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

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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 Pd-Ni@AC 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 Pd-Ni@AC 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 Pd-Ni@AC 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)

Pd@AC PdNi@AC

Pd@AC PdNi@AC

Pd(220)

Intensity (a.u.)

Pd(111)

Intensity (a.u.)

Pd(111)

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2q (degree)

AC Pd-Ni@AC

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 Pd@AC and Pd-Ni@AC NPs and (B) The Raman spectra of Pd-Ni@AC.

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 PdNi@AC 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

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Fig. 2.5 (A) Plot of nH2/nDMAB vs time for the DMAB dehydrocoupling with the existence of Pd-Ni@AC NPs at different catalyst concentrations at 25  0.1oC; (B) % conversion vs time plots for Pd-Ni@AC 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 Pd-Ni@AC 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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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)/DPA@GO 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 Pt@PPY-PANI 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 Pt@fVC 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/TC@GO 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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3

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 (MnF2@N,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 NS@PANI/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

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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 DWCNT@SnO 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 S@S 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 S@S DWCNTs decrease rapidly and disappear when D > 1.5 nm. On the other hand, M@M 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 SWCNT@SiW12O40. 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 core@shell materials of the form MWCNT@TiO2 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  5@10  10 MWCNT system. (B) MWCNT@30-TiO2 system. (C) MWCNT@150-TiO2 system. (D) MWCNT@190-TiO2 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 core@shell 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

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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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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 the resin. Surfactant lowers the surface tension and thereby diminishes the driving force for agglomeration. It is a good laboratory practice to check the possible dispersion influence on the filler morphology (by SEM, TEM, AFM, XRD, and Raman spectroscopy). The surface roughness, sp2 content, and aspect ratio are susceptible to being mechanically changed. Moreover, with different fillers, the dispersion methods should be adjusted to obtain the same degree of dispersion. An innovative dispersion technique was elaborated for MWCNTs that were implemented in epoxy by simultaneously applying ultrasonic waves and shear force generated by an axial flow impeller [84]. The 0.75 wt% load enhanced tensile strength

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by 35%, toughness by 53%, and storage modulus by 35%. A strong anticorrosion effect was also observed. The plasma treatment and wet-chemistry aminefunctionalization of CNTs are also efficient approaches to increase the homogenous dispersion [85]. The enormous role of a filler’s shape, orientation, and concentration on the product properties is worth being underlined. There are many studies comparing what appears to be the same type of filler; however, it was provided by a different supplier or was synthetized in a proper laboratory with the commercial product. Even the smallest change in filler morphology is visible in the macroproperties of materials. In addition to the filler type, its aspect ratio, functionalization, aggregation, specific surface area, and defects need to be considered. Moreover, it was shown that the space orientation is crucial for the value of the fracture energy and the electrical conductivity. The alignment effect might be obtained by applying an electric field or an electromagnetic field [86]. The efficacy of this procedure is strictly related to the filler concentration and diminishes at higher loads. There are also some ex situ techniques such as electrospinning or grafting of CNTs. The aligned dispersion, in comparison to the random one, provides materials with better mechanical resistance and enhanced electrical and thermal conductivity. Such a controlled anisotropy will probably be one of the most explored scientific paths in coming years. Another approach is based on limitation of the agglomeration (especially in graphene-like materials and nanoplatelets that tend to aggregate) in order to obtain the perfect dispersion and isotropic material. Regarding nanocomposites, the concentration of filler is at least one order of magnitude smaller than in classical composite materials. This is an enormous advantage from the point of view of cost or facilitated processing. In addition to that, the specific “nanoeffect” might be observed, which is the existence of local extrema of concentration (minima or maxima) called ONC at which the final material exhibits the stronger enhancement of selected properties. The aim of numerous studies is to find this optima. This is not a banal task due to the lack of a comprehensive model describing the basic reason for observed phenomena. That is why 2D fillers with large robustness are welcomed. This is also an important advantage in the case of continuous polymer processing with methods such as extrusion, in which the filler concentration may change during manufacture. Among other problems to be overcome, increasing viscosity is one of the limiting factors. Various ether compounds added to the bisphenol A might be useful. Phenolic glycidyl ethers are formed in a reaction of condensation between the epichlorohydrin and the phenol group. The differences in resins are due to the number of phenol groups and the structure of the phenol-containing molecule. The low viscosity is needed, for example, for spinning, molding, and 3D printing. With increasing filler content, the passing from Newtonian behavior to shear tinning is observed. For the incorporation of the CNT’s much higher loading (15–36 wt%), a mixed-curing-agent assisted layerby-layer method was successfully adopted [87]. In another study, the resin transfer molding process enabled the controllable alignment of CNTs with loading up to 16.5 wt% [88].

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The flocculation, especially with use of CNTs, is also to be considered. Carbon fillers can be functionalized before integration with the matrix or at the stage of incorporation or epoxy curing. The exposure to UV light and/or an ozone atmosphere is used in order to introduce an oxygen-containing group on the surface [89]. The defects presented in graphene are points of attachments for the functional groups that might be further modified by, for instance, silanation, esterification, or thiolation [90]. Physical functionalization takes place when the supramolecular complexes of graphene are formed by its wrapping around the polymer chain [91]. Moreover, the graphene lattice can consist of not only hexagonal rings, but be built by pentagonal and heptagonal ones. As epoxies are the thermosetting polymers, the curing and postcuring time and temperature are also important and adjustable parameters to be carefully considered. Commonly encountered in the literature, depending on the resin source, there is room temperature processing for 18–22 h and 2–4 h at 80°C, respectively. The exact parameters depend on the resin/hardener pair (such as, for instance, diglycidyl ether of bisphenol A/polyether triamine hardener). With diaminodiphenylmethane (melting point at 94°C), the preheated and degassed material in silicone molds is cured at 90°C then by 150°C (each of the two steps takes 4 h). The stage of mixture degasification is also important. Otherwise, the air bubbles, especially at higher concentrations, will deteriorate the mechanical resistance of the material. The vacuum oven is frequently used. Good results are obtained with such advanced methods as the RIFT (resin infusion under flexible tooling) or VARTM (vacuum-assisted resin transfer molding). In the case of laminate production, the final composite sheet should be done by hot press processing. Finally, the record-breaking load of 68 wt% MWCNTs incorporated in epoxy was obtained by hot-press infiltration through the semipermeable membrane [92]. An impressive Young’s modulus of 36 GPa and maximum electrical conductivity of 37  104 S m1 were achieved. Although there are many parameters to be optimized during the synthesis, the drawbacks related to the additional complications of the system with the introduction of another component are easily overcome by the advantages of the synergy effect. Better methods of nanocomposite synthesis enable further development. In one of the studies, the MWCNTs were functionalized using a glycidyl methacrylate monomer and poly(oxypropylene)diamines. They were mixed together with GNP in a tetrahydrofuran solution with an epoxy oligomer and a curing agent. The best result, from the thermal, electrical, and mechanical points of view, was achieved for the fMWCTs: a GNP proportion equal to 1:9 [93]. The polybenzimidazole-functionalized GNP proved to be better dispersed in the matrix than the raw GNP [94]. Both substances decrease the heat release rate of the curing reaction and provoke the increase of the curing temperature. Nowadays, constant progress is observed in research on the synergy effect due to the use of the hybrid polymer composites with more than one type of carbon nanofiller or mixing various polymers. Sometimes, the additional compound, nanosilica for instance, is introduced into the epoxy before curing and/or synthetized in situ via the sol-gel technique [95]. The nanocarbon/epoxy properties are discussed in the next part of this chapter.

Nanocarbon/epoxy composites: Preparation, properties, and applications

13.4

433

Nanocarbon/epoxy composite properties-the versatility of materials

Nanocomposites reinforced by different nanocarbons exhibit multifunctional properties. Their mechanical, thermal, electrical, and chemical performances as well as their morphology, crystallographic structure, and rheological properties make them abundant in the automotive, aerospace, and construction industries as well as in many advanced applications. Moreover, the epoxy by itself has an affordable cost and a huge flexibility of processing [96]. It is generally observed that, within the increasing linear size of the filler, the concentration to ensure the same effect of enhancement keeps growing [97]. Because of that, the economy of materials is a main argument for the use of nanofillers. What is more, the lower loads facilitate the handling of material during its preparation due to the lower viscosity and better homogenization. The systematic evaluation of the mechanical properties of epoxy resin has been done and is the basis for designing its modifications. For thermosetting polymers, the high cross-link density improves the mechanical performance. However, it also has an undesirable effect on fracture toughness, which remains the most important material drawback. Although it has a variety of favorable mechanical properties, neat epoxy typically exhibits a brittle fracture. The reinforcement of the uniformly dispersed filler [98] is based on the fracture energy dissipation by the interaction with the crack front in order to deflect or bifurcate it, and as a consequence, to disturb its propagation [99,100]. The parabolic and nonlinear fracture patterns are observed [101]. Nanocarbons exhibit a significant capability to enhance the toughness, understood as the capability of the material to absorb the energy before the fracture takes place. The obtaained composites are thus strong, stiff, and resistant at a relatively low weight fraction. The nanocarbons improve epoxy performance in tensile, shearing, and tearing tests. The tensile, bend, creep, fatigue, and hardness testing results are better for the nanocarbon/epoxy material than for the neat resin. The overall effect, such as improvement of the tensile strength and modulus [102] or toughness, depends on the type of carbon filler, its dispersion, and topology [103–116]. As for graphene, the smaller flakes (length, width, thickness) have a stronger positive impact [117,118] due to the smaller stress concentration factor on a wrinkled surface [119]. That is because the smaller difference between the sizes of adjacent compounds limits the strain field. Sometimes the opposite effect is observed as the fracture toughness increases with graphene size [49], indicating that the reinforcement mechanism is more complicated than a simple crack propagation path description. The stress and strain levels in some materials might by monitored by measurements of electrical resistance. This intriguing discovery has been made for the matrix EPON 862 W with 0.5%wt of graphene particles [120]. Probably, the formation of microcracks and the partial debonding of flakes are responsible for that effect. This hypothesis was confirmed by another study [121]. Authors found that the electrical conductivity in graphene/epoxy nanocomposites depends on the level of tensile stress, not the compressive one.

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A similar advantage of nanocarbon fillers that are able to directly solve the particular problems of epoxy properties is observed not only for the mechanical performance. Although intrinsically isolating, epoxy resin might have good electrical properties due to the low percolation threshold of CNTs (0.06 wt% in respect to >3 wt% for GNP). It reflects the degree of connectivity within the composite and influences the conductivity that is expressed by a simple formula: σ ¼ kðm  mcÞβ, where: σ-conductivity, k-empirical constant, m-filler concentration, mc-percolation threshold, and β-critical exponent. For MWCNTs, the values of the electrical percolation threshold vary from 0.0021 wt% up to 5 wt%, depending on the supplier and nanocomposite preparation procedure. In one of the recent studies [86], the influence of the aligned materials (CF and GNP) on the fracture energy and electrical conductivity was tested. The authors described different toughening mechanisms such as debonding of the filler, the energy associated with the frictional pullout of the reinforcements from the matrix, crack bridging, and void growth around the debonded fibers in a process zone. The real mechanism in the composite depends on a type of filler (the crack bridging for GNP and toughening via void growth for CF). A 50% lower percolation threshold and a 40% improvement in fracture energy were due to the alignment of the filler obtained thanks to the alternating current (30 V mm1, 10 kHz). Many concerns related to the enhancement and tailoring of electrical properties are focused on graphene due to its characteristics of effective gauge fields, a Fermi level at the Dirac point, and the improvement of conductivity by the dopant, to list just a few. The best results for graphene/epoxy composite conductivity were observed in the case of mechanic stirring and ball milling at the stage of filler preparation. Discrepancies between the value of percolation thresholds in the case of different properties (electrical, rheological, mechanical) are very understandable due to the special requirements for each type of network [122]. The electrical percolation threshold in the case of CNTs [123–127] is usually obtained with lower loading than others, as conductivity is additionally supported by quantum tunneling. The rheological percolation threshold is defined as the filler load at which the Newtonian matrix changes for a predominant elastic response. Because of the key role of percolation, both the electrical and thermal properties of CNT nanocomposites will obviously depend on the average length of tubes. Besides the percolation threshold, the shape of the percolation curve and the prepercolation values in the so-called dielectric region are also critical factors for the overall material performance. For the narrow transition area, a small perturbation in filler content due to the temperature, pressure, and insufficiently homogenous dispersion has an enormous effect on conductivity. Hybrid fillers (such as CNTs/CB) have in general a wider insulator-conductor transition area, making them less susceptible to processing variances. The modeling of a general percolation threshold drop in a hybrid system based on the well-known theory and verified by the new experimental data is also interesting [128,129].

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The percolation model is also efficient in the description of the thermal energy dissipation and increased flame-retardant properties. The nanocarbon/epoxy material, especially with graphene fillers [130,131], is a promising material for highperformance thermal interface materials, as it has the superior effective thermal conductivity (ETC). Several models describing that phenomenon have been constructed [132–135], giving the conclusion that ETC depends in a nonlinear way on the graphene weight fraction and is strongly related to the aspect ratio and filler orientation [136,137]. From the chemical point of view, one should underline the versatility of the nanocarbon/epoxy materials, the possibility of filler functionalization as a result of surface energy or free dangling bonds at the edges, and material resistance. Fillers additionally improve the matrix performance and curing process by decreasing the heat release ratio. The glass temperature might increase or decrease depending on the level of dispersion. There are two opposite effects of filler on epoxy. On one hand, filler increases the Tg restricting the movements of polymer chains, but on the other hand, the lower cross-linking density decreases Tg. By evaluation of changes in the glass temperature value, one may judge whether the overall effect is dominated by interfacial interactions (with higher Tg). There are numerous studies that compare carbon nanotubes and graphene as epoxy matrices fillers. In terms of the possible number of diverse applications and tuneable properties, graphene outruns other fillers [138]. Its dispersion is obtained by the combination of sonication and mechanical stirring, allowing the further increase of desirable properties making it exfoliated, delayered, and with shorter sheets. The introduction of an organic solvent is also welcome. It is important that in some studies, graphene fillers also exhibit the placebo effect [51, 139–141] or even deteriorate the matrix performance [142–146]. That is why the proper concentration and filler morphology are crucial. The modeling of graphene-interface interactions [24] or the performance related to the number of layers [147] is helpful. The superiority of GNPs over CNTs in the enhancement of mechanical properties might be explained by their higher specific surface area and, because of that, the larger contact with the matrix as well as by stronger adhesion and chain interlocking due to the wrinkling of 2D sheets. For many years, the majority of the research focus was on enhancing the mechanical, electrical, and thermal properties of epoxy due to the added nanocarbon. However, the matrix itself determines the possible range of applications. Commercially available epoxy contains a liquid phase (bisphenol A, its compounds and/or different other substances such as poly(propylene glycol) bis(2-aminopropyl)ether, neopentyl glycol diglycidyl ether) and a curing agent (a blend of aliphatic amines and aliphatic amines adducts, for example based on diethylenetriamine or triethylenetetramine). The compositions and the proportions between the resin and hardener are different. The versatility of the matrix is the advantage during adjusting the optimal properties. For instance, the amount of flexible aliphatic epoxy chains in the system changes the glass transition temperature. It also has serious drawbacks as it makes the data from different studies incomparable. Furthermore, due to the presence of bisphenol A and amine groups, the nanocomposites are not environmentally friendly and biocompatible. With increasing demand for such materials, there is a fast advance in research on

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modification of the matrix. The bisphenol A is known as the endocrine disruptor [148] and probably its presence might be responsible for decreased fertility and morbidity to cancer. The question of whether a bioepoxy would be possible is of significant relevance [149]. According to the research on mechanical (the tensile test of the stiffness and elongation at break), dynamical, thermal (TGA), and electrical properties [150], the bio-based epoxy is not only the inferior alternative to bisphenol A, but a valuable and sustainable material [151]. The comparison has been done for MWCNTs (0.05 wt%, 0.1 wt%, and 0.2 wt%, respectively) in two different matrices. The “classical” epoxy was based on the diglycidal ether of bisphenol A (DGBA). The “bio” material consisted of diglycidyl ether diphenolate n-butyl ester (DGEDPBu). In both cases, the diaminodiphenylmethane (DDM) was used as a hardener. Its aromatic rings work as a steric hindrance and efficiently reduce molecular mobility. The thermogravimetric analysis confirmed a comparable thermal stability of MWCNT/DGEDP-Bu and MWCNT/DGBA. However, the intrinsic viscosity of DGEDP-Bu is higher than for neat DGEBA. Dynamic mechanical analysis (DMA) provided information on both the elastic and viscous nature of the material, the glass transition temperature, the alpha transition temperature, and the storage moduli. The Herschel-Bulkley model is commonly used. At 0.2 wt%, MWCNT/DGEDP-Bu exhibits a 20% higher value of the storage modulus than an unloaded resin and 13% in respect to the DGBA with the same load, which indicates the better structure of the filler network and a slightly more homogeneous state of dispersion in the first case. Moreover, the relative storage modulus enhancement confirms the better interfacial mechanical interaction filler-matrix for DGEDP-Bu. The n-butyl ester might work as a plasticizer. There is also the 1.68 times increase of the yield stress. The percolation thresholds can be achieved with the lower load of filler. Additionally, the hydrophobic side chain of bioepoxy could enhance interactions with nanocarbons. The matrix itself has an advantageous free volume. As a consequence, the MWCNT/DGEDP-Bu may find an application in all crucial fields in which the epoxy is employed, such as the automotive and aerospace industries, microelectronics, and electromagnetic shielding,

13.5

Nanocarbon/epoxy composites applications—main fields of interest

Regarding the outstanding mechanical, thermal, and chemical properties as well as the dimensional stability of epoxy resin and the enhancement in their electrical performances due to the added nanocarbons, one is not surprised to find them in many different applications. The world production of epoxy resins exceeds 2 million tons, with the vast majority (around 90%) of the diglycidal ether of bisphenol A. Let us now consider in detail a few examples of the most popular applications. The epoxy adhesives (structural or engineering) are famous, and not just in the construction industry. They constitute the majority of nanocarbon/epoxy applications and are found in bicycles, boats, skis, snowboards, automobiles, aircraft, and golf equipment. They might be used as structural glue instead of welding or be additionally

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reinforced by Kevlar and boron, glass, or carbon fibers. They are adhesives for a variety of different materials such as wood, metal, glass, stone, and plastics. Moreover, the inherent brittleness of epoxy that limits its application might be overcome by the incorporation of the second-phase component. The fractural toughness of epoxy enhanced by nanocarbons, combined with its intrinsic mechanical properties, make these nanocomposites the ideal material for the aerospace, maritime, automotive, and construction industries. This market that is still growing and seeking innovation is the most important strategic area for nanocarbon/epoxy materials and laminate development, especially taking into account their other desired properties. For instance, such an additional feature as electromagnetic shielding makes them ideal for the aircraft industry and parts of devices operating in intensive electrical fields. The theoretical description of the effect [152] was based on the generalized Maxwell-Garnett Theory [153]. The terahertz time domain spectroscopy is a useful tool to measure the filler impact on electromagnetic shielding and to better understand this phenomena [154]. Nanocarbon/epoxy materials might be used for semiconductor encapsulation or for the production of hardware components. Further, highperformance sport equipment needs materials with a high strength-to-weight ratio. This parameter is also fulfilled by nanocarbon/epoxy, thus one may find them in tennis rackets or the masts of sailing yachts. As epoxy is more heat-resistant than latex- or akryl-based paints, it is often used as a coating (e.g., for washers or concrete floors) or in advanced industrial areas. An important advantage is also its chemical resistance. That might be used in designing the epoxy impregnation for nuclear reactor waste [155]. It is effective in limiting the leaching of radionuclides. Nanocarbon/epoxy is used as an effective coating to protect against diverse external factors such as UV radiation, corrosion, and temperature. Nanocarbons are, in this case, the optimal fillers in order to further extend the favorable properties and enhance the flame retardancy [156–161]. The following fillers were tested: graphene nanosheets [36 sara], organic phosphate functionalized GO [162], phenyl-bis (triethoxysilylpropyl) phosphamide functionalized GO, ZnS, or MoS2 with graphene material [163], fluorinated montmorillonite or ammonium polyphosphate with MWCNTs [164–166], CNTs functionalized by vinyltriethoxysilane [167], and benzalkoniumchloride-N-methyl pryrrolidine-fullerene. The incorporation of nanocarbons results in the inhibition of the flammable drips and in the enhancement of the parameters such as heat resistance, melt flow (decreasing), oxygen index (increasing), and total heat release (reduced). Nanocarbon/epoxy materials are tried and tested as blast-resistant, tribological, anticorrosive, and microwaveabsorbing coatings. The GO transforms epoxy from hydrophilic to hydrophobic, especially when combined with CaCO3 [168]. The GO-CaCO3/epoxy exhibits an enhanced protective performance as the hydrophobic surface reduces the contact surface and exposition time of the degradation medium. Other similar fillers are amino functionalized GO by an aromatic diamine and p-phenylenediamine [169], metronidazole-modified GO [170], and GNP dispersed in water with sodium polyacrylate [171]. Regarding the thermal resistance and conductivity, the nanocarbon/epoxy has a very good ratio of quality to cost. The cured epoxy resin has a better heat conductivity

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than air with thermal properties improved by intrinsically extremely good carbon materials. Using reverse nonequilibrium molecular dynamics (RNEMD), one may simulate the thermal transport across the interface. Its behavior is crucial due to the distinct thermal properties of epoxy and nanocarbons that cause the temperature gradient and associated interfacial thermal resistance. According to that data, proper functionalization might be done in order to reduce the interfacial thermal resistance [172]. Even a 240% increase over the neat epoxy is possible [173], obtaining a thermal conductivity of 0.72 W m1 K1 or more than three times for 10 wt% loading of graphene-like material [174]. In another approach, even 1.7 W m1 K1 was obtained for the 30 wt% of graphene functionalized with methanesulfonic acid/ γ-glycidoxypropyltrimethoxysilane [175]. In order to increase the thermal conductivity, graphene may also be decorated with Ag nanoparticles [176] or form a ball coated by poly-methyl-methacrylate [177]. The desired enhancement results were obtained with GO and a GO-encapsulated boron nitride hybrid material [178], reaching 2.23 W m1 K1 at 40 wt% concentration. Furthermore, the electrical performance of nanocarbon/epoxy materials is an advantage in the microelectronics and electronic industries. For instance, epoxy is used in overmolding integrated and hybrid circuits or transistors and in making printed circuit boards or capacitors and diodes. Depending on the filler, the electric conductivity and dielectric permittivity might be tailored in a specific frequency range, for example from 8 to 20 GHz [179]. A high dielectric constant was also obtained via the homogenous dispersion of GNP of thickness 20–50 nm [180]. The properties favorable for electronic application are obtained also, thanks to the hybrid filler of reduced graphene oxide and inorganic material. Ag/rGO has optimal electrical conductivity equal to 123 S cm1 and for the nanocomposite Ag/rGO/epoxy, the 17.1 S cm1 is achieved [181]. rGO from microwave-assisted chemical synthesis with Fe2O3 particles is also investigated. The crucial part is the orientation of the filler. By alignment obtained due to the use of the AC electric field, further improvement is obtained. The increase of electrical conductivity about 7–10 orders of magnitude with a decrease of the percolation threshold about 50% for 1.5 wt% of GNP and CNTs, respectively, is noted [182]. Due to the presence of graphene-like structures, their applications are also extended to the energy storage and conversion, sensing, and biotechnology fields. The biocompatibility and biodegradability of carbon structures, notwithstanding the additional indispensable tests for specific nanomaterial behavior, are important in implant design or drug targeting. CF/epoxy was used for orthopedic applications to mimic the natural bone and caused extensive osseointegrating bone formation [183]. The GNP/CNT hybrid fillers are used with different matrices in the following applications: hybrid porous electrodes for the new generation of supercapacitors [184,185], electromagnetic shielding devices, dye-sensitized solar cells to enhance the adsorption and charge recombination process [186], new generations of ultrafiltration membranes [187], and gas barriers. One recent development concerns the reversible plasticity shape memory property that might be used in self-healing systems. Authors claimed that the use of MWCNT/epoxy would be a promising alternative to the conventional shape

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memory programming [188], as the temporary shape can be fixed by plastic deformation at the temperature not above but lower than Tg. The carbon filler improved shape fixity, recovery speed, and response temperature. With different MWCNT content (0, 0.5, 1.0, 1.5, 2.0 phr) changing the glass transition temperature, the programming conditions, and the varying transition region, the parameters can be tuned to obtain the desired result. However, even more applications, such as the electronic encapsulating resins, are possible in the case of multicomponent epoxy systems discussed in the following section.

13.6

Multicomponent epoxy systems: Nanocarbons and elastomers/thermoplastics or inorganic compounds

The simultaneous use of micro- and nanoscale fillers is currently a fast-developing research area as one of the most promising ways to substantially enhance the epoxy resin properties. It has been shown many times [189,190] that the incorporation of additional filler enlarges the contact surface area and adhesion that are the critical factors for mechanical parameters such as the fracture toughness, strength, and stiffness. The enhanced mechanical properties of resins are due to the shear bending, nanovoiding, crack pinning, and crack deflection. Those are the mechanisms responsible for the toughness and strength of epoxy in hybrid systems. Taking that into account, there is no need to limit the structure design just to nanocarbon/epoxy. Among macrofillers used in a triple system, the most frequently employed modifiers are liquid rubbers, core shell rubbers, dispersed acrylic rubbers, and thermoplastics. They accompany inorganic and mineral fillers such as nanosilica [191,192], nanoclay (which properties strongly depend on the level of aggregation [174], for instance montmorillonite or hydrotalcite [193]), silicon carbide nanowires (NWSiC) [194,195],a and nanocarbons, mainly CNT or graphene derivatives, talc, calcium carbonate, mica, kaolin, wollastonite, feldspar, alluminiumhydroxide, nd TiO2 [196]. The aim is to improve fracture toughness without a reduction in other material parameters. Although all the mentioned fillers have been proved to be a good reinforcement of the matrix, for obvious reasons, within this section the nanocarbons will be of concern. They are used also together with other inorganic fillers as in CB/silica/epoxy [197–199] or MWCNT/TiO2 with decreasing Tg and CB/clay/epoxy [200]. The incorporation of boron nitride to nanocarbon/epoxy nanocomposites has a favorable effect on electrical properties [201]. For instance, the CNTs have been used together with halloysite (silicate-based) nanotubes. The advantage of that approach is visible in the material toughening that depends on the filler loading, time, method of mixing, and chemical surface treatments [202,203]. There are many studies on MWCNTs as good fillers for epoxy hybrid systems as well as attempts to understand the structure-property relationship [204]. The rubbery domains are smaller as their growth is inhibited by the presence of CNTs, which increases the viscosity. On the other hand, due to the rubber, the dispersion of CNTs is not limited by agglomeration during the cure. Probably the small concentration of CNTs enhances particle nucleation.

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A synergic effect on toughening and a conductive network formation were confirmed. The cure behavior and kinetics (analyzed via DSC) are also considered, observing the decrease of activation energy. In the literature, one may find information about enhanced mechanical properties in the case of the following materials: epoxy/MWCNTS/CTBN (carboxyl terminated poly(butadiene-co-acrylonitrile)), epoxy/MWCNTs-COOH/CTBN [205], and epoxy/MWCNTs-NH2/ATBN (amineterminated butadiene-acrylonitrile) [206]. When modified by carboxylic acid or amino-functionalized, multiwalled carbon nanotubes exhibited better properties as epoxy fillers. The overstoichiometric active hydrogen prevents the increase of cross-link density. Moreover, the carboxyl-functionalized carbon nanotubes accelerate the curing process. The N-octyl-functionalized CNTs (CTNs-No) improve thermal stability in the material epoxy/CNTs-No/CTBN [207]. Graphene derivatives are also largely used with liquid rubbers to form hybrid epoxy systems. GNP/CTBN/epoxy exhibits thermal and mechanical property enhancements [208]. The morphology of graphene nanoplatelets depends on their lateral dimensions. Whereas 5 μm GNP remains uniformly dispersed, the clustering of low dimension ones (< 1 μm) is observed. In a GO/epoxy/liquid polysulfide (LPS) system, the graphene oxide acts as an amphiphilic substance that reduces the interfacial tension [209] and the LPS facilitates the network formation between the nanocarbon sheets and their intercalation. The rubber particles enhance plastic deformation during the fracture and nanofillers have their impact on phase separation. For modification of the polymer matrix, epoxy terminated poly (butadiene-co-acrylonitryle) (ETBN), hydroxyl terminated poly (butadiene-co-acrylonitryle) (HTPB), or hydroxyl terminated liquid natural rubber (HLNR) are also used. At the beginning for use with the epoxy/nanoclay composites, the core-shell rubber toughing agent [210] and acrylic rubber dispersant [211] found their application in nanocarbon/epoxy systems. GNP/epoxy/polysiloxane core-shell hybrid material (CSR) has further increased matrix conductivity, but only for the small addition of CSR [212]. The 3D powdered rubber nanoparticles restrict the agglomeration of chemically reduced graphene oxide incorporated in the epoxy matrix, but also concurrently weakens the toughening efficiency [213]. The soft rubbery core (for example butadiene, siloxane, or acrylate polyurethane) is usually accompanied with a hard shell (like poly (methyl methacrylate)) and particles are formed in emulsion polymerization. To enhance the epoxy performance at low temperature, the polysiloxane CSR was used [214] with promising results, such as promoting cavitation. Finally, the numerous fillers were tested to modify the epoxy/thermoplastics blends with polyamide, poly(ether imide), poly(ether sulfone), poly(ε-caprolactone), acrylonitrile-butadiene-styrene (ABS), acrylic tri-block-copolymer, and poly(styreneb-ethylene oxide) diblock copolymer. The MWCNT/ABS/epoxy [215] toughening mechanism is due to the nanotube pullout, thermoplastic tearing, and cavitation. The CF/poly(ε-caprolactone)/epoxy system [216], able to self-repair, has a smaller domain size than CF/epoxy and thus improves the hardness, toughness, and flexural and tensile strength. A similar positive effect on the modulus has been observed in the case of hydroxylated poly(ether ether ketone) grafted on carboxyl-functionalized MWCNTS and incorporated into the epoxy matrix [217]. An additional phase in MWCNT-COOH/HPEEK/epoxy facilitates the uniform dispersion and reinforces

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interfacial bonding. The acid functionalization was used also to modify graphene nanoplatelets that form EMI shielding material: GNP-COOH/epoxy/BCP (polystyrene-b-poly (ethylene-ran-butylene)-b-polystyrene) [218]. An interesting hybrid system of ATGO/epoxy has been produced in a several-step reaction [219]. The toughening effect of 3-aminopropyltriethoxysilane-functionalized silica nanoparticles coupled with GO (ATGO). The presence of thermoplastics in the epoxy overcomes the inherent brittleness [220]. The proper concentration is needed in order not to obtain the phase-inverted structure with dispersion of thermoset domains in a thermoplastic matrix. The semicrystalline materials such as polyamides are also compatible with epoxy under certain conditions [221]. Hyperbranched polymers (HBP), already tested with TiO2 or Al2O3, are a promising material to be used in a future development of nanocarbon/epoxy composites. Another interesting research direction is the tetrafunctional epoxy resin (tetraglycidyl methylene dianiline) modified with hydroxyl functionalized HBP [222] or GO/HBP [223]. Multicomponent systems enable concurrently tailoring the properties at two different length scales.

13.7

Conclusions and future perspectives

As written before, the nanocarbon/epoxy composites are already a well-known and still growing branch of the materials industry. That makes them being imposed on the market trends and demands. On one hand, there are still important issues concerning synthesis, modeling, and understanding of basic phenomena to be studied. On the other hand, the practical knowledge and advanced models already enable the design of multiphase composed materials, taking advantage of the synergic effect. Combining in a controlled manner the fillers in both the macro- and nanoscale, one may achieve the additional performance of epoxy nanocomposites. The versatility of nanocarbons, especially the graphene and its derivatives, is and probably will be within the coming years an important argument in favor of their use in advanced and intelligent materials. One may also expect the increasing use of 2D materials. Moreover, the challenges of the modern world might be addressed. One of those is directly related to the advancement of biocompatible and biomimetics materials while another is sustainable development without devouring the natural resources of the planet. What are the recent studies concerning that aspect in the field of nanocarbon/epoxy composites and its future perspectives? It might be seen in the literature that the growing interest is related to the environmentally friendly materials. Increasing environmental concerns led to the research on cellulose nanofibers [224], montmorillonite organoclays (MMT), and other well-bonded, uniformly distributed clay agglomerates as fillers. The previously mentioned bio-based epoxy composites with more sustainable matrices than classical bisphenol A ones [225] have been tested. There are still scarce but already produced and available on the market biobased epoxies with different sizes of n-alkyl side chain length and epoxidase vegetable oil derived compounds, for instance linseed (ELO with a very low glass transition temperature), fatty acids (EGO), or soybean (ESO). Furthermore, some of them are mixed with DGEBA in order to tailor the properties to be compatible with a particular filler.

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One of the interesting and recently published articles discusses the use of seashell waste from mollusks as fillers for such an epoxy resin [226]. Seashells are composed mainly from the calcium carbonate and together with bioepoxy matrices may form a renewable material. Material was washed in 4% sodium hydroxide and then grinded by centrifugal milling and subjected to a silane treatment before incorporation into the epoxy. The 50% increase of the flexural modulus and an increase in the glass transition temperature were noted for 30 wt% filler content. The hybrid material nanocarbon/ CaCO3 should be considered as a future step. Although recent years have brought significant improvements in the descriptions of mechanical, thermal and chemical properties of composed materials, there are still some open questions and problems to be resolved, for example: -

How to further improve the interaction between the carbon fillers and epoxy matrix? How to control the filler orientation in the epoxy? How to limit the adverse effect of enlarged viscosity on molding (still one of the most important methods of production)? What is the optimal concentration of filler in the matrix and how to predict it in advance in order to avoid the time- and source-consuming methods of a parametrical study? How to model composed materials with more than one type of filler? What are the interface phenomena and interactions on the functionalized surface of nanocarbon fillers? How to enhance the sustainability of epoxy nanocomposites? How to recycle nanocarbon/epoxy nanocomposites and what is their impact on the environment?

Referring to the last of those points, one may focus on the modeling studies about aging of materials, hydrothermal swelling [227], and the effects of their decomposition under miscellaneous factors. It was proved that the durability of epoxy is increased by added nanocarbon. The main wear mechanisms responsible for nanocomposite structure deterioration are adhesion, abrasion, fatigue delamination, and thermal softening. Environmental factors such as UV light, temperature exposition, and chemical interactions are also not to be neglected. Concerning the ecotoxicity and impact on the environment, the most important seems to evaluate the interactions of particular components and volatile substances that are released during deterioration of the material. The ecotoxicity of nanocarbons is the subject of numerous studies with still no definitive conclusions. In current research, graphene and its functionalized derivatives are considered as nanobiocomposite materials [228]. A further step in the future would be the use of graphene not only for environmentally friendly and biocompatible materials, but also to diminish the pollution problem. Although it may seem science fiction, nanocarbons have already proved to be much more than just an improved substitute for existing materials. They have introduced totally new functional properties. In conclusion, the versatility of nanocarbon/epoxy nanocomposites constantly modified by additional components and perspective studies on their sustainability have to be underlined as the most important factors determining the future direction of their fast development.

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Further reading [229] Zhang J, et al. Polym Sci Part B: Polym Phys 2010;48:417–24.

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rgio Henrique Pezzin†, Christian Matheus dos Santos Cougo*, Se ‡ Wagner Mauricio Pachekoski , Sandro Campos Amico* *Materials Engineering Department, School of Engineering, Federal University of Rio Grande, Porto Alegre, Brazil, †Chemistry Department, Center for Technological Sciences, State University of Santa Catarina, Joinville, Brazil, ‡Mobility Engineering Department, Federal University of Santa Catarina, Campus Joinville, Joinville, Brazil

Chapter Outline 14.1 Introduction 449 14.2 Most used carbon-based nanofillers for multiscale composites 451 14.3 Manufacturing processes for multiscale hybrid composites 452 14.3.1 Thermosetting polymer matrices 452 14.3.2 Thermoplastic polymer composites 454

14.4 Mechanical properties of nanocarbon-based multiscale composites 14.5 Multifunctional characteristics of nanocarbon-based multiscale composites 459 14.6 Trends and future research 462 Acknowledgment 463 References 463

14.1

457

Introduction

The incorporation of reinforcements in polymers to obtain composites has attracted much interest due to the possibility of combining the advantages of reinforcements (e.g., stiffness and stability) with those of polymer matrices (e.g., flexibility and processability). The characteristics of these materials usually include low density and outstanding mechanical properties as well as high dimensional and thermal stability [1,2]. The use of “traditional” polymer composites, that is, fiber/matrix twocomponent systems, as structural materials was the basis of many technological breakthroughs in areas such as aerospace, automobiles, communications, construction, energy, and sports. Fiber-reinforced composites (FRC) have thus been largely studied by the research community in the last six decades [3–7]. Among the most used synthetic fibers, one can mention carbon fibers (CF) and glass fibers (GF), which are embedded into thermoplastic or thermosetting matrices [2,4]. Other relevant synthetic fibers are polymeric fibers such as aramid (AF) and Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00014-6 © 2019 Elsevier Ltd. All rights reserved.

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ultrahigh molecular weight polyethylene (UHMWPE), ceramic fibers such as boron and silicon carbide, and more recently, basalt fibers, although much less has been reported on them in academic studies [4,8,9]. Carbon fibers are mostly polyacrylonitrile (PAN)-based fibers that went through a chemical modification process to obtain a highly oriented structure of bonded carbon atoms. CFs have very good properties such as low weight, dimensional stability, high stiffness, high strength, fatigue resistance, and low coefficient of thermal expansion. They are widely used in advanced composites for numerous applications, such as structural, automotive, and especially aerospace [7,10,11]. GFs are the most used reinforcement in polymer matrix composites, being classified into E-glass, R-Glass, and S-Glass, among others. E-glass is the most common type, mainly due its low cost, relatively high tensile strength (1.70–2.80 GPa), and chemical resistance. Their disadvantages when used in composites include a relatively low modulus (70–90 GPa), high abrasiveness, and low resistance to fatigue. Besides, they are electrically nonconductive and transparent to most electromagnetic radiation [12]. Polyaramid fibers, also known as aramid fibers, consist of poly(p-phenylene terephtlamide) (PPTA) and were developed by DuPont in the 1970s, being commercially introduced in 1972 by the trademark of Kevlar. Their highly oriented macromolecules promote the formation of secondary bonds between polymer chains, making it harder for the polymer chains to slide past each other, thus enhancing their mechanical properties. They present very good mechanical properties and are commonly known for their very high impact and abrasion resistance [13,14]. Recently, fibers obtained from basalt (a volcanic rock found on the surface of the Earth’s crust), mainly composed of SiO2 (40%–60%) and other oxides have come into consideration as reinforcements in composite materials. In addition to their good mechanical properties and excellent heat resistance, these fibers present very good sound and heat insulation and vibration-damping properties [15]. More detailed information about basalt fiber composites can be found in reviews such as those by Fiore [16] and Dhand [17]. Although FRCs have achieved outstanding recognition, some properties are still considered unsatisfactory, especially those more closely related to the matrix, and the lack of some less usual characteristics that may be required in some applications, such as electrical conductivity. Meanwhile, the advances in the field of nanotechnology and the unique attributes of nanomaterials have brought some interesting new possibilities into the materials field. Their use as fillers for polymers has been vastly studied in the past few decades, although the results have been limited in some areas. Thus, it has been proposed to combine these two fields, that is, to use microscale fibers together with nanoscale fillers to achieve composites with a very wide range of characteristics [18–20]. On this context, this chapter focuses on the development and application of synthetic fiber composites containing CNP. It briefly presents the main carbon nanofillers and some important characteristics and reviews some of the manufacturing processes available to obtain multiscale hybrid thermoplastic or thermosetting composites. It also describes the use of these nanofillers to add or improve mechanical-related properties of composites and other properties, such as electromagnetic and thermal, and presents some applications and future trends.

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14.2

451

Most used carbon-based nanofillers for multiscale composites

To discuss in detail the nanofillers available, their synthesis and properties are beyond the scope of this chapter. Nevertheless, the most used CNPs for composites are briefly mentioned in this section. Due to their small size, outstanding mechanical resistance, and high thermal and electrical conductivities, carbon nanoparticles (e.g., graphene, nanoplatelets, nanotubes, nanofibers, nanocarbon black), are mentioned as the ultimate reinforcement for the production of high-performance multifunctional composites. Carbon nanotubes (CNTs) are tubular structures made of carbon atoms with sp2 hybridization [21]. Although very similar to graphite, these structures are highly isotropic, and with really outstanding mechanical strength and toughness, electrical and thermal conductivities, and low density [22]. CNTs can be classified into two groups: single-walled carbon nanotubes (SWCNT), consisting of a single graphite sheet wrapped into a cylindrical tube with a diameter in the 0.7–3 nm range, and multiwalled carbon nanotubes (MWCNT), composed of more than two coaxial cylinders, each rolled out of single sheets, with a 2–40 nm diameter. Generally, MWCNTs are preferred for application in composites because SWCNTs are more expensive and more difficult to disperse. The CNT type, quality, aspect ratio, and impurities are decisive to the final properties of CNT-reinforced composites. There are a great number of papers and reviews in the literature discussing the properties that may be obtained by CNT/ polymeric matrix composites [23,24]. In general, CNTs can provide both intra- and interlaminar reinforcement, thus improving delamination resistance and reducing limitations associated with matrix-dominated properties [25]. Electrical conductivity in CNT nanocomposites is generally explained by the formation of a CNT network, allowing the percolation of electrical currents through the material. Even at low concentrations (550 °C). However, with these methods, the CNPs are, in general, weakly (noncovalently) connected to the fibers and it can be difficult to control their orientation. Even so, significant improvement in storage modulus was reported for a PA-6/GF/MWCNT laminate prepared by deposition of a very low content of oxidized CNT (0.18 wt%) on the fiber [58]. This was credited to a fiber-matrix interconnecting effect, as the GF/MWCNT formed a porous structure that could be

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easily interpenetrated by the polymer, resulting in strong fiber-matrix interfacial adhesion. Dispersion of CNP in the sizing protects the fiber surface and improves stress transfer at the interface [51]. This approach also permits further processing by extrusion, injection molding, compression molding, melt spinning, and other thermoplastic melt processing techniques [49–51]. On the other hand, CVDs or GSDs provide a much better attachment and arrangement of carbon nanotubes around the fibers, but these methods are more difficult to scale up. An interesting effect shown for PP hierarchical composites (CVD growth of MWCNT on CF or GF) is that CNTs act as nucleation sites on the fiber surface, promoting the formation of a transcrystalline layer around the GF, which considerably improves tensile and flexural properties [43,61,69]. Recently, Gonzales-Chi [63] reported the processing of multiscale composites based on PP reinforced with aramid fibers chemically treated with acid solutions and coated with oxidized MWCNT. The results showed an increase in interfacial shear strength (IFSS), suggesting physicochemical interactions among fibers, MWCNT, and PP in addition to mechanical interlocking. More comprehensive information about the manufacturing of hierarchical thermoplastic composites can be found in the review by Dı´ez-Pascual [84].

14.4

Mechanical properties of nanocarbon-based multiscale composites

Efforts to develop composite materials with advanced mechanical properties have been a matter of study for a very long time. But even using the most advanced fibers available, the matrix still has relatively poor mechanical properties that can affect the composite performance in some cases. Thus, the discussed carbon nanofillers may improve the matrix-dominant properties and the fiber/matrix interfacial bonding. Although most studies concentrate on matrix-dominant properties, there are many studies focusing on the influence of the nanoparticles on the tensile, flexural, and compressive behavior of composites. Depending on the manufacturing process and the process parameters, the results may significantly vary because the nanofiller can be placed on the fiber interface, in the matrix, or between the composite ply. Rahmanian [43] studied the influence of CNTs on short GF and CF/PP composites. The CNTs grown on the fiber surface acted to enhance the stress transfer between the fiber and matrix, significantly improving tensile modulus (40% and 57% for GF and CF, respectively) and flexural modulus (36% and 51% for GF and CF, respectively). The interlaminar properties of the composite are perhaps the focus of most papers on multiscale hybrid composites. Pedrazzoli [85] used expanded GNPs (xGNPs) as coupling agents on a GF/PP composite. The xGNPs were introduced in the PP matrix, with and without the use of a compatibilizer (maleic anhydride), and a single fiber fragmentation test was performed. The presence of 7 wt% xGNPs significantly increased the fiber/matrix interfacial adhesion, from 2.7 to 16.4 MPa, with an increase in shear modulus of 44%. With the combined use of the compatibilizer, the increase

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was even higher, obtaining 98% improvement in shear modulus and an interfacial adhesion of 39 MPa. Veedu [86] produced multiscale hybrid composites by growing MWCNTs perpendicular to SiC fiber-woven fabrics through CVD and later infiltrated the reinforcement with an epoxy matrix. They obtained a 348% increase in mode I fracture toughness and 54% in mode II fracture toughness. Ogasawara [87] obtained an improvement of 60% in the interlaminar fracture toughness of CFRP composites by adding 0.5 wt% of fullerenes into epoxy, which was later used to produce CF prepreg. Godara [88] worked on CF prepregs and dispersed 0.5 wt% of MWCNTs, thin MWCNTs (TWCNTs), and amine-functionalized DWCNTs through high-shear calendaring. The multiscale composite presented a substantial decrease in the coefficient of thermal expansion (CTE) and crack initiation and propagation presented a significant improvement, especially for the functionalized nanofillers, mainly due to fiber bridging by the CNTs. For further information on the influence of nanoparticles in interlaminar fracture toughness, one can refer to the review by Tang [89]. The fatigue resistance is another important mechanical property for advanced composites and the possibility to enhance it has received a lot of attention lately. Grimmer [90] studied the effect of 1 wt% of CNTs in a GF/epoxy composite response in a cyclic double cantilever beam flexural test and cyclic mode I delamination. The experiments showed a decrease in the rate of crack propagation with the CNTs, justified by the appearance of nanotube pullout and fracture. Li [91] investigated the creep behavior of a multiscale MWCNT/fiber/matrix composite and found an optimum amount of MWCNTs to decrease the creep strain rate. Indeed, the creep and glass transition temperature (Tg) characteristics are essentially related to the interaction between resin and reinforcement and its influence on the movement of polymer chains. Therefore, it is expected that nanofillers present in the matrix may also affect these characteristics. There are also several studies reporting the synergistic effects between microscale fibers and nanofillers, especially for carbon-based nanofillers and CFs. Using 5 wt% GNPs in a CF/poly(arylene ether nitrile) (PEN) composite, Yang [65] achieved increments in flexural strength of 98.4% and 63.6% and flexural modulus of 4.5 and 1.7 times compared with the bicomponent GNP/PEN and CF/PEN composites, respectively. The impact and postimpact properties of conventional composites can also be enhanced by carbon-based nanofillers. Siegfried [92] used epoxy with 0.25 wt% of pristine, aged, and NH2-functionalized MWCNTs to produce CF woven composites by RTM. The composites were then evaluated in interlaminar shear strength, mode II interlaminar fracture toughness, drop weight, and compression after impact tests. The samples containing CNTs presented a slight increase in mode II fracture toughness. The aged batch used in this study presented a network-like structure of CNT agglomerates in the resin, which was responsible for maintaining CAI strength, differently from the other batches. In addition, all samples containing CNTs presented an increase in the delaminated area but with little change in interlaminar shear strength. Taraghi [93] examined the low-velocity impact response of Kevlar/epoxy composites with CNTs (from 0 to 1 wt%) added through high shear mixing followed by ultrasonication

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in the resin. The study showed an increase of 35% in impact energy absorbed by the composite with 0.5 wt% of CNTs, which decreased for higher CNT content due to possible nanofiller agglomeration. The composite response to high strain rate deformation and the related energy absorption in a ballistic impact, for instance, can also be improved using nanocarbon as filler. When a high-velocity impact occurs, there is a propagation of shockwaves through the material and an amount of energy is converted into vibration. In this case, the presence of the nanofiller can aid in the dissipation of vibration by acting as a network of spring dampers, enhancing the damping capability of the composite [94,95]. Manero [96] reported an increment in the ballistic protection performance of Kevlar 29/epoxy composites filled with different types of nanoparticles. Micheli [97] added 1 wt% of CNTs through solvent ultrasonication to a hybrid Kevlar/CF/epoxy composite and investigated its ballistic performance. By evaluating the damage area caused by the projectile, they concluded that the CNTs improved the ballistic performance, aiding impact energy propagation through the plate. However, the effect of the nanofillers is not always positive. Grujicic [98] investigated the ballistic-protection performance of a GF/poly-vinyl-ester-epoxy (PVEE) composite containing MWCNT in the matrix. The reinforcement consisted of E-glass fiber in the form of mats and the composite was produced through vacuumassisted RTM. In this study, the presence of the CNTs did not show significant improvement in the ballistic performance. This was attributed to the short length of the MWCNTs, which renders the fiber pullout mechanism ineffective regarding energy absorption. This is probably due to the formation of an overlayer on the outer wall of the MWCNTs that is not capable of propagating stress, with a detrimental effect on the absorption of the impact’s kinetic energy (through delamination and/ or fiber pullout) or the mechanism of erosion/fragmentation of the projectile.

14.5

Multifunctional characteristics of nanocarbon-based multiscale composites

A multifunctional composite must combine a load bearing capability with functional properties such as thermal, electrical, electromagnetic, or health sensoring. Because conventional long fiber composites are already capable of sustaining high loads, the multifunctional properties are usually expected to come from the nanofillers. This kind of material is of extreme interest to specialized fields such as aerospace and military because the combination of properties in the same material may bring a significant improvement in efficiency due to the reduction in mass and/or volume of the total system. In particular, the effect of carbon nanofillers on the thermal and electrical properties of conventional fiber-reinforced polymer composites has been largely investigated. Indeed, because carbon-based nanofillers tend to present high thermal conductivity and low (or even negative) CTE, an adequate dispersion/positioning of the nanofiller in the matrix will most likely bring such features. A significant decrease in CTE means that thermomechanical stresses may be significantly reduced. At the same time, the

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thermal conductivity of the composites tends to increase [99]. A well-dispersed network will promote thermal conductivity or stability throughout the whole material, and this is achieved by planning the appropriate alignment, assembling, or positioning of the nanofillers. In fact, it is also possible to produce composites with very distinct thermal characteristics in different directions. Veedu [86] reported a significant increase in through-the-thickness thermal conductivity in a CVD-grown MWCNT/GF/epoxy prepreg composite. Wang [100] found that the inclusion of 3 wt% CNT into low-viscosity polyester/vinyl ester/glass fiber composites resulted in a 1.5-fold increase in thermal conductivity. Noh [101] studied the thermal conductivity of a multiscale composite containing short CF and GNPs in a cyclic butylene terephthalate matrix. The composite was prepared through powder mixing followed by heat compression. The isotropic in-plane thermal conductivity was maximized (183% increase) in the composite containing 5 wt% of CF and 15 wt% of graphene. The synergistic improvement was credited to more efficient thermally conductive pathways and internal structures favoring phonon transport. The synergistic behavior was also mentioned in the study of Yang [65], who revealed that multiscale carbon fiber/poly (arylene ether nitrile) (CF/PEN) filled with graphene (5 wt%) was 80 °C more thermally stable than the PEN/CF bicomponent composite. Carbon nanotubes (1 wt%) were found to cause a 25% reduction in CTE in glassfiber/epoxy composites [102]. Godara [88] evaluated the influence of 0.5 wt% CNTs on the thermal properties of epoxy CF prepregs. They found a significant decrease (32%) in CTE for the functionalized DWCNTs and TMWCNTs, but not for MWCNTs, which was justified by the greater interaction of the DWCNTs and TMWCNTs with the polymer chains due to their reduced size and better dispersion on the matrix. Regarding thermal stability, Yang [65] reported an enhancement when using GNPs in CF/PEN composites. Pedrazzoli [85] also found that the addition of xGNPs promoted a significant increase in both the onset (Td,onset) and maximum (Td,max) degradation temperature of the matrix, which were further increased by using a compatibilizer, probably due to better dispersion and exfoliation of the nanolayers. This showed an interesting thermal shielding aspect. Because most polymer composites lack electrical conductivity or are insulating, except when CFs are used, the addition of carbon nanofillers has been widely used to improve this aspect of the material, as in the work of Veedu [86], who reported a great improvement in through-the-thickness electrical conductivity from 0.075 105 to 0.408 S/cm. Improvement in the electrical conductivity of CNT/glass fiber/epoxy derives from the strong network-forming ability of CNTs above a critical percolation threshold, which is well known in pure nanocomposite systems [99]. The glass-fiber-reinforced epoxy composites containing 0.3 wt% of carbon nanotubes exhibit relatively high electrical conductivity, which enables functional properties such as stress-strain monitoring and damage detection [103]. A high degree of electrical anisotropy has been reported for hierarchical composites based on glass fibers. Conductivity in the primary fiber direction was about an order of magnitude higher than in the transverse direction, reaching values of 1 S/m (from 1011 S/m) at only 0.01 wt% CNT [104]. Gojny [105] also reported

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in-plane conductivity one order of magnitude higher than that out of plane. According to Qiu [102], the improvement in electrical conductivity indicates a way to leverage the benefits of CNTs and opens new opportunities for high-performance multifunctional multiscale composites. Qin [106], incorporating graphene nanoplatelets into a carbon fiber/epoxy interphase, significantly improved the through-the-thickness electrical conductivity by creating a conductive path between the fibers. Carbon black (CB) is one of the most widely used conductive additives in polymers. The overall conductivity of a filled polymer system sharply increases when a sufficient amount of CB, required for the construction of 3D conductive networks, is added. However, the amount of CB required to achieve high levels of composite conductivity is comparatively high. Thus, the combination of carbon black and conductive fillers has been proposed as a way to alter both the percolation threshold and the conductivity levels [107]. The influence of carbon black and recycled short carbon fibers on the electrical properties of unidirectional glass-fiber-reinforced polyethylene has been investigated by Markov [108]. Anisotropy in electrical conductivity was only observed in the percolation threshold range because this threshold is preferentially concentrated along the fibers. Drubetski [107] studied the electrical conductivity and morphology of injection-molded PP-based composites containing CB and CFs, both above their percolation threshold. The results presented similar or even lower values than those systems filled only with CB at the corresponding content, and resistivity of the two-filler systems was always higher than that of systems filled only with CF. In spite of the mechanical disturbances caused by the presence of fibrous and particulate fillers, the coexistence of CB and CF electrically conductive networks supporting each other was confirmed. A similar behavior was found by Shen [109], who prepared high-density polyethylene (HDPE) with CB and CF, and HDPE/PP polymer blends with those fillers. The combined use of CB (ca. 2–37 wt%) and CF (2 wt%) produced a performance inferior to that of HDPE/CB and HDPE/PP/CB, respectively, at the same total filler content. In both composites, electrons are transported over long distances by CFs with little energy loss, whereas CB particles improve the interfiber contact by forming CB particle bridges. The hierarchical design of multiscale high performance composite materials (advanced polymers, fibers, and carbon nanoreinforcements) allows the production of hybrid composites with special radiation shielding properties. Electromagnetic shielding refers to the attenuation, in reflection and/or absorption, of electromagnetic radiation due to the use of a material that acts as a “shield” against it. These materials may be used to protect electrical circuits and equipment that are subject to disturbances due to external electromagnetic radiation that interfere with their operation. As an example, glass fiber composites combined with materials of high dielectric and/or magnetic losses, such as CNT and GNP, can behave as efficient microwave absorbers, being useful for protective structures for radar data transmission antenna systems and in aircraft radomes [76]. The wide range of applications includes materials for electromagnetic shielding (EMI), electrostatic discharge (ESD) protection, special adhesives, and conducting coverings. Silva [76] sonicated CNTs on acetone and deposited the mixture directly onto GF fabrics before molding the multiscale

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composite through RTM. They also investigated the effect of dispersing the CNTs in the resin (also by sonication) prior to the RTM. Reaching up to 4.15 wt% of CNT, the final composite was able to obtain a maximum electromagnetic attenuation of 98.5% in the microwave frequency. A downside in the use of polymeric composites in structural applications has always been the difficulty in monitoring their structural health because composites do not usually show signs of deterioration until just before they actually fail. A possible means of monitoring the actual state of these materials is to use nondestructive evaluation methods such as scan-based techniques, but they present several complications due to the dimensions of the equipment and the limited monitoring capability. An alternative approach rests in monitoring changes in the electrical conductive behavior of the material whenever it has sustained internal or nonvisible damage. Because most polymeric composites lack electrical conductivity, except CF-based composites that present some anisotropic conductivity, it is necessary to use other agents to promote this characteristic. For instance, B€oger [110] incorporated DWCNTs, MWCNTs, and CB into epoxy using three-roll mixing and later produced the tricomponent GF composite through vacuum-assisted RTM. The authors proposed to evaluate composite integrity during interlaminar shear strength as well as static and dynamic tests by measuring its in situ electrical resistance on both in-plane and out-ofplane directions. The method offered a possibility to evaluate damage accumulation in situ during fatigue tests and showed an interesting correlation between the strain state of the composite and its electrical resistance, allowing a more extensive comprehension of its structural integrity. Sebastian [111] produced different sets of CNTgrown GF bundles and introduced them in the composite to act as strain-monitoring systems based on the variation in electrical resistivity response of the bundles with stress. These GF sensors exhibited sensitivity similar to those of conventional metal foil strain gauges, with a prominent advantage of having the sensor embedded in the composite. Nevertheless, further studies to fully comprehend the interaction between tensile, compression, or shear stresses and electrical resistivity response of multiscale materials are still necessary.

14.6

Trends and future research

In these multiscale hybrid composites with carbon-based nanofillers, there is the possibility of taking advantage of the nanofiller’s thermal and electric properties to cure the polymeric matrix in situ. For instance, Xu [112] produced laminates using a single CNT sheet with glass fiber prepregs. The heating of this sheet through an electric current enabled a much faster and more energy-efficient curing process of the prepreg compared to the conventional oven process. Nguyen [113] also reported good results in the use of CNT sheets in a CF/bismaleimide composite to enable its in situ curing. The combined use of carbon-based nanofillers and fibers for multifunctional composites for energy-related devices is getting a lot of attention and is likely to receive even more in the coming years. As their electrical properties seem to show a synergistic effect, these properties being present in a structural material is very interesting

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for several industry fields [114]. Their use for energy storage systems, such as capacitors with great mechanical properties, is another application receiving increasing attention. Islam [115] grafted CNTs onto the surface of CF and evaluated the mechanical and capacitance properties, and found 3.5 times greater specific capacitance with the insertion of CNTs. In another work, Xiong [116] used both EPD and CVD methods to fabricate a 3D GO/CNT/CF laminate for application as an electrode. The material presented a specific capacitance that was four times higher than pure CF. Another field that has been growing related to multiscale hybrid composites refers to the incorporation of a combination of nanoparticles, for example, CNPs with metallic nanoparticles or nanoclays or even hybridization using CNPs alone. For instance, Kwon [117] evaluated both CNT and GNP for the improvement in interfacial adhesion between epoxy and the reinforcement. Gao [118] grafted GO and CNTs onto the surface of CF and found that the presence of the CNPs increased fiber surface area and wettability. The multiscale composite presented superior interlaminar and interfacial shear strength compared to the CF fiber alone. A possible drawback related to these multiscale materials refers to their recycling. Indeed, recycling long fiber composites is already a complex task, and the introduction of nanofillers makes it even more complicated because they usually require special considerations due to potential toxicity and other health-related issues. Therefore, there is a broad field of research for recycling and proper disposal of these [119]. A key point for the commercial success of multiscale hybrid composites is the addition of a functionality without greatly interfering with composite processability. Other mentioned issues are difficulties in CNP production upscaling and the cost-effective manufacturing of CNP/polymer composites. Even so, the future for multiscale composites with carbon nanofillers is promising, and more distinguishable characteristics and applications are certainly to come.

Acknowledgment The authors would like to express their gratitude to CNPq and CAPES/Brazil.

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Yoshiyuki Suda*, Hiroyuki Shima† *Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, Toyohashi, Japan, †Department of Environmental Sciences, University of Yamanashi, Kofu, Japan

Chapter Outline 15.1 Introduction 471 15.2 Experimental setup

473

15.2.1 Sample synthesis 473 15.2.2 Tensile test system 474 15.2.3 Resistivity measurement system 475

15.3 Tensile fracture of CNCs 476 15.4 Spring constant of CNCs 478 15.4.1 15.4.2 15.4.3 15.4.4

Real-time measurement of CNC tensile test 478 Determination of the elastic boundary 479 Comparison to the macroscopic spring theory 479 Estimation of the mechanical strength 480

15.5 Electrical resistivity of CNCs

481

15.5.1 Relationship between the coil diameter and resistivity 481 15.5.2 Temperature dependence of the resistivity 482 15.5.3 Correlation between the mechanical and electrical properties 484

15.6 Summary 484 Acknowledgments 485 References 485

15.1

Introduction

Our surroundings are full of mechanical springs. Your ballpoint pens may have a small spring of about 1 mm that supports the ball of a pen tip. Your private car may be equipped with durable springs at the suspension that relieve the impact from the road surface. Beside of those ordinary-sized springs, there exists an extremely small-sized spring that attracts much attention in the field of materials science. In this chapter, we introduce the nanoscale analogue, called a carbon nanocoil (CNC). A carbon nanocoil is a quasi one dimensional nanomaterial made by carbon only, having a helical geometry composed of thin long wire [1]. Typical thicknesses and coil diameters of CNCs fall within the ranges of 100–400 nm and 400–1000 nm, Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00015-8 © 2019 Elsevier Ltd. All rights reserved.

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respectively [2, 3], and their full lengths are on the order of several tens of micrometers. Electron microscopy measurements [2–6] have revealed that an individual CNC consists of a bundle of helically twisted carbon nanofibers (CNFs) that encapsulate a long, thin, coaxial hollow core [3]. The helical structure of CNCs originates from spatial inhomogeneity in the rate of carbon precipitation over the active surfaces of catalyst particles [4, 6]. Possibly owing to this inhomogeneity, the inside of a CNC is filled with amorphous carbon, that is, a mixture of many small sp2 domains with disordered sp3-bonded carbon. The helical geometry of CNCs implies versatile applications such as microwave absorbers [7] and energy devices [8–10]. Potential nanoscale applications for CNCs include resonant elements, nanosolenoids [11], and field emitters [12]. To exploit their potential utility, the mechanical and electrical properties of CNCs should be clarified. In view of mechanical property assessment, the spring constant and shear stress of CNCs have been evaluated through stress–strain measurements [13–15]. For instance, Chen et al. [13] showed the CNC spring constant to be 0.12 N/m, in which the samples were 10 μm in length and 600 nm in coil diameter; the experiment was conducted using cantilever atomic-force microscopy. Poggi et al. reported the nonlinear response of a coiled multiwalled carbon nanotube under axial compression, revealing a shift in the resonance frequency [16, 17]. After the seminal work, however, experimental efforts have been sparse compared to the intensive computational modeling of the CNC mechanics carried out in recent years [18–20]. In particular, it remains undetermined how CNCs respond to prolonged stretching, that covers initial elastic elongation to large-scale plastic deformation. It is also interesting to investigate the tensile fracture of CNC, the postfracture contraction, and the release of the applied strain, but most of them have been unresolved. On the issue of electrical properties, several research groups have addressed the measurement of the resistivity of CNCs using different approaches [21–24]. Hayashida et al. developed a measurement system by connecting both ends of a CNC to a carbon electrode using an electron beam deposition technique. They suggested that the roomtemperature resistivity of CNCs ranged from 7.1  103 Ω cm to 9.3  103 Ω cm [21]. Low-temperature resistivity measurements were also performed by Chiu et al., who used a four-terminal method [22] to show that the temperature dependence of the CNC resistivity below 280 K is largely attributable to the variable range hopping (VRH) mechanism [25]. Despite the pioneering work described above, the relationship between the pristine (i.e., undeformed) shape of CNCs and their electrical resistivity remains to be addressed [26]. We point out here that subtle variations in the coil geometry would give a significant alteration in the mechanical and electrical properties of CNCs. At the synthesis stage, the coil geometry as well as the internal structure of the constituent fibers are largely governed by spatial inhomogeneity in the precipitation of carbon on catalytic particles. This fact indicates a certain correlation between coil geometry and the degree of structural disorder inside CNCs. Therefore, it is inferred that variation in the coil geometry induces a feasible alteration in mechanical and electrical properties. To verify this conjecture, it is necessary to develop a high-precision measurement system into which an artificially selected CNC with desired coil geometry can be integrated.

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In this study, we established a novel measurement system using a focused ion beam (FIB) technique that enables precise evaluation of the real-time deformation and electrical resistivity of single CNCs. For the mechanical properties, we performed tensile measurements of nine different CNCs with various fiber and coil diameters to apply uniaxial loads. The data obtained by real-time deformation allowed estimations of the spring constant, shear modulus, and ultimate strength of individual CNCs as well as their quantitative dispersion due to sample-dependent inhomogeneity in the atomic structure. We also compared our results to a macroscopic spring theory. For the electrical resistivity, we performed the room-temperature measurements of many CNCs and artificially graphitized CNCs (G-CNCs) with various shapes unveiled a clear relationship between coil diameter and electrical resistivity. It was further revealed that the temperature dependence of CNC resistivity between 4 and 280 K is well described by the VRH theory.

15.2

Experimental setup

15.2.1 Sample synthesis Samples of pristine CNCs and artificially graphitized CNCs (abbreviated by G-CNCs) were synthesized by catalytic chemical vapor deposition (CVD) [3]. The CVD was carried out by the following procedure. After the vacuum-evaporation deposition of a 40 nm thick Sn layer on an Si substrate, 10 μL of an Fe2O3 solution was poured onto the Sn-coated Si substrate. The Fe–Sn catalyst was annealed at 400°C for 5 min. The source (C2H2 at 50 mL/min) and dilution (N2 at 1000 mL/min) gases flowed into a CVD reactor in which a 10  10 mm2 catalytic substrate deposition was carried out at 700°C for 10 min [27, 28]. After the synthesis, a portion of the CNC was graphitized to obtain G-CNCs by heat treatment in an Ar atmosphere at 2873 K for 30 min using a Tammann oven. The coil diameters of the CNCs were 0.3–1.8 μm. Fig. 15.1 shows transmission electron microscopy (TEM; JEM-2100F, JEOL Ltd., Tokyo, Japan) images of an obtained

Fig. 15.1 TEM images and electron diffraction patterns of a single (A) CNC and (B) G-CNC.

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Fig. 15.2 Principle of spring-constant measurements; the top and lateral views are shown.

Nanocarbon and its Composites

Top view

Displacement of spring table Substrate

Applied force

W Probe

Spring table

Spring constant =12 N/m

Lateral view

P

q = 38° a

CNC and G-CNC; the electron diffraction patterns of the same samples are displayed in the insets. The TEM image and ring-shaped diffraction pattern in Fig. 15.1A indicate that the internal structure of the CNCs was mostly amorphous. In contrast, the G-CNCs exhibited evidence of graphitic layers (Fig. 15.2B), and the diffraction pattern of the G-CNCs clearly showed the (002) diffraction peak of graphite. The internal structural order of the CNCs and G-CNCs was examined using a laser Raman microscope (NRS-1000, JASCO, Tokyo, Japan). The intensity ratios of the G band to the D band (IG/ID) were 0.9 for the CNCs and 2.4 for the G-CNCs, indicating that the crystalline order of the G-CNCs was higher than that of the CNCs. The CNCs exhibited a tubular structure in which the tubular walls were composed of many sp2 domains contaminated by patches of amorphous carbon; the structural identification was carried out in our previous work [3] based on a three-dimensional observation system equipped with TEM and Raman spectroscopic analysis.

15.2.2 Tensile test system We conducted the tensile test of CNCs using an FIB system (Quanta 200 3D, FEI, Atlanta, GA, USA) [14, 15]. The FIB apparatus shows enhanced resolution in the beam irradiation, making it possible to irradiate an area of 10 nm in diameter on a substrate with a Ga ion beam. This high-resolution beam irradiation enables us to manipulate precisely an individual CNC; for instance, we can detach a specified (targeted) CNC from the aggregate using the beam irradiation. Besides, using the FIB, we can

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both etch the substrate surface and deposit a conducting thin film on the surface by supplying a Pt-containing hydrocarbon gas, (CH3C5H14)(CH3)3Pt. In actual set up, we first pasted a CNC to a spring table (STFMA, Kleindiek Nanotechnik, Reutlingen Germany), as shown in Fig. 15.2. We next moved the W probe (tip diameter: 500 nm) closer to an aggregation of CNCs, and used a Pt ion beam to adhere the probe to a specified CNC involved in the aggregation. A Ga ion beam was then irradiated at the bottom end of the CNC. Afterward, the W probe to which the CNC had adhered was brought close to the surface of the substrate, and the Pt ion beam was irradiated to the bottom portion of the CNC in order to adhere it to the spring table substrate having a diameter of 13 mm. The tensile tests were carried out for nine CNC samples; for each sample, we increased the distance from the substrate surface to the W probe in a gradual manner. Fig. 15.2 shows the experimental system for measuring the spring constant. The spring table had two leaf springs having a spring constant of 12  0.4 N/m. The spring-table substrate moved with the W probe, as the leaf springs deformed when a tensile load was applied to the CNC. The relation between the applied force and the displacement of the CNC is described by Hooke’s law, given below. P ¼ kδ ¼

k 0 δ0 cos θ

(15.1) 0

Here, P is the applied force, k (k ) is the spring constants of the CNC (spring table), δ 0 (δ ) is the displacement of the CNC (substrate), and θ is the angle of the W probe with respect to the spring-table substrate. The presence of the term, cos θ, on the right side of Eq. (15.1) is attributed to the fact that, in the actual measurement, the W probe was tilted at an angle of 38 degrees relative to the substrate. Therefore, the force detected by the leaf springs was the cosine of the force exerted on the CNC in the direction 0 along the W-probe axis. Once we have measured the values of δ and δ in the tensile test, we can evaluate the spring constant of the CNC, k, using Eq. (15.1). The displacements were evaluated using a scanning ion microscope (SIM) equipped in the FIB apparatus with an accuracy of 90%.

15.2.3 Resistivity measurement system As illustrated in the procedure of the spring constant measurement, we extracted a single CNC from aggregates of CNCs using FIB [29]. For resistivity measurements, we installed the detached CNC in a microscopic electronic circuit. The fabrication procedure is illustrated in Fig. 15.3A. First, a thin Au film (100 nm in thickness) was deposited on a glass substrate using an ion coater; we hereafter refer to it as the “film” electrode. Next, the glass substrate (2–10 μm2) was exposed by a Ga ion beam etching of the Au coating. A single CNC that had been fixed to the W probe was then laid across the hole in the Au film and connected to the film electrode with Pt deposition. Finally, the Au film near one end of the CNC was etched with the Ga ion beam to create an “island-like” Au electrode surrounded by glass substrate, hereafter

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Au film (film electrode) Pt deposition (Point A)

Ion beam

Ion beam

Pt deposition

Pt deposition (Point B) CNC

Au film SiO2

SiO2

SiO2

SiO2

SiO2

(A)

Island electrode

(B)

Fig. 15.3 (A) Electrical circuit fabrication procedure. FIB is used for both Au film etching and Pt deposition. (B) SEM image of the electrical circuit.

referred to as the island electrode. In actual measurements, the island electrode was attached to the probe of a nanomanipulator (MM3A-EM, Kleindiek, Reutlingen, Germany) connected to a low voltage source meter (2401, Keithley Instruments, Cleveland, USA). The film electrode was attached to the stage of a scanning electron microscope (SEM; S-4500 and SU8000, Hitachi High-Technologies Corp., Tokyo, Japan) with silver paste. The I–V characteristics of the circuit were measured in vacuum at room temperature and their linearity was confirmed in the voltage range from 0.5 to 0.5 V. There are two primary advantages to our measuring system: selectability of the coil geometry and perfect adhesion to the electrodes. The selectability is assured by the fact that our FIB-based manipulation technique allows a specific CNC with desired coil geometry to be pruned from the aggregate and integrated into the micron-sized circuit. Furthermore, the FIB technique yields a firm electrical connection between the measurement sample and the electrodes, which guarantees quantitative reproducibility of the electrical resistivity measurements [29].

15.3

Tensile fracture of CNCs

We fix the CNC almost perpendicularly to the Si substrate and perform tensile tests for eight CNCs by gradually changing the distance between the Si substrate and the W probe. Fig. 15.4 shows CNC tensile test micrographs [14]. We carefully maintain constant tensile speeds for all tensile tests. We observe the elongation behavior of CNCs in the FIB instrument using SIM. Fig. 15.4C shows that the CNC coil pitch returns to its original length after fracturing, thus confirming that CNC is a spring. We measure CNC stretch ratios from SIM image pixels and observe the following parameter ranges for the eight fractured CNCs: coil radius (0.7–0.9 μm); coil pitch (1.0–2.3 μm), and fiber diameter (0.3–0.6 μm). The average stretch ratio of the eight CNCs on the verge of fracture is 50%. Fig. 15.5 shows SEM images of the fractured surfaces of CNC #1–6 and #1–7 among eight CNCs. The value of the shear stress multiplication factor Ks of CNC

Fig. 15.4 Scanning ion microscopy images of CNC on an Si substrate in a FIB instrument CNC with (A) relaxed length, (B) maximum stretching, and (C) after fracturing. The CNC fractures after stretching to 150% of its relaxed length.

Fig. 15.5 SEM micrographs of the fractured surfaces of (A) CNC #1–6 and (B) #1–7.

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#1–6 and #1–7 is approximately 1.4 [14]. CNC #1–6 and #1–7 stretch by 50% and 40%, respectively, before fracturing. The red or blue circles in Fig. 15.5 show hollow areas on the fractured sections. We use the measurement data together with the compression mechanism of springs in automobiles [30] to formulate a mechanism for CNC fracture. We consider these hollow areas to be the points where the fracture originates, corresponding to the maximum stress point on the coil surface area. Industrial steel coil springs also exhibit the same fracture mechanism [30], that is, the fracture mechanism is the same for macroscopic coil springs (used for automobiles) and CNCs [14].

15.4

Spring constant of CNCs

15.4.1 Real-time measurement of CNC tensile test Fig. 15.6 demonstrates the time sequence of the elongation ratio of CNCs under axial tension and that of the applied force; the measurement was conducted using a spring table in the FIB chamber [15]. The horizontal axis indicates the elapsed time, and the vertical axes represent the axial strain of the CNC (left) and the applied force (right). In all the tests, the tensile-extension speed was kept as a constant. The three SIM images inserted in Fig. 15.6 help us to understand the geometric evolution of the CNC under a tensile load. Each image corresponds to the initial tension-free state (t ¼ 0 s), the state at the maximum elongation (t ¼ 910 s), or a postfracture state (t ¼ 960 s). In the free state, the CNC showed a coil length of 2200 nm and 14 turns. Immediately after the force was applied, the CNC started elongation; the elongation ratio was found to increase almost linearly with time until fracture. This linearity indicates brittleness (like cast iron or glass) of the CNCs fabricated for these experiments. As the force is applied further, the CNC fractured at 106% strain, at which the applied force reached 14 μN. After fracturing, the CNC rapidly contracted to release the applied strain, and the substrate returned to its original position. At the final (fully relaxed) state (t ¼ 1130 s), the CNC had a coil length of 1910 nm and nine turns; these values At where CNC fracturing occurred

200 180

15

10

160 140 5

120 100 80 0

500

1000 Time (s)

0 1500

Applied force (μN)

220 Elongation ratio (%)

Fig. 15.6 Real-time observation of CNC tensile tests performed in the FIB chamber. The three SIM images offer visualization of variations in the coil geometry over time while under a steadily increasing tensile load.

Preparation and properties of fibrous nanocarbon

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are smaller than their original values as fragments of the CNC were ejected during fracture. The spring constant of the CNCs was measured by observing the movements of the CNC and substrate using micrographs.

15.4.2 Determination of the elastic boundary It should be noted that in the experiment mentioned above, a uniaxial load was applied to a CNC at a constant speed until it fractured. Namely, the CNC under consideration reached a “plastic” deformation region beyond an “elastic” one. Of these two, only the elastic deformation region is the one in which both the spring constant and shear modulus of a spring should be measured [31, 32]. It is thus necessary to determine the elastic limit beyond which the CNC undergoes plastic deformation. To determine the limit, we have cut off the joint between the W probe and CNC and have examined the presence of residual strain in the CNC that had been subjected to axial elongation [15]. The test was carried out by the following procedure. First, the CNC was elongated until the strain reached a certain value, then the probe–CNC jointing was cut off by beam irradiation. After the load was released completely, the CNC pitch distance would recover to the original value as long as it is in the elastic region. We confirmed that this was true at the strain of 10%. However, when the strain reached almost 40%, the CNC did not completely recover even after the load was released. Similar plastic behavior was observed by Chen et al. [13]; they found that even after the uniaxial load was released, the CNC was still approximately 10% longer than its original length. To determine the elastic boundary of the CNCs, we examined the relationship between the applied strain and residual elongation ratios of CNCs after the load release. The result indicated that the CNCs were in the elastic region for elongations up to approximately 15% strain; we thus made the spring constant estimation for CNCs in strain no larger than 10%.

15.4.3 Comparison to the macroscopic spring theory We have revealed that the spring constants of the nine CNCs, derived from the formula k ¼ k’ δ’/δ, ranged from 0.9 N/m at the minimum to 4.8 N/m at the maximum. The average value for the nine CNCs was 1.8 N/m [15], which agrees with results previously reported [13]. It deserves comment that the spring constant depends on the number of turns n. We thus evaluated the normalized spring constant nk of nine CNCs, and obtained an average nk of 16.0 N/m. It is also interesting to note that, in the realm of macroscopic spring theory, the spring constant should weakly depend on the pitch angle of the coil, which varies with its elongation. This poses the question of whether the dependency holds true for their nanoscopic counterparts. Fig. 15.7 presents the comparison between the experimental data for samples #2–3 and #2–9 and the theoretical curves derived from Eqs. (15.2) and (15.3) below. The horizontal axis shows the CNC pitch angle, and the vertical axis shows the spring constant. The original pitch angle of each CNC (α0) was 14 degrees (#2–3) and 28 degrees (#2–9).

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Fig. 15.7 Comparison between experimental and theoretical values of the spring constant for each sample in the elastic region: (A) sample #2–3; (B) sample #2–9. The theoretical and experimental spring constants were well matched.

It follows from Fig. 15.7 that CNC elongation caused a slight increase in the angle and sizeable modulation in the spring constant. The theoretical curves in Fig. 15.7 are based on the equations used to design a macroscopic coil spring [14, 30]: k¼

  Gd 4 ð cos α  cos α0 Þ 3 cos α 1 + ð 1 + ν Þ tan α 0 ð sin α0  sin αÞ 64nR30



(15.2)

1 32PR20

cos 2 α0 ½ð sin α0  sin αÞ + ð1 + νÞf tan αð cos α  cos α0 Þg πd 4 (15.3)

Here, G is the shear modulus of a CNC, R0 is the coil radius, d is the fiber diameter, α0 is the pitch angle at the free length, α is the pitch angle when the CNC is elongated, and ν is the Poisson ratio. The value of n varied among the samples, with n ¼ 21 for sample #2–3 and n ¼ 11 for sample #2–9, for example. We set ν ¼ 0.27 [33] and substituted every geometric parameter as well as the value of P from the tensile measurement into Eqs. (15.2) and (15.3). As a consequence, we obtained the α-dependence of k in the macroscopic realm. The experimentally measured values of the spring constant exhibited sufficient agreement with the theoretically estimated values. These results confirm that Eq. (15.2) accurately describes the mechanical strength of the CNCs despite a large discrepancy in the value of k for CNCs (1.8 N/m) and macroscopic coils (typically tens of N/m). The agreement also indicates that the macroscopic spring theory is applicable to the nanoscopic counterparts.

15.4.4 Estimation of the mechanical strength Because the macroscopic theory applies to CNC elastic deformation, we can evaluate the shear modulus G of CNCs by fitting the experimental data to Eq. (15.2). In the evaluation, we substituted the mean values of α and α0 over the nine CNCs into

Normalized spring constant (N/m)

Preparation and properties of fibrous nanocarbon

50 40

481

Experimental data Theoretical curve G = 50 GPa G = 20 GPa G = 6.43 GPa G = 2 GPa

30 20 10 0 0.0

0.5 1.0 1.5 Coil diameter (μm)

2.0

Fig. 15.8 Correlation between coil diameter and normalized spring constant nk. The solid lines were plotted using the theoretical data as the shear modulus G was changed from 2 to 50 GPa. The value of G ¼ 6.43 GPa was determined by examining when the least mean square value was the smallest.

Eq. (15.2), considering the fact that the variation in the α-dependent term in Eq. (15.2) is negligible within an elastic region. Under the approximation, we derived the optimal value of G for each sample; the results are demonstrated in Fig. 15.8. It shows the correlation between the coil diameter and normalized spring constant nk for the CNC elastic-deformation region. A solid curve indicates the theoretical data obtained from Eq. (15.2). From the fitting results, we concluded the value of G has strong sample dependence; in fact, it ranged from 2 to 50 GPa in the present work. The mean value for the nine CNCs was 6.81 GPa. We have also attempted another method, which is based on a least squares fitting of the nine data points presented in Fig. 15.8 onto a single curve of k ∝ R3 0 . This method gave the result G ¼ 6.43 GPa, which coincides with the former result within an accuracy of 5.9%. The mechanical property to remain undiscussed is the tensile strength of CNCs. The CNC tensile strength is defined by the maximum force applied to the CNC divided by the fiber cross-section at the free length. Our calculation revealed that the average tensile strength of CNCs was 100 MPa, the minimum tensile strength was 35 MPa, and the maximum tensile strength was 197 MPa. These results show that the tensile strength of CNCs widely scatters, even under the same fabrication condition. It is also noteworthy that the linearity between the elongation ratio and applied force was kept just before its fracture, as shown in Fig. 15.6. This persistent linearity is a hallmark of brittle materials, contrary to ductile materials whose stress–strain curves tend to deviate significantly when subjected to large tensile loads [15].

15.5

Electrical resistivity of CNCs

15.5.1 Relationship between the coil diameter and resistivity Fig. 15.9 shows the relationship between the coil diameter and resistivity at room temperature: 15 single CNCs and three single G-CNCs were examined. For reference, the resistivities of four single linear-shaped CNFs with fiber diameters of 407–1258 nm

482

10–1 Resistivity, r (Ω cm)

Fig. 15.9 Relationship between resistivity and coil diameter: 15 CNCs, 3 G-CNCs, and 4 CNFs were examined. The coil diameter which is the length between the centers of fiber cross section was calculated by D-d. D is the outer coil diameter, and d is the fiber diameter. See supplementary materials at https://doi.org/ 10.1063/1.4945724E-APPLAB108-037615 of Ref. [29].

Nanocarbon and its Composites

CNC 10–2

CNF

10–3

G-CNC 10–4 0

500 1000 1500 Coil diameter, D-d (nm)

2000

are also plotted on the vertical axis. The geometric parameters of these samples are listed in our previous report [29]. The resistivity of the CNCs was found to increase with their coil diameter, although certain sample-dependent fluctuation was observed. In contrast, the resistivity of G-CNCs was almost invariant to changes in coil diameter. As a consequence, the resistivity of the CNC with the largest coil diameter was almost two orders of magnitude larger than that of the G-CNCs, as shown in Fig. 15.9. This large discrepancy in the resistivity between CNCs and G-CNCs indicates a significant degree of structural disorder inside the CNCs. An interesting finding was that the resistivity of CNCs with large coil diameter (750–1479 nm) was approximately equal to that of carbon microcoils (CMCs). An earlier work on CMCs [34] reported that CMC resistivities were within a range of 1.0  102 to 3.3  102 Ω cm, in which coil diameters of samples were larger than those of the present CNCs. The fair agreement between the resistivity of the CMCs and our CNCs with large coil diameter implies the presence of an upper limit of resistivity toward which the resistivity of coiled carbons converges at the large limit of the coil diameter. Another intriguing observation was that the resistivity of CNFs with linear shape was insensitive to the change in the fiber diameter. This finding implies robustness in the internal structure of CNFs against variation in fiber diameter, which is in contrast with the high susceptibility of fiber diameter to the degree of internal structural disorder in CNCs.

15.5.2 Temperature dependence of the resistivity The considerable difference between the resistivity of the CNCs and G-CNCs is attributed to structural disorder inside the CNCs. Our results imply that the inside of CNCs with a large coil diameter is filled with a highly disordered carbon network that consists of many small sp2 domains embedded in a sea of sp3-containing amorphous carbon. To verify this theory, we examined the temperature dependence of the resistivity of the CNCs between 4 and 280 K. The VRH model states that [35] the resistivity of disordered materials at low temperature is governed by the phonon-assisted hopping

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of electrons from occupied to unoccupied localized states [36]. As a result, the resistivity obeys the following formula (15.4)   T0 1 d ρðT Þ ¼ ρ0 exp T

(15.4)

Here, d ¼ 4 and T0 ¼TM at moderately low temperature (Mott-VRH) while d ¼ 2 and T0 ¼ TES (6¼ TM) at very low temperature (Efros–Shklovskii (ES)-VRH), indicating a crossover between the two at a certain low temperature. The parameter T0 characterizes the rate of electron hopping between localized states; a large value of T0 corresponds to a low probability for hopping and thus a high resistivity. Specifically in the case of CNCs, localized states are spatially constrained within sp2 domains that are scattered in the disordered host medium. Fitting experimental data to the formula given by Eq. (15.4) yields insight into the internal structures of CNCs as well as the mechanism of electron conduction in these systems. Fig. 15.10 shows the temperature dependence of the resistivity of three single CNCs different from those used to obtain the data shown in Fig. 15.9. Fig. 15.10 shows that all the obtained resistivity data were well fitted by the curve predicted by the VRH theory; the regime in the temperature range of 280–50 K was found to be Mott-VRH while that in the range of 20–4 K was ES-VRH. Interestingly, the resistivity curves shifted in a systematic manner with coil diameter, as plotted in Fig. 15.11A and B. In both temperature ranges (Mott-VRH in Fig. 15.11A; ES-VRH in Fig. 15.11B), the magnitude and slope of the fitting curve grew monotonically with the coil diameter. From the parameter fitting, TM was evaluated to be 81, 70, and 543 K for CNCs with coil diameters of 704, 1001, and 1440 nm, respectively; TES was found to be 1.2  102, 1.1  102, and 0.16 K in the same diameter order. The increasing Fig. 15.10 Temperature-dependent resistivity of three CNCs with different coil diameters.

100

Resistivity, r (Ω cm)

Coil diameter: 1440 nm 10–1

Coil diameter: 1001 nm 10–2

10–3 Coil diameter: 704 nm

10–4 0

50

100 150 200 Temperature, T (K)

250

300

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1.4 × 10–1

Resistivity, log r (Ω cm)

10–1 Coil diameter: 1440 nm

Resistivity, log r (Ω cm)

5 × 10–2 6 × 10–3 Coil diameter: 1001 nm 4 × 10–3

1.6 × 10–4 Coil diameter: 704 nm 1.2 × 10

–4

50 K

1.2 × 10–1 Coil diameter: 1001 nm 8 × 10–3 7 × 10–3

1.9 × 10–4 1.8 × 10–4

280 K 0.20

(A)

0.25

0.30

Temperature, T

–1/4

0.35 –1/4

(K

)

Coil diameter: 1440 nm

0.2

0.40

(B)

Coil diameter: 704 nm

4K

20 K 0.3

0.4

0.5

Temperature, T –1/2 (K–1/2)

Fig. 15.11 (A) Plot of log ρ as a function of T1/4. (B) Plot of log ρ as a function of T1/2. Solid lines in (A) and (B) are the best fit lines derived from the Mott-VRH model.

tendencies both of TM and TES with the coil diameter reflect the fact that the phononassisted hopping of electrons between sp2 domains is suppressed in large-diameter CNCs. We conclude that this suppression was caused by the increase in structural disorder with the coil diameter. Furthermore, positive correlation between the fitting parameter ρ0 in Eq. (15.4) and coil diameter was clearly observed, indicating enhancement in structural disorder in the large-diameter CNCs [29].

15.5.3 Correlation between the mechanical and electrical properties Note that the present results for the coil diameter-dependence resistivity of CNCs are in qualitative agreement with our previous findings on the mechanical properties of CNCs [14]. Our tensile load experiments on CNCs mentioned earlier have shown that the shear modulus of CNCs increases with coil diameter. The positive correlation between the shear modulus and coil diameter was possibly caused by the fact that, in largediameter CNCs, the population of sp2 domains, which are fragile against shear stress, becomes more reduced than that in small-diameter CNCs. The increased volume fraction of sp3-containing amorphous domains to many tiny sp2 domains is thought to result in both the increased shear modulus and increased electrical resistivity of largediameter CNCs.

15.6

Summary

We have developed a precise measurement technique of single CNCs using an FIB and SIM. SIM-based real-time measurements of CNC deformation were conducted to clarify their mechanical responses to prolonged stretching and their essential

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mechanical parameters in elastic and plastic phases. Tensile tests performed on nine different CNCs revealed that the average CNC spring constant was 1.8 N/m. Using a theoretical equation for the design of macroscopic springs, the shear moduli of the nine CNCs were estimated to be 6 GPa on average. We have also demonstrated a precise measurement system for CNC resistivity based on an FIB technique. The system enables us to construct a firm bridge between sample and measurement electrodes. The room-temperature resistivity of CNCs was found to drastically increase with their coil diameter, which implies a reduced sp2-domain population in large-diameter CNCs. Low-temperature resistivity measurements revealed that both VRH parameters, TM and TES, tend to increase with coil diameter. This result supports the proposal that the degree of internal structural disorder should be enhanced for large-diameter CNCs. The systematic investigation of CNC mechanical and electrical properties that we have presented provides essential numeric data requisite for their manipulation. These results will serve as a milestone for developing CNC-based applications in the near future.

Acknowledgments This work was partly supported by Japan Society for the Promotion of Science KAKENHI Grant Numbers 24360108, 25390147, 25630110, 15H03888, 15H04207, and 15K13946; MEXT KAKENHI Grant Number 24110708; the Toyota Physical and Chemical Research Institute; the Asahi Glass Foundation; JGC-S Scholarship Foundation; and Analysis and Development System for Advanced Materials (ADAM), Research Institute for Sustainable Humanosphere (RISH), Kyoto University. We thank to Taiichiro Yonemura, Yasushi Nakamura, and Ryuji Kunimoto of Toyohashi University of Technology, and Tamio Iida of National Institute of Technology, Gifu College to their experimental effort and to Hideto Tanoue and Hirofumi Takikawa of Toyohashi University of Technology, Hitoshi Ue of Tokai Carbon Co., Ltd., Kazuki Shimizu of Shonan Plastic Mfg. Co., Ltd., and Yoshito Umeda of Toho Gas Co., Ltd. for their fruitful discussion.

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[7] Tang NJ, Zhong W, Au CT, Yang Y, Han MG, Lin KJ, et al. Synthesis, microwave electromagnetic, and microwave absorption properties of twin carbon nanocoils. J Phys Chem C 2008;112:19316–23. [8] Hyeon T, Han S, Sung YE, Park KW, Kim YW. High-performance direct methanol fuel cell electrodes using solid-phase-synthesized carbon nanocoils. Angew Chem Int Ed 2003;42:4352–6. [9] Shimizu Y, Suda Y, Takikawa H, Ue H, Shimizu K, Umeda Y. Effective utilization of carbon nanocoil-supported PtRu anode catalyst by applying anode microporous layer for improved direct methanol fuel cell performance. Electrochemistry 2015;83:381–5. [10] Suda Y, Shimizu Y, Ozaki M, Tanoue H, Takikawa H, Ue H, et al. Electrochemical properties of fuel cell catalysts loaded on carbon nanomaterials with different geometries. Mater Today Commun 2015;3:96–103. [11] Yamamoto K, Hirayama T, Kusunoki M, Yang S, Motojima S. Electron holographic observation of micro-magnetic fields current-generated from single carbon coil. Ultramicroscopy 2006;106:314–9. [12] Hokushin S, Pan L, Konishi Y, Tanaka H, Nakayama Y. Field emission properties and structural changes of a stand-alone carbon nanocoil. Jpn J Appl Phys 2007;46:L565–7. [13] Chen XQ, Zhang SL, Dikin DA, Ding WQ, Ruoff RS, Pan LJ, et al. Mechanics of a carbon nanocoil. Nano Lett 2003;3:1299–304. [14] Yonemura T, Suda Y, Tanoue H, Takikawa H, Ue H, Shimizu K, et al. Torsion fracture of carbon nanocoils. J Appl Phys 2012;112:084311. [15] Yonemura T, Suda Y, Shima H, Nakamura Y, Tanoue H, Takikawa H, et al. Real-time deformation of carbon nanocoils under axial loading. Carbon 2015;83:183–7. [16] Poggi MA, Boyles JS, Bottomley LA, McFarland AW, Colton JS, Nguyen CV, et al. Measuring the compression of a carbon nanospring. Nano Lett 2004;4:1009–16. [17] Barber JR, Boyles JS, Ferri AA, Bottomley LA. Empirical correlation of the morphology of coiled carbon nanotubes with their response to axial compression. J Nanotechnol 2014;2014: https://doi.org/10.1155/2014/616240. [18] Wang J, Kemper T, Liang T, Sinnott SB. Predicted mechanical properties of a coiled carbon nanotube. Carbon 2012;50:968–76. [19] Ghaderi SH, Hajiesmaili E. Molecular structural mechanics applied to coiled carbon nanotubes. Comput Mater Sci 2012;55:344–9. [20] Ju SP, Lin JS, Chen HL, Hsieh JY, Chen HT, Weng MH, et al. A molecular dynamics study of the mechanical properties of a double-walled carbon nanocoil. Comput Mater Sci 2014;82:92–9. [21] Hayashida T, Pan L, Nakayama Y. Mechanical and electrical properties of carbon tubule nanocoils. Physica B 2002;323:352–3. [22] Chiu HS, Lin PI, Wu HC, Hsieh WH, Chen CD, Chen YT. Electron hopping conduction in highly disordered carbon coils. Carbon 2009;47:1761–9. [23] Tang NJ, Kuo W, Jeng CC, Wang LY, Lin KJ, Du YW. Coil-in-coil carbon nanocoils: 11 gram-scale synthesis, single nanocoil electrical properties, and electrical contact improvement. ACS Nano 2010;4:781–8. [24] Ma H, Nakata K, Pan LJ, Hirahara K, Nakayama Y. Relationship between the structure of carbon nanocoils and their electrical property. Carbon 2014;73:71–7. [25] Shen JY, Chen ZJ, Wang NL, Li WJ, Chen LJ. Electrical properties of a single microcoiled carbon fiber. Appl Phys Lett 2006;89:153132. [26] Sun Y, Wang C, Pan L, Fu X, Yin P, Zou H. Electrical conductivity of single polycrystalline-amorphous carbon nanocoils. Carbon 2016;98:285–90.

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[27] Suda Y, Maruyama K, Iida T, Takikawa H, Ue H, Shimizu K, et al. High-yield synthesis of helical carbon nanofibers using iron oxide fine powder as a catalyst. Crystals 2015;5:47–60. [28] Xu G, Chen B, Shiki H, Katsumata T, Takikawa H, Sakakibara T, et al. Parametric study on growth of carbon nanocoil by catalytic chemical vapor deposition. Jpn J Appl Phys 2005;44:1569–76. [29] Nakamura Y, Suda Y, Kunimoto R, Iida T, Takikawa H, et al. Precise measurement of single carbon nanocoils using focused ion beam technique. Appl Phys Lett 2016;108:153108. [30] Wahl AM. Mechanical springs. New York: McGraw-Hill; 1963. [31] Timoshenko SP. Theory of elasticity. New York: McGraw-Hill; 1951. [32] Shima H, Sato M. Elastic and plastic deformation of carbon nanotubes. Singapore: Pan Stanford; 2013. [33] Huang WM. Mechanics of coiled nanotubes in uniaxial tension. Mater Sci Eng A 2005;408:136–40. [34] Kaneto K, Tsuruta M, Motojima S. Electrical properties of carbon micro coils. Synth Met 1999;103:2578–9. [35] Mott NF, Davis EA. Electronic processes in noncrystalline materials. 2nd ed. Oxford: Oxford University Press; 197932. [36] Fung AWP, Rao AM, Kuriyama K, Dresselhaus MS, Dresselhaus G, Endo M, et al. Raman scattering and electrical conductivity in highly disordered activated carbon fibers. J Mater Res 1993;8:489–500.

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R.B. Rakhi Chemical Sciences and Technology Division, CSIR–National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Kerala, India

Chapter Outline 16.1 Introduction 489 16.2 Structure of CNTs 490 16.3 Synthesis and growth mechanisms of CNTs

493

16.3.1 Carbon arc discharge 494 16.3.2 Laser vaporization or laser ablation technique 494 16.3.3 Chemical vapor deposition 496

16.4 Purification of CNTs 499 16.5 Properties of CNTs 500 16.6 Preparation of manipulated carbon nanotubes

500

16.6.1 Mechanical manipulation of aligned CNTs by laser pruning 501 16.6.2 CNT/polymer nanocomposites 502 16.6.3 CNT/metal, CNT/metal oxide nanocomposites 502

16.7 Potential applications of CNTs and CNT-based nanocomposites 16.7.1 16.7.2 16.7.3 16.7.4

503

CNTs for hydrogen storage 504 Electrochemical supercapacitors 506 Field emission from CNT-based nanocomposites 509 CNT-based electrochemical biosensors 510

16.8 Conclusions 512 Acknowledgment 513 References 513 Further reading 520

16.1

Introduction

The ability of carbon to assume a wide variety of different structures makes it unique among the elements in the periodic table. Nearly 30 years ago, a new family of carbon cage structures, based on a threefold coordinated sp2 network, was discovered. Fullerene (C60) is the most abundant and best-known member of that family. Carbon nanotubes (CNTs) are perhaps the most exciting addition to the fullerene family. CNTs Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00016-X © 2019 Elsevier Ltd. All rights reserved.

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can be considered as graphitic cylinders having diameters as small as one nanometer, closed at the ends with caps containing both pentagons and hexagons. They are reported to possess exceptional mechanical, structural, and electrical properties that arise from the uniqueness of carbon bonds, their cylindrical symmetry, and their quasi one-dimensional nature. Depending on their structural parameters and the way in which the carbon atoms are arranged, CNTs can either be metallic or semiconducting. This property opens up the exciting prospects of devices and junctions made entirely out of carbon [1–4]. In 1779, graphite was discovered, followed 10 years later by the discovery of diamonds. It was then identified that both these materials belong to the same family of chemical elements. When electron microscopes came into extensive use around 1950, carbon filaments were first observed [5]. In 1985, Kroto, Smalley, and Curl discovered fullerenes [6]. A few years later in 1991, Sumio Iijima discovered CNTs in fullerene soot [7–10]. The fullerene soot was obtained from a carbon arc discharge method similar to the method used for fullerene preparation. In CNTs, carbon atoms are arranged in tubular formations on a nanoscopic level. The microstructure of these materials was studied using a high-resolution transmission electron microscope [9, 11]. CNTs are an entirely new type of carbon fiber that is comprise of coaxial cylinders made of 2–50 graphite sheets [12]. The first observations Iijima made [7, 9, 10, 13] were of multiwalled CNTs (MWCNTs) having more than two coaxial tubes of carbon, and 2 years later single-walled CNTs (SWCNTs) were observed. For the synthesis of SWCNTs, Iijima and Ichihasi used carbon electrodes with a small amount of iron in the carbon arc discharge experiment, and filled the chamber around the carbon arc with methane and argon gas [14]. An SWCNT is a single fullerene molecule that has been stretched out and hence, its length is a million times the diameter [12]. Donald Bethune and colleagues also prepared SWCNTs around this same time [11]. In 1996, Smalley synthesized bundles of SWCNTs for the first time [11, 15]. A perfect CNT can be explained as a smooth cylinder of a rolled-up hexagonal honeycomb lattice of carbon atoms, which is capped at both ends with half a fullerene molecule [8, 11, 16].

16.2

Structure of CNTs

Because of their large aspect ratio, that is, their length-to-diameter ratio, CNTs are considered as nearly one-dimensional structures. CNTs can be broadly classified as SWCNTs and MWCNTs. An SWCNT is similar to a cylinder with only one wrapped graphene sheet. MWCNTs can be considered a collection of concentric SWCNTs. The length, diameter, and properties of these structures differ a lot from each other. !

A single vector called a chiral vector C entirely describes SWCNTs. Two atoms in a planar graphene sheet are chosen, and one is used as the origin. The chiral vector !

C is pointed from the first atom toward the second one (Fig. 16.1) and is defined by the relation: !

!

!

C ¼ na 1 + ma 2

(16.1)

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!

Fig. 16.1 Chiral vector C and chiral angle θ definition for a (2, 4) nanotube on a graphene sheet. Reproduced with permission from Belin T, Epron F. Characterization methods of carbon nanotubes: a review. Mater Sci Eng B 2005;119(2):105–18, © 2005 Elsevier. !

!

Where n and m are integers and a 1 and a 2 are the unit cell vectors of the twodimensional lattice formed by the graphene sheets. The direction of the nanotube axis is perpendicular to the chiral vector. The circumference of the nanotube is equal to the !

length of the chiral vector C . The diameter of the nanotube, d, is given by the corresponding relationship: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a m2 + mn + n2 d¼ π

(16.2)

where a is the length of the unit cell vector and is related to the carbon-carbon bond length acc by the relation: pffiffiffi !   !  a ¼  a 1  ¼  a 2  ¼ acc 3

(16.3)

The carbon-carbon bond length for graphite is acc ¼ 0.1421 nm. For nanotubes, the same value is often used [17]. But probably a slightly larger value of acc ¼ 0.144 nm should be a better approximation due to the curvature of the tube [17–19]. The angle between the chiral vector and zig-zag nanotube axis is the chiral angle θ (Fig. 16.1) and this angle can be defined by: θ ¼ arctan

 pffiffiffi  3m 2n + m

(16.4)

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Nanotubes are usually described by the pair of integers (n,m), which is related to the chiral vector. Based on the values of these integers, CNTs are broadly classified into three categories. When n ¼ m, the nanotube is called the “armchair” type (θ ¼ 30 degrees), and when m ¼ 0, it is the “zig-zag” type (θ ¼ 0 degrees). Otherwise, when n 6¼ m, it is a “chiral” tube, and θ takes a value between 0 and 30 degrees (Fig. 16.2). The value of (n,m) determines the chirality of the nanotube and affects the optical, mechanical, and electronic properties. Nanotubes with j n  mj ¼ 3q are metallic, and those with j n  mj ¼ 3q  1 are semiconducting where q is an integer. The terminating cap of the nanotube consists of both pentagons and hexagons. As the diameter value of the C60 hemisphere (0.7 nm) matches well with the smallest experimental value of the nanotube diameter, the smallest cap that can fit on to the cylinder of the carbon tube is a C60 hemisphere [19, 20]. However, some other reports on nanotubes suggested that SWCNTs can have a theoretical limit of 0.4 nm in diameter (21 23). According to Liang et al. [21], an SWCNT with 0.4 nm diameter can have three possible structures: chiral (4, 2) (diameter ¼ 0.414 nm), zig-zag (5, 0) (diameter ¼ 0.393 nm), and armchair (3, 3) (diameter ¼ 0.407 nm) tubes.

Zig-zag

Armchair

Chiral

Fig. 16.2 Illustration of the three types of CNTs: zig-zag, armchair, and chiral. The chiral angle of the zig-zag tube (n, m) is 0°degrees because either n or m ¼ 0. The chiral angle of the armchair tube (n, m) is 30°degrees because n ¼ m. The chiral tube has n, m integer values between the two extremes. Reproduced with permission from Grobert N. Carbon nanotubes—becoming clean. Mater Today 2007;10(1–2):28–35, © 2007 Elsevier.

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There are different models to explain the structure of MWCNTs based on electron microscopy studies. MWCNTs can be formed from coaxial cylindrically curved, scrolled, or coaxial polygonized graphene sheets [22] (Fig. 16.3). The most popular and widely accepted model for the MWCNT is the coaxial cylindrical model. But MWCNTs having a large tube size are coaxial polygonized. In the case of MWCNTs, only a few possible sequence of (n,m) tubes are available to keep a realistic intershell distance. For carbon materials, the intershell spacing “r” between two successive tubes is in the range from 0.344–0.36 nm for perfect to disordered states, respectively [13, 23–26]. These values suggest a possible dependence of intershell distance on the tube size and an empirical relationship to fit transmission electron microscopy (TEM) experimental data has been given as [27]: r ¼ 0:344 + 0:1exp

c 4π

(16.5)

where c/4π is the radius of the tube. As the tube diameter increases, the intershell distance decreases to 0.344 nm. The theoretical external specific surface area for CNTs was estimated using geometrical calculations by Peigney et al. [28].

16.3

Synthesis and growth mechanisms of CNTs

CNTs are synthesized by various methods such as carbon arc discharge, laser ablation, and chemical vapor deposition (CVD). However, the most critical issue is to identify the synthesis method by which the most economical large-scale production of CNTs

Fig. 16.3 Various models of carbon nanotubes taking into account the experimental measurements: (A) coaxial cylindrically curved, (B) coaxial polygonized, or (C) scroll graphene sheets. Reproduced with permission from Belin T, Epron F. Characterization methods of carbon nanotubes: a review. Mater Sci Eng B 2005;119(2):105–18, © 2005 Elsevier.

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can be achieved. Compared to all other methods, the catalytic CVD technique to synthesize CNTs is simple, inexpensive, and energy-efficient. It can be used for the production of large quantities of CNTs having high purity. In this section, the details of CNT production methods and the mechanisms of CNT growth are outlined.

16.3.1 Carbon arc discharge In the arc discharge method, two graphite electrodes separated by a distance of nearly 1 mm are kept in an inert He atmosphere and a direct current is passed through them. The anode is consumed due to arcing and a cigar-like deposit is formed on the cathode. The outer shell of this deposit is gray and hard, with a black soft inner core that contains MWCNTs, polyhedral particles, and amorphous carbon [29]. SWCNTs may also be obtained but the synthesis of which requires mixed metal catalysts, such as Fe:Co and Ni:Y [30] that are inserted into the anode. SWCNTs are found distributed in the chamber as a fluffy web-like material [30]. In 2006, Ando and Zhao reported the synthesis of SWCNT nets of up to 20–30 cm in length by arc evaporation of a graphite rod containing a pure Fe catalyst in the chamber filled with a mixture of hydrogen and inert gas. Replacement of H2 by He resulted in the formation of MWCNTs with a very thin innermost tube of 70%) yields of SWCNTs. But the significant disadvantages of these two techniques are that both depend on the evaporation of carbon atoms from solid graphite electrodes at temperatures higher than 3000°C, and the tangled nature of as-synthesized CNTs makes the purification process difficult [33].

16.3.3 Chemical vapor deposition The catalytic decomposition of hydrocarbons to produce filamentous carbon was first reported in 1890. In 1988, Endo developed the floating catalyst reactor using catalyst particles of 10 nm in diameter, and this method is widely used for the production of aerosol-based CNTs [32, 39–41]. In the CVD method, MWCNTs and SWCNTs are produced by the pyrolysis of hydrocarbons in the presence of a transition metal catalyst (Fe, Ni, Co). The CVD process involves several steps. In the first step, the catalyst metal particles are coated on a substrate. The substrate is then placed in a furnace, and the catalyst particles are then subjected to a reduction treatment upon heating under typically H2 or NH3. Finally, hydrocarbon gas or CO is let into the furnace. Catalytic decomposition of the hydrocarbon molecules results in the deposition of carbon on the catalyst particles at temperatures ranging from 500 to 1200°C (Fig. 16.6). MWCNTs are mainly synthesized at lower temperatures (500–800°C) in an inert gas atmosphere and SWCNTs at higher temperatures (600–1150°C) in the presence of a mixture of H2 and an inert gas such as Ar [42]. For the synthesis of double-walled nanotubes (DWCNTs), a more complicated catalyst preparation procedure is required. DWCNT samples may also contain SWCNTs and triple-walled CNTs [43]. In plasmaenhanced CVD (PECVD) or plasma-assisted CVD (PACVD), the decomposition of hydrocarbons is aided by plasma. CVD and PECVD are commonly used to grow

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Fig. 16.6 Schematic of the CVD process.

aligned MWCNTs and SWCNTs on various substrates, including Ni, Si [43a], SiO2 [44], and stainless steel [45]. The structure of MWCNTs produced by CVD differs significantly from MWCNTs generated by arc discharge. CVD-grown MWCNTs are less crystalline and exhibit many more defects than MWCNTs grown by the arc discharge method. Therefore, MWCNTs grown by CVD are less straight than MWCNTs generated by arc discharge (Fig. 16.7C and D). Spiral growth of CNTs can also occur in CVD, in some cases, depending on the catalyst [46]. MWCNTs grown by CVD are very long and possess larger diameters of up to 100 nm. The length of MWCNTs grown by CVD can be measured very easily as the nanotubes typically grow perpendicular to the substrate. CVD is probably the most versatile production method for CNTs, especially for generating doped CNTs [44, 46]. The CVD process requires much lower temperatures than the arc discharge and laser ablation processes. The CVD method can grow CNTs of required length directly on a substrate at a desired position. This control permits the integration of the CNT growth into the fabrication processes of microelectronic circuits. Hence, for large-scale production of CNTs at a reduced cost, the CVD method is widely used [32, 33, 47]. Different growth models have been suggested to explain the growth mechanism of CNTs on metal surfaces. A four-step mechanism for the growth of carbon filaments suggested by Baker and Harris [48] is now being applied for CNT growth. In the first step, the hydrocarbon decomposes on the metal surface to release hydrogen and carbon and the carbon dissolves in the catalyst particle. The second step involves the diffusion of the carbon through the metal particle and its precipitation on the rear face to form the body of the filament, in which, diffusion is the rate-determining step. The accumulation of carbon occurs at the front face as the supply of carbon onto the front face is faster than the diffusion through the bulk. The accumulated carbon needs to be removed to prevent the physical blocking of the active surface. In the third step, this is achieved by surface diffusion, and the carbon forms a skin around the main filament body. In the final step, overcoating and deactivation of the catalyst and thereby termination of tube growth will take place.

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Fig. 16.7 TEM images reveal the structural differences of MWCNTs produced via CVD techniques and by arc discharge. (A) TEM image of raw MWCNTs grown by arc discharge, revealing the presence of polyhedral carbon particles. (B) TEM image of purified MWCNTs grown by arc discharge. (C) TEM images of nanotubes generated by pyrolyzing 2-amino-4, 6-dichloros-triazine over laser-etched Co substrates. (D) MWCNTs grown by CVD exhibiting metal particles (40 nm OD) at their ends; inset showing a close-up of the particle containing nanotube tips. Reproduced with permission from Grobert N. Carbon nanotubes—becoming clean. Mater Today 2007;10(1–2):28–35, © 2007 Elsevier.

In the growth mechanism proposed by Dai et al. [49], carbon forms a hemispherical graphene cap called a “yarmulke” on the catalyst particle. The CNT growth starts from the yarmulke. The size of the catalyst particle controls the diameter of the nanotube. This model avoids dangling bonds at all stages of CNT growth [50]. The generally adopted growth model for CNTs is based on the concepts of the VLS (vapor-liquidsolid) theory developed by Wagner and Ellis. In this model, molecular decomposition of hydrocarbon is assumed to occur on one side of the catalyst particle. Carbon diffuses from the side where it has been decomposed to another side, where it is precipitated from solution [5, 48]. The driving force responsible for carbon diffusion within the catalyst particle originates from the temperature gradient created in the particle by exothermic decomposition of the hydrocarbon at the exposed front faces and

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endothermic deposition of carbon at the rear faces, which are initially in contact with the support face [48, 51]. Some other reports show the driving force to be a concentration gradient [5, 52]. The metal-substrate interactions play a significant role in this growth mechanism [48, 53]. Strong interactions lead to root growth whereas weak interactions yield tip growth. The “root growth” model states that the catalyst particles are stuck on the substrate and remain attached at the root of the CNTs during the growth. The “tip growth” model assumes that catalytic particles are detached from the substrate and move upward along with the tip of the growing tubes. Both growth models are schematically shown in Fig. 16.8.

16.4

Purification of CNTs

High-purity and defect-free CNTs are required for various applications based on CNTs. The as-grown CNTs obtained by different synthesis methods contain carbon impurities such as graphite fibers, amorphous carbon, and residual metals from the catalyst used in the production [28]. The systematic characterization of the type and concentration of contaminants present in the as-prepared nanotube sample is an important step involved in the production of high-purity CNTs. For practical CNT applications, these impurities must be removed. For the purification of the as-grown CNTs, various methods have been developed [54, 55]. An air oxidation step is used for the removal of carbonaceous impurities. Centrifugation, filtration, and chromatographic techniques are usually employed for impurity elimination after the CNTs have been treated with acid or oxidizing agents [56]. Based on their oxidation behavior, MWCNTs have been purified by preferential oxidation of the tube tips [57]. Heating in an oxidizing atmosphere can remove the amorphous carbon. Refluxing in concentric acids has been shown to be an effective method for the

Fig. 16.8 Two growth models of filamentous carbon.

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separation of the catalytic impurities [55]. Li et al. have systematically carried out the purification of CVD-grown SWCNTs by different acid oxidation treatments [58]. Of all the purification methods, oxidation followed by acid treatment appears to be effective, as the acid will dissolve the catalytic metallic oxides exposed by the oxidation treatment [55].

16.5

Properties of CNTs

CNTs possess remarkable mechanical and electronic properties that make them useful for various nanomechanical and nanoelectronic device applications. The mechanical properties of a solid mainly depend on the strength of its interatomic bonds. As a result of the carbon-carbon sp2 bonding, CNTs are predicted to have high stiffness and axial strength [33]. The mechanical properties of CNTs can be predicted, based on the knowledge of the known properties of crystal graphite [59]. Diamond has an elastic modulus of 1.2 TPa. Theoretical and experimental studies have indicated that CNTs have an elastic modulus >1 TPa and have mechanical strength that is 10–100 times higher than the strongest steel at a fraction of the weight [60]. CNTs have the highest Young’s modulus of all different types of composite nanotubes such as C3N4, CN, BN, BC3, BC2N, etc. [61]. Experimental results have shown that CNTs have tensile strengths in the range from 11 to 63 GPa [16]. Both SWCNTs and MWCNTs are expected to have large bending constants that depend on Young’s modulus and hence CNTs have been found to be highly flexible [11, 16]. CNTs have unique thermal and optical properties as well. Thermal properties, including specific heat and thermal conductivity of CNTs, are determined primarily by the phonons [33]. Phonons are a result of lattice vibrations observed in the Raman spectra [11, 20]. Especially at low temperatures, the phonon contribution to the thermal properties dominates and is due to the acoustic phonons. Direct information about the type of carriers and conductivity mechanisms can be obtained from the thermoelectric power measurements of the CNT systems (Fig. 16.9). CNTs exhibit exceptional electronic properties. The electronic structure of the CNTs is influenced by the curvature of the graphitic sheets. It has been predicted that a combination of the degree of helicity and the number of six-membered rings per turn around the tube determines the electronic properties and the properties can be tuned from metallic to semiconducting [50]. Theoretical and experimental observations reveal the superior electrical properties of CNTs [62].

16.6

Preparation of manipulated carbon nanotubes

CNTs can be manipulated either mechanically or chemically for making them more suitable for a variety of applications. In mechanical manipulation, by controlling some growth conditions and by laser pruning, the morphology of CNTs can be modified. Mechanical manipulation aims at the application of CNTs as electron field emitters. Covalent and noncovalent sidewall functionalization of CNTs and preparation of

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Fig. 16.9 Raman spectrum of (A) SWCNTs and (B) MWCNTs. Reproduced with permission from Rakhi RB, Sethupathi K, Ramaprabhu S Field emission from carbon nanotubes on a graphitized carbon fabric. Carbon 2008;46(13):1656–63, © 2008 Elsevier.

hybrid nanocomposites of CNTs with other nanomaterials (metals, metal oxides, conducting polymers, etc.) are the chemical manipulation methods. The surface of CNTs can be easily modified with different functional groups carrying specific functionalities through defect and covalent functionalization [63, 64]. The molecules showing a strong affinity with the graphitic surfaces of CNTs can be attached to CNT surfaces by noncovalent functionalization of CNTs, and in this method, the structural integrity of the CNTs will be retained. These hybrid nanocomposites can be used in a variety of applications.

16.6.1 Mechanical manipulation of aligned CNTs by laser pruning A vertically aligned CNT forest can be grown over an Si substrate by plasmaenhanced CVD. In the typical growth process, a thin film of the catalyst particle deposited over the Si substrate will be placed in the PECVD unit for the growth of CNTs. It will be first subjected to hydrogen plasma for 600 s at 750°C in the PECVD unit to reduce the film to metallic nanoparticles. Subsequently, a mixture of C2H2 and H2 gases with flow rates of 15and 60 sccm, respectively, will then be introduced at a temperature of 750°C, a pressure of 1200 m Torr, and an rf plasma of 100 W. An SEM image of the cross-sectional view of an aligned CNT forest of uniform length grown perpendicularly on the Si substrate is shown in Fig. 16.10. The length of the CNT forest is nearly 30 μm. This 3D CNT forest can be used as efficient electron field emitters. The CNT forest can be trimmed into different patterns by a moderate power laser beam.

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Fig. 16.10 (A) SEM image of an aligned CNT forest grown perpendicularly on the Si substrate. Inset of the figure shows a magnified view of a portion of CNTs. (B) Top view of a pattern created on a CNT forest by laser pruning. (A) Reproduced with permission from Rakhi RB, Sethupathi K, Ramaprabhu S. Field emission from carbon nanotubes on a graphitized carbon fabric. Carbon 2008;46(13):1656–63, © 2008 Elsevier.

16.6.2 CNT/polymer nanocomposites CNT/polymer hybrid nanocomposites consist of a polymer or copolymer and CNT nanofillers dispersed in the polymer matrix (Fig. 16.11). Polyaniline (PANI) and polypyrrole (PPy) are two of the most extensively studied conducting polymers, owing to their high electrical conductivity, environmental stability, and relative ease of synthesis. A CNT/conducting polymer composite is considered as excellent electron emitter material. It is also one of the most popular electrode materials for electrochemical energy storage devices. CNT/polymer hybrid nanocomposites can be prepared by an in situ polymerization method. The modification of the CNT surface provides them with new functionalities and helps to enhance the state of the polymer/CNT interfacial region, bringing about the obtainment of nanocomposites with superior properties suitable for advanced applications [65–67].

16.6.3 CNT/metal, CNT/metal oxide nanocomposites Surface modifications of CNTs can also be achieved by decorating the nanotube surface with metal or metal oxide nanoparticles. Loading of the CNT surface with metal/ metal oxides having a low work function results in improvement in the electrical conductivity [68]. Many functional groups will be present on the surface of CNTs after the purification process. Metal/metal oxide nanoparticles can easily be attached to these functional groups by simple chemical processes [66, 69–71]. These composites are widely used for various applications. Metals can also be coated on the outer surface

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Fig. 16.11 SEM image of (A) CNTs, (B) PANI/CNTs, and (C) PPY/CNTs. Reproduced with permission from Rakhi RB, Sethupathi K, Sundara R. Electron field emission properties of conducting polymer coated multi walled carbon nanotubes. Appl Surf Sci 2008;254(21):6770–4, © 2008 Elsevier.

of CNTs by physical deposition techniques. Superconducting nanowires are prepared by continuous coating of the CNT surface with metals such as Mo, Ge, etc. [72] (Fig. 16.12).

16.7

Potential applications of CNTs and CNT-based nanocomposites

CNTs have attracted significant research interest due to their exceptional chemical, physical, and electronic properties that could potentially impact broad areas of science and technology, ranging from nanoscale electronic circuits to mechanically strong nanocomposites. The quantum confinement of electrons normal to the nanotube axis leads to the unique electronic properties of CNTs. These exceptional material

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Fig. 16.12 SEM image of (A) CNTs, and (B) SnO2/CNTs (inset shows HRTEM image). Reproduced with permission from Rakhi RB, Alshareef HN. Enhancement of the energy storage properties of supercapacitors using graphene nanosheets dispersed with metal oxide-loaded carbon nanotubes. J Power Sources 2011;196(20):8858–65, © 2011 Elsevier.

properties open up exciting possibilities for CNTs in nanometer-scale electronic applications. Potential practical applications include chemical sensors [73, 74], field emission materials [69, 75], catalyst support [76], electronic devices [77], high-sensitivity nanobalance for nanoscopic particles [77], nanotweezers [78], reinforcements in high performance composites, nanoprobes in biomedical and chemical investigations, anode for lithium ion batteries [79], nanoelectronic devices [80] supercapacitors [79], and hydrogen storage devices [30]. In the present section, the emphasis has been given to the applications of CNTs in four different areas: hydrogen storage, field emission, supercapacitors, and chemical biosensors.

16.7.1 CNTs for hydrogen storage The ever increasing demand for oil, associated price hikes, and environmental issues keep exerting considerable pressure on an already stretched world energy infrastructure. As the fossil fuels are not renewable, and currently known reserves would not last for longer than a few decades, a significant effort has been made in the development of alternative and renewable energy technologies such as electrochemical and hydrogen energy technologies, biofuels, solar cells, and fuel cells. In the past, these types of energy sources have been marginalized, but as new technology makes alternative energy more practical and competitive in price with fossil fuels, it is expected that the coming decades will usher in a long-expected transition away from oil and gasoline as our primary fuels. One has to look for new energy carriers for automotive transportation due to a decreasing oil supply and increasing demand for a portable fuel. Environmental concerns demand that this new carrier be part of a carbon-free emission cycle. Hydrogen has the potential to be such a carrier, owing to its potential for implementation in a carbon-free emission cycle, abundance, facile synthesis, and high

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120 Hydrogen adsorption isotherms of CNTs obtained from REN2 100

Peq(bar)

80

60 373K 323K 298K 143K 373K 323K 298K 143K

40

20

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

wt%

Fig. 16.13 Hydrogen adsorption isotherms of CNTs (closed symbols indicate purified CNTs and open symbols indicate as-grown CNTs). Reproduced with permission from Rakhi RB, Sethupathi K, Ramaprabhu S. Synthesis and hydrogen storage properties of carbon nanotubes. Int J Hydrogen Energy 2008;33(1):381–6, © 2008 Elsevier.

efficiency when used in a fuel cell [81, 82]. However, safety concerns and the relatively low volumetric density for compressed hydrogen gas in storage tanks demand a better storage system [83, 84] (Fig. 16.13). Pores of a molecular dimension are capable of adsorbing large quantities of gases. The attractive potential of the porous walls leads to the enhanced density of the adsorbed materials inside the pores [85]. CNTs, with their high aspect ratio, hollowness, large surface area, excellent chemical stability, and light mass, are considered ideal candidates for hydrogen storage [85, 86]. The hydrogen adsorption in carbonaceous materials refers to the quantity of hydrogen adsorbed near the solid carbon surface due to the van der Waals interactions between the carbon atoms and the hydrogen molecules. This phenomenon is called physisorption. The quantity of gas adsorbed at a given temperature is only a function of the pressure, and the absorbed gas can be released (desorbed) by decreasing the pressure. This property indicates that physisorption is reversible with pressure. The adsorption can be expressed as a unit of quantity of gas with respect to a unit of quantity of adsorbent. The corresponding units are mole per gram (mole g1) or gram per gram (g g1) or, now the most used, weight % (wt%) [84].

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Reports are available on the hydrogen storage properties of CNTs [86, 87]. Dillon et al. reported a hydrogen adsorption capacity of 5–10 wt% for pure SWCNTs at 133 K [87]. For high-purity crystalline ropes of SWCNT at a pressure >12 MPa and a cryogenic temperature of 80 K, Ye et al. reported a hydrogen adsorption capacity of above 8 wt%. [88] Liu et al. conducted a detailed study on the effects of pretreatments of SWCNTs on their hydrogen adsorption capacity [89]. A report suggests that the hydrogen adsorption capacity of SWCNTs can be enhanced by immersing them in an aqueous solution of HCl and subsequent heat treatment under vacuum conditions [90]. Comparatively, MWCNTs also seem to have an attractive future for H2 storage. Chen et al. reported that alkali doping improves the hydrogen adsorption properties of MWCNTs. Li-doped (653 K) and K-doped (300 K) MWCNTs respectively exhibited a hydrogen intake capacity of 20 and 14 wt% [91]. Gundiah et al. conducted detailed studies on the hydrogen adsorption properties of well-characterized samples of CNTs and reported a maximum storage capacity of 3.7 wt% [92]. The low bulk density of CNTs and the limited amount used could result in large uncertainties in the experiments, especially for high-pressure gas phase storage. Various other factors, such as the unavoidable leaks at high pressures and the influence of temperature variations on pressure, also bring about a wide range of values for hydrogen uptake in CNTs. Hence, extreme care is needed for a systematic study of the hydrogen sorption measurements in CNTs.

16.7.2 Electrochemical supercapacitors Electrical energy storage devices form an integral part in telecommunication devices (cell phones, remote communication, walkie-talkies, etc.), standby power systems, and electric hybrid vehicles in the form of storage components (batteries, supercapacitors, and fuel cells). In the field of energy storage, two main parameters are fundamental for storage devices: the energy density and the power density. The first parameter defines the amount of energy that can be stored in a given volume or weight while the second parameter describes the speed at which energy is stored or discharged from the device. The ideal storage device should simultaneously have both high energy density and high power density. Hence, the integration of conventional primary energy storage units (e.g., batteries and fuel cells) and the electric energy storage devices in the high-power or pulse-power forms (e.g., capacitors) becomes the prime concern in the development of new power systems. On the other hand, the energy densities of conventional capacitors are usually too low to be acceptable for several future applications; the development of capacitors with high energy densities (i.e., supercapacitors) for these applications has become an exciting subject of much research for electrochemical energy storage/conversion systems [93, 94]. Supercapacitors are electrochemical energy storage devices that can be fully charged or discharged in seconds. Due to their higher power density, low maintenance cost, wide thermal operating range, and more extended cycle life compared to secondary batteries, supercapacitors have attracted significant research attention over the past decade. They also possess a higher energy density compared to conventional

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electrical dielectric capacitors. The storage capacity of a supercapacitor depends on the electrostatic separation between electrolyte ions and high surface area electrodes [93]. However, the lower energy density of supercapacitors in comparison with Li-ion batteries is an obstacle in their extensive application. Improvements in the performance of supercapacitors are required to meet the needs of future systems, ranging from portable electronics to hybrid electric vehicles and large industrial equipment. Hence, the need for the development of new electrode materials and advances in our understanding of the electrochemical interfaces at the nanoscale level [100–102] (Fig. 16.14). A supercapacitor comprises two electrodes of high surface area separated by an insulating material that is ionically conducting. Supercapacitors store electrical energy in an electrochemical double layer formed at the electrode-electrolyte interface. A schematic of a supercapacitor is shown in Fig. 16.15. While charging, the positive and negative charges from the electrolyte get accumulated at the surface of the electrodes and compensate for the electronic charges at the electrode surface. Electrodes made with porous materials can provide a massive electrolyte accessible surface area for charge accumulation [95, 96]. Supercapacitors are broadly classified into two categories, based on the charge storage mechanism, as electric double-layer capacitors (EDLC) and pseudocapacitors. In EDLC, carbon-based active materials having a high surface area are used as the electrode materials, and static charge storage occurs at the electrode-electrolyte

Fig. 16.14 Schematic illustration of an electrochemical double-layer supercapacitor (EDLC) and its equivalent circuit in a two-electrode system.

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20 15

1.3 mg cm–2

2.5 mg cm–2

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(B)

50 100 150 200 250 300 350 Time (s)

Fig. 16.15 (A) Cyclic voltammograms measured at a scan rate of 25 mV s1and (B) the chargedischarge profiles collected at a current density of 1.0 A g1 for MnO2/CNT composite electrodes with various mass loadings. Reproduced with permission from Wang K, Gao S, Du Z, Yuan A, Lu W, Chen L. MnO2-carbon nanotube composite for high-areal-density supercapacitors with high rate performance. J Power Sources 2016;305:30–6, © 2016 Elsevier.

interface. Transition metal oxides or electrically conducting polymers are the electrode materials used in pseudocapacitors. Fast and reversible surface or near-surface faradaic reactions for charge storage also contribute to the charge storage in pseudocapacitors [97, 98]. Transition metal oxides as well as electrically conducting polymers can serve as pseudocapacitive active materials. Hybrid capacitors, combining a capacitive or pseudocapacitive electrode with a battery electrode, are the latest kind of EC, which benefits from both the capacitor and the battery properties. An appropriate electrode combination can help increase the cell voltage, further contributing to improvements in energy and power densities. Currently, two different approaches to hybrid systems have emerged: (i) pseudocapacitive metal oxides with a capacitive carbon electrode, and (ii) lithium-insertion electrodes with a capacitive carbon electrode. In comparison with pseudocapacitors, more lifetime, lower cost, and higher power density can be attained for carbon-based double-layer capacitors [95, 96, 99–105]. CNTs with their narrow distribution of mesopore sizes, low resistivity, mechanical strength, high electrolyte accessible surface area, and chemical stability are considered ideal electrode materials for supercapacitors [101]. Supercapacitor electrodes based on CNT-nanocomposites have shown improved capacitive performance as compared to CNT electrodes due to the increased specific surface area resulting from the disintegration of the bundle structure of CNTs in the composites [106–108]. CNTs coated with conducting polymers and metal oxide/MWCNT nanocomposite materials are widely used as efficient supercapacitor electrodes with a large life cycle and high power density [102, 108–115]. Supercapacitors coupled with batteries to combine the best features of a battery and a supercapacitor are considered promising hybrid devices for future energy applications [116–121].

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16.7.3 Field emission from CNT-based nanocomposites Earlier, the thermionic emission mechanism was widely used for the generation of electrons in the electron sources used in information displays [122]. Field emission is considered an alternative to the thermionic emission for the extraction of electrons. In field emission, under the application of a sufficiently high external electric field, electrons near the Fermi level of a metal or semiconductor undergo quantum mechanical tunneling through the energy barrier and escape to the vacuum level [123]. The external applied electric field controls the emission current. Unlike thermionic emission, in this process the heating of the emitter is not required. Field emission is explained using the Fowler-Nordheim (F-N) Theory [124], and the emission current is calculated using the Fowler-Nordheim (F-N) equation:       1:5  106 V 2 2 10:4 6:44  109 Φ1:5 d β exp pffiffiffiffi  exp  I¼A Φ d βV Φ

(16.6)

where I is the current, A represents the dimensions of an area, V is the applied voltage, d the electrode distance, Φ the work function of the emitter tip, and β the field enhancement factor (Fig. 16.16). For a metal with a typical work function and a flat surface, the threshold field is typically around 104 V μm1, which is impractically high. The work function is a characteristic property of a material that cannot be varied significantly [124]. But, the field enhancement factor (β) plays a key role in controlling the value of the threshold field.

Fig. 16.16 (A) Field emission characteristics and (B) Fowler-Nordheim plots of nearly aligned MWCNTs, randomly oriented MWCNTs, and SWCNT field emitters over graphitized carbon fabric. Reproduced with permission from Rakhi RB, Sethupathi K, Ramaprabhu S. Field emission from carbon nanotubes on a graphitized carbon fabric. Carbon 2008;46(13):1656–63, © 2008 Elsevier.

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The larger the β, the higher the field concentration and the lower the effective threshold voltage for emission. Β mainly depends on the shape and size of the field emitter. Sharp tips/protrusions and a high aspect ratio of the emitters improves the β factor. [125]. Field emitters have to be robust to sustain a high electric field and large emission currents. Also, it should be made up of an electrically conducting material. CNTs satisfy these criteria because they possess high chemical and physical stability, high thermal and electrical conductivity, excellent mechanical strength, and a high aspect ratio, which make them suitable for high current-density applications in low or medium vacuum levels [126]. The field emission characteristics of CNTs were first reported in 1995 [127]. Both SWCNTs and MWCNTs have been reported as excellent field emitters at low operating voltages [128, 129]. Even though MWCNT field emitters exhibit high emission stability, they suffer from low emission current due to their small field enhancement factor [130]. CNT field emitters on flexible substrates are widely used in foldable display devices as the source of electrons or as addressing electrodes [131]. Field emission characteristics of CNT emitters can be altered by surface modifications such as focused ion beam irradiation, laser irradiation, patterning, and loading of different nanoparticles on the surface of the CNTs [132–134]. Many reports are available on the field emission properties of different types of CNT-based nanocomposite (metal/CNTs, metal oxide/CNTs, and conducting polymer dispersed CNTs, etc.) field emitters [66, 70, 75, 135, 136].

16.7.4 CNT-based electrochemical biosensors The detection and quantification of various components present in the biological systems are done with the help of electrochemical biosensors. The performance efficiency of a biosensor mainly depends on the physical and chemical properties of the components present in the sensor. Enzymes are biological catalysts. The direct electron transfer of enzymes with electrodes can be used as a measure to study enzyme-catalyzed reactions in biological systems. It can also be used as an electrochemical basis for the investigation of the structure of enzymes, the mechanisms of redox transformations of enzyme molecules, and metabolic processes involving redox transformations. Biosensors are constructed using enzyme-modified electrodes. The enzyme immobilized on an electrode surface must be capable of direct electron transfer and keeping its bioactivity. Unfortunately, due to several factors, it is difficult for an enzyme to carry out a direct electrochemical reaction. Hence, for application in biosensors, the enzymes should be immobilized on the electrode surface to avoid any complications linked to the solution systems [137] (Fig. 16.17). An enzyme-based biosensor is composed of a particular enzyme and a suitable electrochemical transducer. The enzyme must be immobilized on the electrochemical transducer over the electrode surface for retaining its bioactivities and for obtaining its

Preparation and properties of manipulated carbon nanotube composites and applications

Carbonized at 950°C

MWCNT MWCNTs/CSF

Pristine silk fabric

511

Decorated with Pt microspheres

Pt microspheres GOx

Loaded with GOx

GOx loaded Pt-MWCNT/CSF

Pt-MWCNTs/CSF

Fig. 16.17 Preparation process of the Pt-MWCNT glucose sensor fabricated over silk fabric. Reproduced with permission from Chen C, Ran R, Yang Z, Lv R, Shen W, Kang F, et al. An efficient flexible electrochemical glucose sensor based on carbon nanotubes/carbonized silk fabrics decorated with Pt microspheres. Sens Actuators B 2018;256:63–70, © 2018 Elsevier.

direct electrochemical reactions. Inert metals such as platinum or Au and carbonaceous materials such as porous carbon, graphite, carbon fibers, carbon spheres, glassy carbon, and carbon nanotubes (CNTs) are the commonly used electrochemical transducers. These electrochemical transducers also allow for secure enzyme immobilization [138–143]. CNTs possess an excellent electron transfer rate, which is much better than conventional carbon electrodes, and also allows surface chemical reactions for tethering foreign biomaterials such as enzymes and nucleic acids [143]. Composite materials can be prepared by attaching foreign molecules to the functional groups present in CNTs. One can fabricate efficient biosensors by attaching specific enzymes with CNTs (Fig. 16.18). Different groups have reported the fabrication of electrochemical biosensors using nanoparticles (NPs) [144–150]. The ability of carbon nanotubes to promote the electron-transfer reactions of hydrogen peroxide makes them ideal candidates for dehydrogenase- and oxidase-based amperometric biosensors [151–153]. Several successful studies on biosensors constructed with MWCNTs for the detection of substances such as cholesterol [154], glucose [155–157], DNA [146], horseradish peroxidase [158, 159], and hemoglobin [20, 137] have been published. Their performance has been found to be much superior to those of other carbon electrodes with regard to reaction rate, reversibility, sensitivity, stability, and detection limit [145–147].

512

Nanocarbon and its Composites 100 200

150 Current (mA)

Current (mA)

0.5mM 50 0.2mM 0.1mM

100

0

0 0

(A)

50

200

400 Time (s)

600

800

0

(B)

2

4

6

8

10

12

Concentration (mM)

Fig. 16.18 (A) Amperometric response to 0.1, 0.2, and 0.5 mM glucose and (B) The calibration curve of glucose detection of the Pt-MWCNT glucose sensor. Reproduced with permission from Chen C, Ran R, Yang Z, Lv R, Shen W, Kang F, et al. An efficient flexible electrochemical glucose sensor based on carbon nanotubes/carbonized silk fabrics decorated with Pt microspheres. Sens Actuators B 2018;256:63–70, © 2018 Elsevier.

16.8

Conclusions

For more than two decades, CNTs have remained at the forefront of intense research due to their exceptional physical, chemical, and electronic properties. These unique properties arise from their high aspect ratio, closed topology, small dimensions, and lattice helicity and make CNTs ideal for different types of applications ranging from nanoelectronics to superstrong composites used in space applications. CNTs are broadly classified as SWCNTs and MWCNTs. The structure and electronic property of a SWCNT can be explained with the help of the chiral vector. Both chemical and physical routes are used for the synthesis of CNTs. Among the different methods for CNT production, the catalytic CVD technique to synthesize CNTs is a simple, inexpensive, energy efficient, and most economical method for the large-scale production of CNTs. The growth mechanism of CNTs is explained efficiently using the VLS (vapor-liquid-solid) theory. CNTs are widely used in a variety of potential applications such as chemical sensors, field emission, electronic devices, reinforcements in high-performance composites, nanoprobes in meteorology and biomedical and chemical investigations, electrochemical energy storage, and hydrogen storage. Even though the challenges in fabrication may prohibit realization of many of these practical device applications, the fact that suitable surface modifications/manipulations can alter the properties of CNTs can be exploited for more imminent realization of practical devices. In this respect, a combination of CNTs and other nanomaterials, such as nanocrystalline metal/CNTs, nanocrystalline metal oxide CNTs, polymer/ CNTs, and metal filled CNTs, will have unique properties and research is therefore focused on the processing of these manipulated CNTs and their different applications.

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Acknowledgment R.B.Rakhi acknowledges the support of the Ramanujan Fellowship, Department of Science and Technology (DST), Government of India and CSIR-NIIST Thiruvananthapuram, India.

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[110] Xiao Q, Zhou X. The study of multiwalled carbon nanotube deposited with conducting polymer for supercapacitor. Electrochim Acta 2003;48(5):575–80. [111] Pieta P, Obraztsov I, D’Souza F, Kutner W. Composites of conducting polymers and various carbon nanostructures for electrochemical supercapacitors, ECS J Solid State Sci Technol 2, 2013. M3120-M3134. [112] Lota K, Khomenko V, Frackowiak E. Capacitance properties of poly(3,4-ethylenedioxythiophene)/carbon nanotubes composites. J Phys Chem Solid 2004;65 (2):295–301. [113] Wang H, Peng C, Peng F, Yu H, Yang J. Facile synthesis of MnO2/CNT nanocomposite and its electrochemical performance for supercapacitors. Mater Sci Eng B 2011;176 (14):1073–8. [114] Su AD, Zhang X, Rinaldi A, Nguyen ST, Liu H, Lei Z, et al. Hierarchical porous nickel oxide–carbon nanotubes as advanced pseudocapacitor materials for supercapacitors. Chem Phys Lett 2013;561–562:68–73. [115] Park JH, Myoun Ko J. Carbon nanotube/RuO2 nanocomposite electrodes for supercapacitors, J Electrochem Soc 150 (7), 2003, A864–A867. doi:10.1149/1.1576222. [116] Zhang F, Zhang T, Yang X, Zhang L, Leng K, Huang Y, et al. A high-performance supercapacitor-battery hybrid energy storage device based on graphene-enhanced electrode materials with ultrahigh energy density. Energ Environ Sci 2013;6(5):1623–32. [117] Thounthong P, Rael S, Davat B. Control strategy of fuel cell and supercapacitors association for a distributed generation system. IEEE Trans Ind Electr 2008;3225–33. [118] Karden E, Ploumen S, Fricke B, Miller T, Snyder K. Energy storage devices for future hybrid electric vehicles. J Power Sources 2007;168(1):2–11. [119] Dougal RA, Liu SY, White RE. Power and life extension of battery-ultracapacitor hybrids. IEEE Trans Comp Pack Tech 2002;120–31. [120] Conway BE, Pell WG. Double-layer and pseudocapacitance types of electrochemical capacitors and their applications to the development of hybrid devices. J Solid State Electrochem 2003;7(9):637–44. [121] Chu A, Braatz P. Comparison of commercial supercapacitors and high-power lithium-ion batteries for power-assist applications in hybrid electric vehicles: I Initial characterization. J Power Sources 2002;112(1):236–46. [122] Modinos A. Field, Thermionic, and Secondary Electron Emission Spectroscopy, 1984, New York: Plenum Press. [123] Gomer R. Field emission and field ionization. Cambridge: Harvard University Press; 1961. [124] Fowler RH, Nordheim L. Electron emission in intense electric fields. Proc Royal Soc LondSer A 1928;119(781):173. [125] Cheng Y, Zhou O. Electron field emission from carbon nanotubes. Comptes Rendus Physique 2003;4(9):1021–33. [126] de Heer WA, Ch^atelain A, Ugarte D. A carbon nanotube field-emission electron source. Science 1995;270(5239):1179. [127] G.A. Rinzler, Hafner J, Nikolaev P, Nordlander P, Colbert D, E.R. Smalley et al. Unraveling nanotubes-field-emission from an atomic wire, Science 269, 1550–1553. [128] Kyung S-J, Lee Y-H, C-w K, Lee J-H, Yeom G-Y. Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature. Carbon 2006;44(8):1530–4. [129] Jeong HJ, Choi HK, Kim GY, Song YI, Tong Y, Lim SC, et al. Fabrication of efficient field emitters with thin multiwalled carbon nanotubes using spray method. Carbon 2006;44(13):2689–93.

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[130] Sashiro U, Junko Y, Takeshi N, Hiroyuki K, Hiromu Y, Tomotaka E, et al. 39.4: Large size FED with carbon nanotube emitter. SID SympDigest Tech Pap 2002;33(1):1132–5. [131] Choi JH, Park JH, Moon JS, Nam JW, Yoo JB, Park CY, et al. Fabrication of carbon nanotube emitter on the flexible substrate. Diamond Relat Mater 2006;15(1):44–8. [132] Kim D-H, Kim C-D, Lee HR. Effects of the ion irradiation of screen-printed carbon nanotubes for use in field emission display applications. Carbon 2004;42(8):1807–12. [133] Bonard J-M, Weiss N, Kind H, St€ockli T, Forro´ L, Kern K, Ch^atelain A. Tuning the field emission properties of patterned carbon nanotube films. Adv Mater 2001;13(3):184–8. [134] Lee JH, Heo JN, Yi WK, Yu S-G, Jeong TW, Lee CS, Han IT, Kim HJ, Yoo J-B, Kim JM. Field emission of self-assembled carbon nanotubes on triode structure. Elect Lett 2002;38(12):602–3. [135] Morihisa Y, Kimura C, Yukawa M, Aoki H, Kobayashi T, Hayashi S, et al. Improved field emission characteristics of individual carbon nanotube coated with boron nitride nanofilm. J Vacc Sci Technol 2008;26(2):872–5. [136] Yang CJ, Park JI, Cho YR. Enhanced field-emission obtained from NiO coated carbon nanotubes. Adv Eng Mater 2007;9(1–2):88–91. [137] Cai C, Chen J. Direct electron transfer and bioelectrocatalysis of hemoglobin at a carbon nanotube electrode. Anal Biochem 2004;325(2):285–92. [138] Wang J, Mo J-W, Li S, Porter J. Comparison of oxygen-rich and mediator-based glucoseoxidase carbon-paste electrodes. Anal Chim Acta 2001;441(2):183–9. [139] Xu JJ, Chen HY. Amperometric glucose sensor based on glucose oxidase immobilized in electrochemically generated poly(ethacridine). Anal Chim Acta 2000;423(1):101–6. [140] Xiao Y, Ju HX, Chen HY. Direct electrochemistry of horseradish peroxidase immobilized on a colloid/cysteamine-modified gold electrode. Anal Biochem 2000;278(1):22–8. [141] Zheng H, Xue HG, Zhang YF, Shen ZQ. A glucose biosensor based on microporous polyacrylonitrile synthesized by single rare-earth catalyst. Biosens Bioelectron 2002;17 (6–7):541–5. [142] Sotiropoulou S, Gavalas V, Vamvakaki V, Chaniotakis NA. Novel carbon materials in biosensor systems. Biosens Bioelectron 2003;18(2–3):211–5. [143] Lim SH, Wei J, Lin JY, Li QT, KuaYou J. A glucose biosensor based on electrodeposition of palladium nanoparticles and glucose oxidase onto Nafion-solubilized carbon nanotube electrode. Biosens Bioelectron 2005;20(11):2341–6. [144] Zhu ZZ. An overview of carbon nanotubes and graphene for biosensing applications. Nano-Micro Lett 2017;9(3). [145] Yogeswaran U, Chen SM. Recent trends in the application of carbon nanotubes-polymer composite modified electrodes for biosensors: a review. Anal Lett 2008;41(2):210–43. [146] Sanchez-Pomales G, Santiago-Rodriguez L, Cabrera CR. DNA-functionalized carbon nanotubes for biosensing applications. J Nanosci Nanotechnol 2009;9(4):2175–88. [147] Morais PV, Gomes VF, Silva ACA, Dantas NO, Schoning MJ, Siqueira JR. Nanofilm of ZnO nanocrystals/carbon nanotubes as biocompatible layer for enzymatic biosensors in capacitive field-effect devices. J Mater Sci 2017;52(20):12314–25. [148] Lin YH, Yantasee W, Wang J. Carbon nanotubes (CNTs) for the development of electrochemical biosensors. Front Biosci 2005;10:492–505. [149] Balasubramanian K, Burghard M. Biosensors based on carbon nanotubes. Anal Bioanal Chem 2006;385(3):452–68. [150] Ahammad AJS, Lee JJ, Rahman MA. Electrochemical sensors based on carbon nanotubes. Sensors 2009;9(4):2289–319. [151] Vijayalakshmi K, Jereil SD, Alagusundaram K. Electron beam and spray technologies fabrication of hybrid MnO2/CNT/Ta nanocomposite for the electrochemical detection of H2O2. Ceram Int 2017;43(16):14464–72.

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[152] Pang XY, He DM, Luo SL, Cai QY. An amperometric glucose biosensor fabricated with Pt nanoparticle-decorated carbon nanotubes/TiO2 nanotube arrays composite. Sensors Actuat B Cheml 2009;137(1):134–8. [153] Nossol E, Zarbin AJG. A simple and innovative route to prepare a novel carbon nanotube/ Prussian blue electrode and its utilization as a highly sensitive H2O2 Amperometric sensor. Adv Funct Mater 2009;19(24):3980–6. [154] Gopalan AI, Lee KP, Ragupathy D. Development of a stable cholesterol biosensor based on multi-walled carbon nanotubes-gold nanoparticles composite covered with a layer of chitosan-room-temperature ionic liquid network. Biosens Bioelectron 2009;24(7):2211–7. [155] Chen C, Ran R, Yang Z, Lv R, Shen W, Kang F, et al. An efficient flexible electrochemical glucose sensor based on carbon nanotubes/carbonized silk fabrics decorated with Pt microspheres. Sens Actuators B 2018;256:63–70. [156] Yan YM, Baravik I, Yehezkeli O, Willner I. Integrated electrically contacted glucose oxidase/carbon nanotube electrodes for the Bioelectrocatalyzed detection of glucose. J Phys Chem C 2008;112(46):17883–8. [157] Nontawong N, Amatatongchai M, Jarujamrus P, Tamuang S, Chairam S. Non enzymatic glucose sensors for sensitive amperometric detection based on simple method of nickel nanoparticles decorated on magnetite carbon nanotubes modified glassy carbon electrode. Int J ElectrochemSci 2017;12(2):1362–76. [158] Tu XM, Luo SL, Luo XB, Zhao YJ, Feng L, Li JH. Metal chelate affinity to immobilize horseradish peroxidase on functionalized agarose/CNTs composites for the detection of catechol. Sci China Chem 2011;54(8):1319–26. [159] Liu SM, Yuan R, Chai YQ, Su HL. A label-free amperometric immunosensor based on horseradish peroxidase functionalized carbon nanotubes and bilayer gold nanoparticles. Sensors Actuat B Chem 2011;156(1):388–94.

Further reading [160] Tang ZK, Wang N, Zhang XX, Wang JN, Chan CT, Sheng P. Novel properties of 0.4 nm single-walled carbon nanotubes templated in the channels of AlPO4–5 single crystals. New J Phys 2003;5:. [161] Qian WZ, Liu T, Wei F, Wang ZW, Luo GH, Yu H, et al. The evaluation of the gross defects of carbon nanotubes in a continuous CVD process. Carbon 2003;41(13):2613–7. [162] Rakhi RB, Sethupathi K, Ramaprabhu S. Field emission from carbon nanotubes on a graphitized carbon fabric. Carbon 2008;46(13):1656–63. [163] Rakhi RB, Alshareef HN. Enhancement of the energy storage properties of supercapacitors using graphene nanosheets dispersed with metal oxide-loaded carbon nanotubes. J Power Sources 2011;196(20):8858–65. [164] Rakhi RB, Sethupathi K, Ramaprabhu S. Synthesis and hydrogen storage properties of carbon nanotubes. Int J Hydrogen Energy 2008;33(1):381–6. [165] Zhang L, Zhao XS. Carbon-based materials as supercapacitor electrodes. Chem Soc Rev 2009;38(9):2520–31. [166] Wang G, Zhang L, Zhang J. A review of electrode materials for electrochemical supercapacitors. Chem Soc Rev 2012;41(2):797–828. [167] Wang K, Gao S, Du Z, Yuan A, Lu W, Chen L. MnO2-carbon nanotube composite for high-areal-density supercapacitors with high rate performance. J Power Sources 2016;305:30–6.

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Elim Albiter, Jos e M. Barrera-Andrade, Elizabeth Rojas-Garcı´a, Miguel A. Valenzuela Catalysis and Materials Laboratory, ESIQIE-National Politecnic Institute, Zacatenco, Mexico

Chapter Outline 17.1 Nanocarbons and photocatalysis 17.2 TiO2–Nanocarbon 527

521

17.2.1 Fullerenes 528 17.2.2 Carbon nanotubes 532 17.2.3 Graphene 536

17.3 Oxide–Nanocarbon

539

17.3.1 Zinc oxide 539 17.3.2 Copper oxides 540 17.3.3 Tungsten oxide 542

17.4 Chalcogenide-Nanocarbon

542

17.4.1 Cadmium sulfide 542 17.4.2 Other sulfides 545

17.5 MOFs–Nanocarbon 548 17.6 Multicomponent–Nanocarbon 17.6.1 17.6.2 17.6.3 17.6.4

557

Metal oxide 1-NC-metal oxide 2 557 Metal oxide-NC-chalcogenide 557 Semiconductor-NC-metal 560 Semiconductor nanocarbon-MOFs multifunctional materials 561

17.7 Conclusion 571 References 572

17.1

Nanocarbons and photocatalysis

Carbon-based materials have been investigated and used since the second half of the last century as catalysts and tcatalytic supports in a wide variety of chemical reactions of industrial interest [1, 2]. Activated carbon, graphite, and carbon black are the ones that have been used most frequently, followed by, to a lesser extent, glassy carbon, pyrolytic carbon and polymer-derived carbon [3]. Several advantages of carbon-based materials have been reported and their success has been explained in heterogeneous Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00017-1 © 2019 Elsevier Ltd. All rights reserved.

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catalytic reactions, among which the following can be listed: high chemical stability in acid or basic media, low corrosion capability, high thermal stability, hydrophobic character, easy recovery from the reaction mixture, and lower price [1, 4]. However, the discovery of the so-called nanocarbons such as fullerenes (1985), carbon nanotubes (1991), and graphene (2004) has generated a growing interest in the synthesis, characterization, and applications of these materials in new catalytic reactions; nanocarbons can be used in its pristine form or as hybrids with inorganic nanoparticles, polymers, and other materials [4–6]. The concept of nanocarbon (NC) is understood as the ability of carbon atoms to form, under specific conditions, bonds between them via the hybridization of their 2s-2p orbitals, which leads to the generation of sp, sp2, and sp3 hybrid orbitals [7]. These hybrid orbitals are the basis of their evolution toward graphene and diamond, depending on whether three or four bonds are formed with neighboring carbon atoms, respectively [8]. Fullerene (OD), carbon nanotubes (1D), and graphene (2D) are the leading representatives coming from a sp2-hybridization process and, derived from them, are carbon quantum dots, nanohorns, nanofibers, nanoribbons, nanocapsulates, and nanocages, among others [9]. Although the use of carbon materials began with heterogeneous catalysis in the 1970s, from their use in photocatalysis began incipiently in the 1990s, as shown in Fig. 17.1. However, note that an exponential increase in the number of articles is detected in both topics, mostly in the last 5 years. Semiconductor-based photocatalysis plays a significant role in the development of modern technologies, using unlimited solar energy, in topics related to air and water treatment, hydrogen production, self-cleaning surfaces, sterilization, organic synthesis, and the obtention of fuels from carbon dioxide, among others [10, 11]. The heterogeneous photocatalytic reactions are carried out in the presence of a solid (semiconductor), a source of irradiation (UV/vis light), and reactants in the gas or

Fig. 17.1 Number of publications on “carbon materials”/“catalysis”/“photocatalysis” in Scopus on January 2, 2018.

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

523

Fig. 17.2 Most important photocatalytic mechanisms. (A) Water splitting, (B) degradation of pollutant, and (C) CO2 conversion. Based on Zhou P, Yu J, Jaroniec M. All-solid-state Z-scheme photocatalytic systems. Adv Mater 2014;26:4920–35. https://doi.org/10.1002/adma.201400288.

liquid phase. The photocatalytic process begins with the absorption of a photon with energy equal or higher than that of the semiconductor’s band gap, which leads to the generation of charge carriers (i.e., electron-hole pairs) [12]. A series of redox reactions can occur on the semiconductor surface if a suitable scavenger is available for trapping an electron or a hole, leading to some of the most important photocatalytic reactions such as water splitting, degradation of pollutants, and CO2 conversion, as shown in Fig. 17.2 [13]. From a thermodynamic point of view, to carry out any photocatalytic reaction, it is essential that the conduction band (CB) and the valence band (VB) levels of the semiconductor must be more negative or positive than the reduction and oxidation potentials of the corresponding substrates to be converted (Fig. 17.3). Note that in the case of water splitting (Fig. 17.3A), it is necessary to couple the water photooxidation in the VB with the proton photoreduction in the CB to obtain oxygen and hydrogen, respectively. In the case of degradation of pollutants (Fig. 17.3B), this reaction regularly proceeds via OH radicals, which can be generated by oxidation of the hydroxyl ions by holes in the VB or by oxygen reduction with electrons in the CB. This process should be carefully balanced, ensuring that a semiconductor with the capacity to perform both reactions simultaneously is used. Likewise, the CO2 photocatalytic reduction (Fig. 17.3C) requires the coupling of the water photooxidation reaction (VB), which supplies the source of protons and electrons that will be converted in the CB to form products such as CO, CH4, and CH3OH, among others. It is important to note that,

524 Nanocarbon and its Composites

Fig. 17.3 Oxidation and reduction potentials of the different species involved in (A) Water splitting, (B) degradation of pollutants, and (C) CO2 reduction. Based on Li X, Yu J, Wageh S, Al-Ghamdi AA, Xie J. Graphene in photocatalysis: a review. Small 2016;12:6640–96. https://doi.org/10.1002/smll. 201600382.

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525

associated with the thermodynamic aspects, the kinetic aspects are also fundamental to achieve an improved global photocatalytic efficiency [14]. The most important of all is the charge separation/transport kinetics, which can be considered as a controlling step of the global photocatalytic efficiency [14]. At this point, it is where the use of several kinds of heterojunctions (e.g., metal-semiconductor, semiconductor-semiconductor, nanocarbon-semiconductor) that inhibit the recombination of photogenerated charges is justified. Undoubtedly, TiO2 has been the most researched and used semiconductor in some commercial applications due to its high photoactivity and high stability in various reaction media. Also, it is not toxic and has a low cost. However, TiO2 and other semiconductors (e.g., ZnO, WO3, CuxO) have been investigated regarding their surface modification tending to increase their photoactivity, to improve the absorption of visible light, and to modify the reaction mechanism to control the products and intermediates [14]. Fig. 17.4 is an example of a series of surface modifications that have been investigated, mainly with TiO2. Therefore, the hybridization of semiconductors with nanocarbon has been a viable alternative to generate a synergy that leads to the development of new more active, selective, and stable photocatalytic systems. The above can be corroborated by the increase in the number of publications in photocatalytic systems using

Metal deposition

Heterogeneous composites

Pt, Pd, Au, Ag...

CdS, WO3, SnO2, SiO2, Al2O3...

Hybrids with nano-materials CNTs, fullerenes, graphenes, POMs, zeolites

Dye anchoring

TiO2 photocatalyst

Fluoride, phosphate, organic molecules, surfactants, polymers Surface adsorbates

Ru-complex, porphyrins, organic dye

Metal-ion Nonmetal-ion co-doping Doping

Fig. 17.4 Various modification methods of TiO2 photocatalyst. Reproduced with permission from Park H, Park Y, Kim W, Choi W. Surface modification of TiO2 photocatalyst for environmental applications. J Photochem Photobiol, C 2013;15:1–20. https://doi.org/10.1016/j.jphotochemrev.2012.10.001.

526

Nanocarbon and its Composites

Fig. 17.5 Number of publications in photocatalytic systems type nanocarbon-inorganic hybrids.

nanocarbon-inorganic hybrids, as shown in Fig. 17.5. It is worth noting that the use of graphene has had the most significant impact, due to the vertiginous increase in the number of publications from 2010 to the present. The pioneering studies concerning the coupling of photocatalysts with carbonaceous materials were carried out to take advantage of their high adsorption capacity. Consequently, a significant effort was made in research using activated carbon (AC) coupled with TiO2 in different ways: TiO2-loaded AC, powder mixtures of TiO2 with AC, carbon-doped TiO2, and carbon-coated TiO2 [15]. However, nanocarbon compounds showed superior physicochemical properties than AC, such as electric and thermal conductivity, mechanical strength and toughness, and thermal and chemical stability, leading to vigorous and thriving investigations on the synthesis, characterization, and applications of nanocarbon-inorganic hybrids. In this regard, many reports show a higher photocatalytic activity of nanocarbonsemiconductor hybrids compared with bare semiconductors. The improved activity is explained in summary form as follows: (i) nanocarbon can act as a photosensitizer providing additional electrons to the VB of a semiconductor, and (ii) the high conductivity of the nanocarbon allows the extraction and storage of photo-generated electrons [5, 14]. After all, a minimization of the charge-carrier recombination and the creation of an overstructure that favors mass and electron transfer as well as better adsorption of the reactants should also be taken into account. According to the literature, the photocatalytic degradation of pollutants has been the most-studied reaction employing nanocarbon hybridized with oxides and chalcogenides, compared with the photocatalytic water splitting and CO2 reduction. Hence, this chapter will be focused on the recent results and advances in the field of nanocarbon hybridized with the conventional semiconductors (e.g., TiO2, ZnO,

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

527

Fig. 17.6 Chapter structure.

CdS, etc.) and new materials such as MOFs and multicomponents, highlighting the most important aspects reached in recent years in the mentioned photocatalytic (PC) reactions (Fig. 17.6).

17.2

TiO2–Nanocarbon

To date, TiO2 is the most employed material in photocatalysis and shows an excellent potential to be the ideal photocatalyst for a wide range of applications, such as environmental remediation, organic synthesis, water splitting, and CO2 reduction. However, two of its main disadvantages are the poor utilization of the solar spectrum, derived from its large band gap energy, and the fast recombination rate of the photogenerated electron-hole pairs [16]. It has been reported that several nanocarbon materials, such as fullerene, graphene, and carbon nanotubes, improve the absorption of visible light of TiO2 [17–21]. This improvement has been attributed to the formation of chemical bonds between TiO2 and functionalized nanocarbons (TidOdC bonds, Fig. 17.7) [22]. Also, these carbonaceous materials can act as a pool for the photo-generated electrons [14], reducing the recombination rate of electron-hole pairs and improving the photocatalytic performance.

528

Nanocarbon and its Composites

Fig. 17.7 Ti—O—C bonds formed during the synthesis of TiO2–C60 nanohybrids. Based on Mu S, Long Y, Kang S-Z, Mu J. Surface modification of TiO2 nanoparticles with a C60 derivative and enhanced photocatalytic activity for the reduction of aqueous Cr(VI) ions. Catal Commun 2010;11:741–4. https://doi.org/10.1016/j.catcom.2010.02.006.

17.2.1 Fullerenes Fullerenes are well known due to their remarkable physical and chemical properties resulting from their delocalized conjugated structure. Due to their unique electronic structures, C60 and C70 are excellent electron acceptors, making them promising materials to enhance the separation of photo-generated charge carriers in TiO2-based catalysts [16]. Additionally, fullerenes strongly absorb UV light and moderately absorb light in the visible region; thus, they can act as a photosensitizer, donating electrons to a coupled semiconductor. These two characteristics can be further enhanced by the functionalization of fullerenes, derived from the formation of chemical bonds, as mentioned earlier. The observed enhancement in the photocatalytic process has been explained previously in the literature [19, 23]. Briefly, after UV or visible light irradiation, fullerenes are excited from their ground state to a singlet excited state, which then decays to an excited triplet state with a more extended lifetime [24]. It is interesting to note that the excitation of fullerenes improves their capability of accepting or transferring electrons, including up to six electrons [25]. Due to the one-electron reduction potential of fullerenes or their excited states, the photo-generated electrons located in the conduction band of TiO2 could be transferred to these species, producing radical anions. Then, these anions could react with adsorbed species on the catalyst surface. On the other hand, fullerenes may also behave as electron donors, depending on the experimental conditions [26]; this behavior is primarily observed when fullerenes are irradiated with visible light [27–29]. Consequently, it is common to find in the literature that fullerenes can act as electron acceptors, enhancing the photocatalytic performance of the hybrid material due to the reduction of the electron-hole recombination or as an electron donor, enhancing the photocatalytic activity through TiO2 sensitization. These two mechanisms are depicted in Fig. 17.8. Because of their unique behavior when irradiated with UV or visible light, fullerenes have been employed in photochemistry as photosensitizers to produce singlet oxygen, in artificial photosynthesis, and in photochemical solar cells. In recent years, their application, in combination with TiO2, to photocatalytic reactions has been explored. However, the published studies are scarce when compared to other nanocarbons, and there are few reviews that have covered the application of C60

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

529

Fig. 17.8 Photoreaction mechanism of the C70-TiO2 hybrid under UV light (A) and visible light irradiation (B). Reproduced with permission from Wang S, Liu C, Dai K, Cai P, Chen H, Yang C, et al. Fullerene C70–TiO2 hybrids with enhanced photocatalytic activity under visible light irradiation. J Mater Chem A 2015;3:21090–8. https://doi.org/10.1039/C5TA03229F.

fullerene to TiO2 photocatalysis [17, 18]. More recently, C70 fullerene has also been hybridized with semiconductors; however, it is even less studied than C60. A selection of the most representative works regarding the application of fullerenes in TiO2 photocatalysis is presented in Table 17.1. As shown in Table 17.1, fullerenes are not commonly used in the pristine form, but they are functionalized by the introduction of different functional groups. For example, carboxylic groups are introduced into the fullerene structure using mild oxidation with inorganic acids [22, 27] or using organic reactions of cycloaddition [29, 30]. Also, the inclusion of hydroxyl groups has been explored, leading to obtaining the so-called fullerols [31, 32]. As mentioned earlier, the introduction of functional groups accomplishes a better interaction between the fullerene and TiO2, improving the electronic transfer but also improving the solubility in water of these nanocarbons. As a result of the physical and chemical interactions between TiO2 and fullerenes, some of the photophysical properties of the photocatalyst are modified, compared to pure TiO2. For example, Qi et al. found that the band gap energy of C60-TiO2 composites was reduced when the fullerene mass was increased [20]. The authors found a reduction of 0.15 eV when the content of C60 was 4 wt%; it is worthy to note that most published works did not report a modification of the band gap energy [21, 28–30]. However, all authors reported an increased absorption of visible light. The addition of fullerenes also may reduce the photoluminescence (PL) of TiO2-based materials. Cho et al. found that C70 can reduce remarkably the PL of C70-TiO2 composites compared to unmodified TiO2 [29]. The reduction of the PL signal can be attributed to the reduction of the recombination rate of electron-hole pairs, which can lead to an increased photocatalytic performance. The nanocarbon content seems to play an essential role in the photocatalytic activity of fullerene-TiO2 composites. According to the literature, the used content of fullerenes is in the range of 0.1 and 24 wt%, but the highest enhancement in the photocatalytic activity is observed between these values. For example, Qi et al.

530

Table 17.1 Selected publications on the photocatalytic applications of Fullerene-TiO2 hybrids Fullerene C60

Application

Reaction conditions

Commercial C60 Sol-Gel method

Methylene blue degradation

8 W mercury lamp Ccat ¼ 0.4 g L1 NC content ¼ 0.4–4 wt% [C]0 ¼ 5 mg L1

Oxidized commercial C60 Hydrothermal method

Acetone degradation in gas phase

15 W UV lamp Reactor volume ¼ 15 L Catalyst mass ¼ 0.3 g NC content ¼ 0.1–1.5 wt% [Acetone]0 ¼ 400 mg L1

Functionalized commercial C60 Commercial TiO2

Rhodamine B degradation

500 W Xe lamp with a UV cutoff filter (λ >400 nm) Ccat ¼ 1 g L1 NC content ¼ 0.5–3 wt% [C]0 ¼ 10 mg L1

Highlighted properties and results Reduced band gap, depending on C60 content, was observed. TiO2 was coated by C60, and the formation of a heterojunction was observed by HRTEM. The best PC performance was observed with 2 .0 wt% of C60. No modification of BG was observed. The PC performance was increased by the presence of oxidized C60. The catalyst with 0.5 wt% of C60 showed the best activity. The composites presented an enhanced absorption of visible light and they were active under visible light irradiation. The presence of C60 improved the adsorption of RB. The best PC performance was obtained with 1 wt% content of fullerene.

Ref. [20]

[21]

[30] Nanocarbon and its Composites

Preparation method NC/TiO2

Diphenhydramine degradation

Hg lamp with a cutoff filter (λ> 430 nm) Ccat ¼ 1 g L1 NC content ¼ 4 and 12 wt% [C]0 ¼ 100 mg L1 18 W UV lamp Ccat ¼ 0.13 g L1 TiO2 content ¼ 3 and 7.5 wt% [C]0 ¼ 13 mg L1

Functionalized commercial C60 Atomic layer deposition

Methyl orange degradation

Functionalized commercial C70 Hydrothermal method

Methylene blue degradation

Experimental conditions were not provided.

Oxidized commercial C70 Hydrothermal method

Sulfathiazole degradation

300 W Xe lamp with a cutoff filter (λ> 420 nm) Ccat ¼ 1 g L1 NC content ¼ 3–24 wt% [C]0 ¼ 10 g L1

The composite with 12 wt% of NC presented the best PC activity when irradiated with visible or UV light.

[28]

Fullerene was covered with thin layer of amorphous TiO2. The composites were active despite the low content of amorphous TiO2. The PL signal of the composites decreased compared to pure TiO2. The composites were active under visible-light irradiation. Chemical bonding between C70 and TiO2 was evidenced by XPS, FTIR, and Raman spectroscopies. Under UV-light irradiation, the contribution of C70 to the PC activity was negligible, but under visible light, the composite with 18 wt% of C70 showed the best activity.

[31]

[29]

[22]

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

C70

Oxidized commercial C60 Liquid phase deposition method

531

532

Nanocarbon and its Composites

prepared C60-TiO2 hybrids with an NC content of 0.4–4 wt%, and they found the highest activity with a carbon content of 2 wt%. The authors demonstrated, by using DFT calculations, that the interaction between fullerene and TiO2 through the formation of covalent bonds is a key point to enhancing the whole photocatalytic process. Thus, when NC content is increased above a certain threshold, fullerene can form aggregates and clusters on the semiconductor’s surface, increasing the coverage of the surface and the thickness of the fullerene layer [20]. The increased thickness could lead to a reduced NC-TiO2 interaction, and therefore fewer chemical bonds are formed. On the other hand, the increased coverage and the higher thickness inhibit the light absorption on the surface of TiO2. Nevertheless, it is not possible to determine the optimal nanocarbon loading by just comparing the reported results in the literature because this value seems to depend on the experimental conditions or preparation methods used. The fullerene-TiO2 hybrids have been employed in the photocatalytic degradation of water pollutants such as organic dyes, including methylene blue [20], rhodamine B [30], and methyl orange [31]; in the degradation of organic compounds in the gas phase [21, 33]; in the degradation of some emerging pollutants such as diphenhydramine [28] and sulfathiazole [22]; and in the selective oxidation of organic compounds [34]

17.2.2 Carbon nanotubes Carbon nanotubes (CNT) can be classified according to the number of carbon layers in their structure: single-walled nanotubes (SWCNTs), which have one layer of carbon atoms forming a cylinder; and multiwalled carbon nanotubes (MWCNTs), which consist of multiple concentric carbon sheets. Due to their cylindrical morphology, often referred to as a 1D structure [17], CNTs show interesting thermal and electronic properties that make them a promising material with applications in photocatalysis [16]. For example, CNTs can behave as either semiconducting or metallic materials, depending on their morphology; they also have excellent electron conductance [35]. Because of these electrical properties, CNTs can increase the lifetime of the photo-generated charge carriers due to the electron transfer from the CB of TiO2 to the carbon structure, which acts as an efficient electron sink. CNTs can also improve the photocatalytic performance of the TiO2 photocatalyst by providing an increased surface area (200 to 400 m2 g1) [36] and, therefore, more active sites. Also, these nanocarbons can improve the absorption of visible light of TiO2 [17] by modification of its band gap or by sensitization, similar to fullerene enhancement. The described improvements can be influenced by the preparation methods used for the CNTTiO2 hybrids. The reported methods provide reasonable control of the morphology and structure of these materials [37, 38]. For example, Fig. 17.9 shows three common structures of CNT-TiO2 hybrids. The first one (Fig. 17.9A) is composed of nanoparticulate TiO2 and CNT, which is commonly obtained by mechanical mixing. Fig. 17.9B shows TiO2 nanoparticles grown on the CNT surface by a wet chemical route and Fig. 17.9C shows large semiconductor particles wrapped by CNT. It is important

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

533

Fig. 17.9 CNT-TiO2 hybrids structures: (A) A hybrid made up of a random mixture of nanoparticulate TiO2 and CNT, (B) CNT coated with TiO2 nanoparticles, and (C) CNT wrapped around TiO2 nanoparticles. Reproduced with permission from Mallakpour S, Khadem E. Carbon nanotube–metal oxide nanocomposites: fabrication, properties and applications. Chem Eng J 2016;302:344–67. https://doi.org/10.1016/j.cej.2016.05.038.

to note that some morphological aspects of CNTs can have a significant influence on the photocatalytic performance; these include shape, size, aspect ratio, CNT orientation, homogeneity of the hybrid, and volume fraction of CNTs [39]. Besides these aspects, surface modification of CNT plays a key role in enhancing the adsorption of pollutants and, as in the case of fullerenes, increasing the interaction of TiO2 and this nanocarbon. The modification can be achieved by the introduction of hydroxyl, ketone, or acid functional groups on the surface of CNT during purification of the raw materials [39–41]. The CNT loading also has a significant influence in the observed photocatalytic activity. Table 17.2 presents a selection of the most recent works dealing with CNT-TiO2 hybrids, where different CNT loadings have been investigated. The CNT loading range varies from small values as high as 88 wt%, and the most common optimal values are reported ca. 20 wt% [18, 41]. However, this optimum seems to depend on several properties and the morphology of the photocatalyst, and therefore, on the preparation method. For example, in several studies where TiO2 nanoparticles are randomly mixed with CNT, the PC activity can be increased up to loadings of 85 wt% [42, 43]. Some authors have explained this behavior based on the mechanism of PC enhancement, that is, if the CNT is acting as an electron pool or as a photosensitizer [18]. Therefore, it seems that there is a close relationship between the improvement of the PC performance derived from the presence of CNT and the quantity of TiO2. Thus, if CNT is functioning as an electron sink, TiO2 is the photoactive phase in the hybrid. Therefore, it may be useful to have a higher TiO2 amount, which means a higher TiO2 surface exposed. On the other hand, if CNT is behaving as a photosensitizer, it makes sense that the higher activity is achieved under higher loadings of CNT because it is the photoactive phase. The presence of CNT in TiO2-based materials has a significant effect on their properties. For example, the high surface area of these nanocarbons provides improved adsorption of pollutants and a higher number of photoactive sites through the

534

Table 17.2 Selected publications on the photocatalytic applications of Carbon Nanotubes-TiO2 hybrids Preparation method NC/TiO2

Pollutant or cocatalyst

Hydrogen production

Commercial MWCNT Hydrothermal method

Pt

500 W Xe lamp with a cutoff filter (λ >365 nm) Ccat ¼ 0.2 g L1 NC content ¼ 34–88 wt%

Functionalized commercial MWCNT Commercial TiO2

Pt

Commercial MWCNT Sol-gel

Methylene blue

Medium pressure Hg mercury lamp. Ccat ¼ 1 g L1 NC content ¼ 17 wt%. 4 W UV lamp. Ccat ¼ 0.01 g L1 NC content ¼ 24–66 wt%. [C]0 ¼ 10 mg L1

Functionalized commercial MWCNT Sol-gel

Rhodamine B

Pollutant degradation

Reaction conditions

Filtered Xe lamp (λ >300 nm). Ccat ¼ 0.2 g L1 NC content ¼ 5–30 wt%. [C]0 ¼ 10 mg L1

Highlighted properties and results The hybrid with 44 wt% showed the highest production of H2. The electron-hole separation in the materials was evidenced by photocurrent measurements. The interaction between TiO2 and CNT played an essential role in the H2 production rate. The annealing temperature influenced the photocatalytic degradation of methylene blue. The formation of TidOdC bonds was evidenced by XPS analyses. MWCNT enhanced the absorption of visible light of the hybrids.

Ref. [43]

[40]

[45]

[41]

Nanocarbon and its Composites

Application

Rhodamine 6G

Functionalized commercial MWCNT Sol-gel

Tetracycline Pharmaceutical effluents

Catalytic chemical vapor deposition Hydrothermal method

Salicylic acid

125 W Hg lamp. Ccat ¼ 0.25 g L1 NC content ¼ 1–10 wt%. [C]0 ¼ 50 mg L1 6 W UVC lamp (λ > 240 nm). Ccat ¼ 0.1–0.4 g L1 NC content ¼ 0.5–10 wt%. [C]0 ¼ 0.5–30 mg L1 UVA lamp. Ccat ¼ 1 g L1 NC content ¼ 5 wt%. [C]0 ¼ 14 mg L1

A TOC removal of 83% was achieved in the treatment of pharmaceutical effluents. The hybrids showed an inferior PC performance, compared to commercial TiO2. This behavior was attributed to the TiO2 phase composition of the materials.

[44]

[47]

[48]

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

Functionalized commercial MWCNT Hydrothermal method

Reduced band gap, depending on MWCNT content, was observed. The best PC performance was obtained with 20 wt% content of nanocarbon. The PL signal of the hybrids decreased compared to bare TiO2.

535

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Nanocarbon and its Composites

dispersion of TiO2. Also, TiO2 is an n-type semiconductor but, in the presence of CNTs, can behave as a p-type semiconductor [16, 39] because the photo-generated electrons in the CB of TiO2 can be easily transferred to the surface of CNT, leaving an excess of holes in the VB. Some authors have reported that the attachment of CNT to the surface of TiO2 can modify its band gap. Park et al. [41] observed up to 1 eV reduction in the band gap of MWCNT-TiO2 hybrids (see Fig. 17.10B); the authors ascribed this reduction to the interaction of unpaired p electrons in the nanocarbon and the Ti atoms. Finally, the reduction of the electron-hole recombination was recently shown by Natarajan et al. [44] using PL analyses. As can be seen in Fig. 17.10A, the PL signal of the MWCNT-TiO2 hybrids was considerably reduced in comparison to pure TiO2. In recent years, CNT-TiO2 hybrids have been successfully applied to the degradation of organic pollutants in the aqueous phase. The degraded pollutants include several organic dyes, such as methylene blue [45], rhodamine B [41], rhodamine 6G [44], methyl orange [46], and some pharmaceutical drugs such as tetracycline [47] and salicylic acid [48]. The environmental applications of these materials also include the degradation of 4-chlorophenol [49], the inactivation of gram-positive bacteria [46], and the treatment of pharmaceutical effluents [47] where an 83% removal of the total organic carbon (TOC) in the effluent was observed. These hybrids have also been employed in the photocatalytic production of hydrogen, or the water splitting reaction [40, 43].

17.2.3 Graphene Since its discovery in 2004 [50], graphene (GR) has been one of the most-studied nanocarbons. This material is composed of a single layer of interconnected hexagons of carbon atoms, and it is considered as the basic building block of other nanocarbons [51, 52]. For example, it can be enclosed into 0D configurations to obtain fullerenes, rolled into 1D structures to produce CNTs, or stacked merely into a 3D structure to obtain graphite [51]. Owing to its attractive properties, GR has become one of the most promising materials in photocatalysis, and especially in TiO2-based photocatalysis. Graphene has excellent electrical and thermal conductivity, derived from its longrange conjugated structure; it also has a substantial theoretical surface area (2630 m2 g1) [50, 52]. The high electrical conductivity makes GR an ideal electron sink or electron transfer conduit to improve the charge-carrier separation. Its extended surface area favors the dispersion of TiO2, and therefore, it provides a higher quantity of active sites and extends the adsorption of pollutants or reactive species. Finally, GR can enhance the light absorption of the hybrids to the visible range [18]. Nowadays, several methods, including physical or chemical routes, are applied in the synthesis of graphene and its derivatives [53]; however, a modification of the so-called Hummers’ method is the most employed. This method consists in the chemical oxidation of graphite and a subsequent physical exfoliation to obtain graphene oxide (GO) sheets, and a final reduction to produce sheets of reduced graphene oxide (RGO) [53, 54]. GO sheets are hydrophilic due to the presence of hydroxyl, epoxy, and carboxyl functional groups, and therefore, they can generate stable aqueous

Intensity (a.u.)

MWCNT AT TNT 10% MWCNT/TNT

350

375

400

(A)

425 450 475 Wavelength (nm)

500

525

550

4.0 TiO2 precursor CS-TiO2 (0)

3.5

CS-TiO2 (5) CS-TiO2 (10)

3.0

CS-TiO2 (20) CS-TiO2 (30)

(ahn)1/2

2.5 2.0 1.5 1.0

2.66 eV 2.08 eV

0.5

2.55 eV 3.12 eV 2.75 eV

3.30 eV

0.0 2.0

(B)

2.2

2.4

2.6

2.8 hn /eV

3.0

3.2

3.4

3.6

Fig. 17.10 (A) Photoluminescence spectra of MWCNT, AT, TNT, and 10% MWCNT/TNT composites at 320 nm excitation wavelength and (B) plots of the Kubelka-Munk function corresponding to the spectra of MWCNT-TiO2 hybrids. From (A) Park CH, Lee CM, Choi JW, Park GC, Joo J. Enhanced photocatalytic activity of porous single crystal TiO2/CNT composites by annealing process. Ceram Int 2018;44:1641–5. https://doi.org/10.1016/j.ceramint.2017.10.086; and (B) Wang W, Serp P, Kalck P, Faria JL. Visible light photodegradation of phenol on MWNT-TiO2 composite catalysts prepared by a modified sol–gel method. J Mol Catal Chem 2005;235:194–9. https://doi.org/10.1016/j. molcata.2005.02.027.

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Nanocarbon and its Composites

suspensions. Besides, the presence of carboxylic groups allows the anchoring of TiO2 nanoparticles [55], enhancing the transfer of photo-generated electrons or modifying the absorption of visible light of the hybrids, as described earlier. Compared to semimetallic GR, GO is an insulator due to the creation of sp3 hybridization in the carbon network, which interrupts the sp2 conjugated network present in pristine GR [56]. The intrinsic properties of GR can be partly reestablished upon reduction of GO sheets. However, RGO sheets can reaggregate to form graphite due to the removal of the functional groups introduced during the oxidation step of its synthesis [57]. GR and RGO have many advantages compared to other materials used in photocatalysis. For example, GR can be used as a low-cost cocatalyst in many photocatalytic reactions or as a substitute for noble metals, such as Pt or Pd, commonly used in the photocatalytic production of H2 [58]. The band gap of GR and RGO can be tuned, which is an excellent advantage over inorganic semiconductors [13]. For example, Mathkar et al. performed a controlled reduction of GO using hydrazine as a reduction agent [59], achieving a fine control on the band gap of RGO from 3.2 eV to 1 eV (see Fig. 17.11). As one the most studied nanocarbons, GR and its derivatives have been applied to all the reactions covered in this chapter. To date, several reviews have been published covering the photocatalytic applications of these nanocarbons [13, 52, 53, 56, 60–62]. 140

Calculated optical gap

130

Time of hydrazine exposure (h)

120

Tertiary alcohol removal occurs at 108 h

110 100 90 80 70

Reduction of phenol and carbonyl after first 16 h

Reduction of epoxide moiety

60 50 40 30 20 10 0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Optical gap (eV)

Fig. 17.11 Gradual decrease of optical gap derived from the chemical reduction of different functional groups in the graphene oxide structure. Reproduced with permission from Mathkar A, Tozier D, Cox P, Ong P, Galande C, Balakrishnan K, et al. Controlled, Stepwise Reduction and Band Gap Manipulation of Graphene Oxide. J Phys Chem Lett 2012;3:986–91. https://doi.org/10.1021/jz300096t.

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

17.3

539

Oxide–Nanocarbon

17.3.1 Zinc oxide Zinc oxide (ZnO) is an n-type semiconductor with a direct band gap energy of 3.37 eV, very similar to that of TiO2. However, ZnO has a higher photon absorption efficiency showing, in many cases, a better photocatalytic activity than TiO2 [63]. It also has a high oxidizing power while being abundant, nontoxic, and low cost, which makes it a suitable candidate to replace TiO2. However, ZnO has several disadvantages, such as the fast recombination of photogenerated electron-hole pairs, it is only active with UV light, and it suffers from photocorrosion [64]. To solve these limitations, ZnO has been combined with other materials such as metals, semiconductors, and nanocarbons. In the case of NC, an excellent interaction among ZnO, GR, GO, or RGO has been observed, which leads to an improved PC activity compared with bare ZnO [65–84]. Other authors have highlighted the high specific surface area of the ZnOGO hybrids, which favors the adsorption of pollutants [85, 86]. The enhancement of the PC activity can be explained in terms of the high adsorption capacity of the hybrids and their capacity to act as an electron sink, avoiding the recombination of the photo-generated species [68, 79, 85, 87–93]. A schematic view of the ZnO/RGO activation is shown in Fig. 17.12. The ZnO particle size also plays an important role in enhancing the PC performance. For example, GO/ZnO hybrids with a size less than 100 nm presented high stability and photoactivity [67, 77, 94]. Concerning the amount of graphene in the hybrids, it has been reported that, in most cases, the optimal concentration is less than 5 wt% [86, 87, 94, 95]. ZnO photocorrosion is the main disadvantage for its commercial application in photocatalysis. One way to prevent its deactivation is by depositing the ZnO particles on the graphene structure, which could avoid its interaction with the holes and then favor the pollutant photodegradation [91]. Several reviews on ZnO have been published that include the most relevant aspects of NC-ZnO hybrids [96]. In general, most publications report the use of the hydrothermal/solvothermal method for the synthesis of NC-ZnO hybrids, which allows the

Fig. 17.12 Activation of the ZnO particles distributed on CNT using UV light (A) and visible light (B). Based on Mohd AMA, Julkapli NM, Abd HSB. Review on ZnO hybrid photocatalyst: impact on photocatalytic activities of water pollutant degradation. Rev Inorg Chem 2016;36:77–104. https://doi.org/10.1515/revic-2015-0015.

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Nanocarbon and its Composites

inclusion of the particles of ZnO on the surface of graphene [65–84]. Other methods have also been tested with good results, such as chemical deposition [86–88, 93], ultrasonic treatment [90, 91], chemical vapor deposition [85, 92], electrospinning [97], photocatalytic reduction [98], and electrochemical deposition [99]. ZnO-GO photocatalysts have been employed in the reduction of CO2 to obtain fuels. These catalysts presented a specific surface area of 236 m2 g1 and good CO2 adsorption capacity (44.8 cm3 g1 of CO2) [73]. After irradiation with UV-light, this catalyst generated 263 μmol g1 of methanol after 3 h. One of the variables studied in the generation of fuels from the reduction of CO2 was the amount of GO. By increasing the concentration of GO, the generation efficiency decreased. This can be affected by the distribution of the ZnO particles on GO and their activation to form the different active species that govern the CO2 photoreduction. Furthermore, ZnO-RGO hybrids prepared by electrochemical deposition have been evaluated in hydrogen generation, obtaining 28.9 μmol H2 g1 in 2 h, which was 4.5 times greater than that achieved with bare ZnO [100]. In other reports, ZnO particles were combined with GR, GO, and RGO and tested in the degradation of dyes, obtaining a higher photoactivity than bare ZnO. Works employing CNTs as a nanocarbon linked to ZnO are scarce [101–104]. Solgel and reflux methods have been mainly used to prepare CNT-ZnO hybrids [102–104]. A high specific surface area and a small particle size of ZnO chemically bonded with CNT is frequently found [102]. For instance, MWCNT-doped ZnO nanofibers were prepared by electrospinning and evaluated in the MB degradation under UV and visible light. Results showed a higher photocatalytic activity (sevenfold) of the MWCNT-doped ZnO compared to bare ZnO under UV light. Besides, this hybrid was also photoactive under visible light. The authors confirmed the Zn-O-C bond formation, which contributed to a lower bandgap, improving charge carrier transport under UV light irradiation (See Fig. 17.12A) or acting as a photosensitizer under visible light, as shown in Fig. 17.12B. Table 17.3 shows the most recent works related to the ZnO-NC hybrids and their applications.

17.3.2 Copper oxides Copper oxides (CuxO) are p-type semiconductors with a bandgap of 1.21–1.51 eV for CuO and 2.0 eV for Cu2O. They have a high capacity to absorb visible light and notable electrical properties. These oxides have been supported mainly in graphene, and there is, to our knowledge, only one investigation in which Cu2O was supported in CNT [105]. The hydrothermal method has been the most used in the preparation of CuxO-NC hybrids [106–108]. In most cases, CuxO-NC hybrids were evaluated in photocatalytic reactions under visible light irradiation. Particulary, a Cu2O-GR hybrids has presented a suitable interaction and distribution of the oxide on the graphene surface, leading to an efficient charge separation and higher photocatalytic activity in dye degradation [109–111]. A successful integration of Cu2O particles with RGO by using an in situ reduction method has been reported with high performance in the photocatalytic production of hydrogen [112]. On the other hand, stability is a crucial aspect of this type of hybrid system. For this reason, it has been found that the preparation method plays an

Table 17.3 ZnO combined with different nanocarbon and their applications in photocatalysis Preparation method

GO

Applications

Reaction conditions

Hydrothermal

Methylene blue degradation

RGO

Hydrothermal

Methylene blue degradation

G

Chemical

Methylene blue degradation

UV light, 100 mL with 5  105 mol L1 of MB, 80 mg catalyst, 120 min adsorption, 100 min irradiation UV light, 100 mL with 1  105 mol L1 of MB, 20 mg catalyst, 30 min adsorption, 90 min irradiation Visible light, 50 mL with 30 mg L1 of MB, 10 mg catalyst, 30 min adsorption, 130 min irradiation

CNT

Reflux

Methylene blue degradation

RGO

Supercritical CO2

H2 Production

RGO

Hydrothermal

Photoreduction of CO2

UV light, 200 mL with 10 mg L1 of MB, 200 mg catalyst, 60 min adsorption, 30 min irradiation UV light, 60 mL with 0.1 mol L1 of Na2S and 0.05 mol L1 of Na2SO3, 100 mg catalyst, 120 min irradiation UV light, 50 mL with 1 M of NaOH, 100 mg catalyst, CO2 gas was bubbled into the solution for 30 min, 180 min irradiation

Highlighted properties and results

Reference

The degradation rate was 54.3% for ZnO and 98.1% for ZnO/GO; this catalyst had been used five cycles.

[65]

The degradation rate was 40% for ZnO spheres, almost 100% for ZnO/ RGO. The optimal concentration of RGO was 3.5%. The degradation rate was 45% for ZnO, 80% for ZnO/GR. The GR amount was 5%, this quantity of G had the highest photocatalytic activity. The degradation rate was 68% for ZnO, 100% for ZnO/CNT. Small crystalline size and chemical bond between ZnO and CNT. The hydrogen generation was 289 μmol h1 by ZnO/RGO. RGO increased 4.5 times the generation of H2 than ZnO.

[102]

The content of RGO 10% generated more quantity of methanol, the dosage of catalyst was 2 g L1, 263.17 μmol methanol g1 this quantity was five times higher than obtained using ZnO (52.36 mol methanol g1).

[74]

[86]

[102]

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

Nanocarbon

[100]

541

542

Nanocarbon and its Composites

important role. For example, by using the pyrolysis method, a more active catalyst is obtained because the oxide particles grow in a single direction; it can also be distributed more evenly, giving rise to a more stable photocatalytic system [113]. The reported photocatalytic activity in hydrogen production was 19.5 mmol H2 g1 h1 under UV-vis light [113]. The effect of Cu1+ or Cu2+ species in the system of CuxO/graphene was studied in the CO2 photocatalytic reduction to produce methanol [114]. CuO species proved to be the most active by their uniform distribution on the surface of graphene, resulting in a better charge separation under visible light.

17.3.3 Tungsten oxide Tungsten oxide (WO3) has been used to modify the properties of nanocarbon and to obtain new hybrid catalysts with better properties. Tungsten oxide is a semiconductor found in nature, stable at different pH values, with a band gap energy of 2.7 eV, insoluble in water, and is considered as a promising semiconductor in photocatalysis [115]. Due to these properties, it has been used to modify graphene mainly, and CNT [116]. The most commonly used method for synthesizing WO3/GR and WO3/CNT was hydrothermal [115, 117, 118]. These catalysts presented a homogeneous distribution of the WO3 particles on graphene oxide and excellent interaction between the two particles, which leads to an adequate charge separation and therefore is reflected in higher photocatalytic activity. As a result, these hybrids systems presented a good adsorption capacity along with a high photocatalytic activity in the degradation of pollutants [117–119]. WO3-GR hybrids have also been tested in the photocatalytic reduction of CO2, showing a significant selectivity to methane [115]. In this case, the synergy between two components was shown by a significant displacement of the WO3 conduction band to more negative values, making thermodynamically possible the reduction of CO2. Table 17.4 shows some selected examples using NC-metal oxide hybrids and their photocatalytic applications.

17.4

Chalcogenide-Nanocarbon

Chalcogenides are chemical compounds that contain in their structure an element of the group (16 or VI A) of the periodic table. The term chalcogenide is used mainly to refer to some compounds containing sulfides, selenides, and tellurides. The application of this type of semiconductor is extensive and varied, mainly focused on energy storage.

17.4.1 Cadmium sulfide One of the most-studied chalcogenides is cadmium sulfide (CdS). This material is insoluble in water, has a band gap of 2.4 eV, and absorbs energy in the visible region. It has the disadvantage of displaying a high recombination of the photo-generated pair electron-hole and also presents the phenomenon of photocorrosion [120]. However, CdS supported on graphene presented a superior photocatalytic activity than that of

Nanocarbon

Metal Oxide

Preparation method

GR

Cu2O

CNT

Applications

Reactions conditions

CVD

Methyl orange degradation

WO3

Solvothermal

Methylene blue degradation

RGO

Bi2O3

Solvothermal

Methylene blue degradation

graphene

CuO, Cu2O

Covalent grafting

Photoreduction of CO2 to methanol

Visible light, 80 mL with 30 mg L1 of MO, 20 mg catalyst, 30 min adsorption process, 30 min irradiation Visible light, 100 mL with 10 mg L1 of MO, 20 mg catalyst, 60 min adsorption process, 120 min irradiation Visible light, different concentration of MB, 100 mL the solution of MB, 6 h of irradiation Visible light, 45 mL DMF and 5 mL water, degas with N2, CO2 gas was added, 100 mg catalyst, 24 h reaction

GO

WO3

Hydrothermal

Photoreduction of CO2 into CH4

Visible light, 270 mL total volume, 1 mL of water was injected, some quantity of CO2 with high purity was introduced, O2 gas generated, 8 h reaction

Highlighted properties and results The abundance of defects affects the catalytic activity (80% of the MO was degraded). The degradation rate was 25% for WO3, 55% for WO3/CNT.

Reference [109]

[116]

Neutral pH, degradation rate was 55% for Bi2O3, 98% for Bi2O3/RGO.

[180]

The CuO species is more active than Cu2O species. 1282 μmol methanol g1 was obtained. 0.89 μmol CH4 was obtained after 8 h.

[114]

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

Table 17.4 Summary of photocatalytic applications of different NC-metals oxide

[115]

543

544

Nanocarbon and its Composites

bare CdS, which was explained by a better charge separation [120, 121]. For example, for CdS spheres deposited on graphene by a solvothermal method and evaluated in rhodamine-B degradation, a 95% removal of the model compound after 85 min of reaction time under visible light was found [120–122]. The tereftalic acid fluorescence technique was used to check hydroxyl radical generation in CdS and CdS-graphene, giving positive results in both cases. It was also found that due to the VB position of CdS, it is not thermodynamically possible to carry out the photooxidation of water. Therefore, the hydroxyl radicals are generated by the exchange of free electrons in the graphene network reacting with O2 [121–126]. In another report, RGO was hybridized with CdS, proving that by using visible light, the semiconductor was activated. A photocorrosion effect was also detected when measuring 3.5 wt% of Cd2+ in solution at the end of the experiment [127]. In other investigations, CdS was supported on reduced graphene oxide (RGO) employing the solvothermal method [122, 123, 125, 128–131], the combustion method [126], and the gamma reduction method [128]. In all cases, it was found that the separation of the photo-generated charge carriers was more efficient because the electrons generated by the CdS were transferred to the RGO network and this acted as an electron sink (see Fig. 17.13). CdS deposited on RGO was prepared by a high-temperature reaction and tested in hydrogen production [131], obtaining 14.6 mL H2 g1 in 2 h of irradiation with visible light. This photocatalyst was compared with a catalyst synthesized by the hydrothermal method, obtaining only 10.8 mL H2 g1. In this case, it was possible to improve the catalytic activity because the high-temperature reaction produced a better interaction of the CdS particles and the RGO compared to the hydrothermal method. Fig. 17.13 CdS semiconductor impregnated on RGO and the distribution of the charges.

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

545

Fig. 17.14 Schematic illustration of the photocatalytic process on: (A) pure CdS and (B) C60/CdS nanocomposite. Reproduced with permission from Cai Q, Hu Z, Zhang Q, Li B, Shen Z. Fullerene (C60)/CdS nanocomposite with enhanced photocatalytic activity and stability. Appl Surf Sci 2017;403:151–8. https://doi.org/10.1016/j.apsusc.2017.01.135.

Furthermore, CdS particles deposited on C60 evolved 1.7 mmol H2 g1 h1, and the stability of the catalyst was studied at the same time. This proved that when using pure CdS, a significant amount of Cd2+ is produced in solution by photocorrosion. The C60CdS hybrid acted as an excellent electron acceptor and at the same time, as a carrier of the photogenerated electrons. On the other hand, it was evidenced that holes migrate to the surface of the fullerene, thus preventing the CdS photocorrosion (see Fig. 17.14) [132]. With respect to CNT-CdS hybrids tested in photocatalytic hydrogen production, hydrogen evolution under visible light at a rate of 1.8 mmol H2 g1 h1 was found. This results were, again, explained in terms of the improved photo-generated charge separation after the chemical interaction of CNT with CdS [133]. Table 17.5 summarizes representative examples of NC-CdS hybrids applied in photocatalytic reactions.

17.4.2 Other sulfides Sulfides are another quite important member of the chalcogenides family. For instance, copper sulfide (CuS) was supported in GR and RGO and tested in the adsorption and degradation of organic pollutants [134–137]. These hybrids were activated by

546

Table 17.5 Most relevant examples of photocatalytic applications of NC-CdS hybrids Nanocarbon

Preparation method

RGO

Reactions conditions

Solvothermal

Photoreduction of CO2

RGO

Hydrothermal

Rhodamine B degradation

GO and RGO

Impregnation and hightemperature reaction Hydrothermal

Water splitting

Visible light, 80 mg catalyst, 500 mL solution of MB and 100 mgL1 of MB, adsorption process was the overnight; irradiation time was 3 h Visible light, 70 mg catalyst, 70 mL solution of MB and 5 mg L1 of Rhodamine B, adsorption process was 30 min, irradiation time was 60 min Visible light, 250 mL 0.01 M Na2S, 0.004 M Na2SO3, 100 mg catalyst, N2 was bubbled for 60, 120 min irradiation Visible light, 25 mg of catalyst, 50 mL solution lactic acid 10%v and 1 wt% of Pt. Degradation of Rhodamine B: visible light, 20 mg of catalyst, 20 mL solution 10 mg L1 Rhodamine, 30 min adsorption, 40 min irradiation Visible light, 180 mL with 15.13 g of Na2S and 5.67 g of Na2SO3, 100 mg catalyst, 300 min irradiation

Fullerene (C60)

CNT

Hydrothermal

H2 generation and rhodamine B degradation

Water splitting

Highlighted properties and results

Reference

The optimal quantity of RGO was 5wt%, The degradation rate was 94% for CdS/RGO, and 57% of TOC was removed.

[122]

The degradation rate of rhodamine B was 95% after 50 min of irradiation.

[129]

Hydrogen generation was 9.6 mL g1 for 2 h using CdS/ GO and for CdS/RGO was 14.6 mL g1 for 2 h H2 generation was 1.73 mmol h1 g1, the removal% of the dye was 97 in 40 min., and optimal concentration of C60 was 0.4 wt%.

[131]

1.771 mmol g1 h1 of H2 was generated using this catalyst, visible light.

[133]

[132] Nanocarbon and its Composites

Applications

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

547

both visible and ultraviolet light, showing an improved photocatalytic activity superior to that presented by pristine CuS. This was attributed to the enhanced separation of the photo-generated charges in the nanocarbons and a high capacity to adsorb pollutants. Another chalcogenide supported on RGO was bismuth sulfide (Bi2S3), which was synthesized using a poly sodium-p-styrenesulfonate, achieving a homogeneous distribution of Bi2S3 nanoparticles on RGO. The use of this nanocarbon increased the PC activity 3.1 times compared to pure Bi2S3 [138]. Additionally, Bi2S3 flower-like particles supported on RGO were used to decolorize a crystal violet dye, reaching 97% of the dye in 100 min with a percentage of 3 wt% in RGO as the optimal concentration [139]. Besides, indium sulfide (In2S3) was supported on graphene using the hydrothermal method, and its photoactivity was measured in the degradation of methyl orange [140]. In this case, after dye excitation with visible light, the photogenerated electrons were transferred to the chalcogenide and then to graphene, as shown in Fig. 17.15. In another illustrative example, antimony sulfide (Sb2S3) was supported on RGO using the hydrothermal method and activated with visible light. It presented a large specific surface area, high adsorption capacity, and the RGO acted as a charge conduction network, inhibiting the recombination process and thereby increasing efficiency in the discoloration process [141]. Another type of chalcogenide is the ternary one. In all the investigations, zinc and indium were the common elements in the synthesis of the ternary chalcogenides [141–147]. These ternary chalcogenides

Fig. 17.15 Schematic illustration showing the reaction mechanism for photocatalytic degradation. Reproduced with permission from An X, Yu JC, Wang F, Li C, Li Y. One-pot synthesis of In2S3 nanosheets/graphene composites with enhanced visible-light photocatalytic activity. Appl Catal B Environ 2013;129:80–8. https://doi.org/10.1016/j.apcatb.2012.09.008.

548

Nanocarbon and its Composites

were supported in two different nanocarbons, RGO and CNT. In particular, ZnIn2S4 supported on RGO showed a god photon absorption in the visible region with a bandgap energy of 2.34–2.48 eV [142, 143]. It was tested in the degradation of 4-nitrophenol under solar light, presenting high photocatalytic activity and stability. Pure ZnIn2S4 also showed good photoactivity under visible light but with severe photocorrosion. Therefore, the stability of ZnIn2S4 supported on RGO was attributed to the formation of a Zn-O-C covalent bond between two species [142]. Furthermore, a comparison of some nanocarbon structures were mixed with ZnIn2S4 and tested in hydrogen generation. The interaction between the ZnIn2S4 particles and the nanocarbon particles achieved a separation of the photo-generated electron-hole, which generated 2641 μmol H2 g1 h1 [143]. ZnCdS was supported on RGO and CNT [144–146], absorbing energy in the visible region of the spectrum and presenting a lower charge recombination, making their efficiency higher than other chalcogenides. A catalytic formulation type Zn0.83Cd0.17S/ CNT was tested in hydrogen generation, obtaining 5.41 mmol H2 h1 g1. This amount of hydrogen was higher compared to that obtained with other chalcogenide mixtures. It was found that the optimal CNT ratio was 0.25 wt%., which also had a strong effect on the chalcogenide crystal size. [144]. Table 17.6 shows some selected examples of NC-chalcogenides with photocatalytic applications.

17.5

MOFs–Nanocarbon

Metal-organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are composed of metal-based centers (single ions and clusters) linked to organic ligands by strong coordination bonds to form a one-, two-, or threedimensional coordination network [147]. Due to their excellent properties, such as an extremely high surface area (>10,000 m2 g1), tunable pore size, and high pore volume [148], MOFs have attracted significant interest in different applications such as catalysis, gas storage and separation, optics, dye adsorption, biomedical imaging, chemical sensing, and drug delivery [149–151]. In the past few years, several investigation groups have begun to explore their potential as photocatalysts [152–155]. Kuc et al. demonstrated through a theoretical study that MOFs are semiconductors with a band gap between 1.0 and 5.5 eV [156]. Garcia et al. showed the photocatalytic properties of MOF-5, allowing increased attention of the MOFs as photocatalysts (Fig. 17.16) [148, 150, 157]. Various MOFs have been studied as photocatalysts for different applications such as MOF-5 [149], UiO-66 (NH2) [158], NH2-Mil-88 (Fe) [159], and NH2-MIL-68(In), among others [160]. Even though MOFs have good photocatalytic properties, they are inferior to those found for inorganic semiconductors due to the low efficiency for light-to-energy conversion and separation of photogenerated electron-hole pairs. Also, it is well known that some MOFs show instability to thermal treatments, moisture, or chemical agents, which limits their application in different areas. Thus, the scientific community is searching for strategies to improve the stability and photocatalytic activity of MOFs (Fig. 17.17) [161]. Fig. 17.16 shows the number of publications of MOFs and MOFcomposite materials as photocatalysts during the past 9 years.

Nanocarbon

Chalcogenide

Preparation method

RGO

WSe2

Solvothermal

Rhodamine B degradation

GR

CuS

Sol-gel

Methylene blue degradation

RGO

Bi2S3

Hydrothermal

2,4 Dichlorophenol

GO

In2S3

Hydrothermal

Methyl orange degradation

RGO CNT

ZnIn2S4

Microwaveassisted and hydrothermal

Hydrogen generation

Applications

Reactions conditions

Highlighted properties and results

Visible light, 30 mg catalyst, 100 mL solution with 20 ppm Rhodamine B, 30 min adsorption, 0.2 mL H2O2 Visible light, 50 mL solution, 80 ppm MB, 2.5 mL H2O2, 30 min adsorption, 150 min irradiation Visible light, 100 mL with 60 ppm pollutant, 0.08 mg catalyst, 120 min irradiation Visible light, 20 mg catalyst, 20 mL with 25 ppm of MO, 120 min was the adsorption and 120 min irradiation 50 mg catalyst, 80 mL Triethanolamine 10%v, N2 was bubbled

The degradation rate was 60% in 3 h.

[181]

The degradation rate was 93% after 80 min; the size particle was 16 nm, 10 wt% of graphene was the optimal quantity. The degradation rate was 92% after 2 h. The RGO 8 wt% was the optimal concentration.

[135]

Reference

[138]

[140]

The generation of H2 for CNT and RGO were 1601.5 and 2640.8 μmol H2 g1 h1, respectively.

[143]

549

Degradation rate was 98% after 120 min. GO was 1% in the hybrid catalyst.

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

Table 17.6 Some examples of NC-Chalcogenides hybrids and their photocatalytic applications

550

Nanocarbon and its Composites

Fig. 17.16 Number of publications on MOFs (blue) and MOFs composites (red) as photocatalysts (based on Scopus database).

Fig. 17.17 Applications more relevant of nanocarbons-MOFs hybrid materials.

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

551

The integration of a variety of functional materials such as GR, CNTs, metal nanoparticles, nanorods, metal oxides, complexes, and even enzymes has been shown to improve the stability and photocatalytic properties of pristine MOFs. This incorporation can be possible due to the pore nature of MOFs that encapsulates some catalytically active molecules or nanoparticles [162, 163]. The incorporation of nanocarbons in MOFs through different methods (one-step solvothermal, two-step solvothermal, one-step hydrothermal, random mixing, single/multiple-face interaction, etc.) has improved their physicochemical and photocatalytic properties, being of great interest for different applications (Fig. 17.18). Table 17.7 shows the more representative studies of nanocarbon-MOF nanocomposites as semiconductors in different applications. Pi et al. synthesized MWCNT/NH2-MIL-68(In) composite materials by the onestep solvothermal method using MWCNTs treated with HNO3 (70% vol.) [164]. All composite materials showed very similar x-ray diffraction patterns, indicating that the crystalline structure of the parental NH2-MIL-68(In) remained intact. The N2 adsorption-desorption isotherm of the hybrids showed a type I isotherm, characteristic of micropore materials, and small hysteresis loops, indicating the presence of a small number of mesopores for the capillary condensation. The interaction between MWCNTs and NH2-MIL-68(In) allowed a slight increase in their BET surface area. They also observed through XPS a negative shift of 0.4 eV in the in the 3D spectrum of the hybrids, confirming the successful incorporation of MWCNTs in NH2-MIL-68 (In). According to these results, the introduction of MWCNTs in NH2-MIL-68(In) contributes to better photocatalytic properties in the reduction of Cr(VI) through the adding of new mesopores for Cr(VI) diffusion, enhancing the visible light absorption, and decreasing the recombination of photo-induced electrons/holes (Fig. 17.19).

Fig. 17.18 (A) Factors controlling the structural stability of MOFs in aqueous media, and (B) methods used to improve the hydrostability and hydrothermal cyclic stability of MOFs. Reproduced with permission from Kumar P, Vellingiri K, Kim K-H, Brown RJC, Manos MJ. Modern progress in metal-organic frameworks and their composites for diverse applications. Microporous and Mesoporous Mater 2017;253:251–65. https://doi.org/10.1016/j.micromeso. 2017.07.003.

552

Table 17.7 Summary of different nanocarbon-MOFs hybrids as photocatalysts in different applications MOF

Preparation method

Applications

Reactions conditions

RGO (1–10 wt%)

UiO-66 (NH2)

Selfassembly

Reduction of Cr(VI)

λ ¼ 420 nm, 20 mg photocatalyst, 40 mL of 10 ppm Cr(VI) T ¼ 30°C, pH ¼ 2

RGO (1.3–3.2 wt%)

Mil-53 (Fe)

One-step solvothermal

Methylene blue degradation

RGO (1–20 wt%)

NH2MIL-125 (Ti)

One-step solvothermal

Methylene blue Degradation

MWCNTs (4.2–18.9 wt%)

NH2MIL68 (In)

One-step solvothermal

Reduction of Cr(VI)

RGO (50 wt%)

UiO-66NH2

in situ growth and mixing methods

Water splitting

125 W high-pressure Hg lamp (λ ¼ 365 nm) and 250 W (λ > 420 nm) 100 mg photocatalyst 200 mL 30 ppm MB 300 W Xenon lamp (λ ¼ 420 nm), 30 mg photocatalyst 40 mL of MB (5.4  105 mol L1) 500 W Xe lamp, 40 mg photocatalyst 50 mL of a K2Cr2O7 and Cr(VI) (10 ppm) pH to 4.0 Visible light 5 mg of photocatalyst 100 mL MeOH or TEOA

Highlighted properties and results RGO enhanced the visible light absorption and more efficient separation of photogenerated electronhole pairs. RGO separate the photogenerated electrons suppressing electron-hole recombination.

Reference [182]

[163]

The synergistic effect of NH2-MIL-125(Ti) and RGO as well as the Ti3+Ti4+ intervalence electron transfer, was observed. MWCNTs generated new mesopores that facilitate the diffusion of Cr(VI).

[165]

The graphene wellwrapped UiO-66-NH2 octahedrons showed a

[167]

[164] Nanocarbon and its Composites

Nanocarbon

Ultrathin graphene oxide (1–11 wt%)

MIL-88A (Fe)

One-step hydrothermal

Rhodamine B degradation

500 W Xe lamp (λ ¼ 420 nm), 20 mg photocatalyst 50 mL of 10 ppm RhB H2O2 (20 mM) and pH ¼7

superior electron transfer ability that inhibited moreefficient the recombination of electronhole pairs. GO improved the separation of photoinduced electron-hole pairs and generated highly reactive O2 species with enhanced activity.

[183]

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

(sacrificial reagent) Erythrosin B (0.05 g) or Rhodamine B (sensitizer)

553

554

Nanocarbon and its Composites 2.5

2.1

PL-3 PL-2 PL-1 NH2-MIL-68(In)

Absorbance (a.u.)

1.8

(a)

(b)

(c)

1.5

(d)

(Ahv)1/2

2.0 1.5 1.0 0.5 0.0

1.2

1.5

2.0

2.5 hv (eV)

3.0

3.5

0.9 0.6 0.3 0.0 200

(A)

MWCNT PL-3 PL-2 PL-1 NH2-MIL-68(In)

300

400

500 600 Wavelength (nm)

700

800

(B) Fig. 17.19 (A) UV-Vis DRS spectra and (B) Schematic illustration of the reduction of Cr(VI) on PL-1 under irradiation. From Pi Y, Li X, Xia Q, Wu J, Li Z, Li Y, et al. Formation of willow leaf-like structures composed of NH2–MIL68(In) on a multifunctional multiwalled carbon nanotube backbone for enhanced photocatalytic reduction of Cr(VI). Nano Res 2017;10:3543–56. https://doi.org/10.1007/s12274-017-1565-8.

Huang et al. used an easy and straightforward synthesis method for the integration of RGO/NH2-MIL-125(Ti) hybrid nanocomposites for methylene blue (MB) photocatalytic degradation under visible-light irradiation [165]. Pristine NH2-MIL-125 (Ti) was synthesized using tetra-n-butyl titanate as a source of titanium. SEM images of RGO-NMTi-x nanocomposite materials revealed particles slightly agglomerated

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

555

with different surface morphology. The use of RGO as a support severely affected the crystalline structure of MOF NH2-MIL-125(Ti), which was attributed to strong interactions between RGO and NH2-BDC linkers provoking a considerable distortion of the TiO5(OH) paddlewheel and modifying the crystal structure. In this case, hybrid materials showed low specific surface areas compared with the pristine MOF NH2MIL-125(Ti). They also showed that the synergistic effect between RGO and NH2MIL-125(Ti) could lead to a significant reduction in the recombination rate of the photogenerated electron-hole charge carriers, increasing the photocatalytic performance in the degradation of MB (Fig. 17.20). MOFs as photocatalysts for hydrogen production have shown an efficiency much inferior to that of the currently used semiconductors [166]. Therefore, Wang et al. showed that proper surface modification of MOF is crucial for the enhancement of MOF-based photocatalytic properties [167]. They used three ways for synthesizing graphene/UiO-66-NH2 octahedron composite materials: (1) random mixing (RCGO/ U6N), (2) single-face interaction (RDGO/U6N), and (3) multiple-face interaction of graphene and UiO-66-NH2 octahedrons (RGOWU6N). All materials were used in the photocatalytic H2 production in the presence of a sacrificial reagent (MeOH and TEOA) and sensitizer (erythrosin B and rhodamine B) under visible light (Fig. 17.21). All hybrid materials showed X-ray diffraction patterns typical of UiO-66-NH2 octahedral crystals. As a result, the photoluminescence of composite materials showed a better separation of photo-generated electron-hole charge carriers due to the participation of RGO. In addition, fluorescence lifetime experiments showed apparently longer lifetimes for the MOF hybrids. However, RGO-WU6N material synthesized by method (3) with 50 wt% of graphene exhibited higher catalytic activity due to the fact that every face of the UiO-66-NH2 octahedrons was covered with RGO sheets, as shown in the TEM and SEM images of Figs. 17.21C and F). Besides, this

Fig. 17.20 (A) UV-vis DRS spectrum of (a) rGO-NMTi-1, (b) rGO-NMTi-2, (c) rGO-NMTi-3, and (d) rGO-NMTi-4. (B) MO degradation photocatalytic performance of (a) no catalyst, (b) NH2-MIL-125(Ti), (c) rGO-NMTi-1, (d) rGO-NMTi-2, (e) rGO-NMTi-3, and (f ) rGONMTi-4. Reproduced with permission from Huang L, Liu B. Synthesis of a novel and stable reduced graphene oxide/MOF hybrid nanocomposite and photocatalytic performance for the degradation of dyes. RSC Adv 2016;6:17873–9. https://doi.org/10.1039/C5RA25689E.

556

Nanocarbon and its Composites DMF

ZrCl4 + BDC-NH2

+

(a)

DMF

+ GO

UiO-66-NH2

(b)

H

2O

+H

Cl

(A)

(c)

(B) Fig. 17.21 (A) Schematic for the preparation of RCGO/U6N (a), RDGO/U6N (b), and RGOWU6N (c). (B) TEM and SEM images of RCGO/U6N (a) and (d), RDGO/U6N (b) and (e) , and RGOWU6N (c) and (f ). Reproduced with permission from Wang Y, Yu Y, Li R, Liu H, Zhang W, Ling L, et al. Hydrogen production with ultrahigh efficiency under visible light by graphene well-wrapped UiO-66-NH2 octahedrons. J Mater Chem A 2017;5:20136–40. https://doi.org/10.1039/ C7TA06341E.

material exhibited after four reaction cycles a good reproducibility, revealing that the graphene well-wrapped UiO-66-NH2 octahedrons inhibit the electron-hole recombination. They mentioned that the higher catalytic activity shown in the RGOWU6N material can be due to its superior electron transfer ability inhibiting the recombination of electron-hole pairs because every face of the UiO-66-NH2 octahedrons was covered with RGO. MOFs are composed of metallic nodes joined in an ordered fashion by organic linker molecules, which could be used as a precursor to prepare metal oxide nanoparticles and porous metal oxide-carbon hybrids via heat treatment at ambient

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

557

(air or nitrogen) conditions [168–170]. In 2011, Yang et al. were pioneers in the preparation of ZnO nanoparticles and ZnO@C hybrid composites via simple heat treatment of MOF-5 under a variety of atmospheric gaseous conditions [168]. Novel ZnO nanoparticles with 3D cubic morphologies were obtained through heat treatment at 600–700°C for 3 h under air flow while ZnO@C composites were prepared by the same method as those used in the case of ZnO nanoparticles. However, in this case, this was done by using a nitrogen flow. SEM and TEM analysis of ZnO@C hybrid composites showed clusters of ZnO with a hexagonal structure embedded in the carbonaceous matrix (Fig. 17.22). Both materials were used in the adsorption and photocatalytic decomposition of RhB dye under UV irradiation. ZnO nanoparticles showed good photocatalytic activity concerning RhB degradation while ZnO@C hybrid materials showed excellent adsorption capacity of organic dyes.

17.6

Multicomponent–Nanocarbon

Nanocarbon multicomponent mainly include ternary systems with the combinations of semiconductor-NC-semiconductor or semiconductor-NC-metal, where the NC that dominates in most of the reported works is graphene of the GO or RGO type. It has found good compatibility at coupling graphene with semiconductors and metals, leading to functional nanomaterials with high stability, photon absorption in the visible region, and improved electrical conductivity [171].

17.6.1 Metal oxide 1-NC-metal oxide 2 In this category, the following systems have been reported: ZnO-graphene-TiO2, Cu2O-RGO-TiO2, WO3-GO-TiO2, TiO2-RGO-Ag2O, Cu2O-RGO-Bi2O3, or others, including five components, ZnO-Fe2O3-Fe3O4-RGO-Cu. Interestingly, this last novel core-shell (rGO@CuZnO@Fe3O4) structured photocatalyst (Fig. 17.23) was synthesized by a complicated method and evaluated in the CO2 photocatalytic reduction with water using visible light. The primary product of the CO2 photoreduction was methanol with a high yield of 2656 μmol/gcat, and it was easily recovered by an external magnet. This superior photocatalytic activity, compared with the use of GO or without any nanocarbon, was attributed to a better charge separation induced by an sp2 hybridization of the aromatic system in RGO [172].

17.6.2 Metal oxide-NC-chalcogenide This type of heterostructure has been devised to have excellent photon absorption in the visible region, mainly by the chalcogenide, and then to transfer the photogenerated electrons to the metal oxide using a nanocarbon. Such is the case of the systems of TiO2-RGO-CdS, Nb2O5-N-doped graphene-CdS, ZnO-RGOCdS, TiO2-RGO-SnS2, and Fe2O3-graphene-MoS2, among others. Similarly as in the previous case, what is pursued with multiheterostructures is an efficient synergistic effect that manifests itself in a high charge-carrier transport rate. For example, an NiO@Ni-ZnO/RGO/CdS photocatalyst has been synthesized by a multistep

558 Nanocarbon and its Composites

Fig. 17.22 (A–C) FE-SEM images, (D) TEM image, (E) EDS spectrum, and (F) EEL spectrum of ZnO@C hybrid. Reproduced with permission from Yang SJ, Im JH, Kim T, Lee K, Park CR. MOF-derived ZnO and ZnO@C composites with high photocatalytic activity and adsorption capacity. J Hazard Mater 2011;186:376–82. https://doi.org/10.1016/j.jhazmat.2010.11.019.

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

559

Fig. 17.23 Plausible mechanism of CO2 reduction by rGO@CuZnO@Fe3O4. Reproduced with permission from Kumar P, Joshi C, Barras A, Sieber B, Addad A, Boussekey L, et al. Core–shell structured reduced graphene oxide wrapped magnetically separable rGO@CuZnO@Fe3O4 microspheres as superior photocatalyst for CO2 reduction under visible light. Appl Catal, B 2017;205:654–65. https://doi.org/10.1016/j.apcatb.2016.11.060.

method and evaluated on the photocatalytic hydrogen generation reaction under UV and visible light [173]. By comparison to Ni-ZnO-RGO, ZnO-RGO-CdS, or CdS, the multicomponent heterostructure of NiO@Ni-ZnO/RGO/CdS showed tahe higher hydrogen production yield of 824 and 524 μmol h1 under UV-vis and visible light, respectively. These results were explained regarding a synergy between ZnO and CdS enhanced by RGO, which forms a photoexcited carrier transport channel and NiO@Ni acted as a cocatalyst in capturing the photo-generated electrons, as shown in Fig. 17.24. Fig. 17.24 Scheme of photocatalytic reaction process in the NiO@Ni-ZnO/ rGO/CdS heterostructure. Reproduced with permission from Chen F, Zhang L, Wang X, Zhang R. Noble-metal-free NiO@Ni-ZnO/ reduced graphene oxide/CdS heterostructure for efficient photocatalytic hydrogen generation. Appl Surf Sci 2017;422:962–9. https:// doi.org/10.1016/j.apsusc.2017.05.214.

560

Nanocarbon and its Composites

Cr(VI) Bi2S3

Visible light

Potential (eV vs NHE)

–1

0

BiOI O2

O2/•O2-(–0.046)

e–

•O2–

e–

e–

e–

CB: –0.74

e–

CB: –0.39

e–

Cr(III)

e– e–

1

•OH

e–

H+

e–

h+

VB: 1.43 Phenol

•OH/h+

h+

VB: 1.09 h+

h+

2

RGO Degradation products

Fig. 17.25 Proposed reaction mechanism for simultaneous Cr (VI) and phenol removal over Z-scheme BiOI/rGO/Bi2S3 system. Reproduced with permission from Chen A, Bian Z, Xu J, Xin X, Wang H. Simultaneous removal of Cr(VI) and phenol contaminants using Z-scheme bismuth oxyiodide/reduced graphene oxide/bismuth sulfide system under visible-light irradiation. Chemosphere 2017;188:659–66. https://doi.org/10.1016/j.chemosphere.2017.09.002.

An all solid-state Z-scheme system containing bismuth oxyiodide (BiOI) and bismuth sulfide (Bi2S3) supported on RGO with applications in the simultaneous photoreduction of Cr6 and the photooxidation of phenol has been recently reported (See Fig. 17.25) [174]. This photocatalytic system was prepared through an electrostatic self-assembly method with a high photocatalytic activity under visible light, achieving optimal reductive and oxidative efficiencies up to 73% and 95%. According to the results, a fast electron-hole separation occurs between two semiconductors through the surface of RGO.

17.6.3 Semiconductor-NC-metal Metal nanoparticle deposition (e.g., Ag, Au, Pt) on semiconductors results in the formation of a Schottky barrier favoring the transport of photogenerated electrons from the CB of the photoexcited semiconductor to the nanoparticles, and of course, suppressing the recombination pathway [175]. On the other hand, depending on the size of the morphology of the deposited metal NP, Ag, Au, and Cu can present the surface plasmon resonance (SPRE) effect, showing the band gap narrowing to the visible region, and higher efficiency in the transport and distribution of charge carriers [175]. For instance, Ag NP and graphene were coloaded on TiO2 via a surfactant-free solvothermal method and tested in the photocatalytic degradation of paraoxon (an organophosphorus compound) [176]. As a result, a 6 wt% Ag 1 wt% graphene

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

561

Fig. 17.26 Schematic illustration of the activation mechanism of Ag nanoparticle and graphene coloaded TiO2 nanocomposite under visible light irradiation for Paraoxon photocatalytic degradation. Reproduced with permission from Keihan AH, Hosseinzadeh R, Farhadian M, Kooshki H, Hosseinzadeh G. Solvothermal preparation of Ag nanoparticle and graphene co-loaded TiO2 for the photocatalytic degradation of paraoxon pesticide under visible light irradiation. RSC Adv 2016;6:83673–87. https://doi.org/10.1039/C6RA19478H.

deposited in graphene showed the highest photocatalytic activity under visible light, resulting in conversion and TOC removal of the pollutant at close to 100% at 100 min of reaction time. An interpretation of these results included better adsorption of the contaminant and high electron mobility, which decreased the recombination of the photogenerated electron-hole pair, as shown in Fig. 17.26. Table 17.8 summarizes the most relevant examples of nanocarbon-multicomponent systems applied in several photocatalytic reactions. In most cases, the improved adsorption of reactants and remarkable photocatalytic performance with visible light were reported.

17.6.4 Semiconductor nanocarbon-MOFs multifunctional materials Metal oxides (TiO2, WO3, CuOx, SnO2, and ZnO) have been extensively used as photocatalysts in a variety of applications, including photocatalytic degradation of various organic water pollutants, fuel generation through water splitting and carbon dioxide reduction, and CO2 reduction to added-value products. However, some metal oxides such as pure ZnO, CeO2, and TiO2 are almost inactive under visible light illumination (a band gap of 3.2 eV). Also, they have shown a low surface area with a small number of catalytic sites responsible for carrying out the chemical reactions, and in some cases, the fast recombination of photogenerated electron-hole pairs has been observed. Significant efforts are being made to suppress the recombination of photo-generated charge carriers, to decrease the band gap, and to increase the number of catalytic sites and reaction centers. The synthesis of MOF composite materials with

562

Table 17.8 Summary of NC-multicomponent materials as photocatalysts in different photocatalytic applications Nanocomposite

Preparation method

Photocatalytic applications

Highlighted properties and results

References

ZnO/GR/TiO2

Solvothermal

Rhodamine B and industrial dyes

[184]

Cu2O/RGO/ TiO2

Photoreduction

Methylene blue degradation

ZnO/NH2RGO/TiO2 Cu2O/RGO/gC3N4

Hydrothermal

Methyl orange degradation

Self-assembled

Methyl orange degradation

TiO2/RGO/Pd

Rhodamine B degradation

Cu2O/GR/TiO2

Hydrothermal and photodeposition Solvothermal

CdS/GR/TiO2

Hydrothermal

Methylene Blue and p-Chlorophenol degradation

ZnO/RGO/TiO2

Microwave

Reduction of Cr(VI)

MoS2/RGO/ CdS nanorods

Different methods were used

Water splitting

ZGT exhibited a high performance for dye wastewaters. Enhanced dye adsorption/separation of the generated electron-hole pairs and decreased band gap. It was observed excellent dye adsorption and improved charge transport. Improved photocatalytic activity and restrained the recombination of electrons and holes. Pd species acted as an electron acceptor, and RGO presented as high electrical conductivity. Cu2O enhanced the light absorption, and graphene reduced the recombination of carriers. The coupling of graphene expanded photoabsorption range increased the adsorption capacity and efficient separation of electronhole pairs. Increased light absorption intensity and the reduction of electron-hole pair recombination. Effective separation of photogenerated charge carriers led to efficient H2 production.

Rhodamine B degradation

[185]

[186] [187]

[188]

[189]

[190]

[192]

Nanocarbon and its Composites

[191]

Rhodamine B degradation

Improved photogenerated charge carriers transfer and separation at the interface.

[193]

Photodeposition

Methyl orange degradation

[194]

NiO@Ni-ZnO/ RGO/CdS

Multistep

Water splitting

WO3/GO/TiO2

Hydrothermal

Degradation of BPA (2,2-bis (4hydroxy-phenyl) propane)

CdS/RGO/TiO2

Solvothermal

TiO2/CNT/ZnO

Sol-gel

Methylene blue and rhodamine B degradation Methyl orange degradation

ZnO/RGO/TiO2

Hydrothermal

Reduction of Cr(VI)

BiOI/RGO/ Bi2S3 TiO2/GO/Ce

Electrostatic self-assembly Sol-gel method and dip coating

Removal of Cr(VI) and phenol

CuO/GR/TiO2

Hydrothermal

Water splitting

AgVO3/RGO/ Ag

Hydrothermal

Degradation of Bisphenol A

Enlarges the contact area and improved the separation efficiency of photoexcited charge carriers. RGO wide visible-light absorption range and Ni NPs as cocatalysts for capturing photoexcited electrons. The electrons transfer from TiO2 to WO3 to GO, and photogenerated holes transfer from WO3 to TiO2 improved the activity. Enhanced photocatalytic efficiency in the degradation of dyes. The addition of TiO2 and ZnO on MWCNT to promoted dye decomposition increases the photocatalytic activity. Increased light absorption intensity and the reduction of electron-hole pair recombination. RGO was used as an electron “mediator” between BiOI and Bi2S3. Improved the BET surface area and the surface hydroxyl content and induce redshift of the sample to visible light response. The synergistic effect suppressed charge recombination, improve interfacial charge transfer, visible-light adsorption and activity. Exhibited excellent light-trapping ability, absorbance in the visible light region and facilitated charge transfer.

Degradation of formaldehyde

[173]

[195]

[196] [197]

[198] [174]

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

Hydrothermal

TiO2 nanofibers/ RGO/Ag2O TiO2/RGO/Au nanoparticles

[199]

[200]

[201] 563

Continued

564

Table 17.8 Continued Preparation method

Photocatalytic applications

Highlighted properties and results

References

Hydrothermal

Water splitting

[202]

Chemical method at low temperature Dispersion

Methylene Blue degradation

Enhanced the separation efficiency of photogenerated charge carriers. RGO sheets showed efficient separation of electron-hole pairs.

Water splitting

TIO2/RGO/Ag

Via a three-step process Hydrothermal

Cu2O/RGO/ Bi2O3

In situ precipitation

Tetracycline (TC) degradation

TiO2/GR/Bi2O3

Hydrothermal

Rhodamine B degradation

CeO2/NGR/Cu

Ultrasound

Photo-reduction of CO2 to fuel

BiVO4/GO/ Bi2O3

Chemical bath deposition method Hydrothermal

Bisphenol-A (BPA) degradation

Nanocomposite CdS NPs/NGR/ Nb2O5 nanorods CdS NPs/RGO/ ZnO CdS NPs/GR/ MoS2 Cu2O/RGO/Pd

Methylene blue degradation

Photoreduction of CO2 to fuel

2D graphene plays a key role as an efficient electron mediator. Allow the harnessing of the photo-induced charge flow for efficient e-h separation. The synergetic effect of plasmonic Ag and RGO enhanced photoactivity with visiblelight. Z-scheme photocatalytic system favored the transfer of photogenerated electrons and holes toward an effective path. The heterostructures showed an efficient reduction in the recombination of electronhole pairs. NG Cu(II) complex as an artificial enzyme for the reduction of CO2 to methanol fuel was successfully fabricated. GR accelerated the interfacial electrontransfer rate improved separation of photogenerated charge carriers. The restoration of the sp2 hybridized aromatic system in RGO facilitated the movement of electrons.

[204] [205] [206]

[207]

[208]

[209]

[210]

[172]

Nanocarbon and its Composites

CuZnO/RGO/ Fe3O4 microspheres

Water splitting

[203]

Two-step hydrothermal

Water splitting

BiOBr/GR/Er

Hydrothermal

Rhodamine B degradation

CdS/GO/ TAON-Pt

Hydrothermal

Water splitting

Cu2O/RGO/gC3N4

Facile selfassemble approach Liquidexfoliation and solvothermal Hydrothermal and photodeposit Hydrothermal

Methylene blue and methyl orange degradation

Water splitting

Hydrothermal

Rhodamine B degradation

Solvothermal

Water splitting

Photoreduction

Water splitting

SnS2/RGO/ TiO2 TiO2/GR/Pd

ZnS/GR/MoS2 nanosheets Bi2WO6 nanosheets/ RGO/Ag ZnIn2S4/RGO/ MoS2 Nitrogen-doped La2Ti2O7/RGO/ Au NPs

Rhodamine B degradation

Rhodamine B degradation

The photoexcited electrons of CZTS can be readily transported to MoS2 through RGO backbone, reducing the electron-hole pair recombination. Reduced energy band gap and promoted charge separation and transmission over the hybrid photocatalyst. Altered the energy levels of the conduction and valence bands and efficiently lengthen the lifetime of the photogenerated charge carriers. The Cu2O/g-C3N4 heterojunction absorbed visible light region shifted to lower.

[211]

[212]

[213]

[187]

Reduced the recombination of electron/hole pairs and enhanced the rate of electrontransfer for the degradation of dye molecules. Higher available surface area.

[214]

Graphene serves as an excellent electron acceptor and transporter. The effect plasmonic of Ag NPs enhanced the generation and separation of photogenerated charge carriers of Bi2WO6. The photo-generated electrons and holes were suppressed efficiently. Nitrogen doping extends the light absorption range.

[215]

[188]

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

MoS2/RGO/ Cu2ZnSnS4

[216]

[217] [218]

565

Continued

566

Table 17.8 Continued Nanocomposite

Preparation method

Photocatalytic applications

Highlighted properties and results

References

Degradation of different antibiotics such as Tetracycline (TC), Oxytetracycline, Ciprofloxacin, and Doxycycline Water splitting

The high charge carrier mobility of NGQDs and the p-n junction photocatalytic systems, which greatly promoted efficient separation of charge carriers. The interfacial contact between g-C3N4/CdS accelerated the separation and transfer of photoinduced charge carriers and enhanced visible-light absorption. The transfer of conduction band electrons from rutile to anatase improved the charge separation efficiency. The formation of multiheterojunctions promotes the efficient separation of photoinduced electron-hole pairs. high electron conductivity and a low degree of hydrophobicity The synergistic effects played an important role in light absorption, charge separation and transfer, and photo-corrosion. Inhibited the photocorrosion of pure Ag3PO4

[219]

Hydrothermal

RGO g-C3N4/ CdS nanorods/ Pt

Wet-chemical

Anatase TiO2/ RGO/RutileTiO2 Flower-like Bi2O2CO3/GO/ TiO2 Ru/SrTiO3:Rh/ RGO/BiVO4 ZnO/graphene quantum dots/ Cu Ag3PO4/RGO/ BiVO4/Ag

Surfaceassembling strategy Two steps hydrothermal

Water splitting

Photoreduction

Water splitting

Spin-coating and annealing process In situ precipitation and photoreduction Liquidprecipitation

Rhodamine B (RhB) degradation

Hydrothermal

Rhodamine B degradation

g-C3N4/RGO/ anatase TiO2 g-C3N4/RGO/ Bi2MoO6

Methyl orange degradation

Tetracycline degradation

Methylene blue degradation

Improved oxygen-reduction capacity and the formation of hydroxyl radicals driven by the holes in TiO2 Graphene was used as the electron mediator in the Z-scheme system.

[220]

[221]

[222]

[223] [224]

[225]

[226]

[227]

Nanocarbon and its Composites

Quantum dotsBiOI/NGR/ MnNb2O6

Hydrothermal

Methylene blue degradation

TiO2/RGO/Au NPs

Microwaveassisted-and hydrothermal Hydrothermal

Water splitting

Methylene blue degradation

LaMnO3/RGO/ Fe3O4 SnO2/GO/TiO2

Coprecipitation

Methylene blue degradation

Solvothermal

TiO2/RGO/Pt, Pd, Ag, and Au NPs ZnWO4/RGO/ Fe3O4

Solvothermal

Congo red and methylene blue degradation Photoreduction of CO2

Microwave

Methylene blue degradation

WO3/MWCNT/ TiO ZnS/GO/CdS/Pt

Hydrolysis and impregnation Irradiationassisted

Oxalic acid degradation

CdS/RGO/gC3N4

Solvothermal

H2 generation and degradation of Atrazine

MoS2/GR/ Fe2O3

Water splitting

Enhanced photoabsorption range, adsorption capacity and the efficient separation of electron-hole pairs. Au NPs broadens the visible light response of TiO2 due to the surface plasmon resonance (SPR) effect. MoS2 and graphene components extended the light response and acted as a charge transfer medium, respectively. Enhanced the separation efficiency of electron-hole and large surface contact area. Generates more free charge carriers that induce surface chemical reactions. Enhanced utilization of visible light and efficient electron transfer in the noble metaldoped GT nanojunctions. Carrier exploitation efficiently by tolerating the photoexcited electron-hole pairs and thus encouraging oxidative degradation of the pollutants. Active composites were obtained. GO constructed a carrier transport channel between ZnS and CdS to enhance cooperative effects. RGO as a mediator played an important role in accelerating electron transfer in the Z-scheme process.

[190]

[228]

[229]

[230] [231] [232]

[233]

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

TiO2/RGO/CdS

[234] [235]

[236]

Continued 567

568

Table 17.8 Continued Preparation method

Photocatalytic applications

Highlighted properties and results

References

Eu2O3/ Mesoporous Graphene/TiO2

Hydrothermal

4-Chlorophenol degradation

[237]

CdS/GO/ZnO

Hydrothermal

Photocatalytic hydrogen generation and organic dye degradation

MoS2/RGO/ CdS

Photoreduction

Water splitting

CdS QDs/GR/ ZnIn2S4

Hydrothermal

Water splitting

ZnFe2O4/GO/ ZnO

Ultrasound

Methyl orange degradation

MoS2/RGO/ ZnO TiO2/GO/Ce

Hydrothermal

Methylene blue degradation

Sol-gel

Formaldehyde degradation

TiO2/GR/Ag NPs

Solvothermal

Paraoxon pesticide degradation

BiOI/RGO/ Bi2S3 CdS/RGO/ZnO

Electrostatic self-assembly Hydrothermal

Reduction of Cr(VI) and phenol degradation Methyl orange degradation

ZnO/RGO/TiO2

Hydrothermal

Reduction of Cr(VI)

The enhanced surface area with narrow band gap and suppressed electron-hole recombination enhanced OH radical formation. Enhanced surface area and efficient separation of photoinduced charge carriers due to the presence of GO Separation of photoexcited charges by rGO and increased absorption of visible light absorption were observed. Enhancement the ZnIn2S4 photostability, enhanced light absorption and improved separation of photogenerated carriers. The recombination of photo-generated electron-hole utilization of visible light region due to its narrow bandgap. Graphene as a support material enhanced pollutant adsorption and electron transport. The induced redshift of the sample to visible light response. Graphene due to its high electrical conductivity diminishing the recombination rate of the photogenerated pairs. The photoinduced electrons in the conduction band increased its photocatalytic activity. Enhanced photogenerated charge separation, charge transfer and adsorption of dye. Increased light absorption intensity and the reduction of electron-hole pair recombination.

Nanocomposite

[238]

[239]

[240]

[241]

[242]

[176]

[174] [243] [198]

Nanocarbon and its Composites

[199]

Precipitation

Rhodamine B degradation

CdS/RGO/UiO66 ZnO/GO/CuBTC Ce/RGO or GR/ UiO-66

Solvothermal

Water splitting

One-step solvothermal One-step solvothermal

Water splitting Nitroaromatic compounds degradation

Solvothermal

Water splitting

In situ growth and mixing Two-step hydrothermal

Water splitting

MoS2 quantum dots/GO/UiO66-NH2 Pt/RGO/UiO66-NH2 SnO2 NPs/ RGO/UiO-66

Rhodamine B degradation

GO, graphene oxide; GR, graphene; NGR, N-doped graphene; and RGO, reduce graphene oxide.

Effective interaction between RGO and RhB molecules improved the charge carrier separation and photoactivity. Improved the numbers of catalytic sites and minimize recombination of charge carriers. Hradical was encapsulated in the Cu-BTC MOF channel and stabilized. Ce was introduced as “mediator” improving the electron transfer and photoactivity in the visible region. The synergetic effect of MoS2 QDs enhancing photoactivity. Pt cocatalyst for increasing the hydrogen production. Improved RhB adsorption capacity and photocatalytic performance were observed.

[244]

[177] [178] [179]

[245]

[167] [246]

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

BiOI/RGO/AgI

569

570

Nanocarbon and its Composites

graphene oxide, graphene, or carbon nanotubes has allowed the reduction of recombination between photo-induced electron-hole pairs and expanded the photoabsorption range to the visible light region. Recently, the integration of metals or metal oxides into MOF composite materials with graphene oxide (GO), graphene (GR), or carbon nanotubes has shown an improvement in the photocatalytic performance. For instance, cadmium sulfide (CdS) is a most-promising semiconductor for photocatalytic hydrogen production. However, CdS has some drawbacks such as fast recombination of photogenerated electron-hole pairs as well as a low surface area with a small number of catalytic sites. Lin and coworkers designed an UiO-66/CdS/RGO ternary composite for the photocatalytic hydrogen production under visible light [177]. They observed in the pure CdS a deficient hydrogen production; however, when CdS NPs were dispersed into the MOF UiO-66 surface and incorporated RGO, an enhancement in the photocatalytic activity was observed. They explained their results regarding a better dispersion of CdS on UiO-66 increasing the number of catalytic sites and reaction centers, as well as RGO, minimizes the recombination of charge carriers. ZnO is another widely used semiconductor in photocatalysis due to its low cost, chemical inertness, nontoxicity, ready availability, and stability. However, ZnO is limited by a large band gap energy (3.37 eV) and a fast recombination rate of photogenerated electron-hole pairs. Shi et al. synthesized a multicomponent material based in Cu-BTC and a ZnO/graphene oxide system that was used in photocatalytic hydrogen production [178]. They demonstrated that the electrostatic interaction of Cu-BTC with ZnO/GO could encapsulate and stabilize Hradical reaction intermediates, increasing its recombination to form molecular hydrogen. Yang et al. showed the effect of doped metal ions such as Ce in the skeleton of MOF UiO-66 for the photocatalytic reduction of nitroaromatic compounds under visible light [179]. Ce ions were doped into UiO-66 (Zr) nanostructures by the solvothermal method using CeCl2 as a cerium ion precursor. In this case, Ce ions introduced in MOF as an “electron mediator” can improve the electron transfer as well as decrease the recombination of electron-hole pairs (Fig. 17.27). It is important to mention that few studies have been realized using MOF-based multicomponent materials in photocatalytic applications so far. Fig. 17.27 Possible mechanism for the photocatalytic reduction of nitrobenzene over GR/Ce-UiO(10). Reproduced with permission from Yang Z, Xu X, Liang X, Lei C, Gao L, Hao R, et al. Fabrication of Ce doped UiO-66/graphene nanocomposites with enhanced visible light driven photoactivity for reduction of nitroaromatic compounds. Appl Surf Sci 2017;420:276–85. https://doi.org/ 10.1016/j.apsusc.2017.05.158.

Recent advances of nanocarbon-inorganic hybrids in photocatalysis

17.7

571

Conclusion

A large number of nanocarbon-semiconductor (NSH) and multicomponent hybrid materials have been analyzed in terms of the preparation methods used, the photocatalytic reactions in which they have been evaluated, and a possible explanation of their behavior. The review was conducted with three types of nanocarbon: fullerenes, CNTs, and graphene while the considered semiconductors were metal oxides (e.g., TiO2, ZnO, WO3, CuxO), metal sulfides (e.g., CdS, ZnS), and MOFs, among others. Most works were devoted to obtaining active, selective, and stable metal-free photocatalytic systems, which provided, in some cases, a better photocatalytic performance by using NSH. In most cases, the hybrid materials showed a higher adsorption capacity, better UV and visible light absorption, and a higher photocatalytic activity than the bare semiconductor as well as better stability. Even though the number of publications related to the NSH applications in photocatalysis has increased exponentially in the last 5 years, works of fundamental character to understand or design its operation were scarce in this period. Five recurring explanations of the NSH improved properties were noted: (i) better charge carrier separation, (ii) reduction of the band gap value and changes in the CB position of the semiconductor, (iii) evidence of nanocarbon-semiconductor chemical interaction, (iv) charge carrier transfer between them, and (v) nanocarbon photo-sensibilization effect. In particular, graphene (i.e., graphene oxide (GO) and reduced graphene oxide (RGO) were the most nanocarbon sources used in the preparation of NSH. In fact, a few photocatalytic applications of fullerenes or CNT-TiO2 hybrids were reported in comparison with graphene-TiO2. Significant advances in the use of RGO-TiO2 were detected, mainly in photocatalytic hydrogen production and CO2 photoreduction. The most important result obtained with CNT and graphene combined with ZnO was a higher photocatalytic activity and stability and diminishing ZnO photocorrosion. Some reaction mechanisms were proposed to explain the stability of ZnO using the electron and hole transfer mechanism. It seemed that after CNT photosensitization under visible light, electrons are transferred to the ZnO CB and holes from ZnO VB to the CNT, inhibiting ZnO photocorrosion. The use of copper and tungsten oxides combined with RGO played a significant role in the selectivity control for the CO2 photocatalytic reduction while the former produces methanol and the last generates methane. It is worth noting that NC-chalcogenide (e.g., CdS) hybrids were good candidates for hydrogen production under visible light. However, considerable work remains to be done to elucidate the reaction mechanism and to control the stability under different reaction media. A remarkable hydrogen production (2641 μmol H2 g-1 h-1) was obtained with a ternary chalcogenide ZnIn2S4 deposited on RGO. Nanocarbon-MOFs hybrids have been a little-explored field, especially in the case of photocatalytic reactions of hydrogen production or reduction of CO2. According to the reported works, the MOF is activated with visible light and then easily transfers electrons to the nanocarbon favoring the oxygen reduction reaction as well as

572

Nanocarbon and its Composites

increasing its stability. Multicomponent NCS hybrids were a combination of two species (metal oxide, chalcogenide, or metal) with nanocarbon to promote electron transfer between them assisted with nanocarbon. Nevertheless, due to the complexity of the participating components and the different concentrations used for each one, much fundamental work is required to understand their photocatalytic performance.

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Synthesis of nanocarbon– polyaniline composite and investigation of its optical and electrical properties

18

Manoj B. Department of Physics and Electronics, Christ (Deemed to be University), Bengaluru, India

Chapter Outline 18.1 Introduction 589 18.2 Materials and methods

590

18.2.1 Dielectric characterization 591

18.3 Result and discussion

592

18.3.1 Structural analysis 592 18.3.2 FTIR analysis 594

18.4 Electrical characterization–determination of energy gap 18.5 Dielectric characterization 597 18.6 Conclusion 599 References 599

18.1

596

Introduction

Conducting polymers have attracted tremendous attention in the domain of nanocomposites, owing to their high conductivity and redox behavior. Their applications range from display devices to energy conversion to supercapacitors to sensors, to name just a few. Polyaniline (PANI) has the property of fast switching behavior due to its redox states and has a high degree of electrochemical reversibility. Polyanilinebased electrode materials were widely used in supercapacitor/electrochemical capacitors. These materials have a low cost, good thermal stability, a multiredox state, and a high range of electrical conductivity [1–6]. The ability to hybridize carbon-based nanomaterials such as graphene/GO with conducting polymers to form composites has generated great attention and is of great scientific and industrial relevance. In specific, nanocarbons, when compounded with conducting polymers, can boost the electrical conductivity and mechanical strength of the resulting composites. These characteristics enable nanocarbon-based conducting polymer composites to be prime candidates for energy applications and a promising Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00018-3 © 2019 Elsevier Ltd. All rights reserved.

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Nanocarbon and its Composites

pseudocapacitive electrode material for supercapacitors. PANI has excellent environmental stability and biocompatibility [1–4]. Nanocarbon (NC) structures have novel properties such as high electrical conductivity, high surface area, and size-dependent fluorescence, making them a promising candidate for energy storage devices, bioimaging devices, and fuel cells. A composite of NC and PANI will have the dual advantages of both. Another interesting material, graphene oxide, is promising because of its unique structure, low weight, and wide use as a catalyst, gas sensor, fuel cell, and supercapacitor. There are reported studies on the performance enhancement of supercapacitors with multilayer graphene oxide– polyaniline composites [4–7]. The addition of graphene oxide to polyaniline enhances the electrochemical performance of polyaniline to the manifold. The development of an NC–polyaniline composite aims to enhance the unique properties of each material for energy-harvesting applications. The composite material is expected to have a better performance than the original material. These composites can be easily applied to aerospace industry devices such as sensors, radar-absorbing systems, and energy-storage elements. The high fluorescence property makes them a suitable material for use as fluorescent sensors. The tunable dielectric properties make them suitable for stealth applications and energy-storage devices. One can incorporate the NC within a polymer matrix, which can enhance the physical properties of the host polymer. The most suitable graphene derivative for the polymer–graphene composite must be derived from an abundantly available precursor, due to the scalable production and low cost. Coal plays a vital role in the energy needs of our nation. It has a complex structure and has performed sp2 carbon domains of variable size in the nanometer range. These nanocrystallites are linked together in a three-dimensional network of covalent bonds with epoxy, carboxyl, or esters and noncovalent interactions such as hydrogen bonding and π–π interactions. These nano islands can be extracted with the aid of a reactive solvent by simple oxidation or chemical reflux. By controlling the size of a crystallite during the carbonization of the precursor lignite, a series of oxygenated carbon dots with tuned size distribution and fluorescence emission can be obtained [6–12]. The synthesis of the PANI nanocomposite from bituminous coal is an area in research that is least explored. Herein we report the performance of the polyaniline nanocomposites with reduced bituminous coal (RBC) as filler with varying concentration. The impact of the filler concentration in the performance of PANI, such as the conductivity and dielectric properties, is studied.

18.2

Materials and methods

Five grams of finely powdered bituminous coal (BC) are treated with dil.HNO3 (1:5) in distilled water. The solution is sonicated for mixing for 30 min, followed by magnetic stirring at room temperature for 24 h. The mixture is then treated with a 1 M NaOH solution (300 mL) to bring the pH of the solution to 7 using the titration method. The solution was kept for magnetic stirring for 1 h and was centrifuged for 1 h at 11,300 rpm.

Synthesis of nanocarbon–polyaniline composite

591

The supernatant and residue are separated and the residue is kept for dialysis for better separation of the particles. The residue is heated and a final yield of 3.89 g of the nanocarbon (RBC) is obtained. 0.3 M aniline (1.39 mL aniline in 48.6 mL distilled water) and 1 M HCl (8.8 mL HCl in 91.2 mL of distilled water) are added to 0.9 g of GO and sonicated for 1 h. 80 mL of 0.9 M of (NH4)2S2O8 (10.47 g in 50 mL) and 20 mL 3.6 M aniline (9.18 mL in 322 mL) with 20 mL of 1 M HCl are added to the solution and kept for magnetic stirring at room temperature until the solution turns green, indicating the process of polymerization. The sample is centrifuged and the residue and supernatant are separated. The residue is kept for filtration and is washed with distilled water, ethanol, and acetone until the sample becomes colorless. The residue is further heated to remove the water content and the final yield is found to be 2.418 g (PBC-A). The GO–PANI composite is prepared by changing the concentration of GO to 0.45 g to obtain the product PBC-B. The synthesized samples PBC-A and PBC-B along with bituminous coal (BC) and nanocarbon (RBC) are characterized by XRD, FTIR, UV–Vis, and CHNS analysis. The dielectric and AC conductivity of the samples are determined by the impedance analyzer while the energy gap of the material is determined from the UV–Vis spectrum.

18.2.1 Dielectric characterization The prepared pellets of each sample are kept for characterization by analyzing the dielectric properties using a precision impedance analyzer. The dielectric parameter as a function of frequency is described by the complex permittivity [4]. Complex dielectric constant, Ɛðf Þ ¼ Ɛ0 ðf Þ  iƐ00 ðf Þ

(18.1)

where the real part Ɛ0 and the imaginary part Ɛ00 are the components for energy storage and energy loss, f is the applied frequency. Real dielectric constant,  Ɛ0 ¼ Cp ∗ t =Ɛ ∗ A

(18.2)

where Cp is the capacitance in parallel and A is the area of the pellet. Imaginary dielectric constant, Ɛ00 ¼ Ɛ0 = ω∗ Cp ∗ Rp



(18.3)

where Rp is the resistance in parallel. Loss factor/loss tangent, tan δ ¼ Ɛ00 =Ɛ0

(18.4)

AC conductivity σ AC ¼ 2ᴨfƐƐ0 tan δ

(18.5)

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Nanocarbon and its Composites

The energy gap is determined by Tauc’s equation ðαhνÞn ¼ An hν  An Eg

(18.6)

where α ¼ absorption coefficient, which is given by α ¼ log (1/T)/thickness; T ¼ 10 A; ν ¼ applied frequency; A ¼ absorptivity; Eg ¼ energy gap ¼  y intercept/slope; n ¼ 2, direct Eg; 1/2, indirect Eg; 2/3, direct forbidden Eg; 1/3, indirect forbidden Eg.

18.3

Result and discussion

18.3.1 Structural analysis The structural characterization of the precursors and composite was investigated by X-ray analysis and is presented in Fig. 18.1. The diffraction profiles exhibit a clear asymmetric peak around 20 degrees, attributed to the γ peak. It indicates the presence of saturated structures such as aliphatic side chains, attached to the edge of the carbon crystallites. The (002) peak around 25 degrees indicates the spacing of the aromatic ring layer while the (γ) band reflects the packing distance of saturated structures. These observations suggest that the carbon crystallites in the product have an intermediate structure between the graphite and amorphous states called the turbostratic structure or the random layer lattice structure [12–17]. The parameters obtained after deconvolution, as depicted in Table 18.1. The lattice parameters of the samples were calculated by the modified Scherrer formula [8–12]. The lateral sizes (La) and stacking heights (Lc) of the layer structures of the composite range from 1.21 to 1.83 nm and 3.11 to 3.35 nm, respectively. This indicates the formation of a nanocarbon structure in the sample. The stacking height of the PBC-A and PBC-B increased due to the attachment of the nitrogen group to the carbon lattice (Table 18.2). The interlayer spacing (d002) of the crystalline structure ranges from 0.34 to 0.35 nm. The aromaticity (fa) of the virgin coal sample (BC) was 0.21 and it increases to 0.92 after chemical treatment (RBC) and changed to 0.83 with polymerization [8–12]. By analyzing the X-ray spectrum of the PBC-A and PBC-B samples (not shown), it is noticed that the peak position and intensity have substantially changed. This shift is attributed to the PANI–GO interaction and crystallographic sheet changes. The graphite oxide synthesized from coal has oxygen functional groups at its basal planes and edges, providing compatibility with polymer matrices. The GO surface functions as a nucleation site for the polyaniline, owing to the oxygen functionalities on the GO plane. The XRD profile of the GO–PANI composite (PBC-A and PBC-B) shows broadening. This broadening coupled with a decrease in intensity suggests the removal of water from the composite, which suggests the possible interaction between PANI and GO. The X-ray analysis confirmed the structural change of coal to a few-layer reduced graphene and formation of a PANI–graphene oxide composite. The CHNS analysis confirms the improvement of nitrogen content after the polymerization.

800

BC 700

1000

RBC

600 800

Intensity

Intensity

500 600

400 300

400 200 200

100 0

0 10

20

30

40

50

60

10

20

30

40

50

60

Angle (2q)

Angle (2q)

Synthesis of nanocarbon–polyaniline composite

1200

PBC-A

800

Intensity

600

400

200

0 10

20

30

40

50

60

Fig. 18.1 XRD spectra of bituminous coal (BC), reduced bituminous coal (RBC), and polymerized nanocomposite with 0.9 g RBC (PBC-A).

593

Angle (2q)

594

Nanocarbon and its Composites

Table 18.1 Structural parameters elucidated from the XRD analysis Sample

La (nm)

Lc (nm)

d002 (nm)

fa

N

n

BC RBC PBC-A PBC-B

1.21 1.95 1.83 –

3.11 2.78 3.35 20.6

0.347 0.349 0.353 0.334

0.21 0.92 0.83 –

10 9 11 63.42

31.76 25.90 35.21 1287.24

La, lateral dimension in nm; Lc, stacking height in nm; d002, interlayer spacing; fa, aromaticity; N, number of aromatic lamellae; n, number carbon atoms.

Table 18.2 The elemental analysis (CHNS) of the synthesized composite Sample

N% (wt%)

C% (wt%)

S% (wt%)

H% (wt%)

O% (wt%)

BC RBC PBC-A PBC-B

1.52 3.12 8.73 9.29

64.67 55.46 59.41 58.04

1.16 0.57 1.76 2.22

2.60 2.77 3.21 3.97

30.05 38.08 26.89 26.45

The result of the CHNS analysis of the samples is presented in Table 18.2. The nitrogen content of the sample PBC-A is higher after aniline treatment and is attributed to the formation between the nanocarbon–polymer composite. It is surprisingly observed that the polymerization is higher in the PBC-B sample when the concentration of RBC changed to 0.45 g. The increased oxygen content of the sample RBC indicates that chemical oxidation with nitric acid enhances the formation of graphene oxide in coal. From the X-ray and CHNS analysis, the formation of the hybrid composite of PANI–GO is confirmed. The peak broadening is noticed in all the X-ray spectra and is due to the interaction between PANI and GO, which affects the mechanical, electrical, and thermal stability. In order to understand the nature of these interactions, FTIR analysis and dielectric analysis are carried out.

18.3.2 FTIR analysis The FTIR analysis of the virgin and synthesized products is recorded and presented in Fig. 18.2; the corresponding functional groups are identified in Table 18.3. Great similarity is observed between the BC and RBC spectra. PBC-A and PBC-B show different structures, owing to the interaction between PANI and GO. All the samples show the presence of C]C, which is the characteristic of the sp2 hybridized carbon. With treatment, this band is blue shifted to 1567 cm1. IR analysis also confirms the incorporation of a nitrogen group after the in situ polymerization of the PBC sample [12–18]. This finding supports the results of CHNS analysis. In the region from 4000 to 1800 cm1, the spectra showed a broad and intense band due to the stretching of OdH and NdH in the hydroxyl and amine groups. This band

Synthesis of nanocarbon–polyaniline composite

595

Fig. 18.2 IR band assignment of bituminous coal (BC), reduced bituminous coal (RBC), and polymerized composite with 0.9 g RBO (PBC-A).

Table 18.3 FTIR band assignment of PANI–GO composite

S. No.

Wave number (cm21)

Assignment

BC

RBC

PBC-A

PBC-B

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

790 821 1033 1035 1141 1300 1444 1481 1567 1607 1720 3436

δ(C]C) δ(CdH) ν(S]O) ν(S]O) ν(CdO) (CdN) δ(CdH) (C]C) ν(C]C) ν(C]C) ν(CdO) ν(NdH)

– ✓ ✓ – – – – – – ✓ – –

– – – ✓ – – – – – – ✓ –

– ✓ – – ✓ ✓ ✓ ✓ ✓ – – –

✓ – – – – – ✓ – ✓ ✓

may be associated with the hydrogen bond between NH from PANI and possible oxygenated groups from GO. This is very strong in PBC-B, owing to the optimization of PANI to the GO matrix. In the range from 1800 to 1600 cm1, the samples shows unique properties of GO and PANI–GO composites. The π–π* stacking of the aromatic carbon present in the GO is very strong and has a predominant role in sharing

596

Nanocarbon and its Composites

of electron between GO and PANI structure. The bands at 1720 and 1607 cm1 originate due to the C]O stretching of the COOH group and the C]C in the aromatic rings, respectively. These bands are present in both PBC-A and PBC-B nanocomposite. But the majority of the peaks show an increase in intensity in PBC-B (not shown). In the region of 1600–1350 cm1, the bands at 1494 and 1567 cm1 are attributed to the benzenoid and quinoid structures. The 1494 mode is red-shifted in PBC-A compared to PBC-B while the 1559 cm1 mode is blue-shifted along with a reduction in the benzenoid structure. This is attributed to the greater availability of the electron from the graphite oxide in π–π stacking. The mode observed at 1300 cm1 in the spectrum of the composite is owing to the CdN stretching of the secondary amine. This mode is present in PBC-A and is related to the alternation of the benzenoid and quinoid structures. The band at 1141 cm1 arises due to the p electron cloud in the PANI chain. This structure is ideal to evaluate the degree of doping. This effect is more prominent in the composite, favoring better conductivity. The IR analysis also confirms the presence of the CdO and NdO functional groups after polymerization. This result is in support with the result of X-ray analysis. The IR analysis confirms the optimization of the PANI–GO composite at a lower concentration of graphite oxide (RBC).

18.4

Electrical characterization–determination of energy gap

The energy gap of the composite is determined by plotting the Tauc plot from the UV–Vis analysis and is presented in Table 18.4. It is found that the material formed is a direct band gap semiconductor with an energy gap of 3.29–4.27 eV. They are in the range of organic semiconductors or compound semiconductors [5]. With the incorporation of polymer in the graphene oxide matrix, the energy gap is found to be increasing. The bandwidth is controlled by adding the graphite oxide to PANI and is found to be marginally increased with decreasing concentration of GO in the composite.

Table 18.4 Energy gap of the composite from the Tauc plot Sample

Optical energy gap (eV)

BC RBC PBC-A PBC-B

3.29 3.82 4.06 4.27

Synthesis of nanocarbon–polyaniline composite

18.5

597

Dielectric characterization

The real and imaginary parts of the dielectric strength of the samples up to 1 MHz frequency are presented in Figs. 18.3 and 18.4. It is found that the value of dielectric strength lowers with a rise in frequency. Due to the accumulation of charges at the grain boundary, the dielectric constant is more at low frequencies (space charge polarization). With the change in frequency, the dielectric constant decreases because the space charge polarization diminishes gradually; the electronic and atomic contribution dominates [2–4, 18–22]. It is found that the real part of the complex permittivity increases with more concentration of reduced carbon in the composite (0.9 g). The permittivity decreases with a decrease in reduced carbon content. The imaginary part of the complex permittivity shows a different trend with the polyaniline composite. As the concentration of the RGO in the composite increases, the permittivity starts Fig. 18.3 Frequency versus real part of complex permittivity (Ɛ0 ).

Bc

Real part of complex permittivity (e¢)

RBC PBC(A) PBC(B)

60

30

0 2

4

6

Imaginary part of complex permittivity (e ²)

log f (Hz)

Fig. 18.4 Frequency versus imaginary part of complex permittivity (Ɛ00 ).

Bc RBC PBc(A) PBc(B) 800

400

0

2

3

5

4

log f (Hz)

6

7

598

Nanocarbon and its Composites

Fig. 18.5 Variation of frequency with dielectric loss (tan δ).

BC RBC PBC(A)

30

PBC(B)

tan d

20

10

0

2

4

6

log f (Hz)

decreasing. It is observed that the dielectric strength of the material (real and imaginary) increases with the addition of reduced carbon to aniline. Even though there is a change of dielectric strength with frequency at lower range, it reaches a maximum at a frequency of 10,000 Hz and then starts to decrease up to 1 MHz. This material can work as a frequency-dependent organic semiconducting material. The relationship between dielectric loss tan δ and log of frequency is presented in Fig. 18.5. The dielectric loss is the measure of energy dissipation in the dielectric system. It is noticed that tan δ has an inverse relation frequency, which may be attributed to the space charge polarization. The dielectric loss of the PBC-B is substantially high at lower frequency (100 Hz). As frequency increases to a high value (near 10,000 Hz), the value of the dielectric loss decreases to a very low value. This high value of loss factor arises due to the enhanced conductivity of the GO–PANI composite and is an important feature of the percolate composites. It is known that in composites with conductive fillers, the dielectric loss mainly arises due to the leakage current in the composites. Higher conductive fillers could construct a more conductive path, which results in more leakage current and dielectric loss. The frequency dependences of AC-electrical conductivity of the GO and PANI–GO composite are presented in Fig. 18.6. As shown, the conductivity is higher for PBC-A compared to other samples. This improvement of AC conductivity is attributed to the dispersion of PANI in the GO matrix. The increase in conductivity with frequency is due to the hopping of charge carriers through the defect states of the polymer chain [18–22]. This normally appears in a heterogeneous system due to the accumulation of mobile charge carriers in the clusters. It is observed that the AC conductivity is more or less constant with frequency variation. Above 50 kHz, the conductivity starts increasing with applied frequency. This can be attributed to classical hopping and quantum mechanical tunneling of charge carriers. The impedance analysis reveals that the material can be used as a space charge capacitor at low frequency and as a high frequency charge device at high frequency. In bulk, the materials behave like a dielectric material over a wide spectral range.

Synthesis of nanocarbon–polyaniline composite

599

Fig. 18.6 Variation of conductivity (σ AC) with frequency.

BC RBC 0.0004

PBC(A)

sac(S/m)

PBC(B)

0.0002

0.0000

2

3

4

5

6

7

log f (Hz)

18.6

Conclusion

In the present study, a bituminous coal–polyaniline composite was synthesized by in situ polymerization. The obtained composites were investigated by various characterization techniques. The XRD results show that the stacking height of the PBC increased due to the attachment of the nitrogen group to the carbon lattice. The change in the interlayer distance and lateral parameters of the sample (PBC) indicates the functionalization of polyaniline with the graphitic basal plane. The X-ray analysis confirmed the structural change of coal to few-layer reduced graphene and the formation of a polyaniline–graphene oxide composite. The result of CHNS confirmed the presence of organic compounds carbon, hydrogen, nitrogen, and sulfur. The increase in nitrogen in the (PBC) is due to the treatment of RBC with aniline to form the polymer nanocomposite. The bond formation of nitrogen in the sample (PBC) is observed by FTIR analysis. Optical analysis by UV absorption spectra revealed that there is a significant difference in the energy gap and it increases in the composite. The energy gap was found to range from 3.29 to 4.27 eV, which is in the organic semiconductor range. The AC conductivity remains constant at lower frequencies and increases at higher frequencies due to the hopping of charge carriers between localized sites of charges. The imaginary dielectric constants and dielectric loss were found to decrease with an increase in frequency, owing to the space polarization while the real part of the complex dielectric strength remains constant for a wide spectral range.

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Monodisperse PVP-stabilized nanoclusters as highly efficient and reusable catalysts for the dehydrogenation of dimethly ammonia-borane (DMAB)

19

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

Chapter Outline 19.1 Introduction 601 19.2 Experimental 603 19.2.1 19.2.2 19.2.3 19.2.4

Materials and methods 603 Synthesis of Ru/PVP@C 603 Investigation of performances of Ru/PVP@C NPs during DMAB dehydrogenation 603 Reusability examination of Ru/PVP@C 604

19.3 Results and discussion 19.4 Conclusions 607 References 607

19.1

604

Introduction

Hydrogen gas is a clean energy carrier and is harmless to the environment. At the same time, the main obstacle related to hydrogen is storage. Considering recent studies, among H2 sources, amine boranes are important materials because of their higher H2 content, durability, ease of use, and higher solubility at moderate conditions [1–8]. H2 production is very easy with the help of the catalyst, which is suitable for the reaction of amine boranes at room temperature. Until now, different catalysts have been used for the dehydrogenation of amine boranes [9–15]. In recent years, dimethylamine-borane, which is one of the amine borane derivatives, has attracted a great deal of attention due to its various properties such as being solid and nontoxic as well as stable in water and air and harmless to the environment. In recent years, it has been shown that dimethylamine-borane is a model substrate because of its ease of Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00019-5 © 2019 Elsevier Ltd. All rights reserved.

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comprehension compared to other amine boron products, as shown in Scheme 19.1. For this purpose, different heterogeneous and homogeneous materials were tried for the dehydrogenation of dimethylamine-borane [16–21]. Even though very good catalytic activity was obtained using the homogeneous catalysts, as shown in Table 19.1, the development of novel nanocomposites has been emphasized for the improvement of reusability of the catalysts in product isolation for different applications [22, 23, 25, 27, 28, 30–42]. Besides, carbon and polymer-based nanomaterials have also been used for various applications as supporting materials [43–88]. For this purpose, PVP-based, carbon-supported Ru nanoparticles have been synthesized for dimethylamine-borane dehydrogenation, as shown in Scheme 19.1.

Scheme 19.1 The catalytic dehydrocoupling of DMAB((CH3)2NHBH3, DMAB). Table 19.1 Data on the turnover frequencies of other studies on DMAB dehydrogenation Catalysts

Conversion (%)

TOF (h21)

Reference

Ru/PVP@C RhCl3 Pd/C trans-RuMe2(PMe3)4 Cp2Ti [Ir(1,5-cod)m-Cl]2 [Rh(1,5-cod)(dmpe)]PF6 [Rh(1,5-cod)m-Cl]2 [Rh(1,5-cod)2]Otf [RuH(PMe3)(NC2H4PPr2)2] IrCl3 [Cr(CO)5(thf )] Rh(0)/[Noct4]Cl [RhCl(PHCy2)3] RhCl(PPh3)3 (Idipp)CuCl Ni(skeletal) [Cp*Rh(m-Cl)Cl]2 [Cr(CO)5(ɳ 1 BH3NMe3)] Ru(cod)(cot) RuCl3. 3H2O [(C5H3-1,3(SiMe3)2)2Ti]2 [Ru(1,5-cod)Cl2]n HRh(CO)(PPh3)3 trans-PdCl2(P(o-tolyl)3)2 Rh(0)NPs Pt(0)/AA PdCo@PVP

100 90 95 100 100 95 95 100 95 100 25 97 90 100 100 100 100 100 97 40 77 100 70 5 20 100 100 100

561.0 7.9 2.8 12.4 12.3 0.7 1.7 12.5 12.0 1.5 0.3 13.4 8.2 2.6 4.3 0.3 3.2 0.9 19.9 1.6 2.7 420.0 2.5 0.1 0.2 60.0 15.0 330

This study [19] [19] [19] [22] [19] [19] [19] [19] [18] [19] [5] [19] [23] [19] [16] [24] [17] [5] [25] [25] [26] [25] [19] [19] [27] [28] [29]

Monodisperse PVP-tabilized nanoclusters

603

This work describes the use and excellent performance of Ru/PVP@C nanocatalysts in the dehydrogenation reaction of dimethylamine-borane. Herein, the production, stability, and characterization of nanocatalysts by analytical methods such as TEM, HR-TEM XRD, and XPS have been carried out. Then, the prepared nanocatalysts have been used for efficient hydrogen production using dimethylamine borane as a hydrogen-containing compound.

19.2

Experimental

19.2.1 Materials and methods Aldrich supplied dimethylamine-borane, RuCl3, and poly(N-vinyl-2-pyrrolidone). Carbon was supplied from Cabot. The methanol and ethanol used during this study were provided by Merck. Before all glass pieces and other lab materials were washed with large amounts of distilled water, they were cleaned with acetone, then dried. Tranmission electron microscopy images were obtained with the help of a JEOL 200 kV. For XPS measurement, a Specs spectrometer was used. The X-ray diffraction patterns of prepared nanomaterial were analyzed by a Panalytical Empyrean diffractometer. 11B NMR spectra were recorded on a Bruker Avance DPX 400 MHz spectrometer (128.2 MHz for 11B NMR).

19.2.2 Synthesis of Ru/PVP@C At first, poly(N-vinyl-pyrrolidone-based Ru nanomaterials were prepared by ultrasonication and a reduction technique using methanol (20%, volume/volume) and poly (N-vinyl-pyrrolidone (poly(N-vinyl-pyrrolidone/Ru ¼ 7–30mol/mol), as shown before [89]. Later, 1% Ru/C was prepared by mixing a poly(N-vinyl-pyrrolidone-based Ru nanomaterial and carbon (C) in an ultrasonication system. Lastly, the prepared nanomaterials were kept at 70°C for a night.

19.2.3 Investigation of performances of Ru/PVP@C NPs during DMAB dehydrogenation By means of the rate of hydrogen emission, the performance of Ru/PVP@C NPs during washing a dehydrogenation of DMAB was examined. During the study, a coated vessel (50 mL) was put on a stirring machine and the reaction temperature was kept at 25.0  0.1°C by flowing H2O in special parts of the reaction vessel from a bath where the temperature was fixed. After that, the water was poured into a millimetric glass pipe (50 cm in height and 4.0 cm in size) and attached to the dehydrogenation chamber to find the amount of H2 generated. In a sample work, the DMAB was transported into the reaction chamber with 25.0  0.1°C of fixed temperature. After transferring the same amount of Ru/PVP@C, the dehydrogenation of DMAB was begun by covering the reaction chamber. The amount of H2 fabricated was found by writing the H2O

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Nanocarbon and its Composites

change in the column at 1000 rounds per minute. Also, by 11B NMR, the change of DMAB (δ ¼  12.6 ppm) to (Me2NBH2)2 (δ ¼ 5.0 ppm) was discovered, besides measuring the amount of hydrogen gas.

19.2.4 Reusability examination of Ru/PVP@C Some amount of Ru/PVP@C nanomaterials (0.3 mM) was placed to get 20 mL of solution. This mixture and 100 mM DMAB were employed to measure the usage performance of Ru/PVP@C during the reaction of DMAB at 25.0  0.1°C. Several experiments were carried out for this purpose. After the completion of the DMAB change to metaborate, an equal amount of DMAB was suddenly placed again into the reaction chamber. The data were written as % starting performance of Ru/PVP@C versus the number of catalytic cycles during DMAB dehydrogenation.

19.3

Results and discussion

Monodispersed Ru/PVP@C NPs have been characterized by the X-ray photoelectron spectroscopy (XPS), tranmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and X-ray diffraction (XRD) techniques. The particle size, structure, and surface composition of the prepared nanomaterials were characterized by the TEM technique [90–95]. It was found that the prepared Ru/PVP@C NPs had a mean particle size of 3.48  0.35 nm, as shown in Fig. 19.1. The structure of the catalyst prepared in HRTEM is shown in Fig. 19.1. From high-resolution electron microscopy, almost all the particles resembled the spherical shape, and no agglomerate was observed in the synthesized catalyst. A high-resolution electron microscope image for monodispersed Ru/PVP@C NPs was also used to determine the atomic cage fringes. X-ray diffraction was carried out to describe the crystal morphology and size of the prepared nanomaterials [95–101]. The crystal morphology of the prepared

Fig. 19.1 TEM, HR-TEM results, and particle size histogram of the Ru/PVP@C NPs.

Monodisperse PVP-tabilized nanoclusters

605

Fig. 19.2 XRD spectra of Ru/PVP@C.

nanomaterials was found to be fcc structures. The peak shown near 22.3 degrees belongs to the PVP@C, as shown in Fig. 19.2. In addition, by utilizing DebyeScherrer, the crystallite particle size of the prepared nanomaterials was calculated as 3.32  0.25 nm. Moreover, XPS was utilized to describe the structural, physical, and morphological properties of metal in prepared nanomaterials [102–104]. The Shirley-shaped background subtraction technique and the Gaussian-Lorentzian method have been utilized in order to define the oxidation state of the metals. Fig. 19.3 shows that the Ru/ PVP@C nanocomposites are mostly in a metallic state, not oxides, by comparing

Fig. 19.3 (A) 2D and (B) 3D view of X-ray photoelectron spectra of Ru 3p in Ru/PVP@C nanocomposites.

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Nanocarbon and its Composites

the XPS energy values (Ru-3p3/2: 463.3 eV). According to these results, it can be concluded that PVP and carbon supply enough stabilization in order to get the mostly zero oxidation state of metal with the formation of the nanocatalyst. Besides, there are also some oxides such as the Ruthenium (IV) species, as shown in Fig. 19.3, as a conclusion of the oxidation of the metal surface in the catalyst production part. After full characterization of Ru/PVP@C NP, experiments show that the produced Ru/PVP@C nanocatalysts have higher catalytic activity for dehydrocoupling of dimethyl amine borane. The nH2/nDMAB plot against time(s) (Fig. 19.4A at 25.0  0.1°C shows dimethylamine-borane dehydrogenation in the presence of nanocatalysts at various concentrations (0.1, 0.2, 0.3, and 0.4 M). A rapid catalytic dehydrogenation reaction begins instantly without any initiation time and continues throughout the dehydrocoupling of dimethylamine-borane. The nuclear magnetic resonance data demonstrate the conversion of (CH3)2NHBH3 (δ ¼  12.8 ppm) to [(CH3)2NBH2]2 (δ ¼  5.1 ppm), indicating the dehydrocoupling reaction of DMAB. H2 production was observed even at 25°C. Furthermore, the catalytic activity results for the dehydrocoupling of dimethylamine-borane with prepared nanomaterials were obtained at various temperatures, as shown in Fig.19.4B. Fig.19.4B was also utilized to determine the rate constants of H2 produced during the dehydrogenation of DMAB at various temperatures. The Ea (21.73  3 kJ mol1), the activation enthalpy (ΔH# 18.63 kJ mol1), and entropy (ΔS# 153.38 J mol1 K1) were calculated by using the Arrhenius and Eyring diagrams for the dehydrogenation of DMAB (Fig. 19.5A and B). To summarize, it is worth noting that the hydrogen gas (1.0 mol H2/mol of dimethylamine-borane) is fully catalytically spread in a very short period of time with a TOF of 561.0 h1 at 25.0  0.1°C. The TOF data of Ru/PVP@C NPs is approximately 1.7 times higher than the 330 h1 literature data for PdCo@PVP NPs, which is an excellent catalyst to date as shown in Table 19.1. The TOF values of different materials for the dehydrocoupling of dimethylamine-borane were compared in

Fig. 19.4 (A) Plot of nH2/nDMAB versus time(s) for the DMAB dehydrocoupling in the presence of Ru@PVP NPs in different catalyst concentrations at 25.0  0.1°C. (B) Plot of percent conversion of DMAB versus time for Ru/PVP@C (7.5 mol%)-catalyzed dehydrocoupling at various temperatures in THF.

Monodisperse PVP-tabilized nanoclusters

607

Fig. 19.5 (A) Arrhenius and (B) eyring plots for the Ru/PVP@C NP catalyzed dehydrocoupling of DMAB at various temperatures.

Table 19.1. As a conclusion, the prepared nanomaterial is one of the best nanocatalysts that is likely to have sufficient stability of PVP-based carbon used in important catalytic reactions, which leads to particle size reduction and higher performance of the prepared nanomaterials.

19.4

Conclusions

As a result, several important points related to poly (N-vinyl-2-pyrrolidone)-based, carbon-supported Ru nanoparticles for dehydrocoupling of dimethylamine-borane are reported below: l

l

l

l

l

With the help of the ultrasonic reduction technique, the Ru/PVP@C nanocomposition was prepared. The ultrasonication technique increases the distribution of Ru NP on the carbon support. Furthermore, PVP helps to solve the agglomeration problem of Ru NPs. The Ru/PVP@C nanoparticles have excellent catalytic activity and durability for the dehydrocoupling of DMAB. One of the best TOF (561.0 h1) values was obtained for the dehydrocoupling of dimethylamine-borane. Ea was calculated as 21.73  3 kJ mol1 for the dehydrocoupling of dimethyl amine borane with the help of prepared nanomaterials. For fuel cells, Ru/PVP@C can be thought of as a promising catalyst with excellent performance for a very easy application to provide H2 with the help of dehydrocoupling of dimethyl amine borane.

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[68] 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/TC@GO NPs) for Knoevenagel condensation of aryl aldehydes together with malononitrile. Appl Catal Environ 2018;225(5):148–53. [69] Sen B, Kuzu S, Demir E, Akocak S, Sen F. Highly monodisperse RuCo nanoparticles decorated on functionalized multiwalled carbon nanotube with the highest observed catalytic activity in the dehydrogenation of dimethylamine borane. Int J Hydrogen Energy 2017;42(36):23292–8. [70] Sen B, Kuzu S, Demir E, Akocak S, Sen F. Monodisperse palladium-nickel alloy nanoparticles assembled on graphene oxide with the high catalytic activity and reusability in the dehydrogenation of dimethylamine-borane. Int J Hydrogen Energy 2017;42 (36):23276–83. [71] Sen B, Kuzu S, Demir E, Okyay TO, Sen F. Hydrogen liberation from the dehydrocoupling of dimethylamine-borane at room temperature by using novel and highly monodispersed RuPtNi nanocatalysts decorated with graphene oxide. Int J Hydrogen Energy 2017;42(36):23299–306. [72] Sen B, Kuzu S, Demir Eea. Polymer-graphene hybride decorated Pt nanoparticles as highly eficient and reusable catalyst for the dehydrogenation of dimethylamine-borane at room temperature. Int J Hydrogen Energy 2017;42(36):23284–91. [73] Sen B, Kuzu S, Demir E, Yıldırır E, Sen F. Highly efficient catalytic dehydrogenation of dimethly ammonia borane via monodisperse palladium-nickel alloy nanoparticles assembled on PEDOT. Int J Hydrogen Energy 2017;42(36):23307–14. [74] Sen B, Lolak N, Koca M, Savk A, Akocak S, Sen F. Bimetallic PdRu/graphene oxidebased catalysts for one-pot three-component synthesis of 2-amino-4H-chromene derivatives. Nano-Struct Nano-Obj 2017;12:33–40. [75] Sen F, Boghossian AA, Sen S, Ulissi ZW, Zhang J, Strano MS. Observation of oscillatory surface reactions of riboflavin, trolox, and singlet oxygen using single carbon nanotube fluorescence spectroscopy. ACS Nano 2012;6(12):10632–45. [76] Sen F, Boghossian AA, Gibbons BM, Sen S, Faltermeier SM, Giraldo JP, et al. Application of nanoparticle antioxidants to enable hyperstable chloroplasts for solar energy harvesting. Adv Energy Mater 2013;3(7):881–93. [77] Sen F, Ertan S, Sen S, Gokagac G. Platinum nanocatalysts prepared with different surfactants for C1-C3 alcohol oxidations and their surface morphologies by AFM. J Nanopart Res 2012;14:922–6. [78] 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. [79] 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. [80] 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. [81] 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. [82] 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, tertoctanethiol. Energy Fuel 2008;22:1858–64. [83] 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.

Monodisperse PVP-tabilized nanoclusters

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[84] Sen F, Ulissi ZW, Gong X, Sen S, Iverson N, Boghossian AA, et al. Spatiotemporal intracellular nitric oxide signaling captured using internalized, near-infrared fluorescent carbon nanotube nanosensors. Nano Lett 2014;14(8):4887–94. [85] Sert H, Yıldız Y, Okyay TO, Gezer B, Dasdelen Z, Sen B, Sen F. Monodisperse Mw-Pt NPs@VC as highly efficient and reusable adsorbents for methylene blue removal. J Clust Sci 2016. https://doi.org/10.1007/s10876-016-1054-3. [86] Sen S, Sen F, Boghossian AA, Zhang J, Strano MS. The effect of reductive dithiothreitol and trolox on nitric oxide quenching of single walled carbon nanotubes. J Phys Chem C 2013;117(1):593–602. [87] 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. [88] Yıldız Y, Erken E, Pamuk H, Sen F. Monodisperse Pt nanoparticles assembled on reduced graphene oxide: highly efficient and reusable catalyst for methanol oxidation and dehydrocoupling of dimethylamine-borane (DMAB). J Nanosci Nanotechnol 2016;16:5951–8. [89] Baeza JA, Calvo L, Rodriguez JJ, Gilarranz MA. Catalysts based on large size-controlled Pd nanoparticles for aqueous-phase hydrodechlorination. Chem Eng J 2016;294:40–8. [90] Pamuk H, Aday B, Sen F, Kaya M. Pt NPs@ GO as a highly efficient and reusable catalyst for one-pot synthesis of acridinedione derivatives. RSC Adv 2015;5:49295–300. [91] Goksu H, Yıldız Y, Celik B, Yazıcı M, Kilbas B, Sen F. Highly efficient and monodisperse graphene oxide furnished Ru/Pd nanoparticles for the de- halogenation of aryl halides via ammonia borane. Chem Select 2016;(5):953–8. [92] Erken E, Esirden I, Kaya M, Sen F. A rapid and novel method for the synthesis of 5-substituted 1H-tetrazole catalyzed by exceptional reusable monodisperse Pt NPs@AC under the microwave irradiation. RSC Adv 2015;5:68558–64. [93] Erken E, Esirden I˙, Kaya M, Sen F. Monodisperse Pt NPs@rGO as highly efficient and reusable heterogeneous catalyst for the synthesis of 5-substituted 1H-tetrazole derivatives. Cat Sci Technol 2015;5:4452–7. [94] Yin M, Li Q, Jensen JO, Huang Y, Cleemann LN, Bjerrum NJ, Xing W. Tungsten carbide promoted Pd and Pd–Co electrocatalysts for formic acid electrooxidation. J Power Sources 2012;219:106. [95] Wang R, Liao Sand Ji S. High performance Pd-based catalysts for oxidation of formic acid. J Power Sources 2008;180:205–8. [96] Zhang Q, Smith GM, Wu Y. Catalytic hydrolysis of sodium borohydride in an integrated reactor for hydrogen generation. Int J Hydrogen Energy 2007;32:4731–5. [97] Yıldız Y, Okyay TO, Sen B, Gezer B, Kuzu S, Savk A, Demir E, Dasdelen Z, Sert H, Sen F. Highly monodisperse Pt/Rh nanoparticles confined in the graphene oxide for highly efficient and reusable sorbents for methylene blue removal from aqueous solutions. Chem Select 2017;2:697–701. [98] Demirci T, C ¸ elik B, Yıldız Y, Eriş S, Arslan M, Sen F, Kilbas B. One-pot synthesis of Hantzsch dihydropyridines using a highly efficient and stable PdRuNi@GO catalyst. RSC Adv 2016;6:76948–56. [99] Yıldız Y, Kuzu S, Sen B, Savk A, Akocak S, Sen F. Different ligand based monodispersed metal nanoparticles decorated with rGO as highly active and reusable catalysts for the methanol oxidation. Int J Hydrogen Energy 2017;42(18):13061–9. [100] Yıldız Y, Okyay TO, Sen B, Gezer B, Bozkurt S, Baskaya G, et al. Activated carbon furnished monodisperse Pt nanocomposites as a superior adsorbent for methylene blue removal from aqueous solutions. J Nanosci Nanotechnol 2017;17:4799–804.

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€ Dasdelen Z, Sen F. Carbon black hybride material [101] Yıldız Y, Pamuk H, Karatepe O, furnished monodisperse platinum nanoparticles as highly efficient and reusable electrocatalysts for formic acid electro-oxidation. RSC Adv 2016;6:32858–62. [102] Zhang J, Landry MP, Barone PW, Sen F. Molecular recognition using corona phase complexes made of synthetic polymers adsorbed on carbon nanotubes. Nat Nanotechol 2013;8(12):959–68. [103] Koskun Y, Savk A, Sen B, Sen F. Highly sensitive glucose sensor based on monodisperse palladium nickel/activated carbon nanocomposites. Anal Chim Acta 2018. https://doi. org/10.1016/j.aca.2018.01.035. [104] Yıldız Y, Ulus R, Eris S, Aday B, Kaya M, Sen F. Functionalized multi-walled carbon nanotubes (f-MWCNT) as highly efficient and reusable heterogeneous catalysts for the synthesis of acridinedione derivatives. Chem Select 2016;1(13):3861–5.

Nanocarbon-supported catalysts for the efficient dehydrogenation of dimethylamine borane

20

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

Chapter Outline 20.1 Introduction 615 20.2 Experimental 617 20.2.1 Materials and methods 20.2.2 Synthesis of Ru nanomaterials stabilized by GO 617

20.3 Results and discussion 20.4 Conclusions 622 References 622

20.1

618

Introduction

Hydrogen gas is a good alternative for clean energy production, but it is problematic in terms of storage and economics. Recent studies have suggested ammonia boranes (ABs) as alternative hydrogen energy sources because they have a high hydrogen content and are highly soluble and stable in water [1–15]. DMAB (dimethylamineborane), which is an important AB derivative, is a crystalline solid at room temperature, and because decomposition products of DMAB are regarded as an easy substrate, the increased interest in DMAB is greater than that of other ABs. In addition, because DMAB is also nontoxic, most scientists concentrate on this compound as a result of those advantages of DMAB (Scheme 20.1). It is very easy to produce hydrogen using DMABs by means of a suitable catalyst at room temperature. There are lots of catalysts for dehydrocoupling reactions to release H2 from DMAB, as shown in Table 20.1. Mostly, the nanomaterials are preferred as a catalyst because they exhibit good catalytic performance and durability for a model reaction [25–41]. Although the highest activity has been identified in the literature for a homogeneous [(η5C5H3–1,3 (SiMe3)2)2Ti]2 catalyst [21], this work focuses on the development of a new metal nanoparticle catalyst as a recoverable and reusable catalyst for hydrogenation of DMAB. For the preparation of novel nanomaterials, supporting materials are also very Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00020-1 © 2019 Elsevier Ltd. All rights reserved.

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2Me2NHBH3

Catalyst

(Me2N.BH2)2 + 2H2

Scheme 20.1 The catalytic reaction for dimethylamine borane ((CH3)2NHBH3, DMAB) dehydrocoupling. Table 20.1 Different catalyst turnover frequencies (TOF) observed in previous studies for DMAB dehydrogenation Catalysts

Conversion (%)

TOF (h21)

Reference

Ru@GO RhCl3 Pd/C Trans-RuMe2(PMe3)4 Cp2Ti [Ir(1,5-cod)m-Cl]2 [Rh(1,5-cod)(dmpe)]PF6 [Rh(1,5-cod)m-Cl]2 [Rh(1,5-cod)2]Otf [RuH(PMe3)(NC2H4PPr2)2] IrCl3 [Cr(CO)5(thf )] Rh(0)/[Noct4]Cl [RhCl(PHCy2)3] RhCl(PPh3)3 (Idipp)CuCl Ni(skeletal) [Cp*Rh(m-Cl)Cl]2 [Cr(CO)5(ɳ 1 BH3NMe3)] Ru(cod)(cot) RuCl3. 3H2O [(C5H3–1,3(SiMe3)2)2Ti]2 [Ru(1,5-cod)Cl2]n HRh(CO)(PPh3)3 Trans-PdCl2(P(o-tolyl)3)2 Pt(0)/BA Pt(0)/TBA Rh(0)NPs Pt(0)/AA Pt(0)/TPA@AC PdCo@PVP

100 90 95 100 100 95 95 100 95 100 25 97 90 100 100 100 100 100 97 40 77 100 70 5 20 100 100 100 100 100 100

410.01 7.9 2.8 12.4 12.3 0.7 1.7 12.5 12.0 1.5 0.3 13.4 8.2 2.6 4.3 0.3 3.2 0.9 19.9 1.6 2.7 420.0 2.5 0.1 0.2 24.88 31.24 60.0 15.0 34.14 330

This study [4] [4] [4] [7] [4] [4] [4] [4] [16] [4] [17] [4] [13] [4] [18] [19] [3] [17] [20] [20] [21] [20] [4] [4] [22] [22] [23] [15] [2] [24]

important. Generally, graphene, a single layer of sp2-bound carbon atoms with versatile mechanical, electronic, and thermal properties, has emerged as a promising material in various research areas [17,18,42–56]. Due to its different physicochemical properties, it meets a wide range of technological applications such as electronics, organic reactions, solar cells, super capacitors, biosensors, batteries, fuel cells, etc.

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617

[17,18,56–68]. In recent work, two-dimensional carbon-based nanostructures such as graphite, graphite nanoparticles (GNPs), carbon nanotubes, carbon derivatives, and graphene-oxide (GO) have emerged as supporting agents for catalysts [24,69–88]. Here, the activation parameters (Ea, ΔH#, and ΔS#) for the dehydrogenation of dimethylamine-borane were determined with the help of Ru@GO NPs after full characterization of the prepared nanomaterials. Further, the kinetic study of the catalytic reaction was performed depending on the catalyst concentration and temperature. For the characterization of Ru@GO NPs, X-ray diffraction (XRD), Raman, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS) have been performed. In addition, the prepared catalyst was carried out for dehydrogenation of dimethylamine-borane, and TOF (quantifying the specific activity of a catalytic center for the dimethylamine-borane hydrogenation reaction) and activation energy (the amount of energy required to bring the minimum atoms or molecules to a state of chemical conversion or physical transport) were determined.

20.2

Experimental

20.2.1 Materials and methods Aldrich supplied dimethylamine-borane, superhydride, RuCl3, and graphite. Ethanol used during this study was provided from Merck. Before all glass pieces and other lab materials were washed with large amounts of distilled water, they were cleaned with acetone, then dried. Transmission electron microscopy images were performed with a JEOL instrument (200 kV). XPS data was obtained with the help of a Specs instrument. All X-ray photoelectron spectroscopy fitting peaks were found with the help of the Gaussian–Lorentzian method and the Shirley-shaped background subtraction technique. X-ray diffraction peaks were used in order to determine the crystal structure of the prepared nanomaterials with the help of a Panalytical Empyrean diffractometer. 11B NMR spectra were recorded on a Bruker Avance DPX 400 MHz spectrometer (128.2 MHz for 11B NMR).

20.2.2 Synthesis of Ru nanomaterials stabilized by GO An ultrasonic chemical reduction method was used to produce Ru@GO NPs, as shown in our previous studies [25,26]. The performace of the prepared nanocatalyst for dimethylamine-borane hydrogenation and the determination of Ea for Ru@GO NPs are given in detail. For this purpose, by means of the rate of hydrogen liberation, the performance of Ru@GO NPs during the dehydrogenation of DMAB was examined. During the study, a schlenk tube (50 mL) was put on a stirrer and the reaction temperature was kept at 25.0  0.1 °C. After that, water was poured into a millimetric glass pipe (50 cm in height and 4.0 cm in size) and attached to the dehydrogenation to find the amount of H2 generated. In a sample work, DMAB was transported into the reaction chamber with

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Nanocarbon and its Composites

25.0  0.1 °C of fixed temperature. After transferring a certain amount of Ru@GO, dehydrogenation of DMAB was begun by covering the reaction chamber; the amount of H2 fabricated was found by writing H2O change in the column at 1000 rounds per minute. Also, by 11B NMR, the change of DMAB (δ ¼  12.6 ppm) to (Me2NBH2)2 (δ ¼ 5.0 ppm) was discovered, besides measuring the amount of hydrogen gas. For the reusability experiments, several experiments was carried out for this purpose. After the completion of the DMAB change to metaborate, equal amounts of DMAB were suddenly placed again into the reaction chamber. The data was written as % starting performance of Ru@GO versus the number of catalytic cycles during DMAB dehydrogenation.

20.3

Results and discussion

The prepared Ru@GO NPs were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) was used to determine the crystal structure and average crystallite size of Ru@GO NPs. Fig. 20.1A indicated that the crystal morphology of the Ru@GO nanomaterial was found to be fcc. Moreover, the average crystalline particle size of the prepared nanomaterials was calculated as nearly 3.53  0.41 nm. Furthermore, Raman spectroscopy was used to distinguish regular and irregular carbon structures in carbon materials. In Fig. 20.1B, the Raman spectrum of GO and Ru@GO NPs are shown. It can be seen here that the peaks at 1349 cm1 (D band) and 1601 cm1 (G band) are scattering peaks. It is known that the ID/IG ratio (intensity of the D band to the intensity of the G band) can be used to find out the degree or defects of GO. For this purpose, the intensity ratios of the D/G band of GO and prepared nanomaterials were calculated as 1.01 and 1.15, respectively, as shown in

D1349 cm–1

Intensity (a.u.)

Ru(101)

Intensity (a.u.)

C(002)

(a) GO (b) Ru@GO

(b)

(a)

0

(A)

20

40 60 80 Binding energy (eV)

G1601 cm–1

IO/IG = 1.15

Ru@GO

IO/IG = 1.01

GO

100

1200

(B)

1300

1400

1500

1600

1700

Raman shift (cm–1)

Fig. 20.1 (A) The XRD plot of GO and Ru@GO NPs and (B) the Raman spectra of GO and Ru@GO NPs.

Nanocarbon-supported catalysts for the efficient dehydrogenation of dimethylamine borane

619

Fig. 20.1B, which shows increased irregularity in the GO template after functionalization with Ru nanoparticles. The size, structure, and composition of the Ru@GO NPs are shown in Fig. 20.2. They were investigated by TEM-HRTEM analysis and the size of Ru@GO NPs was found to be a mean particle size of 3.43  0.39 nm. From the structure of the catalyst in HRTEM, as shown in Fig. 20.1, almost all the particles resembled the spherical shape, and no agglomerate was observed in the synthesized catalyst. A high-resolution electron microscope image for monodispersed Ru@GO NPs was also used to see the atomic cage fringes. As a result of this figure, the Ru (100) plane of the atomic lattice fringe was found to be 0.23 nm. Besides, X-ray photoelectron spectroscopy (XPS) was used to define the structure of the surfaces and the state of the metals. The Gaussian–Lorentzian method and the Shirley-shaped background subtraction technique have been used to determine the oxidation state of the metals [89–100]. Fig. 20.3 indicated that the Ru@GO nanocomposites are mostly metallic states of Ru, not oxides, by looking at the

Fig. 20.2 The TEM and HR-TEM results and particle size histogram of the Ru@GO NPs.

Fig. 20.3 Ru (3p) XPS spectra and Ru@GO NPs.

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Nanocarbon and its Composites

theoretical XPS energy values (Ru-3p3/2: 463.3 eV). Besides, the binding energy of the Ru 3p3/2 peak was decreased to a smaller energy value, which indicates the formation of Ru nanoparticles within the suggested nanomaterial system. According to these results, it was shown that ruthenium is mostly in a zero oxidation state due to the graphene oxide and its efficient stabilization. Besides, there are also some metal oxides and/or unreduced species like ruthenium (IV) in catalyst as shown in Fig. 20.3. After full characterization of Ru@GO NPs, the prepared NPs have been tried as a catalyst for dimethylamine-borane hydrogenation. For this purpose, as shown in Fig. 20.4A, the dehydrocoupling of DMAB was shown with the help of different concentrations of nanocatalysts at room temperature, and an nH2/nDMAB-time (min) graph is shown. H2 evolution begins rapidly during induction, and the catalytic hydrogenation reaction of DMAB continues until the completion of reaction. Nuclear magnetic resonance analysis showed that the conversion of (CH3)2NHBH3 (δ ¼ 12.7 ppm) to [(CH3) 2NBH2]2 (δ ¼ 5 ppm) was completed in the presence of Ru@GO NPs. In Fig. 20.4B, the H2 production rate constants were found during dehydrogenation and the change in conversion percentage at four various temperatures was shown in the same figure. In Fig. 20.5A, using the Arrhenius equation, Ea was found to be 44.67  2 kJ mol1 and in Fig. 20.5B, using the Eyring plot, the ΔH (activation enthalpy) and ΔS (activation entropy) were calculated to be 42.13 kJ mol 1 and  78.85 J mol1 K1, respectively. In view of this study, a high catalytic activity was observed when Ru@GO NPs (410.01 h 1) was used for dehydrocoupling of DMAB. It should be noted that, with the aid of Ru@GO NPs, H2 gas (1 mol H2/1 mol of DMAB) is completely evolved in the dehydrogenation of the DMAB reaction in a very short time at 25  0.1 °C. As a result, to our knowledge, the TOF value (410.01 h1) of this catalyst is one of the best values of the literature, as shown in Table 20.1. The list of TOF data obtained in different studies for the dehydrogenation

Fig. 20.4 (A) Plots presenting mol H2/mol DMAB versus time (min) for the DMAB dehydrocoupling in the presence of Ru@GO NPs with different catalyst concentrations at 25  0.1°C and (B) plots presenting % conversion versus time (min) for the Ru@GO NPs (7.5% mol) catalyzed dehydrocoupling of DMAB in THF at various temperatures.

Nanocarbon-supported catalysts for the efficient dehydrogenation of dimethylamine borane

621

Fig. 20.5 (A) The Arrhenius plot and (B) the Eyring plot for the Ru@GO NPs catalyzed dehydrocoupling of DMAB at various temperatures.

Fig. 20.6 Plots % conversion versus time graph for Ru@GO NPs (7.5% mol) catalyzed dehydrocoupling of DMAB in THF at room temperature for first and fourth catalytic runs.

of DMAB is shown in Table 20.1. Ru@GO NPs can be thought of as a very efficient nanocatalyst compared to the others in Table 20.1. Furthermore, this catalyst can be isolated and reused when it is used to a significant extent in catalytic reactions. The probable reason for this efficiency may be the cooperative and synergistic effect between Ru and GO in the prepared catalyst. The reusability performance of Ru@GO NPs was also investigated in this study. To do this, DMAB is added several times after each catalysis reaction. At the end of the fourth experiment (Fig. 20.6), Ru@GO NPs retain almost 80% of their initial performance. This decrease can be explained by the passivation of the active sites of catalyst.

622

20.4

Nanocarbon and its Composites

Conclusions

As a result, graphene-oxide stabilized ruthenium NPs were shown to be effective catalysts for dimethylamine-borane hydrogenation, and some of the important results are listed in the following: l

l

l

l

l

l

The ultrasonic chemical reduction method is very effective to produce Ru@GO NPs, which helps the monodispersity of Ru@GO NPs. The results of the studies show that the manufacturing method is very effective in distributing the GOs properly on the Ru material and preventing the Ru@GO agglomeration problem. Ru@GO NPs have superior catalytic performance in dimethylamine-borane dehydrogenation when compared to the literature data. A very good TOF (410.01 h1) value was obtained for dimethylamine-borane dehydrogenation. When Ru@GO nanocatalysts were used, a 44.67 kJ mol1 Ea was calculated for dehydrocoupling of DMAB. The prepared Ru@GO NPs are promising catalytic materials for dehydrocoupling reactions and fuel cell applications.

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[25]

[26]

[27]

[28] [29] [30] [31] [32] [33]

[34]

[35] [36]

[37]

[38]

[39]

[40]

[41] [42]

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catalyst for the dimethylamine borane (DMAB) dehydrocoupling. RSC Adv 2016;6: 24097–102. Celik B, Kuzu S, Erken E, Sert H, Koskun Y, Sen F. Nearly monodisperse carbon nanotube furnished nanocatalysts as highly efficient and reusable catalyst for dehydrocoupling of DMAB and C1 to C3 alcohol oxidation. Int J Hydrogen Energy 2016;41(4):3093–101. Celik B, Baskaya G, Sert H, Karatepe O, Erken E, Sen F. Monodisperse Pt (0)/DPA@ GO nanoparticles as highly active catalysts for alcohol oxidation and dehydrogenation of DMAB. Int J Hydrogen Energy 2016;41(13):5661–9. Amendola SC, Janjua JM, Spencer NC, Kelly MT, Petillo PJ, Sharp-Goldman SL, et al. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int J Hydrogen Energy 2000;25:969–75. Stephens FH, Pons V, Baker RT. Ammonia borane: the hydrogen source par excellence? Dalton Trans 2007;(25):2613–26. Mohajeri N, T-Raissi A, Adebiyi O. Hydrolytic cleavage of ammonia-borane complex for hydrogen production. J Power Sources 2007;167:482–5. Ramachandran PV, Gagare PD. Preparation of ammonia borane in high yield and purity, methanolysis, and regeneration. Inorg Chem 2007;46:7810–7. Xu Q, Chandra M. A portable hydrogen generation system: catalytic hydrolysis of ammonia borane. J Alloys Compd 2007;446-447:729–32. Xu Q, Chandra M. Catalytic activities of non-noble metals for hydrogen generation from aqueous ammonia borane at room temperature. J Power Sources 2006;163:364–70. Umegaki T, Yan JM, Zhang XB, Shioyama H, Kuriyama N, Xu Q. Hollow Ni-SiO2 nanosphere-catalyzed hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage. J Power Sources 2009;191:209–16. Umegaki T, Yan JM, Zhang XB, Shioyama H, Kuriyama N, Xu Q. Preparation and catalysis of poly(N-vinyl-2-pyrrolidone) (PVP) stabilized nickel catalyst for hydrolytic dehydrogenation of ammonia borane. Int J Hydrogen Energy 2009;34:3816–22. Fernandes R, Patel N, Miotello A. Hydrogen generation by hydrolysis of alkaline NaBH4 solution with Cr-promoted Co-B amorphous catalyst. Appl Catal B 2009;92:68–74. Yan JM, Zhang XB, Han S, Shioyama H, Xu Q. Ironnanoparticle-catalyzed hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage. Angew Chem Int Ed 2008;47:2287–9. Yan JM, Zhang XB, Shioyama H, Xu Q. Room temperature hydrolytic dehydrogenation of ammonia borane catalyzed by Co nanoparticles. J Power Sources 2010;195: 1091–4. Yan JM, Zhang XB, Han S, Shioyama H, Xu Q. Synthesis of longtime water/air-stable Ni nanoparticles and their high catalytic activity for hydrolysis of ammonia borane for hydrogen generation. Inorg Chem 2009;48:7389–93. Tong DG, Zeng XL, Chu W, Wang D, Wu P. Magnetically recyclable hollow Co-B nanospindles as catalysts for hydrogen generation from ammonia borane. J Mater Sci 2010;45:2862–7. Patel N, Fernandes R, Guella G, Miotello A. Nanoparticle-assembled Co-B thin film for the hydrolysis of ammonia borane: a highly active catalyst for hydrogen production. Appl Catal B 2010;95:137–43. Yamada Y, Yano K, Xu Q, Fukuzumi S. Cu/Co3O4 nanoparticles as catalysts for hydrogen evolution from ammonia borane by hydrolysis. J Phys Chem C 2010;114:16456–62. Ayrancı R, Baskaya G, Guzel M, Bozkurt S, Ak M, Sen F. Carbon based nanomaterials for high performance optoelectrochemical systems. Chem Select 2017;2(4):1548–55.

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[43] Baskaya G, Yıldız Y, Savk A, Onal Okyay T, Eris S, Sen F. Rapid, sensitive, and reusable detection of glucose by highly monodisperse nickel nanoparticles decorated functionalized multi-walled carbon nanotubes. Biosens Bioelectron 2017;91:728–33. [44] Sen F, Boghossian AA, Sen S, Ulissi ZW, Zhang J, Strano MS. Observation of oscillatory surface reactions of riboflavin, trolox, and singlet oxygen using single carbon nanotube fluorescence spectroscopy. ACS Nano 2012;6(12):10632–45. [45] Sen F, Boghossian AA, Gibbons BM, Sen S, Faltermeier SM, Giraldo JP, et al. Application of nanoparticle antioxidants to enable hyperstable chloroplasts for solar energy harvesting. Adv Energy Mater 2013;3(7):881–93. [46] Sen S, Sen F, Boghossian AA, Zhang J, The SMS. Effect of reductive dithiothreitol and trolox on nitric oxide quenching of single walled carbon nanotubes. J Phys Chem C 2013;117(1):593–602. [47] Sen F, Ulissi ZW, Gong X, Sen S, Iverson N, Boghossian AA, et al. Spatiotemporal intracellular nitric oxide signaling captured using internalized, near-infrared fluorescent carbon nanotube nanosensors. Nano Lett 2014;14(8):4887–94. [48] Abrahamson JT, Sempere B, Walsh MP, Forman JM, Sen F, Sen S, et al. Excess thermopower and the theory of thermopower waves. ACS Nano 2013;7(8):6533–44. [49] Zhang J, Landry MP, Barone PW, Kim JH, Lin S, Ulissi ZW, et al. Molecular recognition using corona phase complexes made of synthetic polymers adsorbed on carbon nanotubes. Nat Nanotechnol 2013;8(12):959–68. [50] Iverson NM, Barone PW, Shandell M, Trudel LJ, Sen S, Sen F. In vivo biosensing via tissue-localizable nearinfrared-fluorescent single-walled carbon nanotubes. Nat Nanotechnol 2013;8(11):873–80. [51] Giraldo JP, Landry MP, Faltermeier SM, McNicholas TP, Iverson N, Boghossian AA, et al. Plant nanobionics approach to augment photosynthesis and biochemical sensing. Nat Mater 2014;13:400–8. [52] Demirci T, Celik B, Yıldız Y, Eris S, Arslan M, Kilbas B, Sen F. One-pot synthesis of Hantzsch dihydropyridines using highly efficient and stable PdRuNi@GO catalyst. RSC Adv 2016;6:76948–56. [53] Yıldız Y, Onal Okyay T, Sen B, Gezer B, Bozkurt S, Baskaya G, et al. Activated carbon furnished monodisperse Pt nanocomposites as a superior adsorbent for methylene blue removal from aqueous solutions. J Nanosci Nanotechnol 2017;17:4799–804. [54] Goksu H, Sert H, Kilbas B, Sen F. Recent advances in the reduction of nitro compounds by heterogenous catalysts. Curr Org Chem 2017;21:794–820. [55] Goksu H, Celik B, Yıldız Y, Kılbas‚ B, Sen F. Superior monodisperse CNT-supported CoPd (CoPd@CNT) nanoparticles for selective reduction of nitro compounds to primary amines with NaBH4 in aqueous medium. Chem Select 2016;1(10):2366–72. [56] Aday B, Pamuk H, Kaya M, Sen F. Graphene oxide as highly effective and readily recyclable catalyst using for the one-pot synthesis of 1,8-dioxoacridine derivatives. J Nanosci Nanotechnol 2016;16:6498–504. [57] Sen B, Lolak N, Koca M, Savk A, Akocak S, Sen F. Bimetallic PdRu/graphene oxide based Catalysts for one-pot three-component synthesis of 2-amino-4H-chromene derivatives. Nano Struct Nano Object 2017;12:33–40. [58] Sen B, Akdere EH, Savk A, Gultekin E, Parali O, Goksu H, Sen F. A novel thiocarbamide functionalized graphene oxide supported bimetallic monodisperse Rh-Pt nanoparticles (RhPt/TC@GO NPs) for Knoevenagel condensation of aryl aldehydes together with malononitrile. Appl Catal B 2018;225(5):148–53.

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[59] Goksu H, Yıldız Y, Celik B, Yazıcı M, Kılbas B, Sen F. Highly efficient and monodisperse graphene oxide furnished Ru/Pd nanoparticles for the dehalogenation of aryl halides via ammonia borane. Chem Select 2016;1(5):953–8. [60] Akocak S, Sen B, Lolak N, Savk A, Koca M, Kuzu S, Sen F. One-pot three-component synthesis of 2-amino-4H-chromene derivatives by using monodisperse Pd nanomaterials anchored graphene oxide as highly efficient and recyclable catalyst. Nano Struct Nano Object 2017;11:25–31. [61] Yıldız Y, Erken E, Pamuk H, Sert H, Sen F. Monodisperse Pt nanoparticles assembled on reduced graphene oxide: highly efficient and reusable catalyst for methanol oxidation and dehydrocoupling of dimethylamine-borane (DMAB). J Nanosci Nanotechnol 2016;16:5951–8. [62] Demir E, Savk A, Sen B, Sen F. A novel monodisperse metal nanoparticles anchored graphene oxide as counter electrode for dye-sensitized solar cells. Nano Struct Nano Object 2017;12:41–5. [63] Goksu H, Yıldız Y, Celik B, Yazici M, Kilbas B, Sen F. Eco-friendly hydrogenation of aromatic aldehyde compounds by tandem dehydrogenation of dimethylamine-borane in the presence of reduced graphene oxide furnished platinum nanocatalyst. Cat Sci Technol 2016;6:2318–24. [64] Aday B, Yıldız Y, Ulus R, Eris S, Sen F, Kaya M. One-pot, efficient and green synthesis of acridinedione derivatives using highly monodisperse platinum nanoparticles supported with reduced graphene oxide. New J Chem 2016;40:748–54. [65] Bozkurt S, Tosun B, Sen B, Akocak S, Savk A, Ebeoglugil MF, Sen F. A hydrogen peroxide sensor based on TNM functionalized reduced graphene oxide grafted with highly monodisperse Pd nanoparticles. Anal Chim Acta 2017;989:88–94. [66] Sen B, Kuzu S, Demir E, Akocak S, Sen F. Monodisperse palladium-nickel alloy nanoparticles assembled on graphene oxide with the high catalytic activity and reusability in the dehydrogenation of dimethylamine-borane. Int J Hydrogen Energy 2017;42 (36):23276–83. [67] Sen B, Kuzu S, Demir Eea. Polymer-graphene hybride decorated Pt nanoparticles as highly efficient and reusable catalyst for the dehydrogenation of dimethylamine-borane at room temperature. Int J Hydrogen Energy 2017;42(36):23284–91. [68] Sen B, Kuzu S, Demir E, Okyay TO, Sen F. Hydrogen liberation from the dehydrocoupling of dimethylamine-borane at room temperature by using novel and highly monodispersed RuPtNi nanocatalysts decorated with graphene oxide. Int J Hydrogen Energy 2017;42(36): 23299–306. [69] 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. [70] 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. [71] Sert H, Yıldız Y, Okyay TO, Gezer B, Dasdelen Z, Sen B, Sen F. Monodisperse Mw-Pt NPs@VC as highly efficient and reusable adsorbents for methylene blue removal. J Clust Sci 2016;27(6):1953–62. [72] Erken E, Yıldız 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.

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[73] Ertan S, Sen F, Sen S, Gokagac G. Platinum nanocatalysts prepared with different surfactants for C1–C3 alcohol oxidations and their surface morphologies by AFM. J Nanopart Res 2012;14:922–6. [74] 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, tertoctanethiol. Energy Fuel 2008;22:1858–64. [75] 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;111:1467. [76] 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. [77] 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. [78] Yıldız Y, Onal Okyay T, Sen B, Gezer B, Kuzu S, Savk A, et al. Highly monodisperse Pt/Rh nanoparticles confined in the graphene oxide for highly efficient and reusable sorbents for methylene blue removal from aqueous solutions. Chem Select 2017;2(2): 697–701. [79] Karatepe O, Yıldız Y, Pamuk H, Eris S, Dasdelen Z, Sen F. Enhanced electrocatalytic activity and durability of highly monodisperse Pt@PPy–PANI nanocomposites as a novel catalyst for electro-oxidation of methanol. RSC Adv 2016;6:50851–7. [80] Gezer B, Okyay TO, Bozkurt S, Baskaya G, Sahin B, Uluturk C, et al. Reduced graphene oxide (rGO) as highly effective material for the ultrasound assisted boric acid extraction from ulexite ore. Chem Eng Res Des 2017;117:542–8. [81] 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. [82] 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. [83] Erken E, Esirden ˙I, Kaya M, Sen F. A rapid and novel method for the synthesis of 5-substituted 1H-tetrazole catalyzed by exceptional reusable monodisperse Pt NPs@AC under the microwave irradiation. RSC Adv 2015;5:68558–64. [84] Esirden ˙I, Erken E, Kaya M, Sen F. Monodisperse Pt NPs@rGO as highly efficient and reusable heterogeneous catalysts for the synthesis of 5-substituted 1H-tetrazole derivatives. Cat Sci Technol 2015;5:4452–7. [85] Pamuk H, Aday B, Kaya M, Sen F. Pt NPs@GO as highly efficient and reusable catalyst for one-pot synthesis of acridinedione derivatives. RSC Adv 2015;5: 49295–300. [86] Yıldız Y, Ulus R, Eris S, Aday B, Kaya M, Sen F. Functionalized multi-walled carbon nanotubes (f-MWCNT) as highly efficient and reusable heterogeneous catalysts for the synthesis of acridinedione derivatives. Chem Select 2016;1(13):3861–5. [87] Yıldız Y, Esirden I˙, Erken E, Demir E, Kaya M, Sen F. Microwave (Mw)-assisted synthesis of 5-substituted 1H-tetrazoles via [3 + 2] cycloaddition catalyzed by Mw-Pd/Co nanoparticles decorated on multi-walled carbon nanotubes. Chem Select 2016;1(8): 1695–701. [88] Yıldız Y, Pamuk H, Karatepe O, Dasdelen Z, Sen F. Carbon black hybride material furnished monodisperse platinum nanoparticles as highly efficient and reusable electrocatalysts for formic acid electro-oxidation. RSC Adv 2016;6:32858–62.

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[89] Baskaya G, Esirden I, Erken E, Sen F, Kaya M. Synthesis of 5-substituted-1H-tetrazole derivatives using monodisperse carbon black decorated Pt nanoparticles as heterogeneous nanocatalysts. J Nanosci Nanotechnol 2017;17:1992–9. [90] 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 Environ 2017;219C:511–6. [91] Sahin B, Demir E, Aygun A, Gunduz H, Sen F. Investigation of the effect of pomegranate extract and monodisperse silver nanoparticle combination on MCF-7 cell line. J Biotechnol 2017;260:79–83. [92] Demir E, Sen B, Sen F. Highly efficient nanoparticles and f-MWCNT nanocomposites based counter electrodes for dye-sensitized solar cells. Nano Struct Nano Object 2017;11:39–45. [93] Sen B, Kuzu S, Demir E, Yıldırır E, Sen F. Highly efficient catalytic dehydrogenation of dimethly ammonia borane via monodisperse palladium–nickel alloy nanoparticles assembled on PEDOT. Int J Hydrogen Energy 2017;42(36):23307–14. [94] 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 Hydrogen Energy 2017;42(18):13061–9. [95] Ayrancı R, Baskaya G, Guzel M, Bozkurt S, Ak M, Savk A, Sen F. Enhanced optical and electrical properties of PEDOT via nanostructured carbon materials: A comparative investigation. Nano Struct Nano Object 2017;11:13–9. [96] Sen B, Kuzu S, Demir E, Akocak S, Sen F. Highly monodisperse RuCo nanoparticles decorated on functionalized multiwalled carbon nanotube with the highest observed catalytic activity in the dehydrogenation of dimethylamine borane. Int J Hydrogen Energy 2017;42(36):23292–8. [97] Eris S, Daşdelen Z, Sen F. Investigation of electrocatalytic activity and stability of Pt@fVC catalyst prepared by in-situ synthesis for methanol electrooxidation. Int J Hydrogen Energy 2018;43(1):385–90. [98] Eris S, Daşdelen Z, Yıldız Y, Sen F. Nanostructured polyaniline-rGO decorated platinum catalyst with enhanced activity and durability for methanol oxidation. Int J Hydrogen Energy 2018;43(3):1337–43. [99] Sahin B, Aygun A, Gunduz H, Sahin K, Demir E, Akocak S, Sen F. Cytotoxic effects of platinum nanoparticles obtained from pomegranate extract by the green synthesis method on the MCF-7 cell line. Colloids Surf B Biointerfaces 2018;163:119–24. [100] 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.

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Vasi Uddin Siddiqui*, Afzal Ansari*, Imran Khan*,†, Weqar Ahmad Siddiqui*, Md Khursheed Akram‡, Anish Khan§,¶, Abdullah Mohamed Asiri¶,k *Department of Applied Sciences and Humanities, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi, India, †Applied Sciences and Humanities Section, University Polytechnic, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India, ‡Applied Sciences and Humanities Section, University Polytechnic, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi, 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, kChemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

Chapter Outline 21.1 Introduction 629 21.2 Synthesis of a nanographene composite ion exchanger 633 21.3 Properties of a nanographene ion exchanger 641 21.4 Applications of nanographene composite ion exchangers 642 21.5 Conclusion 645 References 646

21.1

Introduction

An ion exchanger defines a wider application to some extent, in the form of different types of it. These may be ion exchanger resins (functionalized porous or gel polymer), zeolites, etc. The 21st century has brought scientific attention toward rapid industrialization, advanced agriculture, and a growing world population, which have resulted in the contamination of water, air, soil, and the aquatic ecosystem. With the need to alleviate health and environmental issues, nanotechnology has triggered interest toward its potential with diversified applications in water treatment, energy application sensing, etc. With drastic changes in material properties at the nanoscale, nanomaterials are a promising candidate. As the two-dimensional atomic thickness “magical material” comes, the solution lies with nanotechnology in this carbon form, that is graphene. Its exceptional properties take it in the environment field, it is with Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00021-3 © 2019 Elsevier Ltd. All rights reserved.

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composite in photocatalytic material. It makes the next generation of water treatment membranes and electrode materials for contaminant monitoring or removal [1]. Prior to further discussion, we first discuss the basics of an ion exchanger and its reaction process with some definition. Chen et al. specified the nanographene term with the size of 1–100 nm graphene sheets and 1–5 nm PAHs as “nanographene molecules” with defined chemical structures [2]. Nanographene (NGs), nanographene platelets (NGPs), poly acrylic hydrocarbon (PAH), nanoscale graphene, and graphene quantum dots (GQDs) are referred to as nanographene. Nanographene is graphene at the nanoscale. Three different classes of NGs can be defined according to their edge structures: armchair-edged nanographenes (A-NGs), cove-edged nanographenes (C-NGs), and zigzag-edged nanographenes (Z-NGs), as shown in Fig. 21.1 [3]. Ion exchange material has a diverse range with its diverse application. Without exaggeration, separation technology is the most extensive application of ion exchange material. It possesses different appearances such as natural or artificial, inorganic and polymeric. It might be wood, paper, sand, clay glauconites, zeolite, functional resin or living organisms. Isotopic separation in the nuclear industry was probably the first extensive application in the development of ion exchangers. However, water treatment technology comes first in today’s market in which ultrapure water production

Fig. 21.1 Edge structures of graphene [3].

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631

is the biggest but not the only concern. Highly diluted contaminates from waste streams are the highest benefits of ion exchanger technology. The pharmaceutics and food industries used ion exchange technology in both physical and chemical perspectives. Hydrometallurgy, biochemistry, and biotechnology used ion exchange technology in ways that weren’t just conventional. Controlled drug delivery could be a major application of the ion exchanger in the medical field [4]. Ion exchange is the equivalent exchange of ions between two or more ionized species located in different phases, at least one of which is an ion exchanger. The process takes place without the formation of chemical bonds. The ion exchanger is a phase containing osmotically inactive means such that the carriers cannot migrate from the phase where it is located. Conventionally, ion exchangers are cation exchanger having a fixed negative charge with exchangeable cation. In contrast, an anion exchanger possesses a transferrable anion with an immovable cation. The framework of the cation exchanger may be regarded as a macromolecular or crystalline polyanion and that of an anion exchanger as a polychain. If both types of groups are present in the same polymer, it is called an amphoteric ion exchanger. Polymers bearing such groups are called chelating polymers or chelating resins. There is no strict difference between ion exchange resin and chelating resin because some polymers can act as chelating or nonchelating substances, depending on what the ion exchange resembles. The characteristic difference between these two phenomena is in the stoichiometric nature of the ion exchange. Every ion removed from the solution is replaced by an equivalent amount of another ion of the same sign. In sorption, on the other hand, a solute is usually taken up nonstoichiometrically without being replaced. Along with absorption and adsorption, ion exchange is a form of sorption [4]. An ion exchange reaction carried out in an ion exchanger (solid phase) and a solution phase in reversible interchange between these ions. Ion exchanger being insoluble in in the medium in which reaction is carried out. If ion exchanger F M+ carrying cation M+ as the exchange ions is placed in an aqueous solution phase containing A+ cations, an ion exchange reaction takes place, which may be represented by the following equation for cation exchange: A+ Ð F A + + M+ F M + + Solid Solution Solid Solution Where F is the insoluble fixed anion with F M+, M+ and A+ are counter cations while ions in the solution that bear the same charge as the fixed anion of the exchanger are called coions, although the contribution of the anion is not appreciably extent. Similarly, the anion exchanger reaction is written as follows: A Ð F + A + M F + M + Solid Solution Solid Solution The main fact is that the electroneutrality is always preserved in both the exchanger and solution phases, and this in turn requires that counterions are exchanged in an

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equivalent amount. Three fundamental requirements must be met to confer ion exchange properties upon a material: 1. An inert host structure that allows diffusion of hydrated ions, that is, a hydrophilic matrix. 2. The host structure must carry a fixed ionic charge, termed the fixed ion. 3. The electrical neutrality of the structure must be established by the presence of a mobile ion of opposite charge to that of the fixed ion, called a counter ion [5].

With the growth in worldwide population as well as the Industrial Revolution and urbanization, air, water, and soil have been severely affected by the pollution discharged from households and industries. Plenty of physical, chemical, and biological technologies have been developed for the removal of toxic gases such as NOx (oxides of nitrogen), SOx (oxides of sulfur), CO (carbon monoxide), NH3 (ammonia), heavy metals, and organic as well as biotoxic materials. The adsorption process leads among all technologies available because it is simple, easy, and effective for different types of pollution and does not produce secondary pollution during the process. A large surface area with pore volume and proper functionalities is the key for a good adsorbent. Among activated carbon, clays, zeolites, mesoporous oxides, polymers, and metal organic frameworks, cabanccous-based adsorbent shows great adsorption capacity and thermal stability [6]. Wang et al. summarized the recent progress of photoluminescent GQDs, including the versatile photoluminescence features, key mechanisms, and promising applications such as energy-related applications, biomedical applications for human health, and sensors for environmental applications on a single particle level [7]. Dong et al. has developed a green and facile sensing system for the detection of free residual chlorine in water based on the fluorescence quenching of GQDs [8]. Benı´tez-Martı´nez et al. also studied the GQD fluorescence nanoparticle behaviour along with some exceptional optical and electrical properties and a number of active sites provided by GQD in analytical nanoscience and technology. [9]. Zhu et al. introduced a new green and universal approach for the synthesis of GQDs with 86% high yield with the only byproducts being H2O and CO2. Different and colorful GQDs are also used in fluorescent bioimaging [10]. Liu et al. used the large surface area and strong π-π interaction on the surface of a three-dimensional (3D) graphene oxide nanostructure for the removal of methylene blue and methyl violet through strong π-π stacking and anion-cation interaction with the activation energy of 50.3 and 70.9 KJ/mol [11]. Bacon et al. studied nanometer-sized graphene, that is, GQDs, with respect to its synthesis approach as either top-down or bottom-up, with applications in energy-related fields such as photovoltaics, organic light emitting diodes, and fuel cells in life sciences such as bioimaging, biosensing, environmental monitoring gives the prospects of free chlorine sensor in water, photoluminescence phosphate sensor also [12]. As the consumption of fossil fuel increases, the growth of CO2 concentration in the atmosphere also increases. Balasubramanian et al. give advances in the development of graphene-based adsorbents for CO2 capture [13]. Gahlot et al. synthesized a nanocomposite ion exchange membrane consisting of graphene oxide and sulfonated polyethersulfone (SPES) and found that the effects on water desalination in terms of ionic flux, power efficiency, and current efficiency are 3.51 mol/m2 h, 4.3 kWh/kg, and 97.4%, respectively. This shows better performance and higher

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stability than direct methanol fuel cell (DMFC) and electrodialysis [14]. Zarrin et al. fabricated a functionalized graphene oxide nafion nanocomposite (F-GO/nafion) membrane and functionalized as potential proton exchange membrane (PEM) for low humidity (30%) and high temperature (120°C) for proton exchange membrane fuel cells (PEMFCs) application [15]. Cao et al. synthesized a poly (ethylene oxide)/ graphene oxide (PEO/GO) composite membrane with a thickness of 80 μm and showed the increasing ionic conductivity from 0.086 to 0.134 S/cm at a temperature range from 25°C to 60°C with 100% relative humidity [16]. This chapter will introduce the different types of synthesis methods of nanographene composite ion exchangers as well as the properties and applications in the effective removal of contaminant species from wastewater. The properties and adsorption performance of nanocomposite ion exchangers will be reviewed, including recent efforts focused at improving the purification performance through chemical and biological means.

21.2

Synthesis of a nanographene composite ion exchanger

Until now, two distinct approaches have been used for the synthesis of exfoliation graphite to graphene: the top-down approach and building up graphene from molecular building blocks, that is, the bottom-up approach. Both approaches include the mechanical exfoliation of highly oriented pyrolyzed graphite. HOPG, a solution-based exfoliation of the graphene intercalation compound (GIC), chemical oxidation/exfoliation of graphite followed by reduction of GO and epitaxial growth on the metallic substrate by means of CVD, thermal decomposition of SiC, organic synthesis based on precursor molecule, respectively. PAHs are also typically molecular graphene composed of all sp2 carbon. Two chemical approaches are used to grow PAH into larger graphene: a controlled chemical reaction under mild conditions in solution and thermolysis starting from well-defined carbon-rich precursors. Whereas the fabrication of nanographene by cutting graphene sheets or via CVD fails to precisely control their resulting sizes and configurations, although the bottomup synthesis approach is available in modern synthetic organic chemistry. Most are typically carried out through the intramolecular oxidative cyclodehydrogenation of predesigned, nonplanar precursors that have oligophenylene or partially prefused oligoarylene structures [2, 17, 18]. Nanographene, or extended polycyclic aromatic hydrocarbon (PAH), has witnessed rapid development over the past few years. These developments can be summarized in four categories: (I) the nonconventional method, (II) a structure incorporating seven- or eight-membered rings, (III) selective heteroatom doping, and (IV) direct edge functionalization. On the other hand, a one-dimensional extension of the graphene molecules leads to the formation of graphene nanoribbons (GNRs) with high aspect ratios. GNRs are a longitudinally extended polymeric system made possible through the solution-mediated or surface-assisted cyclodehydrogenation or graphitization of the tailor-made polyphenylene precursor. In contrast to infinite graphene with a zero band gap, structurally confined nanoscale graphene segments, which are called

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nanographene or graphene quantum dots, show nonzero band gaps that are mainly governed by their size and edge configuration [19]. (I) Nonconventional Approaches for Nanographene Synthesis

Dou et al. synthesized m-dimethoxy HBC 3 and the bisspirocyclic dienone 4 unexpectedly during a scholl reaction with 20% yield. The reaction mechanism is shown in Fig. 21.2 [20]. Arslan et al. fabricated an extended or partially fused hexabezocoronene derivative that is based on the benzannulation and cyclodehydrogenation (scholl oxidation) of simple diaryl alkyne. The partially fused derivatives are a new class of contorted aromatic systems with high solubility, enhanced visible adsorption, and reversible redox processes by using benzannulation. They also prepared a polymer poly (phenylene ethynylene). Figs. 21.3 and 21.4 represent both Schemes [21, 22]. A bowl-shaped PAH that more easily accepts electrons than their contorted hexabenzocoronene precursor and associated strongly with C70 synthesis by Whalley et al. with palladium catalyzed chemistry [23]. Hein et al. prepared an aromatic system by benzannulating silyl-protected arylacetylenes [24]. Zhang et al. synthesized threefold symmetrical and highly substituted nanographene or PAH (hexacatahexabenzocoronenes) (c-HBCs) from simple chemicals using the FeCl3 mediated process [25]. Chiu et al. gave a general method for the synthesis of contorted dibenzotetrathienocoronene (c-DBTTC), a tetrathiophene-fused version of contorted hexabenzocoronenes (c-HBC). C-DBTTC displays the flexibility to adopt either the up-down or the butterfly conformation when grown as cocrystals with the size of an electron acceptor such as C60 [26]. Chen et al. reported sulfur containing PAH and hexathienocoronenes (HTCs) 1 that allowed various substituents to be introduced easily. As compared to c-DBTTCs, the tetra-substituted HTCs leave the two α-positions of the annelated thiophene on the anthradithiophene backbone open, allowing further functionalization and polymerization. Fig. 21.5 shows the synthesis Scheme [28].

Fig. 21.2 Reaction mechanism of compound 3 and 4 [20].

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Fig. 21.3 A series of dialkyne compounds 1a–d provides partially fused HBC derivatives 4a–d and fully fused products 5a and 5d using a two-step benzannulation and cyclodehydrogenation protocol [21].

Moreover, Chen et al. gives a formation scheme to Gemini-type amphiphilic hexathienocoronene (HTCGemini), as shown in Fig. 21.6, which owes its amphiphilicity to two hydrophobic dodecyl chains on one side of the HTC core and two hydrophilic triethylene glycol (TEG) chains on the other side. HTCGemini easily forms a stable radical cation, both in solution and in the bulk, upon oxidative doping with nitrosonium tetrafluoroborate (NOBF4) [27]. In Fig. 21.7, the reaction scheme from Pena et al. for the aspect for the use of the palladium catalyst in the cyclotrimerization of arynes and synthesized triphenylenes with [Pd(PPh3)4] catalyst with reagent CsF, Bu4NF, BuLi [29]. In his further extended work, Pena et al. synthesized hexabenzotriphenylene and other strained polycyclic aromatic hydrocarbons by palladium-catalyzed cyclotrimerization of arynes [30]. Polycyclic aromatic hydrocarbon (PAH)/nanographene constitutes a broad family of organic compounds that have been extensively studied in the fields of material science, environmental chemistry, and medicinal chemistry. Romero et al. synthesized ortho-(trimethylsilyl) triphenylenyl triflates. It also a palladium catalyzed [2 + 2 + 2] cycloaddition of benzene [31]. Alonso et al. synthesized a clover-shaped cata-condensed nanographene with 16 fused benzene rings with a palladium catalyst, as shown in Fig. 21.8 [32]. Another clover-shaped nanographene threefold symmetric C78H36 molecule with 22 fused benzene rings is reported by Schuler et al. by using palladium-catalyzed cyclotrimerization of an aryne and a [2 + 2 + 2] cycloaddition synthesis scheme [33].

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Nanocarbon and its Composites

Fig. 21.4 Scheme of benzannulation of each alkyne of a substituted poly (phenyleneethynylene) 1 provides polyphenylene 3 [22]. (II) Structure Incorporating Seven- or Eight-Membered Rings

As two-dimensionally confined structure of graphene, nanographene in principle consists of only six-membered rings. However, microscopy studies have revealed that graphene contains rings of other sizes, including five-, seven-, and eight-membered rings as defects, particularly at the grain boundaries of the graphene sheet shown by CVD. Therefore, extended PAHs containing non six-membered rings can indeed be considered graphene molecules, which allows model studies of defective graphene and may also find application in optoelectronics [19]. Li et al. developed graphene by a CVD growth process on copper foil (25 μm thick) at a temperature up to 100°C using a mixture of methane and hydrogen [34]. Kim et al. synthesized polycrystalline graphene by the Li et al. method and prepared a TEM sample via the direct transfer method. Atomic resolution TEM imaging, electron diffraction, and Raman spectroscopy analysis confirmed the mostly graphene single layer. By using a complementary TEM technique, it was found that a high-angle graphene boundary (GB) consists of an

Fig. 21.5 Synthesis scheme of HTC 1 and fully cyclodehydrogenated HTC 2 based on anthradithiophene-5,11-dione 3 [27].

Fig. 21.6 Molecular structure of HTCGemini [27].

Fig. 21.7 Scheme for the cyclotrimerization of arynes.

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Nanocarbon and its Composites

Ph Ph OTf

Ph

Ph

Ph

Ph

CsF, [Pd(PPh3)4]

OTf

CH2Cl2/MeCN, 40 °C

Ph Ph

Fig. 21.8 Synthesis of cloverphene.

array of alternating pentagons and heptagons without other defect structures such as vacancies. The pentagon and heptagon GB is fairly stable under the electron beam providing its mechanical integrity [35]. Kurasch et al. observed the GB migration in synthesized polycrystalline graphene atom by atom in real time utilizing AC-HRTEM. These findings suggest that graphene may offer the first experimentally accessible platform for in situ atomic level investigation of a host of GB phenomena, including solute drag, Zener pinning, interaction with other lattice defects, and coupling to mechanical stresses [36]. Lahiri et al. grew graphene layers on a 10 mm diameter Ni (111) single crystal wafer and reported the realization of one-dimensional topological defects in graphene containing octagonal and pentagonal sp2 hybridized carbon rings embedded in a perfect graphene sheet. When the surrounding graphene lattice is doped, the defect acts as a quasi one-dimensional metallic wire. The octagonal containing a defect molecule extends the application of graphene as a membrane material for the selective diffusion of atoms of small molecules through an otherwise impermeable graphene membrane [37]. Bunch et al. demonstrated that a monolayer graphene membrane is impermeable to standard gases, including helium. This pressurized graphene membrane is the world’s thinnest balloon and provides a unique separation barrier between two distinct regions that is only one atom thick [38]. Luo et al. explained that the properties of curved π molecules depend on their curvature by synthesizing and characterizing two types of curved π molecules that are π isoelectronic to the planar HBC. The curvature of the π-face plays a role in determining the frontier molecular orbital energy levels and the π-π interaction embedding heptagon in HBC leads to a novel saddle-shaped molecule [39]. Cheung et al. synthesized two heptagonembedded soluble derivatives of C70H26 (1a, b) and C70H30 (2a, b), a new saddleshaped polycyclic arene, as shown in Fig. 21.9. From saddle-shaped diketones (3a, b), it was found that 1b and 3b behaved as p-type semiconductors in solutionprocessed thin film transistor while the amorphous thin film of 2b appeared as an insulator [40]. Kawasumi et al. synthesized a 26-ring C80H30 nanographene that incorporates five- and seven-membered rings and one five-membered ring by stepwise chemical

Nanographene composite ion exchanger properties and applications

639

Fig. 21.9 Molecular structures of saddle-shaped conjugated molecules 1a, b, 2a, b, and 3a–c [37].

methods. This new type of nanocarbon has a grossly wrapped structure and the largest PAH other than fullerene and its derivatives, whose structure (Fig. 21.10) has been determined by X-ray crystallography. Nonhexagonal ring defects, particularly the imposition of seven-membered ring negative curvature, not only cause graphene sheets to wrap but also are predicted to alter their electronic and optical properties. That wrapping causes dramatic improvements in the solubility properties of the material [41]. (III) Selective Heteroatom Doping

Heteroatoms can be incorporated in the graphite lattice either during synthesis or postsynthetic treatment. Wang et al. comprised the in situ and posttreatment approaches for the heteroatom doping synthesis method, including chemical vapor deposition (CVD) and the ball milling in situ method while wet chemical methods, thermal annealing of graphene oxide (GO) with heteroatom precursors, and plasma arc discharge approaches are postsynthetic methods. Significant changes in properties come when graphene is doped with group IIIa elements (B), group Va elements (Na nd P), group VIa elements (O and S), group VIIa elements (F, Cl, Br and I), and other dopants such as Si, H2O, O2, and NO [42]. Maiti et al. summarized some of the precursors used in different processes for heteroatom-doped graphene. Benzylamine, imidazole, ethylene diamine, Fe-phthalocyanine, N, N0 -dimethylformamide, polymers (P4VP, PMPY, PPP, PMV1, PMV1) and ammonia, melamine, acetonitrile, and pyridine at 500–1100°C are some of the N (nitrogen) precursors used in the vapor phase growth process for heteroatom-doped CNTs and graphene. In the postsynthetic annealing

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Fig. 21.10 Molecular structure of C80H30 (4) and its deca-t-butyl derivative C120H110 (8) [41].

process, ammonia is used as the N precursor in which oxidized CNT or graphene oxide are used as the starting material. At 400–1100°C in pristine graphene as a starting material, melamine, polypyrrole, urea, and cyanamine are used as N precursors. In another technique used for N-atom doping with GO as the starting material, urea, ammonia, hydrazine, and dicyandiamide are used for the N precursor. A vapor phase growth process carrying aliphatic or aromatic small molecules or polymers as a carbon source doped Boron with Triphenyl borane, Boron powder, Diborane as B (Boron)-precursor. Boric acid as a B precursor is used in the postsynthetic annealing process. Boron or boron tribromide as the B precursor is used in solution processing with GO as the starting material. P (Phosphorous)-precursor, Triphenyl phosphine used in vapor phase growth process methane, ethane, acetylene, benzene as starting material. In the postsynthetic annealing process, triphenyl-phosphine or 1-butyl-3-ethlyimidazoliumhexafluorophos phate used as the P precursor. Dimethyl sulfide, sulfur powder, thiophene, and benzyl disulfide are also used as the S (sulfur) precursor in various processes of synthesis such as the vapor phase growth process, postsynthetic annealing, etc. [43]. (IV) Direct edge functionalization

Polycyclic aromatic hydrocarbons (PAHs) such as pyrene, triphenylenes, and HBC have attracted much attention as fragments of graphene [44]. By electrophilic substitution of graphene, Tan et al. reported the atomically precise chlorination of a nanographene series with a carbon number ranging from 42 to 222 with a molecular size of 1.2–3.4 nm in high yield [45]. According to Tan et al., sulfur annulation of hexa-peri-hexabenzocoronene (HBC) by thiolation of perchlorinated HBC gave on efficient route to modulate the optical and electrochemical properties [46]. Yamaguchi et al. used iridium catalysis for direct C-H borylation of HBC and synthesized

Nanographene composite ion exchanger properties and applications

641

hydroxy-substituted HBC by oxidation of the boryl group [44]. Ozaki et al. presented APEX (annulative π-extension) methodology, which is a two-step nanographene synthetic method occur at K-resin by Palladium catalyst it unfunctionalized PAH can be directly used for π-component assembly and π-extension without any prefunctionalization [47].

21.3

Properties of a nanographene ion exchanger

Mo et al. designed and synthesized graphene/ionic liquid composite films and investigated five different self-assembled monolayers (SAM) of 1-alkyl-3-(3triethoxysilylpropyl) imidazolium (TSM) salts having different anions [Cl, PF6, SO3, BF4, N(SO2CF3)2] for ion exchange. The results indicated that anions played a great role in determining the graphene surface properties and sensitivity to the solvent system. The effect of the solvent system on the ion exchange ratio on the graphene surface has also been investigated through the anion exchange from Cl to NðSO2 CF3 Þ2  . It was completed in 2 h while the exchange took 24 h in acetone. Meanwhile, the contact angle of ion exchange in pure water was stable at 76 degrees. In contrast, the contact angle of ion exchange in acetone was approximately 65 degrees. The results indicate that the solvent system significantly affected the ion exchanger procedure through the conversion rate and ionic equilibrium [48]. Jang et al. collected the estimated physical constant of carbon nanotubes (CNTs), carbon nanofibers (CNFs), and nanographene platelets (NGPs) from various open literature sources and their own estimations [49] (Table 21.1). Altan et al. studyied the heat epoxy polymer and nanocomposite with 0.5 and 1 wt% of nanographene particles. In Fig. 21.11, a SEM micrograph shows the good dispersion of nanographene particles in the epoxy matrix. The tensile strength increased about 0.5% and 5.2% with the neat epoxy. Stress-strain curves for the neat epoxy and nanographene particle-reinforced epoxy composite can be seen in Fig. 21.12. The result is compatible with the literature in which it is expected that the mechanical properties of the polymer material are generally enhanced by adding nanographene particles [50]. When a graphene sheet is cut into nanofragments, two distinct edge types-zigzag and armchair-are created. Fujii et al. shows the electronic structure of the nanographene or graphene edge, depending on the distinct edge type. Fig. 21.13 shows typical examples of UHV-STM images and scanning tunneling spectroscopy (STS) spectra of hydrogenated graphene edges [51]. Because of the special shape and edge structure, different types of NGs are usually claimed to have a non-Kekule or an open shell structure, which inevitably results in one or more unpaired electrons within the molecule. Thus, Z-NG offers promising potential in electronics and spintronic devices [3]. Gahlot et al. performed the desalination application with a nanocomposite ion exchange membrane (IEM) consisting of GO (0.5, 1.0, 2.5, and 10% w/w). Fig. 21.14 shows the proposed scheme for the water desalination process [14].

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Nanocarbon and its Composites

Table 21.1 Estimated physical constants of CNTs, CNFs, and NGPs [49] Carbon nanofibers

Property

Single-walled CNTs

Specific gravity

0.8 g/cm3

Elastic modulus Strength

1 TPa (axial direction)

Resistivity

5–50 μΩ cm

Thermal conductivity

Up to 2900 Wm/K (estimated)

Magnetic susceptibility

22  106 emu/g (radial) 0.5  106 emu/g (axial)

N/A

Thermal expansion

Negligible in the axial direction

1  106 K1 (HT; axial)

Thermal stability Specific surface area

>700°C (in air); 2800°C (in vacuum) Typically, 10–200 m2/g up to 1300 m2/g

450–650°C (in air)

21.4

50–500 GPa

1.8 (AG)–2.1 (HT) g/cm3 AG ¼ as grown; HT ¼ heat-treated (graphitic) 0.4 (AG)–0.6 (HT) TPa 2.7 (AG)–7.0 (HT) GPa 55 (HT)–1000 (AG) μΩ cm 20 (AG)–1950 (HT)Wm/K

10–60 m2/g

NGPs 1.8–2.2 g/cm3

1 TPa (in-plane) 100–400 GPa 50 μΩ cm (in-plane) 5300 Wm/K (in plane) 6–30 Wm/K (c-axis) 22 9106 emu/g (\ to plane); 0.5 9106 emu/g (jj to plane) 1  106 K1 (in-plane) 29  106 K1 (c-axis) 450–650°C (in air) Typically, 100–1000 m2/g, up to 2600 m2/g

Applications of nanographene composite ion exchangers

As graphene becomes the promising candidate for several applications in diverse fields of technology, likewise nanographene puts its hold on environment and health applications which usually done by filtration process of surrounding in terms of water, air and soil which affected most by the urbanization and industrialization. The scientific community takes concerns with NG and its composites and has used the separation technique as a tool for purification of waste. Water desalination, reverse osmosis and nanofiltration, sorption, and chlorine sensors are the progressive applications done by the NG and its composite. They give better results than traditional adsorbents/ion exchangers. All these processes are part of the ion exchanger techniques used in [8, 13, 14, 16, 52, 53].

Fig. 21.11 (A) SEM micrograph of nanographene particles, (B) SEM micrograph of the neat epoxy, (C) SEM micrograph of 0.5 wt% nanographene particles in epoxy matrix, and (D) SEM micrograph of 1 wt% nanographene particles in epoxy matrix [47].

20 1wt% graphene/epoxy

Stress (MPa)

Neat epoxy

0.5wt% graphene/epoxy

10

0 0

5

10

Strain(%) Fig. 21.12 Stress-strain curves of the samples [47].

15

20

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Nanocarbon and its Composites

Fig. 21.13 (A) Atomically resolved UHV-STM tunneling current image (5.6  5.6 nm2) of an armchair edge. A model of the honeycomb lattice is superimposed on the image to clarify the edge structures. (B) dI/dVs curve from STS measurements taken at the edge shown in (A). (C) Atomically resolved STM image of zigzag and armchair edges (9  9 nm2). (D) dI/dVs curve from STS data at a zigzag edge in (C). Images were acquired with a sample bias voltage of Vs ¼ 20 mV.

Liang et al. synthesized graphene oxide through the thermal exfoliation method and used this graphene oxide membrane for the removal of Ca+2 and Mg+2 ions from water with an excellent efficiency, that is, a 1 mg GO membrane can absorb as much as 0.05 mg Ca+2 ions or a 1 g GO membrane can remove 50 mg Ca+2 ions [54]. Tju et al. studied the Fe3O4/CuO/ZnO composite with various concentrations of NGPs as the adsorbent to degrade the organic dye methylene blue. The results were best with 15 wt% concentration of NGP in a dark alkaline condition, that is, 29 mg/g than 18 mg/g without NGP [55]. Sadegh et al. reviewed nanomaterials as effective adsorbents and their applications in wastewater; they found graphene-based adsorbents to be the best with some limitations [56]. Rashvand et al. synthesized a novel graphenebased magnetic nanocomposite sorbent via a one-step coprecipitation method for the extraction and quantification of targeted pharmaceutical and personal care products (PPCPs), that is, ethyl paraben (Et-P), propyl paraben (Pro-P), butyl paraben (ButP), benzophenone 3 (BZ-3), 4-methylbenzilidine camphor (4-MBC), diclofenac (Dic), and ibuprofen (Ibu), in complex environmental water samples [57]. Xu et al. presented the broader perspective of graphene-based materials for radionuclide from water and wastewater along with heavy metals and came to the conclusion that arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), lead (Pb), and

Nanographene composite ion exchanger properties and applications

645

Fig. 21.14 Schematic representation for graphite to composite membrane preparation [14].

uranium (U) have been studied much more than cobalt (Co), nickel (Ni), antimony (Sb), zinc (Zn), radionuclides, light metalsm and nonmetals ions. Due to a large surface area, an abundant functionalized group, and extremely hydrophilic properties, graphene or graphene oxide as the adsorbent has more advantages [58]. Shabrany et al. also studyied the ZnO/CuO/NGP catalytical activity for the degradation of methylene blue from aqueous solutions. ZnO/CuO with a 10 wt% NGP composite exhibited the highest catalytic activity; it was also found that holes are the main charged carriers on the degradation of MB under visible light and ultrasound irradiation [59]. Firdhouse et al. prepared a low-cost adsorbent using the aqueous extract of Amaranthus polygonoides for the reduction of GO. The synthesized graphene was embedded with silver nanoparticles and Moringa oleifera pulverized seed powder, which possessed better adsorbent properties than conventional activated charcoal in wastewater treatment. This modified graphene was used as an adsorbent for simulated textile, tannery, and paper mill effluents [60]. Li et al. evaluated bird’s nest-like, nanographene-shell encapsulated Si@SiO2 nanoparticles for the Li-ion anode; this revealed a specific capacity of 2634 mAh/g at a current density of 0.2 A/g and an excellent rate and cyclic performance [61].

21.5

Conclusion

Carbon-based graphene nanocomposites are currently controlling the groundbreaking applications in wastewater purification in research fields, all described with an adsorbent, permeable membrane and antimicrobial activity. As we can find, to simplify opened new pathways of purification of wastewater from graphene nanocomposite

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Nanocarbon and its Composites

with combination of various properties as nanofillers. This chapter has highlighted the different types of fabrication processes and the applications of different types of graphene nanocomposites as ion exchangers in microextraction techniques. It is evident from the recent research that four different types of novel preparation methods of graphene nanocomposites via different precursors which is act as a catalyst. Certain modified methods have also been shown for nanocomposite fabrication that are effective for the sorption of different heavy metals from the water. Newly designed and synthesized nanocomposite films play a significance role in determining graphene surface properties and solvent systems through ion exchange. The estimated properties of different types of NG, NGPs, GQT, CNF, and CNT are specific gravity, elastic modulus, strength resistivity, thermal conductivity, magnetic susceptibility, thermal expansion, thermal stability, and specific surface area, respectively. The applications of graphene nanocomposites or nanoparticle sheets have been extensively used in water purification. Recently, graphene nanocomposites have shows very capable application in water purification, that is, water desalination, sorption, catalytic activity, reverse osmosis, nanofiltration, chlorine sensors, and remediation of radionuclide and heavy metal agents in complete water purification systems.

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[35] Graphene P, et al. Grain boundary mapping in polycrystalline graphene. ACS Nano 2011; 5(3):2142–6. [36] Kurasch S, et al. Atom-by-atom observation of grain boundary migration in graphene. Nano Lett 2012;12(6):3168–73. [37] Lahiri J, Lin Y, Bozkurt P, Oleynik II, Batzill M. An extended defect in graphene as a metallic wire. Nat Nanotechnol 2010;5(5):326–9. [38] Bunch JS, et al. Impermeable atomic membranes from graphene sheets. Nano Lett 2008; 8(8):2458–62. [39] Luo J, Xu X, Mao R, Miao Q. Curved polycyclic aromatic molecules that are π-isoelectronic to hexabenzocoronene. J Am Chem Soc 2012;134(33):13796–803. [40] Cheung KY, Xu X, Miao Q. Aromatic saddles containing two heptagons. J Am Chem Soc 2015;137(11):3910–4. [41] Kawasumi K, Zhang Q, Segawa Y, Scott LT, Itami K. A grossly warped nanographene and the consequences of multiple odd-membered-ring defects. Nat Chem 2013;5(9):739–44. [42] Wang X, Sun G, Routh P, Kim D-H, Huang W, Chen P. Heteroatom-doped graphene materials: syntheses, properties and applications. Chem Soc Rev 2014;43(20):7067–98. [43] Maiti UN, et al. 25th anniversary article: chemically modified/doped carbon nanotubes & graphene for optimized nanostructures & nanodevices. Adv Mater 2014;26(1):40–67. [44] Yamaguchi R, Hiroto S, Shinokubo H. Synthesis of oxygen-substituted hexa-perihexabenzocoronenes through ir-catalyzed direct borylation. Org Lett 2012;14(10):2472–5. [45] Tan YZ, et al. Atomically precise edge chlorination of nanographenes and its application in graphene nanoribbons. Nat Commun 2013;4:1–7. [46] Tan YZ, et al. Sulfur-annulated hexa-peri-hexabenzocoronene decorated with phenylthio groups at the periphery. Angew Chem Int Ed 2015;54(10):2927–31. [47] Ozaki K, Kawasumi K, Shibata M, Ito H, Itami K. One-shot K-region-selective annulative π-extension for nanographene synthesis and functionalization. Nat Commun 2015;6. [48] Mo Y, Wan Y, Chau A, Huang F. Graphene/ionic liquid composite films and ion exchange. Sci Rep 2014;4:1–8. [49] Jang BZ, Zhamu A. Processing of nanographene platelets (NGPs) and NGP nanocomposites: a review. J Mater Sci 2008;43(15):5092–101. [50] Altan M, Uysal A. An experimental study on mechanical behavior of nanographene/epoxy nanocomposites. Adv Polym Technol 2016;0:1–6. [51] Fujii S, Enoki T. Nanographene and graphene edges: electronic structure and nanofabrication. Acc Chem Res 2013;46(10):2202–10. [52] Pan S, Deen MJ, Ghosh R. Low-cost graphite-based free chlorine sensor. Anal Chem 2015;87(21):10734–7. [53] Hussain A, et al. Hybrid monolith of graphene/TEMPO-oxidized cellulose nanofiber as mechanically robust, highly functional, and recyclable adsorbent of methylene blue dye. J Nanomater 2018;2018:. [54] Liang J, Huang Y, Zhang F, Zhang Y, Li N, Chen Y. The use of graphene oxide membranes for the softening of hard water. Sci China Technol Sci 2014;57(2):284–7. [55] Tju H, Taufik A, Saleh R. Adsorption of methylene blue using Fe3O4/CuO/ZnO/ nanographene platelets (NGP) composites with various NGP concentration. J Phys Conf Ser 2016;776(1):6–13. [56] Sadegh H, et al. The role of nanomaterials as effective adsorbents and their applications in wastewater treatment. J Nanostruct Chem 2017;7(1):1–14. [57] Rashvand M, Vosough M, Kargosha K. Preparation of magnetic nanographene sorbent for extraction and quantification of targeted PPCPs in environmental water samples. RSC Adv 2016;6(79):75609–17.

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[58] Xu L, Wang J. The application of graphene-based materials for the removal of heavy metals and radionuclides from water and wastewater. Crit Rev Environ Sci Technol 2017;47(12):1042–105. [59] Shabrany H, Tju H, Taufik A, Saleh R. Investigation of ZnO/CuO/nanographene platelets composites catalyst for the degradation of methylene blue from aqueous solution. Mater Sci Forum 2016;864:117–22. [60] Firdhouse MJ, Lalitha P. Nanosilver-decorated nanographene and their adsorption performance in waste water treatment background. Bioresour Bioprocess 2016;3. [61] Li B, Jiang Y, Jiang F, Cao D, Wang H, Niu C. Bird’s nest-like nanographene shell encapsulated Si nanoparticles—their structural and Li anode properties. J Power Sources 2017;341:46–52.

Carbon dots: preparation, properties, and application

22

Selin Sagbas*, Nurettin Sahiner*,† *Faculty of Science and Arts, Chemistry Department, Canakkale Onsekiz Mart University, Canakkale, Turkey, †Nanoscience and Technology Research and Application Center (NANORAC), Canakkale Onsekiz Mart University, Canakkale, Turkey

Chapter Outline 22.1 Introduction 651 22.2 CD preparation methods 652 22.2.1 Top-down methods 652 22.2.2 Bottom-up methods 655

22.3 Applications of CDs 22.3.1 22.3.2 22.3.3 22.3.4 22.3.5

22.4 Concluding remarks References 673

22.1

662

In vivo and in vitro bioimaging 663 Cancer therapy 664 Gene and drug delivery 667 Sensor and biosensors 667 Catalysis and energy 671

672

Introduction

Carbon dots (CDs), also called carbon quantum dots, are a new type of zerodimensional (0D) photoluminescent nanocarbon with a size range less than 20 nm [1–3]. However, it was reported that particle sizes can increase up to 60 nm [4,5]. The photoluminescence CDs were first prepared via purification of single-walled carbon nanotubes, as reported in 2004 by Xu et al. [6]. Then, Sun et al. improved the CDs for strong photoluminescence effects through laser ablation of the carbon target as a mixture of graphite powder and cement [1]. Ever since, there has been growing interest in the fabrication of fluorescent CDs as a latent substitute for toxic metal-based quantum dots (QDs) because of their intriguing properties, including exceptional optical and fluorescence characteristics with high quantum yield (QY), simple and inexpensive preparation methods from renewable sources, high thermal and optical photostability, tunable excitation and emission, easy surface functionalization, and nontoxic nature with high biocompatibility [7,8]. Thus, fluorescent CDs are of great interest in a wide range of applications such as fluorescence imaging, nanocarriers for Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00022-5 © 2019 Elsevier Ltd. All rights reserved.

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drug/gene delivery by means of conjugation and controlled release purposes, medical diagnostics and theranostics, analyte detection, biosensors, optical/electrochemical sensors, light-emitting diodes, energy conversion and storage, electro and photocatalysis, etc. [9–12]. Many synthesis methods of CDs have been developed using the two main classified approaches, top-down and bottom-up [13]. In top-down approaches, CDs are prepared by fragmentation of large carbon materials by using acid oxidation, arc discharge, laser ablation, ultrasonic/electrochemical exfoliation, or hydrothermal/solvothermal exfoliation methods. On the other hand, in the bottom-up approaches, molecular precursor materials are carbonized via hydrothermal, microwave, and thermal pyrolysis methods [14]. The three main governing parameters-the quantum confinement effects, the surface state, and the molecule state-are very important in the design of fluorescent CDs. Fortunately, these parameters can be readily adjusted by changing the synthesis strategy, that is, using different precursors or synthesis methods [15,16]. CDs can be designed to have various functional groups including hydroxyl, carboxyl, carbonyl, ether, and epoxy in addition to their easy functionalization with amine, phosphorous, sulfur, and boron-containing heteroatoms containing functional groups with the different organic, polymeric, and biological materials during the preparation process. Therefore, the fluorescent properties can be tuned by the size and extent of functional groups on the surface of the CDs by using different precursors and synthesis methods [17]. In this chapter, we focus on the synthesis processes using specific precursors, reaction conditions, functionalization, heteroatom doping, and luminescence mechanisms along with their advantages on a wide range of utilizations in different fields such as bioimaging, cancer therapy, drug/gene delivery, sensing, biosensing, energy conversion and storage, and electrocatalytic and photocatalytic applications.

22.2

CD preparation methods

In recent decades, the preparation methods for fluorescent CDs have been improved by employing a wide range of preparation techniques. The most selected synthesis techniques are focused on their excellent optical properties, and as mentioned earlier, the top-down and bottom-up approaches as summarized in Fig. 22.1 have been the most widely used methods for CD preparation.

22.2.1 Top-down methods In the top-down approach, the relatively large materials, including graphene, graphene oxide sheets, carbon nanotubes, carbon fibers, carbon soot graphite, etc., are broken down into small pieces as sp2 fluorescence carbon structures via laser ablation, arc discharge, and acidic, ultrasonic, chemical, hydrothermal, and solvothermal exfoliations [5,12,14]. Although the first reported fluorescent carbon materials were prepared by purification of single-walled carbon nanotubes as a derivative from arc discharge soot [6], there are limited studies about CD preparation by using the arc discharge method, whereas this is a generally used technique in the preparation of carbon nanotubes. Arora et al. indicated that arc discharge is the electrical breakdown of a gas to

Carbon dots: preparation, properties, and application 653

Fig. 22.1 See figure legend on opposite page. (Continued)

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Nanocarbon and its Composites

generate plasma by using electric current with anode and cathode electrodes. The anode is filled with carbon precursors and started to generate plasma with an arc current under a gas media at a high temperature of nearly 4000 K. Then, the carbon vapor aggregated in the gas toward the cathode and cooled down [18]. Sun et al. improved the CDs with strong photoluminescence effects through the laser ablation method of the carbon target as the mixture of graphite powder and cement [16]. The laser ablation technique can involve three steps: (1) the carbon materials absorb the high energy by the laser pulse; (2) electrons are stripped from the atoms through photoelectric and thermionic emission; and (3) a high electric field produces a strong repulsive force between positive ions and solid material, breaking down CDs [19]. In the top-down approach, exfoliation by different techniques such as acidic, ultrasonic, chemical, hydrothermal, and solvothermal processes has been used in the synthesis of CDs. In acid oxidizing exfoliation methods, strong acids such as HNO3, H2SO4, and even KMnO4 have been widely used to exfoliate CDs by the oxidation of carbon materials. Hu et al. reported the oxidizing of coal with H2O2 to prepare CDs to escape the side effects of the strong acids; damage the original structure of graphitic precursors as costly purification and extreme preparation conditions with toxic chemicals [20]. To overcome these types of problems, Lu et al. reported the use of an ultrasonicassisted, liquid-phase exfoliation technique to prepare graphene carbon dots. Briefly, graphite can be well dispersed in organic solvent and the graphite layers cleave apart and are exfoliated by the surface energy for van der Waals forces of graphite layers under the ultrasonication process. This study supported that sonication can enhance the exfoliation effects and dispersion in the organic solvent [21]. Fig. 22.1, cont’d The schematic representations of the CD preparation processes by top-down and bottom-up approaches such as arc discharge, laser ablation, acid oxidizing exfoliation, ultrasonic exfoliation, hydrothermal/solvothermal, microwave-assisted, microreactor, and oil-bath. Data from Arora N, Sharma NN. Arc discharge synthesis of carbon nanotubes: comprehensive review. Diamond Relat Mater 2014;50:135–50; Xiao J, Liu P, Wang CX, Yang GW. External field-assisted laser ablation in liquid: an efficient strategy for nanocrystal synthesis and nanostructure assembly. Diamond Relat Mater 2017;87:140–220; Hu S, Wei Z, Chang Q, Trinchi A, Yang J. A facile and green method towards coal-base fluorescent carbon dots with photocatalytic activity. Appl Surf Sci 2016;378:402–7; Lu L, Zhu, Y, Shi C, Pei YT. Large-scale synthesis of defect-selective graphene quantum dots by ultrasonic-assisted liquid-phase exfoliation. Carbon 2016;109:373–83; Zeng YW, Ma DK, Wang W, Chen JJ, Zhou L, Zheng YZ, et al. N, S co-doped carbon dots with orange luminescence synthesized through polymerization and carbonization reaction of amino acids. Appl Surf Sci 2015;342:136–43; Lu W, Gong X, Nan M, Liu Y, Shuang S, Dong C. Comparative study for N and S doped carbon dots: Synthesis, characterization and applications for Fe3+ probe and cellular imaging. Anal Chim Acta 2015;898:116–27; Rao L, Tang Y, Li Z, Ding X, Liang G, Lu H, et al. Efficient synthesis of highly fluorescent carbon dots by microreactor method and their application in Fe3+ ion detection. Mater Sci Eng C 2017;81:213–23; Liu X, Yang C, Zheng B, Dai J, Yan L, Zhuang Z, et al. Green anhydrous synthesis of hydrophilic carbon dots on large-scale and their application for broad fluorescent pH sensing. Sens Actuators B 2018;255:572–9.

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655

Most synthesis methods employing top-down methods are not suitable for the preparation of large-scale CD production because of their low QY values, special and expensive complicated equipment requirements, and uncontrollable and toxic process conditions [14].

22.2.2 Bottom-up methods In the bottom-up approach, CDs as bulk carbon materials are formed as the precursors change to particle forms via chemical and physical techniques, including hydrothermal, solvothermal, microwave-assisted, and thermal pyrolysis [5,12,14]. Recently, there has been much interest in the development of bottom-up approaches for the preparation of CDs due to the precise control of precursor molecules, ease of techniques, low cost, and practicality and convenience of the procedure with generally nontoxic precursors. Especially, the hydrothermal method is one of the most popular processes employed in CD synthesis because of the simplicity of the synthesis procedure, allowing uniform particle size with high QY. Zhu et al. reported that the highest quantum as high as about 80% of CDs that is almost equal to fluorescent dyes [22]. They used citric acid and ethylenediamine as carbon and nitrogen sources to be utilized in ionization to the condensation, polymerization, and carbonization steps by hydrothermal treatment at 150–300°C for 5 h to prepare polymer-like and carbonaceous CDs. Even the utilization of amino acids such as serine and cystine is reported in the preparation of CDs [23]. As illustrated in Fig. 22.2, amino acids are a polymerization by condensation reaction in the initial stage followed carbonization by hydrothermal treatment. These peptide chains were carbonized with extended reaction times, for example, 5 h

Fig. 22.2 Schematic illustration for the formation of the process of N and S codoped CDs and the TEM images and size distribution graphics. Data from Zeng YW, Ma DK, Wang W, Chen JJ, Zhou L, Zheng YZ, et al. N, S co-doped carbon dots with orange luminescence synthesized through polymerization and carbonization reaction of amino acids. Appl Surf Sci 2015;342:136–43.

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Microwaves

200°C Precursor

160°C 10 min

200°C 30 min

10 min

Fig. 22.3 Schematic diagram of N-doped CDs. From He G, Shu M, Yang Z, Ma Y, Huang D, Xu S, et al. Microwave formation and photoluminescence mechanisms of multi-states nitrogen doped carbon dots. Appl Surf Sci 2017;422:257–65.

at 200°C and nearly 2.5 nm of N and S codoped CDs were prepared with 7% quantum yield [23]. Among the bottom-up approaches, the microwave irradiation process has been more favorable due to rapid synthesis and commercial reasons [24]. The N-doped CD preparation process has three main steps: polymerization, dehydration, and carbonization, which take place by microwave irradiation methods. As demonstrated in Fig. 22.3, amine-rich organic precursors were mixed by general sonication and the cross-linked cluster was prepared via intermolecular and intermolecular dehydration at 160°C for 10 min. These organic clusters have a QY as high as 51.6%, similar to fluorescent dyes with strong blue fluorescence related to rich amine bonds. These amine bonds were hydrolyzed at 200°C and a part of the organic groups was gradually carbonized to generate a carbon core. The QY of these materials was decreased 32.47% because some fluorescence groups were depleted during the carbonization step. As the heating time is increased, more fluorescence groups can be carbonized in the carbon core and the QY further decreases to 4.55%. These results support that reaction temperature and time can play a significant role in the N-doped CD formation as well as their fluorescence properties [25]. In the synthesis of CDs, different types of precursors such as citric acid, urea, glycerol, cysteamine, polyamines, and acrylamide and biological precursors such as carbohydrates, amino acids, milk, lignin, mushroom, etc., were reported by employing hydrothermal/microwave-assisted methods [26–29]. The optical and/or fluorescence properties of the CDs are affected by the choice of precursors as this is directly related to the amount of carbon content and the functional groups. To expand the application areas and render different physicochemical properties, CDs can be prepared by using different functional materials or introducing novel functional groups containing precursors with nitrogen, phosphorus, sulfur, and boron atoms during the synthesis procedures. These functional groups of the precursor molecules that are inherently transferred to surface carbon atoms may allow further postmodification. Therefore, CDs doped with nitrogen (N), sulfur (S), phosphorus (P), and boron (B) atoms can be readily obtained by using amine, phosphonyl, sulfonyl, and boronyl groups containing precursor molecules from organic, polymeric, and biological compounds.

Carbon dots: preparation, properties, and application

657

Many researchers established that the external and internal characteristics, including particle size, crystallinity of the graphitic core, elemental composition, dispersity in different solvents, and the chemical and optoelectronic properties of the CDs, are strongly based on the functional groups of the precursor materials [30]. In the doping process with N, S, P, and B atoms, the photoluminescence, optoelectronic, and sensing abilities have been remarkably enhanced, depending on the adjusted intrinsic electronic structure and the functionalities of the precursor molecules. For example, Zhai et al. prepared fluorescence CDs from citric acid (CA) and ethylenediamine (EDA), diethylenetriamine (DETA), and triethylenetetraamine (TETA) precursors using microwave pyrolysis techniques in 2 min. It was stated that the reaction time has paramount significance on the photoluminescence (PL) of the prepared CDs, and the optimum reaction time was determined as 2 min, as the short irradiation time, for example, up to 2 min, is not enough to complete carbonization to attain the best PL performance. Interestingly, the PL effects were found to decrease for a longer irradiation time than 2 min due to the significant water evaporation that overheated the CDs and destroyed their surface structure [31]. Gao et al. prepared N-doped CDs, S-doped CDs, and N and S codoped CDs using citric acid (CA) as a carbon source and different S- and N-containing material derivatives of amino acids such as N-acetyl-cysteine (L-NAC), L-cysteine (L-Cys), D-cysteine (D-Cys), DL-homocysteine (DL-Hcy), β-cysteamine (β-CA), glutathione (GSH), L-methionine (L-Met), L-lysine (L-Lys), L-isoleucine (L-Iso), L-glutamic acid (L-Glu), L-histidine (L-His), and L-arginine (L-Arg) and synthetic molecules such as 3-mercaptopropionic acid (3-MPA), sublimed sulfur (S), thioacetamide (TAA), and sodium thiosulfate pentahydrate (Na2S2O3) precursors [32]. In the process, citric acid was treated with L-NAC at 70°C for 12 h in the first step, followed by carbonization by hydrothermal treatment at 200°C for 3 h. Various parameters such as the reaction time and temperature, the mole ratio of the precursors, the types of precursors and the doping effects were investigated. As illustrated in Fig. 22.4, the PL intensity, UV-Vis spectra, and QY results for different reaction conditions were investigated, and the results were summarized in Table 22.1. The PL intensity and QYs were increased by increasing the reaction time at 70°C, but it was found that the reaction time was not very effective on the PL performance of L-NAC:CA-derived CDs at high temperature 200°C, as seen in Figs. 22.4A–F. In that study, CA was used as a carbon source and L-NAC was used for doping N and S atoms on the new surface state of the prepared CD. As the L-NAC concentration was increased, the QY was increased from 1.6% to 55.7% because of the doping and surface functionalization of the CD. The low QYs were found as 2.4% and 1.6% for only L-NAC- and CA-derived CDs, respectively. It was assessed that only CA alone as the basic scaffold for CDs as a carbon source is not enough, and the optical properties of CA-derived CDs elucidated the multilevel aspects, the elementary composition, the doped atom content, and the chemical structure of the reactants [32]. In addition, the effects of various types of biological and synthetic reactants that can be used as N and S atom sources that play a significant role on the PL performance were also investigated, and the molecular structure and their QYs are given in Table 22.1.

658

Nanocarbon and its Composites 1000

70°C 3h 70°C 6h 70°C 9h 70°C 12h

70°C 3h 70°C 6h 70°C 9h 70°C 12h

0.4 0.3 0.2

400

(A) 350

400

450

500

45

550

600

(C)

0.0 200 250 300 350

600

0.3

A

400

0.2 0.1

200

(D) 400

450

500

550

Wavelength (nm) 1000

0.0 200 250 300

400

350 400 450 500

0.4

6

8

10

12

Time (h) 60 55 50

40

(F) 2.0 2.5 3.0

3.5 4.0 4.5 5.0

Time (h)

60 40

0.3

A

600

4

2

Wavelength (nm) L-NAC:CA = 0:5 L-NAC:CA =0.5:5 L-NAC:CA =1:5 L-NAC:CA = 2:5 L-NAC:CA = 4:5 L-NAC:CA = 5:5 L-NAC:CA = 12:5

800

PL intensity

600

40

45

(E)

QY (%)

PL intensity

800

500

Wavelength (nm) 200°C 2h 200°C 3h 200°C 4h 200°C 5h

0.4

200°C 2h 200°C 3h 200°C 4h 200°C 5h

400 450

QY (%)

1000

350

50

(B) Wavelength (nm)

0

55

0.1

200 0

60

QY (%)

600 A

PL intensity

800

0.2

20 0

0 2 4 6 8 10 12 L-NAC-to-CA molar ratio

0.1

200

(G)

(H)

0 350

400 450 500 550 600 Wavelength (nm)

0.0 200 250 300 350 400 450 500 Wavelength (nm)

Fig. 22.4 (A) PL spectra, (B) UV-visible absorption spectra, and (C) the QYs of CDs derived form N-acetyl-cysteine:citric acid (L-NAC:CA) prepared at 70°C for various reaction times (3, 6, 9, and 12 h) (ʎex ¼ 340 nm); (D) PL spectra, (E) UV-visible absorption spectra, and (F) the QYs of CDs prepared at 200°C for various reaction times (2, 3, 4, and 5 h) (ʎex ¼ 340 nm). (G) PL spectra and (H) UV-visible absorption spectra of CDs with different L-NAC:CA values (ʎex ¼ 340 nm); the inset shows the QYs of these prepared CDs. Based on Gao F, Ma S, Li J, Dai K, Xiao X, Zhao D, et al. Rational design of high quality citric acid-derived carbon dots by selecting efficient chemical structure motifs. Carbon 2017;112:131–41.

As can be seen from Table 22.1, QYs of citric acid derivate CDs can be organized as N-, S-CDs > N-CDs > undoped-CDs > S-CDs that the QY values of these CDs varied between 1.0% and 55.7%. According to these results, the N atom represents the high influence on the optical properties, and the dNH2 and dRNH forms of reactants are not very effective on the quality of the CDs. However, among the S forms of elements such as dSH, dSR, and dC]S, dSH groups containing reactants have high QYs of 22.7% because of the more reactive nature than the others. In addition, the existence of dRNH together with the dSH groups on the reactant structure is very effective in the optical properties of CDs because of the best motif as dNHdCdCdSH containing small fluorophore molecules.

Table 22.1 The chemical structures and QYs of various CDs from different sources Reactant (QYs)

Chemical structural formula O

CA (1.6%)

OH

Chemical formula

Reactant (QYs)

C6H8O7

Na2S2O3 (1.0%)

HO

Chemical structural formula

Na2S2O3  5H2O

S S

NaO

Chemical formula

ONa • 5H2O

OH

HO

O

O O

3-MPA (1.2%) L-Iso

(10.1%)

O

C3H6O2S

L-Glu

(3.4%)

HS

OH

OH

C6H13NO2

O

L-Lys

(49.8%)

C6H14N2O2

HO

O

O

Urea (7.1%)

O

O

OH NH2

CH4N2O

L-His

O OH NH2

HN H3N

(55.7%)

C6H14N4O2

NH3

H N

β-CA (49.7%)

OH

C2H7NS

H2N SH

O

NH

L-NAC

C6H9N3O2

N

H 2N

(9.8%)

NH2

NH2

(5.1%)

NH2

L-Arg

C5H9NO4

NH2 HO

NH

C5H9NO3S

SH

D-Cys

(50.4%)

C3H7NO2S

O OH

O

H2N

O

SH OH

L-Cys (49.8%)

C3H7NO2S

O OH H2N

GSH (Reduced) (34.8%)

O

C10H17N3O6S

HO HN

SH

O O HS

HN O H2N

DL-Hcy

(22.7%)

C4H9NO2S

NH2 OH HS

(3.1%)

C5H11NO2S

NH2 OH S O

OH

C2H5NS

H2N S

O

L-Met

TAA (6.5%)

660

Nanocarbon and its Composites

Lu et al. also showed the effect of doping on the PL mechanism of CDs, as demonstrated in Fig. 22.5. Undoped as well as N- and S-doped CDs were synthesized using three different precursors-maleic acid, ethanol amine, and ethane sulfonic acid-by using a microwave-assisted pyrolysis method at 700 W for 30 min, as the schematic of the synthesis steps is shown in Fig. 22.5A. The QYs of the prepared CDs were found in the order of N-CDs > undoped CDs > S-CDs, the same as reference 32, and the researchers explained the PL mechanism, as illustrated in Fig. 22.5B. The undoped CDs were labeled as O-states with different types of surface states related to wide range of energy levels to produce a broad UV-Vis absorption and excitation-dependent emission spectra. The N doping leads to new kinds of surface states that are labeled N-states, and electrons trapped by these newly formed surface states could facilitate high quantum yields. On the other hand, S-doped CDs show a low quantum yield due to the elimination of the O-states. Even though the exact mechanism is still unclear, it is reasonable to assume that the doping with nitrogen can

O

OH O C

S

C O

O

S-CDs

SOx-C

OH OH

OH

OH

COOH H2N

HOOC

S-C COOH

N-CDs

NH2

Microware

Microware

Microware

C N COOH

CDs CDs

(A)

CDs Undoped-CDs

Energy

N-states

2 2 2 1

3

1

3

1

3

12000

Undoped-CDs Slope=73480.6 Slope=112766.2 N-CDs S-CDs Slope=40996.4

9000 6000 3000 0 0.00

Ground state

(B)

Undoped-CDs

CDs

N-CDs

O-states

O-states

PL intensity

S-CDs

CDs

CDs

(C)

0.02

0.04

0.06

0.08

0.10

Absorbance

Fig. 22.5 (A) A synthesis route of CDs from maleic acid as the carbon source and ethanol amine and ethane sulfonic acid as N and S sources. (B) PL mechanism of CDs (C) plots of integrated PL intensity of CDs. From Lu W, Gong X, Nan M, Liu Y, Shuang S, Dong C. Comparative study for N and S doped carbon dots: Synthesis, characterization and applications for Fe3+ probe and cellular imaging. Anal Chim Acta 2015;898:116–27.

Carbon dots: preparation, properties, and application

661

introduce the CDs to a new kind of surface state. However, the doping of sulfur will lead to the original surface states being neglected [33]. Although many synthesis methods of CDs are generally performed in the controllable reaction conditions, none of these reactions is designed for large-scale production. Roa et al. designed three types of microreactors to prepare CDs for 5 min by using citric acid and ethylenediamine precursor materials. These CDs were synthesized simply in a short reaction time with a high quantum yield of about 60.1%, and these CDs were utilized as a sensor for Fe3+ ions with the lowest detection limit (LOD) as 0.239 mΜ [34]. Therefore, the use of a microreactor in the preparation of CD synthesis for large-scale synthesis can be applicable for a rapid reaction rate and high quantum yields for potential sensor applications. Additionally, Liu et al. used the green anhydrous method by employing an edible oil bath for the synthesis of CDs. This process is also suitable for large-scale industrial synthesis of fluorescent CDs because of a rapid reaction time as low as 5 min, with large-scale synthesis or green synthesis procedures resulting in strong pH-sensing capabilities [35]. Some uses of various precursors, synthesis methods, reaction conditions, quantum yields, and application areas of various CDs are summarized in Table 22.2. It is apparent from Table 22.2 that the types of precursors, the synthesis processes, and the reaction conditions have significant effects on the size, quantum yields, and application areas of the prepared CDs. Table 22.2 Comparison of precursors, synthesis methods, reaction conditions, quantum yields, and application areas of various CDs Precursors

Synthesis methods

Reaction condition

Quantum Yield

Application area

Ref.

Graphite, cement

Laser ablation

4%



[1]

Single-wall carbon nanotubes Coal

Arc discharge

1064 nm, 10 Hz, 900° C –





[6]

12 h, 80°C



[20]

1 h, 45 W, 59 kHz



Degrading organic dyes Cell imaging

5 h, 150–300°C 5 h, 200°C 10 min, 600 W

80.6%

[22]

6 h, 200°C

15.3%

Sensor, bioimaging Cell imaging Nano drug carrier, bioimaging Sensor for hyaluronic acid

Graphite

Citric acid, ethylenediamine Serine, cysteine Lignin

Mushroom (fungus)

Acid oxidizing exfoliation Solvent exfoliation by ultrasonicassisted Hydrothermal Hydrothermal Microwave

Grill and hydrothermal

7% 43.7%

[21]

[23] [24]

[26]

Continued

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Nanocarbon and its Composites

Table 22.2 Continued Precursors

Synthesis methods

Reaction condition

Quantum Yield

Application area

Citric acid, acrylamide

Hydrothermal pyrolysis

10 h, 180°C

55.4%

1,2Benzenediamine, carbamaldehyde Lysine

Solvothermal, hydrothermal

3 h, 180°C

14.3%

Sensor for Fe+3 and Cr(VI) Ag+ sensing Cell imaging

Microwave pyrolysis Microreactor

23.3%

Hydrothermal

5 min, 750 W 5 min, 160°C 3 h, 200°C

74.15%

Hydrothermal

4 h, 160°C

78%

Citric acid, ethylenediamine Citric acid, ammonium thiocyanate Citric acid, thiourea

60.1%

Bioimaging of cell Cell imaging, Fe+3 dedection Sensor for doxycycline and cell imaging Photocatalysis

Ref. [27]

[28]

[29] [34] [36]

[37]

From Sun YP, Zhou B, Lin Y, Wang W, Fernando KAS, Pathak P, et al. Quantum-sized carbon dots for bright and colorful photoluminescence. J Am Chem Soc 2006;128:7756–7; Xu X, Ray R, Gu Y, Ploehn HJ, Gearheart L, Rker K, et al. Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments. J Am Chem Soc 2004;126:12736–7; Hu S, Wei Z, Chang Q, Trinchi A, Yang J. A facile and green method towards coal-base fluorescent carbon dots with photocatalytic activity. Appl Surf Sci 2016;378:402–7; Lu L, Zhu, Y, Shi C, Pei YT. Large-scale synthesis of defect-selective graphene quantum dots by ultrasonic-assisted liquid-phase exfoliation. Carbon 2016;109:373–83; Zhu S, Meng Q, Wang L, Zhang J, Song Y, Jin H, et al. Highly photoluminescent carbon dots for multicolor patterning sensors, and bioimaging. Angrew Chem Int Ed 2013;52:1–6; Zeng YW, Ma DK, Wang W, Chen JJ, Zhou L, Zheng YZ, et al. N, S co-doped carbon dots with orange luminescence synthesized through polymerization and carbonization reaction of amino acids. Appl Surf Sci 2015;342:136–43; Rai S, Singh BK, Bhartiya P, Singh A, Kumar H, Dutta PK, et al. Lignin derived reduced fluorescence carbon dots with theranostic approaches: nano-drugcarrier and bioimaging. JOL 2017;190:492–503; He G, Shu M, Yang Z, Ma Y, Huang D, Xu S, et al. Microwave formation and photoluminescence mechanisms of multi-states nitrogen doped carbon dots. Appl Surf Sci 2017;422:257–65; Yang K, Liu M, Wang Y, Wang S, Miao H, Yang L. Carbon dots derived from fungus for sensing hyaluronic acid and hyaluronidase. Sens Actuators B 2017;251:503–8; Li C, Liu W, Sun X, Pan W, Wang J. Multi sensing functions integrated into one carbon-dot based platform via different types of mechanisms. Sens Actuators B 2017;252:544–53; Zhong Y, Li J, Jiao Y, Zuo G, Pan X, Su T, et al. One-step synthesis of orange luminescent carbon dots for Ag+ sensing and cell imaging. JOL 2017;190:188–93; Choi Y, Thongsai N, Chae A, Jo S, Kang EB, Paoprasert P, et al. Microwave-assisted synthesis of luminescent and biocompatible lysine-based carbon quantum dots. J Ind Eng Chem 2017;47:329–35; Rao L, Tang Y, Li Z, Ding X, Liang G, Lu H, et al. Efficient synthesis of highly fluorescent carbon dots by microreactor method and their application in Fe3 + ion detection. Mater Sci Eng C 2017;81:213–23; Duan J, Yu J, Feng S, Su L. A rapid microwave synthesis of nitrogen-sulfur co-doped carbon nanodots as highly sensitive and selective fluorescence probes for ascorbic acid. Talanta 2016;153:332–9; Weng C-I, Chang H-T, Lin C-H, Shen Y-W, Unnikrishnan B, Li Y-J, et al. One-step synthesis of biofunctional carbon quantum dots for bacterial labeling. Biosens Bioelectron 2015;68:1–6.

22.3

Applications of CDs

CDs are assumed to be 0D carbon-based particles with a size range of less than 20 nm with high florescence properties. Although up to 60 nm particle sizes have been reported in the literature [5], these particles are generally nanocrystallites or amorphous nanostructures woven with sp2 carbon and oxygen/nitrogen-derived groups [5,16]. Due to their higher photostability, good water solubility, environmentally

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663

friendly nature, the ease of synthesis methods combining the strong fluorescence and optical properties make CDs promising materials with great potential compared to semiconductor quantum dots and toxic organic dyes in the wide range of application fields. Especially, CDs are considered as safe with no vital toxicity on the various cell lines for in vitro applications and have shown excellent biocompatibility for in vivo biomedical applications, including cell imaging, cancer therapy, drug/gene carrier vehicles, diagnosis of diseases, etc. [5,9,11,12,16,22,24,33,35].

22.3.1 In vivo and in vitro bioimaging There has been growing interest in the use of fluorescence CDs in bioimaging of cells and tissues of different living systems for in vivo and in vitro biomedical applications, owing to the excellent biocompatibility, small size (few nm), high PL, and multicolored fluorescence properties of CDs as they can be prepared from a variety of benign sources. In vivo imaging of plant cells is more complicated in comparison with animal cells because of the complex cell structure. The cell walls, chloroplasts, vacuoles, and fibrous tissues and usage process for in vivo visualizing of plants have some important problems. Zhang et al. prepared green N- and S-heterodoped CDs and used them in the in vivo imaging of plants cells [37a]. As demonstrated in Fig. 22.6, strongly (A) blue, (B) green, and (C) red fluorescence images were compared to (D) the bright field image of guard cell-treated N- and S-CDs. These images show that N- and S-CDs in the guard cells exhibit autofluorescence effects by changing filters. In addition, the

Fig. 22.6 Representative laser scanning confocal microscopy images in guard cells: (A) DAPI (blue), (B) FITC (green), (C) R-DIL (red) and (D) bright-field. Representative fluorescence images in root tissues under an excitation wavelength of 405 nm: (E) root hairs, (F) lateral roots, (G) cells of elongation zone, (H) vascular bundles. From Xiao J, Liu P, Wang CX, Yang GW. External field-assisted laser ablation in liquid: an efficient strategy for nanocrystal synthesis and nanostructure assembly. Prog Mater Sci 2017;87:140–220.

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fluorescence microscope images of root tissues such as (E) root hairs, (F) lateral roots, (G) cells of the elongation zone, and (H) vascular bundles treated with fluorescence Nand S-CDs were demonstrated in Fig. 22.6. All these images indicated that N- and S-CDs can successfully penetrate into the plant cells and be used in the in vivo cell imaging for plants [38]. Furthermore, CDs can easily penetrate the cell membrane and act as a fluorescence material to visualize the human and animal cells. Therefore, these can be used in biolabeling, target specific bioimaging, photodynamic therapy, targeted photothermal ablation, bioimaging and biodetection in cancer therapy, gene delivery (nonviral gene vector), and drug delivery applications. In the application area, CDs are generally used in the cancer therapy and gene/drug delivery applications along with bioimaging.

22.3.2 Cancer therapy Several studies have reported that CDs exhibit PL emission in the near-infrared spectral region under the NIR irradiation that can be an advantage for chemo-photothermal (PTT) and photodynamic (PDT) treatment for different tumors [39–41]. Zhang et al. reported the synthesis of magneto-fluorescent CDs derived from Fe-cross-linked chitosan (FeN@CQDs) and conjugated with folic acid as a targeting molecule and riboflavin as well as cross-linking with genipin sequentially to inhibit the toxicity of this material as covered by polymeric nanostructure as GP-Rf-FA-FeN@CQDs as a pH and light-triggered theranostic. In addition, doxorubicin as a cancer drug was further loaded into the polymer spheres via a metal-Dox complex and covalent interactions to prepare GP-Rf-FA-FeN@CQDs-DOX as a targeted drug delivery material. As illustrated in Fig. 22.7A, 60% of DOX was released from GP-Rf-FAFeN@CQDs-DOX at pH 5 for 72 h. Furthermore, the amount of released drug was increased upon the NIR-light irradiation related to pH and light triggered. Therefore, GP-Rf-FA-FeN@CQDs-DOX can be the release of the DOX cancer drug under NIR irradiation at low pH condition. For in vitro applications, GP-Rf-FA-FeN@CQDs-DOX followed by laser irradiation has the most inhibiting effects on the viability of HeLa and HepG2 cancer cells compared with DOX and GP-Rf-FA-FeN@CQDs-DOX and no laser irradiation, as seen in the Figs. 22.7B and C. The fluorescence microscope images of HeLa cells incubated with GP-Rf-FA-FeN@CQDs-DOX were demonstrated in Fig. 22.7D. The cells are seen as a bright green color under the blue laser pulses and the released DOX is aggregated on the cell nuclei and realized as bright red related to PL behavior due to the excellent photostability of CDs [39]. It is also reported that CDs can be used in in vivo cancer treatment [39], and the cellular uptake and intracellular trafficking mechanism of the CDs as a drug carrier vehicle was illustrated in Fig. 22.8A. The cancer drug-loaded CDs can directly pass the cell wall owing to their small sizes of a few nanometers, and release the carried drug to the specific side with pH and photothermal trigger and provide images to determine the guided target delivery by means of their PL properties. The photothermal stimulation of GP-Rf-FA-FeN@CQDs were

Carbon dots: preparation, properties, and application

Fig. 22.7 (A) A Cancer drug (DOX) release from pH and the NIR irradiation triggered GP-Rf-FA-FeN@CQDs-DOX CDs at different pH values (5.0, 6.0, and 7.4), and (B) HeLa, and (C) HepG2 cancer cell viabilities at different concentrations of GP-Rf-FA-FeN@CQDs-DOX with or without irradiation, and (D) Fluorescence microscope images of HeLa cells incubated with GP-Rf-FA-FeN@CQDs-DOX at different excitation wavelength 488, 543 nm, and merged, bright field, respectively. From Zhang M, Wang W, Zhou N, Yuan P, Su Y, Shao M, et al. Near-infrared light triggered photo-theraphy, in combination with chemotherapy using magnetofluorescent carbon quantum dots for effective cancer treating. Carbon 2017;118:752–64. 665

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Fig. 22.8 (A) A schematic diagram for stimuli-responsive drug release using CDs, (B) thermal images of tumor-bearing mice injected with photothermal triggered CDs under NIR irradiation. The digital camera images of (C) mice and (D) tumors after various treatments. From Zhang M, Wang W, Zhou N, Yuan P, Su Y, Shao M, et al. Near-infrared light triggered photo-theraphy, in combination with chemotherapy using magnetofluorescent carbon quantum dots for effective cancer treating. Carbon 2017;118:752–64.

demonstrated by the imaging of tumor-carrying mice under laser irradiation with a different temperature range between 50.2°C and 21°C in Fig. 22.8B. The digital camera images of tumor-carrying mice treated with anticancer drugloaded CDs (GP-Rf-FA-FeN@CQDs-DOX), and the tumor sizes after 16 days were demonstrated in Figs. 22.8C and D. According to these in vivo and in vitro results, the CD-derivated materials can deliver the anticancer drug through the target cells. The drug can be released by NIR irradiation that inhibited the tumor growth as a chemo-photo effect.

Carbon dots: preparation, properties, and application

667

Therefore, it is obvious that CDs with ample design strategies for in vivo cancer treatments offer advanced cancer therapies by monitoring the elimination of cancer cells with a controlled chemical drug release by means of an external trigger.

22.3.3 Gene and drug delivery Cationic CDs have shown great potential as gene carriers and delivery applications because of their ability of electrostatic interaction with positively charged functionalized CDs and negatively charged nucleic acids. Chen et al. prepared positively charged CDs from porphyra polysaccharide and ethylenediamine precursors with a high QY of 56.3% to induce the neuronal differentiation of adult stem cells through nonviral gene delivery [42]. Gene transfection is faster and more efficient in neuronal induction from the adult stem cells by using these plasmid DNA-loaded CDs that can be used in bioimaging, gene delivery, and tissue engineering. Gao et al. reported turn on-off theranostic fluorescent CDs against hyaluronidase (HAase) in cancer cells for self-targeted imaging and drug delivery. Negatively charged CDs were modified with cationic polyethyleneimine (PEI) through electrostatic interaction to prepare P-CDs and functionalized with hyaluronic acid-Doxorubicin conjugate (P-CDs/HA-Dox), as demonstrated in Fig. 22.9 [43]. A P-CDs/HA-Dox nanoprobe can pass into the cells readily with targeting specify to the CD44 receptor on the cancer cell. HA can be degraded to tetra saccharide units in the presence of the HAase enzyme [43]. Therefore, Dox can be released from a P-CDs/HA-Dox nanoprobe into cancer cells because of the enzyme-triggered drug delivery and induce apoptosis in Hela cancer cells. Therefore, this study clearly showed that CDs can be successfully used in the targeted bioimaging and delivery vehicles for image-guided chemotherapy.

22.3.4 Sensor and biosensors Fluorescence CDs can be used as sensors for the detection and identification of a wide range of analytes, that is, cations, anions, drugs, small molecules, and macromolecules, depending on high sensitivity and selectivity, and the easy operation as benign biocompatible, and low-cost device applications [16,44,45]. There are three main strategies to design CDs as a sensor material: (1) As the prepared CDs interact with the analyte, the fluorescence signals could be changed; (2) Specific receptors or special functional groups can be conjugated via postmodification on CDs to generate sensing ability; and (3) Quenchers, fluorophores, and substrates integrations of CDs could be used as sensory materials [16]. The functional groups on the surface can be interacted with several metal ions such as Ag+, Au3+, Fe3+, Cr3+, Cu2+, Eu2+, As3+, Hg2+, Pb2+, Sn2+, Co2+, and their binary and ternary mixtures with nonspecific sensing [20,46,47]. The types of precursors and their surface state can be designated the quenching responsive of CDs to specific analytes. Wang et al. synthesized N-doped CDs from three different amino acids: glycine, lysine, and serine with citric acid as a precursor to investigate the potential use as sensors for metal ions by the change in signal defined as (I0-I)/I0, where I and I0 are the fluorescent intensity in

668

Nanocarbon and its Composites

Endocytosis Endosome

HAase

Drug release

CDs

PEI

P-CDs

Nucleus

HA-Dox P-CDs/HA-Dox CD44

Fig. 22.9 Schematic illustration of the PEI-CDs/HA-Dox and the nanoprobe used for targeted cancer cell imaging and drug delivery. Data from Gao N, Yang W, Nie H, Gong Y, Jing J, Gao L, et al. Turn-on theranostic fluorescent nanoprobe by electrostatic self-assembly of carbon dots with doxorubicin for targeted cancer cell imaging, in vivo hyaluronidase analysis, and targeted drug delivery. Biosens Bioelectron 2017;96:300–7.

the presence and absence of the metal ions, respectively [46]. As shown in Fig. 22.10A, the affinities of the CDs are different against different metal ions, depending on the chelate capability of the different forms and numbers of amino, carboxyl, and hydroxyl groups of amino acid derivates with metal ions. These amino acid-derived CDs can successfully differentiate the complex samples from the metal ion binary and ternary mixtures for all combinations because of the high sensing capabilities, as shown in Fig. 22.10B. Additionally, pH can affect the signal intensity in the sensing ability based on the responsiveness of surface groups of the CDs, as demonstrated in Fig. 22.10C–E. All these results confirmed that CDs have great potential in the detection and identification of metal ions, even from the metal ion mixture. Therefore, they can be conveniently used in the fabrication of sensor arrays [46]. Amin et al. prepared N-doped CDs with a high quantum yield of 21.5% and showed their high sensitivity against tetravalent selenium [48]. They stated that N-doped CDs can be used as controllable fluorescence materials with “on-off-on” fluorescence

100

120 GlyCDs

GlyCDs

LysCDs

100

LysCDs

SerCDs

SerCDs 80 (l–l0)/l0 (%)

40

20

Hg2+

blank Cu2+

Pb2+

(A)

Fe3+ Hg2+

(B)

1.0

1.0 pH=3

0.8

(l0–l)/l0

0.4

0.8

0.2 0.0

0.0 2+

Cu

3+

Eu

3+

Fe

2+

Hg

2+

Pb

pH=7

0.4 0.2 0.0

blank Cr

(D)

GlyCDs LysCDs SerCDs

0.6

0.4 0.2

blank Cr

pH=5

0.6

0.6

3+

1.0

GlyCDs LysCDs SerCDs

(l0–l)/l0

0.8

GlyCDs LysCDs SerCDs

+

Fu3+

+H g 2+ Cu 2 + Fe 3+ +H g2

Eu3+

+

Cu2+

Cu 2

Cr3+

+

0 blank

(l0–l)/l0

40 20

0

(C)

60

Cu 2 + +F e 3+ Fe 3+ +H g2

(l0–l)/l0 (%)

60

Carbon dots: preparation, properties, and application

80

3+

2+

Cu

3+

Eu Fe

3+

2+

Hg

2+

blank Cr3+ Cu2+ Eu3+ Fe3+ Hg2+ Pb2+

Pb

(E)

669

Fig. 22.10 The sensing capability of the glycine, lysine, and serine derivate CDs for (A) different metal ions, and (B) sensing array against Cu2+, Fe3+, Hg2+, and their binary and ternary mixture by means of fluorescence intensity. The effects of pH on the sensing ability (C) pH 3, (D) pH 5, and (E) pH 7. From Wang Z, Chao X, Lu Y, Chen X, Yuan H, Wei G, et al. Fluorescence sensor array based on amino acid derived carbon dots for pattern-based detection of toxic metal ions Sens Actuators B 2017;241:1324–30.

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Nanocarbon and its Composites

Fig. 22.11 Schematic illustration of the sensing process for detection of (A) selenite based on N-doped CDs/Eu3+ and (B) D-penicillamine sensing with CDs in the presence of Cu2+ ions. From (A) Amin N, Afkhami A, Madrakian T. Construction of a novel "off-on" fluorescence sensor for highly selective sensing of selenite based on europium ions induced crosslinking of nitrogen-doped carbon dots. JOL 2018;194:768–77, and (B) Naghdi T, Atashi M, Golmohammadi H, Saeedi I, Alanezhad M. Carbon quantum dots originated from chitin nanofibers as a fluorescent chemoprobe for drug sensing. J Ind Eng Chem 2017;52:162–7.

capabilities, depending on the competitive reactions between selenite and carboxylic acid and the hydroxyl groups of N-doped CDs against Eu3+ ions. The schematic representation of the “on-off-on” sensing process for the detection of selenite based on N-doped CDs/Eu3+ was demonstrated in Fig. 22.11A. As can be seen in Fig. 22.11A, highly fluorescent N-doped CDs were aggregated upon the addition of Eu3+ because of its coordination with the carboxylic acid and hydroxyl groups of the CD surface that act as a bridge between the CDs and the fluorescent effects were diminished. On the other hand, upon the addition of selenite into the N-doped CDs/Eu3+, the aggregation triggered Eu3+ on the N-doped CDs was broken down depending on the strong affinity of selenite on the carboxylic acid and hydroxyl groups of the CDs surface leading to the reoccurrence of the fluorescence effects of CDs [48]. Naghdi et al. also demonstrated the same “on-off-on” fluorescent strategy as depicted in Fig. 22.11B using Cu2+ and D-penicillamine. It is obvious that these studies clearly reveal that the fluorescent effects of CDs from chitin nanofibers was quenched or “turned off” after the addition of Cu2+ ions, whereas the fluorescent was “turned on” again in the presence of D-penicillamine as the Cu2+ ions were bound to D-penicillamine instead of CDs with high affinity [49]. Therefore, using a specific ligand and competitive binding interaction can be used in the design of very specific sensors for biomedical and environmental applications. The utilization of CDs as biosensing devices to recognize specific biological molecules such as glucose, amino acids, peptides, nucleotides, proteins, DNA, vitamins, cells, and bacteria has attracted great attention, especially for clinical sample analysis, early diagnosis of sickness, and so on [16,36,50]. For example, the glucose level in the human body is of vital importance for the treatment of diabetes and/or cancerous diseases [16]. Shen et al. reported the synthesis of fluorescent CDs from phenylboronic acid that has a strong affinity for sugar units and is used for glucose detection. These

Carbon dots: preparation, properties, and application

671

CDs show high selectivity against glucose because of the existence of the boronic acid groups on CDs that can chemically interact with glucose molecules, specifically [16,50]. Kudr et al. also demonstrated the high affinity of N-doped CDs to DNA fragments as a biosensor and analyzed the genomic DNA from PC-3 cells and DNA isolated from melanoma tissues [51]. Moreover, Weng et al. showed the biosensing effects of mannose-modified CDs against bacteria labeling by high selectivity of the CDs that bind to a specific lectin unit of the filegalle of the wild type Escherichia coli K12 strain. In addition, these CDs can be successfully used in the labeling of bacteria by the fluorescence detection method in the real samples, including tap water, apple juice, and human urine [37]. Chen et al. prepared graphene oxide-derived N and S-doped CDs that show more sensitivity and rapid detection of a wide range of biological species, including virus DNA, proteins, and any other analytes via fluorescent quenching and recovering [52]. Consequently, the utilization of different CDs in the recognition of different biomolecules is a viable procedure and offers great advantage over the common diagnostic procedures in many aspects in biomedical applications.

22.3.5 Catalysis and energy More recently, CDs have been used in energy conversion and storage as well as electrocatalytic and photocatalytic devices, owing to their outstanding features such as low cost, broad optical absorbance, high photo and chemical stability, environmental friendless and nontoxicity, and scalable synthesis methods. Xu et al. reported ZnO nanorode-functionalized CDs (ZnO@CDs) as an energy conversion and storage material in photoelectrochemical (PEC) water splitting from solar to hydrogen energy conversion. ZnO@CDs as a photoanode enhanced the PEC activity compared with the bare ZnO nanorodes for solar water splitting, due to the extended spectrum response range improving the photo conversion efficiency. This study pointed out that functionalization of the CD surfaces with photosensitive materials can improve the PEC activity for solar conversion [53]. In another study, sandwiched graphene oxide (GO) with an N and S-codoped CD hybrid composite as a metal-free electrode catalyst was prepared and tested for supercapacitors and fuel cell catalysts [54]. Depending on the electron-rich, heteroatomdoped CDs that are present within the GO layer enhanced the electrocatalytic activity in the oxygen reduction reaction with a 156 F/g capacitance value compared with bare graphene oxide. It is clear that heteroatom-doped CDs and GO composites can be used as energy storage and conversion materials with a synergistic effect [54]. In another study by Zhao et al., the CD-polyaniline hybrid materials as supercapacitor electrodes with a perfect capacitance of 738.3 F/g and high cycle stability of 78% after 1000 cycles were reported. The specific capacitance and cycle stability of bare polyaniline were found as 432.5 F/g and 68%, respectively, after 1000 cycles. It is obvious that CDs can enhance the capacitive performance of the hybrid material [55]. The photocatalytic activity of a CDs/Bi2MoO6 nanosheet composite on the degradation of rhodamine B and methylene blue under visible light irradiation was investigated Sun et al. [56]. The possible heterostructure of a CDs/Bi2MoO6 nanosheet composite

672

Nanocarbon and its Composites Visible light

CB

e – e– e–

e– O2

–0.32 eV

2.73 eV

CQDs/Bi2MoO6

CQDs

O2

H2O OH

2.41eV

h+ h+ h+

h+

VB

Bi2MoO6

Organics

CO2

H2O

Fig. 22.12 The scheme of the possible heterostructure of CQDs/Bi2MoO6 nanosheet and its photocatalytic mechanism. From Sun C, Xu Q, Xie Y, Ling Y, Jiao J, Zhu H, et al. High-efficient one-pot synthesis of carbon quantum dots decorating Bi2MoO6 nanosheets heterostructure with enhanced visible-light photocatalytic properties. J Alloys Comp 2017;723:333–44.

and its photocatalytic mechanism was illustrated in Fig. 22.12. Under visible light irradiation, CDs could be photoexcited from the valance band (VB) into the conduction band (CB). The photo-generated electrons in Bi2MoO6 could be directly transferred to the CDs due to the electric conductivity. Thus, the separation of photo-generated electrons into the holes could be promoted and the degradation efficiency of organic dyes was further improved [56]. Consequently, the utilization of CDs as energy storage or electrochemical or catalyst materials is also very attractive and enables the design of advanced energy materials.

22.4

Concluding remarks

CDs have been promising materials compared to the toxic metal-based quantum dots due to their great optical properties with a highly biocompatible nature making them indispensable materials, especially for biological environments and living organisms. In this chapter, the effects of the synthesis process, precursors, and the with hetero atoms on properties and applications of CDs was revealed. Among the synthesis processes of CDs, the hydrothermal and microwave treatments were found to be the most popular techniques due to their practical, convenient, and inexpensive nature while generally producing nontoxic by-products and uniform particle sizes, offering CDs with excellent optical properties and high quantum yields. Additionally, the PL behavior of the CDs can be controllable by means of the change in the reaction conditions such as the reaction time, reaction temperature, and the types and concentrations of precursor molecules in these methods. The hetero atom doping such as N, S, P, and B of CDs also can be readily accomplished by employing amine, phosphonyl, sulfonyl, and boron groups containing precursor materials from organic, polymeric, and biological compounds that can result in high quantum yields and sensing ability for the

Carbon dots: preparation, properties, and application

673

prepared CDs. The excellent PL and multicolored fluorescence properties, along with their nontoxic nature, CDs will increasing be used in vitro and in vivo bioimaging for plant, animal, and human cells as well as diagnostic purpose for various diseases and cancers for human and animals in the future. In addition to the CD’s natural therapeutically benevolent properties, the ability to carry active agents, genes, cancer drugs and so on to specific targeted sites and deliver these payloads with some external stimuli by means of some external signals, for example, irradiation, pH, and temperature, while monitoring or tracking their movements in vivo makes them indispensable materials as advanced biomedicine materials. Furthermore, the superior recognition capabilities of CDs in biosensors and theranostic applications also make them the favorable choice for the development of new diagnostic and treatment devices in many biomedical and environmental applications as well as the early determination of different kinds of sicknesses and environmental contaminations. Additionally, the use of CDs as catalysts for solar energy application, even supercapacitors and energy storage devices, evidently shows their omnipotent application potential and versatility as adaptable materials in the design of futuristic advanced viable smart systems.

References [1] Sun YP, Zhou B, Lin Y, Wang W, Fernando KAS, Pathak P, et al. Quantum-sized carbon dots for bright and colorful photoluminescence. J Am Chem Soc 2006;128:7756–7. [2] Ponomarenko LA, Schedin F, Katsnelson MI, Yang R, Hill EW, Novoselov KS, et al. Chaotic dirac billiard in graphene quantum dots. Science 2008;320:356–8. [3] Fernando KAS, Sahu S, Liu Y, Lewis WK, Guliants EA, Jafariyan A, et al. Carbon quantum dots and applications in photocatalytic energy conversion. ACS Appl Mater Interfaces 2015;7:8363–76. [4] Liu R, Wu D, Feng X, Millen K. Bottom-up fabrication of photoluminescent graphene quantum dots with uniform morphology. J Am Chem Soc 2011;133:15221–3. [5] Yuan F, Li S, Fan Z, Meng X, Fan L, Yang S. Shining carbon dots: synthesis and biomedical and optoelectronic applications. Nano Today 2016;11:565–86. [6] Xu X, Ray R, Gu Y, Ploehn HJ, Gearheart L, Rker K, et al. Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments. J Am Chem Soc 2004;126:12736–7. [7] Baker SN, Baker GA. Luminescent carbon nanodots: emergent nanolights. Angew Chem Int Ed 2010;49:6726–44. [8] Liu Q, Zhang S, Dai L, Li LS. Nitrogen-doped colloidal graphene quantum dots and their size dependent electrocatalytic activity for the oxygen reduction reaction. J Am Chem Soc 2012;134:18932–5. [9] Peng Z, Han X, Li S, Al-Youbi AO, Bashammakh AS, El-Shahawi MS, et al. Carbon dots: biomacromolecule interaction, bioimaging and nanomedicine. Coord Chem Rev 2017;343:256–77. [10] Lu K-Q, Quan Q, Zhang N, Xu Y-J. Multifarious roles of carbon quantum dots in heterogeneous photocatalysis. J Energy Chem 2016;25:927–35. [11] Shen L-M, Liu J. New development in carbon quantum dots technical applications. Talanta 2016;156-157:245–56.

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Phthalocyanine-nanocarbon materials and their composites: Preparation, properties, and applications

23

Ahmet S¸enocak, Erhan Demirbas¸, Mahmut Durmus¸ Gebze Technical University, Department of Chemistry, Gebze, Turkey

Chapter Outline 23.1 Introduction 677 23.2 Functionalization of nanomaterials with phthalocyanine derivatives

681

23.2.1 Fullerene—phthalocyanines 681 23.2.2 Carbon nanotubes (CNTs)—phthalocyanines 685 23.2.3 Graphene—phthalocyanines 696

23.3 Conclusions 703 References 704

23.1

Introduction

Carbon nanomaterials comprise a diverse range of carbon allotropes with a number of sizes and shapes. This impressive development of new carbon nanoforms has its origin in the discovery of fullerenes by H. Kroto, R. Smalley, and R. Curl; they received the Nobel Prize in 1996 for this discovery [1]. Therefore, it leads to the discovery of other important carbon nanostructures. In the last decade, carbon nanostructures have been utilized in major research areas such as solar cells, catalysts, sensors, and fuel cells because of their unique physical, chemical, optical, and electrical properties. Moreover, they are being used successfully in biomedical and sensing studies [2–4]. Carbon nanostructures show great diversity in structure such as carbon nanotubes (CNTs), graphene, carbon dots, fullerene, carbon nanohorns, and graphene quantum dots (Fig. 23.1). These materials are layered materials and expressed as 0D (fullerene), 1D (one-dimensional nanotube), and 2D (two-dimensional graphene) with respect to the nanoscale range, Co > Cu > Zn > Ni > Mn. The CNTs bonding with M2Pc2 can greatly promote electronic transmission, which significantly improves the performance and the initial voltage of the Li/SOCl2 battery. CV measurements are applied to verify the M2Pc2CNTs possessing catalytic performances and a reasonable mechanism is proposed [56]. Interactions through hydrogen bonding between tetrasulfonate-substituted Cu(II) Pc and oxidized MWCNTs were obtained as electron acceptor components by Hatton et al. Spectroscopic and morphological studies showed that the Pc cores stacked in columns with the MWCNT scaffold (Fig. 23.21) [57]. The CNTs were chemically modified by MPcs for the catalytic activity of CNTs-CONH-MPc, (Fig. 23.22). The MPcs bonded chemically with CNTs and the obtained hybrid materials showed good catalytic activity. The results indicated that all catalysts improved the cell capacity of Li/SOCl2 up to 95.84 mAh (88.49%) [58]. The surface of MWCNTs may be functionalized by a cyclization reaction between nitrile-modified MWCNTs and bis-phthalonitrile in order to obtain high-performance CNT-based polymer nanocomposites (Fig. 23.23). To achieve this, Pcs were coated on the surface of MWCNTs, giving rise to strong interfacial adhesion and good dispersion between MWNCTs and the poly(arylene ether nitrile) (PEN) matrix. The tensile modulus and tensile strength of PEN nanocomposites with Pc-MWCNTs (1:50) increased from 85.6 to 108 MPa and from 2300 to 3350 MPa, respectively. A low rheological percolation threshold of 0.69 wt% was achieved and the value of the dielectric constant of PEN nanocomposites was 3.3 for PEN and 16.6 with Pc-MWCNTs (1:20 wt%) [58]. Table 23.2 illustrated the preparation methods for Pc-CNTs and their applications.

Fig. 23.21 A schematic illustration of the columnar tetrasulfonate-substituted Cu(II)Pcs on MWCNTs.

Phthalocyanine-nanocarbon materials and their composites

Fig. 23.22 The synthesized MTnPc, MPc, and CNTs-CONH-MPc.

695

696

Nanocarbon and its Composites

Fig. 23.23 The synthesized polymeric CNTs-Pc hybrid materials.

23.2.3 Graphene—Phthalocyanines Graphene is an outstanding material because it consists of a 2D single layer of sp2hybridized carbon atoms bonded in a hexagonal honeycomb lattice. Graphene has the same structure of carbon atoms to form carbon nanotubes, but graphene is flat rather than cylindrical. It has exceptional properties, including mechanical and thermal properties. Graphene is also an extremely diverse material, and can be combined with other elements to obtain different graphene-based materials with various superior properties. Graphene and graphene oxide (GO) have many applications such as catalysis, chemical and biological sensing, energy storage, and electronics. A series of coordination and organometallic compounds has been successfully prepared with graphene by covalent or noncovalent modes [23, 54]. Regarding the covalent functionalization of graphene (Fig. 23.24), the absence of strained carbon atoms makes the material less reactive than fullerenes [76]. Electronic interactions between the graphene layers and Pc molecules were determined by the steady-state and timeresolved spectroscopic techniques. Graphene was functionalized with phthalocyanine by a covalent amide and esterification reaction in the literature. The noncovalent functionalization was based on π-π interactions. Cobalt mono carboxyphenoxy phthalocyanine (CoMCPhPc), either covalently linked to graphene oxide nanosheets (GONS), sulfur-doped graphene oxide nanosheets (SDGONS), nitrogen-doped graphene oxide nanosheets (NDGONS), or sulfur/nitrogen codoped graphene oxide nanosheets (SNDGONS) or sequentially added, was used to modify a glassy carbon electrode for detection of hydrogen peroxide (Fig. 23.25). Noncovalent CoMCPhPc-SNDGONS-GCE and covalent CoMCPhPcGCE resulted in good limits of detection and catalytic rate constant values were found 1.58 and 5.44 nM, respectively [77].

Phthalocyanine-nanocarbon materials and their composites

697

Table 23.2 Phthalocyanine-CNTs hybrids and their applications Hybrid material

Description

Application

Ref.

MWCNTs-ZnPc

π-π interaction π-π interaction π-π interaction π-π interaction π-π interaction

ORR electrocatalysis

[59]

Detection of rutin (LOD: 75 nM)

[60]

Cl2 gas sensor (LOD: 0.27 ppb)

[61]

Nitrate reduction (up to 76%)

[62]

Mechanical (>103 MPa), thermal (T > 480°C) and dielectric 0.051 (1 kHz) properties Determination of Isoniazid (LOD: 0.56 μmol L1) ORR activity

[63]

Removal for Cu(II) (30.49 mg g1)

[66]

Giant magnetoresistance

[67]

Detection of reduced glutathione (LOD: 100 μM) Mechanical and electrical properties (3.21  104 S/cm)

[68]

Biomimetic reduction of O2 (Г : 8.87  109 mol cm2) Spin-filter transport and magnetoresistance effects ratio of nearly 100% Fuel cells and batteries (power density of 185 mW cm2) Cl2 sensor (LOD: 0.06 ppb)

[70]

MWCNTs-PdPc Single and MWCNTs-CuPc MWCNTs-CuPc MWCNTs-CuPc

MWCNTs-FePc CNTs-FePc MWCNTs-FePc SWCNTs-TbPc2 MWCNTs-CoPc MWCNTs-ZnPc

DWCNTs-FePc SWCNTs-MnPc

π-π interaction π-π interaction π-π interaction π-π interaction π-π interaction Schiff-base crosslinkages Ionic interaction Covalent

SWCNTs-ZnPc

π-π interaction π-π interaction Covalent

MWCNTs-FePc

Covalent

MWCNTs-FePc MWCNTs-ZnPc

Electron transfer (the higher HOMO and LUMO gaps and electronic transfer efficiencies) Catalytic activity (sustained 12 times)

[64] [65]

[69]

[71] [72] [73] [74]

[75]

Isoniazid is one of the most efficient and widely used drugs for treatment of tuberculosis.

Ragoussi et al. managed to obtain highly exfoliated graphene covalently linked to electron-accepting phthalocyanines (Fig. 23.26A). The functionalization of the nanocarbon surface with alkylsulfonyl phthalocyanines was attained by means of a Click chemistry. The new ensemble was fully characterized by thermogravimetric analysis, AFM, TEM, and Raman as well as ground-state absorption. A series of steady-state

698

Nanocarbon and its Composites

Fig. 23.24 Graphene-Pc nanoconjugate.

and time-resolved spectroscopy experiments resulted in photo-induced electron transfer from the graphene to the electron-accepting phthalocyanines. This was the first example of an electron donor-acceptor nanoconjugate character of graphene [78]. Another photo-induced charge separation was described by Roth et al. The photophysical properties of the resulting nanohybrid were obtained with synthesis of a zinc alkylsulfonylphthalocyanine pyrene conjugate (Fig. 23.26B). The pyrene unit in the conjugate was important for the determination of noncovalently immobilizing phthalocyanines onto the exfoliated graphite. Strong interactions at ground and excited states dominated the electronic properties of the synthesized nanohybrid. As an example, femtosecond pump probe experiments helped generate an ultrafast charge separation [79]. A covalently linked Pc-graphene group was prepared by a Click reaction between phenylethynyl derivatized graphene and an azido-ZnPc. Click modification on a graphene surface with phthalocyanine was investigated for the effect of covalent and noncovalent linking on the carbon nanostructure (Fig. 23.27). The solar cell and ammonia sensor properties of this graphene-ZnPc hybrids were determined [52–54]. The pure polyoxy modified asymmetric zinc(II) phthalocyanine (ZnPc) film without graphene present provided an electrical conductivity of 11.4 mS/cm. This value increased noticeably when reduced graphene oxide was covalently (53.1 mS/cm) and noncovalently (51.7 mS/cm) bonded with ZnPc. In addition, the effects on organic solar cell performance are obtained for rGO covalently and noncovalently functionalized by ZnPc. Their solar cell photo current efficiencies with covalently and noncovalently rGO are found to increase from 3.46% to 4.7%.

Phthalocyanine-nanocarbon materials and their composites

699

Fig. 23.25 Conjugation of cobalt mono carboxy phenoxy phthalocyanine to graphene oxide nanosheets.

Koh and coworkers synthesized iron(II) phthalocyanine (FePc)-GQD conjugates for the energy conversion systems of fuel cells (Fig. 23.28). This material exhibited a greatly enhanced onset potential via a four-electron pathway in an alkaline electrolyte. Moreover, the synthesized electrocatalyst showed distinguished tolerance toward methanol and carbon monoxide [80]. Pyrene-derivatized Zn or Co phthalocyanines (Pcs) and porphyrins (Ps) were immobilized on graphene quantum dots (GQDs) to form GQDs-Pcs and GQDs-Ps hybrids via the π-π stacking interaction method (Fig. 23.29) [81]. The resultant

700

Nanocarbon and its Composites

Fig. 23.26 (A) Procedure for the covalent functionalization of graphene with phthalocyanine [78], and (B) the structure of the phthalocyanine-pyrene conjugate [79].

Phthalocyanine-nanocarbon materials and their composites

Fig. 23.27 Covalently ZnPc bonded reduced graphene oxide (rGO) hybrid material.

701

702

Nanocarbon and its Composites

Fig. 23.28 Structural illustrations of GQD-FePc hybrids.

Fig. 23.29 Pyrene-derivatized Zn(II) and Co(II) phthalocyanines and porphyrins.

hybrids were stable between GQDs and Pcs/Ps. These materials may be important for studies in both fundamental and applied perspectives, owing to the synergistic contributions from the combination of their electronic and optical properties. Preparations of phthalocyanine-graphene hybrids and their applications are shown in Table 23.3.

Phthalocyanine-nanocarbon materials and their composites

703

Table 23.3 Phthalocyanine-graphene hybrids and their applications Hybrid material GrapheneCuPc GrapheneFePc GrapheneCoPc GQDZnPc GrapheneFePc GQDZnPc GrapheneCuPc GrapheneZnPc GrapheneZnPc GQDZnPc GrapheneCuPc GQDCoPc

Description

Application

Ref.

π-π interaction π-π interaction Covalent

Solar cell (PCE: 7.3%)

[82]

ORR catalysts (electron transfer number: 4.05)

[83]

Electrocatalytic (LOD of 0.016 μM for H2O2)

[84]

π-π interaction π-π interaction Covalent

Biothiols fluorescence sensors Cys 0.30, Hcys 0.45 and GSH 0.86 nmol L1 Atomic-scale characterization

[85]

Sensor for Hg2+ (LOD: 0.25 nM)

[87]

π-π interaction π-π interaction Covalent

Photocatalytic activity (degradation efficiency 97%) Optoelectronic (2 A/W) and photocatalytic (3.5 times higher) Solar cell (PCE: 2.2%)

[88]

π-π interaction π-π interaction π-π interaction

FRET (0.81)

[91]

Cl2 sensor (LOD:1.97 ppb)

[92]

Hydrazine hydrate (LOD: 9 μM)

[93]

[86]

[89] [90]

L-cysteine (Cys), DL-homocysteine (Hcy) and glutathione (GSH). Graphene quantum dots (GQDs). F€orster resonance energy transfer (FRET).

23.3

Conclusions

Recent advances in the preparation of covalent and noncovalent fullerene-Pc, CNTPc, and graphene-Pc nanoconjugates and their increasing applications in many areas such as sensors, PDT, fuel cells, solar cells, etc., are highlighted. Pc-nanocarbon conjugates involving the phthalocyanine and nanocarbon moieties have a great success on the chemical protocols toward the preparation of these systems using different reactions such as “Click”, esterification, amidation. Moreover, Pc-nanocarbon hybrid materials offer a myriad of incentives for their incorporation into many related areas due to their extraordinary conductivity, delocalized π electron systems, and stability features. Particularly, photo-induced unidirectional electron transfer processes occur in such Pc-nanocarbon systems, which is widely demonstrated by photophysical

704

Nanocarbon and its Composites

investigations. In this connection, hybrid materials based on the assembling of Pcs with nanocarbon will increase as they lead to novel materials and composites with special optoelectronic features.

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24

Nanocarbon and its composites for water purification

Aftab Aslam Parwaz Khan*,†, Anish Khan*,†, Abdullah Mohamed Asiri*,† *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 24.1 Introduction 711 24.2 Application of carbon nanocomposites for water purification 24.2.1 24.2.2 24.2.3 24.2.4 24.2.5

24.3 In summary References 728

24.1

713

Carbon nanocomposites as adsorption for water purification 713 Carbon and its composites as photocatalysts for water purification 717 Carbon and its composites as desalination for water purification 721 Carbon and its composites as a disinfectant for water purification 724 Carbon and its composites as sensing and monitoring for water purification 727

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Introduction

Clean drinking water is necessary for human life. Right to use to safe water drinking has advanced well ordered and fundamentally finished the last numerous years in relatively all aspects of the world [1]. There is a perfect relationship among inspiring section to secure water and GDP as indicated by capita [2]. In any case, some have assessed that by 2025, the greater part of the world population could be confronting absolute water-based vulnerability [3]. Water plays a vital part in the planet’s economy as it works as a dissolvable for a wide diversity of chemicals and encourages and facilitates industrial cooling and transportation. Water pollution is the tainting of water bodies, for example, lakes, waterways, seas, and groundwater by human exercises. All water contamination influences living beings and plants that live in these water bodies and in all cases the impact is harming the individual species and populace as well as the normal natural groups. It happens when contamination is released straightforwardly or in a roundabout way into water bodies without satisfactory treatment to remove destructive constituents. Water contamination is a significant issue worldwide. It has been stated that it is the main overall cause for death and disease and that it causes the deaths of more than 14,000 individuals daily [4]. Ground and surface water have frequently been worked and directed as particular assets, in spite Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00024-9 © 2019 Elsevier Ltd. All rights reserved.

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of the fact that they are unified. Be that as it may, new innovations are of consistent interest for the reduction and removal of these harmful pollutants so as to enhance personal satisfaction. Nanomaterials have received across-the-board enthusiasm for water purification. Specifically, carbon nanotubes (CNTs), attributable to their large surface area and ratio, have gotten extraordinary consideration for their brilliant nanosorbent properties for separating contaminants from water [5]. CNTs have an excellent combination of mechanical, electrical, and thermal properties that make them a perfect reinforcement for composite material [6, 7]. It is therefore one of the most widely studied and explored elements. [8] It forms numerous compounds with different elements such as H, N, and O, and hence provides the basis of life [9]. CNTs are one-dimensional (1D) allotropes of carbon that are formed by rolling a layer of sp2 hybridized carbon atoms, arranged in a hexagonal network. Their diameter is usually a few nanometers and length can be several millimeters. Depending on the number of layers, they are categorized as single-walled or multiwalled carbon nanotubes (SWCNTs and MWCNTs), as shown in Fig. 24.1. CNTs were first discovered by Ijima in 1991 (MWCNT) and 1993 (SWCNT), respectively [10, 11]. They exist in several allotropic forms such as graphite, diamond, nanotubes, graphene, fullerenes, amorphous carbon, etc. Later on, these CNTs were synthesized by using different routes such as arc discharge, chemical vapor deposition (CVD), spray pyrolysis, electrolysis, hydrothermal and laser ablation, etc. [12]. CVD is considered the best technique that is mostly used for large-scale CNT synthesis. Catalyst-assisted thermal decomposition of hydrocarbons is carried out in the CVD technique. By using this technique, the large-scale production of defect-free CNTs with uniform length and thickness can be carried out [13]. CNTs exhibit various unexpected properties and hence are used in many technological as well as industrial applications because of their unique properties. These properties have made CNTs a promising candidate in the field of energy storage, solar cells, water splitting, flexible displays, sensors, computer chips, biological applications such as cancer treatment, etc. [14]. CNTs show a better performance in the Li-ion battery as an anode material as compared to the conventional graphite electrode. Though the electrical conductivity of CNTs is very good, the low surface area and production cost limits their application in the field of energy storage [15]. Considerable efforts are currently under way to use CNTs effectively for various applications, including nanotechnology, bioengineering, biosensors, and water purification. CNT composites have

Fig. 24.1 Types of carbon nanotubes.

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better mechanical properties with higher electrical conductivity for electrochemical sensor technology to trace and mitigate contaminants, pathogens, and the high complexity of wastewater matrices [16, 17]. Advanced techniques, for example, membrane filtration, ion exchange, and reverse osmosis can be utilized as a part of the treatment and expulsion of contaminants from water [18, 19]. However, the higher cost limits the large-scale use of such treatment strategies in rising countries. Molecules of water move through pores of the nanotube speedier than through different pores of a similar size. The carbon nanotubes are sheets of carbon atoms moved so firmly that only seven water molecules can fit across their distance. Their small size makes them great contenders for isolating particles [20]. Carbon materials, for example, activated carbon (AC) [21] charcoal CNTs [22, 23], have been utilized widely in water purification [24]. Thus, they are irreplaceable segments of all commercial technologies of water [25, 26]. This chapter will present the ebb and flow condition of water’s well being, distinguishing the real contaminant types of concern to people and the difficulties related with the successful removal of these species. The execution of created nanocarbon and graphene materials and their composites for water purification will be investigated. The properties of adsorption, photocatalysis, desalination, disinfection, sensing, and monitoring of these materials will be evaluated, including late endeavors coordinated at enhancing the execution through biological and chemical treatments. An assessment of the appropriateness of these materials for water purification will likewise be discussed.

24.2

Application of carbon nanocomposites for water purification

Various types of carbon nanocomposites are presently being investigated for use in water purification. The prime explanation behind the utilization of nanocomposites in any application lies in their novel properties, which are unique in relation to their counterparts.

24.2.1 Carbon nanocomposites as an adsorption for water purification Adsorption is a standout among the most effective techniques for the elimination of color, smell, and both organic and inorganic contaminations from industrial effluents. Adsorption is viewed as better in water treatment as a result of the accommodation, ease of activity, and effortlessness of the outline. Adsorption tasks misuse the capacity of specific solids specially to focus particular substances starting solution onto their surfaces. The adsorbents of conventional and nonconventional are utilized as a part of this approach. Activated carbon is a widely used conventional adsorbent for this reason due to its broad surface area, high adsorption capacity, microporous structure, and high level of surface reactivity. Nonetheless, its boundless use in wastewater treatment is somewhat limited because of its high cost and poor recovery limit [27–29]. The adsorption display in carbon nanotubes has been occasionally utilized for catching a wide assortment of water pollutants [30–34]. Some CNT properties have

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made them adaptable for adsorptive procedures and of the natural contaminations adsorb at outside surface and internal site of open-finished CNTs. The outside surface of CNTs can be functionalized for appending both natural and inorganic pollutants. MWCNTs generally contribute more pore volume of inward sites than the SWCNTs. SWCNTs are inclined to form bundles as a result of their strong van der Waals forces along the tube length axis that leads to the development of interstitial channels and fringe grooves (a positive influence for adsorption kinetics). In any case, as developed SSA of SWCNTs (400–900 m2/g) and MWCNTs (200–400 m2/g) which is significantly diminished upon aggregation (a negative impact for adsorption kinetics). Real interaction forces between CNT functionalities and water contaminations are covalent bonding, hydrogen bonding, electrostatic interactions, ion exchange, hydrophobic interactions, p-p electron coupling, and mesopore filling. Here, the performance of nanocarbon materials and their composites for water cleansing will be looked into. Salam et al. done the worked of nanocomposite made of MWCNT and chitosan with the ratios 25:75 wt% was readied. The nanocomposite of MWCNT/chitosan was utilized for the expulsion of Zn, Cd, Cu, and Ni ions from an aqueous solution [35]. Kumar et al. proposed a DBSA doped in a nanocomposite of polyaniline/ multiwalled carbon nanotubes that was set up by the method of oxidative polymerization [36]. DP/MWCNTs were observed to be perfect adsorbents for Cr(VI) removal when compared with pristine and oxidized MWCNTs. Tofighy et al. analyzed some divalent heavy metal ions removed from an aqueous solutions by utilizing sheets of CNT. The inclination of adsorption on the oxidized sheets of CNT can be arranged and ordered, seeing that Pb(II) > Cd(II) > Co(II) > Zn(II) > Cu(II), respectively [37]. Stafiej et al. considered that CNTs were utilized as an adsorbent to investigate the adsorption of metal ions such as Co, Cu, Zn, Cd, Pb, and Mn. The conditions of the solution, for example, the pH and concentration were studied. At pH 9, the order of affinity of metals toward CNTs are Cu(II) > Pb(II) > Co(II) > Zn(II) > Mn(II) [38]. As of late, Zhao et al. efforts that carbamazepine (CBZ) removal in water by composites of electroactivated carbon fiberperoxydisulfate. It is astounding to note that, even in recycling ACF 100 times, the CBZ removal rate at 30 min was diminished just marginally from 98.78% to 97.35% in the electroactivated carbon fiberperoxydisulfate process [39]. From Fig. 24.2, the electroactivated carbon fiberperoxydisulfate process can surprisingly upgrade the removal of CBZ in the aqueous solution because of the concurrent adsorption of pollutants and the amplified generation of active radicals on the cathode ACF, and the removal rate of CBZ shows an expanding pattern improvement of the anode potential. For example, the removal rate of CBZ in 15 min expanded from 36.13% to 66.84% when the cathode potential expanded from 3 to 6 V. Jhung et al. portrayed the preparations of Bio-MOF-1-derived porous carbon as strikingly profitable and successfully recyclable adsorbents for water purifying by methods for bisphenol and adsorption [40]. In another examination, different coreshell composites of carbon microspheres at LDH were synthesized through fabrication of the layered double hydroxide (LDH) nanoplatelets on carbon microspheres. After that, they were used as adsorbents to oust 2,4-D from an aqueous solution. The composites exhibit awesome water dispersity because of 3D different leveled circle structure and high affinity due to the carbon cores of organic that have rich hydrophobic

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Fig. 24.2 Proposed mechanism in E-ACF-PDS process. Reproduced from Liu Z, Zhao C, Wang P, Zheng H, Sun Y, Dionysiou DD. Chem Eng J 2018;343:28–36. Copyright (2018), with permission from Elsevier.

compounds. The morphology and microstructures of the composites of carbon microsphere (CMS), CMS@LDH, and LDH were shown as a solid roundabout morphology and a smooth surface showed up in Fig. 24.3 [41]. A recent study completed by Zhu et al. An exceptionally productive and adaptable carbon nanotube/ceramic composite was prepared as a filter for the removal of yeast cells and metal ions from a water system, and also worked in removing particulates from the air. The outcomes demonstrated that the CNT containing the composite filter exhibited a yeast filtration with high efficiency (98%), c. 100% metal ion removal, and superb particulate filtration from air systems. Fig. 24.4 demonstrates the obtained yeast cells (big particles as a bright) connected to surfaces of CNT. It was discovered that yeast cells were caught by the twisted CNT networks at higher magnification [42]. Notwithstanding filling in as a sorbent in favor of organic and inorganic pollutants, the present innovation has utilized CNTs as nanofilters to decrease molecule concentrations in polluted wastewater [43–45]. Like sorbents, the particular selectivity on carbon nanotubes as filters can be controlled through the connection of various functionalities through the pore entrances [46]. In spite of their hydrophobic qualities, carbon nanotubes demonstrated a remarkable execution in water transporting. Molecular dynamics simulations (MDS) demonstrated that the hydrophobic environment of CNT pores makes a weak interaction with water, but along these lines also enabled a quick and nearly frictionless stream of water [47]. Another clarification from Hummer et al. is that the frictionless stream of water is ascribed to the nanoscale repression that prompts the narrowing of the relation’s energy distribution and limits the interaction with water [48]. Aside from that, Fig. 24.5 demonstrates some observed confirmations of pollutant adsorption onto surfaces of CNT. As should be obvious, TEM image of MWCNTs after Pb (II) adsorption

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Fig. 24.3 The SEM image of CMS (A) and (B), CMS@MgAl-LDHs (C) and (D), CMS@NiAlLDHs (E) and (F), and CMS@ZnAl-LDHs (G) and (H) [41].

Fig. 24.4 SEM images of the filtered yeast cells by CNTs at different magnifications. Reproduced from Parham H, Bates S, Xia Y, Zhu Y, Carbon 2013:215–223. Copyright (2012), with permission from Elsevier.

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Fig. 24.5 Adsorption of water pollutants on CNT surfaces. (A) Metal, (B) bacteria, (C) oil, and (D) salt adsorption onto CNT surfaces. Adapted with permission from Das R, Hamid SBA, Ali ME, Ismail AF, Annuar MSM. Ramakrishna S.Desalination2014;354:160; Wang H, Zhou A, Peng F, Yu H, Yang J. J Colloid Interface Sci 2007;316:277–83; Kang S, Herzberg M, Rodrigues DF, Elimelech M. Langmuir 2008;24:6409–13; Gui X, Wei J, Wang K, Cao A, Zhu H, Jia Y, Shu Q, Wu D. Adv Mater 2010;22:617–21; Yang HY, Han ZJ, Yu SF, Pey KL, Ostrikov K, Karnik R. Nat Commun 2013;4:2220, Elsevier, ACS, Wiley and Sons, and Nature.

which isn’t consistent and mostly adsorb at the tips and flawed destinations of the MWCNTs (Fig. 24.5A) [49]. A SEM image of Escherichia coli bacteria presented to SWCNTs plainly proposes the loss of their morphology (Fig. 24.5B) [50]. A sponge-resembling adsorbent of CNT glided on oil-contaminated water and at the same time can remove oil with a bulky adsorption capacity (from 80 to 180 times their own particular weight for an extensive variety of solvents and oils) (Fig. 24.5C) [51], as a salt adsorption by membranes of carbon nanotube (Fig. 24.5D) [52].

24.2.2 Carbon and its composites as photocatalysts for water purification The framework of oxidative degradation is the current advancement strategy for the water purification procedures to debase the natural toxins. The photocatalyst is utilized for the degradation forms, which have the ability to advance the electron level of the material by diminishing the organic matter in the wastewater. In most normal, the advanced oxidation processes is primary and straightforward strategy in the water treatment processes. During the time spent advanced oxidation processes the generation of intermediates of oxygen and hydroxyl radicals included. These procedures utilize conventional oxidants such as H2O2 and/or O3 with extra boosts, for example, ultraviolet light to make highly reactive species such as hydroxyl radicals to oxidize substances. Advanced oxidation processes are fit to oxidize substances such as saturated organic molecules and pesticides, which are extremely hard to treat with

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different techniques. These advanced oxidation processes incorporate H2O2/UV, O3/ UV, H2O2/O3/UV, TiO2/UV, and a vacuum ultraviolet process [53]. The TiO2/CNT nanohybrid materials were synthesized by means of the hydrolysis method by Nguyen and coworkers [54]. The outcomes demonstrate that, compared with TiO2, the nanoparticles of CNTs and TiO2/CNT nanohybrid materials show higher catalytic activities in methylene blue degradation and methylene orange. The observed degradation percentage of TiO2 and CNTs in the proportion of 5/1 were about 33% and 38% for methylene orange and methylene blue, respectively. This can be ascribed to the essentially improved absorption of TiO2/CNT nanohybrids because of the attachment of the nanoparticles of TiO2 on the wall of the carbon nanotubes. The density functional theory estimated that the stability of TiO2/CNT nanohybrids is because of the preferential bonding state connecting TiO2 and CNTs at the edge. A photocatalyst of PANi/CNT/ TiO2 on a glass plate irradiated with visible light was exhibited to degrade DEP by Hung and coworkers [55]. The pot catalysts of PANi/CNT/TiO2 were fabricated by codoping with polyaniline and functionalized CNT (dCOCl and dCOOH) onto TiO2 after a hydrothermal synthesis and sol-gel hydrolysis. Doping of polyaniline brought about the absorption edge potocatalysts moving to 421–437 nm and the shift effect was found in the synthesized hydrothermal process. The greatest DEP degradation of 41.5%–59.0% and 44.5%–67.4% was obtained in the sunlight-irradiated system for both the methods, separately. The ideal pH was obtained at 5 and 7 for the photocatalysts. A InVO4 incorporated with CNT composite nanofibers was synthesized by an electrospinning technique [56]. The as-gathered nanofibers were calcined at 550°C in air to expel polyvinyl pyrrolidone, which could enable InVO4 to crystallize. InVO4 in the composite showed a hollow fibrous morphology and orthorhombic types, and carbon nanotubes were coated with nanofibers of InVO4. The photocatalytic execution of the composites was studied by the rhodamine B degradation under visible light. The nanofibers of CNT/InVO4 in RhB degradation showed a higher photocatalytic activity than InVO4 nanofibers and nanoparticles of CNT/ InVO4. The probable mechanism of photocatalytic degradation of RhB by the nanofibers of CNT/InVO4 under visible light irradiation appears in Fig. 24.6. The nanoparticles of cobalt doped with nickel ferrite were loaded on the CNTs utilization of the microemulsion method [57]. In this study, a functionalized carboxylic group of carbon nanotubes has been made to go about as a help for the beading of the ferrite nanoparticles. Surfactants of SDS ended up utilized as a soft template for controlling the shape of materials. Magnetic investigations of Ni1xCoxFe2O4/MWCNTs were performed utilizing the vibrating sample magnetometer machine, wherein every one of the samples displayed ferromagnetic performance. The magnetization increased with enhanced cobalt concentration, credited to the higher magnetic moment of Co ions as compared to Ni ions. The photocatalytic activity of ferrite/ CNT nanocomposites was studied for the Rhodamine B dye. An efficient photocatalytic inactivation of E. coli K-12 was researched by Shi and coworkers [58] with the use of a chain of Ag/AgX-CNT materials (X ¼ Cl, Br, I) as photocatalysts. The outcomes demonstrated that the visible light driven could completely inactivate photocatalytic 1.5  107 cfu mL1 of E. coli in 40 min, which was better than Ag/

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e–

e–

Visible light

e– e–

VB

H2O Rh B

CB h+

h+

•OH

CNTs CO2+H2O InVO4 nanofiber

e– e–

InVO4

CNTs

Fig. 24.6 Possible mechanism of photocatalytic degradation RhB by CNTs/InVO4 nanofibers under visible light irradiation. Reproduced from Zhang Y, Ma D, Wu J, Zhang Q, Xin Y, Bao N. Appl Surf Sci 2015;353:1260–68. Copyright (2015), with permission from Elsevier.

AgCl-CNTs and Ag/AgI-CNTs. It was discovered that E. coli photocatalytic inactivation was considerably more proficient under visible light at 435 nm and the photogenerated holes assumed a critical part in this system. Furthermore, the stability and mechanism of Ag/AgX-CNTs during photocatalytic bacterial inactivation were too researched, and the outcomes demonstrated that the organic garbage of disintegrated microorganisms might be absorbed on the active sites of the photocatalysts, leading to the decrease of the photocatalytic activity. Fig. 24.7 indicates

Fig. 24.7 Schematic photocatalytic inactivation processes and charge transfer of the Ag/AgBr-CNTs photocatalyst under visible light irradiation. Reproduced from Shi H, Li G, Sun H, An T, Zhao H, Wong PK. Appl Catal B 2014;158:301–7. Copyright (2014), with permission from Elsevier.

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Fig. 24.8 Mechanisms of a potential nanobiohybrid catalyst for water purification. From Das R, Hamid SBA, Ali ME, Ismail AF, Annuar MSM, Ramakrishna S. Desalination.2014;354:160.

the schematic photocatalytic inactivation procedures and the charge transfer of the Ag/AgBr-CNTs photocatalyst. Fig. 24.8 delineates a prototype of a nanobiohybrid catalyst for the monitoring, sensing, and degrading of numerous pollutants in water. The nanobiohybrid could be done by joining CNTs with enzymes through one of the three noteworthy routes, for example, physical adsorption and covalent bonding, carrier free that means crosslinking and encapsulation [59]. Covalent bonding of compounds has been prevalent by inciting the reaction for the amine groups lying on the surface of carboxylic groups with enzymes that could be created by oxidation of carbon nanotubes and the ensuing activation utilizing the chemistry of carbodiimide (Fig. 24.8 II) [60–62]. Carrier-free polymers, for example, chitosan, poly(diallyldimethylammonium chloride), etc., can likewise be utilized to immobilize enzymes resting on CNTs [63, 64]. What’s more, the LBL approach has been embraced for immobilizing compounds utilizing the catalyst’s epitome process. It allows the covering of different enzymes and makes multilayer films of enzymes on CNTs [65], as appeared in Fig. 24.8 III. The covalently bonded enzymes would be better for the nanobiohybrid applications in water purification.

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24.2.3 Carbon and its composites as desalination for water purification There are three essential kinds of water decontamination innovations that are utilized for desalination: (i) membrane technologies, (ii) distillation processes, and (iii) chemical methods. The conventional water purification plants utilize a mixture of these technologies. Membrane technology is today well accepted as the most suitable desalination advancement. Currently, it seems that there is no limit for the future growth of membrane processes. The greater part of membrane transport methods are isothermal and their driving forces are the transmembrane pressures of hydrostatic, electrical potential, concentrations, and so forth. The desalination process is widely used throughout the world and there is a need for more efficient membranes that can separate salt from water. Several approaches have been made for desalination. Recently, CNTs gained more attention in the field of water purification because of their variety of engineering and other applications [66, 67]. Detachment layers in view of CNTs have major potential for water purification. Films, in view of carbon nanotubes stodgy, have been appeared to give closer of water frictionless stream however the nanotubes, while holding critical amounts of salt subsequently offering the likelihood of low-energy desalination [68–71]. The surfaces of the nanoporous membranes of CNT are appropriate for dismissing microscale contaminations and particles in the fluid stage. The hydrophobic empty structures support frictionless development of water particles without the need for any energy-driven power to push water molecules through the hollow tubes. The cytotoxic effects of carbon nanotube layers diminish biofouling and increase membrane life by assassinating and expelling pathogens [72]. The functionalization of carbon nanotube layers specifically dismisses specific pollutant from water fusion. Kristen Prehn et al. portrayed an approach to carbon nanotubes-polymer nanocomposites with potential catalytic use [73]. The procedure starts with the carbon nanotubes flooring was full-fledged on substrate of silicon and little quantity of platinum was spited on CNT slants and the membrane layered was created by lift process. This membrane is given an assemblage with somewhat freestanding nanotubes as well as catalysts. Mohammadi et al. [68] synthesized CNT sheets through chemical vapor deposition and nitric acid oxidizer at a room environment, and after that it worked as a realistic adsorbent for the desalination of salty water. The sheets of the adsorption capacity were analyzed using Langmuir and Freundlich adsorption isotherm models. The outcomes demonstrated that oxidized sheets of CNT can be suggested as a viable adsorbent for the desalination of salty water. It was discovered that the Langmuir and Freundlich adsorption isotherm models coordinate the test data exceptionally well. Lee et al. established an easy process to produce vertically united carbon nanotubes resting on 3D and stretchy stainless steel meshes via the thermal chemical vapor deposition method [74]. A water-oil strainer can be attained by utilized the superhydrophobicity and superoleophilicity of the materials, which keep away water while allowing the penetration of oil. It was found that stainless steel-CNT meshes have the capability for water-oil emulsion and CNTs have a great attraction to oil after that water because of the hydrophobic interaction between CNTs and oil molecules.

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Kim et al. inspected the synthesis and execution of nano-TFN layers, including a thin film layer containing nanosilver and a polysulfone lattice containing CNT [71]. The water take-up, clean water permeability, antibacterial, antifouling, and solute rejection properties for the membranes of n-TFN were examined. The study established to the CNT acid modified and particles of nAg improve the permeability with antifouling properties of skinny nano film composite. Subramanian et al. reviewed the properties and applications of the new method of nanofiltration (NF) for desalination [75]. Thin-film nanocomposite membranes following coating a film by interfacial polymerization method was for desalination performance. It was found that a high flux and rejection salts can be made by (i) the membrane has to be hydrophilic, (ii) pore size reduction, (iii) thickness reduction, and (iv) preheating before the interfacial polymerization. It was also found that the merger of CNT’s better surface properties of the membrane can be used for the desalination process. Goh et al. explained the accessible obstacles and future challenges linked to CNT application in desalination technologies. They discussed the ion and water transport in CNT, and the performance evaluation of CNT for water desalination [76]. They concluded that CNT materials will play a vital function in desalination technology to permit better elasticity and a broader outlook in addressing serious water issues. Hamed Parham et al. demonstrated an easy move to construct very capable and versatile CNT composite strains through direct CNT growth on a porous ceramic medium. The removal of metal ions and yeast cells was assessed from water [77]. It was confirmed that CNT composite filters make use of a broad range of relevances for the removal of heavy metal ions and yeast cells from water. Xiaoshuang Yang et al. presented the preparation of multiwalled carbon nanotube buckypaper for the removal of humic acid from water [78]. In this study, the MWCNT surfaces were modified with carboxylic and hydroxyl groups. The results revealed that the functional groups greater than before the CNTs hydrophilicity and enhanced the removal competence of HA through the buckypaper. Eun-Sik Kim et al. found that multiwalled carbon nanotube fixed LPM demonstrated increased standardized flux than LPM devoid of MWCNTs and also NHPM (MWCNTs entrenched HPM) filtration attained the highest permeate flux as well as the higher organic portion removal [79]. This study evaluated the membrane systems in favor of the removal for extractable organic function from oil sands process affected water. The OSPW of raw was useful to the incorporated membrane scheme which conjugated for the low and high pressure-driven membranes. Another worked, MWCNTs are made out of various layers of sheets of graphene. SWCNTs and MWCNTs were utilized for nonstop water desalination [80–82] and in a roundabout way to remove inconvenient compounds that make desalination difficult [83]. CNTs are entrancing in cutting edge membrane technologies for water desalination because they give the solution as low energy for treatment of water. CNT membranes allow close frictionless water to flow through them with the maintenance of a wide spectrum of water contaminant. The inward hollow cavity of CNTs gives an awesome probability to desalinating water. The high perspective proportions, soft walls of hydrophobic, and internal pore width of CNTs permit the particularly efficient transfer of molecules of water. A few models of CNT-supported membranes appear in Fig. 24.9. The nanotube networks for atomic structures are extremely sensitive to different wet compound

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Fig. 24.9 Structures of some CNT membranes. Shown are (A) cross-sectional scanning electron microscope (SEM) image of a pristine CNT membrane; (B) CNT-based water filter with cylindrical geometry; (C) movement of water molecules through a CNT channel; (D) SEM image of scattered NaCl nanocrystals on CNT membrane surface; (E) movement of pure water molecules through CNT membrane in osmotically imbalanced compartments, and (F) engineered CNT membranes in industrial set up. Adapted with permission from Yang HY, Han ZJ, Yu SF, Pey KL, Ostrikov K, Karnik R. Nat Commun 2013;4:2220; Hilder TA, Gordon D, Chung SH. Small 2009;5:2183–90; Kar S, Bindal RC, Tewari PK. Nano Today 2012;7:385–89;

treatments. Subsequently, some of the time this changes CNT auxiliary properties and even demolishes CNT dividers, solute retention capability, reducing nanotubes for water fluxing, mechanical robustness and enhances water permeability of the membrane [52, 84–86]. The VACNT layers can be prepared via adjusting opposite CNTs with strong filler content (silicon, nitride, epoxy, etc.) between the nanotubes (Fig. 24.10A) [87]. Then again, an MMCNT’s film comprises a few layers of polymers and additional composite materials shown in Fig. 24.10B [88]. The work of these membranes through low energy utilization due to CNT roughness a smaller amount water transport ability through nanotubes for hydrophobic cavity. The material’s membrane is exceptionally sensitive in the direction of the different salt and water pollutants.

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Fig. 24.10 A prototype of a CNT membrane. Shown are trapping of salts and movement of water molecules from salinated water through SWCNT (A) and mixed matrix CNT (B) membranes. Reproduced from Ali ME, Hamid SBA, Ramakrishna S, Chowdhury ZZ. Desalination 2014;336:97–109. Copyright (2014), with permission from Elsevier.

24.2.4 Carbon and its composites as a disinfectant for water purification Water cleansing means the elimination of pathogenic microorganisms. Microorganisms are deactivated, bringing about the end of their development and reproduction. When microorganisms still reside in drinking water, the use of the water will make individuals sick. These dangers with them incorporate the formation of cleansing by products and multidrug bacterial species and have provoked the investigation of cutting edge purification methods [89]. The structures of nanocomposites execute pathogens by freeing toxic chemicals and appeasing cell membrane uprightness on coordinate contact. Now and again, they likewise bring into being reactive oxygen species. The effect of bactericides on metals has been acknowledged since prehistoric times, yet advances in nanotechnology have enhanced their effectiveness and empowered their utilization as a practical disinfectant. CNTs have ended up being extremely successful in expelling bacterial pathogens. Which have been utilized for evacuation of biological polluting influences have gotten unique consideration for their magnificent abilities of expelling biological pollutants from water [90]. CNTs have antimicrobial qualities against an extensive variety of microorganisms,

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including, for example, salmonella, E. coli [91–96], and viral infections [97, 98]. The cyano bacterial contaminants on CNTs are likewise higher when contrast and carbonsupported adsorbents basically because of substantial particular surface area, outer width of CNTs, synthesis of mesoporous level, etc. [99–102]. The methods of eliminating microscopic organisms by CNTs likewise because of the making of oxidative pressure, aggravations to cell membrane, et cetera [103]. Although a few SWCNTs are more impeding against microorganisms than MWCNTs [104], dispersivity of carbon nanotubes is an essential parameter than distance end to end [105] Numerous researchers have experience a high rate of adsorption of microscopic organisms by SWCNTs, notwithstanding their high sorption limits by numerous scientists [106–114]. Although few of other conceivable harmfulness mechanisms have anticipated by the scientists for example, leakage, electron transports inhibition, and cell membrane penetration and making of ROS appeared in Fig. 24.11. A large portion of these mechanisms have not been proofed through experiments [115–117]. Kang et al. have done the prevailing toxicity quality came about because of utilizing SWCNTs comparison to the MWCNTs and furthermore higher toxicity initiated by short, unfastened, and scattered MWCNTs toward microscopic organisms like bacteria [104]. Twofold sword utilizations of CNTs continually entrancing to scientific group. One part of CNTs have antibacterial activities while modified CNTs through various polymer composites go about as inert materials for growth of bacterial cell in MFC. MFC may be defined as a bioelectrochemical framework where current is created by utilizing microorganisms, imitating common bacterial interactions as shown in Fig. 24.12A. The systems consist of anode and cathode electrodes in various

Fig. 24.11 SEM images of bacteria before and after exposures (A) and some possible mechanisms of CNT-mediated bacterial cell death (B). Part A: Adapted with permission from Kang S, Pinault M, Pfefferle LD, Elimelech M. Langmuir 2007;23:8670–73; Liu S, Wei L, Hao L, Fang N, Chang MW, Xu R, Yang Y, Chen Y. ACS Nano 2009;3:3891–3902, American Chemical Society.

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Fig. 24.12 Schematic diagram of a MFC (A) and CNT-textile composite (anode) in MFC (B). Mechanisms of electron transfer and electrodemorphology and orientation for the CNT-textile anode (right), compared with classical carbon cloth anode (left). Part B is adapted with permission from Xie X, Hu L, Pasta M, Wells GF, Kong D, Criddle, Y. Cui, Nano Lett 11 (2010) 291–296, American Chemical Society.

compartments isolated via a membrane. The anodic compartment can be loaded with wastewater, which go about as supplements oxidized by microorganisms prompts CO2, protons and electrons productions. The external membrane for the bacterial cells has protein redox, for example, cytochromes that go about as intermediaries for exchanging the electrons to the electrode. The interaction of an electrode microbe is an essential factor for the rising power density of MFC’s. Xie and researchers fabricated an anode through CNTs for improving their interactions with microbes and increasing power density. A biocompatible synthesis and exceptionally conductive anode via utilizing composites of CNTs-textiles in MFC appeared in Fig. 24.12B [118]. The anode obliged more microbes and henceforth expanded the interactions of electrode bacteria for encouraging mass electron exchange from microorganisms to the anode of CNT materials. Moreover, the anode fabricated brought about a minor resistance of charge-transfer (10-fold) compared with a conventional carbon fabric anode, so this improved current conductivity.

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Liu et al. speculated that the toxicity of a CNT can be subject to its geometrical compositions and surface functionalities [114]. In this way, more investigation is important to obtain the idea of bacterial cell demise when a carbon nanotube is an adsorbent medium and ought to be treated with care.

24.2.5 Carbon and its composites as sensing and monitoring for water purification Quick sensing and observing of follow water pollutants have stayed a basic job utilizing regular sensor advances. What’s more, complexes of waste water mediums increased this trouble on pressing premise. The worldview called to grow very sensitive and quick responsive CNT supported gadgets and sensors. This is a result of some CNT claims to fame, for example, quick electro conductivity, high power of adsorbing with the goal that water contaminations can focus on their surface before detecting and catching. An analyst revealed that conductivity of polyethyleneimine and starchfunctionalized SWNTs was responsive to the [CO2] [119]. Therefore, a film was picked as the selectivity, improving coating for carbon dioxide sensing. SWNTs were amassed on the substrate of a surface acoustic wave, which disposed of the utilization of surfactants and maximized the showing film part to the approaching gas [120]. This sensing technique eliminated the normal issues of hysteresis and changeability because of adsorption of oxygen at the junctions of metal carbon nanotubes in the generally inquired about CNT-FETs. Joined with their little size and radio-frequency task, CNT-coated surface acoustic wave sensors demonstrated a guarantee as a workable solution for transporting and monitoring sensitive wireless gas [121]. In expansion, ionic pollutants-adsorbed CNTs have a known electrical conductance via estimating the proportion between analyte concentrations and current instabilities [122]. Lo´pez and Merkoc¸i planned tyrosinase enzyme integrated CNTs with an epoxy composite electrode and compared with a biosensor based of tyrosinase in light of a composite of graphite epoxy for evaluating phenolic water toxins, for example, catechol [123].

24.3

In summary

Safe and sound water has turned into a focused resource in numerous parts of the world because of increasing population, delayed dry seasons, environmental change, etc. Nanomaterials have one-of-a-kind characteristics such as size, shape, dimensions, and large surface areas that make them especially alluring for wastewater treatment, for example, cleansing, adsorption, and membrane separation. This chapter has demonstrated that wastewater treatment utilizing nanomaterials is a gifted research area for current and future work. One of the promising parts of CNTs and CNT-based composites for water purification is the generally low amount of material necessary to accomplish high filtration and adsorption capacity. For the time being, the mixture of accessible materials of carbon with the other rising nanomaterial sciences will be extremely significant in developing the up and coming age of water filtration and devices of water purification. Indeed, the “nano” era is approaching. CNT-based materials will without a doubt assume a critical part in the field of water purification in the near future.

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[74] Lee CH, Johnson N, Drelich J, Yap YK. Carbon 2011;49:669–76. [75] Subramanian S, Seeram R. Desalination 2013;308:198–208. [76] Sanip SM, Ismail AF, Goh PS, Soga T, Tanemura M, Yasuhiko H. Sep Purif Technol 2011;78:208–13. [77] Parham H, Bates S, Xia Y, Zhu Y. Carbon 2013;54:215–23. [78] Yang X, Lee J, Yuan L, Chae SR, Peterson VK, Minett AI, Yin Y, Harris AT. Carbon 2013;59:160–6. [79] kim ES, Liu Y, EI-Din MG. J Membr Sci 2013;429:418–27. [80] Dai K, Shi L, Zhang D, Fang J. Chem Eng Sci 2006;61(2):428–33. [81] Li H, Zou L. Desalination 2011;275:62–6. [82] Nasrabadi AT, Foroutan M. Desalination 2011;277:236–43. [83] Joseph L, Heo J, Park YG, Flora JR, Yoon Y. Desalination 2011;281:68–74. [84] Hilder TA, Gordon D, Chung SH. Small 2009;5:2183–90. [85] Kalra A, Garde S, Hummer G. Proc Natl Acad Sci 2003;100:10175–80. [86] Kar S, Bindal RC, Tewari PK. Nano Today 2012;7:385–9. [87] Ali ME, Hamid SBA, Ramakrishna S, Chowdhury ZZ. Desalination 2014;336:97–109. [88] Hinds BJ, Chopra N, Rantell T, Andrews R, Gavalas V, Bachas LG. Science 2004;303:62–5. [89] Verbyla ME, Mihelcic JR. Water Res 2015;71:107–24. [90] Savage N, Diallo MS. J Nano Res 2005;7:4–5. [91] Zhu Y, Ran T, Li Y, Guo J, Li W. Nanotechnology 2006;17:4668–74. [92] Deng S, Upadhyayula VKK, Smith GB, Mitchell MC. IEEE Sens J 2008;8:954–62. [93] Nepal D, Balasubramanian S, Simonian AL, Davis VA. Nano Lett 2008;8:1896–901. [94] Upadhyayula VKK, Deng S, Mitchell MC, Smith GB, Nair VK, Ghoshroy S. Water Sci Technol 2008;58:179–84. [95] Upadhyayula VKK, Ghoshroy S, Nair VS, Smith GB, Mitchell MC, Deng S. Res Lett Nanotechnol 2008;156358. [96] Akasaka T, Watari F. Acta Biomater 2009;5:607–12. [97] Brady-Est’evez AS, Kang S, Elimelech M. Small 2008;4:481–4. [98] Mostafavi ST, Mehrnia MR, Rashidi AM. Desalination 2009;238:271–80. [99] Long RQ, Yang RT. J Am Chem Soc 2001;123:2058–9. [100] Yan H, Pan G, Zou H, Li X, Chen H. Chin Sci Bull 2004;49:1694–8. [101] Ye C, Gong Q, Lu F, Liang J. Acta Phys Chim Sin 2007;23:1321–4. [102] de Albuquerque Jr. EC, M’endez MOA, dos Reis Coutinho A, Franco TT. Mater Res 2008;11:371–80. [103] Vecitis CD, Zodrow KR, Kang S, Elimelech M. ACS Nano 2010;4:5471–9. [104] Kang S, Mauter MS, Elimelech M. Environ Sci Technol 2008;42:7528–34. [105] Arias LR, Yang L. Langmuir 2009;25:3003–12. [106] Jia G, Wang H, Yan L. Environ Sci Technol 2005;39:1378–83. [107] Donaldson K, Aitken R, Tran L. Toxicol Sci 2006;92:5–22. [108] Dumortier H, Lacotte S, Pastorin G. Nano Lett 2006;6:1522–8. [109] Endo M, Hayashi T, Kim YA. Pure Appl Chem 2006;78:1703–13. [110] Nuzzo JB. Biosecur Bioterror 2006;4147–59. [111] Niu JJ, Wang JN, Jiang Y, Su LF, Ma J. Microporous Mesoporous Mater. Vol. 100. 2007. p. 1–5 [112] Kar S, Bindal RC, Prabhakar S, Tewari PK, Dasgupta K, Sathiyamoorthy D. Int J Nucl Desalin 2008;3:143–50. [113] Jugan ML, Oziol L, Bimbot M. Sci Total Environ 2009;407:3579–87. [114] Liu H, Ru J, Qu J, Dai R, Wang Z, Hu C. Bioresour Technol 2009;100:2995–3002.

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25

Ultrasonic treatment in the production of classical composites and carbon nanocomposites

Aleksandr Evhenovych Kolosov*, Elena Petryvna Kolosova†, Volodymyr Volodymyrovych Vanin†, Anish Khan‡ *Chemical, Polymeric and Silicate Machine Building Department of Chemical Engineering Faculty, National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute», Kyiv, Ukraine, †National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute», Kyiv, Ukraine, ‡Chemistry Department, Faculty of Science, King Abdulaziz University, Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia

Chapter Outline 25.1 Introduction: Prerequisites for the application of ultrasonic treatment in producing classical composites and carbon nanocomposites 734 25.2 Ultrasonic cavitation treatment for classical epoxy binders and polymer composite materials 737 25.2.1 25.2.2 25.2.3 25.2.4 25.2.5

Ultrasound and ultrasonic cavitation 737 Sonochemistry 739 Radiation of us energy into a low-viscosity liquid 741 Ultrasonic modification of classical liquid epoxy compositions 742 Ultrasonic cavitation processing devices for production of polymer composite materials 744

25.3 Ultrasonic dispersing of nanoparticles in solutions and liquid polymeric media 746 25.3.1 Ultrasonic dispersing of nanoparticles with organic solvents 747 25.3.2 Ultrasonic dispersing of nanoparticles in liquid oligomers 748 25.3.3 Sonication treatment of graphene dispersions 749

25.4 Ultrasonic treatment for preparation of nanosuspensions

750

25.4.1 Method of preparing nanosuspension in the preparation of a nanocomposite 751 25.4.2 Influence of ultrasonic treatment on thermal and rheological properties of suspensions of carbon nanotubes 752

25.5 Ultrasonic treatment in the production of graphene

753

25.5.1 Graphene 753 25.5.2 Ultrasonic treatment in the production of graphene and graphene-containing products 755

25.6 Graphene aerogels: Production methods and operational properties 25.6.1 Aerogels 757 25.6.2 Aerogels based on carbon nanomaterials 757 Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00025-0 © 2019 Elsevier Ltd. All rights reserved.

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Nanocarbon and its Composites 25.6.3 25.6.4 25.6.5 25.6.6

Aerogels based on graphene oxide: Synthesis and properties 759 Ultrasonic effect in the synthesis of hydrogels based on graphene 762 Problem situations in obtaining graphene aerogels 762 Potential applications of aerogels 763

25.7 Epoxy composites based on graphene aerogels with exceptional operational properties 764 25.7.1 Method of direct polymeric infiltration of aerogels 764 25.7.2 Graphene aerogels with adjustable density 765

25.8 Production of classical and nanomodified polymer compositions, prepregs, and composites based on them with ultrasonic treatment 767 25.8.1 Modeling of constructive-technological parameters of forming of classical polymer composites 767 25.8.2 Production of nanomodified thermoplastic composite materials by extrusion method with ultrasonic treatment 768 25.8.3 Method for the preparation of nanomodified epoxy compositions and prepregs on its basis 770

25.9 Conclusions 771 References 773

25.1

Introduction: Prerequisites for the application of ultrasonic treatment in producing classical composites and carbon nanocomposites

At present, in addition to the widespread use of classical polymer composite materials (PCMs) based on well-studied epoxy matrices [1], innovative world technologies for the creation of a new generation of PCMs are being intensively developed. These studies reach the nanoscale molecular level of research into the components that make up PCMs [2]. In connection with this, one of the promising directions in modern science is the production of composites based on polymers filled with carbon nanomaterials, such as carbon nanotubes (CNTs), fibrils, graphene, graphene aerogels (GA), nanoplates, nanofibers, etc. [3]. It is therefore not surprising that, over the next 20 years, many economically developed countries as well as developing countries will associate the further economic growth of their countries with the orientation of industrial sectors to the production and use of precisely nanostructured materials and nanocomposites [4]. Recently, studies related to the use of nanosystems have developed quite actively. To date, three main branches have been outlined in this area of research: (1) from nano- to macrocomposites (obtaining binders, materials from melts, etc. by means of substance synthesis); (2) from macro- to nanosubstances (due to disintegration), and, (3) by using various nanomaterials (nanotubes, fullerenes, etc.) as microadditives for compositional binders [5]. For example, nanotechnology is increasingly used in various technological processes in chemical and other industries, in particular, in chemical and oil and gas engineering. Nanocoatings have broad prospects for increasing the efficiency of the oil and gas complex. The latter is used for hydrophobizing surfaces of a wide range of structures and equipment. For example, these are antistatic coatings for fuel pipelines as well as for various structural metal products. This operation is carried out in order

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to provide these structural elements with chemical resistance as well as water repelling and antifriction properties for a long lifetime of operation. Another area of nanotechnology application in chemical engineering is the preparation of polymer compositions that strengthen stress concentration zones (in the form of holes, cutouts, fillets, thickness differences, etc.) in various power structures. No less effective is the use of such nanomodified polymer compositions for "healing" defects, microcracks, and other damages that occur during the manufacture and operation of such structures as well as to eliminate and seal gaps in the holes and joints of bolted and riveted joints. In addition to the manufacture of multifunctional nanostructured coatings and compositions, nanotechnologies have the prospect of being used in improving the manufacturing technology of various high-strength and corrosion-resistant body and structural elements based on reinforced thermoplastic PCMs, in particular carbon plastics [6]. Moreover, an increasing number of researchers have come to the conclusion that the most promising method for improving the properties of reactoplastic PCMs is their modification by carbon nanostructured components, including CNTs. The latter possess a number of unique properties that allow solving problems that arise in the production of nanomodified (NM) reactoplastic PCMs, that is, NM PCMs. Therefore, in contrast to classical PCMs, the physicochemical properties and technological aspects of the preparation of which have been studied comparatively and comprehensively, NM PCMs are increasingly being considered as their real alternative. This is mainly due to the increased rigidity and strength as well as the electrical conductivity of NM PCMs with a small bulk content of NM fillers. Fullerenes, CNTs, and diamondlike and fullerene-like structures have unique and substantially different physicochemical properties. This allows them to be used as modifiers of polymer binders and to obtain on their basis NM PCMs with wide ranges of performance values [7]. Another trend of the modern development and application of nanomaterials and derivatives based on them is the use of aerogels. Over the huge variety of aerogels, aerogels based on carbon nanomaterials are the most interesting for further study. The latter is a class of ultralight substances in which the liquid phase (at the lattice sites) is completely replaced by a gaseous phase. That is why GA was named a modern ultralight material. It thus (or so) outstripped the air-graphite record, which for a long time retained the “palm tree” in this category. Aerogels are characterized by a whole complex of unique properties. First of all, it has low density, a high specific surface area, and a high hydrophobicity index. No less important indicators of aerogels are low thermal conductivity, high elasticity (the ability to restore shape after multiple compressions and stretching), and the ability to sorb organic liquids. Moreover, depending on the purposes of the application, aerogels based on carbon nanomaterials can exhibit magnetic and electrically conductive properties while retaining the flexibility of their 3D structure. Therefore, it is not surprising that the impressive properties of new aerogels based on graphene are of great interest to scientists around the world. At the same time, it is urgent to find the most effective application of GA in various fields, including environmental protection, medicine, electronics, military, biology, chemistry, and many others. For example, the ability of GA to sorb organic liquids can be used to eliminate oil spills in aqueous media.

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When obtaining NM PCMs, one of the main problems is to ensure high-quality wetting and uniform distribution of the nanofiller in the liquid polymer matrix. On the one hand, nanosuspension is incorporated into the composition of the polymer binder of CNTs for the subsequent fabrication of a nanocomposite on its basis, which substantially increases the strength properties of the finished (polymerized) products. Moreover, the optimal concentration and uniform distribution of CNTs in the binder play a decisive role in the final strengthening of NM PCMs. On the other hand, due to the peculiarities of the nanoparticles used (their propensity to mutual attraction and agglomeration), problematic situations in the production of NM PCMs are de-agglomeration and further dispersion of the used nanoparticles in liquid polymer media. It is obvious that the incorporation of CNTs into the structure of the polymer composite affects not only the structure and properties of the liquid polymer binder, but also the NM PCM as a whole. For example, the dimensions of the agglomerate in nanofluids can significantly affect the thermal conductivity and viscosity of nanofluids and lead to different heat transfer characteristics. Analysis of the literature data on the elastic, strength, rheological, and electrical properties of composites filled with CNTs shows that neglecting the quality of dispersion generates, in the end, a large dispersion of the service (operational) properties of nanomodified PCMs [8]. However, we have to state that at the moment there are no unambiguously and clearly formulated industrial-technological principles concerning the incorporation, distribution, and stabilization of CNT dispersions in NM PCMs. It is known that low-frequency ultrasonic (US) is one of the most common methods of decomposition of CNT agglomerates [9]. In addition, its use facilitates the dispersion of nanoparticles in base liquids in the preparation of nano-based liquids, both on the basis of organic solutions and on the basis of liquid polymers. It is also known that the synthesis of aerogels based on graphene promises the formation of a threedimensional structure under the influence of low-frequency US. Numerous studies have found that the use of US contributes to the intensification of the basic operations of the technological process of producing classical PCMs [10]. This, in turn, leads to an improvement in the operational characteristics of the sonicated classical PCMs and NM PCMs, including sonication (sounding) of the polymeric binders (PBs), impregnation, winding, and dosed application in the preparation of fibrous prepregs. Also, the positive results of using the optimal modes of US treatment are a reduction of the time hardening of composites and obtaining defect-free PCM structures. The above confirms once again that the study of effective methods and the determination of the degree of their influence on the qualitative parameters of the final polymer product as well as the development of hardware-technological schemes for obtaining classical PCMs and NM PCMs are urgent and priority areas of modern research. In turn, the new technological methods developed as a result of the research will undoubtedly find wide application in the production of both structural and functional classical PCMs and NM PCMs. For such materials, reducing the mass of the product due to the improvement of their physicomechanical and operational characteristics is an urgent task for resource and energy saving on an industrial scale [11].

Ultrasonic treatment in the production of classical composites and carbon nanocomposites

737

Thus, the aforementioned brief analysis of the different aspects of molding of US trеаtment in the production of classical PCMs and NM PCMs brings out the actual directions of investigation. The following directions can be identified: l

l

l

l

l

l

Technical means of US cavitation treatment for classical epoxy oligomers (EOs), epoxy compositions (ECs), epoxy binders (EBs), and PCMs based on them. US dispersing of nanoparticles in solutions and liquid polymeric media. Preparation of nanosuspensions for the production of polymeric nanocomposites with US treatment. US treatment in the production of graphene and graphene oxide (GO). Epoxy composites based on GA with exceptional operational properties. Production of NM polymer compositions and prepregs based on them.

The above aspects are briefly described in this chapter.

25.2

Ultrasonic cavitation treatment for classical epoxy binders and polymer composite materials

25.2.1 Ultrasound and ultrasonic cavitation US cavitation is a process that turns off the formation and subsequent activity of formed gas, vapor, or vapor-gas bubbles (cavities) in a liquid medium irradiated with ultrasound (US) [12]. This phenomenon is also attributed to the effects that arise when the formed cavities interact with the liquid medium and with an external acoustic field. According to [13], sound waves propagating in a liquid medium at high intensities lead to alternation of cycles (half-periods) of high pressure (compression) and low pressure (rarefaction); see Fig. 25.1. These half-cycles alternate with velocities depending on the frequency of the external acoustic field. During the low-pressure cycle, high-intensity US waves create small vacuum bubbles or cavities in the liquid. This is due to the expansion of the liquid during the negative half-cycle of oscillations. When these bubbles reach a volume at which they can no longer absorb the energy of the acoustic field, the cavities collapse sharply. And at the interface gas-liquid boundary, the mechanical energy of the US wave is converted into thermal energy. In this case, the temperature and pressure increase many times.

Ultrasonic horn

Compression

Fig. 25.1 US vibrations in a liquid.

Distention

Compression

Distention

Compression

738

Nanocarbon and its Composites Implosion

I

II

III

IV

V

VI

Shell Explosion

Hot gas P = 200 MPa, T = 5000 K VII

v = 280 m/s

VIII

Fig. 25.2 US cavitation, implosion, and explosion in a liquid medium: І—occurrence of cavitation bubbles; ІІ—cavitation bubble grows under the influence of acoustic pressure; ІІІ— cavitation bubble reaches maximum size; ІV—cavitation bubble is compressed; V—cavitation bubble shrinks to a minimum size; VІ—the implosion of a cavitation bubble; VІI—cavitation bubble explosion; VIIІ—multiple recurrences of the cycle.

This results in the explosion of a multitude of bubbles with the formation of an implosion and explosion effect (see Fig. 25.2). The explosion is a chemical reaction that occurs at the highest possible rate, during which a sharp increase in pressure occurs due to a significant amount of hot gases. Explosion processes are always accompanied by the destruction of the internal structure. Implosion is the process that is the reverse of the explosion and means reduction, an inwardly directed explosion, or rarefaction. US cavitation contributes to the creation of unusual physical and chemical conditions, especially in cold liquids. This is due to the fact that, according to [13], very high temperatures (about 5000 K) and pressures (maximum about 200 MPa) are reached in the local zones during implosion, and the heating and cooling rate are >1010 K/s. The implosion of the cavitation bubble also results in the outflow of liquid jets at a speed of up to 280 m/s. In the case of US cavitation with a single bubble, the conditions can be even more extreme. It is understandable (or it is clear), that now (or at present time) check the above declared by the authors [14] theoretical technological parameters of the US cavitation process are practically impossible. Thus, the above effects contribute to the release of considerable energy in the destruction of cavities, the occurrence of local heating, and strong hydrodynamic disturbances (so-called second-order effects). The latter is represented by a complex in the form of microshock waves, cumulative jets, and microstreams of liquid. There are two different types of US cavitation [15]. The first of these is the so-called transient (inertial) cavitation, the nature of which is associated with the formation of the above-mentioned vapor-gas cavities in the liquid. This type of cavitation is characterized by high US intensities I (10 W/cm2). Cavitational bubbles form for a few moments from their original size and are due to collapse at the compression point when the bubble cannot absorb more energy. The second type of cavitation is noninertial (stable) cavitation. It is characterized by fluctuations of long-existing ones, that is, relatively stable vapor-gas bubbles about a certain equilibrium size. The intensity of US is (1–3) W/cm2. If the threshold of inertial cavitation is exceeded, both the above-mentioned types of cavitation can occur simultaneously, especially in view of the fact that the acoustic field in the liquid is usually inhomogeneous [16].

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25.2.2 Sonochemistry We should also mention sonochemistry, which studies chemical reactions when exposed to US. Cavitation-induced sonochemistry provides a unique interaction between energy and matter with hot regions inside the vesicles (droplets). These unusual conditions provide a number of chemical reactions for the synthesis of a wide range of unusual nanostructured materials [17]. The chemical effects of US exposure are not associated with molecular species. The US covers frequencies from about 15 kHz to 1 GHz. At sound velocities in liquids, usually about 1500 m/s, acoustic wavelengths range from 10 to 104 cm, that is, these are not the molecular dimensions of the lengths of acoustic waves. Consequently, at the molecular level, sonochemistry or sonoluminescence cannot explain any direct relationship between the acoustic field and chemical transformations. Also, US irradiation of liquids causes chemical reactions of high energy, often with the emission of light (so-called sonoluminescence). Because cavitation can occur only in liquid media, US irradiation of solids or systems with a solid gas usually does not observe chemical reactions. At the same time, when liquids containing macrosubstances with solid surfaces are irradiated with US, there can be accompanying phenomena [16]. For example, when cavitation occurs near the expanded solid surface of a macrosubstance, the collapse of the emerging vapor-gas cavity is nonspherical and sets in motion high-velocity liquid jets on a hard surface. These jets and associated shockwaves can cause significant damage to the solid surface of the macromatter and even strongly heat it. At the same time, US irradiation of suspensions of liquid powders causes another effect, manifested in the collision of dispersed particles of the suspension with a high speed. Cavitation and shock waves, which are created in such a suspension, can accelerate solid particles to high velocities and promote their intense collision with each other. As a result, a sharp change in the surface morphology, composition, and reactivity of the components of liquid powder suspensions is possible.

25.2.2.1 Sonochemical effects on sol-gel processes for synthesis of polymers Ultrathin nanosized particles, including spherical shape, thin-film coatings, fibers, porous and dense materials as well as aerogel and xerogel, are high-potential additives for the development and production of high-performance materials. Advanced nanostructured materials, including highly porous ultralight aerogels as well as organic and inorganic polymer hybrids, can be synthesized from colloidal suspensions or polymers in a liquid using a sol-gel method. This process is referred to as nanochemistry. The material obtained with the sol-gel method has unique characteristics because it is obtained on the basis of particles of the nanometer range. In general, sol-gel processes are a wet chemical synthesis method for manufacturing an integrated grid (so-called gel) of metal oxide or hybrid polymers. In this case, the conventional salts

740

Nanocarbon and its Composites

of inorganic metals are used as precursors, for example, metal chlorides and compounds of organic metals (metal alkoxides). The sol in the precursor suspension turns into a gel-like diphasic system in the liquid and solid phase. The chemical reactions occurring in the sol-gel process are a sequential chain in the form of hydrolysis, polycondensation, and gelling [16]. During the hydrolysis and polycondensation, a colloid (sol) is formed that consists of nanoparticles dispersed in an (organic) solvent. The existing ash phase is converted into a gel phase. The resulting gel phase is formed of particles that in turn form part of a continuous chain polymer. The shape and size of the latter depend on the chemical conditions of the process as well as the effects of sedimentation and gravity. At subsequent stages of the synthesis process, the gel obtained can be further processed. For example, using precipitation, spray pyrolysis, or emulsion technologies, not only ultrafine but also uniform powders can be obtained. The application of US to sol-gel processes leads to a better mixing and the deagglomeration of the particles. This results in smaller particle size; a spherical, low-dimensional particle shape; and enhanced morphology. Sono-gels are characterized by their density and fine homogeneous structure. These features are created due to the avoidance of the use of solvent during the sol formation, but also and mainly because of the initial cross-linked state of reticulation induced by the US. After the drying process, the resulting sonogels present a particulate structure unlike their counterparts obtained without applying the US, which is filamentous. So-called aerogels, which are characterized by high porosity and extremely low density, can be created by extracting the liquid phase of a moist gel. This is why supercritical conditions (primarily pressure and temperature) are usually required to obtain them. As indicated above, the action of US is especially effective for liquid media. In addition, US treatment ensures highly efficient mixing (homogenization) of the liquid media to be treated. As a rule, in catalytic sol-gel reactions, US is applied to precursors. The materials obtained as a result of such reactions are known as sonogels. At the same time, because of the absence of an additional organic solvent in combination with low-frequency US cavitation, a unique medium for the occurrence of solgel reactions is created. As a consequence, it is possible to create gels with exceptional performance properties, namely high density, fine and homogeneous structure, etc. These properties predetermine the final structure of the functional material formed on their basis. It is known that conventional aerogels consist of a low-density matrix with large empty pores. In contrast to airgel, sonogels are characterized by a thinner porosity, and their pores have a smooth spherical surface. The US is also an effective tool for the synthesis of polymers. During the US dispersion process, the shear forces arising in the liquid polymer environment result in a decrease in the molecular weight and polydispersity of the polymers. In addition, under the influence of US, multiphase systems are very efficiently dispersed and emulsified. As a result, very thin mixtures are obtained. Also, US increases the rate of polymerization compared to polymers with conventional and higher molecular weights with lower polydispersity.

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25.2.3 Radiation of US energy into a low-viscosity liquid The process of radiation of US energy into a low-viscosity liquid (water) with the US concentrators with a cylindrical tip is shown in Fig. 25.3. As a rule, when low-viscosity liquids are scanned, the US intensity І is within 10 W/cm2. The cavitation bubbles created by US waves of this intensity burst during one or more half-periods of tension-compression of the liquid. The gas bubble emerging in the volume of liquid discharge closes quickly enough under the impact of the coming compression period with the formation of shock microwaves (local pressure at the cavitation can reach values of 10–200 MPa). This is accompanied by an increase in the mobility of molecules in the liquid medium and the intensification of structural transformations in it, and also reduces the time of the technological process. The small amplitude of US of the radiating surface of US drives the small number of periodic oscillations of cavitation bubbles. Correspondingly, their small dimensions do not allow creating intense microflows in the entire volume of the technological tank (bath) for oscillating the liquid. In this case, the role of microfluidics when oscillating fluid is insignificant. During scoring, an increase in the amplitude А of US can be achieved up to 50 μm and intensity I up to 100 W/cm2. However, it is impossible to effectively incorporate a wave of such intensity into the liquid, although the efficiency of introducing US energy into low-viscosity liquids (water) is still quite high at the intensity I of US testing up to 20 W/cm2 (Fig. 25.3A and B).

Fig. 25.3 The process of radiation of US energy into the water with the US concentrators: (A) intensity of low-frequency US vibrations І¼ 18 W/cm2; (B) the regime of developed (transient) cavitation; (C) the regime of developed cavitation and the formation of a combined-gas screen (the intensity of low-frequency US vibrations І ¼ 28 W/cm2).

742

Nanocarbon and its Composites

However, with a further increase in the US intensity (more than 20 W/cm2), a strongly pronounced two-phase vapor-gas layer forms on the radiating surface of the US concentrator (Fig. 25.3C). It is because the absorption and scattering of US energy acts as a kind of screen. This screen prevents the passage of US into the liquid. In turn, the sharply reduced load resistance of the liquid medium leads to a mismatch between the “resonant US drive—technological liquid medium” system and the drop in the efficiency of incorporating the US into the liquid. Note that at the present time, there is no satisfactory model of the cavitation region adequately describing its behavior and the behavior of a separate cavitation bubble belonging to it. The behavior of the cavitation region depends on many phenomena and factors: reproduction and coagulation (coalescence) of the bubbles; their interaction; the change in the character of the pulsations of the bubble due to shock waves and sound radiation from neighboring bubbles; change in the average acoustic properties of the medium; microflows within the cavitation region and at the boundary of the bubble; distribution of embryos of cavitation; gas content, etc.

25.2.4 Ultrasonic modification of classical liquid epoxy compositions A somewhat different picture is observed with the introduction of US in EO (Fig. 25.4), that is, with physical modification [18]. The EO brand ED-20 is characterized by high adhesion and cohesion strength, low shrinkage, and manufacturability when applied to the surface of a complex profile. In this case, there is a “dilution” of the sonicated EO as compared to the initial consistency, which can be conditionally divided into three stages. In the beginning, during the first minute of EO sonication (the first stage), the radiating end surface of the concentrator symmetrically forms individual cavitation bands, the shape of which is similar to the “fungus.” During the second minute of scoring (the second stage), further

Fig. 25.4 Origin and development of the US cavitation process in liquid EO brand ED-20, depending on the time τ of its sounding in the low-frequency range by the concentrator US vibrations of cylindrical shape: (A) τ ¼0.5 s; (B) τ ¼60 s; (C) τ ¼120 s; (D) τ ¼ 180 s.

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development of the cavitation process takes place. As a result, there is a significant heating up of the EO, and, accordingly, a decrease in its viscosity. With the further sounding of the EO during the third and subsequent minutes (the third stage), a subsequent decrease in the viscosity of the EO occurs. Also, the process of developed cavitation throughout the volume of EO is spreading as a result of the large values of the amplitudes of the sound pressure. An increase in the intensity І of US leads to an increase in the amplitude value of pressure and the transformation of cavitation bubbles into pulsating bubbles. Numerous bubbles in the volume of EO, formed at this stage, can increase to visible sizes. This allows visual observation of the cavitation strings that move away from the radiating surface of the US concentrator into the liquid volume (Fig. 25.4A). Effective values of amplitude A and intensity I of US vibrations, as a rule, are found in the study of technological and operational characteristics of ECs and EPs. So, the extreme nature of the changes in the above-mentioned dependences of the technological and operational characteristics of ECs and EPs on the parameters of US processing, in particular, on the amplitude A and the intensity of the US testing, was experimentally established. For example, the upper value of the effective range of the amplitude A of sonicated EO is determined by the boundary value when the structure of the EO has not been destroyed and its transformation into a jelly state. Subsequently, the emitting surface of the US drive, which oscillates with a high amplitude as well as large cavitating bubbles, creates intense microflows (microjets) in the bulk of the liquid. If the amplitude of the oscillations A deviates from the optimal value of Аopt, the technological and operational characteristics of the ECs and EPs are changed on their basis. Such a change can be explained as follows. The small value of the amplitude A of the US vibrations is insufficient to achieve the cavitation threshold. An increase in the temperature of the medium when the EO is sonicated leads to the activation of cavitation processes due to the decrease in the viscosity of the medium. In turn, as the amplitude and intensity of the longitudinal US vibrations of the concentrator increase in the EO, processes occur that contribute to the occurrence (or increase) in the number of air inclusions, some of which remain in the volume of EO. This leads ultimately to an increase in defectiveness as well as to a decrease in the strength of solidified EPs. “The sonicating of EO on the effective parameters of the US modification not only leads to an increase in the density of molecular packing, but also to the growth of the effective density of the macromolecular grid. After all, as the degree of cross-linking increases, the number of nodes of the spatial macromolecular grid grows. The latter reinforces the restrictions on free rotation of the chain segments. This may be due to a decrease in the “defectiveness” of the macromolecular grid due to the “improvement” in the structure of the initial EO. The latter, in turn, leads to a corresponding hardening of the solidified Eps as well as to an increase in their performance characteristics (stiffness and glass transition temperature) [19]. In few studies it was shown the effectiveness of the use of US treatment of EOs with variations in the sonicating parameters, such as the frequency range [20], or working pressure [21]. The above-mentioned variation of the parameters of US treatment, carried out experimentally, is aimed at increasing the intensity of the sonicating of liquid

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polymer media. Therefore, it is advisable to use effective sonicating for the processes of preparation (homogenization) of ECs [22], contact US treatment of a preimpregnated glass-fiber filler prior to impregnation thereof [23], intensification of the process of capillary impregnation [24], and US dosing of the content of EC in impregnated fibrous filler [25]. Optimal US treatment at effective regimes promotes hardening of both solidified PBs [26], and formed on their basis of classical PCMs [27].

25.2.5 Ultrasonic cavitation processing devices for production of polymer composite materials Fig. 25.5 shows a “classical” scheme for sonicating a liquid medium by means of a concentrator of longitudinal US vibrations. This scheme is used in the modification of classical and NM reactoplastic binders. Recently, a method for impregnating and dosing the liquid PB onto a long fibrous filler has been applied. This method is realized by bilateral asynchronous contact action of the US vibrators on the surface of the material with the previously applied liquid PB (Fig. 25.6). In this case, a symmetrical feed of the oscillations is provided, with adjustment of their intensity and the feed angle to the surface of this material. US vibrations use magnetostrictive or piezoceramic transducers with a flat radiating plate. The procedure for calculating the metering device for the case of a piezoceramic converter is given in Ref. [28]. This method is carried out as follows. The belt fibrous material 1 passes at a speed v through the envelope roll in the impregnating bath (not shown in Fig. 25.6) with the liquid PB 2 where it is impregnated with this PB. Excess PB 2 is removed using two working tools 3 and 4, which are batch dispensers. Each of these metering devices consists of packages of magnetostrictive (permendur) or piezoelectric material welded (or attached with a threaded joint) perpendicular to the nonworking side of the radiating rectangular plate 5. Dosers 3 and 4 have individual excitation windings of magnetostrictive transducers. Dosers 3 and 4 are located on both sides of the impregnated material 1 and come into contact with it with a metered pressing force P1 and P2. Dosers 3 and 4 are generally pressed against the surface of the impregnated material with different angles γ 1 and γ 2. This changes the direction of movement of the impregnated material 1 after leaving the impregnating bath by an angle β. When pulling the material relative Fig. 25.5 Diagram of the sound of the liquid medium: 1—US generator; 2—frequency meter; 3— magnetostrictive transducer; 4— sonicated liquid; 5—concentrator of longitudinal US vibrations; 6— thermostatic cell.

3 1

2

6

5

4

Ultrasonic treatment in the production of classical composites and carbon nanocomposites ∅



˜

V

3 I2

P2

P1

g1

4 ∅

V





˜



g2

1

745

5

3

V

P1 I1 b

g1 2 1 5

3



g1

ϕ1 ˜



P1 I 1

g2

V

P2 I2 5



ϕ2 ∅

4

Fig. 25.6 To the implementation of a method for impregnating and dosing the PB onto a long fibrous material.

to the two dosers 3 and 4, a predetermined application of the binder is carried out and the required excess thereof is removed. At the same time, the minimum content of air pores in the impregnated material is ensured due to the fact that this is processed from two sides by nonsynchronous US vibrations. Varying the PB content in the impregnated material and removing its surplus are carried out by adjusting the intensities І1 and І2 and the feed angle γ 1 and γ 2 of the oscillations to the surface of the material 1. The voltage bias of the radiating plates 5 of the two dosers 3 and 4 (respectively, φ1 and φ2) is adjustable within the range (0–180 degrees). When the phase shift is 180 degrees, the rectangular plates 5 of the two dosers 3 and 4 operate on the principle of an asynchronous drive. That is, the antinode on one symmetrically located plate coincides with the depression on the second plate, and vice versa. On the width of the radiating plates 5, the oscillations propagate uniformly, and along the length—according to applying the voltage to the excitation windings of the magnetostrictive transducers. That is, an analogy of the physical effect is achieved in the form of peristaltic displacement of liquid and pasty media in the impregnated material due to the action of US. In Ref. [28], an approach to the application of the principles of computer-aided design on the basis of the methodology of structural-parametric geometric modeling was developed. The developed approach provides an algorithm for determining the structural and technological parameters of technical means for forming reactoplastic composite fibrous materials using US treatment. Within the framework of the

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implementation of this approach, a general structural-parametric model for the technological process of preparing long-length PCMs has been developed. The structuralparametric model is presented in the form of three consolidated blocks: (1) US treatment of liquid epoxy resin without hardener and subsequent preparation of PB; (2) impregnation of the fibrous material with a sonicated PB; and (3) dosed application of PB to the impregnated fibrous material. It was investigated that the use of US treatment of ECs reduces the time of their solidification approximately in (2–4) times. The use of US to process the epoxy composites also reduces the time of their solidification by about (2–4) times. In this case, the coefficient of variation of the operational characteristics is reduced by a factor of 2.5–3 times. Also, the wetting power of the polymer matrix increases by 30%–50%, which contributes to a faster and higher quality impregnation. Strengthening of classical armed PCMs, obtained using combined US treatment, is no less than 15%–18%. Note that the effective parameters of the process of US processing (processing time τ, amplitude A, frequency f, intensity I) are determined experimentally in each specific case, for example, using the methods of experimental and statistical modeling.

25.3

Ultrasonic dispersing of nanoparticles in solutions and liquid polymeric media

The ultimate goal of the large-scale theoretical and applied research in the field of polymer nanomaterials science is the creation of a new class of NM PCMs. These materials are obtained by chemical modification, that is, by incorporating nanoparticles, including functionalized ones, into their compositions. Such a modification provides a change in the structure of the polymer matrix, which ultimately leads to a significant improvement in the performance characteristics of the final NM PCMs. It is known that with the optimal incorporation of even small amounts of carbon nanomaterials, the operational and technological properties of polymers change. Namely, their mechanical properties and chemical resistance are improved while the electrical and thermal conductivity are increased, etc. [29]. These additives also allow extending the operating temperature range of the NM PCMs due to the increase in the glass transition temperature. This is due to the fact that the nanoparticles, participating in the formation of the supramolecular polymeric structure, through it seem to have a positive effect on the properties of the created filled composite material. For example, by varying the number of fullerene additives, it is possible to produce elastomers with predetermined physical and mechanical characteristics. The improvement in the properties of a polymer filled with CNTs, apparently, is a consequence of the structural rearrangement of the polymer. Such a restructuring is caused by a change in the concentration of the nodes of the spatial grid of the NM polymer. At present, regardless of the method of modifying the carbon nanomaterials and the nature of the modifier, experiments with carbon materials are mainly carried out using low-frequency US treatment of the reaction mixture. As a result, the processes of disaggregation of carbon nanomaterials intensify and defects are partially formed on their surface [29].

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25.3.1 Ultrasonic dispersing of nanoparticles with organic solvents It is generally accepted that there are two basic methods for dispersing nanoparticles in initial liquids: the so-called one-step method and the two-step method [30]. The twostep method is used primarily because it can be used at a time to obtain a greater amount of nanofluid than a one-step method. Moreover, the two-stage method is suitable for nonmetallic nanoparticles and initial liquids with high vapor pressures. Both methods enhance the chemical activity of the surface of carbon nanomaterials [31]. In the preparation of nanofluids in a two-step process, in the first stage, the nanoparticles are first dispersed in the base liquid. Then, in a second step, the resulting slurry is mechanically treated to reduce the aggregation of the nanoparticles in the suspension. In connection with this, the most widely used is low-frequency US. In this case, as a rule, US-baths, submersibles, and flow-through US dispersers are used to perform low-frequency US treatment, and also high-speed mechanical dispersants are used simultaneously with the US. In laboratory conditions, the sonication of liquid media is produced by a metal tip in glassware to visualize the changes that occur. Nanocomposites based on various polymers (polystyrene, polyethylene, polymethylmethacrylate, etc.) have already been created, and many ways of obtaining them have been developed. The interesting but still little-studied direction in this area is the creation of NM PCMs on the basis of organosilicon polymers. These polymers are distinguished by high radiation, chemical and thermal stability, electrical strength, biological inertness, and compatibility as well as high gas permeability. According to the above properties, these PM PCMs are used in the aerospace industry, the electric power industry, microelectronics, machine building, medicine, and other industries. At the same time, low mechanical characteristics limit their wide application. Therefore, using carbon nanomaterials as fillers of such oligomers, it is possible to create new NM PCMs with improved performance characteristics. In a study [26], the possibility of obtaining stable dispersions of carbon materials in organic solvents and a solution of high molecular weight polydimethylsiloxane rubber SKT (PDMS) was investigated. To prepare the dispersions, organic solvents such as toluene and benzene were used. The choice of the latter was due to the good solubility of PDMS in these solvents as well as the possibility of carrying out in them a chemical modification of dispersions of carbon materials. Samples of carbon materials of PUM and KEM-3 brands were studied. The preparation of dispersions in a solution of PDMS (concentration 0.01% and 0.05% of carbon materials by weight of PDMS) was carried out in two stages. In the first stage, carbon nanomaterials were dispersed in organic solvents using a number of methods: (1) high-speed electromechanical dispersant brand MPW-309 at 14,000 rpm in seven stages of 5 min. each with a break between the stages (5–10) min to cool the dispersion and dispersion chamber; (2) US baths with an operating frequency of 25 kHz in a volumetric flask for 1 h; (3) a submersible US disperser with an operating frequency of 22 kHz in a glass cup in stages by (5–10) min with a break for the addition of toluene in return for toluene, which evaporated. The total dispersion time in the organic solvent was 1 h. In a second step, a solution of PDMS was added to

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the dispersions obtained. Dissolution was carried out with stirring using the MPW-309 electromechanical disperser at minimum speed. To study the uniformity of the distribution and particle sizes of carbon nanomaterials, the optical microscopy method was used. Thin-layer test samples were obtained by depositing a droplet of dispersion of carbon materials in a PDMS solution on a slide, followed by drying in air at room temperature. It was experimentally established that the smallest particle sizes (30–40 μm) are observed when using a submersible US dispersant. When dispersing in the US bath of KEM-3 material, large particles (80–100) μm are produced. At the same time, the carbon material of the PUM brand cannot be dispersed by this method. The use of a high-speed electromechanical dispersant does not allow obtaining a high degree of dispersion—particles of carbon materials have dimensions of (300–400) μm. It was also found that the particle size and distribution in solution is independent of the brand of the organic solvent used (benzene, toluene). It was found that the dispersions obtained using US methods tend to fast (1.5–4 min) aggregation with the formation of flocs, followed by their sedimentation, regardless of the concentration of carbon nanomaterials and the nature of the organic solvent used. However, the aggregated dispersions obtained with the help of a submersible US dispersant are easily restored by their repeated treatment in a US bath. In turn, the incorporation into the sonicated solution of the rubber solution PDMS stabilizes the dispersions of carbon nanomaterials. This is due, in particular, to an increase in the viscosity of the solution being created. The most stable and uniform dispersions are also formed when a submersible US dispersant is used. In conclusion, it should be noted that at present, the US-dispersion method of carbon nanomaterials in organic solvents is used for chemical functionalization (incorporation of modifying groups into their composition). And dispersion of carbon nanomaterials in a solution of rubber PDMS is used to produce carbon-modified polydimethylsiloxane films and coatings [26].

25.3.2 Ultrasonic dispersing of nanoparticles in liquid oligomers Experiments conducted by other researchers confirmed the assumption that it is most advantageous to incorporate nanofiller particles into a less viscous liquid medium [32]. As the filler, the nanodispersed filler of the baked composite was used. The latter consists of oligoelements (minerals, salts, metals). In particular, oligoelements include carbon, bicarbonates, iron, zinc, magnesium, sodium, and manganese. A polymer matrix based on the EO brand ED-20 was used. For cross-linking EC, a low molecular weight polyethylene polyamine PEPA hardener was used, which allows the curing of materials at normal temperatures. PCM was cross-linked by incorporating a curing agent into the composition at a stoichiometric ratio of the components (wt%)–ED-20:PEPA ¼ 100:10. The nanodispersed filler of the baked composite was incorporated into the PB at a content of 0.05 parts by weight per 100 parts by weight EO brand ED-20. In order to achieve an even distribution of this nanodispersed filler in the EO, a technology was developed for incorporating nanoparticles into the EC.

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The technology provides for stage-by-stage mixing with the implementation of temperature-time regimes: (1) preliminary dosing of the oligomer ED-20, heating of the oligomer and nanodispersed filler to a temperature of T ¼ 353  2 K and holding the components at a given temperature for a time t ¼ 20  0.1 min; (2) dosing of the nanofiller, its further incorporation into the EO brand ED-20, as well as the hydrodynamic combination of the oligomer ED-20 and the nanofiller for t ¼ 1  0.1 min; (3) US-treatment of the NM EC during τ ¼ 1.5  0.1 min; subsequent cooling of the sonicated EC to normal temperature during t ¼ 60  5 min.; injection of PEPA hardener and mixing of epoxy binder components for t ¼ 5  0.1 min; (4) curing of the sonicated EC according to the experimentally investigated regime: molding the samples and holding them for t ¼ 12.0  0.1 h at a temperature T ¼ 293  2 K, heating with a velocity υ ¼ 3 o/min to a temperature T ¼ 393  2 K, holding for a time t ¼ 2.0  0.05 h, slow cooling to a temperature T ¼ 293  2 K. In order to stabilize the structural processes in the polymer matrix, the samples were held for a time t¼ 24 h in air at a temperature of T¼ 293 2 K, followed by carrying out experimental tests. The obtained data testify to the expediency of performing US treatment of compositions with nanoparticles to obtain a material with a uniformly distributed structure and improved properties. The process of dispersing nanoparticles in a liquid medium with US vibrations provides cavitation processes as well as a uniform distribution of nanoparticles in the composition. Cavitation processes that occur in the composition during US lead to the activation of epoxy macromolecules, an additional reduction in the viscosity of the system, and also an increase in the temperature of the PB due to the maintenance of mechanical energy in it. This results in a uniform distribution of the nanofiller in the volume of the binder (due to dispersion) as well as the formation of free radicals (due to cavitation processes). Using the method of optical microscopy, the fracture surface of initial and US-modified epoxy matrices and nanocomposites based on them was studied. It has been established that, in US-modified NM PCMs, a flat albeit somewhat corrugated surface is formed in comparison with the untreated US of NM PCMs, in which there are no dark inclusions and craters. This is explained by the fact that due to cavitation processes in the composition during US treatment, nanoparticles are intensively wetted and evenly distributed in the PB. This promotes their active physical interaction with macromolecules already at the initial stage of composite formation. And the subsequent chemical interaction causes the formation of a material with improved properties.

25.3.3 Sonication treatment of graphene dispersions The deterministic degree of dispersion of graphene and graphene oxide is extremely important for the fullest utilization of the potential of graphene with its unique performance characteristics. Therefore, if graphene is not dispersed under controlled conditions, the polydispersity of the resulting dispersion of graphene can lead to unpredictable performance characteristics after it has been

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included in the composition of the corresponding materials and structures. This is due to the fact that the properties of graphene vary depending on its structural parameters [33]. In particular, US treatment is an effective method for weakening interlayer forces and allows precise monitoring of important processing parameters. For GO, which is usually peeled off as single-layer sheets, one of the major polydispersity problems arises from changes in the lateral area (around the edges) of the flakes.It was found that the average lateral dimension GO can be shifted in the range from start 400 nm to 20 μm by changing the initial graphite material and the conditions of US treatment [33]. US treatment of graphene dispersion leading to small and even colloidal suspensions has been demonstrated in a number of studies mentioned in the review [34]. In particular, it was noted that when using US, a stable dispersion of graphene with a high concentration of 1 mg/mL and relatively pure graphene sheets is achieved. And the obtained graphene sheets are characterized by a high electrical conductivity of 712 S/m. Also, the results of the study of infrared Fourier spectra and Raman spectra showed that the US treatment method causes less damage to the chemical and crystalline structures of graphene [34]. The preparation of GO by US treatment will be discussed in the following sections.

25.4

Ultrasonic treatment for preparation of nanosuspensions

Different kinds of treatment are important for the preparation of nanosuspensions, which are nanodispersed systems. In nanosuspensions, the dispersed phase is nanoscale solid particles (nanofillers), and the dispersion medium is a liquid medium. Those nanosuspensions are suspensions of nanoparticles (nanoparticles) in liquid media. Nanosuspensions with particle sizes less than 100 nm are also called colloidal solutions, or sols. Nanosuspensions are characterized by sedimentation and aggregative resistance. Typically, nanosuspensions have very high sedimentation resistance, which, however, can only be detected experimentally in each specific case. When aggregate stability is violated, larger aggregates of nanoparticles are formed as a result of their coalescence (coagulation), which is a negative factor in the formation of NM PCMs. There is a relationship between these two types of sustainability. Thus, for example, a violation of the aggregative stability of nanosuspension leads to a loss of its sedimentation resistance. This is because nanoparticles, increasing in size, show less activity in Brownian motion. Therefore, to increase the aggregative stability in nanosuspension, high-molecular substances are incorporated. The latter form an adsorption layer on the surface of nanoparticles, which prevents them from sticking together. Also, various methods are used that contribute to the disaggregation (disintegration) of the agglomerates formed from the fillers, including US.

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25.4.1 Method of preparing nanosuspension in the preparation of a nanocomposite A number of methods are known for preparing nanosuspension in the preparation of a nanocomposite. For example, for a uniform distribution of a predetermined amount of CNTs by volume of a binder, special mixers with blades and compression chambers are used, also using ionization of nanoparticles according to the patent on invention RU No. 2301771 [35]. The closest method to that of [36] the technical solution is the method for manufacturing the “polymer/CNT” composite according to the invention patent RU No. 2400462 [37], in which an US action on the polymer mixture is used to uniformly distribute the nanoparticles. It is noted that US treatment provides the destruction of agglomerates with CNTs and uniform distribution of agglomerate to an increasingly lesser extent (by size) in the volume of nanosuspension. At the same time, the determination of the CNT dispersion time in the method [37] is not provided. However, an inadequate time of US treatment does not ensure the uniformity of nanoparticle distribution. And with an excessively long process of dispersion, the process of destruction of the longest CNTs can start, which will lead to a decrease in the strength of the NM PCMs manufactured on their basis. The technical task of the invention [36] is the determination of the minimum necessary time for the dispersion of CNTs in a PB to achieve the almost complete dispersion of CNTs. The task is solved due to the fact that the process of US dispersion of CNTs in a PB is performed with simultaneous photoregistration of changes in the intensity of the nanosuspension color. And when the nanosuspension reaches the values of the intensity of the color, they correspond to the values of the normalized degree of dispersion (NDD) in the range from 0.9 to 0.99, and the US effect is stopped. The index of the NDD for a given concentration is determined preliminarily. The US effect of the resulting nanosuspension is carried out with an intensity in the cavitation zone in the range from 1.5 to 2.5 W/cm2. The authors of the method [36] found that the degree of dispersion of CNT nanoparticles at a given concentration of CNTs corresponds to the intensity of the nanosuspension color, which varies with the process of US dispersion. The greatest hardening of the composite occurs when all agglomerates are destroyed, and CNTs are evenly distributed throughout the volume of the binder. In this case, the intensity of the nanosuspension coloration takes the maximum steady-state value for a specific ratio of CNT and binder, and with further US exposure, this index does not change (NDD ¼ 1). It is obvious that immediately after the incorporation of CNTs into the PB, the degree of dispersion is zero because CNTs are incorporated in the form of an agglomerate. As deagglomeration and uniform distribution of particles in the PB, the intensity of the color of the nanosuspension changes from a transparent state first, and then through a gradual turbidity until the color intensity of the steady-state value is reached. Thus, when CNTs are mixed with a PB under conditions of US effect, the NDD changes from 0 to a certain value (maximum of the NDD ¼ 1). Moreover, if the optimal treatment time is exceeded, the agglomerates are no longer destroyed.

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The nanoparticles of CNTs are evenly distributed (i.e., there are no agglomerates in nanosuspension in this case). Continuation of the US treatment process beyond this time value is in vain in terms of achieving better dispersion. It may even be harmful from the point of view of preserving the integrity of CNTs, which can be disturbed during prolonged US exposure. Based on the experimental values obtained, a plot was constructed of the flexural strength of the samples versus the concentration of CNTs at different values of the NDD. It should be noted that method [36] allows us to slightly level the parameters of US impact, which can change the shape of the graph and shift it in time.

25.4.2 Influence of ultrasonic treatment on thermal and rheological properties of suspensions of carbon nanotubes The adherence of nanofluids is very important for determining their operational and, in particular, thermophysical properties. For example, nanofluids with the same nanoparticles and base liquids can behave differently, that is, have different performance properties due to different methods of preparing nanocrystalline materials. This is due to the fact that the sizes of agglomerates formed in nanofluids can significantly affect such important operational properties of nanofluids as thermal conductivity and viscosity. This leads to different heat transfer characteristics. Due to the presence of agglomerates in nanohydrates, US is the most known method of their decomposition. It also facilitates the dispersion of nanoparticles in base liquids. In the investigation [38], the effects of US action on the thermal conductivity and viscosity of suspensions of CNTs (0.5 wt%) in an ethylene glycol solvent-based nanopowder were studied. Considering the fact that the multiwalled (MW) CNT surface is hydrophobic and the solvent ethylene glycol is a polar liquid, gum arabic with a concentration of 0.25 wt% is used as the MWCNT dispersant. Gum arabic was a viscous liquid that solidified in the air. It dissolves well in warm water (in cold water it is much more problematic) with the formation of a sticky weakly acid solution. The use of gum arabic in this case made it possible to better disperse CNTs in an organic solvent. A two-step method of preparing the nanosuspension was used. First, the nanofillers were dispersed in ethylene glycol in a 500-mL glass container into which a rotating mixer was immersed. After the gum arabic was completely dissolved in the solvent, an 0.5 wt% MWCNT was dispersed in the liquid using low-frequency US. The radiation power was 150 W, both in continuous mode and in a pulse mode of 20 modes (0.8 and 3.2 s) at a frequency of 20 kHz. The specific energy of US per minute was 1.8  104 kJ/m3. In addition, there was a correlation between the dimensions of the agglomerate and the length of CNTs under the influence of US treatment. It was also found that with increasing time/energy of US, the thermal conductivity of nanofluids increases nonlinearly. And the maximum increase in thermal conductivity is 23% with the processing time of US 1.355 min. At the same time, the viscosity of nanofluids increases to a maximum at a sonication time of 40 min and then decreases. And at the US treatment time of 1.335 min, the viscosity of the sonicated nanofluid approached the viscosity of a clean (uncharged) base liquid [38].

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It is also noted that the US processing process not only reduces the size of the agglomerates but also reduces the length of CNTs. Moreover, the reduction in the size of the agglomerate is more significant than the decrease in the length of the CNT. Thus, the maximum increase in thermal conductivity and the minimum increase in viscosity are obtained using prolonged US treatment (1.335 min), which, however, is in this case quite an energy-consuming technology.

25.5

Ultrasonic treatment in the production of graphene

25.5.1 Graphene Graphene structurally represents a flat grid of carbon atoms located at the corners of regular hexagons at a distance of 0.1418 nm [39]. In graphene, each carbon atom is connected to three neighboring atoms. Carbon atoms have six electrons, two of which are on ls-orbitals, and four more on hybridized sp3-, sp2-, and sp-orbitals. In graphene, carbon atoms are located on hybridized sр2 orbitals and are bound in the plane by three σ-bonds. And the π-bonds are perpendicular to the plane. In this case, the σ-bonds of graphene are shorter and stronger than in diamond. Therefore, in the plane itself, graphene is more durable than a diamond. The electrons above the plane make graphene electrically conductive, and the interaction of these electrons with light gives black graphene. Graphene is an anomalous material because it has the properties of a metal, although its electronic structure does not correspond to the accepted theory of the structure of metals. To give it a semiconductor property, the substrate effect, the size effect, and the action of the electric field are used. The unusual properties of singlelayer graphenes include direct electron transfer (Klein tunneling) through a potential barrier. The mobility of electrons in graphene at low temperatures reaches 20 m2/(Vs). For comparison, for Si it is equal to 0.15 m2/(Vs), and for GaAs it is 0.85 m2/(Vs). The thermal conductivity of graphene at room temperature is up to 5300 W/(mK). Graphene absorbs only 2.3% of visible light and is an ideal transparent conductor [39]. The features of graphene’s electronic structure make it a ballistic conductor. In such conductors, electrons are able to move at high speed without collisions with the atoms of the material. In this regard, the material has a conductivity of the metallic character and can withstand currents up to 108 A/cm2. This property can be used to create nanoelectronic devices. Graphene is the basic structural element of some allotropes including, besides graphite, CNTs and fullerenes. Used as an additive, graphene can enhance the electrical, physicomechanical, and barrier properties of PCMs at extremely low loadings [40]. Several basic methods for obtaining graphene have been developed [41]: (1) mechanical cleavage from highly oriented graphite (micromechanical and US splitting of graphite); (2) chemical deposition from the gas phase (various methods of epitaxial growth by pyrolysis of hydrocarbons or disproportionation of CO); (3) organic synthesis (catalytic dechlorination of hexachlorobenzene); and (4) chemical method using dispersions. The first group of methods includes micromechanical as well as US splitting of graphite. The second group is represented by various methods of epitaxial

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growth by pyrolysis of hydrocarbons or disproportionation of CO. The third group of methods includes the catalytic dechlorination of hexachlorobenzene. The fourth group of methods includes the production and recovery of GO, the cutting of CNTs by strong oxidants, etc. The "Scotch" method of tearing graphene from graphite gives the purest graphene [39]. However, it is very laborious. With a high yield at a relatively low price, graphene is obtained by reducing graphite oxide in dispersion and liquid-phase decomposition of graphite. Graphene, prepared under harsh chemical conditions, often contains a large number of defects even after recovery, compared to graphenes obtained from other methods.The resulting particles of free graphene are unstable and tend to form aggregates. Through chemical functionalization, it is possible to prevent the aggregation of these particles, in particular by the addition of oxygen-containing groups (carboxyl and hydroxyl). This is achieved, for example, by boiling the particles in a mixture of H2SO4 and HNO3. Carboxylic and hydroxyl groups make graphene hydrophilic; it can then be used for amidation, esterification, the addition of biomolecules, and a number of other reactions. Graphene is characterized by well-studied reactions for fullerenes and CNTs. In addition to amidation and esterification, there is cycloaddition, addition of radicals, partial replacement of C atoms by N or B atoms, etc. Graphene can be alloyed and chemically modified not only by addition reactions. It comparatively simply undergoes decoration with metal and oxide nanoparticles. Here we can also use the analogy with the reactions of CNT decoration. The modification allows regulating the electronic properties of graphene. Thus, graphene fluorides are semiconductors. Graphenes often include not only single-layer flat particles, but also double-layer (digraphene), three-layered, and multilayered particles with a small number of layers. To achieve some practical purposes, these materials have advantages over single-layer graphene—for example, less electrical resistance with a slight deterioration in transparency. Reduction of GO produces graphene-like structures and GO could be an environmentally friendly way for large-scale production of one to few-layer graphitic thin films. The functionalized graphene is capable of forming stable dispersions in water and in organic solvents. Graphene dispersions are also formed with the help of surfactants. It should be mentioned that chemical challenges for large-scale use of graphenes include control of solvation parameters, size distribution, and chemical modifications of these large flexible 2D sheets. Due to its unique structure and properties, graphene has long been a universal nanodimensional building block material for the self-assembly of new materials with new properties and functions [42]. An example for US graphene preparation and its biological is presented in the study [43]. The production of gold-modified binding peptides of biomolecules shows the potential of US irradiation of graphene and graphene composites. Consequently, US seems to be a suitable tool for the preparation of other biomolecules. Other promising uses of graphene inlcude electronics (sensors), medicine, and solar energy (the list of potential applications of graphene is not exhausted by this enumeration). So, graphene is a high-performance material for the electronic sector. Due to the high mobility of charge carriers in the graphene

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grid, graphene is of great interest for the development of fast electronic components in high-frequency technology. As the strongest, most electrically conductive, and one of the lightest and most flexible materials, graphene is a promising material for solar cells, catalysis, transparent and emission displays, micromechanical resonators, transistors, cathodes in lithiumair batteries, ultrasensitive chemical detectors, conductive coatings, and use as an additive in compounds. For example, it is predicted that flexible electronic screens based on graphene can appear the fastest, with the most attractive idea being “electronic paper.” Graphene is so thin that "graphene paint" can act as a protection against rust or as an "electronic ink," or can be added to advanced composite materials to render them impenetrable, conductive, or more durable. Graphene can be used to improve the design and life of solar cells, although there are still many technological barriers in this direction. Graphene as a material is extremely sensitive to the environment, so it can serve as a sensor with one universal device that measures deformation, gas content, magnetism, or pressure. The purity and large surface area of graphene also make it suitable for medical purposes, from drug delivery to the creation of new tissue for regenerative medicine, etc.

25.5.2 Ultrasonic treatment in the production of graphene and graphene-containing products US is a proven alternative for the production of high-quality graphene, including in large industrial quantities [34]. Researchers have developed a number of ways to use US. However, in general, the US production of graphene is a simple one-stage process, which is its obvious advantage. The following is an example of a specific method for producing graphene [44]. Graphite is added to the mixture of diluted organic acid, alcohol, and water. Then the obtained mixture is subjected to US irradiation. In this situation, the acid acts as a "molecular wedge" that separates the sheets of graphene from the "parent" graphite. According to this simple process, a large amount of intact high-quality graphene dispersed in water is created. In a review [34], a scalable and facile technique for noncovalent functionalization of graphene with 1-pyrenecarboxylic acid that exfoliates single-, few-, and multilayered graphene flakes into stable aqueous dispersions is presented. The material based on the use of the exfoliated graphene was demonstrated highly sensitive and selective conductometric sensors whose resistance rapidly changes > 10000% in saturated ethanol vapor. Also, it was declared about possibility of obtaining the ultracapacitors with extremely high specific capacitance ( 120 F/g), power density ( 105 kW/kg), and energy density ( 9.2 Wh/kg). US allows for the preparation of graphenes in organic solvents, surfactants/water solutions, or ionic liquids [34]. This means that the use of strong oxidizing or reducing agents can be avoided. Stankovich et al. [45] produced graphene by an exfoliation process under ultrasonication. The AFM images of graphene oxide exfoliated by the US treatment at concentrations of 1 mg/mL in water always revealed the presence of sheets with uniform thickness 1 nm. These well-exfoliated samples of graphene oxide contained

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no sheets either thicker or thinner than 1 nm. This leads to the conclusion that complete exfoliation of graphene oxide down to individual graphene oxide sheets was indeed achieved under these conditions. Stengl et al. [46] have shown the preparation of pure graphene sheets in large quantities during the production of nonstoichiometric TiO2 graphene nanocomposites. It was made by thermal hydrolysis of a suspension with graphene nanosheets and a titania peroxo complex. From the titania/graphene samples, a 300-μm thin layer on a piece of glass 10  15 cm was created. The pure graphene nanosheets were produced from natural graphite using a high-pressure US reactor at 0.5 MPa. The physical parameters in the US reactor can be accurately controlled, and the assumption is that the concentration of graphene as the dopant will vary from 1% to 0.001%. So, it is possible to produce graphene in a continuous system on an industrial scale. Xu et al. [40] described a convenient one-step method for the preparation of polystyrene-functionalized graphite. In their study, they used graphite flakes and styrene as their main raw material. By US treatment of graphite flakes in styrene (reactive monomer), US irradiation led to the mechanical and chemical exfoliation of graphite flakes into single-layer and low-layer graphene sheets. At the same time, the functionalization of graphene sheets with polystyrene chains was achieved. It was mentioned that the same process of functionalization can be carried out with other vinyl monomers for composites based on graphene [34]. Another type of graphenecontaining product is a graphene ribbon. For example, at widths of about 10 nm or less, the behavior of graphene ribbons is similar to that of a semiconductor because electrons are forced to move longitudinally. Thus, it would be interesting to use CNTs with semiconductor-like functions in electronics (for example, for smaller and faster computer chips). According to an American investigatiors, namely Jiao et al. [47] preparation of graphene nanoribbons bases in two steps. First, they loosened the layers of graphene from graphite by a heat treatment of 1000°C for 1 min in 3% hydrogen in argon gas. Second, the graphene was broken into strips using ultrasonication. The nanoribbons obtained by this technique are characterized by much “smoother” edges than those made by the usual lithographic means. Carbon nanoscrolls are similar to multilayered CNTs. The difference with MWCNTs is the open tips and full accessibility of internal surfaces to other molecules. They can be synthesized wet-chemically by intercalation of potassium graphite, peeling in water, and US treatment of a colloidal suspension. A low-temperature wet chemistry method for producing carbon nanoscrolls is described in the study [48]. This method is based on the use of readily available acceptor graphite intercalating compounds. The initial graphite intercalation compound is first peeled to obtain a monomer suspension of graphene in ethanol. The obtaining suspension is then sonicated, resulting in a suspension of carbon nanostructures. The researchers argue that the developed method does not require the heating and use of an inert atmosphere, which is a clear advantage over previously described methods [49]. It was found that US treatment helps scrolling up graphene monolayers into carbon nanoscrolls. As a result, a high efficiency (about 80%) of the conversion of monolayers into nanoscrolls was achieved that makes the production of nanoscrolls promising for commercial applications.

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Graphene aerogels: Production methods and operational properties

25.6.1 Aerogels The gel is a type of colloidal system that is a slurry of liquid particles in a solid [50]. The solid component in the gel by volume is much less than the liquid component. However, the solid component is represented by nanometer-sized particles. These particles are in contact with each other and form a branched network of chains and sheets that continuously permeates the entire volume of the gel. It is due to this structure that the gel resists fluidity and is gelatinous or even elastic and not liquid. If the liquid phase is completely replaced by a gaseous phase (e.g., air), an airgel is obtained. In an airgel, the solid phase occupies less than 15% of the volume—this is usually about 1% or even less. Thus, airgels are a class of materials representing a gel in which the liquid phase is completely replaced by a gaseous phase. Aerogels are highly porous solid foams that have an interconnected network of thin, continuous walls. Aerogels are characterized by extremely low density and high specific surface area (total surface area per unit mass). Therefore, such materials demonstrate a number of unique properties: hardness, transparency, heat resistance, and extremely low thermal conductivity. At the same time airgels, as a rule, have low strength and low elasticity. Therefore, a hard click on them can lead to a catastrophic destruction of the existing branched spatial network [51]. Typically, two related methods are used to prepare aerogels. The first of these is supercritical drying. If we just dry the gel, then the retreating liquid will pull the net of nanoparticles. Therefore, supercritical drying should be carried out under conditions of no surface tension, that is, when the liquid is in the supercritical state. So, for example, freeze drying results in the transition of the existing solvent from the solid state (ice) directly to gaseous matter, bypassing the liquid phase. Thereby, the porosity of the structure that formed when microparticles of ice form in the airgel structure during its rapid freezing, for example in the volume of liquid nitrogen, does not degrade. Advantages of this method of drying are obvious—the absence of high temperatures on the object, the preservation of the dispersed phase in the structure of the object, and the possibility of using volatile solvents. The method of lyophilization allows obtaining dry tissues, preparations, products, etc., without the loss of their structural integrity and biological activity. For example, during lyophilization, most proteins are not denatured and can persist for a long time with moderate cooling (about 0°C). Moreover, lyophilized tissues and preparations with humidification restore their original properties.

25.6.2 Aerogels based on carbon nanomaterials One of the promising directions in the synthesis of polymeric nanomaterials is the production of macromolecules that are capable of self-assembly like biopolymers (viruses, ribosomes, protein fibers, membranes, enzyme complexes). They are based

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on molecular recognition and ordering of constituent elements with subsequent selfassembly of functional supramolecular structures due to the presence of van der Waals, electrostatic, and hydrogen bonds. For example, protein macromolecules form geometrically regular structures (spirals, rings, etc.) that are packaged in flat layers or tubes. Synthesized macromolecules that are capable of self-assembling into supramolecular tubular structures are transformed into an ordered two-dimensional or columnar liquid-crystalline phase [52]. Depending on the conditions for the anionic copolymerization of styrene and butadiene, directed self-organization of the macromolecules proceeds. The result of this self-organization is the production of block copolymers or copolymers with alternating links with different morphologies (the dimensions of supramolecular structures and domains are up to 0.1 and 0.2 microns, respectively) and various properties. The above phenomenon of self-organization (self-assembly) has long been recognized as one of the main strategies for creating new materials in polymer nanotechnology. In view of the huge variety of airgels, aerogels based on carbon nanomaterials are the most interesting for further study. This is due to the unique properties of carbon aerogels—extremely low density, low thermal conductivity, and high elasticity (the ability to restore the shape after multiple compression and stretching). Prospects for creating ultralight elastomeric airgels have changed with the advent of carbon nanomaterials in the form of CNTs and graphene. Now in the world of science, there are numerous studies on the creation of nanoaerogels from carbon nanomaterials (graphene, CNTs, nanofibers, etc.). According to the high porosity of carbon aerogels, which characterized by low density and high specific surface area, we can register (or ascertain) the presence of electrical conductivity and the ability to sorb organic liquids.. All this forms the prospect of their use as substrates for catalysts, artificial muscles, electrodes for supercapacitors, and gas sensors of sorbents for the liquidation of oil spills. Initially, to create airgels with low density and high elasticity, graphenes and CNTs were used separately. It was found that nanomaterials prepared separately from graphene or separately from CNTs also have their drawbacks. However, due to the fact that the graphene sheets have insufficient bending stiffness, a decrease in the density of graphene sheets impairs the elastic properties of airgels from graphene [53]. At the same time, an airgel based on CNTs has another disadvantage. It is harder, so it does not restore the form after the download is unloaded. This is due to the fact that CNTs under load are irreversibly bent and entangled, and the load is poorly transmitted between them. The most common and interesting for study are the following directions of airgel synthesis: (1) chemical reduction of GO to form a three-dimensional porous structure; (2) composite (hybrid) aerogels based on CNTs and graphene, formed during the chemical reduction of GO; this method is described in more detail in the next subsection; and (3) chemical vapor deposition (CVD), as a result of which superhydrophobic aerogels are formed. This method is also effective for obtaining 3D structures based on carbon materials—aerogels [54].

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Even lighter (less dense) aerogels are obtained by chemical precipitation of the substance, which will serve as the solid phase of the airgel on the previously prepared porous support, which is then dissolved. This method allows you to regulate the density of the solid phase (by controlling the amount of precipitated matter) and its structure (by using a substrate with the required structure). The synthesis of graphene airgel mainly began with the precursor of GO because of its simplicity of obtaining and accessibility. To produce GA, different methods and approaches are used, the most common of which is the approach to self-assembly. Although the process of self-assembly is accompanied by the restoration of GO on the partially restored surface of graphene, as a rule, there are numerous functional groups. These porous graphene-based monoliths adsorbed both oil and water at the same time, decreasing the separation selectivity and efficiency. To adsorb organic solvents from water while repelling water entirely, superhydrophobic graphene-based monoliths should be an ideal alternative. In the synthesis of monoliths based on porous graphene, the material obtained is capable of adsorbing both oil and water simultaneously.As a consequence, the selectivity of their separation decreases and efficiency increases. At the same time, for the ability to adsorb organic solvents from water with the complete hydrophobicity of water, superhydrophobic graphene-based monoliths should serve as an acceptable alternative.

25.6.3 Aerogels based on graphene oxide: Synthesis and properties GA attracted much research attention primarily as an ideal conductive filler that is capable of holding a three-dimensional interconnected network of conductive graphene sheets inside the polymer. In the review article of Kazakh scientists [55], retrospective aspects concerning the synthesis of such materials are presented. It should be noted that in the modern literature devoted to the given direction of polymeric material science, an enormous amount of work is connected with the use of GO as a starting material for the synthesis of aerogels. First of all, the work of researchers describing the production of ultralight and flexible (capable of restoring the old form after mechanical loads) airgels based on GO when it is chemically reduced with ethylenediamine should be noted [56]. According to the published results, a certain amount of the reducing agent was added to the water dispersion GO. The resulting mixture was sealed in a glass vessel and heated for 6 h at 95°C. In this case, the formation of bonds and the overall structure of the hydrogel occurred. In the chemical reduction of GO, functional hydroxy groups are replaced by amine groups. This, in turn, has an effect on the structure of the graphene layers, which form bonds among themselves. For example, a lyophilized unidirectional graphene aerogel (UGA) provides conductivity in composites with an ultralow GA content. The percolation threshold of the resulting nanocomposite depends on many factors, including the aspect ratio and alignment of the GAs.

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According to the second method of synthesis of hybrid airgels, mentioned in the previous subsection, the synergistic assembly of CNTs and graphene allowed the creation of multifunctional ultralight and ultraelastic aerogels. Both materials have many useful properties: low density, high strength and rigidity, high electrical and thermal conductivity, high specific surface, excellent flexibility, and good chemical stability [39]. Therefore, they are promising as building blocks for aerogels, which have a lot of functionality. For example, an airgel with graphene with a density of 5.1 mg/cm3 retains its structural integrity under a load that is more than 50,000 times its own weight, and can also be quickly restored from 80% compression [53]. The obtaining of mechanically strong graphene airgels with large BET surfaces, low thermal conductivity, high thermal stability and electrical conductivity was declared by researchers [57]. For example, the yield strengths and elasticity modulus of GA were in the range of 0.05–0.75 and 0.81–13.84 MPa, respectively. A 75.0-mg GA cylinder with a bulk density of 56.2 mg/cm3 could support at least 26,000 times its own weight. The thermal conductivities and electrical conductivities of GA were in the range of 0.0281–0.0390 (W/mK) and 14.4–53.7 S/m, respectively. The increased characteristics were the result of hydrothermal reduction and subliminal drying with ethanol. Annealing at 1500°C resulted in slightly increased thermal conductivity and further improvement of mechanical properties, oxidation temperature, and electrical conductivity of GA. Such properties make such GA a promising candidate for use in a number of areas, including batteries from sensors, electrodes, light conductors, and insulating materials. When studying the properties of GAs, an important parameter is their hydrophobicity. This is a consequence of the high surface area and the nonpolar carbon structure. Authors of study [58] showed that GA exhibits natural hydrophobicity due to the defectiveness of its surface. After treatment with fluorinated silane, GA begins to exhibit superhydrophobic properties, and the wetting contact angle reaches 160 degrees in contact with water. In view of these specific properties, GA can also be used as a self-cleaning or water-repellent surface that has a low bulk density. Aerogels on the basis of reduced GO with a relative density of 4.4 to 7.9 mg/cm3, which exhibited high compressibility both in air and the volume of organic liquids, were synthesized by researchers [59]. Noteworthy is the fact that the synthesized airgel also exhibits refractory properties, allowing it to regenerate by ordinary annealing. The porosity of the airgel was 99.6%, and the contact angle of its surface with a drop of water was 155 degrees. This allows us to use GA as sorbents for organic liquids with sorption capacity from 100 to 250 g/g, depending on the density and nature of the organic solvent. Due to its hydrophobicity, the airgel is on the water surface and selectively sorbs organic liquids. Subsequently, the sorbed organic liquids can be removed by simply burning the airgel or squeezing. In the study [60], composite aerogels based on graphene and CNTs were obtained by the method of fast microwave irradiation. And in these airgels, graphene layers were "covered" with vertically arranged CNT arrays, which in turn form the overall superhydrophobicity of the material. The resulting graphene airgel [56] was placed in a solution of acetone with ferrocene and dried. At the next stage the dried material was subjected to a rapid microwave irradiation for the growth of CNTs within the airgel structure. It was because of the result of the ferrocene decomposition. The formed iron

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particles and cyclopentadienyl served as the catalyst and carbon source, respectively, for the growth of CNTs within the GA structure. The obtained composite based on graphene and CNT shows high rates and volumes of sorption of oils and petroleum products. Thus, hybrid aerogels based on graphene and CNTs have excellent thermal stability, electrical conductivity, and high absorbing capacity for liquids. For example, for oil, this figure is 215–913 times higher than their own weight, depending on the density of the oil, which is two orders of magnitude higher than that of commercial oil sinks. Due to the superelasticity of the resulting airgel, it can be reused after mechanical extrusion to release the absorbed oil [51]. The authors of [61] explained a similar set of properties by the synergistic interaction of graphene and CNTs. With this combination, the properties of the components mutually complement each other. CNTs covering graphene sheets serve as a bond between adjacent sheets, which improves the transfer of load between them, as well as stiffeners for the graphene sheets themselves. Due to this, the load does not lead to the movement of the sheets relative to each other (as in airgel from pure graphene), but to the elastic deformation of the sheets themselves. Moreover, CNTs fit tightly to graphene sheets, and their position is determined by the position of the sheets. Therefore, CNTs do not experience irreversible deformations and entanglements, nor do they move relative to each other under load, as occurs in an inelastic airgel based on CNTs only. At the same time, the airgel, consisting equally of graphene and CNTs, has the most optimal properties. Simultaneously with the increase in the content of CNTs begin to form “coils”, as in an airgel consisting only of CNTs, which, in its turn, leads to a loss of elasticity of the entire structure.with the increase in the content of CNTs begin to form “coils”, as in an airgel consisting only of CNTs, which, in its turn, leads to a loss of elasticity of the entire structure. The above-described principle of self-organization can also be applied to graphite oxide. Graphite oxides are two-dimensional conjugated amphiphilic polyelectrolytes. They are represented as a hydrophobic component and hydrophilic oxidized groups, which can themselves assemble into a three-dimensional graphene structure [62]. In the study [61], the strategy of the synergistic assembly of scale macroscopic structures (1D, 2D, and 3D) of ultralight aerogels with controlled densities was described. All carbon aerogels with a monolithic three-dimensional structure were synthesized in the form of a matrix with walls of graphite layers and edges consisting of CNTs. The ideal combination of giant-sized graphene layers and the cooperative effect between them and CNTs creates the unique functional properties of the obtained GA, including high elasticity, ultralow density, excellent thermal stability, high sorption activity with respect to organic liquids, and good electrical conductivity. It is known that individual graphene layers, as a rule, tend to irreversible aggregation due to strong π-π stacking and van der Waals forces between the base planes of graphene layers. In view of this, filtration, as an effective way of separating solids from a liquid, was adopted to produce a single-layer GO. This phenomenon was named as self-organization under the influence of a directed flow [63]. Graphene layers were prepared by vacuum filtration of their aqueous dispersions followed by drying. After drying, the individual graphene layers are laid almost parallel, forming compact films with an oriented structure.

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25.6.4 Ultrasonic effect in the synthesis of hydrogels based on graphene It is obvious that other things being equal, US action on liquid media is most effective in comparison with the air environment. In this case, a hydrogel is used as the liquid medium. To obtain a graphene hydrogel, an approach based on the phenomenon of self-organization was developed. The possibility of choosing the shape of the graphene hydrogel by selecting the desired form of the reactor was investigated in study [64]. The 3D architecture of graphene has low density, high mechanical property, thermal stability, high electrical conductivity, and high specific capacitance, making them candidates for potential applications in supercapacitors, hydrogen storage, and catalyst supports. In this paper, it was shown that a variation in the form of the reactor can synthesize graphene hydrogels of various forms, such as a cone, spherical, and pearshaped, only by changing the type of reactor. The process of shrinkage of the formed hydrogel during self-assembly is an isotropic process. This involves the production of graphene hydro- and aerogels with precontrolled forms. In the synthesis of hydrogels based on graphene, a number of studies are devoted to the formation of a three-dimensional structure under the influence of low-frequency US. In colloid chemistry, this process is commonly known as gelation by means of an electrostatic repulsive force between colloidal particles. In the study [65], the influence of US treatment on the process of formation of graphene hydrogel was investigated. The aqueous dispersions of graphene oxide were exposed to US waves. It was found that US reduces the size of graphene layers by grinding them, thus exposing a new edge of the graphene nanolayer, which does not have a stabilizing functional carboxyl group, to the zone of its contact with the edge of the layer of the synthesized material. It was found that US treatment does not affect the chemical functionality of nanolayers of GO because the results obtained were identical before and after treatment. The gelling process is observed after a lapse of 30 min with the formation of a relatively weak hydrogel with a shear modulus of 0.3 kPa. However, an increase in the time of US treatment to 120 min leads to the formation of a more tightly bound hydrogel with a shear modulus of 1.6 kPa. Such an increase in the physical properties of the hydrogel can be explained by the absence of functional stabilizing carboxyl groups on the newly obtained fragment of the graphene layer. It has also been found that the hydrogels obtained have extremely low critical gelling concentrations with US treatment ranging from 0.050 to 0.125 mg/mL.

25.6.5 Problem situations in obtaining graphene aerogels The main problem in obtaining graphene airgels is due to the fact that individual graphene layers tend to have irreversible aggregation due to the strong π-π stacking and the van der Waals forces between the base planes of the graphene layers. Therefore, preventing the aggregation of graphene layers into macroscopic structures is an important task in the synthesis of graphene-based microstructures.

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Despite a number of obvious advantages, the main drawback of the operational properties of airgel until recently was its fragility. Due to this property, airgel cracked under repeated loads. All airgels obtained earlier—from quartz, some metal oxides, and carbon—had this drawback. With the advent of new carbon materials—graphene and CNTs—the problem of obtaining elastic and resistant to the destruction of hybrid airgels was practically solved. Further, the low elastic flexural rigidity of graphene sheets limits the improvement in the elasticity of GA with a decrease in its density. In addition, in CNT-based aerogels, the addition or constant bending of CNTs usually leads to an inefficient transfer of load between CNTs and a significant irreversible deformation of airgels. Although GAs have great potential for molding heat-release composites based on their continuous heat-conducting networks, low density and isotropic GA architecture impede further improvement of the thermal conductivity of composites derived from them. It is especially necessary to note an ecological orientation of the application of graphene materials and derivatives on their basis. In order to overcome the existing problems in this area, in the future, it is necessary to focus on the following main points. First, we need to constantly work on developing new or improving existing techniques for obtaining carbon nanomaterials and derivatives based on them in order to reduce the cost of the final product in the transition to industrial production. Second, it is necessary to develop new, more functional molecules that will have a specific interaction with a certain type of pollutant. This will significantly affect the sensitivity and selectivity of sorbents and sensors. Third, it is necessary to discover and explore more promising methods for the use of carbon nanomaterials and products based on them in the field of environmental protection and in other areas.

25.6.6 Potential applications of aerogels Numerous environmental pollutants, such as toxic gases, heavy metal ions, and organic solvents, are encountered both in air and water, creating a great threat to the ecological balance and human health. For example, heavy metal ions that accumulate in the human body can cause a variety of chronic diseases. Thus, it became necessary to develop simple, sensitive, and inexpensive methods for detecting and removing these contaminants. At present, numerous sensitive sorbents and devices for the detection of environmental pollutants, based on nanomaterials, have been developed. In particular, graphene was obtained, widely known for its unique chemical, thermal, electronic, mechanical, and sorption properties. Heavy metal ions (lead, cadmium, chromium, mercury, copper, arsenic), which pose a serious risk to the environment and human health, should be easily removed from soil and water. In connection with this perspective, the use of graphene and its derivatives having a high surface area and a large number of functional groups enhancing the adsorption or concentration of heavy metal ions is promising. A wide scope for the use of aerogels based on carbon nanomaterials is environmental pollution by organic solvents, oils, dyes, phenol-containing components, and pesticides. These pollutants, due to their extreme toxicity and carcinogenicity, must

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immediately be removed from the contamination sites. A relative new sphere for graphene and its derivatives is the detection of gas molecules. This is necessary for many areas for environmental monitoring in view of their toxic risks. Recently, gas sensors based on graphene have been developed, taking into account its advantages: electron mobility, a large surface-to-volume ratio, and low noise produced. At the same time, the mechanism of detection is mainly associated with a change in the conductivity or resistance of graphene. This is due to a charge transfer between adsorbed gas molecules and graphene layers. Hybrid aerogels (primarily based on graphene and CNTs) can also prove useful in composite materials in which the pores of solid foam are filled with another material to improve their operational properties. Composites based on graphene foam have already found several different applications, such as elastic conductors [54], flexible lithium-ion batteries with high capacity to power flexible electronic devices that can be operated at a high power rate and fully charged in a very short time [66], supercapacitors [67] and materials to facilitate electromagnetic interference [68]. According to the article [68], a lightweight graphene foam composite with a density of 0.06 g/cm3 has been developed. It shows an EMI shielding effectiveness of 30 dB and specific shielding effectiveness of 500 dB cm3/g. These values surpass the best values of metals and other carbon-based composites. Also, the excellent flexibility of this foam composite gives it a stable EMI shielding performance under repeated bending. Also, a brief overview should be noted, which sheds light on new materials used in personal armor systems [69]. In this review, the polymer fibers used in the production of flak jackets were first described, followed by some nanomaterials such as CNTs and graphene with advanced structural and mechanical properties as well as their potential reinforcement of armored composites. Synthesis of composite structures on hybrid airgels can further expand the range of the above applications.

25.7

Epoxy composites based on graphene aerogels with exceptional operational properties

25.7.1 Method of direct polymeric infiltration of aerogels Recently, researchers have developed a number of approaches for the collection of graphene layers in a single monolithic structure. However, the main drawback of the developed approaches is the resulting dense packing of graphene layers. This leads to a decrease in its integral porosity [70]. Given this shortcoming, a very effective approach was suggested by the researchers of Professor Hu’s group [71]. The developed approach consists in the realization of successive stages of functionalization, lyophilization, and irradiation by microwaves. This results in the formation of an electrically conductive and ultralight GA with a porosity of up to 99.8% [56]. Moreover, large voids formed in the structure of a given GA can be filled with ionic polymers. This will ensure the formation of conductive nanocomposites with a low content of filler. Within the framework of the developed approach, a new method

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of direct polymer infiltration was developed. It consists in the simple occurrence of the airgel in the polymer solution. As a result, an ultralight airgel is a framework for providing electrically conductive properties of a PCM with a low graphene content of 0.4 wt%. This approach greatly simplified the process of obtaining polymer composites previously obtained by prolonged US treatment and subsequent mixing. This opens the way for their potential practical application. Epoxy resin (brand E44), hardener (polyamide), and acetone in a certain weight ratio were mixed in a beaker to form a homogeneous solution. After that, ultralight airgels (the technique of which was described in Ref. [56]) were completely immersed in this solution. The solution was then evacuated and heated to 60°C for 1 h until the acetone was completely removed from the mixture. Further GA was removed from the beaker and subjected to vacuum drying for 12 h. Also for comparison, aerogels were synthesized based on CNTs, which were also subjected to polymer infiltration. It was found that the size and morphology of airgels after the incorporation of the polymer changed insignificantly. However, the conductive layers of graphene are well preserved. After complete curing, the samples retained their previous dimensions because the framework, created from graphene layers, retained its shape well. The electrical conductivity of the obtained epoxy composites was measured by two probe methods. Also, to compare the results, the conductivity of the initial epoxy resin was measured, which was 107 S/m, indicating its insulating properties. Epoxy composites based on graphene and CNTs as fillers also show the insulating properties; their conductivity is almost the same as for epoxy resin. Interestingly, the fact that the conductivity of the composite on the basis of ultralight graphene airgel and epoxy resin increased by seven orders, thereby confirming the improvement of the electrically conductive properties of the NM composite [71]. Unidirectional GA (UGA) with adjustable density, the degree of alignment, and electrical conductivity are obtained by changing the average size of precursor GO sheets from 1.1 to 1596 μm2. It has been experimentally established that ultralight (UL)—UGA/epoxy composites obtained by the infiltrating liquid epoxy resin in the ULGA porous structure have a high electrical conductivity of 0.135 S/cm and an ultralow percolation threshold of 0.0066 vol% [72]. Due to the presence of its three-dimensional interconnected network and high level of equalization, UL-UGA increases the fracture toughness of epoxy resin by 69% at a content of 0.9 vol% graphene. Such an effect is presumably due to the arising reinforcement mechanisms, such as crack fixation, its deflection, and interfacial loosening and rupture of graphene.

25.7.2 Graphene aerogels with adjustable density The study [73] focuses on the poor surface hardness of the airgel structure, which limits the use of GO. In turn, the reduced GO (RGO) is obtained by oxidation and reduction of graphite, that is, a kind of graphene. Therefore, RGO has several advantages. Among them are a simple process of preparation, suitability for scalable production, and a relatively low cost of manufacture. An epoxy/RGO composition was prepared by penetrating the epoxy resin into the airgel RGO to increase the surface

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hardness of the airgel. The influence of infiltration time on physical properties, including density, surface hardness, thermal conductivity, and electrical conductivity, was studied. It is declared that the results of the research will provide a way to manufacture light and mechanically improved compositions in the composition of epoxy/RGO. In the study [74], attention is paid to the fact that the low density and isotropic architecture of GAs prevent further improvement of the thermal conductivity of composites based on them. It was found that the thermally annealed at 2000°C hybrid airgel in RGO/boron nitride (BN) has a high efficiency in increasing the thermal conductivity of the epoxy resin. And the resulting nanocomposite has an ultrahigh density with respect to the thermal conductivity of 11.01 W/(mK), which increases the thermal conductivity by 277%. According to the study [75], GAs/organic fibers are obtained by chemical reduction of GO in the presence of organic poly (p-phenylene terephthalamide) fibers (PPTA), followed by freeze drying to enhance the mechanical properties of the threedimensional GAs with aramid fiber. In this case, the thermal annealing of composite airgels is carried out at a temperature of 1300°C. The graphene/carbon fiber aerogels (GCFAs) are declared to have high electrical conductivity and improved compressive properties of enhanced compressive properties. This predetermines the improvement of the physicomechanical and electrical characteristics of epoxy composites based on them. And compared to pure epoxides, the compression modulus, compressive strength, and energy absorption of the GCFA/epoxy composite are increased by 60%, 59%, and 131%, respectively. Ultralarge GO sheets from natural graphite flakes with the maximum area over 10,000 μm2 and a mean area 3400 μm2 at a yield of 40% for GO sheets larger than 2500 μm2 are achieved in the study [76]. It was mentioned that the graphene foam (GF)/epoxy composites consist of a highly porous cellular structure and the 3D interconnected graphene network serves as the channel for uninterrupted movement of charge carriers, achieving a remarkable electrical conductivity of 3 S/cm with only 0.2 wt% GF [77]. The GF/poly(dimethyl siloxane) (PDMS) composites hybridized with CNTs possess a remarkable electromagnetic interference shielding effectiveness of 75 dB. The highly oriented, unidirectional GA/epoxy composites present an excellent electrical conductivity of 2  103 S/cm after incorporating only 0.25 wt% graphene with an extremely low percolation threshold of 0.007 vol%. [78]. In article [79], a highly elastic graphene oxide-epoxy composite aerogel (GOEA) was fabricated by a facile method. According to this method, the mixed suspension of the thermoset epoxy precursors and graphene oxide sheets was freeze dried, followed by a routine curing process. The resulting GOEA with a three-dimensional network structure exhibits a high decomposition temperature (286°C), excellent physicomechanical strength (0.231 MPa), enormously low density (0.09 g/cm3), and high elasticity without significant permanent deformation. The exceptional properties of the obtained GOEA provide the potential for a range of practical applications in energy-absorbing and durable insulation materials. In investigation [80], the effects of as-produced GO and silane-functionalized GO (silane-f-GO) loading and silane functionalization on the mechanical properties of epoxy PCMs are investigated and compared. It has been found that such a silane

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functionalization containing epoxy end groups effectively improves the compatibility between the silane-f-GO and the liquid epoxy matrix. Compared with pure epoxy and GO/epoxy composites, there is an increased storage modulus, glass transition temperature, thermal stability, tensile and bending properties, and fracture toughness. According to the other article [81], high-strength conductive pristine graphene/ epoxy composites were prepared by two simple processing methods—freeze-dry/ mixing and solution processing. PVP-stabilized graphene is aggregation resistant and allows for excellent dispersion in both the resin and final NM PCMs. The superior dispersion quality results in excellent nanofiller/matrix load transfer with a 38% increase in strength and a 37% improvement in elasticity modulus for 0.46 vol% graphene loading. The NM PCMs have an electrical percolation threshold of 0.088 vol%. The freeze-drying method probably is more promising and universal to be used for graphene dispersion in a wide range of other composite precursors.

25.8

Production of classical and nanomodified polymer compositions, prepregs, and composites based on them with ultrasonic treatment

25.8.1 Modeling of constructive-technological parameters of forming of classical polymer composites Currently, the scientific school of the National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute” has developed a wide range of methods for predicting structural and technological parameters and modeling technical means for molding both thermoplastic and reactor-plastic classical composite materials. In connection with the limited volume resources of this chapter, we list only a few. Analysis of the technological process for the production of polymeric products by extrusion blow molding is given in the study [82, 83]. Investigation of the radiation field of a polyethylene terephthalate medium under radiant heating during molding of thermoplastics is described in the studies [84, 85]. Simulation of the process of mixing polymer composites in an extrusion drum mixer is described in the studies [86, 87], and the simulation of the flow of polymer melt at the exit from the extruder working tool in the form of an extrusion die is given in the article [88]. A separate research unit is devoted to modeling the parameters of the process of forming pipe profiles from thermoplastics. An analysis of the existing problematic issues in the formation of corrugating profiles in the production of corrugated tubular products was carried out in the article [89]. In the study [90], the technique of modeling of design parameters of various profiles of corrugations and also their forming equipment is offered. The simulation of the technological process of extrusion welding of layers of corrugated tubular products is described in the study [91]. The study [92] describes the process of extrusion molding of tubular polymer blanks intended for manufacturing corrugated tubular articles. Effective equipment and technologies for connecting and repairing polyethylene pipelines using US modification and heat shrinkage are investigated in a number of

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studies. In particular, in article [93], the aspects of connection and restoration of polymer pipelines for the transportation of low and medium pressure gas are analyzed. Features of molding epoxy repair couplings with a shape memory effect are considered in the study [94]. Various methods of surface treatment (physical and chemical activation) of the surface of polyethylene pipes, with the following use of glassbandage bandages, are described in Ref. [95]. The developed glue-and-adhesive technologies based on epoxy-adhesive compositions and banding are described in the articles [96, 97]. The advantage of using US treatment for molding insulating elements of external facade thermal insulation from PCMs is mentioned in the article [98]. Design of structural and technological parameters of reinforced thermosetting plastics can be carried out on the basis of various approaches. One of them is a compositional structural approach. Within the framework of this approach, parameters of adequate structural models of oriented fibrous composites are determined [99–101], for example, on the basis of a structural analysis of the sections of full-scale composite structures. At the next stages, on the basis of the found parameters of the geometric model, the structural and technological parameters of the process for obtaining thermosetting plastics, for example, the process of "free" impregnation [102], winding [103, 104], and estimation of the stress-strain state of the modeling structure. Thus, it can be stated that a certain scientific reserve has been created for using some of the above approaches to NM PCMs. But the physicochemical, morphological, structural, and other features of the nanofillers are used to impose significant limitations on the free use of approaches developed for classical PCMs. However, as analysis of available research shows, the use of low-frequency US is currently out of the competition when forming both classical and NM PCMs as well as for thermoplastic and thermosetting plastics. Below, the following results of independent researchers once again confirm this postulate.

25.8.2 Production of nanomodified thermoplastic composite materials by extrusion method with ultrasonic treatment As is known, the outstanding properties of polymer nanocomposites are due to the optimal combination of the soft phase (polymer matrix) and the functional phase (dispersed in it by nanofillers). In this case, an effective dispersion of CNTs in a liquid polymer matrix is necessary. The latter, however, is difficult due to the attraction of nanoparticles to other particles with the formation of stable agglomerates. Agglomeration is also facilitated by the lack of affinity between the nanoparticles and the liquid polymer matrix. To solve this problem, the initial components—liquid polymers as well as CNTs— undergo preliminary treatment (physicochemical modification) prior to the manufacture of nanocomposites based on them. The main purpose of this modification is to reduce the difference in free energy between the components composing the nanocomposite. One of the most common methods of physical modification is the use of high-frequency soundwaves during the preliminary processing of both a

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polymer matrix and a nanofiller. In particular, this effect contributes to the improvement of the dispersion of functional CNTs in a liquid polymer matrix. For example, nanoparticles in solution are often pretreated with US. In the review [105], examples are given of such use. It is declared that nanoparticles can be deagglomerated, for example, in a beaker. Such deagglomeration is carried out in an open air system using a US concentrator (probe). As a result of this action, a fluidized air layer is created. After that, the processed nanoparticles can be used in the production of nanocomposites by mixing the melt. It is also effective to use US waves when mixing the melt during extrusion. As a rule, this is done on one (fixed) acoustic frequency. However, the use of several variable frequencies instead of one fixed frequency in the expansion chamber is also declared. This is based on the assumption that there is a specific resonant frequency or frequency spectrum that can be associated with a given polymer chain length (oscillatory motion of the chain mechanism), rather than with US cavitation effects in a liquid polymer medium. However, in connection with the diversification of the distribution of the lengths of polymer chains, a spectrum of different US frequencies is required to change the conformation of such chains from random to extended. Therefore, as soon as the acoustic frequency used for dubbing the liquid polymer is no longer used, the conformation of the polymer chain can be arbitrarily relaxed to random conformation. In connection with this, the management of this mechanism (process) provides directions for improving the mobility of the polymer chain by its determination by CNTs. This, in turn, will contribute to a more efficient dispersion of nanoparticles in a liquid polymer matrix. To test the above hypotheses, a polymer system based on isotactic polypropylene (iPP) with three different melt flow indexes as a matrix and multilayered CNTs (MWCNTc) as functional nanoadditives was investigated. A low-frequency US generator with a power of 750 W was manufactured with the possibility of changing the frequency range with predefined discrete increments. Four different variants were used for manufacturing polymer nanocomposites. According to the first variant, the processing of materials was carried out without US treatment (without US). In the second and third versions, a US generator with a fundamental frequency (20  0.2) kHz was used. In the second variant, a fixed frequency (20  0.1) kHz was used, and in the third variant, frequency scanning was used in the range of 20–50 kHz (with a scanning step of 0.1 and 250 cycles per scan). In these variants, the output power of the radiation was 500 W. In the fourth variant, to reduce the size of the MWCNT agglomerates, they were processed in a fluidized air layer using a US probe. In the first and fourth variants, the materials were not exposed to the US treatment during the extrusion of the polymer melt. As a result of the research, it was found that US-induced extrusion methods affect the morphology and electrical properties of isotactic polypropylenes with different melt flow indexes and NM PCMs with 10 wt% content of multiwalled CNTs using the same polymers. In the study [106], an overview of the current state of the low-frequency US treatment technology used for polymer melts is presented. The scientific and technical aspects of US treatment in the production of thermoplastic polymeric materials are discussed. An analysis of the technological progress achieved shows that the

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mechanism of US action on polymer melts has not been fully studied at present. This, however, is due to fundamental differences, including rheological characteristics, between melts of thermoplastics and liquid thermosets. On the basis of a critical analysis of the state of the matter, some key questions are identified for a deeper understanding of the chemical and physical effects when US is applied to polymer melts.

25.8.3 Method for the preparation of nanomodified epoxy compositions and prepregs on its basis The results described in the previous sections predetermine the effectiveness of using low-frequency US treatment in the production of reactoplastic NM PCMs. In Ref. [107], the method of water filtration for the production of MWCNTs was used. The aqueous suspension was sonicated for 4 h using a US probe of 280 W. The developed US method of impregnating the resin into nanowave MWCNT takes only 4 s. The resulting nanocomposites exhibited ultimate tensile strength (106.8  7.4) MPa. The MWCNT content in the nanocomposites was 26.3 wt%. Also, air voids were eliminated in the material obtained. It is mentioned that for the production of nanocomposites with similar tensile strengths, traditional technologies require an average of 0.5–1 h for resin impregnation. This is an undoubted advantage of the developed method. More difficult is the process of obtaining NM PCMs reinforced with nano- and macrofibers. The first step in this process is the preparation of a liquid NM PB. In the second step, the resulting NM PB is combined with reinforcing (macro) fibrous filler. At the last stage, the prepreg obtained is subjected to formation (storage, hardening, pressing, cooling, cutting, etc.) and production testing. Invention RU No. 2415884—"Method for the preparation of a nanomodified binder, a binder, and prepregs based on it" [108]—refers to the production of composite materials based on binders and fibrous fillers (FF), in particular to the technology and composition of the NM PBs and prepregs based on it. This invention can be used in the aviation industry as well as several other industries, including automobiles, boats, and engineering. The patent on invention RU No. 2278028 [107], on which the method [108] was based, describes the production sequence of the NM PB. The fullerene С60 (0.01 parts by mass), the open CNTs NTA (0.1 parts by weight), the fulleroid multilayer nanomodifier NTC-astralene (0.5 parts by weight), and the amine derivative of fullerene С60 (0.02 parts by weight) were dispersed in an organic solvent. The obtained suspension was subjected to US treatment (frequency—35 kHz, duration—30 min) in a bath of external radiation. Further, the resulting suspension of carbon nanoparticles was incorporated in 100 parts by weight of epoxy amine oligomer brand of ECD, 44 parts of curing agent, and 4,40 -diaminodiphenylsulfone, all were mixed and a final PB was obtained in this way. The patent [107] also discloses a prepreg containing (wt %): a PB—(24–50); NM disperses FF—(50–76). Disadvantages of the invention [109] are determined by the method used to incorporate various nanomodifiers in the form of a suspension in an organic solvent into a viscous EO, in which their aggregation and precipitation (sedimentation) occur as a

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precipitate. The latter is difficult to mix after storage. Therefore, with further redispersion, it is not possible to obtain a PB with a nanolevel modifier. Thus, the drawbacks of the invention [109] include the inability to reredisperse and aggregate the nanomodifier under storage conditions. The aim of the invention [108] is to obtain a nanomodified redispersible PB based on condensation oligomers without aggregating the nanomodifier in storage conditions. The object of the invention [108] is solved by obtaining a concentrate by dispersing the nanomodifier particles in a matrix during US treatment and incorporating said concentrate into an epoxy binder. Moreover, at least one condensation oligomer with a viscosity greater than 0.6 Pa s is used as a matrix and a PB. The US effect in obtaining the concentrate is carried out according to the radiation power from 1 to 5 kW and amplitude from 20 to 80 μm. Particles of at least one nanomodifier selected from the group consisting of nickel, copper, aluminum, and CNTs are used as nanomodifier particles. It is desirable that the nanomodifier particles have an average size not exceeding 50 nm. A nanomodified binder contains the components with the following ratio (% by weight): nanomodifiers—(0.005–0.1), condensation oligomer with a viscosity of more than 0.6 Pa s—the rest. In the prepreg containing the filler and binder, the filler is made of inorganic reinforcing materials in the form of fibers, nonwovens, or fabrics, and impregnated with the above NM PB. Condensation-type oligomers with a viscosity greater than 0.6 Pa s, including EO, phenolic oligomers, polyesters, polyimides, polyamideimides, and polyamides, have functional groups in their structure. The latter can form intermolecular hydrogen bonds with the surface groups of the nanofiller. Therefore, the process of US dispersion of modifier nanoparticles in the presence of oligomers having functional groups leads to the formation of polymer layers on the surface of nanoparticles. These polymer layers will contribute to their stabilization and the production of a nanoparticle concentrate in a polymer matrix that is stable upon storage. It was found that prepregs with nanomodifiers show an improvement in the entire complex of properties from 15% to 30%, depending on the type of condensation oligomer, binder, prepreg, and composite based on it. It is declared that invention [108] allows obtaining composites with a high level of performance properties on the basis of prepregs with a condensingtype thermal condensing oligomer and FF. In this case, the incorporation of nanomodifiers in the form of a concentrate obtained by US treatment during the dispersion process is provided. Therefore, these aspects need special attention and investigation, taking into account existing patent-protected technical solutions [110, 111].

25.9

Conclusions

The survey material in the present chapter confirms that US technology in the production of classical PCMs and carbon NM PCMs is one of the dominant methods for synthesizing new and physical modifications of existing polymer composites to improve their operational (functional) properties. The main physical phenomenon associated

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with US, which has to do with the synthesis of new materials, is low-frequency acoustic cavitation. It manifests itself in the formation, growth, and implosive collapse of bubbles in the liquid. This phenomenon creates extreme conditions inside the collapsing bubble and serves as the source of most sonochemical phenomena in liquids or in liquid solutions with fillers. The main problematic situation in the production of NM PCMs is the need for dispersing (deagglomerating) nanofillers in a liquid matrix. In many cases, US is practically a nonalternative method for solving the above-mentioned situations. Besides, use of intense US allows for the tailoring of unique materials from sol-gel processes. This makes high-power US a powerful tool for chemistry and material research and development. At the present stage, CNTs, graphene, GO, and GAs are the most important carbon nanofillers for constructing functional NM PCMs. At the same time, the use of each of the above-mentioned nanofillers in the polymer composite has its own characteristics. For example, it was found that the addition of graphene to epoxy composites leads to an increase in the rigidity and strength of the material compared to composites containing only CNTs. Graphene is better combined with an epoxy polymer, more effectively penetrating (incorporating) into the structure of the composite. Therefore, according to its unique structure and properties, graphene became a universal nanodimensional building block material for the self-assembly of new materials with new properties and functions. It is noted that the method of freeze drying is more promising and quite versatile so that it can be used for the dispersion of graphene in a wide range of other composite precursors. Another promising type of nanofiller for polymer composites, namely GAs, not only maintains the unique structural advantages of graphene sheets, but also explores outstanding properties, including lower density, excellent electrical conductivity and mechanical strength, and unusual adsorption properties. It is especially necessary to note the most perspective directions of application of carbon nanocomposites as functional materials, namely as substrates for catalysts, artificial muscles, electrodes for supercapacitors, light conductors and insulating materials, sensor batteries, sorbents (environmental protection) and gas sensors, material for bulletproof vests, medicine, and others. Also, graphene-based nanocomposites can be used in the manufacture of aircraft components, which must remain light and resistant to physical impact. Finally, for the sake of fairness, it should nevertheless be noted that in spite of the successes achieved in the synthesis and modification of both existing classical and new NM PCMs, the basic successes in this direction so far fall to the stage of laboratory research. So when going to the industrial scale of production of such materials, among other things, it will also be necessary to take into account the so-called scale effect. That is, with the automatic transfer of the results of laboratory research to the production level, there will certainly be obstacles and "undercurrents" that can not be simulated in "ideal" laboratory conditions. Obviously, given the pace of development of modern science and technology, this is a matter for the near future.

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Yanfeng Ruan*, Wei Zhang*, Jialiang Wang*, Danna Wang*, Xun Yu†,‡, Baoguo Han* *School of Civil Engineering, Dalian University of Technology, Dalian, China, †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

Chapter Outline 26.1 Introduction 781 26.2 Preparation of NCMFCCs 783 26.3 Properties of NCMFCCs 785 26.3.1 Mechanical properties of NCMFCCs 785 26.3.2 Electrical and self-sensing properties of NCMFCCs 791 26.3.3 Other properties of NCMFCCs 794

26.4 Application cases of NCMFCCs 26.5 Summary 797 Acknowledgments 798 References 798

26.1

796

Introduction

Cementitious composites are one of the most consumed infrastructure construction materials in the world because of their abundant resources, mature production process, and strong adaptability. The fabrication, construction, and application processes of cementitious composites will have a big impact on the resources, energy, and environment of the Earth on which we live. As a kind of quasibrittle material, cementitious composites are unable to avoid cracking problems. The emergence and development of cracks will greatly decrease the strength of cementitious composites. Therefore, it can be speculated that they may be confronted with potentially serious damage or fatigue accumulations in severe environments during their service process. In addition, cementitious composites are inanimate materials without “laughing and crying.” Their normal state or abnormal state is very difficult to be obtained by external performance directly. In other words, cementitious composites are short of the ability to Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00026-2 © 2019 Elsevier Ltd. All rights reserved.

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express internal health status. Furthermore, cementitious composites are only required to have sufficient mechanical properties in most instances. With the increasing usage requirements of infrastructure, various functional cementitious composites must come into being to be able to provide functional support of the raw materials. In order to solve the issues/challenges of cementitious composites mentioned above, many experts in the scientific and industrial fields have made great effort to develop cementitious composites with enough strength, excellent self-sensing properties, and functionality. Through these studies and efforts, the various potential advantages of cementitious composites can be dug out, including a reliable carrying capacity; instant and real-time structural health monitoring (SHM) capability; the improvement of infrastructure durability and reliability; the improved ability to resist earthquakes, typhoons, and other natural disasters; and a considerable economic effect by reducing the cost of managing, monitoring, and repairing the infrastructure. It should be noted that cementitious composites mentioned in this chapter are a generalized concept. They can be divided into three categories according to the aggregate composition: concrete (containing coarse and fine aggregates), cement mortar (only containing fine aggregates), and cement paste (containing no aggregates) [1]. In recent years, nanomaterial has attracted more and more attention because of its abundant excellent properties. Many researchers have tried to incorporate various types of nanomaterials into cementitious composites to modify their properties. As a superior nanomaterial, there is no doubt that nanocarbon material (NCM) is also in the filler queue. NCMs. including carbon nanotubes (CNTs), carbon nanofibers (CNFs), and nanographite platelets (NGPs), are promising components for nextgeneration, high-performance structural and multifunctional composite materials. The existing research results have demonstrated the feasibility of NCM enhancing various properties of cementitious composites. Although all of them are nanocarbon materials, CNTs, CNFs, and NGPs possess obvious differences in the composition structure, leading to the differences of the intrinsic physical properties between them. CNTs, first discovered by a Japanese scientist called Iijima in 1991 when he observed the graphite product by arc evaporation, are allotropes of carbon with a hollowly cylindrical nanostructure. The diameter of CNTs is on the order of a nanometer while the length of CNTs is typically on the order of a few hundred micrometers or a millimeter. CNTs can be classified on the number of graphite layers, chirality, conductivity, and sorting conditions. In the field of civil engineering, according to the number of graphite layers, CNTs are always divided into two categories: single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs). The inherent physical properties of CNTs are remarkable. The Young’s modulus of CNTs is 10 times that of carbon fibers (CFs) and the tensile strength of CNTs is 20 times that of CFs. Moreover, the elongation at the break of CNTs can reach 18%. As a kind of quasi onedimensional NCM, CNFs also possess lots of excellent properties. The density of CNFs is low. The strength, modulus, and conductivity of CNFs are high. The aspect ratio and surface area of CNFs are large. The diameter of CNFs varies from 10 to 500 nm, and the length of CNFs varies from 0.5 to 200 μm. NGPs, not as common as the two types of NCMs mentioned above, have also set off an important upsurge in the field of cementitious composite optimization recently. NGPs are formed from

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783

Table 26.1 Properties of various types of NCMs Property

Unit

SWCNTs

MWCNTs

CNFs

NGPs

Aspect ratio Surface area Electrical resistivity Dimensions

None m2/g μΩ cm

1000 >400 5–50

1000 >400 5–50

100–500 200 55

50–300 2630 50 (in-plane)

None

Strength Elastic modulus

GPa TPa

Diameter: 0.7–53 nm Length: 1  50 μm 50–500 1

Diameter: 2–30 nm Length: 0.1 50 μm 10–60 0.3–1

Diameter: 50–200 nm Length: 50 100 μm 2.7–7.0 0.4–0.6

Diameter: 1–20 μm Thickness: 30 nm 10–20 1 (in-plane)

graphite. Multilayer graphenes (MLGs) are one type. MLGs refer to a sort of NCM with graphite-layered structures. Graphene oxide (GO) is another type; it has only a single graphite layer interspaced with oxygen molecules. The corresponding properties of NCMs including CNTs, CNFs, and NGPs are listed in Table 26.1 [2–14]. As described above, NCMs possess numerous excellent properties, of which some representative characteristics are remarkable physical, mechanical, electrical, and thermal properties as well as outstanding chemical and thermal stabilities. Thanks to these known properties, the traditional cementitious composites will be hopefully transformed to possess the service performance required by us. Not only can a single kind of NCM be incorporated to improve one or more properties of cementitious composites, but also several kinds of NCMs can be combined as functional filler to cementitious composites; it depends entirely on the functional requirements for cementitious composites. In other words, the properties of cementitious composites can be optimized and enhanced with the introduction of these functional fillers; the optimized material can be called NCM-filled cementitious composites (NCMFCC). There is no doubt that NCMFCCs will bring a new and promising change to cementitious composites, thus providing infrastructure with excellent properties. Fundamentally, the main function of cementitious composites lies in the applications. Therefore, this chapter focuses on the applications of NCMFCC, depending on the requirements put forward by the applications; discusses the most important factors such as preparation and the mechanical and electrical properties of NCMFCCs; and offers a preview of the follow-up work.

26.2

Preparation of NCMFCCs

During the fabricating process of NCMFCCs, the most important matter to be considered is how to uniformly disperse the NCM in cementitious composites. Because of the van der Waals forces in NCM, homogeneously dispersing NCMs to avoid

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agglomeration is still a difficult problem. In addition, a high quality and uniform NCMFCC is the necessary premise of achieving excellent properties conceived before experiment. The imperfect dispersion of NCMs in cementitious composites may cause defects on the surface or inside the cementitious composites. In this situation, the introduction of functional filler can’t achieve the enhancing effect, and will even have a counterproductive effect on the matrix. Therefore, extensive academic and industrial efforts have been devoted to solving this issue [15]. At present, there are currently two general methods used to disperse NCMs in cementitious composites: physical [16–29] and chemical methods [30–42]. The physical method uses ball milling and ultrasonic or high-shear mixing technologies to mechanically separate the NCM in cementitious composites. Sometimes, a single physical method is applied to disperse NCM while sometimes joint physical methods are adopted. The chemical method is used to decorate the surface of the NCM by creating covalent or noncovalent bonds. In many cases, these two dispersion methods often combine with one other to obtain satisfactory results [17–22]. However, the specific methods mentioned above all have obvious shortcomings. Some need to consume a lot of energy or will generate a lot of noise while others are too complicated to be widely used in engineering fields. Therefore, it’s urgent to use efficient, environmentally friendly, simple, and economical methods to achieve good dispersion of NCMs in cementitious composites. In recent years, with the rapid development of the chemical synthesis process, various surfactants have been researched and manufactured to solve the dispersion issue of NCMs in cementitious composites, of which the most representative one is waterreducing admixtures (including plasticizers and superplasticizers). Many researchers have made many attempts and studies in this area, and they have achieved ideal results to a certain degree. Shah et al. tried to use polycarboxylate-based superplasticizers to assist MWCNTs to evenly disperse in the cementitious composites. They found that the superplasticizers can achieve the desired effect, although MWCNTs possess different dosages and lengths [15, 26]. Huang proposed in his master’s thesis that he had adopted superplasticizer to disperse MLGs in the cementitious composites and achieved proper dispersion [3]. For this key issue of NCMFCC preparation, it’s obviously not enough to conduct experimental attempts and studies. Therefore, some studies made theoretical simulations and mechanism researches. Yazdanbakhsh and his partners conducted a three-dimensional modeling analysis to address this issue. They found that in order to achieve the perfect dispersion of CNTs in the cementitious composites without the decreasing effect of cement particles, cement particles should be guaranteed homogeneously in the cementitious composites without any agglomeration [43]. Of course, according to common sense, it is impossible to satisfy this assumption; what we can do is to achieve infinite approximation as far as possible. Han et al. also made some contributions in this field. They put forward that superplasticizers can effectively disperse different types and concentrations of CNTs and CNFs in cementitious composites, owing to their double-dispersion effect on cement particles and CNTs/CNFs [6]. The introduction of water-reducing admixtures achieves a good dispersion of NCMs in cementitious composites to acquire highquality NCMFCC. At the same time, this method avoids the introduction of additional

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dispersants such as sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and methylcellulose, which will have negative effects on the hydration of cement and the mechanical strengths of cementitious composites. However, this method can only assist the filler dispersion in cementitious composites within a certain degree. There is a certain limited value of the dispersion ability of water-reducing admixtures. Meanwhile, ultrasonic treatment is always required to assist the dispersion effect of water-reducing admixtures, which needs to consume a lot of energy. Nasibulin et al. attempted to utilize common raw materials including cement (clinker), copper powder, fly ash particles, calcinated soil, and sand as the precursor matrix. They tried to grow CNTs/CNFs directly on the surface of the matrix, matrix precursor, or filler particles to fabricate NCMFCCs with high quality, which can be called in situ growing of NCMs [44–47]. The uniform and effective dispersion of NCMs in cementitious composites to acquire remarkable NCMFCCs is still a critical issue. Therefore, it is necessary to research and find out a set of methods to make NCM dispersal in cementitious composites easier as well as more repeatable, stable, controllable, environmentally friendly, low cost, low energy consumption, and large scale.

26.3

Properties of NCMFCCs

26.3.1 Mechanical properties of NCMFCCs As we all know, as basic load-bearing materials, the most concerning and crucial properties of cementitious composites are the mechanical properties. In the existing research results, the studies and efforts on the mechanical properties are the most abundant. Numerous researchers have devoted their time, energy, and intelligence to this field; the corresponding research results are listed in Table 26.2. Studies demonstrated that CNTs will have a big influence on the hydration process and hardness of cementitious composites. The concentration of CNTs as well as their type, surface modification method, and length and diameter will all have obvious influences on the mechanical behaviors of cementitious composites filled with CNTs. The excellent experimental results follow. Al-Rub et al. found that the use of 0.2 wt% CNTs as functional fillers to cementitious composites can achieve a 269% increase in the flexural strength [32]. Nasibulin et al. achieved a 200% enhancement in the compressive strength of cementitious composites by introducing MWCNTs. Regrettably, he and his partners did not give out the specific concentration of MWCNTs in their literature [52]. The cementitious composites filled with 0.3 wt% MWCNTs exhibit a 34.28% increase in tensile strength in a study conducted by Ludvig et al. [54]. The research accomplished by Hunashyal indicated that the addition of 0.5 wt% of MWCNTs gains a 70.9% improvement in tensile modulus [55]. As for Young’s modulus, Ibarra et al. incorporated 0.1 wt% SWCNTs into cementitious composites and gained a 227% increase [30]. Makar et al. discovered that the hardness of cementitious composites filled with 2 wt% SWCNTs can be increased by 600%, compared with that of plain cementitious composites [23]. The properties listed above are many general

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Table 26.2 Enhancement of NCM to mechanical properties of cementitious composites Mechanical behaviors

Enhancement

Concentration of NCM

References

Flexural strength

269% 79%

[32] [48]

300% 144% 160.1% 34.28% 197.2% 70.9% 227% 68% 600% 1.5-fold 149.32% 270% 14% 130% 73% 73% 154% 170% 34.96%

0.2 wt% CNTs 0.5% of nickel-coated MWCNTs 2.0% CNTs 1.0 wt% CNFs 0.05 wt% MLGs 0.02 wt% GO NM MWCNTs 0.5% of nickel-coated MWCNTs 0.4 wt% CNFs 1.0 wt% MLGs 0.02 wt% GO 0.3 wt% MWCNTs 0.02 wt% GO 0.5 wt% MWCNTs 0.1 wt% SWCNTs 1.0 wt% CNFs 2 wt% SWCNTs 5 vol% MLGs 0.5 wt% MWCNTs 0.2 wt% CNFs NM MWCNTs 0.2 wt% MWCNTs 0.1 wt% MLGs 0.1 wt% CNFs 0.04 wt% MWCNTs 0.1 wt% CNFs 0.5 wt% MWCNTs

[11] [53] [51] [54] [51] [55] [30] [10] [23] [56] [57] [31] [58] [34] [59] [32] [34] [32] [57]

60% 20% 16.22% 45.73% 430% 2200% 25% 280% 3400% 1700% 150% 30.7 %

2.0% CNTs 5 vol% MLGs 1% MLGs 5% MLGs 1.0 wt% CNFs 0.04 vol% CNFs 0.1 wt% CNFs 0.04 vol% CNFs 0.04 vol% CNFs 0.04 vol% CNFs 0.2 wt% CNFs 1.0 vol% CNFs

[49] [56] [60] [60] [10] [61] [32] [61] [61] [61] [31] [62]

22% 105%

0.6 wt% MLGs 0.08 wt% GO

[63] [64]

Compressive strength

Tensile strength Tensile modulus Young’s modulus Hardness Fracture toughness Fracture energy Ductility

Modulus toughness Critical opening displacement Damping ratio

Resilience Deflection Stiffness Impact resistance Energy sorption Abrasion weigh loss Peak displacement Ultimate normalized capacity Flexural toughness

32% 250% 82% 184.5% 200% 64.4%

[49] [50] [3] [51] [52] [48]

Note: NM in the table represents the concentration of NCM is not mentioned in the corresponding reference.

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mechanical properties. Moreover, other mechanical behaviors were also tested by some researchers, although they were mentioned in a few references. Luo et al. introduced 0.5 wt% MWCNTs into cementitious composites and found that this concentration of MWCNTs can increase the fracture toughness and critical opening displacement of cementitious composites by 149.32% and 34.96%, respectively [57]. Hlava´cˇek tried to use MWCNTs as fillers to improve the fracture energy of cementitious composites. Finally, it was found that the fracture energy can be increased by 14% [58]. Al-Rub added 0.2 and 0.04 wt% MWCNTs to cementitious composites in his experiment, resulting in a 130% and 154% increase in the ductility and modulus of toughness, respectively [34]. Cui et al. adopted different types of MWCNTs to reinforce the mechanical strengths of cementitious composites; the experimental results are shown in Fig. 26.1. They found that hydroxyl-functionalized MWCNTs feature a better reinforcement effect on flexural/compressive strength compared to carboxyl-functionalized MWCNTs. The best relative/absolute enhancements of 79%/74 MPa and 64.4%/5.6 MPa in the compressive and flexural strength of composites are achieved by incorporating 0.5% of nickel-coated MWCNTs [48]. In addition, the ability of infrastructure to resist natural disasters such as earthquakes and typhoons is also very important. It requires us to enhance the damping property of cementitious composites to consume the energy caused by vibration. Luo et al. discovered that with the addition of 2.0% CNTs, the damping ratio and flexural strength of cementitious composites increase around 60% and 32% as compared to the reference, respectively [49]. Koratkar et al. observed that the addition of CNTs can improve the damping ratio of cementitious composites by 200% compared to the plain ones [65]. There are also numerous studies about the enhancing efforts on the mechanical properties of NCMFCCs by adopting CNFs as the reinforcing fillers. Yazdanbakhsh et al. found that the use of 1.0 wt% CNFs as enhancing fillers to cementitious composites can achieve a 250% increase in the flexural strength [50]. Nasibulina et al. achieved a 300% enhancement in the compressive strength of cementitious composites by introducing 0.4 wt% CNFs into the matrix [12]. Gay et al. found that the addition of 0.2% CNFs per weight of cement resulted in increased splitting tensile strength of 22% in cementitious composites and 26% in composites that also contained silica fume [66]. The cementitious composites filled with 0.1 wt% CNFs exhibited a 73% increase in the ductility in a study conducted by Al-Rub et al. [32]. The research accomplished by Yazdanbakhsh indicated that the addition of 1.0 wt% CNFs gains a 430% improvement in resilience, which is a surprising finding [10]. As for deflection, Peyvandiet al. incorporated 0.04 vol% CNFs into cementitious composites to cooperate with steel fibers and gained a 2200% increase over plain cementitious composites [61]. Al-Rub et al. discovered that the stiffness of cementitious composites filled with 0.1 wt% CNFs can be increased by 25% compared with that of plain ones [32]. Yazdanbakhsh added 1.0 wt% CNFs to cementitious composites in his experiment, resulting in a 68% increase in Young’s modulus [10]. Tyson and his partners discovered a method to increase the fracture toughness of cementitious composites by 270% with the introduction of 0.2 wt% CNFs [31]. Peyvandi tried to use CNFs as strengthening fillers to cementitious composites at the dosage of 0.04 vol%.

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Fig. 26.1 Enhancing effect on the mechanical properties of cementitious composites with different types of multiwalled carbon nanotubes.

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The results showed that the impact resistance, energy sorption, and abrasion weight loss can be increased by 280%, 3400%, and 1700%, respectively [61]. The modulus of toughness of cementitious composites was enhanced by 170% by Al-Rub et al. introducing 0.1 wt% CNFs to the matrix [32]. Tyson et al. found that the peak displacement of cementitious composites can be increased by 150% when the CNF concentration is 0.2 wt% [31]. Howser et al. introduced 1.0 vol% CNFs into cementitious composites and obtained a 30.7% increase in ultimate normalized capacity [62]. As for GO/MLGs, they are also promising fillers to enhance the mechanical properties of cementitious composites [67, 68]. Ranjbar et al. found that using 1.0 wt% MLGs as enhancing fillers to cementitious composites can achieve a 144% increase in the compressive strength [53]. Huang achieved 82% enhancement in the flexural strength of cementitious composites by introducing 0.05 wt% MLGs into the matrix [3]. Singh et al. found that the addition of 0.6 wt% MLGs resulted in increased flexural toughness of 22% in cementitious composites [63]. Cui and her partners tried to adopt NGPs as the enhancing fillers to cementitious composites at the concentration of 5 vol %. The hardness and damping ratio of cementitious composites increases 1.5-fold and 20%, respectively, while the abrasive loss per unit area and abrasion depth of cementitious composites decreases by 71% and 73%, respectively, when compared with pure cementitious composites [56]. The cementitious composites filled with 0.1 wt% MLGs exhibit a 73% increase in the ductility in a study conducted by Fan [59]. Research accomplished by Zohhadi indicated that the addition of 1.0 wt% CNFs gains a 430% improvement in resilience [69]. Lv et al. found that when the GO content is 0.02 wt% in the cementitious composites, the tensile strength, compressive strength, and flexural strength can be improved by 197.2%, 160.1%, and 184.5%, respectively [51]. As for flexural toughness, Lu et al. incorporated 0.08 wt% GO into cementitious composites and gained a 105% increase over plain cementitious composites [64]. Horszczaruk et al. found that the Young’s modulus of reference cementitious composites is in the range of 1–10 GPa while the cementitious composites with 3 wt% GO exhibit the distribution in the range of 5–20 GPa [70]. Ruan et al. tried to introduce MLGs into cementitious composites and found that the damping ratio can be increased significantly. They attributed this phenomenon to the excellent thermal conductivity of MLGs. They also proposed that the damping ratio measured by the time-domain exponential decay method and the frequency-domain half-power bandwidth method is consistent in their experiment [60]. Zheng et al. purposed many mechanisms of graphene-based NCM reinforcing cementitious composites. They thought the water absorption property makes graphene somewhat like a self-curing agent by absorbing water at an early hydration stage to lower the water/cement ratio and releasing water to form self-curing inside in a later hydration stage, as shown in Fig. 26.2 [67, 68]. They also analyzed the mechanism at the chemical level. They considered that hydration products such as Ca(OH)2 tend to grow around graphene sheets due to the nucleation effect, but the existence of graphene sheets also reduces the growth space of the products, which makes the products become smaller and further densifies the composites, as shown in Fig. 26.3 [68]. The studies on the mechanical properties of cementitious composites are quite rich and have real reference values. However, a fair amount of investigation on other

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Fig. 26.2 Schematic diagram of the self-curing effect of graphene in cementitious materials: during early hydration (left); during later hydration (right) [68].

Fig. 26.3 Schematic diagram of the effect of graphene on the hydration product growth around cement particles [68].

aspects is worth doing. Most existing studies only adopted one kind of NCM to explore the reinforcing effects on the mechanical properties of cementitious composites. In the prospective research, two or more kinds of NCMs can be considered to mix and introduce to cementitious composites to study their joint effect. Certainly, this idea may not produce a positive enhancing effect; it can even have a weakened effect.

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It requires us to better select the type and concentration of NCMs. In addition, due to the high price and other factors, the cementitious composites filled with NCM were only made as small components. However, the ultimate aim of cementitious composites is structural applications. Hence, we can try to use NCMFCCs in the appropriate structures when conditions permit. As we know, the life cycle of a structure is fairly long. The long-term enhancing effect of NCMs on the mechanical properties of cementitious composites is also an important matter worth study. Moreover, NCMFCCs can be tested under four-point bending, three-point bending, and other loading forms to explore the abundant mechanical behaviors [71].

26.3.2 Electrical and self-sensing properties of NCMFCCs Like mechanical properties, the electrical conductivity and piezoresistivity of NCMFCCs are also of wide concerned by various researchers. The most important reason for this phenomenon is that the electrical property and piezoresistivity of NCMFCCs are the two most pivotal factors influencing the self-sensing property. In order to acquire stable, repeatable, accurate, and reliable electric signal output by NCMFCCs, extensive studies have been done until now. In the research filed that uses CNTs as filler, Luo and his partners introduced MWCNTs at different concentrations into cementitious composites and found that the resistivity of NCMFCCs decreases with the increase of concentration. When the concentration of MWCNTs is 2%, the resistivity of NCMFCC is only 1.83 kΩ cm [72]. Han et al. found that the cementitious composites possess excellent electrical conductivity and piezoresistivity when filled with CNTs. The enhancing effect on stress/strain sensitivity and electrical conductivity will be limited because of the inevitable inhomogeneous dispersion of CNTs in the matrix [73]. Yu and Kwon discovered that the electrical resistance of the CNT/cement composite changes with the compressive stress level, indicating the potential for using the CNT/cement composite as a stress sensor for civil structures. They also found that dispersion-assistant surfactants could block the contacts among CNTs, thus impairing the piezoresistive response of the composite [74]. Saafi tried to use wireless and CNTs for damage detection in concrete structures. Experimental results showed the electrical resistance decreased as the volume of SNWTs increased from 0% to 1%. A slight decrease was observed when the volume of SNWTs was increased beyond 1% [75]. Luo investigated the stress and strain sensitivities of cementitious composites with the addition of 0.5 vol% CNTs; the corresponding tested values are 0.4% MPa 1 and 54.2, respectively [57]. Luo et al. found that, compared to the plain cementitious composites, the corresponding MWCNTs/cementitious composites increase in electrical conductivity by two orders of magnitude [76]. Han and his collaborators filled cementitious composites with CNTs to explore the self-sensing capability of NCMFCCs in the laboratory and an actual road. Experimental results showed that cementitious composites with CNTs present sensitive and stable responses to repeated compressive loadings and impulsive loadings, and have remarkable responses to vehicular loadings. This indicates the potential for traffic monitoring use, such as in traffic flow detection, weigh-in-motion measurement, and vehicle speed detection [77].

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Han et al. found that the sensitivity of the piezoresistive response largely depends on the type and concentration of CNTs/CNFs [6]. Gao et al. investigated the electrical properties of cementitious composites containing CNFs with different concentrations. They found that the electrical resistance is more sensitive than that of plain cementitious composites, both under compression and tension. The introduction of CNFs to cementitous composites can enhance the electrical properties, indicating it can be used in the cementitious composites for monitoring strain and self-health and evaluating damage [78]. Galao et al. conducted the experiments to investigate the strain and damage-sensing properties on cementitious composites filled with CNFs. The cementitious composites filled with different concentrations of CNFs were fabricated. Experimental results showed cured for seven and 14 days; the cementitious composites did not present strain-sensing properties. However, when the curing age is 28 days, it is possible to adopt CNT-filled cementitious composites as strain sensors under compression [79]. As for the electrical properties of graphene-based materials, some researchers have also devoted their attention to this field. Sun et al. filled cementitious composites with MLGs. The experimental results demonstrated that cementitious composites filled with MLGs possess sensitive piezoresistive effects and stable repeatability to different loading conditions. They also found that the percolation threshold of the electrical resistivity of the cementitious composites filled with MLGs is about 2 vol%, and a second percolation phenomenon appears as the content of the MLGs reaches 9 vol %, as shown in Fig. 26.4 [80]. Sun and his partners also adopted MLGs as the fillers to cementitious composites and found that a fractional change in the electrical resistivity of the cementitious composites filled with 5 vol% of MLGs reaches 15.6% when the compressive stress is 20 MPa [81]. Sedaghat et al. discovered that the electrical conductivity tends to increase with increasing the graphene content in the cementitious composites [82]. Using graphene nanoplatelets (GNP) in cementitious composites to quantify the material damage extent was accomplished by Le and his partners. By adding a sufficient amount of GNP, the cementitious composites can be made to be electrically conductive. Meanwhile, as the GNP amount exceeds a percolation threshold value, the electrical conductivity of the GNP-infused cementitious composites is

Fig. 26.4 Relationship between electrical resistivity and MLG content.

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insensitive to the moisture content, which makes it a reliable damage-sensing material for infrastructure applications [83]. Muhit proposed in his master’s thesis that the addition of 5 wt% GO could decrease 93% of the electrical resistivity of cementitious composites [84]. As mentioned before, most of the studies on the electrical properties of NCMFCCs are limited to using only one kind of conductive fillers. Only a few researchers have tried to incorporate compositing conductive fillers to investigate the enhancing effect on the electrical properties of cementitious composites. Zhang et al. fabricated conductive NCMFCCs with the addition of CNTs/nanocarbon black (NCB) composite fillers made by electrostatic self-assembly. The fillers used in their experiments possess good electrical properties, a low price, and dispersion, which are all benefits desired by structural applications. They thought that the grape cluster structure composed of CNTs (as the stems) and NCB (as the fruit) is beneficial for improving the electrical properties of cementitious composites, as shown in Fig. 26.5. They proposed that the change of distances of the adjacent CNTs and NCB, CNTs and CNTs, and NCB and NCB may lead to connecting or an easy-to-produce tunneling effect. There are more conductive paths that can form to conduct electricity due to the structure of CNTs/NCB composite fillers [85, 86]. Han et al. incorporated cementitious composites with botryoid hybrid nanocarbon materials (BHNCMs). They found that the optimal BHNCMs is 3.38 vol% for the piezoresistivity of the self-sensing cementitious composites with BHNCMs (SCCBHNCMs). The percolation threshold zone of the SCCBHNCMs is from 2.28 to 3.85 vol%. The amplitude of fractional change in resistivity goes up to 70.4% and 28.9%, respectively, under the monotonic compressive loading to failure and under the repeated compressive loading within the elastic regime. The piezoresistive stress/strain sensitivity reaches 3.04%MPa 1/354.28 within the elastic regime [87]. The introduction of NCM into cementitious composites endows conductivity and piezoresistive property to NCMFCCs, which shows the possibility of fabricating NCMFCCs as sensors to monitor the inner health condition of the structure. However, there are still many aspects that need further study in this field. First, as we all know, the temperature, humidity, and service time all will influence the conductivity and piezoresistive property of NCMFCCs; these influencing factors should be considered

Fig. 26.5 The schematic of grape cluster structure composed of CNTs and NCB.

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key points in subsequent studies. Second, at present, most researchers use wired testing methods; wiring in practical applications is not economical and convenient. Therefore, wireless technology should be introduced and improved to solve this issue. Third, the self-sensing property based on the conductivity and piezoresistive property only aims at compression. However, the forced forms (i.e., tension, flexure, shear, and torsion) of a structure are sundry; it is also a very important point to apply NCMFCCs to other kinds of conditions.

26.3.3 Other properties of NCMFCCs In addition to the mechanical and electrical properties, other properties of NCMFCCs, including transport, thermal, smoke detection, and electromagnetic interference (EMI), were also investigated. For example, Han et al. researched the transport properties (i.e., water sorptivity, water permeability, and gas permeability) of CNT-filled cementitious composites. They found that although the additional concentration of the reinforcing filler was small, the water sorptivity coefficient, the water permeability coefficient, and the gas permeability coefficient can be obviously decreased, which suggests the potential of CNTs in improving the durability properties of cementitious composites [88]. Yakovlev et al. discovered that the reinforced nonautoclave cement foam concrete with the incorporation of CNTs can decrease its heat conductivity up to 10%–20% [89]. Shukla et al. used MWCNTs to endow a smoke detection property to cementitious composites. The experimental results showed that the DC transient studies depicted an increase in conductivity when exposed to smoke. It indicated that cementitious composites filled with MWCNTs can be proved to be an asset for smoke-sensing applications [90]. Singh and his partners fabricated Portland cement (PC) filled with MWCNTs. They then discovered that the incorporation of 15 wt% MWCNTs in the PC matrix produced an EMI shielding effectiveness (SE) more than 27 dB in the X-band (8.2–12.4 GHz), and this SE is found to be dominated by absorption [28]. Al-Rub et al. found that the workability of the cement paste can be greatly affected by the untreated CNTs. With the addition of 0.2% untreated CNTs by weight, a very viscous solution when mixed with the water and surfactant can be acquired. Certainly, this phenomenon will affect the shaping process of cementitious composites [32]. Gong et al. discovered that, although the introduction of 0.03% by weight GO sheets into the cement paste can increase the compressive/tensile strength, the workability of the GO-cement composite becomes somewhat reduced [91]. Cui et al. found that, compared with plain cementitious composites, the thermal conductivity of cementitious composites at 1% and 5% levels of NGPs can be increased by 11.3% and 77%, respectively. The specific heat of cementitious composites decreases with increasing content of NGPs. They also tested the electromagnetic wave reflectivity of cementitious composites with NGPs in the 1–18 GHz frequency band. Experimental results showed that the electromagnetic wave reflectivity of cementitious composites at 5% loading of NGPs reaches the minimum value ( 5 dB) in the vicinity of 8 GHz, as shown in Fig. 26.6. The absolute value of reflectivity increases by 38% compared with plain cementitious composites [56]. Zheng et al. believed that the durability of cementitious composites filled with GNP/GO can be improved because of the addition of fillers. With the incorporation of GNP/GO, the micropore structure of the

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Fig. 26.6 Reflectivity of cementitious composites filled with MLGs.

matrix can be enhanced, thus hindering the initiation and propagation of microcracks during the outset. As a result, the macro performances of cementitious composites such as water permeability, gas permeability, and chloride penetration resistance can be significantly optimized [68]. In order to further investigate the mechanism of graphene platelets defending chloride ions in the matrix, Sun et al. have made stupendous efforts. They thought the conductivity of GNP is excellent, thus definitely improving the electric flux in cementitious composites, which is a favorable factor for chloride ions. However, the durability of cementitious composites with GNP is greater than plain ones. Therefore, they conducted the nonsteady-state chloride migration experiments and tested the DRCM (chloride migration coefficient from nonsteady-state migration experiments) of the specimens, from which cementitious composites with 5% GNP have a DRCM value less than 10% of the one of the control sample, meaning the positive effects of the GNP have countered the negative effects. They attributed this experimental phenomenon to the following reasons. On the one hand, GNP can compact the cementitious composites. The density of the NCMFCC increases with the increase of GNP content, which makes it harder for chloride ions to penetrate. On the other hand, the GNP has an extremely large specific surface area, so the GNP inside matrix will absorb chloride ions and act like “filters,” as shown in Fig. 26.7 [68,92]. In addition, Sun and his partners adopted different contents of MLGs

Fig. 26.7 Graphene platelets acting like “filters” for chloride ions [68].

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to cementitious composites (including 1, 2, 5, 9, and 10 vol%). Experimental results showed that with the increase of MLGs, the sensitivity of the thermal-resistance effect of the cementitious composites filled with MLGs presents a decreasing trend. They also found that the best SE of cementitious composites filled with MLGs (10 vol%) is 1.6 times of that of cementitious composites without MLGs. At the same content of MLGs, the highest electromagnetic wave-absorbing performance of cementitious composites can be obtained, which is nearly seven times of that of cementitious composites without MLGs [81].

26.4

Application cases of NCMFCCs

The abundant and excellent properties of NCMs endue various structural applications to cementitious composites. Nowadays, the practical applications of NCMFCCs mainly include two aspects. On one hand, NCMs can be used to improve the mechanical properties of cementitious composites to fabricate structural components with high strength and good durability. On the other hand, the structural application of NCMFCCs mainly concentrates on the electrical properties. Peyvandi et al. conducted an experiment to explore the contributions of MLGs to the durability of dry-cast concrete pipes in an aggressive sanitary sewer environment. MLGs at low dosages (0.05 vol% of concrete) were found to significantly improve the moisture transport performance and acid resistance of concrete. Therefore, the service life can be increased with the introduction of MLGs in sanitary sewer applications [11]. Gao et al. found that with the addition of 1% CNFs, the peak compressive strength of cylinders can be increased by 21.4% compared with the plain ones. They also found that the introduction of CNTs can make a significant contribution to the stiffness improvement of cylinders [78]. Howser et al. observed that the ultimate normalized capacity, deflection, and ductility of a self-consolidating reinforced concrete column all can be improved with the addition of CNFs; the corresponding increasing rates are 30.7%, 34.9%, and 35.1%, respectively [62]. Based on its electrical properties, an NCMFCC can be used in SHM and traffic detection; numerous researchers have conducted experiments in this area. Cui et al. simulated the stress state and strain compatibility of the NCMFCC sensor embedded into a concrete member to provide theoretical support for the design and application of a cement-based sensor. They found that the suitable dimension and voltage electrode separation of the cement-based sensor are 20 20 40 and 10 mm, respectively [93]. Wang embedded self-sensing cement-based sensors with hybrid CNTs and NCB fillers in C30 and C50 concrete columns, respectively. Then he investigated the sensitivity of sensors embedded in a smart concrete column [94]. Han et al. verified the possibility of using self-sensing cementitious composites filled with CNTs to detect various traffic parameters such as vehicle speed, traffic flow, and vehicle weight [77]. Wireless cement-carbon nanotube sensors were embedded into concrete beams and subjected to monotonic and cyclic loading to evaluate the effect of damage on their response in a study by Saffi [75]. Baeza et al. investigated the strain-sensing and damage-sensing functional properties by using CNT-filled cementitious composite

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sensors on a 4-m long conventionally reinforced concrete beam [9]. Thanks to the remarkable piezoresistivity, NCMFCCs have been used in SHM and traffic monitoring successfully [77, 95]. Of course, at the same time as optimizing the applications of NCMFCCs in these two aspects mentioned above, it is also valuable to explore other structural applications. For example, the obvious smoke-detection property of MWCNT-filled cementitious composites offers the potential that this material can be fabricated as a smoke detector [90]. The electromagnetic interference shielding property of NCMFCCs reminds us that we can apply it to military shelters [28]. In addition, NCMFCCs with excellent damping properties can be used to build infrastructure in earthquake-prone areas [49, 60, 65]**. In short, the excellent properties of NCMFCCs give us plenty of possibilities to build smarter and more multifunctional structures.

26.5

Summary

The mechanical behaviors, functional or smart performances, and application cases of NCMFCCs are introduced systematically in this paper. Relying on the inherent excellent physical, chemical, electrical, and thermal properties of NCMs, cementitious composites can be endowed with remarkable mechanical properties such as high strength and damping properties; excellent electrical and self-sensing properties such as high electrical conductivity and high sensitivity of piezoresistivity; and other useful functional properties such as transport, thermal, smoke detection, durability, and electromagnetic interference. Besides, NCMs shows good compound utility with cementitious composites. Therefore, NCMFCCs are expected to be the next generation of multifunctional building materials because of good durability; remarkable mechanical, electrical, and self-sensing properties; and smart and versatile features. The existing studies on various properties of NCMFCC have also confirmed the possibility of applications. NCMFCCs can be used to build infrastructures with high strength and excellent durability, such as dams, offshore construction buildings, and protective shells of nuclear reactors. Thanks to their excellent electrical and self-sensing properties, NCMFCCs can also be used to monitor the health conditions of structures and roads. Developments and studies on the multifunctional properties such as electromagnetic shielding performance and smoke sensitivity demonstrate the potential of NCMFCCs for use in signal shielding facilities and intelligent fire alarm buildings. Although numerous researchers have devoted their efforts to investigate NCMFCCs, there are still a lot of works that urgently need to be implemented. In a future area of research, the homogeneous dispersion of NCMs in cementitious composites remains an important issue, although physical and chemical dispersion methods have been adopted. Moreover, the workability of NCMFCCs needs to be improved. On the basic of this, the investigation of NCM-enhancing concrete can be conducted because most building materials contain coarse aggregates. In addition, theoretical studies, experiments, and simulations can be combined to study the convincing enhancing mechanisms of NCMs on cementitious composites. More in-depth studies are needed to focus on the durability of NCMFCCs as this property is critical.

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NCMFCCs have injected new vitality into the transformation of building materials, yet they also inevitably face some difficulties and challenges. Therefore, a lot of effort should be invested to develop NCMs with remarkable application capabilities in cementitious composites.

Acknowledgments The authors thank the funding support from the National Science Foundation of China (51578110) and the Fundamental Research Funds for the Central Universities in China (DUT18GJ203).

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N. Rajesh Jesudoss Hynes*, R. Sankaranarayanan*, M. Kathiresan†, P. Senthamaraikannan‡, Anish Khan§,¶, Abdullah Mohamed Asiri§,¶, Imran Khank *Department of Mechanical Engineering, Mepco Schlenk Engineering College (Autonomous), Sivakasi, Tamil Nadu, India, †Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai, Tamil Nadu, India, ‡Department of Mechanical Engineering, Kamaraj College of Engineering and Technology, Virudhunagar, Tamil Nadu, 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, kApplied Science and Humanities Section, University Polytechnic, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India

Chapter Outline 27.1 Introduction 806 27.2 Synthesis and characterization of carbon nanotubes

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27.2.1 Synthesis of carbon nanotubes 809 27.2.2 Carbon nanotube characterization 810

27.3 Mechanism behind carbon nanotubes

812

27.3.1 Single-walled carbon nanotubes 812 27.3.2 Multiwalled carbon nanotubes 815

27.4 Composites made of carbon nanotubes 815 27.5 Techniques related to the fabrication of CNT-based composites 27.5.1 27.5.2 27.5.3 27.5.4 27.5.5

818

Friction stiring 818 Plasma-assisted spark sintering 818 Dispersion technique via spreading 818 Mechanical stirring and casting technique 818 Milling technique 819

27.6 Factors and response analysis related to CNT-reinforced aluminum-based metal matrix composites 819 27.6.1 27.6.2 27.6.3 27.6.4

Time consumed for milling 819 Quantity of CNT 820 Microhardness 820 Friction and wear tendency of aluminium/CNT composites 822

Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00027-4 © 2019 Elsevier Ltd. All rights reserved.

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27.7 Factors and response analysis related to CNT-reinforced copper-based metal matrix composites 822 27.7.1 27.7.2 27.7.3 27.7.4 27.7.5 27.7.6

Implication of CNT percentage with the relative density of the MMC 822 Implication of CNT percentage with the hardness (Hv) of the MMC 824 Implication of CNT percentage with the grain size of the MMC 825 Implication of CNT percentage with the strain-hardening exponent of the MMC 825 Implication of CNT percentage with the young’s modulus of the MMC 825 Implication of CNT percentage with the yield (0.2% proof ) strength of the MMC 826

27.8 Conclusion 827 References 828 Further reading 830

27.1

Introduction

The presence of metal matrix composites is tremendous with various reinforcements. But the application of CNT as a reinforcement in the metal matrix composite is still under research, which began a couple of decades ago [1]. Metals dominate as structural materials, even today where CNT-based composites can be a potential alternative in the field of automobiles, aerospace, sports-based industries, and many more. The combination of strength, light weight, and stiffness makes them the most desirable applicants [2]. The area related to functional and structural aspects readily accepted CNTs after their discovery [3]. The exploration of CNTs by Iijima and their extraordinary properties turned industries and researchers toward CNTs [4]. The typical example is the excellent stiffness of the CNTs that is competitive to the diamond and even stiffer than a diamond. The magnitude of Young’s modulus is of the terapascal (TPa) range and the achievement up to 0.06 TPa of tensile strength is possible. The antiphon by CNT toward the deformation is very much impressive. This is one of the desired states for a material, as hard materials fail mostly with 1% strain or less than 1%. This phenomenon results from the spread of defects and dislocations. But this is not so in the case of CNT, as it can withstand tensile strain until 15% [4] before fracture [4]. Moreover, the excellent qualities of CNTs make researchers refer tto hem as the ultimate reinforcement with successful implementation into the metal matrix medium [5, 6]. CNTs are also known for their electronic properties, and those make them an appealing material in the nanotube-based applications and the respective studies. The size along with the symmetric nature of the nanotubes influences its properties. Good symmetric orientation of the nanotube structure with a very small size enhances its lattice, electronic, and magnetic properties. The other beneficial part is the exceptional quantum effects. Experimental results as well as theoretical values proved CNT’s exceptional electronic properties. In case thermal properties are a concern, individual multiwalled CNTs (MWCNTs) possess the extraordinary thermal conductivity of about 3000 W/(m K) whereas single-walled CNTs (SWCNTs) possess 6000 W/(m K). Even at room temperature, aligned SWCNT films hold 200 W/(m K). There are other properties that add values to CNTs with

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

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their incorporation, namely chemical, optoelectronic, electrochemical, and thermoelectric This leads to the analysis of functional aspects and features of the composites that are made with CNTs [4]. The distinct structure of CNTs provides them special properties such as a low density around 2 g/cm3, higher strength around 0.04 TPa, extraordinary chemical stability, superior thermal stability, large aspect ratio in the range of around 100–1000, and excellent elastic modulus around 1 TPa [7, 8]. CNT is called “advanced filler” for composites due to the above combination of material properties [8]. CNT is nothing but graphite in the form of rolled sheets, and appears as a tube. The structure of graphite (CNT) is different from the diamond, as graphite forms a twodimensional sheet with an array of a hexagonal nature, whereas diamond possesses a three-dimensional cubic crystalline construction with a tetrahedron nature. Diamond holds each carbon atom with four immediate neighbors whereas CNT holds three immediate neighbors. The cylindrical shape is derived from the rolling process of graphite sheets. The atomic arrangement plays a vital role in deciding the properties of CNTs and this atomic arrangement is decided by the way it has been rolled. Apart from the atomic arrangement, the nanostructure, length of tubes, morphology, and diameter of the CNT are other influencing factors as far as properties of CNT are concerned. The existence of CNT is in two different modes, namely SWCNT and MWCNT [7]. Tube chirality aids in the description of CNT’s atomic structure. The other term, called helicity, can also be fruitful in the description of the atomic structure of CNTs. !

These can be defined through the vector called the chiral vector (C h), along with the chiral angle (θ). Through Fig. 27.1, it can be visually recognized that the rolling of graphite sheet is in line with dotted lines and the chiral vector comes in contact at

Armchair Zig-zag



r cto

Ch

l ve hira

C



a2



ma2

q, Chiral angle →

a1



na1

Fig. 27.1 Schematic diagram showing how a hexagonal sheet of graphite is rolled to form a carbon nanotube [7].

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the tail. The rollup vector is another terminology given for the chiral vector. The clear understanding of this vector is possible via the following Eq. (27.1) [7]. !

Ch ¼ na1 + ma2

(27.1)

The integers, namely n and m, refer to the number of steps through the hexagonal lat! tice carbon bonds in a ziz-zag manner, whereas unit vectors are mentioned with a 1 and ! a 2, as shown in Fig. 27.1. The angle called the chiral angle decides the quantity of a tube’s twist. The presence of these limitations can be recognized through a 0 degree as well as a 30 degree chiral angle. In specific, 0 degree and 30 degree are termed as zizzag and armchair, respectively, and can be decided on the basis of the geometrical arrangement of the bonds of the carbon located circumferentially around the nanotube. The discrepancy among the ziz-zag and armchair can be visualized in Fig. 27.2. The rollup vector representation of the nanotube for the armchair and ziz-zag are (n,n) and (n, 0), respectively. The diameter of the nanotube can be derived with the help of the rollup vector, as the interatomic spacing is already a known value for the carbon atoms. Consequently, the term called chirality has an influential association with

Fig. 27.2 Illustrations of the atomic structure of (A) an armchair and (B) a ziz-zag nanotube [7].

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the CNT properties. More specifically, the electronic properties have been influenced strongly by the chirality. In general, graphite comes under the semimetal category. But the same can be considered as a metal or semiconductor, decided on by the chirality of the tube [9]. The implication of the chirality was recorded properly through various studies. Yakobson et al. [7, 10] researched the instability condition of the CNT’s outside limits of the linear response. Their research via simulation clearly indicated the capability of the CNTs in terms of resilience. Thus, the sustainability of the CNTs was more even at intense strained conditions. Going further, the structure did even experience any brittleness. As far as elastic stiffness is concerned, the chirality possessed less influence on it. The role of the CNT on the plastic deformation is still critical under the tensile conditions. This was explained through the Stone-Wales transformation and the same is shown in Fig. 27.3. That happens under stressed conditions of the armchair nanotube along the axial direction.

27.2

Synthesis and characterization of carbon nanotubes

27.2.1 Synthesis of carbon nanotubes The different methods through which CNT synthesis can be accomplished have been shown in Fig. 27.4. The very first CNT synthesis was carried out unintentionally by Iijima through arc discharge. But the situation is entirely different for the current scenario as wide varieties of methods are available for CNT synthesis. The categorization of these different methods can be mentioned through CNT’s properties, namely temperature, time, heat source, precursor, mechanism, atmosphere of reactions, etc. The widely accepted methods are laser ablation, arc discharge, and chemical vapor deposition for the synthesis of a carbon nanotube [11–13]. The primary observation was done initially by Iijima over multiwalled nanotubes. The research continued by involving single-walled nanotubes in synthesis within a decade. As mentioned earlier,

5

7

7 5

Fig. 27.3 The Stone-Wales transformation occurring in an armchair nanotube under axial tension.

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Nanocarbon and its Composites

Arc Discharge Sono-chemical or hydrothermal Electrolysis

Laser Ablation

Oxygen assisted

CNT synthesis methods

Microwave plasma

Chemical vapour deposition

Radio-frequency

Thermal

Plasma enhanced

Water assisted

Fig 27.4 Various methods of CNT synthesis [1].

multi- and single-walled carbon nanotubes were synthesized via chemical vapor deposition, laser ablation, gas-phase catalytic growth, arc-discharge, etc [7, 14]. The manufacturing of composites using carbon nanotubes involves a large quantity of CNTs. But a typical quantity of CNTs was not possible to be produced economically through laser ablation or arc discharge methods. Moreover, residues in terms of impurities such as amorphous carbon, nontubular fullerenes, and catalyst particles are also present as part of the outcome in the synthesis of CNTs. This leads to the inclusion of an additional step in the synthesis process for the purpose of purification so that CNTs can be separated from the impurities. But the gas-phase-based synthesis method manufacture CNTs with lower impurities. In addition to this positivity, this method suits the mass production of CNTs. The road doesn’t end here as chemical vapor deposition that is based on the gas-phase method also favors the large-scale production of nanotubes [7].

27.2.2 Carbon Nanotube Characterization The characterization at the micromechanical level for nanotubes possesses real challenges. A further challenge lies in the modeling at the nano level for analyzing the fracture as well as the elastic demeanor. Property measurements cannot be taken directly through micromechanical characterization. The specimen size is restricted to some standard sizes. The ambiguity lies over the data that are taken out of indirect measurement methods. A deficiency of knowledge prevails over the preparation of the specimen for testing and poor control over the distribution as well as alignment of nano-tubes. The characterization is an essential tool in the process of understanding the mechanical-based properties of CNTs; several attempts were made by various researchers in this regard. An inherent thermal vibration’s amplitude and a transmission electron microscope result help in obtaining the modulus of elasticity for the stand-alone multiwalled nanotubes. The value of 1.8 TPa on average was acquired from 11 samples. An atomic force microscope (AFM) can be handy in the process

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of directly measuring the stiffness as well as the strength of the MWCNTs (structurally isolated). Wong and colleagues were first involved in this type of measurement, which was carried out by pinning one end of the nanotube with the surface of the molybdenum disulfide. The tube was given a load with the help of the tip of the AFM. The elastic modulus of 1260 GPa was obtained by measuring the bending force that changed with respect to the displacement as a functional factor through the unpinned portion along its length. The strength average obtained was around 14 GPa under a bending condition [7]. The tendency of SWCNT assemblage is in the form of ropes. Salvetat and team studied those bundles of CNTs by measuring with the assistance of AFM to understand its properties. The analysis showed that the moduli along the axial as well as the shear direction decreases considerably as a CNT bundle’s diameter decreases. This implies the slippage of the CNTs as a bundle structure. Further analysis continued at the AFM for obtaining the elastic strain of the bundle of CNT. The elastic modulus decreased due to the slippage inside the bundle. A tensile test for the CNT was conducted by fixing MWCNT and SWCNT ropes at the two opposite tips of the AFM. Subsequently, the load was applied. MWCNTs experienced the failure via the outermost tube and the failure continued in the form of a pullout that occurred inside the nanotubes. The failure system of the MWCNT is referred to as a sword and sheath telescoping failure and the same is shown in Fig. 27.5. The tensile strength range

Fig. 27.5 Micrographs showing (A) the apparatus for tensile loading of MWCNTs and (B) the telescoping sword and sheath fracture behavior of the MWCNT [7].

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was 11–63 GPa for the outermost layer whereas the elastic modulus range was 270–950 GPa [7]. Similar tests were performed by Xie et al. [15] on the MWCNT under tension. The resulting values from the experiments were 3.6 GPa strength and 450 GPa as modulus. These low values may be due to the occurrences of defects from the chemical vapor deposition-based CNTs.

27.3

Mechanism behind carbon nanotubes

The latest studies have suggested that the exceptional mechanical properties of CNTs such as the potential to withstand fracture strains, the higher modulus of elasticity, and the exceptional elastic strain make it an outstanding material [7]. Similar results were obtained by another theoretical analysis [7, 16, 17], even though the correlation among the experimental as well as theoretical was less in numbers. The discussion of mechanics of the MWCNT and SWCNT was carried out here, which provided the positive results toward the CNTs.

27.3.1 Single-walled carbon nanotubes Many studies related to the mechanical properties of single-walled carbon nanotubes (SWCNT) have been done. Commonly, the vibration modes with long frequency and rigidity of structure possess up to 400 atoms with a minimum count of 100. Empirical Keating Hamiltonian led to this count. The excellence of the SWCNTs was explained through conducting various comparison studies. One such study was the comparison of SWCNT with the iridium beam. The platform of comparison was the bending stiffness for both materials. The continuum Bernoulli-Euler theory related to beam bending was selected to figure out the bending stiffness of the iridium beam. Overney and team came to the conclusion by stating that the rigidity of the bending beam for the CNT exceeds the topmost values of any of the rest of the present materials. In addition, the behavior of the CNTs under the condition of the compressive load was examined experimentally with the aid of a simulation technique called molecular dynamics. The outcome of the simulation deduced the deformation mechanism of SWCNTs where the bent was observed at larger angles. The related experimental as well as theoretical results suggested that CNTs possess exceptional flexibility. To be specific in terms of values, the bending can be reversed entirely for angles above 110 degrees. This phenomenon occurs even though the shape is complex, namely a kink. The diagrammatic visualization is given in Fig. 27.6, which portrays the extraordinary resilience of CNTs in the large strain condition. If the representation of the thickness is mentioned commonly, the real stiffness due to bending for the SWCNT is considerably lower in comparison to the shell model with elastic continuity. The effective stiffness related to CNT bending can be considered as the influencing parameter in the absence of the represented thickness. This concept eases the modification of the equations of the elastic shell, which can be handy for the SWCNT. These calculated results based on the above

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

813

Fig. 27.6 TEM micrograph and computer simulation of nanotube buckling [7].

conceptualization were in line with the outcome of simulations based on molecular dynamics [7]. The implications of the structure of the CNT and chirality on the torsion, tension, and bending was researched by Vaccarini et al. [18]. They concluded their studies by stating that the implications of the chirality were very minimal on the tensile modulus. Nevertheless, there is torsional behavior of an asymmetric nature by the chiral tube in relation to the right and left twist. But this is not the case for the armchair as well as ziz-zag tubes, as there is torsional behavior of a symmetric nature. Lu [16] did a comprehensive examination for the SWCNTs as far as the elastic properties are concerned. The dynamics model of the empirical lattice nature was

814

Nanocarbon and its Composites

adopted by Lu, which had fruitful results for calculating the graphite’s phonon spectrum as well as the elastic properties. The approximation of the atomic interactions of the SWCNT layer was carried out through the summation of harmonic potential pair-wise between atoms in the dynamics model of the empirical lattice nature. The conformal mapping technique can be used for the construction of a local structure for the CNT from the graphite sheet to the cylindrical surface. Lu attempted to analyze the influence of the chirality as well as the size of the CNT on the elastic properties. Lu was also involved in the comparison of CNT with diamond and graphite. The conclusion of the above study suggested the insensitivity of the elastic properties with respect to the chirality and size. The bulk, shear, and Young’s modulus for the CNTs were comparable to diamond. Another study is also in line with Lu’s results with little higher value of Young’s modulus. In contradiction to the findings of Lu, they discovered that the diameter as well as the structure are very sensitive toward the elastic modulus [7]. Along with the distinct elastic properties, the nature of inelasticity has drawn remarkable attention. The simulation of molecular dynamics displays the various morphological patterns when CNT experiences huge deformations. The reversible switching of shape resulted in an unanticipated energy release. The corresponding stress-strain curve was singularity in nature. This can be well elucidated using a continuum shell model. The precise understanding on the demeanor of the CNT outside the linear elasticity is made possible by the above model. The simulation of molecular dynamics for SWCNTs with various temperature and chirality was also executed [17]. This simulation suggested that CNTs possess extraordinary breaking strain and the same decrease as temperature decreases. The dislocation theory was also tested by Yakobson [10] on CNTs for discovering the relaxation mechanism of CNTs under a tensile load condition. CNT symmetry plays a vital role in its yield strength. There was a belief about the existence of intramolecular-based plastic flow. Ropes or well-aligned bundles of SWCNT can be manufactured via arc discharge as well as laser ablation methods. Subsequently, theoretical studies on these CNT bundles have been done. The exploration of elastic buckling of CNT ropes was done through selecting the honeycomb (elastic) model with modifications at the pressurized condition of large magnitude. Easy formulation is available to find the critical pressure with respect to the Young’s modulus of CNT. The ratio between wall thickness and radius also can be calculated with a simple formula. These studies explored the idea that SWCNT ropes are affected by buckling of an elastic nature whenever they experiences large pressure. The elastic buckling of CNT lead to the pressure depended abnormal vibrations and electrical resistivity [7]. Popov et al. [19] researched SWCNT’s elastic properties related to crystal lattices of a triangular nature with the assistance of a model called lattice dynamics of constant force analytically. They also involved armchair as well as zigzag types of CNTs for finding the different elastic constant. In specific, the crystals of CNT were involved. It brought clear findings on the dependency of various factors on the tube radius. This apparently revealed that the tube radius plays a significant role in deciding the bulk modulus, Poisson ratio, and elastic modulus. Even up to 38 GPa of bulk modulus was obtained by the SWCNT crystals with approximately 0.6 nm of radius.

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

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27.3.2 Multiwalled carbon nanotubes The construction of multiwalled carbon nanotubes (MWCNTs) can be achieved by composing a number of single-walled carbon nanotubes of a concentric nature. The van der Waals forces of weak nature hold these nanotubes. As more nanotubes are involved in the MWCNT, the analysis part through modeling becomes complicated. But the bending stiffness and tensile strength constant were found out by Ruoff and Lorents [20] for MWCNTs based on graphite’s elastic properties. The thermal expansion behavior of CNTs is different from the graphite as well as classical carbon fibers where CNTs possess isotropic thermal expansion and the other two possess anisotropic expansion. Nevertheless, CNT thermal conductivity is clearly anisotropic. Moreover, thermal conductivity’s magnitude for CNT is far better than other existing materials. If the parameter’s combination is more, the sensitivity of the elastic properties is low. The typical parameters are the radius of the tube, the quantity of layers, and chirality. But the same elastic properties become equal for all existing nanotubes within the MWCNT if the radius is more than 1.0 nm. It also was found that the influence of van der Waals at the interlayer is negligible toward shear stiffness and tensile strength. There is another mechanism called continuum mechanics that was used first by Govindjee and Sackman [21] to evaluate MWCNT properties. To confirm the validity of the above approach, they employed the Bernoulli Euler bending for obtaining Young’s modulus. The assumptions related to continuum mechanics must be carefully handled, as suggested by Govindjee and Sackman. The dependency of the explicit nature of the properties of materials prevailed over the size, if the assumption was a continuum (cross-section). Another model called elastic shell was employed in the analysis of the implication of van der Waals forces over the buckling (axial) for the double-walled carbon nanotube. The results of theses analyses revealed that there was no progress in the strain resulting from the critical axial buckling. This lead to the proposal of another model called the multiple (column) model. This model incorporates the radial displacements in between the layers that are connected via the van der Waals forces. The same model was utilized to reveal the consequences of the displacement in between the layers on the column buckling. The findings of the above analysis disclosed that displacement in between the layers is of negligible influence until the van der Waals forces become strong [7].

27.4

Composites made of carbon nanotubes

The characterization of CNTs through experiments provided results with differences. But when it was all put together, the experimental as well as theoretical findings disclosed the extraordinary properties of CNTs. These qualities in terms of properties lifted the CNTs to the next level from the research stage to the application platform, and the same was tried in the CNT-based composites. There were several attempts made to produce polymer composites with CNTs as reinforcements. But typical experiments were done on the metal as well as ceramic-based composites with CNT combinations as a reinforcement medium [7].

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Nanocarbon and its Composites

The accomplishment of CNT-reinforced metal matrix composites (MMC) can be done through various methods, as mentioned in the various studies. Liquid-based metallurgy is one of these solutions for producing MMCs. The process called melting eases the mass-scale manufacturing of MMCs through classical casting techniques. Problems exist in this process as dispersion with homogeneity was a challenge and difficult to achieve. The other issue associated with this process, is the creation of harmful products in between the face. The wetting condition also hinders MMC manufacturing through this method. The process called melting has its own subcategories, namely laser deposition, melt stirring, and melt infiltration [22]. Similarly, melt deposits with a disintegration process are fruitful in producing CNT-based MMCs and the same can achieve excellent mechanical properties [23]. Thermal spraying is another distinct technique in the process of manufacturing MMCs with wide applications such as engines for automobiles, turbine blades for aerospace purposes, electronic equipment, and prostheses for orthopedic usage. The coating uses materials in the form of fibers, CNTs, particulates, etc., as mentioned by Bakshi et al. [24]. The CNTs as reinforcements in the drops of powdered aluminum were treated through plasma spraying over the surface of the substrate. The processing techniques do not end here as powder metallurgy is also an option for producing MMCs. In specific, it is the least costly technique. The simplicity and flexible nature of the process make it economical among the contemporary processes. Here, CNTs are dispersed with metal powders for blending that can be achieved through a mechanism called milling. Subsequently, different processes come into the picture, namely isostatic pressing (cold), sintering with plasma spark, isostatic or normal pressing (hot), and compacting as well as sintering. The above processes are kept as the main processes. There are secondary processes that are required for achieving the mechanical deformation. These secondary treatments or processes are mostly executed in the hot environment. Typical processes are rolling (hot), extrusion (hot), and forging (hot). The final output of these hot processes is in the well-consolidated form with a densely packed nature. In general, it cannot be concluded that all processes can achieve the higher mechanical properties. This phenomenon results from the problem called agglomeration that can prevail in the metal matrix reinforced with CNTs. This agglomeration leads to the creation of phases with a harmful nature, which consequently affects the load transfer due to insufficient properties. The high temperature environment creates the above-specified problem. But the reactivity is less for the magnesium composites in the comparison of metals, for example, aluminum [2, 23, 25]. The tendency of agglomeration makes the dispersion as well as incorporation of CNTs difficult in the MMCs. But this negativity can be solved to some extent by involving the manufacturing methods, namely thermal spraying, powder metallurgy, and melting processes [3, 4, 10–32]. The mechanical properties cannot be increased beyond certain levels through the powder metallurgy process in specific cases as a result of agglomeration. Similarly, formed aluminum carbides of a harmful nature cannot always be restricted due to the high temperature environment [2, 25, 26]. For eliminating this problem, a new

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

817

measure was executed that ensures good dispersion and proper alignment of the CNTs in the matrix of MMCs. The reason behind the achievement of good dispersion and proper alignment is the mixture’s rheology and maintenance of process temperature at alower controlled level that is below 200°C. This was experimented with successfully in the polymer matrix composites with CNTs as reinforcements. For example, a polyvinyl alcohol (PVA) matrix with the combination of CNTs as reinforcement materials provided welldispersed and properly aligned composites [27]. A typical example of synthesis flow of a MWCNT-reinforced composite is shown in Fig. 27.7.

Fig. 27.7 Synthesis flow of the Mg-PVA/MWCNTs composites [23].

818

27.5

Nanocarbon and its Composites

Techniques related to the fabrication of CNT-based composites

Aluminum-based MMCs with CNT as reinforcement can be possible by different manufacturing processes, such as sintering (plasma spark), ball milling, and friction stir methods. These methods provide the composites with excellent strength as well as stiffness without compromising the light weight [1, 2]. Typical techniques are discussed below.

27.5.1 Friction stiring The concept behind this method lies behind the rotating tool of a nonconsumption nature and the same is pierced inside the workpiece. Rotational motion generates friction between the tool and job. This friction is the prime reason for heat build-up. The plastic deformation of the material is the consequence of the above occurrences which soften the material and started flowing around the nonconsumable tool. The above proceedings promote good dispersion of the material. Meanwhile, the solid state of the materials prevails that inhibits the formation of deformities [28]. The previous research mentioned that there was an increment in HV hardness through the friction stir technique [1, 29].

27.5.2 Plasma-assisted spark sintering The densification of CNT up to 1% in terms of volume and the relative density of aluminum powder until the 96.8 percentage is achievable through the sintering method. The improvisation in the tensile strength is the reason behind the curtailment of dislocation movements. In other words, tensile strength restricts the plastic deformation whenever experiences stress that is generated while controlling the layer especially at the boundaries [30].

27.5.3 Dispersion technique via spreading The layer formation can be achieved through this technique by laying the number of sheets followed by pressing and rolling, which achieves the bonding in between each layer in the formation of a single layer. This technique increased the tensile strength up to 66%. Meanwhile, the grain structure was refined to around 20 nm. This enhancement in the properties will have an impact on the even distribution of CNTs in the absence of porosity or clusters. Moreover, the above steps enhance the bondage between CNTs and aluminum. Finally, the good properties of graphite can be retained [31].

27.5.4 Mechanical stirring and casting technique This technique starts with the melting of aluminum in the form of a bar. The melting step is achieved through employing a furnace called a muffle. The MWCNT powder in the purified form is dispersed once the required melting of the aluminum bar is

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

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achieved. The dispersion starts mixing with the molten aluminum with the aid of a stirrer. Finally, the casting step begins where the molten aluminum, including the well-mixed MWCNT powder, gets a specific shape by pouring the mixture inside the side, followed by solidification. This technique reduced the tensile strength marginally in comparison to the CNT’s presence with lower quantity. The process is economical for preparing CNT-based MMCs. However, the uniform dispersion of CNTs cannot be accomplished in the complete matrix region. The reason behind the above problem is the uneven flow of the aluminum CNT mixture in the molten form while pouring inside the die. In addition to that, the stirring operation is unable to accomplish its purpose as solidification begins that makes the stirring impractical in the solid form [1].

27.5.5 Milling technique This milling mechanism converts the raw material into a smooth powder form with the aid of a number of small balls that mill the material through collisions among those balls [32]. This conversion into a powder form naturally helps in the even distribution of CNTs in the matrix medium. As a consequence of milling, the damage related to the structure as well as the morphology is minimized [32]. This powder obtained from milling is kept in an inactive environment before being applied in the process called hot extrusion that protects the powder from both oxidation and burning. This milled material is placed in the heated mold with a suitable temperature for extrusion. At the end, the required shape is obtained by extruding the material [1]. The best results in terms of performance on the mechanical ground can be possible through the even dispersion of the CNTs. Excellent bonding between the interlayers is also possible through good dispersion that assists in improving the load transfer from the aluminum matrix to the respective CNTs. The agglomeration phenomenon exists in the process due to the van der Waal forces [33]. However, the economical nature of the process and the easy access to the raw material make this technique a favorite [34]. Many studies have acknowledged the potential benefits of the ball milling process, namely homogeneous MMCs with uniform dispersion of CNTs and a smooth microstructure [33–35]. The good dispersion additionally prevents the powder from severe milling [36]. The possibilities of damage are also present by the aluminum powder on the CNTs during the milling operation [34]. Despite the criticality, the ball milling technique can be considered an excellent solution in handling agglomeration [35].

27.6

Factors and response analysis related to CNTreinforced aluminum-based metal matrix composites

27.6.1 Time consumed for milling The duration of milling is one of the prime influencing factors that affects the morphology of aluminum powder. The greater the milling time, the larger the surface area and the greater the generation of Al2O3. Consequently the properties will vary by an

Nanocarbon and its Composites

Relative increment (%)

820

50 45 40 35 30 25 20 15 10 5 0

Yield strength Ultimate tensile strength

4

6

10 8 Ball milling time (h)

12

Fig 27.8 Relative increments of ultimate tensile strength and yield strength of CNT/Al composites relative to the Al matrix [1, 32].

increment in the hardness level and a decrement in the ductility [37]. The considerable increment in the milling time can even double the modulus of elasticity and hardness at the nano level in comparison to aluminum in the pure form, as stated by M. Raviathul Basariya et al. The influence of the milling time can be explained graphically, as shown in Fig. 27.8. Similar improvements as far as mechanical performance was concerned were mentioned by Yoshida et al. The increment of around 285% was achieved in the performance by keeping the milling process throughout the day just short of 4 h [33]. The duration of 6 h was minimum enough to disperse CNTs in the aluminum matrix. Beyond that, the damage in the CNTs was the output. The tensile as well as yield strength improves when the duration of ball milling is higher. But elongation is a matter of concern that increases initially and shows a decrement [32].

27.6.2 Quantity of CNT The presence of CNTs in terms of amount or quantity inside the composite has significant implications in deciding the properties of the respective composites. Generally, the presence of CNTs in the composite is mentioned by the weight. The strength as well as modulus of elasticity has a direct linkage with the CNT’s presence. But the quantity of CNT cannot be increased beyond the saturation point, as mentioned by A.M.K. Esawi et al. A typical influence of the CNT percentage on the tensile strength is represented graphically in Fig. 27.9.

27.6.3 Microhardness The inclusion of nanotubes in the matrix medium is one of the deciding factors for the hardness of the composites. The marginal inclusion also improves the hardness and the same is realized through a Vickers hardness test. In comparison to aluminum

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

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300 Tensile strength (MPa)

Measured value

250 200 150 100 50 0 Pure

0.5

1 CNT content (wt%)

2

5

Fig 27.9 The effect of CNT content on the tensile strengths of the investigated composites [1, 32].

in the pure form, this small inclusion enhanced the hardness up to 300% [1]. The increment in the hardness contributes a lot in the process of strengthening the composite. This prevents the dislocation. Strain in the lattice structure can be controlled. It can achieve the work hardening during the milling process itself. The probability of agglomeration is less [37]. M. Raviathul Basariya et al. have found the importance of the microhardness through their research. They involved the milling time to analyze the influence of microhardness on the duration of milling. The outcome of the findings revealed that the hardness shows an increasing trend in line with the increase in the duration of milling, as shown in Fig. 27.10. There was considerable increment in the hardness in the initial period of milling. The work hardening phenomenon decreased the rate of increment of the hardness at the longer run. In addition to an increment in

Fig 27.10 Effect on hardness of unreinforced EN AW6082 and composite powders with increasing milling time [1, 37].

Microhardness (HV0.01)

500 400

EN AW6082 EN AW6082 MWCNT

300 200 100 0

10

20

30 40 Milling time (h)

50

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Nanocarbon and its Composites

the hardness, the even dispersion of CNTs in the matrix due to high milling time prevents the deformation of the matrix, which enhances the strength of the composite, as stated by Bustamante et al. [38, 39].

27.6.4 Friction and wear tendency of aluminium/CNT composites The wear property was researched by Esawi et al. [40] through differing the quantity of CNT in the range of 0%–5% in terms of weight. The respective specimens were analyzed by being subjected to various loads and sliding speeds. As the load increased, the wear rate showed the increasing trend. But the coefficient of friction was in a downward trend. The CNTs that were not embedded with matrix, lower the wearing surface against the surface of rubbing. The wear characteristics enhanced a lot as a consequence of the above occurrence. A typical graph of wear rate against the quantity of CNT percentage in terms of weight is shown in Fig. 27.11. A fish bone representation (Fig. 27.12) provides wide knowledge on the possibilities through which wear characteristics, friction tendency, and mechanical properties can be boosted.

27.7

Factors and response analysis related to CNTreinforced copper-based metal matrix composites

27.7.1 Implication of CNT percentage with the relative density of the MMC The implication of CNT percentage with the relative density of the MMC is shown in Fig. 27.13. As the CNT content increases in the copper matrix medium, the relative density showed initial fluctuations and followed the lower percentage with less variation in the higher side of the CNT content in the matrix of the composite. 60

Wear rate (mg/km)

50 40 30 20 10 0

Pure aluminium

1% wt CNT

Fig 27.11 Wear rate versus CNT wt% [1, 40].

2.5% wt CNT

5% wt CNT

Inert atmosphere

1% Stearic acid Toluene 1% Ethanol + Stearic Acid Powder to ball ratio(8:1, 10:1, 5:1)

Methanol

Argon

Improvement in mechanical properties and friction and wear behaviour

CNT/Al composite fabricated by ball milling process

4-8 nm. Multi-walled CNTs Internal diameter CNT Aspect ratio

5% by weight 40 nm. Outside diameter

Single-walled CNTs

2% by weight

140 nm outside diameter

Type of CNT

Diameter of CNTs

CNT content by wt.

823

Fig. 27.12 Fish bone diagram illustrating the parameters that improve the mechanical properties and friction and wear behavior of the aluminum matrix composite [1].

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

Process controlling reagent

Milling time (30 min-50 h)

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Nanocarbon and its Composites

99 Relative density (%)

98 97 96 95 94 93 92 91 90

0

0.02

0.04

0.06 0.08 0.1 CNT volume fraction

0.12

0.14

Fig. 27.13 Effect of CNT content on the relative density of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

27.7.2 Implication of CNT percentage with the hardness (Hv) of the MMC The implication of CNT percentage with the hardness (Hv) of the MMC is shown in Fig. 27.14. As the CNT content increases inside the copper matrix medium, the hardness of the MMC showed the drastic progress, and the respective influence of the CNT content is very much evident here.

190

Hardness (Hv)

185 180 175 170 165 160 155

0

0.02

0.04

0.06 0.08 0.1 CNT volume fraction

0.12

0.14

Fig. 27.14 Effect of CNT content on the hardness (Hv) of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

825

33 32 Grain size (mm)

31 30 29 28 27 26 25 24 0

0.02

0.04

0.06 0.08 CNT volume fraction

0.1

0.12

0.14

Fig. 27.15 Effect of CNT content on the grain size of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

27.7.3 Implication of CNT percentage with the grain size of the MMC The implication of CNT percentage with the grain size of the MMC is shown in Fig. 27.15. As the CNT content increases in the copper matrix medium, the grain size showed the deep initial fall, followed by minor fluctuations and finally the progress in the size of the grains in the composite.

27.7.4 Implication of CNT percentage with the strain-hardening exponent of the MMC The implication of CNT percentage with the strain-hardening exponent of the MMC is shown in Fig. 27.16. As the CNT content increases in the copper matrix medium, the exponent showed drastic progress, and the influence of the CNT content is very much evident here as far as the strain-hardening exponent of the composites is concerned.

27.7.5 Implication of CNT percentage with the Young’s modulus of the MMC The implication of CNT percentage with the Young’s modulus of the MMC is shown in Fig. 27.17. As the CNT content increases in the copper matrix medium, the Young’s modulus of the composite increases with a progressive trend. The influence of the CNT content is very much evident here and improves the mechanical property of the copper matrix-based composites.

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Strain hardening exponent, n

0.38 0.35 0.32 0.29 0.26 0.23 0.2

0

0.02

0.04

0.06 0.08 0.1 CNT volume fraction

0.12

0.14

Fig. 27.16 Effect of CNT content on the strain-hardening exponent of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

Young’s modulus (GPa)

12.5 11.5 10.5 9.5 8.5 7.5 6.5

0

0.02

0.04

0.06 0.08 CNT volume fraction

0.1

0.12

0.14

Fig. 27.17 Effect of CNT content on the Young’s modulus of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

27.7.6 Implication of CNT percentage with the yield (0.2% proof ) strength of the MMC The implication of CNT percentage with the yield (0.2% proof ) strength of the MMC is shown in Fig. 27.18. As the CNT content increases in the copper matrix medium, the yield (0.2% proof ) strength of the composite increases with a progressive trend

Yield (0.2% proof) strength (MPa)

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

827

195 190 185 180 175 170 165 160 155 150 0

0.02

0.04

0.06 0.08 0.1 CNT volume fraction

0.12

0.14

Fig. 27.18 Effect of CNT content on the yield (0.2% proof ) strength of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

similar to the Young’s modulus performance. The influence of the CNT content is very much evident here and improves the mechanical property of the copper matrix-based composites.

27.8

Conclusion

The extraordinary physical as well as mechanical properties of CNTs along with the light weight provide an excellent platform for carbon as an outstanding candidate for composite reinforcement. A broad spectrum of knowledge on CNT-reinforced composites can be acquired through analyzing the combination of thermal and mechanical attitudes of respective composites. However, the analysis and research are taking place at the scale of the nanometer. Because the reinforcement size is in the nano level, the approach toward analysis changes and fresh challenges come into the picture. Those challenges are in terms of characterization, methodology to be adopted, measurement techniques, fracture analysis, etc. Moreover, the atomic level synergy makes it compulsory to find out new mechanisms for conducting analysis. The benefits and challenges prevail simultaneously, for which the latter must be handled effectively to harvest the positivity of the CNTs in applying for composites. The entire route starts with an economic and efficient manufacturing method followed by incorporation and analysis. The quantity of CNT present inside the composite, the duration of milling, and the dispersion quality of the CNT in the matrix are some of the crucial and critical points to be observed for achieving optimum results through the performance of the composites. The same would enhance the strength of the composites.

828

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829

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[40] Bastwros MMH, Esawi AMK, Wifi A. Friction and wear behaviour of Al–CNT composites. Wear 2013;307:164–73.

Further reading [41] Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A. Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties. Mater Sci Eng A 2011;528:6727–32.

Index Note: Page numbers followed by f indicate figures, t indicate tables, and s indicate schemes. A Acoustic insulation, carbon foams, 69–70 Acrylic resin (AR) nanofibers, for VOC removal, 411–412 Activated carbon (AC), 712–713 adsorption of nitrogen oxide, 391–392 aerogel of, 16–17 nanofibers, for VOC removal, 386 Pd-Ni nanomaterials, synthesis of, 28 as photocatalysts, 526 water purification, 713 ADA. See Areal density of aggregates (ADA) Adsorbents activated carbon, nitrogen oxide adsorption, 391–392 aerogel, for wastewater remediation, 11–12 cabanccous-based, 632–633 carbon foams gas, 66–68 liquid, 68–69 volatile organic compounds removal, 402 Adsorption, 11, 713 hydrogen, 504–505, 505f K-ion, 369 of nitrogen oxide, 391–392 of pollutants, 526–527, 532–533, 539, 545–547 volatile organic compounds removal, 422 wastewater remediation, 12–16 Aerogels, 735, 740, 757 carbon nanotubes, 3–7, 4f, 6f for energy storage applications, 7–10 graphene, 3–7, 5f, 762–763 with adjustable density, 765–767 graphene oxide, 759–761 nanodiamond based, 7, 8f polymeric infiltration of, 764–765 potential applications, 763–764 as sensors, 16–19 supercritical drying, 2

types, 3–7 wastewater remediation adsorbents, 11–12 photocatalyst, 12–16, 14–16f AgBr/GN aerogel photocatalyst, 14–15 Agglomeration, homogeneous distribution vs., 183–189 Agilent-E8364B, 220 Ag/rGO/epoxy, 438 Ag/SnO2/graphene nanocomposite, 405–409 Air pollutants, 383–384, 401–402 Allotropes, 172 Alternating current conductivity, 223–224 Aluminium-graphene metal matrix composites (MMC), 156–158, 157f Aluminum flakes, 157 Ammonia boranes (ABs), 615–617 Ammonia gas sensors optical gas sensor characteristics, 243–249 Fabry–Perot interferometer optical fiber, 243, 245f molecular imprint, 247–249 optical spectroscopy, 242–243 Raman studies, 241–242 shield technology, 249–253 surface morphology, 240–241 synthesis, 239–240 Amorphous carbon (a-C), 7, 8f, 173, 494 Amorphous nanocarbon, 239 Amphoteric ion exchanger, 631 Anion exchanger reaction, 631 Anthradithiophene, 634, 637f Antimicrobial activity, of electrospun polymer/CNTs scaffolds, 105 Antimony sulfide (Sb2S3), 547–548 Aramid fibers. See Polyaramid fibers Areal density of aggregates (ADA), 185–186 Armchair, atomic structure, 808f Armchair-edged nanographenes (A-NGs), 630–631, 630f, 644f

832

Armchair nanotube, Stone–Wales transformation occurring in, 809, 809f Arrhenius plot, dimethylamine borane dehydrogenation Ru@GO NPs, 620–621, 621f Ru/PVP@C NPs, 606, 607f Arynes, cyclotrimerization of, 635, 637f Asymmetric supercapacitors, 123 Atomic force microscope (AFM), 810–812 Atomic origin, of magnetic moments, 195 B Benzannulation, 634, 635f hexabezocoronene derivatives, 634, 636f poly (phenylene ethynylene), 634, 636f Bernoulli–Euler theory, 812 β-tricalcium phosphate (β-TCP)/CNF membranes, 110, 110f Bianisotropic metamaterials, 255–257 Biochar in agriculture, 289 biowaste-based charred carbon, 289–291 environmental applications, 289–291, 290f nanocarbon effects, on plant growth, 291–292 BiOI/rGO/Bi2S3 system, 560, 560f Biosensor, 74–75 Biowaste-based charred carbon, 289–291 Bis(dimethylamino)methane (BMAM), 196–197 Bismuth sulfide (Bi2S3), 547 Bisphenol A, 435–436 Bituminous coal (BC), 590, 593f, 595f Blowing method, 44–45 Bond frustration/strain, 233 Buckyball (C60), 172 Bychkov–Rashba (BR) coupling, 238 C Cadmium sulfide (CdS), 542–545, 544–545f, 546t, 570 Carbamazepine (CBZ), 14, 14f, 714 Carbonaceous soot elements, 239 Carbon arc discharge, 494, 495f Carbon-based fertilizers, 293–294, 305f Carbon-based foams as adsorbents gas, 66–68

Index

liquid, 68–69 composite, 54–59, 55–56t doped, 52–53 for energy storage electrochemical, 59–63 thermal, 64–66 for insulation acoustic, 69–70 thermal, 71–73 from nanostructured carbons, 46–51 from polymer precursors blowing method, 44–45 template method, 45 for sensor applications electrochemical, 74–76 electromechanical, 76–79 Carbon blacks (CBs), 451–452, 461 Carbon–carbon bonding, 172 Carbon dots (CDs), 651–652 bioimaging, 663–664 biosensing, 670–671 cancer therapy, 664–667 catalytic mechanism, 671–672 drug delivery, 667 for energy conversion, 671–672 gene transfection, 667 sensing process, 667–671 synthesis process, 652, 653–654f bottom-up approach, 655–661 top-down approach, 652–655 Carbon fiber foams (CFF), 46–47, 46f Carbon fiber/poly (arylene ether nitrile) (CF/PEN), 460 Carbon fibers (CF), 46, 68, 423–425, 450 Carbon fillers, 422, 432 Carbon foam/ordered mesoporous carbon (CF-OMC), 60 Carbon foams, 43–44 cellular structures, 69–70, 70f reticulated structures, 69–70, 70f for sensor applications, 73–79 Carbon foam/silica aerogel (CF-SiA) composites, 72 Carbon magnetism, 195 Carbon microcoils (CMCs), 482 Carbon nanoanions (CNOs), 293 Carbon nanocoil (CNC) electrical resistivity coil diameter, 481–482, 482f

Index

correlation between mechanical and electrical properties, 484 temperature dependence, 482–484, 483–484f resistivity measurement system, 475–476 spring constant, 472 elastic boundary determination, 479 vs. macroscopic spring theory, 479–480, 480f mechanical strength estimation, 480–481, 481f real-time measurement of tensile test, 478–479, 478f synthesis, 473–474 tensile fracture, 476–478, 477f tensile test system, 474–475 transmission electron microscopy, 473–474, 473f Carbon nanocomposites, ultrasonic treatment application in, 734–737 Carbon nanofibers (CNFs), 782–783, 787–789 based carbon foam, 46–47 electrospinning, 110–111, 110f nanocarbon materials for VOC removal, 411–412 physical constant of, 641, 642t Carbon nanoparticles (CNPs), 291–292, 456–457 Carbon nanostructures, 677, 678f Carbon nanotubes (CNTs), 172, 328, 425, 451, 532–536, 677, 760–761, 782–783, 807 aerogels, 3, 4f, 6f aluminum-based MMCs with, 818 applications, 334–335, 503–511 armchair type, 492, 492f based carbon foams, 47–49, 47f based composites, 333–334 carbon arc discharge, 331, 494, 495f characterization, 810–812, 815 chemical vapor deposition, 331, 496–499, 497–498f chiral angle, 491–492, 492f chiral vector, 490–491, 491f composites made of, 815–817, 817f Cu(II)Pc-MWCNT hybrid, 686–688, 687f densification of, 818

833

electrochemical biosensors, 510–511, 511–512f electrochemical supercapacitors, 506–508, 507–508f electronic properties, 500 electrospinning, with natural and synthetic polymers, 100–105 fabrication of CNT-based composites dispersion technique via spreading, 818 friction stiring, 818 mechanical stirring and casting technique, 818–819 milling technique, 819 plasma-assisted spark sintering, 818 functionalization, 685–686 in GF/epoxy composite, 458 and graphene aerogels composite, 4–7, 6f growth mechanisms, 493–499 for hydrogen storage, 504–506, 505f laser ablation technique, 331, 494–496, 495f laser vaporization technique, 494–496 manipulation, 500–503 mechanical properties, 500 multiwalled, 328, 328f, 490–491, 493–494 nanocomposites, field emission from, 509–510, 509f Pc-SWCNT synthesis, 688, 688f photocatalytic degradation of nitrogen oxide, 386–391 physical constant of, 641, 642t π-π stacking, 331 properties, 329, 331–333, 332t, 500 purification, 499–500 reinforced aluminum-based metal matrix composites friction and wear tendency of aluminium/CNT composites, 822, 822–823f microhardness, 820–822, 821f quantity of CNT, 820, 821f time consumed for milling, 819–820, 820f reinforced copper-based metal matrix composites implication of CNT, 822–827, 824–827f single-walled, 328, 328f, 490–491, 494 structures, 490–493, 678–679, 679f synthesis, 331, 493–499, 809–810, 810f

834

Carbon nanotubes (CNTs) (Continued) terahertz detection, 335 thermal and rheological properties of suspensions, 752–753 thermal conductivity, 679 thermal expansion behavior of, 815 in tissue engineering, 95–96, 95f types, 712, 712f van der Waals forces, 331 water purification adsorption, 713–717 desalination, 721–723 disinfection, 724–727 photocatalytic degradation, 717–720 sensing and monitoring, 727 zig-zag type, 492, 492f ZnPc hybrids, 689f, 690, 692f ZnPc-SWCNT, 686, 686f Carbon nanotubes (CNTs)-TiO2 hybrids photocatalytic applications, 533, 534–535t structure, 532–533, 533f Carbon quantum dots, 16 Carboxymethylcellulose binder, 4–5, 6f Carrier-mediated transport (CMT), 314–315 Cassiterite, 364 Catharanthus roseus, MWCNTs-FITC uptake, 310f Cation exchanger reaction, 631 Cavitation-induced sonochemistry, 739 Cellulose aerogels, 11–12 Cellulose nanofibril, 9 Cementitious composites, 781–782 dispersion issue of NCMs in, 784–785 graphene nanoplatelets (GNP) in, 792–793 CF. See Carbon fibers (CF) CF/poly(ε-caprolactone)/epoxy system, 440–441 Chalcogenides-nanocarbon, 542–548, 549t Charcoal (C), 173–174 Charcoal-carbon nanotubes (CNTs), 712–713 Charge-transfer mechanism, 270–271 Chemical bath deposition method, 405–409 Chemically modified graphene (CMG), 129 Chemical vapor deposition (CVD), 453, 457, 636–638, 712–713 carbon nanotubes, 496–499, 497–498f copper- and nickel-based graphene nanocomposites, 162, 162f

Index

graphene foam, 49 heteroatom doping synthesis, 639–640 Chloride ions, filters for, 795f C80H30 nanographene, 638–639, 640f C120H110 nanographene, 638–639, 640f Citric acid (CA), 657 Clausius–Mossotti relation, 223 Cloverphene synthesis, 635–636, 638f CNFs. See Carbon nanofibers (CNFs) CNPs. See Carbon nanoparticles (CNPs) CNTs. See Carbon nanotubes (CNTs) Coaxial cylindrical model, 493 Cobalt mono carboxy phenoxy phthalocyanine, 696, 699f Co-doped mesoporous CF (Co-MCF), 53 Composites. See also specific types of composites ultrasonic treatment application in, 734–737 COMSOL Multiphysics, 256–257 Conduction band (CB), 386, 523–525 Conformal mapping technique, 814 Constrained formation of the fibrous nanostructure process (CoFFiN), 46, 46f Contorted dibenzotetrathienocoronene (c-DBTTC), 634 Contorted hexacatahexabenzocoronenes (c-HBCs), 634 Copper coating characteristics, 249 current–voltage characteristics, 250f, 251t electromagnetic wave interaction, 252f nanocarbon film deposited on, 249f Copper- and nickel-based graphene nanocomposites, 162–166, 162–166f Copper-graphene composites elastic modulus, 165 electrodeposition method, 164–165, 165f flow stresses for, 162–163, 164f grain size, 164–165 hardness, 165 microstructure of, 162–163, 163–164f stress vs. strain plots, 162–163, 164f Copper oxides (CuxO), 540–542 Copper phthalocyanines (CuPcR4), 688–689, 688f Cove-edged nanographenes (CNGs), 630–631, 630f Crack tip opening displacement (CTOD), 189

Index

Cu(II)Pc-MWCNT hybrid, 686–688, 687f Current–voltage measurements, 205–207, 205f CuxO-NC hybrids, 540, 543t CVD. See Chemical vapor deposition (CVD) Cyclodehydrogenation, 634, 635f Cyclotrimerization, of arynes, 635, 637f D Density functional theory (DFT), 358–359 Density of states (DOS), 358–359, 361, 363–364f Deposition method, 47f, 48–49 der Waals interactions, 182 Desalination, 721–723 DFT. See Density functional theory (DFT) Diamond, 172 elastic modulus, 500 nanoporous carbon structures, 373–374, 373f Diamond-like carbon (DLC), 173 Diels–Alder reaction, 681–682 Diglycidal ether of bisphenol A (DGBA), 435–436 Diglycidyl ether diphenolate n-butyl ester (DGEDP-Bu), 435–436 Dimethylamine-borane (DMAB), dehydrogenation of, 601–602, 602s, 602t catalyst, 615–617, 616t catalytic reaction for, 615–617, 616s Ru@GO NPs, 617–621 Ru/PVP@C nanocatalysts, 603–607 Direct current conductivity, 219–220, 249–250 Direct edge functionalization, 640 Disordered length, 233 Dispersion-agglomeration phenomena, 183 Dispersion technique, 430–431 Dispersive Raman spectra, 234f Doped carbon foams, 52–53 Doping process, 52 DOS. See Density of states (DOS) Double-walled carbon nanotubes (DWCNTs), 366–367, 496–497, 678–679, 679f Doxorubicin (DOX), 105, 108–109 DWCNT@SnO, 366 Dyakonov–Perel (DyP) theory, 195

835

Dynamical force constant, 235–236 Dynamic mechanical analysis (DMA), 435–436 E EDLCs. See Electric double-layer capacitors (EDLCs) E-glass, 450 Elastic deformation, carbon nanocoil, 479 Elastin-like polypeptides (ELPs), 54 Electrical circuit fabrication procedure, 475–476, 476f Electrical resistivity, carbon nanocoil coil diameter, 481–482, 482f correlation between mechanical and electrical properties, 484 temperature dependence, 482–484, 483–484f Electric double-layer capacitors (EDLCs), 123, 129–130, 507–508, 507f Electrochemical energy storage, carbon foams, 59–63 Electrochemical sensors biosensor, 74–75, 75t gas sensor, 75–76 humidity sensor, 74 molecule sensing, 75 Electrochemical supercapacitors, 506–508 Electrochemical transducers, 510–511 Electrodeposition method, 164–165, 165f Electromagnetic cloaking bianisotropic metamaterials, 255–257 ferro-nanocarbon split-ring resonators, 255–257, 263–269 Electromagnetic interference shielding (EMI) graphene-like nanocarbons characterizations, 217–222 efficient microwave absorbing properties, 225–229 measurements, 217–222 microwave parameter analysis, 222–225 multifunctional nanocarbons (see Multifunctional nanocarbons) polyurethane nanocomposites, 215–229 shielding parameters, 215–229 Electromagnetic shielding, 461–462 Electromechanical sensors pressure sensor, 76–78, 77f strain sensor, 78–79, 79f

836

Electron phonon coupling (EPC), 233–234 Electron spin resonance (ESR), 195–196, 199–202, 200f, 210f, 249–250 Electrophoretic deposition (EPD), 453 Electrosorption-assisted visible light photocatalytic degradation, 15, 15f Electrospinning process, 93–94, 109–110 of carbon nanofibers, 110–111 concept, 93 of graphene quantum dots, 111 polymer jet, 93 setup, 93–94, 94f Electrospun nanofiber, 99–100 Electrospun polymer/CNTs scaffolds antimicrobial activity of, 105 as drug loading vehicles, 105 Electrospun polymer/graphene scaffolds antibacterial properties of, 108 for drug delivery, 108–109 Electrospun polymeric nanomat carbon nanotubes, 100–105 graphene nanocarbons, 105–109 Elliott–Yafet mechanism, 195 EMI. See Electromagnetic interference shielding (EMI) Energy dispersive X-ray analysis (EDAX), 249 Energy storage amorphous nanoporous carbon, 356–357 carbon foams electrochemical, 59–63 thermal, 64–66 density functional theory, 358–359 electronic structure methods, 357–369 graphene, 341 graphene/VS4 interface, 361, 362f Li batteries, 355–356 core@shell composites, 371–373 polyoxometalates, 368 TiO2/SnO2, 361, 364f metal oxides, 357 MnO2/graphene interface, 358–359 molecular dynamics simulations, 358 MoO3/RGO, 359–360 nanocarbon aerogels for, 7–10 nanostructured carbons, 356 supercapacitors, 355–356 nanocomposites with transition metal oxides for, 357–369

Index

RuO2 metal oxides on, 358 theoretical approaches, 369–374 Enzymes, 510 Epoxy adhesives, 436–437 compositions, nanomodified, 770–771 matrix, nanographene particles in, 641, 643f mechanical properties, 186t, 187f resins, 422 TGA curves, 193f thermal parameters, 194t Epoxy nanocomposites applications, 436–439 effective thermal conductivity, 435 electrical percolation threshold, 434 electrical properties, 434 graphene, 433–434 mechanical properties, 433 surfactant, 430 thermal properties, 435 using nanocarbon fillers, 430–432 Etched N-doped CF (ENCF), 57, 58f European Environmental Agency (EEA), 385 Expanded graphene nanoplatelets (xGNPs), 457–458 Expanded graphite, 182 Explosion, 738, 738f Eyring plot, dimethylamine borane dehydrogenation Ru@GO NPs, 620–621, 621f Ru/PVP@C NPs, 606, 607f

F Fabrication carbon nanotube aerogels, 3, 4f graphene aerogels, 3–4, 5f Fabry–Perot interferometer optical fiber NH3 gas sensors, 243, 245f Fe-doped carbon foams, 53 Fe3O4/CuO/ZnO composite, 644–645 Fermi level, 207, 370–373, 509 Fermi velocity, 233–234 Ferro-nanocarbon (FNC), 253 Ferro-nanocarbon split-ring resonators (FNC SRRs) bianisotropic metamaterials, 255–257

Index

computational electromagnetic, 256–257 configurations, 256f dielectric response, 261 log-log plots, 261f magnetization analysis, 261–263 magnetization hysteresis curves, 262f microwave measurements, 257 modeling and simulation constitutive parameters, 264–265 microwave scattering, 264–265 Nicolson–Ross–Weir formulism, 265–266 retrieval techniques, 267–269 molecular characteristics, 260 morphological analysis, 257–263 preparation, 256 production scheme, 255f Raman analysis, 260, 260f Fiber-reinforced polymer composite (FRP), 333 Field emission, from CNT-based nanocomposites, 509–510, 509f Filamentous carbon, growth models, 498–499, 499f Flexible supercapacitor electrodes (FSCs), 136–140 Fluorescent CDs, 651–652, 667–668 FNC SRRs. See Ferro-nanocarbon split-ring resonators (FNC SRRs) Focused ion beam (FIB) technique, 474–475 Fossil fuels, 504–505 Fourier transform infrared spectroscopy (FTIR), 175, 196–197, 197f, 218f Fowler–Nordheim (F-N) theory, 509, 509f Fractography, 190–192 Fracture toughness, 188–189 Freezing method, 48 Friedele Crafts acylation process, 104 Frozen smoke. See Aerogels FTIR. See Fourier transform infrared spectroscopy (FTIR) Fullerenes, 287–288, 328, 343, 425, 489–490, 528–532, 677–678 isomers of, 678f and phthalocyanine hybrids application, 682, 685t Pc-C60 system, 681–682 silicon naphthalocyanine, 682, 683f synthesis and characterization, 682 in tissue engineering, 98–99

837

Fullerene-TiO2 hybrids applications, 528–529, 530–531t band gap energy, 529 nanocarbon content, 529–532 in photocatalytic degradation of water pollutants, 532 photoluminescence, 529 photoreaction mechanism, 528, 529f Functionalized graphene oxide nafion nanocomposite (F-GO/nafion) membrane, 632–633

G Gas adsorbents, carbon foams, 66–68 Gas sensors, 75–76. See also Ammonia gas sensors GCFAs. See Graphene/carbon fiber aerogels (GCFAs) Gemini-type amphiphilic hexathienocoronene (HTCGemini), 635, 637f GF/poly-vinyl-ester-epoxy (PVEE), 459 GN. See Graphene (GN) GNCs. See Graphene-like nanocarbons (GNCs) GNP/CB/epoxy, 426–428 GNP/CB/MWCNTs/epoxy, 426–428 GNP/CNT hybrid fillers, 441 GNPs. See Graphene nanoplatelets (GNPs) GO. See Graphene oxide (GO) GO-doped poly(lactic-coglycolic acid) (PLGA), 108 GOEA. See Graphene oxide-epoxy composite aerogel (GOEA) GO-grafted PEG (GO-g-PEG), 108 Graphene (GN), 195, 328, 425, 536–538, 679 aerogels, 3–4, 5f, 341, 762–763 with adjustable density, 765–767 applications, 341–345, 342f, 680 based binder-free supercapacitor electrodes, 140 based composites, 340 biomedical applications, 344, 344f bottom-up approach, 337, 338f carbon allotropes, building block of, 329, 330f carbon foam, 49–51, 50f

838

Graphene (GN) (Continued) self-assembly synthesis, 49–50, 50f 3D printing, 50–51, 51f conductive polymer-G nanocomposite supercapacitors poly(3,4-ethylenedioxythiophene), 134–136, 135f polyaniline, 132–133 polypyrrole, 130–132, 132f in supercapacitor applications, 136, 137–138t covalent functionalization, 697–698, 700f electrospinning with natural and synthetic polymers, 106–109 electrospun polymeric nanomat, 105–109 flexible supercapacitor electrodes, 136–140 GQD-FePc compounds, 699, 702f magnetization in, 203–205 metal matrix composites aluminium-graphene, 156–158, 157f copper- and nickel-based, 162–166, 162–166f magnesium-graphene, 158–161, 159–161f nonconductive polymer-G nanocomposite supercapacitors electrolyte, effect of, 129 poly(vinylidene difluoride), 124–126, 125f polytetrafluoroethylene, 126–128 for optical bioimaging, 345 in organic solar cells, 343 oxidation-reduction method, 338 Pc-graphene hybrids and applications, 703t and Pc nanoconjugate, 696, 698f photocatalytic degradation of nitrogen oxide, 392–394 photovoltaic applications, 341, 343f properties, 155, 156t, 338–340, 340t quantum Hall effect, 340 reinforced composites, applications of, 155, 156t reinforcement, research works, 155, 155f self-curing effect of, 789, 790f spintronics, 195–199 structures, 680f synthesis, 337–338, 338f TDAE-treated, 203–205

Index

magnetic ordering, 204f tensile strength, 154–155 for terahertz applications, 344 3D architecture of, 762 in tissue engineering, 95f, 97–98, 97f top-down approach, 337, 338f ultrasonic effect in synthesis of hydrogels based on, 762 ultrasonic treatment in production of, 753–755 Young’s modulus, 154–155 ZnPc hybrids, 698, 702f Graphene bulbs, 343 Graphene/carbon fiber aerogels (GCFAs), 766 Graphene dispersions, sonication treatment of, 749–750 Graphene foam/polydimethylsiloxane (GF/ PDMS), 72–73 Graphene-like nanocarbons (GNCs) average size, 186t characterizations chemical analysis, 175–176 electron spectroscopy, 175–176, 175f FESEM, 178, 221f high-resolution transmission electron microscopy, 178 infrared spectroscopy, 175–176, 175f morphological studies, 178 Raman spectroscopy analysis, 176–178 scanning tunneling microscopy, 178–181 scanning tunneling spectroscopy, 178–181 dispersibility investigations, 183–189 as effective nanofiller, 183 electromagnetic interference shielding characterizations, 217–222 efficient microwave absorbing properties, 225–229 measurements, 217–222 microwave parameter analysis, 222–225 fitted Lorentzian for 2D peaks, 178f flexural properties, 188 fracture toughness properties, 188–189 homogeneous distribution vs. agglomeration, 183–189 interfacial adhesion, 183–189 I–V characteristics, 206–207, 206f

Index

mechanical properties, 186–189 nonmagnetic carbons electron spectroscopy, 197–199 magnetic correlation, 195–199 nitrogen doping chemical analysis, 197–199 spin transport, 195–199 optical imaging, 185–186 physical properties, 192–195 Raman mapping, 184 recorded C 1s spectra, 198f schematic representations, 174f, 229f shielding parameters characterizations, 217–222 efficient microwave absorbing properties, 225–229 measurements, 217–222 microwave parameter analysis, 222–225 softwood charcoal, 174f spin-bath properties anisotropy in g factor, 211–212 linewidth analysis, 210–211 spin transport parameters, 212–214 spin-spin relaxation time, 212f spin transport parameters, comparison, 201t synthesis, 173–174 TDAE-treated, 203–205 tensile properties, 186–188 thermal properties, 192–195 tunneling spectra, 181f vibrating sample magnetometry, 202, 202f Graphene nanoplatelets (GNPs), 426, 451, 792–793 Graphene nanoribbons (GNRs), 182, 633–634 Graphene nanosheet-PANi (GNS-PANi), 133 Graphene nanosheets, 157 Graphene oxide (GO), 3–4, 172–173, 329, 426, 590, 759–761, 782–783 applications, 98, 98f in cell adhesion and differentiation, 97–98 Graphene oxide-epoxy composite aerogel (GOEA), 766 Graphene-PANi nanocomposites, 132–133 Graphene platelets (GPL), 182 Graphene-poly(3,4-ethylenedioxythiophene) (PEDOT) nanocomposites, 134–136, 135f

839

Graphene-poly(vinylidene difluoride) (PVDF) nanocomposites, 124–126 Graphene-polypyrrole (PPy) nanocomposite, 130–132, 132f Graphene-polytetrafluoroethylene (PTFE) nanocomposites, 126–128, 128f Graphene quantum dots (GQDs), 111, 632–633 Graphene sheets/polydimethylsiloxane (GS/PDMS), 72–73 Graphene-WO3 hybrids, 542 Graphite, 172 fillers, 64, 65f hexagonal sheet of, 807f nanoplatelets, 182 nanoporous carbon structures, 373–374, 373f Graphitic foam, 45 Graphitized CNCs (G-CNCs), 473–474, 473f H Hall effect, 229–230 Hall–Petch relationship, 158 Heteroatom doping synthesis, 639 Heterogeneous catalysis, 522 Hexabenzotriphenylene, 635–636 Hexamethylenetetramine (HMT), 76 Hexanitro-metallophthalocyanines, 690–694, 693f Hexa-peri-hexabenzocoronene (HBC), 640–641 Hexathienocoronenes (HTCs) synthesis, 634, 637f High-resolution transmission electron microscopy (HRTEM) dimethylamine borane dehydrogenation Ru@GO NPs, 619, 619f graphene-like nanocarbons, 178 reduced graphene oxide, 236f tellurium-reduced graphene oxide, 236f Homogeneous distribution vs. agglomeration, 183–189 Hooke’s law, 475 HRTEM. See High-resolution transmission electron microscopy (HRTEM) Huisgen cycloadditions, 685–686 Humidity, 74 Humidity sensor, 74 Hummer’s method, 405–409, 536–538

840

Index

Hybrid sp2-sp3 phase carbon network, 173 Hydrocarbon-assisted selective catalytic reduction (HCSCR), 387–388 Hydrogen adsorption isotherms, of CNTs, 504–505, 505f Hydrogen gas, 601–602, 615–617 Hydrogen storage, for carbon nanotubes, 504–506, 505f Hydrophilic nanocarbons, 293 Hydroxyapatite (HAp) particles, 110–111 Hyperbranched polymers (HBP), 441

Liquid oligomers, ultrasonic dispersing of nanoparticles in, 748–749 Liquid-quench method, 357 Lithiation/dilithiation process, 368–369 Lithium–sulfur batteries, 63 Localized density of state (LDOS), 178–179 Lower unoccupied molecular orbital (LUMO), 243 Low-viscosity liquid, radiation of ultrasound energy into, 741–742, 741f

I

M

Implosion, 738, 738f Indium sulfide (In2S3), 547 Insulation, acoustic, 69–70 Interface polarization mechanism, 227, 228f Interfacial shear strength (IFSS), 423–424 Ion exchanger, 629–631 Ionic liquids (ILs), 129

Macroscopic spring theory, 479–480 Magnesium-graphene metal matrix composites (MMC) graphene platelets, 160–161, 160f liquid-state ultrasonic treating, 158–160 manufactureing procedure, 158, 159f microhardness, 161, 161f solid-state stirring, 158, 160 strengthening efficiency, 161, 161f Magneto-fluorescent CDs, 664 Maleic acid, 660, 660f Maxwell–Garnett theory, 437 Mechanical exfoliation, 172–173, 329, 451 Melamine sponge, 52, 52f Membrane technology, 721 16-Mercaptohexadecanoic acid (MHDA), 409–411 Mesocellular silica foam (MSF), 56–57 Metal-graphene interface, 162, 162f Metal matrix composites (MMC), 806, 816 aluminium-graphene, 156–158, 157f copper- and nickel-based, 162–166, 162–166f magnesium-graphene, 158–161, 159–161f Metal-organic frameworks (MOFs) applications, 548 hybrid materials, 550f metal oxides and porous metal oxide-carbon hybrid composites, 558f as photocatalysts, 548, 550f, 552–553t, 555–556 structural stability, factors controlling, 551, 551f Metal oxide (MO), 54, 357, 561–570 Metal oxide-NC-chalcogenide, 557–560 Microwave absorbing properties, 225–229

J Joint density functional theory (JDFT), 358 K Kevlar/CF/epoxy composite, 458–459 L Langmuir and Freundlich adsorption isotherm models, 721–723 Laser ablation technique, 494–496, 495f, 652–654 Laser photon energy, 235f Laser pruning, 501, 502f Latent thermal energy storage (LTES), 64 Lattice dislocation, 158 Left-handed material (LHM), 254 Li-ion battery, 61, 62f, 355–356 core@shell composites, 371–373 Li transport on cathodes for, 359 polyoxometalates, 368 TiO2/SnO2, 361, 364f Linear elastic fracture mechanics (LEFM), 188–189 Liquid adsorbents, carbon foams, 68–69 Liquid epoxy compositions, ultrasonic modification of classical, 742–744, 742f

Index

Microwave irradiation, 656 Microwave parameter analysis, 222–225 MLGs. See Multilayer graphenes (MLGs) MMC. See Metal matrix composites (MMC) MnO2/GN aerogel, 10 MOFs. See Metal-organic frameworks (MOFs) Molecular dynamics (MD) simulations, 194, 358, 375, 422–423 Molecular interactions reduced graphene oxide synthesis, 230–238 tellurium-rGO synthesis, 230–238 Molecule sensor, 75 Mott-VRH model, 477f, 483–484 Multicomponent epoxy systems, 439–441 Multifunctional nanocarbons optical gas sensor characteristics, 243–249 optical spectroscopy, 242–243 Raman studies, 241–242 shield technology, 249–253 surface morphology, 240–241 synthesis, 239–240 Multilayer graphenes (MLGs), 782–783 cementitious composites filled with, 795f Multiscale composites carbon-based nanofillers for, 451–452 manufacturing processes thermoplastic polymer composites, 454–457, 455t thermosetting polymer matrices, 452–454 nanocarbon-based mechanical properties, 457–459 multifunctional characteristics, 459–462 Multishaped graphene fragments, 195 Multiwalled carbon nanotubes (MWCNTs), 48, 182, 309–313, 328, 328f, 451, 490, 532, 712, 712f, 752, 806–807 chemical vapor deposition, 496–497, 498f coaxial cylindrical model, 493 field emitters, 510 gelatin nanofiber, 101 hydrogen adsorption capacity of, 506 mechanism, 815 negatively charged, 309–313 penetration ability, Catharanthus roseus, 309, 310f photocatalysis of NOx Pt particles, 388

841

TiO2, 388–389 physical properties, 332–333, 332t positively charged, 309–313 Raman spectrum of, 500, 501f seed germination, 309–313 silk fibroin mats, 100 stimulating effects for tomato plants, 309, 311f structures, 678–679, 679f surface functionalization, 309–313, 312f sword and sheath fracture behavior of, 811–812, 811f in tissue engineering, 95, 95f VOCs removal, 409–411 MWCNT/fiber/matrix composite, creep behavior of, 458 MWCNTs. See Multiwalled carbon nanotubes (MWCNTs) MWCNT@TiO2, 361–362, 371–372, 533–536

N Nanobiohybrid catalyst, 720, 720f Nanocarbon black (NCB) composite, 793, 793f Nanocarbon material (NCM), 782–784 dispersion issue of, 784–785 properties, 783t Nanocarbon–polyaniline (PANI) composites AC conductivity, 598, 599f applications, 590 bituminous coal, 590, 593f, 595f carbon crystallites, 592 CHNS analysis, 594, 594t dielectric characterization, 591–592, 597–598, 597–599f dielectric loss, 598, 598f electrical characterization, 596 energy gap, 592, 596, 596t Fourier transform infrared spectroscopy, 594–596, 595t, 595f frequency vs. imaginary part of complex permittivity, 597–598, 597f frequency vs. real part of complex permittivity, 597–598, 597f reduced bituminous coal, 590, 593f, 595f structural analysis, 592–594, 594t Tauc plot, 592, 596, 596t

842

Nanocarbon–polyaniline (PANI) composites (Continued) turbostratic structure, 592 X-ray analysis, 592, 593f Nanocarbons (NCs) from carbon to, 172–173 current–voltage characteristics, 250f, 251t deposition at low temperature, 240f dielectric response, 261 disordered 2D carbon networks, 173–181 efficient microwave absorbing properties, 225–229 electromagnetic interference shielding, 215–229, 238–253 electromagnetic wave interaction, 252f FESEM, 241f fracture mechanisms, 190–192 GNCs (see Graphene-like nanocarbons (GNCs)) history, 172–173 HOMO-LUMO scheme, 244f HRTEM, 241f, 259f magnetism (see Nonmagnetic carbons) magnetization analysis, 261–263, 262f microwave parameters, 222–225 morphological analysis, 257–263 NH3 gas sensors, 238–253 optical band structure, 242–243 polyurethane nanocomposites, 215–229 production scheme, 255f SEM, 258f sensor system, 247t structural engineering, nanocomposite approach for, 182–195 surface morphology, 240–241 UV-vis absorption spectra, 243f Nanocoatings, 734–735 Nanocomposites preparation, 751–752 TGA curves, 193f thermal parameters, 194t Nanocomposite supercapacitors poly(3,4-ethylenedioxythiophene), 134–136, 135f poly(vinylidene difluoride), 124–126, 125f polyaniline, 132–133 polypyrrole, 130–132, 132f polytetrafluoroethylene, 126–128

Index

Nanodiamonds (ND) aerogels, 7, 8f in tissue engineering, 99 Nanofiltration (NF), 721–723 Nanofluids, adherence of, 752 Nanographene (NGs), 630–631 armchair-edged, 630–631, 630f cove-edged, 630–631, 630f zigzag-edged, 630–631, 630f Nanographene composite ion exchanger applications, 642–645 direct edge functionalization, 640 nonconventional approaches, 634 properties, 641, 642t selective heteroatom doping, 639 structure incorporation, seven/ eight-membered rings, 636 synthesis, 633–641 Nanographene platelets (NGPs), 641, 642t Nanomodified epoxy compositions, 770–771 Nanomodified polymer composite materials (NM PCMs), 735–736 Nanomodified thermoplastic composite materials, 768–770 Nanoparticles, ultrasonic dispersing of, 746–750 Nanoporous graphene foams (NGF), 49–50 Nanosuspensions, ultrasonic treatment for preparation of, 750–753 Nanotechnology, 327–328 NCM. See Nanocarbon material (NCM) NCM-filled cementitious composites (NCMFCC), 783 application, 796–797 preparation, 783–785 properties, 794–796 electrical and self-sensing, 791–794 mechanical, 785–791, 786t, 788f N-doped carbon foams, 53 N-doped CD preparation, 656, 656f N-doped GF/CNT/MnO2 (NGF/CNT/MnO2), 59–60 n-GNCs. See Nitro-graphene-like nanocarbons (n-GNCs) NH3. See Ammonia gas sensors Nickel-graphene composites electrodeposition method, 165–166 flow stresses for, 164f hardness, 166

Index

microstructure of, 165–166, 166f stress vs. strain plots, 164f Young modulus, 166 Nicolson–Ross–Weir and retrieval techniques, 255, 265–266, 266f NiO@Ni-ZnO/RGO/CdS, 557–561 Nitrogen doping chemical analysis, 175–176, 197–199 exchange interactions, 208f reduced exchange correlations, 207–209 Nitrogen-added carbon foams (NCF), 52, 52f Nitrogen dioxide (NO2), 16–17, 384 Nitrogen-doped GN aerogels, 10 Nitrogen-doped macro/mesoporous carbon foams (N-MMCFs), 53 Nitrogen monoxide (NO), 384 Nitrogen oxide (NOx) adsorption of, 391–392 air pollution, 384, 384f components, 384 control, methods of, 385 fuel, 385 photocatalytic degradation, 385–386 carbon nanotubes in, 386–391 graphenes in, 392–394 prompt, 385 thermal, 385 Nitro-graphene-like nanocarbons (n-GNCs) chemical analysis, 197–199 comparison using FTIR, 196–197 electron spectroscopy, 197–199 vs. graphene-like nanocarbons, 196–197 I–V characteristics, 206–207, 206f preparations, 196 recorded C 1s spectra, 198f spin transport parameters, comparison, 201t vibrating sample magnetometry, 202, 202f NM PCMs. See Nanomodified polymer composite materials (NM PCMs) Nonmagnetic carbons electron spin resonance measurements, 199–202 graphene-like nanocarbons electron spectroscopy, 197–199 magnetic correlation, 195–199 nitrogen doping chemical analysis, 197–199 spin-bath properties, 209–214

843

spin transport, 195–199 graphene spintronics, 195–199 nitro-graphene-like nanocarbons comparison using FTIR, 196–197 preparations, 196 radical spin correlations, 199–202 vibrating sample magneto metric measurements current–voltage measurements, 205–207 nitrogen roles, 207–209 reduced exchange correlations, 207–209 Ruderman–Kittel–Kasuya–Yosida interactions, 203–205 transport characteristics, 205–207 Nylon-6/MWCNT fibers, 104 O Olfactory ensheathing glial cells (OEC), 102 Optical gas sensors, 239 ammonia gas sensors Fabry–Perot interferometer optical fiber, 243, 245f molecular imprint, 247–249 sensing mechanism, 246–247 sensor transfer function, 244–246 Optical micrography, 185–186, 185f Optical spectroscopy, 242–243 Organic solar cells (OSCs), 343 P Paclitaxel (PTX), 96 PAH. See Polycyclic aromatic hydrocarbon (PAH) PANI. See Polyaniline (PANI) Paraoxon photocatalytic degradation, 560–561, 561f PCMs. See Polymer composite materials (PCMs) PDMS. See Polydimethylsiloxane rubber SKT (PDMS) Pd-Ni @AC alloy structures, 29–30, 30f dimethylamine-borane, dehydrogenation of, 28, 32–34, 33f face-centered cubic structure, 30, 31f Gaussian–Lorentzian method, 31–32 high-resolution transmission electron microscopy, 29–30, 30f

844

Pd-Ni @AC (Continued) morphology, 29–30, 30f particle size, 29–30, 30f Raman spectroscopy, 30–31, 31f reusability examination, 29 sodium hydroxide-assisted reduction method, 28–29 synthesis, 28 transmission electron microscopy, 29–30, 30f UV–VIS, 29, 29f X-ray diffraction, 30, 31f X-ray photoelectron spectroscopy, 31–32, 32f zero oxidation state, 29, 29f PELA/CNT fibrous mat, 102 Pentaglycerine (PG), 64–65 Periodontal ligament cells (PDLCs), 101 Perovskite solar cells (PSCs), 343 Peroxyacyl nitrate (PAN), 384 Phase change materials (PCMs), 64 Phase separation, 47, 47f p-Phenylenediamine, 10 Phonons, 500 Photocatalysis, 12–14, 717–720 aerogels, 12–16, 14f nitrogen oxide degradation, 386 carbon nanotubes in, 386–391 graphenes in, 392–394 Photocorrosion, 544–545 Photo resistivity, 233–234 Phthalocyanine and CNTs hybrids, 694, 697t and fullerene hybrids application, 682, 685t Pc-C60 system, 681–682 silicon naphthalocyanine, 682, 683f synthesis and characterization, 682 and graphene hybrids, 696–702, 703t properties, 680–681 structure, 680, 680f Physisorption, 505 Plant growth biochar-derived nanocarbons on, 291–292 nanocarbons, effects of, 293–315, 295–304t Plasma-assisted spark sintering, 818 Plasma-enhanced CVD (PECVD), 496–497, 501

Index

PLLA/fullerene composite, 109 Pollutants, degradation of, 522–525, 523–524f Poly(3,4-ethylenedioxythiophene) (PEDOT), 134–136, 135f Poly (butylene adipate-co-terephthalate) (PBAT), 104 Poly (l-lactic acid-co-3-caprolactone) (PLCL), 101 Poly (lactic acid) (PLA), 101 Poly(p-phenylene terephtlamide) (PPTA), 450 Poly (styrene sulfonic acid) (PSS), 134 Poly(vinyl alcohol) (PVA), 139–140 Poly(vinylidene difluoride) (PVDF), 124–126, 125f Polyacrylonitrile (PAN), 110 Polyacrylonitrile (PAN)-based fibers, 450 Polyamide-6 (PA-6)/GF/MWCNT, 454 m-Poly aminobenzene sulfonic acid (PABSSWCNTs), 315–317 Polyaniline (PANI), 9, 132–133, 589 Polyaniline (PANI)/CNTs, 502, 503f Polyaniline/expanded graphene-oxide (PA/ EGO), 405–409 Polyaniline/polypyrrole/graphene oxide (PANI/PPy/GO), 405–409 Polyaramid fibers, 450 Polycaprolactone (PCL), 107 Polycyclic aromatic hydrocarbon (PAH), 633–634 Polycyclic carbon rings (PCR), 173–174 Polydimethylsiloxane (PDMS), 58–59 Polydimethylsiloxane rubber SKT (PDMS), 747–748 Poly(ether ether ketone) (PEEK)/GF/ SWCNT, 454 Poly (ethylene oxide)/graphene oxide (PEO/GO) composite membrane, 632–633 Polymer composite materials (PCMs), 734 classical, 736–737 nanomodified, 735–736 reinforced thermoplastic, 735 ultrasonic cavitation processing devices for production of, 744–746, 744–745f ultrasonic cavitation treatment for, 737–746 Polymeric foams, 45

Index

Polymeric ionic liquid (PIL), 368 Poly(ε-caprolactone) (PCL)/MWCNT composite, 103, 104f Poly (lactic-co-glycolic acid) (PLGA)/ MWCNT composite, 104 Polyoxometalates (POM), 367–370 Poly(ethylene glycol)-poly(D,L-lactide) copolymer (PELA), 102 Polypyrrole (PPy), 130–132, 132f Polypyrrole (PPy)/CNTs, 502, 503f Polytetrafluoroethylene (PTFE), 126–128 Polyurethane (PUF), 67 Polyurethane/MWCNT composite, 102 Polyvinylpyrolidone (PVP), 27–28, 127–128 POM. See Polyoxometalates (POM) Porous graphene paper, 344 Power transmission, 226t Pressure sensor, 76–78, 77f Pristine NH2-MIL-125 (Ti), 554–555 Pseudocapacitors, 123, 507–508 Pt-MWCNT glucose sensor, 511, 511–512f Q Quenching, 211–212 R Radar cross-section, 215 Radiation, of ultrasound energy into low-viscosity liquid, 741–742, 741f Radical spin correlations, 199–202 Raman spectroscopy, 174, 176–178, 184f, 218f, 235t, 248f chemical bonding, 217 copper-deposited nanocarbons, 242 dispersive, 234f graphene-like nanocarbons, 176f, 184 nanocarbon deposited copper, 242f nanocarbon powder, 242f Ru@GO NPs for dimethylamine borane dehydrogenation, 618–619, 618f Rashba spin-orbit induced topological gap, 229–230 RCGO/U6N, 555–556, 556f Reduced bituminous coal (RBC), 590, 593f, 595f Reduced exchange correlations, 207–209

845

Reduced graphene oxide (RGO), 131, 329, 392–393, 405–409, 536–538, 765–766 chemical analysis, 231f edge structure, 236 electron spin resonance spectra, 236–238, 237f Fourier transform infrared spectroscopy, 232f high-resolution transmission electron microscopy, 236f molecular and spin interactions, 229–238 molecular spintronic parameters, 271–273 operative coupling channels, 238 Raman spectra, 232f, 235t spin dynamic parameters, 237t structure, 233 synthesis of, 230–236 Te chemical state analysis, 231 Reinforced thermoplastic polymer composite materials, 735 Resin infusion under flexible tooling (RIFT), 432 Reverse nonequilibrium molecular dynamics (RNEMD), 437–438 Reversible absorptivity, carbon foams, 67–68, 67f RGO. See Reduced graphene oxide (RGO) rGO@CuZnO@Fe3O4, CO2 reduction, 557, 559f rGO/TiO2 aerogel, 14 Root growth model, 498–499 Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions, 203–205 Ru@GO NPs, for dimethylamine borane dehydrogenation Arrhenius plot, 620–621, 621f Eyring plot, 620–621, 621f Gaussian–Lorentzian method, 619–620 high resolution transmission electron microscopy, 619, 619f H2/mol DMAB vs. time, 620–621, 620f Raman spectroscopy, 618–619, 618f reusability performance, 621, 621f Shirley-shaped background subtraction technique, 619–620 synthesis, 617–618 transmission electron microscopy, 619, 619f

846

Ru@GO NPs, for dimethylamine borane dehydrogenation (Continued) X-ray diffraction, 618, 618f X-ray photoelectron spectroscopy, 619–620, 619f Ru/PVP@C NPs, for dimethylamine-borane dehydrogenation Arrhenius plot, 606, 607f Eyring plot, 606, 607f Gaussian–Lorentzian method, 605–606 high-resolution transmission electron microscopy, 604, 604f investigation, of performances, 603–604 nH2/nDMAB plot vs. time for, 606, 606f reusability examination, 604 Shirley-shaped background subtraction technique, 605–606 synthesis, 603 transmission electron microscopy, 604, 604f X-ray diffraction, 604–605, 605f X-ray photoelectron spectroscopy, 604, 605f

S Saddle-shaped polycyclic arene, 636–638, 639f Scanning electron microscopy (SEM), 174, 179f Scanning tunneling microscopy (STM), 175, 178–181, 180f Scanning tunneling spectroscopy (STS), 175, 178–181 Scattering parameter analysis, 222 Scholl reaction, 634 “Scooter” mechanism, 496 Scotch tape approach, 679–680 S-doped carbon dots, 657, 660–661 Selective area electron diffraction (SAED), 174 Selective catalytic reduction (SCR), 385 Selective noncatalytic reduction (SNCR), 385 Self-assembly synthesis, graphene foam, 49–50, 50f Self-curing effect, of graphene, 789, 790f Self-doped polyaniline (SDPA), 133 Self-propagation high-temperature synthesis, 426, 427f

Index

Self-sensing cementitious composites with BHNCMs (SCCBHNCMs), 793 Semiconductor nanocarbon-MOFs multifunctional materials, 561–570 Semiconductor-NC-metal, 560–561 Sensors biosensor, 74–75, 75t gas, 75–76 humidity, 74 molecule, 75 nanocarbon aerogels as, 16–19 pressure, 76–78, 77f strain, 78–79, 79f transfer function, 244–246 Shielding effectiveness (SE), 226t, 264–265 Shield technology, nanocarbon for coating characteristics on copper, 249 DC conductivity, 249–250 % reflection analysis, 250–252 shielding mechanism, 252–253 Silicon carbide nanowires (NWSiC), 428–429, 429f Silicon naphthalocyanine (SiNC-1), 682, 683f Silk fibroin, 107 Single-walled carbon nanotubes (SWCNTs), 182, 313–315, 328, 328f, 451, 490, 532, 661–662t, 712, 712f, 806–807 aerogels, 9 applications, 334–335 atomic interactions of, 814 chemical vapor deposition, 496–497 endocytosis effect on tobacco, 313–314 fluorophore-labeled, 314–315 hydrogen adsorption capacity of, 506 inside chloroplast of spinach cells, 313 laser-ablation technique, 494–496 mechanism, 812–814, 813f physical properties, 332–333, 332t Raman spectrum of, 500, 501f Rh nanoparticles, photocatalysis of NOx, 386–387 structures, 678–679, 679f in tissue engineering, 95, 95f trapping mechanism, 313, 314f VOCs removal, 409–411 Skin thickness, 223–224 Softwood charcoal, 174f Soil microenvironments, nanocarbons effect on, 315–317, 316f

Index

Sol-gel process nanodiamond aerogels, 7 for synthesis of polymers, 739–740 Solvent-exfoliated graphene (SEG), 393 Sonochemical effects, on sol-gel processes for synthesis of polymers, 739–740 Sonochemistry, 739–740 Sono-gels, 740 Sonoluminescence, 739 Specific capacitance graphene-PANi nanocomposites, 133 graphene-poly(3,4ethylenedioxythiophene) nanocomposites, 134–135 graphene-poly(vinylidene difluoride) nanocomposites, 125–126 graphene-polytetrafluoroethylene nanocomposites, 126–127 graphene-PPy composites, 131–132, 132f Spin-bath properties, GNCs, 210–211 anisotropy in g factor, 211–212 linewidth (ΔHpp) analysis, 210–211 spin transport parameters, 212–214 Spin interactions reduced graphene oxide synthesis, 230–238 tellurium-rGO synthesis, 230–238 Spin-lattice relaxation, 212–214 Spin-orbit coupling constant (SOCC), 213–214, 213t Spin-orbit (SO) interactions, 195, 212–214 Spin-spin interactions, 195 Spin-spin relaxation, 212–214 Spin transport, 195–199, 201t Spintronics, 195–199 Split-ring resonators (SRRs), 253 Spring constant, of carbon nanocoil, 472 elastic boundary determination, 479 vs. macroscopic spring theory, 479–480, 480f mechanical strength estimation, 480–481, 481f real-time measurement of tensile test, 478–479, 478f SRRs. See Split-ring resonators (SRRs) Stone–Wales transformation, 809, 809f Strain sensor, 78–79, 79f Stress transfer, 158 Sulfides, 545–548

847

Sulfonated polyethersulfone (SPES), 632–633 Supercapacitors, 7, 123 nanocomposites with transition metal oxides for, 357–369 poly(3,4-ethylenedioxythiophene), 134–136, 135f poly(vinylidene difluoride), 124–126, 125f polyaniline, 132–133 polypyrrole, 130–132, 132f polytetrafluoroethylene, 126–128 RuO2 metal oxides on, 358 Surfactants, 409–411, 430 SWCNTs, 785–787. See Single-walled carbon nanotubes (SWCNTs)

T Tannin, 69–70 Tauc plot, 592, 596, 596t TDAE-treated graphene, 203–205 Tellurium-reduced graphene oxide (Te-rGO) bond molecular environment, 232 chemical state analysis, 231 edge structure, 236 electron spin resonance spectra, 236–238, 237f Fourier transform infrared spectroscopy, 232f high-resolution transmission electron microscopy, 236f molecular and spin interactions, 229–238 Raman spectra, 232–236, 232f, 235t spin dynamic parameters, 237t structure, 233 synthesis of, 230–236 Temperature dependent resistivity, carbon nanocoil, 482–484, 483f Tensile fracture, carbon nanocoil, 476–478, 477f Tensile fracture surface, of nanocomposites, 190, 190f Te-rGO. See Tellurium-reduced graphene oxide (Te-rGO) Ternary chalcogenides, 547–548 Tetrabenzotriazacorroles (TBC), 682, 684f Tetrahydrofuran (THF), 196 Tetrakisdimethylamino-1, 2-dioxetane (TDMD), 196–197

848

Tetrakis(dimethylamino)ethylene (TDAE), 196–199 Tetramethylhydrazine (THM), 196–197 Tetramethyloxamide (TMO), 196 Tetramethyl urea (TMU), 196 Thermal conductivity, 71, 71f Thermal insulation, carbon foams, 71–73, 71f Thermal spraying, 816 Thermoplastic composite materials, nanomodified, 768–770 Thermoplastic polymer composites, 454–457, 455t Thermoplastic polyurethane (TPU), 105 Thermoset epoxy precursors, 766 liquid, 769–770 plastics, 768 Thermosetting polymer matrices, 452–454 3D chitosan/CNTs aerogel, 18–19 Tin oxide, 364 TiO2/fGO composites, 394 TiO2/graphene composites, 392–393 TiO2 nanoparticles, 371–372, 372f TiO2 photocatalyst, 525, 525f TiO2/single-walled CNT aerogels, 15–16, 16f Tip growth model, 498–499 Tissue engineering (TE) carbon nanotubes, 95–96 electrospinning process, 93–94, 109–110 of carbon nanofibers, 110–111 of graphene quantum dots, 111 electrospun nanofiber for, 99–100, 112–115t electrospun nanomat carbon nanotubes, 100–105 graphene nanocarbons, 105–109 fullerenes, 98–99 graphene compounds, 97–98, 97–98f nanodiamonds, 99 Toroidal shape samples, 221–222 Transmission electron microscopy (TEM), 184 of amorphous carbon, 7, 8f dimethylamine borane dehydrogenation Ru@GO NPs, 619, 619f Ru/PVP@C NPs, 604, 604f Pd-Ni @AC, 29–30, 30f Tungsten oxide (WO3), 542

Index

U UGA. See Unidirectional graphene aerogel (UGA) UiO-66/CdS/RGO, 570 Ultimate tensile strength (UTS), 186–188 Ultrasonic cavitation, 737–738, 738f devices for production of PCMs, 744–746 treatment for classical epoxy binders and PCMs, 737–746 Ultrasonic dispersing of nanoparticles, 746–750 with organic solvents, 747–748 of nanoparticles in liquid oligomers, 748–749 Ultrasonic effect, in synthesis of hydrogels based on graphene, 762 Ultrasonic modification, of liquid epoxy compositions, 742–744, 742f Ultrasonic treatment application, 734–737 extrusion method with, 768–770 in production of graphene, 753–755 and graphene-containing products, 755–756 Unidirectional graphene aerogel (UGA), 759 US Environmental Protection Agency (EPA), 385 V Vacuum-assisted resin transfer molding (VARTM), 432 Vacuum-impregnated composite, 64 Valence band (VB), 523–525 van der Waals forces, 815 Variable range hopping (VRH), 472 Vector network analyzer (VNA), 220, 222f Vibrating sample magnetometry (VSM), 195–196, 209 current-voltage measurements, 205–207 magnetic correlation parameters, 203t magnetometric analysis, 202–209 nitrogen roles, 207–209 reduced exchange correlations, 207–209 Ruderman–Kittel–Kasuya–Yosida interactions, 203–205 transport characteristics, 205–207 Vibration spectroscopy, 232

Index

Volatile organic compounds (VOCs), 384 chemical structure, 402–403, 404–405t human health, effects on, 402–403, 404–405t removal adsorbents, 402 carbon nanofiber-based nanocarbon materials, 411–412 carbon nanotube-based nanocarbon materials, 409–411 graphene-based nanocarbon materials, 405–409 sources, 401–402, 403f toxicity, 402–403 VSM. See Vibrating sample magnetometry (VSM) W Wastewater remediation, aerogels adsorbents, 11–12 photocatalyst, 12–16, 14–16f Water-channel protein (LeAqp2) genes, 309–313 Water desalination process, 641, 644–645, 645f Water purification, carbon nanotubes adsorption, 713–717 desalination, 721–723 disinfection, 724–727 photocatalytic degradation, 717–720 sensing and monitoring, 727 Water-soluble carbon dots (wsCDs), 306–308 Water-soluble carbon nanoonions (wsCNOs), 305–308 in gram plants, 305, 307f seed to seed cycle, 306–308, 308f Water-soluble carbon nanotubes (CNTs) capillaries, 294–305

849

inside roots, 306f multiwalled carbon nanotubes, 294–305 single-walled carbon nanotube trapping, 314f water uptake, 294–305 Water-soluble fullerenes, 308 Water soluble nanocarbons, 293–315 Water splitting, 522–525, 523–524f X X-ray diffraction (XRD) dimethylamine borane dehydrogenation Ru@GO NPs, 618, 618f Ru/PVP@C NPs, 604–605, 605f Pd-Ni @AC, 30, 31f SnO2, 364–365, 365f X-ray photoelectron spectroscopy (XPS), dimethylamine borane dehydrogenation Ru@GO NPs, 619–620, 619f Ru/PVP@C NPs, 604, 605f Y Yarmulke, 498–499 Z Zeolite (ZP) foams, 67 Zigzag-edged nanographenes (Z-NGs), 630–631, 630f, 644f Zinc oxide (ZnO), 539–540, 539f Ziz-zag nanotube, atomic structure, 808f ZnO@C composites, 556–557 ZnO-GO photocatalysts, 539–540 ZnO/RGO activation, 539, 539f ZnPc hybrids, 689f, 690, 692f Zr-doped graphitic CFs, 53