Nanocarbon and its composites : preparation, properties and applications [First edition] 9780081025109, 0081025106, 9780081025093

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Nanocarbon and its composites : preparation, properties and applications [First edition]
 9780081025109, 0081025106, 9780081025093

Table of contents :
Content: 1. Nanocarbon aerogel composites 2. Highly active and reusable nanocomposites for hydrogen generation 3. Carbon foam preparation and applications 4. Electrospun polymeric nanocarbons nanomats for tissue engineering 5. Graphene and polymer composites for supercapacitor applications 6. Interface engineering in nanocarbon - metal oxide hybrid and their applications 7. Nanocarbon: preparation and performance evaluation for EMI shidling, cloak, spintronics, and hybrid capacitor applications 8. Nanocarbon and biofeature 9. Nanocarbon and its composites: preparation, properties and applications 10. Nanocarbon as electrode materials for supercapacitor 11. Nanocarbon composite for poisonous gas degradation 12. Nanocarbon composites for detection of volatile organic compounds 13. Nanocarbon epoxy composites preparation properties and applications 14. Nanocarbon tri-component composites with synthetic fibers 15. Preparation and properties of fibrous nanocarbon 16. Preparation and properties of manipulated carbon nanotubes composites and applications 17. Recent advances of nanocarbon and its composites in photocatalysis 18. Synthesis of Nanocarbon Polyaniline Composite and Investigation of its Optical and Electrical Properties 19. Monodisperse PVP stabilized Nanoclusters as Highly Efficient and Reus able Catalyst for the Dehydrogenation of Dimethyl Ammonia- Borane (DMAB) 20. Nanocarbon supported catalysts for the efficient dehydrogenation of dimethylamino borane 21. Nanographene composition exchanger properties and applications 22. Carbon dots: preparation properties and application 23. Phthalocyanine-Nanocarbon Composites: Preparation, Properties and Applications 24. Nanocarbon and its composite for water purification 25. Ultrasonic tr tment in the production of classical composites and carbon nanocomposite 26. Nanocarbon material-filled cementitious composites for construction applications 27. Synthesis, Properties & Characterization of Carbon Nanotube Reinforced Metal Matrix Composites

Citation preview

Nanocarbon and its Composites

Woodhead Publishing Series in Composites Science and Engineering

Nanocarbon and its Composites Preparation, Properties, and Applications

Edited by

Anish Khan, Mohammad Jawaid, Inamuddin & Abdullah Mohamed Asiri

An imprint of Elsevier

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom © 2019 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-08-102509-3 (print) ISBN: 978-0-08-102510-9 (online) For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Gwen Jones Editorial Project Manager: Ali Afzal-Khan Production Project Manager: Swapna Srinivasan Cover Designer: Victoria Pearson Typeset by SPi Global, India

Dedication

The editors are honored to dedicate this book to the family members of Dr. Anish Khan

List of contributors

Mohamed Shaaban Abdel-wahab Hassan Center of Nanotechnology, King Abdulaziz University, Jeddah, Saudi Arabia Jaideep Adhikari Dr. M.N. Dastur School of Materials Science and Engineering, Indian Institute of Engineering Science and Technology, Howrah, India Akil Ahmad Department of Chemical Engineering, Howard College Campus, University of KwaZulu-Natal, Durban, South Africa; School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia Md Khursheed Akram Applied Sciences and Humanities Section, University Polytechnic, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi, India Burcu Akyıldız Sen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar University, K€ utahya, Turkey Elim Albiter Catalysis and Materials Laboratory, ESIQIE-National Politecnic Institute, Zacatenco, Mexico Ashwini P. Alegaonkar Department of Chemistry, Savitribai Phule Pune University (formerly Pune University), Pune, India Prashant S. Alegaonkar Department of Applied Physics, Defence Institute of Advanced Technology (DIAT), Pune, India Othman Y. Alothman Department of Chemical Engineering, College of Engineering, King Saud University, Riyadh, Saudi Arabia Ahmed Alshahrie Center of Nanotechnology; Physics Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia Sandro Campos Amico Materials Engineering Department, School of Engineering, Federal University of Rio Grande, Porto Alegre, Brazil Mohammad Omaish Ansari Center of Nanotechnology, King Abdulaziz University, Jeddah, Saudi Arabia

xvi

List of contributors

Shahid Pervez Ansari Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India Afzal Ansari Department of Applied Sciences and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi, India Mohammad Asad Chemistry Department, Faculty of Science, King Abdulaziz University; Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia Abdullah Mohamed Asiri Chemistry Department, Faculty of Science, King Abdulaziz University; Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia Ayşenur Ayg€ un Sen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar University, K€ utahya, Turkey Busra Balli Sen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar University, K€ utahya, Turkey Mohamed Abou El-Fetough Barakat Department of Environmental Sciences, Environment and Arid Land Agriculture, King Abdulaziz University, Jeddah, Saudi Arabia; Central Metallurgical R&D Institute, Cairo, Egypt Jose M. Barrera-Andrade Catalysis and Materials Laboratory, ESIQIE-National Politecnic Institute, Zacatenco, Mexico Mehmet Durmus Calisir TEMAG Labs; Department of Nanoscience and Nanoengineering, Istanbul Technical University, Istanbul; Department of Nanotechnology Engineering, Recep Tayyip Erdogan University, Rize, Turkey Christian Matheus dos Santos Cougo Materials Engineering Department, School of Engineering, Federal University of Rio Grande, Porto Alegre, Brazil Ana Karina Cuentas-Gallegos Instituto de Energı´as Renovables, Universidad Nacional Auto´noma de Mexico, Temixco, Mexico Agnieszka Da˛browska University of Warsaw, Faculty of Chemistry, Laboratory of Molecular Interactions, Warsaw, Poland Anindya Das Dr. M.N. Dastur School of Materials Science and Engineering, Indian Institute of Engineering Science and Technology, Howrah, India Erhan Demirbaş Gebze Technical University, Department of Chemistry, Gebze, Turkey

List of contributors

xvii

Buse Demirkan Sen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar University, K€ utahya, Turkey Mahmut Durmuş Gebze Technical University, Department of Chemistry, Gebze, Turkey Nestor David Espinosa-Torres Instituto de Energı´as Renovables, Universidad Nacional Auto´noma de Mexico, Temixco, Mexico Hasan Fouad Department of Biomedical Engineering, Helwan University, Cairo, Egypt Alfredo Guillen-Lo´pez Instituto de Energı´as Renovables, Universidad Nacional Auto´noma de Mexico, Temixco, Mexico Baoguo Han School of Civil Engineering, Dalian University of Technology, Dalian, China Mohammad Jawaid Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Selangor, Malaysia; Department of Chemical Engineering, College of Engineering, King Saud University, Riyadh, Saudi Arabia Kathiresan Marimuthu Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai, Tamil Nadu, India Anish Khan Chemistry Department, Faculty of Science, King Abdulaziz University; Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia Imran Khan Department of Applied Sciences and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi; Applied Sciences and Humanities Section, University Polytechnic, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India Aftab Aslam Parwaz Khan Chemistry Department, Faculty of Science, King Abdulaziz University; Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia Asma Khatoon CLEAR, Ibnu Sina Institute for Industrial and Scientific Research, UTM, Skudai, Malaysia Ali Kilic TEMAG Labs, Istanbul Technical University, Istanbul, Turkey

xviii

List of contributors

Aleksandr Evhenovych Kolosov Chemical, Polymeric and Silicate Machine Building, Department of Chemical Engineering Faculty, National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute», Kyiv, Ukraine Elena Petryvna Kolosova National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute», Kyiv, Ukraine Ramar Kumar Department of Environmental Sciences, Environment and Arid Land Agriculture, King Abdulaziz University, Jeddah, Saudi Arabia R. Kumar Department of Mechanical Engineering, Vels Institute of Science, Technology & Advanced Studies, Pallavaram, Chennai, India Esra Kuyuldar Sen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar University, K€ utahya, Turkey David Lokhat Department of Chemical Engineering, Howard College Campus, University of KwaZulu-Natal, Durban, South Africa Manoj Balachandran Department of Physics and Electronics, Christ (Deemed to be University), Bengaluru, India L.M. Mejı´a-Mendoza Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, United States Jesu´s Mun˜iz Instituto de Energı´as Renovables, Universidad Nacional Auto´noma de Mexico; CONACYT-Universidad Nacional Auto´noma de Mexico, Temixco, Mexico Nur Dilara Ozturk TEMAG Labs; Department of Nanoscience and Nanoengineering, Istanbul Technical University, Istanbul, Turkey Wagner Mauricio Pachekoski Mobility Engineering Department, Federal University of Santa Catarina, Campus Joinville, Joinville, Brazil Sergio Henrique Pezzin Chemistry Department, Center for Technological Sciences, State University of Santa Catarina, Joinville, Brazil Mohd. Rafatullah School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia Rajesh Jesudoss Hynes Navasingh Department of Mechanical Engineering, Mepco Schlenk Engineering College (Autonomous), Sivakasi, Tamil Nadu, India

List of contributors

xix

Raghavan Baby Rakhi Chemical Sciences and Technology Division, CSIR–National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Kerala, India Miguel Robles Instituto de Energı´as Renovables, Universidad Nacional Auto´noma de Mexico, Temixco, Mexico Elizabeth Rojas-Garcı´a Catalysis and Materials Laboratory, ESIQIE-National Politecnic Institute, Zacatenco, Mexico Yanfeng Ruan School of Civil Engineering, Dalian University of Technology, Dalian, China Naheed Saba Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Selangor, Malaysia Selin Sagbas Faculty of Science and Arts, Chemistry Department, Canakkale Onsekiz Mart University, Canakkale, Turkey Prosenjit Saha Dr. M.N. Dastur School of Materials Science and Engineering, Indian Institute of Engineering Science and Technology, Howrah, India Nurettin Sahiner Faculty of Science and Arts, Chemistry Department; Nanoscience and Technology Research and Application Center (NANORAC), Canakkale Onsekiz Mart University, Canakkale, Turkey Sabyasachi Sarkar Nanoscience and Synthetic Leaf Laboratory at Downing Hall, Center for Healthcare Science and Technology, Indian Institute of Engineering Science and Technology, Shibpur, India Aysun Şavk Sen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar University, K€ utahya, Turkey Bet€ ul Şen Sen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar University, K€ utahya, Turkey € Ozde Şen Sen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar University, K€ utahya, Turkey Fatih Şen Sen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar University, K€ utahya, Turkey Ahmet Şenocak Gebze Technical University, Department of Chemistry, Gebze, Turkey

xx

List of contributors

Senthamaraikannan Planichamy Department of Mechanical Engineering, Kamaraj College of Engineering and Technology, Virudhunagar, India Siti Hamidah Mohd Setapar CLEAR, Ibnu Sina Institute for Industrial and Scientific Research, UTM, Skudai, Malaysia Hiroyuki Shima Department of Environmental Sciences, University of Yamanashi, Kofu, Japan Vasi Uddin Siddiqui Department of Applied Sciences and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi, India Weqar Ahmad Siddiqui Department of Applied Sciences and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi, India Sumit Kumar Sonkar Department of Chemistry, Malaviya National Institute of Technology, Jaipur, India Elena Stojanovska TEMAG Labs, Istanbul Technical University, Istanbul, Turkey Yoshiyuki Suda Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, Toyohashi, Japan Miguel A. Valenzuela Catalysis and Materials Laboratory, ESIQIE-National Politecnic Institute, Zacatenco, Mexico Volodymyr Volodymyrovych Vanin National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute», Kyiv, Ukraine Jialiang Wang School of Civil Engineering, Dalian University of Technology, Dalian, China Danna Wang School of Civil Engineering, Dalian University of Technology, Dalian, China Xun Yu Department of Mechanical Engineering, New York Institute of Technology, New York, NY, United States; School of Mechanical Engineering, Wuhan University of Science and Technology, Wuhan, China Wei Zhang School of Civil Engineering, Dalian University of Technology, Dalian, China

Preface

Nanocarbons are a growing field of interest for designing next-generation functional materials for quantitative and qualitative, chemical and biological moieties as well as having implications for medicine, the environment, and energy. The field of carbon nanotechnology, where scientists have remarkable knowledge about how to manipulate atoms to achieve desirable properties and applications, has boomed in the present scientific world. As we know that atoms have unique properties like magnets to attract negative to positive, that behavior of the atom benefits making a machine to use one atom possible at once involving least numbers of the atoms. Nanocarbons are very promising due to their unique features and highly tailorable combination properties such as high porosity, large specific surface area (greater interaction zone), chemical inertness, radiation stability, high electrical conductivity (fast electron transfer), high toughness, and greater modulation properties. It was the result of controlled and vital nanocarbon manipulation to develop and create the nanomachine. This book gives a broad updated survey and information on major innovations in the field of nanocarbons and their composite preparations, properties, and applications, particularly in broadway. It also provides a reference material for future research in carbon-based materials, which are much in demand due to the sustainable, recyclable, and eco-friendly methods for highly innovative and applied materials. This book aims to cover a wide aspect of properties and applications of the carbon base. The chapters provide cutting-edge, up-to-date research findings on the use of carbon-based materials, different applications to achieve the material’s characteristics, and significant enhancements in physical, chemical, mechanical, and thermal properties. Recent and up-to-date topics of nanocarbon composites, namely, nanocarbon aerogels, nanocarbon for hydrogen generation, carbon-based foams, nanocarbon for tissue engineering, nanocarbon in a supercapacitor, structural engineering and spintronics, nanocarbons in agriculture, nanocarbons in sensors, fibrous nanocarbons, nanohybrids and photocatalysts, ion-exchange, carbon dots, nanocarbons for water purification, and nanocarbons for construction applications are covered in this book. We hope that these chapters contributed by experts in the nanocarbon field will provide a positive stimulus for the engineers, researchers, and educators in different parts of the globe. We are highly thankful to all the authors from different parts of world that contributed chapters in this edited book and supported it by providing valuable ideas and knowledge. Our appreciation goes out to all the authors for their excellent

xxii

Preface

contributions as well as their acceptance of editorial suggestions to help shape this book. We are also grateful to the Elsevier, United Kingdom, support team, especially Simon Holt, Ali Afzal-Khan, and Swapna Srinivasan for helping us finalize this book. Anish Khan Jeddah, Saudi Arabia Mohammad Jawaid Serdang, Malaysia Inanmuddin Jeddah, Saudi Arabia Abdullah Mohamed Asiri Jeddah, Saudi Arabia

Nanocarbon aerogel composites

1

Mohammad Omaish Ansari*, Rajeev Kumar†, Shahid Pervez Ansari‡, Mohamed Shaaban Abdel-wahab Hassan*, Ahmed Alshahrie*,§, Mohamed Abou El-Fetough Barakat†,¶ *Center of Nanotechnology, King Abdulaziz University, Jeddah, Saudi Arabia, † Department of Environmental Sciences, Environment and Arid Land Agriculture, King Abdulaziz University, Jeddah, Saudi Arabia, ‡Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India, §Physics Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia, ¶ Central Metallurgical R&D Institute, Cairo, Egypt

Chapter Outline 1.1 Introduction 1 1.2 Different types of nanocarbon aerogels 1.2.1 1.2.2 1.2.3 1.2.4

3

CNT aerogels 3 Graphene aerogels 3 CNT and graphene aerogels composite 4 Nanodiamond based aerogels 7

1.3 Nanocarbon aerogels for energy storage applications 7 1.4 Nanocarbon aerogel adsorbents for wastewater remediation 11 1.5 Nanocarbon aerogel photocatalyst for wastewater remediation 12 1.6 Nanocarbon aerogels as sensors 16 1.7 Conclusion and future research 19 References 20

1.1

Introduction

Aerogels are a special class of materials that has been used for space travel since the 1960s, but nowadays they are finding interesting applications in all industrial sectors [1]. Aerogels possess a specific geometrical structure and thus are not a specific mineral or material with a set chemical formula [2]. These are extremely branched materials and the structure is solid foam with high porosity and connectivity. Due to this high connectivity, it can take many different shapes and forms. Until now the majority of the works on aerogels have been done on silica-based materials, but other materials such as semiconducting metal oxides, polymers, noble metals, etc., have also been found to form aerogels [3–5]. Aerogels consist of very little solid material and internally the structure is nothing but air. This unique structural composition gives it a ghostly appearance, and hence it is also called frozen smoke [6]. Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00001-8 © 2019 Elsevier Ltd. All rights reserved.

2

Nanocarbon and its Composites

Kistler first introduced the concept of aerogels by using supercritical drying conditions to remove the liquid by air in a wet gel; his work was published in the journal Nature in 1931 in just a half-page [7]. He showed that, at a certain critical point, the liquid phase can be completely removed without disturbing the structure and the formation of liquid-vapor interfaces. Also, the capillary forces that subsequently collapse the drying gel to xerogel can be avoided. Due to this supercritical drying condition, the aerogel retains high porosity, low density, and high surface area [8]. The Kistler method of preparing aerogels was cumbersome, and this field largely went uninvestigated until the 1960s when Teichner and Nicolaon prepared aerogels by what is now commonly known as the sol-gel process. The technique developed by Teichner and Nicolaon eliminated the time-consuming salt removal and solvent-exchange steps. They succeeded in producing silica aerogels by using silicon alkoxide and thus trigging the aerogel synthesis in hours rather than days [9]. This advancement in technique triggered research in the synthesis and design of a wide variety of simple and complex aerogel structures, including inorganic materials (SiO2, TiO2, ZnO, ZrO2) [10], noble metals (Ag, Au) [11], organic materials (i.e., resorcinol-formaldehyde, polyimide, polystyrene, conducting polymers such as polyaniline or polypyrrole, etc.), [12] carbonaceous materials (i.e., charcoal, carbon black, carbon nanotubes (CNT), graphene (GN)) [13,14], semiconductor chalcogenide (i.e., ZnS, PbS, CdS, CdSe, PbTe) [15], natural-based aerogels (i.e., cellulose and proteins) [16,17], and, more recently, SiC-based aerogels [18]. Besides the interesting properties of these single-component aerogels, their composite with a specific component has often conferred an additional functionality such as, for example, high mechanical strength, good hydrophobicity, and catalytic features in comparison to the pristine materials, which makes them applicable to high-performance applications in various sectors such as energy harvesting, environmental remediation, sensors, etc. [19–22]. Among a large number of aerogels, the carbon material-based aerogels are rather promising as they have the possibility of a large number of tunable diverse morphologies as well as hierarchical porosity, a large specific surface area, and the interconnected network that endows it with high electrical properties [23]. Carbon aerogels consisting of CNT, GN, etc., have been largely employed in various fields as they possess a high surface area to form the electric double layer and have good electrochemical oxidation/reduction stability [24]. The high porosity and possibilities of chemical functionalization make it a promising material for the adsorptive removal of pollutants and the sensing of volatile organic compounds [25,26]. In view of all these, the present chapter gives a brief overview of different types of nanocarbon aerogels and their composites, their structural morphology, and the synthesis and major developments in their applications in different fields. The compiled work does not cover all the developments in the above-mentioned fields, but it gives a basic understanding of the major developments and progress in this field. For the sake of convenience, the chapter is divided into different categories, depending on the field of application.

Nanocarbon aerogel composites

1.2

3

Different types of nanocarbon aerogels

Nanocarbon materials consist of CNT, GN, nanodiamond (ND), fullerenes, etc. There are different ways in which these nanocarbon materials and aerogels have come together. First, aerogels can be made using these carbonaceous materials or different types of aerogels can be reinforced with embedded nanocarbon materials. Nanocarbon aerogels have the advantage of having diverse macroscopic morphology, hierarchical porosity, and a large surface area. The interconnected framework of three-dimensional (3D) carbon gives them excellent electrical properties, which make them a promising material for a variety of applications [23].

1.2.1

CNT aerogels

Silica, polymers, carbonaceous materials, metal, and metal oxides have been used to fabricate aerogels [27–29], but fabrication of pure carbonaceous materials such as CNT aerogels has been a tedious job for scientists, who have achieved only modest success so far [30,31]. Ya-Li Li et al. [32] showed that the gaseous phase during CNT synthesis can be wound into continuous fibers of endless length. It was found that the continuous fibers formed an aerogel and gave an appearance of elastic smoke. The continuous spinning is possible in the case of a variety of carbon sources such as methanol, ethanol, ethers, acetones, etc. The structure of CNT aerogels is widely dependent on the carbon sources and alcohol and ketones gave continuous fibers while carbon particles, thick fibers, or a mixture of both was obtained in the case of mixed hydrocarbons such as petroleum. A report from Shen et al. [33] used simple CNT raw powders dispersed in water with surfactant sodium dodecyl benzene sulfonate to form aerogels. The optimum ratio of CNT, surfactant, and water under ultrasonic waves coagulated into gel, which was washed and dried under supercritical conditions to give CNT aerogels. This simple fabrication methodology has been presented in Fig. 1.1.

1.2.2

Graphene aerogels

GN aerogels are generally produced from graphene oxide (GO) [34] due to its high functionality and ability to form uniform aqueous dispersion. Due to the presence of oxygen-containing groups in the basal planes and edges, it can covalently react with different materials, which gives the possibility of synthesizing new materials with a completely different set of properties [34]. The GO can be reduced into GN aerogel by hydrothermal methodology, reducing agents, or a combination of both. The GN aerogel is a highly porous and extremely lightweight material consisting of aggregated sheets of GN; it also possesses high mechanical strength [35]. Xu et al. [36] showed that GN can be self-assembled into highly porous macroscopic GN architectures by a simple heating of GO under hydrothermal methodology. The aerogels showed high mechanical properties and conductivity in the range of 5  103 S/cm due to the generation of a conjugated structure upon reduction. The advantage of this methodology

4

Nanocarbon and its Composites

Using 1% PVA solution (hot) Standing at room temperature for 2 days

Ultrasonic wave 4h

Solvent exchange 90 °C

SDBS + H2O Gelation Solution

5 loops 1 day/loop

Hydrogel

Hydrogel

CNT

R

w

w=0

Pure ethanol G SCFD

Aerogel

Centrifugation

Solvent exchange Alcogel

Hydrogel

Fig. 1.1 Fabrication of CNT aerogels. Figure taken with permission from Shen Y, Du A, Wu X-L, Li X-G, Shen J, Zhou B. Low-cost carbon nanotube aerogels with varying and controllable density. J Sol-Gel Sci Technol 2016;79(1):76–82.

in contrast to others is that the formation of a macroporous gel-like structure and a reduction occur simultaneously [37,38]. Cheng et al. [13] showed that further hightemperature treatment of GN aerogel by hydrothermal methodology at 1500°C resulted in improvement in the conductivity, mechanical properties, oxidation temperature, and electric conductivity of the GN aerogels. Fig. 1.2 represents the fabrication of a GN aerogel by hydrothermal methodology and its further treatment at 1500°C, which gives a GN aerogel with improved properties. GO can also be reduced by chemical as well as electrochemical methods using hydrazine, vitamin C, sodium ascorbate, mercaptoacetic acid, mercaptoethanol, etc. [39,40]. However, these reduction methodologies often lead to a less-porous GN aerogel due to high stacking between the GN sheets.

1.2.3

CNT and graphene aerogels composite

Polymers have been widely used as binding materials to withstand the 3D structures of carbonaceous aerogels [41]. CNT aerogels with a sodium carboxymethylcellulose binder showed a low density of 32.7–77.7 mg/cm3, high mechanical strength, electrical conductivity of 0.034–0.162 S/cm and ultralow thermal diffusivity of 2.09–4.54 mm2/s. The binder plays a very important role for the formation of aerogels. The SEM images of carboxymethylcellulose binder freeze dried under liquid

Nanocarbon aerogel composites

5

180 °C

Supercritical drying

1500 °C

12 h

270 °C, 8 MPa

1h

COH

(A)

(B)

(C)

(D)

Fig. 1.2 Synthesis of GN aerogels. (A) GO aqueous dispersion, (B) GN hydrogel, (C) dried GN aerogels, and (D) annealed GN aerogels. Figure taken with permission from Cheng Y, Zhou S, Hu P, Zhao G, Li Y, Zhang X, Han W. Enhanced mechanical, thermal, and electric properties of graphene aerogels via supercritical ethanol drying and high-temperature thermal reduction. Sci Rep 2017;7(1):1439.

nitrogen shows the formation of spindly stripes due to its polymer chain structure. Thus, it can be inferred that the binder and CNTs entangle with each other to synergistically construct the hierarchical microstructures of the CNT aerogels [42]. Fig. 1.3 shows the role of the carboxymethylcellulose binder in the formation of bindersupported CNT aerogels. The entrapment of CNT into dendritic polymers was also reported by Zhang et al. [43] to get highly functionalized 3D hydrogel networks. The individual nanotubes were assembled with poly(amido amine) in the presence of Fe3O4 nanoparticles to give the 3D poly(amido amine) entangled CNT and Fe3O4 composites. Similarly, a lot of progress has been made in producing a variety of CNT-based aerogel composites as porous supporting binders that can be easily tailored to meet needs by incorporating different metals, metal oxides, etc. [44,45] GOs have been widely utilized to prepare GN aerogel composites with metal, metal oxides, polymers, etc. The dispersion of the metal, metal oxides, polymers, or their precursors in the GO dispersion and their further hydrothermal reduction with or

6

Nanocarbon and its Composites

Fig. 1.3 Preparation procedures of the CNT aerogels (A), SEM images of CNT aerogels-0 M (B), CNT aerogels-0.25 M (C), CNT aerogels-0.5 M (D), CNT aerogels-0.75 M (E), CNT aerogels-1 M (F), CNT aerogels-0.75 MH (G), a critical micelle concentration monolith prepared by freeze-drying the critical micelle concentration solution (inset shows the molecular chain structure of the critical micelle concentration) (H), distribution of C, O, and Na elements in CNT aerogels-0.75 M aerogel sheets (I), and microstructure schematic prepared aerogels (J). Insets in (B), (D), and (F) are magnified sheets of corresponding aerogels. The 0 M, 0.25 M, 0.5 M, and 0.75 M represent the mass ratio of the critical micelle concentration solution to the CNT paste in the CNT slurries. For comparison, distilled water was added into CNT aerogels-0.75 M’s slurry, making the CNT concentration decrease from 2.86 to 1.50 wt%, and the fabricated aerogel was labeled as CNT aerogels-0.75 MH. Figure taken with permission from Dong L, Yang Q, Xu C, Li Y, Yang D, Hou F, Yin H, Kang F. Facile preparation of carbon nanotube aerogels with controlled hierarchical microstructures and versatile performance. Carbon 2015;90:164–171.

without a reducing agent has been widely utilized to get a wide variety of GN aerogel composites. Guo et al. [46] hydrolyzed ZrOCl2.8H2O in a GO/dimethyl formamide solution to form the gel precursors, which was followed by supercritical drying with CO2 and carbonization in argon to give GN/ZrO2 aerogels. Peng Lv et al. [47] deposited polyaniline by electrochemical methodology on superelastic GN aerogels to get GN/polyaniline aerogel composites. Similarly, a ternary composite of GN aerogels

Nanocarbon aerogel composites

7

can be prepared by mixing two different materials in the GO dispersion followed by the hydrothermal reduction of GO. The self-assembled GN/polyaniline/Co3O4 ternary hybrid aerogels by Lin et al. [48] utilized a universal strategy. First, a GO/polyaniline composite was prepared by in situ polymerization. A subsequent self-assembly of the GO/polyaniline composite in the presence of GO/cobalt salts hybrid was done by hydrothermal treatment to fabricate a novel 3D macroporous framework of GN/polyaniline/Co3O4. Similarly, different combinations of polymers (polypyrrole, polythiophene, nonconducting polymers) and nanomaterials (metal/metal oxides) can be used to prepare the GN aerogel with interesting properties [49–52].

1.2.4

Nanodiamond based aerogels

The ND aerogel is expected to have a wide tunable optical index of refraction ( 1 < n < 2.4), which can be used for synthesizing antireflection coatings as well as electrical field emission applications while maintaining its low density along with the high surface area and self-supporting aerogel morphology. The synthesis of the ND aerogel at high temperature and pressure from an amorphous carbon aerogel was demonstrated by Pauzauskiea et al. [53] using a laser-heated diamond anvil cell. Chemically inert neon was used to prevent the collapse of the structure by filling the aerogel’s void volume. The microscopic images revealed that the highly porous aerogel morphology of the precursor was well preserved in the ND aerogel, and the electron diffraction confirmed the conversion from amorphous carbon to cubic diamond. Fig 1.4 shows the TEM images and diffraction patterns of the ND aerogel. The sol-gel process using a reaction between resorcinol and formaldehyde molecular precursors gave macroscopic ND aerogel materials with high surface areas based on acid-catalyzed condensation reactions in a polar aprotic solvent (acetonitrile). The large pore size obtained in this methodology suggests a wide range of applications of ND aerogels in energy storage, catalysis, pharmaceuticals, etc. [54]. The ND aerogel composite was reported by Roldan et al. [55] by a similar reduction of GO in the presence of ND, as discussed earlier. The support of ND on GN prevented the restacking of GN and the agglomeration of ND at the same time.

1.3

Nanocarbon aerogels for energy storage applications

Supercapacitors have emerged recently as a sustainable, low-cost, and high-power energy source for applications in devices needing uninterruptible energy requirements, such as electrical vehicles and renewable energy systems [56]. Carbon materials exhibit lower energy density than pseudocapacitors and thus pseudocapacitors have attracted much attention in recent years. In order to utilize the beneficial properties of carbonaceous materials, hybrid electrode materials have been proposed [57]. The mesoporous morphology of carbon materials has been largely employed as electrode-active materials due to their high surface area. However, these porous structures suffer from defects such as a complicated and disordered porous structure, which affects the electron transfer and thus limits the performance of the devices [58, 59].

Fig. 1.4 Transmission electron micrographs of the amorphous carbon precursor and recovered diamond aerogel. (A and B). Bright-field transmission electron micrographs of amorphous aerogel (A) and recovered aerogel (B). Samples are supported on lacy carbon. Scale bars: 200 nm. (C and D) Corresponding electron diffraction from precursor and recovered material, respectively (In D the fundamental beam is blocked). (E and F) High-resolution images of carbon and diamond aerogel microstructure. Nanocrystalline lattice fringes visible in F (arrowheads) correspond to the (111) plane of a cubic diamond with a lattice spacing of ˚ . Scale bars: 5 nm. 2.06 A Figure taken with permission from Pauzauskie P J, Crowhurst J C, Worsley M A, Laurence T A, Kilcoyne A D, Wang Y, Willey T M, Visbeck K S, Fakra S C, Evans W J. Synthesis and characterization of a nanocrystalline diamond aerogel. Proc Natl Acad Sci 2011;108 (21):8550–8553.

Nanocarbon aerogel composites

9

In the viewpoint of the above-mentioned problems, carbon material aerogels (CNT, GN, and ND) that are composed of 3D networks of interconnected nanoparticles offer high porosity with large pore volumes, high surface areas, and tunable porosity. Binderless single-walled CNT aerogels prepared by Katherine L. Van Aken et al. [60] showed high ratios of surface area to volume and strength to weight. The singlewalled CNT aerogels showed high capacitive performance over 10,000 cycles as well as an impressive performance at high charge and discharge rates, which was attributed to its accessible pore networks and enhanced electronic and ionic conductivities. CNT aerogel composites with metal and their salts also showed promising energy storage applications. Acid treated CNTs in combination with V2O5 sol was successfully fabricated into aerogels by Wu et al. [61]. Due to their hierarchical porous structure, high specific surface area, and good electrical conductivity, the V2O5CNT-based supercapacitors demonstrated extremely high specific capacitance, high power and energy densities, and excellent cycling stability. A similar high performance has also been reported for cobalt disulfide nanoparticles/GN/CNT aerogels and 3D NiCo2O4/multiwalled CNT nanocomposite aerogels prepared by a supercritical CO2 drying method [62, 63]. Zheng et al. [64] showed high energy-storage performances in their highly flexible aerogels consisting of cellulose nanofibril, reduced GO [32], and CNT fabricated without any binders, current collectors, or electroactive additives. Because of the porous structure of the cellulose nanofibril/rGO/CNT aerogel electrodes and the excellent electrolyte absorption properties of the cellulose nanofibril present in the aerogel electrodes, the resulting flexible supercapacitors exhibited a high specific capacitance (i.e., 252 F g1 at a discharge current density of 0.5 A g1) and a remarkable cycle stability (i.e., more than 99.5% of the capacitance was retained after 1000 charge-discharge cycles at a current density of 1 A g1). Furthermore, the supercapacitors also showed extremely high areal capacitance, areal power density, and energy density (i.e., 216 mF cm2, 9.5 mW cm2, and 28.4 μWh cm2, respectively). Instead of rGO, when polyaniline was used, a high specific capacitance of 791.13 F/g was obtained at 0.2 A/g. The polyaniline nanoparticles deposited on the surface of cellulose nanofibril and CNTs connected to form a continuous network, which increased the charge mobility, hence resulting in high electroactivity [65]. The rGO aerogels show very high supercapacitive performance in aqueous electrolytes due to their plentiful mesopores and macroporous structures. Weijiang Si et al. [66] calculated their specific capacitance in KOH and H2SO4 to be 211.8 and 278.6 F g1, respectively. Thus, the mesoporosity highly affects the conductance and supercapacitive nature of electrodes. The GN aerogels synthesized by Hummers’ method by thermal treatment of GO with resorcinol-formaldehyde gel was found to possess pore volume and surface area of 2.96 cm3/g and 584 m2/g, respectively. Due to that, a high electrical conductance of 87 S/m was obtained [67]. Zhang et al. [37] successfully obtained 512 m2/g surface area and high electrical conductivity of 102 S/m. These highly conductive aerogels exhibited a specific capacitance of 128 F/g.

10

Nanocarbon and its Composites

Functionalized GN such as nitrogen-doped GN aerogels functionalized with melamine showed high specific capacitance of 170.5 F g1 at 0.2 A g1, along with the high charge/discharge cycling stability [68]. The p-phenylenediamine functionalized GN aerogel by Habib Gholipour-Ranjbar et al. [69] showed porous 3D structures and with an increase in the content of p-phenylenediamine, an increase in the surface area was observed. Their study revealed an excellent capacitance of 385 F g1 at a discharge current density of 1 A g1, along with the exceptionally high cyclic stability by displaying a 25% increase in specific capacitance after 5000 cycles. The aerogels with larger pore size showed enhanced supercapacitive performance compared with the aerogels with smaller pore size. Metal/metal oxide composites with GN aerogels allow the access of electrolytes inside the porous aerogel structures, which results in enhanced diffusion and migration of electrolyte ions during the rapid charge/discharge process. The 3D MnO2/GN aerogel by Ji et al. [70] showed a high specific capacitance of 200 F g– 1 . As p-phenylenediamine helps in the formation of a porous 3D structure, the p-phenylenediamine functionalized MnO2/GN aerogel showed excellent performance of 26.2 Wh kg1 energy density and high cyclic stability after 5000 cycles. The enhanced performance is due to the improved electrolyte accessibility owing to high porosity and utilization of the highest surface area of the electrode materials. Due to its porous structure, cellulose yielded high porosity in the cellulose/GN aerogels, which is expected to provide efficient migration of electrolyte ions and electrons. Zhang et al. [71] found the specific capacitances of cellulose/GN aerogels to be 300 F/g at a scan rate of 5 mV/s. Conducting polymers such as polyaniline and polypyrrole have also shown superior electrochemical properties. This excellent electrochemical performance is due to the synergistic contribution of the local conductivity of GN layers sandwiched between polyaniline layers and the long-distance conductivity of 3D GN frameworks. Qu et al. [72] showed a high specific surface area of up to 337 m2/g for symmetric and asymmetric all-solid-state supercapacitors, and the aerogels delivered areal capacitances up to 453 and 679 mF/cm2, respectively, which was found to be superior to most of the GN/GO or polyaniline-based aerogels [73]. GN/polypyrrole nanoparticle hybrid aerogels with a 3D hierarchical porous structure exhibited high specific capacitance (418 F g1) at a current density of 0.5 A g1, extremely outstanding rate capability (80%) at various current densities from 0.5 to 20 A g1, and good cycling performance (74%) after 2000 cycles [49]. Ye et al. [74] showed that polypyrrole nanotubes not only provided a large accessible surface area for fast transport of hydrate ions but also acted as spacers to prevent the restacking of GN sheets. Their 3D hierarchical GN/polypyrrole aerogels showed high specific capacitance up to 253 F g1, good rate performance, and outstanding cycle stability. Thus it can be concluded that GN aerogels have a large number of possibilities for chemical functionalization and composite fabrication with metal/metal oxides, polymers, etc., that can give different type of GN-based aerogels with hitherto unreported properties. These materials are easy to fabricate and hold tremendous potential in solving the energy crisis of the present era.

Nanocarbon aerogel composites

1.4

11

Nanocarbon aerogel adsorbents for wastewater remediation

The growth of industrialization and various human activities are responsible for the release of different types of organic, inorganic, and biological contaminants into surface water. To avoid health problems, the elimination of these contaminants is necessary. Adsorption is the well-known method for the removal of impurities present in wastewater. The low operation cost and easy handling of the process make this method most promising. Until now, a lot of the adsorbents have been investigated for environmental remediation applications and many commercially available sorbent materials such as biomass, activated carbons, metal oxides, minerals and clays, polymers, and so forth have been used. An ideal adsorbent must have the following properties: l

l

l

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l

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Low cost and high adsorption capacity. High surface area, porosity, and large pore volume. High chemical and physical stability. Easy to modify. Easy separation from aquatic medium. Ability to attain ultralow contaminant levels in treated streams. High reusability capacity.

Among the various types of adsorbents, the aerogel adsorbents are getting the attention of researchers while having most of the aforementioned properties of an ideal adsorbent. Aerogel microstructures usually are hydrophobic in nature, having welldefined 3D structures. The exceptionally high surface area, porosity, low density, and flexible surface chemistry of the aerogels makes them an effective and efficient sorbents for the scavenging of pollutants from aqueous solutions [75]. The major advantage of aerogel as an adsorbent is its macro size that can be easily removed from an aqueous solution compared to the nanostructured powdered materials. In the past, different types of aerogels have been prepared and used for the removal of various kinds of contaminants from wastewater such as carbons, metal oxides, polymers, and so forth. Currently, carbon-based aerogel adsorbents are getting much attention due to their high surface area and high adsorption capacity. Several studies showed that powdered carbons such as activated carbon, CNT, and GN are efficient adsorbents for the removal of organic, inorganic, and biological contaminates from wastewater. The physical and adsorption properties of the aerogels highly depend on the method of synthesis and the parent materials used for the preparation. For example, Wang et al. [76] and Li et al. [77] prepared a cellulose-based carbon aerogel adsorbent and investigated its application for the removal of dyes and metals from an aquatic medium. Wang et al. [76] used a combination of aqueous LiOH/urea/H2O (4.6:19:76.4w/w) and different wt% of cellulose. The cellulose hydrogel was exchanged with deionized water, ethanol, and t-butanol followed by N2 freeze drying. The obtained cellulose aerogels were heated under N2 flow at 800°C for 1 h to convert it into a carbon aerogel. The surface area by the Brunauer-Emmett-Teller technique (SBET) of the carbon aerogel was 500 m2/g, the mesopores were 30–50 nm, and the density was 20mg/cm3.

12

Nanocarbon and its Composites

The adsorption capacities of the carbon aerogels for different dyes were in the range of 195–1947 mg/g, higher than other types of activated carbon. The adsorption capacity of the carbon aerogels was 801 and 182 mg/g for the Cu (II) and Sn (II), respectively, while carbon aerogels were inactive for Zn (II) and Ni (II) adsorption. Compared to Wang et al. [76], cellulose-based carbon aerogels prepared by Li et al. [77] showed different properties and adsorption efficiency. Li and coworkers [77] used the NaOH/urea/H2O (7:12:81), NaCl, and isooctyl alcohol ether phosphate. The obtained gel was freeze dried at 55°C for 48 h followed by carbonization at 600°C for 2 h. However, the SBET of the carbon aerogel was higher (725.12 m2/g) compared to the Wang et al. [76] study (SBET ¼ 500 m2/g), but the adsorption capacity of the carbon aerogel was much lower (55.25 mg/g). By comparing these studies, it can be concluded that the adsorption properties of the aerogel highly depend on the method of preparation and applied experimental conditions. Besides the carbon, CNTs and GO have been widely used as adsorbents due to their high surface area, low density, and abundant oxygeneous functional groups. However, the agglomeration is the biggest problem associated with CNTs and GO. Making the 3D aerogel framework of the GO, CNTs, and GO/CNTs nanocomposite may be the best solution to prevent the agglomeration and improve the surface area and porosity. Wan and coworkers [78] prepared the ultralow density (6.2–2.8 mg/cm3) GN-CNTs aerogel and applied it as an adsorbent for the removal of oil, organic dyes, and solvents. The results showed that the prepared aerogel was highly efficient and can adsorb 100–270 times of its own weight, depending on the density of the adsorbed organics. Moreover, the efficiency of the aerogel was not changed up to 10 cycles. The hybrid aerogel based on the GN sheet and nanoribbons was synthesized by Wang et al. [79]. The synthesized hybrid aerogels showed the largest adsorption capacity for the organic solvent and oils, as shown in Fig. 1.5. The aerogel was able to adsorb 100–350 times its weight with excellent recyclability.

1.5

Nanocarbon aerogel photocatalyst for wastewater remediation

Photocatalysis is an advanced oxidation process that has received significant attention due to its low energy requirements and easy operation to break down the organic contaminants into mineral products. Due to the abundance of solar light, semiconductorbased heterogeneous photocatalysis is considered one of the most encouraging technologies for resolving environmental contamination. Although photocatalysis is an advanced and effective technology, there are some problems related to the photocatalyst materials, such as: l

l

l

l

l

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Most of the semiconductor materials are not visible light active or show poor activity. High band gap energy. Fast electron-hole (e/h+) pair recombination rate. Agglomeration of nanoparticles. Difficult in separation of catalyst after photocatalysis from aqueous solution. Reusability.

Nanocarbon aerogel composites

(A) Normalized adsorption capacity

e

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e ton Ace oil sel Die ene Xyl e uen Tol il no bea y o S F DM form oro Chl 0

(B)

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Oil adsorption capacity (g g )

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1.0 0.8 0.6 0.4 0.2 0

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Fig. 1.5 (A) GO-GN nanoribbons aerogel adsorbing diesel oil dyed with Oracet Blue B. (B) Adsorption capacity of the GO-GN nanoribbons aerogel for various organic liquids. (C) Recyclability of the GO-GN nanoribbons aerogel tested with hexane. The adsorption capacity after 10 cycles is normalized by the initial weight gain. Figure taken with permission from Wang C, He X, Shang Y, Peng Q, Qin Y, Shi E, Yang Y, Wu S, Xu W, Du S. Multifunctional graphene sheet-nanoribbon hybrid aerogels. J Mater Chem A 2014;2(36):14994–15000.

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Nanocarbon and its Composites

Fig. 1.6 Schematic diagram of the proposed mechanism of carbamazepine photodegradation. Figure taken with permission from Nawaz M, Miran W, Jang J, Lee D S. One-step hydrothermal synthesis of porous 3D reduced graphene oxide/ TiO2 aerogel for carbamazepine photodegradation in aqueous solution. Appl Catal B Environ 2017;203: 85–95.

The nanocarbon-based semiconductor materials are gaining the attention of researchers to overcome the aforementioned problems related to the pure metal semiconductors. Carbon-based aerogels showed good results not only in terms of high visible light photocatalytic activity but also led to the reduction in the band gap energy, slower e/h+ pair recombination rate, and hence better recovery and reusability of the catalyst. Due to the unique properties of 1D CNTs and 2D GN, such as physicochemical, electrical, and optical properties, the semiconductor aerogel composite photocatalysts have been widely synthesized and applied for the degradation/reduction of the various types of pollutants [34, 80]. GN and metal-based composite aerogels have been synthesized successfully by the self-assembly of GO nanosheets with π–π staking and the H-bonding between sheets, and then applied as the photocatalysts. Nawaz and coworkers [81] synthesized the 3D aerogel of rGO/TiO2 and its application was investigated for the photocatalytic degradation of carbamazepine in an aqueous solution. The composite aerogels not only showed higher adsorption but also showed two times higher photocatalytic properties than the pure TiO2. The rGO helped in the charge separations by acting as an electron sink and the amounts of the rGO and TiO2 in the rGO/TiO2 aerogel played a vital role in the improvement of the photocatalytic activity. Wang et al. [82] suggested that the excessive concentration of the rGO in the composite may promote the recombination of the photogenerated e/h+ pair by acting as a kind of e/h+ pair recombination center. Thus, excessive amounts may show a negative impact on the adsorption and photocatalysis. In agreement with this, Nawaz et al. [81] also reported that rGO/TiO2 aerogel prepared in the ratio 1:2 (rGO:TiO2) showed the best degradation efficiency and reusability over the five successive cycles. A proposed mechanism for the degradation of carbamazepine is shown in Fig. 1.6. A novel 3D AgBr/GN aerogel photocatalyst was synthesized by uniform distribution of the AgBr nanoparticles throughout the porous structure of GN aerogel. [83].

Nanocarbon aerogel composites

15

Compared to the pristine AgBr, the photocatalytic degradation efficiency and reusability of the AgBr/GN aerogels was much higher, not only for the oxidative degradation of methyl orange but also towards the reduction of Cr(VI) to Cr(III). The authors reported three possible reasons for the higher photocatalytic activity of the AgBr/GN aerogels compared to the pure AgBr: (i) High adsorption capacity due to large BET surface area. (ii) Strong interaction between the AgBr and GN aerogels, which reduces the recombination of the e/h+ pair and enhances photocatalytic activity. (iii) High stability of the AgBr/GN aerogels.

Besides the GN aerogels, CNTs and carbon aerogel have also been explored for wastewater decontamination. Carbon aerogels show high adsorption due to high surface area and good conductivity that favors the charge separation between the semiconductor and carbon aerogel. Wang et al. [84] prepared a ternary composite aerogel of Cu2O/TiO2/carbon aerogel for the degradation of 2,4,6-trichlorophenol. The degradation efficiency of 2,4,6-trichlorophenol and total organic carbon removal were 96.3 and 91.3%, respectively, in 5.5 h on a prepared Cu2O/TiO2/carbon aerogel. A proposed mechanism for the electrosorption-assisted visible light photocatalytic degradation of 2,4,6-trichlorophenol is shown in Fig. 1.7. The high surface area and the large electron storage capability of the single-walled CNTs are considered the ideal support materials for the synthesis of the novel photocatalyst. Park et al. [85] synthesized TiO2 decorated on the single-walled CNTs

Fig. 1.7 A proposed mechanism for the electrosorption-assisted visible light photocatalytic degradation of the 2,4,6-trichlorophenol on the Cu2O/TiO2/carbon aerogel electrode. Figure taken with permission from Wang Y, Zhang Y-n, Zhao G, Tian H, Shi H, Zhou T. Design of a novel Cu2O/TiO2/carbon aerogel electrode and its efficient electrosorptionassisted visible light photocatalytic degradation of 2, 4, 6-trichlorophenol. ACS Appl Mater Interfaces 2012;4(8):3965–3972.

16

Nanocarbon and its Composites

Fig. 1.8 Schematic illustration of the synthesis process for TiO2/single-walled CNT aerogels. Figure taken with permission from Park H A, Liu S, Salvador P A, Rohrer G S, Islam M F. High visible-light photochemical activity of titania decorated on single-wall carbon nanotube aerogels. RSC Adv 2016;6:22285–22294.

(Fig. 1.8) and investigated the degradation of methylene blue under visible light irradiation. After incorporation of TiO2 onto the single-walled CNT surface, a significant reduction in band gap energy from 3.2 (TiO2) to 2.6 eV was observed. The authors reported that the degradation rate of TiO2/single-walled CNT aerogels was approximately twice that of the reported TiO2/CNT composites.

1.6

Nanocarbon aerogels as sensors

Previously, it has been discussed that aerogels are extremely low density materials with a very high surface area. Various forms of aerogels (carbon, silicon, metal oxides, organic polymers, etc.) have been produced for practical application due to these interesting properties. Carbon quantum dots have also gained attention due to their peculiar properties such as low toxicity and environmentally friendly nature in comparison to the heavy metal quantum dots, along with good electrochemical properties, photoluminescence, unique opticals, etc. [26, 86]. The importance of nitrogen dioxide (NO2) detection is due to its harmful effects to the respiratory systems of humans and animals. NO2 is a typical model pollutant for sensing materials and is one of the most prominent air pollutants as well as a source of particulate matter in the atmosphere, for example, PM2.5, acid rain, and photochemical smog. Besides NOx, volatile organic compound exposure to humans is also very harmful and causes various respiratory diseases [87, 88]. Therefore, in order to monitor air quality and as a precautionary warning, volatile organic compound sensors are much needed. The typical designs of different gas sensors are mainly based on adsorption of analytes and their detection on the surface of the sensor. With the sensing of analytes being a surface phenomenon, the surface area of the sensor therefore plays an important role in sensor design. With aerogels being extremely low density solids with a very high surface area, they have good potential to be utilized in chemical or vapor sensors [67, 89]. Wang et al. constructed an NO2 gas sensor based on carbon quantum

Nanocarbon aerogel composites

17

dots-aerogel hybrid materials. The group functionalized silica aerogel with branched polyethylenimine-capped quantum dots and observed its fluorescence quantum yield more than 40%. The prepared hybrid aerogel (porous quantum-aerogel) exhibited excellent fluorescence activity in the solid state. The fluorescence activity of the hybrid aerogel was quenched selectively and sensitively by the NO2 gas, and therefore offers its potential for a gas-sensing application [26]. While Dolai et al. [90] have reported sensor based on fluorescent carbon quantum dots embedded in silica aerogel for sensing different volatile organic compounds. The carbon quantum dots show a highly sensitive (to local environments), broad-ranged, excitation-dependent emission spectra and offer possible sensing applications [91, 92]. They also found a distinct shifting and quenching phenomenon in the fluorescence signals of the carbon quantum dot aerogel. Thubsuang et al. [93] reported a highly sensitive vapor sensor based on carbon aerogel derived from polybenzoxazine. The aerogel of activated carbon is a nanoporous material with high pore volume (57 cm3/g) and surface area (920 m2/g). The activated carbon aerogel/polybutadiene composite exhibited a good response of 11.2 and 6.7 towards toluene and n-hexane, respectively. The aerogel also exhibited good recovery as the electrical resistance came back to the original value within minutes when exposed to nitrogen gas. GN and GN-based materials hold technological promise in the areas of energy storage, electronics, composites, actuators, and sensors. Individual GN sheets possess a number of remarkable properties, including extremely low electrical and thermal resistivity, large carrier mobility, high surface area, and exceptional mechanical elasticity [94–96]. GN-based sensors have also been made on the basis of changes in their properties such as electrical conductance, etc. Samsonau et al. [97] fabricated a sensor based on GN film (1.5 nm) for the detection of NO2. The film’s thickness displayed high chemical sensitivity towards NO2 to tens of parts per billion. The sensitivity is attributed to the sufficient concentration of a hole-like carrier through the electron transfer from a p-type electron withdrawing GN to NO2 acceptors adsorbed on GN [98, 99]. Yavari et al. [100] studied the sensing characteristics of NO2 and NH3 in mechanically robust and porous GN foams having interconnected network. NO2 adsorption caused an increase in conductance while NH3 adsorption reduced conductance during the detection process, arising from a decrease in the number of charge carriers on GN. The GN foam-based sensor exhibited much better sensitivity than commercial polymer sensors [101], which is due to the effective transport of the charge carrier between the high porosity foam, few-layer GN component, and the adsorbed gas. Xu et al. [102] reported very good catalytic performance of a GN-based sensor for NO reduction. They used a film of GN/Au composite immobilized with hemoglobin as a sensor that responded sensitively for NO with a detection limit of 1.2  108 mol L1. Guo et al. [103] designed a very stable biomimetic sensor by using pyrenebutyric acid functionalized GN film. It was observed to exhibit excellent sensitivity toward nitric oxide to be utilized for real-time nitric oxide detection, which is released from living cells [104]. Alizadeh and Ahmadian [105] reported an ammonia sensor based on a GN aerogel treated with thiourea. On exposure to ammonia, the sensor exhibited a change in the electrical conductivity, which was the basis of the sensing mechanism. They

18

Nanocarbon and its Composites

hydrothermally prepared GN hydrogel and heated it in the presence of thiourea to obtain the GN aerogel. The aerogels so prepared exhibited a higher surface area (389 m2 g1), better selectivity, higher sensitivity, and a shorter response time. The response of the sensor was found to be linear towards ammonia in the range of 0.02–85 ppm and a detection limit up to 10 parts per billion was obtained. Preparation conditions (amount of thiourea) were also found to affect the sensing properties of the resultant aerogel. Jeong et al. [106] prepared a 3D aerogel consisting of molybdenum disulphide (MoS2) and a GN-based glucose electrochemical biosensor. The 3D MoS2/ GN aerogel provided a larger surface area that effectively assists the enzyme immobilization, and additionally provides an electrically conductive and continuous framework of GN sheets. The electrode made of this aerogel exhibited a low detection limit and rapid response for glucose sensing. Liu et al. [107] synthesized 3D GN aerogel/ZnO composite-based NO2 sensors that exhibited good response and fast and effective response/recovery behavior, even at room temperature. The homogeneously anchored ZnO spheres on the GN surface and the 3D GN offered interconnected macroporous networks, which was attributed to the enhancement of the sensing response of the sensor. Li et al. [108] developed a two-step strategy to hydrothermally prepare 3D SnO2/ rGO composites and use them for NO2 detection. It was found that the products prepared from different tin salts exhibited different sensing performances for NO2 detection. GN-based aerogels generally are weak and delicate; therefore, it is difficult to utilize them as flexible sensors for strain. It is also noticed that the π–π stacking and the van der Waals forces between two different GN sheets cause brittleness in the structure of the aerogel. To improve the mechanical properties of such aerogels, Yuan et al [109] prepared GN/alginate aerogels that possessed excellent porosity (99.61%) and very low density (6–7 mg/cm3). These aerogels also exhibited excellent durability, excellent bending, and compression sensitivity. These properties of the aerogels are attributed to the presence of a strong hydrogen bonding between the GN and sodium alginate. An et al. [110] fabricated a GN aerogel having a 3D nanostructure using a microextrusion printing technique for electronic sensor devices. Multiresponse, multifunction, and high integration are the critical pursuits of advanced electronic wearable sensors. The printed GN aerogel patterns exhibited excellent electrical conductivity while the 3D nanostructure of aerogel offered multidimensional deformation-sensing responses that are suitable for the wearable flexible electric sensor. These may also be utilized for complicated movement sensing. Therefore, these printed aerogel sensors offer promising applications in gesture language analysis and auxiliary devices for deaf-mute communication. Qi et al. [111] reported the first aerogel as a vapor sensor based on CNTs and cellulose. The sensing characteristics of the aerogels for vapors were observed as a change in the electrical conductivity upon exposure to volatile organic compounds such as ethanol, methanol, toluene, etc. These aerogels exhibited high sensitivity, rapid response, and good reproducibility towards different vapors. The sensing property of these aerogels is enhanced due to their 3D and porous structure, which

Nanocarbon aerogel composites

19

in turn provide a surface for the analyte to be detected. Li et al. [112] prepared a 3D chitosan/CNTs aerogel with the wide tips of CNTs emerging on the surface of chitosan/CNTs aerogel nanosheets by the ice-templating method. The unique design of the aerogel was utilized for biosensing applications, which exhibited good specificity toward dopamine even at an extremely low concentration (0.3 nM). The uniqueness of the surface of the nanosheets with wide CNT tips makes it an incubative bed for in situ adsorbed growth of nanostructures. Cu2O/CuO chipped core-shell nanoparticles were in situ grown onto the soft nanosheets by simple adsorption of Cu2+ and used later for nonenzymatic detection. Gao et al. [113] composited a 3D and ultralight aerogel (density 2.52 mg cm3) containing GN and poly(dopamine) modified CNTs. These aerogels exhibited excellent electrical conductivity, repeatable compressibility, and offered promising applications in strain-sensitive functional devices.

1.7

Conclusion and future research

This chapter summarizes the major developments in the nanocarbon aerogels comprising GN, CNT, ND, and their composites, with an emphasis on fabrication techniques and properties as well as their potential applications in energy and environmental remediation sectors. The aerogel composites containing GN, CNT, ND, and polymers/metal/metal oxides exhibit remarkable strength and porosity, which might open new applications in the field of environmental remediation, energy storage, nanotechnology, electronics, medicine, etc. However, much attention needs to be paid to the synthesis mechanism for their cost-effective production and to understand the different parameters which needs to be conditioned for their application in the various discussed fields. With the deepening energy crisis, there is a pressing need to develop advanced and upgraded capacitors (super/pseudo) by minimizing the defects caused by disordered porosity and therefore, newer fabrication methods for enhancing the electronic transport in the nanocarbon-based aerogels is an important area of research. These nanocarbon-based aerogels should be further studied and their photocatalytic performances should be improved to efficiently degrade more toxic and harmful compounds in the environment. The nanocarbon aerogels showed excellent properties for the removal of water contaminants and could be used for commercial applications. However, enough work has been done to probe the water purification application of the carbon aerogels, so more research work should be focused on the following areas: (i) Production of low-cost, carbon-based aerogels for commercial use with low adverse effect on environment during synthesis. (ii) The mechanism of wastewater purification by carbon aerogels should be explored in detail. (iii) Most work has been focused on synthetic wastewater. Therefore more research should be focused on investigating the application of the aerogels on real wastewater to optimize the operational parameters.

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Nanocarbon and its Composites

(iv) The application of the carbon aerogels should be tested on a pilot scale and by column process. (v) Green synthesis of aerogels with high stability, efficiency, and good reusability using environmentally friendly techniques. (vi) Simple routes for mechanically robust aerogels which can resist shrinking upon repeated usage.

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Highly active and reusable nanocomposites for hydrogen generation

2

Betu€l S¸en, Esra Kuyuldar, Buse Demirkan, Aysun S¸avk, Ays¸enur Aygu€n, Fatih S¸en Sen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar University, K€utahya, Turkey

Chapter Outline 2.1 Introduction 27 2.2 Experimental methods

28

2.2.1 Materials and methods 28 2.2.2 The synthesis of Pd-Ni nanomaterials decorated by AC 28 2.2.3 Reusability examination of Pd-Ni @AC 29

2.3 Results and discussion 2.4 Conclusions 34 Acknowledgments 35 References 35

2.1

29

Introduction

There is a growing interest in alternative novel materials for many types of applications [1–18]. Those applications are fuel cells, organic synthesis, solar cells, electrochemical sensors, fluorescence sensors, drug delivery, dye removal, etc. [7, 19–43]. One of them is also the efficient storage of hydrogen. For this purpose, a variety of materials such as amine-borane are considered leading candidates for chemical hydrogen storage [44–53]. More importantly, recent studies have suggested amine-borane adducts as an alternative energy carrier [54–65]. Of particular interest, the catalytic dehydrogenation of dimethylamine-borane (DMAB) potentially releases 3.5% H2 by weight. Furthermore, it is very easy to generate hydrogen by using DMABs at room temperature if there is an appropriate catalyst. A quick literature survey shows that many materials were tried as catalysts for dehydrocoupling of dimethylamine borane [66–76]. For instance, catalytic activities in the formation of hydrogen from the Pd-Co NPs stabilized by polyvinylpyrolidone (PVP) and dehydrogenation of DMAB at 25  0.1°C have recently been reported [48]. In this paper, instead of PVP, activated carbon (AC) was proposed as a stabilizer for transition metal NPs. In the Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00002-X © 2019 Elsevier Ltd. All rights reserved.

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dehydrogenation of DMAB, the high catalytic performance of these Pd-based bimetallic alloy NPs prompted us to test another Pd-based bimetallic nanocatalyst (palladium-nickel nanoparticles, Pd-Ni NPs) in the dehydrogenation of DMAB. For this purpose, we report the production, characterization, and catalytical performance of monodispersed Pd-Ni NPs stabilized by AC in terms of dehydrogenation, kinetic studies, and reusability of DMAB under mild conditions. To get good stability, nanocatalyst production was performed by coreducing both metals, employing the sodium hydroxide-assisted reduction method. Later, ultraviolet-visible (UV–Vis) spectroscopy, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy were used in order to describe the prepared nanomaterials. Then, one equivalent of H2 per dimethylamine borane was generated with the help of Pd-Ni NPs stabilized by AC. The turnover frequency was found to be 316.41 h1 for dehydrocoupling of dimethylamine borane. Also, the kinetic parameters of dehydrocoupling of dimethylamine borane were examined with the help of different temperatures and catalyst amounts.

2.2 2.2.1

Experimental methods Materials and methods

Aldrich supplied dimethylamine-borane, NiCl2, K2PdCl4, and activated carbon. The C2H5OH and water used during this study were provided by Merck and a Milli Q-pure machine, respectively. Before all glass pieces and other lab materials were washed with large amounts of distilled water, they were cleaned with acetone and then dried. Transmission electron microscopy images was obtained by a JEOL 200 kV. For X-ray photoelectron spectroscopy (XPS) analysis, a Specs spectrometer (Kα lines of Mg (1253.6 eV, 10 mA)) was used. XRD analysis was performed with the help of a Panalytical Empyrean instrument. UV-Vis analyses were taken by a Perkin Elmer Lambda 750. A 200–900 nm was selected to gather the data and 1 cm quartz cell was employed.

2.2.2

The synthesis of Pd-Ni nanomaterials decorated by AC

The Pd-Ni nanoparticles supported by AC were synthesized by a facile sodium hydroxide-assisted reduction method. Typically, 100 mg of activated carbon powder in a two-necked, round-bottomed flask was ultrasonically dispersed in 2.5 mL of ethanol and subsequently mixed with an aqueous solution of NiCl2 and PdCl2 with desired concentrations. The resultant aqueous suspension was further homogenized under sonication for 30 min. Then, 12 mg of NaBH4 dissolved in 1.0 mL of 3.0 M NaOH solution was added into the above-obtained solution with vigorous shaking, resulting in the generation of the catalyst as a dark suspension. Other kinetic and catalytic investigations for the [email protected] NPs were given in detail in our previous papers [77–89]. The dehydrogenation of DMAB was performed in a typical jacketed reaction flask connected to the water-filled cylinder glass tube under a dry nitrogen atmosphere.

Highly active and reusable nanocomposites for hydrogen generation

2.2.3

29

Reusability examination of Pd-Ni @AC

For the reusability experiments of Pd-Ni @AC, the solid mixture at the end of the dehydrogenation reaction was precipitated with cold hexane (10 mL, added under N2 atmosphere) and the supernatant solution was removed by filtration. The solid was further washed with hexane (3  20 mL) and dried under vacuum, giving the isolated colloid as a dark brown powder.

2.3

Results and discussion

The monodisperse Pd-Ni @AC nanoparticles were characterized by using UV-VIS, XRD, TEM, HRTEM, and XPS. For this purpose, after the NiCl2 and PdCl2 were reduced together by using the sodium hydroxide-assisted reduction method, the resulting mixture was refluxed for 3 h, then the color of the material was changed. Here, the color implies the reduction of Pd2+ and Ni2+ ions to the zero oxidation state of metals. To see this, a UV–VIS investigation was performed (Fig. 2.1) and the d–d transitions belonging to the Pd2+ and Ni2+ ions exhibited the reduction of all cations. Moreover, TEM analyses were accomplished to determine the size, morphology, and composition of the Pd-Ni @AC NPs (Fig. 2.2). The average particle size was measured as 3.55 0.42 nm. HR-TEM results indicating the morphology of the NPs can be seen in Fig. 2.2. It was shown that the particles were mostly spherical and no agglomerations were seen for the prepared catalyst. It can also be seen from the HRTEM image that monodispersed Pd-Ni @AC NPs had (Fig. 2.2) 0.21 nm atomic lattice fringes, which is a bit smaller than nominal Pd (111) spacing (0.22 nm) and indicates the alloy formation of Pd-Ni @AC [9]. Additionally, an EELS line profile also confirms that Pd-Ni @AC NPs had alloy structures as well as the ratio of Pd:Ni in this

Fig. 2.1 UV-Vis absorption spectra of the aqueous solutions of Pd+2, Ni+2, and [email protected] NPs.

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Fig. 2.2 (A) Transition electron micrograph, (B) HR-TEM image, (C) The EELS line profile scanned on the arrow shown in HR-TEM image, (D) Particle size histogram of [email protected] NPs.

alloy structure. The ratio of Pd:Ni was found to be 1:1, also confirmed by an ICP analysis (Pd52Ni48). Besides, an XRD analysis was performed to define the crystal morphology of the prepared nanocomposites. As shown in Fig. 2.3A, a face-centered cubic structure was determined for the prepared material. Moreover, slightly shifted diffraction peaks can be seen in Fig. 2.3A; this confirms the alloy formation of Pd-Ni @AC NPs. Furthermore, here the peak at around 25.7 degree was attributed to AC. In the XRD patterns for Pd-Ni, no significant diffraction peaks of Ni species were detected due to relatively strong signals for Pd species. In addition, in Fig. 2.3A, the peaks at 2θ ¼ 40.1, 46.6, 68.1, and 82.1 degrees correspond to the crystal planes of prepared nanomaterials. Moreover, the crystalline particle size was calculated to be 3.72  0.42 nm with the help of the Debye-Scherrer [90–96]. The particle size of Pd-Ni @AC is in good agreement with TEM results. Raman spectroscopy was also used in the present study to discriminate the ordered and disordered carbon structures in the carbonaceous materials. In Fig. 2.3B, the

31

20

40

(A)

36

38 40 2θ (degree)

60

42

44

Pd(311)

34

Pd(200)

[email protected] [email protected]

[email protected] [email protected]

Pd(220)

Intensity (a.u.)

Pd(111)

Intensity (a.u.)

Pd(111)

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2q (degree)

AC [email protected]

D 1580 cm–1

Intensity (a.u.)

1350 cm–1

G

ID /IG : 0.90

ID /IG : 0.53

1200

(B)

1300

1400 1500 1600 Raman shift (cm–1)

1700

1800

Fig. 2.3 (A) The XRD of [email protected] and [email protected] NPs and (B) The Raman spectra of [email protected]

Raman spectra of AC and Pd-Ni @AC are shown. The peaks at 1350 and 1580 cm1 are the remarkable scattering peaks in this figure. It is known that the ID/IG ratio is the intensity ratio of the D to G band and can be employed to find the degree of modification or defects in the AC. In the present study, the ID/IG values of AC and Pd-Ni @AC were found to be 0.53 and 0.90, respectively, which indicate the increasing disorder in the AC lattice after functionalization with Pd-Ni. The X-ray photoelectron spectroscopy data for Pd and Ni were studied using the Gaussian-Lorentzian method [97–105]. The relative intensity of the species was evaluated by counting each peak’s integral after smoothing and subtracting the Shirleyshaped background. The binding energies (0.3 eV) were determined by referencing

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Nanocarbon and its Composites

Fig. 2.4 Pd 3d (A) and Ni 2p (B) XPS spectra of [email protected] NPs.

the C 1s peak at 284.6 eV in the XPS spectra. The XPS results are shown in Fig. 2.4. After comparing the experimental binding energies (Pd-3d5/2 was 335.6 eV and Ni-2p3/2 was 856.3 eV), it was understood that Pd and Ni at the surface were not oxides, but were mostly metallic. For the Ni binding energy, the shift of the 2p3/2 peak to the lower energy indicated an alloying process of Pd-Ni. These findings showed that Ni and Pd existed as elements in the Pd-Ni @AC NPs produced in the present study rather than O2-containing oxide compounds. Some metal oxides in the prepared nanomaterials can be a possible surface oxidation or chemical sorption of oxygen during the fabrication procedures. After the completion of characterization of Pd-Ni @AC with the help of various techniques, the catalytic performances of Pd-Ni @AC were evaluated for DMAB dehydrogenation. The experiments showed that the Pd-Ni @AC NPs are highly efficient catalysts for DMAB dehydrogenation. In Fig. 2.5A, the graph of nH2/nDMAB versus time can be seen, indicating the DMAB dehydrogenation while there were nanocatalysts at different amounts at 25.0 0.1°C. Hydrogen evolution starts rapidly and continues until the reaction is completed. The nuclear magnetic

Highly active and reusable nanocomposites for hydrogen generation

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Fig. 2.5 (A) Plot of nH2/nDMAB vs time for the DMAB dehydrocoupling with the existence of [email protected] NPs at different catalyst concentrations at 25  0.1oC; (B) % conversion vs time plots for [email protected] NPs (7.5% mol) catalyzed dehydrocoupling of DMAB in THF at different temperatures; (C) Arrhenius plot; and (D) Eyring plot.

resonance analysis showed that the conversion of (CH3)2NHBH3 (δ ’ 12.6 ppm) to [(CH3)2NBH2]2 (δ ’ 5.1 ppm) occurred completely. This indicates that DMAB dehydrocoupling (at 1.0 equiv. H2 generation) can happen at room temperature. Fig. 2.5B was depicted to determine the hydrogen production rate constants throughout the DMAB dehydrogenation. In this figure, the change of conversion percentage at different temperatures (20oC, 25oC, 30oC, and 35oC) was displayed. By using the rate constants and Fig. 2.5C, the Arrhenius plot, the Ea was found to be 46.99  2 kJ mol1. With the help of Fig. 2.5D, the Eyring plot, the Δ H (activation enthalpy) and Δ S (activation entropy) were calculated to be 45.17 kJ mol1 and 68.82 J mol1 K1, respectively. Furthermore, in the DMAB dehydrocoupling’s transition state, an associative mechanism was observed by looking to the activation entropy and the activation enthalpy values. To sum up, in this study, a high catalytic activity was observed when Pd-Ni @AC NPs (316.41 h1) were utilized for the DMAB dehydrocoupling. It should be stated that with the help of Pd-Ni @AC NPs, hydrogen gas (1 mol H2/1 mol DMAB) was completely emitted in the DMAB dehydrogenation within a short time at 25  0.1oC. Pd-Ni @AC NPs can be considered a good nanocatalyst because they are isolable and

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Nanocarbon and its Composites

100

% conversion

100

96

90

85

83

81

80 60 40 20 0 1

2

3

4

5

6

run

Fig. 2.6 Plots % conversion versus time graph for [email protected] NPs (7.5 % mol) catalysed dehydrocoupling of DMAB in THF at room temperature for first and sixth catalytic runs.

reusable when utilized in catalytic reactions substantially. The probable reason is the stability of AC and the cooperative and synergistic impact of Pd and Ni in the present study’s catalyst system. Besides, the reusability of the AC-supported Pd-Ni NPs was investigated, as shown in Fig. 2.6. To do that, DMAB was introduced subsequently after the first catalysis reaction in the DMAB dehydrogenation. At the end of the sixth experiment, Pd-Ni @AC NPs maintained 81% of their initial performance. In the DMAB dehydrocoupling, the probable reason for the catalytic activity decrease can be the passivation, as there are increased amounts of nanoparticles on surfaces, and therefore active site accessibility becomes low. The aggregation of NPs was shown after six cycles of the catalytic experiment because, even after six cycles, the catalyst retains its initial content, as found by the ICP analysis (12.95% metals based) study.

2.4

Conclusions

In conclusion, AC-decorated Pd-Ni NPs were shown to be a highly efficient catalyst for DMAB dehydrogenation and the significant steps related to its fabrication, analytical examination, and utilization are as follows: l

l

l

l

l

l

The sodium hydroxide-assisted reduction method was utilized to fabricate Pd-Ni @AC NPs. The results showed that the fabrication method was very operative to distribute Pd-Ni NPs uniformly on the AC material and to prevent the agglomeration problem of Pd-Ni NPs. Pd-Ni @AC NPs had good catalytic performances in DMAB dehydrogenation compared to the literature data. A high TOF (316.41 h1) value was achieved for DMAB dehydrocoupling. 46.99 3 kJ mol1 of Ea was calculated for dehydrocoupling of dimethylamine borane when Pd-Ni @AC nanocatalysts were used. According to these results, Pd-Ni @AC NPs are promising materials and can be used in many catalytic applications as a nanocatalyst.

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Acknowledgments The authors would like to thank DPU-BAP (2014-05 and 2015-50) for financial support.

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[88]

[89]

[90]

[91]

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[93]

[94]

[95]

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and dehydro-coupling of dimethylamine-borane (DMAB). J Nanosci Nanotechnol 2016; (6):5951–8. Erken E, Yildiz Y, Kilbas B, Sen F. Synthesis and characterization of nearly monodisperse Pt nanoparticles for C1 to C3 alcohol oxidation and dehydrogenation of dimethylamine-borane (DMAB). J Nanosci Nanotechnol 2016;16:5944–50. € Erken E, Sen F. Monodisperse Pt (0)/[email protected] Celik B, Başkaya G, Karatepe O, nanoparticles as highly active catalysts for alcohol oxidation and dehydrogenation of DMAB. Int J Hydrog Energy 2016;41:5661–9. Sen B, Kuzu S, Demir E, Akocak S, Sen F. Polymer-Graphene hybride decorated Pt nanoparticles as highly eficient and reusable catalyst for the dehydrogenation of dimethylamine-borane at room temperature. Int J Hydrog Energy 2017. https://doi. org/10.1016/j.ijhydene.2017.05.112. Sen B, Kuzu S, Demir E, Akocak S, Sen F. Highly monodisperse Ru-Co nanoparticles decorated on functionalized multiwalled carbon nanotube with the highest observed catalytic activity in the dehydrogenation of dimethylamine borane. Int J Hydrog Energy 2017. https://doi.org/10.1016/j.ijhydene.2017.06.032. Celik B, Kuzu S, Erken E, Sert H, Koskun Y, Sen F. Nearly monodisperse carbon nanotube furnished nano catalysts as highly efficient and reusable catalyst for dehydrocoupling of DMAB and C1 to C3 alcohol oxidation. Int J Hydrog Energy 2016;41:3093–101. Sen F, Karatas Y, Gulcan M, Zahmakiran M. Amylamine stabilized platinum (0) nanoparticles: active and reusable nano catalyst in the room temperature dehydrogenation of dimethylamine-borane. RSC Adv 2014;4:1526–31. € Başkaya G, Sert H, Kalfa OM, Sen F. New Pt (0) Erken E, Pamuk H, Karatepe O, nanoparticles as highly active and reusable catalysts in the C1–C3 alcohol oxidation and the room temperature dehydro-coupling of dimethylamine-borane (DMAB). J Clust Sci 2016;27(1):9–23. C¸elik B, Erken E, Eriş S, Yıldız Y, Şahin B, Pamuk H, Sen F. Highly monodisperse Pt (0) @AC NPs as highly efficient and reusable catalysts, the effect of the surfactant on their catalytic activities in room temperature dehydro-coupling of DMAB. Catal Sci Technol 2016;6:1685–92. Karatepe O, Yıldız Y, Pamuk H, Eris S, Dasdelen Z, Sen F. Enhanced electro catalytic activity and durability of highly mono disperse [email protected] nanocomposites as a novel catalyst for electro-oxidation of methanol. RSC Adv 2016;6:50851–7. Yıldız Y, Kuzu S, Sen B, Savk A, Akocak S, Sen F. Different ligand based monodispersed Pt nanoparticles decorated with rGO as highly active and reusable catalysts for the methanol oxidation. Int J Hydrog Energy 2017;42(18):13061–9. Dasdelen Z, Yıldız Y, Eris S, Sen F. Enhanced electrocatalytic activity and durability of Pt nanoparticles decorated on GO-PVP hybride material for methanol oxidation reaction. Appl Catal B Environ 2017;219C:511–6. Eris S, Dasdelen Z, Sen F. Enhanced electrocatalytic activity and stability of monodisperse Pt nanocomposites for direct methanol fuel cells. J Colloid Interface Sci 2018;513:767–73. Eris S, Daşdelen Z, Sen F. Investigation of electrocatalytic activity and stability of [email protected] catalyst prepared by in-situ synthesis for Methanol electrooxidation. Int J Hydrog Energy 2018;43(1):385–90. Eris S, Daşdelen Z, Yıldız Y, Sen F. Nanostructured Polyaniline-rGO decorated platinum catalyst with enhanced activity and durability for Methanol oxidation. Int J Hydrog Energy 2018;43(3):1337–43.

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[96] Ozturk Z, Sen F, Sen S, Gokagac G. The preparation and characterization of nano-sized Pt-Pd/C catalysts and comparison of their superior catalytic activities for methanol and ethanol oxidation. J Mater Sci 2012;47:8134–44. [97] Sen B, Akdere EH, Savk A, Gultekin E, Goksu H, Sen F. A novel thiocarbamide functionalized graphene oxide supported bimetallic monodisperse Rh-Pt nanoparticles (RhPt/[email protected] NPs) for Knoevenagel condensation of aryl aldehydes together with malononitrile. Appl Catal B Environ 2018;225(5):148–53. [98] Sen F, Ertan S, Sen S, Gokagac G. Platinum nano-catalysts prepared with different surfactants for C1-C3 alcohol oxidations and their surface morphologies by AFM. J Nanopart Res 2012;14:922–6. [99] Sen F, Sen S, Gokagac¸ G. Efficiency enhancement of methanol/ethanol oxidation reactions on Pt nanoparticles prepared using a new surfactant, 1, 1-dimethyl heptanethiol. Phys Chem Chem Phys 2011;13:1676–84. [100] Sen F, Sen S, Gokagac G. High performance Pt nanoparticles prepared by new surfactants for C1 to C3 alcohol oxidation reactions. J Nanopart Res 2013;15:1979. [101] Sen F, Gokagac G. Different sized platinum nanoparticles supported on carbon: an XPS study on these methanol oxidation catalysts. J Phys Chem C 2007;111:5715–20. [102] Sen F, Gokagac¸ G. Activity of carbon-supported platinum nanoparticles toward methanol oxidation reaction: role of metal precursor and a new surfactant, tert-octanethiol. J Phys Chem C 2007;11:1467–73. [103] Sen F, Gokagac¸ G. Improving catalytic efficiency in the methanol oxidation reaction by inserting Ru in face-centered cubic Pt nanoparticles prepared by a new surfactant, tert-octanethiol. Energy Fuel 2008;22:1858–64. [104] Sen F, Gokagac G. Pt nanoparticles synthesized with new surfactans: improvement in C1-C3 alcohol oxidation catalytic activity. J Appl Electrochem 2014;44(1):199–207. [105] Sen S, Sen F, Gokagac¸ G. Preparation and characterization of nano-sized Pt-Ru/C catalysts and their superior catalytic activities for methanol and ethanol oxidation. Phys Chem Chem Phys 2011;13:6784–92.

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Elena Stojanovska*, Mehmet Durmus Calisir*,†,‡, Nur Dilara Ozturk*,†, Ali Kilic* *TEMAG Labs, Istanbul Technical University, Istanbul, Turkey, †Department of Nanoscience and Nanoengineering, Istanbul Technical University, Istanbul, Turkey, ‡Department of Nanotechnology Engineering, Recep Tayyip Erdogan University, Rize, Turkey

Chapter Outline 3.1 Introduction 43 3.2 Carbon-based foams: Types and preparation methods

44

3.2.1 Carbon foams from polymer precursors 44 3.2.2 Carbon foams from nanostructured carbons 46 3.2.3 Doped and composite carbon foam structures 51

3.3 Applications of carbon-based foams 3.3.1 3.3.2 3.3.3 3.3.4

59

Carbon foams for energy storage 59 Carbon foams as adsorbents 66 Carbon foams for insulation applications 69 Carbon foams for sensor applications 73

Conclusion 79 References 81

3.1

Introduction

By definition, carbon foam is a “porous carbon product containing regularly shaped, predominantly concave, and homogeneously dispersed open or closed cells (that) interact to form a three-dimensional array throughout a continuum material of carbon, predominantly in the nongraphitic state” [1]. Its porosity as the main characteristic is defined by the so-called cells, which actually are the voids (pores) surrounded by carbon walls [2]. Besides the cellular foam structures, reticulated carbon foams in which the walls are actually interconnected struts also exist [3]. In general, carbon walls consist of graphitic or nongraphitic carbon derived from a polymeric carbon precursor. However, with the discovery of nanostructures such as nanotubes and graphene, new forms of carbon foams have been developed [4, 5]. Based on the production process and the starting material, the pores inside the foam structure may differ in size and be open (interconnected with each other) or closed [3, 6]. Most of the studies on pure carbon foams report carbon foams with open and macrosized pores. However, this is not the case when doping elements are introduced into its structure or when an Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00003-1 © 2019 Elsevier Ltd. All rights reserved.

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activation process is applied, which results in the formation of meso- and microporous foam structures [7, 8]. With their solid matrix and porous structure, carbon foams are characterized by low density, sound and electromagnetic wave absorption, thermal and electrical conductivity, and tailorable mechanical properties [9]. All these properties, which can be tailored using different raw materials and production methods, and the possibility of combining them with other elements and/or compounds into unique composite structures make them suitable for a wide range of applications. Carbon foams were reported for the first time in the middle of the 20th century, and since then, their development and application areas have seen steady growth [10]. Despite the general idea of carbon foams being made of a polymer precursor, today we can also find foams made of other carbon-based structures (such as carbon nanofibers, carbon nanotubes, or graphene) that are generally not described as carbon foams. Therefore, we use the term “carbon-based foams.” Divided into two main parts, this chapter gives insight into the area of carbon-based foams. In the first section, the different types of carbon-based foams are described, based on the carbon material they are made of and the methods used for the production of each type. The second part of the chapter collects the recent progress of the four application areas of carbon-based foams: energy storage, adsorption, insulation, and sensors.

3.2

Carbon-based foams: Types and preparation methods

This part of the chapter describes the general production methods of carbon foams obtained from polymer precursors as well as the methods to produce foams using different carbon nanostructures. The methods used to produce doped and composite carbon-based foams are also discussed here.

3.2.1

Carbon foams from polymer precursors

Thermal treatment of organic compounds in an inert atmosphere is the general approach for producing carbon materials. However, in order to achieve the desired porous structure, as in the case of carbon foams, “holes” have to be created within the polymeric precursor before or during the carbonization process. In this regard, there are two main approaches to obtain carbon foams from polymeric carbon precursors: blowing or the template method [2]. The blowing method can be either internal or external, depending on the type of blowing agent used to form the pores. For the former, the precursor is first kept at an elevated temperature and under pressure. Under such conditions, the precursor releases gases that form bubbles, which further coalesce and form a cell [9]. During the foaming process, different parameters such as temperature, initial pressure, pressure drop rate, and solvent and/or gas proportions have to be controlled in order to obtain the foam with desired cell structure [11]. The as-formed foam is further carbonized in order to get the final carbon foam [12]. A similar blowing mechanism occurs when the carbon precursor is mixed with a chemical that

Carbon-based foams: Preparation and applications

45

releases gases during the heat treatment of the mixture. Here, the type and amount of blowing agent and other additives play an important role in the formation and properties of the cells that form the foam structure [9, 13, 14]. Different types of thermoplastic materials such as pitches, phenolic resins, tannin, and sucrose are used in this method [2, 14–16]. Unlike the blowing method, where the formation of bubbles cannot be precisely controlled, the template method allows foams with well-controlled porosity and cell structure [17] where the type of template dictates the structure of the foam that will be obtained. In this regard, several approaches using foam or particle templates have been proposed. Polymeric foams (melamine foam, melamine-formaldehyde foam (MF), polyurethane foam (PU), etc.) can be directly carbonized or impregnated into resins and subsequently carbonized to form carbon foam [18–20]. Cross-linked protein-based hydrogels obtained by the freeze-drying method can also serve as a template, which after carbonization can be functionalized [21]. As-produced carbon foams generally have an open cell structure, and their porosity and density strongly depend on the template used. In another approach, sacrificial particles with different shapes and sizes are first well mixed together with the carbon precursor and then removed [17, 22]. A wide range of inorganic and organic templates has been used for the formation of porous carbon structures with uniform and well-defined pore type and size. Among the inorganic templates, zeolite, mesoporous silica, mesocellular aluminosilicate foam (AlMCF), or nickel particles are the most widely used [17, 22]. While the inorganic templates are removed after carbonization by etching with a suitable solvent, the polymeric types usually are selected in a way to be removed during the carbonization process due to their lower thermal stability. In this group, polystyrene spheres, poly(methyl methacrylate) (PMMA) fibers, or sol-gel polymer blends such as PAN/PVA (polyvinyl alcohol) have been proposed so far [17, 23]. In this type of synthesis, carbon can be obtained by different polymer precursors such as poly(acrylonitrile) (PAN), poly(furfuryl alcohol), poly(vinylpyrrolidone) (PVP), furfuryl alcohol (FA), or phenol-formaldehyde resins (PF) [17, 22, 23]. CFs from polymer precursors can be classified as graphitic and nongraphitic according to the degree of graphitization. Graphitic foam has superior mechanical properties as well as thermal and electrical conductivities. In general, these foams are derived from graphitization of coal or pitch at around 2400°C. These kinds of CFs can be used as a heat sink material for the cooling of power electronics because of their light weight (0.2–0.3 g/cm3) and high thermal conductivity (180 W/mK) [24]. Nongraphitic foam is prepared by carbonization of organic polymers such as phenol formaldehyde [25, 26], sucrose [27], and fluorinated polyimide [28]; it is then used as a thermal insulation material because of its low thermal conductivity. Among them, the phenolic resins are the most suitable precursor material because of their excellent flame resistance and low toxic gas emission during carbonization. Additionally, a high self-ignition temperature (>480°C) and thermal stability over a broad temperature range make this precursor favorable for thermal insulation applications [29]. However, phenolic resin-based CFs have inferior mechanical properties than others due to the formation of glassy carbon during the carbonization process [30].

46

3.2.2

Nanocarbon and its Composites

Carbon foams from nanostructured carbons

The fast development of nanomaterials in the last few decades has brought new opportunities in the area of carbon foams. The discovery of unique nanocarbon structures and their large-scale production led the development of carbon-based foams with unique characteristics. Carbon nanofibers (CNF), carbon nanotubes (CNTs), or graphene (G) with their peculiar physical, chemical, and electrical properties enabled the formation of foam structures with the controlled density, pore size, flexibility, and electrical conductivity required for high-tech applications.

3.2.2.1

Carbon nanofiber-based carbon foam

Carbon fiber foams (CFF) are formed from carbon fibers with diameters from several tens up to a few hundreds of nm. The constrained formation of the fibrous nanostructure process (CoFFiN) is the widely used method to produce CFF [31, 32]. The CoFFiN process relies on carbon fiber growth as a consequence of the decomposition of the gaseous carbon into a closed mold in the presence of a catalyst. The catalyst is placed in a mold and heated until a certain temperature at atmospheric pressure while the gaseous carbon precursor is purged. A variety of bulk carbon foams can be obtained with this method by modifying different parameters such as temperature, catalyst type, and gaseous precursor (one or a mixture of several gaseous carbons). The main advantage of this process is the possibility of obtaining CFF with a specific shape and morphology at a low temperature (usually below 700°C). Using this production process, Atwater et al. [31] prepared a high-density nanoscale nonwoven carbon structure (Fig. 3.1) with high stability. The obtained low strain value clearly shows the influence of foam density on its mechanical and electrical properties. In another study, Luhrs et al. [32] developed a foam containing intertwined fibers and voids. Due to this specific morphology, the foam showed good compressibility and viscoelastic properties. During loading, it exhibited linear stress-strain behavior similar to elastic materials while, when the load was released, it

Fig. 3.1 (A) As-prepared carbon fiber foam produced by CoFFiN process; (B) CFF under bending forces; and (C) SEM image showing the morphology of CFF. Adapted with permission from Atwater MA, Mousavi AK, Leseman ZC, Phillips J, Direct synthesis and characterization of a nonwoven structure comprised of carbon nanofibers, Carbon 57 (2013) 363–370.

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demonstrated the stress-strain behavior of a viscous material. Furthermore, the foam showed hydrophobic properties as well as high thermal and electrical conductivity that were affected by the fiber length.

3.2.2.2 Carbon nanotube-based carbon foams The production of carbon nanotube (CNT) network foams can be achieved by two ways: phase separation or deposition on a template [4, 33]. Phase separation can be divided into two subgroups such as foaming (or gas-liquid phase separation) and freezing (solid-liquid phase separation) techniques, as shown in Fig. 3.2A [4]. In the gas-liquid phase separation method, the CNT/polymer suspension is bubbled, followed by separation of the foamed part from the suspension. Worsley and colleagues used this method to obtain stiff, low-density, and electrically conductive

Fig. 3.2 Schematic representation of the two main preparation methods of CNT foam: (A) phase separation method and (B) deposition method. Adapted with permission from Nakagawa K, Foam materials made from carbon nanotubes, In S. Bianco (Ed.), Carbon nanotubes—from research to applications, InTech: Rijeka; 2011, pp. 313–334.

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carbon nanotube foam [34]. They used sol-gel precursors that formed carbon nanoparticles after carbonization. The enhanced properties of the foam were the result of these carbon nanoparticles that acted as a binder for the carbon nanotubes. The carbon nanoparticles not only supported the structure but also enhanced the electrical conductivity and enabled the formation of foam in different shapes. The prepared foam containing a high amount of CNT reached a strain of 76% during the loading-releasing process, which is about 12 times stiffer than that of silica. On the other hand, during the freezing method, the CNT/polymer suspension is frozen in order to form ice that separates the CNTs into a network after sublimation [35, 36]. It is important to note that the velocity of the suspended particles, the freezing front velocity, and the freezing rate influence the foam microstructure [4]. The foam can be prepared by contact freezing or immersion freezing techniques that determine the microstructure of the foam. A foam obtained by the contact freezing technique has cylindrical pores while the CNT foam after immersion has linear monolith pores. Moreover, faster cooling leads to smaller pores in the foams [4]. Other parameters that influence the properties of the CNT foam in the freezing process are the temperature, the pressure, and the amount and type (i.e., single or multiwalled CNTs) of CNT [37]. Higher temperatures lead to lower cell density. However, increasing the temperature does not have a significant influence on the wall thickness of the composite. Adding more multiwalled carbon nanotubes (MWCNT) decreases the pore size [37] and their distribution in the polymer matrix affects the homogeneity of the pore structure. Furthermore, increasing pressure generally reduces the pore size and cell-wall thickness of the composite. However, over a certain level, that makes the structure denser where the cells start to merge [37]. It is important to control each parameter during the synthesis because the volume expansion of the final composite is affected by the pore size and number of nucleation sites. When porous media is used as the substrate for the CNT deposition, the process is called the deposition method. The deposition of CNTs can be conducted by immersion of the foam template into a CNT suspension or by deposition of the tubes at high temperatures using carbonaceous gas as a precursor. Both methods require further treatment in order to remove the template or to make a composite foam structure. In the first case, a heating or freezing treatment may be applied while for the later, chemical etching with a suitable solvent is used to remove the template (or catalyst) [33, 38]. In this regard, Jin and colleagues prepared three-dimensional (3D) CNT/PAN nanofiber composite materials with excellent mechanical properties and high adsorption capacity [38]. They used the electrospinning method to produce a PAN nanofiber and an ethanol bath as a collector. A subsequent freeze-drying step enabled a self-assembly process for the production of 3D PAN nanofiber foam. Later, the 3D nanofibrous structure was immersed into a CNT solutions with a different concentration and again freeze dried to obtain the final 3D CNT/nanofiber foam structure. On the other hand, Paul et al. [39] used microwave plasma-assisted chemical vapor deposition (CVD) to prepare highly porous foam. They used nickel foam as a template for the CNT growth and methane gas as the carbon precursor. The nickel foam was first coated with three layers of titanium (Ti), aluminium (Al), and iron (Fe), which served as a catalyst for the controlled growth of the MWCNTs. The resultant foam had a hierarchically

Carbon-based foams: Preparation and applications

49

graphitic microstructure with very high porosity and ultralow density. Activated CNT foam was prepared by Wang and colleagues [40]. They first used the low-temperature method in the presence of metal magnesium and nickel foam to convert CO2 into CNT foam. Then they applied thermal treatment in the presence of potassium hydroxide to form the final microporous CNT foam with a specific surface area of 1487 m2 g1.

3.2.2.3 Graphene-based carbon foam Low weight, high porosity, and high conductivity are the main properties of graphene foam (GF). Its porous structure allows good mixing with nanoparticles, polymers, and other materials, forming a composite structure with high mechanical stability. Additionally, the production methods used for its synthesis are facile and reproducible. Generally, GF is produced by the CVD method. However other methods such as self-assembly synthesis at room temperature, solvothermal synthesis in the presence of nanoparticle or foam templates, and a 3D printing technique have also been investigated. The mechanism of the CVD process can be described as the growth of foam on a metallic template [5]. The process starts when nickel (Ni) or copper (Cu) foam (the most commonly used templates) is heated to about 1000°C at a constant rate under an argon and hydrogen atmosphere [41]. Then, methane (CH4) or another carbon source is purged into the system at the same temperature for a certain time to form a foam with the desired thickness. After the system cools down to room temperature, the foam is first coated with a temporal protective polymer layer (such as PMMA) in order to protect its degradation during the next step, which is the removal of the Ni skeleton with an acidic media. As the last step, the temporal polymer coating is removed, also by etching with a suitable solvent. The properties of the GF produced by this method are affected by the carbon source, the temperature and the time at which the carbon source is purged, the template material, the template etching agent, and the cooling velocity [42, 43]. The type of carbon source and the time it is purged determine the number of graphene layers as well as the thickness and density of the final foam structure. In general, increasing the carbon source causes an increase of graphene layers and foam density. It has been reported that increasing the CH4 concentration from 0.7 to 4.0 vol% increases the number of graphene layers from three to 27 and the foam density from 1.3 to 6.8 mg/cm3 [43]. Regarding the templates, it can be said that the Ni skeleton is more convenient to form GF than Cu foam under the acetone etchant ambient [44]. Huang and colleagues used graphene oxide sheets and modified metal oxide nanospheres to prepare graphene nanofoam [45]. Graphene sheets, which are hydrophobic in the center and hydrophilic at the edges, were mixed with methane-modified silica (SiO2) spheres into a proper solution for a self-assembly synthesis at room temperature (Fig. 3.3). Subsequently, calcination and etching were applied to solidify the structure and to remove the SiO2 particles. As a final product, they obtained a graphene foam with average pore wall thickness of 0.89 nm, surface area of 851 m2 g1, and pore volume of approximately 4.28 cm3 g1. The pore size difference was the result of the presence of silica spheres and their agglomeration. In order to

50

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: JCH3 modified silica sphere

Graphene oxide

+

I

Assembly

II Calcination/HF etching

NGFs

GO/silica spheres composite

Fig. 3.3 Graphene foam obtained via self-assembly synthesis with nanoparticle templates. Adapted with permission from Huang X, Qian K, Yang J, Zhang J, Li L, Yu C et al., Functional nanoporous graphene foams with controlled pore sizes, Adv Mater 24 (2012) 4419–4423.

prevent agglomeration and to obtain foam structure with uniform pore size, methyl functionalized silica nanoparticles were synthesized and used as templates. The nanoporous graphene foams (NGF) with a pore size of about 58 and 94 nm were produced by using modified silica particles with a diameter of 60 and 120 nm, respectively. Additionally, the authors also showed that metal oxide nanoparticles can be tied into the nanoporous material, which amplifies the application of NGF in different areas [45]. Self-assembly of the graphene network can also be achieved by keeping the graphene-based solution in a closed vessel at elevated temperature. For such hydrothermal synthesis, Song et al. [46] prepared a graphene oxide (GO)/CNT solution and kept it in an autoclave at 250°C for 36 h. During that time, the GO and CNT were coupled into a 3D network that, after freeze drying, formed the macroporous foam structure. In another approach, graphene paste was cast onto a sacrificial nickel foam and dried under vacuum [47]. The foam structure obtained after removal of the nickel template had high specific surface area as well as loose and uniform pores as a result of its gauze-like morphology formed of interconnected graphene layers, wrinkles, and ripples. Another interesting graphene nanofoam preparation technique is 3D printing [48] by using a template, agent (catalyst), and carbon source for foaming, and a CO2 laser for sintering the template/carbon source. The process is conducted in the following steps: i) Carbon source (ex. sucrose) is attached to the template. ii) Template/carbon source layer is exposed to a CO2 laser that moves at high speed and sinters the template/carbon source layer, hence leaving a carbon layer on the template surface. iii) Graphene layer formation by fast cooling. iv) Etching of the skeleton and drying.

Carbon-based foams: Preparation and applications 10.6 mm laser

Laser conversion

Ni powder (particle size 2–3 mm) Sucrose

Ni Sucrose over Ni particle

51 Repeat

Graphene/Ni

Adding another Ni/sucrose layer

Ni/sucrose layer

3D printed GF

1.Ni etching & purification 2.Critical Point Drying

Fig. 3.4 Schematic illustration of the production method of 3D-printed graphene foam. Adapted with permission from Sha J, Li Y, Villegas Salvatierra R, Wang T, Dong P, Ji Y et al., Three-dimensional printed graphene foams, ACS Nano 11 (2017) 6860–6867.

For formation of more graphene layers after the first sintering, a second template/carbon source is added and the same steps are repeated. A schematic illustration of the whole process is given in Fig. 3.4. During the process, the laser cutter, the laser duty cycle, and the rastering speed are important parameters to be controlled in order to obtain the desired foam. Different foam shapes can be produced by using different laser cutters. To obtain a higher energy input, the laser duty cycle should be increased while the speed should be decreased. Sha et al. [48] reported optimum values of 2% for rastering speed and 100% for the cycle. The 3D printed GFs had an average electrical conductivity of 8.7 S cm1, a storage modulus of nearly 11 kPa, a damping capacity between 0.13 and 0.06, and stable structural integrity that could stand mechanical loading for more than 70,000 cycles without collapsing. Foam porosity was approximately 99.3% and its density nearly 0.015 g cm3 [48].

3.2.3

Doped and composite carbon foam structures

Carbon foams with high pore volumes and hierarchical porous structures that are lightweight and have good thermal and electrical conductivity have serious potential applications as catalyst supports, energy storage electrodes, insulation materials, and adsorbents. Depending on the demanded application, CFs have to meet certain criteria. The structural (pore size, distribution, open/close pore structure, pore density), physical properties (electrical and thermal conductivity, foam density, etc.), and mechanical properties of carbon foams are mainly determined by the precursor used and the production parameters (temperature, pressure, foaming process). However, the properties of the CF can be further improved by doping with heteroatoms and/or by the formation of composite foam structures after blending with other inorganic or organic compounds [9, 49].

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3.2.3.1

Nanocarbon and its Composites

Doped carbon foams

Doping is a process of intentional placement of proper heteroatoms into the main structure in order to modify its physical, chemical, and/or mechanical properties. Doping of a porous structure can be performed by three main methods: (1) carbonization of the dopant-containing precursor (such as melamine sponge) [50], (2) carbonization of dopant and carbon precursor together [51, 52], and (3) reaction of porous carbon materials with a gaseous doping precursor [53]. According to the literature, nitrogen [50] and boron [52, 54] are the elements used most often in studies for doped CFs, even though zirconium- [55], cobalt- [8], or gold-doped [56, 57] CFs for specific applications can also be found. The production of nitrogen-added carbon foams (NCF) using melamine sponge as a precursor and N source is generally achieved by single-step pyrolysis. Xiao et al. [50] produced freestanding, hydrophilic nitrogen-doped CFs with direct carbonization of a commercial melamine sponge, (Fig. 3.5A). After carbonization, the samples showed an obvious volume shrinkage compared to the original melamine sponge. In general, the melamine sponge is composed of numerous branched fibers with smooth surfaces (Fig. 3.5B) and after carbonization, it retains the interconnected network, as shown in Fig. 3.5c. However, carbonization at 800°C makes the surface of NCF fibers rougher due to the formation of an N-doped carbon nanosheet layer (Fig. 3.5D). The self-template approach using carbon and metal precursors with or without additional foaming agents has been used for doping CFs with N, B, Zr, or Co [8, 51, 52, 55]. Arami-Niya et al. [51] used banana peels (BP) modified with zinc nitrate, 2-aminophenol, and furfural to produce N-doped activated CFs via the self-template method. The furfural-aminophenol resin used for modification of BPs behaved as a foaming agent during the carbonization process due to devolatilization and polymerization, which leads to the formation of CF. On the other hand, nontreated BPs formed only a porous carbon structure. Carbonization temperature and time played an important role in the control of N-content and porosity of the final product. Using a similar approach, Narasimman et al. [52] produced boron-doped CF from molten sucrose using boric acid as a foaming agent and boron precursor. Formation of B-O-C

Fig. 3.5 (A) Real image and (B–C) SEM and (D) HRSEM images of melamine foam and nitrogen-doped carbon foam. Adapted with permission from Xiao K, Ding LX, Liu G, Chen H, Wang S, Wang H, Freestanding, hydrophilic nitrogen-doped carbon foams for highly compressible all solid-state supercapacitors, Adv Mater 28 (2016) 5997–6002.

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cross-links as a result of a condensation reaction between B-OH and C-OH groups of boric acid and molten sucrose, respectively, was thought to be the main reason for increment in the char with increased boric acid concentration. The cell size and oxidation resistance of B-CF increased with the increase of boron content at an expense of a reduction in the density and compressive strength. There was an increment in pore size and reduction in pore density with increasing boric acid content as well. Additionally, there were microcracks in the cell walls of carbon foams prepared at a boric acid concentration of 4 wt% and above, which explains the reason for the lower compressive performance of these foams. In another study, Li et al. [55] prepared Zr-doped graphitic CFs with an open-cell structure using a molten naphthalene-based mesophase pitch (MP) and ZrOCl2 as carbon and Zr precursors, respectively. The open porosity and nonuniform pore size of the doped CF were attributed to the increased viscosity due to the polycondensation reaction between ZrCl4 (obtained by decomposition of ZrOCl2) and MP, which inhibited the coalescence of bubbles. While Lu et al. [8] synthesized Co-doped mesoporous CF (Co-MCF) using a novel trinuclear cobalt-oxo cluster (Co-OXO), resole resin, polyoxypropylene/polyoxyethylenetriblock copolymers and NaOH as cobalt, carbon precursors, template, and catalyst, respectively. Co-MCF was obtained after carbonization at 900°C of the thermally polymerized mixture from these materials. The as-obtained Co-MCF had a highly ordered mesostructure and uniform pore size, plus improved wettability, an increased graphitization degree, and pseudocapacitative properties. A template-free, two-step method including hydrothermal and thermal treatment processes was used by Ma et al. [53] for production of Fe- and N-doped carbon foams. They used sucrose, melamine, and ferric chloride (FeCl3) as carbon, nitrogen, and iron sources, respectively. The N-doped foam was first produced by a self-assembly hydrothermal synthesis in acidic conditions. Then, the foam was mixed with FeCl3 and subsequently carbonized to get the Fe- and N-doped CFs. In order to compare the 3D effect of CF on the oxygen reduction rate (ORR) and other properties, they also produced Fe-N doped carbon powders using the same method without the hydrothermal step. The CF obtained with a three-dimensionally interconnected hierarchical pore structure exhibited a BET surface area (676 m2 g1) that was about two times larger, a higher ORR (onset potential of 41 mV positive), and methanol tolerance than Fe, N-doped carbon particles, and undoped CFs. A three-step method consisting of oil-in-water (O/W) emulsion polymerization, carbonization, and subsequent activation was used for the synthesis of nitrogen-doped macro/mesoporous carbon foams (N-MMCFs) [58]. In this study, melamine (C3H6N6) is used as a nitrogen source and KOH as the activation agent. The N-MMCFs obtained consisted of uniformly distributed macropores of 0.2 μm and mesopores of 2.6–4.0 nm whereas MMCF without melamine had nonuniformly distributed macropores of 0.8 μm. Formation of homogeneous mesopores in N-MMCF was attributed to a more stable and higher dispersion degree of the emulsions as a result of H-bond formation between the amino group of melamine and the hydroxyl group of surfactant molecules on the oil/aqueous interface. N-MMCF also showed higher specific surface area and pore volume than MMCF-3 and the nonactivated samples.

54

3.2.3.2

Nanocarbon and its Composites

Composite carbon-based foams

Designing a composite structure of metal oxides or sulfides and highly conductive, stable, and porous CF can provide materials with the increased surface area and catalytic properties for several electrochemically based applications such as electrodes for sensors, energy storage devices, etc. In general, metal oxide (MO) materials are synthesized on presynthesized or commercial carbon foams by hydro- or solvothermal treatment with their precursor solutions and a subsequent annealing procedure. Depending on the duration of the thermal reaction time, the precursor and additives used, and the surface treatment of the carbon foam, the resulting MO structure can be in different shapes such as nanosheets (NS), nanorods (NR), nanoflowers (NF), etc. Such composite structures were synthetized using either polymer- or nanostructurebased CFs together with cobalt oxide (Co3O4), nickel cobaltite (NiCo2O4), manganese oxide (MnO2), iron oxide (Fe2O3), cobalt nickel sulfide (NiCo2S4), zinc oxide (ZnO), manganese (II) phosphate (Mn3(PO)4), molybdenum disulfide (MoS2), etc. as metal components [20, 59–69]. As summarized in Table 3.1, in most of the cases, the polymer-based CFs used as a frame for the formation of such composites are usually obtained by direct carbonization of the polymer foam template while the nanostructured CFs are produced by the CVD method using the sacrificial metal template. Infiltration of the metal precursor into CF followed by annealing is another approach for the preparation of composite MO CFs. Kim et al. [96] reported a highly efficient biosensor based on coentrapment of Fe3O4 magnetic nanoparticles (MNP) into the pores of mesoporous CF prepared by this method. They first synthesized mesocellular carbon samples by employing mesocellular silica foam as a hard template. In order to produce MNP-CF composite structure, Fe(NO3)3 was impregnated into the pores of CF and subsequently annealed. After that, GOx was immobilized in the remaining pore space by using glutaraldehyde as a cross-linker. Increasing MNP and GOx enzyme loading caused a reduction in BET surface area and total pore volume of the pure CF, as expected. TEM, XRD, and magnetic analyses showed that MNPs were formed in the pores of the foam with a highly crystalline structure and superparamagnetic feature at room temperature. A novel bio-inspired approach to synthesize 3D porous graphitized carbon scaffolds containing metal fluoride nanoparticles ([email protected],F–C) was reported by Lu et al. [21] The authors first prepared elastin-like polypeptides (ELPs) [containing FLAG tags [(VPGIG)2VPGKG(VPGIG)2]16 CDYKDDDDKL (named ELK16-FLAG)] by lyophilization in order to get a cross-linked 3D hydrogel template. After adsorption of manganese and fluorine ions on the surface of the hydrogel, a heat treatment in an inert atmosphere was applied. The annealed foam contained pores with diameters of about 500 nm, and their surface was homogenously covered with nanoparticles of about 50 nm. Using a similar approach, the same group of authors also prepared open cell MoO2/Mo2C/N composite CF [80]. In another study, Fang et al. [97] used the freeze-drying method to impregnate Ge nanoparticles into N-doped CF (Ge-C/ NCF). For this composite foam production, first a GeO2/polyvinyl alcohol (PVA) solution was prepared. After that, the presynthesized melamine-based NCF was soaked into the solution and subsequently freeze dried. At the end, the foam samples were annealed at 800°C to get the composite foam.

Carbon-based foams: Preparation and applications

55

Table 3.1 Composite carbon-based foams and their production methods Carbon foam type Carbon foam Carbon foam

Doping/ composite material

Carbon foam

N-doped N-doped, MnO2 N-doped

Carbon foam

N-doped

Carbon foam Carbon foam Carbon foam

N-doped Fe, N-doped B-doped

Graphitic carbon foam Carbon foam

B-doped B-doped

Graphene foam Carbon foam

B, N and B,N co-doped B-doped

Carbon foam

Zr-doped

Carbon foam

Co-doped

Carbon foam Carbon foam

Ag-doped Ge, N

Carbon foam

Co3O4

Carbon foam

MnF2, N, F

Carbon foam

CNT, MgO

Carbon foam Carbon foam

Carbon foam

Li7La3Zr2O12 MoO2, Mo2C, N Mesoporous silica Fe2O3

Carbon foam

Silica aerogels

Carbon foam

K2Ti6O13

Carbon foam

Preparation method

Ref.

Carbonization of melamine sponge Etching of melamine sponge with KMnO2 and carbonization Oil-in-water (O/W) emulsion polymerization and carbonization Modification of banana peels with N-precursor and carbonization Solvothermal treatment and pyrolysis Hydrothermal treatment and carbonization Foaming, carbonization and thermal treatment of boron-coal precursor mixture Pyrolysis and thermal treatment of boroncoal precursor mixture Foaming and carbonization of molten sucrose with boric acid CVD

[50] [70]

Carbonization of boron and carbon precursors Carbonization and graphitization of Zr-carbon precursor Polymerization and carbonization of Co and carbon precursors Polymerization and carbonization Carbonization of polymer template, freeze drying, and annealing Carbonization of polymer template and solvothermal treatment Carbonization of coated freeze-dried hydrogel Carbonization of polymer template immersed in CNT/MgO solution Carbonization of dried porous gel Carbonization of freeze-dried hydrogel

[54]

Carbonization of resin-impregnated template Foaming and carbonization of Fe2O3 powder and CF precursor Synthesis of silica aerogels via sol-gel method on CF Carbonization of K2Ti6O13 whiskers and CF precursor

[58] [51] [71] [53] [72] [73] [52] [74]

[55] [8] [75] [76] [77] [21] [78] [79] [80] [81] [82] [83] [84] Continued

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Nanocarbon and its Composites

Table 3.1 Continued Carbon foam type

Doping/ composite material

Carbon foam

MWCNT

Carbon foam Graphene foam Graphene foam Graphene foam Graphene foam Graphene foam Graphene foam Graphene foam Graphene foam Graphene foam Graphene foam Graphene/ CNT hybrid foam Graphene foam Graphene foam Graphene/ CNT hybrid foam Graphene foam

Preparation method

Ref. [29]

Ni+2 ZnO nanowire

Carbonization of MWCNT and CF precursor Carbonization of polymer template CVD

[85] [86]

Ag

Freeze-drying method

[87]

ZnO nanorod

CVD and hydrothermal treatment

[65]

TiN

[88]

MoS2

Electrospinning and liquid-phase photopolymerization Hydrothermal treatment

[89]

NiO

Hydrothermal reflux treatment

[90]

SnO2

Hydrothermal treatment and freeze drying

[91]

PDMS

Infiltration of polymer and CVD

[92]

PDMS, Ni

Infiltration of polymer and CVD

[93]

PDMS

CVD and infiltration of polymer

[94]

N, MnO2

CVD and hydrothermal treatment

[66]

Mn3(PO4)2

CVD and hydrothermal treatment

[67]

MoS2

CVD and hydrothermal treatment

[68]

Ni(OH)2, Fe2O3

CVD, electrodeposition, and hydrothermal treatment

[69]

MoO3

Solvothermal treatment and carbonization

[95]

Metal oxide CFs can also be prepared by carbonization of the CF precursor with metal oxide nanostructures [82, 84] and foams [81] or heat treatment of coated CF with metal oxide slurry [54]. In order to prepare the Fe2O3/CF composite by the former method, Lee et al. [82] added iron (III) oxide powder into a mixture of CF precursor, organic acid, and hydrocarbon as curing and foaming agents, respectively.

Carbon-based foams: Preparation and applications

57

A porous silica-carbon nanocomposite foam (MSCF) was prepared by impregnating furfuryl alcohol as a carbon source into the mesocellular silica foam (MSF) template, followed by polymerization of the carbon source with the aid of a small amount of organic acid and carbonization in an inert atmosphere [81]. Using this approach, the mesoporous size can be easily controlled by changing the pore size of the MSF template while hydrophilicity and conductivity can be changed by adjusting carbon-silica ratios. In a similar approach, Wang et al. [54] used either H3BO3 or B2O3 as additives for doping of the CF and for the formation of the boron coating layer. In their study, the phenolic resin was first obtained by mixing the proper amount of phenol and formaldehyde in the presence of NaOH. The as-prepared resin was mixed with a certain amount of boron precursor and hollow microsphere, followed by curing and carbonization in order to get B-modified CF. Later on, they applied a B2O3-based slurry on the surface of the synthesized B-modified CF with a brush and did annealing at 700°C. All modified/coated samples exhibited higher oxidation resistance than pure CF. This effect was attributed to the formation of a melt glass layer on the surface of CFs that prevents oxygen penetration at high temperature. The CF coated and modified with 7 wt% B2O3 (BO-7) showed the highest stability with only 17.4% mass loss, which was four times lower than that of pure CF. Additionally, oxidation resistance increased with increasing boron content for all doped samples. As another approach for composite CFs, KMnO4 was used as an etchant and catalyst precursor in a two-step process (Fig. 3.6) [70]. The authors first etched the melamine sponge (MS) with a KMnO4 solution, which led to the formation of etched N-doped CF (ENCF) consisting of exfoliated carbon fibers with a higher surface area than the MS. In addition, they obtained MnO2 after pyrolysis of KMnO4, impregnated MS. During the pyrolysis, KMnO4 employed two functions: as a graphitization catalyst that improves the degree of graphitization (the ratio of G mode to D mode was increased from 0.801 to 0.967 with the etching process) and as an electroactive substance that improves the pseudocapacitance of electrode materials. As expected, the ENCF had a higher electrical conductivity than that of the untreated NCF carbonized at 900°C sample (3.8 S cm2) while still maintaining its outstanding mechanical properties. CF can also be used as a 3D supporting matrix for high surface area inorganicorganic composite materials. Such a hybrid structure, consisting of silver nanosheets (Ag NS) grown on polyaniline (PANI)-coated CF (Ag [email protected]/CF), was prepared by Xu et al. [49]. For that purpose, PANI was directly synthesized on the surface of the melamine-based CF. Later on, the PANI/CF was immersed in a hydrazine solution and then into an AgNO3 aqueous solution containing succinic acid. In the final hybrid material, PANI was coated on the surface of CF in the form of nanobars while Ag was formed on the PANI layer in the form of NS. The study showed that silver growth time and the introduction of succinic acid had great importance in the formation of interlaced silver NS with full coverage on the 3D scaffold. The increasing growth rate resulted in a highly covered surface and thicker NS while the presence of succinic acid provided the formation of AgNS instead of agglomerated Ag NPs. Regardless of the polymer-based carbon foams, those based on CNT or graphene are also widely used as a matrix for composite structures with both organic and

58

Nanocarbon and its Composites

Fig. 3.6 (A-1  C-1) Schematic illustration of the formation of ENCF and structure evolution images: (A-2) MS, (B-2) MS after etching, and (C-2) ENCF. Adapted with permission from Xiao K, Zeng Y, Long J, Chen H, Ding LX, Wang S et al., Highly compressible nitrogen-doped carbon foam electrode with excellent rate capability via a smart etching and catalytic process, ACS Appl Mater Interfaces 9 (2017) 15477–15483.

inorganic compounds. In this case, a multistep synthesis consisting of a combination of several production methods is employed. Among them, the CVD and hydro/ solvothermal methods are mostly used not only for preparation of the CF matrix but also for the synthesis of the metal/metal oxide or other carbon components [76, 86, 93]. Chen et al. [76] reported a flexible, lightweight 3D CNT-GF composite structure covered with porous α-Fe2O3 nanorods. They first synthesized the GF via the CVD method by employing Ni foam as a sacrificial template. Second, they homogeneously covered the surface of the GF with metallic Ni and Co particles by the hydrothermal method and used them as a catalyst for CVD growth of CNT. Then, FeOOH nanorods were grown on the surface 3D CNT-GF by the solvothermal method. Finally, the Fe2O3/CNT–GF was obtained by annealing at 400°C for 1 h in an air atmosphere. Employing the bottom-up method for the growth of CNT and Fe2O3 endowed rigid contact at the interface of Fe2O3-CNT and CNT-GF, and prevented their agglomeration. Polymer-filled GFs (graphene foam) were prepared by Zhao et al. [41]. The authors first used CVD for the preparation of GF. Then, polydimethylsiloxane (PDMS) was infiltrated into the graphene foam, followed by vacuuming in order to remove bubbles and curing at 80°C. Due to the high porosity of the GF, the polymer was able to fully penetrate not only into the GF network but also between the graphene layers of the foams’ arm. Another composite foam structure was prepared by immersing

Carbon-based foams: Preparation and applications

59

macroporous sponges into CNT/GO suspension [98]. These sponges are used to prepare an all-carbon foam that contains all three types of carbonaceous materials (CNTs, GO, and carbon). The production method included three steps: (1) impregnation of CNT/GO into a sponge using a highly dispersed CNT/GO solution containing a silica-based template, (2) carbonization, and (3) etching of the silica-based template. The obtained 3D all-carbon foam had well-dispersed graphene CNT into the mesoporous carbon network as well as bimodal pores with a size of 5.1 and 2.7 nm and surface area of 1286 m2/g.

3.3 3.3.1

Applications of carbon-based foams Carbon foams for energy storage

Energy storage systems allow energy to be stored and used when needed. Based on the type of energy being stored and the principle on which it is stored, a system can be categorized into mechanical, electrochemical and/or electrical, chemical, thermal, and thermochemical [99]. Among them, electrochemical and thermal energy storage systems are the two main groups in which carbon foams can be used. In the following part, recent developments of some of these energy storage systems will be reviewed and the importance of the incorporation of carbon-based foams in their structure will be discussed.

3.3.1.1 Electrochemical energy storage The multifunctionality of C-based foams and their suitability for electrochemical energy storage have been proved in many studies. These studies include the application of the different types of C-based foams as electrodes into several electrochemical energy storage technologies such as supercapacitors [22, 66, 98] and Li-based [100] or Na-ion [101] batteries. The increased porosity and high electrical conductivity of CFs are the main advantages that make them suitable for this application. These characteristics allow easier and faster ion and electron transport throughout the electrode, which increases the rate capability of the electrode while their structural stability improves the life cycle of the energy storage device. Additionally, the possibility to embed electrochemically active materials into the porous foam structure enables the formation of high-capacity electrodes that affects both the energy and power density of the device. In the following section, we offer a brief review of C-based foams that have been applied as electrodes for electrochemical energy storage. Among the different C-based foams, CNT- and GO-based foams provide high surface areas and electrical conductivity, which are the main requirements for capacitative energy storage [102]. Therefore, neat, activated, and/or porous CNT/GO foams are among the most widely investigated electrodes for supercapacitor application. In this regard, an N-doped GF/CNT/MnO2 (NGF/CNT/MnO2) composite was prepared by the two-step CVD method during which graphene foam and CNT cover were formed, followed by hydrothermal synthesis of MnO2 nanosheets [66]. Such foam was used as an electrode for an aqueous supercapacitor exhibiting capacity of

60

Nanocarbon and its Composites

about 284 F/g (at 2 mV/s) in a three-electrode system. The authors showed that increasing the amount of MnO2 positively affects the capacity and life cycle of the composite foam. The composite foam containing 70% MnO2 not only exhibited high capacity retention, but also an increased capacity after 15,000 cycles. This effect was attributed to the increased porosity caused by the repeated insertion and extraction of the electrolyte ions, which resulted in increased distance between the adjacent MnO2 nanosheets within the foam structure. A similar graphene/CNT aerogel with high MnO2 or PPy (polypyrrole) loading (denoted as 3DGA/CNT/MnO2 and 3DGA/ CNT/PPy) was prepared by Pan et al. [103]. This was synthesized in a three-step synthesis consisting of graphene aerogel synthesis by the hydrothermal and freeze-drying methods, growth of CNT from a PVP precursor, and finally, growth of MnO2 by a hydrothermal method or pyrrole polymerization. The two electrodes were used as a pair in a flexible all-solid-state asymmetric supercapacitor exhibiting gravimetric energy and power densities of 18.42 Wh/kg and 2.32 kW/kg, respectively. Manganese phosphate (Mn3(PO4)2) graphene foam composite electrodes synthesized by the hydrothermal method using CVD-growth GF as the substrate were used against a bio-based active carbon electrode [67]. Such an asymmetric cell showed 96% capacity retention after 10,000 cycles at a current of 2 A/g gravimetric, which corresponded to 5.4 Wh/kg and 1.215 kW/kg energy and power densities. The hydrothermal method was also used by Masikhwa et al. [68] for the synthesis of MoS2 graphene foams (MoS2/GF). The composite MoS2/GF electrode with optimum GF loading allowed chemical stability of the MoS2 active material and fast charge transfer during cycling. It exhibited a capacitance of 220 F/g at 2 A/g in a 0.5-V operating voltage window. An aqueous asymmetric supercapacitor in which one electrode was the MoS2/GF showed a maximum energy of 16 Wh/kg, corresponding to a power of 0.76 kW/kg when operating at a voltage of 1.4 V and a current of 1 A/g. An asymmetric supercapacitor with self-supported carbon foam/ordered mesoporous carbon (CF-OMC) as the negative electrode and carbon foam/NiCo2O4 nanosheets as the positive electrode showed higher energy density (47.8 Wh/kg) [20]. Additionally, the device exhibited an extremely stable capacitance of about 125 F/g over prolonged cycling (10,000 cycles) at current density 1 A/g. Both electrodes were prepared on a carbonized mesoporous foam as a frame. The ordered mesoporous carbon structure of the negative electrode was prepared by carbonization of the CF coated with a polymer precursor solution while the NiCo2O4 structure was grown via hydrothermal treatment. Using similar production methods, NiCo2S4 nanosheets grown on N-doped CF in a full cell against OMC N-doped CF showed enhanced rate capability and almost the same energy and power [62]. An Ni-Fe asymmetric electrochemical cell using GF/CNT was shown to be an effective approach for fabrication of a flexible and rechargeable energy storage device [69]. GF/CNT was used as support for Ni(OH)2 and Fe2O3 deposition. The two hybrid electrodes were later assembled into a flexible cell with energy density in the range of thin film batteries. In another study, activated 3D CNT foam was used as an electrode in a symmetric supercapacitor cell and as an anode in a half-cell Li-ion battery [40]. In both cases, the electrode showed stable cycling and rate capability as a result of the presence of micro- and mesopores in the structure, which, in combination with the macropores

Carbon-based foams: Preparation and applications

61

of the CNTs, provides a larger area for charge storage and decreased ion diffusion paths. Supercapacitor electrodes with high cycling stability at high currents were prepared by synthesis of all-carbon foam consisting of three different carbons: GO, CNT, and carbon [98]. As-prepared electrodes exhibited a large surface area and bimodal mesoporous structure, hence high conductivity that influenced the electrochemical performances of a three-electrode cell. A template synthesis of graphene foam using sucrose as a carbon source and reinforced with CNTs was used by Sha et al. [104] for preparation of ultrastrong foam. The mechanical properties of the so-called rebar graphene were tested under a load 8500 times its weight, after which the 62.8 mg sample recovered 75% of its initial height. This strength of the foam was attributed to the partial bonding between the CNTs, the graphene sheets, and carbon shells that formed the foam structure. Its structural stability was also confirmed by dynamic mechanical analysis. This foam was further applied as an electrode in a symmetric Li-ion capacitor, where it showed stable electrochemical performances and energy density of about 6 Wh/kg. An asymmetric Li capacitor using MoO3/graphene nanosheet (GNS) foam as the negative electrode and N-doped GNS foam as the positive electrode delivered higher energy (18 Wh/kg) and power (0.18 kW/kg) as a result of the hybridization of two electrodes with a large surface area and controlled pore size and network structure [95]. The electrodes were prepared by the activation of a previously carbonized PANI/GO composite and the hydrothermal treatment of GO in an Mo-based solution. The multifunctionality and the potential of different carbon-based foams in the area of lithium energy storage were shown in several studies [77, 100]. Carter et al. [100] prepared CNT foam via template synthesis (Fig. 3.7) and used it as an electrode in an ionic-liquid based supercapacitor, a Li-ion battery, and a Li-air battery while Smirnova et al. [18] prepared polymer-based elastic CF by a simple carbonization process and investigated the effect of anode thickness (14 mm and below 5 mm) on the electrochemical performances of a half-cell Li-ion battery. The study demonstrated that thinner anodes exhibit higher discharging capacities and lower irreversible capacity due to the lesser surface area exposed to the electrolyte, faster ion diffusion, and electron transfer. The anodes with a thickness of 2.8 mm showed a capacity of about 350 mAh/g (at 0.12 A/g), which was higher than that of the thick anode (225 mAh/g) at the same current density. On the other hand, CNT foam obtained by microwave plasma CVD delivered a capacity of about 600 mAh/g (37.2 mA/g) due to the presence of submicron channels into the foam structure that improves the electrode/electrolyte contact and increases the ability for Li-ion intercalation [39]. Different carbon foams doped with nitrogen (N), germanium (Ge), Fe2O3, and Co3O4 were also tested as anodes for Li-ion energy storage [21, 76, 77, 105]. A bio-based 3D porous matrix was directly assembled from a protein hydrogel template [21]. The template was synthesized from polypeptides containing metal binding motifs and was doped with N, F, and MnF2 nanocrystals. Such an electrode exhibited high cycling stability due to its unique porous structure. Carbonized melamine foam with subsequent dip-coating into a Ge-precursor solution or hydrothermal treatment into a Co-based solution were used for the synthesis of 3D Ge/C N-doped CF or ultralight flower ball-like Co3O4 CF, [76, 77, 105], even though both electrodes showed

62

Nanocarbon and its Composites

(A) D

EP

SWCNT–Ni foam Freestanding SWCNT foam

Ni dissolution SWCNT-NMP

(B)

1 µm

40 µm

(C) 10,000 Total device mass

Power density (W/kg)

Supercapacitors Li-air batteries

1000

100

Li-ion batteries 10 0.1

1

10

100

1000

10,000

Energy density (Wh/kg)

Fig. 3.7 (A) Real images of CNT foam and the steps of its preparation method; (B) SEM images of CNT foam; and (C) Ragon plot showing the energy and power densities of CNT foam used as supercapacitor, Li-ion, and Li-air battery. Adapted with permission from Carter R, Oakes L, Cohn AP, Holzgrafe J, Zarick HF, Chatterjee S, et al.. Pint, solution assembled single-walled carbon nanotube foams: superior performance in supercapacitors, lithium-ion, and lithium-air batteries, J Phys Chem C 118 (2014) 20137–20151.

Carbon-based foams: Preparation and applications

63

high rate capability and excellent cycling stability due to the presence of voids that buffered the volume expansion of Ge and the increased conductivity of the oxide anode. On the other hand, high cycling stability and moderate rate capability were achieved when using porous α-Fe2O3 nanorods on CVD-prepared CNT/GF [105]. Lithium-sulfur batteries are another energy storage technology in which carbon foams can be applied. Here, they are used as cathodes with high sulfur loading, which enables high sulfur utilization and thus an efficient battery with enhanced energy and life cycle. For this purpose, flexible carbon foam obtained after calcination of melamine (MFC) was prepared by An et al. [106]. The ultralight hollow network was then loaded with sulfur by the dip-coated method and tested. The results showed that this cathode containing about 75% sulfur can provide cycling stability due to its ability to absorb a large number of polysulfides and the ability to buffer the volume expansion during cycling. The addition of GO or rGO into the carbonaceous network additionally improved the capacity of the cathode. Even higher sulfur loading (79%) but lower cycling stability was achieved in a high areal Li-S battery that was produced using CNT foam [36]. The foam was produced via the solvent exchange method while sulfur was added by vapor-phase infiltration. Carbon foam doped with magnesium oxide (MgO) was prepared by Xiang et al. [78] by immersing melamine foam into an Mg-nitrate/CNT solution and subsequent carbonization to form CNT/MgO CF. Further sulfurization of the CNT/MgO CF was achieved by liquid phase infiltration of S/CS2 solution into the carbon foam. When tested as cathodes in an Li-S half-cell, the 3D interconnected macroporous hybrid networks were shown to be able to effectively store more active sulfur and to absorb the electrolyte. Besides, they also demonstrated the ability for fast electron transport due to their increased conductivity. The cathode assembled as a flexible pouch cell showed high rate capability and cycling stability in both the flat and bent states. A carbonized gel containing lanthanum (La), zirconium (Zr), aluminum (Al), and niobium (Nb) precursors and citric acid was used as carbon support for sulfur [79]. This Li7La3Zr2O12/ CF composite electrode was assembled into an all-solid-state Li-S battery exhibiting high capacity and excellent cycling stability at elevated temperatures. Lu et al. [80] annealed lyophilized protein that was previously dip-coated in an Mo-precursor solution to prepare MoO2/Mo2C/N CF. Such carbon foam is comprised of pure MoO2 and Mo2C phases and a hierarchical micro- and mesoporous structure. The open porous structure of the foam allowed efficient bifunctional catalytic activity for oxygen evolution and reduction, increased electrode-electrolyte interface, and high electrical conductivity, which resulted in the formation of an Li-oxygen battery with high capacity and cycling stability. As an alternative technology that could replace Li-ion batteries, sodium-ion batteries require nongraphitic carbon-based electrodes that can accommodate the larger Na ion. Therefore, carbon foams with a disordered structure may be potential anodes or cathodes. Acid-functionalized commercial nanocellular carbon foam and activated nanocellular carbon foam were proposed by Shao et al. [101] as free-standing cathodes in an Na-ion half-cell. Between them, the CF with an acidic carbonyl (dC]O) functional group on its surface showed higher affinity for surface redox reactions with Na ions, and hence higher gravimetric capacity.

64

3.3.1.2

Nanocarbon and its Composites

Thermal energy storage

Carbon-based foams in latent thermal energy storage (LTES) devices play an important role due to their high thermal conductivity. They are generally used as supporters for phase change materials (PCMs) in order to improve their thermal conductivity, and thus to enhance the thermal storage and energy distribution of the device. PCMs are substances that, when going through a change in their physical state, are able to absorb or release large amounts of latent heat [107]. They are widely used active materials for passive thermal energy storage (TES) embedded into a wide range of matrixes, including foams. Thermal energy storage devices containing a large amount of PCM and constant thermal conductivity over prolonged charge-discharge periods are still a big challenge [108]. In this aspect, CF as a matrix allows high PCM-to-CF volume ratio and high thermal conductivity, which increases the storage capacity of the device. A TES device with maximum storage capacity and life cycle can be obtained only by optimized PCM/foam structures and types. Therefore, in the following part, we discuss the effect of foams’ porosity and density, the addition of fillers and dopants that increase thermal conductivity, and the effect of the production method of the TES device. A carbon foam carrier for paraffin, as a phase change material (PCM), was prepared by Xiao et al. [109]. They impregnated commercially available carbon foam into paraffin with and without the assistance of a vacuum in order to investigate the impregnation ratio. Their results showed that a vacuum-impregnated composite has a larger amount of paraffin that later influences the density, the specific heat capacity, and the thermal conductivity of the composite. Additionally, vacuum-assisted impregnation allows more uniform paraffin distribution within the foam structure, which leads to uniform heat transfer in the composite. The presence of carbon, due to the presence of CF, decreases the onset of the melting temperature of the paraffin while increasing its freezing point, which results in a faster and longer effect of the PCM. As a result, the capability of the paraffin/CF composite to exchange thermal energy with its surroundings increases. In order to improve the thermal conductivity of CFs, Jana et al. [15] studied the effect of the addition of graphite fillers with different particle sizes. They used tannin and graphite powders together with a blowing agent for the preparation of the graphite/carbon foam. The addition of filler increased the density of the foam due to the increased closed porosity as well as the increased disorder of the foam structure, which resulted in increased thermal conductivity. The study (Fig. 3.8) also showed that graphite particles with a smaller size have a higher contribution to this effect because they produce foams with lower porosity. A template synthesis of graphite foam was used to produce 3D phase change composite foam [110]. Here, erythritol was used as a bio-based organic PCM and was impregnated into the carbonaceous foam. The tests conducted demonstrated that the composite foam has high compressive strength and long-term compatibility between the two components as a result of the increased thermal conductivity and the porosity of the graphite foam. While vacuum infusion is suitable for highly porous foams, other methods are needed when denser foams have to be filled with PCM. For that purpose, Hu et al. [108]

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Fig. 3.8 SEM images of (A) neat carbon foam and carbon foam filled with (B) SFG6, (C) SFG15, and (D) SFG55 graphite powder. (E) Thermal conductivity of carbon foams filled with different amounts of graphite powder. Adapted with permission from Jana P, Fierro V, Pizzi A, Celzard A, Thermal conductivity improvement of composite carbon foams based on tannin-based disordered carbon matrix and graphite fillers, Mater Des 83 (2015) 635–643.

developed a device that allows liquid PCM to flow through the foam and hence to fill it. Using the device, they infused graphite foam (GF) with pentaglycerine (PG) and studied its thermal energy storage capability and duty cycle (charge-discharge time ratio) by measuring the transient temperature distribution within a sample. The samples were subjected to a heat flux with high and low boundaries in order to examine the heating and cooling behavior of the composite foam. The results showed that PG/GF can effectively store thermal energy (almost 100% of its theoretical value) and that they can retain that capacity over a large number of heating/cooling cycles with a 31% duty cycle. Gimenea et al. [111] prepared graphite foams infiltrated with inorganic PCM and showed the effect of the porosity, energy density, and thermal conductivity on the thermal properties of the composite foam. They tested thermal energy storage devices made of graphite foams with different porosity and thermal conductivity. In general, foams with higher porosity allow more PCM to be infiltrated at the expense of the thermal conductivity of the composite due to the lower amount of carbon. Therefore, these composite foams exhibit a higher storage capacity but lower discharge rates. The study also showed that the addition of a PCM layer on the surface of the composite foam does not change the discharge time of the TES device containing a carrier with high thermal conductivity. Several studies conducted by Paul et al. [112, 113] investigated the thermal properties of boron-nitrogen-doped carbon foam. They used microwave-assisted thermochemical surface treatment to modify the structure of crystalline and amorphous carbon foams, and hence to form a B-C-N foam. The addition of CdB, CdN, BdN bonds resulted in significant enhancement of the thermal properties of the foam, such as methanol desorption enthalpy, thermal stability, and conductivity. This was

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especially prominent in the sample with a crystalline structure. Further improvement of these properties was achieved by decorating the surface of the foam with graphene petals via microwave plasma CVD before the incorporation of boron and nitrogen [113].

3.3.2 3.3.2.1

Carbon foams as adsorbents Gas adsorbents

Carbon materials, especially carbon foams with an interconnected, hierarchical pore structure (macro-, meso-, and micropores) and high surface area, are one of the most suitable materials for adsorption/desorption and storage of various gases (CO2, water vapor, toluene, etc.) or ions (Pb, Cu, Cr, Ni, etc.). Additionally, their excellent chemical and thermal stability as well as their rigid and uniform structures make them an attractive adsorbent in severe conditions [114]. The adsorption efficiency of carbon materials is directly related to their pore structure and distribution. While macroand mesopores provide easy and fast transport of the species, micropores ensure a large surface area for interaction [114]. The type of precursor, additives, synthesis conditions (temperature, pressure, etc.), and posttreatment (activation with CO2, air, etc.) are important parameters for determination of the pore structure, surface chemistry, and adsorption capacity [25]. CO2 capture and storage are critical technologies when it is considered that CO2 from fossil fuels accounts for about 60% of the global greenhouse gases (GHG) emitted nowadays [115]. The currently used technology for capturing CO2, especially in power plants, is based on aqueous amine. This means it has drawbacks such as the toxic and corrosive nature of the amine solutions as well as high power consumption [116]. Among the new technologies, carbon foams as porous solid sorbents are considered as one of the most promising alternatives for CO2 adsorption. Besides their rigid, sponge-like structure and high stabilities, they can also be produced at low cost and can be easily regenerated with very small energy consumption [7]. According to the reports, 3–4 mmol g1 have been estimated as the optimal adsorption capacity for CO2 practical applications of CFs [116]. Recently, Rodriguez et al. [7] performed a comprehensive study in order to determine the influence of each individual parameter on the CO2 uptake of volatile bituminous coal-based CFs. They worked on the effect of carbonization temperature and the addition of different amounts of ZnCl2 or KOH as activation agents on the pore structure of the CF. Activation agents were added into the coal precursor before the foaming process, which led to increased micropore volume and subsequently enhanced adsorption efficiency. In the case of the activation with ZnCl2, a spherical structure was obtained, whereas the addition of KOH resulted in more irregular and elongated pores. The most homogeneous micropore size distribution (centered at 0.71–1.3 nm) with a micropore volume of 0.25 cm3 g1 and the highest surface area (866 m2 g1) was obtained from the CF activated at 500°C with ZnCl2 (with activation agent-to-coal mass ratio (AA/C) of 2). However, CF activated with KOH (at AA/C 0.5) at 800°C showed the highest adsorption capacity (2.8 mmol g1 at

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25°C, 1 bar versus 2.1 mmol g1 for CF activated with ZnCl2). The authors found that there is a direct relation between the adsorption capacity and the volume of the micropores. Additionally, carbonization at a higher temperature also changed the surface chemistry from acidic to basic and made the micropore surfaces hydrophobic, which increases the affinity to CO2 molecules. Saini et al. [117] worked on the selective adsorption properties of micro/mesoporous CF monoliths that were prepared using polyurethane (PUF) and zeolite (ZP) foams as sacrificial templates. The produced CFs had high BET surface areas (283 m2 g1 for PUF-based CF and 420 m2 g1 ZP-based CF), pore volumes (0.14 cm3 g1 for PUFbased CF and 0.2 cm3 g1 ZP-based CF), and mechanical strength. When adsorption capacities were measured at a high pressure of 900 kPa and 25°C, PUF-based CF showed the highest adsorption capacities for all the gases (3.6, 1.8, and 0.8 mmol g1 for CO2, C2H4, and C2H6 gases, respectively). The selective adsorption profiles of the prepared foams were investigated by using CO2/C2H6 and C2H4/C2H6 mixtures at different pressures. Although both PUF- and ZP-based CFs showed increased selectivity with increasing pressure, PUF-based CF exhibited more lucrative selectivity values. This could be attributed to the highly porous structure of ZP-based CF, which causes the slow diffusion of molecules through the micropores and thus shows low selectivity value. In another study, Ohta et al. [28] examined the water-vapor adsorption/desorption behavior of microporous CF that was prepared from a fluorinated polyimide and melamine foam as a template. As shown in Fig. 3.9A, there was a linear relation between micropore volume and absorptivity. The authors reported that the adsorbed amount of water is three times higher after activation of the CF at 400°C for 1 h in air atmosphere (Fig. 3.9B). The CF carbonized at 1000°C had the highest micropore volume

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(0.6 mL/g) and surface area (1700 m2 g1), plus it showed an absorptivity of about 40% mass fraction for a micropore volume. A comparison study for toluene removal efficiency between larch sawdust-based CF and commercial activated carbon fiber was reported by Liu et al. [114] According to the study, carbon foam had a semiclosed, honeycomb-like structure after carbonization at 400°C for 1 h and a network structure was formed after additional activation with KOH at 700°C. The produced CF showed relatively higher adsorption kinetics (80% and 95% of the toluene were absorbed within 10 and 30 min, respectively) than the commercial activated carbon fiber (30% and 45%). The adsorption capacity of the CF was calculated as 0.4834 mmol g1. Although the BET surface areas were almost the same for the two samples (1187 and 1160 m2 g1 for commercial-activated carbon fiber and CF, respectively), the large difference in adsorbent capacity is thought to be due to the microporous structure of the CF.

3.3.2.2

Liquid adsorbents

Among several water treatment technologies such as precipitation, ion-exchange, adsorption, or membrane separation used for removing heavy metals from wastewater, adsorption is widely used due to the convenience of the operation. Carbon foams are effective absorbents of several heavy metals (Zn, Cd, Cu, Cr, Pb, etc.) known as toxic substances when their concentration in water is high. Therefore, CFs can also find application in the field of wastewater treatment. Phenolic resin-based carbon foam with an open-cell structure and specific surface area of 458.59 m2 g1 was prepared as an adsorbent for removing heavy metals from aqueous solutions [25]. Batch experiments of mixed solutions with the same initial concentration (50 mg L1) showed removal ratios of 19.83%, 34.35%, 59.82%, and 73.99% for copper, zinc, cadmium, and lead, respectively (Fig. 3.10). The maximum adsorption capacity of the CF determined by the SIPS model was 491 and 247 mg g1 for lead and copper, respectively. Pb showed the highest removal efficiency because of its higher atomic weight and electronegativity. The study also reports the reusability of the adsorbent after chemical and electrical regeneration methods for copperadsorbed CF. For the chemically regenerated adsorbent, the removal efficiency was more than 75% during five cycles. The increase of the efficiency in the second cycle was noted as a result of the removal of cations (such as Ca, Na, etc.), which were residues from the foaming process. The decrease of the removal efficiency in the following cycles was attributed to the structural deterioration of the adsorbents due to continuous regeneration. On the other hand, the regeneration efficiency of the electrical method decreased more rapidly, to a value of 11.77% at the end of the fifth cycle because of the continuous accumulation of copper on the negative electrode plate and higher debris formation in the electrical regeneration method. In another study, Lee et al. [82] analyzed the effect of Fe2O3 addition into phenolic resin-based CF on the removal efficiency of cationic (Cu, Ni) and anionic (Cr) heavy metals from synthetic and real metal plating wastewater baths. The addition of Fe2O3 increased the specific surface areas from 458.59 to 545.99 m2 g1 and did not make a noticeable difference in the average pore diameter and volume. A faster removal of

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Fig. 3.10 Removal ratios of heavy metals (Cu, Zn, Cd, and Pb) by carbon foam in mixed solutions. Adapted with permission from Lee CG, Jeon JW, Hwang MJ, Ahn KH, Park C, Choi JW et al., Lead and copper removal from aqueous solutions using carbon foam derived from phenol resin, Chemosphere 130 (2015) 59–65.

chromium compared to that of the ordinary CF was obtained with the addition of Fe2O3, but with a similar and slightly slower removal of copper and nickel, respectively. The similar removal rates and efficiencies indicate that the adsorption of Cu and Ni often occurs in a fraction of carbon foam. But Cr, which produces chromate and dichromate anions in the wastewater, shows higher affinity for iron oxide compared with carbonaceous adsorbents. The removal efficiency for Cr and Cu from a realistic wastewater was found to be higher than the synthetic one. This effect was attributed to the difference in the chemical composition between the real and synthetic wastewater baths.

3.3.3

Carbon foams for insulation applications

3.3.3.1 Acoustic insulation The combination of chemical, electrical, and thermal properties of carbon have been attractive for many application areas. Despite these properties, researchers also proved the acoustic properties of carbon [118, 119]. However, compared to the extent of the studies on energy storage and foreign matter absorption properties of carbon foams, their sound absorption properties have been scarcely investigated. In this regard, Celzar’s group investigated the acoustic properties of carbon foams with reticulated and cellular structures [120, 121]. The group used tannin as the main component for obtaining carbon foam via the chemical-blowing method. The open cell carbon foams showed high air flow resistance potential but low sound absorption due to the high

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density of the foam. A significant increment of the sound absorption coefficient was achieved by applying the double porosity concept, for which carbon foams were perforated [120]. Recently, they studied the dependence of sound absorption on the bulk density, cell size, and connectivity of the foam [121]. The results indicated that cellular CFs show better sound absorption properties at lower frequencies and vice versa for the reticulated CFs (Fig. 3.11). Moreover, the permeability of the former CFs depends on the window (small holes that connect neighboring cells) size while for the later CFs, it depends on the cell size. In another study, mesophase pitch-based carbon foam and composite graphite foam were proposed for acoustic insulation [122, 123]. The highly porous and low-density carbon foam obtained from naphthyl-synthesized mesophase pitch showed good absorption properties even at low frequencies [122]. An even higher absorption coefficient at frequencies below 400 Hz was obtained with a GF/CNT/poly(dimethylsiloxane) (PDMS) composite [123]. This was attributed to the synergetic effect of several mechanisms, including the viscous loss during the movement of the soundwaves, the high damping properties of the PDMS matrix, the semiopen structure of the composite, and the dissipation of the sound waves through the graphene layers of the GF and their conversion into heat.

Fig. 3.11 SEM images (up) and absorption coefficient-frequency graph (down) of (A) cellular and (B) reticulated vitreous carbon foams. Adapted with permission from Letellier M, Ghaffari Mosanenzadeh S, Naguib H, Fierro V, Celzard A, Acoustic properties of model cellular vitreous carbon foams, Carbon 119 (2017) 241–250.

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3.3.3.2 Thermal insulation Thermal conductivity (λ) describes the transport of heat energy through a body of mass as a result of the temperature gradient. In porous solid phases, heat flow is limited by several factors such as the low volume fraction of the solid phase (low foam density, cell density, and porosity), cell morphology (size of the foam cell) and heat transfer coefficient of the raw material, and/or the enclosed gas encapsulated in the closed cells. In these porous structures, when parameters such as λ and cell density are low, they can reduce the heat flow by the restriction of convection [124, 125]. Depending on the thermal conductivity values, the materials can be divided into two categories: conductors and insulators. Based on this, materials with thermal conductivity >10 W/mK are known as a good thermal conductor while materials with thermal conductivity 480°C) and high thermal stability and thus are the favorite material for thermal insulation applications. Increasing their pore size or combining them with other highly porous materials such as silica aerogels leads to even more enhanced thermal insulation properties. The sound absorption properties of carbon foams are also of interes; however, a very limited number of research articles can be found on this topic. Because of the interconnected, hierarchical pore structure (macro-, meso-, and micropores) and high surface area, CF is one of the most suitable classes of materials for adsorption or storage of various gases and ions. Parameters such as pore size distribution, pore structure, or wettability, which can be controlled

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with the choice of precursor materials and production method, directly affect the adsorption properties of CFs. There are various reports that show the ability of CFs for removing heavy metals (Zn, Cd, Cu, Cr, etc.) from wastewaters, capturing CO2 from the atmosphere, or the storage of C2H4. Getting accurate results is vital for detecting target molecules or materials in sensor applications. To achieve this purpose, the sensor material should have a high surface area, conductivity, specific sensitivity, response, recovery time, and reproducibility. For these reasons, carbon-based foam structures are attractive for this application area as well. In this regard, two main types of CF-based sensors were developed: electrochemical (humidity sensor, biosensor, and gas sensor), and electromechanical (pressure sensor and strain sensor). In conclusion, carbon foam structures are an interesting type of materials that have been most widely used as gas adsorbents until now. However, nowadays the studies are mostly concentrated on the research of energy storage (either electrochemical or thermal) and various types of sensing devices. Depending on the requirements of these specific application areas, the properties of CFs can be accordingly tailored via various techniques. Hence, in the future, we can expect superior performance on such advanced applications.

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Electrospun polymeric nanocarbon nanomats for tissue engineering

4

Anindya Das, Jaideep Adhikari, Prosenjit Saha Dr. M.N. Dastur School of Materials Science and Engineering, Indian Institute of Engineering Science and Technology, Howrah, India

Chapter Outline 4.1 Introduction 92 4.2 Electrospinning process 93 4.3 Different types of nanocarbons used in tissue engineering

94

4.3.1 Carbon nanotubes 95 4.3.2 Graphene compounds 97 4.3.3 Other nanocarbons 98

4.4 Electrospun nanofiber materials for tissue engineering 99 4.5 Electrospun polymeric nanomats containing different nanocarbons for tissue engineering 100 4.5.1 4.5.2 4.5.3 4.5.4 4.5.5

Electrospun nanomat containing carbon nanotubes 100 Electrospun nanomat containing graphene nanocarbons 105 Electrospinning with other forms of nanocarbons 109 Electrospinning of carbon nanofibers for biomedical applications 110 Electrospinning of graphene quantum dots for biomedical applications 111

4.6 Review of some related works 4.7 Conclusion 116 References 116 Further reading 122

111

Abbreviation TE PLGA PLLA PLCL CS PVA PCL CMWCNT PHB PANI

tissue engineering Poly(lactic-co-glycolic acid) poly(L-lactide) poly (l-lactic acid-co-3-caprolactone) chitosan poly(vinyl alcohol) poly(ε-caprolactone) Carboxyl multi-walled carbon nanotube Polyhydroxybutyrate polyaniline

Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00004-3 © 2019 Elsevier Ltd. All rights reserved.

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PNIPAm-co-MAA DCM DMF SF BMSC F-MWCNT SMWCNT PBAT HA CA PU GO RGO nHA PLA PVP PEO PEG GO-g-PEG THF hMSCs HFIP P34HB TPU BC PVC AD

4.1

poly(N-isopropyl acrylamide)-co-methacrylic acid dichloromethane dimethylformamide silk fibroin bone marrow-derived mesenchymal stem cells functionalized multiwalled carbon nanotube superhydrophilic multi-walled carbon nanotubes poly (butylene adipate-co-terephthalate) hydroxyapatite cellulose acetate polyurethane graphene oxide reduced graphene oxide nano hydroxyapatite polylactic acid polyvinyl pyrrolidone polyethylene oxide poly(ethylene glycol) GO grafted PEG tetrahydrofuran human mesenchymal stem cells 1,1,1,3,3,3-hexafluoroisopropanol poly(3-hydroxybutyrate-co-4-hydroxybutyrate) Thermoplastic polyurethane bacterial cellulose poly vinyl chloride Average diameter

Introduction

Tissue engineering (TE) is a suitable method for replacing biological tissue with synthetic materials. Actually, tissue engineering covers the broad range of principles of material science, chemistry, biology, physics, etc., for developing a biological substitute that would restore and maintain the original function of replaced tissues and also help to improve organ or tissue function. The main challenges of these fields are to repair the damaged tissue and find solutions for new tissue creation [1]. The cell and substrate interaction is very important for tissue engineering. Engineered scaffolds can temporary replace living tissue [2,3], providing adequate mechanical strength with acceptable physiological function. Moreover, the material should be biocompatible in nature. Electrospinning is one of the most prominent methods for developing porous scaffolds for various applications in TE. In fact, the electrospun nanofibers containing nano- to microscale diameters, high surface area, and high porosity with good mechanical properties have been extensively used in different biological fields. Electrospun fibrous scaffolds with similar features to the extracellular matrix (ECM) can provide a suitable environment for the growth and proliferation of different cells and tissues [4].

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For improving the physicochemical properties of polymeric electrospun structures, several modifications can be processed, such as postprocess surface modifications, blends with other polymers, use of nanofillers, etc. The different types of nanocarbons are very useful filler materials for their nanoscale structures, light weights, superior electrical mechanical properties and they are used as filler materials in various polymer solutions for developing hybrid scaffolds. Therefore, electrospun polymer/nanocarbon fibrous scaffolds have gained huge potential for their broad applications in TE. The combination of various nanocarbons such as carbon nanotubes [5–18], graphene [19–31], nanodiamonds [32,33], and fullerenes [34,35] within the polymeric matrix can actually modify the mechanical strength and conductivity while also improving the bioactivity of the electrospun nanofiber matrix. Such nanocarbon-containing fiber scaffolds have now became very prominent materials for their application in TE, drug delivery, and other biomedical fields [36].

4.2

Electrospinning process

The initial electrospinning concept was first patented in 1902 [37,38] by Morton and Cooley through two separate works. Thereafter, the process was further enriched and modified by Formhals [39,40]. The primary concept of electrospinning is based on applying an electric field between the ground collector and a positively charged polymer solution loaded in a syringe attached with a needle. The polymer solution is subsequently charged by applied voltage and pumped at a constant rate on a metallic collector. As a result, a polymer jet will be created when the electric charge of the polymer solution overcomes the surface tension of the liquid. The jet of polymer solution will start to eject and flow, following a bending and spiral path from the tip of the needle in the presence of increasing applied voltage. The electrospinning solution consists of a volatile solvent. During electrospinning, the solvent evaporates and the polymers in the form of solid fibers get deposited on the grounded metallic collector [41,42]. Finally, the electrospun mat fibers are ready to use in different TE applications. The structure and morphology of the electrospun mat depends on several factors, such as: (a) concentration of the polymer solution, (b) molecular weight of the used polymer, (c) flow rate of the polymer solution through the syringe, (d) the applied voltage, (e) the distance between the syringe needle and collector, and (f ) the choice of solvent or polymer, etc. All these factors are quite essential for the development of uniform nanofiber mats with no bead formation. The uniform nanomat is a very useful environment for cell growth and proliferation [43]. The main schematic diagram of the electrospinning process is shown in Fig. 4.1. The electrospinning process has gained much attention for its varying submicron thickness, high surface area, high length/diameter ratios, good mechanical properties, etc. Basically, the polymers with good chemical, physical, mechanical, and electrical properties are converted into nanofibers through electrospinning. With advancements in nanotechnology, researchers are able to develop nanofibers through

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Fig. 4.1 Schematic diagram of the basic setup of the electrospinning process: (A) vertical setup (B) horizontal setup. Reprinted with permission from Bhardwaj N, Kundu SC. Electrospinning: a fascinating fiber fabrication technique. Biotechnol Adv 2010;28:325–47.

electrospinning, combining major scientific disciplines for advanced application in biomedical science [44].

4.3

Different types of nanocarbons used in tissue engineering

The nanocarbons are basically nanostructured carbon materials having a nanoscale dimension, structures, and it functional properties that depend on its nanoscale features. The nanocarbon family (Fig. 4.2) includes all carbon nanotubes (CNTs), graphene and its derivatives, nanodiamonds, fullerene, carbon nanofibers [45,46], etc. The nanocarbons have huge applications in different fields such as biotech,

Electrospun polymeric nanocarbon nanomats for tissue engineering

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Fig. 4.2 Schematic illustration of some nanocarbon structures. Reprinted with permission from Su DS, Perathoner S, Centi G. Nanocarbons for the development of advanced catalysts. Chem Rev 2013;113:5782–816.

nanomedicine, photonics, polymer composite, etc., for their excellent thermal electrical conductivity as well as their good mechanical strength and light weight [47].

4.3.1

Carbon nanotubes

Carbon nanotubes are actually cylindrical fullerenes with a nanoscale dimension in which carbon atoms are arranged as a hexagonal lattice [48]. Carbon nanotubes (CNTs) can be synthesized by chemical vapor deposition by heating the catalyst material in a furnace with hydrocarbon gas (acting as a carbon source) flowing through the reactor. The catalyst materials are typically transition metal nanoparticles, mainly responsible for nanotube growth. The growth of the organized nanotube occurs from the specific catalytic site on the surface. On the basis of the growth pattern, carbon nanotubes mainly exist in two forms: single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). SWCNTs are chemically inert in nature due to covalent bonding between the sp2 bonded carbon atoms on the side walls of the nanotube. SWCNTs can be formed by rolling a graphene sheet (simply a honeycomb structure) into a cylindrical shape along different lattice vectors. The multiwalled carbon nanotubes (MWCNTs) are made of layers of SWCNTs with an inner diameter of 1–3 nm and an outer diameter of 2–100 nm. The conductive nature of the carbon nanotube, whether it acts as a metal or a semiconductor, mainly depends on chirality (the angle between the hexagons and the tube axis) [48]. The CNTs possesses good mechanical, thermal, and electrical properties. They have an elastic modulus very close to pure diamonds, thermal stability around

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2800°C in vacuum, electrical conductivity around three times higher than copper wires, and thermal conductivity two times higher than diamond. The CNTs have wide applications in nanoscience, including material chemistry, solid-state physics, surface chemistry, and nanomaterials in biological systems [48]. CNTs have now become the most prominent nanomaterials in various fields of biomedical applications for their nanosize, good flexibility, thermal stability, favorable electromagnetic properties, and good biocompatibility. They have various useful and broad applications in cancer therapy, photothermal therapy, local drug delivery, gene therapy, immune therapy, etc. CNT-based drug delivery systems have shown good biocompatibility and nontoxicity compared to chemotherapy, which has severe side effects [49]. For example, one chemotherapy drug, paclitaxel (PTX), can be incorporated into polyethylene glycol chains with the assistance of SWCNTs, forming a PTX-SWCNT conjugate system that acts as a tumor-suppressing agent in breast cancer. Also, the extent of PTX uptake by cancer cells is increased many times when delivered through SWCNTs. Also, this drug delivery system does not have toxic effects on a person’s normal organs [49,50]. Also, poly ethylene glycol (PEG)-grafted SWCNTs and MWCNTs act as good carriers for drug molecules. Both PEG-grafted SWCNTs and MWCNTloaded PTX drugs are highly efficient for killing HeLa and MCF-7 cancer cells. Thus, CNTs are very effective for cancer detection, treatment, and diagnosis [49, 51]. Though SWCNTs are very useful materials for drug delivery and tumor treatment, they possess low solubility, which is the main problematic area for cancer treatment. So, surface functionalization of these nanotubes may be a good solution for overcoming this problem. One of the widely accepted functionalization processes of SWCNTs is to treat with an acid mixture (HNO3:H2SO4 ¼ 1:3) and reflux for 2 h at around 120° C. After this acid treatment, the attachment of the carboxylic acid group (dCOOH) on the surface results in the formation of the hydrophilic structure of SWCNTs. These carboxylated SWCNTs can be attached to several molecules such as lectins, carbohydrates, antibodies, glycoproteins, etc., for further modification of their physical and chemical properties, which allows them to bind to cancer cells [52]. The two coupling agents EDC (3-dimethylaminopropyl-N-ethyl carbodiimide hydrochloride) and NHS (N-hydroxysuccinimide) are used to attach FITC (fluorescein isothiocyanate)-labeled lectin-HPA (Helix pomatia agglutinin) to the SWCNTs for synthesis of the SWCNT-lectin-FITC conjugate system for cancer therapy. The lectin-FITC attached SWCNTs emit green spectra under a confocal microscope. It is also observed that both HPA-FITC and HPA lectin-FITC-SWCNT conjugated materials are able to bind with human breast cancer cells (MCF-7) [52]. Both SWCNTs and MWCNTs can be used as bone substitutes as well as in different implant materials without any host rejection problems. It has been also investigated that nanocomposites made of CNTs with different natural and synthetic materials act as good substrates for cellular attachment, growth, and proliferation. The important characteristics of CNTs such as cell tracking abilities, sensing of cellular behaviors, enhancing tissue regeneration, and containing a large surface area that is capable of immobilizing DNA, proteins, etc., make them promising materials for tissue engineering. The main disadvantages of CNTs are their low solubility in aqueous media, nonbiodegradability, and toxicity to the human body. The CNT particles under 100 nm are more toxic to the lungs and are prone to escape from the host immune defenses [53].

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4.3.2

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Graphene compounds

The graphene has a honeycomb-like carbon sheet structure. It is a single-layer, twodimentional material containing layers of carbon forming a six-membered ring (honeycomb) structure. However, there is also bilayer graphene that can be synthesized by accomodating up to 10 layers of graphene. Both single and bilayer graphene have extraordinary electronic properties [54]. Due to their higher biocompatibility, high surface area, and excellent electrical conductivity, graphene materials have become promising materials for various biomedical fields, including drug delivery and tissue engineering. Particularly, graphene-based materials have wide applications in cardiac, neural, bone, skeletal muscle, and skin/adipose tissue engineering [55], as represented in Fig. 4.3. For their remarkable physical and chemical characteristics, they are used in a broad range of applications which includes cancer therapy, medical imaging, tissue engineering, antibacterial agents, antiviral materials, nanocarriers for drug delivery, etc. [56]. Graphene substrates are nontoxic as well as biocompatible for human osteoblast and mesenchymal stromal cells. Graphene films have potential application in engineering new osteoconductive/inductive implants. Graphene substrates are now considered as the most prominent material for implant engineering as well as other medical applications [57]. The graphene compounds can be modified using different materials, which may help to functionalize them for their better application in TE. Amphiphilic polyethylene glycol can functionalize reduced graphene oxide nanoribbons for attachment with peptides to target the integrin receptor on the human glioblastoma cell line. So, this type of PEGylated graphene nanoribbon is useful for cancer cell imaging and photothermal therapy [58]. Highly oxygenated graphene oxide (GO) samples show good cytocompatibility in nature while helping to promote cell adhesion and differentiation. The graphene oxide-silica nanohybride can be achieved by loading GO and silica in the solid state.

Fig. 4.3 Different application of graphene in TE. Reprinted with permission from Shin SR, Li YC, Jang HL, Khoshakhlagh P, Akbari M, Nasajpour A et al. Graphene-based materials for tissue engineering. Adv Drug Deliv Rev 2016;105: 255–74.

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Fig. 4.4 Schematic overview of various applications of graphene-based nanomaterials. Reprinted with permission from Goenka S, Santa V, Sant S. Graphene-based nanomaterials for drug delivery and tissue engineering. J Control Release 2014; 173:75–88.

The excellent enhancement of mechanical and thermal properties was observed in graphene oxide-silica nanohybride composite materials [59]. The rolled GO foams could be utilized in the directional growth of neural fibers. Neural growth was found to occur under electrical stimulation. The lamininfunctionalized graphene oxide foam surface is hydrophilic, resulting in the proliferation and differentiation of human neural stem cells through pores and the interface of the rolls [60]. The structure property relationship of graphene and graphene oxide has been summarized in Fig. 4.4 along with their possible application areas [61].

4.3.3

Other nanocarbons

Some other forms of nanocarbons that have huge applications in tissue engineering are fullerenes and nanodiamonds. Fullerenes are a hexagonal and pentagonal cage-like structure of carbon atoms. The first-discovered fullerenes contain 60 carbon atoms joined with each other, forming a hollow sphere structure that is analogous to a nanosized soccer ball. All carbon atoms in fullerenes remain in the sp2 hybridized state and contain an electron-attracting effect. Fullerenes are strong reactive-like alkenes. The functionalization process of fullerenes is easy because of their involvement in various reaction such as cycloaddition, nucleophilic and electrophilic substitution, etc. [62].

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C60 fullerenes are now widely used in several biological applications. Fullerenebased nanocarriers may be useful for biomedical applications. Unmodified C60 fullerenes are not toxic to cells. More accurately, unmodified C60 fullerenes are easily taken up by normal and malignant cells. They do not alter cellular morphology or cytoskeletal organization nor do they inhibit cell proliferation [63]. Fullerenes are mainly biologically inert unless functionalized with some specific functional groups. Fullerenes can also be absorbed by HeLa cells (cancer cells). The different groups of folic acid, L-arginine, and L-phenylalanine are used to functionalize the C60 fullerenes for delivery to the cells. Also, pyrrolidinium substitute and mono cationic fullerenes C60 are good mediators of killing cancer cells [64]. Nanodiamonds are mainly nanosized diamond particles first produced in the 1960s. However, at the end of the 1990s, they became widely applicable in various fields for their interesting properties [65]. Nanodiamonds have become an important material for biological applications such as labeling and drug delivery in tissue engineering, particularly for their small size and surface structure with high thermal conductivity and chemical inertness. The nanodiamonds can be synthesized through various processes such as shock-wave transformation of graphite into nanodiamonds, detonation of certain explosives, and destruction of bigger diamond crystals. Furthermore, good biocompatibility and low cytotoxicity enable nanodiamonds to be potentially used in tissue engineering. They are well suited for adsorbing certain biomolecules such as cytochrome c, myoglobin albumin proteins, etc. Poly-Llysine-coated nanodiamond particles help to detect DNA oligonucleotides, a few bacterial species, etc. Nanodiamonds can act as a good reservoir for genes and help in drug delivery to living cells. Antibodies and fluorescence-labeled nanodiamonds are also very helpful for detecting cancer cells [66]. Nanodiamonds are potential elements for therapeutic treatment, including breast and liver cancer treatments. The properties of nanodiamonds are influenced by different postsynthetic treatments (incorporating some functional groups on nanodiamonds, that is, the functionalization process), which help to modify the surface for better performance in biological and other fields. Some functional groups such as carboxyls (dCOOH), amino, thiol, and halogens can functionalize nanodiamonds. Recently, nanodiamonds have been incorporated into various substrates to obtain a composite structure for several tissue engineering applications. Octadecylamine (ODA)modified nanodiamonds and poly (l-lactic acid) composites were applicable in bone tissue engineering. Nanodiamond-incorporated piezoelectric polymers such as poly (vinylidene fluoride) composites have broad application in TE [67].

4.4

Electrospun nanofiber materials for tissue engineering

The electrospun fibers have shown great potential in biomedical engineering, including skin, blood vessels, the nervous system, bone tissue engineering, and drug delivery vehicles, due to their high surface area and structures that mimic an extracellular matrix (ECM) [4,5].

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Some growth factors/signaling molecules and mineral-incorporated fibers are really helpful for repair and regeneration of both hard and soft musculoskeletal tissues. Electrospun nanofibrous scaffolds are considered good carriers for such growth factors, which may stimulate osteoblast-associated gene expression for bone regeneration [4]. Also, various drugs ranging from antibiotics to anticancer agents as well as DNA, RNA, proteins, and growth factors can be loaded to various electrospun fiber mats for tissue engineering as well as drug delivery applications [5].

4.5

Electrospun polymeric nanomats containing different nanocarbons for tissue engineering

In this section, the work based on the electrospun scaffold using different forms of nanocarbon structural materials such as CNTs and graphene will be presented.

4.5.1

Electrospun nanomat containing carbon nanotubes

Nowadays, the uses of CNTs with a combination of different polymers have shown attractive properties in biomedical applications. Recently, CNT-incorporated electrospun scaffolds have been extensively used in various TE fields such as the regeneration of neural cells [6,7], bone tissue [8], and muscles [68,69]. Several natural polymers have also been used as a matrix in electrospun mats such as silk [70,71], gelatin [72], etc., Also, some synthetic polymers are applicable for CNT dispersion, including polyurethane [10,11], polycaprolactone [12,13], poly(lactic-co-glycolic acid) (PLGA) [15], PLCL [poly (l-lactic acid-co-3-caprolactone)] [73], polylactic acid (PLA) [74], etc. The various characteristics of electrospun polymeric mats such as physical mechanical properties, electrical conductivities, and biocompatibility of fibrous mats get improved with the incorporation of CNTs. Also, incorporated CNTs within polymeric mats can induce the regeneration of different tissues through cell growth and proliferation.

4.5.1.1

Electrospinning of carbon nanotubes with natural and synthetic polymers

Pan et al. [70] have successfully synthesized microcomposite fibers using regenerated silk fibroin and functionalized MWCNTs through electrospinning. The mechanical properties of the composite fibrous mats were also enhanced by the incorporation of very small amounts (0.25–1 wt%) of MWCNTs. The well-dispersed MWCNTs within the electrospun matrix act as a reinforcing agent for the modification of electrospun-regenerated silk fibroin. Electrospun-regenerated silk fibroin mats with incorporated MWCNTs show good biocompatibility in nature and have no cytotoxicity for cell growth and proliferation. Gandhi et al. [71] successfully regenerated silk protein from cocoons of Bombyx mori to produce random and aligned nanofibers through electrospinning. This

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electrospun silk nanofiber is reinforced with 1% CNT for making nanocomposites for the improvement of mechanical and electrical properties. Ultimately, fewer than 100 nm fibers were produced through this process. The electrospun silk CNT composite nanofibers showed many times higher strength and toughness compared to silk fibers alone. Also, crystallinity and electrical conductivity were both increased for silk CNT nanofibers compared to silk nanofibers without CNTs. The electrospun gelatin MWCNT composite fibrous scaffolds are considered prominent materials for the growth of myoblasts. Incorporated MWCNTs within a gelatin nanofiber actually enhance the myotube formation and also activate the mechanotransduction related genes. The presence of MWCNTs within the gelatin nanofiber improved the mechanical properties of the fiber. Such gelatin/MWCNT hybrid scaffolds are now considered useful materials for skeletal muscle cellular organization. Hence, gelatin/MWCNT scaffolds can induce cellular growth and muscle tissue formation [72]. The electrospun poly (lactic acid) (PLA) also has various applications in TE because of its biocompatibility and controlled degradation properties. There was enhancement in conductance and tensile strength with the addition of multiwalled carbon nanotubes (MWCNTs) within electrospun poly (L-D-lactic acid) (PLA) nanofibers. The fiber diameter was reduced up to 70% with the addition of MWCNTs due to the increased surface charge density of PLA/MWCNT composite scaffolds [74]. CNTs act as additives to improve the physicochemical properties of materials. Electrospinning is also helpful for developing porous scaffolds using poly(l-lactic acid) and 4-methoxyphenyl functionalized MWCNTs for neuronal cell growth and proliferation. The covalent functionalization of CNTs allows them to be well dispersed within the electrospun matrix [75]. The MWCNT-coated electrospun PLCL [poly (l-lactic acid-co-3-caprolactone)] nanofibers were promising materials for the neural regeneration process. The MWCNT-coated PLCL scaffolds could induce the adhesion and proliferation of neural cells for facilitating neurite outgrowth. Cell transplantation applications can be served with the help of these MWCNT-coated nanofibrous scaffolds in neural tissue engineering [73]. A new type of electrospun composite membrane can be developed using poly (L-lactic acid), multiwalled carbon nanotubes (MWCNTs) and hydroxyapatite (HA) for fabricating various biomaterials to meet diverse clinical requirements. The incorporated hydroxyapatite MWCNT nanoparticles were actually found to be well dispersed within the poly (L-lactic acid) fiber. Through cytological research, it was revealed that PLLA/MWCNTs/HA composite membranes helped to enhance the adhesion and proliferation of periodontal ligament cells (PDLCs) and also inhibited the adhesion and proliferation of gingival epithelial cells as compared to the control group. Also, in an in vivo experiment, it was observed that animals survived without any local complications after periodontal ligament cells (PDLCs) seeded in a PLLA/MWCNTs/HA composite were implanted into the leg muscle pouches of immunodeficient mice. Histological examinations showed that PDLCs attached on these composite membranes with no obvious inflammation in the implant areas [76].

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The doping of the poly (L-lactic acid) (PLLA) polymer with SWCNTs is favorable for nerve tissue engineering. The electrospun PLLA/SWCNT composite scaffolds promote adhesion, growth, survival, and proliferation of olfactory ensheathing glial cells (OEC), which are the most prominent cell line used for the treatment of nerve injuries. OECs are actually found to be well attached and distributed on SWCNT/ PLLA scaffolds [68]. One of the most prominent ways to load a high amount of CNTs into an electropsun mat while also minimizing their cytotoxicity is entrapping CNTs in fiber through a coaxial and blend electrospinning process. Actually, the blend fibers possess slightly higher conductivity compared to coaxial fibers with the same CNT loadings. Up to 6% CNTs can be loaded within a poly(ethylene glycol)-poly(D,L-lactide) copolymer (PELA) fiber through blend and coaxial electrospinning for the creation of a synthetic microenvironment, which helps to improve the function of cardiomyocytes while also improving the electrical conductivity of the substrate. The conductive networks are formed in the electrospun PELA/CNT fibrous scaffolds with incorporation of 3% CNTs. It could induce the cell elongation, increase the production of sarcomeric α-actinin and troponin I, and promote the synchronous beating of cardiomyocytes. The electrospun PELA/CNT fibrous mat showed a high degree of alignment under an SEM micrograph (Fig. 4.5A and B), and TEM images (Fig. 4.5C and D) showed the bulk distribution of CNTs in a blend fiber. Actually, both blend and coaxial electrospinning are used to control the distribution of CNTs within fibers [77]. Fig. 4.5C and D showed the TEM images of fibrous mats, indicating a bulk distribution of CNTs in blend fibers whereas the coaxially electrospun fiber showed a coresheath structure with the entrapment of CNTs in the fibers. Actually, the coaxial fibers were gray (Fig. 4.5D) but the blend electrospun fiber showed a black appearance (Fig. 4.5C). The biological results revealed that high amounts of CNT loading within the PELA fiber helped to induce cell elongation and cell viability while maintaining the synchronous beating behavior of cardiomyocytes, etc. [77]. The electrospun nanofiber of polyurethane with incorporated MWCNTs shows higher biocompatibility. The fibroblast growth on the MWCNT-embedded polyurethane matrix described their potential application in biomedical science. The tensile strength and mechanical properties of fiber increased with an increase in MWCNTs within the nanofiber [9]. The electrospun nanofiber scaffolds made of MWCNTs and polyurethane exhibit higher fibroblast cell migration, cell adhesion, and proliferation while also showing good cell aggregation. Besides these, cells (fibroblasts) cultured on the nanofiber scaffolds of polyurethane/MWCNTs released large amounts of collagen proteins as compared to cells on the other substrates. So, this polyurethane/MWCNT composite nanofibrous mat is now one of the essential substitute materials used for tissue repair and regeneration [10]. Electrospun polyurethane/MWCNT composites actually have a positive effect on the growth and proliferation of human umbilical vein endothelial cells. These types of aligned composite nanofibers can induce extracellular signals to stimulate cell growth and proliferation while helping to secret extracellular collagen and maintaining anticoagulant function. Several cell division control proteins such as Rac, Cdc 42, and Rho can be activated by a polyurethane/MWCNT composite fiber [11].

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Fig. 4.5 (A,B) SEM morphology, (C,D) TEM images of blend (A, C) and coaxially electrospun fibers (B, D) containing 5% CNTs. Insets in c and d show the physical appearance of the fibrous mats obtained. Reprinted with permission from Liu Y, Lu J, Xu G, Wei J, Zhang Z, Li X. Tuning the conductivity and inner structure of electrospun fibers to promote cardiomyocyte elongation and synchronous beating. Mater Sci Eng C. 2016;69:865–74.

The fabricated electrospun poly(ε-caprolactone) (PCL)/MWCNT composite nanofibrous membrane possesed high porosity and nanodiameter distribution. Also, the PCL/MWCNT nanocomposite had good degradation and biocompatibility properties. The average diameter of the PCL/MWCNT nanofiber decreased with the addition of CNTs. With the addition of 0.5% of MWCNTs, the average diameter of the PCL/MWCNT nanofiber decreased to the 52–244 nm range [12]. It was also found that PCL nanofibrous membranes were not able to enhance blood coagulation after the addition of MWCNTs. The blood-clotting behavior of the PCL/ MWCNT membrane is shown below. Both the glass and PCL/MWCNT nanofibrous scaffolds were incubated with blood up to 50 min. It could be seen that blood incubated with the PCL/MWCNT nanofiber mat showed a higher absorbance value compared to glass at each different interval of time (Fig. 4.6). No significant difference of clotting time was found for the PCL with different increasing concentration of MWCNTs. So, from the below figure it was concluded that the PCL/MWCNT nanofiber membranes possessed good anticoagulant properties [12].

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Fig. 4.6 Clotting time of electrospun PCL–MWCNT nanofiber membranes. Reprinted with permission from Meng Z X, Zheng W, Li L, Zheng Y F. Fabrication and characterization of threedimensional nanofiber membrance of PCL-MWCNTs by electrospinning. Mater Sci Eng C 2010;30:1014–21.

Based on all these properties, it can be concluded that electrospun PCL/MWCNT nanofibrous scaffolds may be a promising material for tissue engineering applications [12]. Both functionalized and unfunctionalized MWCNTs are used as reinforcing materials in PCL/MWCNTs nanocomposite fibers. One of the functionalized processes is to introduce aromatic amine (COC6H4-NH2) groups on the side walls of CNTs through the Friedele Crafts acylation process. A functionalized MWCNT/PCL nanocomposite is more thermally stable and contains less beaded structure than an unfunctionalized MWCNT/PCL composite. Chemical modification of the MWCNT surface causes more compatibility with PCL [13]. The poly (butylene adipate-co-terephthalate) (PBAT) is also a very prominent material for the bone regeneration process in spite of having poor mechanical resistance. This problem can be solved by adding a small amount of super hydrophilic MWCNT (0.1–0.5 wt%) as a reinforcing agent to the PBAT for developing electrospun PBAT/MWCNT nanocomposites. Superhydrophilic MWCNTs help to enhance the tensile strength (from 1.3 to 3.6 MPa) of PBAT fiber. PBAT/MWCNT composite materials can provide a good matrix for MG63 cell migration and differentiation. Therefore, this type of composite material proves to be noncytotoxic in nature and may be allowed in a wide range of biomedical applications [14]. The fabrication of poly (lactic-co-glycolic acid) (PLGA)/MWCNT composite fibrous scaffolds through electrospinning has attracted tremendous attention for vast applications in skeletal muscle tissue engineering. The presence of MWCNTs modulates the tensile strength and electrical conductivity of PLGA fibers. These PLGA/ MWCNT fibrous scaffolds are cytocompatible in nature and induce the proliferation of C2C12 cells. Moreover, these PLGA/MWNT composite scaffolds promoted more mature myotube formation compared to PLGA scaffolds without MWCNTs [15]. A study attempted to use electrospun nylon-6/MWCNT fibers as a restorative material for dental applications. The resin composite is optimized by altering electrospun parameters and observed to have improved flexural strength. “Fiber alignment” and “nanotube concentration” are the two main process variables in the research [16].

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Antimicrobial activity of electrospun polymer/CNTs scaffolds Bacterial infections and colonization are the major problems arising in every implant surface. Hence, there is increasingly high demand for antibacterial coatings. One such inexpensive process for developing an antibacterial coating is to fabricate an electrospun mat with incorporated varieties of antibacterial agents. Such types of coatings have exhibited potential applications in biomedical fields, including tissue engineering, drug delivery vehicles, etc. Almost all carbon-based nanomaterials are actually cytotoxic to bacteria. The CNT-loaded electrospun mats have potential applications for various antibacterial coatings. Schiffman et al. synthesized SWCNT (diameter 0.8 nm) incorporated electrospun polysulfone (PSf ) mats that are found to be flexible and composed of continuous randomly oriented fibers. The increased loading amount of SWCNTs from 0.1 to 1 wt% within electrospun PSf mats is found to decrease the viability of gram-negative bacteria (Escherichia coli) from 18% to 76%. So, the PSf/SWCNT mats are now considered to be very effective for antibacterial coatings [17]. CNTs are well known for their excellent antibacterial properties, though their applications are limited in various medical devices for the cell toxicity and aggregation of CNTs in the polymer matrix. Shia et al. proposed one environmentally friendly method in which CNTs retain their antibacterial function through electrospun thermoplastic polyurethane (TPU). The prepared TPU/CNT electrospun nanofibers are found to be effective for their antibacterial function. Polyethylene glycol (PEG) can be grafted onto electrospun TPU through UV photo-graft polymerization, producing TPU-g-PEG/CNTs electrospun nanofibers that exhibit good bactericidal properties and nontoxicity to human cells, like CNTs alone. Ultimately, the TPU-g-PEG/CNT nanofibers showed excellent antibacterial properties and hemocompatibility [18].

Electrospun polymer/CNTs as drug loading vehicles Doxorubicin (Dox) is well known and widely accepted as an anticancer drug. Dox can be loaded in CNTs and electrospun with a polymeric solution for fabricating drug/ CNT/polymer composite nanofibrous materials. These types of composite materials may be used as therapeutic agents for local chemotherapy without containing severe side effects. The electrospun PLGA/Dox/CNT composite scaffolds were found to be very effective for inhibiting the growth of cancer cells (HeLa cells) [78]. Both the PLGA polymer and MWCNTs have the ability to carry the drugs. The eletrospun composite materials consisting of MWCNTs and PLGA can be used for the controlled and prolonged release of doxorubicin. The doxorubicin-loaded MWCNT/PLGA composite nanofibers were biocompatible in nature and gained enhanced mechanical properties. Therefore, Dox/CNT/PLGA composite nanofibrous mats have potential applications in local chemotherapy and tissue engineering [79].

4.5.2

Electrospun nanomat containing graphene nanocarbons

The various applications of graphene-based materials have already been discussed in previous sections. Among the graphene-based materials, graphene oxide (GO) has received much more attention for its excellent physical mechanical and electrical

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properties, and is widely used in electrospun nanofibers for improving their thermal, mechanical, and electrical properties [36]. Some other important properties of graphene oxides are better biocompatibility and unique physical-chemical properties for their wide apllicability in tissue engineering. Furthermore, the high hydrophilicity of graphene oxide ensured by the presence of oxygen moieties and wrinkled texture makes their surface rougher in nature, which is advantageous for better cell attachment and proliferation. Also, graphene oxides with a wrinkled texture are found to induce cell signaling and differentiation [80].

4.5.2.1

Electrospinning of graphene with natural and synthetic polymers

Electrospun nanofibrous scaffolds with ultrafine and uniform fibers can be prepared with the incorporation of less GO into different polymer solutions such as chitosan, polyvinyl pyrrolidone, and polyethylene oxide. The GO-reinforced nanofibers actually show enhanced mechanical strength with a controllable water permeability property that is close enough to match the requirements of natural skin. Also, this GO-containing nanofiber mat is found to be a good matrix for cell attachment and proliferation. The presence of a certain quantity of GO (1.5%) within the chitosan nanofiber membrane increases the wound-healing ability. Moreover, the presence of GO within the electrospun nanofiber helps to increase the strength with better permeability, better cell attachment, and absence of scars and inflammation in the implanted site due to antibacterial activity [19]. Recently, natural biopolymers with nanocarbon-based materials have gained much attention for artificially developing temporary skin grafts. This type of temporary skin graft can mimic the physicomechanical properties of natural skins, offering a superior acute wound healing effect, antibacterial activity, etc. Such skin grafts that mimic natural skins can be prepared by electrospinning chitosan-polyvinylpyrrolidone solutions containing GO nanosheets. GO provides good viability of human skin fibroblast cells. Over 99% of wound healing occurs after 21 days, containing 1.5 wt% GO within the polymeric nanofiber. In presence of GO, cells are more prone to adhere with the mats indicating good interactions of cells with the fiber [20]. The physical and mechanical properties of nanofiber composites can also be immensely modified by the addition of GO within the nanofiber mat. Recently, nanofibers containing chitosan with bacterial cellulose have gained much attention for containing wound dressing ingredients and potentially suitable materials for skin tissue engineering. Bacterial cellulose has now become one of the best choices for developing nanofiber mats for its important properties in biomedical applications including biocompatibility, hydrophilicity, biodegradability, transparency, high crystallinity, good mechanical properties, and stability at wide temperature ranges. The average size of the electrospun nanofiber composite consisting of chitosan/bacterial cellulose gets reduced with the addition of GO. Also, decreases in hydrophilicity and water vapor permeability of the electrospun nanofiber have been observed with the addition of GO [21].

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GO coated on electrospun silk fibroin nanofibrous mats is useful in tissue engineering applications. Silk fibroin is one of the interesting polymers that can combine well with graphene and is well known for its good biocompatibility. Fibroin protein with a secondary molecular structure (beta sheet) can combine well with graphene, forming hybrid film formats for cell growth. The electrospinning silk fibroin mats with graphene base material composite scaffolds have a high potential for cell growth and proliferation. The GO coating on the fibrous mat can be achieved through the electrochemical deposition process. Coating with various graphene compounds reduces the elongation capacity of the silk fibroin electrospun fibers, whereas the low content of GO and the high content of RGO (reduced graphene oxide) help to increase the elastic modulus and strength of the fiber [22]. Some of the synthetic polymers are used in combination with GO for developing an electrospun fibrous scaffold used in bone tissue engineering applications. With the addition of GO, RGO within the electrospun mats of polycaprolactone (PCL), poly lactic acid (PLA), and poly(lactic-co-glycolic acid) (PLGA) is found to increase the physicochemical properties and biocompatibility while making them more suitable for biological applications, especially in TE. However, one of the major limitations of PCL application is poor mechanical performance due to the low melting point (50–60°C), which may affect its further application in TE and other biomedical fields. To overcome this problem, various approaches–including blending, copolymerization, and making nanocomposites with PCL–should be processed for improving its properties. Incorporation of GO as well as RGO within the PCL may be one solution for better function. Electrospun poly (ε-caprolactone) (PCL) nanofibers containing both GO and RGO possess increased mechanical and tensile strength, which is supported by increased relaxation time and molecular orientation of the PCL chains. There is remarkable increased tensile strength and modulus of PCL nanofibers with the incorporation of GO and RGO [23]. The addition of GO not only improves the mechanical strength of the PCL nanofiber but also increases its biocompatibility for making it a suitable material for bone tissue engineering applications. The presence of graphene oxide nanosheets in PCL fibers increases its degradation rate and promotes in vitro biomineralization, indicating its good bioactive nature. Furthermore, the adhesion and proliferation of MG63 cells are found to be increased in GO-incorporated PCL nanofibers [24]. The addition of GO nanoplatelets (GOnPs) within the bioactive polymer helped to enhance its conductivity, dielectric permittivity, and biocompatibility. The rise in conductivity and dielectric permittivity of GO/PCL composite is favorable for stimulating multinucleated myotube formation, which is essential for regeneration of functional skeletal muscle. Mesenchymal stem cells derived from human cord blood were found to be differentiated to skeletal muscle cells on both thin GO nanosheets and GO/PCL composite meshes. Thus, both GO sheets and GO/PCL fibrous composite meshes might be potential substrates for human skeletal muscle tissue regeneration [25]. Having good biocompatibility, GO and nano hydroxyapatite are incorporated within the electrospun PLA nanofiber mat. In recent years with advancements in nanotechnology, ceramic nanomaterials—especially hydroxyapatite (HA) with a chemical composition of Ca10(PO4)6(OH)2—have shown excellent biocompatibility,

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bioactivity, and chemical similarity to human bone tissue; they are widely used as reinforcement material in polymeric mats. The presence of GO and nanohydroxyapatite within the PLA fiber mat increases its tensile strength, elastic modulus, and water uptake abilities while also enhancing the adhesion and proliferation of fibroblast cells [26]. Also, GO-incorporated nanofibrous mats help to induce gene expression and osteogenic differentiation of mesenchymal stem cells. The GO-doped poly(lactic-coglycolic acid) (PLGA) nanofiber scaffolds prepared via electrospinning accelerated the human MSC adhesion and proliferation as well as induced the expression of alkaline phosphatase and Col I (osteogenic marker genes), which are responsible for the proliferation and differentiation of human mesenchymal stem cells toward osteoblast for bone regeneration [27]. It has also been reported that GO can be dispersed in electrospinning solvent PLA by surface grafting with poly (ethylene glycol). This PEGylated graphene oxide has various applications in cell imaging and drug delivery. The surface grafting of GO with PEG can be carried out through esterification between the carboxylic groups of GO and the hydroxyl groups of PEG. These GO-grafted PEG (GO-g-PEG) materials are found to be well dispersed in water, organic solvents, and nonpolar solvents and possess no significant toxicity effect for cells. GO-g-PEG can actually improve the thermal stability of the PLA solvent and also improve interfacial adhesion with PLA. The PLA/GO as well as the PLA/GO-g-PEG composite fiber mat showed improved mechanical strength and the capability of supporting NIH 3T3 cell growth and attachment [28].

Antibacterial properties of electrospun polymer/graphene scaffolds Graphene oxides have excellent antibacterial properties. Graphene and its derivatives are beneficial for their wound healing and antiinfective characteristics. Also, GO can be doped with silver and some other materials to enhance its antibacterial effects. The antibacterial activity of fiber containing GO and RGO is also greatly enhanced against some bacteria such as Escherichia coli, Staphylococcus aureus, etc. Electrospun gelatin/CS/HA nanofibrous incorporating either GO or RGO show good antibacterial properties and protein adsorption capabilities [29]. The presence of GO within the PLA/PU nanocomposite enhances antibacterial activity against gram-positive Staphylococcus aureus and gram-negative Escherichia coli. The addition of GO actually inhibited the attachment and proliferation of bacterial cells, thus resulting in a PLA/PU/GO composite with good antibacterial activity and suggesting that such a nanocomposite can be used as good material for TE [30]. The incorporation of GO in PLA/PU can reduce the growth of bacteria up to 100% within 24 h.

Electrospun polymer/graphene scaffolds for drug delivery Ardeshirzadeh et al. [31] successfully incorporated doxorubicin (DOX) within the electrospun PEO/chitosan/GO nanofibrous scaffolds. DOX released from this scaffold was found to be strongly dependent on pH. The higher release of the entrapped drug was found in pH 5.3 compared to pH 7.4. The DOX-loaded PEO/chitosan/GO

Electrospun polymeric nanocarbon nanomats for tissue engineering

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nanofibrous scaffolds showed more cytotoxicity compared to free DOX toward cancerous cells. So the PEO/CS/GO/DOX scaffolds could be considered as promising drug delivery vehicles.

4.5.3

Electrospinning with other forms of nanocarbons

Having proper biocompatibility and a nontoxic nature, nanodiamonds were incorporated into PLGA for developing PLGA-nanodiamond composite scaffolds, which showed improved mechanical properties. Both PLGA as well as PLGA-ND membranes were composed of straight and randomly oriented fibers and diamond nanoparticles relatively homogeneously distributed within the electrospun PLGA matrix. The PLGA-ND membranes were the more material cluster type and had higher density and lower porosity compared to the PLGA membrane. This PLGA-ND nanofibrous membrane actually induced the attachment, spread, and proliferation of human osteoblast-like MG-63 cells and showed no considerable inflammatory response. Therefore, the PLGA-ND scaffold could be used as a carrier for cells in bone tissue engineering [32]. Mahdavi et al. [33] reported that electrospun chitosan-based nanofiber mats containing bacterial cellulose and medical grade nanodiamonds (up to 3 wt%) were useful materials for skin tissue engineering. The uniform distribution of diameters and a decrease in the size of electrospun fibers were observed with the introduction of diamond nanoparticles. Also, the hydrophilicity and water vapor permeability of the mats was modified with the addition of nanodiamonds that are favorable for cell attachment. The permeability of electrospun mats was found to decrease with increasing ND concentration. The permeability of the mat depends on the size and pores present within the fibrous mat. As the fibrous mat containing ND showed a closed-packed structure and finer fiber, the permeability of the ND-containing fiber mat was reduced. The introduction of ND within the CS/bacterial cellulose nanofiber mat not only enhanced the mechanical strength of the fibrous mat but also helped to decrease the hydrophilicity and water permeability. So, ND/cellulose/CS could be a promising material for wound dressing and tissue engineering applications [33]. Recently, multifunctional electrospun composite scaffolds have gained much attention in drug delivery vehicles, bioimaging, and TE applications. Electrospun PLLA nanofibers with integrated water soluble fullerene nanoparticles have become particularly promising materials for tissue engineering, bioimaging, and drug delivery. The reported PLLA/fullerene composite fibers are smooth and uniform in nature with the average fiber diameter in the range from 300–600 nm. The core-shell structure of PLLA/fullerenes shows an excellent hydrophilic surface in which watersoluble fullerene nanoparticles are encapsulated within the electrospun PLLA nanofiber. In vitro human liver carcinoma cells cocultured on PLLA/fullerenes revealed that a large number of fullerene nanoparticles were released from the composite nanofiber and entered into the human liver carcinoma cells for bioimaging. The electrospun PLLA/fullerene composite also showed good biocompatibility and low cytotoxicity [34].

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Nanocarbon and its Composites

The drug paclitaxel and water-soluble fullerene C70 nanoparticles both can be successfully loaded to PLLA via electrospinning. The controlled release of paclitaxel from the PLLA/fullerene composite nanofiber actually inhibited the proliferation of human liver carcinoma cells without altering the cytotoxicity effect. Also, the release rate of paclitaxel could be controlled by the incorporated C70 nanoparticle content within the composite nanofibrous scaffold, that is, the drug release rate became faster with an increase of C70 content [35].

4.5.4

Electrospinning of carbon nanofibers for biomedical applications

Polyacrylonitrile (PAN) is used as a precursor for obtaining carbon nanofibers (CNFs). Generally, electrospinning of PAN is followed by sintering of the prepared mats to acquire carbon nanofibers. The application of carbon nanofibers in biomedical fields is achieved by incorporating several additives in the polymeric solution [81]. One such approach is the incorporation of calcium nitrate tetrahydrate as the calcium source and triethyl phosphate as the phosphorus source in the PAN solution. After processing, the carbon nanofibers are decorated with β-tricalcium phosphate (β-TCP) nanoparticles with tunable degradation ability and excellent biocompatibility. The schematic process outline is represented in Fig. 4.7 [45]. In another approach, the hydrophobicity of the CNFs is controlled with the incorporation of hydroxyapatite (HAp) particles. The HAp particles are biomimetically synthesized on the surface-activated CNFs under simulated body fluids (SBF). The obtained CNF/HAp composites possess strong interfacial bondings and high mechanical strength, which make them a potential material to be used in bone tissue

Fig. 4.7 Schematic illustration for the preparation of β-TCP/CNF membranes and their tailoring to short ones in HCl. Reprinted with permission from Liu H, Cai Q, Lian P, Fang Z, Duan S, Ryu S et al. The biological properties of carbon nanofibers decorated with β-tricalcium phosphate nanoparticles. Carbon 2010;48:2266–72.

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engineering [46]. Bone regeneration material with a CNF fiber can be synthesized in the method described earlier with PAN. However, electrospinning and sol-gel techniques are combined in order to incorporate bioglass nanoparticles in the carbon nanomats for bone tissue engineering applications [82]. Ag-Pt bimetallic nanoparticle decorated CNF can be used as a biosensor for the selective detection of dopamine (DA) in the presence of uric acid (UA) and ascorbic acid (AA). A PAN/polyvinylpyrrolidone (PVP) blend solution is used for electospinning, which is followed by water extraction and heat treatment to convert PAN nanofibers to CNFs with a nanoporous structure [83].

4.5.5

Electrospinning of graphene quantum dots for biomedical applications

Quantum dots are inorganic nanocrystal particles within the size range of 1–10 nm that exhibit size-dependent optical and electrical properties due to quantum confinement. Recently, QDs have been capped with novel ligands with an aim to develop less toxic QDs. Bioimaging and biosensors are the two main applicable areas of QD-based nanocomposites [84,85]. Graphene quantum dots (GQDs) have several potential applications in fabricating biosensors. Researchers have tried to obtain a three-dimensional structure from a nanofibrous membrane of GQDs by electrospinning. Water-soluble GQDs are electrospun directly with the assistance of polyvinyl alcohol (PVA). The prepared electrochemical biosensor is intended to identify hydrogen peroxide (H2O2) and glucose [86]. Similarly, H2O2 released from the prostate cancer cell (PC-3) can be identified by fabricating a biosensor from rGO QDs/ZnO hybrid nanofibers utilizing electrospinning. Nylon 6/6 nanofibers fabricated though electrospinning are used as templates for fabrication of the biosensors. The efficacy of the biosensors is tested under the stimuli of several corresponding anticancer drugs (apigenin, antisense CK2α, etc.) [87].

4.6

Review of some related works

Bone, cartilage, vascular, muscle, neural, and skin tissues are some of the major tissue engineering sectors where modifications are adapted on a regular basis. The present section deals with tabulating (Table 4.1) some related works incorporating different members of the carbon family through electrospinning for healing and modification of the targeted applications. Nanotubes, nanosheets, and graphene oxide can be integrated within the polymeric structures with an aim to improve the mechanical, electrical, and overall biocompatibility of the materials. Several kinds of electrospinnable natural and synthetic biopolymers are dissolved in suitable solvents that are coupled with different sources of carbons for advanced applications in TE field.

112

Table 4.1 Electrospun nanofibrous scaffolds containing nanocarbons Additives (polymer/solvent)

Result parameter

Applications

Target tissue

Ref.

SWCNT

CS-PVA/acetic acid

SWCNT homogeniously distributed within matrix

Neural

[69]

MWCNT

PVA-CS/acetic acid

Fibroblast

[88]

CMWCNT

PHB

Good dispersibility, compatibility, hydrophilicity

Osteoblast cells

[89]

MWCNT

PANI-PNIPAmco-MAA/HFIP/ DMF

Noncross-linked PVA/CS/ MWCNT, AD  157 40 nm, porosity (%) 584.47, density 0.43 cross-linked PVA/CS/MWCNT, AD 170 43 nm, porosity (%) 57  6.00; density 0.44 Tensile strength 4.64 MPA, fracture energy 109.73 kJ/ m2 AD 500–600 nm

Improved mechanical and elecrtrical property, high tensile strength, growth and proliferation of brain derived cells, that is, nontoxic and potentially biocompatible Good mechanical property, enhanced protein adsorption ability, well attachment and proliferation of mouse fibroblast cells

Bone

[90]

MWCNT

PCL/DCM:DMF

Good conductive and mechanical property Excellent growth and proliferation of mice fibroblast cells Improved conductivity, elastic modulus and yield stress, good biocompatibility Developing bioartificial muscle, creation of nanoactuator for skeletal muscle tissue engineering

Skeletal muscle

[91]

Unaligned fiber, fiber diameter: 1.032  0.601 μm (PCL) 1.704 1.452 μm (PCLMWCNT)

Nanocarbon and its Composites

Base materials

MWCNT

Collagen-PCL/ chloroformmethanol PLGA/TCM, DMF

Fiber diameter 564 nm

Good aligned and well dispersed of MWCNTs within fiber Random fiber with smooth and beadless fiber morphology Highly aligned fiber with few beads formation, AD: 730 190 nm, 610 120 nm, 530 210 nm

MWCNT

PLGA/SF/catalpol/ HFIP

FMWCNT

PLLA-PCL/ chloroform, methanol

SMWCNT (0.1–0.5 wt %) CS/ MWCNT

PBAT/chloroformDMF

Fiber with bead formation diameter 148–250 nm

CA/acetone + N,Ndimethylformamide

MWCNT (3%)

PU/acetdimethyllamide

Porous 3D, smooth surface of CA nanofibrous mats, diameter 305  128 nm Good alignment of fiber, D 300–500 nm

MWCNT (1%)

PLA/gelatin/DCM + DMF

Random fiber, diameter: 2.08 0.13 μm

MWCNT (1–5 wt%)

PLA/DMF

Randomly oriented nanofiber, AD 247–312 nm

Nerve

[92]

Bone

[93]

Nerve

[7]

Stem cells

[94]

Bone

[14]

Improved mechanical property. Growth, proliferation, and spreading of rat fibroblast Enhancement of endothelial cell growth and proliferation with nonthrombogenic phenotype Improved mechanical and electrical properties Good biocompatibility, nongenotoxic or cytotoxic to human chondrocyte cells Improved osteoblast functions. Osteoblasts in the samples grew along the electrical current direction

Fibroblast

[95]

Endotheliam, vascular

[96]

Cartilage

[97]

Bone

[8]

Continued

113

Promote nerve regeneration, increased electrical conductivity, biocompatible and nontoxic, improved hydrophilicity Increased thermal stability, mechanical properties. Proper adhesion and growth of BMSCs Higher porosity and mechanical properties. Differentiation of stem cells (adipose tissue) into neural-like cells Increased electrical conductivity, mechanical properties, tensile strength Improved biodegradation, nontoxic effects on proliferation of adiposederived stem cells Improved mechanical strength and good cytocompatibility in nature

Electrospun polymeric nanocarbon nanomats for tissue engineering

CSWCNT

114

Table 4.1 Continued Base materials

Additives (polymer/solvent)

Graphene (0%–4%)

Applications

Target tissue

Ref.

SF/HFIP

Random fiber, AD 1013–1236 nm

Bone

[98]

Graphene nanosheet (0.3–2%w/ w) GO (1%– 3%)/nHA (15%) GO (0.5%– 2%)

PCL/Chloroform

Random orientation of composite fiber, AD 1–8 μm

Muscle

[99]

PLA/DCM-DMF (75:25)

AD decreased on addition of GO. AD: 412  240 nm

Bone

[26]

PCL/DMF

Random fiber, D¼ 0.2–2.5 μm

Bone

[24]

GO (1%– 2%), GO-g-PEG

PLA/chloroform

Randomly distributed fiber

Bone

[28]

GO (4 wt %)

PLGA/Collagen/ HFIP

Ramdom fiber, D 100–950 nm

Muscle

[100]

GO (1 wt %)

PLGA/tussah silk/ HFIP

Uniform, smooth nanofiber

Improved electroactivity and mechanical properties. Also supported the growth and expansion of rat bone mesenchymal stem cells Excellent biocompatibility, conductivity, elastic mechanical properties. Good myoblast cell adhesion and spreading Higher biocompatibility. Increased tensile strength, elastic modulus, adhesion and growth of osteoblast Better protein adsorption ability, improved mechanical properties and bioactivity Improved mechanical, thermal property, better wettability, good cytocompatibility Nontoxic to fibroblast cells Increased hydrophilicity and biocompatibility. enhanced attachment, proliferation of C2C12 skeletal myoblasts Improved mechanical properties, adhesion proliferation of mesenchymal stem cells Enhanced hydrophilicity, protein adsorption ability

Bone

[101]

Nanocarbon and its Composites

Result parameter

PLGA-RGD peptide

Random fiber, AD 558 nm

GO

P34HB

Smooth and random fiber. Diameter 490–1560 nm

GO

PLLA/HFIP

Uniformly aligned aminolyzed nano fiber. AD 0.68 0.12 μm

GO (0.5%– 2%)

TPU/DMF

AD of fiber 295–397 nm

GO (0.5–5wt %) GO, RGO

PVA/H2O

Random fiber with beads like defect, D: 300–500 nm

PVC/DMF

Random fiber. The average thickness of the graphene oxide sheets 1.0 nm

Thermally and mechanically stable. Promotes the attachment and proliferation of vascular muscle cells Enhances hydrophilicity, mechanical properties, osteogenic differentiation, bone regeneration Promoted Schwann cell growth. Proliferation and differentiation of pheochromocytoma cells and induced neurite growth Increased tensile strength, Young’s modulus, hydrophilicity Enhanced fibroblast proliferation and human umbilical vein endothelial cell attachment Improved mechanical strength. Good attachment and proliferation of osteoblastic cells Excellent biocompatibility, high mechanical strength, Good cellular adhesion, proliferation of human mesenchymal stem cell

Muscle

[102]

Bone

[103]

Nerve

[104]

Vascular tissue

[105]

Bone

[106]

Bone marrow

[107]

Electrospun polymeric nanocarbon nanomats for tissue engineering

GO

115

116

4.7

Nanocarbon and its Composites

Conclusion

Electrospun nanofibrous scaffolds are now considered as very useful substrates in tissue engineering in connection with their high surface areas, porous structures, good interaction with various types of cells, and mimicking the structures of an extracellular matrix found in living systems. The adhesion, growth, and proliferation of different tissue-type cells including bone, nerves, muscles, skin, etc., are also observed on electrospun fibrous mats. The incorporation of various types of nanocarbons is considered as good filler material for substantial improvements in the physical, chemical, and electrical properties of fibrous scaffolds to ensure the required effectiveness as advanced biomaterials. Among the nanocarbons, CNTs, graphene, and its derivatives are the most widely investigated nanocarbons compared to other forms of nanocarbon materials used in natural as well as synthetic polymers for developing electrospun hybrid scaffolds. Particularly, CNTs and GO both are found to be very promising candidates for regenerating cartilage, bone, nerve, and skin tissues. The incorporated CNTs, graphene compounds, and nanodiamonds within nanofibrous mats help to increase biocompatibility, cellular attachment, and the proliferation and differentiation of various tissues/cells. The CNTs, GOs, and fullerenes containing electrospun scaffolds have shown great potential as drug delivery vehicles and possess excellent wound healing properties. Some well-known antibacterial drug loaded polymer/nanocarbon hybrid matrixes have shown remarkable antibacterial properties against a few bacteria. Also, fullerene-based scaffolds are applied in the identification of cancer cells through the bioimaging process.

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[64] Mroz P, Pawlak A, Satti M, Lee H, Wharton T, Gali H, Sarna T, Hamblin MR. Functionalized fullerenes mediate photodynamic killing of cancer cells: type I versus type II photochemical mechanism. Free Radic Biol Med 2007;43(5):711–9. [65] Danilenko VV. On the history of the discovery of nanodiamond synthesis. Phys Solid State 2004;46(4):595–9. [66] Krueger A. New carbon materials: biological applications of functionalized nanodiamond materials. Chem Eur J 2008;14:1382–90. [67] Nunes-Pereira J, Silva AR, Ribeiro C, Carabineiro SAC, Buijnsters JG, LancerosMendez S. Nanodiamonds/poly(vinylidene fluoride) composites for tissue engineering applications. Compos B 2017;111:37–44. [68] Kabiri M, Oraee-yazdani S, Dodel M, Hanaee-ahvaz H, Soudi S, Seyedjafari E, et al. Cytocompatibility of a conductive nanofibrous carbon nanotube/poly(L-lactic acid) composite scaffold intended for nerve tissue engineering. EXCLI J 2015;14:851–60. [69] Shokrgozar MA, Mottaghitalab F, Mottaghitalab V, Farokhi M. Fabrication of porous chitosan/poly(vinyl alcohol) reinforced single-walled carbon nanotube nanocomposites for neural tissue engineering. JBiomed Nanotechnol 2011;7:1–9. [70] Pan H, Zhang Y, Hang Y, Shao H, Hu X, Xu Y, et al. Significantly reinforced composite fibers electrospun from silk fibroin/carbon nanotube aqueous solutions. Biomacromolecules 2012;13:2859–67. [71] Gandhi M, Yang H, Shor L, Ko F. Post-spinning modification of electrospun nanofiber nanocomposite from Bombyx mori silk and carbon nanotubes. Polymer 2009;50:1918–24. [72] Ostrovidov S, Shi X, Zhang L, Liang X, Kim SB, Fujie T, et al. Myotube formation on gelatin nanofibers—multi-walled carbon nanotubes hybrid scaffolds. Biomaterials 2014;35:6268–77. [73] Jin GZ, Kim M, Shin US, Kim HW. Effect of carbon nanotube coating of aligned nanofibrous polymer scaffolds on the neurite outgrowth of PC-12 cells. Cell Biol Int 2011;35(7):741–5. [74] McCullen SD, Stano KL, Stevens DR, Roberts WA, Monteiro-Riviere NA, Clarke LI, et al. Development, optimization, and characterization of electrospun poly(lactic acid) nanofibers containing multi-walled carbon nanotubes. J Appl Polym Sci 2007; 105:1668–78. [75] Vicentini N, Gatti T, Salice P, Scapin G, Marega C, Filippini F, et al. Covalent functionalization enables good dispersion and anisotropic orientation of multi-walled carbon nanotubes in a poly (L-lactic acid) electrospun nano fibrous matrix boosting neuronal differentiation. Carbon 2015;95:725–30. [76] Mei F, Zhong J, Yang X, Ouyang X, Zhang S, Hu X, et al. Improved biological characteristics of poly(L-lactic acid) electrospun membrane by incorporation of multiwalled carbon nanotubes/hydroxyapatite nanoparticles. Biomacromolecules 2007;8(12):3729–35. [77] Liu Y, Lu J, Xu G, Wei J, Zhang Z, Li X. Tuning the conductivity and inner structure of electrospun fibers to promote cardiomyocyte elongation and synchronous beating. Mater Sci Eng C 2016;69:865–74. [78] Yu Y, Kong L, Li L, Li N, Yan P. Antitumor Activity of doxorubicin-loaded carbon nanotubes incorporated poly(lactic-co-glycolic acid) electrospun composite nanofibers. Nanoscale Res Lett 2015;10:343. [79] Qi R, Tian X, Guo R, Luo Y, Shen M, Yu J, Shi X. Controlled release of doxorubicin from electrospun MWCNTs/PLGA hybrid nanofibers. Chin J Polym Sci 2016;34 (9):1047–59.

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[80] Wang Z, Tonderys D, Leggett SE, Williams EK, Kiani MT, Steinberg RS, et al. Wrinkled, wavelength-tunable graphene-based surface topographies for directing cell alignment and morphology. Carbon 2016;97:14–24. [81] Zhang L, Aboagye A, Kelkar A, Lai C, Fong H. A review: carbon nanofibers from electrospun polyacrylonitrile and their applications. J Mater Sci 2014;49:463–80. [82] Yang Q, Sui G, Shi YZ, Duan S, Bao JQ, Cai Q, et al. Osteocompatibility characterization of polyacrylonitrile carbon nanofibers containing bioactive glass nanoparticles. Carbon 2013;56:288–95. [83] Miao YE, Huang Y, JI S, Tiju WW, Liu T. Electrospun carbon nanofibers decorated with Ag Pt bimetallic nanoparticles for selective detection of dopamine. ACS Appl Mater Interfaces 2014;6:12449–56. [84] Yu WW, Chang E, Drezek R, Colvin VL. Water-soluble quantum dots for biomedical applications. Biochem Biophys Res Commun 2006;348:781–6. [85] He X, Ma N. An overview of recent advances in quantum dots for biomedical applications. Colloids Surf B Biointerfaces 2014;124:118–31. [86] Zhang P, Zhao X, Ji Y, Ouyang Z, Wen X, Li J. Electrospinning graphene quantum dots into a nanofibrous membrane for dual-purpose fluorescent and electrochemical biosensors. J Mater Chem B 2015;3:2487–96. [87] Yang C, Hu LW, Zhu HY, Ling Y, Tao JH, Xu CX. rGO quantum dots/ZnO hybrid nanofibers fabricated using electrospun polymer templates and applications in drug screening involving an intracellular H2O2 sensor. J Mater Chem B 2015;3:2651–9. [88] Liao H, Qi R, Shen M, Cao X, Guo R, Zhang Y, et al. Improved cellular response on multiwalled carbon nanotube-incorporated electrospun polyvinyl alcohol/chitosan nanofibrous scaffolds. Colloids Surf B Biointerfaces 2011;84:528–35. [89] Zhijiang C, Cong Z, Jie G, Qing Z, Kongyin Z. Electrospun carboxyl multi-walled carbon nanotubes grafted polyhydroxybutyrate composite nanofibers membrane scaffolds: preparation, characterization and cytocompatibility. Mater Sci Eng C 2018;82: 29–40. [90] Sharma Y, Tiwari A, Hattori S, Teradaa D, Sharma AK, Ramalingam M, et al. Fabrication of conducting electrospun nanofibers scaffold for three-dimensional cells culture. Int J Biological. Macromolecules 2012;51:627–31. [91] McKeon-Fischer KD, Flagg DH, Freeman JW. Coaxial electrospun poly(ε-caprolactone), multiwalled carbon nanotubes, and polyacrylic acid/polyvinyl alcohol scaffold for skeletal muscle tissue engineering. J Biomed Mater Res A 2011;99A:493–9. [92] Yu W, Jiang X, Cai M, Zha W, Ye D, Zhou Y, et al. A novel electrospun nerve conduit enhanced by carbon nanotubes for peripheral nerve regeneration. Nanotechnology 2014;25:165102. [93] Zhang H. Electrospun poly (lactic-co-glycolic acid)/multiwalled carbon nanotubes composite scaffolds for guided bone tissue regeneration. J Bioact Compat Polym 2011;26 (4):347–62. [94] Liao GY, Zhou XP, Chen L, Zeng XY, Xie XL, Mai YW. Electrospun aligned PLLA/ PCL/functionalised multiwalled carbon nanotube composite fibrous membranes and their bio/mechanical properties. Compos Sci Technol 2012;72:248–55. [95] Lou Y, Wang S, Shen M, Qi R, Fang Y, Guo R, et al. Carbon nanotube-incorporated multilayered cellulose acetate nanofibers for tissue engineering applications. Carbohydr Polym 2013;91:419–27. [96] Han Z, Kong H, Meng J, Wang C, Xie S, Xu H. Electrospun aligned nanofibrous scaffold of carbon nanotubes-polyurethane composite for endothelial cells. J Nanosci Nanotechnol 2009;9:1400–2.

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[97] Markowski J, Magiera A, Lesiak M, Sieron AL, Pilch J, Blazewicz S. Preparation and Characterization of Nanofibrous Polymer Scaffolds for Cartilage Tissue Engineering. J Nanomater 2015;2015:9. [98] Yang Y, Ding X, Zou T, Peng G, Liu H, Fan Y. Preparation and characterization of electrospun graphene/silk fibroin conductive fibrous scaffolds. RSC Adv 2017;7:7954–63. [99] Patel A, Xue Y, Mukundan S, Rohan LC, Sant V, Stolz DB, et al. Cell-instructive graphene-containing nanocomposites induce multinucleated myotube formation. Ann Biomed Eng 2016;44:2036–48. [100] Shin YC, Lee JH, Jin L, Kim MJ, Kim YJ, Hyun JK, et al. Stimulated myoblast differentiation on graphene oxide-impregnated PLGA-collagen hybrid fibre matrices. J Nanobiotechnol 2015;13:21. [101] Shao W, He J, Sang F, Chen L, Cui S, Ding B. Enhanced bone formation in electrospun poly(L-lactic-co-glycolic acid)-tussah silk fibroin ultrafine nanofiber scaffolds incorporated with graphene oxide. Mater Sci Eng C Mater Biol Appl 2016;62:823–34. [102] Shin YC, Kim J, Kim SE, Song SJ, Hong SW, Oh JW, et al. RGD peptide and graphene oxide co-functionalized PLGA nanofiber scaffolds for vascular tissue engineering. Regen Biomater 2017;4:159–66. [103] Zhou T, Li G, Lin S, Tian T, Ma Q, Zhang Q, et al. Electrospun poly(3-hydroxybutyrateco-4-hydroxybutyrate)/graphene oxide scaffold: enhanced properties and promoted in vivo bone repair in rats. ACS Appl Mater Interfaces 2017;9:42589–600. [104] Zhang K, Zheng H, Liang S, Gao C. Aligned PLLA nanofibrous scaffolds coated with graphene oxide for promoting neural cell growth. Acta Biomater 2016;37:131–42. [105] Jing X, Mi HY, Salick MR, Cordie TM, Peng XF, Turng LS, et al. Electrospinning thermoplastic polyurethane/graphene oxide scaffolds for small diameter vascular graft applications. Mater Sci Eng C 2015;49:40–50. [106] Qi YY, Tai ZX, Sun DF, Chen JT, Ma HB, Yan XB, et al. Fabrication and characterization of poly (vinyl alcohol)/graphene oxide nanofibrous biocomposite scaffolds. J Appl Polym Sci 2013;127:1885–94. [107] Jin L, Zeng Z, Kuddannaya S, Yue D, Bao J, Wang Z, et al. Synergistic effects of a novel free-standing reduced graphene oxide film and surface coating fibronectin on morphology, adhesion and proliferation of mesenchymal stem cells. J Mater Chem B 2015;3:4338–44.

Further reading [108] Bhardwaj N, Kundu SC. Electrospinning: a fascinating fiber fabrication technique. Biotechnol Adv 2010;28:325–47.

Graphene and polymer composites for supercapacitor applications

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Busra Balli, Aysun S¸avk, Fatih S¸en Sen Research Group, Biochemistry Department, Faculty of Arts and Science, Dumlupınar University, K€utahya, Turkey

Chapter Outline 5.1 Introduction 123 5.2 Graphene-derived materials—Polymer composite supercapasitors 124 5.2.1 Nonconductive polymer-G nanocomposite supercapacitors 124 5.2.2 Conductive polymer-G nanocomposite supercapacitors 129

5.3 Flexible supercapacitor electrodes 136 5.4 A different approach: Graphene-based–binder-free supercapacitor electrodes 140 5.5 Conclusion 140 Acknowledgments 141 References 141 Further reading 151

5.1

Introduction

Supercapacitors are classified into three classes according to their charge storage mechanism: (i) electric double-layer capacitors (EDLCs), (ii) pseudocapacitors, and (iii) asymmetric (hybrid) supercapacitors. EDLCs are-based on nonfaradaic processes, for example the charge storage is-based on the absorption of ionic species and the charged particles at the interface of the electrode/electrolyte. Energy storage in pseudocapacitors is-based on highly reversible Faradaic processes such as redox reactions between the surfaces of electrochemically active materials and the electrolyte. They generally are comprised of metal oxides or hydroxides. Finally, asymmetric supercapacitors have various types of electrode materials that have improved capacitance, raising the operating voltage and energy density. These have been developed to overcome the problems of low capacitance, high resistivity, and yield loss because of poor cyclability with metal oxides and hydroxides. Nanomaterials offer very promising solutions to the ongoing problems of the world. Fuel cells [1,2], catalyst materials [3–8], thermopower applications [9], capacitors [10], solar cells [11,12], and sensors [13–16] are some examples of these solutions. Especially, nanoparticles are used in a wide variety of applications [17–28]. Materials are also used corporately to combine their superior properties. Metal-metal Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00005-5 © 2019 Elsevier Ltd. All rights reserved.

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combinations, polymer-metal combinations, and some hybrids with carbon-based materials are used for a variety of nanomaterial applications [29–38]. Composite materials are widely studied in developing appropriate materials for asymmetric supercapacitors such as conductive polymers and other electronically active material composites. Graphene and graphene oxide [18,19,39–41], carbon nanotubes (CNTs) [42–47], activated carbon (AC) [48, 49], vulcanized carbon (VC) [50,51], carbon black [52,53], and graphene-derived materials that have different structures and morphologies such as reduced graphene oxide [54–59], graphene nanosheets, graphene nanoribbons, and graphene nanoplatelets may be thought of as active materials for different applications. Graphene-derived materials provide an electronically active network as well as high conductivity, excellent surface area, high mechanical strength, and flexibility.

5.2

Graphene-derived materials—Polymer composite supercapasitors

More recently, graphene has attracted much attention owing to its superior mechanical, electrical, and thermal properties as a two-dimensional (2D) building block of new materials. However, making use of these attributes into new materials requires developing methods to assemble carbon flakes that are a single atom thick into macroscopically ordered structures. Also, these macroscopic structures should have superior electrical, mechanical, and thermal properties to take part in various devices used in different applications such as solar cells and supercapacitors. Integration of graphene nanostructures into a polymer matrix is one of the facile methods for using these nanostructures in supercapacitor devices.

5.2.1 5.2.1.1

Nonconductive polymer-G nanocomposite supercapacitors PVDF-G nanocomposite supercapacitors

Poly(vinylidene difluoride) (PVDF) is a thermoplastic fluoropolymer that shows high chemical resistivity and thermal stability while having unique mechanical properties, electronic activity, and excellent aging resistivity among all the other thermoplastics. The unique mechanical properties of PVDF are based on its semicrystalline structure [60,61]. Poly(vinylidene difluoride) (PVDF) consists of a CH2-CF2 unit, as shown in Fig. 5.1. This polymer has the typical stability of fluoropolymers but interactive groups lead to a superior polarity. This high polarity gives the polymer superior properties such as good chemical and oxidative resistances, poor hydrophilicity, and a significant swelling property in the electrolyte [62]. Due to all these advantageous properties, PVDF is employed in membranse [63], sensors [64], energy harvesting [65], and many other applications. Graphene-PVDF composite structures are used in graphene-based supercapacitor devices on the current collectors to provide the electrode attribute. To compose the graphene-reinforced PVDF electrodes, graphene-based nanostructures, such as graphene nanoplates and graphene oxide sheets, are dispersed in appropriate

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Fig. 5.1 Chemical structure of PVDF.

organic solvents with the use of some surfactants, if required. PVDF solutions at different ratios by weight are prepared separately by mixing PVDF into an organic solvent. To constitute the composite solution, the prepared two solutions are mixed. The final solution has to be stirred thoroughly for full dispersion of graphene-based nanostructures. This composite solution is coated on a substrate that will be used as a supercapacitor electrode. At this stage, drop casting, spin coating, and several coating techniques can be used. While composing the supercapacitor electrode, the PVDF ratio is a crucial parameter in a composite solution. Because if the PVDF content is inadequate, it cannot provide enough binding strength between the graphene nanostructures and the substrate. In other respects, too much PVDF content decreases the conductivity of the material and the energy carrier capability of the supercapacitor. To keep the detrimental properties of the current material at a minimum, a certain amount of conductive nanomaterial can be added to the composite solution. As shown by some previous studies, carbon nanofillers take part in preventing aggregation and agglomeration of graphene nanostructures in the composite, helping to provide high electrical conductivity and improve the rate capability and cycle durability of the material for supercapacitor applications [66,67]. These conductive carbon nanofillers are generally dispersed onto graphene nanostructures. Chieh et al. reported that the conductive nanomaterials in the composite are nitrogen-doped reduced graphene oxide (NrGO), carbon nanotubes (CNTs), and carbon black (CB). Supercapacitor electrodes are produced by mixing these conductive and active carbon nanostructures with PVDF. By using these nanocomposite electrodes as both anode and cathode into two electrodes, such a symmetric supercapacitor in the 1 M KOH aqueous electrolyte shows a high specific capacitance (227 F g 1 at 20 mVs 1), fast rate capability (83% capacitance of current density 1 mA cm 2 at current density 5 mA cm 2), low

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resistance (0.98 Ω), and excellent cycling stability (87% capacitance retention after 10,000 charge/discharge cycles) [68]. Dong et al. reported a method for producing a composite material for use as a supercapacitor electrode. This electrode consists of MnO2 nanoparticles, graphene nanosheets as active materials, and PVDF as a binder. In order to make a comparison, graphene nanosheets were used, and the weight of graphene in the composite binder is approximately 0, 1, 5, and 10 wt%. The supercapacitor that uses the composite electrodes in 1 mol L 1 Na2SO4 electrolyte has been examined with cyclic voltammetry (CV) and galvanostatic charge-discharge, and exhibits superior electrochemical performance with maximum specific capacitance of 220 F g 1 at 5 wt% graphene mass ratio. So, it is concluded that mixing the graphene nanosheets in the composite binder significantly affected the electrochemical activities of the electrode. The optimum graphene mass ratio is determined as 5 wt% for a maximum specific capacitance of 220 F g 1 [69]. Considering the characteristic properties of PVDFs, the modifying agent that contains fluorine ended groups to functionalize the graphene nanosheets would produce enhanced compatibility and miscibility between the graphene nanosheets and the PVDF. Besides, the PVDF also includes fluorine groups and the same molecular chains are more likely to be intertwined with each other [70].

5.2.1.2

PTFE-G nanocomposite supercapacitors

Polytetrafluoroethylene (PTFE) is another type of fluoropolymer that is also most widely used for composing supercapacitor electrodes. It includes more fluorine atoms than PVDF and is dispersed in water, ethanol, and isopropanol [71–73]. In the supercapacitor industry, the PTFE matrix use is very common while producing graphene nanostructure-reinforced polymer electrodes, due to the cost efficiency of using water as a solvent; also, using an organic solvent such as water does not lead to safety problems. This polymer is usually preferred as a matrix material when constituting graphene-based composite electrodes for its two main advantages: low reactivity and high adhesion to active components, for example, graphene nanosheets and graphene oxide nanosheets [74]. Different matrix materials can give rise to differences in performance of supercapacitor devices. In a comparison study, Abbas et al. reported that matrix materials used in the same amount in electrodes (PVDF versus PTFE) affect the capacitance of the overall supercapacitor [75]. These electrodes include active carbon materials and a NaNO3 aqueous electrolyte. An electrode that comprises the PVDF matrix is characterized as less porous than the PTFE matrix electrode. The capacitors are charged to 1.6 V, the PVDF-AC-based capacitor reaches 104 F g 1, and the PTFE-AC-based one reaches 116 F g 1 capacitance values. So, the more porous AC-PTFE supercapacitor shows a higher capacitance performance due to its higher microporous structure and volume [73]. The PTFE can cause a decrease of conductivity in the electrode by resisting the flow of the aqueous electrolyte ions into the micropores due to its hydrophobicity

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and insulation. So, this resistivity in the electrode results in reducing the energy density and specific capacitance in the supercapacitor [75,76]. A supercapacitor electrode comprises different compounds joined at different ratios. These compounds include a material that is active and conductive, an electrolyte, and an electrode material such as PTFE. Kotok et al. reported that for the nickel hydroxide and graphite-based supercapacitor using the 6 M potassium hydroxide, the maximum specific capacitance and energy density are achieved using 3 wt% of PTFE in the composite when the graphite and nickel hydroxide particles are using 16 wt% and 81 wt%, respectively [76]. During working 1 wt% PTFE containing electrode, its specific capacitance initially reached maximum (40 mA cm 2) value and then decreased abruptly. While working 2 wt% PTFE containing electrode, its specific capacitance value was increasing gradually and then reached maximum value (40 mA cm 2). After that, this value was not changed significantly and then somewhat dropped. And during working 3 wt% PTFE containing electrode, its specific capacitance initially was increased and then a significant decrease was observed after reaching of its specific capacitance value. The insufficient matrix polymer content indicates the dependence of the specific capacitance on the cycle number under the maximum current density for graphene-PTFE electrodes. From the analysis of these relationships, it can be concluded that during cycling under high current densities, a decrease of specific capacitance relies on the evolution of large volumes of oxygen, which causes it to fall off the active mass [76]. According to the results of these studies, the amount of polymer matrix significantly affects the overall capacitance, especially in high rate charge-discharge regimes that are typical in supercapacitors. Polyvinylpyrrolidone (PVP) is offered as a “greener” alternative to generally be utilized as a binder for supercapacitor electrodes instead of polyvinylidenedifluoride (PVDF) or polytetrafluoroethylene (PTFE). The most important benefits of using PVP are that it is nontoxic and soluble in ethanol and it is proper for spray coat or drain cast activated carbon (AC) electrodes directly on a current collector such as aluminum foil. When the PVP is compared to PTFE or PVDF, the results show that the pore volume is significantly higher and therefore the specific surface area is significantly larger [77]. The wettability of the electrode material in an aqueous system depends mainly on the hydrophilic/hydrophobic nature of the material. During electrode preparation, only H2SO4 or KOH solutions can come in contact with the electrode; do not use neutral aqueous electrolytes (KCl or Na2SO4). In order to improve the hydrophilicity of the electrode material, a small amount of PVP (3%) was added in addition to PTFE. The electrode material has a 151-degree contact angle value without containing PVP. The addition of superhydrophobic PVP decreased the contact angle value of the electrode material to 22 degrees. This drastic decrease means the electrode material now has a very hydrophilic nature. PVP is preferred due to its good film formation, adhesion, hydrogen bonding, and water-absorption properties [78,79]. Carbonaceous materials provide not only a high specific surface area but also the high conductivity, which are two important parameters of supercapacitor electrode materials. The threedimensional structure supplies a higher surface area for the electrode materials. It is

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Fig. 5.2 Scanning electron microscopy image obtained from (GNS:CB:PTFE) 90:10:10 (A), 60:30:10 (B) and 0:90:10 (C) electrodes; (D) XRD pattern obtained from graphene. Reprinted from Robat Sarpoushi M, Nasibi M, Moshrefifar M, Mazloum-Ardakani M, Ahmad Z, Reza Riazi H, Electrochemical investigation of graphene/nanoporous carbon black for supercapacitors, Mater Sci Semicond Process 33:89–93. Copyright (2017) with permission from Elsevier.

clearly seen in Fig. 5.2A that, while SEM images of the mixture of GNS and PTFE without carbon black indicate a highly flat surface, after addition of carbon black to the composite, significant changes have been observed on the surface of novel materials. Besides, as shown in Fig. 5.2B, the addition of NCB particles (as shown in Fig. 5.2C) will change the specific properties of the prepared materials, such as surface area, reversibility, etc. Fig. 5.2D gives the X-ray diffraction pattern for graphene nanosheets in order to determine the distance between the sheets. This distance has ˚ , which indicates the formation of a multilayered graphene been found to be 3.36 A sheet [80].

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5.2.1.3 Effect of electrolyte type and concentration For the supercapacitor performance, the type, composition, and concentration of the electrolyte is as important as the electrode materials. There are different sorts of liquids used as an electrolyte such as ionic liquids (ILs), organic electrolytes, graphene quantum dots, and aqueous electrolytes to widen the electrochemical stability range of supercapacitors [81–83]. Interactions between the graphene-based nanostructures reinforced polymeric composite electrode and the electrolyte affect the electrochemical performance of the supercapacitor. Especially when using ionic liquids as an electrolyte, the functional oxygen containing groups on graphene-based nanostructures on the electrode can enhance the interfacial interactions between the electrolyte and the electrode [79]. As reported in various studies, the electrolyte concentration strongly affects the electrochemical efficiency and capacitance of the supercapacitor [84]. To illustrate this, when the electrolyte concentration is high, the ion transport within the electrode layer is easier, and this induces an effective build-up for the double layer. Nevertheless, when the electrolyte concentration is too high, the ion activity is reduced because of less water hydration, resulting in decreased ion mobility. So, an optimized electrolyte concentration is investigated. In general, the most popular electrolyte is aqueous Na2SO4 for aqueous electrolyte-based supercapacitors [75]. Stoller et al. [85] carried out a study using chemically modified graphene (CMG) as the conductive material, PTFE as the matrix material, and different electrolytes such as potassium hydroxide, which supplies the pne of the highest capacitance value as compared to others. However, this issue has not been well explored for electrodes that comprise graphene nanosheets or nanostructures, and hence further investigation and exploration are needed.

5.2.2

Conductive polymer-G nanocomposite supercapacitors

As discussed previously, polymer matrix materials—namely polymer binders—are one of the most important parts of supercapacitor electrodes that are comprised of a graphene-derived nanostructure-polymer composite. Yet these polymer binders have some disadvantages because of their insulating nature. As detailed in the previous part, the nonconductive binders can give rise to a decrease in conductivity, and therefore decrease the energy density and overall capacitance of the supercapacitors. The working principle of supercapacitor devices, including graphene-derived nanostructure-polymer composite electrodes, is mainly based on the electrical double layer (EDL) capacitance. The mechanism of ion absorption and desorption to the electrical double layer contributes to the charge and discharge of EDLC. By applying voltage to the facing electrodes, ions are drawn to the surface of the electrical double layer and the EDLC is charged. Conversely, they move away when discharging EDLC. This shows how EDLC is charged and discharged. In this type of capacitor, the capacitance mechanism relies on the surface of the composites with the help of various materials, that is, conducting polymers or conductive nanostructures of carbon derivatives such as graphene and carbon nanotubes [86–88]. Besides all these, conducting polymers are a convenient and functional alternative for use as a polymer

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binder in electrodes. Electrically conductive polymers are widely studied as a polymer matrix or binder in supercapacitor electrode materials due to their being ideal candidates for an ideal matrix material, thanks to their features such as facile thin film fabrication, ease of processability, light weight, and elasticity [89]. It is seen when a retrospective research is carried out that various conductive polymers were employed in the supercapacitor electrodes such as polyindophenine [90], polythiophene [91], p-phenylenevinylene (PPV) [92], polyaniline (PANi) [93–95], and polyprrole (PP) [96]. Among these polymers, the PANi and PPy are the most proper and promising materials for use in electrodes as a matrix material due to their high conductivity, ease of synthesis, low cost, fast redox reaction capability, and higher energy density [97]. Besides, in supercapacitor electrodes, PANi and PPy are employed in low and neutral pH solutions [89,98]. The practical specific capacitance values of the graphene-based capacitors in aqueous solutions vary within the 135–264 F g 1 range, despite the fact that the theoretical specific capacitance values are calculated as 550 F g 1. Because of the graphene nanostructure’s tendency toward stacking and agglomeration, its theoretical capacitance is not calculated. Conductive polymers also have been used for restraining the agglomeration of graphene and its derivatives and increasing their specific capacitance by this way [99]. On the other hand, there are some problems while using these electrically conductive polymers. These include the considerable volume change during the repeated intercalation and the depletion of ions in reactions occurring during charge and discharge. This problem has largely decreased the mechanical stability of the supercapacitor in the service because of the expansion and contraction of molecular chains during ion doping/dedoping [100,101]. In order to overcome this problem, nanostructured materials that show a high surface area, high mechanical strength, electrical conductivity, and chemical stability, such as carbon nanotubes, graphene oxide or graphene nanosheets, are widely used to reinforce the polymer binder in electrodes. This application efficiently improves the overall performance of the supercapacitor [102–104].

5.2.2.1

Graphene and PPy nanocomposites

Polypyrrole (PPy) is an ideal electroactive material for electrode materials, especially in asymmetric supercapacitors, due to its being environmentally friendly and demonstrating good stability as well as providing facile synthesis at large-scale production and low cost [90]. Nevertheless, pure PPy exhibits weak cycling performance because of the volume change caused by the doping-dedoping of ions during charging and discharging [90–92]. It’s a great challenge to fabricate high-performance, flexible, solid-state supercapacitors with high areal and volumetric energy storage capability, superior conductivity, robust mechanical flexibility, and long-term stability. There have been efforts and attempts to fabricate with these standards, including PPy synthesis with various morphologies and techniques and synthesizing high performance composite materials composed of carbonaceous materials such as carbon nanotubes (CNTs) [105], activated carbon (AC) [106], and graphene [93,94,96].

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A number of studies have been carried out that focus on the capacitors used as electrodes for composites composed of carbonaceous materials and conductive polymers. In these studies, the comparison of Ppy synthesis methods (chemical or electrochemical), improving the polymer surface morphology, and the effect of carbonaceous materials on the performance of electrodes in the supercapacitor are the mainstays and motivations of studies. The chemical synthesis method is more preferred, because it provides a facile synthesis with the advantages of large-scale production of the material with low cost [107]. In the recent literature, it is mostly reported that chemical polymerization methods are generally applied for producing the graphene-PPy composite. These chemical methods involve complicated multiple-step procedures as well as an oxidant requirement such as ammonium persulfate (APS) or FeCl3 [108–110]. There is a promising and noteworthy system for producing graphene-PPy composites, and that is the electrochemical method. This method works without the need for any oxidant or reductant chemical during production. Therefore, the system remains clean and the produced composites are free from pollution. Besides, the produced composites are generally able to be electrochemically measured and also used as electrode materials without any further process step or operation. As reported in some studies, graphene-PPy composite electrodes have common specific capacitances in the range of 200–400 F g 1 and show greatly improved electrochemical stability [107,111–116]. The exceptionally high specific area of graphene (2630 m2 g 1) provides effective background for execution in supercapacitor applications. Because it has high specific capacitance of 154.1 F g 1 and high energy density of 85.6 Wh kg 1 in an ionic liquid electrolyte [117–122]. Whereas a typical AC-based capacitor with a specific surface area in the range of 1000–2000 m2 g 1 shows a gravimetric capacitance of 100–120 F g 1. By the way, a multiwalled CNT-based capacitor with a specific surface area of 430 m2 g 1 has a specific capacitance and energy density of 113 F g 1 and 0.56 Wh kg 1, respectively, in H2SO4 electrolyte [123]. There are some problems while using graphene, which tends to agglomerate in most solvents due to its inherently hydrophobic nature [83,124]. This property of graphene leads to obstruction of the ionic accessibility of an asymmetric supercapacitor [97]. This problem can be surmounted by using reduced graphene oxide (rGO) instead of graphene, which has a few oxygen groups that could prevent the restacking of graphitic sheets. The rGO also shows excellent electrical conductivity of up to 1.28 S/m at 6 wt% [125] and is appropriate to be employed in supercapacitor electrodes by compositing electrochemically active materials. Composites in which graphene and metal oxides are used corporately in conducting polymers have been studied for electrodes, reported as having 480 F g 1 specific capacitance in recent years [123,124,126]. Graphene-PPy composites produced by chemical polymerization of pyrrole onto graphene nanostructures exhibit utmost specific capacitance of 482 F g 1 at 0.5 A g 1 current density [123] and 267 F g 1 at 0.1 A g 1 current density [124]. The other graphene-PPy composites produced by the electropolymerization of polypyrrole onto graphene exhibit an utmost specific capacitance of 1510 F g 1 [127]. These comparative results demonstrate that the capacitance of

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Fig. 5.3 (A) The specific capacitance curves of PG10:1GE and PPy at different current densities. (B) Specific capacitance retention ratio of PG10:1 and PPy as a function of cycles at the current density of 1.8 A g 1. Reprinted from Liu Y, Wang H, Zhou J, Bian L, Zhu E, Hai J, Tang J, Tang W; Graphene/ polypyrrole intercalating nanocomposites assupercapacitors electrode, Electrochim Acta 112:44–52. Copyright (2017) with permission from Elsevier.

graphene-PPy nanocomposites is strongly dependent upon the production methods of composites as well as the morphology of composites. As clearly demonstrated in Fig. 5.3, the capacitance of the nanocomposite of graphene/polypyrrole with a mass ratio of 1:10 at all current densities (0.45–3.17A g 1) is 430 F g 1 higher than that of pure PPy and 500 F g 1 higher than that of pristine graphene at the same current density, which is ascribed to the synergistic contribution of graphene and PPy. As also seen in Fig. 5.3, the specific capacitance of 650 F g 1 at 0.45 A g 1 for this composite is higher than all specific capacitances for graphene/PPy composites reported so far in the literature [123,128,129], perhaps thanks to the different synthesis method of composites leading to the optimized morphology of this nanocomposite [130].

5.2.2.2

Graphene and PANI nanocomposites

Among conducting polymers, PANI is expected to be a good candidate for this purpose due to its high capacitance, low cost, and environmental stability [131–133]. Nevertheless, it is difficult to get the high cycle stability of a device with only the utilization of PANI due to the weakness of mechanical stability during the chargedischarge of the supercapacitor. For improving the energy storage performance of supercapacitors, hybrid electrode materials have been fully utilized for their advantages. Various literature studies have been published focusing on hybrid electrode materials, but there was little attention paid to graphene-PANi composites. From our viewpoint, graphene-PANi nanocomposites will be a future trend due to their superior and unique properties [134]. Numerous experimental studies concerning various graphene/PANI composites have been carried out. For instance, comparison of the morphology and structure of PANi and graphene-PANi composites, comparative

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133

analysis of the production method of graphene-PANi composites, and the effects on pH control of the supercapacitor system have been investigated in these studies. When we compare the effect of PANI structures and nanocomposite structures, three-dimensional (3D) structures such as 3D porous structures [135], sandwiched structures [136], tremella-like spheres [137], spheres [138], and hollow spheres [139] show far better electrochemical performance compared to a lamellar structure. Stanciu et al. reported that high specific surface area (891 m2 g 1) and specific capacitance (257 F g 1 at 0.1 A g 1) were obtained when PANi nanoparticles were used as a spacer to get a sandwich-type graphene-PANi composite [136]. Liu et al. have synthesized tremella-like PANI/graphene spherical composites by applying the selfassembly method of graphene nanosheets. In this structure, the specific capacity of 497 F g 1 at 0.5 A g 1 was obtained thanks to the 3D porous spherical architecture of graphene PANi nanocomposites [137]. It’s reported that its microsphere structure is very initiative for energy storage. When the current density is 0.5 A g 1, the PANI/ G-MS exhibits the maximum specific capacitance of 596.2 F g 1, higher than those of PANI/GO-MS (494.6 F g 1), PANI/GO (379 F g 1), and pure PANI (333 F g 1), respectively. Apparently, the introduction of GO and the spherical structure are advantageous for energy storage [138]. While a freestanding graphene nanosheet-PANi (GNS-PANi) composite film exhibits a 740 F g 1 specific capacitance at 0.5 A g 1, a 3D skeleton network of graphene nanosheet-wrapped PANi nanofibers can reach 921 F g 1 specific capacitance [140]. Even though such high specific capacitance values were reported in these studies, the 3D structures with macropores in micrometer size could not initiate to develop the volumetric energy storage. Hence, for use as an electrode material, producing ultrathin GNS-PANi layers with an appropriate porous morphology in an ultrathin structure is a great challenge. To achieve a solution for this problem, a method was reported that shows a route for producing composites using PANi in different structures and graphene nanosheets together. To summarize, by a two-step hydrothermal assistant chemical oxidation polymerization process, some ultrathin composite layers (10–20 nm thickness) were produced by using graphene nanosheets and PANi nanofibers. The specific capacitance of the composite layers reaches as high as 532.3 F g 1 at the scan rate of 2 mV s 1. Also, even if the scan rate is at 50 mV s 1, the specific capacitance can still reach as high as 304.9 F g 1 with good cyclic performances. This result proves that the production method greatly improves the specific capacitance, rate capability, and cycling stability of the composite [141]. Different production methods can be utilized such as chemical polymerization, electrochemical polymerization, or noncovalent functionalization for enhanced electrochemical properties while producing graphene-PANi composites [97]. It should be noted that, in comprising a supercapacitor, PANi and PPy are used in low pH and neutral electrolyte solutions [98]. Hence, PANi and its derivatives should work smoothly under neutral pH conditions [142]. Among the PANi derivatives, selfdoped polyaniline (SDPA) exhibits electrochemical activity over a wide pH range, even in a neutral and basic solution, thanks to the presence of functional groups carrying a negative charge (such as COO and SO3 ) that act as proton generators and dopants with no need for external ions [89].

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5.2.2.3

Nanocarbon and its Composites

Graphene and PEDOT nanocomposites

Poly(3,4-ethylenedioxythiophene), namely PEDOT, was first developed by a group of scientists at Bayer AG research laboratories in Germany in the 1980s [143]. It is possible to reach as high as 500 S cm 1 conductivities in its doped states [86,144,145]. PEDOT is a conductive polymer characterized by a small theoretical gravimetric capacitance (210 F g 1) [146] in comparison with other conductive polymers. Nevertheless, it displays high conductivity, low oxidation potential with a wide electrochemical window (1.2–1.5 V), chemical and thermal stability, and high charge mobility, which results in fast electrochemical kinetics. However, the disadvantage of PEDOT for use in supercapacitor electrodes is its high molecular weight, which causes low specific capacitance. In the composite production process, in order to overcome the aggregation problem of graphene-derived nanostructures, the electropolymerization method is employed. Also, poly (styrene sulfonic acid) (PSS) is used for enhancing the water solubility of PEDOT [147]. In this method, the PSS and ethylenedioxythiophene monomers are mixed in an aqueous solution. Then, graphene-derived nanostructures and oxidants [148–150] are added, respectively. Oxidant acts as an initiative of polymerization and composites are obtained after completion of polymerization. By incorporating of graphene and PEDOT, the electrical conductivity, energy storage and mechanical properties of PEDOT are greatly improved. Graphene has furnishing pathways for percolation and propagation, hence enhancing the entire electrical and mechanical properties of PEDOT’s behavior. Lehtimaki et al. reported the capacitances of the devices that consist of both PEDOT and PEDOT/GO. This is illustrated clearly in Fig. 5.4 [87]. Within the conductive polymer composites, graphene plays crucial roles. One of these is preventing the composite from structural damage born of the volumetric changes of PEDOT while charging and discharging of the supercapacitor by forming a heterogeneous internal structure. Another is making the composite more conductive with higher internal conductivity than PEDOT:PSS. Another benefit of graphene is giving the composite a 3D morphology. The specific capacitance and energy harvesting are significantly improved due to its large surface area in its 3D morphology [151]. According to the investigation carried out by Wilamowska et al., electrodeposited pEDOT/rGOx composite electrodes exhibit approximately 10 mF cm 2 when deposited with the charge of 0.2 C cm 2 and approximately 30 mF cm 2 deposited with the charge of 0.8 C cm 2. In another study, Damlin et al. reported that graphene oxidePEDOT composite electrodes produced by the electrodeposition method by cyclic voltammetry from a suspension of GOx in ionic media have a specific capacitance of 39 mF cm 2; it increased to 43 mF cm 2 by the electrochemical reduction of graphene oxide [145]. There are also different studies carried out under different conditions in terms of the production methods of composites. Alvi et al. reported in their study in 2011 that a graphene-PEDOT nanocomposite fabricated with the chemical oxidative polymerization method exhibits 374F g 1 specific capacitance value at maximum (Alvi et al. [148]).

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135

Fig. 5.4 SEM images of PEDOT/GO (A) and PEDOT/rGO (B) composite films with 1000  magnification. Images (C) and (D) show the cross-section with 500  magnification; CV curves of supercapacitors, normalized to voltage sweep rate: (E) PEDOT, (F) PEDOT/GO, (G) PEDOT/rGO. The axis and curve labels are the same for all graphs. Reprinted with permission from Lehtim€aki S, Suominen M, Damlin P, Tuukkanen S, Kvarnstr€om C, Lupo D. Preparation of supercapacitors on flexible substrates with electrodeposited PEDOT/graphene composites. ACS Appl Mater Interfaces 2015;7 (40):22137–22147. Copyright (2015) American Chemical Society.

Besides, a study conducted by Alabadi et al. in 2016 reported a 296 F g 1 specific capacitance graphene oxide-(Thiophene-2,5-diyl)-co-(benzylidene) composite produced by the in situ polymerization method [152]. The differences in capacitance values obtained for various pEDOT/GOx and pEDOT/ rGOx composites demonstrate the importance of production process optimization

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in terms of the composition of the initial solution, the concentration of substrates, the solvent, the compatibility of the production method with materials, etc. [145].

5.2.2.4

Comparison and summary for graphene and conducting polymer nanocomposite supercapacitors

All examined studies show that graphene and graphene-derived nanostructures have tremendous effects on improving the properties of conductive polymers such as electrical properties and structural stabilities. The composite morphology created by the graphene also greatly influences the composite capacitance performance. Various factors affect the composite performance, such as the polymerization method employed in composite production, the intrinsic properties of the conductive polymer, the type and concentration of the electrolyte, the dispersion or aggregation state of the graphene nanostructures, and the use of a type of graphene nanostructure derivative. To sum up, it should be noted that the properties of composites can easily be enhanced with the help of a conductive polymer. For instance, supercapacitors comprised of a graphene-PPy composite electrode exhibit a worse cycling performance because the PPy films are more porous than the others because of larger particles formed in the PPy. In addition, the internal conductivity of the conductive polymer used as an electrode material greatly affects the ESR of the supercapacitor. PEDOT is the most conductive polymer among the polymers studied in this review. PPy and PANi follow, respectively. When the composite is produced by mixing graphene into polypyrrole with a surface area of 37.7 m2 g 1g, the surface area of this composite becomes 260 m2 g 1. When the composite is produced with a well-controlled pore structure, the specific capacitance reaches 267 F g 1 but the pseudocapacitance effect is more improved in supercapacitors with electrode graphene-PANi nanocomposites than the other graphene-conductive polymer electrodes. To sum up, graphene-PANI nanocomposite electrodes have 291.4 F g 1 capacitance, 64.2 (Wh kg 1) energy density, and 11.5 (kW kg 1) power density values. Also, graphene-PEDOT nanocomposite electrodes have 286.3 F g 1, 63.1 Wh kg 1, and 9.1 Wh kg 1; and graphene-PPy nanocomposite electrodes 214.2 F g 1, 47.2 Wh kg 1, and 8.5 Wh kg 1 as the capacitance, energy density, and power density values, respectively [153]. Graphene-conductive polymer composites used in supercapacitor applications were summarized in Table 5.1.

5.3

Flexible supercapacitor electrodes

Recently, most electronic products have tended to be light, ultrathin, and flexible, reflecting the arrival of the era of portable, wearable, and flexible electronics that have a wide application area such as artificial smart skin, implantable medical devices, bendable displays and smart card, mobile phones, military garment devices, heart-rate sensors and monitors, pedometer devices, and various military, medical, and fitness applications. Because all these applications and devices requires aesthetic appeal

Table 5.1 Comparison of the supercapacitor performances based on graphene/conducting polymers composite materials Electrolyte

Specific capacitance

Capacitance retention

Energy/power density

ESR

PPy/ Graphene PPy/ Graphene PPy/ Graphene

1 mol/L KCl (aq) 1 M H2SO4 (aq) 2 M H2SO4 (aq)

285 F g 1 at a current density of 0.5 A g 1 482 F g 1 at current density 0.5 A g 1 417 F g 1 at a scan rate 10 mV s 1

92% after 800 cycles 95% after 1000 cycles 90% after 500 cycles

NG

514 S m

PPy/ Graphene PPy/ Graphene

1 M H2SO4 (aq) 3M NaClO4(aq)

492 F g 1 at a current density of 0.2 A g 1 350 F g 1

PPy/rGO

2 M H2SO4 (aq) 3 mol/L KCl (aq)

500 F g 1 at a current density of 5 A g 1 280 F g 1 at a scan rate of 100 mV s 1

70% after 1000 cycles Not notable change after 1000 cycles 72% after 5000 cycles NG

255 F g 1 at a scan rate of 50 mV s 1

PANi/ Graphene PANi/ Graphene

LiClO4NaClO4 ionic liquid 1 M H2SO4 (aq) 1 M H2SO4 (aq)

PANi/ Graphene

1 M H2SO4 (aq)

PPy/ Graphene nanosheets PPy/ Graphene

233 F g

1

210 F g 1 at a discharge rate of 0.3 A g 1 340 F g 1 at a current density of 0.25 A g 1

12.7 kW kg

1

1

1.39 Ω 1

Year

Ref.

2010

[111]

2011

[123]

2011

[124]

6.56 S cm



2012

[109]

NG

NG

2013

[113]

NG

2.7 Ω

2013

[108]

NG

0.3 Ω

2014

[115]

NG

NG

NG

2017

[99]

NG

NG

0.36–0.51 Ω

2009

[154]

71% after 800 cycles

NG

4.0 103 S m

2010

[136]

90% after 4200 cycles

7.56 Wh kg 1 at the power density of 3149 W kg 1

0.76 Ω

2013

[155]

1

137

94.93 Wh kg 1 at power density of 3797.2 W kg 1 NG

Graphene and polymer composites for supercapacitor applications

Materials

Continued

Table 5.1 Continued Specific capacitance

Capacitance retention

Energy/power density

PANi/GO

1M Na2SO4

79.5 F g 1 at current density 0.2 A g 1

80.6% after 1000 cycles

PANi/ Graphene

1 M H2SO4 (aq)

82 F g 1 at a scan rate of 20 mV s 1

100% after 1000 cycles

PANi/ Graphene sheets PANi/ Graphene 3D framework PANi/ Graphene

PVAH2SO4 (aq)

573 F g 1 at current density 0.5 A g 1

1 M H2SO4 (aq)

1024 F g 1 within the pot. Window of 150–800 mV

86.5% after 5000 cycles

H3PO4/ PVA

261 F g 1 at a current density of 0.38 A g 1

89% after 1000 cycles

PEDOT/GO PEDOT/ rGO

NaCl(aq)

14 mF cm 18 mF cm

PEDOT/ rGO PEDOT/GO

0.1 M KCl

7.2 F cm

2 M KOH (aq)

296 F g 1 at a current density of 0.3 A g 1

1 M KCl

100.3 mF cm 0.5 mA cm 2

PEDOT/ GO-CNT

2 2

3

2

at

5.4% after 2000 cycles 6.8% after 2000 cycles 76% after 1000 cycles 91.86% after 4000 cycles 97.5% after 5000 cycles

ESR

Year

Ref.

2.2 Wh kg 1 at power density of 90.3 kW kg 1 2.4 Wh kg 1 at a power density of 124 kW kg 1 19.97 Wh kg 1 at power density of 7.61 kw kg 1 3 kW kg 1 at a power density of 120 Wh kg 1

0.84–1.16 Ω

2014

[156]

0.28 Ω

2014

[157]

3.06 Ω

2015

[133]

10 Ω

2016

[158]

23.2 Wh kg 1 at a power density of 399 W kg 1 0.7 J cm 3 1.2 J cm 3

6.4 Ω

2014

[159]

43 Ω 25 Ω

2015

[87]

NG

0.12–0.14 Ω cm

2016

[145]

148 Wh kg 1 at a power density of 41.6 W kg 1 4.4 mWh cm 2 at a power density of 4.0 mW cm 2

33 S cm

2016

[152]

2016

[160]

NG

1

2

Nanocarbon and its Composites

Electrolyte

138

Materials

Graphene and polymer composites for supercapacitor applications

139

and multifunctionality, the new electronic devices must be ultrathin, lightweight, flexible, foldable, twistable, and stretchable. Thereby, the rising demand for these soft electronic devices requires flexible batteries and supercapacitors having the abovementioned characteristics [161,162]. Therefore, it is critical to fabricate FSCs that are lightweight and have high mechanical durability with high electrochemical performance via inexpensive and facile methods [163]. To date, much progress has been made for the construction of FSCs using materials based on graphene, metal oxides, and conductive polymers as active materials deposited on various flexible substrates, including plastics, metal sheets, textile fibers, and metal wires [164,165]. Lately, important studies have been carried out to improve the flexible supercapacitors depending upon conductive polymers and electrically active carbon materials such as active carbon [166,167], carbon nanotubes [135,160,168], carbon quantum dots [169], and graphene-derived materials such as graphene nanosheets [136,154,170]. Ultrathin graphene papers and films can be produced by different methods. Ghoniem et al. reported an electrode which contains a high quality rGO film supported over flexible PET substrate produced by low cost single-step fabrication technique. When the rGO film was examined as an EDLC electrode and current collector for the deposition of amorphous pseudocapacitive MnO2 thin film, it was seen that the rGO/PET and MnO2/rGO/PET electrodes exhibited specific capacitances of 82 and 172 F g 1, and retained 98% and 95% of their initial capacitances after 2000 cycles, respectively, at 1.0 mA cm 2 current density [171]. PET served as a mechanical support to the graphene film, but it cannot help the capacitance effect of this structure. Furthermore, there are some issues about the restacking of graphene [113,172–176]. While materials such as carbon nanotubes, graphene-derived materials, and conductive polymers have been shown to be effective as standalone materials as flexible supercapacitor electrodes, a composite nanostructure will, of course, result in improved capacitance performances. To this end, the most promising strategy is to combine a highly conductive material with a pseudocapacitive material (i.e., a conducting polymer), forming composite structures with optimized performance. A composite film or paper comprised of graphene and a conducting polymer can be used to construct a flexible supercapacitor. This device can be used without losing the device integrity in various applications that require excessive bending or twisting. Another intriguing application of flexible supercapacitors is reported in a study carried out by Chen et al. According to this investigation, the strength, toughness, and capacitive performance of graphene-based fibers is significantly enhanced by adding hydrophilic poly(vinyl alcohol) (PVA) into spinning dope—a nonliquid-crystalline graphene oxide (GO) dispersion—before wet spinning. When the structure and properties of the resulting PVA/graphene hybrid fibers are systematically investigated, it is seen that the hybrid fiber with a PVA/GO weight ratio of 10/90 possesses a capacitance of 241 F cm 3 in 1 M H2SO4. A solid-state yarn supercapacitor assembled from these fibers exhibits a device energy of 5.97 mWh cm 3 and features excellent flexibility and bending stability. This device is strongly suitable to be integrated into soft

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electronics [177]. A different example of graphene-conductive polymer composites of reduced graphene oxide nanoscrolls embedded in a polypyrrole matrix is reported by Atri et al. The composite electrode exhibits the specific capacitance of 55.3 F g 1 at the scan rate of 5 mV s 1, which is high enough among various composites mentioned in this review [178].

5.4

A different approach: Graphene-based–binder-free supercapacitor electrodes

Due to its hierarchical sheet structure, graphene provides more space than other porous materials thanks to the structure that the graphene creates. Due to these properties of graphene-based structures, they are more advantageous than other activated carbons regarding easy ion access, facilitating faster ion absorption and desorption, etc. The direct use of graphene usually leads to inadequate capacitive behaviors because of agglomeration during the production of macroscale structures from graphene nanostructures. Due to this, studies in this area are conducted by using graphene with other hierarchical materials acting as spacers to enlarge the basal spacing between the graphene sheets. But Yang and Zou made graphene films as binderfree electrodes as a capacitor material. They produced flexible and robust materials that could be directly used as electrodes without any additional binders or conductive additives by a five-step method. This method includes preparing GO by a modified Hummer’s method, sulfonating the GO solution, reducing the GO solution to get a stable graphene suspension, vacuum filtrating the graphene suspension, and finally, removing the filtrating membrane to obtain a stand-alone graphene film electrode. The G film exhibited high SC, long cycle life, high rate performance, and high power and energy densities. These advantages recommend the prepared free-standing G films as promising electrode materials for application in supercapacitors [179,180].

5.5

Conclusion

The developments of graphene/polymer composites in supercapacitors in recent years have showed that the promising features of these materials are not only enhanced capacitance but also a large surface area, higher electrical conductivity, and mechanical enhancement of capacitance applications. When we compare the matured technologies of energy storage devices and asymmetric supercapacitor technology, it is clearly seen that much more research and development needs to be done in asymmetric supercapacitor device technology. The most important issue is increasing the operational voltage and working potential of supercapacitors in this regard to enhance the capacitance properties. These will provide enhancements in energy density and so result in commercial fabrication. Finally, much further study, research, and development of enhanced composites are needed, including the morphology of materials, electrolytes, and additives on the capacitance. Last of all, graphene/polymer composites are good candidates for supercapacitor applications

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to improve current supercapacitor technologies. It is clearly seen that graphene/ polymer composite-based supercapacitors will be commercialized and be widely used in the near future.

Acknowledgments This research was supported by Dumlupinar University Research Funding Agency (2014-05 and 2015-35). The partial support by Science Academy and FABED are gratefully acknowledged.

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[138] Chang KH, Hu CC, Chou CY. Textural and capacitive characteristics of hydrothermally derived RuO2xH2O nanocrystallites: independent control of crystal size and water content. Chem Mater 2007;19(8):2112–9. [139] Wang HW, Hu ZA, Chang YQ, et al. Design and synthesis of NiCo2O4-reduced graphene oxide composites for high performance supercapacitors. J Mater Chem 2011;21 (28):10504. [140] Hu N, Zhang L, Yang C, et al. Three-dimensional skeleton networks of graphene wrapped polyaniline nanofibers: an excellent structure for high-performance flexible solidstate supercapacitors. Sci Rep 2016;6(1):19777. [141] Wang R, Han M, Zhao Q, et al. Hydrothermal synthesis of nanostructured graphene/polyaniline composites as high-capacitance electrode materials for supercapacitors. Sci Rep 2017;7(174):44562. [142] Karyakin AA, Maltsev IA, Lukachova LV. The influence of defects in polyaniline structure on its electroactivity: optimization of “self-doped” polyaniline synthesis. J Electroanal Chem 1996;402:217–9. [143] Jonas F, Schrader L. Conductive modifications of polymers with polypyrroles and polythiophenes. Synth Met 1991;41(3):831–6. [144] Mastragostino M, Arbizzani C, Soavi F. Conducting polymers as electrode materials in supercapacitors. Solid State Ionics 2002;148(3–4):493–8. [145] Wilamowska M, Kujawa M, Michalska M, Lipiska L, Lisowska-Oleksiak A. Electroactive polymer/graphene oxide nanostructured composites; evidence for direct chemical interactions between PEDOT and GOx. Synth Met 2016;220:334–46. [146] Lota K, Khomenko V, Frackowiak E. Capacitance properties of poly(3,4-ethylenedioxythiophene)/carbon nanotubes composites. J Phys Chem Solids 2004;65(2-3):295–301. [147] Groenendaal BL, Jonas F, Freitag D, Pielartzik H, Reynolds JR. Poly(3,4-ethylendioxythiophene) and its derivatives: past, present, and future. Adv Funct Mater 2000;12(7):481–94. [148] Alvi F, Ram MK, Basnayaka PA, Stefanakos E, Goswami Y, Kumar A. Graphenepolyethylenedioxythiophene conducting polymer nanocomposite-based supercapacitor. Electrochim Acta 2011;56(25):9406–12. [149] Yoo D, Kim J, Kim JH. Direct synthesis of highly conductive poly(3,4-ethylenedioxythiophene):Poly(4-styrenesulfonate) (PEDOT:PSS)/graphene composites and their applications in energy harvesting systems. Nano Res 2014;7(5):717–30. [150] Gao Y. Graphene and polymer composites for supercapacitor applications: a review. Nanoscale Res Lett 2017;12(1):387. [151] Bagri A, Mattevi C, Acik M, Chabal YJ, Chhowalla M, Shenoy VB. Structural evolution during the reduction of chemically derived graphene oxide. Nat Chem 2010;2(7):581–7. [152] Alabadi A, Razzaque S, Dong Z, Wang W, Tan B. Graphene oxide-polythiophene derivative hybrid nanosheet for enhancing performance of supercapacitor. J Power Sources 2016;306:241–7. [153] Punya A. Development of nanostructured graphene/conducting polymer composite materials for supercapacıtor applıcations by Punya A. Basnayaka in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Mechanical Engineering College of Engineering University of South Florida, 2013. [154] C P, Anodic S, Wang D, et al. Fabrication of graphene/polyaniline composite paper via in situ anodic electropolymerization for high-performance flexible electrode. ACS Nano 2009;3(7):1745–52.

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[155] Li L, Raji ARO, Fei H, Yang Y, Samuel ELG, Tour JM. Nanocomposite of polyaniline nanorods grown on graphene nanoribbons for highly capacitive pseudocapacitors. ACS Appl Mater Interfaces 2013;5(14):6622–7. [156] Yu P, Zhao X, Huang Z, Li Y, Zhang Q. Free-standing three-dimensional graphene and polyaniline nanowire arrays hybrid foams for high-performance flexible and lightweight supercapacitors. J Mater Chem A 2014;2(35):14413–20. [157] Xu Y, Hennig I, Freyberg D, et al. Inkjet-printed energy storage device using graphene/ polyaniline inks. J Power Sources 2014;248:483–8. [158] Kulkarni SB, Patil UM, Shackery I, et al. High-performance supercapacitor electrodebased on a polyaniline nanofibers/3D graphene framework as an efficient charge transporter. J Mater Chem A 2014;2(14):4989–98. [159] Xie Y, Liu Y, Zhao Y, et al. Stretchable all-solid-state supercapacitor with wavy shaped polyaniline/graphene electrode. J Mater Chem A 2014;2(24):9142–9. [160] Zhou H, Zhai HJ, Han G. Superior performance of highly flexible solid-state supercapacitor-based on the ternary composites of graphene oxide supported poly(3,4-ethylenedioxythiophene)-carbon nanotubes. J Power Sources 2016;323:125–33. [161] Shieh JY, Tsai SY, Li BY, Yu HH. High-performance flexible supercapacitor-based on porous array electrodes. Mater Chem Phys 2017;195:114–22. [162] Zhu Y, Ye X, Tang Z, Wan Z, Jia C. Free-standing graphene films prepared via foam film method for great capacitive flexible supercapacitors. Appl Surf Sci 2017;422:975–84. [163] Ramadoss A, Yoon KY, Kwak MJ, Kim SI, Ryu ST, Jang JH. Fully flexible, lightweight, high performance all-solid-state supercapacitor-based on 3-Dimensional-graphene/ graphite-paper. J Power Sources 2017;337:159–65. [164] Zhang YZ, Wang Y, Cheng T, Lai WY, Pang H, Huang W. Flexible supercapacitorsbased on paper substrates: a new paradigm for low-cost energy storage. Chem Soc Rev 2015;44(15):5181–99. [165] Chen T, Dai L. Flexible supercapacitors-based on carbon nanomaterials. J Mater Chem A 2014;2(28):10756. [166] Balducci A, Bardi U, Caporali S, Mastragostino M, Soavi F. Ionic liquids for hybrid supercapacitors. Electrochem Commun 2004;6(6):66–570. [167] Zhao W, Zhang M, Pan P, et al. Design and fabrication of flexible supercapacitor devices by using mesoporous carbon/polyaniline ink. Surf Coat Technol 2017;320:595–600. [168] Meng C, Liu C, Chen L, Hu C, Fan S. Highly flexible and all-solid-state paperlike polymer supercapacitors. Nano Lett 2010;10(10):4025–31. [169] Zhao Z, Xie Y. Enhanced electrochemical performance of carbon quantum dotspolyaniline hybrid. J Power Sources 2017;337:54–64. [170] Zhou H, Zhai HJ. A highly flexible solid-state supercapacitor-based on the carbon nanotube doped graphene oxide/polypyrrole composites with superior electrochemical performances. Org Electron 2016;37:197–207. [171] Ghoniem E, Mori S, Abdel-Moniem A. Low-cost flexible supercapacitors-based on laser reduced graphene oxide supported on polyethylene terephthalate substrate. J Power Sources 2016;324:272–81. [172] Zhao X, Hayner CM, Kung MC, HH K. Flexible holey graphene paper electrodes with enhanced rate capability for energy storage applications. ACS Nano 2011;5 (11):8739–49. [173] H L, Y H, V P, Q C, P S, C L. MnO2 nanoflake/polyaniline nanorod hybrid nanostructures on graphene paper for high-performance flexible supercapacitor electrodes. J Mater Chem A 2015;3(33):17165–71.

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[174] Moon YS, Kim D, Lee G, et al. Fabrication of flexible micro-supercapacitor array with patterned graphene foam/MWNT-COOH/MnOx electrodes and its application. Carbon NY 2015;81(1):29–37. [175] Dong X, Wang J, et al. Supercapacitor electrode-based on three-dimensional graphenepolyaniline hybrid. Mater Chem Phys 2012;134(2-3):576–80. [176] Sawangphruk M, Srimuk P, Chiochan P, Krittayavathananon A, Luanwuthi S, Limtrakul J. High-performance supercapacitor of manganese oxide/reduced graphene oxide nanocomposite coated on flexible carbon fiber paper. Carbon NY 2013;60 (0):109–16. [177] Chen S, Ma W, Xiang H, et al. Conductive, tough, hydrophilic poly(vinyl alcohol)/ graphene hybrid fibers for wearable supercapacitors. J Power Sources 2016;319:271–80. [178] Atri P, Tiwari DC, Sharma R. Synthesis of reduced graphene oxide nanoscrolls embedded in polypyrrole matrix for supercapacitor applications. Synth Met 2017;227:21–8. [179] J Y, L Z. Graphene films of controllable thickness as binder-free electrodes for high performance supercapacitors. Electrochim Acta 2014;130:791–9. [180] Selvakumar D, Alsalme A, Alswieleh A, Jayavel R. Freestanding flexible nitrogen doped-reduced graphene oxide film as an efficient electrode material for solid-state supercapacitors. J Alloys Compd 2017;723:995–1000.

Further reading [181] Tsay KC, Zhang L, Zhang J. Effects of electrode layer composition/thickness and electrolyte concentration on both specific capacitance and energy density of supercapacitor. Electrochim Acta 2012;60:428–36. [182] Giorgi L, Antolini E, Pozio A, Passalacqua E. Influence of the PTFE content in the diffusion layer of low-Pt loading electrodes for polymer electrolyte fuel cells. Electrochim Acta 1998;43(24):3675–80.

Graphene-based nano metal matrix composites: A review

6

Rajesh Jesudoss Hynes Navasingh*, Ramar Kumar†, Kathiresan Marimuthu‡, Senthamaraikannan Planichamy§, Anish Khan¶,k, Abdullah Mohamed Asiri¶,k, Mohammad Asad¶,k *Department of Mechanical Engineering, Mepco Schlenk Engineering College (Autonomous), Sivakasi, Tamil Nadu, India, †Department of Mechanical Engineering, Vels Institute of Science, Technology & Advanced Studies, Pallavaram, Chennai, India, ‡ Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai, Tamil Nadu, India, §Department of Mechanical Engineering, Kamaraj College of Engineering and Technology, Virudhunagar, India, ¶Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia, kCenter of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia

Chapter Outline 6.1 Introduction 153 6.2 Graphene 154 6.3 Manufacturing and testing of GRMMC

156

6.3.1 Aluminium-graphene MMC 156 6.3.2 Magnesium-graphene nanocomposites 158 6.3.3 Copper- and nickel-based graphene nanocomposites 162

6.4 Conclusions 166 References 167 Further reading 169

6.1

Introduction

Generally, the idea to improve the mechanical strength of materials has been achieved effectively by introducing second-phase reinforcement particles into the materials. Many researchers all over the world have utilized this approach to enhance mechanical characteristics such as tensile strength, compression strength, bending strength, toughness, hardness, etc. Although the above research efforts have provided the highly desirable output, the development of cost-effective manufacturing techniques of metal matrix composites is still an unresolved problem. In this context, searching for new reinforcing materials as nanofillers in metal-matrix composites has vital importance. For examining new reinforcement materials, the following key elements Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00006-7 © 2019 Elsevier Ltd. All rights reserved.

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have greatly influenced the role of nanofillers as reinforcing elements in the metal matrix composites. Reinforcement materials must have greater mechanical properties. It should have high surface area and high aspect ratio. It must have strong binding with the metal matrix after inclusions. It should be dispersed homogeneously during reinforcing of particles in the metal matrix and should not create an agglomeration structure. v. It should be easily purchased in a cost-effective manner.

i. ii. iii. iv.

In these situations, the manufacturing of metal matrix composites with an improved mechanical strength denotes the subjects of intense research efforts in nanotechnology. For instance, within the approach in question, ceramic nanoparticles and nanofibers, metallic nanoparticles, and carbon nanotubes have been effectively exploited as reinforcing nanoinclusions.

6.2

Graphene

Graphene is a novel two-dimensional (2D) carbon thick sheet with remarkable mechanical strength such as a Young’s modulus of 1 TPa and tensile strength of 130 GPa (Fig. 6.1). Moreover, due to its honeycomb structure, high strength with low weight, and electrical and thermal properties, this material has been used by many researchers

Fig. 6.1 Mother of all graphitic forms [1].

Graphene-based nano metal matrix composites: A review

155

CVD graphene with stitched grain boundaries exhibit 90% of the strength of theoretical pristine graphene Graphene supercapacitor breaks storage record Roll-to-roll production of 30 square inches of graphene

Publication Numbers

First report of ‘Scotch Tape’ graphene exfoliation

Nobel prize awarded in 2010 to Geim and Novoselov for their work on 2D atomic crystals in 2005

12000

Ohta and coworkers report electronic control of bilayer graphene

8000

4000

13 20

12 20

11 20

10 20

09 20

08 20

07 20

06 20

05 20

20

04

0

Publication Year

Fig. 6.2 Graphene publication timeline in past years [9].

to produce novel products [1–7]. In recent years, researchers have fabricated the nanobased GRMMC and compared with the unreinforced metals [8]. Fig. 6.2 shows the research work related to graphene-based reinforcement, which includes functionalizing and controlling the graphene films on the metal matix and discovering the applications of graphene reinforcement. In the year of 2004, according to the database of the web of science, there were 164 research journals reported. Likewise, in 2010, there were 3,671 research papers reported in the Thomson Reuters source. But with these extensive benefits, reinforcement materials are clearly demonstrated and especially used in industries such as aerospace and automobiles. However, it is very difficult to achieve the enhanced mechanical performances using reinforcement materials alternation, deformation, and heat treatment processes. The metal matrix along with carbon (C), alumina (Al2O3), silicon carbide (SiC), boron carbide (B4C), and carbon nanotube (CNT) reinforcements has been roughly reported [10–13]. At present, there is a big challenge in developing the homogenous graphene-dispersed composite without changing or damaging the structures of the metal matrix. Graphene nanoparticles (GNPs) in the metal matrix may be suitable inclusions for the next generation of nanocomposites. Table 6.1 provides some of the significant properties of graphene. For the graphene-reinforced composites (Table 6.2), the applications mostly are in the biomedical and electrical areas. Some of the aluminum metal matrix composite is manufactured and most of the work is related to the Al-graphene composite for application in the automobile fields.

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Table 6.1 Graphene properties Properties

Graphene

Reference

Tensile strength Specific surface area Young’s modulus Coefficient of thermal expansion Transmittance Thermal conductivity Resistivity Electron mobility

130 GPa 2630 m2g1 0.5–1 TPa 6  104/K >95% for 2 mm thick film 5.3  103 Wm1K1 106 Ω cm 1500 cm2 V1 s1

[14] [14] [15] [15] [15] [16] [16] [16]

Table 6.2 Applications of graphene-reinforced composite Composition

Properties and applications

Reference

Pt-Graphene Al/Pd/Pt

Good capacitor Used as catalytic methanol oxidation— Methanol Biosensors, biodevices and DNA sequencing applications Bioelectroanalytical chemistry applications DNA specific sequence applications Li-ion battery applications Li-ion battery applications Strengthening of composite applications Higher Vickers hardness and lower failure strain High strength metal matrix composite Superior electrical conductivity and hardness

[17] [18]

Biosensor applications

[28]

Au-Graphene

Co-Graphene Si-Graphene Al powder-graphene Mg-Graphene Cu-graphene composite foil Au-Graphene-HRP— CS

6.3

[19–22]

[23] [24] [25] [26] [27]

Manufacturing and testing of GRMMC

The different methods for manufacturing of metal matrix composites (MMC) with the reinforcement of graphene have discussed in a detailed manner. This reinforcement has the structure of a sheet or nanoplatelets because some difficult technological issue occurred. As a result, limited studies were reported until now. Some of the previous research works reported the different manufacturing methods are given below [29–50].

6.3.1

Aluminium-graphene MMC

Wang et al. [51] fabricated graphene-based nano MMC using a powder metallurgy technique and reported enhanced strength as compared to the unreinforced nano

Graphene-based nano metal matrix composites: A review

157

MMC. They have followed the steps in order to manufacture the graphene-based nano MMC. In the first step, the preparation of an aqueous solution with dispersed graphene oxide nanosheets has been done. Typically, this sheet has less than a 1.5-nm thickness. In the second step, aluminum flakes are prepared by hydrophilic PVA membranes. Usually, these flakes have a 2D planar morphology with a thickness of 2 μm. In the third step, graphene oxide nanosheets are absorbed at the surfaces of the aluminum flakes. Thus, the aluminum flakes with graphene nanosheets have been dispersed homogeneously at the surfaces. In the third step, the graphene-based aluminum metal matrix composite powders have been compacted and consolidated so as to manufacture the required nanocomposite. This composite consists of an aluminum matrix reinforced by homogeneously dispersed graphene nanosheets. In the fourth step, the consolidation occurs through sintering of compacted billets of graphene-aluminum composite powders in an argon gas atmosphere at a temperature of 580°C for 2 h and consequent hot extrusion at 440°C with an extrusion ratio of 20:1. They have performed tensile tests for machined composites with dimensions of 5 mm in diameter and 25 mm gauge length, which consists of an Al-matrix reinforced by 0.3 wt% graphene nanosheets. The result was a tensile strength of 256 MPa and 13% elongation, which is shown in Fig. 6.3A. This shows a 62% higher tensile strength and two times lower elongation than the tensile strength of the unreinforced composite (154 MPa) and the elongation (27%) of the pure Al matrix. They have also reported that the enhancement in strength owing to the reinforcement of 0.3 wt% graphene nanosheets is most significant when compared to other reinforced graphene nanosheets in aluminum composites. At the same time, the stated improvement by 62% is much less than expected from theoretical approximations of the potential effects of the reinforcement of graphene nanosheets on the mechanical characteristics of nanocomposites. They have concluded that reinforcement by graphene nanosheets is most effective for the aluminum matrix and has enormous potential for applications.

Engineering stress e (MPa)

250

(A)

0.3wt%GNS/Al

(B)

GNSs

200 150

Al

100 nm

100 50 0 0.00

5 mm

0.05

0.10

0.15

0.20

0.25

0.30

Engineering strain e

Fig. 6.3 (A) Tensile properties of graphene-Al composite, (B) fracture surface of graphene nanosheets pulled out [51].

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The important effects of reinforcement of graphene nanosheets are given below. a) Grain refinement. b) Dislocation strengthening. c) Stress transfer.

First, because graphene nanosheets hamper grain boundary migration, grain growth is hindered in metal-graphene nanocomposites, therefore they tend to have lower grain sizes than their metal counterparts free from graphene. As per the following Hall-Petch relationship, the strength of materials is expressed: σ ¼ σ 0 + kd  ⁄ 1

2

where σ is the strength, σ 0 is the dislocation friction stress, k is a material constant, and d is the mean grain size. By that relationship, the strength of materials rises with decreasing grain size. But this factor has not played a major role in studies of the aluminum matrix with 0.3 wt% graphene nanocomposites because of a negligibly low fraction of grain boundaries hampered by such a low amount of graphene nanosheets. Second, such nanosheets resist lattice dislocations whose slip indicates the dominant mode of deformation in metals. The destruction of a lattice dislocation slip by graphene nanosheets significantly donates to strengthening of aluminium-graphene nanocomposites. The third strengthening mechanism is the stress transfer. This mechanism efficiently contributes to the experimentally documented dramatic strengthening of aluminiumgraphene nanocomposites. This statement is supported by experimental observations of pronounced dimples at fracture surfaces (Fig. 6.3B). These dimples are prolonged along with the direction of tensile load and certain graphene nanosheets being drawn out at the dimple edges (Fig. 6.3B).

6.3.2

Magnesium-graphene nanocomposites

Chen et al. [52] manufactured magnesium matrix nanocomposites reinforced by graphene platelets using a combined technique of liquid-state ultrasonic treating and solid-state stirring. This platelet consists of a number of graphene layers that have approximate thickness of 10–20 nm, and the length and width are less than 14 μm. Although the in-plane strength of platelets is less than that of single-layer graphene sheets, the chemical stability of the platelets is attractive for use as a reinforcement in aluminum matrix nanocomposites. The schematic arrangement of the method used is shown in Fig. 6.4. The two stages are followed during manufacturing of graphene-based magnesium nanocomposites. In the first stage, graphene platelets are passed into the magnesium melt at 700°C, along with the process of ultrasonic treatment. Then, by the action of a high-power ultrasonic probe, the homogeneous dispersion of graphene platelets into the molten magnesium is obtained. Moreover, the ultrasonic treatment was provided to the magnesium melt for 15 min after the feeding of graphene platelets. Fig. 6.4A

Transducer

Thermocouple

Booster Nanoplatelets feeding system

CO2+FS6 gas flow

As-cast plate

Mg melt

(A)

(B)

Furnace and crucible Vertical force Translation

FSP tool

Graphene-based nano metal matrix composites: A review

Niobium Probe

Rotation Advancing side

Z

Shoulder

Y X

Leading edge Stirred Region Trailing edge

(C)

Retreating side Pin

Fig. 6.4 Procedure for manufactureing magnesium-graphene nanocomposites [52]. 159

160

Nanocarbon and its Composites

shows the dispersion of the nanoparticles effectively in a liquid state [7]. Finally, the composite system was obtained as a plane mold cast. After the solidification process, the graphene platelet-reinforced magnesium plate with a thickness of 6 mm results, as shown in Fig. 6.4B. From the result of the characterization of the structure of the magnesium-graphene mold structure, it was observed that some micrometer-sized aggregates exist in the nanocomposite. For this reason, it was necessary to use the second stage of processing in order to attain a better uniform dispersion of the nano reinforcement. At this stage, the solid-state stirring treatment was employed with the nanocomposite. This treatment helps to achieve an effective dispersion of the reinforcement in composites in their solid state. Fig. 6.4C demonstrates the details of solid-state processing. A rotating pin tool with a 5-mm diameter is inserted into the plate and moves across the plate. This tool is rotated at a speed of 1800 rpm and has a travel speed of 25 mm min1, which leads to stirring the materials along the travelling path in the solid state, as shown in Fig. 6.4C. By using the above two stages, Chen et al. [52] manufactured a uniformly distributed and dispersed graphene platelet in the magnesium matrix nanocompoiste. Furthermore, it has been demonstrated that the bonding between platelets and the magnesium matrix is good in the developed nanocomposite. A microstructural investigation revealed dispersed graphene platelets in the magnesium matrix composite. Fig. 6.5A and B shows that the micrometer-sized aggregates

Fig. 6.5 SEM images of the as-cast ultrasonic processed plate (A and B) and the ultrasonic processed + solid state stirred sample (C and D) at low and high magnification [52].

Graphene-based nano metal matrix composites: A review

161

exist in the magnesium matrix. Fig. 6.5C shows the uniformly distributed graphene platelets in the magnesium matrix composite and Fig. 6.5D shows the very well dispersed graphene platelets in the magnesium matrix composite. They have also measured the microhardness of the magnesium-graphene nanocomposite. A 66-kg mm2 microhardness was revealed for the magnesium 1.2 wt% graphene platelets. This microhardness of the magnesium-graphene nanocomposite is 78% higher than the unreinforced pure magnesium (37 kg mm2). Also, that data was compared with the literature presenting outputs of other experiments regarding the fabrication of magnesium matrix composites. It was concluded that the dispersion of inclusions of graphene platelets in the magnesium matrix by using the above method is the most significant method for strengthening, as compared to reinforcement by either carbon nanotubes or metallic nanoparticles. The discussed advantages of graphene platelets as reinforcing elements in magnesium matrix nanocomposites are demonstrated by comparison in both magnitude and efficiency of the strengthening between the magnesium graphene nanocomposite and the magnesium matrix nanocomposites reinforced by carbon nanotubes and nanoparticles (Fig. 6.6). The strengthening efficiency S is expressed as follows S¼

Mc  Mm VMm

90 80 70 60 50 40 30 20 10 0

(A)

70 GNP Al2O3

Strengthening efficiency

Hardness increase (%)

where Mc and Mm represent the microhardness of a magnesium matrix composite and the microhardness of pure magnesium, respectively; and V is the volume fraction of dispersed reinforced graphene elements. From Fig. 6.6, it was noticed that the magnesium matrix composites reinforced by graphene platelets represent a novel nanocomposite along with enhanced mechanical properties.

SiC SiO2

Y2O3 CNT 0

1

2 3 4 Reinforcement (vol.%)

GNP 60 Al2O3

50 40

Y2O3 SiC

30 CNT

20

SiO2

10 0

5

(B)

0

1

2 3 4 Reinforcement (vol.%)

5

Fig. 6.6 (A) Hardness increase and (B) strengthening efficiency of graphene nanoplatelet and other nanoscale reinforcements in magnesium-based nanocomposites [52].

162

6.3.3

Nanocarbon and its Composites

Copper- and nickel-based graphene nanocomposites

Kim et al. [53] manufactured copper- and nickel-based graphene nanocomposites using the chemical vapor deposition method and determined the strength characteristics. Fig. 6.7 shows the manufacturing steps followed by the chemical vapor deposition method. Graphene is initially developed using chemical vapor deposition and moved onto the vanished metal thin film on an oxidized silicon substrate. Then, this layer is detached, and the next metal thin film layer is evaporated. By recapping the metal deposition and graphene transfer procedures, the copper-graphene nanolayered composites are manufactured with different repeated metal thicknesses of 70, 125, and 200 nm, and the nickel-graphene nanolayered composites are manufactured with repeated metal thicknesses of 100, 150, and 300 nm. The copper-based graphene nanolayered composites and nickel-based graphene nanolayered composites consist of repeated layer thicknesses of λ¼ 70 nm and λ ¼ 100 nm, respectively. The developed nanolayered composites exhibit strengths of 1.5 and 4.0 GPa, respectively. It was denoted that this method was used to achieve the highest value of strength than the other methods. The chemical vapor deposition method permits one to manufacture metallic nanolayers with graphene interfaces having typically a single (85%) atomic layer structure, but in some places, a bilayer structure was noticed (Fig. 6.8A). Kim et al. [53] investigated the mechanical characteristics using nanopillar compression tests. In order to conduct the mechanical compression test, nanolayered metal graphene nanopillars are machined using a milling method. It has the dimensions of 200 nm in diameter and a 400–600 nm height (Figs. 6.8B–C and 6.9A–B). Typical stress-strain curves for nickel and copper graphene nanolayered composites with various values of the repeat layer spacing λ are shown in Fig. 6.9C and D. The flow stress at 5% plastic strain as a function of λ for nickel- and copper-based graphene

Fig. 6.7 Schematic arrangement in production of a metal-graphene multilayer [53].

Graphene-based nano metal matrix composites: A review

163

Fig. 6.8 Transmission electron microscopy analysis of the copper-graphene nanolayered composite. (A) Microstructural of a metal-graphene interface, (B) microstructural image of a copper-graphene nanopillar, and (C) microstructural image of a copper-graphene nanopillar after deformation [53].

nanolayered composites is shown in Fig. 6.9E and F, respectively. It shows the trend that the flow stress enhances in linear with decreasing the metal layer thickness. Especially, the flow stress has its maximum values of 1.5 and 4.0 GPa for a coppergraphene composite with λ¼ 70 nm and a nickel-graphene composite with λ ¼ 100 nm, respectively. Kim et al. [53] determined the strengthening effects of the nanolayered composite and the geometry of the nanopillar in the case of metal-graphene nanolayered composites on the basis of the experimental data stated in the previous research. It was concluded that the strengthening effect of graphene reinforcement in the metal-graphene composites is much more significant than the combined effects of the nanolayered structure and the nanopillar geometry.

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After

Before

(B) 4

100 nm 150 nm 300 nm

3 2 1 0

0.1 0.2 True strain

5% flow stress (GPa)

(C)

3 –0.537

2 150 200 250 300 Layer spacing (nm)

2

(D)

(F)

70 nm 125 nm 200 nm

1

0

0.3

Ni-graphene

4

100

(E)

True stress (GPa)

5

5% flow stress (GPa)

True stress (GPa)

(A)

0.1 0.2 True strain

2

Cu-graphene

1.5 1

0.5 50

–0.402

100 150 200 250 Layer spacing (nm)

Fig. 6.9 Microstructure of copper-graphene nanopillar (A) before compression testing, and (B) after compression testing. Stress versus strain plots for (C) nickel-graphene and (D) coppergraphene. The flow stresses for (E) nickel-graphene and (F) copper-graphene nanolayered composites [53].

The important reason for the superior strength of the metal-graphene nanolayered composites signifies the role of graphene interfaces as extremely effective obstacles for lattice dislocation slip. This declaration was demonstrated by the evidence of experimentation in the microstructure of the copper-graphene nanolayered nanopillar (Fig. 6.8C). Pavithra et al. [27] focused their research on the fabrication and mechanical testing of copper-graphene nanocomposite foils. The copper-graphene nanocomposites with a copper-matrix having an average grain size of 1.2  0.4 μm were manufactured by the pulse reverse electrodeposition method. Fig. 6.10 shows the production of copper-graphene nanocomposites by the pulse reverse electrodeposition method. Graphene and graphene-oxide sheets with 1–5 atomic layers are located at grain

Graphene-based nano metal matrix composites: A review

(A)

+

A

165



Anode

Cathode

Suspended Graphene Copper ion Electrolyte

(C) Pulse reverse electrodeposition

if t (ms)

if

i (amp/cm2)

i (amp/cm2)

(B) Direct current electrodeposition

ir Fon Foff RonRoff

t (ms)

(E)

(D)

Fig. 6.10 (A) Procedure of electrodeposition, (B) and (C) schematic representation of the current waveforms and the codeposition of copper and graphene, (D) and (E) copper-graphene nanocomposite foils prepared [27].

boundaries and serve as effective inhibitors of grain growth in the copper-graphene nanocomposites. The hardness and elastic modulus of the copper-graphene nanocomposites were determined in a nanoindentation test and compared with the corresponding mechanical characteristics of pure copper foils (with an average grain size of 1.3  0.3 μm) fabricated by the pulse reverse electrodeposition method. It was experimentally reported that the hardness and elastic modulus of the coppergraphene composite have values of 2.5 and 137GPa, respectively, whereas pure electrodeposited copper specimens have a hardness of 1.2 GPa and an elastic modulus of 116 GPa. Also, Pavithra et al. [27] observed grain growth processes in the pure copper and copper-graphene nanocomposite specimens under 30 min annealing in the atmosphere of argon gas at 300°C. They noticed that grain size in the copper-graphene composite in practice does not change after the annealing treatment, whereas grain size of pure copper displays increases from 1.3 μm to around 10 μm. These experimental data reveal the drastic hampering effect of graphene on grain growth in metal-matrix nanocomposites. In these conditions, it was concluded that metal matrices in metal graphene nanocomposites typically have a lower grain size and thereby higher strength, as compared to the corresponding characteristics of pure metals. Kuang et al. [54] used the electrodeposition method to manufacture nickelgraphene nanocomposite films. The nanocomposite films are characterized by 0.12 wt% graphene fraction with graphene nanoinclusions well dispersed in the form

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Fig. 6.11 Microstructural images of the nickel-graphene composites: (A) plane view, (B) crosssectional view, (C) the energy dispersive spectrometry (EDS) spectrum of the selected area in (B), (D) high-resolution transmission electron microscopy image of the composites [54].

of multilayered sheets in the nickel matrix. Moreover, microstructural studies revealed that graphene sheets in the nickel-matrix tend to be curled and entangled together (Fig. 6.11). The hardness and Young modulus of the nickel-graphene composite specimens were determined in nanoindention tests. These mechanical characteristics of the nickel-graphene composites were found to be dramatically improved as compared to those of pure electrodeposited nickel. So, the hardness and Young modulus of the nickel-graphene composite have values of 6.85 and 252.76 GPa, respectively, whereas pure electrodeposited nickel is specified by the hardness of 1.81 GPa and a Young’s modulus of 166.70 GPa. Also, they have revealed that the increase in the hardness is mostly related to the role of graphene inclusions as effective stoppers of lattice dislocation slip.

6.4

Conclusions

Thus, the latest and very effective method to manufacture metallic materials with high strength, hardness, and Young’s modulus is to implant graphene platelets and sheets of

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a few layers in metallic matrices. Especially, metal-graphene nanocomposites with low volume/weight fractions of graphene inclusions exhibit dramatically enhanced strength and hardness. The manufacturing of metal-graphene nanocomposites for high strength, hardness, and elastic properties is in its infancy. In the near future, it is logical to expect explosive progress in fundamental science, fabrication, and applications of such nanocomposites whose potential to transform so many technologies is tremendous.

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[19] Xu C, Wang X, Zhu J. Graphene metal particle nanocomposites. J Phys Chem C 2008;112(50):19841–5. [20] Song B, Li D, Qi W, Elstner M, Fan C, Fang H. Graphene on Au (111): a highly conductive material with excellent adsorption properties for high-resolution bio/nano detection and identification. ChemPhysChem 2010;11(3):585–9. [21] Gong J, Zhou T, Song D, Zhang L. Mono dispersed Au nanoparticles decorated graphene as an enhanced sensing platform for ultrasensitive stripping voltammetric detection of mercury(II). Sensors Actuators B Chem 2010;150(2):491. [22] Du M, Yang T, Jiao K. Immobilization-free direct electrochemical detection for DNA specific sequences based on electrochemically converted gold nanoparticles/graphene composite film. J Mater Chem 2010;10:9253–60. [23] Yang S, Cui G, Pang S, Cao Q, Feng UKX, Maier J, Mullen K. Fabrication of cobalt and cobalt oxide/graphene composites: towards high-performance anode materials for lithium ion batteries. ChemSusChem 2010;3(2):236–9. [24] Chou SL, Wang JZ, Choucair M, Liu HK, Stride JA, Dou SX. Enhanced reversible lithium storage in a nanosize silicon/graphene composite. Electrochem Commun 2010;12(2): 303–6. [25] Ebinezar B. Analysis of hardness test for aluminum carbon nanotube metal matrix and graphene. Ind J Eng 2014;10(21):33–9. [26] Chen LY, Konishi H, Fehrenbacher A, Ma C, Xu JQ, Choi H, Xu HF, Pfefferkorn FE, Li XC. Novel nano processing route for bulk graphene nanoplatelets reinforced metal matrix nanocomposites. Scr Mater 2012;67(1):29–32. [27] Pavithra CLP, Sarada BV, Rajulapoti KV, Rao TN, Sundararajan D. A new electrochemical approach for the synthesis of copper-graphene nanocomposite foils with high hardness. Sci Rep 2014;4:4049. [28] Zhou K, Zhu Y, Yang X, Luo J, Li C, Luan S. A novel hydrogen peroxide biosensor based on Au–graphene–HRP–chitosan biocomposites. Electrochim Acta 2010;55(9): 3055–60. [29] Walker LS, Marroto VR, Rafiee MA, Koratkar N, Corral EL. Toughening in graphene ceramic composites. ACS Nano 2011;5(4):3182–90. [30] Kvetkova L, Duszova A, Hvizdos P, Dusza J, Kun P, Balazsi C. Fracture toughness and toughening mechanisms in graphene platelet reinforced Si3N4 composites. Scr Mater 2012;66:793–6. [31] Porwal H, Grasso S, Reece MJ. Review of graphene–ceramic matrix composites. Adv Appl Ceram 2013;112(8):443–54. [32] Centeno A, Rocha VG, Alonso B, Ferna´ndez A, Gutierrez-Gonzalez CF, Torrecillas R, Zurutuza A. Graphene for tough and electroconductive alumina ceramics. J Eur Ceram Soc 2013;33:3201–10. [33] Lui J, Yan H, Jiang K. Mechanical properties of graphene platelet-reinforced alumina ceramic composites. Ceram Int 2013;39(6):6215–21. [34] Nieto A, Lahiri D, Agarwal A. Graphene NanoPlatelets reinforced tantalum carbide consolidated by spark plasma sintering. Mater Sci Eng A 2013;582:338–46. [35] Ramirez C, Miranzo P, Belmonte M, Osendi MI, Poza P, Vega-Diaz SM, Terronez M. Extraordinary toughening enhancement and flexural strength in S3N4 composites using graphene sheets. J Eur Ceram Soc 2014;34:161–9. [36] Fan Y, Estili M, Igarashi G, Jiang W, Kawasaki A. The effect of homogeneously dispersed few-layer graphene on microstructure and mechanical properties of Al2O3 nanocomposites. J Eur Ceram Soc 2014;34(2):443–51.

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[37] Koltsova TS, Nasibulina LI, Anoshkin IV, Mishin VV, Kauppinen EI, Tolochko OV, Nasibulin AG. New hybrid copper composite materials based on carbon nanostructures. J Mater Sci Eng B 2012;2:240–6. [38] Nasibulin AG, Koltsova TS, Nasibulina LI, Anoshkin IV, Semencha A, Tolochko OV, Kauppinen EI. A novel approach to composite preparation by direct synthesis of carbon nanomaterial on matrix or filler particles. Acta Mater 2013;61:1862–71. [39] Ajayan PM, Schadler LS, Braun PV. Nanocomposite science and technology. Weinheim: Wiley-VCH Verlag; 20031–76. [40] Tjong SC. Novel nanoparticle-reinforced metal matrix composites with enhanced mechanical properties. Adv Eng Mater 2007;9(8):639–52. [41] Bakshi SR, Lahiri D, Agarwal A. Carbon nanotube reinforced metal matrix composites - a review. Int Mater Rev 2010;55(1):41–64. [42] May P, Khan U, O’Neill A, Coleman JN. Approaching the theoretical limit for reinforcing polymers with graphene. J Mater Chem 2012;22:1278–82. [43] Koltsova TS, Nasibulin AG, Tolochko OV. Novel composite materials copper—carbon nanofibers. Sci Tech Bull St Petersburg State Polytech Univ 2013;106(3):125–31. [44] Armstrong RW. 60 years of Hall-Petch: past to present nano-scale connections. Mater Trans A 2014;55(1):2–12. [45] Veprek S. Structural nanocrystalline materials: fundamentals and applications. Cambridge: Cambridge UniversityPress; 2007. [46] Pande CS, Cooper KP. Nanomechanics of Hall–Petch relationship in nanocrystalline materials. Prog Mater Sci 2009;54:689–706. [47] Li X, Cai W, An J, Kim S, Nah J, Yang D, Piner R, Velamakanni A, Jung I, Tutuc E, Banerjee SK, Colombo L, Ruoff RS. Large-area synthesis of high-quality and uniform graphene films on copper foils. Science 2009;324:1312–4. [48] Greer JR, Nix WD. Nanoscale gold pillars strengthened through dislocation starvation. Phys Rev B 2006;73:245410. [49] Yu Q, Shan Z-W, Li J, Xiao L, Sun J, Ma E. Strong crystal size effect on deformation twinning. Nature 2010;463:335–8. [50] Chang S-W, Nair AK, Buehler MJ. Nanoindentation study of size effects in nickel– graphene nanocomposites. Philos Mag Lett 2013;93(4):196–203. [51] Wang J, Li Z, Fan G, Pan H, Chen Z, Zhang D. Reinforcement with graphene nanosheets in aluminum matrix composites. Scr Mater 2012;66(8):594–7. [52] Chen L-Y, Konishi H, Fehrenbacher A, Ma C, Hu G-Q, Choi H, Hu H-F, Pfefferkorn FE, Li X-C. Novel nanoprocessing route for bulk graphene nanoplatelets reinforced metal matrix nanocomposites. Scr Mater 2012;67(1):29–32. [53] Kim Y, Lee J, Yeom MS, Shin JW, Kim H, Cui Y, Kysar JW, Hone J, Jung Y, Jeon S, Yan SM. Strengthening effect of single atomic- layer graphene in metal–graphene nanolayered composites. Nat Commun 2013;4:2114. [54] Kuang D, Xu L, Liu L, Hu W, Wu Y. Graphene–nickel composites. Appl Surf Sci 2013;273:484–90.

Further reading [55] Hwang J, Yoon T, Jin SY, Lee J, Kim T-S, Hong SH, Jeon S. Enhanced mechanical properties of graphene/copper nanocomposites using a molecular-level mixing process. Adv Mater 2013;25:6724–9.

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[56] Randviir EP, Brownson DAC, Banks CE. A decade of graphene research: production, applications and outlook. Mater Today 2014;17(9):426–32. [57] Bartolucci SF, Paras J, Rafiee MA, Rafiee J, Lee S, Kapoor D, Koratkar N. Graphene– aluminum nanocomposites. Mater Sci Eng A 2011;528(27):7933–7. [58] Tang Y, Yang X, Wang R, Li M. Enhancement of the mechanical properties of Graphene– copper composites with graphene–nickel hybrids. Mater Sci Eng A 2014;599:247–54.

7

Nanocarbons: Preparation, assessments, and applications in structural engineering, spintronics, gas sensing, EMI shielding, and cloaking in X-band

Ashwini P. Alegaonkar*, Prashant S. Alegaonkar† *Department of Chemistry, Savitribai Phule Pune University (formerly Pune University), Pune, India, †Department of Applied Physics, Defence Institute of Advanced Technology (DIAT), Pune, India

Chapter Outline 7.1 Background information: From carbon to nanocarbon 172 7.2 Graphene-like nanocarbons: The disordered 2D carbon network

173

7.2.1 Synthesis of GNCs 173 7.2.2 Characterizations on GNCs 174

7.3 Nanocomposite approach for structural engineering

182

7.3.1 GNCs as effective nanofiller 183 7.3.2 Dispersibility investigations: Homogeneous distribution versus agglomeration and interfacial adhesion of GNCs 183 7.3.3 Fracture mechanisms using fractography 190 7.3.4 Thermal and physical properties 192

7.4 Magnetism in otherwise nonmagnetic carbon: Defect as a source 7.4.1 7.4.2 7.4.3 7.4.4

195

Spin transport and magnetic correlations in GNCs and nitro-GNCs: Graphene spintronics 195 Radical spin correlations: Electron spin resonance measurements and analysis 199 Magnetometric analysis by VSM 202 Spin-bath properties of GNCs 209

7.5 Electromagnetic interference shielding: Countermeasures against radar seekers in X-band 215 7.5.1 Shielding parameters of GNC/polyurethane nanocomposites 215

7.6 Molecular and spin interactions: Tellurium and reduced graphene oxide 7.6.1 Synthesis of reduced graphene oxide and tellurium-rGO 230 7.6.2 ESR studies of rGO and Te-rGO 236

7.7 Multifunctional nanocarbons: NH3 gas sensors and EMI shielding 7.7.1 7.7.2 7.7.3 7.7.4

Synthesis 239 Surface morphology of nanocarbons 240 Raman studies 241 Optical spectroscopy: Optical band structure of nanocarbon 242

Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00007-9 © 2019 Elsevier Ltd. All rights reserved.

238

229

172

Nanocarbon and its Composites 7.7.5 Optical gas sensor characteristic 243 7.7.6 Nanocarbon for shield technology 249

7.8 Electromagnetic cloaking and metamaterials (left-handed medium) 7.8.1 7.8.2 7.8.3 7.8.4

7.9 Concluding remarks and work scheme References 273 Further reading 285

7.1

253

Ferro-nanocarbon split-ring resonators: Bianisotropic metamaterial 255 Microwave measurements on FNC SRRs 257 Morphological analysis of NC and FNC 257 Modeling and simulation: FNC SRRs 263

270

Background information: From carbon to nanocarbon

In the periodic table, the element carbon occupies the sixth position and is one of the most versatile atoms. It has four valence electrons that offer a multitude of chemical bonds. In all organic elements, carbon-carbon bonding is the key component for the basis of life. One of the reasons that living matter has such an extraordinary richness of different molecules is probably due to the inherent bonding scheme of carbon. Carbon atoms form minerals, allotropes, crystals, jewels, etc., in addition to living matter. Naturally, atomic carbon has a tendency to form numerous multiatomic structures with different molecular configurations, called allotropes. Until recently, only two allotropes were well known: (i) graphite and (ii) diamond. In graphite, each carbon has bonded with three neighbors and they are in a two-dimensional (2D) layered format, whereas in diamond, the coordination number is four for each atom, and they form a threedimensional (3D) crystal. These are very profuse forms of carbon and are very much appreciated, especially diamonds. As an unexpected surprise, in recent years, scientists were impressed more by two new allotropes: fullerenes (discovered in 1985) and carbon nanotubes (CNTs) (1991). In these structures, the carbon-carbon bonds are in a bent or curved format. In the former, a soccer ball-shaped molecule—the so-called buckyball (C60)—is formed while in the latter, carbon atoms form a long cylindrical tube. Both are in the nanometer size and range. The two discovered artificial allotropes provided a wealth of knowledge in basic science to researchers and were found to be useful in many applications. For example, C60s are used in creating new medications such as antioxidant drugs while CNTs are basic constituents of transparent conducting films, dyesensitized solar cells, and flat panel screens and also seem to be very promising for compact electronic circuits, etc. The researchers were of the opinion that not much more could come from carbon, but it has surprised us once again! Graphene, a one atom-thick, planer honeycomb crystal lattice, has excellent physical, chemical, and mechanical properties [1–6]. Since its discovery, a considerable amount of work has been carried out on the fabrication of layered graphene by micromechanical/mechanical exfoliation from graphite [1], the epitaxial route [7], chemical vapor deposition (CVD) [8], solvothermal production [9], liquid phase exfoliation [10], microwave-assisted exfoliation [11], and other oxidative techniques [12,13]. Layers of graphene synthesized using colloidal suspension [14] are realized to be both

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scalable and inexpensive. In this, graphene is synthesized via a reduction process of graphene oxide (GO) or, alternatively, from an intercalated graphitic compound such as expanded graphite. Graphene therefore produced from the reduction of GO consists of a significant percentage of oxygen as well as defects [15]. Whereas graphene synthesized from expanded graphite consists of a low density of defects with a small production yield [16]. The graphene produced from the above-mentioned two methods have distinctly different physical and chemical properties. The major techniques mentioned are involved in graphene production with a rich sp2 phase. From the point of view of fundamental sciences and technological applications, studies on disordered solids are of utmost importance. In graphene, disorder could be in the form of external (extrinsic) defects, intrinsic defects, and/or a hybrid sp2-sp3 phase carbon network. The presence of such defects may lead to dramatic modifications in the physicochemical-mechanical properties [17–20] of 2D graphene layers due to a change in the short-range interactions in the honeycomb lattice network [21,22]. Such defects are useful in a wide range of applications [17,19,23–25]. The reports on the presence of sp2 and amorphous carbon (a-C) together showed interesting properties of the mixed structure such as an improvement in elastic properties [26,27]. Diamond-like carbon (DLC)/GO showed a significant improvement in Young’s modulus, hardness, elastic recovery, and electrical conductivity of DLC [26]. However, such mixed structures are difficult in synthesis while scalability is another challenge. The current chapter is divided into nine sections and several sections. The second section deals with synthesis and characterizations on graphene-like nanocarbons (GNCs) while the third deals with a filler approach of GNCs in epoxy. The fourth involves magnetism in nitro-GNCs; the fifth electromagnetic interference (EMI) shielding; the sixth molecular-spin interactions in tellurium-reduced graphene oxide (Te-rGO); the seventh multifunctional nanocarbons for EMI and gas sensing; and the eighth a metamaterial-inspired cloaking approach. The chapter ends with concluding remarks and a work scheme.

7.2

Graphene-like nanocarbons: The disordered 2D carbon network

Herein, we show the synthesis of a biphase, sp2-sp3 bonded, disordered, few-layer graphene-like nanocarbon (GNC) network obtained from soft wood charcoal. The samples were studied using a number of characterization techniques. The analysis revealed that sp2 chains and polycyclic carbon rings (PCR) are formed during the synthesis process in the host ta-C medium, generating a hybrid sp2/sp3 phase with a few layers of GNC with native disorder. [28] The reported method is inexpensive, time effective, and easy with good practical yield.

7.2.1

Synthesis of GNCs

GNCs were obtained from the precursor of charcoal (C) admixed with KNO3 and sulfur (stoichiometric ratio  85:10:05, C:KNO3:S). The admixed material in the powder form was cast into a large number of pellets (1 g) of dimensions 15  5 mm (dia. 

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thick.), and dried in an oven @80°C for 6 h. One pellet at a time was taken in a glass plate, detonated under the atmospheric conditions, and allowed for the onset of fireassisted combustion in 20–30 s, leading to the formation of a carbonaceous ash column with smoke. The obtained column was crushed and weighted, which was observed to be decreased to  35–40 mg. The ash powder was immersed in deionized water/acetone (8:2, v/v), sonicated (1 h), and vacuum filtrated (by poly tetra fluoro ethylene (PTFE), pore size 1.2 μm). The process was repeated 3–4 times to remove inorganic impurities, K, S, and other oxide-based elements. The intermediate stage was termed as as-synthesized samples. They were subjected to intercalation, followed by the annealing process. The intercalation route was adopted to etch a-C and to separate the conjugated layers of GNCs. The intercalation was done at room temperature using H2SO4 (98%): HNO3 (60%), (4:1, v/v) for 48 h at constant stirring. In this process, nitronium ion (NO2 + ) is formed due to the dissociation of HNO3, which acted as a weak etchant for a-C and an oxidizing agent [29] leading to CO2 and N2 evolution. The formed NO2 + selectively attacks sp3 sites. Further, the presence of H2SO4 is necessary to form transient species such as C-S-On and C-N-On, leading to the creation and enrichment of the sp2 phase. After this, samples were washed repeatedly in a 5 mM NaOH solution and deionized water. The process was repeated until the pH of the wash water reached neutral. The obtained samples were dried for 12 h at 100°C. Following this, the samples were annealed at 1000°C for 60 s in a force convection oven under inert atmosphere conditions. During annealing, the reduction of CdSdOn and CdNdOn occurs with the evolution of species such as NOn, and SOn. In the formed sp2 chains, the amount of disorder seemed to be changed. These chains are transformed into a PCR. Annealing and onset have provided strength to intra-PCR coupling with the ordered states. Thus, the state of hybridization of the reduced carbon atoms seems to be changed from sp3 to sp2 [30,31]. A schematic representation of the formation of the GNC sheets is shown in Fig. 7.1.

7.2.2

Characterizations on GNCs

All three staged (as-synthesized, intercalated, and annealed) samples were subjected to various characterizations such as Raman spectroscopy, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy/selective area electron diffraction (HRTEM/SAED) for stage-wise analysis.

Annealing

D

1000 °C

n Charcoal

Imperfect sp2 / sp3 chains and polycyclic rings (PCR)

Graphene-like nanocarbon sheet

Fig. 7.1 Schematic representation of the process of formation of a GNC sheet from softwood charcoal.

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The Raman measurements were performed using a plane-polarized Raman spectrometer. The Raman spectra were recorded at 514 nm using a Horiba LABRAMHR spectrometer in which the objective was fixed at 4  with a 325 nm notch filter. The SEM images were recorded using a JSM-6360A (JEOL) system operated at 20 kV, whereas images for the HRTEM/SAED were recorded using a JEM-2100F (JEOL) system operated at 200 kV. Scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS) were done to explore the electron transport properties of the obtained samples. For this purpose, a 200 μL solution of the GNC sheets dispersed in IPA was dropcoated on the HOPG substrate using a micropipette. A freshly cut Pt-Ir tip was used for these measurements. Nanosurf STM was used to image the samples. The STM imaging was recorded in a constant current mode with the tunneling current of magnitude  1 nA at bias voltage  0.1 V. The compositional analysis of all stages was done using Fourier transform infrared spectroscopy (FTIR, PerkinElmer, FTIR 1605 spectrometer) over 400–4000 cm1 and X-ray photoelectron spectroscopy (XPS, Omicron ESCA Probe, Omicron Nanotechnology system).

7.2.2.1 Chemical analysis: Electron and infrared spectroscopy Fig 7.2A shows the FTIR of GNCs. Typical bands appeared at 3430 cm1 (OdH stretching), 1241 cm1 (CdOH stretching), and 1088 cm1 (CdOdC stretching), indicating the presence of hydroxyl and epoxide in GNCs. The bands at 2925 and 2853 cm1 correspond to asymmetric and symmetric CdH stretching, respectively, while the band that emerged at 1628 cm1 corresponds to CdC stretching. The presence of functional groups is further verified by ESCA results. For ESCA analysis, in terms of contributions from individual components representing various species, each obtained peak was fitted by a combination of components by minimizing the total squared error (least squared error) of the fit. Individual components were denoted by a convolution of a Lorentzian function representative of the lifetime broadening and a Gaussian function to account for the instrumental resolution. The Gaussian broadening was kept the same for different components. A Shirley background function is considered to account for the inelastic background in the ESCA spectrum.

Fig. 7.2 Recorded FTIR and C-1s ESCA spectrum for GNCs. Recorded (A) FTIR and (B) C-1s ESCA spectrum for GNCs.

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Fig. 7.2B shows the recorded ESCA peak of C-1s recorded for GNCs. C-1s for GNCs appeared at 285.33 eV with FWHM  2.50 eV and intensity  3  105 counts per second (cps). The atomic % (at%) of carbon is estimated at 93%. C-1s is deconvoluted into three components, C1 at 289.20 eV is ascribed to sp2 carbon, C2 (286.23 eV) for sp3, and C3 (285.07 eV) to CdC of sp3 origin. The compositions of C2 and C3 are 40.45 and 43.23 at.%, respectively. These two peaks are the representative peaks of the GNC phase. The overall range of peak intensity of the C-1s peak, recorded for GNCs, indicated that the p-type dopant also exists due to physically adsorbed oxygen moiety. The dopant exists in the form of a bound Dirac hole. GNCs contain native oxygen 6.75 at.%. ESCA analysis indicated that GNCs contained 93 at.% carbon and 6.75 at.% oxygen sites. The exchange interactions are dominant at the oxygen site that exchanges the pair hole; they are also dominant at the sp2/sp3 interface too, due to the availability of a dangling electron bound to the π-state. From ESCA we have seen that the population of sp3 content is relatively higher than sp2.

7.2.2.2

Analysis by Raman spectroscopy

Intensity (a.u.)

Fig. 7.3 shows recorded Raman spectrum for (a) as-synthesized, (b) intercalated, and (c) annealed samples. The inset in Fig. 7.3 shows a spectrum recorded for softwood charcoal (precursor). From the inset, the peaks at  1354 and  1591 cm1 are indicated, respectively, for the D and G bands, which are typical peaks of charcoal. For all processed samples, five prominent peaks were observed. Their position and intensity

Intensity (a.u.)

2D ta-C

1000

D1

D2 G

1500



1000

1500 2000 2500 Raman shift (cm–1)

3000

I(D1)/I(G) ~ 0.9

(C)

I(D1)/I(G) ~ 1.1

(B)

I(D1)/I(G) ~ 1.3

(A)

2000

2500

3000

Raman shift (cm–1)

Fig. 7.3 Raman spectrum recorded at excitation wavelength 514 nm for (A) as-synthesized, (B) intercalated, and (C) annealed samples. The inset shows the Raman spectrum recorded for the precursor of GNC sheets, that is, softwood charcoal.

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were found to vary with subsequent posttreatment on as-synthesized samples. In plot (a), a sharp peak at 1025 cm1 is attributed to the ta-C phase [32]. For plot (b), a shift in ta-C from 1025 to 999 cm1 is observed, which is further downshifted from 999 to 985 cm1 for annealed (plot (c)) samples with a substantial decrease in intensity. The decrease in intensity of the ta-C peak was observed with posttreatment on as-synthesized samples, indicating the amount of ta-C sites is eventually reduced with posttreatment; however, ta-C exists throughout. In (a), for D-peak, a sharp doublet (D1 at 1346 and D2 at 1378 cm1) is observed with the emergence of a small G-peak at  1600 cm1. The analysis of features indicated that the formation of sp2 bonded linear chains and the PCR process is initiated during combustion, but with structural and topological distortion in the form of bond angle, length, and delocalized states around sp2 chains/PCR clusters. There are contributions from stretching modes of chains/PCR and residual ta-C. The emergence of the D doublet is twofold: (i) superimposed stretching modes of sp3 and imperfect sp2 chains and (ii) weakly coupled vibration modes of PCR [31–33]. The shift is observed for D1 and D2, respectively, seen in (b) and (c). The stretching modes of D1 and D2 are suppressed steadily with the evolution of a G-band associated with prominent enhancement in the breathing mode of sp2 chains and PCR. The steady increase in G-band intensity is consistent with the mechanism discussed above. A sequential downshift with posttreatment in the G band is observed from 1600–1560 cm1, approximately. The G-band consists of an additional weak shake-up feature D (indicated by the arrow in Fig. 7.3), which became more evident in posttreated spectra. The G-band corresponds to in-plane vibration of the sp2 chains with E2g symmetry. The D peak is attributed, selectively, to the presence of PCRs, which are at the formative stage and serve to lower the symmetry of the quasiinfinite lattice. The fundamental difference in the behavior of the D and D0 peak is observed and attributed to the different Raman-allowed modes from which they emerged [34]. Generated sp2 bonded PCR can be considered as a part of a disordered graphitic super lattice, both electronically and vibrationally. The shape of the 2D peak changed significantly with an increase in the number of layers in GNCs. The 2D peak was observed at  2890 cm1 (as synthesized) and shifted up to  2860 cm1 (annealed). The 2D in bulk graphite is decomposed into two components; however, in graphene there is only one component. In the reported literature, the 2D component of the bilayer was fitted to four Lorentzian components, whereas the tri-layer was fitted to six Lorentzian components [35,36]. These four components in the bilayer graphene were explained in terms of the double resonance Raman scattering process, which raises the electronic structure of the graphene layers. In Fig. 7.4, there are 4–5 components in the 2D peak that are assigned to GNC layers, typically 2–5 [36–38]. The ratio of intensity of the D and G peaks is related to disorder or defects present in the sample. We have taken the ratio of intensity of peak D1 to G, due to a dominating linear component. In Fig. 7.3, the magnitude of ratio I(D1)/I(G) is  1.3 for the as-synthesized sample and decreases to  0.9 (in posttreatment). In the analysis, charcoal, after combustion, is transformed into hybrid, mixed carbon network having both sp2 and sp3 components. The presence and emergence of G and 2D peaks with the subsequent processing signifies the evolution of a phase of carbon that is a combination of amorphous and graphene-like carbon with a 2D

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Fig. 7.4 Fitted Lorentzian for 2D peaks of (A) intercalated and (B) annealed samples. The 2D peak was obtained from Fig. 7.3.

planer structure. The material obtained in this process is GNC sheets and not graphene that contains hybrid carbon rather than a pure sp2 network compounded with local disorder.

7.2.2.3

Morphological studies: FESEM and HRTEM

Fig. 7.5A–C shows recorded SEM images for a few GNC layers. The area of GNC sheets is varied, typically 10–20 μm2. Recorded HRTEM images at different portions of GNCs are seen Fig. 7.5D–F. Image (D) shows overall topology whereas image (E) shows folding/bending edges and (F) corresponds to the SAED pattern. Image (F) shows the presence of two blur diffraction rings (arrows). The rings in such patterns are observed for carbon samples that typically contain the ta-C phase [39,40]. Their presence is consistent with the analysis presented before. HRTEM images recorded for annealed samples are shown in Fig. 7.5G–I. It seems that annealed sheets are quite thin and almost transparent to the incident electron beam, as observed in Fig. 7.5G. Image (I) is a recorded respective SAED pattern for annealed samples. Comparing images (F) and (I), the diffraction intensity and the number of rings are observed to be increased in the case of an annealed sample. After annealing, a short range ordering is possibly seen that is absent for intercalated GNCs.

7.2.2.4

Scanning and tunneling microscopy and spectroscopy

Fig. 7.6 shows a typical atomic resolution image of the final product, that is, the annealed sample. Attempts to image as-synthesized and intercalated samples were unsuccessful. Getting an atomic resolution image after postprocessing itself proves that the samples are clean at the atomic scale, enabling a controlled analysis of structural properties. Fig. 7.6B shows the zoomed portion of Fig. 7.6A. The image clearly revealed the atomic structure and different types of ring defects present therein. In some areas of the image, we see slight distortion in the honeycomb

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Fig. 7.5 SEM images of GNC sheets of (A) intercalated sample, and (B, C) annealed sample. For the intercalated sample (D, E) HRTEM images, and (F) SAED pattern. Similarly, for annealed sample (G, H) HRTEM images and (I) SAED pattern.

structure, whereas in Fig. 7.6C, some rings are in toto distorted. The overall disorder is also compounded with the recorded Fourier transform of topographic images (area 5  5 nm2) taken at different locations. Fig. 7.6D–F showed a recorded transform of annealed samples. From Fig. 7.6D, a hexagonal networked lattice is resolved; however, in Fig. 7.6E and F, the sharp Bragg spots recorded were not so clear. In order to investigate the effect of lattice distortion onto the localized density of the state (LDOS), we compared typical tunneling spectroscopy data obtained from different regions of the atomic arrangements. One can obtain valuable information about LDOS using dI/dV curves obtained from STS measurements. The dI/dV at V ¼ 0 quantifies LDOS at the Fermi energy level. We present the atomic resolution images of the GNC sheets and their tunneling spectroscopy. Fig. 7.7 on the right inset shows dI/dV with a bias voltage in eV. The recorded spectrum of a typical LDOS that was obtained on the area in Fig. 7.6D did not show much variation in different portions of the image. However, a resonance peak near the conduction band minima has been observed in all the

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Nanocarbon and its Composites

Fig. 7.6 Recorded STM images of the GNC sheets for (A) 5  5 nm2 area, (B) zoomed portion of (A), resembling the hexagonal rings of the GNC sheets with distortion, (C) zoomed portion of the more disordered atomic arrangement of atoms, (D)–(F) Recorded Fourier transform spectra of the atomic arrangements in different images of 5  5 nm2 area (D) the hexagonal arrangement in the reciprocal lattice space, (E) and (F) the recorded Bragg spots are not clear.

recorded spectra, which is attributed to the electron tunneling due to the disordered states compounded with interface defects because of oriented spin polarizability [39,40]. The current measurements were carried out at room temperature; the effect of different defect sites was not observable in the recorded dI/dV plots. It has been reported that the amount of disorder depends on the starting carbon material and the synthesis approach [19]. Monolayer graphene LDOS is linear in energy that vanishes at the Dirac point. Hence, linear LDOS vanishing at the Dirac point can be used

Nanocarbons: Preparation, assessments, and applications

1.75

Normalized (dI / dV)

1.00

1.0 Normalized (dI / dV)

1.25

0.25 Normalized (dI / dV)

1.50

181

0.20

0.15

0.10

0.05 –0.4

–0.2

0.0 eV

0.2

0.8 0.6 0.4 0.2 0.0 –0.9 –0.6 –0.3

0.4

1

0.75

0.0 eV

0.3

0.6

0.9

2 3

0.50

4 5

0.25

0.00 –5

–4

–3

–2

–1

0

1

2

3

4

5

eV

Fig. 7.7 Variation in the tunneling spectra of GNC sheets after adding a number of layers on the substrate. The inset shows that the peak related to the tunneling of electrons to the disordered states is present in all the layers. The right side inset is the averaged normalized dI/dV as a function of bias voltage, V, obtained in different areas of the posttreated samples.

to identify the signature of graphene decoupled from the substrate [41]. The inset in Fig. 7.7 is V-shaped but does not vanish at the Dirac point. This will not be observed if two or more layers are coupled together because the interlayer coupling produces additional states leading to finite LDOS [42]. From this, we concluded that the graphene layers are more coupled to the substrate. In order to confirm this, measurements were performed on a typical multilayered GNC sheet sample and a set of conductance spectra was recorded on each sample. In Fig. 7.7, the shape of the spectra changed significantly as the number of layers increased. It completely deviated from the linear energy spectra. As the number of layers became large, the atomic resolution topographic image could not be obtained (profile not shown here). Clearly, the spectra also reflected the ta-C nature (energy-independent behavior) recorded for the topmost layer. To this endeavor, GNCs prepared by the above methodology have been integrated for revealing mechanical and thermal properties in the form of fillers in the host epoxy resin matrix. In the following section, the utility of GNCs as an effective nanofiller for improving the mechanical and thermal properties of polymer at low weight fractions has been demonstrated [43].

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7.3

Nanocarbon and its Composites

Nanocomposite approach for structural engineering

Graphene possesses an exceptionally high in-plane elastic modulus (1 TPa), high strength (130 GPa) [6], and high specific surface area (>2000 m2/g) [44] due to its 2D single atom-thick sheet-like character. It has high thermal conductivity (5000 W/m K) [5] and thermal stability [45] and excellent electron mobility even at room temperature (105 cm2/V s) [46]. Such exclusive properties make it an ideal filler material for developing polymer-reinforced nanocomposites [47,48]. They find a multitude of applications in conducting media [45], active transparent electrodes [49], high strength materials [48,50], EMI shielding [51], etc. Further, graphene-type nanofillers such as expanded graphite [52] graphite nanoplatelets [53,54], GO [55,56], and graphene nanoribbons (GNR) [57] have extensively been used as reinforcing elements in various polymer matrices with variable weight fractions up to 5 wt%. In such a compounded system, the formation of agglomeration as a result of poor dispersion of nanofillers in the polymer matrix limits the transfer of its properties to the host medium [58]. Aggregation is ascribed to strong interlayer physical forces such as van der Waals interactions between filler sheets and its poor interfacial adhesion with the host polymer. Chemical functionalization has been carried out to address these issues [50,59]. The mechanical properties are one of the most deliberated phenomena in epoxy composites due to their widespread applications from aerospace to windmills. There are consistent efforts in this context to reduce the amount of filler content in epoxy in order to minimize the nanoparticle agglomeration and to achieve superior dispersion and improvements in mechanical properties [48,50,60]. The aggregate performance of nanocomposites is a manifestation of the combo of various mechanical parameters, such as tensile strength modulus, flexural strength modulus, fracture toughness, etc. In this, fracture toughness is the most critical parameter, particularly for structural applications. There has been considerable effort to improve the toughness and to reveal the fracture behavior of epoxy composites [55,58,61]. The advances in mechanical parameters of materials are highly influenced by filler physical and chemical properties such as geometry, surface area, and surface condition, that is, morphology [48,62], surface chemical functionality [63], interface chemistry [64], and aggregation tendency [58]. In a study, Rafiee et al. [48] compared the mechanical parameters of nanocomposites using various fillers such as graphene platelets (GPL), single-walled carbon nanotubes (SWCNT), and multiwalled carbon nanotubes (MWCNT) into epoxy. The highest improvement was observed for GPL among the other fillers used. The Young’s modulus, tensile strength, fracture toughness (KIC), and critical strain energy release rate (GIC) of 0.1 wt% GPL composites were found to be increased by 30%, 40%, 53%, and 126%, respectively, with respect to pure epoxy. Bortz et al. [55] reported the effect of GO incorporation on properties of epoxy in which the flexural strength and modulus, KIC and GIC, showed a monotonic increase with weight fractions between 0.1 and 1 of GO; whereas the tensile parameters showed improvements that were, however, inconsistent [55]. At 1 wt% GO loading, the flexural strength and modulus were found to improve by 23%, 12%, 63%, and 111%, respectively. In another study, Tang et al. [58] investigated the effect of filler dispersion on properties of

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183

graphene/epoxy composites in which poorly dispersed reduced GO composites showed pedestrian performance compared to highly dispersed composites (52%) in KIC with 0.2 wt% filler [58].

7.3.1

GNCs as effective nanofiller

In the above studies, graphene and its derivatives are found to be superior nanofillers for improving the mechanical properties of polymers compared to other allied carbon fillers with filler content typically 0.05 wt% or more [54,57,58]. So far, the mechanical properties and fracture mechanisms in epoxy composites at a very low content (0.01 wt%, though the values are higher than that of virgin values. The GNCs/composites showed larger plastic deformation at the crack tip compared to pure epoxy. The plasticity at the crack tip was further investigated through fractography on SENB specimens. The results are discussed in the subsequent section.

190

7.3.3

Nanocarbon and its Composites

Fracture mechanisms using fractography

The fracture mechanism of the nanocomposites can be revealed using fractography analysis. SEM micrographs of the fracture surface of tensile specimens are shown in Fig. 7.11. In 0.01 wt%, the individual sheets are found dispersed with effective uniformity and appear to be coated with epoxy (Fig. 7.11A). This indicates a good interfacial adhesion between the epoxy/GNC interface. At 2 wt% composite, the agglomeration of a few GNCs layers is distinctly visible in a typical specimen location, as shown in Fig. 7.11B. These aggregates are acting as stress concentration zones in composites. Due to the formation of aggregates, the individual conjugated GNC surface may not be in direct contact with the epoxy. Hence, effective reinforcement has not been realized, leading to the mechanical properties being below expectations at higher weight fraction nanocomposites as compared to the lower weight fraction counterparts. This fact can be supported by Fig. 7.11A, which shows a type of pull-out feature of a mono-GNC-layer from an aggregate in 2 wt% nanocomposite. The average size of the GNC layer (range 1–10 mm) in a nanocomposite is found to be reduced more than the pristine GNC layers (range 10–20 mm) [56]. This is due to the fact that GNCs might undergo breakage of sheets during sonication [50]. Low-magnification SEM images are shown in Fig. 7.12A and B, indicating the fracture surface of SENB specimens for epoxy and composites, respectively. The source of the crack or stress concentration regime is clearly visible at the middle of the notch in all these micrographs (Fig. 7.12A and B). Several tracks of cracks can also be seen in the micrographs, propagating in the direction away from the notch front. From micrographs, it is evident that the fracture occurred mainly due to the action of both normal and shear stresses. As observed in Fig. 7.12A and B, the appearance of the lines that are not normal to the macroscopic notch is the indication of shear stresses [70]. At the microscopic level, this is thought to be the mixed-mode (normal and shear) fracture, even though the fracture test was performed using Mode I (tensile stress in the direction orthogonal to the face of the notch). In the notch vicinity or at the

Fig. 7.11 SEM image of tensile fracture surface of nanocomposite (A) 0.01 wt% (scale bar: 300 nm), (B) 2 wt% (scale bar: 1 mm).

Nanocarbons: Preparation, assessments, and applications

Fig. 7.12 SEM images of the fractured surface of mode 1 fracture toughness specimens for (A) epoxy, (B) 0.01 wt% nanocomposites, at lower magnification (scale bar: 1 mm). SEM micrographs of (C) epoxy (scale bar: 200 mm) and (D) nanocomposite show microscopic features near the precrack and fracture surface interface at higher magnification (scale bar: 100 mm). Schematic representation of crack propagation at the vicinity of notch (E). 191

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Nanocarbon and its Composites

crack tip, the surface appears to be more ductile compared to the distance away from the notch in all the cases. The surface appears flat and indicates a cleavage-like feature away from the notch, which indicates unstable crack propagation with no gross plastic deformation followed by fracture. Relatively higher magnification micrographs at the vicinity of the notch are shown in Fig. 7.12C and D. The river-bed marks indicate the formation of microcracks and their growth directions in the plane of fracture for epoxy (image (C)) and nanocomposites (image (D)). As evident, the roughness of the nanocomposite surface is much higher than that of neat epoxy, which indicates a higher surface area in the former case. Further, the areal density of microcracks in the case of nanocomposites is significantly higher than that of neat epoxy. The 3D images of Fig. 7.12C and D highlight the surface roughness obtained by image processing. The microcrack undergrowth of a parabolic shape bulging out of the crack path indicates the deflection of the crack [54,70]. The twists and tilts in cracks are also distinctly observable in Fig. 7.12D. These features are more prominent in the fracture surface of nanocomposites with respect to virgin epoxy. Features clearly suggest that crack deflection is mainly due to obstacles rendered by GNCs in the path of crack propagation. Higher crack deflection leads to higher energy absorption [55]. So, the nanocomposites are found to absorb more energy to fracture as compared to virgin epoxy. This phenomenon is verified by the higher values of KIC and GIC displayed by the nanocomposites as compared to epoxy (Fig. 7.12C). The possible fracture mechanism in the SENB specimen is schematically illustrated in Fig. 7.12E, where the crack initiation zone and its propagation are shown [70].

7.3.4

Thermal and physical properties

Fig. 7.13A presents the thermogravimetric analysis (TGA) profiles for GNCs, epoxy, and nanocomposites. The weight loss of GNCs between 300°C and 850°C is 50 wt% (inset (ii) of Fig. 7.13A), which is related to the removal of oxygen functionalities and the thermal degradation of GNCs. In epoxy and nanocomposites, the thermal decomposition processes mainly occurred between 376°C and 442°C (Fig. 7.13A). Table 7.2 shows thermal properties achieved via TGA curves. The onset at degradation temperature (Tonset) for epoxy is found to be 376°C. The Tonset is increased by  27°C at 2 wt% nanocomposites. Similarly, T50% (temperature at 50% wt loss) and Td,max (maximum degradation temperature) are increased with an increase in the GNC wt fraction in the matrix. For 2 wt% GNCs, the maximum increase in T50% and Td,max is found to be 27°C and 29°C, respectively (Table 7.2). The residual char yield (CY) at 600°C for epoxy, 0.005, and 2 wt% are found to be, respectively, 6%, 9%, and 15%. This is apparently due to the effective reinforcement of GNCs in the epoxy matrix and their effective bonding with the matrix, as discussed earlier. Tan δ versus temperature curves for pristine epoxy and GNC composites are presented in Fig. 7.13B. The peak of the tan δ curve is ascribed to the second-order transition or glassy transition temperature (Tg) of the polymer. The pristine epoxy showed a relaxation peak at 85°C, which corresponds to its Tg. Interestingly, the symmetric

Nanocarbons: Preparation, assessments, and applications

193

100

100

80

Epoxy

95

0.005 wt%

90

0.01 wt%

85

(i)

80 300 320 340 360 380 400 420

0.05 wt% 1 wt%

60

2 wt% 100

(ii) 40

20

Weight (%)

Weight (%)

0.5 wt%

90

14.82% at 600 °C

80

2 wt% GNC

70

GNC

60 Residual: 49% at 850 °C

50 0

0

200 400 600 Temperature (°C)

100

200

(A)

800

Residual: 5.97% at 600 °C 300

400

500

600

Temperature (°C) 1.0 Epoxy 0.005 wt% 0.01 wt%

0.8

0.05 wt% 0.5 wt%

tan d

0.6

0.4

0.2

0.0 60

(B)

70

80

90

100

110

Temperature (°C)

Fig. 7.13 (A) TGA curves of pure epoxy and nanocomposites. The inset (i) shows expanded area near the Tonset and inset (ii) is TGA of GNCs, respectively. (B) tan δ as a function of temperature for epoxy and nanocomposites obtained from DMA.

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Nanocarbon and its Composites

Table 7.2 Thermal parameters of epoxy and nanocomposites obtained from TGA and tan δ curves

Sample Pure epoxy 0.005 wt% 0.01 wt% 0.05 wt% 0.5 wt% 1 wt% 2 wt%

Tg

Tonset (°C)

T50% (°C)

Td,max (°C)

CY (wt%)

Tg1

376

396

413

5.97

85.0

378 383 390 395 400 403

401 405 408 412 420 423

417 418 425 433 440 442

9.63 11.35 11.55 12.10 13.72 14.82

86.4 81.1 80.6 84.0 – –

Tg2

87.9 90.4 91.1 – –

single peak as seen in the case of epoxy became asymmetric and broad with the addition of GNCs and resulted in two peaks when curve-fitted with the Gaussian function. These peaks are assigned to Tg1 and Tg2, which resemble swift chain mobility due to slackly cross-linked domains (at low temperature) and slower chain mobility due to highly cross-linked domains (at high temperature), respectively [43,71]. The data are presented in Table 7.2. The Tg showed moderate increases in the case of nanocomposites; particularly the Tg2, which increased about 5°C in the case of 0.05 wt%. The increase in Tg with the inclusion of nanofillers is often associated with a hindrance in mobility of polymeric chains [43,72]. It is reasonable to state that the observed increase in Tg (refer to Tg2 for composites) is likely due to a good degree of dispersion and robust interfacial adhesion at the polymer/GNCs interface, which may cause a hindrance in polymer chain mobility. In contrast, the decrease in Tg (refer to Tg1 for composites) is likely due to disordered polymer networks, which might arise due to filler agglomeration [43,71]. Molecular dynamics simulation studies have suggested that the surface geometry of the nanofillers influences polymer chain mobility [73]. Therefore, the defected, warped, and wrinkled surface of nanocarbon (see Fig. 7.8) may also contribute in decreasing chain mobility (consequently, an increase in Tg) through mechanical interlocking with the matrix. To summarize, we have demonstrated mitigation in mechanical as well as thermal properties at low weight fractions of GNCs in host epoxy. They are exfoliated into individual sheets in epoxy at lower weight fraction ( 0.01 wt%. The areal aggregate density increases with GNC wt. fraction, which adversely affected the mechanical properties. The mechanical parameters such as tensile, flexural, and fracture toughness showed significant enhancements due to GNC incorporation, irrespective of its content in epoxy. Optimum reinforcement is obtained at 0.01 wt% nanocomposites, presumably due to the cooperative effects of a large surface area, uniform dispersion, and good interfacial adhesion. The thermal and physical properties are enhanced considerably with GNC. Mechanisms for fracture energy

Nanocarbons: Preparation, assessments, and applications

195

absorption in nanocomposites are mainly governed by the formation of more microcracks and their deflection, as compared to neat epoxy. On account of these properties, GNCs could be promising nanofillers for structural applications. In another study, GNCs have been evaluated for their spin properties and magnetic correlations [74], as described in the following section.

7.4

Magnetism in otherwise nonmagnetic carbon: Defect as a source

Magnetic materials are universal components in today’s technology. Currently used metal magnets involve partially filled d- or f-band atoms. Pure carbon is strongly diamagnetic, consisting of s and p electrons. In recent years, reports on the existence of ferromagnetic, antiferromagnetic, and paramagnetic ordering in carbon have become the focus of several investigations. Carbon magnetism is controversial, intriguing, and originates due to a size reduction of the system. A large number of experimental attempts have been made to demonstrate allotropes of carbon such as fullerenes [75], highly ordered pyrolytic graphite [76], carbon nanofoams [77], and nanodiamonds [78] as magnetic materials. Graphene is not an exception [79,80]. The magnetic moments in graphene emerge due to zigzag edge states [81], topological disorders [76], unsaturated dangling bonds [82], mixed sp2/sp3 interconnected phases [79], etc. Moreover, multishaped graphene fragments such as triangular hexagonal nanoislands [83], ribbons [84], nanoflakes [85], and fractal carbon [86] have shown high-spin ground states and behaved as artificial ferromagnetic atoms. To provide atomic-level understanding of the observed magnetism in carbon, numerous theoretical studies have been performed [87–89]. The studies showed that isolated vacancies and chemisorption of foreign atoms near vacancies could induce strong local magnetic moments [90]. The atomic origin of magnetic moments has three principal sources: (i) the spin with which electrons are endowed, (ii) the orbital angular momentum of electrons about the nucleus, and (iii) the change in orbital momentum induced by external perturbations. The first two factors give spin-spin and spin-orbit (SO) interactions, and the third measures the strength of spin-orbit coupling. In graphene, SO interaction couples π and σ bands. The principal parameter governing the usability of graphene in magnetic applications is spin-lattice relaxation time, Tsl. The relaxation of spin coupled to its lattice depends on broken inversion symmetry [91] and the presence of heterostructure [92] in the two-dimensional graphene superlattice. The ElliottYafet mechanism [93] explains the former case, and the latter is based on the Dyakonov-Perel (DyP) theory [94].

7.4.1

Spin transport and magnetic correlations in GNCs and nitroGNCs: Graphene spintronics

For investigation into spin transport and magnetic correlation parameters of GNCs doped with nitrogen, magnetic measurements were carried out using electron spin resonance (ESR) and vibrating sample magnetometry (VSM). This analysis underlines

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Nanocarbon and its Composites

that spin transport is somewhat hindered and magnetic correlations are reduced due to the presence of nitrogen,. Further, it plays an important part in the exchange coupling of the hole to the electron onto the carbon network at substitutive native oxygen sites. This is also supported by chemical analysis. On increasing nitrogen content, I-V measurements showed that the second-order resistance is affected considerably.

7.4.1.1

Preparations of N-GNCs

GNCs are doped with nitrogen with tetrakis(dimethylamino)ethylene (TDAE). A finely dispersed suspension of GNCs was prepared in 25 mL of tetrahydrofuran (THF) and a measured quantity (1%) of TDAE was added; it was kept stirred for 8  10 h at room temperature. After vacuum filtration, the obtained product is termed N-GNCs, which is further characterized. Both GNCs and N-GNCs were weighed (0.1  0.5 mg) and sonicated well in THF for a period of 15 min to obtain better dispersion. These suspensions were used for I-V measurements.

7.4.1.2

Comparison using FTIR: GNCs and N-GNCs

Fig. 7.14 shows the FTIR spectrum for (a) GNCs and (b) N–GNCs. In profile (a), mainly three bands were observed below 3000 cm1. In GNCs, no bands above 3000 cm–1 were observed, indicating that GNCs contain no hydroxyl (dOH) or amino (dNH2) groups and bands between 1500 and 1600 cm1 could be attributed to the conjugated double bond C]C. The profile (b) for N-GNCs is different than that of profile (a). The complete assignment of 108 fundamental vibration modes of TDAE is a difficult task; therefore, we have focused our attention mainly on the CdN bond stretching modes where different charge states are expected from different fragments [95]. The emergence of a broad peak at 3433–3335 cm–1 (indicated by the open circle) is assigned for dNdH amines/amides due to one of the fragments of the TDAE, which is dimethylamine (DMA). The bands observed at 3000–2650 cm–1 correspond to the CdH stretch while that at 3350–3310 cm–1 is for N-heterocyclic compounds. Additionally, it would be reconfirmed with the dC]NdH mode that appeared at 1641cm–1 and RdNH2 and ArdNH2 in plane bending between 1599 and 1641 cm–1, attributed to the fragment of tetramethyl urea (TMU). The small peak appearing at 983 cm–1 (indicated by an arrow) may be due to out-of-plane bending of CH2 twisting. The shakeup feature at 1599 cm–1 is attributed to the fragment of the tetramethyloxamide (TMO). Additionally, a band appearing at 1599 cm–1 confirms ArdNHd bonding with the aromatic ring. The presence of different fragments due to the stretching of CdN bonds is observed. The band at 1450–1350 cm–1 is due to N2C]CN2 while for NdCH3, the bands are observed at 1372–1382 cm–1, which existence of fragments of tetrakisdimethylamino-1, 2-dioxetane (TDMD), and tetramethylhydrazine (THM). The aromatic CdN stretching appeared between 1350 and 1265 cm–1. The bands at 1641 and 1599 cm–1 due to the presence of disubstitution of amines to the sp2 bonded carbon network and 1448 and 1408 cm–1 are stretch vibrations assigned to the geminal dimethyl group. The band appearing at 1350 cm–1 is for dCOdCH3

Nanocarbons: Preparation, assessments, and applications

H3C

90

197

O CH3 N N CH3 CH3

(–NH2)

Intensity (a.u.)

(b) N

CH3

H3C

N

N N

60 H3C

CH3

(a) O H3C

30

N N

CH

CH3

700

CH3

O

1400

2800

3500

–1

Wave number (cm )

Fig. 7.14 FTIR spectrum recorded for (a) GNCs and (b) N-GNCs.

on the zigzag carbon of graphene, which is attributed to the asymmetric bending mode appearing at 1448 cm–1 attached with the bis(dimethylamino)methane (BMAM). Herein, we can conclude that different functional groups attached to the zigzag chains of graphene edges or the honeycomb network are fragments of TDAE. Our further studies show that fragments via radicals played important roles in the charge transfer process, resulting in reduced ordering in N-GNCs [96].

7.4.1.3 Chemical analysis of nitrogen doping in GNCs by electron spectroscopy Chemical analysis was carried out to identify σ and π bonds in N-GNCs; they are compared with bonds in GNCs. In ESCA, the spectrum is deconvoluted [97]. Fig 7.15 shows the ESCA peak of C 1s recorded for (a) GNCs and (b) N-GNCs. In GNCs, the C 1s core peak is located at 285.33 eV, which is further shifted to a lower binding energy at 284.66 eV after the incorporation of nitrogen with a reduction in width at half-maximum (FWHM) 2.50–2.33 eV for N-GNCs. The peak intensity is also reduced from 3.35  105 cps to 3.20  104 cps for N-GNCs, confirming the successful doping of nitrogen in GNCs [97]. The added amount of TDAE in the GNC suspension negligibly small, so the atomic percentage of carbon estimated in both samples is 93.00 at.%. For GNCs, the C 1s peak is deconvoluted into three peaks, and the C1 at 289.20 eV is attributed to O] CdOH for sp2 carbon, the C2 peak at 286.23 eV is assigned to the sp3-based dCdOdCd group, and C3 at 285.07 eV corresponds to CdC of sp3 origin.

C-1s

3.34 ´ 105

6.48 ´ 105

Intensity (in Cps)

6.48 ´ 105

3.30 ´ 105

6.47 ´ 105

3.28 ´ 105

6.47 ´ 105

C2

3.24 ´ 105 3.22 ´ 105

6.47 ´ 105

C1-sp2

294

292

290

288 286 284 282 Binding energy (in eV)

280

278

276

274

(C)

412

410

408

406 404 402 Binding energy (in eV)

400

398

396

C-1s 2.50 ´ 104

3.00 ´ 10

4

N-1s

2.45 ´ 104

2.80 ´ 104

2.40 ´ 104

2.60 ´ 10

2.35 ´ 104

4

N1

2.30 ´ 104

2.40 ´ 10

4

C3

2.20 ´ 104

2.25 ´ 104

C2

2.20 ´ 104

C1

2.00 ´ 10

4

2.15 ´ 104 2.10 ´ 104

1.80 ´ 104

2.05 ´ 104 292

290

288

286

284

Binding energy (in eV)

282

280

278

2.00 ´ 104 410

(D)

408

406

404 402 400 398 Binding energy (in eV)

Fig. 7.15 Recorded C 1s spectra for (A) GNCs and (B) N-GNCs N-1s (C) for GNCs, and (D) N-GNCs by ESCA technique.

396

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Nanocarbon and its Composites

Intensity (in Cps)

N1

6.46 ´ 105

3.20 ´ 104

(B)

N3

6.47 ´ 105

3.20 ´ 105

1.60 ´ 104 294

N2

6.47 ´ 105

C3

3.26 ´ 105

296

N-1s

6.48 ´ 105

3.32 ´ 105

(A)

198

6.48 ´ 105

3.36 ´ 105

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199

The composition % of C1, C2, and C3 moiety is 9.32, 40.45, and 43.23 at.%, respectively. In the case of N-GNCs, the C1 peak located at 286.21 eV is attributed to CdNH2, which is an amine-based moiety having an sp3 state of hybridization [98] with composition 31.32 at.%. The composition of CdNH2 is peak C2 at 284.99 eV and C3 at 284.04 eV are assigned to CdC with the compositions 40.21 and 32.09 at.%, respectively. These peaks have similar characteristics to those of C-1s peaks. The reduced oxygen is utilized for the radicalization/fragmentation of TDAE. This is consistent with our FTIR. Thus, ESCA analysis indicates that chemical reduction takes place after treatment of GNCs with TDAE. The at% of nitrogen in GNCs is 2.19 at% and after doping nitrogen, it is increased to 9.20 at%. For GNCs, the main peak of nitrogen N-1s appears at 400.45 eV, along with a satellite feature N1 at 405.06 eV. After treating GNCs with TDAE, the main peak is shifted to a lower binding energy at 399.89 eV and the satellite feature disappears. The negative chemical shift in the main peak of nitrogen is 0.45 eV due to the transfer of charge from GNCs to TDAE, indicating a p-type dopant from GNCs. GNCs are electron-rich due to delocalization of the π-electrons. The decrease in oxygen content indicates that the reaction takes place at the carboxyl and epoxy sites. Nitrogen in the form of amine (dNH2) is introduced in the sp2 network. Nitrogen becomes bonded by accepting a p-type dopant from GNCs. The exchange interaction is leading at oxygen sites that interchange the couple hole, replace oxygen, and vary the state of hybridization from sp2 to sp3. As a result, the overall sp3 content is increased substantially. The modification could bring changes in the makeup of exchange correlations and spin transport in GNCs.

7.4.2

Radical spin correlations: Electron spin resonance measurements and analysis

ESR measurements were carried out to investigate spin transport at a microwave frequency of  9.1 GHz (X-band) at room temperature. The static magnetic field is swept over the range 300  370 mT gradually at a spectrum-point time constant of 0.1 s and 6 kHz was the amplitude of modulation frequency. The other parameters such as field center, ESR line width sensitivity, resolution, and maximum microwave output are 336 mT, ΔH ¼ 0.05 mT, 7.0  109 spins/0.1 mT, 2.35 μT, and 985 1000 μW, respectively. For the GNCs and N-GNCs, the first derivative of the paramagnetic absorption signal was recorded. Among low and high concentration samples, the highest was taken from a batch of N-GNCs. Fig. 7.16A shows the room-temperature ESR spectra for GNCs. The first absorption derivative, dY/dH, as a function of applied field shows a symmetric absorption peak. For GNCs, symmetric and homogeneous broadening is observed. In general, the ESR line shape gives information about magnetic interactions in the measured sample. The line width, peak-to-peak distance, and Hpp have an azimuth angular dependence, θ. So it has the orientation of an applied field with respect to the orientation of the GNC planes. For the bulk powder specimen, Hpp(θ) ¼ sin2(θ)Hk + cos2(θ)H?. For GNCs, line width is found to be 0.6756 mT. The measured Hpp consists of

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Nanocarbon and its Composites

Fig. 7.16 Typical electron spin resonance spectra recorded at room temperature. (left panel) Comparison of (a) GNC and (b) N-GNC line widths; (right panel) enlarged feature for N-GNCs, indicated by an arrow.

contributions from Hk as well as H?. From spin dynamics, the prospective important parameter is the spin-spin relaxation time, Tss, correlated by Hpp ¼ (1/γ eTss), where γ e is the gyromagnetic ratio for electrons with a value of 1.760859  1011/s T. For GNCs, the value Tss is 0.8406 ps. The further magnitude of the g-factor is 1.99685, at which the resonance has occurred under an applied microwave frequency and characterizes the magnetic moment, μ, and γ e, associated with unpaired electrons in the material. The estimate of the g-factor is found to be less than 2.0023, which is the g-factor for a typical nondegenerated Pauli gas. This directs that the local magnetic environment in GNCs is specifically different than the conventional magnetic material. It is also clear that spin does not originate from transition metal impurities, but only from carbon-inherited spin species due to the small values of Hpp and a minor deviation in the g-factor. Furthermore, the key factor in governing spintronic usability is the spin-lattice relaxation time, Tsl, which describes the variation of the nonthermal spin state around the lattice. The magnitude of Tsl is computed as (1/Tsl) ¼ (28.0 GHz)/ THpp (where T is in Kelvin) and is found to be 1.586 ns. The theoretical estimate for Tsl for materials spintronic application is 1 100 ns; however 60  150 ps are the experimental spin-transport measurements for graphene [99]. Using the theory of spin relaxation and spin transport, the parameters were calculated. Fig. 7.16B shows an enlarged ESR spectrum for N-GNCs. A comparison of obtained parameters is given in Table 7.3. A comparative study of N-GNCs with GNCS shows that spin transport is modified and reflected in measured relaxation parameters. Magnitudes of Tss and Tsl are increased, respectively, to 1.2297 ps and 2.3201 ns. Thus, electron spin takes more time to regain its original state. The possible reason is hindrance offered by a donor-loaded nitrogen group. The extent of the SO coupling constant, Li, for N-GNCs is reduced to 16.0 from 22.12 meV as that of GNCs. Basically, in graphene, Li originates through intrinsic, Bychkov-Rashba (BR, related to structural symmetry break), and ripples (related to inevitable wrinkles/folding edges). The theoretical estimation for the intrinsic SO coupling constant is between 1 μeV and 0.2 meV, and for BR, 10  36 μeV and V1 nm1. For the intrinsic component, when μ~≫ Γ,

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201

Table 7.3 Comparison of spin transport parameters for GNCs and N-GNCsa Parameters

GNCs

N-GNCs

Resonance magnetic field (Hr) g-Factor Peak-to-peak (ΔHpp) Spin-spin relaxation time (Tss) Spin-lattice relaxation time (Tsl) Δg Band width (Δ) Spin-orbit coupling constant (Li) Spin relaxation rate (Γ spin) Momentum relaxation rate (Γ) Pseudo chemical potential (μ) Density of states (ρ) Pauli-spin susceptibility (χ at 300 K) ESR line limit of detection

327.163 mT 1.99685 0.6756 mT 0.8406 ps 1.586 ns 5.45  103 4.06 eV 22.12 meV 7.81  108 eV 2.63 meV 1.94 eV 0.07469 stats/eV atom 4.80  107 514.81  LD

327.020 mT 1.99856 0.4618 mT 1.2297 ps 2.3201 ns 4.24  103 3.79 eV 16.07 meV 5.35  108 eV 2.97 meV 1.79 eV 0.06892 stats/eV atom 4.43  107 580.55  LD

a

Measurements were performed at 300 K.

Γ(intrinsic)  L2i /Γ, which is an Elliot-Yafet-like result [93], where μ~is the pseudochemical potential, Γ is the momentum relaxation rate, and Γ(intrinsic) is the intrinsic momentum relaxation rate. Further, the ripple relaxation contribution becomes dominant only when Γ ≫ μ~. The computed values of Γ and μ~for our systems indicate that intrinsic and BR could be the operative channels. But one can neglect the intrinsic term because disorder is present in GNCs. Once a vacancy is generated, the carbon atom that is the nearest neighbor to a vacancy is displaced from its equilibrium position. Using the theory of spin relaxation from experimentally obtained ESR lines, the spin transport parameters were computed for maximum concentration ( 0.5 mg/ mL for GNCs and N-GNCs). As an effect, the system with a vacancy undergoes a Jahn-Teller distortion and breaks the honeycomb symmetry. The quantity of sites with damaged spatial inversion symmetry is high in GNCs compared to pure graphene. In the case of pure graphene, when Δ ≫ Li, where, Δ, is band gap, the SO interaction couples π and σ band. Hence, in GNCs the principal component in Li is BR with π-σ band distortion. The decrease in the extent of Li may be attributed to a reduction in mid gap between the π and σ bands, which is due to alterations at the broken symmetry sites. This perturbation is due to the nitrogen-loaded donor moieties. This is because of their strong tendency to donate wandering electrons via a charge-transfer interaction with the sp2 network. Further spin-relaxation time is calculated using Tss ¼ Ncol τ, where Γ  1/ τ, τ is the momentum scattering rate and Ncol is the spin-flip scattering probability (meV ps) of the spin density wave propagating in crystal. [100]. Using this relation of GNCs, the value of Ncol is estimated to be 2.22 meV ps and for N-GNCs, the collisional probability is boosted to 3.65 meV ps. A similar consistency is found with

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variations of density of states ρ(μ~,Γ spin) in both systems, where Γ spin is the spinrelaxation rate. It is remarkable that enormous deviations have been observed between theoretical and experimentally derived values of spin transport parameters for GNC systems. The Pauli spin susceptibility, χ spin, obtained via the ESR technique (given in Table 7.3) as well as from VSM measurements are well matched.

7.4.3

Magnetometric analysis by VSM

A VSM is used to measure the magnetic behavior of magnetic materials and works on the principle of mutual inductance. In order to enhance the sensitivity of the measurements, the VSM system was calibrated properly as well as tuned for background level noise reduction. The diamagnetic correction in each sample was done. For GNCs and N-GNCs, measurements were performed at room temperature, and the field was swept from 1.5  104 to +1.5  104 Oe. Fig.7.17 shows VSM spectra for (a) GNCs and (b) NGNCs. From obtained spectrum of VSM, saturation magnetic moment Ms, effective magnetic moment μeff, coercive field Hc, remanence field Mr, and squareness ratio Mr/Ms were obtained and compared for both systems. Due to the presence of exchange anisotropy, a shift in the hysteresis loop is predicted while the field is applied in a reverse direction. For GNCs, the curve shows saturation behavior while no clear saturation behavior drift is in N-GNCs. The extent of Ms is calculated at  1.02 A m2 for GNCs, which is three orders or more in magnitude than reported in the literature [101]. For N-GNCs, in order to estimate Ms, the geometrical projection of the M-H curve is taken on the magnetic moment axis. This methodology is followed due to the absence of a saturation magnetic moment feature. The value of Ms is reduced 3.8 times and is estimated to be  0.27 A m2 N-GNCs, after doping. The effective magnetic moment μeff is computed from values of Ms obtained for both samples. The Bohr magneton, μB, physical constant for μ of electrons per single atom and  1 μB per electron is an intrinsic electron magnetic dipole moment [102,103]. The study of μB allow us to know how many μB a single carbon atom

Fig. 7.17 Vibrating sample magnetometric spectra recorded for (a) GNCs and (b) N-GNCs at 300 K. Inset shows zoomed portion of M-H loop as indicated by an arrow. Ellipse shows region of magnification by VSM.

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203

Table 7.4 Magnetic correlation parameters obtained after analyzing M-H loops of VSM recorded for GNCs and N-GNCsa Parameters

GNCs

N-GNCs

Saturation magnetic moment (Ms) Effective magnetic moment (μeff) χ pauli (at 300 K) Coercive field (Hc) Remanence field (Mr) Magnetic hardness (MH) Squareness ratio (Mr/Ms)

1.02 A m2 0.198 1.63  105 50.477 mT 0.302 A m2 423.0 mT/A m2 0.296

0.27 A m2 0.053 1.17  106 22.849 mT 0.098 A m2 481.5 mT/A m2 0.363

a

Measurements were performed at 300 K.

contains. The computation of μB was done based on two basic assumptions: (i) distortion in the unit cell for both systems is neglected, and (ii) an equal flake area for GNCs and N-GNCs. The carbon atom concentration is worked out as CC ¼ [4/ (32)1/3]a2, where a is the CdC bond length in graphene; the value of a is ˚ . Using this, the magnitude of CC is estimated to be 3.81  1015 atoms/cm2. 1.421 A Further, the effective magnetic moment is computed as μeff ¼ (Ms/W)(V/NC), where W (in grams) is the weight of samples subjected to VSM, NC is the number of carbon atoms in the active layer, and V is the effective volume of the GNC layer. The thick˚ . The value of NC is given by NC ¼ CCA, where ness of the carbon layer is taken as 3.5 A A is the area of the flakes. Using a SEM study, the area of the flakes is roughly calculated to be 20 μm2 [104], so, NC is estimated to be 7.62  1010 carbon atoms. The values of W were in the range of 47 51 mg. Thus, the magnitude of volume V is computed as  7  1013 cm3. The value of μeff is found to be 0.198 μB for GNCs and 0.053 μB for N-GNCs. Thus, the μeff calculation shows that for a cluster of 1000 carbon atoms, there are  200 interacting spins for GNCs, which also sheds light on the reduced number of carbon atoms to 50 for N-GNCs. This indicates the number of electrons participating in the spin interaction is reduced after nitrogen doping. The μeff and χ spin values are interconnected as μeff ¼ 2.83(χ spin T)1/2μB [93]. For room temperature measurements, the magnitude of χ spin (GNCs) and N-GNCs are 1.63  105and 1.17  106, respectively. The calculated values for χ spin by the VSM and ESR showed a slight variation in magnitude and the difference was found to be two orders of magnitude for GNCs while one order of magnitude variation for N-GNCs; this may be dependent on the accuracy and precision of the systems used. The other parameters obtained by VSM are listed in Table 7.4.

7.4.3.1 Magnetization in graphene: Ruderman-KittelKasuya-Yosida interactions Fig. 7.18 shows the probable mechanism of magnetic ordering in TDAE-treated graphene [105]. Fig. 7.18A shows the structure of TDAE the molecule whereas Fig. 7.18B shows the graphene network with fragments of TDAE. As discussed in earlier sections, it was mentioned that fragments are formed from TDAE, which could

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Nanocarbon and its Composites

Fig. 7.18 Schematics of magnetic ordering in TDAE-treated graphene in which (A) TDAE molecule, (B) its attachment with graphene, and (C) ordering state.

be accountable for the magnetic ordering in GNCs, as shown in Fig. 7.18C. The fragments of TDAE consist of wandering electrons that attach to the graphitic network where already localized electrons are present. At this point, its charge transfer takes place. It would be more interesting to know how wandering electrons generate spin ordering in localized electrons, and what kind of coupling exists between the wandering and localized electrons. More precisely, we can say that, in general, magnetism requires an unpaired electron with a lattice atom that carries a net nonzero magnetic moment and is coupled via an exchange interaction with other electrons. In a graphitic sp2 network, every sp2 carbon atom has one pz orbital and a π-electron. These π-electrons are symmetrically bound to a hole and a half field. In the magnetization process of the sp2 carbon network, electron-electron interactions show a key role [106,107]. Such interactions are called Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions [108–110]. The RKKY interaction involves indirect exchange interactions induced by wandering electrons in a host material between two localized spins. Such interactions are short range and repulsive in nature where mobile charge facilitates magnetic coupling [31,106,107,111]. The interaction of wandering and localized electrons takes if they occupy the pz atomic orbital in the host lattice site, cloaking the Columbic repulsion. The required condition for finite temperature magnetic ordering is onsite Columbic repulsive energy, U, between wandering and

Nanocarbons: Preparation, assessments, and applications

205

localized electrons, and must be less than the hopping length, t, (expressed in hopping energy) of the wandering electrons. In other words, one can say that magnetic interaction between wandering and localized electrons takes place with the cloaking of Columbic repulsion. This hypothesis is applicable for magnetization in TDAE-treated GNCs and mainly depends upon the ratio U/t [112]. In the host site, the probability of finding random localized electron spin is modulated by the charge-transferred itinerant electron spin. As a result, the spin polarization of the conduction electron in the host graphene matrix takes place and could be expressed by a spin wave. The modulated spin wave gets transmitted as a damping oscillation to the next lattice point, where the itinerant electron spin resides with the separation of the lattice constant. If the phase of the spin wave matches at each lattice point, then all the localized electron spins align to the same direction, achieving a ferromagnetic spin ordering in the TDAE-treated graphene.

7.4.3.2 Current-voltage measurements: Transport characteristics To perform I-V measurements, a standard cyclic voltmeter setup was used. Fig. 7.19A shows a photograph of the I-V measurement setup, and a schematic of the assembly is shown in Fig. 7.19B. The assembly consists of a computer-controlled potentiostat and a galvanostat having a four-electrode assembly. For measurements, out of four electrodes, one electrode was disabled. One was used as a reference electrode connected to the potentiostat via a current meter and voltmeter. The important features of the system are: (i) sweep voltage can vary from 10 V to +10 V, having a compliance voltage 30 V with a maximum current 2 A; (ii) the current can vary from 1 A to 10 nA with voltage accuracy

Fig. 7.19 (A) Photograph of I-V measurement setup. (B) Schematic assembly details, showing the cell arrangement of electrodes, circuitry arrangements for current, voltage meter, potentiostat, etc.

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Nanocarbon and its Composites

0.2%; (iii) the voltage resolution was 0.3 mV with current accuracy 0.2%; (iv) the resolution for current was 0.0003% over the measured current sweep range; and (iv) the input impedance was greater than 1 TΩ with potentiostatic bandwidth 1 MHz. I-V measurements were performed on all N-GNC samples. Fig 7.20 describes measured I-V characteristics for GNCs and N-GNCs for 0.1  0.5 mg/mL. The I-V profile recorded for GNCs is linear, more or less parallel to the voltage axis, and passes through the origin. The resistance R1 computed for GNCs was 82.36  0.0121 MΩ (as shown in panel B by an arrow). The profiles are distinctly different and show nonlinear behavior in contrast to the linear I-V characterP istic of GNCs. The profiles were simulated by use of the equation: V ¼ C + 3i¼1 Ri I i . The intercepts for GNCs have a positive magnitude and vary from 1.804  108 1.404  109 to 3.820  108  2.770  109. For N-GNCs, the value of resistance as a function of concentration for higher orders is shown in panel B. The nonlinearity is found up to the third order. Panel B shows resistance as a function of variable TDAE loading in GNCs. Profiles R1, R2, and R3, show coefficients of the first-order, secondorder, and third-order resistance term. The variations in R1 and R2 are positive in magnitude and almost similar, whereas R3 is negative. However, the value of R1 is reduced with respect to the resistance of GNCs. The decrease in R1 is from 4.410  0.079 to 3.290  0.079 MΩ, from a lowest to highest nitrogen content. This change seems to be small compared to the change of R2 with subsequent increase in TDAE. There is one order of magnitude variation in R2 from low to high content. Moreover, the increase is around 2.5 times more with respect to the R2 value at lowest. This shows that the higher-order term R2 is dominant over R1. R2 is a coefficient of I2. The second-order

1.0 ´ 10 (GNCs) –1.0

–0.5

(v)

2.5 ´ 1012

(iv) (iii) (ii)

2.0 ´ 1012

(i)

1.5 ´ 1012

0.0 0.0

Voltage (V) 0.5

1.0

–1.0 ´ 10–7 –2.0 ´ 10–7 –3.0 ´ 10–7 –4.0 ´ 10–7

(A)

R2

1.0 ´ 1012

–7

Resistance (in W)

2.0 ´ 10–7

Current (I)

3.0 ´ 10

–7

GNCs 108

107

(i) 0.1 mg/ml (ii) 0.2 mg/ml (iii) 0.3 mg/ml

R1

106

(iv) 0.4 mg/ml

0.0

(v) 0.5 mg/ml

0.1

0.2

0.3

0.4

0.5

Loading of TDAE in GNCs (mg/ml)

(B)

Fig. 7.20 I-V characteristic for (A) N-GNCs in THF ranging from 0.1 to 0.5 mg/mL, The horizontal profile near the x-axis is I-V characteristic for GNCs. (B) Resistance R as a function of TDAE loading in GNCs (milligrams per milliliter). Arrow in panel B indicates first-order resistance, R1 of GNCs ( 82.36 MΩ). Resistance of the liquid medium (THF)  48.80 MΩ.

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207

term in the I-V measurements pertains to nonlinear scattering effects [113]. The cubic term is negative, and the negative resistance R3 seems to be apparent and carries no physical meaning. The observed contribution could be due to nitrogen dopants in GNCs. The observed decrease in R2 is indicative of a decrease in ρ calculated for N-GNCs at the highest TDAE loading. The obtained results are opposite to the theory and experimental findings reported before for nitrogen-doped graphene. The overall idea of an intrinsic magnetic carbon structure is still at the infancy stage of research, and much effort is needed for a better understanding of the mechanism of magnetic ordering in carbon nanosystems. Analysis by ESR and VSM revealed that spin transport and exchange interactions occurring in GNCs are modified after the incorporation of nitrogen. Specifically, the SO couples electronic states with opposite spin projections in π and σ bands. This is indeed reflected in the variation in magnitudes of Li and ρ obtained for GNCs and N-GNCs. Li is connected to the inter σ-π band separation via μ (chemical potential) while the physical entity ρ is associated with the density of states of carriers at the Fermi level. The questions to be asked are (i) how to quantify σ-π variations and (ii) whether it has implications on the electrical transport of the system. By the use of chemical analysis, one may shed light on the first issue, whereas the second could be addressed by electrical measurements.

7.4.3.3 Reduced exchange correlations: Role of nitrogen Fundamentally, magnetism requires an unpaired electron with a lattice atom that carries a net nonzero magnetic moment. These moments can be coupled through an exchange interaction with other electrons. The observed decrease in μeff shows that the moments, coupled through interactions, are reduced. Electron-electron and latticeelectron interactions play important roles in the magnetization of honeycomb carbon [31]. In such a network, every sp2 unit contains a half-empty pz orbital and a half-filled localized π-electron. The π-electron is symmetrically bound to a hole. They form a bound pair. Furthermore, TDAE is an organic material that shows a strong tendency to donate electrons via a charge-transfer mechanism due to the presence of nitrogen [30]. Nitrogen plays a crucial role by donating one of the electrons from its lone pair. The donation is exchange-based, and exchange takes place with a half-empty pz orbital. The itinerant electron is bound and spin-coordinated with a localized π-electron at the carbon lattice. The coupling takes place in the screen-Columbic repulsive environment. This introduces distortion in laterally oriented σ-bonds. The proposal is based on the calculation of percent variation (%var) of Tss and Tsl computed for GNC and N-GNC samples. Percent variation is given by %var ¼ [T(GNCs)  T(NGNCs)]  100. The Tss %var is  46% and for Tsl it is  74%, computed across GNCs to N-GNCs. This indicates that percent variation for spin-lattice interaction is dominant over spin-spin interaction. This is possible, in fact, in graphene; each Dirac cone ˚ apart. The lateral is positioned, longitudinally, at a distance of CdC, that is, 1.421 A 2 σ-band, connecting the nearest two sp carbons, is an immediate neighbor of the bound pairs of the Dirac cone. During exchange coupling, the perturbation offered by the uncoordinated itinerant electron to the empty pz orbital could distort the σ-bond

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Nanocarbon and its Composites

Dirac cones

Localized-p-electron

s-distortion s-bond

Bound hole

Itinerantelectron Bound hole

(A)

Itinerant-electron

(B)

Fig. 7.21 Schematic representation of exchange interaction between itinerant electron and halffilled localized π-electron. (A) The coupling takes place by replacing bound hole. The interaction is short-range and Coulombically repulsive, which could distort lateral σ-bonds. (B) The itinerant electron is donated by nitrogen of TDAE to sp2 carbon network of GNCs.

sensitively. The schematic in Fig. 7.21 panel a shows the attack of the itinerant electron on the hole, and panel B indicates σ-bond deformation. As a result, the strength of SO coupling and carrier concentration at the Fermi level could be affected. The clue obtained by ESR, VSM, and ESCA analysis is sufficient to address the second issue raised above. Current-voltage (I-V) measurements were performed in order to explore modification in the transport characteristics of GNCs after doping with nitrogen. In general, I-V measurements performed by reported techniques need complex lithographic processing, contact electrodes, positioning of graphene, etc. Furthermore, the electrical transport through the contact electrode is also a crucial issue. In such measurements, the carrier transport is constrained by the dimension of the contact point and the mean free path of the electrons at the graphene-contact electrode interface. In the present study, a simple approach has been adopted to measure the I-V characteristics of the samples. The electrical transport measurements were performed in a liquid phase after dispersion of GNCs in an appropriate suspension. The variation in dispersibility as a function of time was studied separately, and the details are outside the scope of the present discussion. Nonetheless, the analysis revealed that GNCs (N-GNCs) contain  3.25 (1.45) at.% native oxygen. The oxygen moieties are in the form of carboxylic and epoxy groups. These groups, in general, enhance the Lewis acidity of the dispersing medium (THF), assisting the dispersion for a longer lifetime. Thus, the dispersion of samples was found to be uniform and homogeneous for a period of 3 weeks in the THF solution [114]. The magnitude of resistance of the THF solution at a fixed electrode configuration is 48.80  0.56 MΩ, which is much better as compared to other dispersing media studied therein. Hence, THF was selected as the dispersing medium for electrical measurements.

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In summary, GNCs were doped with nitrogen by use of TDAE for varied concentration. The magnitude of the g-factor indicates that spin originates from only carboninherited species in GNCs. The increased value of Tsl after doping indicates that the induced nonthermal equilibrium spin state takes a longer time to decay in the presence of nitrogen. The operative mechanism was the Elliot-Yafet SO interaction in GNCs. The density of states was reduced. The spin-flip probability, Ncol, was found to be enhanced after doping. To obtain magnetic correlation parameters, vibrating sample magneto metric (VSM) measurements were carried out over 1.5 kOe, at room temperature. The magnetic moment was reduced for N-GNCs. Thus, for a cluster of 1000 carbon atoms, there were  200 interacting spins for GNCs. After nitrogen doping, the number was reduced to  50 spins for the same number of carbon atoms. The exchange couple hole replaces oxygen and modifies the state of hybridization to sp3. As a result, the population of the sp3 content is increased appreciably. The coupling occurs under a Columbic repulsive environment, distorting σ bonds. Clearly, a charge-transfer mechanism between GNCs and TDAE is responsible for bringing a change in the makeup of exchange correlations and spin transport in GNCs.

7.4.4

Spin-bath properties of GNCs

The spin transport parameters have been revealed in more detail in which ESR measurements were carried out over 123–473 K [115]. The paramagnetic resonances arising from the conduction electron in carbon could be harnessed for processing information in the form of strings of quantum bits (qubits) [116]. These electrons diffuse in and out of the surface of the carbon and would have a decisive effect on the transport of information for storage and secured communication [117]. The conduction electrons are assumed to be mobile like free particles and collective electron magnetic moments are treated as the bath of free particle carrying quantized spin moments. In general, such quantum systems communicate with their environment via up or down orientations of spin [118]. The spin degrees of freedom of the electron bath endowed to its lattice is on the talking term and could be exploited, primarily, using spin-orbit (SO) interactions [119]. However, crucial requirements on such spin baths are quantum entanglement [120], spin-phonon coupling [121], weak decoherence [122], and a hyperfine interaction with surrounding nuclear spin [106]. In carbon, especially graphene, the SO interaction has intrinsic [123], Bychkov-Rashba [124], ripple [119], and extrinsic contributions [125]. These interactions are supposed to be weak in the μeV regime [126], due to the low atomic number of carbon. However, the interest of the whole analysis will lie not in the diffusion effect itself but in quantifying properties of such a spin bath. In this section, we present the behavior of such a spin bath embedded in a GNC super lattice. The electron spectroscopy for chemical analysis indicated that the composition of sp2:sp3 was 2:3 and GNCs mainly contained oxygen attached to the sp3 moieties. The spin transport data was obtained over 123–473 K, using an ESR spectroscopic technique, at the X-band. Analysis revealed that feeble magnetization exists due to a puddling effect that leads to weak delocalization of π-electrons. Practically, no effect of oxygen 2p-orbital quenching had been observed due to the O impurity that

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2.5k (i) 473 K

2.0k

(ii) 423 K (iii) 373 K

A

(iv) 323 K

1.0k

(v) 298 K 500.0 (ii) 0.0

1.50

(viii)

(iii)

(vi) 273 K

(i)

1.45

500.0

(vii) 223 K

1.40

(ix)

A/B

Magnetic field (mT)

1.5k

–1.0k

(vii)

1.30

–1.5k

B

1.35

(ix) 125 K

(vi)

1.25 1.20

–2.0k 325

(viii) 173 K

(v)

150 200 250 300 350 400 450 Temperature (m K)

326

327

(iv) 328

329

dY/dH Fig. 7.22 ESR dispersion derivatives (dY/dH) as a function of the magnetic field for GNCs over the measured temperature range (i)–(ix) 473–125 K. The spectra show unsymmetrical Dysonian absorption derivative line-shape on low field side. The arrows show the magnitude of A and B. The inset shows variations in the Dysonian lineshape ratio as a function of temperature.

existed in GNCs. Nonetheless, the π-electron delocalization is sufficient to offer desired spin degrees of freedom to the spin bath of GNCs. Understanding spin transport in GNCs may provide clues to design a spin bath with tunable spin degrees of freedom; this is essential for solid state quantum bits and for future quantum computational devices. The first derivative of the paramagnetic absorption signal was recorded for the GNCs. Fig. 7.22 shows the room-temperature ESR spectra for GNCs. The first absorption derivative, dY/dH, as a function of the applied field shows a symmetric absorption peak. The inset shows the Dysonian ratio (A/B) computed for Δ Hpp over the measured temperature range.

7.4.4.1

Linewidth (Δ Hpp) analysis

The Δ Hpp has an azimuth angular dependence (θ), that is, the orientation of the applied field with respect to the orientation of the carbon planes of the samples. Because measurements were conducted on the bulk powder specimen, then: Δ Hpp(θ) ¼ sin2(θ)Hk + cos2(θ)H?. Thus, the measured Δ Hpp consists of contribution

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from parallel as well as perpendicular field components. The obtained and estimated values of the transport parameters are almost constant over the measured temperature range. Hence, in subsequent discussions, the temperature-averaged magnitude of measured and estimated physical quantities is quoted (with SD) using the symbol T. The value of T is 0.5693  0.03275 mT for GNCs. To describe the width of ESR lines, there are two common shapes: Gaussian and Lorentzian. The spectra are found to be asymmetric for GNCs and mixed Lorentzian-Gaussian. The magnitude of obtained T indicates that there are more than two components of spin-lattice relaxation time (Tsl) involved in one overall line. The strength of Tsl is different and reflected in the asymmetric line shape. The asymmetry arises due to charge inhomogeneities, so-called puddles, due to weak delocalization of π-states from bound π-electrons. The charge inhomogeneities over the volume of the sample exceed the natural line width, (1/γTsl), where γ is the gyromagnetic ratio. These puddles prevent the relaxing electron from reaching the Dirac point, and they have the average minimal charge density as 109 cm–2. The spins associated with these charge inhomogeneities, in various parts of the sample, find themselves in a different field strength, and the resonance is narrowed in an inhomogeneous manner. The variations in A/B with temperature indicate that the conductivity of the samples varies with temperature [127].

7.4.4.2 Anisotropy in g factor Another prominent component is the magnitude of the g-factor, at which the resonance has occurred under applied microwave frequency. The g-factor characterizes the magnetic moment and the gyromagnetic ratio associated with unpaired electrons in the material. If the angular momentum of a system is solely due to spin angular momentum, the tensor g-factor should be isotropic with the value 2.00232. Any deviation from this value involves contributions of: (i) orbital moment from the excited state and (ii) spin-spin interaction. However, the orbital moments interact strongly with the crystalline fields and become decoupled from the spin, a process called quenching. The more incomplete the quenching, the farther the g-factor from the free electron value and results into the effective g-factor. The magnitude of the effective g-factor is estimated for the GNCs. The effective g-factor is observed to be less than 2.00232. The difference Δg for the system is computed. The value of T is found to be 0.00453  4.73  10–5 for GNCs. For GNCs, there are 6–7 oxygen atoms per 100 carbon atoms. Even though the isoelectronic carbon contains 6–7 at% oxygen, the anisotropy in the g-factor is too small. One can see that the values of Δg are unchanged up to the third decimal place, that is,  0.004. The variations have been observed from the third decimal place onward. Thus, data indicate slightly anisotropic magnitudes of g-factors of the measured GNC system. Thus, the spin originating from the oxygen impurity has a weak contribution to the spin bath of the carbon-inherited spin species [104]. The values also indicate that the local magnetic environment in the carbon system is distinctly different than that existing in a conventional magnetic material. Moreover, it gives a clue that the species responsible for the intense line is carbon. Thus, the contribution of orbital moments and spin-spin interactions from

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oxygen species could be neglected and the overall strength of this component seems to be less in GNCs. To evaluate the contribution of each component, we have analyzed spin-spin and spin-orbit interactions, as discussed later:

7.4.4.3

Spin transport parameters: Spin-spin and spin-lattice relaxation, spin-orbit coupling

To estimate them, the magnitude of Δ Hpp bares important information about the spin dynamics of the system, specifically Tss, which corresponds to electron spin-spin relaxation time. Due to external perturbations, the deformed spin system regains the state of equilibrium “up” or “down” over the characteristic time scale and is termed as the spin-spin relaxation time. The entity Δ Hpp and Tss are correlated by the equation Δ Hpp ¼ γ :T1 ss , where γ e has magnitude 1.760859  1011/s T for electrons. e The variations in Tss, as a function of temperature, for GNCs are shown in Fig. 7.23. The magnitude of Tss, at a temperature window of 300–375 K is found to be greater than 11.01  0.60 ps. As one moves away from this window, Tss decreases by one order of magnitude. It is interesting to note that Tss varies where T is temperature (Table 7.6). The principal parameter governing spintronic usability is spin-lattice relaxation time (Tsl), which quantifies the variation of the nonthermal spin state around the lattice. For spintronic applications, the theoretical estimate of Tsl for graphene is 10–100 ns [128], whereas the experimental spin-transport measurements showed that it is as short as 60–150 s [129]. The magnitude of Tsl is computed using a variation in Tsl as a function of temperature for the measured carbon systems is shown in the inset 11.5

30

11.0

20

Tss (in ps)

15

Tsl (in ns)

25

10

10.5

Temperature (K) 5 100 150 200 250 300 350 400 450 500

10.0

9.5 100

150

200

250 300 350 Temperature (K)

400

450

500

Fig. 7.23 Variations in spin-spin relaxation time (Tss) as a function of temperature for GNCs. Arrows indicate change in Tss as temperature of spin bath moves away from room temperature window of 75 K. Inset shows change in spin-lattice relaxation time (Tsl) with temperature.

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Table 7.6 Variation in spin-orbit coupling constant (Li) over measured temperature range Temperature (in K)

Li (in meV)

123 173 223 273 298 323 373 423 473 T

18.68 18.96 18.80 18.19 18.59 18.07 18.10 18.07 18.55 18.49  0.32

of Fig. 7.23. For GNCs, the magnitude of < Tsl > is 18.75  6.99 ns. However, one can see that, over the measured temperature range, Tsl varies by fivefold, that is, from 5 to 25 ns, and the change is almost linear. Basically, the electron spin relaxes by transferring energy selectively to those lattice modes with which they resonate. The resonant modes are on talking terms with the spins. And one can modify their cross-talk by introducing breaks in the symmetry inversion (i.e., disorder) or an adatom in the sp2 carbon network. However, over the temperature, the resonant levels seem to be somewhat broadened for GNCs. In principle, the orbital and the spin angular moment have been considered separately; it is important to know the extent to which these are coupled. As a first approximation, the two may be considered independently, later introducing a small correction to account for the so-called spin-orbit (SO) interaction. The pure, radical-free, carbon system has essentially zero orbital angular momentum; the SO interaction is usually very small for such systems. Hence, for most purposes, attention may be focused wholly upon the spin angular momentum. However, SO interaction must necessarily be included in a discussion of the ESR behavior, as presented later. SO interaction is one of the most prominent modes of spin relaxation. There are three principal sources of SO coupling in graphene: intrinsic, Bychkov-Rashba (BR, related to structural symmetry break), and ripples (related to inevitable wrinkles/folding edges). However, it is not possible to estimate the contribution of each component experimentally. Theoretical estimates for intrinsic SO coupling range from 1 μeV–0.2 meV and BR 10–36 μeV/V/nm. Until recently, it has been reported that curved carbon surfaces could have zero-spin splitting with SO coupling up to 3.4 meV [87]. The quantification of SO interaction could   be done using the correlation Δ g ¼ α LΔi , where α is the band structure dependent constant  1, and Li is the spin-orbit coupling constant (SOCC). From the estimated value of Δ g (in Table 7.5) and obtaining the magnitude of π-π* bandwidth, Δ, one can estimate Li. A correlation of Δ Hpp and optical bandwidth (Δ) has already been noted. The values of Δ for GNCs are measured to be 3.380 eV. The existing knowledge about the SO interaction in carbon systems is not yet complete [89]. From the

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Table 7.5 Estimated Δg for GNCs Temperature (in K)

Δg

123 173 223 273 298 323 373 423 473 T

0.00457 0.00458 0.00459 0.00449 0.00453 0.00447 0.00452 0.00464 0.00449 0.00453 4.73  10–5

The Δg is difference between g-factor for nondegenerated Pauli gas to effective g-factor obtained for the samples at the resonance field. The last row T indicates the temperature averaged value of Δg.

perspectives of spintronic applications, it is important to understand the role of electronic spin in an sp2 + sp3 network bonded environment of disordered GNCs. The value of < Li > T is 18.49  0.32 meV, for GNCs. The ESCA analysis showed that the at.% of native oxygen is 6.75%. However, the orbital angular momentum contribution seems to be fully quenched from the spin bath of GNCs. The magnitude of Li is found to be three orders larger than reported previously [89]. The GNCs are a disordered hetero structure sp2 + sp3 network, in contrast to ordered sp2 graphene carbon. The observed variations could also possibly be attributed to different spin and orbital angular momentum contributions of carbon in ordered and disordered sp2 honeycomb networks, leading to entirely different types of exchange interactions [119]. Thus, the analysis of spin-spin (Tss), and spin-orbit (Li) together with spin-lattice interactions (Tsl) indicates that spin degrees of freedom seem to be somewhat enhanced in GNCs. For realistic applications, a spin bath with tunable spin degrees of freedom is advantageous to manipulate, flip, and toggle the spin density wave. Thus, the SO interaction that couples electronic states with opposite spin projections in different bands typically identified as spin-up and spin-down [89] seems to be stronger in GNCs. In summary, spin bath parameters were analyzed for GNCs over 125–475 K using the X-band ESR technique. The analysis of spin-spin and spin-orbit coupling together with spin-lattice interactions indicated that spin degrees of freedom seem to be somewhat enhanced in GNCs. For realistic applications, spin bath with tunable spin degrees of freedom is advantageous to manipulate, flip, and toggle the spin density wave without decoherence. The radical spin associated with free electrons in nanocarbon is useful to absorb microwave radiation for electromagnetic shielding applications. In subsequent sections, another interesting application aspect of GNCs has been revealed as an electromagnetic radiation absorber [130].

Nanocarbons: Preparation, assessments, and applications

7.5

215

Electromagnetic interference shielding: Countermeasures against radar seekers in X-band

It is of great tactical importance to handle the signature of an object at a short-range tracking (8 12 GHz, X-band) threat from a seeker projectile [131]. Such a tracking signature involved combination of parameters like physical optics, geometry, and, importantly, reflection abilities of any target; quantified in terms of the shielding effectiveness (SE) and, popularly, known as the radar cross-section. Broadly speaking, it is a ratio (measured in decibels) that accounts for the reflected-to-incident power of the radiation from an object and depends on incident signal wavelength (λ) to target dimension (d) correlation. The condition d < λ tends to Rayleigh scattering, d > λ yields diffraction/specular scattering, and d  λ represent Mie scattering [132]. Due to the arbitrary shape and size of the hostile target, it is challenging to control individual components selectively and tunably. In order to enhance the transmission loss for incident signals, a surface coating with specific constitutive parameters (permittivity and permeability) could be a viable option to attenuate the electric/magnetic field at its interface with a minimum amount of reflection. The incident energy then can be mostly spent in absorption within the skin surface, causing a field/surface interaction in terms of motion of the mobile charge carriers and polarization. Metallic encasings were preferred before [133] in such applications. However, there were several reasons to depart from the metallic option, including the expensive cost, high density, tendency for corrosion, and, recently, the fact that polymer-based nanocarbon composites are being considered as a strong alternative.

7.5.1

Shielding parameters of GNC/polyurethane nanocomposites

For hybrid coatings, SE depends specifically on cooperative interactions between the filler and host matrix. Dimensions, charge mobility, and structural robustness are particularly essential for fillers. For a polymer host, the important properties are conductivity, matrix density, design flexibility, and environmental compatibility. Due to their premium properties, nanocarbon-like CNTs and graphene in polymer as nanocomposites were reported to be effective shielding constituents in the 8–12 GHz regime [134–138]. For multiwalled CNTs incorporated in an ethylene tertiary polymer [139], PMMA [140], and PS (polystyrene) [141] , the SE was reported, respectively, as 28 (3.2), 40 (10), and 66 dB (20). Bracketed numbers indicate filler loading with maximum weight with a shield thickness of the 2  2.5 mm range. The values of SE were reported to be 20 (7) and 85 dB (0.5) with thickness  2 mm, respectively, for PS [142] and carbon foam [143] incorporated with CNTs. For PANI [144], PU [145], and epoxy [146] dispersed with single-walled nanotubes, the values of SEs were, respectively, found to be 30 (25), 17 (20), and 30 dB (15) with a variable range of shielding thickness from 0.5–1.0 cm. Moreover, graphene-based nanocomposite shielding has been the focus of several recent investigations [147]. Graphene/PANI [144], graphene/nitrile-rubber [148], and graphene/epoxy [149] showed SE values, respectively, of 35 (33), 57 (10), and 21 dB (8.8) with thickness  5 mm

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of the shield. Wherein graphene incorporated in PVDF [150], PMDS [151], and PMMA [152] foam showed a magnitude of SE, respectively, of 20 (5), 20 (1.8), and 33.3dB (0.8). Literature also exists on incorporating other types of nanocarbon in PANI [153], PP [154], and acrylonitrile [140] with reported SE 20–50 dB range at different shielding thicknesses. In this work, a typical interface polarization mechanism is recognized between GNCs (1 25 wt%) and PU to mitigate the shielding parameters for PU in the 8–12 GHz radar region. The scattering study is presented in light of interface bonding at GNCs/PU, GNCs dispersibility, and morphology/rheology of nanocomposites, using vibration spectroscopy. The microwave measurements were performed on toroidal-shaped samples to determine the permittivity of the real and imaginary part, the alternating current (ac) transport, skin depth, transmission coefficient, S21, and shielding efficiency. The details of the polarization mechanism are discussed. Broadly, the required PU thickness for about  40% loss in S21 is more than a centimeter whereas almost 99.9% loss is recorded for a millimeter-thick PU at 25 wt% loading of GNCs. An injection molding technique has been adopted for the preparation of nanocomposite samples using a cylindrical die. In brief, the methodology followed is sealing and simultaneous heat-pressing of the composite slurry at 120°C and loaded  100 kg/cm2, adiabatically, in the hydraulic press, as shown in Fig 7.24.

Fig. 7.24 (Upper panel) a typical production batch (A) initial phase of the mixture (liquid state), (B) gel-type paste, and (C) thick paste, in the mortar and pestle (mixing and grinding  3 h to obtain paste). (Lower panel, D) fabricated toroidal shaped specimen (dimensions: i.d., 3 mm; o.d., 7 mm; and height, 6 mm; postproduction edge polishing).

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7.5.1.1 Characterizations and measurements FTIR and Raman Spectroscopy Chemical bonding. Characteristic FTIR and respective Raman fingerprints recorded for PU, GNCs, and 1–25 wt% GNC/PU nanocomposite samples are shown in plots A  D of Fig. 7.25. Fig. 7.25 indicate band assignments for PU, revealing characteristic inelastic vibration modes (in cm1) for urethane amide (CdN) at 1184 and amide III at 1271, δ(CH) amide III at 1315, and δ(CH2) νsym for NdCdO at 1441. The corresponding Rayleigh active amide band is broad and present at 1196. Amide III and δ(CH) are completely merged at 1296. The emerging feature at 1515 is assigned to ν(Ar) of amide II ν(CdN) + δ(NdH) at 1615 to ν(Ar) and at 1657 to ν(Ar) amide I ν(CdO). Between 3000 and 2700, the modes appeared are attributed to the CdH (sp3 str) band, regardless of their nature to the rest of the PU macromolecule. Fig. 7.25B is a typical band at 1088 cm1 indicating an epoxy presence (CdO str) group in virgin GNCs. At  2925, 1628, and 1300 are the bands allotted to CdH (sp3 str), C]C, and CdC stretching vibrations for GNCs, respectively [155,156]. The resonant modes associated with GNCs are assigned for D-peak (in split form), G-band, and a broad 2D-peak; details in Ref. [117]. Fig. 7.25C and D are recorded spectra for 1 and 25 wt% GNC/PU nanocomposite systems, respectively. They displayed significant alterations to their parent counterparts. In plot (C), the emergence of a wide peak, at the 1150–1400 interval has merged all active inelastic modes of GNCs and PU. The band ν(CdN) + δ(NdH) is existent at 1511 for 1 wt% but terminated, thereafter, with successive GNC loading. The modes related to ν(Ar), the amide backbone of PU, are invariant. The Raman active CdH str mode is changed markedly for PU, with 1 wt% GNC presence. A corresponding high intensity for GNCs compared with PU is indicative of hydrosorption site availability in GNCs. For the transmittance loss of the IR bands, the characteristic trend observed is inconsistent with the change observed for Raman active modes in all samples, mostly. Broadly speaking, with GNC incorporation in PU, any change in the band associated amide backbone, that is, ν(Ar), is peripheral. For these peaks, neither intensity nor vibration frequency is observed to be varied, indicating that the single-bonded backbone of the polymer matrix mostly remains intact even though GNCs are added. The summary of observations are: (a) for nonplanar, hydrogen-bonded amide III δ(CH) peaks at 1315 and 1500cm1 (dNdHd), there is a decrease in Raman peak intensity, (b) doubly bonded oxygen (C]O @1700 cm1), δ(CH2) νsym N]C]O at 1441cm1, nitrogen (C]Nd at 1250 cm1) is modified, and (c) sp3 CdH str Raman active modes at 3000 cm1 are vanishing. Hydrogen bonding in the host matrix is modified heavily by GNCs with a change in doubly bonded moieties. Among all, the groups dNdHd, N]C]O, and dC]Nd are nitrogen-based moieties having the electrondonating capacity, contributing to GNC bonding via hydrosorption. Due to π-conjugation, in this the double-bonded sites are more reactive, and the effect is dominant at these sites, probably. The nature of hydrogen is, in general, tricky because it is neither electronegative nor positive. The intermediate conclusion is the nature of the interaction is lying at double-bonded sites, dominantly, with hydrosorption in origin.

Fig. 7.25 Typical FTIR and Raman spectra (excitation wavelength: 785 nm) recorded for the systems. (A) PU, (B) GNCs, (C) 1 wt%, and (D) 25 wt% GNC/PU nanocomposite samples. For PU the peak indexing is (i) Urethane amide (Raman active), CdN (IR), (ii) Urethane amide III (both Raman and IR), (iii) δ(CH) urethane amide III (IR), (iv) δ(CH) ν N C O (Raman), (v) ν(Ar)-urethane amide II ν(CdN) + δ(NdH) (IR), (vi) ν(Ar) (both IR and Raman), and (vii) ν(Ar) urethane amide I ν(C O) (both IR and Raman). The peak positions are indicated in the text.

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Direct current conductivity The transport properties of PU and GNC/PU composite samples were determined by a prototype two-probe method in coaxial configuration using a standard Keithley 6487 picoammeter/voltage source equipped with the data acquisition software. Recorded variations in dc conductivity (σ dc) with respect to GNC weight fraction ( p) are shown in Fig 7.26. Below the weight percent threshold of  5.0, the conductivity showed a dramatic decrease of  7 orders of magnitude, indicative of the fact that above this threshold, the percolating network is formed in the PU matrix. Due to an increase in the number of hydrosorpted conducting sites, the observed variations are seen. The power law shown in the inset of Fig. 7.26 is broadly obeyed by the electrical conductivity [157]. σ α ðP  Pc Þγ

(7.2)

where σ is conductivity of composite, p, GNCs wt. fraction, pc, percolation threshold, γ, critical exponent. Herein, we used weight fraction values of GNCs instead of 10–5 10–6 10–7

–5.2 –5.6

–9

10

log (s)

Conductivity (S/m)

10–8

–10

10

–6.0 –6.4

10–11 –6.8 10–12 –7.2 10–13

–0.2

0.0

0.2

0.4

0.6

0.8

1.0

log ((p–pc)/pc)

10–14 0

5

10

15

20

25

GNCs (wt%)

Fig. 7.26 Variations in dc conductivity (σ dc) (in logarithmic scale) as a function of weight fraction ( p) of GNCs in the PU matrix. Measurements were performed using a standard two-probe technique at room temperature. Inset: log-log profile for σ dc vs log((p  pc)/pc). The straight line in the inset is fitted using least-squares methods for the obtained data using Eq. (7.1) returning the best fit values pc  5.0 wt% and γ ¼ 1.69 (correlation factor, 0.02).

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Nanocarbon and its Composites

volume fraction, as the density of GNCs is approximately estimated. The GNC/PU nanocomposite conductivity agrees well with the percolation behavior for log(σ) with log((p  pc)/pc), predicted by Eq. (7.2). An excellent fit is obtained for the data having a correlation factor of 0.02, with a profile as a straight line and pc  3.0 wt% and γ ¼ 1.69. Relatively at the lower side, that is, 3.0 wt% GNCs, the percolation threshold is seen, attributing to efficient GNC dispersibility into the host matrix. In the reported literature, theoretical values for γ in case of a three-dimensional (3D) percolating network are observed to be varied from 1.6–2.0 [158] while for carbonaceous composites, the exponent values are, experimentally, reported to be varied from 0.7 to 3.1 [159–161]. In the subsequent section, the analysis of X-band measurements has been presented, which resembles the discussions presented above.

Morphological studies FESEM (Zeiss ΣIGMA) was used to study the surface morphology of PU, and GNC/ PU nanocomposites were investigated at beam voltage 5 kV. Using the cryofractured technique, the samples were prepared in which one PU sample was immersed at a time into the liquid nitrogen bath for about 5 min and allowed to reach the sample nearing liquid nitrogen temperature. The sample was subsequently taken out and instantly broken. GNC/PU nanocomposite samples were fractured in similar fashion. Prior to FESEM imaging, the sample surface was subjected to gold coating, and the process was carried out using a standard sputtering technique. The surface morphology of the cryofractured samples (left panel) and magnified region (right panel) are shown typically in Fig. 7.27. As compared to the morphology of GNC-incorporated PU, the surface morphology of PU is found to be distinctly different. The uncorrugated zones in PU are observed as well as the microvoids that inherently exist in the host matrix. The synthesis route of PU has the origin of such microvoids, which were formed by onset hot press curing of the samples. Gaseous species responsible for out-diffusion during the curing could be responsible to form these microvoids with random size distribution. They are mostly spherical in shape and a few were found to be elliptically shaped, coupled to each other and thought to be formed due to the local fugacity and rheology of the polymer matrix. The homogeneous morphology of a typically magnified position of the host matrix is seen in Fig. 7.27E and surrounded by the microvoids in Fig. 7.27A. The morphological change associated with low weight % GNCs, such as 1, of the homogeneous portion of the matrix is seen in Fig. 7.27B and F.

Microwave measurements Using a vector network analyzer (VNA, Agilent-E8364B) equipped with the coaxial transmission waveguide (HP broad frequency range coaxial 7 mm airline (8505160007)) in the frequency range of 8.2 12.4 GHz, microwave measurements on PU and GNC/PU samples were carried out. A schematic sketch of the distinctive electromagnetic shielding measurements setup is shown in Fig. 7.28. VNA was started about 2 h earlier to the measurements for stabilizing the microwave source. Errors due to isolation, directivity, load match, and source match, etc., are minimized by full two-port calibration of the VNA performed on the test specimen along

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Fig. 7.27 Typical FESEM micrographs recorded for (A) PU, (B) 1 wt%, (C) 10 wt%, and (D) 25 wt% GNCs incorporated in PU. The rectangle coupled to the arrow indicates the respective magnified regions showed in images (E)(H).

with the calibration, performed in both forward and reverse directions. From the measured scattering parameters, the complex permittivity (ε0  jε00 ) and S21 parameters for composites were determined by standard Agilent software module 85071, based on the procedure given in the HP product note [162].

Toroidal shape sample preparation The stipulations of the waveguide were the toroidal-shaped samples with inner diameter (i.d.) 3 mm and outer diameter (o.d.) 7 mm for X-band measurements. A die assembly has been designed and developed in order to fabricate such samples. The details of (a) design and fabrication of the die, (b) preparation of the nanocomposite

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VNA

Torroidal shape samples

Matched load

RF source

Incident plane wave

Transmitted plane wave

Fig. 7.28 Schematic representation of the electromagnetic scattering measurement setup comprised of the vector network analyzer, a RF source, matched load, and the sample under test in the coaxial transmission waveguide. The frequency range X-band, 8.2  12.4 GHz.

paste, (c) the adiabatic hot-press technique, and (d) cutting and edge polishing protocols have been provided elsewhere.

7.5.1.2

Analysis of microwave parameters

Analysis of Scattering Parameters: Real and Imaginary Parts of Permittivity For the attenuation of an incident electromagnetic wave, three mechanisms are basically responsible: (a) reflection, (b) absorption, and (c) multiple internal reflection losses at the interface due to conductive fillers or porosity of the materials. Analyzing permittivity response, we determined the absorption properties of the test specimens in the X-band. For real (ε0 ) and imaginary (ε00 ) counterparts of the permittivity function, plots (A) and (B) of Fig. 7.29 re the recorded frequency response spectra, respectively, for PU and GNC/PU nanocomposites with variable GNC wt.%. In both real and imaginary parts of the permittivity, a monotonic increase has been observed over the frequency zone with the increase in GNC content.

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Fig. 7.29 Recorded complex microwave scattering data over the measured frequency regime: (A) real (ε0 ) and (B) imaginary (ε00 ) parts of the permittivity spectra for PU and GNC/PU nanocomposites. The numbers to the right side in each profile indicate the weight percent of the GNCs.

For the real part of permittivity, the response is nearly frequency-independent. The flat response showed that the PU as well as the nanocomposite is a class of non-Debye solid [163]. Broadly, the frequency dependence of permittivity comes from the polarization mechanism via the Clausius-Mossotti relation. However, small percentage variations in the real part of the permittivity (i.e., ε0 %) have been estimated for all samples. At actuals, differences are estimated over the full range frequency. The ε0 %  1.03, for PU, whereas for 5 wt%,  1.11, resembling closely that of PU. The ε0 % is found to be saturated to  16 with an increase in the loading of GNCs from 5 to 15 wt%. The percentage variation is negative, 3.17 thereafter, and 15.00, respectively, for 20 and 25wt% GNC/PU nanocomposites. With rapid saturation in both directions, the flipping of magnitude of ε0 is indicative of the flip in the operative dipole moment. Gradual trendy variations in frequency have been observed and measured for the imaginary part of permittivity. A small variation is observed at higher loadings, for, for example, 10, 15, and 25 wt% samples. Over the frequency regime, both real and imaginary parts of the permittivity are found to be varied marginally. The comparison on a relative scale has been made across the categories of the samples by taking an average at log-normal scale (indicated in Fig. 7.28). At 25 wt%, the value of the real part of permittivity is 9.31  0.03. PU offers low (real) permittivity 1.84  0.01 in contrast with an increase  5 . The imaginary part of permittivity is 1.29  0.03 (for 25 wt%), which indicates an increase by a factor of  30 with respect to the base value of PU (0.043  0.006). Due to an increase in ac conductivity by enhancing the active modes of charge transfer polarization via GNCs in PU, on a relative platform, increases in both the real and imaginary parts of the permittivity are observed. Loss Tangent, Alternating Current Conductivity, and Skin Thickness. Using the relation tan δe ¼ ε00 /ε0 , the electrical losses inside the nanocomposite material under

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Table 7.7 Magnitudes of measured loss tangent, tan δe, and alternating current conductivity, σ ac, for PU and different weight percent GNC/PU nanocomposites under testinga and CdNattached to GNCs Samples

1 5 10 15 20 25

PU wt% wt% wt% wt% wt% wt%

tan δ (rad)

σ ac (S/m)

0.024 0.003 0.039 0.005 0.061 0.005 0.081 0.011 0.110 0.002 0.120 0.007 0.140 0.003

0.248  0.135 0.511  0.255 1.261  0.515 2.894  1.548 4.801  1.215 5.706  2.076 7.288  2.740

The effect seems to be dominant at double-bonded GNCs sites, due to π-conjugation with the host matrix. a The σ ac is increased linearly by a factor of 30 with subsequent GNCs incorporation up to 25 wt%.

the test condition are quantified by calculated loss tangent (tan δe). In Table 7.7, the magnitudes of the average tan δe (rad) measured for the systems are provided. The loss component for PU is 0.024  0.003. For the 25 wt% value, it is recorded to be 0.140  0.003. The tan δe is increased by six times. Basically, tan δe is the quadrature part of the polarizability vector component in line with the incident electromagnetic field. Greater field losses are indicative of an increase in field arrest inside the nanocomposite material. So, after incorporating GNCs in the host matrix, the field losses seem to be increased, marginally. Using, σ ac ¼ 2πfε0ε00 , relation between the ε00 parameter and the ac conductivity (σ ac) of a dielectric material could be evaluated, where, σ ac is measured in Siemens per meter, ε0 is the free space permittivity (8.854  1012 F/m), and f is the applied frequency in hertz. For PU, the value of σ ac is 0.248  0.135, which is increased linearly by a factor of 30 times with sequential incorporation of GNCs until 25 wt%. As discussed, the observed increase is due to donor-loaded nitrogen sites such as dNdHd, N]C]O. The incident EM field interacts with mobile charge carriers in the host medium to generate charge transfer displacement current, quantified as the skin depth, which is a thickness parameter (δ). This effect is responsible for coupling incident field wiggles with charge carriers and a decisively set thickness of the shielding nanocomposite. The σ ac is related to the magnitude of δ, given by: δ ¼ (1/(πfμ0μrσ ac)1/2), in which μ0 is the free space permeability (4π  107 H/m) and μr is the relative permeability for GNC/PU nanocomposites,  1. Moreover, the skin thickness estimates the distance over which the field intensity decreases to 1/e of its original value (0.3678 mW) under the condition σ ac ≫ 2πfε0ε0 . In Fig. 7.30, the estimated values of skin depth in terms of thickness as a function of GNC weight percent are shown, indicating that the magnitude of the skin thickness, δ, for PU 10.44  2.1 mm, whereas 5.91  1.09 mm is for the 3 wt% GNC. The estimated values showed 40% reduction in skin thickness of the sample. With a subsequent increase in GNC weight percentage in PU (25wt% GNC), the thickness of shielding is reduced to 1.88  0.25 mm. Reduction in the skin thickness is indicative of absorption of microwave power, generating hindrance to free charge carrier propagation. The higher attenuation microwave power shows the ability

225

10

2.0

8

1.5

6 1.0 4

Average (e²)

Average (e¢)

Nanocarbons: Preparation, assessments, and applications

0.5 2 0.0 0

5

10

15

20

25

GNCs (wt%)

Fig. 7.30 Estimated magnitudes of real ε0 and imaginary ε00 parts of the permittivity (average at logarithmic-normal scale) as a function of GNC weight percent.

to block the charge carrier across the shielding thickness compounded with efficient coupling between field wiggles to the matrix. The amount of microwave absorption has been quantified from the scattering data, given in a subsequent section.

7.5.1.3 Efficient microwave absorbing properties The question to be asked is how much has the level of an incident power (or power flux density) decreased after passing the specimen under test? The mode of measurement is typical transmission measurements (scalar S21 measurements) and obtained decibel value can provide the answer. In Table 7.8, the calculation of percentage values is presented to their power relationship. The penetrating power is reduced down to 1%, at  20 dB shielding. To calculate the dB value, the following equation is used to compute SE: SE PT ¼ 10 log PI dB

(7.3)

where incident power, PI, and transmitted power, PT. The S21 are computed as shown in Fig. 7.31A, as having a flat response consistent with the variations in permittivity. The S11 and S12 data are not presented. Moreover, S21 alone is insufficient for claiming total shielding effectiveness (SETO), but uniform spread over frequency is indicative of operative mechanism; as explained earlier. The plot of Fig. 7.31B, is the SE due to transmission (SET), measured using Eq. (7.3) for the estimated effective transmittance loss. For PU, the SET is highest and gradually decreases with filler wt%. The magnitude of SET for PU is 2.34 dB

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Nanocarbon and its Composites

Table 7.8 Relationship between shielding effectiveness (se) and power transmission (%) SE (dB)

Power transmission (%)

SE (dB)

Power transmission (%)

0 1 2 3 4 5 6 7 8 9 10 11

100 81 62.80 50 40 31.60 25 20 16 12.50 10 7.90

12 13 14 15 16 17 18 19 20 25 30

6.25 5.00 4.00 3.13 2.50 2.00 1.56 1.20 1.00 0.316 0.1

0 0 –4

PU 1 5

–5

10

–10 SET (dB)

S21 (dB)

–8 15

–12

–20

20

–16

–15

–25

–20 25

–30 8

(A)

9

10

11

Frequency (GHz)

12

13

0

(B)

5

10

15

20

25

GNCs (wt%)

Fig. 7.31 (A) Variation in scalar S21 parameters measured in dB unit. Response is plotted over 8.2  12.4 GHz for PU and variable weight percent of GNCs in composite. (B) Computed shielding effectiveness due to transmission loss, SET.

and found to be 10.76 dB on the percolation threshold and progressively, the value is enhanced to 26.45 dB at 25 wt% GNC/PU composites, maintaining an identical trend as that of S21. So, we have plotted the average change showing the magnitude of SET is about 12 times less than 25 wt%. Interestingly, the percent variation in transmission loss has been estimated. The amount of transmission loss is dictated by the extent of microwave propagation in a medium. In the following section, discussions on the amount of microwave propagation with percent transmittivity have been presented. For a polymeric medium, the % transmittivity values obtained provide an idea about electromagnetic behavior in a composite material under test at low microwave electric conductivity. From Fig. 7.32, one can see that  40% transmission loss

Nanocarbons: Preparation, assessments, and applications

227

100 12 90 10 80 8 70 6 60 4 50

Transmission loss (in %)

Thickness of nanocomposite (in mm)

14

2 40 0 0

5

15 10 GNCs (wt%)

20

25

Fig. 7.32 Variation in thickness measured (mm) and transmission loss (%) as a function of weight percent of GNCs in PU. Measurements on base PU are also indicated for comparison.

requires a thickness >10 mm for virgin PU, whereas GNC addition in it improves the magnitude of the transmission loss. The thickness of the composite is less than 5 mm, typically, at optimum percolation threshold  5 wt% and with transmittivity addition of GNCs like 25 wt%, almost 99% transmittivity is lost at a lesser thickness value  1 2 mm, indicative that transmittivity addition of GNCs is beneficial because of inherent functionality generating interfacial interactions achieving major loss in transmittivity at low thicknesses, relatively. The embedment of uncorrugated zones in PU with the GNC flakes is seen, including microvoids in Fig. 7.33 in which the amount of filler seems to be insufficient at this concentration level to be distributed uniformly. The coverage of GNCs on microvoids gets denser as their weight percent in PU is increased, subsequently. Interface Polarization Mechanism: Based on the analysis of microwave scattering parameters, the interface polarization mechanism is proposed, showing the amount of dipolar/orientational (OP, radical), atomic (AP), and number of electron donor (EP) constituent moieties available. They are almost identical in quantity and charge accumulation-based polarization remains constant. This leads to a nearly constant real permittivity response for frequency range for any sample. Three shallow relaxation peaks at  12 (EP),  10 (AP), and  9 (OP) GHz are seen for PU in which OP is fading by loading GNCs. This behavior is consistent with vibration spectroscopy discussions presented before. Saturation of radical sites via GNC hydrosorption is seen. For EP (12 GHz), at a low concentration of GNCs, up to  5 wt%, the peak disappears. This region of loading is, importantly, estimated as the percolation threshold, as discussed before in describing σ dc, with efficient dispersion. Following this, the loss peak emergence at high

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Nanocarbon and its Composites

Fig. 7.33 Scheme of interface polarization and variable contribution of polarization modes in PU and GNC/PU nanocomposites. AP, atomic polarization (urethane amide III δ(CH), at  1315 cm1, dNdHd at 1500 cm1); EP, electronic polarization indicating doubly bonded oxygen (C]O, dC]N]Od) and nitrogen (dC]Nd) sites; OP, radical polarization associated with stereo regular -GNCs-H mode.

concentration of GNCs is indicative of no dissimilar amount of EP related to doubly bonded oxygen (C]O, dC]N]Od) and nitrogen (dC]Nd) sites, whereas AP (at 10 GHz) with PU is due to segmental motion. The generation of a variable strength conducting path (shown by small dashed arrows) possibly couple the incident EM field wiggles to the matrix. The arrows at the bottom indicate incident radiation whereas the broken arrows at right (brown) indicate power transmittivity, S21. The computed transmission loss in percent is represented by a number. Image (A) shows (microvoids) hollow spheres in PU and (B) and (C) low and high wt% GNCs in PU, respectively, showing full GNC coverage in voids. The conducting path strength is high, leading to the dissipation of field wiggles, effectively, at higher GNC loading urethane amide III δ(CH) (at  1315 cm1) and dNdHd (1500 cm1), acts as a backbone and remains invariant even after GNC loading. A corresponding interface polarization scheme is shown in Fig. 7.34. The associated relaxation time τr,OP  60 fs for -GNCs-H stereoregular radical modes whereas the host matrix segmental dynamics have τr,AP, magnitude,  40–50 fs. The τr,OP and τr,EP are absent at low weight fraction (up to  5 wt%). The absence is related to aggregates and their interface, having polarization within the periphery of aggregates. Below the percolation threshold, their absence is indicative of homogeneous GNC dispersion of GNCs in PU having relaxation, occurring via mostly AP, which resembles σ dc data analyzed. The value of σ dc is in the range of 1012  1013 S/m, typically, below the percolation threshold, with estimated activation energy  94 meV of charge carriers pertaining to AP. Beyond the threshold value, dipole and electronic contributions emerge having, respectively, activation energies  93 and 95 meV. A gradual increase of GNCs in PU provides a carrier transport path to the incident EM field to get coupled to the host matrix via generating polarization currents. as shown in Fig. 7.34, schematically. Image (A) in Fig. 7.34 shows PU matrix microvoids with penetration of the incident EM field (shown by an arrow at the bottom), having small transmittivity  40%. Whereas theaddition of GNCs at low percent has increased the loss, doubly, by  80%. Hence, it cannot provide an effective conducting

Nanocarbons: Preparation, assessments, and applications

229

Fig. 7.34 Schematic representation indicating the role of GNCs (hollow rectangles, B and C) in PU to enhance shielding effectiveness, whereas, (A) shows microwave interaction in absence of GNCs.

path (Fig. 7.34B). At higher weight percent, such as more than 20 wt%, the transmission loss almost reached 99.9% (Fig. 7.4C). Due to dissipation of the wiggling field into the surface, transmittivity is hindered severely by generating a polarization current via GNCs, consistent with the discussion presented earlier. The existence of channels like OP and EP is attributed to polarization related with homogeneous distribution of GNCs in PU and around the microstructure interface. In summary, the analysis of microwave data provided basic insight into the behavior of the EM field in heterogeneous ponderable media. The CdH, NdH, and CdO nonplanar bonds in PU, decisively, played a pivotal role in modifying transport parameters, thereby getting hydrosorpted with sp3 CdH sites of GNCs. The percolation threshold (at  5 wt%) is in good agreement with the literature report [164]. It suggests superior dispersibility of GNCs in PU. In the host, the active modes of polarization are modified by GNCs, especially at the skin void. Loaded GNCs provide a transport path to EM wiggles by producing mobile charge carriers along amide III polymer segments near microvoids. The coupling occurs via various modes of polarization, resulting in dramatic enhancement in transmittivity loss. The loss improves, for PU, from  40 to 99.9% with a reduction in coating thickness from a centimeter scale down to millimeter, at 25 wt% GNCs. The GNC/PU nanocomposites are promising for building shielding patterns, especially at short-range projectile tracking. In another study, reduced graphene oxide (rGO) has been synthesized and studied for its molecular and spin interaction comparatively by doping tellurium [165]. In a subsequent section, their molecular spin interactions are presented.

7.6

Molecular and spin interactions: Tellurium and reduced graphene oxide

In our day today, the inherent spin moment of an electron plays an important role. In the field of spintronics, handling spin states is important for applications such as data storage, molecular sensing, and power applications. For such applications, graphene is

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Nanocarbon and its Composites

well known as a pertinent medium for spin transport [166], predominantly its properties such as weak spin orbit connection, coherent spin diffusion length, and absence of hyperfine interactions within the sp2 superamolecular network [167]. The utility of graphene in spin-based logic devices and circuits is attributed to slow spin lifetime, higher Fermi carrier velocity, and linear electronic dispersion [168]. From a spintronics perspective, spin moments tailored in a hexagonal crystal structure have a vital role. Such spin moments are generated via vacancy defects, the addition of light or heavy adatoms or molecules within the layers, and inherent edge states that are significant to tune the spin parameters [169]. In many reports, hydrogen doping in a graphene sublattice showed alteration in the spin flip cross-section, which leads to the strong enhancement in spin-orbit coupling [170]. In reports on CeH and CeF, the exchange interactions show frustrated spin and antiferromagnetically ordered molecular quantum spin-liquid [171]. In the case of graphene as a cobalt adatom, the AA and AB sublattice showed antiferromagnetic and spin-compensated magnetic couplings [172]. In similar reports for nitrophenyl and aniline-functionalized graphene, magnetic anisotropy [173] was shown. Further, in graphene molecular doped HNO3, NO2 and Cu2O revealed remarkable suppression in spin relaxation for the gated on/off voltage characteristics [174,175]. In another study using the ab-initio approach, the electronic band structure parameters of manganese, chromium, and vanadium substituted graphene were studied and compared for their exchange strengths with spatial separation [176]. A DFT study showed multiferroic BaMnO3 in graphene-induced spin splitting of Dirac cones that exhibited a quantum Hall effect and Rashba spin-orbit induced topological gap [177]. In general, a distinct spin property in graphene is due to the presence of a dopant or defect and the spin interactions are highly dependent on the nature of the vacancy or adatom. The molecular interactions at the spin level for rGO with tellurium (Te) have not garnered much attention yet. Te bonding among chalcogenides in its radical state and structure make it interesting [178] to study molecular spintonics within an rGO framework. This is the focus of the current and subsequent sections. At the electronic level, the integrated density of states, chemical potential, spin scattering rates, spin orbit coupling, spin splitting band width, and exchange spin interactions are modified due to alterations in both spinlattice and spin-spin by Te.

7.6.1

Synthesis of reduced graphene oxide and tellurium-rGO

Initially, GO is prepared using graphite powder (standard purity 99.99% Aldrich) using a modified Hummer’s process. The dried GO power is weighed and reduced with hydrazine hydrate [14,179] in an equal volume ratio. The prepared rGO and incorporation of Te metal were done in situ (1–11 w/w%). It is observed that subsequent enhancement in Te wt% (like 3, 5, 7, 9, 11) has no significant effect on the structure-property relationship of the composite system. It has been analyzed extensively by FTIR and Raman spectroscopy. Henceforth, the main focus is on the 1 wt% Te-rGO system.

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231

7.6.1.1 Chemical state analysis of Te in rGO using electron spectroscopy Fig. 7.35 shows the fitting of C-1s for rGo and Te-rGO, which were fitted into several components. Fig. 7.35A directs the presence of five functional components in carbon such as the CdC ring, CdN, C in CdO, carbonyl group, and C(O)O at binding energies 284.6, 285.9, 286.0, 287.8, and 289 eV, respectively. In Fig. 7.35B, the corresponding changes are shown. The decrease in values in intensity at 286.0 and 287.4 eV showed a reduction in oxygen functional groups present in rGO. The peak at 289 eV in rGO significantly disappears in Te-rGO, indicating substantial removal of the C(O)O group. The sharpening and increase in intensity of the CdC peak at 284.6 eV indicate recovery of sp2 carbon that was ruptured during the oxidation process. The ratio C-to-O is decreased for Te-rGO 2.36 from 6.06 for rGO. This indicates a reduction in oxygen proportion with an increase in sp2 content and suggests enhancement of π bond character in Te-rGO [180].

Fig. 7.35 Chemical analysis of C-1s for (A) rGO and (B) Te-rGO by electron spectroscopy.

232

7.6.1.2

Nanocarbon and its Composites

Bond molecular environment of Te in rGO: Vibration spectroscopy

Fig 7.36A shows the FTIR spectrum for rGO and Te-rGO. In the FTIR spectrum of rGO, residual oxygen-related functional groups are trapped within the conjugated layers within the surface or at the edges. In Te-rGO, almost all these groups are almost vanished. The CdH (in-plane bending modes) are observed at 664 and 1525 cm1. The bands at 2312 cm1 are attributed to C]C, which remains unchanged for both the systems. The bands associated to CdOdC (epoxides), C]O (carbonyl), and C] O (carboxylate), observed respectively at 1210, 1370, and1730 cm1, are wiped out absolutely in Te-rGO.

7.6.1.3

Raman analysis

Raman analysis provides useful information on the structure and details of molecular environments such as Fermi velocity of charge carriers, electron-phonon coupling, and dynamic force constants. Fig. 7.36 (A) FTIR and (B) Raman spectra (532 nm) for rGO and Te-rGO. Corresponding inset shows 2D deconvolutions for number of layers.

Nanocarbons: Preparation, assessments, and applications

233

Structure of rGO and Te-rGO: Disordered length, LD, Bond frustration/strain Fig.7.36B shows the recorded Raman spectra for rGO and Te-rGO at 532 nm photo excitation. The inset shows the corresponding 2D peak deconvolutions. For rGO, the D peak is at 1354.58 cm1 whereas the G peaks were observed at 1595.12 cm1. On Te addition, a slight shift in vibration modes was observed. The D peak has upshifted to 1359 cm1 while the G peak downshifted to 1590 cm1. The resultant shift indicates bond strain and frustration at both sites due to Te. This was further investigated using Raman dispersion studies. The deconvolutions for the 2D peak showed that the number of layers is reduced in Te-rGO. The magnitude of ID/IG for rGO was 1.11, which is increased to 1.41 in Te-rGO for 532 nm excitation. This is indicative of lowering defect concentration with an increase in the number of sp2 rings in Te-rGO [181]. Fig.7.37 shows Raman dispersive studies carried out by multiphoto excitation wavelength for samples of rGO and Te-rGO. Dispersion for the G-peak in both cases is almost upshifted while in the case of rGO, the D-peak was dispersed by 100 cm1 and a reduction in dispersion was observed by Te-rGO 60 cm1 over the photon energy. From this it is clear that the surface and edge of rGO have been modified by the presence of Te. In Fig. 7.38, the variations in IIDG ratio with excitation energy are shown for (a) rGO and (b)Te-rGO. In general, the ratio varies linearly with crystallite size, La, and nonlinearly by changes in photon energy. Due to the variable G-band Raman cross-section and respective bond polarization strength, the strain on bond length would vary nonlinearly with photon energy [32,182–184]. At 2.41 eV was found to be the threshold value below this photon energy that the non-sp2 fraction participated in the scattering process. Above this threshold, non-sp2 sites were ceased due to an enhancement in the overall sp2 content in Te-rGO. The was computed using: parameters like crystalline length La,   10 4 ID La ðnmÞ ¼ 2:4  10 λL , where λL, laser excitation (514 nm). Further, La is IG linked to optical band width, Eg, given by: Eg ¼ 2γ La0a , where a0, CdC spacing ˚ , γ, nearest energy interaction parameter (1 eV). Further, ID/IG is correlated 2.46 A ID CðλL Þ to interdefect distance LD, by a modified Tuinstra-Koenig relationship: ¼ 2 , IG LD where, C(λ) is phenomenological parameter  102 nm2 (at 514 nm) and, LD, in terms   1014 of defect density nD, one can write nD cm2 ¼ 2 . The other key parameter, termed LD as electron phonon-coupling (EPC), plays an important role in estimating the photoconductivity and dynamics of carriers and photo-excited state dynamics of the two systems.

Electron phonon coupling (EPC), Fermi velocity (VF), and photo resistivity In this section, EPC, VF, and photoconductivity have been discussed, particularly at 2.41 eV photo-excitation energy. The G peak gives us information regarding EPC by using the line width of the optical phonon (both longitudinal and transverse modes). The phonon-phonon interaction with other elementary excitations gives rise to line

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Nanocarbon and its Composites

r-GO

(g) (f) Intensity (a.u.)

(e) (d) (c) (b) (a)

1200

1300

D 1400

G 1500

1600

1700

Raman shift (cm−1)

(A)

Te-rGO

(g)

Intensity (a.u.)

(f) (e) (d) (c) (b) (a)

1200

(B)

1300

1400

1500

1600

1700

Raman shift (cm−1)

Fig. 7.37 Dispersive Raman spectra at (a) 325, (b) 442, (c) 457, (d) 488, (e) 514, (f ) 532, and (g) 632 nm. Left (A) pan rGO and right (B) Te-rGO.

width. The nature of excitations is unharmonic, which can be neglected. The phonon line width is estimated by measuring the FWHM of the G peak and the magnitude for rGO is 42.26 while it is increased to 57.37 cm1 for Te-rGO. Similarly, the calculated γ EPC is shown in Table 7.9. The γ EPC calculated using Fermi velocity VF (m/s) by pffiffiffi 2 3 a0 EPCΓ2 EPC ¼ , where μ is reduced mass of the carbon sublattice, EPCΓ 2, theγ 4 μ VF2 ˚ )2). The photo-resistivity is calcuoretical Γ-phonon-branching parameter (47 (eV/A 1 mV F 2 lated using σ ¼ b nD , where, m, mass of carbon expressed in atomic mass 144 ne2 unit, n, number of carbon atoms/cm2 (3.81  1015), e, electronic charge, (1.62  10– 15 stat Coulomb), b, Burger’s vector at defect sites, equal to, a0, and nD as computed, 4  1021 nD in Ω cm. above. This simplifies to σ  n

Nanocarbons: Preparation, assessments, and applications

Photons

(a) 2

235

ID as a IG function of laser photon energy. Profile (a) rGO and (b) Te-rGO. Left bottom schematic: Raman scattering at defect sites, Right top: modification by Te scatters.

Detector

Fig. 7.38 Estimated

(b)

ID/IG

1.5 Te in rGO Photons

Detector

1 rGO Defect sites

2

2.5

3 Energy (eV)

3.5

4

Dynamical force constant, kq The dispersive Raman technique can be used to study variations in dynamical force constant, kq, with photon energy. The magnitude of kq is estimated for rGO and Te-rGO and their magnitudes are listed in Table 7.9. It is calculated by the equation pffiffiffiffiffiffiffiffiffiffi ωq¼ kq =μ, where μ, is the reduced atomic mass of carbon. It is seen that Te s selectively reacts on the non-sp2 carbon fraction, which is present as C(O)O and contributes in modifying Raman scattering process (inset, Fig. 7.38). This results into the increase in the degree of crystallinity and π-π* interactions because of reduction in Eg and disorder [185]. As a result, the nature of hybridization at the Te sites changes, which alters the spatial arrangements of C-lattices and, consequently, the surrounding electron distribution. The change in electron density redistribution around C-Te changes the dynamical variable of the force constant, kq. The calculations show that Te generates a bond frustration of 3% at the adatom site and a strain of 7% at C(O)O, which expresses improvement in the mechanical character of Te-rGO. The edges of rGO

Table 7.9 Enlisted dynamic molecular parameters estimated using Raman spectroscopy for rGO and Te-rGO Parameter (unit) Crystalline length (nm) Optical bandwidth (meV) Interdefect distance (nm) Defect density (cm2) Electron phonon coupling (cm1) Fermi velocity(m/s) Photoresistivity (μΩ cm) Force constant (cm1 eV1)

La Eg LD nD γ EPC VF σ kD (D site) kG (G site)

rGO

Te-rGO

10.3 2.5 9.0 1.22  1015 38 3  106 3.28 53.30 61.1

22.4 1.1 7.5 1.72  1016 53.37 1.5  106 0.23 41.2 36.4

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Nanocarbon and its Composites

Fig. 7.39 HRTEM images of edges recorded for (A) rGO and (B) Te-rGO. Inset (both images) indicate details of Raman (2.41 eV) G-band. Arrows in (B) shows Te clustering at the edges.

play a very important role because of the presence of oxygen atoms and electronic and states symmetry.

Edge structure: Electron microscopy and Raman Fig. 7.39 shows the edges of rGO and Te-rGO recorded using HRTEM. For rGO, the edge was 5 nm and enhanced to 10 nm in Te-rGO. It can be said that Te atoms modified the nature of conjugation and the folding of the edges of rGO and might be precipitated at the edges, as indicated by arrows in (B). This has an implication on overall symmetry and localized electron states in Te-rGO. This generates a Te-modified edge that can be confirmed with the emergence of additional features in G in the form of additional an shake-up peak at the D0 1620 cm1 stretching of bonds. The intensity of D0 majorly depends on the number of edges, their orientation, the angle with which they get polarized, and the length scale of the edge, etc. [186]. The D0 is dispersive, existing for arm-chair and zigzag edges that involved double-resonance K-K0 scattering like D. The reduction in intensity is indicative of modifications in the crystalline and disorder length, as seen in Table 7.9. By and large, the p-orbitals of Te atoms carry axial vector charge distribution with respect to carbon chain atoms that seems to be acting as a bridging element for long-range transport of mobile charge carriers.

7.6.2

ESR studies of rGO and Te-rGO

ESR provides information on the spin dynamics of mobile charge carriers for rGO and Te-rGO. The transport data at R.T. has been deconvoluted for their line widths (Δ Hpp), shapes, and anisotropy in g-factor (Δ g) to calculate spin-spin (Tss), and spin-lattice (Tsl) relaxation times [74]. Using line width, shape, etc., the spin dynamics parameters were estimated and given in Table 7.10  [74].  dY Fig. 7.40 shows ESR dispersion derivatives with a magnetic field for dH (a) rGO and (b) Te-rGO. The measurement of Δ Hpp shows inhomogeneous broadening and a change in magnitude for both systems is attributed to (i) modified Tsl,

Nanocarbons: Preparation, assessments, and applications

237

Table 7.10 Spin dynamic parameters for rGO and Te-rGO Specimen Parameters (unit) Line width (mT) Spin-spin (ps) Spin-lattice (ns) g-Factor anisotropy Spin splitting-width (eV) Spin-orbit coupling (meV) Momentum conjugate (μeV) Pseudo-chem.-potential (eV) Integrated density of states (ρ) (states per eV-atom) (in %)

ΔHpp Tss Tsl Δg Δ Li Γ  μ ρ

rGO

Te-rGO

38.963 3.3 5.38 05.50 93.30 103 4.77 445.04  7.3 0.138 2.3876 15%

61.223  2.27 11.00 15.71 61.70  103 3.15 225.20  3.7 0.315 1.8249 50%

Fig. 7.40 Recorded ESR spectra for (A) rGO and (B) Te-rGO at 300 K.

(ii) charge puddles. The puddles over the volume of the sample exceed the natural line 1 width, , where, γ is the gyromagnetic ratio for the mobile electron. The puddles γTsl somewhat constrain the relaxing electron from reaching the Dirac point, and have average minimal charge density 109 cm–2. Due to the presence of p orbitals of Te, the spins associated with the puddles in different parts of the sample face different field strengths. As a result, the resonance response broadened in an inhomogeneous fashion. This was reflected in the Δg change that underline the facts: (i) the contribution of orbital moments, and spin-spin interactions were increased in rGO, by Te,

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Nanocarbon and its Composites

(ii) orbital moments of Te p-orbitals also contributed, vectorially, and (iii) the spin response of extrinsic Te to the broken inversion symmetry of the rGO sublattice seems to be somewhat different. This has implications on spin scattering parameters such as spin orbit coupling, its momentum conjugate, and the integrated density of states. The comparison of computed values of spin parameters indicates that intrinsic and Bychkov-Rashba (BR) are the operative coupling channels available for rGO and the extrinsic coupling component get added vectorially by p-Te orbitals. In summary, the reduction of GO plays a crucial role in the retaining perfect structure, properties, and in turn the applications of reduced GO (rGO), such as molecular spintronic. To conclude, the modifications in molecular spintronic parameters of rGO have been demonstrated by the facile addition of Te atoms. The chemisorbed Te modified the hybridization nature, altered the spatial arrangement of carbon, and resulted in electron density redistribution at C-Te. It generated 3% bond frustration at the Te position, and 7% stress surrounding the sp2 superlattice, consequently altering the electronic properties. The presence of Te enhanced the crystalline length by a factor of two and decreased the interdefect distance from 7 to 10 nm by enriching the overall sp2 content. The defect density was reduced by one order of magnitude and photo bandwidth by 50% due to an increase in π-π* interactions. The reduction in photo resistivity by Te was responsible for offering the strength to couple the mobile charge carriers to phonons. Details of the molecular spintronic parameters are discussed. At the spin level, Te improved the integrated density of states by three times on the adatom site, which had implications on the reduction in chemical potential from 2.4 to 1.8 eV. The rate at which spin scattering occurred in rGO was enhanced by Te due to weakening in the spin-orbit coupling from 450 to 225 meV. Te depletes the spin splitting bandwidth by dropping anisotropy in spin exchange interactions. Both spin-lattice and spin-spin interactions were retarded, respectively, by nearly two to three times. The comparison of the spin parameters indicated that intrinsic and Bychkov-Rashba coupling are the operative channels in rGO in which the extrinsic coupling component got added, vectorially, by Te. The integrated carrier density associated with these sites participated in the spin resonance process by inhomogeneous broadening of line width in addition to the inherent spin coupling present in rGO. Our study suggested that Te-rGO is a viable medium for molecular spintronics. To this endeavor, a spherical nanocarbon has been synthesized and studied for its shielding and gas-sensing applications [187].

7.7

Multifunctional nanocarbons: NH3 gas sensors and EMI shielding

In the military and civil domains, the need for multifunctional materials, especially for gas sensing and EMI shielding, is of great importance. Recently, nanocarbon-like and graphene, rGO, and CNTs have become popular sensing probes for gas detection [188], particularly for ammonia (NH3). They were reported to show good sensitivity toward NH3 due to their structurally small size, large specific surface area [189], and outstanding electronic properties such as high electron mobility [190] and sensitivity

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239

to electrical perturbations through NH3 molecules. At room temperature, rGO has been reported to be an NH3 gas sensor [191]. For NH3 sensors, Cui et al. developed silver nanocrystal-incorporated nanotubes of carbon at room temperature [192]. For an rGO sensor probe, Ghosh et al. found NH3 sensitivity in the range of 200–2800 ppm, [193]. In a review study of sensors for gas detection, Mao et al. focused on encounters and openings of nanocarbon-based materials [194]. Further, in today’s fast-developing society, the importance of EMI shielding has increased several fold due to the dependence of electronics and the growth of radio frequency radiation sources. At microwave frequencies, the EM radiation tends to interfere with radars and electronics, which is assumed to be strategically adverse in defense and hazardous in the civil sector [164]. The temperature dependence of permittivity and multiregion microwave absorption of ultrathin graphene composites [195] and of nanoneedle-like ZnO [196] was studied by Cao et al. In another study, tunable EM reduction in the microwave region and the capability of magnetic nanoparticle-decorated rGOs were reported by He et al. [197]. Thinnest, lightweight and efficient microwave attenuation performance of rGO as carried out by Wen et al. [198]. Broadly, the emphasis is on graphene-based nanocarbon material in the reported studies, correlating dielectric, EMI shielding, and microwave-absorption performance [199,200]. There is another class of carbon, that is, the amorphous nanocarbon popularly known as carbonaceous soot element, Such nanocarbon exists in the form of agglomerated particles with a diameter in the range 10–50 nm. They have interesting physical properties because they are neither graphite nor diamond character of carbon [201], with the degree of sp2 graphitic ordering ranging from nanocrystalline graphite to glassy carbon. Depending upon the formation of the amorphous phase, one can have DLC as an amorphous carbon with a mixture of sp3 and sp2 phases in composition. Their bond molecular environment that consists of s and p states is the basic element which makes this material interesting. They have significantly different behavior and properties change dramatically with the composition of sp2 and sp3. They are light in weight, easy to synthesize, cost effective, stable at high temperatures, and ecofriendly. The focus of the current section is to evaluate the performance of such nanocarbons obtained by pyrolysis of camphor at low temperature for NH3 gas sensing and EMI shielding applications. The fabricated nanoparticles consisted of sp2 + sp3 carbon having a bond disorder in terms of unsaturated dangling bonds, vacancy mediated defects, etc. The nanocarbons were integrated with an optical-based gas sensor. The performance characteristic of the designed sensor was investigated for full-scale detection, response function, and limit of detection. The evaluation of nanocarbon for EMI shielding was demonstrated by analyzing dielectric function, dc and ac conductivity, skin depth, and % reaction.

7.7.1

Synthesis

Analytical grade, commercially available 1,7,7-trimethyl-bi-cycloheptan (C10H16O, camphor) has been taken as a starting material for nanocarbon film synthesis.

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Copper boat

LN2bath Nano carbon

Camphor combustion

Fig. 7.41 Scheme of nanocarbon deposition at low temperature.

At 77 K, deposition was carried out under normal atmospheric conditions. A rectangular boat (dim. 5  3  1 cm3) was prepared in order to deposit nanocarbon onto copper of thickness 25 mm and purity 99.8%. Initially, the substrate was subjected for sonication at a constant frequency 30 kHz at 50 mW of power. For cleaning purposes, the boat was immersed in methanol and kept in a water bath for 15 min at room temperature for sonication. This followed the drying process by an IR heating lamp (power 500 W) for 1 h. Prior to deposition, a stainless steel spatula was mounted onto a fixed platform and the starting material was kept on the rectangular side of the spatula, which was vertically movable via a moving clamp coupled to a stand. After camphor loading onto the specula, the boat (poured with liquid nitrogen) was clamped and brought in the vicinity of the precursor pellet that had been subjected to combustion for deposition. Postdeposition, the base of the boat on which the thin film was deposited had been cut into the dimensions of 2.5  1.5 cm2 for characterizations on deposited material, as seen in Fig. 7.41.

7.7.2

Surface morphology of nanocarbons

Fig. 7.42 shows the typical (A) HRTEM and (B) FESEM images of nanocarbon indicating a spherical shape of coagulated obtained nanocarbon. Deposited amorphous carbon soot was in the form of spheres of 40–50 nm. HR imaging showed that the nanocarbon consisted of concentric shells with shell discontinuity and amorphous zones at numerous sites. The degree of amorphous content seems to be more prominent, particularly at overlayer (outermost) shells. At certain sites, highly crystalline outer shells have been observed detached from the nanocarbon core region, connected via uncrystalline carbon zones. Mostly, crystallinity was a prominent feature in prepared nanocarbon. TEM gives two-dimensional imaging; however, it seems that nanocarbons were interconnected. Our SEM results enriched TEM findings, showing coagulated and connected nanoparticles, as seen in Fig. 7.42B. It formed threedimensional networks with nearly homogeneous particle size distribution. Being in

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Fig. 7.42 Recorded images (A) HRTEM and (B) FESEM of nanocarbon.

powder form, it can easily be transformed into a coating on a desired substrate. Such high surface area with a spatial, homogeneous, and mix-phased nanocarbon structure was found to be advantageous for molecular level gas sensing and EMI shielding.

7.7.3

Raman studies

The proportion of sp2 and sp3 fractions in nanocarbon brings a great amount of versatility to their physical properties. Raman scattering is a unique tool to probe sp2 and sp3 fractions and fundamentally depends directly on the number of sites and content. Briefly, Raman spectroscopic analysis of nanocarbon is presented in this section in light of morphological studies. Fig. 7.43A shows a recorded spectrum that consisted of two peaks, D and G, assigned to the sp3 and sp2 phases of nanocarbon. The emergence of these phases is indicative of a large disorder and peculiar characteristics of amorphous carbon. To quantify disorder, a curve fitting was carried out in terms of spectroscopic parameters such as peak position, width, line shape (i.e., Gaussian, Lorentzian, or a mixture of both), and band intensity. The result of this line decomposition is seen in Fig. 7.43. Several fittings were tried, leaving all spectroscopic parameters free to progress, and the best fitting was invariably obtained for our spectrum. The D-peak was composed of three components, namely D1 at 1393, D3 at 1534, and D4 at 1182 cm1. In this, D1 and D4 were Gaussian and D3 narrow Lorentzian. The D4 is superimposed molecular vibrations of sp2 +sp3 bonds [202], D1 ta-C, and D3 a-sp2 phase [203,204]. The G-band curve fitting is, identically, shown having a narrow line width at 1606 cm1 and broad at 1726 cm1 (D2) assigned to the breathing mode of the nanocarbon shell. The result of this fitting was in good agreement with the literature work [205], indicating the appearance of the G mode close to the main E2g band of the crystalline graphite, which was further confirmed in favor of a rich sp2 crystalline environment supported by a narrower line width of G compared to the D-band for nanocarbon [202,206]. The G-band had higher intensity than the D-band and has a dependence

1392.77

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Copper

1543.65 1610.35

654.27

(A) 155.59

Intensity (a.u.)

1181.54

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1605.59

Nanocarbon and its Composites

1534.12

242

1500

D2

2000

Fig. 7.43 Recorded Raman spectra for (A) nanocarbon powder, (B) nanocarbon deposited copper. Inset shows vibration modes of metallic copper. Arrows indicate changes in vibration modes of copper by nanocarbon (λ  457 nm).

between ratio ID/IG (1.5), which is inversely proportional to the nanocrystalline planar size La 3 nm [207, 208]. Fig. 7.43B displays the Raman spectrum recorded for copper-deposited nanocarbon in which the Raman fingerprint modes of copper 200 and 500 cm1 seem to be suppressed heavily while catalyzing the nanocarbon. The corresponding ID/IG ratio was 2.3 with La 2 nm showing variations in the amount of graphitization when nanocarbon is deposited onto condensed copper (77 K). Perhaps La was incrementally enhanced in freestanding nanocarbon, revealing vibration modes of copper interacting with the modes of nanocarbon. Interestingly, the sp3 sites have only σ-states while the sp2 sites also possess π-states and have implications on their optical band structure via total energy, charge density, occupancy, and polarizability to generate optically active photo-excited states.

7.7.4

Optical spectroscopy: Optical band structure of nanocarbon

In the present section, we have demonstrated applications of the obtained nanocarbon for optical-based gas sensing and EMI shielding. One needs to know the optical bandwidth and the density of the available photo-excited state to utilize them for optical

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25

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0 200

(A)

250 300 Wavelength (nm)

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400

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(B)

2

3

4 E (eV)

5

6

Fig. 7.44 Recorded UV-vis absorption spectra of nanocarbon colloidal. Plot (A) absorbance (a) vs. wavelength and (B) (αhν)2 versus energy. The dotted lines indicate tangent to Tau curve.

applications. UV-visible together with PL spectroscopy provides information about band gap and excited states. Fig. 7.44 shows the recorded UV-visible spectrum of fabricated nanocarbon in a nonaqueous medium. A variation in normalized absorbance (a) as a function of wavelength (in nm) is seen in plot (A) with the appearance of a sharp peak at 207 nm and a shoulder extended in a broad range 225–400 nm. The sharp peak was attributed to π-σ transitions with broad feature to π-π*. The plot (B) is a Tauc graph showing (αhν)2 versus energy. The extrapolation of a straight line to (αhν)2 ¼ 0 axis gives the value of the band gap [209]. The constructed tangents (red-dotted lines) to the curve indicated the respective optical band gap for π-σ and π-π* electronic environments. The band gap was 4.5 eV, having a midgap of 2.8 eV. Interestingly, the optical charge carriers have a midgap state that was nonradiative in nature and have photo carrier excited states [206]. PL emission is shown in Fig. 7.45A. It shows two excited states, one at 2.27 and the other at 1.6 eV lower than the midgap state along with nonradiative fluoroexcited states. From the dangling edges of sp2 + sp3 heterostructures, the contribution of fluoroexcited states emerges. The lower unoccupied molecular orbital (LUMO) band, having both s and p orbitals, contains a total of eight electrons per Brillouin zone for nanocarbon [210]. The corresponding band scheme is shown in Fig. 7.45B. Several states exist between the HOMO-LUMO gap having wide dispersion of oscillator strength, especially active in the infrared and midinfrared region, making it useful for optical applications [211].

7.7.5

Optical gas sensor characteristic

For study, the nanocarbon specimen was deposited onto an optical fiber used for gassensing applications. A schematic representation of the Fabry-Perot interferometer optical fiber NH3 gas sensor is shown in Fig. 7.46. In this, the incident midinfrared radiation (λ  1510–1590 nm) from the source gets bifurcated from a 3 dB coupler into the ratio of 50:50. Among that, 50% of light travels

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Intensity (a.u.)

HOMO

mid-gap 2.8 eV 4.5 eV

LUMO 200

400 600 800 Wavelength (nm)

1000

(A)

Fluoro-excited 2.27, 1.6eV

1200

(B)

Fig. 7.45 (A) PL spectrum recorded for nanocarbon at excitation wavelength of λ  300 nm, (B) HOMO-LUMO scheme of nanocarbon.

through SMF-28. It’s one end was perfectly cleaved orthogonal to the fiber axis. The light from the fiber end reflects back, giving rise to an interference pattern and the cleaved fiber-end was extremely sensitive to the reflection of light. In Fig. 7.46, the magnified portion of the fiber tip decorated with nanocarbon is seen in schematics. The tip is so sensitive that, on interaction with NH3 molecules, the optical band gap of the tip coating changes with a change in the interference pattern. The remaining 50% amount of light passes via optical spectrum analyzer (OSA), coupled to a PC-controlled unit that displays sensory output in terms of power and wavelength shift, as seen in Fig. 7.47. The spectrum was, initially, recorded for the tip in the absence of NH3 and then 3 ppm gas was insufflate into the chamber while spectrum and onset were recorded, iterating the process from 3–3000 ppm. At each iteration, the response/recovery was monitored and recorded. What we observed is with a subsequent increment in ppm level, there was a systematic shift in wavelength displayed on OSA. The sensing measurements were carried out at room temperature and reliability as well as reproducibility in the sensor response has been noted. In Fig. 7.48A, the wavelength shift (Δλ) with molar concentration (C in ppm) for ammonia gas is plotted at various probe/gas interaction times of 60–180 s. At 3000 ppm, the wavelength shift in the sensor was 1.66 nm for 180 s, whereas at 3 ppm, the shift was 80 pm for 180 s. The sensor showed a prominent wavelength shift as the ammonia concentration enhances from 0 to 3000 ppm. Notably, all measurements were perfectly reproducible.

7.7.5.1

Sensor transfer function

The response of stimuli as a function of input can be quantified by the transfer function of any sensory system. Transfer function characteristics were simulated with the nature of Δλ-C for the measured time interval. For temporal variations up to 60 s,

Nanocarbons: Preparation, assessments, and applications

Source

245

3 dB coupler SMF-28

Gas outlet

OSA

Optical fiber

PC control

Sensing head

Gas inlet

Nano-carbon NH3

Fig. 7.46 Elements of Fabry-Perot interferometer optical fiber gas sensor set up consisted of SMF-28, 3 dB coupler, OSA, gas sensing chamber, and PC control unit.

Fig. 7.47 Sensing response of nanocarbon tip to NH3 molecules, for a period of 180 s, at molecular concentrations ranging from 3 to 3000 ppm.

the function took the simple form Δ λ ¼ KeC/M, where M is interaction volume and K is sensitivity in the form of molar interaction length and concentration (C). The simulated curve parameters in this time domain showed a change in λ of the order of 0.8–1.0 nm, affecting carbon molecules of the order of 1000–1500 with sensitivity for molecular ammonia 7–8 atomic distances of the carbon superlattice. For higher

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Fig. 7.48 (A) Response function characteristics of wavelength shift (Δλ) with molar concentration (C in ppm) for different time period, and (B) Functional response of the sensor in terms of (Δλ) and time at different ppm levels.

temporal portions, the statistical interaction scenario between the ammonia/nanocarbon system became more complex, particularly due to other physical components that started dominating the interaction. They are diffusion of ammonia in the interior of individual nanoparticles, mutual changes in chemical potential (μ) and physisorption potential (ε) experienced by the ammonia/carbon ensemble, onset, within interaction volume, mismatch in the molecular vibration, etc. higher ammonia diffusivity increased the number of carbon atom that were participating in interaction (estimate 5000–6000). This had an effect on both molar length and sensitivity in which sensitivity improved marginally to 10–15 molar length. For a still higher time domain, Q the Δλ-C transfer function took the form: Δλ ¼ 3i, j¼1 KeC=Mj in which molar interactions were 103–4 and beyond. However, the set up transfer function characteristic is, strictly, at 300 K and departure from ambiance introduces adiabatic perturbative terms of higher order, which has potential dependence on the thermodynamics of the interacting system [212]. A variation in Δλ, is seen in Fig. 7.48B as a function of time for both response and recovery. The empirical analysis of the curve indicated that it was composed of response function that is expressed as a linear and impulse 

Q3 ðμ_ E_ 1 + ε_ μ_ 1 Þ Kj B interaction time, B is the diffusivity Δλ ¼ i, j¼1 Ki T + T 3 1  exp Kj (cm2 s1) of the NH3 molecule, ε and μ, respectively, the rate of change of physisorption and chemisorption potential. The recovery and response time of about 5 and 8 s was noted, respectively, and the performance characteristics of our sensor are shown in Table 7.11.

7.7.5.2

Sensing mechanism

The nanocarbon is composed of an sp2 +sp3 carbonaceous phase in which the sp3 phase was distributed inhomogeneously and isotropically. At the interface of the sp2 +sp3 phase, the coordination number of carbon atoms connecting two phases is

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Table 7.11 Performance sheet of nanocarbon sensor system Parameters

Performance and range

Gas selectivity Full-scale detection (instrumental) Pulse rise time Rise response Pulse decay time Decay response Limit of detection (intrinsic) Time Molar transfer function

NH3 (assessed and evaluated) 3–3000 ppm 5s Linear 8s Impulse 1–5 ppb per 10 cycles (irreversible) 0–180 s Q Δλ ¼ 3i, j¼1 KeC=Mj  

Q ðμ_ E_ 1 + ε_ μ_ 1 Þ KB Δλ ¼ 3i, j¼1 Ki T + Tj3 1  exp Kj

Temporal transfer function Temperature range

300  10 K

disproportionate and leads to one undercoordinated electron. There were π-π* conjugated electrons in the sp2 zone, revealing a vacancy mediated disorder in nanocarbon. At the vacancy site, there was nonuniform electron sharing between three uncoordinated carbon atoms, hence the charge-deficient two carbon atoms played the role of acceptor moiety, wherein the third carbon was the donor. The undercoordinated electrons, π-π* conjugated electrons, and vacancy sites acted as the photo-carriers that participated in the optical excitation process, resulting in the behavior of nanocarbon as an n-type photoconductor. The molecular NH3 on interaction with nanocarbon, transiently, changed the midgap, fluoro-excited gap, and, consequently, the optical gap. In ammonia, nitrogen carries a lone pair of electrons that gets attracted to these charge-deficient carbon atoms by molecular interactions. In NH3, the hydrogen atom physically adsorbs at the interface of sp2 + sp3. Depending upon the stereoregular configuration of sp2 +sp3, there could be the possibility of complete charge transfer because the ammonia/nanocarbon interactions are statistical in nature. This will have an indentation on the molecular vibrations of nanocarbon analyzed before. Interestingly, the variations have been observed in post NH3 sensed nanocarbon revealed by Raman spectroscopy.

7.7.5.3 Molecular imprint of NH3 on nanocarbon probe There are significant changes observed in Fig. 7.49B for the post ammonia-treated nanocarbon sensing probe when compared to its pristine counterpart (plot (A)). The G-peak was reduced and D was increased, compounded with broadening, which is indicative of a reduction in crystallite size. The G-peak was shifted from 1606 to 1598 cm1 after gas sensing. The downshift was attributed to enhancement in bond-angle disorder at graphitic sites [213]. The vanishing D4 and D3 and partial disappearance of D2 suggested that sp2 + sp3sites and zones related to amorphous carbon

1000

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D2 –1726.29

–1605.59

(A)

1500

2000 –1597.90

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–1374.97

Intensity (a.u.)

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D4

D1

–1534.12

Nanocarbon and its Composites

–1392.77

248

D1

1500

(B) G

2000

Raman shift (cm–1) Fig. 7.49 Recorded Raman spectrum of (A) nanocarbon decorated on optical fiber tip, (B) post NH3 sensing (λ  457 nm). This will have indentation on molecular vibrations of nanocarbon, analyzed before. Interestingly, the variations have been observed in post NH3 sensed nanocarbon revealed by Raman spectroscopy.

were altered. There was an indication of increased unsaturated dangling bond character in the nanocarbon by submerging D3 and D4 into D1. The sensing action of the nanocarbon probe, though, seems to be completely reversible, macroscopically. However, at the molecular level, the imprint of ammonia onto nanocarbon is identified, evidently, by Raman analysis. In this, the sum of molecular contribution of ta-C moieties, that is, D4 + D3, was about 7  103 that got lost during one sensor cycle. For the sp2 environment, about 3  103 dangling carbon bonds were modified at a sensing level of 3–3000 ppm toward the end of several cycles. The rupture at sp2 was nearly half than at sp3. One can investigate the effect per individual cycle, though the cumulative data is shown at the end of several cycles. The result projected that the disordered carbon in the nanoshell forms interstitial defects leading to molecular imprints [205,213–215]. The reduction in La from 3 (freestanding nanocarbon) to 1 nm (post gas-treated) corresponds to the change in a second maximum in the graphite vibrational

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249

density of states near the M point of the first Brillouin zone boundary, which became prominent in small graphite crystallites per NH3 treatment. This resulted in a lack of a long-range translation symmetry, which led to a breakdown of the k-momentum conservation rule [216, 217].

7.7.6

Nanocarbon for shield technology

In both military and civil sectors, shielding an object from incoming EM radiation is important. It could be achieved by reflection and absorption of radiation by proper coating, thereby acting as a shield against the penetration of radiation.

7.7.6.1 Coating characteristic of nanocarbon on copper In Fig. 7.50A, one can see a recorded SEM cross-sectional view of nanocarbon film onto copper foil (25 mm) having thickness 60 μm. Mostly, the coating was uniform; however, it did have columnar deposition inhomogeneity at certain places. Generally, most of the coating was continuous laterally and longitudinally and at some places the peeling effect was observed. The elemental composition of deposited nanocarbon was studied by energy dispersive X-ray analysis (EDAX). In Fig. 7.50B, the peak associated with oxygen, carbon, and copper is seen, including the table (inset) showing elemental composition. The oxygen 12 at.% in high amount was attributed to the presence of a native oxide layer on copper.

7.7.6.2 DC conductivity The EM rays in the X-band, particularly, interact with nanocarbon material in the form of various losses such as hysteresis, electric conduction, and ESR. Nanocarbon is nonmagnetic in nature, so eddy current and ESR are the dominant effects in the microwave region of absorption. We have not carried out any study on ESR in the current work. To quantify eddy losses, the electrical conductivity of nanocarbon was measured and

Fig. 7.50 (A) FESEM cross-section view of nanocarbon film deposited onto copper, (B) recorded EDAX spectrum and inset in (B) shows elemental composition.

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Nanocarbon and its Composites

100

Copper Nano-carbon

80 Voltage (mV)

Fig. 7.51 Measured current-voltage (I-V) characteristic of nanocarbon and copper foil. Inset shows typical photograph of samples with electrical connections and contact developed.

60 40 20 Copper

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compared with copper. Reflection is the primary mechanism of EMI shielding, which depends on the conductivity, both ac and dc, of the shielding material. EMI shielding depends on the strength of interaction between dynamic charge carriers (electrons or holes) of the shield coating to the incident EM signal. Shielding with a good amount of electrical conductivity is one of the necessary requirements in EMI shield technology. Reflection loss, specifically, is the function of ratio of σ r/μr where, σ r, conductivity relative to copper and, μr, relative magnetic permeability [218]. In Fig. 7.51, one can see the measured I-V characteristics of nanocarbon vis-a-vis copper. l , where, l, length of transThe conductivity was estimated using σ dc ðS=mÞ ¼ mhw port channel, m, slope, h, thickness, w, width of the channel. Table 7.12 shows I-V measurement parameters estimated for both systems.

7.7.6.3

% reflection analysis

The % reflection in Fig. 7.52 shows obtained data for both systems in the X-band regime. The amount of reflection from nanocarbon was recorded to be 85%, comparable to copper (95%). For EMI shielding coating, the reflection, including reflection from the surface and interface scattering, will enhance with increasing conductivity of the coating. From Table 7.12, the magnitude of σ dc for nanocarbon was 10 times less compared to copper. Another important estimated parameter is the skin depth, δ, the qffiffiffiffiffiffiffiffiffiffi 1 where extent to which incident radiation interacts with material, given by δ ¼ πf μσ dc f, frequency bandwidth, μ ¼ 1 (for copper). At 10 GHz, the value of δ is 10 and 40 nm for copper and nanocarbon, respectively. For both copper and nanocarbon, reflection loss caused due to eddy current is more due to skin effect [219, 220]. In Fig. 7.53, one sees the imaginary permittivity ε00 for copper and nanocarbon. The reflection loss of microwave radiation is mainly due to the impedance matching condition and the change of EM parameters of complex permittivity [221]. For the imaginary part of complex permittivity, the free Fermi electrons play an important role due to good electrical conductivity [218,222,223]. Therefore, rendering

Nanocarbons: Preparation, assessments, and applications

251

Fig. 7.52 Recorded % reflection for both the systems, in X-band.

1.00

% Reflection

0.95 0.90 0.85 0.80

Copper Nano-carbon

0.75 9

8

10 11 Frequency (GHz)

12

Table 7.12 I-V measurement parameters for copper and nanocarbon Parameters

Copper

Nanocarbon

Slope (m) Thickness (h) Width (w) Length (l) dc Conductivity (σ dc) Skin depth (δ)

0.16013 25 μm 2 cm 2 cm 2.5  105 S/m 11 nm

0.90712 60 μm 2 cm 2 cm 1.8  104 S/m 42 nm

Imaginary permittivity (e″)

80

Fig. 7.53 Recorded imaginary permittivity, ε00 , as a function of frequencies, exhibiting monotonic relationship between frequency and ε00 .

× 103 Copper Nano-carbon

60

40

20

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10 11 Frequency (GHz)

12

252

Nanocarbon and its Composites 00

σ ac to the free electric theory, ε00 could be obtained by using ε  2πE [224]. The relation 0f 00 showed that σ ac plays the dominating role in variations for ε . Notably, the σ ac of copper was marginally high compared to nanocarbon. This change was due to the bond environment of the copper and nanocarbon medium.

7.7.6.4

Shielding mechanism

Incident wave

95%Reflected wave

s total

The schematics of the interaction mechanism of incident EM with a nanocarbon and copper medium is seen in Fig. 7.54. For metallic copper, the conduction mechanism is due to free mobile charge carriers. There exist both migrating and hopping conductions in nanocarbon due to heterostructured sp2 + sp3 carbon zones. Thus, the incremental total σ ac in nanocarbon concentric shells enhances the imaginary permittivity ε00 , resulting in more reflection, comparable to copper. But, the incident EM of 15% is reduced within the nanocarbon layer due to a larger penetration depth of EM in nanocarbon compared to copper. The incident electric field component and the onset interaction get coupled strongly with the molecular electric field of the sp2 σ-bond, resulting in a larger dissipation of field in terms of heat within nanocarbon. Thus, the comparative microwave reflection property of nanocarbon with copper, particularly in the X-band region, is advantageous. As a metal, copper possesses high density while being a corrosive and commercially expensive material for shielding technology. In contrast, the obtained carbon is featherweight, ecological, shows high temperature stability, is synthesized in one step, and is cost effective. Thus, nanocarbon is well suited for EMI shielding over copper. In summary, the utility of nanocarbon for effective gas sensors and efficient EMI shielding applications has been demonstrated. Nanocarbon was, initially, obtained by the combustion of the camphor precursor and assessed for the structure-property

Nano-carbon Copper

1/e 11 41

Copper

Skin depth (d, nm) 85%

Copper

Eddy current

Hopping and migrating conduction

Fig. 7.54 Schematic illustration showing electromagnetic wave interaction of nanocarbon and copper with differing skin depth level in X-band.

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relationship using HRTEM, FESEM, Raman (λ  547 nm), UV-visible, and PL spectroscopy. In morphological studies, obtained structures were three-dimensionally interconnected spherical nanocarbon, consisted of an sp2 +sp3 phase. In this, the sp3 phase was inhomogeneously distributed within the nanosphere, especially at the surface. The ID/IG, ratio estimated, using Raman, was 1.5 for nanocarbon. In Tauc analysis, the optical band gap of nanocarbon was 4.6 eV with several photoexcited states. This made nanocarbon useful for gas-sensing applications. The sensor performance was analyzed for wavelength shift with a change in molecular concentration and time at 300 K. A statistical model is presented for NH3/nanocarbon interaction, showing the physicochemical nature. The molecular configuration of the sp2 + sp3 phase having undercoordinated π-π* conjugated electrons and vacancy sites made a nanocarbon n-type photoconductor, whose bandgap modifies, transiently, on interaction with ammonia. The contribution of sacrificed sp3 was 7  103, whereas for sp2 3  103 in sensing 3–3000 ppm NH3 molecules. The rupture of sp2 sites was half that of sp3. For EMI shielding, the effectiveness of nanocarbon was quantified by estimating σ dc, ε00 , σ ac, δ, and % reflection derived from S-parameters. The magnitude of σ dc, for nanocarbon was about 10 times less than copper, engaging incident radiations four times higher in the skin of nanocarbon. On interaction with nanocarbon, components of the incident electric field seem to be coupled intensely to the σ-bonded, in-plane, sp2 molecular field. Because these were in-plane bonds directed along the carbon network, the excitations of σ-electrons bridged to two carbon atoms accommodate interaction energy to dissipate within carbonaceous shells in the form of heat. The incident field was capable of rotating three atoms branched to central sp3 carbon. As a result, the overall % reflection was 85% for nanocarbon, which is comparable to copper (95%). This makes nanocarbon coating useful for radar shielding material in the X-band. Our result showed the multifunctional character of nanocarbon obtained by facile combustion of the hydrocarbon precursor. The doping of nanocarbon spheres by cobalt impurity [225] added specific functionality to the nanocarbon spheres when transformed into split-ring resonators (SRRs). The nanocarbons incorporated with cobalt impurity were termed ferronanocarbon (FNC). In the following section, the metamaterial-inspired properties of such FNCs are discussed.

7.8

Electromagnetic cloaking and metamaterials (left-handed medium)

As discussed earlier, there are typically four modes of threats from a radar seeker. Namely, they are surveillance and guidance as well as the locked-in and homing modes. Strategically, the tracking trail of a target is assumed to be a hostile activity that can be counteracted using various means, such as radar-absorbing material, booster fragmentation, jammers, chaff, decoys, etc. This would generate a clutter in the radar signature by shielding or shadowing the target object [226]. Target invisibility in the form of an EM cloak, in recent years, has emerged as a promising technique to shield the object completely [227].

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In order to make the target obscure, the paths of EM radiation are adjusted within or nearby the target material. However, for this purpose, permeability (μ) and permittivity (ε) (constitutive parameters) of the material play important roles. One needs to arrange these parameters spatially in a specific way [228,229]. Then, such architecture transforms the coordinates of parameters to cuddle the incident EM space volume field into a shell form surrounding the concealing volume, and then generates a cloak. In order to achieve this, a cellular pattern has to be produced that obeys a specific scat , where a is periodic dimension of the pattern, λ, incident tering condition: a≪λ ¼ 2πc ω wavelength, ω, frequency of radiation, and c, velocity of light [230]. This will enable the incoming EM waves to get compounded to the cellular pattern. Depending upon the dynamics of charges within the cellular pattern the field cascade the magnetic or electric dipole, at the cell. This would cause asymmetric reflection, asymmetric transmission, or complete concealing of the EM fields. Broadly speaking, the EM cloak comes under the category of metamaterial. Metamaterials are exotic, having peculiar properties such as negative constitutive parameters not necessary for cloaking. However, they are useful for other interesting wave phenomena such as backward direction wave propagation, zero-point phase velocity, etc., to offer a negative refractive index to the medium. Formally, Veselago [231] proposed the notion of metamaterials, popularly known as a negative index material (NIM), or left-handed material (LHM) [232]. Under normal conditions, for a ponderable medium to get transformed into a metamaterial character is a great challenge. This is due to the restrictive set of values of constitutive parameters offered by a normal medium, which mainly happens due to the myopic response of atoms and molecules to incident EM radiation. Recent advancements in this field showed that the behavior for a cellular architecture could be analyzed by studying the number of parameters, such as constitutive tensor values, chiral parameters, nonreciprocity conditions, forward/backward reflections, scattering (S)-parameters (or impedances), scattering power, reflection coefficients, magnetoelectric coupling factors, refractive index, and many more, to classify them as an anisotropic, bianisotropic, and/or chiral metamaterials. By studying a wired metallic medium, the first theoretical realization of LHM occurred in which the permittivity of the medium was estimated to be negative due to the artificial electric plasma properties of the wired material [233]. A pair of metal SRRs showed the magnetic plasma properties of the cellular medium to have negative permeability. Smith et al. in 2001 demonstrated the first artificial LHM by combining metal wires and SRRs, in which the phenomenon of the negative refraction was confirmed [234]. In both theoretical exploration and experimental study, LHM has been the focus [235–239] in recent years, including the discovery of perfect and superlenses [240]. By a macroscopic composite of periodic or aperiodic structure, the elements of LHM could, artificially, be fabricated; their function is due to both the cellular architecture and the chemical composition. LHM has inherent disadvantages such as lossy character, narrow bandwidth, metal composition, etc. Particularly, metal SRRs are corrosive and inflexible in nature. The EM properties of nonmetal-based nanocarbon SRR composition is not revealed in the literature. They are simple to assemble and pattern while being relatively cost effective compared to lithographically obtained metal SRRs.

Nanocarbons: Preparation, assessments, and applications

7.8.1

255

Ferro-nanocarbon split-ring resonators: Bianisotropic metamaterial

In this, we have shown ferro-nanocarbon (FNC) split-ring resonators (SRRs) as a bianisotropic LHM. They were demonstrated as an EM cloak having operation in a narrow range of the 8.5–10 GHz (X-band) regime. The FNC was prepared by the atmospheric combustion of camphor with cobalt (Co) present at variable wt% [225]. The prepared FNCs were studied for their structure-property relationship using Raman, VSM, electron microscopy, and dielectric relaxation spectroscopy. In the analysis, the enhanced internal magnetic fields due to the formation of Co-sp3 phase and the emergence of low-frequency polarization modes were found to be advantageous to implement FNC as SRRs. They offered peculiar characteristic of constitutive parameters extracted using Nicolson-Ross-Weir and retrieval technique revealing bi-anisotropy and LHM behavior. The concealing of the radiated field at SRRs has been confirmed by the computational EM work showing a cloak-like response over 8.5–10 GHz. The theoretical result resembles the experimentally obtained scalar microwave scattering parameters. The NC/FNC specimen was synthesized as per the description in Section 7.6.1.1 and samples were subjected to characterization techniques, as described in the preceding sections. Dielectric and VSM were two more techniques we used and their details are given below (Fig. 7.55): (a) Dielectric measurements:

To measure the permittivity and conductivity of the samples over 10–30 MHz, the dielectric relaxation studies were performed using a dielectric spectrometer (Novocontrol broadband) equipped with an analyzer (Alpha-A) interfaced to the sample cell that was used along with Win Fit software.

Fig. 7.55 (A) Fabricated NC and FNC precursor with various wt% of Co(C2H3O2)2, (B) production scheme of NC/FNC.

256

Nanocarbon and its Composites

(b) VSM studies:

Using 16 T PPMS-VSM, Quantum Design, the magnetization measurements were performed. The magnetometer had a dc sensitivity 105 emu at 1 T over the temperature range 2–300 K with field sweep 100 Oe/s. The thermomagnetic measurements (cooling/heating) were carried out at a rate of 1.5 K/min.

7.8.1.1

Preparation of NC and FNC split-ring resonators

For this, the FR4 dielectric substrate was prepared into 2.286 (A)  1.016 (B)  0.1 (D) cm3 dimension (Fig. 7.57) by cutting. The composite of NC/FNC was made with the help of readily available, standard colloidal silver liquid for imprinting SRRs onto the substrate. The fabricated SRRs consisted of a planar set of two concentric conducting rings with inner ring diameter 5 mm and outer 8 mm (thickness  100 μm), having a gap (split, g ¼ 1 mm) on each ring opposite to each other. Fig. 7.56 shows coordinate variables along with the geometry and sample axis of the FNC SRR unit cell. It was assumed that three coordinate axes e^1 , e^2 , e^3 of the unit cell were oriented in the z, x, and y directions, respectively.

7.8.1.2

Computational electromagnetic

To simulate a field scattered from SRRs, the RF module of COMSOL Multiphysics 5.2 was used by employing the harmonic propagation analysis mode and parametric solver in the X-band regime. In our analysis, the longitudinal configuration was assumed for the propagation of the wave. The configuration was homogenized and completely fills the cross-section of the waveguide. The fundamental mode (TEz10) Fig. 7.56 Configurations of principal axis and geometry of the FNC SRR unit cell. The field directions and propagation vector k. Lm ¼ 8mm, w ¼ 0.5 mm, g ¼ 1 mm, d ¼ 1 mm (thickness of the FR4 substrate), a ¼ 22.86 mm (height), b ¼ 10.16 mm (width), c ¼ 9.78 mm (distance from the input and output port).

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257

was propagating in the z direction from air (∂y∂ ¼ 0) and only TEzm0 modes were supported by the longitudinal configuration changing with x and y to simulate a radiated field around SRRs and microwave (scattering) S-parameters.

7.8.2

Microwave measurements on FNC SRRs

Experimental X-Band (8–12 GHz) measurements were performed using a PNA network analyzer (Agilent, N5222A), equipped with waveguide (dim. 2.3  1.1 cm2), for measuring S-parameters of NC/FNC SRRs. The setup is shown in Fig. 7.28. The S-parameters were measured using this value, and further values of permittivity and permeability were obtained using the Nicolson-Ross-Weir method. Further, using a self-consistent approach, other parameters such as n, refractive index, z+ws, z ws,forward and backward normalized wave impedances, permeability, permittivity, S11, and S21 were extracted by estimating the Г 1, Г 2 reflection coefficients, and the T propagation factor, the β0z phase constant in the z direction, using more advanced retrieval technique.

7.8.3

Morphological analysis of NC and FNC

SEM exhibits the features of NC and FNC. In Fig. 7.57A, it is observed that NC clusters are randomly distributed on SiO2. The individual cluster of NC seems to have less aggregation compared to FNCs, as seen in Fig. 7.57B–D. Though not quantified, analytically, the cluster size is increasing gradually. This indicates that, in FNCs with a higher concentration of Co, the tendency of formation of NC aggregates is enhanced. The clusters are random in shape and size, having an arbitary number of FNCs connected. It also indicates the formation of larger 3D networks with higher Co wt%. As such, it is challenging to comment on the crystallinity of NC and FNCs using SEM; however, the nature of amorphization can be qualitatively understood by studying HRTEM. The HRTEM image analysis in Fig. 7.58 indicated that the structures are spherical, coagulated NC. The deposited spherical NC soot having dimensions 40–50 nm, consisted of concentric shells and at several sites a large amount of the amorphous phase was observed. Especially at higher Co wt%, the top spherical surface was observed to be crystalline compared to the core amorphized zone. At 20 wt% FNC, a clear demarcation between surface crystalline and core amorphous zone was observed. At several sites, we have seen crystalline outer shells separated from core FNCs by an amorphous zone. The FNCs formed three-dimensional structures with homogeneous particle size distribution. The analysis of electron microscopy revealed that the obtained particles had homogeneous isotropic structural properties. They consisted of hybrid phases, that is, crystalline as well as amorphous distributed uniformly within the structures. Such material can easily be transformed into a coating on the desired substrate. Electron microscopy provides qualitative information about crystallinity, Raman spectroscopy is a versatile tool to quantify the phase separation in carbon. The great versatility of FNC arises from the strong dependence of the physical properties on the ratio of sp2 and sp3 bonds.

258

Co ~ 0.5

C 84 at %

0 2 4 6 8 Full Scale 149050 cts Cursor: 0.000 30 nm

10

0 2 4 6 8 Full Scale 62902 cts Cursor: 0.000

EHT = 5.00 kV

Signal A = InLens

Date :18 Aug 2016

WD = 6.0 mm

Mag = 200.00 K X

Photo No. = 17478

30 nm

4

6

8

Signal A = InLens

Date :18 Aug 2016

Mag = 200.00 K X

Photo No. = 17481

0 2 4 6 8 Full Scale 53057 cts Cursor: 0.000

10

EHT = 5.00 kV

Signal A = InLens

Date :18 Aug 2016

WD = 6.3 mm

Mag = 200.00 K X

Photo No. = 17484

30 nm

Signal A = InLens

Date :18 Aug 2016

Mag = 200.00 K X

Photo No. = 17487

10

EHT = 5.00 kV WD = 6.0 mm

Fig. 7.57 (A) SEM at high magnification of NC randomly dispersed on silica and (B), (C), (D), respectively, 3, 5, 20 wt% of Co in NC (scale bar 30 nm). The inset shows EDAX of corresponding samples. Elemental composition in at % is indicated for carbon and Co.

Nanocarbon and its Composites

2 30 nm

EHT = 5.00 kV WD = 6.3 mm

Co ~ 0.7

Co ~ 0.6

0

10

Nanocarbons: Preparation, assessments, and applications 259

Fig. 7.58 Recorded HRTEM images for (A) NC, (B) 3, (C) 5, and (D) 20 wt% FNC. The inset shows corresponding SAED pattern of NC and FNC.

260

7.8.3.1

Nanocarbon and its Composites

Molecular characteristic of NC and FNC: Raman analysis

Fig. 7.59A shows the recorded Raman spectrum of NC, which consisted of two peaks, D and G. They were, respectively, assigned to the sp3 and sp2 phases of NC. The emergence of these phases is a peculiar characteristic of amorphous carbon indicative of a large amount of disorder [74]. To quantify this, a curve fitting was carried out in terms of spectroscopic parameters such as peak position, peak width, line shape (i.e., Gaussian, Lorentzian, or a mixture of both), and band intensity using Labspecs 5.0 software. The result of the line decomposition is indicated in Fig. 7.59. Several fits were tried, leaving all spectroscopic parameters free to progress and the best fitting was invariably obtained for all recorded spectra. For NC, the D-peak was deconvoluted for two components at 1313.31 and 1353.97 cm1 having effective peak width 110 cm1. In the case of FNC, the major change that has been observed in recorded spectra was variations in line shape and width of the D-peak, as seen in Fig. 7.59B. For FNC, the effective width was increased to 150 cm1. It consisted of a D-peak doublet appearing at 1303.99 and 1352.09 cm1. However, the G-peak appearing in NC and FNC was found to be invariant in peak position and width, respectively, recorded to be 1590 and 60 cm1. Broadly, it seems that Co incorporated in the sp3 zone of NC. The packing fraction of sp3 is low compared to the sp2 network. Due to this, the incorporated Co gets an opportunity to migrate into the interstitial space of the sp3 zone, available within NC. This has implications on the molecular and, consequently, dielectric characteristics of NC and FNC. The dielectric characteristic is associated with the relaxation behavior of NC in terms of motion of sp2 and sp3 molecules, which subsequently may get modified by adding Co. Because most of the carbonaceous medium exhibits more than one dielectric molecular relaxation region, generally, no single molecular model is adequate to describe the behavior over a wide frequency and temperature range. Here, we have conducted frequency relaxation study mainly.

Fig. 7.59 Raman spectra recorded for (A) NC and (B) FNC 20% at 532 nm excitation wavelength.

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261

7.8.3.2 Dielectric response of NC and FNC To study molecular relaxation in NC and FNC, an equal amount of powder was mixed with a polymer whose relaxation behavior is known and nonoverlapping with our NC [241]. Fig. 7.60A shows variation in permittivity (in arbitrary units) as a function of frequency. It indicates that very prominent low-frequency orientational polarization appeared between mHz to Hz. The observed peak could be attributed to, mainly, the chair-to-chair motion of sp2 segments of NC. A broadening of the low-frequency peak with a subsequent increase of Co% in NC, and finally for FNC 20% the emergence of a broader shoulder peak was observed. Fig. 7.60B shows variations in σ ac as a function of frequency, which corresponds to the dielectric loss component. At higher frequency of range from kHz to MHz, no prominent change has been observed for NC and FNCs. It can be said that the electronic polarization is not affected as significantly as that of the orientation polarization. This is in line with the Raman data analyzed, indicating the interaction of Co in NC is mainly with sp3 carbon affecting its molecular environment . Because Co is magnetic in nature, as with changes in like dielectric properties, one needs to investigate the magnetic parameters of FNC. This is important from the viewpoint of effective permeability that depends on the magnetization of the medium.

7.8.3.3 Magnetization analysis: NC and FNC Fig. 7.61A shows M-H curves recorded for NC and FNC. The hysteresis curve recorded for NC exhibits almost the least saturation behavior. For estimation of the magnetic moment, Ms, the M-H curve was plotted. The magnitude of Ms was estimated to be 3.1  103 emu/g for NC. On increasing wt% of Co in NC, the value of Ms was increased. For FNC 20%, the computed Ms value is 2.03 emu/g, three times more than NC. The effective permeability, μeff, was calculated using Ms, and it is 0.0015 μB for NC and 10 μB for FNC 20% [74]. In permeability, an increase of four orders of magnitude difference is found. This showed that the extent of the internal magnetic field was more than a thousand atoms in FNC, whereas in NC, the field dies

Fig. 7.60 Recorded log-log plots for (A) permittivity and (B) ac conductivity (σ ac) as a function of frequency in the range 10 mHz–30 MHz.

262

2.5 Magnetization (emu/g)

T = 300 K

2.0

NC FNC 3% FNC 5% FNC 20%

1.5 1.0 0.5

(A)

0.0 0.00

0.12 H/T (kOe/K)

0.18

0.24

0.065

2.16 Magnetization (emu/g)

0.06

FNC 20% 0.060

2.15

0.055

NC

Moment (emu/g)

Fig. 7.61 (A) Magnetization hysteresis curves (M-H) measured for NC and FNC at 300 K. Inset photograph shows the response of FNC 20% to permanent magnet, (B) M-T curve recorded for NC (inset) and FNC 20%.

Nanocarbon and its Composites

Temperature (K)

0.050 10

20

30

40

50

60

70

2.14 ZFC FC

2.13 10

(B)

20

30 40 50 Temperature (K)

60

70

Fig. 7.62 Schematic representation of decay and rise in effective permeability with superlattice distance for NC and FNC.

by a factor of (1/6)th within the carbon sublattice. The schematic shown in Fig. 7.62 indicates variations in effective permeability as a function of lattice distance. The thermomagnetic analysis, carried out using FC-ZFC measurements, on NC (inset) and FNC 20% is shown in Fig. 7.61B. From the inset, one can see that in the NC medium, the exchange anisotropy NC medium was less compared with FNC 20%. It can be said that an inherent increase in exchange anisotropy is attributed

Nanocarbons: Preparation, assessments, and applications

263

to structural defects (vacancies), topological disorder, and impurities influenced. Typically, their existence is in the form of the radical spin moment available on the carbon atom. Subsequently, these structural inhomogeneities are randomly distributed with smaller concentration, the interaction between them is not direct, and a carbon sublattice mediator assists them. The encaged Co in FNC is responsible for the induced dipole moment and plays the crucial role as a mediator. Dominant exchange interaction between the magnetic moments is due to Coulomb interaction between itinerant sp3 electrons and magnetic dipole. As a result, significant irreversibility has been observed, producing large exchange anisotropy in FNC 20%. It would be of interest to investigate the nature of magnetization, the range of ordering, and the correlation of ordering; however, it is not in the purview of the present discussions. Besides this, the Co-sp3 molecular magnetic environment is accountable for producing a low-frequency dipolar field that is highly anisotropic, inhomogeneous, and scattered within the sp2 nanocarbon framework. Favoring of atomic and molecular characteristics of FNC is beneficial, however, for building material with peculiar electric and magnetic properties, especially at frequencies in the gigahertz range as it provides limiting values of constitutive parameters. To exploit naturally unavailable properties of FNC, they are implemented in the form of SRRs, as described in the experimental section.

7.8.4

Modeling and simulation: FNC SRRs

The transformation of FNC into SRRs is a process to reposition the atoms of the original concept with the structure on a larger scale. The periodic structure has a unit cell of a characteristic dimension a, which obeyed the scattering wave relation a ≪ λ ¼ 2πC/ω. The electromagnetic resonance of FNC SRRs has been investigated numerically. For this, port boundary conditions were placed on the input and output boundaries of the waveguide. For the input, the incident transverse electric field (TE10 mode, E) obeys Maxwell’s wave equation:     1 2 jT — E  — —  E  k 2 Er  E ¼ 0, ωσ μr

(7.4)

where, μr, relative permeability, Er, relative permittivity, jT, total current, σ, ac conductivity. The dispersion of incident wave vector, k, is given by, k2 ¼ μr εr ω2 + μr ωσ,

(7.5)

The total time reversal electric field solution is given by, n a o n a o +t , Ei, r ¼ E1 exp iω  t + E2 exp iω C C

(7.6)

264

Nanocarbon and its Composites

Fig. 7.63 Simulated electric field, in x-y plane, by FNC 20% SRRs unit cell indicating variations in the topology of the field in which (A), (B), (C), and (D) are, respectively, for 8, 8.5, 10, and 12 GHz response frequency.

where, Ei, r is total electric field, and E1 and E2 corresponding incident and reflected components, respectively. The related boundary condition used in terms of the Poynting vector (dimension: energy/area  time) is given by,

2 dST Ei, r  E1  E1 Ei, r  E2 ¼ + 2 ,

2

E1

E2 dΩ

(7.7)

where, dΩ, is the volume element through which scattering occurs at SRRs. In the above equation, the first term is the input and the second term with the output port. The port boundary automatically determined the reflection and transmission characteristics in terms of the S-parameters. In Fig. 7.63, the electric field distribution in the x-y plane is simulated for a typical FNC 20% SRRs unit cell, that indicated squeezing and localization of the incident field around the cell. It has frequency dependence, which comes through the capacitive action that was generated by SRRs. On interaction with the incident field, the structure acts as an infinitely conducting cylinder in the high-frequency limit that generates oppositely directed alternating currents, due to a split moving within the ring structure. The curl of currents was responsible for producing a net dipole moment vector orthogonal to the plane of the ring. Thus, the imprinted SRRs together with the split acted as an LC circuit [242,243]. Over the bandwidth (8–12 GHz), the resonance response due to the dipole moment was observed to be varied, as seen in Fig. 7.63.

7.8.4.1

Microwave scattering and constitutive parameters

Further, the simulated scalar S-parameters for FNC 20% SRRs is shown in Fig. 7.64A. The magnitude of S-parameters in terms of shielding effectiveness (SE) measured in dB [130] is given by:

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265

Fig. 7.64 (A) Simulated and (B) experimental S-parameters for FNC 20% SRRs for X-band region. Inset indicates corresponding simulated and experimentally fabricated SRRs unit cell.

 SEðdBÞ ¼ 10 log

 PT , PI

(7.8)

From computational techniques, the S11 parameter was well below 20 dB at 8.5 GHz, whereas at the same frequency, the S21 was nearing 0 dB. This indicated that, at this frequency, the SRRs were fully transparent to the incident radiation by squeezing the field within the unit cell. Fig. 7.64B shows the average of the experimental microwave scattering S-parameters obtained statistically for FNC 20% SRRs. There is a marked difference between measured and simulated S-parameters of FNC SRRs, which could be attributed to factors such as the design of the unit cell, the field received and interacted within the cell, and response of boundaries and interfaces. In a simulated cell, these factors could be operative at the optimum level in contrast to the practical one. Qualitatively, the field received in the simulated cell would experience no corner reflection and losses within annular structures of SRRs. Further, the simulated cell is strictly a homogeneous medium with a mismatch at the edges and the split region. In the practical cell, the condition of homogeneity of the medium would be somewhat graded due to the presence of magnetic impurity in the inherent dielectric carbon network. Such a locally inhomogeneous electromagnetic medium cannot be simulated that effectively. As a result, one can see the variations in simulated and experimental S-parameters; however, the similarity in the S11 and S21 cut-off region appeared at 10.5 GHz.

7.8.4.2 Nicolson-Ross-Weir formulism In the current study, we have used two port rectangular waveguide methods in which the S11 and S22 parameters were, invariably, symmetric. This is due to the fact that the incident magnetic field perpendicular to the plane containing the SSRs ring will induce magnetic excitation in the ring along the z axis, producing electric dipole along the x axis. Further, the dipole field perpendicular to the slit axis, that is, the x axis will cause charges of opposite polarities to accumulate over the ring, yielding a magnetic dipole symmetric along the z axis. Thus, for the TEz10 mode, the scattering S11 and S22 will be symmetric. In order to calculate the total loss of incident

266

Nanocarbon and its Composites

radiation in FNC SRRs, we have plotted j S11 j2 + j S21 j2 as a function of frequency and provided below. The medium is less lossy over 8–10 GHz compared with the high-frequency regime. Fig. 7.65A and B shows estimated values of constitutive parameters by the Nicolson-Ross-Weir method for NC and FNC over the X-band region. The overall response of permittivity is negative with a cutoff at high frequency for all systems and an almost similar trend is observed for recorded permeability. In general, the response of the dipole, atom, and electron to the harmonically oscillating electromagnetic field is such that the charge segregation is in the same direction as that of the oscillating field below the resonance frequency. Above resonance, the charge lag occurs due to the mass involved in the segregation of polar moieties harmonically bound within the medium [244]. The observed negative response suggests that FNC is LHM in nature in the SRRs configuration. Further, the Nicolson-Ross-Weir formalism is based on an estimation of the constitutive parameters for homogeneous isotropic materials directly by examining the measured S-parameters [245]. However, this approach has some challenges for studying metamaterial cellular elements due to limitations on the homogenization of scattered electromagnetic fields [246].

Fig. 7.65 Extracted (A) permittivity (ε), (B) permeability (μ), for NC and FNC by NicolsonRoss-Weir method, (C) permittivity, refractive index (n) inset, and (D) permeability real and imaginary parts, total normalized wave impedance (z), for FNC 20% SRRs obtained by retrieval technique.

Nanocarbons: Preparation, assessments, and applications

267

7.8.4.3 Retrieval technique: Extraction of other scattering parameters In a cellular metamaterial structure, the electromagnetic response depends on three factors: (i) production of the magnetic dipole by generation of a circulating surface current due to magnetic field interaction of the incident wave with the cell, (ii) generation of electric dipoles by induction of charge densities with opposite polarities due to accumulation of charges at corners/edges of the cell, and (iii) magnetoelectric field coupling. The interaction transforms the metamaterial property from anisotropic to bianisotropic or to chiral behavior, resulting in asymmetric reflection and transmission, respectively. Mostly, metamaterials exhibit a bianisotropic property when transformed into SRRs. The bianisotropic medium can be specified by analyzing constitutive, chirality, and magnetoelectric parameters. They have different forward and backward scattering parameters/impedances, a wider stop band transmission spectrum, they differ in forward and backward powers, an have variable reflection coefficient and magnetoelectric coupling factors. It is difficult to estimate all of them merely using S11, S22, and S21. However, in the the present communication, we have estimated a few of them by the retrieval technique [247] using the following equations: Sb11 ¼

Г1 ð1  T 2 Þ ; 1  Г1 Г 2 T 2

Sb21 ¼ Sb12 ¼ Г1 ¼

Г 2 ð1  T 2 Þ 1  Г 1Г 2T2

T ð1  Г 1 Г 2 Þ 1  Г 1Г 2T2

+ zws 1 ; + zws + 1

z ws ¼

Sb22 ¼

Г2 ¼

μ1 β0z ; βsz  ik0 ξ0

(7.9)

(7.10)

z μ1 β0z ws  1 + ;zws ¼  βsz + ik0 ξ0 zws + 1

T ¼ e +iβsz L ;βsz ¼ nβ0z

(7.11)

(7.12)

where b denotes bianisotropic feature of FNC SRRs, in Eqs. (7.9) and (7.10). The intermediate variables, Г1, Г2 reflection coefficients, T propagation factor, β0z phase constant in z direction, z+ws, z ws forward and backward normalized wave impedances, n refractive index were computed for longitudinal wave propagation configuration with Lm ¼ 8 mm, for SRRs. From Eqs. (7.9)–(7.12), it has been noted that, for two port rectangular waveguide measurement magnetoelectric coupling, ξ0 ¼ 0, and Г1 ¼ Г2, Sb11 ¼ Sb22. In order to extract ε, μr, μi, one has to determine z+ws, z ws from the above equations.

+ zws

¼

^2 

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi V2 V V ffi 2 4 1 3 V ; 2 1

z ws

V + zws + 4 V ¼ + 1 + zws 4

(7.13)

268

Nanocarbon and its Composites

^

 2    ¼ Sb21  1  Sb11 1  Sb22 ;

^

1

    2 ¼ 1 + Sb11 1 + Sb22  Sb21 ;

^

3

^

ξ0 ¼

 +   z ins β0z zws ws ; + + z k0 zws ws

μi ¼ zn;

z¼

0 ε¼

1B 2 @ξ + μr 0

2

4

  ¼ 2 Sb11  Sb22

¼

Sb11  Sb22 Sb11 + Sb22

  ik0 ξ0 + μr ¼ zws ns + β0z

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 + S11 Þ2  S21 2 ð1  S11 Þ2  S21 2 μr 2 1 β μi 0x C A 2

(7.14) (7.15)

(7.16)

(7.17)

β0z 2 ns 2 + k0

(7.18)

Using Eqs. (7.13)–(7.18), the variations in extracted parameters can be studied as a function of frequency. In addition, the values of S11 and S21 obtained from (7.13)– (7.15) as a function of frequency are provided in supporting information in Fig. 7.66. Fig. 7.65C and D typically show the extracted ε, μr, μi, n, z parameters for FNC 20%. One can see that there is a variation in the nature of profiles obtained by both methods. The study of constitutive parameters of FNC SRRs by the retrieval technique is in its infancy stage. The results of S- and constitutive parameters, broadly, suggest that the higher anisotropy and out-of-phase response of the Co-sp3 dipolar field within the supramolecular carbon domains segregate the charge in opposite directions to the oscillating incident field. Further, computationally, FNC SRRs created a resonance condition at 8.5 GHz, and a tradeoff 10 GHz (experimentally resembling as well). In this

Fig. 7.66 Total loss as a function of frequency.

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subband regime, the average dipolar field gets coupled strongly to the incident field in a somewhat reverse fashion. This sets a gradient in constitutive parameters that made FNC as a LHM to generate a cloak-like response, particularly between 8.5 and 10 GHz, when imprinted as SRRs. The field distribution incident on the surface of SRRs was transported uniformly across the dielectric slab and generated a cloak-like effect in this frequency regime, as seen in Fig. 7.66. Our discussions revealed tha the heterostructure molecular environment present in FNC is responsible for behaving like an electromagnetic cloak in the narrow X-band regime that squeezes the incident field effectively and brings out the possibility to make the object invisible. The transparency of an object to the radar threat spectrum at guidance and tracking range is a strategically important application. In summary, in this section, we have studied the electromagnetic character of FNC SRRs in the X-band (8–12 GHz) region. The FNC SRRs act as a bianisotropic LHM generating a cloak-like response between 8.5 and 10 GHz, as revealed by analysis of the constitutive parameters. Using a camphor precursor (1,7,7-trimethyl-bicycloheptan, C10H16O), by incorporating the variable wt% (3–20) of Co(C2H3O2)2, FNC was synthesized. In the analysis, FNCs were a 40–50 nm spherical, self-assembled, interconnected three-dimensional nanocarbon network with sp2/sp3 heterostructure molecular environment in which Co was encaged within sp3 phases. The molecular relaxation studies on NC and FNC clearly indicated a prominent low frequency (mHz to Hz) orientation polarization attributed to chair-to-chair motion of the sp2 segment for NC, which was disappeared systematically with increasing Co content. The amount of magnetization was increased from 3.1  103 (NC) to 2.03emu/g (FNC 20%) with a dramatic enhancement in effective magnetic permeability by 7000 times. The exchange anisotropy of the medium was high due to random volume distribution of Co substitutional impurities that interacted indirectly with the carbon lattice network. This has implications for microwavescattering properties of FNC when transformed into SRRs, as studied experimentally. The numerous parameters were extracted such as scattering (S), permittivity, permeability, forward/backward scattering impedances, magnetoelectric coupling factors, refractive index, phase constant, etc., using the Nicolson-Ross-Weir and retrieval methods. In addition, S-parameters and field profiles were simulated, indicating concealing of the radiated field at FNC SRRs with S11 around 20 and S21 0 dB, at 8.5 GHz. In the mechanism, the incident electromagnetic wave, especially the magnetic field component, extended along the y axis, and interacted with SRR structures oriented in the xy plane in waveguide configuration. It generated the circulating surface currents in structures, producing a magnetic dipole due to the ferrocarbon phase in FNC. Moreover, rich π-electron characteristics associated with sp2 bonding induced charge densities with opposite polarities at the corners and edges of the ring structure, developing an electrical dipole that got coupled with the incident field. This made FNC SRRs a bianisotropic LHM generating a cloak-like response at 8.5–10 GHz. The obtained SRRs were nanocarbonbased material, noncorrosive, flexible, environmentally friendly, and inexpensive. The FNC SRRs showed the emerging possibility to generate basic building blocks for a narrow bandwidth X-band cloak.

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7.9

Nanocarbon and its Composites

Concluding remarks and work scheme

The work presented herein has a large amount of variety that ranges from structural applications to spintornics and gas sensing to EMI shielding/cloaking. However, the central part of the work is to prepare nanocarbons with a specific structural-property relationship. A number of nanocarbons were prepared, mostly by route chemical techniques such as GNCs, rGO, NC, and FNCs and subjected to numerous characterizations. The analysis provided insight into their utility and important clues to integrate them in various applications. A typical application is demonstrated (in the below scheme), indicating their usage in mechanical, spin devices, gas sensing, and EMI shielding and metamaterial cloaking in the X-band. Synthesis of GNCs was carried out that contains mixed phase, sp2-sp3 bonded, few atom layers, and disordered carbon network. The blend of softwood charcoal (C), potassium nitrate (KNO3), and sulfur (S) (stoichiometric ratio  85:10:05, C:KNO3:S) was subjected to the combustion procedure at atmospheric conditions. The obtained, as-synthesized samples were employed to intercalation, using the mixture of H2SO4 (98%):HNO3 (60%) for 48 h at room temperature, followed by annealing at 1000°C. All stage samples were studied using Raman spectroscopy, and SEM and HRTEM/SAED for optical and morphological properties, respectively. The electron transport properties of the obtained samples were examined by (STM/STS) techniques. The composition was investigated using Fourier transform infrared and XPS techniques. Raman analysis showed variations in the intensity in sp2, ta-C, graphene (2D), and disordered (D), indicating gradual evolution of GNCs with the presence of both sp2 crystalline and sp3 amorphous nature. The nature of the 2D peak indicated the presence of 2–5 GNCs layer with structural disorder. SEM area  10–20 μm2 and morphology was close to disordered graphene. HRTEM and SAED results revealed short-range ordering after annealing. The tunneling spectra showed V-shaped LDOS with peak present near the minima, supporting the presence of local disorder. The resonance peak near Fermi energy is indicative of applications of GNCs in organic magnetism. In the next section, we demonstrated advances in mechanical and thermal properties at low weight fractions of (0.01 wt%) GNCs in epoxy. With GNCs exfoliated to individual sheets below 0.05 wt% and above 0.01 wt%, the aggregate density increases, adversely affecting the mechanical properties. Mechanical parameters such as tensile, flexural, and fracture toughness were significantly improved; however, the optimum reinforcement was 0.01 wt% nanocomposites, presumably due to the cooperative effects of a large surface area, uniform dispersion, and good interfacial adhesion. The thermal and physical properties were improved considerably with GNC. Fracture energy absorption mechanisms in nanocomposites are mainly governed by the formation of more microcracks and their deflection as compared to neat epoxy. On account of these properties, GNC could be a promising nanofiller for structural applications. In this, GNCs were doped with nitrogen using TDAE with variable concentration 0.1–0.5 mg/mL. In ESR, a number of spin dynamic parameters were obtained, such as effective, g-factor, spin-spin, Tss, spin-lattice, Tsl, relaxation, spin-orbit coupling, Li,

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momentum relaxation rate, Γ, effective chemical potential, μ~, spin DOS, ρ, spin-flip probability, and Ncol, indicating the profound impact of loading N into GNCs. In parametric magnetic correlation analysis, Ms values, effective magnetic moments, and μeff were computed. Broadly, it indicated that, for a cluster of 1000 carbon atoms, there were  200 interacting spins for GNCs. After N doping, the number was reduced to  50 spins for the same number of carbon atoms. The values obtained for spin susceptibility χ spin by the VSM technique showed slight variation in magnitude of χ spin computed by ESR. Clearly, a charge-transfer mechanism between GNCs and TDAE is responsible for bringing a change in the makeup of exchange correlations and spin transport in GNCs. Subsequently, EMI shielding studies were carried out on GNC/PU nanocomposites at variable 1–25 wt% of GNCs, over the 8.2–12.4 GHz (X-band) regime. The focus was: how much is transparency of the composite material to the incident signal? Microwave scattering data analysis provided basic insight into the behavior of the microwave field in a heterogeneous composite medium. The nonplaner CdH, NdH, C]O of PU played a decisive role in modifying dc and ac conducting properties by getting hydrosorpted with sp3 C-H sites of GNCs. The active modes of polarization in the host were modified by GNCs. The mechanism suggested that GNCs provide a conducting path to the incident field wiggles by generating mobile charge carriers along urethane amide III segments around the microvoids. The loss improved, for PU, from 40% to 99.9% with a reduction in coating thickness from a centimeter scale down to millimeter, at 25 wt% loading of GNCs. The GNC/PU nanocomposites are promising for building shielding patterns, especially at short-range projectile tracking. In the next section, modifications in molecular spintronic parameters of rGO were demonstrated by the facile addition of Te atoms. The Te-modified hybridization nature altered the spatial arrangement of carbon and resulted in electron density redistribution at C-Te. It generated 3% bond frustration and 7% stress surrounding the sp2 superlattice, consequently altering electronic properties. Details of the molecular spintronic parameters are discussed. Our study suggested that Te-rGO is a viable medium for molecular spintronics. Further, the utility of nanocarbon for an effective gas sensor and efficient EMI shielding applications have been demonstrated. The nanocarbons, obtained by the combustion of camphor, were assessed using HRTEM, FESEM, Raman, UV-visible, and PL spectroscopy. In the analysis, the structures were 3D interconnected nanocarbon having sp2 +sp3 phase. Tauc analysis revealed the existence of multiphoto-excited states, making CNS useful for gas-sensing applications. The performance characteristics of the sensor were studied, for wavelength, change in molecular concentration, and time at room temperature. In EMI shielding, the utility parameters of nanocarbon wwereas quantified by estimating the σ dc, ε00 , σ ac, δ, and % reflection derived from S-parameters. In the analysis, the σ dc for nanocarbon was 10 times less than copper, engaging incident radiation four times higher in the skin of nanocarbon. Overall, the % reflection was 85% for nanocarbon, which is comparable to copper (95%). This makes nanocarbon coating useful for radar shielding material in the X-band. Our results showed the multifunctional characteristics of nanocarbon obtained by facile combustion of the hydrocarbon precursor. In the

272 Nanocarbon and its Composites

Fig. 7.67 Crux of the work presented.

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metamaterial study, we have studied the EM character of FNC SRRs in the X-band (8–12 GHz) region. The FNC SRRs acted as a bianisotropic LHM, generating a cloak-like response between 8.5 and 10 GHz, as revealed by analysis of the constitutive parameters. Initially, FNC was synthesized using a camphor precursor by incorporating variable wt% (3–20) of Co(C2H3O2)2. The obtained FNC was subjected to a number of studies, including VSM. In the analysis, the nanocarbon network contained Co encaged within sp3 phases having a prominent low frequency (mHz to Hz) orientation polarization attributed to the chair-to-chair motion of the sp2 segment. Magnetization was increased from 3.1  103 (NC) to 2.03 emu/g (FNC 20%) with dramatic enhancement in effective magnetic permeability by 7000 times. This has implications on the microwave-scattering properties of FNC when transformed into SRRs, as studied experimentally. The numerous parameters were extracted such as scattering (S), permittivity, permeability, forward/backward scattering impedances, magnetoelectric coupling factors, refractive index, phase constant, etc., using the NicolsonRoss-Weir and retrieval methods. The incident EM wave, especially the magnetic field component, extended along the y axis and interacted with SRR structures oriented in the xy plane, generating circulating surface currents in structures producing a magnetic dipole due to the ferrocarbon phase in FNC. It induces charge densities with opposite polarities at the corners and edges of the ring structure, developing an electrical dipole that got coupled with the incident field. This made FNC SRRs a bianisotropic LHM generating a cloak like response at 8.5–10 GHz. The obtained SRRs were nanocarbon-based material, noncorrosive, flexible, environmentally friendly, and inexpensive. The FNC SRRs showed the emerging possibility to generate basic building blocks for a narrow bandwidth X-band cloak (Fig. 7.67).

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8

Prospects of nanocarbons in agriculture

Sumit Kumar Sonkar*, Sabyasachi Sarkar† *Department of Chemistry, Malaviya National Institute of Technology, Jaipur, India, † Nanoscience and Synthetic Leaf Laboratory at Downing Hall, Center for Healthcare Science and Technology, Indian Institute of Engineering Science and Technology, Shibpur, India

Chapter Outline 8.1 Introduction 287 8.2 Biochar 289 8.2.1 Biowaste-based charred carbon 289 8.2.2 Effect of biochar-derived nanocarbons on plant growth 291

8.3 Nanocarbons on plant growth 293 8.3.1 Water-soluble nanocarbons 294

8.4 Effect of nanocarbons on soil microenvironments 8.5 Conclusion 317 Acknowledgments 318 References 318

8.1

315

Introduction

Curiosity about the spectroscopic signature of interstellar dust in the universe led to the discovery of the third allotrope of carbon as fullerenes [1]. The discovery of fullerenes triggered the exploration of carbon in the nanodomain, leading to the characterization of diverse versions of nanocarbons in different shapes and sizes. This constitutes a family of nanocarbons, having the multiwalled carbon nanotubes (MWCNTs) [2–4], carbon nanoonions (CNOs) [5–10], single-walled carbon nanotubes (SWCNTs) [11], graphene/graphene nanosheets [12–15], photoluminescent carbon dots (CDs) [16–19], graphene quantum dots (GQDs) [20], carbon nanorods [21–23], carbon nanodiamonds [24], carbon nanohorns [25], and carbon nanocubes [26]. Such material in gram quantity was mostly synthesized by graphitic arc-based experiments [2, 5, 11, 17, 27, 28] and was further modified, decorated with hydrophilic surface functionalities to investigate the properties of its soluble form. Nanocarbons comprising only elemental carbon can be differentiated based on possessing the varied structure and bonding between the carbon atoms. As all the nanocarbons are comprised of the hydrophobic carbon network or cluster, therefore not being soluble in water, these versions are constrained to be used in biology. Smalley predicted that if these nanocarbons can have Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00008-0 © 2019 Elsevier Ltd. All rights reserved.

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hydrophilic groups attached, then such derivatives of nanocarbons may have significant applications in the field of biological sciences [29, 30]. Since their discovery, it has been a prevalent practice to purify the crystalline nanocarbons with dilute hydrochloric acid [31–33] and nitric acid [34–36] to free these from contaminants such as metallic impurities and amorphous carbon. On longer treatment with dilute nitric acid, the crystalline form also got sporadic incorporation of carboxylic acid groups on their surfaces, which are normally removed by thermal annealing treatment. Thus, the robust hydrophobic nanocarbons have been shown to incorporate hydrophilic groups by the simple process of oxidative derivatization [6–8, 14, 15, 18, 21, 23, 37–43]. Such attempts with subsequent variations in the concentration of nitric acid yielded high density carboxylated group attachments on the surface or periphery of nanocarbon materials, rendering these fairly dispersible or even soluble in water [6–8, 14, 15, 18]. Such creation of soluble derivatives opened immense possibilities for the use of these nanocarbons to probe living objects of diverse kinds. The use of water-soluble or dispersible nanocarbons in exploring fauna and flora kingdoms slowly started to begin with cautionary steps mainly based on the issue of toxicity [44–55]. There were sporadic attempts to use nanocarbon derivatives in biomedical fields, mainly for imaging purposes. This is because of the surface passivation of these nanocarbons introducing hydrophilic groups that made them self-fluorescent [8, 15, 16, 56, 57]. However, out of curiosity, people used pure CNTs in cell studies and concluded that CNTs are toxic to living cells. The problem that was never addressed at that time is the nature of cell damage caused by these pure CNTs. The toxic effect could be chemical, physical, or biochemical in nature and in most cases, the physical interaction between hard CNTs with cell membranes caused cell damage. Such physical damage could be avoided by using soluble and soft versions of nanocarbons. These are in general named as defective nanocarbons possessing the high density surface defects [9, 15, 21, 34, 42]. Based on such criteria, the use of nanocarbons in the plant kingdom has been slowly developed. This chapter is based on such attempts to introduce nanocarbons in assisting the growth of the plant kingdom [34, 55, 58–65]. It is very relevant now as everyone understands that the increase in agricultural production for feeding a fastgrowing global population is the most significant challenge today. For the present and future demands of food [66–71], the use of newer sustainable materials and technologies [10, 72–77] is very desirable for increasing crop or plant productivity. To achieve this goal, presently we are in the state of using synthetic fertilizers [78–84] for improving crop productivity. But it is now known that the use of synthetic fertilizers is remarkable only for a short period of time. The long-run applications of synthetic fertilizers are showing adverse effects compared to organic based fertilizer like biochar [78, 81, 85–89]. Especially in deteriorating the natural composition of the soil [80, 82–84] and killing valuable microorganisms [90–92] and causing water pollution after discharge into water streams [79]. Presently, the use of sustainable organic-based fertilizers or manure is gaining popularity as some people believe that food grown with such a natural product would be better. Compared to synthetic fertilizers [78, 81, 91–94], the organic-based fertilizers show biocompatibity [95–97] and higher efficiencies, which make them more suitable materials for long-term agriculture applications as fertilizers. So organic-based

Prospects of nanocarbons in agriculture

289

fertilizers are being explored to increase the yield of agricultural output. However, organic fertilizers or manures are relatively costly compared with synthetic fertilizers. Along these, recent studies pointed out the benefits of age-old practices regarding the use of charred carbon materials from biowaste as a promoter of soil health in the name of “biochar” [86–89, 98–121]. Principally, biochar is a carbonaceous material mainly composed of carbon and partly with oxygen that has a good level of biological and chemical stability [104, 107, 120]. This is made by the simplest method of charring the residual waste biomass in open and in air-exposed atmospheres [101, 102, 115, 122, 123]. Thus swapping from synthetic chemical-based fertilizers [73, 78, 81, 91–94] to organic fertilizers [85, 124] or carbon-based [86–89, 111, 112, 125] to nanocarbon-based fertilizers [10, 36, 58, 61, 64, 126–130] could be a worthy option. As biochar is in principle of plant origin and it has recently been shown that it predominately contains graphene oxide [131], it can be extended to natural-based fertilizers. Also, with it being nanosized, this could handle the future positive consequences of carbon nanotechnology [10, 36, 65, 73, 74, 126] in the fields of agriculture and plant biotechnology [68, 74] to enhance crop health and increase productivity [10, 126]. So, the soluble versions of nanocarbons may challenge organically grown plant products, once the myth of toxicity can be completely erased. Such nanocarbons can be the solution that not only boosts plant growth but also helps generate the concept of nanofertilizers [132]. It has been argued that the slow and sustained release of micro and essential nutrients to young plant saplings by nanocarbons and nanocarbonbased composites may lead to plant growth, resulting in good and healthy flowering that yields the best possible fruit or crop. To achieve this, the negative impacts of nanocarbons in plants concerning its dose-dependent toxicity [44–55] must be addressed.

8.2

Biochar

The significant uses of biochar in agriculture are very simple to understand, as the precise information is available from long-known ancient agricultural practices concerned with the use of biochar [86–88, 100, 120] in the soil for increasing fertility [98, 101, 102], sequestration [98, 102, 113], restoration of soil carbons, and removal of hazardous chemicals [118, 121].

8.2.1 Biowaste-based charred carbon At present, biochar has been explored in various fields based on its immense potential [98, 105, 107, 120, 121, 133], as displayed in Fig. 8.1A that showing the environmental applications of biochar [87, 107, 114, 120]. During the process of charring or pyrolysis of biowaste in open air, many oxygen-bearing groups were impregnated on the carbonaceous surface of biochar, particularly the carboxylic acid and hydroxyl groups. There are several participating sites for the ion-exchange type of responses displayed by these charred species, as shown in Fig. 8.1B [87]. Chen et al. demonstrated in their representative work that the surface carboxylic acid groups were close

290

Nanocarbon and its Composites

Fig. 8.1 A schematic illustrating (A) biochar as a platform carbon material for various potential applications [107]; (B) pH-dependent dissociation of acid/base groups on the biochar surface. Reprinted with permission from (A) Liu W-J, Jiang H, Yu H-Q. Development of biochar-based functional materials: toward a sustainable platform carbon material. Chem Rev 2015;115 (22):12251–12285; (B) Chen Z, Xiao X, Baoliang Chen, Zhu L. Quantification of chemical states, dissociation constants and contents of oxygen-containing groups on the surface of biochars produced at different temperatures. Environ Sci Technol 2015;49:309–317.

Prospects of nanocarbons in agriculture

291

3 to the negative moieties (ClO 4 [87, 134] and PO4 [87]) via hydrogen bonding and once the species became deprotonated due to the change in soil pH, these groups then combined with the metal ions (Al, Cd, and Pb) [108, 116]. The study from Sarkar and coworkers [73] described the nanosignificance of the biochar where they have shown the presence of spherical nanocarbons in the biochar.They correlated the benefit of the biochar by high degree of surface functionalization which create defects on its surface and hence makes it highly porous. The presence of such ionic materials provides the active sites for the ion exchange reactions to control the uptake and release of several ions [73]. These studies successfully correlate the positive effects of the conventional technique of “charring the waste” (residual crop, waste root ends, straws, etc.) after harvesting the main crop; such burning to create biochar has been directly implicated with nanocarbons [73]. The elemental compositions of nanocarbons are very similar to the biochar and the difference could be related to the variation in the presence of hydrophilic groups per unit mass of the product. Being loaded with limited hydrophilic groups, biochar remains sparsely soluble in water and thus it remains in the soil for years with very slow aerial oxidation under a natural environment. So, to extend the extraordinary results with biochar, nanocarbons can achieve more if the simplest surface modification introducing more units of electrophilic groups per unit mass renders these appreciably soluble in an aqueous medium. Essentially, the introduction of more electrophilic groups is related to more changes of sp2 hybridization of carbon centers to the sp3 type of hybridization in the nanocarbon framework. This would allow more volume with porosity on the carbon network, which would help to absorb nutrients or even water in appreciable amounts. In comparison to synthetic fertilizers, nanocarbon-based fertilizers cause a slow and steady nutrient release that results in more sustained availability of nutrients, which can replenish soil structure and plant growth [98, 102], Such nanocarbon-based fertilizers show excellent properties related to balancing the soil pH [87, 108] and helping in the retention of water [102] and nutrients [73, 98, 102]. Because of a very high surface area [102, 121] to volume ratio, such material can significantly retain more water that could be released when needed by the plant [102]. So, the charred carbon deals with both the classical (black carbonaceous mass left in the field after burning the plant waste) [100, 104, 120, 135] and the laboratory made nanocarbons, both are showing the enhancement in the crop productivity.

8.2.2 Effect of biochar-derived nanocarbons on plant growth The surface of biochar and biochar-derived nanocarbons acts like a storage house of nutrients, including micronutrients. It was further reported that biochar derived CNPs show controlled release of nutrients with essential time of retention [73]. The surface defects associated with the presence of functional groups are directly involved with controlling the release of the nitrogen-comprising nutrient, like the cationic (NH+4 ) and anionic (NO 3 ) in the plant. This representative study is then correlated with the results in accordance with the use of biochar for the controlled release of the nutrients [73]. As described in Fig. 8.2. A-F the wheat seeds were germinated under different concentration (10, 20, 40, 50, 80, 65, 100 and 150 mg L1) of soluble CNPs and the maximum growth was noticed in the plants treated with 50 mg L1 concentration

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Nanocarbon and its Composites

(C)

(D)

(E)

(F)

3

rCNPs wsCNPs

2

1

0

(G)

24

72

120 Time (h)

168

240

Concentration of [NO3]− ions released (10−5 M)

(B)

Concentration of [NH4]+ ions released (10−5 M)

(A)

3

rCNPs wsCNPs

2

1

0

(H)

24

72

120 Time (h)

168

240

Fig. 8.2 Germinated seeds (A) control (0 mg L1) with wsCNPs of concentration; (B) 10 mg L1; (C) 20 mg L1; (D) 40 mg L1; (E) 50 mg L1; and (F) 150 mg L1 (these are not imaged from the same distance and the figures compare only the variation of shoots in germination); Release of adsorbed ions from rCNPs and wsCNPs at different time intervals: (G) NH+4 ; (H) NO 3 ions. Reprinted with permission from Saxena M, Maity S, Sarkar S. Carbon nanoparticles in ‘biochar’ boost wheat (Triticum aestivum) plant growth. RSC Adv. 2014;4:39948–39954.

wsCNPs. So, the exact concentration-dependent studies are needed to understand the dose-dependent effect. They proposed a threshold concentration from the variable concentration window for wsCNPs to show the prominent optimized concentration. Exceeding this concentration, the effect of high concentration of carbon-based fertilizers leading to plant senescence is observed. Importantly, they showed by their model experiment that both the materials as raw CNPs (rCNPs) (just like as collected biochar) and the water-soluble version of CNPs as wsCNPs were capable of absorbing the nutrient molecules [73]. The slow release with time of cationic and anionic nutrients from wsCNPs is displayed in Fig. 8.2G and H.

Prospects of nanocarbons in agriculture

8.3

293

Nanocarbons on plant growth

Presently, the nature of nanocarbon–plant interaction is well supported by several reports. The simplest versions of hydrophilic nanocarbons, also known as watersoluble versions of nanocarbons [10, 36, 73, 130], can easily enter the seeds of the gram [10, 36] and wheat plants [126, 136, 137] to show the increase in the overall growth. Tripathi et al. described [132] that the wood wool (basically a wood) on charring formed carbon nanoanions (CNOs) and its water-soluble version is capable enough to increase the overall yield of the first-generation gram seeds with the additional advantage of having higher concentrations of micronutrients in the seeds. Such effort would be helpful to enrich cereal food grain with desired micronutrients for better health. Studies [30, 60, 63] have shown the physical presence of nanocarbons inside the tracheal elements of the xylem vessels, which are responsible for the conduction of water inside the plants. Scanning electron and transmission microscopic images [10, 36, 129, 130] showed that the growth-stimulating effects of the watersoluble nanocarbons are related to their water solubilization and high density surface defects [10, 36, 73, 130, 137]. Such a high degree of surface defects can easily be correlated with losing crystalline nature, resulting in high surface area like biochar. Khodakovskaya et al. reported that the increase in the gene expression can be correlated to the activity of the aquaporin [61, 126, 128, 138, 139]. Such a study complimented the belief that nanocarbons in the xylem vessels help the water conduction efficiency in plants. Thus the water-soluble and functionalized versions of nanocarbons can smoothly travel through the microchannels in plants whereas nonfunctionalized nanocarbons cannot be easily transported inside. Being insoluble, they are stuck and by physical force pierce the channel lines and cell lines of living plants manifesting toxic properties. For the CNTs, both SWCNTs and MWCNTs [136, 140, 141] can enter the seed coat of the tomato [126, 138, 139, 142]; these were shown to increase the growth of plants (tomato) [126] and tobacco cells [128]. However, the initial interaction of these crystalline nanocarbons with a swelled seed coat may assist in the germination process. Giraldo et al. reported a significant study related to the increase in the photosynthetic efficiency in the chloroplasts in spinach [65]. A few more reports are also available for the potential uses of several nanocarbons on other plants [34, 55, 58–65]. Studies related to the continuous tracking of the movement of nanocarbons inside the plant body and, at the end, their accumulation are available. Presently, the physical presence of nanocarbons inside plant parts is seen by various spectroscopic and microscopic techniques. The much-used spectroscopic techniques include Raman [61, 142], FTIR [34, 58], and microscopic techniques including fluorescence [36, 130, 136], confocal [74, 136], transmission electron microscopy [90, 143, 144], scanning electron microscopy (SEM) [62], atomic force microscopy (AFM) [145], and confocal scanning laser microscopy (CSLM) [130, 145]. Because of the presence of the extra cell wall in a plant, the internalization and the movement of the nanocarbons inside the plant system are not straightforward to trace like the approach used for mammalian cells. The uptake, movements, or translocation of different shaped nanocarbons purely depends on several factors. Among these, the important factors are the shape and size of the nanocarbons nature of surface

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functionalization, growth media, aggregation of nanocarbons, exposure time, water solubility and most importantly on the plant species. The type of nanoparticles, their physical presence, and the technique used to characterize those with positive impact inside plants are thoroughly reviewed, as briefed in Table 8.1. Generally, the nanocarbons suspended in aqueous growth medium are easily taken up by the plant via routine activity, such as the uptake of other essential nutrients. Nanocarbons taken by the plants were mostly been found inside the xylem vessels. Significant observations were made with the use of surface-functionalized nanospecies that showed reduced toxic effects of nanocarbons in plants [10, 36, 73, 126, 138–140, 142]. The surface-functionalized nanocarbons can easily cross the cell walls because of the softness and fluidity in the aqueous medium to utilize channels defined to allow the transport of materials [136, 140, 141]. On the other side, hard nonfunctionalized versions of the nanocarbons would physically interact with softer cell membranes of plants and eventually pierce the cell walls, causing cell death. Comparatively, the functionalized nanocarbons are fairly soluble or dispersed in the water, showing enough softness due to high surface functionalization. Thus nanocarbons can easily cross plant cell walls and significantly articulate in the overall growth of the treated plants by promoting the process in seed germination [61, 126, 127, 138, 139], gene regulation [10, 34, 35, 60, 61, 126, 128, 138, 139], and water uptake [10, 36, 130], leading to an increase in biomass resulting in the overall productivity of the plants [10, 126] and in enriching soil microenvironments [126, 132] as well. Such positive effects of carbon-based fertilizers such as biochar are shown in Fig. 8.3.

8.3.1 Water-soluble nanocarbons 8.3.1.1 Water-soluble carbon nanotubes Sarkar and coworkers were the first to document the positive impact of the watersoluble version of MWCNTs as wsCNTs on gram plants [36]. They used hard MWCNT and made them heavily derivatized, introducing peripheral carboxylic and hydroxyl groups to create water-soluble CNTs (wsCNTs). The gram plants treated with such wsCNTs showed increased overall growth with an increase in the length of shoot, the number of roots, and the branches. The increase in the overall growth was related to the increase in the water uptake efficiency of treated plants compared to control. Between the control and treated plant, this observation can be related to the better supply of nutrients and micronutrients in the treated plant with the uptake of more water. This can thus refer to the enhanced movement of the water-soluble nutrients with wsCNTs. They proposed a possible mechanism (Fig. 8.4A) related to the formation of smaller capillaries via the embedding of wsCNTs inside the larger capillaries as the tracheal elements of xylem vessels. This is somewhat like aligning the wsCNTs one on another from the head because of the large potential drift generated at the process of transpiration. The newer capillaries generated by the wsCNTs could help in enhancing the water uptake and retention properties of the plant. That can directly facilitate the movements of water-soluble nutrients required by the plant. They have successfully confirmed the proposed mechanism by using fluorescence

Table 8.1 Effects of nanocarbons on plant growth

Type of nanocarbons Water soluble

wsCNTs Or wsMWCNTs

Presence of nanocarbons inside the plant parts

Functionalized/ nonfunctionalized

Concentration of nanocarbons

Growth medium

Gram

Functionalized with conc. nitric acid

3 and 6 μg mL1

DI water

Mustard

Functionalized with 2 M nitric acid

2.3 and 6.9 μg mL1

DI water

Lumen of tracheal elements of xylem vessels Shoots and roots

Alfa-alfa, Wheat

Acid functionalized

40–2560 mg L1

DI water

Root surface

Tomato

Functionalized with different groups (COOH, acetone and CH2,3, poly-ethylene glycol (PEG))

40 μg mL1

MS medium

Root cells

Wheat

Oxidization with hydrochloric acid (HCl)

10, 20, 40, 80, and 160 μg mL1

DI water

Cellular cytoplasm of root cells

Plants/cells

Effect on plants Increased 1. overall growth 2. water uptake Increased 1. seed germination rate 2. shoot and root length Increased 1. seed germination 2. root elongation Increased 1. fresh biomass 2. seed germination 3. expression of water channel genes Increased 1. root growth 2. vegetative biomass

References [36]

[34]

[60]

[137]

Continued

Table 8.1 Continued

Type of nanocarbons

Plants/cells

Functionalized/ nonfunctionalized

Concentration of nanocarbons

Growth medium

Presence of nanocarbons inside the plant parts

Rice

Functionalized with nitric acid

50 μg mL1

MS basal medium

Root surface

Barley, corn, soybean

Functionalized with carboxyl groups

100 and 200 μg mL1

MS medium

Seed surface and can even penetrate seed coat of all plants, corn leafs, and shoots of soybean

Effect on plants 3. cell elongation 4. concentration dependent increase in dehydrogenase activity Increased 1. root and shoot length 2. seedling growth 3. seed germination rate Increased 1. seed germination rate 2. expression of water channel genes (aquaporins, PIP, TIP, SIP) 3. in corn highest (i) total fresh shoot weight

References

[35]

[61]

SWCNTs

Hybrid Bt cotton

Oxidized with H2SO4 + HNO3 (3:2)

20, 40, 60, 80, and 100 μg mL1

MS medium



Fusarium graminearum and Fusarium poae

Functionalized with carboxyl groups

62.5–500 μg mL1



Spore surface

Nicotiana tobacum L.cv. Bright Yellow (BY-2) cells

Functionalized with a concentrated mixture of H2SO4/HNO3

0.08 mg mL1

DI water

Vacuoles

(ii) leaf length 4. in soybean longer root system Increase in 1. shoot and root length 2. height 3. number of roots, leaves, and bolls per plant 4. boll size Antifungal activity– inhibition of 1.water uptake 2 induced plasmolysis SWCNTs enter via fluidicphase endocytosis

[59]

[146]

[136]

Continued

Table 8.1 Continued

Type of nanocarbons

Effect on plants

Functionalized/ nonfunctionalized

Concentration of nanocarbons

Growth medium

Functionalized with a concentrated mixture of H2SO4/ HNO3 (3:1) Functionalized with mild nitric acid treatment

0.4 mg mL1

DI water

Vacuoles, cytoplasm, and cell nucleus

SWCNT-FITC enters the cell wall

[141]

50 μg mL1

MS medium

Root, leaves, and fruits

[138]

Fusarium graminearum and Fusarium poae

Functionalized

62.5–500 μg mL1



Spore surfaces covered with SWCNT

Gram

Functionalized with conc. nitric acid

10, 20, and 30 μg mL1

DI water

Tracheal elements of xylem vessels.

Increased 1. vegetative biomass 2. expression of various genes Antifungal activity 1. reduced water uptake 2. caused plasmolysis Increased 1. vegetative mass 2. fruit production Increased 1. yield per plant 2. individual weight, size of seeds

Plants/cells Catharanthus roseus

Tomato

wsCNOs

Presence of nanocarbons inside the plant parts

Cotyledon/seed coat

References

[146]

[10]

[132]

wsCDs

Wheat

wsCNPs

Wheat

CSCNTs

Arabidopsis thaliana

Functionalized with conc. nitric acid Functionalized with HNO3 and water (in 1:1 ratio)

Functionalized with H2SO4 + HNO3 (3:1)

150 μg mL1

DI water

Root region of plants

10, 20, 40, 50, 80, 100, and 150 mg L1

DI water

50 μg mL1

MS medium with 1 μM of 2,4-diphenoxy acetic acid and kinetin.

Xylem vessels, show controlled release of nutrients and ions Structures of plant tracheids

3. protein content 4. stored electrolytes 5. metallic micronutrients Increased in shoot and root length Increased in overall growth of plant

Building the heterogeneous deposition of tracheary elements of plants and participate in nonenzymatic polymerization process

[73]

[130]

[145]

Continued

Table 8.1 Continued

Type of nanocarbons

Plants/cells

Functionalized/ nonfunctionalized

Concentration of nanocarbons

Growth medium

Presence of nanocarbons inside the plant parts

GO sheets

Wheat (Triticum durum cv. Yallaroi)

Functionalized

20 mg L1

DI water

Desired slow and sustained release of micronutrients (Zn and Cu) by GO micronutrient fertilizers

GN, GO





0.05–0.5% w/w



Cogranulation of GN and GO sheets with MAP fertilizer enhanced mechanical strength, resistant to abrasion, impact resistance, diffusion rate of phosphorous

Effect on plants Grain dry mass was higher only for soil treated with Zn–GO, and nutrient uptake was higher for soil treated with Zn– GO and Cu– GO, slow release rate of Zn and Cu ions (55%) after 72 h –

References [147]

[148]

Functionalized

C60(OH)24–26 Fullerol

Arabidopsis thaliana

Synthesized through an alkali route

100 and 200 mg L1

MWCNTs

Catharanthus roseus

Functionalized

0.04 mg mL1

Tomato

Slightly functionalized

50 μg mL1

Minimal medium (1 mM KCl, 1 mM CaCl2; solidified with 1% phytoagar) MS medium with minimal organics supplemented with 1 μM 2,4-diphenoxy acetic acid and 1 μM kinetin MS medium

lower with MAP-GN and MAP-GO fertilizers –

Shorterplastids, nucleus, and vacuoles Largerendoplasmic reticulum Fruits, root, and leaves

Increased in the length of hypocotyls

[51]

Plant protoplasts adopted endosomeescaping uptake mode (nontoxic internalization) of MWCNTs Increased 1. seed germination 2. biomass 3. relative transcriptive 4. water channel protein,

[140]

[138]

Continued

Table 8.1 Continued

Type of nanocarbons

Non functionalized

MWCNTs

Plants/cells

Functionalized/ nonfunctionalized

Concentration of nanocarbons

Growth medium

Presence of nanocarbons inside the plant parts

Tomato

Slightly oxidized

50 and 200 μg mL1

Agar MS medium

Surface of fruits and flowers.

Tobacco

Purified with HCl

5100 and 500 μg mL1

MS medium with 0.8% agar

Cells

Ryegrass, rape, corn, cucumber

Pristine

2000 mg L1

DI water and Zn+2 solution

Different seed soaking techniques resulted in different effect

Effect on plants Les.564.1.S1heat shock protein 90, and stress protein Increased 1. number of seeds 2. flowers 3. fruits 4. plant height Increased 1. cell growth 2. fresh biomass 3. expression of aquaporin genes (NtPIP1), cell division genes (CycB), and cell wall extension genes (NtLRX1) Increased in the root length

References

[126]

[128]

[55]

SWCNTs

Maize

Pristine

10, 20, and 40 mg L1

Nutrient agar gel medium

Seed surface

Corn

Pristine

500, 1000, and 5000 mg kg1

DI water



Onion, Cucumber

Non functionalized

56, 315, and 1750 mg mL1

DI water

Tomato

Non functionalized

50 μg mL1

1% agar (Murashige and Skoog) MS medium

Surface of secondary roots and root hairs as compare to main roots Leaf

Rice

Purified and pristine form

50 μg mL1

MS basal medium

Germination site, in root surface and shoot tissues

Increase 1. water absorption 2. plant biomass 3. concentration of Fe, and Ca nutrients Increased in the 1. net growth 2. biomass Increased in the root elongation

[64]

Increased 1. vegetative biomass 2. seedling growth Increased 1. root and shoot length 2. seedling growth 3. seed germination rate

[142]

[149]

[62]

[35]

Continued

Table 8.1 Continued

Type of nanocarbons

Plants/cells

Functionalized/ nonfunctionalized

Concentration of nanocarbons 1

Fullerene (C60)

Rice

Pristine form

50 μg mL

[C60(OH)20] Fullerol

Bitter melon

Pristine form

0.943, 4.72, 9.43, 10.88, and 47.2 nM

Growth medium

Presence of nanocarbons inside the plant parts

MS basal medium

Roots surface

Milli-Q water

Stem, leaf, petiole, flower, and fruit

Effect on plants Increased 1. root and shoot length 2. seedling growth 3. seed germination rate Increased 1. biomass yield 2. fruit number 3. fruit weight 4. fruit length 5. plant water content, two anticancer phytomedicine and two antidiabetic phytomedicine

References [35]

[58]

Prospects of nanocarbons in agriculture

305

Fig. 8.3 Schematic diagram supporting effects of the organic charred carbon as biochar and different types of nanocarbons in the plants and soil environment.

microscopic images, as shown in Fig. 8.4B and C and by SEM microscopic pictures, as illustrated in Fig. 8.4D and E showing the absence and presence of well-aligned and embedded wsCNTs in the control and treated samples, respectively.

8.3.1.2 Water-soluble carbon nanoonions and carbon dots Sonkar et al. described the beneficial role of wsCNOs in gram plants in terms of the increased productivity regarding harvesting more fruits (gram seeds) per plant, as shown in Fig. 8.5. The overall growth of gram plants was related to the increased water conduction properties of the plant facilitating the transport of micronutrients. Recently, Tripathi et al. reported a detailed comparative analysis of the micronutrient content in the first-generation gram seeds (FGSs) obtained just after harvesting [132]. Gram plants were treated with three different concentrations of wsCNOs for a short period of time to restrict the accumulation of wsCNOs in fruits.

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Nanocarbon and its Composites

By the force of absorption

CNT forms the “large capillaries”

Carbon nanotube in random arrangement

Introduction of ‘‘large capillaries” inside tracheary elements

= Water conduction

(A)

= Water molecule

Conduction takes place

(B)

+

Arrangement of CNT inside tracheary elements

Tracheid Vessel Tracheary elements

Tracheid + vessel

(C)

200 nm

(D)

Mag = 50.00 K X

EHT = 10.00 kV

Date :2 Nov 2010

WD =

Signal A = InLens

Time :11:04:41

4mm

100 nm

Mag = 100.00 K X

EHT = 10.00 kV

Date :29 Oct 2010

WD =

Signal A = InLens

Time :11:29:16

3mm

(E)

Fig. 8.4 (A) Schematic diagrams of alignment of wsCNTs inside roots. (B and C) Fluorescence images showing the transverse section (T.S.) and longitudinal section (L.S.) [128] of CdS-treated root; FESEM images of the L.S. of root; (D) without and (E) with wsCNTs, white arrows show its alignment inside the xylem vessels of root; inset shows zoomed of nicely aligned wsCNTs. Reprinted with permission from Tripathi S, Sonkar SK, Sarkar S. Growth stimulation of gram (Cicer arietinum) plant by water soluble carbon nanotubes. Nanoscale 2011;3:1176–1181.

In a continuation of their earlier findings, a full life cycle analysis related to the “seed to seed” cycle study was carried out and the result is shown in Fig. 8.6 [132]. A significant and substantial increase was observed in the concentrations of stored micronutrients (copper, zinc, molybdenum, iron, manganese, and nickel) in seeds obtained from the wsCNO treated plants compared to the seeds obtained from control plants. Tripathi et al. demonstrates the interaction of water-soluble carbon dots

Prospects of nanocarbons in agriculture 307

Fig. 8.5 (i) Phenotypic images of gram plants treated with different concentration of wsCNOs after 10 days; (A) control; (B–D) plants treated with 10, 20, and 30 mgmL1 wsCNOs, respectively. (ii) Phenotypes of gram seeds (fruits) for the control (A), and under varying concentrations of wsCNOs (B) 10, (C) 20, and (D) 30 mgmL1 of wsCNOs. (iii) Shows the weight of fruits per plant in terms of SE (Reprinted with permission from Sonkar SK, Roy M, Babara DG, Sarkar S. Water soluble carbon nano-onions from wood wool as growth promoters for gram plants. Nanoscale 2012;4(24):7670–7675). Histogram: (iii) effect of wsCNOs concentrations; (iv) total protein contents of FGSs with  SE (number of seeds—25); (v) electrical conductivity from stored electrolytes in FGSs with  SE (number of seeds—15) along with the given significance (p < 0.005). Reprinted with permission from Tripathi KM, Bhati A, Singh A, Sonker AK, Sarkar S, Sonkar SK. Sustainable changes in the contents of metallic micronutrients in first generation gram seeds imposed by carbon nano-onions: a life cycle seed to seed study. ACS Sustainable Chem Eng 2017;5(4);2906–2916.

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Fig. 8.6 Schematic diagram showing the effect of wsCNOs on the growth as overall productivity and metallic micronutrient concentration compared to control. Reprinted with permission from Tripathi KM, Bhati A, Singh A, Sonker AK, Sarkar S, Sonkar SK. Sustainable changes in the contents of metallic micronutrients in first generation gram seeds imposed by carbon nano-onions: a life cycle seed to seed study. ACS Sustainable Chem Eng 2017;5:2906–2916.

(wsCDs) with wheat plants under the influence of light and dark conditions of its growth. They also showed the increase in the growth of the root and shoot of plants using wsCDs [130].

8.3.1.3 Water-soluble fullerenes In comparison to CNTs, there are only limited reports available showing the positive interactions of plants with fullerenes (functionalized versions as fullerol [C60(OH)20]). Kole et al. used the seeds of a bitter melon and showed the significant effects of fullerol on the growth of those seeds [58]. They observed that the fullerol-treated plants were shown to have increased in the biomass and water content of the seeds while the phytomedicine content of the fruit also gets changed. The fullerol-treated fruits show improved length, weight, and amount of fruit with the increase in the overall yield (in terms of fruits). Nair et al. showed the improvement in the germination rate of 60% with rice when treated with C60 [35]. As discussed earlier, hydrophobic nanocarbons (in the present case C60) physically interact with the swelled seed coat to facilitate germination. Another study described by Gao et al. showed the positive interactions of polyhydroxyfullerols on Arabidopsis thaliana seeds [51].

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8.3.1.4 Multiwalled carbon nanotubes Being the first version of nanocarbons with low cost and easy access, MWCNTs have been the most explored nanocarbons used for the study on plant–nanocarbon interaction. Serag et al. reported the differential penetrating ability of MWCNTs into the cell membranes and protoplasts of Catharanthus roseus, using confocal microscopy and HRTEM analysis [140]. They reported that the penetration ability of MWCNTs was considerably dependent upon their size, as shown in Fig. 8.7A. The shorter FITC-labeled MWCNTs (30–100 nm) showed an increased infiltration capacity and were found distributed in the vacuole, plastids, and nucleus of the cells, as demonstrated in Fig. 8.7B and C while larger FITC-labeled MWCNTs (>200 nm) were found in other subcellular structures such as the endoplasmic reticulum and mitochondria of the Catharanthus cells. MWCNTs were internalized by labeling the endosomal membrane with FM4–64 dye, which is a robust marker for endocytosis. Only a few endosomes having a larger size were observed to entrap MWCNTs-FITC and show fluorescence. TEM observations (Fig. 8.7D–F) clearly indicated the presence of isolated MWCNT-FITC conjugates inside the cells while the presence of larger aggregates or bundles was observed outside the cells [140]. The uptaking of MWCNT-FITC conjugates via the endosome escaping route was suggested to endow with a method for further use in delivery techniques of biomolecules within the cells in a more precise way. Khodakovskaya and coworkers studied the interaction of MWCNTs with diverse plant cells [10, 61, 126, 128, 138, 139]. In their study they reported that the plants treated with MWCNTs exhibited a significant gain in overall growth, such as plant height, the number of leaves, flowers, and fruits, and the overall size of the fresh fruit (Fig. 8.8). It was proposed that the plant reproductive system was activated by the use of MWCNTs, which consequently led to overall increased productivity [126]. In another case, MWCNTs were supplied to plants by two different ways, either added via an air sprayer or directly mixed in the growth medium [61]. The MWCNTs exhibited more pronounced stimulating effects for treated tomato plants associated with the faster rate of germination, growth activation, and phenotype variation. MWCNTs have also been reported to activate the expression of many stressresponsive tomato genes [126, 138, 139]. It was explained that MWCNTs stimulated the water channel gene expressions that have a critical role in the seed germination process. MWCNT-exposed tomato plants showed the upregulation of stress-related and water-channel protein (LeAqp2) genes in roots and leaves. Significant differences were identified in 91 and 49 transcripts in leaves and roots, respectively, for 16 genes of known functions in MWCNT-exposed plants [138]. Genetic analysis was adapted to confirm the activation of LeAqp2 gene expression and enhanced the expressions of mitogen-activated protein kinase with MWCNT-exposed seedlings. The surface functionalization of MWCNTs significantly influenced their ability to cross the cellular membrane. For instance, seed germination and plant growth rates were more pronounced in oxidized MWCNT (o-MWCNT)-treated seeds than seeds treated only with nonfunctionalized MWCNTs [34]. Zhai et al. demonstrated the effect of neutral (p-MWCNTs), positively charged (NH2-MWCNTs), and negatively charged (COOH-MWCNTs) MWCNTs on the growth of maize and soybean plants [63]. The growth effects were found to be more significant for negatively

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Fig. 8.7 Uptake of MWCNTs-FITC by the protoplasts of Catharanthus roseus: (A) confocal microscopy images of the protoplasts incubated with MWCNTs-FITC conjugate, scale bar is 20 μm; (B) histogram showing the length dependent distribution of MWCNTs-FITC inside the protoplast after 3 h incubation; (C) schematic model of cell showing the uptake and distribution of MWCNTs-FITC based upon length of nanotubes; (D–F) TEM characterization showing the cellular distribution of MWCNTs-FITC inside the protoplasts; (D) localization of MWCNTs-FITC inside the cell vacuole (black arrows); (E) inside the plastids (green arrow). The dark rounded structure represents the starch granules; (F) inside the cell nucleus (red arrow) and blue arrow indicates the nuclear membrane. The scale bars are of 500 nm for (D–F). Reprinted with permission from Serag MF, Kaji N, Gaillard C, Okamoto Y, Terasaka K, Jabasini M, et al. Trafficking and subcellular localization of multiwalled carbon nanotubes in plant cells. ACS Nano 2011;5:493–499.

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Fig. 8.8 Effect of carbon nanotubes watering (50 and 200 μg mL1) on development (CNT) supplied with tomato fruits: (A) average number of flowers; (B) average number of fruits; (C) size of fruits, and (D) number of seeds per fruit were measured for CNT-exposed plants (CNT 50 and CNT 200), plants exposed to activated carbon (AC), and unexposed tomato plants (control) at stage of mature (red) fruits. Each data point is the average of 20 individual measurements. Thus, vertical bars indicate SE (n ¼ 20). Reprinted with permission from Khodakovskaya MV, Kim B-S, Kim JN, Alimohammadi M, Dervishi E, Mustafa T, et al. Carbon nanotubes as plant growth regulators: effects on tomato growth, reproductive system, and soil microbial community. Small 2013;9:115–123.

charged MWCNTs compared to neutral or positively charged MWCNTs. Longer (50–1000 nm) MWCNTs were reported to accumulate in the roots while shorter ones (50–100 nm) accumulated in the stem and leaves. The significant effect of surface functionalities of nanocarbons with five different types of MWCNTs (MW1 to MW5) were investigated with tomato plants, as shown schematically in Fig. 8.9A [139]. The change in phenotypes for MWCNT-exposed plants was observed continuously up to 41 days and photographed sequentially after 1 week, 4 weeks, and 41 days, as shown in Fig. 8.9B–D, respectively. MW2- and MW3-treated plants showed two times higher in the biomass than control plants while an increase in the biomass of tomato plants treated with MW4 and MW5 was less pronounced, but higher than the control and MW1. That can be correlated with the enhanced expression of water channel protein (LeAqp1), as shown in Fig. 8.9E. They reported that MWCNTs activated the water channels (aquaporin) and major gene regulators responsible for cell division and extension [139]. Enhancement (55%–64%) in the fresh biomass and overall growth of tobacco cells was observed for MWCNTs in comparison with the activated carbon (AC), which stimulates only 16% cell

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Fig. 8.9 (A) Schematic diagrams showing the experimental conditions of various surface functionalities of MWCNTs; (B) Phenotypic effects of tomato seedlings treated with different surface functionalities (MW1–MW5) of MWCNTs (40 μg mL1) with control sample after 1 week; (C) after 4 weeks; (D) after 41 days of incubation; (E) water channel protein (LeAqp1) expression analysis by Western blot after 8 and 41 days of incubation of tomato plants grown on standard MS medium (control). Reprinted with permission from Villagarcia H, Dervishi E, Silva KD, Biris AS, Khodakovskaya MV. Surface chemistry of carbon nanotubes impacts the growth and expression of water channel protein in tomato plants. Small 2012;8:2328–2334.

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growth. MWCNT-treated tobacco cells showed a 35-fold increase of the CycB (cell cycle progression) gene expression within 6 h of incubation. Additionally, the study showed a significant increase in the expressions of the aquaporin gene (NtPIP1) with MWCNT-exposed tobacco cells [128]. Similarly, Mondal et al. reported a comparative study for the beneficial effect of MWCNTs before and after oxidation (o-MWCNTs) with nitric acid on the germination and growth of mustard plants [34]. On comparison between o-MWCNTs and MWCNTs, o-MWCNTs showed more prominent effects, even at lower concentrations, because of an increase in moisture content and water uptake.

8.3.1.5 Single-walled carbon nanotubes Strano and coworkers reported the passive transport and localization ability (irreversibly inside the lipid envelope) of SWCNTs inside the chloroplast of spinach cells (Spinacia oleracea L.), as shown in Fig. 8.10A [65]. Once assembled inside chloroplasts, SWCNTs were found to add new functional properties in chloroplast-based photocatalytic complexes. Assimilations or interactions of the SWCNTs with chloroplasts is of great significance. Chloroplasts are a unique source of chemical energy and exhibit great potential as an alternative energy source if they can be appropriately engineered for long-term, stable photosynthesis ex vivo. The physical presence of SWCNTs inside the chloroplasts was detected by using near infrared optical imaging. An SWCNT-coupled chloroplast showed three times increased activity of photosynthetic machinery via increasing the rate of electron transport. An increase in the electron transport process was immediately connected with the absorption of more photons [65]. SWCNT-coupled chloroplast provides suitable electronic band gaps for the enhanced conversion of absorbed energies into excitations, which facilitated the electron transport mechanism and consequently improved the power of solar energy conversion. SWCNTs at 2.5 mg L1 concentration significantly increased the electron transport rates up to 49% in vitro. In vivo 2.5 mg L1 and 5 mg L1 of SWCNT-treated leaves showed 27% and 31% enhancement of the electron transport process, respectively, in contrast to control leaves, as displayed in Fig. 8.10B and C. SWCNTs coupled with ceria significantly decreased the concentration of reactive oxygen species (ROS) 21.4% by converting the hydroxides and superoxide radicals into their respective ions that consequently prevent the damage of photo pigments and photoproteins. Fang and coworkers reported the ability of SWCNTs as nanotransporters (concerning intracellular labeling, imaging, and genetic transformation) for intact plant cells [136]. They investigated the temperature-dependent endocytosis effect of oxidized SWCNTs (SWCNTs) on tobacco (Nicotiana tobacum, bright yellow (BY-2)) cells. Conjugates of SWCNTs with fluorescein isothiocyanate (FITC) and FITClabeled single-stranded DNA (FITC-ssDNA) were fabricated via sonication and the efficiency of SWCNT-FITC and SWCNTs-FITC-ssDNA conjugates was further investigated. BY-2 cells, when incubated with SWCNT-FITC, exhibited better fluorescence in contrast to cells treated with only FITC molecules. More importantly, the ss-DNA-FITC conjugate molecule alone was not able to penetrate the cell walls. But

Nanocarbon and its Composites

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Fig. 8.10 (A) Mechanism of SWCNT trapping; SWCNTs transport through the chloroplast double membrane envelope via kinetic trapping by lipid exchange; (B) maximum electron-transport rates in extracted chloroplasts and leaves were quantified by the yield of chlorophyll fluorescence; (C) electron-transport-rate light curves indicated enhanced photosynthesis above 100 μmol m2 s1 for 5 mg L1 SWCNTs leaves. Reprinted with permission from Giraldo JP, Landry MP, Faltermeier SM, McNicholas TP, Iverson NM, Boghossian AA, et al. Plant nanobionics approach to augment photosynthesis and biochemical sensing. Nat Mater 2014;13:400–408.

conjugated with SWCNTs, as SWCNT-ss-DNA-FITC, they can cross the cellular barrier, confirming the ability of SWCNTs in the delivery of macromolecules. SWCNT conjugates showed their preferences over the differential localization within a cell because fluorescence signals from SWCNT-FITC were localized at the vacuoles and cytoplasmic localization for the SWCNT-ss-DNA-FITC conjugates. Neither conjugate exhibited any toxicity (in terms of cell death and changes in normal cytoplasmic fluidity), even after an incubation period of 24 h. Serag et al. demonstrated the tracking of SWCNTs in various cell components based on the fluorescence recovery measurements after photo bleaching (FRAP) [141]. Discrimination between fluorophores and fluorophore-labeled SWCNTs was proposed as

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attributed on the basis of relative differences in mobility. Short SWCNTs and Catharanthus roseus cells were used to explain the facilitation and inhibition of SWCNTs across the cell wall. It was proposed that the intake, consumption, and distribution of SWCNTs inside cells can be easily controlled via the surface functionalities and provide their eliminations at minimum toxicity. SWCNTs in this study were used as carrier-mediated transport (CMT) to target the specific molecules inside cells. The FITC-functionalized SWCNTs (SWCNTs-FITC) were localized inside cell vacuoles, based on the photo-bleaching measurements by fluorescence recovery of suspended cells.The vacuolar uptake of SWCNTs-FITC conjugate were restricted by the probenecid. Then the cytoplasm-accumulated SWCNT-FITC conjugates, which further increases the possibility of the conjugate being taken as the nuclear uptake. After the FRAP experiment, FITC was released from the SWCNT-FITC conjugate. The basis for releasing the FITC molecule is a strong bond, formed by negatively charged SWCNTs and positively charged nucleoplasmic protein. The free FITC accumulated inside the nucleolus. intracellular transport of SWCNTs-FITC is shown inCanas et al. reported that seeds with smaller sizes were greatly affected by nanocarbons, more so than larger seeds [62]. Khodakovskaya et al. reported the effect of SWCNTs, QDs, and mixed conjugates such as SWCNT-QD composites on tomato plants [142]. Only SWCNT-treated plants were able to reach a larger biomass in comparison with QDs and SWCNT-QD treated plants. The chlorophyll content was observed to decrease 1.5-fold along with a 4-fold reduction in root system biomass in SWCNT-QD treated plants [142].

8.4

Effect of nanocarbons on soil microenvironments

Apart from offering many positive solutions, the persistent impact of nanocarbons on the soil microenvironment is crucial for their long-term applications. More significantly, under optimized conditions, nanocarbons can directly be used in the soil by merely watering to benefit the concerned plants. Studies on the impact of nanocarbons on a soil microenvironment (soil microbial community) related to their different shapes, sizes, and surface functionalities are very limited but showed huge differences [146–148, 150]. Alterations in soil microenvironments directly affect the nutrient cycle and its ability toward the sensor for heavy metal contamination and antimicrobial agents [149]. Studies related to enzyme activities are assumed as potential indicators of soil microbial function and are applied for the investigation of soil microenvironments [151, 152]. Shrestha et al. reported the impact of MWCNTs with varying concentration from 10 to 10,000 mg kg1 on soil microorganisms [153]. Oleszczuk et al. reported the phytotoxicity of sewage sludge containing CNTs at various levels [154]. Chung et al. proposed the short-term effects of MWCNTs on the activity and biomass of microorganisms in two different types of soil via an incubation study [151]. Li and coworkers studied the effect of MWCNTs on the structure and diversity of the bacterial community in activated sludge [155]. A study by Khodakovskaya et al. revealed that tomato plants grown in soil supplemented with CNTs via watering have a substantial effect on plant phenotype and on the composition of soil microbiota [126]. A comparative analysis of species-level phenotypes (OTUs) for treated CNTs and control experiments is shown in Fig. 8.11A. The effect of CNT exposure with varying

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Fig. 8.11 (A) The analysis of common and different OTUs among control and CNT-treated samples obtained from pyrosequencing data. The upper numbers indicate the number of OTUs and numbers in the blank are total read numbers of each OTU. Most of the sequences were common among the three CNT-treated soil samples. Compositions of the bacterial community in control soils compared with those of CNT-treated soils. Relative abundances of phyla (B) and classes (C) were calculated from pyrosequencing data. The phyla and classes were selected by the abundance >0.5% in any of three soil samples. Reprinted with permission from Khodakovskaya MV, Kim B-S, Kim JN, Alimohammadi M, Dervishi E, Mustafa T, et al. Carbon nanotubes as plant growth regulators: effects on tomato growth, reproductive system, and soil microbial community. Small 2013;9:115–123.

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concentration on the composition of bacterial communities at each phylogenetic level in three different types of soil is shown in Fig. 8.11B and C. Likewise, Goyal et al. reported the impact of raw SWCNTs in a soil microbial community, showing that even at short exposure, they significantly affected the microbial community in complex environmental systems [156]. In a similar study, Jin et al. reported that high concentrations of SWCNTs significantly reduced the soil’s microbial enzyme activity and biomass [157]. Turco and coworkers reported the essence of raw (contains metal impurities) and purified SWCNTs functionalized with polyethylene glycol (PEG-SWCNTs) or m-poly aminobenzene sulfonic acid (PABS-SWCNTs) on microorganisms in two dissimilar types of soil [158]. The effects of carboxylic group-functionalized SWCNTs on fungal and bacterial microbial communities presented in soil were investigated by Rodrigues et al. [159]. Similarly, the impact of fullerenes on soil microorganism has been investigated by several researchers with varying results. Tong et al. studied the impact of fullerenes on soil microorganisms and observed that a dose of 1000 μg g1 of granular C60 slightly affected both gram-positive and gram-negative bacteria in the soil [152]. The proportion of gram-negative bacteria was 5% higher than the control sample. Johansen et al. reported the impact of pristine C60 on soil bacteria and protozymes via the analysis of total respiration, biomass, number, and diversity of bacteria and protozymes [160]. Nyberg et al. investigated the effect of C60 fullerenes on anaerobic bacteria based upon methanogenesis by monitoring the production of CO2 and CH4 [78]. A possible explanation for these differential interactions can be ascribed on the basis of different synthetic routes, which cause changes in various physiochemical properties and thus show varying results based on soil microorganisms.

8.5

Conclusion

The beneficial effect of biochar in its use in the plant kingdom has been known for >1000 years. Recent research has shown that the microsized biochar also contains appreciable amounts of the oxidized derivative of assorted nanocarbons comprised of graphene oxide, oxidized CNTs, carbon nanoonions, and smaller versions of these such as carbon quantum dots. The surface derivatization, under burning of the biowaste in air, incorporates several oxo functionalities in this hydrophobic carbon to transform these to being hydrophilic in nature. Thus pyrolyzed carbonaceous material becomes porous and partially soluble in water. These have a great property to trap nutrients and micronutrients available in the soil. On entering the root of young saplings, these release essential ingredients to facilitate healthy plant growth. Also, these can trap heavy metals under favorable pH of the soil and thus, by arresting such toxic metal ions, may prevent these from interacting with safe plant life. Along with this property, these carbon materials can absorb water to retain it and can slowly release this to young plants when needed; they are therefore beneficial in arid zones or places with water scarcity. Therefore, nanocarbon derivatives as a composite with an optimum fertilizer level can be used as promoters for the plant. Such a composite can compete with natural fertilizers, manure, or organic fertilizers, as here the release of

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nutrients and micronutrients will be slow and as per the need of the young sapling to facilitate its normal and healthy growth. It is to be noted that the commercial production of nanocarbon derivatives will be dirt cheap from biowaste to be used in the composite. This will avoid the excessive use of synthetic chemical fertilizers to save plants and more so will protect the soil structure and its inhabitants. Furthermore, by exploiting the self-fluorescence property of some of these nanocarbons in the domain beyond a plant’s autofluorescence, the health of a plant can be monitored using the selective fluorescence of the nanocarbon as a sensor. In this process, specific drugs in effective doses can also be delivered to the affected part of a plant for its healthy recovery. Finally, it is to be clearly understood that nanocarbons such as carbon nanotubes, when initially used to probe a plant system, have been procured as defect-free carbon nanotubes. These are very hard in nature and readily pierce the cell wall of any biomaterial. The gross toxicity of nanocarbons is thus based on such physical damage. The use of soft, derivative nanocarbons that are dispersible or even soluble in water has recently been used to avoid such physical damage in the plants. Furthermore, the dose-specific use of these hydrophilic nanocarbons could avoid any deleterious effects.

Acknowledgments S.K.S. thanks DST (SB/EMEQ-383/2014) and CSIR (01(2854)/16/EMR-II) for the financial support and S. S. thanks SERB-DST for the Ramanna Fellowship.

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9

N. Saba*, M. Jawaid*,†, H. Fouad‡, Othman Y. Alothman† *Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Selangor, Malaysia, †Department of Chemical Engineering, College of Engineering, King Saud University, Riyadh, Saudi Arabia, ‡ Department of Biomedical Engineering, Helwan University, Cairo, Egypt

Chapter Outline 9.1 Introduction 327 9.2 Carbon nanotubes 330 9.3 Carbon nanotube synthesis 331 9.4 Carbon nanotube properties 331 9.5 Carbon nanotube-based composites 9.6 Carbon nanotube applications 334 9.7 Graphene 335 9.8 Graphene synthesis 337 9.9 Graphene properties 338 9.10 Graphene-based composites 340 9.11 Graphene applications 341 9.12 Conclusion 345 Acknowledgment 346 References 346

9.1

333

Introduction

Nanotechnology is a developing field that brings materials into the nanoscale level and applies them in the interdisciplinary sciences [1–3]. Nanomaterials are categorized into three groups: nanotubes, nanoparticles, and nanolayers, depending on the number of measurements of the dispersed particles that are in the nanometer range (10–9 m) [4]. The diverse nanofillers include carbon nanotubes (CNTs), nanoclays, nanooxides, gold, silver, zinc, copper, and organic nanofillers such as cellulosic nanofibers and cellulosic nanocrystals [5,6], all shown to have a wide range of applications. However, a marked and rapid growth of interest has been shown by the scientific and engineering communities in the organic [7,8] and carbon-based nanomaterials. Over the last two decades, carbonaceous nanofillers such as graphite, diamond, fullerene, and CNTs have established widespread research. They are Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00009-2 © 2019 Elsevier Ltd. All rights reserved.

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challenging due to their superior behaviors and interesting applications over other materials [9]. Among these engineered three-dimensional (3D) CNT and 2D graphene honeycomb lattice structure nanomaterials is one of the most promising functional materials, utilized in various fields due to its positive features including the properties of thermal, electrical, and mechanical strength as well as elasticity [2, 10]. CNTs and fullerenes are the allotropes of carbon characterized by a hollow structure and extraordinary thermal, electrical, and mechanical properties. Spherical fullerenes are also called buckyballs whereas cylindrical ones are known as nanotubes. The walls of these structures consist of a single layer of carbon atoms called graphene [11]. Although carbon is ubiquitous in nature, CNTs are a man-made form of carbon [12]. Among them, CNT possess better structural and fascinating properties which attracted it utilization and opened up a broad range of possible studies and functional applications [13,14]. The development and characterization of inorganic hybrids consisting of metal oxide (MO) and CNTs are gaining attention, in terms of superior electronic, optical, and mechanical properties [9,15,16]. The discovery of CNTs perhaps contributed to the nanotechnology revolution, owing to their superior thermal, physical, optical, and electrical properties as well as a remarkably high thermal conductivity [17]. A Japanese scientist Iijima is known for his discovery of CNTs. However, according to Monthioux and Kuznetsov [18], CNTs were discovered much earlier than the 1990s. Conceptually, CNTs are classified as single-walled carbon nanotubes (SWCNTs), and are made by rolling a graphene sheet into a seamless cylinder. CNTs with a multiwalled configuration are called multiwalled carbon nanotubes (MWCNTs), and are formed by more rolledup graphene sheets [19]. MWCNTs were first discovered in 1951 by Russian scientists Radushkevich and Lukyanovich [20]. Forty years after the discovery of MWCNTs, another type of CNT with a single wall, also known as an SWCNT, was discovered by Iijima and Ichihashi [21] in 1993 [17]. CNTs classified as SWCNTs and MWCNTs are displayed in Fig. 9.1 [22]. Arc discharge, laser ablation, and chemical vapor deposition (CVD) as well as diffusion and premixed flame methods are the major synthesis methods for SWCNTs and MWCNTs [23,24].

Fig. 9.1 CNT classification [22].

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The properties of CNTs are a consequence of their structure. SWCNTs may be zigzag, armchair, or chiral in their structure [25]. SWCNTs can be either metallic or semiconducting, a property determined by the atomic arrangement (chirality) and nanotube diameter. The roll-up vectors (n, m) of the cylinder describe the electrical properties of SWCNTs [26]. Metallic SWCNTs have roll-up vectors such as n  m ¼ 3q while semiconducting SWCNTs have n  m 6¼ 3q (here q is any integer/zero). If m ¼ 0, the nanotubes are called zigzag. If n ¼ m, the nanotubes are called armchair and the rest are called chiral [22]. CNTs can be dissolved into various solvents to enhance the thermal and physical properties of the solvent for various applications. CNTs possess exceptional chemical, adsorption, electrical, magnetic, and mechanical properties, which make them indispensable for various engineering applications [12,27]. However, the outer wall of pristine CNTs is hydrophobic and chemically inert to the base fluid [28]. Apart from that, the large surface area of CNTs leads to a high level of van der Waals interaction between nanotubes, which results in aggregation during synthesis [17]. CNTs are a part of extensive and multidisciplinary research and have often been designated as the most researched materials of the 21st century [15]. Even though many nanostructures are currently under investigation, the area of CNTs remains very active in different fields, including analytical science [29,30]. Graphene is the basic unit that forms 2D-carbon materials, which can be warped into zero dimensional (0D) fullerenes, curled into 1D CNTs, and stacked into 3D graphite and hence regarded as the mother building block of other carbon allotropes, represented in Fig. 9.2 [31]. Graphene and its derivatives possess a fascinating combination of physical, mechanical, photonic, and optoelectronic properties. Currently, graphene is mainly based on graphite, including the mechanical exfoliation method, liquid phase stripping, oxidation-reduction, and CVD [32,33]. Graphene oxide (GO, an oxidized single-layered or multilayered graphene) and reduced graphene oxide (rGO) are practically the most-studied graphene derivatives [3,34]. It has been reviewed that graphene can be processed as nanoribbons, platelets, foams, and even as quantum dots for immense use in semiconductors, energy storage devices, hydrogels, and biological applications [35]. Remarkably, based on the high surface area, scalable production, tunable surface chemistry, noncorrosive properties [1], and the presence of oxygen-containing functional groups at the surface of CNTs and graphene nanomaterials, the adsorption performance are generally better than other conventional adsorbents, such as zero valent iron, iron oxide, zeolite, silica, titanium dioxide, chitosan, and polymer [1,36,37]. More notably, the functionalized derivatives of CNTs and graphene with high surface area and adsorption sites are also proposed to remove heavy metals via adsorption, such as Hg(II), Pb(II), Cr(VI), Cd(II), As(III)/(V), Co(II), U(VI), and other metal ions [1,38,39]. Graphene and CNTs are mainly used for the preparation of flexible conductive collectors and are used for the additive of anode and cathode materials, which can help to reduce the internal resistance of the battery. With the development of the flexible electronics industry, the fabrication of high-performance FEES devices becomes a reality to be used in the field of portable electronic products based on CNTs and graphene [33].

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3D Graphite

Graphene

Stack

Roll 1D

2D

Carbon nanotube

Fullerene 0D

Wrap

Fig. 9.2 Graphene, the building block of carbon allotropes [31].

However, scientific and technological research face challenges with the durability, functionality, and performance of advanced nanomaterials, which are increasingly important in environmental applications [1,40]. As the pure nanomaterials have some limitations, carbon-based nanocomposites or nanohybrids with more and controlled functionalities are attracting attention for environmental and energy-storage applications [41]. Recently, covalently bonded 3D hierarchical CNT sponges having a 3D-CNT ultrastructure [42,43] in the macrostructure of a porous sponge or aerogel have shown an intriguing combination of hyperelasticity, ultralight weight, excellent electrical conductivity, controllable viscoelastic properties, excellent plastic behavior, and superhydrophobicity [44,45], compared to their pristine counterpart [10,46] fabricated via the CVD method [42,43]. Such ultra- and macrostructure CNT-based 3D assemblies can be used for applications ranging from environmentally friendly reusable sor bent to aerospace structures as energy absorption materials [42,47].

9.2

Carbon nanotubes

The discovery of CNTs represents a major breakthrough in nanotechnology development [48]. It was originally described in 1991 by Sumio Iijima [49] as having unique and remarkable properties [50]. This triggered one of the most widely studied subjects in the field of physics as well as electronic and photonic engineering [22,51,52].

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On the basis of the number of tube layers, CNTs can be divided into three categories: SWCNTs, double-walled carbon nanotubes (DWCNTs), and MWCNTs [52,53]. Rolling a graphene sheet into a cylinder results in SWCNT. On the other hand, the arrangement of concentric graphene cylinders with an interlayer space of 0.34 nm and a diameter typically of the order of 10–20 nm leads to the formation of MWCNTs [22,30]. The electrical conductivity of CNTs ranges from metallic to semiconducting based on the diameter and the rolling angle, which also impart chirality in the tubes [11].

9.3

Carbon nanotube synthesis

CNTs are generally produced using very useful and widespread methodologies. The most popular methods applied for the synthesis include arc discharge vaporization [54], laser ablation/evaporation [55], CVD [56], the floating catalyst method, lowtemperature solid pyrolysis, the ion bombardment growth method, the electrolytic method, and the polymer preparation method [57]. All these methods allow to some extent the synthesis of moderately high quantities of CNTs with a relatively precise number of layers [58]. The arc discharge, laser ablation, and CVD methods require a carbon source, a heat source, and inert gases such as He, N2, and CF4 for avoiding the oxidation of CNT [24] to achieve the desired operating temperature [9,59,60]. Thus, the obtained material is not ready for applications as it contains contaminants [61,62]; interestingly, contaminations depend greatly on the applied synthesis method. Research findings revealed that the main contaminants can be amorphous carbon, graphene flakes, metallic catalysts, and their supports. However, after purification, CNTs can be functionalized in order to get specific properties required for a given application [12]. Interestingly, variations in the methodology for obtaining CNTs give rise to different structures in terms of their dimensions, that is, diameter and length, their alignment (zig-zag, armchair, or chiral), the number of walls, and the presence of carbonaceous and metallic impurities inside them [30]. In addition, new structures with fascinating geometries, such us cup-stacked carbon nanotubes, carbon nanohorns, carbon nanotori, or carbon nanobuds, have been obtained [63]. Therefore, controlling the production process of CNTs is of great importance because these characteristics will define the properties of the final product and, ultimately, their applicability in any field, such as analytical chemistry [64]. As the introduction of CNTs in analytical chemistry involves their ability to interact with analytes through different types of interaction, that is, π-π stacking, van der Waals forces, and hydrogen bonding, combined with their large surface area that can facilitate the adsorption of analytes in a selective and reproducible manner [30].

9.4

Carbon nanotube properties

CNTs are the strongest and most typical nanocarbon material ever discovered by human beings [65,66]. They are the allotrope of carbon and they show unique and

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excellent physical, chemical, electrical, mechanical properties as well as extraordinary electronic, high thermal conductivity, and environmental stability [65], due to the combination of their small size, cylindrical structure and immense surface area, making them promising for various applications worldwide [11]. The unique 1D nano structure endows individual CNTs with superior physical properties, such as a high thermal conductivity of 3500 W/m K, a charge mobility of 10,000 cm2/V s, a Young’s modulus of 1 TPa, and a theoretical specific surface area of 1315 m2/g [67,68]. CNTs can carry a current density as high as 109 A/cm2 because of the interplay of high mechanical strength, high thermal conductivity, and extremely low electrical resistiv ity [65]. The CNT’s interesting chemical and electrical properties make it suitable and extremely useful when integrated in sorbents for both preparative and analytical separations as well as in electrochemical sensors and transducers. It also opens up new approaches to the full integration of a sample pretreatment with detection, providing exceptional possibilities for further miniaturization and automation [69], leading to its importance in analytical chemistry and electroanalytical schemes [30]. The unique electrical properties of CNTs are largely attributed to their 1D characteristics and the peculiar electronic structure of graphite. Furthermore, the manner in which the SWCNT is rolled from a graphene sheet strongly dictates its electronic properties [52,70]. It is worth mentioning that they are capable of superconductivity at low temperatures [71]. The comparative physical properties of MWCNT and SWCNTs are listed in Table 9.1 [9]. SWCNTs show excellent chemical stability, good mechanical strength, and a range of electrical conductivity properties. MWCNTs show metallic electronic properties similar to metallic SWCNTs [72], which in some respects makes them more suitable for electrochemical applications. The optical properties exhibited by MWCNTs are less striking than SWCNTs, so they are used as a delivery system for large biomolecules, including plasmids (DNA), into cells. Both types of CNTs contain a 1D structure and exhibit excellent properties such as good electrical conductivity, strong adsorptive ability, and excellent bioconsistency. These properties enable CNTs to carry high currents with negligible heating [29,73,74]. Interestingly, a CNT’s presence in electrochemical detectors and sensors is highly generalized, owing to their

Table 9.1 Physical properties of MWCNTs and SWCNTs [9] Properties

SWCNTs

MWCNTs

Specific gravity (g/cm3) Elastic modulus (TPa) Strength (GPa) Electrical conductivity (S/cm) Electron mobility (cm2/V s) Thermal conductivity (W/m K) Coefficient of thermal expansion (103 K1) Thermal stability in air (°C) Resistivity (μΩ cm)

0.8  1.4 50–500 102–106  105 6000 >1.1 600–800 50–500

53 degrees in Fig. 10.5C curves for 15 and 18 mL, a greater number of peaks and a better definition of the same is observed. Fig. 10.5D displays simulated XRD patterns for SnO2 tetragonal phase (red line) and cubic phase (blue line), and experimental for 18 mL and 800°C with

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˚ (green line), modeled with PDF-4+. A goodness of match a crystal size of 191 A (GOM) value of 3079 was obtained with respect to cassiterite in the tetragonal phase for sample 18 mL/800°C with Sieve+ tool and the Hanawalt search/match method. Samples at 600°C were modeled in order to find possible contaminants that could explain why small peaks in 2θ > 53 degrees are absent. Double-walled carbon nanotubes (DWCNTs) are carbon nanostructures that are the best known currently. A DWCNT is constituted of two CNTs, see Fig. 10.6 where a [email protected] is depicted. In functionalized nanostructures, DWCNTs could present eccentricity, due to attraction between the two nanotubes and mainly due to the proximity of other types of molecules. In MWCNTs, this eccentricity is normally observed only in two or three external layers. The simplest structure for studying the effects of interwall coupling on many properties, including the physical, electronic, and optical properties of CNTs, is of course DWCNT. In comparison with single-walled carbon nanotubes (SWNTs), DWCNTs have a better stability and higher mechanical strength and they present stimulating electronic and optical properties. Even though DWCNTs were discovered in 1991, the first synthesis was not published until 1998. DWCNTs have received moderate attention until the recent synthesis and purification of high-purity samples [49].

Fig. 10.6 (A) SEM image of DWCNT. (B) Low-resolution TEM image of DWCNT. (C) Sketch formation of carbon nanotubes. DWCNT contains two concentric SWNTs. Each SWNT is a molecular cylinder conceptually rolled from graphene along a chiral vector. (D) Calculated DOS for model DWCNT arm chair/zig-zag. (E) Calculated DOS for model arm chair/armchair. Based on Nestor David Espinosa-Torres, Alfredo Guillen-Lo´pez, J. Martı´nez-Jua´rez, J.A.D. Herna´ndez de la Luz and Jesu´s Mun˜iz. Theoretical study on the electronic structure nature of single and double walled carbon nanotubes and its role on the electron transport, to be published.

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Big efforts have been extensively applied to study the interwall interactions through theoretical calculations. Liang and coworkers have developed a tight-binding model to explain the electronic structure of the commensurate DWCNT and they established that the energy band structure of a DWCNT is modified by both the DWCNT mean value of the inner and outer tube diameters and the interwall coupling strength [50]. The band gap of a semiconductor [email protected] DWCNT is reduced with an increase of the interwall coupling tp, a constant describing the π-electron hopping between the two walls, in the interval of 0 < tp < 0.4 eV. By example, the interwall coupling of graphite is 0.35 eV, corresponding to the distance between layers of 0.344 nm. The interwall coupling is inversely proportional to the difference in radius of the two walls. As the diameter increases, the energy gaps of [email protected] DWCNTs decrease rapidly and disappear when D > 1.5 nm. On the other hand, [email protected] DWCNTs (where M stands for metallic) remain gapless, independent of tp and D. This prediction was reinforced by DFT [51]. Furthermore, the discovery of other composite materials that improve the electrochemical capacity, such as carbon/polyoxometalates [23] (see Fig. 10.7), may combine the chemical reactivity of polyoxometalates (POM) and the electronic properties of nanocarbons. The properties of the resultant composites have been used in catalysis, energy conversion and storage, molecular sensors, and electronics [52–55]. In this respect, POMs are widely reported in the literature [56, 57] with different symmetries. Particularly, the Keggin-type structure with formula [XM12O40]3 is also called a heteropolyacid (XM12, as an abbreviation). In this notation, X may be a nonmetal such as Si or P; M represents a metal such as W or Mo. These metal oxides are able to produce multielectronic redox reactions that may be applied on Li-ion batteries [58, 59] and supercapacitors [60]. POMs present a wide potential to be used in energy storage devices, but because they are cluster molecules, no constant conductivity is expected. Considering their solubility, it is necessary that POMs are anchored to a solid substrate [19]. Consequently, redox-active POMs attached to carbon substrates may improve energy storage capabilities. The use of AC [61], CNTs, and graphene [60] has facilitated the anchoring of POMs on carbon substrates.

Fig. 10.7 [email protected] Adapted from Guillen-Lo´pez A, Espinosa-Torres ND, Cuentas-Gallegos AK, Robles M, Mun˜iz J. Understanding bond formation and its impact on the capacitive properties of SiW12 polyoxometalates adsorbed on functionalized carbon nanotubes. Carbon 2018;130:623–35. 10. 1016/j.carbon.2018.01.043. Available from: http://www.sciencedirect.com/science/article/pii/ S0008622318300526, with permission from Elsevier and Copyright Clearance Center. License No. 4297770330764.

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The anchoring of POMs to the carbon surface is tentatively achieved through chemisorption and an apparent covalent bonding is present. Furthermore, this interaction is also assisted by functional groups [60, 62] and the role of porosity [60, 61]. In this regard, the introduction of POMs has allowed the facile Li-ion diffusion in Li-ion batteries as compared to pristine carbon. In this sense, POMs represent a potential candidate that may replace the use of RuO2 as a pseudocapacitive material [63]. The fundamental understanding of the atomistic interactions present in this type of composite material may give us insights into the design of carbon electrodes, as was previously suggested by Genovese and Lian [60]. The systematic combination of experimental/theoretical research may be a key to open disruptive avenues to improve energy storage devices. The ab initio investigations on POMs supported on carbon substrates are in due course. That is, a theoretical work was performed on the study of the PMo12 POM cluster supported on a CNT [57], where a tetrabutylammonium (TBA) works as the linker. The ground-state geometry was found and the DOS indicates a change to the metallic character in the composite system, which allows the material to be used on energy storage applications. In another work [64], the PW12 cluster was studied after its anchoring to a graphene substrate, and a noncovalent attraction was identified. Along this same line, Yang et al. [65] discovered that a polymeric ionic liquid (PIL) presented the role of an interfacial linker between a graphene layer and PMo12. This was also assessed by impedance spectroscopy (EIS) by showing that PIL increases redox reactions of the POM by inducing charge transfer and strengthening the specific capacitance of the composite. On the other hand, the study of functional groups and solvents that induce POM anchoring on carbon substrates has also been the subject of intense research. For instance, Rozanska et al. [66] performed a DFT study where POMs uptake hydroxyl groups and trigger H2O formation as well as a covalent-type bonding at the cristobalite surface/POM interface. With respect to the role of the solvent, Aparicio-Angle`s et al. [67] carried out a hybrid DFT/MD study where an SiW12 cluster was studied on a silver surface. In this case, the presence of the solvent appeared in the spontaneous reduction of the SiW12. An interesting theoretical modeling has also been performed on the mechanism responsible for the observed high specific capacitance of metal oxides deposited on carbon. That is, the lithiation/dilithiation process and the redox reactions in a Li-ion battery appear to dominate the overall capacity in the devices. Nevertheless, no fundamental study has been provided to elucidate the phenomena involved in this process. From this motivation, Jeong et al. [68] performed a series of MD simulations at the reactive force field level. In the model, they considered Ni oxide nanocrystals deposited on a graphene sheet, which after lithiation form core-shell structures Ni/ LiO2. After the process of dilithiation, the formation of molecular clusters of NiO is achieved. This is in agreement with the experimental results where optical spectroscopy shows that the valence of Ni2+ changes to Ni0 during the lithiation/dilithiation process and to Ni2+ at the end of the cycle. The methodology developed by Jeong et al. may be reproduced with different metal oxides in order to gain insights into the in silico design of novel energy storage materials. In this work, the MD simulations were performed with the NVT methodology and with a Nose-Hoover thermostat.

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The computations were performed with previously tested known reactive potentials [69, 70] by using ReaxFF [71]. Recently [5], other studies have emerged where the synthesis of a novel carbon material and a theoretical effort to understand the mechanism behind energy storage are both present. That is, [72] synthesized a novel carbon material where nitrogen atoms may be readily attached. Besides, the material presents a high surface area and a uniform microporous structure, which makes it suitable for a supercapacitor electrode with high energy and power density. A DFT study on a reduced carbon model system was carried out at the B3LYP/6-31G(d) level of theory, and the adsorption of an isolated K-ion was also modeled at the same level of theory. The computations suggested that the energy storage performance on this material might be understood from an augmented wettability and increased adsorption energy of K-ion adsorption on the carbon surface doped with nitrogen atoms. Such results may give us a priori information to decide on the use of heteroatom doping to tune the capacitance of carbon materials. This combined methodology may also provide new pathways to achieve massive production of doped carbon.

10.3

Recent contributions in theoretical approaches

From the experimental perspective, Cuentas-Gallegos et al. [73] have detected that the presence of some functional groups (namely φ-NH2 and φ-OH) anchored on carbon matrixes actively cooperate in the adhesion of POM clusters such as PMo12 [73]. Nevertheless, no modeling on these composite systems was available. In this regard, Mun˜iz et al. [74] performed a theoretical study using DFT where graphene was considered as a model system to simulate an isolated location on a nanoporous carbon. Additionally, the presence of functional groups and the PMo12 cluster was also considered in the calculations, as depicted in Fig. 10.8. Consequently, it was possible to simulate a localized region of an energy storage device electrode. In this study, the nature of bonding between PMo12 and carbon was analyzed where the functional groups were involved. The presence of a noncovalent/ electrostatic interaction was evidenced, with a π  π stacking between PMo12 and carbon ruling the attraction. Calculations suggest that φ-NH2 and φ-OH functional groups induce a covalent-type bonding with PMo12 at a geometrical arrangement where the POM cluster lies above the functional group. This behavior was addressed as the mechanism that explains the empirical insight stating the facile adsorption of POMs on the carbon structure. Furthermore, Mun˜iz et al. stated that the composite formed by the functional groups φ-NH2 and φ-OH along with the PMo12 cluster is responsible for the rising of new pathways where ions are allowed to trace in electrochemical cells. These results may give new insights into the design of analogous composite materials with Keggin structures adsorbed with different functional groups adhered to carbon substrates. In this study, two approaches were considered in the methodology; that is, the computations were performed with and without periodic boundary conditions at the PBE level [27] of theory in order to elucidate generalities that may be further implemented to reduce computing time. Furthermore, the presence

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Fig. 10.8 Model systems of PMo12 cluster anchored on: (A) a top configuration at a functionalized graphene; (B) a side configuration at a functionalized graphene. Reproduced from Mun˜iz J, Cuentas-Gallegos AK, Robles M, Valdez M. Bond formation, electronic structure, and energy storage properties on polyoxometalate-carbon nanocomposites. Theor Chem Acc 2016;135(4):92. 10.1007/s00214-016-1855-3, with permission from Springer Nature and Copyright Clearance Center. License No. 4264950764198.

of nominal charges on the POM systems and the corresponding neutralization with counterions were both considered because this represents the most likely state of the POM at an electrolyte solution. The counterions were simulated with three Na+ cations by using a technique previously implemented by Kostyrko et al. [75]. After giving attention to this issue, a benchmark study was performed by Mun˜iz et al. [76] into a series of POM systems attached to a carbon substrate. The clusters PdMo12, RuNb12, SiMo12, PMo12, and SiW12 were studied (see Fig. 10.9). Particularly, this work reports the energetic stability and existence of PdMo12 and RuNb12 systems, which have not been previously synthesized. In this work, the systems under study were fully optimized using two approaches. In the first, the POM clusters were fully relaxed with formal charges, and their neutralization as a second approach was also considered. This was performed as a benchmark to explore a route to effectively model the intermolecular bonding at the composite. The computations reveal a noncovalent bonding of the electrostatic type with the cooperation of vdW dispersion forces. It was also shown that the DOS is strengthened around the Fermi level at all composite systems under study. Mun˜iz et al. assigned this behavior as the available trajectories that ions are permitted to track at solid-state devices (a battery or supercapacitor, for instance). The observed increase intensifies the density current values and also the pseudocapacitive properties found in pristine carbon systems. According to their schemes, Mun˜iz and coworkers showed that SiWO12 and RuNb12 clusters are potential candidates to be anchored on carbon matrices where the presence of functional groups may be optional and they may be considered as possible composites to be implemented on electrodes of an energy storage device. In this study, DFT at the PBE level was used with a contribution coming

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Fig. 10.9 [XM12O40]n anchored to a graphene sheet at (A–C) C3-orientations. B, H, and T stands for bridge, hollow, and top orientations with respect to the C atoms on the substrate. (D–F) S4-orientations with B, H, and T configurations. (G and H) Lateral views of C3 and S4 orientations of POM with respect to graphene. Reproduced from Mun˜iz J, Celaya C, Mejı´a-Ozuna A, Cuentas-Gallegos AK, Mejı´a-Mendoza LM, Robles M, et al. First principles study on the electronic structure properties of Keggin polyoxometalates on carbon substrates for solid-state devices. Theor Chem Acc 2017;136 (2):26. 10.1007/s00214-017-2049-3, with permission from Springer Nature and Copyright Clearance Center. License No. 4264970496687.

from vdW effects, namely PBE + vdW functional, while at those computations where counterions were included to neutralize the overall charge, the PBE functional as implemented on the SIESTA computational code [77] was used. Mulliken charges analysis and density difference isosurfaces were also computed in order to gain more insight into the electronic structure properties. As was previously suggested, composite [email protected] materials of the form [email protected] represent a tested option to be implemented as an electrode material for Li-ion batteries due to the increased specific capacity when it is further compared to the individual components, that is, MWCNTs and TiO2. Several features seem to be

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present in the rising of the phenomena at the interface of the metallic oxide and the carbon substrate, such as the level of disorder at the oxide and the thickness of the shell. In this sense, Mun˜iz et al. [78] performed a series of calculations with a combined methodology where MD simulations and DFT calculations were both involved. The aim of this study was to understand the origin of bonding at the composite material and also to elucidate the role of the degree of disorder that is induced when the graphene surface and TiO2 interact to form the composite material. The size of the shell thickness was varied by changing the number of TiO2 nanoparticles and each of the systems was simulated using MD at the ReaxFF level with reactive potentials. Besides, the MWCNT was modeled using a double-walled CNT to simplify computations. An optimal configuration was readily found where the core remains virtually intact through the simulation. It was also found that at different shell thicknesses, the geometry of the MWCNT at the core was fully distorted and the graphene-like structure was completely lost, depicted in Fig. 10.10. Consequently, the shell size is crucial to model the chemical capacity and it is intimately involved in the increasing of electronic states around the Fermi level, as is

Fig. 10.10 Atomistic interaction of TiO2 nanoparticles deposited on an MWCNT model at 400°C. All results were found with nanoparticles ranging from 0 to 190. Note that the carbon core is virtually unaltered at a intermediate configuration. (A) 5  [email protected]  10 MWCNT system. (B) [email protected] system. (C) [email protected] system. (D) [email protected] system. Reproduced from Mun˜iz J, Cuentas-Gallegos AK, Robles M, Valdez M. Bond formation, electronic structure, and energy storage properties on polyoxometalate-carbon nanocomposites. Theor Chem Acc 2016;135(4):181. 10.1007/s00214-016-1855-3, with permission from Springer Nature and Copyright Clearance Center. License No. 4265230635865.

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suggested by the DOS. Remarkably, the stability of the MWCNT is preserved at a certain shell thickness and the DOS at the Fermi level is maximum. This may be considered as a signature to find composite materials where the increasing of this property may be associated with stability at the core and also as available pathways that ions may cross during the charge/discharge cycles in a Li-ion battery. This hybrid methodology may give insights into the structural design to improve materials intended to be implemented at [email protected] composites on Li-ion batteries. Such analysis may also be extended with different metallic oxides supported on MWCNTs or other configurations where carbon is involved. Considering the design of electrodes for energy storage devices, Mejı´a-Mendoza et al. [14] performed a theoretical study on the in silico design of nanoporous carbon structures at different densities, which may be directly implemented as supercapacitor or battery electrodes. Such nanoporous structures were obtained from diamond and graphite as precursor models (see Fig. 10.11). The study was aimed to relate the thermodynamical and structural properties of a series of carbon samples that were

Fig. 10.11 Molecular representations of nanoporous carbon structures obtained from diamond and graphite as precursors in a heating-quenching procedure. Nanoporous structure found at (1) a density ρ ¼ 0.71g/cm3 and a Quench-Rate QR ¼ 38.48  1012 K/seg; (2) ρ ¼ 1.03g/cm3 and a Quench-Rate QR ¼ 35.52  1012 K/seg; (3) ρ ¼ 1.37g/cm3 and a Quench-Rate QR ¼ 41.44  1012 K/seg. Reproduced from Mejı´a-Mendoza LM, Valdez-Gonzalez M, Mun˜iz J, Santiago U, Cuentas-Gallegos AK, Robles M. A theoretical approach to the nanoporous phase diagram of carbon. Carbon 2017;120:233–43. 10.1016/j.carbon.2017.05.043, with permission from Elsevier and Copyright Clearance Center. License No. 4265250336178.

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computationally obtained. The work was performed using MD simulation with the aid of a Tersoff many-body potential. Mejı´a-Mendoza et al. considered a quench-rate process applied to a series of 1750 carbon structures at a temperature interval ranging from the critical to the triple-point densities of carbon. The initial carbonaceous samples were based on the geometries of graphite and diamond with different densities. The final structures obtained after the quenching rates were systematically characterized by considering structural properties such as surface area, RDF, structure factors, and free volumes. A correlation between potential energy and the degree of sp3 hybridization was found. The latter may be directly implemented to assess structural phases by using clustering methodologies. In this regard, they basically identified two structural phases on the series, namely sponge-like and graphite-like configuration; an unstable structure was also characterized. Based on the calculations, a close correspondence with available experimental results such as the work of Dash et al. [79] may be readily verified. The relevance of the scheme proposed by Mejı´a-Mendoza and coworkers relies on the issue of computational cost, which was tested to be lower than other theoretical methodologies available and applied to the study of nanoporous carbon structures (such as the work of Pikunic et al. [80] and Ranganathan et al. [81]) at a reasonable dependability.

10.4

Perspectives for future development and conclusion

The study of materials for energy storage devices is a current subject of intense research and it requires a sustainable cooperation among multidisciplinary groups where theory meets experiment to design novel materials that comprise the basic understanding of the components of the device, that is, electrodes, electrolytes, and binders, among others. In this review, we have covered theoretical works that mainly comprise density functional calculations in order to understand the properties of functionalized carbon surfaces and their role in the development of carbon-based electrodes for batteries and supercapacitors. In this matter we can recommend: (a) Determination of the nature of bonding between metallic oxides and carbon substrates using specialized methodologies such as Bader’s Atoms in Molecules method [82] and NBO [83], among others. In this regard, it is possible to predict a covalent bonding or weaker interactions, such as hydrogen-type or electrostatic attractions. This may give insights into the origin of charge retention at the interface of metal oxide/carbon. (b) Estimation of the electronic transport properties at the interface of metallic oxides/carbon substrates using nonequilibrium Green’s functions. This may assess the degree of conductivity present from the carbon surface to the metal oxide, which it has shown to be highly sensitive to the geometry of the carbon surface; that is, such electron transport may be directed from metal oxide to carbon if the carbon surface is planar [76] or from carbon to metal oxide if it is curved [23]. The quantification of this phenomena may provide some light into the nature of redox reactions present in devices such as Li-ion batteries. (c) Localization of ion trajectories through the interface of metallic oxides/carbon by using ab initio MD simulations, which represent a scheme to be implemented in a variety of

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geometries and components because charge retention is a condition that depends on the time that an ion may spend on certain locations at the interface. Due to the computational cost that such calculations may imply, it is recommended to perform them with model systems limited to well-defined periodic conditions and a reduced number of atoms.

Also, at the end of our review, we emphasize the importance of the atomic topology that with the electrode must have; in that spirit we think that the correlation between pyrolyzed carbon obtained by biomass and the theoretical search of nanoporous structures using an MD level of theory, we can resume de following: (a) The process to obtain sustainable carbon structures coming from biomass is a technique that may directly be implemented on the design of electrodes in an energy storage device. Such process may be modeled by using MD simulations with classical force fields such as that of Tersoff [14] or by using semiquantum reactive potentials such as those implemented on ReaxFF. Such carbon substrates are nanoporous structures that may be obtained from sustainable feedstock such as pyrolized biomass. The heating of the molecular content of such precursors may further be simulated with such techniques in order to find carbon nanoporous surfaces that may be structurally characterized with known methodologies [14]. This perspective may give the opportunity to generate a large database to provide the necessary conditions that a nanoporous carbonaceous substrate should have to give acceptable performance on an energy storage device. (b) Finally, we consider that the MD simulation of supercapacitors with such nanoporous carbon structures with aqueous or polymeric ionic liquids (PIL) is also of high relevance because with the simulation, it is possible to control the known variables such as carbon density at the electrode, pore size, and degree of hybridization contents. On the other hand, the nature and concentrations of the electrolyte may also be controlled and interchanged with the carbon electrodes at the simulation. Using these parameters, it may be possible to predict capacitances and the structural features of the electrodes. Such analysis may be used as a tool to design novel supercapacitors or batteries to reduce the costs of fabrication in the laboratory.

Acknowledgments Jesu´s Mun˜iz wants to acknowledge the support given by Ca´tedras-Conacyt (Consejo Nacional de Ciencia y Tecnologı´a) under Project No. 1191; the computational infrastructure provide by Laboratorio Nacional de Conversio´n y Almacenamiento de Energı´a (CONACYT) under Project No. 270810, and the Supercomputing Department of Universidad Nacional Auto´noma de Mexico for the computing resources under Project No. LANCAD-UNAM-DGTIC-310. Nestor David Espinosa-Torres is grateful for the Postdoctoral Scholarship provided by CONACYT with Project No. 229741. Alfredo Guillen-Lo´pez wants to acknowledge the PhD scholarship provided by CONACYT with No. 306891. L.M. Mejı´a-Mendoza acknowledges the support given by CONACYT-FONCICYT under contract FONCICYT/51/2017. The authors would like to acknowledge the financial support given by DGAPA (Direccio´n General de Asuntos del Personal Academico) under Project No. IG100217.

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11

Nanocarbon composites for poisonous gas degradation

Shahid Pervez Ansari*, Anish Khan†,‡, Abdullah Mohamed Asiri†,‡ *Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India, †Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia, ‡Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia

Chapter Outline 11.1 Introduction 383 11.2 Methods of control 385 11.3 Photocatalysis: An important technique for NOx degradation 11.4 Carbon nanotubes in the photocatalysis of NOx 386 11.5 Adsorption of NOx over CNT 391 11.6 Graphenes in photocatalysis of NOx 392 11.7 Conclusions 394 References 395

11.1

386

Introduction

One of the most serious challenges before modern human society and the scientific community is to control and degrade the increasing entry of pollutants into the atmosphere and environment. In the present day, urban environments are facing very serious issues caused by air pollution, which is due to exhaust gases from vehicles and burning of fuels in different industries. This results in a major environmental issue on the global level. Its impact could be sensed on human beings, animals, plants, and the climate. The main sources of pollutants in the atmosphere are different man-made and natural processes such as anthropogenic activities (social and industrial), forest fires, soil erosion, volcanic eruptions, and other sources, in particular emissions from human actions. In view of the intensity of the issue, various air pollutants have been recognized for their alarming effects. On the basis of their origin, these air pollutants may further be divided into two main groups: (i) primary pollutants, which directly enter the atmosphere from the emitting sources, such as carbon monoxide (CO), nitrogen oxide (NOx), sulfur dioxide (SO2), and particles such as PM10 and PM2.5; and (ii) secondary pollutants, which are formed as a result of chemical reactions of primary pollutants and Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00011-0 © 2019 Elsevier Ltd. All rights reserved.

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other species present in the atmosphere, such as HNO3, H2SO4, and tropospheric ozone (O3) [1–4]. Nitrogen oxides will be considered as model compounds in our discussion. These are produced naturally due to volcanic activity and the decomposition of organic matter as a result of microbial and solar action; they are transported into the atmosphere by the process of diffusion. However, a significant amount of NOx is contributed from human activities. The most common components of nitrogen oxides are nitrogen monoxide (NO) and nitrogen dioxide (NO2). Thermodynamically, NO is an unstable molecule having an enthalpy of formation of 90.2 kJ mol1. Therefore, its decomposition into elements is thermodynamically favored below the temperature of 1000°C. However, because of a high activation energy barrier (335 kJ mol1), it is kinetically hindered and the use of catalysts becomes indispensable. The presence of NOx in the atmosphere has caused various adverse and harmful effects. The NOx is also responsible for grave environmental problems such as depletion of the ozone layer, photochemical smog, tropospheric ozone, and global warming caused by N2O while NOx also contributes to acid rain together with sulfur oxides. As far as human health is concerned, NOx affects in several ways: as the amount of NOx increases and it interacts with lung tissue, it causes diseases such as bronchitis, emphysema, etc. (Fig. 11.1). Besides the above problems, the interaction of NOx with other air pollutants, such as volatile organic compounds (VOC) present in the atmosphere, forms peroxyacyl nitrate (PAN) or nitrous acid, which are 10 times more toxic to humans than NOx. Moreover, the presence of ozone in the troposphere is also harmful to the eyes and lungs [3, 5–7].

Air pollution

NO

NOx NO2

Acid rain

Global warming Human diseases Sources

Problems

Fig. 11.1 Problems and sources associated with NOx air pollution [5].

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Therefore, it has become obligatory to control the amount of NOx in working places and urban atmospheres. In support of these measures, the US Environmental Protection Agency (EPA) and the European Environmental Agency (EEA) have recommended that the hourly NOx limit in air be 0.1 and 0.2 ppm, respectively. However, it is quite difficult to maintain the recommended concentration of NOx in big cities. At some places, it is urged that the area’s administration support new NOx standards for the short term, which is in the range of 50–75 ppb [8, 9]. This has raised interest in the research community to study new actions to effectively combat NOx pollution. Two important entry points of NOx in the atmosphere are industries and the transportation sector; the latter is the major contributor of NOx in urban environments. There are three types of NOx: fuel NOx, prompt NOx, and thermal NOx. As a result, three reaction mechanisms have been suggested for the formation of nitrogen oxides (NOx) during combustion. Fuel NOx is the result of direct oxidation of fuel having organo-nitrogen compounds. During combustion of the fuel, the ring structures break and species such as CN NH2, HCN, or NH3 are formed; on reaction with O2, these produce NOx, reaction between nitrogen and oxygen at high temperature form thermal NOx while the prompt NOx are produced by reaction of hydrocarbon radicals with nitrogen [5, 10–12]. l

l

N2 + CH $ HCN + N

(11.1)

N2 + C $ CN + N

(11.2)

N + O2 $ NO + O

(11.3)

11.2

Methods of control

To control and manage NOx emissions in the environment, various methods have been tried, tested, and developed. These methods are divided into two categories: (i) primary methods that prevent the formation of NOx at the source point, like NOx removal carried out inside a combustion zone (e.g., a furnace) without the need for another reactor, and (ii) secondary methods that reduce the level of existing NOx. Some of the most commonly used methods include selective noncatalytic reduction (SNCR), selective catalytic reduction (SCR), and other absorption of NOx, NOx storage and reduction (NSR), phase separation, NO conversion into HNO3, or ozone injection [12–14]. However, no method was able to control and reduce NOx to the desired limits in the urban centers. Photocatalysis is considered an innovative and promising technique to remove undesirable gaseous compounds for urban localities. Yu et al. [15] suggested that the mechanism of photocatalytic degradation of NOx should include three steps: (i) adsorption of reactant gas on the photocatalyst surface; (ii) generation or recombination of e–h+ pairs; and (iii) oxidation of NO and reduction of water.

386

11.3

Nanocarbon and its Composites

Photocatalysis: An important technique for NOx degradation

Among the advanced oxidation processes, photocatalytic oxidation utilizing semiconducting metal oxides as a photocatalyst (TiO2, ZnO, WO3, etc.) is very promising for the photodegradation of many pollutants in industrial wastewater [16]. When the semiconducting catalyst is irradiated with radiation of a suitable wavelength, for example, photons of energy equal to or higher than the band gap of the photocatalyst, these photons excite electrons in the valence band (VB), which jump into the conduction band (CB); as a result, holes are left in the valence band. Therefore, two simultaneous reactions are expected: the oxidation reaction and the reduction reaction due to photogenerated holes and photogenerated electrons, respectively. The photogenerated holes and water molecules or hydroxide ions (OH) react to produce hydroxyl radicals ( OH). The pH of the reaction medium affects the process and rate of generation of these radicals. The target pollutants that are already adsorbed over the photocatalyst surface get oxidized by the OH radical. While the photoexcited electrons are transferred to the conduction band (CB) and generate the OH radical, these photoelectrons can also form a highly reactive superoxide radical ion (O 2 ) by reacting with the O2 molecule, which can oxidize the pollutant molecules [17]. The kinetics of the photocatalytic reaction are affected by some important factors, which include the type of semiconductor, light radiation, the pH of the medium, the concentration of pollutants, the reaction temperature, etc. [18, 19]. The rate of photodegradation can be enhanced if the electron–hole recombination is reduced, the agglomeration of particles is reduced, and the adsorption of pollutants on the catalyst is enhanced. The photocatalytic efficiency of the photocatalyst can be improved by taking the following controlling steps [20, 21]: l

l

l

l

l

l

l

l

A nanosized photocatalyst should be used due to its larger surface area. Space charge separation should be induced by generating structural defects that in turn reduce the electron–hole recombination. A single semiconductor should be doped with metal or other semiconductor to enhance the photocatalytic properties. Alumina, clay, silica, or zeolite should be added as a cosorbent.

Nanocarbon-based composites have recently attracted the attention of the scientific community due to their intrinsic properties that have been endowed as a result of the incorporation of nanocarbon. There are several reports of nanocarbons being utilized as photocatalysts in the degradation of pollutants, especially NOx [3, 5, 22–24].

11.4

Carbon nanotubes in the photocatalysis of NOx

Vermisoglou et al. [25] evaluated the photocatalytic activity of the Rh nanoparticle doped single-walled carbon nanotubes (SWCNT) functionalized using polyethylene glycol toward NOx reduction. They also studied gravimetric adsorption to understand the NO adsorption on the active surface sites wherein they determined the desorption

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of oxygen. They provided valuable information to understand the optimum reaction parameters to remove NO efficiently. These data include reaction kinetics, the onset of NO scission temperature, the life of the photocatalyst, the rate of abatement of NO, etc. They performed their photocatalytic degradation of NOx by varying the feed compositions (partial pressures, temperature) and operating conditions (presence/absence of other gases such as CO, hydrocarbons, oxygen). The above Rh-based nanostructured photocatalyst exhibits enhancement in photocatalytic activity, even at lower temperatures, as compared to the previous reports for other catalytic systems of the type Rh/support (Rh/Al2O3, Rh/SiO2, and Rh/MWCNTs). Higher surface area, higher particle dispersibility, and a high number of surface functionalities are mainly responsible for the above enhancement in the photoactivity [26]. Properties such as porosity, conductivity, metallic, or semiconducting behavior that depend on the chirality of SWCNTs compared to MWNTs play a crucial role when these nanocarbons are acting as a supporting component during heterogeneous catalysis while also affecting catalytic activity [27]. Some researchers have reported on the resistant nature of the CNTs against oxidation in the presence of oxygen at 450°C and that they contribute as Rh(0) stabilizers only [28, 29]. It has also been indicated by several studies that at certain times, CNTs may partially oxidize, provided that a lesser amount of reducing agent is available. As a result of this oxidation reaction, the efficiency of NO abatement is positively affected and favors the oxygen desorption from metal particles [30–32]. Recently, Vermisoglou et al. [25], while dealing with the nanocarbon materials as supports in preparation of the catalyst for the depletion of NO, suggested the following reaction mechanism [29, 31, 32]: θ + NOðgÞ $ θ  NO

(11.4)

θ  NO + θ ! θ  N + θ  O

(11.5)

2θ  N ! N2ðgÞ + 2θ

(11.6)

2θO + C ! CO2ðgÞ + 2θ

(11.7)

θ  O + C ! COðgÞ + θ

(11.8)

where Eq. (11.1) relates adsorption and Eq. (11.2) relates dissociation of NO on the active sites (θ) of the catalyst. Eqs. (11.3)–(11.5) relate the pathways of desorption for nitrogen and oxygen in the form of N2, CO, and CO2. Due to the rise in the amount of NOx, techniques other than photodegradation are also explored. Selective catalytic reduction using hydrocarbon as a reductant (HCSCR) is considered a promising solution to control the rise of NOx. However, a single catalyst for this purpose is of little use due to the fact that the operating temperature range of a diesel engine lies between 150°C and 500°C. The wide range of temperature limits the activity of a single catalyst over the entire range. Therefore, in order to manage this problem, a dual catalyst system is considered as a potential solution. The two

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catalysts are placed in parallel and gas switching is applied to ensure the appropriate utilization of the catalyst, which typically depends on the temperature of the exhaust gas. In these binary catalyst systems, one component operates in the temperature range of 150–300°C while the other component operates in the temperature range of 300–500°C. Ag/Al2O3 or In2O3/Al2O3 are reported as high temperature catalysts having satisfactory activity, durability, and selectivity [33]. Among lower temperature range catalysts, platinum and like metals supported on activated carbon are reported to be the most active HCSCR catalysts [34, 35]. Unfortunately, the activated carbon supports tend to burn in oxidizing environments at unacceptably low temperatures. Jimenez et al. [36] reported hydrocarbon-assisted selective catalytic reduction of NOx (HCSCR) using a dual catalyst system to limit the emissions of NOx from the diesel engines. These catalysts consist of a distinct formulation to work satisfactorily in low and high temperature range formulations. Pt particles supported on multiwalled carbon nanotubes (MWCNTs) exhibited better reduction activity for NOx as compared to Pt/Al2O3. The MWCNT support is also a better resistance to oxidation than activated carbon. The NOx reduction efficiency of the catalyst based on Pt and MWCNTs could also be enhanced if the MWCNT support is refluxed in a 1:1 mixture of HNO3 and H2SO4, followed by metal deposition. On treating MWCNTs with acid, the Brønsted acidity of MWCNTs increases in addition to the partial oxidation of hydrocarbon. Further improvements in the HCSCR performance of MWCNT-based catalysts may also be made by using a PtdRh alloy (3:1) as the support material. Santillan-Jimenez et al. [36] prepared highly dispersed Fe2O3 and CNT-based catalysts by a simple ethanol-assisted impregnation method wherein the size of the Fe2O3 nanoparticles was 4.8 nm. A synergistic effect was observed in the catalyst system. For example, the iron oxide exhibited improved adsorption, redox property, and activation ability of NOx due to the excellent electron transfer in CNTs. These catalysts showed NO conversion higher than 90% in the temperature range 200–325°C, due to its large surface area, fine dispersion of α-Fe2O3 nanoparticles, and interaction with CNTs. Liu et al. [37] studied NO removal in a fixed-bed reactor wherein multiwalled carbon nanotube (MWCNT)-coated TiO2 was used as the photocatalyst (MWCNTs/TiO2). The results of their experiments showed that at a lower initial concentration of NO, its removal was conducive; however, in the presence of SO2, denitration was hindered. On the other hand, the presence of O2 and H2O favor the photocatalytic denitration of NO. At optimal conditions (73 mg m3 NO, 8% O2, and 5% H2O), a removal efficiency of 46% was obtained. Based on their photocatalytic denitration experiments conducted in different conditions (UV, silica gel, TiO2, MWCNTs/TiO2) and observations, Liu et al. also proposed a mechanism for the denitration reaction. Under UV irradiation and in the absence of a photocatalyst, some reactions were reported to cause 16% degradation. The photochemical degradation mechanism under UV irradiation and without a photocatalyst resembles closely that of photochemical smog. Photolysis of NO causes it to form active O, which in turn reacts with O2 to give O3. As a result, NO was oxidized to a higher valence state. The UV irradiation-induced reaction mechanism is given below [38]:

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NO2 + hυ ! NO + O

(11.9)

O 2 + O ! O3 + O + M

(11.10)

NO + O3 ! NO2 + O2

(11.11)

O + NO + M ! NO2 + M

(11.12)

O + NO2 + M ! NO3 + M

(11.13)

O3 + NO2 + M ! NO3 + O2 + M

(11.14)

Reaction mechanism in the presence of a catalyst [39, 40]: TiO2 + photon ðhυÞ ! e + h +

(11.15)

h + + OH !  OH

(11.16)

e + O 2 ! O 2 

(11.17)

NO +  OH ! HNO2

(11.18)

HNO2 +  OH ! NO2

(11.19)

NO2 +  OH ! NO3 

(11.20)

NO + O2  ! NO3 

(11.21)

The mechanism of photocatalytic oxidation of NO proposed by Balbuena et al. is given below [3]. The efficiency of the photocatalytic degradation was highest in the MWCNT/TiO2 catalyst as compared to the bare TiO2 and photolysis in the absence of any catalyst. The improvement in the efficiency of the MWCNT/TiO2 catalyst is due to the (i) good conductivity of MWCNT, which offered alternative pathways of electron transfer via TidOdC bonds, causing it to increase the life of the electron–hole pair [41]; (ii) smaller size of MWCNTs/TiO2, which provided a larger surface area in a particular volume. According to Devi et al. [42], in such cases, volume recombination dominates the e–h+ recombination. The large size of the particles will provide a larger diffusion path and consequently, favor more e–h+ recombination. However, if the size of the particle is small, there will fewer chances of e–h+ recombination; and (iii) narrow energy band gap of MWCNTs/TiO2, which has its adsorption edge shifted to a longer wavelength, due to which there are more chances of e–h+ pair generation at the same UV intensity. Jimenez et al. [43] studied the selective catalytic reduction of NOx on functionalized MWCNT-supported metal catalysts and assisted by hydrocarbons. According to their transient studies experiments, a characteristic volcano-shaped

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curve for NOx conversion was obtained. The temperature of the maximum conversion (Tmax) was taken as a point of reference to our current discussion; below Tmax, the surface of the catalyst was covered by a hydrocarbon species, which suppresses the reduction activity of NOx. Above the Tmax, O2 adsorption was prevalent and favors oxidation of both the NO and the hydrocarbon species. Temperature programmed desorption (TPD) studies for Pt–Rh/fMWCNTs and Pt/fMWCNTs suggested that the adsorption of the hydrocarbon species is stronger on the PtdRh alloy compared to that on the Pt while the adsorption of NO was stronger on Pt compared to that on the alloy. Burch et al. [44] proposed the HCSCR reaction in 1994 and suggested that the NO gets adsorbed on reduced Pt sites and forms nitrogen and oxygen ad-atoms. The Nads species combine to given N2 or react with NO to give N2O while hydrocarbon reacts with Oads and undergoes a combustion reaction, then gives the reduced Pt sites and closes the catalytic cycle. NO + θ ! Nads + Oads

(11.22)

Nads + Nads ! 2θ + N2

(11.23)

Hydrocarbon + Oads ! 2θ + CO2 + H2 O

(11.24)

In many cases, the above NO decomposition mechanism was opposed by some supporters of mechanisms in which the hydrocarbon species reacts with Oads atoms to yield oxygenated surface intermediates, which further react selectively with the available (adsorbed or gas phase) NOx to produce the HC-SCR reaction products [45]. NO + θ ! Nads + NOads

(11.25)

HC + θ ! HCads

(11.26)

HCads + NOads ! CHONads

(11.27)

CHONads + O2 ! θ + CO2 + H2 O + N2

(11.28)

Beyer et al. [46] studied the removal of NOx (NO and NO2) over rhodium particles supported on carbon nanotubes (Rh/CNT) in the presence and absence of oxygen. They reported that the stoichiometric oxidation of the carbon support takes place by adsorbed oxygen, which in turn is derived from the scission of NO by a rhodium catalyst; it is also a very important step to achieve the steady conversion of NOx. Rh/ CNT catalysts exhibited high activity toward NO and NO2 reduction, especially in the absence of excess oxygen, wherein the carbon support acted as a reducing agent. The generally accepted mechanism for decomposition of NO on rhodium surfaces is given below: where ‘θ’ shows a coordinatively unsaturated metal site. θ + NO $ θ  NO

(11.29)

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θ  NO + θ ! θ  N + θ  O

(11.30)

2θ  N ! N2 + 2θ

(11.31)

θ  N + θ  NO ! N2 O + 2θ

(11.32)

2θ  O ! O2 + 2θ

(11.33)

Inderwildi et al. [47] suggested that, as the coverage of oxygen increases, the heat of NO adsorption on the surface of rhodium decreases; however, NO adsorption is thermodynamically favorable over O2 adsorption because of its endothermic nature at higher O2 coverage. At rhodium surfaces, NO may dissociate, even at low temperatures (300 K) and low NO coverage [48, 49]. The effects of other species present on the surface of the catalyst and on the selectivity of N2 desorption for the formation of N2O or N2 are still under investigation. From their TPD experiments, Aryafar and Zaera [50] found two peaks for desorption of N2 from metal (Rh) surfaces. The first peak maximum supports second-order kinetics that shifted from 700 to 560 K with increasing surface coverage while the second peak maximum favors first-order kinetics at 460 K and was observed when the monolayer coverage was higher than 0.2 [51, 52]. Significant desorption of O2 from the surface of Rh occurs at temperatures above 800–1100 K (depending on the surface coverage) [53]; therefore, that makes O2 desorption the rate-determining step. As the desorption of O2 takes place at a higher temperature, the desorption of O2 at the operating temperature is negligible or very low. Therefore, this causes catalyst deactivation due to adsorbed O2 on the surface, and this case may be more prominent if there is excess O2 in the feed. Wang et al. [54] studied the reduction of NO over two types of hydrogenated CNTs (hydrogenated CNTs and Pd/hydrogenated CNT). They suggested that the stored hydrogen and the CNT support act as reducing agents for NO. Luo et al. [28] reported the higher conversion of NO over the Rh/CNT catalyst as compared to Rh/g-Al2O3, due to the better stabilization of Rh (0) on CNT. It is therefore suggested that the CNT plays a crucial role in the catalytic degradation of NO using catalysts as a noble metal supported on CNT, that is, Rh/CNT [31]. Tang et al. [29] also studied the decomposition of NO on Rh/CNT and suggested that the CNT is resistant to oxidation.

11.5

Adsorption of NOx over CNT

During the recent past, major efforts were dedicated to the development of technologies to control the emissions of NOx from the combustion of fossil fuel. The most commonly used adsorbents to remove NOx at low temperatures were activated carbon, FeOOH dispersed on active carbon fiber, and ion exchange zeolites. The highly effective adsorption of NO can be observed over activated carbon due to the presence of surface functional groups, although the amount of adsorbed species may not be significant [55].

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Long and Yang [56] suggested that the CNTs could also be utilized as an efficient adsorbent for the removal of NO by adsorption. The amount of NOx adsorbed on CNT was 78 mg/g. Adsorption of NOx on CNT may be due to the electronic properties, surface functional groups, and unique structures of CNTs. When O2 and NO pass through the CNT, NO gets oxidized to NO2 and gets adsorbed on the surface of nitrate species. Mochida et al. [57] supported the adsorption over the CNT surface and also suggested the oxidation of NO, which forms NO2 over activated carbon at room temperature.

11.6

Graphenes in photocatalysis of NOx

The discovery of graphene, a 2D material having exciting properties, has been a key material of attraction in the scientific community for its possible application in different fields [58]. In our present concern, it has also been thoroughly studied for the preparation of composite catalysts with different semiconductors, for example, TiO2 [59], ZnO [60], and CdS [61], to enhance the photocatalytic activity of the single catalyst/ semiconductor. Zhang et al. first demonstrated the photocatalytic activity of graphene-based semiconductor composites [62], followed by Ng [63], Shen et al. [64], and Zhou et al. [65] prepared graphene–TiO2 hybrid materials by single step hydrothermal and suggested enhanced photocatalytic activity of the composite catalyst for organic molecule degradation. Most of the studies have recognized the role of graphene as an electron reservoir that possesses high conductivity, which is possible due to its 2D geometry and excellent electron transport [66, 67]. When graphene with very few defects or defect-free graphene is used as a support material for a semiconducting metal oxide such as TiO2, the photogenerated electrons can easily be accepted from the TiO2/graphene interface, stored, and shuttled away from the same position. The interfacial contact between the graphene sheets and the semiconductor nanoparticles plays a very important role in effectively transferring the photo-induced electrons [68, 69]. This decreases the e–h+ recombination and the lifetime of e–h+ gets extended due to their separation; this considered an important factor for the improved photocatalytic activity of TiO2/graphene nanocomposites [66, 70]. The purity and type of graphene depend on the preparation procedure adopted. Typically, pure graphene is obtained by exfoliating graphite in some organic solvent or water, through proper modification and use of suitable surfactants that stabilize the graphene layers in the dispersion [71]. Graphene can also be obtained by the oxidation of graphite to graphite oxide, followed by a reduction reaction to get reduced graphene oxide (rGO). This route is highly preferred due to the presence of covalently bonded oxygen functionalities on the surface of graphite oxide sheets that favor the processing of the intermediate and the synthesis of various composites, including TiO2/graphene photocatalysts. Though the rGO in the majority of reports has been considered as a substitute for graphene, it lacks the purity of ideal graphene and possesses more structural defects and compromised electrical properties [72]. The electronic conductivity of the rGO depends on the reduction level and is typically less compared to that of the ideal graphene in a single layer, which is nonoxygenated and defect-free [73, 74].

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Therefore, it could be suggested that performance and photoactivity vary with the type of graphene and may be different for the same application, that is, a TiO2-based photocatalyst. Graphene oxide has also been reported to act as a photosensitizer and increase the photoactivity of the catalyst in visible light [75]. Trapalis et al. [76] coupled TiO2 with two different graphenes, that is, (i) graphene stabilized by a surfactant and obtained by exfoliating graphite, and (ii) reduced graphene oxide obtained via oxidation of graphite, followed by reduction. These TiO2/graphene composites were investigated for their efficiency to remove NOx photocatalytic degradation from ambient air. Two types of trends were found for the TiO2/graphene composites for the photocatalytic removal of NOx. At first, the photocatalytic efficiency was enhanced in UV light for TiO2/ssG and TiO2/rGO, exhibiting maximum efficiency at a low loading of graphenes (ssG and rGO). This improvement in the photocatalytic activity is attributed to the interaction developed between graphene sheets and TiO2 nanoparticles, with the former acting as an electron trap in case of ssG and a photosensitizer for rGO. The second case, the low release of NO2, was observed in the case of the TiO2/graphene catalysts, which is due to the higher affinity of NO2 with graphene. Therefore, the TiO2/graphene catalyst acted in two ways to control and degrade the NOx in the atmosphere. It was also reported that porosity was affected by the presence of both the surfactant and the graphenes while the band gap energy (Eg) of the material was affected only by the presence of graphene and not by the surfactant. Liang et al. [77] demonstrated that solvent-exfoliated graphene (SEG) possesses comparatively low defect density and better electrical conductivity compared to rGO. The nanocomposites (TiO2/SEG) with low graphene defect densities exhibit a higher photocatalytic performance in case of reduction of CO2 to CH4 compared to that of rGO/TiO2 nanocomposites. The superior electrical transport property in SEG with lower defects favors the effective diffusion of photoexcited electrons to the adsorbed CO2 through the longer mean free paths, and therefore, facilitates the photoreduction of CO2 effectively. Therefore, it could be suggested that by minimizing the defects in graphene, its electrical conductivity can be improved, which in turn improves the separation and transportation of the photo-induced charge carriers within graphene. Consequently, the photoactivity of graphene-based composite photocatalysts will be enhanced. In contrast to the above discussion, the lower photocatalytic efficiency of a TiO2-surfactant stabilized SEG catalyst is reported for NOx removal as compared to that of TiO2/RGO [78]. This decrease in photocatalytic efficiency is due to the presence of residual surfactant in the catalyst. The presence of residual surfactant hinders the role of pure graphene (highly conductive) in the composite photocatalysts. It is therefore‘ crucial to carefully optimize the process while using the SEG with improved electrical conductivity to make composite photocatalysts. Tang and Cao [79] reported studies based on density functional theory for the interactions of nitrogen oxides NOx (x ¼ 1, 2, 3) and N2O4 with graphene and graphene oxides (GOs). According to them, the NOx adsorption on GO was stronger compared to that on graphene because of the active defect sites on GO, such as the –OH and >CO functional groups and the carbon atom near these groups. These active defect sites enhance charge transfers from NOx to GO by increasing the binding energies, which

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led to the chemisorption of gas molecules. The NOx interacts with GO via various functional groups present on graphene, which can form hydrogen bonds between – OH and NOx (–OH……O(N)) and new weak interactions C…..N and C…..O, in addition to the abstraction of H to form species such as nitrous acid and nitric acid. The spin-polarized density of states suggests a strong hybridization of frontier orbitals in NO2 and NO3 with the electronic states around the GO’s Fermi level, and results in a strong acceptor doping by these molecules and effective charge transfers from molecules to GO, compared to adsorptions of NO and N2O4 on GO. Seifvand and Kowsari [80] studied the photocatalytic activity of nanostructured composites that consist of TiO2 and graphene oxide functionalized by a Co–imidazole complex (fGO).The Co–Im complex was attached to the GO by a covalent bond between Im with functional groups present on graphene. The photocatalytic activity of the TiO2/fGO composite was tested for photocatalytic degradation of NOx and CO under UV irradiation. The TiO2/fGO composite photocatalyst exhibited higher photocatalytic activity for NOx (51%) than for CO (46%). The increase in photocatalytic activity is attributed to a decrease in the band gap of the catalyst and an increase in the sensitivity to visible light irradiation (λ > 400 nm) due to the incorporation of FGO in the TiO2. In addition to the above improvements, the photo-induced electrons move easily around fGO, causing an efficient charge separation and delay charge recombination. The above-discussed improvements along with the increase of the reactant adsorption at the catalyst surface are the important factors for effective photocatalytic operation. In the fGO composite photocatalyst, the attachment of Co–Im on the GO surface plays a very important role in preventing e–h+ recombination by transferring the photogenerated electrons to the conducting band of the GO from the Co–Im complex, thus the charge separation is achieved. In the photocatalytic mechanism of the TiO2/ fGO nanocomposite, the role of cobalt is as a complexing metal and the pollutant gases act as coordinating agents, as per the following chemical equations: 

   Coð ImÞ4 Cl2 + 2NO ! Coð ImÞ4 ðNOÞ2 Cl2

(11.34)



   Coð ImÞ4 Cl2 + 2CO ! Coð ImÞ4 ðCOÞ2 Cl2

(11.35)



   Coð ImÞ4 ðNOÞ2 Cl2 + O2 ! Coð ImÞ4 ðNO2 Þ2 Cl2 ðNO ! NO2 Þ

(11.36)



   Coð ImÞ4 ðNOÞ2 Cl2 + O2 ! Coð ImÞ4 ðNO3 Þ2 Cl2 ðNO2 ! NO3 Þ

(11.37)



   Coð ImÞ4 ðCOÞ2 Cl2 + O2 ! Coð ImÞ4 ðCOÞ2 Cl2 ðCO ! CO2 Þ

(11.38)

11.7

Conclusions

Nitrogen oxides (NOx) are very harmful to human beings and therefore must be controlled in the environment. Various techniques have been used to degrade or control

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NOx, most importantly, photocatalysis using semiconducting metal oxides. The photocatalytic activity of the catalyst could be enhanced by using nanocarbons; that is, CNTs and graphenes act as support materials for the semiconducting metal oxide catalyst. CNTs and graphenes work in multiple ways such as the adsorption of the pollutant, the delayed recombination of the photogenerated e–h+, and the special affinity toward the product of photocatalytic reactions.

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Akil Ahmad*,†, David Lokhat*, Siti Hamidah Mohd Setapar‡, Asma Khatoon‡, Mohd. Rafatullah† *Department of Chemical Engineering, Howard College Campus, University of KwaZuluNatal, Durban, South Africa, †School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia, ‡CLEAR, Ibnu Sina Institute for Industrial and Scientific Research, UTM, Skudai, Malaysia

Chapter Outline 12.1 Introduction 401 12.2 Toxicity and health effects of VOCs 402 12.3 Nanocarbon-based composite materials 403 12.3.1 Graphene-based nanocarbon materials for VOC removal 405 12.3.2 Carbon nanotube-based nanocarbon materials for VOC removal 409 12.3.3 Carbon nanofiber-based nanocarbon materials for VOC removal 411

12.4 Conclusion and future prospects Acknowledgment 412 References 413

12.1

412

Introduction

With rapid industrialization, agriculture and domestic activities, and unplanned urbanization, the pollution level of the environment is increasing day by day and has led to the deterioration of air quality. In most of the metro cities of the world, air pollution has reached alarming levels that have adverse effects on human health. The most common air pollutants are carbon dioxide, carbon monoxide, methane, chlorofluorocarbons, and VOCs (such as methanol, ethanol, isopropanol, acetone, butanol, 1,2-trichloroethylene, 1,2-dichlorobenzene, chlorobenzene, xylene, and toluene), which are mainly present in the air in the form of gases and vapor [1–10]. Apart from these, thousands of other organic compounds come under the definition of volatile organic compounds (VOCs) and cause air pollution in the troposphere. These compounds (such as aldehydes, aromatic compounds, polycyclic aromatic hydrocarbons, alcohols, and ketones) have a direct effect on human health and the ecological environment and are considered as carcinogenic or neurotoxin materials. The major Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00012-2 © 2019 Elsevier Ltd. All rights reserved.

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sources of VOCs are industrial and residential coal burning, aerosol sprays, biomass burning, vehicle exhaust, gasoline vapor, pesticides, wood preservatives, paint, asphalt and the petrochemical industry. These VOCs are responsible for indoor and outdoor air pollution because they are the essential part of many products and materials [11–15]. Due to the VOC’s toxic and harmful impacts (such as carcinogenic, mutagenic, and neurotoxic actions) on human health, it is very important and urgent to eliminate these toxic materials from the environment. For this, we need to develop an efficient and economical technique for the removal of toxic VOCs from indoor and outdoor environments. Various recovery and destruction techniques such as adsorption [16–18], condensation [19–21], absorption [22–24], membrane separation [25–28], incineration [29–32], photocatalytic oxidation [33–36], ozone catalytic oxidation [37–40], plasma catalysis [41–43], and biological degradation [44–46] have been attempted by researchers and scientists to reduce the level of pollutants. Among all the reported methods, the adsorption process is an easy, simple operation and an economical method for the elimination of VOCs due to the low energy cost and material cost [47–50]. Several adsorbents such as silica gel, zeolites, granular activated carbons, activated carbon fibers, clays, sludge, natural fibers, and alumina have been used for the removal of VOCs [51–60]. Among all these adsorbents, nanocarbon-based materials such as carbon nanotubes, graphene, nanohorns, nanofibers, etc., have special characteristics (high surface area, electron and thermal mobility, high durability, mechanical strength, and excellent selectivity) that make them very efficient and promising materials in environmental applications for the removal of indoor and outdoor pollutants [61–69]. These nanocarbon materials (with one dimension in the nanometer range) have the potential to interact with VOC analytes better than other traditional adsorbents due to excellent and efficient high-energy adsorption sites. In this chapter, the detailed synthesis and application of the carbon-based nanoparticles have been reviewed. The excellent properties of nanocarbon-based materials have prompted an interest in developing methods for environmental applications to remove VOCs from indoor and outdoor areas. The diverse applications of nanocarbon-based materials are discussed and their use for the removal of VOCs is explained herein. Sources of VOCs by anthropogenic and industrial origins and their detection by nanocarbon composites are shown in Fig. 12.1.

12.2

Toxicity and health effects of VOCs

VOCs have adverse and severe effects on both the ecological environment and human health. Many of the organic compounds, including alcohols, ketones, aldehyde, polycyclic aromatic hydrocarbons, aromatic compounds, etc., are very toxic and cause carcinogenic and mutagenic effects on human health [70]. The most common VOCs are methanol, ethanol, acetaldehyde, formaldehyde, isopropyl alcohol, propylene, ethylene, acetone, pyrene, benzene, toluene, carbon tetrachloride, chlorobenzene, dichloromethane, etc., and they are mainly found and generated by burning biofuels, coal, and oil as well as in preservatives, cosmetics, antiseptics, petroleum products, paints, adhesives, biowaste decompositions, pharmaceuticals, perfumes, synthetic

Nanocarbon composites for detection of volatile organic compounds

403

Nano carbon composites

Several health effects of VOCs on human

Tumors, Skin cancer, change in skin pigmentation

Asthma

Graphene, Carbon nanotubes and nanofibers for detection of VOCs

A breath of fresh air

Sources of VOCs Nano carbon composites

Fig. 12.1 Sources of VOCs by anthropogenic and industrial origins and their detection by nanocarbon composites.

resins, etc. These VOCs have very adverse, acute, and toxic effects on human health such as throat and eye irritation, CNS depression, nasal tumors, carcinogens, dizziness, pulmonary damage, leukemia, headache, nausea, etc. [71–84]. Table 12.1 presents the chemical structure of some of the common VOCs and their effects on human health.

12.3

Nanocarbon-based composite materials

Recently, nanosized carbon composites have been extensively used in wide application due to their remarkable structural and physical properties that make them attractive as building blocks in the field of nanotechnology [85]. These materials can be considered suitable candidates for the detection of environmental toxic pollutants due to their good conductivity, excellent mechanical properties, chemical stability, and large specific surface [86–89]. Carbon is a unique element in the periodic table due to the existence of a variety of polymorphs. These allotropes such as graphite, diamond, lonsdaleite, and fullerene, among others, are composed entirely of carbon but have different physical structures. Most of the natural and synthetic substances consist of carbon atoms, such as single crystals of diamond and graphite. Due to their excellent physical and chemical properties, such as the ease of incorporation of metal oxides on their surface, high temperature stability, high porosity, and high proportion of active sites, these are good materials in various applications. The discovery of fullerenes in 1985 opened the door for carbon research and gained the attention for application throughout the world. The first HR-TEM of carbon nanotubes (CNTs) was observed in 1991. This work was highlighted when R.F. Curl, H. Kroto, and R.E. Smalley got the Nobel prize in 1996 in chemistry for this work. Later, in 2010, physicists A. Geim and K. Novoselov received the Physics Nobel prize for reporting the

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Table 12.1 Chemical structure of VOCs and potential effects on human health Metals

Chemical structure

Acetone

Potential health effects Eye and nasal irritation, loss of strength, mood swings, nausea, dizziness, carcinogenic effects

O

Headache, dizziness, drowsiness, confusion, tremors and loss of consciousness, cancer and aplastic anemia, eye and skin irritant, chronic lymphocytic leukemia, non-Hodgkin’s lymphoma and multiple myeloma

Benzene

Skin and eye burns, irritation in eyes, nose, and throat with narcosis or anesthesia, pulmonary edema, burning sensation, coughing, wheezing, laryngitis, shortness of breath, headache, nausea, and vomiting

Butanal O

Carbon disulfide

S

Dichlorobenzene

C

S

Causes irritation in eyes and respiratory tract. High-level exposure or inhalation caused liver damage, burning pain in stomach, nausea, vomiting, and diarrhea. Hemoglobin may change to methemoglobin, resulting in dusty skin color; may damage the liver and kidneys

Cl

Cl

Ethanol

Cancer and nervous system damage as well as psychiatric problems such as depression, liver and cardiovascular disease, anxiety, and antisocial personality disorder

HO

Formaldehyde

Problems in breathing and chest pains. Nausea, vomiting, dizziness, fatigue, headache, mood changes, lethargy, blurred vision, delirium, and convulsions after acute exposure

H O

Formaldehyde exposure causes irritation in eyes, nose, and throat. It may cause occupational asthma and lung and nasopharyngeal cancer

H

Terpenes



Toxic effects such as seizures, nausea and vomiting, central nervous system (CNS) depression, respiratory disorders

Nanocarbon composites for detection of volatile organic compounds

405

Table 12.1 Continued Metals

Chemical structure

Potential health effects

Toluene

Causes irritation on skin, eyes, and respiratory tract, sore throat, dizziness, and headache

Xylene

Depression of the central nervous system, with symptoms such as headaches, dizziness, nausea, and vomiting

Chloroform

Effects on the liver include hepatitis and jaundice, central nervous system depression, and irritability

Cl Cl Cl

Carbon tetrachloride

Cl Cl

Cl Cl

Inhalation and oral exposure causes headaches, weakness, lethargy, nausea, and vomiting. Acute exposures of carbon tetrachloride cause liver and kidney damage in humans

two-dimensional graphene material (single-layer graphene). Due to increasing demand for interdisciplinary research, these materials have gained attention in science and technology for their wide and specific applications in various fields. In this chapter, we will briefly discuss the main nanocarbon-based materials such as graphene, carbon nanotubes, nanofibers, nanohorns, etc. Different nanocarbon materials, properties, and detection of VOCs are depicted in Fig. 12.2.

12.3.1 Graphene-based nanocarbon materials for VOC removal Graphene-based nanocarbon materials are now considered as new and dynamic nanosized materials in the field of environmental remediation. This is the newest allotrope of carbon with an sp2 hybridized carbon atom and arranged as a honeycomb structure. Its unique and novel properties such as high mechanical strength, optical, electrical conductivity, large surface area, excellent carrier mobility, and thermal and chemical properties make it a promising material for various applications in many technologies [90–93]. For the preparation of graphene-based composites (GO/ZnONR/GO, GO/ZnO-NR and ZnO-NR), a chemical bath deposition method was used by Vessalli et al. 2017 [94]. The synthesized materials were characterized on the basis of scanning electron microscopy, Raman spectroscopy, and energy dispersive x-ray spectroscopy. These materials were used as a sensor for sensing various VOCs such as ethanol, methanol, acetone, and benzene in a wide concentration range

406

Nanocarbon and its Composites

High surface area Highly porous High tensile strength, mechanical and electrical properties Properties

C

C Types

Detection Nano carbon composites

C

C Types Single or

C

VOCs C

Alcohols Aldehyde Alkenes Aromatic compounds Halogenated VOCs Ketones Polycyclic aromatic hydrocarbons

multiwalled carbon nanotubes Fullerenes Carbon nanofibers Graphene

Fig. 12.2 Types of nanocarbon materials, properties, and detection of VOCs.

(10–500 ppm). It was observed that 450°C is the optimum working temperature for all sensors. In this study, the authors reported three types of sensors: GO/ZnO-NR, ZnONR, and GO/ZnO-NR/GO. From the results, the response of GO/ZnO-NR to acetone, benzene, ethanol, and methanol was 6.97, 1.97, 6.46, and 6.61, respectively, while the response of ZnO-NR to acetone, benzene, ethanol, and methanol was 12.52, 1.37, 11.38, and 5.27, respectively, and of the GO/ZnO-NR/GO sensor was 6.99, 1.27, 5.96, and 1.36, respectively. It was observed that the response to benzene was poorer as compared to other gases in all the cases. Owing to the functional group present on GO, it exhibited good selectivity and was responsible for the adsorption of gaseous analytes. It was observed that the abundant amount of oxygen (O2) present on composite materials was responsible for the sensor response. The GO/ZnO-NR/GO composite was successfully applied for the sensing of ethanol, methanol, acetone, and benzene analytes and opened the door for the sensing performance of one-dimensional ZnO/GO nanocomposites. In 2017, Li et al. prepared a novel polyaniline/polypyrrole/ graphene oxide (PANI/PPy/GO) material via in situ electrochemical deposition for the detection of VOCs [95]. A solid-phase microextraction gas chromatography-mass spectrometry (SPME-GC-MS) method was used, which is simple and sensitive for the detection of VOCs in the headspace gas of lung cell lines. A scanning electron microscope was used to assess the surface morphology of the PANI/PPy/GO

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composites. A rough and wrinkled multilayer structure was observed, which is an exclusive characteristic of graphene oxide. From the results, composites showed high surface area and excellent mechanical and thermal stability as well as high extraction efficiency. Some analytical parameters such as linearity range, limit of detection (LOD), correlation coefficients (R2), and interday and intraday relative standard deviations (RSD) were evaluated to check the validity of the method. This method showed a wide linear range from 0.002 and 40 μg L1 with good correlation coefficients (R2) from 0.9916 to 0.9999. It was observed that the method exhibited a low detection limit ranging from 1.0 to 12 ng L1. Experiments were carried out by spiking a known amount into all samples. It was found that recoveries ranged from 88% to 110%, which shows the accuracy of the developed analytical method. From the results, it was concluded that the method is simple, reliable, and efficiently applicable for the determination of VOC analytes in trace amounts in various complex gaseous samples. A graphene film was synthesized by the chemical vapor deposition (CVD) method and applied for the sensing of VOCs [96]. Various VOCs such as butanol, isopropanol, acetone, and ethanol were analyzed at room temperature. In the gas-sensing procedure, the interaction of the surface of the adsorbent and adsorbate molecules plays a great role. VOCs interacted with the –COOH group via a hydrogen bond or van der Waals interaction present on graphene moieties. It was easy to desorb the VOCs from the graphene surface because the –COOH group involved in the interaction is very weak. From the results, it was observed that the sensor exhibited good and high sensitivity toward ethanol. Good reproducibility, reversibility, and rapid responserecovery showed at room temperature by a developed sensor for the detection of butanol, isopropanol, acetone, and ethanol. In 2016, Ge et al. synthesized an Ag/SnO2/ graphene nanocomposite by a wet-chemical method [97]. Various VOCs such as ammonia, furan, chlorobenzene, formaldehyde, and acetone were tested and among all analytes, the Ag/SnO2/graphene nanocomposites showed a high response toward acetone. A wide range (0.005–3000 ppm) of response was reported for the sensor and the lowest detection concentration of 0.005 ppm was achieved for acetone. The operating temperature was optimized and 300°C was determined to be the best operating temperature for taking the responses. From the results, it was concluded that Ag/SnO2/ graphene nanocomposites can be applied for the wide concentration range of VOCs. Polyaniline/expanded graphene-oxide (PA/EGO) composite materials were synthesized by Konwer et al. in 2017 [98]. An in situ chemical oxidative polymerization method was used to synthesize PA/EGO composites in an acidic medium. Scanning electron microscopy, electrical conductivity, and x-ray diffraction techniques were used to characterize the prepared materials. Different concentrations of VOCs (acetone, chloroform, carbon tetrachloride) were examined. From the results, it was found that the PA/EGO composite is highly sensitive for chloroform vapors (ΔR/Ro ¼ 32.5–36.5) as compared to other VOCs, carbon tetrachloride (ΔR/ Ro ¼ 19–24.5), and acetone ((ΔR/Ro ¼ 18–24). The limit of detection for three VOCs was studied for concentrations up to 2000 ppm. As compared to pure polyaniline, PA/EGO showed good response time and reversibility, which proved that PA/EGO can be used as sensor materials for the detection of VOCs. Graphene oxide (GO) and reduced graphene oxide (rGO) were synthesized by using the modified Hummer’s

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method. It was observed that this method is very effective for the fabrication of GO and rGO from precursor material graphite [99]. A sheet-like morphology for GO and rGO was exhibited by using transmission electron microscopy. Fourier transform infrared spectra confirmed the presence of –OH (bending and stretching peak– 3432.8 cm1) and C]O (stretching vibration–1643.1 cm1) groups on the surface of GO and rGO. With the help of Raman spectra, it can be observed that the absence of the 2D band showed the removal of oxygenated functional groups. From the BET analysis, a large surface area and high adsorption capacity were observed for rGO (292.6 m2/g) as compared to GO (236.4 m2/g). Prepared adsorbent material was applied for the removal of benzene and toluene and the adsorption capacity of rGO for benzene and toluene was found to be 276.4 and 304.4 mg/g, respectively, at room temperature and normal pressure. On the other hand, GO showed the least adsorption capacity for benzene and toluene. The excellent adsorption/elution behavior of rGO for benzene and toluene makes it an efficient material for the detection of VOCs at the ppm level. Titania-reduced graphene oxide nanocomposites were synthesized by using the hydrothermal method, which is considered an ecofriendly and facile procedure [100]. The characterization parameter of nanocomposites such as crystalline structure, morphology, chemical bonds, porosity, etc., were examined. Photocatalytic activity was carried out under UV and visible light irradiation. At an optimum flow rate of 17 mL min1 with 30 mg of the coated composite, a 70% conversion was achieved under visible light irradiation. From the results, it was found that low doses of composite materials would be an efficient photosensitizer and electron acceptor for the degradation of VOCs at normal conditions. The gravure technique was employed for the preparation of a thin film of WO3/reduced graphene oxide (GO) nanosheets decorated with Pt nanoparticles [101]. Two particle size of graphene (large-sized reduced GO microsheets (GMs) and small-sized reduced GO nanosheets (GNs)) were taken to study the effect of particle size on the performance of gas detection. At the optimum temperature of 200°C, the WO3/Pt-GNs nanocomposite as a sensor showed a good selectivity and a high response of 12.2 to 10 ppm acetone with fast response/ recovery time (14.1/16.8 s) but very low response below 3 for 10 ppm of ethanol, benzene, formic acid, methanol. and n-butanol. The detection limit was observed to be 600 ppb for acetone. The presence of large p-n junction active sites at the nanocomposite interface shows the high efficiency and good sensing behavior. From the results, it was concluded that WO3/Pt-GNs could be an efficient material for fast, selective, and sensitive determination of VOC pollutants in the future. A nanocomposite synthesized by using reduced graphene oxide (rGO) and platinum (Pt) was applied as a catalyst for the complete oxidation of benzene [102]. The prepared nanocomposite was characterized on the basis of Raman spectra, BET method, X-ray diffraction, TEM, and HRTEM. From the results, it was found that with increasing the rGO concentration by a certain amount, the catalytic activity increases. However, when it increases to 1% of rGO concentration, no significant enhancement was observed on the catalytic performance. A density functional theory was employed for the elimination of VOCs through a graphene-based adsorbent [103]. Transition metals—palladium (Pd), platinum (Pt), silver (Ag), and gold (Au)—were doped on the surface of graphene to prepare an efficient adsorbent for the detection of VOCs

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such as benzene, furan, pyrrole, pyridine, and thiophene. Among all adsorbents, the Pt-doped graphene adsorbent showed high efficiency and was more suitable for the removal of VOCs. The binding of benzene, furan, and pyrrole with adsorbents favors the π-Int mode. On the other hand, pyridine and thiophene favor X-Int ligation. This work suggested that the interaction of benzene, furan, and pyrrole with transition metal composites depends on the p-orbitals of carbon atoms. On the other hand, pyridine and thiophene interactions are eased by the sp2-hybridized orbitals of heteroatoms. Graphene-based adsorbents were prepared by microwave irradiation (rGOMW) and microwave irradiation and KOH activation (rGOMWKOH) [64]; these materials were applied for the removal of VOC pollutants. In this study, scanning electron microscopy, X-ray photoelectron spectroscopy, N2 adsorption-desorption isotherms, and Raman spectroscopy were used to characterize the morphology and chemical composition of the adsorbent materials. The maximum volume capacity of rGOMWKOH, for toluene and acetaldehyde samples at 30 ppmv concentration, was found to be 3510 mm3/g and 630 mm3/g, respectively, while for the rGOMW adsorbent, the maximum volume capacity was found to be 1710 and 405 mm3/g for toluene and acetaldehyde, respectively. The adsorption efficiency for the toluene and acetaldehyde samples was found to be 98% and 30%, respectively. In the case of toluene, high adsorption efficiency was reported, which is possible due to the interactions between the π-electron-rich pristine graphene and the aromatic ring of toluene molecules. From this study, it was concluded that graphene-based adsorbents could be more efficient for the removal of nonpolar aromatic pollutants rather than polar pollutants. A solvothermal method was employed for the synthesis of graphene/metalorganic composites and characterized the material on the basis of scanning electron microscopy, FTIR, and nitrogen adsorption [104]. At the optimum condition, the adsorption capacity for benzene and ethanol was found to be 72 and 77 cm3 g1, respectively. The surface area and pore volume for the adsorbent were measured and it was found that the adsorption behavior of GO/MOF composites for VOCs is influenced by these parameters.

12.3.2 Carbon nanotube-based nanocarbon materials for VOC removal The unique properties of carbon nanotubes (both single-walled and multiwalled) such as chemical and thermal stability, unique morphology, relatively high reactivity, and large specific surface area, make them very efficient for the removal of VOCs as pollutants from the environment [105–107]. A highly efficient nanocomposite was synthesized for the detection of aromatic (benzene and toluene) and nonaromatic (ethanol, methanol, and acetone) toxic gases [108]. It was prepared by incorporating 16-Mercaptohexadecanoic acid (MHDA) on gold-decorated multiwalled carbon nanotubes (MWCNT/Au) by the self-assembly technique. Various characterization techniques such as transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to know the size, structure, and chemical composition of synthesized nanocomposites.

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At room temperature, the detection of aromatic and nonaromatic volatile compounds was investigated for both MWCNT/Au and MWCNT/Au/MHDA nanocomposites. The incorporation of MHDA on the MWCNT/Au surface encouraged the sensitivity (102%  ppm1) and high sensitivity was observed for nonaromatic VOC ethanol (525.3) and methanol (108.9) While relatively low sensitivity was found for the MWCNT/Au, a high response and better sensitivity were achieved for MWCNT/Au/ MHDA nanocomposites, due to the carboxylic functional group present in MHDA. A spray layer-by-layer method was employed for the preparation of ecofriendly nanocomposites by incorporating different surfactants (sodium deoxycholate (DOC), sodium dodecylbenzenesul-fonate (SDBS), 1-hexadecyl trimethyl ammonium bromide (CTAB), benzalkonium chloride (BnzlkCl), and tritonx-405 (TX405)) on the carbon nanotube (CNT) surface for the detection of various VOCs [109]. In this work, the author’s selected anionic, cationic, and nonionic surfactants were fixed with a CNT to check the selectivity and sensitivity toward the various VOCs (methanol, ethanol, water, acetone, chloroform, and toluene). From the results, it was concluded that a surfactant CNT-based sensor could be used as an efficient and potential material for the detection of various VOCs, including lung cancer biomarkers. Liu et al. (2011) developed a highly potent sensor by fixing nonpolymeric materials (tricosane and pentadecane) onto single-walled carbon nanotubes (SWNTs) and applied this material for the detection of polar (1,2,4-trimethybenzene) and nonpolar (decane) VOC molecules [110]. The SWNTs functionalized with tricosane showed that an increase in resistance to 2.3% for decane and 7.2% for 1,2,4-trimethybenzene while 3.3% for decane and 5.1% for 1,2,4-trimethybenzene was detected by pentadecane-functionalized SWNTs. From the results, tricosane-functionalized SWNTs show good sensitivity for the polar volatile organic compounds. The sensitivity of tricosane-functionalized SWNTs was found to be 21% for 1,2,4-trimethybenzene and 6.9% for decane. After absorption, they can donate electrons to the SWNTs. The developed method could be used as a simple and noninvasive mechanism for the analysis of VOCs of lung cancer via breathing. A nanocomposite film based on multiwalled carbon nanotube functionalized polymethylmethacrylate (PMMA:f-MWCNTs) groups was synthesized by ultrasonication and centrifugation [111]. This prepared sensor was selectively applied for the detection of various VOCs such as methanol, ethanol, isopropanol, acetone, butanol, 1,2-trichloroethylene, 1,2-dichlorobenzene, chlorobenzene, xylene, and toluene. The response of the sensor was observed to be 17, 14.5, 0.9, 6.24, 0.27, 13.95, 1.88, 0.58, 0.05, and 0.098 for methanol, ethanol, isopropanol, acetone, butanol, 1,2-trichloroethylene, 1,2-dichlorobenzene, chlorobenzene, xylene, and toluene with a response time (min) of 1, 2, 3, 2, 3, 3, 4, 5, 4, and 7 min, respectively. High response was observed in the case of methanol, which may be due to the small size and electronegative (-OH) group present in the molecules. The application of the sensor was carried out at room temperature and a high regeneration cycle was observed. A nanocomposite sensor was prepared by electropolymerization of carboxylatedSWNTs doped with poly(3,4-ethyle-nedioxythiophene) and poly(styrene sulfonic acid) and applied for the elimination of VOCs (methanol, ethanol, and methylethyl ketone) from industrial manufacturing [112]. Cyclic voltammetry and field-effect transistor characterization techniques were employed to confirm the presence of the

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PEDOT:PSS coating on SWNTs. Detection limits for methanol, ethanol and methyl ethyl ketone were found to be 1.3%, 5.95%, and 3%, respectively. The developed sensors showed a good response over a wide concentration range and exhibited enhanced sensitivity.

12.3.3 Carbon nanofiber-based nanocarbon materials for VOC removal Carbon nanofibers possess unique chemical and physical properties such as excellent mechanical strength and high electrical and thermal conductivity, which make them an efficient material for various applications. It could be passed on to a wide range of matrices such as elastomers, ceramics, thermoplastics, thermosets, and metals. It has a unique surface state and due to this, the surface functionality of the nanofibers is easy to tailor or engineer [113, 114]. Homemade electrospun nanofibers, polystyrene (PS) nanofibers, acrylic resin (AR) nanofibers, and PS-AR composite nanofibers with Tenax TA were employed for the removal of VOCs, including hydrocarbon, cyclic, alcohol, ketone, ester, aromatic, and chlorinated compounds [115]. A preconcentration method was carried out for the determination of these model analytes using different synthesized sorbents. Five mg of sorbents was taken for the adsorption process. Polystyrene (PS) nanofibers, acrylic resin (AR) nanofibers, and PS-AR composites exhibited high adsorption capacity for ethanol of 2.46, 24.7, and 67.9 μg/5 mg, respectively. All the analytes showed good linearity and regression coefficients (R2) in the range of 0.988–0.999. From the results, it was concluded that electrospun nanofibers have the potential to eliminate VOC samples from environments. Ju and Oh (2017), prepared an activated carbon nanofiber by electrospinning and subsequent thermal treatment [116]. This nanofiber was prepared by blending with polyacrylonitrile (PAN) and cellulose acetate (CA). The adsorption capacity for PC10, PC09, PC08, and PC07 was found to be 65 g/100 g, 66 g/100 g, 72 g/ 100 g, and 67 g/100 g, respectively, at optimum temperature 25°C. A hydrothermal method was employed for the preparation of TiNF/ACF porous composites [117]. A large number of functional groups (–COOH, –NO3 and –OH) are present on the surface of composite materials as active sites, which makes them excellent photocatalytic adsorbents for VOC removal. Various characterization techniques such as field emission scanning electron microscope, energy-dispersive spectrometry, transmission electron microscope, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy were used to know the morphology, particle size, and functional group present in the nanocomposites. The adsorption efficiency for toluene was found to be 98.9% at 1 μm), might be distinguished depending on the size of material gathered. This is just the conventional and rough division. Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00013-4 © 2019 Elsevier Ltd. All rights reserved.

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For the proper choice of matrix and filler for nanocomposites, epoxy and carbon structures both are highly recommended as popular, available, and well studied over the last 30 years in terms of their physical and chemical properties, their degradation in time, manufacturing techniques, and dedicated equipment, synthesis, and production. The main advantage of polymer modification by carbon structures is related to the enhancement of selected mechanical and electrical properties. Epoxy resins, although exhibiting many outstanding properties, are highly brittle structures with low toughness and impact resistance. Carbon fillers are proven to be an effective method of inhibiting the crack initiation and propagation. Moreover, beyond the percolation threshold, they enable the conductivity in this intrinsically isolating material. The sharp insulation-conductor transition is observed. That leads to further applications in the highly demanding automotive, maritime and aerospace industries. Taking into account the variety of epoxy resins and nanocarbon fillers in the market that can be further modified, the number of possible materials seems to be unlimited and to design the new product, one may choose from a continuum of properties. Favorable synergic effects between the filler and matrix might be enlarged by providing a stronger interaction at the constantly increased interface. It is obtained by use of nanomaterials. Regarding the nanofiller properties, many researchers wonder why dimensionality matters [2], pointing mainly to the following phenomena: -

the increase of surface-to-volume ratio. the large specific surface area and high fraction of the reactive surface atoms. the quantum effects and quantum confinement. the local nonlinear behavior due to the defects or reaction kinetics.

Those interactions specific for the nanoscale (domination of van der Waals and weak forces) together with the surface enlargement [3] and the phenomena from surface science are the basis of the model construction for nanomaterial properties and behavior. In contrast to one-phase homogenous bulk materials, composite materials represent a major challenge in model constructing and theoretical studies. There is no one objective reference composite material, as its properties are always related with the synthesis method, load, and the form of the final structure. One may list the properties of classical bulk material (steel, copper, etc.) but in the case of composed phases, there is always the range of expected value and almost each composite should be treated individually. The same problem is encountered in aging modeling and in the development of recycling procedures. From the large-scale industry point of view, the lack of a simple and easily adjustable model is a serious disadvantage. However, one may treat versatility as a significant advantage in the material design. One of the solutions in theoretical modeling is the use of computer simulations to understand the physical properties of polymer nanocomposites [4]. Different-sized molecular dynamics (MD) simulation methods are currently mostly applied to investigate the relationship between the structure and performance of composed materials. It is one of the best approaches to model the viscoelastic complex systems. The MD methods are used, for example, to determine the influence of CF on the epoxy matrix. The most reliable and accurate quantum mechanical ab initio methods do not depend

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on empirical force-field parameters. Unfortunately, as they are computationally expensive, they are used on a small scale, which means only for local phenomena (e.g., the defects). The key task is to determine the microstructures of fillers and polymers and the interactions between them to understand the macroscopic behavior [5]. The MD method describes the movement of atoms and the geometry of molecules. Among calculated properties, one may find thermal and electrical conductivity, diffusion and thermal expansion coefficients, glass transition temperature, and mechanical modulus. The list of used methods includes the Monte Carlo simulation, quantum dynamics (ab initio based on the density functional theories), molecular dynamics (classical, Metropolis, coarse-grained, kinetic, or off-lattice), mesoscopic multiscaled and continuum (macroscopic scale) methods, and the force field. Running Monte Carlo simulations, the percolation phenomena might be described in detail, whereas the finite element algorithm may provide the information on the macroscale. Despite the fast progress, the methods for the prediction of thermal conductivity and electronic properties still need to be improved. Providing the good agreement reached between the simulation and experimental results, the approach may be used for predictions of performance, to explain the real systems, or to supply information on how to improve the laboratory techniques. In recent studies, it was possible, for instance, to find the ductile deformation response in epoxy, the correlation between fracture toughness and chain-filler molecular architecture [6], to describe the epoxy properties [7,8], or to model the performance of carbon fillers [9,10]. The type of filler and its geometry determine the structure of the cross-linked epoxy network. The key seems to be the interface and the contact zone between the filler and matrix. Modeling and understanding the phenomena in that region are crucial for the proper description of enhancement due to filler incorporation and, as a consequence, enables the optimization of phr and the type of carbon material added. In one of the recent studies [11], the molecular dynamics and molecular mechanics simulations were used to investigate the radial mass density profiles, the free volume on the filler surface, and the local cross-link distribution. This study revealed the clear dependence of the filler-matrix interaction on the degree of cross-linking. With increasing crosslink conversions, the interfacial adhesion between the filler and matrix is reduced. The proposed micromechanics-based multiinclusion model gives a prediction for the thermomechanical property of the composite. Although this particular work concerned the silica filler, one may explore this direction for research on nanocarbons. Recently, the comprehensive theoretical approach has been elaborated for singlewalled carbon nanotube interaction and performance in the epoxy matrix [12]. Defects should also be considered [13]. The vacancies degrade the interfacial shear strength, whereas the Stone-Wales defects promote it and the presence of the adatoms has no effect. The interfacial properties have been assessed not only theoretically but also in many laboratory studies [14] as they determine the performance of the whole material. It was proved already in the 1970s (in the case of polyester and fiber glass) that the stronger interfacial bonding between the filler and matrix would increase material mechanical resistance. Dating back to the 1990s, the general description of carbon fiber and epoxy interface was studied [15]. As the surface modifications of filler tailor

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the interfacial adhesion with the polymer, there have been many attempts to coat the fiber or change its structure, wettability, orientation, or distribution. For instance, the polar functional groups such as dCO, dOH, dCOH, and dCOOH generally enhance the mechanical resistance of the composite as they bind carbon fiber to the matrix via van der Waals (sometimes also hydrogen) bonding. The fact that the dOH and dCOOH groups may form a covalent bond with the epoxy matrix is largely used in filler design. Functionalities on the filer surface are more important for the rheological percolation than the physical bridging of polymer chains. Also, the high aspect ratio of CNTs enables better interfacial bonding. The energy adsorption and dissipation mechanism is responsible for the mechanical resistance of the material. There is still a lack of knowledge at the microscale about the fracture toughness and shear strength. The surface chemistry and delamination zone are studied by XPS, TEM, SEM, AFM, TGA, and fractographic analysis or via acoustic probing while IR or Raman spectroscopy is useful in a functional group’s qualitative description. The interfacial shear strength (IFSS) is the common parameter used to characterize the interface. It might be evaluated experimentally by the fiber pull-out test, the fiber push-in test, the fragmentation test, or the microbond test. Nowadays, the number of attempts to create epoxy nanocomposites with more than one nanocarbon filler is constantly growing. That is due to further enlarging their capability of efficient stress transfer as the calculation models enable more precise description of local and nonlinear effects at interlayers and interconnects. Finally, the intermediate materials can improve the transverse strength that used to be the weakest point of composite materials with carbon fibers. Despite its limitations, the capabilities of theoretical modeling increase constantly, creating new opportunities for the material industry. One of those is designing multicomponent systems. There is no need to limit oneself to just one filler. In the case of epoxy resin matrices, the hybrid materials already exist and perform in a very promising way. They may be obtained by the incorporation of more than one filler to the polymer matrix, by mixing different polymers or as a result of both approaches. Combining different fillers results in a further enhancement of properties as it is an efficient way to ensure the proper dispersion or to create the bridges to lower the percolation threshold. Stronger interfacial interactions and the enhancement of a chain’s 3D orientation or synergy effect enable the synthesis of unique materials. The synergy effect is described as a total effect greater than the sum from components. Otherwise, it is the exception from the rule of mixtures [16]. Let us now consider the possible nanocarbon fillers to be used.

13.2

Nanocarbons—overview of possible fillers

Among many carbon nanostructures, a few have been successfully implemented in composite materials. They might be classified as fullerenes (CFu), carbon nanotubes (CNTs), carbon (nano)fibers (CNF), and graphene and graphene-like materials (e.g., expanded graphite EG, graphene oxide GO, or graphene nanoplatelets GNP). Each class denotes the whole family of structures with a variety of adjustable properties.

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Despite the use of only one type of substance, carbon, there are numerous possible final 3D, 2D, or even 1D materials. The “classical” 3D materials such as carbon black, synthetic and natural graphite, and carbon fibers (for example, pitch-based) or their modifications (short carbon fibers, SCF) were commonly used as reinforcements for epoxy thermosetting polymers. Also, anthracite is considered a material with properties that are in between less-ordered carbons and graphene-like structures. Fullerenes were discovered in 1985 and earned the Nobel Prize for chemistry in 1996. They were initially considered for applications in tribology, solar cells, superconductors, electronics, batteries, energy storage, photovoltaics, and medicine (as virus inhibitors). They were used to modify the thermomechanical properties of epoxy resins, leading to improvements in friction, resilience, toughness, strength, and hardness as well as elastic, shear, and Young’s modulus. The failure mechanism was changed from brittle to ductile. Although the discovery of fullerenes paved the way for new concepts in materials design and changed it to the “bottom-up” approach, the real nanocarbon popularity was yet to come. This further step has been done due to carbon nanotubes, which were the next nanocarbon structure to be discovered, this time in 1991 (although predicted much earlier) [17]. Their applications and production yields overcame the fullerenes; this was noticeable even in comparison to the most popular polymer matrix reinforcement, carbon fibers (CF) [18]. The rising attention has been drawn to their applications in many different fields, such as sports equipment, the aerospace industry, and even medicine. That was due to the fact that CNTs are better than CF for mechanical resistance [19], surface energy, and wettability. They are known for exceptional stiffness, flexibility, strength, low density, a high aspect ratio, and a low electrical percolation threshold [20,21]. CNT/epoxy still has the stable position on the market [22]. One may distinguish the single-walled carbon nanotubes (SWCNTs) or the multiwalled carbon nanotubes (MWCNTs) with their special variant-the double-walled carbon nanotubes (DWCNTs). SWCNTs are flexible whereas MWCNTs are stiff and rigid. The properties of CNTs depend also on their aspect ratio [23]. The high point of the “carbon revolution” dawned with the discovery of graphene in 2004 (earnimg the Nobel Prize for physics in 2010) and, more precisely, with the development of its large-scale production [24]. The term graphene was introduced in 1987 and its theoretical structure was already described [25] in the first half of the previous century. The material exhibits potentially superior mechanical [26], electrical, chemical, magnetic, and thermal [27] properties that are revealed in polymer composites. It also has the large surface to thickness aspect ratio, sp2 hybridization, and a honeycomb crystal lattice [28]. Graphene outruns the other nanocarbons [29,30] as it incorporates not only one 2D material but also the numerous and versatile derivatives. It also brings the concept of 2D materials, which will be a rapidly growing field of interest and research in years to come [31]. In the family of graphene structures [32], one may distinguish, among others, the expanded graphite (EG) [33], graphene oxide (GO) and graphene nanoplatelets (GNP) [34,35], graphene nanoribbons from unzipped CNTs (GNR) or graphene nanosheets [36] that already have found the most interest in the epoxy composite materials [37]. They make at least some of the outstanding features of the ideal 2D infinite plane of graphene come true.

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Graphene oxide (GO) is a convenient filler because of its large surface area, high modulus, good thermal conductivity, favorable optical properties, low cost and fast synthesis. Moreover, GO is rich in possible surface functional groups: carboxyl, epoxide, and hydroxyl. It can interact strongly with both polar and nonpolar polymers and has hydrophilic edges and a hydrophobic plane inside. As GO might be obtained in many diverse methods, it is a legitimate approach to check its appropriateness for a particular application. The comparison between Brodie’s and Hummers’ oxidation products reduced in a second step at different temperatures (700°C, 1000°C, 2000°C, respectively) has been done [38]. In conclusion, GO-Hummers reduced at low temperature are suitable as reinforcing fillers, whereas GO-Brodie enhances the conductivity. In the case of graphene nanoplatelets, the large surface, metallic bonds, and Van der Waals interactions are used to enhance the filler-matrix contact. GNP is prepared by many methods, including from graphite through intercalation followed by exfoliation due to thermal treatment [37], by microwave irradiation, or by thermal expansion of oxidized flakes. It may be functionalized in many ways [3,39,40]: by methanesulfonic acid/c-glycidoxypropyltrimethoxysilane, in a Bingel reaction, amine-functionalized, or grafted 4,40-methylene diphenyl diisocyanate. A one-step synthesis of hybrid graphene-like materials and inorganic compounds (MgO, TiO2, SiC, etc.) or their nonstoichiometric derivatives is possible by the SHS process (Self-propagation High-temperature Synthesis) (Fig. 13.1). This is the exothermic combustion synthesis between the reducer and oxidant that might be used, for example, to obtain graphene flakes from GO. Graphene derivatives are also used as fillers, together with CNTs [41] or carbon black [42]. Those multicomponent systems enable us to trigger the properties, taking advantage of the synergy effect typical for hybrid materials. In that manner, the resistance to external factors is maintained and the weight reduced. The more expensive components can be replaced in part without deteriorating the properties that additionally help the product on the market. Moreover, the CB tightens the fractal-like network created by CNTs, lowering in that manner the percolation threshold. The presence of carbon black particles reduces the tunneling distance for CNTs [43] and creates new conductive pathways. Among different tested hybrid carbon systems, one may find CB/MWCNTs/epoxy (0.0025–0.6 wt%) [44] with an increase in the electric conductivity by even six orders of magnitude (up to 2.75  107 S cm1 for 0.2 wt% CB and 0.2 wt% CNTs [45]) and the considerable improvement of the impact resistance (from 4.82 to 7.57 k Jm2); GNP/CB/epoxy (9:1); and GNP/CB/MWCNTs/epoxy (7:1:2) [46]. Combining carbon black with graphene enhances the CB dispersion and the level of GNP exfoliation, preventing the sheet stacking. The mix of nanocarbons was also used to modify other polymers than epoxy: polypropylene, high density polyethylene or polypropylene/ethylene-propylene-diene monomer, styrene butadiene rubber, styrene, polyvinylidene fluoride, poly(ethersulfone), polyamide 6,6, polycarbonate, cyanate ester, polyetherimide, polysulphones, and polyethersulphones. CB is also mixed with carbon fibers for macrocomposites. Fibrous carbon facilitates the longdistance charge transfer, whereas carbon in the form of particles creates the interconnections between fibers and builds the local pathways. That mechanism, observed in

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Fig. 13.1 The SHS reaction chamber and the propagation of the combustion wave during the reaction.

HDPE [47], can describe the situation in the other matrix. Carbon black works as nodes between fibers, enabling their proper orientation. However, the better effect is observed in the case of CNT/GNP hybrids [48,49]. A cosupporting network of both fillers prevents agglomeration and enhances the electrical and mechanical properties of polymers. Both SWCNTs and MWCNTs are used. The highest conductivity was obtained for GNP:SWCNTs ratio 3:1 and 1:1 for MWCNTs 1 wt%/GNP 1 wt% (4.7  103 S cm1). MWCNTs (0.1–1 wt%)/GNP (up to 2 wt%)/epoxy [50].

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MWCNT-COOH/few-layer graphene filler increases the thermal (up to 0.295 W m1 K1) and electrical conductivity of epoxy resin. The presence of GNP enhances the interfacial heat exchange. The same effect, but with a different concentration depending on the diameter of the tubs, might be obtained for MWCNTs. In another study, vertically aligned carbon nanotubes were grown directly on GNP in a catalytic CVD process [51] to form the hybrid/epoxy material. Also, EG added to MWCNTs results in a significant enhancement of the electrical conductivity. That effect might be explained by: -

EG nanosheets working as bridges and facilitating the conducting network construction. The block of tubes aggregation due to the large diameter to thickness aspect ratio of EG. Strong van der Waals interaction between fillers.

Moreover, in the presence of the MWCNTs, the π-π interaction between graphite sheets decreases and so does the tendency of EG platelets to agglomeration. By contrast, the thermally reduced graphene oxide (trGO) caused the decrease of the electrical conductivity in the MWCNT/epoxy composite [52]. Probably, the trGO flakes hindered the agglomeration and reduced the number of contacts in the network. In one of the newest (2018) studies [53], the comparison of nanocarbon performance as epoxy resin fillers in the function of their different diameter has been done. It is proved that growing dimensionality (from 1D CNTs, via 2D GNP, up to 3D graphite) decreases the intrinsic viscosity and increases the electrical percolation threshold, optimal nanomaterial concentration (ONC), and robustness. This last parameter is the full-width at half maximum (FWHM) of the curve presenting the enhancement of selected property for different filler concentration in the epoxy. The higher value is desirable as it indicates the bigger tolerance in adjusting the right nanocarbon concentration. GNP has it higher than other tested structures. The reinforcement efficiency might be described as the ratio of the enhancement in percent to loading in percent of weight. It is the parameter to quantify the mechanical enhancement effect and decreases about one order of magnitude with the increasing dimensionality. As nanocarbons of different dimensionalities exhibit contradictory characteristics, for instance for 1D the favorable percolation threshold and ONC are contrasted by high viscosity and low robustness, the figure of merit (FOM) might be a useful tool in material design. The FOM, by definition, is the product of robustness and reinforcement efficiency divided by relative viscosity [54] that makes the handling and processing of the mixture more difficult. The relative viscosity is fitted to the semiempirical Krieger-Dougherty model. The FOM is a simple and fast way to compare different fillers in terms of their reinforcement, percolation, rheological behavior, and processing. Moreover, due to the variety of discovered nanocarbons, still commonly used CF/ epoxy nanocomposites were upgraded by adding the third phase of nanoparticle material. There are various methods of carbon fiber surface modification reported in the literature [55–80], in particular sizing, covalent and noncovalent functionalization, acid treatment (the mixture of sulfuric and nitric acids), ozone treatment, chemical vapor deposition (CVD), electrophoretic deposition (EPD), repeated chemical grafting, liquid phase deposition, and thermal, plasma, or electrochemical treatment. In that manner, the addition of nanoparticles, CNTs, or GO and its derivatives is

Nanocarbon/epoxy composites: Preparation, properties, and applications

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possible. All those methods result in functionalization, triggering interfacial connections and, as a consequence, in the further enhancement of material performance, especially from a mechanical point of view. That is due to the increasing roughness and improving fiber adhesion to the polymer. On the other hand, the undesired disorder at the surface and the material deterioration caused by different thermal coefficients are observed. Nanoparticles create bridges between the fiber and matrix and bond noncovalently. The nanocarbons have a stronger effect if they are on the CF surface than in the matrix. The optimal amount is also important to create locally the 3D structure, as too small CNT deposition works as a defect. The oxidized MWCNTs work as a shielding transition layer that minimizes the stress concentration and prevents the crack to deteriorate fiber. The enhancement of properties is due to the CNTs at the filler surface that enable more efficient energy dissipation. The matrix toughness might be enlarged by adding some rubber and hydroxyl terminated polybutadiene. Sometimes together with carbon fillers, additional particles are added, for instance ZnS or the silane coupling agent, for a better interphase microstructure. The amine functional group additionally facilitates epoxy curing. An interesting and scalable approach of GO incorporation was based on slow CF passing through the emulsion of the aqueous solution of GO in epoxy and final drying at 100°C. Another idea on how to suppress delamination focuses on the thermoplastic film in between the carbon/epoxy composite [80]. Also, silicon carbide nanowires (NWSiC) and silica nanoparticles from combustion synthesis were successfully incorporated in the EG/ epoxy system in order to prevent the carbon lattices from aggregating (Fig. 13.2).

Fig. 13.2 The variety of nanofiller morphologies: expanded graphite, graphene flakes, and nanowires of SiC with silica preventing agglomeration of carbon structures.

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Nanocarbon and its Composites

However, in all referred methods, the synthesis becomes more complicated (by at least one more step added). The challenges related to nanocomposite production will be discussed in the following section.

13.3

Epoxy nanocomposite preparation-challenges and opportunities of using nanocarbon fillers

In the case of nanocomposites, the fabrication method determined the properties of the final products [81]. As this contains various steps of preparation, there is an exponentially increasing number of parameters to consider. That is why the elaboration of the synthesis protocol is important. Depending on the final application, each time the proper structure design and basic modeling are needed [82]. The progress in theoretical predictions based on the numerical calculations enables more rapid and cost-effective new material design. In many cases, there are the variations of the most popular and already tested approaches. In order to ensure a homogeneous filler dispersion in the matrix, the following methods are used: – – – – – – – – – – – – –

three-roll milling (calendaring process), ball milling, planetary mixer with zirconia balls, sonication, sonication combined with the calendaring process, sonication followed by freeze drying, high-shear mixing, centrifugal mixing, use of an ultrasonic homogenizer, ultrasonication followed by a high-speed shear mixer, mechanical stirring, magnetic stirring, overhead stirrer.

This is a list of just a few of them and their modifications. Sonication and mechanical stirring were proved to be, in combination, one of the best methods for graphene filler [83]. Sometimes dispersive substances [53] and surfactants are added (e.g., solventfree acrylate copolymer) together with the filler to facilitate its incorporation before curing th