All-carbon Composites and Hybrids 1839161760, 9781839161766

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All-carbon Composites and Hybrids

All-carbon Composites and Hybrids Edited by

Oxana V. Kharissova Universidad Auto´noma de Nuevo Leo´n, Mexico Email: [email protected] and

Boris I. Kharisov Universidad Auto´noma de Nuevo Leo´n, Mexico Email: [email protected]

Print ISBN: 978-1-83916-176-6 PDF ISBN: 978-1-83916-271-8 EPUB ISBN: 978-1-83916-272-5 A catalogue record for this book is available from the British Library r The Royal Society of Chemistry 2021 All rights reserved Apart from fair dealing for the purposes of research for non-commercial purposes or for private study, criticism or review, as permitted under the Copyright, Designs and Patents Act 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry or the copyright owner, or in the case of reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Whilst this material has been produced with all due care, The Royal Society of Chemistry cannot be held responsible or liable for its accuracy and completeness, nor for any consequences arising from any errors or the use of the information contained in this publication. The publication of advertisements does not constitute any endorsement by The Royal Society of Chemistry or Authors of any products advertised. The views and opinions advanced by contributors do not necessarily reflect those of The Royal Society of Chemistry which shall not be liable for any resulting loss or damage arising as a result of reliance upon this material. The Royal Society of Chemistry is a charity, registered in England and Wales, Number 207890, and a company incorporated in England by Royal Charter (Registered No. RC000524), registered office: Burlington House, Piccadilly, London W1J 0BA, UK, Telephone: þ44 (0) 20 7437 8656. Visit our website at www.rsc.org/books Printed in the United Kingdom by CPI Group (UK) Ltd, Croydon, CR0 4YY, UK

Preface This book covers the hybrids/composites based on carbon–carbon allotropes. Currently, carbon in its zerovalent state is one of the most investigated elements of the Periodic Table because of its unique ability to form C–C covalent bonds in different hybridizations (sp, sp2, sp3). This well-known fact causes a grand variety of carbon allotropes, both classic, such as diamond or graphite, and the relatively recently discovered nanostructures graphene or carbon nanotubes. Carbon allotropes have very distinct properties, ranging from 3D super-hard diamond crystals containing tetrahedral sp3 carbon atoms to soft graphite containing sp2 carbon atoms constructed from 2D graphene sheets united by van der Waals forces, including the 1D carbon nanotubes possessing outstanding mechanical, electrical, and other properties, and the 0D fullerenes, nanoonions, nanodots, or nano-diamonds, among other lesser-common carbon architectures. It is known that carbon allotropes can form their composites or hybrids because of the spontaneous formation of covalent or van der Waals bonds between carbon atoms, initially belonging to the precursors. The C–C hybrid materials are mainly composed of graphite, graphene, their oxidized (graphene or graphite oxides) and reduced (reduced graphene oxide, rGO) forms, fullerenes, quantum dots, carbon nanotubes, carbynes, carbon nanorings/nanotori, xerogels and aerogels, among others. These composites of different carbon allotropes represent 3D structures, bonded with covalent bonds or van der Waals forces. Such combination of different dimensionalities and reactivities frequently leads to materials with unusual properties. As a result of carbon–carbon hybrid formation, their counterparts can remain practically unchanged or distorted in distinct grades, maintaining distinct reactivity and availability for further functionalization.

All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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Synthesis methods for C–C hybrids vary, using direct interaction of different nanocarbons or their in situ preparation from their hydrocarbon precursors by spray pyrolysis, CVD and solvothermal methods, redox processes, ultrasonication, etc. Therefore, the produced hybrids can possess unusual properties due to the simultaneous presence of carbon structures of distinct dimensionality and reactivity. Their properties are frequently not a sum of those of their counterparts and can be varied upon addition or elimination of oxygen-containing and other groups. These hybrids have several current and possible applications in the areas of catalysis, sensors, supercapacitors, as well as can be applied for environmental remediation. Carbon is an outstanding element of the Periodic Table, so many research groups worldwide work in the area of carbon allotropes. Taking into account that about 500 still undiscovered carbon nanoallotropes have been theorically predicted and reported, a more extensive application of DFT and related methods is desirable to propose the existence of other C–C combinations, suitable for practical uses. We believe that further studies of nanosized C–C combinations can lead to novel emerging materials and technologies and hope that this book will be useful for researchers, professors, and engineers, working in the areas of nanotechnology, carbon allotropes, bionanotechnology and nanochemistry, organometallic chemistry, catalysis, nano-medicine, and materials chemistry, among others. Editors Oxana V. Kharissova, Universidad Auto´noma de Nuevo Leo´n, Mexico and Boris I. Kharisov, Universidad Auto´noma de Nuevo Leo´n, Mexico

About the Editors Dr. Oxana V. Kharissova (born in 1969 in Ukraine, former USSR, has lived in Mexico since 1995, and naturalized in Mexico in 2004) is currently a Professor and Researcher at the Universidad Auto´noma de Nuevo Leo´n (UANL). Degrees: M.Sc. in 1994, in crystallography from Moscow State University, Russia, and a Ph.D. in Materials from the Universidad Auto´noma de Nuevo Leo´n, Mexico. Memberships: National Researchers System (SNI, Level II), Materials Research Society, Mexican Academy of Science. She is the co-author of 10 books, 12 book chapters, 105 articles, and holds eight patents. Specialties: Materials, nanotechnology (carbon nanotubes, graphene, nanostructurized metals, fullerenes), microwave irradiation and crystallography; nanotechnology-based methods for petroleum treatment. Dr. Kharissova holds the awards ‘‘Flama, Vida y Mujer 2017’’ and ‘‘Tecnos’’ (2004). She is an expert of the National Council for Science & Technology of Mexico (Conacyt).

All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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Dr. Boris I. Kharisov (born in 1964, in Russia, has lived in Mexico since 1994, and naturalized in Mexico in 2003) is currently a Professor and Researcher at the Universidad Auto´noma de Nuevo Leo´n (UANL). He took part in the liquidation of the consequences of the Chernobyl accident, working in the contaminated zone in 1987. Degrees: M.Sc. in 1986, in radiochemistry and a Ph.D. in inorganic chemistry in 1993, from the Moscow State University, Russia; Dr. Hab. in physical chemistry in 2006 from Rostov State University, Russia. Specialties: Materials chemistry, coordination and inorganic chemistry, phthalocyanines, ultrasound, nanotechnology, chemical treatment of petroleum, environmental remediation. Memberships: Mexican Academy of Science, National Researchers System (SNI, Level III). He is the co-author of 15 books, 186 articles, 13 book chapters, and holds eight patents. Co-editor: Three invited special issues of international journals. He is the member of the Editorial board of four journals. His biography was published in: ‘‘Who is Who in the World’’, ‘‘Outstanding People of the Twentieth Century’’, and so on.

Contents Section 1: Graphite-, Graphene- and Graphene Oxide-based Hybrids Chapter 1 Hybrids of Graphite, Graphene and Graphene Oxide ´ximo Oliva Gonza ´lez, Oxana V. Kharissova, Cesar Ma Cynthia Estephanya Ibarra Torres, Boris I. Kharisov and Lucy T. Gonzalez 1.1 1.2

Introduction Graphite Hybrids 1.2.1 Composites of Graphite (Graphite Oxide) with Carbon Nanotubes 1.2.2 Other Graphite–Carbon Composites 1.3 Graphene and Graphene Oxide Composites 1.3.1 Graphene–Carbon Nanochain Composites 1.3.2 Graphene (Graphene Oxide)–Carbon Nanofiber Composites 1.3.3 Graphene–Fullerene Composites 1.3.4 Graphene Hybrids with Carbon Nanocages 1.3.5 Graphene(Graphene Oxide)-nanodiamond Composites 1.3.6 Other Graphene–Carbon Composites References Chapter 2 Production of Carbon Nanostructure/Graphene Oxide Composites by Self-assembly and Their Applications ´n and M. Ortega-Lo´pez R. Ortega-Amaya, M. A. Pe´rez-Guzma 2.1

Introduction

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3 4 4 10 13 13 15 16 21 22 25 28

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All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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2.2

GO Synthesis Methods 2.2.1 GO Chemical Structure 2.2.2 GO Functionalization 2.2.3 GO Self-assembly 2.3 Carbon-based Composites (GO, CQD, and CNT) 2.3.1 Graphene Oxide–Carbon Quantum Dot Composites 2.3.2 Reduced Graphene Oxide–Carbon Nanotube Composites 2.4 Conclusion References

33 33 35 36 37 37 44 47 47

Section 2: Carbon Nanotube Composites Chapter 3 Synthesis of Carbon Nanotube/Graphene Hybrids by Chemical Vapor Deposition Zhi Liu, Hua-Fei Li, Shuguang Deng and Gui-Ping Dai 3.1 3.2

3.3

3.4

Introduction Preparation of Carbon Nanotube/Graphene Hybrids 3.2.1 Vacuum Filtration Method 3.2.2 Layer-by-layer Self-assembly Deposition 3.2.3 Solution Method 3.2.4 Electrophoretic Deposition 3.2.5 Multi-step Chemical Vapor Deposition 3.2.6 One-step Chemical Vapor Deposition Effect of Experimental Parameters of the CVD Technique 3.3.1 Effect of Catalyst 3.3.2 Effect of Carbon Source 3.3.3 Effect of Growth Temperature and Growing Time 3.3.4 Effect of Carrier Gas Application Prospects of Carbon Nanotube/ Graphene Hybrids 3.4.1 Carbon Nanotube/Graphene Hybrids in Fuel Cells 3.4.2 Carbon Nanotube/Graphene Hybrids in Transparent and Flexible Electrodes and Field-effect Transistors 3.4.3 Carbon Nanotube/Graphene Hybrids in Supercapacitors

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55 57 58 58 59 59 59 61 61 61 62 63 64 65 65

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3.4.4

Carbon Nanotube/Graphene Hybrids in Lithium Batteries 3.5 Further Prospects and Conclusions References Chapter 4 Design of Graphene/CNT-based Nanocomposites: A Stepping Stone for Energy-related Applications Waseem Raza 4.1 4.2

Introduction Synthesis Method for Graphene/CNT Hybrids 4.2.1 Chemical Vapor Deposition 4.2.2 Electrophoretic Deposition 4.2.3 In Situ Reduction 4.3 Recent Growth in Energy-related Applications of Graphene/CNT Hybrids 4.3.1 Supercapacitors 4.3.2 Fuel Cells 4.4 Conclusion Acknowledgements References Chapter 5 One-dimensional Carbon Nanotube Decorated Two-dimensional Reduced Graphene Oxide Composite: Insight from Synthesis to Application in Dye Sensitized Solar Cells Khursheed Ahmad and Shaikh M. Mobin 5.1 5.2

Introduction Dye Sensitized Solar Cells 5.2.1 Fabrication of Dye Sensitized Solar Cells 5.2.2 Components of Dye Sensitized Solar Cells 5.3 Recent Advances in Counter Electrodes 5.4 Conclusions and Future Prospects Acknowledgements References

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Section 3: Composites of Carbon Nanodots and Quantum Dots Chapter 6 Carbon Dot-based Composites: Recent Progress, Challenges and Future Outlook L. C. Sim, S. S. Terng, J. Y. Lim, J. J. Ng, W. C. Chong, K. H. Leong and P. Saravanan 6.1

Introduction to Carbon Dots

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6.1.1 6.1.2

Graphene Quantum Dots (GQDs) Carbon Nanodots (CNDs) and Carbon Quantum Dots (CQDs) 6.1.3 Carbonized Polymer Dots (CPDs) 6.2 Recent Progress in CD-based Composites 6.2.1 CQD-based Composites 6.2.2 GQD-based Composites 6.2.3 CND and CPD-based Composites 6.3 Challenges 6.4 Conclusion and Future Perspectives Acknowledgements References Chapter 7 Carbon Dots Derived from Natural Carbon Sources: Preparation, Chemical Functionalization, Characterization, and Applications Monikankana Saikia and Binoy K. Saikia 7.1 7.2

116 121 124 126 126 130 131 134 135 136 137

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Introduction CQD Synthesis Techniques 7.2.1 Top-down Approach 7.2.2 Bottom-up Approach 7.2.3 Post-synthetic Variations 7.3 Characterization and Properties of CQDs 7.3.1 Structural Characterization 7.3.2 Photophysical Characterization 7.4 Applications of CQDs 7.4.1 Sensors 7.4.2 Bio-imaging 7.4.3 Drug-delivery 7.4.4 Catalysis 7.4.5 Optronics 7.5 Carbon Nanoparticle Allotropes 7.6 Summary and Future Outlook References

142 144 144 146 147 151 151 153 157 157 159 160 160 161 162 163 166

Chapter 8 Composites of Carbon Nanodots for Hydrogen Energy Generation Biswajit Choudhury

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8.1 8.2

Introduction Carbon Nanodots

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8.3

Fabrication of C-nanodot Composites and Their H2 Evolution Performance 8.3.1 Carbon Dot–Carbon Nitride Composite (CDs/CN) 8.3.2 Graphene Quantum Dot–Graphene Composite (GQDs/G) 8.3.3 Graphene Quantum Dot–Graphitic Carbon Nitride Composite (GQD/CN) 8.3.4 Carbon Nitride Quantum Dot–Graphene Nanocomposite (CNQD/G) 8.4 Conclusion References

175 175 184 187 190 193 193

Section 4: Fullerene Clusters Chapter 9 Clusters of Fullerenes Klavs Hansen and Henning Zettergren 9.1 9.2 9.3

Introduction Fullerene Building Blocks Production, Geometry and Stability 9.3.1 Production 9.3.2 Geometric Structures 9.3.3 Stability: Theory 9.4 Fusing Clusters 9.4.1 Coalescence of Laser Heated Fullerenes 9.4.2 Low Energy Bi-molecular Collisions 9.4.3 Femtosecond Light-induced Fusion 9.5 Ion-cluster Collisions 9.5.1 Multiply Charged Clusters 9.5.2 Molecular Growth Processes Acknowledgements References

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Section 5: Other Exclusive Carbon–Carbon Nanocomposites Chapter 10 Less-common Carbon–Carbon Nanocomposites Cynthia Estephanya Ibarra Torres, Oxana V. Kharissova, ´ximo Oliva Gonza ´lez and Boris I. Kharisov Cesar Ma 10.1 10.2

Introduction CNT Hybrids with Non-graphene Nanocarbons 10.2.1 Carbon Nanotubes Containing Carbyne

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10.2.2

CNT Composites with Carbon Nanofibers (CNFs) 10.2.3 Other CNT Hybrids 10.3 Hybrids of Nanoballs and Nanospheres 10.4 Nanoring (Nanotori) Composites 10.5 Xerogels on the Basis of Carbon–Carbon Composites 10.6 Amorphous and Glassy Carbon Composites References

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Section 6: Polymer Composites of Carbon–Carbon Hybrids Chapter 11 Advances in Polymeric Nanocomposites Incorporating Graphene–Fullerene and Graphene Oxide–Fullerene Hybrids Ayesha Kausar 11.1 11.2 11.3

Introduction Carbon Nanomaterials: Graphene and Fullerene Carbon Nanomaterial Hybrids as Nanobifillers: Graphene–Fullerene Hybrids 11.4 Aspects of Polymer/Graphene–Fullerene and Polymer/Graphene Oxide–Fullerene Nanocomposites 11.4.1 Polymer/Graphene–Fullerene Nanocomposites 11.4.2 Polymer/Graphene Oxide–Fullerene Nanocomposites 11.5 Prominence and Future Visions of Graphene–Fullerene-based Nanomaterials in High Performance Applications 11.6 Summary References Chapter 12 Mechanical Properties of Graphene–Carbon Nanotube Reinforced Hybrid Polymer Nanocomposites Sushant Sharma and Bhanu Pratap Singh 12.1 12.2 12.3

Introduction to Polymer Nanocomposites Nanofillers 12.2.1 Carbon-based Nanofillers Graphene–CNT Hybrid Nanofiller 12.3.1 Synthesis of Graphene–CNT Hybrid Nanofiller

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Forms of Graphene–CNT Hybrid Nanofiller 12.3.3 Synthesis of Graphene–CNT Polymer Nanocomposites 12.3.4 Mechanical Properties of Graphene–CNT Reinforced Nanocomposites 12.4 Summary and Conclusions References

294 297 300 306 308

Section 7: Characterization and Identification Methods Chapter 13 Raman Spectroscopy Characterization of Carbon Materials: From Graphene to All-carbon Heterostructures Alexandre Merlen, Josephus Gerardus Buijnsters and Cedric Pardanaud 13.1

Basic Principle of Raman Spectroscopy 13.1.1 Context 13.1.2 Raman Effect in Solids 13.2 Raman Spectroscopy of Carbon Allotropes 13.2.1 Graphene and Related Materials 13.2.2 Low-ordered Carbons 13.2.3 Diamond (Single-crystal and Polycrystalline) Materials 13.3 All-carbon Heterostructures 13.3.1 Class 1: The Final Spectrum is the Sum of Each Different Allotrope Contribution 13.3.2 Class 2: Slight Changes in the Sum 13.3.3 Class 3: New Modes Are Present Acknowledgements References Chapter 14 Final Remarks Oxana V. Kharissova and Boris I. Kharisov Final Remarks Subject Index

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Section 1: Graphite-, Graphene- and Graphene Oxide-based Hybrids

CHAPTER 1

Hybrids of Graphite, Graphene and Graphene Oxide ´XIMO OLIVA GONZA ´LEZ,a OXANA V. KHARISSOVA,a CESAR MA CYNTHIA ESTEPHANYA IBARRA TORRES,a BORIS I. KHARISOV*a AND LUCY T. GONZALEZb a

´noma de Nuevo Leo ´n, San Nicola ´s de los Garza, NL, Universidad Auto ´xico; b Departament of Chemistry and Nanotechnology, Tecnolo ´gico de Me ´xico Monterrey, Monterrey, NL, Me *Email: [email protected]

1.1 Introduction Carbon is one of the most interesting elements in the periodic table due to the ease with which it forms C–C covalent bonds in various hybridizations (sp, sp2, sp3). This characteristic gives carbon the ability to generate countless allotropes whose properties vary depending on the morphology and hybridization of the compounds, a very clear example of this can be found when we examine the properties of two of the best-known allotropes of carbon, diamond and graphite; 3D diamond crystals are super hard and contain hybridizing tetrahedral carbon atoms sp3, on the other hand, graphite is soft and contains hybridizing carbon atoms sp2. Currently, carbon allotropes have been part of numerous studies, for which different ways of classifying them have been generated, among the most common are based on their dimensionality, morphological characteristics or by the hybridization of carbon atoms. There is a large number of investigations that focus on synthesizing materials that are formed by the combination of carbon allotropes of nanometric sizes, using a wide variety of methods in order to take advantage of All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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their synergistic properties to implement them in different applications. In this section we will focus on analyzing the different carbon composites reported in the literature, focusing mainly on those that contain graphite, graphene and graphene oxide as main components.

1.2 Graphite Hybrids Graphite is one of the most abundant allotropes of carbon in nature and the most used in our day to day through its best-known application as part of pencils, however, we rarely mention its electrical properties, which allow it to be used as an electrode. The properties of graphite are due to the fact that it is made up of carbon atoms with hybridization sp2 constructed in the form of 2D sheets, which are stacked on top of each other and joined by van der Waals forces. Additionally, this carbon allotrope is one of the cheapest and easy to modify, which makes it an ideal component for manufacturing of low-cost carbon composites.

1.2.1

Composites of Graphite (Graphite Oxide) with Carbon Nanotubes

Carbon nanotubes (CNTs) can be divided into two types: single-walled carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs). Regardless of the form in which the CNTs are, they are the nanostructures that have attracted the most attention of researchers along with graphene. For this reason, graphite/CNT composites are one of the most commonly found in literary reports. Some of the most promising applications where these composites have been implemented are in electroanalysis of bioactive molecules, battery electrodes, supercapacitors, sensors etc.12 The synergies that exist between the 2D structures of the graphite sheets and the 1D structures of CNT allow the resulting composite to have improvements in some of its properties such as electrical conductivity, adsorption capacity, mechanical and tribological properties.3 Furthermore, graphite/CNT composites can be manufactured using a wide variety of methods, among which chemical vapor deposition (CVD) stands out, as it is the most common method found in the literature. The selective synthesis of SWCNTs/graphite composites on nickel foam was carried out in different ways, the first by CVD from acetylene gas a carbon precursor (eqn (1.1)).4 Other synthesis methods are also available, for instance, graphite fiber composites with SWCNTs (this process could be scaled up after additional improving efforts and adaptation to the existing manufacturing process) were obtained by SWCNTs air-spraying onto the surface of graphite/epoxy prepreg.5 3H–CRC–H-6C þ 3H2

(1.1)

SWCNTs/graphite composites out-of-plane electrical conductivity was improved by 144% for 2 wt.% SWCNT samples compared to samples without SWCNTs. The composite of treading CNTs and coated graphite was obtained as a result of the pyrolysis of CNT/polyaniline composites at 1500 1C,

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presenting a specific orientation relationship between the CNTs axis and graphene layers,6 with an angle of 1101 between them. This synthesis method can be applied to fabricate high-performance carbon materials via the alignment of graphene layers. Some reports established several intriguing differences between graphite oxide (GrO) and reduced graphite oxide (rGrO) behavior in carbon nanotubes, which have direct effects on the electrical conductivity of the composite.7 In particular, the presence of electrically insulating GrO within a SWCNT network (Figure 1.1) strongly enhances electrical conductivity, meanwhile rGrO, even though electrically conductive, suppresses electrical conductivity, revealing the ‘‘indirect’’ role of the oxide groups. These groups, being in GrO within the SWCNT/GrO composite act through electronic doping of metallic SWCNTs. The following two factors controlling electronic transport were proposed: a) In SWCNT networks: high intrinsic conductivity of SWCNT versus poor coupling between nanotubes. b) In rGrO networks: good coupling between the sheets versus poor intrinsic conductivity within the sheet. Graphite/CNT composites alone, as well as their derivatives with metal oxides, are mainly used for the manufacture of Li-based storage equipment and supercapacitors. In one of them, the graphite/CNT compound (Figure 1.2), the CNTs are dispersed evenly on the surface of the graphite sheet when they are mixed together.8 For a three-phase-containing hybrid (Figure 1.3), consisting of carbon nanotubes (CNTs), and commercial graphite particles, and graphene oxide sheets, prepared using ultrasound with strongly oxidizing reagents such as KMnO4 and H2O2, it was revealed9 that, after compositing with graphite and CNTs, the graphene oxide maintains its typical wrinkled paper-like structure. The graphite particle and CNTs are covered by GO sheets, and the CNTs are

Figure 1.1

Transmission electron microscopy (TEM) micrographs of the CNT/GrO composite seen from the top (a) and from the side (b), in addition, the backings show the diffraction of electrons from both perspectives.7 Reproduced from ref. 7 with permission from Elsevier, Copyright 2014.

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Chapter 1

Scanning electron microscope (SEM) micrographs of graphite, CNT and their composite. (a) CNT, (b) CNT treated at 200 1C in air, (c) CNT treated at 200 1C in vacuum, (d) graphite, and (e and f) CNT/graphite composite with 5 wt.% CNT treated at 200 1C in vacuum.8 Reproduced from ref. 8 with permission from Elsevier, Copyright 2008.

randomly aligned to form a conductive bridge. This composite was found to have a very high reversible Li-storage capacity of 1172.5 mA h g1 at 0.5C (1C ¼ 372 mA g1), exceeding the theoretical sum of capacities of the three ingredients. This composite can be directly used as a binder-free anode material, possessing excellent electrochemical characteristics. However, despite the good conductivity that graphite/CNT composites have presented, these materials present some long-term drawbacks in the

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Figure 1.3

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SEM micrographs of (a) graphite, (b) CNT, (c) graphene oxide, and (e) graphene oxide/graphite/CNTs with the conductive additive composite (GGCC). TEM images of (d) graphene oxide and (f) GGCC composite.9 Reproduced from ref. 9 with permission from Elsevier, Copyright 2014.

manufacture of batteries, the two most relevant problems are structural pulverization and the instability of the solid electrolyte interface,1 which are the result of constant volume changes during charge/discharge cycles. One way to avoid structural spraying problems is by adding spacers between the

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Figure 1.4

Chapter 1

Scheme that exemplifies how CNTs work between the graphite sheets of the composite in the charge/discharge processes.1 Reproduced from ref. 1 with permission from Elsevier, Copyright 2019.

graphite sheets (Figure 1.4). In this case, a layer of CNTs of micrometric lengths are grown, which are interlaced on the surfaces of the graphite flakes, this allows the CNTs to function as an effective conductive medium and as a volumetric change buffer, due to these structural characteristics, the composite after being subjected to 1500 charge/discharge cycles, exhibits a reversible capacity of 234 mA h g1 at 2 A g1 and it retains 97% of its capacity, which is better than graphite flakes alone.1 As an example of a complex composite of ‘‘graphite/CNTs/metal oxide’’, we have an ultra-thin 3D graphite film (UGF)/CNT uniformly covered by NiO nanofilms10 manufactured by the method of the CVD and deposited by electrodeposition. The composite material exhibited improved lithium storage properties as anode materials for lithium-ion batteries. A similar composite with multiple layers of graphite, composed of CNT/graphite/zinc oxide was used for supercapacitor electrode materials.11 The structural deterioration that graphite undergoes when it is applied in energy storage systems, which is due to charge/discharge cycles, also occurs when graphite is used as a thermo-conductive material due to the fact that it is subjected to heating/cooling cycles. This translates into a bad cyclical residence (elastic deformation), so using graphite/CNT composites, where the CNTs are oriented vertically between the graphite sheets,12 is a good solution to the structural deterioration of graphite alone. In addition, it provides the composite with a high value of thermal conductivity of the transversal plane (k>) and a low density (1.67 g cm3), however, it was found that despite the good thermal conductivity of CNTs, increasing the concentration of CNTs in the composite can decrease the heat transfer capacity of the material, so 60% by weight of CNT gives the material good stability characteristics to function as a thermo-conductor. A different perspective to solve the problem of brittleness in thermoconductive elastic materials based on carbon allotropes is to use graphite welds that function as connectors and allow reducing the stress caused by volumetric change (Figure 1.5). An example of this is the graphite welds that were placed on CNTs sponges,13 these welds, which are found discontinuously throughout the structure, promote the transfer of stresses between the joints and generate an interconnected network that improves the heat conduction of

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Figure 1.5

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Schematic illustration showing the manufacturing steps of the CNT/ polydimethylsiloxane (PDMS) compound with graphite welds and an example of how graphite interacts with CNTs is shown in the inset.13 Reproduced from ref. 13 with permission from Elsevier, Copyright 2019.

the material. In addition, the composite presented good resilience, allowing it to withstand up to 100 000 compression cycles over a wide temperature range, permitting it to be a promising material for the development of flexible sensors. Another application field is in the sensor area, the implementation of composites in the biosensors area offers an attractive proposition due to the low costs of starting materials. A clear example of this is the electrodes made from pencil graphite powder, CNT and silicone oil (Figure 1.6),2 that with simple surface modification using sodium lauryl sulfate, which is an anionic surfactant, granted the material the ability to be sensitive to species such as riboflavin, obtaining satisfactory values of reproducibility, selectivity and stability. In addition, the composite presented a detection limit below 1.16108 M, even in the presence of interfering molecules like dopamine. Graphite oxides are also promising candidates for the production of sensors, for example, the b-cyclodextrin/GrO/CNT composite was used14 to recognize three kinds of biomolecules (dopamine, thioridazine, L-tyrosine) and presented an excellent supramolecular recognition capability, being compared to individual CNT and b-cyclodextrin/CNT composites. The main advantage of these composites is the ease with which they can be functionalized, that is to say, altering the group with which the composite is functionalized can allow it to be sensitive to other molecules.15 Carbon composites are also widely studied for innovation in the field of catalysis, more specifically as particle supports with catalytic capacities such as molybdenum disulfide,16 which can be used to catalyze the hydrogen evolution reaction. The interest in using graphite/CNT composites stems from the stability and ability of these materials to increase the number of active sites of the material.

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Figure 1.6

SEM micrographs of (a) bare graphite paste electrode, (b) CNT/graphite composite paste electrode, (c) anionic surfactant sodium lauryl sulfate modified CNT/graphite composite paste electrode and (d) electrochemical performance.2 Reproduced from ref. 2 https://doi.org/10.1016/j.jsamd.2019.11.001 under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/ by/4.0/.

Another way in which these hybrids can be applied is by adding them to polymers. For instance, the graphite/CNT hybrid formed a composite (Figure 1.7) with ultrahigh molecular weight polyethylene (UHMWPE)17 possessing a segregated structure, in which the graphite/CNT hybrid is selectively distributed at the interfaces of UHMWPE domains to form interconnected networks. This graphite/CNT/UHMWPE composite exhibited highly conductive networks and was used to develop efficient EMI shielding materials. Another example of a triple system is the silicon/graphite/CNTs composite material, prepared by the mechanical method of ball milling and further annealing,18 exhibiting an initial specific discharge capacity of 2326 mA h g1. The improved electrochemical properties were explained due to the CNTs uniform dispersion in the inter-space of Si and graphite materials.

1.2.2

Other Graphite–Carbon Composites

Insertion of other carbon allotropes as dopants into the graphite structure can lead to considerable changes of the original graphite support. Thus, for the sandwich-like graphite–fullerene composites, prepared via a solution

Hybrids of Graphite, Graphene and Graphene Oxide

Figure 1.7

11

(a) Schematic for the fabrication of segregated CNT/graphite/hybrid loaded ultrahigh molecular weight polyethylene (UHMWPE). SEM images of (b) pure UHMWPE and (c) 2.0 wt % CNT/graphite/hybrid coated UHMWPE complex granules. (d) Digital images of the CNT/graphite/ UHMWPE with wafer shape.17 Reproduced from ref. 17 with permission from American Chemical Society, Copyright 2018.

mixing/evaporation method, the fullerene doping into graphite was found to have minimal effect on the graphite electrical properties, but significantly improved its electromagnetic waves absorption properties,19 probably derived from the way in which these allotropes are inserted between the graphite sheets and from how they modify the surface of the composite. The main applications of graphite hybrids with other ‘‘carbon/carbon’’ combinations are in supercapacitor manufacturing. For instance, a composite of a commercial graphite, carbon black and graphene oxide was synthesized by mixing these components and used to prepare the anode slurries to be applied as a high capacity and binder-free material directly in lithium-ion batteries.20 GO was responsible for binder and lithium storage functions, the graphite for conductivity and capacity, and the carbon black, being dispersed uniformly between the GO sheets, for enhancing conductivity. This composite showed excellent rate capability and cycle performance, among other characteristics. Carbon nanofiber/graphite-felt composites (Figure 1.8) were synthesized by catalytic CVD (CCVD) through the growth of carbon nanofibers (CNFs) on the surface of the graphite microfibers of the felt.21 Both the apparent density and the compressive strength varied with the CNF yield (w/w), reaching maximum values with a CNF yield of about 6. Excessive growth of CNFs could destroy the felt framework. A large external surface area, high mechanical strength, and distinct mesoporous character make them attractive as catalysts for the

12

Chapter 1

oxidative dehydrogenation of ethylbenzene to styrene, possessing much greater catalytic stability than activated carbon. As an example of another catalytic use, the carbon nanofiber/graphite-felt composite was applied as a catalyst support for a high loaded iridium catalyst (30 wt.%) for hydrazine

Hybrids of Graphite, Graphene and Graphene Oxide

13

22

catalytic decomposition (eqn (1.2)–(1.4)), showing high thermal conductivity and strong mechanical resistance. This catalyst is resistant to the great increase in pressure and it can be compared to an industrial one based on Ir supported on a high surface area alumina. In addition, carbon nanofibers (CNFs) and reduced graphite oxide (rGrO) nanocomposites with distinct composition (10–40% CNFs) were offered for removal of different ions by electrosorption.23 These composites have different surface areas and capacitance, indicating that both the electrode conductivity and porosity are important for the electrochemical performance. N2H4 ¼ 4 NH3 þ N2

(1.2)

3 N2H4 ¼ 3 N2 þ 6 H2

(1.3)

4 NH3 þ N2H4 ¼ 3 N2 þ 8 H2

(1.4)

1.3 Graphene and Graphene Oxide Composites Graphene is one of the most surprising modern carbon allotropes because it has excellent conductivity and mechanical resistance characteristics, this allotrope consists of sheets of carbon atoms whose thickness is the size of an atom, which is why there are currently a great variety of studies that seek to use graphene as a component for the manufacture of modern materials that help the development of new technologies. This includes the formation of composites using unmodified graphene, graphene oxide (GrO) or reduced graphene oxide (rGrO). The industrial-scale production of graphene still remains a problem for researchers, however, it is expected that in some years’ time the production of graphene in large quantities will be a reality; therefore it is important to take into account the new materials that use this allotrope of carbon as a component. Then, we will review the advances in graphene composites that science has produced in recent years.

1.3.1

Graphene–Carbon Nanochain Composites

Among hybrids with 1D nanoobjects, the simplest case corresponds to the graphene–carbon nanochains. This way, the composite of sandwiched graphene–carbon nanochain webs, prepared by in situ polymerization and Figure 1.8

SEM micrographs of graphite fibers and carbon nanofibers illustrating the evolution of CNF/graphite-felt composite synthesis: (a) graphite fibers of felt; (b) graphite fibers deposited by nickel precursor; (c) graphite fibers covered by carbon filaments having been grown for 3 h; (d) graphite fibers covered by filaments with carbon agglomerates within felt after reaction for 8 h; (e) fleecy filaments grown on the surface of graphite fibers; (f) high-magnification image of carbon filaments showing the nanofiber structure; (g) high-magnification image of carbon agglomerates showing the nanofiber structure; and (h) amorphous carbon in agglomerates.21 Reproduced from ref. 21 with permission from Elsevier, Copyright 2006.

14

Chapter 1

Hybrids of Graphite, Graphene and Graphene Oxide

15

24

subsequent carbonization, possesses high conductivity and is useful for Li-ion batteries, showing, after 50 cycles, a charge capacity of 1103.2 mA h g1 at 0.05 A g1. In its nano–microstructure, carbon nanochain webs were found to be inserted between graphene layers to form sandwiched plates, where carbon nanochain webs derived from PPy act as the isolator to prevent the overlap of graphene layers. Large surface for the deposition of carbon nanochain webs and high conductivity is precisely provided by these graphene layers. This composite consists of the uniform granules of 10 mm diameter, made of thin plates within a lot of grids. 1D carbon nanochains are dispersed on the surface of graphene films, being coated by graphene to form sandwiched plates. The atomic content of N in carbon nano webs is 12.08%, much higher than that in other N-doped carbon materials, and graphene/carbon nano webs have a content of 5.11%. Lithium ions can reversibly enter and exit in this structure, demonstrating an excellent rate performance as an anode and high reversible Li-storage capacities.

1.3.2

Graphene (Graphene Oxide)–Carbon Nanofiber Composites

Carbon nanofibers (CNFs) represent very small cylinder-like nanometerscale structures consisted of graphene layers.25 We note that, despite a series of similarities between CNFs and CNTs, the first type of nanoobjects received considerably less attention from researchers. However, some of their composites with graphene and graphene oxide (GO) are known; almost all of them are described by authors as 3D freestanding materials and have mainly electrochemical device applications, including more complex composites, such as, for example, graphene-coated carbon nanofiber/sulfur composite materials for battery applications (Figure 1.9).26 Thus, the freestanding material on the basis of highly conductive rGO sheets with tightly intertwined mechanically stable CNFs scaffolds was prepared by one-step carbonization/reduction of cellulose/graphene oxide mats at 800 1C.27 CNFs act as nanospacers in the interpenetrated CNF/rGO network, which enhances the excellent volumetric electrochemical performance of supercapacitors. This mesoporous material, having well-interconnected graphene layers, can

Figure 1.9

Structure design and morphology of CNF interpenetrated graphene (CNFIG). (a) Schematic illustration of the synthesis of ultra-stable CNFIG architecture based on CNFs and graphene sheets; SEM images of (b) CNFs and (c) GO sheets; SEM images of CNFIG architectures at (d) low, (e) medium, and (f) high magnifications; (g) schematic representation of the compressible ability of CNFIG; (h) digital photos of the CNFIG architecture being compressed; (i) stress-strain curves of CNFIG at the 1st, 20th, and 30th cycles.26 Reproduced from ref. 26 https://doi.org/10.1038/s41467-019-11925-z under the terms of the CC BY 4.0 license http://creativecommons.org/ licenses/by/4.0/.

16

Chapter 1

be used as a supercapacitor composite electrode material, showing very promising volumetric values of capacitance, energy and power density. Similar 3D freestanding graphene hydrogel/carbon nanofibers composites, where CNFs are uniformly embedded into graphene nanosheets, were obtained28 by chemical reduction of a GO/CNF mixture at low temperature. The products can be used as an electrode directly without using any conductive additives or binders. One more related example corresponds to freestanding CNF/graphene nanosheet composites, which were fabricated using a membrane–liquid interface culture method with further carbonization.29 The CNFs and GNs are uniformly dispersed in a 3D conductive architecture, providing good structure stability, fine flexibility, and large specific surface area, being very promising superior supercapacitor electrodes30 for high-performance energy storage devices. It is worth mentioning that some reports indicate that the addition of nanostructured cellulose in any of its forms to GO/CNF composites has allowed improvements in the development of flexible supercapacitors that can retain up to 95% of their charge after being subjected to 2000 charge/ discharge cycles. In addition, they also have a maximum energy density of 60.4 mW h cm2, even in conditions where the material was flexed,31 and these qualities are extremely attractive for the manufacture of components that are part of portable electronic devices. Another potential application of graphene/CNF composites is in the area of biomedicine, since some studies ensure that the application of these nanostructures in polymeric matrices does not generate cytotoxicity.32 Furthermore, the addition of these carbon allotropes has a positive influence on the compression performance, thermal behavior, wettability and cell adhesion of the composites formed. A clear example in biomedicine is the composite formed by oxidized carbon nanofibers (O-CNF) and GO incorporated33 into the mineralized hydroxyapatite (Ca10(PO4)6(OH)2, (M-HAP)). The resulting product was found to be similar to natural bone (M-HAP) with high mechanical strength and corrosion protection improved by GO. This composite can be considered as a potential candidate for orthopedic and dental applications, unlike hydroxyapatite, which is not suitable to use in bone tissue engineering applications.

1.3.3

Graphene–Fullerene Composites

Fullerenes are carbon molecules with a morphology similar to a sphere that is hollow inside. These molecules can be presented with different amounts of carbon atoms as C60, C70, C80, etc. Since their discovery in 1985, they have attracted the interest of scientists because they have high stability, low density and are very electronegative.34 Some theoretical studies of functional density (DFT) show that the formation of composites made up of graphene and fullerene molecules, present a physisorption behavior with adsorption energies of 0.7 to 1.2 eV. The main interactions that occur between these two materials are from van der Waals

Hybrids of Graphite, Graphene and Graphene Oxide

17

forces. In addition, their union is also governed by permanent electrostatic Coulombic interactions that contribute at least 31% to their stability.35 This last interaction is due to the intrinsic polarizability of fullerenes, and this information allows us to better understand the synergies presented by the composites that will be seen in this section. Graphene nanobuds, which are nothing more than hybrid architectures produced from the union or fusion of fullerene molecules on a defective graphene sheet, have been successfully simulated using molecular dynamics.36 The results obtained showed that the tensile strengths were above 50 GPa and the elastic moduli were observed to degrade by a certain amount, though still remaining high. The Young’s muduli were calculated to be in the range from 0.43 to 0.77 TPa, depending largely on the structure configuration. In such C60/graphene composites, the C60 molecules are attached onto the graphene surface.37 Due to good characteristics at specific capacitance, it can be considered as a promising material for supercapacitor electrodes. Such graphene-based supercapacitor electrodes are indeed very attractive due to their chemical inertness, featuring high surface area and high electrical conductivity. Due to these and other useful properties, several theoretical investigations have been carried out on the elucidation of possible structural features and the stability of distinct graphene/fullerene composites, using fullerenes with sizes different from C60. Thus, sandwiched graphene–fullerene composites were considered in numerical simulations using grand canonical Monte Carlo calculations by forming covalent junctions between graphene layers and randomly dispersed fullerene units.38 Different fullerene types (i.e. C180, C320 and C540; their approximate radii are 0.6, 0.8 and 1.0 nm, respectively) that were considered in these simulations as the sandwich core can be seen in the Figure 1.10. This type of nanoarchitecture is similar to some that were already mentioned. In this case, the fullerenes would function as spacers between the graphene sheets, thus preventing their agglomeration. The properties presented by this composite make it a

Figure 1.10

The proposed atomistic models: (a) graphene–fullerene (C180) composites, (b) graphene–fullerene (C320) composites and (c) graphene–fullerene (C540) composites.38 Reproduced from ref. 38 with permission from Elsevier, Copyright 2016.

18

Chapter 1 39

promising candidate as a material for hydrogen storage. The result is that the hydrogen storage performance of sandwiched graphene–fullerene composites (maximum 5 wt.% at 77 K and 1 bar) can be improved by appropriate selection of fullerene types; in addition, their hydrogen adsorption capacity can also be enhanced considerably via Li doping. The key features of this structure are as follows: 1) H2 molecules can easily enter the nanostructure and pass between the graphene layers and fullerenes. 2) Interlayer spacing and pore size could be easily adjusted to increase the accessible surface area. 3) The first layer, formed by the hydrogen molecules, has a high molecular density of nitrogen and is thermodynamically stable, due to the interactions of graphene with the gas. 4) Fullerenes promote the formation of a stable bilayer, however, the presence of fullerenes has repercussions, reducing the space in which nitrogen can be stored.40 A DFT study was also carried out to elucidate the electronic structure of porous graphene (PG)/fullerene composites.41 It was revealed that smaller sized composites contribute to the efficient charge separation upon photoexcitation and, so, these composites may be utilized for designing solar cells, unlike the larger ones. These systems could be useful in many promising directions, such as electronics, photovoltaics, and optoelectronic devices. In addition, two different hybridized fullerene–graphene composites (Figure 1.11), a) C60 fullerenes physisorbed on a graphene monolayer and b) graphene nanoribbons functionalized with fullerene molecules such as C20, C60, C70 and C60 derivative PCMB ([6,6]-phenyl-C61-butyric acid methyl ester), were DFT-studied.42 It was revealed that the C60 adsorption on a graphene layer does not modify its low energy states, but it strongly influences its optical spectrum, introducing new absorption peaks in the visible energy region. In addition, the longitudinally polarized electro-absorption spectrum was found to be enriched with many new transitions coming from nanoribbon–fullerene hybridized states. Practically, such hybrid rGO/fullerenes nanocomposites were obtained (Figure 1.12) via slow diffusion of a GO suspension in 2-PrOH through C60 solution in m-xylene.43 In this case, nanowires of 200–800 nm in length that exhibited p-type semiconducting behavior are formed due to p–p interactions between rGO and fullerene molecules. An electron transfer between counterparts and hole transport through rGO was proposed. It should be noted that various functionalized graphene nanobuds are known, for instance those containing pyrene units, attached to C60,44 water-soluble fullerol/graphene nanobuds (Figure 1.13),45 and organic-functionalized pristine graphene/fullerene hybrids.46 In several reports it is mentioned that the addition of fullerenes causes a macro-scale structural arrangement of the layers of rGO that are usually disordered. This gives us the possibility of having a multiscale control of some

Hybrids of Graphite, Graphene and Graphene Oxide

19

Figure 1.11

Schematic view of different complexes of fullerenes physisorbed onto a graphene monolayer and armchair graphene nanoribbons. On top graphene/C60 and graphene/C60 and at the middle from left to right: graphene/ C20, graphene/C60, graphene/[6,6]-phenyl-C61-butyric acid methyl ester (PCMB) and graphene/C70. At the bottom the coordination modes for each system are shown.42 Reproduced from ref. 42 https://doi:10.3390/nano7030069 under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

Figure 1.12

Schematic illustration and SEM micrographs (a–e) showing the formation steps of rGO/C60 wires.43 Reproduced from ref. 43 with permission from American Chemical Society, Copyright 2011.

20

Chapter 1

Figure 1.13

Synthesis and tentative structure of the water-dispersible graphene/fullerol hybrid.45 Reproduced from ref. 45 with permission from Elsevier, Copyright 2017.

Figure 1.14

Schematic of the decolorization mechanism of dyes catalyzed by pure RGO and RGO/C60 hybrid fibers.47 Reproduced from ref. 47 with permission from Elsevier, Copyright 2019.

morphological parameters of the composite and being able to form new architectures, which facilitate the transfer of electrons, that could be applied in the catalytic decomposition of dyes (Figure 1.14).47 Furthermore, the high chemical reactivity of the curved sp2 carbon atoms in C60 in the nanowires

Hybrids of Graphite, Graphene and Graphene Oxide

21

rGO–C60 led to a selective covalent attachment of organic groups onto a fullerene molecule without affecting the aromatic system of graphene. This also works for nanobuds of carbon nanotubes, where the properties of nanobuds are not a sum of the properties of individual components. Such waterborne conductive graphene nanobuds could be used as sensors, thin films, fillers, and conductive inks, as well as an effective Li absorption material in batteries. On the other hand, the incorporation of fullerene/GO composites in polymeric matrices usually improves the elasticity properties of the resulting material, resulting in an increase in the Young’s modulus of up to 25 times in the resulting composite.48 However, other tests show that there is a decrease in other mechanical properties, such as the resistance of materials under both static and shock wave loads, so that the increase in Young’s modulus is mostly associated with the formation of an additional network of composites due to the formation of covalent bonds between fullerenes and GO, which does not interact directly with the polymer network.

1.3.4

Graphene Hybrids with Carbon Nanocages

Materials with 3D structures are usually highly studied in applications such as energy storage, since these structures give them great stability for the storage of charges in their interior. This is the case for carbon allotropes known as nanocages (CNCs), which are mesoporous cage-like nanostructures, made up of a graphene layer with a regular frame; some other applications in which they have been successfully implemented include hydrogen production and storage, catalysis, sensing, and drug and gene delivery.49 However, most of these three-dimensional carbon-based materials tend to have disordered structures, so some research focuses on creating threedimensional structures based on composites of different carbon structures, but even so, composites of ‘‘carbon cage – another carbon form’’ are practically unknown (except for fullerene hybrids). Some of the graphene/CNC composites reported in the literature applied to energy storage systems, more specifically as a component of lithium–sulfur batteries, are used as anodes that function as a host for the ions. Results of these composites can be seen enhanced with nitrogen, by providing a better charge coupling and greater stability of the material, such is the stability of this anode that allowed the battery to support 5000 cycles with a fading rate of only 0.066% per cycle and a good rate ability (880 mA h g1 at 2 1C).50 For its part, the modification at the nanometric level of some composites, makes it possible to provide favorable results to improve their application as part of energy storage devices, a clear example of this is an architecture inspired by the plant philodendron hederaceum, synthesized using the method of CVD in a fluidized bed reactor. The formation of this composite is based on vertically stacking the porous graphene sheets and the CNCs, in such a way that they are aligned and interconnected.51 This architecture allows the material to accelerate the transport of electrons. The connections formed in situ between the individual units of the CNCs are robust, and this morphological aspect makes it a

22

Chapter 1

Figure 1.15

TEM (a) and HRTEM (b) images of CNCs; (c) SEM image of GO; (d) TEM image of rGO/CNCs.52 Reproduced from ref. 52 with permission from Elsevier, Copyright 2015.

material with durable electrochemical properties. In addition, the pores of the graphene sheets facilitate filtration electrolyte and ion transport. In monitoring applications, there are reports of stable composites (Figure 1.15) formed by CNCs with rGO, prepared in situ by solvothermal reaction at 180 1C for 5 h, from CNCs obtained by Hummers method and GO.52 This composite was applied for the electrochemical detection of catechol (CC) and hydroquinone (HQ). The detection limits (S/N ¼ 3) for HQ and CC were found to be 0.87 and 0.40 mM, respectively.

1.3.5

Graphene(Graphene Oxide)-nanodiamond Composites

Nanodiamonds (NDs) are not structures that are usually used as composites with graphene, but they can be applied in field-effect transistors and other high current devices,53 as well as in catalysis. Thus, the composites of rGO modified with ND particles were obtained by heating aqueous GO suspensions, after 3 h ultrasonication for better exfoliation, with NDs at different ratios (1/1, 4/1, 10/1 and 20/1) at 100 1C for 48 h.54 The molar ratio of OH active groups on NDs is regarded to be sufficient for efficient GO reduction. The resulting rGO/NDs composites were found to be stable for 1 week in EtOH, acetone, CH3CN, DMF, and N-methyl-2-pyrrolidone, meanwhile showing a low dispersibility in THF, DMSO, water, and toluene. It was

Hybrids of Graphite, Graphene and Graphene Oxide

23

established that the electrochemical behavior (best for a GO : ND ratio of 10 : 1) depends on the initial GO/ND ratio used for the formation of the nanocomposites, being useful in the field of supercapacitors. Hybrid materials consisting of graphene and NDs can potentially display not only the individual properties of ND and graphene but also those resulting from synergism when they closely interact. Thus, ND/FLG (FLG: 2D support, few-layer-graphene, from 5 to 20, ND: nano-spacer for partly preventing the re-stacking of the FLG sheets) composite, prepared by mixing a suspension of FLG in ethanol with NDs, was further used as a stable metalfree catalyst for the steam-free direct dehydrogenation of ethylbenzene to styrene.55 The deactivated catalyst can be easily regenerated by oxidative treatment in mild temperature conditions. The same authors reported more complex systems for the same catalytic purpose: the exfoliation (17% yield) of few-layer-graphene (FLG) under 5 h sonication time of expanded graphite in water, carried out using GO as a surfactant, was used56 as a template for decoration with NDs, resulting in 3D-laminated sandwich-like nanostructures FLG–GO@NDs, containing spherical NDs (4–8 nm in diameter), homogeneously distributed on their surface. The GO double role was established, being responsible for the exfoliation of expanded graphite and incorporation of NDs for the formation of composites, concentrating ND nanoparticles on the surface of GO. This metalfree catalyst showed excellent performance in the dehydrogenation of ethylbenzene (35.1% of ethylbenzene conversion and 98.6% styrene selectivity). The deactivated catalyst can be efficiently regenerated by air calcination at 400 1C. Furthermore, defective ND/graphene composites were applied to prepare an atomically dispersed metal catalyst (Pd, Figure 1.16), which was used for selective hydrogenation of acetylene in the presence of abundant ethylene.57 The catalyst was shown to be stable, having high ethylene selectivity (90%) and high conversion (100%). Its structure, containing atomically dispersed Pd atoms on graphene through the formation of Pd–C bonds, allowed it to avoid formation of unselective subsurface hydrogen species. Ethylene is easily desorbed and not hydrogenized to undesired ethane. The high selectivity of the acetylene hydrogenation reaction is attributed to the competition of ethylene desorption at the catalytic active sites of Pd1/ND/graphene. On the other hand, the composites of rGO/NDs have been used to form part of polymeric matrices such as Polyaniline (PANI).58 These composites using conductive polymers are especially popular for creating electrodes and have better electrical conductivity compared to polymers without carbon allotropes (Figure 1.17). Furthermore, the synergy between these components promotes ion diffusion and structural stability during charge/discharge processes,59 which translates as an improvement in capacitive performance in general. Another similar example is the implementation of GO/ND composites in polymeric matrices of polymeric poly (diallyldimethylammonium chloride), in order to be used for thermal management applications.60 The method selected for the synthesis of the composite was

24

Figure 1.16

Chapter 1

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Pd1/ND/graphene at low (a and b) and high (c and d) magnifications; STEM (e) and HAADF-STEM (f) images of Pdn/ND/ graphene. (The inset in (b) is a diamond diffraction rings image; the atomically dispersed Pd atoms in (d) are highlighted by the white circles; and Pd nanoclusters in (f) are highlighted by the yellow circles).57 Reproduced from ref. 57 with permission from American Chemical Society, Copyright 2018.

Hybrids of Graphite, Graphene and Graphene Oxide

Figure 1.17

25

SEM images of (a) PANI; (b) rGO/PANI; (c) cNDs/rGO(1 : 4)@PANI; (d) cNDs/rGO(1 : 10)@PANI; (e) cNDs/rGO(1 : 20)/PANI; (f) cNDs/ rGO(1 : 50)@ PANI.58 Reproduced from ref. 58 with permission from Elsevier, Copyright 2019.

vacuum-assisted self-assembly. This composite exhibits thermal conductivity in the upper plane of 16.653 W m1 K1 which is about 80 times higher than that of traditional pure polymers. Furthermore, the maximum heat release rate of the management nanocomposite paper was only 99.16 W g1 at 439.2 1C. Two other less common applications of rGO/NDs composites in polymeric matrices are the production of ultra-sensitive humidity sensors61 and the manufacture of filtration membranes. In this last application, the addition of rGO/NDs to polyvinyl resins (PVC) improves the properties of hydrophilicity, porosity and surface roughness compared to membranes without the addition of these carbon nanostructures. In addition, the material could be subjected to higher water flows for purification, reaching flows of up to 200 Lm2 h1.62 Additionally, the mechanical aspects can also be positively affected by the addition of rGO/NDs, as has already been observed with other polymeric phenolic resin composites.63

1.3.6

Other Graphene–Carbon Composites

So far in this chapter, the most common graphene composites and carbon allotropes found in the literature have been analyzed. However, there are still many carbon allotropes whose implementations in composites with graphene are less frequent, which is why in this section we will make a compilation of these composites. Some of the carbon nanostructures that we will

26

Chapter 1

include in these sections are nanodots, nanorods, nanoonions, carbon black, nanoflowers, microporous carbon, and amorphous carbon. Microporous carbon/graphene composites were prepared directly from coal tar pitch and GO by KOH activation.64 These composites possess two advantages: abundant micropores for ion storage and incorporated graphene for high electron conduction, exhibiting excellent cycle stability and high specific capacitance. In core–shell graphene nanosheets (GNs) and amorphous carbon (AC) composites (GNs@AC), synthesized by a chlorination method, the GNs act as a shell, possessing high surface area, corrosion resistance, and excellent conductivity, and as a protecting coating to alleviate the degradation of the amorphous carbon core.65 The amorphous carbon nanoparticles are covered with a few layers of GNs, yielding a core–shell GNs/AC architecture. Upon further homogeneous deposition of Pt nanoparticles, the resulting Pt/GNs/AC catalyst displayed considerably higher stability and activity in comparison with the commercial Pt/C catalyst. This activity was attributed to the good conductivity of graphene that was also effective in inhibiting migration and aggregation of Pt nanoparticles by coating the amorphous carbon core. This composite also has promising applications in polymer electrolyte membrane fuel cells. Furthermore, studying the effect of reaction temperature in the range of 400 to 1000 1C on the synthesis of graphitic thin film on a nickel substrate, it was established that temperature was critical in the production of multilayer graphene (MLG) and AC films: the hybrid films of MLG and amorphous carbon can prepared at 600 1C, while MLG and AC film can be CVD-obtained at 800 1C and 400 1C, respectively.66 These films are promising materials for low-cost and simple carbon-based solar cells, which can be easily assembled. In addition, the thin (o15 layers) MLG microsheets and carbon nanoflowers (CNFl) form (Figure 1.18) a carbon–carbon nanocomposite, which has potential for increased capacity for lithium-ion insertion.67 In its structure, the interlayer distance in the sheets was increased by 12% compared to graphite. It was proven that graphene layers in the MLG/CNFl composite were rotated to each other. The CNFls were tightly bound with the MLG sheets and present both as small agglomerates and separate particles. On the other hand, the addition of carbon nanostructures, such as nanodots and nanorods, generates composites capable of being used as components in electrothermal heaters, due to the fact that they reduce the formation of fractures derived from the volumetric changes of the material. Quantum dots, for their part, improve the natural conductivity of graphene, creating materials that can be used as composite films with thermal conductivity values of 1.9786 W m1 K1 and electrical conductivity of 2.0534 S cm1,68 which are much higher than the values of the non-film material. Carbon nanodots by themselves on graphene can be used both for electrode manufacture69 as well as for some existing catalyst improvement, because they provide an increase in the surface area of the material and also allow one to increase the amount of active sites in catalysts, as demonstrated

Hybrids of Graphite, Graphene and Graphene Oxide

Figure 1.18

27

Formation mechanism of the multi-layer graphene (MLG)/carbon nanoflower CNFl composite. At low annealing temperatures, evaporative dissociation of the precursor particles created suitable conditions for nucleation of the SiC crystals (1). The growth of the MLG (2) both from the precursor particles and the SiC crystals was observed at this stage. At temperatures below 2200 1C, Si–C cores were visible in the precursor particles (3) but above 2200 1C they disappeared and CNFl were formed. After annealing at 2600 1C the dissociation of the SiC crystals was almost complete, resulting in the formation of the MLG/CNFl composite.67 Reproduced from ref. 67 with permission from Elsevier, Copyright 2015.

by the composites of nanodots on graphene hydrogels synthesized by a onestep hydrothermal method.70 In the case of nitrogen-doped nanoonion/rGO composites, they are applied with the purpose of generating better counter electrodes for solar cells. The synergy of both nanostructures added to the nitrogen groups, improve the long-term stability of the electrode. In addition, the device in which it was used showed an improvement in its energy conversion efficiency of 10.28% compared to similar platinum counter electrode devices,71 which can be attributed to the fact that the counter electrode formed by the composite presents more electrocatalytic active sites for the reduction of I3 to I. Finally, there are the composites made up of graphene/carbon black doped with nitrogen that is used as an electrocatalyst of the oxygen reduction reaction, to improve the efficiency in microbial fuel cells (MFC).72 Electrochemical tests demonstrate that this carbon composite can catalyze the oxygen reduction reaction in a neutral pH medium through a four-electron pathway with a positively changed starting potential, an improved current density and a reduced load transfer resistance. These results were attributed to the increase in surface area, the presence of abundant mesopores and the high nitrogen content, and in addition, the maximum power density of the air-cathode MFC using the composite as a cathode electrocatalyst, reached 936 mW m2 which is much higher than that of NG alone.

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CHAPTER 2

Production of Carbon Nanostructure/Graphene Oxide Composites by Self-assembly and Their Applications ´Nc,d AND ´REZ-GUZMA R. ORTEGA-AMAYA,*a,b M. A. PE a ´ PEZ M. ORTEGA-LO a

´n y de SEES, Electrical Engineering Department. Centro de Investigacio Estudios Avanzados del IPN, Av. IPN 2508, Col. San Pedro Zacatenco, ´noma Mexico City 07360, Mexico; b Physics Deparment. Universidad Auto Metropolitana, Unidad Iztapalapa, Av. San Rafael Atlixco 186, Col. Vicentina, Mexico City 09340, Mexico; c Nanoscience and Nanotechnology ´n y de Estudios Avanzados del IPN, Av. IPN Program. Centro de Investigacio 2508, Col. San Pedro Zacatenco, Mexico City 07360, Mexico; d Chemistry ´noma Metropolitana, Unidad Iztapalapa, Deparment. Universidad Auto Av. San Rafael Atlixco 186, Col. Vicentina, Mexico City 09340, Mexico *Email: [email protected]

2.1 Introduction The hybridization of the carbon 2s2p orbitals into the spn (n ¼ 1, 2, 3) hybrids allows for the formation of a great variety of carbon allotropes. As discussed in ref. 1 and 2, the sp2-bonding carbon atoms lead to a great variety of structurally complex carbon nanostructures exhibiting exotic physical properties. Among them, fullerenes, carbon nanotubes, graphene, and graphene-related nanomaterials are the most intensively studied. All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

31

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Chapter 2

Graphene-related nanomaterials include graphene oxide (GO), graphene quantum dots, and graphite quantum dots (Figure 2.1). All of them are currently studied as emerging multifunctional materials to fabricate a great variety of GO-based micro- and nano-structures for advanced applications in the technological areas of electronics, environmental remediation, storage and production of energy, biomedical sensors and implants, etc.3–7 The increasing interest in GO-derived materials has been fueled by the synergic effect between the GO unique chemistry and the development of cost-effective methods to produce GO at a mass produced level.8 On the other hand, the self-assembly process is a valuable bottom-up technique to easily produce macro-sized composites. The chemically prepared GO sheet self-assembly can easily be promoted in solution only by changing the pH or by adding nanoparticles or molecules. Resulting in 1D, 2D, and 3D nano- and micro-composites being obtained. A number of reports on the GO sheets chemistry produced by the Hummers’ method indicated that individual GO sheets have plenty of hydrophobic basal planes and hydrophilic oxygen-containing groups, including epoxy, hydroxyl, carboxyl, and carbonyl groups, which provide opportunities for further covalent or non-covalent functionalization to change the original functionality. Hence, GO is frequently considered either as a multifunctional material or as a valuable raw material to develop a number of nano- or micro-sized n-dimensional materials (n ¼ 1, 2, and 3) by several methods, including printing, hydrothermal, and self-assembly-based ones. Additionally, GO sheets or GO-preformed nanostructures can be combined with organic or inorganic molecular species or nanostructures for

Figure 2.1

Graphene related materials.

Production of Carbon Nanostructure/Graphene Oxide Composites

33

fabricating GO-based nanocomposites. Furthermore, GO-sheets have been proven as a suitable platform to be decorated and as a template to grow 2D inorganic chalcogenides. As such, GO-derived materials encompass a great variety of materials that display useful physicochemical properties for electronics, environmental remediation, storage and production conversion of energy, biomedical sensors and implants, etc.

2.2 GO Synthesis Methods Currently, chemically-prepared GO is the preferred precursor in the synthesis of graphene-based materials.9,10 It can be obtained by delamination of previously oxidized graphite flakes or natural graphite powder.11 The first reports on graphite oxidation by Schafhaeutl (1840),12 Brodie (1859)13 and L. Staudenmaier (1898) date back to the 18th century.14 However, the chemical reaction yield leading to graphite oxidation was rather low, and it was not until 1959 that Hummers and Offeman proposed a chemical route to efficiently oxidize graphite by using a mixture of sulfuric acid (H2SO4), sodium nitrate (NaNO3), and potassium permanganate (KMnO4).15 The Hummers and Offeman protocol, nowadays denominated as ‘‘the Hummers’ method to prepare GO’’, allows a faster and more efficient oxidation of the graphite. After the oxidation process, the obtained product is cleaned and then ultrasonically agitated to break off the graphite oxide particles into individual sheets (GO) and other graphitic nanostructures comprising unexfoliated and partially exfoliated graphite. Then an additional process is required to separate the different graphitic species. It should be mentioned that ultrasonic agitation is not the only technique that can be used in the graphite oxide exfoliation. Other possibilities are mechanical, thermal exfoliation, or a combination of these.16 More recently, some authors have proposed important modifications to increase the efficiency of the Hummers’ method.17 The more significant proposals are: adding a pre-treatment stage (pre-oxidation) prior to the KMnO4 oxidation reaction and avoiding NaNO3;18–20 increasing the amount of potassium permanganate, excluding NaNO3 and using phosphoric acid instead;17,21–23 performing the Hummers’ reaction in the absence of NaNO3;24 and varying the sulfuric acid concentration.25,26 Other approaches use oxidizing agents like benzoyl peroxide,27 potassium dichromate K2Cr2O7,28,29 a mixture of perchloric and nitric acids and potassium chromate,30 and potassium ferrate (K2FeO4),31,32 instead of NaNO3. Despite the significant changes that have been proposed with respect to the original Hummers proposal, to date, the Hummers’ method is still the basis for preparing graphene oxide (Figure 2.2).

2.2.1

GO Chemical Structure

In recent years, significant efforts have been directed toward elucidating the chemical structure of GO produced by the Hummers’ method. This has been

34

Chapter 2

Figure 2.2

Scheme of graphite exfoliation to obtain graphene oxide.

Figure 2.3

GO Lerf–Klinowski model.23 Reproduced from ref. 23 with permission from American Chemical Society, Copyright 1998.

hard work as the exact identity and distribution of oxygenated functional groups strongly depend on the oxidation procedure,17 the starting carbon source,18 and the purification protocols. Several models have been proposed to describe the structure and chemical composition of GO. Among them,19,20 the outstanding Lerf and Klinowski model describes the structure of moderately oxidized GO, assuming a graphene sheet containing oxygenated groups that are randomly distributed, epoxides and hydroxyls on the graphene sheet basal plane, as well as carbonyls and carboxylic groups, mainly located at the graphene sheet edge.21 Later, the Lerf–Klinowski model was extended by Wei Gao et al.22 by incorporating five- and six-membered-ring lactols and esters of the tertiary alcohols. Recent experimental studies, using modern sophisticated spectroscopy techniques, have validated the earlier proposed models for the GO chemical structure, especially that of Lerf and Klinowski, in which oxygen binds C to form CQO bonds preferentially on the graphene sheet edge.23 To date, the Lerf and Klinowski model continues to be the basis for representing the structure of graphene oxide.24–26 Additionally, a plausible model was also proposed for highly oxidized GO sheets (Figure 2.3).27

Production of Carbon Nanostructure/Graphene Oxide Composites

2.2.2

35

GO Functionalization

The functionalization or the surface chemical modification of a nanoparticle consists in the covalent or non-covalent attaching of specific atomic or molecular species to the nanoparticle surface. In carbon nanostructures, it was observed that, in contrast to the non-covalent functionalization method, covalent functionalization is stable and commonly involves oxidation of the graphitic matrix in the presence of a strong acid and oxidants.28 Conveniently, oxygenated functional groups attached to the carbon structures (derived from the oxidation process) make these highly hydrophilic and susceptible to further functionalization.29 GO is obtained by chemical exfoliation of graphite under strongly oxidative processes in aqueous medium,8 so that the final product comprises GO nanosheets with a disrupted conjugation, containing a great variety of the oxygenated chemical groups anchored to the graphene plane and the sheet edge.30,31 rGO is obtained from GO by a reductive process.32 Conveniently, both GO and rGO interact with various materials such as polymers,33–35 metal,36,37 and metal oxide nanoparticles38,39 easily. Oxygenated functional groups are highly hydrophilic and allow the use of various approaches already developed in chemistry in order to functionalize both the GO and rGO.40 This peculiar feature of GO makes it a truly versatile material to produce novel nanocomposites aiming to combine carbon nanostructures and graphene oxide properties to produce advanced materials with useful physicochemical properties for application in Li-ion batteries, photovoltaics, optoelectronics, supercapacitors, drug delivery, sensors, photocatalysis, etc.7,41,42 In general terms, the GO functionalization can be classified into four types of reactions. Namely: (I) nucleophiles (e.g. amines or hydroxyl groups) covalently bind to the carboxylic acid groups located principally at the edges of the GO sheets. (II) Covalent attachment at epoxy groups on GO basal planes through ring-opening reaction of amines. (III) Non-covalent functionalization of GO based on van der Waals (polymers, surfactants, and other small molecules) and p–p type interactions (polyaromatic hydrocarbon derivates) and (IV) functionalization of rGO through diazonium reaction, cycloaddition, etc.43,44

2.2.2.1

Non-covalent Interaction (Physical Adsorption)

Non-covalent functionalization is a useful strategy to improve the solubility and processability of carbon materials without affecting their exceptional physical properties.45 This method is based on the adsorption of molecules (surfactants, biomacromolecules, or wrapping with polymers) on the surface of carbon nanomaterials that occurs via weak interactions such as p–p stacking, van der Waals forces, electrostatic forces, hydrogen bonding, hydrophobic interactions, and CH–p interactions.46,47 Conveniently, the attractive forces in combination with the repulsive forces provided by the adsorbed surfactants stabilize the carbon materials.48

36

Chapter 2

GO and rGO can be functionalized through non-covalent interactions forming self-assembled multifunctional materials.46 The amphiphilic character of GO that involves its non-oxidized aromatic domains (p-conjugated, that allow p–p type interactions) and the presence of oxygen functional groups (that produce a negative surface charge) allows it to interact with a wide range of molecules or other materials, in several different ways.49–51 Positively charged molecules or nanoparticles can be easily adsorbed on GO/rGO via electrostatic interaction.52 Several authors reported non-covalent functionalization of GO/rGO with metal nanoparticles such as Ag, Au, and Pt through electrochemical deposition53 and in situ reduction methods,54 among others.

2.2.2.2

Covalent Interaction

Covalent functionalization of graphene oxide is an effective method to modify and modulate graphene oxide properties through different anchored functional groups.51 GO covalent functionalization involves the formation of covalent bonds of specific molecular species (free radicals, dienophiles or organic molecules) to the atoms present in GO sheets (CQC bonds or oxygen-containing functional groups) to alter the physicochemical properties.46,55,56 Several authors have reported on the covalent functionalization of graphene oxide sheets with various molecular species, finding that the physicochemical properties such as electrochemical potential (the Fermi level of a solid), adsorption ability, chemical stability, biocompatibility, and dispersibility vary according to the nature of the bound molecule to the surface.57–60 It is especially important to note that this type of variation allows the production of graphene oxide-based composite materials with useful physicochemical properties for a specific application.61 Among other factors, GO applications are highly dependent on the interactions of materials with it; therefore, understanding the interaction between GO and materials is fundamentally essential.62–64 To date, a few excellent reviews have summarized the mechanisms of a variety of the covalent functionalization approaches of GO in detail.65 The covalent functionalization of graphene oxide has been investigated using a significant number of molecular species including aliphatic and aromatic isocyanate derivatives,66,67 octadecylamine (ODA),68–71 3-aminopropyltriethoxysilane (APTS),72,73 thionyl chloride (SOCl2),74 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC),75 etc. The covalent functionalization of graphene oxide substantially modifies its physicochemical properties. Studies indicate that fundamental properties such as the band structure (hence the electrical–optical properties) of graphene oxide can be altered by covalent functionalization leading to an insulating-semiconductor transition.76

2.2.3

GO Self-assembly

Self-assembly of nanosized materials is a powerful bottom-up strategy to build macroscopic structures.77 The amphiphilic character of GO makes

Production of Carbon Nanostructure/Graphene Oxide Composites

37

it a versatile material to be self-assembled into a variety of hierarchical n-dimensional (n ¼ 1, 2 and 3) structures.78 Aqueous dispersions of GO have been used as a precursor to prepare self-assembled two-dimensional thin films and monolithic three-dimensional pure GO or GO combined with organic and inorganic nanostructures for water remediation, sensing, catalysis, photovoltaic films, and biomedical applications. The amphiphilic properties of the Hummers-prepared GO sheets arise from the presence of both unoxidized hydrophobic graphene-like regions on the basal plane and hydrophilic oxygen-containing groups attached to the graphene sheet edge, both formed during graphite oxidation.11 In water-dispersed GO sheets, the functional groups become electrically charged depending on the pH of the aqueous medium.79 These dispersions may be easily destabilized by a slight pH change or by adding powdered or colloidal nanoparticles. The so-formed flocs are comprised of pure selfassembled GO or nanoparticle-decorated self-assembled GO, respectively.80 The GO self-assembly process has been observed to occur by destabilizing the GO dispersion, in hydrothermal processes and in the presence of interfaces such as the liquid–air, liquid–liquid, and liquid–solid interfaces.81 Two-dimensional GO self-assembled thin films were prepared by spin-coating, dip-coating, vacuum filtration, and Langmuir–Blodgett (LB) assembly. Three-dimensional GO structures are usually fabricated by lyophilization of GO gels.82 The above-mentioned GO-based assembled materials may be obtained in a wide variety of forms including membranes,83 films,84 sponges,42,85 hydrogels,80,86,87 foams,88 nanofibers,89 etc. During the self-assembly process, GO sheets interact with each other through electrostatic, van der Waals, and p–p stacking interactions and hydrogen bonding. When ions, molecules, nanoparticles, or polymers participate in the process, the functional oxygenated groups play the key role because they might exchange electrical charge with these chemical species.78

2.3 Carbon-based Composites (GO, CQD, and CNT) GO as a 2D material can be decorated due to its large surface area, the functional groups present on the surface, and its possibility to be manipulated to form composite materials that can be produced by several methods. In general, we can classify them into in situ and ex situ processes (Figure 2.4),7 and there is also the possibility of make mixtures among them. The final product comprises hierarchical 1D, 2D, and 3D structures and macroscopic composites (fibers, thin films, membranes, papers, and porous materials).

2.3.1

Graphene Oxide–Carbon Quantum Dot Composites

There are few reports about CQD–GO composites, where graphene oxide is used as the support having a high surface area, biocompatibility, tunable

38

Figure 2.4

Chapter 2

Production methods of carbon-based nanocomposites.

conductivity, flexibility, and surface functionalization that can lead to high sensitivity and selectivity. Nevertheless, GO re-staking is promoted for the van der Waals and p–p interactions, which in solution leads to the agglomeration and precipitation which is undesirable for biological applications and also decreases the surface area, compromising device performance. To overcome the re-staking and maximize the surface area, molecules or inorganic nanoparticles may be employed. In recent years, CQD has been used taking advantage of their low production cost and their unique properties such as low toxicity, biocompatibility, tunable conductivity, remarkable optical properties, and good electron donors and acceptors. The GO–CQD composite production methods include hydrothermal, colloidal, and electrochemical processes that allow one to obtain different structures

Production of Carbon Nanostructure/Graphene Oxide Composites

Figure 2.5

39

TEM images of a GO sample after being reduced via a CA aqueous solution. (a and b) show rGO sheets without and with rGOQD respectively. The inset in (b) displays the lattice fringes of rGOQD. (c and d) display, respectively, low- and high-magnification TEM images of the rGOQD. (d) provides details of the interplanar spacing of the rGOQD. Reproduced from ref. 100 with permission from IOP Publishing Ltd, Copyright 2016.

such as thin films and electrodes formed by 3D structures. GO–CQD-based composites have been used in supercapacitors, catalysis, biological sensors, bioimaging, and bioremediation (Figure 2.5). The recent progress has been focused on the development and study of composites based on graphene oxide/reduced graphene oxide–carbon dots (GO/rGO–CD) for electronic applications. In the case of supercapacitors, Chen et al.82 synthesized through an electrochemical method a composite with hierarchical porosity based on graphene quantum dots (GQD) decorated on three-dimensional reduced graphene oxide (3D-rGO) slices as the building scaffolds for high performance supercapacitors. They found that the electrochemical assembly of the GQD on the 3D-rGO proceeded smoothly and led to the formation of a uniform film on the surface of the 3D-rGO. The results indicated that the fabricated GQD/3D-rGO composites as electrodes for symmetrical supercapacitors exhibited high stability and a high specific capacitance of 268 F g1, which was much higher than that of pure 3D graphene electrodes (136 F g1). Later, Xu et al.90 investigated binder-free supercapacitors composed of graphene quantum dot (GQD)-anchored reduced graphene oxide (rGO) nanosheets. There were several studies of GO and GQD

40

Chapter 2

composite (GO/GQD) electrodes with different mass ratios of GO to GQD (6 : 1, 3 : 1, 1 : 1). The results showed that the gravimetric specific capacitance values of the rGO/GQD-3 : 1 composite electrode (296 F g1 at 0.5 A g1) were superior to those of the rGO/GQD-1 : 1 electrode. They reported that important factors to obtain a high capacitance performance are the mass ratios of rGO/GQD composites and their morphological characteristics. Additionally, due to the large specific surface area of GQD, the interfacial property among GO can be enhanced remarkably. Moreover, Feng et al.86 fabricated a flexible solid-state supercapacitor composed of reduced graphene oxide hydrogel/carbon dot (rGH/CD) films. The three-dimensional (3D) interconnected network rGH/CD composite electrode was obtained through a hydrothermal process. They reported an areal specific capacitance of 394 mF cm2, high gravimetric specific capacitance (264 F g1 at 1 A g1), excellent cycling stability (9.1% deterioration after 5000 cycles), larger energy density (35.3 W h kg1), in addition to high power density (516 W kg1) for a 130 mm thick rGH/CD electrode. Additionally, they also reported that the carbon dots in the reduced graphene oxide hydrogel provided a large specific surface area, contributed to electron transport, and reduced the charge transfer resistance as well as internal resistance of the electrode materials. More recently, Luo et al.91 synthesized graphene quantum dot (GQD)/threedimensional reduced graphene oxide (3DrGO) composites for supercapacitors by a hydrothermal method. GQD were fabricated by chemically oxidation and cutting carbon fibers. They reported that the composites of GQD (40 wt%) and rGO can achieve 22% higher specific capacitances (242 F g1) compared to pristine 3DG electrodes (198 F g1) because of the larger specific surface area and better electrical conductivity. The experimental results also indicated that after 10 000 charge–discharge cycles, the capacitance of the GQD/3DrGO composites remained at 93% of the initial value. On the other hand, in the study to boost the electrochemical performance of 3D reduced graphene oxide (3DrGO), Yuan et al.92 employed, as an efficient nano-enhancer, nitrogen-doped carbon dots (N-CDs). The N-CDs/ 3DG interconnected framework nanocomposites were synthesized through a one-step hydrothermal method using graphene oxide (GO) dispersed in a N-CD aqueous suspension. They reported that when used as a binder-free electrode for supercapacitors, the maximum specific capacitance and areal capacitance at a N-CD/3DG nanocomposite current density of 0.5 A g1 were 338 F g1 and 604 mF cm2 respectively, which are notably higher values than those for pristine 3D graphene. The experimental results also indicated that nitrogen-doped carbon dots can improve the rGO performance for highperformance electrochemical energy storage (Figure 2.6). In the case of biosensors, GO CQD composites are typically used as a dopamine (DA) detector. DA is a neurotransmitter in the human brain, that coexists with ascorbic acid (AC) and uric acid (UA), which have similar oxidation potentials as DA therefore resulting in interference during the detection. Graphene materials are therefore studied, since they are able to modify the properties of the working electrode by enhancing the redox

Production of Carbon Nanostructure/Graphene Oxide Composites

Figure 2.6

41

A scheme of the supercapacitor properties, an SEM image of the composite, and a scheme showing the graphene oxide decorated with CD. Reproduced from ref. 92 with permission from Elsevier, Copyright 2019.

response of DA. On the other hand, CD can increase the charge storage capability allowing signal amplification. Regarding specific composites, Hu et al.93 used a rGO–CD film as a DA sensor. The rGO–CD was obtained by a hydrothermal process, and some of the resulting solution was deposited into a bare glassy carbon electrode (GCE) with a micro-injector. This hybrid material displayed a DA detection range of 0.01–450 mM, a detection limit of 1.5 nM, and the electrode was stable for several detection cycles. Subsequently, Fang et al.94 produced a microelectrode formed by carbon fibers decorated by GO and CQD. They tested the electrode as a dopamine (DA) sensor. The composite was fabricated by forming the CF by protrusion of carbon fiber through a capillary tube, then the GO and CD were electrochemically deposited onto the carbon fiber to obtain rGO/CF/CD. The performance of the microelectrode was tested by differential pulse voltammetry with a three-electrode system with an auxiliary electrode of platinum wire, a reference electrode of AgIAgCl, and a working electrode of composite material. The electrode had linear DA detection in the range of 0.1–100 mM, a detection limit of 0.02 mM and was stable during 10 repeated DA detection cycles. More recently, Wei et al.95 produced an electrode formed by GO–CD electrochemically deposited on the surface of GCE, to detect AA, DA, and UA simultaneously. The electrode had linear DA detection in the range of 1–10 mM, a detection limit of 1.34 mM and was highly stable. Another possibility for GO–CD application is in the oxygen reduction reaction (ORR), which is important in fuel cells where the conversion of chemical energy of H2 or an alcohol is converted into electric energy.96 In the OOR, metals such as Pt are suitable, but nowadays the metal-free oxygen reduction reaction has become important due to the low-cost production method and in particular, it is known that carbon materials change the chemisorption mode of O2 and/or reduce the ORR potential, enhancing its catalytic performance. Meanwhile, GQD act electrochemically at the edge planes instead of the basal plane, enhancing the high selectivity and stability

42

Chapter 2 97

in alkaline media. For instance, Fei et al. synthesized self-assembled boron- and nitrogen-doped graphene quantum dots/reduced graphene oxide (BN-doped GQD/rGO) hybrid nanoplatelets by hydrothermal reaction and post-annealing treatment for the oxygen reduction reaction (ORR). The coal and graphite were chosen as inexpensive raw materials to obtain graphene quantum dots and GO, respectively. This hybrid material exhibited abundant edges and doping sites as well as a large surface area and good electrical conductivity, features derived from the combination of components. They found that hybrid nanoplatelets exhibited excellent electrocatalytic activities for the ORR, with B15 mV more positive onset potential and similar current density to commercial Pt/C. Besides, Zhang et al.98 synthesized nitrogen-doped graphene quantum dots (N-GQD) anchored on thermally-reduced graphene oxide (rGO) as an electrocatalyst for the oxygen reduction reaction (ORR). N-GQD/rGO hybrid catalysts were synthesized by using dopamine (nitrogen-rich precursor) and GO (electrocatalyst support). The N-GQD (2.5–8.5 nm) were grown in situ on thermally-reduced graphene oxide. They reported that the N-GQD/rGO catalyst exhibited comparable ORR activity, better durability as well as superior tolerance to the methanol crossover effects compared to the Pt/C catalyst. In another study, Vinoth et al.99 used the same hydrothermal technique to develop nitrogen-doped graphene quantum dots (N-GQD) decorated on a three-dimensional (3D) MoS2–reduced graphene oxide (rGO) framework. The 3D N-GQD/MoS2–rGO nanohybrid was then used as an efficient electrocatalyst for the oxygen reduction reaction (ORR) under alkaline conditions. Due to the chemical interaction between the N-GQD and MoS2–rGO, as well as the increased surface area and pore size of the N-GQD/MoS2–rGO nanohybrid, the ORR onset potential was increased to þ0.81 V versus the reversible hydrogen electrode (RHE). They also reported that the N-GQD/MoS2–rGO nanohybrid displayed an enhanced tolerance to methanol (3.0 M) under alkaline conditions (Figure 2.7). We produced a GOQD–rGO composite100 and analyzed the mechanism formation of GOQD during the graphene oxide reduction, by citric acid in aqueous media at room temperature, proposing a green and simple one-step method to produce rGOQD/rGO composites. Moreover, Yu101 examined the electron transfer in carbon nanodot (CND)–graphene oxide (GO) nanocomposites without linker molecules, using steady-state and time-resolved spectroscopy. The GO was chosen as carrier acceptors as it is an excellent combination of properties and processability. In their study, significant quenching of CND–GO nanocomposite fluorescence was observed, indicating ultrafast electron transfer from CNDs to GO (time constant of 400 fs). They also reported that CND–GO nanocomposite could be an excellent candidate for hot carrier solar cells. Kumawat et al.102 explored a hybrid material of GO/GQDs, prepared by electrostatic layer-by-layer assembly using a polyethylene imine (PEI) bridge. They tested the hybrid material, as well as the individual GO and GQD in cell imaging, and photothermal and oxidative stress in breast cancer cells. GQD were synthesized by the hydrothermal

Production of Carbon Nanostructure/Graphene Oxide Composites

Figure 2.7

43

Electrochemical response and comparison between N-GQD/rGO and Pt/C. Reproduced from ref. 98 with permission from John Wiley & Sons, Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

carbonization of L-glutathione, GO was obtained by the Hummers method. Solutions of GO and PEI (ratios from 1 : 1 to 1 : 4 GO:PEI) were mixed, the obtained solution was washed and centrifuged to remove excess PEI, and finally, a solution of GQD was added, which was stirred under ambient conditions, after a centrifugation process was done to remove the unlinked GQD. The hybrid material displayed a stable fluorescence and a rise in the cytotoxic and photothermal activities on cancer cells. Finally, Zhang et al.103 replaced the commonly used catalytic metals (Pd, Au, AuAg, FePd, etc.), with GQD supported on an rGO matrix with a 3D composite; this composite was tested in the reduction of nitroarenes, where the zigzag edges and defects present on rGO and the larger periphery area of the GQD act as catalytic active sites. The composite was prepared by a hydrothermal route, the product obtained consisted of a 3D material. The 3D materials as well as individual rGO and GQD were tested as catalytic materials in the reduction of nitroarene with NaBH4 as the reducing agent. They found that all composites had higher catalytic activity than the rGO or GQD, and that the better results were for the GOQD/rGO 1 : 4 weight ratio. Finally, it is important to note that the catalyst turnover frequency (TOF), this is the number of molecules of nitroarene that can be reduced by 1 g of catalyst per unit of time, was higher compared with Ag/CA or Au/CA systems (Figure 2.8).

44

Chapter 2

Figure 2.8

Bioimages of fibroblast and cancer cells with contrast agent (GO–PEI– GQD, GQD) and control. Reproduced from ref. 102 with permission from Elsevier, Copyright 2019.

Figure 2.9

Scheme of the formation of the composite rGO, N-CNT, and SEM images of the composite. Reproduced from ref. 110 with permission from the Royal Society of Chemistry.

2.3.2

Reduced Graphene Oxide–Carbon Nanotube Composites

Graphene oxide and CNT have been studied with great interest for the last decade due to their unique physicochemical properties derived from their structural features. The combination of these two carbon nanomaterials gives rise to composites in which the GO can be the matrix for the nanostructures, or both materials can be immersed in a matrix of a metal or polymer, which provides the opportunity to combine the properties of the graphene oxide and CNT with those of other components, increasing the number of possible applications,104–106 which includes supercapacitors, ion batteries, water remediation, and thermal interference materials (Figure 2.9).

Production of Carbon Nanostructure/Graphene Oxide Composites

45

In regard to supercapacitors, electrochemical electrodes, and ion batteries, Yang and co-workers107 have shown that electrochemically reduced graphene oxide/CNT composite films can be fabricated by an electrochemical method for electrochemical energy storage applications. They informed that the GO can be effectively reduced with the assistance of carbon nanotubes (CNT). Moreover, the resulting rGO/CNT composite films could then be used as active materials for electrochemical energy storage applications. Experiments indicated that the mass ratio of the GO/CNT composite was a factor that strongly affected the specific capacitance. In their study, the specific capacitance (279.4 F g1) of the rGO/CNT composite electrode remained above 90% after 6000 cycles. Interestingly, Liu et al.106 reported an asymmetric fiber-shaped supercapacitor based on two core sheathed electrodes assembled using a single carbon nanotube fiber as the core and three-dimensional (3D) porous reduced graphene oxide (rGO) or carbon nanotubes/polypyrrole as the sheath obtained through one-pot electrochemical deposition. The resulting CNF/rGO composite negative electrode exhibited abundant pore architecture that enlarged the ion accessible surface area, benefiting the good capacitive performance and cyclic stability of the carbon composite electrode. Apart from the solution-based self-assembly method, vacuum filtration is another common technique used by other researchers. For instance, Sammed et al.108 synthesized a carbon nanotube and reduced holey graphene oxide film (RHGOF) sandwich structure by using the combination of vacuum filtration technique and chemical vapor deposition for binder-free electrodes. They determined that the binder-free porous hybrid electrode has advantages over pristine graphene, such as higher ion diffusion rate, longer diffusion length, and larger ion accessible surface area. At 0.5 A g1, the current density of 3D sandwich RHGOF/CNT hybrid materials can achieve a high specific capacitance of 557 F g1. For electrochemical applications, Luo et al.109 developed and investigated a reduced graphene oxide-coated a-helical carbon nanotube (HCNT) network which was synthesized through a solution-based self-assembly method. This composite displayed a 3D ferroconcrete-like structure for use as an effective sulfur host that consists of 1D HCNT and 2D rGO nanosheets. The results demonstrated that rGO plays a dual role in HCNT interacting with the GO network to be wrapped in GO sheets. Where the rGO sheets can coat HCNT to form a 1D/2D HCNT/rGO heterostructure and provide a coating that protects S. Also, they reported that there is a notable improvement in the electrochemical performance, when it is used as a cathode the HCNT/rGO/S composite exhibits an initial discharge capacity and stable reversible capacity of 1196 mA h g1 and 1016 mA h g1 after 200 cycles at 0.1 C, respectively. In order to obtain a host material for energy conversion and storage, Yan et al.110 combined N-doped CNT with few-layer reduced graphene oxide (Figure 2.9). The resulting bead-like nitrogen-doped CNT/graphene composites (NCNT/G) were synthesized by direct pyrolysis of N-rich melamine through a CVD technique. The melamine was utilized as a carbon and nitrogen source, where GO acts as a substrate. The results further revealed that N-doped CNT are vertically organized on the RGO

46

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nanoplates to create a potentially useful 3D structure. They reported, as expected, that nitrogen atoms in the carbon framework can not only induce structural defects and active sites but also provide benefits to the surface wettability and improved electrical conductivity. In 2019, Shi et al.111 fabricated graphene/CNT–S/graphene/CNT integrated cathodes. They reported 3D cross-linked GO/CNT aerogels obtained by freeze drying, that were reduced to 3D G/CNT aerogels by self-propagating combustion, for use as a sulfur host and ultralight internal current-collector interlayer in ultrahigh volumetric-energy-density lithium sulfur batteries. The results showed that the hybrid material can serve as a superior sulfur host. They reported a volumetric capacity of 1841 A h L1 and volumetric energy density of 2482 W h L1, both of which were the highest values for LiS batteries reported to date. Other applications including the study of the aqueous behavior of rGO, thermal interface materials, and membranes for water remediation are described below. In the study to develop and understand rGO–CNT hybrid materials, Azizighannad112 investigated the aqueous behavior of rGO–CNT which were synthesized through controlled in situ reduction of GO using nascent hydrogen. They determined that oxygen content could be controlled by a suitable synthesis process to form hybrids with levels ranging from 26 to 2%. Additionally, they also observed that the rGO–CNT hybrid material behaved strongly dependent on the level of reduction; the GO character was predominant at a low level of reduction, while as the rGO–CNTs reduction increased, the CNTs character was the most important. Additionally, the experimental results indicated an increase in the hydrophobicity from 3.2 to 7.4% of the rGO–CNT hybrid material as oxygen concentration decreased from 26 to 2%. Additionally, Yuan et al.113 synthesized a promising thermal interface material for thermal packaging applications from CNTs and reduced graphene oxide. The layered structure rGO/CNT composite film was obtained through thermal annealing of the film prepared by vacuumassisted flow filtration of the GO/CNT suspension. CNT among the reduced graphene oxide layers were used to improve the mechanical and thermal properties of the layered structure rGO/CNT composite films. In their study, compared with the rGO film, they found that the 1 wt% CNT reduced the Young’s modulus by 93.3% and increased the tensile strength of the rGO/CNT composite film by 60.3%, which could significantly enhance its flexibility. Moreover, Yang et al.114 developed a 3D all-carbon nanofiltration (NF) membrane with abundant two-dimensional (2D) nanochannels, consisting of multi-walled carbon nanotubes interposed between layers of graphene oxide, linked by diallyldimethylammonium chloride (PDDA), and this composite was tested for chemical separation applications. This promising membrane through electrostatic interactions can physically sieve antibiotic molecules. In their study, for a membrane thickness of 4.26 mm, the adsorption was 99.23% for tetracycline hydrochloride (TCH), and water permeation was 16.12 L m2 h1 bar1. In addition, the results indicated that the multi-walled carbon nanotube/GO composite could be useful for other applications as the dye methylene blue (MB) was also removed by 83.88% (Figure 2.10).

Production of Carbon Nanostructure/Graphene Oxide Composites

Figure 2.10

47

3D carbon nanofiltration membrane of MWCNT-GO linked by PDDA and the efficiency of the composite at different pH and in different solutions. Reproduced from ref. 114, https://doi.org/10.1186/s11671-018-2555-9 under the terms of the CC BY 4.0 license, http://creativecommons.org/ licenses/by/4.0/.

2.4 Conclusion This chapter reviewed the most important aspects involved in the preparation of micro-sized n-dimensional (n ¼ 1, 2, 3) graphene oxide-based self-assembled composites. The preparation methods as well as the GO chemical structure were concisely described. It was noted that the Hummers method produces amphiphilic GO sheets whose physicochemical properties might be tailored by varying the oxidation degree of the GO sheet or by functionalizing it. The unique chemical structure of GO promotes its easy self-assembly and offers the option for further functionalization. Hence graphene oxide may be cataloged as a multifunctional material. In fact, the GO assembled structures and composites already have real applications, as indicated by the described applications for the composites.

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80. M. J. Nine, T. T. Tung and D. Losic, in Comprehensive Supramolecular Chemistry II, ed. J. L. Atwood, Elsevier, Oxford, https://doi.org/10.1016/ B978-0-12-409547-2.12634-4, ch. 9.04 – Self-Assembly of Graphene Derivatives: Methods, Structures, and Applications, 2017, pp. 47–74. 81. J.-J. Shao, W. Lv and Q.-H. Yang, Adv. Mater., 2014, 26, 5586–5612. 82. Q. Chen, Y. Hu, C. Hu, H. Cheng, Z. Zhang, H. Shao and L. Qu, PCCP, 2014, 16, 19307–19313. 83. L. C. Cotet, K. Magyari, M. Todea, M. C. Dudescu, V. Danciu and L. Baia, J. Mater. Chem. A, 2017, 5, 2132–2142. 84. M. M. Barsan, M. David, M. Florescu, L. T - ugulea and C. M. A. Brett, Bioelectrochemistry, 2014, 99, 46–52. 85. V. Chabot, D. Higgins, A. Yu, X. Xiao, Z. Chen and J. Zhang, Energy Environ. Sci., 2014, 7, 1564–1596. 86. H. Feng, P. Xie, S. Xue, L. Li, X. Hou, Z. Liu, D. Wu, L. Wang and P. K. Chu, J. Electroanal. Chem., 2018, 808, 321–328. 87. M. I. Lescano, A. Gasnier, M. L. Pedano, M. P. Sica, D. M. Pasquevich and M. B. Prados, RSC Adv., 2018, 8, 26755–26763. 88. H. S. Ahn, J.-W. Jang, M. Seol, J. M. Kim, D.-J. Yun, C. Park, H. Kim, D. H. Youn, J. Y. Kim, G. Park, S. C. Park, J. M. Kim, D. I. Yu, K. Yong, M. H. Kim and J. S. Lee, Sci. Rep., 2013, 3, 1396. 89. Z. Tian, C. Xu, J. Li, G. Zhu, Z. Shi and Y. Lin, ACS Appl. Mater. Interfaces, 2013, 5, 1489–1493. 90. Y. F. Xu, X. Li, G. Hu, Y. Luo, L. Sun, T. Tang, J. Wen, H. Wang and M. Li, Int. J. Electrochem. Sci., 2017, 12, 8820–8831. 91. P. Luo, X. Guan, Y. Yu, X. Li and F. Yan, Nanomaterials, 2019, 9, 201. 92. G. Yuan, X. Zhao, Y. Liang, L. Peng, H. Dong, Y. Xiao, C. Hu, H. Hu, Y. Liu and M. Zheng, J. Colloid Interface Sci., 2019, 536, 628–637. 93. S. Hu, Q. Huang, Y. Lin, C. Wei, H. Zhang, W. Zhang, Z. Guo, X. Bao, J. Shi and A. Hao, Electrochim. Acta, 2014, 130, 805–809. 94. J. Fang, Z. Xie, G. Wallace and X. Wang, Appl. Surf. Sci., 2017, 412, 131–137. 95. Y. Wei, Z. Xu, S. Wang, Y. Liu, D. Zhang and Y. Fang, Ionics, 2020, DOI: 10.1007/s11581-020-03703-5. 96. X. Tong, Q. Wei, X. Zhan, G. Zhang and S. Sun, Catalysts, 2017, 7, 1. 97. H. Fei, R. Ye, G. Ye, Y. Gong, Z. Peng, X. Fan, E. L. G. Samuel, P. M. Ajayan and J. M. Tour, ACS Nano, 2014, 8, 10837–10843. 98. B. Zhang, C. Xiao, Y. Xiang, B. Dong, S. Ding and Y. Tang, ChemElectroChem, 2016, 3, 864–870. 99. R. Vinoth, I. M. Patil, A. Pandikumar, B. A. Kakade, N. M. Huang, D. D. Dionysios and B. Neppolian, ACS Omega, 2016, 1, 971–980. ´rez-Guzma ´n 100. R. Ortega-Amaya, Y. Matsumoto, A. Flores-Conde, M. A. Pe ´pez, Mater. Res. Express, 2016, 3, 105601. and M. Ortega-Lo 101. P. Yu, X. Wen, Y.-R. Toh, Y.-C. Lee, K.-Y. Huang, S. Huang, S. Shrestha, G. Conibeer and J. Tang, J. Mater. Chem. C, 2014, 2, 2894–2901. 102. M. K. Kumawat, M. Thakur, R. Bahadur, T. Kaku, R. S. Prabhuraj, A. Ninawe and R. Srivastava, Mater. Sci. Eng., C, 2019, 103, 109774.

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Section 2: Carbon Nanotube Composites

CHAPTER 3

Synthesis of Carbon Nanotube/ Graphene Hybrids by Chemical Vapor Deposition ZHI LIU,a HUA-FEI LI,b SHUGUANG DENG( 0000-0003-28923504)*c AND GUI-PING DAI( 0000-0001-9208-9994)*a a

Department of Chemical Engineering, School of Environmental and Chemical Engineering, Nanchang University, Nanchang 330031, Jiangxi, China; b Institute for Advanced Study, Nanchang University, Nanchang, Jiangxi 330031, China; c School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287, USA *Emails: [email protected]; [email protected]

3.1 Introduction Graphene, a 2D honeycomb lattice architecture, is composed of sp2 hybridized single-layer carbon atoms1,2 (see Figure 3.1a). Consequently, graphene exhibits a lot of extraordinary chemical and physical properties attributed to the stable hexagon structure. To date, graphene is the lightest and thinnest two-dimensional material in the world, with a specific surface area of up to 2630 m2 g1 and a thickness of only 0.335 nm.3 Furthermore, an increasing number of research results reveal that graphene has great application prospects in the field of nano-devices such as in supercapacitors,4 lithium batteries,5 field-effect transistors,6 biosensors7 and so on. Single-layer or multi-layer graphene curls around the center at a certain angle, forming the seamless, hollow carbon nanotubes (the typical 1D carbon nanomaterials) (see Figure 3.1b). All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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Figure 3.1

Chapter 3

Schematic diagrams of (a) carbon nanotube and (b) graphene.

Carbon nanotubes display the following merits: outstanding mechanical strength, high electrical conductivity, good stability, large specific surface area, and excellent optical and thermal properties,8–12 and play an increasingly important role in aerospace, medical, and health, electronic equipment, transportation, machinery, and other fields. As the new nanocarbon materials, carbon nanotubes, and graphene are sought after for many scientific research works on account of their unique properties. However, in the practical synthesis process, their physical and chemical properties will be decreased due to inevitable stacking and self-aggregation caused by intermolecular van der Waals forces.13,14 Therefore, three-dimensional carbon hybrids (carbon nanotubes/graphene) have gradually received more and more attention during recent years. It is hoped that by constructing a threedimensional (3D) hierarchical structure, the shortcomings of individual carbon nanomaterials could be effectively mitigated. In addition, nanocarbon materials of different dimensions could be synergistically integrated, thereby generating unique physical and chemical properties. This kind of 3D nanomaterial usually possesses a unique hierarchical structure, resulting in wider application prospects such as in field-effect transistors,15,16 sensors,17,18 fuel cells,19,20 batteries,21,22 supercapacitors,23,24 and so on. The basic principle of the CVD technique in the fabrication of nanocarbon materials on the metal substrate is that at sufficient temperature, appropriate carbon precursors are introduced to decompose in the CVD vacuum furnace, and in the annealing process, carbon atoms are rearranged under the effect of catalysts to form a nanocarbon structure.25 Due to the low synthesis temperature, low cost, simple operation, and large-scale preparation, the CVD method is considered as the most promising method for large-scale synthesis of graphene, carbon nanotubes and 3D carbon nanotube/graphene hybrids. In addition, a large number of techniques for synthesizing 3D carbon nanotubes/graphene materials have been reported. For instance, vacuum filtration method,26,27 layer-by-layer self-assembly deposition,28,29 solution method,30,31 and electrophoretic deposition.32

Synthesis of Carbon Nanotube/Graphene Hybrids by Chemical Vapor Deposition

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Compared with other synthesis methods, the CVD method can successfully produce three-dimensional carbon nanomaterials with a stable threedimensional structure, mechanical strength, and intercommunication without using toxic and polluting chemical reagents and solutions.33 In this chapter, we aim to briefly summarize the relevant techniques used for 3D nanotube/graphene hybrids in recent years, especially, CVD methods. Furthermore, the advantages and deficiencies of the various techniques will be investigated in detail. Finally, concluding remarks and perspectives are offered and we hoped this chapter will shed light on performance applications, recent development trends, and further challenges of 3D nanotube/ graphene hybrids.

3.2 Preparation of Carbon Nanotube/Graphene Hybrids To facilitate the fine arrangement of 1D CNTs and 2D graphene and build the 3D hierarchical structure, several techniques and methods have been developed. Up to now, approaches for preparing carbon nanotube/graphene hybrids mainly include five parts: vacuum filtration method, layer-by-layer self-assembly fabrication, solution method, electrophoretic deposition, and in situ chemical vapor deposition growth (as shown in Figure 3.2). Among them, the vacuum filtration method, layer-by-layer self-assembly fabrication, and solution method can be classified as facile and simple mixing processes of graphene and CNTs, which are the early research studies on the construction of 3D carbon nanotube/graphene hybrids. This kind of self-assembly approach mainly relies on the electrostatic interaction of CNTs

Figure 3.2

Classification of 3D carbon nanotube/graphene hybrids synthesis techniques.

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and graphene. This merit of the preparation method is facile, inexpensive, scalable, efficient, and does not require a sophisticated transfer process compared to the CVD technique. The CVD technique is a simple and effective method for the in situ synthesis of 3D carbon composites with reasonable stability and mechanical strength. In this preparation method, a large number of three-dimensional composites are synthesized at different temperatures by using different carbon precursors, and this new type of carbon nanocomposite has great potential and prospects in the future. CVD technology can generally be categorized into two different approaches: multi-step and one-step chemical vapor deposition.

3.2.1

Vacuum Filtration Method

With regard to the vacuum filtration method, vacuum filtration is a simple film-forming technique in the preparation of high-performance 3D carbon nanotube/graphene films. The vacuum pump is the main instrument used in the field of carbon nanotube/graphene thin film. The first step of the 3D film formation is to prepare a stable dispersion of graphene and CNT. Then the mixed suspension is vacuum filtered using a membrane. Finally, the hybrid film is removed from the filter membrane and transferred to the corresponding substrate by washing. Different concentrations of suspensions can be prepared by the vacuum filtration method. The 3D hybrid film thickness also has obvious advantages: simple operation, uniform film thickness, and high material utilization rate. Nevertheless, the disadvantage is that the area of the hybrid is limited by the area of the filter paper and the obtained hybrids are generally thick and have poor performance; in particular, poor light transmittance.26,27

3.2.2

Layer-by-layer Self-assembly Deposition

The layer-by-layer self-assembly deposition is based on the principle of electrostatic attraction between the target compound and the functional groups on the substrate surface. That is, the mechanism is based on the mutual adsorption of different dimensional nanocarbon materials with opposite charges on the surface. In recent years, this technique has been used for synthesizing carbon nanotube/graphene hybrids by utilizing electrostatic attraction from pre-treated graphene and CNTs. Yu et al.34 firstly reduced (graphene oxide) GO and obtained PEI-modified graphene by using hydrazine and a certain amount of polyurethane (PEI) (as a stabilizer). The hybrid is synthesized by self-assembly deposition of acidified multi-walled carbon nanotubes and PEI-modified graphene. The carbon nanotube/graphene film has an internally cross-linked network nanostructure with nanopores and at the scanning speed of 1 V s1, the capacitance is 120 F g1, meaning the hybrid has good application prospects in supercapacitors.

Synthesis of Carbon Nanotube/Graphene Hybrids by Chemical Vapor Deposition

3.2.3

59

Solution Method

Vincent et al.31 reported the synthesis of hybrids comprised of carbon nanotubes and chemically converted graphene by using low-temperature solution processing. GO powder and acidified carbon nanotubes were dispersed in anhydrous hydrazine. Then the mixed dispersion was treated with ultrasound and deposited on different substrates by a rotating coating method. The solution-based method preserves the intrinsic properties of both components and does not require any surfactants or a sophisticated transfer process during the whole process.

3.2.4

Electrophoretic Deposition

Electrophoretic deposition is a simple and economical deposition technique to fabricate 3D carbon nanocarbon hybrids. The main equipment used in electrophoretic deposition is an electrophoresis tank and it has outstanding merits: fast deposition rates, good uniformity, and easy control of film thickness. In recent years, electrophoretic deposition technology has been applied to the preparation of carbon nanotubes/graphene. The key procedure in the preparation of hybrid is to prepare uniformly dispersed carbon nanotube/graphene solution. To move towards the electrode under the action of an applied electric field, the stabilized suspension and the colloidal particles in the suspension must contain a charge. To solve this problem, we usually treat them with an acid or alkali so that they form active functional groups (carboxyl or amino groups). Shi35 and Su36 successfully fabricated a carbon nanotube/graphene hybrid by repeated ultrasonic processing and the effect of electrophoretic deposition under an external electric field.

3.2.5

Multi-step Chemical Vapor Deposition

As a sophisticated synthesis technique for both laboratory research and industry production, the CVD technique has attracted tremendous research attention during recent decades. Thus it is always used to facilitate the synthesis of CNTs, graphene, and hybrids, leading to the 3D hierarchical composite. A conventional CVD (shown in Figure 3.3) device generally consists of four parts: CVD furnace, carrier gas, substrate, catalyst, and precursor. Under the action of a suitable carrier gas (H2, Ar, N2) and catalysts (Fe,15 Ni,37 Cu,13 Co38 or their mixture39,40), the carbon atoms from the precursor are rearranged to form a carbon nanostructure on the corresponding substrate through high-temperature annealing. Multi-step chemical vapor deposition is a typical method to achieve controllable configurations of 3D nanostructures composed of CNTs and graphene. Recently Yan et al.82 in our group reported a novel and simple route using CVD methods to fabricate bead-like nitrogendoped CNT/graphene composites via simple pyrolysis of the N-rich melamine in the presence of graphene oxide (GO) as a substrate using a Mn–Ni–Co ternary catalyst and the whole process is divided into two steps. In this work,

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Figure 3.3

Schematic diagrams of CVD technique.

Figure 3.4

Scheme for the in-situ synthesis of covalently bonded graphene/singlewalled carbon nanotube hybrid.

GO is used as a potential platform for nucleation and a substrate to support Mn–Ni–Co ternary oxides (MNCO) by combining a facile co-precipitation reaction at first. Then 3D bead-like N-doped CNT/graphene structures are prepared via direct pyrolysis of N-rich melamine by a CVD method. A solution method and CVD techniques are utilized respectively, resulting in more consumption of cost and time. Furthermore, Zhu et al.33 fabricated a seamless three-dimensional carbon nanotube/graphene hybrid material by a two-step CVD process. The whole growth strategy could be divided into two steps: in the first, the graphene is grown on a copper foil at 1000 1C by using CH4 or PMMA as a carbon source, and secondly, the CNT carpet is grown directly out of the graphene under the iron catalyst and alumina buffer layer which are deposited on the graphene in series by electron beam (e-beam) evaporation at a lower temperature of 750 1C by utilizing the hydrocarbon: C2H4 or C2H2, resulting in the in situ synthesis of covalently bonded graphene/single-walled carbon nanotube hybrid (as shown in Figure 3.4). Multi-step chemical vapor deposition in situ synthesis of graphene/carbon nanotube composites is very uniform and easy to control, resulting in good repeatability compared to the facile mixing methods. Additionally, the obtained 3D nanostructure could be effectively controlled by varying the relevant parameters without sacrificing their properties.

Synthesis of Carbon Nanotube/Graphene Hybrids by Chemical Vapor Deposition

3.2.6

61

One-step Chemical Vapor Deposition

Recently, the one-step CVD method for preparing carbon nanotube/graphene composites reduces the reaction steps, decreases reaction time, and reduces the production cost, which is conducive to mass production of such nanocomposites. Zhu et al.41 successfully fabricated the carbon nanotube/graphene hybrids by a one-step chemical vapor deposition process by using a mixed catalyst of MgO (acts as the template for the growth of graphene) and Fe/MgO (facilitates the growth of single or double-walled CNTs). And the result illustrates that the 3D hybrids have hierarchical porous structures owing to the composition of graphene with the CNT network. Interestingly, the graphene to CNT ratio and shape of the hybrid could be conveniently changed by adjusting the fabrication environment (e.g. the MgO to Fe/MgO catalyst ratio). And Li et al.83 firstly reported a novel and facile one-step process using template-directed chemical vapor deposition (CVD) to fabricate highly nitrogen-doped 3D N-doped carbon nanotube/N-doped graphene architecture (N-CNTs/N-graphene) utilizing melamine as a single carbon source and nickel foam (NF) as the catalyst and substrate respectively. First, when the temperature was below 300 1C, melamine was absorbed and uniformly distributed on the surface of NF. Between 300 1C to 600 1C, melamine gradually decomposed to carbon nitride and released NH3 which enables the growth of CNTs. With the increasing temperature, layered graphitic carbon nitride deposits on the surface of NF and gradually decomposes to graphene under the effective etching process of H2 at about 800 1C. Simultaneously, carbon nanotubes grow on the surface of the graphene layers catalyzed by the Ni nanoparticles produced in the etching process of NH3 resulting from the decomposing of melamine and 3D hierarchical N-CNT/N-graphene is obtained (illustrated in Figure 3.5). Although this onestep process effectively decreases the consumption of power and reduces the reaction steps, the relevant growth mechanism of three-dimensional carbon nanomaterials by a one-step CVD method needs to be further studied.

3.3 Effect of Experimental Parameters of the CVD Technique 3.3.1

Effect of Catalyst

There is no doubt that the catalyst is essential to the morphology, nanostructure, and applications of 3D nanomaterials in the conventional CVD method. Single transition metals (Fe, Co, Ni, Cu, (palladium) Pd,42 (ruthenium) Ru43) are utilized to catalyze the CVD process of graphene, CNTs, and hybrids. Particularly, due to the low cost and availability, Fe, Co, Ni, and Cu have become the common catalysts for the growth of carbon nanostructures compared to other metals. Furthermore, to combine the 1D CNTs and 2D graphene and obtain hybrid sp2 carbon with an ordered 3D nanostructure, a single transition metal substrate is not enough for the CVD growth procedure. Yan et al.40

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Figure 3.5

Schematic diagram of the N-CNTs/N-graphene material synthesized by onestep CVD method. Reproduced from ref. 14 https://doi.org/10.3390/nano8090700 under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

synthesized bead-like nitrogen-doped CNT/graphene hybrid via a simple CVD method by using a Mn–Ni–Co ternary catalyst. The Mn–Ni–Co ternary catalyst not only facilitates the growth of CNTs but is also essential to the reduction of GO. Additionally, the metal nanoparticle also has an important role in the growth of different dimensional nanocarbon materials. In CVD methods, spin-coating,44 template etching,14,21 and electron evaporation33 are the typical ways to obtain the metal nanoparticle for the growth process. Under the catalysis of a mixed catalyst of MgO and Fe/MgO, Zhu et al.41 successfully synthesized the composite of CNTs and graphene and the change of mixed catalysts has a great influence on the graphene to CNT ratio. Additionally, Nguyen et al.6 successful grew thin CNT-graphene hybrid films in situ via one-step CVD on copper substrates that had been coated with thin iron catalyst films of various thicknesses and the various densities of catalyst nanoparticles had a different effect on the diameter, density, and quality of the CNTs of 3D carbon hybrids (see Figure 3.6).

3.3.2

Effect of Carbon Source

Early research studies on the source of carbon hybrids were focused on hydrocarbon compounds such as CH4,45,46 C2H2,33,39 C2H4,47 C3H8,48 and so on. However, solid feedstock (camphor,49 melamine,40 Prussian blue50) and liquid carbon sources (toluene,42 ethanol,44 pyridine51) are increasingly utilized for the carbon sources as the feedstock for the basic supply of 3D architecture according to the relevant reports. Nevertheless, the relevant

Synthesis of Carbon Nanotube/Graphene Hybrids by Chemical Vapor Deposition

Figure 3.6

63

Typical FESEM images of 3 nm thick iron on copper after CVD (a), 3 nm thick iron on G-100 (b) and Cu substrate (c) after annealing at 780 1C, 1.5 nm (d), 3.0 nm (e) and 4.5 nm (f) thick iron on G-100 after CVD. Insets in (d), (e) and (f) are their corresponding FESEM images at high magnifications. Reproduced from ref. 6 with permission from The Royal Society of Chemistry.

CVD synthesis of graphene requires high annealing temperature, typically 1000 1C.52,53 A low-temperature facile and versatile method to grow graphene is desirable based on the CVD growth route by using solid and liquid carbon sources, which are more convenient, economical, environmentally-friendly, and feasible for industrial application. Because of the quick diffusivity of carbon atoms through metal catalysts and covering on the surface at lower temperatures, liquid and solid feedstocks are more suitable for the growth of 3D CNT/graphene hybrids at a lower temperature. Moreover, according to the relevant reports, liquid and solid carbon sources, the overall nucleation barrier and dehydrogenation barrier are much lower than that of gaseous carbon sources during the dehydrogenation process. Li et al.14 fabricated a 3D nitrogen-doped carbon nanotube/graphene on the nickel foam surface. During the CVD synthesis, graphene and carbon nanotubes are simultaneously synthesized by using melamine as the solid carbon and nitrogen source at 800 1C. There is no doubt that structural defects will be inevitably formed during the CVD process, resulting in the significantly limited quality of the synthesized nanocarbon structure. A recent article54 illustrated that the heterogeneous solid carbon source, polycyclic aromatic hydrocarbons (containing a mixture of aromatic and aliphatic carbon), effectively mitigate defect formation during the low-temperature CVD synthesis process.

3.3.3

Effect of Growth Temperature and Growing Time

The nanostructure and morphology of 3D carbon nanomaterials can not only be directly determined by the catalyst but can also be decided by changing

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growth temperature and time in the CVD method. Graphene–carbon nanotube hybrid materials were fabricated on copper foil decorated with silicon nanoparticles (Si NPs) in the study by Dong.44 Interestingly, the density of CNTs in the 3D graphene–carbon nanotube hybrid can be effectively controlled by the size of the silicon nanoparticles and the growth temperature. Moreover, the growing temperature has a great influence on the property (p-type field-effect characteristics and conductivity) of the obtained sample. Carbon nanotube/ graphene hybrids are directly grown under the catalysis of cobalt catalystcoated nickel foam by one-step ambient pressure CVD at different growing times from the recent work. In order to understand the relationship between the growth time and the sample, the experiment was carried out at a different growing time and the result illustrates that the I2D/IG, the number of layers of graphene, graphene weight, and specific capacitance could be controlled by adjusting the CVD growing time.55 Adjusting the growing time contributes to the optimal synthesis conditions, yet the relevant reason is seldom explained and the mechanism needs further research.

3.3.4

Effect of Carrier Gas

Argon (Ar) or nitrogen (N2) (which is responsible for introducing the carbon source into the CVD furnace and protecting the whole reaction process under a suitable flow rate) and hydrogen (H2) (which has multifunctional effects in a practical CVD environment) is the usual carrier gases for the synthesis of nanocarbon materials in the high-temperature annealing process. Typically, H2 can effectively remove the surface oxide layer of impurities and defects on the surface of the growth substrate before the growth process.16,56 Moreover, it also facilitates the reduction of GO and the metal oxide catalysts at high temperatures during the graphene and CNT synthesis process.40 For instance, it was reported by Kim et al. that two coinstantaneous processes appear during the preparation of 3D graphene/CNT hybrids.84 The pyrolysis of methane occurs with the increasing temperature and then facilitates the CNT growth out of islands of metal nanoparticles. Simultaneously, the hydrogenation process appears on the surface of the graphene sheet (as shown in Figure 3.7) and the graphene sheet was transformed into CH4 under the effect of H2 etching at the surface of the connection with the catalyst nanoparticles (Ni nanoparticle þ C graphene þ2H2-Ni þ CH4). As for graphene/CNT hybrids, compared to the CVD fabrication of pure 1D or 2D nanocarbon materials, H2 is essential for building three-dimensional hierarchical carbon nanostructures by a gas etching process. Kim et al.13 successfully fabricated a 3D graphene/CNT structure under the flow rate of CH4, Ar, and H2, and the hybrid structures could be optimized by careful optimization of the gas flow rates of CH4 and H2 via controlling the simultaneous, competing reactions of CNTs formation and hydrogenation which suppresses CNTs growth via a H2 etching process. The result implies that the H2 etching process effectively optimizes the 3D nanocarbon hybrids by controlling the density of CNTs grown on the surface of the graphene sheet.

Synthesis of Carbon Nanotube/Graphene Hybrids by Chemical Vapor Deposition

Figure 3.7

65

Schematics illustrating the growth of CNT on planar graphene by H2 etching. Reproduced from ref. 85 http://dx.doi.org/10.5772/intechopen.89671 under the terms of the CC BY 3.0 license https://creativecommons.org/ licenses/by/3.0/.

In addition, the relevant report further confirms that H2 has an important effect on the growth of CNTs during the 3D carbon nanostructure. The result illustrated that H2 saturation suppresses the catalytic activity of the nanoparticle by the etching process and it could also indirectly alter the morphology by adjusting the flow rate of H2 in the CVD technique.57

3.4 Application Prospects of Carbon Nanotube/ Graphene Hybrids Carbon nanotube/graphene composites synthesized by the CVD method are very uniform, and their composition is easy to control and repeat and they have attracted continuous attention in the application of photoelectric devices, energy storage, and other fields. In particular, it is used in the fields of fuel cells, transparent and flexible electrodes, field-effect transistors, supercapacitors, and lithium batteries. With further research, improving the electrical conductivity and mechanical strength of the composites will increase their application value.

3.4.1

Carbon Nanotube/Graphene Hybrids in Fuel Cells

Owing to the unique physical, chemical, and electrical properties, graphene sheets show higher ORR electro-catalytic performance and electrochemical

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surface areas than the commercial Pt/C catalyst. However, because of graphene’s morphological characteristics, Pt-graphene electrodes have an obstacle in the mass transfer of the chemical reactants and products to and from the active sites.59 Hence, a new strategy for the mass transfer of Pt-based electrodes is required. Pt-CNT/graphene electrodes act as an effective Pt-3D nanocarbon ORR catalyst with better thermal stability and durability, and unique 3D network structure induced by the CNTs acted as a pathway for mass transfer of the chemical reactants and products and as an electrical bridge resulting in the higher electrochemical performance (lower ORR charge transfer resistance and higher power density) for fuel cells than did Pt-graphene cathodes, indicating 3D nanocarbon hybrid materials are becoming increasingly competitive in fuel cell applications.60,61 Furthermore, the special CNT/graphene material can be directly used as a kind of durable and efficient non-precious metal oxygen reduction reaction (ORR) electrocatalyst and the ORR activity of this kind of catalyst is more favorable compared with the commercial Pt-based catalyst.62 Wang et al. proposed a facile and efficient method for the fabrication of Fe/N-doped CNTs on graphene sheets. When the hybrid is applied in a microbial fuel cell, the Fe/N-doped nanocarbon hybrid possesses superior ORR electron transfer (3.91  0.02) and a better maximum power density of 1210  23 mW m2, which is much higher than the Pt/C electrode (1080  20 mW m2), demonstrating that the Fe/N-doped nanocarbon hybrid is a type of promising highly efficient catalyst that can enhance the ORR performance of microbial fuel cells (illustrated in Figure 3.8).63

Figure 3.8

The comparison of multicycle power density and polarization curves among Pt/C-MFC and Fe–N/G-MFCs. Reproduced from ref. 63 with permission from The Royal Society of Chemistry, Copyright 2018.

Synthesis of Carbon Nanotube/Graphene Hybrids by Chemical Vapor Deposition

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Carbon Nanotube/Graphene Hybrids in Transparent and Flexible Electrodes and Field-effect Transistors

Due to remarkable mechanical, electrical, thermal, and optical properties, low dimensional nanocarbon materials including carbon nanotubes and graphene have recently received a great deal of attention for potential uses in transparent and flexible nanoelectronics.64,65 Furthermore, 3D nanocarbon hybrid nanostructures have been applied in two applications: transparent and flexible electrodes and field-effect transistors (FETs). As for transparent and flexible electrodes, indium tin oxide (ITO) is the most common material in solar cells, organic light-emitting diode panels, and touch panels. Considering the sustainability, price concerns, optical transmittance, and sheet resistance, the carbon nanotube/graphene hybrid may be a good candidate for transparent and flexible electrodes. CNT/graphene hybrids were prepared from the related report16 and the hybrid was transferred onto a polyethylene terephthalate (PET) film as transparent and flexible electrodes and the experimental results illustrate that the CNT/graphene hybrid shows better optical transmittances (96.4%) than that of bilayer graphene (about 95.4%) and lower sheet resistance (300 O sq1) which presumably originates from the low contact resistance between graphene and CNTs in the hybrid compared with the ITO (about 90%) and previous hybrid materials (Ag nanowire/graphene hybrids (96.5%; 800 O sq1),66 indicating that the CNT/graphene hybrid could be applied to the alternative materials of flexible and transparent electrodes on PET films, owing to the high conductivity and flexibility of graphene with CNTs. Besides, compared to other semiconducting materials, such as pure 1D or 2D nanocarbon materials and organic semiconductors, 3D CNT/graphene hybrids have been receiving tremendous attention for application in flexible electronics because they are technically superior in terms of carrier mobility and chemical stability in FETs. 3D CNT/graphene FETs containing a mixture of metallic and semiconducting nanotubes are greater than that of graphene-based FETs which are hampered by the semi-metallic nature of graphene sheets.67 Based on the transfer characteristics for CNT/graphene hybrid FETs, the relevant report achieved an improved on/off ratio and on-state current compared with pristine graphene and CNTs. Additionally, the CNT-graphene transistors68 showed good overall performance, such as an operation voltage ofo5 V, on/off ratio ofB104, field-effect mobilities (m) of 80 cm2 V S1, and a transparency ofB84%. A series of results demonstrated that such flexible and transparent CNT/graphene thin-film transistors are potential high-performance FETs.

3.4.3

Carbon Nanotube/Graphene Hybrids in Supercapacitors

Generally, based on the energy storage mechanism, supercapacitors can be classified into two categories: Electrical double-layer capacitor and pseudocapacitor and the properties of the two types of supercapacitors largely

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depend on the properties of the electrode materials. Owing to the desired physical and chemical properties, low cost, ease of processability, relatively inert electrochemistry, and controllable porosity, carbon-based materials ranging from activated carbons to CNTs, graphene are the most widely used electrodes.69–71 Nevertheless, the aggregation and overlapping that happened in graphene flakes or CNT bundles lead to a loss of available surface area and a reduction in supercapacitor performance.72 To overcome the aforementioned weakness of aggregation and overlapping, CNT/graphene composites have been fabricated towards three-dimensional nanostructures. By constructing the efficient electron transport between CNTs and graphene during the charge–discharge process, the CNT/graphene sample maintained a rectangular shape (490.3 mF cm2 capacitance) throughout the testing duration at exceedingly high scan rates of over 300 mV s1 and the 3D nanocarbon hybrid electrode exhibits potential as a supercapacitor with a greater capacitance than the graphene-based electrode, indicating that the special CNT/graphene structure generates a stable, low resistance electric interconnection between graphene and it has the potential application for high-performance supercapacitors from the relevant study.13 Zhang et al.38 also reported a CVD strategy to prepare 3D CNT/graphene nanostructures that have been successfully applied as electrodes of supercapacitors. The supercapacitor exhibits excellent electrochemical performance (a specific capacitance of 385 F g1 at 10 mV s1 in 6 M KOH solution and a capacitance increase of ca. 20% of the initial capacitance after 2000 cycles) due to the unique structure which endows the high-rate transportation of electrolyte ions and electrons and comprehensive utilization of pseudo and double-layer capacitance. Chen et al.73 synthesized a three-dimensional seamless graphene/carbon nanotube (G/CNTs) hybrid by directly growing aligned CNT arrays from the surface of graphene in foam and the hybrid was used for the electrode of solid-state multifunctional photosensitive supercapacitors. Furthermore, the obtained supercapacitors exhibited higher photo-responsive properties (more capacitive and less resistive) compared with supercapacitors based on bare graphene or bare CNT electrodes (shown in Figure 3.9). As promising electrode materials, metal oxides (e.g. Co3O4,74 NiO,75 RuO2,76 MnO277) have also received considerable interest in virtue of the low cost, high theoretic capacitance, good cycle stability, and environmental friendliness. By combing metal oxide nanostructures with conductive carbon materials such as CNTs, graphene, or 3D hybrids, the poor electrical conductivity and densely packed structure of metal oxides which limit application within the development of high-performance supercapacitors could be effectively solved, obtaining better capacitance performance with higher specific capacitance, power density, and energy density. The related study23 demonstrated that a higher specific capacitance (250 F g1), lower internal resistance (1.25 ohm), and good cycling stability at a current density of 1.0 A g1 electrode of supercapacitor compared to graphene–Ni, CNT–graphene could be fabricated by combing the MnO2 with CNT/ graphene to form a 3D nanocomposite (shown in Figure 3.10).

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Figure 3.9

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(a) CV curves, (b) GCD curves and (c) Nyquist plots of the supercapacitor under simulated solar light illumination with different power densities, respectively. (d) The dependence of capacitance and series resistance of the supercapacitor on the power densities of simulated sunlight. Reproduced from ref. 73 with permission from The Royal Society of Chemistry, Copyright 2019.

In addition, it also provided a new approach for us to synthesize novel nanocomposites with a better comprehensive electrochemical performance by combing metal particles with 3D nanocarbon hybrids.

3.4.4

Carbon Nanotube/Graphene Hybrids in Lithium Batteries

CNTs and graphene with unique electrical, thermal and mechanical properties are considered as suitable electrode materials of high-performance lithium-ion batteries. However, graphene nanosheets and CNTs tend to agglomerate in the synthesis process, which induces the loss of the merits of graphene-based electrodes and CNT-based electrodes of lithium-ion batteries, leading to poor cycling performance, low-rate capability, and large contact resistance. The relevant work39 demonstrates that a 3D nanocarbon hybrid electrode could significantly enhance electrical conductivity and chemical stability by accelerating the electron collection and transport around the cycling process. For example, the 3D CNTs/graphene hybrid electrode in a previous work78 exhibited good electrochemical performance: a high initial reversible specific capacity (439 mAh g1 at a

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(a) J–V responses of the graphene–Ni, CNT–graphene–Ni and MnO2– CNT–graphene–Ni hybrids. (b) CV curves of MnO2–CNT–graphene–Ni electrodes at different scan rates. (c) CV curves. Reproduced from ref. 23 with permission from The Royal Society of Chemistry, Copyright 2014.

current density of 372 mA g1), a high capacity (429 mAh g1 was maintained after 100 cycles) and a high coulombic efficiency of 98.5%. Therefore, integrating different dimensional nanocarbon materials is an effective route to fabricate high-performance Li-ion batteries with fast ion/electron transfer and higher Li storage capability in the future. Due to the advantage of abundant element, low cost, environmental benignity, and high theoretical specific capacity (1672 mAh g1, fivefold that of the current traditional cathode materials based on transition metal oxides or phosphates), sulfur is a promising cathode material of Li–S batteries and it has attracted increased attention from a large number of research groups.79 Nevertheless, the performance of Li–S batteries partly decreased in practical applications due to the problems of low electrical conductivity of sulfur, dissolution of polysulfides in the electrolyte, and volume expansion of sulfur during discharge, leading to poor cycle life, low energy efficiency, and low specific capacity.80 To solve this problem, various nanocarbon materials (such as multi-walled CNTs, graphene, and graphene oxide) have been synthesized and utilized as electrode materials in Li–S battery applications. CNT/graphene hybrids are considered as an excellent cathode of

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high-rate performance for Li–S batteries attributed to the special 3D architecture that enables rapid electron transfer between CNTs and graphene and effectively buffers volume changes during the cycling process. Sun et al.81 successfully achieved super-aligned CNT/graphene hybrids and the obtained S–CNT/G nanocomposite exerted improved mechanical performance and favorable electrochemical characteristics compared to the S–CNT composite (a high discharge capacity of 1048 mA h g1 at 1 C with capacity fade as low as 0.041% per cycle over 1000 charge–discharge cycles and excellent highrate and long-term cycling performances (see Figure 3.11). Furthermore, nitrogen-doped CNT/graphene hybrids are also suitable as the cathode of Li–S batteries. Su et al.50 reported the synthesis of N-doped graphene–CNT hybrids for Li–S batteries and the materials possessed outstanding electrochemical performance, including a high reversible capacity (1221 mAh g1 at a rate of 0.2 C), excellent rate capability (458 and 220 mA h g1 at rates of 5 and 10 C, respectively), and excellent cycling stability (321 and 164 mAh g1 at 5 and 10 C after 1000 cycles) owing to the nitrogen doping process which introduces more defects and active sites to the carbon structure and effectively traps lithium polysulfides on electroactive sites within the cathode.

Figure 3.11

(a) Rate performance, (b) galvanostatic charge–discharge curves, (c) cycle performance at 1 C, (d) percentage of active sulfur with increased cycle numbers at 1 C of the S–CNT and S–CNT/G composites. Reproduced from ref. 81 with permission from The Royal Society of Chemistry, Copyright 2015.

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3.5 Further Prospects and Conclusions Carbon nanomaterials such as graphene, carbon nanotubes have attracted more and more attention since they were discovered due to their excellent mechanical strength, good thermal properties, and unique electrical properties. With further research, these new carbon nanomaterials have been found to have great potential in hydrogen storage, energy storage, batteries, biology, and other fields. Moreover, by integrating carbon nanomaterials of different dimensions into new three-dimensional carbon nanomaterials, not only can the stacking phenomenon between graphite layers be effectively alleviated, but they can also show more excellent physical and chemical properties. At the same time, they have attractive prospects in many fields, especially in the field of new energy. At present, the relevant mechanism of synthesizing three-dimensional carbon nanomaterials by the CVD method is still very much lacking, and the relevant growth mechanism needs to be further studied in depth. Furthermore, the carbon nanotube/graphene composites synthesized by CVD methods have their shortcomings and deficiencies, in general, a simple and scalable CVD method for the preparation of three-dimensional carbon nanocomposites with controllable structures is still of great significance in industrial production. Further research and exploration are needed for a better simple and feasible method to prepare high-quality carbon nanotube/graphene composites. With the continuous exploration and innovation of researchers in this field, the synthesis methods of carbon nanotube/ graphene composites will become more diversified and easier to perform, achieving better results in the fields of energy, environment and photoelectric devices and development in other fields.

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CHAPTER 4

Design of Graphene/CNT-based Nanocomposites: A Stepping Stone for Energy-related Applications WASEEM RAZAa,b a

Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India; b Department of Materials Science and Engineering, WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany Email: [email protected]

4.1 Introduction It is well known to all that carbon is one of the fascinating and essential elements for life and that it plays a key role in the development of human society on this planet. In the early eighteenth century, wood fuel was the major power source for humans, after which coal appeared to be the main energy source before the discovery of the steam engine which delivered an industrial revolution. After that fossil fuels and oil became the main global energy sources. Therefore, consistently, carbon has been an appealing material for scientists, engineers, chemists, materials, physicists, and biologists. It is not only a very important part of life but also plays a key role in the formation of carbon nanostructures providing remarkable chemical and physical properties. Carbon is an exclusive element that can occur in different allotropic forms such as zerodimensional (0D, fullerenes), one dimensional (1D, CNT), two dimensional (2D, All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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Schematic illustration of (a) 0D fullerene, (b) 1D graphene nanoribbon, (c) 1D CNT, (d) 2D graphene sheet, (e) 3D graphite, (f) 3D CNT networks, (g) 3D graphene and vertical CNT hybrid, (h) 3D graphene vs. horizontal CNT hybrid, and (i) 3D graphene in schwarzite form showing heptagonal rings (highlighted).2 Reproduced from ref. 2 with permission from American Chemical Society, Copyright 2014.

graphene sheet), and three dimensional (3D, graphene, and CNT hybrid) forms, which have diverse physical and chemical properties as shown in Figure 4.1.1–3 However, diamond and graphite are excellent and naturally existing allotropes of carbon, although their properties are very different from each other. Diamond is the hardest and most transparent material as well as an electrical insulator, whereas graphite is soft and black as well as a good electrical conductor. Moreover, diamond is made up of exclusively large crystals and sp3 hybridization of tetrahedral carbon, whereas graphite consists of 2D sp2 stacked graphene monolayers which are bound by van der Waals forces. Fullerene (C60) was the first carbon allotrope to be identified by Kroto et al. in 1985, but later on, other fullerenes were also detected such as C20, C70, and other big species, however, C60 is the most extensively used and designed material.4–6 Fullerenes (C60) are the smallest recognized stable material made up of a sequence of pentagon and hexagon sp2 carbon atoms which gives spherical patterns. Fullerenes (C60) are soluble in organic solvents especially in toluene, and the border between nanomaterials and molecules also cleared the gate toward a novel class of carbon allotropes at the nanoscale level. Six years later, CNT was reported by Iijima, which was another leading step toward designing carbon nanomaterials, followed by the architecture of single-walled carbon nanohorns, bamboo-like nanotubes, and onion-like carbon spheres.7–9 Graphene is the thinnest material. It is a one atom thick planar sheet of sp2 carbon atoms covalently bonded in a honeycomb crystal, which is the essential unit of graphite, experimentally reported by Boehm et al. in 1962 and characterized by Novoselov and Geim in 2004.10–12 However, the bonding between the carbons of graphene is highly robust along with weak van der Waals interactions, having sC–C bonds

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in-plane and out of plane. Graphene has drawn considerable attention in several fields as a perfect material for energy-related applications due to its impressive thermal conductivity, mechanical flexibility, electronic properties, and optical properties as well as large surface area.13,14 Moreover, a large number of active sites and good conductivity are promising for electron transfer in energy-related applications, and it can be folded into tube graphene known as carbon nanotubes (CNTs). CNTs are a 1D allotrope of carbon and were highly studied materials before the discovery of graphene, which are a smooth cylindrical hollow structure consisting of one or more layers of graphene. Therefore, the performance of CNTs is also almost similar to graphene in that it has a high Young’s modulus, large tensile strength, good electric current, large thermal and electrical conductivity, and promising catalytic properties.15 The properties of CNTs may be semiconducting or metallic due to a deep dependence on the chirality of the hexagonal lattice of graphitic carbon.16 However, both CNTs and graphene are not free from drawbacks, therefore the pure form of CNTs and graphene is not suitable for energy-related applications. One of the big obstacles of graphene sheets is that they restack together to form graphite as an outcome of strong van der Waals interactions and durable p–p stacking consequently limiting the complete utilization of the surface area. Moreover, electrical conductivity is found to be very low out of plane as compared to in-plane conductivity, and out of plane electron transfer is also poor due to their vast sheet facet ratio and non-bonding van der Waals interactions in the transverse direction. To overcome these inherent problems of CNTs and graphene, to perfectly exploit, and to further explore, the start-of-the-art service of CNTs and graphene is important. Engineering the conductive spacers such as CNTs between graphene layers via designing a hybrid is an appealing approach. Therefore, this chapter will discuss the path toward the design and architecture of 3D graphene/CNT hybrids for energy-related applications, which highlights a novel research theme in the field of synthesis and applications. Moreover, this hybrid material is durable, sustainable, robust, rigid, and highly conductive due to the innate synergistic effects of both conductive carbon materials as well as acquiring the original properties of both graphene and CNTs.

4.2 Synthesis Method for Graphene/CNT Hybrids Various methods have been described in the literature for the synthesis of CNTs and graphene such as arc-discharge deposition, laser ablation, and chemical vapor deposition (CVD), exploitation of graphite, solvothermal approach, segregation growth method, thermal treatment, plasma treatment, hydrazine hydrate, fossil hydrocarbon, waste material, and so on. However, CNTs and graphene can be synthesized using gaseous fossil fuels via the action of thermal decomposition of hydrocarbons in the presence of catalysts. Figure 4.2 exhibits the synthesis of CNTs and graphene from various naturally appearing precursors such as liquid and gaseous hydrocarbon waste in the presence of various metal catalysts at a high temperature. However, carbon in a zerovalent state is one of the most explored elements of the

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Figure 4.2

Chapter 4

Schematic illustration of the synthesis of CNT and graphene using natural liquid and solid hydrocarbon waste.17 Reproduced from ref. 17 with permission from Elsevier, Copyright 2015.

periodic table due to its exclusive capability to form C–C covalent bonds via different hybridizations such as sp, sp2, and sp3 for extensive areas of energyrelated applications. Therefore, 3D hybrid heterostructures can be applied to various energy-related applications due to their fascinating properties and by combining the synergistic effects of both 2D graphene and 1D CNTs. Moreover, in hybrid heterojunction CNTs the spacing of the layers between graphene sheets can be increased to provide a platform for electron transfer. Furthermore, the specific surface area of graphene is found to be higher – but layers tend to restack, which lower the specific surface area of graphene but increase the electronic conductivity. Therefore, designing and engineering a hybrid is an impressive means to achieve a heterojunction with large specific surface area and high conductivity by introducing CNT into graphene layers using in situ and ex situ methods. There are three types of CNT/graphene hybrid composites that are possible; 1) horizontal distribution of CNTs on graphene sheets (type 1), 2) vertical growth of CNTs on graphene sheets (type 2), and 3) wrapping of the CNTs with graphene (type 3), as shown in Figure 4.3. Most of the CNT/graphene hybrid composites have been constructed in the manner of type 1, where there is no crystallographic control of the distribution of CNTs on the extended surface of graphene sheet even in terms of

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Figure 4.3

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Schematic presentation of the distribution of CNTs on the graphene sheet design of CNT/graphene hybrid composites.15 Reproduced from ref. 15 with permission from Elsevier, Copyright 2020.

orientation and development. Although, the equal support between CNTs and graphene plays a key role in controlling the properties of the hybrid composite, which enhances the activity of the heterojunction for energyrelated applications. However, the type 2 approach is one of the most appealing strategies for the vertical growth of CNT on graphene surfaces using the catalyst for engineering the 3D nanostructure. Synergistic coordination of the vertical expansion of CNTs in between the graphene layers is only possible after exfoliation of graphene (GO), reduced graphene oxide (RGO), or graphite, which provides outstanding out of plane and in-plane properties due to excellent C–C bonding between the graphene and CNTs. To achieve the type 3 approach is a little bit tricky due to the wrapping type of interaction between the CNTs (dominated) and graphene (dominated), showing exclusive properties such as enhancement in the oscillating frequency of the nanomotor and reduction in the damping coefficient. Different synthesis methods have been reported for the design and engineering of CNT/graphene hybrid heterojunctions with various architectures and different physical and chemical properties. Herein, we will only discuss the main approaches such as the chemical vapor deposition (CVD), in situ reduction (ISR), and electrophoretic deposition (EPD) methods for the design and engineering of CNT/graphene hybrid composites.

4.2.1

Chemical Vapor Deposition

In the CVD approach, the defined surface of the substrate was covered by a fascinating thin film of performing or decomposing volatile precursors. Therefore, the CVD strategy has been applied for the construction of CNT/graphene hybrid composites due to its easy handling and capacity to designing and manufacture all carbon-based composites and provide covalent bonding between CNTs and graphene as well as enhanced charge transport and separation due to p–p interaction.18 Kondo et al. suggested a novel scheme for designing and engineering carbon composite hybrids for the first time using the CVD approach in the presence of multiwalled graphene and

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Figure 4.4

Chapter 4

(a) Schematic illustration for the construction of CNT/graphene hybrid composites. (b and c) Scanning electron microscopy (SEM) images of multilayer graphene/MWCNT hybrid composites designed and engineered using a CVD approach.15 Reproduced from ref. 15 with permission from Elsevier, Copyright 2020 and from ref. 19 with permission from IOP Publishing, Copyright 2008 The Japan Society of Applied Physics.

aligned multiwalled CNT (MWCNT).19 The carbon hybrid was constructed using a bilayer made up of cobalt (Co) film on titanium nitride (TiN), which was deposited on a silicon substrate and placed in a low-pressure chamber in the presence of acetylene and argon gases. In this process, the multilayers of graphene were transferred to the substrate from the Co film, after which the Co film turns into particles, and MWCNT was also manufactured from Co particles by the tip growth method resulting Co particles remaining between the graphene and CNT multilayers as shown in Figure 4.4(a). It can be seen from Figure 4.4(b and c) that vertically aligned CNTs were grown on the substrate, and a flat-film-like architecture was established on the top of the CNT bundle. Yu et al. reported the manufacture of CNT/graphene hybrid composites using the CVD method, where graphene was cultivated on the surface of alumina followed by CNTs designed on the surface of the alumina with the help of an iron catalyst and the connection between them through sp2 hybridization.20 Sandwich-like graphene/CNT hybrid composites were synthesized using an FeMO catalyst at a high temperature by cultivating CNTs followed by graphene.21 The construction of the CNT/graphene hybrid was reported by Ozkan et al. using Fe-nanoparticle decorated Cu film substrates where CNTs underlined graphene via a one-step CVD approach.22 Chen et al. described the preparation of CNT/graphene hybrid composites using Si-nanoparticle coated Cu foil via the cultivation of graphene on Cu foil uniformly followed by growing of CNTs on Si nanoparticles uniformly on the surface of graphene film using a

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Figure 4.5

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Schematic presentation of a CNT/graphene hybrid using a one-step CVD approach using (a) Si nanoparticle coated Cu foil, (b) mixed catalysts of MgO and Fe–MgO, and (c) a FeMgAl LDH substrate.27 Reproduced from ref. 27 with permission from Elsevier, Copyright 2019.

one-step CVD approach as shown in Figure 4.5(a).23 The construction and architecture of CNT/graphene hybrid composites were achieved using magnesium oxide (MgO) and a Fe/MgO mixed catalyst through a one-step CVD method for getting uniform distribution of graphene and CNTs through a MgO template for graphene cultivation, whereas Fe nanoparticles on the MgO act as a catalyst for the growth of CNTs as shown in Figure 4.5(b). The preparation of CNT/graphene hybrid composites was done using methane as a carbon source at a high temperature (950 1C), where decomposition of a methane source of graphene was cultivated on the surface of a FeMgAl LDH substrate, whereas CNTs were regularly grown on both sides of the graphene plane as shown in Figure 4.5(c).24 However, the construction of CNTs/graphene was also achieved using nickel foam followed by nitrogen plasma treatment via a one-step CVD approach at a higher temperature (800 1C).25 Peng et al. reported the synthesis of graphene/CNTs using a GO/CNT precursor in water containing hydrochloric acid solution at 80 1C for 6 h, where the formation of an efficient bridge via neighboring CNTs leads to charge transport through strong p–p interactions.26

4.2.2

Electrophoretic Deposition

EPD is an efficient method for the synthesis of graphene-based composites in which electric field-based particles induced from suspension to the

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substrate due to excellent, simple, and low-cost access. The EPD approach is a combination of a two-step process such as an electric field that is applied to charge the graphene suspension to run the progress of the graphene flakes followed by the deposition of graphene on the surface of the electrode. Graphene/CNT hybrid composite films with different thicknesses were synthesized using an indium tin oxide coated polyethylene terephthalate substrate and a DC voltage of 1 to 5 V, through carboxy-functionalized singlewalled CNTs and oxygen functionalized graphene.28 Seo et al. reported the synthesis of graphene/MWCNT using nickel nitrate salt as the charge carrier, where graphene sheets were exfoliated, whereas MWCNT was faced with strong carboxylic acid in the presence of a constant supply of 100 DC V for 10 min.29 However, the EPD method can be used with the CVD method jointly for the construction and design of graphene/CNT hybrid composites. Zhang et al. reported the manufacture and engineering of graphene/CNT hybrid transparent conductive films by combining the EPD with the CVD approach.30 In this process, CNTs were deposited on the Cu substrate using electroplated EPD, whereas the uniformly grown graphene sheets were achieved using CVD for the design of CNTs/graphene.

4.2.3

In Situ Reduction

Generally, the ISR approach is a single-step reduction process, which has been used for the design and engineering of graphene/CNT hybrid composites. In this approach, CNTs have been dispersed in a solution of GO followed by the reduction of these solutions using appropriate methods to achieve the desired composite. A remarkable porous graphene/CNT hybrid was prepared by injecting CNT into graphene layers using ethylene glycol as a reducing agent in the presence of MWCNT and GO.31 Woo et al. demonstrated the synthesis of graphene/CNT hybrid composites using hydrazine and ammonia as a reducing agent by introducing CNT bundles into the ultrathin graphene layers.32 The reduction of GO/CNTs was also achieved to obtain the porous hybrid of wrapped CNTs and RGO using ultrasonication.33 Kim et al. reported the manufacture and architecture of 3D graphene/CNT hybrids using a relatively low cost and facile approach by maintaining the intrinsic properties of graphene using ethylenediamine as the reducing agent.34 First of all, CNTs were ultrasonically dispersed in sodium dodecyl-benzene sulfonate (SDBS) and cetyl-trimethyl-ammonium bromide (CTAB) followed by addition of graphene in the presence of ethylenediamine as shown in Figure 4.6(a and b). The suspension was heated at 95 1C for 6 h in a closed vial for the manufacturing of graphene/CNT hybrids as shown in Figure 4.6(c). However, p–p synergy was induced by SDBS/CNT dispersion, whereas CTAB/CNT dispersion provides ionic interaction. It can be seen from Figure 4.6(d) that GO can be dispersed well into the water leading to the production of reduced GO, and p–p stacking cooperation provides a self-assembly mechanism for 3D graphene engineering due to the steric hindrance effect of reduced GO sheets. Moreover, besides the use of reducing agent, other methods such as flashlight irradiation and

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Figure 4.6

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Chemical commutation of graphene/CNT with (a) CTAB, (b) SDBS surfactant, (c) the synthesis process for graphene/CNT hybrids, and (d) the self-assembly mechanism for designing 3D graphene.34 Reproduced from ref. 34 https://doi.org/10.3390/nano8090694 under the terms of the CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

electrochemical reduction have been applied for the design of graphene/CNTs by the reduction of a GO/CNT composite.35,36 However, the flashlight irradiation approach has been proven to be a green and straightforward approach for obtaining surface attached CNTs with an outstanding hydrophobic activity. On the other hand, the electrochemical reduction approach is a simple, versatile, and fast method for achieving graphene/CNT hybrid composites.

4.3 Recent Growth in Energy-related Applications of Graphene/CNT Hybrids Due to the exclusive properties of 1D CNTs and 2D graphene such as magnetic, optical, chemical, extensive surface area, immense conductivity, and strong chemical and mechanical stability, the carbon–carbon hybrid has been extensively designed for different energy-related applications. The architecture and design of 1D CNTs and 2D graphene into 3D graphene/CNTs hybrid composites with a logical junction not only combine the excellent properties of CNTs and graphene but also import further synergistic properties. Therefore, graphene/CNTs offer excellent conductivity, outstanding specific capacitance, superior electrochemical characterization, huge flexibility, rapid transport kinetics, great mechanical stability, and so on. Hence, the graphene/CNT hybrid composite is a very interesting material for various applications in different fields, particularly in energy-related fields.

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4.3.1

Chapter 4

Supercapacitors

Supercapacitors have been receiving extensive research attention due to their large energy density, quick charging process, ultra-long and excellent cycle efficiency, eco-friendly nature, and one of the most crucial energy storage technologies.37,38 However, electrode materials are one of the most important factors to achieve strongly efficient supercapacitors, and carbon-based candidates such as CNTs and graphene are the most prominent. Therefore, graphene/CNT hybrid composites serve as a start-of-the-art for electrode materials for supercapacitors due to their huge surface area to volume ratio, high conductivity, superb strength to weight ratios, and outstanding stability.39,40 Moreover, CNTs can link the graphene sheets resulting in multidimensional electron transport pathways and they can also stop the stacking of the graphene layers leading to the simple availability of electrolyte ions and a reduction in the transport distances between the bulk electrode and electrolyte. However, in supercapacitors, the CNT/graphene hybrid can be used in two forms such as an electrode for electrical double-layer capacitors (EDLCs) and remarkable conductive scaffolding with massive surface area for packing of pseudocapacitive metal oxides and sulfides for achieving outstanding capacitance. Fan et al. described the synthesis of a 3D CNT/graphene sandwich hybrid composite with CNT pillars between graphene layers for the first time as supercapacitor electrodes.41 The exclusive 3D structure provides a huge rate of electrons and electrolyte ion transportation and exhibits a specific capacitance of 385 F g1 at 10 mV s1 in 6 M KOH solution. Moreover, the value of capacitance enhanced by 20% as compared to the original one, and even after 2000 cycles it had excellent electrochemical stability. The logical 3D graphene/ CNT was synthesized by controlling the vertical lengths of CNTs at the graphene surface and reducing the poor CNT-metal electrode contact, low surface area usage availability as well as post-transfer difficulties.42 Yang et al. reported the engineering and manufacture of CNT/graphene hybrid-based supercapacitors using chemical vapor deposition at atmospheric pressure with the growth of a CNT forest on the graphene layers to reduce the selfaggregation of both graphene and CNTs.43 The results exhibited an excellent specific capacitance of 653.7 mF cm2 at 10 mV s1 with outstanding stability with a fast charging–discharging capacity of around 75%. Lin et al. demonstrated the design and manufacture of 3D graphene/CNT carpet hybrid-based micro-supercapacitors in situ on nickel current collectors to maintain marvelous interfacial electrical conductance.44 Figure 4.7(a) exhibits the designed and engineered structure of 3D graphene/CNT carpet hybrids where the graphene layers are situated on nickel pillars followed by the addition of CNTs and iron/alumina catalysts. The corresponding SEM micrograph of the as-prepared 3D graphene/CNT carpet hybrid is shown in Figure 4.7(b). Moreover, CNTs were developed vertically on the graphene surface at different times (1, 2.5, and 5 min) to explore the influence of the height of the CNT carpet on the electrochemical activity of microsupercapacitors as shown in Figure 4.7(c–e).

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Figure 4.7

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(a) Schematic illustration of the constructed graphene/CNT carpet hybrid, (b) the corresponding SEM image of the manufactured graphene/ CNT carpet, cross-sectioned SEM images of the CNT carpet cultivated on graphene layers for (c) 1 min, (d) 2.5 min, and (e) 5 min.44 Reproduced from ref. 44 with permission from American Chemical Society, Copyright 2012.

The electrochemical impedance spectroscopy (EIS) analysis of the asfabricated hybrids was carried out to calculate the alternative current (ac) and impedance phase angles of micro-supercapacitors with a CNT carpet cultivated for different durations of time (1, 2.5, and 5 min). It can be seen from Figure 4.8(a) that the phase angle designed at 1 min growth of CNTs was found to be 81.51 at 120 Hz, whereas it decreased to 73.41 for 5 min growth of CNTs at 120 Hz which may be due to the longer length of ion diffusion and higher electrical resistance between the tip and tube base. The specific areal capacitance (CA) was found to be 230 mF cm2 for 1 min cultivation, 470 mF cm2 for 2.5 min cultivation, and 662 mF cm2 for 5 min growth at 120 Hz as a function of frequency, as shown in Figure 4.8(b). However, real and imaginary capacitances are key parameters used to define the performance of supercapacitors, which are calculated from impedance data. Therefore, the specific real (C 0 ) and imaginary (C00 ) capacitance at 120 Hz have been calculated and found to be excellent, as shown in Figure 4.8(c). The relaxation time constant (t0) was also calculated from the graph of C 0 and C00 as a function of frequency from the C00 at maximum frequency (f0) and found to be t0 (0.82 ms) for the microdevice with the CNT carpet grown for 1 min, 1.78 ms for 2.5 min growth, and 2.62 ms for 5 min growth. The extra small t0 value for the microdevice with the CNT carpet cultivated for 1 min explains the ultrafast ion adsorption/desorption. The calculation of power performance of the prepared materials is of prime importance as supercapacitors have been used to transport energy or power,

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Figure 4.8

Chapter 4

(a) Comparison of impedance phase angles of the prepared material with a commercial aluminum electrolyte capacitor (AEC) at 120 Hz. (b) Plot of specific areal capacitance as a function of frequency. (C) Plot of specific real and imaginary capacitance as a function of time for the cultivation of CNT carpet for 1, 2.5, and 5 min and (d) discharge current densities as a function of scan rate.44 Reproduced from ref. 44 with permission from American Chemical Society, Copyright 2012.

which can be calculated using cyclic voltammetry (CV). Figure 4.8(d) exhibits the discharge current densities at various scan rates from 0.1 to 400 V s1 for the CNT carpet cultivated for 2.5 min in an aqueous electrolyte of 1 M Na2SO4, which are found to be linearly dependent on the scan rates. Pseudocapacitive metal sulfides and oxides have been attracting tremendous attention as supercapacitor electrode materials due to their high theoretical specific capacitance, but the power density is very low due to small conductivities. Therefore, mixing of a graphene/CNT hybrid with a wide surface area and huge conductivity could help to wipe out these drawbacks by supporting the pseudocapacitive materials. Zhu et al. described the design of MnO2-supported CNT/graphene hybrid electrodes on nickel foam for the construction of a highly conductive network for outstanding activity of supercapacitors.45 The constructed device exhibits excellent cyclic stability with 82% activity after 3000 charge–discharge cycles with a magnificent specific capacity of 251 F g1 at a current density of 0.1 A g1. The MoO3 nanoplate-loaded CNT/graphene hybrid was synthesized for supercapacitor electrodes, which displays outstanding electrochemical activity with an excellent specific capacitance of 1503 F g1 at 1 A g1 and marvelous stability of 96.5% after 10 000 cycles.46 Li et al. designed the electrode material for supercapacitors based on sulfonated MnO2

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Figure 4.9

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Procedure for the design of binder-free MnO2–3D-CNT/graphene/Cu electrodes.48 Reproduced from ref. 48 with permission from Elsevier, Copyright 2019.

CNTs/graphene hybrids with an excellent specific capacitance of 336.4 F g1 at a current density of 0.5 A g1 as well as outstanding stability of 91.3% after 10 000 cycles.47 The marvelous activity of the as-prepared electrode material for supercapacitors may be due to the synergistic effects of the graphene/CNT hybrid with high electronic conductivity and MnO2 pseudocapacitance. Bi et al. described the design and architecture of a MnO2/3D-CNT/graphene hybrid material on a copper foil substrate using CVD followed by thermal decomposition of a manganese acetylacetonate precursor.48 In an a typical process, multilayers of graphene were cultivated on Cu foil at 1000 1C for 15 min in the presence of a mixture of CH4 (10 sccm) and H2 (300 sccm) followed by the cultivation of CNTs under H2 (210 sccm) and C2H2 (2 sccm) for 15 min using an Fe catalyst and the Al2O3 layer of a buffer as shown in Figure 4.9. The prepared 3D CNT/graphene/Cu hybrid was dissolved in ethylene glycol containing Mn(acac)3.4H2O using an ultrasonication method followed by thermal treatment in a tube furnace at 410 1C under an air atmosphere. The SEM images of the engineered 3D-CNT/graphene/Cu exhibit tightly packed bundles of CNTs and the tip of CNT bundles covered with Fe/Al2O3 catalyst, indicating a tip-growth mechanism as shown in Figure 4.10(a and b). However, the CNT bundles turn hard and crude after the addition of MnO2 into CNT/graphene/Cu due to diffusion of the MnO2–3D-CNT/graphene/Cu sample as shown in Figure 4.10(c and d). The addition of MnO2 into CNT/graphene/Cu can provide quick transfer of electrolyte ions during the electrochemical procedure. The presence of metal ions and their distribution were investigated using elemental mapping and EDX analysis indicating that the decomposition of MnO2 was perfectly systematic in 3D-CNT/graphene as shown in Figure 4.10(e and f). To confirm the behavior of the designed hybrid, CV analysis was carried out at different scan rates of 10, 20, 50, 100, and 200 mV s1 with a voltage window of 0.6–0 V, and the CV curves are rectangular shapes with a few humps of 3D

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Figure 4.10

SEM images of 3D CNT/graphene/Cu (a and b), SEM images (c and d), elemental mapping (e and f), and EDX analysis of the MnO2-3D-CNT/ graphene/Cu sample.48 Reproduced from ref. 48 with permission from Elsevier, Copyright 2019.

CNT/graphene/Cu and MnO2–3D-CNT/graphene/Cu indicating a double-layer capacitive behavior as shown in Figure 4.11(a and b). However, the area of the rectangular curve for MnO2–3D-CNT/graphene/Cu is much larger than that for 3D-CNT/graphene/Cu indicating outstanding capacitive activity as well as maintaining rectangular-like curves at a very high scan speed around 200 mV s1, implying that the pseudocapacitive reaction is fast and reversible. Figure 4.11(c and d) shows the triangular-like shape of the charge and discharge curves for 3D-CNT/graphene/Cu and MnO2–3D-CNT/graphene/Cu, further confirming the double-layer capacitive behavior. Moreover, the charge– discharge curves of MnO2–3D-CNT/graphene/Cu demonstrate the contribution toward pseudocapacitance due to the long triangular shape of the curve with a large distortion of the electrode, due to the redox reaction. The specific capacitance for 3D-CNT/graphene/Cu and MnO2–3D-CNT/graphene/Cu was determined at different current densities using galvanostatic charge–discharge (GCD) analysis, and found to be 42.3 F g1 and 365 F g1 at a current density of 1 A g1 respectively, and shown in Figure 4.11(e). However, the Nyquist plots for 3D-CNT/graphene/Cu and MnO2–3D-CNT/graphene/Cu are shown in Figure 4.11(f) indicating the presence of Rs which gives information on series resistance at the interface of the electrode and current collector. The charge transfer resistance (Rct) is derived from Figure 4.11(d) and found to be 0.23 O for MnO2–3D-CNT/graphene/Cu, indicating faster and easier transfer of electrons and electrolyte ions.

4.3.2

Fuel Cells

Fuel cells have been proven to be an auspicious energy storage device with high conversion efficiency, high energy density, low discharge activity, and supplying

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Figure 4.11

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CV curves at various scan rates (a and b), charge–discharge curves (c and d), specific capacitance at various current density (e), and Nyquist curves (f) in 0.1 M Na2SO4 for 3D-CNT/graphene/Cu and MnO2–3D-CNT/graphene/ Cu.48 Reproduced from ref. 48 with permission from Elsevier, Copyright 2019.

eco-friendly chemical energy to electrical energy, transport vehicles and portable electronics. However, the high cost and poor stamina of electrocatalysts limit their practical applications. The precious metal (Pt) deposited on a carbon support like CNTs and others have been used as electrocatalysts and considered as an outstanding material for fuel cells as well as receiving great attention. However, the catalytic activity and durability of Pt/CNTs are not so satisfactory due to the poor electrical, mechanical, and structural properties of CNTs. Hence, tremendous attempts have been dedicated to engineering the start-ofthe-art carbon supports for designing high-performance electrocatalysts. Among them all, graphene/CNTs hybrid composites have a strong potential in constructing high-performance fuel cell applications due to their corrosion resistivity, huge electrical conductivity, and large surface area. Herein, we will discuss proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) on graphene/CNT hybrid composites. Yun et al. described the design of PEMFC using a porous graphene/CNT hybrid composite cathode and composite exhibiting a lower charge transfer resistance as compared to the Pt–graphene cathode.49 In order to develop a high-performance PEMFC, Du and co-workers fabricated a graphene/CNT

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Figure 4.12

Chapter 4

(a) XRD patterns of Pt/rGO-PMWCNT with different mass ratios, (b–d) FESEM images of hybrid Pt/rGO-PMWCNT with different mass ratios, and (d) Nyquist graph of hybrid Pt/rGO-PMWCNT with different mass ratios. Reproduced from ref. 52 with permission from Elsevier, Copyright 2018.

hybrid composite using a two-step CVD method, which exhibited an outstanding power density of 1072 mW cm2.50 Pham et al. described the synthesis of a novel hierarchical graphene/CNT support for the Pt catalyst for PEMFC using a two-step process using a thermal chemical vapor deposition technique.51 A 3D structured Pt/rGO-polyethyleneimine-multiwalled CNT (PMWCNT) hybrid composite was prepared by Yan et al. for PEMFC.52 They synthesized GO using a modified Hummer’s method, and MWCNT was functionalized by polyethyleneimine (PEI) by mixing and stirring the aqueous solution for 24 h at 60 1C. Different ratios of GO-PMWCNT (1/3, 1/1, 3/1) were synthesized by stirring both the GO and PMWCNT at 50 1C for 12 h in the presence of 0.1 M HCl solution. Pt-decorated rGO-PMWCNT was prepared using microwave treatment. Figure 4.12(a) shows the XRD patterns of Pt/rGOPMWCNT at different mass ratios of rGO, which indicate the successful deposition of Pt on rGO-PMWCNT. It can be seen from Figure 4.12(b–d) that Pt/rGO-PMWCNT exhibits the worm-like MWCNTs well dispersed between rGO surfaces, forming a 3D structure. Nyquist plots were obtained for Pt/rGO-PMWCNTs with different mass ratios as shown in Figure 4.12(e). The figure shows that the diameter of the semicircle presents the charge transfer resistance, which is almost in a similar range except for Pt/rGO. However, the different mass ratios of rGO and PMWCNTs disturb the electrochemical active surface area (ECSA), and durability and electrochemical cell activity, which is improved by the hybridization of appropriate ratios of rGO and multiwalled CNTs, and the best performance was achieved at a 1 : 1 ratio of rGO and multiwalled CNTs.

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Jha et al. described the design of PtRu–graphene/multiwalled-CNT hybrid nanocomposites for DMFC as the cathode electrocatalyst, exhibiting a power density of about 68 mW cm2.53 Kwok and co-workers demonstrated the engineering of DMFC as a porous anode based on RuPt core–shell nanoparticle decorated CNT/graphene hybrid composites. The manufactured DMFC shows a specific power density of 10.15 mW mg1 due to the synergistic effect of graphene and CNTs.54 Liu et al. reported the architecture of a 3D sandwiched structure of highly active and stable Pt nanoparticle decorated graphene with intercalated CNTs using electrostatic self-assembly for the methanol oxidation reaction in a fuel cell.55 CNTs work as spacers between the graphene sheet and hinder the restacking, which increases the utilization of Pt (114%) for DMFC. Kwok et al. described the manufacture of anode materials for a direct microfluidic fuel cell (DMFC) based on RuPt core–shell decorated graphene/CNT hybrid composites.56 The synthesis of RuPt–graphene/CNT hybrids was achieved using a hydrothermal method at 120 1C for 6 h. Typically, a RuPt nanoparticle solution was mixed with graphene and CNTs by an ultra-sonication method, where nanoparticles would be adsorbed on the graphene sheet and cultivated in the gel during the reduction process of graphene. The performance of the porous anode was analyzed using a designed microfluidic fuel cell (MFC) exhibiting orthogonal flow as depicted in Figure 4.13(a). SEM and transmission electron microscopy (TEM) images of a RuPt–graphene/CNT hybrid composite reveal that the porous anode was manufactured by interconnected sub-ten micro-meter pores and also indicate the presence of graphene and CNTs. The elemental mapping also indicates the presence of Ru, Pt, carbon, and

Figure 4.13

(a) MFC showing the flow direction of the electrolyte, (b) SEM image, (c) TEM image, (d) high-resolution TEM image, and (e–f) elemental mapping of the RuPt–graphene/CNT hybrid composite.57 Reproduced from ref. 56 with permission from Elsevier, Copyright 2018.

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oxygen as shown in Figure 4.13(b–h). The MFC was manufactured for both co-flow and counter flow, where the flow of anolyte was from the porous anode reaching the catholyte in a face-to-face arrangement. The distance between the two electrodes was 1000 mm, whereas the thickness of the micro-channel was found to be reduced to 500 mm during co-flow through orthogonal flow leading to a non-straight line between the anolyte and catholyte in the mid-channel as shown in Figure 4.14(a). The open current–voltage (OPV) of the MFC was found to be 0.5 V with a 500 mm channel when operated in 1 M methanol with 1 M KOH anolyte and catholyte and it significantly enhanced to 0.8 V with a 1000 mm channel. However, the power density was found to be enhanced by 10% from 9 mW mg1 to 9.92 mW mg1 catalyst, whereas electrolyte flow was found to be enhanced from 50 ml min1 to 100 ml min1 as shown in Figure 4.14(b). Further enhancement in the flow rate up to 100 ml min1 does not give any specific change. Figure 4.14(c) indicates the effect of the loading amount of catalyst in the MFC from 0.5 to 2 times, where the specific power output decreased from 16.6 mW cm2 to 9.3 mW cm2 for half the catalyst loading whereas it enhanced to 23.2 mW cm2 for twice the catalyst loading on the anode. The output power enhanced by 39.8% only for the doubled loading of catalyst in MFC as shown in Figure 4.14(d). Therefore, the loading of catalyst at the anode is a key parameter for obtaining high power density by wasting too much catalyst in practical applications.

Figure 4.14

MFC activity at different (a) channels, (b) flow rates of electrolyte, (c) loading of catalyst, and (d) the single electrode performance for the different catalyst loading in (c).57 Reproduced from ref. 56 with permission from Elsevier, Copyright 2018.

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4.4 Conclusion The connection between human beings and materials is one of the excellent successes in the expansion of nanotechnology and material science. Scientific approaches allow the matter to expose itself more notably through the craft of nanotechnology. Therefore, a great endeavor has been carried out by the scientific community to expose and exploit the matter more logically. However, use of carbon (CNTs and graphene)-based materials in a diverse field of energy conversion and storage has established their importance due to their excellent Young’s modulus, large surface area, great thermal and chemical stability, good electrical and thermal conductivity, huge mechanical flexibility, and so on. Therefore, considerable attention has been focused on the design and engineering of CNT/graphene hybrid composites for energy-related applications. Moreover, the hybrid of graphene and CNTs can conquer the innate deficiency of the individual component as well as carry forward the inherent merits. The fixing of CNTs into a hybrid can stop the aggregation of the graphene sheet and further improve the existing properties with the help of a synergistic effect. Hence, the desired synergistic effect enhancement can be achieved by adopting suitable graphene sheets and CNTs with different walls. The architecture and engineered CNT/graphene hybrid composites with a smooth C–C junction acquire the excellent properties of single 1D CNTs and 2D graphene as well as build some state-of-the-art functions which result in their being auspicious materials for energy-related applications. Therefore, this article offers inspiration for scientists for further achieving great quality graphene/CNT hybrid composites for the energy-related applications.

Acknowledgements Financial support for this research project from the Science and Engineering Research Board (SERB) by DST, Government of India, New Delhi, for providing the National Post-Doctoral Fellowship to Waseem Raza (Scheme No: PDF/2016/001471) and from the Department of Chemistry, Indian Institute of Technology Delhi is gratefully acknowledged.

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CHAPTER 5

One-dimensional Carbon Nanotube Decorated Two-dimensional Reduced Graphene Oxide Composite: Insight from Synthesis to Application in Dye Sensitized Solar Cells KHURSHEED AHMAD AND SHAIKH M. MOBIN* Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Khandwa Road, Indore 453552, M.P, India *Email: [email protected]

5.1 Introduction Energy demand is one of the major concerns for the next generation of humans.1–7 Although different energy sources have been found, few of them are considered as highly efficient energy sources. Among them, solar energy is the most efficient renewable energy source which can satisfy the energy requirements.8–17 Solar energy can be utilized by a photovoltaic device which directly generates electricity upon the consumption of solar energy.18–34 These photovoltaic devices are also known as solar cells. In the last few decades, different types of solar cells (amorphous, crystalline, polycrystalline, organic, All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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perovskite, dye sensitized, polymer and quantum dot) have been developed.35–40 Dye sensitized solar cells (DSSCs) were developed by Gratzel and co-workers in 1991.3 DSSCs have been proven to be highly efficient energy conversion devices. DSSCs also have many advantages such as being cost effective, having a simple fabrication process, wide material sources, good flexibility, multi-color transparency and so on. Light sensitizers play a vital role in the performance of the DSSCs. Previously, metalloporphyrin dyes, metalfree organic dyes and Ru/Co based dyes have been utilized as light sensitizers for the construction of DSSCs. The best power conversion efficiency (PCE) of 13% was achieved for DSSCs.41 However, although DSSCs have shown excellent performances, the presence of precious metal platinum-based counter electrodes increased the cost of the DSSC devices. Hence, it is important to achieve low-cost non-precious metal-based counter electrodes. In this regard, Jin et al.42 employed a MnO2 nanotube decorated reduced graphene oxide (MnO2/rGO) composite as a counter electrode material. The fabricated DSSCs showed a PCE of 5.4%. Ahmad et al.2 used a hydrothermal method to prepare MnO2 nano-rods for DSSC applications. The constructed DSSC device exhibited the best PCE of 4.1% with a good open circuit voltage. Zhang et al.43 constructed novel rGO/Mn3O4 based counter electrodes for DSSCs. An improved PCE of 5.9% was achieved. In another work, MnO2 coated carbon nanofibers were also employed as low-cost counter electrodes.44 An excellent PCE of 8.86% was achieved which is comparable with the Pt-based DSSCs.44 Bora et al.45 introduced carbon black/polyaniline nanotube-based counter electrodes for DSSCs and obtained a PCE of 6.68%. Carbonaceous materials (diamond, graphite, amorphous carbon, graphene and carbon nanotubes) have been widely explored in different applications. Graphene and carbon nanotubes have a good conductive nature and excellent electrical and chemical properties. In previous years, reduced graphene oxide (rGO) and carbon nanotubes (CNT) have been explored as cost effective counter electrode materials for DSSC applications. In this chapter, we review the recent advances in the development of rGO/CNT-based counter electrode materials for DSSC applications.

5.2 Dye Sensitized Solar Cells In this section, we describe the fabrication and working principle of DSSCs.

5.2.1

Fabrication of Dye Sensitized Solar Cells

Fluorine doped tin oxide (FTO) glass is widely used as the working substrate to fabricate a photoanode. Typically, titanium dioxide (TiO2) paste is deposited onto the FTO glass using a glass rod followed by annealing at 500 1C for 30 min. Subsequently this prepared photoanode is dipped in dye solution for 24 h. Meanwhile, platinum is deposited on ITO glass to prepare the counter electrode. Finally, both the electrodes are clipped together in a sandwich-like structure and a liquid electrolyte is injected between the two electrodes.

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Figure 5.1

5.2.2

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Schematic diagram of DSSCs.

Components of Dye Sensitized Solar Cells

There are four different components in DSSCs as listed here: i) ii) iii) iv)

Photoanode Dye Electrolyte Counter electrode

Solar light strikes the surface of the constructed DSSC device. This light creates the electron–hole pairs in the dye. This electron is injected into the photoanode (FTO/TiO2) and travels to the counter electrode through load. The use of redox electrolyte regenerates the dye molecules (Figure 5.1). Generally, Pt is used as the counter electrode material whereas iodide/ triiodide is used as the redox electrolyte.

5.3 Recent Advances in Counter Electrodes Pt has been proven to be a highly efficient counter electrode material but due to its preciousness and high cost its use is limited in DSSCs. It is necessary to determine other non-precious and low-cost counter electrode materials. In this section we describe the use of rGO/CNT composites as cost effective and Pt-free counter electrode materials for DSSC applications. Yeh et al.46 synthesized core–shell hetero-structures of multi-walled carbon nanotube/graphene oxide nanoribbons (MWCNT@GONR) and multi-walled carbon nanotube@reduced graphene oxide nanoribbon (MWCNT@rGONR). Yeh et al.46 utilized microwave-assisted synthesis step/chemical reduction approaches for the synthesis of MWCNT@rGONR. The recorded Raman spectra of the synthesized MWCNT@GONR and MWCNT@rGONR are shown in Figure 5.2A. The Raman spectra of the MWCNT@GONR showed a relatively high D/G value which may be attributed to the presence of a high density of defects. Furthermore, to investigate the electro-catalytic properties of MWCNT and MWCNT@rGONR, the authors recorded the cyclic voltammetry (CV) curves in the presence of iodide/triiodide redox solution.

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Raman spectra (A) of MWCNT@GONR and MWCNT@rGONR. CV (B) and J–V curves (C) of GNP, MWCNT and MWCNT@rGONR. Reproduced from ref. 46 with permission from American Chemical Society, Copyright 2014.

The recorded CV results suggested the good electro-catalytic properties of the synthesized MWCNT and MWCNT@rGONR. The following reactions ((5.1) and (5.2)) occur during the CV measurements. 3I-I3 þ 2e

(5.1)

I3 þ 2e -3I

(5.2)







The authors also recorded the CV curve for the commercially available graphene nano-powder (GNP) for comparison purposes. The CV results showed the relatively poor electro-catalytic behavior of the GNP. Further DSSCs were fabricated using GNP, MWCNT and MWCNT@rGONR as costeffective counter electrode materials. The photocurrent–voltage (J–V) curves of the fabricated DSSCs were also recorded and are presented in Figure 5.2C. The GNP-based DSSCs showed a PCE of 4.86% whereas the MWCNT-based device exhibited a PCE of 5.93%. However, an enhanced PCE of 6.91% was obtained using MWCNT@rGONR. This enhanced PCE was attributed to the synergistic effects and better electro-catalytic properties of the MWCNT@rGONR. The obtained PCE of 6.91% is interesting and showed the potential of low-cost counter electrode materials for practical purposes.

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This PCE can also be improved by utilizing other Pt-free counter electrode materials. In this regard, Battumur et al.47 developed DSSCs using different counter electrodes consisting of multi-walled carbon nanotubes (MWNT) and graphene nano-sheets (GNS). The recorded SEM picture of the GNS is shown in Figure 5.3a, whereas the SEM picture of the MWNT is presented in Figure 5.3d. The TEM image and SAED pattern of GNS are presented in Figure 5.3b and c, respectively. Further authors also recorded the XRD patterns of the GNS and MWNT. The XRD data of the GNS and MWNT are presented in Figure 5.4A. The XRD patterns of the GNS and MWNT showed the presence of phase purity. The counter electrodes with different percentages (20, 40, 60 and 80%) of GNS were prepared. Furthermore, electrochemical investigations were also carried out to check their electro-catalytic activity for DSSC applications. The counter electrode consisting of MWNT (60%) and GNS (40%) exhibited the better electro-catalytic behavior. Furthermore, DSSCs devices were fabricated using different counter electrodes. The J–V curves of the constructed DSSCs devices with Pt, GNS, MWNT and different percentages of GNS (20, 40, 60 and 80%) are presented in Figure 5.4B. The fabricated DSSCs with a Pt counter electrode showed the highest PCE of 5%. However, the lowest efficiency of 1% was observed for the DSSCs having GNS as the counter electrode. The 3% PCE was obtained using MWNT as the counter electrode. Furthermore, it was observed that the PCE increases when the percentage of GNS decreases. The improved PCE of 4% with an open circuit voltage of 770 mV was obtained for DSSCs having a counter electrode consisting of MWNT (60%) and GNS (40%).

Figure 5.3

SEM images of GNS (a) and MWNTs (d). TEM image (b) and SAED pattern (c) of GNS. Reproduced from ref. 47 with permission from Elsevier, Copyright 2011.

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Figure 5.4

XRD (A) of GNS and MWNT and J–V curves of the fabricated DSSCs with different counter electrodes (B). Reproduced from ref. 47 with permission from Elsevier, Copyright 2011.

Figure 5.5

Schematic diagram showing the fabrication process of RGO films. Reproduced from ref. 48 with permission from Elsevier, Copyright 2019.

The observations revealed that the counter electrode consisting of MWNT (60%) and GNS (40%) has a better electro-catalytic activity and synergistic effects. The obtained PCE of 4% for the counter electrode consisting of MWNT (60%) and GNS (40%) is comparable with the Pt-based counter electrode. In another work, Zhao et al.48 synthesized RGO by the in situ thermal conversion of graphene oxide films. Typically, the authors prepared the GO film on an FTO substrate and annealing (300 1C) in of the GO/FTO yielded RGO film. This obtained film was denoted as RGO1. The authors also obtained the RGO films by directly using RGO solution. This prepared film was denoted as RGO2 (Figure 5.5).

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The surface morphological properties of RGO1 and RGO2 were investigated using SEM and TEM analysis. The formation of RGO1 and RGO2 was confirmed by XPS and Raman analysis. The SEM and TEM results of RGO1 and RGO2 suggested the presence of sheet-like surface morphology. Furthermore, electrochemical measurements were performed to check the potential of RGO1 and RGO2 as counter electrode materials for DSSC applications. The CV curves of Pt, RGO1 and RGO2 were recorded in acetonitrile solution (0.1 M LiClO4, 10.0 mM LiI, and 1.0 mM I2). The recorded CV curves are presented in Figure 5.6A. The CV curves show the two oxidation and two reduction peaks. The Pt electrode exhibits better electro-catalytic behavior compared to the other two electrodes. However, RGO1 shows remarkably good electrocatalytic activity compared to RGO2. Furthermore, DSSCs were fabricated using three different counter electrodes (Pt, RGO1 and RGO2). The photovoltaic performance of the fabricated DSSCs was investigated by recording J–V curves under 1 sun conditions. The recorded J–V curves of the DSSCs with three different counter electrodes (Pt, RGO1 and RGO2) are depicted in Figure 5.6B. The highest PCE of 7.53% along with open circuit voltage of 734 mV was obtained for DSSCs with the Pt counter electrode. The lowest PCE of 1.20% was observed for the RGO2 counter electrode-based DSSCs. However, an improved

Figure 5.6

CV curves (A) of Pt, RGO1 and RGO2 in electrolyte solution (0.1 M LiClO4, 10.0 mM LiI, and 1.0 mM I2). J–V (B) and IPCE (C) curves of the fabricated DSSCs with different counter electrodes (Pt, RGO1 and RGO2). Adapted from ref. 48 with permission from Elsevier, Copyright 2019.

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PCE of 6.38% was obtained for the RGO1 counter electrode-based DSSCs device. The incident-photon-to-current-conversion (IPCE) was also recorded. The IPCE curves of the fabricated DSSCs with different counter electrodes (Pt, RGO1 and RGO2) are presented in Figure 5.6C. The highest IPCE value of 83.1% was observed for the Pt counter based DSSCs, whereas 75% was obtained for the RGO1 counter electrode-based DSSCs. In 2019, Yu et al.49 developed DSSCs and achieved a remarkable PCE of 10.56% using graphene/carbon nanotubes as the counter electrode. In this work, Yu et al.49 utilized single-walled carbon nanotubes and graphene as efficient and cost-effective electrode materials for the counter electrode. The authors adopted a hydrothermal and freeze-drying approach to obtain the 3D graphene/carbon nanotube (GCT) composite. The structural properties of the GCT were determined by SEM and TEM methods. The SEM and TEM results showed the presence of carbon nanotubes on graphene sheets. The formation of GCT was confirmed by XRD and Raman analysis. The specific surface area of the GCT was also studied. Furthermore, the electro-catalytic activity of the rGO, Pt and GCT were investigated by recording CV curves in iodide electrolyte solution. The CV curves of the rGO, Pt and GCT in iodide electrolyte are presented in Figure 5.7. The obtained results showed the better electro-catalytic activity of GCT compare to the rGO or Pt. Furthermore, DSSCs were fabricated using rGO, Pt and GCT as counter electrode materials. The photovoltaic performance of the constructed DSSCs was investigated under 1 sun conditions. The recorded J–V curves of the DSSCs fabricated with rGO, Pt and GCT counter electrodes are presented in Figure 5.8a. The Pt counter electrode-based DSSCs exhibited a PCE of 7.64% while rGO-based DSSCs showed a PCE of 7.25%. In the case of GCT counter electrode-based DSSCs, the highest PCE of 9.24% was achieved (Figure 5.8a). The IPCE spectra of the rGO, Pt and GCT

Figure 5.7

CV curves of RGO, Pt and GCT in the presence of iodine electrolyte solution. Adapted from ref. 49 with permission from Elsevier, Copyright 2018.

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Figure 5.8

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J–V curves (a) and IPCE (b) curves of the rGO, Pt and GCT counter electrode-based DSSCs. J–V curves (c) of the DSSCs fabricated with GCT and GCT-F. Reproduced from ref. 49 with permission from Elsevier, Copyright 2018.

counter electrode-based DSSCs are displayed in Figure 5.8b. The DSSCs with the GCT counter electrode showed better IPCE values compared to the other DSSC devices. Furthermore, the J–V curve of the GCT counter electrodebased DSSCs was also recorded by using a mirror (this mirror was kept perpendicular to the light direction below the cell). This was denoted as GCT-F and the recorded J–V curve is presented in Figure 5.8c. The obtained results showed an improved open circuit voltage which resulted to an improved PCE of 10.56%. The UV–vis transmittance spectra of the FTO and GCT are presented in the inset of Figure 5.8c. This shows the good transmittance of the GCT.

5.4 Conclusions and Future Prospects Dye sensitized solar cells are highly efficient and cost-effective photovoltaic devices. The counter electrode of the dye sensitized solar cells plays a vital role. Initially, precious metal (Pt)-based counter electrodes were developed for dye sensitized solar cell applications. However, the limited availability and high cost of Pt restricted its use for practical applications. Thus, Pt-free counter electrodes were developed for dye sensitized solar cells. Previously, different electrode materials such as metal oxides, polymers, and hybrid

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composite materials were utilized as counter electrode materials for dye sensitized solar cells. The hybrid composite of graphene/carbon nanotubes has also been employed as cost-effective counter electrode materials. The following strategies may be useful to further improve the efficiency of the dye sensitized solar cells: a) The performance of dye sensitized solar cells can be further improved by developing new counter electrode materials or photoanodes. b) Two-dimensional (2D) and carbon-based materials possess excellent conductivity which may be useful to construct highly efficient counter electrodes for dye sensitized solar cells. c) A counter electrode with graphitic carbon nitride may be a suitable electrode material for dye sensitized solar cells. d) The hybrid composite of graphitic nitride with transition metal oxide may improve the efficiency of dye sensitized solar cells. e) All-carbon composites (graphitic carbon nitride/graphene or carbon nanotubes/graphitic carbon nitride) may also be used as low-cost counter electrode materials for dye sensitized solar cell applications. f) Novel photoanodes would also be useful to further improve the electron transporting activity which will enhance the photovoltaic performance of the dye sensitized solar cells. g) The use of novel and solid-state electrolytes will also be of great importance for dye sensitized solar cells. h) The design and synthesis of ruthenium-free dye would be useful for the development of low-cost dye sensitized solar cells for practical applications.

Acknowledgements K.A. thanks UGC, New Delhi, India for RGNFD. S.M.M. acknowledges the Discipline of Chemistry, IIT Indore. S.M.M. would like to thank SERB-DST (Project No. CRG/2020/001769), BRNS (Project No. 58/14/17/2020-BRNS), CSIR (01(2935)/18/EMR-II), New Delhi, India, and IIT Indore, India for financial support.

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4. X. Fang, T. Ma, G. Guan, M. Akiyama, T. Kida and E. Abe, Effect of the thickness of the Pt film coated on a counter electrode on the performance of a dye-sensitized solar cell, J. Electroanal. Chem., 2004, 570, 257–263. 5. K. Ahmad and S. M. Mobin, Graphene oxide based planar heterojunction perovskite solar cell under ambient condition, New J. Chem., 2017, 41, 14253–14258. 6. C. W. Kung, H. W. Chen, C. Y. Lin, K. C. Huang, R. Vittal and K. C. Ho, CoS acicular nanorod arrays for the counter electrode of an efficient dye-sensitized solar cell, ACS Nano, 2012, 6, 7016–7025. 7. F. Gong, H. Wang, X. Xu, G. Zhou and Z.-S. Wang, In situ growth of Co0. 85Se and Ni0. 85Se on conductive substrates as high-performance counter electrodes for dye sensitized solar cells, J. Am. Chem. Soc., 2012, 134, 10953–10958. 8. K. Ahmad, S. N. Ansari, K. Natarajan and S. M. Mobin, A Two-Step Modified Deposition Method Based (CH3NH3)3Bi2I9 Perovskite: Lead Free, Highly Stable and Enhanced Photovoltaic Performance, ChemElectroChem, 2019, 6, 1192–1198. 9. K. Ahmad, P. Kumar and S. M. Mobin, A Two-Step Modified Sequential Deposition Method-based Pb-Free (CH3NH3)3Sb2I9 Perovskite with Improved Open Circuit Voltage and Performance, ChemElectroChem, 2020, 7, 946–950. 10. K. Ahmad, S. N. Ansari, K. Natarajan and S. M. Mobin, Design and Synthesis of 1D-Polymeric Chain Based [(CH3NH3)3Bi2Cl9]n Perovskite: A New Light Absorber Material for Lead Free Perovskite Solar Cells, ACS Appl. Energy Mater., 2018, 01, 2405–2409. ¨tzel, Dye-sensitized solar cells, J. Photochem. Photobiol., C., 2003, 11. M. Gra 4, 145–153. 12. K. Ahmad and S. M. Mobin, Organic–Inorganic Copper (II)-Based Perovskites: A Benign Approach toward Low-Toxicity and Water-Stable Light Absorbers for Photovoltaic Applications, Energy Technol., 2020, 8, 1901185. ¨tzel, Solar energy conversion by dye-sensitized photovoltaic cells, 13. M. Gra Inorg. Chem., 2005, 44, 6841–6851. 14. S. S. Kim, Y. C. Nah, Y. Y. Noh, J. Jo and D. Y. Kim, Electrodeposited Pt for cost-efficient and flexible dye-sensitized solar cells, Electrochim. Acta., 2006, 51, 3814–3819. 15. Q. Liu, J. Wu, Z. Lan, M. Zheng, G. Yue, J. Lin and M. Huang, Preparation of PAA-g-PEG/PANI polymer gel electrolyte and its application in quasi solid state dye-sensitized solar cells, Polym. Eng. Sci., 2015, 55, 322–326. 16. K. Ramalingam, S. Panchu, A. S. Salunke, K. Muthukumar, A. Ramanujam and S. Muthiah, Free-standing graphene/ conducting polymer hybrid cathodes as FTO and Pt-free electrode for quasi-state dye sensitized solar cells, ChemistrySelect, 2016, 1, 4814–4822.

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17. K. Imoto, K. Takahashi, T. Yamaguchi, T. Komura, J. I. Nakamura and K. Murata, High performance carbon counter electrode for dyesensitized solar cells, Sol. Energy Mater. Sol. Cells., 2003, 79, 459–469. 18. K. Suzuki, M. Yamaguchi, M. Kumagai and S. Yanagida, Application of carbon nanotubes to counter electrodes of dye-sensitized solar cells, Chem. Lett., 2002, 32, 28–29. ¨llen, Transparent, conductive graphene 19. X. Wang, L. Zhi and K. Mu electrodes for dye sensitized solar cells, Nano Lett., 2008, 8, 323–327. 20. X. Zhang, M. Zhen, J. Bai, S. Jin and L. Liu, Efficient NiSe-Ni3Se2/ graphene electrocatalyst in dye-sensitized solar cells: the role of hollow hybrid nanostructure, ACS Appl. Mater. Interfaces., 2016, 8, 17187–17193. 21. M. Y. Yen, C. C. Teng, M. C. Hsiao, P. I. Liu, W. P. Chuang, C. C. M. Ma and C. H. Tsai, Platinum nanoparticles/graphene composite catalyst as a novel composite counter electrode for high performance dye-sensitized solar cells, J. Mater. Chem., 2011, 21, 12880–12888. ¨tzel, Conversion of sunlight to electric power by nanocrystalline 22. M. Gra dye-sensitized solar cells, J. Photochem. Photobiol., A., 2004, 164, 3–14. 23. T. N. Murakami, S. Ito, Q. Wang, M. K. Nazeeruddin, T. Bessho, I. Cesar ¨tzel, Highly efficient dye-sensitized solar cells based and M. Gra on carbon black counter electrodes, J. Electrochem. Soc., 2006, 153, A2255–A2261. 24. G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese and C. A. Grimes, Use of highly-ordered TiO2 nanotube arrays in dye-sensitized solar cells, Nano Lett., 2006, 6, 215–218. 25. Z. Jin, M. Zhang, M. Wang, C. Feng and Z.-S. Wang, Cobalt selenide hollow nanorods array with exceptionally high electrocatalytic activity for high-efficiency quasisolid- state dye-sensitized solar cells, J. Power Sources., 2018, 378, 475–482. 26. G. R. Li, F. Wang, Q. W. Jiang, X. P. Gao and P. W. Shen, Carbon nanotubes with titanium nitride as a low-cost counter-electrode material for dye-sensitized solar cells, Angew. Chem., Int. Ed., 2010, 49, 3653–3656. 27. W. J. Lee, E. Ramasamy, D. Y. Lee and J. S. Song, Efficient dye-sensitized solar cells with catalytic multiwall carbon nanotube counter electrodes, ACS Appl. Mater. Interfaces., 2009, 1, 1145–1149. 28. J. D. Roy-Mayhew, D. J. Bozym, C. Punckt and I. A. Aksay, Functionalized graphene as a catalytic counter electrode in dye-sensitized solar cells, ACS Nano., 2010, 4, 6203–6211. 29. C. J. Liu, S. Y. Tai, S. W. Chou, Y. C. Yu, K. D. Chang, S. Wang and T. W. Lin, Facile synthesis of MoS2/graphene nanocomposite with high catalytic activity toward triiodide reduction in dye-sensitized solar cells, J. Mater. Chem., 2012, 22, 21057–21064.

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30. W. S. Hummers Jr and R. E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc., 1958, 80, 1339. 31. F. Gong, Z. Li, H. Wang and Z.-S. Wang, Enhanced electrocatalytic performance of graphene via incorporation of SiO2 nanoparticles for dye-sensitized solar cells, J. Mater. Chem., 2012, 22, 17321–17327. 32. Y. Xue, J. Liu, H. Chen, R. Wang, D. Li, J. Qu and L. Dai, Nitrogen-doped graphene foams as metal-free counter electrodes in high-performance dye-sensitized solar cells, Angew. Chem., Int. Ed., 2012, 51, 12124–12127. 33. Z. Li, F. Gong, G. Zhou and Z.-S. Wang, NiS2/reduced graphene oxide nanocomposites for efficient dye-sensitized solar cells, J. Phys. Chem. C., 2013, 117, 6561–6566. 34. H. Liu, Z. Jin, Y. Su and Y. Wang, Visible light-driven Bi2Sn2O7/reduced graphene oxide nanocomposite for efficient photocatalytic degradation of organic contaminants, Sep. Purif. Technol., 2015, 142, 25–32. 35. M. Zhang, Z. Jin, C. Feng, M. Wang and Z.-S. Wang, Phenyl and thienyl functionalized imidazolium iodides for highly efficient quasi-solid-state dye-sensitized solar cells, J. Mater. Chem. A., 2017, 5, 16976–16983. 36. X. Jiang, H. Li, S. Li, S. Huang, C. Zhu and L. Hou, Metal-organic framework-derived Ni–Co alloy@ carbon microspheres as highperformance counter electrode catalysts for dye-sensitized solar cells, Chem. Eng. J., 2018, 334, 419–431. 37. K. Imoto, K. Takahashi, T. Yamaguchi, T. Komura, J. I. Nakamura and K. Murata, High performance carbon counter electrode for dyesensitized solar cells, Sol. Energy Mater. Sol. Cells, 2003, 79, 459–469. 38. W. Hong, Y. Xu, G. Lu, C. Li and G. Shi, Transparent graphene/PEDOT– PSS composite films as counter electrodes of dye-sensitized solar cells, Electrochem. Commun., 2008, 10, 1555–1558. ¨tzel, Electrochemical impedance 39. Q. Wang, J. E. Moser and M. Gra spectroscopic analysis of dye-sensitized solar cells, J. Phys. Chem. B., 2005, 109, 14945–14953. 40. T. Liu, X. Mai, H. Chen, J. Ren, Z. Liu, Y. Li and Z. Guo, Carbon nanotube aerogel–CoS2 hybrid catalytic counter electrodes for enhanced photovoltaic performance dye sensitized solar cells, Nanoscale, 2018, 10, 4194–4201. 41. S. Mathew, A. Yella and P. Gao, et al., Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers, Nat. Chem., 2014, 6, 242–247. 42. P. Jin, X. Zhang, M. Zhen and J. Wang, MnO2 nanotubes with grapheneassistance as low-cost counter-electrode materials in dye-sensitized solar cells, RSC Adv., 2016, 6, 10938–10942. 43. Q. Zhang, Y. Liu, Y. Duan, N. Fu, Q. Liu, Y. Fang, Q. Sun and Y. Lin, Mn3O4/graphene composite as counter electrode in dye-sensitized solar cells, RSC Adv., 2014, 4, 15091–15097. 44. L. Li, Q. Lu, J. Xiao and J. Li, et al., Synthesis of highly effective MnO2 coated carbon nanofibers composites as low cost counter electrode for efficient dye-sensitized solar cells, J. Power Sources, 2017, 363, 9–15.

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45. A. Bora, K. Mohan, P. Phukan and S. K. Dolui, A low cost carbon black/ polyaniline nanotube composite as efficient electro-catalyst for triiodide reduction in dye sensitized solar cells, Electrochim. Acta., 2018, 259, 233–244. 46. M.-H. Yeh, L.-Y. Lin, C.-L. Sun, Y. A. Leu and J.-T. Tsai, et al., Multiwalled Carbon anotube@Reduced Graphene Oxide Nanoribbon as the Counter Electrode for Dye-Sensitized Solar Cells, J. Phys. Chem. C, 2014, 118, 16626–16634. 47. T. Battumur, S. H. Mujawar, Q. T. Truong, S. B. Ambade, D. S. Lee, W. Lee and S.-H. Han, et al., Graphene/carbon nanotubes composites as a counter electrode for dye-sensitized solar cells, Curr. Appl. Phys., 2012, 12, e49–e53. 48. G. Zhao, C. Feng, H. Cheng, Y. Li and Z.-S. Wang, In situ thermal conversion of graphene oxide films to reduced graphene oxide films for efficient dye-sensitized solar cells, Mater. Res. Bull., 2019, 120, 110609. 49. F. Yu, Y. Shi, W. Yao, S. Han and J. Ma, A new breakthrough for graphene/carbon nanotubes as counter electrodes of dye-sensitized solar cells with up to a 10.69% power conversion efficiency, J. Power Sources., 2019, 412, 366–373.

Section 3: Composites of Carbon Nanodots and Quantum Dots

CHAPTER 6

Carbon Dot-based Composites: Recent Progress, Challenges and Future Outlook L. C. SIM,*a S. S. TERNG,a J. Y. LIM,a J. J. NG,b W. C. CHONG,a K. H. LEONGb AND P. SARAVANANc a

Department of Chemical Engineering, Lee Kong Chian Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Jalan Sungai Long 9, Bandar Sungai Long, 43000 Kajang, Selangor, Malaysia; b Department of Environmental Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, Kampar, Perak, Malaysia; c Department of Environmental Science and Engineering, Indian Institute of Technology(ISM), Dhanbad, Dhanbad-826004, Jharkhand, India *Email: [email protected]

6.1 Introduction to Carbon Dots The discovery of nanomaterials has had an impactful effect on technology development in numerous fields such as electronic, water treatment, health science and petrochemical. The carbon dot is a new generation of carbon nanomaterial which was first accidentally discovered by Xu et al. in 2004 when they purified single-walled carbon nanotubes (CNT) from arc discharge soot through electrophoresis.1 These nanomaterials of different molecular weights with a maximum lateral dimension of less than 18 nm and vertical size of around 1 nm, emitted green-blue, yellow and orange colors in suspension form. Since then, CDs have been extensively studied by scientists to break through their current limit with higher efficiency, better All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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accuracy and greater speed for various applications. Typically, CDs are a 0D carbon-dominated material, most of them present in extremely small average size of less than 10 nm with a conjugated p structure. Its centric carbon core is covered with an abundance of functional groups like carboxyl (–COOH), carbonyl (–CO) and hydroxyl (–OH) as a result of the oxidation process.2 Therefore, the properties of the CDs could be easily modified or tuned by surface engineering to achieve different energy gaps, hence they exhibit distinct photoluminescence and optical properties.3 Compared with other carbon materials such as carbon nanotubes (CNT), fullerenes and graphene which require synthesis methods that are tedious and high in cost, CDs can be produced via facile preparation methods on a large scale. The former macro-sized carbon materials have poor water solubility and low fluorescence.4 On the other hand, CDs exhibit superior light stability against blinking and photo-bleaching, and higher photoluminescence quantum yields compared with semiconductor quantum dots and traditional dyes.5 Its conjugated p structure accompanied by quasimolecules, sub-fluorophores, defect or doping with other elements enables the formation of diverse energy levels instead of a continuous band gap. Hence diversifying their applications in various industries.6 Other advantages of CDs include being environmentally friendly, low toxicity, water soluble, high stability, strong absorption, biocompatibility and good thermo stabilization. In view of their flexibility and unique character, CDs are widely investigated in numerous application fields including biomedicine, biosensing, photovoltaic devices, catalysis, light-emitting diodes (LED) and development of total antioxidant capacity.7,8 The research of CDs has gradually matured and this new carbon material has been extended to a big family. Reviewing the physical structure and chemical properties of the CDs, the family members of CDs can be categorized into graphene quantum dots (GQDs), carbonized polymer dots (CPDs) and carbon nanodots (CNDs).9 Their unique molecular structures lead to distinct photoluminescence mechanisms such as confinement quantum effect, edge effect, molecular state, surface state and cross-link enhanced emission (CEE) effect. Generally, CDs can be produced using ‘‘topdown’’ and ‘‘bottom-up’’ approaches. The top-down approach is a technique used to breakdown the macro-sized carbon materials into nano-size. The methods used include arc discharge, electrochemistry, laser ablation, nanometer etching and chemical oxidation. On the other hand, the bottomup approaches are solvothermal, hydrothermal, electrochemical carbonization, microwave irradiation and thermal decomposition.10

6.1.1

Graphene Quantum Dots (GQDs)

Graphene is one of the most exciting materials discovered in the broad family of carbon materials. Although having wide application in a variety of areas, its zero band in nature has restricted its usage in nanodevices. Significant breakthrough took place when the fragmentation of 2D graphene

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sheets into 0D GQDs was discovered. The GQDs retain the graphene lattices and functional groups in the interlayer defect and edges.5 The high ratio of edge than the basal plane of GQDs contributes to the unique properties of GQD of having an edge effect. Besides, the quantum confinement effects of GQD are mainly contributed by the size and defect on the plane resulting in conjugated p-domains as the fluorescence centers.11 Hence, GQDs with lateral dimension smaller than 10 nm and less than ten graphene layers shows excellent quantum confinement and edge effects.10 These marvelous properties of GQDs have widened their usage in various applications. Distinct from CDs and CPDs, GQDs consist of one or several layers of graphene which can be easily identified with selected area electron diffraction (SAED) pattern and atomic force microscopy (AFM) analysis.12 The high-resolution transmission electron microscopy (HRTEM) analysis shows an average N doped GQD size of 3.8  0.5 nm. The lattice spacing of 0.24 nm which corresponds to the (1120) lattice fringe of graphene indicated the crystalline nature of hexagonal sp2 carbon sheets. This revealed that the N doped GQDs have a graphite-like structure. The SAED pattern showed a hexagonal structure and this was significant for few-layered GQDs, and this finding could be further proven via the AFM analysis. Theoretically, a single layer of graphene is 0.34 nm in height.13 Therefore, the height profile of 1.2  0.3 nm suggested a lamellar structure of GQDs between 1 to 3 layers. The higher the graph the more layers of graphene structure in the GQDs. For instance, an AFM image with a height profile between 1.1 nm and 2.0 nm indicated a three- to five-layered structure of the GQDs.14 The top-down methods used to produce GQDs in previous studies include electrochemical treatment of graphene oxide,15 one-step hydrothermal treatment of graphene oxide,16 acid treatment of carbon fibers,17,18 laser ablation19,20 and electron beam lithography.21,22 However, getting a controlled size of GQDs is a challenging task. An electrochemical method was employed to control the size and photoluminescence performance of GQDs by varying materials and process parameters such as electrode material, type of electrolyte, concentration of electrolyte, applied current with time, reaction time and reaction temperature. In addition, this method is rather economic and can be easily scaled up for bulk production.15,23 Therefore, it is one of the most widely studied GQD fabrication techniques among the top-down methods. Ahirwar et al.24 successfully synthesized GQDs of 2–3 nm by balancing the reaction between citric acid and alkali hydroxide in the electrochemical process. The molar ratio of citric acid and NaOH of 1 : 3 was used as the electrolyte while graphite rods were used as the anode and cathode. When current was applied, free OH ions  were generated in the electrolyte.23 There were attracted to the anode and oxidized on the defect sites of the defect-induced graphite rod. As a result, exfoliation of the graphite rod at the anode happened, followed by C–C cleavage and hence GQDs were produced. Later, Kalita et al.15 investigated the breakdown of graphene oxide using a three-electrode system with lithium perchlorate in propylene carbonate as

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the electrolyte to obtain GQD with uniform size of 3–5 nm and thickness of 2–10 nm under room temperature. Graphene oxide first underwent oxidative cleavage at different oxidation periods (2, 8, 12 and 16 h) with þ1.05 V as shown in Figure 6.1. The bonding between the sp2 carbon atoms was broken down and more defect sites were created. Subsequently, the highly oxidized graphene oxide underwent reduction by 1.05 V for 3 h to facilitate sizecontrolled GQDs. Although previous study by Shinde and Pillai25 reported that oxidation period was inversely proportional to the GQD size, there was no significant difference in size at varied oxidation time in this study, suggesting short oxidation time was adequate for the oxidative cleavage process. Besides, GQDs in the size range of 3–8.2 nm were produced at 90 1C while 23 nm sizes were obtained at room temperature by using multi-walled CNT as the starting material, indicating temperature is a size determining factor in the electrochemical method.25 On the other hand, the bottom-up approach makes use of the selfassembly of organic carbon precursors such as citric acid, oxalic acid, polyethylene glycol, glucose, tris(hydroxymethyl) and trisodium citrate. Hong et al.26 prepared blue photoluminescence GQDs using trisodium citrate as the carbon precursor. The precursor was first pyrolyzed at 200 1C for 4 min, followed by centrifugation and ultrafiltration to obtain GQDs with molecular weights of 3–10 kDa. Chai et al.27 used sugarcane bagasse as the carbon precursor to produce GQDs, sugar and porous carbon simultaneously using a hydrothermal-carbonization process. The GQDs with an average size of 2.26 nm and graphene structure were mainly derived from the lignin and polysaccharides contained in the bagasse. Among the carbon precursors, citric acid has been widely investigated using a hydrothermal method28,29 and pyrolysis method.30,31 In an in-depth study by Qu et al.29 citric acid was identified as a better candidate compared to glucose and tris(hydroxymethyl) as the GQDs produced from these precursors demonstrated relatively low photoluminescence quantum yields of 6% and 16%, respectively, after undergoing the same hydrothermal treatment. The great performance of GQDs from citric acid could be explained by the sixmembered ring structure formed by the aggregation of citric acid monomer during the reaction.28 Pyrolysis and hydrothermal methods are among the widely investigated bottom-up approaches for the synthesis of GQDs due to their simplicity, convenience and cost effectiveness. Other than the selection of effective carbon precursor, reaction time has a significant effect on the GQD photoluminescence intensity. This is similar to the fabrication of other CDs.32 Ogi et al.28 investigated the transient nature of the nanomaterial from citric acid and urea in each reaction stage. This study gave an insight to the changes of molecular structure of GQDs at different reaction times. The color of the solution changed from transparent to pale orange over 90 min, and further became deep orange and finally turned black after 600 min. The photoluminescence intensity of the GQDs varied with the reaction time and the GQDs at 90 min showed the highest photoluminescence quantum yield of

Formation of GQDs from graphene oxide by an electrochemical approach. Adapted from ref. 15 with permission from Elsevier, Copyright 2019.

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Figure 6.1

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32.6%. Nuclear magnetic resonance (NMR) analysis revealed that initially citric acid amide was formed as illustrated in Figure 6.2, subsequently further self-assembling into nanosheet structures upon dehydration and deammoniation between the functional groups of the intermolecular compounds. Finally, GQDs as the condensation product of citric acid amide with high luminescence were formed at the end of the hydrothermal process. Though, as time progressed decomposition of urea happened due to high reaction temperature and hence reduced the photoluminescence intensity. Furthermore, the ratio of starting materials is a crucial factor in determining the crystallinity, functional group distribution and optical properties of the GQDs. In a study by Gu et al.33 thermal pyrolysis of citric acid and urea in a 3 : 1 ratio showed the highest production yield and quantum yield of 43.wt% and 22.2%, respectively.

Figure 6.2

Formation of GQDs from citric acid and urea in a hydrothermal process. Adapted from ref. 28 with permission from the Royal Society of Chemistry.

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6.1.2

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Carbon Nanodots (CNDs) and Carbon Quantum Dots (CQDs)

CNDs are quasi-spherical nanoparticles having a particle diameter less than 10 nm. They exist either in an amorphous or crystalline structure depending on their carbon clusters which are sp3 or sp2, respectively.34 These structures mainly originate from the synthesis method. The CNDs with amorphous structure are normally carbonized in high temperature and undergo surface passivation to stabilize the optical properties. Their PL originates from defect sites and the emission bandwidth is normally wider than the crystalline CNDs. On the other hand, CNDs with crystalline structure are also named as carbon quantum dots (CQDs). CQDs have high crystallinity quality structures and their particle size has a significant influence on the quantum confinement effect. These are their main differences from the amorphous CNDs.35 The top-down approaches used to produce NCDs are electrochemical, laser ablation and high energy ball milling. Hu et al.36 combined laser ablation and surface passivation by immersing graphite flakes into a poly(ethylene glycol) solution. This one-step reaction produced CQDs with sizes of 3.2, 8.1, and 13.4 nm after 4 h laser irradiation by tuning the laser pulse widths to 0.3, 0.9, and 1.5 ms, respectively. The highest fluorescence quantum yield was 12.2% from the smallest CQDs. Wang et al.37 performed ball milling at 500 rpm for 50 h in a suspension of activated carbon and potassium hydroxide. The high energy ball milling produced CQDs with unique dual-wavelength photoluminescence and electrochemiluminescence properties. Later, Devi et al.23 employed a facile electrochemical exfoliation method to produce CQDs at various reaction times and obtained the CQDs with an average size of 7 nm. However, the above methods have rarely been studied and reported for the synthesis of CNDs (referring to both amorphous and crystalline CNDs) in recent years. Based on the literature, we noticed that electrochemical and laser ablation methods are more commonly used to produce GQDs. On the other hand, hydrothermal, solvothermal, chemical vapor deposition and microwave-assisted methods are among the bottom-up approaches that are commonly used to produce CNDs.38–41 Molecular state is the photoluminescence center in CNDs formed by the organic fluorophore. It typically exists in bottom-up preparation methods where small molecules are carbonized during the synthesis process. The CNDs with photoluminescence related to the molecular state normally demonstrate high quantum yield.42 The relation of molecular state and photoluminescence was investigated by Pan’s group43 by using methyl red (MR) and 2-(4-dimethylaminophenylazo) benzoic acid as the precursor via a hydrothermal pyrolysis method. The CNDs synthesized with different temperatures (140, 160, 180 and 200 1C) revealed molecular state-controlled emission as the quantum yield decreased with increasing hydrothermal temperature. The absorption peaks of MR at 300, 335 and 538 nm in the UV-visible absorption spectra weaken gradually with increasing temperature.

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This could be explained as the photoluminescence of the CNDs originated from the fluorophore of the MR, and they were consumed at higher temperature, leading to decrement of quantum efficiency. Surface passivation and doping are normally introduced to enhance the fluorescence of CNDs. A surface state emissive trap will dominate the emission when the CNDs are excited by light and this is also called surface state-controlled photoluminescence. A higher degree of modification results in more defects and hence improves the fluorescence. This modification is also common for the synthesis of GQDs.42 An interesting study was done by Gyulai et al. to prepare CQDs with different fluorescence properties using a few preparation approaches with microwave and solvothermal methods.39 The use of microwaves is simple, fast and can be completed within a few minutes.44 The CQDs formed using water as the medium in the microwave heating method (CQD2) were 1.7 nm, whilst average sizes of 1.2 nm and 0.9 nm were formed in dimethyl sulfoxide (DMSO) using reflux (CQD3) and microwave methods (CQD4), respectively. The higher boiling point of DMSO (180 1C) favors carbonization leading to compact particles. Furthermore, the carboxylic functional group was reduced under high temperature, resulting in less negative surface charge. On the other hand, the highly negatively charged CQD2 contained the lowest carbon and highest oxygen amount, due to the presence of carboxylic- and amide-type oxygen. CQD4 had the largest carbon content, attributed to the presence of an enlarged carbon core. Hence, the CQDs hydrophilicity decreased with CQD2, CQD3 and CQD4. Furthermore, the CQDs formed in DMSO appeared to be less polar than those using water as the medium. This study demonstrated the relation of the CQD surface chemistry with their chemical properties. In addition, the functional groups at the shell part of the CQDs showed significant influence towards their luminescence quantum yield. The doping of heteroatoms such as N, S and P into the carbon cluster can be performed by the addition of urea, sulfuric acid, ethylenediamine and phosphoric acid during the synthesis process. Figure 6.3 shows the formation of bare CQDs and its corresponding P, S and N structures in NaOH solution.45 The carbon precursor undergoes inter-molecular dehydration, carbonization, and condensation reaction to form CQDs. At the end of the reaction, CQDs of aromatic structure with a conjugated system (sp2 carbon state with C–C and CQC bonds) are formed, hydroxyl and carboxyl are attached at the edge and numerous sp3 carbon (C–C–OH/–C–O) areas are available for surface passivation. Chemical groups such as –NH2, –SO2, –HSO3, and –H2PO4 are connected to the carbon bonds with –C–N and –NH2 attach at the edges. P and S atoms are likely attached at the sp2 carbon conjugated structure resulting in edge defect as shown in Figure 6.3. The introduction of ammonia, sulfuric acid, phosphoric and NaOH reduced the amount of –COOH at sp2-conjugated p-domains, leading to non-radiative recombination centers. This study reported that CQDs doped with S demonstrated the highest quantum yield, followed by N and P doping with 29.7%, 18.7% and 10.3%, respectively. These values were higher than the

Carbon Dot-based Composites: Recent Progress, Challenges and Future Outlook

Figure 6.3

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Synthesis of bare CQDs, P-CQDs, S-CQDs and N-CQDs. Adapted from ref. 45 https://doi.org/10.1186/s11671-019-3079-7 under the terms of the CC BY 4.0 license http://creativecommons.org/licenses/by/4.0/.

bare CQDs of 9% without functional group modification. Another study by Cheng et al.46 also proved that the quantum yield and generated yield of CNDs improved from 0.7% and 0.9% to 13.3% and 14.3%, respectively after N and S doping. Natural resources and waste biomass materials such as juice,47 eggs,48 food waste49 and tea leaves32 have been widely used as carbon precursors for the synthesis of CNDs. Cheng et al.46 fabricated CNDs from cellulose-based biowaste of willow catkin using a simple one-step combustion method. The willow catkin was burned at high temperature to form ashes prior to the dialysis process to obtain CNDs. To reduce energy consumption, many studies have been focused on a hydrothermal carbonization method and hence it has been known as a facile and green method. Besides, different biomass as the carbon source demonstrated diverse fluorescence intensity. Zhu et al.32 compared the properties of CQDs synthesized from tea leaves and peanut husk using a hydrothermal method. Although having a similar spherical shape and size of 7 to 9 nm, the CQDs from tea leaves showed greater functional groups of –COOH and –OH in the FTIR spectrum as well as higher fluorescence intensity. However, the biomass-based CNDs

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normally show low quantum yield with unclear morphology and a crystalline structure.

6.1.3

Carbonized Polymer Dots (CPDs)

A new category of CDs has recently been identified and named as PDs or carbonized polymer dots (CPDs). This nanoparticle possesses both polymer and carbon hybrid structures with a carbon core at the center and a highly crosslinked polymer structure at the surface as shown in Figure 6.4. The sources of precursor can be from small molecule, polymer or monomer containing amino, carboxyl or hydroxyl groups. The carbon core could be completely carbonized, partially carbonized or cross-linked. The stability of CPDs was said to be better than polymer owing to the carbonization. Furthermore, their compatibility is better than CNDs due to the polymer chains.6 Fluorescence CPDs have been reported on both p-conjugated and non-conjugated polymer backbones, depending on the types of precursors, resulting in the wide characteristics of CPDs.50 The synthesis of CPDs was mainly produced by a bottom-up approach instead of top-down method which is more complicated. The bottom-up approach used to synthesize CPDs includes hydrothermal, solvothermal or microwave methods.5 The topdown approach typically produces CNDs or GQDs with graphite structure and no obvious polymer chains. They are further functionalized with polymer or organic molecules to transform into CPDs. Carbonized polymer dots may be distinct from the other CDs due to their unique properties and fluorescent origin. They revealed strong emission which is uncommon for molecular structures with typical conjugated fluorophores and subfluorophores. Perhaps the enhanced fluorescence comes from the highly cross-linked polymer structure of CPDs and this unusual emission is known as the cross-link enhanced emission (CEE) effect.6 The addition of non-conjugated polymer during the synthesis process decreases the vibration and rotation in the crosslinked CPDs, amplifies the

Figure 6.4

Synthesis and structure of PDs. Adapted from ref. 6 with permission from American Chemical Society, Copyright 2019.

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CEE effect and hence enhances the fluorescence intensity. Besides, the fluorescence behavior of the crosslinked CPDs is also influenced by pH and temperature as these parameters would affect the rigidity of the structure.42 Vallan et al.50 produced highly fluorescent non-conjugated CPDs using an in situ functionalization method at room temperature by controlling the polycondensation of citric acid and ethylenediamine using different coupling agents. The activation of carboxylic acid with N,N-diisopropylcarbodiimide (DIC) and N-(3-Dimethylamino-propyl)-N 0 -ethylcarbodiimide hydrochloride (EDC) showed a higher quantum yield (QY) of 17.7% and 8.6%, respectively owing to the stable and rigid conformation during the polymerization process. On the other hand, hexafluorophosphate azabenzotriazole tetramethyl uronium (HATU) and thionyl chloride (SOCl2) showed an extremely low QY of 2.2% and 4.0%, respectively revealing an inefficient polymerization process. The intramolecular hydrogen bonding and electrostatic interactions were explained as the main factors contributing to the high conformational rigidity of the CPDs. The addition of functionalization agents during the polymerization process further improved the optical property of the CPDs. The addition of rigid and bulky groups of amines showed low fluorescence emission while thiol functionalization showed higher fluorescence results. This study demonstrated a highly versatile and simple method for the synthesis of fluorescence CPDs at room temperature using various coupling agents and functionalization agents. An improved version of the hydrothermal synthesis method is proposed by Xia et al.51 for the first time by incorporating additional polymerization and carbonization in the synthesis process (HAPC). The main advantage of using the HAPC method over the hydrothermal, condensation crosslinking and carbonization method (HCCC) is that it can achieve high yield production as the reverse reaction of hydrolysis does not happen and most of the monomers would turn into CPDs. Monomer and different initiators first undergo hydrothermal treatment to form a polymer cluster. At the same time dehydration and crosslinking took place, forming a hydrophobic environment at the carbon core and a hydrophilic part outside. Subsequently, the polymer cluster was carbonized to form CPDs. The size of CPDs formed with 2,2 0 -azoisobutyronitrile (AIBN) and potassium peroxydisulfate (KPS) was 4.56 nm and 2.81 nm, respectively and the size of CPDs decreased with higher dosage of KPS. Besides, the degree of carbonization could be verified via HRTEM and X-ray photoelectron spectroscopy (XPS) analysis. Carbonized polymer dots with a high carbonization degree showed an apparent lattice structure with smaller size while the XPS analysis showed higher content of C–C/CQC which was attributed to graphite carbon. The graphite carbon acted as a physical crosslinking point, enhancing the fluorescence emission rate. The study demonstrated that charge-control-size of CPDs and fluorescence properties could be achieved with proper selections of initiator and dosage. Carbonized polymer dots also can be produced by breaking down plastic film around its melting point using a simple heating process. Using plastics as the starting material for the top-down method can reduce the step to functionalize the CNDs. Plastics such as polystyrene (PS), polyethylene (PE),

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polypropylene (PP) and polyvinyl chloride (PVC) consist of 38.4%–92.3% of long-chain carbon and hence can be used as the precursor.8 The long-chain carbons undergo polymerization, structural changes and bind with atoms such as oxygen, nitrogen, hydrogen to form new functional groups. Aji et al.52 reported that the higher the heating temperature, the greater the number of carbon chains broken down hence the smaller size of CPDs formed. The study showed that CPDs with average size of 15 nm, 11 nm and 8 nm were observed in TEM after heating PP plastic waste with a temperature at 200 1C, 250 1C and 300 1C, respectively. The CQO bond observed in fourier-transform infrared spectroscopy (FTIR) spectra revealed the attachment of oxygen on the carbon, affecting the photoluminescence emission intensity. It was interesting to note that two main emission peaks were found in the photoluminescence emission spectra with 410 nm (3.03 eV) and 440 nm (2.83 eV). This rare phenomenon is known as stimulus emission where recombination happened in two electron emissions after absorbing UV energy.

6.2 Recent Progress in CD-based Composites This section highlights the recent progress of CD-based composites in the synthesis, properties and applications in the field of disease detection, optoelectronics, photovoltaics, photocatalysis, etc.

6.2.1

CQD-based Composites

According to Xia et al. (2019),5 CQDs are spherical and exhibit apparent crystal lattices and chemical groups on the surface. The tuning of CQD size can control the wavelength of photoluminescence due to its quantum confinement effect and intrinsic state luminescence.5,53–57 Recently, many efforts have been devoted to combine CQDs with a wide range of photocatalysts such as Bi2O3,58,59 g-C3N4,60 TiO2,61,62 ZnFe2O4,63 ZnO,64,65 Fe2O3,66 Co3O467 and BiOX (XQCl, Br and I).68–70 The composites of CQDs were widely applied for photocatalytic application such as the removal of anti-inflammatory drug,71 dye pollutants58,59,72 and H2 evolution.62 Some researchers reported the usage of CQD-based composites as electrode materials for alkaline batteries66 and as a detection probe for anticancer and antibiotic drugs.67 CQD-based composites can be prepared via a one-step or multi-step synthesis approach. In a one-step approach, all precursors for the preparation of CQDs, photocatalyst and doping agent are mixed together in one pot. Yun et al. reported a one-step hydrothermal method to prepare N-CQDs/rGO/ Fe2O3 composite using Ketjen black as the raw materials, and a high yield of more than 40% was achieved. The obtained composite was used as an electrode for alkaline aqueous batteries. It exhibited ultrahigh specific capacity, admirable rate property, and superior cycling performance. This is owing to the 3D network structure of the composite aerogel with large specific surface area and hierarchical porous structure, excellent electronic conductivity of CQDs, high-capacity of Fe2O3 and highly conductive rGO.66

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A multi-step approach was widely reported by many researchers in which the pre-made CQDs are mixed together with the photocatalyst or precursor of photocatalyst. Zhang and co-workers used this method to prepare CQDs/ TiO2/Fe2O3 through a multi-step hydrothermal method. The pre-made TiO2 nanoparticles and CQD solution were mixed with the precursor of Fe2O3, then placed into a Teflon lined autoclave for hydrothermal treatment. The existence of CQDs could enhance visible light absorption and boosted the photodegradation rate of methylene blue (MB) dye up to 86.5% in 180 min.73 Very recently, CQDs were combined with Bi2O3 to obtain CQDs/a-Bi2O358 and CQDs/Bi2O3x.59 In Sharma’s work, CQDs/a-Bi2O3 was prepared using a template-free sonochemical method. The narrowed band gap of CQDs/a-Bi2O3 (2.49 eV) led to much-improved photodegradation of indigo carmine (IC) dye (86%) and levofloxacin (79%) than that of pure a-Bi2O3 (57%)58. In Xian’s work, CQDs/Bi2O3x composites were prepared in three steps. First, Bi2O3 and CQDs were prepared through a polyacrylamide gel route and hydrothermal route, respectively. It was followed by the reduction by NaBH4 to produce Bi2O3x with surface oxygen vacancies and then mixed Bi2O3x with CQDs solution to produce the final product. The surface oxygen-vacancy states in Bi2O3x could narrow the band gap of Bi2O3 from 2.89 eV to 2.83 eV and trapped the electrons to prolong the lifetime of charge carriers. The CQDs/ Bi2O3x composite with optimized concentration of NaBH4 (3 mmol L1) and volume of CQDs solution (15 mL) achieved the highest degradation efficiency of Acid orange 7 (AO7) (B97%) after 60 min photoreaction.59 Recently, many efforts have been devoted to combine CQDs and graphitic carbon nitride (g-C3N4) to generate CQDs/g-C3N4 composites. Graphitic carbon nitride can be combined with CQDs since it has a strong affinity towards the attachment of CQDs and prevent the CQDs from dissolving in solutions. The presence of various types of functional group in g-C3N4 provides large surface support for CQDs.74 CQDs with a large cross-section of up-conversion absorption act as spectral converters which can be ascribed to the high separation rate between the CQDs and g-C3N4. Hence, the contradiction between chemical reaction dynamics and optical absorption can be solved by a varied range of emission spectra.75 The Time-Resolved Photoluminescence (TRPL) analysis revealed that electron transport was decelerated by the excessive loading of CQDs.60 This is because when the CQD loading was in excess, the trap state was created at the interface between CQD and g-C3N4. An optimized amount of CQDs could accelerate the electron transfer due to its unique electron transportability that can act either as electron donor or electron acceptor molecules in solution.76 The water splitting is a promising technology for large scale hydrogen production with low production cost. Qu et al. (2018)77 combined carbon dots with g-C3N4 through one-step homogeneous thermal pyrolysis by melting citric acid (CA) and urea powders together under controlled temperature. It is found that the CQDs increased the charge separation efficiency of about 96% with the photoelectron transfer rate of 3.07109 s1, which significantly enhanced the hydrogen production performance. According to Wang et al. (2017),78 the

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composite with 10 wt% loading of CQDs showed the 2.2 mmol/h of H2 production rate that is 4.4 fold higher than that of g-C3N4 (0.5 mmol h1). The tubular CQDs with g-C3N4 heterostructures (CCTs) were produced by Wang’s group through thermal polymerization of freeze-dried urea and CQDs precursor.79 The CCTs exhibited the highest H2 production rate of 3538.3 mmol (g h)1 with 10.94% of quantum yield while CQDs/g-C3N4 could only reach a production rate of 2002.4 mmol (g h)1. In Li’s work, they synthesized carbon nitride nanosheets (CNNS) and CQDs composites with 17.5% content of CQDs using a hydrothermal method. The hydrogen evolution is 3 times higher than that of pure CNNS.80 Apart from g-C3N4, CQDs were also combined with a photoanode material such as bismuth vanadate (BiVO4) because CQDs could act as a superfast charge tunnel for optoelectronic devices.81 Ye et al. (2017)82 designed an integrated CQDs/BiVO4 photoanode by embedding an ultrathin interlayer of isolated CQDs between the BiVO4 core and the two OEC shells (NiOOH/ FeOOH). Oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) occurred at the NiOOH/FeOOH/CQD/BiVO4 (NFCB) photoanode and Pt cathode, respectively. The electron and hole pairs generated from both of BiVO4 and CQDs were directed to the respective photoanode and cathode for photoelectrochemical (PEC) water splitting. The NFCB photoanode recorded excellent photocurrent density of 5.99 mA cm2 at 1.23 V vs. RHE under AM 1.5G in KH2PO4 aqueous solution without a hole scavenger (pH ¼ 7). By combining CQDs with alkaline niobates (KNbO3) at a mass ratio of 1.5 : 0.5, Qu’s group found that the degradation rate of crystal violet dye and hydrogen evolution amount was 70% and 468.72 mmol (h g)1, respectively. The performance of CQDs/KNbO3 was much better than pure KNbO3 (41.5% and 245.52 mmol (h g)1) because CQDs trapped more electrons and converted the absorbed visible light to ultraviolet light.83 The CQD-based photocatalysts were also widely applied for the degradation of organic pollutants, photocatalytic conversion of CO2 with water, photocatalytic disinfection, etc. For example, Ke et al. (2017)84 has studied the performance of N-doped CQDs/TiO2 in methylene blue (MB) degradation. It was reported that the degradation efficiency was 90% within 120 min, which was 3.6 times higher than pristine TiO2. Moreover, Yu and Kwak (2012)85 also conducted research regarding the photocatalytic degradation of MB by using the CQD-incorporated mesoporous hematite composites (CDs/MH). The photodegradation efficiency of MB was reported to be 97% within 90 min. It was reported that the surface area increased to 187 m2 g1 and the charge recombination rate reduced. Besides, the photocatalytic activity of CDs/Fe2O3 composites against benzene gas was discussed by Zhang et al. (2011).86 In their research, the degradation efficiency of benzene gas was found to be improved from 37% (pure Fe2O3) to 80% (CDs/Fe2O3) due to the excellent UCPL of the CDs which promotes formation of photogenerated electron-holes pairs. Besides, it was reported that the conjugate network structure enhanced the adsorption properties due to the p-p interaction between the CDs and benzene molecules. Li and co-workers (2019)87 developed a carbon layer (CL) to cover

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and protect the composite of CQDs/Cu2O to improve the photocatalytic performance for CO2 reduction. The formation of CL@CQDs/Cu2O involved three steps: The Cu2O particles were firstly produced from glucose, NaOH and Cu2SO4 through ultrasonic treatment. At the same time, CQDs were formed from the excessive glucose and NaOH solution. Next, a carbon layer formed on the surface of CQDs and Cu2O after a hydrothermal method. The prepared sample exhibited a high yield of methanol (249 mmol g1) after 2.5 hours reaction because CL was able to enhance light absorption, to facilitate electron transfer and to protect Cu2O from photocorrosion.87 Morphology control is an alternative method to maximize photon utilization efficiency and photoactivity. According to Zhang et al. (2019),88 perovskite material ZnSn(OH)6 (ZSH) with a yolk–shell structure was expected to display multi-reflection of light property, organic adsorption and porous structure.89,90 They embedded CQDs into ZSH with a yolk–shell structure through a hydrothermal method (Figure 6.5) to fabricate ZSH@CQDs, demonstrating 70.4% of RhB degradation efficiency in 8 h and 100% disinfection of Staphylococcus aureus in 16 h. Another group incorporated CQDs into the host matrix of oxidized porous silicon (PSiO2) for the fluorescence sensing of trypsin and adenosine triphosphate (ATP). The resulting nanostructured hybrid improved the interaction between the CQDs and adsorbed analytes and thus showed higher and real-time sensitivity towards the detection of small molecules.91 Prasath’s group92 prepared nanostructured bismuth oxide (Bi2O3) and milk-derived CQDs composite and used it as an anode material in lithium-ion batteries. They claimed that CQDs increased the conductivity of the oxide matrix and reduced the interfacial resistance, leading to a superior discharge capacity of 1500 mA h g1.

Figure 6.5

Controlled fabrication process for three morphologies of ZSH@CQD composites. Adapted from ref. 88 with permission from Elsevier, Copyright 2019.

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GQD-based Composites

According to Zhang et al. (2020), GQDs possess higher crystallinity than CQDs which helps to promote superior electron mobility and prolong the lifetime of charge carriers. There is a lack of studies on the photocatalytic application of GQDs compared to solar cells, biosensing and bioimaging. In Zhang’s work, they mixed polyvinylidene fluoride (PVDF) and polyvinyl pyrrolidone (PVP) with tetrabutyl titanate (TBT) to form a nanofiber film, namely PVDF(TBT)/ PVP(TBT). This film was then deposited with GQDs via hydrothermal reaction using glucose. The obtained [PVDF(TBT)/PVP(TBT)-GQDs] film was porous with large specific surface area (16.037 m2 g1). It could degrade 97.16% Rhodamine B within 210 min under visible light irradiation.93 Another Zhang’s group incorporated GQDs on the hollow TiO2 nanosphere (H-TiO2) which had a large specific surface area (223.48 m2 g1) and highly exposed active sites.94 The prepared GQDs (B5 nm) were rich with oxygencontaining functional groups and abundant defect edges which enhanced the visible light absorption of H-TiO2 and prolonged the lifetime of electron–hole pairs. As a result, the GQDs/H-TiO2 composite was able to degrade 96.9% of RhB compared to that of pure H-TiO2 (33.3%) in 180 min. Kumar et al. (2018)95 decorated ZnO nanorods with GQDs and found that 2 wt% of GQDs showed the highest degradation efficiency (B95%) for MB and carbendazim (CZ) within 70 min under sunlight irradiation. Such good results were attributed to the effective charge separation, wide solar spectrum harvesting and high specific surface area (353.447 m2 g1). The tertiary composite of GQDs/ Mn-N-TiO2/g-C3N4 (GQDs/TCN) was firstly discovered by Nie et al. (2018)96 for photocatalytic degradation of p-nitrophenol (4-NP), diethyl phthalate (DEP) and ciprofloxacin (CIP) coupled with simultaneous H2 production. The optimal loading amount of g-C3N4 achieved 89% removal rate of 4-NP and H2 evolution rate of 0.87 mmol L1 g1 after a 2 h reaction. Recently, GQDs were also used as a conductive filling material and structuraldirecting agent to fine-tune the morphology of MnCo2O4.5 composites into nanosphere, nanoneedle or nanoarray for high-performance supercapacitors.97 They found that 40 mg of GQDs induced the growth of nanoneedle composites and increased the capacitance to 1625 F g1 at 1 A g1, which was four times higher than that of the pure MnCo2O4.5 nanosphere electrode (368 F g1 at 1 A g1). The homogeneous distribution of GQDs created a conductive network to improve charge transfer for the best electrochemical performance. Tam’s group98 successfully fabricated the GH-BGQD composite by anchoring borondoped GQDs onto graphene hydrogel through one-step hydrothermal treatment of glucose, boric acid and graphene oxide. The obtained electrocatalyst possessed a three-dimensional (3D) structure, high porosity and high specific surface area which provided more active sites for better electrolyte mass transport and ion diffusion. A current density of 10 mA cm2 at a cell voltage of 1.61 V was recorded in a water electrolysis cell using GH-BGQD electrodes. The GH-BGQD electrodes achieved excellent long-term durability and stability compared to that of commercial Pt/C and Ir/C catalysts. The water electrolysis cell was

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incorporated with Zn–air batteries using GH-BGQD2 electrodes. The generation of O2 and H2 bubbles on both GH-BGQD2 electrodes proved the successful operation of water electrolysis driven by Zn–air batteries.98 In Luo’s work, GQDs were prepared through chemical oxidation of carbon fibers and then incorporated into three-dimensional graphene (3DG) through one-step hydrothermal treatment.99 It was then applied as an electrode material to improve the performance of supercapacitors. When the GQDs to GO feeding ratio was optimized at 40%, the specific capacitance of 242 F g1 was more than 22% that of pure 3DG due to the higher specific surface area and electrical conductivity. The formation of oxygenated groups during one-step hydrothermal treatment promoted strong interaction between GQDs and 3DG. This resulted in a superior cycling stability by maintaining the capacitive retention rate at 93% after 10 000 circles. Graphene quantum dot-based composites were also applied in biomedical sensors. Cui et al. (2019)100 developed a ‘‘turn-on’’ magnetic fluorescent biosensor, called the aptamer@Fe3O4@GQD@MoS2-based nanosurface energy transfer (NSET) assay for rapid detection of circulating tumor cells (CTCs). First, electrochemically prepared GQDs were modified with magnetic agent and aptamer [epithelial cell adhesion molecule (EpCAM) receptors] to form an aptamer@Fe3O4@GQD complex which could act as a donor. This complex was then assembled on molybdenum disulfide (MoS2) nanosheets which act as fluorescence quencher. The developed NSET was able to identify and detect tumor cells (both low- and high-EpCAM-expressing cells) within 15 min due to the presence of aptamer. This system could also detect up to ten tumor cells in human blood with a linear detection range 2–64 nM, and the limit of detection (LOD) was 1.19 nM. The presence of magnetic Fe3O4 resulted in an average of 90% capture efficiency. The sensing mechanism of this system is shown in Figure 6.6.

6.2.3

CND and CPD-based Composites

According to Xia et al. (2019),5 CNDs show the characteristics of high carbonization degree, insignificant crystal lattice structure and polymer features and existence of some chemical groups on the surface. Unlike CQDs, the photoluminescence of CNDs originates from the defect or surface state and subdomain state within the graphitic carbon core instead of the quantum confinement effect of the particle size.101–103 In 2018, Majumdar et al. demonstrated magnetically recoverable iron oxide-carbon nanodots (Fe3O4-CNDs) for one-pot synthesis of quinazolinones with alcohols and 2-aminobenzamindes in the presence of tert-butyl hydroperoxide (TBHP). Polyethylene glycol-200 (PEG-200) was used as the starting material to prepare CNDs through microwave irradiation. The peroxidase activity of CNDs might influence the decomposition of TBHP into radicals which could stabilize the surface of Fe3O4. These radicals had a longer lifetime to catalyze the tandem reaction resulting in the formation of the quinazolinone derivatives in high yields.104

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Sensing mechanism of aptamer@Fe3O4@GQD@MoS2-based nanosurface energy transfer (NSET) assay for rapid detection of circulating tumor cells (CTCs). Adapted from ref. 100 with permission from Springer Nature, Copyright 2019.

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Recently, Meng and co-workers constructed a highly electrical and thermal conductive 3D chiral-structured graphene-based framework (GO-CNC) which exhibited the steady temperature of 315 1C and maximum heating rate of 44.9 1C s1 at an input voltage of 10 V. As shown in Figure 6.7, cellulose nanocrystalline (CNC) aligned as chiral spirals to connect the graphene oxide (GO) layers while CNDs were incorporated inside the composite as conductive nanofillers. Carbonized CNC nanorod (CNR) linked graphene nanosheets (rGO) and formed a rGO-CNR skeleton to promote effective electron and phonon transport in the direction of in-plane and through-plane. As a result, the prepared film (rGO-CNR-CDs) achieved outstanding thermal and electrical conductivity of 1978.6 W m1 K1 and of 2053.4 S cm1, respectively.105 Juang et al. (2018)106 employed bamboo-derived CNDs as cathode interfacial layers to improve the performance of organic photovoltaics (OPVs). The embedded CNDs could smooth the surface, alter surface energy and reduce the work function (WF) of the ZnO layer. The changes of morphology and energy level alignment at the interface led to efficient electron transfer and extraction, achieving power conversion efficiencies (PCEs) of 9.6% with a fill factor of 72.8%. Aziz et al. (2019)107 synthesized direct band gap polymethyl methacrylate (PMMA) and CND nanocomposite films using a solution cast technique for applications in optoelectronic devices. The addition

Figure 6.7

Schematic assembly of the hierarchical graphene-based composite film intercalated by CNDs. Adapted from ref. 105 with permission from Springer Nature, Copyright 2019.

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of CNDs modified the electronic structure of pure PMMA film and further reduced the band gap energy of the film. A prominent broad-band around 480 nm was observed in the photoluminescence (PL) spectra, indicating that the PMMA/CNDs nanocomposite film had a well-defined excitonic emission feature. Such good results in PL and UV–vis absorption characteristics showed that this nanocomposite film was suitable for use in future optoelectronic devices. As mentioned by Xia et al. (2019),5 the CPDs exhibit a polymer and carbon hybrid structure consisting of abundant functional group/polymer chains on the surface and a carbon core. The lack of study in the influence of carbonation degree to chemical structure and PL mechanism has motivated Xia’s group to fabricate CPDs and polyvinyl alcohol (PVA) nanocomposites.51 The CPDs were obtained using hydrothermal addition polymerization and carbonization (HAPC) method in which the initiator (2,2 0 -azoisobutyronitrile and potassium peroxydisulfate), monomer (acrylamide) and crosslinking agent (N,N 0 -methylenediacrylamide) were three main parts of addition polymerization. The obtained CPDs possessed a high yield rate (ca. 85%) and high photoluminescence quantum yields (PLQY) (ca. 45.58%). The CPDs solution was mixed with PVA water to obtain CPDs/PVA nanocomposites which promoted dual-mode emission of fluorescence and phosphorescence for advanced anti-counterfeiting.

6.3 Challenges One of the key challenges facing researchers to realize the application of CDs (GQDs, CPDs and CNDs) on an industry scale is to obtain high product yield and high quality of CDs with low production cost. The physicochemical property of CDs is influenced by quantum confinement effect and quantum size effect which could be tuned by particle size,108 presence of edge states45 and intrinsic molecular structure using different fabrication methods. For example, GQDs were produced on a large scale from waste biomass and rice husk biomass using pyrolysis and hydrothermal treatment, respectively.109,110 Wang’s group110 obtained high-quality GQDs with a yield of B15 wt% and quantum yield of B8.1%. In Jing’s work,111 they reported a facile, large scale and low-cost method to synthesize CQDs from biomassderived hydrochar through mild oxidation of NaOH/H2O2 solution. An ultrahigh CQD production yield of 76.9 wt % and high quantum yield of 22.67% was obtained. Tang et al. (2019)45 found that the presence of functional groups such as –NH2, –SO2, –HSO3, and –H2PO4 CQDs at the edge of CQDs could improve a fluorescent quantum yield up to 29.7%. The origin of the photoluminescence properties of CDs is not fully understood although some basis has been proposed such as the size distribution of CDs, structure of nanoparticles and different distribution of emissive trap sites. Some researchers disagreed with the presence of UCPL property in CDs by claiming that the regularly cited UCPL properties might originate from the standard fluorescence excited by the leaking component

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in the monochromater of the fluorescence spectrophotometer or secondorder diffraction light of wavelength l/2.113 There can be mistakes in interpreting the enhanced photocatalytic performance of CD-modified photocatalysts which could be due to the red shift of light absorption edge but not the UCPL. A series of spectral analyses have been carried out by Gan’s group to answer the question of whether there is real UCPL in GQDs. Their work revealed that no real UCPL was detected in GQDs under excitation of a xenon lamp. They agreed with Tan’s comments that the detection of UCPL in GQDs was due to the second-order diffraction light of wavelength l/2 and the real UCPL could only be observed under excitation of a pulsed laser.114

6.4 Conclusion and Future Perspectives Carbon dots will be used as worthy nanomaterials due to its unique nature and various properties. The potential of coupling with other materials to form composites will further enhance its quantum confinement effects that are will lead to enhanced performance. This is mainly attributed to the enhancement in the light absorption especially in the visible and infrared region. Moreover the quantum confinement generates multiple excitons and boosts the excited charges separation. Therefore, CD-based composites will have an optimistic potential growth in addressing various severe environmental remediation including the new generation of water splitting and energy conversion. To realize the application of CD-based composites in practical production, a lot of opportunities can be explored in the near future. In-depth research studies on the fundamental characteristics of CD-based composite is an interesting area to be explored when it comes to the identification of the potential growth of CDs with various synthesis routes. Moreover, the influence of the size, shape and growing spots of CDs will further enhance the quantum effect and achieving a higher quantum yield. Improving the visible light absorption properties of CDs will also posses a higher visible–NIR light activity in wide bandgap semiconductors such as TiO2, ZnO and Fe2O3. Therefore, developing new synthesis methods could result in a more promising potential growth of CD-based composites. The opportunities lie in extending the application in the field of more complex organic pollutant degradation apart from the simpler organic dye pollutants. The CD-based composites can be applied to the degradation of other organic pollutants such as pesticides, antibiotics, endocrine disturbing chemical which is available in large quantities in the environment. Research on the environmental acceptance ability of CD-based composites should be explored to understand its adaptability to the environment to shift from laboratory research to potential practical application. It is understood that the catalytic activity is much dependent not only on its properties but also the actual environmental conditions such as the pH, contaminant, fluctuation of temperature and others. Until today, there is no relevant research on such topic and it would be a novel breakthrough if the CD-based

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composites can be encapsulated to overcome the actual environmental condition. Besides, expanding the application to heavy metal removal should be a future focus in using such composites. This is indeed another novel area to be explored as the presence of heavy metal in our environment is alarming. Exploring this opportunity will lead to other research on the recovery and environmental risk assessment because the removal of heavy metal could lead to the secondary pollutant. Hence, CD-based composites will be the ideal selection due to its non-toxicity characteristics. Similarly, the properties of CDs could address the shortcoming in the field of energy generation such as low energy density, slow reaction kinetics and access of scarcity. Therefore this opens up an alternative solution with the incorporation with CD-based nanomaterials to then improve the quantum yield with the extraordinary optical properties that will be useful in energy storage and conversion systems. Meanwhile, in the field of optoelectronic applications, CDs can fully utilise its potential of high surfaces area, the longer life span of the electron and hole pairs, strong fluorescence and tunable bandgap potential will show high prospects in developing optoelectronic applications such as solar cells, photodetectors, light-emitting diodes, etc. Furthermore, analyzing the characteristic of the redox reaction of the CD-based composites could enhance the relationship between the redox reaction with the reaction kinetics energy. This will enable one to explore further the contribution of different energy bands and the mobility of the electron in the composite. Although many reports have been published on the oxidation reactions and reduction reactions with the presence of electron–hole pairs, further studies are needed to evaluate these findings on the kinetics and mechanism model in the future. This provides a better understanding of the electron and hole pairs movement during the redox reaction to further enhance its ability to address various environmental and energy conversion issues. As a conclusion, this chapter reviews in-depth details on the types of CD materials and their functions as base materials in the various types of composites. Based on our detailed literature studies, it reveals that CD-based materials portray various distinct advantages such as strong light absorption especially in the visible and NIR region, possess large surface area, enhanced charge separation, etc. This indicates the strong potential of CD materials as a good candidate to enhance the performance in various applications such as environmental, biomedical, optoelectronic devices, energy conversion and photovoltaic system.

Acknowledgements The research was supported by the Ministry of Higher Education (MoHE) Malaysia through Fundamental Research Grant Scheme project FRGS/1/ 2019/TK10/UTAR/02/5.

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CHAPTER 7

Carbon Dots Derived from Natural Carbon Sources: Preparation, Chemical Functionalization, Characterization, and Applications MONIKANKANA SAIKIAa,b AND BINOY K. SAIKIA*a,b a

Coal & Energy Group, Materials Science and Technology Division, CSIR-North East Institute of Science and Technology, Jorhat-785006, Assam, India; b Academy of Scientific and Innovative Research (AcSIR), Ghaziabad-201002, India *Emails: [email protected]; [email protected]

7.1 Introduction Carbon dots (C-Dots)/carbon quantum dots (CQDs) are a broad collection of fluorescent, nanosized carbon particles of zero-dimension with sizes less than 10 nm having high quantum yield (QY). Carbon dots (C-Dots) were first prepared by the purification of single-walled carbon nanotubes via preparative electrophoresis in 2004.1 Since then, concerns regarding the sp2 hybrid carbon particle have been mainly focused on their interesting properties, including excellent optical and fluorescence characteristics with high quantum yield (QY), simple and economical preparation techniques from renewable resources with All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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strong thermal and optical photostability, adjustable excitation and emission, tailorable surface functionalization with good biocompatibility and low toxicity. Due to the broad range of applications such as bio-imaging, bio-sensors, nanocarriers for drug/gene delivery, optical sensors, photocatalysis, energy conversion and storage, light-emitting diodes, etc., fluorescent C-Dots has proved to be an interesting chapter in the field of research.2–4 Nevertheless, Carbon dots (C-Dots) have significant features as it is made of carbon which is an abundantly available and usually non-hazardous element which is known as the origin of life. Various CQD synthesizing methods were established using both the top-down and bottom-up classification approaches. In the top-down method, CQDs are prepared by using arc-discharge, laser ablation, chemical oxidation, hydrothermal/solvothermal, and ultrasound/electrochemical exfoliation methods by disrupting bulk carbon resources. But in the bottom-up methods, different molecular carbon precursors are used for preparation by microwave, hydrothermal and pyrolysis methods via carbonization.5 The key factors essential for designing fluorescent CQDs are quantum confinement effects, surface state, and molecular state. By modifying the synthesis approach that is, by utilizing different resources or synthesis procedures, these factors can be easily altered. Tuning of the emission properties depends mainly on the conditions of synthesis comprising of the synthesis methods, variation in the starting materials, post-fabrication techniques with variation in time and temperature, heteroatom doping, and surface functionalization/passivation. The CQDs can be functionalized by various functional groups like carbonyl, carboxyl, epoxy, ether, and hydroxyl including heteroatom doping/co-doping with nitrogen, sulfur, fluorine, boron, selenide, and phosphorous to alter the optical and electronic performance of CQDs.6 Various carbon resources including natural sources such as fruits, vegetables and their peels, leave extract,7 waste sources such as waste kitchen materials, waste paper,8 and animal-derived materials such as eggshells, silkworm chrysalis, etc9 have been utilized for the fabrication of CQDs. Moreover, coal, petroleum coke, graphite, activated carbon, and some nano-materials like carbon nanotube and nanodiamonds can also be utilized for the synthesis of CQDs.10 As natural sources are of low-cost and are environmentally friendly, it is advantageous to use these materials as a root for the synthesis of CQDs. In this overview, the key information on C-Dots along with their luminescence mechanism is summarized prior to the progress of C-Dots in preparation techniques, amazing physical and electronic features, and applications in sensing, catalysis, bio-imaging, and optronics. However, the current chapter will mainly focus on the synthesis of C-Dots from various natural sources and a brief idea regarding GQDs and g-CNQDs has also been furnished. We also discuss in-depth the current progress in doping and co-doping C-Dots and the formation of nano-composites and nano-hybrids along with their preparation technique, and their various feasible applications. We hope this analysis will offer some valuable insights to encourage more exciting research on C-Dots in the near future.

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7.2 CQD Synthesis Techniques In the last few decades, luminescent CQD fabrication techniques have been enhanced through the use of a variety of preparation procedures. The preparation techniques are generally categorized by top-down and bottomup approaches. Focusing on their exceptional optical properties, these methods are considered to be the most selective ones for its preparation.

7.2.1

Top-down Approach

The top-down method concerns the synthesis of CQDs by the breakdown of bulk carbonaceous resources including graphite, graphene, graphene oxide sheets, coal, petroleum coke, soot, carbon fibers, CNTs, etc., through arcdischarge, laser ablation, chemical oxidation, ultrasonic treatment, electrochemical oxidation, hydrothermal, and solvothermal synthesis techniques.2,6,11 Some of the significant top-down techniques are summarized below.

7.2.1.1

Laser Ablation

Many researchers used a laser ablation technique as a standard method to fabricate CDs from various natural sources in a controlled manner. It has been established that the ablation of a graphite target generates carbon nanoparticles under definite circumstances. By modulating the factors of the laser ablation technique such as pulse duration, laser wavelength, irradiation time, laser fluence, and spot size, different characteristics of CDs can be established. Nguyen et al.12 described the synthesis of size variable CDs from graphite powders through laser ablation by varying the time of irradiation. CDs with improved and intense photoluminescence properties were also prepared from a combination of cement and graphite powder using a laser ablation technique by Sun et al.13 Hu et al.14 conveyed a single-step fabrication of CDs with particle size varying from 3 to 13 nm by a laser beam of 1064 nm from graphite flakes in a solution of polymer by altering the pulse time from 0.3 ms to 1.5 ms. Li et al.15 synthesized photoluminescence CQDs by using carbon nanoparticles as a precursor in an organic solvent by laser irradiation technique. The TEM images of the synthesized CQDs displayed that its outermost part was amorphous while the inner portion resembled a hollow core with polygonal carbon structure. Besides, no functionalization after synthesis is needed in this case to achieve highly luminescent CDs. Laser ablation is enhanced by acetone-like solutions, in which laser reacts with the solvent to dissociate into radicals and leads to binding of these radicals to the surface of the nanoparticles, resulting in surface functionalization.

7.2.1.2

Electrochemical Oxidation

The electrochemical oxidation technique is a top-down approach that utilizes an electrochemical cell where the oxidation/reduction reaction occurs

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under the effect of current imposed between two electrodes separated by the electrolyte. Zhou et al.16 reported the first fabrication of blue fluorescent graphitic CDs through an electrochemical oxidation technique by using multi-walled carbon nanotubes (MWCNTs) as an electrode. Li et al.17 prepared fluorescent CDs by using graphite rods as electrodes with an electrolyte of a mixture of NaOH and EtOH, but no fluorescence is observed in acidic medium. Moreover, CDs with varying size were prepared by using graphite rods as the carbon source.18,19 Thus, CNTs or graphite was extensively examined in this technique due to easy handling and gentle experimental procedure. It should be mentioned that the synthesis of CDs largely depends on the electrolyte property and working factors and the size of CDs can also be controlled and adjusted by altering the applied voltage.20

7.2.1.3

Chemical Oxidation

Chemical oxidation is one of the frequently used top-down fabrication methods for the production of CQDs, due to significant advantages such as cost-effective, easy size control, high quantum yield, and purity. Bulky carbon structures can be exfoliated into surface-functionalized nanoparticles by chemical oxidation methods in which hydrophilic groups such as hydroxyl or carboxyl groups are added to the surface of the CQDs to enhance the water solubility and fluorescence features. Peng et al.21 prepared CQDs from various carbohydrate sources such as starch, sucrose, and glucose by treating with H2SO4 under constant stirring for 40 mins by successively adding H2O. Activated carbon can also be used as a source for the fabrication of fluorescent CQDs by refluxing with HNO3.22 Moreover, CQDs have been synthesized from carbon nanomaterials like SWCNTs, MWCNTs, and C60 by refluxing with a mixture of 3 : 1 ratio of sulfuric acid and nitric acid at 140 1C for 7.5 h.23 Researchers studied the synthesis of CDs by the oxidation of coal with H2O2, minimizing the use of toxic and environmentally harmful acids/chemicals and providing potential for the large-scale synthesis of CDs.10,24 It was reported by Wu et al.25 that CQDs can be synthesized from petroleum coke by acid oxidation with a mixer of concentrated HNO3 and H2SO4 followed by hydrothermal treatment of the synthesized CQDs with ammonia solution for the production of N-CQDs. Yang et al.26 synthesized doped CQDs through the acid treatment of Chinese ink-derived carbon nanoparticles proceeded by hydrothermal reaction for the doping of N, S, and Se-doped CQDs. These doped CQDs shows tunable fluorescence behavior with high quantum yield and fluorescence lifetime.

7.2.1.4

Ultrasonic Treatment

Ultrasonic treatment is a very effective approach since sound energy in the form of waves is used for the breakdown of bulky carbon structures. In order to solve the problems arising from the use of hazardous chemicals, Lu et al.27 reported the synthesis of graphene quantum dots (GQDs) by exfoliation of graphite by dispersing in an organic solvent via ultrasonication irradiation.

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This report proved that exfoliation effects can be improved by the use of ultrasonic sound waves. Nitrogen-doped CDs have been synthesized from ascorbic acid and NH3 through an ultrasonic-assisted method as reported by Wang et al.28 It was reported by Dang et al.29 that polyamide oligomer resin can be used for the synthesis of highly functionalized and fluorescent CDs by an ultrasonic method. Moreover, Li et al.30 mentioned the synthesis of luminescent CQDs from Chinese anthracite coal by a simple one-step technique, in which coal was treated with concentrated HNO3 and further sonicated for 24 h at 140 1C to yield CQDs with a diameter of 2.04 nm. Blue-fluorescent CQDs synthesized from low-grade high-sulfur sub-bituminous coal by wetchemical oxidation followed by ultrasonic treatment were described by Das et al.10 However, blue-fluorescent CQDs were also fabricated from high-grade coals like anthracite and bituminous coals by chemical oxidation followed by the ultrasonic-assisted method with lower QY, as reported by Saikia et al.31

7.2.2

Bottom-up Approach

The bottom-up approach mainly deals with the synthesis of CQDs by carbonization and polymerization of a sequence of tiny molecules. Here, in this section, carbon sources such as natural products, polymers, carbohydrates, etc. used to fabricate CQDs by various processes such as hydrothermal/solvothermal reaction, microwave method, and plasma treatment are discussed.8,32,33

7.2.2.1

Hydrothermal Treatment

Hydrothermal treatment is the most regularly used technique to synthesize C-Dots/CQDs from precursors such as natural products, carbohydrates, polymers, etc. as it is simple, environmentally friendly and cost-efficient. This process yields products with uniformly dispersed particles and size showing good quantum yield. In this technique, carbon-based precursors are mixed with water or other solvents that are shifted to a Teflonlined stainless steel autoclave and placed in an oven/furnace to react at high temperatures that fused to form CQDs with particle sizes less than 10 nm. Several natural precursors such as Saccharum officinarum,34 orange peel,35 cabbage,36 natural aleo,37 sweet potato,38 and ocimum sanctum leaves39 were used to synthesize CQDs. CQDs were also prepared by utilizing the sideproducts obtained from a bio-refinery process via a hydrothermal process, with strong compatibility, photo-stability, and sustainable features as reported by Huang et al., 2019.40 These synthesized CQDs showed bluish-green fluorescence having a quantum yield of 13%. Also, it was reported by Lu et al.41 that tunable inner frameworks of fluorescent CQDs can be adjusted and synthesized from carbon and nitrogen-rich biomolecules by hydrothermal condensation. Likewise, abundant carbon resources such as lowrank subbituminous coal, petroleum coke, and coal-derived humic acid are some cheaper sources from which high-value carbon nano-materials i.e., carbon quantum dots, were synthesized by the hydrothermal technique.42

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Besides, this technique proves to be an important approach to develop and produce CQDs by doping/functionalizing to alter their structure, size, and optical properties as discussed in subsequent sections.

7.2.2.2

Microwave Irradiation

The microwave irradiation technique is a rapid and cost-efficient process to fabricate CQDs by using the specific power of microwave radiations into the precursors/reaction mixture. Zhu et al.43 for the first time effectively synthesized photoluminescent CQDs by carbonizing with variable quantities of polyethylene glycol (PEG-200) with various saccharides, such as fructose and glucose, by using microwave treatment. Similarly, Chandra et al.44 reported the microwave synthesis of CQDs by using eco-friendly polysaccharides like starch, chitosan, and alginic acid, etc., separately mixed with polyethylene glycol and agitated for 5 min at 450 W to yield the final CQDs. CQDs were also synthesized from other carbon sources such as citric acid,45,46 sucrose,47 glucose,48 etc. Besides, Pires et al.49 in their work explained that CQDs were synthesized by two-step routes. Firstly, cashew gum was microwave irradiated at 800 W for about 30–40 min which was further dissolved in water and centrifuged. Secondly, polymerization resulted in the formation of blue fluorescent CQDs with a particle diameter of 9 nm. It was also reported by Gu et al.50 that luminescent nitrogen-doped C-Dots were fabricated from natural lotus roots by microwave-assisted irradiation at 800 W for 6 min with further centrifugation, filtration and dialysis with 1 KDa dialysis membrane.

7.2.3

Post-synthetic Variations

The chemical and electronic structures of CQDs could be further changed to control the ensuing characteristics. There are two approaches that have been implemented: heteroatom doping and surface modification. Their optical properties can be modified through chemical doping and functionalization for various definite implementations. This may also lead to a rise in the quantum yield (QY) of the fabricated CQDs.

7.2.3.1

Single-heteroatom Doping

Doping is a commonly used process to alter the photoluminescence (PL) features of synthesized fluorescent substances by tuning the electronic structures. Several techniques are available for the heteroatom doping of various elements such as N, S, B, Se, and P to adjust the optical behavior of synthesized CQDs. By using heteroatomic reactants, heteroatoms can be conveniently inserted into CQDs. The photoluminescent properties of the CQDs can also be modified by the facile co-doping method. As it was reported in several research works that the quantum yield of CQDs is improved by doping nitrogen, as doping contributes to the upward shift in the Fermi level and conduction band electrons. The external

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chemical reactivity and electronic features were efficiently altered by N-doping. Only because the nitrogen atom has the same dimensions as carbon atoms and due to the strong bonding between the valence electrons, has the study of N-doped CQDs become very encouraging in recent years.5,51–53 N-doped CQDs can be produced by using inorganic chemicals like NH3,54,55 and NaNH2;56 organic compounds like amino acids,58 and N,N-dimethylformamide;57 and natural products like bombyxmori silk,59 strawberry juice,60 grass,61 and barley.62 Some researchers reported the fabrication of N-doped CDs by using konjac flour as a carbon precursor via pyrolysis treatment, which resulted in less toxic and highly fluorescent N-doped CDs with a quantum yield of 22% and 7.03% nitrogen content.51 Wu et al.59 reported the single-step hydrothermal synthesis of N-doped CDs by agitating natural protein-bombyxmori silk (B18% nitrogen content) at 180 1C for 3 h. This resulted in particles with sizes ranging from 4–7 nm with 10.45% N content and displayed a photoluminescent quantum yield of 13.9%. Manoj et al.63 synthesized surface-functionalized luminescent organic semiconductor dots by chemical oxidation of natural carbon sourcelignite by oxidizing with nitric acid. Tadesse et al.64 and co-workers reported the fabrication of blue fluorescent water-soluble N-doped CQDs with a quantum yield of 31% by using lemon juice as the carbon source and ethylenediamine as the co-reactant through hydrothermal treatment. Besides, Yang et al.65 explored the large-scale production of photoluminescent doped-CQDs by using Chinese ink as a precursor. The carbon nanomaterials achieved from Chinese ink were further treated to attain oxidized CQDs which were then in situ doped by reacting with DMF via a single-step hydrothermal process that resulted in blue-fluorescent N-doped CQDs with a higher lifetime and quantum yield of about 39%. It is seen that sulfur doping is also performed to enhance the fluorescence properties and to extend their applications in different fields.66–68 This is reported to be a simple and cost-effective way of fabricating sulfur-doped carbon quantum dots by using sucrose as the precursor through acidic carbonization.66 It was established by Yang et al.,67 that sulfur-doped CQDs with a QY of 32% were synthesized by using cellulose fibers as a carbon source and H2SO4 is used for carbonization and as a doping agent. Besides, Hu et al.68 explored the conversion of waste frying oil to value-added S-doped CDs by directly carbonizing waste oil with H2SO4 (concentrated, 98%). Also, Yang et al.26 explored the large-scale production of fluorescent doped-CQDs by using Chinese ink as the carbon source. The nanomaterials attained from Chinese ink were further treated to yield oxidized-CQDs which were then reacted with NaHS solution through a hydrothermal process which resulted in S-doped CQDs with green emission under a UV source.

7.2.3.2

Co-doping Heteroatoms

Co-doping of heteroatoms started gaining considerable interest, as the synergistic influence amongst the doped heteroatoms on CQDs can establish

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a specific electronic structure. The subsequent sections review the significant effects concerning the co-doping of CQDs. Nitrogen and sulfur co-doped CQDs synthesized from natural resources have been keenly studied by several researchers and are established to improve the fluorescence characteristics. Sun et al.69 studied the synthesis of fluorescent N and S co-doped CDs using human hair as a low-cost natural carbon source. H2SO4 (concentrated) was used to carbonize the hair fibers, and the maximum QY was found to be 11.1%. It was extensively explained by Zhao et al.70 that garlic can be used as a source for the fabrication of blue fluorescent N, S co-doped CDs possessing 6.9% N and 1% S (QY 17.5%) via hydrothermal carbonization at 200 1C for 3 h. Furthermore, Sun et al.71 synthesized N S co-doped CDs by hydrothermal treatment of garlic along with the addition of ethylenediamine, Na2S9H2O, and Na2SO4, respectively, at 200 1C for 6 h to enhance the fluorescence as well as QY. The quantum yield of 20.5% was found to be the highest when ethylenediamine was added to the reactant. It should be mentioned that N and S doped CDs yield higher QYs than those synthesized from waste biomass, peels of pomelo, and grass, due to lower N and S contents. Zhang et al.72 prepared N and S co-doped CDs from Nannochloropsis bio-crude oil by microwave treatment followed by H2SO4 carbonization. They reported that the QY of CDs formed after microwave treatment was 6.92% while it was increased to 13.71% when carbonized by H2SO4. Similarly, Ye et al.73 prepared self-co-doped N, S-CQDs from various natural precursors that are rich in C, N, S, and O such as egg yolks, egg whites, feathers, and pigeon’s manure by pyrolysis carbonization at 300 1C for 3 h to yield the final product with a QY of 16.34, 17.48, 24.87, and 33.50%. In another study, Cheng et al.74 prepared blue-fluorescence emitting N, S co-doped CQDs with a quantum yield of 13.3% from willow catkin, a biowaste, through a combustion process in the presence of urea and H2SO4, and these co-doped CQDs were used for the detection of Fe31 ions. Analogously, Das et al.75 reported the conversion of biowaste-Allium sativum peels to N and S co-doped green-fluorescence emitting CDs (PL QY 12.4%) via a pyrolysis technique. Doping of nitrogen and phosphorus atoms into C-Dots results in codoped C-Dots with improved properties and PL/quantum yield, and as such increases their potential applications. Shi et al.76 reported the fabrication of blue fluorescence emitting N, P-co-doped CDs by using glucose as the carbon precursor, and NH3 and H3PO4 as dopants, and they were synthesized via a hydrothermal process at 160 1C for 5 h. These co-doped N, P-CDs were used for the detection of Fe31 ions with the lowest detection limit of 1.8 nM. Bao et al.77 proposed an easy and cheap route for the synthesis of fluorescent N and P co-doped CQDs from Eleocharis dulcis juice through hydrothermal treatment by adjusting the temperatures for 5 h at 90 1C, 120 1C, and 150 1C, respectively. The synthesized co-doped CQDs displayed navy blue, blue, and cyan luminescence emission under a UV source with a maximum QY of 11.2% shown by the CQDs synthesized at 120 1C. Moreover, it was explained by Babar et al.78 that dextrose can be utilized as a carbon precursor, and

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ammonia and phosphorus pentoxide as nitrogen and phosphorus sources which act as dopants while synthesizing N, P-co-doped CDs. The co-doped CDs are treated with nitric acid (concentrated) to make them water-soluble to act as good sensors towards trinitrophenol, a harmful explosive. Moreover, it was explored that kitchen waste onion peels can also be used for the fabrication of nitrogen, sulfur, and phosphorus co-doped carbon dots by a simple microwave process.79 The peels were dispersed in 70% ethanol and then allowed to react in a microwave oven to yield blue-emitting CDs under UV irradiation. Similarly, Yuan et al.80 reported the synthesis of self-doping N, S, P-co-doped CQDs from foxtail millet by the hydrothermal process (120 1C for 5 h) showing bluish-green fluorescence emission under a UV source with a 21.2% QY. Due to the electron-withdrawing capability of the doping atoms, the active areas in the surface of CQDs may be stabilized. Furthermore, multi-doping of CQDs may also lead to stabilization of excitons that changed the electronic arrangement giving rise to improved recombination yield.

7.2.3.3

Hybrids of CQDs

In recent years, much attention has been focused on developing new and innovative hybrid carbon consisting of CQDs and inorganic nanosized centers. The fabricated nanohybrid composite materials enhance the optical and physical properties of the CQDs, and this resulting nanohybrid promises to have diverse applications such as sensing, photo-degradation, hydrogen production, drug delivery, etc. Gogoi et al.81 used chitosan-derived CDs to prepare hydrogel films by mixing agarose and CDs along with 0.1 N NaOH followed by microwave treatment for 30 s. This hybrid film was used for the exploration of heavy metal ions (Pb21, Fe31, Mn21, Cu21, Cr61) and as a membrane for their separation. Sun et al.82 reported the hydrothermal preparation of CDs from carrots, which were then combined with Nile blue and polyethyleneimine to form a nanocomposite through electrostatic interactions via the ultrasonic treatment and this composite was used for sulfide ion sensing. The hybrid composite from CDs was synthesized from amino acid and citric acid with Au-nanoparticles that was used for Ag1 ion sensing in the existence of glutathione.83 When Ag1 ions are introduced, the CDs/AuNPs hybrid converts its color from red to blue with 50 nM estimation limits. Jlassi et al.84 reported on the synthesis of carbon dots from petroleum coke residue in the existence of NH3 through a hydrothermal method. These CDs were satisfactorily utilized to prepare hybrid hydrogel composite films with chitosan for the removal of heavy metals such as Cd21 from wastewater. CDs (3 wt%) were mixed with chitosan (10 wt%) in 0.1 M CH3COOH (30 1C) to form a hydrogel which was distributed and allowed to dry for 24 h at 80 1C. Moreover, Mosconi et al.85 reported on the synthesis of fluorescent N-doped-CQDs which were used as a precursor to develop hybrid-polymer composites of polyester, polyamide, polyurea, etc. which have been applied for various applications such as in the photochemical reaction. With the

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help of this hybrid, Ag ions can be reduced to Ag nanoparticles within the polymer matrix under a UV source, and benzylalcohol is oxidized to benzaldehyde under visible light. Sun et al.86 fabricated (CQDs)/Bi2MoO6 nanocomposites via single-step hydrothermal treatment of glucose-based CQDs with Bi2MoO6 nanosheets that show high photocatalytic activity under visible light towards the degradation of rhodamine B and methylene blue. Moreover, Martindale et al.87 synthesized CQDs by thermal breakdown of citric acid at 180 1C for 40 h, which was then mixed with Ni catalyst to develop a hybrid composite that acts as a photosensitizing agent towards H2 production. Li et al.88 synthesized CQDs from which they fabricated a hybrid composite of carbon-packed ruthenium nanoparticle catalyst for hydrogen evolution reaction. In that work, CQDs were prepared from ginkgo leaves as the precursor via a hydrothermal method, and then RuCl3 and CQDs were reacted to yield the hybrid product. Additionally, Guldi et al.89 explained the formation of hybrid complexes between electron donors, CDs, and electron acceptors, perylenediimides, through electrostatic and p–p interactions. Similarly, Sun et al.90 prepared the hybrid supramolecular complexes by mixing CDs derived from D-glucose and L-glutamic acid, with doxorubicin to develop a CD-derived drug release system.

7.3 Characterization and Properties of CQDs By using natural carbon resources as a precursor, various CQDs with different features can be produced by the above described synthesis techniques. Besides the synthesis of a huge number of CQDs, they possess an almost similar structural or photophysical properties. C, H, N, and O are the basic structural elements that constitute the CQDs derived from natural resources that exist in different forms such as biomolecules, carbohydrates, protein, fossil fuels, etc. This section focuses on and explains concisely a few key features of CQDs.

7.3.1

Structural Characterization

High resolution-transmission electron microscopy (HR-TEM) is the most important technique for observation of nanosized CQDs by determining their morphology, composition, crystallinity, and size distribution. Figure 7.1a represents the TEM image of lemon juice derived C-Dots that are spherical shaped in structure with uniform distribution, and the inset indicates the HR-TEM image with interplanar fringes of 0.21 nm. On the other hand, the atomic force microscope (AFM) specifies the surface morphology of the C-Dots. Figure 7.1b signifies the AFM micrographs of the C-Dots signifying the width to be about 1.5 nm and the inset describes the related height profiles. Moreover, X-ray diffraction (XRD) is another technique to have an idea for structural elucidation. The XRD patterns of C-Dots exhibit an amorphous structure as the carbon atoms are distorted. The patterns generally show a wide diffraction peak with 2y values within 20–251.

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(a) TEM and HRTEM (inset) images of the C-Dots. (b) AFM micrographs of the C-Dots (inset: height profile along the line). (c) XRD pattern and (d) Raman spectrum of C-Dots. Reproduced from ref. 91 with permission from the Royal Society of Chemistry.

Figure 7.1c depicts a typical XRD pattern with a peak at 251 indicating an interlayer spacing of 0.32 nm. Another important technique for structural identification is Raman spectroscopy. As shown in Figure 7.1d, C-Dots possess two peaks at 1344 and 1598 cm1 for the D-band and G-band, respectively, in the Raman spectra that corresponds to the disordered carbon and sp2-hybridized carbon networks.91 From the intensity ratio ID/IG, the degree of crystallinity can be evaluated which can be compared with the results obtained from TEM and AFM. A typical example of self-doped CDs by using Enteromorpha prolifera green algae as a carbon precursor is reported elsewhere.92 The Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) are used for the identification of functional groups on the surface and the chemical structure of the biomass-derived CDs. The XPS spectrum as in Figure 7.2a shows major peaks for S2p, C1s, N1s, and O1s at 169.2, 284.8, 399.9, and 532.4 eV, respectively, which indicate the presence of various functional groups such as C–C, C–O, C–S, C–N, CQC, CQN, C–N–C, N–(C)3 and C-SOx at different bonding modes (see Figure 7.2b, c, and d). The FTIR spectrum of biomass-derived

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Figure 7.2

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XPS spectra of the synthesized CDs: (a) Survey XPS spectrum; Deconvoluted spectra of (b) C1s; (c) N1s; (d) S2p, respectively. Reproduced from ref. 92 https://doi.org/10.1038/s41598-017-04754-x under the terms of the CC BY 4.0 license http://creativecommons.org/ licenses/by/4.0/.

self-doped CDs signifies the presence of O–H, C–H CQO/CQC, C–O–C, and C–S functional groups at 3417, 2939, 1637, 1398, and 1317 cm1, respectively. Similar investigations were also made by several other researchers regarding the synthesis of carbon quantum dots.42,70,79,93

7.3.2

Photophysical Characterization

The photophysical properties/optical properties of carbon quantum dots were generally characterized by using UV–vis absorption spectroscopy and fluorescence spectroscopy.

7.3.2.1

UV–Vis Absorption

In general, the UV–vis absorption spectrum of CQDs displayed a major absorption peak in the ultraviolet (UV) range that stretches to the visible region. Nevertheless, the UV spectrum still varies with regards to the precursors used and synthetic methods. Figure 7.3a portrays representative UV–vis absorption spectra along with blue-emitting CDs under UV irradiation (inset). Most of the studies regarding CQDs exhibit two major absorption peaks, one observed within 230B280 nm is due to p–p* transitions of CQC bonds, while the other is observed within 300B330 nm is due

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(a) UV–vis absorption spectrum of CDs. Photographs of the as-prepared CD suspension under visible light (inset, left) and under UV light with 365 nm UV light (inset, right). (b) Emission spectra with increasing excitation wavelengths from 300 nm to 460 nm and excitation spectrum of the CD suspensions. Reproduced from ref. 92 https://doi.org/10.1038/s41598-017-04754-x under the terms of the CC BY 4.0 license http://creativecommons.org/ licenses/by/4.0/.

to n–p* transitions of CQO bonds.31,75,92–94 Moreover, in some cases a shoulder at about 300 nm is reported due to n–p*/p–p* transitions of COO/C–O–C bonds.95 For instance, Yao et al.96 prepared CDs from waste crab shell and achieved a UV spectrum with a slight peak at 350 nm due to binding of excited energy of the surface groups. In another study on the green synthesis of S, N Co-doped CDs from biomass sources, Hu et al.97 obtained UV–vis spectra with a peak at 333 nm due to the binding of excited state energy. Besides, Figure 7.4a and c represent the UV–vis absorption spectra of CQDs synthesized from anthracite and bituminous coal via ultrasonic-assisted wet-chemical method; and from sub-bituminous coal, petroleum coke, and coal-derived humic acid via hydrothermal treatment, respectively. The bands observed at around 250–270 nm and a tailoring band at around 300–320 nm correspond to p-p*and n–p* electron transition.31,42

7.3.2.2

Fluorescence (FL) Emission

Fluorescence is one of the most interesting characteristics of CQDs. A distinct feature of fluorescence is that emission wavelength and intensity is dependent on the excitation wavelength. The emission wavelength in most situations is greater than the associated wavelength of absorption, suggesting that the emission energy is less than the absorption energy. A variety of potential hypotheses exist regarding the emission property, which comprises several factors such as the impact of size, surface irregularities, rate of oxidation, surface states/group/doping etc. which endow to the whole phenomenon.42,98 Xu et al.92 prepared biomass-derived CDs from Enteromorpha prolifera and examined their optical properties. It was reported that FL spectra showed excitation dependent behavior with maximum emission at

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(a) UV–vis absorption spectra and (b) FL spectra (excited at 300 nm) of CQDs synthesized from anthracite and bituminous coal. (c) UV–vis absorption spectra and (d) FL spectra (excited at 320 nm) of CQDs synthesized from sub-bituminous coal, petroleum coke and coal-derived humic acid. (a and b) Reproduced from ref. 31 with permission from Elsevier, Copyright 2019. (c and d) Reproduced from ref. 42 with permission from Elsevier, Copyright 2020.

450 nm when excited at 370 nm as referred to in Figure 7.3b. Excitation dependent CQDs with maximum emission at 460 nm representing blueemission (maximum QY 14%) was also reported by Das et al.93 from subbituminous low-quality Indian coals and they also mentioned that the FL spectra were red-shifted along with their change in intensity as the excitation wavelength is increased, as reported by other researchers.99–102 As Chen et al.103 mentioned, there are some disadvantages of these CDs regarding their applicability. As for instance different monochromatic sources for excitation and the point that higher wavelength emissions have lesser intensities are the unfavorable issues regarding excitation dependent CQDs. On the other hand, CDs showing FL properties with excitation independent behavior involve complex processes or low QY products as was stated by Maiti et al.104 Chatzimitakos et al.105,106 reported the synthesis of excitation

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independent CQDs from citrus peels (QY of 16.8% and 15.5% for C. sinensis and C. limon) via carbonization and fingernails of human (QY 42.8%.) by carbonization followed by acid treatment. In the above mentioned synthesis of N, S-doped CQDs from human nails there are two emissions with maximum emission at 380 and 430 nm when excited at 320 and 250 nm, respectively. The emission at 380 nm is mainly due to n–p* transitions, whereas the other subsequent emission results from the heterogeneous surface organic fragments causing intermediary states amongst HOMO and LUMO orbitals. Figure 7.4b and d are representations of the FL spectra of CQDs synthesized from anthracite and bituminous coal via an ultrasonicassisted wet-chemical method and from sub-bituminous coal, petroleum coke, and coal-derived humic acid via hydrothermal treatment, respectively. In Figure 7.4b, maximum fluorescence emission is seen at around 400 nm corresponding to blue emission when excited at 300 nm.31 However, in Figure 7.4d, maximum emission intensity is centered at nearly 450 nm indicating blue-fluorescence when excited at 320 nm.42 Such significant changes in the FL properties are due to variation in the functional groups, surface irregularities, particle sizes and the rate of oxidation. Depending on the energy/frequency of the photon emitted, FL emission may be referred to as down-conversion or up-conversion. Down-conversion FL correspond to one higher-frequency photon from which two lowerfrequency photons are emitted. It is to be noted that this form of FL is the most popular among CDs. All the above-mentioned examples regarding the FL emission of CDs belong to this type. While up-conversion FL corresponds to the emission of a photon of higher energy by the association of two photons of lower energy. However, such reports regarding these FL properties in CDs are very scanty. CDs with both down and up-conversion FL properties synthesized from sweet pepper were reported by Yin et al.107 For up-conversion, emission wavelength was observed within 400 and 600 nm when excited within 780 and 900 nm, while for down-conversion the FL emission was observed at 450 nm when excited at 360 nm. Moreover, CQDs with a specific color and longer wavelength emission such as green,108 yellow,25,100,108,109 orange,91 and red91 could also be achieved from different green sources.

7.3.2.3

Photostability of CQDs

The key factor for the use of CQDs in various applications is based on their photo-stability under different conditions. Synthesized CQDs are highly photo-stable even after exposure to UV light (365 nm) for 2 h;37 irradiation with a Xe lamp of 150 W for 2–3 h,105,106 and 500 W for 7 h.60 It was examined that no photobleaching of CQDs occurred even after their irradiation signifying the high photostability of the CQDs. Furthermore, it was reported that the fluorescence emission of CQDs showed slight variation when it is kept at room temperature for 72 h, while at ambient conditions lyophilized CQDs can be kept for more than six months without any change in emission

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intensity. Tadesse et al. explored that N doped CQDs have no notable effect on the FL emission intensity even after irradiating under UV radiation for 45 minutes and storing for 3 months. Meanwhile, less than 10% reduction in FL intensity was observed when N, S co-doped CDs were kept under UV source for 6 h (360 nm) as was established by Cheng et al.74 On the other hand, the FL emission of CQDs is pH-dependent. Due to the presence of functional groups such as carboxyl, hydroxyl, and amino groups, the conditions of CQDs varies as a function of pH of the solution. Highly pHsensitive CDs were synthesized from shiitake mushrooms as reported by Wang et al.110 The fabrication of CDs from rose-heart radish at pH 2–7 displays stable fluorescence.111 CDs with stable FL at pH 1–12 can also be synthesized from lemons.112 However, no effect of pH was noticed on the FL intensity of N, S-co-doped CDs in some of the studies.74 But in some instances, it was clearly observed that N-doped CQDs increase their FL emission as the pH gets reduced from 6 to 2, while its FL emission is declined as the pH gets increased from 8 to 12.64 As CDs are highly sensitive to pH, researchers established a pH probe that could be used for various applications. Moreover, it was also stated by Chatzimitakos et al.105 that CQDs are even stable in high salt-containing solutions. The biomass-derived CQDs have no substantial change in their fluorescence intensity even when the concentration of NaCl was altered from 0–6 M (o10%) and 0.25–2 M, respectively.64,74 Thus, we can mention that the FL emission intensity of CQDs is not affected by the ionic strength of a solution and so it can be utilized for different applications having objections regarding higher ionic strength.

7.4 Applications of CQDs As CQDs are tiny particles with a large surface to volume ratio and high surface functionality, they are highly reactive. Owing to their high photostability with unique fluorescence properties and easy tuning of the fluorescence emission spectra of CQDs, they are a promising material for various applications based on the synthesis from natural sources. In the subsequent sections, we will emphasize the analytical applications such as sensing, catalysis, bioimaging, and optronics that have been established using CQDs from natural resources.

7.4.1

Sensors

Fluorescent CQDs have been employed as sensors for the identification of various metal ions, chemical species, and biological molecules based on their degree of sensitivity and selectivity. The species that are to be detected interacts with the surfaces of CQDs through several mechanisms, for instance, photo-induced electron and charge transfer, resonance energy transfer, and inner filter effect causing variations in their fluorescence properties.113 The surface functional groups of CQDs are used for the

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detection of ions including Fe , As , Cr , Co , Cu , Ag , Hg , Sn21, and Pb21, and biological compounds such as vitamin B12, glucose, galactose, and L-cysteine.6,114,115 In the context of Fe31 ion sensing, several research groups used doped and un-doped CQDs.38,71,73,74,80,110,111 Cheng et al.74 fabricated N,S-doped CDs from willow catkin, a biowaste, that have been used for Fe31 ion detection with a linear range of 40–700 mM and a detection limit of 0.03 mM. It was also mentioned that the quenching effect of Fe31 is due to the transfer of electron from the LUMO state of N,S-doped CDs to partially filled 3d orbitals of Fe31 ions. Moreover, they also successfully analyzed an in vitro cellular study to identify Fe31 ions in HeLa cells (see Figure 7.5). The identification of Fe31 ions in human blood and urine respectively was also studied by using biomass-derived CDs.99,105 Similarly, several studies regarding Hg21 ion probes utilizing CDs were also reported.50,60,62,64,73 Even though several studies were carried out for the detection of Hg21, Lu et al.116 established a sensitive probe utilizing CDs synthesized from pomelo peel. The established probe has the lowest detection limit reported for Hg21 with 0.23 nM detection limit, and a linear range of 0.5–10 and 500–4000 nM. They also effectively identified Hg21 ions in lake water. Likewise, very few research groups have evaluated Cu21

Figure 7.5

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31

21

21

1

21

(a) Cell viability assays of HeLa cells after 24 h treatment with different concentrations of N,S-doped CDs. (b) The bright and (c–f) dark field confocal laser scanning microscopy images of HeLa cells labeled with N,S-doped CDs (40 mg mL1) under different conditions: (c) only N,Sdoped CDs, (d) N,S-doped CDs þ 10 mM Fe31, (e) N,S-doped CDs þ 20 mM Fe31, and (f) N,S-doped CDs þ 40 mM Fe31 (inset: schematic illustration of fluorescence quenching of N,S-doped CDs with Fe31 ions). Reproduced from ref. 74 with permission from Elsevier, Copyright 2019.

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46,61,63,109

probes using CDs. Lignite was used as a carbon source to prepare CDs that have been used to establish a sensitive copper probe with a detection limit of 0.0089 nM.63 Moreover, human fingernails were also used for the synthesis of N, S-co-doped CDs that are a sensitive probe towards the detection of Cr (VI) ions with the lowest detection limit of 0.3 nM and linear range of 1.7–67.5 nM through an inner filter and static quenching effect.117 Likewise, Das et al.93 used coal-derived CQDs as a FL sensing probe for the determination of Ag1 ions with a limit of detection (LOF) of 0.19 mmol L1. Also, waste biomass-derived CQDs complexed with Eu31 was utilized as a probe for fluoride sensing via an on-off-on mechanism.94 Different foods colorants such as tartrazine (LOD 73 nM; 60 nM),37,105 sunset yellow (LOD 0.1 nM),117 and carmine (LOD 0.16 mg L1)118 can also be detected by using CD-based fluorescent probing systems. Likewise, there are several established novel techniques to identify organic chemicals, for instance, organophosphorous pesticides, nitrophenols, methylene blue from different CDs derived from paper ash,119 bamboo tar,120 and lychee seeds121 respectively. The mushroom and papaya-derived CDs were also used as FL probes for the detection of hemin and E. coli.101,110 Using the ‘‘off–on’’ approach, it was revealed that the fluorescence of N-doped CDs was quenched by Fe31 ions and FL was recovered by adding L-lysine.51 Thus, all these illustrate the advantages of designing fluorescent probes using CDs derived from different natural resources.

7.4.2

Bio-imaging

CQDs possess significant in vivo and in vitro applications in the areas of fluorescent bioimaging of cells and tissues because of their specific properties like low toxicity, biocompatible, highly water soluble, and multicolored fluorescence. In recent years, a large number of research groups have fabricated CQDs for bioimaging from various natural sources.51,64,91,97,106 It was stated that N-doped CDs were applicable for bio-imaging of the human lung cancer cell line (A549) and images of different colors were observed at different excitation wavelengths.59 Bankoti et al.79 synthesized N, S, P co-doped CDs fabricated from onion peelings that were found to be cyto-compatible when applied for in vivo cell imaging in MG63 and HFFs cells. Likewise, N,Sdoped CDs produced from a biomass source have proven to be biocompatible for in vitro cell imaging and toxicity studies towards HeLa cells (cancer cell line) with a maximum concentration of 300 and 400 mg mL1 respectively (see Figure 7.5).74,92 Crab shell-based Gd31-doped CDs were combined with folic acid for definite directing of folate receptor to positive HeLa cells and negative HePG2 cells.96 This study provides a new vision for the advancement in using green CDs for definite cell targets. Another study on the cytocompatibility of coal-derived CQDs in L6, HeLa, PC3, and MDAMB 231 cells and cell imaging was also performed in L6 cells that suggests that CQDs can be used as a favorable source for fluorescence imaging.93 Moreover, Park et al.122 explored an in vivo mouse study for the distribution of mango-based

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CDs having different color emission with different particle sizes and found that the large particles assembled in the liver while the smaller sizes were assembled in the urinary bladder. Thus, CDs have been effectively used as fluorescence cell imaging probes.

7.4.3

Drug-delivery

CQDs are highly stable, biocompatible, fluorescent materials enabling them to be a novel vehicle for the loading and discharging of drugs. Ding et al.123 reported the preparation of CDs from DNA isolated from E. coli which in turn indicated that CDs could work as a fluorescent drug-transport vehicle in the near future. Shrimp-based N-doped CDs have been used as a traceable drug delivery vehicle with the capability to transport boldine drug to MCF-7 cells (cancer cell).124 Similarly, hydrothermally prepared fluorescent CDs from pasteurized milk were prepared and satisfactorily used as feasible drug vehicles with the ability to deliver Lisinopril drug to HeLa cells.125 However, there has not been much research done on the development of biomassderived CDs for use as a vehicle in drug delivery, so new approaches are warranted in this direction in the near future.

7.4.4

Catalysis

Catalysis is one of the futuristic applications of CQDs. Owing to its specific fluorescent and photoelectron transfer characteristics, CQDs derived from natural sources can be thought of as a dynamic material in the design of photocatalysts. Researchers have also studied the doping of CQDs to modify their electronic structure with other chemical species to form composites which can be applied as a photocatalyst for the degradation of dyes and H2 generation via photo-splitting of H2O. Doping of CQDs can alter the band gap as well as increase the QY and in turn, ensure the catalytic activity of CQDs. Zhu et al.126 prepared bifunctional CDs from soy milk that showed fluorescence properties along with excellent electrocatalytic performance for oxygen reduction. A hybrid CD/g-C3N4 system was made to study photocatalytic H2 generation, which was found to be 88.1 mmol h1 times higher than bulk g-C3H4.127 Li et al.88 synthesized biomass-derived CDs from which they fabricated a hybrid composite of carbon-packed ruthenium nanoparticle catalyst (Ru@CQDs) for the hydrogen evolution reaction. The electrocatalytic study of the Ru@CQDs catalyst possesses good catalytic performance with 0 mV onset overpotential along with a Tafel slope of 47 mV decade1 and excellent stability. Tyagi et al.128 prepared lemon peel-based CQDs that were restrained over TiO2 nanofibers to design TiO2–CQDs composites. Methylene blue has been used as an ideal contaminant and the photocatalytic performance of TiO2–CQDs composites is about 2.5 times better than TiO2 nanofibers. Our research group also synthesized CQDs from coal, petroleum coke, and coal-derived humic acid. These CQDs exhibit excellent

Carbon Dots Derived from Natural Carbon Sources

161 

1

photocatalytic activity via the generation of electron (e )–hole (h ) pairs on their surface and showed good degradation of 2-nitrophenols with an efficiency of 80.79%.42 Also, the synthesis of CDs from bitter apple peel and palm powder respectively established photocatalytic activity towards the degradation of crystal violet.129,130

7.4.5

Optronics

Optronics is a significant area of application of CQDs. CQDs derived from natural resources are recently used in energy storage and conversion devices, due to their exceptional properties such as high stability, strong absorbance, eco-friendliness, and cost-effectiveness. The FL quenching mechanism was used to efficiently enhance the conversion efficiency of CD-sensitized solar cells.131 The monkey grass-derived CDs successfully confirm the recommended opinion that they are liable for increasing FL quenching and is proved to be relevant for improving the activity of other FL nanodot sensitizer solar cells. Figure 7.6a displays a graphical diagram showing the quenching behavior of an excited electron by an electron acceptor and donor. Briscoe et al.132 prepared waste biomass-derived CDs with surface functionalization. These CDs can be used as a sensitizer for ZnO nanorods that can be applied in nano-structured solar cells. They have also mentioned that binding of CDs to the surface of ZnO and transport of charge in the layer depends on the functionality of CDs, which finally impacts the activity of the solar cells. Likewise, Marnovich et al.133 also

Figure 7.6

(a) Schematic representation of a photo-excited electron quenched by an electron acceptor (Route II) and donor (Route III). (b) Emission spectrum of the white LED, inset: a photograph of the LED operated at 3.2 V. (c) CIE 1931 chromaticity diagram of the white LED. (a) Reproduced from ref. 131 with permission from Elsevier, Copyright 2015. (b and c) Reproduced from ref. 136 with permission from the Royal Society of Chemistry.

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developed lobster shell-based CQDs that can be used to sensitize TiO2-derived nano-structured solar cells. Besides carboxylic acid, amine functionalized CQDs are highly advantageous for the activity of solar cells. Sarswat et al.134 established a technique to synthesize multi-colored, fluorescent CDs from the waste remains of food, beverages and combustion and consequently these CDs were utilized for the production of light emitting diodes (LEDs). Depending on tint and Commission Internationale d’Eclairage (CIE) data, these LEDs are classified to be a bluishwhite light releasing type. Similarly, fluorescent CQDs were prepared via the hydrothermal treatment of glucose and polyethylene glycol 200 as the carbon source from which white LEDs with color coordinates of (0.32, 0.37) were designed.135 Similarly, Feng et al.136 designed white LEDs from cokederived CQDs with a CIE coordinate of (0.31, 0.35), which can ultimately be used for the improvement of photonic devices. Figure 7.6b and c represent the emission spectrum of white LED along with the CIE chromaticity diagram. As white LEDs can save a huge amount of energy, the traditional diodes comprised of rare earth metals have disadvantages because of their cost and toxicity. However, CQDs derived from natural sources are costeffective, less toxic, eco-friendly and have high QY and as such, they can replace conventional white LEDs.

7.5 Carbon Nanoparticle Allotropes In this chapter, we have mainly reviewed the synthesis, properties, and applications of CQDs, however a brief note on graphene quantum dots (GQDs), and graphene nitride carbon quantum dots (g-NCQDs) is also provided below. Graphene quantum dots (GQDs) are a novel form of zero-dimensional fluorescent nanoparticles regarded as tiny fragments of graphitized planes that result in quantum-size and confinement effect within 3–20 nm particles. The quantum confinement effect and variability of the sp2 sites existing in GQDs enable its optical features largely reliant on its size so that band gap energy could be adjusted by modifying its size.137 Various green carbon sources have been used for the fabrication of GQDs including glucose,138 cow’s milk,139 starch,140 coal tar pitch,141 graphene oxide,142 and cotton cellulose,143 among others. Due to their extraordinary properties such as biocompatible, water-soluble, tunable FL, higher surface to mass ratio, electrical conductivity and transparency, GQDs also find notable applications in the area of sensing, catalysis, display, photonics, energy conversion and storage, etc.137,144 In the meantime, GQDs have potential applications in the area of biomedicine, generally for in vitro/in vivo bioimaging, drug delivery, theranostics, and near-infrared (NIR) sourceinitiated therapy.145 Besides, graphitic carbon nitride quantum dots (g-CNQDs) are polymeric substances comprising of C, N, and some H contaminants that are linked through tri-s-triazine-based arrangements. By correlating to the carbon nanoparticles, CNQDs are also observed to be surface-functionalized with

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163

electron rich properties. They have gained much interest regarding their interesting properties, such as biocompatibility, low toxicity, watersolubility, photostability, and strong fluorescence with good QY, so g-CNQDs can be used as a favorable candidate for multicolor cell imaging, bio-sensing, drug delivery, theranostics, and catalysis.146,147 Furthermore, g-CNQDs can form nanocomposites with other nanomaterials and can offer specific features in photocatalysis, and initiate other nanomaterials to improve their activity. The photocatalytic performance of g-CNQDs has been extensively utilized for waste degradation, disinfection, and design of solar cells.148 The synthesis of g-CNQDs with a QY of 15.7% was also reported by hydrothermal treatment of human urine as a raw precursor.149 These g-CNQDs were utilized as fluorescent probes for multicolor bio-imaging. Soon, the nanocomposites of g-CNQDs will be a promising candidate for various applications. It should be mentioned that several researchers are currently focusing on the design of g-CNQD composites with CQDs/GQDs and their related applications. Table 7.1 summarizes the sources with different techniques used for the synthesis of C-Dots/CDs/CQDs along with their sizes, quantum yield and applications.

7.6 Summary and Future Outlook In this chapter, the current progress in the field of CQDs, emphasizing their synthesis from different natural sources along with their chemical modifications, and structural, physical, and optical properties have been summarized. CQDs have revealed an explicit perspective in the fields of chemical and biological sensing, in vitro and in vivo bio-imaging, drug delivery, catalysis, light emitting devices, energy conversion and storage devices. Also, the cost and availability of the raw material should be of concern in synthesizing CQDs. Even though a lot of effort has been dedicated to the improvement of the applications of CQDs from new natural resources, there remain some challenges that need to be overcome. In the future, CQDs with near-infrared (NIR) absorbance and emission should be the focus, as NIR light is favored over UV–vis light, as UV–vis light is too short for various biomedical uses and it damages proteins, inhibiting the penetration of visible light for bio-imaging, while NIR light has lower biological toxicity and can enter into tissues. It is also observed that not much research work has been done on the fabrication of different light emitting diodes from CQDs derived from natural sources; it should be a concern and focus to design different low-cost light emitting diodes. Moreover, the utilization of CQDs in photodynamic therapy, drug delivery, and hydrogel nanocomposites for in vivo bio-imaging and tissue engineering must be investigated. In spite of unusual and prominent applications in different fields, some properties of CQDs are still uncertain. Detailed and systematic R&D is needed in the future to clarify the probable complexities and notable applications.

164

Table 7.1 Summary of available C-Dot synthesis techniques along their properties and applications. Method

Size (nm)

QY (%)

Saccharum officinarum Cabbage Aloe Sweet potato Ocimum sanctum leaves Bio-refinery side product Sub-bituminous coal Petroleum coke Coal-derived humic acid Bombyxmori silk Lemon juice

Hydrothermal Hydrothermal Hydrothermal Hydrothermal Hydrothermal Hydrothermal Hydrothermal Hydrothermal Hydrothermal Hydrothermal Hydrothermal

3 2B6 5 2.5–5.5 1–4 1–6,1–3 3.2–5.4 3.5–6.5 4.5–6.5 4–7 1–6

5.67 16.5 10.37 8.64 9.3 13 21.95 10.91 2.38 13.9 31

Grass Garlic

Hydrothermal Hydrothermal

3–5 10.7

Dried shrimps Mushroom

Hydrothermal Hydrothermal

6 2–6

Papaya Konjac flour Allium sativum Willow catkin Lychee seeds Human fingernails Glucose Cashew gum

Hydrothermal Pyrolysis Pyrolysis Combustion treatment Carbonization Carbonization Microwave Microwave, polymerization

2–6/8–18 3.37 2 7.3 1.12 3.5 1–7 9

Applications

Cellular imaging of bacteria and yeast Bio-imaging Detection of tartrazine Cell-imaging, Detection of Fe31 Pb21 ions and live cell imaging Cell imaging and in vivo bio-imaging Photo-degradation of 2-nitrophenols Photo-degradation of 2-nitrophenols Photo-degradation of 2-nitrophenols Bio-imaging Detection of mercury(II) ions and live cell imaging 2.5–6.2 Detection of Cu21 17.5 Cellular imaging and free radical scavenging — Bio-imaging and drug delivery 5.5 Multicolor imaging of HeLa cells, detection of hemin 18.39–18.98 Detection of Fe31, E. coli 22 Detecting Fe31 12.4 Solar conversion, in vitro cell labeling 13.3 Detection of Fe31, bio-imaging 10.6 Detection of methylene blue 81.4 Detection of Cr(IV) and bio-imaging 12.7 Self-imaging, non-viral gene delivery 8.7 —

References 34 36 37 38 39 40 42 42 42 59 64 61 70 124 110 101 51 75 74 121 117 48 49

Chapter 7

Source

50 44 44 44 79 106

0.53, 0.20

—

31

8.5–11.5

4.45

Detection of fluoride ions

94

8–12

26.20

Detection of fluoride ions

94

8–11

13.80

Detection of fluoride ions

94

2.5–6

32

Detection of Fe31

67

8.7, 15.8 1–3

5, 2.7 —

25 24

17 6.5 2–10

— 9.2 11.1

— Photocatalytic activity methylene blue and methyl orange Detection of Cu21 Light emitting devices Bio-imaging

Microwave Microwave Microwave Microwave Microwave Microwave

9.41 2–10 2–4 1–2 2–4 2.2

Algae Sub-bituminous coal

Microwave Ultrasonic-assisted wetchemical oxidation

4 1–6, Bioimaging, silver ion detection 3–9, 2–11

13.71

1.5–4.5 Anthracite and bituminous coals Sugarcane bagasse

Petroleum coke Coal

Ultrasonic-assisted wetchemical oxidation Ultrasonic-assisted wetchemical oxidation Ultrasonic-assisted wetchemical oxidation Ultrasonic-assisted wetchemical oxidation Acid hydrolyzed, hydrothermal Acid oxidation, hydrothermal Acid oxidation

Lignite Coke Human hair

Acid-oxidation Chemical oxidation Acid treatment

Taro peels Garlic peels Cellulose fibers

19 — — — 42.8

72 3, 4, 8, 14

93

Carbon Dots Derived from Natural Carbon Sources

Detection of Hg21and cell imaging Photoluminescence (PL) properties Photoluminescence (PL) properties Photoluminescence (PL) properties Bio-imaging Detection of sunset yellow and bioimaging Bio-imaging 2–5.5,10–30

Lotus roots Chitosan Starch Alginic acid Onion peels Human fingernails

63 136 69

165

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CHAPTER 8

Composites of Carbon Nanodots for Hydrogen Energy Generation BISWAJIT CHOUDHURY Institute of Advanced Study in Science and Technology (IASST), (An autonomous Institute Under DST, Govt. of India), Paschim Boragaon, Vigyan Path, Guwahati-35, Assam, India Emails: [email protected], [email protected]

8.1 Introduction Our uninterrupted global energy requirements are fulfilled by the burning of coal. However, the same energy source is responsible for the emission of poisonous gases of sulfur, nitrogen, and carbon into the environment. These emissions are profoundly detrimental for every single living dweller on earth. Researchers are considering water as an alternate, environmentfriendly energy source to combat these issues associated with coal energy. Breaking up of water generates only hydrogen and oxygen. The production of these gasses is accelerated in the presence of solar light, electric current, and the aid of an electrode catalyst. In 1972, Fujishima and Honda successfully dissociated water into H2 and oxygen (O2) by shining UV light on a TiO2 electrode submerged in water.1 The photonic efficiency of TiO2 is strong under UV light but weak in the visible region. TiO2 reflects the useful 95% of visible to NIR of solar constituents back in to the atmosphere. From the earliest discovery until now, many publications have appeared on water splitting using new types of electrode materials that can harvest the entire spectrum of solar light. In this chapter, we discuss the fabrication of carbon nanodot-based composite nanostructures with graphene and graphitic carbon nitrides All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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(g-C3N4 or CN). 0D carbon nanodots comprising of CDs, GQDs, and CNQDs are the materials of choice for making carbon nanodot-based metal-free photocatalysts. These 0D nanostructures are coupled with other one-dimensional (1D) and two-dimensional (2D) carbon-based nanomaterials for forming all carbon nanocomposites. These carbon nanodots are stabilized over 2D graphene, graphitic carbon nitride nanosheets via p–p stacking interactions, van der Waals interactions, or an electrostatic self-assembly process.2–5 0D/2D carbon-based composites have abundant surface reactive sites and functional groups for stable dispersion in water. CDs and GQDs have –OH, –COOH functional groups, while downsized CNQDs have –OH, –COOH groups, and cyano groups on the surface.6,7 In comparison to graphene, GQDs provide a higher fraction of reactive edges. Thus, in GQD/graphene, GQDs accumulate photons and generates free electrons, while graphene functions as an electron collecting layer.8–9 Similarly, CN is an excellent photocatalyst for water reduction in the blue region. C-nanodots because of their excellent upconversion property extend the photon absorption range in CN from the near-visible to NIR region of illumination.5,6,10 These 0D/2D carbon-based heterostructures form a type II heterojunction with interfacial carrier transfer and separation as a requirement for photocatalysis.11 GQDs provide support to CN for stabilized dispersion in water and act as an intercalating agent for bulk CN exfoliation.12

8.2 Carbon Nanodots CDs are 0D carbon nanoparticles having a size less than 10 nm. CDs are a promising alternative to inorganic semiconductor quantum dots because of their lower toxicity, solubility, biocompatibility, and environment-friendly nature. The first report of the accidental discovery of fluorescent carbon nanoparticles is dated back to 2004 when Scrivens and his team were conducting electrophoretic isolation of single-walled carbon nanotubes (SWNT) from arc-discharge of carbon soot.13 They observed that a fraction of the isolated SWNT suspension contained fluorescent carbon nanoparticles. These carbon nanoparticles had a lateral dimension of 18 nm and a height of 1 nm. In 2006, Sun and his co-workers synthesized nanoscale carbon particles by a laser ablation method.14 Polyethylene glycol was used for the surface passivation of these carbon nanoparticles to produce fluorescent CDs. From 2006 up until now, a plethora of publications has become available on the synthesis of CDs for various applications. Primarily, there are two methods for the fabrication of CDs: top-down and bottom-up. The top-down process involves fractionation of large-sized carbon materials, such as carbon soot,13 graphite,15 carbon nanotubes,16 by arc-discharge,13 laser ablation,14,15 and electrochemical ways.16,17 Laser ablation and arc-discharge methods of the fabrication of CDs have already been mentioned in the reports of Scrivens et al.13 and Sun et al.14 The electrochemical fabrication of CDs is performed in a three-electrode electrochemical cell. One of the reports shows electrolysis of a medium containing alcohol, water, and NaOH at a current density of 15–100 mA cm2.18

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After performing the reaction for 4 h, CDs are collected by adding excess ethanol to drive out NaOH. The bottom-up strategy involves carbonization of small organic molecules by solvothermal/hydrothermal methods, and microwave irradiation.19 The thermal decomposition of citric acid prepares pristine CDs at a high temperature (4180 1C). In some cases, more than one precursor is used. For example, citric acid and precursors containing nitrogen (N), boron (B), and phosphorus (P), e.g., urea, boric acid (H3BO3), and phosphoric acid (H3PO4), are mixed and treated hydrothermally to obtain N, B, and P-doped CDs.20 Water-soluble CDs are synthesized by the ultrasonic treatment of a mixture of activated carbon and hydrogen peroxide.21 A green approach for CDs synthesis is extraction from fruit and vegetables, such as guava, spinach, and peas.22 The resultant CDs have abundant C, N, and O-related surface functional groups. The physiochemical properties of these CDs can be altered by adding surface functional groups, doping with hetero atoms such as N and P, and forming composites with other carbonaceous nanomaterials. CDs display unique optical properties. CDs show p-plasmon absorption, which covers the entire solar spectrum spanning from the UV–vis to the NIR. The photoluminescence of CDs can be tuned from visible to NIR by changing the size, shape, and content of the surface functional groups.23 A unique optical property shown by CDs is fluorescence upconversion in which the CDs absorb NIR light and emit photons in the visible region. This property makes CDs a useful photosensitizer for catalytic applications.24 Other metal-free analogs of CDs are GQDs and CNQDs. CDs and GQDs are structurally analog. Both of them have a carbon cluster with sp2 core bonding surrounded by oxygenated functional groups.25 While CDs are prepared from small organic molecules, GQD preparation involves molecules with benzene structures, e.g., graphene and graphene oxide (GO). The core is more crystalline in GQD than that in CD. CNQDs have an s-heptazine ring with sp2 C–N bonding.26 All three forms of carbon nanodots show strong light absorption properties, thus making them suitable for energy applications. CDs are used as a photosensitizer in photocatalytic reactions. These different 0D forms of CDs can be combined with other dimensional forms of carbon, such as graphene oxide (GO) and graphitic carbon nitrides (CN), to form various carbon nanocomposites. One major area of applications of these carbon composites, which we discuss in this chapter, is photocatalytic H2 evolution.

8.3 Fabrication of C-nanodot Composites and Their H2 Evolution Performance 8.3.1

Carbon Dot–Carbon Nitride Composite (CDs/CN)

Hydrothermal,3,27,28 ultrasonication,5,29 and thermal decomposition30–34 are some of the methods adopted to deliver well-interconnected CDs/CN composites. In the hydrothermal method of composite fabrication, CDs and

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ultrathin CN nanosheets were suspended in ethanol, stirred for sufficient mixing, and finally treated hydrothermally at 120 1C for 2 h.3 The pristine CDs are obtained by the hydrothermal treatment of rapeseed flower pollen bees, and ultrathin CN nanosheets are obtained by the thermal oxidation of a solution-dried mixture of dicyandiamide and ammonium chloride. The composite has a fixed amount of CN nanosheet with a different fraction of CDs. The composite forms a type-II van der Waal heterostructure which is a requirement for efficient photocatalytic systems.3 Lv et al.27 fabricated CDs/CN composites by ultrasonic treatment of an ethanol–water dispersion of CDs and CN for 30 min, followed by hydrothermal treatment at 180 1C for 4 h. Hydrothermal treatment of an aqueous mixture of catechol, ethanediamine results in CDs. CN nanosheets are collected from the dispersion of the ultrasonic treatment of bulk CN. Wang et al.,28 instead of using the as-prepared CDs, used the precursors for CDs for mixing with CN. In their approach, a certain amount of L-ascorbic acid is mixed with CN in a water–ethanol mixture followed by hydrothermal treatment at 180 1C for 4 h. In this way, CDs with an average size of 5 nm are well-dispersed over the CN nanosheet. The reaction scheme is shown in Figure 8.1. The interfacial connection occurs between acidic enol groups of L-ascorbic acid and amino groups present at the edges of CN.

Figure 8.1

The schematic shows the fabrication of CDs/CN. Thermal polymerization of (a) melamine to (b) CN. (c) Complexation of L-ascorbic acid with CN to prepare (d) CDs/CN. Reproduced from ref. 28 with permission from the Royal Society of Chemistry.

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Figure 8.2

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Scheme showing the ‘‘spot heating’’ approach for the synthesis of CD-decorated CN nanosheets. Reproduced from ref. 29 with permission from John Wiley & Sons, Copyright 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

An electrostatic self-assembly strategy has been adopted to construct better interfacial contact between anionic CDs and protonated CN nanosheets.5 The method involves the ultrasonic mixing of CDs and CN. CDs prepared by an electrochemical method contains an abundance of –COO and –OH groups. Similarly, protonated CN is synthesized by acid refluxing of bulk CN. Ultrasonic mixing of CDs and CN result in electrostatic self-assembly of CDs over CN. Zhang et al.29 reported an ultrasonic cavitation mediated ‘‘spot-heating’’ approach for getting well-dispersed CDs over CN. The fabrication scheme is shown in Figure 8.2. In their approach, the interlayer spacing in CN is enlarged by intercalating SO42/NO3. When water is added to this acid-treated CN, an enormous amount of heat is generated, which helps layer exfoliation. The separated sheets are freeze-dried, and an intense ultrasonic vibration is passed through. The extreme heat produced by ultrasonication fragments nitrogen and cyano groups leaving only carbon on the disrupted sites. It finally leads to CDs with a size of 2 nm deposited over 1.5 nm thick CN nanosheets.29 Thermal polymerization is one of the most sought after strategies for the fabrication of CDs/CN.30 Amine-functionalized CDs are prepared by hydrothermal treatment of citric acid and ethylenediamine. These aminefunctionalized CDs are mixed with urea in water. This mixture was kept in N2, followed by freeze-drying. The freeze-dried mixture was subjected to calcination with the temperature slowly increasing from room temperature to 550 1C. An exciting outcome of this synthesis is that the aminefunctionalized CDs develop amide bonding. The amide bonding gives a tubular morphology to the CN. The tubular morphology is retained only up to a certain weight percentage of CDs over CN. If the amount of CDs crosses an optimum level, the tubular morphology collapses, giving the CN a sheet-like structure again.30 Guo et al.35 synthesized CDs/CN composites by thermal polymerization of CDs and melamine at 450 1C. In the synthesis, the

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released NH3during melamine calcination provides an inert atmosphere for the growth of CDs (Figure 8.3). This thermal co-polymerized system is stabilized through strong electrostatic interaction. Wang et al.31 considered both in situ and ex situ approaches to fabricate CDs/CN composites. In the in situ method, a certain amount of glucose and urea is added into a crucible and dissolved in water. Water is evaporated from the mixture by drying at 80 1C followed by calcination at 550 1C. In this method, glucose serves as the source for CDs and urea as the source for CN. The composite is prepared with varying glucose content. In the ex situ approach, they synthesized CDs by hydrothermal treatment of glucose at 180 1C for 6 h. The as-prepared CDs are mixed with urea in a crucible, dried in an oven, and finally calcined to get the CDs/CN composite. The in situ process gives more uniform distributions of CDs over CN, whereas the ex situ method gives large particle agglomeration.31 Unlike the report of Wang et al.,31 Zhou et al.32 showed the use of citric acid instead of glucose for thermal polymerization with urea. They calcined the mixture of urea and citric acid at 550 1C for 4 h. However, under TEM, they could not identify the deposition of any CDs over CN. They mentioned that the preparation leads to the clubbing of the N-doped graphitic sp2 cluster with the aromatic ring of CN. The N doping occurs because of the condensation copolymerization of –COOH groups of citric acid and –NH3 groups of urea. For composite making, Zheng et al.33 used an as-prepared solution of CDs for mixing with urea. This urea-CDs mixer is calcined at 550 1C for 3h. The final product after calcination is collected after washing and drying. The composite shows a broad absorption covering the entire solar light spectrum from visible to NIR. In a template-mediated growth approach for the composite34 molten cyanamide was added to silica colloid with different amounts of CDs and heated at 50 1C for water evaporation. The transparent gel was calcined at 550 1C for 3.3 h. The template is finally removed by treating with 4 M NH4HF2. The mesoporous CN shows a pore diameter of 12 nm and acquires a surface area of 133.691 m2 g1. Bulk CN offers a surface area of only 16.74 m2 g1. This template-mediated mesoporous CN provides a high surface area for CD decoration. CDs decorated over an ultrathin CN (UCN) nanosheet shows excellent efficiency in H2 evolution under visible light, whereas CDs deposited over bulk CN (BCN) show poor performance.3 Under visible light, the hydrogen evolution rate (HER) for UCN/CDs is 88.1 mmol h1, whereas, for BCN/CDs this value is 46.3 mmol h1. An optimum loading of 0.2% CDs over UCN gives the best photocatalytic results. Under visible light, CN gets excited, and the photoexcited electrons channel to the CDs favoring carrier separation. Under monochromatic radiation of 465  20 nm, the sample shows HER of 17.6 mmol h1. A pristine UCN shows weak efficiency towards HER under visible light. The composite uses the upconversion properties of CDs and absorbs visible light for photocatalysis. In the upconversion process, CDs absorb light at a long wavelength and emits at a shorter wavelength, which further channels to the CB of CN. The UCN/CDs-0.2% shows excellent

Composites of Carbon Nanodots for Hydrogen Energy Generation

Synthesis protocol for CD-decorated CN nanosheets. Reproduced from ref. 35 with permission from the Royal Society of Chemistry.

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photocatalytic stability with a rise in H2 evolution for 4 h of illumination. Thus, CN nanosheets exploit their upconversion properties and work as a visible light active photosensitizer in CD/CN composites.33,35 In the CDs/CN composite, a change in the loading amount of CDs over CN help to realize the optimum loading for better photocatalytic performance. An increase in CDs is reported to increase the visible light absorption with a sufficient narrowing in the band gap. The shifting in the VB and CB edge positions result in the narrowing in the band gap.36 Density functional theory (DFT) provides the interlayer mechanism for the enhanced H2 evolution in trigonal/hexagonal CD-decorated CN. The composite forms a type-II van der Waals heterojunction. The study shows that the energy levels of trigonal CDs lie near the CB of CN with an energy gap of 0.29 eV, whereas hexagonal CDs form energy states close to the VB with a gap of 1.45 eV from the CB level.4 As the CD concentration increases, a progressive red-shift in the band gap is monitored. Feng et al.37 studied the influence of functional groups such as –OH, –COOH, and –CHO present on CDs on H2 production. The type of functional groups determines the relative shifting in the VB and CB position and shows the direction of electron flow. In the case of –CHO attached to CDs/CN, the direct electron transfer occurs from CN to CDs. Thus, the electrons on CDs reduce water to H2, while holes in CN lead to the oxidation of H2O. However, in the case of hydrogen (H) attached to the CDs/CN composite, a direct charge excitation from H-CDs to CN favours the H2O reduction process. The holes in CDs lead to water oxidation.37 CDs help in the enhancement of visible light absorption in the CDs/CN composite. However, an excess dose of CDs serves as carrier recombination sites and lowers the photocatalytic activity. Li and Zhou38 show that the CN nanotube decorated with 0.5%CDs shows HER of 382 mmol h1 g1. Bare CN produces only 123 mmol h1 g1 of H2 (Figure 8.4a). The surface of the nanotube provides catalytically active sites, and the tubular morphology

Figure 8.4

(a) The rate of H2 evolution of CN nanosheets decorated with different amounts of CDs. (b) Stability test for CN/CDs-0.5. The red line represents the stability test for newly synthesized samples. The blue line is for half a year-old sample. Reproduced from ref. 38 with permission from American Chemical Society, Copyright 2018.

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Figure 8.5

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(a) Stability performance test for pristine CN and CDs/TCN composite for 20 h of operation. (b) Wavelength-dependent absorption spectra and AQE determination for the composite. Reproduced from ref. 30 with permission from John Wiley & Sons, Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

triggers fast diffusion of photoexcited carriers from the inner to the outer surface of the nanotube. The composite shows high stability in H2 production for a continuous 12 h of operation (Figure 8.4b). CDs decorated over tubular CN (TCN) show far more superior H2 production activity than the electrostatically bound CDs/CN nanosheet. CDs/TCN can effectively produce 3538.3 mmol h1 g1 of H2 with an apparent quantum yield (AQY) of 10.94% at 420 nm. The CN nanosheet with CDs produces only 2002 mmol h1 g1 of H2 (Figure 8.5a). The AQY is 10.94% at 420 nm, which decreases toward the longer wavelength (Figure 8.5b).30 CDs/CN composites obtained by spot heating of CN nanosheets with the aid of ultrasonic cavitation are capable of functioning dual roles in photocatalysis.29 Bisphenol is an aquatic hazard. When an aqueous bisphenol solution is irradiated, the composite degrades bisphenol and releases H2 by water splitting. CDs/CN can release 152 mmol g1 h1. Therefore, a concurrent process occurs in which the catalyst effectively degrades bisphenol and splits water into H2. Wang et al. compared the H2 evolution activity between in situ and ex situ prepared composites.31 In the in situ prepared composite with varying glucose content, it is seen that HER increases with an increase in glucose (G) content from G(0.25) to G(0.5). A decrease in HER is noticed at a higher loading of glucose. A comparison made between in situ and ex situ prepared photocatalysts reveals a higher HER in the prepared in situ system. During the in situ thermal polymerization process of glucose and urea, interfacial contact is established between –OH groups of glucose and –NH3 of urea. This leads to a better charge transfer through the interfacial connection. It appears that the polymerization time has a profound impact on the HER of the composite. An increase in the thermal polymerization time from 2 h to 8 h, reduces the HER activity of CN/G(0.5). Thermal polymerization releases NH3 from glucose, which shields the CDs from burning. However, a longer time for polymerization diminishes the amount of evolved NH3 and starts burning CDs. CN/G(0.5) is a good HER photocatalyst as revealed from 4 h of uninterrupted HER.31

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Zhou et al. changed glucose with citric acid and fabricated the composite structure. The process lead to a condensation polymerization of –COOH groups of citric acid and –NH3 of urea and results in N-doped sp2 graphitic clusters over CN. They measured H2 evolution under visible light with 10 vol.% triethanolamine (TEA) as the hole sacrificial agent and 3 wt.% Pt as the co-catalyst. CN containing 20 mg of glucose releases 64 mmol h1 of H2, which is nearly 4.3 times more than the 15 mmol h1 in CN. The composite catalyst is stable for four cycles of reaction. The photocurrent response is more robust in the composite than in pristine CN, indicating a facile charge excitation and separation. Instead of a single element doping, doping of more than one element is reported to benefit the HER process. Si et al. coupled N, P co-doped carbon nanoparticles (NPCP) with CN.39 Hydrogen evolution is studied under visible light with triethanolamine (TEOA) as a sacrificial donor. A comparison is made between Pt-loaded and unloaded samples. The Pt-loaded samples are Pt-g-CN, Pt-CN/NPCP-x (x ¼ 0.05, 0.5, 2.5 mg), Pt-CN/PCP-x, Pt-CN/CP-x, where x ¼ 0.05, 0.5, 2.5 mg. The samples without Pt are CN/NPCP-0.5, CN/PCP-0.05, and CN/CP-0.05. The H2 evolution for CN is 2511 mmol g1 after 3.1 h of irradiation. The evolved H2 increases in the composite and reached a maximum of 6200 mmol g1 for Pt CN/NPCP-0.5. The evolution of H2 reduces to 4448 mmol g1 in Pt-CN/NPCP-2.5. Similarly, Pt-CN/PCP-0.05 shows 5357 mmol g1, and Pt-CN/CP-0.05 is 2181 mmol g1. Interestingly, while Pt-CN gives 2511 mmol g1 H2, pristine CN without Pt releases only 39 mmol g1. Similarly, CN/NPCP-0.5, CN/PCP-0.05, and CN/CP-0.05 generate 1011, 1076, and 12 mmol g1 of H2. The composites’ surface contributes to the overall H2 generation to a greater extent than the photoinduced charge separation.39 Wang et al.28 measured photocatalytic H2 evolution under UV light (365 nm) with 10 vol.% aqueous lactic acid solutions as the sacrificial agent. CN/CDs-10 wt.% shows an H2 production rate of 2.2 mmol h1, whereas CN shows only 0.5 mmol h1 H2 production. Furthermore, an increase in CD-loading to 50 wt.% and 100 wt.% results in a decrease in H2 evolution to 1.5 mmol h1 and 0.75 mmol h1, respectively. When Pt is used as a co-catalyst, the HER for 10wt.% CDs/CN increases to 183 mmol h1. For Pt-CN, the HER is 85.4 mmol h1. When the evolution is measured in water without any sacrificial agent but with Pt as the co-catalyst, the production rate of H2 is 2.75 mmol h1. Pt-CDs/CN-10 wt.% generates 48.1 mmol h1 of H2 under 420 nm visible excitation. Photoexcitation results in the generation of photogenerated carriers in the CB of CN. These carriers transfer to CDs. The electron reduces H1 to H2.28 During the photoelectrochemical (PEC) water splitting experiment, photogenerated H2O2 poisons the pristine CN and diminishes its activity in H2 production. However, when CDs are added to CN, the former decomposes H2O2, thus recovering the photocatalytic activity of the composites.36 The H2O2 decomposing activity of pure and CDs/CN is monitored by determining the photocurrent’s temporal change. Upon shining light and with an increase of time, the dark current decreases and reaches the value zero (Figure 8.6–1). The reduction in photocurrent upon longer light exposure

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Figure 8.6

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Temporal change of photocurrent of CN and CN/CDs composites under dark and light. Photocurrent for (a) pristine CN, (b) CN/CDs-0.05, (c) CN/ CDs-0.05-H2O2, and (d) recovered CN/CD-0.05. (b) Photocatalytic H2 evolution of CN/CD-0.05 in a reaction medium with 100 mmol of H2O2. (c) The ratio of H2 production and H2O2 decomposition. Reproduced from ref. 36 with permission from Elsevier, Copyright 2018.

time signifies the poisoning effect of the produced H2O2. When CDs are added, there is a fluctuation in current but it does not shift to zero (Figure 8.6–2). The fluctuation in photocurrent indicates balancing between the produced and eliminated H2O2. When this added H2O2 interacts with CN at the initial reaction time, it poisons CN and reduces the photocatalytic activity indicated by the reduced photocurrent (Figure 8.6–3). However, as the reaction continues for a longer duration, CDs decompose H2O2 and regain an oscillating photocurrent (Figure 8.6–4). Photocatalytic H2 evolution is measured with TEOA as a hole scavenger to prevent H2O to H2O2 transformation. The poisoning effect of H2O2 is reflected in H2 production. In the absence of any evolved H2O2, the H2 evolution recorded was 27.6 mmol h1. After adding 100 mmol H2O2, H2 evolution reaches 8.5 mmol h1 (Figure 8.6b). With the reaction time, two-third of CDs are utilized for H2 production, and one third is used for H2O2 decomposition (Figure 8.6c).36 Theoretically, the H2 evolution mechanism in the CDs/C3N composite has been described, and the mechanism is depicted in Figure 8.7.40 Photoexcitation of the CDs/C3N layer generates energetic electrons in the inner CDs, while holes are accumulated on the C3N layer. The holes on the outer C3N layer induce proton production. Following a concentration gradient, the protons diffuse through the outer to the inner layer of C3N passing a low

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Figure 8.7

The structural models of (a) C3N and (b) carbon quantum dots (CQDs or CDs). (c) The mechanism of H2 production in the C3N/CDs composite. Reproduced from ref. 40 with permission from the Royal Society of Chemistry.

potential barrier of 2.3 eV. These protons reach the reduction sites, interact with electrons and produce H2 on the inner-side of the CDs. The C3N layer does not allow oxygen-related products produced during water splitting to reach CDs. Once H2 is produced over CDs, the evolved gases cannot move back to the C3N layer. It is due to the large barrier height of 4.5 eV at the CDs/C3N interface.36

8.3.2

Graphene Quantum Dot–Graphene Composite (GQDs/G)

An active NGQD/GO composite photocatalyst is fabricated by mixing NGQD, GO, and 3 wt.% Pt. Initially, N doped graphene (N–G) is synthesized by passing NH3 gas through GO at 500 1C for 3 h. This N–G is further oxidized in concentrated HNO3 for 12 h to obtain NGQD. In NGQD, N atoms are distributed on the graphene periphery forming pyridine or pyrrolic functionalities.9 One of the reports shows that the oxidation process of N–G leads to the generation of NGQD on the graphene oxide sheet.41 They have shown that a separate addition of GO is not necessary. The nitrogen-doped graphene oxide quantum dots (NGO-QDs) acquire both n-and p-type conductivity and form a p-n junction. The composite possesses a band gap of 2.2 eV, which is quite suitable for the H2 evolution reaction. In the composite, Pt acts as a co-catalyst for H2 evolution.41

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One-pot hydrothermal synthesis of GQD/graphene. Reproduced from ref. 42 with permission from John Wiley & Sons, Copyright 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

An in situ deposition of boron-doped GQD (BGQD) on the graphene sheet is accomplished by the hydrothermal method. The process involves first mixing of glucose, boric acid, and a few mL of H2SO4 in water.42 The mixture was sonicated for a few minutes to ensure complete dispersion of the components. To this dispersion, GO was added, and the final mixture is treated hydrothermally at 180 1C for 24 h. The product was soaked in deionized water (DI) for 12 h for the removal of impurities. GO is finally reduced by the treatment with hydrogen iodide (HI) at 40 1C for 5 h (Figure 8.8). The final composite of the BGQD/graphene aerogel is collected by washing and vacuum drying. The boron content in the composite is changed from 1.8 to 5.4% by varying the boric acid concentration. GQDs act as an intercalation surfactant and leads to exfoliation of bulk graphite powder to 2D nanosheet.43 GQDs are intercalated on the edges of the graphite and provide stable aqueous dispersion to graphite and its subsequent conversion to graphene sheets. The process leads to 0D/2D van der Waals heterostructures. The composite has –OH functionalized GQD, which is synthesized by hydrothermal treatment of nitronapthelene in NaOH at 200 1C for 8 h. N and S-doped GQD is prepared by replacing NaOH in the medium with ammonia and thiourea. Graphite powder was added to 50 mL dispersion of GQD and sonicated for 2 h. The final product of the GQD/graphene nanosheet is obtained by centrifugation. GQD/graphene composites function as an efficient photocatalyst for H2 evolution reaction. One study shows H2 evolution from aqueous TEOA solution with Pt as a co-catalyst. The experiment is performed in the range of 420–800 nm.9 The photocatalytic measurement shows a slight difference in the HER between GQD and NGQD/GO composite. A pristine NGQD shows HER of 10 mmol h1, whereas NGQD/GO shows HER of 10.8 mmol h1, respectively. The AQY measured at 420 nm is 14.8% for NGQD and 16% for NGQD/GO. NGQD/GO shows excellent photocatalytic stability up to 96 h of illumination. In the case of NGQD, the HER reduces by half after 96 h of irradiation (Figure 8.9a).9 The mechanism for H2 production involves photoinduced carrier generation in NGQDs and subsequent transfer to GO (Figure 8.9b). NGO-QDs forming an n-p type of heterojunction shows an enhanced photocatalytic H2 evolution under visible light without the aid of any sacrificial agent or co-catalyst. The H2 evolution with NGO-QDs for 9 h

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(a) Amount of H2 evolved as a function of reaction time for NGQDs and NGQD/GO composites. (b) Transfer of photogenerated electrons from NGQDs to the reduction sites of GO with subsequent H2 production. Reproduced from ref. 9 with permission from Elsevier, 2016. (c) A Z-scheme electron and hole transfer mechanism at the n–p interfacial junction in NGO-QDs. H2 is evolved on the p-type and H2O oxidation occurs on the n-side. Reproduced from ref. 41 with permission from John Wiley & Sons, Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

irradiation is 5.5 mmol, which is twice that of GO-QDs.41 The heterojunction forms a Z-scheme photocatalytic system. In NGO-QDs, small sp2 clusters act as an interfacial junction for majority electron and hole recombination from the n and p-type of domains. The Z-scheme recombination leaves excess electrons on the CB in the p-side and holes on the VB of the n-side. The electrons facilitate H2 production, and holes lead to water oxidation (Figure 8.9c). H2 evolution performance of the van der Waal system is compared with their pristine and doped counterparts by measuring the Tafel slopes.43 Nitrogen and sulfur are co-doped into GQD. NS-GQD/graphene has a comparatively smaller Tafel slope of 71.2 mV dec1 in comparison to pristine GQD (395.6 mV dec1), GQD/graphene (167.3 mV dec1), N-GQD/graphene (119.1), and S-GQD/ graphene (98.5 mV dec1). A small Tafel slope signifies an efficient H2 evolution performance. In the van der Wall structure, photogenerated holes migrate from GQD to graphene, while photogenerated electrons move from GQD to graphene. NS-GQD/graphene shows a strong photoresponse without any external bias voltage. Moreover, the Faradaic efficiency shown by NS-GQD/graphene

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is 95.7%. The Schottky barrier height in pristine GQD/graphene is 0.32 eV, whereby GQD is a p-type side. However, after doping of N and S, GQD transforms from p-type to n-type with a further reduction in the Schottky barrier height. Thus, a reduced barrier height speeds up the electron transfer across the barrier.43

8.3.3

Graphene Quantum Dot–Graphitic Carbon Nitride Composite (GQD/CN)

GQDs and mesoporous graphitic CN (mpg-CN) are coupled by approaching the solvent evaporation technique. A homogenous mpg-CN solution is first prepared by dissolving mpg-CN into ethanol under sonication.44 The ethanolic dispersion of GQD and CN are mixed and stirred at 80 1C for 12 h. The evaporation of ethanol leaves behind the composite catalyst. For the pristine GQD preparation, a certain amount of pyrene is mixed with hot HNO3 at 80 1C for 12 h. The product tri-nitropyrene (TNP) is washed and dispersed in NaOH. The alkaline dispersion is hydrothermally treated at 200 1C for 12 h. Finally, hydroxylated GQDs are obtained by membrane filtration. Pristine mpg-CN is synthesized by heating a mixture of cyanamide and SiO2 at 60 1C for 2 h. Finally, calcination of the resultant mixture at 550 1C in N2 flow results in mpg-CN.44 Carbonyl and carboxylic acid groups on the GQD surface are attached with sulfur-decorated carbon nitride nanosheets in a hydrothermal route of fabrication. Sulfur doped CN is dispersed in GQD solution and hydrothermally treated at 230 1C for 3 h. A stronger interfacial connection is established by calcination of the composite at 400 1C for 1 h in Ar.45 A dispersion containing the prepared GQD and CN are treated hydrothermally at 150 1C for 4 h to obtain GQD-decorated CN nanorods (Figure 8.10). N-doped GQD-decorated porous CN composites are fabricated by hightemperature calcination. In this process, N-GQD and porous-CN are dispersed in water and evaporated at 60 1C.46 The dispersion was calcined in air at 500 1C and 580 1C for 2 h to obtain the composite photocatalyst. While pristine CN shows a band gap of 2.74 eV, the band gap is reduced to 2.48 eV after loading a maximum of 1.5 wt.% of GQD onto CN. The introduction of carbonrelated sub-band states is responsible for an effective lowering in the band gap.4 Instead of taking a prepared CN nanosheet for making the composite with NGQD, the precursor for CN, such as urea, is also taken in an in situ approach.47 Aqueous dispersion of urea and NGQD are mixed and heated at 100 1C for the evaporation of water. The solid powder is taken in a crucible and calcined at 550 1C. The high-temperature calcination converts urea to CN, which is firmly held onto NGQD. Pristine NGQDs are prepared by hydrothermal treatment of a citric acid and urea mixture. NGQDs contain –NH2, –COOH, and –OH, surface functionals, which provide GQD with a better dispersion in water. These NGQDs have a size of 8 nm and contain nearly 1–3 layers. In the NGQD-CN composite, the N-lone pair electrons on NGQD form a p–p–p network channel with CN thus providing easy transport of photogenerated carriers.47

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(a) Fabrication method for sulfur-doped CN and GQD hybrid (s-g-C3N4@GQD) catalyst. (b) STEM images of the composite and the corresponding (c) elemental mapping of carbon, nitrogen, and sufur. Reproduced from ref. 45 with permission from the Royal Society of Chemistry.

CN nanosheets show weak ultrasonic dispersion in water even after a prolonged sonication of 12 h. After adding GQDs, CN forms a stable dispersion in water. A stable p–p interaction occurs between graphene layers and CN nanosheets. The electrostatic interaction occurs between –OH groups on GQD with terminal –HN groups on CN.48 The GQD/CN composite forms a p-n heterojunction if P is doped into GQD. The electron-donating nature of –OH groups on GQD induces an n-type conductivity into GQD. However, after surface P doping, P binds with the –OH groups on GQD. The PQO bonds, thus formed, have an electron-withdrawing nature, thus giving p-like characteristics to GQD. Similarly, by its n-type behavior, CN nanosheets combine with GQDs to form a p-n heterojunction.48 Instead of GQD, if P is doped into tubular-CN (TCN), one of the C-atoms in the s-heptazine ring is replaced with P forming P–N bonding.49 P-TCN forms a stable dispersion with GQDs in aqueous solution. The final GQD/P-TCN composite is obtained by a freezedrying approach marinating 24 h reaction duration. Photocatalytic H2 evolution is measured for thermally calcined composites as well as for physically mixed NGQD-CN. The NGQD(1.66%)/CN composite

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47

shows HER of 139.6 mmol h under visible light. It is around 7.7 times higher than that of pristine CN (18.2 mmol h1). Physically mixed NGQD-CN shows 19.3 mmol h1 HER, which is similar to that of pristine CN. GQD (1.66%)/CN shows HER of 54.9 mmol h1. It is less than that of NGQD/CN. Photocatalytic HER for NGQD(1.66%)/CN is 21.3 mmol h1 under 420  15 nm irradiation. GQD (1.66%)/CN shows HER of 10.8 mmol h1 and physically mixed NGQD (1.66%) þ CN shows HER of 6.6 mmol h1 and pristine CN shows 5.6 mmol h1. NGQD-CN shows absorption above 500 nm; hence, excitation above 520 nm is chosen. Pristine CN shows less than 0.5 mmol h1 HER. The physically mixed one is also inert because of the weak electron transfer. GQD1.66/CN shows 2.8 mmol h1 at 520 nm  15 nm and 0.6 mmol h1 at 550  15 nm excitation. The HER shown by NGQD1.66/CN is 11.1 mmol h1 at 520  15 nm and 4.2 mmol h1 at 550  15 nm.47 P-doped TCN(PTCN) shows HER of 64.2 mmol h1. It is 6 times higher than that of bulk CN (BCN) 11.8 mmol h1.49 The composite PTCN/GQD-0.15 shows HER of 112.1 mmol h1 (Figure 8.11a). It is more than 9 times that of BCN.

Figure 8.11

(a) The H2 production rate shown by BCN, PTCN and PTCN/GQDs with different amounts of GQD loading. (b) Stability test for 5 cycles. (c) Photocurrent response recorded for the samples under visible light. (d) Nyquist plot showing the lowest charge transfer resistance in PTCN/GQDs. Reproduced from ref. 49 with permission from John Wiley & Sons, Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Higher loading of GQD decreases the rate of H2 evolution in the composite because of the decrease in the reactive catalytic sites.48 The photocatalytic stability in the composite remains even after running for 5 cycles (Figure 8.11b). The samples also show a stronger photocurrent response than that of bulk CN (Figure 8.11c). In PTCN, P forms a P–N bond with the triazine ring by replacing one of the C-atoms from the triazine ring. By P-doping, we expect to have better electron delocalization over the composite system. The electrochemical impedance spectra show a fast charge transfer in the composite catalyst (Figure 8.11d).49 Because of the upconversion properties, GQD serves as an excellent photosensitizer in CN photocatalysis. The electronic coupling can result in a narrowing in the band gap. Hence, even though CN has an absorption edge in the visible region, it shows HER of 1.01 mmol h1 g1.50 HER increases to 2.18 mmol g1 h1 after incorporating NGQD. The composite photocatalyst shows 5.25% apparent quantum efficiency (AQE) at 420 nm. The contribution of PL upconversion is evident from the photocatalytic results above 590 nm. Pristine NGQD and CN are inefficient to produce H2 above 590 nm, but the composite does produce H2. The GQD content of 10 wt.%, 15 wt.%, and 20 wt.% causes a rise in the HER from 8 mmol g1 h1, 13.3 mmol g1 h1, to 10.31 mmol g1 h1. It thus confirms that GQD absorbs at 600–800 nm and emits at 400–600 nm. The emitted light is reabsorbed by CN to perform the H2 evolution.50

8.3.4

Carbon Nitride Quantum Dot–Graphene Nanocomposite (CNQD/G)

In CNQD-graphene composites, monolayer CNQD serves many benefits compared with that of bulk CNQD. Monolayer CNQD firmly holds onto graphene through p–p stacking interactions. In CNQD/graphene composites, CNQD provides numerous catalytic active N-sites and suffers less resistance for electron collection from graphene.51,52 Most of the reports on the fabrication of CNQD–graphene composites are based on the hydrothermal method.6,7,51–53 Monolayer CNQD with a thickness of 0.4 nm and diameter 31 nm are synthesized via a modified Hummers method.51 The process includes oxidation and exfoliation of bulk CN, followed by the removal of unexfoliated CN from monolayer CNQD. An aqueous dispersion of graphene oxide and CNQD are mixed and treated hydrothermally at 220 1C for 3 h to decorate CNQD over the graphene sheet. CNQD/graphene forms an interconnected three-dimensional (3D) porous architecture.51 The fabrication process is shown in Figure 8.12. Instead of taking aqueous dispersions of both CNQDs and graphene, an alcoholic dispersion of GO and an aqueous dispersion of CNQDsare treated hydrothermally at 180 1C for 24 h. The final product is obtained by freezedrying at 48 h to obtain CNQD dispersed over 3D graphene.52 Atomicallythin CNQD/Graphene composites can also be obtained by calcination of the product obtained from hydrothermal treatment under an Ar atmosphere at 400 1C for 2 h. Ultrasonication treatment of bulk CN fabricates monolayer

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Scheme showing the fabrication process of CNQD/graphene composites. Reproduced from ref. 51 with permission from the Royal Society of Chemistry.

CNQD. In the composite, the CNQD layer has a thickness of 0.4 nm, and the graphene sheet has a thickness of 1 nm.7 Zhao et al.53 decorated boron (B)-doped CNQD over a CN/graphene composite by hydrothermal treatment. BCNQD, protonated CN nanosheets, and GO are mixed in water and ultrasonically treated. The final dispersion is subjected to hydrothermal treatment at 150 1C for 2 h. The BCNQD/CN/ graphene possesses a higher surface area (52.1 m2 g1) than that of CN/GO (43.1 m2 g1), and CN (32.94 m2 g1).53 Huang et al.6 synthesized CNQD/CN/ GO composites. The group adopted a compressed molding method for CNQD preparation. CNQD/GO composites were prepared by hydrothermal treatment of an aqueous dispersion of GO and CN at 180 1C for 6 h. CNQD/ CN/GO was prepared by adding CNQD solution on a CN/GO dispersion and allowed to stay for 10 h. Finally, freeze-drying results in the formation of the CNQD/CN/GO ternary composite.6 CNQD/graphene is an excellent choice as a catalyst for H2 evolution. Its efficiency as an electrode for H2 evolution is evaluated by measuring the overpotential and Tafel slope. The monolayer composite shows an overpotential of 150 mV at a current density of 10 mA cm2.7 In comparison, CN nanosheets conjugated with graphene show a much larger overpotential (Figure 8.13a). The electron transfer from CNQD to graphene is a fast

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process, as evident from impedance measurements (Figure 8.13b). CNQD/ graphene acquires a small Tafel slope (53 mV dec1). In comparison, pristine CNQDs, graphene, CN nanosheets, and CN/graphene show Tafel slope values of 185 mV dec1, 167 mV dec1, 164 mV dec1, and 138 mV dec1 (Figure 8.13c).7 Chronoamperometric measurement shows no apparent loss in current density after 10 h of continuous running. The experiment and theory complement the evolved hydrogen gasses with nearly 100% Faradaic efficiency (Figure 8.13d). Photocatalytic H2 evolution is measured for CNQD/CN/GO ternary composites under full solar light as well as under selected 700 nm excitation with 2 wt.% Pt and anhydrous ethanol as the sacrificial agent.6 Pristine CN, CNQD, and graphene oxide aerogel show HER of 76.92 mmol h1, 6 mmol h1, and 60.61 mmol h1, respectively. A sharp rise in the HER to 617.59 mmol h1 occurs in CN/GO composites. The addition of CNQD to the CN/GO composite increases the HER to 1230.72 mmol h1. At 700 nm excitation, CN does not show any H2 evolution activity. The HER in CNQD@CN-GO is 2.3 mmol h1. The AQY of CNQD@CN/GO at 420 nm is 13%, at 450 nm it is 2%, at 650 nm it is 0.07%, and at 700 nm it is 0.116%. Under solar light, the strong photocatalytic activity in CN/GO is due to the light absorption, charge excitation, and interfacial carrier separation. However, when CNQD is added to CN/GO, an additional activity, known as luminescence upconversion, contributed to the enhanced photocatalytic activity. CNQDs show upconversion emission

Figure 8.13

(a) Polarization curve for HER at a scan rate of 10 m V s1. (b) Electrochemical impedance analyzer and (c) Tafel plot for CN nanosheets, CNQDs, graphene (G), CNQDs þ G, CN/G and CNQDs/G in 0.5 M H2SO4. (d) Chronoamperometric response of CNQDs/G at 0.11 V vs. RHE. Reproduced from ref. 7 with permission from American Chemical Society, Copyright 2018.

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between 350–550 nm upon excitation by light of a wavelength of 650–800 nm. At 725 nm excitation, no fluorescence is seen in CN/GO, but the light is measured at 450 nm in CNQD/CN/GO. The light emitted by CNQD is reabsorbed by CN for creating electron–hole pairs. Hence, the luminescence upconversion in CNQD is also responsible for the strong photocatalytic activity and high AQY at 700 nm.6

8.4 Conclusion We have summarized the fabrication of carbon nanodot composites with graphene, graphitic carbon nitrides, and their applications in the solar powerdriven water reduction reaction. Graphene provides an excellent charge transport channel for carriers but displays weak visible light absorption activity. Similarly, graphitic carbon nitrides possess a semiconducting bandgap but suffer from poor absorption of visible light and rapid carrier recombination. Carbon nanodots comprising carbon dots, graphene quantum dots, and graphitic carbon nitride quantum dots possess a large amount of surface functional groups, good dispersion in water, and an excellent electron relay center. The unique photoluminescence upconversion properties make them a superb photosensitizer for broadband solar light harvesting. Thus, carbon nanodot-based composites show an efficient photocatalytic hydrogen evolution reaction in the visible and near infra-red region.

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34. Y. Wang, F. Wang, Y. Feng, Z. Xie, Q. Zhang, X. Jin, H. Liu, Y. Liu, W. Lv and G. Liu, Dalton Trans., 2018, 47, 1284. 35. Y. Guo, P. Yao, D. Zhu and C. Gu, J. Mater. Chem. A, 2015, 3, 13189. 36. D. Qu, J. Liu, X. Miao, M. Han, H. Zhang, Z. Cui, S. Sun, Z. Kang, H. Fan and Z. Sun, Appl. Catal., B, 2018, 227, 418. 37. J. Feng, G. Liu, S. Yuan and Y. Ma, Phys. Chem. Chem. Phys., 2017, 19, 4997. 38. L. Li and X. Zhu, ACS Appl. Nano Mater., 2018, 1, 5337. 39. Y. Si, Z. Lv, L. Lu, M. Liu, Y. Wen, Y. Chen, H. Jin, J. Liu and W. Song, Appl. Surf. Sci., 2019, 491, 236. 40. X. Wang, X. Jiang, E. Sharman, L. Yang, X. Li, G. Zhang, J. Zhao, Y. Luo and J. Jiang, J. Mater. Chem. A, 2019, 7, 6143. 41. T. Yeh, C. Y. Teng, S. J. Chen and H. Teng, Adv. Mater., 2014, 26, 3297. 42. T. V. Tam, S. G. Kang, M. H. Kim, S. G. Lee, S. H. Hur, J. S. Chung and W. M. Choi, Adv. Energy Mater., 2019, 9, 1099945. 43. Y. Yan, D. Zhai, Y. Liu, J. Gong, J. Chen, P. Zan, Z. Zeng, S. Li, W. Huang and P. Chen, ACS Nano, 2020, 14, 1185. 44. J. Liu, H. Xu, Y. Xu, Y. Song, J. Lian, Y. Zhao, L. Wang, L. Huang, H. Ji and H. Li, Appl. Catal., B, 2017, 207, 429. 45. C. Xu, Q. Han, Y. Zhao, L. Wang, Y. Li and L. Qu, J. Mater. Chem. A, 2015, 3, 1841. 46. C. Wang, Y. Zhou, Y. Sun, G. Guo, Q. Fu, Z. Xiong and Y. Liu, Diamond Relat. Mater., 2018, 89, 197. 47. Z. Mou, C. Lu, K. Yu, H. Wu, H. Zhang, J. Sun, M. Zhu and M. C. Goh, Energy Technol., 2019, 7, 1800589. 48. J. Qian, J. Yan, C. Shen, F. Xi, X. Dong and J. Liu, J. Mater. Sci., 2018, 53, 12103. 49. Y. Gao, F. Hou, S. Hu, B. Wu, Y. Wang, H. Zhang, B. Jiang and H. Fu, ChemCatChem, 2018, 10, 1330. 50. J. P. Zou, J. C. Wang, J. Luo, Y. C. Nie, Q. J. Xing, X. B. Luo, H. M. Du, S. L. Luo and S. L. Suib, Appl. Catal., B, 2016, 193, 103. 51. X. Wang, L. Wang, F. Zhao, C. Hu, Y. Zhao, Z. Zhang, S. Chen, G. Shi and L. Qu, Nanoscale, 2015, 7, 3035. 52. H. Yuan, J. Liu, H. Li, Y. Li, X. Liu, D. Shi, Q. Wu and Q. Jiao, J. Mater. Chem. A, 2018, 6, 5603. 53. J. Zhao, Y. Liu, Y. Wang, H. Li, J. Wang and Z. Li, Appl. Surf. Sci., 2019, 470, 923.

Section 4: Fullerene Clusters

CHAPTER 9

Clusters of Fullerenes KLAVS HANSEN*a AND HENNING ZETTERGRENb a

Center for Joint Quantum Studies and Department of Physics, School of Science, Tianjin University, 92 Weijin Road, Tianjin 300072, China; b Department of Physics, Stockholm University, SE-106 91 Stockholm, Sweden *Email: [email protected]

9.1 Introduction Carbon is the element that is represented in the largest number of chemical compounds, naturally occurring or produced by humans. Most of these also contain other types of atoms, primarily hydrogen, but often also oxygen, nitrogen, sulfur and sometimes atoms of metallic elements in biological molecules. In the realm of nanoscale materials, pure carbon substances have appeared in no less than three new forms discovered during the last few decades; closed cage fullerene molecules,1 carbon nanotubes,2 and monolayer graphitic graphene sheets,3 representing zero, one and twodimensional all-carbon materials. Both the carbon nanotube and graphene allotropes are subjects of intense research due to their special electronic properties and their potential for use in electronic devices. The fullerene molecules have found applications in chemically modified forms where added functional groups have been used to tailor functions, both as nanomedicine4 and for light harvesting molecules in solar cells.5 In a more fundamental scientific context, advances in spectroscopic technology have lead to the identification of both neutral6 and cationic7 fullerenes in space, boosting the interest in their formation and destruction processes, and their potential roles as building blocks of interstellar dust grains. All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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The carbon fullerene molecules are fairly resilient and can aggregate to form weakly bound clusters of intact molecules. This opens the possibility for creating size-selected carbon compounds at a size that is large compared to most molecular sizes but still small on the scale of the increasingly smaller components in electronic nano-devices. The formation, structure, energetics and observed reaction channels of such clusters will be presented here.

9.2 Fullerene Building Blocks Fullerene molecules come with an even number of atoms. The class of molecules is named after the architect Buckminster Fuller who designed buildings with a supporting frame of hexagons and pentagons. The term therefore refers to the geometric structures composed of hexagons and pentagons, and not specifically to the carbon variety, although this is the only element for which macroscopic amounts of molecules with fullerene structures have been produced. Carbon fullerenes have been observed in gas phase and molecular beams from sizes of C20 and up. No upper limit has been established. Fullerenes were predicted theoretically years before their discovery,8,9 but it was only when they were discovered in molecular beams1 that general interest arose. The interest exploded when a method for production of macroscopic amounts was developed a few years later,10 and it was further boosted by the discovery that solid fullerites, as the fullerene molecule crystals are called, become superconducting when intercalated with six alkali atoms.11 The fullerene production method discovered is based on an arc discharge between carbon electrodes, yielding between 10 and 20% fullerenes in the produced soot, mainly in the form of C60, with a smaller amount of C70, and small amounts of higher fullerenes, mainly C76, C78, and C84.12 The carbon nanotubes were discovered on inspection of the spent electrodes from this process.2 Carbon nanotubes are presently produced with the help of designed catalysts, and the formation is reasonably well understood in general terms, albeit not well controlled in all details.13 In contrast, the precise production mechanism of the fullerenes is not entirely understood, although empirical rules allow for optimization of production yields. They may form from smaller bowl-or ring-shaped clusters by aggregation of stray carbon atoms or molecules.14 The addition channel was very convincingly demonstrated to be present even for the otherwise closed fullerene structures15 with a 13C-isotope addition technique. The possibility of production of the particularly stable species along the ‘shrinking hot giant road’, i.e. a top-down approach, was given in ref. 16. In our opinion the production most likely involves both aggregation and shrinking, akin to other forms of nucleation, but with attraction points at sizes corresponding to the most stable species. The selection of specific sizes will be assisted by efficient cooling through the strongly size- and

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structure-dependent thermal radiation which is emitted from fullerenes at excitation energies encountered in their production process.17 The directional covalent bonds of carbon atoms restrict the possible geometries of the molecules strongly. Representing the carbon atoms as the vertices in a molecule, the bonds as the edges and the surfaces spanned by the position of the atoms as the surfaces, the number of pentagons and hexagons in any fullerene composed of only these two surface geometries can be calculated with Euler’s formula, f  e þ v ¼ 2,

(9.1)

where f, e, and v are the numbers of faces, edges, and vertices. The number of vertices v ¼ N (the number of carbon atoms), edges e ¼ 3N/2 and faces f ¼ p þ h (the sum of the number of pentagons and hexagons), together with N ¼ 5p/3 þ 2h, gives the number of pentagons as 12, irrespective of the size of the molecule. It also follows that fullerenes have an even number of atoms. The number of hexagons is N/2  10, and the smallest fullerene is therefore the 20 atom-all-pentagon truncated icosahedron, which has been observed in the gas phase.18 The presence of pentagons introduces a positive curvature in the carbon skeleton, which is the geometrical reason the molecules can be closed structures. Also geometries with heptagons are allowed by Euler’s theorem and by carbon chemistry. A heptagon introduces a negative curvature, i.e. a saddle point geometry. To cancel this in a closed structure, each heptagon requires the existence of an additional pentagon. As both add to the stress of the structure and raise the energy, heptagons tend to be annealed out in the highly energetic fullerene production processes. They do occur in carbon nanotubes, where they will adjust the diameter of the tube in conjunction with a pentagon partner. The number of possible isomers of fullerene structures can be calculated numerically with methods based on their topological classification. The counting can be done with the Spiral code.19 For C60 the number of distinct hexagon and pentagon containing isomers is 1812. A small subset of isomers have no adjacent pentagons, and N ¼ 60 and 70 are the two smallest fullerenes that have any such isomer, with one each. It is a striking fact that when nuclear magnetic resonance studies on the naturally occurring 13C isotope were performed, the signal indicated only one type of site for the carbon atoms in C60, demonstrating that all sites were equivalent with respect to the chemical environment, and hence that only the single isolated pentagon species was present in the sample.20,21 Only for this structure are all sites equivalent. The experiment was performed with the natural abundance of the carbon isotopes. This contains about 1% of 13C, making the average content of that isotope in C60 close to one atom. The high stability of neutral C60 has been rationalized in terms of the isolated pentagon rule.22 The rule states that fullerenes with isolated pentagons are the lowest energy isomers. The price of two adjacent pentagons in a fullerene cage has been calculated to approximately 1 eV of

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strain. The rule is also reflected in the mass spectra of the soot produced in the arc discharge where C60 is the most and C70 is the second most abundant molecule. 12 isolated pentagons account for 60 bonds, and for C60 the remaining bonds are each shared between two hexagons, connecting two pentagons. The lengths of these two types of bonds are quite similar, albeit not identical, 1.46 Å for the bonds shared by pentagons and hexagons, and 1.40 Å for the hexagon bonds that connect two pentagons.24 These values are fairly size-independent. For C60 the distances of all carbon atoms to the center of the molecule are identical, and the molecule is for many practical purposes spherical. The icosahedral symmetry becomes more pronounced for very large fullerenes, and the radial distribution of atomic positions is no longer a delta-function for these. Nevertheless, the spherically averaged radii of most fullerenes encountered in practice are close to proportional to N1/2, and their surface areas are therefore approximately proportional to the number of atoms they contain and the volume of the carbon skeleton proportional to N3/2. The carbon skeleton of C60 has a radius of 3.5 Å. The radius of the molecule is close to 5 Å when interacting with another C60, with the added value due to the electron spill-out (see Figure 9.4). The electron affinities of a number of fullerenes have been measured to be 2.5 to 3 eV, with values for C60 of 2.68 eV25,26 and a similar value for C70. Although high in an atomic physics context, these electron affinities are similar to those of metal clusters of similar geometric sizes. For comparison, the work function of fullerite, the FCC crystal structure bulk matter of C60, is 4.7 eV,27 a little more than half the C60 ionization energy of 7.6 eV. The difference between the molecular ionization energy and the bulk work function, which parallels metals with their delocalized electrons, may suggest a strong degree of delocalization of electrons in the bulk fullerite, and a dissolution of the fullerene structure. However, the integrity of the molecules is conserved in the crystals to a large extent, as can be seen from the fairly low binding energy of the fullerene monomers to the fullerite crystals (1.6 eV28) compared with the lowest binding energy of the carbon atoms in the C60 molecule, which is 10.7 eV for C2 emission.29 The fullerene clusters show the same behavior as the fullerites in this respect and can likewise be considered as composed of intact molecules. Indirect evidence for this is seen when studying the fragmentation pattern of the clusters (see below).

9.3 Production, Geometry and Stability 9.3.1

Production

The molecular building blocks can be vaporized from an oven up to temperatures of a little over 500 1C. At higher temperatures they tend to polymerize in the crucible, a process which is possibly catalyzed by metals. The vapor pressure of C60 is 0.3 Pa at that temperature.30 Embedding such an oven into a liquid nitrogen-cooled inert atmosphere, usually helium,

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provides enough cooling to cause aggregation of the molecules and form the clusters: nC60-(C60)n

(9.2)

The reaction is schematic as written and does not require an n-body collision. Rather, the process is expected to be a sequential accumulation of molecules and/or small clusters on larger clusters. The presence of internal vibrational degrees of freedom into which the heat of formation can be dissipated allows the process to occur as a two-body collision. The cooling gas will remove this dissipated energy from the cluster before it can induce a thermal breakup. The lowest molecular vibrational frequency is that of the quadrupole deformation and has a value of 272 cm1 (¼0.034 eV), which should be compared with the binding energy of 0.28 eV for (C60)2. Another interesting comparison is the intermolecular vibrational frequency. For the dimer it has a quantum energy of 0.031 eV, as calculated with the intermolecular Girifalco potential (see below). The aggregation process is irreversible in these sources, as witnessed by the mass spectra measured with single photon ionization at above-threshold energies, often provided by an F2 excimer laser with a photon energy of 7.9 eV, compared with the 7.6 eV ionization energy of both C60 and C70. Figure 9.1 shows an example of such a mass spectrum.31 The above refers to cluster formation in the gas phase. It should be mentioned that clusters may also be formed in solution. This idea was used to explain anomalies in the solubility of fullerenes in organic solvents.32

Figure 9.1

A mass spectrum of (C60)n, produced cold and one-photon-ionized with 7.9 eV photons, followed by measurement in a time-of-fight mass spectrometer. Data from ref. 31.

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Clusters in solution exist in completely equilibrated systems in contrast to the gas phase production which will often produce clusters that are in internal thermal but not chemical equilibrium. The equilibrium situation in the liquid may facilitate applications, although the transfer from liquid to gas phase, which is often the preferred state for applications, presents a challenge.

9.3.2

Geometric Structures

An aggregation process involving loss of molecules would introduce structure in the abundances that reflect the size-dependent cluster stability. If a cold source is used to produce the clusters, the loss can alternatively be induced by exposure of the clusters to a laser pulse with photons with energies below the ionization threshold. Enough absorbed energy will cause unimolecular reactions which, with their stability dependent rate constants, will cause structure related stability to develop. Figure 9.2 gives an example of such a spectrum.31

Figure 9.2

A mass spectrum of initially cold (C60)n clusters ionized with a single 7.9 eV photon and heated with a subsequent nanosecond laser pulse of 4.03 eV photons and measured in a time-of-flight mass spectrometer. The strong and broad peak is the n ¼ 13 cluster. The heating causes substantial loss of single C60 molecules and acts as a developing agent of any contrasts in the size-to-size binding energies of the clusters. The large and asymmetric width of the n ¼ 13 peak is due to the fact that the evaporative process continues during acceleration in the time-of-flight mass spectrometer and hence smears the flight time of the detected cluster. The data were published in ref. 31.

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The appearance of a strong peak at n ¼ 13 in this spectrum gives a hint about the structure of the cluster. n ¼ 13 is the first closed shell of the packing of spheres into Mackay icosahedra. This was first seen in physical systems for rare gas clusters.33 This packing gives closed structures when the number of monomers equals 13, 55, 137, or ns ¼ 10K3/3  5K2 þ 11K/3  1, where K is the positive integer shell number.34 These shell closings provide particularly stable structures, which is reflected in the dissociation energies of the clusters. The dissociation energies (differential binding energies) determine evaporation rates, and laser heating and the resulting evaporation therefore enhances the intensity of the closed-shell cluster with 13 molecules, corresponding to the shell K ¼ 2 (the monomer is defined as shell number 1). Structures of this kind are generated by spherical and fairly sharp intermolecular potentials, and this indeed is the intermolecular potential for clusters of fullerenes (see Section 9.3.3). The packing of the molecules therefore closely resembles that of hard spheres. A contributing factor for the creation of the icosahedral shell structure for the small sizes here is that the C60 molecule is practically spherical. The C60 fullerite, with its FCC lattice structure, has a phase transition at 249 K, above which the molecular rotations are released.35 Below the transition temperature, the granularity of the fullerene shell is sufficient to lock the molecules into a specific angular configuration. Whether or not a similar locking is present in the clusters is unknown, but for most purposes the molecules behave as spheres. The assignment of the strong peak at n ¼ 13 to the icosahedral shell structure is confirmed by the observation of an intense peak at n ¼ 55, which is the next closed shell, as well as an intense peak at n ¼ 19.36 The n ¼ 19 cluster is a subshell closing, composed of an n ¼ 13 core plus six fullerene molecules, capping one of the vertices with one each in the five sites created by three molecules and the last in the crown of this ring of five.37 The assignment to the icosahedral symmetry is supported by theoretical calculations with the Girifalco potential.38 In addition to the binding energies of the ground state configuration, the entropies associated with different numbers of equivalent sites for different cluster sizes should be considered at the finite excitation energies used to develop the shell structure. This question was investigated for Ar and Xe clusters in ref. 39. The main result was that the shell amplitude was suppressed with a significant amount even for the clusters with relatively few monomers here. However, both n ¼ 13 and n ¼ 19 remained shell and subshell closings. Although these results are not directly transferable (cf. the difference between Ar and Xe clusters), the trend for fullerene clusters should be similar and the observed high intensity peaks for small clusters can with some confidence be taken as evidence for their geometric structure. For larger clusters, the entropic effects will complicate the interpretation of the mass spectra. They may also have interfered with the conclusions drawn from the quasi-equilibrium study in ref. 40 concerning the geometric structures at elevated temperatures, where only the energetics were considered. We also note that the frequency factor in the Arrhenius expression

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Figure 9.3

A mass spectrum of mixed (C60)n(C70)m clusters produced in a cold source from the arc discharge extract, and ionized and heated as in Figure 9.2. The fine structure of the peaks is caused by clusters of different mixtures of C60 and C70, grouped with the same total number of molecules, n þ m. Data from ref. 31.

for the breakup of (C60)2, 41010 s1, seems unrealistically small, and may suffer from the equilibration issues reviewed in ref. 41 and analyzed in detail in ref. 42. The dimer dissociation energy of 0.28 eV (for the neutral) appears to be quite realistic, though. By application of the laser heating technique, the contrast between the peaks of n ¼ 13 and n ¼ 14 can be increased up to a factor of eight.43 A calculation of the evaporative cascade shows that this contrast is produced by a difference in dissociation energy between (C60)13 and (C60)14 of only 20%. The surprisingly large contrast is caused by the large heat capacity of the clusters. The connection between heat capacities and abundance contrasts is well understood and is discussed in detail in ref. 44 and 45. It is of interest to explore the sensitivity of the shell and subshell closings to the nature of the components. Figure 9.3 shows two mass spectra similar to those of Figures 9.1 and 9.2 but prepared with a mixture of C60 and C70 in the ratio of 4 : 1, extracted from the soot produced in the arc discharge process and used without any further purification. Apparently, mixing C70 and C60 does not greatly affect the shell closings at these relatively small sizes. This result is confirmed by the theoretical calculations in ref. 46.

9.3.3

Stability: Theory

Standard Density Functional Theory (DFT) methods provide accurate results for the properties of isolated fullerenes, but they are not well-suited for

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describing the weak dispersion forces holding the fullerenes together in neutral clusters. More advanced quantum chemical calculation methods such as time-dependent DFT47 are thus needed for accurate predictions of cluster stabilities. Unfortunately these methods are extremely computationally demanding and thus only feasible for small clusters. One approach to simplify calculations relies on the assumption that the temperature is high enough for the molecules to rotate freely such that the interaction between neighboring fullerene molecules can be described by a spherical potential. An example is the Girifalco potential, where the individual fullerenes are treated as rigid spheres consisting of a uniform density of carbon atoms.48 Using a well-established expression for the interaction energy between two carbon atoms, this gives an analytical expression for the inter-fullerene potential that has been successfully used for a wide range of applications. The Girifalco potential for the neutral C60 dimer system is shown in Figure 9.4. The binding energy is 0.277 eV, which is in excellent agreement with a simple counting of bonds dividing the sublimation energy of bulk C60 fullerite31 and with the experimental dissociation energy of 0.275 þ 0.08 eV reported in ref. 40. The equilibrium distance is 19.0 a0 (10.1 Å). The potential depth is two orders of magnitude larger than that of two carbon atoms interacting by weak van der Waals forces.48 It is also unusually short ranged. The two half-minimum points of the potential are separated by 0.16 times the equilibrium distance. For comparison, the analogous number for the Lennard–Jones potential is 0.31 (see Figure 9.4). The short range results in a contribution from the second and greater nearest neighbor interactions that accounts for only a few percent of the total interaction energy in clusters. This means that for many applications one can assume that the fullerenes only interact with their nearest neighbors. Another commonly used two-body potential is the so-called PPR-potential, which is based on a fit to results from time-dependent DFT calculations.47 This potential is less steep than the Girifalco potential at short interfullerene distances (see Figure 9.4), and has been shown to give improved descriptions of fullerite properties such as e.g. its molar volume as a function of pressure.47 When singly charged, the dimer system becomes more stable and is bound by 0.372  0.08 eV according to the experiment reported in ref. 40. This is in good agreement with model calculations where the charge is localized on one of the fullerenes and the interaction is described as arising from the polarizability of metal spheres with the same radii,49 and also with results from more advanced quantum chemical DFT calculations where corrections due to dispersion interactions are a key ingredient.50 The equilibrium distance is essentially unaffected and a single charge is thus not expected to affect the geometries of the clusters. However, for a pair of charged molecules the global minimum on the potential curve is at infinite separation. The local minimum for the doubly charged cluster is already very shallow,49 and such systems will fission due to the Coloumbic repulsions. This is a general feature for any charge state, and

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Lennard Jones (LJ) Pacheco Prates Ramalho (PPR) Girifalco

Interaction energy [eV]

0.4

0.2

0

-0.2

8

10

12

14

16

18

20

Center-center distance [Å] Figure 9.4

Model interaction energies for C60 dimers as a function of the distance between the two molecular centers.

there is a smallest cluster size (appearance size) for which a cluster in a given charge state may survive (see Section 9.5.1). The precise shape of the inter-molecular potential has consequences for the existence of liquid fullerite,51,52 and therefore also for the existence of the finite size analog of melting in the clusters. No liquid phase of fullerites has been observed, although some numerical studies have suggested that it exists in a narrow temperature interval. Short range potentials as the interfullerene one will in general support more metastable states than long range potentials, i.e. local minima on the potential energy surface with energies above the ground state energy. This usually favors the presence of liquid state. However, the analysis in ref. 51 suggests that the short range of the potential can also make the liquid phase thermodynamically unstable, and hence explain the apparent absence of a liquid state.

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In yet another twist on this question, finite size systems tend to have reduced melting points, in particular systems in gas phase, and it is therefore possible that a liquid or pseudo-liquid phase of clusters of fullerenes exist.53

9.4 Fusing Clusters Clusters of fullerenes are thermodynamically unstable, like clusters in general, but unlike atomic clusters they are also unstable in the absolute sense, because the monomers are only metastable, at least for sufficiently large clusters. The lowest free energy state of bulk carbon under ambient conditions is graphite. The energy of a carbon atom in the C60 fullerene carbon allotrope is 0.40 eV above that of a carbon atom in graphite. The spontaneous conversion of a macroscopic piece of fullerite into graphite is prevented by the energy barrier separating the two structures, making the fullerene molecules and fullerite stable on long time scales at sufficiently low temperatures. Excitation of a fullerene cluster can produce one of two main results. Either the clusters will disintegrate into the fullerene molecule components, or they will fuse into larger, more tightly bound clusters. The dominant emission process of a fullerene cluster is loss of intact fullerene molecules. The internal conversion can result in an amorphous structure or a graphene sheet, or it can fuse into a larger fullerene. Most of these reaction channels are easily distinguished in molecular beam experiments. Emission of an intact fullerene is clearly observable as loss of the corresponding mass. The fusion products are distinguished by the subsequent post-process evaporation of small fragments. Fusion into a graphene sheet or an amorphous cluster appears as a mass spectrum with no preference for odd or even numbers of carbon atoms, whereas fullerene products only appear with even number of carbon atoms. The composition of the products is typically only measured after a fraction of a microsecond, and it is possible that highly excited non-fullerenes are converted into fullerenes between production and observation. If a fullerene is not the lowest free energy state initially, it may become so by evaporative loss of carbon atoms in what is effectively an annealing process. The possible net processes are summarized as (assuming for simplicity only C60 in the cluster):

where

P j

jmj ¼ m

(C60)n-(C60)n þ hv

(9.3)

(C60)n-(C60)n1 þ C60

(9.4)

(C60)n-(C60)nþ

(9.5)

þe



(C60)n-C60n2m þ mC2

(9.6)

ðC60 Þn ! Cn60m þ m1 C þ m2 C2 þ . . . ;

(9:7)

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The reaction in eqn (9.3), assumed to be thermal here, is expected to be the dominant channel only on very long time scales due to its expected small rate constants, corresponding to times much longer than seconds. It will only act as a cooling channel and will be disregarded in the following. The reaction in eqn (9.4) has already been illustrated above for the determination of the geometric structure. This is also a relatively low energy reaction, due to the activation energy which is on the order of 1 eV, corresponding to reactions on ms to ms time scales at room temperature. The systematics of the time scales for these two classes of reactions have been discussed in ref. 54, to which the reader is referred for details on their branching ratios etc. The thermionic emission in eqn (9.5) requires very high temperatures and occurs for fullerene clusters only in quasi- or non-equilibrium situations. The quasi-equilibrium situations will be treated in more detail in Section 9.4.3 on femtosecond processes. The reactions in eqn (9.6) and (9.7) describe the creation of a giant fullerene and a graphene sheet/amorphous carbon cluster, respectively. As mentioned, the two are distinguished experimentally by the even number of carbon atoms in the fullerene fragments, in contrast to the non-fullerene products which can be both odd- and even-numbered. In addition to the primary processes, these fusion reaction products, be they fullerenes or another allotrope, can emit thermal photons and/or electrons. The question of which reaction occurs is a question of both the amount of excitation energy, how fast it is introduced into the cluster and on the method of excitation. This section reviews work on the fusion, including the electron emission that accompanies it for certain types of excitation.

9.4.1

Coalescence of Laser Heated Fullerenes

Fullerenes were discovered experimentally in a laser ablation (Smalley) source.55 After macroscopic amounts of the molecules became available, this type of source was also used with C60 replacing the graphite as ablation feedstock. The most gentle operation of the source involves use of a cooling gas, usually helium. Although the cooling is quite efficient, the ablation of molecules still involves highly excited species during the initial phase, which lasts on the order of a microsecond, after which the ablated plasma is quenched to much lower terminal temperatures. During the hot plasma phase fullerenes will react uni- or bi-molecularly as Cn þ Cm-Cn1m

(9.8)

Cn-Cn2 þ C2

(9.9)

where n and m are both even integers. As mentioned, the loss of C2 is characteristic for fullerenes and is caused by the stability of the fullerene product molecule. C2 emission requires breaking a number of carbon bonds

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and the presence of the channel is indicative of a very high effective temperature, between 3000 and 4000 K.56 These reactions were observed in ablation experiments with C60, C70 and the mixed unpurified C60, C70 soot extract.57 With the right choice of helium pressure, both the fullerenes and the C2 molecules will be confined to a small volume, and C2 can reattach to a fullerene in the reaction Cn þ C2-Cn12

(9.10)

Figure 9.5 shows an example of the ablation products from a film of C60, measured in a time-of-flight mass spectrometer. As seen from the spectrum, both coalescence and loss/attachment can repeat several times before the product is quenched. In contrast to the single C60 signal from the ablation source, where the anion channel is the most intense, the coalescence signal is not present for anions. The high temperature causing the C2 fragmentation will induce a rapid thermal emission from any anion fusion product that may be formed. The intensity profile across the mass regions that correspond to a specific number of coalesced molecules, as for example between 1200 and 1700 u in Figure 9.5, can be explained by assuming that the coalescence of intact fullerenes occurs in parallel with a continuous loss and recapture of C2

Figure 9.5

Mass spectrum of C60 coalesced after laser ablation. The inset shows a schematic drawing of the laser ablation source and the extraction region of the time of flight mass spectrometer. Reproduced from ref. 57 with permission from Springer Nature, Copyright 1992.

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units. Yields can be calculated in quantitative agreement with the experimentally observed intensities when the two rates are assumed proportional, indicating that they have similar time dependencies. This in turn indicates that they have similar energetic barriers, which further suggests that coalescence is activated by a partially open molecular cage, similar to a geometry that allows C2 emission as the next step. The fullerenes produced in these experiments were found to be coalesced into very stable species, which was documented by experiments where they were collided with a silicon surface and found to remain intact up to fairly high collision energies.59 The production of these stable fullerene species captures the behavior of one end of the parameter space of the coalescence process. As shown with ion mobility measurements and reported in ref. 60, the fusion products of 2C60, 2C70 and C60C70 can, in addition to fully fused fullerene structures, also have a dumbbell geometry with cages linked by only a few bonds, similar to the structure shown in Figure 9.11. The spectrum shown in Figure 9.5 is likely to be dominated by the completely fused coalescence products, because of the extensive fragmentation and recapture of C2 units, which is a fingerprint for fullerenes and not likely to happen for dumbbell structures. The observation of anionic coalescence products in ref. 60 also contrasts with their absence in the coalescence spectra produced as the one shown in Figure 9.5, and indicates that the dumbbell products are generated at a lower internal energy. A hint on the formation process of the two different kinds of coalescence products was provided in ref. 61, which reported a difference in yield and mass distribution of coalescence products depending on the degree of oxidation of C60 molecules desorbed from a surface. The results showed that a reactive species, such as a fragmentation product, enhanced the coalescence yield. A similar conclusion was reached for the coalescence of iridiumcontaining fullerenes in ref. 62. In both cases the production of significant amounts of fullerene fragments is a strong indication that the outcome of the process are completely fused fullerene molecules, similar to the experiments in ref. 57. In contrast, Zhu et al. reported odd-numbered fusion products after ablation of C60 with a chemically added functional group,63 indicating a dumbbell structure of the products, produced by a linking involving the added functional group. A C60-graphite co-evaporation experiment64 demonstrated another example of linking fullerene cages. The experiment is interesting for two reasons. First of all because the linking occurred in a cold atmosphere (liquid nitrogen temperature), indicating the absence of any reaction barrier for carbon atom addition to the fullerene, in analogy to the experiment of Dunk et al. which showed a similar result for C2 addition to a fullerene cage.15 Secondly because exposure of the fused molecules to high fluence laser excitation induce fragmentation through two types of channels. In one, two large fragments appear, corresponding to breaking of the single C atom bond. In the other, less intense, a significant amount of C2 loss occurs, signaling the fragmentation of a completely fused fullerene product.

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9.4.2

213

Low Energy Bi-molecular Collisions

A more specific fusion process is provided by collisions between gas-phase fullerenes and fullerene ions accelerated electrostatically. Such collision þ þ studies of C60 on C60 and C70, and of C70 on C70 were reported in ref. 65. Charged fullerenes were accelerated to keV energies and collided with the target gas-phase fullerene molecules in a heated cell. The pressure in the cell was kept low to ensure single collision conditions, and the acceleration voltage was varied to study center-of-mass collision energies in the 50–250 eV range. This made the experiment exact with respect to collision partners and in terms of center-of-mass collision energy, modulo the thermal speed in the collision cell. In addition to scattering, the main detected events were fusion and fragmentation of the fusion products. A threshold for fusion was observed at collision energies of 60 to 80 eV in the center of mass frame. The cross section was found to increase rapidly at the threshold and to fall off again above the threshold. Overall the fusion cross section was measured to be significantly smaller than the geometric value. The two observations combined are consistent with at least a fraction of the collisions resulting in dumbbell structures that decayed into essentially intact fullerenes before detection. However, some amount of the fusion products were detected as molecules with masses significantly above the separate projectile and target masses and closer to the mass of a fused product, and hence must be fusion products. A mass deficit was present and found to increase with collision energy, indicating a post-fusion loss of C2 molecules and that this loss increased with the collision energy. The variation corresponded roughly to a linearly decrease in measured mass with collision energy. This behavior is expected for an approximately constant value of the dissociation energy for C2 emitted sequentially and in statistical processes, and occurring between the fusion in the center of the experimental chamber and the detection at the periphery. The dissociation energies extracted were based on a schematic model and used a frequency factor which is now known to be much too small,66 and also the radiative cooling was ignored, but the overall trend is robust to changes in these parameters, and the qualitative understanding of the fusion process remains valid.

9.4.3

Femtosecond Light-induced Fusion

When exposed to light pulses of short duration, on the order of hundred femtoseconds or shorter, molecules and clusters are excited to a state where most of the excitation energy resides in the electronic degrees of freedom for some hundreds of femtoseconds. During this phase, which lasts until the energy is dissipated into vibrational motion, the systems are highly reactive. For isolated molecules and clusters, this is manifested in thermal electron emission at an extremely high effective temperature.67–69 A large number of results of experiments involving femtosecond optical excitation have been

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rationalized in terms of this phenomenon, including fluence dependencies, ion yields, and post-ionization fragmentation patterns. An analysis of the results for single molecule C60 is given in ref. 70. For clusters of fullerenes, excitation by this method will create an electron plasma where the integrity of the individual molecules is seriously challenged and a rearrangement of bonds can take place, converting clusters of intact molecules into lower energy species. These species may be both amorphous carbon, graphite sheets, or larger fullerenes. In ref. 71 experiments were reported in which fullerene clusters produced in a gas aggregation source were exposed to the light from a 150 fs TiSa solidstate laser with photon energies of 1.6 eV. As for single fullerenes, the laser pulse readily causes ionization of the clusters. Also the single molecules ionize, and do so with little fragmentation and delayed ionization, features that otherwise both show up very prominently in nanosecond belowthreshold excitation experiments of fullerenes. In addition to an intense channel showing a major amount of loss of intact molecules, clusters are observed to fuse, a process which is accompanied by a significant loss of carbon atoms. A part of the spectrum with fusion products is shown in Figure 9.6. These products are fused into fullerene structures, as witnessed by the loss of C2 molecules in the fragmentation from the Cn60 molecules to the measured fragments that are tens of atoms smaller. The number of atoms lost from the fullerenes can be related

Figure 9.6

Mass spectrum of charged coalescence products after exposure of (C60)n clusters to a 150 fs pulse of 1.6 eV photons. Fusion products up to five fused molecules can be seen. The arrows indicate the masses of the two lowest order intact fusion products. The molecules in all peaks contain an even number of carbon atoms and must therefore be fullerenes. The data are published in ref. 71.

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to the threshold for fusion measured in the single collision experiments in ref. 65 and gives a quantitative agreement.72 The branching between fullerene molecule loss and fusion remains unknown, though, both experimentally and theoretically. An interesting case of coalescence for geometrically fixed fullerenes was reported in ref. 73 where molecules inserted into a carbon nanotube were exposed to an intense and highly energetic electron beam, which also acted as the beam of an electron microscope. The practically equidistant C60 coalesced in a process that produced connections with a negative curvature between the rudiments of the positive curvature fullerene cages. The coalescence processes in this experiment are likely to be of the same nature as the ones induced by light pulses, due to the excited, quasi-equilibrium state of the electron system.

9.5 Ion-cluster Collisions Most of the reactions described above involve a high degree of, albeit not complete, equilibrium. Collisions with highly energetic keV-ions probe processes from highly non-equilibrium to near-equilibrium situations. Here the charge, mass, and velocity of the ions may be tuned to study intrinsic cluster properties and to induce different types of intra-cluster bond formation processes. Such studies have demonstrated that the way energy is deposited plays an essential role for the outcome of the collisions, as opposed to the dominant role played by simply the amount of energy in purely statistical processes. When ions are highly charged, reactions in the collisions are dominated by distant electron transfer processes where little energy is transferred to the clusters. This allows one to probe the fragmentation dynamics of multiply charged clusters, the ultimate Coulomb stability limits (appearance sizes), and intracluster charge communication. Cluster ionization in collisions with low charge state projectiles are, on the other hand, associated with small impact parameters leading to strong cluster heating. Here, the projectile mass and velocity determine the intricate balance between energy transfer due to Rutherford-type scattering processes (nuclear stopping) and electronic excitations as the ion penetrates the molecular electron clouds in the cluster (electronic stopping). To our knowledge, so far all experimental studies of ion impact on clusters of fullerenes have been carried out at the ARIBE facility in Caen, France. Here we will highlight some of the results from these studies together with those from related theoretical work.

9.5.1

Multiply Charged Clusters

In an ion-fullerene cluster collision study 400 keV Xe201 were collided with clusters produced in an aggregation source.74 A typical mass spectrum is shown in Figure 9.7. In addition to the prominent charged monomer

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Figure 9.7

Table 9.1

Chapter 9

Mass spectrum from 400 keV Xe201 þ (C60)n collisions. The inset shows the distribution of charged clusters, where the arrows indicate positions of odd-numbered doubly charged clusters. Reproduced from ref. 74 with permission from the American Physical Society, Copyright 2003. 74 Measured appearance sizes (nexp ) as a function of cluster charge (q) and app 75 the corresponding results from the contact sphere model (nCS ), the app LD 75 LC 76 liquid drop model (napp ), localized charge model (napp ), and DFT 77 calculations (nDFT ). app

Q

nexp app

nCS app

nLD app

nLC app

nDFT app

2 3 4 5

5 10 21 (33)

7 13 23 31

9 15 23 32

5 10 23

2

peaks, the spectrum shows singly and doubly charged clusters. The size distribution of these peaks reflect both the broad size distribution of neutral clusters exiting the source and their ionization and subsequent survival. Such spectra allow the determination of the smallest cluster size (napp) for a given charge state (q), denoted by the appearance size. Here it is observed to be napp ¼ 5 for q ¼ 2. Different source conditions allowed the observation of the appearance size of higher charge states. The experimental result of such measurements are summarized in Table 9.1, together with results calculated with different models. Two of the models (in ref. 75 and 76) consider the electrostatics of charged spherical monomers, whereas another model in ref. 75 treats the whole cluster as a liquid drop.

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The model in ref. 76, which considers the binding provided by the nearest neighbors and requires that charged monomers are not neighbors, reproduces the observed appearance sizes quite well. Recent quantum chemical DFT calculations by Huber and co-workers provide more detailed information on the potential energy landscapes for the decay of fullerene cluster dications.77 These calculations predict (C60)25 þ to be metastable, with a significant energy barrier for fragmentation and which may therefore survive on the experimental microsecond timescales. Smaller clusters are much less stable when doubly charged and are thus likely to decay on such timescales. In more recent experiments, it has been shown that the pentamer is also the smallest cluster size for dianions, (C60)2, formed in helium droplets.78 From a chemical point of view, this may appear surprising since the bonding situation for dianionic and dicationic systems are markedly different. Their similar stabilities suggest that the Coulomb forces outcompete other interactions, and explains why the simple models work remarkably well for such systems. In the study reported in ref. 74, no signs of magic numbers reflecting geometrical shell effects were observed in the distribution of singly (positively) charged clusters. As these are formed in the most distant electron transfer collisions, it was concluded that the internal energies are too low to induce evaporation processes which tend to generate high intensities for the most stable cluster sizes. For q ¼ 2 a slightly enhanced intensity is observed for n ¼ 13, while significantly stronger intensities are seen for n ¼ 13, 19 and 23 for q ¼ 3, and for n ¼ 25 and 29 for q ¼ 4. This is consistent with an increased heating of the cluster with the degree of ionization. The observed magic numbers agree with those reported for singly charged species produced by means of other ionization agents (see above), suggesting that the geometrical structures are not seriously affected by the excess charges. Neutral clusters may be viewed as small pieces of fullerite material where the individual fullerenes are only weakly interacting with their neighbors. When ionized, the situation changes dramatically as the charge communication becomes high within the cluster. This was first reported in ref. 74 where, with the help of coincidence detection techniques, it was demonstrated that collisions where the clusters have been ionized q times yield q product ions. The charge must thus have been redistributed across all cluster constituents before fragmentation. This is consistent with results from time-dependent quantum dynamical simulations of þ þ C260þ þ C60-C60 þ C60 collisions which show that electron transfer occurs for fullerene–fullerene center–center distances that exceed the 10 Å dimer equilibrium distance.79 These results are in contrast to those from highly charged ion impact on weakly bound Ar clusters.80 For those, the charge stays localized to a few constituents from which the electrons have been captured, consistent with the much smaller electron mobility and delocalization of bulk argon compared to fullerite.

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Further information about the charge communication timescales was obtained in experimental studies of multiply charged fullerene dimers produced by highly charged ion impact.81 An entirely unexpected odd–even effect was observed in the ionization yields (see Figure 9.8), which varied with a period of two, in addition to a less surprising smooth decrease with charge state. It is unexpected because the sequence of dimer ionization energies varies smoothly with charge state. However, a rapid charge redistribution within the cluster can explain the observation, as follows: In most collisions the projectile comes closer to one of the two fullerene molecules from which the first electron is captured. An electron is then rapidly transferred within the dimer such that the projectile again can capture an electron from a neutral fullerene closest to its path. The energy required to capture the second electron (and hence the capture distance) is then similar to that of the first electron. However, the third electron is significantly more strongly bound as it corresponds to the ionization energy of a singly charged fullerene in the vicinity of another singly charged fullerene. In a similar fashion as in the capture of the first two electrons, the fourth electron is almost as easily captured as the third one due to the rapid charge communication such that the projectile again ‘‘sees’’ a singly charged fullerene. The step-wise trend is repeated along the ionization ladder and gives rise to the observed even-odd effects in the ionization yields. This mechanism requires that the electron transfer between the two fullerenes occur during the collision and hence on sub-femtosecond timescales.

Figure 9.8

The yield of different charge states in collisions of di-molecular clusters of C60 (red squares) or mixed C60 and C70 (blue triangles). The dashed curve is the pure C60 divided by the relative C70 abundance in the source material. Reproduced from ref. 81 with permission from AIP Publishing LLC, Copyright 2007.

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Another intriguing result from studies of fullerene dimers is that a substantial fraction of the electrostatic potential energy created during the multiple ionization is converted into internal fullerene excitation during the subsequent Coulomb explosion process.50,81 Figure 9.9 shows the measured kinetic energy releases (solid circles) in such proþ q2 þ cesses, (C60)q2 þ -Cq1 60 þ C60 , as a function of the dimer charge (q1 ¼ q2 for even q and q1 ¼ q2 þ 1 for odd q). The kinetic energy release follows an increasing trend reflecting the increased Coulomb energy following ionization. The corresponding results from DFT calculations (solid squares) and a simple model where the fullerenes are treated as charged metal spheres (open squares) are both significantly higher. The two models assume that the dimers and the reaction products (intact monomers) are internally cold, and the difference compared to the experimental results are therefore attributed to excitation energies of the individual fullerenes. The dissipation mechanism is not established at the time of writing. One possibility is a direct conversion of the potential energy into intramolecular vibrational excitations. Such vibrational excitations play key roles in fullerene–fullerene fusion processes, and have been rationalized in terms of a simple model where the fullerenes are described as two collinear colliding springs interacting via an additional massless spring.82 Another possible dissipation mechanism is excitation of the surface plasmon

Figure 9.9

þ q2 þ Kinetic energy releases in (C60)q2 þ -Cq1 reactions as functions 60 þ C60 of the dimer charge (q1 ¼ q2 for even q and q1 ¼ q2 þ 1 for odd q). Experimental results81 are shown as solid circles together with results from a simple electrostatic model (open squares)81 and from DFT calculations (solid squares).50 The energy dissipated into the separating monomers is indicated by the arrow. Reproduced from ref. 50 with permission from AIP Publishing LLC, Copyright 2009.

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resonance. Although this resonance is centered at 20 eV for C60, it stretches down into the visible part of the spectrum and is responsible for the strongly enhanced thermal radiation of the fullerenes.83 This broad and intense profile may possibly provide a doorway state for further dissipation.

9.5.2

Molecular Growth Processes

Light keV-projectiles such as e.g. H1 and He21 transfer energy predominantly to the molecular electron clouds as they penetrate the fullerene cages in the cluster. The excitation energy is then rapidly redistributed among the cluster constituents before the cluster decay, resulting in the emission of intact fullerenes that are colder than those stemming from collisions of the same projectile ions with isolated fullerene monomers. The cluster environment is thus protecting the molecules from damage by this evaporative cooling. However, atoms may also be promptly knocked out in low impact parameter collisions between the projectile and individual carbon atoms in the cluster. The cross section for such nuclear stopping processes is low for light projectiles because the collisions must be close to head-on to eject a bound atom, and the projectiles will mainly hit a single carbon atom. As a consequence, the most likely collision products are C59 and C58 fragments. The presence of C59 is a clear experimental signature of a non-statistical fragmentation process.84 Such products were observed in an early experiment  where 50 keV C60 projectiles were collided with He and Ne gases.85,86 For these knockout processes, it is of little consequence where the charge is residing, and a similar result for this reverse collision is as expected. Knockout fragments are typically more highly reactive than those produced by statistical fragmentation after e.g. photon or electron impact and may play important roles in interstellar chemical reaction networks. A key question here is if they are formed sufficiently cold to be stable against a secondary fragmentation. This calls for future studies at cryogenically cooled storage rings or traps that allows monitoring the cooling dynamics in yet unexplored time domains (up to minutes), where a weak radiative cooling can stabilize the ions. When formed inside clusters, knockout fragments may react with neigboring molecules leading to efficient molecular growth processes. The key to their survival is a sufficiently rapid dissipation of the collision generated electronic excitation energy into the surrounding cluster constituents so that the newborn molecules are formed sufficiently cold not to spontaneously decay. Such a process was observed in 22.5 keV He21 impact on clusters of fullerenes.87 The top panel in Figure 9.10 shows the mass spectrum for all collision events leading to one or several product ions, while the bottom panel shows the mass spectrum for events correlated with one or several þ þ intact C60 ions. Broad C60 peaks dominate in both panels, showing that most

Clusters of Fullerenes

Figure 9.10

221

Mass spectra recorded in 22.5 keV He21 þ (C60)n collisions.87 The top panel includes all events yielding charged collision products (total spectrum), while the lower panel shows the events measured in coinciþ dence with one or several intact C60 ions. The inset in the lower panel shows that there is a peak at 119 carbon masses per atomic unit of charge, which is a clear signature of bond forming reactions leading to þ dumbbell shaped C119 molecules (see the main text). Reproduced from ref. 87 with permission from the American Physical Society, Copyright 2013.

collisions lead to cluster fragmentation in which the individual fullerene ions are cold enough to survive. In the lower panel, measured by requiring coincidence with a charged C60 molecule, a distribution of three peaks is clearly seen, corresponding to 118, 119, and 120 carbon masses per atomic unit of charge. An odd number of carbon atoms is a signature of knockout processes and the odd number molecule of 119 carbon atoms is attributed to þ C59 þ C60 bond formation inside the clusters, leading to formation of the þ dumbbell-shaped C119 dimers. þ þ The molecular dynamics study of the formation of such C118 , C119 , and þ C120 dumbbells in ref. 88, based on a DFT-enhanced tight-binding method, þ þ suggests that covalently bound dumbbell-shaped C119 and C118 molecules indeed can be formed in this type of process following single or double atom þ knockouts, while the formation of C120 dumbbells is associated with

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prohibitively high reaction barriers. The minimum energy structures of the dumbbells are shown in Figure 9.11. The 120 carbon atom peak is instead ascribed to weakly bound, intact fullerene dimers, (C60)2þ , remaining after fragmentation of larger clusters. In contrast to light ions, heavy ion impact leads to severe damage of the individual cluster building blocks in impulse-driven reactions. This ignites molecular growth of a broad range of species inside the clusters, as observed in collisions between 400 keV Xe201 ions and clusters of fullerenes.89 Figure 9.12 shows the mass spectra from such collisions. The distributions in red and blue are the results from small and large fullerene clusters, corresponding to a low (T ¼ 530 1C) and high (T ¼ 590 1C) cluster aggregation

Figure 9.11

þ þ þ The most stable C118 , C119 , and C120 dumbbells according to the DFT calculations in ref. 1. The red color highlights the sp3 carbon atoms and the point groups are given in parentheses. Reproduced from ref. 88 with permission from the American Physical Society, Copyright 2014.

Figure 9.12

Mass spectra from collisions between 400 keV Xe201 ions and clusters of fullerenes recorded with low (rlow, source temperature T ¼ 530 1C) and high (rhigh, T ¼ 590 1C) monomer densities in the cluster aggregation source, yielding smaller and larger clusters, respectively (see the text). Reproduced from ref. 89 with permission from AIP Publishing LLC, Copyright 2010.

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source temperature. These two source temperatures produced cluster size distributions up to about 15 and 40 fullerene masses, respectively. For the smaller cluster case, the peaks in the mass spectrum are attributed to distant electron transfer collisions leading to single ionization of the monomers in the target (C60 and C70), singly charged dimers and trimers, and doubly charged pentamers, possibly followed by loss of intact molecules in an evaporation cascade. Collision products from the larger clusters show a pattern that is similar to the one seen for the excitation with a sub-ps laser pulse (see Section 9.4.3). The main difference is the presence of strong peaks of unfragmented (C60)qn þ , with q ¼ 1, 2 in the figure. These ions likely originate in the smaller cluster component which is also present in this distribution, albeit with smaller relative intensities. Another difference is that the fragmentation of the reacted species is more extensive and reaches down to 76, 78, and 84 carbon masses, which are also intense peaks in the raw ¨tschmer–Huffman production method. The material extract from the Kra presence of even numbered carbon clusters over a broad mass range is clear evidence of a complete fusion of molecules with an excess amount of energy, and the presence of known stability patterns in the fragment products is an indication of well-annealed coalescence products undergoing delayed, thermally activated dissociative loss of C2. The actual formation mechanisms are still not precisely known, but the measured kinetic energy releases of the growth products point at both bottom-up and top-down processes being important.89 The elucidation of this process will provide a better understanding of fullerene coalescence in general and also of the production of the individual molecules. An intermediate mass projectile experiment where Ar21 was collided with fullerene clusters at a (lab) energy of 13 keV, yielded a broader distribution of þ þ þ þ 90 covalently bound systems: C116 , C118 and C119 , and C179 . The difference to a helium projectile spectrum is ascribed to the higher probabilities for multiple carbon atom knockouts for the heavier projectile. The study also showed that much less energy, on average, is transferred for 22.5 keV He21 than for 13 keV Ar21-projectiles. This readily explains the observations that þ the C60 ions emitted in cluster fragmentation processes are colder and more often stay intact in the former case. þ þ Interestingly, the spectra also show peaks corresponding to C61 and C62 , which are attributed to absorption of knocked-out single C-atoms and/or þ C2 molecules into individual C60 molecules. The addition of small carbon molecules to a closed cage is consistent with the observation of direct growth of fullerenes upon exposure to carbon vapor reported in ref. 15, and suggests that a bottom-up closed network growth is a viable route for fullerene growth. In a more recent study, Delaunay et al.91 studied collisions with 3 keV Ar1 projectiles where the typical energy transferred through nuclear scattering processes has its maximum and where it is significantly higher than the electronic stopping contribution. This energy is ideal for the purpose of

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Clusters of Fullerenes

225

studying knockout driven reactions and for comparisons with results from classical molecular dynamics simulations of full collision sequences including bond forming reactions. In the simulations, the actual collision occurs within a few fs and all cluster atoms are then followed up to 10 ps after the projectile was fired. A comparison between the experimental and the simulated mass spectra is shown in Figure 9.13. The upper (a–b) and middle panels (c–d) are the measured spectra with the cluster aggregation source producing small (T ¼ 780 K) and large (T ¼ 850 K) clusters, respectively. The corresponding mass spectra from the MD-simulations of (C60)24 clusters are shown in the lower panels (e–f), displaying the same features as the experimental mass spectra. Some differences in the intensity distributions are expected because of the difference in simulation and experimental timescales (up to 10 ps vs. ms), which allow secondary statistical fragmentation processes to occur in the experiments. The overall good agreement between the experimental and MD results suggests that the reaction products indeed stem from knockout driven reactions, and that the structures and reaction pathways contributing to the different peaks in the measured mass spectrum may then be identified from the simulation results. In the monomer region (left column of Figure 9.13), there are peaks separated by one carbon atom ranging in size from C54 to C64. As discussed above, an odd-number of carbon atoms is a fingerprint for knockout processes and the masses larger than C60 are here results of small knockout fragments being absorbed by neighboring intact molecules in the cluster. The latter (e.g. C61) and the large knockout fragments (e.g. C59) may in turn react with neigboring molecules forming covalently bound dimer systems shown in the right column of Figure 9.13. Simulations with clusters containing 13 and 24 fullerenes show that the formation of covalently bound systems containing more than 60 carbon atoms is highly efficient in these collisions and amounts to about 70% of the geometrical cross section. A selection of such growth products is shown in Figure 9.14. Most of them have sizes in the range shown in Figure 9.13, but there are also examples where significantly larger species are formed. These are a mixture of aliphatic and aromatic structures, as shown to the far right in Figure 9.14. Knockout processes have also been observed as the main

Figure 9.13

Details of the mass spectra measured recorded in 3 keV Ar1 þ (C60)n collisions.91 Upper row (a–b): Neutral target distribution with smaller clusters (oven temperature, Toven ¼ 780 K). Middle row (c–d): Neutral target distribution with larger clusters (oven temperature, Toven ¼ 850 K). Bottom row (e–f): Spectrum from classical molecular dynamics simulations of 3 keV Ar collisions with clusters containing 24 fullerene molecules. Left column (a, c, and e): Mass region in the vicinity of the C60 monomer. Right column (b, d, and f): Mass region close to the mass of two C60 molecules (120 carbon masses per atomic unit of charge). Reproduced from ref. 91 with permission from Elsevier, Copyright 2018.

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Figure 9.14

Chapter 9

A selection of molecular growth products from MD simulations of 3 keV Ar impact on clusters of fullerenes.91 Note the different scale for the structure containing 1294 carbon atoms. Reproduced from ref. 91 with permission from Elsevier, Copyright 2018.

driving mechanism for molecular growth in collision with clusters of PAHs and hydrocarbons, and in mixed clusters of PAHs and fullerenes.92,93 For lighter projectile ions as e.g. carbon, nitrogen, and oxygen, the cross section for such processes is peaking at the typical velocities for those ions in e.g. supernova shock waves. Further combined experimental and theoretical studies are thus key to gauge the significance of impulse-driven reactions in interstellar environments and planetary atmospheres.

Acknowledgements The authors thank Olof Echt for carefully reading the manuscript. KH would like to acknowledge the collaboration of colleagues and students at UCLA, the Niels Bohr Institute, the Max Born Institute, Aarhus University, Gothenburg University, and the stimulating atmosphere at the Tianjin University’s Center for Joint Quantum Studies. HZ acknowledges colleagues at Stockholm University and the research groups in Caen and Madrid, and financial support from the Swedish Research Council (contract no 2016-04181) and from the project grant ‘‘Probing chargeand mass-transfer reactions on the atomic level’’ (2018.0028) from the Knut and Alice Wallenberg Foundation. This publication is based upon work from COST Action CA18212 – Molecular Dynamics in the GAS phase (MD-GAS), supported by COST (European Cooperation in Science and Technology).

References 1. H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl and R. E. Smalley, Nature, 1985, 318, 162. 2. S. Iijima, Nature, 1991, 354, 56–58.

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Section 5: Other Exclusive Carbon–Carbon Nanocomposites

CHAPTER 10

Less-common Carbon–Carbon Nanocomposites CYNTHIA ESTEPHANYA IBARRA TORRES, ´XIMO OLIVA GONZA ´LEZ OXANA V. KHARISSOVA, CESAR MA AND BORIS I. KHARISOV* ´noma de Nuevo Leo ´n, San Nicola ´s de los Garza, NL, Universidad Auto ´xico Me *Email: [email protected]

10.1 Introduction Carbon materials have been extensively studied since, due to their properties, they can be applied in a wide variety of fields. In the case of carbon hybrids and composites, characteristics of the individual components are synergistically combined to produce materials with exceptional properties, such as high electrical conductivity, large surface area, high porosity, excellent stability, among others. Carbon–carbon hybrids and composites have proven to be ecologically viable, efficient, and economical materials. Thus, they have a wide variety of applications, such as in sensors (for environmental or biological purposes), electrochemical applications (fuel cells, batteries, supercapacitors, etc.), catalysts (for example water splitting), and electronic devices. In this chapter, synthesis methods used to obtain carbon–carbon composites (which are based on CNTs, nanospheres or nanoballs, nanorings (nanotori), amorphous and glassy carbon), properties obtained due to the conformation of the material (including their structures, bonds, interactions, among other important factors) and their potential applications will be analyzed. All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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10.2 CNT Hybrids with Non-graphene Nanocarbons 10.2.1

Carbon Nanotubes Containing Carbyne

Experimentally, confined carbyne protected by DWCNTs has been synthesized by a very high temperature (900–1500 1C) and high vacuum (o8107 mbar) procedure, finding that the optimal diameter of the DWCNTs to work as nanoreactors and stabilize carbyne chains was B0.71 nm; when the diameter is very large, the long linear carbon chains (LLCC) disintegrate. By this method it was possible to obtain chains up to 797 nm of length (more than 6000 carbon atoms).1 Also, optical band gaps and resonant differential Raman cross sections were measured, obtaining band gaps of 2.253–1.848 eV (with a linear relation with Raman frequency)2 and resonant differential Raman cross section of (0.7–1.1)1022 cm2 sr1, therefore this material is the strongest Raman scatterer reported so far.3 This synthesis method represents a breakthrough for carbyne bulk production. Several theoretical methods were applied for CNTs–carbyne systems.4 The ab initio DFT molecular dynamics calculations were used to study the high temperature behavior and initial stages of melting of fullerene C60, carbon nanotubes, carbyne chains, and carbon nanotube-carbyne composites in a temperature range of 3000–5000 K (Figure 10.1). Carbyne chains were found to be in a quasi-1D liquid state inside the nanotube, a quasi-2D liquid state, and early melting. Also, the computational algorithm, developed5 for the investigation of superconductivity in various thin SWCNTs, allowed prediction of the Tc (superconducting transition temperature, maximum 60 K in this case), taking into account the effect of radial pressure, symmetry, chirality and bond lengths. In the case of a linear carbon chain embedded in the center of (5,0) SWCNTs, their strong curvature in the presence of the inner carbon chain provides an alternative path to increase the Tc of this carbon composite by a factor of 2.2 with respect to the empty (5,0) SWCNTs. The inner carbon nanowire is twisted from linear to a zigzag form in order to

Figure 10.1

55 nanotube with 9-atom carbyne chain. Image after 4 ps of annealing at 3000 K. Chain is continuous most of the time and stays away from tube. Reproduced from ref. 4 with permission from Elsevier, Copyright 2018.

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minimize its energy. The covalent bond distances of the zigzag carbon nanowire are 1.578 Å and 1.404 Å. Using full atomistic modeling of sequential carbon reactions carbyne synthesis inside SWCNTs (Figure 10.2) was studied to determine the optimal conditions of temperature and confinement (nanotube diameter) to obtain LLCC. The resulting chain lengths are related to synthesis conditions, with the longest chain (23 carbon atoms) obtained at 750 K using (6,6) CNT (diameter of 0.81 nm). The conditions found as optimal in this study differ from the experimental results shown previously using DWCNTs, which could be explained due to the magnification of the confinement effect due to the double carbon layer and the stiffness of the DWCNTs.6 Recently, confined carbyne–polymer nanocomposites have been synthesized. Carbyne reinforced DWCNTs were obtained by high vacuum alcohol Chemical Vapor Deposition and high temperature high vacuum annealing, and polymer nanocomposites were prepared basically by dispersion of confined carbyne (or DWCNTs for comparison) in a modified bisphenol-A epoxy resin and a curing agent mixture, followed by a curing procedure. Carbyne-containing nanocomposites showed an improvement in mechanical and electrical properties compared to those containing DWCNTs, the tensile strength increased 5.6% (from 64.0 to 67.6 MPa), elastic modulus by 9.7% (from 3.39 to 3.72 GPa), failure strain by 9.9% (from 7.27 to 7.99%), fracture toughness by 13% (from 0.77 to 0.87 MPa m1/2) and electrical conductivity increased 14 times (from 0.07 to 0.98 Sm1).7 This modest improvement in properties can be a good starting point for future research on carbyne composites and their potential applications.

10.2.2

CNT Composites with Carbon Nanofibers (CNFs)

A low-temperature CVD method was applied for preparation of CNT/CNF composite films having a network structure with optimal pore-size distribution.8 This characteristic strongly depends on the graphite substrate and vitally affects its performance as an electrosorptive electrode material for desalination. Despite the similarity of dimensionality of the CNT and CNF, their combination can lead to an improvement of several properties, in particular the electrochemical ones. Thus, the nanocomposites of graphitic nanofibers (GNFs) and CNTs, prepared via CVD method and being assembled in a symmetric two-electrode system, showed a very good cycling stability of 96% after 10 000 charge/discharge cycles and other excellent characteristics.9 This high performance can be related with the CNTs skeleton, providing channels for charge transport, and GNFs, responsible for sufficient accessible sites for charge storage. Other hybrid materials with excellent electrochemical properties and their applications can be analyzed (see Table 10.1). Other interesting applications of CNT composites with CNFs include, for example, the use of hybrid 3D Ni-MWCNTs/CNFs such as sarin gas sensors, showing high sensitivity for dimethyl methyl phosphonate (a sarin gas simulator), high stability of sensing response and rapid response-recovery,

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Figure 10.2

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Snapshots of the formation process. (a) Initial configuration of the constructed model. (b) Minimizing the system, carbon atoms attracted to the sidewalls, some local bond events. (c) First significant segment formations. (d–h) Continued bonding events and primary chain growth. Reproduced from ref. 6 with permission from Elsevier, Copyright 2019.

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237 10

thus it is considered viable as a sarin sensing material. Also, CNTs/CNFs composites can be used as electrocatalysts for the oxygen evolution reaction (OER) due to their high-density of active sites, high surface area, good electrical properties and low cost.11,12

10.2.3

Other CNT Hybrids

Hybrids of MWCNTs and nanodiamonds are also known,18,19 where electrical and thermal properties of MWCNTs are united with the inertness and hardness of nanocrystalline diamond (NCD).20 Obtaining materials with improved properties with potential application in electronics, field emission, wear-resistant coatings, electrochemistry, among others. Certain attention is paid to the nanoreactors21–23 on the CNT basis, as reviewed in ref. 24. Among other composites, we note C60/CNT hybrids (nanobuds), discovered relatively recently by Nasibulin and Kauppinen.25 This structure seems like fusion of a cylinder with a sphere; there is a covalent bond between outer sidewalls of the nanotube and the fullerene. In this material, fullerenes are covalently bonded to the outer sidewalls of the underlying nanotube.26 Consequently, NBs exhibit properties of both carbon nanotubes and fullerenes. Carbon nanobuds can exist in various types of structures, all being stable. One NB geometry can be transformed to another kind and this process can be monitored and visualized by transmission electron microscopy (TEM).27 The fullerenes can be converted into tubular branches via treatment of the electron beam irradiation. SWCNT samples with NBs were fabricated by an aerosol-assisted CVD (carbon source: an aerosol of toluene with 5 wt.%; synthesis conditions: 800 1C, 15 min), collected on a substrate, placed at the end of the reactor, dispersed in acetone, sonicated and transferred onto TEM grids. The resulting products (NBs) contained fullerenes of various sizes on the SWCNTs. The electron beam provides enough energy to allow the fullerene to be transformed into a tube-like structure before collapsing entirely. To obtain fullerene–SWCNT hybrids without SWCNT structural damage, a reductive functionalization route was implemented using alkali metals as reducing agents (Figure 10.3). Obtained hybrid materials have been synthesized from SWCNTs and endohedral Sc3N@C80, this material being the first that has used Sc3N@C80 as the building block. This hybrid has potential applications in electronic devices, solar cells, or advanced sensors.28 Amorphous carbon hollow nanocubes (ACF) were obtained from ZIF67 nanocubes. Co/Co2P@ACF/CNT hybrid material was obtained by an etching process and heat treatment. This material presents good electrocatalytic properties due to the multiple synergistic effects between Co/Co2P and ACF/CNT, thus it is presented as an applicable material as an electrocatalyst for the hydrogen evolution reaction (HER).29 Trilayered CNT/MoSe2/amorphous carbon hybrid materials have been obtained by a solvothermal method to form MoSe2 nanosheets covering CNTs, followed by a glucose-coated CNT/MoSe2 hydrothermal process and

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Table 10.1

Synthesis and electrochemical properties of CNT/CNF materials.

Final product Branched carbon nanotube/ carbon nanofiber composite CNT/graphite nanofiber nano-composites

Carboxylated multiwalled carbon nanotubes/carbon nanofiber composite

CNT/CNF composite

CVD method

Application Super-capacitor electrodes

Electrochemical properties

Ref. 1

Specific capacitance: 207 F g Cycling stability: 95.6% (after 5000 charge/discharge cycles) Catalytic CVD method Symmetric Specific capacitance: 270 F g1 (Co/MgO catalyst) super-capacitors Cycling stability: 96% (after 10 000 charge/discharge cycles) Deliver maximum specific energy: 72.2 W h kg1 (at power density of 686.0 W kg1) Microbial fuel Specific capacitance: 1.722 F cm2 Polyacrylonitrile (PAN)/CNT cells anode solution was prepared followed Delivered maximum power density: by an electrospinning procedure 362  20 mW m2 and thermal process. Catalytic current: 148 mA cm2 Exchange current density i0: 6.3105 A cm2 Internal resistance: B40 O Apparent capacitance: 0.68  0.11 Super-capacitor PAN/CNT solution was prepared microbial fuel cell F cm2 followed by an electrospinning cathode procedure and thermal process. Maximum power density: 306  14 mW m2 Exchange current density: 13.68 A m2 Internal resistance: 0.18 O cm2 Energy storage Specific capacitance: 61.1 F g1 Carbonization of PAN fibers (obtained by electrospinning) to form electrode CNF skeletons, followed by CNT growth on CNFs and KOH activation

13 14

15

16

17 Chapter 10

Active carbon nanotubecarbon nanofiber hierarchical hybrids

Reaction conditions/main steps

Less-common Carbon–Carbon Nanocomposites

Figure 10.3

239

Synthetic route of CNT-endohedral fullerene hybrids. Reproduced from ref. 28 with permission from American Chemical Society, Copyright 2019.

heat treatment to form CNT/MoSe2/C material. In this way, the active material MoSe2 is protected from direct exposure to the electrolyte and agglomeration and restacking are prevented during its application as an anode for sodium-ion batteries.30

10.3 Hybrids of Nanoballs and Nanospheres Composites based on carbon nanospheres/nanoballs (except the fullerene composites above) with other nanocarbons are rare, representing mainly graphene and CNT hybrids, as well as more complex transition metalcontaining composites, having catalytic applications. Thus, a sandwichlike composite of the reduced graphene oxide (rGO) and hollow N-doped carbon nanospheres (N-HCNSs, sub-40 nm diameter) were produced from 2D arrangements of triblock copolymer micelles on GO sheets by carbonization (Figure 10.4).31 GO (prepared via the chemical exfoliation of graphite) instead of graphene was used as the initial 2D free-standing substrate, since the GO contains abundant functional groups, which can be easily modified. The formed hybrid structures were shown to have a better performance for oxygen reduction compared to physically mixed rGO and N-HCNSs without chemical bonding. Further interaction with a uniformly distributed iron dopant yielded composites possessing high N-doping (6.5 at.%) and high surface area, showing good performance as electrocatalysts in the oxygen reduction reaction (ORR). Another representative example is a composite on the basis of Pt3Ni alloy nanoparticles supported on sandwich-like graphene sheets/carbon nanospheres/graphene sheets substrate (Pt3Ni-C/rGO), solvothermally processed in DMF.32 CNSs were found to be homogeneously

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Figure 10.4

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Synthetic process of the preparation of 2D Fey-N-HCNS/rGO-T nanosheets. Reproduced from ref. 31 with permission from American Chemical Society, Copyright 2018.

dispersed in the matrix of exfoliated graphene sheets. Non-agglomerated Pt3Ni nanoparticles were distributed on the graphene surfaces. The product found applications as a catalyst for methanol oxidation, showing a mass activity 1.3-times higher than that of commercial Pt/C (20 wt.%) and 1.7-times higher than that of Pt3Ni NPs on rGO alone. The effect of prevention of restacking/refolding of the graphene sheets by the insertion of CNs into the graphene matrix is offered, among others, as responsible for superior electrocatalytic activity. Other applications include uses as supercapacitors33,34 or ultra-high energy supercapacitors35 and determination of doxorubicin (an anti-cancer drug).36 Hierarchical 3D nanostructures, carbon nanospheres hanging on carbon nanotubes (CNSs/CNTs, where the CNTs are 200 nm in diameter), were prepared by CVD, allowing easy structural control of the reaction products.37 Carbon spheres were found to be homogeneously distributed on the CNT walls. This composite was found to have supercapacitor properties, whose principle of function is based on the hanging CNSs for ion accumulation and the CNT backbone providing path channels for faster ion diffusion. This kind of CVD process is normally one-step, as, for example, the formation of CNSs/CNTs from acetylene as the carbon precursor and iron nitrate as the catalyst precursor, when CNTs are formed at 700 1C from C2H2 and then CNs at 900 1C from a C2H2 þ N2 þ H2 mixture.38 As elucidated in a previous report,39 the nanospheres, coating CNTs, can be composed of several nanoparticles of graphene stacks and are seamlessly connected with graphene stacks. Carbon nanospheres can be formed in three steps: a) the formation of graphene nanoparticles on CNT surfaces and silicon substrates, b) the migration of active hydrocarbon groups towards the surface of the CNT deposition zone at high temperature, and c) the formation of carbon nanospheres by the aggregating hydrocarbon active groups. Hard carbon spheres interconnected by carbon nanotube composites have been synthesized by carbonizing CNTs supported 3-aminophenol-formaldehyde resin spheres. These materials can be applied as anodes for sodium-ion batteries, since they have a large capacitive contribution due to the high number of

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pores in the composites. As a result of high-pore volume there is abundant space for Na1 diffusion and storage.40 Another application for hybrid materials of CNTs–CNSs is electromagnetic-interference shielding. In this case, a CNT–necklace-like hybrid was obtained, where conducting pathways for electrons is provided by a necklace-like nanostructure (Figure 10.5).41 Some other CNS composites have been used in electrochemical applications. Hollow carbon spheres (HCSs)@Fe3O4@carbon nanodisc composites (Figure 10.6) have high structural stability, high-rate cycling performance (specific capacity of 595 mAh g1 after 800 cycles at 1 A g1) and improved electrical conductivity. These properties make it viable for highvolume expansion anodes in Li-ion battery applications.42 Also, to use as a lithium-ion battery anode, walnut core-like hollow carbon micro/nanospheres (WCSs) supported SnOx@carbon composites have been developed. WCSs@SnOx@C synthesis consisted basically in resorcinol formaldehyde resin (RF)/SiO2 composite carbonization and etching to obtain WCSs, hydrothermal treatment of WCSs, potassium stannate and glucose suspension and, final carbonization (Figure 10.7). This material presents high cycling performance (853 mAh g1 after 400 cycles at 200 mA g1).43 To obtain efficient supercapacitors, a nitrogen-rich carbon nanosheet (derived from dicyandiamide and glucose) and hollow sphere composite were fabricated. This composite presents high specific capacitance of 425 F g1 at 1 A g1 and, at a high current density (80 A g1) maintains 248 F g1.44 Carbon nanospheres coated on a carbon fiber paper composite can be used to prepare electrodes for electrochemical determination of progesterone. By a Differential Pulse Voltammetry (DPV) method, a linear relationship between the anodic peak current intensity and progesterone concentration

Figure 10.5

CNTs–CNSs hybrid structure and schematic of the mechanism of multilevel EMI shielding. (Wave dispersion on CNT–necklace-like hybrid 3D nanostructures.) Reproduced from ref. 41 with permission from Elsevier, Copyright 2018.

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Figure 10.6

Fabrication process of HCSs@Fe3O4@C nanodiscs. Reproduced from ref. 42 with permission from Elsevier, Copyright 2019.

Figure 10.7

The preparation and lithium storage schematic of the WCSs@SnOx@C composite. Reproduced/Adapted from ref. 43 with permission from Elsevier, Copyright 2019.

was found. This method presents a sensitivity of 19 590 mA mM1 cm2. In addition to its high sensitivity, this method is highly selective and free from interference.45

10.4 Nanoring (Nanotori) Composites Carbon–carbon composites of ring- and torus-like carbons are known for combinations with graphene and CNTs and are mainly studied theoretically,

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with the exception of rare examples. Thus, a 3D-bridged carbon nanoring (CNR)/graphene hybrid paper, in which the CNRs are covalently bonded to the graphene sheets, was fabricated by the intercalation of a polymer carbon source (PMMA) and metal (Ni) catalyst particles.46 The CNRs were grown through thermal annealing in situ in the confined intergallery spaces between graphene sheets. This CNR/graphene hybrid paper shows an excellent heat spreading ability. A molecular dynamics simulation method was applied to a hybrid structure formed by two graphene nanoribbons and carbon nanorings (CNRs).47 This composite with two linkers revealed the highest thermal conductivity of 68.8 Wm1 K1, being 11 times higher than that of overlapped graphene sheets without CNRs. In these CNR–graphene hybrids, the density and the diameter of CNR are the most important parameters responsible for the thermal transport. For the CNR–graphene hybrid structure, the interactions between adjacent graphene sheets are significantly strengthened by the cross-linked CNRs.48 When the diameter of CNRs is large or the CNR linkers are dense, the tensile strength of the hybrid reaches the maximum. Interactions between CNR and SWCNT were also studied by MD simulations,49 revealing that the CNR can spontaneously insert into the hollow interior of the SWCNTs to collapse to a linked double graphitic nanoribbon and wrap in a helical manner around a tube (in a spiral form), or to form a DNA-like double-helix. This behavior is a result of the van der Waals interactions. It was suggested that this spiral form of the CNR takes the least amount of energy and takes up the least space. As straight CNTs can contain carbynes (see the section above on CNT–carbyne interactions), the toroidal carbon nanotubes (carbon nanotori) can also encapsulate a single symmetrically-located carbon atomic chain.50 It was suggested that for the carbon nanotorus, synthesized from a perfect CNT, the chain is centrally located, and the cross-sectional radius r of the nanotorus is larger than 3.17 Å. In the case of the chains inside CNTs, it was found that they are energetically favorable in (5,5) and (10,0) tubes, but not in a (4,4) tube. N,O-co-doped graphene nanoring–integrated N-doped carbon box composites were prepared by pyrolysis and acid etching of precursors. The resulting hollow graphene nanorings provide a high quantity of active sites for HER, OER, or ORR. Due to its structure, accelerated mass and charge transport were obtained. This material can be applied as air-cathode electrocatalysts for rechargeable Zn–air batteries (maximum power density of 111.9 mW cm2) and bifunctional electrocatalysts for water splitting.51

10.5 Xerogels on the Basis of Carbon–Carbon Composites Carbon Xerogels (CX) are solids with complex 3D structures based mainly on CNT and graphene, prepared by slow drying of gels at room temperature or methods that do not cause sudden contractions in their structure, this

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allows the material to present a large surface area and high porosity. In the literature these materials have been used as anodes for sodium ion batteries (SIB), removal of pollutants in water, supercapacitors, catalysis, etc.52 Despite their good results in these applications, their high manufacturing cost compared to other materials with similar properties, have made CX unattractive to be scaled at an industrial level.52 As an example of graphene-based xerogels, the synthesis of graphene/CX composites was carried out by an ionic liquid template method.53 In this synthesis, graphene was prepared by graphitizing the organic interlayer of dodecylamine by Fe species; then RF þ Na2CO3 þ BMImBF4 (ionic liquid) were added into the mixture with further hydrothermal treatment, drying and carbonization. The as-made xG/CXs showed a greatly improved performance in terms of the cycle stability, the specific capacitance, and the rate capability. This type of CX composite can be used to remove contaminants in water, for this it is necessary to make some modifications to the surface of these materials causing the appearance of acidic, basic active centers or a combination of weak/basic acid. Some forms to generate these active sites are: a hydrothermal method with dilute nitric acid, gas treatments with dilute ammonia and then oxygen, and thermal modification.54 Some more common hybrid CX variants are CX/graphene oxide (GO), CX/micronized graphite (MG) and CX/carbon black (CB), these variants were specially manufactured to improve the conductivity of CX alone, in order of creating components for aqueous supercapacitors.55 All the hybrid CX presented improved electrical conductivities compared to the non-hybrid CX. Among these materials, the CX/GO stood out showing energy and power densities of 14 Wh kg1 and 42 000 W kg1, respectively. Representing a 16 and 97% improvement over pristine carbon xerogel but a more dramatic 143 and 409% improvement over commercially available materials. Other carbon/carbon composite xerogels were synthesized (matrix phase ¼ RF; disperse phase ¼ cotton fibers) by a vacuum drying technique, meanwhile a carbon cryogel was prepared by a freeze-drying method.56 It was revealed that the vacuum drying can decrease the pore shrinkage despite the gas–liquid interface. The porous properties of the carbon xerogel and the carbon cryogel were found to be quite similar. Finally, multi-carbon composites were fabricated by addition of graphite (G) and carbon black (CB) to a high surface area activated carbon (HSAC).57 The synthesis of CX from the multiphase carbonization method, starting from polymeric three-dimensional structures such as resorcinol/formaldehyde resin, can present several carbon phases such as amorphous and graphitic carbon,58 depending on the relationship that these faces present in the CX. The CX will present meso or macroporosity, covering a wide range of average meso or macropore sizes from 10 nm to 3000 nm. In addition, it was observed that depending on its pore size, the conductivity of the material was modified, with values ranging from 2 Scm1 for materials with a pore size of 10 nm up to 18 Scm1 (Figure 10.8) for materials with larger pore sizes.

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Figure 10.8

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SEM micrographs of the CXs: (a) CX-10; (b) CX-100; (c) CX-350; (d) CX-700; (e) CX-1000; (f) CX-3000. Reproduced from ref. 58 with permission from Elsevier, Copyright 2019.

We note that this research area is not yet well-developed; further efforts are needed, in our viewpoint, for creation of cheap metal-containing C–C xerogels for catalytic and absorption purposes, in particular for in situ oil solidification and recovery in the sites of oil spills.

10.6 Amorphous and Glassy Carbon Composites Amorphous carbon (AC) has been extensively studied as a matrix to generate hybrid composites,59 some of the most common components for the manufacture of these hybrid composites are metallic nanoparticles such as

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(a–b) TEM micrographs and (c) SAED pattern of G5. (d–e) TEM micrographs and (f) SAED pattern of S5. (g–h) TEM micrographs and (i) SAED pattern of P5. Reproduced from ref. 60 with permission from Elsevier, Copyright 2019.

gold, silver and palladium (Figure 10.9).60 These composites with metals are usually used as electrodes, for example, amorphous carbon electrodes with gold nanoparticles exhibit an energy density of 7.1 Wh kg1. For the amorphous carbon (AC), its composites are known to be formed for nanodiamond (ND),61 graphene, and carbon nanotubes. These hybrids are mainly produced by similar techniques (CVD/pyrolysis), and the majority of these products possess excellent electrochemical properties. Thus, the 10 nm size nanodiamond particles were embedded in an amorphous carbon matrix via double-assisted hot filament CVD at a substrate temperature of 800 1C from CH4 : H2 (1–99% ratio) forming films,62 including the steps of diamond nucleation, growth of both diamond and AC, suppression of the diamond growth and its further re-nucleation. A 3D skeleton core–shell amorphous porous carbon/multilayer graphene was CVD-prepared from a mixed gas of CH4 and H2 on Ni foam at 650–1000 1C (Figure 10.10).63 The formed material was found to have not only outstanding mechanical and electrical properties of the multilayer

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Figure 10.10

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(a) SEM micrographs of top view, (b) SEM micrographs of highmagnification top view, (c) SEM micrographs of the cross-sectional view of 3-D APCF grown at 650 1C (CH4/H2 flow: 30 sccm/40 sccm for 20 min) under ambient pressure, (d) SEM micrographs of top view, (e) SEM micrographs of high-magnification top view, (f) SEM micrographs of cross-sectional view of 3-D MGF grown at 900 1C (CH4/H2 flow: 180 sccm/50 sccm for 20 min) and 6.5 Torr, (g) SEM micrographs of top view, (h) SEM micrographs of high-magnification top view, and (i) SEM micrographs of cross-sectional view of the 3-D MGF/APCF hybrid structure grown at 900 1C (CH4/H2 flow: 5 sccm/180 sccm for 20 min) at ambient pressure. Reproduced from ref. 63 with permission from American Chemical Society, Copyright 2017.

graphene, but also the mesoporous characteristics of the amorphous carbon, and can be used as anodes for lithium ion batteries. After additional incorporation with NaBiO3, for the formed NaBiO32H2O/MGF/APCF hybrid, superior electrochemical activities above 2 V can be achieved with a discharge capacity ofB300 mAh g1. Carbonizing a CNT/polyaniline composite, a nitrogen-rich carbon nanotube/amorphous carbon (CNT/C) composite was prepared.64 The product retained a mesoporous CNT structure as its backbone, meanwhile the N-rich polyaniline-derived carbon formed a thin amorphous

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coating on the CNT surface. Both the CNT/C composite and pristine CNT showed an excellent cyclability at 1 C charge/discharge over 600 cycles, among other useful characteristics. It was suggested that the mesoporous tubular architecture of the CNT, its relatively higher graphene interlayer distance (0.342 vs. 0.335 nm for graphite) and the nature of charge storage mechanism, appear to play a crucial role in realizing high-rate capability of CNT-based electrodes in comparison with conventional graphite. In addition to the pure carbon–carbon hybrids above on the amorphous carbon basis, presence of a third component (a metal salt) in their composites generally improves desirable properties. Thus, anchoring Co1xS and NiS nanocrystals on amorphous carbon-coated MWCNTs using CNTs@C as template via a facile hydrothermal/solvothermal route, the formed CNTs@C@Co1xS and CNTs@C@NiS hybrids allowed us to accommodate the volume change and enhance the electrical conductivity in lithium-ion batteries.65 Also, the highly conductive 3D interlinked amorphous carbon nanotube (ACNT)/reduced graphene oxide (RGO)/BaFe12O19 composite, composed of dense intertwined ACNT and graphene with quantities of dihedral angles, was directly prepared by a self-propagation combustion process. High conductivity of its networks would lead to energy dissipation in the form of heat through molecular friction and dielectric loss. Another example of the development of a hybrid composite focused on creating materials with better conductivity properties for battery manufacturing, is the composite made up of silicon nanoparticles that are coated with different carbon phases (amorphous carbon, order carbon, dual carbon [amorphous/organized]) (Figure 10.11).66 The amorphous carbon coating shows better conductivity, while the ordered carbon coating exhibits good tolerance to volume variation. Interestingly, the dual carbon (amorphous/ ordered) coating structure simultaneously exhibits the two advantages that the amorphous and ordered carbon coating structures have. The dual carbon coated composite featured an eclectic initial discharge capacity of 3300 mAhg1 to the current density of 0.36 Ag1 and more improved cycle performance, which achieves a specific discharge capacity of 650 mAhg1 after long 500 cycles in the voltage range of 0.01 a 1.5 V. Performance can be further improved by optimizing the dual carbon structure. On the other hand, amorphous carbon composites are so flexible that they can lead to the manufacture of composites with three carbon phases, an example of this, is the manufacture of microspheres composed of reduced graphene oxide/graphitic carbon/amorphous carbon activated with KOH (Figure 10.12),67 synthesized by spray drying at the pilot scale. When evaluating its characteristics for potential application as supercapacitors electrodes, it was shown that the composite presents an improved cyclic stability, exhibiting a retention capacity of 94.7% after 10 000 cycles to 10 mA g1. Therefore, the developed compound outperforms other carbon materials and graphene oxide compounds. Carbon–carbon composites on the basis of glassy carbon are rare, although their properties seem promising. Thus, graphite/glassy carbon

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Figure 10.11

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TEM micrographs of carbonized SiNPs-PANI (a) and (b); carbonized SiNPs-CA (c) and (d); carbonized SiNPs-PANI-CA (e) and (f). Reproduced from ref. 66 with permission from Elsevier, Copyright 2019.

composites useful as self-lubricating materials, showing low friction coefficients and little influence by the sliding velocity, being compared with glassy carbon and graphite alone. Low gas permeability of a coating prepared based on a similar dense composite, where small graphite particles are wrapped by a glassy carbon matrix and act as a second phase, can also have useful applications. On the other hand, there are also some electrodes that are formed by means of graphite coated by vitreous carbon composites with some metallic nanoparticles to improve the conductivity of the material.68

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Formation mechanism of AC-GC-rGO-a composite microspheres using the spray drying and chemical activation process. Reproduced from ref. 67 with permission from Elsevier, Copyright 2019.

Other glassy carbon composites, containing other carbon components, usually form part of polymer-based triple systems, such as, for example, acrylonitrile styrene acrylate copolymer/graphite/carbon black composite or conductive polymer composites. These last ones were fabricated from mixed graphite and carbon black, incorporated in a polypropylene matrix for the fabrication of an electrically conductive polymer composite plate.

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Section 6: Polymer Composites of Carbon–Carbon Hybrids

CHAPTER 11

Advances in Polymeric Nanocomposites Incorporating Graphene–Fullerene and Graphene Oxide–Fullerene Hybrids AYESHA KAUSAR Nanosciences Division, National Center For Physics, Quaid-i-Azam University Campus, Islamabad, Pakistan Email: [email protected]

11.1 Introduction Carbon nanomaterials have gained research attention since the discovery of the fullerene and carbon nanotubes.1–3 They have truly exclusive properties and are in demand to be produced on the industrial scale for various purposes. Carbon nanomaterials have been applied in a wide range of industries such as electronics, energy devices, defense relevant areas, environmental aspects, and biomedical appliances. They have been used successfully alone as nanofillers. However, when two nano-sized carbon materials are combined, they may be termed as a carbon nanobifiller hybrid.4,5 The carbon nanobifiller hybrids of almost all types of nanocarbons have been developed and used for various purposes. These nanostructures are often called nanobifiller hybrids. The synthesis, properties, and applications of nanobifiller hybrids have been studied. The nanobifiller hybrids of graphene have been All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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developed as the graphene–carbon nanotube, the graphene–nanodiamond, graphene–montmorillonite, and other nanobifiller hybrid forms. Graphene is a two-dimensional nanocarbon nanomaterial.6,7 It is made up of sp2 hybridized carbon atoms arranged in a hexagonal fashion. The modified forms of graphene are graphene oxide (GO) and reduced GO. GO can be prepared from an inexpensive material like graphite. It can then be converted to graphene to yield a costless route. The modified graphene nanostructures may have a very high electrical conductivity of 27 Sm1. Moreover, graphene has a unique mechanical strength, exclusive electrochemical properties, high thermal stability, high thermal conductivity, and other physical properties. Fullerene is a zero-dimensional carbon nanomaterial.8,9 The basic structure of fullerene is similar to that of graphene i.e. made up of sp2 hybridized carbon atoms arranged in a hexagonal manner. Fullerene molecules exist in various forms depending upon the number of carbon atoms involved in its structure. The fullerene molecule with sixty carbon atoms is called Buckminster fullerene or fullerene C60. It has a hollow symmetrical ball-like structure. It is the most commonly studied form of fullerene molecules. The fullerene C60 molecule has a diameter of 0.7 nm. It is made up of 12 pentagons and 20 hexagons to form a cage-like structure. This cage-like structure often encapsulates an atom, a charge species, or gas molecules. The fullerene molecules with the encapsulation are often known as endohedral fullerenes. The graphene–fullerene nanobifiller hybrid is an important derivative of graphene and fullerene.10,11 The graphene–fullerene nanobifiller hybrid can be developed using modified forms of the graphene or fullerene. Graphene has a nanosheet-like structure, while fullerene appears as a spherical ball. However, other shapes of fullerenes are also observed other than the spherical form. During the combination of graphene or fullerene and the formation of the graphene–fullerene nanobifiller hybrid, the balls may be intercalated between the parallel sheets of the graphene. This process is complicated and various techniques have been proposed and applied to form graphene–fullerene nanobifiller hybrids. The hybrid nanostructure may have a very high specific surface area, unique electrical conductivity, thermal transport, magnetic features, thermal properties, electron affinity, and other essential physical properties. The graphene–fullerene nanobifiller hybrid has been applied in a multitude of advanced applications including composite formation. The graphene– fullerene nanobifiller hybrid has been filled in the polymers to form the nanocomposites. The polymer/graphene–fullerene hybrids have been successfully designed using various methods. However, sometimes the graphene–fullerene nanobifiller hybrid may be self-associated to form aggregates in the polymer matrices. The aggregation may occur due to the van der waals forces and the p–p stacking interactions. The polymer/graphene– fullerene hybrids have revealed the mechanical strength, the thermal stability, the electrical conductivity, the non-flammability, the tribological features, and the anti-corrosion performance. The polymer/graphene– fullerene hybrid nanocomposites have scope for a wide range of current and

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future applications. However, fewer studies have been reported for the actual application of polymer/graphene–fullerene hybrid nanocomposites. The most likely applications are in aerospace, automotive structures, energy devices, and biomedical appliances. In this chapter, the essentials and the potential prospects of graphene–fullerene nanobifiller hybrid and polymer/ graphene–fullerene hybrid nanocomposites are discussed. Furthermore, the fundamentals and state-of-the-art improvements in the field of graphene– fullerene nanobifiller hybrid and polymer/graphene–fullerene hybrid nanocomposites are comprehended. Future growths in this field must focus on the use of graphene–fullerene nanobifiller hybrid and polymer/graphene– fullerene hybrid nanocomposites in advanced structures and investigations on the structure–property relationships of these nanomaterials. For these graphene–fullerene nanobifiller hybrid materials, thermoplastic, thermoset, as well as conjugated polymers have been used. The materials have different properties depending upon the design and graphene–fullerene nanobifiller hybrid content and nanobifiller dispersion.

11.2 Carbon Nanomaterials: Graphene and Fullerene Graphene is a unique nano-sized carbon allotrope (Figure 11.1). Owing to hexagonally arranged sp2 hybridized carbon atoms, graphene possessed exceptional electrical, optical, thermal, and other physical and structural properties.5,12,13 It has a two-dimensional sheet-like structure, a high surface area and remarkable electron transportation properties. In addition, thermal

Figure 11.1

Structure of graphene, graphene oxide, and fullerene.

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transportation has also been observed. The mechanical properties of graphene have also been found to be extraordinary and useful for practical applications.14 Different bottom-up and top-down approaches have been used to form graphene and it possesses several advantageous features.15 It is considered to be the strongest material on earth so far. A single graphene nanosheet has a Young’s modulus of 1 TPa. The ultimate strength is 4100 GPa. Graphene has a high thermal conductivity of 5000 W m3 K1. Among the bottom-up methods used for the formation of graphene, the plasma method, chemical vapor deposition, epitaxial growth, and other methods have been used. Chemical vapor deposition is a successful technique to form graphene through a bottom-up approach. A top-down method has been used to form graphene starting from graphite. The graphite method involves the production of graphene oxide and then conversion into graphene. The top-down and bottom-up methods must be selected carefully to design graphene with a small size and perfect single nanosheet structure, with a high surface area, and an optimum aspect ratio. Graphene oxide is the oxidized form of graphene. It possesses several essential functional groups on its surface.16 Graphene has numerous oxygen-containing groups such as hydroxyl, epoxide, carbonyl, and carboxylic groups.17 Graphene oxide is a hydrophilic carbon material. The hydrophilic nature of this carbon material is obviously due to the presence of oxygen-containing groups. The Brodie method, Staudenmaier method, Hummer’s method, and modified Hummer’s method have been used to form graphene oxide from a graphite precursor. This is an inexpensive way to form graphene oxide. Graphene can be considered to be a robust and transparent nano-membrane made up of carbon atoms. In graphene oxide, this robust and transparent nano-membrane has several groups on the surface.18 The functional groups are used to make the graphene nanosheet flexible and hydrophilic. Graphene oxide can be prepared on a large or commercial scale using the inexpensive methods mentioned above.19 It can be used as a low-cost nanomaterial and a source for graphene. It has also been used itself to form polymer/graphene nanocomposites.20 Graphene oxide has also been used as an important nanofiller for polymers. Fullerene is also an important nanomaterial that has been discovered and explored.21,22 It is also a nano-allotrope of carbon. It has a zero-dimensional ball-like or spherical or round nanostructure. The hollow ball-like structure resembles a hollow soccer ball. However, fullerene molecules may assume a slightly ellipsoidal or tube shape of various sizes. The carbon nanotube can be considered as the tube form of fullerene. This is the reason why the nanotubes are also called buckytubes. Fullerene may consist of different numbers of carbon atoms ranging from 24 carbon atoms to several carbon atoms. The most common fullerene form is fullerene C60. Such fullerene molecules are usually formed by the combination of pentagons and hexagons.23,24 The fullerene molecules may have highly symmetrical configurations. Fullerene may show chiral behavior such as in C76, C78, C80, C84, etc. Such fullerenes exist in various enantiomeric forms.

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The enantiomeric forms of fullerenes have potential for various advanced applications. Fullerene molecules form hollow nanostructures, which are able to encapsulate different molecules including the charges and gases. Some big fullerene molecules have also been reported, for example, fullerenes may have more than 300 carbon atoms in their structure.25,26 The fullerite name is often given to the bulk solid form of fullerenes. Both graphene and fullerene are important forms of the nanocarbons. Fullerene and graphene have unique nanostructures and a range of useful properties and applications for advanced materials, electronics, energy, and nanotechnology.

11.3 Carbon Nanomaterial Hybrids as Nanobifillers: Graphene–Fullerene Hybrids There are range of carbon nanofillers available such as graphene, fullerenes, carbon nanotube, nanodiamond, etc. The carbon nanostructures have been combined to gain the benefits of two or more nanocarbons.27,28 Among these nanocarbon materials, graphene is an important carbon nanofiller. Graphene has been successfully combined with the other nanocarbons to form graphene-based nanofillers. Table 11.1 shows the carbon nanobifillers based on graphene nanofillers. The table shows that graphene may be combined with various nanocarbons to form graphene–carbon nanotubes, graphene–fullerene, graphene–nanodiamond, graphene–carbon black, and graphene–carbon nanofiber nanobifiller hybrids. The nanostructure based on the graphene nanobifiller can be designed and controlled using varying methods and techniques. The mechanisms behind the structural alterations of the nanocarbon entities have been studied to reveal its structure, properties, and applicatons. However, the nanocarbon combination mechanism is quite complicated to understand. The combination of fullerene with graphene is rather intricate to be comprehended in detail and to understand all the mechanisms involved in this combination.29,30 A thermal activation process has been used to transform graphene and fullerene into a graphene–fullerene nanobifiller nanostructure.31,32 The molecular dynamics simulations have been used to study the formation and structure of the graphene–fullerene nanobifiller. Different methods have been used to form the graphene–fullerene nanobifillers. Sometimes, metal aggregates or metal clusters (in simple words metal nanoparticles) have been used as catalysts to develop graphene–fullerene nanobifiller nanostructures. Table 11.1

Carbon nanobifillers of graphene.

Carbon nanofillers

Nanobifiller

Graphene

Graphene–carbon nanotube Graphene–fullerene Graphene–nanodiamond Graphene–carbon black Graphene–carbon nanofiber

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For coupling with graphene, endo fullerenes with encapsulated atoms have also been used. As discussed above, due to its endohedral nature, graphene may encapsulate various metals, charges, and gaseous specious in its hollow core structure. Graphene–fullerene alterations have been used for various technical compliances. These nanobifiller nanostructured hybrids have been used in magnetic devices and nanoelectronic devices.33 The graphene– fullerene nanobifiller hybrid has been formed both through van der Waals interactions and covalent bonding.34,35 Though chemical interactions are more probable to develop a stable graphene–fullerene nanobifiller hybrid. For the development of covalent interactions between the graphene-fullerene nanobifiller hybrid, it is essential to chemically modify the graphene molecules and fullerene nanostructures. The modified nanostructures can be reacted to form the graphene–fullerene nanobifiller hybrid.36 In this regard, the well-known density functional theory has been used to elucidate the interactions between the graphene and fullerene nanostructured hybrid.37,38 This theory has helped to reveal the structure and the molecular mechanisms of the nanofillers hybrid.37,38 The geometry of the nanostructures is essential to understand the combination of these entities. In the graphene–fullerene nanobifiller hybrid, it is also important to comprehend the binding energetics responsible for holding the nanostructures together.39 Lebedeva et al.40 used nickel particles to develop graphene–fullerene nanobifiller hybrids. In this regard, the endo-fullerene containing Ni clusters have been used. The molecular dynamics simulations have been applied to study the structure and interactions in the graphene–fullerene nanobifiller hybrid. Use of the endofullerene helped to perform the controlled fabrication of graphene–fullerene nanobifiller hybrids.40,41 The activation energies of the nanostructures were also reduced in the formation of the graphene–fullerene hybrid to 1.5–1.9 eV. The conversion of graphene and fullerene to a graphene–fullerene nanobifiller hybrid may require a high temperature in the range of 700–800 K. Ozturk et al.42 developed a layered nanostructure based on graphene and fullerene. The spherical fullerene molecules were found to be sandwiched between the parallel graphene nanosheets. The graphene–fullerene nanobifiller sandwiched structure has shown good hydrogen adsorption properties. These properties were observed due to the optimum structure porosity present because of the development of the layered and encapsulated structure. The graphene–fullerene nanobifiller hybrid has been investigated and applied for the hydrogen storage capacity. The three-dimensional atomistic model was used to study the chemical linkages between graphene and fullerene. The covalent fusion of graphene and fullerene has enhanced the homogeneity of the nanostructure. Owing to the uniform nanostructure, the graphene– fullerene nanobifiller hybrid may not only store the hydrogen or gases but may also hoard lithium charges. So, such nanostructures can be useful for gas-capturing applications and Li-ion battery uses. The sandwiched graphene–fullerene nanocomposites have been investigated through various methods.43 The most important is the morphology studies to find out the orientation of graphene and fullerene in the hybrid nanostructure. In this

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Figure 11.2

263

The proposed atomistic model of the sandwiched graphene–fullerene C180 nanocomposite. Reproduced from ref. 42 with permission from Elsevier, Copyright 2016.

regard, a pillared graphene nanostructure has been proposed.35,44,45 There are always empty spaces found between the graphene pillars.46–48 The graphene forms nanosheets as pillars and the spherical balls are placed or sandwiched among the graphene layers. Figure 11.2 depicts the sandwiched graphene– fullerene C180 nanocomposite nanostructure. The model consists of four nanosheets with sandwiched fullerene molecules. Formation of the endohedral fullerene is given in Figure 11.3. The cavity in the fullerene is used to store gaseous species and charge entities. In this way, the lithium charges can be easily loaded and stored in the endohedral fullerene (Figure 11.4). Such behavior is obviously useful for lithium-ion battery applications. The graphene–fullerene nanobifiller hybrid has been used to design lithium-ion batteries.

11.4 Aspects of Polymer/Graphene–Fullerene and Polymer/Graphene Oxide–Fullerene Nanocomposites 11.4.1

Polymer/Graphene–Fullerene Nanocomposites

Polymeric nanocomposites have been studied with graphene nanofiller.49,50 In this context, the in situ polymerization and solution mixing methods have been used. The polymer/graphene nanocomposites have been applied in electronics,51 energy storage devices,52 conducting materials,53 solar cell electrodes,54 and electrochromic devices.55 The fullerene C60 nanomaterial

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Figure 11.3

Endohedral hole at the fullerene–graphene junction region. Reproduced from ref. 42 with permission from Elsevier, Copyright 2016.

Figure 11.4

Lithium-doped sandwiched graphene–fullerene C180 structure in different perspectives. Reproduced from ref. 42 with permission from Elsevier, Copyright 2016.

Figure 11.5

TEM micrographs of: (A) graphene dispersion in tetraethylene glycol diacrylate (TEGDA) (0.033 wt%); (B) fullerene dispersion in TEGDA (0.05 wt%); and (C) graphene–fullerene dispersion in TEGDA (0.035/0.035 w/w). Reproduced from ref. 63 with permission from Elsevier, Copyright 2015.

has been used with polymers.56,57 Various methods have been used in polymer/fullerene nanocomposites.58,59 The polymer and fullerene-based nanocomposites have been applied in photovoltaics and supercapacitors.60–62 Alzari et al.63 filled the tetraethylene glycol diacrylate (TEGDA) matrix with graphene, fullerene, and graphene–fullerene

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nanobifiller hybrids. The morphologies of the TEGDA/graphene, TEGDA/fullerene, and TEGDA/graphene–fullerene nanobifiller hybrid have been explored. Figure 11.5 shows the transmission electron micrographs (TEM) of graphene, fullerene, and graphene–fullerene nanobifiller hybrid. The graphene micrograph has shown a transparent nanosheet nanostructure in the matrix. The fullerene particles seem to be dispersed in the micrograph presented. The TEM micrograph of graphene–fullerene-dispersion in the TEGDA matrix showed homogeneous dispersion properties. Liu et al.64 prepared fullerene C60 and functional graphene nanofillers. Fullerene C60, functional graphene, and fullerene C60-functional graphene hybrids were used to form epoxy-based nanocomposites. The morphology, anti-corrosion, and tribological performance of the epoxy/fullerene C60, epoxy/functional graphene, and epoxy/graphene-fullerene nanobifiller nanocomposites have been studies. The nanomaterials were loaded in epoxy resin with 0.25–1 wt% content. Figure 11.6 shows the formation of the epoxy/functional fullerene C60. The same route was used to form the epoxy/ functional graphene nanocomposite. Figure 11.7 shows the dispersion of the epoxy/functional fullerene C60 and the epoxy/functional graphene nanocomposite. The dispersion process affects the corrosion phenomenon occurring in these nanocomposites. The nanofillers act as a barrier towards the diffusion of the corrosive species through the epoxy medium. The graphene–fullerene nanobifiller hybrid shows better dispersion and zig-zag diffusion paths for

Figure 11.6

Preparation process of epoxy/functional fullerene C60 composite coating. Reproduced from ref. 64 with permission from Elsevier, Copyright 2016.

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Figure 11.7

Schematic of epoxy composite coatings with functional fullerene C60 (a) and functional graphene (b) during the corrosion process. Reproduced from ref. 64 with permission from Elsevier, Copyright 2016.

Table 11.2

Electrochemical parameters for the bare and coated substrate obtained from polarization curves. Reproduced from ref. 64 with permission from Elsevier, Copyright 2016. Icorr (A cm2)

Sample Pure epoxy Epoxy/functional Epoxy/functional Epoxy/functional Epoxy/functional

6

C60 0.25 wt% C60 1 wt% graphene 0.25 wt% graphene 1 wt%

8.110 3.7106 3.7106 2.8106 2.5106

Vcorr (mm per year) 10

3.910 1.01010 9.61011 7.61011 5.91011

Z (%) 61.9 82.8 82.4 86.6 88.3

the corrosive species. The anti-corrosion of the epoxy/fullerene C60, epoxy/ functional graphene, and epoxy/graphene–fullerene nanobifiller hybrid have been studied. Tafel polarization has been used as a useful method to study the corrosion behavior of various composite and nanocomposite materials. In this method, the material is tested against corrosive solution and the corrosion resistance performance is determined. The Tafel polarization curves of epoxy/fullerene C60, epoxy/functional graphene, and epoxy/ graphene-fullerene nanobifillers in 3.5 wt% NaCl solution have been considered for the study (Table 11.2). The corrosion current density (Icorr), the corrosion rate (Vcorr), and the anti-protection efficiency (Z) have been evaluated.65 The Icorr revealed lower values due to lower dissolution of the metal ions.66,67 The epoxy/functional graphene coatings have shown a higher anti-corrosion efficiency and barricade performance.68,69 It has been observed that the performance of the nanobifiller hybrid coatings depend on the nanoparticle shape, the nanofiller aspect ratio, the nanofiller surface area, and most importantly the

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70

filler dispersion. Chakravarty et al. developed the porous graphene– fullerene nanobifiller hybrid. The nanostructure was filled in the polymer and tested for optical properties, optoelectronic features, and solar cell application. The graphene–fullerene nanobifiller hybrid has shown the better charge separation competence. Several graphene–fullerene nanobifiller hybrid nanostructures have been proposed for various technical applications including energy devices, batteries, and solar cells. An important application of the graphene–fullerene nanobifiller is in the polymeric nanocomposite.

11.4.2

Polymer/Graphene Oxide–Fullerene Nanocomposites

Polymer and graphene oxide (GO) have been amalgamated to form polymer/ GO nanocomposites. These nanocomposites have been synthesized through the melt method, solution blending, and in situ polymerization. The polymer/graphene oxide nanocomposites have been used in photovoltaics, electronics, microelectronics, and energy related device applications. The graphene oxide and the intercalated fullerene nanostructures have been produced in various attempts.71 The epoxy matrix has been filled with the graphene–fullerene nanobifiller hybrid. The graphene oxide-grafted fullerene has been reported and filled with poly(3-hexyl)thiophene.72 In solar cells, poly(3-hexyl)thiophene acts as a donor whereas fullerene acts as an acceptor. This kind of solar cell has been prepared with the poly(3hexyl)thiophene and graphene–fullerene nanobifiller hybrid. The solar cell with the graphene–fullerene nanobifiller hybrid has a higher power conversion efficiency than the nanomaterial with single nanofillers. In solar cells, there is photoinduced electron transmission from graphene to fullerene.73 In polymer-based solar cells, the graphene–fullerene nanobifiller hybrid acts as an electron donor to the polymers.74–76 Kumar et al.77 formed phenyl[C61]butyric acid methyl ester (PCBM)-based nanocomposites with graphene oxide. The study was performed to develop the covalent linking between the PCBM and the graphene nanofiller. The phenyl[C61]butyric acid methyl ester was processed using various reagents and chemicals to develop phenyl[C61]butyric hydrolyzed acid (PCBA), phenyl[C61]butyric acyl chloride (PCB-Cl), and phenyl[C61]butyric graphene oxide (PCBGO) (Figure 11.8). Thermogravimetric analysis was performed on the modified graphene oxide (mGO) and PCBGO (Figure 11.9). The inclusion of the phenyl[C61]butyric nanostructure with graphene oxide has lowered the thermal stability compared with neat mGO. During the degradation process, bond splintering was observed for PCBM at 900 1C. Wang et al.78 developed the epoxy and graphene–fullerene nanobifiller hybrid-based nanocomposites. The inclusion of the 1.0 wt% graphene– fullerene nanobifiller hybrid has been used to enhance the nonflammability properties of the epoxy matrix. The flame hindering efficiency of the epoxy has been increased using the graphene–fullerene nanobifiller hybrid. Various polymers have been used with the

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Figure 11.8

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Synthesis of phenyl[C61]butyric hydrolyzed acid (PCBA) (i) glacial acetic acid:HCl (4:1), phenyl[C61]butyric acyl chloride (PCB-Cl), (ii) CS2, thionyl chloride, and phenyl[C61]butyric graphene oxide (PCBGO), and (iii) mGO, water, and triethyl amine. Reproduced from ref. 77 with permission from Elsevier, Copyright 2016.

graphene–fullerene nanobifiller hybrids. The resulting polymer/graphenefullerene hybrid has the superior morphology, optical features, conductivity, anti-corrosion, tribological properties, optical properties, thermal stability, non-flammability, and dispersion. These polymer/graphene–fullerene hybrid materials have been used in solar cells, energy appliances, electronics, thermally stable materials, mechanically robust materials, and nonflammable materials. Consequently, the polymer/graphene–fullerene nanobifiller hybrids need to be applied in aerospace, automobile, and biomedical fields.

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Figure 11.9

269

TGA thermograms of mGO and PCBGO under a nitrogen atmosphere. Reproduced from ref. 77 with permission from Elsevier, Copyright 2016.

11.5 Prominence and Future Visions of Graphene– Fullerene-based Nanomaterials in High Performance Applications Table 11.3 demonstrates the essential details and characteristics of the polymer and graphene–fullerene nanobifiller-based nanomaterials. The nanofillers such as graphene, GO,79 carbon nanotube, fullerene,80 nanodiamond,81,82 and their nanobifiller hybrids83–85 have been prepared. The polymer/nanobifiller hybrids have been used in several essential technical applications.86,87 The carbon materials have shown potential for photovoltaics or solar cells. Especially, the graphene–fullerene nanobifiller hybrids have been used to form polymer/graphene–fullerene nanobifiller-based nanomaterials for solar cell applications. The polymer/graphene–fullerene nanobifiller hybrid nanocomposites have been coated on the ITO substrate for photovoltaic applications. The polymer/graphene–fullerene nanobifiller hybrid nanocomposites have shown a visible increase in the power density and power conversion efficiency of the materials. For the solar cells, the various modified graphene–fullerene nanobifiller materials are needed to further enhance the solar cell efficiency. The graphene–fullerene nanobifiller hybrid has been filled in various polymers. However, the conjugated polymer/ graphene–fullerene nanobifiller hybrid has shown a quite better response

270 Table 11.3

Chapter 11 Features of polymer nanomaterials.

and

graphene–fullerene

nanobifiller-based

Matrix

Nanobifiller

Property

Ref.

Tetraethylene glycol diacrylate Epoxy

Graphene–fullerene

Morphology/dispersion

63

Graphene

64

Epoxy

Fullerene C60

Epoxy

Graphene–fullerene

Polymer

Graphene–fullerene

Phenyl[C61]butyric acid methyl ester Epoxy

Graphene oxide–fullerene

Morphology, anti-corrosion, tribological performance Morphology, anti-corrosion, tribological performance Morphology, anti-corrosion, tribological performance Optical properties, optoelectronic features, solar cell applications Thermal stability

Graphene oxide–fullerene

Non-flammability

64 64 70 77 78

for photovoltaic applications. For example, the design of polythiophene/ graphene–fullerene nanobifiller hybrids is found to be useful and applicable. Other polythiophene derivatives have also been used with the graphene–fullerene nanobifiller hybrid for solar cells. The optoelectronic properties of the polythiophene/graphene–fullerene nanobifiller hybrid are essential for certain advanced high performance applications. These materials are useful in organic electronic devices. These materials may develop a useful interface for charge or photon transport. This kind of conducting polythiophene/graphene–fullerene nanobifiller hybrid has also found scope due to its anti-corrosion properties. It has been observed that highly conducting carbon materials are always suitable for corrosion resistance. The morphology of polymer/graphene–fullerene nanobifiller hybrid nanocomposites is also useful to be explored to gain the precise design knowledge for the end application. However, several useful application areas of the polymer/graphene–fullerene nanobifiller are still unexplored owing to the smaller amount of research undertaken on the graphene–fullerene nanobifiller hybrid compared with the graphene or fullerene singly-oriented nanobifillers. In short, graphene–fullerene nanobifiller hybrids have a range of advanced technical applications in the energy, electronics, biomedical, and other fields. Electrodes based on graphene–fullerene nanobifiller hybrids can be developed. The use of graphene–fullerene nanobifiller hybrids may considerably enhance the power density, conductivity, capacitance, power conversion efficiency, and cycling ability of the materials. Dye-sensitized solar cells can be developed using polymer and graphene-fullerene nanobifiller hybrids. Consequently, the photovoltaic efficiency of future devices can be enhanced several times using graphene–fullerene nanobifiller hybrid materials. Graphene–fullerene nanobifiller hybrids and their derivatives can be applied in gas sensors. Conducting polymers and graphene–fullerene

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nanobifiller hybrid-based gas sensors may show high gas adsorption and selectivity towards various gases. The polymer and graphene–fullerene nanobifiller hybrids need to be exploited in electronic devices. Moreover, the polymer and graphene-fullerene nanobifiller hybrids may be applied in biomedical-related applications including drug delivery, biosensing, and cell imaging. Although, the use of graphene–fullerene nanobifiller hybrids is crucial in these advanced applications. Nevertheless, graphene–fullerene nanobifiller hybrid functionalization, hybrid processing, and application challenges need to be addressed to attain the full potential of graphene– fullerene nanobifiller hybrids in these fields.

11.6 Summary This chapter presents the state-of-art of graphene, fullerene, and graphene– fullerene nanobifiller hybrids. The behavior of graphene, fullerene, and graphene–fullerene nanobifiller hybrids has been observed in various matrices. The graphene–fullerene nanobifiller has been designed for various technical purposes. Graphene, fullerene, and graphene–fullerene nanobifillers have been filled in the polymers. In polymer matrices, the graphene–fullerene nanobifiller hybrid may form van der Waals interactions, other physical associations, and covalent bonding. The compatibility between the graphene–fullerene nanobifiller hybrid and the polymers have improved the characteristics of the matrices. The desirable optical, electrical, thermal, mechanical, and other properties of the hybrids have been affected by the graphene–fullerene nanobifiller hybrid structure. These polymer/graphene–fullerene nanobifiller hybrids have been employed in solar cells, energy storage, electronics, and other technical applications. Further efforts are needed for the functionalization of graphene and fullerene to form compatible nanostructures for high performance applications.

References 1. Y. Gogotsi and V. Presser, Carbon nanomaterials, CRC press, 2013. 2. L. Dai, D. W. Chang, J. B. Baek and W. Lu, Carbon nanomaterials for advanced energy conversion and storage, Small, 2012, 8(8), 1130–1166. 3. A. Kausar, Carbon nano onion as versatile contender in polymer compositing and advance application, Fullerenes, Nanotubes, Carbon Nanostruct., 2017, 25(2), 109–123. 4. A. Kausar, A Study on Poly (vinyl alcohol-co-ethylene)-graft-Polystyrene Reinforced with Two Functional Nanofillers, Polym.-Plast. Technol. Eng., 2015, 54(7), 741–749. 5. A. Kausar, Enhanced electrical and thermal conductivity of modified poly (acrylonitrile-co-butadiene)-based nanofluid containing functional carbon black-graphene oxide, Fullerenes, Nanotubes, Carbon Nanostruct., 2016, 24(4), 278–285.

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25. B. Yadav and R. Kumar, Structure, properties and applications of fullerenes, Int. J. Nanotechnol. Appl., 2008, 2(1), 15–24. 26. P. Weis, F. Hennrich, R. Fischer, E. K. Schneider, M. Neumaier and M. M. Kappes, Probing the structure of giant fullerenes by high resolution trapped ion mobility spectrometry, Phys. Chem. Chem. Phys., 2019, 21(35), 18877–18892. 27. A. Kausar, Review of fundamentals and applications of polyester nanocomposites filled with carbonaceous nanofillers, J. Plast. Film Sheeting, 2019, 35(1), 22–44. 28. A. Kausar, Polyacrylonitrile nanocomposite with carbon nanostructures: a review, Polym-Plast Tech Mat, 2019, 58(7), 707–731. ´nchez-Sa ´nchez, et al., Fullerenes from 29. G. Otero, G. Biddau and C. Sa aromatic precursors by surface-catalysed cyclodehydrogenation, Nature, 2008, 454(7206), 865–868. 30. A. Chuvilin, U. Kaiser, E. Bichoutskaia, N. A. Besley and A. N. Khlobystov, Direct transformation of graphene to fullerene, Nat. Chem., 2010, 2(6), 450–453. 31. I. Lebedeva, A. Knizhnik, A. Bagatur’yants and B. Potapkin, Kinetics of 2D–3D transformations of carbon nanostructures, Phys. E Low Dimens. Syst. Nanostruct., 2008, 40(7), 2589–2595. 32. I. Lebedeva, A. Knizhnik and B. Potapkin, The kinetics of carbon nanostructure 2D–3D transformation, Russ. J. Phys. Chem. B., 2007, 1(6), 675–684. 33. N. A. Poklonski, E. F. Kislyakov and S. A. Vyrko, et al., Magnetically operated nanorelay based on two single-walled carbon nanotubes filled with endofullerenes Fe@ C 20, J. Nanophotonics, 2010, 4(1), 041675. 34. F. Du, D. Yu, L. Dai, S. Ganguli, V. Varshney and A. Roy, Preparation of tunable 3D pillared carbon nanotube–graphene networks for highperformance capacitance, Chem. Mater., 2011, 23(21), 4810–4816. 35. G. K. Dimitrakakis, E. Tylianakis and G. E. Froudakis, Pillared graphene: a new 3-D network nanostructure for enhanced hydrogen storage, Nano Lett., 2008, 8(10), 3166–3170. 36. S. Laref, A. Asaduzzaman and W. Beck, et al., Characterization of graphene–fullerene interactions: Insights from density functional theory, Chem. Phys. Lett., 2013, 582, 115–118. 37. L. Liu, M. Krack and A. Michaelides, Density oscillations in a nanoscale water film on salt: Insight from ab initio molecular dynamics, J. Am. Chem. Soc., 2008, 130(27), 8572–8573. 38. S. Laref, Y. Li, M.-L. Bocquet, F. Delbecq, P. Sautet and D. Loffreda, Nature of adhesion of condensed organic films on platinum by firstprinciples simulations, Phys. Chem. Chem. Phys., 2011, 13(25), 11827– 11837. 39. Y. Dappe, J. Ortega and F. Flores, Intermolecular interaction in density functional theory: Application to carbon nanotubes and fullerenes, Phys. Rev. B, 2009, 79(16), 165409.

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CHAPTER 12

Mechanical Properties of Graphene–Carbon Nanotube Reinforced Hybrid Polymer Nanocomposites SUSHANT SHARMAa,b AND BHANU PRATAP SINGH*a,b a

CSIR-National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi-110012, India; b Academy of Scientific & Innovative Research (AcSIR), India *Emails: [email protected]; [email protected]

12.1 Introduction to Polymer Nanocomposites Polymeric composite materials have been employed for a number of advanced technological applications like biomedical, electronics, structural composites, sports goods, automobiles, aerospace, energy, protection gears etc. due to their vast range of properties.1–8 Incorporation of different reinforcement can attribute different composites having different traits and applications. In polymeric composites, fillers play an important role in transforming the desirable properties of polymer and tumbling the cost of the composite. Conventionally, polymeric composites were reinforced with micrometer scale fillers such as calcium carbonate, glass blends, kaolin and talc (mostly minerals and glass based) to improve the mechanical properties of polymeric composites.9 These properties can be better governed by controlling the volume fraction, shape and size of the filler particle. Furthermore, these properties can be improved by using high aspect ratio fillers. All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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In this direction, nanomaterials are potentially adopted as a filler material due to their high specific surface areas of more than 1000 m2 g1. High aspect ratio provides large surface-to-volume ratio, these surfaces are inherently high energy sites.10,11 When these high surface energy nanofillers are reinforced in the polymer matrix, they result in improved interfacial properties. According to polymer nanocomposite theory, strong interfacial properties improve overall mechanical properties due to enhanced interfacial bonding.12 In this direction, different types of carbon nanomaterials having different spn hybridizations are used as reinforcement. Well known are diamond and tetrahedral amorphous carbon as sp3 hybridized, while sp2 hybridization is present in graphite, fullerene, graphene, CNTs, and linear carbon chain of sp hybridized carbyne.13–16 The experimental research in this field has focused profoundly on the synthesis of graphene, CNTs, nanodiamonds and carbyne by using various techniques. Out of these nano-forms of carbon, graphene and their reforms and CNTs and their reforms are widely used as reinforcement in almost every structural polymer matrix. It is because, apart from a high aspect ratio, these graphitic nanomaterials also possess superior mechanical, electrical and thermal properties as well.17–21 Figure 12.1

Figure 12.1

Ashby plot of tensile strength plotted against density comparing the mechanical properties of conventional polymer composites with CNT and graphene-based composites. The envelopes for engineering materials are shown as shadows in the background.22 Reproduced from ref. 22 with permission from Elsevier, Copyright 2009.

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illustrates the Ashby plot for representing the potential of CNT and graphene as reinforcement in polymer composites. Due to their superior mechanical properties (tensile strength B800–900 GPa and modulus B1 TPa), they are frequently used as reinforcement in various forms (short discontinuous particulate filler or continuous fiber filler). Although the stiffness of these materials are extremely high, owing to a nearly defect-free graphitic structure, but reinforcement of these structures in the polymer matrix is an extremely challenging task. Primarily, both graphene and CNT in particulate filler form possess large dimension (lateral size of the graphene and length of CNT), generally reaching micrometers or sometime millimeters also. These short particulates are associated with poor load carrying capacity due to agglomeration during composite fabrication by dispersion technique. Secondly, the surface morphology of graphene and CNTs are inheritedly smooth, which inhibit the strong bonding between matrix and fiber, leading to a poor interfacial loading capacity during mechanical deformation.22,23 Poor interfacial problem is a major roadblock in carbon nanofiller reinforced composites and researchers have figured out the chemical functionalization to attach some necessary functional group on their surface.24,25 But, in the case of graphene and CNTs, chemical functionalization can affect their inherit properties.26 Thus, for conserving the graphitic structure of carbon nanofillers and simultaneously improving the state of dispersion in the polymer matrix, a unique technique was adopted by many research groups. In which hybrid 3D nanofiller was prepared from 1D CNTs and 2D multilayered graphene. The long convoluted CNTs uniformly bridge between the adjacent graphene layers and result in a 3D high surface area nanofiller, which when reinforced into polymer synergistically improve the mechanical properties of the polymer composite.27–29 The study of the mechanical properties of hybrid 3D graphene/CNT (GCNT) nanofiller reinforced composites is rapidly attracting attention from both academia and industrial communities and therefore requires more insight of this hybrid carbon nanofiller and their reinforced composites. This chapter brings updated information dealing extensively with the fabrication of hybrid nanofiller reinforced composites and their mechanics. In the first three sections of the chapter, properties and preparation techniques of different carbon nanofillers are elaborated, which also involves the details on hybrid carbon nanofillers. Furthermore, the synergistic effect of individual carbon nanofillers in polymer nanocomposites will be discussed in detail. Finally, the reinforcement effect of 3D hybrid carbon nanofillers on interfacial and mechanical properties will be discussed, which cover the present and future scenario of hybrid carbon nanofiller reinforced nanocomposites.

12.2 Nanofillers The science of filler reinforced composites has established that the size of the filler material has a profound effect on the resulting properties of the

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composite. This is because the surface interaction with polymer matrix, adhesion, polymer chain motion, dispersion, interaction etc. depends on the size of the particles. As some of these effects dominate the reduction in the size of the filler and they cause an intense effect on the nanoscale. When the nanofiller surface area to volume ratio is high, it drastically affects the various interface dependent properties like electrical resistivity, catalytic reactivity, chemical reactivity, gas storage, polymer adhesion etc. Also, phenomenon like quantization of energy, quantum confinement, electromagnetic forces and molecular motion are very dynamic at the nanoscale (Figure 12.2). Resultantly, these phenomenons give rise to intermolecular bonding, hydrogen bonding, van der Waals interactions, catalysis, surface energy, etc. These prevalent effects are the basis of nanotechnology. When a polymer matrix combined with the reinforcement material having one or more dimensions is in the nanoscale, a 3D structure is formed which is termed as a nanocomposite. Dimensionally, these reinforcement materials can be categorised as 1D, 2D and 3D in which one, two and three dimensions respectively are in the nanoscale (o100 nm).30,31 Nanocomposites were initially used in 195932 but appeared as a research material, when Toyota attempted to exfoliate clay nanofillers in nylon 6.33,34 The study demonstrated the effect of reinforcing the nanofiller on various mechanical properties of polymer-based composite and further used that composite material for tyre applications. Afterward, extensive research in polymer nanocomposites, nanofillers and their potential applications has been

Figure 12.2

Anomaly for nanoscale; starting with hair to the water molecule.

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Figure 12.3

Classification of nanofillers.

carried out globally. In polymer nanocomposites some theories are very essential and must be taken into consideration as they directly affect the properties of the end composite. Depending on the desired properties of composites for particular application, a different combination of nanofiller, polymers and processing variables were used in the past few decades. Figure 12.3 describes another parameter to distinguish nanofiller i.e. organic and inorganic filler which may be 1D, 2D or 3D.30 Organic filler includes polymer nanofibers, natural fibers and natural clay, while inorganic nanofiller involves nanoclays, various metal particles, metal oxides and most importantly carbon nanofillers. According to the properties required in polymer nanocomposites, the selective size, shape, volume fraction and state of dispersion will be considered. Now, areas involving graphene and other carbon nano-materials are become the focus point of various research groups.

12.2.1

Carbon-based Nanofillers

Carbon is the most versatile element of the periodic table which offers matchless traits and therefore is in great demand nowadays. It is the most abundant material in the earth’s crust and also in the universe after hydrogen, helium and oxygen. From a structural point of view it can exist in both amorphous and crystalline forms. Carbon deployment for the insulator

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diamond, layered graphite, nanosize fullerene, monolayer graphene, nanotubes, long atomic chain carbyne, coal lamp black etc. depend upon atomic arrangement and type of hybridization. Diamond, fullerene, graphene, and carbon nanotube are crystalline carbon nanofillers while coal, charcoal and lamp black are amorphous forms of carbon. All allotropic forms of carbon are solids under normal conditions, with graphite being the most thermodynamically stable form. Carbon having atomic number six, belongs to the group IV family in the periodic table. A carbon atom possesses six electrons and therefore the electron configuration 1s2 2s2 2p1x 2p1y in its ground state (lowest energy state). There is a large energy difference between the 1s and 2s orbital and a very small difference in energy between 2s and 2p orbitals. The 1s2 orbital contains two strongly bound core electrons. The remaining four weakly bound electrons occupy the 2s2 2p2 valence orbitals. The valence electron in the crystalline phase tends to appear as 2s 2px 2py and 2pz, which is helpful in forming the covalent bond with other carbon atoms to form different carbonaceous structures. The energy difference between 2s and 2p in carbon is small (B4 eV) compared to the binding energy of the chemical bonds, the electronic wave function of these four electrons can eagerly mix with each other, hence to increase the binding energy of the C atom, the occupational state changes to 2s and three 2p. Here, the general mixing of 2s and 2p atomic orbitals is termed as hybridization. The mixing of 2s electron with one, two and three 2p electrons is called spn hybridization (n ¼ 1, 2, 3) which is responsible for the different crystal structures of carbonaceous materials. Though carbon has been known for its various forms, during the last three decades its two graphitic forms carbon nanotubes (CNTs) and graphene have been investigated a lot for various technological applications.

12.2.1.1

Carbon Nanotubes

During 1950, Roger Bacon from Union Carbide observed linear hollow nanowhiskers with carbon layers separated by the same spacing as the palmer layer of graphite during carbon fiber analysis. Furthermore, in the 1970s Morinobu Endo from the University of Orleans, observed hollow carbon fibers under a high-resolution transmission electron microscope, which was produced by pyrolysis of benzene and ferrocene at 1000 1C. The global interest for carbon-based nanomaterials came after Sumio Ijima synthesized hollow nanocarbon by an arc discharge technique in 1991.15 CNTs are the thinnest tubular one-dimensional nanomaterial humans have ever made, which is nothing but a rolled-up structure of graphene. If the seamless roll is made up of monolayer graphene it is termed as a singlewall carbon nanotube (SWCNT) and if it is made up of multilayer graphene it is called a multiwalled carbon nanotube (MWCNT). Their diameter ranges B0.4–3 nm for single-wall and B1.4–100 nm for multiwall CNTs.35,36 They are chemically and thermally very stable and therefore used in a wide range of advanced applications.37–39 Generally, these nanomaterials possess a high aspect ratio (length to diameter 41000) therefore their properties change

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with structure. Some of CNTs distinguishing properties are high strength, superior toughness and excellent thermal and electrical conductivity.41,42 Figure 12.4 represents the high-resolution TEM and molecular representation of SWCNTs and MWCNTs. There are plenty of techniques which are currently available for synthesising CNTs. The main CNT production techniques are arc discharge, laser ablation, chemical vapour deposition, high-pressure carbon monoxide process, and fluidized bed.44–46 The arc discharge and laser ablation use a solid-state carbon source which is vaporized at higher temperature, while on the other hand chemical vapour deposition requires fluidic or gaseous carbon sources to flow at moderate temperature.47 These CNT production techniques require a large amount of energy, closely optimized parameters and generate large amounts of undesirable by-products, which hinder their large scale production and also adversely affect the environment. To overcome these problems and to match the yield with industrial requirement the high-pressure carbon monoxide (HiPco) process and fluidized bed CVD techniques are used.41,48,49 CNTs are endowed with unusual combinations of material properties which are very close to their theoretical values, such as high strength and

Figure 12.4

High-resolution TEM and molecular structural illustration of different CNTs showing diameters of (a) single-wall CNTs and (b) multiwall CNTs.43 Reproduced from ref. 43 with permission from Elsevier, Copyright 2010.

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modulus, excellent thermal and electrical properties and low density. The outstanding mechanical properties of CNTs originate from their geometrical structure and C–C bonds. The carbon–carbon basal plane bond is the strongest bond in nature which delivers a high Young’s modulus and tensile strength to CNTs. Various efforts have been made to establish the mechanical, electrical and thermal properties of CNTs experimentally and theoretically. From the studies, it is established that the Young’s modulus and tensile strength have reached the values of approximately 1.4 TPa and 100 GPa at 20–30% elongation. Specific gravity of any material plays an important role in the design of any application. The specific gravity of CNTs is as low as upto 0.8 which make them very useful for a variety of lightweight high-strength applications. Therefore, these high-strength low-density CNTs are the ideal candidate for reinforcement in composite materials.50 As well as mechanical properties, CNTs also possess excellent electrical and thermal transportation properties. The thermal and electrical conductivity at room temperature have been determined as 3000 W mK1 and 10 000 S cm1 respectively, which enforce the CNTs to introduce multifunctional properties in polymer nanocomposites for various applications.51,52

12.2.1.2

Graphene and Graphene Oxide

In 2010, Andre Geim and Konstantin Novoselov showed that carbon in such a flat form has exceptional properties that originate from the remarkable world of quantum physics. Ten years after receiving the Nobel Prize in physics for the thinnest 2D material graphene, the global interest in this wonder material has increased rapidly.53 Graphene is a 2D crystal of one-atom thick layer of sp2 hybridized carbon, arranged in a defined honeycomb lattice structure.18,48 It is the basic building block of graphitic materials of all other dimensionalities such as fullerene, CNTs and graphite. The 2D structure of graphene’s crystal lattice and unique band structure give rise to a range of exceptional physical, mechanical, chemical and transportation properties.54–57 Some of these established properties are thermal conductivity B5000 W mK1, electron mobility at room temperature 250 000 cm2 Vs1, surface area 2630 m2 g1, modulus of elasticity B1 TPa and electrical conductivity 240 000 S m1.9,17–19,58 These outstanding properties make this material suitable for a wide range of applications including high-end composite materials, fieldeffect transistors, electrical devices, strain sensors, hydrogen storage, super capacitors, solar cells etc. But, the implementation of graphene in the aforementioned applications is still a challenging task as the most important problem associated with the production of high quality and well-defined graphene in bulk. Another important parameter, which is essential to take advantage of the unique properties of this novel material in bulk, is adequate dispersion. Therefore, various strategies have been followed to take care of these disadvantages, which will be discussed in the upcoming sections of this chapter. There are several techniques available to fulfill the

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demand of both industries and academia. Few of these strategies have already been proposed in the literature and a number of them are employed by industrial manufacturers. Mainly, these strategies involve, mechanical exfoliation,59 liquid phase exfoliation,60,61 electrochemical exfoliation,62,63 chemical vapour deposition technique,64,65 chemical reduction of graphene oxide66 and epitaxial growth using SiC.67,68 Out of these, mechanical, liquid phase and electrochemical exfoliation are top-down approaches. While chemical vapour deposition and epitaxial growth over SiC substrates are bottom-up approaches to prepare high-quality graphene. Intensive research efforts have been made to improve the quality and yield of graphene in the past few years. Each production technique attributes different characteristics to the end product and have different up-scaling probabilities. The draw back and advantage of each production technique have been nicely summarized by Raccichini et al.62 Their study represented the clubbed view of different synthesis techniques and their associated traits i.e. graphitic quality, production cost, scalability, yields, etc. The characteristic properties of output product and application for which the graphene will be used, decides the selection of the technique. These 2D materials show considerable promise for structural composite applications due to their exceptional strength, elastic modulus and structural flexibility caused due to their chemical bond strength and minimal atomic thickness. However, the ultimate performance of these graphene reinforced composites depend on their interfacial properties, graphene morphology including shape, size and number of chemical defects present on the surface. For instance, oxidized sp3 carbon atoms and vacancies present in the graphene sheet can hamper its mechanical properties, but can improve its interaction with polymer matrices, thus enhancing the effective load transfer and leading to a most effective mechanical reinforcement.69 Thus chemical defects at the nanoscale can control the mechanical properties at the macroscale and it is possible to obtain the high-yield monoatomic soluble 2D sheets up to several micrometers by a chemical oxidation route. GO can be synthesised by strong oxidation and chemical exfoliation of graphite using strong oxidants like sodium nitrate, sulfuric acid, and potassium permanganate, which chemically attach the stable graphitic structure using reactive dimanganese heptoxide (Mn2O7).70 Electrochemical techniques are also used to enhance the chemical reactivity and hence the exfoliation of graphene in the solution. This technique yields the high degree of exfoliation of graphite in a few minutes by using electrolyte solution less troublesome than the strong acid. Another important advantage of this approach is that it allows precise control of the sheet oxidation, giving sheets that are larger and more soluble than those obtained by ultrasound exfoliation, but have less defects than the GO sheets produced by the Hummers method.71,72 The high processability of GO is due to the presence of various hydrophilic moieties over its surface (hydroxyl, carboxyl and epoxy groups) that improves the interaction with the polymer matrix and also enhances the state of

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dispersion, which is not possible in the case of high quality graphene. However, the presence of these chemical defects and vacancies destroyed the inherit transportation properties and also drastically affected its mechanical properties. To overcome this problem along with maintaining the state of dispersion, a hybrid structure has been introduced, which will be discussed in detail in the upcoming sections of this chapter.

12.2.1.3

Carbon Nanofibers

Carbon fibers were first prepared by Thomas Edison in 1879 by carbonization of natural fiber (cotton and bamboo strands) for energy conversion and storage, sensing device and reinforcement of composite. In 1889, Hughes and Chamber filed a patent regarding filamentous carbon nanofibers (CNF) via gas (mixture of methane hydrogen) pyrolysis. The real application of these nanofibers came later when their structural and morphological analysis was done using electron microscopy. The first electron micrograph of CNF was captured in the early 1950s by the Soviet scientists Radushkevich and Lukyanovich, showing 50 nm diameter hollow graphitic carbon fibers. Furthermore, in 1970, the Japanese researcher Morinobu Endo, reported the discovery of CNF, which were mostly hollow tubes. Later in 1980, researchers worked in the direction of synthesis and properties of these fibrous materials for various advanced applications. CNF is the kind of nanoscale fiber, which is a quasi-one-dimensional carbon material having dimensional ranges between the carbon nanotube and carbon fiber.73 Its diameter generally ranges between 10 and 500 nm and its length is in the range of 0.5 to 200 mm. Structurally, it can be categorized as a hollow nanofiber and solid nanofiber and possesses a high degree of crystallinity and therefore is widely used as reinforcement and is ideal for electrical and thermal conductivity. They are light in weight, easy to process and shape and have excellent corrosion resistance properties, therefore are widely used as reinforcement. The structural defects depend on its diameter and decreases with respect to the diameter, hence improving the mechanical properties. They also possess high flexibility. When the diameter decreases, its aspect ratio is increased which improves the adhesion with polymer matrix, hence it allows the fibers to maintain the aspect ratio without bending or breaking. CNF has a low density of 1.3–2 g cm3, high tensile strength of 3–500 GPa and high elastic modulus of 240–1500 GPa. The thermal conductivity ranges between 1950–6000 W mK1 and electrical conductivity is in the range of 1000–10 000 S m1. The properties of CNFs have a wide range of variation due to the difference in the structure. If we compare the mechanical properties of CNF reinforced polymer composite with previously discussed CNT reinforced composite, we will observe that their mechanical properties are low compared to CNT reinforced composites. The main reason behind it is the better intrinsic properties of the CNTs over CNFs. Therefore a great deal of effort has been

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made for the production of CNFs with advanced mechanical properties which are desired to formulate multifunctional CNF reinforced composites. There are several techniques available nowadays to produce CNFs with a different shape and structure, which generally involve chemical vapour deposition, electrospinning templating, drawing, and a phase separation route. CVD, assisted with thermal or plasma-based pyrolysis is the most common technique to produce vapour grown carbon nanofibers (VGFCFs). Herein, gaseous molecules are decomposed at elevated temperature and carbon atoms are deposited over a metallic catalyst and subsequent growth of fibers takes place. This complete process involves various sub-processes to make hollow fiber-like gas decomposition, carbon deposition, fiber growth, fiber thickening and graphitization. The growth mechanism of CNF depends on the metallic particle and carbon feedstock flow rate and depending on that the CNF can have shell, fish bone, plate and amorphous structures.74,75 Electrospinning is another commercially used technique for the preparation of CNFs, mostly polymeric nanofibers. Due to its ease of control and environmental compatibility, it is considered as a flexible and powerful approach to produce polymer or composite nanofibrous mats, with a fiber diameter in the submicron to nanometre range.76,77 There are various parameters which govern the properties of final CNFs, such as the type of polymer solution, solvent, capillary size flow rate, working distance collecting speed, and applied voltage. There are other different techniques available to produce solid or hollow CNF such as drawing, template synthesis, phase separation and self-assembly.78–81

12.2.1.4

Carbon-based Hybrid Nanofiller

A combination of more than one nanofiller is termed as a hybrid nanofiller, when these nanomaterials are reinforced in the polymer matrix, it not only delivers their individual properties to the matrix but is also helpful in increasing the inherited properties of the matrix. Generally, it is observed that the polymer nanocomposite reinforced with low concentration single nanofiller can improve the inherited properties of nanocomposites, but have aggregation problems after certain concentration nanofillers fail to deliver their properties effectively. Additionally, this agglomeration can also affect the processability of the nanocomposite, which directly controls their properties. Therefore, hybrid carbon nanomaterials which are a combination of nanofiber, graphene, GO, nanodiamonds, and CNT have been extensively prepared and reinforced in the polymer matrix for various advanced applications. Out of these hybrid nanofillers 2D graphene and their reforms along with 1D CNT are widely studied combinations of hybrid nanofiller for improving the mechanical properties of polymer nanocomposites. Graphene and MWCNTs together form an intercross linking arrangement with each other which helps in transferring load transfer effectively to the polymer matrix.

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Effectiveness of hybrid nanofiller reinforced composites depend upon the ratio of the nanofiller, physical properties of individual nanofiller, type of interaction between nanofillers, and concentration of nanofiller in a matrix system. Substantial research has been conducted to determine the effect of these parameters on the mechanical performance of the nanocomposites which will be discussed in the upcoming sections of this chapter.

12.3 Graphene–CNT Hybrid Nanofiller The poor dispersion of carbon nanomaterials and the weak interfacial interaction between the nanofillers and polymer matrix greatly limit the reinforcing efficiency of nanofillers in polymers. The major limitation with graphene and CNT are restacking and agglomeration respectively due to van der Waals forces between the carbon nanofillers, which reduces the effectiveness of the nanofiller in the composites.82,83 The incorporation of hybrid nanofiller can overcome the limitation and lead to the improvement in mechanical, electrical and thermal properties of nanocomposites, in comparison with the single nanofiller reinforced composite. There are various mechanisms studied so far for the fabrication of hybrid graphene–CNT structures, which will be discussed in the upcoming sections. Among the 2D graphene-based materials, GO is more effective in improving the reinforcement efficiency due to its sheet-like structure and presence of oxygen functional group on both the surfaces of the sheet, which provides versatile sites for adhesion. Grafting polymer molecules and functional groups is the best way to enhance the compatibility and state of dispersion of GO in the host polymer.84,85 Although attaching the functional group through covalent bonding can strengthen the interfacial properties, the polymer on low modulus GO may sometimes reduce the intrinsic properties of GO. On the other hand, CNT is the noteworthy nanofiller for reinforcement in the polymer matrix due to ultrahigh elastic properties. Various studies have been reported on the interaction between CNT and adjacent graphene in which CNTs are bridged between the graphene layers and inhibits their restacking, resulting in increased active surface area for adhesion with the polymer matrix. This puts forward a new technique to improve the dispersion of graphene along with CNTs in the polymer matrix. Hybrid graphene–CNTs are typically bonded by p–p interactions, which can prompt functionalization due to the difference in the physical characteristics. Apart from this non-covalent p–p interaction, there are other covalent and hydrogen bonds that are also used to form the hybrid structure of graphene and CNT,86–88 which collectively possess high surface area for adhesion, superior mechanical, electrical and thermal properties.

12.3.1

Synthesis of Graphene–CNT Hybrid Nanofiller

Depending on the type of graphene forms and carbon nanotubes, there are various techniques available to prepare hybrid graphene–CNTs. Graphene

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nano-platelets are slightly different from GO and possess low dispersibility due to non-availability of oxygen carrying functional groups. Graphene nano platelets derived from the chemical reduction of GO are termed as reduced GO or rGO. Various methods used to prepare hybrid graphene–CNT nanofillers are described below.

12.3.1.1

Solution Processing

Solution processing is the simplest technique to prepare hybrid graphene– CNT nanofiller. In this technique, chemically converted GO sheets and treated or pristine CNTs are dispersed in anhydrous hydrazine, which result in a stable suspension of graphene and CNTs. In this technique GO sheets are also reduced to rGO. The stable suspension of graphene nanosheets and CNTs can transform into thin sheets of hybrid nanofiller by a dip-coating technique or can be transformed into hybrid nanofiller by evaporating the solvent. Furthermore, these hybrid nanofiller sheet and nanofillers are directly used in direct applications for sensing, electronic devices, and nanocomposites.89–91 In hybrid graphene–CNT filler tortuous CNTs attached themselves at the available active sites of the graphene sheet and by a bridging action between two adjacent sheets provide the surplus surface area for more adhesion. In this way, both 2D graphene sheets and 1D CNTs help each other to maintain the state of dispersions in the polymer nanocomposites. Furthermore, the mechanism of interaction between two nanofillers depends on the size of the graphene sheets and CNTs, availability of functional group on the surface and the dispersion technique. In this direction, various researchers gave several mechanisms of interaction. Li et al. revealed the mechanism of GO–CNT interaction by adopting a computational approach since directly imaging the GO–CNT structure is experimentally difficult. In particular (10, 10) SWCNT and GO are used for molecular dynamics (MD) simulation and screen shots of the MD simulation are represented in Figure 12.5 (a and b). When GO attached to the CNT surface, the intermolecular forces between them become stronger, the GO molecule begins to curl up and wrap around the CNT and forms a scrolled structure.82 Furthermore, the binding energy between GO–CNT complexes was calculated to confirm the interaction. Apart from this, few researchers have postulated that hybrid graphene– CNTs prepared by this technique possess the 3D nanostructure in which convoluted CNTs spread uniformly over the surface of the graphene sheet and form a grass mat structure. Patole et al. used this technique to prepare the thin sheet of graphene and CNTs for electrode applications.92 They used the graphene crystals and CNT in a 1 : 1 ratio and prepared a free-standing thin sheet of hybrid nanofiller as represented in Figure 12.6c. Similarly, our group has also prepared a hybrid nanofiller using this technique and further used it as reinforcement in a acrylonitrile butadiene styrene (ABS) polymer.93–98 After conducting static and dynamic mechanical analysis it was found that the optimized wt% of the hybrid nanofiller has reached 7%

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Figure 12.5

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MD simulation frames of hybrid graphene–CNT (a) at time intervals from 0 to 500 ps, (b) side-view of the scrolled structural conformation at 500 ps82 (Reproduced from ref. 82 with permission from the Royal Society of Chemistry), and (c) free standing thin sheet of hybrid nanofiller in which hybrid graphene and convoluted CNTs form a grass mat structure.92 Reproduced from ref. 92 https://doi.org/10. 1038/s41598-018-30009-4 under the terms of the CC BY 4.0 license http://creativecommons.org/licenses/by/4.0/.

which delivers uniformly dispersed network of CNT and GO in ABS and synergistically improving the mechanical properties.28 Tung et al. used chemically converted graphene and pre-treated carbon nanotubes to form the hybrid nanofiller. With the help of spin coating and dip coating a thin layer of hybrid nanofiller was deposited over the substrate and taken out as optically transparent and electrically conductive thin films of SWCNTs– rGO.99 Huang et al. formed a hybrid film with a tuneable work function (Fw) that was used to fabricate electrodes for inverted PVs, and exhibited muchimproved efficiencies.100 Besides the graphene–CNT hybrid 3D nanofiller as discussed above, multi-component hybrids have also been reported in the open literature using a similar technique. Either CNT or graphene are modified101,102 or a third component is incorporated to further enhance the functionalities of the hybrid material.103–105

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Figure 12.6

(a) SEM micrograph of the as-grown CNT–G hybrid hetrostructure on SiO2/Si substrate, and (b) schematic of the graphene–carbon nanotube hybrid film synthesis process.133 Reproduced from ref. 133 with permission from the Royal Society of Chemistry.

12.3.1.2

Layer-by-layer (LBL) Deposition

Layer-by-layer deposition is a unique technique which only facilitates hybrid nanofiller in a thin film form. LBL is used to deposit multicomponent films with a microstructure that can be controlled finely at the nanoscale. This technique comprises plunging a negatively (or positively) charged substrate in an oppositely charged polyelectrolyte (PE) which is adsorbed onto the substrate. After reaching equilibrium, the substrate is removed, washed and dried. Furthermore, it is immersed in opposite polarity polyelectrolyte to form thin films of nanofillers one by one.106 These steps are repeated until the desired thickness is achieved. The thickness of the deposited film depends on the concentration of nanofiller in the polyelectrolyte, the ionic strength, assembling temperature and pH.107 One of the major advantages of the LBL technique is the high level of dispersion of nanoparticle in the composite due to direct absorption of the nanoparticle from a homogeneous electrolytic suspension to a solid-state film without phase segregation.108 Initially the LBL technique focused on electrostatic interaction, and then researchers worked on the development of a multilayer composite based on charge-transfer interaction,109 hydrogen bonding,110 coordinate bonding111 and covalent binding.112

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Kim et al. prepared a double-layer structure of an ultrathin transparent hybrid film of MWCNTs and rGO over a Si substrate using electrostatic adsorption.106 It was observed that the hybrid conducting film of rGO–MWCNT was transparent, which confirms the fine control of uniform deposition of the nanofiller. Furthermore, the dual layer hybrid structure showed excellent adhesion strength, which is an important parameter for practical applications. Hong et al. used a simple LBL technique to prepare the MWCNTs–rGO hybrid thin films on silicon and quartz substrates.113 They prepared the multilayer structure from a positively charged MWCNTs suspension and a negatively charged rGO suspension alternatively. Electrical conductivity and optical properties were controlled by the number of layers and graphitization. A similar technique was adopted by Yu et al. for LBL deposition of hybrid rGO and MWCNT hybrid film over a flexible substrate. In this work, a stable suspension of positively charged polymer-bonded graphene nanosheets was prepared by in situ reduction of exfoliated graphene and negatively charged MWCNT suspension by acid oxidation. The obtained hybrid films had well interconnected network carbon nanostructures which were well-defined and nonporous, enabling fast ion diffusion and were promising for supercapacitor electrode applications.

12.3.1.3

Vacuum Filtration

Vacuum filtration is the simplest way to prepare the hybrid graphene–CNT nanofiller-based free standing sheets. In this technique, a suspension of constituting carbon nanofillers are prepared by an external source of energy using an ultrasonic bath, probe sonication, homogenization, high rotation mixing, etc. The homogeneous suspension was then filtered over some porous cellulose or polymeric membrane. The segregated hybrid nanofiller membrane was then removed sophisticatedly and used for various applications. CNT-based free standing sheets have also been prepared by similar techniques114 and have been used in various applications such as in electrodes for lithium ion batteries,115–119 supercapacitors,120 thermoelectrics,121 photo detectors122 and reinforcements as conducting,123 structural8,124 and ballistic composites.125 Khan et al. dispersed the SWCNTs and graphene nanosheets in N-methyl pyrrolidone (NMP) using external agitation followed by vacuum filtration with film thickness varying from 100 to 500 mm. It was found that the mechanical properties of the hybrid sheets were better than those of other SWCNT or graphene sheets. The hybrid composite sheet also possessed the higher electrical conductivity.126 Similarly, Tang et al. explored the same technique to fabricate the paper-like composite structure from few-layered graphene and MWCNTs having varying ratios of nanofillers. A synergistic effect is achieved between 2D few-layered graphene and 1D MWCNTs.127 Aside from this graphene–CNT sandwiched structure formed by filtration,128 CNTs can be grown between the graphene layers and distributed uniformly over the entire surface. Recently, Jyoti et al. prepared MWCNTs,

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functionalized CNTs (FCNTs) and graphene oxide–carbon nanotube (GCNTs) hybrid Bucky paper (BP) via vacuum filtration followed by hot compression molding.93

12.3.1.4

Chemical Vapor Deposition (CVD)

Numerous efforts have been made to prepare 3D hybrid nanostructures of graphene–CNT or rGO–CNT by chemical vapour deposition, in which 2D graphene sheets play the role of substrate for the growth of 1D CNTs.129,130 This technique provides uniform growth of CNTs onto the surface of the graphene sheets via strong interactions, which ultimately avoids restacking of graphene sheets and provides high stability to the hybrid material. One of the major advantages of this technique is that it is able to make 3D hybrid nanofiller with a well-defined hierarchical structure. Chen et al. fabricated the hybrid multilayer graphene–CNT structure by in situ growth of CNT over GO using a CVD technique. The chemical vapour reduction and deposition were carried out at 500 1C for different growth times (1, 2 and 5 min) to alter the CNT length, and the best performance was achieved in the case of 2 min growth time. Hence, the key factor for controlling the properties of graphene–CNT hybrid grown by a CVD process is tuning the time duration of the growth process.131 Fan et al. prepared a 3D G–CNT structure by growing CNT pillars inbetween graphene layers using a CVD technique. For preparing the hybrid structure, a paper-like structure of graphene was prepared first using a filtration technique followed by CVD growth to prepare the double-layered sandwich structure for flexible supercapacitor applications.128 In this direction, our group has also reported a similar technique for large-scale production of a three-dimensional carbon nanotube pillared graphene hybrid structure. Herein, a novel synthesis process for large-scale in situ growth of a 3D CNT pillared graphene hybrid nanostructure network (GCNT) was fabricated where simultaneous reduction of GO and formation of CNT pillared on the RGO sheet was carried out in a single-step in a CVD setup. Furthermore, this hybrid material was used for quenching the luminescence of Rhodamine organic dye and simultaneously displays its own defect-induced strong photoluminescence (PL).132 A slightly different approach was adopted by Das et al. in which a hetrostructure consisting of a graphene layer on top of a CNT array was prepared using CVD. Figure 12.6 indicates that the hybrid structure of graphene on top of a CNT array could be transferred to various polymer substrates and reveals apparent variation in electrical conductivity in tension and compression conditions.133

12.3.2

Forms of Graphene–CNT Hybrid Nanofiller

The emergence of advanced technological devices with shape amenability and high mobility has simulated the development of flexible hybrid materials for power sources to bring revolutionary changes to daily life.

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The conventional electronic devices having rigid geometries and bulky size limits their functionalities in advanced flexible devices. Therefore, in the past decades research interest has been amplified in the development of reforming the hybrid nanomaterial to macroscale flexible materials such as hybrid fibers, free-standing sheets and foams. In this direction, Sun and Peng developed the hybrid nanostructure composite fiber of graphene and CNT by using a biscrolling method, which possess excellent high-tensile strength, electrical conductivity, and electrocatalytic activity.137 An aligned CNT sheet was prepared by a forest spinning technique and furthermore it was impregnated by graphene oxide solution (0.025 wt%) in water and ethanol (volume ratio of 1 : 20). The prepared fiber possesses superior flexibility (knot structure), mechanical (630 MPa), and electrical (450  20 S cm1) properties, which makes it a suitable candidate for wearable electronics. Due to these outstanding properties, these fibers were used in flexible dye-sensitized solar cells and electrochemical supercapacitors. Figure 12.7 (a and b) represent the detailed morphology of hybrid graphene–CNT twisted fibers which have a tensile strength of 300 MPa and electrical conductivity of 105 S m1. The mechanically strong hybrid composite fibers further demonstrated their application in lighting an electric lamp.134 Chong et al. reported the first-ever graphene-based fibrous rechargeable batteries fabricated by flexible rGO/CNT/S composite fibers using a wet-spinning protocol.138 The liquid crystalline behaviour of highconcentration GO sheets facilitates the alignment of rGO/CNT/S composites, enabling rational assembly into flexible and conductive fibers as lithium–sulfur battery electrodes. Similarly, Zan et al. and Cheng et al. have also developed the superelastic hybrid graphene–CNT fibers for flexible wearable energy storage devices.139,140 Another important macroscopic form in which these hybrid graphene– CNT fillers can be reformed is a flexible free-standing thin sheet. Fan et al. prepared the porous graphene–CNT (p-GC) hybrid papers by using vacuum filtration and a sacrificial template technique.135,141 Initially a homogeneous suspension of exfoliated GO and CNT (1 : 1) was prepared, which was further mixed with monodispersed PS sphere (400 nm) suspension (60 mL of 0.5 mg mL1 GO–CNT suspension drop by drop). The resulting mixture was vacuum filtered over a poly (vinylidene fluoride) (PVDF) membrane to obtain the free-standing hybrid GO/CNT/PS sheet, which was heated at 800 1C in nitrogen medium to get the p-GC hybrid papers. Due to the high surface area of this hybrid porous paper, it was used in highperformance flexible supercapacitors. Hu et al. fabricated the free-standing hybrid paper by filtration of an aqueous suspension of graphene nanosheets and CNTs.142 In the hybrid paper CNTs were randomly dispersed between graphene nanosheets and due to its 3D hybrid structure it possesses superior mechanical and electrochemical properties and therefore can be used as current collector and binder-free anodes for lithium ion batteries. Similarly, Jun et al. and Lu et al. also prepared the free-standing 3D hierarchical graphene–CNT hybrid paper by adopting a vacuum

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(a) TEM image of flexible hybrid graphene CNT fiber having excellent mechanical and electrical properties which demonstrate their applicability in electronic purposes by lighting a lamp.134 Reproduced from ref. 134 with permission from the Royal Society of Chemistry. (b) Schematic diagram of the fabrication of flexible hybrid paper including a digital image.135 Reproduced from ref. 135 with permission from the Royal Society of Chemistry. (c) Steps of fabrication of hybrid foam by a two-step CVD technique including SEM and digital images of the prepared hybrid foam.136 Reproduced from ref. 136 with permission from the Royal Society of Chemistry.

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filtration technique and studied the mechanical and thermal properties for various advanced applications.86,143 Another lightweight macroscopic form of hybrid graphene CNT is freestanding foam, which is an aerogel porous conducting network of graphene and CNTs. As they are made up of an aerogel of graphene and CNTs, they are super hydrophobic and superoleophilic in nature and can be useful for reusable oil separation applications. In this direction, Dong et al. prepared flexible free-standing graphene CNT hybrid foam using a two-step CVD technique.144 In the first step 3D graphene was CVD-grown using nickel foam as the substrate followed by subsequent nickel removal by HCl etching, and in the second CVD, 1D CNTs were grown over the surface of monolithic graphene foam (Figure 12.7(c)). Similarly, Cohn et al. developed a flexible hybrid foam of graphene and CNT for lithium ion battery anodes. This hybrid foam has a high lithium ion storage capacity which is close to the most ideal bulk material. It was helpful in achieving the ultrahigh storage capacities in hybrid structures (2640 mAh g1, 0.186 A g1), and also to maintain excellent rate performance (236 mAh g1, 27.6 A g1).136

12.3.3

Synthesis of Graphene–CNT Polymer Nanocomposites

The reinforcement of a variety of carbon nanomaterials for the production of nanocomposites is already rife in academia and research. The main objective during the fabrication of carbon nanofiller-based nanocomposites is to ensure adequate dispersion of nanofiller within the matrix. A uniform dispersion will promote the mechanical reinforcement efficiency of the filler in the matrix. Moreover, the interfacial properties between carbon nanofiller and matrix along with the aspect ratio of the nanofiller played an important role for designing the polymer-based nanocomposite. Herein we discuss the number of preparation strategies which are commonly used to achieve the mechanically strong nanocomposites such as (a) melt processing; (b) solvent processing; (c) in situ polymerization; (d) electrospinning. (a) Melt processing is one of the most economical and environmentally friendly techniques to prepare the polymer nanocomposites and is therefore widely used in large industries. The compounding is generally achieved in a single or twin-screw extruder where the polymer and the nanoparticle mixture are heated to form a melt. Various stresses and forces (shear stress, elongation stress, hydrodynamic forces of resin, etc.) act together to break the agglomerates of nanofillers and facilitate homogeneous dispersion in the viscous polymer matrix. Higher shear rates provide better dispersion and improve the throughput of the process. Another important advantage of this process is that it does not incur the organic solvent during processing and therefore maintains the mechanical properties of the base polymer system. The compounded nanofiller–polymer melt is further transfered to injection molding, blow molding, and profile extrusion for further processing.

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Most of the work reported in the literature have used thermoplastic polymers such as low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), polymethyl methacrylate (PMMA), polyamides, polyesters, and polycarbonate (PC).28,145–147 With this technique Jyoti et al. prepared hybrid nanocomposites of the ABS nanocomposite of different loading of GO and CNT and determined the synergistic effect of both the nanofillers on mechanical, electrical, rheological and electromagnetic shielding properties.28,148,149 From these experimental studies, it was concluded that graphene–CNT hybrid carbon nanofiller simultaneously helped in improving the interfacial properties by uniform dispersion. It was also observed that GCNT required greater concentration compared to GO/rGO and CNT to form a percolation network in ABS during melt blending. (b) Solvent processing involves mixing a carbon nanofiller-based suspension with a polymer which is already in solution form or can be mixed with the carbon nanofiller suspension by simple shear mixing or ultrasonication technique. Furthermore, this suspension is casted into a mold of the desired shape and dimensions. The added solvent is removed during polymer mixing and the remaining is removed during casting. This technique is very useful for almost all kind of polymeric materials such as thermoplastic, thermosetting and elastomer. Polymers such as PMMA, poly(vinyl alcohol), PS, PE, polyethylene oxide (PEO) and epoxy with carbon nanofiller have been widely processed using this technique.150–153 In general, solution processing provides adequate dispersion of nanofiller and is quite versatile, since the different combinations of solvent and polymer matrix can be processed. The lower viscosity of polymer matrix in solvent along with external mechanical agitation aids in better dispersion of the filler. This technique is not free of drawbacks; some problems are associated with the use of solvent such as toxicity of the solvent, complete removal of the solvent, damaging the inherited properties of polymer and thus the final product. Verma et al. prepared the graphene–CNT hybrid polyurethane nanocomposite by a solution blending technique and took advantage of the synergistic effect of graphene and CNT in improving the EMI shielding.89 It was concluded that the hybrid graphene–CNT nanofillers were capable of reinforcing with high concentration (10 wt%) compared to other individual carbon nanofillers and synergistically improve the shielding properties. Similarly, Bansal et al. prepared graphene–CNT reinforced PS film by using a drop casting technique and determined its mechanical and thermal properties.153 From this study it was observed that nanocomposite films harness the properties of rGO and CNT in a single structure and help in improving the thermomechanical properties.

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(c) In situ polymerization: In this technique carbon nanofillers are mixed uniformly in the monomer or primary polymer and a polymerization reaction is performed afterwards to arrest the nanofiller network in the nanocomposite. This technique facilitates strong interfacial adhesion between the matrix and nanofiller. This process allows grafting of the nanofiller on the polymer with or without functionalization and hence improves the compatibility between two component systems. During polymerization, increased viscosity of the matrix limits the loading fraction and processing of the composite.154 In an example that highlights the advantages of this specific method, Patole at al. prepared the self-assembled hybrid nanocomposite of graphene–CNT/ polystyrene using water-based in situ micro emulsion polymerization.155 In this hybrid nanocomposite film graphene sheets provide the total surface area for adhesion with the matrix, while CNT acts as a wire between the large graphene pads in the mechanically strong polystyrene film and hence the total sheet resistance decreases. Similarly, Cho et al. fabricated the nylon 6, 6-based hybrid nanocomposite reinforcing GO and CNT.156 During the polymerization reaction, two immiscible phases were used in which the first phase comprises of polyvinylpyrrolidone (PVP) surfactant containing adipoyl chloride with dispersion of GO and CNTs and the second phase was hexamethylenediamine. It was observed that the acyl-chloridefunctionalized GO in PVP dispersed uniformly, leading to strong interfacial interaction between carbon nanofiller and nylon 6, 6. Moreover, the functionalization technique enabled the formation of strong interfaces between the filler and the matrix and therefore helped in improving the mechanical properties. In a series of hybrid nanofiller reinforced composite, graphene–CNT has been proved to function in two distinct roles during polymerization; first it catalyzes the dehydrative polymerization due to the presence of functional groups, and secondly the hybrid structure further helped in dispersion even at the higher viscosity reaction phase. (d) Electrospinning is an alternate technique used to form the hybrid carbon nanofiller reinforced composite fiber. The diameter of spun composite fiber lies in the range of tens of nanometer to hundreds of micrometer. The electrospinning set-up includes an electrode connected to a high voltage power supply that is inserted into a syringe-like container containing the polymeric solution and a static or rotating collector connected to other electrodes. The syringe metallic capillary can be mounted vertically, horizontally, or tilted at a certain defined angle.157,158 Nanofiller reinforced polymers in the syringe at the end of capillary upon application of high voltage become charged and further increment of voltage, induces the charge on the surface of the liquid. Mutual charge repulsion leads to development of a force directly opposite to the surface tension. A jet of polymer comes out when the applied electric field overcomes the

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surface tension of the liquid polymer. The increased electric field causes the hemispherical drop of polymer at the tip of the capillary to elongate and form a conical shape known as the Taylor cone. Within a few centimeters of travel from the tip, the discharged jet undergoes bending instability (Raleigh instability) and begins to whip and ruptures into bundles of smaller fibers. Along with bending instability or Raleigh instability the ejected polymer also faces elongation which reduces the diameter and increases the length of the spun fiber.159

12.3.4

Mechanical Properties of Graphene–CNT Reinforced Nanocomposites

The reinforcement of high modulus carbon nanofiller either in individual form or hybrid form in a low modulus matrix can lead to significant reinforcement effect and therefore anticipated the enormous amount of research in the field of mechanical properties and nano-mechanics of these composites. CNT, graphene and reforms and their combinations have been widely scrutinized to develop polymer nanocomposites and multiscale composites with improved mechanical, electrical and thermal properties. Earlier it was expected that the parallel use of CNT and graphene would have a synergistic effect on the mechanical properties of nanocomposites, which was confirmed experimentally28,160–163 and theoretically.164,165 The effect of reinforcement on nanocomposites can be elaborated by typical stress–strain curves obtained during mechanical testing. There are several parameters which can affect the mechanical properties of nanocomposites includes structure of constituting CNTs or graphene, preparation technique, state of dispersion in the matrix, filler matrix interaction, and filler orientation. With the reinforcement of nanofiller elastic modulus, yield strength and resilience increases while straining percentage before failure reduces. The nanoreinforcement obviously improves the load carrying capacity by limiting the propagation of fracture crack but on the other hand nano filler reinforcement increases the stiffness of the matrix system and reduces the straining capability.29 Therefore, researchers have worked in the direction of a uniform network of nanofiller of optimized concentration in the matrix system. Theoretically, Liu et al. investigated the interfacial mechanical properties of hybrid graphene–CNT reinforced composites using molecular dynamics simulations. In this study two different types of hybrid nanofiller, i.e. p–p stacked hybrid and covalently-bonded hybrid, as reinforcements were investigated. From the analysis, it was observed that there are various physical parameters of CNTs (diameter, length, chirality and orientation) which affect the mechanical interfacial properties of the composite and with the same parameters of CNT, the interfacial mechanical properties of p–p stacked hybrid graphene–CNT was superior to the covalent bond hybrid graphene– CNT.165 The graphene and CNT are the basic building blocks of any nanocomposite system, and recently integration of both the nanofillers has

Graphene–Carbon Nanotube Reinforced Hybrid Polymer Nanocomposites

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stimulated research interest. Xu et al. investigated the mechanical and thermal properties of 3D interconnected graphene–CNT hybrid nanofiller molecular dynamics (MD) simulations.164 In this study, they investigated the mechanical and thermal properties of the hybrid nanofiller by considering it as a 3D network comprised of graphene and CNTs, termed ‘‘pillaredgraphene systems’’. From the simulation it was observed that the Young’s moduli was 0.9 TPa which was equivalent to pristine graphene and CNT. The tensile strength was 65–72 GPa with a fracture strain of 0.09. The stress– strain curve represents the linear behaviour, which could be useful for various nanomechanics applications where linear response is required. Furthermore, the reinforcement effect of this hybrid nanofiller was also evaluated by a molecular dynamic simulation technique. Zhang et al. simulated the behaviour of graphene–CNT hybrid nanofiller reinforced composites under tensile loading and determined the interfacial strength at the matrix–matrix interface, and filler–matrix interface. From the analysis, it was found that the peak strength of the stress–strain evolution in the matrix is lower than the peak strength of the filler–matrix interface. Hence, it was concluded that the damage zone was always associated with the matrix. It was also analysed that there was a polymer phase found around graphene and CNT having some thickness and was generally termed as the interface zone. This zone possesses a higher value of density and strength compared to the normal polymer matrix phase. Therefore, the damage path always avoids the interface zone.166 Polymer nanocomposites with improved mechanical properties can be formulated by uniform dispersion of high aspect ratio mechanically strong nanofiller within the matrix system. There are lots of experimental studies carried out to enhance the mechanical properties of the base polymer matrix by reinforcing the graphene–CNT hybrid nanofiller, and few of them are elaborated in this section. Recently, Li et al. prepared the TPU-based nanocomposite film using solution casting in which a 3D hybrid nanofiller of graphene and CNT were used as a reinforcement.167 Due to the establishment of a uniform interconnected structure of graphene–CNT within the base polymer matrix, the mechanical properties like yield strength and toughness increase simultaneously and reach the maximum values of 69.5 MPa and 246.2 MJ m3 respectively. The significant improvement in mechanical properties was attributed to homogeneous dispersion of graphene–CNT and the unique energy dissipation by nanofiller at different stages of fracture due to tensile deformation. The hybrid nanofiller consisted of multiple strong interfacial interactions which involve p–p interactions and covalent bonding which provide a simple and efficient approach for developing strong and ductile polymer nanocomposites. There are lots of parameters which are controlled while preparing these 3D nanostructures of graphene and CNTs, such as the form of graphene (i.e. graphene nanosheets, graphene nanoparticles, graphene oxides or reduced graphene oxides), ratio of the graphene nanoparticles and CNTs, and the processing conditions.168–170 Bagotia et al. prepared the hybrid nanocomposite with

302

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improved mechanical and electrical properties by reinforcing graphene– MWCNT hybrid nanofillers. From the experimental study, it was found that the graphene–CNT in the optimized ratio of 1 : 3 gave the maximum mechanical and electrical conductivity when reinforced 10 phr in polycarbonate/ ethylene methyl acrylate (95/5 w/w). The maximum tensile strength and modulus reached were 98.17 MPa and 338.5 MPa, respectively. This multifunctional hybrid nanocomposite possesses the maximum electrical conductivity of 0.1913 S cm1 which was suitably used in electromagnetic interference shielding with a shielding effectiveness of 34 dB.147 Kadambinee et al. suggested that 1D CNT and 2D graphene in an optimized ratio formed a hybrid 3D carbon nanofiller which possesses the superior interfacial adhesion due to the formation of high surface-area nanofiller. In this experimental study rGO and MWCNT were mixed together to form a hybrid nanocomposite. The tensile strength and modulus of rGO– CNT hybrid nanofiller reinforced composites are far better than individually reinforced rGO or MWCNT-based composites, which was due to the synergistic effect of both the nanofillers.171 Merely improving the dispersion and surface-area by using the hybrid nanofiller is not sufficient to improve the interfacial adhesion with the polymer matrix. Researchers are also looking for some other ways to improve the interlocking between the filler and matrix such as taking advantage of the functional groups. Tan et al. mixed the functionalised graphene and functionalised CNTs into a poly (styrene-b-butadiene-b-styrene) (SBS) matrix, where the ratios of individual fillers were changed and their effects were studied.172 The prepared GO and CNTs were functionalised with 1-bromobutane (C4H9Br) and HNO3. The uniaxial tensile test showed that f-CNT was efficient in improving the modulus while f-GO significantly improves the tensile strength and elongation at break. Wang et al. adopted a facile approach of functionalization for preparing the hybrid GO/CNT hybrid nanofiller. In this approach GO and acid-treated CNTs (CO) were chemically bonded with an amide group and a hybrid cross-linked structure of GOCO was prepared. The GOCO reinforced polyimide (PI) nanocomposite was prepared by a solution casting approach and further subjected to a uniaxial tensile test.173 The amide bond cross-linked GO/CNT (1.1 wt%) endowed the PI matrix with a dramatic increment in tensile strength (118%), modulus (94%), fracture toughness (138%) and electrical conductivity (11 orders), due to the effective stresstransfer at the interface between PI matrix and nanofillers as well as at the interface between GO and CNT. Zhang et al. also used the nitric acid treated MWCNTs and GO for preparing hybrid carbon nanofiller. Hydrazine is added in the dispersed suspension of GO and CNT for in situ reduction of GO and to obtain a hybrid of r-GO and functionalized CNTs. Due to the synergistic interaction of the two kinds of nanofillers, the tensile strength and Young’s modulus of the resulting PVA nanocomposite filled with only 0.6 wt% G–CNT hybrids were significantly improved by about 77% and 65%, respectively.161 In this direction, our own group has also worked extensively and established the individual and synergistic effect of nanofiller on

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inherited properties of the matrix system. Jyoti et al. prepared the 3D hybrid carbon nanofiller by using GO and f-CNT and used this hybrid nanofiller to prepare the ABS-based thermoplastic nanocomposite. The intensive study is carried out on the effect of hybrid nanofiller on mechanical, nanomechanical, thermal, electrical and rheological properties of ABS nanocomposite and based on these studies the relation between various processing parameters with intrinsic nanocomposite properties is established.28,93,148,149,174 Information regarding the mechanical properties of graphene–CNT reinforced nanocomposites is given in Table 12.1. Apart from playing an important role in improving the mechanical properties of polymer nanocomposites they are also useful in improving the interfacial properties of multiscale composites. Multiscale composites have different failure mechanics as compared to nanocomposites. In the case of nanocomposites, a crack initiates and propagates in the matrix system and is hindered by the nanofiller. While in the case of multiscale composites the crack generated at the interface of the elastically different fiber and polymer matrix was tackled by the bridging effect of nanofillers. Most recently, Cho et al. prepared the hybrid carbon fiber (CF)/polyamide 66/AGO–CNT multiscale composite by an in situ polymerization technique. In this work, CF cored composite tow was prepared by adopting a facile wet coating technique in which a homogeneous optimized suspension of acyl chloridefunctionalized GO (AGO) and CNT (2 : 1) dispersed in dichloromethane/ adipoyl chloride was used to coat the plasma treated CF as represented in Figure 12.8a. The optimized hybrid filler-reinforced PA66/CF (1.5 mg mL1) composites showed improvements in the interfacial shear strength, tensile strength and storage modulus of 160%, 136% and 300%, respectively, compared to the control sample, as well as high damping properties due to an enhanced transcrystalline interphase. The improvement in the mechanical properties were attributed to hydrogen bonding, excellent state of dispersion and mechanical interlocking induced by the hybrid carbon nanofiller during in situ interfacial polymerization. Similarly, Hua et al. prepared the multiscale composite in which hybrid carbon nanofiller and glass fiber (GF) were used as nano and microscale reinforcement. Initially, the study postulates the different roles of constituent carbon nanofillers (i.e. GO and CNT) in improvising the interfacial properties between GF and matrix system and further proved this by conducting various characterization techniques. From the study, it was concluded that CNTs were effective in enhancing interfacial bonding due to their anchoring role while they are easy to strip off from fiber surfaces; comparatively, GO was less effective in enhancing interfacial bonding but can be tightly adhered on fiber surfaces due to its encapsulating role and simultaneous grafting of CNT and GO on to the GF, synergistically combining their advantages (Figure 12.8b).175 Therefore, the transverse tensile strength (indication of interfacial normal bond property of GF/epoxy composites) was greatly enhanced by the GO/CNT hybrid coating and represented the improvement of 128% over GF/epoxy interfacial normal bond strength. Recently, our group prepared multiscale

Mechanical properties of various graphene–CNT hybrid nanofiller-reinforced polymer nanocomposites (T.S ¼ Tensile strength, Y.M ¼ Young’s Modulus, T ¼ Toughness, F.T ¼ Fracture toughness, F.S ¼ Flexural strength, F.M ¼ Flexural modulus, and E.B ¼ Elongation at break). Preparation Technique

Loading Ratio (G : C)

Nanofillers

Polymer Matrix

Epoxy–GO and carboxyl–CNT

Thermoplastic polyurethane

Solution casting

1:1

Graphene nanoribbons and MWCNTs Chemically-reduced GO and MWCNTs Reduced GO and f-MWCNTs

Thermoplastic polyurethane Thermoplastic polyurethane Poly(vinyl alcohol)

Solution casting Solution casting

Unzipping of CNTs 1:1

Solution casting

2:5

Graphene nanoplatelets and MWCNTs rGO and MWCNTs

Epoxy

3-roll mill calendaring and molding Solution casting

1:9

Multi-graphene platelets (MGPs) and MWCNTs Graphene nanopowder and MWCNTs Graphene nanoplatelets and MWCNTs GO and MWCNTs

Poly(methyl methacrylate) Epoxy

Mechanical properties

Ref. 167

1:1

T.S ¼ 69.5  5.7 MPa Y.M ¼ 88.6  8.7 MPa T ¼ 246.2  6.3 MJ m3 1 T.S ¼ 88.0 MPa Y.M ¼ 59.7 MPa 0.5 T.S ¼ 60.8 MPa Y.M ¼ 40.0 MPa 0.6 T.S ¼ 96.7  1.7 MPa Y.M ¼ 2.54  0.05 GPa 0.5 F.T ¼ 0.85 MPa m1/2 F.M ¼ 3.15 GPa rGO ¼ 0.5CNT ¼ 0.3 T.S ¼ 28 MPa Y.M ¼ 2.1 GPa 1 T.S ¼ 64.5 MPa Y.M ¼ 3.35 GPa 0.5 vol% T.S ¼ 62 MPa Y.M ¼ 2.5 GPa 0.3 T.S ¼ 71.2 MPa

5:3

Loading wt% 1

9:1

10 : 1

1.1

Graphene nanoplates and MWCNTs

Epoxy

High-rotation mixing and molding

9:1

0.5

Graphene nanoplatelets and f-MWCNTs

Epoxy

Mechanical stirring and molding

1:1

1

1:4

T.S ¼ 161.97 MPa Y.M ¼ 4.79 MPa F.T ¼ 30.2 MPa T.S ¼ 89.5 MPa Y.M ¼ 3.83 GPa E.B ¼ 2.9% T.S ¼ 73.4 MPa F.S ¼ 125.7 MPa F.M ¼ 2.71 GPa

169 168 161 177 171 27 178 179 173 180 181

Chapter 12

Shear mixing and moulding Epoxy Sonication and spin-coating Poly(acryloyl chloride) Molding and Epoxy Aromatic polyimides Solution casting

304

Table 12.1

Polypropylene

Solution blending and 3 : 5 injection molding

0.8

T.S ¼ 34.2 MPa Y.M ¼ 420 MPa

182

Epoxy

3 roll-milling and molding Mechanical stirring and molding

CVD growth 5:1

0.5

183

0.24

Graphene and MWCNTs

Polycarbonate/ ethylene methyl acrylate (95/5 w/w) Silicone rubber (VMQ)

Melt blending

1:3

10 phr

T.S ¼ 69 MPa Y.M ¼ 3.11 GPa T.S ¼ 50 MPa Y.M ¼ 1.9 GPa F.T ¼ 80 MPa T.S ¼ 98.17 MPa Y.M ¼ 338.5 GPa

Solution mixing and casting

1:1

0.75

Solution mmixing

1:1

0.5

4:1

5 vol%

Polyethylene terephthalate (PET)

Compounding and injection molding Extrusion and injection molding

1:1

3

GO and thin-walled carbon nanotubes (TWCNTs)

Polyurethane

Solution casting

3:7

0.1

GO and CNTs

Polyimide (PI)

In situ polymerization 3 : 1

1

py-rGO and py-MWCNTs

Polyimide (PI)

In situ polymerization 9 : 1

1

Functionalized graphene (f-G) and f-MWCNTs

Poly(ether sulfone) (PES)

Solution casting

10

GO and MWCNTs GO and MWCNTs Graphene nanoplatelets and MWCNTs Graphene nanoplatelets and MWCNTs

Epoxy

Ultra-high molecular weight polyethylene Polypropylene

1:1

T.S ¼ 0.67  0.03 MPa Y.M ¼ 0.43 MPa E.B ¼ 194  4% T.S ¼ 39.3 MPa Y.M ¼ 638 MPa T.S ¼ 37 MPa

147

185 186 187 188

189 190 191 192

305

T.S ¼ 57.4  9.4 MPa Y.M ¼ 2.3  0.6 MPa F.S ¼ 84.5  13.1 MPa F.M ¼ 3.4  0.2 MPa E.B ¼ 3.0  1.2% T.S ¼ 55 MPa Y.M ¼ 32 MPa E.B ¼ 780% T.S ¼ 95.77 MPa Y.M ¼ 1.7 GPa E.B ¼ 20% T.S ¼ 581 MPa Y.M ¼ 31 GPa T.S ¼ 78.45  1.14 MPa Y.M ¼ 3607.98  66.21 GPa

184

Graphene–Carbon Nanotube Reinforced Hybrid Polymer Nanocomposites

Exfoliated graphite nanoplatelets (xGnPs) and MWCNTs Graphene nanoplatelets and MWCNTs GO and MWCNTs

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composites by reinforcing aramid fiber in polycarbonate. Aramid is a highly crystalline and smooth textured fiber, which is generally associated with poor interfacial load carrying properties. To overcome this problem, three different types of nanofillers (i.e. 1D CNT, 2D GO, and 3D GCNT) were grafted over aramid fiber and their interfacial properties were determined by conducting static and dynamic mechanical testing at uniaxial loading conditions. The 0.2 wt% of CNT, GO, and GCNT network aramid reinforced multiscale composites showed B20%, B26% and B32% increment in tensile strength respectively, as compared to the baseline composite (i.e. without any nanofiller).163 The hybrid GCNT nanofiller inclusion demonstrated the synergistic load transfer between adjacent aramid yarns in inter-wrap, interweft and wrap-weft direction by bridging action (Figure 12.8c), and results in increased tensile properties along with superior interfacial adhesion. From Table 12.1, it is observed that graphene–CNT hybrid nanofiller was incorporated with a variety of matrix systems as reinforcement and the majority of these studies have shown the significant improvement in the mechanical and related intrinsic properties. Effectiveness of the hybrid carbon nanofiller reinforcement in composites indicated by the (1) ratio of carbon nanofiller, (2) weight fraction of nanofiller, (3) active functional group and (4) processing technique. The aforementioned table covers all the aspects of hybrid carbon nanofiller reinforced nanocomposites in which graphene and CNTs synergistically improve the loading capability of the composites. It is also observed that in some cases due to hybrid nanofiller reinforcement, the intrinsic mechanical properties such as yield strength, Young’s modulus, and toughness increase but the straining capability (elongation at break point) reduces due to stiffening of the polymer matrix.

12.4 Summary and Conclusions The mechanical properties of hybrid carbon nanofiller reinforced polymer nanocomposites have been discussed in this work. It has been found that the possibilities and capabilities of this hybrid carbon nanofiller for advanced engineering applications are practically endless due to the synergetic effect attributed to the presence of multifunctional CNT and graphene reforms. The majority of the production techniques and properties of CNT, graphene, GO and hybrid nanofiller of graphene and CNT have been elaborated, while once again it has been unveiled how important it is to apply studies of 1D CNT and 2D graphene to form 3D G-CNT and further their beneficial effects. A detailed investigation on thte mechanical properties of graphene–CNT reinforced hybrid nanocomposites and their corresponding effect on multiscale composites have been discussed. The effect of using CNT, graphene reforms and hybrid graphene–CNT nanofiller on the stresstransfer efficiency between filler and matrix and the respective response over elastic modulus and yield strength have been exhibited clearly. Although CNT and graphene reforms showed significant progress in mechanical

Graphene–Carbon Nanotube Reinforced Hybrid Polymer Nanocomposites

Figure 12.8

307

(a) Illustration of the mechanism of interphase reinforcement in PA66/ CF/AGO–CNT composites through hydrogen bonding and mechanical interlocking effect.176 Reproduced from ref. 176 with permission from Elsevier, Copyright 2020. (b) HRTEM image of the GO/CNT hybrid coating layer on glass fiber.175 Reproduced from ref. 175 with permission from Elsevier, Copyright 2017. (c) Schematic of the bridging mechanism by 1D CNT, 2D GO and 3D GCNT nanofillers for improving the load carrying capacity.163 Reproduced from ref. 163 with permission from Elsevier, Copyright 2020.

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properties, their uniform dispersion is a challenging task. Therefore, graphene–CNT hybrid nanofiller seeks great attention to overcome this problem. These hybrid nanofillers greatly improved the mechanical, electrical and thermal properties by improving the state of dispersion in the matrix system. It should be mentioned that there are various challenges to be faced before industries can proceed with the implementation of hybrid nanofiller for several engineering applications. For example, high yield and high quality of constituting CNTs and graphene reforms, synthesis of a well cross-linked 3D structure of graphene–CNT, appropriate reformation of these hybrid nanofillers for desired application, and processing conditions during their use as reinforcement. Based on the findings presented earlier, the aforementioned problems and their solutions are discussed in detail. From the experimental and theoretical description, it is concluded that high aspect ratio, 3D hybrid carbon nanofiller is useful for the successful development of advanced nano and multiscale composites.

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Section 7: Characterization and Identification Methods

CHAPTER 13

Raman Spectroscopy Characterization of Carbon Materials: From Graphene to All-carbon Heterostructures ALEXANDRE MERLEN,*a JOSEPHUS GERARDUS BUIJNSTERSb AND CEDRIC PARDANAUD*c a

´riaux Microe ´lectronique Nanoscience de Provence, IM2NP, Institut Mate ´s d’Aix Marseille et de Toulon, site de UMR CNRS 7334, Universite ´ de Toulon, France; b Department of Precision and l’Universite Microsystems Engineering, Research Group of Micro and Nano Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, ´, CNRS, PIIM UMR 7345, 13397, The Netherlands; c Aix-Marseille Universite Marseille, France *Emails: [email protected]; [email protected]

13.1 Basic Principle of Raman Spectroscopy 13.1.1

Context

Raman spectroscopy is a non-destructive analysis technique which is particularly well suited to characterization of carbon (nano)materials. It is highly sensitive to carbon–carbon bonds and is able to provide a wealth of information about their structure. If one wants to understand the Raman spectra of the various carbon nanoforms1 as well as all-carbon heterostructures, one

All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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has to understand first the Raman spectrum of graphene and diamond as they represent the pure allotrope cases. The aim of the following part is to briefly give a first context and the main physical ideas behind normal and resonant Raman scattering but is not to give a complete lesson on Raman spectroscopy theory. The evolution of the use of Raman spectroscopy to study materials, and more precisely carbon allotropes and their hybrids, has been correlated to the evolution of experimental techniques. The historical milestones (experimental, theoretical, and instrumental) on the Raman effect have been summarized in ref. 2, and the first part of this chapter has been directly adapted from ref. 2. The early history of the Raman effect can also be found in ref. 3, and more information on the instrumental aspects can be found in ref. 4. Very briefly, Raman spectroscopy was first performed with molecules in liquids and then in gas (the first measurements were done unsuccessfully in the gas phase from 1924 to 19285,6). The study of crystals was not easy until the advent of the laser in the 1960s due to both low Raman scattering cross sections and because of competitive absorption of light. Only transparent samples like diamond and CdS, with a large volume probed, were reported in the period of 1930–1960.7,8 Lasers, contrary to the mercury lamp previously used, offered many advantages such as: high power, monochromaticity, and coherence, thus opening the era of studying solids. In parallel, progress had been made in electronics so that photomultipliers were used first before the CCD (charged coupled devices) cameras.9 The latter were invented in 1970, based on semiconductor technology arranged in arrays and applied first in the field of Raman spectroscopy for solids in 1987 for characterizing ultrathin organized layers of organic films.10 Raman spectrometers are nowadays routinely used and are present in many characterization labs across the world.

13.1.2

Raman Effect in Solids

Raman photons are created by a fluctuating electric dipole in the scattering medium (which could be in the form of a gas, a liquid or a solid), by the simultaneous action of the incident light beam and the elementary excitations of this medium, leading to an induced polarization moment. For ordered solids, the elementary excitation of the crystal, called the phonon, has a crystal momentum q and a corresponding frequency oq for each value of q (the dependency between oq and q is called the dispersion relation and is represented in the q-space called the Brillouin zone). As the system is isolated, two conservation rules (total energy and total -momentum) apply, involving the scattered light frequency and wavevector k . Due to these conservation rules, the geometry of the experiment determines orientation and magnitude of the scattering wave vector. In experiments nowadays, the backscattering geometry (i.e. reflection geometry, see Figure 13.1) is routinely used in labs. Here, the crystal symmetry11 and orientation relative to the polarization direction of the incoming light, both play a role in the Raman band intensity.12 However, whatever the symmetry, one photon and one phonon can interact

Raman Spectroscopy Characterization of Carbon Materials

Figure 13.1

321

Experimental set-up with a backscattering geometry.

only close to the center of the Brillouin zone. This is because of geometry and the respective values of q and k . As |q | varies from 0 to something close to 1/a, a being the typical lattice parameter (E1 Å), and as |k | is close to 1/l, l being the wavelength of the incident photon (E500–600 nm in most experimental setups), conservation rules can be satisfied only when |q | is close to 1/l, i.e. close to the center of the Brillouin zone. The Brillouin zone could be investigated by means of Raman scattering by using higher order processes (i.e., interaction involving more than just one phonon), or by other processes like the double resonance process that occurs close to a specific high symmetry region in the Brillouin zone of graphene and related materials.

13.2 Raman Spectroscopy of Carbon Allotropes Figure 13.2 displays the Raman cross-sections of typical graphene-based materials compared to some relevant other molecules and reference materials. As an illustration, diamond has a lower cross-section than graphite (roughly 60 times), but it does not mean diamond will necessarily be hard to observe compared to graphite. The number of vibrators under the laser beam is another aspect to bear in mind. It is governed by optical properties of the material probed. For example, diamond is an insulator and thus it is transparent in the visible range, whereas graphite is a semi-metal and not transparent in the visible range. The in-plane size of the laser spot focused by the microscope objective is of the order of 1 mm2 (it depends on the numerical aperture of the objective lens and the laser wavelength). For diamond, the depth probed is roughly 1 mm whereas it is roughly 10 times less for graphite. As a result, the volume probed is 10 times larger for diamond than for graphite, meaning that in the same acquisition conditions,

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Figure 13.2

Chapter 13

Raman cross section of carbonaceous materials compared to reference materials.2 Sources: diamond,13 graphene,14 nanographite,15 benzene,16 silicon17 and SERS.18 Note that the Raman cross section of solids can sometimes be difficult to obtain because of substrate effects that can give rise to interference effects, which depend on the thicknesses and optical indexes, as it was shown for graphene19,20 and earlier for amorphous carbon.21

the intensity of a diamond sample will be the same order of magnitude as for graphite. Band intensities can be modified if the electronic structure of the materials is modified, but also because of reflection losses or interference effects.19–21 In some cases, cross-sections become large because of some resonance conditions with the laser wavelength, as in the case for amorphous carbons or C60 for some modes, as will be discussed later. We will see that some bands are related to the amount of disorder. For example, in 2007, Pimenta et al. reported that the resonant Raman behaviors in nanographite-based systems allow better characterization of their disorder.22

13.2.1 Graphene and Related Materials 13.2.1.1 Graphene, Graphite and Multi-layer Graphene Many reviews can be found on Raman spectroscopy applied to graphene and related materials. We advise the reader to study the one by Ferrari et al.23 and its useful 13 pages of supplementary information, the ones by Beams et al.24 and Wu et al.25 or the one we wrote in 2017.2 The Raman spectrum of graphite (displayed in Figure 13.3) or graphene (displayed in Figure 13.4a) should display only two Raman active modes. These are one vibrational mode corresponding to the sliding of planes between each other atB42 cm1, which is called the C band27,28 and is often not

Raman Spectroscopy Characterization of Carbon Materials

Figure 13.3

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Raman spectra of graphite and disordered graphite recorded at 514 nm.2

easily accessible in standard set-ups that start generally at E100 cm1 due to the use of filters to attenuate the laser signal from reaching the detector, and a band at 1582 cm1 corresponding to the stretching mode in planes, called the G band.29 This G band corresponds to both longitudinal and transversal optical modes at the center of the Brillouin zone. The width of this G band due to electron–phonon coupling has been evaluated to be 11.5 cm1,30 phonon–phonon scattering being responsible for extra 4–5 cm1 broadening. Graphite consists of stacked graphene layers. Depending on the number of layers and the way graphene layers are stacked, the Raman spectrum could vary. For example, the C band position varies from 31 cm1 for bilayer graphene to 44 cm1 for graphite (Figure 13.4c).25 It is sensitive to the number of stacked layers up to NE20, as displayed in Figure 13.4c, whereas the G band only weakly depends on it. As the G band is very sensitive to strain and doping, its position is not used to determine the number of layers. Several other bands can be found in the spectrum of defect-free graphite and graphene. The most intense is called the 2D band, close to 2700 cm1, but its wavenumber depends linearly with the energy of the laser used (the dispersion rate is E100 cm1 eV1), which is counter intuitive. The 2D band height is generally one third of the G band’s height for graphite but can be close to 3 times the G band height for graphene, or even higher (up to 16 times). This 2D band is always present in graphite and graphene, even if there is no defect. Its shape depends on the number of layers and the way they are stacked. For monolayer graphene on a substrate, it has a single Lorentzian shape and it could be the composition of two Lorentzians for suspended monolayer graphene or graphene in poor interaction with the substrate (due to some process involving asymmetry in the double resonance mechanism explained below and that is modified by the presence of interaction between the layer and the substrate31). For well-stacked layers, four or more bands can be observed, due to the peculiar electronic structure of these

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Figure 13.4

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Raman spectra of graphene (recorded at 514 nm). (a) Graphene on silicon. (b) 2D band shape of stacked layer graphene (adapted from ref. 26), graphite and turbostratic graphite. (c) C band frequency with the number of stacked layers.

materials that we do not outline in detail here.32 The shape of the 2D band is thus a way to obtain information on the number of layers in multilayer graphene (up to NE10).26 However, note that when graphene layers display weak interaction between each other,33 their Raman spectra are not composed of several bands but could be very close to a Lorentzian shape, so the 2D band shape criterion has to be considered with care. For example, turbostratic graphite could give a Lorentzian shape, with a width that could reach 60–70 cm1,34,35 as shown in Figure 13.4b. The electronic structure could however change drastically depending on the way the two layers are stacked. This is the case of twisted bilayer graphene (tBLG) that became a model system for strongly correlated electrons36 and supports superconductivity behavior.37 Being able to control the value of the twist angle is then an important task, as the conductivity properties are angle dependent.

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Scanning tunneling microscopy (STM) is a good tool to measure it as stacking ´ patterns. Raman spectroscopy also helps in determining two layers give moire this twist angle. First, the electronic structure of van Hove singularities (vHS) depends on this twist angle, so does the corresponding band gap that falls in the visible range. As a consequence, the Raman process is resonant. The G band intensity could then reveal the presence of a peculiar vHS.38 The 2D band width and position also strongly depend on this twist angle.

13.2.1.2

Double Resonance Mechanism

When there are defects, other bands can be seen (D, D 0 , D00 , . . .). Some involve the presence of defects (like D and D 0 ), others do not need defects to be explained. The common behavior between these D, D 0 , D00 , . . . bands is that they originate from interactions close to the K point of the Brillouin zone, where there is no electronic gap between the valence and conduction bands,39 and electronic resonance could occur whatever the incident photons energy. The mechanism leading to these D, D 0 , D00 , . . . bands is called the double resonance mechanism and it can involve two phonons or one phonon and a defect, for example.29 It could also involve more particles, but the intensities will be smaller.40 The double resonance mechanism, the main mechanism explaining most of the unusual Raman features of graphene-related materials, uses a 4th order perturbation theory.32,40,42–44 It is described in four steps. First step: an incoming photon with an energy ho0 is absorbed, hereby exciting an electron/ hole pair (symbolized by the orange spot in Figure 13.5). Second step: charge carriers could be scattered by many phonons with different possible vibrational energies and wavevectors. Third step: selected phonon could be scattered back by a different phonon or a defect. Fourth step: the electron/hole pair recombines. Conservation rules on energy and momentum are applicable during the overall process. Many different scatterings can occur, however only the resonant ones will modify the Raman cross section. Steps one and two are resonant, whereas steps three and four are not. For step one, changing the energy of the incident photon will select another phonon that will maximize the Raman cross section, leading to the observed dispersion of the D and 2D bands. This is not strictly speaking a selection rule, but the denomination found in the literature is a ‘‘quasi selection rule’’. The intravalley process, using scattering by a defect and a phonon, will give rise to the D 0 band (Figure 13.5b). The intervalley process between K and K 0 points in the Brillouin zone will give rise to the D band if the excited electron is scattered by a phonon and a defect (Figure 13.5c), or to the 2D band if the scattering by the defect is replaced by a scattering back with another phonon (Figure 13.5d).

13.2.1.3

Doping and Strain Effects on Graphene

Intentional doping of the graphene layer (or the presence of non-controlled adsorbed impurities45) modifies its Raman spectrum.46–49 Playing with the

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Figure 13.5

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Double resonant scattering for the valence and conduction bands, adapted from ref. 2 and 41. (a) G band: Electron–hole excitation followed by non-resonant phonon scattering. (b) D 0 band: Intravalley process. (c and d) D and 2D bands: Intervalley processes.

electron doping means changing the position of the Fermi level that will play a role in the double resonance mechanism: doping with electrons or holes can increase or decrease the 2D band position, and both diminish I2D/IG.47 The effects of doping were reviewed in 200742,48 and in 2015.24 Mechanical strain also affects the Raman spectrum.50 The 2D band position plotted as a function of the G band position has been successfully used to disentangle electron or hole doping from macroscopic strain effects (for doping higher than E1012 cm2).51–53 On a set of data, a slope close to 2 in that plot reveals a purely macroscopic strain effect, whereas a slope close to 0.7 reveals pure doping and intermediate values exposed to a combination of the two effects that can be disentangled using basic algebra.

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13.2.1.4

327

Single-walled Carbon Nanotubes

Graphene is a building block for many carbon-based materials. We have seen previously that stacking individual (graphene) layers will give rise to multi-layer graphene and eventually graphite, depending on the way layers are stacked. Some other examples of Raman spectra of graphene-based materials (nanotubes and fullerenes) are displayed in Figure 13.6. Rolling up of graphene results in the formation of carbon nanotubes.54,55 The way these structures are rolled up is called the chirality of the tube56,57 and it defines several properties of the tubes (e.g., diameter and band gap that influences the metallic or semi-conductor nature of the nanotubes). The electronic density of states of single-walled nanotubes (SWNTs) exhibits van Hove singularities, the chirality defining the allowed electronic transition between them with a photonic excitation. All the allowed electronic transitions can be plotted versus the diameter of the SWNT, and this plot is called a Kataura plot.58 As Raman scattering from SWNTs obeys a resonant mechanism (the Raman cross-section being extremely high contrary to their IR signal59), the Kataura plot directly indicates which kind of tube is resonant for a given photonic excitation wavelength. As a consequence, the typical first-order Raman spectrum of SWNTs is divided in three parts. The first one is the low frequency region which is associated to Radial Breathing Modes (RBM). The frequency of these modes is directly related to the diameter of the tubes. One can find a review on RBM published in ref. 60. In the second part, the D band (around 1300 cm1) related to defects (see the next section) can be found. In the third part, the G band (around 1550 cm1), which is similar to the G band of graphite and graphene, is located. If the resonant tubes are metallic, the G band exhibits an asymmetric profile close to a Breit–Wigner–Fano

Figure 13.6

Raman spectra of single-walled carbon nanotube and C60 (recorded at 514 nm). Adapted from ref. 2.

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(BWF) profile. This feature is coming from a specific interaction between the phonons and the electronic continuum62 and is not present for semiconducting tubes.

13.2.1.5

Fullerenes

Another form of graphene-related material is fullerenes. These carbon molecules were discovered in 198563 and exhibit pentagonal and hexagonal rings which leads to their curved shape, like a soccer ball. They have been extensively studied using Raman spectroscopy.64,65 Briefly, fullerenes exhibit specific Raman modes that can be easily identified. For example, as shown in Figure 13.6 for the case of C60, the modes labeled Hg(1) to Hg(8) rise at 272, 433, 709, 772, 1099, 1252, 1425 and 1575 cm1, respectively, and the modes labeled Ag(1) and Ag(2) rise at 496 and 1470 cm1. The intensity of the Ag(2) mode at 1470 cm1 is much more intense than the other modes due to a vibronic coupling that enhances its Raman intensity for laser energy close to 2.6 eV.66 This band intensity can be used as a probe of the coupling between C60 and its environment. C60 can be polymerized under UV radiation,67 leading to the rise of low vibrational modes at 118 cm1, which correspond to bonds between C60 in the solid phase. This polymerization can also be induced under high pressure.68

13.2.2

Low-ordered Carbons

The Raman spectra of a wide variety of disordered carbons are displayed in Figure 13.7 in order to provide an overview showing that spectroscopic parameters vary qualitatively. The Raman spectra displayed on this figure were obtained from highly oriented pyrolytic graphite, nanographite, and amorphous carbon (a-C). First, one can see that the G band broadens from

Figure 13.7

Raman spectra of disordered carbons recorded at 514 nm.

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graphite to amorphous carbon and its position varies from 1582 cm for graphite, blueshifts to E1600 cm1 for nanographites and then redshifts down to 1550 cm1 (lower values are also reported) for amorphous carbons. The D band increases and then decreases with an increase of disorder (see ref. 2 for a better description of the state of the art on this topic). The 2D band intensity, compared to the G band intensity can vary from 3 to 1/3 from graphene to graphite, respectively. When disorder increases, the 2D band broadens, overlapping with other bands and nearly disappears. For amorphous carbon, the intensity compared to that of the G band is lower than 6%. There are also several weaker bands (2 to 5% of the G band intensity), at 2450, 3240 and 4300 cm1, respectively. Most of the weak bands are listed in ref. 69 for graphite. In the range E2000–3000 cm1, they are due to two-phonon processes explained in the framework of the double resonance mechanism, and named 2D, D þ G, D þ D 0 , 2D 0 , and so forth. D þ G and D þ D 0 bands are very close and cannot be disentangled for each case (see the discussion on this point in ref. 70). Considering the less intense bands, a D00 band is present in the shoulder of the D band of very defective samples. It is always needed when fitting (see our review paper2). The band at 2450 cm1 has been attributed recently to a D þ D00 band by Couzi et al.71

13.2.2.1

Graphene Oxide and Reduced Graphene Oxide

Graphene oxide (GO) is the oxidized form of graphene. It is an attractive 2D material as it is easy to produce and inexpensive. By reducing graphene oxide, one obtains reduced graphene oxide (rGO). This offers the possibility to tune optical, mechanical and electronic properties, which could be useful in electrochemistry.72 Interest in research involving GO/rGO is devoted to control partial reversibility of the processes, and many methods that lead to different kinds of GO/rGO have been tested.73 GO is found to be graphene with defect-free regions of few nanometers in size surrounded by defective areas containing topological defects like pentagons and heptagons.74 C/O ratios of 2/1 to 4/1 are typically produced and can sometimes reach even higher values (12/1–246/1).73 Despite the large variety of possibilities (surface coverage, chemical functions containing oxygen atoms) the corresponding experimental Raman spectra are quite poor. In most of the cases they are composed of broad D and G overlapping bands plus a set of broader bands centered close to 2900 cm1, near the 2D band (the 2D, D þ G and 2G bands). This could be surprising but in fact this is due to the high Raman cross-section of the graphene skeleton which dominates, hiding all the possible modes predicted by some calculations.75 However, some indirect information could be obtained by analyzing the D, G and 2D band spectroscopic parameters. As an example, changes in peak area ratios (D/G and 2D/G) under a variation of laser power intensity that reduces the GO (Figure 13.8) show a direct correlation with electrical resistance of rGO samples.70

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Figure 13.8

Raman spectra of GO and rGO, obtained by tuning the power of the laser from 0.1 to 10 mW. Data recorded at 514 nm.70

13.2.2.2

Amorphous Carbons

Most of the work cited on Raman spectroscopy of amorphous carbons76 comes from the papers by Ferrari et al. that have been cited thousands of times. Their four landmark papers were published from 2000 to 2005 and altogether combine a comprehensive view of the understanding about Raman spectroscopy of amorphous carbons.77–80 Amorphous carbons generally contain sp3, and sp2 carbons and heteroatoms (such as hydrogen). sp3 carbons determine the mechanical properties (hardness, density), whereas the sp2 aromatic clusters determine the optical properties (energy gap), mainly due to the p/p* bonds with the energy gap in the IR-visible-UV range, depending on their size. Adding hydrogen, and organizing the structure by heating the sample, can change the optical properties together with the structure81–89 and Raman spectroscopy can help in discerning the changes. For example, hydrogen changes the electronic structure, which leads to a Raman resonance mechanism that can be observed, helping in quantifying the amount of hydrogen in the amorphous carbons.90 Depending on the amount of aromatic/aliphatic sp2/sp3 carbons and hydrogen atoms, several kinds of amorphous carbons can be distinguished: a-C, ta-C, a-C:H, and taC:H. The t stands for tetracoordinated as these carbons contain generally close to 70% of sp3 carbons. Raman spectra of amorphous carbons generally display two broad overlapped G and D bands, the D band being less intense, or disappearing when the amount of sp3 carbons is higher. The D band position displays the wavelength dispersion as in graphene but shifted depending on the kind of amorphous carbon. For the G band, it depends on the local degree of order.

Raman Spectroscopy Characterization of Carbon Materials

Figure 13.9

331

Raman spectra of amorphous carbon (a-C:H with about 30 at.% H) obtained with different laser wavelengths.2

Figure 13.9 displays the Raman spectra of one a-C:H film (H being close to 30 at.%) but recorded with 5 different laser wavelengths, ranging from 266 to 633 nm.91 The dependence of the spectral shape with the laser wavelength92 can be explained mainly by the fact that the sample is composed of a distribution of aromatic domains which display local electronic structures. The wavelength used is resonant with one kind of environment that appears stronger in the spectrum. Ferrari et al. proposed a ‘‘three stage’’ model that is based on the ordering of the sp2 phase going from nanocrystalline graphite (nc-G) to highly disordered amorphous carbon, explaining the changes in the spectroscopic parameters.78 Note that for a high number of sp3 carbons, a T band can be observed when using UV laser light. It has been found that the G band width, FWHMG, is correlated to the sp3 content and the linear dispersion of the G band position of as-deposited samples correlates to the H content.93 Thus, a good way to represent the data is to plot the G band position as a function of FWHMG, as was done for several wavelengths in ref. 87 and 91. Figure 13.10 displays such a plot for different kinds of heated amorphous carbons plus nanocrystalline graphite (nc-G). If one uses FWHMG as an indicator of local disorder close to sp2 bonds in the material (which can be related to the size of the clusters15,79 and/or to the sp3 content close to sp2 bonds93), one can use this parameter to have an idea of where the sample is situated in Ferrari’s ‘‘three stage model’’. With this in mind, nc-G is more ordered than a-C:H which are themselves more ordered than ta-C:H and ta-C. Each kind of carbon draws its own line when heated, but all these lines tend to converge in a region close to 100 cm1.

13.2.3

Diamond (Single-crystal and Polycrystalline) Materials

Single-crystal diamond is a homopolar solid that crystallizes with the wellknown covalent structure with each carbon atom in four-fold coordination

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Figure 13.10

Chapter 13

Plots of G band position vs. FWHMG for nc-G and different amorphous carbons. Data were recorded with l ¼ 514 nm.2

from sp3 hybridization. The Raman spectrum of pure monocrystalline diamond is dominated by a single sharp line around 1332 cm1 from the zone center optical phonon (T2g mode). Its line width (full-width half-maximum (FWHM) B1.2 cm1 at room temperature) increases with the concentration of point defects. This first-order diamond mode has been extensively reviewed,94,95 and was effectively used to measure the temperature of a diamond or the pressure that the solid is subjected to.96–98 In polycrystalline diamond films, defects and other non-diamond parasitic phases give rise to extra features in the Raman spectrum besides the first-order Raman line at 1332 cm1. A sloping background due to photoluminescence is frequently recorded as well, usually attributable to N–V complex optical centers or the incorporation of Si or metal impurities. Nowadays, Raman scattering is routinely used to characterize the phase purity of chemical-vapor deposited polycrystalline diamond, and multiwavelength Raman spectroscopy is particularly suited for that purpose. As an example,35 we will here consider the multi-wavelength Raman spectra (325, 514, 633 nm) recorded from a nanocrystalline diamond (NCD) film grown by hot-filament assisted chemical vapor deposition;99 see Figure 13.11. The diamond film material is composed of nano-sized diamond grains (10–30 nm) and is further characterized with a significant fraction (B5–10%) of (disordered) sp2 carbon residing at the grain boundaries. As a result, the Raman spectra are dominated by the D and G bands observed around 1321 and 1583 cm1, respectively. Other features appear at 1150 cm1 and 1480 cm1 and may be related to trans-polyacetylene (t-PA). The latter contributions will not be further discussed here, and the reader is referred to earlier studies.78,100,101 When observing Figure 13.11, it is immediately evident that the overall shape of the Raman spectra is strongly dependent on the excitation wavelength. The predominance of sp2 related signals can be explained by the fact that sp2 carbons are relatively strong Raman scatterers in the visible range as compared to diamond

Raman Spectroscopy Characterization of Carbon Materials

Figure 13.11

333

Multi-wavelength Raman spectra of a nanocrystalline diamond film grown by hot-filament assisted chemical vapor deposition. The black line indicates the position of the first-order diamond mode at 1332 cm1. Spectra have been normalized to the G band height. Adapted from ref. 35.

(see Figure 13.2). In addition, the strong absorption of visible light by the sp2 carbon present in the film reduces the interaction volume between the sample material and the laser radiation, and consequently the diamond intensity diminishes. Note that the diamond line intensity increases relatively to the G band intensity with decreasing the wavelength of the laser used.35 With 633 nm laser wavelength, the characteristic diamond peak is only observed as a very weak kink at 1332 cm1. A slightly more pronounced diamond signal is detected when using 514 nm laser light, whereas a dominant diamond peak is observed in the UV Raman spectrum (325 nm). On the other hand, the intensity of the D band diminishes relatively to that of the G band when decreasing the laser wavelength according to the behavior known for disordered nanocrystalline graphite.88 In 2010, Klauser et al. reported a multi-wavelength, visible-Raman study of structurally different nanocrystalline diamond films grown by using a similar method.101 They demonstrated that specific features of the diamond phase (e.g., grain size) as well as the grain boundaries (e.g., relative content and composition of t-PA and amorphous carbon phases) can be extracted from the Raman spectra, if correlated with data derived from atomic force microscopy, X-ray diffraction, and ellipsometry measurements. Next to sp2 content, the level of doping is of high practical importance for diamond materials. In particular, the incorporation of boron into the diamond lattice renders diamond an attractive semi-conductor. At very high boron concentrations (above about 31020 cm3), metallic conductivity on a boron impurity band is reached. A detailed interpretation of the Raman spectra of B-doped diamond (BDD) remains complex, but good progress was made in recent years. Some important insights will be summarized here. The Raman spectrum is sensitive to the carrier concentration and increasing

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asymmetry of the zero phonon line with increasing hole concentration is typically observed.94 This asymmetry is attributed to a Fano resonance effect.102 For boron-doped diamond films with metallic conductivity, the following Raman features are typically distinguished: two wide asymmetric bands centered around 455 cm1 and 1200 cm1, respectively, and the diamond’s zone center phonon line, which is redshifted relatively to the intrinsic diamond line (1332 cm1). In some cases, a dip trough (or antiresonant line) centered around 1345 cm1 is observed as well.103 The origin of the broad band at B455 cm1 is controversial, although it is frequently attributed to boron dimers or boron–carbon vibrational modes. The band at 1200 cm1 is distinctly attributed to a carbon–carbon vibration mode of the disordered diamond lattice.103 Interestingly, Kumar et al. observed that important information on the doping-induced changes could be obtained by using Raman excitations of varying wavelengths between 325 nm and 488 nm.104 Fano parametrization of various boron-doped diamond samples indicated that the downshift and broadening of the diamond line are due to phonon confinement, electronic Raman interaction and lattice expansion due to the boron doping.103,104 Over the last decade, significant progress in the Raman spectroscopy analysis of diamond nanoparticles is reported as well. A good overview of the contributions and weaknesses of the method in the characterization of the diamond cores and surface features of detonation nanodiamonds can be found in ref. 105 and the references therein. It should be noted that, at present, the interpretation of the Raman spectra is still debated heavily and that for a complete description of the particles the Raman technique should be combined with other characterization methods, such as high-resolution transmission electron microscopy and infrared spectroscopy.

13.3 All-carbon Heterostructures We will now consider the Raman response of all-carbon hybrids. As for any carbon material, Raman spectroscopy offers several advantages and its general interest for the study of carbon heterostructures has already been demonstrated.106 Nevertheless, there might be some tricks, and the reader should first consider if Raman spectroscopy is really the appropriate tool for the characterization of their hybrid. In addition, the choice of excitation wavelength is also a key parameter, and if possible, the measurements should be preferentially performed with various laser sources. We will first consider the general Raman response of carbon hybrids. In most cases, the final spectrum is a mere sum of the signature of each constituent. This feature is characteristic of a low interaction. This means that no (or very weak) chemical bonding is formed between the carbon allotropes. As Raman spectroscopy is very sensitive to electronic properties, this also means that no charge transfer occurs. The sum is determined by the relative Raman cross-sections, as reported in Figure 13.2. In some cases, the

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difference in cross-section is so important that the final spectrum is dominated by a single constituent. Obviously, in this case, Raman spectroscopy cannot bring much information and other techniques should be considered for the characterization of this kind of hybrid. To avoid such a situation, the idea is to tune the excitation wavelength to play with the relative dependence of Raman cross-sections with the wavelength. For instance, if we consider a hybrid with sp2 and sp3 carbon heterostructures, the Raman spectrum using visible excitation will be dominated by the sp2 component and the sp3 signature will be hardly visible. Using a UV laser, the Raman cross-section of sp3 carbon is significantly increased and it is now possible to detect its contribution. As mentioned above, the choice of the laser is thus a key point. In other cases, the original Raman responses of each compound are very similar and the identification of each contribution requires a careful deconvolution of all modes. This usually happens when the spectral signatures are dominated by D and G bands, which is rather common in most carbon structures. If so, Raman spectroscopy must be performed with a sufficiently high spectral resolution to perform the deconvolution. Once again, it might be interesting to tune the excitation wavelength in relation with each compound cross-section, which will facilitate the identification of each contribution. If the interaction between the carbon structures increases compared to the previous situation, the final Raman spectrum remains the sum of both contributions but some slight shifts and changes in peak intensity are observed. As regards shift, we can consider mainly two situations. In the first case, the interaction induces strain. This happens for instance if chemical bonds are formed between structures characterized with different carbon– carbon bond lengths. This situation is similar to the strain induced in heteroepitaxy. As Raman spectroscopy is very sensitive to strain, this scenario will induce shifts in the final spectrum. Basically, for a tensile strain, this shift is towards lower wavenumber, the opposite for a compressive strain. In the second case, a charge transfer (see previous parts) induces shifts in the Raman signature. This feature is very common when two heterostructures are mixed, in particular if they have very different electronic properties. Obviously, charge transfer and strain have similar consequences in a Raman spectrum, a point that potentially can induce misinterpretation and mistakes. For most carbon heterostructures, Raman shifts are induced by a charge transfer mechanism, but the reader should keep in mind that a contribution from strain is also possible. It is very difficult to quantitatively discriminate between those two features but some methods have been proposed, in particular for graphene.51 As mentioned above, it is also possible to observe changes in relative intensity of Raman modes. A typical case is the increase of the D band intensity. As explained earlier, the intensity of this band is directly related to defaults in the sp2 structure. The chemical process for the preparation of the heterostructure can induce a larger number of defects increasing the D band intensity. This feature is commonly observed in carbon hybrids.

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Up to now, we have not mentioned the appearance of new bands in the Raman spectrum. This is not the case for most carbon hybrids but for a small number of them new modes may appear. They have generally a weaker intensity compared to the original modes but their presence is a sign of a specific interaction. As such, they can offer novel information and their detection should be a priority. Their position can be predicted by numerical calculations, but with a very weak intensity their detection is a real experimental challenge. Once again, the careful choice of the excitation wavelength is essential. Finally, we will now consider that the final Raman signature of all-carbon hybrids belongs to one of the following three classes: – Class 1: The final spectrum is the sum of each different allotrope contribution, with intensity ratio governed by their relative crosssections. – Class 2: The final spectrum is the sum of each different allotrope contribution with slight shifts mainly due to charge transfer between the compounds and with changes in intensity ratio. Changes in relative intensity are also possible. – Class 3: New modes are present in the final spectrum due to specific interactions between the allotropes. We will review each class separately in more detail below.

13.3.1

Class 1: The Final Spectrum is the Sum of Each Different Allotrope Contribution

This situation is the most common. The simplest case corresponds to the detection of the Raman spectrum of only one compound, without any change in its spectral signature. This is what Woszczyna et al. observed for a heterostructure composed of amino-terminated carbon nanomembrane and single-layer graphene for which only the signature of graphene is visible.107 In addition, its spectrum is almost identical to isolated graphene, except for a change in the full width at half maximum of the G band. It is clear that for this situation, Raman spectroscopy cannot supply much relevant information. Another example of class 1 hybrids is the nanobud. This heterostructure is formed when a fullerene is covalently bonded to the outer surface of a singlewall carbon nanotube. Its typical Raman spectrum is shown in Figure 13.12. Due to the resonance mechanism, the carbon nanotube has a much higher Raman cross-section compared to the fullerene and the spectrum is logically dominated by its modes. Low-frequency RBM D and G bands are thus easily observed. Nevertheless, the Ag(2) mode from C60 is also present, located between the D and G bands and with a very low intensity. A very weak peak at around 270 cm1 is also present and related to the Hg(1) mode of C60.108 The final spectrum is a mere sum of the contributions from C60 and SWNT,

Raman Spectroscopy Characterization of Carbon Materials

Figure 13.12

337

Typical Raman spectrum of nanobud carbons recorded at 532 nm. Adapted from ref. 109.

confirming that nanobuds belong to class 1. Notwithstanding, firstprinciples calculations have suggested that new Raman modes should be observed, depending on the final geometry, but their experimental observation has not been reported yet. This is also the case for the graphene/nanotube hybrid for which the 2D band is larger110 and can be decomposed in a contribution centered at 2660–2670 cm1 from the carbon nanotube (CNT) and another one, with a lower intensity, located at 2700 cm1 and attributed to graphene.111 This is a typical case of a spectrum requiring a careful deconvolution for the identification of each contribution. Nanotube/carbon nanofiber composites also belong to this first class. They are characterized by the standard D and G bands112,113 but the identification of each contribution is hardly possible. Maybe the measurement of the 2D band could answer this question but it has not been reported in the literature to our knowledge. Other carbon hybrids are also characterized by the D and G bands: nanographene/carbon nanofibers,114 graphene/carbon nanochain,115 graphene oxide/nanotube,116 and reduced graphene oxide nanoribbon/nanotube.117 In most cases, the authors do not go further than the ID/IG ratio calculation, highlighting the presence of defects in the hybrid structure. As explained in the previous sections, it would be possible to get much more information by simply broadening the spectral range, increasing the spectral resolution (to detect subtle shifts) and using different excitation wavelengths.

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Chapter 13

Class 2: Slight Changes in the Sum

An example from this second class is the C60/graphene composite. Its Raman spectrum is shown in Figure 13.13 in comparison with pure graphene. Once again, we can notice immediately that the contribution from C60 is very weak, the Ag(2) mode being hardly visible.119 The main point is the shift of both G and 2D peaks of graphene. Jnawali et al. reported that this shift is due to hole doping.118 First-principles calculations have confirmed that the electronic properties of such hybrids can be tuned depending on whether the C60 is simply covalently bonded to graphene or fused onto the monolayer.120 It is thus clear that the electrical interaction between fullerenes and graphene is strong and Raman spectroscopy is a simple and fast method for its characterization. The charge-transfer mechanism can also induce disappearance of some modes. This is the case for carbon nanodot/nanotube hybrids: Strauss et al. observed that for SWNTs with two RBM modes only one is present after the integration of the nanodots.121 They suggested that this feature is induced by a shift of the resonance energy confirming the charge injection into the valence band of CNTs. The shifts of both G and 2D bands confirmed this hypothesis. As mentioned above, in some cases the vibrational signature of one compound cannot be detected due to its too small Raman cross-section. This is the case for instance with graphene/nanodiamond composites. Diamond has a low Raman efficiency compared to graphene and the final Raman spectrum is purely composed of the D, G and 2D bands from graphene. Nevertheless, the G band is slightly shifted due to doping.122 In the

Figure 13.13

Typical Raman spectrum of a C60/graphene composite recorded at 532 nm, adapted from ref. 118. Data corresponding to graphene are added as a reference.

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specific case of boron-doped nanodiamond with graphene, Sankaran et al. detected a slight contribution at around 480 cm1 associated to boronmodified carbon nanosheets.123 To enhance the contribution of the lower cross-section compound, the simplest way is to tune the excitation wavelength. Varga et al.124 used this method with diamond/carbon nanotube composites, with four excitation wavelengths: 785, 633, 442 and 325 nm. The diamond sp3 signature at E1330 cm1 could be observed only with the UV excitation (325 nm). For the other wavelengths, the spectra were dominated by D and G bands of sp2 carbon. This observation confirms the crucial role of the excitation wavelength. This point is usually not taken into account as most of the Raman measurements of carbon hybrids reported in the literature are performed with only one excitation wavelength.

13.3.3

Class 3: New Modes Are Present

Very few carbon hybrids belong to the third class. It must be said that the new modes usually have a weak intensity and their detection remains an experimental challenge. Peapods are very similar to nanobuds, except that the fullerenes are inside the tube and no specific bond is formed. At first sight, their Raman spectrum is the sum of the signature of the fullerene and the tube.65 The Raman signature has been theoretically predicted125 and the authors did not mention the appearance of specific modes. Nevertheless, Zou et al.126 observed new modes using a near-infrared excitation source (830 nm), whereas those features were not observed with a visible source (488 nm). Once more, this result demonstrates the crucial role of the excitation wavelength in Raman experiments. As those modes are localized between the RBM and the D bands, they called them Intermediate Frequency Modes (IFM). They can be seen in Figure 13.14. The authors proposed to associate those modes to

Figure 13.14

The intermediate frequency modes of a peapod recorded at 830 nm, adapted from ref. 126. Modes associated to the tube, the fullerene and specifically to the heterostructures are evidenced.

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pentagonal and hexagonal radial vibrations and related their spectral signature to specific tube-pentagon and tube-hexagon interactions depending on the temperature. Based on this hypothesis, the authors concluded that C60 molecules are localized inside the tube with a preferential hexagon-tohexagon orientation. This particular study showed that Raman spectroscopy of carbon hybrids, used with the appropriate conditions, can supply much more information than the simple presence of unidentified defects alone.

Figure 13.15

The three different classes of Raman signature of all-carbon heterostructures and associated examples. Typical Raman spectra adapted from ref. 109, 110, 112, 114, 118, 121, 122, 124 and 126 are shown for each specific case.

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In conclusion, Figure 13.15 summarizes the three different classes of Raman response of all-carbon heterostructures. The association of each carbon hybrid to its specific class here is simply based on the available literature. In the above, we have clearly demonstrated that the Raman analysis requires specific experimental conditions (high resolution, broad spectral range, various excitation wavelengths). In the future, new carefully planned measurements gathering all those conditions could lead to more precise Raman spectra and it would then be possible to confirm, or when needed to change, the class associated to each different hybrid.

Acknowledgements CP wants to acknowledge Raul D. Rodriguez for sharing the original data published in ref. 70.

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Final Remarks OXANA V. KHARISSOVA AND BORIS I. KHARISOV* ´noma de Nuevo Leo ´n, San Nicola ´s de los Garza, Universidad Auto ´xico NL, Me *Email: [email protected]

Final Remarks A host of classic and some less-common synthesis methods have been applied for the synthesis of carbon–carbon composites. In particular, these hybrids have been produced by high-temperature techniques such as CVD/ pyrolysis/carbonization (for instance, CNT/GO by ethanol decomposition, rGO/CNFs from cellulose/graphene oxide mats, or graphite/CNTs from CNT/ polyaniline composites), their combination with next-step nitrogen-plasma treatment (G/CNTs), or by middle-temperature methods under pressure (solvothermal techniques, G/CNS/G). Another group of methods is solutionbased (solution mixing/evaporation, graphite–fullerene composites; slow diffusion of a GO suspension in 2-PrOH through C60 solution in m-xylene, rGO–C60). Some processes need subsequent oxidation and reduction steps, such as, for example, the preparation of a SWCNT/G composite through the intermediate formation of graphite oxide and graphene oxide. Also, techniques such as air-spraying and flash light irradiation methods are sometimes applied. The resulting 3D hybrids possess novel properties, which are frequently not a sum of those of their counterparts, in particular improved electrical conductivity (for example, for graphite by 144% for 2 wt% SWCNT samples compared to samples without SWCNTs), better ‘‘solubility’’ (example: CNDs can behave as dispersing agents for CNTs solubilization), improved gas adsorption capacity and metal insertion ability. These properties can be All-carbon Composites and Hybrids Edited by Oxana V. Kharissova and Boris I. Kharisov r The Royal Society of Chemistry 2021 Published by the Royal Society of Chemistry, www.rsc.org

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varied upon addition or elimination of oxygen-containing and other groups. For instance, the presence of electrically insulating GrO within a SWCNT network strongly enhances electrical conductivity, while rGrO, even though electrically conductive, suppresses electrical conductivity, revealing the ‘‘indirect’’ role of the oxide groups. To elucidate in detail these and other effects, DFT calculations are frequently applied, for instance, for predicting the graphene–fullerene proposed hydrogen storage performance. As a result of carbon–carbon hybrid formation, the counterparts can remain practically unchanged (for instance, the GO in some composites can maintain its typical wrinkled paper-like structure) or distorted in distinct grades. It is also known that different parts of a hybrid system possess distinct reactivity, such as, for example, the case of fullerene and nanotube sections in the nanobuds. In addition, the types of carbon allotropes, such as graphene, carbon nanotubes, activated carbon, and carbon quantum dots, have been used as effective co-catalysts to enhance the photocatalytic activities of semiconductors, making them widely used for photocatalytic energy generation and for degradation of pollutants. Their combinations could be useful in this, and in other areas, leading to a wide range of novel applications of carbon allotropes and carbon-based nanomaterials. Among the described hybrids with other carbon allotropes, those of MWCNTs, and especially graphene, are obviously of utmost interest to researchers. On the basis of the novel properties mentioned above, a series of new applications have been proposed for these composites. Among these uses, Li-ion insertion and storage (for example, into the composite of MLG microsheets and carbon nanoflowers) and the area of supercapacitors are the most frequently mentioned, showing very promising volumetric values of capacitance, energy, and power density, particularly for flexible devices. Another large field of application is the catalysis of distinct important industrial processes, such as, for example, steam-free direct dehydrogenation of ethylbenzene to styrene (GO/ND), selective hydrogenation of acetylene in the presence of abundant ethylene (ND/G for Pd catalyst preparation), or hydrazine catalytic decomposition (carbon nanofiber/graphite-felt composite for a highly loaded Ir catalyst). Analytical and sensor-based techniques are also in progress using the C–C composites, such as those for the recognition of three kinds of biomolecules (dopamine, thioridazine, L-tyrosine), or for the electrochemical detection of catechol, hydroquinone, and uric acid, as well as for the removal/remediation of different ions by classic sorption methods and electrosorption. Less frequently mentioned applications are those using specific composites as lateral heat spreaders (G/CNFs) for commercial portable electronics, also as thermal absorbers (MWCNT/GO), and for orthopedic and dental applications (CNFs/GO).

Subject Index AC. see ascorbic acid (AC) AFM. see atomic force microscopy (AFM) all-carbon heterostructures, Raman spectroscopy of, 334–341 class 1, 336–337 class 2, 338–339 class 3, 339–341 amorphous and glassy carbon composites, 245–250 amorphous carbons, 330–331 ascorbic acid (AC), 40 atomic force microscopy (AFM), 117, 151 b-cyclodextrin/GrO/CNT composites, 9 bio-imaging, carbon quantum dots in, 159–160 carbon allotropes, Raman spectroscopy of, 321–334 diamond (single-crystal and polycrystalline) materials, 331–334 doping and strain effects on graphene, 325–326 double resonance mechanism, 325 fullerenes, 328 graphene, 322–325 graphite, 322–325 low-ordered carbons, 328–331 multi-layer graphene, 322–325 single-walled carbon nanotubes, 327–328

carbon-based hybrid nanofillers, 288–289 carbon-based nanofillers, 282–289 carbon–carbon nanocomposites, 233–250 amorphous and glassy carbon composites, 245–250 CNT hybrids with nongraphene nanocarbons CNT composites with carbon nanofibers, 235–238 CNTs–carbyne systems, 234–235 nanoballs/nanospheres hybrids, 239–242 nanoring (nanotori) composites, 242–243 xerogels, 243–245 carbon dot-based composites, 115–136 carbonized polymer dots, 124–126 carbon nanodots, 121–124 carbon quantum dots, 121–124 challenges to, 134–135 future perspectives of, 135–136 graphene quantum dots, 116–120 recent progress in CND-based composites, 131–134 CPD-based composites, 131–134

350

carbon dot-based composites (continued) CQD-based composites, 126–129 GQD-based composites, 130–131 carbon dot–carbon nitride composite (CDs/CN), 175–184 carbon dots (C-Dots). see carbon quantum dots (CQDs) carbonized polymer dots (CPDs), 124–126 carbon nanocages (CNCs) graphene hybrids with, 21–22 carbon nanodots (CNDs), 121–124, 174–175 composites, for hydrogen energy generation, 173–193 carbon nanofibers (CNFs), 11–13, 15–16, 287–288 CNT composites with, 235–238 carbon nanoparticle allotropes, 162–163 carbon nanostructure/graphene oxide composites, production of, 31–47 carbon nanotube/graphene hybrids preparation of, 57–61 electrophoretic deposition, 59 layer-by-layer self-assembly deposition, 58 multi-step chemical vapor deposition, 59–60 one-step chemical vapor deposition, 61 solution method, 59 vacuum filtration method, 58 synthesis by chemical vapour deposition, 55–72 carbon source, effect of, 62–63 carrier gas, effect of, 64–65

Subject Index

catalyst, effect of, 61–62 field-effect transistors, 67 fuel cells, 65–66 future prospects, 72 growing time, effect of, 63–64 growth temperature, effect of, 63–64 lithium batteries, 69–71 supercapacitors, 67–69 transparent and flexible electrodes, 67 carbon nanotubes (CNTs), 32, 283–285 double-walled, 235 with graphite, composites of, 4–10 hybrids with non-graphene nanocarbons CNTs–carbyne systems, 234–235 multi-walled, 4, 82, 84, 103, 145, 283, 284, 288, 293, 348 single-walled, 4, 145, 243, 283, 284, 327–328, 347, 348 carbon nitride quantum dotgraphene nanocomposite (CNQD/G), 190–193 carbon quantum dots (CQDs), 121–124 applications of, 164–165 bio-imaging, 159–160 catalysis, 160–161 drug delivery, 160 optronics, 161–162 sensors, 157–159 carbon nanoparticle allotropes, 162–163 characterization and properties of photophysical characterization, 153–157 structural characterization, 151–153 derived from natural carbon sources, 142–165

Subject Index

hybrids of, 150–151 synthesis techniques bottom-up approach, 146–147 post-synthetic variations, 147–151 top-down approach, 144–146 carbon source, effect on chemical vapour decomposition, 62–63 Carbon Xerogels (CX), 243–245 carbyne contained CNTs, 234–235 carrier gas, effect on chemical vapour decomposition, 64–65 catalysis, carbon quantum dots in, 160–161 catalyst, effect on chemical vapour decomposition, 61–62 C-Dots. see carbon dots (C-Dots) CEE. see cross-link enhanced emission (CEE) chemical oxidation, 145 chemical vapor deposition (CVD), 4, 21 catalytic, 11 graphene–CNT hybrid nanofillers, 294 multi-step, 59–60 one-step, 61 synthesis of CNT/graphene hybrids by, 55–72, 81–83 carbon source, effect of, 62–63 carrier gas, effect of, 64–65 catalyst, effect of, 61–62 field-effect transistors, 67 fuel cells, 65–66 future prospects, 72 growing time, effect of, 63–64 growth temperature, effect of, 63–64 lithium batteries, 69–71 supercapacitors, 67–69 transparent and flexible electrodes, 67

351

CNCs. see carbon nanocages (CNCs) CND-based composites, 131–134 CNDs. see carbon nanodots (CNDs) CNFIG. see CNF interpenetrated graphene (CNFIG) CNF interpenetrated graphene (CNFIG), 14–15 CNFs. see carbon nanofibers (CNFs) CNT/graphite/zinc oxide composites, 8 CNT/MoSe2/amorphous carbon hybrid, 237, 239 CNT/PDMS composites, 9 CNT/polyaniline composites, 4–5 CNTs. see carbon nanotubes (CNTs) Co/Co2P@ACF/CNT hybrid, 237 co-doping heteroatoms, 148–150 counter electrodes, recent advances in, 101–107 covalent interaction, 36 CPD-based composites, 131–134 CPDs. see carbonized polymer dots (CPDs) CQD-based composites, 126–129 CQD–GO composites, 37–44 CQDs. see carbon quantum dots (CQDs) cross-link enhanced emission (CEE), 124, 125 CVD. see chemical vapor deposition (CVD) CX. see Carbon Xerogels (CX) DA. see dopamine (DA) density functional theory (DFT), 16, 18, 180, 206–207, 219, 262 DFT. see density functional theory (DFT) diamond (single-crystal and polycrystalline) materials, 331–334 dimethyl sulfoxide (DMSO), 122 direct methanol fuel cells (DMFCs), 91, 93–94 DMFCs. see direct methanol fuel cells (DMFCs)

352

Subject Index

EDLCs. see electrical double-layer capacitors (EDLCs) EIS. see electrochemical impedance spectroscopy (EIS) electrical double-layer capacitors (EDLCs), 86 electrochemical impedance spectroscopy (EIS), 87 electrochemical oxidation, 144–145 electrophoretic deposition (EPD), 59, 83–84 EPD. see electrophoretic deposition (EPD) Euler’s theorem, 201

FT-IR. see Fourier transform infrared (FT-IR) FTO. see fluorine doped tin oxide (FTO) fuel cells (FCs) direct methanol, 91, 93–94 graphene/CNT-based nanocomposites in, 65–66, 90–94 proton exchange membrane, 91–92 fullerenes, 259–261, 328 building blocks, 200–202 clusters, 199–226 fusing clusters of, 209–215 femtosecond lightinduced fusion, 213–215 laser heated fullerenes, coalescence of, 210–212 low energy bi-molecular collisions, 213 geometric structures of, 204–206 ion-cluster collisions molecular growth processes, 220–226 multiply charged clusters, 215–220 production, 202–204 stability of, 206–209 structure of, 259

FCs. see fuel cells (FCs) femtosecond light-induced fusion, 213–215 FETs. see field-effect transistors (FETs) field-effect transistors (FETs) carbon nanotube/graphene hybrids in, 67 FL. see fluorescence (FL) emission fluorescence (FL) emission, 154–156 fluorine doped tin oxide (FTO), 100 Fourier transform infrared (FT-IR), 152–153

GCE. see glassy carbon electrode (GCE) g-CNQDs. see graphitic carbon nitride quantum dots (g-CNQDs) glassy carbon electrode (GCE), 41 GNP. see graphene nano-powder (GNP) GNs@AC composites, 26 GNS. see graphene nano-sheets (GNS) GO. see graphene oxide (GO) GOQD–rGO composite, 42, 43 GO/rGO–CD composites, 39–40

DMSO. see dimethyl sulfoxide (DMSO) dopamine (DA), 40, 41 doping effects on graphene, 325–326 double resonance mechanism, 325 double-walled carbon nanotubes (DWCNTs), 235 drug delivery, carbon quantum dots in, 160 DSSCs. see dye sensitized solar cells (DSSCs) DWCNTs. see double-walled carbon nanotubes (DWCNTs) dye sensitized solar cells (DSSCs), 99–108 components of, 101 fabrication of, 100

Subject Index

GQD-based composites, 130–131 GQDs. see graphene quantum dots (GQDs) graphene, 259–261, 285–287, 322–325 carbon nanobifillers of, 261–263 doping and strain effects on, 325–326 multi-layer, 322–325 structure of, 259 see also individual entries graphene/carbon nanochain composites, 13–15 graphene/CNT-based nanocomposites, design of, 77–95 recent growth in energy-related applications, 85–94 fuel cells, 90–94 supercapacitors, 86–90 synthesis method, 79–85 chemical vapour deposition, 81–83 electrophoretic deposition, 83–84 in situ reduction, 84–85 graphene–CNT hybrid nanofillers chemical vapour deposition, 294 synthesis of layer-by-layer deposition, 292–293 solution processing, 290–292 vacuum filtration, 293–294 graphene–CNT reinforced hybrid polymer nanocomposites, mechanical properties of, 278–308 graphene–CNT hybrid nanofillers forms of, 294–297 synthesis of, 289–294 nanofillers, 280–289 carbon-based nanofillers, 282–289 synthesis of, 297–300

353

graphene–fullerene-based nanomaterials in high performance applications, prominence and future visions of, 269–271 graphene–fullerene hybrids, 261–263 graphene nanobuds, 17 graphene nano-powder (GNP), 102 graphene nano-sheets (GNS), 103, 104 graphene oxide (GO), 32, 285–287, 329–330 chemical structure of, 33–34 functionalization of covalent interaction, 36 non-covalent interaction (physical adsorption), 35–36 self-assembly of, 36–37 structure of, 259 synthesis methods, 33–37 graphene oxide/CNC composites, 21–22 graphene oxide/CNF composites, 15–16 graphene oxide/ND composites, 22–25 graphene quantum dot–graphene composite (GQDs/G), 184–187 graphene quantum dot–graphitic carbon nitride composite (GQD/CN), 187–190 graphene quantum dots (GQDs), 32, 39–40, 116–120, 162 graphite, 322–325 graphite/CNT composites, 4–10 graphite/CNTs/metal oxide composites, 8 graphite/CNT/UHMWPE composites, 10, 11 graphite/fullerene composites, 10–11, 16–21 graphite hybrids, 4–12 graphite quantum dots, 32

354

graphitic carbon nitride quantum dots (g-CNQDs), 162–163 growing time, effect on chemical vapour decomposition, 63–64 growth temperature, effect on chemical vapour decomposition, 63–64 HCNT/rGO/S composites, 45 high resolution-transmission electron microscopy (HR-TEM), 117, 151 hollow carbon spheres (HCSs)@Fe3O4@carbon nanodisc composites, 241 HR-TEM. see high resolutiontransmission electron microscopy (HR-TEM) Hummers’ method, 32, 33, 37 hydrogen energy generation, CND composites for, 173–193 fabrication carbon dot–carbon nitride composite (CDs/CN), 175–184 carbon nitride quantum dot–graphene nanocomposite (CNQD/G), 190–193 graphene quantum dot– graphene composite (GQDs/G), 184–187 graphene quantum dot–graphitic carbon nitride composite (GQD/CN), 187–190 hydrothermal treatment, 146–147 incident-photon-to-current-conversion (IPCE), 106 in situ reduction (ISR), 84–85 ion-cluster collisions molecular growth processes, 220–226 multiply charged clusters, 215–220

Subject Index

IPCE. see incident-photon-to-current-conversion (IPCE) ISR. see in situ reduction (ISR) laser ablation, 144 laser heated fullerenes, coalescence of, 210–212 layer-by-layer (LBL) deposition graphene–CNT hybrid nanofillers, 292–293 self-assembly, 58 LBL. see layer-by-layer (LBL) deposition Lerf–Klinowski model, 34 lithium batteries, carbon nanotube/ graphene hybrids in, 69–71 low energy bi-molecular collisions, 213 low-ordered carbons, 328–331 M-HAP. see mineralized hydroxyapatite (M-HAP) microwave irradiation, 147 mineralized hydroxyapatite (M-HAP), 16 MLG/CNFl composites, 26, 27 MnO2–3D-CNT/graphene/Cu hybrid, 89–90 molecular growth processes, 220–226 multi-layer graphene, 322–325 multiply charged clusters, 215–220 multi-step chemical vapor deposition, 59–60 multi-walled carbon nanotube/ graphene oxide nanoribbons (MWCNT@GONR), 101–102 multi-walled carbon nanotubes (MWCNTs), 82, 84, 103, 283, 284, 288, 293, 348 chemical oxidation, 145 electrochemical oxidation, 145 with graphite, composites of, 4 MWCNTs. see multi-walled carbon nanotubes (MWCNTs)

Subject Index

nanoballs/nanospheres hybrids, 239–242 nanodiamonds (NDs), 22–25 nanofillers, 280–289 carbon-based, 282–289 carbon-based hybrid, 288–289 classification of, 282 nanoring (nanotori) composites, 242–243 NDs. see nanodiamonds (NDs) nitrogen-doped nanoonion/rGO composites, 27 NMR. see nuclear magnetic resonance (NMR) non-covalent interaction (physical adsorption), 35–36 non-graphene nanocarbons, CNT hybrids with CNT composites with carbon nanofibers, 235–238 CNTs–carbyne systems, 234–235 nuclear magnetic resonance (NMR), 120 one-dimensional CNT decorated 2D reduced GO composite, 99–108 counter electrodes, recent advances in, 101–107 dye sensitized solar cells, 100–101 future prospects of, 107–108 one-step chemical vapor deposition, 61 optronics, carbon quantum dots in, 161–162 ORR. see oxygen reduction reaction (ORR) oxygen reduction reaction (ORR), 41, 42, 66 PCE. see power conversion efficiency (PCE) PEMFCs. see proton exchange membrane fuel cells (PEMFCs) photostability of CQDs, 156–157

355

polymer/graphene–fullerene nanocomposites, 263–267 polymer/graphene oxide–fullerene nanocomposites, 267–269 polymer nanocomposites, 278–280 porous graphene (PG)/fullerene composites, 18 power conversion efficiency (PCE), 100 production methods, of carbonbased nanocomposites, 38 proton exchange membrane fuel cells (PEMFCs), 91–92 Pt/rGO-polyethyleneiminemultiwalled CNT hybrid, 92 Raman spectroscopy of all-carbon heterostructures, 334–341 class 1, 336–337 class 2, 338–339 class 3, 339–341 basic principle of context, 319–320 effect on solids, 320–321 of carbon allotropes, 321–334 diamond (single-crystal and polycrystalline) materials, 331–334 doping and strain effects on graphene, 325–326 double resonance mechanism, 325 fullerenes, 328 graphene, 322–325 graphite, 322–325 low-ordered carbons, 328–331 multi-layer graphene, 322–325 single-walled carbon nanotubes, 327–328 reduced graphite oxide (rGrO), 5, 13, 15, 329–330 functionalization of, 35 non-covalent interaction (physical adsorption), 36

356

reduced holey graphene oxide film (RHGOF), 45 reversible hydrogen electrode (RHE), 42 rGrO. see reduced graphite oxide (rGrO) rGO/CNT composites, 44–47 rGO/fullerenes nanocomposites, 18–21 rGO/NDs composites, 25 RHE. see reversible hydrogen electrode (RHE) RHGOF. see reduced holey graphene oxide film (RHGOF) SAED. see selected area electron diffraction (SAED) selected area electron diffraction (SAED), 117 sensors, carbon quantum dots in, 157–159 single-heteroatom doping, 147–148 single-walled carbon nanotubes (SWCNTs), 234–235, 243, 283, 284, 327–328, 347, 348 chemical oxidation, 145 with graphite, composites of, 4, 5 solution method, 59 solution processing, 290–292 strain effects on graphene, 325–326

Subject Index

supercapacitors, graphene/CNTbased nanocomposites in, 67–69, 86–90 SWCNT/GrO composites, 5 SWCNTs. see single-walled carbon nanotubes (SWCNTs) 3D CNT/graphene/Cu hybrid, 89, 90 3D N-GQD/MoS2–rGO nanohybrid, 42 transparent and flexible electrodes, carbon nanotube/graphene hybrids in, 67 UA. see uric acid (UA) ultrasonic treatment, 145–146 uric acid (UA), 40 UV–vis absorption, 153–154 vacuum filtration method, 58 graphene–CNT hybrid nanofillers, 293–294 WCSs@SnOx@C synthesis, 241 XPS. see X-ray photoelectron spectroscopy (XPS) X-ray diffraction (XRD), 151–152 X-ray photoelectron spectroscopy (XPS), 152 XRD. see X-ray diffraction (XRD)