4 Basic Principles and Challenges of Gas Sensors The sensing mechanism of gas sensors is mainly attributed to the carriers (electrons or holes in MOS based gas sensors, protonation or deprotonation in CP) transfer due to surface reactions or adsorptions of analyst gas molecules on the sensing materials [54]. Therefore these carriers transfer changes the resistance (or conductance) in gas sensors. This should be noted that the origin of the change in the electrical resistance in chemiresistive sensors depends on the type of semiconducting material (n or p). Although MOSs have relatively higher gas response than other materials, they have some disadvantages such as low sensitivity, poor selectivity, and high operating temperature. On the other hand, carbon-based gas sensing materials can be operated at the room temperature and have a high surface area, but they have also big disadvantages such as low response and low reproducibility [55]. We could see in the literature and reported researches that, these disadvantages of the old gas sensing designs could be overcome by MOFs-based gas sensors and their derivatives with other materials. Among different heterostructures of MOFs with other materials, MOF@G nanocomposites are the best candidate for gas sensing applications due to several reasons, which will be addressed later in the chapter.

4.1 Principle of Charge Transfer in MOF@Gs Configuration There are three mechanisms for charge transfer between graphene sheets and MOFs composites [56]: (i) Porosity-involved charge transfer mechanism This mechanism could be defined as a charge transfer through a space that involves an electron/hole transfer, in which the porosity of the MOF facilitates the charge transport behavior (Fig. 3a). (ii) Electro delocalization mechanism The redox-active of MOF is the principle of this mechanism. In fact, based on this mechanism charge transports via a bond is highly delocalized systems. Graphene as a conductor plays as the redox-active metal centers role in such configuration. It should be taken into account that organic ligand helps to charge delocalization, and the redoxinactive metal only plays as structural support. Usually, charge transfer mediated by π-stacking in metal-organic interface, and in some cases, the conducting polymer features π–π stacking. In MOF@Gs, such as [Ni3 (HITP)2 ] and [Ni3 (BHT)2 ], the electron delocalization through the 2D sheets involves strong π-conjugation [57]. Therefore, the significant overlap of the metal and ligand frontier orbital facilitates conductivity (Fig. 3b). (iii) Donor–acceptor mechanism

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Fig. 3 The possible charge transfer mechanisms in MOF@Gs, a porosity-involved charge transfer mechanism, b electrode localization mechanism, and c a donor–acceptor mechanism [56] (Reproduced with permission from D. Durgalakshmi, et al., Graphene–Metal–organic framework-modified electrochemical sensors, graphene-based electrochemical sensors for biomolecules, Chapter 11, Elsevier Publisher (2019) 275–296). In this figure VB, CB, LUMO, HOMO, and EF are valence band, conduction band, lowest unoccupied molecular orbital, highest unoccupied molecular orbital, Fermi energy, respectively

This mechanism is based on charge transport by the donor-acceptor mechanism. In this mechanism, the degree of charge transfer between the donor (D) and the acceptor (A) is generated by the electronic coupling between the highest occupied molecular orbital (HOMO) of donor and the lowest unoccupied molecular orbit (LUMO) of the acceptor. Such charge transfer causes a significant ground state partial ionicity (Fig. 3C). Therefore, in such configuration, lattice energies, bond lengths, optical band gap, and conductivities of the metal-organic salts will determine the degree of charge transfer. In addition to the above mechanisms, it should be considered that the mechanisms for the measurement of composite layers are driven by the main components. In the case of MOF composites, the main components are selected from metal oxides, carbon composites, nitrides, etc. The n-type mechanism can be explained in n-type materials susceptible to MOS nanostructures by the charge-space layer state. When the carbon-based materials are exposed to the gas, some transient bonds may be formed

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due to their electronic charge distribution, resulting in the donation/deprivation of electrons (inside the carbon) or change the electron hopping currents. In addition, the metal nodes and active functional organic groups in MOFs can act as the effective adsorption sites for the gas sensing. The redox reaction of transition metals in MOFs and volume change of MOFs during exposure to the gas can also affect the conductivity of MOFs [58, 59].

5 Synthesis Methods of MOF@G Usually, for the synthesis of MOF@G nanocomposites, the graphene is dispersed directly within the reaction mixture of the MOF soluble precursors, at the proper temperature conditions needed for the synthesis of the MOF. The weak dispersive interactions between the organic linker molecules or the formed nuclei of the MOF nanocrystallites and graphene sheets lead to self-assembly and buildup of the MOF atop the graphene sheets. This interaction is theoretically weaker compared to the direct covalent or coordination bonding in other methods utilizing graphene oxide (GO), however, it is more potentially promising due to retaining the pristine high electrical conductivity of graphene as compared Therefore, this should be noted that the synthesis conditions might strongly affect the nature of the interactions between the MOFs and the graphene in the MOF@G nanocomposites. Consequently, different synthesis strategies have been developed toward achieving composites with better properties. In this section, some of the synthesis techniques for the synthesis of MOF@G nanostructures are presented.

5.1 Electrophoretic Deposition Electrophoretic deposition (EPD) is a low-cost technique used to grow nanostructures on conductive surfaces. This technique can be chosen for fabricating different forms of MOF@G nanocomposites with complex multifunctional nanoarchitectures, which could contribute to the selective extraction of gas molecules based on using the affinity between gas molecules and metal ions [56]. In this method, the process is achieved by the movement of charged particles in a dispersed nonpolar solvent toward the conductive electrode under an applied electric field. I. Hod et al. reported the ability of the EPD method to deposit of four representatives MOFs: the Zr-based materials, NU-1000, and UiO-66; the iconic Cu-based MOF, HKUST-1; and the aluminum-based form of MIL-53 on FTO substrate [60]. This deposition technique can produce a varying range of morphologies with high surface porosities, highly homogeneous, and good chemical stability. These properties are highly affected by deposition conditions such as applied electric field strength, type of solvents, and so on.

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5.2 In situ Syntheis Method In this method, the graphene is directly mixed with the soluble precursors of the MOF materials at the room temperature conditions, where uniformal suspension of the graphene is achieved through ultrasonication. As the reaction proceeds for the MOF synthesis, favorable dispersive interactions between the early formed nuclei of the MOF crystallites and the suspended graphene occurs. Such interactions can lead to self-assembly process where the MOF crystallites are uniformly deposited atop graphene sheets. Recently, Hassan et al. used such a method to synthesize monolithic composite of graphene and the Cu-based MOF (HKUST-1), which the SEM image of this composites is shown in Fig. 4 [61]. The ability to attain monolithic, moldable, conductive, and microporous composite material through this synthetic strategy is promising for applications in devices based on MOFs. The ability to fine-tune the composition of this type of composites, in terms of the MOF-to-G ratio, can then produce a range of composites with variable surface area and electrical conductivity (Fig. 5). One major strength point of this synthetic strategy is the flexibility in terms of the MOF composition and topology, where another MOF, the Zr-based UiO-66-NH2 MOF was also successfully deposited on G utilizing this in situ synthesis [62]. Key to success of this synthetic approach is the utilization of mechanical stirring of the dispersed G flakes and the MOF precursors’ solution, to avoid phase segregation and to induce the hetero nucleation on the G surface. The UiO-66-NH2@G conductivity was measured using two different techniques, the conductive atomic force microscopy (C-AFM) and the four-probe method (Fig. 6). The conductivity of the UiO-66-NH2 @G was found to be 11.8 S m−1 , as compared to 4-probe measurement on powder sample showing conductivity of 6.6 × 10−4 S m−1 .

Fig. 4 a The synthesis scheme for the monolithic, moldable form of HKUST-1@G (showing a molded pellet) and b SEM image of the HKUST-1@G showing the distinctive continuum monolithic morphology of the MOF, covering the G flakes (some flakes were made visible by the sample grinding) [61] (Reproduced with permission from M. H. Hassan, et al.; Electrically Conductive, Monolithic Metal–Organic Framework Graphene (MOF@G) Composite Coatings, ACS Appl. Mater. Interfaces 2019 116 6442–6447)

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Fig. 5 a–d N2 sorption isotherms for the monolithic HKUST-1 and its composites with G (G wt% indicated within each isotherm), e showing controllable loading of G within the composites, but retaining the overall MOF microporosity and pore size distribution [61] (Reproduced with permission from M. H. Hassan, et al.; Electrically Conductive, Monolithic Metal–Organic Framework Graphene (MOF@G) Composite Coatings, ACS Appl. Mater. Interfaces 2019 116 6442–6447)

Fig. 6 a, b C-AFM measurement of electrical conductivity of the UiO-66-NH2 @G showing a conductivity of 11.8 S m−1 , as compared to c 4-probe measurement on powder sample showing conductivity of 6.6 × 10−4 S m−1 [62] (Reproduced with permission from M. H. Hassan, et al.; Probing the conductivity of metal-organic framework-graphene nanocomposite. Materials Letters, 249 (2019) 13–16)

This method clearly demonstrated the benefits of merging the desirable electrical conductivity of G and the enhanced microporosity of the MOF, where the minimal effect on the surface area was attained by efficient compounding of the two. The UiO-66-NH2 @G composite containing 37 wt% G demonstrated a surface area of 1103 m2 /g, significantly higher than the surface area of G used in this study

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(450 m2 /g), while not significantly diminished as compared to the pristine MOF showing 1256 m2 /g. This synthetic method has several advantages include being (i) versatile pathway to induce electrical conductivity while retaining the pristine MOF’s chemical composition and topology, (ii) can produce tunable surface area-conductivity composites by adjusting the MOF-to-G ratio in the synthesis, (iii) retains the intrinsic properties of the graphene (most importantly its enhanced electrical conductivity) and the MOF (especially pore size distribution, cage functionality, and surface area).

5.3 Microwave-Hydrothermal Method To date, microwave-hydrothermal method or hybrid hydrothermal synthesis as a facile and one-pot synthesis technique has been widely used for the fabrication of high crystalline nature and high purity MOF@G composite materials [63, 64]. In this method, the nanomaterial synthesis is greatly dependent on the solubility of the reaction species in a water medium at the desired pressure. The hydrothermal synthesis has been hybridized with microwaves toward increasing the reaction kinetics, which can provide MOF@G nanocomposite materials in a very short time with low power consumption and is environmentally safe. Using organic matter in this technique can produce highly stable nano regime MOF@Gs frameworks. There are some works which reported that synthesizes of MOF@G composite materials such as MIL-100(Fe) or HKUST-1) modified with graphene [63–65].

6 MOF@G Composites for Gas Sensing Application This is should be noted that there are not the giant volume of reports published on gas sensing field using MOF@G material, but this section tries to introduce some of the new application for MOF@G in the gas sensing area.

6.1 MOF@G Composites Based H2 Gas Sensor Hydrogen is a colorless, flammable, tasteless, and odorless gas, which poses several hazards to human safety, from potential explosion and fires when mixed with air to presence as an asphyxiant gas in its pure form. Therefore, the fast detection of this gas has been one of the biggest challenges in the gas sensing field in recent years. Recently, Fardindoost et al. reported research about the application of CoMOFs@GO nanocomposites as a hydrogen gas sensor [66]. However, they used Pt as a catalyst to improve the sensing performance of the nanocomposites. In fact, they tried to create a p–p heterojunction of GO–Co3 O2 in a MOF structure. Because it

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has been proven that such heterojunction could enhance hydrogen sensing [66]. The most important role of the Pt element in this composite was as an active catalyst for selective reaction to hydrogen gas due to GO properties, which can absorb any gas and causes a non-selectivity property for a gas sensor device. In addition, the Pt causes to decrease the working temperature of gas sensing. Figure 7a, b shows the response of Co-MOFs@GO and Pt spattered Co-MOFs@GO composites to 15000 ppm hydrogen gas. The response graph of the sensor to different hydrogen gas concentrations from 700 to 35000 ppm, which is below the threshold value (40000 ppm) and it has been repeated five times during 30 d for each hydrogen concentration, is shown in Fig. 7c. As shown, the response graph presents a linear increase in response by increasing H2 concentration, which could be due to a high specific surface area for gas adsorption by the sensor. Graphene usually absorbs most of the gases, so in order for the sensor to be able to detect hydrogen gas with greater sensitivity, a Pt layer can be used on the composite by a sputtering method, which has a high sensitivity to detect hydrogen gas. The selective response of the /PtCo-MOFs@GO/Pt nanocomposites in exposure to several gases is shown in Fig. 7d. In addition, Fig. 7d shows that the fabricated sensor has a high sensitivity to humidity, which indicates interference of moisture on the prepared sensor is non-ignorable at 15 °C.

Fig. 7 a Responses of Co-MOF@GO, b Pt sputtered Co-MOF@GO, c in several stable cycles to 15000 ppm H2 , responses of Pt sputtered Co-MOF@GO in exposure to different H2 concentrations, and d responses to H2 , CH4 , CO, methanol, acetone, and humidity [66] (Reproduced with permission from S. Fardindoost, et al.; Hydrogen sensing properties of nanocomposite graphene oxide/Co-based metal organic frameworks (Co-MOFs@GO), Nanotechnology 29 (2017) 015501)

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Fig. 8 Schematic illustration of H2 reaction with Pt sputtered Co-MOFs@GO [66] (Reproduced with permission from S. Fardindoost, et al.; Hydrogen sensing properties of nanocomposite graphene oxide/Co-based metal organic frameworks (Co-MOFs@GO), Nanotechnology 29 (2017) 015501)

The hydrogen sensing mechanism according to the MOF structure and Pt role, as well as the gas sensing device, is shown schematically in Fig. 8. This schematic reveals the GO role as a 2D structures with the high surface area of GO as an adsorption site for both the H2 and O2 gases. The electronic conductivity of GO helps the charge carriers to diffuse to the semiconductor component, easily. Furthermore, Co particles have been quite evenly dispersed between GO sheets in 3D MOG@G structure that prevents their accumulation.

6.2 MOF@G Composites Based H2 S Gas Sensor Hydrogen sulfide (H2 S) is a colorless, corrosive, flammable, toxic gas that is naturally released in crude oil, natural gas, or wastewater treatment, which can cause annoying, malignant problems even in the very low concentrations [67]. A Low H2 S concentration (above 2–5 ppm) is very helpful for the human respiratory tract and the exposure above 1000–2000 ppm results in immediate death [68]. Thus, it is a big challenge to continuously monitor such low H2 S concentrations in the industry. One of the MOF@G composites introduced for RT sub-ppb H2 S RT gas sensors is

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SnO2 /rGO/PAni composites [69]. These nanocomposites were made using the in situ polymerization technique on flexible polyethylene terephthalate (PET) substrate with inter-digital electrodes (IDE) (Fig. 9a, b). SnO2/rGO/PAni sensor response was high (23.9–200 ppb for H2 S), while the detection limit was 50 ppb. In addition, the sensor shows great stability and long-term stability (Fig. 9c, d). The mechanism of SnO2 /rGO/PAni assay toward H2 S gas was its heterogeneous nature because of the high surface area of SnO2 /rGO/PAni film, the chemical adsorption of oxygen on the surface of SnO2 hollow spheres. The experimental results indicated that the polymerized SnO2rGO/PAni nanocomposite is tightly wrapped in situ and forms a porous nanostructure. Furthermore, the high surface area of the

Fig. 9 The preparation process of a SnO2 hollow spheres and b in situ polymerized SnO2 /PAni/rGO film sensor. c Response-recovery curves of the SnO2 , SnO2 /PAni, SnO2 /rGO, and in situ polymerized SnO2 /rGO/PAni sensors toward H2 S. d Long-term stability of the SnO2 /rGO/PAni sensor toward 200 ppb, 1 ppm, and 5 ppm H2 S [69] (Reproduced with permission from D. Zhang, et al.; Flexible and highly sensitive H2 S gas sensor based on in situ polymerized SnO2 /rGO/PAni ternary nanocomposite with application in halitosis diagnosis, Sensors & Actuators: B. Chemical 289 (2019) 32–41)

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nanocomposite can invigorate the adsorption and desorption of gas molecules. In addition, the p–n heterogeneity acts in this structure as a signal amplifier, enabling this sensor to reach sub-ppb-level sensitivity for H2 S tracking detection.

6.3 MOF@G Composites Based Ammonia Gas Sensor Ammonia (NH3 ) is also a colorless, pungent gas with high toxicity, and inflammable (approximately 15–28% in air), which is produced a by-product of the chemical engineering processes, fossil fuel combustion, and so on. Furthermore, its low concentrations (50–100 ppm) can cause side effects on the skin and eyes, but inhaling its high concentrations can lead to fluid accumulation in the lungs and pulmonary edema [70]. Therefore, accurate detection of low NH3 concentrations is essential for safety in environmental monitoring or chemical control in medical and health regulation. Y. Yin et al. reported synthesis Cu–BTC on nanocomposite of polypyrrolenanofiber-coated reduced graphene oxide (PPy–rGO) for ammonia detection [71]. Figure 10 shows the SEM and TEM images of different products, which were synthesized in this research. The PPy–rGO nanocomposites were decorated with nanoparticles of Cu–BTC [Cu3 (BTC)2 (H2O)3 , where BTC is short for benzene-1,3,5-tricarboxylate] via a hydrothermal process combined with in situ chemical polymerization, which is a new MOF@G composite for NH3 gas sensing (Fig. 10).

Fig. 10 SEM images of a rGO, b polypyrrole-nanofiber-coated reduced graphene oxide (PPy– rGO), c Cu–BTC, and d Cu–BTC/PPy–rGO; TEM images of e PPy–rGO and f Cu–BTC/PPy– rGO [71] (Reproduced with permission from Y. Yin, et al.; Inducement of nanoscale Cu–BTC on nanocomposite of PPy–rGO and its performance in ammonia sensing, Materials Research Bulletin 99 (2018) 152–160)

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Fig. 11 a Response of the Cu–BTC/PPy–rGO-based sensor to different concentrations of NH3 at room temperature. b Response of the Cu–BTC/PPy–rGO-based sensor to various gases at a fixed concentration of 50 ppm. c Response of the Cu–BTC/PPy–rGO-based sensor to 50 ppm of NH3 for five adsorption and desorption cycles. d Stability of the Cu–BTC/PPy–rGO-based sensor upon exposure to 50 ppm of NH3 for 15 days at room temperature [71] (Reproduced with permission from Y. Yin, et al.; Inducement of nanoscale Cu–BTC on nanocomposite of PPy–rGO and its performance in ammonia sensing, Materials Research Bulletin 99 (2018) 152–160). Here ppm is “parts per million”

As can see in Fig. 11a–d, owing to the synergistic effect between Cu–BTC and PPy–rGO, the MOF@G nanocomposite exhibited a good response to NH3 at RT and fast response and recovery rates, high sensitivity, good reproducibility, and acceptable long-term stability [71]. As a means of sensing the ammonia, the mechanism of the Cu–BTC/PPy–rGO nanocomposite assay is that the nanoscale Cu–BTC quantities allow the gas to be adsorbed because of increasing the surface area and PPy–rGO, as a high gas content, and forms copper nanoparticles and signal amplification due to the unique electrical property. Given the synergistic relationship between copper and BTC and PPy–rGO, the sensor shows a copper-BTC/PPy–rGO nanocomposite-based sensor and offers fast recovery speed, high sensitivity, good reproducibility, and long-term stability.

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6.4 MOF@G Composites Based Acetone Gas Sensor By identifying biomarker gases such as acetone in human exhaled breath, it is understandable to early diagnose human diseases such as diabetes [72]. In addition, acetone gas detection with the lowest detection rate (0.2–0.5 ppm) is an important factor in health care programs. ZnO/S, N:Graphene/Polyaniline Quantum Dots (ZnO/S, N:GQDs PAni) Triplicate nanohybrid are also of the new MOF@G composites presenting propitious candidates for room temperature. An in situ polymerization pathway for the synthesis of this composite was reported by D. Zhang et al. [69, 73]. The composite was also synthesized on a PET substrate. According to Fig. 12a–d, the MOF@G sensor showed a high response (2% at 500 ppb), fast response/recovery time (15/27 s), stable reproducibility, dynamic resistance transfer at 50 ppm of gas acetone, outstanding long-term stability, and ppb-level sensitivity (LoD = 0.1 ppm) to detect the acetone at room temperature. Figure 13 demonstrates the summer mechanism of measuring the acetone gas with this MOF@G.

Fig. 12 a Response of the ZnO/S, N: GQDs/PAni sensor toward acetone at room temperature of 25 °C. b Dynamic response-recovery curves. c The sensor response as a function of gas concentration. d Dynamic resistance transition toward 50 ppm of acetone gas [73] (Reproduced with permission from D. Zhang, et al.; Metal-organic frameworks-derived zinc oxide nanopolyhedra/S,N: Graphene quantum dots/polyaniline ternary nanohybrid for high performance acetone sensing, Sensors & Actuators: B. Chemical 288 (2019) 232–242). Here ppm is “parts per million”

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Fig. 13 a, b Mechanism diagram of the ZnO/S, N: GQDs/PAni nanocomposite toward acetone sensing. c p–n heterojunction in the ZnO/S, N: GQDs/PAni nanocomposite [73] (Reproduced with permission from D. Zhang, et al.; Metal-organic frameworks-derived zinc oxide nanopolyhedra/S,N: Graphene quantum dots/polyaniline ternary nanohybrid for high performance acetone sensing, Sensors & Actuators: B. Chemical 288 (2019) 232–242)

To increase the properties of acetone gas from MOF@G nanocomposites for the active sites and the p–n heterojunction between n-type ZnO and p-type PAni/S, N: GQD and decrease the activation energy and necessary enthalpy, requires the absorption of the acetone gas molecules. D. Ding et al. [74] have used the MOF@G composites as acetone sensors. They mixed a hydrothermal and a simple chemical method to achieve three-dimensional (3D) functional GO hydrogels (FGH) decorated with MOF Zolytic imidazolate

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Fig. 14 a The schematic illustration of preparation procedure of ZIF-67/FGH (functionalized threedimension (3D) GO hydrogels) composites by a one-step self-assembly process. b SEM image of polyhedral ZIF-67 crystals. c SEM image of ZIF-67/FGH composites. The inset is TEM image of ZIF-67/FGH composites [74] (Reproduced with permission from D. Ding, et al.; Chemically functionalized 3D reticular graphene oxide frameworks decorated with MOF-derived Co3 O4 : Towards highly sensitive and selective detection to acetone, Sensors and Actuators B 259 (2018) 289–298)

Framework-67 (ZIF-67) Co3 O4 -derived nanostructures, in which the Co3 O4 nanostructures were uniformly distributed in 3D FGH frameworks [74]. Figure 14a shows a schematic map of ZIF-67/FGH synthetic composites which shows how Co2+ ions are adsorbed on the defected sites of FGH. In fact, what caused the formation of ZIF-67 crystals was the electrostatic bonding of Co2+ to FGH with negatively charged and then organic ligands (2methylimidazole, Hmin). As it is clear from Fig. 14b, ZIF-67 crystals have polygon morphology. Figure 14c shows the 3D FGH frames decorated with ZIF-67 crystals. Column 14 (c) demonstrates a TEM image of ZIF-67/FGH composite structures. As can be clearly observed, the ZIF-67 polygon is engraved by FGH frameworks. Figure 15 shows the gas measurement properties of these composites. Figure 15a indicates the operating temperature of the sensors to find the optimized gas response if exposed to the same 25 ppm acetone concentration. As gas-sensitive materials for acetone, these composites showed a very high response (Rgas /R0 = 81.2) to 50 ppm acetone, which was ∼20 times higher than pristine Co3 O4 film, a short response time and a distinctive choice over other perverse gases. The gas sensing mechanism for Co3 O4 /FGH composites has changed in the electrical conductivity described for different interactions of acetone vapor and sensor surfaces including the adsorption chemistry, surface reaction, and desorption processes. It is known that Co3 O4 is a p-type semiconductor in which the holes carry the main charge. According to the following equation [74]

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Fig. 15 a Histogram plots of the steady-state response of Co3 O4 , Co3 O4 /FGH (functionalized three-dimension (3D) GO hydrogels), FGH to 25 ppm acetone in air at temperature from 190 to 280 °C. b Dynamic response characterization of Co3 O4 , Co3 O4 /FGH, FGH to acetone in air at 250 °C in the concentration range of 1–50 ppm. c The response of Co3 O4 , Co3 O4 /FGH, and FGH to different concentrations of acetone. d Selectivity characteristics of Co3 O4 /FGH toward acetone (25 ppm) with interfering gases of ethanol (100 ppm), carbon dioxide (250 ppm), oxygen (250 ppm), ammonia (100 ppm), and methane (250 ppm). e Dynamic resistivity transient of Co3 O4 /FGH at 25 ppm acetone. f Long-term acetone sensing stability tests of Co3 O4 /FGH in a concentration range of 1–50 ppm [74] (Reproduced with permission from D. Ding, et al.; Chemically functionalized 3D reticular graphene oxide frameworks decorated with MOF-derived Co3 O4 : Towards highly sensitive and selective detection to acetone, Sensors and Actuators B 259 (2018) 289–298)

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O2(ad) + 2e− → 2O− (ad)

(1)

if this p-type semiconductor was in the background of the air, the oxygen molecules would be adsorbed on the surface of nanostructures and ionized by trapping free electrons into oxygen (O− ) ions and creating a layer with a space charge. When a reducing gas (e.g., acetone vapor) is adsorbed on the surface of Co3 O4 , the next reaction shows that the oxygen ions (O adsorption) can interact with the acetone [74] CH3 COCH3 (gas) + 8O− → 3CO2 + 3H2 O + 8e−

(2)

It leads to the production of CO2 , H2 O, and electrons. Eventually, these electrons returned to the conductive band of p-type Co3 O4 to facilitate increased resistance upon exposure to acetone gas, which was consistent with the acetone sensing of Co3 O4 results. The increase in the gas sensing performance of the Co3 O4 /FGH compound toward acetone could be caused by the unique 3D porous structure and modulation of the electrical transport properties of the Co3 O4 /FGH junctions. In addition, a large amount of free gas access to acetone to reach Co3 O4 can easily be provided by the unique porous structures of 3D FGH frameworks, thereby increasing the level of Co3 O4 interaction with acetone.

7 Future Prospects In the past decades, significant studies have been achieved regarding the synthesis and fabrication of nanomaterials for the gas sensing application. However, the main challenge for researchers is the finding of material or structures that can be used for the fabrication of stable, high selective, rapid, and high response gas sensors. Among all solutions for achieving more practical nanomaterials for gas sensing applications, the idea of fabricating a type of composite combining MOFs and other materials especially graphene seems to be the most efficient choice. In this chapter, we have discussed the recent advancements in the field of graphene-based MOFs for gas sensing applications. We discussed the importance, challenges, synthesis methods, and gas sensing application toward the high applicable gas sensors. The main focus of gas sensing is to develop a fast, high sensitive sensing analysis that can be stable with a high detection limit of accuracy for specific gas molecules. In fact, we opted to focus the interest on microporous material which is amenable to modular tuning through judicious selection of the starting building blocks, in this vision for the structure of the chapter, we outlined the current state of the art in composites of microporous solids and graphene, outlining the wide futuristic horizon for similar material which can be made more porous and tunable to maximize specific host-guest interactions. Graphene and its derivate possess excellent properties and with the combination of these materials with MOFs, many defects of MOFs as the gas sensor materials

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can be overcome, which the composites exhibit an improved performance and their recyclability in many applications. Although many works in MOF/graphene-based materials has been achieved, there is still some critical point that needs to be noted: • Trying to study on different MOF for composition with graphene • Study on type of MOF structure toward better employing of the composites • Overcome on limitation is seriously limited to MOF/graphene application that is caused with environmental conditions. • Study on better synthesis methods provides flexible choices for gas sensing applications Although maybe there are still some challenges that need to be overcome, composites of MOF/graphene-based materials lead to superior achievements in gas sensing devices, but in order to meet these criteria, great efforts must be made to the characterization, synthesis of MOF/graphene-based materials in order to acquire reliable gas sensors in the futures. In addition, it is anticipated that more 2D materials such as silicene and boron nitride, which are graphene-like materials as well as 2D TMDs, can improve gas sensing performance of semiconductors in the close future. Acknowledgements R. Yousefi would like to acknowledge I.A.U, Masjed-Soleiman Branch and Shiraz University for their financial support in this research work.

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Chapter 6

Recent Advances on UV-Enhanced Oxide Nanostructures Gas Sensors Nirav Joshi, Vijay K. Tomer, Ritu Malik, and Jing Nie

1 Introduction The industrialization in the last decade is increasing at a rapid rate, and air quality monitoring is a biggest concern for environment that people are facing [1–3]. For the benefit of society, detection of hazardous gases and environmental monitoring is essential. However, the rapid expansions of petrochemical industries have a negative aspect as well, that is, the transmission of gases that pollute the environment and pose a high risk to the living species [4, 5]. Thus, there is a need for reliable gas detectors to monitor the gas concentration in the atmosphere so that timely steps can be taken to control the pollution [6]. Real-time monitoring of such toxic gases in private houses, workplaces, and public places is necessary and to manage such a strong network needs reliable, robust sensors [7, 8]. In this context, chemiresistive gas sensors based on semiconductor metal oxide nanostructures (e.g., ZnO, SnO2 , WO3 , TiO2 , etc.) have been studied extensively for detection of various hazardous gas analytes because of their reasonably good sensitivity, stability, possible miniaturization, and convenience of operation [9–11]. The simple device operation and mechanism of semiconductive thin films were presented by Seyama et al. [12] and still it is used to oxide nanostructures. He used a simple chemo-resistive device to detect propane gas using ZnO thin films; the response was higher than the thermal conductivity detector; however, the operating temperature was 485 °C. Presently, room-temperature gas sensing is now ubiquitously required given the need to observe and regulate the release of flammable gases associated with pollution and power plant processes [13, Author Contributions All the authors have contributed equally to this work N. Joshi (B) São Carlos Institute of Physics, University of São Paulo, CP 369, São Carlos, São Paulo 13560-970, Brazil e-mail: [email protected] V. K. Tomer · R. Malik · J. Nie Berkeley Sensor & Actuator Center (BSAC), University of California, Berkeley, CA 94805, USA © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_6

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14]. In such a case, to improve the quality of the sensor devices in terms of sensitivity, selectivity, and reproducibility, research is performed using functional nanomaterials, mainly derived from Material Science and Technology [15, 16]. Nanostructured zinc oxide (ZnO) and tin oxide (SnO2 ) have been generating tremendous interests due to their wide-bandgap, fundamental significance, and potential applications, especially as gas sensor devices. Nevertheless, the operation temperature is still a challenge. The major limitations of traditional metal oxide-based gas sensors are high operating temperature and selectivity toward target gas which can minimize the lifetime of the sensor and stability [17, 18]. In this way, the UV light illumination is an alternative and efficient way to provide more activation for the chemical reactions to operate the sensor at room temperature. This means that to lower power consumption and the ability to work at much lower temperatures, one of the alternatives is to activate the sensor under light source. Recently, few efforts have been made to achieve selectivity toward target gases by tuning a UV wavelength of excitation. Saura et al. investigated that the selectivity of the sensor can be tuned by UV irradiation. Dravid et al. [19] proposed a UV-enhanced room-temperature mechanism toward H2 gas, where the author explained the interaction of photo-induced holes with adsorbed oxygen species (O2− ), causing the desorption of oxygen from the surface of ZnO. Similarly, [20] demonstrated the ZnO–SnO2 heterojunctions to detect ozone under continuous UV illumination, which showed excellent sensing properties compared to the dark condition. Figure 1 shows the picture of the chemiresistive gas sensor by Taguchi and several research papers cited per year on a light-activated metal oxide gas sensor [21]. To address these shortcomings such as selectivity and operation temperature, functional novel materials such as 2D materials, hierarchical oxide nanostructures, and heterostructures have been paid attention recently due to their outstanding electrical and mechanical properties for gas sensors [22–29]. However, in this chapter, our main emphasize is on zinc oxide nanostructures due to its outstanding popularity, and the main reason is its multifunctionality in various applications ranging from optoelectronics, photocatalysis, to gas sensing, owing to their high mobility of electrons, cost-effective and environmental friendliness [30–32]. Several ZnO nanostructures including nanoparticles, nanotubes, nanorods, nanosheets, nanoflowers, nanowires, 150

Sum of Times Cited per Year

Sum of Times Cited per Year

100

50

0 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019

Year

Fig. 1 a Schematic illustration and picture of Taguchi chemiresistive gas sensor (Image has been reproduced with permission and cited in this chapter) b Research article cited per year on lightactivated metal oxide gas sensor (Web of Science, accessed on 14 August 2019)

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Table 1 Room-temperature gas sensing properties of ZnO nanostructures Nanostructures

Synthesis method

Target gas

Concentration (ppm)

Sensor response

tres /trec

Reference

Nanorods

Hydrothermal

H2 S

Nanowires

Electrospinning

Ethanol

100

~35

~20/–min

[33]

78

9/12 s

Thin films

Spray pyrolysis

H2

[34]

150

63

320/200 s

[35]

Nanorods

Wet chemical route

NH3

200

24.1

239/398 s

[36]

Nano flower

Sol–gel technique

Methanol

100

51.94

14.6/15.3

[37]

Nanowires

Drop-cast

NO2

Coral rock

Solvothermal method

HCHO

32

72/69 s

[38]

23.2

–

[39]

1

20 600

Nanostructure

Sol–gel

Acetaldehyde

100

33

228/14

[40]

Nanorod arrays

Microwave hydrolysis

CO

100

81.1

/2.5 min

[41]

Nanoparticles

Chemical method

Acetone

100

90

67/189 s

[42]

Nanorod

Hydrothermal

Ozone

2.56

26/72 s

[43]

Nanorod

Hydrothermal

LPG

500

60

50–70 s

[44]

Nanowires

Chemical vapor Deposition

NO

10

46

–

[45]

Nanosheets

Hydrothermal method

CH4

50

63.45

21/30 s

[46]

80

and core-shell have been used to detect different types of dangerous gases at room temperature. Table 1 summarizes room-temperature ZnO nanostructures-based gas sensors.

2 UV-Enhanced Gas Sensor and Sensing Mechanism Using Oxide Nanostructures Metal oxide-based traditional gas sensors offer low cost, relatively small size, simple setup, and high sensitivity but selectivity and the high working temperature are still a challenge and also available only with silicon-based platforms. Metal oxide-based gas sensors under the influence of gases will undergo the following changes: 1. Surface conductance changes due to electron transport process 2. Bulk conductance changes as an effect of the ion transport process. When the sensor is in contact with environmental oxygen, it adsorbs on the metal oxide surface. For metal oxide surfaces, these oxygen surface states capture electrons from the conduction band of n-type materials or valence band of p-type materials. The various states of oxygen adsorption are as follows [47]:

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O2gas → O2ads ◦ O2gas + e− → O− 2ads < 100 C ◦ O2gas + e− → O− 2ads < 100 C − − ◦ O2− ads + e → 2Oads 100 − 300 C 2− − ◦ O− ads + e → Oads > 300 C

Two major factors govern the response of a sensor to gas, i.e., a number of active sites available for oxygen to adsorb on the surface and reactivity of the target gas with the sensing element [48–50]. The quantity of adsorbed oxygen depends on the sensor surface area, particle size, and operating temperature. Nanostructured sensing materials led to noteworthy improvements in terms of sensor response/sensitivity due to their high surface area and chemical interaction with intrinsic oxygen vacancies and conduction channels [51–53]. However, discrimination toward specific analytes is still elusive.

3 UV-Enhanced Gas Sensor Mechanism In the gas sensing measurement under UV light illumination, there are various sensing reactions taking place, but the important one is electrons and hole creation by the interactions with photons at the surface of metal oxides. Figure 2 shows the possible gas sensing mechanism based on the surface adsorption theory and the band theory of the sensors under UV light irradiation. It can be seen that oxygen species are adsorbed

Fig. 2 Photo-activated sensing mechanism under UV irradiation. (Image has been reproduced with permission and cited in this chapter)

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on metal oxide surfaces in the form of O2− and O− ; however, a minimal amount of surface interaction happens with these molecules in dark conditions, which results in a slight change in the conductivity (Fig. 2a). UV-LED provides a higher number of active sites on the surface for chemical activity and also increases the number of charge carriers in the conduction band. Therefore, the adsorption capacity is enhanced due to a higher number of electrons (Fig. 2b). When the target gas is exposed on the surface of the material, causing the gas molecules to react with the adsorbed oxygen, conductivity changes due to change in the number of electrons (Fig. 2c). At the end, it recovered entirely and reached its initial state when it comes in contact with oxygen (Fig. 2d). The sensing mechanism under UV light illumination has been demonstrated in several studies but very few reports show the sensing mechanism with clear band bending theory or with the change of Schottky barrier height. Ehsan Espid et al. have reported a detailed review on UV-LED photo-activated chemical gas sensors and they explained the sensing mechanism for n-type and p-type semiconductors for oxidizing and reducing gases under UV illumination [54, 55].

4 UV-Enhanced Gas Sensor Using Oxide Nanostructures In the last few years, oxide nanostructures have been paid attention because of their various applications in photocatalysis, solar cells, and gas sensors [56–58]. These nanostructures have been prepared by numerous chemical and/or physical processes. Controlling the reproductive synthesis of these nanostructures is still challenging to achieve for technological applications. Moreover, most of these techniques not only require specific equipment but also have a constraint in controlling the size, shape, and composition of the nanoparticles [59–61]. To overcome these constraints, wet chemical approach using organic solvents under exclusion of water is being advantageous owing to the role of organic components in the reaction parameters. These organic components work as an oxygen-supplying agent in metal oxide fabrication and strongly influence the size, shape of nanoparticles, and surface properties; and, in specific instances, even composition and crystal structure were also affected. To date, various functional nanomaterials have been used in gas sensor devices, e.g., metal oxides, silicon nanowire field-effect transistors (SiNW FETs), carbon nanotubes (CNT), carbon black-polymer composites, and several others [62–64]. Metal oxides, which rely on high-temperature modulation to gain high sensitivity and selectivity, have been widely used. However, temperature cycling decreases the sensor life and makes the system more complex. These problems may be overcome with UV light irradiation to activate chemical reactions at room temperature, thus allowing for operation at much lower temperatures and with less energy consumption. A few efforts have been made to achieve selectivity toward target gases by tuning the UV wavelength of excitation [19]. Nanostructured zinc oxide (ZnO) has been generating tremendous interests due to its wide-bandgap, fundamental significance, and potential applications, especially as gas sensor devices. Nevertheless,

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Table 2 Summary of ZnO based gas sensing properties under UV light illumination Material

Light source (Wavelength)

Synthesis techniques

Target gas

Sensor response

Reference

ZnO

UV (365 nm)

Atomic layer deposition

NO2

103.8

[65]

ZnO

UV (365 nm)

Hydrothermal

O3

44

[18]

Au-ZnO

UV (365 nm)

Hydrothermal

O3

108

[18]

NiPc-ZnO

UV (365 nm)

Hydrothermal

O3

3.27

[43]

ZnO

UV (365 nm)

Screen-printing

HCHO

1.5

[66]

ZnO

UV (365 nm)

Screen-printing

H2

19%

[19]

Ni doped ZnO

UV (365 nm)

Electrospinning

HCHO

532.7%

[67]

Ag/ZnO

UV (365 nm)

Electrochemical deposition

H2

50%

[68]

ZnO

UV (365 nm)

Carbothermal

Ethanol

424%

[69]

selectivity is still a challenge. Most studies have focused on improving their sensitivity, and fewer efforts are in place to improve their selectivity. Many researchers have demonstrated gas sensing properties under UV illumination to enhance sensitivity and selectivity. Table 2 shows the gas sensing properties of ZnO-based gas sensor under light illumination. For zinc oxide (ZnO)-based gas sensors, the chemisorbed oxygen species are difficult to remove at low temperature; however, under illumination, the photon energy is equal or higher than the bandgap of ZnO (3.37 eV), and as a result, electrons can quickly be excited from valence band to the conduction band and generate photo-induced hole-electron pairs in ZnO. So, due to the built-in electric field, photo-induced electrons diffused in ZnO, while photo-induced holes will react with chemisorbed oxygen, causing the desorption of oxygen from the ZnO surface. As a result, current in ZnO-based sensor increases due to the diminishment of the depletion layer and unpaired electrons in the conduction band. Upon exposure to gas analytes, adsorption–desorption will increase under UV illumination. As a consequence, a strong sensor response was observed with fast recovery under UV illumination at low operating temperatures. Recently, Joshi et al. [18] reported flexible gas sensor made with pure and gold-modified ZnO nanorods on a flexible substrate (biaxially oriented polyethylene terephthalate-BOPET) to detect ozone gas at room temperature under UV light illumination. Figure 3 shows the flexible ZnO and Au-ZnO gas sensor under UV illumination for selective sensing toward ozone gas. The ZnO nanorods were grown in situ using the hydrothermal method onto the flexible BOPET substrate and their structural morphology analyzed using scanning electron microscopy. It shows that ZnO NRs are randomly oriented, with an average diameter varying between 70 and 300 nm with a length of 2 µm. Thin layer of (~10 nm) gold has been deposited using thermal evaporation on prepared ZnO samples. To understand the gas sensing behavior, first, pristine ZnO samples were analyzed under dark and UV illumination

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30 ppb

100 80 60

1 ppm

NH3

10 ppm

CO2

10 ppm

20

10 ppm

40 2000 ppm

Sensor Response(Rg/Ra)

120

CO

NO2

CH2O

0

120

ZnO Au-ZnO @30 ppb under UV-LED

Sensor Response (Rg/Ra)

100

BOPET

O3

80 60 40 20 0 200

250

300

Time (sec)

350

400

Fig. 3 UV-enhanced gold-modified ZnO NRs ozone sensing at ppb level (Adapted with permission from [18] Copyright 2019 Springer)

by exposing 30 ppb of ozone. However, there was no response under dark condition, while UV-LED illumination provides more energy at room temperature to release chemisorbed oxygen species and induces a fast adsorption–desorption process. Sensor properties in terms of selectivity and sensitivity are usually enhanced by the surface modification of noble metals such as Pd, Au, Pt and also by incorporating of dopants, single and binary metal oxides [70–75] and conducting polymers [76–83]. Herein, the ZnO sample has been modified with a ~10 nm layer of Au, which improves charge carrier transport on the ZnO surface and enhances the overall conductivity of the sensing films. The selectivity and sensor response are enhanced for the following possible reasons: (i) on the metal oxide surface, a nano-Schottky barrier formed at the interface is modulated due to gas adsorption, resulting in sensor response improvement and (ii) chemisorbed oxygen increases due to spillover effect and provides more active catalytic sites. Room-temperature sensor response for ozone gas has been observed in the range of 30–570 ppb. It has been noted that the Au-modified ZnO sensor has a higher sensor response than pristine ZnO. Furthermore, Au-modified ZnO exhibited excellent reliability and reproducibility of the sensor under UV light irradiation. Fast response and recovery speed are critical parameters of the sensor; it has been observed that Au-modified ZnO NRs revealed a stable response–recovery behavior. After exposure of 30 ppb ozone, the corresponding response and recovery times were 13 and 29 s, respectively. Selectivity

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histogram (Fig. 3) shows that compared to different toxic gases, Au-modified ZnO NRs showed strong sensor response toward ozone due to the catalytic role of Au in catalytic dissociation of molecular oxygen species and electronic sensitization in reaction kinetics. The complete gas sensing mechanism of oxide nanostructures are still not clear; however, there is a consensus view from previous reports and based on the theory of charge carrier exchange between bulk metal oxides and adsorbed gas species. As discussed above, the gas sensing mechanism, Joshi et al. has explained the detail gas sensing mechanism of Au-modified ZnO nanostructure under UV light which is as follows: At room temperature without UV illumination, O2 − species are chemisorbed on ZnO surface and transferred the electrons from the conduction band. Ionic species of oxygen molecules at room temperature are formed according to the following reaction: O2 (g) → O2 (ads)

(1)

O2 (ads) + e− → O− 2 (ads)

(2)

On the surface of ZnO nanorods, electrons are consumed and form a depletion layer, and as a result, electrical resistance increased. Under the UV light irradiation, a large number of electron-hole pairs were generated due to the higher energy than the ZnO bandgap and these photo-generated electrons and holes recombine and react with oxygen species on the surface of ZnO nanorods and as a result width of the depletion layer is decreased. Contrarily, photo-generated electrons will contribute to diminishing the depletion layer width and electrical resistance. h+ (hυ) + O2− (ads) → O2 (g)

(3)

Upon exposure to oxidizing gas (ozone), gas molecules adsorb on the ZnO surface and release the photo-generated electrons from the surface and increase the depletion layer in ZnO nanorods as shown Fig. 4. The electrical resistance is thus increased because of electrons participating in the below reaction. For gold-modified ZnO nanorods, dissociation of ozone gas enhanced due to the high catalytic activity of gold and ozone gas molecules split over the ZnO surface. In conclusion, the number of electrons increased via the transfer process from gold to the conduction band of ZnO, as consequences sensor response enhanced because of strong chemisorption and dissociation of gas molecules. O3 (g) + e− (hυ) → O2 (des) (hυ) + O− (ads) (hυ)

(4)

6 Recent Advances on UV-Enhanced Oxide … In the dark

hv

Under UV

Electron Hole

151 Oxidizing Gas Under UV

Depletion layer Conducting Channel

Oxidizing Gas

Fig. 4 Schematic illustration of the ozone gas sensing mechanism for the UV-enhanced ZnO nanorods (Adapted with permission from [18] Copyright 2019 Springer)

5 UV-Enhanced Gas Sensor Using Hybrid Materials Recently, the combination of hybrid nanostructures using metal oxide with other nanostructure materials has been used frequently to enhance the sensing performance in terms of selectivity and recovery speed [84–87]. For example, combining metal oxide and conducting polymers such as ZnO-polypyrrole and ZnOpolyaniline exhibits selective sensing toward target gas but their response kinetics remain unchanged. Similarly, heterostructures of combining n- and p-type metal oxides offer a vital path to achieve the desired chemical and physical properties of individual components in one system. Compared to single oxides, it provides more active sites and improved stability at the interface of p/n junction. At the equilibrium, the internal electric field makes more negative charges at the p-type region and positive charges at the n-type region. In this situation, a combination of n-n and n-p metal oxides for gas sensing application can equalize the Fermi level with a wide depletion region to achieve high sensitivity with fast response speed. Similarly, metal oxide/organic semiconductors such as ZnO-metal phthalocyanine have been explored for interface study, solar cell, diode applications but not much for the gas sensing application. Joshi et al. have demonstrated the heterostructures of ZnO nanorods with nickel phthalocyanine which enhances the selectivity for ozone and improves the response speed. Further investigation on the photo-activated gas sensing behavior of the fabricated metal oxide heterostructures was carried out by Luis et al. [20] for ozone detection. He proposed ZnO–SnO2 heterojunctions prepared via a facile hydrothermal technique that is operating under 325 nm UV-LEDs with 200 µW irradiances for ozone gas detection and room temperature. They varied the composition of ZnO and SnO2 and optimized the best possible composition for further sensing measurements. They investigated that the 50Zn50Sn sample shows

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Fig. 5 a Sensor response for 50Zn50Sn material as a function of ozone level under UV illumination at room temperature b Selectivity histogram of 50Zn50Sn material (Adapted with permission from [20] Copyright 2017 Elsevier)

excellent sensitivity toward ozone gas even at a low ppb level (20 ppb) with total reversibility (See Fig. 5a). Nevertheless, the prepared heterojunction demonstrated excellent selectivity toward ozone compared to other toxic gases such as NH3 , CO, and NO2 (Fig. 5b). It is essential to mention that above the threshold limit (120 ppb) ozone gas is dangerous to humankind; therefore, the prepared heterojunctions are a great potential for detection of ozone at a low level at room temperature using UV illumination. Other than these metal oxides, some 2D materials have emerged as candidates for gas sensors using the UV-LED concept. For instance, a single element graphene gas sensor is promising for the discrimination of multiple analytes. However, this remains at a seminal stage as the device performance is limited and misclassification is frequent due to the lack of a viable approach to differentiate between similar device responses. To address these shortcomings, the fabricating heterostructure/heterojunction using different nanostructures is favorable to evolve highly selective and sensitive gas sensors through the synergetic effects of hybrid nanomaterials. They increase the adsorption capability by providing more electrons to the active surface and reduce the rate of recombination. In terms of UV-enhanced hybrid gas sensors, [88] reported UV-activated room-temperature gold-modified rGO/ZnO nanocomposite for hydrogen detection. Herein, they fabricated rGO/ZnO nanosheet with gold modification by pulsed laser ablation and a sputtering method. The chemical elements comprising the Au-decorated rGO/ZnO heterostructure can be seen in XPS spectra (Fig. 6a). The survey spectra reveal the presence of desired elements without impurities. The binding energy of Au 4f is resolved into doublets at binding energies of 84.0 and 87.7 eV in Fig. 6b being attributed to Au 4f7/2 and Au 4f5/2 . As revealed from the SEM micrograph (Fig. 6c) that nanosheets of rGO/ZnO samples are very thin with an oval shape and their magnified area (Fig. 6d) demonstrates the typical dimensions and thickness around 327, 179, and 11 nm, respectively. After the modification of gold (Fig. 6e), a wide variety of sizes of Au/rGO/ZnO nanosheets

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Fig. 6 a XPS survey spectra of Au/rGO/ZnO heterostructure b high-resolution XPS spectra for Au 4f c, d low and high magnification SEM image and dimension analysis of rGO/ZnO heterostructure e, f low and high magnification SEM image and dimension analysis of Au/rGO/ZnO heterostructure g–i sensor response comparison of pure ZnO, rGO/ZnO, and Au/rGO/ZnO sensor under dark and UV irradiation as a function of operating temperature. (Adapted with permission from [88] Copyright 2019 Elsevier)

were observed and their long diagonal, edge, and thickness were estimated as 716, 376, and 11 nm, respectively (Fig. 6f). Figure 6g–i demonstrates the sensing behavior of pristine ZnO, rGO/ZnO nanosheets, and Au-decorated rGO/ZnO heterostructure to 500 ppm of H2 as a function of operating temperatures under dark and UV illumination. It can be seen that compared to pristine ZnO, the sensor response is high, with lower operating temperatures for rGO/ZnO sensors. So, heterostructures decrease the operating temperature and improve the sensitivity. Optimized temperature and sensor response of ZnO nanorods appear as 250 °C and 13%, respectively, while rGO/ZnO’s optimized temperature and sensor response are observed as 150 °C and 35% under UV illumination. Interestingly, operating temperature for gold-modified rGO/ZnO heterostructure is shifted toward RT under UV illumination with higher sensor response with excellent reproducibility than pristine and rGo/ZnO nanosheets.

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6 Future Research Perspective and Conclusion Light-activated gas sensors are promising alternatives to achieve desired sensing performance at a lower temperature; however, high power and costly LEDs generally even out some factors of nanostructured gas sensors. Recently there are some efforts made to apply small power LEDs with the gas sensors. Therefore, combining gas sensors and light sources at one place is favorable for next-generation electronic devices. The fundamental gas sensing mechanism of UV-enhanced gas sensors need in-depth research to understand the surface interactions of metal oxides. This will help to get a clear idea that what is the role of each element and how they interact on the surface so accordingly researchers could modify the nanostructure materials to achieve the desired parameters of end-chain products. Besides, some factors are always kept in mind such as humidity, wavelength, temperature, and sensor design, to get desirable gas sensing performance. In this chapter, we demonstrated recent advancement on ZnO and their heterostructures, and possible sensing mechanisms and practical challenges of photo-enhanced gas sensors were discussed. In Summary, UV-enhanced light source could contribute to achieving gas sensing performance at room temperature by producing more photo-induced electron-hole pairs, and soon, they will play a significant role in microelectronic devices. Acknowledgements NJ wants to acknowledge the Brazilian funding agencies: São Paulo Research Foundation-FAPESP (2014/23546-1, 2016/23474-6). RM is thankful to UC Berkeley for providing visiting scholar supports. VKT is thankful to the United States-India Education Foundation (USIEF) for Fulbright-Nehru award (Award No: 2308/FNPDR/2017). JN is grateful to the National Natural Science Foundation of China (61603349). Conflicts of Interest The authors declare no conflict of interest.

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Chapter 7

Hierarchical Oxide Nanostructures-Based Gas Sensor: Recent Advances Sudip K. Sinha, Shashank Poddar, and Subhas Ganguly

1 Introduction In recent times, mankind has put a special emphasis in certain crucial areas, for instance, environment, biomedical, and safety/security which are proved to be increasing threat to our lives. In particular, with the fast expansion of industrial outputs as well as urbanization, air pollution, which largely attributed to vehicle emission and factory release, has turn into an immense risk to mankind and civilization. In this connection, it is important to understand that an effusion of toxic, combustible, and volatile gases could lead to a massive loss of human life and valuable assets. Therefore, it is imperative to understand at present that the instantaneous and faultless identification of those hazardous vapors or gases by the application of chemical or gas sensors is in acute need for the society. It was Seiyama who first discovered that the electrical conductivity of zinc oxide thin films could be used as a sensing layer to detect various gases in diverse challenging atmospheres [1]. The sensing material he fabricated has been successfully operated at 485 °C for the detection of propane gas and the results confirm that it was about ~100 times higher compared to other sensors used in that era. Since then, numerous semiconductor functional materials based on wide band gap MOS has been attracted widespread attention as gas sensors owing to their superior sensitivity, rapid response/recovery time, small detection limit, selectivity, stability, reduced cost, and simplicity in its fabrication [2–4]. Among the vast categories of functional material, nano-structured materials offers exceptional advantages as compared to others since they possess superior mechanical, optoelectronic, electrical, catalytic, and magnetic characteristics. In addition to these features, one of the unique advantages of these classes of materials is their larger surface area per unit volume/mass (surface area/volume). It has been seen in S. K. Sinha (B) · S. Poddar · S. Ganguly Department of Metallurgical and Materials Engineering, NIT Raipur, Raipur 492010, Chhattisgarh, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_7

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Fig. 1 Internet search results based on Scopus database for various metal oxide semiconductors devoted to gas sensing applications. Inset image shows contribution from p-type MOS sensors only

numerous applications that the physicochemical as well as optoelectronic behavior completely changes and a completely different set of mechanism become active when materials are in nanometer scale. As already mentioned, the specific surface area in addition to surface/volume ratio enhances significantly once the materials fall in the nanometer size range. The creation of number of electrons/holes and their electronic mobility are drastically different in semiconductor a nano-structured material which is a direct influence of the size and morphology of these materials [5]. All the abovementioned advantages together with a well crystalline assemblage ease of doping in particular with various noble metals, and simplicity in production, directly influences the development, and subsequent commercial production of various nanocrystalline materials in the field of chemoresistive sensors for toxic and hazardous gas detection. Figure 1 shows the online search outcomes on publications dedicated to “gas sensors” only for the last one decade. According to the web search results obtained from Scopus database on May 2019, a total number of “12407” articles accessible on the oxide semiconductor-based gas sensors (the keywords used for the search were the “SnO2 + gas sensor”, “ZnO + gas sensor”, “NiO + gas sensor”, etc.). It is exciting to note that the number of articles specifically on p-type oxide semiconductors gas sensors (i.e., NiO, CuO, Co3 O4, and Cr2 O3 ,) is only a meager 13.23% (Fig. 1, inset) of these 12407 articles available on web database during that timeframe.

2 Definition of Hierarchical Nanostructures “Hierarchical” materials is considered as a universal term which encompasses both homogeneous or heterogeneous materials system and with a domain size ranging from macroscopic to atomic or molecular sizes [6]. Hierarchy in macroscopic level can be found in various materials system. As a classic example, Eiffel Tower located

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Fig. 2 A collection of 3-D hierarchical nanostructures. a 3-D nanorod–nanoparticle assembly. b 3D nanorod–nanosheet assembly. c 3-D nanorod–nanobrush assembly. d 3-D urchin. e 3-D hollow nanosphere assembly

in Paris, might be mentioned here which is inspired by macroscopic hierarchical structure. Structural hierarchy is often found in various polymers, composite materials, biological, organometallics, fibrous, crystalline, and mesoporous materials for diverse applications [7]. Therefore, structural hierarchy can be a blessing if it can be utilize to produce novel functional materials with superior properties depending on diverse field of engineering applications. In this connection, hierarchical nanostructures have been demonstrated as a superior class of materials for specific applications for example, energy, sensors, environment, biomedical etc. Typically, hierarchical nanostructures are special class of materials which is created by a unique three-dimensional (3-D) self-assembly of basic configuration (nanowires, particles, nanorod, nanotube, nanoplates, or sheets, etc.) of various materials in nano regime. By achieving this special morphology and hierarchical architecture, materials in nanoscale can perform superior gas detection owing to its enhanced surface area and faster rate of gas/vapour diffusion. In addition, the lesser propensity for agglomeration is an extra advantage of this morphology as compared to other low dimensional nanostructures. Figure 2 shows schematic representation of a variety of 3D hierarchical nanostructures used specifically for gas sensing applications.

3 Synthesis Methods for Hierarchical Nanostructures Hierarchy oxide-based nanostructures have been found to control many applications since the last decade or so. This special form of nanoarchitecture is not only applicable in the field of gas sensing but also in catalysis, energy-based materials, electrochemical sensors, lithium ion battery applications, and many others. As already mentioned, nanostructure materials in general offers a superior surface area to volume ratio which shows lot of promise for using this unique materials in the field of gas sensors. However, the efficiency of the gas sensors can be improved manifold by developing MOS with special hierarchical morphology in the nanoscale range [8].

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Several synthesis methods have been opted so far to develop different class of hierarchical oxide nanostructures. Some of the notable methods to synthesize hierarchy nanostructure have been predominantly classified into several categories [8, 9]. The details of these methods will be discussed in subsequent sections. For example, Zhu et al. [10] used hydrothermal method to develop bismuth-doped tin oxide hierarchy nanostructure. In their study, 3-D flower-shaped hierarchy nanostructure was uniformly synthesized by doping Bi ions. Moreover, improved recovery and response time has been observed for HCHO gas sensing sensitivity at a working temperature of 170 °C. Zhang et al. [11] synthesized novel oxide-based hollow hierarchy structure to improve the thermal properties and to maintain the thermal stability at a higher temperature. Here, distribution of platinum nanoparticles was incorporated in between the hierarchy mesoporous silica and titania nanoplatelets. This novel hierarchy hollow oxide structure was prepared by a facile solvothermal technique which acts as nanocatalyst with great thermal stability when dehydrogenated with propane. Huang et al. [12] developed novel zirconia titania gold-based nanocatalyst hollow spheres by applying hydrothermal synthesis method. Three to five nanometers of gold nanoparticles were dispersed into zirconia–titania hollow structure. It has been found that the addition of gold nanoparticles with composite ZrO2 –TiO2 significantly increases the catalytic activity. Wang et al. [13] synthesized porous SnO2 nanotubes by turning SnS2 nanosheets into tin oxide nanotubes with the help thermal oxidation. Moreover, the developed nanotubes were topped with Au nanoparticles. Addition of gold nanoparticles over SnO2 nanotubes improves the gas sensitivity toward acetone gas. Zhang and co-worker [14] spearheaded the work by incorporating quantum dots along with the zinc oxide to improve the properties of gas sensing applications. In their work nanocage, zinc oxide hierarchy structure with quantum dots has been developed resulted in enhancement in their sensitivity to detect the ethanol gas with 25 ppb detection limit. Thus, the large surface area of quantum dots is mainly responsible for excellent gas sensing properties. Zinc oxide (ZnO) is the most studied nanostructure in the past few decades. This metal oxide 3-D hierarchy nanostructure has wide applications as battery electrodes, emitters, lithium-ion batteries, electrochemical sensors in addition to photo catalysis, and solution phase synthesis route I the facile way to synthesis ZnO 3-D hierarchy nanostructure [15]. Figure 3 shows the schematic illustration of some widely used physicochemical methods to produce hierarchical oxide nanostructures.

3.1 Combined Growth Processes (a) Layer-by-layer approach Layer-by-layer assembly technique is a flexible way to make thin films with desired control in film thickness in nanoscale regime. Development of nanofilm with controlled dimension from this technique has great significance when it comes to it potential application. Tailoring the nanofilm thickness with precise uniformity has

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Fig. 3 Popular synthesis method for synthesizing hierarchical oxide nanostructures

illuminated many technological areas especially in the field of engineering [16]. Du et al. [17] fabricated metal oxide hollow structure in a novel way to develop tin oxide hollow spheres with the assistance of silica templates and redox reaction. In order to synthesize tin oxide hollow spheres structure researchers applied the technique of layer-by-layer coating approach. Additionally, thickness and size of the shell has been controlled by this facile approach. Moreover, polyelectrolyte has been tried to transform the surface behavior of silica hollow spheres with the use of electrostatic force within the charged particles. Thus, large surface area of polycrystalline tin oxide hollow structure leads to improvement in the performance of lithium-ion battery. Caruso et al. [18] developed iron oxide-based nanoparticle with the help of the LBL technique. Researchers in their experiment used magnetic iron oxide (Fe3 O4 ) nanoparticle coated with polystyrene (PS) followed by mixing those oxide particles in the aqueous solution containing polyelectrolyte which gets absorbed on the surface of the coated nanoparticle. These colloidal particles in the nanoscale regime could find a lot of potential for diagnostic purposes when an external stimulus (e.g., external magnetic field) is applied. (b) Spray Pyrolysis This process is flexible, high production rate, and cost-effective method to create nanoporous oxide structure. However, this process depends on certain parameters like pyrolysis temperature, flow rate, precursor aerosol solution, and atomizer genre [19]. Although, it is widely known that 3-D hollow hierarchy structure has potential application in gas sensing properties owing to its higher surface area. In order to achieve good gas sensing properties, Cai et al. [20] adapted this technique to synthesize comb-like nonstructural zinc oxide/tin oxide (ZnO/SnO2 ) hollow cylinder.

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Epitaxial growth of SnO2 has been formed over ZnO nanorod assembly which was confirmed through characterization. Furthermore, from an application point of view, rapid response to ethanol gas of 100 ppm at 275 °C has been detected. Singkammo and co-workers [21] prepared a novel structure by applying a unique flame spray technique. Nanoparticles of doped SnO2 by varying Sm2 O3 in different weight percentage by using singe step flame spray pyrolysis method to examine the catalytic property for the detection of ethylene oxide for gas sensing performance.

3.2 Vapor Phase Growth One-dimensional (1-D) functional nanostructural metal oxides like nanorods, nanobelts, nanosheets, nanowires, and nanoribbons are possible to synthesize with the application of vapor phase growth model. Synthetic method to get such oxide nanostructure includes vapor phase growth technique which is classified into certain groups. However, thermal evaporation is one of the vapor growth phase techniques which is an efficient and facile approach to obtain such nanostructure. In this process, vaporization of starting materials has to be carried out at elevated temperatures followed by condensation to get final product [22]. Thus, a different mechanism of vapor phase growth has been studied to achieve 1-D nanostructure. As such three governing mechanisms have been studied by the researchers that include (i) vapor liquid solid (VLS) based (ii) oxide-based (iii) vapor-solid growth (VS) mechanism. Yildirim et al. [23] used vapor liquid solid growth mechanism to develop copper doped zinc oxide nanonails with cap diameter and prismatic shaft diameter of 350 nm and 550 nm respectively which connected along with the 250 nm of the cylindrical neck. Characterization results confirm that the copper ions are evenly distributed over the entire surface of ZnO nanonails. In another study, Yildirim et al. [24] have developed a novel nano architecture based on comb-like morphology. Zinc oxide nanocomb structure was prepared by mixing nanoparticle of zinc oxide with the carbon-based allotrope graphene nanoplatelets. These novel teeth like structure have been fabricated over the Au coated silicon wafer. Yang et al. [25] developed zinc oxide nanowires by facile vapor transport condensation technique. This ZnO nanowire has been synthesized by applying the VLS mechanism. Zhang et al. [26] fabricated ZnO network like nanowires for sensing application. The excellent gas detection properties have been achieved to detect the CO gas at ppm level. Tellurium is one of the elements in the period table that comes under the category of p-type semiconductor. Moreover, it has some special properties to detect toxic gases in a potential way at room temperature. Her et al. [27] adopted the method of vapor transport via vapor-solid growth mechanism. In their study, researchers have grown the nanotubes of tellurium over the nanowires of tin oxide (SnO2 ) which was successfully done by vapor-solid growth mechanism. Decoration of Te nanotubes over tin oxide nanowires leads to an enhancement of its reactive surface exposed in direct contact and leading to its amplification in the gas sensing properties. Dai et al. [28] synthesized ZnO nanorods with tetrapod-like structures and it was all done by vapor-solid growth mechanism.

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2–3 μm of nanorods tetrapod length was achieved with a nucleus size of 70–150 nm. Sun et al. [29] have developed branched SnO2 nanobelts/nanowires over the surface of single crystal tin oxide nanobelts. Moreover, intricacy to make such nanostructure has been reduced down by the utilization of catalyst free (VS) growth mechanism. Vomiero et al. [30] with the support of catalyst used vapor solid growth mechanism and vapor–liquid–solid condensation technique fabricated In2 O3 nanowires over alumina substrate. Pd catalyst was used for the growth of wires through VLS condensation. It was found that at higher operating temperature this nanostructure could possibly detect acetone. Tharsika et al. [31] developed SnO2 -core/ZnO-shell with the application of thermal evaporation technique. It was observed by them that at 400 °C, tin oxide nanowires were capable of sensing ethanol (C2 H5 OH) gas as low as 20 ppm. Sensing response toward ethanol gas enhanced because of tin oxide encircled by zinc oxide nanostructure [31]. Yan et al. [32] designed novel In2 O3 nanotowers by applying chemical vapor deposition (CVD) technique which could be potentially used in functional nanodevices.

3.3 Chemical Techniques (a) Sol-gel Technique: This is also one of the alternative methods to develop hierarchy nanostructure for gas sensing application. Moreover, this method is reliable and does not include any high production cost. The sol-gel method can be used at lower operating temperature to get a quality product [33]. Dong et al. [34] fabricated tin oxide (SnO2 ) interwoven- a tube-like hierarchy nanostructure film. However, they combine two methods, sol-gel, and biotemplate eggshell to obtain such a hierarchy. The sensitivity of nanostructure SnO2 film for LPG and ethanol was found to be good at an operating temperature of 300 °C and 270 °C respectively. As -synthesized structure was calcined to improve the crystallinity in oxide films. Deng and co-workers [35] synthesized copper oxide (CuO) hierarchical flower-like nano-morphology by a simple sol-gel scheme followed by electrospinning. Additionally, CuO nanoflower gas response and recovery time were recorded to be 1.9 and 8.4 s respectively. Besides, the detection limit was significantly enhanced against formaldehyde (HCHO) gas from 1.37 to 50 ppb at 250 °C. Song et al. [36] synthesized hollow SnO2 based porous hierarchy structure via sol-gel process. In their study, researchers utilized natural butterfly wings as templates. 3-D hierarchy structure of butter fly wings contains chitin compound that makes it a potential candidate for functional templates. Moreover, chitin compound can be thermally eliminated by further calcination. Thus, a porous hollow SnO2 hierarchy elevates the sensitivity of oxide nanostructure against detection of ethanol and formaldehyde gas. (b) Electrospinning Technique: Electrospinning is a unique alternative method to create uniform nanostructure. This method is facile and easy to operate. Furthermore, this method depends on the application of high voltage source which is needed to get nanofibres from the prepared solution. Moreover, it works on

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the principle of electrostatic effect [37]. Li et al. [38] used a co-axial electrospinning technique to make core-shell nanofibres of Au doped SnO2 indium oxide. Response and recovery time for acetone sensitivity was improved up to much extent when compared to Au doping into SnO2 –In2 O3 nanofibres only. Thus, promising properties of such core-shell nanofibre structure were confirmed through characterization. M.A Kanjwal et al. [39] developed Co3 O4 and ZnO hierarchical nanoarchitecture by applying combined techniques of electrospinning along with a hydrothermal method. Colloidal solution was made of cobalt acetate and polyvinyl alcohol by mixing with zinc nanoparticles. Addition of zinc nanoparticles in the form of powder increases the surface roughness. Moreover, encourages formation of nanobranches of zinc oxide. (c) Solvothermal Self Assembly Route: This method is one of the simplest chemical routes to synthesis hierarchical nanostructure. Moreover, it can be operational at lower temperature [8]. Solvothermal or hydrothermal route is widely used versatile techniques to develop nanostructure/nanohollow hierarchy structure. Multilayered tin oxide hollow microspheres fabricated along with carbonbased composites in tin tetrachloride (SnCl4 )—sucrose aqueous solution via hydrolysis. Furthermore, heat treatment at 500–600 °C was done to eliminate core polymeric parts (Yang et al.) [40]. Zhang et al. [41] synthesized 3-D ZnO hollow structure microspheres accompanied with self assemble ordered nanorods of 50 nm diameter by their novel approach. In their study, researchers give their novelty in ethylene glycol solution added with some alkali material that plays a vital role to get hollow microspheres of 3-D zinc oxide. Thus, oxide with nanorod assembly provides superior surface area/volume ratio which has a potential application to detect ethanol (C2 H5 OH) and ammonia (NH3 ) gas in an efficient way. Xu et al. [42], made In2 O3 porous flower like nanostructure with ethylene glycol and l-lysine solution. At lower temperature results showed better gas sensing quality for NO2 detection. Li et al. [43] fabricated hierarchical nanoflower metal oxide SnO/SnO2 structure for the detection of formaldehyde vapor. As synthesized composite nanoflower showed outstanding gas sensing response to the environment. Moreover, this structure is highly effective to potentially detect vapor for gas sensing application. This novel structure has been found to detect 80.9 ppm formaldehyde and the detection limit is up to 8.15 ppb at an optimum operational temperature of 120 °C. Guan et al. [44] applied solvothermal technique to develop SnO2 nanostructure architecture by incorporating nitrogen for acetone gas sensor application. This investigation shows that the absorption of nitrogen increases the gas sensitivity with acetone detection limit of 7 ppb.

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4 Development of Chemical Sensors via Hierarchical Oxide Nanostructures Materials in various dimensions and structural morphology, ranging from micro to meso to nanoscale, have been explored in the area of chemical sensing technology. The development of sensing materials based on metal oxide semiconductors is directly associated to various prospects found in multiple dimensions that is obtained in today’s nanoworld. Design of materials by manipulating material up to molecular level is the underlying reason for the development of nanomaterials for innumerable functional applications. Scientists and researchers have used this strategy to obtain highest level of electro-optical properties to be applied in numerous modern devices and instruments. In the same approach, nanomaterials have shown tremendous prospect in design and fabrication of chemo-resistive sensor devices with multiple scope of use. The primary advantage of using materials in nanoscale dimensions arises owing to its miniature size, lighter weight, lesser power consumptions, superior sensitivity, and high level of selectivity. The extraordinary enhancement in sensitivity is primarily credited to the reactions to the outermost surfaces owing to their enormous surface/volume fraction and negligible diameter equivalent to Debye length [45]. Nanostructured materials, especially in one dimension also offers the additional advantages of greater aspect ratio (i.e., size limitation in x, y coordinates), superior crystallinity, and better integration ability. Among the popularly adopted metal oxides, In2 O3 -, SnO2 -, ZnO-, and TiO2 -based nanostuctures shows extreme promises since they possess superior physical, electrical, and chemical properties which are extensively required for efficient detection of chemical species in gaseous form. During the last few decades, tremendous growth has been seen in the field of chemical sensors specially those which are based on 1-D nanostructure, the limitation of selectivity and lack of stable materials for detecting gaseous elements at room temperature with good reproducibility is yet to fully achieve for the scientific community. Low-dimensional hierarchical nanostructures have the advantages of enhanced surface reaction zones with larger number of active sites which directly attributes to its superior sensing properties. For example, urchinlike 1-D ZnO hollow hemispheres [46], hydrothermally prepared 3-D urchin-like α-Fe2 O3 nanostructures [47], Au-modified 3-D flower-like ZnO structures [48] and CuO-decorated ZnO [49] are some recently studied hierarchical nanostructures for the recognition of various chemical and gaseous elements hazardous to the atmosphere. Apart from the surface area factor of the binary oxide based hierarchical nanostructures, various hetero-junction based hybrid nanocomposites (metal oxidemetal oxide combination or doped MOs) are extremely effective from sensitivity point of view by precisely influencing the thickness of surface depletion layers of the reactive elements. Recently, Lou et al. [50] have developed an original hierarchical heterostructure with an arrangement of α-Fe2 O3 nanorods/TiO2 nanofibers which exhibits a branch-like architecture by a facile 2-step process which combines the electrospinning method with conventional hydrothermal process. Figure 4 shows the

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Fig. 4 High-resolution FESEM micrographs of the α-Fe2 O3 /TiO2 heterostructures: a, b panoramic and c, d magnified. Adapted with permission from [50]. Copyright (2013) American Chemical Society

micrographs of the heterostructure aggregates have been depicted in FESEM. The nanofibers are distributed in branchlike manner with average dia. ~600 nm. Higher magnification field Emission scanning electron microscope (FESEM) image exhibits well-aligned protrusions from the main stem (Fig. 4c–d). These α-Fe2 O3 /TiO2 -based branch-like hierarchical nanostructures are excellent candidate materials for chemical sensor applications because of their well aligned multifaceted branch-like architecture and possessing rough surface with innumerable molecular-level active sites for electron–matter transactions as compared to individual nanostructures. The beneficial effect of this specially fabricated nanoarchitecture assembly of αFe2 O3 /TiO2 heterostructures can be seen from their improved trimethylamine (TMA) sensing performances as compared to the pristine counterparts. These branch-like hierarchical nanostructures show a response of 13.9–50 ppm TMA vapour and a remarkably fast response and recovery time of 0.5 s and 1.5 s, respectively. A detailed investigation of the sensing mechanism shows that a combined use of TiO2 nanofibers and α-Fe2 O3 nanorods is the prime cause for the drastic improvement in sensitivity of TMA vapours. The detailed sensing mechanism can is explained in Fig. 5a, b.

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Fig. 5 a, b A representative diagram of sensing mechanism of the bare TiO2 nanofibers and the α-Fe2 O3 /TiO2 hierarchical heterostructure. Adapted with permission from [50]. Copyright (2013) American Chemical Society

5 Controlling Parameters Affecting Gas Sensitivity 5.1 Effect of Nano-building Block Dimensions Applications of engineering materials in nanoscale dimension in various fields of science and technology have been given special impetus all over the world, especially in the twenty-first century. Undoubtedly, most of the discovered materials, either in pure or combined form, have shown tremendous promises in various disciplines. Materials when scale down to nanoscale regime, demonstrate different physical and chemical characteristics as compared to its bulk counterpart. It is not only its changing structure and morphology that decides the end property, but also the underlying physics of electronic transactions plays a controlling role in this aspect. In the field of gas sensors also, suitable utilization of materials in nanoscale dimensions as potential sensing element have found to be extremely beneficial in contrast to the bulk assemblies to enhance its detection ability in a particular environment. However, it has been seen from various research results that nanostructures in all form does not have the same capacity to efficiently participate in the gas sensing reaction or improving its sensitivity. The general characteristics of any nano-structured materials display a packing or agglomeration tendency under any physicochemical circumstances and this primarily arises since there is a usual reduction of the total surface energy in this

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configurations. Nano-structured materials, when transformed into such a dense and impenetrable configuration are found to be unfavourable from gas diffusion standpoint and therefore the inherent sensing performances are found to be substantially lower in all practical applications. To overcome this difficulty, metal oxide-based and/or other materials in such magnitude has been successfully transformed into various 3-D hierarchical assemblies by using nanowires, nanorods, nanosheets, nanobelts, nanospheres, and some other form of nanostructures as basic building blocks [51–53]. These state of the art morphologies contribute in two ways, for example, the enhancement in reactive surface area in addition to formation of a porous morphology with 3-D pores are uniformly distributed through the entire assembly. These two factors are directly associated with the increasing sensitivity (first factor) of the nanostructures as well as drastic improvement in sensor response/recovery times by increase in diffusivity of the identified chemical species in gaseous form. The effect of HCHO sensing properties of Y3+ -doped SnO2 hierarchical flowerlike nanostructures at different operating temperature has been studied by Zhu et al. [54]. Low- and high-resolution SEM images of pure and Y3+ -doped SnO2 has been shown Fig. 6a–d. It is clearly understood by analyzing these nanostructures, produced by a simple one-step hydrothermal process that they are fashioned in 3-D hierarchical

Fig. 6 The FESEM images at low- and high-magnification for a, b pure SnO2 and c, d Y-doped SnO2 . Adapted with permission from [54]. Copyright (2013) Elsevier

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Fig. 7 a Effect of working temperatures on the sensitivity of pure and Y-doped SnO2 nanostructures to 50 ppm HCHO. b The dynamic response behaviour recovery curves in the 1–100 ppm concentration range of HCHO. The dynamic sensing plots to 25 ppm HCHO for c pure SnO2 d and Y-doped SnO2 . e Effect of HCHO concentrations (1–5000 ppm) on sensitivity of pure and Y-doped SnO2 nanostructures. f The calibration curves of the same data in the lower concentration range of 1–200 ppm. Adapted with permission from [54]. Copyright (2013) Elsevier

flower-type morphology. The average diameter of these nanopetals ranges within 1– 1.5 μm and it clearly displays homogeneous and sufficient yield of the as deposited products. They also extensively study the HCHO sensing characteristics for these nanostructures ate different experimental conditions. It can been from their study that the incorporation of trivalent Y3+ cations into the SnO2 host lattice in addition to its unique hierarchical flower-like nanostructure morphology which consists few layers of coarse and porous flakes are the primary reasons to its outstanding improvement in sensitivity. The detailed sensor properties have been adopted in Fig. 7a–f. The effect of gas working temperature (100–300 °C), HCHO concentration (in ppm), and response/recovery behaviour has been explained in this plots for Y3+ -doped SnO2 in comparison to pure SnO2 . The optimum working temperature for pure and Y 3+ -doped hierarchical flower-like SnO2 nanostructures is 180 °C with maximum sensitivity values of 6.7 and 18, respectively. Earlier, Wang et al. [55] have successfully synthesized α-Fe2 O3 hierarchical nanostructures by simple solvothermal method and shown the ethanol sensitivity. The drastic enhancement in the ethanol vapor detection capacity of these novel α-Fe2 O3 nanostructures has been ascribed due to its higher reaction zones on the exposed surfaces between these flower-like hierarchical morphology and host gases. The specific surface area of the specimens were calculated by the Brunauer–Emmett–Teller (BET) method via N2 adsorption desorption isotherm and is found to be 107 m2 g−1 , and this value is way greater than of the compact α-Fe2 O3 structures (17 m2 g−1 ).

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The α-Fe2 O3 hierarchical nanostructures constructed with several nanosheets which create in the fabrication of a loose and porous nanostructure in contrast to the dense nanoparticles of α-Fe2 O3 . Consequently, these distinctive assemblies of nanostructures can effectively deliver an enormous surface/volume ratio, which subsequently contribute to the diffusion of gases or vapours and their mass transport. Finally, a large number of atomic oxygen from ambience is captured on its surface leading to its rapid diffusivity to the external vacancies. The overall benefit arises owing to the fast and efficient capture of electrons from the conduction band of α-Fe2 O3 by this molecular oxygen to convert it into oxygen ions (O− , O2− ). By using a simple microwave-assisted technique Yin et al. [56] have recently synthesized a hierarchical CuO–reduced graphene oxide (rGO) hybrid nanostructure assembly, consists of CuO nanoparticles attached to rGO nanosheets to create a p–p heterojunction structure. This rare combination consists of two different low band gaps (Eg ) reactive species where CuO nanoparticles with dimensions of 4–11 nm are consistently attached on rGO surfaces. The CuO–rGO-based hierarchical chemoresistive sensors demonstrate superior response and selectivity toward H2 S gas at low concentration range (1–10 ppm) while maintaining low operational temperatures (50–150 °C.) The intrinsic reason for the massive improvement in sensitivity lies in the fact that the CuO nanoparticles in such a fine scale (4–11 nm), are uniformly attached on rGO surfaces and immobilize them, thereby developing vast number of heterojunctions.

5.2 Effect of Porosity Within Hierarchical Oxide Nanostructures The first generation metal oxide chemoresistive gas sensors were typically made up of dense and compact layer of sensing materials in order to maintain the design flexibility. Consequently, in this robust design configuration, the gas molecules are unable to disperse into the core of the active sensing layer. Therefore, researchers have tried to develop novel techniques to synthesize a new set of hierarchical nanostructures with the combined effect of enlarged exterior region and a network of porous internal structure systematically distributed within it. For example, Wang et al. [57] have produced a special class of SnO2 microspheres using a single-step hydrothermal technique with varying specific surface region and a uniform distribution of pores with different size range. The as-synthesized porous hierarchical microspheres (Ms) of SnO2 (SnO2 -Ms) are excellent candidate material for sensing acetone, ethanol, and H2 S. The gas sensitivity of these hollow SnO2 hollow microspheres is much superior to the solid nanospheres of same specimen at various operating temperature. Figure 8 illustrates scanning electron microscopy (SEM) and transmission electron microscopy (TEM) representations of various solid and hollow SnO2 microspheres with specific reaction times. SEM images show that the wall thickness and pore size of microspheres depends on reaction conditions. The TEM images

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Fig. 8 SEM and TEM images of the hollow/solid hierarchical SnO2 microspheres (inserted image is in FESEM mode). Adapted with permission from [57]. Copyright (2013) American Chemical Society

in Fig. 8a–c shows points out toward the transformation of the spherical structures form solid to hollow ones. The gas sensing theory in this specially fabricated hierarchical nanostructures is initially dominated by the surface chemical reaction (SCR) controlled mechanism at lower temperatures. However, the sensing mechanism is transformed into a gas diffusion controlled mechanism at relatively higher temperatures. Figure 9 describes the entire process schematically [57]. From our previous knowledge, it is accepted that the molecular diffusion of gaseous species is directly associated with the geometric mean free path of the molecules (GMFP, λ) assuming this to be Knudsen diffusion. Here the three SnO2 microspheres offers a mean pore size range from 1 to 100 nm. The diffusion mechanism applicable in these pores is known as “Knudsen” diffusion. The mean free path of the gas molecules or GMFP is same at a particular temperature and/or concentration, for a specific target gas resulting in a faster molecular diffusion for the larger pore size (Fig. 9a) specimens as compared to the smaller ones (Fig. 9c). In another attempt, Su et al. [58] have synthesized ultrathin hierarchical porous p-type Co3 O4 nanosheets for superior acetone sensing. The application of a structure sensitive agent, that is, glucose, has transformed the agglomerated Co3 O4 nanosheets into flower-like porous nanostructures by hydrothermal method. The porous hierarchical Co3 O4 nanostructures possess a very high specific surface area of 80.8 m2 g–1 , which is believed to be the key reason for its extraordinary gas detection ability. Acetone vapor response of 48.1 has been obtained in the best stoichiometric combination in this study, while the peak response and recovery times are 18 s and 13 s, respectively, at 130 °C. In addition, this fascinating nanostructure offers a long-term sensing stability and decent selectivity. The limit of detection (LOD) for acetone vapor has been found to as low as 200 ppb.

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Fig. 9 Effect of changing pore size from a big, b middle, and c small in close proximity with the target gas is schematically described. Adapted with permission from [57]. Copyright (2013) American Chemical Society

Zhang et al. [59] have synthesized porous Zn2 SnO4 /SnO2 -based hierarchical microspheres by a hydrothermal process for establishing the excellent sensitivity of triethylamine in contrast to similar other investigated gases at 280 °C. In addition to its outstanding sensitivity, other notable features of these nanostructures include broader linearity response at low ppm level, extraordinary selectivity, and quick reaction time. In addition to the creation of an n–n heterojunction at the interfacial region of these two dissimilar materials possessing two separate work functions, the sensing mechanism is also largely dominated by the formation of reactive sites which assists in faster surface-controlled gas diffusion for enhanced sensitivity. Wang et al. [60] have successfully produced 3-D hierarchical metal–metal oxides (Co/Al2 O3 and Co3 O4 /Al2 O3 ) hybrids with peony-like morphology where identical nanosheets form its basic building blocks. This unique composition with mesoporous

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structure offers enormous surface area. Consequently, once the ethanol vapour is introduced, it diffuses within the exposed surface as well as the internal reaction sites of the structure instantly. Further, the adsorbed oxygen ions are combined with this organic vapour and subsequently discharge the trapped electrons and merge with the free holes, with the overall enhancement of resistance. This novel Co3 O4 /Al2 O3 -based mesoporous morphology displayed an excellent response time of 1 s for 50 ppm of ethanol vapor and a sensitivity of 8.9.

6 Hierarchical Oxide Nanostructures: Gas Sensing Properties Hierarchical oxide nanostructure has been classified by their dimensions. Moreover, higher dimension of such nanostructure decides their surface area which attributed to the gas sensitivity. As such 1-D nanorods, nanotubes, nanoneedle, nanowires, nanotowers, 2-D nanosheets, nanobelts, and 3-D nanoflower, urchin like nanoarchitecture structure has been studied by the researchers [8, 61]. Xu et al. [62] designed a novel 3-D hierarchical nanostructure by applying Ga2 O3 nanorods over the nanoarchitecture of In2 O3 . They used the facile vapor–liquid–solid (VLS) process in combination with oxide-assisted growth model. Zhang et al. [63] synthesized 1-D functional nanofibres of SnO2 –ZnO for the formaldehyde detection. Both hydrothermal and electrospinning route has been utilized to get such active oxide nanofibres for better gas sensitivity. Additionally, at 150 °C, the gas response for 200 ppm formaldehyde was found to be 12.2. Qin et al. [64] applied a superficial hydrothermal process to make square shape hierarchy SnO2 nanowires sphere like structure. Moreover, excellent sensitivity toward acetone has been detected. The response time and recovery time for acetone sensitivity for 20 ppm was 7 s and 10 s, respectively. Wang et al. [65] reported dual-shell hollow microspheres of cobalt oxide with assembled nanorod with the help of carbon microspheres precursor. This novel core–shell structure of cobalt oxide-based hollow structure with nanorod assembly gives an improvement in the capacitive performance as well as good electrochemical properties. Hence, the nanorod assembly might be the reason for the enhancement of electrochemical performance. Table 1 elaborates gas sensing properties of some latest developed hierarchical oxide nanostructures.

7 Sensing Mechanism of Hierarchical Oxide Nanostructures Since the invention of gas sensor technology by Seiyama in early 1962s, there has been a sustainable effort to improve its detection power to a particular gas in both adverse and favorable conditions. Metal oxide-based chemoresistive gas sensors

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Table 1 Sensitivity, recovery/response time, temperature effect of hierarchical oxide nanostructures S. no.

Hierarchical oxide nanostructures

Sensitivity (Ra /Rg )

Recovery/Response time

Working temperature (T op )

References

1

α-Fe2 O3 /TiO2

13.9–50 ppm

0.5 s/1.5 s

250 °C

[66]

2

SnO/SnO2 nanoflowers

80.9 (50 ppm)

–

120 °C

[67]

3.

Zn2 SnO4 nanospheres

23.5–50 ppm

–

180 °C

[68]

4.

SnO2 spheres

7.7–33.1 (50 ppm)

1s

600 °C

[69]

5.

SnO2 –La2 O3 nanowires

57.3 (100 ppm)

3.1–3.3

400 °C

[70]

6.

In2 O3 hierarchy nanostructure

2.16–3.81 (10–50 ppm)

4–8 s

400 °C

[71]

7.

In2 O3 hollow hierarchy nanostructure

1.99–2.79 (10–50 ppm)

5–10 s

400 °C

[71]

8.

Cu2 O/CuO hierarchy microspheres

–

10/15 s

320 °C

[72]

10.

α-Fe2 O3 flower like nanostructure

2.6 (50 ppm)

0.5/1 s

280 °C

[55]

11.

Bi-doped SnO2 flower like nanostructure

36.2 (100 ppm)

8/10 (50 ppm)

170 °C

[10]

Ra = electrical resistance of the sensor in air; Rg = electrical resistance in the presence of the test gas mixed in air

have shown tremendous growth in sensitivity and response behavior to a specific toxic gas/vapour by using selected dopants, with the grain size reduction of thin film structures, by optimizing its working temperature or humidity level or all of them. However, the governing mechanism of enhanced sensitivity of any of these MOSbased resistive sensors is practically same. The fundamental gas sensing mechanism is explained by the surface ionosorption [73] model as follows: First, the MOS-based (n-type) gas sensor, for example, ZnO, SnO2 , Fe2 O3 etc., absorbed atmospheric oxygen molecules onto its surface. These O2 molecules transformed to various ionic oxygen species (O− , O2− , O− 2 which forms between 50 and 400 °C) onto the conduction band of the sensor surface by an electronic transport process and thereby increasing the width and height of the potential barrier at the gas–MOS interface. The overall effect is the sudden enhancement of electrical resistance from its base value. P-type semiconducting materials act exactly in the reverse way.

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The room temperature oxygen adsorption process is explained by the following reactions: O2 (gas) → O2 (ads)

(1)

O2 (ads) → O− 2 (ads)

(2)

Thereafter, when the bared sensors comes in contact to reducing gas such as H2 , CH4 , CO, ethanol, or acetone, etc., the chemi-adsorbed O− on the sensors surface instantly reacts with these reduced gases. The electrons trapped within the conduction band are free to move to the conduction band. Subsequently, the thickness and height of the potential barrier reduced which causes the resistance drop of the sensors. The surface chemical reaction-based gas sensing mechanism has been extended to any variety of hierarchical (single material) nanostructures also. Here also, the fundamental sensing mechanism is typically reliant on the defect density and the ease of electron transfer on the semiconductor surface. It has been identified that p-type oxide materials shows superior properties for selectively detecting some specific gaseous materials as compared to the n-type oxide semiconductors. With this indication, the idea of developing heterostructured gas sensors has been generated by combining both p- and n-type (p–n, n–n, or p–p heterojunction) semiconductor materials to achieve their maximum functional properties. A combination of two dissimilar metal oxide aggregate in hierarchical form offers an extremely fascinating and distinct heterojunction interface with porous and open structure that allows ease of access to the target gas molecules. The enhanced sensitivity of the acetone sensing of the p-NiO/n-ZnO heterojunction nanofibers is explained by the formation of a p–n junction within p-type NiO and n-type ZnO nanostructures in addition to the usual absorption theory (discussed above) of gas sensitivity. In order to bring the Fermi levels in the same platform of these two dissimilar semiconducting species with holes (in NiO) and electrons (in ZnO) as majority carriers, flow of electrons occurs from ZnO to NiO, while that of holes occur from NiO to ZnO. Therefore, band bending at the interface of p–n heterojunction and the development of an electronic depletion and accumulation layer over the surface of ZnO and Ni, respectively, can be seen. Overall, the size of the depletion layer in n-type ZnO nanostructures enhances in contrast to pristine ZnO which results in increase in resistance for the p-NiO/n-ZnO heterojunctions aggregates. In view of the above phenomenon, it is evident to declare that the sensing mechanism for this type of dissimilar semiconductor metal oxide heterojunction arrangements are controlled by the formation of depletion layers from two separate sources: (i) reaction of ambient oxygen with semiconductor surface and a depletion or space charge layer formation by electron transfer from surface layer and (ii) an additional increment in width of the depletion layer by the transport of electrons/holes in opposite sides of the p-NiO/n-ZnO heterojunctions.

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Once the sensing device is in direct contact with the H2 gas or ethanol vapor, the chemisorbed O− species releases its electrons to the conduction band of ZnO (with a decrease in band gap) via a redox reaction, with subsequent reduction in the overall resistance of the composite structure. Therefore, it might be construed that the design and formation of the p-NiO/n-ZnO heterostructures has a significant contribution toward its superior sensitivity for toxic and hazardous gaseous species. Similar trend in gas sensing mechanism is found in n–n or p–p homojunction for both hierarchical and film-based nanoaggregates.

8 The Selectivity Issue In spite of numerous advantages, oxide-based hierarchical nanostructures suffer from one major drawback, which is its poor “selectivity” with high sensitivity. This parameter denotes the ability of a gas sensor to spot the existence of a specific analyte in an atmosphere where other gases coexist. Therefore, the parameter “Selectivity” is often very strenuous to accomplish under standard environmental conditions. At present, there are two common methods available to improve the selectivity in such sensors. The first approach is to develop materials with the ability to detect a specific analyte while retaining almost no sensitivity to other gaseous species in the same mixture under normal atmospheric conditions. It has been well understood that the unique adsorption power and/or chemical reactivity sensing materials toward target gases is responsible for such discrimination. In order to achieve a high level of selectivity through novel materials design, optimization of working temperature, incorporation of dopants and modulating its composition and stoichiometry are the crucial factors to consider. The second approach consists of constructing devices by means of sensor arrays. Here, N-different signals are sent from N-sensors with distinct configurations. Such devices are known as electronic noses (or e-noses) where specifically designed sensor arrays are used to selective detection of gaseous species by a pattern recognition technique. An artificial neural network-based method is used to achieve this. This modern technique is found to be a superior technique for selectively detecting toxic and hazardous gases in a wide variety of applications such as chemical and food processing industries, agricultural sector, biomedical applications, environmental sector, and many other technical areas. In a sense, an e-nose is analogous to a much superior version of human sniffers. Nevertheless to these untiring efforts, it is almost impossible to achieve absolute selectivity by these techniques in real atmospheric conditions. Among the numerous attempts to improve sensitivity by using materials with novel morphology and compositions, some are explained as follows: Zhang et al. [74] have developed novel brush-like B-SnO2 –ZnO-based hierarchical oxide nanostructures by a simple two-step hydrothermal method, in addition to its pure counterparts. These sensors along with pure SnO2 nanowires and ZnO nanorods were tested to 1 ppm NO2 , acetone, C7 H8 , H2 S, CHCl3 , NH3 , and ethanol at 150 °C, respectively. The B-SnO2 –ZnO-based hierarchical oxide nano-structured

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sensors show much superior selectivity to NO2 gas as compared to the pristine materials. It is clear from Fig. 10, that the brush-like B-SnO2 –ZnO-based sensor shows a sensitivity of 25.5–1 ppm NO2 , whereas the same material shows a sensitivity of less than 1 while exposed to other six gases. Septiani et al. [75] have successfully synthesized 3-D wool ball-like hierarchical zinc oxide (ZnO) (Fig. 11) nanostructures by a simple solvothermal process. These novel nanostructures were subsequently converted into a composite assembly with the addition of MWCNTs (multi-walled carbon nanotubes) as an external reagent. This fascinating morphology with a composition of ZnO: MWCNTs = 10: 1 shows an excellent detection capacity of toxic SO2 gases at the most favourable working temperature (300 °C).

Fig. 10 a Dynamic response study of B-SnO2 –ZnO hierarchical nano-structured sensors for NO2 sensing from 5 ppb to 10 ppm. b Linear relationship of the as-developed sensor to NO2 vapor within the 5 ppb–10 ppm range. c Comparison of selectivity of pure ZnO nanostructures, pure SnO2 nanostructures, and B-SnO2 –ZnO hierarchical nano-structured sensors for various test gases at concentration of 1 ppm. Adapted with permission from [74]. Copyright (2018) Elsevier

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Fig. 11 FESEM images of a 3-D spherical woollen ball-type ZnO structure synthesized by solvothermal technique. FESEM images of identical ball-like ZnO morphology at b low and c high magnification and synthesized at different experimental conditions (the plate-like sub-units formed by numerous nanoparticles is depicted in inset). TEM images at d low and e high magnifications. f HRTEM images of the porous spherical woollen ball-type ZnO nanostructures. g A model depicting the mechanism of formation of the ZnO precursor particles is shown here schematically. Adapted with permission from [75]. Copyright (2018) Elsevier

It is interesting to note, that this composite nanostructure displays outstanding selectivity to SO2 and the tested sensitivity value is almost 7–14 times superior than the other toxic gases at identical test conditions. The superior selectivity and sensitivity of this composition is primarily attributed (a) to its ability to form p–n heterojunctions at the ZnO–MWCNTs interfaces and (b) the improved adsorption ability for O2 and SO2 molecules which arises due to its much higher surface area of the composite as well as formation of lattice defects by the chemical affinity of MWCNTs. Earlier, Ponzoni et al. [76] have successfully synthesized the WO3 -based meshlike nanowire networks by a catalyst-free vapor–solid process involving metallic tungsten as micron range powder particles at 1400–1450 °C in oxygen atmosphere. This novel hierarchical nanowire networks exhibit excellent sensitivity for the detection of NO2 gas in the concentration up to a minimum of 50 ppb. The selectivity of this composition is found to be above average at various working temperatures when tested for gases like NH3 , CO, and H2 S. In an attempt, Li et al. [77] have synthesized ZnO-based single-crystalline hierarchical porous microstructures and demonstrated

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their superior sensitivity toward 50–2000 ppb triethylamine (TEA). This effort has also resulted in achieving an exceptional selectivity toward TEA in a mixture of other commonly found volatile organic compounds (acetone, ethanol, ammonia, etc.). As already mentioned, selectivity enhancement has remain one of the significant obstacles which need to overcome by scientists and engineers and this issue has been a area of concern to the entire research community dealing with gas sensors.

9 Future Trends It is simple to understand now that the use of gas (chemical) sensors for the identification of toxic and hazardous gases arise from industries, human consumption of fossil fuels, or any other sources, make a crucial impact in our day to day life for several reasons. Therefore, the productive exploitation of the fundamental properties of a range of sensing materials to attain superior performance to detect these environmentally harmful gases is an area of research interest to scientists and academicians. Since the first demonstration of change in electrical conductivity of ZnO upon exposure to external stimuli or gases by Seiyama in the year 1962, a great deal attempt has been made so far for the improvement of sensing performance of metal oxide-based or any other type of gas sensors. It is traditionally said that the “4S” sensor performances, that is, “selectivity,” “sensitivity,” faster response/recovery rate or “speed” to detect a particular gas, and long-term “stability” of performance should be met in order to fabricate the best sensing device for this purpose. Therefore, efforts are necessary for further improvement in gas sensing abilities by the fabrication of various novel hybrid materials, or using sensor arrays or any other technique, keeping the above-mentioned four pillars of sensing in mind. Among the few selected challenges needs to be overcome for the best possible sensing device to construct, higher operating temperature possess substantial difficulty for its permanent stability. Low-temperature detection of toxic and hazardous gas is feasible by the use of 2-D metal oxide/sulfides/nitride-based nanostructures (e.g., graphene, BN, MoS2 , WS2 , MoSe2 , etc.) by forming various fascinating nano-architectures. Nevertheless, these 2-D nanostructures show in inadequate selectivity as compared to others. Therefore, substantial effort must be given in coming days in order to achieve superior selectivity for a specific gas while using these new age materials. Apart from the above-mentioned research gaps found in the 1-D, 2-D, or other form of metal oxide-based nanostructure in gas sensor applications that limits it further development, it has been also seen that substantial work has not been done in finding the “lower limit of detection” for a particular gas. Therefore, any special class of material in pure or hybrid form that agree to the detection of gases at extremely low concentrations at or only marginally higher than ambient temperature is of immense significance not only to industrial or household applications, but also for clinical aspects.

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Lower working temperature of any type of gas sensor has the additional advantage of low power consumption while making it a stable and long-lasting device. In the coming days, special categories of gas sensors might be extensively use in extreme environmental conditions such as in defence applications or in space explorations. Based on the above-mentioned discussion, it must be come into general consensus that the development of sophisticated and multifaceted sensor configurations is a must in order to prevail over the limitations of existing gas sensors for the betterment of mankind.

10 Conclusions With the fast development and growth of industrial sector globally during last few decades, the formation and uneven distribution of toxic, inflammable, and hazardous gases in earth’s atmosphere has possess a long-term risk for the entire human race. Therefore, detection of these toxic and perilous substances has becoming an important aspect for all of us not only for industrial or domestic reasons, but also for automotive sector, space vehicles, air-conditioning, quality control for chemical and food processing reasons, and breath examination for clinical purpose. Keeping this requirement in mind, chemoresistive gas sensors has become a significant topic of interest since the dawn of this century for successful detection of volatile and toxic gases that exists around us. Metal oxide gas sensors (MOS) are found to be suitable candidate materials for the easy detection of this unwanted species due to their outstanding sensitivity, cost-effectiveness, faster response time, excellent repeatability, and device portability. Last few decades’ advancement in developing various modern and sophisticated techniques and equipments have enabled us to synthesize nano-structured materials in diverse morphology such as spheres, fibers, tubes, wires, belts, hollow particles, etc. Among these unique nano-architectures, hierarchically assembled pure or hybrid assembles are found to be one of the most lucrative option and therefore have been extensively studied in diverse technological applications including gas sensors. Researchers have adopted novel techniques to achieve the best possible combination of hierarchically assembled nanostructures for the detection of unwanted chemical species in gaseous form available in environment and consumed by mankind. Their extensive study has demonstrated that this unique hierarchical morphology in nanoscale dimensions offer incredible potential for the fabrication of ultrasensitive chemoresistive gas sensor devices. The sensing mechanism, however, does not differ much when compared to other form of nanostructures. The advantage arises since these nanostructures offer huge number of active sites for gas absorption and hence, increases its sensitivity in large amount compared to the other form of nanoarchitectures. In spite of sustainable research effort, the “4S” challenges in sensing platform are not fully achieved by using this unique morphology in pure or hybrid form.

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Therefore, extra efforts must be made in prospect to meet industry demand which can revolutionize the field of chemical sensors for the welfare of human civilization. Acknowledgements The authors would like to express their endless gratitude to the editorial staffs, reviewers, and others who actively participated in this book chapter. The authors would like to express their sincere thanks to NIT Raipur for providing the basic infrastructural facilities to complete this assignment.

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Chapter 8

Reduced Graphene Oxide (rGO)-Based Nanohybrids as Gas Sensors: State of the Art Bhagyashri Bhangare, Niranjan S. Ramgir, K. R. Sinju, A. Pathak, S. Jagtap, A. K. Debnath, K. P. Muthe, and S. W. Gosavi

1 Introduction The air contains the mixture of various gaseous elements which are natural and artificial too. Among them some are vital to our life, whereas some of them are fatal to the human health. The oxygen and humidity are the vital gases and need to monitored, while hazardous gases should be well below the safety limits. Nowadays, industrialization has been increased in larger extent which results in the use of toxic and explosive materials. It often leads to release of gaseous entities into atmosphere. The basic need of the gas sensors comes in the field of medical institutions, agriculture, national aeronautics, space administrations, food quality control and research institutions. The significant use of gas sensors in medical diagnostics and environmental safety demands the fulfilling of the 4-S selection criterion wherein each S stands for the remarkable sensitivity, selectivity, stability and the suitability. Along with this the fast response/recovery, low detection limit is a prerequisite. Metal oxide semiconductors (MOS) are the ancient and promising candidates for the gas sensing application over the past decades [1]. However, the fulfilment of the commercial requirements 4-S criterion brings the challenges in MOS in gas sensors. In MOS, the sensing behaviour is mainly from the contribution of surface morphology and heterostructure interface properties. The most commercial MOS-based sensors needs the high selectivity, low ppb level detection, longer lifetime, good repeatability and low power consumption. These inorganic semiconductors require the additional thermal energy to activate the surface adsorption/desorption of the gas molecules. The B. Bhangare · N. S. Ramgir (B) · K. R. Sinju · A. Pathak · A. K. Debnath · K. P. Muthe Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India e-mail: [email protected] B. Bhangare · S. W. Gosavi Department of Physics, Savitribai Phule Pune University, Pune 411008, India S. Jagtap Department of Instrumentation Science, Savitribai Phule Pune University, Pune 411008, India © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_8

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operating temperature of the MOS-based sensors is particularly high, i.e. in the range of 200–300 °C. It makes MOS unsuitable for the battery-operated applications due to high power consumption [2]. Although, the higher working temperatures influence the kinetics and electron mobility, it may often lead to the fusion of grain boundaries and consequently degrading the sensor response and stability of the sensor films [3]. The nanohybrids (NHs) of organic–inorganic materials have given a new alternative in the gas sensing area and overcome the drawbacks of MOS. ‘Nanohybrids’ refers to the atomic or molecular level mixture of different materials leading to the formation of heterojunctions. The properties of the heterojunction interfaces can easily be tuned by changing the chemical as well as structural properties of the nanomaterials. Also, engineering of the surface, shape, size, interface and compositions can lead to the tailoring of the response characteristics of the gas sensor. This can further enable the enhanced charge transport, decrease in depletion region and operating temperatures along with the fast response and the recovery. In addition, the catalytic behaviour of the noble metal sensitizers/additives can be utilized to enhance the sensor characteristics. To realize a commercially viable sensor, the satisfaction of the ‘4-S’ selection criteria is demanded.

2 General View 2.1 Current State of the Art in rGO Nanohybrids-Based Gas Sensor Gas sensors are the devices used for the detection of toxic and flammable gases. The graphene-based NHs and nanostructures have been widely used in gas sensing application, (Fig. 1) due to their high surface area and atom thick layer of sp2 hybridized carbon atoms. In order to enhance the gas sensor outputs, the various approaches such as functionalization with noble metals, NHs with metal oxides (p/n) and use of different detection techniques have been employed. The use of noble metal nanoparticles such as Pt, Pd, Au and Ag improves the sensor response by providing the extra adsorption sites along with the low activation energy. The miniaturized form of gas sensors can be achieved by microfabrication techniques and micropatterning. The most popular configurations of miniaturized devices of gas sensors include field effect transistors (FET), CMOS, diodes and MEMS. The GO-based flexible sensor arrays has been used to monitor the humidity [4, 5], NO2 , NH3 [6] and CWAs [7]. In addition, rGO NHs have been used for the discrimination of disease biomarkers (VOCs) using e-nose [8], as well as for breath analysis [9, 10].

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Fig. 1 Graphene-based gas sensor devices, state of the art, a application areas and b flexible devices. Reprinted with permission from Eric Singh et al. Appl. Mater. Interfaces 9 (2017) 34544–34586, Copyright @ ACS

2.2 Synthesis of rGO Graphene is a 2-D material that consists of sp2 -hybridized carbon atoms arranged in hexagonal ring. It is also known as the parent material of all graphitic domains. Its excellent surface, magnetic, chemical and mechanical properties has leads to its prominent use in energy storage devices, functional composites and optoelectronics. These properties can be further tailored using different synthesis approaches and characterization tools [11, 12]. Graphene can be derived in the form of single- as well as biand few-layers and accordingly, its physicochemical properties are distinct from that of graphite. In order to obtain high-quality and scalable graphene, different synthesis techniques have been investigated and optimized [13]. They are often classified into two approaches, namely bottom-up and top-down. The top-down approach involves breaking and separation of the stacked graphite layers to yield single as well as multilayers of graphene, whereas bottom-up approaches involve synthesis of graphene from carbon source such as graphite. Among the bottom-up routes, chemical vapour deposition (CVD) and epitaxial growth have resulted in high-quality graphene but the techniques suffers from the limitation of poor scale up due to high cost and bulky instrumentation. On the other hand, top-down approach gives large-scale production of graphene. The graphene derived from top-down approach use GO as the main component and hence the presence of some of the oxygen functionalities differs the rGO properties (chemically derived graphene) from that of the pristine graphene. For potential application of rGO, the synthesis methods are still needed to be developed in order to obtain the improved quality of graphene along with reproducibly. The most commonly used method for the synthesis of rGO is the oxidation–reduction (top-down) approach, commonly referred to as the modified Hummers’ method [14]. On the other hand, depending on the reducing sources, the synthesis approaches are

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Fig. 2 Top-down synthesis of rGO using different routes

named as chemical, photoreduction, green approach, electrochemical and hydrothermal (Fig. 2). By using appropriate reducing agent, the chemical reduction of GO can be carried out, whereas the UV source irradiation also results in the reduction of GO. In hydrothermal route, the reaction temperature itself acts as the reducing agent. By applying sufficient amount of electrical potential or by using biological entities such as plant extract and bacteria, the rGO can be derived in electrochemical and green synthesis approaches, respectively.

2.3 Importance of Chemically Derived Graphene The chemically derived graphene consists of different chemical structures due to presence of oxygen functionalities such as epoxides, alcohols and carboxylic acids. In GO, the different oxygen functional groups lead to the electrically insulating behaviour; however, chemical reduction can restore the conductivity by removal of oxygen functionalities and hence restore the aromatic sp2 -hybridized carbons. The chemical reduction method does not yield pure graphene, as some of the oxygen groups still remain on the basal planes and at the edges. Thus, rGO is conductive in nature with chemically active defect sites making it a promising active material for sensor application. The presence of oxygen functionalities gives active sites for polymer functionalization thereby increasing the hydrophilic nature and hence could also act as a good candidate for humidity sensor application. In addition, the chemical functionalization of rGO with metal and metal oxide nanoparticles readily allows the detection of harmful gases in the parts-per-billion (ppb) range.

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Fig. 3 Different approaches for the synthesis of rGO nanohybrids

2.4 Synthesis of rGO Nanohybrids Similar to the rGO, the nanohybrides of rGO with metal, polymer and metal oxides can easily be derived. Depending on the gas sensor engineering, the electrical properties and surface modification of the rGO nanohybrids has been tailored. Different approaches used to fabricate the rGO nanohybrids are shown in Fig. 3. By varying the reaction parameters such as exposure time, temperature, sensitizers and precursors the gas sensing properties of the nanohybrids can be enhanced. Till date the defect engineering is proved to be an efficient way to tune the characteristics such as sensor response, selectivity and stability of gas sensors. The defects created due to vacancies, doping and surface modification of rGO nanosheets plays an important role in gas sensing. The NHs of rGO have been derived by photoreduction, liquid phase exfoliation and hydrothermal route gives an alternative of the reducing agent-free synthesis of rGO NHs [15–19].

2.5 Classification of rGO Nanohybrids and Their Use in Gas Sensing Application The different configuration of graphene-based gas sensors such as field effect transistors (FETs), surface acoustic waves (SAW), optical and chemiresistive sensors are studied for commercial applications. The NHs materials are the combination of nanoscale organic and inorganic counterparts, in which the electronic/chemical interaction at the molecular level results in the unique interface properties. The inorganic nanostructures present in the rGONHs are in the form of three-dimensional (3-D), two-dimensional (2-D), one-dimensional (1-D) or zero-dimensional (0-D). The pristine graphene has capability to detect the gas analytes at parts per billion (ppb) levels and is comparable to the most of reported gas sensors [20]. However, the experimental and theoretical approaches have shown that the pristine graphene is able to respond towards number of analytes such as NO2 , NH3 and CO2 . Further, the gas adsorption process often leads to the formation of specific traps which scatters the charge carriers in graphene thereby affecting the charge carrier’s mobility. Also, depending upon the different substrates used, the graphene shows p or n-type behaviour [21, 22].

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The combination of noble metal, semiconductors and polymers with the rGO are classified as follows, • • • •

Noble Metal/rGO nanohybrids Metal Oxide (0-D, 1-D, 2-D, 3-D)/rGO nanohybrids Ternary rGO-based nanohybrids Polymer/rGO nanohybrids.

2.5.1

Noble Metal/rGO Nanohybrids

The unique chemical and physical properties of the noble metal nanostructures make them promising functional materials for the gas sensing applications [23]. Considering the scarcity and high cost, the integration of noble metals with the other economical materials helps to minimize its consumption. Till date, the organic and inorganic materials with noble metal sensitizers have been studied for their potential applications in various fields such as catalysis, energy storage and gas sensing [24– 26]. The catalytic behaviour of noble metal nanoparticles improves the adsorption of gas molecules by creating the active sites on the surface of sensing material. This further lowers the activation energy of the sensing materials and enhances the sensor response due to spillover effect [27]. Furthermore, the noble metal decorated rGO structures are sensitive and selective owing to the synergistic effect as a result of nanohybrid configuration. The Fermi level of the noble metals lies above that of the semiconductors and hence electron transfer to the semiconductors takes place till the Fermi levels of both becomes equal. This further leads to the formation of Schottky-type barriers at the interfaces and plays role to prevent the electron-hole pair recombination. Figure 4 shows the response curves towards NH3 , NO2 and H2 of the rGO NHs with Ag [28], Sr [29]

Fig. 4 a rGO/Ag nanohybrids for NH3 gas sensor. Reprinted with permission from the S. Cui et al., J. Chem, Analyst, 138 (2013) 2877, Copyright @ RSC. b NO2 gas sensor using rGO-Sr. Reprinted with permission from A. Mukherjee et al. Mater. Res. Express 6 (2019) 065611 (11), Copyright @ IOP Scienceand. c Pt-P/rGO-based H2 gas sensors. Reprinted with permission from the Y. Peng et al. RSC Adv. 6 (2016) 24880–24888, Copyright @ RSC

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Table 1 The metal NPs/rGO-based gas sensors Sr. no.

Materials

Analytes

SR

Res/Rec time

References

1.

Pt/Single layer graphene

H2 (150 °C)

5.3% (10000 ppm)

6/69 s

[31]

2.

Pt/rGO

H2 (150 °C)

14% (1000 ppm)

2–6 min

[32]

3.

Ag/sulfonated rGO

NO2 (RT)

45% (50 ppm)

12/20 s

[33]

4.

Pt/3-D graphene

H2

115.1% (1000 ppm)

9/10 s

[34]

5.

Pd/rGO

H2 (25 °C)

28% (1000 ppm)

700 s

[35]

and Pt [30], respectively. Table 1 list various metal-doped rGO nanohybrids-based gas sensors.

2.5.2

Metal Oxide Semiconductor (MOS)/rGO Nanohybrids

The MOS has been mostly used in past years for commercial gas sensing applications and mainly works on the principle of colorimetric, acoustic, optical and gas chromatography. These approaches have the limitations of low sensitivity, poor selectivity, unsuitability for miniaturization and high power consumption. As an alternative, the carbon-based materials such as carbon nanotubes (CNTs), charcoal and graphene have gained the importance due to large surface area which effectively improves the sensor response. Among them, the 2-D materials are able to screen the charge fluctuations as compared to 1-D material and hence graphene is considered as the most suitable for gas sensing applications (Fig. 5). The nanowires (NWs), nanotube (NTs) and nanorods (NRs) are the 1-D or quasi1-D metal oxide nanostructures. These 1-D nanostructures have large surface areato-volume ratios. The metal oxides in the form of 0-D [42–46], 1-D [47–50], 2-D [51–55] and 3-D [56–63] have been studied and listed in Table 2. Further, the unique structural characteristics such as the compatibility with the Debye screening length in lateral dimensions helps to improve the sensitivity.

2.5.3

Ternary rGO based Nanohybrids

The ternary nanocomposites of rGO mainly includes the nanohybrids of metal oxide, noble metal dopants and rGO. The noble metal nanoparticles react with the adsorbed oxygen species. In SnO2 /rGO nanohybrids, the doping of Pt, Pd, Ag, Ni and Au helps to reduce the operating temperature and also tune the sensitivity towards various analytes with chemical and electrical sensitization. The addition of noble metals enhances the electronic conductivity as well as catalytic activity of the nanohybrids (Table 3).

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Fig. 5 The band diagram for the metal oxide/rGO nanohybrids for NO2 gas sensing. Reprinted from the permission of B. Bhangare et al. Appl. Surf. Sci. 487 (2019) 918–929, Copyright @ Elsevier

Table 2 The metal oxide/rGO-based gas sensors Sr. no.

Materials

Analytes

SR

Res/Rec time

References

1.

SnO2 /rGO

H2 (150 °C)

87.2 (atmosphere H2 100%)

1.71 ppm

TFB (Amperometry)

RT

[104]

2.

Hexane

Advanced Gastric Cancer (AGC)

RGO (SERS)

[105]

costly and non-portable. As an alternative, graphene-based chemiresistive gas sensors exhibits high sensitivity, lower detection limit and portable. In addition, it is a non-invasive and rapid detection tool for breath analysis using the gas sensor device and hence gained prime importance. Among the number of VOCs, the acetone is a known biomarker for diabetes, whereas the production of ketones is the indication of insufficient insulin. Table 9 lists some of the biomarkers that are prominent in the corresponding diseases.

5.2.2

Flexible and Wearable Electronics

The wearable electronic sensor devices have been used for the realtime health information monitoring and hence are primarily defined in medical field for the purpose of point-of-care applications. In flexible gas sensors, the graphene film can be transferred on to the flexible substrates such as poly(dimethyl siloxane) (PDMS) [106], poly(ethylene terephthalate) (PET) [107–109], poly(ethylene naphthalate) (PEN), poly(methyl methacrylate) (PMMA) [110] and polyimide (PI) [111]. The flexible and wearable [112, 113] gas sensors devices mostly operate at room temperature and hence accordingly, lowers the power consumption. Further, the miniaturized devices avoid the complexity in circuitry (Fig. 9 and Table 10).

5.3 Chemical Warfare Agent’s Detection The chemical warfare agents (CWAs) are the chemical weapons used for mass destructions, with highly toxic and fast reactive nature which cause the irreversible effects/lethal. The use of chemical warfare agents (CWAs) and biological warfare agents (BWAs) has been banned in 1925 by adopting the Geneva protocol. The nerve agents can inhibit the enzyme activities which are irreversible, so cause the death. The highly toxic and hazardous nature of CWAs restricts its use for laboratory testing. So, the alternatives with similar chemical structures called simulants are used for laboratory confirmations [117]. In comparison with the pristine metal oxide nanostructures, the carbon-based nanostructures and nanocomposites have paved more significant

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Fig. 9 a Flexible ammonia sensor using S and N co-doped graphene quantum dots/polyaniline hybrid operated at RT (PET substrate). Reprinted from the permission of J. N. Gavgani et al., Sens. Actuators B 229 (2016) 239–248, Copyright @ Elsevier, b rGO-decorated cotton yarn robust NO2 sensor, Reprinted from the permission of Yong Ju Yun et al., Scientific Reports 5:10904, Copyright @ Nature. c rGO film deposited on flexible PET substrate for NH3 gas sensing application, Reprinted from the permission of Pi-Guey Su et al., Sens. Actuators B 190 (2014) 865–872, Copyright @ Elsevier. d Wearable H2 S gas sensor using intense pulsed light-reduced graphene oxide (IPL-rGO) sensor, Reprinted from the permission of S.-J. Choi et al., NPG Asia Materials 8 (2016) 8, 1–10, Copyright @ Nature

Table 10 The rGO nanohybrids-based flexible gas sensors Sr. no.

Material (Substrate)

Analyte

SR

LOD

References

1.

SnO2 /rGO/PANI (PET)

H2 S (Halitosis)

23.9 (200 ppb)

50 ppb

[114]

2.

MoS2 /rGO (PET)

NO2

26% (5 ppm)

0.15 ppm

[107]

3.

rGO/nylon-6 NFs

NO2

13.6% (ppm)

1 ppm

[115]

4.

rGO (ECAD)

Isoprene

2% (50 ppm)

5 ppm

[116]

contribution in highly sensitive detection of CWAs. The current research focus on the detection as well as detoxification of CWAs using various nanostructures. At the laboratory scale the testing of CWAs is one of the risk factors; hence, to lower the poisoning effect, the low concentration detection is of prime need. Amongst all

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Table 11 The characteristics of the various carbon-based nanohybrids towards CWAs Sr. no.

Materials

Analytes

1.

rGO

DMMP

2.

Au/rGO

Arsine

3.

PANI/rGO

DMMP

LOD

3 ppb

SR

OT

References

137% (20 ppm)

25 °C

[118]

100% (1 ppmv)

130 °C

[119]

6.6% (300 ppb)

RT.

[120]

the nanomaterials, the carbon-based nanostructures such as CNTs, Polymers, C60, graphene are known to be a promising candidate due to high surface area, good conductivity and low operating temperature. The current state of art in the carbon-based materials includes the development of highly sensitive and reliable devices for the commercial deployment (Table 11).

6 rGO Nanohybrids for Room Temperature Gas Sensing Application The gas sensors operated at room temperature allows us to explore the materials in nanodimension which avoids the high temperature operations in gas sensing. Room temperature gas sensors have numerous applications for societal benefits. For example, sub-parts per million-level detection of ammonia gives the diagnosis of kidney disorders. For such applications, room temperature-based wearable devices are needed for in situ medical diagnosis [121]. The wearable sensors can be directly attached to human body or embedded in body and expected to play a major role in the field of healthcare monitoring. In addition, it also helps in all those sectors related to the Internet of Things (IoT). Graphene being optically transparent, thermally stable, mechanically strong, lightweight, flexible, stretchable and biocompatible. It fulfils the criteria’s required for room temperature-operated flexible and wearable sensors for monitoring and detection of toxic gases which are hazardous to human health [122].

7 Different Approaches to Enhance Sensor Characteristics 7.1 Doping/Functionalization 7.1.1

Noble Metal/rGO Interface

In solids, the electronic energy levels/energies such as vacuum level, Fermi level, ionization energy or electron affinity and work function plays crucial role in charge

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Fig. 10 Effect of noble metal sensitizers in gas sensing mechanism

carrier transport and device control [123]. These energy levels quantify the dependence of different sensor characteristics such as sensitivity, selectivity and operating temperature. These characteristics are mainly depending upon surface engineering which include structural, morphological and chemical composition of the nanomaterials [124]. The small chemical change on the surface of nanomaterials gives the significant change in its work function values. During gas sensing, the adsorption of gas molecules on the surface of graphene-based hybrids causes change in surface dipole moment, electron affinity and surface work function (SWF). In principle, the monitoring of work functions and Fermi levels helps to understand the nature of interaction and associated sensing mechanisms. In noble metal/rGO nanohybrids, the immobilization of noble metal nanostructures on the surface leads to alteration of Fermi levels [125]. During sensing, the trend observed in work function, carrier concentration and resistance of the rGO nanohybrids due to noble metal doping is shown in Fig. 10 (Table 12). The changes in work function can be further used to rectify the metal–semiconductor interfaces by studying the band bending diagram [136] as shown in Fig. 11. When metal work function (φm ) is higher than that of graphene (φg ), (φm > φg ), the electrons transfer from graphene to metal, till the fermi levels aligned to equilibrium. This results in upward band bending and formation of depletion layers at the interface. For φg > φm, the electrons are transferred from metal to graphene and energy bands bends downwards resulting in the formation of accumulation layer.

7.1.2

MOS/rGO Interface

Chemiresistive metal oxide gas sensors have been studied extensively for the wide range of applications. The higher operating temperature of these metal oxides results in power consumption and affects the long-term stability. As an alternative, the

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Table 12 Gas sensing performance of rGO NHs at room temperature Sensing materials

Analyte Gas

Concentration (ppm)

Sensor response

Response/ recovery

References

ZnO–rGO

NO2

5

25.6%

165 s/499 s

[43]

SnO2 :rGO

NO2

5

3.31

135 s/200 s

[46]

ZnO NRs/rGO

NO2

1

119%

75 s/132 s

[126]

In2 O3 –rGO

NO2

30

8.25

4 min/24 min

[127]

CNT–rGO

NO2

10

20%

60 min/>60 min

[111]

SnO2 QWs/rGO

H2 S

50

33

2s

[128]

PANI–rGO

NH3

50

59.2%

18 min/4 min

[129]

ZnO NW–rGO

NH3

50

19.2%

φm

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nanohybrid of metal oxides with graphene has gained more attention. Furthermore, the p-type graphene and n/p-type metal oxides give the formation of p–p, n–n and p–n junctions at the interfaces. As a result, the nanohybrids exhibit enhanced performances than those of the pristine materials. The synergistic effect between rGO and MOS has resulted in improved sensing characteristics. The role of graphene is briefly mentioned as follows; i.

During the chemical synthesis, graphene effectively controls the size as well as the morphology of the MOS nanoparticles. ii. Improved conductivity of nanohybrids results in rapid charge transfer in sensing surface. iii. Immobilization of MOS nanoparticles on the surface of graphene sheets helps to lower the aggregation of graphene and increase the stability of nanohybrids. iv. The formation of p–n junctions between p-type graphene and n-type metal oxides alters the space-charge layers at the interface and accordingly, enhances the sensing characteristics. In conclusion, the decoration of rGO surface with MOS nanoparticles increases the overall active sensing surface area and gas molecule adsorption. For example, in SnO2 /rGO nanohybrids the formation of p–n junction at the interfaces of SnO2 (n-type) and rGO (p-type) leads to the formation of depletion layer. In presence of oxidizing gases, the Fermi level shifts towards the valence band and increases the conductivity of rGONHs [42, 46, 137–139]. While in presence of reducing gases, few electrons are donated to rGO and Fermi level shifts towards conduction band which results in the decreased conductivity of nanohybrids [61, 140–142].

7.2 Induced Structural/Chemical Defects The structural and chemical imperfections in solid-state materials are called as defects which possibly degrades the performance of the nanomaterials. In nanomaterials, defects play important role as they can be exploited in order to generate novel, innovative properties. The 2-D nanomaterials such as graphene and graphene nanoribbons (GNRs) are no exception. In graphene, the structural (sp2 -like), topological (sp2 -like) defects and doping/functionalization (sp2 -, sp3 -like), vacancies/edge-type defects (non-sp2 -like) [143] are present. These defects help to alter the physicochemical properties of the graphene or rGO. In order to confirm the role of oxygen functional groups, the systematic investigation towards the reducing and oxidizing gases have been confirmed using first principle DFT studies [144]. The presence of oxygen moieties on the graphene surface not only provides active sites, but also acts as nucleation and growth centres for metal and MOS nanoparticles [145] (Figs. 12 and 13). During the chemical synthesis of rGO, the doping and functionalization of other molecules and atoms efficiently modify the electrical and chemical properties of graphene [146]. As compare to pristine graphene, the modified graphene (P-, B-,

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Fig. 12 Illustrations of a point defects and b line defects in graphene, LD and La denotes the inter defect distance and in-plane crystallite size of graphene, for pristine graphene, L D → ∞, L a → ∞ while for defected graphene L D → 0, L a → 0. Reprinted with permission from L. G. Cancado et al., 2 D Mater. 4 (2017) 025039, Copyright @ IOP science

Fig. 13 Chemical defects a Different oxygen functionalities in GO, (1) COOH group on the basal plane (edge), (2) CO group on the basal plane(edge), (3) OH group on the basal plane (edge), (4) OH group perpendicular to the basal plane, (5) epoxy group perpendicular to the basal plane. b Defects due to the eight different adsorption positions (OP). Reprinted with permission from L. G. Cancado et al., 2 D Mater. 4 (2017) 025039, Copyright @ IOP science

N-doped graphene) have shown more affinity towards CO, NO, NO2 and NH3 gas molecules [147, 148]. To create the structural/chemical defects in graphene, various approaches such as ion irradiation, UV illumination, chemical modification have been extensively utilized [149–151]. In graphene-based devices, the implementation of defects in graphene can modify the conductivity which is crucial for the creation of electronic junctions [152].

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7.3 Irradiation of rGO Nanohybrids The irradiation of graphene and other nanomaterials is proved to be one of the effective methods to incorporates the number of structural and chemical defects. It further enhances the sensor response by availing the plenty of adsorption sites for gas molecules. The various irradiation sources such as ultraviolet light, visible light, oxygen plasma, electron beam, ions, ozone and γ-rays. In rGO, the irradiations cause the structural defects and oxygen vacancies which helps to improve the sensing properties. The irradiation of graphene sheets in water promotes the oxidation at the basal planes of graphene. On the other hand, the irradiation of e-beam on the surface of graphene sheets not only enhance the sensor response, but creates the structural defects such as nanopores, hillocks, and vacancies [153]. It is essential to note that, the vacancies play important role to detect the gas molecules with good sensitivity and selectivity [154]. In order to incorporate the defects, the techniques such as ozone irradiation [155], e-beam irradiation [156] and O2 plasma irradiation [154] have been used (Fig. 14 and Table 13). Fig. 14 Different irradiation sources to generate defects in the graphene-based nanohybrids

Table 13 The irradiated rGO nanohybrids for gas sensing applications Sr. no.

Materials (Analyte)

Operating temperature

Irradiation source

Sensor response

References

1.

Graphene (Acetone)

RT.

UV

7% (900 ppb)

[157]

2.

ZnO/rGO (NO2 )

300 °C

Microwave

12.57 (1 ppm)

[158]

3.

SnO2 /rGO (Ethanol)

175 °C

UV

150 (200 ppm)

[159]

4.

Graphene (NO2 )

RT.

Ozone

17% (200 ppb)

[160]

5.

rGO (NO2 )

RT.

Laser beam

1.27 (20 ppm)

[161]

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8 Challenges in rGO Nanohybrid Gas Sensors The pristine graphene is chemically inert and so have poor selectivity, low sensitivity and slow response/recovery time. On the other hand, the chemically derived graphene, i.e. rGO contains number of oxygen functional groups which act as the active sites to anchor the functional groups as well as nanoparticles. This covalent anchoring of the nanoparticles contributes to detect the specific gas molecules and hence helps to improve the selectivity. One of the major limitations is to derive the single layer graphene which also affects the advancement in the graphene-based materials. The single layer graphene-based gas sensors faced the limitations due to complexity to transfer the deposited graphene on to the suitable substrate. Additionally, the functionalization with the suitable dopant has become a tedious job while depositing the single layer graphene, the chemically derived graphene gives an alternative over the single layer synthesis methods. The CVD grown single layer graphene gas sensors are not suitable for mass production. As graphene is highly sensitive materials, which shows the cross sensitivity towards number of gases and hence there is need to tune the selectivity of the rGO nanohybrids.

9 Conclusion and Future Scope The gas sensor is an important technology in human as well as environmental safety applications. The rGO NHs have gained the potential applications in advanced gas sensors due to lower detection limit, noticeable and astonishing sensor response with comparatively low operating temperature. The flexibility to tune the selectivity with the help of heteroatomic doping brings the best alternative to the semiconductor metal oxides. The new inputs such as post thermal treatments, flexible electronics and defects generation brings the new insights in the rGO-based gas sensors due to optimized stability, robust devices and improved sensor response along with lower operating temperatures. The formation of heterojunctions of rGO with semiconductor metal oxides and noble metals helps to reduce the activation energy of the gas sensor. The flexible and wearable gas sensors added a new trend in the advances in flexible electronics and hence are contributes majorly in human safety, environmental safety and biomedical applications. To realize a practical application these sensors needs to be studied extensively towards their fulfilment of sensitivity, selectivity, stability and suitability. The sensor characteristics of all the rGO NHs sensors able to follow the above-mentioned selection criteria and hence became an important class of materials for practical applications. Apart from this, effect of various sensitizers, namely Au, Cu, Fe and Ag plays a major role in the sensor’s response kinetics. In addition, the detailed theoretical investigations are still needed in order to understand the reaction kinetics and sensing mechanisms in rGO NHs. Nowadays, the configurations such as FETs have gained an important insight into point of care application. Hence, the upcoming

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research can be able to bring the advantages over the conventional sensor device configurations. Overall the future of the rGO-based gas sensors is bright and soon a complete commercial level sensors is expected.

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Chapter 9

Graphene–Polymer-Modified Gas Sensors Flavio M. Shimizu, Frank Davis, Osvaldo N. Oliveira Jr., and Seamus P. J. Higson

1 Introduction Over the past few decades, a range of gas sensor technologies have been developed for reliable monitoring of air quality [1]. These gas sensor technologies are now a crucial part of various fields such as food safety, medical, and automobile applications [2–4]. For these purposes, functional nanomaterials such as oxide nanostructures and 2D materials have been widely used for gas sensors due to their high surface area and excellent electrical properties [5–7]. Among these materials, the traditional sensors are made of semiconductor metal oxides because of their low cost, simple operation, and high sensitivity [2, 8, 9]. Recent studies show that nanostructured oxide semiconductors, in the form of wires, particles, belts, etc., have better sensitivity to different gases owing to their large surface-to-volume ratio. However, there are limitations in using oxide semiconductors for commercial sensors because of high working temperatures and, most notably, lack of selectivity. The first metal oxide-based commercialized gas sensor was introduced by Figaro in 1981 to detect hazardous gases [10]. Recently, graphene has emerged as a promising candidate for gas sensors since its discovery in 2004 [11]. To achieve high signal-to-noise detection ratios, graphene provides high carrier mobility and density with low intrinsic noise. For instance, a single element graphene gas sensor is promising for the [7] discrimination of F. M. Shimizu (B) Brazilian Nanotechnology National Laboratory (LNNano), Brazilian Center for Research in Energy and Materials (CNPEM), Campinas, SP 13083-970, Brazil e-mail: [email protected] F. Davis · S. P. J. Higson Department of Engineering and Applied Design, University of Chichester, Bognor Regis, West Sussex PO211HR, UK O. N. Oliveira Jr. São Carlos Institute of Physics, University of São Paulo, CP 369, São Carlos, São Paulo 13560-970, Brazil © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_9

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multiple analytes. However, this remains at a seminal stage as the device performance is limited and misclassification is a regular occurrence due to the lack of viable approaches to differentiate between similar device responses. The first gas sensing investigation on graphene was performed by Novoselov et al. [12]; however, there is no systematic research to achieve high sensitivity and selectivity using graphene and its derivatives. Nallon et al. [13] demonstrated a 24-element graphene sensor array for selective vapor classification with stable operation. However, increasing the number of sensing elements in the array increases power consumption, complicates the device circuitry, and increases the risk of overfitting the data analyzed, which can adversely affect classification accuracy. There are issues with pristine graphene which contains relatively few active groups on the graphene surface; this can limit the chemisorption of gas molecules onto the surface as well as lowering the recovery speed. The surface modification of graphene is an alternative way to enhance the recovery speed and selective sensing toward a target gas. In this regard, conducting polymers are an ideal choice because of their electrical conductivity changes when they come in contact with analyte gases [14– 19]. For instance, electron acceptor gases, such as NO2 and Cl2 , can remove the electrons from the conducting polymers, which results in an enhancement of the conductivity of a p-type conducting polymer [19, 20]. On the other hand, for reducing gases like NH3 , H2 S, etc. the conductance of p-type conducting polymers will decrease. Exactly the opposite will take place for the n-type conducting polymers. The conducting polymer-based sensors have distinct advantages in terms of easy processing, low cost, and room temperature operation. However, similar to their inorganic counterparts they, too, suffer from lack of specificity, in addition to sluggish response and recovery. To overcome these problems, various strategies have been employed including the modification of molecular structure, doping, and nanostructuring the conducting polymer. Recently, some research has been developed toward a new concept of organic/inorganic hybrid films so that the best properties of the materials, namely, graphene and polymers, could be utilized to obtain highly selective gas sensors. Graphene–polymer composites have been recognized with a range of sensing application (Fig. 1) due to outstanding electrical properties and high electron mobility at room temperature. Within this chapter, we will provide a general overview of the graphene–polymerbased gas sensor detection methods, various gas sensor technologies, and the key parameters to improve the gas sensing performance. Last but not least, we will describe possible challenges and future outlooks to provide new insights into the possible development of these kinds of hybrid materials for gas sensor application.

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Fig. 1 Types of sensors based on graphene–polymer nanocomposites

2 Methods of Detection In order to understand the gas sensing mechanism and sensing performance of graphene–polymer-modified gas sensors, it is important to classify the sensing methods and their advantages and disadvantages. Figure 1 shows the classification of sensing technologies based on graphene–polymer sensors. In the last two decades, various detection methods have been utilized for graphene– polymer nanocomposites including chemoresistive, surface acoustic wave (SAW) sensors, electrochemical gas sensors, optical gas sensors, and quartz crystal microbalance sensors, each with unique principles of operation, facile fabrication, and prospective applications. The most common procedures are resistive and optical due to their low cost and facile operation; however, long-term stability and operation temperature is still an issue for these methods.

2.1 Chemoresistive Gas Sensors Most of the gas sensors are resistive based as their sensing mechanism is based on the change of resistance under the interaction with the target gas. In brief, when the target analytes and gas-sensitive materials are in contact, gas molecules are adsorbed on the surface of graphene–polymer composites, resulting in changes in resistance. There are many factors affecting the mechanism such as material type (either ntype or p-type), analyte gas (oxidizing or reducing gas), operation temperature, and humidity. The response can be measured qualitatively and quantitatively depending on the change in resistance value. The combination of two materials such as n–n type, p–p type, or n–p type has attracted the interest of many research groups. In this regard, Zhang et al. [21] have demonstrated a graphene oxide/polypyrene (GO/PPr)

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composite with gas sensing properties toward toluene. It showed a fast, linear, and reproducible response with high sensitivity. Recently, Myungwoo et al. [22] explored the charge transfer processes between graphene and polymer upon CO2 exposure. They reported a room temperature sensing of CO2 utilizing an applied bias of 0.1 V under controlled humidity conditions and obtained a conductivity change of 32% at 5000 ppm of CO2 . Basically, the nanocomposites have been shown to have an important synergistic effect on exposure to target gases because of excellent electrical properties and selective sensing compared to pristine graphene [23]. Some research is still going on to improve the gas sensing performance of pristine graphene and its composites because the gas molecules adsorb weakly on the graphene surface since there are no dangling bonds. However, gases adsorb more strongly on doped graphene, making it more sensitive to gases like CO, NO2 , and NO. The main problem in the graphene-based sensor is the recovery speed; the sensor needs longer periods to reach its baseline. To overcome this problem, Huiliang et al. [24, 25] demonstrated a novel AC phase sensing approach to detect chemical vapors. They reported that the recovery speed of the phase-based sensing approach is ten times faster than the conventional DC resistance measurement schemes at room temperature.

2.2 Quartz Crystal Microbalance (QCM) Sensors Amongst all the mentioned sensors, QCM sensors have received intensive attention due to their excellent sensing performance with high sensitivity at low working temperature and low cost, easy installation, and inherent ability to monitor analytes in situ and real time. Compared to other techniques, QCM sensors could measure nano-scale changes in mass on the quartz crystal surface based on the Sauerbrey equation by recording its frequency shifts. QCM provides a higher level of information than other detection methods. They can give information about molecule interaction with surface and also measurement of mass and density, while this kind of information cannot be obtained from sensors developed with other detection methods [26]. The basic, operating principle of QCM is that quartz crystals are subjected to electrical or mechanical stress; QCM measures the mass variation per unit area by measuring the change in frequency of the quartz crystal resonator. In 2016, Yang and He [27] fabricated GO functionalized QCM resonators for HCHO sensing. The response toward HCHO was significant and frequency shifted up to 39 Hz in 60 s for 1.7 ppm of formaldehyde concentration. It demonstrated a stable sensing response up to 100 days with high reproducibility. Similarly, Huang et al. [28] fabricated poly(vinylpyrrolidone) (PVP)/reduced graphene oxide (RGO) nanocomposites for NO2 sensing using quartz crystal microbalance. The gas sensing responses were measured at room temperature in air toward NO2 ranging from 60 to 100 ppm. The sensor showed stable and linear sensing behavior toward NO2 and selectivity compared to other gases such as NH3 , CO, and CO2 .

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3 Gas Sensing with Graphene–Polymer Composites The most commonly employed materials utilized within composites of graphene– polymer gas sensors are summarized in Fig. 2. As will be discussed, some devices were prepared using single-layer graphene and its oxidized and reduced forms in the presence of conducting polymers such as poly(pyrrole) (PPY), poly(aniline) (PANI), poly(3,4-ethylenedioxythiophene) (PEDOT), and poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS). In the majority of cases, the monomers are in situ polymerized on graphene surface through oxidative methods [29], and interestingly the same polymerization process may act as reducing agent to produce RGO [30].

3.1 Hydrogen Hydrogen gas (H2 ) is the most promising clean-energy source [31]. However, its use is still limited by several safety issues related to spontaneous ignition and degradation of many types of steels during a possible leak. The limit of ignition for hydrogen in contact with air is 4.0% [32, 33] since its minimum ignition energy (MIE) is below 0.02 mJ [31]. Hydrogen is colorless, odorless, and tasteless meaning it cannot be

CH3 O OH

OH

oxidation

reduction

OH

OH

OH

O

OH

OH H3C

O

CH3

H2C

graphene

reduced graphene oxide (RGO)

graphene oxide (GO) H3C

H N

H3C

n CH3

N H

n

CH3

PSS

poly(pyrrole) (PPY)

O NH

NH

H3C CH3

H3C

NH

poly(aniline) (PANI)

NH

n

O

O

S

O

O

●

S O

O

S

S

+ O

O

S

●

PEDOT

SO3-

SO3H

SO3H

SO3H

SO3-

O

S

+ O

n CH3

O

poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)

Fig. 2 Chemical structures of conducting polymers and graphene derivatives employed to fabricate nanocomposites

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detected by humans; thus, the creation of reliable and real-time monitoring devices is essential for its practical use. Among the various materials employed in the development of gas sensors, the use of conducting polymers becomes interesting because their electrical properties are affected via redox reactions and charge transfer upon exposure to H2 [34]. To further improve its sensitivity, composites of conducting polymers and graphene have been investigated and applied as gas sensors. Composites can be prepared by in situ polymerization of aniline monomer in the presence of reduced graphene oxide (rGO) under acidic conditions. Al-Mashat et al. reported the use of rGO sheet, poly(aniline) (PANI) nanofibre, and rGO/PANI nanocomposite gas sensors obtaining resistivity changes of 0.83, 9.38, and 16.57%, respectively, upon exposure to 1% of H2 in synthetic air at room temperature. Hydrogen gas is a reducing agent that in contact with graphene causes depletion of holes from the valence band (increasing the resistance of the material). Meanwhile, for PANI, a chemisorption process occurs at the charged amine nitrogen that induces a change in PANI back to its polaronic state and consequently decreases the resistance of the polymer. Since the resistance decreased for both PANI and rGO/PANI gas sensors, we may conclude that the response of nanocomposite is dominated by the chemisorption process [35] and that it is an n-type semiconductor. This behavior was modified by increasing the concentration of reduced graphene oxide in rGO/PANI nanocomposites making it a p-type semiconductor. Now, the resistance increased when in contact with reducing hydrogen gas because of holes depleted from the valence band. This, however, does not lead to an enhancement in the resistivity change, 5%, under exposure to 1% of H2 . However, doping the nanocomposite with palladium nanoparticles allowed the sensor to reach a response of 25% toward 1% of H2 , with response/complete recovery times of 20–50 s and limit of detection of 0.01 vol.% [36]. Interestingly, in a nanocomposite comprised of graphene and poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), it was observed that graphene oxide provides superior sensing performance than its reduced form toward hydrogen gas at 100 ppm [34]. A sophisticated device was proposed by Hong et al. comprising poly(methyl methacrylate) (PMMA) polymeric membranes coating single-layer graphene (SLG) prepared by a CVD technique and doped with palladium (Pd) nanoparticles. The rationale for employing a non-conducting polymer is based on selective filtration membrane effect as a consequence of free volume of the PMMA matrix, which allows penetration of only H2 molecules (kinetic diameter of 0.289 nm). The success on achieving selective sensor is because the kinetic diameter of interferents such as nitrogen dioxide (NO2 = 0.4 nm), carbon monoxide (CO = 0.33 nm), and methane (CH4 = 0.38 nm) are larger than H2 molecules. PdNP/SLG composite in the absence of PMMA membrane showed a response to all interferents as demonstrated in Fig. 3a. In Fig. 3b, it is shown that PMMA/PdNP/SLG gas sensor is not affected by the presence of interferents and exhibited a response of 66.37–2% of H2 gas within 1.81 min and complete recovery of 5.52 min at room temperature [37].

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Fig. 3 Relative resistance changes for a PdNP/SLG gas sensor forward 10% CH4 (red solid line), 0.5% CO (green solid line), 0.05% NO2 (black solid line), and 0.025% H2 (blue solid line). b PMMA/PdNP/SLG gas sensor forward 10% CH4 (red solid line), 0.5% CO (green solid line), 0.05% NO2 (black solid line), 0.025% H2 (blue solid line), and mixture gases of 0.025% H2 and 0.05% NO2 (purple solid line), respectively. Reproduced from [37] with permission of the American Chemical Society. Copyright 2015 American Chemical Society

3.1.1

Hydrogen Sulfide

Hydrogen sulfide (H2 S) is produced in daily life within industrial production and natural degradation of organic matter (e.g., foodstuffs). However, depending on its concentration, it is a highly toxic gas that can harmfully affect multiple metabolic systems of the human body even at quite low concentration. Thus, rapid identification and detection of H2 S are critically important [38]. To ensure the freshness of foodstuffs, smart food packages have been intensively investigated. Shu et al. reported a user-friendly protocol, through coupling a light-emitting diode (LED) with modified paper electrode whose reading response was performed with a smartphone-based visual sensing platform. A conductive aerogel Cux O-PPY was loaded into a graphene oxide framework to obtain Cux O-PPy@GO nanocomposites; these were employed to coat the paper substrate. From electrical resistivity measurements, the response was estimated to be 12% at 50 ppm of H2 S, and the sensor could be utilized within a concentration range of 2–200 ppm with response and recovery times of 40 and 80 s, respectively. After optimizing experimental condition, the LED was positioned in the

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paper electrode and then the varying intensity of LED light under H2 S atmosphere could be monitored by simple smartphone imaging and color analysis. A limit of detection (LOD) of 8.6 μM (0.6 ppm) was determined. Knowing that H2 S gas is released during the egg spoiling process, the system was further assessed as smart food package of eggs and relative intensity varied with time, indicating the successful application of the sensor [39]. Since food packages are plastic-based, then flexible sensors must be developed. Zhang et al. synthesized a tin oxide/reduced graphene oxide/polyaniline (SnO2 /rGO/PANI) nanocomposite sensor for H2 S gas, fabricated onto flexible interdigitated electrodes. A response of 9.1% at 0.1 ppm was obtained, and the calculated LOD, 0.05 ppm, is 1 order of magnitude lower than previous studies. Since people with halitosis exhaled more than 0.1 ppm of H2 S gas, the sensor was assessed on exhaled breath of 12 healthy people and 12 persons with halitosis. Principal component analysis was successfully employed for pattern recognition of healthy and halitosis groups, which could be an interesting alternative for diagnostic in human exhaled gas samples [40].

3.2 Carbon Dioxide/Monoxide Anthropogenic emissions of carbon dioxide (CO2 ) and carbon monoxide (CO) originating mainly from incomplete combustion of carbonaceous materials have been constantly increasing every year. Carbon dioxide is a major greenhouse gas and can cause ocean acidification. Carbon monoxide is a leading cause of unintentional and suicidal poisoning deaths since it severely affects health by forming carboxyhemoglobin (COHb), thereby hampering the oxygen carrying capacity of the blood [41, 42]. Conductivity-based gas sensors were developed using three-dimensional (3D) porous graphene aerogel (GA) prepared by chemical reduction of GO with L-ascorbic acid (L-AA) followed by freeze drying and then coated by in situ polymerization of poly(aniline)(PANI) and poly(pyrrole) (PPY). The conductivity of GA, GA/PANI, and GA/PPY composites are similar, 4.17 × 10−2 , 1.92 × 10−2 , and 1.18 × 10−2 S cm−1 , respectively. Upon exposure to CO2 gas at flow rate of 200 mL min−1 over a period of 60 min, GA and GA/PPY composites demonstrated negligible variations while GA/PANI conductivity decreased to 5.05 × 10−6 S cm−1 . Recovery experiments demonstrated stable response under dynamic relative humidity of 1– 60% and after three cycle measurements at 50 °C, and room temperature negligible variation were observed in 5 replicates [43]. A sophisticated system was assembled by Son et al. using a graphene-based electrode prepared by low-pressure chemical vapor deposition (CVD) onto an SiO2 /Si substrate and further functionalized with poly(ethyleneimine) (PEI), poly(ethylene glycol) (PEG), and mixed PEI/PEG solutions via drop-casting. Chemoresistance experiments showed negligible response to pristine and PEG-functionalized device, a slight difference for PEI–graphene device, 0.4%, and a clear response of 11.5% to PEI/PEG-co-functionalized graphene devices toward 5,000 ppm of CO2 at room temperature. This demonstrates that PEG does

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not contribute to detection and only assists PEI sensing by enhancing protonation of amine moieties during exposure. The sensitivity of device was further improved to 32% by increasing the humidity to 60% [22]. Regarding carbon monoxide gas sensors, Naikoo et al. reported a ternary nanocomposite composed of zeolite-X (Na-X), reduced graphene oxide (rGO), and poly(pyrrole). The performance of Na-X/rGO/PPY ternary nanocomposite was compared to Na-X/PPY and PPY sensors. It was found that the addition of zeolite and graphene not only accelerated the response times to 600, 660, and 720 s, but also enhanced the response to CO gas, 14.9, 9.8, and 6.3% at 5 ppm, respectively. These improvements are strictly related to the increase of surface area correlated by number of nanomaterials employed in the composite. Concerning the sensor stability, experiments showed a small variation of ~7% in response but with time a significant loss in the response [44].

3.3 Nitrogen Oxides and Ammonia 3.3.1

NO2

The problem of excessive anthropogenic emission of NO2 into the environment arises mainly from combustion processes (heating, power generation, and vehicle engines). Epidemiological studies have associated the long-term exposure to NO2 gas with respiratory diseases and complications such as bronchitis, increase in childhood asthma, and reduced lung function [45]. Therefore, many composites of graphene and polymers are promising candidates for the development of NO2 gas sensing [46–49]. In early work, a single layer of graphene oxide (GO) was deposited on a substrate by the Langmuir–Blodgett technique [50, 51] and subsequently reduced by thermal treatment at 180 °C for 4 h under steam. Poly(3,4-ethylenedioxythiophene) (PEDOT) was then polymerized in situ onto the GO films followed by different heating procedures (1, 5, 8, and 15 °C/30 s) which afforded different porosities to the composite film. Compact films were obtained at slow rates (1–5 °C) and bubble-like nanostructures were achieved at higher rates as consequence of rapid solvent evaporation. Compared to pristine reduced graphene oxide, the sensitivity of RGO/PEDOT is slightly higher, but porous RGO/PEDOT film (pore size of 40 nm) enhances it by 2 orders of magnitude (41.7% at 20 ppm). Despite the sensor being affected by humidity, it maintains repeatability after five cycles and is selective to NO2 even in the presence of SO2 , Cl2 , H2 S, CO, and NO [51]. The humidity problem was further reduced by Dunst et al. who employed numerical simulation to determine the best design of interdigitated electrode combined with a heater for gas sensing, and then further modified through ablation laser patterning. PEDOT/RGO nanocomposite films were then deposited by electrochemical polymerization and reduction onto the interdigitated electrode. At an optimum working temperature of 80 °C, after short-term annealing at 170 °C for 20 min, the devices kept the selectivity to NO2 gas, and they were only slightly affected even when

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exposed to humidity levels up to 60%. However, at room temperature the response to humidity was much higher, up to sevenfold which could compromise sensor reliability [49]. In later work, the effects of annealing process and long-term storage upon response were investigated. At 200 °C, a temperature at which reduction process predominated led to the removal of oxygen functional groups and an improvement in electrical contact within the composite. At 300 °C, they observed that the sensing layers are sensitive to oxidation process which hampers NO2 detection by reducing the signal. Surprisingly, long-term experiments revealed an increase in electrical resistance of the PEDOT/RGO composite within a period of 6 h. This is probably caused by changes in PEDOT because the same did not occur for GO and RGO sensors [52]. This phenomenon was further investigated by the same group via Raman spectroscopy. Characteristic Raman bands of PEDOT increased in intensity and shifted toward higher wavenumbers, whereas GO bands remained unchanged. It revealed that long continuous exposure to NO2 leads to PEDOT oxidation, not degradation, which is partially reversible after regeneration [53]. Another derivative, poly(3-hexylthiophene) (P3HT), was also utilized within a graphene composite NO2 gas sensor. In early work, the composite was prepared by in situ polymerization of 3-hexylthiophene in the presence of different amounts of reduced graphene oxide, generating RGO-P3HT composites containing 1.5, 3, 5, 8, and 10% m/m. After deposition on flexible electrodes, the composites were evaluated toward NO2 gas and 5%RGO-P3HT film presented the highest response with 26.36% change in conductivity at 10 ppm NO2 . This is similar to the response obtained by Yang et al. with RGO/PEDOT composite in the same conditions, R ≈ 30% [51]. The highest response to NO2 gas was obtained by Yang and Katz who prepared P3HT-ZnO@GO nanocomposite heterostructures by a simple chemical reaction. ZnO-NH2 + nanoparticles and GO were mixed in aqueous solution for 1 h via magnetic stirrer, then the as-prepared ZnO@GO was mixed with different weight fractions of P3HT (0–80%) dissolved in chlorobenzene solution by sonication. Thin films were prepared by spin-coating technique to construct organic field-effect transistors (OFETs) for conductance experiments. After 5 min exposure to NO2 gas, a conductance change of 210% was obtained toward 5 ppm NO2 at room temperature to 60%P3HT-ZnO@GO, which is higher than 60%P3HT-ZnO and 60%-P3HT-GO composites [48].

3.3.2

Ammonia

Naturally produced in our environment and widely employed in industries such as refining, cleaning, manufacture, and manufacture of nitrogenous fertilizers and chemical substances, ammonia (NH3 ) plays a vital role in our daily life. Despite that, it is a harmful gas and may cause respiratory disease or human body irritation, e.g., eyes, skin, and throat depending on concentration and exposure time. US Occupational Safety and Health Administration (OSHA) has set a limit of 25 ppm in the workplace during an 8 h shift and a short-term limit (15 min) of 35 ppm to avoid harm to health. However, it is impossible for humans to detect ammonia below

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50 ppm [54–56]. That makes urgent the development of low-cost ammonia gas sensor devices that are reliable and operate at room temperature. Graphene, metal oxide, and polymers have been intensively investigated as active layers for such device; however, many materials suffer from poor high-temperature operational stability, sensitive, and selectivity. Graphene–polymeric composites are potential candidates for gas sensors because they operate at room temperature. A summary of ammonia gas sensors is presented in Table 1, so we will discuss only the most interesting advances of this application. Sensitive and selectivity problems are still a challenge, and to solve this problem graphene–polymers composites have been proposed. Huang et al. first proposed the composite of poly(aniline) nanoparticles anchored on the surface of reduced graphene oxide. GO was initially reduced and functionalized with manganese dioxide that then served as an active site and oxidant for aniline polymerization. After the reaction, the excess of manganese ions, aniline monomer, and acid solution was removed by filtering and washing the obtained rGO-PANI composite. This gave a conductivity response toward ammonia gas of 59.2% (50 ppm) which was 10- and fourfold superior to graphene or PANI sensors, respectively [29]. Later, the same group employed a different strategy to reduce graphene oxide, while PANI was being anchored on its surface, by employing aniline monomer to reduce graphene oxide by oxidative polymerization. Acid-doped (with hydrochloric acid), de-doped (with ammonium hydroxide) PANI attached to rGO and free rGO (after PANI removal with N,N-dimethylformamide) were applied toward ammonia gas sensing. Despite the highest response being attained by acid-doped PANI/rGO with a ~47.6% conductivity change at 50 ppm ammonia, its recovery to initial state was not possible because the doping effect is removed by exposure. In this case, free rGO with a 37.1% response under the same experimental condition was considered the most suitable for gas sensing because it demonstrated complete recovery [58]. Concerning the organization of PANI on GO surface, Wei et al. reported a controllable in situ polymerization method to obtain vertically aligned PANI nanorod arrays grown on the surface of GO nanosheets (PNrA/GO) but without considering GO reduction. Although the response was only 14.63% to 50 ppm of ammonia, a wide detection range could be obtained (1–6400 ppm) with short response/recovery times of 61/10 s, respectively [59]. The authors also proposed a schematic illustration of the sensing mechanism corroborated through first-principle calculations and experimental observation. Alternatively, poly(pyrrole)-graphene composites demonstrated superior performance compared to PANI composites. Initially, pyrrole was electropolymerized on CVD-grown graphene (G(CVD)), allowing the obtaining of PPY-G(CVD) composites within a few minutes. The chemoresistive device afforded a response of 1.7% to 1 ppm of ammonia gas within only 2 min of exposure [65]. This performance was surpassed by a three-dimensional reduced graphene oxide structure (inspired by olfactory cells of mice) prepared by a hydrothermal method. The high porosity afforded a uniform distribution of in situ polymerized PPY nanoparticles on the 3D-rGO surface, as schematically represented in Fig. 3a. The response of the

SPRAY

CuTSPc@3D-(N)GF)/PEDOT:PSS

PEDOT:PSS:GO (0.04 wt%)/n-GaAs

TiO2 @PPY–GN

TiO2 /GO/PANI

PANI/GO/ZnO

PANI/GO/PANI/ZnO

IDE

Diode

Chemoresistor

Chemoresistor

Chemoresistor

IDE

LAYER-by-layer

Nanoemulsion

ISCOP

ISCOP

–

–

–

–

ISCOP

rGO-doped PEDOT-PSS

IDE

GO/PMMA

rGO/p3ht

IDE

Chemical oxidative polymerization

Hydrothermal

PPy/3D-rGO

PANI-GO

–

IDE

N-GQDs/PANI

IDE

ISCOP ISCOP

ISCOP

rGO/PANI

IDE

Electropolymerization

PNrA/GO

IDE

ISCOP

PPY/rGO

Aniline reduced GO

IDE

ISCOP ISCOP

PPY + G(CVD)

Graphene/PANI

IDE

Chemoresistor

rGO-PANI

IDE

Synthesis method

IDE

Sensing material

Electrode

Ammonia and amines gas sensors

Table 1 Summary of the graphene-polymer-based ammonia and amines gas sensors

50–500

100–1000

5–300

1–200

1–1000

11000

2–30

0.33–5

1–5

1–10

30–1000

10–50

0.3–15

10–1500

12–1500

1–6400

20–50

1–6400

5–50

Range (ppm)

29/100

1.31/300

110/100

102.2/50

194/20

8/50 and 91/1000

58/30

1010/5

1.7/1

50/10

87/1000

7.15/10

9/10

37/100

0.08/100

5.6/10

20/20

3.65/20

31/10

Response (%) (ppm)

23

–

5

1

1

10

–

0.33

1

–

–

–

0.3

–

10

–

–

1

–

LoD (ppm)

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

Temp. (°C)

30/38.31

2/164

32/17

36/16

95/121

138/63

180/180

5/20

120/300

–

60.3/170.4

(continued)

[73]

[72]

[71]

[70]

[69]

[68]

[67]

[66]

[65]

[64]

[63]

[55]

[62] 141/488

[61] −/1326

[60]

[59]

[58]

[57]

[29]

References

900/126

10/29

61/10

1080/−

50/23

1080/−

Resp./rec. time (s)

230 F. M. Shimizu et al.

S, N: GQDs/PANI

(GO)-PANIHs

5 wt% rGO-PPy

10C-PPy@SLG

(GO/PAH)n /PSS/PAH

IDE/PET

Chemoresistor/PET

PET

PET

Single yarn

Layer-by-layer

ELECTROPOLYMERIZATION

ISCOP

ISCOP

Chemical oxidative polymerization

ISCOP

ISCOP

–

Inkjet-printing

ISCOP

ISCOP

ISCOP

Synthesis method

5–100

0.0001–1

1–10

0.5–100

1–1000

0.1–100

10–100

30–1500

25–1000

200–1800

50–200

0.2–50

5–10

Range (ppm)

20.75/100

8/1

50/10

31.8/100

42.3/100

53/100

344.2/100

116.38/1000

9.6/500

194/20

325/200

2.8/0.2 and 59.1/50

2.78/10

Response (%) (ppm)

1.5

0.00004

–

0.05

–

–

–

–

–

–

–

0.2

–

LoD (ppm)

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

Temp. (°C)

68/–

1/10

–

102/186

115/44

36/18

20/27

462/600

180/300

95/101

2/–

–

8/33

Resp./rec. time (s)

[84]

[83]

[64]

[82]

[81]

[80]

[79]

[78]

[77]

[76]

[75]

[30]

[74]

References

3D-(N)GF): three-dimensional nitrogen-doped graphene-based framework; CuTSPc: copper (II) tetrasulfophthalocyanine; G(CVD): graphene grown by chemical vapor deposition; GN: graphene nanoplatelets;GO: graphene oxide; GQDs: graphene quantum dots; Hy: hydrazine; IDE: interdigitated electrode; ISCOP:in situ chemical oxidative polymerization; PMMA: poly(methylmethacrylate); n-GaAs: n-type gallium arsenide; N-GQDs: N-doped graphene quantum dots; PANI: poly(aniline); PANIHs: rambutan-like polyaniline hollow nanosphere; PNrA: PANI nanorod arrays; PPANI: PANI nanoparticles; FPANI: PANI nanofiber; PEDOT-PSS: poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate); PY: pyrrole; PPY: poly(pyrrole); RGO: reduced graphene oxide; SLG: single-layer graphene ; SnO2 : tin (IV) oxide;TiO2 : titanium dioxide; ZnO: zinc oxide

PPANI/rGO-FPANI

Graphene-PEDOT:PSS

IDE/PET

Chemoresistor/PET

poly-GO (0.04 wt%)/BiVO4

Diode

N-GQDs/PEDOT-PSS

RGO–MWCNT–ZnO/PPy

Chemoresistor

rGO–PANI

Hy-RGO, Py-RGO, Py-RGO/PANI, and Py-RGO/PPy

IDE

Chemoresistor/PET

SnO2 -rGO-PANI

Chemoresistor

IDE/PET

Sensing material

Electrode

Ammonia and amines gas sensors

Table 1 (continued)

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PPY/3D-rGO nanocomposite was 4–5 times higher than raw PPY and rGO, achieving conductivity changes of 1010% at 5 ppm of ammonia, Fig. 4b. Moreover, high selectivity could be observed when exposed to other gases, Fig. 4c. Polymeric thiophenes such as P3HT [55] and PEDOT [63] were also investigated as graphene nanocomposites, but poor sensitivity and selectivity were reported. To enhance its properties, ternary heterostructures were prepared. Copper(II) tetrasulfophthalocyanine supported on a 3D nitrogen-doped graphene-based framework and PEDOT-PSS composite was deposited onto gold-interdigitated electrodes and applied to ammonia gas sensing [68]. A response of 8% to 50 ppm of NH3 and 91% to 1000 ppm was determined, which is only ~5% higher than obtained with rGO-doped PEDOT-PSS [63]. A remarkable enhancement was, however, obtained utilizing a PEDOT:PSS:GO (0.04 wt%)/n-GaAs Schottky diode sensor. The diode was comprised of a layer of PEDOT:PSS deposited onto the n-type GaAs film. Subsequently, the PEDOT:PSS pores were filled with different ratios of graphene oxide (GO) nanosheets, and the optimum condition is 0.04 wt%. A sensitivity of 194% at 20 ppm of ammonia was achieved with response and recovery times of 95 and

Fig. 4 a Schematic formation mechanism for the 3D crumpled PPY/3D-rGO nanocomposite; b response comparison of bare PPY, 3D-rGO, and PPY/3D-rGO nanocomposite sensor to varying concentration of NH3 at room temperature; ca dynamic response of PPY/3D-rGO nanocomposite sensor to different gases at room temperature. Reproduced from [66] with permission of Elsevier. Copyright Elsevier 2019

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121 s, respectively. This device maintained its stability for up to 30 days and presented an excellent selectivity to ammonia upon exposure to other gases (methanol, ethanol, acetone, propanol, chlorobenzene, CO2 , and CO) [69]. Surprisingly, a diode structure built with different polymeric layers: PTFE, PVDF, and PANI, filled with different ratios of GO nanosheets (0.02, 0.04, 0.06,0.08 wt%) which were deposited onto BiVO4 film, provided the same performance (194% at 20 ppm) as the previously described diode gas sensors in the presence of PANI polymer filled with 0.04 wt% GO nanosheets [76]. Another series of ternary nanocomposites composed basically of conducting polymer and graphene have been reported in the literature with the addition of metal oxides, polymer, and carbon nanotubes [30, 70–75]. Titanium dioxide was utilized to decorate PPY-GN (graphene nanoplatelets) [70] and GO/PANI [71] nanocomposites that under room temperature provided a response of 102.2% and ~49%, respectively, toward 50 ppm of ammonia. Both nanocomposites presented a strict selectively to NH3 gas, but the stability of TiO2 @PPY-GN when assessed for 15 days decreased by 10.2% of its initial value and the TiO2 /GO/PANI, after 60 days, decreased by 16.7% of its initial value in first 15 days and then remained almost constant. In the case when zinc oxide was utilized as a nanocomposite component with PANI/GO, the response values were low, being 1.31% at 300 ppm [72] and 29% at 100 ppm of NH3 gas. Also, aniline could be polymerized in situ on tin dioxide (SnO2 )-rGO composite at 5–10 °C, which afforded a response of 2.78% to 10 ppm with rapid response/recovery times of 8/33 s, respectively [74]. Ly and Park prepared ultrathin films using the Langmuir–Schaeffer (LS) technique which enabled the control of film organization at molecular level. In their study, hydrazine (HY) and pyrrole (PY) were employed as reducing agents to prepare HyRGO, PY-RGO, PY-RGO/PANI, and PY-RGO/PPY nanocomposite LS films. Using these materials, the best results were obtained with a PY-RGO/PANI device that allowed the detection at very low concentration (0.2 ppm) with a response of 2.8% [30]. Among the development of next-generation sensors, researchers are developing flexible sensors for portable, wearable, and body-attachable devices to be applied in health care and environmental monitoring [64, 85, 86]. The most common flexible devices are produced by the inkjet-printing technique due to its advantages of simplicity, low-temperature processing, and pattern generation precision [77]. Seekaew et al. prepared an electronic ink composed of a graphene dispersion and PEDOT:PSS to print a film onto flexible interdigitated electrodes which could then be applied to ammonia gas sensing. However, this reached a response of only 9.6% at 500 ppm. Interestingly, it was observed in bending experiments that higher bending angles increased the response, and at 70° Rflat = 9.6% and Rbending = 15.8%. This effect was attributed to an enhanced swelling of the composite that facilitated ammonia molecules’ diffusion [77]. Yoon et al. developed a flexible and transparent electrode, initially modified with a single layer of graphene as a template for well-ordered deposition of camphor sulphonic acid-doped PPY by an electrochemical polymerization technique using a conventional three-electrode system. Molecular ordering was characterized by grazing-incidence wide-angle X-ray diffraction (GIWAXD)

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for films with 1–70 deposition cycles. It was observed that 1–2 cycles of deposition are not enough to cover the entire electrode surface because of the formation of disordered structures. However, films with 10 cycles of deposition, named as 10CPPy@SLG, exhibited a well-ordered structure with flat orientation parallel to the substrate. For thicker films, the structure collapses and both well-ordered and lateral stacking peaks were observed. Therefore, the 10C-PPy@SLG sensor demonstrated the highest sensitivity for ammonia gas due to enhanced charge mobility within the well-ordered film, and the limit of detection was 0.03 ppb to NO2 (oxidizing gas, decreased resistance) and 0.04 ppb NH3 (reducing gas, increased resistance) with rapid response and recovery times of 1–2 and 7–10 s. Bending experiments also demonstrated that after 50 bending cycles the response decreases by ~10% [83]. In the field of wearable devices, electronic textile devices are an interesting alternative in the creation of novel devices. Su and Liao proposed the use of single yarn as a substrate to deposit layer-bylayer (LbL) films of poly(allylamine hydrochloride)(PAH), poly(4-styrenesulfonic acid)(PSS), and graphene oxide to form (GO/PAH)n /PSS/PAH composite (n = 1, 2, 3) for ammonia gas sensing. LbL technique is advantageous because of its low cost, simplicity (requires only beakers and electrolyte solutions), and allows the control of film architecture at molecular level [87]. For (GO/PAH)1 /PSS/PAH, the highest response is 8.45% at 5 ppm of NH3 , but its behavior is affected when the humidity is higher than 70%.

3.4 Organic Solvents Organic solvents are widely used in a variety of household, industrial, and medical environments. Unfortunately, many of these materials can be both damaging to health and to the environment. They can be toxic, carcinogenic, flammable, or explosive. To prevent such adverse effects, the detection and monitoring of these compounds are necessary. Because of its high surface area and aromatic structure, graphene is of interest as a potential sensor for binding volatile organic compounds, especially aromatic compounds although it is not limited to these types of organics. Many organic species could potentially adsorb onto graphene and interact with it via such interactions as C-H–π or π–π interactions, thereby affecting the electronic structure and properties of the graphene. Much research has been done on this topic and has been reviewed elsewhere [88] so we will discuss some of the most recent results. One issue with graphene is its readiness to interact with many different species making the determination of any single species challenging. However, this may not be an issue where simple detection of inimical compounds is desired, for example, detection of leaks in an oil refinery or other chemical plant. Graphene-based sensors have been used to detect a number of species, for example, layer-by-layer films of graphene oxide with poly(diallyldimethyl ammonium chloride) could be deposited onto interdigitated gold electrodes [89], and the graphene oxide then reduced to

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graphene using hydrazine. These sensors gave electrical responses to a range of analytes including hydrocarbon and other common organic solvent vapors and showed promise as non-selective chemical sensors. The use of polymethyl methacrylate (PMMA) has been shown to affect the performance of graphene-based sensors. Thin films of graphene and PMMA/graphene composites could be laminated onto a polymer substrate and their conductivity determined [90]. The graphene sensors responded to dichloromethane, acetone, chloroform, or benzene; however, the graphene/PMMA composites showed a 3 times higher response to dichloromethane than an unmodified graphene sensor as well as displaying much lower responses to the other solvents. This demonstrated that the polymer conferred a chemical selectivity on the sensor; this is thought to be due to the dichloromethane causing higher swelling on the polymer than the other solvents. Limits of detection for dichloromethane of GR/PMMA and graphene sensors were 72.46 and 175.27 ppm, respectively. Patel et al. [91] utilized PEVA-graphene nanocomposite threads to enhance the response time and sensitivity toward benzene vapor. They analyzed the gas sensing behavior by annealing the PEVA-graphene nanocomposite threads at ≥80 °C and observed that limits of detection were improved after the annealing treatment. The optimized result demonstrated the selective sensing of benzene at 5 ppm with limits of detection estimated to be as low as 1.5 ppm. PMMA was also mixed with graphene and cast onto platinum electrodes and assessed as a chemoresistive sensor for a variety of vapors [92]. The highest response was for formaldehyde; this being caused by direct interaction of the formaldehyde with the graphene surface; use of PMMA also greatly lowered interference from water vapor. The sensor gave a linear response to formaldehyde between 0.05 and 5.0 ppm with a detection limit of 10 ppb. Other workers drop-cast graphene/PMMA composites onto screen-printed electrodes and measured their conductivity [93]. The resultant sensors were shown to respond to a range of organic vapors with tetrahydrofuran giving the greatest response and hexane the lowest of the VOCs measured. Again, polymer swelling was proposed as the major contributor to sensitivity. Methane is a highly flammable and explosive gas used for heating and other domestic purposes as well as being a potential hazard in situations such as mining or wherever organic materials are being decomposed such as in waste landfill sites. It is also a greenhouse gas, being 25 times more potent than carbon dioxide. Graphenebased materials have been used in the determination of methane. Laser-induced graphene could be deposited onto a polyimide film and then decorated with palladium nanoparticles [94]. This could then be covered with a porous polyvinylidene fluoride/ionic liquid pseudo-electrolyte layer, and the resultant electrodes (Fig. 5) shown to be capable of detecting methane down to levels of 9 ppm. In other works, a SnO2 /reduced graphene oxide composite was synthesized by a hydrothermal method and then incorporated into a polyaniline matrix [95]. The electrical resistance of the resultant material was demonstrated to vary with the presence of methane gas, and a sensor could be developed which could determine methane between 100 and 10000 ppm. Composites of graphene with conductive polymers have also been studied. Polyaniline has been widely used within chemical sensing applications due to its

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Fig. 5 Schematic of the construction of a laser-induced graphene electrochemical sensor for methane. Reproduced from [94] with the permission of the American Chemical Society. Copyright American Chemical Society 2019

ease of deposition by chemical or electrochemical methods and can easily form composites with graphene. For example, polyaniline/polyethylene oxide/graphene oxide composites could be electrospun into nanofibres, and the graphene oxide then partially or completely reduced to graphene [96]. These fibers could then be deposited across interdigitated microelectrodes and shown to respond to alcohol vapor with the partially reduced graphene oxide composites giving the highest responses. An array of electrodes coated with either the polymer doped with either graphene oxide or partially or completely reduced derivatives was shown to act as an electronic nose, capable of distinguishing between methanol, ethanol, and propan-1-ol vapors. Aniline could be mixed in solution with graphene nanoplatelets and then polymerized with ammonium peroxydisulfate to give nanocomposites which could be spin-coated onto glass substrates [97]. Spectroscopic analysis showed the formation of charge-transfer complexes between the polyaniline and graphene. The resultant films showed fast and recoverable conductivity changes upon exposure to benzene and toluene vapors. A similar polymerization procedure could be used to formulate polyaniline/ZnO nanoparticle composites with sulfur- or nitrogen-doped graphene which were deposited onto a flexible polyethylene terephthalate substrate with interdigitated electrodes [98]. This was shown to act as a sensor for acetone vapor with good repeatability, high sensitivity and stability, and rapid response to acetone vapor at ppb levels. An alternative conducting polymer that has been used in conjunction with graphene is poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT). For example, N-doped graphene quantum dots could be mixed with PEDOT in water and cast onto silicon substrates patterned with interdigitated gold electrodes [99]. The conductivity of these sensors was affected by solvent vapors (methanol, ethanol, and acetone) with methanol giving the highest response. Rapid (12 s) and reversible (32 s) responses for all these vapors were observed between 1 and 1000 ppm. Much lower responses were observed for polymer films without graphene. Similarly, graphene oxide/PEDOT mixtures could be spin-coated across interdigitated gold electrodes and shown to be capable of detecting methanol vapor at levels as low as 35 ppm [100]. Nanowires of PEDOT/graphene oxide composites could be fabricated using

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soft lithographic methods [101]. Arrays of these could be deposited and were exposed to different solvent vapors. By using different polymer/graphene oxide mixtures along with principal component analysis and linear discriminant analysis, discrimination between different solvent vapors (ethanol, n-hexane, acetone, and p xylene) could be attained. Electrospinning could also be used to formulate nanofibres of polystyrene/poly hydroxybutyrate composites which could be doped with graphene and tetraphenyl porphyrin [102]. These were deposited onto interdigitated electrodes and exposed to toluene or acetic acid vapor. Since graphene and porphyrins are both flat, aromatic systems they interact readily with each other, similar behavior has been observed for layer-by-layer films of graphene and phthalocyanines [103]. The presence of the porphyrin led to the formation of thinner, smoother fibers and displayed enhanced responses to the solvent vapors, while the effect of humidity was lowered compared to fibers without porphyrin. The responses were enhanced by factors of 32 and 18 for acetic acid and toluene, respectively, reducing their detection limits to 1 and 3 ppm.

3.5 Humidity Humidity plays an important role in our environment, and its precision monitoring has vital importance in industries, agriculture, health care, etc. Hence, the challenges to create such devices must demonstrate high sensitivity, low hysteresis, and rapid response and be constructed by a simple and feasible process. Among a number of methods to detect humidity, the quartz crystal microbalance (QCM) gravimetric system has attracted attention because of its capability to sense vapor mass change at sub-nanogram level [104]. Yuan et al. modified quartz crystal electrodes with different levels of GO concentration (0.02, 0.1, 0.5, and 2 mg/ml) and poly(ethyleneimine) (PEI) through a facile spraying method. The best results were obtained using 0.1 mg/mL GO/PEI films from which a hysteresis of 0.54%RH, limit of detection of 0.00691%RH (namely, 2.12 ppm) and a sensitivity of 27.3 Hz/%RH could be obtained. Moreover, after 30 days the sensor response dropped by only 15.6% [104]. Using a similar strategy, Zhang et al. employed GO/SnO2 /PANI nanocomposites for the spray modification of quartz crystal electrodes. This novel architecture allowed them to obtain a sensitivity of 29.1 Hz/%RH with low hysteresis and shorter response and recovery times (7 and 2 s, respectively) [105]. The same group also attempted to use layer-by-layer techniques to increase molecular level organization; using (PANI/GO)n /(PSS/PDDA)2 nanocomposite multilayer films (n = 3, 5 7), the sensitivity, however, did not surpass the previous studies (20.0 Hz/%RH) [106]. In general, the sensing mechanism of humidity sensors is based on the adsorption and desorption processes of water molecules onto the functional groups of GO (hydroxyl, carboxyl, and epoxy) and conducting polymers (amine, sulfur, etc.). More specifically, chemisorption process occurs when a water monolayer is initially formed, and then subsequently physisorption arises from the deposition of a second

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Fig. 6 Schematic of humidity sensing at PANI/GO films. Reproduced from [106] with the permission of Elsevier. Copyright Elsevier 2018

layer of water molecules through the action of double hydrogen bonding. At high humidity, a second-layer physisorption of water molecules is formed through the action of single hydrogen bonding [106], as depicted in Fig. 6. Impedimetric systems were assessed by Li et al., utilizing crosslinked and quaternized poly(4-vinylpyridine) (QC-P4VP)/RGO bilayers deposited onto goldinterdigitated electrodes which afforded a high sensitivity of 2.1–0.18% RH with a hysteresis of ~4. RH (8.3-fold higher than that obtained by Yuan et al.). Wang et al. proposed to calculate the capacitance values from impedance data at a frequency of 100 Hz to assess PPy/GO nanocomposites deposited on Ni/Cu interdigitated electrodes for use as a humidity sensor. Here, a sensitivity of 1670.3 pF/%RH could be reached, and no hysteresis was observed between 11 and 43% RH from adsorption–desorption experiments, but at 67% RH hysteresis is 1.12% [107]. Interestingly, Pang et al. developed a wearable chemoresistive humidity sensor based on foamed GO, PEDOT:PSS, and colloidal Ag nanocomposites deposited on PET substrate for respiration monitoring. This device was capable of detecting physiological activities including water loss during fast, normal, and deep breathing period which could indicate dehydration symptoms of patients [108].

3.6 Nerve Agents Nerve agents are highly toxic chemicals based on organophosphorus compounds that act by disrupting nervous system and bodily functions by inhibiting tissue cholinesterase. Originally having pesticides/insecticide applications, they were developed for lethal military weapon purposes due to their high toxicity. The two main classes of nerve agents are G agents (sarin, tabun, and soman), based on alkyl esters of methylphosphonofluoridic acid or of dialkylphosphoramidocyanidic acid that act via

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inhalation; and V agents(VX), being alkyl esters of S-dialkylaminoethylmethyl phosphonothiolic acid formulations that act by way of skin penetration and by inhalation of aerosol [109, 110]. To aid the security of society, the creation of devices capable to detect these gases is of utmost importance. In this field, Yu et al. developed a PANI-based gas sensor to detect dimethylmethylphosphonate (DMMP), a simulant of the nerve G-agent sarin; however, although the minimum detectable level was 5 ppb, it was limited to short time exposure [111]. Later, this problem was addressed by the addition of 0.28% (wt) graphene and 13.1% (wt) of poly(ethylene oxide) (PEO) as a dispersing agent to PANI nanofibres. The compounded nanocomposite could be screen-printed onto cellulosic paper. The adhesion of the nanocomposite film to the substrate was enhanced by initially depositing a 5-μm-thick film of poly(vinylbutyral-co-vinyl alcohol-co-vinyl acetate) (P(VB-co-VAco-VAc)) copolymer onto cellulose. The resulting nanocomposite decreased the minimum detectable level to 3 ppb of DMMP gas, with response and recovery time of 2 and 35 s. After repetitive sensing and stability experiments, it was demonstrated that this sensor maintained their initial signal response when exposed to 300 ppb over a long time period [112].

4 Summary and Conclusion The development of graphene–polymer composite-based gas sensors devices has progressed much within the last two decades. Interesting approaches to solve sensitivity and selectivity problems have been demonstrated during composite preparation. These include tailoring electrical properties during in situ polymerization or by utilizing the oxidative polymerization process to reduce graphene oxides. Some devices have been shown to be capable of operating for long periods with low hysteresis. To reduce the need for various sensors in the same device, some gas sensors have also demonstrated that they are not affected by temperature (up to 70 °C) or changes in humidity. Despite the successful examples aforementioned, these gas sensors have not been commercialized as yet. We hope within this work to have stimulated researchers to combine the already acquired knowledge with new work to develop a new generation of gas sensors by taking advantage of nanotechnology and the Internet of Things (IOT). Acknowledgements This work was carried out with financial assistance from the Brazilian funding agencies: São Paulo Research Foundation—FAPESP (2013/14262-7) and National Council for Scientific and Technological Development—CNPq. Nirav Joshi for his helpful contribution. Conflicts of Interest The authors declare no conflict of interest.

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Chapter 10

Functionalization of Graphene and Its Derivatives for Developing Efficient Solid-State Gas Sensors: Trends and Challenges Debanjan Acharyya and Partha Bhattacharyya

1 Introduction Graphene is used as a promising gas sensing material due to its several unprecedented advantages like (i) highly sensitive detection of target species at trace (ppb) or even molecular level; being a zero bandgap material, small change in the environment alters the active donor and acceptor level at the “Dirac point” of graphene [1]. Therefore, due to the infinitesimal change in the free charge carriers (as a result of gas adsorption/desorption on it), it causes a noticeable change in the output voltage/current/resistance [2]; (ii) very high sensitivity; due to its 2-D crystalline structure, all of its atoms are on the surface, which eventually offers very high surface-to-volume ratio for gas adsorption and desorption [3]; (iii) fast detection of target gases even at room temperature [2]; due to exceptionally high carrier mobility (10,000–50,000 cm2 V−1 s−1 at room temperature) and low electrical noise at room temperature, electron/hole transfer between measuring electrodes during the adsorption/desorption process was found to be very fast [4, 5]. However, intrinsic graphene is a perfect hexagon of carbon atoms and possess several disadvantages as a gas sensing material because (i) it has no functional groups (which are considered to be the active sites for gas adsorption/desorption) [2]; (ii) zero bandgap, and therefore due to very low intrinsic resistance for detection of reducing vapor (where sensor resistance decreases with exposure to gas) pristine graphene is not suitable [6]; (iii) it is hydrophobic in nature, and therefore, incorporation of functional groups employing low-cost aqueous solution-based chemical synthesis is not possible [7–8]. In this context, graphene oxide (GO), oxidized form of graphene, can be used as a popular gas/vapor sensing material as GO contains highly dense oxygen-containing functional groups (carboxyl, carbonyl, hydroxyl, and epoxy) attached with the carbon D. Acharyya · P. Bhattacharyya (B) Department of Electronics and Telecommunication Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, India e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_10

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lattice [9]. Nevertheless, such functional groups introduce a hydrophilic nature in GO and increase the bandgap [10, 11]. As a hydrophilic material, GO offers stable suspensions in aqueous media from where graphene oxide sheets can be exfoliated by simple ultra-sonication and subsequently functionalized by foreign materials [12]. However, graphene oxide (GO), even though contains several pendant oxygen functional groups, is electrically insulating (as high defect density in the carbon structure significantly reduces the extraordinary electronic and mechanical properties of pristine graphene) and impedes its use as a resistive gas sensor [13, 14]. However, restoring the physicochemical properties of graphene by means of reduction in GO has widely been considered as a promising approach for the graphene derivative-based sensor development [10, 15]. More importantly, the reduced graphene oxide (rGO) offers an altered chemical structure from pristine graphene, having residual oxygen content, carbon vacancies, and clustered carbon structures (i.e., pentagons/heptagons) but at the same time possess comparable carrier mobility and conductivity as that of the pristine graphene [16]. Derivatives of graphene (e.g., GO, rGO) offer high surface adsorption/desorption energy barrier and poor selectivity, which impede its use in practical gas sensing applications [17, 16, 18]. Therefore, in order to improve the gas sensing performance of graphene derivatives, researchers have further explored chemical or physical functionalization approach for graphene/graphene derivatives with nanomaterials (in particular, polymers, metals, and metal oxides) such that a site-selective enhanced gas adsorption/desorption can be achieved [3, 19, 20]. Nevertheless, such functionalization also enhances surface-to-volume ratio; hence, surface adsorption/desorption energy barrier reduced significantly. Since the discovery of metal oxide-based gas sensors, the disadvantage of high operating temperature, resulting in high power consumption and short lifetime, remains a key bottleneck [2]. In this context, the functionalization of graphene/graphene derivatives using metal oxides not only retains the individual advantages, but they may also bring forth additional synergistic properties which are desirable and advantageous for gas sensing performance [21]. Moreover, functionalization with n-type metal oxide with p-type graphene/graphene derivative results in the formation of a p–n junction which may exhibit promising sensing performances than that of the individual material(s) as well as metal oxide-based p–n homojunctions [22, 23]. Further, in case of metal nanoparticle functionalization, Fermi level shift in graphene/graphene derivatives is observed which depends on the work function difference between graphene and the metal [24]. Such Fermi level shift can enhance the gas adsorption/desorption. On the other hand, metal NPs act as a catalysis to enhance the selectivity toward particular species [25, 26]. Moreover, functionalization of graphene with polymer has been considered as the most promising approach among all the functionalization techniques, as polymer can tune both functional groups and conductivity of graphene/graphene derivatives through the formation of strong covalent bonds [27]. Therefore, the strong electronic interaction between polymer and graphene/graphene derivatives enhances the charge transport in the presence of target gas; hence, enhanced sensing performance is observed [28, 29]. In addition, incorporation of selective functional groups (e.g., carboxyl, amine, etc.)

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on the basal plane or edge sites of graphene during the synthesis of graphene/graphene derivatives was also investigated to obtain enhanced sensing performance. Recently, 2D materials are considered as the emerging class of nanomaterial for gas sensing application due to their unique thickness-dependent physical and chemical properties and very high surface-to-volume ratio. 2D materials can easily be fabricated as chemiresistive field-effect transistors (FETs), which inevitably offer low power consumption and long-term stability. So far, several 2D nanomaterials like graphene (and its derivatives), metal di-chalcogenides (like MoS2 , WS2 , MoSe2 , VS2 , and SnS2 ), hexagonal boron nitride (h-BN), and black phosphorus (BP) have been employed as promising gas sensing materials [7, 17]. Among these 2D materials, graphene/graphene derivative is considered as the most favorable material for the functionalization by foreign elements due to the following reasons: (1) the hexagonal graphene structure offers ultrathin 2D nanostructure which is an ideal platform for anchoring organic/inorganic nanomaterials of various sizes and shapes [15] and (2) the graphene structure supports the interfacial electron transfer generated by anchoring organic/inorganic materials and hence, stable hybrid structure can be obtained [30]. So far, a large number of strategies have been developed to functionalize the graphene/graphene derivative surface (by organic/inorganic nanomaterials) toward the development of efficient gas sensors (discussed in a subsequent section) [31], (Yin et al. [32]). The discussions on other (than graphene and its derivatives) 2 D materials and its functionalization for gas sensor application are beyond the scope of this particular book chapter. The progress on the functionalization of graphene (or its derivatives) has been addressed in a scattered fashion in several reviews focusing either on synthesis and characterization or on the applications like supercapacitors, electrochemical actuators, biological sensors, etc. [17, 8]. On the other hand, several review reports on the graphene-based hybrid gas sensor have also been reported [2] in the recent past. However, a comprehensive review focusing on the technological advancement of the gas sensors due to the improvement caused by the functionalization of graphene (or its derivatives) is still missing and is a timely demand, which is the background for the motivation of this particular chapter. In this book chapter, in the beginning, the functionalization routes of graphene and graphene derivatives relevant to the development of gas sensors have been discussed with emphasis on the advantages/prospects offered by such functionalization(s). The state-of-the-art progress of functionalized graphene/graphene derivative-based gas sensor devices emphasizing the performance improvement quotient of the different sensors in terms of sensitivity, selectivity, robustness/stability, and transient characteristics, by correlating the corresponding mechanism, has been discussed subsequently. Finally, the existing challenges in order to realize practically feasible functionalized graphene/graphene derivative-based gas sensor system and its possible technological solution along with the roadmap of the future research direction are presented followed by the concluding remarks.

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2 Functionalization Strategies of Graphene for Developing Gas Sensors Functionalization of graphene surface for the gas sensor applications can be achieved by introducing transition metal nanoparticles, polymers, or other suitable modifiers [33]. In general, functionalization schemes can be broadly divided into two categories: (i) covalent and (ii) non-covalent [27]. The covalent functionalization is usually achieved via metal doping and reaction with oxygen-containing functional groups which eventually modulate the graphene structure [34]. On the other hand, the non-covalent methods appear to be more versatile and promising one as natural hexagonal structure of graphene remains unaffected [27, 35]. Moreover, non-covalent methods are categorized as functionalization technique which employs hydrogen bonding, van der Waals force, coordination bonds, electrostatic interaction, and/or π–π stacking interactions [30]. The interactions originated from non-covalent functionalization are relatively weak compared to the covalent ones although multiple non-covalent bonds working in concert can lead to highly stable functionalization [36]. In addition, non-covalent bonds are easy to achieve (through hydrothermal or dip coating method) over the entire graphene surface [37]. In summary, when the electrical conductivity and surface properties of graphene are of prime importance, non-covalent functionalization methods are usually preferred over the covalent ones [27]. In contrast, when the stability and the strong mechanical properties of modified graphene are of key concern, covalent functionalization is usually chosen [3]. Graphene edges having dangling bonds are more reactive than that of the ones in basal plane, having strong covalent bonds with highly delocalized π electrons on the sp2 -hybridized carbon atoms [38]. Thus, the dangling bonds at the graphene edge sites can be used for both covalent and non-covalent functionalizations toward various functional groups and foreign elements [39]. Among the functionalization schemes, oxidation of graphene for the formation of graphene oxide followed by the reduction of that GO to produce rGO was found to be a very popular method [20]. The GO sheet offered reactive carboxyl groups at the edge and epoxy and hydroxyl groups on its basal plane, which can be derived employing facile solution-based oxidation process of graphite with strongly oxidizing reagents (e.g., KMnO4 , HNO3 , and/or H2 SO4 ) [36]. On the contrary, reduction of GO by strong reducing reagents (e.g., hydrazine hydrate) is employed to obtain rGO, which offers functional groups as well as better conductivity and mobility compared to that of GO [20]. In addition, thermal and electrochemical reduction of GO was also found to be the efficient route for the formation of rGO sheets [3]. Contemporary research reveals that, the key motivation of functionalizing graphene and graphene derivatives with foreign materials for the development of efficient gas sensors is usually implemented by either of the following approaches: (i) functionalization via direct doping/binding/loading of graphene lattice and (ii) modification via oxygen-containing functional groups. For the first approach, theoretical studies showed that the band structure of the graphene can be engineered

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by doping [40]. Liu and his co-workers [3] also experimentally verified such theoretical prediction. Functionalization by metal and metal oxide NPs on graphene can be achieved in two different ways, viz., (i) in situ binding [30] and (ii) postimmobilization (ex situ hybridization). Post-immobilization includes mixing of separate solutions of graphene nanosheets and pre-synthesized NPs. In this method, at first, metal oxide NPs are synthesized and then metal oxide NPs are loaded on graphene/graphene derivatives for functionalization. In such process, covalent C–C coupling or non-covalent π–π stacking or reactions are responsible for the formation of functionalized graphene sheets [33]. However, such post-immobilization suffers from non-uniform coverage and, hence, low repeatability of functionalized graphene sheets [41]. Electrodeposition of metal nanoparticles on graphene/graphene derivatives was found to be an efficient route for graphene/graphene derivatives’ functionalization [3]. In this approach, modification via residual functionalities can form both covalent and non-covalent bonds with graphene structure. Nevertheless, polymers, organic, and inorganic salts are also very popular for the introduction of oxygencontaining group on the graphene/graphene derivatives surface. For example, Hisao et al. [42] experimentally established that functionalization of graphene via the functional groups (such as carboxylic acid-terminated polymer precursors) offered greater thermal stability over that via free radical grafting methods (e.g., via amine groups).

3 Sensing Mechanism of Functionalized Graphene/Graphene Derivative-Based Gas Sensors The sensing mechanism of graphene/graphene derivative-based solid-state gas sensors relies on the change of conductance upon exposure to gas/vapor molecules which get adsorbed on the surface of graphene and subsequently, varies the carrier concentration/carrier density of the graphene (or its derivative) surface [2, 14]. Explicitly, in the exposure of oxidizing gas/vapors (like NO2 ) molecules, it (oxidizing gas/vapors) withdraws electrons from the graphene derivative surface; therefore, the hole concentrations (i.e., majority carrier) on the graphene derivative surface (considered as a p-type semiconductor in nature) increase due to lower recombination probability and hence, resistance decreases significantly [43]. On the contrary, in the presence of electron-donating gas/vapor molecules (reducing gases/vapors like H2 , ethanol, and NH3 ), electron–hole recombination increases which decreases the effective hole concentrations on the graphene derivative surface; hence, increase in resistance value is observed [8]. For polar molecules (e.g., methanol), the sensing mechanism can be explained from the scattering effect of carriers caused by the presence of absorbents [44]. Adsorbents may cause scattering of electrons or holes, and consequently decrease carrier mobility and hence conductance of graphene [45, 46]. Moreover, the adsorption of gas/vapor molecules on graphene surface is possible in both the ways. On the one hand, adsorption on low-energy binding sites (i.e., sp2 -bonded carbon on the basal plane) leads to room temperature or low-temperature

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sensing performance [24]. On the other hand, the adsorption on higher energy binding sites (on the edge or basal plane) such as defects, vacancies, and functional groups on the surface of graphene which results in higher change in conductance level, thereby enhanced sensitivity [47]. As described by Lee et al. [48], response amplitude (i.e., absolute change in resistance/voltage/current value due to the exposure of target species) (Ar ) of graphene surface (toward reducing and oxidizing gases) as a function of defect types can be described by the following equation:  Ar (sensitivity) = A f 1 − e

−

t−t0 τf



  t−t0 + A s 1 − e − τs

(1)

where A is the amplitude, t 0 is the time when the target gas pulse is turned on, and τ is the response time constant for saturation. The subscripts f (fast) and s (slow) are related to low-energy and high-energy binding sites, respectively. However, in pristine graphene surface, covalent bonding between target gas/vapor molecule and graphene results in poor recovery characteristics of the pristine graphene-based gas sensors [49]. To mitigate such problems, functionalization of graphene surface is a popular approach. Functionalization of graphene by organic or inorganic molecules/agents enhances the recovery characteristics as target species attached to the oxygen-containing functional groups (on graphene derivatives) employ weak non-covalent bonding, which can easily be removed than that of covalently bonded (relatively stronger bonds) target species [4, 34, 35]. For example, noble metal NPs (viz., NPs of Pt, Pd, Au, and Ag) were successfully dispersed on the graphene sheet which resulted in enhanced and selective gas response [20, 33, 41]. Functionalization of graphene/graphene derivatives by metal oxide nanoparticles (NPs) is also found to be beneficial for improving sensing characteristics as the incorporation of metal oxide NPs on the surface of graphene leads to junction formation (having low barrier height) between graphene derivatives and metal oxides. Such shallow localized junctions facilitate fast carrier transport due to low barrier height and high mobility of carriers in graphene/graphene derivatives [10]. On the other hand, for polymer-functionalized graphene surface, sensing mechanisms are governed by two major factors, viz., (i) the interaction of gas/vapor molecules alters both carrier density and mobility in polymer surface; (ii) the polymer chain transformation (e.g., swelling and doping) due to interactions with gas/vapor species, resulting in a change of conductance [50, 51]. Adsorption/desorption behavior of target species on the polymer-functionalized graphene/graphene derivatives surface was explained by the Langmuir–Henry–Clustering model [1, 48]. Employing such model, Tung et al. [1] calculated the target vapor concentration from the response magnitude (AR ) of polymerized graphene surface. In addition, limit of detection can also be predicted from the following equation:      b L f  − f AR = + k H f + f − f  f n 1 + bL f

(2)

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Fig. 1 Bar diagram representation of a distance (Å), b adsorption energy (eV), and c charge transfer (electron) for NO2 , NH3 , CO, and H2 O gas molecule at edge –COOH (carboxyl group) in plane position or the basal plane. Reproduced from Maity et al. [53] with permission from IEEE

where bL is the Langmuir affinity constant, k H is the Henry diffusion parameter,  n is the average number of solvent molecules per cluster, B is the extra clustering coefficient, f is the solvent fraction over which clustering takes place, and f is the vapor fraction over which Langmuir’s diffusion is replaced by Henry’s diffusion. By introducing different functionalized groups and vacancies on the pristine graphene surface, Lee et al. [48] calculated the different gas adsorption/desorption parameters including adsorption energy of pristine and functionalized surface for the adsorption of oxidizing and reducing gases (NO2 and NH3 were used as test gases). From the DFT calculation, they showed that adsorption of oxidizing gas on the pristine and functionalized surface (except carbonyl-functionalized surface) is more favorable than that of reducing gas. In addition, it is also evident from their results that the sp3 defects have negligible effect on gas sensing performance, while vacancy defect significantly enhances the sensing performance toward both oxidizing and reducing gases even higher than that of graphene surface functionalized with different functional groups (e.g., epoxide, carbonyl, ether, etc.). In the recent past, the impact of oxygen functional groups on the basal plane and edges (of graphene) for different gas interactions was theoretically investigated by Maity et al. [52]. It is revealed from their computational results that the presence of carboxyl (–COOH) group enhanced the sensing toward NH3 , CO, and H2 O, while hydroxyl (–OH) group is the most favorable one for NO2 gas adsorption/desorption. The comparative results are depicted in Fig. 1a–c.

4 Functionalized Graphene/Graphene Derivative-Based Gas Sensors 4.1 Metal Oxide Nanoparticle-Functionalized Graphene/Graphene Derivative-Based Gas Sensors Pristine graphene/graphene derivative surface suffers from poor gas/vapor sensing performance due to insufficient amount of adsorption/desorption centers (for

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target species) on its surface. However, incorporation of metal oxide on the graphene/graphene derivative surface leads to increase in effective surface area as well as adsorption/desorption centers, which inevitably enhance the gas/vapor sensing performance. In addition, metal oxide incorporation also reduces the agglomeration tendency of graphene/graphene derivative surface [10]. However, from the functionalization (of graphene/graphene derivatives) point of view, metal oxide NPs are considered as efficient nanostructures (for graphene), as the size and concentration of NPs can easily be tuned to obtain optimized sensing performance for the functionalized sensor. During the functionalization, metal ions (of metal oxide) are adsorbed by the hydrolysis–condensation reaction with already adsorbed oxygencontaining functional groups on graphene derivative surface. Such attachment can be tuned by altering the NPs size and concentration which is also known as “steric hindrance effect” [54]. For example, the size of NPs was tuned employing “steric hindrance effect” to obtain optimized ppb level benzene sensor reported by Meng et al. [54]. The enhanced gas sensing performance due to metal oxide NPs functionalization of graphene/derivatives has been discussed employing heterojunction formation between n-type metal oxide and p-type rGO, followed by fast carrier transport through these shallow localized junctions due to low barrier height and high mobility of carriers in graphene/graphene derivatives. It is revealed from the earlier reports that p-type rGO forms low barrier Schottky junction (i.e., barrier height of ~0.1– 0.5 eV) with most of the n-type metal oxides (e.g., SnO2 , ZnO) [10]. For example, Mao et al. reported that the work function of n-type SnO2 is higher than that of rGO; therefore, electron transfers from rGO to SnO2 and subsequently, low Schottky barrier height of 0.2 eV was formed at the rGO-SnO2 junction. It is also found that initial resistance (after exposed to air) of the metal oxide functionalized rGO structure is much lower (due to the presence of conducting rGO) than that of pristine metal oxide nanostructures. In addition, wider electron depletion region at the meal oxide side (of p-rGO-n- metal oxide junction) is formed in functionalized structure (as the carrier concentrations in rGO are several orders of magnitude higher than that of metal oxide). Therefore, such active junction property is dramatically modulated in the presence of both oxidizing and reducing target species, and hence drastic change in device resistance is observed. For example, Mao et al. [55] reported on the gas sensing performance of SnO2 NPs-functionalized rGO surface. Schematic of the device structure and corresponding sensing system is depicted in Fig. 2a, b. The result portrays that due to the functionalization of rGO by SnO2 NPs, the conductance of SnO2 NPs decreased almost two times. Moreover, the enhancement in selectivity toward NO2 was explained through p–n junction formation between rGO (p-type) and SnO2 (n-type) NPs which effectively modulates the electronic-transfer process during the adsorption/desorption process. Due to the formation of such p–n junction, in the presence of oxidizing gas (like NO2 ), more electrons are attracted from the rGO toward SnO2 which results in shifting of Fermi level (of rGO) toward the valence band and thus conductivity of the sensor layer increases [55]. On the other hand, with exposure to NH3 (as an electron donor), due to hole depletion at the junction, fewer electrons are injected into the rGO, resulting in a higher decrease

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Fig. 2 a Schematic of the SnO2 NCs functionalized rGO layer-based sensor device structure. b Schematic of the sensor testing system of the SnO2 NCs functionalized rGO layer-based sensor device (Reproduced from Mao et al. [55] with permission from The Royal Society of Chemistry). c Schematic representation of a possible sensing mechanism of the nickel oxide-reduced graphene oxide (NiO-rGO) nanocomposite exposed to H2 gas and d energy band diagram of the NiO-rGO and the hole transfer between NiO and rGO (E CB: Conduction band Energy level, E VB : Valance band energy level, E f : Fermi energy level) (Reproduced from Ren et al. [57] with permission from Elsevier). e Schematic illustrations of the NO2 sensing mechanism in Co3 O4 (Cobalt oxide) NPs-functionalized rGO surface (Reproduced from Chen et al. [58] with permission from Elsevier)

in conductivity compared to that of the pristine rGO. It is worth mentioning that such sensor showed improved sensing performance toward oxidizing gas than that of reducing gas. Similar kind of observation was also reported by Zhang et al. [43] in SnO2 NPs-functionalized rGO surface. Another popular n-type metal oxide NPs, i.e., ZnO, was also employed to functionalize rGO and reported as highly sensitive and fast responsive NO2 sensor at room temperature [56]. Recently, p-type metal oxides such as NiO and Cu2 O were also employed to functionalize rGO layer. Similar to the observation for n-type metal oxide, junction having very low barrier height is also formed between p-type metal oxide and p-rGO. Ren et al. [57] stated that, since the valence band in NiO (−4.64 eV) is lower than that of rGO (−4.40 eV), the holes of NiO will transfer toward rGO to obtain an aligned Fermi level, and hence, formation of lower barrier height for hole (Bp ) at the junction

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is observed. Moreover, with exposure to reducing gas (i.e., H2 ), adsorbed oxygen ions on the sensing surface react with H2 and release the electron. Consequently, these electrons get transferred to the conduction band of NiO, which further initiate the transfer of holes from rGO to valance band of NiO to maintain the charge neutrality. Therefore, the device resistance increases significantly in the presence of reducing species even at room temperature. Similar type of observation was also reported by Kamal [59] for NiO functionalized graphene surface. In another endeavor, Zhou et al. [60] prepared p-type-Cu2 O NPs with a diameter of ~3 nm which were uniformly grown on graphene sheets for functionalization. The graphene sheets facilitated the controlled nucleation of Cu2 O NPs by acting as a hexagonal molecular template and subsequently reduce the nucleation energy. Moreover, at room temperature, the Cu2 O/graphene surface exhibited promising sensitivity toward H2 S, even at low concentration (lowest detection limit was reported to be 5 ppb) [60]. The selectivity toward H2 S was also found to be appreciable in comparison to NH3 , H2 , CH4 , and C2 H5 OH. The functionalization of rGO by metal oxide NPs also leads to improved morphology in terms of large specific surface area, more conductive path, and more active sites which results in enhanced sensing performance than that of pristine graphene/graphene derivatives. For example, Chen et al. [58] observed improved room temperature NO2 sensing performance based on Co3 O4 NPs-functionalized rGO surface compared to that of pristine rGO. They also demonstrated that sticking and agglomeration probability of pristine rGO was reduced due to the intercalation of Co3 O4 NPs between rGO layers [58]. In this functionalized structure, Co3 O4 NPs behave like nanopillars between rGO layers, resulting in a 3-D microporous structure [58] (as shown in Fig. 2c). In such microporous layer, target gas can diffuse in X-Y-Z directions (for pristine rGO surface it is only X-Y) driven by the capillary force and thereby, sensitivity improves significantly. In addition, the strong coupling effect between Co and graphene inevitably enhances the oxygen reduction ability as adsorption of atmospheric oxygen on the graphene matrix and Co makes the CoO more non-covalent ionic bonding. In this scenario, with the exposure of NO2 , electrons can be indirectly extracted from the graphene through the bridging oxygen, which leads to significant decrease in the resistance in the exposure of NO2 than that of pristine graphene counterpart. In addition, Co3+ centers in graphene layer also offered lower adsorption energy and favorable adsorption centers for NO2 molecules. Moreover, the resistance was not recoverable even after withdrawal of NO2 pulse. However, with exposure to methanol and ethanol for withdrawal of the pulse the resistance returned to the baseline value although the sensitivity was found to be very poor (~9.7% toward 1000 ppm methanol). A schematic of the sensing mechanism is depicted in Fig. 2d, e. In another work, α-Fe2 O3 NPs-functionalized rGO surface was found to be highly sensitive toward ethanol [61]. Enhanced sensing performance of the α-Fe2 O3 NPs-rGO sensor was illustrated employing highly conducting “electric bridge” formation by rGO between adjacent α-Fe2 O3 NPs. Gas/vapor sensing performance of metal oxide NPs-functionalized graphene/graphene derivative-based sensors are summarized in Table 1. In case of metal oxide NPs-functionalized gas sensors, sensing performance is enhanced compared to that of pristine one due to

NO2

Benzene

NO2

NO2

NO2

Ethanol

H2 S

H2

ZnO NPs-rGO

SnO2 NPs-rGO

SnO2 NPs-rGO

SnO2 NPs-rGO

Co3 O4 NPs-rGO

αFe2 O3 -graphene

Cu2 O-rGO

NiO NPs-rGO

50 °C

RT

280 °C

RT

RT

30 °C

210 °C

RT

Operating temperature

0.5–10%

5–100 ppb

1–1000 ppm

60 ppm

1–100 ppm

2–500 ppm

5–100 ppb

1–25 ppm

Detection range

28 s at 1% H2

450 s 60 ppm

Rair /Rgas = 88%

(Rgas −Rair )/Rair = 64% at 1% H2

65 s at 100 ppm

I air /I gas = 2.8 at 100 ppm

2–3 min at 100 ppb

177 s at 5 ppm

Rair /Rgas = 4.63 at 5 ppm

(Rgas −Rair )/Rair = 38% at 100 ppb

1–3 min

(I gas −I air )/I air = 98% at 100 ppb

4–5 s at 1000 ppm

165 s at 5 ppm

Rair /Rgas = 25.6% at 5 ppm

Rair /Rgas = 30 at 1000 ppm

Response time

Response magnitude

146 s at 1% H2

3–4 min at 100 ppb

6–7 s at 1000 ppm

–

3–4 min at 100 ppm

510 s at 5 ppm

–

499 s at 5 ppm

recovery time

Ren et al. [57]

Zhou et al. [60]

Liang et al. [61]

Chen et al. [58]

Mao et al. [55]

Zhang et al. [43]

Meng et al. [10]

Liu et al. [56]

References

Rair resistance in air, Rgas resistance in gas/vapor, I air current in air, I gas current in gas/vapor, RT: room temperature, ppm: parts per million, ppb: parts per billion

Target gas

Materials

Table 1 Sensing performance metal oxide nanoparticle-functionalized graphene/graphene derivative-based gas sensor

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the formation of low barrier Schottky junction between n- or p-type metal oxide and p-type graphene/graphene derivatives. Such junction facilitates faster carrier transport between measuring electrodes; hence, response time and recovery time are often found to be faster. However, the selectivity of such metal oxide NPs-functionalized gas sensor was found to be poor in most of the cases. For example, SnO2 NPsfunctionalized graphene/graphene derivative-based gas sensors are found to offer high response magnitude toward both H2 (reducing species) and NO2 (oxidizing species) [10]. To circumvent such selectivity issue, metal NPs were introduced and discussed in the following section.

4.2 Metal Nanoparticle-Functionalized Graphene/Graphene Derivative-Based Gas Sensors Poor selectivity (toward particular target species) is the major concern for the metal oxide NPs-functionalized graphene/graphene derivative-based gas sensors. To enhance the selectivity of the graphene/graphene derivatives surface, metal NPs functionalization was found to be a promising route. Metal nanoparticles (NPs) can be categorized as (i) catalytic metal (e.g., Pt, Pd, Ag) and (ii) non-catalytic metal (e.g., Al, Cu) [62]. In case of metal oxide nanostructure-based gas/vapor sensor, incorporation of catalytic metal inevitably enhances the sensing response toward particular target species (owing to catalytic dissociation of target species) [63]. On the other hand, non-catalytic metals are used (for metal oxide-based sensor) as an electrode due to its passive nature toward the target species [64]. However, such categorization is not exactly applicable for metal NPs-functionalized graphene/graphene derivative-based gas sensors. In case of metal NPs graphene/graphene derivative-based sensors, both the catalytic and non-catalytic metals can enhance the sensitivity toward a particular species [65]. Here, sensing mechanism is predominantly governed by the work function difference between metal and graphene/graphene derivatives (discussed in the subsequent section). Moreover, catalytic dissociation of particular target species on catalytic metal is considered as an additional effect reinforcing the effect mentioned earlier. Functionalization of graphene with metal nanoparticle can be obtained by anchoring metal nanoparticles to the surface of graphene, GO, or rGO employing both covalent and non-covalent bonding techniques (Yin et al. [32]). In particular, it is experimentally proved that the presence of structural defects and oxygen-containing functional groups make GO and rGO an ideal template/matrix for the nucleation and growth of metallic nanostructures of Pd, Pt Au, and Ag [33, 35, 66, 67]. Since both the metal nanoparticles and graphene have excellent electron transfer properties and metal NPs enhance catalytic dissociation toward particular target species, therefore, the functionalization of graphene with catalytic metal NPs is considered to be a useful strategy for the selective detection of targets species [66–69]. In general, functionalization of graphene with above-mentioned metal nanoparticles is obtained

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Fig. 3 Schematic illustration of the fabrication steps of Ag functionalized rGO-based sensor devices and the subsequent sensing measurements. Reproduced from Cui et al. [74] with permission from The Royal Society of Chemistry

via the reduction of metallic salts using well-known chemical agents such as sodium borohydride, sodium citrate, and ethylene glycol [4, 22, 36, 70]. Such functionalization relies on the fact that the negatively charged functional groups on the reduced graphene oxide can accelerate the nucleation of positively charged metallic ions from the metallic salts (as a precursor), resulting in the controlled growth of metal nanoparticles on the rGO surface [33], (Yin et al. [32]). Further, functionalization of graphene with metal NPs can also be accomplished employing chemical binders (e.g., o-nitroaniline to 1,2-benzenediamine) which have a strong affinity toward the graphene surface via π–π stacking [71]. In an endeavor, Li et al. [72] introduced an innovative “green synthetic” method for the deposition of metal NPs (like Au, Ag, Pt, and Pd) employing reduction and stabilization agent as bovine serum albumin (BSA) and glucose, respectively. Besides the chemical reduction methods, mentioned above, electrochemical functionalization technique was also found to be very useful as size and density of the metal NPs can easily be tailored by adjusting the intensity of the applied electrochemical voltage/current [33, 66]. For example, Claussen et al. [73] employed the electrochemical technique to tune the Pt NPs size toward functionalization of multi-layered graphene nanosheets. The schematic illustration of the fabrication steps of rGO/Ag-based sensor devices is shown in Fig. 3. To study the effect on gas sensing performance resulting from graphene functionalization with metal NPs, several (i) catalytic (e.g., Pd, Pt, Au, Ag) and (ii) noncatalytic metals (e.g., Al, Co, Ni) were tested for experimental as well as theoretical validation [35, 36]. Nevertheless, several attempts were also observed to find out the most promising metal to obtain the best sensing performance toward different target species. In summary, it is revealed that the selection of metal NPs as a functionalizing agent plays a crucial role to enhance the sensing performance toward reducing or oxidizing species by catalytic dissociation. More elaborately, metals having very high work function (larger than 5 eV, like Pt, Pd) increase electron transfer from graphene/graphene derivatives to Pd NPs (i.e., p-doping effect in graphene); hence, increased amount of holes in graphene/graphene derivative surface is observed. Such phenomenon inevitably enhances the sensing performance toward reducing species (i.e., H2 , NH3 ) [6, 30]. On the other hand, for the metal, having lower work function (like Al, Ag) hole depletion region at the interface is formed due to electron

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transfer from Al NPs to graphene/graphene derivatives (i.e., n-doping effect) and hence, enhanced sensing performance toward oxidizing species was observed. For example, Cho et al. [6] observed enhanced sensing performance toward NH3 for Pd–graphene structure, while Al–graphene structure enhanced the sensing performance toward NO2 . The sensing results are depicted in Fig. 4a–d. In another work, Giovannetti et al. [36] employed density functional theory to compare the electron transport behavior for different metals (viz., Al, Ag, Cu, Au, and Pt) functionalized graphene surface, separately. Theoretical calculation confirmed that n-type doping can be obtained employing Al, Ag, and Cu metals, while p-type doping is achievable through Au and Pt (where metal work function is larger than 5.4 eV). Moreover, they also showed that functionalization with Pt offered the maximum Fermi level shift (~0.5 eV; with respect to the conical point) and hence, the best sensing performance can be obtained (depicted in Fig. 4e, f). Such theoretical observations were also experimentally authenticated by Chu et al. [75] and Shafiei et al. [76] for Pt metal NPs considering H2 as the test species. Shafiei et al. [77] deposited graphene-like nanosheets on SiC substrate by spray coating of graphene oxide followed by a reduction of the same using hydrazine vapor to obtain rGO. Subsequently, Pt NPs were deposited on the rGO surface employing e-beam evaporation technique. Such sensor offered low-temperature (100 °C) H2 sensing performance. On the other hand, Chu et al. [75] investigated the H2 detection using Pt functionalized epitaxial graphene. Here, the Pt layer acted as a p-type dopant and increased the conductance of rGO layer. Moreover, the enhanced H2 sensing performance was achieved as hydrogen atoms dissociated on Pt surface due to “spillover” effect [75]. Consequently, such dissociated H2 atoms get adsorbed on the edge and on the basal plane of rGO through the hydrogen bonding. Such hydrogenated form of graphene derivative offered increased work function of 4.97 eV, which is 0.2 eV greater than pristine graphene [8]. Such increased work function increases the work function difference between graphene derivatives and Pt and hence, larger resistance change was observed compared to pristine graphene derivative-based sensors. To cope with the present technological need of wireless sensor system, Lee et al. [78] fabricated radio-frequency identification (RFID)-based wireless H2 sensor employing Pt functionalized rGO sensor. It is also reflected from their work that due to the strong interactions between the Pt functionalized rGO surface and H2 molecules, change in electrical resistivity of the patch antenna was significantly improved (than that of pristine one), which eventually shift the reflectance of the RFID sensor tag and result in efficient hydrogen detection in wireless mode. “Holey rGO” (hrGO) (“holey graphene/graphene derivative is formed by removing a large number of atoms from the graphitic plane to produce holes distributed on and through the atomic thickness of the graphene sheets” [79, 80] was first synthesized by Vedala et al. [79] employing enzymatic oxidation of GO followed by a chemical reduction of the same with hydrazine. In the next step, hrGO flakes were further functionalized with Pt NPs employing pulsed electrodeposition technique and FET-based sensor was developed. The experimental results revealed that Pt/hrGO-based sensor offered highly selective H2 sensing performance at room temperature than that of Au/ hrGO and bare hrGO-based ones. The high sensitivity of Pt-hrGO hybrid sensors was correlated with (i) the increased edge-to-plane ratio, (ii)

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Fig. 4 a–d Gas sensitivity of graphene, Pd/graphene, and Al/graphene devices. a Transient response upon 1.2 ppm of NO2 gas. b Transient response upon 100 ppm of NH3 gas. c Sensitivity under NO2 concentration from 1.2 to 5 ppm. d Sensitivity under NH3 concentration from 5 to 100 ppm. All gas sensing tests were performed at the operating temperature of 150 °C (Reproduced from Cho et al. [6] with permission from The Royal Society of Chemistry). e Schematic illustration of interface dipole and potential step formation at the graphene–metal interface. f Calculated Fermi energy shift (EF ) with respect to the conical point, EF and change in the work function as a function of W M -W G , the difference between the pristine metal (W M ) and graphene (W g ) work functions (d is the equilibrium separation between graphene and metal sheets and “Z d is the effective distance between the charge sheets on graphene and the metal, Z d < d; as most of the charge is located between the graphene layer and the metal surface,” E F denotes Fermi level energy) (Reproduced from Giovannetti et al. [65] with permission from The American Physical Society)

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the presence of functional oxygen groups, and (iii) the Pt NPs decoration. In addition, the DFT calculation revealed that the “spillover” of the hydrogen from Pt NPs to the functionalized edges of graphene is also an energetically favorable process [79]. Quite similar to the case of Pt functionalization, Pd NPs also promotes the selective dissociation of H2 molecules [81]. Such dissociation facilitates the formation of a dipole on Pd surface and consequently decreases the work function of Pd. Hence, at the Pd–graphene derivative junction, the electrons are transferred to graphene derivative and deplete the majority carriers (i.e., hole) of graphene derivative, leading to an increase in resistance of the same. Moreover, the concentration and size of Pd NPs in rGO layer were found to be the crucial factor to obtain optimized sensing performance. Therefore, in several reports, researchers were found to improve the sensing performance by tuning the size/amount of Pd NPs. For example, Alfano et al. [25] tuned the amount of Pd NPs on graphene to obtain optimized H2 sensing performance at room temperature. They concluded that α- to β-phase transformation of palladium hydride layers (with Pd loading above 5 mg) is responsible for such enhancement in H2 sensing performance. In another work, Shin et al. [82] electrodeposited flower-like Pd nanoclusters on CVD-grown graphene. In their experiment, the population density of the Pd nanoclusters was controlled by adjusting the concentration of 1,5-diaminonaphthalene (DAN) (as a surfactant) during synthesis. Finally, for the Pd nanocluster having size of 300 nm, low ppm (0.1 ppm) H2 detection was observed. Not only the size and the concentration of Pd NPs, but also the layers in graphene sheets were also found to enhance the H2 sensing performance (than that of single-layer graphene–Pd structure) [83]. However, higher work function of metal not always confirms the best sensing performance toward all the reducing species. Higher catalytic dissociation of metal NPs toward particular target species also enhances the sensing performance. In addition, a comparative study on the functionalization of graphene with different metal NPs (i.e., Ag, Au, and Pt) for sensing ammonia (NH3 ) was systematically investigated by Karaduman et al. [84]. They showed that sensor functionalized by Ag NPs offered higher sensitivity, selectivity, better response time/recovery time, and stability toward NH3 than that of sensors functionalized by Au or Pt NPs. According to Karaduman et al. [84], the Ag NPs promote catalytic dissociation for NH3 and forms active radicals (acting as an efficient electron sink) due to its (Ag) large “Helmholtz double layer”. Ag NPs tend to form Ag2 O in air ambient which inevitably increases the depletion layer thickness at Ag–graphene interface. On the contrary, in the exposure of NH3 net charge transfer from NH3 to Ag NPs reduce the oxidation state of Ag, which results in increased hole depletion (which makes the graphene less p-type), and thereby higher change in conductivity occurs (compared to the pristine graphene). In a similar study, Cui et al. [85] and Song et al. [86] also found the Ag NPs functionalization enhances the NH3 sensing performance. They showed that the amount and size of Ag NPs on the rGO surface are the crucial factors to obtain optimized gas sensing performance [86]. On the other hand, Ag NPs-functionalized sulfonated rGO (Ag-S-rGO) surface was also found to enhance the sensing performance toward oxidizing species like NO2 as well as reducing species (like NH3 ). Huang et al. [87] developed a flexible NO2 sensor employing sulfonation of reduced graphene oxide

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(S-rGO) followed by functionalization of the same with Ag NPs. They also postulated that –SO3 H functional groups (having lone pairs of electron) of Ag-S-rGO surface reduce the adsorption/desorption energy barrier for the target species, while Ag promotes the catalytic dissociation toward NO2 and NH3 , and hence higher sensitivity was observed for both NO2 and NH3 [87]. Gas/vapor sensing performance of metal NPs-functionalized graphene/graphene derivatives sensors are summarized in Table 2. It can be envisaged from the above discussion that the introduction of catalytic/non-catalytic metal NPs eventually enhances the selectivity of the device toward a particular species. However, due to the formation of rectifying/non-Ohmic junction between metal NPs and p-rGO, carrier transfer kinetics is hindered and thus, such sensor often suffers from poor transient characteristics (compared to metal oxide NPs-functionalized sensor) as well as high operating temperature (due to the lower surface-to-volume ratio than that of metal oxide NPs-functionalized sensors).

4.3 Polymer-Functionalized Graphene/Graphene Derivative-Based Gas Sensors In connection with the previous section, it is revealed that the metal NPsfunctionalized sensors offer promising selectivity, due to catalytic dissociation of metal NPs toward particular target species. However, such sensor often suffers from sluggish transient characteristics as well as high operating temperature. In this context, polymer-based gas sensors usually offer room temperature sensing performance due to its highly porous interconnecting structure coupled with low surface adsorption energy toward the target species. In addition, polymer-based sensor is preferable for flexible or wearable electronics (if deposited on a flexible substrate) which are in line with the demands of future generation bio-compatible electronics [29]. However, response time and recovery time of polymer-based sensor are usually very sluggish. To mitigate this problem, polymer-functionalized graphene structure is found to be a very effective way to enhance the sensitivity as well as transient characteristics toward a particular species [28]. The graphene/graphene derivative can tune both the structural morphology and electrical conductivity of the polymer concomitantly. Nevertheless, during functionalization, polymer can also introduce functional groups (on the graphene/graphene derivative structure), which are favorable for gas molecule adsorption [10]. The significant improvement in gas sensing characteristics of polymerized graphene-based sensors is due to the following reasons: (i) covalent π–π interaction between polymer and graphene forms active π electron cloud which promote charge interaction between target gas and sensing surface [28, 50]; (ii) due to porous structure of polymer, the effective surface area of polymerized graphene was found to be very high which inevitably increases the adsorption/desorption of target gas, and hence higher sensitivity was obtained [51]; (iii) for reducing gas, polymer offers de-protonation which also decrease the adsorption energy of the surface and thereby room temperature sensing performance was improved [90].

H2

H2

H2

H2

NO2

NH3

NO

NH3

NO2

NH3

NH3

Pt-rGO

Pt-rGO

Pd-rGO

Pd-rGO

Pd-rGO

Al-rGO

Pd-rGO

Ag-rGO

Ag-S-rGO

Ag-rGO

Ag-rGO

RT

RT

RT

RT

RT

150 °C

150 °C

RT

100 °C

RT

175 °C

Operating temperature

10–1250 ppm

0.1–15 ppm

0.5–50 ppm

2500–10000 ppm

2–420 ppb

5–100 ppm

1.2–5 ppm

1000 ppm

40–8000 ppm

1–50 ppm

1%

Detection range

Response time 2–3 min – 7 min at 8000 ppm 40 s at 1000 ppm 5 min at 1.2 ppm 4 min at 100 ppm 71 s at 156 ppb 6 s at 10000 ppm 12 s at 50 ppm 5 s at 1 ppm 118 s at 1.25%

Response magnitude (Rgas −Rair )/Rair = 4.8% at 1% H2 Rair /Rgas = 9.8% at 50 ppm (Rgas -Rair )/Rair = 77% at 8000 ppm (Rgas −Rair )/Rair = 26% at 1000 ppm (Rair −Rgas )/Rair = 3% at 1.2 ppm (Rgas −Rair )/Rair = 2 at 100 ppm (Ggas −Gair )/Gair = 9% at 156 ppb (Rgas −Rair )/Rair = 18% at 10000 ppm (Rair −Rgas )/Rair = 74.6% at 50 ppm (Rgas −Rair )/Rair = 6.9% at 1 ppm (Rgas −Rair )/Rair = 67 at 1.25%

54 at 1.25%

6 s 1 ppm

6 s at 50 ppm

10 s at 10000 ppm

191 s at 156 ppb

16 min at 100 ppm

15 min at 1.2 ppm

490 s at 1000 ppm

5 min at 8000 ppm

–

3–4 min

Recovery time

Song et al. [86]

Karaduman et al. [84]

Huang et al. [87]

Cui et al. [85]

Li et al. [89]

Ao et al. [88]

Cho et al. [6]

Alfano et al. [25]

Johnson et al. [83]

Lee et al. [78]

Chu et al. [75]

References

Rair resistance in air, Rgas resistance in gas/vapor, Gair conductance in air, Ggas conductance in gas/vapor, RT: room temperature, ppm: parts per million, ppb: parts per billion

Target gas

Materials

Table 2 Sensing performance metal nanoparticle-functionalized graphene/graphene derivative-based gas sensor

262 D. Acharyya and P. Bhattacharyya

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For the functionalization of graphene with a polymer, solution mixing, melt mixing, and in situ polymerization methods are considered to be the most common approaches [10]. Besides, pulverization, heterocoagulation, and freeze-drying were also employed [29]. Graphene/graphene derivatives in polymer layer also act as conducting filler to modulate conductivity of the polymer layer. It is also reported that polymer increases the tunneling probability between neighboring graphene sheets and enhance the electron transport kinetics [91]. Filler concentration (i.e., graphene) can be tuned to achieve complete conductive path in a polymer matrix for different polymers [28]. This process is known as “percolation of polymer” [50]. From the family of polymers, conjugated polymers, like polyaniline (PANI), and polypyrrole (Ppy), poly(diallyl dimethyl ammonium chloride) (PDAC), polythiophene (PTh), and poly(3,4-ethylene dioxythiophene) (PEDOT), which possess πconjugated carbon chains that have been investigated intensely as a promising functionalization agent for graphene/graphene derivatives, due to their promising gas sensing properties [1]. In addition, these polymers offer various excellent properties, such as high porosity, environmental, and mechanical stability. Bai et al. [92], Wu et al. [93] and Guo et al. [94] developed graphene/PANI nanocomposites by in situ polymerization and found that the PANI functionalized sensor exhibited promising sensing toward reducing species. Such enhancement toward reducing species is due to the fact that the change in conductivity of the hybrid in the exposure of reducing species was due to the strong hydrogen bonding between reducing species and PANI by alteration of H-bonding between graphene derivatives and PANI [95]. After the removal of reducing species, reformation of the H-bonding between graphene derivatives and PANI restored and hence, the conductivity returned to its baseline value. For example, Guo et al. [94] synthesized graphene/PANI composite films for fabricating flexible and transparent NH3 sensors compatible with foldable and wearable electronic devices. In addition, it is also reported that due to functionalization of PANI, higher specific surface area was obtained for graphene/PANI composite films that can be achieved (up to 47.9 m2 /g) compared to that of PANI (~41.01 m2 /g). Apart from NH3 , graphene/PANI structure also offered promising sensitivity toward H2 [96], methanol [95], and toluene [97]. Similar to the PANI functionalization, Ppy [98], PTh [99], PEDOT [100], and PDAC [101, 102], functionalizations were found to offer better sensitivity toward reducing species, especially toward organic vapors (e.g., methanol) due to the π–π electron transfer between graphene and organic vapors. In another endeavor, Jang et al. [98] compared the sensing properties of the Ppy functionalization with different graphene derivatives such as GO and rGO. The Ppy/GO offered poor sensing performance than the Ppy/rGO as the electron transfer between Ppy and GO was relatively difficult, due to the poor interfacial affinity and the lower electrical conductivity of GO in the presence of target species. In addition, rGO played an important role in tuning the morphology of the Ppy structure also. Ppy was uniformly and thinly coated on rGO structure, and Ppy/rGO hybrids offered lesser aggregation tendency than that of GO/Ppy structure. On the other hand, a comparative investigation by Bai and co-workers [103] revealed that GO/Ppy structure offered the most promising NH3 sensing performance than that of GO/Ppy, GO/PEDOT, and GO/PANI structures.

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The higher sensing performance of the GO/Ppy sensor is attributed to the ultrathin nature of Ppy compared to other polymers. From the above discussion, it is imperative to mention that polymer-functionalized graphene/graphene derivatives exhibit better sensitivity toward reducing species. However, to enhance the sensing performance toward oxidizing species, co-functionalization using two different polymers was also found to be an effective route. Employing amalgamation of amine-rich polyethyleneimine (PEI) and polyethylene glycol (PEG), the number of protonated amine groups in PEI is significantly enhanced, which leads to a strong n-type doping effect on graphene [104]. Therefore, in the exposure of CO2 (electron absorber), dramatic decrease in aforementioned doping effect leads to large change in the resistance of PEI/PEG-co-functionalized graphene structure, which results in better sensitivity toward CO2 than that of the pristine one [104]. In addition, corresponding sensing mechanism is pictorially depicted in Fig. 5a, b. It is worth mentioning that tailoring of polymer nanostructure is also established to be a promising approach

Fig. 5 a, b CO2 sensing mechanism of a PEI- and b PEI/PEG functionalized graphene devices (Reproduced from Son et al. [106] with permission from Springer). c Gas selectivity of bare rGO and porous PEDOT/rGO-based device to 1 ppm different gases (Reproduced from Yang et al. [9] with permission from Royal Society of Chemistry). d Sensing response of rGO:PIL/PEDOT compositebased chemiresistor sensors upon periodic exposure to different analytes (PIL: poly ionic liquid, PEDOT: poly 3,4-ethylene dioxythiophene, PEI: polyethyleneimine, PEG: polyethylene glycol) (Reproduced from Tung et al. [91] with permission from Elsevier)

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for the improvement of response time and recovery time of the polymer functionalized gas sensors. For example, porous PEDOT/graphene structure offered faster response time and recovery time toward NH3 than that of continuous non-porous film of PEDOT/graphene structure [105] (as shown in Fig. 5c). Although the functionalization of graphene with polymer improves the transient kinetics of the sensor compared to the pristine polymer and pristine graphene/graphene derivative-based ones, such improvement is not really very significant in a true sense. In this context, ionic liquid of polymer can be employed as the linking agent between polymer and graphene/graphene derivatives which leads to fast response time and recovery time. For example, Tung et al. [91] developed graphene/PEDOT/PIL composite film-based chemical sensor, where rGO was stabilized and functionalized in a liquid suspension of poly(ionic liquids) (PILs), i.e., poly(1-vinyl-3-methylimidazolium). In such a structure, PIL acts as bridging molecules between PEDOT and rGO and enhances the carrier mobility [91]. Such rGO–PIL/PEDOT-based film sensor inevitably showed enhanced sensing performance with fast response time (e.g., ~20 s in the exposure of 2.5 ppm methanol) and recovery time (e.g., ~49 s in the exposure of 2.5 ppm methanol) toward different VOCs such as methanol, ethanol, acetone, methyl acetate, dimethylsulfide, and toluene than that of pristine rGO-based sensor (as shown in Fig. 5d). It is necessary to mention that in such composite structure, PEDOT acts as an absorbent layer for target vapor molecules. On the other hand, both PIL and rGO improve the carrier transport efficiency. Similar kind of observation was also reported (Ji et al. [107]) for poly(sodium styrene sulfonate) ionic liquid (PSS)/graphene-based structure, where benzene sensing performance and transient characteristics were found to be enhanced due to introduction of ionic liquid of polymer. In another work, Tung et al. [108] used PILs for the functionalization of graphene and developed CNT/PIL/rGO composite structure. Here, PIL serves both as a stabilizer and as a linker for assembling CNT onto the rGO surface [91]. This sensor exhibited promising ppb level sensing response for the detection of VOCs presented in exhaled breath (e.g., methanol, chloroform, benzene). Such promising performance of the sensor is attributed to the PIL-controlled enhanced electron transfer capacity between the hybrid 1D (i.e., CNT) and 2D (i.e., rGO) carbon and availability of additional active binding sites on CNT and rGO. It is often speculated that the functionalized polymer-based gas sensors can seldom be used for commercial applications as stability and reproducibility of the polymer matrix are poor in these hybrid structures. However, different polymers (e.g., polycaprolactone, polyepichlorohydrin polyvinyl alcohol, polyisobutylene, etc.) functionalized graphene nano-platelet-based sensor array was fabricated by Wiederoder et al. [109] which is commercially available for the selective detection of the chemical warfare agents. With the help of innovative data pre-processing methods including vector normalization, Z-scale, and Kalman filter, such sensing array can efficiently detect unique response signature of multiple chemical warfare agents such as acetone, hexane, toluene, and trimethyl phosphate. In another endeavor, polymerfunctionalized graphene-based sensing array was developed by Mackin et al. [110]

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Fig. 6 a Complete measurement system and sensor array insert, b system overview (μC: microcontroller, EGFET: Extended gate field-effect transistor), c graphene sensor diagram (S/D contact: Source/Drain contact, SU-8 is the epoxy-based negative photoresist), d microscope image of graphene sensor with channel region outlined in white (dashed), e microscope image of graphene sensor array, and f trans-impedance amplifier schematic. Reproduced from Mackin et al. [110] with permission from Royal Society of Chemistry

employing 5,1 0,15,20-Tetrakis-(pentafluorophenyl) porphyrinatocobalt (III) perchlorate (Co(tpfpp)ClO4 ) functionalized graphene structure. They also developed a novel sensing architecture employing such polymer-functionalized graphene which can detect NH3 in a rapid and convenient fashion (Fig. 6). The mean correlation coefficient between the sensor response magnitudes of the same set of sensor devices was found to be 0.999 which confirmed the highly consistent nature of sensor responses and excellent reproducibility of functionalized sensor [110]. Gas/vapor sensing performance of polymer-functionalized graphene/graphene derivative-based sensors is summarized in Table 3. In summary, polymer-functionalized graphene/graphene derivative-based sensor offers very high sensitivity even at room temperature but suffers from sluggish response time and recovery time (than that of metal oxide NPs and metal NPs-functionalized sensors) due to several number of percolated layered structures (of polymer). Therefore, to obtain high response magnitude (at room temperature) and fast response kinetics at the same time, formation of ternary structure (metal oxide NPs metal–NPs graphene or polymer–metal NPs graphene) is discussed in subsequent section.

NH3

NO2

NH3

Methanol

CO2

H2

NH3

NH3

NH3

DMMP vapor

Porous PEDOT/rGO

Porous PEDOT/rGO

PANI/graphene

rGO/PIL/PEDOT

rGO-PEI/PEG

PANI/graphene

rGO/Co(tpfpp) ClO4

rGO/aryl propargyl ether

rGO-polypyrrole

rGO/phenylenediamine

RT

RT

RT

RT

RT

RT

RT

RT

RT

RT

Optimum temp.

5–80 ppm

1 ppb–50 ppm

20–100 ppm

20–160 ppm

60–1000 ppm

500–5000 ppm

2.5–1000 ppm

1–6400 ppm

500 ppb–20 ppm

200 ppb–10 ppm

Detection range (ppm)

Response time ~280 s % at 10 ppm ~360 s at 2 ppm ~50 s at 20 ppm ~20 s at 2.5 ppm ~160 s at 5000 ppm ~35 s at 1000 ppm ~ 60 s at 5000 ppm ~78 s at 20 ppm ~43 s at 20 ppm ~1020 s at 20 ppm

Response magnitude (Rgas −Rair )/Rair = 4.02% at 10 ppm (Rair −Rgas )/Rair = 19.8% at 2 ppm (Rgas −Rair )/Rair = 3.65% at 20 ppm (Rgas −Rair )/Rair = 3.6% at 2.5 ppm (Rair −Rgas )/Rair = 12.8% at 5000 ppm (Rgas −Rair )/Rair = 16.5% at 1000 ppm (Gair −Ggas )/Gair = 12.8% at 5000 ppm (Rgas −Rair )/Rair = 60% at 20 ppm (Rgas −Rair )/Rair = 22% at 20 ppm (Rgas −Rair )/Rair = 8% at 20 ppm

~135 s at 20 ppm

121 s at 20 ppm

~260 s at 20 ppm

–

~ 65 s at 1000 ppm

~74 s at 5000 ppm

~49 s at 2.5 ppm

~23 s at 20 ppm

~140 s at 2 ppm

~125 s % at 10 ppm

Recovery time

Hu et al. [113]

Hu et al. [112]

Khurshid et al. [111]

Mackin et al. [110]

Al-mashat et al. [96]

Son et al. [104]

Tung et al. [91]

Wu et al. [93]

Yang et al. [100]

Yang et al. [105]

References

Rair resistance in air, Rgas resistance in gas/vapor, I air current in air, I gas current in gas/vapor, Gair conductance in air, Ggas conductance in gas/vapor, RT: room temperature, ppm: parts per million, ppb: parts per billion

Gas/vapor

Sensing materials

Table 3 Sensing performance of polymer-functionalized graphene/graphene derivative-based gas sensor

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4.4 Metal, Metal Oxide, Polymer-Functionalized Graphene/Graphene Derivative-Based Ternary Hybrid Gas Sensors Binary hybrid structure based on graphene–metal oxide or graphene–polymer was found to improve the sensing parameters partially. That is, among response magnitude, response time, recovery time, and selectivity, only one or two can be improved at best. Therefore, to achieve promising sensitivity and transient response without compromising the selectivity feature, such binary hybrid gas sensors were further functionalized by catalytic noble metals which lead to the formation of ternary hybrid structure of metal NPs graphene–metal oxide or metal NPs graphene–polymer sensors [98]. Such metal NPs incorporation is beneficial for the following reasons; (i) metal NPs promote both electronic and chemical sensitizations; (ii) metal NPs increase the charge carrier concentrations of the semiconducting oxide or polymer layer, which promotes the electron transfer during the interaction with gases [114]; (iii) large number of oxygen vacancies on metal oxide/polymer layer and carboxyl group in graphene structure can be introduced by metal NPs functionalization [73]; and (iv) during one-pot hydrothermal synthesis process metal can also restrict the growth of metal oxide structure, leading to very small nanocrystal/nanofiber formation which can eventually increase surface-to-volume ratio of the sensing surface [28]. However, theoretical calculation followed by experimental validation is needed to justify the exact role of double heterojunctions (present in the ternary hybrid structure) in the improvement of selectivity and sensitivity toward a particular species. Ternary hybrid structure of metal NPs-functionalized graphene/graphene derivative metal oxide is beneficial to obtain enhanced sensing performance in two ways; On one hand, graphene and metal oxide enhance the sensing kinetics and sensitivity, respectively, and on the other hand, metal NPs increase the selectivity toward a particular target species due to catalytic dissociation. Thus, keeping the same graphene/graphene derivative-metal oxide structure, the sensitivity of the ternary structure can be tuned simply by functionalization with different catalytic metal NPs. For example, Russo et al. [114] observed selective H2 sensing performance in Pt NPs-functionalized rGO-SnO2 NPs-based sensor (the selectivity study revealed that the sensor was insensitive to the other reducing and oxidizing gases like CO, NO2 , and methane). On the other hand, due to functionalization with In, rGO-SnO2 NPs-based sensor offers selectivity toward oxidizing species (e.g., NO2 ) [74]. A comparative theoretical analysis on gas sensing performance for different metal NPs (i.e., Au, Pd, and Pt) functionalized rGO-SnO2 surface revealed that the oxidation of CO is most favorable on Au-functionalized surface (as shown in Fig. 7a, b). As reaction barriers for the CO (employing DFT calculations) were found to be 0.21, 0.87, and 1.20 eV, for Au, Pd, and Pt catalysts, respectively [115]. Corresponding sensing performance and sensing mechanism of the ternary devices are depicted in Fig. 7c, d.

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Fig. 7 a Dynamic response curves of pristine SnO2 , 0.44 wt% rGO–SnO2 , 1 wt% Au-functionalized SnO2 nanofibers, and 1 wt% Au-functionalized 0.44 wt% rGO–SnO2 nanofibers at the CO concentration of 2 and 5 ppm, respectively, at 400 °C. b Summary of sensor responses with respect to CO gas. c, d Schematic illustration of sensing mechanism of c pristine SnO2 NFs and d Aufunctionalized rGO nanosheets-SnO2 NFs (ϕ is the work function and χ is the electron affinity) (Reproduced from Kim et al. [115] with permission from The Royal Society of Chemistry). e Dynamic normalized resistances of the Au-functionalized rGO-loaded ZnO NFs sensor to different concentrations of CO, C6 H6 (Benzene) and C7 H8 (Toluene) gases. f Corresponding calibration curves. g Dynamic normalized resistances of the Pd-functionalized rGO-loaded ZnO NFs sensor to different concentrations of CO, C6 H6 , and C7 H8 gases. h Corresponding calibration curves (Reproduced from Abideen et al. [116] with permission from Elsevier). i Schematic of the Pd/rGO/TiO2 NT ternary hybrid gas sensor device structure (Reproduced from Ghosal and Bhattacharyya [117] with permission from Elsevier)

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Apart from SnO2 , other metal oxides and rGO hybrid surface were also functionalized employing different catalytic metal NPs. For example, Abideen et al. [118] experimentally showed that Pd functionalization on rGO/ZnO nanofibers surface offered better sensing performance toward polar hydrocarbons (i.e., benzene). On the contrary, Au functionalization enhanced the sensing performance toward non-polar species reducing species (i.e., CO) (shown in Fig. 7e, f). A schematic of sensing mechanism is depicted pictorially in Fig. 7g, h. Similarly, Ghosal and Bhattacharyya [117] also demonstrated enhanced methanol (i.e., reducing vapor) sensing performance for Pd-functionalized rGO-TiO2 nanotubes structure (as shown in Fig. 7i). In recent past, bi-functionalized graphene oxide surface by Ir NPs and p-Co3 O4 fibers was found to offer ppb level selective sensing performance toward acetone vapor [119]. Such sensor also offered selective detection of acetone from the exhaled breath, which might be used as a marker for the diagnosis of diabetes. Apart from the metal oxide–rGO surface, polymer–rGO layer was also functionalized by metal NPs. For example, volatile organic compounds’ sensing performance of Ag NPs and poly (ionic liquid) (PIL) functionalized rGO film was investigated by Tung et al. [120]. Experimental results revealed that the sensitivity order toward polar vapors was as follows: methanol > ethanol > methyl acetate > acetone > water [120]. It is worth mentioning that the higher sensitivity was observed toward polar vapors compared to non-polar vapors. In another endeavor, Zou et al. [121] introduced Pd NPs on the PANI/graphene surface and obtained enhanced H2 sensing performance. Ternary polymer–metal oxide–graphene-based hybrid device was also investigated as a gas sensor. Gas/vapor sensing performance of metal NPs, metal oxide NPs, and polymer-functionalized graphene/graphene derivative-based ternary hybrid gas sensors are summarized in Table 4.

4.5 Functionalization During the Synthesis of Graphene Derivative-Based Gas Sensors The electronic and gas sensing properties of graphene can be engineered not only by functionalization with (i) metal oxide NPs or (ii) metal NPs or (iii) polymers but also by introducing defects/functionalization groups into the basal plane or edge sites of graphene/graphene derivatives during the synthesis (in situ) of graphene derivatives [53]. Such selective and intentional incorporation of different functional groups in graphene/graphene derivative structure was found to be beneficial for selective and enhanced gas/vapor sensing performance. Several theoretical reports were found on this topic. In summary, it is speculated that the carboxylic functional group (–COOH) at the edge of armchair graphene nanoribbon can enhance both sensitivity and selectivity toward NO [125]. Omidvar and Mohajeri [125] theoretically demonstrated that the –COOH and –CN functionalization at the edge of armchair graphene nanoribbon (AGNR) enhance the sensitivity toward NO and O2 gas, respectively. In addition, –COOH functionalized AGNR offered the lowest exothermic adsorption

H2

Methanol

Methanol

Ethanol

CO

NO2

Benzene

CO

Pd NPs Polyaniline-rGO

Ag-PIL-rGO

Pt-SnO2 -rGO

Au-SnO2 NPs-rGO

Au-SnO2 Nanofiber-rGO

In-SnO2 NPs-rGO

Pd-ZnO nanosheets-rGO

Au-ZnO nanosheets-rGO

400 °C

400 °C

RT

110 °C

110 °C

110 °C

RT

RT

Optimum temp.

1–5 ppm

1–5 ppm

1–100 ppm

2–5 ppm

1–1000 ppm

10–500 ppm

1–200 ppm

10–2000 ppm

Detection range (ppm)

Response time ~20 s at 2000 ppm ~10 s at 200 ppm

~6 s at 100 ppm ~155 s at 100 ppm ~114 s at 5 ppm ~547 s at 100 ppm ~378 s 5 ppm ~285 s 5 ppm

Response magnitude (Rgas −Rair )/Rair = 51% at 2000 ppm (Rgas −Rair )/Rair = 53% at 200 ppm (Rair )/Rgas = 130 at 100 ppm (I air −I gas )/I air = 255% at 100 ppm Rair /Rgas = 27.4 at 5 ppm (Gair −Ggas )/Gair = 11.1% at 100 ppm Rair /Rgas = 24 at 5 ppm Rair /Rgas = 35 at 5 ppm

~114 s 5 ppm

~146 s 5 ppm

–

~121 s at 5 ppm

~57 s at 100 ppm

~21 s at 100 ppm

~102 s at 200 ppm

~50 s at 2000 ppm

Recovery time

Abideen et al. [81]

Abideen et al. [124]

Cui et al. [74]

Kim et al. [115]

Meng et al. [123]

Peng et al. [122]

Tung et al. [120]

Zou et al. [121]

References

Rair resistance in air, Rgas resistance in gas/vapor, I air current in air, I gas current in gas/vapor, Gair conductance in air, Ggas conductance in gas/vapor, RT: room temperature, ppm: parts per million, ppb: parts per billion

Gas/vapor

Sensing materials

Table 4 Sensing performance of metal NPs, metal oxide NPs, and polymer-functionalized graphene/graphene derivative-based ternary hybrid gas sensors

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Fig. 8 Schematic illustration of the formation of the 3D FrGOH (hydroquinone functionalized mesoporous rGO hydrogel) in a one-step, hydrothermal self-assembly process and subsequent application of the synthesized FrGOH in gas sensing. A microheater is integrated into the substrate of the gas sensor to improve the selectivity, response, and signal recovery simultaneously (Reproduced from Wu et al. [127] with permission from The Royal Society of Chemistry)

energies (−29 kcal mol−1 ) which leads to high sensitivity toward the NO molecule [125]. Sun and Bai [47] theoretically demonstrated that introduction of H atoms in graphene structure improves the sensing performance toward CH4 and CO2 by lowering the diffusion coefficients of the sensing for these gases. In another endeavor, the influence of –COOH, –OH, and C–O–C groups along with “Stone–Thrower–Wales (STW)” and “Inverse STW (ISTW)” defects (at the same side and at the either side of the graphene surface) on the gas sensing performance was investigated by Lalitha and Lakshmipathi [126]. The “Stone–Thrower–Wales (STW) defect” is a topological defect on carbon hexagonal structure arising from bond rotation and rearrangement of four six-membered rings into two pentagons and two heptagons. These structural defects are generally formed during graphene synthesis, and such defects can also be introduced by means of electron irradiation [126]. The same authors also theoretically demonstrated that the ISTW sheet is more reactive to the functional groups than the STW sheet. Among the in situ functionalization routes chemical functionalization was found to be popular one due to its unique advantages like cost-effectiveness, reliability, and huge options for chemical reagents. Wu et al. [127] reported that inorganically functionalized (by hydroquinone) 3-D rGO hydrogel-based sensor offered twofold higher sensitivity toward both NO2 and CO2 than that of pristine rGO [127] (schematic of sensor fabrication steps are depicted in Fig. 8). In addition, the sensor offered sig-

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nificantly faster recovery with a potential for a ppb level sensing. Authors demonstrated that the hydroxyl groups introduced by hydroquinone molecules play a pivotal role in enhancing the NO2 sensing performance. Firstly, the hydroxyl groups act as the active sites for the adsorption/desorption for NO2 molecules as hydroxyl group functionalization reduced the interaction distance between carbon basal plane and the NO2 molecules. Secondly, DFT calculation revealed that binding energy between hydroxyl group functionalized graphene and the NO2 molecule (218.5 meV) is much larger than that of pristine one (87 meV). Such enhancement in binding energy is due to the formation of hydrogen bonds between the NO2 molecule and the hydroxyl groups (O–N = O… –OH). Thirdly, rotation and relaxation of hydroxyl groups during the adsorption/desorption of NO2 molecules was observed which results in the increased sensitivity [127]. In several reports, organic molecules were also reported to be used for functionalizing graphene toward the development of high-performance gas sensor [128, 129]. Non-covalent chemical functionalization of rGO by cyclodextrin was reported by Nag et al. [128] employing organic molecule as a linker, in order to construct a supramolecular assembly. They reported that the cyclodextrin provides selective π–π interaction between different target molecules and rGO. In another endeavor, Kang et al. [129] developed a highly sensitive and wearable gas sensors employing as functionalization agent, i.e., heptafluorobutylamine (HFBA), 1-(2-methoxyphenyl) piperazine (MPP), and 4-(2-keto-1-benzimidazolium) piperidine (KBIP). Their gas sensors exhibited higher sensitivity toward NH3 and NO2 (even at low concentration like 5 ppm) than chemically (by hydrazine) reduced graphene oxide. They demonstrated that organic molecules introduced specific functional groups which have distinctive sensing characteristics owing to the different number of lone pair of electrons [129, 130]. In another experiment, Yu et al. [131] observed significant enhancement in response time and recovery time toward ammonia due to the functionalization of rGO with 1,8,15,22-tetra-(4-tert-butylphenoxyl)metallophthalocyanine (TBPOMPc, M = Cu, Ni, and Pb) via a solution-based selfassembly method. A comparative study revealed that the response magnitude toward NH3 followed the order of rGO/TBPOPbPc > rGO/TBPOCuPc >rGO/ TBPONiPc [131]. The enhancement of the NH3 -sensing performance by TBPOPbPc is attributed to the self-assembly behavior of a rigid phenoxyl-substituted group of TBPOPbPc. The rigid structure of TBPOPbPc on one hand effectively prevents the intermolecular aggregation behavior and, on the other hand, offers higher specific surface area, which is beneficial for the physical adsorption and diffusion of NH3 molecules and thus, fast response time and recovery time can be achieved [131]. Gas/vapor sensing performance of functionalized graphene derivative-based gas sensor (where graphene derivatives were functionalized during (in situ) synthesis process) is presented in Table 5.

NH3

NH3

NH3

Phenoxyl Substituent reduced rGO

Fluorinated rGO

Organic molecule functionalized rGO

RT

RT

RT

Optimum temp.

25 ppm

100 ppm

0.3–3200 ppm

Detection range (ppm)

Response time ~34 s at 50 ppm ~840 s at 100 ppm ~210 s % at 25 ppm

Response magnitude (Rgas −Rair )/Rair = 8% at 50 ppm (Rgas −RN2 )/R N2 = 7% at 100 ppm (Rgas −Rair )/R air = 2.6% at 25 ppm

~ 2000 s % at 25 ppm

~17 s at 100 ppm

~31 s at 50 ppm

Recovery time

Duy et al. [133]

Park et al. [132]

Yu et al. [131]

References

Rair resistance in air, Rgas resistance in gas/vapor, Gair conductance in air, Ggas conductance in gas/vapor, RT: room temperature, ppm: parts per million, ppb: parts per billion

Gas/vapor

Sensing materials

Table 5 Sensing performance of functionalized graphene derivative-based gas sensor where graphene derivatives were functionalized during synthesis process

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5 Technological Challenges and Constraints Functionalization of graphene and its derivatives (like GO or rGO) offers enormous possibility for the development of efficient gas sensor devices due to their unique and controllable surface (structural), electrical, mechanical, and thermal properties with appropriate tunability [134] . However, the key challenges are as follows: (1) Functional groups on the graphene play an important role in gas sensing. During functionalization of graphene/graphene derivatives, control of process parameters is required at atomic level to obtain reliable and repeatable sensor devices. However, a clear gap between experimental and theoretical efforts is still there which needs a bridging. Very few reports described the experimental and theoretical finding side by side. Most of the experimental findings are explained intuitively. On the other hand, theoretical reports are based on unrealistic approach and not amenable with the current fabrication technology. (2) Functionalization agent (i.e., metal NPs, polymer, etc.) can interact with graphene or its derivatives through various binding processes (e.g., covalent and non-covalent bonding); consequently, they may attach to graphene at edges and/or at basal plane and/or at defective sites. During the functionalization process, edges and defects on graphene/graphene derivatives behave very differently compared to the saturated hexagonal structure of the graphene. Therefore, amount, size, density, and crystallographic orientation of functionalization agent on graphene also influence surface property of the resultant hybrid structure. However, controlling the functionalization parameters remains a challenging issue for the functionalized graphene surface. The technology of producing large area, single-layer and high-purity graphene with low cost is in its infancy, which is restricting the controlled production and hindering the commercial application of functionalized graphene in the field of gas sensor devices. (3) The presence of functionalization agent on the graphene surface can prevent the agglomeration and restacking tendency of the pristine graphene. However, long-term stability of the hybrid structure still remains a crucial challenge. For example, during target gas/adsorption and desorption, charging and discharging process of the active surface results in loss and agglomeration of the functionalization agent. Such phenomenon may lead to detachment of the functionalization agent leading toward the performance degradation of the device particularly in long term. (4) Functionalized graphene-based sensor is highly sensitive to environmental change as graphene surface is prone to adsorb ambient water molecule easily. It is also reported that pristine graphene/graphene derivatives can be used as a promising humidity sensor [135]. Therefore, performance variation for the functionalized graphene surface in different humid environments is a great challenge. (5) Fortunately, functionalized graphene surface (adopted by judicious siteselective functionalization technique) proved its prospective candidature for the development of selective gas/vapor sensor.

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6 Conclusion In this book chapter, recent advancements, trends, and challenges of the functionalized graphene and graphene derivatives (GO and rGO) based gas/vapor sensor devices, developed employing different functionalization routes (e.g., functionalization with metal or metal oxide NPs, polymer, etc.), have been discussed elaborately. Different functionalization strategies of functionalizing graphene/graphene derivatives for improved gas/vapor sensor applications can be broadly categorized into two types: (i) covalent and (ii) non-covalent functionalization techniques. In summary, non-covalent approach significantly influences the electrical conductivity and surface properties of the device, while better stability and/or mechanical properties of the sensor can be achieved through the covalent functionalization. Moreover, it is imperative to mention that the sensors based on functionalized graphene/graphene derivatives offer higher sensitivity and selectivity compared to its pristine counterparts. In general, for the development of gas sensor device, functionalization of graphene/graphene derivatives surface can be achieved with (i) noble metal nanoparticles, (ii) metal oxide nanoparticles, (iii) polymer, (iv) both metal and metal oxide NPs or both metal NPs and polymer, and (v) functionalization by other means (such as incorporation of selective functional group(s) during in situ functionalization). More elaborately, in case of metal oxide NPs-functionalized based gas sensor, enhanced sensing performance was correlated to the low barrier Schottky junction formation between n- or p-type metal oxide and p-type graphene/graphene derivatives, which eventually enhances carrier transport kinetics due to high mobility of carrier in graphene/graphene derivatives. Increase in sensitivity of such sensor is achieved at the cost of poor selectivity. On the other hand, metal NPs-functionalized sensors were found to offer promising selectivity, due to catalytic dissociation of metal NPs toward particular target species. However, such sensor often suffers from poor transient characteristics (compared to metal oxide NPs-functionalized sensor) as well as high operating temperature. On the other hand, polymer-functionalized graphene/graphene derivative-based sensor shows high sensitivity even at room temperature but suffers from sluggish response time and recovery time than that of metal oxide NPs and metal NPs-functionalized sensors. Moreover, formation of ternary structure (metal oxide NPs metal NPs graphene or polymer–metal NPs graphene) offers enhanced sensitivity, selectivity, and fast transient kinetics. Besides incorporation of foreign material, modification by incorporating different functional groups on basal plane or edge sites of graphene/graphene derivatives during the synthesis (in situ) of graphene derivatives is also proved beneficial to enhance the sensing performance toward a particular target species. However, such in situ functionalizing usually employs expensive techniques and often suffers from poor repeatability than that of other functionalization methods. However, despite plenty of numbers of open challenges, functionalized graphene-based sensors indicate toward a very bright and exciting future. In the days to come, functionalized graphene-based efficient, cost-effective gas sensor devices will have a potential to grab the lion share of the commercial market.

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Chapter 11

Hybridized Graphitic Carbon Nitride (g-CN) as High Performance VOCs Sensor Prashant Kumar Mishra, Ritu Malik, Vijay K. Tomer, and Nirav Joshi

1 Introduction The modernization of society and industrialization growth has caused the emission of extremely harmful VOCs in indoor climate which is triggering immense loss of natural atmosphere and leading to many health-related fatal diseases for humans [1–4]. As most of our quality time is spent in the indoor climate, the human naively inhales abundant toxic and hazardous gases thereby risking their health and life [5– 8]. In the recent years, the quality of our direct indoor environment has depreciated exponentially due to the excessive use of VOCs emitting household products such as adhesives, paints, varnishes, wallpapers, vinyl flooring, wood furniture cosmetics, and carpets, which is the prime source of sick-house syndrome [9, 10]. Toluene is a colorless volatile liquid with a pungent, benzene-like odor and is commonly used in the dye, rubber, paint, printing, chemical, and related consumer products. [11, 12]. Toluene significantly affects the central nervous system and causes reduction in thinking, memory, and muscular abilities. Studies also confirmed that toluene has played a vital role in malfunctioning the color vision and hearing abilities [13, 14]. The inhalation of toluene vapors in minor quantity ( 4.4 °C. Similar to toluene, formaldehyde is also a toxic gas which is colorless, flammable, strong-smelling liquid and is used to design pressed-wood products such as plywood, fiberboard, permanent-press fabrics, glues and adhesives and paper product coatings are the main sources of formaldehyde emission in the indoor climate [1, 2]. Inhalation of formaldehyde causes rapid chemical changes to the human body affecting the upper respiratory tracts which can be seen as the common symptoms like nausea, watery eyes, burning sensations, coughing, wheezing and skin irritations, etc. [15, 16]. Besides, formaldehyde has also been known to affect the lymphatic and hematopoietic systems, while it’s prolonged exposure can cause cancer as well. There are myriads of other VOCs such as acetone, ethanol, propanol, n-butanol, etc. released due to common household activities/consumers in the indoor climate and are equally responsible for deteriorating the indoor air contamination levels. Thus, to deal with the “sick-house syndrome” has become the utmost need of the hour and attracted significant interest to develop the new sensing technologies. Owing to this, advanced sensing materials were developed and commercialized for the detection of variety of gases present in the local climate [3, 6–10, 17]. Working on the different transduction principles, the newly developed multifunctional nanomaterials not only offers the improved selectivity, better sensitivity, good stability, excellent reusability, and rapid response but also enhances the device portability [18–21]. Apart from the conducting polymers and semiconducting metal oxides [22–27], 2D materials such as molybdenum disulfide (MoS2 ), graphene, tungsten disulfide (WS2 ), phosphorene, and carbon nitride have attracted excessively in last one decade for development of chemical sensors due to their high surface to volume ratio, improved quantum hall effect, better electron transfer rate and excellent thermal stability [28–33]. Among 2D materials, graphite-phase, polymeric carbon nitride (g-C3 N4 or gCN) has received remarkable scientific attention due to its facile synthesis, high chemical stability, and narrow bandgap (2.7 eV), etc. [34–37]. The great flexibility in structural modulation of g-C3 N4 by chemical functionalization or by doping has received noteworthy focus as multifunctional material toward electrocatalysis, pollutant degradation, water-splitting, and sensing applications. In addition, the rise of g-CN as a sustainable and environmental friendly organic semiconductor is furthermore favored by its earth-abundant “carbon” and “nitrogen” constituent elements [38–40]. Owing to its low specific surface area (~11 m2 /g) and low reactivity, the usage of g-CN in designing chemical sensors remained on backseat only until nanocasting process using mesoporous silica templates (SBA-15, KIT-6) were introduced for designing the ordered mesoporous g-CN with high surface/volume ratio

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and meso-ordered channels [3, 23, 41]. Due to their excellent permeabilities, transmission, functionalization, and diffusion of charge carriers on the sensor’s surface have generated extensive opportunities toward production of efficient and advanced gas sensing technologies [42, 43]. Moving in this direction, researchers have fabricated 3D mesoporous g-CN materials and modified them with various metal(s) and metal oxide(s) for enhancing their sensing attributes related to sensitivity, selectivity, response/recovery time, hysteresis, and stability [29, 44].

2 Mesoporous In(III)-SnO2 /g-CN Nanohybrids Based Toluene Gas Sensor SnO2 is an important n-type semiconductor with wide bandgap (~3.6 eV) and has been widely utilized with other 2D/3D materials based nanocomposite to form highly selective, sensitive, and stable gas sensors [45– 49]. Analyzing the success accomplished by SnO2 based 2D/3D materials nanocomposites in gas sensing applications with refinement of selectivity in sensor than traditional metal oxide based gas sensors, the group of Malik et al. prepared nanocomposite of mesoporous In(III)-SnO2 and g-CN using templated inversion of KIT-6 material for selective detection of trace levels of VOCs (toluene, n-butanol, benzene, xylene, and ethyl acetate) [50]. The results indicated outstanding response and selectivity of In(III)-SnO2 /g-CN nanocomposite to toluene gas at much lower temperature. In addition, the nanocomposite demonstrated excellent stability, highly reversible response, rapid response/recovery, and excellent cross-sensitivity to toluene. The small-angle X-ray scattering (SAXS) patterns for KIT-6, In(III)-SnO2 , g-CN, mpg-CN, SnO2 /mpg-CN, In(III)-SnO2 /mpg-CN in 2θ range of 0.5–3° are shown in Fig. 1A. It is quite clear from the figure that finely resolved Bragg’s reflection peaks are present at 2θ = 0.84° which is corresponding to (211) plane of the KIT-6 template [3, 23, 51]. Furthermore, two weak shoulders can be also observed in the theta range of 1−2° which represents the Ia3d symmetry and presence of ordered network of channelized pore of KIT-6 template. The KIT-6 templated materials (mpgCN, In(III)-SnO2 , SnO2 /mpg-CN, In(III)-SnO2 /mpg-CN) demonstrated a sharp peak around 2θ ~ 0.84° which represented a structure similar to mesoporous KIT-6 [23, 52]. However, the shoulder peaks disappear which is due to loss in mesoporosity in the nanocasted materials. Additionally, the close observation of Fig. 1A, reveals the shifting in (211) reflection peak toward higher Bragg’s angles (with reference to (211) peak for KIT-6 template) for nanocasted sample. This is due to the decreasing X-ray scattering contrast between pore channels and the mesoporous framework. Figure 1B portrays the x-ray diffraction (XRD) profiles in the 2θ range of 10°– 70° of as-synthesized compositions. The presence of a broad hump (2θ = 22°) in KIT-6 represents the characteristic of non-crystalline materials, whereas the CN based compositions the peaks demonstrated at 2θ = 12.5° and 27.5° corresponds

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Fig. 1 A SAXS and B XRD spectra of (a) KIT-6, (b) In(III)-SnO2 , (c) g-CN, (d) mpg-CN, (e) SnO2 /mpg-CN and (f) In(III)-SnO2 /mpg-CN; C N2 adsorption-desorption isotherms curves and D BJH Pore size distribution of (a) KIT-6, (b) mpg-CN, (c) SnO2 /mpg-CN, (d) In(III)-SnO2 /mpg-CN and (e) In(III)-SnO2 . Adapted with permission from Ref. [50]. Copyright 2018 Elsevier

to the (100) and (002) lattice planes of CN representing tri-s-triazine and stacked aromatic heptazine sheets, respectively (JCPDS Card No. 087-1526). For samples containing SnO2 , the well-resolved peaks were observed at 2θ = 26.9°, 34.2°, 38.2°, 52.3°, 54.9°, 57.8°, 62.2°, 64.6°, and 66.2° corresponding to (110), (101), (200), (211), (220), (002), (310), (112), and (301) lattice planes of SnO2 crystal structure (JCPDS Card No. 03-1116) [53–56]. Additionally, for samples In(III)-SnO2 and

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In(III)-SnO2 /mpg-CN, the weak diffraction peaks represent the body-centered cubic (bcc) structure of In2 O3 (JCPDS Card No. 06-0416). To measure the specific surface area and pore size distribution, N2 adsorption-desorption isotherms were performed. The N2 sorption isotherms for KIT-6 in Fig. 1C reveals the presence of type IV isotherms and H1-type hysteresis associated with the capillary condensation inside the mesoporous channels [57–59]. The steep increment in the sorption curves beyond relative pressure of P/Po = 0.6 demonstrates the mesoporous nature and narrow pore size distribution for sample KIT-6 Fig. 1C. For nanocasted compositions the isotherm type remains the same; however, the hysteresis loop changes to H3 type due to the interparticle porosity and non-uniform pores [60–62]. The conventional g-CN being non-porous in nature demonstrated a total surface area of ~11 m2 /g. Figure 1D stands for pore size distribution plots of mesoporous compositions. Figure 2a represents the response of the prepared materials toward toluene gas (50 ppm) in the temperature range of 90–350 °C. A monotonic increase in response

Fig. 2 a Responses of as-prepared materials to 50 ppm toluene gas at different operating temperatures (90–350 °C) and b responses of the sensors to toluene in the range from 1–500 ppm at their respective optimized operating temperature; c responses of the In(III)-SnO2 /mpg-CN toward varied concentration of toluene gas (1–100 ppm) in the temperature range from 90 to 200 °C; d Responses of In(III)-SnO2 /mpg-CN to 50 ppm test VOCs at 200 °C. Adapted with permission from Ref. [50]. Copyright 2018 Elsevier

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with increase in operating temperature was observed for all materials after which the response begins to decrease. The mpg-CN shows a maximum response (R) of 21.8 at the temperature of 250 °C which is approximately twice than that of g-CN (R = 11.3 at 275 °C). The reason for almost double response of mpg-CN than g-CN is the enhanced surface area of mpg-CN. For SnO2 loaded mpg-CN, the working temperature reduces to 225 °C while a maximum response of 46.9 was observed. Furthermore, with in In(III)SnO2 /mpg-CN, the optimized operating temperature decreases to 200 °C and a maximum response of 61.4 toward was obtained. Although, a steep fall in the response was observed for In(III)-SnO2 /mpg-CN below 150 °C, yet, the toluene could be easily detected at sub 100 °C. Figure 2b demonstrates the response of the prepared materials toward toluene gas (1–500 ppm) at their respective optimized operating temperature. The In(III)-SnO2 /mpg-CN material shows significantly higher response to toluene and excellent linearity even in 1–50 ppm concentration range than other studied materials herein. The response of In(III)-SnO2 /mpg-CN sensor toward varying concentration of other test VOCs at 200 °C in Fig. 2c shows that the response of In(III)-SnO2 /mpg-CN sensor was comparatively higher during the entire concentration range. The response of In(III)-SnO2 /mpg-CN sensor at varied temperature range (90–200 °C) in Fig. 2d shows that a little response was observed in the 1–50 ppm range (inset) at 90 °C operating temperature. With the increase in the operating temperature from 90–200 °C, a clear increase in the response was also observed. The response/recovery curve for In(III)-SnO2 /mpg-CN nanocomposite to toluene gas (50 and 1 ppm concentrations) at 200 °C in Fig. 3a, b illustrates a fast response/recovery time of 4/2 s for 50 ppm and 8.5/7 s for 1 ppm toluene gas at 200 °C.

Fig. 3 Single loop response and recovery curve of In(III)-SnO2 /mpg-CN sensor to a 50 ppm and b 1 ppm toluene gas at 200 °C. Adapted with permission from Ref. [50]. Copyright 2018 Elsevier

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The sensing mechanism for In(III)-SnO2 /mpg-CN starts from its exposure to hot air in gas chamber at 200 °C which eventually leads to the adsorption of oxygen molecules on the sensor surface. The free electrons (e− ) present in the conduction band of g-CN and SnO2 were trapped and forms negatively charged oxygen species (O− , O2− , O2 − , and O2 2− ) which forms a depletion region on the surface of sensor [10, 34–36, 63]. When the material contacted the toluene gas, the molecules adsorbed on these negatively charged oxygen sites which oxidized the sensor surface causing the electrons to get back to the conduction band, thereby increasing the electrical conductivity of the sensor. The presence of semiconducting SnO2 and mpg-CN heterostructures causes the transportation of electrons across two homojunction (SnO2 ↔ SnO2 and mpg-CN ↔ mpg-CN) and one heterojunction (SnO2 ↔ g-CN). The schematic of the sensing mechanism for In(III)-SnO2 /mpg-CN sensor is portrayed in Fig. 4. The work function (WF ) of the material in the nanocomposites governs the electronic movements on the sensor’s surface, wherein the electrons transfer from the lower WF material to that of higher WF material to acquire an equilibrium position and to produce a new Fermi energy level. In the present case, due to the lower value of WF of g-CN (4.3 eV) than SnO2 (4.9 eV), the electrons get relocated from the conduction band of g-CN to SnO2 by band bending, therefore, creating a potential barrier between the heterojunction [45–49]. These potential barriers impede the electron movement across the hybridized sensor and the electrons present on the sensor surface turn out to be the prominent sites for the adsorption of oxygen species and extensively increases the response of sensor. Additionally, the sensor response is particularly affected by the toluene gas properties and its molecular interaction with the adsorbed oxygen species on the sensor surface. At first, the sensor is exposed to hot air inside the chamber which causes the electrons carried away from the conduction band of mpg-CN. During the process, the atmospheric oxygen is adsorbed on the sensor surface thus causing a reduction in electrical conductivity of the sensor (as O2gas + Sads ↔ O2ads + 2e− ↔ 2O− ads ), where Sads denotes the surface adsorption

Fig. 4 A schematic of toluene sensing mechanism by In(III)-SnO2 /mpg-CN sensor. Adapted with permission from Ref. [50]. Copyright 2018 Elsevier

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site. Thecontact of sensor to the toluene gas results in its interaction with adsorbed  2− in order to yield CO2 and H2 O and frees up electrons as or O oxygen 2O− ads − → 7CO + 4H C7 H8 + 18O− 2 2 O + 18e [64, 65]. ads The presence of In2 O3 nanoparticles also enhances catalytic oxidation and chemical sensitization processes and strengthens the active oxygen species on SnO2 /mpgCN surface [66]. In addition, the concentration of oxygen vacancies is increased because of the presence of In3+ nanoparticles in SnO2 (Sn4+ ) which is due to the enhanced rate of adsorption-desorption of molecular oxygen over the sensor surface which dissociates oxygen molecules to produce active chemisorbed oxygen ions (O− ) with the help of e− species captured from the conduction band of g-CN and SnO2 [47–49, 67]. Therefore, it can be concluded that the excellent and stimulating semiconducting properties of 2D mesoporous cubic mpg-CN results in enhancing the response of In(III)-SnO2 /mpg-CN based sensor toward detection of toluene at much lower operating temperature.

3 Mesoporous Pd-WO3 /g-CN Nanohybrids Based Formaldehyde Gas Sensors Tungsten trioxide (WO3 ) is a widespread n-type semiconductor material due to its excellent electronic properties for gas sensing applications [68–70]. The properties such as a wide range of bandgap, low-cost, non-toxic, environmentally friendly, easy to synthesize, make it more attractive toward an ideal sensing material [71–73]. In this context, the group of Malik et al. had synthesized ordered Pd-WO3 doped mesoporous g-CN nanocomposites for selective sensing toward common volatile organic compounds using mesoporous silica (KIT-6) as a hard template [74]. The optimized Pd-WO3 /m-CN based sensor showed the excellent temperature-dependent sensing properties to flammable VOCs (Acetone, Ethanol, Toluene, and formaldehyde) with fast response and recovery speed. Figure 5A indicates the structural analysis using X-ray diffraction for KIT-6 and prepared nanohybrids such as Pd-WO3 , m-CN, WO3 /m-CN, and Pd-WO3 /m-CN. It can be observed the broad hump in the XRD plot due to the amorphous KIT-6 silica. Also, two major peaks have been identified with carbon nitride (CN) contents with nanohybrids that clarifies the arrangement of tri-s-triazine and heaping of aromatic heptazine sheets [3, 23, 38]. The XRD plot demonstrates the orthorhombic phase of WO3 and also metallic (Pd0 ) state can be observed due to Pd nanoparticles in PdWO3 and Pd-WO3 /m-CN. Figure 5B showed the small-angle x-ray spectra (SAXS) for nanohybrids and it can be seen that sharp reflection at 2θ = 0.84° (211) and two significant peaks ((321) and (332) planes) of KIT-6 [75–80]. Interestingly, only once characteristic peak (211) can be observed at around 2θ = 0.85° rest of the peaks had vanished in all nanohybrid samples which might be the reduction in the mesoporous ordering. Figure 5C displays the brunauer-emmett-teller (BET) surface area analysis for all the prepared nanohybrids. It can be seen that type IV isotherms of KIT-6

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Fig. 5 A XRD; B SAXS patterns; C N2 adsorption-desorption isotherms illustrating the shape of hysteresis loop and surface area and D BJH Pore size distribution for determining the pore diameter of (a) KIT-6, (b) m-CN, (c) WO3 /m-CN, (d) Pd-WO3 /m-CN, and (e) Pd-WO3 . Adapted with permission from Ref. [74]. Copyright 2018 The Royal Society of Chemistry

from the Nitrogen adsorption-desorption isotherms with hysteresis loop of H1 type according to the International Union of Pure and Applied Chemistry [81–84]. Also, there is an abrupt steep at P/P0 = 0.6 in the isotherm of mesoporous KIT-6 which confirms the narrow pore size distribution with mesoporous nanostructures. In the Nanohybrid samples, it can be observed that H3 type hysteresis loop is due to the presence of reduced interparticle porosity and non-uniform pores for the replicated structure. Figure 5D shows the pore size distribution curves of all nanohybrid samples with an average pore diameter (Dp ) ~ 8.4 nm for the KIT-6 template. It can be seen that nanocasted samples display smaller surface area/volume ratio and low porosity as compared to the pristine KIT-6. While in m-CN and WO3 /m-CN nanohybrids, the pore diameter decreases after the impregnation of WO3 and Pd nanoparticles. Figure 6a indicates the sensor response (R) as a function for the operating temperature of g-CN, m-CN, Pd-WO3 , WO3 /m-CN, and Pd-WO3 /m-CN nanohybrids toward

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Fig. 6 a Responses of g-CN, m-CN, Pd-WO3 , WO3 /m-CN, and Pd-WO3 /m-CN materials to 25 ppm formaldehyde at different operating temperatures (40–300 °C); b responses of the PdWO3 /m-CN to 25 ppm test VOCs at their optimized temperature; c responses of the Pd-WO3 /m-CN to all the test VOCs in the range from 1–500 ppm at 120 °C; d the response and recovery times of mesoporous Pd-WO3 /m-CN to all the tested VOCs (25 ppm) at 120 °C; e typical real time response curves of Pd-WO3 /m-CN sensor when exposed to different concentrations of formaldehyde (1– 100 ppm) at a working temperature of 120 °C in dry and humid ambient (95%RH) and f response for Pd-WO3 /m-CN under fluctuating %RH conditions toward 25 ppm formaldehyde gas at 120 °C. Adapted with permission from Ref. [74]. Copyright 2018 The Royal Society of Chemistry

25 ppm formaldehyde gas. It can be seen that the typical “increase → maximum → decay” pattern for all the samples in the range of 40–300 °C. At 200 °C, mesoporous CN shows the sensor response of 4.5 which is higher than the g-CN material (2.3); this is due to the mesoporous design which can fasten the diffusion process and also more surface-active sites in m-CN material. The hybrid nanostructure of m-CN with WO3 nanoparticles exhibits the enhancement in the sensor response (19.4) toward formaldehyde at 140 °C compared to the pristine m-CN sample. Furthermore, Pd decorated WO3 /m-CN structures are sensitive and selective toward formaldehyde with an increased response of ~24.2 at 120 °C owing to the synergistic effect as a result of nanohybrid configuration. Also, the Pd-WO3/ m-CN sensor showed a high response toward 25 ppm of formaldehyde at lower than 100 °C which makes them more favorable toward low-temperature sensors. Figure 6b shows the response of Pd-WO3 /m-CN was investigated on exposure to 25 ppm of all VOCs exposure as a function of temperatures (40–300 °C). Sensor response is found to increase with temperature with a maximum and further decrease due to desorption of the chemisorbed oxygen is enhanced more than the

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reaction rate, which leads to a decrease in response, considered the best-operating temperature. The best optimum temperature observed for toluene gas is 160 °C and 200 °C for acetone and ethanol, respectively. These results also demonstrate the clear discrimination toward target gases for the Pd-WO3 /m-CN sensor. Figure 6c displays the selectivity behavior of the Pd-WO3 /m-CN sensor for all volatile organic compounds in the range of 1–500 ppm. It can be observed that the sensor response of formaldehyde vapors was higher compared to other VOCs. Response and recovery transient behavior (Fig. 6d) were observed for all VOCs (25 ppm) for Pd-WO3 /mCN at 120 °C. From the response/recovery plot, it can be seen that compared to other VOCs formaldehyde (6.8/4.5 s) shows fast response and recovery behavior for the Pd-WO3 /m-CN sensor. Temperature and humidity are the vital parameters to understand the sensing performance in terms of stability and reproducibility. Figure 6e shows the transient response/recovery curves under high humid condition (95%RH) to understand the formaldehyde detection in the range of 1–100 ppm at 120 °C. The swiftly varying the transient curve emphasizes the total reversibility of the sensor by following the quick swapping between air and VOC due to the mesoporous structure which releases formaldehyde molecules from the surface to attain its original state in all the concentration ranges. Thus, results indicated that the high sensor response under the humid conditions is favorable and recommended that the sensor can work perfectly under wet conditions too. Similarly, Fig. 6f shows the sensing performance of Pd-WO3 /m-CN with high stability of sensor response under different humid conditions toward 25 ppm formaldehyde gas at 120 °C. Many factors govern the enhanced sensing performance of Pd-WO3 /m-CN for the detection of VOCs which include the mesoporous structure, formation of heterojunction, presence of noble metals (Pd0 nanoparticles), m-CN matrix, and the gas analytes behavior. The mesoporous Pd-WO3 /m-CN sensor shows the fast adsorption-desorption for target analytes which results in rapid response and recovery speed with excellent sensing performance [85, 86]. The development of heterojunction in the Pd-WO3 /m-CN sensor plays a vital role in enhancing the sensing properties toward different analytes. Combination of two different work function materials to form a hybrid material, unidirectional flow of electrons occurs from lower work function material (4.3 eV) to higher side (4.8 eV) to equalize their Fermi levels. Then at the equilibrium state, due to the band bending, an additional depletion layer is generated in the vicinity region of the heterojunction interfaces [38, 74]. The graphical illustration of the formaldehyde sensing mechanism using the Pd-WO3 /m-CN sensor is shown in Fig. 7. Modification of mesoporous m-CN with Pd-WO3 plays a significant role in enhancing the sensing performance of the Pd-WO3 /m-CN sensor. It can be observed that compared to pure Pd-WO3 , Pd-WO3 /m-CN shows a better response toward formaldehyde. So basically, mesoporous m-CN helps in easy transmission of charge carriers across the sensor surface of WO3 and noble metal particles improve the response and selective behavior due to the catalytic oxidation and chemical sensitization by changing the surface energy through “spill-over” effect on the surface of the Pd-WO3 /m-CN sensor [87–90]. And sensitization of Pd NPs can increase the overall rate of adsorptiondesorption of molecular oxygen and further increases the conductivity of the sensor [91]. It can be noted that sensing performance can be influenced by many factors

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Fig. 7 A schematic of formaldehyde sensing mechanism and band energy diagram for Pd-WO3 /mCN sensor. Adapted with permission from Ref. [74]. Copyright 2018 The Royal Society of Chemistry

such as surface interaction of adsorbed oxygen ions, bond dissociation energy, and partial pressure [22, 92–95]. Based on the above results, the proposed gas sensing mechanism of Pd-WO3 /m-CN is as follows: Under ambient conditions, Pd-WO3 /mCN adsorbs oxygen from the air and creates the surface states which allow the electron to be excited from the valance band of m-CN and electrical conductivity of the sensor is reduced. When the sensor is exposed to the reducing formaldehyde (HCHO) gas,  these reducing gas molecules can react with the adsorbed oxygen O− ads and the trapped electrons will be simultaneously released back to the conduction band [96– 99], which significantly reduces the height of the potential barrier and the width of the electron depletion layer at the interfaces of the Pd-WO3 /m-CN sensor, resulting in a conductivity enhancement. The different optimized functioning temperature for toxic gases can be elucidated based on C-H bond dissociation energies (BDE) of the organic molecules. Here, the BDE of toluene (89.9 kcal/mol) and acetone (94 kcal/mol) is higher than the formaldehyde (85 kcal/mol) so, at low operating temperature 120 °C, less energy is needed to dissociate formaldehyde molecules [100–103]. So, compared to earlier reports, prepared nanohybrid shows promising sensing behavior to detect VOCs at low temperatures, and therefore it is most favorable for the development of next-generation gas sensors.

4 Conclusion and Future Outlook This chapter describes the effectiveness of hard template synthesis scheme to synthesize the ordered mesoporous In(III)-SnO2 —loaded and Pd-WO3 —loaded g-CN based nanocomposites for trace level detection of toluene and formaldehyde in the indoor climate at much lower operating temperatures. Owing to its 3D cubic ordered

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mesoporous structure with high surface to volume ratio, the nanohybrid sensor exhibited advanced sensing characteristics toward toluene and formaldehyde. The existing idea in this chapter can be utilized to design and fabricate the highly efficient, low-temperature functioning all-in-one sensor for environmental monitoring applications. The results also embarked the outstanding low temperature sensing performance which can be utilized to precisely detect the toxic VOCs present in the indoor climate and will permit the researchers to manufacture ultramodern miniaturized hand-held sensors with improved gas sensing ability. Acknowledgements RM is thankful to UC Berkeley for providing visiting scholar supports. VKT is thankful to United States-India Education Foundation (USIEF) for Fulbright-Nehru award (Award No: 2308/FNPDR/2017). NJ wants to acknowledge the Brazilian funding agencies: São Paulo Research Foundation-FAPESP (2014/23546-1, 2016/23474-6).

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50. Malik R, Tomer VK, Chaudhary V, Dahiya MS, Nehra SP, Duhan S, Kailasam K (2018) A low temperature, highly sensitive and fast response toluene gas sensor based on In(III)-SnO2 loaded cubic mesoporous graphitic carbon nitride. Sens Actuators B 255:3564–3575 51. Tomer VK, Devi S, Malik R, Nehra SP, Duhan S (2016) Fast response with high performance humidity sensing of Ag-SnO2 /SBA-15 nanohybrid sensors. Microporous Mesoporous Mater 219:240–248 52. Kleitz F, Choi SH, Ryoo R (2003) Cubic Ia3d large mesoporous silica: synthesis and replication to platinum nanowires, carbon nanorods and carbon nanotubes. Chem Commun 2003:2136 53. Malik R, Rana PS, Tomer VK, Chaudhary V, Nehra SP, Duhan S (2016) Visible lightdriven mesoporous Au-TiO2 /SiO2 photocatalysts for advanced oxidation process. Ceram Int 42:10892–10901 54. Zhang D, Sun Y, Li P, Zhang Y (2016) Facile fabrication of MoS2 -modified SnO2 hybrid nanocomposite for ultrasensitive humidity sensing. ACS Appl Mater Interfaces 8:14142 55. Malik R, Tomer VK, Rana PS, Nehra SP, Duhan S (2015) Effect of annealing temperature on the photocatalytic performance of SnO2 nanoflowers towards degradation of Rhodamine B. Adv Sci Eng Med 7:448–456 56. Malik R, Tomer VK, Rana PS, Nehra SP, Duhan S (2015) One-pot hydrothermal synthesis of porous SnO2 nanostructures for photocatalytic degradation or organic pollutants. Energy Environ Focus 4:340–345 57. Tomer VK, Devi S, Malik R, Duhan S (2016) Chapter-7, Mesoporous materials & their nanocomposites. In: Nanomaterials and nanocomposites, Wiley-VCH Verlag, Germany, pp 223–254. ISBN 9783527337804. 10.1002/9783527683772.ch7 58. Duhan S, Tomer VK (2014) Chapter-6, mesoporous silica: making “Sense” of sensors. In: Advanced sensor and detection materials. Wiley-Scrivener, USA, pp 149–192. https://doi. org/10.1002/9781118774038.ch6 59. Nepak D, Tomer VK, Kailasam K (2018) Chapter 3, Carbon Nitrides (g-C3N4) and Covalent Triazine Frameworks (CTFs). In: Metal-free functionalized carbons in catalysis: synthesis, characterization and applications. Royal Society of Chemistry, UK, pp 67–101. https://doi. org/10.1039/9781788013116-00067 60. Tomer VK, Malik R, Jangra S, Nehra SP, Duhan S (2014) One pot direct synthesis of mesoporous SnO2 /SBA-15 nano-composite by the hydrothermal method. Mater Lett 132:228–230 61. Duhan S, Tomer VK, Sharma AK, Dehiya BS (2014) Development and properties Study of micro-structure silver-doped silica nanocomposites by chemical process. J Alloys Comp 583:550–553 62. Duhan S, Dehiya BS, Tomer VK (2013) Microstructure and photo-catalytic dye degradation of silver- silica nano composites synthesized by sol-gel method. Adv Mater Lett 4:317–322 63. Malik R, Tomer VK, Rana PS, Nehra SP, Duhan S (2016) Facile preparation of TiO2 / SnO2 catalysts using TiO2 as an auxiliary for gas sensing and advanced oxidation processes. MRS Adv 1(46):3157–3162 64. Wang L, Wang S, Xu M, Hu X, Zhang H, Wang Y, Huang H (2013) A Au-functionalized ZnO nanowire gas sensor for detection of benzene and toluene. Phys Chem Chem Phys 15:17179 65. Duhan S, Tomer VK (2014) Chapter-7, Advance electronics: looking beyond silicon. In: Advanced energy materials”. Wiley-Scrivener, USA, pp 295–326. https://doi.org/10.1002/ 9781118904923.ch7 66. Xu W, Li J, Sun J (2015) Fabrication of monodispersed hollow flower-like porous In2 O3 nanostructures and their application as gas sensors. RSC Adv 5:81407 67. Qi Q, Zhang T, Liu L, Zheng X (2009) Synthesis and toluene sensing properties of SnO2 nanofibers. Sens Actuators B 137:471 68. Shorie M, Kumar V, Kaur H, Singh K, Tomer VK, Sabherwal P (2018) Plasmonic DNA hotspots made from tungsten disulfide nanosheets and gold nanoparticles for ultrasensitive aptamer-based SERS detection of myoglobin. Microchim Acta 185:158

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69. Malik R, Chaudhary V, Tomer VK, Nehra SP (2017) Nanocasted synthesis of Ag/WO3 Nanocomposite With Enhanced Sensing And Photocatalysis Applications. Energy Environ Focus 6:43–48 70. Malik R, Rana PS, Tomer VK, Chaudhary V, Nehra SP, Duhan S (2016) Nano gold supported on ordered mesoporous WO3 /SBA-15 hybrid nanocomposite for oxidative decolorization of Azo dye. Microporous Mesoporous Mater 225:245–254 71. Malik R, Chaudhary V, Rana PS, Tomer VK, Nehra SP, Duhan S (2016) Nanostructured WO3 /SnO2 and TiO2/SnO2 heterojunction with enhanced photocatalytic activity. Energy Environ Focus 5:108–115 72. Tomer VK, Duhan S (2015) Highly sensitive and stable relative humidity sensors based on WO3 modified mesoporous silica. Appl Phy Lett 106:063105 73. Tomer VK, Duhan S, Sharma AK, Malik R, Jangra S, Nehra SP (2015) Devi S (2015) Humidity sensing properties of Ag0 nano particles supported WO3-SiO2 with super rapid response and excellent stability. Eur J Inorg Chem 31:5232–5240 74. Malik R, Tomer VK (2018) Cubic mesoporous Pd-WO3 loaded graphitic carbon nitride (gCN) nanohybrids: Highly sensitive and temperature dependent VOCs sensors. J Mater Chem A 6:10718–10730 75. Tomer VK, Malik R, Chaudhary V, Baruah A (2019) Chapter 14, Noble metals-metal oxide nanohybrids in humidity and gas sensing applications. In: Mohapatra S, Nguyen TA, Nguyen P (eds) Noble metal-metal oxide hybrid nanoparticles: fundamentals and applications. Woodhead Publishing, Elsevier, UK, pp 283–302. https://doi.org/10.1016/b978-0-12-814134-2. 00014-0 76. Poonia E, Duhan S, Kumar K, Kumar A, Jakhar S, Tomer VK (2019) One pot hydrothermal synthesis of ordered mesoporous SnO2 /SBA-16 nanocomposites. J Porous Mater 26(2):553– 560 77. Jangra S, Chhokar V, Tomer VK, Sharma AK, Duhan S (2016) Influence of Functionalization Type on Controlled Release of Emodin from Mesoporous Silica. J Porous Mater 23:1047–1057 78. Adhyapak PV, Meshram SP, Tomer VK, Amalnerkar D, Mulla IS (2013) Effect of preparation parameters on the morphologically induced photocatalytic activities of hierarchical zinc oxide nanostructures. Ceram Int 39:7367–7378 79. Tomer VK, Duhan S (2015) Nano titania loaded mesoporous silica: preparation and application as high performance humidity sensor. Sens Actuators B 220:192–200 80. Tomer VK, Duhan S, Adhyapak PV, Mulla IS (2015) Mn loaded mesoporous silica nanocomposite: a highly efficient humidity sensor. J Am Ceram Soc 98:741–747 81. Tomer VK, Adhyapak PV, Duhan S, Mulla IS (2014) Humidity sensing properties of Agloaded mesoporous silica SBA-15 nano composites prepared via hydrothermal process. Microporous Mesoporous Mater 197:140–147 82. Tomer VK, Duhan S, Malik R, Nehra SP, Devi S (2015) A novel highly sensitive humidity sensor based on ZnO/SBA-15 hybrid nanocomposite. J Am Ceram Soc 98:3719–3725 83. Tomer VK, Jangra S, Malik R, Duhan S (2015) Effect of in-situ loading of nano Titania particles on structural ordering of mesoporous SBA-15 framework. Colloids Surf A Physicochem Engg Aspects 466:160–165 84. Tomer VK, Duhan S, Sharma AK, Malik R, Nehra SP, Devi S (2015) One pot synthesis of mesoporous ZnO–SiO2 nanocomposite as high performance humidity sensor. Colloids Surf A Physchem Engg Asp 483:121–128 85. Tomer VK, Duhan S (2015) In-situ synthesis of SnO2 /SBA-15 hybrid nanocomposite as highly efficient humidity sensor. Sens Actuators B 212:517–525 86. Malik R, Tomer VK, Chaudhary V, Dahiya MS, Rana PS, Nehra SP, Duhan S (2016) Facile synthesis of hybridized mesoporous Au@TiO2 /SnO2 as efficient photocatalyst and selective VOC sensor. ChemistrySelect 1:3247–3258 87. Poonia E, Mishra PK, Poonia V, Sangwan J, Kumar R, Rai PK, Malik R, Tomer VK, Ahuja R, Mishra YK (2019) Aero-gel based CeO2 nanoparticles: Synthesis, structural properties and detailed humidity sensing response. J Mater Chem C 7:5477–5487

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88. Poonia E, Mishra PK, Poonia V, Sangwan J, Kumar R, Rai PK, Tomer VK (2018) Aero-gel assisted synthesis of anatase TiO2 nanoparticles for humidity sensing applications. Dalton Trans 47:6293–6298 89. Poonia E, Dahiya MS, Tomer VK, Kumar K, Kumar S, Duhan S (2018) Humidity sensing behavior of tin-loaded 3-D cubic mesoporous silica. Physica E 101:284–293 90. Chaudhary V, Malik R, Tomer VK, Nehra SP, Duhan S (2016) Enhanced relative humidity sensing performance using TiO2 loaded SiO2 nanocomposite. Energy Environ Focus 5:234– 239 91. Chi X, Liu C, Liu L, Li Y, Wang Z, Bo X, Liu L, Su C (2014) Tungsten trioxide nanotubes with high sensitive and selective properties to acetone. Sens Actuators B 194:33 92. Baruah A, Chaudhary V, Malik R, Tomer VK (2019) Chapter 17, Innovations in Nanotechnology for Wastewater Treatment. In: Ahsan A, Ismail AF (eds) Nanotechnology in water and wastewater treatment. Elsevier, UK, pp 323–338. https://doi.org/10.1016/b978-0-12-8139028.00017-4 93. Joshi N, Silva LF, Jadhav HS, Shimizu FM, Suman PH, M’Peko JC, Orlandi MO, Mastelaro VR, Oliveira ON (2018) Yolk-shelled ZnCo2 O4 microspheres: Surface properties and gas sensing application. Sens Actuators B 257:906–915 94. Joshi N, Silva LF, Jadhav H, M’Peko JC, Torres BBM, Aguir K, Mastelaro VR, Oliveira ON (2016) One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structures. RSC Adv 6:92655–92662 95. Joshi N, Shimizu FM, Awan IT, M’Peko JC, Mastelaro VR, Oliveira ON, Silva LF (2016) Ozone sensing properties of nickel phthalocyanine: ZnO nanorod heterostructures. IEEE Sens: 1–3 96. Lai X, Shen G, Xue P, Yan B, Wang H, Li P, Xia W, Fang J (2015) Ordered mesoporous NiO with thin pore walls and its enhanced sensing performance for formaldehyde. Nanoscale 7:4005–4012 97. Singh H, Tomer VK, Jena N, Bala I, Sharma N, Nepak D, Sarkar AD, Kailasam K, Pal SK (2017) A porous, crystalline truxene-based covalent organic frameworks and its applications in humidity sensing. J Mater Chem A 5:21820–21827 98. Liu J, Li X, Chen X, Niu H, Han X, Zhang T, Lin H, Qu F (2016) Synthesis of SnO2 /In2 O3 hetero-nanotubes by coaxial-electrospinning method for enhanced formaldehyde response. New J Chem 40:1756–1764 99. Zhang Y, Liu Q, Zhang J, Zhu Q, Zhu Z (2014) A highly sensitive and selective formaldehyde gas sensor using a molecular imprinting technique based on Ag–LaFeO3 . J Mater Chem C 2:10067–10072 100. Palke WE, Kirtman B (1988) The C-H bond energy of formaldehyde. Chem Phy Lett 148:202– 204 101. Wu YD, Wong CL, Chan KWK, Ji GZ, Jiang XK (1996) Substituent effects on the C−H bond dissociation energy of toluene. A density functional study. J Org Chem 61:746–750 102. Bordwell FG, Harrelson JA (1990) Acidities and homolytic bond dissociation energies of the αC-H bonds in ketones in DMSO. Can J Chem 68(10):1714–1718 103. Malik R, Tomer VK, Mishra YK, Lin L (2020) Functional gas sensing nanomaterials: a panoramic view. App Phys Rev 7:021301

Chapter 12

Graphene Oxide (GO) Nanocomposite Based Room Temperature Gas Sensor Umesh T. Nakate, Sandip Paul Choudhury, Rafiq Ahmad, Pramila Patil, Yogesh T. Nakate, and Yoon-Bong Hahn

1 Introduction Room temperature gas sensing is one of the major requirements of today’s sensor technology since it simplifies the device configuration. The processing time for sensor fabrication can also be reduced which contributes to large scale production. Since the heating source as well as monitoring not required in the sensor device, it brings the cost of the production and sale down. The major advantage of room temperature sensors is that it avoids the possibility of fire accidents due to heating. The power consumption to operate the gas sensor device will also be reduced significantly. Semiconducting metal oxides such as ZnO, SnO2 , TiO2 , and WO3 have been extensively studied for room temperature gas sensor applications. Carbon nanostructure like graphene and carbon nanotubes (CNT) having single wall (SW) or multi U. T. Nakate (B) · Y.-B. Hahn School of Semiconductor and Chemical Engineering, Solar Energy Research Center, Jeonbuk National University, Deokjin-gu, Jeonju-si, Jeollabuk-do 54896, Republic of Korea e-mail: [email protected] S. P. Choudhury School of Chemistry and Chemical Engineering, Shandong University, No. 27 South Shanda Road, Jinan 250100, China Department of Science and Humanities, NIT Nagaland, Chumukedima, Dimapur 797103, Nagaland, India R. Ahmad Centre for Nanoscience and Nanotechnology, Jamia Millian Islamia (A Central University), Jamia Nagar, New Delhi 110025, India P. Patil Defence Bioengineering and Electromedical Laboratory, Bengaluru 560093, India Y. T. Nakate School of Physical Sciences, Kavayitri Bahinabai Chaudhari North Maharashtra University, Jalgaon, Maharashtra, India © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_12

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wall (MW) are also being reported for room temperature gas sensing. Further, the transformation of graphene into graphene oxide (GO) has been tested for gas sensing properties. GO was observed as one of the potential materials for high-performance sensor applications. The conductivity of GO can be enhanced by chemical reduction process. The reduced graphene oxide (rGO) is a more effective sensor material for room temperature operated gas sensing. These individual sensor materials are having a limitation of poor selectivity which could be overcome by specific nanocomposites. This chapter covers various properties of GO such as physical, chemical, and electronic which makes it an ideal candidate for gas sensor application. The variety of synthesis methods for GO and its composites were summarized. The sensing mechanism for GO-based nanocomposites was explored. The GO-based sensor materials for different gases/vapors reports are discussed. The limitations and challenges of GO-based sensing materials for future room temperature operated sensors are discussed.

2 Properties and Applications of GO When the existence of a material like graphene was realized [1], it has found its applicability in fields like supercapacitors, electronic devices, photocatalysts, lithium battery electrodes, and gas sensors [2–4]. The wide application can be attributed to a number of factors including specific surface area (2630 m2 /g) and high carrier mobility at room temperature [5]. Graphene oxide has exceptional mechanical, electronic, thermal, and optical properties which can be attributed to its 2D sp2 -bonded structure [1]. GO consists of aromatic (sp2 ) and aliphatic (sp3 ) regions, which increases its horizon of applications because of the possibility of types of interactions. GO has tremendous scope in the field of material science such as display/e-papers, flexible/lightweight electronics, catalyst, polymer composite, bio-derivatives, energy materials, and sensors because of the chemical, mechanical, electrical, and thermal property that it possesses. GO can be chosen as an efficient system to evenly disperse size-controlled catalytic nanomaterials. Individual graphene oxide structure and their doping with metal oxides is suitable for room temperature gas sensing. The performance of the material is highly desirable in terms of responses, recovery time, and stability. Carbon-based materials like graphene, graphene oxide, and reduced graphene oxide are one of the most widely explored materials for gas sensing, catalysis, anti-bacterial agents, DNA sequencing applications [6–8]. What makes them a suitable candidate can be attributed to the properties they hold. The improved electron transport properties, good adsorption of gas molecules, and superior signal to noise ratio, all add to the properties to that takes graphene oxide ahead in the race of finding a desirable gas sensing material [9]. Implementation of graphene oxide in electronic circuits has gained more popularity with the introduction of Si-based technology which encourages advanced lithographic techniques [10, 11].

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2.1 Physical Properties The mechanical property of graphene oxide makes it an ideal candidate in integrated electronic devices. The strength is provided because of its 2D structure. Graphene FET is designed on a flexible substrate which results in a wearable device [12, 13]. A graphene-based FET, however, has a restriction in terms of selectivity [14]. The mechanical feature can be modified depending on the need, by changing the type, and amount of functional group coverage on GO. For a few layers of GO thickness, Young’s modulus increases to 200 GPa [15–17]. A monolayer of GO has larger modulus compared to paper GO. The mechanical properties can also be modified by doping or making GO composites. The mechanical property is influenced by the functional group attached to the GO sheets. It was studied that when a functional group like –OH and –COOH is present in an extended amount, Young’s modulus reduces drastically. Graphene oxide has light-absorbing property which can find its application in photovoltaics [18]. A single layer of GO is transparent and has significant optical transmittance in the visible range. This is attributed to its atomically thin nature [19]. By controlling the extent of reduction and thickness of GO, its optical transmittance can be tuned. Fluorescence property is a major property which differentiates GO from graphene. Graphene does not display fluorescence due to the absence of bandgap [20]. The chemical structure of GO favors broad fluorescence with peak wavelength in the range of 600–700 nm because of the π-π* transition. Visible photoluminescence was also observed for GO which can also be enhanced by chemical reduction [21]. The broad photoluminescence indicates bond modification in the GO plane resulting in more scattering. Weak blue to ultraviolet fluorescence for GO thin films is observed when exposed to ultraviolet radiation [22, 23]. When the sp2 system is introduced by reduction treatment, the PL intensity also changes. Nano and picosecond pulses are used for analyzing the NLO properties of GO at 532 [24, 25]. For picosecond pulses the Z-scan shows 2 photon absorption process more in comparison to nonlinear absorption. But for nanosecond pulse, excited state adsorption is dominating. When zinc phthalocyanine (PcZn) functionalized GO, it displays larger extinction coefficients and broader optical limiting performance [25]. This is attributed to the covalent bond of GO and PcZn. When GO–Fe3 O4 composite is considered, the NLO properties are improved bearing an absorption value of 400 nm compared to pristine GO in a similar water concentration [26]. When GO, which is thermally unstable, is subjected to high temperature, it decomposes vigorously [27]. Further work showed that if the decomposition is carried out in purified naphtha, within 100–200 °C, water and CO2 are generated in substantial amount. GO displays hygroscopic nature [27]. The absorbed water content, when exposed to air, can be removed easily by normal heating. Hence, water absorption and release is a reversible process. During the absorption, it is assumed that water layers are within the GO layers. It is noticed that the hk0 plane remains intact during water absorption, hence the swelling of GO is one dimensional in nature. The stability of

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GO was tested in solutions of different pH, salt systems, and solutions of different ionic strengths. The stability of GO was good in natural and synthetic surface waters.

2.2 Chemical Properties Graphene oxide is synthesized by chemical exfoliation of graphite. A strongly acidic solution is used as the solvent for combining graphene sheets and oxygen [28]. The chemical property of graphene oxide allows it to be separated into individual sheets. Chemical changes in the environment bring changes to graphene for a number of reasons. Firstly, the electron mobility and transport at room temperature remains ballistic up to a distance of 3 μm. Secondly, each of the carbon atoms on the graphene surface provides a high specific surface area. Hence the electron transport on the graphene surface behaves sensitively to the molecules which it adsorbs. Finally, because of the low electrical noise attributed to the unique crystal structure, graphene oxide has a high electrical conductivity. When GO is treated with reducing agents like ammonium sulphide/potassium, tin chloride/copper chloride, it becomes a black residue and forms graphite [27]. It was also seen that the process was reversible and GO can be obtained again even after reduction [29]. The amount of oxygen to be removed from GO depends on the reducing agent where Fe(II) chloride, ~68% of O2 can be removed, hydrazine hydrate can remove ~82% O2 and H2 S can remove ~91% [30]. NMR spectroscopy has revealed that treating GO with iodide has an impact on GO. It results in selective extraction of the epoxide oxygen. But new peaks are also observed in the NMR spectrum; at ~110 ppm and ~160 ppm. These peaks indicate the formation of phenols in GO. When GO is treated with boiling nitric acid, there were no changes observed in GO [30]. Bases, however, interact with GO in varied ways. Dilute NaOH causes osmotic swelling of GO. Alkali/alkaline earth acetate solutions can remove hydrogen cations which leads to GO-alkali salts. But the reaction is reversible. A solution of NaCl becomes acidic when GO is kept in it for a long time [31]. Protons are released from GO, and sodium ions are absorbed as a result of which graphitic acid sodium salt is formed. This property is attributed to the presence of COOH groups at the edge positions of GO. The stability of GO in acids or bases depends on the temperature at which it is treated. The carbon lattice is not modified by NaOH or HCl at a low temperature of 10 °C [32]. However, at a temperature of 40 °C for base, the system is ruptured. The stability is also dependent on the method by which GO is synthesized. DFT calculation revealed that due to local distortions, the adsorption energy for one epoxide and hydroxyl group are −4.72 eV and − 9.34 eV, respectively [33]. These functional groups when symmetrically distributed on the surface of GO can stabilize the system. If the epoxide and hydroxyl group combine together, they form chains with sp2 carbon regions [33, 34]. As the number of functional groups increases, more graphene will be oxidized. Once GO is fully oxidized it becomes energetically more favorable [35].

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2.3 Electronic Properties Graphene has a high-quality crystal lattice and displays superior ballistic charge transport and reduced noise. The possibility of operating graphene oxide-based gas sensors at room temperature makes them cost effective as the setup needs low power for operation. The low power consumption also opens avenues for remote operation. The 2D structure of graphene makes it easy for Hall measurement and four-probe experiments since the contact resistance is minimum. The study can be conducted on a focused active area which is difficult in case of 1D material. The first report of graphene in FET was published in 2004 [36], and subsequently, many reports followed showing significant improvements [37, 38]. In more work conducted in the later years, graphene-based memory devices were designed [39]. When gas molecules even at room temperature interact with graphene, the electrical properties are modified. This property makes graphene and its derivates a reliable candidate for gas sensing applications. One of the initial studies by Schedin et al. [2], the adsorbed gas molecules on the surface of graphene changes its resistance by affecting the local carrier concentration. rGO also is tested for improved gas sensing applications, because of the defects and vacancies it has during the reduction process [40]. The presence of oxygen functional group on the surface of rGO initiates electron transfer from rGO to functional groups, resulting in the holes to become the majority charge carriers. This shows that rGO now acts as a p-type semiconductor [41, 42]. But graphene-based sensors have the drawback of long recovery time [39] because the interaction between graphene and gas molecules is either via Van der Waals force or covalent bonds [43]. The raw GO synthesized experimentally is an insulator having a minimum sheet resistance of 1010 ohm/sq. This is attributed to the sp3 hybridized carbon attached to the groups containing oxygen [35, 41]. When the GO is reduced to rGO, the sheet resistance reduces to improve the conductivity. Experimental result shows that rGO has a sheet resistance of 106 ohm/sq [41]. The reduction treatment employed like chemical and thermal methods also reduces the sheet resistance compared to the non-reduced GO [44]. The thermal graphitization technique is used to obtain GO films having sheet resistance from 102 to 103 ohm/sq and a transmittance of 80% for 550 nm of light. Also, it was observed that the resistance has a direct variation with the annealing temperature [45]. Important applications of GO depend on how conducting the sample is. When reduced, the resistivity decreases and the conductivity increases. When GO is reduced, the oxygen vacancies are created as the functional groups vanishe, but at the same time sp2 carbon increases resulting in enhanced conductivity. The reduction time and temperature also dictate the conducting property of GO [46]. The conductivity is directly proportional to the reduction time and temperature until it reaches a saturation value. The amount of oxidation influences the bandgap of GO sheets and is a good way to vary the same for electronic applications [47]. When femtosecond laser pulses are used for reduction, the amount of oxygen in GO can be varied by tuning the laser power [48]. It was observed that the bandgap varied from 2.4 to 0.9 eV when the

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output power was modulated from 0 to 23 mW. The structure of GO and position of the functional group also decide the electronic structure. When GO has an armchair structure there exists a possible bandgap opening, but for a zigzag edge, the behavior is more metallic. Further theoretical studies have been considered to determine the bandgap of GO in the presence of epoxide and hydroxyl groups. The results were similar to that of oxygen variation. The density of states images shows that the bandgap reduces from 2.8 to 1.8 eV when the amount of functional group covering the surface is reduced from 75 to 50% [39]. GO also displays the electromechanical effects [49]. When GO is constructed along a single axis, CO hybridization weakens resulting in release of more electrons which in turn reduces the bandgap. It is observed that for an ordered GO, the bandgap is reduced to 0.61 eV from 1.41 eV with variation in tensile strain from 0 to 10%. In case the amorphous GO, with tensile strain from 0 to 8%, the reduction in bandgap is from 1.03 to 0.78 eV.

3 Synthesis Methods of GO and Its Composites 3.1 GO Synthesis GO, an amazingly attractive sensing material, has been extensively studied by researchers since its first synthesis using simple exfoliation of graphene by wellknown Hummers method [50]. Since then, different methods have been utilized to prepare the GO as shown in Fig. 1 [51]. In these methods, initially bulk graphite powder is chemically treated in acids (i.e., H2 SO4 , HCl, and HNO3 ) followed by dispersion in basic solutions (i.e., KMnO4 , NaNO3 , and KClO3 ) yields single-layer form (GO) of graphite [27, 50, 52–54]. The GO “single-layer form of graphite” have attached oxygenous functional groups (i.e., hydroxyl, epoxy, carboxy, carbonyl, and

Fig. 1 Scheme showing synthesis methods of GO and their applications [51]

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phenol groups) on the basal planes and edges, which makes them a considerable potential material for attaching other functionalities and sensing different gases.

3.2 Metal Oxide Graphene Oxide Nanocomposites Compared to GO, metal oxide-GO nanocomposites have the advantage of higher specific surface area, which results in enhanced gas-sensing performance. This enhancement in gas sensing is attributed to the oxidation/reduction reactions that occur on the metal oxide surface and changes the electric charge carriers [55]. Detailed synthesis methods of metal oxide-GO nanocomposites were reviewed by Khan and co-workers [56]. In brief, they have divided metal oxide loading/modification on graphene in two different ways (i.e., in-situ and ex-situ binding) (Fig. 2). In ex-situ methods, presynthesized metal oxide nanomaterials are mixed with the surface-functionalized graphene nanosheets. The functionalization step is crucial to enhance the solubility of graphene and synthesis of metal oxide-GO nanocomposites. However, in-situ synthesis method, metal oxide-GO nanocomposites are prepared in a simultaneous reduction of GO and metal precursors (i.e., AgNO3 , HAuCl4 , H2 PdCl6 , K2 PtCl4 ) with reductants (i.e., NaBH4 , hydrazine hydrate, amines) [57–60]. Based on the use

Fig. 2 Schematic representation of metal oxide-GO nanocomposites preparation using in-situ and ex-situ hybridization methods [56]

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of metal oxide-GO nanocomposites, metal oxide nanomaterial and metal precursors are selected and their composites are synthesized.

3.3 Conducting Polymer Graphene Oxide Nanocomposites The conducting polymers have highly π-conjugated polymeric chains with metal-like magnetic, electronic, and optical properties. They possess high electrical conductivity, superior electrochemical activity, and larger surface area, which make them ideal for making nano-composite material with GO for many applications [61–63]. On the other hand, GO also provides high mechanical strength and thermal conductivities along with high electrical conductivities and specific surface area. Thus, incorporating GO into conducting polymers can enhance the mechanical/electrical properties. Different synthesis methods for preparation of conducting polymer-carbon nanomaterials (i.e., carbon nanotubes, GO) have been reported [61]. Mainly, these methods are categorized into chemical (in-situ chemical polymerization and solution mixing) and electrochemical-based methods (Fig. 3). In in-situ chemical polymerization, monomer and carbon nanomaterial (i.e., GO) with different weight ratios are mixed in a suitable solvent followed by evaporation of the solvent to form nanocomposite of conducting polymer-GO (Fig. 3a). However, in mixing method (Fig. 3b), polymers and carbon nanomaterial (i.e., GO) are dissolved in organic solvent and sonicated to make nanocomposite. The electrochemical polymerization (Fig. 3c) is a rapid process as compared to the chemical polymerization. Also, it forms better nanocomposite with controlled uniformity.

Fig. 3 Schematic illustration of preparation methods of conducting polymer-carbon nanomaterials based nanocomposites: a in-situ chemical polymerization, b sonication, and c electrochemical process [61]

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4 Working Mechanism of GO-Based Gas Sensor Most of the reported gas sensors mainly operate on resistive changes [64–66] when the resistance of the sensing material changes as it comes in contact with the gas to be detected. (a) Sonicated mixture of monomer and carbon nanoparticles (b) Polymer dissolved, mixed and sonicated + carbon nanoparticles (c) Electrochemical-based method Such sensors are simple to fabricate and the signal measurement is straight forward. The sensors based on graphene have a similar working mechanism where the changes are measured by direct resistance change or from the current/voltage graph plotted using the contacts on the surface. When the gas to be detected gets adsorbed on the graphene oxide surface, the conductance changes. The interaction mechanism varies depending on the composition of the gas. An adsorbate like H2 O is inert in nature and do not incorporate impurity states in graphene oxide. Such adsorbates modify the conductance by electron redistribution on the graphene oxide surface [67]. Gases like NO2 [66, 68], halogens, or alkali are non-inert and hence chemically active. Such materials behave as temporary dopants to bring about a change in the electron/hole concentration. Adsorbates like H and OH radicals may result in covalent bond with graphene [67] and hence modify the conducting properties. Graphene oxide shows p-type behavior, hence the exposure to varied gases displays different conducting behavior. Gases such as NO2 extracts electrons and increases the conductance of graphene oxide. But gases like NH3 reduces conductance [69]. The interpretation of gas sensing has different takes on it [70, 71]. Two prominent models are oxygen iono-sorption and oxygen vacancies. The mechanism of oxygen adsorption on the surface and subsequent reaction upon exposure to CO reducing gas can be described by the following reaction schemes: O2 (gas) + e− ↔ O− 2 (ads) 2− − − O− 2 (ads) + e ↔ O2 (ads) ↔ 2O (ads)

CO + O− (ads) → CO2 + e− − 2CO + O− 2 (ads) → CO2 + e

GO and CNT have many similarities, hence a sensing mechanism model for CNT will be in close proximity to that of GO. Hence an accepted model by Lee et al. [72] for CNT can be taken as reference for the parameters involved in sensing by GO. The mathematical model can be written as

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G(t) = G max

cK    1 + cK 1 − exp − 1+cK kt K

(1)

where G and Gmax represent the conductance change and maximum conductance change, K is the binding equilibrium constant, gas concentration is represented by c, k is surface reaction rate. If the change in current I is considered to be directly proportional to G by ignoring other constants involved, a simplified equation for current can be written as I (t)/I0 = A exp(−Ct) + D

(2)

where I 0 is the measured current before exposure to gas, A, C, and D are constants that are dependent on sensor properties, gas concentration, and behavior of gas molecules to be sensed. As explained by Ganhua et al. [73], the NH3 sensing by GO is governed by two types of mechanisms denoted as M1 and M2. M1 and M2 compete in terms of response and the output is a combination of the two which is linear in nature. When values A, C, and D are assigned suitable values, M1 and M2 can be determined to simulate the experimental results. As per the model, M1 is responsible for an increased conductance on exposure to gas and the expression for the normalized change in current due to M1 is M1: I (t)/I0 = 5−4 exp[−0.033 · (t − 600)]

(3)

M2, on the other hand, decreases the conductance and in terms of current change can be written as M2: I (t)/I0 = exp[−0.012(t − 600)]

(4)

M2 dominates the process of sensing while M1 is considered negligible. But considering M2 is important, as in case of neglecting the same, an abnormal behavior may be observed. Including both M1 and M2 to explain the sensing phenomenon gives a good theoretical fit to the experimental graphs. Yong Zhou et al. [74] proposed a hybrid sensing mechanism for rGO thin films for room temperature gas sensing. The sensing process was a combined effect of intraand inter-sheet behavior of the rGO sheets. With deposition of more rGO sheets, the inter-sheet distance increased which in turn increased the electric resistance. This was attributed to the inter-sheet electron tunneling being more difficult [75].

5 GO-Based Gas Sensors Single-layer graphene has challenges when considered for mass production. Also, pure graphene does not have the bandgap for a semiconductor gas sensor [43]. GO when synthesized from graphene has bandgap for semiconducting applications. GO

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can be employed in gas sensing applications by integrating modifications. Based on its semiconducting nature, a resistive type gas sensor can be designed. When the morphology and structure are modified, GO can be more responsive to certain gases. Metaloxide is a proven material for gas sensors and hence when combined with graphene gives different genres of gas sensing materials.

5.1 Based on Semiconductor Interfaces rGO can form interfaces like heterojunctions when combined with metal oxides. rGO being a p-type semiconductor can form a p–n heterojunctions when combined with SnO2 [76]. There are changes incurred in the position of conduction band, valence band, and Fermi level. The work function of rGO is less than SnO2 , hence electron transfer is from the former to the later. Since the SnO2 –rGO has a low Schottky barrier (0.2 eV), the electron transition takes place at room temperature. The p–n semiconductor has a lower resistance than SnO2 since rGO has a high conductivity. This is one of the major attributes which results in room temperature detection of gases. Also, the electron depletion layer of SnO2 –rGO is larger compared to SnO2 . The ZnO–rGO sensor [77] displays superior response/recovery time compared to pure rGO. The sensor shows good sensitivity to even a low concentration of NH3 because of the p–n heterojunction of ZnO–rGO. As demonstrated by Chen et al. [76], SnO2 –rGO also showed a good response to NH3 with acceptable response/recovery time at room temperature. rGO has a very long recovery time at room temperature, hence when combined with SnO2 or ZnO, this drawback is covered efficiently. Hydrothermally synthesized CuO was combined with rGO to form CuO–rGO gas sensors [78]. The heterojunction shows high sensitivity and better response/recovery time toward CO sensing compared to pure rGO. Also, when CO is exposed to CuO at room temperature, it shows no response. CuO–rGO forms a p–p junction which is the major reason for improved sensing behavior. Since the work function of CuO is less than rGO, the electron flows from the former to the latter. This happens till the Fermi level is equalized resulting in better sensing for CO at room temperature. The work function related sensing response was also observed for the p–n junction formed when SnO2 –rGO was combined [79]. Liu et al. conducted the work where rGO–ZnO was synthesized by growing ZnO on rGO thin films [80]. The sample showed room temperature response of 9.61 to NO2 with a recovery time of 15 s. Pure ZnO and rGO had long recovery time and response of 6.2 and 1.2 s. In another work, because of the p–n junction of the CeO2 –rGO sensor, it responded with sensitivity 8.2 times higher than that of pristine rGO at a room temperature [81]. Fe2 O3 –rGO synthesized by hydrothermal method detected NO2 displaying high sensitivity and good response time [82]. The sensor being applicable at room temperature showed good selectivity for NO2 , since for other gases the working temperature needs to be raised.

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5.2 Based on Change of Morphology and Structure Some of the governing properties for an efficient gas sensor are conductive pathways, large surface area, and more active sites which can be achieved by modification of the morphology and structure of the sensing material. Graphene on its own has the drawback of getting agglomerated. The same can be avoided when combined with SnO2 nanoparticles [79]. The well-distributed graphene can now provide conductive pathways for the electrons and enabling room temperature gas sensing. Graphene also adds to more adsorption sites for better sensitivity. ZnO despite being a wide bandgap semiconductor and a good sensing material has poor selectivity [83]. A 3D ZnO–rGO aerogel structure was synthesized by Liu et al. using solvothermal method [84]. The sensor displayed response to 50 ppm NO2 with good response/recovery time compared to pure rGO. Also, the selectivity was appreciable. When the same study was conducted by physically mixing ZnO and graphene dispersion, the sensing results were not up to the mark due to agglomeration of ZnO nanoparticles and graphene sheets. The same group in another work showed the superior sensing performance toward NO2 for 3D SnO2 -graphene aerogel compared to 2D SnO2 -graphene sensor [85]. TiO2 is another gas sensing wide bandgap semiconductor but the operating temperature is above 200°C. Research conducted on composite of TiO2 and graphene also has stability issues. rGO decorated TiO2 nanocrystals worked as ultrafast NH3 sensor [86]. The two different morphologies developed in TiO2 –rGO are rGO on TiO2 nanoparticle surface or rGO generating bridges at the nanoparticle interfaces. Due to the later morphology, the resistance dropped drastically making room temperature sensing possible. The former morphology assisted in increasing the number of adsorption sites. Compared to pristine rGO, TiO2 –rGO displayed better selectivity to NH3 because of the absorbing property of acidic TiO2 surface. GO is deposited on TiO2 (on IDEs) and heated to synthesize TiO2 –rGO which displays 1.5 times better response than rGO at room temperature [87]. The response of SnO2 is not enhanced enough for low concentration of test gas [88, 89]. But an encouraging result was observed by Lin et al. for SnO2 -graphene synthesized by hydrothermal method using GO and SnCl2 [90]. GO acted as a template to influence the preferential growth of SnO2 and helping to avoid SnO2 nanoparticle agglomeration. The composite sensor responded to 10 ppm of NH3 with response/recovery time less than 1 min. In2 O3 –rGO was synthesized by hydrothermal method by Yang et al. [91]. The sensor responded to 5 ppm of NO2 with a sensitivity of 37.8%. In addition, excellent selectivity and stability were observed at room temperature. Graphene increases the conductivity while In2 O3 does not allow rGO to agglomerate which in turn increases active sites and specific surface area. Cu2 O–rGO nanorods were developed by Meng et al. [92] using hydrothermal method and microwave. After annealing the hybrid showed a porous structure and high surface to volume ratio. Because of the porosity, the gas diffused more easily for better response. The response showed a linear behavior toward NH3 .

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5.3 Based on Coupling Effect Between Graphene and Metal Oxides It is believed by some researchers that heterojunction formation because of graphene is not the correct way to explain improved sensing properties for MOS-graphene duo [93]. Also, it might not be the case that the morphology or structure of sensing material is modified due to graphene. A third proposed mechanism is the synergetic coupling effect between metal oxide and graphene. The improved sensing results may be attributed to the chemical bonds between metal oxides and graphene. The claim is supported by XPS results. A room temperature sensor based on WO3 was developed to detect NO2 . Sol-gel technique was used to synthesize the WO3 nanosphere covered with graphene sheets. WO3 and graphene individually do not show any response to NO2 room temperature. The reason is the C–O–W chemical bond between WO3 and graphene as confirmed by Raman and XPS studies which results in an effective charge transfer. After reacting with NO2 the work function of WO3 resonates with graphene which facilitates the easier movement of electrons for conduction. Hydrothermal method was employed to fabricate the Co3 O4 -graphene gas sensor [94]. The XPS study established Co–O–C bond formation at Co3 O4 and graphene interface. When the electron transfers through the bond when exposed to NO2 as the test gas, it increases the width of the hole accumulation layers and hence improving the sensitivity. Electrospinning method was used to synthesize rGO nanofibers coated Co3 O4 . The sample shows excellent sensitivity to even low concentrations of ammonia. A response time of 4 s and improved selectivity was also observed. The result was attributed to the strong affinity of rGO toward NH3 and the chemical bond between rGO and Co3 O4 . The chemical bond C–O–Sn confirmed by XPS for SnO-graphene sensors was responsible for a good response toward 50 ppm of NH3 [95]. Similar encouraging results were observed for sensing NO2 by NiO-graphene sensor because of the Ni–O–C bonds [96]. In the cases mentioned above, the main reason behind improved sensing was the bridge that was formed for electron conduction due to the chemical bond formed between the graphene based composites.

5.4 NO2 Sensing Nitrogen dioxide (NO2 ) is one of the most toxic and hazardous gases which is present in the environment. The NO2 gas usually produced as by-product in high-temperature chemical reactions takes place at various places such as combustion in vehicles, chemical industries, power plants, etc [97]. It has a quite adverse effect on human health as well as an ecosystem. The exposure to NO2 gas with a low concentration of 3 ppm to the human body can cause irritation to lungs and eyes, weakens the immunity against respiratory infection, and fatality chances [98]. The prolonged NO2 exposure with high concentration can promote to carcinogenic effect or leads to cancer. The

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NO2 gas is the primary reason to form acid rain, ozone as well as photochemical smog in the atmosphere. Taking in the account all these harmful effects of NO2 gas, US Environmental Protection Agency has declared exposure limit at 53 ppb [99]. GObased nanocomposite are being widely studied for NO2 sensing at room temperature. The NO2 sensing performance parameters using GO-based nanocomposites at room temperature are summarized in Table 1. It is observed that rGO-based ternary compound is an efficient room-temperature sensor for NO2 and exhibits high response (Fig. 4).

5.5 NH3 Sensing Ammonia (NH3 ) is one of the widely produced and used chemicals in the world. Nearly 2.1–8.1 Tg amount of NH3 is discharged in the environment via various processes. The most common sources of NH3 are nitrate salts, nitrogen fixation, chemical combustions, and chemical industries, etc. It is toxic in nature and has a severe impact on respiratory tracks. The increasing respiratory health-related issues are of serious concern since inhaling NH3 gas amount than natural level can lead to death. As per the occupational safety and health administration (OSHA) NH3 exposure time controlled to 8 h–10 min time for 25 and 35 ppm concentration, respectively, at the working place. Hence, there is urgent need to develop a high-performance NH3 sensor with real time detection at room temperature. The rGO-based nanocomposite material was reported for NH3 sensing at room temperature. rGO-based materials are also observed to display good selectivity toward NH3 (Fig. 5). The performance of rGO-based nanocomposite sensor for room temperature NH3 gas sensing is shown in Table 2.

5.6 Volatile Organic Compound Vapors Volatile organic compounds (VOCs) having boiling points range from room temperature to ~260 °C are considered as major pollutants in total air pollution [130]. VOCs is a class of air pollutants which is expelled out as by-product in chemical reactions from laboratories. The VOCs can also be generated when volatile organic liquids vaporize at room temperature on exposure to external atmosphere. The effect of VOCs on the ecosystem or environment is decided by the type of VOC and its process of emission. Among various VOCs, some are abundant and toxic in nature such as formaldehyde, phenol, acetone, methane, etc. These VOCs are primary concerns for the researchers and must be monitored. Table 3 lists some rGO-based nanocomposite sensors for room temperature VOCs sensing.

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Table 1 Summary of rGO-based nanocomposite sensor for room temperature NO2 gas sensing Material

Response time (s)

Synthesis method

Gas response S (%)

rGO–Co3 O4

90

Modified hummer and hydrothermal

26.8a

Cu modified carbon-rGO

500

Modified hummer and chemical

66

Concentration (ppm)

References

5

[94]

37*

50

[100]

Modified hummer and one-pot process

14b

1

[101]

126

Modified hummer and hydrothermal

8.2c

5

[82]

Pd/SnO2 /rGO

13

Modified hummer and wet chemical

3.92c

1

[102]

SnO2 –rGO

14

Modified hummer and hydrothermal

3.8c

1

[103]

6

Modified hummer and hydrothermal

149b

20

[104]

rGO/Fe2 O3

32

Modified hummer and hydrothermal

3.86#

5

[105]

In2 O3 /rGO

1100

Modified hummer and hydrothermal

1098#

1

[106]

220

Modified hummer and thermal treatment

6227b

60

[107]

SnO2 –rGO

45

Modified hummer and hydrothermal

1.38c

5

[108]

ZnO/rGO

25

Modified hummer and chemical

9.61#

50

[80]

Ag–SnO2 /rGO

49

Modified hummer and wet chemical

2.1c

5

[109]

CuO/rGO

rGO/Fe2 O3

WO3 /S–rGO

NiO–SnO2 –rGO

(continued)

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Table 1 (continued) Material

Response time (s)

Synthesis method

Gas response S (%)

Fe3 O4 /rGO

29

Modified hummer and solvothermal

35.6*

Ni(OH)2 /rGO

32

Modified hummer and reflux

65*

ZnO/rGO

75

Modified hummer and solution process

8

Concentration (ppm)

References

97

[110]

100

[111]

119*

1

[112]

Modified hummer and chemical

2.25c

5

[113]

90

Modified hummer and microwave assisted hydrothermal

16.5a

1

[114]

In2 O3 /rGO

240

Modified hummer and hydrothermal

8.25#

30

[115]

rGO/WO3

540

Modified hummer and 1 pot polyol process

769a

5

[116]

rGO/CNT/SnO2

In(OH)3 /rGO

Value approximated from a graphical plot S∗ =

Rg −Ra Ra

× 100%, S# =

Rg Ra ,

Sa =

Ra −Rg Ra

× 100%, Sb =

Ig −Ia Ia

× 100%, Sc =

Ra Rg

6 Limitations and Challenges For effective use of GO as gas sensor, its properties must be tuned and controlled. Inclusion of the functional group is one of the most important factors. Analysis of proper functionalization can be conducted using characterization like FT-IR spectroscopy, Raman spectroscopy, and XPS. The sensor must be stabilized which must be achieved during the synthesis process [140]. Stabilization can also be carried out at constant temperature, keeping a note of the electric resistance [141]. It is expected and also observed that room temperature operation of sensor devices can be battery operated. But a sensor operating at higher temperature adds to the effectiveness as the adsorption/desorption process is faster. But it is also observed that at a temperature above 200 °C, the functional groups get destroyed [142]. The temperature of 150 and 120 °C was observed to be appropriate temperature for sensing NO2 and

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Fig. 4 Comparison of the sensitivity of the as prepared samples to different concentrations of NO2 [107]

Fig. 5 Response of the Cu2 O/rGO-based sensor to various tested gases at room temperature [92]

H2 [142, 143]. The selectivity of GO might not be as good as its sensing properties. The selectivity can be improved when GO is functionalized/hybridized with other active materials. The selectivity of CO is good for graphene when it is functionalized with N [144]. A possible solution might be to use filters to improve selectivity by removing the other interfering gases or in other words masking the presence of other gases. As per the discussion in the chapter and Tables 1, 2, and 3, it can be concluded

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Table 2 Summary of rGO-based nanocomposite sensor for room temperature NH3 gas sensing Material

Response time (s)

Synthesis method

Gas response S (%)

PANI/rGO

1326

Modified hummer and chemical

13*

15

[117]

78

Modified hummer and click reaction

63*

20

[118]

GO–CuFe2 O4

3

Modified hummer and combustion

2.35*

5

[119]

GO@PANI hollow sphere

102

Modified hummer and in-situ oxidation

31.8#

100

[120]

Pd–TiO2 –rGO

184

Modified hummer and 1 pot polyol

14.9a

10

[121]

7

Modified hummer and reduction

11*

5

[122]

ZnO/PANI/GO

38

Modified hummer and polymerization, chemical

4#

100

[123]

PANI/GO/ZnO

5

Modified hummer and polymerization, chemical

0.5d

600

[124]

Polypyrrole/rGO

120

Modified hummer and oxidation

50*

10

[125]

SnO2 NRs/rGO

8

Modified hummer and hydrothermal

1.3c

200

[76]

PANI/SnO2 /rGO

5

Modified hummer and polymerization, chemical

0.83d

10

[126]

TiO2 /GO/PANI

32

Modified hummer and polymerization, chemical

110#

100

[127]

4

Modified hummer and electrospinning

53.6e

50

[128]

420

Modified hummer and 1 pot proute

7.6a

5

[129]

55

Modified hummer and spray

0.62*

10

[87]

Aryl fluoride/GO

Pt–rGO

rGO–Co3 O4

Pd/SnO2 /rGO TiO2 /rGO

Concentration (ppm)

References

(continued)

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Table 2 (continued) Material

Response time (s)

Synthesis method

Gas response S (%)

Concentration (ppm)

References

rGO–TiO2

10

Modified hummer and reflux

3.5*

30

[86]

ZnO–rGO

78

Modified hummer and chemical

1.2*

10

[77]

Cu2 O NPs/rGO

28

Modified hummer and microwave-assisted hydrothermal

104*

200

[92]

Value approximated from a graphical plot S∗ =

Rg −Ra Ra

Sd =

Ia −Ig Ig ,

× 100%, S# = Se =

Rg −Ra Rg

Rg Ra ,

Sa =

Ra −Rg Ra

× 100%, Sb =

Ig −Ia Ia

× 100%, Sc =

Ra Rg

× 100%

Table 3 Summary of rGO-based nanocomposite sensor for room temperature VOCs sensing Material

Response time (s)

Synthesis method

Gas response S (%)

Concentration (ppm)

References

rGO/In2 O3

2 Trimethylamine

Modified hummer and hydrothermal

9.3a

100

[131]

SnO2 @rGO-PANI

360 Methane

Modified hummer and in-situ chemical oxidation

26.1*

100

[132]

rGO/ZnO

20–30 Chloroform

Modified hummer and sol airbrush Tech.

2.5*

40

[133]

rGO/SnO2

145.8 phenol

Modified hummer and synthetic process

1.3#

0.06

[134]

rGO/TiO2

70 Formaldehyde

Modified hummer and spray

0.4*

0.5

[135]

TiO2 /rGO

18 Methanol

Modified hummer and electro-deposition

96.93a

800

[136]

SnO2 –rGO

120–140 Acetone

Modified hummer and hydrothermal

2.19a

10

[137]

SnO2 –rGO

78–97 Ethanol

Modified hummer and hydrothermal

3.8*

100

[138]

rGO/ZnO

28 Formaldehyde

Modified hummer and solution process

5.2*

10

[139]

Ig −Ia Ia

× 100%, Sc =

Value approximated from a graphical plot S∗ =

Rg −Ra Ra

× 100%, S# =

Rg Ra

, Sa =

Ra −Rg Ra

× 100%, Sb =

Ra Rg

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that GO or rGO adds to the improvement of different materials to act as a gas sensing material. GO or rGO when used individually for sensing gases, does not display a response as enhanced as in cases of appropriately doped or composite samples. Also, the response and recovery time is not conducive unless structure and bandgap are engineered by using specific synthetic methods or addition of foreign material to GO or rGO.

7 Conclusion In summary, GO has remarkable physical and chemical properties which makes it future potential material for advanced gas sensor devices. The electronic tunability of GO to rGO as well as active surface sites confirm the room temperature operation of the sensor with enhanced performance. The various possible synthesis routes for GO and their nanocomposites have been discussed. The probable gas sensing mechanism of GO-based sensor materials have been elucidated. The recent progress and challenges of GO-based room temperature gas sensors have been discussed. GO-based nanocomposites are being developed for room temperature gas sensor applications. There is plenty of scopes to work on GO nanocomposites in order to achieve high selectivity and response with rapid recovery time. Acknowledgements This research work is supported by the National Leading Research Laboratory program via National Research Foundation (NRF) (NRF-2016R1A2B2016665) Republic of Korea and funded by the Ministry of Science, ICT and Future Planning.

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Chapter 13

Carbon Nanotube Based Wearable Room Temperature Gas Sensors Abhay Gusain

1 Introduction The nanomaterials are an important class of materials with at least one-dimensional (1D) size in 1–100 nm range and zero-dimensional (0D) shape, e.g. quantum dots, and one-dimensional based nanostructures [1, 2]. The quantum dots have been used as structural component materials for various applications like digital memory modules, quantum lasers and optical sensors [1]. As the CNTs were discovered, many CNT properties were found to be significantly different from those of the quantum dots, which led to the achievement of many applications through CNTs. The CNT is also a carbon form, intermediate between other materials—graphite and fullerenes. The CNTs have unique physicochemical properties required for large application range, which include, but not limited to, polymer/catalyst additives, the lighting component used in cathode ray autoelectron emission, flat displays, gas discharge tubes in telecommunication networks, electromagnetic wave absorption/screening, energy conversion and storage like anodes for lithium battery, hydrogen storage, composite materials in fillers/coatings, nanoprobing devices, sensors, supercapacitors etc. [1]. The applications of CNTs for gas sensors have been reported earlier in detail, but these studies have focused on broad application areas of CNTs for gas sensors, and recent studies in this area need to be addressed [3]. The focus of this chapter, therefore, will be concentrated on the reported applications of the CNTs in different ways for developing chemical gas sensors, more specifically, flexible and wearable gas sensors. It has been reported that the modification of CNTs into its structures can be achieved through many ways, which ultimately affects the gas sensor response to the investigated gas [1]. Although 2D planar surface materials like graphene are more flexible than the 1D structured CNTs, which makes graphene better processable for integration into the fabricated electronic devices, the growth of a large number of A. Gusain (B) Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, São Carlos, SP 13566-590, Brazil e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_13

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graphene layers by some methods is impossible to control and is unsuitable for largescale production [4]. In addition, the integration of CNT with other material types, like metal oxides, polymers etc., can also be achieved for changing its properties as well as the gas sensing mechanisms [5, 8, 9, 11, 12–17]. Although these types of materials have also been used without CNT for gas sensing [6, 7], the addition of CNT in these materials in different forms has been shown to improve mechanical properties required for fabrication of flexible sensors [8, 9]. For example, the key metal oxide gas sensors have been demonstrated due to their significantly low power consumption. The thin/thick-film technology based semiconductor oxides have been further employed as less-cost sensing components for toxic gas detection/monitoring [10]. These materials are highly and fastly (tens of seconds) sensitive to large gas range and are highly long term stable [10]. The gas sensing mechanism in these materials is based on metal oxide electrical conductivity change due to catalytic reduction/oxidation reactions, occurring at the metal oxide surface due to the toxic gases and oxygen. The metal oxide electronic structure and morphology, the chemical composition and crystal structure control these reactions [10]. However, application of CNT-integrated metal oxides in the mechanically stable flexible sensors has also been demonstrated [11]. Similarly, other types of materials like conducting polymers have also been shown to be used in the gas sensing applications, and these materials can also be further modified with CNTs to improve their gas sensing properties [12, 13]. Table 1 shows the wide range of sensors developed using the CNTs in different materials for fabrication of flexible and wearable gas sensors, and with different performance parameters. Table 1 Summary of reported sensing properties of flexible and wearable gas sensors based on CNTs composite materials S. no.

Type of CNT

Composite material

Gas under detection

Detection limit (ppm)

Sensitivity (%)

Response and recovery time (s)

Year

References

1.

SWCNTs

ZnO quantum dot

C2 H5 OH

500

10.9

992, 301

2018

[5]

2.

SWCNTs

AgNPs

NO2

0.2

17

120, 450

2018

[8]

3.

SWCNTs

–

DMMP

1

3.6

–

2010

[9]

4.

SWCNTs

PVA

H2 O

–

2400

40

2017

[12]

5.

SWCNTs

Fe2 O3

NH3

10

1.5

250, 300

2017

[11]

6.

MWCNTs

–

NO2

10

–

–

2015

[14]

7.

MWCNTs

PMS

C7 H8

10

–

–

2017

[15]

8.

MWCNTs

EC, PEO, PVP

C2 H5 OH

–

1

–

2017

[15]

9.

MWCNTs

PAH

NO2

1

19.10

–

2009

[16]

10.

MWCNTs

HPMC, PMVEMA, PVP

VOCs

0.12

1.2604

–

2019

[17]

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2 Brief History CNTs were first synthesized by S. Iijima, Japan, in 1991 [18]. The CNTs have shapes of long cylindrical structures with one to several nanometers diameters and several microns length consisting of one/multiple hexagonal graphite planes rolled within these structures. Their surface has regular carbon hexagons [1]. The CNT shape influences their properties and thus, influences the performance parameters of the chemical gas sensors [1]. Further, the CNT synthesis conditions determine different types of CNT structures—one/multilayered tubulenes with open/closed ends. The cylindrical surfaces containing the interconnected carbon atoms within a single hexagonal cell network, i.e. the sp2 -network determines tubulene structure [1]. Further, chirality of the tubulene is determined by the mutual orientation of the hexagonal network and the longitudinal axis of CNT. The chirality is determined by a set of two integers, n and m, locating the hexagon that will match after nanotube rolling with the hexagon at the origin of coordinates. The chirality is also uniquely determined by the orientation angle or the chiral angle , between the directions of nanotube rolling and common edge between two adjacent hexagons [1]. Further, the multiple nanotube rolling options which do not change the hexagonal network structure are of potential interest. The angles,  = 0° and 30° correspond to the (n, 0) and (n, n) chiralities at these desired rolling directions. This angle is important in determining the CNT electrical properties, like metallic/semiconductor conductivity [1]. Most nanotubes show semiconductor conductivity, with 0.1–0.2 eV band gap. However, many types of electronic devices can be achieved by varying the CNT band structure [1]. The CNTs are categorized into two types—the achiral CNTs, with a screw symmetry, and chiral ones, with a cylindrical symmetry [1]. The achiral tubulenes are further divided in two types—zig-zag nanotubes, with two edges of each hexagon parallel to the cylinder axis (Fig. 1a), and arm-chair nanotubes, with two edges of each hexagon perpendicular to the cylinder axis (Fig. 1b).

Fig. 1 Idealized models of a zig-zag and b arm-chair monolayer nanotubes. Reproduced with permission from Elsevier (Ref. [1]). https://doi.org/10.1016/j.moem.2017.02.002, https:// creativecommons.org/licenses/by/4.0/

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3 CNTs-Based Gas Sensors One of the important CNT features unique for the gas sensing applications is large CNT unit surface, which in turn, determines their electrochemical and adsorption properties. Since CNTs are surface structures, their whole weight distribution is along the surface of their layers. Thus, the large surface area provides high adsorption capacity as well as excellent sensitivity to adsorbed atoms/molecules on the CNT surface. This provides the immense possibility of designing CNT based chemical gas sensors. For example, the application of transparent CNT networks has been reported to develop a room temperature healable/flexible gas sensor device [19]. This was achieved by layer-by-layer assembly of transparent healable polyelectrolyte films and CNT networks deposited on healable substrates. These CNT films were reported to have better network structures on these substrates, by bringing the separated areas of the CNT layer back into contact through the self-healing layer lateral movement. The films also showed comparatively high charge carrier mobility as well as large surface-to-volume ratio, with the room temperature gas sensing response to 5 ppm ammonia (NH3 ). Such flexible and wearable gas sensors based on CNTs are described in detail in the following sections. In order to improve the sensing properties of the gas sensors based on CNTs, two different types of the CNTs—single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs), are used along with other different types of materials like metal oxides, quantum dots, metal nanoparticles, nanoribbons and other nanomaterials to form their composites that are used for gas sensing. In case of SWCNTs, the wall structure consists of a single graphite sheet closed in a tubular shape, while in case of MWCNTs, graphite sheets are each closed into a tubular shape one within the other [20]. These CNTs composites gas sensors and the various reported flexible and wearable gas sensors are discussed below.

4 SWCNTs Based Gas Sensors This section describes some of the SWCNT based room temperature gas sensors in order to describe some of the mechanisms of these gas sensors, followed by SWCNT based flexible and wearable gas sensors. These SWCNT based gas sensors have been reported to be fabricated using the SWCNT along with the nanomaterials and metal oxides in order to improve the sensing properties of the gas sensors.

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Choi et al. have demonstrated the application of platinum nanoparticles (Pt NPs)decorated SWCNTs, deposited using sputtering and then treated thermally in atmosphere, in order to fabricate high sensitive/selective room temperature NO2 gas sensors with 2 ppm detection limit [21]. The influence of the temperature on the sensing properties of these sensors was also determined. However, it was also demonstrated that despite better sensitivity to NO2 at 100 °C relative to 25 °C, these sensors are also applicable to other gases as well at room temperature. The effects of NO2 gas on the SWCNT were also demonstrated. As oxygen species are adsorbed on the SWCNTs surface and then ionized to O2 − , O− or O2− (Fig. 2a), the valence band electrons of SWCNTs are extracted by these species, and this leads to an increase in the SWCNTs conductivity. As the oxidizing gases, such as NO2 are introduced, this process occurs and expands the SWCNT hole accumulation layer. Simultaneously, the Pt catalyst effect (Fig. 2b), improves NO2 selectivity of these sensors, as the presence of more active sites offered by the Pt NPs leads to more electron transfer from SWCNTs to Pt NPs. Further, in the case of Pt NPs-decorated SWCNTs, an easy dissociation of NO2 molecules and subsequent migration to the SWCNT surface expands the hole accumulation layer. Hence, this leads to more enhancement of NO2 selectivity. Similarly, the application of Pt NPs-doped SWCNTs networks for the chlorine (Cl2 ) sensors has also been demonstrated by photoreduction, using an ultraviolet (UV) irradiation method [22]. These sensors showed a 32% response for 0.1 ppm Cl2 gas at room temperature. Further, the application of a composite of conducting polyaniline and SWCNT for 5 ppm NH3 and CO gas sensors at room temperature has also been reported [23]. These composites were formed by depositing polyaniline using drop-cast for warping the polyaniline covered SWCNT. In addition to the nanoparticles, the application of novel chemically sensitive cadmium arachidate (CdA)/SWCNT composite coatings for fabricating better performing opto/chemical sensors has also been demonstrated [24]. These coatings improve the adhesion of the sensor to the composite fiber surface, with improved robustness and sensitivity of the sensors, many times compared to directly coated SWCNT. The application of such materials like nanoparticles can also be extended to the fabrication of SWCNT based flexible and wearable gas sensors. In case of flexible and wearable gas sensors, Gao et al. have demonstrated fiber gas sensors for the wearable electronics, with ZnO quantum dot-decorated SWCNT as sensing material [5]. These flexible fiber gas sensors were demonstrated to be room temperature operable, highly sensitive, having good recovery time, highly long-term stable, bendable and mechanically robust. The wearable smart face masks were also developed by the integration of these sensors, with variable selectivity for C2 H5 OH, HCHO and NH3 gases. In order to fabricate the sensors, flexible substrates based on nylon were incorporated with sensing material and selective detection features were combined to fabricate a wearable gas sensor using LEDs with different colors. In order to demonstrate the flexibility of these sensors, gas sensing was also demonstrated at different bending angles. A small current change at these angles, from 0° to 90°, was found (Fig. 3a, b) for the 500 ppm NH3 . Further, after many sequential bending–straightening cycles of 0, 100, 200, 500 and 1,000, the gas sensor was found to be extremely mechanically stable (Fig. 3c, d). The washing of sensor in

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Fig. 2 a Schematic diagrams of the sensing mechanism of a pure SWCNT sensors and b Pt NPsdecorated SWCNT sensors in air and NO2 , respectively. Schematic diagram of the Pt NPs-decorated SWCNT sensor. Reproduced with permission from Elsevier (Ref. [21])

water, after 0, 10, 20, 50 and 100 times, also showed no changes (Fig. 3e). In addition, after 5 week atmospheric exposure, the sensor performed up to 85% of its initial value, indicating good long-term stability. The sensing mechanism of these sensors for different gases was shown to be dependent on the type of the reaction between the sensing material and the gas. For NH3 gas, it was shown that the easy charge transfer

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Fig. 3 a The M-GS sensor resistance in air for different bending angles. b Four sensing cycles of the device to 500 ppm NH3 under different bending degrees. c I–V curves of the device after bending for 0, 100, 200, 500 and 1,000 cycles. d SEM images of device before and after bending 1,000 times. e Response curves of device after being washed with water. Different colors of LED lights corresponding to the injection of different gases. f Electrical resistance change of device over 1–5 weeks. Reproduced with permission from Springer Nature (Ref. [5])

from NH3 molecules to the surfaces of CNTs causes a change in the conductivity of the CNTs. In case of HCHO gas, the sensing mechanism was attributed to the negligible change in the bandgap of the SWCNTs when HCHO molecules were absorbed onto the surfaces of the SWCNTs, which results in donation of electrons. In case of C2 H5 OH gas, it was shown that ZnO quantum dots reduce the defects of SWCNTs. Therefore, a greater number of electrons are adsorbed by the oxygen ions trapped in the surface state of the particles, which reduces the charge density of the particles. Flexible chemical sensors usually require transferring of the fabricated layers or complete device onto specially applicable flexible substrates, which are then attached to the target objects. Such transferring can limit the practical applications of sensors [8]. In order to address this issue, a gas sensor array fabricated by spraying material based on silver nanoparticles all-carbon hybrid nanostructures, along with metallic SWCNTs as interdigital electrodes and silver nanoparticles/reduced graphene oxide as sensing layers has been demonstrated to allow direct device fabrication on various targeted objects like leaf, silk and portable sticker (Fig. 4a–c) [8]. A complete flexible

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Fig. 4 Photographs of the gas sensor array fabricated on a leaf, b silk stitched onto the lab coat, and c portable sticker attached on the human body, respectively. Dynamic responses of the samples fabricated on d leaf, e silk and f portable sticker, respectively. Reprinted with permission from Ref. [8]. Copyright (2016) American Chemical Society

device was formed by using these SWCNTs as conducting electrodes and AgNPsdecorated reduced graphene oxide as sensing layers. The sensor showed sensing response of 6–20 ppm for NO2 gas, with high mechanical stability for around 3000 bending cycles and response limit up to 0.2 ppm at room temperature. The sensitivity of around 3.3 for the metal electrode-based sample and 13 times for the non-AgNPdecorated sample was demonstrated. A high scale compatibility and suitability on the planar/nonplanar structures was also demonstrated. The devices fixed on a lab coat or the human body showed stability in their sensing performance, showing that these sensors can be used practically in wearable/portable areas. In addition, the leaf, silk and portable sticker sensors showed high response as well as reversible behavior (Fig. 4d–f). The leaf sensor demonstrated the fastest response as well as recovery speed, whereas the silk sensor showed the highest response. The sensing mechanism was mainly attributed to the direct charge transferring between NO2 molecules and sensing materials with the absorption of the target gas on the sensor. With rGO behaving typically as p-type semiconductor and the oxidizing NO2 acting as an electron acceptor, NO2 absorption increases the hole density and electrical conductivity. Wang et al. have demonstrated SWCNT assembly of flexible thin film gas sensors to detect the nerve agent stimulating material dimethyl methylphosphonate (DMMP) vapors at room temperature (Fig. 5a) [9]. These sensors were shown to be highly mechanically flexible, with a low 5.6% deviation in the response value under bending condition in a 5 mm radius glass tube. The sensors detected low DMMP concentration up to 1 ppm, and showed a 3.6% resistance change, 0.232 ± 0.007 ppm−1 response and 0.996 linearity for 1–40 ppm response range. Even

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Fig. 5 a The optical micrograph of the flexible assembled SWCNT sensor on PI substrate. b and c SEM images of the assembled SWCNT films on the flexible PI substrate at low and high magnification, respectively. Reproduced with permission from Elsevier (Ref. [9])

with exposure to known interfering vapors, these sensors showed higher sensitivity to DMMP, making them potential portable on-site detectors. Their higher sensitivity was attributed to their high surface area, as the fabricating technique allowed the formation of uniform grown SWCNT bundles between electrodes (Fig. 5b, c). The sensing mechanism was attributed to the interconnections between SWCNTs in the thin film which provide regular pathways for charge carriers. With DMMP acting as highly electron donating material, electron transfer occurs from DMMP to the p-type nanotubes, which decreases hole numbers in the CNTs when the sensor is exposed to DMMP vapors. This increases the SWCNTs electrical resistance in the thin film and thus the nominal resistance of the SWCNT-based sensor. Zhou et al. have demonstrated wearable humidity detector for smart wearable electronic textiles, with potential applications in the management of wounds, skin pathologies or cloth microclimatic control (Fig. 6a–c) [12]. The humidity sensor was demonstrated by using SWCNT/PVA based filament structures of different weight ratios with large strength of nearly 750 MPa and high toughness of 4300 J g−1 energy to break. It was shown that the conducting nanotube networks within the filaments for the sensing can be changed through adjustment of the intertube distance. This was done through PVA molecular chain swelling due to the water molecules absorption,

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Fig. 6 a The fabricated SWCNT/PVA filaments used to stitch different patterns onto a cotton cloth. b Magnified image of one type of pattern shown in a indicated by a blue dashed line. c Photo of the SWCNT/PVA pattern swelled in boiling water. d and e Variations in the electrical resistance of the SWCNT/PVA filaments with SWCNT-to-PVA weight ratios of 1:1 and 1:5 under different RH conditions. f Resistance reversibility and response time of the 1:5 SWCNT/PVA fiber sensor at 25 °C. g The tensile strength and humidity sensitivity (with relative humidity varying from 60 to 100%) of SWCNT/PVA fiber sensors as compared to the literature data for other film- and fibertyped humidity sensors. Reprinted with permission from Ref. [12]. Copyright (2016) American Chemical Society

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with two times increase in SWCNT/PVA filament diameter as compared to that under dry conditions. In order to demonstrate the flexibility of the sensors, the stitching of fiber sensor on a hydrophobic textile material was done, and it was shown that after water absorption, its electrical resistance enhanced significantly (Fig. 6d). Further, these wearable sensors with SWCNT/PVA of 1:5 weight ratio showed large sensitivity to large relative humidity (Fig. 6e). A high sensitivity as well as short response time was also shown (Fig. 6f). Also, the application of these sensors for water leakage monitoring and their superior performance relative to other sensors was also demonstrated (Fig. 6g). Further, the application of SWCNT films with high surface area, and directly synthesized using CVD, has been demonstrated for fabricating flexible gas sensors for NH3 , NO and NO2 gases [11]. These films with high residual catalyst amount have also been used for direct fabrication of high area, flexible/wearable sensors for these toxic gases [11]. For this, a film with composite of SWCNT and Fe2 O3 was fabricated for H2 S and NO2 sensing [11]. The improved performance of these gas sensors was attributed to uniform Fe2 O3 nanoparticle formation in the film, with no requirement for chemical functionalization/doping. Further, the repeatability of the stable gas sensing of the sensors under large angle bending conditions on flexible substrates was also demonstrated, with potential applications in portable, wearable environmental monitoring devices. The sensing mechanism was attributed to the electron adsorption on the Fe2 O3 nanoparticle surface due to the reaction between H2 S gas molecules and oxygen ions, and then transfer to the SWCNT film. This process resulted in higher electron/hole recombination and hole reduction in the SWCNT film, which increased the sensor resistance. The flexibility of these sensors was demonstrated by attaching a composite film in the flexible gas sensor, bent to large angles, and then converted to straight shape (Fig. 7a). The gas-sensing behavior of the SWCNT/Fe2 O3 composite film was demonstrated using 20 ppm H2 S exposure at room temperature. It was shown that such repeated bending of the sensor did not show degradation in the sensor structure (Fig. 7b) and the sensor showed small change in response in both conditions. In addition, earlier method for flexible sensor fabrication using deposited SWCNT films on adhesive polydimethylsiloxane (PDMS) substrates has also been demonstrated [25]. However, the reduced device performance in this case was shown in bending condition. Similarly, SWCNTs based flexible/transparent sensor with superior performance has been demonstrated [26]. The SWCNT films were spray coated on transparent/flexible plastic substrates, and subsequently AuNPs decoration was done [26]. The detection limit of 255 ppb at room temperature was shown, with fast response to NH3 . However, these sensors did not show full recovery at room temperature. However, CNTs show limited selectivity in their analyte interaction. Therefore, CNT functionalization is important to improve sensitivity/selectivity toward target analytes. It has been reported that such functionalization also improves the solution processability of these otherwise nonsoluble sensing materials [27]. Various approaches, classified as noncovalent and covalent modifications, have been reported

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Fig. 7 Response of SWNT-Fe2 O3 composite film gas sensors under different bending angles upon exposure to 20 ppm H2 S. a Illustration of gas sensor bended to 0°, 90° and 180°. b Response of gas sensor under bending from 0° to 180° and returned to 0° again. Reproduced with permission from Elsevier (Ref. [11])

for the functionalization of CNTs [27]. Some approaches such as anchoring of chemical groups, macromolecules or biomolecules provide a function for selective identification, interaction or reaction with analyte. Further, small molecule adsorption or polymers/biomolecules wrapping on the CNT surface provides the noncovalent functionalization. The covalent functionalization involves reactions for covalent attaching of strong and stable chemical groups to the CNT conjugated surfaces [27]. In addition, SWCNT synthesis with control over the structure has been reported as important issue because of chirality dependent metallic/semiconductor behavior of SWCNTs, with most prepared SWCNTs are a semiconducting/metallic nanotube mixture [28]. Therefore, to make CNTs for their potential applications in electronic devices, it is important to realize selective growth of pure metallic or semiconducting SWCNTs, and even SWCNTs with specific chiralities. However, MWCNTs can be produced on a large scale with low cost, but their large scale use and importance in applications is still limited [28].

5 MWCNTs Based Gas Sensors This section describes some of the MWCNT based room temperature gas sensors in order to describe some of the mechanisms of these gas sensors, followed by MWCNT based flexible and wearable gas sensors. These MWCNT based gas sensors have been reported to be fabricated using the MWCNT along with the nanomaterials and metal oxides in order to improve the sensing properties of the gas sensors.

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Balazsi et al. have shown the application of novel CNT/hexagonal tungsten oxide (hex-WO3 ) hybrid composites for room temperature NO2 gas sensors [29]. In addition, the application of metal nanoclusters with the CNTs for improved gas sensing response of the films up to 100 ppb NO2 was also reported. This resulted in reducing the temperature range of sensitivity of the nanocomposites. The sensitivity of hex-WO3 to NO2 was shown to be in the 150 and 250 °C temperature range, while the composites were shown to have sensitivity at room temperature. The sensing mechanism was attributed to the modification of the semiconductor behavior of the active sensing layer due to the nanotube addition to hex-WO3 . While hex-WO3 is an n-type semiconductor, the composite acts as a p-type material, with decrease in resistance due to NO2 . Similarly, the active sensing layers for NO, CO and NH3 gas sensing applications have been shown to be achieved by addition of drop-casted oxygen plasma functionalized MWCNTs to WO3 [10]. It was reported that MWCNTs enhance the WO3 surface area which improves the room temperature sensing of these toxic gases. For example, in case of CO gas, the gas response of 0.4% was reported up to 10 ppm of the gas at room temperature. The sensing mechanism was attributed to the formation of the hetero-structure n-WO3 /p-MWCNTs at the tungsten oxide/carbon nanotubes interface due to n-type semiconductor behavior of WO3 films and p-type semiconductor behavior of MWCNTs films. The adsorption at the surface of carbon nanotubes modifies the depletion layer at the n-WO3 /p-MWCNTs hetero-junctions, which results in the modulation of the depletion layer at the surface of WO3 grains. Chen et al. have shown the application of a simple solution processable method to fabricate highly selective room temperature NH3 gas sensor based on few nanometer size graphene quantum dots (GQDs) formed by using MWCNTs through ultrasonication treatment [30]. The room temperature gas sensing response of these sensors has been shown to be 14.9% for the 10 ppm NH3 . The gas sensing mechanism of GQDs was attributed to the adsorption/desorption of gas acting as donor/acceptor on the GQDs surface and edge, leading to GQD conductivity change. One of the interesting features of the CNTs is the synthesis of different dimensional materials like 1D graphene nanoribbon (GNR) through various routes, and their subsequent applications in the gas sensing with better sensing performance parameters. Cho et al. have demonstrated the application of GNR and CNTs, chemically functionalized with aminopropylsilane (APS) molecules, for improved gas sensing performance [31]. The GNR was synthesized through chemical oxidation of MWCNT, resulting in GNRs (Fig. 8). The GNR-APS sensor was shown to exhibit response up to ppb levels of NO2 . It was shown that the improved sensing response was due to the higher electron densities in the HOMOs of GNR-APS and addition of more adsorption sites. Further, the application of functionalized Ag NC–MWCNTs for sensitive, fast and highly stable room temperature NH3 gas sensors has also been demonstrated [32]. A simple mini-arc plasma method was used to synthesize Ag NCs, which were further incorporated on MWCNTs. It was demonstrated that the Ag NC–MWCNTs sensors showed a linear IV response (Fig. 9a, b) to 1% NH3 at room temperature, with an enhanced sensitivity ∼9% (Fig. 9c), fast response ∼7 s and full recovery

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Fig. 8 Schematic of the synthesis of graphene nanoribbon (GNR) and GNR functionalized with aminopropylsilane (GNR-APS). Reprinted with permission from Ref. [31] Copyright (2016) American Chemical Society

Fig. 9 a I–V characteristics of MWCNTs before and after Ag NCs decoration. b I–V characteristics of Ag NC–MWCNT hybrid sensors in airflow and in 1% NH3 flow. c The room temperature dynamic sensing response (R/R) before and after Ag NCs decoration. d Five sensing cycles of the Ag NC– MWCNT hybrid sensor to 1% NH3 , indicating a good stability. Reprinted with permission from Ref. [32]. Copyright (2016) American Chemical Society

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in atmosphere (Fig. 9c, d). The gas sensing mechanism of these sensors has been attributed to the charge transfer from NH3 to Ag NC–MWCNTs, which leads to the conductance change in the sensing material. The application of such sensing materials has also been demonstrated by using low oxygen-functionalization of MWCNTs with SnO2 , WO3 or TiO2 for NO2 detection [33]. The sensors based on hybrid SnO2 /MWCNTs films were demonstrated for sensitive room temperature NO2 sensing. In addition, these sensors showed full recovery and low response and recovery times. It was reported that the formation of depletion layers at the metal oxide grains surface and at the n-metal oxide/p-MWCNT interface leads to NO2 gas sensing response. The application of such materials like nanoparticles can also be extended to the fabrication of MWCNT based flexible and wearable gas sensors. In case of flexible and wearable gas sensors, Chuang et al. have demonstrated a system chip with array of room temperature sensors for gas sensing of dichloromethane, acetonitrile, 2-chloroethyl ethyl sulfide, and dimethyl-methyl phosphonate (DMMP) [34]. This was achieved by fabricating multistack composite sensors using different functional polymers, by solution drop casting method. For the DMMP gas, a 1.6% response was shown for 43 ppm. The sensing mechanism was attributed to the dipole–dipole electrostatic force between polar analytes and polymer alkyl side chains which modify the polymer molecules, thus modify the hopping distance and associated activation energy. Similarly, wireless NO2 sensor for low concentration has been demonstrated [14]. An oxygen plasma method was used for outer functionalization of MWCNTs, without affecting CNTs. However, this leads to sidewall grafting of carbonyl groups, which provides highly sensitive and selective room temperature NO2 gas sensors. It was shown that for higher oxygen plasma treatment duration, NO2 response was better due to interaction between gas and CNTs. It was shown that upon adsorption, charge transfer from the CNT to NO2 molecule decreases the resistance of the film. Chiou et al. have demonstrated a polymer/MWCNT composite based integrated flexible gas sensor, using wireless communication/interface technology, for wearable gas sensing system to study air pollution [13]. For this, gas sensor array was fabricated with polymer/MWCNT composite films, deposited on polyimide flexible substrate by drop casting method. Subsequently, for acquiring signal, interdigitated electrodes and multichannel sensor boards connected to a wireless transceiver, were used. In addition, a display screen using a smartphone application was used for multichannel sensing responses. The advantages of low weight, less cost, highly integrated sensors, wireless telecommunication and real-time functioning were demonstrated by using this system. Similarly, flexible, fast and highly sensitive room temperature hydrogen gas sensors, based on MWCNTs incorporated with Pd nanoparticles, have been demonstrated [35]. Novel flexible room temperature NO2 gas sensors have been demonstrated by the layer-by-layer MWCNT self-assembly on plastic substrates [16]. In order to achieve this, first, a negatively charged organic monolayer (3-mercapto-1-propanesulfonic

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Fig. 10 On-body measurement for human odor. a Photograph of a wearable electronic nose on a subject’s armpit, integrating chemical sensor array, and wireless circuit. b Schematic diagram of proposed method using a sensor array to detect emitted VOCs. Reproduced with permission from Elsevier (Ref. [17])

acid sodium salt; MPS)-based substrate was fabricated on PET substrate. Subsequently, poly(4-styrenesulfonic acid-co-maleic acid)/poly(allylamine hydrochloride) (PSSMA/PAH) bilayers and MWCNT multilayers were deposited on this substrate. A flexible thin film sensor with high sensitivity, good linearity and a rapid response was demonstrated. Similarly, a novel flexible H2 gas sensor has been demonstrated by the layerby-layer self-assembly of functionalized MWCNTs on a PET substrate [36]. It was shown that a self-assembled Pd-based material was deposited on MWCNTs thin film to form an MWCNT-Pd thin film for the gas sensing application. The flexible room temperature H2 gas sensor was demonstrated to show a good response. Wang et al. have demonstrated a passive wireless CO2 sensor based on surface acoustic wave (SAW) device and CNT nanocomposite thin film [37]. With exposure to pure CO2 , the electrical resistance and frequency for nanocomposite coated SAW sensor changed. However, the sensor performance was degraded by environmental humidity. The lowest detection limit for these sensors was shown as 1% CO2 concentration, with 0.0036% frequency change. In addition, wireless module was tested and showed potential transmission distance at preferred parallel orientation. Flexible chemical sensors for an electronic nose for skin odor organic component detection have also been demonstrated (Fig. 10a) [17]. In order to achieve this, a gas sensor array based on functionalized CNTs was fabricated (Fig. 10b). The signal processing for skin odor monitoring by the electronic nose was conducted to achieve highly accurate odor identification and contrast response. Similarly, flexible polymer/MWCNT composite sensor array based on ethyl cellulose (EC), polyethylene oxide (PEO) and polyvinylpyrrolidone (PVP) polymers for wearable gas sensors system has also been demonstrated [15]. The polymer/MWCNT

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Fig. 11 a Configuration of the flexible gas sensor array. The inset shows the sensing electrode (top electrode) and the heater (bottom electrode); and b the cross-sectional schematic structure of the single gas sensor (Ref. [15])

composite sensing film membranes were fabricated by a drop casting method to fabricate the two-layer gas sensing structure. A cost effective flexible gas sensor array, shown in Fig. 11, with three different types of polymer/MWCNT composite sensing films, was fabricated by flexible printed circuit industry technologies. Each type of the selected polymer was arranged in one of the rows in the matrix. The fabricated flexible gas sensor array was demonstrated to show excellent flexibility.

6 Conclusion and Perspectives This chapter presents the reported applications of the CNTs in different ways for developing chemical gas sensors, more specifically, flexible and wearable gas sensors. The flexible and wearable gas sensors discussed are categorized two different types of the CNTs based sensors: SWCNTs based sensors and MWCNTs based sensors. These types of CNTs are used along with other different types of materials like metal oxides, quantum dots, metal nanoparticles, nanoribbons and other nanomaterials to form their composites that are used for gas sensing. These CNTs are reported to improve the gas sensing as well as mechanical properties of the gas

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sensors. Despite the improved gas sensing and mechanical properties, there are several challenges and issues that need to be addressed. For example, there is a need to develop sensors that are small and light, with improved mechanical properties, as the rigidity of conventional chemical sensors also leads to poor mechanical resiliency against repeated complex deformations. In addition, in extreme cases, stretchability is not enough and the wearable device is desired to possess self-healing ability in order to quickly recover itself after mechanical damage. Such smart wearable chemical sensors that have autonomous self-healing ability will have a major impact in such devices as they will self-repair upon mechanical damage, which will thus allow uninterrupted sensing. Stability is another major issue faced by almost all forms of chemical sensors, and particularly wearable ones, as wearable sensors are exposed to varying temperature, pH, ionic strength, humidity or pressure during prolonged indoor and outdoor activities. Further, with the growing demand for wearable sensors, the demand for relevant power sources also grows in order to meet the increasing demands for detection of multiple parameters simultaneously, performing complex data analysis and communicating with other sensors, devices and data transmission, leading to the development of low-power energy efficient devices, compact, energy dense wearable power sources and low-power-consuming electronics. Acknowledgements Abhay Gusain is awarded FAPESP Postdoctoral fellowship grant (2017/07635-2) by FAPESP, Brazil. Prof. Paulo B. Miranda, IFSC, USP Sao Paulo, Brazil is acknowledged for his support.

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Chapter 14

Recent Advances in Functionalized Micro and Mesoporous Carbon Nanostructures for Humidity Sensors J. Sharath Kumar, Naresh Chandra Murmu, and Tapas Kuila

1 Introduction The design and synthesis of carbonaceous materials possessing ordered porous structures are one of the hottest topics in materials science. The matchless properties such as high specific surface area, large pore volume, high electrical conductivity, wellordered and controllable porous structure, exceptional chemical and mechanical stability with tunable morphology attract them in a variety of fields [1–4]. Generally fullerenes, CNT, graphene, and ordered nanoporous (micro and mesoporous) carbon are majorly applied in environmental (absorbent, purification of water and gases) and energy applications [5, 6]. Synthesis of graphene, CNT, and fullerenes is a sophisticated process, which may be a huddle in exploiting its application to its full potential. This permits the use of highly ordered porous carbon to its full potential since its preparation and modification are quite simpler and possess properties better than other carbonaceous material. Additionally, the pores in the porous carbon can host organic, inorganic, and bio-materials which considerably enhances its performance in numerous applications. Highly ordered nanoporous carbon materials are in use from many years in the fields like catalysis, adsorption and separation, energy storage & conversion and biomedical applications. One of the first reports on preparation of highly ordered nanoporous carbon was via nanocasting technique. This approach has led to the ordered nanoporous carbon with tunable pore size (2–50 nm) and high surface areas [1, 7]. This development led to the discovery of many porous materials such as polymers, metals, metal oxides, etc. which are now considered as potential game-changers in numerous technologies used currently [1, 7]. Nano-hard templating technique was also used in the production of nanoporous carbon materials using J. Sharath Kumar · N. C. Murmu · T. Kuila (B) Surface Engineering and Tribology Division, Council of Scientific and Industrial Research-Central Mechanical Engineering Research Institute, Durgapur 713209, India e-mail: [email protected]; [email protected] Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_14

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phenolic resin as the carbon source [8, 9]. Highly ordered porous carbon synthesized through hard template method shows surface areas more than 2000 m2 g−1 , high pore volume of 3.0 cm2 g−1 , and tunable pore diameter with excellent electrical conductivity. The properties and performance of ordered nanoporous carbon can be controlled by incorporating organic and inorganic moieties in the pores. Introduction of foreign atoms like B, N, P, S, O, etc. in the framework of carbon can not only tune the morphology but also enhance the electrical properties and surface area which in turn elevates the catalytic and sensing properties. Therefore, research on the development of ordered nanoporous carbon with different functionalities is actively and rigorously progressing in catalytic and sensing applications [10–13]. Sensing is an important area in the modern science and technology as it plays a crucial role in our day-to-day life. In recent years, the improvement in the sensor technology has been driven by high-speed processing, power and cost-effective microelectronics and miniaturization [14–24]. Humidity can be defined as the amount of water (vapour) in air/gas. Humidity sensors play an important role in domestic, industrial, agricultural, biomedical, and automobile sectors [25]. Nowadays, it is highly essential to monitor, detect, and control the humidity in the vicinity precisely [26]. The necessity of protection against environmental condition leads to extensively develop humidity sensors. Advancements in the humidity sensors encompass enriched efforts in the improvement of transducer properties such as sensing element, design, mechanism, and fabrication [27–33]. The chapter focusses on different fabrication techniques, materials, application, and mechanism of various humidity sensors. It will also cover the recent research and development of humidity sensors for various technological and potential applications. Each field requires different operating conditions and thus, different sensing materials are required accordingly. The basic parameters required to understand and compare various humidity sensors have been discussed in detail. It also discusses the types of humidity sensors available and how they are similar or different from each other. Finally, it has been concluded by discussing the possible future scope for attaining better sensors.

2 Basics and Measuring Parameters Humidity can be defined as the amount of water vapour present in the atmosphere. Humidity parameters are represented in diverse ways depending on the technique of measurement performed. Commonly represented are relative humidity (RH), parts per million (PPM), by volume, by weight, and dew/frost point (D/F PT). Among the above discussed ones the latter three are concerned with absolute humidity, they can even be called as the subclass of absolute humidity. Relative humidity is defined as the ratio of partial pressure of water vapour to the saturation vapour pressure at any temperature and generally expressed in percentage (%). As this is temperature dependent, this is a relative measurement. Dew point is the temperature at which the water vapour condenses to liquid and is >0 °C. Frost

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point is the temperature at which the water vapour condenses to ice and is 950 °C under inert atmosphere to produce evenly adhered cubic TiC NPs on the carbon framework. Another way is to introduce oxygen functionalities over the surface of carbon via oxidation and utilize these oxygen moieties for further modification [107–116]. Surface properties can be refined by treating oxygen functional groups present over porous carbon with thionyl chloride to produce carboxyl groups [117]. It is seen that the morphology and the porosity were vastly damaged during oxygenation [118, 119]. Functionalization with organic moieties is another approach where organic monomers were introduced by controlled impregnation, which subsequently polymerize via cross-linking [120, 121]. The surface of the porous carbon can be modulated by introducing functional group of choice. This will render bifuntionality; higher electric conductivity from porous carbonaceous framework; and reactivity from the functional groups over the carbon framework. The structure of the porous carbon can be retained and the catalytic properties can be elevated. Solvent free technique can also be useful where in situ reactions are possible [122–125]. Covalent grafting of aryl group substituted at the 4-positions (ArR, R = Cl, CO2 R’, alkyl) over the surface of carbon can be performed by the introduction of these groups just after impregnation of carbon precursors before carbonization [53, 54]. The higher surface area of the porous carbonaceous material was available for grafting which resulted in higher grafting density of 0.9–1.5 μmol m−2 [53]. Functionalization via in situ grafting of the porous carbon resulted in the decrease of pore size at least by 1–1.5 nm leading to the decrease of the total pore volume and the BET surface area significantly. Thus, the pore size of the porous carbonaceous material can be easily manipulated by surface functionalization. During surface modification, there is absorption/reaction involved which is primarily helpful in the shrinkage of the pore size. Porous carbon was functionalized via reduction of diazonium salt and sulfonic acid-containing aryl groups. The surface-treated material can be used in esterification and condensation reaction due to the presence of reactive H+ ions [126]. When surface groups are not directly involved in the surface modification of porous carbon then controlled oxidation is a great option to introduce oxygen moieties. Now these groups can be made use for surface modification via various modes of interactions such as covalent, electrostatic and hydrogen bonding. Furthermore, controlled oxidation increases the wettability of the carbon pores with organic solvents which are found useful in enhancing the pore volume and surface area. The extent of oxidation greatly depends on the reaction condition and the oxidation capability of the oxidizing agent [108–110]. Mild oxidizing agents can be used for tuning the surface of the carbon. All the oxygen moieties are not equally stable, therefore, it is very necessary to know the type of group to introduce well before the start. Additional pores are created due to the removal of oxygen functional groups when it undergoes heat treatment. The other way of altering the surface texture of carbon framework is through KOH activation. This etches the framework to create additional pores resulting in the improvement of pore volume and surface area significantly resulting disorder in

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the porous structure [127]. The alkali metal can be easily removed by simple washing. Sulfonic groups can be introduced to obtain sulfonated porous carbon. Porous carbonaceous materials prepared by carbonization of starch or other higher polysaccharides can be suspended in sulfonic acid at elevated temperature to produce sulfonate functionalized porous carbon [128]. Another simple way is to make the fumes of sulphuric acid with the carbon inside an autoclave [129]. Surface modification drastically changes the properties at the surfaces. One such change is halogenation, where fluorine modification can be carried out which can produce hydrophobic surface. This can be achieved by flowing F2 gas over the sample for a few days in temperature the range of 25 to 250 °C [130]. The flowing F2 gas reacts with the C–H hydrogens and the unsaturated bonds of carbon. Porous carbon can also be treated with diluted fluorine gas (4% in helium) for about 4 days at room temperature and a gradual degradation in structure was observed [130]. The modified porous material changed from black to brown and to white with increasing temperature and the end product lost its structural architecture and porosity. The structure retained in the case of modified porous carbon prepared at room temperature. The degradation in structure at elevated temperature is possibly due to the reactivity of the fluorine with carbon. Modification of porous carbon material to convert into a superhydrophobic material is another interesting approach [131]. Water contact angle of superhydrophobic material is generally greater than 150° [132–134]. In addition to achieving superhydrophobic surface, the contact area at the substratewater interface should be minimum and increased at the water-air interface [134]. Superhydrophobic materials could be accomplished by bonding fluorine-rich organic groups of carbon surfaces. Earlier studies showed that the superhydrophobicity and hydrophobicity can be achieved by altering the structural parameter without disturbing the chemical composition [133]. Superhydrophobicity can be achieved by treating porous carbon material with strong acids such as sulphuric and nitric acids followed by modification with fluoroalkylsilane [CF3 (CF2 )7 CH2 CH2 Si(OCH3 )3 ] [131]. Thus, the obtained superhydrophobic carbon exhibited a contact angle of 150.2° with retention in the porous architecture. The best possible way to access the surface functional groups on the porous carbon is by chemical grafting. In most of the case, the carbon surface consisted of some sort of pre-existing functionality which can be easily changed through organic reactions. In another approach, the functional groups are directly attached to the carbon surface [94]. The functional groups generated by the oxidation can be treated with thionyl chloride to produce acyl chloride group which can be further used to attach Schiff’s base ligand by esterification [135]. The major advantage of this functionalization is that the Schiff’s base allows metal complexation. CMK-1 was treated with tetraethylenepentamine to develop porous carbon capable of absorbing Cu2+ . The porous carbon absorbed 0.4 mmol Cu2+ per gram. The unfunctionalized carbon did not absorb detectable Cu2+ [135]. Hydrophilic porous carbon with pre-existing surface functionalities was treated with chloropropylamine under reflux condition overnight to produce primary amine groups. The amine coverage over the surface of porous carbon was >4 mmol g−1 . The presence of excess oxygen functionalities

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facilitates to incorporate varities of functional groups on the surface of carbon. Diazonium compounds were used to functionalize the surface of porous carbon materials [53, 54, 136–139]. Electrochemical technique is another way of surface modification which has attracted significant research interest among the researchers and has been studied over a few decades [136, 140–145]. One of the first reports appeared in the early 2001 where a column was packed with porous graphitized carbon and used as a working electrode in an electrochemically modified liquid chromatography apparatus [145]. Acetonitrile was used as the solvent with diazonium salts in it; a negative potential was preferred to minimize the concentration of diazonium slats over the surface of porous carbon. The radicals produced by the diazonium salts were attached to the carbon via C–C bond. Another report showed the use of covalently modified porous graphitized carbon where they used diazonium salt-based ionic liquid [p-butylbenzenediazonium][bis(trifluoromethane sulfonyl)amide] [146]. The ionic liquid served as both reactant and solvent. The functionalization density increased from 3.38 to 6.07 μmol m−2 when the surface modification technique followed was changed from chemical to electrochemical. The technique for directly functionalizing the porous carbon materials has also been developed. Activated carbon was allowed to react with solvent-free alcohols, amines, or thiols to develop organic groups with the unsaturated carbon. The reaction took place at moderate conditions of 400 °C. Since then various types of IMOs such as tin oxide (SnO2 ), tungsten oxide (WO3 ), titanium oxide (TiO2 ), and iron oxide (Fe2 O3 /Fe3 O4 ) have also been widely investigated, and utilized for detecting various gas species [5, 9, 12]. Sensors based on IMOs operate at elevated temperature (100–400 °C) which leads to higher power consumption and poor sensor stability [3, 10, 13–15]. Gas sensors having room temperature operation with high sensitivity and good selectivity have great practical significance owing to many unique advantages such as very low power consumption, small size, cost effectiveness and simple electronics circuitry [10, 14]. A. Kumar · V. Singh Department of Physics, Chaman Lal Mahavidyalaya, Haridwar 247664, India M. Murali Krishnan (B) Department of Physics, M S Ramaiah University of Applied Sciences, Bangalore 560058, India e-mail: [email protected] S. Samanta · N. S. Ramgir Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_15

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Organic conducting polymers (OCPs), for example, polypyrrole (PPy), polyaniline (PANI), polythiophene and their derivatives have been widely employed as an active gas sensing layer [8, 16–19]. In contrast to IMOs, these polymers are capable to detect the gas species with high sensitivity, fast response and complete recovery even at room temperature. Furthermore, OCPs also own various unique benefits such as mechanical flexibility, lightweight, and tunable chemical/electrical properties [18, 20]. Therefore, OCPs are good materials alternative to conventional IMOs for developing the efficient and low-cost room temperature chemiresistive gas sensors. Among OCPs, PANI has been intensively used as gas sensing materials because of many advantages which include easy synthesis, high sensitivity, good stability, and interesting doping/de-doping properties [8, 21, 22]. It has also been established that PANI-based nanocomposite prepared by incorporation of secondary inorganic elements show superior gas sensing performance over pure PANI due to synergic/complementary effects [4, 14, 23]. This chapter will summarize the fundamental properties of pristine PANI. In addition, chemiresistive gas sensing properties of PANI and functionalized PANI with TiO2 nanoparticles (NPs) are also discussed.

2 Chemical Properties of PANI PANI is the oxidative polymeric product of aniline under acidic conditions and formerly identified as aniline black in 1862 [24]. The chemical structure of PANI is shown in Fig. 1. In this chemical structure, indexes a and b (with a = 1 − b) denote the reduced and oxidized state, respectively, while X represents the degree of polymerizations. PANI has mixed oxidation states and comprises of two different structural units referred to as reduced benzenoid and oxidized quinoid [22, 25, 26]. Thus, PANI can exist in three different oxidation states which are described below and their corresponding chemical structures are presented in Fig. 2a–c [22, 26, 27]. Emeraldine (ED): This form corresponds to a = 0.5 and known as intermediated oxidation state. It comprises an alternative chain of benzenoid and quinoid units with ratio 1:1. Leucoemeraldine (LB): This form is obtained when 1 − a = 0, which is fully reduced or deoxidized state (benzenoid). It comprises of benzenoid rings bounded together by amine groups (–NH–).

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Pernigraniline (PB): This form corresponds to 1 − a = 1 and it is the completely oxidized state (quinoid). In this structure, imine group (–N=) binds benzenoid and quinoid rings together. Among these, ED is the most stable state of PANI and it is normally referred to as emeraldine base, while its doped or protonated form is known as emeraldine salt [25, 27]. Normally, the oxidation state of ED is unidentified, i.e., deviates from the theoretical value of 1 − a = 0.5 and has been obtained with a = 0.4–0.6 according to synthesis conditions [25].

3 Electrical and Chemiresistive Properties of PANI Generally, polymeric or organic materials exhibit insulating behavior owing to the inherent property of charge localization in C–C and C–H covalent bonds. In order to make polymers electrically conductive, alternating single and double bonds called conjugation must be present in the backbone chain of the polymer [20, 27]. Conjugation allows the charge carriers to move over the conjugation length rather than being strongly localized as in the case of C–C or C–H covalent bonds. Like other OCPs, the backbone chain of PANI contains π -conjugated electron system, which is extended to the nitrogen pz -orbitals and carbon rings [20, 28]. The conductivity associated with conjugation is extremely low therefore intrinsic charge carriers need to be generated to make the PANI highly conductive. In order to generate the charge carriers, electrons are removed or inserted into the OCPs via doping process. There are principally two types of doping process used to enhance the conductivity of the OCPs to metallic or semiconductor level: oxidative or p-doping (removal of the electrons) and reductive or n-doping (addition of electrons) [18, 28]. It has been demonstrated that PANI is a unique type of OCP as it can also be doped/de-doped under acidic/basic environment (known as non-redox process), which is attributed to the presence of –NH group in repeating unit of PANI

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[22, 29, 30]. This process involves the transfer of hydrogen ions (H+ ) rather than the exchange of electrons from backbone chain of PANI and induces the protonation at the nitrogen atom at the –NH site [21, 26]. This doping process does not change the number of π -conjugated electrons present in the polymer backbone. It has been reported that conductivity increases significantly from insulator to metallic level (by 10–11 orders of magnitude) via protonic acid treatment, e.g., the undoped ED base known as insulating form of PANI having the conductivity 1 S/cm) and its value is determined by the amount of doping that can be controlled by adjusting the pH of acid [18, 33]. The maximum conductivity is attained when the all the nitrogens of imine (–NH) group (equal to half of the total nitrogen atoms in the PANI) are doped [21, 31]. It has also been reported that the same magnitude of enhancement in the conductivity could be achieved by oxidation of the amine or completely reduced form of PANI (polyleucoemeraldine base); however, if oxidation level exceeds more than 50% the conductivity begins to decrease [18, 34]. Therefore, the increase in the conductivity of PANI is attributed to both factors: (i) degree of oxidation and (ii) degree of protonation. The overall conductivity of the polymers is determined by the ability to transport the charge carriers along the conjugation present in backbone chain and interchange hoping or jumping of charges between the polymer chains [31]. ED salt form has extended π -conjugation in the PANI backbone chain. Therefore, it exhibits the highest conductivity among the different forms of PANI [30]. The oxidation/reduction of PANI creates the charge carriers in the form of radical ion (cation or anion) and dication (or dianion) known as polarons and bipolarons, respectively, which permits the movement of free and mobile charge carriers along backbone chain [21, 33–35].

4 Chemiresistive Gas Sensor and Its Key Parameters A chemiresistive gas sensor exhibits a change in resistance in response to the change in nearby chemical environment such as variation in the concentration of gases or other chemical species [2, 14]. Performance of sensing material can be evaluated by response curve that can be obtained by time-dependent variation in the resistance or conductance as a function of the concentration of chemical species (as shown in Fig. 3) [2]. The essential parameters which recount the performance of gas sensors may be extracted from the response curve. These parameters are given below [4, 14]: Response and Sensitivity: The response (R) and sensitivity (S) of a gas sensor at a particular temperature may be defined by the following equations [4, 36, 37]:

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Fig. 3 Typical response–recovery curve for a chemiresistive sensor indicating time-dependent variation of resistance/conductance

Resistance or conductance

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where Rg , and Ra are the resistance in presence of test gas and air (or base value), respectively. Rs is the resistance corresponding to the saturation level. Response time (t res ) and recovery time (t rec ): Time taken by the sensor to reach the 90% of the saturation value of the resistance or conductance from the base value (Ra ) on the exposure the target gas is reported as t res , whereas t rec represents time required to recover the 90% of the initial or base resistance once the target gas is removed (as can be seen in Fig. 3) [38, 39]. It has been reported that mobility of sensing material influences the t res as well as t rec , and their values will be smaller if the mobility is higher [40, 41]. Selectivity: It is the ability of a sensor to differentiate the gas which is being analyzed in the presence of other interfering gases [14, 36].

5 PANI-Based Chemiresistive Sensors We have discussed earlier that conductivity of PANI depends on the degree of oxidation as well as on the degree of protonation. Hence, PANI-based gas sensors show excellent response to the various reducing/oxidizing gases, acidic/basic environment, and volatile organic compounds, and have been widely exploited for advancement of room temperature gas sensors [20, 31]. PANI-based gas sensors can be generally classified into two categories: (i) Pristine PANI-based gas sensors

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(ii) PANI nanocomposites-based gas sensors.

5.1 Pristine PANI-Based Sensors These sensors are based on pure or intrinsic PANI, no secondary elements are added. Researchers have developed various methods to create different nanostructures of pure PANI for improving the gas sensing properties. These nanostructures possess extremely high surface-to-volume ratio, hence, provides large number of interaction sites for gas adsorption as compared to the bulk PANI. Sadek et al. synthesized the doped and de-doped PANI nanofibers and fabricated conductometric H2 gas sensors using nanofiber thin films [42]. These films were exposed to the different concentrations of H2 gas and their resistances were found to decrease upon the exposure of H2 gas. Authors demonstrated that H2 gas molecules induce the protonation in nitrogen atoms which generates delocalized charge carriers in the backbone chain of PANI. Therefore, the conductive of nanofibers sensor films increases (resistance decreases) upon the exposure of H2 gas. The sensitivity (defined as S = Ra /Rg ) of doped and de-doped sensor films were determined to be 1.11 with t res = 32 s and 1.07 with t res = 28 s toward 1% H2 at room temperature, respectively. However, the resistive responses of these nanofibers in the presence of other gases have not been studied. In another work, a detailed investigation on utilization PANI nanofibers as an active chemiresistive gas sensing materials was carried out by Virji et al. [31, 43] they used organic–aqueous interfacial polymerization method for synthesizing PANI nanofibers. They have demonstrated that a porous sensing layer can be fabricated using these PANI nanofibers, which evidently allows the gas molecules to diffuse in/out more rapidly as well as provide large surface area for gas interaction as compared to PANI conventional film (Fig. 4a, b). The authors also observed that nanofiber film displayed better value of S and lower value of t res than that of conventional PANI films. Moreover, the response of nanofiber and pure films corresponding to different thicknesses upon were also investigated and results are presented in the Fig. 4c, d. It is fascinating to note that the sensing characteristics of nanofiber film was found to be independent of film thickness, while the performance of pure film gets lower with increasing the thickness. Authors also examined chemiresistive responses of PANI nanofibers film for other gases (HCl, N2 H4, CHCl3, and CH3 OH) and similar results were obtained [43]. The better performance of PANI nanofiber film is ascribed to porous nature of interconnected nanofibers’ network as shown in Fig. 4a. Different response mechanism have been presented corresponding to different gas species, i.e., acid doping for HCl, base de-doping for NH3 , reduction in case of N2 H4 swelling induced by CHCl3, and lastly, a change in the conformation of the polymer chain stimulated by CH3 OH [21, 43]. Wang et al. investigated the NH3 gas sensing response of a chemical sensor based on PANI–nanoframeworks comprising of several intercrossing nanowires directly grown on Pt electrodes via electrochemical polymerization route [44]. The sensor

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fabrication process and surface morphology of PANI–nanoframeworks are shown in Fig. 5a–d. This sensor was also found to be responsive to NH3 , C2 H5 OH and HCl. An increase in the resistance by 1.2 orders of magnitude was noticed when it was exposed to 100 ppm of NH3 . However, upon the exposure of same concentration of HCl gas, the resistance was found to decrease by 4 orders of magnitude. The fabricated sensor also displayed high reversibility/reproducibility in responses curve as presented in Fig. 6a, b. Results are shown only for the cyclic exposure 0.5 ppm of NH3 and saturated C2 H5 OH vapor. Moreover, the authors performed pH sensing experiments with 15 aqueous NaCl solutions of different pH values showing PANInanofibers capability for pH sensing. In another work, non-lithographic deposition process was employed by Craighead his co-workers for designing a chemical sensor based on PANI single nanowire for detection of NH3 [45]. High surface-to-volume ratio of PANI–nanowire makes it a good candidate for chemiresistive gas sensing applications. This sensor could detect the presence of NH3 gas down to 0.5 ppm with rapid response. Moreover, the authors noticed that PANI nanowires exhibit 30 times higher sensitivity as compared to the traditional PANI film. Yang and Liau [46], demonstrated a straightforward approach for preparing the nanostructured PANI films from polystyrene [PS]–PANI core-shell particles. Two

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Fig. 5 a Optical microscope image of the electrode and b magnified image of PANI–nanoframework electrode junction (PNEJ), c SEM images of PNEJ, and d the magnified image of the region marked in (c) (Reprinted with permission from Ref. [44] © 2004 American Chemical Society)

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Fig. 6 a Reproducible responses of PNEJ to cyclic exposure of (a) 0.5 ppm NH3 and b saturated ethanol vapor (Reprinted with permission from Ref. [44] © 2004 American Chemical Society)

different approaches were employed for preparing the nanostructured PANI and corresponding films were named as P and F films. Nanostructure corresponding to P film was obtained by removing the PS cores after immersing the core–shell particles dried on a substrate into tetrahydrofuran (THF). The second kind of nanostructure (F film) was prepared by heating the core–shell at 140 °C for the duration of 10 min

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before extracting the PS by THF. The chemiresistive responses of these films were examined by exposing them to dry gas flow, C2 H5 OH, HCl, and NH3 . The authors noticed that P film is better than F film for sensing application, ascribed to the tight packing of PANI shells in it. Upon the exposure of NH3 , HCl, and C2 H5 OH, response curves of these films showed similar trend as displayed by PANI–nanofibers. However, P film exhibited higher sensitivity with shorter value of t res as over PANI– nanofiber synthesized by Wang et al. [44, 46].

5.2 PANI Nanocomposite (PANI/TiO2 )-Based Gas Sensor As discussed above, the pristine PANI nanostructures are promising materials and have the capability to detect various chemical species even at room temperature. Nevertheless, there are many drawbacks with the use of pristine PANI in room temperature chemiresistive gas sensing applications including relatively low reproducibility, shifts in base resistance, the effect of moisture, or humidity as well as poor selectivity [4, 47–49]. Consequently, during past two decades, studies were largely focused to overcome the restrictions associated with use of pure PANI as sensing material based by functionalizing the PANI with secondary components such as inorganic metal oxide NPs metallic or bimetallic NPs, carbon-based compounds such as carbon nanotubes (CNT), graphene, and chalcogenides [4, 14, 49]. However, in this section, we will discuss the gas sensing characteristics of nanocomposites synthesized by PANI and TiO2 NPs. It has been reported that TiO2 NPs exhibits n-type conductivity, therefore p–n junction is formed on adding them into the PANI matrix. Tai et al. investigated the NH3 chemiresistive gas sensing properties of PANI/TiO2 nanocomposite, and pristine PANI films [50]. The authors observed that PANI/TiO2 film sensor showed higher sensitivity with shorter values of t res /t rec over pristine PANI film. Upon the exposure of 23 ppm of NH3 , the sensitivity of PANI/TiO2 and PANI film sensors were determined to be 1.67 and 0.49, respectively. PANI/TiO2 film was tested upon repeated cycles of NH3 for 23 ppm, obtained results reveal high reproducibility (Fig. 7a). The authors also examined the room temperature dynamic response at humidity level of 55% for different concentrations level of NH3 gas as shown in Fig. 7b. It is obvious that PANI/TiO2 displays quick and reproducible response with excellent constancy in base resistance even at higher concentration of NH3 . Furthermore, PANI/TiO2 film was found to be less sensitive to humidity and response remained stable even after 20 days [50]. The authors examined the surface morphology of PANI/TiO2 and pure PANI films by SEM and the corresponding results are presented in Fig. 7c, d. It can be seen that both films have mesh-like porous structure. However, the size of pores in PANI/TiO2 is much larger than that of the size of pores in pristine PANI film which enables the gas molecules to diffuse rapidly and allow them to penetrate more deeply [50]. Therefore, better chemiresistive gas sensing performance has been displayed by PANI/TiO2 film over the pristine PANI. Moreover, it has been demonstrated that a p–n junction is formed at

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Fig. 7 a Reproducible response (at 23 ppm) to NH3 and b transient response curve of PANI/TiO2 thin film sensor, c scanning electron microscope image of PANI/TiO2 and d pure PANI thin films (Reprinted with permission from Ref. [50] © 2007 Elsevier)

PANI/TiO2 interface hence electrons can transfer from n-type TiO2 to PANI creating a positively charged depletion layer on TiO2 side. Hence, the activation energy and enthalpy of physisorption for NH3 gas get decrease thereby improving the gas sensing performance of PANI/TiO2 film [4, 51]. In other work, a comparative study of room temperature NH3 gas sensing of PANI/TiO2 , PANI/SnO2 , PANI/In2 O3 and pure PANI films were carried out by Tai et al. [52]. The authors demonstrated that the core–shell structure of PANI–metal oxides NPs can be obtained via in situ self assembly approach, in which PANI serve as shell of nanocomposite covering metal oxides NPs (Fig. 8a). The resistive response curves measured at room temperature upon the exposure of 1 ppm of NH3 are sown in Fig. 8b. Evidently, PANI nanocomposite exhibited a better sensing response than pure PANI. Moreover, PANI/TiO2 film showed the highest sensitivity and lowest t res ~ 2–3 s among these nanocomposites. Whereas t res was determined to be ~60 s for pure PANI. The optimum sensing characteristics of PANI/TiO2 is attributed to the close matching of energy levels of PANI and TiO2 [52]. Long-term stability tests were also performed by the authors and the obtained results disclosed that PANI nanocomposite films did not show any appreciable change in response over a period of 30 days, while pure PANI response got diminish in first 15 days as displayed in Fig. 8c. These composites films exhibited very low response to CO gas and nearly no response toward H2 gas, indicating higher selective of the films toward NH3 [52].

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Fig. 8 a TEM image of pure PANI and core–shell structure of PANI/IMO nanocomposites with different IMOs, b response–recovery behavior for the fabricated films to 1 ppm of NH3 gas at room temperature and c long-term response behavior of the fabricated films to 23 ppm NH3 (Reprinted with permission from Ref. [52] © 2010 The Chinese Society for Metals. Published by Elsevier Ltd.)

Pawar et al. [53] prepared PANI/TiO2 nanocomposite films with varying concentration of TiO2 NPs (0–50% wt.). They investigated the room temperature NH3 sensing performance of PANI/TiO2 (50% wt.) film (as it displayed the maximum crystallinity) along with the pure PANI and TiO2 films. Authors observed that PANI/TiO2 (50% wt.) film showed better sensitivity as compared to pure PANI and TiO2 films. Furthermore, sensitivity values for concentration of 100 ppm of CH3 OH, C2 H5 OH, NO2, and H2 S were found to be much lower as compared to sensitivity value for NH3 concentration of 20 ppm, which indicates PANI/TiO2 are extremely selective toward NH3 in the presence of other interfering gases. In an additional work, Gong et al. fabricated an ultrasensitive p–n junction NH3 gas sensor by enchasing PANI NPs or nanograins with TiO2 microfibers [54]. The image of TiO2 microfibers and PANI nanograins/TiO2 microfibers junction are presented in Fig. 9a and b, respectively. Authors examined the current response curves of p–n junction sensor, which are shown in Fig. 9c. Notably, this sensor could able to detect NH3 concentration down to 50 ppt of NH3 , which is approximately 1000 times superior to the best sensor made of pure PANI [54, 55]. This is ascribed to electric current switching due to the quick increase in the resistance of p–n heterojunction when NH3 gets absorbed by the PANI NPs. Sensitivity values were determined to be 0.004, 0.009, and 0.018 corresponding to the exposure of 50, 100, and 200 ppt of NH3 gas. Moreover, the authors demonstrated that this sensor displayed excellent

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Fig. 9 High-magnification SEM images of a TiO2 microfibers and b TiO2 microfibers enhanced with PANI nanograins, c response curve for enhance TiO2 nanofibers for different NH3 gas concentrations and d sensing reproducibility to NH3 gas at 10 ppb (Reprinted with permission from Ref. [54] © 2010 American Chemical Society)

reversibility against exposure of NH3 and the corresponding results can be seen in Fig. 9d. PANI/TiO2 nanocomposite has been prepared using one-step hydrothermal process by Zhu et al. [56], who examined its NH3 gas sensing performance at room temperature. Authors observed that PANI/TiO2 sensor could detect the NH3 down to 0.5 ppm and displayed good linearity relationship in the range from 0.5 to 100 ppm. This sensor was successively exposed to 50 ppm of NH3 (four times) at room temperature, corresponding sensitivity values were determined to be 2.59, 2.6, 2.56, and 2.56, respectively, evidencing its good reproducibility. The sensor also showed very good selectivity for NH3 that was tested with seven other interfering gases. Recently, a PANI/TiO2 nanocomposite was prepared by Gao et al. [57] by depositing the layer of PANI of (5 nm) on TiO2 NPs layer (20 nm) and employed for room temperature detection of NH3 gas. Authors observed that PANI/TiO2 film displayed better performance in terms of sensitivity, t res /t rec and selectivity as compared to pure PANI film.

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All PANI/TiO2 nanocomposites sensors displayed the p-type behavior, i.e., their resistance values increases upon the exposure of NH3 gas which is ascribed to the extraction of protons (deprotonation) from –NH site of PANI by NH3 gas molecules forming more favorable NH4 + results in a decrease in density of holes. The resistance recovers to its initial value when exposed to air, which is attributed to decomposition of NH4 + into NH3 and H+ [4, 51]. All the results discussed above evidently indicate room temperature gas sensing performance of PANI (sensitivity, selectivity as well as long-term stability) can be improved by fictionalization of PANI with TiO2 NPs.

6 Conclusions Chemiresistive gas sensing properties of PANI have been effectively studied and showed good response at room temperature. Nonetheless, pristine PANI itself suffers from the problems of poor selectivity, lack of long-term stability, and reproducibility. In order to eradicate these problems, PANI nanocomposites have been synthesized by incorporation inorganic metal oxide NPs into PANI matrix. The progress on development of PANI/TiO2 -based nanocomposites for chemiresistive gas sensing applications has also been discussed. PANI/TiO2- based sensors displayed better sensitivity and good selectivity over pristine PANI-based sensors, and exhibited remarkable long-term stability in their responses. Hence, it has been established that PANI/TiO2 nanocomposites can be a good candidate for the development of efficient chemiresistive gas sensors with room temperature operation.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

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Waghuley SA, Yenorkar SM, Yawale SS, Yawale SP (2008) Sens Actuators B Chem 128:366 Nicolas-Debarnot D, Poncin-Epaillard F (2003) Anal Chim Acta 475:1 Dunst KJ, Cysewska K, Kalinowski P, Jasi´nski P (2016) IOP Conf Ser Mater Sci Eng 104:12028 Park S, Park C, Yoon H (2017) Polymers 9:155 Fratoddi I, Venditti I, Cametti C, Russo MV (2015) Sens Actuators B Chem 220:534 Dhawan SK, Kumar D, Ram MK, Chandra S, Trivedi DC (1997) Sens Actuators B Chem 40:99 Al-Mashat L, Shin K, Kalantar-zadeh K, Plessis JD, Han SH, Kojima RW, Kaner RB, Li D, Gou X, Ippolito SJ, Wlodarski W (2010) J Phys Chem C 114:16168 Sharma S, Suryanarayana M, Nigam A, Chauhan A, Tomar L (2009) Catal Commun 10:905 de Albuquerque JE, Mattoso LHC, Faria RM, Masters JG, MacDiarmid AG (2004) Synth Met 146:1 Shimano JY, MacDiarmid AG (2001) Synth Met 123:251 Vyas S, Shivhare S, Shukla A (2017) Int J Res Sci Innov IV:86 Le T-H, Kim Y, Yoon H (2017) Polymers 9:150 Varela-Álvarez A, Sordo JA, Scuseria GE (2005) J Am Chem Soc 127:11318 Chiang J-C, MacDiarmid AG (1986) Synth Met 13:193 Huang J, Virji S, Weiller BH, Kaner RB (2004) Chem Eur J 10:1314 Huang W-S, Humphrey BD, MacDiarmid AG (1986) J Chem Soc Faraday Trans 1 Phys Chem Cond Phases 82:2385 Khalid M, Honorato AMB, Varela H, Polyaniline: synthesis methods, doping and conduction mechanism. http://dx.doi.org/10.5772/intechopen.79089 Baughman RH, Wolf JF, Eckhardt H, Shacklette LW (1988) Synth Met 25:121 Scotto J, Florit MI, Posadas D (2018) Electrochim Acta 268:187 Joshi N, da Silva LF, Jadhav H, M’Peko J-C, Millan Torres BB, Aguir K, Mastelaro VR, Oliveira ON (2016) RSC Adv 6:92655 Kumar A, Samanta S, Ramgir N, Singh A, Debnath A, Muthe KP, Barshilia H (2017) Sens Lett 15:104 Verma VK (2009) Earth system sciences: felicitation volumes in honour of Professor VK Verma. Concept Publishing Company Wetchakun K, Samerjai T, Tamaekong N, Liewhiran C, Siriwong C, Kruefu V, Wisitsoraat A, Tuantranont A, Phanichphant S (2011) Sens Actuators B Chem 160:580 Singh A, Samanta S, Kumar A, Debnath AK, Prasad R, Veerender P, Balouria V, Aswal DK, Gupta SK (2012) Org Electron 13:2600 Kumar A, Debnath AK, Samanta S, Singh A, Prasad R, Veerender P, Singh S, Basu S, Aswal DK, Gupta SK (2012) Sens Actuators B Chem 171–172:423 Sadek AZ, Wlodarski W, Kalantar-Zadeh K, Baker C, Kaner RB (2007) Sens Actuators, A 139:53 Virji S, Huang J, Kaner RB, Weiller BH (2004) Nano Lett 4:491 Wang J, Chan S, Carlson RR, Luo Y, Ge G, Ries RS, Heath JR, Tseng H-R (2004) Nano Lett 4:1693 Liu, Kameoka J, Czaplewski DA, Craighead HG (2004) Nano Lett 4:671 Yang L-Y, Liau W-B (2009) Mater Chem Phys 115:28 Matsuguchi M, Okamoto A, Sakai Y (2003) Sens Actuators B Chem 94:46 Talegaonkar J, Patil DR (2016) Int J Eng Res V5 Sen T, Mishra S, Shimpi NG (2016) RSC Adv 6:42196 Tai H, Jiang Y, Xie G, Yu J, Chen X (2007) Sens Actuators B Chem 125:644 de Lacy Costello BPJ, Evans P, Ewen RJ, Honeybourne CL, Ratcliffe NM (1996) J Mater Chem 6:289 Tai H, Jiang Y, Xie G, Yu J (2010) J Mater Sci Technol 26:605 Pawar SG, Chougule MA, Patil SL, Raut BT, Godse PR, Sen S, Patil VB (2011) IEEE Sens J 11:3417 Gong J, Li Y, Hu Z, Zhou Z, Deng Y (2010) J Phys Chem C 114:9970

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Chapter 16

Conducting Polymer Nanocomposite-Based Gas Sensors Kalpana Madgula and L. N. Shubha

1 Introduction—Conducting Polymers Quickly expanding human demands and digital revolution, accelerate our capability to create new, and advanced materials that can power industries from energy to manufacturing. In this context, the research in material science and nanotechnology, is directed towards the control of three-dimensional structures, beyond basic chemical structures, in order to create a novel functional material. One such wonder material polymer, is a macromolecule consists of repeating structural units that are connected by covalent bonds. Though polymers existed for many years, they are considered to be non-conducting or insulating materials due to unavailability of free electrons and were used to coat or cover metallic cables to insulate them. In spite of their low conductivity, efforts have been made to increase the conductivity. In 1971, Shirakawa first reported that acetylene can be polymerized to give a freestanding film with mechanical properties [1]. Later, MacDiarmid and Heeger showed that when polyacetylene was exposed to oxidizing agents such as iodine and arsenic pentafluoride (AsF5 ) it became conducting and thus has raised conductivity by many orders of magnitude 103 –104 S/cm [2]. However, the instability of polyacetylene on exposure to air opened the research interest toward various other CPs [3–7]. All the conducting polymers were characterized with the combination K. Madgula (B) Department of Chemistry, St. Francis College for Women, Uma Nagar, Begumpet, Hyderabad 500016, Telangana, India e-mail: [email protected] L. N. Shubha Department of Electronics, St. Francis College for Women, Uma Nagar, Begumpet, Hyderabad 500016, Telangana, India e-mail: [email protected] K. Madgula Product Development, SAS Nanotechnologies LLC, Delaware Technology Park, 550 S College Avenue, Newark, DE 19716, USA © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_16

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of properties like electrical, magnetic, electronic, and optical properties as in metals and mechanical and processable properties as found in conventional polymers [8]. The term “conducting polymers” (CP) broadly refers to the materials that possess the exclusive property of electronically conducting, including the first-generation materials derived from the dispersion of conductive fillers (e.g., carbon black or metal powders/fibers) [9] in polymeric binder and the second generation involved, inherently conducting polymers (ICP) which are also referred as “synthetic metals” that themselves conduct electric current, without the intervention of any additives. In the case of ICPs, the conduction path is provided by the contribution from delocalized electron cloud and/or non-bonding electrons throughout the chain of molecules. Conducting polymers, i.e., ICPs or CPs [10, 11] also referred as the conjugated polymers possessing some of the interesting electrical and optical properties, owing to the delocalization of electrons in a continuously overlapped pi (π) orbital along the polymer backbone. These appealing and unusual optoelectronic properties of CPs allow them to be used for various applications [12, 13] including protecting metals from corrosion [14], sensing devices [15], light emitting displays [16], artificial actuators, all plastic transistors, and non-linear optical devices [17].

1.1 Structure and Properties CPs are the organic compounds that have an extended π orbital and conjugated carbon system [18, 19]. A necessary condition for a polymer (CP) to be intrinsically conducting (Table 1) seems to be its conjugated structure with alternate single and double bonds or conjugated system coupled with atoms (e.g., N, S) that contribute π orbitals for a continuous orbital overlap. The contribution from both the charge carriers and further the orbital system that makes these carriers move freely are Table 1 List of typical conductivities of some common conducting polymers and their repeat units; S-Siemens, cm-centimeters. Adapted with permission from Ref. [14]

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required for polymers to be electronically conducting which is identical to the free electron movement associated to high conductivity of metals. Thus, the conjugated structure in CPs can meet the requirement by a continuous pi orbital overlap in the polymer backbone and the conjugated polymers in their pristine state, behave like semiconductors or insulators. The energy gap can be >2 eV, could make too high for thermally activated conduction. Delocalization of conjugated states arises due to the resonance-stabilized structure of the conducting polymer, where the delocalization and the bond change increase the energy gap in CPs. Among the notable features of CPs is tuning the conductivity of polymers by proper doping [20]. The conductivity also depends on the type of dopant and its concentration. Table 1 lists conductivities of some conjugated polymers and their repeat units. Among the CPs the polyacetylene, with its simple conjugated structure and attractive properties has been widely studied as a model for other electronically conducting polymers [21]. Various theories [22] have been proposed for conductivity, physical and chemical properties of above listed polymers. Most of the organic polymers, in their undoped state, have no intrinsic charge carriers and have a rather low conductivity, and the required charge carriers are provided, when an electron is removed from the valence band by oxidation (p-doping, with electron acceptors) or is added to the conducting band by reduction (n-doping, with electron donors) does the polymer become highly conductive. Polyacetylene is the first organic conjugated polymer where conductivity of the sample increased due to doping [23]. The doped conjugated polymers can be dedoped using suitable reagents; hence, doping/dedoping is a reversible process. Further, polymer doping results into conjugation defects, such as solitons, polarons, or bipolarons in the polymer chain, that are responsible for the conductivity. A polaron is a radical cation that is delocalized over some monomer units, and when doping is increased, two polarons can combine to form a bipolaron. Also, the conductivity of organic polymers can be regulated from insulating to semiconducting and further to highly conducting polymers by changing the doping levels [24–26]. Figure 1 illustrates the mechanism of band structure in standard polymer, undoped and doped conducting polymers [26]. During doping process, an organic polymerpossessing conductivity in the range of 10−10 –10−5 S/cm−1 is changed to a polymer in a “metallic conducting” regime (1–104 S/cm−1 ). Though the highest value of Fig. 1 Mechanism of band structure in standard polymer, undoped and doped conducting polymers [26]

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conductivity (>105 S/cm−1 ) reported to date, is in the case of iodine-doped polyacetylene and other CPs it is upto 103 S/cm−1 . This conductivity is sufficient to fabricate electronic devices like, diodes transistors and light-emitting diodes. Of all those CP materials, the most common and important polymers, polyaniline (PANI), polypyrrole, (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) and their hybrids with heterogenous species as the as the active layers has been discussed in detail in this chapter [27, 28].

1.2 Fabrication Strategies The ease of synthetic methodologies to make conducting polymers and their hybrids [29, 30] through chemical or electrochemical processes, and their molecular chain structure which can be modified conveniently by copolymerization or structural derivations along with good mechanical properties, allow a facile fabrication for sensor applications. The general physical properties depend on the polymer chain size and length and also described in terms of molecular weight [31]. The chemical properties depend on the polymer chains and solubility in aqueous solutions. Chemical polymerization is beneficial for large-scale manufacturing at low cost, whereas electrochemical polymerization can offer the advantage of in situ deposition on to an electrode, that can be fabricated as sensor device. CPs are mostly synthesized by using oxidizing agents or in the presence of metal salts that can act as both oxidizing and doping agents to initiate the polymerization yielding polymers directly in a conductive state. Some common oxidizing agents (p-type dopant) are HClO4 , FeCl3 , AsF5 , I2 , NH4 BF4 , SO3 CF3 , HCl, HNO3 , H2 SO4 , H3 PO4 , and some reductants are Li, K, Na, etc. (n-type dopants) [32–38]. Nanotechnology advances are employed to produce nanostructured CPs, by the use of template [39] during the polymerization that include hard template, soft template and template-free synthetic methods. However, it is sometimes difficult to distinguish the hard and soft template approaches. Various parameters and their combination, like type of surfactants, pH, temperature, and structure-directing agent, provide immense possibilities to fabricate CP nanostructures with controlled morphology.

1.3 Effect of Morphology CPs can be synthesized chemically or electrochemically from monomers to produce CPs in various forms such as powder form, films deposited on substrate, colloidal solutions, stand-alone membranes, or water-soluble polymers. An improved control of film thickness and morphology that produce uniform polymers for some desired application, can be achieved by electrochemical polymerization in comparison to

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chemical oxidation or polymerization. Precise control of structure and morphology of CPs during the synthesis also makes them soluble in common solvents and thermoplastic that contribute to the overall performance of related sensors. Different morphologies of conducting polymers, such as nanowires [40, 41], nanotubes [42, 43], and nanofibers [44] are being studied and reported. Single-step fabrication of a highly porous PANI nanofiber vapor sensor by UV irradiation of a precursor solution [44] onto an interdigitated electrode array is reported by Li et al. CP nanoparticles [45] and other nano-structured materials [46] such as tubes, rods, thimbles, and belts also have been reported. Several techniques like solution casting [47], thermal evaporation [48], dropcoating [49], dip coating, and Langmuir–Blodgett technique [50] are used to prepare polymer gas-sensing films. Electropolymerization method of synthesis is preferred [51] because reaction conditions such as the polymerization potential, solvent, supporting electrolyte, and substrate materials can be tailored to obtain desired properties. As for sensing applications, 1D conducting polymer nanostructures, including nanowires, nanofibers, and nanotubes have larger surface area compared to their bulk counterparts [52] and offer enhanced sensing performance to produce amplified sensitivity and real-time response due to the enhanced interaction between CP, analyte molecules [53]. However, in contrast to inorganic nanomaterials, they have limitations, such as instability at the nanometer scale, owing to the presence of covalent bonds that makes them unstable [54]. As reported by Xu et al. [55], a facile and effective route to prepare highly ordered, monolayer 1D PEDOT nanowires (NWs) is by using LB technique, which is by wetting Al2 O3 membrane (anodic aluminum oxide, AAO) template method. The resulting PEDOT–surfactant complex at air/water interface has the capability of selfassembly and form stable float layer that has collapse pressure more than 50 mN/m. These NWs (film) are transferred onto the interdigitated electrode (or substrate) to use them as sensing material for NH3 and HCl gases, at low concentration ( DMSO > ROHs > aldehydes > Ketones > benzene derivatives > alkanes and low sensitivity (DL ≥ sub-micrograms) hydrocarbons such as heptane and hexane. A highly sensitive nitro explosive vapor detection system that works at RT was reported by MR Eslami et al. [114] and was prepared by polypyrrole–bromophenol (PPy-–PB) deposited in the form nanospheres and nanorods on quartz crystal microbalance (QCM) gold electrode in direct, single electrochemical method. A series of QCM sensors modified with PPy doped with different bromine containing anions and the regular chloride dopant were prepared and used for sensing of nitro explosives. The design entailed a PPy–BPB modified QCM sensor with very low limit of detections for nitro explosives including; TNT (500 ppt), PETN (800 ppt), RDX (1 ppb), and HMX(2 ppb). The sensor presented to be stable, with fast response time (5 s for TNT and 7–10 s and 15–25 s for other explosives) with recovery time 10 s and readily reversible at room temperature. Hydroxylated PEDOT nanotubes (HPNT) were used [115] to detect dimethyl methyl phosphonate (DMMP), which is a most common stimulant for nerve agent sarin. Axially aligned HPNTs are produced by electrospinning process under magnetic field and then integrated into the conductometric substrate. The response time of HPNTs was found to be 3–25 s toward DMMP with sharp increase in resistance along with detection limits up to 10 ppt. as illustrated in the Fig. 15a, the other organophosphorus compounds also responded to HPNTs at concentrations of 1 ppb. Figure 15 b, also represents the interaction of HPNTs with DMMP by forming hydrogen bonding. Although, all the stimulants were interacted with hydroxylated

Fig. 15 Sensing performance of chemical nerve agent sensor based on hydroxylated PEDOT nanotubes (HPNTs) a Histogram showing the response of HPNTs toward similar organophosphorus compounds at 1 ppb (TCP, MDCP, DMMP, TMP); b 3D graphics showing the formation of hydrogen bonds between nerve agent stimulant compounds and HEDOT; (c) Principal components analysis plot using response intensity inputs from four different conducting polymer nanomaterials (two different HPNTS, pristine PEDOT nanotubes, and PPy nanotubes) to the 16 analytes (including DMMP): each analyte concentration was fixed at around 4 ppm. With permission from Ref. [87], Copyright 2012, American Chemical Society

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PEDOT through hydrogen bonding, HPNTs exhibited unique selectivity to DMMP. Figure 15c also depicts intensity plots of interference due to 15 other potential volatile organic compounds, for four different conducting polymer nanomaterials, at each analyte concentration fixed at 4 ppm. The flexibility of HPNTs and its durability under mechanical deformation, provides a novel route to fabricate a wearable nerve agent sensor.

3.2 CP–Organic (Graphene, RGO, CNTs, etc.) Hybrids CP-based organic hybrids include potential components of carbon nanomaterials such as carbon nanofibers, carbon nanotubes (CNTs), graphene, graphene oxide (GO) and rGO. Graphene is one atom thick sheet of carbon atoms with 2D hexagonal crystal structure that has shown promising applications [115]. Graphene and its derivatives are most studied for their unique features with a large surface-to-volume ratio, that offer improved adsorption capacity for gas molecules and strong surface activities [116]. Further, graphene exhibits an excellent electron mobility (200 000 cm2 V−1 s−1 ) large surface area and a high carrier density of 1012 cm2 , and it has low resistivity at room temperature. Graphene sheet offers an advantage of creating a change in resistance with low electrical noise when a small amount of gas is adsorbed on its surface. Similarly, other carbon nanostructures, class of CNTs (carbon nanotubes)-based CP composites are categorized for their superior mechanical strength, high electrical conductivity with few limitations like poor processability and lack of chemical stability [117, 118] as explained in detail in the following research articles. Kwon et al. [119] developed Ammonia sensors from flexible membranes of Poly(3,4-ethylenedioxythiophene) nanotubes (PEDOT NTs) were prepared by using a facile approach of vapor deposition polymerization (VDP)-mediated electrospinning. Core part is electrospun PVA nanofibers (NFs) and using VDP method EDOT monomer was adsorbed on to the template of elctrospun PVA to form f PVA/PEDOT coaxial nanocable. Further, PEDOT NTs membrane is obtained from PVA/PEDOT NCs by removing PVA NFs (core) in distilled water NFs as shown in the Fig. 16 SEM images. By using PVA/PEDOT coaxial NCs and PEDOT NTs, NH3 gas was detected in the range of 1–100 ppm (Scheme and Fig. 16a, b, and c). Low detection limits of 5 ppm, fast response time (less than 1 s), and recovery time (30 s for PEDOT NTs; 50 s for PVA/PEDOT NCs) was reported. It was concluded that in the case of PEDOT NT membrane higher the thickness more the sensitivity, for 60 ppm of NH3 gas. The PEDOT hybrids gained attention due to superior conductivity, good stability including other electrochemical and spectroscopic properties associated with its low band gap, electrochromic, and antistatic nature. A low cost CO2 sensor was fabricated by inkjet printing technique using PEDOT:PSS on a PET substrate [120] by Ando et al. The deposition of double layer of PEDOT:PSS and graphene on interdigitated electrodes (IDE) printed onto a PET substrate. Further, IDE was made from

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Fig. 16 The SEM and TEM (inset) images of a the pristine PVA nanofibers with a diameter of 100 nm, b PVA/PEDOT nanocables with a diameter of 140 nm, and c PEDOT nanotubes with a diameter of 140 nm The above figure displays the FE-SEM and TEM images of PVA NFs, PVA/PEDOT coaxial NCs and PEDOT NTs membranes. As shown in the inset TEM images, the diameter of PVA NFs ws ca. 100 nm (Fig. a) and the PEDOT sheath thickness of PVA/PEDOT coaxial NCs and PEDOT NTs was ca. 20 nm (Fig. b and c). The PEDOT sheath was smooth and thin due to the characteristics of VDP including the uniform and smooth coating. PEDOT NTS were fabricated by washing PVA/PEDOT coaxial NCs with distilled water. Moreover, the morphology of PEDOT NTs did not collapse without reduction of outer diameter after washing (Fig. c). With permission from [90]; Copyright 2010

printing of a conductive pattern of silver nanoparticle solution using a commercial Ink jet printer. The electrical conductivity of graphene due to the adsorption of CO2 molecules at 30 °C, is measured as 45μOhm/ppm and a sensitivity of 100 ppm (Fig. 17). An rGO/PEDOT composite film made by electrochemical method was reported by K Dunst et al. [121] for sensing NO2 gas. The sensing device was alumina substrate with gold interdigitated, where the sensing film was prepared on sensing platform by the electro polymerization of EDOT monomer and graphene oxide solution

Fig. 17 a Schematic illustration for the interaction of PEDOT nanotubes membrane with ammonia gas. b A calibration curve of bulk PEDOT films, PVA/PEDOT nanocables membrane, and PEDOT nanotubes membrane as a function of NH3 vapor concentration (R0 values of the PEDOT NTs and PEDOT NCs are 3 × 103  and 3 × 103 ); —Ohm; PVA—polyvinyl alcohol; PEDOT— poly(3,4-ethylenedioxythiophene); NTs—Nanotubes; NC—Nanocable; NH3 —Ammonia (With permission from Ref. [119], copyright 2010)

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and reduction in 0.1 M KCl. PEDOT/graphene or rGO composites show very little mass loss at high temperatures (about 300 °C).and as compared to PEDOT/PSS (polystyrene sulfonate). Huang et al. [122] fabricated ammonia (NH3 ) gas sensor based on rGO–PANI hybrids. rGO–MnO2 hybrids acted as both templates and oxidants for polymerizing aniline monomer, and thus formed PANI NPs could anchor on the surface of rGO sheets. Recently, Lee, et al. [123] reported PANI–rGO nanocomposite films as sensitive membranes to detect NH3 gas at RT. PANI nanospheres are homogeneously distributed on both the surfaces of rGO nanosheets. PANI–rGO nanocomposite sensors are operated at room temperature with improved performance for a 15 ppm NH3 is 13% and 22.1 min. It was reported that incorporating more rGO content reduces the sensitivity in the final nanocomposite and attributed to high surface area of rGO, but the responsivity can be reduced by incorporating more rGO content and PANI/rGO weight ratio should be less than 125. The lowest detectable limit of NH3 gas concentration was 0.3 ppm. Roy et al. [124] reported the PANI-coated multiwalled carbon nanotube (MWNT) composite synthesized by chemical oxidative polymerization process for the CO gas sensing. A patterned IDE was used to spin coat the PANI–MWCNT film. This composite showed maximum response to 500 ppm CO with lowest response toward other compounds such saturated 2-propanol, acetone, ethanol, methanol, NH3 , etc., indicating the selectivity toward the CO at RT.

3.3 CP Hybrids with 2D TMDs (Two-Dimensional Transition Metal Dichalcogenides) The need for high selectivity and low or room temperature operation demand the continued exploration of new material combinations and devices for efficient gas sensing [125]. Two-dimensional materials are playing significant role because of their specialized properties such as large number of available reactive sites (due to high surface area), tunable band gap size, high absorption coefficient, that makes them efficient materials for gas sensing, catalysis, and energy storage technologies. Due to low electrical noise and high electrical conductivity, a significant change in electrical conductivity is achieved on exposure to gas molecules followed by change in carrier concentration [126]. From the past few decades, ultrathin 2D nano-structured metal chalcogenides (TMDs) have given substantial consideration for the applications [127] in supercapacitors, battery materials, sensors, and in most of the other electronic devices, because of their distinctive compositional and unique features as reported in the number of reviews and research articles [125, 128, 129, 130]. Transition metal dichalcogenides (TMDs) are a class of materials with the general formula of MX2 , where M indicates a transition metal element (including Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, etc.), and X represents a chalcogen (Se, S, or Te)

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[131]. Though bulk materials of TMDs are long known, isolation of their 2D- layered structure where a plane of metal atoms covalently bonded to the chalcogen atoms. TMDs are part of a variety of materials and the layered inorganic equivalents of graphene, with notable examples—MoS2 , WS2 , MoSe2 , WSe2 , ReS2 , and ReSe2 (S—sulphides and Se—selenides of Molybdenum, Tungsten, and Rhenium, respectively) as well as layered metal oxides (MoO3 and SnO2 (Sn–Tin)), layered group III–VI semiconductors (GaS, GaSe, and SnS2 Ga—gallium), phosphorene, h-BN (Boron Nitride), etc., that contributed to current research due to their thicknessdependent physical and chemical properties. They also have large surface-to-volume ratio like graphene (Sect. 4.2), also possess these semiconducting properties with an appropriate bandgap, which can be tuned to control transport characteristics to achieve improvement in the sensing capabilities of final device especially suitable for outstanding properties in diverse applications including energy conversion and storage, sensor, thermoelectric devices, memory devices, and biomedical devices [132]. Apart from their inherent properties, other material properties also can be tuned by combining them with other nanomaterials [133, 134] resulting into hybrid nanocomposites. The final properties of CP/2D nanocomposites not only depend on morphology but also on the adopted synthetic strategy, that efficiently allows interaction among the components to provide synergistic effect in the final product. The advancement in the fabrication technologies facilitates to adopt synthetic procedures with varying morphologies, properties for desired applications. Among various in situ and ex situ methods (like one-pot synthesis, electrophoretic, solution mixing, etc.) to synthesize CP/2D composites [131, 133], the most common being in situ polymerization approach involve addition of CP monomers (e.g., PPy and PANI) and 2D materials (nanosheets and nanostructures) into the host dispersion and polymerization was initiated by the addition of oxidant, and the final product is obtained after purification and the factors like concentration of monomer, dopant, oxidant to monomer molar ratio, temperature, and reaction time will decide the morphology of final nanocomposite. A few applications of CP/TMD composites with respect to gas sensors has been reported. Wang et al. [135] reported the MoS2 /PANI composite has been for mercury (Hg) vapor sensing. The sensor showed fast response and recovery not only to a wide range of concentrations of elemental mercury vapor and good selectivity over common gases like NO2 , SO2 , and NH3 in flue gas at room temperature. This resistive Hg sensor has exhibited special features like enhanced sensing performance, reduced time of response and recovery with long-term stability. As compared to pristine CP, their doped, and functionalized materials with graphene, GO or rGO, metals and metal oxides, 2D TMDs are proven to be advantageous [131] in improving gas-sensing performance.

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3.4 Introduction to Flexible Gas Sensors The ability to fabricate CP as thin film sensors and on substrates gives them an edge. One of the first reports have been made by Nylander et al. [27], who measured ammonia vapor by using polypyrrole-impregnated filter paper and later extended the same device to measure NO2 and H2 S under suitable conditions. Recent years have witnessed the development of low-cost electronics and sensors by the use of graphic industry processes that exploit innovative materials and flexible substrates such as paper, textiles, and plastics (PET). In comparison with conventional electronics, based on silicon, the printed devices offer economical and multidimensional benefits with disposable devices [135, 136]. Major techniques used to fabricate printed sensors, are screen printing and inkjet printing. Screen printing is based on the use of masks to deposit a thick film, in which masks and material waste is generated, whereas the inkjet printing, uses a contactless printing to produce rapid prototyped electronic components. Present trend of utilizing the combination of functional CPs, for example, conductive silver NPs [137–139] with materials like PEDOT:PSS, PANI being used widely as suitable materials to inkjet printer [140]. Flexible devices [141] would also play a critical role to shape the irregular surfaces for the applications in fabrics and food packaging.

4 Summary and Conclusions In this book chapter, CP-based sensors, that convert a chemical interaction into an electrical output, covering a broader application, have been effectively presented. The review also discussed the research trends and directions in the development of CP hybrids for their use as gas/vapor sensors at room temperature, and in detecting target gases that are important to environment, human health, and industry. Additionally, the morphology and structural diversity of CP nanostructures offered advantages with regard to selectivity, fast response, and recovery times with improved stability is also being illustrated.

5 Future Prospects The continued advancement in the nanotechnology has facilitated the improvement in response, recovery times, and sensitivity of sensors. In a complex environment, and in the presence of mixture of gases, selectivity for a particular target gas is still a major challenge and the mechanism involved in sensitivity, selectivity, and response time with respect to the morphology of polymer nanostructures need to be understood. The reproducibility, accuracy, and sensitivity could be a crucial step for smart and advanced sensors that are flexible, wearable materials and devices. Gas

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sensors made of CP nanocomposites with the combination of graphene and 2D TMDs [134], are being reported in the literature, and has some limitations, with respect to sensing mechanism, synthetic strategies for dispersion and enhanced efficiency in the composite.

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123. Lee CT, Wang Y (2019) High performance room temperature NH3 gas sensors based on polyaniline-reduced graphene oxide nanocomposite sensitive membrane. J Alloy Compd 789:693–699 124. Roy A et al (2018) Polyaniline-multiwalled carbon nanotube (PANI-MWCNT): room temperature resistive carbon monoxide (CO) sensor. Synth Met 245:182–189 125. Joshi N, Hayasaka T, Liu Y, Liu H, Oliveira ON Jr, Lin L (2018) A review on chemiresistive room temperature gas sensors based on metal oxide nanostructures, graphene and 2D transition metal dichalcogenides. Microchim Acta 185:213. https://doi.org/10.1007/s00604-018-2750-5 126. Yang S, Jiang C, Wei S (2017) Gas sensing in 2D materials. Appl Phys Rev 4. https://doi.org/ 10.1063/1.4983310 127. Yang F et al (2019) Recent progress in two-dimensional nanomaterials: synthesis, engineering, and applications. Flat Chem 18:100133 128. Liu X, Ma T, Pinna N, Zhang J (2017) Two-dimensional nanostructured materials for gas sensing. Adv Funct Mater 27:1702168. https://doi.org/10.1002/adfm.201702168 129. Yang S, Jiang C, Wei S (2017) Gas sensing in 2D materials. Appl Phys Rev 4:021304. https:// doi.org/10.1063/1.4983310 130. Varghese SS, Varghese SH, Swaminathan S, Singh KK, Mittal V (2015) Two-dimensional materials for sensing: graphene and beyond. Electronics 4:651–687. https://doi.org/10.3390/ electronics4030651 131. Wang QH et al (2012) Nat Nanotechnol 7:699–712 132. Sajedi-Moghaddam A, Saievar-Iranizad E, Pumera M (2017) Two-dimensional transition metal dichalcogenide/ conducting polymer composites: synthesis and applications. Nanoscale 9:8052 133. Ko KY et al (2016) Improvement of gas-sensing performance of large-area tungsten disulfide nanosheets by surface functionalization. 10:9287–9296 134. Wang D et al (2013) Anal Methods 5:6576–6578 135. Yang S, Jiang C, Wei SH (2017) Gas sensing in 2D materials. Appl Phys Rev 4:021304 136. Abad E, Zampolli S et al (2007) Flexible tag micro lab development: gas sensors integration in RFID flexible tags for food logistic. Sens Actuators B Chem 127(1):2–7 137. Mäntysalo M et al (2009) Capability of inkjet technology in electronics manufacturing. In: Electronic components and technology conference 138. Zheng L, Rodriguez S, Shao B (2008) Design and implementation of a fully reconfigurable chip less RFID tag using Inkjet printing technology. In: 2008 IEEE international symposium on circuits and systems (ISCAS 2008) 139. Amin Y et al (2009) Inkjet printed paper based quadrate bowtie antennas for UHF RFID tags. In: 2009 11th International conference on advanced communication technology (ICACT 2009) 140. Andersson H et al (2012) Inkjet printed silver nanoparticle humidity sensor with memory effect on paper. IEEE Sens J 12–6:1901–1905 141. Andò B, Baglio S (2011) Inkjet-printed sensors: a useful approach for low cost, rapid prototyping. IEEE Instrum Meas Mag 14(5):36–40

Chapter 17

Calixarene-Based Gas Sensors Frank Davis, Seamus P. J. Higson, Osvaldo N. Oliveira Jr., and Flavio M. Shimizu

1 Introduction In the early twentieth century, Leo Baekeland reacted phenol with formaldehyde to form the thermoplastic “Bakelite.” This presence of three reactive sites ortho and para on the phenol ring led to a high degree of cross-linking—and therefore attempts were made to synthesize linear analogues. One way to achieve this was to block the para position of the phenol ring with, for example, a t-butyl group. However, this instead led to cyclic oligomers of the phenol units linked with methylene bridges. Much of the early work on calixarene synthesis, modification, structure, and applications has been summarized by a series of monographs by Gutsche and other workers [1–8]. Although initially mixtures of oligomers were produced, eventually synthetic schemes were developed which allowed production of pure oligomers, especially when the para substituent was t-butyl. Cyclic oligomers containing 4, 6, or 8 phenolic units were the ones most easily synthesized [1], though oligomers containing up to 90 units have been synthesized [9]. These are usually named as calix[n]arenes where n is the number of phenolic moieties in the macrocycle. Other phenolic units have been utilized; resorcinol or pyrogallol, for example, can be condensed with aldehydes to give calix[n]resorcinarenes or pyrogallenes (where n is usually, but not always, four) [1–3, 7, 10]. Typical structures are shown for a calix[4]arene and a calix[4]resorcinarene in Fig. 1. F. Davis · S. P. J. Higson Department of Engineering and Applied Design, University of Chichester, Bognor Regis, West Sussex PO211HR, UK O. N. Oliveira Jr. São Carlos Institute of Physics, University of São Paulo, CP 369, São Carlos, São Paulo 13560-970, Brazil F. M. Shimizu (B) Brazilian Nanotechnology National Laboratory (LNNano), Brazilian Center for Research in Energy and Materials (CNPEM), Campinas, SP 13083-970, Brazil e-mail: [email protected] © Springer Nature Singapore Pte Ltd. 2020 S. Thomas et al. (eds.), Functional Nanomaterials, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-15-4810-9_17

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Fig. 1 Structures and X-ray crystallographic models of a t-butyl calix[4]arene and b tetra-undecyl calix[4]resorcinarene

The structures of both calix[4]arenes and calix[4]resorcinarenes can be likened to a vase or bowl (leading to Gutsche’s proposal of calix for their name) and contain an internal pore within their macrocyclic structure. This raises the possibility of using these moieties as chemical sensors, since their open structure would allow diffusion of guests into and throughout the pores. The smallest, simplest calixarenes (such as in Fig. 1a) display a rigid cone conformation with all the –OH groups together forming strong hydrogen bonds with each other. Various molecular conformations exist, including cones, baskets, or more complex configurations such as double cones and pleated loops [1–3, 8]. Potential interactions between host and guest include hydrogen bonding with the phenolic groups, π–π interactions, etc. Calixarenes are versatile, can be synthesized in a variety of macrocyclic ring sizes and both phenolic and aromatic units can be modified chemically, allowing tailoring of the macrocycle properties. Calixarenes are usually stable chemically and thermally, soluble in many

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solvents, easily processed, and relatively inexpensive compared with other receptor species. Within this chapter, we will concentrate on the use of calixarenes as gas sensors. There is a vast literature on the synthesis, structures, chemical modification, and binding to calixarenes and related compounds; this is outside the scope of this chapter and has been reviewed elsewhere [1–8, 10]. Air pollution and its effects on public health is becoming a major cause for concern. Exposure to toxic chemical species can be disastrous in nature, such as in Bhopal where the release of methyl isocyanate caused thousands of deaths and hundreds of thousands of nonfatal injuries. Continuous exposure to much lower levels of pollutants can lead to respiratory illness and cancer. Toxic compounds commonly released into the environment encompass greenhouse gases, ozone, carbon monoxide, ammonia and other amines, formaldehyde, and hydrocarbon compounds. Many of these can be highly injurious to human health and may cause environmental damage [11]. The detection, quantification, and monitoring of these species has become the subject of much recent research [11]. This has led to the development of gas sensors based on several techniques, as is the case of calixarene-based sensors for gases and vapors [11–14]. Calixarenes are especially suited for use as sensors for the reasons described below: (a) Their porous structure means that gases can diffuse freely within thin films of these species, facilitating rapid detection even at low concentrations. (b) They form pre-organized structures which aid specificity toward target guest molecules. (c) Many functional groups can be incorporated as receptor sites, again increasing selectivity and sensitivity. (d) Various calix sizes and shapes can be obtained, allowing tailoring of receptor cavities to allow effective encapsulation of guests. (e) The stability and solubility of many calixarenes enable them to be easily formulated as thin films by solution casting, spin coating, thermal evaporation, and Langmuir–Blodgett methods.

2 Methods of Detection Only a brief description of the most common methods of detection will be given here. These methods include quartz crystal microbalance (QCM), surface plasmon resonance (SPR), and surface acoustic wave (SAW), but many other optical and electrical methods have also been used to interrogate these materials. In QCM, gold or platinum electrodes are deposited onto a thin quartz disc as shown in Fig. 2. Passing an oscillating electrical current between these electrodes causes the quartz disc (which is piezoelectric) to vibrate, producing an acoustic wave perpendicular to the current direction [15]. What makes this suitable for sensing is that the frequency of the oscillation is dependent on the mass of the crystal, including anything adsorbed on the surface. Therefore, if a QCM is modified by a thin film of

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Quartz Disk Au or Pt Electrical Contacts

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Fig. 2 A schematic of a a QCM crystal b a SAW device (reproduced with permission from Sensors [16])

calixarene, if this film then binds any guest molecules, its mass must increase with a concurrent change in the frequency which is easily detected [15]. SAW devices are made of piezoelectric materials deposited onto interdigitated metal electrodes. When subjected to an AC voltage, an acoustic wave is generated at the substrates surface [16] and can be converted back to an electrical signal. The presence of a sensing film and any binding interactions perturb this wave and the resultant electrical output can be processed. An extensive review on SAW sensors has been recently published [16]. Surface plasmon resonance is an optical technique shown schematically in Fig. 3. The basic sensor is a thin metal film either deposited onto a glass prism or more commonly onto a glass slide pressed against the prism. When the metal film (usually gold about 40 nm in thickness) is irradiated by a laser, at a critical angle, energy is adsorbed and used to create surface plasmons which then propagate along the metal surface, causing a drop in reflected laser intensity (Fig. 3). The plasmons are not located just at the surface; they extend somewhat into the immediate vicinity, falling off exponentially as the distance from the metal surface increases. This renders the actual adsorption of light sensitive to the conditions immediately adjacent to the surface. The presence of a thin film at this surface will affect the position and intensity of the adsorption of the reflected laser light, as will adsorption of any guest molecules to the film [17]. SPR has become widely used for analysing interactions of this type as demonstrated by the range of commercial SPR systems available [17]. Incident Light

Metal Film

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Intensity Reflected Prism Light Slide Calixarene Film Angle of Reflection

Fig. 3 Surface plasmon resonance schematic and typical output

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Other optical techniques can be used for sensors containing calixarenes. For example, although simple calixarenes tend to be white powders, the basic frame can be modified with chromophores to give color changes on response to binding of vapors. Calixarenes have also been deposited onto optical fibers and changes in their refractive index on guest binding can be monitored.

3 Gas Sensing with Calixarenes 3.1 Hydrogen Hydrogen has been considered as a replacement for hydrocarbons as transport fuel due to its relative environmental friendliness. Since hydrogen is not only gaseous but also highly explosive, its detection becomes crucial. Complexes of hydrogen with calixarenes can be formed at 298 K but high pressures (31 bar) are required [18]. Research appears to be more focused toward the use of calixarenes as hydrogen storage materials rather than as sensors; recent results in this field have been reviewed elsewhere [11]. However, composite materials including calixarenes have been utilized in hydrogen sensing. Calixarenes modified with phosphonic acid groups were incorporated into composites with palladium nanoparticles and either carbon nanotubes or carbon nano-onions [19]. These electrically conductive composites could be dropcast from solution onto interdigitated electrodes and used as electrical sensors for hydrogen; the nanotube composite showed higher sensitivity than the nano-onion composite although both could determine hydrogen in the range 0.1–10%. A composite of calixarene sulfonic acid with 12-phosphatotungstic acid could be used as a membrane within a potentiometric sensor and could determine hydrogen levels from 0.01 to 0.2% and was unaffected by the presence of up to 200 ppm carbon monoxide [20]. Also, TiO2 nanorod or nanotube arrays were modified by three calixarenes (Fig. 4) [21], and their electrical resistance decreased upon exposure to hydrogen. The authors report that calixarenes enhance gas diffusion into the arrays, allowing

Fig. 4 Molecular structures of calixarenes 1–3 used in formation of sensing arrays (a). Depiction of 1@TiO2 NRs (b) and 1@TiO2 NTs (c), rod/tube tip functionalization is depicted for clarity. Reproduced from [21] with permission from the Royal Society of Chemistry

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fast, recoverable, and reproducible detection of hydrogen at room temperature with the t-butyl-substituted calixarene being the most effective.

3.2 Carbon Dioxide/Monoxide Carbon dioxide is a target of great interest; it is an asphyxiant and potent greenhouse gas. Levels of CO2 above 1000 ppm are considered too high for continuous exposure and acute effects begin to be felt above 40,000 ppm [11]. Most sensors for this gas are infrared-based sensors but other gas sensing technologies are used. QCM sensors made with calixarenes mixed with carbon nanotubes cast onto quartz crystals showed varying responses to CO2 , with a substituted t-butyl calix[4]arene being the most sensitive [22]. The sensors displayed much lower responses to oxygen, air, and carbon monoxide. QCM crystals were modified using a dropcast film of t-butyl calix[4]arene substituted with ferrocene groups [23], with the resultant system showing sensitivity toward carbon monoxide and dioxide. Calixarene showed higher sensitivity to CO2 than CO and better sensitivity than t-butyl calix[4]arene without the ferrocene groups, although it should be noted that no real assessment of the limits of detection for these gases was undertaken. Calix[4]arenes modified with pyridine, nitropyridine, oligosaccharide, and other groups were cast onto QCM crystals and assessed for their sensitivity to carbon monoxide [24]. Adding Fe3+ ions to the calixarenes before casting greatly improved their response, in some cases by a factor of 200. The calixarene modified with pyridine groups showed the highest sensitivity—in its iron-doped form displaying a detection limit of 12.5 ppm CO. The multiamino functionalized calix[4]arenes in Fig. 5a have also been utilized for carbon dioxide capture with the number and position of the calixarene substituents being shown to greatly affect the binding of CO2 [25]. The tetraamino-substituted calixarenes shown formed a highly stable 1:2 adduct with CO2 , yielding a dicarbamate complex (Fig. 5b), whereas a 1,3-diaminocalix[4]arene formed a less stable 1:1 adduct. Solid films of these materials on QCM devices were used as carbon dioxide sensors. The best material for response and reversibility appeared to be the diaminocalixarene (4); Fig. 5c shows the responses of this and a monoaminocalixarene upon exposure to 20 ppm CO2 . Other researchers demonstrated adsorption of carbon dioxide in t-butyl calix[4]arene [26] or para-acyl calix[4]arenes [27] but no detailed investigation has been made of their sensitivity toward the gas. Similarly, p-tetranitrocalix[4]arene could be adsorbed onto silica and then incorporated into a cellulose acetate membrane, the resultant composite being selective toward carbon dioxide although no assessment of its sensor applications were made [28].

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Fig. 5 a Structures of some of the calixarenes used in the study, b reaction of calixarene 4 with CO2 to form a 1:2 adduct, c QCM responses traces of calix[4]arenes 8 and 4 to CO2 at 20 ppm. Reprinted (adapted) with permission from [25]. Copyright (2011) American Chemical Society

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3.3 NOx Another group of gases of interest is the nitrogen oxides. Some binary oxygen– nitrogen compounds are of commercial and environmental importance, most of them being greenhouse gases and which could have other health and environmental implications. Dinitrogen monoxide (N2 O) is used as an anesthetic gas, being also a greenhouse gas. There are few materials that form complexes with this gas [11]. A so-called molecular capsule material consisting of two calix[4]resorcinarene units bridged by three methylene units has been shown to reversibly form an adduct with several gases, including N2 O [29]. Nitric oxide (NO) is a colorless odorless gas, which is also a stable free radical. It is an intermediate in industrial processes and can be generated in lightning storms. In humans and other mammals, it is an important signaling molecule within several physiological processes [30]; the discovery of the role of nitric oxide within cardiovascular signaling led to the award of the 1998 Nobel Prize in Physiology or Medicine. Nitric oxide forms deep purple colored 1:1 complexes with the 1,3-conformer of t-butyl calix[4]arene methyl ether [31] with a high binding coefficient (k = 5 × 108 mol−1 ) when the gas is bubbled through a solution of the calixarene in dichloromethane. X-ray diffraction and nuclear magnetic resonance (NMR) studies demonstrate that the NO molecule is oxidized to NO+ and is encapsulated deep within the calixarene cavity. The formation of highly colored complexes between NO+ and several calix[4]resorcinarene octamethyl ethers has also been observed [32]. A patent has been filed for the use of calixarenes bound to a metal–carbon alloy which produces an electrical response when exposed to NO in human breath [33]. Nitrogen dioxide (NO2 ) is a red-brown gas, often existing as a mixture with its dimer N2 O4 . It is a major atmospheric pollutant, contributing to smog and acid rain and is an intermediate in the synthesis of nitric acid. Calixarene ethers interact with nitrogen dioxide, causing the gas to disproportionate and the formation of colored calixarene/NO+ complexes [34]. Oligomeric versions of these calixarenes were shown to have nanotube-like structures. When exposed to NO2 /N2 O4 , with the nitrous oxide disproportioning (to NO+ NO3 − ), it leads to encapsulated NO+ [35] within the tubes. A similar behavior was noted for calixarene propyl ethers, and addition of nitrogen dioxide lead to reversible formation of highly colored calixarene– NO+ complexes [36] with association constants of approximately 104 . Micromolar levels of NO2 /N2 O4 could be detected in solution and the calixarenes could also be attached to polyethylene glycol to give materials that in solution or solid state gave colorimetric responses to the gas. Many calixarenes and similar materials such as hemicarcerands [37] capable of binding NO2 /N2 O4 have been synthesized. Calixarenes were attached to silica gel supports and used to trap NO2 /N2 O4 with a visual response [38]. Solutions of alkylated calixarenes in the ionic liquid 1-butyl-3-methylimidazolium bis(perfluoroethylsulfonyl)imide were shown to give strong color changes upon exposure to NOx. Cellulose could also be added to this solution and the resultant mixture cast as membranes which gave a strong colorimetric response to NO2 , with this

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reaction being able to be reversed by exposure to water vapor [39]. Tetrahexylated 1,3-alternate or cone calixarenes were also deposited on a silica chromatography plate and interrogated using a fiber-optic-based photodetector. Exposure to NO2 led to color changes, allowing measurements of the vapor as low as 0.54 ppm [40]. Flushing with air reversed this change although repeated exposures led to loss of sensitivity, probably due to nitration of the calixarene [40] or by oxidation [41]. Other substituted calix[4]arenes could be immobilized on Merrifield resin or polyethylene glycol supports to give solid state sensors for NO2 [42]. Single-walled carbon nanotubes were immobilized across interdigitated gold microelectrodes and then calixarenes absorbed onto these from solution via π–π interactions to give chemiresistor sensors capable of detection of NO2 , exposure to the vapor led to an increase in conductance [43]. Limits of detection as low as 25 ppb could be determined. Calixarenes have also been used as composites with porphyrins, in these cases the calixarene does not actually give an active response to the vapor, detection is obtained by interaction of the porphyrin with the gas. However, the calixarene acts as a porogen, facilitating gas transport throughout the film and increasing sensitivity. For example, an octa-carboxylate-substituted calix[8]arene was assembled as a Langmuir–Blodgett film along with a porphyrin and shown to give rapid (10–15 s) response to NO2 between 0.13 and 4.6 ppm [44]. The film followed simple Langmuir kinetics, and it was found that pre-exposure of the film to toluene vapor greatly slowed its response, possibly due to toluene being adsorbed into the calixarene pores and blocking diffusion. The presence of the calixarene prevented aggregation of the porphyrin in the film and improved their sensing ability with films 20 monolayers thick displaying optimal sensitivity and speed of response [45]. The same porphyrin/calix[8]arene combination was used by other workers where porphyrin layers could be deposited with and without layers of calixarene mixed with polymethyl methacrylate (PMMA) on top of them [46]. The porphyrin layers were sensitive to NO2 and organic acid vapors; however, when the calix/PMMA layer was added the selectivity for NO2 over the other gases greatly increased since the calix acts as a size selective layer.

3.4 Acidic Gases and Formaldehyde Acidic vapors have been detected by calixarene-based systems. Polyaniline is well known to respond to the presence of acidic or alkaline vapors; however, it can be difficult to process due to poor solubility. A composite material consisting of polyaniline and a phosphorylated calix[4]resorcinarene (where the calix acts as a solubilizing agent and a porogen) could be deposited as an LB film onto gold electrodes [47]. The resultant films (usually 20-layer thick) were insulating on deposition but their conductivity increased (up to 0.5 S m−1 ), with films containing 20% calix[4]resorcinarene having the highest conductivity. Films containing 60% calix responded well to HCl vapor, even at levels of 20 ppm. Porphyrins were also deposited onto substrates and then t-butyl calix[4, 6 or 8]arene/PMMA films assembled on top of them [48].

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Exposure to acetic, butyric, or hexanoic acid caused changes in the porphyrin spectra, while the calixarenes acted as size-selective barriers, allowing differentiation between the acidic vapors. Formaldehyde is an organic pollutant given off by many manufactured products such as MDF, plywood, fabrics, and cleaning products. The World Health Organization recommend that formaldehyde in air should not exceed 0.08 ppm over a 30-min period due to is toxic, allergenic, and carcinogenic effects. Sensors capable of determining these levels have been constructed using the colorimetric reaction with acetylacetone [49]. QCM crystals were coated with either t-butyl calix[4]arene or calix[4]resorcinarenes based on resorcinol, 2-methyl resorcinol, or pyrogallol and shown to bind formaldehyde [50]. The pyrogallol-based tetramer was the most sensitive, capable of determining formaldehyde from 109 to 2721 ppm in air with good reproducibility, stability, and reversibility.

3.5 Ammonia and Amines Ammonia and the related amines are known pollutants as well as intermediates in many chemical and biological processes. The Haber process, for example, is used to synthesize ammonia from nitrogen and hydrogen, with the ammonia produced having industrial uses such as fertilizer manufacturing. Ammonia and its related amines often have adverse toxicological and environmental effects, concentrations of ammonia higher than 25 ppm have been classified as dangerous to humans. Sensors for ammonia and other amines have been developed [51], including with calixarenes. Since these vapors are basic in nature, they can deprotonate the acidic phenolic groups in calixarenes, and ammonium ions can also form complexes with the electronrich aromatic rings. Polyaniline/calixarene LB films display electrical responses on exposure to HCl vapor [47]. The same films when constructed using 20% calixarene were conductive and displayed reproducible drops in conductivity when exposed to ammonia vapor at levels as low as 5 ppm. The authors also report that exposure to 100 ppm phenylamine caused drops in conductivity two orders of magnitude greater than similar levels of ammonia, possibly due to greater interaction of the relatively hydrophobic amines with the calixarenes in the film. Calixarenes substituted with four 4-nitrophenyl-3’-hydroxy azobenzene groups (Fig. 6a) could be complexed with lithium ions and then mixed with polyvinyl chloride and dip coated onto an optical fiber [52]. Exposure to ammonia vapor led to a distinct color change from yellow to red and allowed the detection of NH3 down to levels as low as 5 ppm. Later work used filter paper strips infused with the calixarene [53] and demonstrated optical responses to trimethylamine, producing visible color changes in less than 2 min. Water-soluble calix[4]resorcinarenes bearing sulfonate groups have been complexed with dimethyl didodecyl (or dioctadecyl) ammonium surfactants to create novel liquid crystal phases (Fig. 6b), with these having been shown to give optical responses to ammonia vapor [54]. Immediate increases in UV/Visible absorption were observed on exposure to ammonia vapor (Fig. 6c, d).

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Fig. 6 a Structures of the sulfonated calix[4]resorcinarene/surfactant complexes b proposed lamellar liquid crystal structures for the complexes c UV spectra of CalixB–DiC16 after exposure to NH3 and HCl vapor, inset shows expanded spectra between 275 and 400 nm. d Change in absorption at 310 nm, e Adsorption of methyl orange and methyl red into Calix B/DiC16 films and effect of ammonia vapor. Reproduced from [54] with permission of the Royal Society of Chemistry

This behavior was reversed by exposure to HCl vapor. These complexes are also capable of adsorbing indicator dyes such as methyl orange or red [54], with enhanced visual responses to ammonia vapor (Fig. 6e, f). The response to ammonia vapor is often a function of the deprotonation of the phenolic hydroxyls reacting with the basic ammonia. Therefore, it is logical that substituted amino compounds will also display similar responses. Amines are used in industrial and medical fields and therefore their monitoring is of importance. They are often the product of organic decomposition processes. A hydroxyazobenzenesubstituted calixarene adsorbed onto filter papers was used to detect amines released by decomposing fish [55]. During fish decomposition volatile amines such as ammonia, methyl- and dimethylamine were released, reacted with the hydroxyl on the azobenzene group and caused increases of adsorption at 500–510 nm. Different fish (cod and whiting) gave different responses and the sensitivity of the dye could be

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modified by binding lithium ions. Other workers bridged the 1,3-positions of t-butyl or phenylmethyl-substituted calixarenes with cystine containing units [56], with the resultant range of materials being able to be adsorbed as monolayers onto gold-coated QCM crystals and shown to respond to amine vapors. At low levels (c. 5 × 10−5 mol L−1 ), the responses to n-butyl amine were greater than for isobutyl amine and the least response was observed for t-butylamine; however, at higher levels (c. 2.5 × 10−4 mol L−1 ), these trends reversed. Calixarenes substituted on the upper rim with one or two nitrophenylazo substituents and the lower rim hydroxyl groups either unmodified or reacted to give benzoyl esters could be spin-coated onto glass substrates and showed color changes from yellowish to dark red on exposure to amines [57]. The highest sensitivities amongst the compounds tested were for hexylamine (600 ppm), more so than for shorter or branched amines. This is thought to be that at low concentrations the least hindered amine adsorbs fastest, whereas at higher concentrations amine polarity dominates. Ethylene diamine gave higher responses than ethylamine. The addition of a second nitrophenylazo group enhanced sensitivity, whereas substitution of the hydroxyl groups with benzoate units greatly lowered it. Three calixarenes could be cast onto acoustic wave sensors and their responses to the aromatic amine pyridine vapor determined [58]. Tetra-undecyl calix[4]resorcinarene was the most sensitive material, being capable of determining levels of pyridine with linear responses in the range 0.040–8.000 mg L−1 with a detection limit of 0.008 mg L−1 , although interferences from other organic compounds were noted. A bilayer system of 11-mercapto undecylamine/t-butyl calix[6]arene was adsorbed onto a gold-coated interdigitated transducer and used to detect propylamine with a detection limit of 60 ppm [59]. Composite Langmuir–Blodgett films made up of zinc or manganese porphyrins with a carboxylic acid-substituted calix[8]arene had less aggregation and had enhanced lifetimes compared to pure porphyrin LB films [60]. These composite films showed clear spectral changes upon exposure to several amines. Interestingly, opposite effects were observed, the zinc porphyrin adsorption displayed a blue shift, whereas the manganese porphyrin displayed a red shift. The zinc porphyrin displayed a higher response than the manganese, secondary amines gave higher responses than primary or tertiary and increasing amine size reduced response, perhaps indicating that larger amines have more difficulty penetrating the LB film. Other composites include cast films of single-walled carbon nanotubes with tetraundecyl calix[4]resorcinarene [61] to give a field effect transistor sensor capable of determining volatile amines. Ammonia, trimethylamine, and dimethylamine were all investigated; limits of detection were 0.6 ppm, 0.3 ppm and 0.4 ppm, respectively, with trimethylamine displaying the highest sensitivity. Although not sensing per se, recent work has demonstrated that amphiphilic calix[4]arene derivatives can be used as stationary phases within gas chromatography, being capable of separating isomers of toluidine [62].

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3.6 Organic Solvents Organic solvents are used in a variety of household, industrial, and medical environments. This leads to issues as many of these compounds are either flammable, can form explosive mixtures with air and have other undesirable medical and environmental effects. Detection and monitoring of these compounds is necessary in order to mitigate these effects. We have split these solvents into three groups for this review, chlorinated and aromatic solvents will be discussed later. Acetone has many industrial uses as well as being the most common solvent for nail varnish removal. The t-butyl calix[4, 6, and 8]arenes and tetraethyl calix[4]resorcinarene were deposited onto acoustic wave sensors and exposed them to 21 organic vapors [63]. The most sensitive material was the resorcinarene and it gave the best responses to acetone, with significant responses to 1,4-dioxane, acetonitrile and acetate esters. Much lower responses were observed for other hydrocarbons, chlorinated, and aromatic solvents. Acetone could be determined between 0 and 2280 ppm with detection limit of 1.25 ppm. X-ray crystallographic studies showed the resorcinarene formed a 1:1 complex with acetone. A calix[4]resorcinarene modified by a Mannich reaction (Fig. 7a) was deposited as an LB film on a QCM crystal [64]. This film gave responses to organic solvents, ethanol giving the strongest response. A urea and siloxane-modified calix[4]arene (Fig. 7b) formed a monolayer onto a glass slide via reaction of the siloxane group. When combined with a fluorescent pyrenyl or terthiophene dye this resultant system displayed a selective fluorescent response to tetrahydrofuran vapor, even in the presence of benzene or toluene vapors [65].

Fig. 7 Structures of a Mannich-substituted calix[4]resorcinarenes, b siloxane- and urea-modified calixarenes, and c aza-bridged calixarene, both for THF sensing (reproduced from [11] with permission from Springer). d structure of dicyclodipeptide-bearing calix[4]arenes and e differential responses to (R)- and (S)-methyl lactate at 303 K (reproduced from [75] with permission from Elsevier)

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Cast films of azocrown-bridged calix[4]arenes were used as fluorescent sensors for solvents [66] (Fig. 7c). Formation of J-aggregates of the calixarenes was observed in the film and again high selectivity for THF detection was observed, with large increases in fluorescence possibly due to disassembly of the aggregates. In another interesting work, a family of calixarenes bearing chiral dicyclodipeptide moieties (Fig. 7d) was synthesized and cast onto QCM crystals [75]. These calixarenes were shown to adsorb methyl lactate vapor; however; what is of interest is that due to the chirality of calixarene, there is discrimination between the two enantiomers of the adsorbent. Figure 7e demonstrates how the R-enantiomer of methyl lactate is adsorbed much more strongly than the S-enantiomer. As for detection of alcohol vapors, bridged calix[4]arenes were deposited onto QCM crystals, and a calixarene with a 1,3-conformation exhibited preference for linear alcohols, whereas branched alcohols gave lower responses [67]. This was thought by the authors to be a combination of hydrogen bonding and steric hindrance. Thin films of calix[4, 6, or 8]arene or C-tetra-pentyl calix[4]resorcinarene (Fig. 8a) could be deposited onto silicon substrates with these films leading to interference colors when illuminated by a light source [68]. Analysis of these films by a camera allowed determination of their red–green–blue components; these were found to change when the films were exposed to alcohol vapors. The resorcinarene film was the most sensitive and could distinguish between ethanol, propanol, and pentanol. The same group used the same compounds as sensing films combined with SPR detection [69]. The four calixarenes were thermally evaporated as 100-nm-thick films onto an SPR plate and imaged. The calixarene films showed differing responses upon exposure to ethanol vapor. A combination of the optical technique with QCM measurements for these macrocycles was employed to develop a model for ethanol adsorption [70]. Films were deposited by spin-coating, spraying, LB method, and thermal evaporation. The proposed model is that the calixarenes form thin porous films with nanocavities stochastically distributed throughout the film. Adsorption of ethanol is proposed to take place on the inside of these pores. The t-butyl calix[4]arene films appeared to be the least porous (~25% of the film is pore), which explains its corresponding low ethanol adsorption. Calix[6 and 8]arenes have pore fractions of ~36% and ~44%, respectively, and corresponding higher ethanol adsorption. The tetra-amyl calix[4]resorcinarene has a pore fraction of ~40%. Pores sizes vary, when modeled as spherical, the calix[6 and 8]arene and tetra-amyl calix[4]resorcinarene have pores with radii of 5.7 nm, 2.8 nm, and 2,4 nm respectively. The authors also point out that the structure could of course change as ethanol is adsorbed. The same three calixarenes along with C-tetraethyl calix[4]pyrogallene were deposited onto QCM crystals and shown to respond to ethanol vapor. Pyrogallene was the most sensitive with good adsorption and desorption kinetics, while X-ray crystal studies demonstrated formation of hydrogen-bonded complexes in the solid state [71]. QCM crystals were coated with tetra-C-ethyl resorcinarenes (Fig. 8b, c, d) or t-butylcalix[4]arene, with calix[4]resorcinarene (Fig. 8b) being the most sensitive with much stronger responses toward isopropanol compared to methanol and ethanol [72]. This is attributed to C–H–π bonding between the methyl groups of the isopropanol and the aromatic rings of the resorcinarene. Other organic compounds

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Fig. 8 Structures of some calix[4]resorcinarenes and calixarenes used for solvent detection. Figures 8m, n reproduced from [94]

have been examined as targets for calixarene-based sensors. A Japanese patent reports on QCMs coated with a tetra-allyl calixarene with esterified –OH groups combined with polydimethyl siloxane [73]. The resultant composite was shown to respond to acetone, toluene, and ethyl acetate vapors. QCM crystals coated with calixarenes were used to detect organic vapors, especially compounds with acidic C–H bonds such as nitromethane and acetonitrile [74].

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3.7 Chlorinated Compounds Adsorption of chlorinated vapors has been studied in dropcast and LB films of calixarenes and calix[4]resorcinarenes deposited onto QCM crystals [76]. Some of the calix[4]arenes displayed a high sensitivity for chloroform. Other vapors also had effects and, for mixed vapors, the response was often greater than for the same concentration of any of the components, possibly indicating that different guests are bound in different locations and manner by the calixarene matrix. Thin films (10–100 nm) of t-butylcalix[4 and 6]arene were deposited onto QCM and SAW devices [77]. Films of t-butylcalix[4]arene showed good response to few ppm levels of vapors including acetone, toluene, and halogenated compounds such as chloroform, perchloroethylene (used as a cleaning fluid), and methylene chloride with lower responses for carbon tetrachloride and methanol. Similar responses were observed for calix[6]arene but with higher responses for toluene, especially over saturated compounds such as cyclohexane. The same group also utilized vase-shaped substituted resorcinarenes (such as in Fig. 8e) spin-coated onto SAW devices. These were exposed to chlorinated and aromatic compounds and showed especially high affinities, depending on the sensing moiety used, toward perchloroethylene, xylenes, and pyridine [78]. Levels down to 2.5 ppm could be ascertained. Due to its toxic, carcinogenic and anesthetic properties, chloroform is one target of interest. The group in Baleiksir has undertaken studies of gas sensing properties of calixarenes and calixresorcinarenes. In an attempt to deduce the effect of cavity size, LB films of t-butylcalix[4, 6, and 8]arene were deposited onto QCM crystals. The films were then exposed to 20–100% saturated chloroform, toluene, benzene, and ethanol vapors [79]. In all cases, chloroform gave the highest response and ethanol the lowest. Flushing with dry air restored the original baseline. The calix[8]arene gave the highest and fastest response (a few seconds), possibly because of its larger cavity size leading to greater porosity within the film. The same group also studied calix[4]arenes bearing 0, 2 (in the 1,3-positions), 3 or 4 t-butyl groups at the upper rim [80]. These displayed different isotherms at the air-water interface, the presence of the t-butyl groups increasing the surface area per molecule as expected. On exposure to chloroform (20–100% saturated vapor) the calixarene bearing three t-butyl groups gave a higher response than that bearing two, with the tetra and unsubstituted calixarenes having almost the same lower response. Again, benzene, toluene, and ethanol all gave lower responses than chloroform with the tri-substituted calixarene demonstrating the best selectivity for chloroform. Similar studies were undertaken using calix[6arene with either six or no t-butyl substituents at the upper rim, both these compounds could be deposited as LB films onto QCM, although transfer ratios declined as the films became thicker [81]. Both films responded to 20–100% saturated vapors of chloroform, benzene, toluene, and ethanol, again with chloroform giving the strongest response. Both calixarenes gave rapid, reproducible QCM responses with the unsubstituted calix[6]arene giving a better response than the substituted one, possibly due to easier access of the solvent to the cavity. Theoretical molecular dynamics calculation studies [82] on the

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complexation of p-chloro-trifluoromethyl-benzene with t-butyl and unsubstituted calix[4]arene indicate that formation of complexes is tens of times faster for the unsubstituted calix[4]arene. Substituting the –OH group with a short alkyl carboxylic acid chain was also possible [83] with the resultant calix[8]arene forming LB films. Deposition onto SPR chips allowed its sensing abilities to be determined, once again displaying a large, reversible effect upon exposure to chloroform and lower effects for benzene, toluene, and ethanol. As an alternative to the LB method, plasma deposition (0.6 mbar operating pressure, 2.0 kV DC, and 19 kV pulsed voltages) could be utilized to form thin (several nm) films of the same carboxylic acid-substituted calixarenes above as well as tetra-undecyl calix[4]resorcinarene [84]. UV spectra confirmed the chemical structure had not been overly altered by the deposition process. QCM and SPR studies confirmed both films again showing fast, reproducible, and reversible responses to chloroform. An amphiphilic calix[4]resorcinarene bearing phenyl substituents on the linking groups and substituted with cationic ammonium groups in the resorcinol 2-position (Fig. 8f) could be successfully deposited as an LB film onto QCM and SPR devices [85]. SPR data gave a thickness per layer of 1.14 nm and a refractive index of 1.6. These films showed fast and recoverable responses to the organic vapors mentioned above and again gave much larger responses for chloroform. Recent work demonstrated that similar calix[4]resorcinarene bearing uncharged diethylamino substituents (Fig. 8g) could be deposited using the LB technique to give films with thickness values of 1.14 nm and refractive indices of 1.6–1.9 depending on film thickness, with these films also being shown by SPR to be highly sensitive and selective for chloroform [86]. LB films of a calix[4]resorcinarene with undecyl substituents on the linking groups and ammonium groups in the resorcinol 2-position (Fig. 10h) were used in SPR studies [87]. These films had similar properties (layer thickness 1.04 nm, refractive index 1.4) and a rapid, reversible response to chloroform, with smaller response toward benzene, toluene and ethanol. Spin-coated calixarenes modified using phosphonic acids at the upper rim and bridging aza-crown ethers at the lower rim (Fig. 8i, j, k) displayed a high preference for chlorinated solvents, especially chloroform over benzene, toluene, or ethanol [88]. The calixarene shown in Fig. 8k displayed the highest sensitivity. Temel et al. employed calix[4]arenes with different lower and upper rim substituents which were dropcast onto QCM crystals [89]. These films were sensitive to many solvents (acetone, acetonitrile, carbon tetrachloride, chloroform, dichloromethane, DMF, 1,4-dioxane, ethanol, ethyl acetate, xylene, methanol, nhexane, and toluene). The highest responses were obtained for dichloromethane with other chlorinated solvents such as chloroform and carbon tetrachloride showing significant responses to acetone, hexane, and toluene. The highest sensitivity was shown by a calix[4]arene bearing both amino and imidazole groups which had a detection limit of 54.1 ppm for dichloromethane. Composite materials made from gold nanoparticles and calixarenes were deposited using the Langmuir–Schaeffer method and shown by SPR to respond to organic vapors with a preference for dichloromethane [90] and high sensitivity and reversibility. In another study,

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calix[4]resorcinarene-based cavitands were synthesized which were substituted with alkyl sulfide groups (such as in Fig. 8l) [91]. These could be deposited onto gold surfaces using the Au–S interaction and were shown by thermal desorption experiments to bind organic guests with a very high selectivity for perchloroethylene.

3.8 Aromatic Compounds Since calixarenes are based on aromatic macrocycles, it appears obvious that they will show high affinity for organic solvents such as BTEX (benzene, toluene, ethyl benzene, and xylene) and other aromatic compounds. Aromatic compounds such as these are used within the chemical and petrochemical industry, are highly flammable and can have highly adverse health and environmental effects. Selectivity is an issue in that it proves very difficult to distinguish one aromatic compound from another, although the presence of any of them usually indicates that there is an issue. We have already mentioned [76] work which showed sensitivity of LB films of calixarenes to organic compounds which included toluene, with the best responses coming from calix[6 and 8]arenes compared to calix[4]arene and resorcinarenes. Likewise, a Mannich-substituted calix[4]resorcinarene (Fig. 9a) was sensitive to

Fig. 9 a Schematic illustration of the experimental setup used for single LPG and LPG arrays (reproduced from [100]), b dynamic change of CA[4] sensor during the spray paint experiment (reproduced from [99]), and c structure of the calixarenes used in this work (n = 4 or 8)

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ethanol and also shown to respond to benzene, toluene, ethyl benzene, and cumene [64]. Cast and LB films of t-butylcalix[6]arene could be deposited onto QCM crystals and shown to respond to aromatic benzene derivatives [92]. The LB films gave higher collapse pressures and better deposition when the subphase pH was increased from 6.5 to 10.5. Response to vapors increased as pH increased from 6.5 to 9.5 due to better deposition; however, response at 10.5 decreased, possibly due to tighter packing of calixarene molecules. Cast films were more sensitive than LB films, probably because they had a more open porous structure. Calix[4]arenes substituted with 2-ethylhexyl groups at the lower rim and 3-mercaptopropyl groups at the upper rim could be assembled onto the surface of gold nanoparticles using the strong Au– S interaction [93] with an average of 29 calixarenes on each nanoparticle (average diameter 2.1 nm). These composites could be spin-coated from THF onto QCM crystals and were shown to give good responses to toluene vapor. Two azobenzene-substituted calix[4]resorcinarenes (Fig. 8m, n) could be spincoated onto gold-coated glass slides and examined by SPR [94]. Typical films thicknesses were 12 nm with the films being shown to respond to benzene vapor. Compound 8n was the most sensitive and gave linear responses to benzene from 0 to 400 ppm. LB films of two calix[4]resorcinarenes substituted with either phosphonate or azo-groups could be deposited onto QCM crystals or gold-coated slides (for SPR) [95]. Samples were exposed to benzene, toluene, and other vapors and both study methods demonstrated that adsorption depended on the condensed vapor pressures of the adsorbates rather than their structure. This was interpreted by assuming that the vapors undergo capillary condensation within the nanoporous matrix of the films, leading to film swelling and changes in its refractive index. In another piece of work, an LB film of t-octylcalix[8]arene substituted with 3-aminoproyl groups at the lower rim was shown to respond to all of the BTEX vapors [96]. SPR studies showed that the films exhibited swelling behavior and changes in refractive index on vapor adsorption. The adsorption kinetics appeared to follow Fick’s second law and higher sensitivities were observed than for the calix[4]resorcinarenes previously mentioned. Benzene was shown to penetrate the calixarene matrix faster than its substituted analogues. Recent work has discussed the deposition of calixarene films on fiber-optic gratings. Laser irradiation can be used to construct grating within fiber-optic cables. Long-period gratings (LPG) can be constructed by irradiating an optical fiber with a UV laser, causing periodic variations of refractive index within the cable (Fig. 9). These then have a transmission spectrum consisting of a series of resonance bands, the intensity of which can depend on coupling between the fiber-optic core and its immediate environment. If the core is coated with a film, changes in thickness and refractive index will affect this spectrum. An LPG was coated with an LB film of tetra-undecyl calix[4]resorcinarene and then its spectrum interrogated [97]. Exposure to organic vapors causes changes in refractive index allowing their determination. Toluene and benzene gave greater responses than cyclohexane or hexane. Levels of toluene down to 231 ppm could be determined.

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Alternating layer-by-layer mesoporous films of anionic silica nanoparticles/cationic poly(allylamine hydrochloride) could be deposited onto an LPG (5 layers of each) [98]. These films were then infused by soaking in aqueous solutions of anionic p-sulfonato calix[4 or 8]arene. The composite films were placed in a chamber with a piece of freshly painted cardboard. Volatile compounds given off by the paint could be detected by the sensor, steady state was achieved after 30 min and reversed to baseline values 3 min after exposure to clean air. The calix[4]arene gave a higher response than the calix[8]arene. The same system was used with the LPG designed to operate at its phase matching turning point to provide the highest sensitivity [99]. Again, sensitivity was shown toward the organic vapors described above with toluene giving the best response as well as for paint VOCs (Fig. 9b). Different responses were found for the two sensors when exposed to can paint and spray paint, indicating differing sensitivities to different aliphatic and aromatic VOCs in the paint samples. Further work employed three gratings etched into a single optical fiber, one capable of determining temperature, one to determine humidity, and one as above to determine the presence of volatile organic compounds [100], with the temperature and humidity sensors notably giving responses comparable with commercial sensors. The calix[8]arene-based volatile organic compound sensor could detect chloroform, toluene, benzene, and acetone vapors with toluene giving the highest response.

3.9 Other Compounds Inert gases are among the most difficult species to detect. However, calixarenes have been shown to be capable of adsorbing such gases. Polymeric calix[4 and 6]arenes could be deposited onto QCM crystals and shown to respond to the anesthetic noble gas xenon [101]. Both calixarenes and calixarene substituted polyurethanes gave responses to xenon pulses, the best being a permethylated t-butylcalix[4]arene which demonstrated rapid responses and excellent reversibility. A second uncoated QCM could be used to subtract out effects of temperature and humidity. Thiacalix[4]arenes, where the bridging methylene group is replaced by a sulfur atom, were shown to form complexes with cobalt atoms [102]. When combined with chiral camphoric acids, these systems formed a variety of architectures such as grids, 2D polymers, or cages. The cage form strongly adsorbed nitrogen gas, whereas the other two forms showed minimal adsorption. Many other materials have been detected using calixarenes. A surface acoustic wave detector modified with a calix[6]arene was used to determine anisole vapor [103], with good recovery, reproducibility, and stability. A German patent describes the incorporation of selective porous substances including calixarenes into polymer membranes such as Viton or silicone [104]. These selectively adsorbed the analyte and can then be heated to release it allowing its detection at a gas sensor. A Japanese patent describes the use of calixarenes substituted with p-sulfonate groups modified as alkyl ammonium salts [105], these were coated as a thin film onto electrical

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gas sensors and could detect toxic volatile organic compounds. Other workers [106] employed calixarenes and resorcinarenes for their sensitivity to vapors (aromatics, chlororganics, ketones, and alcohols) and studied the effects on sensitivity of the various hosts. For almost all the analytes, the most effective host was t-butylcalix[8]arene where the hydroxyls had been substituted with diethoxyphosphoryl groups. Organic field effect transistors were fabricated to determine pesticide levels [107]. A field effect transistor contained a dielectric layer of t-butylcalix[6]arene, spin-coated as a 50-nm-thick layer on Si/SiO2 and then coated with a thin film of 6,13-bis(triisopropylsilylethynyl]pentacene. This system responded to methylparaben vapor (which is believed to bind to the calixarene) with drops of up to 60% in drain current in 1 min. Levels of the pesticide as low as 1 ppb could be detected.

4 Calixarene Analogues Calixarenes are of course not the only cavitand-type molecules suitable for gas sensing. In fact, many other molecules of similar porous structure are suitable for gas and vapor sensing. Although a detailed description of these systems is outside of the scope of this chapter, a brief mention will be given. Gas sensing using a variety of cavitands has been recently reviewed [108]. Pillarenes are a variation on calixarene-type structure in that they are based on hydroquinone-type units rather than phenol or resorcinol (Fig. 10a). The cyclic pentamer and hexamer (n = 5 or 6) are the most common forms. These again display a cavitand-type structure and have several applications [109]. For example, amine substituted pillarenes could be dissolved in water and underwent reversible interactions with carbon dioxide, with a transformation from vesicles to micelles when bubbled with CO2 [110]. Cyclodextrins are cyclic polysugars which occur in various sizes, but most commonly have 6, 7, or 8 sugar units within the macrocycle (the cyclic hexamer is shown in Fig. 10b). These adopt a three-dimensional vase-like structure which can encapsulate a variety of guests [8]. Initial work was on solution-based binding; however, it proved possible to modify the cyclodextrins to make them more hydrophobic and then brush solutions of them onto QCM crystals. This allowed construction of sensors capable of detecting benzene vapor in a range of 0.08–400 mg L−1 [111]. An electronic nose was also developed with reduced graphene oxide which had been modified with a range of chemically substituted cyclodextrins using pyrenyl adamantine as a linker group [112]. The resultant array could detect aromatic vapors which are markers for lung cancer. Chromatropic acid can be condensed with formaldehyde to give a cyclic tetramer (Fig. 10c) [113]. This molecule can be self-assembled into layer-by-layer thin films along with a cationic polymer and has been shown to undergo strong color changes upon exposure to ammonia gas or solution as well as n-butylamine vapor. Cyclotriveratylenes are cyclic trimers based on substituted 1,2-hydroxybenzene [8]. They are shallow dish-shaped molecules which tend not to show as high a binding affinity

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Fig. 10 Structure of a a pillar(5]arene, b a cyclodextrin, c cyclotetrachromotropylene, d a cryptophane, and e a cucurbituril

as calixarenes. However, one interesting series of compounds is the cryptophanes, made by coupling together two cyclotriveratylenes as shown in Fig. 10d. Since a detailed discussion of these compounds is outside the field of this chapter, only a few examples will be given. Early work used films of a carboxylic acid substituted cryptophane [114] coated onto a QCM, with responses to ammonia vapor from 10 to 200 ppm and minimal interference from H2 S, CO2 , CH4 , or N2 O. However, results could be affected by temperature and humidity. One of the simplest cryptophanes is the so-called cryptophane A (Fig. 10 R = OCH3 ). When deposited as a mixed LB film with phospholipids or porphyrins onto a piezoelectric substrate [115], this system had strong specific

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responses to methane. In [116], an evanescent wave optical fiber was modified with a polymeric cladding containing cryptophanes, thus obtaining specific responses to methane from 2 to 8%. Much work has been done on the incorporation of xenon into cryptophanes [8]. The interest in this is because xenon can be hyperpolarized, where over 50% of the molecules exist in a high-spin state, making it suitable for magnetic imaging and for studying gas flow in lungs [117]. Cryptophane A forms strong complexes with xenon in solution and the solid phase, meaning it could possibly be used as a sensor for this gas [118]. Cucurbiturils (Fig. 10e) are another family of macrocyclic species based on the condensation reaction between urea, glyoxal, and formaldehyde. The hexamer is usually the most common product but pentamers heptamers and octamers can also be easily synthesized. The synthesis and structure of these materials has already been reviewed [8] for their use in molecular recognition [119]. Cucurbiturils were capable of adsorption of a variety of gases within their porous structure with the smaller cucurbiturils being the most effective. It should be stressed that not a great deal of work has been done on these systems as sensors, possibly due to their much lower solubility than other macrocycles [119]. Two materials that could be adsorbed into the hexamer were sulfur hexafluoride [120] with a very high association constant (31,000 M−1 ) from aqueous solution and acetylene with the solid cyclic hexamer being capable of adsorbing 6.1% of its own weight of acetylene vapor [121]. A fluorescent dye/cucurbituril hexamer complex in aqueous solution was shown to bind hydrocarbons bubbled from the gas phase through the solution [122], leading to displacement of the dye and a decrease in fluorescence. Binding constants for linear and branched hydrocarbons were obtained.

5 Conclusions Within this chapter, we have discussed a wide range of calixarenes and similar structures for gas and vapor sensing. However, yet commercial devices using these materials have not been developed and several issues need to be addressed before they can come to market. Suitable transducer methods which convert the binding event to an easily measurable signal need to be developed. These exist under laboratory conditions but some of them such as SPR may be too expensive for commercial devices. QCM and electrical sensors may be more economical. Any sensors must be robust to environmental conditions, for example, they must be unaffected by changes in temperature and humidity as well as vibration. One method of doing this is to utilize two sensors, one modified with the sensing coating and one not. The sensing material must also not deteriorate over time. Fortunately, many calixarenes are highly stable chemically so this may not be an issue. For monitoring environmental conditions over time, the sensors must be easily regenerable and not require any special treatment after detection of absolute. Many of the calixarenes are suitable since just removing the analyte is enough, the binding event is spontaneously reversible, and the sensor can undergo many detection events

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without a decrease in performance. The two main issues are sensitivity and selectivity. In the case of sensitivity, this does depend somewhat on the application. Sensors can be used just to give notice of an incident such as a spillage or leakage, these often lead to localized high levels of a dangerous substance and are more easily detected. However, for constant environmental monitoring, the levels required to be measured are much lower, often defined at ppm levels by the World Health Organization. In the case of many of the systems described above, these levels have not been attained so far. As for selectivity, again in the case of spills this is less of an issue as there will be high levels of analyte and only background levels of potential interferents. For environmental monitoring, the system is much more problematical. The calixarene sensing material needs to ideally only bind to the chosen analyte and nothing else, but this can be difficult to achieve. Since many of the calixarenes form nanoporous structures capable of encapsulating many compounds, especially organic vapors, distinguishing between them is difficult. Sometimes just being able to detect a family of materials will be enough. For example, all the BTEX vapors are damaging so a sensor which responds to all the compounds would be suitable, especially in a location such as an oil refinery, there may be no need to distinguish between say benzene and toluene. Another possible approach is to have an array of differing sensors with different binding characteristics combined with pattern recognition technology to attempt to measure levels of more than one analyte simultaneously. With all these issues to be addressed, it is obvious that using these systems for selective detection of low levels of environmental contaminants is not yet attainable. However, simpler systems to detect spillage and leakage events are feasible. The binding abilities, chemical and physical stability and ability to be used as a porous platform to attach highly specific chemical-binding agents means that these systems do show the potential for being used as the basis of low-cost, sensitive, and specific sensors for a range of environmental pollutants and other gases and vapors. Acknowledgements This work was carried out with financial assistance from the Brazilian funding agencies: São Paulo Research Foundation—FAPESP (2013/14262-7) and National Council for Scientific and Technological Development—CNPq.

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